Potassium alum thermal decomposition study under non-reductive and reductive conditions

Abstract

Potassium sulfate (K2SO4) is a very important compound, mostly used as nutrient for plant growth. Potassium sulfate production can be accomplished through the Mannheim process. For potassium bearing silicate minerals, such as glauconite, one alternative is acid leaching followed by selective precipitation and thermal decomposition of potassium alum (KAl(SO4)2). This chemical process is responsible for the formation of soluble K2SO4 and insoluble Al2O3, which can be later separated after solubilization in water and filtration. In this pyrometallurgical reaction, the temperature control is very important. Through the addition of a reducing agent, the decomposition temperature could be significantly reduced. In the present work, the thermal behavior of synthetic samples of hydrated potassium alum (KAl(SO4)2·12H2O) is appreciated through thermogravimetric analysis (TGA), both in the absence as well as in the presence of a reducing agent (charcoal) under inert atmosphere (nitrogen) and dynamic analysis. The addition of a stoichiometric amount of the reducing agent stimulated considerably the decomposition, which started at a lower temperature in comparison with the pure alum sample. Based on the XRD characterization of selected samples, it is suggested that the decomposition process should happen in at least two stages, with Al2(SO4)3 as one of the intermediate reagents. After full decomposition, only Al2O3 and K2SO4 have been identified, as expected based on thermodynamic simulations. Finally, it was demonstrated that the K2SO4 formed could be totally transferred to aqueous solution after a solubilization carried out at 363K for two hours; the remaining solid was characterized as pure aluminum oxide (Al2O3).