minerals Review Challenges Related to the Processing of Fines in the Recovery of Platinum Group Minerals (PGMs) Kirsten C. Corin 1,* , Belinda J. McFadzean 1, Natalie J. Shackleton 2 and Cyril T. O’Connor 1 1 Centre for Minerals Research, Chemical Engineering Department, University of Cape Town, P Bag X3, Rondebosch, Cape Town 7700, South Africa; [email protected] (B.J.M.); [email protected] (C.T.O.) 2 Minerals Expertise Tech Pty Ltd., Germiston 2007, South Africa; [email protected] * Correspondence: [email protected] Abstract: In order to increase the recovery of PGMs by flotation, it is necessary to optimise the liberation of the key minerals in which the platinum group elements (PGEs) are contained which include sulphides, arsenides, tellurides, and ferroalloys among others, while at the same time ensuring the optimal depression of gangue minerals. In order to achieve this, comminution circuits usually consist of two or three stages of milling, in which the first stage is autogeneous, followed by ball milling. Further liberation is achieved in subsequent stages using ultra-fine grinding. Each comminution stage is followed by flotation in the so-called MF2 or MF3 circuits. While this staged process increases overall recoveries, overgrinding may occur, hence creating problems associated with fine particle flotation. This paper presents an overview of the mineralogy of most of the more significant PGM ores processed in South Africa and the various technologies used in comminution circuits. The paper then summarises the methodology used in flotation circuits to optimise recovery Citation: Corin, K.C.; McFadzean, of fine particles in terms of the collectors, depressants, and frothers used. The effect of entrainment, B.J.; Shackleton, N.J.; O’Connor, C.T. slimes coating, changes in rheology caused by the presence of a significant amount of fines and of Challenges Related to the Processing chromite recovery is addressed. of Fines in the Recovery of Platinum Group Minerals (PGMs). Minerals Keywords: fines; PGMs; mineralogy; comminution; flotation; rheology 2021, 11, 533. https://doi.org/ 10.3390/min11050533 Academic Editors: Lev Filippov and 1. Introduction Daniel Fornasiero The issues related to fine particle flotation have been extensively studied (e.g., [1,2]). Received: 23 April 2021 It is generally accepted that many of these problems are owing to the low mass and Accepted: 17 May 2021 high surface area of fine particles and a number of contributory factors that are usually Published: 18 May 2021 interrelated [3]. All of these factors, which will be discussed below, have the potential to affect the flotation recovery and grade of the valuable product. Publisher’s Note: MDPI stays neutral Gangue slimes can also create problems by coating fresh mineral surfaces, and this with regard to jurisdictional claims in may be detrimental, in particular, for fines flotation [4]. Surface coatings of value bearing published maps and institutional affil- minerals by hydrophilic species reduces flotation recovery in all size fractions, but particu- iations. larly for fine particles. Surface coatings affect fine particles more than coarse articles most likely due to the relative surface areas—fine particles are rendered hydrophilic by a lesser degree of surface coating. It has however been shown that it is possible that the problems associated with fine Copyright: © 2021 by the authors. flotation may be mitigated by improved circuit design (e.g., [5,6]). In the processing of Licensee MDPI, Basel, Switzerland. PGMs, these include the widespread use of flow sheets which take into account multi- This article is an open access article ple liberation/beneficiations and are referred to as MF (mill-float) 2 or MF3 flow sheets. distributed under the terms and Figure1 shows a typical MF2 flowsheet. The advantages of such circuits are that they conditions of the Creative Commons reduce the propensity of over-grinding. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Minerals 2021, 11, 533. https://doi.org/10.3390/min11050533 https://www.mdpi.com/journal/minerals MineralsMinerals 20212021,, 1111,, 533533 22 ofof 1717 FigureFigure 1.1. TypicalTypical MF2 MF2 circuit circuit used used in in treatment treatment of PGMsof PGMs in Merensky, in Merensky, Upper Upper Group Group Chromitite Chromitite No.2 (UG2), No.2 and(UG2), Platreef and Platreef ores [7]. ores [7]. These improved processes are aimed at managing the problem of increasing liberation whileThese not excessively improved processes grinding finer.are aimed As has at beenmanaging shown the by problem Jameson of [ 8increasing], large particles libera- withtion while a high not % surfaceexcessively liberation grinding are finer. able toAs float has asbeen fast shown as small by particlesJameson with[8], large a lower parti- % surfacecles with liberation. a high % surface liberation are able to float as fast as small particles with a lower % surfaceIn a study liberation. of the effect of milling circuits for a UG2 ore, it has been shown that on a size-by-size basis, the inclusion of additional mill-float stages results in increased PGM In a study of the effect of milling circuits for a UG2 ore, it has been shown that on a recovery for all size classes, and interestingly, the highest recoveries were achieved in the size-by-size basis, the inclusion of additional mill-float stages results in increased PGM finest size fraction [9]. This study also showed that liberation of PGM particles in the very recovery for all size classes, and interestingly, the highest recoveries were achieved in the fine −10 µm fraction, rather than overgrinding, was the dominant factor at this very fine finest size fraction [9]. This study also showed that liberation of PGM particles in the very grind and that fine, liberated particles are more floatable than locked or partially liberated fine -10 µm fraction, rather than overgrinding, was the dominant factor at this very fine particles in the typically optimum size range (e.g., 38–75 µm). Such observations emphasize grind and that fine, liberated particles are more floatable than locked or partially liberated the critical importance of identifying the degree of liberation in various stages of the circuit particles in the typically optimum size range (e.g., 38–75 µm). Such observations empha- rather than the particle size. It was proposed that liberation of valuable minerals was size the critical importance of identifying the degree of liberation in various stages of the more efficient under attrition breakage than impact breakage in the finest size fraction and circuit rather than the particle size. It was proposed that liberation of valuable minerals thus stirred milling proved more efficient at liberating valuable minerals in the finest size was more efficient under attrition breakage than impact breakage in the finest size fraction fraction but still had the potential to result in over-grinding of liberated particles. and thusExcessively stirred milling fine grinding proved may more lead efficient to higher at liberating chromite valuable recoveries, minerals which isin knownthe finest to besize via fraction entrainment. but still Thishad the is problematic potential to sinceresult it in is over-grinding necessary to ensureof liberated that theparticles. chromite concentrationExcessively in thefine concentrate grinding may is less lead than to higher≈3%so chromite that it does recoveries, not negatively which is impact known the to downstreambe via entrainment. smelting This process. is problematic It has been since observed it is necessary that increasing to ensure both that solids the percentage chromite andconcentration frother dosage in the resulted concentrate in a non-selective is less than ≈3% increase so that in it solids does recoverynot negatively owing impact to entrain- the mentdownstream which is smelting a non-selective process. mechanism It has been basedobserved on thethat particle increasing size both and densitysolids percentage as well as theand water frother recovery dosage to resulted the concentrate in a non-selective [10]. increase in solids recovery owing to en- trainmentA further which problem is a non-selective when considering mechanism fine particlebased on flotation the particle is that size the and large density specific as surfacewell as areathe water of fine recovery particles to increases the concentrate the amount [10]. of reagent that can adsorb at the surface. Thus,A a further large proportion problem when of the considering reagent is fine consumed particle by flotation the very is that fine the particles. large specific When presentsurface area in limited of fine amount, particles thereincreases may the not amount be enough of reagent reagent that available can adsorb to float at the the surface. larger particles,Thus, a large which proportion results in of decreased the reagent recovery. is consumed Moreover, by the especially very fine in particles. the case of When sulphide pre- mineralssent in limited such asamount, those associated there may with not be PGMs, enou rapidgh reagent oxidation available of the to mineral float the might larger occur, par- renderingticles, which the results mineral in non-floatable decreased recovery. under the Mo conditionsreover, especially used for in their the flotation. case of sulphide mineralsThe presentsuch as those paper associated reviews the with challenges PGMs, rapid arising oxidation from the of the presence mineral of might fines inoccur, the processingrendering the of