A potential method for arsenic removal from copper concentrates

Minerals Engineering 20 (2007) 26–33 This article is also available online at: www.elsevier.com/locate/mineng

A potential method for arsenic removal from copper concentrates Ivan Mihajlovic a,*, Nada Strbac a, Zivan Zivkovic a, Renata Kovacevic b, Mirjana Stehernik b a

Technical Faculty in Bor, University in Belgrade Vojske, Jugoslavije 12,19210 Bor, Serbia and Montenegro b Copper Institute Bor, Zeleni Bulevar 35, 19210 Bor, Serbia and Montenegro Received 6 January 2006; accepted 12 April 2006 Available online 8 June 2006

Abstract This paper presents results for a project demonstrating a new technology for arsenic removal from copper concentrates. The problems concerning pyrometallurgical processing of copper concentrates with high arsenic content are presented in the first part of the paper. Possible solution of the problem by leaching of natural enargite crystals with sodium hypochlorite, under alkaline oxidizing conditions, with enargite converted into crystalline CuO and the soluble arsenic forming AsO3 4 was experimentally investigated and results presented in the second part of the paper. Kinetic parameters were calculated for both isothermal roasting and enargite leaching. According to the Arrhenius diagrams, the activation energy of the process, conditions was calculated. The kinetic equation for h under isothermal i2   1=3 desulfurization during isothermal roasting was found to be: 1  ð1  aÞ  t, with activation energy of ¼ k  t ¼ 5:0593  exp 4571 T 38 ± 3 kJ/mol. Kinetic equation describing arsenic removal, during same process, was: ð1  2=3aÞ  ð1  aÞ2=3 ¼ k  t ¼ 2:7322   4330 exp T  t, with activation energy 36 ± 5 kJ/mol. For the process of enargite leaching kinetic equation was:ð1  2=3aÞ  ð1  aÞ2=3 ¼  3608 k  t ¼ 837:87  exp T  t, with activation energy of 30 ± 1 kJ/mol. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Sulfide concentrates; Leaching; Roasting; Arsenic; Hypochlorite

1. Introduction Arsenic is one of the most common toxic impurities found in copper concentrates. The main As-containing mineral species, in copper concentrates, are enargite and luzonite, while realgar and arsenopyrite are also present in lesser amounts. Unfortunately, the prevalence of enargite among the copper-bearing minerals and as a result the relatively high arsenic content in the concentrates substantially reduces their economic value, owing to the hazardous emissions generated from pyrometallurgical processing. The amount of arsenic removed during the process of

*

Corresponding author. Tel./fax: +381 30 424 547. E-mail address: [email protected] (I. Mihajlovic).

0892-6875/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2006.04.006

arsenic-bearing concentrate roasting is very high. Arsenic, as well as its oxides, are highly volatile and leaves the reactor as off-gas constituents. Thus, in unfavorable metal market conditions, direct roasting is uneconomical because gas cleaning facilities demands are too expensive. In order to minimize the problems associated with the processing of these very hazardous fumes, the arsenic content in copper concentrates must be reduced to low levels (usually to <0.5% As) and such levels are difficult to obtain by differential flotation some sulfide deposits (Vinals et al., 2002). In an attempt to solve this problem, we have explored the possibility of hydrometallurgical technology for dissolving the arsenic from the concentrate prior to pyrometallurgical processing. Two techniques for arsenic removal from enargite are presented in the literature (Cureli et al., 2005):

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(1) Alkaline leaching of energite concentrates using sodium sulfide solutions after mechanical activation via fine grinding (Balazˇ et al., 2000). (2) Leaching of natural enargite crystals with sodium hypochlorite under alkaline oxidizing conditions with enargite converted into crystalline CuO and the arsenic solubiles forming AsO3 4 (Vinals et al., 2002). It was decided to evaluate the possibility of applying the second method, because it is attractive in terms of its potential application on a commercial scale.

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2. Experimental methods Large natural crystals of enargite from Bor copper mine (Serbia and Montenegro), ore deposit ‘‘H’’, were chosen for this study. Mineralogical investigations were conducted using X-ray diffraction with the atomic powder diffractometer ‘‘PHILIPS’’ APD SYSTEM PW 1710, under the following conditions: 2h range (5–90°), velocity 0.05°/s, Cu anticathode with 40 A current and voltage of 35 V. ICP-AES analyses, on the atomic emission spectrometer (model

Fig. 1. Experimental setup for the roasting process described: (1) syringe filed with solid CaCl2 (first) and KMnO4 with KOH solution (second), (2) electric-resistant furnace, (3) pyrometer and voltmeter for measuring and regulation of temperature, (4) silica tube in which crucible with investigated sample is placed, (5) water cooled spiral condenser, (6) crucible with mineral sample, (7) absorption tube filed with aqueous solution of hydrogen peroxide, (8) magnetic stirrer, (9) automatic burette filed with NaOH standard solution, (10) control absorption tank and (11) vacuum pump.

Table 1 Composition of the starting natural enargite sample Components

Cu

As

S

Fe

Al2O3

SiO2

Ba

Mn

Sb

Ge

Pb

Sn

Ti

Ca

V

Zn

Rest

Contents, %

26.25

10.34

19.48

1.62

3.18

38.12

0.64

0.004

0.04

0.06

0.0054

0.011

0.052

0.014

0.008

0.15

0.025

Fig. 2. XRD patterns of a representative sample of enargite mineral from ore deposit ‘‘H’’.

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Plasma Vision 3410+ARL), were used to ascertain the purity of the starting sample as well as the composition of the products of reaction with % of errors expected ±0.05%. Isothermal investigations were performed using an electric resistance furnace with thermostatic control. A measured volume of air was introduced into the reaction area, while gaseous product of reaction (mostly SO2), were passed from the furnace tube to absorption tubes, filed with an aqueous solution of hydrogen peroxide, producing sulfuric acid. Gaseous arsenic compounds (As4O6) were removed from the gas stream in the spiral condenser located in front of the absorption tubes, where As4O6(g) was transformed to As2O3(s). Sulfuric acid produced was reacted with measured standard solution of sodium hydroxide in the presence of indicator for the purpose of calculation of sulfur content and hence the degree of desul-

furization during oxidation roasting was calculated with % of errors expected ±0.1%. The amount of sulfur reacted with oxygen during process of oxidative roasting was calculated using equation %S ¼

V NaOH  M NaOH  ES 100 ;  m 1000

ð1Þ

where: ES – sulfur equivalent, m – mass of the sample, VNaOH – volume of NaOH spent during titration, MNaOH – NaOH molarity. The degree of desulphurization was calculated using the equation

Fig. 3. Amount of: (a) sulfur, (b) arsenic reacted with oxygen during oxidative roasting of enargite (D – degree of desulfurization, XAs – arsenic reacted, s – time).

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%D ¼

%S  100; %S1

ð2Þ

where: %S – amount of sulfur reacted with oxygen during process of oxidative roasting, calculated using Eq. (1), %S1 – amount of sulfur present in starting sample. Schematic diagram of the experimental setup for roasting process is presented in Fig. 1.

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The degree of desulphurisation was calculated using Eq. (2) and was used for determination of the kinetic parameters of oxidation, using Sharp’s isothermal model (Sharp et al., 1966) described later. All the kinetic experiments were performed with particle sizes – 100 lm. Leaching of enargite samples was conducted in a 1 dm3 three-neck tank with condenser, mechanical stirrer and ultra-thermometer. The leaching kinetic experiments were performed at nearly constant hypochlorite concentration (0.3 mol/dm3 NaCIO) by using a large solution volume

Fig. 4. Linearisation of experimental data points using chosen kinetic equation: (a) for desulfurization and (b) for arsenic removal during enargite roasting process.

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(800 cm3) and a small amount of solid (0.5 g). Leaching solutions contained 5 g/dm3 NaOH, with pH = 12 in the starting solution. The progress of the reaction was determined by analyzing arsenic by inductively coupled plasma emission spectroscopy. According to the reaction stoichiometry, the fraction of the enargite reacted was determined as a function of arsenic extracted, XAS. Residual solids were characterized using XRD. 3. Results and discussion Table 1 shows the composition of the samples used. Diffractometric analysis (XRD) (Fig. 2) shows that starting sample contains a significant concentration of enargite. Oxidative roasting was performed by introducing a predetermined volume of air into the tube furnace reaction space. Kinetic analysis under isothermal conditions during enargite roasting was investigated both with respect to S and As removal during the process. Roasting temperatures were in the range of 400–800 °C. The following reactions (Smirnov and Tihonov, 1966) describe the chemistry of enargite oxidative roasting:

Civilized Software). This method for selection of kinetic equation for optimal fit of experimental data points was described previously (Minic´ et al., 2005; Mihajlovic´ et al., 2004). Standard deviation was calculated for all nine equations using the formula sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n X 1 2 ð5Þ ðy M  y F Þi ; SD ¼  n i¼1 where: SD – Standard deviation; n – number of data points regarded; yM – values of straight-line points obtained using MLAB software iteration model; yF – values for linear experimental data obtained using one of the kinetic equations proposed by Sharp. Using equation D3: [1  (1  a)1/3]2 = k Æ t for desulfurization and D4: (12/3a) – (1a )2/3 = k Æ t, for arsenic removal; where: a – degree of reaction, t – time, k – rate constant. Best linearisation of experimental data was performed (i.e. minimum standard deviation for all the

– When roasting enargite in oxidative atmosphere at temperatures up to 193 °C oxidation can be described as: 4Cu3 AsS4 þ 13O2 ¼ 6Cu2 S þ 10SO2 þ 2As2 O3

ð3Þ

– In the temperature interval 193–550 °C, oxidation is carried out as: 4Cu3 AsS4 þ 13O2 ¼ 6Cu2 S þ 10SO2 þ As4 O6

ð4Þ

Cu2S formed will oxidize to CuO. – Part of the As2O3 present, during roasting is transformed to less volatile arsenic (V) oxide, (As2O5) which, forms copper arsenate 3CuO–As2O5. – At 315 °C As2O5 dissociates to As4O6 and O2 (Mihajlovic´ et al., 2005). Analysis of kinetic parameters for the process of oxidative roasting of investigated samples, under isothermal conditions, was done using the method proposed by Sharp et al. (1966). Results presenting the degree of desulfurization as a function of time at different roasting temperatures are given in Fig. 3(a), while results presenting amount of arsenic, removed as As4O6 (XAS) are presented in Fig. 3(b). According to the results presented in Fig. 3 it is obvious that the maximum desulfurization is obtained at 800 °C after 30 min and maximum value is 78%. Also 81% of arsenic is removed at 750 °C, after 30 min. Linearisation of experimental results, presented in Fig. 3, was tested using nine different kinetic equations, proposed by Sharp et al. (1966). The criteria for accepting an equation as best for linearisation of the experimental data was the least standard deviation of linear data in comparison with linear fitting of experimental data obtained by iteration program that MLAB software provide (MLAB,

Fig. 5. Arrhenius diagram for the process of enargite oxidative roasting constructed from the angle of: (a) S and (b) As removal during the process.

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isotherms), Fig. 4. According to the Sharp’s theory, equation D3 describes three-dimensional diffusion, spherical symmetry, and is Jander’s equation, while equation D4 represents three-dimensional diffusion, spherical symmetry, and is Ginstling–Braunstein’s equation (Sharp et al., 1966). From the slopes of the linearized isotherms, rate constants were determined and an Arrhenius diagram, Fig. 5, constructed. According to the Arrhenius diagrams, the activation energy of the process, under isothermal conditions was calculated. The kinetic equation for desulfurization was found h i2   to be: 1  ð1  aÞ1=3 ¼ k  t ¼ 5:0593  exp 4571  t, with T

31

activation energy of 38 ± 4 kJ/mol. Kinetic equation 2=3 describing arsenic removal is: ð1  2=3aÞ  ð1  aÞ ¼   t, with activation energy 36 ± k  t ¼ 2:7322  exp 4330 T 3 kJ/mol. The % of errors expected was calculated using error in the slopes of the relevant regression lines for each Arrhenius plot. From the results shown in Fig. 3(b) it is obvious that 81% of arsenic present volatilises during the process of oxidative roasting. Bearing in mind that amount of 10–20 t/ day of arsenic comes with the concentrate to the smelter plant, the amount of arsenic emitted is huge, demanding expensive gas cleaning equipment. This kind of As behaviour during pyrometallurgical processing is a fact well

Fig. 6. XRD patterns of a sample of solid leach residue.

Fig. 7. Influence of temperature on arsenic removal efficiency versus leaching time.

Fig. 8. Linearisation of experimental data points using chosen kinetic equation for arsenic removal during enargite leaching process.

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known for a long time, but still the problems arising from that fact are always present. This is the main reason why in

this paper investigations were done towards developing a procedure for arsenic removal prior to pyrometallurgical processing. 3.1. Leaching experiments Leach tests were conducted in a 1 L thermostated reactor containing NaClO solution mechanically stirred at 500 rpm. Leaching temperatures were in the range: 25–60 °C and times up to 120 min. Leach residues were also analyzed using XRD, Fig. 6, to determine the chemical and mineralogical changes undergone during hypochlorite leaching. Data obtained were found to agree with literature reports (Vinals et al., 2002, 2003) concerning tenorite (CuO) formation detected as the oxidation products of enargite. Thus, under these conditions the leaching reaction can be written as Cu3 AsS4 þ 11NaOH þ 35=2NaClO ¼ 3CuO þ Na3 AsO4 þ 4Na2 SO4 þ 11=2H2 O þ 35NaCl

Fig. 9. Arrhenius diagram for the process of enargite leaching.

Fig. 10. The process proposed technology scheme.

ð6Þ

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standard procedure that needs not to be specially explained. Copper concentrate after arsenic removal can also be treated pyrometallurgically in standard procedure.

Table 2 Selectivity of leaching reagent Mineral

% Reacted

Enargite Cu3AsS4 Realgar As2S2 Chalcopyrite CuFeS2 Chalcocite Cu2S Covellite CuS

98.93 96.6 16 5.38 31.68

4. Conclusion

The influence of temperature is shown in Fig. 7 where arsenic removal efficiency is plotted versus time for varying solution temperatures. For the results presented in Fig. 7, the procedure for obtaining kinetic parameters was the same as presented for the roasting process. Using Sharp’s method of half time of reaction, described above, kinetic equation: D4: (1  2/ 3a) – (1  a )2/3 = k  t was chosen. Using the chosen equation, for linearisation of experimental data points, Fig. 8 was constructed. Using data in Fig. 8, the following Arrhenius diagram was constructed, Fig. 9. According to the Arrhenius diagram, the activation energy of the process, under isothermal conditions was calculated. Kinetic equation was found  to be: ð1  2=3aÞ ð1  aÞ2=3 ¼ k  t ¼ 837:87  exp 3608  t, with activation T energy of 30 ± 1 kJ/mol. For enargite leaching using hypochlorite solution to be applied in the practice of copper concentrate leaching, it is important to know behaviour of other minerals possible to be present in copper concentrate during the process. The results of the selectivity of leaching reagent investigations are summarized in Table 2. The % reacted was determined after individual leaching of natural minerals presented in Table 2. Experiments for copper concentrate leaching, containing those mineral species, would be the subject of a future publication. According to the results obtained during the experiments and presented above, following scheme for process proposed can be proposed, Fig. 10. Arsenic can be stabilized by direct precipitation of the leach solution with Ca(OH)2. The reaction that occurs is (Kucharsky and Szafirsky, 2002) Na3 AsO4 þ CaðOHÞ2 þ H2 O ¼ CaHAsO4 þ 3NaOH

33

ð7Þ

CaHAsO4 obtained precipitates and can be safely removed after liquid/solid separation, phase 3 on Fig. 10. Hypochlorite solution can be returned to the process. Reaction rates are fast (in transition area with activation energy of 30 kJ/mol. After 120 min, at temperature 60 °C, almost all the enargite reacted (99%). This leaching reaction could be the basis of the process for removing As from copper concentrates. Copper converted to CuO, can be easily utilized by leaching with H2SO4 solution, which is a

Selective alkaline-oxidizing leaching of enargite in hypochlorite media can be used for arsenic removal from copper concentrates. Leaching reaction described could be the basis of a new process for removing As from copper concentrates and can be used on a commercial scale. The work presented in this paper can be treated as an beginning of investigation of this possibility for treating copper concentrates from Bor copper mine (Serbia and Montenegro), ore deposit ‘‘H’’, which contain large amount of As. Acknowledgements Research funded by Serbian Ministry of Science and Environment, as the part of the Project No.: 6743. References Balazˇ, P., Achmovicˇova, M., Bastl, Z., Ohtani, T., Sanches, M., 2000. Influence of mechanical activation on the alkaline leaching of enargite concentrate. Hydrometallurgy 54, 205–216. Cureli, L., Ghiani, M., Surracco, M., Orru, G., 2005. Benefication of gold bearing enargite ore by flotation and As leaching with Na-hypochlorite. Minerals Engineering 18 (8), 849–854. Kucharsky, M., Szafirsky, B., 2002. An investigation on stabilization of arsenic removed from blister copper. In: Palfy, P., Vircikova, E. (Eds.), Proceedings of the V International Conference Metallurgy, Refractories and Environment. Kosice, Stara Lesna, Slovakia, May 13– 16. ˇ ivkovic´, Z ˇ ., 2004. Determination of kinetic Mihajlovic´, I., Sˇtrbac, N., Z parameters of As2S2 oxidation process using MLAB software. In: Markovic, Z. (Ed.), Proceedings of 36th International October Conference on Mining and Metallurgy. Bor Lake, Serbia and Montenegro, pp. 375. ˇ ivkovic´, Zˇ., Ilic´, I., 2005. Kinetics and Mihajlovic´, I., Sˇtrbac, N., Z mechanism of As2S2 oxidation. Journal of the Serbian chemical society 70 (6), 869–877. ˇ ivkovic´, Z ˇ ., 2005. Thermal analysis Minic´, D., Sˇtrbac, N., Mihajlovic´, I., Z and kinetics of the copper-lead matte roasting process. Journal of Thermal Analysis and Calorimetry 82 (2), 383–388. MLAB, Civilized Software, Inc. Available from: . Sharp, H.J., Brinduy, W.G., Harahari, N.B., 1966. Journal of the American Ceramic Society 49, 379. Smirnov, V.I., Tihonov, A.I., 1966. Roasting of copper ores and concentrates. Metallurgy, Moscow (in Russian). Vinals, J., Roca, A., Benevente, O., Hernandez, M.C., Herreros, O., 2002. Removal of Arsenic Selenium and Tellurium from Base Metal Concentrates. In: Palfy, P., Vircikova, E. (Eds.), Proceedings of the V International Conference Metallurgy, Refractories and Environment. Kosice, Stara Lesna, Slovakia, May 13–16, pp. 481–486. Vinals, J., Roca, A., Hernandez, M.C., Benevente, O., 2003. Topochemical transformation of copper oxide by hypochlorite leaching. Hydrometallurgy 68, 183–193.