Factors influencing the release rate of uranium, thorium, yttrium and rare earth elements from a low grade ore

Abstract

This paper presents data from laboratory leaching of a mineralogically complex low grade ore containing uranium (U), thorium (Th), yttrium (Y), rare earth elements (REEs) and accessory pyrite. The study examines the influence of varying the leaching protocol on the rate of release of U, Th, Y and REEs. Leaching protocols were designed to simulate a range of heap/in situ stope leaching scenarios. Protocol variants included flushing frequency, leachate recycle, nutrient (9K salts) addition and ferric sulphate addition to cells. Maximum extractions over 52weeks were: 58% U, 50% Th, 36% Y and 45% of the REEs; observed in cells flushed fortnightly with a ferric sulphate lixiviant (0.5g/l, pH 3.5). Flushing with tap water once monthly resulted in second highest observed extractions (57% U, 6% Th, 27% Y, 37% REEs) with the exception of Th, where the lower release was due to insufficiently low pH for Th mobilisation. Pourbaix plots indicate that redox potentials within the leaching system were buffered by coffinite dissolution. The high redox potentials, acidic pH and elevated sulphate concentrations required for metals dissolution were found to be a consequence of (a) direct addition of ferric sulphate as a lixiviant or (b) in situ generation of acidity and ferric sulphate via microbially-mediated oxidation of accessory pyrite. Generation and maintenance of acidic sulphate-rich interstitial water appear to control the dissolution of Th, Y and REEs in addition to U. The presented data highlight that significant U, Th, Y and REEs can be mobilised from rock materials by the addition of a ferric sulphate lixiviant, furthermore periodic flushing with water alone is sufficient to achieve similar release of U, Y and REEs where accessory pyrite is available in the reacting system. These findings have significant implications for recovery of valuable elements from low grade and marginal ores or materials previously considered as wastes.