Factors influencing the release rate of uranium, thorium, yttrium and rare earth elements from a low grade ore D.J. Sapsford a , R.J. Bowell b , J.N. Geroni a,⇑ , K.M. Penman a , M. Dey b a Cardiff School of Engineering, Cardiff University, The Parade, Cardiff CF24 3AA, UK b SRK Consulting, Churchill House, Churchill Way, Cardiff CF10 2HH, Wales, UK article info Article history: Received 7 May 2012 Accepted 22 August 2012 Available online 5 October 2012 Keywords: Uranium Rare earth elements Leaching Eh/pH control 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 exam- ines 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 (9 K salts) addition and ferric sulphate addition to cells. Maximum extractions over 52 weeks were: 58% U, 50% Th, 36% Y and 45% of the REEs; observed in cells flushed fortnightly with a ferric sulphate lixiviant (0.5 g/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 excep- tion 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. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The efficiency of leaching of elements such as uranium and Rare Earths (REEs) is a major factor in determining the economic viabil- ity of mining low grade ores. In cases where uranium concentra- tion is too low to be economical to extract alone the recovery of REEs as a leaching by-product can boost profitability. Such ores may contain a variety of complex REE bearing mineral phases as well as uraninite and pitchblende and while the leaching kinetics of uraninite have been widely studied, the interrelationship be- tween complex mineralogy, mineral liberation and leaching behav- iour in low grade mixed ore deposits is not yet well defined and often site specific. The processes which operators seek to maximise during mineral extraction operations by contrast need to be minimised with re- spect to natural weathering of waste rock piles which can lead to the release of uranium and other heavy metals into the environ- ment with implications for eco-toxicity. Understanding the pro- cesses occurring within rock piles is therefore essential not only in terms of maximising the profitability of mining operations, but also in reducing their environmental impact over the long term. 1.1. Uranium chemistry Uranium is most commonly found in the +4 oxidation state as UO 2 (uraninite and pitchblende being different crystalline phases with the same chemical composition). Hexavalent uranium is read- ily solubilised in acidic solution forming a uranyl cation (Eq. (1)) with tetravalent uranium requiring oxidation, often by Fe(III) liber- ated from accessory pyrite or added during leaching (Eq. (2)). UO 3 þ 2H þ ! UO 2þ 2 þ H 2 O ð1Þ UO 2 þ 2Fe 3þ ! UO 2þ 2 þ 2Fe 2þ ð2Þ In sulphate systems a soluble complex is formed under mildly acidic conditions (Eq. (3)) with hydrolysis favoured above pH 5–6 resulting in the formation of an insoluble hydroxyoxide precipitate (Eq. (4)). 0892-6875/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2012.08.002 ⇑ Corresponding author. Tel.: +44 (0)7971 087398; fax: +44 (0)29 20874716. E-mail address: geronijn@cf.ac.uk (J.N. Geroni). Minerals Engineering 39 (2012) 165–172 Contents lists available at SciVerse ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng