Decomposition of monazite concentrate in sulphuric acid

Abstract

ABSTRACTA sulphuric acid bake-leach process was investigated for the extraction of rare earth elements (REEs), uranium, and thorium from a monazite concentrate. X-ray diffraction analysis showed the formation of water-soluble sulphates that readily dissolved during the water leach step. Nearly complete extraction of REEs, uranium, and thorium was achieved when the concentrate was baked at 250°C for 4 h with a sulphuric acid to concentrate (S/C) weight ratio of 4:1. At the 1:1 sulphuric acid to concentrate ratio ∼65% of REEs were leached while only ∼2–5% of uranium and thorium were extracted. Temperature (180–250°C) had little effect on the extraction of REEs, but greatly affected the extraction of thorium and uranium. In general, the extraction of thorium and uranium decreased with an increase in temperature. The effect of particle size (48–137 µm) was also tested and found to have little effect on the extraction of REEs. Acid consumption was calculated and found to increase with longer baking times (0.3–0.8 g acid/g concentrate). The major mineralogical phases formed because of the sulphation reaction included potassium cerium sulphate hydrate (2K2SO4·Ce(SO4)2·2H2O), lanthanum sulphate hydrate (La2(SO4)3·2H2O), sodium praseodymium sulphite hydrate (NaPr(SO3)2·2H2O), and neodymium sulphate hydrate (Nd2(SO4)3·5H2O), all of which were water-soluble.