Thermal behaviour of mixtures of an alunite ore with K2CO3, CaCO3 and Ca(OH)2

Abstract The thermal behaviour of alunit/K 2 CO 3 , alunite/CaCO 3 and alunite/Ca(OH) 2 mixtures has been established. DTA coupled with XRD analysis has allowed appraisal of the main reactions that occur when the mixtures are roasted up to about 1000°C. The main products of roasting are K 2 SO 4 , CaSO 4 and other sulphur compounds and aluminates which retain the SO 3 deriving from the alunite. The products are formed between 700 and 800°C. The aim of the work was to assess the feasibility of a thermal process for use in the case of certain kaolin deposits whose exploitation is impossible at the present time because they are contaminated with alunite (KAl 3 (SO 4 ) 2 (OH) 6 ). This mineral causes great difficulty when attempting to use these kaolin ores because kilning drives off SO 3 which corrodes the furnaces and is environmentally harmful. The K 2 SO 4 and CaSO 4 produced by roasting the indicated mixtures can be leached with water and dilute HCl respectively and the former can be used as fertiliser. The leaching residue, with its reduced sulphur content, can then be employed by the ceramics and building industries.