Heterocoagulation and chemical transformations in aluminum hydroxide-magnesium hydroxide suspensions
Abstract
The behavior of mixtures of magnesium hydroxide and aluminum hydroxycarbonate gels was evaluated both in terms of initial aggregation of the two solids and the formation of a mixed magnesium aluminum hydroxycarbonate phase. Surface chemistry was found to be an important factor governing the stability of such systems. Magnesium hydroxide gels were found to consist of plate-like crystallites having very high isoelectric points ($>$pH 12). Aluminum hydroxycarbonate gels were found to consist of smaller particles and have variable isoelectric points (with an approximate range of pH 6 to pH 9) which was related to the presence carbonate anion in potential-determining sites in the gel structure. Desorption of carbonate anion from the gel structures is enhanced by dilution with water, increased pH or the presence of magnesium hydroxide. The immediate interaction which takes place between magnesium hydroxide and aluminum hydroxycarbonate particles in mixed suspensions was found to result from two processes. Heterocoagulation due to electrostatic attraction occurs as the resultant pH of mixtures falls between the isoelectric points of the two types of particles. Also, dissolution of aluminum hydroxycarbonate particles releases aluminate anions which are also attracted to the magnesium hydroxide surface. The two processes lead to the formation of an aluminum hydroxycarbonate coating on the magnesium hydroxide particles. The coating thus formed is so complete that the magnesium hydroxide surface is occluded and mixtures tend to demonstrate surface chemical properties akin to those of an aluminum hydroxycarbonate suspension. Formation of the mixed magnesium aluminum hydroxycarbonate crystal hydrotalcite was seen to take place directly on the magnesium hydroxide surface in mixtures of aluminum hydroxycarbonate and large magnesium hydroxide crystals. Formation of hydrotalcite in colloidal mixtures of magnesium hydroxide and various aluminum hydroxides appears to be accelerated by conditions which promote magnesium hydroxide solubilization. The process also occurred more rapidly in systems where complete surface coverage of magnesium hydroxide by the aluminum hydroxide phase did not take place. Therefore, coating of aluminum hydroxycarbonate on the magnesium hydroxide particles may serve to slow formation of the mixed crystal.
Degree
Ph.D.
Advisors
Hem, Purdue University.
Subject Area
Pharmaceuticals|Chemistry
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