Biohydrometallurgy in Turkish gold mining: First shake flask and bioreactor studies H. Ciftci, A. Akcil ⇑ Department of Mining Engineering, Mineral Processing Division (Mineral-Metal Recovery and Recycling Research Group), Suleyman Demirel University, TR32260 Isparta, Turkey article info Article history: Received 4 July 2012 Accepted 18 March 2013 Keywords: Biooxidation Hydrometallurgy Biotechnology Environmental Cyanidation abstract The first laboratory tests on biooxidation and cyanidation of gold ores in Turkey were carried out using samples of the Copler Gold Mine. Over a 3 year R&D test period, mixed bacterial/archaeal cultures improved biooxidation of the Copler ore. The highest sulphide oxidation of 87.35% over 432 h was achieved in shake flasks in the presence of the mixed culture (MODM: Sulfolobus acidophilus and Sulfol- obus thermosulfidooxidans). Bioreactor tests resulted in greater dissolution rates for iron and arsenic than did shake-flask tests, which led to a greater extent of sulphide oxidation within a shorter period of time. The maximum sulphide oxidation in the bioreactor tests was 97.79% after 240 h when the EXTM (Acidi- anus brierleyi and Sulfolobus metallicus) mixed culture was used. After the biooxidation experiments with solids contents of 10% and 20% (w/v), the gold recovery from the oxidised ore was lower than that achieved in the presence of 5% solids (w/v) because the extent of sulphide oxidation was reduced as the pulp density increased. A strong correlation between the sulphide oxidation and gold recovery was also established. The highest gold recovery of 94.48% was achieved during cyanidation from the biooxi- dised ore produced from the experiment conducted using the EXTM mixed culture. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Submicroscopic gold, which is typically trapped in a matrix of arsenopyrite/pyrite, cannot be effectively recovered, even when extensive fine-grinding processes are applied. To reduce the initial investment cost necessary for the construction a large-scale facil- ity, the ore is typically crushed and milled, and a flotation concen- trate is produced because gold is mostly found in the arsenopyrite/ pyrite fraction. A pre-treatment process must be applied to the flo- tation concentrate before cyanide leaching. For the pre-treatment process, the flotation concentrate can be heated at 600–700 °C or treated with an acidic or basic solution under high pressure and temperature using an autoclave. Instead of using these methods, which are expensive or, in the case of heating, causes environmen- tal problems, a biooxidation pre-treatment process was used (Komnitsas and Pooley, 1989). The biooxidation pre-treatment process is used to break up the crystal structures of minerals that contain gold. Thus, the contact area between cyanide and gold is increased, and the gold is more efficiently dissolved. This process can be performed under atmospheric pressure and at low temper- atures, especially for low-tonnage production; furthermore, this process is more economical and compatible with environmental legislation. The use of microorganisms in the preliminary prepara- tion processes of gold recovery from refractory gold ores began in the 1980s (Rawlings, 1998). Although the biological processes used in the recovery of metals have been well known for centuries, the term ‘‘biohydrometallur- gy’’ first appeared in 1972 in a manuscript titled ‘‘biohydrometal- lurgy of cobalt and nickel’’ (Torma, 1972). Biohydrometallurgy encompasses bioleaching, biooxidation, mineral biotechnology and biomining. The dissolution of sulphidic metals by biological processes is more environmentally friendly than other conven- tional chemical processes (Brierley and Brierley, 2001; Akcil, 2004; Ehrlich, 2004). The mesophilic iron- and sulphur-oxidising bacteria are widely used in the bioleaching/biooxidation process for the oxidation of sulphidic ores and flotation concentrates. Depending on the mineral, chemical attack occurs through a combination of ferric iron and acid (protons), whereas the role of the microorganisms is to generate the ferric iron and the acid. This strategy for metal recovery is known as bioleaching because the metal is solubilised in the process (Rawlings et al., 2003). Convinc- ing evidence showing that microbes were active participants in the leaching of copper and some other metals from ores was not obtained until the middle of the twentieth century. This discovery led to a concerted effort to identify these microorganisms and their mode of action (Ehrlich, 2004). Recently, metal recovery using biological processes (bioleaching and biooxidation) is more economical and compatible with wastes from production processes and low-grade ores than other meth- ods, and interest in metal recovery through the use of biological processes is increasing (Brierley, 2010). The microorganisms used in these processes take on the role of a catalyst task in the leaching of ore. For this reason, the leaching process in the presence of 0892-6875/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2013.03.020 ⇑ Corresponding author. Tel.: +90 246 211 1321; fax: +90 246 2370859. E-mail address: ataakcil@sdu.edu.tr (A. Akcil). Minerals Engineering 46-47 (2013) 25–33 Contents lists available at SciVerse ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng