A Rheological Investigation of the Behaviour of Two Southern African Platinum Ores ⇑ M

Minerals Engineering 49 (2013) 92–97 Contents lists available at SciVerse ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng A rheological investigation of the behaviour of two Southern African platinum ores ⇑ M. Becker a, , G. Yorath a, B. Ndlovu a,b, M. Harris a, D. Deglon a, J.-P. Franzidis a a Centre for Minerals Research, Department of Chemical Engineering, University of Cape Town, South Africa b Julius Kruttschnitt Mineral Research Centre, University of Queensland, Australia article info abstract Article history: With the installation of ultrafine grinding on many platinum operations in southern Africa, there were Received 21 February 2013 concerns as to whether this would cause rheologically complex behaviour during the subsequent flota- Accepted 7 May 2013 tion of the ore. Rheologically complex behaviour refers to the non-Newtonian behaviour experienced Available online 5 June 2013 by some suspensions, associated with exponential increases in yield stress and viscosity with increasing solids content. This is attributed to particle size and solids concentration effects, surface chemistry, and Keywords: mineralogy. In this study, the rheological behaviour of two different platinum ores; a western limb UG2 Rheology ore and a Great Dyke platinum ore were investigated and compared with that of single mineral studies of Platinum ores the major gangue minerals of platinum ores (chromite, orthopyroxene, plagioclase and talc). The results Phyllosilicate minerals show that Great Dyke ore is considerably more rheologically complex than UG2 ore. Great Dyke flotation concentrate shows high yield stress and viscosity at low solids concentrations (>20 vol.% solids). Should the ROM ore in a Great Dyke flotation operation suddenly show significant changes in ore mineralogy, the rheological properties of the slurry should be considered since they may be detrimental to the overall performance of the operation (e.g. loss of recovery through poor gas dispersion). In contrast, the rheological behaviour of UG2 flotation samples shows little cause for concern for the plant operator. Comparison of the pure mineral samples shows that the complex rheological behaviour of the Great Dyke ore may be attributed to the high degree of low temperature alteration and the formation of phyllosilicate minerals such as talc, more than particle size effects. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction and is found in the Main Sulfide Zone (MSZ) and the Lower Sulfide Zone (LSZ) (Oberthür et al., 2004). 1.1. Southern African platinum ores In the last two to three decades, PGM concentrators have pre- dominantly processed the Merensky Reef, which is a pegmatoidal The Bushveld Complex in South Africa and the Great Dyke in pyroxenite ore with a base metal sulfide (BMS) content of Zimbabwe are two of the primary sources of the platinum group 1 wt.%. This is simpler to process than other platinum ores on ac- elements (PGE – Pt, Pd, Ir, Ru, Ro, and Os). The global market for count of the slightly coarser average grain size of the PGM, their these precious metals includes a variety of applications such as strong association with the base metal sulfides (flotation recovery the catalytic, electronic, automotive and even medical industries of PGM due to their association with floatable BMS) and the low (Cawthorn, 2010). These precious metals occur in a host of plati- chromite content of the ore (<5 wt.%). Since then, there has been num group minerals (PGM) that are often associated with the base a shift to process other PGM ores: UG2, Platreef and Great Dyke, metal sulfides (BMS) in three discrete reefs within the Bushveld all of which come with their own set of challenges (Rule, 2010). Complex: namely the Merensky Reef, the UG2 Chromitite and the These include the liberation and flotation recovery of ultra-fine Platreef. Typically, the PGM in these three reefs are fine to very fine PGM particles (Rule and Schouwstra, 2011), processing of a dual grained and only present in g/t concentrations (Holwell and Mc density ore (e.g. UG2; Mainza et al., 2005), minimising the grade Donald, 2007; Kinloch, 1982; Mc Laren and De Villiers, 1982; of Cr2O3 in the concentrate due to its detrimental effects on the Schouwstra et al., 2000). Similarly to the Bushveld Complex, the smelter efficacy (Barnes and Newall, 2006) and minimising the economic PGE mineralisation in the Great Dyke is stratabound amount of floatable gangue in the concentrate so as to increase smelter capacity (Mkhize and Andrews, 2011). With the installation of stirred milling on many PGM operations, the issue of poor PGM liberation has essentially been ad- ⇑ Corresponding author. Tel.: +27 21 650 3797; fax: +27 21 650 5501. E-mail address: [email protected] (M. Becker). dressed. A PGM flow sheet may use stirred milling in two 0892-6875/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2013.05.007 M. Becker et al. / Minerals Engineering 49 (2013) 92–97 93 applications: traditional ultra-fine grinding (UFG 80% < 20 lm) is supported by fundamental studies which have been conducted that seeks to improve concentrate grade in the cleaning stages on pure phyllosilicate mineral suspensions, reporting significantly and main stream inert grinding (MIG 80% < 45 lm) which seeks higher viscosities and yield stresses in the presence of phyllosili- to liberate the valuable PGM locked in silicates (Rule, 2010). Sev- cate minerals (particularly swelling clays and serpentine minerals) eral concerns have arisen with the installation of this technology compared to non-phyllosilicate mineral suspensions (e.g. quartz) on PGM ore. One of these is whether there will be an increase in (Ndlovu et al., 2011a,b). Often broadly classified as ‘clays’, this class the amount of naturally floating gangue (composite orthopyroxene of phyllosilicate minerals is closely associated with several pro- and talc particles, Becker et al., 2009; Jasieniak and Smart, 2009) cessing issues such as reduced flotation rates (Ralston and Fornase- resulting in significant dilution of concentrate grade. This is partic- rio, 2006), complex tailings treatment (de Kretser et al., 1997) and ularly so for Great Dyke ores that show considerable low temper- pumping challenges (Dunn, 2004). ature, late stage alteration associated with the formation of Non-Newtonian and rheologically complex behaviour in the hydrophobic talc (Li et al., 2008). Another concern is whether the processing of Platreef PGM ore has been attributed to the presence decrease in average particle size associated with fine grinding will of phyllosilicate minerals such as talc and serpentine (Burdukova cause rheological problems in subsequent downstream ore pro- et al., 2008; Shabalala et al., 2011). These minerals generally form cessing. It has been well established that suspension rheology is through low temperature alteration processes that convert anhy- strongly affected by the nominal particle size as well as the distri- drous minerals such as orthopyroxene (MgSiO3) to talc (Mg6Si8- bution of particle sizes in the mineral suspension (Boger, 1999; O20(OH)4)), or olivine (Mg2SiO4) to serpentine (Mg3Si2O5(OH)4). Farris, 1968; Tangsathitkulcahi, 2003). Little, however, is known about the rheology of other (non-phyllosilicate) minerals, specifically orthopyroxene (inosilicate), plagio- 1.2. Effects of rheology on processing clase (tectosilicate) and chromite (oxide) which are the major gangue constituents of UG2 ore. In general, ultrafine particles lead to rheological difficulties that may often manifest in complications during ore manageability and 1.3. Objective processing. Typical processing challenges associated with rheological complexities include the retardation of grinding within the The primary objective of this study is to compare the rheologi- ultrafine regime, due to viscosity effects (Shi and Napier-Munn, cal behaviour of two different platinum ores, a western limb UG2 2002). In flotation, high pulp viscosities can result in poor gas dis- ore and the Great Dyke ore with that of the constituent minerals persion, increased turbulence damping, bubble coalescence and of the ores. In order to do so, rheological measurements of typical high gangue recovery, and can influence froth stability and mobil- feed, concentrate and tailings of the two ores were performed to ity (Bakker et al., 2010; Farrokhpay, 2012; Genc et al., 2010; Schu- ascertain the changes in yield stress and viscosity with increasing bert and Bischofberger, 1978). In the case of poor gas dispersion, percentage solids. In this paper, these measurements are compared the small bubbles generated within the vicinity of the impellor with those of single mineral studies of orthopyroxene, plagioclase, are not efficiently dispersed throughout the cell by bulk fluid flow chromite and talc to determine the effect the minerals have on the due to the high (apparent) viscosity of the slurry. This inhibits bub- rheology. An additional objective is to identify streams which show ble-particle contact, which is the fundamental mechanism of flota- complex rheology that may cause concern regarding any detrimen- tion. Cavern formation refers to the formation of a region of yielded tal effects it will have on the performance of the flotation fluid within the vicinity of the impeller of the flotation cell, whilst concentrator. the rest of the slurry remains stagnant. Poor gas dispersion and cavern formation usually occur at high volumetric solids concentrations (Bakker et al., 2010;

Load more