Electrorefining Electrolyte from Copper Plant Dust

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1 Materials Transactions, Vol. 43, No. 3 (22) pp. 532 to 536 c 22 The Japan Institute of Metals Electrorefining Electrolyte from Copper Plant Dust Bipra Gorai, Ranajit Kumar Jana and Zahid Husain Khan National Metallurgical Laboratory, Non Ferrous Process Division, (Council of Scientific and Industrial Research) Jamshedpur-8317, India An attempt was made to develop a suitable process for the recovery of copper from the dust sample of copper plant by sulphuric acid leaching with the idea that the resulting leached copper sulphate solution could be used directly as electrolyte in the copper electrorefining plant. XRD analysis of dust sample showed that copper was mainly present as sulphide and sulphate forms. Sulphuric acid leaching of the sample was carried out varying different parameters. The maximum copper recovery of 75% was achieved with the leaching parameters 353 K temperature, 4 h duration, 1:1 solid:liquid ratio and 1% H 2 SO 4 concentration. Roasting followed by leaching experiments were also carried out. It was found that leaching of the roasted (873 K) sample in sulphuric acid medium yielded maximum copper recovery of 95%. Leaching kinetics of roasted product indicated that mixed control i.e. ash diffusion and chemical control reaction was prevalent. XRD analysis of the roasted sample showed that copper sulphide was converted to oxide which probably increased the recovery of copper. It was also found that composition of the leach liquor obtained in optimised condition was comparable with the composition of electrolyte of operating electrorefining plant. (Received September 21, 21; Accepted January 21, 22) Keywords: copper dust, leaching, roasting, electrorefining 1. Introduction In conventional pyrometallurgical route, copper extraction is generally carried out in three steps: roasting, smelting and converting. During these processing steps considerable amount of copper loss occurs in the form of dust from the roaster, smelter and converter. It has been found that for processing of 1 tonnes of concentrate 7 1 tonnes of dust is formed. 1) In order to ensure efficient recovery and collection of dust all the plants have dust collecting systems like collection of dust beneath flues, expansion chambers, baloon flues, boiler settling, filtration through cloth bags in a bag house, electrostatic precipitation in cottrell treaters etc. So far as its treatment is concerned, it depends upon the composition of the dust. 2) Usually the dust is simply charged back to the smelting circuit. In this process the total production capacity of the plant reduces. Also such dust is a serious problem in terms of pollution and storage because of its high content of metal in the form of water soluble compounds and its low apparent density. There are several methods for copper plant dust processing either by hydrometallurgical route or by pyrohydrometallurgical route. Hydrometallurgical process using pressure leaching of smelter flue dust with sulphuric acid have been reported. 3, 4) Kruesi and Kruesi 5) described the leaching of copper containing material, scrap, ore, dust etc. with cupric tetra amine sulphate lixiviant and copper was recovered from leachate by electrolysis. Dust from smelter and converter of Huelva copper plant, Spain has been processed by leaching in hydrochloric acid-sodium chloride medium. 6) Fumes of Port Kembla copper smelter, flue-gas dust at the Alaverdi copper chemical plant, dusts of flash smelting furnace, copper processing flue dust, reverberatory dust and primary smelter dust have been processed hydrometallurgically to recover copper. 7 12) In pyro-hydrometallurgical process, dust sample is roasted in controlled atmosphere prior to leaching. The roast- ing is carried out to convert sulphide minerals of copper to its sulphate and/or oxide forms, which are easily leachable in dilute sulphuric acid. 13) The roasting is generally done between 773 and 1123 K depending upon wheather the need is partial roasting, dead roasting or sulphatizing roasting. 1) During roasting copper and iron sulphides are converted to their sulphates and oxides and some of the reactions are as stated below. CuFeS 2 + 4O 2 CuSO 4 + FeSO 4 (1) 2CuS + 7/2O 2 CuO CuSO 4 + SO 2 (2) 2CuFeS /2O 2 2CuO + Fe 2 O 3 + 4SO 2 (3) Apart from the roasting temperature, the partial pressures of O 2 and SO 2 in the roasting furnace are important parameters which governs the type of roasted product produced. The idea about the required values of these parameters for a particular product formation can be obtained from the predominance or stability diagram. 14) After successful roasting of the dust sample, the sulphate and oxidic products can be leached in dilute sulphuric acid without any difficulty as generally faced in the case of sulphide minerals. Although some literature on hydrometallurgical and pyrohydrometallurgical routes for processing of dust sample from various copper plants are available, no work has been reported on generation of required grade of leach liquor to be directly used in the copper electrorefining tank of the plant. In the present investigation, some dust sample from Indian copper plant has been taken up for these studies. Both the sulphuric acid leaching and roasting followed by sulphuric acid leaching routes have been carried out with the objective that the resulting leach copper sulphate solution can be used directly as electrolyte in the copper electrorefining plant. The details of the studies undertaken in this direction have been described in this paper. Corresponding author: bipragorai@yahoo.com

2 Electrorefining Electrolyte from Copper Plant Dust Materials & Methods The dust sample was collected from Indian Copper Complex, Ghatsila. The samples were ground to 14 µm and mixed thoroughly. Then the representative samples were prepared by cone and quartering method for XRD and chemical analysis. The chemical analysis of the sample was done by conventional method and by atomic absorption spectrophotometer. The XRD analysis of the sample were performed using CoKα target in a SEIFEERTS Diffractometer (PTS 33). Leaching tests were carried out on 1 g scale in conical flask using a temperature controlled water bath provided with the wrist action shaking facility. Roasting of the sample was done in a Raising Hearth Furnace on 1 g scale. Distilled water and LR grade sulphuric acid were used for all the leaching experiments. Chemical analysis of the leach liquors were done by conventional method as well as by atomic absorption spectrophotometer. 3.2 Sulphuric acid leaching Effect of acid concentration The water leaching of dust sample did not produce good recovery of copper as most of the copper was in sulphide and brochantite forms. Hence, sulphuric acid leaching tests were carried out to improve the dissolution of copper. The results of leaching of dust sample in different sulphuric acid concentrations are depicted in Fig. 2. The dissolution of copper increased with acid concentration and with 1 vol% H 2 SO 4, the copper recovery was about 55%. On increasing the concentration of acid beyond this, recovery of copper did not improve much. So 1 vol% sulphuric acid was taken as the optimum acid concentration for the further leaching tests. 3. Result & Discussion The chemical analysis of copper dust sample is shown in Table 1. It contained about 37% of copper apart from considerable amount of iron, silica, alumina and sulphur. The XRD studies of the sample (Fig. 1) showed that copper in the sample was present in sulphide, brochantite and sulphate forms. Therefore, water leaching of the sample was first done to dissolve copper sulphate from the sample. 15) Later on, all the leaching tests were carried out in sulphuric acid so that resulting copper sulphate solution can be used as electrolyte in the copper electrorefining tank. The results of the various sulphuric acid leaching tests are discussed in the following sections. 3.1 Water leaching The dissolution of copper in water leaching is shown in Table 2. It can be seen that maximum recovery of copper in water leaching was about 23%, which implied that about one fourth of the total copper present in the dust sample was probably in the form of copper sulphate which was easily soluble in water. Table 2 Fig. 1 Time of leaching in h XRD of copper dust sample. Recovery of copper in water leaching. % recovery of Cu Conditions: Temp = 33 K, Solid:Liquid = 1 : 15 Table 1 Composition of the Dust Sample. % by mass 65 Cu 37. Co.56 Fe SiO Al 2 O 3.38 S 8.97 Ag.16 Cd.6 Zn.17 Bi. Pb.45 Ni.79 Fig Temp = 33K Solid:Liquid = 1: H 2 SO 4 (v/v)( ) Effect of H 2 SO 4 concentration on recovery of copper.

3 534 B. Gorai, R. K. Jana and Z. H. Khan 8 353K K 318K 33K 45 Temp. = 33K Solid : Liquid = 1:1 Conc. of H 2 SO 4 = 1 (v/v) Time, t / h Solid : Liquid = 1:1 Conc. of H 2 SO 4 = 1 (v/v) Time, t / h Fig. 5 Recovery of copper with time at different temperatures. Fig. 3 Effect of time on recovery of copper. Table 3 Water leaching of roasted product. 7 3 Conc.of H 2 SO 4 = 1 (v/v) Temp = 33K Time of roasting in h % recovery of copper in leaching Roasting conditions:temp = 873 K, Temp = 33 K, Liquid:Solid = 1 : 1 2 Fig Liquid : Solid ratio Effect of Liquid:Solid ratio on recovery of copper Effect of time The effect of leaching time on recovery of copper is shown in Fig. 3. It can be seen that with the increase in leaching time copper dissolution increased and after 4 h its recovery was about 55%. With further increase in leaching time there was not much improvement in copper dissolution Effect of liquid:solid ratio Effect of Liquid:Solid ratio on copper recovery was studied in the range of 5 to 2, which is shown in Fig. 4. It can be seen that the dissolution of copper was about 55% at the S:L ratio 1:1. With further increase in the ratio to 1:2 there was marginal improvement in copper recovery Effect of temperature It was seen that the highest recovery of copper at room temperature was about 55%. So leaching test at elevated temperature was carried out to evaluate its effect on recovery of copper. The result is depicted in Fig. 5. Leaching temperature was varied from 33 to 353 K. It was seen that with the increase in leaching temperature from 33 to 353 K the copper recovery increased from 55 to 75%. 3.3 Roasting followed by leaching of dust sample As sulphuric acid leaching of dust sample did not show good recovery of copper, attempt was made to roast the dust sample varying the time and temperature of roasting prior to leaching. The temperature of roasting was varied from 873 to 173 K and roasting time was varied from 1 to 4 h Water leaching of the roasted product The dust sample which was roasted at 873 K was leached with water and the maximum recovery of copper in water leaching was found to be 24% (Table 3) when roasting was carried out for 1 h. As the time of roasting was increased the poor copper recovery was observed. This implied that copper minerals were converted to oxide rather than sulphate which was not easily soluble in water Sulphuric acid leaching of the roasted product The effect of time and temperature of roasting on recovery of copper in sulphuric acid leaching is depicted in Fig. 6. It can be seen that with the increase in time of roasting from 1 to 4 h at 873 K (roasting temperature), recovery of copper in sulphuric acid leaching increased from 75 to 95%. The higher recovery of copper after roasting may be due to the fact that during roasting operation copper sulphide was converted to oxide which was amenable to dissolution in sulphuric acid. It was also confirmed from the XRD analysis (Fig. 7) of the roasted sample that mostly copper oxide formation took place in roasting at 873 K. From predominance area diagram, 14) it is expected that at lower temperature (< 973 K) CuSO 4 formation is favoured. However, due to low sulphur content of the dust sample, the sulphur dioxide partial pressure was low, which probably did not favour the CuSO 4 formation in the present case. It was also observed that with the increase in roasting temperature from 873 to 173 K the recovery of copper decreased from 95 to 82%. This may be due to the for-

4 Electrorefining Electrolyte from Copper Plant Dust K 973K.8 Recovery of Cu (%) K Solid : Liquid = 1:1 Temp. = 33K Conc. of H 2 SO 4 = 1% (v/v) Time of Roasting, t / h 1-(1-X)^1/3/t (1-X)^2/3+2(1-X)/t Fig. 6 Effect of time and temperature of roasting on recovery of copper in H 2 SO 4 leaching. Fig. 9 Leaching kinetics of copper at 318 K. leaching kinetics of copper following the most wide spread shrinking core model for fluid-solid reaction was tested. The 16, 17) standard equations of this model tested were xαt (4) 1 (1 x) 2/3 αt (5) 1 (1 x) 1/2 αt (6) 1 (1 x) 1/3 αt (7) 1 3(1 x) 2/3 + 2(1 x)αt (8) Fig. 7 XRD of roasted copper dust sample. Solid:Liquid = 1:1 Conc. of H 2 SO 4 = 1 Temp. = 318K (v/v) Time, t / min Fig. 8 Recovery of copper with time in leaching of roasted product at 873 K for 4 h. mation of copper ferrite at higher roasting temperature which was difficult to solubulise in dilute sulphuric acid. 3.4 Kinetics of sulphuric acid leaching of roasted product Studies on the dissolution rate of copper at higher leaching temperature (318 K) were also made using roasted product of 873 K. The results have been shown in Fig. 8. In first half an hour, the copper dissolution was very fast with recovery about 86% and thereafter the dissolution rate became slow. The {1 (1 x) 1/3 }/tα{1 3(1 x) 2/3 + 2(1 x)}/t (9) Where x is the fraction of copper leached with respect to time t and α is the proportionality. It was finally found eq. (9) which is normally described as mixed control i.e. chemical and ash diffusion controlled kinetics gives an excellent fit to our data. The mixed control kinetics was derived for the leaching of chrysocolla 18) and was also used by Choi and Sohn 19) in the leaching of sea nodules. The leaching kinetics of roasted copper plant dust sample following the mixed control kinetics is depicted in Fig. 9. The mixed controlled reaction kinetics was probably due to the presence of some amount of copper ferrite in the roasted product. Copper ferrite leached as follows CuOFe 2 O 3 + 2H + Cu 2+ + Fe 2 O 3 + H 2 O (1) The Fe 2 O 3 phase provided a diffusion barrier with a moving reaction boundary and contributed to the diffusion controlled reaction along with the chemical controlled reaction of copper oxide present. 3.5 Composition of leach liquor The composition of the roasted product after roasting at 873 K for 4 h is shown in Table 4. The composition of the leach liquor obtained in the leaching of this roasted product and the composition of electrolyte used in electrorefining plant are given in Table 5. It was seen that the concentration level of sulphuric acid and copper in leach liquor were comparable with the electrolyte used in electrorefining plant. As far as the impurities level are concerned it can be seen that the composition of Fe, Ni, and Bi are

5 536 B. Gorai, R. K. Jana and Z. H. Khan Table 4 Composition of Roasted Dust Sample. % by mass Cu Co.574 Fe SiO Al 2 O Ag.16 Cd.61 Zn.174 Bi.613 Ni.81 Pb.46 Table 5 Comparison of leach liquor obtained and electrolyte used in copper electrorefining tank. Leach solution of roasted product Concentration Electrolyte used in electro-refining tank (ICC, Ghatsila) H 2 SO 4 18 kg/m 3 18 kg/m 3 Cu 32 kg/m kg/m 3 Fe.9 kg/m kg/m 3 Co Trace Ni.5 kg/m kg/m 3 Bi.55 kg/m 3.2 kg/m 3 Temp = 33 K, Liquid:Solid = 1 : 1, Time = 4 h, Roasting conditions: Temp = 873 K, far lower than the permissible limit. So the leach solution produced can be directly used as electrolyte in the copper electrorefining section of the plant without further purification. 4. Conclusions In an attempt to recover copper from the dust sample of copper plant, various studies were made and the findings were as given below (1) In water leaching as well as sulphuric acid leaching tests of the dust sample at room temperature, copper recovery was poor. At 353 K copper recovery was 75% in sulphuric acid medium. (2) To improve the copper recovery, the dust sample was roasted and was subsequently leached in sulphuric acid which gave the following results. (i) With the increase in roasting temperature from 873 K to 173 K the copper recovery in leaching decreased. (ii) The maximum leaching recovery of 95% Cu was obtained with the dust sample roasted at 873 K for 4 h. (iii) The leaching of roasted product followed the mixed controlled kinetics i.e. ash diffusion and chemical control. (3) The composition of leach liquor of the roasted sample was comparable with the electrolyte used for the electrorefining tank and therefore, the leach liquor can be directly used in the electrorefining plant. REFERENCES 1) W. H. Dennis: Metallurgy of the Nonferrous Metal, 2nd ed., (Sir Issac Pitman & Sons, London, 1961) pp ) J. Newton and C. L. Wilson: Metallurgy of Copper, (J. Wiely & Sons, London, 1942) pp ) K. Jia Jun and C. Chia Yung: Hydrometallurgy 12 (1984) ) J. D. Prater and J. S. Wells: US Patent, , July 19 (1978). 5) W. H. Kruesi and P. R. Kruesi: US Patent, 53325, March 5 (1993). 6) C. Nunez, F. Espiell and A. Roca: Hydrometallurgy 14 (1985) ) D. E. Giles and A. Boden: Proc. Inst. Min. Met., 262 (1977) 39. 8) A. R. Sarkisyan, A. S. Burnazyan, O. A. Crigoryan, E. E. Torosyan and M. A. Arutyunyan: Chem. Abstr. 82 (1975) 11559n. 9) E. Mohri and M. Yamada: World Mining and Metal Technology, Vol. 1, Alfred Weiss (Ed), (American Institute of Mining, Metallurgical and Pertoleum Engineers, New York, 1976) pp ) K. S. Gritton, D. K. Steele and J. E. Gebhardt: Proc. 2nd Int Symp Recycling of metals and Engineered Materials, Virginia, USA, Oct (199) p ) R. Hanks, J. Vander Zel, P. Chesney and G. B. Harris: Inst. Min. Met., (June 1979) pp. C99 C16 12) W. E. Anable, J. I. Paige and D. L. Paulson: Copper recovery from primary smelter dust, U.S. Bur. Mines Rep. Invest. no (1981). 13) A. K. Biswas and W. G. Davenport: Extractive metallurgy of Copper, 2nd ed. (Oxford, Pergamon, 198) pp. 61, ) M. Nagamori and F. Habashi: Metall. Trans. 5 (1974) ) P. C. Hayes: Process Selection in Extractive Metallurgy, (SBA Publication, Calcutta, 1987) p ) D. Georgiou and V. G. Papangelakis: Hydrometallurgy 49 (1998) ) H. Y. Sohn and M. E. Wadesworth: Rate Processes of Extractive Metallurgy, (Plenum Press, New York, 1979) pp ) S. L. Pohlman and F. A. Olson: Proc. Solution Mining Symposium, F. F. Alpan, W. A. McKinney and A. D. Pernichele, eds. Soc. Min. Engr. AIME, Dallas Texas, February (1974) Soc. Min. Engr, AIME, New York (1974) pp ) K. S. Choi and J. W. Sohn: Proc. 1st ISOPE-Ocean Mining Symposium, Tsukuba, Japan, Nov , (1995) pp

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