Self inhibition of aluminum leaching from coal fly ash by sulfuric acid

The coal fly ash (CFA) that is produced by coal fired power plants in Israel is alkaline and contains aluminum that can be leached by different acids. In this work, the mechanism of aluminum leaching from CFA by sulfuric acid is considered. It is shown that higher CFA content, which indicates higher solid to Liquid ratio in the leaching suspension, decreases the fraction of leached aluminum by means of sulfuric acid. This behavior constitutes a new unexplained phenomenon, which could not be explained by analysis of the mass action law of the dissolution reactions, but rather by mass transfer considerations. It is shown that the leaching process involves a self inhibition mechanism due to the precipitation of calcium sulfate on the surface and within the CFA particles. The effects of CFA content, acid concentration, temperature, and pre-leaching conditioning, upon leaching rates and yields, were tested. Increasing the acid concentration produces two opposing effects simultaneously. An increase in concentration of the hydronium ion enhances the dissolution of aluminum, whereas the increase in concentration of the sulfate and the dissolved calcium ions intensifies the formation of calcium sulfate precipitates. These precipitates hinder mass transfer across the surface of the CFA particles, and in this sense they generate a self inhibition effect. Conditioning of the CFA with hydrochloric acid at pH 4 removes 65% of the calcium. Consequently, the conditioned CFA can be leached more efficiently with sulfuric acid. This higher leachability is Linked to the reduction in calcium sulfate precipitation on the CFA. (C) 1999 Published by Elsevier Science S.A. All rights reserved.