FFR_chap04.txt

CHAPTER 4
TECHNOLOGY OF AQUEOQOUS SUSPENSIONS*
4-1. SUSPENSIONS AND THEIR APPLICATIONS IN REAcTORst

4-1.1 Introduction. With the inception of the aqueous homogeneous
power reactor program at Oak Ridge National Laboratory in 1949, the
primary choice of fuel was highly enriched UQsS04 solution. Use of en-
riched uranium alleviated to some extent the need for strict neutron
economy, but it was found that at high temperature (250 to 300°C) U02804
solutions were more corrosive than pure water and were subject to a high-
temperature instability. As a result, a secondary development effort was
initiated at ORNL to determine the potentialities of suspensions of solid
uranium compounds as reactor fuels. The principal efforts were directed
at forms of UO3, because it was believed that under reactor operating
conditions the trioxide would be the stable oxidation state. Considerable
progress was made in studies of the oxide and its slurries, and in develop-
ment of equipment for circulating the slurries at concentrations of several
hundred grams per liter in 100-gpm loops at 250°C. In addition, a criticality
study was carried out with enriched UO3 - H2O In water to obtain assur-
ance that local fluctuations in concentration or settling would not unduly
affect nuclear stability [1].

In 1955, the UO3 work was set aside so that effort could be concentrated
on ThO2 suspensions, which are at the present time believed to be the only
suitable fluid homogeneous fertile material for use in an aqueocus homo-
geneous thorium breeder. The ultimate of this effort at ORNL has been set
at a two-region, ThOg, homogeneous, power breeder.

In the following sections of this chapter a detailed account is given of
the studies on UOsz slurries and ThO» slurries, and a description of the
present state of knowledge of their production, properties, and utilization.
The discussion will be based largely on work done in the United States, but
it should be kept in mind that studies on fuel- and fertile-material sus-
pensions have been conducted in other countries—in particular, in the
Netherlands and in Great Britain—and that exchanges of concepts and
data have aided the U.S. efforts.

*By J. P. McBride and D. G. Thomas with contributions from N. A. Krohn,
R. N. Lyon, and L. E. Morse, Oak Ridge National Laboratory.
iBy R. N. Lyon.
128
4-1] SUSPENSIONS: THEIR APPLICATIONS IN REACTORS 129

4-1.2 Types of suspensions and their settled beds. Two-phase systems
of solids in liquids may be classified in several ways. On the basis of size
of particle a phase is said to be colloidal when it is sufficiently finely di-
vided to permit the surface attraction forces of the particles to exert a
strong influence on the mechanical properties of the material as a whole.
If, in addition, the particles are dispersed in a liquid and are sufficiently
small so they diffuse throughout the liquid due to their Brownian motion
in a normal gravitational field, they are referred to as sols. Sols are not
resolved in an ordinary microscope but are usually recognizable in an
ultramicroscope. The particles of a sol are usually less than 0.5 mjcron in
length for materials of density near that of water, while particles in a ThO»
sol are usually less than 0.05 micron. Particles in a sol may join to form a
random network of some strength having a semisolid appearance and called
a gel, or they may coalesce into loose and relatively independent clouds of
joined particles referred to as flocs. Suspensions of flocs or of particles which
are large enough to settle are referred to as slurries.

In some sols the particles are stabilized by the preferential attraction of
the suspending liquid to the particles’ surface. These are referred to as
lyophillic sols. In other sols the thermodynamically stable condition is a
flocculated or a gelled state, but the particles are held apart by electro-
static forces produced by ions which collect on and near the surface of the
particles. Sols of elements, oxides, and salts (including the oxides of
uranium and thorium) are generally of the latter type and are referred to
as lyophobic sols.

Although dispersions having particle sizes greater than 0.5 micron do
not form sols, since they are too large and have too large a mass to be ap-
preciably affected by Brownian motion, the particle surfaces may exhibit
some colloidal properties which are most pronounced when the particles
are very close together. The magnitude of these forces is such that spherical
particles of ThO2 which are 10 to 15 microns in diameter appear to show
only slight tendency to flocculate, while cubic or platelet forms of ThO» and
U0z - H20 of 1 or 2 microns on a side do show a marked tendency to
flocculate.

When slurries having particles that are either relatively large or have a
high ionic charge on their surface (and hence have little tendency to floc-
culate) settle, the settled bed density approaches about 50 to 709, of the
particle density. The bed resuspends only slowly and is not easily de-
formed rapidly. An example of such a bed is settled sand. In general, such
beds may exhibit dilatancy, which means that the bed must expand to be
deformed, and the apparent viscosity of the bed increases as the rate of
shear increases. ThO: spheres of more than 5 to 10 microns settle to beds
of this type.

Flocculated slurries settle to a concentration at which the flocs become
130 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHAP. 4

joined, and from that point the particles are in part supported by indirect
contact with the walls and bottom of the container through the floc strue-
ture. The resulting settled bed may continue to compact indefinitely at a
slower and slower rate. Such beds behave more or less in a plastic fashion,
and may even exhibit a pronounced yield stress (i.e., shear stress required
before an appreciable deformation rate is initiated).

In UO3 - H20 slurries of concentrations up to several hundred grams per
liter, the yield stress is less than 0.1 1b/ft? and the slurries are almost of
Newtonian character. A breeding blanket requires ThOs slurries con-
taining 500 to 1500 grams of thorium per liter, and in these concentrations
the yield stress varies from 0 to well over 1 1b/{t?, depending in part on the
concentration, on the shape and form of the oxide particle, and on the
presence or absence of certain additives. Settled beds of both ThO2 and
U0z - H20 may be either colloidal and plastie, or much more dense, non-
colloidal, and apparently dilatant.

4-1.3 Engineering problems associated with colloidal properties. The
colloidal behavior of some slurries offers three types of problems: high
vield stress, caking, and sphere-forming tendencies. To these may be
added a general instability in the colloidal behavior which changes with
time, chemical treatment, and general previous history. A high yield stress,
in turn, offers three main engineering problems: high velocity required to
produce turbulence, a tendency to plug tubes, and a tendency to increase
the difficulty of mixing in a large blanket or reactor vessel.

A cake 1s defined as an accumulation of particles on part of the surface
of the system in so dense and rigid a form that it cannot be deformed with-
out fracture. A mud is a similar dense accumulation of greater yield
strength than the circulating slurry but which can be deformed without
fracture of the aceumulation.

Caking and mud formation have occurred occasionally in circulation
loops, causing plugging of tubes, hydraulic or mechanical unbalance in a
centrifugal pump, and drastic reduction in heat transfer to or from the
walls. These phenomena appear to be due to compaction of flocculated
slurry under the influence of stresses due to flow. The rigidity of a cake
or mud appears to be inversely related to the particle size.

Muds have been observed in both ThOs and UQ3 - H20 platelet slurries.
Cakes have been observed in ThO: slurries. In one case, a - to 2-in.
layer of ThO; cake was built up on essentially all parts of a 3-in. 200-gpm
circulating system. The cake resembled chalk in strength and consistency.
It had a density of about 5.5 g/ce.

Sphere formation occurs when a circulating slurry contains very fine
particles and appears to resemble the formation of a popcorn ball [2].
Spheres ranging in size from about five to several hundred microns in
4-1] SUSPENSIONS: THEIR APPLICATIONS IN REACTORS 131

diameter have been made. Prolonged circulation causes an equilibrium
size to be reached which depends on the circulating conditions and the
starting material. Spheres have been formed from certain types of ThO2
in suspension, but no spheres or cakes have been observed in circulating
UO3 - HoO. This may be due to the fact that the greater solubility of
UO3 - H20 prevents its remaining as extremely fine particles.

Since cake, mud, and sphere formations appear to be a result of colloidal
behavior, effective control of the colloidal behavior of a slurry will prob-
ably control the formation of such aggregates.

Plastic materials which exhibit a high yield stress require a high velocity
before they become turbulent. It is not uncommon for ThOz slurries to
require 30 to 40 ft/sec velocity for the onset of turbulence. (It is of interest
to note that velocity by itself appears to be the most important criterion
for whether a given plastic will be in laminar or turbulent flow [3]—as op-
posed to the product of tube diameter and velocity, which is the cor-
responding criterion for a given Newtonian liquid.) Turbulence is, of
course, important in maintaining the suspension and in providing good
heat transfer.

Control or elimination of colloidal, flocculating properties of slurries
can be accomplished by additives, particle-size control, or particle-shape
control. Electrolyte additives which attach to particle surfaces may pro-
vide so strong a charge that particles cannot approach to form a floc or
gel. In true lyophobic sols, the most effective additives are often those
which produce ions of atoms or radicals of the same type as those com-
posing the particle. Tor example, ThO2 or Th(OH)s sols of up to
4000-g/liter concentration are easily made by the addition of Th(NO3)s
solution to freshly prepared Th(OH)s. On a somewhat similar basis, addi-
tives which may form partially ionized or lyophillic surface compounds are
often effective. Very small additions of H2C204, Na2SiO3, NazPOy,
and NaAlO» have been found effective at room temperatures in producing
free-flowing Newtonian slurries from high-yield-stress ThO2 muds. In
most cases the effect is lost at elevated temperatures. However, coating of
the particles with a silicone compound and firing to convert it to 5102 has
produced slurries which appear to remain unflocculated at temperatures
up to 300°C [4].

NasHPO4 or NaHoPO4 added to UOg - H20 platelet slurries has pre-
vented the formation of muds in regions where they normally form. The
latter additive is preferred, since NasHPO4 solution appears to attack
stainless steel in the presence of oxygen at elevated temperatures [5].

Evidence indicates that as the particle size increases the colloidal effects
become reduced. In the case of UO3 - H20 an equilibrium size is reached
due to the continuous abrasion of the particles and subsequent recrystalli-
zation. In ThOg, and probably in UQOg slurries, the crystals are much more
132 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHAP. 4

resistant to abrasion, but at the same time the recrystallization in solution
is essentially nil. ThO2 produced by calcination of a plate or cube form of
Th(C204)2 retains the plate or cube form, but the particle is composed
of smaller crystals of ThO.. Violent agitation causes crystals in the
particles to break apart, after which they are free to exhibit the col-
loidal behavior of the finer particles. The higher the calcination temper-
ature, the larger the ThOz crystals. By calcination at 1600°C, crystals
in excess of 0.25 micron are produced, whereas caleination at 650°C gives
crystals of 50 to 100 A. In both cases, the over-all particle size may be of
the order of 0.5 to 5 microns. Kven the material calcined at the higher
temperature exhibits a discouragingly high yield stress, however, at the
preferred concentration for a two-region breeder blanket (~1500 g/liter
at room temperature). Part of the difficulty may be due to a possible sys-
tematic arrangement of charges on the particle surface which causes actual
attraction and binding of particles into an unusually strong floc in a pre-
ferred orientation [6].

If spherical particles are used, the amount of possible common surface
between particles is limited and the permanence of mutual attachment is
correspondingly limited. In addition, if the surface is essentially uniform,
the charge arrangement might tend to repel rather than attract other
particles. Furthermore, a spherical shape permits the particles to be
larger without excessive abrasion.

Spheres made in circulating systems, as mentioned above, exhibit
Newtonian flow properties at concentrations up to about 3500 to 4000
g/liter at room temperature. At the present time they are rather friable,
although their density is of the order of 8.5. Calcining the spheres at very
high temperatures in furnaces, oxyacetylene flames, or electric arcs gives
them considerably greater integrity while retaining their noncolloidal
properties. However, at high firing temperatures (1800°C) a tendency of
the spheres to break up, owing presumably to internal stresses, has been
noted [7]. Dense spheres have been produced in small quantity by spraying
a Th(OH)4 gel which is subsequently hardened, dried, and fired.

Thus it appears that the colloidal behavior of ThO. slurries may be
minimized through the use of spherical particles, larger particles, coating
the particles with silica or some other compound, or by the use of some as-
yet-unperfected additive.

4-1.4 Engineering problems not associated with colloidal properties.
Sedimentation. One of the principal noncolloidal problems encountered
with suspensions or slurries is sedimentation which, in a flowing system, is
offset by any upward component of liquid velocity. By definition, in
idealized laminar flow in a horizontal conduit there is no upward velocity
component and the rate of settling should proceed at the same rate as in a
4-1] SUSPENSIONS: THEIR APPLICATIONS IN REACTORS 133

stagnant vessel. In a vertical tube with laminar flow, particles tend to be
more concentrated near the center of the tube [8]. One possible explanation
may be that the particles spin in the velocity gradient near the wall in a di-
rection which would cause them to move toward the center of the tube as
the liquid moves past them. It appears possible that a similar effect could
reduce the sedimentation rate in a horizontal tube. In an inclined tube
the solids will collect on the lower side of the tube, while a channel of low
solids content will appear along the upper side. The resulting radial vari-
ation in density and possibly in viscosity distorts the normal parabolic
velocity profile and complicates computation of the local sedimentation
rate.

In turbulent flow, fluctuating radial velocities will tend to cause diffusion
from more concentrated regions to regions of lower concentration in
competition with the settling due to gravity. Although a relatively strong
diffusion tendency exists across the bulk of the conduit, the diffusion rather
suddenly begins to be damped near the wall, although some random radial
veloeity fluctuations may occur essentially up to the wall. The distance
from the wall at which damping begins to become pronounced is of the
order of about 1 mm for water for velocities at 1 ft/sec, in tubes larger than
about L to % in. in diameter, and it is generally recognized by hydrody-
namicists as being approximately proportional to v/u, where u, is the
mean velocity of the fluid and » is the kinematic viscosity, about
10~3 ft2/sec. Since particles in the slurries under discussion are of micron
size, rather than large fractions of millimeters, those particles which find
themselves well inside this layer above a horizontal surface may tend to
build up a sediment which becomes the solid surface from which the more
or less damped layer must be measured. This process can continue until
the diameter and velocity are reduced to the point where flow becomes
laminar and the tube is choked off completely, or until the local shear
stress becomes high enough to drag the particles along and an equilibrium
bed thickness is approached. At a given distance from the wall in the
damped region the local velocity of the liquid is proportional roughly
to the square of the mean velocity through the conduit, and the radial
velocity fluetuations in the damped regions vary in about the same pro-
portion. Therefore the mean stream velocity at which sediment tends to
accumulate is rather sharply defined for a given slurry.

It follows directly that a slurry which is flowing horizontally cannot
keep its particles in suspension unless the flow is turbulent, or unless it
is an extremely stiff locculated mud, and even in turbulent flow a minimum
velocity may be required to prevent the accumulation of a sediment along
the bottom of a tube or conduit. Such sedimentation can occur in a reactor
blanket vessel in regions where the net velocity is extremely low.

Abrasion. A second problem is that of abrasion, which is more serious
134 TECHNOLOGY OF AQUEOUS SUSPENSIONS {cHAP. 4

in the case of thorium slurries than uranium slurries. UOg - H20O crystals
are relatively soft, and when they strike a stainless-steel wall, they tend
to break without damaging the surface of the wall. ThO; particles, on the
other hand, appear to be sufficiently hard to abrade the protective oxide
film on stainless steel and perhaps to abrade the base metal. Continuous
removal of the film exposes the bare metal to corrosive attack by the hot
water and, in some cases, causes very serious attack. The attack is most
severe 1n local recirculation regions associated with flow separation and
in regions of sudden acceleration and direction change, such as in orifices,
pump impellers, and pump seal rings. Materials such as zirconium and
titanium, which form very hard oxide films, and essentially noncorrodible
metals such as gold and platinum show more resistance to ThQ-» slurries
than do stainless steels. Reduection of particle size, use of round particles
rather than sharp-cornered particles, and design of components and
piping to avoid regions of high velocity or high acceleration will reduce
attack.

4~1.5 Systems and components for using slurries in reactors. The pre-
ceding discussion implies several general conelusions regarding systems and
components for using slurries. For example, the over-all system should be
kept as free of extraneous circuits and secondary lines as possible. If
possible, the system should always tend to drain into a sump whenever
circulation stops, to prevent plugging by settled beds. Smaller side lines
should, where possible, be attached to the top of a horizontal run of the
main system to allow solids to settle into the main stream, and a minimum
size for smaller lines should be established based on the expected strength
of any reasonably conceivable settled bed. In ThO: or UQg2 systems, all
elbows should be of at least moderately large radius, and sudden constric-
tions such as orifices should be avoided.

Mechanical pumps for ThO2 or UOs must be leaktight, and should be
capable of handling the hard abrasive particles; this includes adequate
hydraulic design, the use of particularly resistant materials in regions of
high fluid acceleration or high velocity, and either very abrasion-resistant
bearings or essentially complete isolation of the bearings from the slurry.
Valves must be designed to operate in spite of the abrasive nature and the
settling or compacting properties of the solids; they must be leaktight to
the outside. The trim must be unusually abrasion- and corrosion-resistant
to ensure continued internal leaktightness.

Pressure-sensing instruments should not, in general, include long blind
passages of small diameters, which might easily become plugged: in ThOq
slurries, flowmeters should not include rapidly moving parts in contact
with slurries, unless bearings which are not affected by the abrasive action
of the solids are used.
4-2] URANIUM OXIDE SLURRIES 135

All vessels should be provided with a means of resuspending solids which
have settled to the bottom. In some vessels, a simple mechanical agitator
is sufficient. In others, steam or gas sparging can be used. In still other
cases, more sophisticated systems may be required involving, for example,
injection of an external liquid or slurry stream to induce strong internal
recirculation currents.

The following sections of this chapter represent a brief, condensed
progress and status report of the work on suspensions. This effort is con-
tinuing at an accelerated rate as their potentialities are becoming more
clearly recognized and as the problems and difficulties are becoming more
rapidly overcome.

4-2. UraNiUM OXIDE SLURRIES*

4-2.1 Introduction. Preliminary studies on uranium oxide slurries for
use in a plutonium-producer reactor were carried out in the period 1940-
1944 by Vernon, Hickey, Huffman, and others, first at Columbia University
and later at the University of Chicago as a part of the Manhattan Project.
This program was discontinued before the feasibility of uranium oxide
shurries could be established, but a large backlog of information on the
properties and slurry behavior of the uranium oxides was obtained. These
studies are reported in detail by Kirschenbaum, Murphy, and Urey [9] in
a still secret volume of the National Nuclear Energy Series (I1I, 4-B)
which should soon be declassified. In 1951 work on the development of
uranium oxide slurries was revived, primarily at the Oak Ridge National
Laboratory. These studies were terminated in 1953 before a satisfactory
slurry was developed. The results are reported by Blomeke [10] and, in a
1955 Geneva paper, by Kitzes and Lyon [11]. Since 1953, emphasis on a
slurry fuel has centered on the development of a thorium-uranium oxide
slurry [12,13].

4-2.2 Chemical stability of uranium oxides. Both the early Manhattan
Project work [9] and the ORNL work [10] indicated that uranium tri-
oxide would be the probable stable form of uranium oxide under the
radiolytic gas formed by the radiation-induced decomposition of water in a
reactor. Uranium dioxide in an aqueous slurry at 250°C was oxidized to
uranium trioxide in the presence of oxygen overpressure and even in the
presence of excess hydrogen gas. The extent of this oxidation depended on
the oxygen pressure, and seemed to be independent of the partial pressure
of hydrogen (Table 4-1). The extent of oxidation of U3Og to uranium tri-
oxide depended on both temperature and oxygen pressure. The presence

*Information taken from reports by J. O. Blomeke (Ref. 10) and A. 8. Kitzes
and R. N. Lyon (Ref. 11).
136 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHAP. 4

TaBLE 4-1

OxipATION OF [UQs SLURRIES upON HEATING UNDER
VARYING PARTIAL PRESSURE OoF HYDROGEN AND OXYGEN

 

 

 

 

 

 

 

 

 

 

 

Heating conditions (Gas pressure Uranium
oxidized,
Temp., °C Time, hr Hs, psi O3, psi %o

200 48 63.5 31.8 64.9

250 16 202 7.5

16 378 82 .2

24 70 61.5 78.4

24 70 175 91.4

TaBLE 4-2

Repuction oF UQOs3 SLurgries AT 250°C UNDER
VARYING PArTIAL PrEssurEs oF HYDROGEN AND OXYGEN

 

 

 

 

(Gas pressure Uranium
Heating time, hr reduced,
H., psi O3, psi %
20 263 26.3 0.5
2 378 — 1.39
24 70 35 0
68 527 26.3 0.4

 

 

 

 

 

 

of a partial pressure of hydrogen did not seem to markedly inhibit the
oxidation (Table 4-2). When a slurry of UO3 - H20 rods prepared by ther-
mal decomposition of uranium peroxide in water was heated at 250°C
under varying pressures of hydrogen and oxygen, it was unchanged in the
presence of a stoichiometric mixture of hydrogen and oxygen in the ratio
of water. It was only very slightly reduced by a tenfold excess of hydrogen
over the stoichiometric (Table 4-3). Reduction of the UO3z and UzOs
under pure hydrogen atmospheres was quite slow, although freshly oxi-
dized uranium species formed by treatment of UQgz with peroxide were
rather readily reduced with hydrogen [9].

4-2.3 Crystal chemistry of UO3. Uranium trioxide in an aqueous slurry
can exist as one of three hydrates, depending on the temperature at which it
4-2] URANIUM OXIDE SLURRIES 137

—

7

", Bipyramids //
/ '

/ -
’/ \ N //(///Rod lets

Platelets - - ;Z

 

- /
—
F1a. 4-1. UO3-H:0 crystal habits.
Air HQO
UOy - 4HpO mmmmmm )O3 eemm—). |)C3 - HyO
300°C 200-300°C (Rod|efs)
185-200°C Pulvenzed
H20 UOs3 ++
UQg ++ Ho0O
Pulvenzed 200-300°C
Air UOQ++ H2O
UO2(NO3) ) e YO ; wemmm—) ———— |0 . HyO
300°C 185-300°C (Plo?elets) 185-300°C " {Bipyramids}

Fia. 4-2. Preparation of UOs-H 0.

is maintained. In the earlier work [Ref. 9, pp. 45-49, 127-131] the mono-
hydrate was shown to be the stable form between 100 and 300°C. Four
crystalline modifications of the monohydrate were described: the a form,
“large six-sided orthorhombic tablets”; 8, “small six-sided orthorhombic
tablets’’; v, “rhombic(?) hexagonal rods”; and 6, “triclinic crystals with a
very complicated x-ray pattern.” The a and 3 were stable in water below
185°C and the v and ¢ above 185°C.

ORNL studies on preparation of uranium oxide hydrates agreed in
general with those reported in Ref. 9. The most important exception was
the inability to prepare a triclinic crystal resembling the 6-UQO3 - H2O.
Attempts to prepare this modification resulted in the formation of bipyra-
mids or platelets, depending on the conditions. Three allomorphic modi-
fications of the monohydrate were obtained, depending on the mode of
preparation and treatment (Figs. 4-1 and 4-2). A rodlet form of UO3 - H20
was produced when the anhydrous trioxide, formed by heating uranium
peroxide at 300°C, was heated in water at 185 to 300°C (Fig. 4-2). The
rodlets were also prepared by autoclaving at 250°C for at least 16 hr a slurry
of uranium peroxide containing less than 50 ppm uranyl nitrate as an im-
purity. A platelet form of UO3 - H2O appeared when the trioxide, made
138 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHAP. 4

TABLE 4-3

OxipaTioN oF UszOg SLURRIES oN HEATING FOR 24
Hours UNDER VARYING P.RTIAL PRESSURES OF
HybproGceN aND OXYGEN

 

 

 

 

Gas pressure Uranium
Temp., °C oxidized,
Hg, psi Og, psi Yo

150 51.8 25.9 63.2
170 59.5 29.8 71.7
200 63.5 31.8 90.1
295 67 33.5 92.6
250 — 35 88.0
250 175 35 85.1
250 70 17.5 75.2
250 70 175 96.8

 

 

 

 

 

 

by decomposition of uranyl nitrate at 300 to 400°C, was hydrated at 185
to 300°C. Pulverized uranium trioxide rodlets digested at 200 to 250°C
converted to the platelet form, whereas pulverized uranium trioxide
platelets digested at 150 to 200°C transtormed into rodlets. Crystals which
resemble truncated bipyramids were formed when either rodlets or platelets
were heated with water containing several hundred parts per million of
uranyl lons.

The rodlets were bright yellow in color, normally I to 5 microns in di-
ameter and 10 to 30 microns long; the platelets were pale yellow in color,
6 to 50 microns on edge and about 1 micron thick; the bipyramids were
also pale yellow in color and several hundred microns along each edge.

The rodlets appeared to be the same material as a y-UO3 - H20 re-
ported in Ref. 9 as having an orthorhombic structure. Unfortunately, cell
dimensions were not given in this reference, and it was impossible to
establish the identity without question. Zachariasen [14] reported the
cell dimensions of two different UO3 - H20 crystals but gave no information
concerning the chemical history of his samples. He indexed both of these
structures as orthorhombic and called them a— and 5-UOs3 - H20, inde-
pendently of the nomenclature of Ref. 9. From the cell dimensions given
by Zachariasen the positions of all possible lines in the x-ray diffraction
patterns were calculated, thus permitting a comparison to be made with
material prepared in the present studies. It was established from this that
the rods gave the same x-ray diffraction pattern as Zachariasen’s
a-UQ3 - H20 and that the platelets had the structure of his —UO3 - H20.
4-3] THORIUM OXIDE AND ITS AQUEOUS SUSPENSIONS 139

4-2.4 UO; - HpO slurry characteristics. Easily suspended slurries were
prepared of both the rodlets and platelets. On the other hand, the bi-
pyramids, because of their size, required violent agitation to keep them in
suspension. With rodlets or platelets, slurries could be prepared which
were dispersed and kept in suspension, by mild agitation, at both room
temperature and at higher temperatures, even though settling occurred in
stagnant water.

Shurries of the rodlets were pumped satisfactorily at temperatures below
200°C [15]. Slurries of the platelets, although easily pumped, had a tend-
ency to form soft cakes on the pipe walls at temperatures above 200°C [16].
The influence of the trace quantities of nitrate impurities which remained
in the “'purified”’ oxides was not investigated, however.

The solubility of pure UO3 - H20 in pure water is less than 10 ppm at
room temperature and is also low at high temperatures. As a result, pure
UO3 - HoO slurries were essentially neutral. The presence of soluble
uranyl salts of strong acids lowered the pH of the slurry, however, and
increased the solubility of the oxide. In the preparation of UOg by the
pyrolysis of UO2(NO3)e, for example, residual nitrate could not readily
be removed by a simple washing step, and slurries of such oxides released
nitrate when the crystals were broken down in a pumping system. Under
extreme conditions the increased uranyl ion coneentration in the supernate
caused serious crystal growth and the formation of hard cakes in stagnant
regions [17].

4-2.5 Zero-power reactor tests. The microscopic inhomogeneity of en-
riched rodlet slurry fuel was found to offer no serious difficulty in the
operation of a zero-power homogeneous slurry reactor [9]. In this reactor,
suspension was established by a propeller type of mixer located near the
bottom of the vessel. The reactor was extremely stable at any given stirrer
speed. Changing the stirrer speed produced a change in nuclear reactivity
which was attributed to a redistribution of the oxide when the stirrer
was moving slowly, and to a change in the shape of a vortex type of con-
cavity in the slurry when the stirrer was moving rapidly.

4-3. PREPARATION AND CHARACTERIZATION OF THORIUM OXIDE
AND ITS AQUEOUS SUSPENSIONS®

4-3.1 Selected properties of thorium oxide. Thorium oxide is a white,
granular, slightly hygroscopic solid with a fluorite structure (lattice
constant — 5.5859 4 0.0005) [18] and an x-ray density of 10.06. The
Chemical Rubber Handbook of Chemistry and Physics [19] gives 10.03 as
the density of thoria. Foex [20] gives pycnometric densities for thorium

*By J. P. McBride.
140 TECHNOLOGY OF AQUEQUS SUSPENSIONS [cHAP. 4

powders, prepared by firing the hydroxide, which increased with increasing
firing temperature (8.6 at 450°C; 9.4 at 725°C; 9.7 at 910°C), approaching
the x-ray density asymptotically. Foex also noted that the density of a
compacted bed of the thoria powder (3000 kg/ecm? pressures) increased
with firing temperature but remained much lower than the actual powder
densities, the ratio of pycnometric density to bed density changing from
1.46 to 1.40 over the firing-temperature range of 250 to 1000°C. Thoria
powders obtained from oxalate thermal decomposition had pycnometric
densities almost 1dentical with those prepared from the hydroxide for the
same firing temperature [21]. The melting point of thorium oxide has
been reported [22] to be 3050 4 25°C, and the boiling point has been
estimated [23] at 4400°C.

The bibliographies of reports available from the AEC on thorium oxide
in the list appended to this chapter provide sources of more detailed
information.

4-3.2 Preparation of thorium oxide. The principal method of preparing
thorium oxide for use in aqueous slurries has been the thermal decomposition
of the oxalate. Thorium oxalate, precipitated from thorium nitrate solution,
is erystalline, easy to wash and filter, and the oxide product is readily dis-
persed as a slurry. In addition, the oxide particle resulting from oxalate
thermal decomposition retains the relic structure of the oxalate, and hence
the particulate properties are determined by the precipitation conditions.
The mechanism by which the thermal decomposition takes place has been
quite widely investigated [24-26]. The following is proposed by D’Eye
and Sellman [26] for the thermal decomposition:

ThOs 4 2C0O2 + 2CO
ThC00: 5500

Th(CO3)2 + 2CO

Th02 + 2002

Properties of thorium oxide prepared by the thermal decomposition of
oxalate are discussed in detail in Articles 4-3.3 and 4-3.4.

A satisfactory oxide has also been prepared by the hydrothermal decom-
position of thorium oxalate as an aqueous slurry in a closed autoclave at

300°C{27]: Unidentified organic

and inorganic gases

>175°C
Th{(C204)2 + excess HoO ——> Th(OH)+ 4 2H2C>04

ThO;z - 2H20
4-3] THORIUM OXIDE AND ITS AQUEOQOUS SUSPENSIONS 141

This preparation is characterized by a very small particle size, approxi-
mately 0.02 micron, and low bulk density, and very closely resembles the
oxide from the thermal decomposition of oxalate after the latter has been
pumped at elevated temperatures.

A third method for the preparation of slurry oxide is the thermal decom-
position of thorium formate [28]. In this procedure, thorium nitrate in
solution is decomposed on adding it to concentrated formic acid at 95°C
[29,30]. The precipitated thortum formate is washed free of excess acid
and decomposed by calcination at 500 to 800°C. The oxide from the formate
procedure is similar in its slurry behavior to that produced by thorium
oxalate thermal decomposition; however, less is known about its handling
characteristics. Because of this, the oxalate preparation method is pre-
ferred at the present time.

Experience on the preparation of oxide by the direct calcination of
Th(NO3)4 is limited. The nitrate decomposes at about 250°C, but firing to
500°C is necessary to remove the last traces of nitrogen oxide decomposition
products. The hydrated salt goes through a plastic stage during caleination,
and the resulting oxide is sandlike and difficult to slurry. In the absence
of a grinding and size-classification step, direct caleination of the nitrate
in a batch process does not appear to be a promising preparation method
for preparing oxide for slurry.

An interesting method for producing submicron-size thorium oxide
directly from thorium nitrate is that developed by Hansen and Minturn
[31]. Their method consisted of the combustion of an atomized solution of
thorium nitrate in an ethanol-acetone mixture and collection of the resulting
thorium oxide smoke.

Micron-size thorium oxide may also be prepared by the hydrothermal
decomposition of a thorium nitrate solution at 300°C. The product from
the preparation is a free-flowing powder [32]. At temperatures much below
300°C the rate of hydrolysis is quite slow.

Brief studies made with thorium hydroxide indicated [33] that it is
probably not a good source material for the production of slurry oxide. As
precipitated from nitrate solution, the hydroxide formed a bulky precipitate
which was hard to filter and wash, was amorphous to x-rays, and contained
considerable nitrate impurity. Drying at 300 to 500°C yielded a crystalline
oxide product which was difficult to slurry. Autoclaving a slurry of the
hydroxide (without previous drying) at 250°C gave a bulky slurry (settled
volume 300 to 500 g Th/liter) exhibiting a characteristic ThO» x-ray
diffraction pattern.

4-3.3 Large-scale preparation of thorium oxide. In the present method
(Fig. 4-3) [34] for making thorium oxide in a pilot plant operated by the
Chemical Technology Division at Oak Ridge National Laboratory, 1 M
142

 

Precipitation
10-15°C
Oxalate Addition, 3 hr
Digestion At 75°C—6 hr

 

TECHNOLOGY OF AQUEOUS SUSPENSIONS

First Addition 125 gal

e ————— A —
1.0 M Thorium Nitrate

Second Addition 275 gal

 

 

 

 

 

 
 
 
 
 
   

 

 

 

P ————
1.0 M Oxalic Acid

 

[cHAP. 4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   
  

 

 

 

   
     

 

280 b/doy " Ciassfeaton
thori . 110 Ib/day i
orium ' 85% Yield Dewatering
! \ Supernate
(SDIxalcne : Redispersion I 65 ib/day
urry | and ! 85 % Yield
Demineralizer Water Wash | | Decantation
3 Times, 25 gal Each ' {2 Times) . Waste Filtrate
: Recyc||e 170 gal 10-3 g Th/liter THO,
: Hee
Thorium Oxalate Cake - | supernate ! Cakes
i : - !
i D'SZ?‘:'OF' 170 gal. 2.8x10-3M Oxalic Acid
Filtration . | Decantation | | 5 liter/kg of THO,, Per
100 Ibs/day {4 Times) Dispersion
LT T T :
i Third
Waste Filtrate ) T 1600°C  |Waste Heel 11% Calcination
475 gal, 1073 g Th/liter Caldined (s re) | 650°C, 13/4 hr
THO 400 tb/day
First Calcination 2
Stepwise
180°C, 2 hr Second
380°C, 2 hr o ) Calcination
450°C, 1-3/4 hr | 650°C Calcined  |1600°C, 4 hr 4
90 Ib/day Thorium Oxide 35 Ib/day S;ro uc:
97 % Yield 96 % Yield ipmen
% Yie AL 85 % Yield

Fic. 4-3. Thorium oxide pilot plant chemical flowsheet. Percent yields based
on initial thorium input.

solutions of thorium nitrate and oxalic acid are mixed in an agitated tank
with controlled temperature, addition rate, and order of addition. In the
first step, all the thorium nitrate is added to the precipitator, after which
the oxalic-acid solution is added over a period of 3 hr with the reagents
held at 10°C by external cooling. The slurry of precipitated thorium oxalate
is digested for 6 hr at 75°C and then pumped to a vacuum filter where the
solid is separated from the mother liquor and washed three times with
demineralized water. The oxalate cake on the filter is air-dried and is then
loaded on trays for the first caleination.

In the first calcination the air-dried thorium oxalate is heated succes-
sively at 180°C for 2 hr, at 380°C for 2 hr, and at 650°C for 1.75 hr. The
material is then packed on a tray for the second calcination, and heated
at 1600°C for 4 hr.

The 1600°C-calcined thorium oxide normally contains about 109% of
particles larger than desired (>5 microns). These oversize particles are
removed by classification, i.e., by suspension of the thorium oxide in oxalic-
acid solution (pH 2.6) to a ThO2 concentration of 100 to 200 g/liter. The
suspension is stirred, and the mixture is then allowed to stand for 5 min
4-3] THORIUM OXIDE AND ITS AQUEOUS SUSPENSIONS 143

before the supernate is decanted. Coarse material (5 to 7 microns) is
separated by setting the supernate withdrawal rate at 0.5 in./min liquid
level drop. The withdrawn thorium oxide is dispersed and decanted again
twice to ensure removal of oversized particles. The thorium oxide that
settles to the bottom is also redispersed and decanted three more times to
separate the considerable fraction of the fine particles that settle with the
heel or are imperfectly dispersed. This procedure removes nearly all the
thorium oxide smaller than 5 microns, and the final product contains only
1 or 29, of particles greater than 5 microns. This material is then refired at
650°C to decompose the oxalic-acid dispersant before being used in en-
gineering studies.

Oxide prepared in this way has an average particle size of 1 to 3 microns
and has handled well in high-temperature engineering loop tests at slurry
concentrations as high as 1500 ¢ Th/kg H2O. Removal of the oversize
particles has decreased the erosive attack on loop components to essen-
tially what would be observed with water alone (see Section 6-7). At
1500 g Th/kg H»0), slurries of average particle sizes = 1 micron have moder-
ately high vield stresses (0.5 to 1 lb/ft?). Lower-yield-stress slurries are
obtained with the larger particles.

Previous engineering experience with slurries of oxide prepared similarly
but with final firings at 650 and 800°C [35] showed them to possess an
extremely high yield stress at concentrations greater than 750 g Th /kg H,0,
and an occasionally bad caking characteristic [36]. Firing at 1600°C
appears to have in large part removed or substantially diminished the
caking tendency [37].

4-3.4 Characterization of thorium oxide products. Although thorium
oxide 1s a very refractory substance, it is well known that such properties
as its density, catalytic activity, and chemical inertness depend on the
conditions of its formation. With particular references to preparation from
the oxalate, the firing temperature has a marked effect on the ease of forma-
tion of colloids [38]. Beckett and Winfield [25] concluded from electron
micrographs of oxide residues that the initial oxalate erystal imposes on
the residual oxide a mosaic structure of thin, spongy, microerystalline
laminae all oriented in very nearly the same direction. Foex [20], investi-
gating the rate of change in density as a function of the firing temperature
for oxide prepared from the hydrous oxide, associated the density change
with crystallite growth among closely joined crystallites and observed that
no sintering of particles seemed to take place below 1000°C.

Oxide products from thorium oxalate decomposition are normally char-
acterized by their behavior as slurries. In addition, they have been char-
acterized by means of electron micrograph pictures, their nitrogen
adsorption surface areas, particulate properties as measured by sedimenta-
144 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHAP. 4

tion* [39], and average x-ray crystallite size by x-ray diffraction line
broadening{ [41].

Effect of preparation variables on the particulate properiies of thorium oxide.
The effects of thorium oxalate precipitation temperature, calcination
temperature, and calcination time on oxide properties were initially in-
vestigated by Allred, Buxton, and McBride [42]. Oxalate was precipitated
at 10, 40, 70, and 100°C from a 1 M thorium-nitrate solution by dropwise
addition of oxalic-acid solution and vigorous stirring. The precipitates
were fired at 400°C for 16 hr and successively at 500, 650, 750, and 900°C
for 24 hr. Eleetron micrographs of the oxide products showed particles of
the approximate size and shape of the original oxalate particles from which
they were formed. The particles of oxide prepared from 10°C-precipitated
material were approximately 1 micron in size and appeared quite uniform;
those from the 40°C material were 1 to 2 microns in size and less uniform.
A marked increase in particle size was observed for the oxide particles
prepared from the 70°C- and 100°C-precipitated materials, which were
4 to 7 microns in size. There was no change in particle shape or average
particle size and no evidence of sintering as the firing temperature was
increased from 400 to 900°C. Micrographs of shadow-cast oxides showed
that the particles prepared from oxalate precipitated at 10°C were almost
cubic in shape, with an edge-to-thickness ratio of about 3:2, and that those
from the 100°C material were platelets with an edge-to-thickness ratio of
6:1 (Fig. 44). The mean particle sizes determined by sedimentation
particle-size analyses were in good agreement with the data from the elec-
tron micrographs (Fig. 4-4).

Table 4-4 shows typical data obtained with the 10°C-precipitated ma-
terial. Included in Table 4-4 are the results of additional firings up to
1600°C. No increase in average particle size was noted even up to 1600°C.
However, in all oxide preparations, there was about 10 w/o above
5 microns in particle size.

*A radioactivation method for sedimentation particle-size analysis of ThO2 was
developed at ORNL [39]. The oxide was activated by neutron irradiation, dis-
persed at < 0.5 w/o concentration in a 0.001 to 0.005 M Nay4P207 solution and
allowed to settle past a scintillation counter connected to a count-rate meter and
a recorder. The secintillation activity, being proportional to thoria concentration,
was analyzed in the usual manner, using Stokes’ law, to give the size distribution
data. Independently, an analogous method for use with UO2 powders was devel-
oped at Argonne National Laboratory [40].

$The x-ray crystallite (as opposed to the actual oxide particle, which may be
composed of a great many crystallites in an ordered or disordered pattern) is de-
fined as the smallest subdivision of the solid which scatters x-rays coherently. The
crystallite size can, in principle, be determined from the width of the x-ray diffrac-
tion peak, the width being greater the smaller the average crystallite size [41].
4-3] THORIUM OXIDE AND ITS AQUEOUS SUSPENSIONS 145

   
   

(o). Cubic Shape From 10°C l;recgpit;:!ed Oxalate

{b) Platelet hcpe From 70° C Pre":ipi.fuléd Oxalate

Fia. 4-4. Particle shapes of thorium oxide prepared from oxalate thermal de-
composition.

Additional studies on the effects of the chemical and physical variables
of the batch oxalate precipitation step on the particulate properties of
thorium oxide from oxalate thermal decomposition were carried out by
Pearson, et al. [34,43]. In addition to precipitation temperature, the effect
of reagent concentration, stirring rate, reagent addition rate, and digestion
time were investigated. Most of the precipitations were made by adding
oxalic-acid solution to the thorium nitrate solution, which appeared to
give an oxide product of smaller average size and fewer oversize particles
than the reverse. Rather than being added dropwise, the oxalic acid was
146 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHAP. 4

TaBLE 4-4

CHARACTERISTIC PRrROPERTIES OoF THORIUM OXIDE FROM
OxALATE THERMAL DECOMPOSITION

(Precipitation temperature 10°C. Average particle size for all firings was between
1.1 and 1.4 microns.)

 

 

 

Final firing . . Average x-ray Specific
Firing time, e
temperature, crystallite size, surface area,
o hr ;
C A m2/’g
400 16 61 35.0
500 24 78 40.0
650 24 143 25.0
750 24 250 13.0
900 24 550 6.3
1000 12 803 4.3
1200 12 1100 3.0
1400 12 2000 2.4
1600 12 2000 1.0

 

 

 

 

 

 

introduced under pressure into the thorium nitrate solution through a
capillary tube projecting beneath the surface of the nitrate solution, the
tube exit being directly above the agitator blades.

In these experiments [34,43] it was found that the conditions for pro-
ducing oxide with uniform* particles of 1 micron average size and a low
percentage of particles greater than 5 microns were 1 M Th(NO3)s and
H.C204 concentrations, a 10°C precipitation temperature, a high rate of
oxalic-acid addition, and vigorous stirring. A draft tube with a variable
opening placed around the stirrer permitted a further control over the
average particle size, larger particles (2 to 4 microns) being produced by
increasing the rate of recirculation of the precipitating system through the

*Tt, is common practice to present the particle-size distribution in the form of a
plot of the logarithm of the particle size versus the cumulative weight percent
undersize on a scale based on the probability integral. If the particle-size data
follow a logarithmic probability distribution (as they usually do reasonably well
for most ThO; preparations), the resulting plot is a straight line and the steeper
the slope of the line, the less uniform the material. The 509 size in such a plot
is the geometric mean particle diameter (d,). The geometric standard deviation
(o,) is equal to the ratio of the 84.139 size to the 509 size (also 509 size: 15.87%
size) [44]. For thorium oxide prepared from oxalate precipitated at 10°C, o, was
1.2 to 1.4; og increased with increasing precipitation temperature.
4-3] THORIUM OXIDE AND ITS AQUEOUS SUSPENSIONS 147

 

 

 

 

D, micron
0.001 0.01 0.1 1.0 10
0 =T !1%11 TTTTI T T TO T TS
— AN =
— "\ p
L q\‘ );590
. AR N
o 0 AR =
T Or e\, -
s F {61\ 1 \ o —
g — S_(F F')E A a %4& —
8 | AN,
b
‘g 1= \%O |
I = X =
— &100° \ g 61
- ® 70° “ o D |
- O 40 V P Df
= N
& D-16 {Standard Oxide)
oL LU LT L mmi
10 100 1000 10,000 100,000

Average X-Ray Crystallite Size, A

F16. 4-5. Relationship between average crystallite size and specific surface area
for thorium oxide prepared by the thermal decomposition of thorium oxalate
(400 to 900°C firings).

draft tube. Decreasing the concentration of the oxalic-acid solution in-
creased the average particle size (0.7 to 0.8 micron at 1.5 M and 2 to
3 microns at 0.5 M) but did not affect the size distribution, which appeared
to be primarily temperature-dependent. The shape also changed, with
decreasing reagent concentration, from a cube to a platelet. Only the
particles about 1 micron in size were cubic in shape. Long digestion times
seemed to reduce localized sintering effects in high-fired oxides (1400,
1600°C) and hence lowered the fraction of oversize particles.

Effect of preparation variables on x-ray crystallite size and specific surface
areas of thortum oxide from oxalate thermal decomposition. Specific surface
areas of oxide prepared from oxalate thermal decomposition were much
larger than could be anticipated from the particle size, and decreased with
increasing firing temperatures (Table 4-4). Crystallite sizes as measured
by x-ray diffraction line broadening increased with increasing firing tem-
perature and corresponded very closely to the particle sizes estimated
from the specific surface areas (Fig. 4-5) [42]. The product of the surface
area in m?/g and the crystallite diameter in angstroms, at least for oxide
fired at =900°C, was approximately constant and equal to 3.6 X 103.

While the crystallite size was determined primarily by the firing tem-
perature, a relationship between crystallite size and firing time was also
148 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHAP, 4

established [42]. Log-log plots of crystallite size versus firing time for
oxides at various firing temperatures fit an equation of the form

D = tagl4=BIT)

where D is the crystallite diameter, ¢ the time, and 7' the absolute tem-
perature; «, A, and B are constants. The constant o appears to be char-
acteristic of oxide prepared by the thermal decomposition of thorium ox-
alate. For D in angstroms and ¢ in hours, the equation becomes

D= t0‘146(10'3515_ 5482/ T).

The temperature-dependent function, e!4~ 2/ is typical for rate processes
requiring an energy of activation, the constant B being equal to AH/R,
where AH is the heat of activation. The heat of activation was determined
to be 10.97 keal/g-mole [42].

Ozxides from the hydrothermal decomposition of thorium oxalate. Oxides
prepared by the hydrothermal decomposition of the oxalate [27] at 300°C in
a closed autoclave were found to be markedly different in their characteristic
properties from the thermally prepared materials. The precipitation tem-
perature of the oxalate had no effect on the final shape or size, and all
evidence of the original oxalate structure had disappeared. Sedimentation
particle-size analyses indicated particle sizes between 0.5 and 1 micron.

60 T

 

50 —

  
  

Thermally Decomposed
Fired at Designated Temp. tor 24 hr.

N
O
\

L)
<
T
|

Hydrothermally Decomposed
at 300°C for 24 hr,, then Fired
at Temp. for 24 hr. Period.

Specific Surface Area, m2/fg

N
o
\

 

 

0 1 1 l |
200 400 400 800 1000 1200 1400
Temperature,*C

Fi6. 4-6. Effect of oxalate decomposition method on thorium oxide surface area.
(Prepared from oxalate precipitated at 100°C.)

 
4-3] THORIUM OXIDE AND ITS AQUEOUS SUSPENSIONS 149

The hydrothermal oxides were composed of crystallites 200 A in size which
did not grow on subsequent firing at temperatures up to 900°C. Further-
more, the specific surface area of the hydrothermally prepared oxide
decreased less with increasing firing temperature than did the surface area
of the oxide from oxalate thermal decomposition, and was much larger
for the higher firing temperatures (Fig. 4-6).

Lffect of high-temperature waler on oxide properties. Experiments have
been carried out [45-48] which show that treatment with high-temperature
water has practically no effect on the characteristic properties of oxide
itself. Prolonged heating in water at temperatures up to 300°C did not
change the crystallite size, bulking properties, or abrasiveness of thorium
hydroxide caleined at 500 and 650°C [45], and tests showed no evidence
of hydrate formation [46,47]. Lack of crystallite growth probably indicates
an extremely low solubility of thorium oxide in water at 300°C. When
slurries of oxide caleined at 650, 750, 900, and 1000°C were heated overnight
at 300°C m the presence and absence of as much as 10,000 ppm of SO,
(pH about 2), there was no increase in the average sedimentation particle
size or x-ray crystallite size [49]. _

Effect of pumping on oxide properties. Oxides with particle size much
greater than 1 micron are degraded to an average particle size much less
than 1 micron on pumping at elevated temperatures,* while oxides com-
posed of cubie particles of about 1 micron show little change [50]. Surface
area increases of from 16 m?/g to 30 m2/g have been noted for some pumped
oxides, the latter figure being almost the theoretical maximum for the
measured x-ray crystallite size [51].

Pumping does not affect the average x-ray crystallite size of slurry
oxides [52]. Also, oxides which have been pumped as slurries, dried, and
then calcined, show relatively little cerystallite growth. From these con-
siderations it would seem that the crystallite size as measured by x-ray
diffraction line broadening represents the ultimate limit of the attrition
process due to pumping.

4-3.5 Sedimentation characteristics of thorium oxide slurries. All tho-
rium oxide-water slurries, except the very dilute suspensions, in the absence

*While most of the pumped slurries have been slow settling, composed of small
particles (=1 micron), and have vielded bulky sediments, on occasion the small
particles resulting from the attrition of the original slurry particles reagglomerated
to form large spheres 10 to 50 microns in size. This resulted in a slurry which set-
tled rapidly to a dense but easily resuspended bed. Sphere formation appeared
to be the property of specific oxide preparations and was observed most often with
800°C-fired material. Preparing the oxide in a particle size which does not degrade
on pumping {~ 1 micron) and firing at 1600°C to improve particle stability appears
in the initial pumping studies to have largely removed the problem.
150 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cuar. 4

0.74

 

0.12

0.10

©
O
@

Initial Settling Rate, ¢m/sec
o
o
o

0.04

0.02

 

 

 

0 | |

200 400 600 800
Oxide Concentration, g Th/kg HpO

Fra. 4-7. Effect of oxide calcination temperature and slurry concentration on
the room temperature settling rates of aqueous thorium oxide slurries.

of a dispersing agent exhibit “hindered” settling. All particles settle at a
constant rate, and there is a well-defined interface between the suspended
particle and the supernatant. This form of settling is called ‘“‘hindered”
because there is mutual interference of the particles in their motion, and
Stokes’ law does not apply to the settling of each particle.

Usually three zones of settling are observed: the initial zone during which
the motion of the interface from 1ts initial level is uniform (hindered settling)
and rapid; a compressive zone during which the motion of the interface
is also uniform, but much slower; and a final stationary state. The first
settling zone terminates when the interface reaches the settled mass of
flocs. The shurry concentration (grams of thorium per liter) at the
point of transition between the initial settling zone and the compressive
zone is termed the critical density or the critical concentration. The slurry
concentration in the final stationary state is called the settled concentration
(sometimes the bulk density). The compressive zone is characteristic of a
flocculated material and shows the rate of compaction of the settled bed
4-3] THORIUM OXIDE AND ITS AQUEOUS SUSPENSIONS 151

under the force of gravity. With discrete particles, or with ThOs in the
presence of a dispersing agent (0.005 M NasP207), the particles settle
directly into a permanently settled bed, and there is no compressive zone.

Room-temperature sedvmentation characteristics. 'The room-temperature
hindered-settling rates (for a given slurry concentration), critical concen-
trations, and settled concentrations of thorium oxide slurries (unpumped)
all increased with increasing firing temperature up to 1000°C of the oxide.
In addition, the hindered-settling rates also increased with decreasing
slurry concentration (see I'ig. 4-7) [53]. It should be noted in I'ig. 4-7
that all slurries are in compaction in the 500 to 800 g Th/kg H-O con-
centration range.

Table 4-5 shows the combined effects of slurry temperature and oxide
calcination temperature on the hindered-settling rate, Uy[50]. From the-
oretical considerations the product Uou, where u is the viscosity of water,
should remain constant for an oxide over a series of settling temperatures,
provided that no change has occurred in the particulate or dispersive char-
acteristics of the oxide. Ugu does remain fairly constant over the temper-
ature range 27 to 98°C. That changes do occur in oxide properties,
however, with increasing calcination temperatures up to 1000°C is shown
by the increase in the Upu product with calcination temperature. The
trend appears to reverse with the 1300°C-fired material, which also shows
a larger change in the Ugu product with temperature than do the lower-
fired materials.

High~-temperature sedimentation characteristics.  Slurry settling rates
at temperatures in excess of 100°C have been obtained in quartz tube
8 mm in diameter [54]. These data, obtained with a slurry of thorium
oxide prepared by a 650°C calcination of thorium formate [55], indicated
that the slurry was already in the compaction zone of settling above
500 g Th/kg H»0 at 200 to 300°C. At a concentration of 1600 g Th/kg H»0,
no settling occurred at temperatures above 100°C. The small diameter
of the tube probably affected the concentration at which the slurry went into
compaction.

Data on the sedimentation characteristics of thorium oxide slurries at
elevated temperatures in stainless-steel autoclaves, § in. in inside diam-
eter,* have been obtained by an x-ray adsorption technique [56].
Standard x-ray film was transported at a controlled speed past a vertical
slot in a lead shield behind which a bomb containing the settling slurry
and an x-ray source were placed. A typical radiograph of a setthing slurry
at an initial concentration of 250 g Th/kg HoO at 205°C is shown in
Ing. 4-8.

*This diameter should have no effect on slurry hindered-settling rate certainly up
to 400 g Th/kg H20 concentration and possibly even higher.
TABLE 4-5

ErrecT oF SLURRY TEMPERATURE AND OXIipE CALCINATION

TEMPERATURE ON SETTLING CHARACTERISTICS OF THO: SLurries, 500 g Th/kg H.0

(Uo = hindered settling rate, cm/sec)

 

Settling characteristics of oxides calcined at indicated temperature

 

 

 

 

 

 

 

 

 

 

 

 

Slurry Viscosity u
temp., of H20, 650°C 800°C 900°C 1000°C 1300°C
°C centipoise
[70 U()/J. Uo Uo,u, ('0 [,'o,u [‘Yo ('o,u. ('0 (.Y()/J
27 0.8545 0.031 0.029 0.038 0.032 0.06 0.05 0.10 0.08 0.03 0.03
50 0. 5494 0.045 0.025 0.07 0.036 0.09 0.047 0.15 0.08 0.07 0.04
75 0.3799 0.07 0.027 0.12 0.044 0.12 0.044 0.27 0.10 16 0.06
98 0.2899 0.10 0.028 0.16 0.048 0.15 0.042 0.34 0.10 0.20 0.06

 

 

 

 

 

¢Sl

SNOISNAdSAS SA0d00V J0 ADOTONHDIL

b "dVHD]
4-3] THORIUM OXIDE AND ITS AQUEOUS SUSPENSIONS 153

 

[ T | T T T T ! [ f I | I T T T !

Initial Concentration—250g Th/kg H0O

Temperature—205°C

Subsidence, cm

 

 

0 5 10 15 20
Time, sec

Fiac. 4-8. Typical high-temperature settling curve obtained with x-ray apparatus.

Temperature, °C

 

   

 

    
    
 

 

 

 

75 100 150 200 250 300
10 ‘ ¢
$
o
2
5
» W
=
S
o
11 microns
1.0 1.0
“J
2
e
£
w
- 2.6 microns
2
5 ]
o
o
£ ]
3
A
oA _
1.0 microns ]
0.7 microns ]
! !
0.01 f i ‘
30 26 22 18
T x10% (oK)

Fic. 4-9. Temperature-particle size effects on the settling rate of thorium oxide
sturries: 250 g Th/kg H»0.
154 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHAP. 4

 

   
 

 

 

 

5.0
\ \ I 1 I
™~
\ ~
NG N ]
~
~\‘\\\
20 ~ ]
\\
« 250°C
2
20 E-\\\ N |
E *' S~ —]
2 | ~ ]
2
© L
B ]
@ 200°C
= — —
S
T
8 — |
2
v
0.5 — ]
150°C
0.2 — |
100°C
0 200 400 600 800 1000 1200

Initial Concentration, g of Th/kg of H,0

F1g. 4-10. Effect of slurry concentration on settling rate.

The combined effects of temperature and particle size on slurry settling
rates for 250 g Th/kg H20 slurries of oxides fired at 800 to 900°C are
shown in I'lg. 4-9. Since the systems were flocculated, the hindered-
settling rates are much greater than those predicted from the mean particle
sizes by a simple application of Stokes’ law. The settling rate-temperature
dependence curve for the slurries containing the larger particles closely
paralleled the curve for the fluidity of water. Hence it may be assumed
that for these slurries little or no change occurred in the flocculating
characteristics with increasing slurry temperature. For the slurries con-
taining the smaller particles the curve is steeper, showing an apparent
increase in agglomerate size or density with increasing temperature.

The effect of slurry concentration on the settling characteristics of a
slurry at elevated temperatures is illustrated in Fig. 4-10. The bulk of this
slurry (>80 w/0) was made up of spherical agglomerates 10 to 15 mi-
crons in size. The data indicate that the slurry settling rate is an exponential
function of the concentration and has the form
4-3] THORIUM OXIDE AND ITS AQUEOUS SUSPENSIONS 155

Temperatures,®C

 

50 /5 100 150 200 250 300

10 = [ \ T T T m LY
- 18 s
O

- 46
5 C\)‘qe — ‘;'6
- o 4 2
" 2
[ N 1 3
w

Initial Settling Rate, cm/sec

 

0.02 — .

 

 

001 | | | 4 1 |
3 29 27 25 23 21 19 17

171 x10%, ok !

 

Fia. 4-11. Effect of firing temperature on high-temperature settling rates.
250 g Th/kg H0; particle size 1 micron; prepared from 10°C precipitated oxalate.

n%rzza(],

where Uy is the measured sedimentation rate, U, is the settling rate at
infinite dilution where Stokes’ law should govern the particulate settling, C
is the slurry concentration, and a is the slope of the logarithmic settling
rate-slurry concentration curve. Extrapolating the straight-line portion
of the curves to zero concentration and assuming an agglomerate density
of 5.0 g/ce, Stokes’ law particle diameters were calculated at the various
temperatures and were found to be approximately the same, again illus-
trating a lack of change in slurry flocculation characteristics with increasing
slurry temperature. The calculated particle diameters at 100, 150, 200,
and 250°C were 40, 45, 45, and 44 microns, respectively, far greater than
those obtained by sedimentation particle-size analysis in dilute suspension.

1
156 TECHNOLOGY OF AQUEOUS SUSPENSIONS [crAP. 4

 

CE T TN T T T T T T TTTTH

Initial Concentration — 250g Th/Kg H,0 i

——\/\ 250°C

FTTTT
|

t

|

  

§ 1.0 = —]
E — ]
g — ]
T [— —
& | —
g’ | 10 H250 4 Titration 200°C N
kS 8
A 6
0. 4 150°C
2

FTTTTIH

L

100°C

L Ll L PP PP [P

10 100 1000 10,000
ppm Sulfate
(Based on Dry Solids)

F16. 4-12. Effect of thorium sulfate on hindered settling rates of oxide slurries.

.02

 

 

 

The effects of oxide firing temperatures of 900 and 1600°C on slurry
settling rates at elevated temperatures are shown in Fig. 4-11. Slurry
concentrations were 250 g Th/kg H»0, and the oxide was prepared from
the 10°C-precipitated oxalate {cubic particles, ~1 micron). Settling data
obtained on the slurries after they had been pumped at elevated tempera-
tures are also included. The slurry of higher-fired material shows much
higher settling rates. Pumping does not greatly affect the settling rates of
slurries of either oxide above 150°C. The temperature dependence of
settling rates roughly follows the change in water fluidity, but the curve
for the 1600°C-fired pumped material deviates considerably below 150°C
and is much flatter.

Effect of additives on settling rates. The fluidity of concentrated non-
Newtonian slurries can be increased by the use of additives. Of particular
importance is a knowledge of the effect of temperature on the action of such
additives. Both sulfate and sodium silicate additions, either as thorium
sulfate or sulfuric acid, were investigated and found to change markedly
the settling rates and handling characteristics [50] of thoria slurries, the
relative effect of any additive concentration on a given slurry depending on
the slurry temperature.

Sodium silicate is a well-known deagglomerator, as well as a wetting
agent. Its addition to thick concentrated slurries at room temperature
increases their fluidity to that approaching water. It also markedly im-
proves their heat-transfer properties for certain flow conditions [58].

The effect of thortum sulfate additions on the high-temperature sedimen-
tation properties of a thoria slurry (250 g Th/kg H20) composed of spheri-
cal agglomerates approximately 15 microns in average size is shown in
4-3] THORIUM OXIDE AND ITS AQUEOUS SUSPENSIONS 157

10

 

\ l

—_— ‘ 300°C

\E

 

 

250°C

N

150°C

 

] ili IHIT

Settling Rate, cm/sec

[ L Ib1]

\ lllllil

 

 

 

100°C
|

Slow—wle—-0.007 —]
1 1000 10,000
5i04 Concentration ppm, based on ThO,

 

0.01

 

Fia. 4-13. Effect of sodium silicate on the hindered settling rates of oxide slurries.
250 g Th/kg H20, d, = 1 micron.

Fig. 4-12. An abnormal increase in the hindered-settling rate in the tem-
perature region of 150 to 200°C was obtained upon the addition of between
500 and 1000 ppm of sulfate (based on ThO3) and again at about 5000 ppm
of sulfate. The concentration region of 2000 to 3000 ppm of sulfate appears
to be one of a relatively low settling rate and good temperature stability.
It is of interest to note that in the operation of a high-temperature loop,
abnormal pump power demands at 200°C were observed when pumping
slurry containing 1000 ppm, and that increasing the sulfate concentration
to between 2000 and 3000 ppm removed the difficulty and permitted
operation at 300°C- [54].

Also appearing in Fig. 4-12 is the sulfuric acid titration curve obtained
with the standard slurry. It should be noted that the sulfate concentration
regions of temperature instability bracket the break in the pH curve and
the region of temperature stability occurs between pH 6 and 7.

Figure 4-13 shows the effect of sodium metasilicate additions on the
settling rate at elevated temperatures of a 250 g Th/kg H20O slurry of
800°C-fired oxide which had been micropulverized to an average particle
size of 1 micron. At silica eoncentrations of 5000 to 30,000 parts StO2 per
million ThOs, the settling rates are reduced (by comparison with the pure
slurry) at all temperatures up to 250°C, but the effect is more pronounced
at the lower temperatures.

It would appear from the studies carried out so far that the relative
dispersion effect for any additive concentration depends markedly on the
158 TECHNOLOGY OF AQUEOQOUS SUSPENSIONS [cHaP. 4

slurry temperature. The more pronounced effects are observed at tempera-
tures below 200°C. Above 250°C the effect of the additive becomes less
pronounced and sometimes even negligible from the point of view of its
effect on the settling rate. It may be, however, that at high temperatures
the additive could change the viscous properties of the slurry in a dynamic
system and not affect its settling rate in a quiescent state (essentially zero
shear stress) at the same temperature.

4-3.6 Status of laboratory development of thorium oxide slurries. Tho-
rium oxide prepared by the thermal decomposition of thorium oxalate ap-
pears suitable for a reactor slurry at concentrations up to 1500 g Th /kg HO.
Study of the preparation variables has indicated that a considera-
ble control can be exercised over the propertics of the slurry oxide.
Little is known as to what physical and chemical properties of thorium
oxide are important in determining its handling characteristics in water
at high temperatures, and studies are being made to determine these
properties. Sulfate and silicate additives have been shown in settling studies
to have a marked effect on the dispersion characteristics of slurries at
temperatures below 200°C, but at reactor temperatures the effect of the
additive on the settling rate diminishes and may be negligible. The effect
of additives on the rheological properties of slurries at reactor temperatures
has not yet been determined. Attempts are being made to obtain slurries
of ideal rheological characteristics by the preparation of oxide of controlled
particle size, shape, and surface activity.

4-4. ENGINEERING PROPERTIES®

4-4.1 Introduction. The major difference in deseriptions of the engi-
neering properties of aqueous suspensions (compared with aqueous solu-
tions) arises from the fact that suspensions may exhibit either Newtonian
or non-Newtonian laminar-flow characteristics. The consequences of the
possibility of these two different types of behavior modify conventional
heat transfer, fluid flow, and sedimentation correlations, and are important
in the design of large systems for handling slurries.

The magnitude of the effects that can be observed with non-Newtonian
slurries s illustrated in Iig. 4-14, where the critical veloeity for the onset
of turbulence is shown to be a strong function of the slurry yield stress and
almost independent of coefficient of rigidity and pipe diameter [59]. The
usefulness of laminar-flow measurements in characterizing different
suspensions, as well as the application of these constants to a variety of
correlations, will be given 1n the following sections.

*By D. (. Thomas.
4-4]

ENGINEERING PROPERTIES

159

 

20

Critical Velocity, ft/sec
o
E

 

I

    

n=l2ep

I l

 

 

0.5

1.0

Ty, Yield Stress, Ib/#t?

Fia. 4-14, Effect of slurry physical properties on velocity for onset of turbulence

p=100 Ib/ft3, D=1 to 24 in.

SpEciFic-HEAT CONSTANTS FOR THE

TABLE 4-6

OxipeEs oF Uranium aND THORIUM [61]

(Cp=a+ (b X 1079)T + ¢ X 105/T2)

 

 

 

 

 

 

 

Material a b c Terp. range, °K
H>0O 11.2 7.17 — —
ThO2 16.45 2.346 —2.124 to 1970
U0: 19.20 1.62 —3.957 to 1500
U305 (65) (7.5) (— 10.9) —
UO3 22.09 2.54 —2.973 to 900

 

 

 
160 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHAP. 4

4—4.2 Physical properties. The heat capacity of suspensions of solids is
commonly taken as the sum of the heat capacities on a weight basis of the
liquid and the solid at the bulk mean suspension temperature, each multi-
plied by its respective weight fraction in the suspension [60]. Specific-heat
values for pure thorium and uranium oxides are given in Table 4-6.

Thermal conductivity data for mixtures of solids have been correlated
[62] using Maxwell’s [63] equation:

Do + kp — 26(ko — k)

ko= ko 5p— ey + (ko — ky)

(4¢-1)

for the electrical conductivity of a two-phase system. This equation was
subsequently used to correlate conductivity data for suspensions of solids
in a gel [64]. However, the thermal conductivity of suspensions has not
been shown to be independent of the rate of shear [65]. The thermal
conduectivity of sintered thorium oxide having a bulk density of 8.16 g/cc
was found to decrease from 6.0 to 2.5 Btu/(hr) (ft) (°F) as the tempera-
ture was increased from 140 to 500°C [66,67].

Suspensions of solids in liquids may be either Newtonian or non-New-
tonian, depending primarily on particle size and electrolyte atmosphere
around the particles [68]. Newtonian and non-Newtonian materials are
classified and compared by means of shear diagrams in which the rate of
shearing strain, dv./dr, is plotted against the shear stress, 7. Newtonian
fluids are characterized by a shear diagram in which the rate of shearing
strain is directly proportional to the shear stress, as shown in Fig. 4-15, the
viscosity being given by:

o= —ge7/(dv,/dr), (4-2)

where the coefficient of viscosity, u, is independent of the rate of shearing
strain. On the other hand, non-Newtonian fluids have a variable viscosity
that is a function of the rate of shear and in some cases of the duration of
shear. Detailed discussions of non-Newtonian materials are available
elsewhere [69-72].

Einstein [73] has shown that the viscosity of dilute suspensions of rigid
spherical particles is a function of the volume fraction of solids in the
suspension and is independent of particle size, as shown in Eq. (4-3):

ps = (14 2.5¢), (4-3)

where ¢ is the volume fraction of solids in the suspension. It was assumed
that the system was incompressible, that there was no slip between the
particles and the liquid, no turbulence, and no inertia effects, and that the
ngacroscopic hydrodynamic equations held in the immediate neighborhood
4-4] ENGINEERING PROPERTIES 161

Newtonian
Material

 

  
 
       
     

Bingham Plastic
Material

dv _ a
3—7(7 Ary)

1

Pseudo-Plastic
Material

Dilatant
Material

, Rate of Shearing Strain, sec

dv
dr

 

 

 

/
/
!
|
!
l

 

7, Shear Slress,lblh2

F1c. 4-15. Classification of Newtonian and non-Newtonian materials by shear
diagram.

of the particles. Einstein’s treatment for dilute suspensions has been
extended by Guth and Simha [74], Simha [75], de Bruyn [76], Saito [77],
Vand [78], and Happel [79] to suspensions of higher concentrations. In
all these treatments the hydrodynamic interaction between particles was
considered, the extension usually taking the form of terms proportional
to ¢2 and ¢3, the terms to be added to the Einstein term 2.5¢. These
results are summarized in Table 4-7.

Theoretical treatments have also included such nonspherical particles
as ellipsoids [85] and dumbbells [86] and an empirical relationship has
been determined for rod-shaped particles [80]:

ts = (1 + 2.5Fp + 8F?¢* + 40F3¢?), (4-4)

where F is dependent upon the axial ratio, but not upon size or concentra-
tion of the rods. Values of F, determined with a Couette viscosimeter, are
given in Table 4-8.

The viscosities of suspensions of UQOsz - HzO rods and platelets with
uranium concentrations of up to 250 g/liter were measured [87] with a
modified Saybolt viscosimeter at temperatures from 30 to 75°C. There was
no detectable difference in viscosity between the slurries of rods and those
of platelets at these uranium concentrations. The viscosity values given in
 

 

 

162 TECHNOLOGY OF AQUEOUS SUSPENSIONS fcaAP, 4
TaBLE 4-7
CoMPARISON OF THEORETICAL AND EMPIRICAL EXTENSION OF
THE EINSTEIN RevLaTioN To HicHErR CONCENTRATIONS
(All relations are of the form p,= u(l+ A:1¢+ A202+ A3¢3)
fer-
Author Refer Ay As Aa Comments
ence
Vand [78] | 1.5 7.349 0 Considered mutual hydro-
dynamic interaction and
collisions between parti-
cles and pairs of particles
Guth and
Simha [74] | 2.5 14.1 0 Considered mutual hydro-
dynamic interaction; neg-
lected formation of pairs
de Bruyn {(76] | 2.5 2.5 2.5 | Considered only mutual hy-
drodynamic interaction
Saito [77] | 2.5 2.5 2.5 | Considered only mutual hy-
drodynamic interaction
Gosting and
Morris [84] | 3.35 0 0 Very dilute
Oden [83] | 2.5 30to60 | O Sulfur sols
Boutaric and | [82] | 2.5 75 0 As2S3 sols
Vuillaume
[80] | 2.5 9tol3} O Glass spheres in water
Eirich
[81] | 2.5 7.17 16.2 | Glass spheres in ZnlI,-
Vand water-glycerol solutions
Happel [79] | 5.5¢* — — | Each particle confined to a

 

 

 

 

 

 

 

 

 

 

cell of fluid bounded by
frictionless envelope

 

* = Interaction factor, 1.00 at ¢ = 0; 4.071 at ¢p = 0.50.

 
4-4] ENGINEERING PROPERTIES 163

TaBLE 4-8

VaLvues or CorrecTioN FacTor, F, ¥oRr
ErreEcT oF AxiarL Ratio or Robps
IN VISCOSITY OF SUSPENSIONS

 

 

 

L/D F
D 2.1
11 2.25
17 2.60
23 4.20
25 5.60
32 7.0
50 11.0
75 22
100 32
140 50

 

 

 

 

Ref. 87 were used to determine the value of the shape factor # in Eq. (4-4),
which was derived for rod-shaped particles. The value of I for the
U03 - HoO data is 2.4 + 0.7, corresponding to an L/D of about 14 (from
Table 4-8). This agrees very well with the dimensions reported for the
rodlets of from 1 to 5 microns diameter and 10 to 30 microns long [87].
(The dimensions for platelets were 6 to 50 microns on edge and about
1 micron thick.)

Powell and Eyring [88] have applied the theory of absolute reaction
rates to arrive at a suggested general relation between the shear stress and
the shear rate for non-Newtonian fluids:

L 1
_cdr_*_bsmh (adr) (4-5)

Tt is found that in the range of most common interest, 10? < (dv/dr) < 107,
sinh ™[ (1/a)(dv/dr)]= 6.4 4 3.5 for a variety of ThOs slurries [89] and does
not change rapidly with changes in dv/dr. Thus 1/b sinh™[(1/a)(dv/dr)]
is in effect 7, in the Bingham equation [90] for an idealized plastic:

dv ge . B
dr"" n (T Ty)' (4 6)
164 TECHNOLOGY OF AQUEOUS SUSPENSIONS {cHAP. 4

For convenience, the apparent yield stress, 7, and the coefficient of
rigidity, 7, will be used to characterize different uranium and thorium
oxide slurries [72].

If 1t 1s assumed that the particles in a flocculated non-Newtonian slurry
stick together in the form of loose, irregular, three-dimensional elusters in
which the original particles can still be recognized, and further that the
yield stress is a manifestation of the breaking of these particle-particle
bonds, then for constant isotropic bond strength the yield stress should be
proportional to the cube of the volume fraction solids. Since the shear
forces are exerted in a plane, the yield stress should also be proportional
to the number of particles per unit area, and hence, for constant volume
fraction solids, the yield stress should be proportional to the reciprocal of
the square of the particle diameter.

Data on the yield stress and coefficient of rigidity as a function of
concentration for three particular uranium oxide preparations [91] are
summarized in Table 4-9. The yield stress-volume fraction solids data
may be expressed by a relation of the form

Ty = ]€1¢4. (4"7)

Values of ky for the three different oxides are shown in Table 4-9. The
data for the coeflicient of rigidity may be fitted by a relation of the form

N = u explka2¢], (4-8)

using values of ks given in Table 4-9.

TaBLE 4-9

RuroLocic PrRoPERTIES OF NON-NEWTONIAN
UranivM OXIDE SLURRIES

 

 

 

 

Particle-size
distribution k= ¥
1 1 )
Oxide k= 0/1) P
D,, ¢ ib/ft2
o microns
U0, 1.7 1.4 1.8 150
U;0g 2.0 1.3 2.2 230
UO3zH20 1.9 1.2 2.2 430

 

 

 

 

 

 
TaBLE 4-10

RurorLocic PrRoOPERTIES OF NON-NEWTONIAN THORIUM OXIDE SLURRIES

 

Oxide designation

 

Particle-size

 

 

 

 

 

L Ty
Calcination Agitation distribution ky= l}%@ ka = (},‘?,
tempsgtule, othod Dur}a tion, Teonip" . _DP’ b /ft2
1 microns
S-59 650 Pump 325 290 2.7 0.030 2.4 1100
200A-1 800 Pump 234 300 2.9 0.58 1.8 470
200A-11 800 Pump 900-1800 300 2.8 0.75 1.4 550
W-30 1600 Waring blendor 0.5 50 1.9 1.0 1.5 145
LO-25-S 1600 None — — 1.5 1.6 1.2 100
200A-14 1600 Pump 3787 300 1.8 1.4 0.8 60
LO-25-1 1600 None - — 1.7 2.0 1.0 44
LO-22 1600 Mikro-pulverizer 4 passes 30 1.7 2.4 1.2 33

 

 

 

 

 

 

 

 

 

 

SHILUNIONd HNIHTANIONT (5

ol
166 TECHNOLOGY OF AQUEOUS SUSPENSIONS [caAP. 4

 

 

 

 

 

T T T T T T,
— — 3
— —z
S e
2 — ]
= -
- — —
o~ | — —
: e
g -
5 05— -
£ | ]
o
= |
oo 0.2 — _
Qo
>
k—
o1 - _
0.05 |— ]
002 _
0.01 || L
001  0.02 0.05 0.1 0.2 0.5 1

Velume Fraction Solids, @

Fic. 4-16. Effect of particle diameter and volume fraction solids on ThOj3 slurry
yield stress.

The yield stress and coefficient of rigidity as a function of volume
fraction solids for a variety of different ThOs slurries [89] are given in
Table 4-10. The yield stress-volume fraction solids curves can be fitted
by a relation of the form

T, = k3> (4-9)

The coeflicient of rigidity-volume fraction solids curves can be fitted by
a relation of the form

n = u exp(kao). (4-10)

The data given in Table 4-10 were obtained with ThO2 slurries having
a pH less than 6 and whose rheological constants were relatively insensi-
4-4] ENGINEERING PROPERTIES 167

 

 

 

 

 

 

 

- 1
T T ] \| — ]'°
/-/- :
. —0.5
— | -
S -
I —1o02
| .
—0.1 2
| = g
—] a
— 2
vy
| —0.05 ¢
] -
| =
3
o™
< 002 |— I | —o02 %
2 . 3
ol | | :
2 o001} r =001
vy L ] o
- | | 1 <
> 0005 —{0.005 3
v I | 1 8
[ ] s
c
0.002 |— ‘ —o.v02 -2
Q
i Region of Newtonian I o
0.00 |— | Behavior i —0.00t
0.0005 |— l I —0.0005
- .
0.0002 | | | —0.0002
oo L1 L o
0 10 20 30 40 50 60 70 80 90 100

Oxalic Acid/mole Th02 ,millimales

Fic. 4-17. Effect of electrolyte on ThOg slurry yield stress.

tive to dilution and reconcentration and therefore could be considered as
having a similar and reproducible electrolyte atmosphere associated with
the particles [89]. On the basis of the above considerations a plot of k3
(Eq. 4-9) versus mean particle diameter was found to fall on a line of
slope minus two on a log-log plot, as suggested by the plausibility argu-
ments given above. Therefore the data of Table 4-10 were plotted as
TyDg versus volume fraction solids. All points fell within the two lines
shown in Fig. 4-106. It is believed that the spread of data is largely due
to effects of the electrolyte atmosphere [89], since the deviations from
the mean particle size were similar. The influence of small quantities of
electrolyte on the yield stress of a particular ThOgz slurry [92] is shown
in Fig. 4-17. Similar behavior for particulate systems has been described
elsewhere [68,93].
168 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHaP. 4

Preliminary measurements [94] of rheologic properties of ThO» slurries
at temperatures up to 290°C indicate that the yield stress is essentially
independent of temperature (430%), whereas the coefficient of rigidity
decreases with temperature, although not to the same extent as water.

4-4.3 Fluid flow. The pressure drop due to friction for viscous flow of
Newtonian fluids through pipes is given by the Poiseuille equation:

Ap = 32uLV /g.D2 (4-11)

The same equation may be used for non-Newtonian suspensions, pro-
vided that the “apparent’ viscosity, m,, is substituted for the viscosity.

Buckingham [95] has presented a mathematical relationship for the flow
of Bingham plastics in circular pipe:

8V/g.-D = (1/7)(ru —(4/3)7,+ (1/3)75/73). (1-12)

For large values of 7., the last term of Eq. (4-12) becomes small, and the
resulting expression for the shear stress at the wall, 7,, when combined
with Eq. (4-13),
7w = DAp/4L, (4-13)
gives
Ap =329 LV /g.D2 4 (16/3)r, L/D (4-14)

for the pressure drop of a Bingham plastic in laminar flow.

Hedstrom [96] has proposed a simple eriterion to distinguish between
laminar and turbulent flow of Bingham plastic materials. The Reynolds
number at which turbulence sets in is determined by the intersection of
a parametric curve, defined by

Nue = g.p 7,D% /9%, (4-15)

1 _f_

_ 1 Ny 1 Nt
Ngr. 16 6 (Nge)?

T3 PNr)®

 

 

(4-16)

and the turbulent Newtonian friction curve on a Fanning friction factor-
Reynolds number plot, provided that the Reynolds number is defined as
DVp/n. The usefulness of the Hedstrom concept has been demonstrated
by several investigators [97,98]. Figure 4-18 is a plot of the solution of
Eqs. (4-15) and (4-16) superimposed on a friction factor-Reynolds number
diagram for Newtonian fluids flowing in smooth tubes.

In general, non-Newtonian fluids behave similarly to Newtonian fluids
in the turbulent flow region in that they exhibit relatively constant ap-
4-4] ENGINEERING PROPERTIES 169

 

  
   

 

 

 

0
10
I I T T | 7 ]
9_ _ 9cp7yDy
107= Hedstrom No. =Ny, =——3— —
»
~ 107! —
5 ]
W
£ —
c
s _
£
- _
c
£
102 ]
03 bl ]
102 103 104 109 106 107

DV
Reynolds Number, Ng, =

 

F1a. 4-18. Friction factor-Reynolds number diagram for Bingham plastic slur-
ries in smooth pipes.

 

 

 

 

[ T i 1
N ]
- N
20 —
o - ]
|’
c [ —
2
% — ]
& \\
2 | Newtonign —aN ]
‘E Laminar S
c
S. Flow
= 10 [ ~—— Commercial Pipel- ]
| "'-___ ]
| . ‘:::_'— - e
bl N
- Smooth Tube.f |
103 | L1 | L1t | P11 | L
102 103 104 105 106
DVp
Nrer =~

Fig. 4-19. Friction factor-Reynolds number data for ThO¢ slurries in turbulent
flow. 7,=0.0751b/ft? =209 cp.
170 TECHNOLOGY OF AQULOUS SUSPENSIONS [cHap, 4

parent viscosity. Alves, Boucher, and Pigford [69] indicate that, in the
absence of data, the conventional friction-factor plot may be used to predict
turbulent pressure drop to within 4259, provided that the Reynolds
number is evaluated by using the coefficient of rigidity or viscosity at
infinite shear and that the density is taken as that of the slurry. However,
1t has been shown that a large amount of turbulent pressure-drop data
taken with both Newtonian and non-Newtonian slurries can be correlated
using the usual friction factor-Reynolds number plot provided the density
is taken as that of the slurry and the viscosity as that of the suspending
medium [99-105]. Data obtained [106] with two different ThO». slurries
using three different tubes are shown in Fig. 4-19. As can be seen, the tur-
bulent friction-factor line is below the smooth-tube Newtonian line at low
Reynolds numbers and approaches the smooth-tube line at high Reynolds
numbers.

Vanoni [107], who reviewed the literature on sedimentation transporta-
tion mechanics through 1953, has pointed out that although a quantitative
description of the phenomena was unavailable at that time, it was clear
that sediment movement is intimately associated with turbulence.
Subsequent work either has been largely of an empirical nature [108-110]
or has unquestioningly accepted and used [111-113] generalized flow rela-
tions which have been developed for homogeneous Newtonian fluids
[114]. Undoubtedly the presence of particulate matter in the flowing
stream will exert a perturbing influence on the flow pattern at velocities
near drop-out, and a quantitative solution to the problem must include at
least an estimate of this effect [115].

It has been proposed [116] that the velocity below which particulate
matter will be deposited on the bottom of horizontal pipes from a Bingham
plastic suspension corresponds to the critical veloeity for the onset of tur-
bulence. This velocity may be calculated approximately by setting the
modified Reynolds number (obtained by using the apparent viscosity
o =n(1+ g.D7,/60V) from Eq. 4-14 instead of viscosity as usually de-
fined) equal to 2100 and solving for the velocity:

 

 

2100n 21()Un (4)(2100);;673, -

Resuspension velocities* for one particular slurry [117] in a §-in. glass pipe
are given in Table 4-11 together with the rheological properties [118] and
the eritical Reynolds number calculated with the resuspension velocity.

*The resuspension velocity corresponds to the velocity at which a moving bed
disappears as the mean stream velocity 1s increased.
4-4] ENGINEERING PROPERTIES 171

TABLE 4-11

REesuspeEnsioN VevLocity ror BingHaAM Prastic TaOs SLURRIES

 

Critical Reynolds

Concentration, Resuspension number,
i’e ¢ Th Ty K velocity, V.,
) -— 2 ’ 7
& kg H20 b/t ‘P fps DVep

n{l+ (g. D 7,/6nV.)]

 

 

2.44 1645 0.48 5.9 7.4 2220
2.29 1490 0.35 5.0 6.3 2040
2.17 1325 0.25 5.1 5.5 2050
2.05 1180 0.19 4.6 5.2 2240
1.94 1030 0.12 4.2 4.5 2380
1.84 910 0.098 | 3.9 3.7 1920
1.76 825 0.065 | 3.6 3.5 2350
1.65 690 0.040 | 3.1 3.7 3460

 

 

 

 

 

 

 

 

As can be seen, the critical Reynolds number is very close to the proposed
value [116] of 2100. Additional data on a variety of slurries and different
tube diameters are being obtained [119] to further substantiate Iiq. (4-17).

4—4.4 Hindered-settling systematics. The hindered-settling velocity of
slurries may be expressed as a coefficient times the Stokes’ law settling
rate. Table 4-12 summarizes [120] typical coeflicients obtained in the-
oretical and empirical investigations. A plot of these coeflicients versus
porosity showed that they are substantially in agreement. Steinour [121]
introduced the concept of immobilized water in his treatment of flocculated
suspensions and showed that by defining

__immobilized fluid volume
solid volume

 

(4-18)

and making appropriate corrections to the volume fraction solids term in
his empirical equations, a good correlation could be obtained for all ma-
terials that were studied. Typical values of « for flocculated [120,121]
ThOs slurries are @ ~ 1 to 25, which corresponds to floe densities of from
1.3 to ~bg/cc.

Hindered-settling studies [120] with ThO-= slurries having yield-stress
characterization constants, k3, from 50 to 500, in containers having diam-
eters from 1.6 to 10.25 em, showed that for containers having depth greater
than six times the diameter, the onset of compaction was a function of the
TapLE 4-12

HiNDERED SETTLING OF SUSPENSIONS

2 —_ -
UO = Qp_(ep—p)gr' (/rs = CIIO

 

 

 

 

 

 

 

18 i
) . L Reference
C* Porosity Origin
range of C .
Author No.
gt-65 0.33-1.0 (a) Richardson, Zaki (1954) [123]
€2 1018219 0.65-1.0 (a) Steinour (1944) [122]
1

8 rt (a/Ry?*—1 dh o . o
0Jo T 2a/R)* — (4/3)(¢/R)2— 3 1In (R/a) b 0.5-0.95 (b} Richardson, Zaki (1954) [124]

1 0810 (e) Burgers (1942) [125]
14+ 6.875(1 —¢)
eZexp [— 2.5(1 —€}/1 — §3(1 — €)] 0.51.0 (¢} Hawksley (1950) [126]

8
4+ 3/49)(1 — e)[l T 3] 0.51.0 (b) Brinkman (1947) [127]
0.106 /(1 — €) 0508 (d) DallaValle et al, (1957) [128]3
0.123 €*/(1 —¢€) (.5-0.785 () Steinour (1944) [122]
&
-
(1— e)[2k + i 6_ E:l 0.5-1.0 (d) Loeffler, Ruth {1953) [129]
0.10 (El—_wei)g (e) Powers (1939) [130]
0.5-0.82

0.123(e — wi)? w,; = 0.27 to 0.35 for vari- {e) Steinour (1944) [131]
(1—w)?(1—¢) ety of emery powders

 

 

 

 

 

 

*The term e refers to volume fraction liquid. (a) Experimental settling rate. (b) Drag theory. (¢} Stokes’ law using physical properties of suspension for p and pu.

) Empirical — hydraulic radius. (e) Empirical — hydraulic radius for flocculated systems,

GLI

SNOISNAJISAS SA0TADY J0 XHDOTONHOAL

¥ "dVHO]
4-4] ENGINEERING PROPERTIES 173

 

WO T T T T T T

1.25

oS —
~ |=
o O

o
tn
o

Hindered Settling Rate, cm/sec

0.25

 

 

 

 

0 10 20 30 40 50 &0 70 80 20
Angle of Inclination from Vertical, deg

Fig. 4-20. Effect of angle of container inclination on ThQ: slurry hindered
settling rate. Slurry concentration 300 g Th/kg H20, container diameter 1.63 in.

slurry yield stress, slurry concentration, and container diameter, the par-
ticular relation being

2
D = (0.07 + 0.02)ks 1_—(1%3@5. (4-19)

where the concentration term, ¢., is the value for the onset of compaction.

Settling-rate data obtained [122] in inclined tubes showed a maximum
at an angle of about 50° from the vertical, with typical results being given
in Fig. 4-20. The spread in the data is largely due to the difficulty in dis-
cerning an interface due to supernate rushing upward as the thoria
settles out.

4-4.5 Heat transfer. Grigull [131] has presented a theoretical treatment
of heat transfer to pseudoplastic and Bingham plastic non-Newtonian
fluids for laminar flow through tubes. Theoretical treatments of laminar
heat transfer to pseudoplastic materials have been given for a variety of
boundary conditions [132-134]. Pigford [135] has shown that, for Bing-
ham plastics, the laminar isothermal coefficient should increase less rapidly
than the 1/3 power of the mass flow rate and the magnitude of these
174 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHAP. 4

coefficients should be increased by the factor [14 (1/9)(7./7,)], approxi-
mately. Bailey [136] substituted Eqs. (4-13) and (4-14) into Eq. (4-6)
to obtain an approximate expression for the velocity gradient and then
substituted that expression into Leveque’s solution [137] for the case of
constant wall temperature and uniform fluid temperature at the entrance
to the tube, to obtain:

@__ Vpch 1/3 -gTyD 1/3 .
- —1.615( - ) (1+24nV) : (4-20)

 

 

Experimental data [106] are in substantial agreement with Eq. (4-20).
The principal uncertainty in non-Newtonian heat transfer in the tran-
sition and turbulent region is the criterion for the onset of turbulence and
range of the transition region. Data taken in fully developed turbulent
flow with a variety of solid-liquid suspensions may be correlated satisfac-
torily with the Dittus-Boelter equation (or variations of it), with some un-

certainty about the best viscosity to use in the correlation [138-142].
10-2

 

T T T T T T 111N T

C.14

ovey” 02
n

      

=o.027(

  
    
  

L]

DIA., L/D o, Ty
in gmfec thfsq ft op
00318 378 1.67 0.066 2.8
1.67 0.045

|

 

 

 

   

—_

 
 

fi = 4.01og (Np, v/ £)~0.407

 

 

[ L LTI |1 Pl

| | {111
103 2 3 456784 2 3 4 5678 5 2 3 456788

 

Reynolds Number, Np = D_:p

Fia. 4-21. Heat-transfer and fluid flow characteristics of ThO; slurries.
4-4] ENGINEERING PROPERTIES 175

 

F1e. 4-22. Slurry blanket mockup.

Figure 4-21 gives experimental data [106] for heat transfer to the ThO»
slurry with which the pressure-drop measurements of IFig. 4-19 were made.
Comparison of the heat-transter and pressure-drop data shows that al-
though the onset of turbulence in heat transfer occurs at Reynolds numbers
greater than the 2100 expected for Newtonian fluids, it corresponds exactly
with the experimental eritical Reynolds number for the onset of turbulence
obtained from the pressure-drop measurement (which can be predicted
approximately by the Hedstrom criterion). The transition region then
extends to Reynolds numbers a factor of four or five greater than the
eritical, as is the case with Newtonian materials. Heat transfer to ThO»
slurries in fully developed turbulent flow is the same as that predicted by
the usual Newtonian correlations [143] to within the precision of the ex-
perimental data. The very interesting question as to whether a suspension
has more desirable heat-transfer characteristics than a pure liquid, as
indicated by the results of Orr and DallaValle [139], or whether the yield
stress of & Bingham plastic decreases the heat transfer coefficient, as sug-
gested by Lawson [142], remains unanswered by these results [106] on
ThO¢ slurries.
176 TECHNOLOGY OF AQUEQOUS SUSPENSIONS [cHaP, 4

4-5. OpERATING ExPERIENCE WITH THE HRE-2 SLURRY BLANKET
TesT IFaCILITY*

4-5.1 Introduction. The purpose of constructing the HRE-2 slurry
blanket test facility was to determine whether the blanket system installed
in the HRII-2 for use with solutions could be used with slurry and, if so,
what modifications would be required. The facility, often referred to as
the blanket mockup, was a full-scale replica of the HRT blanket circulating
system with respect to the reactor-vessel dimensions, piping size, circulating
pump, and heat-exchanger tubing size. The pressurizer and piping con-
figurations were not the same, and the high-pressure heat exchanger con-
tained only one-fifth as many tubes as the HRE-2 heat exchanger. A
general view during the last stages of constructions [144] is shown in
Fig. 4-22,

The system was divided into a high-pressure and a low-pressure section,
since the reactor vessel available was a steel prototype, good for only a few
hundred psi, and could not be used during circulation and heat-transfer
tests at high pressure. A schematic drawing [145] of the slurry blanket
mockup system is given in Fig. 4-23. The slurry stream entered the
60-in.-diameter mockup pressure vessel (which contained a stainless steel
replica of the HRIS-2 core) through two inlet nozzles arranged to direct the
flow in opposite directions on either side of the core inlet, with the expecta-
tion [146,147] that the flow pattern in the blanket would consist of two
vortices which would rotate in opposite directions in parallel vertical planes,
thus preventing deposition of sediment on the bottom of the pressure
vessel. The slurry was removed at the top of the vessel by putting a shroud
around the core outlet and removing the fluid through this shroud. It was
also expected [146,147] that as the slurry moved across the top of the core
vessel to the shrouded outlet, the rotating motion produced by the inlet
nozzles would set up a free vortex in a horizontal plane, the accompanying
inerease in angular velocity of the fluid as it moved toward the axis tending
to sweep the top of the core vessel free of solids. The design of the inlet and
outlet nozzles was based on successful tests with an 18-in. model of the
vessel [146]. Slurry leaving the pressure vessel flowed to the circulating
pump, a Westinghouse type 230A canned-motor pump having a design
point of 230 gpm at 50-ft head and a working pressure of 2000 psi which
produced a pipeline velocity of 10 ft/sec at its nominal capacity. I'rom the
pump, the slurry stream entered the pressurizer, a 6-in. schedule-160
vertical pipe, with an over-all height of 14 {t 0 in. above the inlet. Entering
flow was directed down and out of the pressurizer with a reducing tee, and

*By D. G. Thomas.
4-5] THE HRE-2 SLURRY BLANKET TEST FACILITY 177

Yent
o E

Steam or
J Nitrogen

—
L |

Pressure Balancing Valves

-

Level
Controller Pressurizer

3 %10 Kw

    
  

Condensate
Hold
Tank ’

 
 

  
   
 
 
 

Thermal
Flowmeter

L

{ Rotameter

 

 

 

 

 

 

 

 

 

 

 

Flow
Nozzle
‘ | | ‘ i
o 1 /! — ||
4 Kw
WA oD
Feed, Letdown & Dump —e—=— 4 Kw W

 

 

F1a. 4-23. Slurry blanket test system.

baffles were installed above the inlet to damp out any vortices. System
pressure was maintained by a 10-kw heater located just below the liquid
line on the top of the pressurizer. Slurry flowed to the bottom of the mockup
pressure vessel from the pressurizer. The main loop auxiliaries consisted of a
slurry feed system and a letdown and dump system, which provided
system versatility during startup and shutdown. A detailed description
of the system is given in reference [145].

4-5.2 Operation of blanket pressure vessel mockup system. Initial ex-
periments [145] in the 60-in.-diameter vessel (duplicating the HIRE-2)
at 170 and 200°C showed that a scale-up based on maintaining equal
superficial vertical velocity at the equator and equal inlet nozzle velocities
was nadequate to maintain a uniform suspension. At 170°C, a sharp
concentration gradient was found, and above 180°C most of the slurry
charge remained essentially stagnant in the blanket, with quite dilute
slurry circulating through the piping loop. The increase in settling rate
with temperature is believed to account for the effect of temperature on
slurry distribution.
178 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cuap, 4

As a result of the initial tests the following changes were made in the
mockup system [ 148, 149]:

(1) The circulating pump was reassembled with a 300-gpm stainless-
steel impeller.

(2) The blanket inlet nozzles were redesigned, with the nozzle diameter
being reduced from 2.16 in. to 1.50 in,

(3) A steam sparging system was installed just above the inlet nozzles to
ald in maintaining the slurry in suspension during a dump.

Although these changes are relatively minor in nature, they resulted in
significant improvement in blanket operation [150]. Hydraulic measure-
ments with water showed that the loop flow was increased from 230 to
350 gpm, the head developed was increased from 50 to 56 ft of fluid, the
pressure drop through the blanket went from 7.3 to 33 ft of fluid, and the
loss across the blanket inlet nozzles went from 5.5 to 25 ft of fluid. During
system operation at 200°C with a slurry made from ThOg calcined at 800°C,
samples were withdrawn from 36 different locations in the blanket vessel.
The mean value of the concentration was 610 g Th/kg H2O with a devia-
tion at the 95% level of 75 ¢ Th/kg H20. The average concentration of the
loop circulating stream was 617 g Th/kg H20, and an average blanket
vessel concentration of 655 g Th/kg H20 was obtained from a gamma-ray
transmission scan of the vessel. Only about 40 kg (out of a total charge of
1000 kg) was unaccounted for by inventory on the system at this time.
This material appeared to be more or less stagnant in the blanket region on
top of the core vessel and at the bottom of the pressure vessel. The evidence
for this was as follows:

(1) The blanket samples indicated high-conc¢entration regions on the
top of the core vessel and on the bottom wall of the pressure vessel.

(2) Gamma-ray transmission scans of the blanket vessel indicated the
highest concentration at the top of the core vessel.

(3) Addition of uranium tracer to the circulating stream indicated that
10 to 15 kg of ThO: was immobilized in the blanket and was only slowly
mixing with the circulating stream; this was supported by examination
of the slurry remaining in the system at the conelusion of the test, when a
pure white layer containing no uranium was found to be partially covered
over with a yellow layer containing a substantial quantity of uranium,
both layers being found on the top of the core vessel.

During this operation at a flow rate of 350 gpm the temperature of the
system was raised and lowered at will between 150 and 200°C with no sig-
nifieant change in circulating concentration. That this marked improve-
ment in operating characteristics over the initial tests was due to the small
system changes and not duc to a change in slurry properties was indicated
by the results from operation with the electrical frequency of the pump
motor reduced from the normal 60 cycles to a value giving pump flow
4-6] RADIATION STABILITY OF THORIUM OXIDE SLURRIES 179

characteristics similar to those in the initial tests. Reduetion of the fre-
quency to 12 cycles with a blanket temperature of 190°C gave a circulating
concentration of 265 versus 625 g Th/kg H-O charged. The blanket
samples averaged 495 g Th/kg Ho0O. These results are quite similar to
those obzerved in the initial tests.

Operation of the steam sparging system during the dumping operation
at the end of the run allowed recovery of all but 50 liters of the slurry from
the blanket (total blanket volume, 1600 liters), compared with about
200 liters remaining at the end of the dump after the initial tests.

During operation with redesigned inlet nozzles, a number of experiments
were run in which the pump was shut down for periods up to 3 hr. In every
case the slurry settled rapidly to a concentration of approximately
800 g Th/liter and then gradually compacted to a concentration of ap-
proximately 1500 g Th/liter. On restarting the pump, no difficulty was
experienced in resuspending the slurry, and the original slurry distribution
was re-established in approximately 2 min. This proved to be one advantage
of operating with a highly flocculated ThO3 slurry compared with operation
with a deflocculated slurry that would settle to much more dense beds
that are correspondingly more difficult to resuspend.

Among the problems that have been more clearly defined as a result of
the tests on the blanket mockup are:

(1) The necessity of establishing the effect of settling rate on the slurry
distribution.

(2) Determination of the scale-up laws for applying small- or intermedi-
ate-scale results to full-scale systems.

(3) Evaluation of the danger of possible boiling at the core wall due to
high heat flux and low fluid velocities.

4. RapratroNn Stasinrty oF TaorivM OxXIipDE SLURRIES¥

4-6.1 Introduction. FExperiments have been carried out at the Oak
Ridge National Laboratory as part of a continuing program to determine
the effect of radiation on the physical properties of aqueous suspensions of
thorium oxide. Since changes in particle size, surface properties, and vis-
cosity of the suspension might have a deleterious effect on the operability
of a homogeneous-reactor slurry system, these properties were examined
in detail.

Suspensions of thorium oxide and thorium oxide containing 0.5 mole 97
of either natural or highly enriched UO3z were irradiated in the Low In-
tensity Test Reactor (LITR) at a thermal-neutron flux of 2.7 X 1013
neutrons/(em?)(sec).  Although more than 40 irradiations were carried
out, no significant changes in the properties studied were noted [151].

*By N. A. Krohn and J. P. Mc¢Bride,
180 TECHNOLOGY OF AQUEQUS SUSPENSIONS [cHAP. 4

 

 

inches

Fi1c. 4-24. Parts and assembly of in-pile autoclave for slurry irradiations.

4-6.2 Experimental technique. The experiments were carried out in
small, eylindrical, stainless steel autoclaves such as shown in Fig. 4-24.
Thermocouples welded into the bottom closure and a 20-mil-1D steel capil-
lary in the top permitted a continuous measurement of temperature and
pressure. Continuous stirring was accomplished by means of a dashpot
type stirrer of Armco iron clad with stainless steel, with a stainless steel stir-
ring head. The capacity of the autoclave with stirrer was approximately
14 ml.

The stirrer was made to reciprocate at 3 to 4 cycles/sec by alternately
energizing two solenoid coils of double glass-insulated aluminum wire
wrapped around the autoclave body. The timing unit consisted of a multi-
vibrator circuit whose frequency of oscillation and cycle time division
could be controlled by varying resistances in the circuit. The timer unit,
in turn, controlled the grids of two pairs of thyratrons which furnished
power to the solenoids. Stirrer operation was monitored by a tickler coil
connected to an oscilloscope.

The radiation facility consisted of a double-walled aluminum tube ~7/
in. in inside diameter which extended from the top of the LITR through
approximately 20 ft of water into the reactor core. The autoclave was
lowered into this tube on a bundle of wires containing the electrical leads,
4-6] RADIATION STABILITY OF THORIUM OXIDE SLURRIES 181

thermocouples, and pressure capillary. During reactor shutdown the auto-
claves were maintained at 300°C by the heat generated by the stirring
coils. Heat was removed during reactor operation by air which flowed
down the tube, over the autoclave, and back through the annulus pro-
vided by the double-walled tube,

The uranium-bearing oxides were prepared by wet-autoclaving mixtures
of ThO. and UOjz (about 909 enriched uranium) at 300°C or by co-
precipitating thorium and uranous oxalates and calcining to the oxide.
The autoclaves were loaded with 5 ml of slurry at room temperature, which
expanded to fill half of the autoclave at 300°C. When uranium-bearing
slurries were irradiated, either palladium oxide or molybdenum oxide was
added to catalyze the recombination of radiolytic hydrogen and oxygen
(see Section 4-7). In some experiments an oxygen overpressure of 200 to
250 psi was added at room temperature.

Viscosity measurement. The versatility of the timing device deseribed
above made possible the measurement of relative slurry viscosity both
in-pile and out-of-pile. It was found that the time it takes the stirrer to
reach its maximum height for a fixed solenoid current is dependent on the
viscosity of the autoclave contents. The “rise time”’ was determined by
adjusting the frequency and load division so that the upper solenoid was
de-energized the instant the oscilloscope indicated that the stirrer had
reached the top of its travel. The rise time in seconds was then obtained
by dividing the load division by the frequency.

Calibration curves of rise time versus viscosity were made using a
silicone oil of known viscosity at various temperatures. Low viscosity
points were obtained using water and air.

4~6.3 Irradiation results. Viscosities, x-ray crystallite sizes, particle
size, and settled concentrations appeared unaffected by the irradiation, as
indicated by the data shown in Table 4-13. Measurements made on ir-
radiated and nonirradiated materials were the same within the limits of
error. The mean particle sizes were in general not changed significantly;
however, in two cases, involving samples cooled for 10 and 11 months, a
large Increase in mean particle size was observed. It is possible that the
long cooling period allowed hard agglomerates to form which were not
broken up by the brief shaking prior to opening of the autoelaves. One ex-
periment cooled 12 months did not show an agglomeration effect. In
general, the irradiated slurries poured readily from the autocluve, and no
tendency toward caking was observed even on samples irradiated in settled
condition for 10 days.

Chemical and radiochemical analyses of both phases of the irradiated
slurries showed the bulk of the fission produets, protactinium, and uranium
to be associated with the solids. Only cesium appeared in the supernatant
182 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHAP.

TABLE 4-13

ErrEcT OF RADIATION ON THE PROPERTIES
or THorruM OXIDE SLURRIES

Conditions: 300°C, 2.7 X 1013 neutrons/(ecm?)(sec)

 

 

 

ThO2 + 0.5% ThO2+ 0.5%
Property ThO¢ nat. UOg(a)O U2350) ;™ ¢

Precipitation temp., °C 40 40 10
Calcination temp., °C 900 900 900
Radiation time, hr. 175-300 151-172 168-314
Slurry concentration, 750 750 750

g Th/kg H20
Viscosity, centistokes

Control 16 — 4-7

During irradiation 16 — 5
X-ray crystallite size, A

Original 350-464 350 —

Irradiated 315-540 295-350 —
Settled conc., g Th/liter

Control 1100 1500 1400

Irradiated 1400 1600-1700 1400-1500
Mean particle size, microns'®

Control 1-2 2.8 2.1-2.3

Irradiated and cooled up

to 6 months 1-1.8 1.1-314 1.8

Irradiated and cooled

10-12 months — — 15

 

 

 

 

 

 

(a) 650°C-fired ThO2 wet-autoclaved at 300°C with UQj3 - H20; mixtures refired
at 900°C.

(b) Prepared as in (a) and also by thermal decomposition of the coprecipitated
thorium-uranous oxalates.

(c¢) Measured by sedimentation in dilute suspension dispersed with 0.005 A
N&4P207.

(d) The result in two experiments; a third test showed no change in average
particle size.
4-7] CATALYTIC RECOMBINATION OF RADIOLYTIC GASES 183

in significant amounts. Strontium appeared to absorb less on the higher-
fired materials. The ruthenium analyses were inconsistent, the probable
result of the perchloric acid dissolution treatment used.

Total corrosion-product pickup was similar to that obtained in out-of-
pile experiments except with the slurries containing sulfate, which showed
higher corrosion-product pickup under irradiation. All of the iron and
most of the nickel and chromium were associated with the slurry solids.
The greater part of the corrosion-produet pickup resulted from abrasive
attack by the slurry solids under the action of the stirrer.

1-7. CaTarLyTic RECOMBINATION OF RapronyTric (GASES IN AQUEOUS
THoriuM OXIDE SLURRIES¥

4-7.1 Introduction. Radiolytic decomposition of the aqueous phase of a
thorium oxide slurry blanket will produce a stoichiometric mixture of
deuterium and oxygen which must be recombined. Total recombination
may be accomplished external to the blanket system by suitable methods.
It would be advantageous, however, to recombine the gases internally to
minimize both reactor control problems accompanying bubble formation
and engineering problems associated with external recombination. The
magnitude of the internal-recombination reaction desired may be judged
from the estimate that an aqueous thorium oxide-uranium slurry breeding
blanket may produce from 2 to 3 moles of 1T and 1 to 1.5 moles of O2 per
hour per liter of slurry by radiolytic decomposition, assuming a G-value
of 1T and an average blanket flux of 6 X 102 neutrons/(¢m?)(sec). This
is within a factor of 2 of the decomposition one would expect for a solution
at the same power density.

Work on the development of a catalyst for use in thorium oxide slurries
to recombine the radiolytic gases has been carried out as a part of the
Iomogeneous Reactor Project at the Oak Ridge National Laboratory [152].
More recently, Westinghouse {(Pennsylvania Advanced Reactor Project)
undertook similar studies. The experimental approach at both labora-
tories has been similar, and the results are in reasonable agreement. While
sufficient data have been obtained to assure that a eatalyst can be used in
both thorium and thorium-uranium oxide slurries for complete internal
radiolytic-gas recombination, specific conditions for the most efficient
catalyst preparation and use have not as yet been established.

*By L. E. Morse and J. P. Mc¢Bride.
tMolecules of water decomposed per 100 ev of encrgy dissipation in the slurry.
184 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHaP, 4

4-7.2 Experimental techniques and method of analysis. The out-of-pile
tests were carried out in ~15-ml stainless-steel autoclaves provided with a
thermocouple well and a capillary pressure connection through the top
closure. The autoclaves were approximately half filled with the slurry con-
taining an appropriate catalyst, closed, and charged with 550 psi oxvgen
and then 900 psi hydrogen from regular high-pressure cylinders. To
minimize corrosive attack at temperature, the oxygen charged was in
excess of the stoichiometric 1:2 ratio to hydrogen. The autocluves were
raised to temperature in an appropriate furnace, mechanically agitated,
and the decrease in pressure with time was followed continuously by ap-
propriate instrumentation.

For a stoichiometric mixture of hydrogen and oxygen, the rate of pres-
sure decrease was proportional to the total gas pressure in excess of a
certain minimum pressure (i.e., steam and inert gases). A plot of pressure
data in the differential form, AP/Af versus P, resulted in a straight line,
the slope of which, k., was a measure of the experimentally observed first-
order reaction rate,

While the experimental data are insufficient to establish firmly that the
reaction rate is first order, it is convenient to present the data in this form
and assume a first-order dependence on hydrogen partial pressure:*

dly,
dt
where Py, is the hydrogen partial pressure, ¢ the time in hours, and %,
the observed reaction rate constant (hr—!). The moles of Hs reacted per
hour per liter of slurry, dn/dt, for any given hydrogen partial pressure
may be calculated, assuming the ideal gas law, from

dn_ kel Vy
dt — RT V.

 

= kfl'])sz

 

where V; and V, are the gas and slurry volumes and R and 7 are the gas
constant and absolute temperature. The recombination rates calculated
in this way are conservative in that only the hydrogen removed from the
gas phase is taken into account, and dissolved and adsorbed gases are
ignored. The method of analysis is convenient, however, and permits the
evaluation of the relative activity of the various slurry and catalyvst
systems.

The results of the out-of-pile tests were evaluated on the ability to at-
tain a reaction rate equivalent to the consumption of 2 moles of Ho per
hour per liter of slurry at 280°C and a hydrogen partial pressure of 100 psi.

*The homogeneous catalysis of the hydrogen and oxygen reaction in the case
of solutions is first order with respect to the hydrogen partial pressure (sce Ar-
ticle 3-3.4).
4-7] CATALYTIC RECOMBINATION OF RADIOLYTIC GASES 185

4-7.3 Catalytic activity of thorium and thorium-uranium oxide slurries.
Experiments with several different samples of ThO2 alone showed that
none of the aqueous slurries prepared with the pure oxide possessed the
desired degree of catalytic activity for the stoichiometric gas mixture.
Reaction rates in the region of 300°C were equivalent to less than 0.1 mole
of Hs consumed per hour per liter of slurry at a partial pressure of 100 psi
Ho. The higher reaction rates appeared to be associated with the slurries
prepared with oxides having higher surface areas.

A small catalytic effect for the stoichiometric gas reaction was obtained
by incorporating uranium in the aqueous ThO: slurries; however, the
reaction rates remained much too slow to be useful. These tests were carried
out with aqueous slurries of simple oxide mixtures, as well as with a mixed
oxide prepared by calcining the coprecipitated thorium-uranous oxalates.
Heating the slurry of the latter preparation under a small hydrogen partial
pressure at 280°C produced a marked but temporary increase in catalytic
activity which progressively diminished as further gas-recombination ex-
periments were carried out at higher temperatures [that is, dn/dt (at Pn,
= 100 psi) = 1.42 at 137°C; 0.65 at 154°C; 0.08 at 282°C].

4-7.4 Survey of possible catalysts. The primary criterion, other than a
satisfactory catalytic activity, for a catalyst to be used In an aqueous
thorium oxide slurry blanket is that it have a low thermal-neutron ab-
sorption cross section. A convenient estimate of the allowable concentra-
tion is provided by the rule that the total cross section of elements other
than thorium which are in the blanket slurry should be less than 109 of
that of the thorium itself.

On this basis, scouting experiments were carried out with copper sulfate,
copper chromite, copper oxide, copper and nickel powders, silver carbonate
(reduces to metal at elevated temperature), vanadium oxide, ceric oxide,
palladium oxide, and molybdenum oxide. Of these only the silver, vana-
dium, palladium, and molybdenum oxide showed sufficient activity at
reasonable concentrations. Bilver and vanadium were rejected because of
their higher thermal-neutron cross section; palladium appeared susceptible
to poisoning, particularly at lower temperatures, <120°C, and subsequent
development effort was concentrated on molybdenum oxide.

The preliminary catalyst evaluation was carried out with slurries of
thorium oxide fired at 900°C. Subsequent experience with molybdenum
oxide has indicated that it is inactive with low-fired oxides, and its activity
at least at low concentrations is decreased by the presence of uranium
oxide (see the following discussion). Hence in slurry systems using low-
fired thorium oxide or thorium-uranium oxides, silver, palladium, and
platinum, which are active in these slurries, may prove to be useful [154].
186 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHaP, 4

4-7.5 Molybdenum oxide as a catalyst. The molybdenum oxide cata-
lyst used in the initial scouting studies of its catalytic activity was prepared
by caleining ammonium paramolybdate at 480°C for 16 hr. It was added
to the slurry by dry-mixing 900°C-fired ThQgz, UO3 - H20, and the MoOj3
and then slurrying the mixture in water. Prior to its use in recombination
experiments, the slurry was heated under Oy at 280°C. Recombination
data obtained with slurries prepared in this way indicated that reaction
rates in the range of 1 to 6 moles of H» per hour per liter were obtained in
slurries which were heated for 1 hr at 280°C with hydrogen at as low a con-
centration as 0.025 m MoOs;. Lower reaction rates (0.03 to 1.8 moles of
Ho> per hour per liter) were measured for slurries not treated with Ho.

It was found that the catalytic activities of the activated slurries were
independent of the method used to prepare MoOj. Also, experiments
with MoO3 indicated that it was not stable under the experimental con-
ditions and that this form of the oxide is not the very active catalytic
species produced when the slurries were heated with Ho.

The addition of sulfate up to 10,000 ppm SO4= (based on thorium) did
not appear to impair the catalytic activity of the above slurry containing
0.05 m MoO3s. Corrosion vroducts resulting from the attack on the
reaction vessel at the highest sulfate concentration greatly decreased the
recombination rate. Similar slurries to which a ferric oxychloride sol
(1500 to 1600 ppm Fe/Th) was added showed decreased catalytic activities
(approximately one-tenth the iron-free rate), but reducing the iron by
treatment with hydrogen gave catalytic activities equal to or greater than
those of the iron-free systems. The addition of 634 ppm rare-earth oxides
(based on Th)* had no effect on the catalytic activity.

Subsequent experiments indicated that in slurries of mixed thorium-
uranium oxides, the order of addition of the slurry solids, the method of
incorporating the uranium, and particularly the oxide firing temperature
and time were important. It was necessary in some cases to fire mixed
oxides containing 0.5 mole 9} uranium as high as 1000°C for as long as
16 hr to give an active slurry with the molybdenum oxide. Lower firing
temperatures or a shorter firing time at 1000°C did not give an active
slurry.

Oxides fired above 1000°C (4 hr at 1200 or 1600°C) gave very active
slurries. Table 4-14 gives the data obtained with slurries of simple mix-
tures of 1600°C-fired thorium oxide and UQOjz - H2O and with a 1600°C-
fired mixed oxide prepared from the coprecipitated oxalates. Both prepara-
tions gave excellent combination results at 0.05 m MoQOj3 both before and

*Estimated steady-state concentration of fission-product oxides to be produced
in the thorium oxide by irradiation at a flux of 5 X 10'3 neutrons/(cm?)(sec) and
continuous blanket processing on a 250-day cycle [155].
4-7] CATALYTIC RECOMBINATION OF RADIOLYTIC GASES 187

TasLe 4-11

ReactioNn RaTes oF SToicHIOMETRIC Hse-Os MIXTURES
IN THORITM OXIpDE-URANIUM OXIDE SLURRIES

 

i Reaction rate, moles Ho/hr (liter)*
Oxide used for makeup of

500 g Th/kg H20 slurry

 

Slurry as prepared | He-activated slurryt

 

 

Reaction temperature, 250°C

 

10°C coprecipitated oxalates
(U/Th=0.005) calcined at 1600°C
for 24 hr 0.2 0.2
MoO3 additions, m:
0.012 6.86 11.0
0.024 6.40 11.0
0.036 11.5 18.1
0.048 15.3 17.1
0.06 15.2 15.0

 

 

Reaction temperature, 291°C

 

‘ThOs caleined at 1600°C for 4 hr 0.02 0.02
0.5 mole 9, U added as UO3 - H20 0.02 0.40
MoQ3; additions, m:
0.012 0.08 0.05
0.024 0.10 0.05
(G.036 0.24 0.62
0.048 1.04 3.46
0.06 2.53 4.16

 

 

 

 

 

*At 100 psi partial pressure of hydrogen.
Slurry heated with hydrogen (250 psi at 25°C) for 2 hr at 270°C.

after treatment with He, but at low molybdate concentrations the simple
mixture showed lower activity. Apparent over-all activation energies of
13 to 16 kecal/mole were calculated for the hydrogen-treated slurries con-
taining 0.05m MoQO3 from an Arrhenius plot of the observed reactign
rate constants.

Investigations of MoQOjs chemistry by the Houdry Process Corpora-
tion [156] (for the PAR Project) have shown that MoOs3 reacts in high-
temperature water with 650°C-fired thorium oxide and uranium oxide.
The failure of some lower-fired material and thorium-uranium mixtures
188 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cuaP. 4

to give active slurries may be the result of chemical reaction of the molyb-
denum oxide to form catalytically inactive species.

4-7.6 In-pile studies.* Radiolytic-gas production and recombination
rates were determined in the ORNL Graphite Reactor using a slurry of
ThO2 containing approximately 2.8% uranium which was approximately
93% enriched in U235, The mixed oxide was prepared by coprecipitation
of thortum and uranous oxalates at 10°C followed by calcination at 650°C.

Gas production rates were calculated from the initial pressure rise ob-
served in the autoclave at low temperatures, where the reverse reaction
rate could be neglected. The measured production rates were 1.6 X 1074,
3.3 X 1074 and 4.9 X 10™* moles of Hy per hour, respectively, for slurry
concentrations of 250, 500, and 750 g Th/kg HoO. The G-value for
water decomposition, assuming an average neutron flux of 4.2 x 10
neutrons/(¢cm?)(sec), an energy deposition in the slurry of 170 Mev/fission,
and a fission cross section of 380 barns, was 0.8 molecule per 100 ev.

Gas recombination rates were measured by determining the equilibrium
pressures in excess of steam for various temperatures (Fig. 4-25). Table
4-15 lists rate constants, k4 (hr™!), calculated from the equilibrium pres-
sures and gas production rates, assuming first-order dependence, and
making the necessary corrections for volume changes with temperature.
From an Arrhenius plot an apparent activation energy of 16.2 keal/mole
was obtained. There is some evidence which indicates that the action of
the stirrer increases the reaction rate either by forming catalytically active
corrosion products or by providing reactive steel surfaces.

TABLE 4-15

Gas ReEcoMBINATION RATE CONSTANTS FROM
EQuiLiBRIUM PRESSURES IN AN IRRADIATED
TrortUM-UrANIUM OXIDE SLURRY

 

 

 

Temperature, °C k., hr1
282 0.45
275 0.26
250 0.15
235 0.10
200 0.03

 

 

 

 

 

*Written by N. A. Krohn.
4-7] CATALYTIC RECOMBINATION OF RADIOLYTIC GASES 189

Temperature, °C

 

 

 

 

 

5 300 250 200
2x 10 — T T T T i
103 |
.; 1}
a
Ej‘ —
2
2
& 3 —~
o
E
3 —
8
Tg
o _ |
L
2 |
102 | | | ;
1.7 1.8 1.9 2.0 2.1 2.2
1/Tx 103, 9k-1

F1g. 4-25. Equilibrium radiolytic gas pressure, excluding steam pressure, in
irradiated thorium oxide slurries.

In all the slurry stability tests in the Low Intensity Test Reactor (see
Section 4-6) where enriched uranium was present, sufficient catalyst was
added to prevent any net radiolytic-gas production. Both PdO and MoOj3
were used for this purpose. No radiolytic gas (<25 psi) in excess of steam
pressure was observed in these experiments. In the tests where only ThOs
was used, no catalyst was necessary.

In an LITR experiment [157] carried out with a 1000 g Th/kg H20 slurry
of 1300°C-fired thorium-uranium oxide (U23%/Th = 0.005), suflicient
catalyst (0.02 m MoOs) was added to give a small partial pressure of
radiolytic gas. A G-value (calculated as above) for gas production of 0.6
molecule of Hz per 100 ev was obtained. From the equilibrium gas pres-
sures at 250 and 280°C, first-order rate constants for gas recombination of
4,98 and 7.14 hr~! were calculated. These values compare well with rate
constants of 4.95 and 8.75 hr~1! obtained in out-of-pile gas-recombination
experiments made with a similar slurry of the same oxide at the indicated
temperatures.
190 TECHNOLOGY OF AQUEQOUS SUSPENSIONS [cHAP, 4

BiBLioGrarHY

Thorvum and Thorium Ozride

Thorium, A Bibliography of Unclassified Report Literature, USAEC Report
TID-3309, Technical Information Service Extension, AEC, October 1956.

Davi, Lore 8. (Comp.), Thorium, A Bibliography of Unclassified Litera-
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Prater, W. D. et al. (Comps.), Thortum, A Bibliography of Published Litera-
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Sacns, Frances (Comp.), Literature Search on Selected Properties of Thorium
Ozxide, USAEC Report AECD-3423, Union Carbide Nuclear Co., 1952,

SweetoN, F. H. (Comp.), Thorium Oxide, Literature Survey of Preparation
and Properties of, USAEC Report CF-55-8-11, Oak Ridge National Laboratory,
1955.

Thortum and Thorium-Ozide Chemistry

Roppen, C. J. and Warr, J. C., Thorlum, in Analytical Chemistry of the
Manhattan Project, ed. by C. J. Rodden, National Nuclear Energy Series,
Division VIII, Volume 1. New York: McGraw-Hill Book Co., Inec., 1950,
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192 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHAP, 4

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194 TECHNOLOGY OF AQUEOUS SUSPENSIONS [cHAP, 4

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