Diethylenetriamine depression of Cu-activated pyrite hydrophobised by xanthate

Minerals Engineering 57 (2014) 36–42

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Diethylenetriamine depression of Cu-activated pyrite hydrophobised by xanthate Eric A. Agorhom ⇑, W. Skinner, M. Zanin Ian Wark Research Institute, The ARC Special Research Centre for Particle and Interfaces, University of South Australia, Mawson Lakes Campus, Adelaide, SA 5095, Australia

a r t i c l e

i n f o

Article history: Received 24 August 2013 Accepted 6 December 2013 Available online 28 December 2013 Keywords: Flotation Depression Diethylenetriamine Surface analysis Pyrite

a b s t r a c t In copper sulphide flotation, copper adsorbs on pyrite through superficial oxidation of the copper minerals (e.g. chalcopyrite) which promotes pyrite flotation in the presence of xanthate. This ‘‘inadvertent’’ activation of pyrite by copper ions is undesirable in copper sulphide flotation. In order to minimise this effect, depressants are used to suppress the effect of the activating ions. The effect of diethylenetriamine (DETA) in different combinations (under aerated and non-aerated conditions) on Cu-activated pyrite hydrophobised by xanthate was examined using flotation, spectroscopic and solution analyses, at pH 10. The results showed that DETA affects the flotation behaviour and surface chemistry of pyrite. However, high dosages are required. The depression action of DETA on Cu-activated pyrite was attributed to both the removal of surface copper to form soluble Cu–DETA complex in solution and competition for Cu sites on the activated pyrite surface. The significant depression of pyrite in the presence of DETA under the aerated condition was due to increased amount of iron oxy–hydroxides (Fe–O/OH), copper oxides and Cu(I)–DETA hydrophilic species. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Chalcopyrite and gold are the main valuable mineral phases in most polymetallic sulphide ores, with pyrite being the main sulphide gangue mineral. Chalcopyrite and gold are mostly associated with pyrite in these ores; therefore, their economical extraction requires a selective depression of pyrite. The major problem associated with chalcopyrite and pyrite selectivity is due to accidental activation of pyrite by dissolved Cu2+ or Pb2+ ions from complex sulphide minerals through oxidation/dissolution reactions, which may be enhanced by galvanic interactions. This enhances pyrite interaction with thiol collectors and hence promotes its floatability. In most processing plants, sodium cyanide is employed to depress pyrite (by removing Cu2+ ions from its surfaces) under alkaline conditions. The environmental and health hazards associated with cyanide usage make it an unsafe approach to depress sulphides in selective flotation. As an alternative to cyanide, sulphur–oxy depressants, in the form of sul2 2 phite (SO2 3 ), bisulphite (HSO3 ), metabisulphite (S2 O5 ) or sulphur dioxide (SO2 ) have been used in depressing pyrite, sphalerite and 2 galena (Grano et al., 1997a, 1997b; Khmeleva et al., 2002, 2003; Shen et al., 2001; Chander and Khan, 2000; Bulut et al., 2011). The general mechanism of sulphide mineral depression using sulphite ions involves the formation of metal sulphite hydrophilic ⇑ Corresponding author. Tel.: +61 883023692. E-mail address: [email protected] (E.A. Agorhom). 0892-6875/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2013.12.010

species, decomposition of xanthate and consumption of oxygen (reduction in Eh) (Misra et al., 1985; Yamamoto, 1980; Miller, 1970). Despite numerous studies on the mechanism of sulphite ions in pyrite depression, little effort has been made to understand the effect of diethylenetriamine (DETA) in selective depression of pyrite in sulphide minerals. DETA is a polyamine and soluble in water and alcohol (Weast, 1985). Generally, polyamines are excellent complexing or chelating agents. The application of DETA in flotation is to depress sulphide minerals and has been used extensively in pyrrhotite rejection in Ni–Cu ore flotation. It has been observed by Falconbridge Ltd., that DETA was only effective in rejecting pyrrhotite in the presence of SO2 (Kelebek et al., 1995). The mechanism of DETA in the rejection of pyrrhotite was linked to the removal of activating ions (e.g. Cu2+ and Ni2+). The removal of activating ions from pyrrhotite surfaces in the presence of DETA resulted in a decreased xanthate adsorption and hence reduced its floatability (Yoon et al., 1995). In another related operation (Inco operations), it was observed that DETA was only effective when pyrrhotite was extensively oxidised. This is because the activation product on the pyrrhotite surface (CuS species) is insoluble in the reducing condition which is not soluble in DETA (Yoon et al., 1995). However, when pyrrhotite is oxidised, the activation product is converted to oxide/hydroxides of copper and nickel, which are more soluble in DETA. The effective role of DETA in depressing pyrrhotite has led to an investigation into its applicability in depressing other sulphide minerals like pyrite. Sui et al. (1998) have shown that DETA

E.A. Agorhom et al. / Minerals Engineering 57 (2014) 36–42

can be a good depressant for pyrite. The depression mechanism of DETA on pyrite involves the removal of activating ions (in this case, Pb2+) and the formation of a hydrophilic Pb–DETA complex which adsorbs back onto the pyrite surface to compete with xanthate for adsorption sites. Although the use of DETA for depressing pyrite is as good as sulphur–oxy depressants, limited work has been conducted on this depressant (Sui et al., 1998; Zanin and Farrokhpay, 2011). In this study, the effect of DETA in combination with aeration of Cu-activated pyrite in flotation was investigated. Surface (X-ray spectroscopic, XPS and EDTA extraction technique) and solution (Ultraviolet visible spectroscopic, UV–visible and inductively coupled mass spectroscopy, ICP-MS) analytical techniques were used to understand the mechanisms of DETA on Cu-activated pyrite depression.

2. Materials and methods 2.1. Chemical composition A high grade pyrite sample was obtained from a copper mine in Peru. The chemical/elemental compositions of the pyrite sample analysed by inductively coupled plasma mass spectroscopy (ICPMS) is shown in Table 1.

37

was examined, at pH 10. The experimental procedure is shown schematically in Fig. 1. 2.4. Rest potential measurement The rest potential for the pyrite in the pulp during conditioning and flotation was measured using a TPS 90-FLMV pH/Eh/DO combined meter. The pH and Eh probes were calibrated using pH buffer solutions of 7 and 10 and Light’s solution (Light, 1972). 2.5. X-ray photoelectron spectroscopy (XPS) The X-ray spectroscopy (XPS) measurements were performed using a Kratos Axis-Ultra X-ray photoelectron spectrometer equipped with a delay-line detector. A monochromatised Al Ka X-ray source was used, operating at 300 W and the spectrometer analysis area was 0.3  0.7 mm dimensions. The broad scan survey and high-resolution spectral data were processed using CasaXPS version 2.3.5. The Cu-activated pyrite samples were taken from the flotation experiments (Fig. 1) and the following sample streams (Py + Cu + X, Py + Cu + X + 500 g/t DETA, Py + Cu + X + Air and Py + Cu + X + Air + 500 g/t DETA) were analysed. For the XPS analysis, each sample was analysed for Cu, Fe, S, O, C and N in terms of their surface atomic concentration (.%). High resolution scans were also collected of the C 1s, O 1s, Fe 2p, Cu 2p and S 2p photoemission lines.

2.2. Reagents 2.6. EDTA extraction technique All the chemical reagents were of analytical grade. Potassium amyl xanthate (CH3CH2OCS2K, abbreviated PAX) and methyl isobutyl carbinol (MIBC) were used as collector and frother, respectively. PAX and MIBC were supplied by Orica Mining and Cytec Chemicals, respectively. Copper sulphate (CuSO4) was used as an activator for pyrite during the conditioning time. Diethylenetriamine (NH2–CH2–CH2–NH–CH2–CH2–NH2, abbreviated DETA, 99% pure) supplied by Sigma–Aldrich, and air were used as depressants. The pulp pH was kept constant at 10 by means of lime and hydrochloric acid additions. Ethylene diaminetetraacetic acid di-sodium salt (abbreviated EDTA, 99% pure), supplied by AJAX CHEMICALS. The reagents concentrations used were; CuSO4 (200 g/t), PAX (100 g/t), MIBC (40 g/t), DETA (200 and 500 g/t) and air (2 dm3/ min for 5 min). Each reagent was prepared as 1% solution strength before used in the flotation process.

Ethylene diaminetetraacetic acid (EDTA) was used to extract metal oxidation products (e.g. oxide/hydroxide, sulphate, carbonate, etc.) from the pyrite surface (Kant et al., 1994; Rumball and Richmond, 1996; He, 2003), in the following: Py + Cu + X, Py + Cu + X + 500 g/t DETA, Py + Cu + X + Air and Py + Cu + X + Air + 500 g/t DETA sample streams. A pulp volume of 0.1 dm3 was mixed with a 3% AR grade EDTA solution and conditioned for 5 min while purging with nitrogen to prevent further oxidation of the pyrite surface. The EDTA solution was purged with nitrogen for 10 min before extraction to remove residual oxygen. The amount of surface oxidation products extracted by EDTA was measured in solution by inductively coupled plasma mass spectroscopy (ICP-MS). 2.7. UV–visible spectroscopy

2.3. Sample preparation and flotation 3

The experiment was carried out in a 0.5 dm Gliwice mechanically agitated flotation cell. Ceramic agate mortar and pestle were used to grind the pyrite sample to produce particles of d80 = 38 lm. The particle size distribution of the feed pyrite was determined by Malvern MasterSizer (Malvern Instrument Ltd., UK). The ground products were divided into 50 g each, kept under a desiccator to remove all moisture content before storing in a freezer below 4 °C to minimise surface oxidation. After grinding, 50 g of the sample was pulped in the flotation cell and agitated for 5 min before reagent conditioning. In these tests, the effect of depressant, DETA (at different dosages) under aerated and non-aerated conditions

An Evolution 201 UV–visible spectrophotometer was used to determine the concentration of xanthate remaining in solution before and after treatment with DETA. Identifying the peaks (for xanthate and DETA) in the process water was difficult due to the presence of other mobile ions, hence demineralised water at neutral pH was used for all the experiments. A 0.1 dm3 beaker containing demineralised water was maintained at neutral pH 7 prior to potassium amyl xanthate addition. All spectra were recorded over the range 200–400 nm. In addition to xanthate in solution, adsorption spectrum was recorded for Cu-activated pyrite induced by xanthate in the presence of DETA. 3. Results

Table 1 Chemical composition of the pyrite sample.

3.1. Flotation

Elements (wt.%)

Pyrite

Fe

S

Cu

Ca

Si

Pb

Mg

Zn

Mn

Py

44.9

53.5

0.13

0.44

0.69

0.02

0.08

0.18

0.02

>98

3.1.1. Effect of DETA and aeration The effect of DETA on pyrite rejection was studied under two different test conditions; (1) non-oxidising (or non-aerated) and (2) oxidising, (or aerated) condition. The experimental results

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E.A. Agorhom et al. / Minerals Engineering 57 (2014) 36–42

DETA (200 g/t, 500 g/t) 3 min Air, 2 dm 3/min (5 min) PAX (100 g/t) 2 min

CuSO4 (200 g/t) 3 min

MIBC (40 g/t) 1 min

Feed

Tail

d80 = 38 µm

Fig. 1. Schematic representation of the experimental procedure for studying the effect of DETA in single mineral pyrite flotation.

under non-aerated condition are presented in Fig. 2a. The results showed that pyrite recovery without air and DETA was high (92 wt.%) in the presence of xanthate. However, under aerated condition, pyrite recovery decreased from 92 wt.% to 65 wt.% (Fig. 2b). This can be attributed to the formation of ferric hydroxide species which passivate pyrite surface and prevent adsorption of both Cu and xanthate (Owusu et al., 2013). Addition of 200 g/t DETA without aeration also decreased pyrite recovery substantially (from 92 to 44 wt.%). Increased DETA concentration (500 g/t) further decreased pyrite recovery, from 92 to 33 wt.% (Fig. 2a). The effect is enhanced when aeration and DETA are used in combination (Fig. 2b). This shows that DETA can depress Cu-activated pyrite flotation induced by xanthate. However, high dosages are required, which is consistent with other studies in the literature (Sui et al., 1998). DETA addition prior to collector (PAX) resulted in drastic reduction in pyrite recovery, from 92 to 13 wt.% (Fig. 2a). This can be attributed to both the removal of activated copper and adsorption of DETA onto the active sites of pyrite to compete with xanthate. The latter can lead to the formation of Cu–DETA hydrophilic species on pyrite surface and thus hinder the formation of Cu–xanthate (hydrophobic entity) for improved pyrite flotation.

The depression of pyrite in the presence of DETA is also described by the initial rate constants, k0, under the experimental conditions (Table 2). In the absence of air and DETA, the initial flotation rate constant of pyrite was high (1.06 min1). However, in the presence of air, 200 g/t DETA and 500 g/t DETA, the initial pyrite flotation rate constants decreased to 0.61 min1, 0.29 min1 and 0.20 min1, respectively. Similar lower flotation rate constant of pyrite (0.02 min1) was observed when DETA was added before xanthate conditioning. Also, lower pyrite flotation initial rate constants were observed when DETA was applied after aeration of the pulp and this could be responsible for the significant depression of pyrite (Table 2). The results indicate that DETA addition and aeration decreased pyrite recovery significantly than either DETA or air alone. 3.1.2. Rest potentials The open-circuit or rest potential measured during the flotation involving DETA is shown in Fig. 3. Addition of DETA decreased the rest potential by 35 mV (from 257 to 222 mV). The decreased rest potential may be due to the ability of DETA to form chelates with Fe2+ ions resulting in their stability. This may slow down

100

100

(a)

(b)

Py+Cu+X+Air Py+Cu+X+Air+200 g/t DETA

80 Py+Cu+X Py+Cu+X+200 g/t DETA Py+Cu+X+500 g/t DETA Py+Cu+500 g/t DETA+X

60

40

Pyrite recovery (%)

Pyrite recovery (%)

80

Py+CU+X+Air+500 g/t DETA

60

40

20

20

0

0 0

2

4

6

8

10

0

2

4

6

8

10

Flotation time (min)

Flotation time (min)

Fig. 2. Effect of DETA on depression of Cu-activated pyrite induced by xanthate (PAX) at pH 10 under (a) non-aerated condition and (b) aerated condition. [CuSO4] = 200 g/t, [PAX] = 100 g/t, [DETA] = 200 g/t and 500 g/t and aeration rate was 2 dm3/min for 5 min.

Table 2  The initial rate constants of Cu-activated pyrite after conditioning and flotation with DETA at different dosages and conditions (aerated and non-aerated), k0 ¼ k Rmax . Rate constant

Xanthate

X + 200 g/t DETA

X + 500 g/t DETA

500 g/t DETA + X

X + Air

X + Air + 200 g/t DETA

X + Air + 500 g/t DETA

k0 k

1.06 ± 0.03 1.16 ± 0.03

0.29 ± 0.01 0.69 ± 0.01

0.20 ± 0.01 0.64 ± 0.01

0.02 ± 0.01 0.14 ± 0.01

0.61 ± 0.02 0.95 ± 0.02

0.18 ± 0.01 0.67 ± 0.01

0.06 ± 0.01 0.48 ± 0.01

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E.A. Agorhom et al. / Minerals Engineering 57 (2014) 36–42

Open circuit potential (mV SHE)

260 250 240 230 220 210

Open-Circuit Potential (mV SHE)

200 190 0

CuSO4

Xanthate

Aeration

DETA st

DETA ed

Condition Fig. 3. Open-circuit or rest potential after DETA addition at pH 10. [DETA] = 500 g/t, [PAX] = 100 g/t, [CuSO4] = 200 g/t and [Air] = 2 dm3/min for 5 min.

the oxidation process of xanthate to dixanthogen, which is the main xanthate product responsible for pyrite flotation. 3.2. Surface/solution chemical effect 3.2.1. XPS study The results in Fig. 4a show the XPS analysis of Cu-activated pyrite induced by xanthate before and after conditioning with DETA, under non-aerated condition. After contacting DETA with Cu-activated pyrite, it was observed that the amount of Cu on pyrite surface decreased considerably. About 75% of the Cu was removed from pyrite surface in the presence of DETA. This suggests that DETA is an effective Cu deactivator. The surface exposure of N on pyrite concomitantly increases after contacting with DETA. This finding suggests that DETA does not only remove copper from the activated pyrite but also adsorb onto the pyrite surface, perhaps displacing and/or preventing further interaction with collector (xanthate). The surface Cu exposure was too low after DETA treatment for the collection of a high resolution Cu 2p spectrum, however the Cu 2p3/2 component acquired in the survey spectrum appeared to have broadened somewhat on the high binding energy side. No Cu(II) satellite contributions could be discerned, which means that some DETA remains bound to Cu sites at the surface in a Cu(I)–DETA form. This would seem quite plausible as a similar broadening, to higher binding energy, of the Cu 2p3/2 has been observed for amine adsorption on fine Cu powders (Unwin et al., 2008). The concentration of Fe in the presence of DETA decreased significantly for the Cu-activated pyrite. This may be due to

passivation (masking) of surface Fe by increased amount of C and N. The reduction in Fe could also be due to the formation of aqueous complex of DETA with oxidised ferrous iron (Kelebek, 1996). This condition may slow down the rate of the oxidation of pyrite resulting in the formation of ferric hydroxyl xanthate, which is one of the species responsible for impacting hydrophobicity of pyrite for improved flotation. Under the aerated condition (Fig. 4b), DETA removes Cu from the pyrite surface and, in the process, collector is removed. It is suggested that DETA competes with collector-Cu, displacing the collector (due to decrease in C concentration) and increase in both Fe and S. It is also observed that some of the DETA binds with Cu on the surface (appearance of significant N) oxidising it to Cu(II) in the process. Consequently, the concentration of the surface copper responsible for collector adsorption decreased significantly (i.e. from 3.4 to 0.9 .%). After conditioning, the aerated Cu-activated pyrite with DETA, the surface Fe exposure increases significantly, and this may be due to the relative contribution of iron from oxidised species (Fe–Ox) to the total surface Fe. Clearly, the use of DETA after aeration has a greater effect on the surface chemistry of pyrite particles compared to either DETA or air alone. In the presence of air and DETA, more iron oxide (Fe–Ox), copper oxide/ hydroxides and Cu–DETA species (increased amount of N) were formed on the pyrite surface increasing the surface hydrophilicity and hence decreasing pyrite floatability (Fig. 2b). The Fe 2p and Cu 2p XPS spectra in Fig. 5a and b further confirm the presence of iron oxy–hydroxide and copper oxide/hydroxide species on the surface of the Cu-activated pyrite in the presence of air and DETA.

3.2.2. EDTA extractable surface metal species To further understand the mechanism of DETA depression of Cu-activated pyrite under aerated and non-aerated conditions, EDTA extraction was performed on the aerated and non-aerated samples before and after DETA addition. Table 3 shows the amount of Cu and Fe oxidation species extracted by EDTA solution. For the non-aerated condition, the amount of EDTA extractable copper increased (from 0.032 to 0.038 mg/g) while the amount of EDTA extractable iron decreased (from 0.086 to 0.074 mg/g) after DETA addition, as shown in Table 3. A similar trend in terms of the concentrations of Cu and Fe oxide/hydroxide species was observed when DETA was added after aeration (Table 3). With respect to copper oxide/hydroxide species, the EDTA extraction and XPS results correlate for both test conditions. This confirms the observation made under the aerated condition that adsorption of DETA on pyrite surface results in subsequent copper oxidation, from

50

50 Py+Cu+X+Air Py+Cu+X+Air+500 g/t DETA

40

Atomic Concentration (%)

Atomic Concentration (%)

Py+Cu+X Py+Cu+X+500 g/t DETA

(a) 30

20

10

0

Cu

Fe

O

C

Surface Species

S

N

40

(b) 30

20

10

0

Cu

Cu(I) Cu(II) Fe Fe-Ox

O

C

S

N

Surface Species

Fig. 4. Surface species present on Cu-activated pyrite induced by xanthate in the presence of 500 g/t DETA under (a) non-aerated and (b) aerated conditions. ‘‘Py’’ and ‘‘X’’ denote pyrite and xanthate, respectively.

40

E.A. Agorhom et al. / Minerals Engineering 57 (2014) 36–42

Fe 2p

Fe(II)-S

Fe(II)/Fe(III)-O

Cu 2p

Cu(I)-S

Py+Cu+X+Air

(a)

(b) Py+Cu+X+Air Cu(II)-O

Py+Cu+X+Air+500 g/t DETA Py+Cu+X+Air+500 g/t DETA

Fig. 5. Fe 2p (a) and Cu 2p (b) XPS spectra of Cu-activated pyrite in air and air plus DETA.

Table 3 Surface Cu, S and Fe oxidation species present on the pyrite surface as extracted by EDTA for Cu-activated pyrite in the presence of DETA under non-aerated and aerated conditions. Test condition

Xanthate X + DETA X + Air X + Air + DETA

Metal oxidation species extracted by EDTA (mg/g of pyrite) Cu

Fe

0.033 ± 0.002 0.038 ± 0.002 0.029 ± 0.002 0.036 ± 0.002

0.086 ± 0.002 0.074 ± 0.002 0.098 ± 0.002 0.069 ± 0.002

Cu(I) to Cu(II) (Fig. 4b). Also, the reduction in the oxidised Fe concentration in the presence of DETA suggests a possible complexation of Fe(II) with DETA. Thus, we therefore propose that adsorption of DETA onto pyrite surface through Cu–DETA complex formation promotes surface copper oxidation. 3.2.3. UV–visible study The adsorption spectra for potassium amyl xanthate (PAX) in solution, in the absence and in the presence of DETA, have been recorded at pH 7 as shown in Fig. 6a. In solution, the xanthate peak at 301 nm (Hao et al., 2000, 2008; Khmeleva et al., 2002; Montalti et al., 1991) was identified but disappeared from solution in the presence of Cu-activated pyrite. This shows a complete adsorption of xanthate onto the pyrite surface. The addition of DETA to PAX, alone, did not consume xanthate. In the presence of xanthate-trea-

ted Cu-activated pyrite, the effect of DETA is to form a soluble product (at peak 250 nm) suspected to be a Cu(DETA)2+ complex (Fig. 6b). Kelebek (1996) showed, for the Cu–Fe–S–DETA system at pH 7, that the most dominant solution species was Cu(DETA)2+ (Fig. 7). In order to confirm the presence of Cu(DETA)2+, a mixture of Cu and DETA in 1:2 M ratio was prepared and analysed by UV–visible spectroscopy. The identified peak correlates with the peak obtained when Py + Cu + X was treated with DETA (Fig. 6a). This finding supports the observations of earlier workers that DETA can desorb Cu from pyrrhotite surfaces (Xu et al., 1997). Also, conditioning of Cu-activated pyrite with DETA before xanthate addition shows little or no loss of xanthate from solution (Fig. 6b). This observation, together with the XPS surface analysis, shows adsorption of DETA onto the active sites of pyrite through complex formation with copper with a much higher affinity than xanthate. This is consistent with similar observation made on Pb-contaminated pyrite interaction with DETA (Sui et al., 1998). Therefore, this result indicates that DETA can both desorb Cu and adsorb on more ‘‘robust’’ Cu sites on pyrite surface. This correlates well with the flotation depression of pyrite in the presence of DETA (Figs. 2, 4 and 5). 4. Discussion 4.1. DETA and aeration In the present study, it has been demonstrated that DETA can depress the flotation of copper-activated pyrite induced by 5

5

Absorbance

Xanthate (301 nm)

3

(b)

X X+DETA DETA Cu+DETA Cu+DETA+X

4

Absorbance

(a) 4

Cu-DETA (250 nm) 2

1

0 200

DETA (303 nm)

250

300

Wavelength, nm

350

Xanthate (301 nm)

3

X+DETA X DETA+X

2

Cu-DETA (250 nm)

1

400

0 200

250

300

350

400

Wavelength, nm

Fig. 6. UV absorbance spectra of potassium amyl xanthate in solution (a) and Cu-activated pyrite hydrophobised by PAX (b) in the presence of DETA at pH 7. For the Cu–DETA complex, Cu and DETA were mixed in 1:2 M ratios. [DETA] = [Cu] = 1% solution strength.

E.A. Agorhom et al. / Minerals Engineering 57 (2014) 36–42

Cu-species disrtibution (%)

100

Cu2+

80

CuOHDETA+

CuDETA2+

60

40

20

CuHDETA3+ Cu+

41

pyrite) or by deliberate activation of pyrite with copper sulphate in porphyry copper–gold flotation. The latter is mostly done to increase gold recovery as a result of enhanced pyrite flotation in the rougher stage. However, in the cleaner stage, where selectivity between chalcopyrite, gold and pyrite is required to improve copper and gold grades, pyrite liberation and depression becomes crucial. The findings from this study suggest that DETA can be an advantageous depressant to reject Cu-activated pyrite hydrophobised by xanthate. Furthermore, not only will DETA be able to depress pyrite but may also clean chalcopyrite surface of oxidation products (e.g. Cu(OH)2) due to its affinity for copper ions (Fig. 7). This will enhance subsequent chalcopyrite interaction with collector and thus improve copper recovery and grade, and selectivity over pyrite (Kelebek, 1996).

0 2

4

6

8

10

12

pH Fig. 7. Species distribution of Cu in Cu–Fe–S–DETA system, [DETA] = 103M (modified after Kelebek, 1996).

xanthate at a pH 10. The flotation results in Fig. 2a and b indicate that DETA is even more effective depressant for pyrite activated by Cu and xanthate when used in aerated condition than in non-aerative condition. However, higher dosages of DETA are required. Oxidation of Cu-activated pyrite before DETA addition proved to be the most effective mechanism of pyrite depression in the presence of DETA. Cu(II) is reduced, during the adsorption process, to Cu(I) via sulphur oxidation to form a Cu(I)S surface species (Voigt et al., 1994). Specifically at the surface of pyrite, and in the vicinity of the reaction, this may be represented by (von Oertzen et al., 2007): þ FeS2 þ 3xCu2þ þ 4xH2 O ! Cu3x FeSð2xÞ þ xSO2 4 þ 8xH

ð1Þ

This (Cu–S species) then interacts with the collector for improved pyrite flotation. The Cu–S activation product is observed to be highly stable and insoluble in DETA, however oxidation/aeration of the Cu-activated pyrite transforms this product to oxides/ hydroxides which are more soluble in the presence of DETA. A similar observation was made by Yoon et al. (1995), where depression of pyrrhotite was more effective under aerated or oxidative conditions than non-aerated condition. This is further confirmed by the XPS and EDTA extraction results (Fig. 4a and b and Table 3), which show that the significant depression of pyrite by DETA and aeration may not only be due to the removal of copper ions but also to increased oxidation products (such as Fe–Ox, Cu(OH)2) on the pyrite surface. The XPS results (Fig. 4a and b) have shown, under the experimental conditions studied, that DETA can desorb Cu from the pyrite surface and also adsorb strongly to Cu sites in competition with xanthate. Since the activating Cu (Cu–S species) is the main species which enhances xanthate interaction and adsorption on pyrite surface, its removal can lead to reduced pyrite surface hydrophobicity and hence pyrite depression. This is in agreement with Sui et al. (1998) but in contrast to other observations made on the DETA depression mechanism of pyrrhotite (Yoon et al., 1995; Xu et al., 1997). The results from UV–visible spectroscopy (Fig. 6a and b) may further explain the reason for the decreased pyrite recovery in the presence of DETA. It has been demonstrated (XPS and UV–visible results) that DETA can compete with xanthate at the pyrite surface as well as to form soluble Cu–DETA complexes. 4.2. Process significance Accidental activation of pyrite occurs in most copper flotation plants via superficial oxidation of the copper minerals (e.g. chalco-

5. Conclusions DETA depressed xanthate-induced Cu-activated pyrite at pH 10 (d80 = 38 lm). The depression of pyrite was more pronounced in the presence of DETA at higher dosage (500 g/t DETA). However, almost complete depression of Cu-activated pyrite by DETA was observed when the pyrite was oxidised by purging with air (2 dm3/ min for 5 min) before DETA addition. Spectroscopic and solution studies revealed that the depression of pyrite in the presence of DETA was due to complexation with Cu in solution or its adsorbed state (whether copper hydroxide or xanthate). The XPS, UV–visible and EDTA extraction results have demonstrated under the experimental conditions studied that the following mechanisms are responsible for pyrite depression: (i) Copper removal/deactivation (whether copper sulphide or xanthate) from the pyrite surface. (ii) Formation of Cu(I)–DETA complex on pyrite surface and consequently increased the oxidation of surface copper (mainly copper oxide). The fundamental findings (DETA depression effect) can be applied to pyrite depression in pyritic copper–gold ore flotation and, in particular, in the post-regrind depression of pyrite for gold recovery. Acknowledgements The financial support from AMIRA International and the industry sponsors of the P260F project is strongly acknowledged. The first author would like to acknowledge the scholarship he received from the University of South Australia, Ghana government, and the University of Mines and Technology, Ghana. References Bulut, G., Ceylan, A., Soylu, B., Goktepe, F., 2011. Role of starch and metabisulphite on pure pyrite and pyritic copper ore flotation. Proc. Physiocochem. Probl. Miner. Process. 48 (1), 39–48. Chander, S., Khan, A., 2000. Effect of sulphur dioxide on flotation of chalcopyrite. Int. J. Miner. Process. 58, 45–55. Grano, S.R., Johnson, N.W., Ralston, J., 1997a. Control of the solution interaction of metabisulphite and ethyl xanthate in the flotation of the Hilton ore of Mount Isa Mines Limited, Australia. Miner. Eng. 10 (1), 17–45. Grano, S.R., Prestidge, C.A., Ralston, J., 1997b. Solution interaction of ethyl xanthate and sulphite and its effect on galena flotation and xanthate adsorption. Int. J. Miner. Process. 52, 162–186. Hao, F.P., Silvester, E., Senior, G.D., 2000. Spectroscopic characterization of ethyl xanthate oxidation products and analysis by ion interaction chromatography. Anal. Chem. 72 (20), 4836–4845. Hao, F., Davey, K.J., Bruckard, W.J., Woodcock, J.T., 2008. Online analysis for xanthate in laboratory flotation pulps with a UV monitor. Int. J. Miner. Process. 89, 71–75. He, S., 2003. Depression of pyrite in the flotation of copper ore. PhD Thesis, University of South Australia, Australia.

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