A comparison of pyrrhotite rejection and passivation in two nickel ores T. Chimbganda, M. Becker ⇑ , J.L. Broadhurst, S.T.L. Harrison, J.-P. Franzidis Minerals to Metals Initiative, Department of Chemical Engineering, University of Cape Town, Rondebosch 7701, South Africa article info Article history: Received 14 August 2012 Accepted 28 March 2013 Available online 2 May 2013 Keywords: Acid rock drainage Pyrrhotite rejection Pyrrhotite passivation Polyethylene polyamines abstract The non-stoichiometric sulfide mineral pyrrhotite (Fe1-xS) occurs almost ubiquitously inter-grown with the principal nickel mineral, pentlandite ((Fe,Ni)9S8). During Ni processing, pyrrhotite is generally rejected to the tailings stream by flotation to produce a low tonnage, high grade (Ni) smelter feed and reduce SO 2 emissions. In this study, the effect of different pyrrhotite flotation rejection strategies (artifi- cial oxidation and TETA: SMBS addition) are evaluated on a magnetic (Ore A) and non-magnetic (Ore B) pyrrhotite ore to determine if either may effectively depress and potentially passivate the pyrrhotite sur- face during flotation to produce benign tailings without compromising pentlandite recovery. For both ores, the best pyrrhotite rejection (pentlandite/pyrrhotite recovery) was obtained using TETA: SMBS. Dif- ferences in the flotation performance of the two ores are considered more a function of BMS content, lib- eration and ore handling rather than a difference in sulfide passivation from the inherent pyrrhotite mineralogy (magnetic vs non-magnetic pyrrhotite). Pyrrhotite passivation could possibly provide a means of rendering the tailings non-reactive and thus mitigate acid rock drainage (ARD) formation. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Pyrrhotite is a sulfide mineral found in a number of ore depos- its; lead–zinc, nickel–copper and platinum group elements (PGE). It has a non-stiochiometric composition, Fe 1x S where x varies from 0 (FeS) to 0.125 (Fe 7 S 8 ); the chemical formula can also be ex- pressed as Fe n1 S n with n P 8. The dominant nickel mineral in sul- fidic deposits is pentlandite, (Fe,Ni) 9 S 8 , which almost occurs ubiquitously inter-grown with pyrrhotite. Pyrrhotite occurs in two different forms commonly known as magnetic (Fe 7 S 8 ) and non-magnetic (Fe 9 S 10 , Fe 11 S 12 ) pyrrhotite which exhibit varying physio-chemical properties. In many nickel processing operations, pyrrhotite is rejected to the tailings stream by flotation. This is due to the fact that in most magmatic nickel sulfide deposits, pyrrhotite is significantly more enriched than pentlandite. Hence sending low grade Ni concen- trates through to the smelter results in the inefficient use of smel- ter capacity and energy. Of even greater importance though is the fact that sending unwanted pyrrhotite to the smelter results in undesirable SO 2 emissions. Conversely, improved pyrrhotite rejec- tion produces an improved pentlandite grade which, in turn, re- duces both the total energy requirements and harmful gaseous emissions (Evans et al., 2011). It is thus hardly surprising that the efficient depression of pyrrhotite during flotation of nickel sul- fide ores has been the focus of much research and development over the last few decades (see for example Kelebek, 1993, 1995; Kerr, 2002; Kim, 1998; Lan et al., 2002; Lawson, 2005; Legrand et al., 2005a, 2005b; Prestige et al., 1993; Senior et al., 1995; Wells, 1997). Of specific relevance to this study is the depression of pyr- rhotite by means of either air oxidation, or the application of poly- ethylene polyamine chelating agents. Due to the reactive nature of pyrrhotite, its surface is oxidized rapidly upon exposure to air (Miller et al., 2005), producing a hydrophilic ferric oxyhydroxide, FeOOH, layer (Ekmekci et al., 2010; Mycroft et al., 1995; Pratt et al.,1994). This passivating layer inhibits attachment of the sulfide collector to the surface of the pyrrhotite particles during flotation; thus resulting in pyrrhotite rejection (Kelebek, 1993). Studies by Kelebek (1993) have indi- cated that the depression of pyrrhotite increases as the oxidation period increases. When using oxygen as a depressant in the flota- tion of nickel sulfide ores, however, it is important to sufficiently depress pyrrhotite without compromising the recovery of pent- landite (Wells, 1997; Kelebek, 2007). This can be achieved by tak- ing advantage of the different rates of oxidation of pyrrhotite and pentlandite (Legrand et al., 2005a, 2005b). According to Legrand et al. (2005a), pyrrhotite oxidizes more rapidly than pentlandite, with an oxidation period of 30 min being required to achieve the same extent of pentlandite oxidation as that obtained for pyrrho- tite after a period of only 5 min. Studies by Becker et al. (2010a) indicated that the rate of pyrrhotite oxidation in the presence of oxygen is also likely to be dependent on the pyrrhotite type. In accordance with these studies, non-magnetic pyrrhotite takes up oxygen at a slower rate, and can thus be considered as less reactive, than magnetic pyrrhotite. 0892-6875/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2013.03.031 ⇑ Corresponding author. Tel.: +27 21 650 3797; fax: +27 21 650 5501. E-mail address: megan.becker@uct.ac.za (M. Becker). Minerals Engineering 46–47 (2013) 38–44 Contents lists available at SciVerse ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng