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Control of grinding conditions in the flotation of chalcopyrite and its separation from pyrite
Int. J. Miner. Process. 69 (2003) 87 – 100 www.elsevier.com/locate/ijminpro
Control of grinding conditions in the flotation of chalcopyrite and its separation from pyrite Yongjun Peng a, Stephen Grano b,*, Daniel Fornasiero b, John Ralston b b
a COREM, 1180, rue de la Mine´ralogie, Quebec City, QC, Canada G1N 1X7 Ian Wark Research Institute, The ARC Special Research Centre for Particle and Material Interfaces, University of South Australia, Mawson Lakes, Adelaide, SA 5095, Australia
Received 23 April 2002; accepted 10 September 2002
Abstract A specially designed mill which allowed the control of pH throughout grinding was used to study the effect of grinding conditions on chalcopyrite flotation and chalcopyrite separation from pyrite. The mechanism of galvanic interaction between minerals and grinding media was investigated by ethylene diamine-tetra acetic acid disodium salt (EDTA) extraction and X-ray photoelectron spectroscopy (XPS) measurements. Chalcopyrite flotation was strongly dependent on both iron oxidation species and metal deficiency on the chalcopyrite surface. Iron oxidation species from grinding media played a dominant role in depressing chalcopyrite flotation, while metal deficiency from chalcopyrite oxidation improved chalcopyrite flotation. Therefore, chromium grinding medium produced a higher chalcopyrite recovery than mild steel grinding medium while gas purging during grinding had little effect on chalcopyrite flotation. Chalcopyrite separation from pyrite was affected by the activation of pyrite flotation by copper species dissolved from chalcopyrite. Grinding media had a large effect on the reduction of copper(II) to copper(I) on the pyrite surface. The reducing grinding condition generated by mild steel medium favoured formation of copper(I) sulphide phase, which resulted in high pyrite activation. Thus, chromium medium produced better chalcopyrite selectivity against pyrite than the mild steel medium. D 2003 Elsevier Science B.V. All rights reserved. Keywords: chalcopyrite; pyrite; grinding; flotation; copper activation; galvanic coupling
* Corresponding author. Fax: +61-8-8302-3683. E-mail address: [email protected] (S. Grano). 0301-7516/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 7 5 1 6 ( 0 2 ) 0 0 11 9 - 9
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1. Introduction It has been known for a long time that grinding conditions have a significant influence on the subsequent flotation of sulphide minerals. For chalcopyrite flotation, there is agreement that mild steel grinding medium results in a lower chalcopyrite recovery than stainless steel grinding medium in the presence or absence of collectors (Rao et al., 1976; Forssberg et al., 1988; Van Deventer et al., 1991). The depression of chalcopyrite flotation by mild steel medium was attributed to the iron hydroxide species on chalcopyrite surface, which was closely related to the anodic iron oxidation (Yelloji Rao and Natarajan, 1988). The rate of anodic iron oxidation was found to decrease by nitrogen purging during wet grinding of chalcopyrite minerals (Natarajan, 1996). However, whether nitrogen purging during grinding with iron grinding media can improve chalcopyrite flotation needs to be confirmed. Although mild steel grinding medium is detrimental to chalcopyrite flotation, it is favoured for chalcopyrite selectivity against pyrite. Yuan et al. (1996) found that chalcopyrite selectivity against pyrite was reduced when the minerals were ground in an oxidising environment with a stainless steel medium, but was restored when ground in a reducing environment with mild steel medium. A similar observation was found by Van Deventer et al. (1991). Clearly, it is desirable to investigate the effect of grinding with low iron content media on chalcopyrite separation from pyrite. Yuan et al. (1996) also indicated that air-sparged mild steel grinding gave no advantage in selectivity compared with ordinary mild steel grinding. However, oxygenation during conditioning and flotation is considered an important factor in the selectivity of flotation operations. As reported by Houot and Duhamet (1990) and Kuopanportti et al. (2000), oxygenation had a favourable effect on the separation of chalcopyrite from pyrite and the oxygen demand increased with the proportion of pyrite in mineral systems. In this study, a specially designed mill which allowed the control of chemical conditions during grinding was used. The effects of two types of iron media, mild steel and 30 wt.% chromium (with approximate 70 wt.% iron), and three types of purging gases (nitrogen, air and oxygen) on chalcopyrite flotation and chalcopyrite separation from pyrite were investigated.
2. Experimental 2.1. Materials and reagents Chalcopyrite and pyrite samples were obtained from Willyama, Earth History Supplies, NSW, Australia and Huanzala Mine, Peru, respectively. The samples were crushed through a rolls crusher and then screened to collect the + 0.6 3.2 mm particle size fraction. The processed samples were then sealed in polyethylene bags. The chemical composition of these samples analysed by ICP-MS is shown in Table 1. Two types of grinding media were used. The tapered cylinder mild steel grinding medium was supplied from Pasminco Mining, Elura, Australia and the spherical 30 wt.% chromium grinding medium from Magotteaux, Australia. The specific surface areas of
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Table 1 Chemical composition of the chalcopyrite and pyrite samples Minerals
Elements present (wt.%) Si
Fe
Ca
Mg
Mn
S
Pb
Cu
Zn
Chalcopyrite Pyrite
0.50 0.69
27.84 44.93
0.15 0.45
0.09 0.08
0.04 0.02
39.29 53.51
0.03 0.02
31.91 0.13
0.15 0.18
mild steel and chromium grinding media are 6.32 10 5 and 3.55 10 5 m2/g, respectively, calculated by the measurements of the total surface areas of 4 kg of grinding media. Sodium diisobutyl dithiophosphate (DTP, AR grade) and Dowfroth 250 (polypropylene oxide methanol, AR grade) were used in the experiments as collector and frother, respectively. Copper nitrate (Cu(NO3)2, AR grade) were used to introduce copper species during grinding. The pH was adjusted by the addition of AR grade NaOH solutions. Deionised water was used in all experiments. High purity nitrogen and oxygen from BOC Gases, Adelaide, Australia, were used throughout the study. 2.2. Mineral grinding and flotation Chalcopyrite or pyrite single mineral (100 g) or chalcopyrite –pyrite mixture (50 g each) was combined with 400 cm3 of water and ground with 4 kg of grinding medium in the specialised mill for 30 min so that 90 wt.% of the particles were less than 53 Am in diameter. The specific surface areas of mineral particles measured by BET after grinding are shown in Table 2. The mill was originally designed by Cases et al. (1990). During grinding, slurry was pumped through a monitoring cell where the pH was monitored and fixed at 9.0 by adding NaOH solution continuously. The slurry in the monitoring cell was purged with different gases to change the oxidation conditions in the mill. Samples for EDTA extractions and XPS analyses were obtained from the cell at the completion of grinding. After grinding, the pulp was transferred to a 1.5-dm3 flotation cell, conditioned with pH modifier, collector and frother. Four flotation concentrates were collected after cumulative times of 0.5, 2.0, 4.0 and 8.0 min at an air flow rate of 2.5 dm3 min 1. The flotation froth was scraped every 10 s. After grinding, the system was open to the air whilst the pH was fixed at 9.0. In chalcopyrite – pyrite mixture mineral experiments, the chalcopyrite Table 2 Specific surface areas of mineral particles after grinding Grinding medium
Mineral
Specific surface area (m2/g)
Mild steel 30 wt.% Chromium Mild steel 30 wt.% Chromium Mild steel 30 wt.% Chromium
Chalcopyrite Chalcopyrite Pyrite Pyrite Pyrite + Chalcopyrite Pyrite + Chalcopyrite
3.40 3.39 1.96 1.92 2.45 2.04
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recovery was calculated from the assay of copper, while pyrite recovery was calculated by the difference in mass and chalcopyrite recovery. The Eh (pulp potential) was measured at room temperature during grinding and conditioning using a HANNA meter combined with a platinum electrode and expressed relative to the standard hydrogen electrode, SHE. 2.3. Techniques 2.3.1. EDTA extraction A 3 wt.% solution of AR grade ethylene diamine-tetra acetic acid disodium salt (EDTA) was made up and a sodium hydroxide solution was used to adjust the pH to 7.5. 95 cm3 of the EDTA solution was placed in a vigorously stirred reaction vessel and purged with nitrogen for 5 min. A 5 cm3 of slurry sample from the mill discharge was frozen in liquid nitrogen first and then added to the EDTA solution, followed by 5 min of conditioning. Nitrogen was continuously purged throughout. The slurry was then filtered through a 0.45-Am Millipore filter. The filtrate was analysed for iron and copper by inductively coupled plasma (ICP) atomic emission spectrophotometry (Spectroflame M). The solids were retained to obtain the sample dry weight, enabling calculation of the mass of metal oxidation species per unit mass of solid (Rumball and Richmond, 1996) and per unit area of solid surface. 2.3.2. X-ray photoelectron spectroscopy (XPS) The species on the surface of chalcopyrite after different grinding were examined by XPS. XPS measurements were carried out with a Perkin-Elmer Physical Electronics Division (PHI) 5100 spectrometer with a Mg Ka X-ray source operating at 300 W and with a pass energy of 18 eV. The pressure in the analyzer chamber was 10 8 Torr during analysis. The energy scale was calibrated using the Fermi edge and 3d5/2 line (BE = 367.9 eV) for silver, whilst the retardation voltage was calibrated using the position of the Cu 2p3/2 peak (BE = 932.67 eV) and the Cu 3p3/2 peak (BE = 75.13 eV). The measurements were performed at a take-off angle of 45j. The slurry samples were washed with water of pH 9.0 to remove any species from solution and introduced into the fore-vacuum of the spectrometer as concentrated slurries. The samples were first examined in survey mode to identify all the elements present, then the various elemental regions were scanned in order to extract information on chemical bonding and oxidation stages. Details of this procedure have been reported previously by Smart (1991).
3. Results and discussion 3.1. Effect of grinding conditions on chalcopyrite flotation The effect of grinding media and gas purging on chalcopyrite flotation is shown in Fig. 1. It can be seen that 30 wt.% chromium medium produced a higher chalcopyrite recovery than mild steel medium. This is consistent with reports in literature that mild steel medium depresses chalcopyrite flotation (Rao et al., 1976; Forssberg et al., 1988; Van
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Fig. 1. Effect of grinding media and gas purging on chalcopyrite recovery as a function of flotation time: (dashed lines) mild steel medium; (solid lines) 30 wt.% chromium medium; (5) nitrogen purging; (D) air purging; (o) oxygen purging.
Deventer et al., 1991). Meanwhile, for both mild steel and 30 wt.% chromium grinding media, gas purging had little effect on chalcopyrite flotation. The samples from the mill discharge were analysed by EDTA extraction. The amount of extracted iron and copper is shown in Table 3. The EDTA extracted copper originated from the oxidation of chalcopyrite, whilst the EDTA extracted iron originated from the oxidation of chalcopyrite and grinding media. According to the mixed-potential theory (Fontana and Greene, 1978), when two materials are coupled, the oxidation rate of the more active material will increase and the oxidation rate of the less active material will decrease. It is well known that chalcopyrite is much more noble than the iron grinding media (Rao et al., 1976; Kocabag and Smith, 1985). As a result, during grinding, chalcopyrite oxidation is less pronounced than the medium oxidation and most EDTA extracted iron should emanate from the grinding media. This is in agreement with the results in Table 3 showing a much higher amount of EDTA extracted iron than copper. From the amount of EDTA extracted copper, more chalcopyrite was oxidised after grinding with 30 wt.% chromium medium than with mild steel medium. Oxygen purging increased chalcopyrite oxidation as expected. Eh values at the completion of grinding are also indicated in Table 3. A 30-wt.% chromium medium and oxygen purging resulted in higher Eh values, suggesting more oxidising grinding conditions, which is consistent with the higher amount of EDTA extracted copper. Table 3 Effect of grinding conditions on the amount of EDTA extracted iron and copper in chalcopyrite single mineral experiments Grinding condition (medium; gas)
Eh (mV)
Extracted Fe (mol/m2 mineral)
Extracted Cu (mol/m2 mineral)
Mild steel; oxygen Chromium; oxygen Chromium; nitrogen
195 220 135
1.19 10 4 3.26 10 5 1.05 10 5
1.07 10 6 1.42 10 6 9.14 10 7
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As discussed previously, iron oxidation species on chalcopyrite surfaces play the dominant role in depressing chalcopyrite flotation. It can be seen that mild steel medium produced a much higher amount of EDTA extracted iron than 30 wt.% chromium medium, in agreement with the lower chalcopyrite recovery by mild steel medium. However, oxygen purging also produced a higher amount of EDTA extracted iron than nitrogen purging, but the flotation results in Fig. 1 were similar after grinding with oxygen or nitrogen purging. Obviously, chalcopyrite flotation depends on the hydrophilic iron oxidation species as well as some hydrophobic oxidation species on the chalcopyrite surface. The same samples for EDTA extraction were also analysed by XPS. The type of surface species and their concentrations on chalcopyrite are reported in Table 4. As expected the iron concentration on chalcopyrite surface is higher with mild steel medium than with 30 wt.% chromium medium, and with oxygen purging than with nitrogen purging. The Fe2p spectra are shown in Fig. 2 and indicate that an iron oxyhydroxide layer is present on the chalcopyrite surface. This is shown by a broad band in the Fe2p spectrum near 711– 712 eV and near 726 eV, while the iron associated with sulphur as in chalcopyrite occurs at 708 eV (Buckley and Woods, 1984). It can be seen that the relative intensity of the iron oxyhydroxide band is higher when mild steel medium was used. However, this intensity decreased after chalcopyrite was ground with 30 wt.% chromium medium, especially under nitrogen purging. This observation is consistent with the amount of EDTA extracted iron in Table 3. The S2p spectra are shown in Fig. 3. According to Buckley and Woods (1984), the S2p spectrum of chalcopyrite is composed of three components and each component is made of a doublet. The peak near 161 eV is attributed to sulphide of unoxidised chalcopyrite. The two higher binding energy components around 162 and 164 eV are due to metaldeficient sulphide and polysulphide, respectively. Peaks for sulphate and thiosulfate at higher binding energies around 168 eV (Buckley and Woods, 1984) are present when oxygen was purged during grinding for both mild steel and 30 wt.% chromium grinding media. Fig. 3 shows that 30 wt.% chromium medium produced more sulphate on the chalcopyrite surface than mild steel medium, whilst oxygen purging produced more sulphate and polysulphide than nitrogen purging in agreement with the EDTA exaction results. Although the type of gas used during grinding produced similar chalcopyrite recoveries, the type and proportion of the surface oxidation products were different. Higher proportions of polysulphide, sulphate and ferric oxide/hydroxide were formed on chalcopyrite surface with oxygen purging than with nitrogen purging. These results Table 4 Concentration (at.%) of the elements measured by XPS on the surface of chalcopyrite Grinding condition (medium; gas)
Atomic concentration of element (%) Fe
S
Cu
O
Mild steel; oxygen Chromium; oxygen Chromium; nitrogen
12.8 6.3 4.7
36.2 43.6 58.4
2.6 2.4 2.3
48.4 47.7 34.6
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Fig. 2. Fe2p XPS spectra of the chalcopyrite surface after grinding: (a) mild steel medium with oxygen purging; (b) 30 wt.% chromium medium with oxygen purging; (c) 30 wt.% chromium with nitrogen purging.
indicate that chalcopyrite flotation is closely dependent on iron oxidation species, which depress chalcopyrite flotation, and metal deficient sulphide, which increases chalcopyrite flotation. Similar observations have been found by Fairthorne et al. (1997) who demonstrated that the hydrophobicity of chalcopyrite will increase with the formation of a sulphur-rich surface but will be offset by the precipitation of hydrophilic metal hydroxide species. 3.2. Effect of grinding conditions on chalcopyrite separation from pyrite The effect of grinding conditions on the flotation of chalcopyrite and pyrite in mineral mixture experiments is indicated in Fig. 4. It can be seen that in agreement with the single mineral flotation results (Fig. 1), 30 wt.% chromium medium produced higher chalcopyr-
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Fig. 3. S2p XPS spectra of the chalcopyrite surface after grinding: (a) mild steel medium with oxygen purging; (b) 30 wt.% chromium medium with oxygen purging; (c) 30 wt.% chromium with nitrogen purging.
ite recovery than mild steel medium and gas purging had little effect on chalcopyrite flotation. Furthermore, mild steel medium produced higher pyrite recovery than 30 wt.% chromium medium with gas purging during grinding having no or little influence on pyrite flotation. Therefore, higher chalcopyrite selectivity against pyrite was produced by 30 wt.% chromium medium than by mild steel medium. Yelloji Rao and Natarajan (1989) reported that galvanic interactions between a noble mineral and an active mineral markedly affected the floatability of the noble mineral, but had a minimal effect on the floatability of the active mineral. The chalcopyrite flotation rate constant and maximum recovery in the experiments of chalcopyrite single mineral and chalcopyrite – pyrite mixture were calculated by fitting the experimental recovery data to the first order rate equation (Lynch et al., 1981): Recovery ðtÞ ¼ Rmax ð1 ekt Þ
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Fig. 4. Effect of grinding media and gas purging on the separation of chalcopyrite from pyrite: (empty symbols) mild steel medium; (solid symbols) 30 wt.% chromium medium; (squares) nitrogen purging; (triangles) air purging; (circles) oxygen purging.
where Rmax (%) is the maximum recovery at infinite time and k (min 1) is the first order rate constant. The rate constant and maximum recovery are shown in Table 5. After pyrite addition, the chalcopyrite flotation rate constants decreased, but the maximum recoveries remained within experimental error. It was also noted that pyrite exhibited a different flotation behaviour after mixed with chalcopyrite. As shown in Fig. 5, pyrite single mineral displayed a poor floatability with less than 10% recovery after 8 min of flotation. However, pyrite flotation was increased after addition of chalcopyrite, especially with the mild steel medium (Fig. 4). Samples of chalcopyrite– pyrite mixture ground in different conditions were conditioned with EDTA solutions. The extracted amounts of iron and copper are shown in Table 6. Higher iron oxidation production was produced with mild steel medium than with 30 wt.% chromium medium and with oxygen purging than with nitrogen purging. A comparison of the amount of extracted iron in Tables 6 and 3 reveals similar trends and only a small increase in iron oxidation after chalcopyrite was mixed with pyrite. This increase in surface iron oxidation may contribute to the lower chalcopyrite flotation rate constant observed in chalcopyrite – pyrite mixture experiments. Attention now turns to the activation of pyrite flotation by chalcopyrite. Two different mechanisms for the increased floatability of pyrite after chalcopyrite addition have been proposed (Ekmekcß i and Demirel, 1997): (i) increased proportion of elemental or metalTable 5 Chalcopyrite flotation rate constant and maximum recovery in single mineral and mixture mineral experiments Grinding condition (medium; gas)
Rate constant (min 1)
Recovery maximum (%)
Cp
Cp + Py
Cp
Cp + Py
Mild steel; oxygen Mild steel; nitrogen Chromium; oxygen Chromium; nitrogen
1.6 1.2 2.1 2.4
1.1 1.1 1.6 1.9
75 76 87 88
75 77 86 86
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Fig. 5. Pyrite single mineral flotation as a function of flotation time, with and without Cu2 + addition, under oxygen purging: (o) without Cu2 +; ( ) with 1.5 10 3 M Cu2 +; (dashed lines) mild steel; (solid lines) 30 wt.% chromium medium.
.
deficient sulphur on the pyrite surface and (ii) copper activation. Regarding mechanism (i), because chalcopyrite is more active than pyrite, ferric hydroxide may be reduced to a more soluble iron species, ferrous hydroxide, and thus elemental or metal-deficient sulphur will be generated on pyrite surface, which results in an improved pyrite flotation in the presence of chalcopyrite. A similar mechanism should also occur for pyrite in the absence of chalcopyrite in this study as both the mild steel and 30 wt.% chromium grinding media are more active than pyrite. However, Fig. 5 shows that pyrite flotation was poor in absence of copper. This concluded that the improved pyrite flotation in the experiments of chalcopyrite – pyrite mixture is not due to elemental or metal-deficient sulphur but rather to copper activation of pyrite flotation. It is generally accepted that the formation of a new copper sulphide phase on the pyrite surface through the reduction of copper(II) to copper(I) is responsible for the activation of pyrite while sulphide ions are oxidised (Bushell and Krauss, 1962; Weisener and Gerson, 2000). Laajalehto et al. (1999) proposed that a chalcopyrite-like phase (CuFeS2) might be formed during the activation of pyrite because the copper-activated pyrite exhibited the typical S2p XPS spectrum of copper sulphides (e.g. chalcocite or chalcopyrite) and similar electrochemically controlled xanthate adsorption behaviour as chalcopyrite. A comparison of the amount of EDTA extracted copper in Tables 6 and 3 shows that a much lower amount of copper hydroxide Table 6 Effect of grinding conditions on the amount of EDTA extracted iron and copper in chalcopyrite – pyrite mixture experiments Grinding condition (medium; gas)
Extracted Fe (mol/m2 mineral)
Extracted Cu (mol/m2 mineral)
Mild steel; oxygen Chromium; oxygen Chromium; nitrogen
1.25 10 4 4.43 10 5 1.58 10 5
2.57 10 7 5.78 10 7 2.86 10 7
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Fig. 6. Eh during grinding of pyrite in the presence of chalcopyrite (D) or Cu(II) (5) under oxygen purging: (dashed lines) mild steel; (solid lines) 30 wt.% chromium medium.
was extracted from the chalcopyrite – pyrite mixture than from chalcopyrite in the absence of pyrite even if considering that the amount of chalcopyrite in the mixture was half that in single mineral. Therefore, it appears that the improvement in pyrite flotation in the presence of chalcopyrite is due to copper activation. This was further confirmed by pyrite flotation in the presence of copper(II). In Fig. 5, 1.5 10 3 mol/l of copper(II), based on the amount of EDTA extracted copper in Table 3, was added during pyrite grinding. It can be seen that addition of copper(II) had exactly the same effect on pyrite flotation as chalcopyrite. Pyrite flotation was improved in the presence of copper(II), especially after grinding with mild steel. Fig. 6 shows the Eh during grinding as a function of grinding time. In the presence of chalcopyrite or copper(II), mild steel medium always produced lower Eh values than 30 wt.% chromium medium. Voltammetric studies have shown that activation of pyrite by copper is strongly dependent on the redox potential in the activating solution and that increases in oxidising potential inhibit copper uptake (Richardson et al., 1996). This may explain why pyrite flotation was increased further after grinding with mild steel medium in the presence of copper(II) in Fig. 5. Since chalcopyrite displayed the same effect on pyrite flotation as copper(II), the effect of grinding media on copper(II) reduction could be studied by analysing the pyrite surface after grinding in the presence of copper(II) ions. Table 7 shows that 30 wt.% chromium medium resulted in slightly more copper on the pyrite surface than mild steel medium. However, it is copper(I) which is responsible for pyrite activation. In order to quantify the amount of copper(I) on the pyrite surface, Cu2p Table 7 Concentration (at.%) of the elements measured by XPS on pyrite surface in the presence of 1.5 10 3 M Cu(II) Grinding condition (medium; gas)
Atomic concentration of element (%) Fe
S
Cu
O
Mild steel; oxygen Chromium; oxygen
20.8 20.3
20.7 26.4
1.7 2.1
56.8 51.2
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Fig. 7. Cu2p XPS spectra of the pyrite surface in the presence of 1.5 10 3 M Cu2 +: (a) mild steel medium with oxygen purging; (b) 30 wt.% chromium medium with oxygen purging.
XPS spectra were fitted as shown in Fig. 7. The relatively smaller intensity of the broad band observed around 943.4 eV is attributed to copper(II) shake-up satellites and the peak of 934.4 eV, attributed to cupric oxide or hydroxide indicates that the surface copper, is mainly in a cuprous state with the most intense band at around 932.3 eV (Fairthorne et al., 1997). The percentage of copper(I) on pyrite surface after grinding with mild steel medium and 30 wt.% chromium medium is 79% and 54%, respectively, indicating that more pyrite activation occurred with mild steel medium than with 30 wt.% chromium medium in agreement with the flotation results in Fig. 5.
4. Conclusion Grinding conditions had a significant effect on chalcopyrite flotation. This effect was closely associated with the presence of iron oxidation species and metal deficient sulphide present on the chalcopyrite surface. Iron oxidation species from grinding media played a dominant role in depressing chalcopyrite flotation, whilst the presence of metal deficient sulphide improved chalcopyrite flotation. As a result, 30 wt.% chromium medium produced a higher chalcopyrite recovery than mild steel medium, whilst gas purging during grinding had little effect on chalcopyrite flotation for the same grinding medium. When chalcopyrite was mixed with pyrite, the amount of iron oxidation species from grinding media was increased, which resulted in a lower rate of chalcopyrite flotation.
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Pyrite flotation was significantly improved especially after grinding with mild steel medium. This improvement in pyrite flotation was due to copper(II) activation. Grinding media had a large effect on copper(II) reduction on pyrite surface. Mild steel medium caused more reducing grinding condition and less pyrite oxidation than 30 wt.% chromium medium. Thus, more copper(II) species were reduced to copper(I) species on the pyrite surface, which resulted in more pyrite activation by mild steel medium. Consequently, 30 wt.% chromium medium produced better chalcopyrite selectivity against pyrite than mild steel medium.
Acknowledgements Financial support for this work from AMIRA International as well as scholarships to Y. Peng from the University of South Australia are gratefully acknowledged. Pasminco Mining, Elura, Australia and Magotteaux, Australia are thanked for the supply of grinding media.
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Richardson, P.E., Chen, Z., Tao, D.P., Yoon, R.H., 1996. Electrochemical control of pyrite activation by copper. In: Woods, R., Doyle, F.M., Richardson, P. (Eds.), Electrochemistry in Mineral and Metal Processing, vol. IV. Pennington, New Jersey, pp. 179 – 190. Rumball, J.A., Richmond, G.D., 1996. Measurement of oxidation in a base metal flotation circuit by selective leaching with EDTA. Int. J. Miner. Process. 48, 1 – 20. Smart, R.St.C., 1991. Surface layers in base metal sulphide flotation. Min. Eng. 4, 891 – 909. Van Deventer, J.S.J., Ross, V.E., Dunne, R.C., 1991. The effect of milling environment on the selective flotation of chalcopyrite from a complex sulphide ore. Proc. XVII Int. Min. Proc. Cong. Dresden, pp. 129 – 140. Weisener, C., Gerson, A., 2000. Cu(II) adsorption mechanism on pyrite: an XAFS and XPS study. Surf. Interface Anal. 30, 454 – 458. Yelloji Rao, M.K., Natarajan, K.A., 1988. Influence of galvanic interactions between chalcopyrite and some metallic materials on flotation. Miner. Eng. 1, 281 – 294. Yelloji Rao, M.K., Natarajan, K.A., 1989. Electrochemical effects of mineral – mineral interactions on the flotation of chalcopyrite and sphalerite. Int. J. Miner. Process. 27, 279 – 293. Yuan, X.M., Pa˚lsson, B.I., Forssberg, K.E., 1996. Flotation of a complex sulphide ore: II. Influence of grinding environments on Cu/Fe sulphide selectivity pulp chemistry. Int. J. Miner. Process. 46, 181 – 204.
Control of grinding conditions in the flotation of chalcopyrite and its separation from pyrite Yongjun Peng a, Stephen Grano b,*, Daniel Fornasiero b, John Ralston b b
a COREM, 1180, rue de la Mine´ralogie, Quebec City, QC, Canada G1N 1X7 Ian Wark Research Institute, The ARC Special Research Centre for Particle and Material Interfaces, University of South Australia, Mawson Lakes, Adelaide, SA 5095, Australia
Received 23 April 2002; accepted 10 September 2002
Abstract A specially designed mill which allowed the control of pH throughout grinding was used to study the effect of grinding conditions on chalcopyrite flotation and chalcopyrite separation from pyrite. The mechanism of galvanic interaction between minerals and grinding media was investigated by ethylene diamine-tetra acetic acid disodium salt (EDTA) extraction and X-ray photoelectron spectroscopy (XPS) measurements. Chalcopyrite flotation was strongly dependent on both iron oxidation species and metal deficiency on the chalcopyrite surface. Iron oxidation species from grinding media played a dominant role in depressing chalcopyrite flotation, while metal deficiency from chalcopyrite oxidation improved chalcopyrite flotation. Therefore, chromium grinding medium produced a higher chalcopyrite recovery than mild steel grinding medium while gas purging during grinding had little effect on chalcopyrite flotation. Chalcopyrite separation from pyrite was affected by the activation of pyrite flotation by copper species dissolved from chalcopyrite. Grinding media had a large effect on the reduction of copper(II) to copper(I) on the pyrite surface. The reducing grinding condition generated by mild steel medium favoured formation of copper(I) sulphide phase, which resulted in high pyrite activation. Thus, chromium medium produced better chalcopyrite selectivity against pyrite than the mild steel medium. D 2003 Elsevier Science B.V. All rights reserved. Keywords: chalcopyrite; pyrite; grinding; flotation; copper activation; galvanic coupling
* Corresponding author. Fax: +61-8-8302-3683. E-mail address: [email protected] (S. Grano). 0301-7516/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 7 5 1 6 ( 0 2 ) 0 0 11 9 - 9
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1. Introduction It has been known for a long time that grinding conditions have a significant influence on the subsequent flotation of sulphide minerals. For chalcopyrite flotation, there is agreement that mild steel grinding medium results in a lower chalcopyrite recovery than stainless steel grinding medium in the presence or absence of collectors (Rao et al., 1976; Forssberg et al., 1988; Van Deventer et al., 1991). The depression of chalcopyrite flotation by mild steel medium was attributed to the iron hydroxide species on chalcopyrite surface, which was closely related to the anodic iron oxidation (Yelloji Rao and Natarajan, 1988). The rate of anodic iron oxidation was found to decrease by nitrogen purging during wet grinding of chalcopyrite minerals (Natarajan, 1996). However, whether nitrogen purging during grinding with iron grinding media can improve chalcopyrite flotation needs to be confirmed. Although mild steel grinding medium is detrimental to chalcopyrite flotation, it is favoured for chalcopyrite selectivity against pyrite. Yuan et al. (1996) found that chalcopyrite selectivity against pyrite was reduced when the minerals were ground in an oxidising environment with a stainless steel medium, but was restored when ground in a reducing environment with mild steel medium. A similar observation was found by Van Deventer et al. (1991). Clearly, it is desirable to investigate the effect of grinding with low iron content media on chalcopyrite separation from pyrite. Yuan et al. (1996) also indicated that air-sparged mild steel grinding gave no advantage in selectivity compared with ordinary mild steel grinding. However, oxygenation during conditioning and flotation is considered an important factor in the selectivity of flotation operations. As reported by Houot and Duhamet (1990) and Kuopanportti et al. (2000), oxygenation had a favourable effect on the separation of chalcopyrite from pyrite and the oxygen demand increased with the proportion of pyrite in mineral systems. In this study, a specially designed mill which allowed the control of chemical conditions during grinding was used. The effects of two types of iron media, mild steel and 30 wt.% chromium (with approximate 70 wt.% iron), and three types of purging gases (nitrogen, air and oxygen) on chalcopyrite flotation and chalcopyrite separation from pyrite were investigated.
2. Experimental 2.1. Materials and reagents Chalcopyrite and pyrite samples were obtained from Willyama, Earth History Supplies, NSW, Australia and Huanzala Mine, Peru, respectively. The samples were crushed through a rolls crusher and then screened to collect the + 0.6 3.2 mm particle size fraction. The processed samples were then sealed in polyethylene bags. The chemical composition of these samples analysed by ICP-MS is shown in Table 1. Two types of grinding media were used. The tapered cylinder mild steel grinding medium was supplied from Pasminco Mining, Elura, Australia and the spherical 30 wt.% chromium grinding medium from Magotteaux, Australia. The specific surface areas of
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Table 1 Chemical composition of the chalcopyrite and pyrite samples Minerals
Elements present (wt.%) Si
Fe
Ca
Mg
Mn
S
Pb
Cu
Zn
Chalcopyrite Pyrite
0.50 0.69
27.84 44.93
0.15 0.45
0.09 0.08
0.04 0.02
39.29 53.51
0.03 0.02
31.91 0.13
0.15 0.18
mild steel and chromium grinding media are 6.32 10 5 and 3.55 10 5 m2/g, respectively, calculated by the measurements of the total surface areas of 4 kg of grinding media. Sodium diisobutyl dithiophosphate (DTP, AR grade) and Dowfroth 250 (polypropylene oxide methanol, AR grade) were used in the experiments as collector and frother, respectively. Copper nitrate (Cu(NO3)2, AR grade) were used to introduce copper species during grinding. The pH was adjusted by the addition of AR grade NaOH solutions. Deionised water was used in all experiments. High purity nitrogen and oxygen from BOC Gases, Adelaide, Australia, were used throughout the study. 2.2. Mineral grinding and flotation Chalcopyrite or pyrite single mineral (100 g) or chalcopyrite –pyrite mixture (50 g each) was combined with 400 cm3 of water and ground with 4 kg of grinding medium in the specialised mill for 30 min so that 90 wt.% of the particles were less than 53 Am in diameter. The specific surface areas of mineral particles measured by BET after grinding are shown in Table 2. The mill was originally designed by Cases et al. (1990). During grinding, slurry was pumped through a monitoring cell where the pH was monitored and fixed at 9.0 by adding NaOH solution continuously. The slurry in the monitoring cell was purged with different gases to change the oxidation conditions in the mill. Samples for EDTA extractions and XPS analyses were obtained from the cell at the completion of grinding. After grinding, the pulp was transferred to a 1.5-dm3 flotation cell, conditioned with pH modifier, collector and frother. Four flotation concentrates were collected after cumulative times of 0.5, 2.0, 4.0 and 8.0 min at an air flow rate of 2.5 dm3 min 1. The flotation froth was scraped every 10 s. After grinding, the system was open to the air whilst the pH was fixed at 9.0. In chalcopyrite – pyrite mixture mineral experiments, the chalcopyrite Table 2 Specific surface areas of mineral particles after grinding Grinding medium
Mineral
Specific surface area (m2/g)
Mild steel 30 wt.% Chromium Mild steel 30 wt.% Chromium Mild steel 30 wt.% Chromium
Chalcopyrite Chalcopyrite Pyrite Pyrite Pyrite + Chalcopyrite Pyrite + Chalcopyrite
3.40 3.39 1.96 1.92 2.45 2.04
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recovery was calculated from the assay of copper, while pyrite recovery was calculated by the difference in mass and chalcopyrite recovery. The Eh (pulp potential) was measured at room temperature during grinding and conditioning using a HANNA meter combined with a platinum electrode and expressed relative to the standard hydrogen electrode, SHE. 2.3. Techniques 2.3.1. EDTA extraction A 3 wt.% solution of AR grade ethylene diamine-tetra acetic acid disodium salt (EDTA) was made up and a sodium hydroxide solution was used to adjust the pH to 7.5. 95 cm3 of the EDTA solution was placed in a vigorously stirred reaction vessel and purged with nitrogen for 5 min. A 5 cm3 of slurry sample from the mill discharge was frozen in liquid nitrogen first and then added to the EDTA solution, followed by 5 min of conditioning. Nitrogen was continuously purged throughout. The slurry was then filtered through a 0.45-Am Millipore filter. The filtrate was analysed for iron and copper by inductively coupled plasma (ICP) atomic emission spectrophotometry (Spectroflame M). The solids were retained to obtain the sample dry weight, enabling calculation of the mass of metal oxidation species per unit mass of solid (Rumball and Richmond, 1996) and per unit area of solid surface. 2.3.2. X-ray photoelectron spectroscopy (XPS) The species on the surface of chalcopyrite after different grinding were examined by XPS. XPS measurements were carried out with a Perkin-Elmer Physical Electronics Division (PHI) 5100 spectrometer with a Mg Ka X-ray source operating at 300 W and with a pass energy of 18 eV. The pressure in the analyzer chamber was 10 8 Torr during analysis. The energy scale was calibrated using the Fermi edge and 3d5/2 line (BE = 367.9 eV) for silver, whilst the retardation voltage was calibrated using the position of the Cu 2p3/2 peak (BE = 932.67 eV) and the Cu 3p3/2 peak (BE = 75.13 eV). The measurements were performed at a take-off angle of 45j. The slurry samples were washed with water of pH 9.0 to remove any species from solution and introduced into the fore-vacuum of the spectrometer as concentrated slurries. The samples were first examined in survey mode to identify all the elements present, then the various elemental regions were scanned in order to extract information on chemical bonding and oxidation stages. Details of this procedure have been reported previously by Smart (1991).
3. Results and discussion 3.1. Effect of grinding conditions on chalcopyrite flotation The effect of grinding media and gas purging on chalcopyrite flotation is shown in Fig. 1. It can be seen that 30 wt.% chromium medium produced a higher chalcopyrite recovery than mild steel medium. This is consistent with reports in literature that mild steel medium depresses chalcopyrite flotation (Rao et al., 1976; Forssberg et al., 1988; Van
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Fig. 1. Effect of grinding media and gas purging on chalcopyrite recovery as a function of flotation time: (dashed lines) mild steel medium; (solid lines) 30 wt.% chromium medium; (5) nitrogen purging; (D) air purging; (o) oxygen purging.
Deventer et al., 1991). Meanwhile, for both mild steel and 30 wt.% chromium grinding media, gas purging had little effect on chalcopyrite flotation. The samples from the mill discharge were analysed by EDTA extraction. The amount of extracted iron and copper is shown in Table 3. The EDTA extracted copper originated from the oxidation of chalcopyrite, whilst the EDTA extracted iron originated from the oxidation of chalcopyrite and grinding media. According to the mixed-potential theory (Fontana and Greene, 1978), when two materials are coupled, the oxidation rate of the more active material will increase and the oxidation rate of the less active material will decrease. It is well known that chalcopyrite is much more noble than the iron grinding media (Rao et al., 1976; Kocabag and Smith, 1985). As a result, during grinding, chalcopyrite oxidation is less pronounced than the medium oxidation and most EDTA extracted iron should emanate from the grinding media. This is in agreement with the results in Table 3 showing a much higher amount of EDTA extracted iron than copper. From the amount of EDTA extracted copper, more chalcopyrite was oxidised after grinding with 30 wt.% chromium medium than with mild steel medium. Oxygen purging increased chalcopyrite oxidation as expected. Eh values at the completion of grinding are also indicated in Table 3. A 30-wt.% chromium medium and oxygen purging resulted in higher Eh values, suggesting more oxidising grinding conditions, which is consistent with the higher amount of EDTA extracted copper. Table 3 Effect of grinding conditions on the amount of EDTA extracted iron and copper in chalcopyrite single mineral experiments Grinding condition (medium; gas)
Eh (mV)
Extracted Fe (mol/m2 mineral)
Extracted Cu (mol/m2 mineral)
Mild steel; oxygen Chromium; oxygen Chromium; nitrogen
195 220 135
1.19 10 4 3.26 10 5 1.05 10 5
1.07 10 6 1.42 10 6 9.14 10 7
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As discussed previously, iron oxidation species on chalcopyrite surfaces play the dominant role in depressing chalcopyrite flotation. It can be seen that mild steel medium produced a much higher amount of EDTA extracted iron than 30 wt.% chromium medium, in agreement with the lower chalcopyrite recovery by mild steel medium. However, oxygen purging also produced a higher amount of EDTA extracted iron than nitrogen purging, but the flotation results in Fig. 1 were similar after grinding with oxygen or nitrogen purging. Obviously, chalcopyrite flotation depends on the hydrophilic iron oxidation species as well as some hydrophobic oxidation species on the chalcopyrite surface. The same samples for EDTA extraction were also analysed by XPS. The type of surface species and their concentrations on chalcopyrite are reported in Table 4. As expected the iron concentration on chalcopyrite surface is higher with mild steel medium than with 30 wt.% chromium medium, and with oxygen purging than with nitrogen purging. The Fe2p spectra are shown in Fig. 2 and indicate that an iron oxyhydroxide layer is present on the chalcopyrite surface. This is shown by a broad band in the Fe2p spectrum near 711– 712 eV and near 726 eV, while the iron associated with sulphur as in chalcopyrite occurs at 708 eV (Buckley and Woods, 1984). It can be seen that the relative intensity of the iron oxyhydroxide band is higher when mild steel medium was used. However, this intensity decreased after chalcopyrite was ground with 30 wt.% chromium medium, especially under nitrogen purging. This observation is consistent with the amount of EDTA extracted iron in Table 3. The S2p spectra are shown in Fig. 3. According to Buckley and Woods (1984), the S2p spectrum of chalcopyrite is composed of three components and each component is made of a doublet. The peak near 161 eV is attributed to sulphide of unoxidised chalcopyrite. The two higher binding energy components around 162 and 164 eV are due to metaldeficient sulphide and polysulphide, respectively. Peaks for sulphate and thiosulfate at higher binding energies around 168 eV (Buckley and Woods, 1984) are present when oxygen was purged during grinding for both mild steel and 30 wt.% chromium grinding media. Fig. 3 shows that 30 wt.% chromium medium produced more sulphate on the chalcopyrite surface than mild steel medium, whilst oxygen purging produced more sulphate and polysulphide than nitrogen purging in agreement with the EDTA exaction results. Although the type of gas used during grinding produced similar chalcopyrite recoveries, the type and proportion of the surface oxidation products were different. Higher proportions of polysulphide, sulphate and ferric oxide/hydroxide were formed on chalcopyrite surface with oxygen purging than with nitrogen purging. These results Table 4 Concentration (at.%) of the elements measured by XPS on the surface of chalcopyrite Grinding condition (medium; gas)
Atomic concentration of element (%) Fe
S
Cu
O
Mild steel; oxygen Chromium; oxygen Chromium; nitrogen
12.8 6.3 4.7
36.2 43.6 58.4
2.6 2.4 2.3
48.4 47.7 34.6
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Fig. 2. Fe2p XPS spectra of the chalcopyrite surface after grinding: (a) mild steel medium with oxygen purging; (b) 30 wt.% chromium medium with oxygen purging; (c) 30 wt.% chromium with nitrogen purging.
indicate that chalcopyrite flotation is closely dependent on iron oxidation species, which depress chalcopyrite flotation, and metal deficient sulphide, which increases chalcopyrite flotation. Similar observations have been found by Fairthorne et al. (1997) who demonstrated that the hydrophobicity of chalcopyrite will increase with the formation of a sulphur-rich surface but will be offset by the precipitation of hydrophilic metal hydroxide species. 3.2. Effect of grinding conditions on chalcopyrite separation from pyrite The effect of grinding conditions on the flotation of chalcopyrite and pyrite in mineral mixture experiments is indicated in Fig. 4. It can be seen that in agreement with the single mineral flotation results (Fig. 1), 30 wt.% chromium medium produced higher chalcopyr-
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Fig. 3. S2p XPS spectra of the chalcopyrite surface after grinding: (a) mild steel medium with oxygen purging; (b) 30 wt.% chromium medium with oxygen purging; (c) 30 wt.% chromium with nitrogen purging.
ite recovery than mild steel medium and gas purging had little effect on chalcopyrite flotation. Furthermore, mild steel medium produced higher pyrite recovery than 30 wt.% chromium medium with gas purging during grinding having no or little influence on pyrite flotation. Therefore, higher chalcopyrite selectivity against pyrite was produced by 30 wt.% chromium medium than by mild steel medium. Yelloji Rao and Natarajan (1989) reported that galvanic interactions between a noble mineral and an active mineral markedly affected the floatability of the noble mineral, but had a minimal effect on the floatability of the active mineral. The chalcopyrite flotation rate constant and maximum recovery in the experiments of chalcopyrite single mineral and chalcopyrite – pyrite mixture were calculated by fitting the experimental recovery data to the first order rate equation (Lynch et al., 1981): Recovery ðtÞ ¼ Rmax ð1 ekt Þ
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Fig. 4. Effect of grinding media and gas purging on the separation of chalcopyrite from pyrite: (empty symbols) mild steel medium; (solid symbols) 30 wt.% chromium medium; (squares) nitrogen purging; (triangles) air purging; (circles) oxygen purging.
where Rmax (%) is the maximum recovery at infinite time and k (min 1) is the first order rate constant. The rate constant and maximum recovery are shown in Table 5. After pyrite addition, the chalcopyrite flotation rate constants decreased, but the maximum recoveries remained within experimental error. It was also noted that pyrite exhibited a different flotation behaviour after mixed with chalcopyrite. As shown in Fig. 5, pyrite single mineral displayed a poor floatability with less than 10% recovery after 8 min of flotation. However, pyrite flotation was increased after addition of chalcopyrite, especially with the mild steel medium (Fig. 4). Samples of chalcopyrite– pyrite mixture ground in different conditions were conditioned with EDTA solutions. The extracted amounts of iron and copper are shown in Table 6. Higher iron oxidation production was produced with mild steel medium than with 30 wt.% chromium medium and with oxygen purging than with nitrogen purging. A comparison of the amount of extracted iron in Tables 6 and 3 reveals similar trends and only a small increase in iron oxidation after chalcopyrite was mixed with pyrite. This increase in surface iron oxidation may contribute to the lower chalcopyrite flotation rate constant observed in chalcopyrite – pyrite mixture experiments. Attention now turns to the activation of pyrite flotation by chalcopyrite. Two different mechanisms for the increased floatability of pyrite after chalcopyrite addition have been proposed (Ekmekcß i and Demirel, 1997): (i) increased proportion of elemental or metalTable 5 Chalcopyrite flotation rate constant and maximum recovery in single mineral and mixture mineral experiments Grinding condition (medium; gas)
Rate constant (min 1)
Recovery maximum (%)
Cp
Cp + Py
Cp
Cp + Py
Mild steel; oxygen Mild steel; nitrogen Chromium; oxygen Chromium; nitrogen
1.6 1.2 2.1 2.4
1.1 1.1 1.6 1.9
75 76 87 88
75 77 86 86
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Fig. 5. Pyrite single mineral flotation as a function of flotation time, with and without Cu2 + addition, under oxygen purging: (o) without Cu2 +; ( ) with 1.5 10 3 M Cu2 +; (dashed lines) mild steel; (solid lines) 30 wt.% chromium medium.
.
deficient sulphur on the pyrite surface and (ii) copper activation. Regarding mechanism (i), because chalcopyrite is more active than pyrite, ferric hydroxide may be reduced to a more soluble iron species, ferrous hydroxide, and thus elemental or metal-deficient sulphur will be generated on pyrite surface, which results in an improved pyrite flotation in the presence of chalcopyrite. A similar mechanism should also occur for pyrite in the absence of chalcopyrite in this study as both the mild steel and 30 wt.% chromium grinding media are more active than pyrite. However, Fig. 5 shows that pyrite flotation was poor in absence of copper. This concluded that the improved pyrite flotation in the experiments of chalcopyrite – pyrite mixture is not due to elemental or metal-deficient sulphur but rather to copper activation of pyrite flotation. It is generally accepted that the formation of a new copper sulphide phase on the pyrite surface through the reduction of copper(II) to copper(I) is responsible for the activation of pyrite while sulphide ions are oxidised (Bushell and Krauss, 1962; Weisener and Gerson, 2000). Laajalehto et al. (1999) proposed that a chalcopyrite-like phase (CuFeS2) might be formed during the activation of pyrite because the copper-activated pyrite exhibited the typical S2p XPS spectrum of copper sulphides (e.g. chalcocite or chalcopyrite) and similar electrochemically controlled xanthate adsorption behaviour as chalcopyrite. A comparison of the amount of EDTA extracted copper in Tables 6 and 3 shows that a much lower amount of copper hydroxide Table 6 Effect of grinding conditions on the amount of EDTA extracted iron and copper in chalcopyrite – pyrite mixture experiments Grinding condition (medium; gas)
Extracted Fe (mol/m2 mineral)
Extracted Cu (mol/m2 mineral)
Mild steel; oxygen Chromium; oxygen Chromium; nitrogen
1.25 10 4 4.43 10 5 1.58 10 5
2.57 10 7 5.78 10 7 2.86 10 7
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Fig. 6. Eh during grinding of pyrite in the presence of chalcopyrite (D) or Cu(II) (5) under oxygen purging: (dashed lines) mild steel; (solid lines) 30 wt.% chromium medium.
was extracted from the chalcopyrite – pyrite mixture than from chalcopyrite in the absence of pyrite even if considering that the amount of chalcopyrite in the mixture was half that in single mineral. Therefore, it appears that the improvement in pyrite flotation in the presence of chalcopyrite is due to copper activation. This was further confirmed by pyrite flotation in the presence of copper(II). In Fig. 5, 1.5 10 3 mol/l of copper(II), based on the amount of EDTA extracted copper in Table 3, was added during pyrite grinding. It can be seen that addition of copper(II) had exactly the same effect on pyrite flotation as chalcopyrite. Pyrite flotation was improved in the presence of copper(II), especially after grinding with mild steel. Fig. 6 shows the Eh during grinding as a function of grinding time. In the presence of chalcopyrite or copper(II), mild steel medium always produced lower Eh values than 30 wt.% chromium medium. Voltammetric studies have shown that activation of pyrite by copper is strongly dependent on the redox potential in the activating solution and that increases in oxidising potential inhibit copper uptake (Richardson et al., 1996). This may explain why pyrite flotation was increased further after grinding with mild steel medium in the presence of copper(II) in Fig. 5. Since chalcopyrite displayed the same effect on pyrite flotation as copper(II), the effect of grinding media on copper(II) reduction could be studied by analysing the pyrite surface after grinding in the presence of copper(II) ions. Table 7 shows that 30 wt.% chromium medium resulted in slightly more copper on the pyrite surface than mild steel medium. However, it is copper(I) which is responsible for pyrite activation. In order to quantify the amount of copper(I) on the pyrite surface, Cu2p Table 7 Concentration (at.%) of the elements measured by XPS on pyrite surface in the presence of 1.5 10 3 M Cu(II) Grinding condition (medium; gas)
Atomic concentration of element (%) Fe
S
Cu
O
Mild steel; oxygen Chromium; oxygen
20.8 20.3
20.7 26.4
1.7 2.1
56.8 51.2
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Fig. 7. Cu2p XPS spectra of the pyrite surface in the presence of 1.5 10 3 M Cu2 +: (a) mild steel medium with oxygen purging; (b) 30 wt.% chromium medium with oxygen purging.
XPS spectra were fitted as shown in Fig. 7. The relatively smaller intensity of the broad band observed around 943.4 eV is attributed to copper(II) shake-up satellites and the peak of 934.4 eV, attributed to cupric oxide or hydroxide indicates that the surface copper, is mainly in a cuprous state with the most intense band at around 932.3 eV (Fairthorne et al., 1997). The percentage of copper(I) on pyrite surface after grinding with mild steel medium and 30 wt.% chromium medium is 79% and 54%, respectively, indicating that more pyrite activation occurred with mild steel medium than with 30 wt.% chromium medium in agreement with the flotation results in Fig. 5.
4. Conclusion Grinding conditions had a significant effect on chalcopyrite flotation. This effect was closely associated with the presence of iron oxidation species and metal deficient sulphide present on the chalcopyrite surface. Iron oxidation species from grinding media played a dominant role in depressing chalcopyrite flotation, whilst the presence of metal deficient sulphide improved chalcopyrite flotation. As a result, 30 wt.% chromium medium produced a higher chalcopyrite recovery than mild steel medium, whilst gas purging during grinding had little effect on chalcopyrite flotation for the same grinding medium. When chalcopyrite was mixed with pyrite, the amount of iron oxidation species from grinding media was increased, which resulted in a lower rate of chalcopyrite flotation.
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Pyrite flotation was significantly improved especially after grinding with mild steel medium. This improvement in pyrite flotation was due to copper(II) activation. Grinding media had a large effect on copper(II) reduction on pyrite surface. Mild steel medium caused more reducing grinding condition and less pyrite oxidation than 30 wt.% chromium medium. Thus, more copper(II) species were reduced to copper(I) species on the pyrite surface, which resulted in more pyrite activation by mild steel medium. Consequently, 30 wt.% chromium medium produced better chalcopyrite selectivity against pyrite than mild steel medium.
Acknowledgements Financial support for this work from AMIRA International as well as scholarships to Y. Peng from the University of South Australia are gratefully acknowledged. Pasminco Mining, Elura, Australia and Magotteaux, Australia are thanked for the supply of grinding media.
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