CA2003669A1

CA2003669A1 - Process for the production of dispersible boehmite

Info

Publication number: CA2003669A1
Application number: CA002003669A
Authority: CA
Inventors: Neil Brown
Original assignee: Vereinigte Aluminium Werke AG
Current assignee: Vereinigte Aluminium Werke AG
Priority date: 1988-12-03
Filing date: 1989-11-23
Publication date: 1990-06-03
Also published as: AU4572589A; DE3840862A1; EP0372210A1; JPH03115118A

Abstract

ABSTRACT
The present invention is directed to a process for producing a finely divided dispersible boehmite (alpha-alumina monohydrate) from coarse grain particles of aluminum trihydrox-ide. The aluminum trihydroxide is subjected to calcination in an amount sufficient and for a time sufficient to produce very fine boehmite particles embedded in a matrix chi-alumina, with the matrix material dissolved in either an alkali or acid solution, thereby liberating finely divided boehmite particles of high purity which can readily be formed into aqueous dispersions which are stable and do not gel on standing for long periods of time.

Description

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) ZC~3fi~i9 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.

X(~3~9 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.

2cn3~;~i9 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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:` .' ' .
' `

~()3~.9 - 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 ~C~)36fiS

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 .

~C~)36fi9 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-ZC~136fi9 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.

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.

ZC~)3~;~i9 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 '` ' .

ZC~)3fi69 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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Claims

1. A process for the production of dispersible boehmite particles which comprises the steps of calcining gibbsite particles at temperatures above 350°C 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%.

2. The process of claim 1 wherein the matrix material is essentially chi-A1203 and is essentially free of gibbsite, gamma-A1203 and alpha-A1203.

3. The process of claim 1 wherein the dissolving step is conducted at a temperature ranging between about 60 C and about 120-C, for a time sufficient to produce a product which has a boehmite content of at least 95%.

4. The process of claim 1 wherein the alkali solution is a sodium hydroxide or sodium aluminate solution.

5. The process of claim 1 further comprising dispers-ing the recovered boehmite particles in an aqueous acid solution containing at least 0.01 moles of a monovalent acid per mole of boehmite.

6. The process of claim 5 wherein from 0.01-0.10 moles of a monovalent acid is added per mole of boehmite.

7. The process of claim 5 wherein said monovalent acid is from the group consisting of nitric acid, hydrochloric acid, acetic acid, and formic acid.

8. The process of claim 1 wherein the dispersible boehmite particles obtained have a particle size in the range of 0.01-0.50µm and a soda content in the range of 0.01-0.10%
by weight.

9. The process of claim 1 wherein the recovered boehmite has a specific surface area Or about 30-70 m2/1.

10. The process of claim 1 wherein the gibbsite particles are calcined in an amount sufficient to achieve an L.O.I. in the range of about 7-10%.

11. The product produced by the process of claim 1.

12. Dispersible boehmite crystals having an average particle size of 0.01-0.50 µm, an L.O.I. of less than 10%,a specific surface area in the range of 30-70 meters squared per gram, and a soda content in the range of 0.01-0.10% by weight.