The effect of varying pulp reagent chemistry on the flotation performance of a South African PGM ore

Minerals Engineering 95 (2016) 155–160

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The effect of varying pulp reagent chemistry on the flotation performance of a South African PGM ore T.M. Moimane, K.C. Corin ⇑, J.G. Wiese Centre for Minerals Research, Department of Chemical Engineering, 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 5 April 2016 Revised 27 June 2016 Accepted 2 July 2016

Keywords: Flotation reagents Froth flotation Froth phase Ionic strength Pulp chemistry

a b s t r a c t The pulp chemistry plays a pivotal role during the extraction of commodities in the PGM industry as they employ the froth flotation separation process, which is based on the selective alteration of surface properties between the valuable minerals and undesired gangue. The chemical environment is intricate owing to the multiple and various surface reactions occurring at the mineral surface, and the increased ionic concentrations due to the practice of water recycling. Thus, a holistic understanding of the pulp chemistry is sought. By conducting bench scale flotation tests at varying concentrations of flotation reagents and typical ions present in plant water, it is possible to better understand the role of pulp chemistry on metallurgical plant performance. The effects of such variations, which modify the chemical environment of the pulp and impact plant performance, are discussed in this paper. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The South African Bushveld Igneous Complex, which consists of three main ore bodies, namely the Merensky, the UG2 and the Platreef, hosts the largest PGM reserves on a global scale (Cawthorn, 1999; U.S. Geological Survey, 2015). The Platinum Group Minerals (PGM), together with the Base Metal Sulphides (BMS) present in the ores, are extracted by the froth flotation separation technology which relies heavily on the surface chemistry of the valuable mineral particles and gangue. With the ever increasing demand for precious metals for applications in catalysis, medicine, fuel cells and other industries (Gupta et al., 2014), and the depletion of high-grade ores, it is imperative to gain a fundamental understanding of the process in an effort to optimise the metallurgical circuits for a better performance. The chemistry of the pulp is of crucial importance as it determines the particle surface properties which are essential for a successful separation between the valuable mineral particles and gangue. Collectors are added to selectively impart/enhance hydrophobicity onto the valuable mineral particles, and depressants are added to render the gangue particles hydrophilic. Upon attachment to the air bubbles, the collector-coated valuable mineral particles are transported to the froth phase and recovered, and the gangue particles remain submerged in the pulp and disposed in the tailing stream. Of particular importance is a stable ⇑ Corresponding author. E-mail address: [email protected] (K.C. Corin). http://dx.doi.org/10.1016/j.mineng.2016.07.002 0892-6875/Ó 2016 Elsevier Ltd. All rights reserved.

froth, which is sought, for successful transportation and recovery of the valuables, and this is achieved by addition of frothing agents. Due to the complexity of the process owing to multiple surface reactions taking place, competitive and interactive effects among the reagents, as well as secondary effects in addition to the primary roles of the reagents, it is often imperative to take a holistic approach in evaluating the behaviour of reagents in flotation (Bradshaw et al., 2005). Moreover, depending on the nature and adsorption characteristics of a reagent in question, pulp conditions can affect its adsorption onto the mineral surface. Therefore change in chemical pulp conditions owing to the practice of water recycling, which leads to build-up of pollutants such as organics, reagent residuals, and dissolved ions in the process (Chen et al., 2009; Levay et al., 2001; Rao and Finch, 1989), at concentrator plants can affect the behaviour of the flotation reagents. From the onset, UCT developed synthetic plant water (SPW) containing only inorganic ions, with total dissolved solids (TDS) of 1023 mg/ L, to mimic industrial plant water and to understand the behaviour of reagents under such conditions when conducting laboratory scale experiments (Wiese et al., 2005). The concentration of the ions in the process steadily increases with time, and it is thus important to understand the behaviour of the reagents under elevated concentrations of the ions. Therefore this is a flotation chemistry study that seeks to gain a holistic understanding of the pulp chemistry and its influence on the metallurgical performance of a PGM-bearing ore from the Merensky reef, with particular emphasis on the dosages of the

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chosen flotation reagents; collector, depressant and frother, and the ionic strength of the plant water. 2. Experimental details Shown in Table 1 are the two synthetic plant water types; 1 SPW, the standard synthetic plant water, which was used as the base plant water, and 5 SPW, the higher ionic strength plant water used to simulate accumulation of ions in the plant water. They were prepared by dissolving various chemical salts in distilled water to achieve the concentration of the ions as shown in the table. A sample of a typical Merensky ore was obtained from the Bushveld Igneous Complex of South Africa in the North West province. The bulk sample was crushed, blended, riffled and split using a rotary splitter into 1 kg portions. The 1 kg samples were milled in synthetic plant water (at the required ionic strength), with the collector added to the mill, using an Eriez laboratory stainless steel rod mill at 66% solids to achieve a grind of 60% passing 75 lm. This grind was chosen on the basis that it matches the primary rougher grind at operations processing this type of ore (Wiese, 2009). The milled slurry was transferred to a 3 L modified Leeds batch flotation cell, and synthetic plant water (at the required ionic strength) was added to achieve a pulp density of 35% solids. The cell was fitted with a variable speed drive and the pulp level was controlled manually by addition of synthetic plant water. The impeller of the flotation cell was set to a speed of 1200 rpm. Air was then introduced into the cell, at a flow rate of 7 L/min, which was sustained throughout the test. A constant froth height of 2 cm was sustained throughout the test by addition of synthetic plant water at the required ionic strength. Concentrates were collected into a pan by scraping off the froth at intervals of 15 s for collection times of 2, 4, 6 and 8 min. A feed sample was drawn before and a tails sample was drawn after each test. Water usage was monitored throughout the test. Feeds, concentrates and tails were filtered, dried and weighed before analysis. All batch flotation tests were conducted in duplicate in order to minimise the error, and the reproducibility was found to be within acceptable limits as required by the UCT standard batch flotation procedure (Manono, 2012). The frother Dowfroth 250, a polyglycol ether frother of molecular weight 264 g/mol which was supplied by Betachem in concentrated form, was added at dosages of 50 and 60 g/t. The polysaccharide depressant, guar gum (Sendep 348) with molecular weight 239,000 g/mol and 92% purity, was added at dosages of 100 and 300 g/t, and was supplied by Senmin. A xanthate collector, sodium isobutyl xanthate (SIBX) with molecular weight of 171.1 g/mol and purity of 90%, which was also supplied by Senmin was dosed at 50 and 150 g/t. Copper and total nickel analysis of all samples was carried out using a Bruker S4 Explorer X-ray Fluorescence (XRF) spectrometer. Sulphur analysis was carried out using a LECO DR423 sulphur analyser. It should be noted that the analytical technique for the nickel content measurement measures the total nickel present in the ore, and owing to the significant amount of nickel associated with the gangue, the recovery of nickel as an indication of the sulphide mineral pentlandite cannot be entirely accurate. However this value can be used when comparing sulphide nickel recoveries for the

same ore. It has also been assumed that the analysis of the sulphide minerals; chalcopyrite and pentlandite, recovered in the concentrates serves as a proxy to the PGM recovery due to the strong association of the sulphides with PGMs in these type of ores (Wiese et al., 2005). 3. Results and discussion This section presents results and discussion of the effects of varying pulp chemistry on the solids, water, copper and nickel recoveries and grades. It should be noted that the test conditions on the graphs (Figs. 1–3) are in chronological order, i.e., the test condition 50 SIBX, 100 Guar, 50 D250, 1 SPW, designates test condition 1, and that they are abbreviated for ease of reference. For example, 50 SIBX refers to 50 g/t of the collector sodium isobutyl xanthate, 100 Guar refers to 100 g/t of the depressant guar gum and 50 D250 refers to 50 g/t of the frother Dowfroth 250. 3.1. The effect of varying the pulp chemistry on solids and water recoveries It is clear from Fig. 1 that generally, under 1 SPW, increasing the collector dosage decreased both solids and water recoveries. Water recovery at a fixed froth height gives an indication of the change in froth stability (Wiese, 2009). Thus it is evident from the figure that under 1 SPW increasing the collector dosage from 50 g/t to 150 g/t decreased the stability of the froth (as seen by reduction in water recovery), and ultimately reduction in solids recovery. The effect of particle hydrophobicity is well documented in literature, and it has been consistently reported that highly hydrophobic (contact angle: h > 90°) particles have an impact of destabilising the froth phase (Bradshaw et al., 2005; Hadler et al., 2005; Schwarz and Grano, 2005; Johansson and Pugh, 1992; Dippenaar, 1982), and ultimately may lead to reduction in valuable mineral recovery. It can thus be postulated that increasing the collector dosage from 50 g/t to 150 g/t led to particle hydrophobicity above the optimum particle hydrophobicity (90°), after which point any further increase in particle hydrophobicity starts causing bubble rupture – froth destabilisation. Although the collector-coated particles were observed to destabilise the froth, as seen by the reductions in both solids and water recoveries, as a result of increasing the collector dosage under 1 SPW, the effect was less prominent at higher frother dosage of 70 g/t. This is an indication of a competitive phenomenon between the frother molecules and collectorcoated particles on the froth, as the role of the former is to stabilise the froth, while the latter have the ability to act as froth breaking agents, when ‘’too’’ hydrophobic. Therefore at sufficiently high frother concentration in the pulp, a froth stable enough to resist the effect of the highly hydrophobic particles can be induced, and consequently bubble rupture will be minimal, and hence adequate recovery of particles can be achieved. On the contrary, generally, both solids and water recoveries were seen to increase when the collector dosage was increased under high ionic strength plant water, 5 SPW, as shown in Fig. 1. This indicates that the destabilisation effect of the resultant highly hydrophobic collector-coated particles on the froth was minimised or counteracted. Thus under 5 SPW the froth was sufficiently stable to resist any change (destabilisation) owing to the dynamic forces

Table 1 The concentration of the ions present in the two synthetic plant waters used in this study. Plant water

Ca2+ (ppm)

Mg2+ (ppm)

Na+ (ppm)

Cl (ppm)

SO24 (ppm)

NO3 (ppm)

NO2 (ppm)

CO23 (ppm)

TDS (ppm)

1 SPW 5 SPW

80 400

70 350

153 765

287 1435

240 1200

176 880

– –

17 85

1023 5115

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1000

50 45

800

40

700

35

600

30

500

25

400

20

300

15

200

10

100

5

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Final solids recovery, g

900

Final water recovery, g

5 SPW

1 SPW

0

16

50 150 50 150 50 150 50 150 50 150 50 150 50 150 50 150 SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX 100 Guar

300 Guar

100 Guar

50 D250

300 Guar

100 Guar

70 D250

300 Guar

100 Guar

50 D250

300 Guar

70 D250

Water 390.3 259.4 364.7 248.8 549.8 526.5 410.4 394.6 529.7 562.9 472 479.4 840.8 770.5 659.3 703.2 Solids 32.70 27.43 15.85 13.55 38.90 37.23 16.89 17.47 31.82 35.88 18.28 18.6 45.38 46.29 21.54 23.68

Water

Solids

Fig. 1. Final solids and water recoveries for all tests. Error bars represent standard deviation between duplicate tests.

3.5

80

1 SPW

5 SPW 3

60

2.5

50 2 40 1.5 30 1

20

0.5

10 0

Final copper grade, %

Final copper recovery, %

70

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

50 SIBX

150 SIBX

50 SIBX

150 SIBX

50 SIBX

150 SIBX

50 SIBX

150 SIBX

50 SIBX

150 SIBX

50 SIBX

150 SIBX

50 SIBX

150 SIBX

50 SIBX

150 SIBX

100 Guar

300 Guar

100 Guar

50 D250

300 Guar

100 Guar

70 D250

300 Guar

100 Guar

50 D250

0

300 Guar

70 D250

Cu Recovery 71.07 66.99 68.58 64.82 72.68 70.62 68.78 65.23 70.89 70.68 68.75 68.24 74.21 72.28 70.24 71.41 Cu Grade

1.42

1.61

2.69

3.09

1.23

1.21

2.61

2.34

Cu Recovery

1.41

1.28

2.46

2.28

1.11

1.06

2.11

1.97

Cu Grade

Fig. 2. Final copper grades and recoveries for all tests. Error bars represent standard deviation between duplicate tests.

imposed by the highly hydrophobic collector-coated particles. Therefore this behaviour of the process under 5 SPW can be attributed to the ability of the ions present in the process water at high concentration to act as frothing agents, thereby inducing a more stable froth by reducing bubble size and retarding bubble coalescence (Corin and Wiese, 2014; Wang and Peng, 2014; Castro et al., 2013; Manono et al., 2012; Kurniawan et al., 2011; Quinn et al., 2007), and subsequently allowing for improved recovery of particles. Moreover, the behaviour of the system subject to varying the collector dosage under 5 SPW as opposed to that observed under 1 SPW can possibly be due to the chemistry of the collector in the pulp. SIBX, a xanthate collector, is ionic in nature, and forms a xanthate and sodium ions in water, and the former is responsible

for imparting hydrophobicity onto the surface of mineral particles through either chemisorption or physisorption, depending on the form of xanthate species that is formed as a result of the pulp pH (Fuerstenau, 1978). By virtue of chemistry, oppositely charged particles will tend to attract each other when brought into close contact, and likewise like charged particles will repel each other. So it can be postulated that accumulation of the cations (such as Ca2+, Mg2+, Na+) in the pulp induced prominent electrostatic interactions with the xanthate ions, and these interactions disturbed the action of collector adsorption onto the mineral surface, and hence the resultant collector-coated particles were not sufficiently hydrophobic to cause bubble rupture. Consequently, under 5 SPW the forces imposed by the collector-coated particles on the froth were minimal, and hence no reductions in recoveries were

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60

7

1 SPW

5 SPW 6 5

40 4 30 3 20 2 10

0

Final nickel grade, %

Final nickel recovery, %

50

1

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

0

50 150 50 150 50 150 50 150 50 150 50 150 50 150 50 150 SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX SIBX 100 Guar

300 Guar

50 D250

100 Guar

300 Guar

70 D250

100 Guar

300 Guar

50 D250

100 Guar

300 Guar

70 D250

Ni Recovery 51.57 47.22 47.48 47.12 50.95 50.09 45.06 45.26 50.26 50.67 48.23 48.28 52.64 49.60 46.26 46.94 Ni Grade

2.78 2.99 4.96 6.04 2.26 2.28 4.57 4.57 2.77 2.38 4.57 4.39 2.00 1.91 3.59 3.27

Ni Recovery

Ni Grade

Fig. 3. Final nickel grades and recoveries for all tests. Error bars represent standard deviation between duplicate tests.

observed, as shown in Fig. 1. This phenomenon could also be due to precipitation of the ions onto the mineral surface, thereby hindering the collector molecules from fully adsorbing, and therefore increasing the collector dosage under the high ionic strength plant water having a minimal effect on the particle hydrophobicity. Moreover, Fig. 1 shows that increasing the depressant dosage decreased both solids and water recoveries under the two synthetic plant water types owing to the reduction in the amount of naturally floatable gangue (NFG) particles reporting to the froth. In addition to this, the reduction of the NFG particles, which possess froth-stabilising properties, causes froth destabilisation (Wiese et al., 2011; Wiese, 2009; Bradshaw et al., 2005; Martinovic et al., 2005) which may result in reduction of the valuable mineral particles reporting to the concentrate. Unlike with the collector, no evident change in behaviour was observed with the depressant owing to change in water quality. This is expected since guar is a non-ionic depressant, and therefore its adsorption behaviour is not expected to be significantly affected by changes in ionic strength of the process water (Parolis et al., 2008; Khraisheh et al., 2005; Wang et al., 2005; Morris et al., 2002). Increasing both the frother dosage and ionic strength of the plant water increased froth stability as seen by the increase in both solids and water recoveries. This increases the amount of gangue reporting to the concentrate, and will lead to a reduction in concentrate grade. However, it is worth noting that solids recoveries were not improved when the ionic strength was increased under the test condition 50 SIBX, 100 Guar, 50 D250 (1&9). This observation is counter-intuitive in that it is well-documented in literature that the presence of ions at high concentrations in the pulp induces a more stable froth which allows for high recoveries, as it was also seen with the other tests. Therefore this is regarded as a discrepancy which can be attributed to experimental errors. 3.2. The effect of varying the pulp chemistry on copper and nickel grades and recoveries Shown in Fig. 2 is the final copper grade and recovery, which represents the flotation behaviour of the sulphide mineral chalcopyrite in the ore, for all tests conducted. As it has been observed in Fig. 1 that increasing the collector dosage from 50 g/t to 150 g/t

had an effect of decreasing recoveries, due to a reduction in froth stability, under 1 SPW, it is also shown for copper (Fig. 2) that recoveries decreased (2–4%, on average) as the collector dosage was increased. This figure further demonstrates that the highly hydrophobic collector-coated particles had a minimal effect on the froth phase under 5 SPW, and hence on mineral recovery, as shown by the copper recovery. While copper recovery decreased with increased depressant dosage due to the depression of the froth-stabilising NFG particles, it was also increased with an increase in both frother dosage and ionic strength of the plant water, indicating a more stable froth. The stability of the froth phase is of crucial importance as this phase plays a pivotal role in determining the final grades and recoveries of the valuable minerals. While a more stable froth may improve valuable mineral recovery, this usually occurs at the expense of the concentrate grade. This is because a more stable froth allows for high water recovery, which is proportional to nonselective recovery of particles by the entrainment mechanism (Ekmekçi et al., 2003; Yang and Aldrich, 2006; Boylu and Laskowski, 2007). Therefore increasing both the frother dosage and ionic strength of the plant water induced a more stable froth which increased the non-selective recovery of particles, and led to a dilution of the concentrate. This is evident for copper in Fig. 2, however, the changes were minimal. Similarly, the concentrate grade was improved by increasing the depressant dosage in the pulp. Nickel grade and recovery results, which represent the flotation behaviour of the sulphide mineral pentlandite in the ore, subject to varying the pulp chemistry conditions are presented in Fig. 3. It is interesting to note that nickel recovery, under 1 SPW, only decreased (with 4.35%) at lower depressant dosage (1 & 2) when the collector dosage was increased, and there was a negligible change at high depressant dosage of 300 g/t (3 & 4; 5 & 6; 7 & 8). However, copper recovery (Fig. 2) was shown to be negatively affected when collector dosage was increased across the spectrum of the other conditions under 1 SPW. By virtue of copper recovery being negatively affected by change in froth stability, owing to the destabilisation by the highly hydrophobic particles, across the spectrum of the tested conditions, it then infers that the flotation response of pentlandite (as

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represented by nickel) cannot be attributed to a froth phenomenon. Moreover, it was also observed that increasing the frother dosage at high depressant dosage (300 g/t), under both plant water types, had a negligible effect on nickel recovery. These findings illustrate the susceptibility of the sulphide mineral pentlandite to high depressant dosages, and it can be postulated that the pentlandite particles were not sufficiently liberated in the ore. Therefore the presence of the depressant at high concentrations in the pulp had the effect of depressing the composite pentlandite-gangue particles by adsorbing on the gangue portion of the particle, and as a result any changes in pulp chemistry will have a negligible effect on the floatability of pentlandite. This susceptibility of pentlandite to depressant dosages is a welldocumented phenomenon in literature (Corin and Wiese, 2014; Manono, 2012; Corin et al., 2011; Wiese, 2009; Wiese et al., 2007; Bradshaw et al., 2005). This behaviour could also be attributed to the fact that high depressant dosages led to a drastic reduction in froth stability, and any effort to counteract this effect will not bear a significant effect on the flotation response of minerals with slower flotation kinetics like pentlandite. Increasing the ionic strength of the plant water generally increased the nickel recoveries. There was a prominent increase in nickel grade when the depressant dosage was increased, however, there were minimal reductions owing to increasing both frother dosage and ionic strength of the plant water, as shown in Fig. 3.

4. Conclusions This study sought to provide some insights on the role that pulp chemistry plays during the extraction of the valuable minerals from the Merensky reef, using copper and nickel grades and recoveries – representing the flotation behaviour of the sulphide minerals chalcopyrite and pentlandite, respectively, as the metallurgical indicators. It was also assumed that these should serve as a proxy to the flotation behaviour of the Platinum Group Minerals due to the strong association with the base metal sulphides in these particular ores (Wiese et al., 2005). The following are the main observations and conclusions that can be drawn from this investigation:  It was shown that increasing the collector dosage from 50 g/t to 150 g/t led to particle hydrophobicity above the optimum particle hydrophobicity, beyond which particles start causing froth destabilisation. It is therefore important to determine the optimum particle hydrophobicity in order to yield maximum recoveries, and to optimise reagent costs.  The behaviour of the xanthate collector, which is ionic in nature, was shown to be affected by changes in the ionic strength of the plant water, whereby the froth destabilisation by the collectorcoated particles was observed to be minimal under increased ionic strength plant water. This suggests interactions between the xanthate ions and the cations, and a competitive adsorption of these ions on the mineral surface. Absorption studies should be conducted on the xanthate type collector at different ionic concentrations in order to further elucidate whether accumulation of the typical ions in the process owing to water recycling can impose any effect on the behaviour of this collector type in flotation of these type of ores.  The intended objective of increasing frother dosage on increasing recoveries was achieved for copper across all the spectrum of tested conditions. This was however never achieved for nickel particularly at high depressant dosage, demonstrating the susceptibility of pentlandite to high depressant dosages and suggesting the presence of insufficiently liberated pentlandite particles.

159

 No apparent effects were observed on the behaviour of the depressing and frothing agents as a result of increased ionic strength of the plant water. Increasing the ionic strength increased the frothability of the system – as seen by an increase in solids, water, copper and nickel recoveries. However, the effect on decreasing the concentrate grade was minimal. Thus deviation in the plant water make up did not impose adverse effects, on overall, on the metallurgical performance of this ore, and therefore process water with comparable ionic strength should not have detrimental effects during the extraction of the commodities in the Merensky PGM industry, or any other operations processing similar ores. It is however imperative to understand the behaviour of the chemical environment at even higher ionic concentrations, and also in the presence of other pollutants in the plant water.  Full mineralogical and liberation analysis of the feed ore, at the chosen grind, will be key in completing the understanding of the pulp chemistry, and hence the fundamental mechanisms that govern the separation in the flotation system.

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