Studies on amorphous aluminium hydroxide precipitates (gels)

Summary

Amorphous (and partly micro-crystalline) aluminium hydroxide precipitates were prepared by controlled hydrolysis of aluminium isopropoxide and aluminium sulphate solutions: the kinetics of their dissolution in well-stirred sodium hydroxide solutions (0.25 to 3 g ion/l) in large excess was studied at 5° to 35°C.

The reaction is chemically rate-controlled in dilute hydroxide solution. The dissolution of the non-aggregated 30–40 per cent (amorphous) material follows a four-third order mechanism with respect to powder weight; the aggregated material reacts far less rapidly.

The rate constants for initial dissolution vary linearly with mean ionic activity of the hydroxide solution.

The mechanism of dissolution is similar to that of the dissolution of crystalline boehmite; for this reaction, the rate-controlling step is a reaction between reactive AlOOH sites on the powder surface and hydroxyl ions adsorbed near these sites.

For the dissolution of the most amorphous (unaggregated) material,k _w1(ata _±=1) at 5°C=6.0g^−1/3hr⁻¹,k _{w 1} at 20°C=22.4 g^−1/3hr⁻¹.E _act=14.500 cal/mole.

Micro-crystalline material reacts at one-third to one-half the above rates andE _act=16.000–17.000 cal/mole.