Thermochemical purification of talc with ammonium sulphate as chemical additive

Abstract

This study reports on the thermochemical reactivity of talc with ammonium sulphate, a low-cost, recyclable chemical additive commonly used in the thermochemical extraction of strategic metals from low-grade ores and industrial and mine residues. The talc sample used in this study contained 70% talc, 15% lizardite and 10% kaolinite, following the removal of carbonate minerals by HCl leaching as a pre-concentration step. The solid product obtained by thermochemical treatment followed by aqueous leaching contained high-grade purified talc (>98%) as the principal mineral phase. It was depleted of lizardite and kaolinite, which had reacted with ammonium sulphate to form water-soluble hydrated ammonium metal sulphates. A comparative analysis of the TGA profiles of the concentrated parent sample, the solid product and the solid residue highlighted an identical mass loss between 850 °C and 1000 °C, which is the temperature range at which the dehydroxylation of talc occurs. This indicated minimal, if any, structural denaturation of the talc phase following thermochemical treatment with ammonium sulphate under the experimental condition studied. This process may therefore represent a promising technology for the purification of talc under the right market conditions, provided the mineral co-exists with phases featuring a high degree of reactivity with ammonium sulphate under thermal conditions.