Covellinisation of copper sulphide minerals under ...

Hydrometallurgy 137 (2013) 1–7

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Covellinisation of copper sulphide minerals under pressure leaching conditions Antoni Muszer a, Jerzy Wódka b, Tomasz Chmielewski b,⁎, Sabina Matuska b a b

University of Wroclaw, Institute of Geological Sciences, pl. Uniwersytecki 1, 50-137 Wroclaw, Poland Wroclaw University of Technology, Faculty of Chemistry, Division of Chemical Metallurgy, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland

a r t i c l e

i n f o

Article history: Received 18 July 2012 Received in revised form 15 February 2013 Accepted 18 March 2013 Available online 10 April 2013 Keywords: Copper concentrate Pressure leaching Covellinisation

a b s t r a c t A series of laboratory experiments have been performed on pressure leaching in oxygenated sulphuric acid solutions of commercial copper flotation concentrate produced by Lubin Concentrator (KGHM, Poland). Major copper sulphides: chalcopyrite, bornite and chalcocite were reported to undergo phase conversion to covellite (CuS), which was found to be the most stable sulphidic form of copper. The formation of copper sulphide (covellite) appeared to be a significant step in the examined pressure leaching process. Copper present in the flotation concentrate was observed to be solubilised in the leaching solution only after the conversion of all copper sulphide minerals into the covellite phase. Permeable, openwork texture of covellite formed during the copper sulphide conversion facilitates easy leaching and transport of the leaching medium to the leaching surface, effecting leaching of the remaining minerals in the solid feed. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Treatment of copper flotation sulphide concentrates by smelting, converting and electrorefining has been dominating the World's copper industry for technical and economic reasons. However, research and development for hydrometallurgical alternatives to traditional pyrometallurgical processes has remarkably intensified in the recent years, due to declining copper ore grade, increasing ore complexity and shrinking resources. A wide range of chemical and biological processes for copper recovery from concentrates are available (Dreisinger, 2006; Goble, 1981; Gupta and Mukherjee, 1990; Habashi, 1999, 2005, 2007; Jansen and Taylor, 2000; Marsden, 2007; Marsden and Wilmot, 2007a, b,c,d; Peacey et al., 2003; Ramachadran et al., 2007). Chmielewski (2012) discussed a possible role of hydrometallurgy in more effective processing of polymetallic concentrates and by-products of Polish copper industry. The hydrometallurgical processes are recognised as effective in the leaching of copper from polymineral and chalcopyrite concentrates, purifying the leach solutions (PLS) using modern separation processes, mainly solvent extraction, and recovering a high value, high purity copper metal product. Copper hydrometallurgy has been extensively studied as an alternative way to smelting. A great attention has been paid to the chalcopyrite concentrate treatment, since this mineral is most abundant among copper-bearing sulphides. The challenge of process development for chalcopyrite leaching in sulphate media is generally to leach chalcopyrite quickly and completely. However, to overcome slow and incomplete leaching of chalcopyrite at a lower temperature, two problems ⁎ Corresponding author. Tel.: +48 71 320 38 94; fax: +48 71 320 24 44. E-mail address: [email protected] (T. Chmielewski). 0304-386X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2013.03.010

must be solved: formation of passive films on the chalcopyrite surface and potential blocking and wetting of chalcopyrite by liquid elemental sulphur. Polish hydrothermal copper ore deposits located in the Legnica– Glogow Copper Basin exhibit specific, complex, polymetallic and polymineral composition (Konstantynowicz-Zielinska, 1990; Pieczonka et al., 2001; Rydzewski, 1996). In some areas chalcocite and bornite are generally the dominating copper-bearing sulphides with chalcopyrite and covellite as minor components (Polkowice–Sieroszowice and Rudna) while the Lubin area copper deposit exhibits distinctly elevated content of chalcopyrite. Mineralogical composition of the Polish copper ores is very advantageous for hydrometallurgical treatment, because chalcocite and bornite are better leachable, in contrary to chalcopyrite, being the most refractory in leaching. Additionally, the Polish copper ores contain pyrite and marcasite, which are well known as electrochemical activators in sulphide leaching (Chanturija and Vigdergauz, 2009; Dixon and Mayne, 2007; Holmes and Crundwell, 1995; Majima and Peters, 1968; Mehta and Murr, 1983; Nowak et al., 1984). The sulphide minerals in the Lubin Concentrator flotation products form a vast number of intergrowths, creating intermineral galvanic cells (Chmielewski and Kaleta, 2011), which remarkably elevate the rate of the leaching process. Based on the existing facilities and announced world-wide expansion of KGHM, the annual mines' production capacity in the period of 2009–2014 is expected to grow at an average rate of around 4.4% per year to reach 24.2 million tons in 2014, an increase of around 4.7 million tons (24%) from that in 2009. Of the total increase, the copper-in-concentrate capacity is expected to increase by 3.8 million tons (4.7%/year) to reach 19 million tons and solvent extraction– electrowinning (SX–EW) production by 850,000 tons (3.7%/yr) to

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A. Muszer et al. / Hydrometallurgy 137 (2013) 1–7

reach 5.2 million tons. Most of the new mine projects and expansions are located in Brazil, Chile, Congo, Mongolia, Peru, the United States and Zambia, which together account for around 3.4 million tons (72%) of the projected copper production increase during this period. The main objective of this work was a mineralogical analysis, at the micro level, of the mechanism of leaching of copper minerals from the polymineral sulphide industrial concentrate at elevated temperature and pressure. The performed investigations allowed, for the first time, to describe and characterise processes inside the leaching autoclave on the basis of standard quantitative and qualitative analyses of ore minerals. The experimental recognition of the mechanism of phase conversions during pressure leaching is expected to be very helpful in controlling the leaching process and in the selection of the optimum leaching conditions. Phase conversion had already been investigated, observed and described (Buerger, 1940; Djurle, 1958; Ross, 1953). The phase conversions of copper minerals both in simple (CuS–Cu2S) and in more complex structures (Cu–S–Fe, Cu–S–Sb, Cu–S–As) are mainly regarded with model systems. In the natural systems are observed simple copper sulphides (covellite CuS, chalcocite Cu2S), complex sulphides (chalcopyrite CuFeS2, bornite Cu5FeS4), and hardly ever transient phases (djurleite Cu1,97S, digenite Cu1,8S, anillite Cu1,75S, geerite Cu1,6S, spionkopite Cu1,4S, yarrovite Cu1,12S, Djurle, 1958; Goble, 1981). Copper sulphides crystallise in specific hydrothermal conditions, frequently slightly reductive or slightly oxidative and covellite is known as the most stable form (Ramdohr, 1975; Uytenbogaardt and Burke, 1971). The structure of these sulphides was well recognised and described in literature (Buerger and Wuensch, 1963; Djurle, 1958) in particular for chalcocite and digenite (Will et al., 2002; Buerger, 2006). For the explanation of mineral conversion in hydrometallurgical leaching systems the most important observations are made by Bartlett et al. (1986), Bartlett (1992) and Viñals et al. (2004). The authors investigated the phase transformation of copper or zinc sulphides at various conditions and indicated some kinetic aspects in hydrothermal environments. In this paper the observations and results of earlier works were utilised and respected, but due to very complex polymetallic and polymineral compositions of copper flotation concentrate from Lubin Concentrator (KGHM, Poland) it was necessary to verify experimentally and analyse phase conversions of copper sulphides on solid/liquid interface in conditions similar to pressure leaching in autoclave.

Table 1 The copper and accompanying metals content in Lubin copper concentrate. Metal

Cu

Fe

Zn

Ni

Co

Ag

As

Pb

Corg

Content, %

16.6

7.00

0.947

0.047

0.120

0.088

0.31

2.67

10.1

the solution was chemically analysed for the Cu, Fe, Zn, Ni, Co, As, and H2SO4 contents. Due to complications related with the reactions inside the reactor and with difficulties in the determination of the balance of copper mineral content, the samples taken at temperatures from 100 °C to 160 °C appeared to be not useful for explanation of the phase conversions. We observed an increasing content of covellite in comparison to other minerals but it was impossible to find out the quantitative relations between them. Only for samples taken at 180 °C it was possible to perform quantitative and qualitative characterisations of the phase conversions taking place in the reactor. The first sample of the slurry (at time zero) was taken when the temperature reached 180 °C, just before the introduction of oxygen to the reactor. To identify the main mineral phases in the feed and in samples taken during leaching at 180 °C, they were prepared and polished for microscopic examinations in the reflected light. The specimens were made according to a standard procedure required for metal ores and powdered materials. The grinding of the specimen was carried out using synthetic corundum powders, whereas polishing was completed by means of polishing cloths with abrasive surfaces for use for polishing metallographic samples (DP–Mol, DP–Dur, DP– Nap) of strictly selected grade of diamond paste provided by Struyers. The mineralogical microscopy investigations of the polished specimens taken in the course of leaching were performed by means of the Optiphot 2-Pol Nikon microscope. The applied procedure for detection and identification of minerals was based on physical and optical properties of minerals, widely described in the literature (Criddle and Stanley, 1993; Muszer, 2000; Picot and Johan, 1982; Pracejus, 2008; Ramdohr, 1975; Uytenbogaardt and Burke, 1971). For determination of mutual qualitative and quantitative relations between metal-bearing minerals and barren material a planimetric analysis of the microscopic images was carried out using computer program “Lucia M”. Additional electron RTG microanalyses were carried out by means of the JEOL JSM-5800LV scanning electron microscope equipped with an X-ray spectrum analysis program, to confirm optical microscopy identification.

2. Experimental 2.2. Concentrate characterization 2.1. Leaching procedure A copper sulphide commercial concentrate produced by Lubin Concentrator (ZWR Lubin, KGHM Polska Miedź SA) was used as a feed for the pressure leaching experiments. Leaching was carried out in a 2-litre titanium autoclave manufactured by Parr Instruments, equipped with a Teflon stirrer and stirring rate controller. The leaching tests were conducted at temperatures of 100, 120, 140, 160, and 180 °C. In each experiment 135 g of dry concentrates was introduced to the reactor in the form of water slurry and 0.8 l of solution acidified by means of sulphuric acid with an initial concentration of 120 g/l. After preliminary, total decomposition of carbonate matter (calcium and magnesium carbonates) with H2SO4, the autoclave was sealed off and purged 3 times with nitrogen. After temperature reached 100 °C, the nitrogen was removed from the reactor and the slurry was heated up. When the required level of temperature was reached, oxygen was introduced to the reactor. The oxygen partial pressure was kept constant at the level of 5.0 atm. He stirring rate was 400 rpm. In the course of leaching at each temperature the samples of the slurry were taken systematically (every 10 or 30 min). For each sample, after phase separation, the solid (from 2.62 to 3.23 g) was used for mineralogical qualitative and quantitative examinations whereas

A commercial sulphide flotation concentrate being a final polymineral and polymetallic product of Lubin Concentrator (ZWR Lubin, KGHM “Polska Miedz” SA) has been used in the pressure leaching

7,3%

with quartz

with carbonates

34,2%

and clay minerals with carbonates

58,5%

Fig. 1. Distribution of intergrowths of sulphide minerals in Lubin concentrate.


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Table 2 The content of sulphides in the pressure leaching feed sample. Mineral

Bornite

Chalcocite digenite

Chalcopyrite

Pyrite marcasite

Covellite

Sphalerite

Tennantite

Galena

Total

% mass

10.98

5.96

12.05

5.03

0.61

1.48

0.81

3.06

39.98

Fig. 2. Particles of sulphides prior to pressure leaching. Reflected light, without analyser.

experiments (Table 1). The low grade concentrate (16.6% Cu) is hardly acceptable in current smelting operations. High contents of carbonate matter (23.12%) and organic carbon (10.1%) make the leaching feed very specific and unsuitable for direct smelting. Such unfavourable composition of the copper concentrate requires a specific treatment and hydrometallurgy is an alternative. It was established that 66.15% of the metal sulphides in the examined concentrate was in the form of intergrowths. Copper sulphides were predominantly in the form of intergrowths with carbonates and clay minerals (Fig. 1). Chalcopyrite, chalcocite, digenite, pyrite and marcasite were detected as primary minerals in the concentrate feed sample. Moreover, galena, sphalerite, covellite, and tennantite were detected as secondary minerals (Table 2, Fig. 2). Stromeyerite and cobaltite–gersdorfite were identified as accessory minerals with contents below 0.01% by mass. The content of metal-bearing minerals was found to be 39.98% by mass. Dominating barren minerals are calcite and dolomite, forming intergrowths with clay minerals. The content of quartz did not exceed 9.51% of the concentrate. Examination of the metal content in the copper sulphides, accomplished by means of microanalysis with scanning electron microscopy indicated, that the content of cobalt in selected bornite particles was

Table 3 The content of sulphides (in mass%) in solid samples during pressure leaching at 180 °C. Ore minerals

Feed

Bornite Chalcocite, digenite Chalcopyrite Pyrite, marcasite Covellite Sphalerite Tennantite Galena Sulphides in total

10.98 5.96 12.05 5.03 0.61 1.48 0.81 3.06 39.98

Leaching time (in minutes) 0

10

20

30

60

90

120

150

180

210

240

1.26 8.20 4.84 4.00 10.94 1.16 0.72 2.52 33.64

1.25 6.63 4.41 3.86 13.08 0.77 0.74 2.59 33.34

1.21 3.38 4.03 3.67 17.29 0.17 0.59 2.33 32.68

1.16 2.84 2.84 3.22 18.70 0.13 0.52 1.93 31.34

0.31 0.00 0.68 3.22 24.27 0.11 0.10 0.86 29.55

0.17 0.00 0.51 0.93 24.51 0.08 0.08 0.51 26.79

0.16 0.00 0.43 1.02 22.57 0.00 0.07 0.47 24.72

0.14 0.00 0.43 1.01 22.49 0.00 0.07 0.47 24.61

0.13 0.00 0.46 1.00 21.63 0.00 0.07 0.46 23.75

0.16 0.00 0.43 0.86 19.01 0.00 0.00 0.43 20.89

0.13 0.00 0.45 0.99 16.85 0.00 0.00 0.11 18.53

Mineral in the solid, wt %

20 chalcocite-digenite chalcopyrite

15

pyrite-marcasite covellite sphalerite

10

tennantite galena

5

Content of metal-bearing minerals, % wt.

40

a

25

b 35 30 25 20 15 10 5 0

0 0

30

60

90

120 150 180 210 240

Leaching time, min

0

30

60

90

120 150 180 210 240

Leaching time, min,

Fig. 3. The changes of mineralogical phases (a) and content of metal-bearing minerals (b) in the concentrate during pressure leaching at 180 °C.

4


within the range from 0.01 to 1.11%, whereas the content of nickel was below 0.1%. The pyrite framboidic particles, cemented with copper sulphides, exhibited the content of cobalt up to 6.15% by mass. Microanalyses of the chalcopyrite particles showed that the cobalt content was from 0.1 to 1.13%. Similarly, the content of cobalt and nickel in marcasite did not exceed 1.12% by mass. The content of nickel in the isomorphic series of cobaltite–gersdorfite reached 10.56%. Cobaltite–gersdorfite was also found in aggregates with chalcopyrite as very fine, below 15 μm, hipoautomorphic grains or crystals as well as in aggregates with bornite and sphalerite. 3. Results and discussion The total content of metal-bearing minerals in the examined sample of copper concentrate during pressure leaching at 180 °C was observed to decrease from 39.98% to 18.53% by weight after 240 min of leaching (Table 3). This corresponded to a 53.65% of overall reduction in sulphide content in examined pressure leaching process (Table 3, Fig. 3). However, the content of chalcocite–digenite initially increased from 5.96% in feed to 8.20% (sample 0, Table 3) before the introduction of oxygen at 180 °C and then decreased to 0% after 60 min of leaching. Simultaneously, the content of covellite remarkably increased from 0.61% in feed to its maximum 24.51% after 90 min of leaching. In the initial stage of pressure leaching, a formation of Cu sulphides in the mineral phase conversion is observed as a dominating process. The key observation in this stage of process was mineralogical alterations of copper sulphides without the presence of soluble copper in the solution. It was found that at 180 °C, leaching of copper was not observed within the initial 90 min of the process (Fig. 4). No copper was analytically detected in the leaching solution. Simultaneously, the amount of metal-bearing minerals in the solid decreased from 39.98% to 26.79% in total, when the leaching of copper starts (Fig. 3). It can be subjected to only the solid phase conversion of copper minerals. Moreover, at the initial stage pyrite–marcasite, sphalerite, galena and tennantite undergo either to chemical leaching or to chemical conversion (PbS to PbSO4, Table 3). Galvanic effect between the various sulphides can also be considered for the polymineral leaching feed.

100oC 120oC

25

140oC

Fig. 5. Openwork, permeable texture of covellite (blue) formed during conversion of chalcopyrite (yellow). Reflected light, without the analyser.

The solid conversion of copper sulphide minerals was remarkable even without the presence of oxygen (Table 3; see feed and sample 0). After temperature reached 180 °C, the major copper sulphide minerals: chalcopyrite and bornite started to convert to covellite and chalcocite–digenite (Table 3 — sample 0, Fig. 3). The phase transformation process took place after about 1 h, along with a decreasing content of all metal-bearing minerals. It was microscopically detected that in the first period of leaching the chalcopyrite particles were surrounded with a porous structure of covellite crystals (Fig. 5). Simultaneously, there was no copper detected in the solution. The observed increase of chalcocite and covellite content with simultaneous decrease of other metal sulphide content unequivocally shows that the phase transformation during pressure leaching takes place in the solid state both without the oxygen and in the presence of oxygen. The content of covellite in the initial stage of leaching was found to increase from 0.61% (feed) to 10.94% (Table 3, sample 0, without oxygen). Simultaneously, the content of chalcopyrite decreased by 61.49%, whereas the content of bornite by 88.51%. It is unquestionable that the observed increase of covellite content is directly correlated with the decrease of chalcopyrite and bornite contents.

160oC 180oC

Cu concentration, g/L

20

15

10

5

0 0

30

60

90

120

150

180

210

240

Leaching time, min Fig. 4. Copper concentration vs. leaching time for different leaching temperatures.

Fig. 6. The grain of a primary bornite at the conversion stage to chalcocite and finally to covellite. Reflected light, without analyser.


Fig. 7. Final effect of phase alterations during pressure leaching (blue colour indicates openwork permeable covellite aggregate). Reflected light, without analyser.

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However, the analysed process is based not only on a simple phase alteration. It is due to also a simultaneous increase of chalcocite– digenite, which grew by 37.56%. During the microscopic observation of the polished specimens one can see that bornite is the first copper sulphide which exhibits covellinization in the initial period of pressure leaching. Bornite, in the intermediate step of chalcocitisation, changed its structure. The evidence for this is the microscopically observed existence of transitional phases at the boundary between bornite relicts and newly created chalcocite phases (Fig. 6). The final product of such a progressive phase transition is covellite. Before copper ions were detected in the solution, chalcocite was completely converted to covellite (Table 3, Fig. 3). After 90 min of the process, the copper solubilisation was observed to occur (Table 3, Fig. 3). The principal copper minerals contained in the leaching feed were detected to be converted into covellite (Figs. 6, 7). Simultaneously, the contents of the remaining sulphide minerals: pyrite, marcasite, sphalerite, galena and tennantite evidently decreased (Fig. 3). In particular, the behaviour of pyrite–marcasite is not clear. From the beginning of the leaching process its content decreased slightly and it was observed to decrease considerably when copper starts to be leached after 90 min. When the content of covellite in the leached solid reached its maximum equal to 24.51%, the copper concentration in the solution started to increase (between 90 and 120 min of leaching,

Fig. 8. A model of phase conversion for a grain containing different copper sulphides during pressure leaching (explanations in the text).

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Fig. 3). The relative content of covellite in minerals increased from 1.52% in the feed to as high as 91.48% in the solid after 90 min of leaching. From the microscopic observation of the mineralogical alterations in the samples taken during leaching (Fig. 3) and their correlation with the leaching kinetics (Fig. 4) one can state that the phase transformation of copper sulphides during pressure leaching goes through only one mechanism. This can be presented in the form of a model of phase conversion for a grain containing different copper sulphides (Fig. 8). The grain consists of bornite, chalcopyrite and chalcocite (Fig. 8a, b). Initially, chalcopyrite converts its structure to covellite. Simultaneously, the bornite aggregates, if present at the grain's edge, are converted into chalcocite–digenite texture through intermediate steps. Chalcopyrite is the only copper sulphide which forms a kind of openwork, with a permeable texture, during its conversion to covellite (Fig. 7). This specific texture enables the transport of reactants to the surface of leached mineral (Fig. 8c). The permeable and openwork texture of the external covellite layer effectively guarantees the required rate of reactants transported to and from the reaction surface and enables successive phase conversion. Bornite, which is inside the particle, converts to chalcocite. As a result of intensive transportation of ions inside the permeable covellite texture, both primary and secondary chalcocite grains can be converted into openwork and permeable covellite (Fig. 8e, f). 4. Conclusions The observed phase alterations of copper sulphides during pressure leaching of sulphide copper concentrates clearly indicate complexity of hydrometallurgical pressure leaching of the polymineral and polymetallic sulphidic feed. From the presented quantitative and qualitative mineralogical analyses we can unequivocally notice that the complex copper mineral phases behave in a very specific way. Chalcopyrite (CuFeS2) and bornite (Cu5FeS4), sulphides with elevated contents of copper and sulphur, are subjected to specific phase alterations during leaching. Chalcopyrite is very rapidly converted into covellite in the first step of covellinization. The openwork and permeable texture of newly formed covellite enables the transport of reactants to other leached minerals disseminated in the chalcopyrite phase. Bornite, during the transformation to chalcocite, forms a compact and non-permeable texture which protects the transport of reactants to disseminate inside bornite minerals such as Ni–Co sulphides. Covellinisation of copper sulphides under the pressure leaching conditions, mainly including formation of openwork and permeable texture of covellite was found to take place both without and in the presence of oxygen in the reactor. The observed conversion can be considered as the most important process in the pressure leaching of copper sulphides. The permeable texture of newly formed covellite, exhibiting a high reaction surface, accelerates leaching of the remaining sulphides of copper and accompanying metals disseminated in the mineral phase. Permeable, openwork texture of the covellite, formed during copper sulphides conversion, becomes easily leachable and facilitates the transport of the leaching media to the reaction surface, effecting leaching of the remaining minerals in the solid feed. The fast destruction and collapse of the open work texture of the leached minerals are unfavourable for the leaching rate. Consequently, the process leads to a remarkable decrease in the leaching rate and formation of covellite and chalcopyrite massive structures which are difficult to leach. A control of both the phase conversion and formation of open work permeable textures enable more effective optimisation of parameters of pressure leaching of the polymineral sulphide feed. Acknowledgements This work was carried out in the frame of HYDRO project (Polish NCBiR project contract ZBP/56/66309/IT2/10). Authors acknowledge

the financial support given to this project by the NCBiR (National Center for Research and Development) under the IniTech Enterprise. We also wish to thank our various partners of the project for their contributions to the work reported in this paper.

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