An investigation of copper and selenium recovery from copper anode slimes

International Journal of Mineral Processing 124 (2013) 75–82

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An investigation of copper and selenium recovery from copper anode slimes Yasin Kilic ⁎, Guldem Kartal, Servet Timur Istanbul Technical University, Department of Metallurgical & Materials Engineering, 34469 Maslak, Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 6 August 2012 Received in revised form 15 April 2013 Accepted 20 April 2013 Available online 29 April 2013 Keywords: Anode slime Recovery Dissolution Selenium Copper

a b s t r a c t In this study, an alternative method was investigated for the recovery of copper and selenium from copper anode slimes, supplied from Sarkuysan Co. in Turkey. The proposed process was composed of two hydrometallurgical steps; decopperization in H2SO4 medium and dissolution of selenium into NaOH solution. The effects of oxygen flow rate and solution temperature on the decopperization of anode slimes were examined in H2SO4 medium and copper recovery was achieved with an efficiency of 94%. The X-ray diffraction (XRD) analysis revealed Cu2Se as a remaining phase of copper in anode slimes after the decopperization process. Following the decopperization stage, anode slimes were treated with NaOH solution in order to dissolve selenium. The influences of NaOH concentration, temperature and various types of oxidants on selenium dissolution were inspected. The uniqueness of this technique is that it provides to have an efficiency of selenium recovery as high as 86.8% in an alkaline solution (4 M NaOH) under atmospheric conditions without requiring an autoclave. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Anode slimes are valuable by-products in the electrolytic copper refining process. Impurities in anode slimes can be classified into the groups based on their electrochemical potentials and chemical behaviors (Table 1). Copper anode slimes from electrolytic copper refineries are now supplying the source of most of the world's selenium due to its significant contents considering high costs and technical difficulties of extraction from its ores. Many process combinations, namely pyro–hydro, hydro– pyro, hydro–hydro, and hydro–electro metallurgical methods have been experienced for the recovery of copper and/or other valuable metals from the slimes (Fig. 1). The primary goal of these arrangements can be categorized in two ways; the recovery of copper and then the recovery of valuable target metals, or the recovery of all valuable elements at the same time (Morrison, 1963; Scott, 1990; Langner, 1997; Hoffmann, 2000; Davenport et al., 2002; Amer, 2003; Chen and Dutrizac, 2005; Rao, 2006; Ojebuoboh, 2007). Decopperization of anode slimes is the first stage of selenium recovery processes. Several techniques have been developed for decopperization, and the most popular ones are by dissolving after sulfatizing or oxidizing roasting, by leaching in concentrated sulfuric acid solution or by dissolving under oxygen pressure (Hoffmann, 2000; Rao, 2006). During or after decopperization, selenium can be recovered by either pyrometallurgical or hydrometallurgical methods. Pyrometallurgical techniques like sulfatizing and oxidizing roasting

⁎ Corresponding author. Tel.: +90 212 285 71 35; fax: +90 212 285 34 27. E-mail address: [email protected] (Y. Kilic). 0301-7516/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.minpro.2013.04.006

generate Se vapor which needs to be captured as SeO2 (Plitzko and Treiber, 2005). On the other hand, hydrometallurgical processes are more promising compared to pyrometallurgical options in terms of cost effectiveness and environmental concerns. Alkali leaching under pressure or atmospheric conditions, wet chlorination, solvent extraction, and vacuum distillation have been so far developed in hydrometallurgy to recover selenium (Morrison, 1963; Cooper, 1990; Heimala et al., 1977; Hait et al., 2002; Amer, 2003) The aim of the present study is to develop an alternative recovery method for both selenium and copper by designing an economical and most importantly continuous system with high efficiency. The effects of process parameters were examined in a systematic manner. In addition, the kinetics of selenium dissolution was investigated to determine a proper reaction kinetic model. Gold and other PGMs were enriched in the anode slimes during the process.

2. Materials and methods A sample of anode slimes (Courtesy of Sarkuysan Copper, Turkey) was first ground to below 300 μm and then dried to analyze chemical composition by atomic absorption spectroscopy (AAS). The chemical composition of the homogenized sample is given in Table 2. Additionally, the XRD analysis revealed that Cu2Se, SiO2, PbSO4, and SbAsO4 were the main phases of the homogenized slime (Fig. 2). Two hydrometallurgical procedures were applied to recover selenium from the slime; decopperization in H2SO4 medium and subsequently dissolution of selenium into NaOH solution. The characterized slime samples were put into the glass reactor then heated up to a desired temperature which was stabilized with a contact thermometer

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Table 1 Classification of elements and components in anode slimes (Yildirim, 1982; Yavuz and Ziyadanogullari, 2000; Davenport et al., 2002; Amer, 2003; Chen and Dutrizac, 2005). Classification

Elements

Form/phase

Explanation

Nobler than copper ⁎Basic metals with E0 values 0 closed to E0Cu2+/Cu Basic metals with relatively lower electrode potentials

Ag, Au and Pt-group Bi, As and Sb

Elemental Mixed oxides with lead oxides or arsenates

Most valuable part

Pb, Sn, Ni, Co, Fe and Zn,

PbSbO2, PbAsO2, SnO2, NiO and Fe2O3, PbSO4 and Sn(OH)2SO4

Copper

Cu

Elemental

Chalcogen

S, Se and Te

Insoluble compounds with copper, i.e. Cu2Se, Ag2Se, Cu2Te, Ag2Te, and Cu2S

- Coming directly from anodes as insoluble compounds - Reacting with the electrolyte to produce insoluble compounds Disproportion of Cu2O according to the below reaction 2Cu+ → Cu + Cu2+ leading to noteworthy copper losses

0 ⁎ E0 = the standard potential relative to the standard hydrogen electrode; E0Cu2+/Cu = the standard potential of copper.

Anode Slime

Sulfatizing Roast

Oxidizing Roast

Aeration Leach H2SO4

Se 4+ Reduction SO2

H2SO4 Leach

Oxidizing Roast

Sulfation Leach H2SO4

H2SO4 Leach (O2 Pressure)

Caustic Pressure Leach

Soda Roast

Se +4 Reduction SO2

Sulfatizing Roast in SO2 + O2

Dore Furnace

TBRC

Fig. 1. Flow diagram of different processes for the treatment of copper anode slimes (Cooper, 1990).

(±1 °C). The stirring speed of prepared solutions was kept constant at 800 rpm. AR-grade chemical reagents were used in all experiments. In decopperization, 100 g of slime samples was treated with 500 ml solution comprised of 53.26 ml H2SO4 (98 g/L) and 0.5 ml HCl (1.189 mg/L) which was added to prevent Ag dissolution; whereas in selenium dissolution, 50 g of decopperized slime samples was dissolved in the NaOH solution ranging in concentration from 0.36 to 4 M with the constant liquid/solid ratio of 10:1. Chemical analyses of the solutions were carried out by AAS to measure the contents of copper and selenium; and the residual solid samples were characterized by X-ray diffraction (XRD) to determine the main phases. The proposed procedure for the treatment of copper anode slime to recover both copper and selenium is summarized in Fig. 3. The dissolution conditions of copper and selenium were optimized according to the

characterization results of samples taken before and after each dissolution steps. 3. Results and discussions 3.1. Recovery of copper The homogenized and characterized anode slime was used for the first step of recovery process; decopperization. Effects of relevant parameters such as; oxygen requirement, oxygen flow rate and reaction temperature are reported below. 3.1.1. Necessity of oxidative condition The required amount of oxygen to dissolve copper in the slime was initially examined at constant parameters: 80 °C temperature, 2 M

Table 2 Chemical analysis of anode slime used in decopperization studies. Elements

Weight [%]

Cu Se Pb S Sn Ag Au Te Fe Ni As Sb Ba Bi Zn Cl Mg Others (Si, Mn, O2, Sr…etc.)

25.80 4.68 12.93 8.95 8.1 2.80 0.23 0.90 0.67 0.29 3.93 0.99 0.68 0.15 0.35 1.35 1.53 24.5 Fig. 2. X-ray pattern of the homogenized slime.

Y. Kilic et al. / International Journal of Mineral Processing 124 (2013) 75–82

Anode Slime

Grinding

Dewatered

< 300 µm

Chemical Analysis & XRD

Characterization

H2SO4

Decopperization

77

Oxygen or Air

HCl

Filtration

Solution

Electrolysis

COPPER

(Can be used as electrolyte)

Filter Cake

Characterization

Dissolution of Selenium

NaOH

Chemical Analysis & XRD

Solution

Reduction & Rafination

SELENIUM

Filter Cake

Reductive Smelting

METAL

(Include precious metals)

Slag Fig. 3. Flow chart of the proposed procedure for the recovery of copper and selenium.

H2SO4 + 0.5 ml HCl (1.189 mg/L) acid solution, 800 rpm stirring speed, 1:5 S/L ratio. In this series of experiment, oxygen was also used instead of air in order to shorten the leaching time and to increase the recovery rate of

copper in the slime. The effect of oxygen on the dissolution of copper is given in Fig. 4. As shown clearly, the sufficient amount of oxygen was essential to dissolve copper with relatively high recovery rate. The maximum

100 Atmospheric conditions Air (2 L/min)

Cu Recovery [%]

95

Oxygen (2 L/min)

90

85

80

75 0

20

40

60

80

100

120

Time [min] Fig. 4. Effect of oxidative conditions on the dissolution of copper from anode slime. [80 °C, 2 M H2SO4 + 0.5 ml HCl, 800 rpm, 1:5 S/L ratio].

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95

Cu Recovery [%]

90

85

80

75

2 L/min 3 L/min

70

4 L/min Athmospheric cond.

65 0

20

40

60

80

100

120

Time (min) Fig. 5. Effect of oxygen flow rate on the dissolution of copper from anode slime. [80 °C, 2 M H2SO4 + 0.5 ml HCl, 800 rpm, 1:5 S/L ratio].

recovery of copper at ~ 94% was obtained in the presence of oxygen at 2 L/min flow rate.

A negligible amount of selenium (~%0.20 of total selenium) was dissolved from the slimes at the given conditions.

3.1.2. Effect of oxygen flow rate The influence of oxygen flow rate on the recovery of copper was investigated in a range of 2–4 L/min. During the experiments, 100 g of slime samples was leached with the same solutions (2 M H2SO4 + 0.5 ml HCl) at 80 °C. The flow rates below 2 L/min were not studied due to low recovery efficiencies (Yildirim, 1982). 2 L/min of oxygen flow rate was found as a sufficient and an ideal value for the highest dissolution of copper, namely ~ 94% (Fig. 5). Oxygen flow into the solution initially caused a significant contribution on the copper dissolution compared to the one obtained in atmospheric condition. However, further increase in oxygen flow rate did not promote the dissolution; conversely, it had a slightly negative influence on the copper recovery (i.e., ~94% and ~92% of copper recovery were accomplished after 2 h of treatment at 2 and 4 L/min of oxygen flow rate, respectively). This unexpected situation was thought to have occurred because of the turbulence which might be triggered by high stirring speed (800 rpm); nonetheless, it was not investigated in detail during the study.

3.1.3. Effect of temperature The effect of temperature on the dissolution of copper was investigated at the temperature ranges of 25–90 °C in H2SO4 solution. In this series of experiments, initial sulfuric acid concentration and oxygen flow rate were kept constant at the value of 2 M and 2 L/min, respectively. The recovery efficiency of copper as a function of time is given in Fig. 6 at various temperatures. The high Cu recovery rate observed in the first 5 min was possibly attributed to the green copper hydroxide film formation on the longtime stored slimes. The extraction of copper was increased with an increasing temperature until 60 °C. Beyond this point, temperature did not lead to any significant change in the dissolution rate of copper, considering the slight difference of extraction rate obtained at the temperature range of 60 to 90 °C. It could be deduced that 93% of soluble copper could be recovered at 60 °C and above. However, the rest of the experiments were carried out at constant temperature of 80 °C for the comparison of obtained results. The results of XRD and chemical analyses of decopperized anode slimes revealed that regardless of the leaching parameters

95

Cu Recovery [%]

90

85

80 25 °C

60 °C

80 °C

90 °C

70 °C

75

70 0

20

40

60

80

100

120

Time (min) Fig. 6. Effect of temperature on dissolution of copper from anode slime. [2 M H2SO4 + 0.5 ml HCl, 2 L/min O2, 800 rpm, 1:5 S/L ratio].

Y. Kilic et al. / International Journal of Mineral Processing 124 (2013) 75–82

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Fig. 7. X-ray pattern and chemical composition of decopperized anode slime. Fig. 9. X-ray pattern and chemical composition of the residue after the dissolution of selenium.

(i.e., temperature and flow rate), the dissolution of copper became limited beyond 90 min due to the insoluble intermetallic copper compound; Cu2Se (see Fig. 7). This outcome is also consistent with the research conducted by Chen and Dutrizac (2005).

3.2.1. Effect of NaOH concentration The effect of alkali concentration on the extraction of selenium was inspected in the concentration range of 0.36 to 5 M NaOH solution. The rate of selenium dissolution increased significantly with increasing NaOH concentration from 0.36 to 4 M. The recovery rate of selenium in 0.36 M NaOH was too low (~3%) as seen in Fig. 8. This illustration clearly exhibits the diverse influence of NaOH concentration at various process durations and it is also commercially significant for industrial applications in two ways: (i) Estimating the efficiency of selenium recovery in a given process time and NaOH concentration and (ii) deciding on the process parameters for selenium recovery. Selenium recovery rate reached its maximum value; ~ 86% in 180 min at 4 M NaOH, and higher concentrations did not increase recovery efficiencies. The XRD pattern and chemical analyses revealed Ag(Sb)Se(Te) as the insoluble selenium phase after the leaching process (Fig. 9).

3.2. Recovery of selenium The decopperized anode slime was treated with NaOH solution in the second stage to get selenium based on the dissolution reactions (Eqs. (2) and (3)). Se þ 2NaOH þ O2 →Na2 SeO3 þ H2 O

ð2Þ

Se þ 2NaOH þ 3=2O2 →Na2 SeO4 þ H2 O:

ð3Þ

The composition of reaction products depends not only on the oxidation behavior of different selenium compounds, but also on the oxidation capability of the oxidant. In this part of the study, the decopperized anode slime with the composition presented in Table 3 was used to investigate the effects of critical parameters; namely NaOH concentration, temperature and different oxidant additions on the efficiency of Se recovery.

Concentration of NaOH [M]

5

45 min 30 min

3.2.2. Effect of temperature The influence of leaching temperature on selenium extraction was inspected at temperatures ranging from 40 to 90 °C and at constant process parameters: 4 M NaOH solution with 2 L/min O2 flow. Apparently,

90 min

60 min

120 min

150 min

180 min

15 min

4

150 min

30 min 120 min

90 min

45 min 60 min

150 min

15 min

3 180 min

30 min 45 min

120 min

90 min

10 min 60 min

2 30

35

40

45

50

55

60

65

70

75

80

85

Se Recovery [%] Fig. 8. Effect of NaOH concentration on the dissolution of selenium. [80 °C, 2 L/min O2, 800 rpm, 1/10 S/L ratio].

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100 90

40 °C

50 °C

60 °C

70 °C

80 °C

90 °C

80

Se Recovery [%]

70 60 50 40 30 20 10 0 0

20

40

60

80

100

120

140

160

180

Time [min] Fig. 10. Effect of temperature on the dissolution of selenium. [4 M NaOH, 2 L/min O2, 800 rpm, 1/10 S/L ratio].

The plot of 1 − 3(1 − α) 2/3 + 2(1 − α) with respect to process time, t exhibits almost linear correlation (R2 > 0.95) at various leaching temperatures (see Fig. 11) which apparently signified the mechanism of dissolution reaction of selenium as a diffusion-controlled reaction. The rate constants (k) for different temperatures (T) were calculated from the slopes of the straight lines in Fig. 11. Values of ln k were plotted versus 1 / T and the activation energy (Q) of selenium dissolution was determined as 62 kJ/mol (Fig. 12).

the increase in leaching temperature boosted the dissolution of selenium considerably (Fig. 10). Approximately 86% of selenium recovery was achieved at 80 °C and above. There was a negligible difference between 80 °C and 90 °C of leaching temperature in terms of recovery rate. The leaching kinetics of selenium dissolution was analyzed according to the Shrinking-core model which is the most accepted model for the kinetics of fluid–solid reactions (Hait et al., 2002; Amer, 2003). Four basic equations of this model were tested to determine the suitable model for this study. After examining the experimental results, it was found that the ash diffusion control model (Eq. (4)) was the most appropriate one for the kinetics of selenium dissolution under the studied conditions. 1−3ð1−αÞ

2=3

þ 2ð1−αÞ ¼ −kt

3.2.3. Effect of different oxidant additives The influences of different oxidants, namely potassium permanganate, hydrogen peroxide, oxygen and combinations of them, were studied to determine the importance of oxidant for the dissolution of selenium. This series of experiments was carried out at the same temperature, 60 °C in order to eliminate the risk of H2O2 decomposition at elevated temperatures. The variation of selenium recovery with respect to the additions of different oxidants is given in Fig. 13.

ð4Þ

where α is the fraction reacted at reaction time, t.

0.5

1-3(1-x)2/3+2(1-x)

0.4

40 °C

50 °C

60 °C

70 °C

80 °C

90 °C

0.3

0.2

0.1

0 0

20

40

60

80

100

120

140

160

Time [min] Fig. 11. Variations of 1 − 3(1 − α)2/3 + 2(1 − α) versus time, t at different temperatures, T.

180

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81

-5.5 -6 -6.5 -7

ln k

-7.5

Slope = -EA/R -8 -8.5 -9 -9.5 -10 2.7

2.75

2.8

2.85

2.9

2.95

3

3.05

3.1

3.15

3.2

3.25

1/T x 103[K-1] Fig. 12. ln k versus 1 / T.

70

60

Se Recovery [%]

50

40

30

20

10

0 0

20

40

60

80

100

120

140

160

180

Time [min] Fig. 13. Variations of selenium recovery with respect to the addition of different oxidants. [4 M NaOH, 60 °C, 800 rpm, 1/10 S/L ratio].

The highest dissolution rates were obtained with the usage of both oxygen and the combination of KMnO4 and oxygen. Even though KMnO4–oxygen combination exhibited superior efficiency

Table 3 Chemical analysis of decopperized anode slime used in selenium dissolution. Elements

Weight [%]

Cu Se Pb S Sn Ag Au As Sb Ba Cl Mg Fe Ni

1.50 8.15 20.30 11.49 13.00 4.48 0.45 3.65 0.68 1.40 2.29 2.90 0.20 0.18

values compared to the pure oxygen in the first 120 min of leaching, the final selenium recoveries were almost equal at steady state. The addition of H2O2 decreased the dissolution rates of selenium due to the reducing characteristic of H2O2 at high pH. The reduction mechanisms could be explained by the following equilibriums (Pourbaix, 1974) (Eqs. (5)–(8)): 2−

SeO4

2− SeO3

þ

−

þ

−

2−

þ 2H þ 2e →SeO3 þ H2 O þ 6H þ 4e →Se þ 3H2 O

− 2HO− 2 →O2 þ 2H2 O þ 2e þ

2−

−

þ

ð5Þ ð6Þ

ð7Þ



SeO4 þ 6HO2 þ 8H →Se þ 3O2 þ 6H2 O ðTotal ReactionÞ:

ð8Þ

4. Conclusions In this study, the recovery of copper and selenium from copper anode slimes was investigated in a laboratory scale and the results of this research could be summarized as the following;

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• The presence of oxygen or other oxidants was essential for dissolving copper and selenium with relatively high recoveries. • The extraction of copper and selenium from anode slimes was increased with increasing leaching temperature. However, the dissolution rate of copper and selenium did not significantly change at and above 60 °C and 80 °C, respectively. • Oxygen was the most suitable oxidant for the recovery of both copper and selenium. Flow rate of oxygen at 2 L/min was sufficient to achieve the maximum dissolution of copper and selenium at 94% and 86%, respectively. • Selenium recovery reached its maximum (~ 86%) using 4 M NaOH solution. • The leaching conditions for extracting 94% of copper were optimized as 2 M H2SO4 solution, 2 L/min O2 flow rate and 80 °C leaching temperature for 120 min of leaching time. • The ideal conditions for extracting 86% of selenium were determined as 4 M NaOH, 2 L/min O2 flow rate and 90 °C leaching temperature for 180 min of leaching time. References Amer, A.M., 2003. Processing of copper anodic-slimes for extraction of valuable metals. Waste Manag. 23, 763–770. Chen, T.T., Dutrizac, J.E., 2005. Minerological characterization of a copper anode and the anode slimes from the La Caridad copper refinery of Mexicana de Cobre. Metall. Mater. Trans. 36B, 229–240.

Cooper, W.C., 1990. The treatment of copper refinery anode slimes. J. Miner. Met. Metal. Mater. Soc. 42 (8), 45–49. Davenport, W.G., King, M., Schlesinger, M., Biswas, A.K., 2002. Extractive Metallurgy of Copper, fourth ed. Pergamon, USA. Hait, J., Jana, R.K., Kumar, V., Sanyal, S.K., 2002. Some studies on sulfuric acid leaching of anode slime with additives. Ind. Eng. Chem. Res. 41, 6593–6599. Heimala, S.O., Hyvarinen, O.V.J., Kinnunen, J.P.E., Tiitinen, H.A., 1977. Hydrometallurgical Process for the recovery of valuable components from the anode slimes produced in the electrolytic refining of copper, US. Patent 4002544. Hoffmann, J.E., 2000. Process and engineering considerations in the pressure leaching of copper refinery slimes. Proc. EPD Congr, pp. 397–410. Langner, B.E., 1997. Selenium. In: Habashi, F. (Ed.), Handbook of Extractive Metallurgy, vol. 3. Wiley-VCH, Weinheim, pp. 1557–1563. Morrison, B.H., 1963. Recovery and separation of se and te by pressure leaching of copper refinery slime. Proc. Int. Symp. Unit Processes in Hydrometallurgy, pp. 227–249. Ojebuoboh, F., 2007. Selenium and tellurium from copper refinery slimes and their changing applications. Proc. Eur. Metall. Congr., 2, pp. 571–585. Plitzko, C., Treiber, B., 2005. Selenium — an interesting but undervalued by-product of copper production. Proc. Eur. Metall. Congr., 1, pp. 193–209. Pourbaix, M., 1974. Atlas of Electrochemical Equilibria in Aqueous Solutions. NACE International Cebelcor, Houston0915567989. Rao, S.R., 2006. Resource recovery from process wastes. In: Rao, S.R. (Ed.), Resource Recovery and Recycling from Metallurgical Wastes, pp. 421–426. Scott, J.D., 1990. Electrometallurgy of copper refinery anode slimes. Metall. Mater. Trans. 21A, 629–635. Yavuz, O., Ziyadanogullari, R., 2000. Recovery of Gold and Silver from Copper Anode Slime. Sep. Sci. Technol. 35 (1), 133–141. Yildirim, G., 1982. Assessment of Selenium and Tellurium Rich Copper Anode Slime. (PhD. Thesis) Department of Metallurgical & Materials Engineering in ITU, Istanbul.