Effect of reducing grinding conditions on the flotation behaviour of low-S content PGE ores Heikki Miettunen a,⇑ , Risto Kaukonen b , Kirsten Corin c , Satu Ojala a , Riitta L. Keiski a a Mass and Heat Transfer Process Laboratory, Department of Process and Environmental Engineering, University of Oulu, FI-90014, Finland b Department of Geosciences, University of Oulu, FI-90014, Finland c Centre for Minerals Research, Department of Chemical Engineering, University of Cape Town, Rondebosch 7700, South Africa article info Article history: Available online 23 April 2012 Keywords: Ni–Cu–PGE Pulp properties Low sulphide Grinding atmosphere Grinding media Magotteaux mill abstract The effect of grinding environment on the flotation behaviour of sulphide–poor platinum group element (PGE) ores was studied with 3 different ores containing Ni, Cu and PGE. The sulphur content of the inves- tigated ore samples varied from 0.3% to 0.6%. Grinding tests were performed with a Magotteaux Mill Ò which is instrumented for measurement and monitoring of pulp properties. Flotation tests were done with a 5L Magotteaux float cell. Pulp potential, pH and dissolved oxygen (DO) levels were measured con- tinuously during grinding. Pulp oxidation–reduction potential (ORP) and DO were adjusted by the addi- tion of CO 2 ,N 2 and nitric acid. Iron or 30% chrome containing balls were used in grinding. In this study, a number of grinding and flotation tests were done with natural ore samples into which 1 g of synthetic sperrylite was added. This study presents some aspects of the effect of a reducing grinding environment on the flotation response of low-S content PGE containing ores. Highly reducing grinding environment created with CO 2 was found to have a positive effect on flotation behaviour of PGE minerals from low- S content ores. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Sulphide ores are conventionally concentrated by grinding in steel rod and ball mills followed by flotation. During wet grinding sulphide minerals form galvanic couples with the grinding media in which sulphide minerals with potentials much more noble than the grinding media result in accelerated corrosion of the grinding media. Galvanic coupling between sulphide minerals and grinding media leads to the formation of iron hydroxide coating on the min- eral surface (Adam and Iwasaki, 1984). Oxidised iron species may inhibit interaction of sulphide minerals with xanthates, which are usually used as collectors in sulphide mineral flotation. Huang and Grano (2006a,b) refer to a large number of studies in the past which have shown that galvanic interactions between mineral sur- faces and grinding media have a depressing effect on the flotation performance of the sulphide minerals. Prevailing knowledge to ex- plain this is that galvanic interactions result in precipitation and adsorption of hydrophilic iron species on the mineral surface. Accelerated corrosion of the grinding media and the formation of hydroxyl ions from the reduction of oxygen close to the sulphide mineral surface are considered to be the main reasons for this phenomenon. Platinum and palladium arsenides and tellurides are often re- ported to be common in flotation tail samples in platinum mines. Shackleton et al. (2007b) have investigated the flotation behaviour of platinum and palladium arsenides using synthetic arsenide min- erals. Synthetic minerals offer a way to fundamentally study the behaviour of pure minerals. In this study the effect of grinding environment on the flotation behaviour of synthetic sperrylite was investigated with one ore sample where synthetic sperrylite was added. 2. Galvanic coupling, corrosion and flotation behaviour of minerals Gu et al. (2004) have divided galvanic coupling in sulphide min- eral flotation systems into three types: sulphide mineral-sulphide mineral–water system; sulphide mineral-steel ball-water system and sulphide mineral-sulphide mineral-collector system. In a sul- phide mineral–sulphide mineral galvanic couple system the min- eral with the lower rest potential causes anodic oxidation and the mineral with the higher rest potential is passivated leading to oxygen reduction. In a sulphide mineral-steel medium system the oxidation of steel is expected to allow the hydrophilic iron oxi- dation products to interact with the sulphide mineral surface which has adverse effects on sulphide mineral flotation. The rest potentials of some common sulphide minerals and mild steel med- ia are presented in Table 1. 0892-6875/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2012.03.029 ⇑ Corresponding author. Tel.: +358 8 5532366. E-mail address: heikki.miettunen@oulu.fi (H. Miettunen). Minerals Engineering 36–38 (2012) 195–203 Contents lists available at SciVerse ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng