Modelling of Dehydroxylation and Desulphuration Kinetics of Haft Sandogh Alunite Ore

Solubility of alunite increases in acids and bases by calcinations. This process includes two main steps which are dehydroxylation and desulphuration. In this paper, after characterization of Haft Sandogh alunite ore, kinetics of dehyroxylation and desulphuration were studied. According to the results of thermogravimetric and weight loss, dehydroxylation of alunite occurs at 508 to 577˚C and desulphuration takes place at temperatures above 680˚C. It was observed that the dehydroxylation and desulphuration reaction rates increased with increasing temperature. Different kinetics models were used to evaluate kinetics data. By regression analysis and normalized standard deviation (NSD), the Avrami model was found to give the best fit to kinetic data. The reaction rate of both calcination processes were found to be less than one (i.e., n<1). This finding along with the high activation energy value (i.e., 117.4 kJ mol-1 for dehydroxylation and 215.4 kJ mol-1 for desulphuration) indicate that both reactions are controlled by chemical mechanism.