Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng A new thiosalt depuration bioprocess for water- recycling in metallic sulphide mineral processing Alfonso Mazuelos ⁎ , Nieves Iglesias-González, Cristina Montes-Rosúa, Juan Lorenzo-Tallafigo, Rafael Romero, Francisco Carranza Department of Chemical Engineering, Faculty of Chemistry, University of Seville, Spain ARTICLE INFO Keywords: Thiosalts Thiosulphate Sulphite Acidithiobacillus Bio-oxidation Flotation ABSTRACT Economic and environmental imperatives force the metallic sulphide processing industry to re-use liquors that normally contain thiosalts; this processing entails difficulties in operation control and losses in metal-extraction performance. Bio-oxidation presents a promising option for thiosalt removal. Hitherto, this method has been tested to treat synthetic liquors containing only one thiosalt, either tetrathionate or thiosulphate, usually at low concentration (< 1 g/L). In this article, bio-oxidation is studied of synthetic and real liquors containing mixtures of these thiosalts at concentrations close to 4.5 g/L. Coexistence of thiosulphate and tetrathionate in an acidic medium leads to strong and irreversible inhibition phenomena (performance losses close to 80%). In order to solve this problem, a previous treatment is proposed consisting of the 8-hour aeration of inlet solution after adding up to 20 mg/L and pH 1.5 of Cu 2+ and sulphuric acid, respectively, resulting in the complete removal of thiosulphate and sulphite. Pre-treated real liquors were fed into a 1 L discontinuous stirred tank bioreactor and a 1 L continuous flooded packed-bed bioreactor, which were successfully started up and operated, thereby ob- taining bio-oxidation rates close to the highest rates found in the literature with synthetic liquors. 1. Introduction Mineral processing by milling and flotation requires a high con- sumption of water. Cost effectiveness and impact on the conservation of freshwater resources, according to sustainability policies and goals, force this industry to re-use process liquors, thereby decreasing the demand for primary water supply. Recycled liquors mainly come from classification ponds, tailings dams, and solid/liquid separation units containing dissolved and sus- pended substances (residual flotation reagents and ions and colloids coming from the ore). Water recycling leads to modifications of feed- water quality which hinders the operation control and even induces loss in flotation performance (Rao and Finch, 1989; Liu et al, 2013; Bicak et al, 2018; Guerrero-Flores et al, 2018; Muzinda and Schreithofer, 2018). Thiosalts are usually present in recycled liquors of metallic sulphide ore processing. Thiosulphate (S 2 O 3 2- ) and polythionates, mainly tet- rathionate (S 4 O 6 2- ), are generated in milling and flotation circuits, usually in alkalinity conditions, by the partial oxidation of sulphides (Rolia and Chakrabarti, 1982; Silver, 1985). These anions can interact with added reagents, thereby affecting flotation selectivity (Rao and Finch, 1989; Kirjavainen et al, 2002; Petrus et al, 2012; Shengo et al, 2014; Ozturk et al, 2018). Thiosalts are metastable ions which consume O 2 and finally generate H 2 SO 4 through spontaneous oxidation and disproportionation reactions (Silver and Dinardo, 1981; Suzuki, 1999; Druschel et al, 2003). These reactions are catalysed by metal ions and microorganisms of the Acidithiobacillus genus, usually present in mining environments. Discharging effluents containing thiosalts exert an in- direct environmental impact in receptor aquatic bodies undergoing delayed dissolved O 2 depletion and progressive acidification (Silver and Dinardo, 1981; Chanda et al, 1984; Silver, 1985; Kuyucak and Yaschyshyn, 2007; Ghosh and Dam, 2009; Dopson and Johnson, 2012). The complete removal of thiosalts in process liquors can become considered a critical target in the design and operation of flotation plants. Lagooning is the current, more extensively applied procedure. This passive depuration method is simple and inexpensive because it is based on the natural oxidation of thiosalts. Due to its low efficiency, this process requires huge ponds, which produce a visual impact and poor management of the territory. Facing the impossibility of tem- perature control, its applicability is limited in cold climates due to re- duced bacterial and chemical activity (Kuyucak and Yaschyshyn, 2007; Montes-Rosúa et al, 2018). Active treatments based on the controlled https://doi.org/10.1016/j.mineng.2019.106031 Received 27 May 2019; Received in revised form 6 September 2019; Accepted 15 September 2019 ⁎ Corresponding author. E-mail address: mazuelos@us.es (A. Mazuelos). Minerals Engineering 143 (2019) 106031 Available online 18 September 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved. T