Note: Descriptions are shown in the official language in which they were submitted.
2cn36~,s
PROCE8~ FOR ~RE PRODUCTION OF DI~PER~IBLE BOEHMITE
CROSS-REFERENCE TO REL~TED APPLICATION
FIELD OF TIIE INVE~TION
This invention relates to a process for converting
~ aluminum trihydroxide into boehmite*particles suitable for *~
- ~ ~';~ forming aqueous dispersions which are stable, do not gel, and
retain a high percentage of the particles in suspension.
BACKGROUND OF THE INVENTION
Particulate boehmite is used in the glass, catalyst and
ceramic industries. Commercially, boehmite particles are pre-
~ pared by digestion of aluminum trihydroxide ~gibbsite, chemi-
: ~"~ 15 cal formula: A1203.31l2O) in water at temperatures of 200-250 C.
'I~ For example, U.S. Patent No. 3,954,957 to Xoenig describes a
process for the production of boehmite pigment starting from
gibbsite. The process involves preliminary grinding oE gibb-
site to an average particle size of 1-3 microns, and digesting
the ground gibbsite in the presence of a controlled amount of
mineral acid at temperatures between about 180--250-C for 0.5 -
120 minutes. This leads to the production of a boehmite pro-
duct of uniform particle size having a particle size range of
between about 0.2 - 0.7 microns.
~) (alpha-alumlna monohydrate)
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It is also known that when crystallized gibbsite, ob-
tained from the well-known Ba~er Process for producing alumina
from bauxite, is heated to a temperature in the range of 120~C
to 300 C, conversion of the gibbsite to measurable amounts of
boehmite occurs, particularly if the heating is rapid and
coarse gibb~ite particles are used. (See, for example, Oxides
and ~Iydroxides of Aluminum Tecllnical Paper No. 19, Alcoa
Research Laboratories, 1972.) This is known as the solid state
reaction since it takes place in the absence of added water.
The boehmite produced by solid state reaction is embedded in a
matrix of activated material, whi{~-H~e~ ~ ~e-rem~e~-~n-a
~B~_ ~e~pe~Y~ v ~n~e~s~h~smi-t~. In the discussions
~9 that follow below, the term ~activatedn refers to the non-boeh-
mite components of the heated gibbsite material.
In U.S. Patent Application serial number 013,009, a
method is disclosed for producing crystalline boehmite of fine
particle size which has a substantially higher than expected
surface area, providing a new particular boehmite for use as a
filler grade pigment.
In addition to producing fine particle size filler
~B grade pigment, dispersions or suspensions of a~u~lnu~-~oRohy-boe ~ te
~9~9 ~r~e in water are also needed. Generally, dispersions or
suspensions in water are initially formed using peptizing
agents such as monoprotic acids (HCl, HNO3) or bases such as
sodium hydroxide. Such dispersible aluminas are preferably
based on boehmite which forms more stable dispersions with
dilute peptizing agents than do other aluminas, such as
aluminum trihydroxides or gamma alumina.
A number of methods can be used for preparing disper-
sible boehmite particles, such as hydrolizing organic aluminum
~B~ compounds, for example as disclosed in U.S. Patent No. 2,636,8-
~989 65, neutralizing solutions of sodium aluminate with sulfuric
acid, for example as in U.S. Patent No. 2,590,833, or through
hydrothermal conversion of aluminum trihydroxide, as for
- 35 example in U.S. Patent No. 4,117,105.
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- 3 - 23828-52
An important property of the dispersion is the ability
to maintain a particular viscosity at a desired level for as long
as possible. In other words, it would be undesirable to have
boehmite solids settling out, thinning the solution, or to have
the boehmite gel and thicken the dispersions. Generally, simply
placing boehmite in a solution of water does not maintain the
required viscosity over time. Consequently, a number of methods
have been developed for stabilizing boehmite dispersion such as
those disclosed in United States Patent No. 4,179,940, United
States Patent No. 4,191,737 and European Patent No. 0 025 817 Bl.
However, a method for directly obtaining boehmite dispersions with-
out requiring additional processing steps or additives to control
stability would be preferred.
It therefore is an object of the present invention to
also provide a method for the production of dispersible boehmite
particles which form stable suspensions, retaining a high degree
of solids in suspension without gelling over time.
SUMMARY OF THE INVENTION
The present invention provides a process for the pro-
duction of dispersible boehmite particles which comprises the steps
of calcining gibbsite particles at temperatures above 350C to
produce boehmite embedded in a matrix material, dissolving the
matrix material in a heated alkali or acid solution, and recovering
boehmite particles having an L.O.I. of less than 10~.
In a preferred embodiment the matrix material is
essentially chi-A12O3 and is essentially free of gibbsite, gamma-
A12O3 and alpha-A12O3.
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- 4 - 23828-52
According to the present invention, a method for pro-
ducing a dispersible boehmite is disclosed which converts relative-
ly inexpensive coarse aluminum trihydroxide to finely divided
particles of boehmite which can be readily formed into stable
aqueous dispersions which do not gel, without utilizing any addi-
tives or additional processing steps. The process comprises the
steps of calcining gibbsite particles to produce boehmite embedded
in an activated matrix (chi-alumina), dissolving the chi-alumina
in a heated alkali or acid solution to liberate the boehmite par-
ticles contained therein, separating the boehmite particles from
the liquid phase and forming a slurry of the particles in water to
which an acid peptizing agent is added. It has been found that
by heating the starting gibbsite to a temperature above 350C and
heating long enough to lower the L.O.I. to less than 10%, prefer-
ably 7-10%, from the initial gibbsite L.O.I. of approximately 35%,
the recovered boehmite becomes exceptionally fine and free of
impurities, forming dispersions without additives or additional
processing steps. Essentially pure crystallized particles of
boehmite are obtained, which have a specific surface area in the
range of about 30-60 m2/g, a particle size in the range of about
0.01-0.5 ~m and a soda content, measured as Na2O, within the range
of about 0.01-0.10% by weight.
In a preferred embodiment the dissolving step is con-
ducted at a temperature ranging between about 60C and 120C for
a time sufficient to produce a product which has a boehmite content
of at least 95%.
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- 4a - 23828-52
sRIEF D RIPTION OE THE DRAWINGS
Figure 1 is a graph depicting the properties of
calcined glbbsite.
Figure 2 is a flow sheet of the steps for preparing a
dispersible boehmite according to the present invention.
Figure 3 shows electron micrographs of the resultant
product boehmite (a) (5,000X magnification) and (b) (lO,OOOX
magnification).
DETAILED DESCRIPTION OF THE INVENTION
The process for the production of dispersible boehmite
in accordance with the present invention, starts with the coarse
gibbsite particles of the Bayer process, and subjects them to
light calcination (i.e., heating to a high temperature but below
the melting or fusing point, causing a partial loss of moisture).
The gibbsite (aluminium trihydroxide) from the Bayer
Process, consisting for example, of gibbsite particles (90% greater
than 45 ~m in size) is subjected to light calcination, typically
in a rotary oven at temperatures of greater than about 350C, (for
rotary oven calcination) for a period long enough to lower the
loss on ignition (L.O.I.) of the 'activated' gibbsite to less than
10%, but optimally 7%-10%. The higher the temperature, the shorter
the retention time required. Under these conditions, the lightly
calcined material contains about 30% of crystalline boehmite, as
identified by X-ray diffraction, embedded in a matrix of which
approximately half is chi-alumina (a thermodynamically unstable
transition form of alumina containing appreciable amounts of
hydroxide ions) with the remainder being material o~ such small
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particle size as to render it non-identifiable by X-ray dif-
fraction. The activated matrix material contains little or no
gibbsite, alpha-A1203 or gamma-A1203, none of which have the
enhanced solubility in caustic or acid liquors characteristic
of the activated matrix material of the present invention.
The properties of the activated gibbsite under various
calcination conditions are presented below in Table 1 and ~B
graphically in Figure 1. 29~8
.
Image
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Table 1 shows that the boehmite formed during calcina-
tion is formed relatively quickly. Furthermore, the amount of
boehmite is relatively constant at 28-31% in the temperature
range of 300-400-C. The results also demonstrate the
importance of controlling the L.O.I. of the calcined gibbsite.
Figure 1 graphically demonstrates the relationship
between the specific surface area, solubility and L.O.I. of the
boehmite-containing 'activated' gibbsite. Referring to Figure
1 it can be observed that the maximum points in solubility and
specific area curves do not coincide. The maximum solubility
occurs at 10-13% L.O.I., while the maximum surface area is
obtained at 6-8% L.O.I. Without wishing to be bound by theory,
it is believed that at approximately 400 C, the boehmite
initially formed begins to transform to gamma-alumina, which is
less soluble in either an alkali or acid solution than the
activated matrix material that forms directly from the initial
gibbsite calcination.
The process for producing the dispersible boehmite
generally follows the process previously described in U.S.
patent application serial no. 013,009, commonly assigned
herewith, for converting coarse particles of aluminum trihy-
droxide into fine boehmite particles of an average size from 1-
2 um and having a specific surface area in the range of 20-40
m2/g with a soda content in the range of 0.25-0.50% by weight.
However, several of the processing steps must be modified to
produce the dispereible boehmite as hereinafter discussed.
The conditions for producing fine boehmite particles
are represented graphically in Fig. 1 and correspond to the
lower maximum at 10-13% on the L.O.I. axis. These conditions
approximate calcining aluminum trihydroxide at about 350- for
20 minutes. The maximum in the curve signals the beginning of
the conversion of chi-alumina to a species having a lower
specific surface area and lower solubility in caustic liquor.
For producing the dispersible boehmite, we refer now to the
second ~naximum shown in Fig. 1, i.e., that corresponding to 7-
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10% on the L.O.I. axis. The continuing increase in specific
surface area betwe~n the two maxima, despite the falloff in
alumina in solution, is generally believed due to the physical
alteration Or the boehmite which accompanies the conversion to
gamma alumina and therefore has no influence on the solubility
characteristics of the calcined material. Consequently,
boehmite particles embedded in the chi-alumina calcined at
400-C for 30 minutes, to give an L.O.I. in the range of about
7-10, are significantly finer than the boehmite particles
present in the material calcined at 350-C for 20 minutes which
give an L.O.I. in the range of 10-13%. Consequently, on
removing the activated matrix material by dissolution in either
an alkali or acid solution, the boehmite particles liberated
are very finely divided, consisting of particles in the size
range of about 0.01-0.50 um. Surprisingly, a substantial
amount of these particles remain in aqueous dispersion when a
peptizing agent such as an acid is added the~reto. Generally, a
monovalent acid may be used as the peptizing agent, such as,
for example, nitric, hydrochloric, acetic or formic acid. A
flow sheet of this process is shown in Figure 2, with typical
operating data.
Figures 3A and 3B are scanning electron micrographs of
a dispersible boehmite obtained according to the present method
at 5,000X and lO,OOOX magnification.
Image
x~n3~ s
Referring to Table 2, data is shown which was obtained
for boehmite particles liberated from calcined aluminum
trihydroxide using 5N sodium hydroxide (Tests 1-5) and 18N
hydrochloric acid (Tests 6 and 7), with a dissolution period of
six hours at about 95-C. From the results shown, it was found
2B~ that calcin-~ing aluminum trihydroxide at 400-C at 30 minutes
~989 led to the highest percentage of fine boehmite remaining in
aqueous dispersion after a period of 21 days. This long-lived
disper6ion was accomplished without utilizing additives or any
other processing steps.
The dissolution of the matrix material could also have
been carried out in sodium aluminate or a Bayer liquor as
previously described in the process for the production of
boehmite. While such a solution might have been thought to be
required to guarantee against dissolution of the boehmite
during removal of the activated matrix material, it was found
that this was not necessary when dissolving the matrix material
tJ~ associated with boehmite obtained from calcin~ing the gibbsite
~989 at the higher temperatures which achieve an L.O.I. value of
~ 20 less than 10%. Only in test no. 1, where the calcina~ing
2 ~o~- temperature was about 320-C with an L.O.I. of 18%, was there
~98q any significant loss of boehmite due to dissolution in the hot
caustic solution. Consequently, temperatures below about 350-C
are to be avoided as these produce boehmite of the wrong
particle size for dispersion. It was also found that the
amount of boehmite obtained in tests 2-7, i.e., after dissolu-
tion Or the activated matrix material, agreed with the percent
boehmite available as described by x-ray diffraction analysis.
Another advantage of producing dispersible boehmite i~
in the low value of impurities, particularly soda content.
Very low soda values were obtained, particularly in test nos. 3
and 7, which i6 indicative of the relea6e of the soda which
would otherwise remain trapped at internal 6urfaces and
boundaries within larger boehmite particles.
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11
Exam~le
Boehmite was obtained by calcining aluminum trihydrox-
ide at 400 C for 30 minutes, with dissolution in 5N sodium
hydroxide for 6 hours at 95 C. The boehmite was recovered by
filtration. The boehmite was dispersed in water at an amount
of about 10-30% by weight, at various acidities ranging from
0.01-0.10 moles nitric acid per mole boehmite. The boehmite
was added to the dispersing solution already containing the
appropriate amount of acid. After mixing for about 30 minutes,
viscosities were measured using a HAAKE falling-ball vis-
cosimeter. The suspensions were set aside undisturbed for 23
days with the amount of boehmite remaining dispersed after 23
days subsequently determined by decanting each dispersion from
the sediment at the bottom of the test flask.
Table 3
Viscosity ~ehavior of Boehmite DisPersions
SU~T A~ 23 DAYS
Acid Added~x~mite in Viscosity~x~mite in Viscosity
(moles HNO3 perDispersion at 22-C Dispersion at 22-C
mole ~x~mite) (%) (cps) (%) (~rc)
10.0 1.36 5.89 1.20
0.01 20.0 1.42 10.9 1.23
30.0 1.79 15.0 1.32
10.0 1.46 ~.75 1.40
0.05 20.0 3.00 19.8 3.56
30.0 7.90 29.8 9.47
10.0 2.60 9.94 2.50
0.10 20.0 13.3 19.6 9.89
30.0 -* 28.8 -~
* Outside measurement range of instrument
Referring to Table 3, there is shown that for a given
percent solids, there is a minimum acid level which result~ in
practically all the boehmite particles remaining in dispersion.
None of the dispersions gelled during the 23-day period.
Therefore, the process of the present invention provides
dispersible boehmite particles which are stable for an extended
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12
period of time, do not gel, and consequently remain easy to
handle and are therefore suitable for use in applications
requiring dispersible alumina. Moreover, since no additional
processing steps such as digesting with hot water, sparging
with carbon dioxide gas or autoclaving are required, the
dispersions are more easily and readily produced in high
quantity yet at low cost.
The lnvention has been described above with reference
to preferred embodiments. It would be obvious to one of
ordinary skill in the art that many additions, substitutions
and/or deletions can be made without departing from the scope
of the invention as claimed below.
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