In the production of alumina (Al2O3) from clays by acid leaching, thermal activation by calcination is applied widely as a pre-treatment to improve the dissolution of aluminum. Previous studies have focused only on the thermal behavior of high-purity kaolinite and pyrophyllite, individually. However, thermal activation of complex clay ores containing several silicate minerals and their effect on aluminum extraction have not been studied. The purpose of the present study was to characterize the thermal behavior of a clay ore containing mainly pyrophyllite (Al2Si4O10(OH)2), kaolinite (Al2Si2O5(OH)4), muscovite (KAl2(AlSi3O10)(OH)2), quartz (SiO2), and kyanite (Al2SiO5) from the Pütürge clay deposits (Malatya, Turkey) for possible use in alumina (Al2O3) production by acid leaching. The ore and its calcination products obtained at various temperatures were characterized with respect to their mineral paragenesis, crystal structure, surface morphology, and thermal, calorimetric, and aluminum dissolution properties in order to understand the changes that occurred. Aluminum recovery in the leach solution increased in direct proportion to the dehydroxylation degree (Dtg) of the ore as the calcination temperature increased to 900°C. A maximum aluminum extraction of 90.57% was achieved by leaching of the product from calcination at 900°C. Aluminum extraction decreased sharply above that temperature, even though Dtg increased a little. By increasing the calcination temperature; the structures of pyrophyllite, kaolinite, and muscovite were destroyed by dehydroxylation, resulting in the exfoliation of the mineral layers, thus, a mixture of dehydroxylated phases formed. Depending mainly on the temperature range at which each of the dehydroxylated phases is durable, aluminum could be leached to some extent. The sharp decrease in the extraction of aluminum, iron, and potassium at higher temperatures was attributed to compaction of previously exfoliated layers of the minerals through re-crystallization to form mullite-like structures which seemed insensitive to acid attack during the leaching.
