Effect of surface oxidation on the flotation response of enargite in a complex ore system

Effect of surface oxidation on the flotation response of enargite in a complex ore system

Minerals Engineering 119 (2018) 149–155 Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/min...

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Minerals Engineering 119 (2018) 149–155

Contents lists available at ScienceDirect

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

Effect of surface oxidation on the flotation response of enargite in a complex ore system

T



Maedeh Tayebi-Khoramia, , Emmy Manlapiga, Elizaveta Forbesb, Mansour Edrakic, Dee Bradshawd a

Julius Kruttschnitt Mineral Research Centre, The University of Queensland, Indooroopilly, QLD 4068, Australia CSIRO, Mineral Resources, Clayton, VIC 3168, Australia; c Centre for Mined Land Rehabilitation, The University of Queensland, St Lucia, QLD 4072, Australia d Minerals to Metals Initiative, Department of Chemical Engineering, University of Cape Town, Rondebosch, South Africa b

A R T I C L E I N F O

A B S T R A C T

Keywords: Surface chemistry Galvanic interaction Enargite Copper sulphide minerals Oxidation EDTA Flotation Tampakan

In previous study, the selective separation of enargite from complex ore samples in a batch flotation system under controlled pulp potential was investigated (Tayebi-Khorami et al., 2017b). It has been found that it is possible to make a separation between enargite and the other copper minerals in a real ore system using pulp potential control. In this study, the effect of the surface chemistry on the floatability of enargite and pyrite in complex ore systems were investigated. Based on flotation behaviour, EDTA (Ethylenediaminetetraacetic acid) extraction, pulp potential measurement and pure mineral study, it was found that the galvanic interactions between sulphide minerals, and pulp potential conditions, determined the flotation performances of the ore samples. It was also concluded that enargite had the lowest rest potential compared to the other sulphide minerals, which caused strong galvanic interaction between enargite and pyrite. This study has an important implication in sulphide flotation where enargite is present in the ore samples.

1. Introduction Sulphide minerals are readily oxidised in the presence of aerated aqueous conditions due to the electrochemical interactions that occur between sulphide minerals and the solution species. Oxidation of their surfaces is an important phenomenon, especially in the flotation process, as the floatability of sulphide particles is controlled by the level of surface oxidation (Fullston et al., 1999c; Peng and Zhao, 2011; Jacques et al., 2016). Slight surface oxidation can increase the floatability of sulphide minerals by forming elemental sulphur or polysulphides (reactions (1) and (2)). In acidic conditions:

MS → M n+ + S0 + ne−

(1)

In alkaline conditions:

MS + nH2 O→ M(OH)n + S0 + nH+ + ne−

(2)

where M is metal ion (Rao et al., 1992; Ralston et al., 2005). Any of these equations can cause the mineral surface to become hydrophobic to a certain degree, which is the basis of collectorless flotation (Heyes and Trahar, 1984; Rao et al., 1992). The oxidation of chalcopyrite, for



instance, causes the dissolution of Cu and Fe ions and produces a polysulphide layer on the mineral surface in alkaline conditions. The collectorless floatability of chalcopyrite is attributed to the presence of this polysulphide layer (Walker et al., 1989; Woods, 1976; Smart et al., 1998; Smart, 1991). However, under highly oxidative conditions, sulphate or thiosulphate would be formed (reactions (3) and (4)) which causes a lack of hydrophobicity and reduces the floatability of sulphide minerals and their selectivity (Senior and Trahar, 1991; Rao et al., 1992; Kant et al., 1994; Clarke et al., 1995; Rumball and Richmond, 1996; Ekmekçi and Demirel, 1997; Greet and Smart, 2002).

S0 + 4H2 O→ SO24− + 8H+ + 6e−

(3)

and/or

2S0 + 3H2 O→ S2 O32 − + 6H+ + 4e−

(4)

Several surface chemistry techniques are available for the study of the oxidation of sulphide minerals such as ToF-SIMS (Time of Flight Secondary Ion Mass Spectroscopy), XPS (X-ray Photoelectron Spectroscopy), and EDTA (Ethylenediaminetetraacetic acid) extraction technique. EDTA has been used to determine the extent of oxidation of

Corresponding author at: SMI - Julius Kruttschnitt Mineral Research Centre (JKMRC), University of Queensland, 40 Isles Rd, 4068 Brisbane, Australia. E-mail address: [email protected] (M. Tayebi-Khorami).

https://doi.org/10.1016/j.mineng.2018.01.024 Received 13 July 2017; Received in revised form 19 December 2017; Accepted 18 January 2018 0892-6875/ © 2018 Elsevier Ltd. All rights reserved.

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minerals in the flotation pulps. It has the ability to extract metal oxidation products from the surfaces of minerals (Rao and Leja, 2004). Previous researchers have mentioned the ability of EDTA to solubilise surface oxidised products, such as oxides/hydroxides, sulphates, and carbonates, without dissolving the metal sulphide (Kant et al., 1994; Rumball and Richmond, 1996; Greet and Smart, 2002). Shannon and Trahar (1986) found that the removal of surface species by EDTA increased the floatability of chalcopyrite. Senior and Trahar (1991) studied the effect of the addition of EDTA on the interactions of lead and zinc hydroxide with chalcopyrite in the absence and presence of xanthate. They showed that the precipitated metal hydroxides on the surface of chalcopyrite prevented its collectorless flotation and reduced its recovery in collector-induced flotation. They also observed that the effect of xanthate on the floatability of chalcopyrite depends on the type of metal hydroxide. Rumball and Richmond (1996) studied the extent of oxidation of the sulphide minerals by leaching slurries with an EDTA solution. They suggested that chalcopyrite released iron but not copper (Rumball and Richmond, 1996). Kant et al. (1994) used the EDTA extraction technique in a study of sulphide mineral surfaces and found also that EDTA did not extract copper from chalcopyrite. Previously, Tayebi-Khorami et al. (2017a,b) investigated the separation of enargite from other copper sulphide minerals in real ore systems by controlling the potential of the pulp. Two ore samples were selected from the Tampakan copper-gold deposit located in the Philippines. A composite of several high arsenic-containing drill core intersections for a high arsenic sample (HAS) and a composite of some low arsenic-containing drill core intersections for a low arsenic sample (LAS), were selected to provide a range of arsenic levels (TayebiKhorami et al., 2017a). Comprehensive size-by-size chemical and mineralogical analyses were performed on the HAS and the LAS samples. It was observed that enargite was the only arsenic-copper mineral present in both samples and arsenic in the HAS sample was two times higher than that for the LAS, assaying about 230 ppm. The other non-enargite copper minerals (NECu) were bornite and chalcopyrite. The amount of copper in other non-enargite copper minerals (NECu) was similar in both samples (circa 0.56%) (Table 1). The sulphide gangue mineral was pyrite with practically two times higher in the HAS sample than that for the LAS sample. Gangue mineralogy mainly comprised of quartz, diaspore, muscovite, pyrophyllite, and kaolinite. The size-by-size mineral distributions in the ground product showed that the amounts of fine (−11 µm) particles were noticeably greater for enargite than for NECu, indicating that the enargite tends to be ground more readily than the other copper sulphide minerals. The mineral grain size data showed that enargite had a finer grain size distribution compared to the other copper minerals in both samples. Moreover, it was observed from the mineral locking data that enargite was more associated with pyrite in the HAS sample compared to the LAS sample. More information regarding the mineralogy characteristics of the HAS and the LAS samples is available in Tayebi-Khorami et al. (2017a). Flotation tests were performed at several pulp potential values (−200, 0, +200, and +400 mV SHE) on both the HAS and the LAS samples. The flotation data were analysed to determine the floatability of enargite, NECu, and pyrite under these controlled pulp potential conditions (Fig. 1). For the LAS sample, the results confirmed that it is possible to separate enargite from non-enargite copper minerals (NECu) at a reducing potential (−200 mV SHE) and pH 11. For the HAS sample no separation between enargite and NECu was observed at reducing

potentials and enargite in the HAS sample showed very low recovery variations as the Eh was changed. It was observed that even for liberated enargite, its recovery was still low for all Eh ranges in the HAS sample. In addition, it was observed that pyrite showed high recovery at pH 11 and Eh values below +400 mV SHE, while it was highly depressed at Eh +400 mV SHE in both samples without the addition of any chemical depressant. More details about the selective flotation of enargite from copper sulphides in the HAS and the LAS ore samples can be found in Tayebi-Khorami et al. (2017b). One possible reason for the different response of enargite in the HAS sample could be the presence of oxidation products on the surface of the particles. In the present paper, surface chemistry studies were undertaken using an EDTA extraction technique for both the HAS and the LAS samples to evaluate the amount and type of metal ions on the mineral surface and the possible reasons for different flotation results. 2. Experimental procedure 2.1. Sample preparation Similar to the previous papers by Tayebi-Khorami et al. (2017a,b) the high arsenic sample (HAS) and the low arsenic sample (LAS) from Tampakan deposit were selected for this study. A Roclabs laboratory jaw crusher followed by a roll crusher in closed circuit with a sieve was used for crushing the HAS and the LAS drill cores to 100% finer than 1.7 mm. The crushed drill cores were blended to form a composite HAS and LAS sample, and then split into sub-samples of 500 g by a rotary splitter using standard methods and stored in a dry cupboard to prevent further oxidation. For each test, the sub-sample of 500 g crushed ore (HAS or LAS) was mixed with Brisbane tap water in an iron ball mill at a solid-liquid ratio of 2:1, i.e. 67% solids by weight and ground using stainless steel balls to achieve the P80 of 90 µm. The mill was cleaned by grinding a sample of quartz for 5 min before each test. 2.2. EDTA extraction tests Ethylenediaminetetraacetic acid tri-sodium salt (Na3EDTA) as an AnalaR product was used to solubilise metal sulphide oxidation products from the surface of particles and to determine the type and amount of oxidation species formed on the minerals surface. Two sets of EDTA extraction tests were conducted. The schematic of the experimental procedure is shown in Fig. 2:

• The first set of tests was conducted on the HAS and the LAS samples, which were ground to P of 90 µm (Mill discharge). • The second set of tests was performed on the conditioning stage of 80

the flotation tests (Flotation feed).

For the first set of tests, after grinding the sample of the HAS or the LAS to P80 of 90 µm, the sample was weighed and leached by 5% solution Na3EDTA for 10 min using an electrical stirrer. The slurry was filtered using a Whatman number 40 filter paper and then fine filtered using a 0.45 µm Millipore filter. The solubilised metals were determined by Inductively Coupled Plasma (ICP) Atomic Emission Spectrophotometry (AES). For the second set of tests, as details described in Tayebi-Khorami et al. (2017b), four different pulp potential (Eh) values of −200, 0, +200, and +400 mV SHE were selected and the experiments were conducted at these Eh values for both the HAS and the LAS samples in a 3 dm3 modified laboratory stainless steel Denver cell. This cell was fitted with pulp level control, an automatic titration control unit for control and monitoring pH and pulp potential, and a gas on/ off control. The pH was measured with a Radiometer glass electrode, and calibrated using standard pH 7 and pH 10 buffer solutions before each test. The pulp potential was measured with a high impedance differential voltmeter using polished platinum electrode and an Ag/AgCl reference

Table 1 The head assays of HAS and LAS samples. Sample

Cu (%)

As (ppm)

S (%)

Fe (%)

NECu (%)

HAS LAS

0.64 0.57

233 114

2.96 1.91

2.91 1.87

0.58 0.54

150

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LAS 100

90

90

80

80

70

70

Recovery (%)

Recovery (%)

HAS 100

60 50 40

60

50 40

30

30

20

20

10

10 0

0

-300 NECu

-200

-100 Pyrite

0

100 200 Eh (mV SHE)

Enargite

300

400

-300

500

Non-Sulphide Gangue Minerals

NECu

-200

-100 Pyrite

0

100 200 Eh (mV SHE)

Enargite

300

400

500

Non-Sulphide Gangue Minerals

Fig. 1. The recovery of NECu, Enargite, Pyrite, and NSG after 10 min flotation at different Eh conditions for the HAS (left) and the LAS (right) (Tayebi-Khorami et al., 2017b).

Fig. 2. Schematic of the EDTA extraction test procedure.

3. Results

electrode, and was checked using a standard ferric-ferrous solution. Measured potentials were converted to the standard hydrogen electrode (SHE) scale by the addition of 0.2 V. By using this equipment, it was possible to continuously monitor and control the pH and Eh. The pulp was deoxygenised by bubbling the nitrogen through the pulp for the tests being performed at Eh −200 and 0 mV SHE. Make-up water used in the tests was Brisbane tap water. After the ground slurry (P80 of 90 µm) was transferred to the flotation cell, water was added to reach the target level (15.7% w/w), and pH adjusted to 11 using NaOH. An automatic titrator was used to deliver dilute solutions (2.5% w/w) of sodium hypochlorite (NaClO) as an oxidising agent and sodium dithionite (D.T) (Na2S2O4) as a reducing agent to set and maintain the Eh at the test value. The pulp was conditioned for 10 min. A fresh sample of potassium ethyl xanthate (KEX) as a collector was then added and the pulp was conditioned for a further two minutes. In this stage, a 200 ml sample was syringed from the pulp for the EDTA extraction test and then the impeller was turned off. The 200 ml collected sample was weighed and leached by 5% solution Na3EDTA for 10 min using an electrical stirrer, same as for the first set of tests. After filtering using Whatman number 40 filter paper and 0.45 µm Millipore filter, the filtrate was submitted to the laboratory for measuring the solubilised metals assays by ICP-AES. EDTA tests were repeated several times at each Eh condition to establish reproducibility. The results were so close, indicating that the procedure had good reproducibility and the relative standard deviation were reasonable, not exceeding 5%. The percentage of oxidised mineral present in a pulp from EDTA extraction data was calculated from the following equation, which was developed by Rumball and Richmond (1996):

% EDTA extractable M =

Mass of M in EDTA solution × 100 Mass of M in solids

The filtrate of the EDTA dissolution tests showed a darker greenish colour for the HAS sample compared to the LAS sample in all sets of the experiment.1 The greenish colour can be due to the mixture of yellowish colour from iron hydroxide and bluish colour from copper hydroxide (Fig. 3). The results in Fig. 4 represent the amount of Cu, Fe, and As extracted by EDTA from the mill discharge and flotation feed tests at different Eh values for both samples. It should be noted that water chemistry tests were also conducted on these samples before adding EDTA which showed that the level of metal (Cu, Fe, and As) ions in solution was less than 1 mg/L. Therefore, the metal values determined from EDTA extraction tests are extracted from surface products.

3.1. Copper Copper is the predominantly extracted metal. There is a general upward trend in Cu extraction towards the higher pulp potential levels. He (2006) also observed that the surface copper oxidation species (extracted by EDTA) increased with increasing Eh values. The EDTA extracted copper originates from the oxidation of copper minerals including chalcopyrite, bornite, and enargite. It can be seen that the amount of extracted copper is higher in the HAS sample over the entire Eh range. The copper oxidation species might not be critical in the flotation of copper minerals; however, they can activate some other minerals present in the ore samples such as pyrite or non-sulphide gangue minerals, which then affect the selectivity of the separation.

(5)

where M is the metal under study.

1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.

151

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Enargite Recovery (%)

M. Tayebi-Khorami et al.

HAS

LAS

100 90 80 70 60 50 40 30 20 10 0

Fig. 3. Filtrates of the EDTA dissolution tests on the HAS and the LAS samples.

HAS

0 +200

LAS

-200

+200

+400

+400

-200

0.0

0.2

0.4

0.8

1.0

Fig. 5. Correlation between oxidised arsenic and flotation recovery values at different Eh conditions (mV SHE) for both samples.

The EDTA extractable iron after grinding (mill discharge) is about 2%. It indicates that the pulp entering the flotation circuit contained elevated levels of EDTA extractable iron. Interestingly, the levels of EDTA extractable iron decreased through the conditioning stage. The sharp increase in EDTA extractable iron in the grinding circuit may be due to the rapid oxidation of pyrite or other copper/iron sulphides in the grinding circuit. One possible reason for the decrease in the amount of EDTA extractable iron may be the further oxidation of the iron species to higher level iron oxides that are more resistant to extraction and are insoluble in EDTA (Rumball and Richmond, 1996; Greet et al., 2006). There is a downward trend in Fe extraction as the Eh is increased as also observed by Rumball and Richmond (1996). They found that iron hydroxyl species, which was present on particle surfaces, formed in the first stages of flotation, and had a significant effect on galena floatability. They mentioned that ferric salts might be insoluble in EDTA. Thus as the Eh of a pulp increase and ferrous salts are oxidised to ferric salts, the amount of EDTA extractable iron decreases (Rumball and Richmond, 1996). Another important observation is that the percentage of EDTA extractable iron in the mill discharge tests is higher in the HAS sample than in the LAS sample. However, it sharply decreased in the conditioning stage for the HAS sample and hence the level of extractable iron in the conditioning stage of the flotation tests is lower for the HAS sample than for the LAS sample.

that the enargite in the initial samples was not oxidised. However, the level of EDTA extractable arsenic increased through the conditioning stage meaning that more enargite became oxidised during the conditioning stage which was then dissolved by EDTA. The important point is that the amount of arsenic extracted by EDTA is much higher (2–6%) in the HAS sample compared to that in the LAS sample. This can be seen as evidence for the role of oxidation in the poor recovery of enargite from the HAS sample. The only other explanation that can be inferred is that the mineral is being dissolved by EDTA. However, that would apply to both ores, and there would be little difference between the EDTA extractable arsenic from the HAS and the LAS samples. Therefore, this explanation cannot be correct. A plot of EDTA extractable arsenic in the flotation feed corresponding to enargite recovery (Fig. 5) also shows that arsenic recovery is lower in the HAS sample which has the higher amount of EDTA extractable arsenic.

4. Discussion It was observed in Fig. 1 and Tayebi-Khorami et al. (2017b) that the floatability of pyrite was higher at Eh values lower than +400 mV SHE compared to those expected from the pure mineral studies by other researchers. The pyrite flotation recovery decreased sharply at Eh values of higher than +400 mV SHE. On the other hand, enargite recovery in the HAS sample was much lower than that in the LAS sample. In this section, the electrochemical reactions that caused these phenomena are discussed in view of the EDTA extractions results.

3.3. Arsenic The EDTA extractable arsenic after grinding is negligible showing

LAS

EDTA extractable Cu, As, and Fe (%)

HAS

EDTA extractable Cu, As, and Fe (%)

0.6

EDTA Extractable As (%)

3.2. Iron

7.0

6.0 5.0 4.0 3.0 2.0 1.0 0.0

Mill Discharge Cu

0

Fe

As

-200

0

+200 Eh ((mV SHE))

7.0

6.0 5.0 4.0

3.0 2.0 1.0 0.0

Mill Discharge

+400

Cu

Flotation Feed

Fe

-200

As

Fig. 4. Results from EDTA tests on mill discharge and flotation feeds at different Eh.

152

0

+200 Eh (mV SHE)) (

Flotation Feed

+400

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4.1. Pyrite floatability 4.1.1. Effect of oxidation on pyrite depression The pyrite flotation recovery decreased sharply at Eh values of higher than +400 mV SHE. It can be due to the oxidation of the hydrophobic elemental sulphur into the hydrophilic species such as sulphate according to reaction (6) (Rao and Finch, 1988). Moreover, it was observed that the Fe extracted by EDTA decreased as the Eh increased toward the more oxidising conditions (Fig. 4). It can be concluded that at the highly oxidising conditions the pyrite surface was strongly oxidised and covered with iron oxide/hydroxide/sulphate according to the reactions (7) and (8) (Ekmekçi and Demirel, 1997). These hydrophilic species prevent collector adsorption, and as a result, pyrite recovery has decreased at values above Eh +400 mV SHE.

FeS2 + 3.5O2 + H2 O→ Fe2 − + 2SO24− + 2H+

(6)

Fe2 − + 0.5O2 + H+ → Fe3 − + 0.5H2 O

(7)

Fe3 − + 3H2 O→ Fe(OH)3 + 3H+

(8) Fig. 6. Pyrite recovery as a function of Eh in the presence or absence of 1.5 × 10−3 mol/ dm3 copper ions, collector Sodium isopropyl xanthate, modified from (Peng et al., 2012).

These observations of pyrite flotation in oxidising conditions are in good agreement with previous studies. Several research studies have been conducted on the pyrite depression by controlling the electrochemical conditions of the flotation pulp. The decrease in the floatability of pyrite with increasing pulp potential was found to be due to the precipitation of the hydrophilic species such as iron hydroxide at high potentials. At highly oxidising potentials, ferrous ions released by oxidation of pyrite have low stability and quickly oxidised to the ferric oxy-hydroxide species which are hydrophilic, and thus decrease the pyrite floatability and eventually cause its depression (Chander, 1988; Trahar et al., 1994; Ekmekçi and Demirel, 1997; GuLer et al., 2013; Yang et al., 2016). 4.1.2. Effect of copper ion on pyrite activation The presence of dissolved Cu ions may affect the floatability of other sulphide minerals such as pyrite; the floatability of pyrite increases due to the reaction with copper ions on the surface and formation of CuS according to the reaction (9) (Chandra and Gerson, 2009; He, 2006; Huang and Grano, 2006; Peng and Grano, 2010; Trahar et al., 1994; Yoon et al., 1995).

FeS2 + Cu2 − → CuS + Fe2 − + S0

(9)

Fig. 7. Cu species distribution at different pH, from Yang et al. (2016).

2+

Yoon et al. (1995) found that the activation of sulphides by Cu was strongly dependent on the redox potential of the pulp. In a study by Peng et al. (2012) the effect of electrochemical potential on pyrite flotation in the presence and absence of 1.5 × 10−3 mol/dm3 copper ions was investigated (Fig. 6). They observed that in the absence of copper ion pyrite showed very poor floatability at different Eh values while in the presence of copper ions the floatability of pyrite increased significantly. They also observed that at reducing conditions the recovery of the activated pyrite was higher than that in the oxidising condition; the maximum recovery of activated pyrite was achieved at Eh −185 mV SHE. According to these observations, it was concluded that increasing pulp electrochemical potential caused an increase in the formation of Cu(OH)2 and a decrease in the concentration of Cu+ on the pyrite surface. He (2006) also observed the same results and concluded that the maximum pyrite recovery obtained at reducing Eh range was due to the increased adsorption of copper. It was also observed that the rate of the copper adsorption decreases at more oxidising conditions because copper has to penetrate into the surface hydroxide layer (He, 2006; Moslemi and Gharabaghi, 2017). Chandra and Gerson (2009) stated that Cu2+ and Cu(OH)2 are the two main Cu species in solution for the activation of sulphide minerals and can cause loss of selectivity through reaction with xanthate. Ekmekci et al. (2006) discussed that the Cu(OH)2 and Cu(OH)+ are dominant compounds at pH 9. This is consistent with the copper species

as a function of the pH diagram by Yang et al. (2016) in a specific copper concentration (Fig. 7). At higher pH values, Cu(OH)2 as a dominant species would be precipitated on the mineral surfaces (valuable and silicate minerals) and cause the activation of that mineral. Based on the Cu extracted results from the EDTA tests, it is most likely that the pyrite activation during conditioning stage was due to the reaction with copper ions on its surface, from the copper sulphide oxidation, which resulted in high recovery of pyrite at a pulp potential lower than +400 mV SHE.

4.2. Enargite floatability Iron in the samples of this study is from the iron-bearing sulphide minerals in the ore. However, it also may originate from galvanic interactions occurring between sulphide minerals during milling. The oxidation present in the ball mill before grinding is insignificant because the mill was cleaned by grinding a sample of quartz for 5 min prior to the each test. Stainless steel media was also used to reduce the galvanic interactions between sulphide minerals and media during grinding. In addition, the mineral samples were stored in a dry cupboard to prevent oxidation. Galvanic interactions can reduce the selectivity in sulphide minerals 153

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polysulphide. In addition, a small amount of iron as an impurity was observed on the mineral surface which can cause the galvanic interactions in these samples (Fullston et al., 1999a,b). Fornasiero et al. (2001) examined the separation of enargite and tennantite from other copper sulphide minerals (chalcocite, covellite, and chalcopyrite) by flotation in mixed mineral systems. The XPS results confirmed that the surface of the arsenic minerals was more oxidised than the non-arsenic minerals (Fornasiero et al., 2001). Lattanzi et al. (2008) mentioned that at alkaline conditions, the oxidation of enargite is fast with the formation of a copper-depleted layer and Cu-O species on its surface. XPS study of the enargite surface at alkaline pH also indicated the presence of cupric oxide and arsenic oxide (Lattanzi et al., 2008). In another detailed study of the electrochemical behaviour of enargite, a passivation layer was found to form over a broad potential range (−660 to +1070 mV SHE) and that the main oxidation product found on the mineral surface was copper hydroxide (Cu(OH)2) (Guo and Yen, 2014). The modal mineralogy of the HAS and the LAS samples showed that there was a greater proportion of pyrite in the HAS sample compared to that of the LAS sample. It is therefore anticipated that the flotation of the HAS sample would be affected by the higher amount of pyrite present in this ore sample. Further, from the flotation study on the HAS and the LAS sample, it was observed that enargite showed lower recovery in the HAS sample compared to that in the LAS sample and its recovery was not changed as the Eh was increased. In addition, the EDTA experiment on the HAS and the LAS sample showed a higher amount of extracted Cu and As in the HAS than in the LAS sample. Based on the previous studies by other researchers (Cheng and Iwasaki, 1992; Rao et al., 1992; Pauporte and Schuhmann, 1996; Fullston et al., 1999a,b; Hu et al., 2009; Chen et al., 2014) and the flotation and EDTA results obtained in this study, it can be concluded that enargite had the lowest rest potential compared to other main sulphide minerals presented in the HAS and the LAS ore samples. The rest potential of the main sulphide minerals present in the HAS and the LAS ore samples follows the order:

Table 2 Galvanic series of some sulphide minerals, modified from Rao et al. (1992) and Hu et al. (2009). Mineral

Rest potential (V SHE)

Pyrite Marcasite Chalcopyrite Sphalerite Covellite Bornite Galena Argentite Stibnite Molybdenite

0.66 0.63 0.56 0.46 0.45 0.42 0.4 0.28 0.12 0.11

in flotation. Many researchers have observed the galvanic interactions between sulphide minerals in flotation pulp that can significantly affect the flotation behaviour of minerals of interest (Martin et al., 1989; Ekmekçi and Demirel, 1997; Peng et al., 2003b; Huang and Grano, 2005; Azizi et al., 2013; Rabieh et al., 2016). In the case of mineralmineral interactions, a mineral with higher potential acts as a cathode, while a mineral with lower potential serves as an anode (Table 2). For example, pyrite/chalcopyrite interactions, chalcopyrite oxidises and loses electrons while pyrite accepts the electrons. These electrons transfer to oxygen of the water, forming OH− ions. The formation of hydrophilic OH− ion and reduced potential consequently depress the sulphide minerals flotation (Martin et al., 1989; Cheng and Iwasaki, 1992). Rao and Natarajan (1990) suggested that there is a special distribution of the iron species among the minerals present in a slurry depending on the relative electrochemical activity of the minerals. The iron distribution is more favoured on more noble minerals relative to the active minerals. Owusu et al. (2014) also observed that the Fe2+ ions released into the solution would rapidly oxidise to Fe3+ and formed Fe(OH)3 precipitates on the mineral surface under strong alkaline environments (Owusu et al., 2014). Bradshaw et al. (2006) mentioned that the galvanic interactions between sulphide minerals affects their flotation behaviour by the formation of iron hydroxy species on their surface. Jacques et al. (2016) stated that the galvanic interactions and redox reactions in a slurry are the main factors causing the precipitation of iron oxyhydroxy species at the sulphide minerals surfaces, which lead to poor floatability of sulphide minerals. Pauporte and Schuhmann (1996) investigated the electrochemical characteristics of natural enargite under alkaline conditions and determined the rest potential of enargite of 0.28 V SHE in the presence of bubbling oxygen without xanthate and 0.21 V SHE in the presence of xanthate. In a study by Fullston et al. (1999c) the zeta potential of the copper sulphide minerals (chalcocite, covellite, chalcopyrite, bornite, enargite, and tennantite) was investigated and the oxidation rates compared. It was stated that the oxidation rate of the copper sulphide minerals at pH 11 follows the order:

pyrite > chalcopyrite > bornite > enargite Pyrite is the most cathodic sulphide mineral in the current study and would draw electrons from all of the other sulphide minerals in the ore. As enargite has the lowest rest potential of the other copper minerals in the HAS, a stronger galvanic interaction happens between enargite and pyrite than between other copper minerals and pyrite. Consequently, strong oxidation occurs on enargite, which produces a hydrophilic oxidation layer on the enargite surface that depresses its flotation along with a significant amount of copper ions, which is sufficient to activate pyrite. This galvanic interaction between enargite and pyrite caused the poor recovery of enargite in the HAS sample. On the other hand, it is generally agreed that iron hydroxide species are hydrophilic and are detrimental to the flotation of sulphide minerals (Grano et al., 1997; Peng et al., 2003a,b; Greet, 2009). However, these hydrophilic species did not depress the flotation of NECu minerals in the HAS sample. It has also been found by some researchers that in the presence of the hydrophilic iron hydroxides, the flotation recovery of very fine valuable minerals would be affected detrimentally to a greater extent than the intermediate size fractions (Johnson, 2005; Grano, 2009; Peng and Grano, 2010; Chen et al., 2014). From the mineralogical characteristics of the HAS it was observed that about 22% of enargite reported to the finest size fraction −11 μm. In addition, enargite had the lowest grain size distribution amount of all the copper sulphide minerals (P80 of 27 μm) (Tayebi-Khorami et al., 2017a). It is therefore concluded that the very fine grain size of enargite in the HAS ore sample had a critical role on its floatability. Overall, the results demonstrate that the separation of enargite from other copper sulphide minerals is highly affected by the presence and amount of pyrite and is depended on the minerals’ electrochemical properties.

chalcocite > tennantite > enargite > bornite > covellite > chalcopyrite It can be seen that enargite is more electrochemically active than other copper minerals in the HAS and LAS ore samples (bornite and chalcopyrite), so it is anticipated that enargite flotation will be more susceptible to the grinding condition than other copper flotation. The surface oxidation of enargite was studied by Fullston et al. (1999a,b) which showed the amount of copper dissolved from the enargite was much higher than that of arsenic. XPS studies showed that even with weak oxidants such as dissolved oxygen, two oxidation layers form on the surface of the enargite: a thin layer comprised of copper and arsenic oxide, or hydroxide, and a layer made of metal-deficient sulphide and/or 154

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5. Conclusions

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• Enargite had the lowest rest potential compared to other main sul-

phide minerals present in both a HAS and a LAS ore sample from Tampakan deposit. The rest potential of the main sulphide minerals present in the HAS and the LAS ore samples follows the order:

pyrite> chalcopyrite > bornite>enargite

• Based on enargite surface studies by other researchers and the re-

• •

sults obtained from the current work, it is concluded that the strong galvanic interactions between enargite and pyrite in the HAS sample caused the oxidation of enargite and its poor floatability and consequently poor selectivity with respect to other copper minerals in the HAS sample. The galvanic interaction also produced copper ions in the flotation pulp, which activated the pyrite and resulted in high recovery of pyrite at a pulp potential lower than +400 mV SHE. The decrease in the floatability of pyrite with increasing pulp potential (higher than 400 mV SHE) was found to be due to the precipitation of the hydrophilic iron hydroxide on pyrite’s surface.

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