Role of water in the aging of aluminum hydroxide suspensions.

The rate of acid neutralization of anhydrous suspensions of amorphous aluminum hydroxide in methanol did not change during aging. At low water content, when all of the water was adsorbed, an initial decrease in the rate of acid neutralization was observed but no further change occurred during aging. When enough water was present to exceed the adsorptive capacity and produce water in the bulk methanol, the initial rate of acid neutralization decreased and a decrease in the rate of acid neutralization also occurred during aging. The decrease in rate of acid neutralization during aging is believed to be due to polymorphic transformation of the amorphous aluminum hydroxide and aggregation. The change caused by aggregation could be reversed by dehydration or sonication, but the change due to polymorphic transformation could not be reversed.

Effect of shear on the apparent viscosity of amorphous carbonate ion containing aluminum hydroxide suspensions.

The application of shear to carbonate ion containing aluminum hydroxide suspensions caused a change in the apparent viscosity by two possible mechanisms: change in the surface charge because of desorption of specifically adsorbed carbonate ion, and aggregate dispersal and formation of more extensive particle networks. The desorption of specifically adsorbed carbonate ion is related to the expansion of the air-liquid interface during shear. Shear-inducing processing equipment which generates a minimal amount of new air-liquid interface was found to produce the least change in pH and, consequently, in surface charge. However, viscosity increases caused by aggregate dispersal and formation of more extensive particle networks may occur without a shear-induced change in surface charge.