Nickel and cobalt precipitation from sulphate leach liquor using MgO pulp as neutralizing agent P. Oustadakis, S. Agatzini-Leonardou, P.E. Tsakiridis * National Technical University of Athens, Department of Mining and Metallurgical Engineering, Laboratory of Metallurgy, 9, Iroon Polytechniou Street, 157 80 Zografou, Athens, Greece Received 1 September 2005; accepted 3 November 2005 Available online 11 January 2006 Abstract The research work presented in this paper determined the conditions at which nickel and cobalt can be recovered, as easily filterable precipitates, from leach liquors, which are produced by sulphuric acid leaching of nickel oxide ores, at atmospheric pressure. More spe- cifically, nickel and cobalt were precipitated as hydroxides at pH = 9, using MgO pulp as a neutralizing agent, by 99.9% and 99.0%, respectively. At this pH value, manganese, which was contained in the leach liquor, was precipitated having a concentration of about 3% in the final mixed precipitate. Statistical design and analysis of experiments were used in order to determine the main effects and interactions of the precipitation factors, which were the pH at equilibrium and the addition of seed material. The characterization of the produced mixed precipitate was carried out by X-ray diffraction, TG–DTA, scanning electron and optical microscopy and particle size distribution analyzer. The main mineralogical phases in the precipitate were theophrastite (Ni(OH) 2 ) and brucite (Mg(OH) 2 ). The microscopic observation revealed that the presence of brucite was, mainly, due to the un-reacted MgO pulp, during the precipitation process. This brucite content acted as seed material for nickel precipitation. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: pH control; Precipitation; Hydrometallurgy 1. Introduction A novel integrated hydrometallurgical method has been developed and patented in order to treat low-grade nickel oxide ores efficiently and economically. It involves heap leaching of the ore by dilute sulphuric acid at ambient tem- perature, purification of the leach liquors and recovery of nickel and cobalt by solvent extraction and electrowinning (Agatzini and Dimaki, 1991; Agatzini et al., 1999; Tsakiri- dis and Agatzini, 2004; Agatzini and Karidakis, 2000). A typical composition of the pregnant solution pro- duced from heap leaching of Greek low-grade nickeliferous laterites with sulphuric acid is the following: Ni 2+ = 5.0 g/l, Co 2+ = 0.6 g/l, Fe 3+ = 22.0 g/l, Al 3+ = 6.0 g/l, Cr 3+ = 1.0 g/l, Mn 2+ = 0.8 g/l and Mg 2+ = 5 g/l (Agatzini and Dimaki, 2000). After the removal of iron, chromium and aluminum by chemical precipitation (Agatzini et al., 1999), the leach liquor contains nickel, cobalt, magnesium and manganese, which can be separated and recovered by solvent extraction, with suitable solutions for nickel and cobalt electrowinning (Tsakiridis and Agatzini, 2004). An alternative process for the nickel and cobalt recovery comprises their precipitation as hydroxides. These precipi- tates can be either further treated in refineries or fed into converters of nickel pyrometallurgical plants. The present paper presents the results of a research study involving the recovery of nickel and cobalt from purified heap leach liquors by hydrolytic precipitation, using MgO pulp as a neutralizing agent. Currently, hydrometallurgical processing of laterite ores consists of high-pressure acid leach (HPAL) where the ore 0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.11.006 * Corresponding author. Tel.: +30 210 7722234/7722179; fax: +30 210 7722218. E-mail address: ptsakiri@central.ntua.gr (P.E. Tsakiridis). This article is also available online at: www.elsevier.com/locate/mineng Minerals Engineering 19 (2006) 1204–1211