Interactive effects of the type of milling media and copper sulphate addition on the flotation performance of sulphide minerals from Merensky ore Part I: Pulp chemistry

Int. J. Miner. Process. 78 (2006) 153 – 163 www.elsevier.com/locate/ijminpro

Interactive effects of the type of milling media and copper sulphate addition on the flotation performance of sulphide minerals from Merensky ore Part I: Pulp chemistry D.J. Bradshaw a,*, A.M. Buswell a, P.J. Harris a, Z. Ekmekci b a

University of Cape Town, Mineral Processing Research Unit, Department of Chemical Engineering, Rondebosch 7701, Cape Town, South Africa b Hacettepe University, Department of Mining Engineering, Beytepe 06532, Ankara, Turkey Received 19 October 2005; accepted 21 October 2005 Available online 27 December 2005

Abstract It is well known that the chemical environment determines the success of the flotation process, however its characterisation and control is difficult to achieve. This paper, as two parts, Part I and Part II, evaluates the use of various measurements and their interpretation to gain an understanding of the influence of varying parameters such as the type of milling media and copper sulphate addition on the flotation performance of sulphide minerals from a platinum group mineral (PGM) bearing Merensky ore. It shows the complexity of interpretation and the importance of analysing flotation performance holistically. Part I focuses on the pulp chemistry and mineral potential measurements have been used to show the differences in the response of the various mineral electrodes to different conditions. The final flotation recoveries of the sulphide minerals in the ore followed the same trend as the decrease in mineral potential due to collector addition viz. chalcopyrite N pentlandite N pyrrhotite. Type of milling media and copper sulphate addition slightly affected the mineral electrode potential and flotation recovery of chalcopyrite. Addition of copper sulphate increased the recovery of pentlandite and particularly pyrrhotite due to activation by copper (II) ions. The copper activation mechanism was likely to be in the form of initial adsorption of copper hydroxide followed by reduction to Cu+ at the surface. However, the changes in flotation performance of the different minerals in the ore could not be completely described by the electrochemical changes, demonstrating the limitations of these measurements. Part II addresses the effect of froth stability as demonstrated by the variations in the mass and water recovery data resulting from the different milling conditions and addition of copper sulphate which emphasised the importance of considering the froth phase in the evaluation of flotation data. D 2005 Elsevier B.V. All rights reserved. Keywords: milling media; copper sulphate; sulphide minerals; pulp chemistry

1. Introduction The flotation process is widely used for the beneficiation of sulphide ores. However, due to the complex-

* Corresponding author. Tel.: +27 21 650 3797; fax: +27 21 650 5501. E-mail address: [email protected] (D.J. Bradshaw). 0301-7516/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2005.10.004

ity and interactions of the physical and chemical parameters involved, the characterisation and control of the process is inadequate, even after many years of extensive research and development. It has been recognised that the gap between plant practice and fundamental understanding must be closed for any major breakthroughs or improvements to be made (Fuerstenau, 1995). The question of the feasibility of the chemical control of flotation was asked by Woodcock as far

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back as 1970. The answer was then, and still is, an only qualified bYesQ (Jones, 1991; Woodcock and Jones, 1970a,b). For the chemistry of flotation to be controlled and appropriately manipulated, an understanding of the roles and interactions of all the factors affecting both the pulp and the froth zone is required. Sulphide minerals are semiconductors and oxidise in the presence of water and oxygen via a coupled electrochemical reaction with the reduction of oxygen. The adsorption of xanthate collectors onto sulphide minerals also occurs via an electrochemical mechanism. Therefore, the controlling mechanism for the electrochemical mechanism is the mineral/solution interface potential. The electrochemical potential (also known as the oxidation–reduction potential (ORP) or redox potential) of a solid/solution interface is determined by the presence of oxidising and reducing species in solution. In flotation pulps the situation is complicated by the presence of more than one dominant redox couple, such as Fe3+/ Fe2+ and O2/H2O, and a mixed potential exists (Rand and Woods, 1984). In a mixed potential system, electrodes made of different materials can yield different values depending on the thermodynamics and kinetics of the specific reactions on the electrode surface. Thus, it is most appropriate for the electrode to be constructed from a mineral being concentrated in the pulp. However, this is not always suitable as the minerals are susceptible to oxidation, corrosion and poisoning. Therefore, the ORP is generally measured with an inert electrode such as platinum or gold and reported as relative to the standard hydrogen electrode (SHE), which has been shown to approximate the solution pulp potential (Labonte and Finch, 1988). In essence this means that the actual potential measured is the overall solution potential and not the specific mineral potential and rather indicates oxidising or reducing conditions of the pulp. However, it must be noted that the metal electrodes may also be subjected to poisoning mainly at high sulphide ion concentration (Zhou and Chander, 1991), but considerably lower than the mineral electrodes. The type of milling media affects flotation behaviour of sulphide minerals substantially through the galvanic interactions that occur between the milling media and sulphide minerals. The use of mild steel milling media in place of stainless steel milling media has the effect of creating a more reducing pulp environment as well as reducing its dissolved oxygen content. The reduced pulp potential (ORP) and dissolved oxygen (DO) levels can cause the following: i) The formation of iron hydroxy species: The corrosion of mild steel iron into solution occurs and at

alkaline pHs iron hydroxy species are formed. These hydrophilic species may coat the mineral, reducing floatability (Forssberg et al., 1993). The presence of hydrophilic colloidal metal hydroxides has been reported to stabilise the froth zone (Bikerman, 1953). A more stable froth zone would increase the froth recovery of both valuable minerals and unwanted gangue. Thus, the use of mild steel may result in increased recovery and decreased grade of valuable minerals. ii) A reduction in the mineral surface oxidation: In an excessively oxidising environment the surface of sulphide minerals oxidise to form hydrophilic species, which may interfere with the collector– mineral reaction and thereby reduce mineral floatability and resulting flotation performance. A slightly reducing environment would thus reduce mineral surface oxidation and may benefit flotation performance, particularly for PGM ores containing very small amount of sulphide minerals. iii) Reduced collector oxidation: A reducing environment lowers the solution potential and if this is below the potential required for dithiolate formation, thiol collector oxidation will not occur (Yoon and Basilio, 1993). As dithiolate formation is coupled with oxygen reduction, lower dissolved oxygen levels may also reduce dithiolate formation. If dixanthogen is critical to the hydrophobicity of mineral surface, flotation performance will be adversely affected. The overall effect of the change in milling media will depend on the dominance of the competing reactions. In the case of platinum group mineral (PGM) bearing ores, such as Merensky ore, the majority of the valuable PGMs are associated with the sulphide minerals; chalcopyrite, pentlandite and pyrrhotite. However, the combined sulphide grade of the ore is typically less than 1%. The process is in essence a bulk sulphide float with the overall flotation performance comprising of the combined performance of the individual minerals and the aim is to optimise the total sulphide recovery. The flotation performance of different sulphide minerals varies, with pyrrhotite being ranked the most difficult to float, and in many plants the greatest losses of PGMs are associated with poorly floating pyrrhotite (Hochreiter et al., 1985; Goodall, 1995). One of the major reasons for the addition of copper sulphate is to activate the pyrrhotite, however at pH values of 9, no electrochemical reaction was observed by Buswell and Nicol (2002).

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The objective of this investigation was to evaluate the information gained from the different measurement techniques obtained in batch flotation tests of Merensky ore with respect to the elucidation of the effects of changing parameters on the pulp chemical environment and the flotation performance of the different sulphide minerals present. The pulp chemical environment was varied by using mild steel rods in place of stainless steel rods as the milling media (MS vs. SS) and with the use of the activator, copper sulphate as nil or 50 g/t. The measurements made were dissolved oxygen (DO) using a membrane electrode, the oxidation–reduction potential (ORP) using a Pt–Ag/AgCl electrode, mineral potentials using chalcopyrite, pentlandite and pyrrhotite stationary electrodes (Buswell et al., 1998), as well as ex situ measurements at equivalent flotation conditions of residual xanthate in solution and the extent of ethylenediamine tetra acetic acid (EDTA) complexation with the metal hydroxides. Flotation performance has been considered in terms of the grade vs. recovery relationship of the different sulphide minerals in the ore. Part I of this paper evaluates the contribution and value as well as limitations of electrochemical measurements in assessing flotation performance and the influence of different types of grinding media on flotation of Merensky PGM ore. Part II of this paper emphasises the importance of the contribution of the froth structure due to changing froth stability in analysing batch flotation data for the same set of tests. 2. Experimental details 2.1. Materials Merensky ore sample, obtained from the Bushveld Igneous Complex in South Africa, was used in this investigation. The sulphide grade was about 1% and was made up of 0.3% iron sulphides [approximately 0.06% pyrite (FeS2) and 0.24% pyrrhotite (Fe1.13S)], 0.4% pentlandite (Ni0.5Fe0.5S) and 0.2% chalcopyrite (CuFeS2). The remaining 99% of the ore consisted of non-sulphide gangue minerals, pyroxene, feldspar and small amounts of talc. 2.2. Pulp chemistry Stationary chalcopyrite and pyrrhotite electrodes were made with pure mineral samples from Wards Natural Science Institute and a similar electrode was made with synthetic pentlandite from Johnson-Matthey (Buswell et al., 1998; Buswell and Nicol, 2002). Mineral potentials were measured relative to the Ag/AgCl

155

electrode (+ 0.207 V vs. SHE) and were logged continuously throughout the pulp conditioning stages. Mineral electrodes were removed during the flotation stage. All potentials reported in this paper have been converted to the standard hydrogen electrode (SHE) scale. Dissolved oxygen (DO), (YSI membrane electrode), pulp potential (EPt), (Pt–Ag/AgCl electrode), pH (Glass combination electrode) and temperature were logged continuously using an automatic recording device called TPS meter from TPS Pty. Ltd. throughout the conditioning and flotation stages. The pulp temperature, approximately 24 8C, was similar in all flotation tests. 2.3. Batch flotation For each test, 1.1 kg ore sample was milled in a Sala laboratory rod mill at 60% solids for 12 min to obtain a particle size distribution of 45% passing 75 Am and transferred to a 3 l modified Leeds cell with water added to adjust the pulp density to 30% w/w. The pH was not modified but was recorded continuously and was constant at pH = 9 due to the natural buffering effect of the ore. Where included in the reagent suite, 50 g/t copper sulphate was added and conditioned for 5 min. 30 g/t each of sodium isobutyl xanthate (SIBX) and SK5 (a dithiophosphate collector) were added as collectors together with 15 g/t SF7000 as frother and conditioned for 5 min. These reagents were supplied by Senmin Mining Chemicals and were used as received. 10 ml of a 1% solution (100 g/t) of IMP4, a modified guar depressant supplied by Trohall, was the last reagent added for depression of talc and conditioned for 1 min. Five flotation concentrates were collected after 1, 3, 5, 10 and 20 min and the mass and water recovery data collected. The reproducibility of the tests was deemed within acceptable limits for batch flotation test work. All recovery and grade results presented are an average of duplicate experiments. The measurements of water recovery and mass yield were used as indicators of the nature of the froth zone. An increase in the water recovery and mass yield indicates an increase in the stability of the froth. Analysis of the flotation samples for copper and nickel was performed by digestion and AA analysis and Leco was used for sulphur analysis. The grade and recovery for chalcopyrite and pentlandite are given in terms of copper and nickel assays. It should be noted that some nickel is associated with the gangue minerals and therefore the sulphide nickel recoveries would actually be higher than the total nickel recoveries reported here. Since there were minerals other than

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Table 1 Summary of flotation results obtained for batch flotation tests with Merensky ore after milling in mild steel (MS) or stainless steel (SS) milling media, with copper sulphate added as specified (nil, CuSO4) Milling media

SS MS SS MS

Copper sulphate addition

Mass

Water

Copper

Yield

Rec.

Rec.

Grade

Rec.

(%)

(g)

(%)

(%)

(%)

Nil Nil 50 g/t 50 g/t

3.79 4.51 3.87 4.89

763 927 558 846

85.6 81.7 85.7 87.1

1.39 1.24 1.47 1.18

72.5 72.3 73.7 80.0

pyrrhotite (Fe1.13S) and pyrite (FeS2) containing iron, the iron sulphides were calculated from the residual sulphur remaining after subtracting sulphur in chalcopyrite (CuFeS2) and pentlandite (Ni0.5Fe0.5S) from the total sulphur assayed. The elemental balance was between 90% and 110% for all tests and the relative standard error of the flotation tests was below 5% and thus the quality of the data was deemed within acceptable limits for batch flotation test work. All recovery results presented are an average of duplicate experiments. 3. Results

100 90 80 70 60 50 40 30 20 10 0

Iron sulphides

Gangue

Grade

Rec.

Grade

Rec.

(%)

(%)

(%)

(%)

2.97 2.64 2.88 2.54

42.0 35.3 76.7 71.5

4.3 3.1 7.8 5.9

3.27 3.87 3.19 4.11

changes in both froth stability and surface properties of the sulphides. Table 1 shows that for all conditions iron sulphide recovery was lower than that of chalcopyrite and pentlandite, but addition of copper sulphate increased the recovery considerably. Fig. 1 shows the recovery vs. time of the copper, nickel and iron sulphide minerals obtained for the condition of SS milling media with no copper sulphate addition and illustrates the different performance of the minerals. It can be seen that chalcopyrite (copper) was the fastest floating mineral and achieved the highest final recovery (85.6%). The final recovery of pentlandite (nickel) was 72.4%. The iron sulphide recovery of only 42.0% was substantially lower than the other sulphides. In flotation of PGM ores from the Merensky reef, a xanthate type collector is always used as the primary collector in conjunction with a secondary collector such as dithiophosphate (DTP). DTPs are identified as a weaker but more selective collector than xanthate and with some frothing behaviour. The use of DTP as a second collector has been shown to improve collector effectiveness through both synergistic effect and froth stabilisation. Wiese et al. (2005a) have reported that the role of DTP in the flotation of a Merensky ore was to increase frothability and to enhance flotation of pyrrho-

0.3

Potental (V, SHE)

Recovery (%)

A summary of flotation results is shown in Table 1. Flotation performance is measured in terms of recovery and grade. The water and mass recovery data were used as indicators of the froth characteristics. The use of MS milling media increased the water and hence mass recoveries. The increased mass yield obtained with MS milling media was accompanied by a reduction of the grades of all sulphides, and a corresponding increase in gangue recovery. The effect of copper sulphate addition was to reduce water recoveries, but to increase mass yields, indicating

Nickel

Copper Nickel Iron Sulphides

0

5

10

15

Collector addition

0.25

Pentlandite

0.2

Chalcopyrite Pyrrhotite

0.15 Exanthate/dixanthogen

0.1 0.05 0

20

Flotation Time (minutes) Fig. 1. Flotation recovery vs. time after stainless steel (SS) milling and no copper sulphate addition.

0

2

4

6

8

10

12

14

Conditioning Time (minutes) Fig. 2. Variations in mineral potentials during conditioning stage after stainless steel (SS) milling and no copper sulphate addition.

D.J. Bradshaw et al. / Int. J. Miner. Process. 78 (2006) 153–163

0.3

Start of Flotation

7

SS

6

Copper sulphate

0.25

5

ECp (V, SHE)

Dissolved Oxygen (ppm)

8

MS

4 3 2 1

157

Collector addition

0.2 0.15 Exanthate/dixanthogen

0.1 SS, CuSO4 SS MS, CuSO4 MS

0.05

0 0

5

10

15

20

25

30

0 0

Time (minutes)

tite when used with xanthate and copper sulphate. There was no enhanced sulphide recovery when using DTP over that obtained when using xanthate. Therefore, adsorption of collectors on the sulphide minerals present in Merensky ore was discussed based on the adsorption behaviour of SIBX. The standard equilibrium potential of isobutyl xanthate/dixanthogen couple is reported as 0.128 V, which correcting for a concentration of xanthate of 10 4 M, results in an equilibrium potential of 0.108 V (Winter and Woods, 1973). Fig. 2 shows the mineral potentials measured in the pulp during the conditioning time of the flotation tests at the same conditions (SS milling media with no copper sulphate addition). The mass ratio of pyrrhotite to pyrite in the ore was approximately 4 : 1. Therefore, for simplicity only the pyrrhotite electrode potential measurements were used in the evaluation of the flotation behaviour of iron sulphides. It can be seen that the potentials of all minerals lie anodic to the potential required for the oxidation of xanthate to dixanthogen (0.108 V). The magnitude of the mineral potentials recorded are in the following 0.4

EPt (V, SHE)

SS, CuSO4 SS MS, CuSO4 MS

Collector addition

0.3 0.25

8

0.1 0 4

6

8

14

0.2 0.15 Exanthate/dixanthogen SS, CuSO4 SS MS, CuSO4 MS

0.1 0.05

0.05 2

12

Collector addition

Copper sulphate

0.25

Exanthate/dixanthogen

0

10

order; pentlandite N chalcopyrite N pyrrhotite. The magnitude of the change in potential on collector addition differs for different minerals, showing the reactivity of the mineral surface with collector ions. The highest change was observed with chalcopyrite showing the highest degree of reactivity with the collector. In the case of pyrrhotite only small changes were measured. Figs. 3–7 show the various measurements made to characterise the chemical aspects of the flotation pulp at the different conditions, represented dMST and dSST for mild steel and stainless steel milling media and dCuSO4T to indicate that 50 g/t copper sulphate has been added. The dissolved oxygen (DO) measurements in Fig. 3 show that the effect of milling in MS media was to remove almost all the DO from the flotation pulp. It can be seen that once the air was turned on at the start of flotation, the DO levels were similar for both milling media. The addition of copper sulphate had no effect on the DO of the flotation pulp. Fig. 4 shows that the ORPs measured for the pulp with ore milled in SS milling media with Pt–Ag/AgCl 0.3

0.2 0.15

6

Fig. 5. Chalcopyrite electrode potentials measured during conditioning time after mild steel (MS) and stainless steel (SS) milling media with 50 g/t copper sulphate addition as specified (CuSO4).

EPn (V, SHE)

Copper sulphate

4

Time (minutes)

Fig. 3. Dissolved oxygen levels measured during flotation tests with ore milled with mild steel (MS) and stainless steel (SS) milling media.

0.35

2

10

12

14

Time (minutes) Fig. 4. Platinum electrode potentials measured during conditioning time after mild steel (MS) and stainless steel (SS) milling media with 50 g/t copper sulphate addition as specified (CuSO4).

0 0

2

4

6

8

10

12

14

Time (minutes) Fig. 6. Pentlandite electrode potentials measured during conditioning time after mild steel (MS) and stainless steel (SS) milling media with 50 g/t copper sulphate addition as specified (CuSO4).

D.J. Bradshaw et al. / Int. J. Miner. Process. 78 (2006) 153–163

0.3 0.25

EPo (V, SHE)

Collector addition

Copper sulphate

SS, CuSO4 SS MS, CuSO4 MS

0.2 0.15 0.1

Exanthate/dixanthogen

0.05 0

0

2

4

6

8

10

12

Nickel Grade (%)

158

14

10 9 8 7 6 5 4 3 2 1 0

Time (minutes)

0

Fig. 7. Pyrrhotite electrode potentials measured during conditioning time after mild steel (MS) and stainless steel (SS) milling media with 50 g/t copper sulphate addition as specified (CuSO4).

electrode (EPt) were higher than those measured in MS milling media. The oxidising effect of copper sulphate addition was shown by a slight increase in EPt. The collector addition decreased EPt, and although this measurement records the reaction of the collectors with the platinum electrode, it has been shown to approximate the effect of the thiol collectors on the solution pulp potential (Labonte and Finch, 1988). Fig. 5 shows that there was little change in the chalcopyrite mineral potential measurements for the four conditions with the chalcopyrite electrodes and the potential increase resulting from copper sulphate addition remained constant. In the case of pentlandite mineral electrodes (Fig. 6), however, potentials measured for the tests with the use of MS milling media were lower than those obtained for SS milling, although they still remain well above the xanthate–dixanthogen couple. On the other hand, Fig. 7 shows that the potentials measured with pyrrhotite electrode were substantially reduced with MS milling media to below the equilibrium potential of dixanthogen formation (0.108 mV). The changes in pyrrhotite potential due to collec-

20

40

60

80

100

Nickel Recovery (%) Fig. 9. Nickel grade vs. recovery obtained for flotation tests with ore milled with mild steel (MS) and stainless steel (SS) milling media with 50 g/t copper sulphate as specified (CuSO4).

tor or copper sulphate addition were negligible. These measurements demonstrate the differences in response of the different mineral electrodes to the changes in operating conditions. Figs. 8–10 show the effect of the changes in conditions on the flotation performance of chalcopyrite (copper), pentlandite (nickel) and pyrrhotite (iron sulphides), respectively, as represented by the grade vs. recovery relationships. Initially, the highest copper grade vs. recovery relationship was obtained for the ore milled in SS milling media with no copper sulphate added (Fig. 8). Copper grades were higher with SS milling media, which related to the lower mass pull. Copper sulphate addition did not increase the recovery obtained with SS milling. However, after milling in MS milling media, the addition of copper sulphate caused the final copper recovery to increase from 81.7% up to 87.0%. The highest nickel recovery (80.0%) was obtained in the case of MS milling media in the presence of copper sulphate (Fig. 9). The use of SS milling media with 14

Iron Sulphide Grade (%)=

4.5 4

Copper Grade (%)

SS, CuSO4 SS MS, CuSO4 MS

3.5 3 2.5 SS, CuSO4 SS MS, CuSO4 MS

2 1.5 1 0.5 0

12 10 8 6 SS, CuSO4 SS MS, CuSO4 MS

4 2 0

0

20

40

60

80

100

Copper Recovery (%) Fig. 8. Copper grade vs. recovery obtained for flotation tests with ore milled with mild steel (MS) and stainless steel (SS) milling media with 50 g/t copper sulphate as specified (CuSO4).

0

20

40

60

80

100

Iron Sulphide Recovery (%) Fig. 10. Iron sulphide grade vs. recovery obtained for flotation tests with ore milled with mild steel (MS) and stainless steel (SS) milling media with 50 g/t copper sulphate as specified (CuSO4).

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copper sulphate addition decreased the initial grade and recovery values, but without any significant effect on the final recovery. The final grades with MS milling were lower than those obtained after SS milling irrespective of copper sulphate addition. The flotation performance of iron sulphides was affected by the operating conditions to a greater extent than the other minerals. Fig. 10 shows the dramatic effect of copper sulphate addition on recovery of iron sulphides in both milling conditions and that the best performance was obtained for SS milling media with copper sulphate addition. Although the initial recoveries were higher with the MSCuSO4 test, the final recovery of 77.7% was approximately 5% higher with SSCuSO4. In the absence of copper sulphate, the lowest grade and recovery values were obtained with MS milling. 4. Discussion The magnitude of mineral potentials were measured in the following order; pentlandite N chalcopyrite N pyrrhotite (Fig. 2), and correlated to the catalytic activity for oxygen reduction for these minerals (Rand, 1977). The floatability of sulphide minerals present in Merensky ore for all conditions applied can be ranked as chalcopyrite N pentlandite N pyrrhotite pyrrhotite (Fig. 1), which is different from the order of mineral potentials. However, it was found that the floatability of sulphide minerals followed the trend of the decrease of potential due to collector addition (chalcopyrite N pentlandite Npyrrhotite). This may be interpreted as an increase in the extent of breactionQ between the collector and mineral surface. The greater extent of the breactionQ with xanthate corresponded to a higher rate of flotation for chalcopyrite as shown in Fig. 1. In the case of pentlandite and pyrrhotite, only dixanthogen formation at the surface is expected to lead to flotation (Hodgson and Agar, 1989), whereas chemisorbed xanthate and copper xanthate might be the additional products on chalcopyrite. The use of mild steel milling media in place of stainless steel milling media has the effect of creating more reducing pulp environment (Fig. 4), mainly due to high Fe2+ / Fe3+ ratio and consumption of dissolved oxygen (Fig. 3). The corrosion of mild steel releases iron into solution and at alkaline pHs iron hydroxy species are formed. Table 2 shows the measurements of residual xanthate measured in solution and metal hydroxide complexes with EDTA from the ore surface at conditions equivalent to flotation conditions, using the method of Clarke et al. (1995). Although the total

159

Table 2 Measurements of residual xanthate in solution and, iron and copper levels on the mineral surface as complexed with EDTA Milling media

Copper sulphate addition

Residual xanthate in solution (mmol/l)

Iron abstracted (mg/g)

Copper abstracted (mg/g)

SS MS SS MS

Nil Nil 50 g/t 50 g/t

38.60 33.14 0 0

0.046 0.44 0.055 0.48

0.0014 0.0008 0.0037 0.0019

amount of copper abstracted by EDTA was very low in all of the tests, it could be seen that there was less abstraction of copper from the ore with MS milling media than SS milling media. The increase in the EDTA soluble iron content of the ore after MS milling was approximately 10 times higher than that with SS milling. The increased quantity of iron abstracted from the mineral surface confirmed the presence of iron hydroxy species on mineral surface. These hydrophilic species may coat the mineral surfaces and reduce mineral floatability as seen by the decrease in the final recoveries where no copper sulphate was added (Figs. 8–10). The residual xanthate in solution was measured using UV–Visible Spectrophotometer at 301 nm wavelength. The results showed that when copper sulphate was added, no residual xanthate remained in solution. Copper sulphate addition obviously consumed the xanthate available, possibly by chemisorptions of xanthate ions onto copper activated minerals or by formation of copper(II)-xanthate precipitates in the bulk solution. These precipitates may also physically coadsorb onto the chemisorbed xanthate layer on the mineral surface and improve the hydrophobic character of the minerals. At later stages, the initial form of unstable copper(II)xanthate may decompose into more stable copper(I)xanthate and dixanthogen forms (Allison et al., 1972; Fuerstenau, 1982; Wang and Forssberg, 1989). The most pronounced effect of copper(II)-xanthate formation was observed with pyrrhotite flotation. Pyrrhotite recovery increased dramatically after the addition of copper sulphate irrespective of type of milling media which can be attributed to the formation of strongly hydrophobic copper-xanthate species at the surface (Fig. 10). Fig. 4 shows that the EPt measurements in the pulp with ore milled with SS milling media were higher than those measured with MS milling media. However, the EPt measured after MS milling in this study (EPt = 200 mV) was well above the pulp potential values reported in literature for complex sulphide ores milled with MS milling media (Kelebek, 1993; Yuan et al., 1996; Free-

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man et al., 2000; Ekmekc¸i et al., 2003). Since EPt is determined by the rate of galvanic interaction between the milling media and sulphide minerals, the difference in EPt values was considered to be due to the difference in the sulphide mineral content of Merensky ore (1%) with those reported in the literature (minimum 50%). Copper ions added as CuSO4d 5H2O would be in the form of Cu2+ and the reduction of Cu2+ to Cu+ would be account for the increase in potential of the electrodes. This increase may indicate the adsorption of copper ions on the surfaces of the sulphide minerals by an electrochemical mechanism at pH = 9. In addition, at pH 9, copper ions are in the form of CuOH+ and Cu(OH)2 according to the solubility diagram (Fuerstenau and Fuerstenau, 1982). Therefore, copper-hydroxy species may also adsorb on the surface by physical adsorption (Allison, 1982). These two different types of reactions may take place successively at the mineral surface (Pugh and Tjus, 1987; Prestidge et al., 1997; Weisener and Gerson, 2000). After adsorption of copper-hydroxy species, the initially adsorbed copper ions at the mineral surface may be reduced from Cu2+ to Cu+ form, resulting in an increase in potentials of the electrodes. The collector addition reduced the EPt showing the effect of thiol-collector addition on the pulp potential. It must be noted that in effect this measurement actually records the reaction of the collectors with the platinum electrode, which has been shown to approximate the mixed potential of the pulp (Rand and Woods, 1984; Labonte and Finch, 1988). It can be seen that the EPt of all tests (Fig. 4) was anodic to the potential required for the oxidation of xanthate to dixanthogen (Ex /x2 = 0.108 V). On the addition of xanthate one would expect to see a shift in the mixed potential to more cathodic potentials in line with the predictions of the mixed potential model. However, although the measured EPt values were more anodic than the Ex /x2 and the formation of dixanthogen was favoured, a slight reduction in EPt was observed after addition of collector both in the presence and absence of copper sulphate. The residual xanthate measurements in the absence of copper sulphate (Table 2) were in correlation with EPt measurements showing a significant portion of xanthate ions was not oxidised to dixanthogen. This was attributed to the fact that the sulphide mineral content of Merensky ore is only 1% and oxidation of xanthate to dixanthogen is rather slow in the absence of sulphide minerals, particularly pyrite, which acts as catalyst (Majima, 1971). On the other hand, there was no residual xanthate in solution after addition of copper sulphate because of the formation of Cu(II)-xanthate species.

The mineral potential measurements showed that the change in conditions had the least effect on chalcopyrite and this corresponded to the little effect on its flotation response. The lower copper recovery with MS milling media and no copper sulphate addition may be attributed to coating of the surface by hydrophilic iron hydroxy species and metallic iron. Addition of copper sulphate counteracted the negative effect of iron hydroxy species and increased the final copper recovery in MS milling. EPn measurements showed that flotation of pentlandite (in the potential range measured in this study, Fig. 6) was not affected by electrochemical conditions of the pulp. The lowest EPn value measured was about 0.125 V with MS milling. The potential difference of 75 mV between SS and MS milling did not significantly change the surface oxidation of pentlandite and hence its flotation behaviour. The increase in the recovery of nickel in the presence of copper sulphate with MS milling was due to the activation of pentlandite by copper ions (Fig. 9). Recently, Malysiak et al. (2002) reported that the interaction between copper (II) ions and pentlandite surfaces at pH 9 is most likely to be chemical rather than electrostatic, based on the results of zeta potential and ToFSIMS measurements. Formation of copper (II)-xanthate species has been observed on pentlandite surface, which increased both the flotation rate and recovery of pentlandite. However, the recovery of nickel in MS milling with copper sulphate addition was higher than SS milling with copper sulphate addition. Although iron hydroxy species may also coat pentlandite surface, as observed with chalcopyrite after MS milling, enhanced copper activation and increasing froth stability were considered as the main reason for higher nickel recovery in MSCuSO4 test. The electrode potential of pyrrhotite (EPo) with MS milling was slightly lower than Ex /x2 couple (Fig. 7) which suggests that the lower recovery and grade (Fig. 10) obtained with MS milling could be attributed to decreased hydrophobicity due to the lack of dixanthogen on the surface. As EPo increased to about 150 mV with SS milling, the recovery of iron sulphides increased by only 4% and reached to 42%. However, copper sulphate addition, with little effect on the EPo, increased both rate and recovery of iron sulphides irrespective of milling media type showing that copper activation is of vital importance for the flotation of iron sulphides in Merensky ore at pH 9. There are contradictory results in the literature about copper activation of iron sulphides in alkaline solutions. Some of them (Nicol, 1984; Iwasaki, 1988) concluded that activation

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was ineffective in alkaline, pH 9, solutions. While the others (Leppinen, 1990; Senior et al., 1995; Yoon et al., 1995; Kelebek et al., 1996; Wiese et al., 2005b,c) indicated that activation could be achieved at pH 9, and subsequent recovery of pyrrhotite was enhanced. The results of the present investigation were in agreement with the later reports, i.e. copper does activate pyrrhotite in alkaline solutions. Although the activation mechanism is still open to debate, and is not expected to be electrochemical (Buswell and Nicol, 2002) the slight increase in the electrode potential after copper sulphate addition showed that the activation may occur by attachment of copper initially in the form of basic complexes or cupric hydroxide on the surface of iron sulphides (Allison, 1982; Finkelstein, 1998; Weisener and Gerson, 2000). As it was described above, in the following stages cupric xanthate species may decompose into more stable cuprous-xanthate and dixanthogen forms. The nature of activation mechanism was likely to be chemical, as it was for pentlandite. Although more fully analysed and discussed in Part II, (Ekmekc¸i et al., 2005—this issue) the water and mass recovery data given in Table 1 can be used as indicators of the froth characteristics. The decrease in the water recovery was considered as a result of reduced stability of the froth zone. The data showed that while MS milling increased the froth stability probably due to presence of metallic iron and iron hydroxides (Bikerman, 1953; Van Deventer, 1998; Freeman et al., 2000), addition of copper sulphate reduced the stability, particularly with SS milling media. The destabilisation effect of copper sulphate addition was counteracted with the presence of metallic iron and iron hydroxides obtained with MS milling. The variations in the water recovery data and hence in stability of froth phase emphasised the importance of the froth phase in evaluation of flotation data. 5. Conclusions Although Merensky ore is sometimes classified as a complex sulphide ore in reference to its variety of sulphide minerals, its behaviour is entirely different because of its very low sulphide mineral content of approximately 1% relative to that of N 50% for classical complex sulphide ores. The very low sulphide mineral content resulted in electrode potential measurements using platinum and different mineral electrodes being substantially higher than that reported in the literature after MS milling. The final flotation recoveries of chalcopyrite, pentlandite and pyrrhotite followed the same trend as the

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decrease in mineral potential resulting from collector addition viz. chalcopyrite N pentlandite N pyrrhotite. The chalcopyrite mineral electrode showed the strongest interaction with the thiol collectors and this interaction was not affected by changing milling media or copper sulphate addition. The flotation performance of chalcopyrite was the least affected of the sulphide minerals by the changing conditions, which corresponded to little change in the mineral potential measurements. The lowest potential measurement of the pentlandite electrode was obtained with MS milling media and was also well above the potential of the xanthate/ dixanthogen couple. Besides, the potential difference of 75 mV for pentlandite electrode was not enough to alter the flotation performance of pentlandite. Addition of copper sulphate increased the recovery of nickel with MS milling due to activation by copper ions. Although the same copper activation mechanism was also considered to occur with SS milling, the reduced stability of the froth zone upon copper sulphate addition might be the reason for lower nickel recovery with SSCuSO4 test (see Part II, Ekmekc¸i et al., 2005—this issue). In the case of pyrrhotite, the potentials obtained using MS milling media dropped below that necessary for dixanthogen formation and hence reduced the final recovery slightly lower than that with SS milling. Addition of copper sulphate dramatically increased pyrrhotite flotation performance, which demonstrated that copper activation of pyrrhotite had occurred at alkaline pHs but that the nature of activation mechanism was not electrochemical and likely to be chemical, as it was for pentlandite. The measurements made in this investigation have contributed to increased understanding of the changes to the pulp chemical environment arising from changing milling media and copper sulphate addition and to the effect on the flotation performance obtained with the different minerals in a PGM ore, but have also demonstrated the limitations of the individual measurements due to both to the very low sulphide mineral content of the ore and the evidence of a range of reactions occurring, both chemical and electrochemical. The water and mass recovery data showed that while MS milling increased the froth stability probably due to presence of metallic iron, addition of copper sulphate reduced the stability, particularly with SS milling media. The destabilisation effect of copper sulphate addition was counteracted with the presence of metallic iron with MS milling. The variations in the water recovery data and hence in stability of froth phase emphasised the importance of the froth phase in eval-

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