yb

frically,

Quantitative mineralogyOrthopyroxeneTalc

aluamis thedof

o decouple the NFG from entrained gangue recovered in otation, a series of

in decreasing order of abundance. Orthopyroxene

ica is a lm2 (Eaplatin

Int. J. Miner. Process. 93 (2009) 246255

Contents lists available at ScienceDirect

International Journal o

j ourna l homepage: www.e lssignicant proportion of the economy to the country and as such, anunderstanding of the intricacies related to the mineralogy of these oresis of great importance. Typically, orthopyroxene and plagioclase are thedominant silicate minerals in the run of mine ore and invariably occurwith one another in awide variety of textures. Lesser clinopyroxene andoccasional olivine as well as other hydrous silicate minerals (e.g. mica,talc, serpentine and clay minerals) also occur. The Merensky Reefcontains only minor proportions (b1wt.%) of the base metal sulphides;pyrrhotite, chalcopyrite and pentlandite, which are generally associatedwith the economic platinum group minerals (Lee, 1996; Schouwstra

aremore commonlyknownasnaturallyoatinggangue(NFG) isneeded.Talc is a naturally hydrophobic mineral which is known to contribute tothe amount of NFG (e.g. Shortridge, 2002). Since it is generally onlypresent in small quantities in theMerensky Reef and the volume of NFGrecovered during otation is usually quite signicant for Merensky ores(Wiese et al., 2005), theremust be additional mineralogical componentscontributing to NFG. Since orthopyroxene is one of the predominantgangue minerals in the Merensky Reef ore, it of interest to review itsotation properties. In the review of Lotter et al. (2008) on orthopyr-oxene, themechanisms of entrainment, composite particles, inadvertentet al., 2000; Cawthorn et al., 2002).Typically the platinum group minerals and

theMerensky Reef are recovered by froth otat

Corresponding author. Centre for Minerals ReseaEngineering, University of Cape Town, Private Bag, RSouth Africa. Tel.: +27 21 650 3797; fax: +27 21 650 5

E-mail address: megan.becker@uct.ac.za (M. Becker)

0301-7516/$ see front matter 2009 Elsevier B.V. Adoi:10.1016/j.minpro.2009.10.004umgroupmineral (PGM)UG2 Reef and Platreef.ore deposits represents a

can be minimised, this sometimes occurs at the expense of the recoveryof the valuable mineral (Wiese et al., 2007). Therefore, a goodunderstanding of the deportment of these undesirable minerals, whichore deposits including the Merensky Reef,Mining and mineral processing of these three1. Introduction

The Bushveld Complex in South Afrstretching over approximately 65000 kand is host to some of theworld'smajoreven though the particles recovered are classied as liberated or high grade middlings. On closerexamination, these orthopyroxene particles show a preferential surface association to talc and vice versawhich is consistent with the low temperature alteration of anhydrous orthopyroxene to a hydrous phase,namely talc. These composite orthopyroxene particles with partial talc rims are therefore thought to be oneof the main contributors to NFG in the Merensky Reef ore.

2009 Elsevier B.V. All rights reserved.

ayered igneous intrusionles and Cawthorn, 1996)

controlled by the addition of various reagents such as collectors; whichpromote valuable mineral recovery and depressants; which inhibit theotation of undesirable minerals. Although by controlling the dosage ofthese polymeric depressants the recovery of the undesirable mineralsbase metal sulphides inion. Flotation is generally

ion activation anfor the recoverinadvertent actived (e.g. Nagaraj aJasieniak and Smexposure of Mg aconcentrate relaspectra, they attr

rch, Department of Chemicalondebosch, 7701, Cape Town,501..

ll rights reserved.is however, by far the greatest diluent of concentrate gradeMerenskyFlotationconcentrate samples have been analysed in this study using quantitative mineralogical analysis methods.Results show the major diluents of concentrate grade are orthopyroxene, talc, clinopyroxene and plagioclaseKeywords: University of Cape Town tMineralogical characterisation of naturallMerensky Reef ore otation

M. Becker a,, P.J. Harris a, J.G. Wiese a, D.J. Bradshawa Centre for Minerals Research, University of Cape Town, Rondebosch, Cape Town, South Ab Now at Julius Krutschnitt Mineral Research Centre, University of Queensland, Indooroopi

a b s t r a c ta r t i c l e i n f o

Article history:Received 15 July 2009Received in revised form 28 September 2009Accepted 3 October 2009Available online 27 October 2009

During the recovery of the vore by otation, a certainconcentrate. Although talcproportion of NFG recoveraccounted for by the massoatable gangue in

Brisbane, Australia

able platinum group minerals and base metal sulphides in the Merensky Reefount of naturally oatable gangue (NFG) is recovered, which dilutes thee only silicate mineral well known for it naturally oatable properties, thein batch otation tests of the Merensky ore, however, cannot be solelypure talc present in the feed ore. Using methodology developed at the

f Mineral Processing

ev ie r.com/ locate / i jm inprod naturally oating gangue were discussed to accounty of orthopyroxene to the concentrate, althoughation appears to have been the most widely investigat-nd Brinen, 1996; Malysiak et al., 2004). More recently,art (2009) showed using ToF-SIMS that the surfacend Si was greater for gangue particles in a Merenskytive to a tailings sample. In conjunction with XRDibuted this to be due to the preferential association of

orthopyroxenewith talc. This is consistent with the study of Becker et al.(2006) on the mineralogical characteristics of a Merensky feed ore, thatsuggested that the presence of composite orthopyroxene and talcparticles were most likely to constitute naturally oating gangue.

There have been a number of studies which have incorporatedprocess mineralogy into the interpretation of otation performancesuch as Becker et al. (2008), Dai et al. (2008) and Triffett and Bradshaw(2008). The benets obtained by commissioning process mineralogystudies are not only encompassedby the improvedunderstandingof thesystem, but also include tangible outcomes such as improved recovery,circuit design and selectivity (Lotter et al., 2002; Nel et al., 2005;

bulk samples were crushed, blended, rifed and split using a rotarysplitter into 1 kg samples at the Centre for Minerals Research at theUniversity of Cape Town prior to batch otation tests. Batch otationtests were conducted using the standard procedure as outlined inWiese et al. (2005) at a grind of 60% passing 75 m. Four successivetimed concentrates were collected (noted as C1, C2, C3 and C4hereafter). The reagents used in batch otation tests are given inTable 1. Batch otation tests were conducted at two conditions; withand without a modied guar gum depressant from which theproportion of NFG can be calculated according to the methodologygiven in Wiese et al. (2009) as illustrated in Fig. 1. Each test conditionwas conducted in quintuplicate in order to produce sufcientconcentrate mass for mineralogical analysis.

Feed and concentrate samples were subsequently submitted foranalysis at Mintek in Johannesburg on the Impala QEMSCAN.A common feed sample was analysed in four size fractions; namely+5/20; +20/38; +38/75 and +75 m using the BulkMineralogical Analysis (BMA) routine in QEMSCAN (Gottlieb et al.,2000; Goodall et al., 2005). This routine consists of the linear analysisof sections and is routinely applied to determine bulk modalabundances in metallurgical samples. Due to the limitations of the

Table 1Summary of the reagent dosages used in batch otation tests (shown for the tests withdepressant addition).

Type Name Dosage (g/t)

Frother DOW 200 40Depressant Stypres 504 300Collector SIBX 100Activator CuSO4

247M. Becker et al. / Int. J. Miner. Process. 93 (2009) 246255Charland et al., 2006; Lastra, 2007). Therefore, the objective of thisprocessmineralogy study is to use quantitativemineralogical analysis asa tool to investigate and identify the mineralogical nature of naturallyoating gangue in Merensky Reef ore otation. Based on methodologydeveloped at the Centre for Minerals Research at the University of CapeTown in order to discriminate between oatable and entrained gangueusing the mass and water recovery in batch otation tests of Merenskyore (Wiese et al., 2009), it is proposed that one can make an in depthcharacterisation of the mineralogy of the NFG recovered duringotation, with the use of a suitably designed experimental programme.According to the method given in Wiese et al. (2009), the recovery ofNFG isminimised at high depressant dosages (N300 g/t depressant) andis at a maximum in the absence of depressant. By comparison of themineralogy of concentrates produced from batch otation tests at theabove mentioned conditions, it is possible to evaluate the nature of theparticles contributing to NFG using quantitative mineralogical analysismethods (QEMSCAN).

2. Experimental details

Merensky Reef ore was sourced from the Impala Platinummine asrun of mine ore and has been used in an extensive programmeinvestigating the oatability of this ore (e.g. Wiese et al., 2005). TheFig. 1. Calculated oating gangue for batch otation tests of Merensky ore with and withoutWiese et al (2009). The standard error in the analysis is also shown.instrument, the 5 m size fraction, representing 12.3wt.% of thetotal could not be analysed. Concentrates from repeat batch otationtests were combined and thenwet screened to the same size fractions.Only samples from the +20/38 and +38/75 m size fractionwere selected for further QEMSCAN analysis. The choice of sizefractions for QEMSCAN analysis was based on selection of particles inthe optimum size range for true otation with minimal contributionfrom entrainment (Savassi, 1998). Concentrate samples were runusing the particle mineralogical analysis (PMA) routine on QEMSCAN.This routine consists of an area scan of closely spaced points ofsections and is routinely used for detailed particle mapping todetermine the textural features of particles. Data validation wasperformed based on the correlation of the QEMSCAN results withchemical assays obtained using ICP-OES at Mintek, or AAS and a LecoSulphur Analyser at UCT. No platinum group element analyses wereperformed in this study.

Given the similarity in bulk chemical composition and the backscattered electron (BSE) level between talc (Mg6Si8O20(OH)4) andorthopyroxene ((MgFe)2Si2O6), the methodology used for mineralidentication with QEMSCAN analysis was appropriately tailored toexploit distinguishing characteristics of the two minerals. In addition,the boundary phase particle processor facility available in theQEMSCAN iExplorer software was utilised to reprocess isolated and

300 g/t guar depressant addition based on the methodology described by Wiese ) and

misidentied single pixels of talc into orthopyroxene and vice versa.Use of these precautions and the correlation of QEMSCAN particleimages with typical intergrowth textures observed between ortho-pyroxene and talc in optical microscopy are factors which allow thenecessary condence to be placed in the correct identication of theseminerals.

3. Results

3.1. Feed characterisation

The composition of run of mine Impala Merensky feed is shown inTable 2 where it is apparent that the ore is dominated by the silicateminerals, plagioclase (43.7wt.%) and orthopyroxene (34.2wt.%) withlesser clinopyroxene (5.8wt.%) and talc (1.9wt.%). All other silicateminerals such as olivine and mica constitute less than 2.5wt.% ofthe ore. The ore is fairly enriched in the oxide minerals (i.e. chromite;7.4wt.%) and the sulphide mineralogy mainly consists of pentlandite(0.3wt.%), chalcopyrite (0. 2wt.%) and pyrrhotite (0.4wt.%). Theaverage grain size of pyroxene at this grind is ~24 m and plagioclase,~29 m. Their similarity is most likely due to the absence of the

masses of orthopyroxene were fairly similar between the +38/75and +20/38 m size fractions.

The variation in clinopyroxene recovery as shown in Fig. 3 tends toshow a similar trend to orthopyroxene in that there is a consistentincrease in recovery with increasing otation time. However, both theabsolute grade and recovery of clinopyroxene is considerably lower;the highest grade of clinopyroxene in the concentrate is only 9.6wt.%(no depressant, C4, +20/38 m fraction), and the cumulative massrecovery of clinopyroxene is ~2.9 g (+38/75, +20/38 m frac-tions) in the tests with no depressant addition. With the addition ofdepressant the mass recovery of clinopyroxene was reduced to lessthan 0.3 g (+38/75 m fraction). For both sets of tests, cumulativeconcentrate masses of orthopyroxene were fairly similar betweenthe +38/75 m and +20/38 m size fractions.

Fig. 4 shows that similarly to orthopyroxene and clinopyroxene,plagioclase mass recovery show a consistent increase in mass withincreasing otation time. However, in contrast to the pyroxenes, theeffect of depressant addition on plagioclase recovery is notably smaller(e.g. with depressant: 2.6 g plagioclase; no depressant: 1.3 g plagioclasefor +38/75 m fraction). In addition, it appears that the plagioclase

Oxides 0.3 0.5 1.1 1.5 0.6 1.0 1.5 1.9Calcite 0.1 0.1 0.1 0.3 0.1 0.1 0.2 0.3Other 0.1 0.1 0.2 0.2 0.1 0.1 0.2 0.2Total 100 100 100 100 100 100 100 100

No depressantMass (g) 12.27 9.54 11.61 9.35 10.18 9.83 9.96 7.90% of total conc mass 19.5 13.7 14.1 12.8 16.2 14.1 12.1 10.8Pentlandite 21.0 12.4 3.1 1.0 17.9 8.7 1.8 0.7Pyrrhotite 7.8 7.2 7.9 7.0 5.9 5.7 5.1 4.1Chalcopyrite 15.3 1.4 0.5 0.3 12.2 0.8 0.2 0.1Pyrite 5.1 0.6 0.1 0.1 3.5 0.4 0.1 0.0Other sulphides 0.7 0.5 0.3 0.3 0.7 0.4 0.3 0.2Orthopyroxene 27.0 46.3 56.3 60.0 34.4 52.8 60.8 61.9Clinopyroxene 4.5 6.9 8.1 8.4 5.3 8.0 9.2 9.5Talc 9.6 12.5 10.0 8.2 11.3 11.6 8.4 6.7Chlorite 0.8 1.2 1.2 1.1 1.0 1.3 1.4 1.5Serpentine 0.8 1.2 1.3 1.4 1.0 1.4 1.6 1.6Olivine b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1Amphibole 0.4 0.5 0.7 0.6 0.4 0.6 0.6 0.6Plagioclase 4.4 5.9 6.9 7.6 3.8 5.1 7.2 9.1Mica 0.7 1.0 1.2 1.3 0.7 1.0 1.0 1.2Quartz 0.8 1.1 1.2 1.2 0.6 0.8 1.0 0.9Oxides 0.8 1.0 1.0 1.2 0.9 1.1 1.2 1.4Calcite 0.2 0.3 0.2 0.2 0.2 0.2 0.2 0.2Other 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1Total 100 100 100 100 100 100 100 100

Key silicate minerals of interest are shown in bold.

248 M. Becker et al. / Int. J. Miner. Process. 93 (2009) 246255pegmatoidal pyroxenite layer in the Merensky Reef ore type presentin the Impala mine lease area (Leeb-Du Toit, 1986). Typical alterationminerals of the anhydrous ferromagnesian silicate minerals (olivine,orthopyroxene) such as talc, serpentine and chlorite tend to havemuch ner average grain sizes (37 m).

3.2. Concentrate characterisation

The mineralogy of the concentrates as determined by QEMSCANfor tests conducted with and without depressant addition is given forthe +20/38 and +38/75 m size fractions in Table 3. Unwantedsilicate minerals recovered in the concentrates from the batch otationtests under all the conditions are dominated by four different minerals;namely orthopyroxene, clinopyroxene, plagioclase and talc and theseare themineralswhichwill be focussed upon. It is apparent fromTable 3that the grade of orthopyroxene is by far the greatest (up to 62wt.%) andin order to put this into a mass recovery perspective, results are shownin Fig. 2 in terms of the proportion of orthopyroxene recovered given ingrams. Fig. 2 shows that the amount of orthopyroxene recovered issignicantly greater for batch otation tests without depressant (N19 gorthopyroxene) than for those with depressant (b0. 8 g orthopyrox-ene), illustrating the enormity of the role which depressant additionplays during otation. For both sets of tests, cumulative concentrate

Table 2Quantitative mineralogy of the Impala Merensky Reef feed ore given in weight %.

Mineral wt.%

Pentlandite 0.3Pyrrhotite 0.4Chalcopyrite 0.2Pyrite 0.1Other sulphides b0.1Orthopyroxene 34.2Clinopyroxene 5.8Talc 1.9Chlorite 0.9Serpentine 2.1Olivine 0.5Amphibole 0.3Plagioclase 43.7Mica 0.8Quartz 0.6Oxides 7.4Calcite 0.2Other 0.7Total 100.0Key silicate minerals of interest here are shown in bold.Table 3Quantitative mineralogy of the +38/75 and +20/38 m size fractions ofconcentrates obtained from batch otation tests of Impala Merensky Reef ore givenin wt.%, with and without depressant (300 g/t guar) addition.

Mineral +38/75 m +20/ 38 m

With depressant C1 C2 C3 C4 C1 C2 C3 C4

Conc mass (g) 5.42 3.49 1.88 1.82 4.11 2.37 1.37 1.49% of total conc mass 22.4 17.9 10.3 8.3 17.0 12.2 7.5 6.8Pentlandite 39.8 46.1 28.1 13.2 41.7 45.1 20.9 6.7Pyrrhotite 11.1 18.7 26.3 30.6 7.7 18.6 29.3 26.6Chalcopyrite 28.4 8.5 5.2 2.3 28.1 7.2 2.9 0.9Pyrite 9.1 3.5 1.9 0.5 8.1 2.8 0.9 0.3Other sulphides 1.3 1.7 1.7 1.5 1.3 1.9 1.8 1.4Orthopyroxene 2.2 5.0 9.7 15.1 3.2 6.4 13.7 23.7Clinopyroxene 0.9 2.2 3.9 5.8 1.0 2.3 4.5 6.0Talc 0.4 0.8 1.5 3.3 0.4 1.4 4.1 8.2Chlorite 0.3 0.5 0.7 1.0 0.3 0.6 0.9 1.2Serpentine 0.1 0.2 0.4 0.6 0.2 0.4 0.8 1.1Olivine b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1Amphibole 0.2 0.4 0.6 0.8 0.1 0.4 0.7 1.0Plagioclase 5.3 10.5 16.5 20.8 6.6 10.5 15.9 18.2Mica 0.1 0.3 0.4 0.7 0.1 0.3 0.6 0.8Quartz 0.5 0.9 1.6 1.8 0.3 0.7 1.1 1.2grade is actually enhanced with depressant addition (e.g. Table 3; with

Fig. 2. Cumulative mass of orthopyroxene (g) recovered for the four successive concentrates collected in batch otation tests of Merensky ore at conditions of with and withoutdepressant (300 g/t guar), shown in the +38/75 and +20/38 m size fractions.

249M. Becker et al. / Int. J. Miner. Process. 93 (2009) 246255depressant: 5.320.8wt.%; no depressant: 4.47.6wt.% for +38/75 m fraction). In general, lower plagioclasemass recovery occurredfor the ner size fraction (Fig. 4).

The effect of depressant addition on the mass recovery of talc isnoted to be fairly signicant and caused a decrease from 4.3 g talc (Nodepressant) to 0.1 g of talc (With depressant) for the +38/75 mfraction as shown in Fig. 5. A lower nal mass recovery of talc wasobtained for the ner size fraction in the tests with depressant additionand vice versa for the tests with no depressant addition.

3.3. Floatable gangue

On the basis of the methodology previously described by Wieseet al. (2009), the amount ofoatable gangue canbe estimated for batchotation tests once the relative contributions from the valuableminerals (i.e. sulphide otation) and entrainment have been ac-counted for. The amount of NFG recovered in a batch otation test

(Fig. 1) with depressant addition is relatively negligible (b2 g NFG

Fig. 3. Cumulative mass of clinopyroxene (g) recovered for the four successive concentratedepressant (300 g/t guar), shown in the +38/75 and +20/38 m size fractions.recovered for the entire oat), whereas in the absence of a depressantup to 38 g of gangue is naturally oatable (Note: these masses areappropriate for a 1 kg batch oat, whereas concentrate massesrepresent the equivalent of a 5 kg feed sample). Given the differencein the absolute amount of oatable gangue recovered between thetest conditions with and without depressant, one can evaluate thedifference in the nature of the mineralogy of the material recovered inorder to determine what type of particles constitute NFG from amineralogical perspective. It should be recognised that even thoughthe size fractions selected for analysis should have minimal contribu-tion from entrainment (Savassi, 1998), there will always be someminor degree of entrainment of ne particles due to the dynamics ofthe otation process.

By examinationof thedifferences in recovery of the silicateminerals inthe +20/38 and +38/75 m size fractions from the otation testsconducted with and without the addition of depressant as shown inFigs. 25, it is apparent that the silicate minerals which contribute the

most tooatable gangueareorthopyroxeneand talc. Sinceorthopyroxene

s collected in batch otation tests of Merensky ore at conditions of with and without

Fig. 4. Cumulative mass of plagioclase (g) recovered for the four successive concentrates collected in batch otation tests of Merensky ore at conditions of with and withoutdepressant (300 g/t guar), shown in the +38/75 and +20/38 m size fractions.

250 M. Becker et al. / Int. J. Miner. Process. 93 (2009) 246255is not naturally oatable, it is of interest to explore the liberation andassociation relationships of orthopyroxene to determinewhether there isany indication of a preferential association that may lend it someoatability.

3.4. Mineralogical characterisation of oatable gangue

During the grinding process, particles are produced which arevariably liberatedwith respect to themineral of interest. It is noted thatin process mineralogy, the denition of liberation is based on area,whereas surface association is related to the perimeter of the grain orparticle of interest. Free surface is dened as the association of the grainperimeter with background. Where relatively simple mineral texturesexist, liberation and free surface coincide, but in some scenarios such asthat illustrated in Fig. 6, these twomeasurementsmay be quite differentand can lead to misinterpretation (Lastra, 2002; Pooler et al., 2009).

Comparison of the liberation characteristics of orthopyroxene

recovered in the concentrates as illustrated in Fig. 7 shows that for

Fig. 5. Cumulative mass of talc (g) recovered for the four successive concentrates collected(300 g/t guar), shown in the +38/75 and +20/38 m size fractions.conditionswithandwithoutdepressantaddition, theparticles recoveredexist as locked (030area %), low grade middlings (3060area %), highgrade middlings (6090area %) and liberated (N90area %). Thesignicant contrast in mass recovery of the orthopyroxene particlesrecovered in the twodifferent sets ofotation tests is alsomost apparentin Fig. 7. It is noted that for the tests without depressant, themajority ofthe particles are high grade middlings or liberated. Although thisinformation is only shown for the+38/75 msize class, thebehaviouris similar for the+20/38 m fraction. For talc however, it appears thatboth liberated and unliberated talc are recovered during otation(Fig. 8). Although not illustrated here, talc particles in the+20/38 mfraction also behave in a similar manner.

In order to investigate further the nature of these orthopyroxeneand talc particles which are causing considerable dilution to concentrategrade, the associations of the different liberation classes of the particlescan also be evaluated. The surface association of orthopyroxene is shownin Fig. 9 for particles from Concentrate 3 only, where the following

subdivisions are used: Surface association with background (i.e. free

in batch otation tests of Merensky ore at conditions of with and without depressant

concentrates have been omitted for simplicity, but show similar trends.Results shown for the tests conducted with no depressant addition(Fig. 9), illustrate that the mass of orthopyroxene associated withbackground is by far the greatest, especially for liberated orthopyroxene(3.0 g). The second most common association of orthopyroxene is withthe alteration minerals, which predominantly represents talc. Bothliberated (0.6 g) and high grade middlings (0.4 g) orthopyroxeneparticles show a signicant association to the alteration minerals. Theremaining associations of orthopyroxene are encompassed by itsassociation to clinopyroxene and other. The surface associations of talcin Concentrate 3 illustrated in Fig. 10 accordingly show the preferentialassociation of talc to orthopyroxene, especially in the no depressant tests.The preferential association of talc to orthopyroxene occurs for locked(0.2 g), low grade middlings (0.1 g) and high grade middlings particles(0.08 g).

4. Discussion

In the otation of theMerensky Reef ore, the four dominant silicateminerals recovered which dilute concentrate grade are orthopyrox-ene, clinopyroxene, talc and plagioclase of which only talc is known tobe hydrophobic and naturally oatable. Comparison of the concen-trate mineralogy from batch otation tests conducted with theaddition of depressant in order to minimise the recovery of NFGwith batch otation tests with no depressant addition, where all theNFG is recovered, shows that orthopyroxene and talc are the majorminerals which constitute NFG.

The results presented also clearly show that plagioclase, one of

251M. Becker et al. / Int. J. Miner. Process. 93 (2009) 246255Fig. 6. Schematic to illustrate the difference between liberation and free surfacemeasurements. The composite particle between mineral A and B, represents orthopyr-oxene (A)with talc (B) rimming,whereas the composite particle betweenC andD, ismoretypical of an unliberated sulphide (C) hosted by a silicate gangue mineral (D).surface which a bubble can attach to), with the base metal sulphides(pentlandite, pyrrhotite, chalcopyrite etc), with pyroxene (clinopyrox-ene), plagioclase, alteration minerals (talc, chlorite and serpentine) andother (all remaining minerals). Surface associations for the other

Fig. 7. Liberation of orthopyroxene particles in the +38/75 m fraction in successive concdepressant shown as the mass of locked (030area %), low grade middlings (3060area %)the other dominant gangueminerals recovered during otation, doesnot contribute to NFG since the concentrate grade of plagioclase ishigher for tests conducted with depressant addition relative to thosewhere no depressant is added (Table 3). In addition to the fact thatsome of the plagioclase particles recovered during otation containunliberated chalcopyrite that render them oatable, it also suggestssome depressant selectivity i.e. the guar depressant is preferentiallyadsorbed on talc and pyroxene particles, and that plagioclaseparticles remain relatively free of depressant. Although the work of

entrates from batch otation tests performed with (a) and without (b) the addition of

, high grade middlings (6090%area %) and liberated (N 90area %) particles.

s frorade

252 M. Becker et al. / Int. J. Miner. Process. 93 (2009) 246255Martinovic et al. (2005) showed no signicant difference betweenthe natural oatability of orthopyroxene and plagioclase for a pure

Fig. 8. Liberation of talc particles in the +38/75 m fraction in successive concentrateshown as the mass of locked (030area %), low grade middlings (3060area %), high gmineral system, more recent work by Schreithofer (Unpubl.)suggests that selective adsorption of polymeric depressants doesindeed take place for gangue minerals in Merensky Reef ores.

Fig. 9. Surface association of the different liberation classes of orthopyroxene shown for Condepressant. Orthopyroxene particles are classied as locked (030area %), low grade middAssociation with pyroxene represents clinopyroxene, association with BMS represents the baThe remaining minerals are classed as other. Only particles from the +38/75 m size fraIt has also been shown that the majority of the orthopyroxenereporting to the concentrate in the no depressant otation tests are

m batch otation tests performed with (a) and without (b) the addition of depressantmiddlings (6090area %) and liberated (N90area %) particles.classied as liberated or high grade middlings (Fig. 7). However,according to the surface association data, these same orthopyroxeneparticles have a distinct association with the alteration minerals and in

centrate 3 from batch otation tests performed with (a) and without (b) the addition oflings (3060area %), high grade middlings (6090area %) and liberated (N90area %).se metal sulphides, whereas alteration minerals represent talc, serpentine and chlorite.ction are illustrated.

Fig. 10. Surface association of the different liberation classes of talc shown for Concentrate 3 from batch otation tests performedwith (a) andwithout (b) the addition of depressant.Talc particles are classied as locked (030area %), low grade middlings (3060area %), high grade middlings (6090area %) and liberated (N90area %). Association with pyroxenerepresents both orthopyroxene and clinopyroxene, association with BMS represents the base metal sulphides, whereas alteration minerals represent talc, serpentine and chlorite.

ze fr

253M. Becker et al. / Int. J. Miner. Process. 93 (2009) 246255particular, the naturally oating talc (Fig. 9). Themanifestation of this isin the form of partial talc rims surrounding the orthopyroxene particlessimilar to those illustrated in the QEMSCAN particle images in Fig. 11.

The remaining minerals are classed as other. Only particles from the +38/75 m siThe association of talc and pyroxene within these Merensky ores is notentirely unexpected given that talc is a low temperature alteration

Fig. 11. Selected QEMSCAN particle images from the +38/75 m fraction recovered in baillustrate the type of composite orthopyroxene and talc particles which represent NFG.product of anhydrousmagnesiumsilicateminerals (Hemley et al., 1977;Nesbitt and Bricker, 1978; Viti et al., 2005). Both olivine andorthopyroxene fall into this category of minerals, although given the

action are illustrated.greater abundance of orthopyroxene in the feed and concentratesrelative to olivine, the focus here is on orthopyroxene. In general, the

tch otation tests with no depressant addition (Concentrate 3). The QEMSCAN images

254 M. Becker et al. / Int. J. Miner. Process. 93 (2009) 246255alteration reaction of orthopyroxene occurs to form a hydrous mineralwith similar Si/Mg ratio as orthopyroxene (OPX: Si/Mg ~1; Talc: Si/Mg~1.33). Similarly, the alteration reaction of olivine follows a pathway toproduce a hydrous silicatewith similar Si/Mg ratio (Olivine: Si/Mg ~0.5;Serpentine: Si/Mg ~0.66). Various authors including, Farquhar (1986),Zingg (1996), Penberthy and Merkle (1999), Li et al. (2004) havereported the alteration of orthopyroxene to talc within the BushveldComplex. Other accounts of talc rimming in the Bushveld Complexinclude those of Gottlieb and Adair (1991) who documented talcrimming of chromite. Penberthy and Merkle (1999) also described aminor alteration of clinopyroxene to talc in UG2 ore, although the mostcommon alteration product of clinopyroxene was amphibole. Thealteration reaction of orthopyroxenewouldproceedvia the formation oftalc along rims and fractures of the orthopyroxene and would result incomposite orthopyroxenetalc particles with partial talc rims such asthose particles illustrated in Fig. 11.

Fig. 11 also shows that only a veryminor proportion of talc rimmingis needed to produce a naturally oatable orthopyroxene particle. Thismay account for the reason why so much liberated orthopyroxene isrecovered during otation. Bearing in mind that the resolution ofautomated SEM type instruments (QEMSCAN, MLA) is governed by thesize of the electron beam and its interaction volume, precisemineralogical analyses cannot be guaranteed for phases less than5 m (Gottlieb et al., 2000). Therefore, the presence of submicron talcrims or partial coatings on liberated orthopyroxene particles cannot bedetected using this type of instrumentation and is where surfacetechniques such as ToF-SIMS may be more appropriately used.Accordingly, the ToF-SIMS study of Jasieniak and Smart (2009) showedelevated Mg and Si concentrations on gangue particles in a Merenskyconcentrate relative to those in the tailings. In combination with theXRD results from their study, Jasieniak and Smart (2009) similarlysuggested that the talc was preferentially associated with orthopyrox-ene and may account for the natural oatability of these particles.

Additional evidence to support the argument that the associationof orthopyroxene with talc is critical to NFG, can be found in the studyof Brough (2008) and Becker et al. (2008) on the geometallurgy ofdifferent Merensky Reef ore types at Northam platinum mine. In thisstudy, the batch otation response of three different Merensky Reeftypes was compared. It was shown that the plagioclase-rich NP2 Reefhad the least amount of oatable gangue compared to the Normal andP2 Reef types. The Normal and P2 Reef types were characteristicallyrich in orthopyroxene and associated ferromagnesian minerals andshowed pervasive alteration of orthopyroxene by minerals such astalc, serpentine and chlorite.

5. Conclusions

This paper has demonstrated the value which quantitativemineralogical analysis (QEMSCAN) can add in characterising andinterpreting the characteristics of naturally oating gangue (NFG) inthe otation of Merensky Reef ores. Mineralogical analysis of theconcentrate recovered in batch otation tests shows that middlingsand liberated orthopyroxene, aswell as both liberated and unliberatedtalc are the dominant particles that constitute NFG. The preferentialassociation of orthopyroxene with talc and vice versa is noted. This ismanifested as orthopyroxene with partial rims of talc. The sheervolume of liberated orthopyroxene recovered with no signicantassociation to talc, is most likely due to the presence of submicron talccoatings that cannot be resolved with QEMSCAN. This suggests thatonly very minor amounts of talc associated with orthopyroxene arenecessary to cause an orthopyroxene particle to be naturally oatable.

The need to interpret quantitative mineralogical data in terms ofboth liberation and surface association has been shown in order toavoid misinterpretation, given that otation is based upon the surfaceproperties of the particle of interest. This is of particular relevance

when complex ore textures such as partial rimming exist.Mineralogical analysis of concentrate samples from otation testswith depressant addition relative to those with no depressant, hasshown that the oatability of plagioclase is relatively enhanced dueto the addition of a depressant and suggests that some depressantselectivity may occur.

Acknowledgements

Our appreciation goes to Impala Platinum for their support andfunding of this research as well as to MINTEK for the technical supportduring QEMSCAN analyses. Members of the UCT Reagent ResearchFacility are also acknowledged for their continued support of this ongoingresearch: Anglo Platinum, Impala Platinum and Lonmin Platinum.

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255M. Becker et al. / Int. J. Miner. Process. 93 (2009) 246255

Mineralogical characterisation of naturally floatable gangue in Merensky Reef ore flotationIntroductionExperimental detailsResultsFeed characterisationConcentrate characterisationFloatable gangueMineralogical characterisation of floatable gangue

DiscussionConclusionsAcknowledgementsReferences