Method of producing an alumina dispersible at a pH greater than 8

Abstract

A method of making an alumina including providing an alumina slurry, aging the slurry, adding a tricarboxylic acid to the aged alumina slurry, further aging the slurry, and spray drying, the method being characterized by the addition of a dicarboxylic acid either at the same time as the tricarboxylic acid, or after the second aging and before the spray drying. The resulting alumina is dispersible at a pH greater than 9.5 above 95% and has a viscosity below 0.4 Pa.Math.S for 10 wt % sols.

Claims

1. A method of making an alumina which is highly dispersible at a pH of greater than 8 including the following steps: i) providing an alumina slurry which includes aluminum oxyhydroxide, aluminum oxide, aluminum hydroxide, or mixtures thereof; ii) aging the alumina slurry to form an aged alumina slurry including alumina having a crystallite size of from 38 to 450 Å for the 120 crystal plane; iii) adding a tricarboxylic acid to the aged alumina slurry to form an acid modified slurry; iv) aging the acid modified slurry at a temperature of between 75° C. and 125° C. to form a product slurry; and v) spray drying the product slurry to form the alumina with a dispersibility at a pH of greater than 8 above 90% and a viscosity below 1 Pa.Math.S for 10 wt. % sols, the method being characterised by the addition of a dicarboxylic acid either in step iii) of the process with the tricarboxylic acid or by adding the dicarboxylic acid to the product slurry after step iv) before spray drying in step v).

2. The method of claim 1, wherein the alumina slurry includes Boehmite, Bayerite, Gibbsite, gamma-alumina, transitional (delta-theta) aluminas and mixtures thereof.

3. The method of claim 1, wherein the alumina slurry has a pH of 6-10.

4. The method of claim 1, wherein the alumina slurry is aged by heating to a temperature of 95-220° C. for a period of from 30 minutes to 8 hours.

5. The method of claim 4, wherein after aging, the alumina slurry includes alumina having a crystallite size of from 40 to 180 Å for the 120 crystal plane.

6. The method of claim 1, wherein the tricarboxylic acid includes citric acid, isocitric acid, aconitic acid, tricarballylic acid, trimesic acid and mixtures and/or derivatives thereof.

7. The method of claim 1, wherein once the tricarboxylic acid is added (either alone or with the dicarboxylic acid), the pH of the acid modified slurry is between pH of 1 and 6.

8. The method of claim 1, wherein the acid modified slurry is aged for a time period of between 10 minutes to one hour.

9. The method of claim 8, wherein the acid modified slurry is aged at a temperature of between 85° C. and 115° C.

10. The method of claim 1, wherein the dicarboxylic acid includes malonic acid, succinic acid, gluatric acid, adipic acid, pimelic acid, maleic acid, fumaric acid, glutaconic acid, muconic acid, citraconic acid, mesaconic acid, or mixtures thereof.

11. The method of claim 9 wherein the dicarboxylic acid is added to the aged alumina slurry at the same time as the tricarboxylic acid by premixing the acids.

12. The method of claim 9 wherein the dicarboxylic acid is added to the aged alumina slurry sequentially with the tricarboxylic acid.

13. The method of claim 9 wherein the dicarboxylic acid is added to the product slurry.

Description

EXAMPLES

(1) The invention will now be described by way of non-limiting examples and a FIGURE, where:

(2) FIG. 1 shows the viscosities of alumina dispersions at 5 and 10 wt % as per Examples 1 and 2, respectively, and Comparative Example 2 demonstrating the effect that dicarboxylic acid addition as per the invention has on the viscosity of the product slurry.

(3) Dispersibility is measured by first producing an alumina dispersion at the indicated pH. This is done by adding 10 wt. % alumina to an aqueous solution of ammonium hydroxide (having a pH of 10). The pH is adjusted using a concentrated solution of ammonium hydroxide. The dispersion is stirred for 30 minutes. The dispersion is then centrifuged for 30 minutes following which the supernatant is decanted off. Any residual powder is dried at 120° C. and massed. The dispersibility is calculated by subtracting the mass of the residue after drying from the mass of the powder added, and then dividing by the mass of the powder added and finally multiplying the result by 100.

(4) Viscosity is measured by first producing an alumina dispersion at the indicated pH and a solids loading of 10 wt. % This is accomplished by adding the alumina to an aqueous solution of ammonium hydroxide and adjusting the pH using ammonium hydroxide as required. The resulting slurry is then stirred for 30 minutes. A small amount of slurry is then transferred to the base plate of a TA instruments DHR2 rheometer which is temperature equilibrated at 25° C.

(5) The 40 mm flat plate geometry is lowered to the requisite gap and any slurry pushed from the gap trimmed. If the slurry added is insufficient to completely fill the area under the plate, the plate is raised and additional slurry added. The instrument is initiated with a shear rate of 100 s.sup.−1 and the viscosity recorded.

Example 1—Preparation of a Malonic/Citric Acid Modified Alumina

(6) A Boehmite slurry was produced via the hydrolysis of aluminum alkoxides and hydrothermally aged for 2 hours at 120° C. to obtain an alumina having a crystallite size of 95 Å (120 crystallite plane). 5 wt. % citric acid and 3 wt. % malonic acid (weight percentage on the basis of alumina content) were premixed in water and then added to the boehmite slurry to form an acid modified slurry. The acid modified slurry was aged for 1 hour at 105° C. to form a product slurry and the product slurry was then spray dried and product collected.

Example 2—Preparation of a Citric Acid Modified Alumina, Post Modified with Malonic Acid

(7) A Boehmite slurry was produced via the hydrolysis of aluminum alkoxides and hydrothermally aged for 2 hours at 120° C. to obtain an alumina having a crystallite size of 95 Å (120 crystallite plane). 5 wt. % citric acid (weight percentage on the basis of alumina content) was added to the boehmite slurry to form an acid modified slurry. The acid modified slurry was then aged at 105° C. for 1 hour to form a product slurry. The product slurry was then modified by the addition of 3 wt. % malonic acid and stirred for 30 min. The modified product slurry was then spray dried and product collected.

Comparative Example 1—Example 1 without the Aging Step IV)

(8) A Boehmite slurry was provided as per Example 1 and hydrothermally aged to obtain an alumina having a crystallite size of 95 Å (120 crystallite plane). 5 wt. % citric acid and 3 wt. % malonic acid (weight percentage on the basis of alumina content) were premixed in water and added to the boehmite slurry to form an acid modified slurry. The acid modified slurry was then spray dried and the product collected.

Comparative Example 2—Preparation of a Citric Acid Modified Alumina

(9) A Boehmite slurry was provided as per Example 1 and hydrothermally aged to obtain an alumina having a crystallite size of 95 Å (120 crystallite plane). 5 wt. % citric acid (weight percentage on the basis of alumina content) in water was added to the boehmite slurry to form an acid modified slurry. The acid modified slurry was aged for 1 hour at 105° C. and then spray dried and the product collected.

Comparative Example 3—Example 2 with the Malonic Acid Aged with the Alumina and Citric Acid Added Post-Aging

(10) A Boehmite slurry was provided as per Example 1 and hydrothermally aged to obtain an alumina having a crystallite size of 95 Å (120 crystallite plane). 3 wt. % malonic acid (weight percentage on the basis of alumina content) was added and the boehmite slurry which was then aged at 105° C. for 1 hour. The slurry was then modified by the addition of 5 wt. % citric acid and stirred for 30 min to form a product slurry. The product slurry was then spray dried and the product collected.

(11) The results are included in Table 1 hereunder and some illustrated in FIG. 1.

(12) TABLE-US-00001 TABLE 1 Viscosity of 10% sol Dispersibility at pH 10 Sample @ pH 10 (30 min) Example 1 97.5% 0.156 Pa .Math. S Example 2 .sup. 98% 0.180 Pa .Math. S Comparative Example 1 Gelled Gelled Comparative Example 2 95.5%  1.3 Pa .Math. S Comparative Example 3 Gelled Gelled

(13) As illustrated by the results in Table 1, citric acid modified boehmites (Comparative Example 2) are indeed highly dispersible, however the viscosity of the materials, 1.3 Pa.Math.S, is too high for use in many potential applications. The co-addition of malonic acid with the citric acid to the slurry before hydrothermal aging (Example 1) produces a material with slightly higher dispersibility (97.5%) and a much lower viscosity (0.156 Pa.Math.S). This is sufficiently fluid to be used in many applications. Alternatively, the malonic acid can be added to the alumina slurry after it has been aged with the citric acid (Example 2), without any negative effects compared to Example 1. In fact the materials produced are nearly identical with respect to dispersibility and viscosity.

(14) The order of addition and aging of these materials is an important step in the production of a highly dispersible material capable of producing low viscosity high pH sols. In the case where the alumina is not aged in the presence of the acids (Comparative Example 1) the material gels and does not produce a dispersion. When the malonic acid alone is aged with the alumina, and the citric acid added post-aging (Comparative Example 3) the material also gels when dispersed at a pH of 10.

(15) FIG. 1, further shows the difference in sol viscosity for a dispersion produced at 10 wt % and pH 10 using the samples produced in Examples 1 and 2 compared to Comparative Example 2 under the same conditions. The samples prepared using both malonic and citric acids have sol viscosities substantially lower than that of the sample produced using citric acid alone. After 30 minutes the viscosity of the sol produced using the material from Example 1 has a viscosity of 0.156 Pa.Math.S, while that from Example 2 a viscosity of 0.180 Pa.Math.S, both of which are substantially lower than that of the sol prepared from the material in Comparative Example 2, which had a viscosity of 1.3 Pa.Math.S.