Minerals Engineering 70 (2015) 92–98

Contents lists available at ScienceDirect

Minerals Engineering

journal homepage: www.elsevier .com/ locate/mineng

The interactive effects of chemical and process parameterson the flotation performance of a UG2 ore

http://dx.doi.org/10.1016/j.mineng.2014.08.0160892-6875/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding. Tel.: +27 216505528.E-mail address: Belinda.mcfadzean@uct.ac.za (B. McFadzean).

B. McFadzean ⇑, S. Pani, J. Wiese, C.T. O’ConnorCentre for Minerals Research, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa

a r t i c l e i n f o

Article history:Received 20 June 2014Accepted 26 August 2014Available online 29 September 2014

Keywords:Interactive effectsFlotation performanceFactorial design

a b s t r a c t

A major challenge in the processing of PGM-bearing UG2 ore is the fact that it is comprised largely ofchromite (60–90% by volume). Chromite is generally not naturally floatable but significant quantitiesreport to the concentrate by entrainment. This is a serious problem for the downstream smelting process,which is generally constrained to a grade of about 3% chromite. Four of the most accessible levers for thecontrol of flotation performance which are available to a plant metallurgist are frother and depressantdosage, air flow rate and froth height. The effects of each of these factors on the flotation performancehave been extensively studied, but interactive effects are not well understood. This study presents theresults of an investigation into the interactive effects of these parameters using a factorial experimentaldesign approach. The flotation tests were carried out using a 2 m high continuous column flotation cellwhich facilitated the generation of a deep froth, since this was the phase of most interest. The resultswere analysed using statistical analysis software that fitted a regression model to the data for each ofthe outputs. These results highlight the interactive effects of these variables on the flotation performanceand illustrate the complexity that is required in process control.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

South Africa is the largest producer of platinum in the world(Cawthorn, 1999). About 75% of the world’s platinum reservesare present in the Bushveld igneous complex, which consists ofthree main ore bodies, viz. the Merensky reef, the UG2 reef andthe Platreef. The Merensky reef is comprised largely of orthopyrox-ene (�60%), plagioclase feldspar (�20%), pyroxene (�15%),phlogopite (�5%) and some olivine. The base metal sulfides andassociated platinum group minerals are contained within thesegangue minerals. The Platreef consists of a complex collection ofpyroxenites, serpentinites and calc-silicates. The base metalmineralisation and PGE concentrations are irregular in value anddistribution (Schouwstra et al., 2000).

The mineral composition of UG2 ore is different from the otherores in that it is dominated by hydrophilic chromite gangue (60–90% by volume), with some silicate minerals (5–30% pyroxeneand 1–10% plagioclase). The chromite reports to the concentrateby mechanical entrainment rather than true flotation and thisresults in a linear relationship with water recovery (Engelbrechtand Woodburn, 1975; Bishop and White, 1976). Large amounts

of chromite in the concentrate have severe implications in PGMsmelting because of spinel formation and decreased capacity inthe furnace (Ekmekci et al., 2003). Process parameters such as frothheight, superficial air velocity, frother concentration and depres-sant dosage all play major roles in the recovery of chromite tothe concentrate (Yoon and Luterell, 1989; Harris et al., 1963;Jowett, 1966; Sadler III, 1973; Ekmekci et al., 2003; Valenta,2007; Hay and Roy, 2010).

There have been a number of studies conducted on the effects ofindividual parameters on the flotation performance of a UG2 ore(Ekmekci et al., 2003; Valenta, 2007; Hay and Roy, 2010; Hay,2010). It is well known that an increase in froth height will leadto drainage of water from the plateau borders and bubble thinfilms, leading to a lower degree of entrainment (Savassi et al.,1998; Ekmekci et al., 2003). Ekmekci et al. (2003) found that anincrease in froth height decreased the grade of Cr2O3 in the concen-trate. However, they pointed out that this would also affect thePGM recovery – a response that they did not investigate. The effectof froth height on PGM recovery is not well studied. Alvarez-Silvaet al. (2014) showed that differences in froth height had little effecton PGM recovery. However, the two froth heights that were testedwere both very deep and the critical point in PGM losses may havebeen expected at shallower froth depths. A meticulous plant-scaleCCRD study conducted by Harris et al. (2013) showed that PGM

B. McFadzean et al. / Minerals Engineering 70 (2015) 92–98 93

recoveries were dependent on froth depth as well as air flow rate.The PGM recoveries fell sharply with an increase in froth depthonly at high air flow rates. At lower air flow rates, there was littledifference in recovery with increasing froth depths.

An increase in air flow rate has been shown to result in adecreased froth residence time of air and greater entrainment(Savassi et al., 1998). It is not as straightforward to quantify asthe change in froth height, since a change in air flow may resultin a change in froth structure. Naturally, an increase in air flow ratewill also increase the pulp phase kinetics. An increase in frotherdosage will result in an increase in water recovery and an increasematerial recovered by entrainment. This results in decreasedgrades of valuable minerals. However, work by Wiese et al.(2010) has shown that there can also be a substantial increase invaluable mineral recovery with increasing frother concentrations.Increasing depressant dosages increases the grade of concentrate,but may also decrease the recovery of valuable minerals (Wieseet al., 2009; Wiese et al., 2010). However, in PGM flotation, depres-sant is often used to regulate the mass pull. In this regard, there is asubstantial interaction between frother and depressant, with lowerfrother dosages required at lower depressant dosages (Harris et al.,2013). From the above, it is clear that there are many competinginteractions occurring during flotation and it is important to takethese interactive effects into account when optimising a flotationcircuit. A factorial design is a useful tool for the study of the effectof the various process parameters. In the factorial design approachthe interdependency of process variables can be studied with tar-geted responses (Cochran and Cox, 1990; Araujo and Brereton,1996).

The objective of the current study is to investigate the interac-tive effect of process parameters on the recovery of chromite and,to a lesser extent, PGMs in UG2 ore flotation. A regression modelwas fitted to the experimental results while varying the processparameters of froth height, superficial air velocity, depressant dos-age and frother dosage. This allowed an optimum set of conditionsto be predicted and targeted such that chromite grade andrecovery were minimised, while PGM grade and recovery weremaximised.

BMS, 0.16Other, 0.63

2. Material and methods

2.1. Materials

A PGM bearing UG2 ore from the western limb of the Bushveldcomplex, South Africa, was used in this study. The run of mine(ROM) sample was crushed to less than 1 mm and split into1.3 kg sub-samples. These samples were milled to 60% passing75 lm using synthetic plant water in a stainless steel rod mill.The particle size distribution of the feed is shown in Fig. 1. The

0

10

20

30

40

50

60

70

80

90

100

1 10 100 1000

Cum

ula�

ve %

pas

ssin

g

Par�cle size (μm)

Fig. 1. Particle size distribution of the feed.

reagent suite of collector (SIBX), depressant (guar gum) and frother(Dow200) were used in this study. The feed sample was analysedand characterized by QEMSCAN. The bulk mineralogy of the ROMsample is shown in Fig. 2. It consisted mainly of chromite(52.44%), with significant amounts of orthopyroxene (23.27%)and plagioclase (15.94%). Talc comprised just over 2% and basemetal sulphides comprised a small amount (0.16%). Chemicalassays showing the PGM, Cr2O3, copper and nickel content of thefeed sample are shown in Table 1.

2.2. Method

2.2.1. Column flotationA laboratory column flotation cell adapted from Xu et al. (1996)

was used for the study. The column was made of transparent Plexi-glass, with an internal diameter of 4.6 cm and a height of 200 cm.The pulp/froth interface was held constant to within 1 cm by usinga pressure transducer, located 80 cm from the top, which con-trolled the tailings pump rate. The slurry and air are co-fed nearthe bottom of the column via an external sparger consisting ofthree feed ports for, respectively, the air, feed and recirculatedslurry. The addition of synthetic air was controlled using a needlevalve. Bubbles were formed by shear resulting from passing therecycled slurry and air across a porous pipe at high pressure inthe sparger. Recirculation of the slurry at the same level as the feedpoint increased the generation of bubbles and provided therequired energy input to ensure bubble-particle collisions. Theair flow rate was measured with a rotameter which was regularlycalibrated using a soap bubble meter. A schematic diagram of thecolumn rig is shown in Fig. 3.

The feed (d60 = 75 lm) was slurried with synthetic plant waterto achieve a solids concentration of 16%. The feed mass flow rate tothe column was kept constant at 160 g/min. The collector wasadded first to the feed tank, followed by the depressant and finallythe frother. Each reagent was added 2 min after the other to allowsufficient conditioning time. Finally, the slurry was kept in suspen-sion for 15 min before being fed to the column. The conditionedslurry was fed to the column at a constant flow rate of 1 L/min.The column was operated in closed circuit until steady state wasachieved which was assumed to take the equivalent of 2–3 resi-dence times. After steady state was reached, the column was oper-ated in open circuit for 7–9 min (i.e. 2–3 residence times). Sampleswere taken from the feed tank and simultaneously from thetailings and concentrate streams for precisely two minutes each.Five such samplings were made during the full test. A test was con-sidered acceptable when the overall solids mass balance for theentire run was within 10% (i.e. concentrate + tailings vs feed). The

Orthopyroxene, 23.27 Clinopyroxene,

2.36

Olivine, 1.10

Amphibole, 0.88

Serpen�ne, 0.07

Talc, 2.07

Chlorite, 0.40

Plagioclase, 15.94

Mica, 0.57

Chromite, 52.44

Fig. 2. Bulk mineralogy of the UG2 feed.

Table 1Chemical Assay of the ROM sample given as a mass percent (PGM in g/t).

PGM (g/t) Cr2O3 (%) Cu (%) Ni (%)

3.76 23.12 0.04 0.14

Table 2Factors and levels for factorial design.

Parameters Frothheight(cm)

Superficialair velocity(cm/s)

Depressant dosage(guar gum)(g/t)

Frother dosage(DOW 200)(ppm)

Levels (Min.) 18 0.5 100 10Levels (Max.) 30 1.5 300 30

94 B. McFadzean et al. / Minerals Engineering 70 (2015) 92–98

concentrate and tailings samples were weighed before and afterdrying to determine the solids and water recovery in each sample.Chromite (expressed as Cr2O3) and PGE assays were determinedusing acid digestion followed by atomic absorption spectroscopy(AAS) and fire assay methods, respectively.

2.2.2. Experimental designA factorial experimental design was used to identify the effects

of various process parameters. The responses used were chromiteand PGM grade and recovery, respectively and the solids and waterrecovery. The experiments were carried out by using four factors attwo levels. These are given in Table 2.

Design Expert 8.0 (Design Expert 8, 2011) software was used forthe regression analysis, statistical and optimisation calculations.Design Expert uses the method of least squares to fit a linear modelto the data. An output to the data is an analysis of variance(ANOVA) table where the main effects are tested for significanceby the F-test.

Five centre point experiments were performed to check for cur-vature and reproducibility. No curvature was found in any of themodels. The relative standard error of solids and water recoverywas found to be less than 2%, that of chromite recovery was6.5%, chromite grade was 8.8%, PGM recovery was 2.8% and PGMgrade was 4.2%.

3. Results

3.1. Cr2O3. recovery and grade

Chromite is the most abundant gangue mineral present in theUG2 ore and it is well known that it is recovered into the concen-trate by entrainment only. There are reports that it may appear tobe recovered by true flotation and this is ascribed to talc rimming.Mineralogical analysis however showed no evidence of talc rim-ming. Fig. 4 shows the grade and recovery of chromite (Cr2O3) ateach of the conditions investigated. It is clear that superficial airvelocity (SAV) and froth height (FH) both have a significant effecton chromite recovery and grade. These effects are well illustrated

Fig. 3. Schematic diagram of the column fl

by comparing the effect of increasing superficial air velocity, whileholding all the other parameters constant. Likewise, the chromitegrade and recovery increased sharply with an increase in superfi-cial air velocity when all other conditions remained constant.Depressant dosage had a negligible effect on chromite recoverybut did affect the chromite grade significantly. From Fig. 4 it canbe seen that the chromite grade increased from 4% to 12% withan increase in depressant dosage from 100 to 300 g/t. The reasonfor this will be discussed later. The recovery of chromite increasedslightly with an increase in frother concentration but chromitegrades did not show any dependence on frother concentration.

Factorial design methods are most suitable in attempting tounravel the relationships between the various parameters andthe indicated responses. The experimental data was consequentlyanalysed using factorial design methods and the significance ofthe factors analysed using ANOVA. Table 3 shows the analysis ofvariance for each contributing factor to the regression equation.The confidence levels were set at 90%. Values outside this levelwere discarded.

From Table 3 it can be observed that the superficial air velocityhad the most significant impact on chromite recovery, followed bythe effect of and froth height and frother concentration. Theseobservations are based on the relative contributions shown inthe final column. The depressant dosage had little effect on chro-mite recovery (1.3%) and was therefore not incorporated into themodel. No significant interactive effects were observed. An interac-tive effect exists when differences in one factor depend on the levelof another factor. The best fit to the experimental data is given bythe regression equation shown in Eq. (1).

Cr2O3Recovery ¼ 0:62� 0:13FHþ 0:30SAVþ 0:077FC ð1Þ

where FH, SAV and FC represent froth height, superficial air velocityand frother concentration, respectively. The R2 value for the regres-sion equation was found to be 0.82 which indicates that the pre-dicted and observed values were fairly similar. From this equation

otation rig (Alvarez-Silva et al., 2014).

Fig. 4. Results of chromite (Cr2O3) recovery and grade with different conditions of various process parameters. FC = frother concentration (ppm), DD = depressant dosage(g/t), SAV = superficial air velocity (cm/s), FH = froth height (cm).

Table 3Summary of major effects of variables and interactions on chromite recovery and grade at the 90% confidence level.

Response Term P value Prob > F Effect on response % Contribution

Chromite recovery Froth height 0.0085 Negative 13Superficial air velocity 0.0001 Positive 67Frother concentration 0.0927 Positive 4.3

Chromite grade Froth height 0.0020 Negative 8Superficial air velocity 0.0002 Positive 18Depressant dosage 0.0001 Positive 41Frother concentration 0.0038 Positive 6Superficial air velocity and depressant dosage 0.0003 Negative 15

B. McFadzean et al. / Minerals Engineering 70 (2015) 92–98 95

it can be seen that Cr2O3 recovery decreases with increasing frothheight and relatively more significantly with increasing superficialair velocity. Frother concentration has a small positive effect onCr2O3 recovery.

In the case of chromite grade, the best fit to the experimentaldata is given in Eq. (2). It can be seen that all the individual factorshad a significant effect on the grade. Moreover, interactive effectswere also observed. The depressant dosage (DD) had relativelythe greatest positive effect on the grade of Cr2O3 in the concentratefollowed by superficial air velocity (SAV). However the interactionbetween depressant dosage and superficial air velocity had asignificant negative effect on chromite grades.

Cr2O3Gradeð%Þ ¼ 11:68� 0:85FHþ 1:30SAVþ 1:96DD

þ 0:76FC� 1:19SAV � DD ð2Þ

where FH, AV, DD, FC stand for froth height, superficial air velocity,depressant dosage and frother concentration, respectively. Each fac-tor and their respective interactions contribute significantly to theCr2O3 grade as summarised in Table 4.

Fig. 5(a) and (b) shows two examples of three-dimensionalplots obtained from Eqs. (1) and (2). Fig. 5(a) shows the effect ofsuperficial air velocity and depressant dosage on chromite recov-ery. This Figure illustrates that depressant dosage had essentiallyno effect on chromite recovery, but that the recovery increasedwith increasing superficial air velocity. The increase in superficialair velocity had the same effect on chromite recovery at all levelsof depressant dosage. Fig. 5(b) shows the effect of depressant dos-age and superficial air velocity on the chromite grade, while frotherconcentration and froth height are held constant at their midpointlevels. There is a minimum chromite grade at low depressant

dosages and superficial air velocities. At high superficial air veloc-ities, depressant dosage had a relatively minor effect on chromitegrade. However, at low superficial air velocities there was amarked increase in chromite grade with increasing depressant dos-age. Likewise, at high depressant dosages superficial air velocityhad no effect on chromite grade.

Fig. 5(c) and (d) shows the effect of depressant dosage and frothheight on Cr2O3 recovery and grade at the mid-levels of frotherconcentration and air flow rate. Increasing froth height causes adecrease in chromite recovery at both high and low levels ofdepressant dosage. Depressant dosage has little effect on chromiterecovery. Froth height, however, has more of an effect of chromitegrade at low depressant dosages than at high depressant dosages.In the case of chromite grade, depressant dosage now plays a rolein that the chromite grade increases as the depressant dosageincreases.

Fig. 6 shows chromite recovery as a function of water recovery.There is a reasonably good linear relationship between these twoparameters passing through the origin and this is indicative ofthe fact that the chromite was recovered by entrainment ratherthan as a result of true flotation. The slope of the line indicates that,in this system, 0.0036 g of Cr2O3 will be recovered per gram ofwater.

3.2. PGM recovery and grade

Table 4 summarizes the major effects of variables and interac-tions on PGM recoveries and grades. The table shows that thePGM recovery is most favourably affected by superficial air velocity(SAV) and most negatively affected by froth height. PGM grade, on

Table 4Summary of major effects of variables and interactions on responses at the 90% confidence level.

Response Term P value Prob > F Effect on response % Contribution

PGM recovery Froth height 0.0002 Negative 27Superficial air velocity 0.0001 Positive 49Superficial air velocity and depressant dosage 0.0055 Negative 11

PGM grade Froth height 0.0005 Positive 25Superficial air velocity 0.0001 Negative 40Superficial air velocity and frother concentration 0.0010 Positive 21

Fig. 5. Effect of superficial air velocity and depressant dosage on (a) chromite recovery and (b) chromite grade. Effect of depressant dosage and froth height on (c) chromiterecovery and (d) chromite grade at constant mid-point levels of the other two variables.

y = 0.0036xR2 = 0.82

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 50 100 150 200 250 300 350 400

Cr2O

3 Re

c (g/

min

)

Water Rec (g/min)

Fig. 6. Chromite recovery as a function of water recovery.

96 B. McFadzean et al. / Minerals Engineering 70 (2015) 92–98

the other hand is negatively affected by superficial air velocity(SAV) and positively affected by an increase in froth height. Bothgrade and recovery have parameters which interact to give com-plex responses. These observations are shown in Eqs. (3) and (4)which show the best fit to the experimental data in the case ofPGM recovery and grade.

PGM Rec ¼ 69:21� 0:86FHþ 1:16SAV� 0:55SAV � DD ð3ÞLnðPGM gradeÞ ¼ 4:66þ 0:29FH� 0:36SAVþ 026SAV � FC ð4Þ

where FH, SAV and FC represent froth height, superficial air velocityand frother concentration, respectively.

Fig. 7(a) and (b) provide illustrative plots of the effects of depres-sant dosage and air flow rate on the PGM recovery and grade whenthe frother concentration and froth depth are kept constant at theirmidpoints. PGM recovery showed a marked increase with superfi-cial air velocity at low depressant dosages, but less of an effect athigh depressant dosages. PGM grade decreased uniformly withincreases in superficial air velocity both at high and low depressantdosages. Depressant dosage had little effect on PGM grade.

4. Discussion

This study showed that the superficial air velocity appeared toplay the most dominant role in affecting both chromite grade

B. McFadzean et al. / Minerals Engineering 70 (2015) 92–98 97

and recovery and PGM grade and recovery. Increases in superficialair velocity caused increases in chromite grade and recovery,which is undesirable in PGM flotation. However, this should be bal-anced against the fact that an increase in superficial air velocityalso produced increases in PGM recovery although this alsoresulted in a decrease in grade. Chromite is known to be recoveredby entrainment and not by true flotation. This is evident fromFig. 6, which shows the linear relationship between chromite andwater recovery. Thus, an increase in superficial air velocity isexpected to cause an increase in water recovery (and, therefore,entrainment) by the theories of entrainment such as the wakeand bubble swarm theories and boundary layer effects (Gaudin,1957; Moys, 1978; Smith and Warren, 1989). Although no size-by-size analyses were carried out in the present study it is worthmentioning that the entrained chromite would mostly be inthe <25 lm fraction since it has a relatively high density. Fig. 1shows that about 25% of the total feed is in the <25 lm fraction.Alvarez-Silva et al. (2014) found that, at the same grind size as thisstudy, about 20% of the chromite was in the <25 lm range. Thechromite recovered to the concentrate belonged overwhelminglyto this size fraction. The factorial design experiments also showedthat chromite and PGM responses were affected by interactionsbetween air flow rate and the other variable parameters. Due tothe interactive effects of superficial air velocity and depressantdosage, the chromite grade showed a greater relative increase withincreasing superficial air velocity at low depressant dosage than athigh depressant dosage (Fig. 5). At high depressant dosagesmore of the naturally floating gangue is depressed and this wouldresult in an increase in the grade of the chromite in theconcentrate. This is less evident at high air flow rates since entrain-ment would be more significant in terms of gangue recovery ofboth silicates and chromite than true flotation. Similarly the PGMrecovery is unaffected by depressant dosage at low air flow ratesbut decreases with increasing depressant dosage at high air flowrates. This may be due to changes in froth structure, since bothincreased air flow rate and increased depressant dosage may beexpected to destabilise the froth due to turbulence in the case ofair flow rate and reduced froth-stabilising gangue, in the case ofdepressant dosage. This would result in a loss of the valuableminerals.

Froth height was another significant factor affecting both chro-mite and PGM responses. It was expected that an increase in frothheight would allow for drainage of water and gangue particles andcause a decrease in both chromite recovery, especially given theirrelatively high density, and grade (Finch et al., 1989; Neethlingand Cilliers, 2002). This was, indeed, observed. However, theincreased froth retention time of air allows for more bubble rup-turing within the froth and also results in the dropback of valuable

Fig. 7. The effect of depressant dosage and air flow rate on (a) PGM recovery and (b) P

particles. Thus, it was observed that an increase in froth heightcaused a decrease in PGM recovery (Table 5). The interactiveeffects observed in Eq. (3) caused differing chromite graderesponses as a function of froth height at the different levels ofother variables. For example, the effect of froth height on chromitegrade was greater at low depressant dosages than at high depres-sant dosages (Fig. 5). The interactive effects of froth height withother parameters in the PGM recovery response was extremelysmall (3%) and had little impact on the PGM recovery.

Frother concentration was the other parameter having an effecton chromite recovery (Eq. (1)). The effect of frother on chromiterecovery was relatively minimal within the range of frother con-centrations chosen for this study. It was shown in preliminaryexperiments carried out prior to this study that frother concentra-tion could appear to play a significant role if the range chosen waswide enough. However, all parameters were chosen such that theywere within plant-scale ranges. An increase in frother concentra-tion resulted in an increase in chromite recovery and grade. Theincrease in recovery was similar at all levels of the other parame-ters. The water recovery also increased with an increase in frotherconcentration, which was consistent with the increase in entrainedgangue (chromite) recovery. However, the increase in grade withincreasing frother concentration was different for different levelsof the other responses. For example, chromite grade increased atincreasing dosages of frother at low superficial air velocities butremained the same at high superficial air velocities. This may havebeen due to a changing froth structure or froth rheology thataltered the amount of chromite that could be entrained. Frotherconcentration had no direct effect on PGM grade or recovery andhad limited interactive effects with other parameters.

Depressant dosage, while having no effect on the chromiterecovery, had a marked effect on the grade. As mentioned above,this was due to the depression of other naturally floatable gangue,which caused the grade of chromite in the concentrate to increase.Thus, an increase in depressant dosage resulted in an increase inchromite grade, the relative amounts of which varied accordingto the levels of the other parameters due to interactive effects.Depressant dosage, perhaps somewhat surprisingly, had no effecton PGM grade. The depressant effect was obviously not enoughto be noticeable in the PGM grade.

The linear relationship between water recovery and chromiterecovery confirmed the fact that chromite was recovered mainlyby entrainment. Regression analysis showed that 0.0036 g ofCr2O3 was recovered per gram of water. This is in good agreementwith the data from Ekmekci et al. (2003) where, although they donot determine such a relationship, a simple calculation indicatesthat the amount of Cr2O3 recovered per gram of water was in theregion of 0.003 g.

GM grade, at constant mid-point levels of froth depth and frother concentration.

Table 5Solutions to the models showing the constraints placed on the responses and theimportance given to each response.

Lowerlimit

Upperlimit

Importance(1–3)

Optimisedvalue

Cr2O3 recovery (g/min) 0.05 1.4 3 0.2Cr2O3 grade (%) 4.4 15.6 3 5PGM recovery (%) 58 82 3 69PGM grade (g/t) 42 354 3 187

Solutions for the optimised valuesFroth height (cm) 30Superficial air velocity (cm/s) 0.5Depressant dosage (g/t) 100Frother concentration (ppm) 26

98 B. McFadzean et al. / Minerals Engineering 70 (2015) 92–98

It is necessary to know at what levels of the four variableparameters to operate in order to optimise operations for the bestPGM grade and recovery, with the lowest chromite grade andrecovery. This can be done by using the models to predictresponses based on various levels of air, froth depth, frother anddepressant dosage. Table 5 shows the best levels of process param-eters to be used in order to obtain the lowest levels of chromitegrade and recovery together with the highest levels of PGM gradeand recovery, depending on the constraints which are put on theoptimisation. Different solutions in terms of variable operatingparameters would be obtained depending on the operating philos-ophy of each individual operation and the importance placed oneach of the responses. For example, if an operation had a stringentsmelter contract, then greater importance would be placed onchromite grade and recovery and less on PGM grade and recoverywhereas if the company were not smelter-constrained, then theoptimisation may be entirely PGM recovery-driven.

5. Conclusion

This study has shown that, of the four process parametersinvestigated, superficial air velocity was the dominant parameteraffecting the recovery of chromite and PGM’s. The depressant dos-age had a strong influence on chromite grade rather than chromiterecovery and was not a significant factor in PGM grade or recovery.Froth height was also a strong contributor to both chromite andPGM grades and recoveries. Interactive effects indicated that theresponses were complex and not the same at different levels ofparameters.

From the perspective of concentrator operations the importantaspects which this study has highlighted are:

1. The sensitivity of the flotation performance to superficial air veloc-ity: This emphasises the need to optimise and control air flowrate on flotation concentrators. Experience is that operationsseldom determine this parameter for each cell. These resultsshow how important it is to measure and calibrate the SCADAair flow rate to a known superficial air velocity.

2. The sensitivity of the flotation performance to froth depth: This isthe most accessible lever that a metallurgist has to control theflotation performance and it is important that cell levels are cal-ibrated against SCADA levels and optimised through cell-by-celloptimisation processes.

3. Interactive effects are important when optimising plant flotationperformance: Grades and recoveries will be different at differentlevels of interacting parameters. For example, chromite grade

increases with increases in superficial air velocity at lowdepressant dosages but remains the same at high depressantdosages (Fig. 6).

4. This type of factorial design study provides a significant amountof information for relatively few tests and indicates how bene-ficial it could be if undertaken on concentrators. The informa-tion obtained from such a study can then be used to targetspecific operating strategies as outlined in Table 5.

Acknowledgments

The authors wish to acknowledge funding from Anglo AmericanPlatinum, Impala Platinum, Lonmin, Nkomati, the University ofCape Town and the National Research Foundation. They are espe-cially grateful to Lonmin for providing the ore and the assays forthis study.

References

Alvarez-Silva, M., Wiese, J., O’Connor, C.T., 2014. An investigation into the role of thefroth phase in the flotation of UG2 ore using a laboratory column flotation cell.Miner. Eng. 55, 125–131.

Araujo, P.W., Brereton, R.G., 1996. Experimental design II. Optimization. TrACTrends Anal. Chem. 15 (2), 63–70.

Bisshop, J.P., White, M.E., 1976. Study of particle entrainment in flotation froths.Trans. Inst. Min. Metall 85, C191–C194.

Cawthorn, R., 1999. The platinum and palladium resources and the bushveldcomplex. S. Afr. J. Sci. 95, 481–489.

Cochran, W.G., Cox, G.M., 1990. Disefios Experimentales, 2nd ed. Trillas, Mkxico.Design Expert 8, U.M., 2011. Design Expert User Guide, s.l.: Start Ease.Ekmekci, Z., Bradshaw, D.J., Allison, S.A., Harris, P.J., 2003. Effects of frother type and

froth height on the flotation behaviour. Miner. Eng. 16, 941–949.Engelbrecht, J.A., Woodbum, E.T., 1975. The effect of froth height, aeration rate and

gas precipitation on flotation. J.S. Afr. Inst. Min. Metall. 76, 125–132.Finch, J.A., Yianatos, J., Dobby, G., 1989. Column Froths. Miner. Process. Extract.

Metall. Rev.: An Int. J. 5 (1–4), 281–305.Gaudin, A.M., 1957. Flotation, 2nd ed. McGraw-Hill, New York.Harris, C.C., Jowett, A., Ghosh, S.K., 1963. Analysis of data from continuous. Trans.

Am. Inst. Min. Eng. 226, 444–447.Harris, A., Venkatesan, L., Greyling, M., 2013. A practical approach to plant scale

optimization. J. SAIMM 13, 263–272.Hay, M.P., 2010. A case study of optimising UG2 flotation performance part 2:

Modelling improved PGM recovery and Cr2O3 rejection at Northam’s UG2concentrator. Min. Eng. 23, 868–876.

Hay, M.P., Roy, R., 2010. A case study of optimising UG2 flotation performance. Part1: Bench, pilot and plant scale factors which influence Cr2O3 entrainment inUG2 flotation. Min. Eng. 23, 855–867.

Jowett, A., 1966. Gangue mineral contamination of froth. British Chem. Eng. 2 (5),330–333.

Moys, M.H., 1978. A study of a plug-flow model for flotation froth behavior. Int. J.Miner. Process. 5 (1), 21–38.

Neethling, S.J., Cilliers, J.J., 2002. The entrainment of gangue into a flotation froth.Int. J. Miner. Process. 64, 123–134.

Sadler III, L.Y., 1973. Dynamic response of the continuous mechanical froth flotationcell. Trans. Am. Inst. Min. Eng. (254), 336–343.

Savassi, O.N., Alexander, D.J., Franzidis, J.P., Manlapig, E.V., 1998. An empiricalmodel for entrainment in industrial flotation plants. Miner. Eng. 11 (3), 243–256.

Schouwstra, R.P., Kinloch, E.D., Lee, C.A., 2000. A short geological review of thebushveld complex. Platinum Metals Rev. 44, 33–39.

Smith, P.G., Warren, L.J., 1989. Entrainment of particles into flotation froths. In:Frothing Flotat. Gordon and Breach, NewYork, pp. 123–145.

Valenta, M.M., 2007. Balancing the reagent suite to optimise grade and recovery.Miner. Eng. 20, 979–985.

Wiese, J., Harris, P., Bradshaw, D., 2009. Optimising collector performance in theproduction of high grade PGM concentrates. s.l., In: Proceedings SMEmeeting.

Wiese, J.G., Harris, P.J., Bradshaw, D.J., 2010. The effect of increased frother dosageon froth stability at high depressant dosages. Miner. Eng. 23, 1010–1017.

Yoon, R.H., Luterell, G.H., 1989. The effect of bubble size on fine particle flotation.Miner. Process. Extract. Metall. Rev.: An Int. J. 5 (1–4), 101–122.