The leaching and adsorption of gold using low concentration amino acids and hydrogen peroxide: Effect of catalytic ions, sulphide minerals and amino acid type J.J. Eksteen ⇑ , E.A. Oraby Department of Mining Engineering and Metallurgical Engineering, Western Australian School of Mines, Curtin University, GPO Box U1987, Perth, WA 6845, Australia article info Article history: Received 8 May 2014 Accepted 29 August 2014 Keywords: Gold Glycine Heap In situ Leaching abstract The leaching of gold using alkaline amino acids–hydrogen peroxide solutions at low concentrations has been studied. The application of alkaline amino acid–hydrogen peroxide system may offer an alternative and environmentally benign process for gold leaching, particularly in the context of leaching low grade gold ores in an in-situ or in heap leach processes. In the presence of an oxidant or oxidants, it was found that amino acids can dissolve gold at alkaline condition at low and moderate temperature. Heating the leach solution between 40 and 60 °C was found to enhance the gold dissolution significantly in alkaline amino acid–peroxide solutions. It was also found that gold dissolution increases by increasing amino acid concen- tration, peroxide and pH. Amino acids acts synergistically to dissolve gold. Although glycine showed the highest gold dissolution as a single amino acid compared to histidine and alanine, histidine was found to enhance gold dissolution when used in equimolar amounts with glycine. The presence of Cu 2+ ion enhances gold dissolution in the glycine–peroxide solutions. The process will propose an environmentally benign process for gold treatment in order to replace the use of cyanide in heap or in-situ leaching. In the presence of pyrite, the amount of gold leached was lower due to the peroxide consumption in sulphide oxidation. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The current alternative lixiviants to cyanide pose many chal- lenges. Despite copper ammoniacal-sodium thiosulphate being the main contender by different studies (Sparrow and Woodcock, 1995; Aylmore, 2001, 2005; Jeffrey, 2001; Breuer, 2002; Fleming et al., 2003; Chandra and Jeffrey, 2004; Oraby, 2010; Oraby et al., 2010), it is expensive, it has a complex leaching chemistry leading to the formation of polysulphide intermediates, it is very pH sensi- tive (labile) and it is applicable to a limited number of gold ores. It cannot economically be produced at site, it requires complex downstream separation and it is not biodegradable. Despite these challenges, thiosulfate has been applied in pilot plant since the 1980s, and has been used in heap leaching processes by Newmont Gold (Wan et al., 1994). Industrially, Barrick Gold has implemented a calcium thiosulphate retrofit at their Nevada operations (Choi, 2013). A development of in situ gold leaching using ferric EDTA and ferric oxalate complexes in thiosulfate solutions has been studies by Heath et al. (2008). However, in thiosulfate-iron (III) system the operating pH must be controlled, which increases the complexity of the application of this system in heap or in situ leaching. In addition, it was found that in the presence of thiourea the gold oxidation rate increases (Chandra and Jeffrey, 2004). How- ever, thiourea has been identified as a potential carcinogen and its use has been met with scepticism. In the main thiosulfate–copper–ammonia system, the presence of ammonia can cause environmental safety concerns of the sur- rounding environment by the evaporation of ammonia (Aylmore, 2005; Aylmore and Muir, 2001). Ammonia poses environmental and toxic problems both as a gas in air and as a soluble solution. The threshold limiting value (TLV) for ammonia gas, in air, is 14 mg/m 3 (Gos and Rubo, 2000), which is a very similar to that of HCN (15 mg/m 3 ). In additions, the US National Institute for Occupational Safety and Health (NIOSH) recommend an airborne exposure limit of 25 ppm (18 mg/m 3 ) averaged over a 10-h work- shift with a limit of 35 ppm (27 mg/m 3 ) not to be exceeded during any 15 min work period (NIOSH, 2000). The application in situ leaching (ISL), or heap leaching, or vat leaching, of complex and low-grade ores has a significant potential because it eliminates the need for hauling/hoisting large amounts of ore from underground and open pit operations (for ISL) or it can eliminate extremely energy inefficient grinding (by rather http://dx.doi.org/10.1016/j.mineng.2014.08.020 0892-6875/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +61 (0)8 9266 4419; fax: +61 (0)8 9266 1597. E-mail address: jacques.eksteen@curtin.edu.au (J.J. Eksteen). Minerals Engineering 70 (2015) 36–42 Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng