Phase transformation and non-isothermal kinetics studies on thermal decomposition of alunite

Alunite was considered as a potential alternative resource for potassium and alumina production. The disintegration of alunite was a principal step to extract valuable components and therefore was highly relevant to great and efficient utilization. In this research, the phase transformation and kinetics of alunite during thermal decomposition were examined. The results showed that increasing the calcination temperature was beneficial to the decomposition of alunite. The soluble potassium salt K2SO4 was recovered by water leaching after calcination. The recovery ratio of K reached 83.9% after calcined at 900 °C for 2 h, and the purity of K2SO4 was 83.77%. A crystal structure disintegration mechanism of KAl3(SO4)2(OH)6 was proposed on the basis of the phase transformation sequences characterized by XRD and FTIR. Dehydroxylation was attributed to the breakage of the AlOH and (Al)OH bonds. [AlO4] tetrahedrons in alunite were transformed into [AlO6] octahedrons. The S-O linkages between [SO4] tetrahedron and [AlO6] octahedron were broken during desulphation. Then the staged kinetics of alunite decomposition was studied by thermo-gravimetric analysis using the Kissinger-Akahira-Sunose method. The apparent activation energies of dehydroxylation and desulphation were determined.