Alumina dispersion behavior

A method is provided for forming stable dispersions of boehmite alumina in water containing a peptizing acid by mixing said alumina with dilute aqueous acid solutions and treating the alumina prior to dispersion by heating at temperatures of from about 250.degree. F. to 700.degree. F. and pressures of from about 10 psig to about 2000 psig in the presence of up to 80 weight percent of water based upon the total alumina for a period of time sufficient to stabilize the alumina to the extent necessary. Dispersions can be prepared which will remain fluid up to days longer than will those prepared from the untreated alumina.

This invention relates to a method for stabilizing alumina slurries against 
viscosity increases. More particularly, this invention relates to a method 
for increasing the fluid life of alumina slurries by heat treating alumina 
under pressure in the presence of water, then forming alumina slurries 
using the recovered alumina and normal peptizing agents. 
In this application and claims, dispersion will mean the apparent uniform 
distribution of alumina particles throughout a dilute acidic solution. A 
stable dispersion will mean a dispersion in which the alumina particles 
remain homogenously dispersed throughout their observed lifetime without 
the use of agitation to keep the particles suspended and the dispersion 
remains fluid. A dispersion that gelled, even though the alumina particles 
remained homogenously distributed, would not be considered stable because 
gelling is a special case of flocculation. These definitions are derived 
from An Introduction to Clay Colloid Chemistry, Second Edition, H. van 
Olphen, pages 16-17, and 27-28, John Wiley and Sons, New York, 1977. 
Aluminas are utilized commercially by placing such aluminas into 
dispersions or suspensions (sometimes called slurries in the art) through 
the use of peptizing agents such as an acid. Examples of such acids are 
hydrochloric acid and nitric acid. Once in a suspension or slurry state, 
these aluminas are commercially used and widely varying applications such 
as frictionizing paper surfaces, fiberglass surfaces, and metal surfaces. 
Alumina is also used as an antistatic and soil protection agent on wool, 
nylon, and acrylic carpets. Alumina is used as a dispersion agent in rug 
shampoos as well as an antistatic or anti-soil agent. Alumina also finds 
use as binders for vacuum cast alumina silica fibers, as a sintering aid, 
and for coating ceramic monoliths for use in auto exhaust catalysts. 
Usually, such dispersible aluminas are of the boehmite structure which 
tend to form more colloidal aqueous dispersions with dilute peptizing 
agents such as acids than do other aluminas such as alpha alumina 
trihydrates. However, unless stabilized in some fashion, many dispersions 
tend to very rapidly form thick gellatinous materials (or gels) and thus 
become unuseable for their commercial applications. It is of great 
importance to maintain the fluid lifetime of a prepared alumina dispersion 
at the desired level for as long as possible. Factors effecting the useful 
lifetime of these dispersions are acid concentration, type of acid 
employed, the type of alumina employed, and the alumina concentration. 
The chemical and physical structure of the boehmite aluminas was not well 
understood for many years. Because "well crystallized" boehmite (also 
known as alpha alumina monohydrate) had been found to have the emperical 
formula of Al.sub.2 O.sub.3.H.sub.2 O it was believed to be a different 
material than psuedoboehmite alumina (also referred to as microcrystalline 
boehmite and gelatinous boehmite) because this latter material contained 
excess water in comparison to the emperical formulation B. R. Baker and R. 
M. Pearson have recently proposed a model in Water Content of 
Psuedoboehmite: A new model for its structure, Journal of Catalysis, 33, 
1974, pages 265-278, which shows these aluminas are both of the same 
physical and chemical structure but differ in water content because of 
their relative crystallite size. While these aluminas are of the same 
family, the smaller crystallite psuedoboehmite (hereafter PB) aluminas are 
much more preferred in applications related to surface phenomena such as 
catalysts and adsorption applications. While dispersions of both the well 
crystallized and PB aluminas can be made we have found that they behave 
differently. The PB aluminas tend to disperse readily, are prone to gel, 
and do not settle. On the other hand the well crystallized aluminas tend 
to behave more as a suspension and tend to show different concentration 
zones in the dispersion, settle out, and resist gelling. We believe that 
well crystallized aluminas do not gel because their dispersed particle 
size is many times larger than the PB aluminas. X-ray diffraction shows 
that common PB aluminas have crystallite sizes in the 25 to 100A range. 
These aluminas, when processed by the techniques of this invention exhibit 
crystallite sizes of 25 to 400 depending on the autoclave temperature and 
time. A well crystallized boehmite alumina prepared by the method 
described in U.S. Pat. No. 4,117,105 in comparison, was greatly in excess 
of 500A. 
The particles of PB aluminas as produced are agglomerations of 
crystallites. In order to disperse these aluminas the agglomerates must be 
reduced to a sufficiently small particles size that such particles stay 
suspended. For some PB aluminas this is readily accomplished by using a 
monovalent acid (HCl, acetic, formic, nitric) as a peptizing agent. 
Divalent and trivalent acids lead to non-dispersing flocculants. Once 
peptized these aluminas readily form a colloidal-like dispersion. However, 
even at relatively low alumina concentrations (.about.5% PB) these 
dispersions may begin to thicken and will eventually gel. Because PB 
aluminas have a plate like crystal structure and because of the behavior 
of the gel, we believe that the crystallites form a face-to-edge 
arrangement or face-to-edge flocculated and aggregated arrangement. If the 
face-to-edge bonding is strong, the gels formed will solidify and be 
difficult to break, but if weak they will break up on agitation and the 
gel will return to a liquid form (thixotropic type gels). In the process 
of this invention we have observed that the bonding force forming the gel 
has been reduced. 
In a dispersion the tendency for the particles to flocculate and form a gel 
or alternatively fall out of suspension is a function of the solids 
concentration, the size of the dispersed particles, and the amount and 
kind of peptizing agent used. With increasing solids concentration the 
probability of particles approaching sufficiently close to be mutually 
attracted and becoming agglomerates increases. For dispersions of equal 
concentration the probability of agglomeration increases with decreasing 
particle size, since more particles are present. This agglormerization 
process will result in either gelling or the particles settling out of 
suspension. It is well known that for dispersions of the same 
concentration that the amount of light that can be transmitted through the 
dispersion increases with decreasing particle size. We have determined by 
this technique that in dispersions of PB alumina particle size decreases 
with increasing levels of acid peptization and with time. We have also 
found that the stronger the acid used as peptizing agent, the higher the 
degree of peptization and the shorter the fluid life. 
If the level of peptizing agent is too low, a portion of the alumina will 
not be sufficiently reduced in size to stay suspended and will settle. The 
FIGURE shows that dispersibility of the alumina increases rapidly, with 
increasing acid concentration, then levels off. Since increased acid 
levels promote gelling, it is common practice to sacrifice some alumina to 
sediment to achieve a longer fluid life. 
In addition, small amounts of salts in the dispersion are reported to 
increase the thickness of boehmite alumins dispersions significantly, as 
set forth in Baymal.RTM. Colloidal Alumina, section 2 "Physical and 
Chemical Behavior" DuPont Chemical Company product brochure. Addition of 
colloidal silica is reported to reduce the thickening of colloidal 
aluminas (Alumina as a Ceramic Material, American Chemical Society, 1970, 
Gitzen, page 113). 
In copending Ser. No. 948,124 filed Oct. 10, 1978 now U.S. Pat. No. 
4,191,737 granted Mar. 4, 1980, alumina slurries or suspensions were 
taught to be stabilized against viscosity increases by treating with water 
which had been heated to temperatures of from about 40.degree. C. to about 
100.degree. C., then cooling, decanting the water and recovering a wet 
cake. Water was then added to the recovered wet cake alumina to obtain a 
slurry with a desired alumina content and CO.sub.2 was sparged through the 
finished slurry. 
In U.S. Pat. No. 4,186,178, alumina dispersions were taught to be 
stabilized against thickening and gelling by digesting the dried alumina 
powder in hot water for a time sufficient to stablize the alumina. The 
digested alumina was recovered and dispersed in water containing a small 
amount of peptizing agent. 
Many of these prior art inventions were useful where alumina slurries are 
utilized and only a portion of the alumina needed to be truly dispersed, 
while the balance could be kept mechanically suspended by agitation. All 
the prior art method incur one or more undesirable limitations. For 
example, the treatment processes result in a wet cake which is difficult 
to handle. In addition, considerable amounts of water are required in the 
treatment process, and if carbon dioxide is used, considerable amounts of 
carbon dioxide are likewise necessary. The treated alumina from the prior 
art processes may require filtering and/or washing before being used in 
the preparation of the dispersion. In addition, continuous stirring may be 
required to keep all the alumina suspended since only a portion may be 
truly dispersed. Continual bubbling of CO.sub.2 through the dispersion or 
slurry may also be needed. The instant invention, in contrast, provides a 
method wherein the alumina is truly dispersed and no mechanical agitation 
is required to keep the alumina suspended. In addition, even longer fluid 
life is obtained than provided by those processes of the prior art. 
Thus it would be of great benefit to provide a method for obtaining a true 
alumina dispersion having a long suspension life before gelling. 
It is therefore an object of the present invention to provide a method for 
making a true alumina dispersion which has a long fluid life. Other 
objects will become apparent to those skilled in this art as the 
description proceeds. 
It has been discovered in accordance with the instant invention that stable 
dispersions of boehmite aluminas in water can be obtained by mixing 
alumina having a crystallite size of from 25 to 400 angstroms with dilute 
aqueous acid solution and treating the alumina prior to dispersion by 
heating to temperatures of from about 250.degree. F. to about 700.degree. 
F. and pressures of from about 10 pounds per square inch gauge (psig) to 
about 2000 psig in the presence of from about 10 to about 80 weight 
percent of water, based upon the total alumina, for a period of time 
sufficient to stablize the alumina to the extent necessary. The alumina 
must be free of sulfate and/or other impurities which cause flocculation 
and precipitation in dispersions. The present invention affects aluminas 
itself and not contaminants. 
In carrying out the process of the instant invention, the time of heating 
can range from about 5 minutes to about 24 hours, although from about 1 
hour to about 8 hours is preferred. The aluminas tested herein are derived 
from the hydrolysis of aluminum alkoxide, although aluminas from other 
sources can be used if significant levels of flocculating contaminants are 
not contained in the alumina and the alumina has a crystallite size of 
from 25 to 400 angstroms. The preferred method of carrying out the process 
of the instant invention is to heat the aluminas at temperatures of from 
about 250.degree. F. to about 500.degree. F. in the presence of from about 
10 to about 30 weight percent water. As this heating in the presence of 
water is done in the sealed autoclave, an autogeneous pressure will 
result. Such a pressure is normally quite sufficient for the stabilization 
of the aluminas described herein. However, extra outside pressure can be 
added as desired. Normally, however, such pressure is not necessary. 
Once recovered from the autoclave, alumina is dispersed at concentrations 
from 5 to 45 weight percent in an aqueous acid solution containing from 
about 0.4 to about 2.0%weight percent monovalent acid. These acids can be 
those well known in the art as represented by nitric, hydrochloric, 
acetic, formic, and in general, monovalent acids. 
The aluminas obtained from the process of the instant invention maintain 
true dispersions for great lengths of time. The physical details of the 
structure alteration in the alumina are not known; however, without being 
bound by any theory, we believe that one or more properties are related to 
the improved fluid life. 
Crystallite Size - is important, since X-ray diffraction shows that 
autoclaved aluminas have progressively larger crystallite size with 
increasing autoclave temperatures and time. In the dispersion process a 
significant amount of the alumina particles are peptized to individual 
crystallites. The larger crystallites are less prone to agglomerate and 
form a gel than smaller crystallites. 
Chemical alterations are important, since thermogravometric analysis (TGA) 
indicates increasing loss of chemically bound water with increasing 
autoclave temperatures. This change in chemistry appears to lessen the 
degree of attractive forces existing between the face and edges of the 
crystallite particles. 
Degree of peptization is important, since light transmittance measurements 
indicate increasing size for the dispersed particles as increasing 
autoclave temperatures are used. This increased size appears to be related 
to the autoclaved alumina not being reduced to aggregates (of 
crystallites) that are as small as those obtained when unautoclaved 
alumina is dispersed. 
The instant invention is more concretely described with reference to the 
examples below wherein all parts and percentages are by weight unless 
otherwise specified. The examples are provided to illustrate the instant 
invention and not to limit it. 
Several experiments were carried out showing the effect of autoclaving 
time, autoclaving temperature and water content upon the process of the 
instant invention. These experiments were generated using from 1000 to 
1500 grams of alumina (CATA `SB`, trademark of and sold by Conoco Inc.) 
which were placed in a 1 gallon autoclave. Distilled water was then added 
to the autoclave, the amount of water normally being equivalent to about 
10 weight percent of the alumina. The autoclave was sealed and brought to 
treatment temperature. The rate of the temperature rise was about 
10.degree. F. per minute. At the desired autoclaving temperature, the 
affect of increasing periods of time were tested. At the end of the 
autoclaving period, the autoclave was allowed to cool to 225.degree. F. At 
225.degree. F. the autoclave was vented to allow the steam to escape to 
prevent the alumina from picking up moisture and becoming a wet cake. 
After cooling to room temperature, the alumina was removed from the 
autoclave as a dry, flowable powder. 
The dispersions tested in the following examples were prepared by first 
measuring the autoclave alumina for water content. Normally the water 
content was from 22.5 to 25 weight percent, the latter being typical for 
CATA `SB` alumina as manufactured). If the water content was outside 
the range, the formula concentration was adjusted so that all dispersions 
would be made on the same basis (each dispersion made at a given 
concentration would contain the same amount of Al.sub.2 O.sub.3). A 
dispersing solution having the desired acid concentration was measured by 
volume and the alumina was poured into the acid solution. During this 
addition, the alumina was continuously stirred with a low shear mixer. 
Mixing was continued for 24 hours after alumina was added to the solution. 
Viscosity and pH were measured at the end of the mixing period. All 
dispersions were checked visually every day to see if such dispersions 
were still fluid. At the end of 30 days the amount of alumina actually 
dispersed was determined by decanting the dispersion from the sediment of 
the alumina which had settled to the bottom of the container. This 
determination could not be made for dispersions that had gelled. 
Samples of precursor (unautoclaved) and autoclaved alumina were examined by 
X-ray diffraction (XRD), thermal gravimetric analysis (TGA), and light 
transmittance (LT) to determine what chemical and physical changes had 
occurred to the alumina.

Example 1 shows the effect of autoclaving time upon the process of the 
instant invention. 
EXAMPLE 1 
Samples of boehmite alumina (CATA `SB`) were autoclaved at 250.degree. 
F. for varying periods of time. The autoclave charge consisted of alumina 
and distilled water equal to 10% of the weight of alumina. After 
autoclaving, the alumina was dispersed at 42 weight percent concentration 
level in 1.4 weight percent nitric acid solution. Table 1 contains a 
summary of the test results. 
TABLE 1 
______________________________________ 
Autoclave Time (hrs.) 
0 2 6 24 
Dispersion Data 
1 hr. 
pH -- 4.0 4.1 3.0 
viscosity (cps) gel 1102 10.60 
55 
3 hrs. 
pH -- -- 4.2 3.5 
viscosity (cps) -- gel 8200 120 
______________________________________ 
Viscosity measurements were made using a Brookfield viscometer. The data 
indicates that at constant temperature the increasing autoclave time 
increases dispersion stability. 
The effect of the amount of water used was tested in Example 2. 
EXAMPLE 2 
Samples of CATA alumina were autoclaved as described in Example 1 except 
the ratio of water to alumina was varied. Test dispersions were made from 
the autoclave treated alumina. Test results showed that increasing the 
amount of water improved the dispersion stability of the alumina. However, 
the treated alumina resulting from higher water contents (above 30% water) 
appeared lumpy and clay-like. This consistency made subsequent handling of 
the treated alumina more difficult although dispersions showed improved 
stability over the untreated precursor alumina. 
TABLE 2 
______________________________________ 
Water (as wt % of alumina) 
10 20 40 80 
Dispersion Data: 
1 hr. 
pH 4.0 3.8 3.9 3.9 
Viscosity (cps) 1102 800 20 25 
3 hr. 
pH -- 3.8 4.1 4.0 
Viscosity (cps) gel 2200 25 25 
______________________________________ 
The effect of autoclaving temperature is set forth in Example 3. 
EXAMPLE 3 
Experimental runs were carried out as set forth in the previous examples 
except that all runs were made for 3 hours and the temperature was varied 
over the range of 250.degree. to 500.degree. F. using 10 weight percent 
water. Test dispersions of the treated alumina were dispersed in acid 
solution. Results of the test are set forth in Table 3. 
TABLE 3 
______________________________________ 
Temperature of Run (.degree.F.) 
250 300 350 400 450 500 
Dispersion Data: 
1 hr. 
pH 4.0 3.0 2.1 1.7 1.5 1.3 
Viscosity (cps) 
180 78 65 168 gel gel 
3 hr. 
pH 4.1 3.1 2.1 1.7.sup.(1) 
Viscosity (cps) 
532 212 65 3140 
______________________________________ 
.sup.(1) After 25 Hrs. 
The data of Example 3 shows that increasing autoclave temperature improved 
dispersion stability up to a point. However, once that point is reached, 
it appears that dispersions ability is detrimentally effected. All 
dispersions carried out have shown that there is an optimum acid level 
which will result in nearly all alumina being dispersed and remaining 
dispersed for some period of time. At the concentration levels tested (42 
percent alumina) it has been found that when the acid level is below the 
optimum, less alumina will be dispersed (more will settle out). If the 
acid level is increased above optimum, fluid life of the dispersion 
decreases. 
Samples of the aluminas autoclaved at 450.degree. and 500.degree. F. were 
dispersed in acid solutions of various concentrations. In all dispersions 
tested, the amount of nondispersed alumina was very small (about 5% of the 
alumina) but was noted to be getting larger at the lowest acid 
concentrated tested. 
It was observed that alumina autoclaved in the above experiments did not 
appear to be receiving uniform heat treatment. This was observed since 
alumina samples from the same autoclave experiment have different 
dispersion behaviors unless the alumina was uniformly blended before 
dispersion testing. 
Cause of this non-uniformity was believed to be due to the inherent poor 
heat transfer of alumina powder, allowing some alumina to receive more 
heat treatment than alumina at other positions of the autoclave. 
Therefore, a new agitator was designed so that the powder would receive 
more mixing action during autoclaving. After installation of the agitator, 
the material received a more uniform heat treatment. 
EXAMPLE 4 
After the improved agitation was installed, additional autoclave runs and 
dispersions were made. Autoclave temperatures of 250.degree. and 
300.degree. F. and dispersion concentrations of 10 to 30 weight percent 
alumina were tested. Tests were carried out as described in Example 1. The 
samples were observed daily for 30 days, viscosity being measured every 7 
days. 
These tests showed that for given acid concentrations, the amount of 
alumina that will disperse is enhanced by increasing autoclave treatment 
temperatures. The data also showed the fluid life of the dispersions is 
greatly improved by autoclave treatment, particularly for higher alumina 
concentrations (30% and higher). Finally, the data showed the dispersions 
made from autoclave treated alumina which gelled were thixotropic; that 
is, with the application of shear, they again became fluid. This is in 
distinct contrast to typical boehmite aluminas which have formed gels 
which remain in such a state even with agitation. The results of these 
experiments showing non-autoclaved alumina, autoclaved for 3 hours at 
250.degree. F., and alumina autoclaved 3 hours at 300.degree. F. are shown 
in Tables 4, 5, and 6, respectively. 
TABLE 4 
______________________________________ 
Not Autoclaved 
Concen- Vis- 
Nitric Percent tration cosity 
Acid of of Final 
after 
Concen- Alumina Dispersion 
30 days 
Alumina Treatment 
tration Dispersed (%) (cps) 
______________________________________ 
A. 10% Alumina in 
Acid Solution 
Mixture 0.2 60 6.0 
0.4 93 9.3 
0.6 98 9.8 -- 
0.8 96 9.6 520 
B. 15% Alumina in 
Acid Solution 
Mixture 0.2 23 3.5 -- 
0.4 82 12.4 -- 
0.6 93 13.9 -- 
0.8 96 14.5 -- 
C. 20% Alumina in 
Acid Solution 
Mixture 0.2 16 3.2 -- 
0.4 42 8.4 -- 
0.6 85 19.0 10,000 
0.8 93 18.6 -- 
1.0 93 18.7 410 
D. 30% Alumina in 
Acid Solution 
Mixture 0.6 (5) -- gel 
0.8 (5) -- gel 
1.0 (5) -- gel 
1.2 (5) -- gel 
1.4 (5) -- gel 
______________________________________ 
TABLE 5 
__________________________________________________________________________ 
Autoclaved 3 Hours at 250.degree. F. 
Nitric Concen- 
Acid Percent tration 
Viscosity 
Concen- 
of of Final 
after 
tration 
Alumina Dispersion 
30 days 
Alumina Treatment 
(Wt %) 
Dispersed 
(%) (cps) 
__________________________________________________________________________ 
A. 
10% Alumina in 
Acid Solution 
Mixture 0.2 73.7 7.3 -- 
0.4 98 9.8 -- 
0.6 98 9.8 -- 
0.8 
B. 
15% Alumina in 
Acid Solution 
Mixture 0.2 89 13.4 -- 
0.4 97 14.6 -- 
0.6 98 14.6 75 
0.8 
C. 
20% Alumina in 
Acid Solution 
Mixture 0.2 66 13.2 -- 
0.4 95 18.9 -- 
0.6 98 19.6 -- 
0.8 100 20 &gt;10,000* 
D. 
30% Alumina in 
Acid Solution 
Mixture 0.6 72 21.6 930 
0.8 93 27.9 250 
1.0 10,000+* 
1.2 gel (&lt; 1 day)* 
1.4 
__________________________________________________________________________ 
*Thixotropic- 
TABLE 6 
__________________________________________________________________________ 
Autoclaved 3 Hours at 300.degree. F. 
Nitric Concen- 
Acid Percent tration 
Viscosity 
Concen- 
of of Final 
after 
tration 
Alumina Dispersion 
30 days 
Alumina Treatment 
(Wt %) 
Dispersed 
(%) (cps) 
__________________________________________________________________________ 
A. 
10% Alumina in 
Acid Solution 
Mixture 0.2 89 8.9 -- 
0.4 99 9.9 -- 
0.6 97 9.7 -- 
0.8 
B. 
15% Alumina in 
Acid Solution 
Mixture 0.2 96 14.4 -- 
0.4 99 14.9 -- 
0.6 99 14.9 75 
C. 
20% Alumina in 
Acid Solution 
Mixture 0.2 
0.4 89 17.7 -- 
0.6 97 19.4 -- 
0.8 99 19.9 375* 
1.0 100 20.0 10,000+* 
D. 
30% Alumina in 
Acid Solution 
Mixture 0.6 
0.8 93 28 -- 
1.0 98 29.5 10,000+* 
1.2 gel (2 days)* 
1.4 -- -- gel (11/2 hrs)* 
__________________________________________________________________________ 
*Thixotropic- 
In these tables the percent of alumina dispersed is that alumina in the 
dispersion which did not settle out. Concentration of final dispersion is 
that alumina in the final dispersion after setting unstirred for 30 days. 
A dash in the viscosity column indicates that the viscosity was below 
measurable limit, that is, water thin. In Table 4 some samples gelled in 
less than 1 day when 30% alumina was used and the amount of alumina 
dispersed could not be determined. In Tables 5 and 6, those numbers marked 
with an asterisk indicate thixotropic dispersions. 
EXAMPLE 5 
Samples of alumina were autoclaved at varying temperatures using the 
procedure described in example 3. These samples and the precursor were 
examined by TGA, X-Ray diffraction and light transmittance through 
attempted 10 wt% dispersions. Table 7 clearly shows that as autoclaving 
temperatures are increased, chemically bound water is lost, both the 
crystallite and dispersed particle sizes are increased and the 
dispersibility of the alumina is increased. 
TABLE 7 
______________________________________ 
XRD TGA 
Auto- Crystallite 
Chemically 
Disper- 
Light 
clave.sup.(1) 
Size (A) Bound H.sub.2 O 
sibility.sup.(2) 
Transmittance.sup.(3) 
Tempera- 
Reflection 
Removed (wt % (% of 
ture (.degree.F.) 
020 021 (Wt %) Al.sub.2 O.sub.3) 
Total Light) 
______________________________________ 
Precursor 
32 46 0 77 87 
250 51 91 2.4 95 81 
300 51 102 2.9 93 23 
350 74 117 3.8 94 23 
400 116 205 4.1 85 4 
______________________________________ 
.sup.(1) All autoclave runs were 3 hours at temperature 
.sup.(2) Alumina was added to 0.4 wt % HNO.sub.3 to make a 10 wt % 
mixture. The mixture was then stirred and centrifuged. The amount of 
alumina that settled out was determined and the % dispersed calculated as 
DISPERSIBILITY (wt %) =- 
##STR1## 
.sup.(3) Light transmittance through the dispersions were measured with a 
Bausch & Lomb model Spectronic 88 Spectrophotometer, wavelength 800 
nanometers. 
EXAMPLE 6 
Samples of alumina were autoclaved at varying temperatures using the 
procedures described in Example 3. A 10 wt% alumina dispersion of these 
samples and unautoclaved precursors in 0.5 wt% nitric acid solution were 
made and examined by light transmittance over a 3 day period. Table 8 
shows that light transmittance and hence degree of peptization increases 
with time, meaning that the dispersed particle size also decreases with 
time. 
TABLE 8 
______________________________________ 
Pre- 
Autoclave Temperature (.degree.F.).sup.(1) 
cursor 250 300 350 400 
______________________________________ 
Alumina Dispersed (wt %).sup.(2) 
91.3 96.9 94.9 95.9 87.8 
Light Transmittance 
(% of Total).sup.(3) 
At Time 
3 minutes 82.0 60.5 46.0 10.0 0.2 
1 hour 84.2 69.7 54.5 15.5 0.5 
2 hour 85.5 72.2 60.5 18.2 0.8 
3 hour 86.2 75.0 64.2 21.0 1.0 
4 hour 86.9 76.0 65.7 22.2 1.2 
5 hour 87.5 76.5 67.2 23.5 1.6 
6 hour 87.8 77.3 68.5 24.0 1.9 
7 hours 88.2 77.8 68.5 25.2 2.0 
24 hours 89.3 80.1 74.2 31.1 4.0 
48 hours 90.2 81.0 75.1 34.1 5.2 
72 hours 91.0 81.2 76.0 36.4 6.0 
______________________________________ 
.sup.(1) All autoclave runs 3 hours temperature 
.sup.(2) Alunima was added to 0.5 wt % HNO.sub.3 to make a 10 wt % 
mixture. The mixture was stirred and then centrifuged. The amount of 
alumina that settled out was determined and the % dispersed calculated as 
##STR2## 
.sup.(3) Light transmittance through the dispersions were measured with a 
Bausch & Lomb, Model Spectronic 88, spectrophotometer. Wavelength 800 
nanometers. 
Thus the instant invention provides a vastly improved method for 
stabilizing alumina dispersions for greatly increased lengths of time 
while allowing the alumina to be easily handled, constant attention is not 
required except in the case of thixotropic dispersions which can be easily 
converted to fluid dispersions under agitation. The alumina obtained is in 
the form of a dry powder, is easily handled, the pre-treatment is simple 
and does not require exotic equipment. 
While certain embodiments and details have been shown for the purpose of 
illustrating this invention, it will be apparent to those skilled in this 
art that various changes and modifications may be made herein without 
departing from the spirit or scope of the invention.