J. Cent. South Univ. (2012) 19: 71-76

DOI: 10.1007/s11771-012-0974-9

Oxygen pressure acid leaching of Gacun complex Cu concentrates

XU Bin(徐斌)^{1, 2}, ZHONG Hong(钟宏)³, JIANG Tao(姜涛)¹

1. School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China;

2. Sichuan Xinyuan Mining Limited Liability Company, Chengdu 610000, China;

3. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: The treatment of the Gacun complex Cu concentrate with high contents of Pb, Zn, Ag, etc by oxygen pressure acid leaching was studied. It is unusual that tetrahedrite, whose treatment was rarely studied, is the primary copper mineral of the concentrates. Most of silver also occurs in the mineral. The optimum operating parameters of oxygen pressure acid leaching were established by conditional tests. Pilot scale test was carried out under the parameters, and the leaching rates of copper and zinc are as high as 97.10% and 89.83% while lead and silver are transformed into sulfate and sulfide respectively and stay in leaching residue. The copper and zinc in lixivium were reclaimed by extraction-electrowinning and purification-electrowinning, respectively, and the lead and silver in the residue were reclaimed separately by chloride leaching and thiourea leaching. The extraction rate of copper achieves 96%, and the leaching rates of lead and silver reach 90% and 95%, respectively.

Key words: complex Cu concentrates; oxygen pressure acid leaching; synthetic reclaiming; tetrahedrite

1 Introduction

The Gacun multimetal mine lies in Sichuang province of China. The average grades of Cu, Pb, Zn and Ag of the complex ore are 0.5%, 3%, 5% and 0.02%, respectively. The ore is somewhat similar with Japanese black ore, whose disseminated grain size of minerals is so fine that it is difficult to get qualified copper, lead, and zinc concentrates by flotation. At present, the mining and processing system of Gacun mine has been constructed and separate Cu, Pb, and Zn concentrates are produced for sale by differential flotation. However, the Cu concentrate is unqualified because of its low Cu grade and high Pb, Zn, As and Sb contents [1].

The Gacun Cu concentrate contains appreciable arsenic and antimony impurities, so it is not suitable for treatment by traditional pyrometallurgical methods. Fluidized roasting is not an appropriate method either because over 10% lead content of the concentrates will result in its agglomeration [2]. The study of biological oxidation leaching is mainly focused on chalcopyrite [3-5] and rarely involves tetrahedrite. Cu recovery from chloride leaching of tetrahedrite under normal temperature is low [6]. Slurry electrolysis is difficult to achieve industrialization [7-9]. The study of oxygen pressure acid leaching is mainly focused on chalcopyrite [10-11], in which there are three main methods: sulfation roasting followed by pressure leaching, sulfidation roasting followed by pressure leaching and direct pressure leaching [12-14]. The direct oxygen pressure acid leaching of chalcopyrite can be classified into moderate and high temperature regimes [15]. Moderate temperature process takes use of fine grinding and moderate temperature, typically 140-150 °C, in which specific reagents are added to minimize wetting of the chalcopyrite surface by molten sulfur, promoting the leaching kinetics. More than half of sulfide sulfur is transformed into sulfur and the sulfate content of the leaching residue is low. High temperature process (200-230 °C) has rapid leaching kinetics, in which the entire sulfides are transformed into sulfate, leading to higher oxygen consumption and higher acidity in lixivium. The Gacun copper concentrates are treated by direct oxygen pressure acid leaching in the regime of moderate temperature, and most of sulfide sulfur in the concentrates is oxidized into sulfur and the heat generated by the oxidation can be utilized effectively during the leaching. There is not sulfur dioxide released, so the process is more environmentally friendly and the choice of factory location is also more flexible without the restrictions of sulfuric acid sales. The technologies and chemical principle utilizing Gacun complex copper concentrates by oxygen pressure acid leaching are studied in this work.

2 Experimental

2.1 Materials

The Cu concentrate used in this work was taken from Gacun Mine, and its chemical composition is given in Table 1. The main valuable elements are Ag, Cu, Pb and Zn. Mineralogical analysis of the concentrates was performed by quantitative XRD, and the result is given in Table 2. The main copper minerals are tetrahedrite, tennantite and chalcopyrite, and the primary lead and zinc minerals are galena and sphalerite, respectively. Except for a trifle of argentite, there is not any other special silver-bearing mineral identified under the microscope. Table 3 and Table 4 list the chemical phase analysis result of silver and the granularity distribution of the concentrate, respectively. As listed in Table 3, most of silver occurs in tetrahedrite.

Sulfuric acid (98%), hydrochloric acid (36%), thiourea, sodium chloride, ferric sulphate, and sodium lignin sulfonate used in these experiments are all chemically pure, and 260^# kerosene, Acorga M5640 extractant and bottle oxygen are all industrial products.

Table 1 Chemical composition of Gacun complex Cu concentrates (mass fraction, %)

Table 2 Mineralogical composition of Gacun complex Cu concentrates (mass fraction, %)

Table 3 Chemical phase analysis result of Ag in Gacun complex Cu concentrate

Table 4 Granularity distribution of Gacun complex Cu concentrate (volume fraction, %)

2.2 Experiment flow and equipment

The flowsheet of oxygen pressure acid leaching process is given in Fig. 1. The Cu concentrate was treated by oxygen pressure acid leaching in autoclave after being finely ground. And then, Cu and Zn were reclaimed from lixivium, and Pb and Ag were reclaimed from leaching residue.

First of all, a certain quantity of ore sample was slurried with diluted sulfuric acid at the set liquid-to- solid ratio, and then the slurry was added into autoclave. After the charging, the autoclave was sealed and heated in succession. When the temperature arrived at the set value, oxygen was led in and the partial pressure of the oxygen was adjusted to desired value, which was maintained constant for the whole duration of the experiment. At the end of the experiment, the autoclave was rapidly cooled with cold water and then the oxygen flow was shut down. The slurry was filtered and the residue was washed adequately with deionized water. The lixivium and residue were collected for chemical analysis, and the leaching rates of copper and zinc were calculated from the results of residue analysis.

Fig. 1 Flowsheet of synthetic reclaiming technique of oxygen pressure acid leaching

Concentrate grinding was done in a rod mill with the size of 200 mm (diameter) × 250 mm (length), of which the filling ratio was 40%. A 2 L autoclave was used in conditional tests and a 10 L one was used in the pilot scale test. The mixershafts, paddles and cooling coils of the two autoclaves were all made of titanium. The inner pots for each were titanium-coated. Extraction was done in mixer settlers made of organic glass and feed liquor was added with metering pump. Common filtration and drying facilities were also used in these experiments.

3 Results and discussion

3.1 Oxygen pressure acid leaching

3.1.1 Principle of leaching

The main minerals in the Cu concentrate are galena, sphalerite, tetrahedrite, tennantite, chalcopyrite and pyrite. In the reactions (1)-(6), the sulfide sulfur was oxidized into sulfur and the metal elements as Pb, Zn, Cu, etc were transferred into sulfates during the leaching. SbO₂ and AsO₂ generated separately by reactions (3) and (4) were oxidized into H₃SbO₄ and H₃AsO₄, which would react continually with Fe³⁺ and Ca²⁺ ions in the solution and stayed in the leaching residue in the stable forms of antimonate and arsenate. Fe²⁺ produced by reactions (5) and (6) was first oxidized into Fe³⁺, and then Fe³⁺ would oxidize sulfides such as galena and sphalerite, whilst Fe³⁺ itself was reduced into Fe²⁺. The reactions (5)-(8) formed a cycle and these reactions took place ceaselessly, so the reactions (1)-(5) were accelerated dramatically. Furthermore, in reaction (9), certain sulfides were directly oxidized into sulfates without the generation of sulfur during the leaching. The Me in the reactions (8) and (9) stands for Pb, Zn, Cu, etc.

PbS+H₂SO₄+0.5O₂→PbSO₄+S⁰+H₂O                        (1)

ZnS+H₂SO₄+0.5O₂→ZnSO₄+S⁰+H₂O                        (2)

Cu₁₂Sb₄S₁₃+12H₂SO₄+10O₂→

12CuSO₄+4SbO₂+13S⁰+12H₂O                            (3)

Cu₁₂As₄S₁₃+12H₂SO₄+10O₂→

12CuSO₄+4AsO₂+13S⁰+12H₂O                            (4)

CuFeS₂+2H₂SO₄+O₂→CuSO₄+FeSO₄+2S⁰+2H₂O     (5)

FeS₂+H₂SO₄+0.5O₂→FeSO₄+2S⁰+H₂O                      (6)

2FeSO₄+H₂SO₄+0.5O₂→Fe₂(SO₄)₃+2S⁰+H₂O           (7)

MeS+Fe₂(SO₄)₃→MeSO₄+2FeSO₄+S⁰                       (8)

MeS+2O₂→MeSO₄                                                     (9)

3.1.2 Conditional tests

Ground concentrate of 200 g together with 0.5 g sodium lignin sulfonate added to minimize wetting of the mineral surface by molten sulfur was slurried with diluted sulphuric acid in each test, and then the slurry was charged in autoclave. Conditional tests were carried out under the fixed operating parameters with stirring speed of 850 r/min and liquid-to-solid ratio of 4. Figure 2 shows that the extraction of copper increased moderately while that of zinc increased dramatically with the grinding time up to 20 min, and 99.7% of the concentrate was less than 44 μm at that time by sieve analysis. As Fig. 3 showed, both extractions of copper and zinc rose slowly in the initial H₂SO₄ concentration range of 120-150 g/L. It was seen in Fig. 4 that temperature had significant effect on both extractions of copper and zinc in the temperature range of 130-150 °C and the improvement of zinc leaching was more remarkable relative to the copper with the increase of temperature. The extraction of copper and zinc both had a mild increase when the oxygen partial pressure rose from 1.0 to 1.2 MPa. However, there was no further increase when the pressure was higher than 1.2 MPa, as observed in Fig. 5. Figure 6 shows that both extractions of copper and zinc went up moderately in the leaching time range of 1.5-2.5 h. From the above, the optimum leaching parameters were established as grinding granularity of 99.7% less than 44 μm, initial H₂SO₄ concentration of 150 g/L, 150 °C, oxygen partial pressure of 1.2 MPa and leaching time of 2.5 h.

Fig. 2 Effect of grinding time on leaching rates of Cu and Zn under condition of initial H₂SO₄ concentration of 160 g/L,  160 °C, oxygen partial pressure of 1.4 MPa and leaching time of 2 h

Fig. 3 Effect of initial acid concentration on leaching rates of Cu and Zn under condition of grinding time of 20 min, 160 °C, oxygen partial pressure of 1.4 MPa and leaching time of 2 h

Fig. 4 Effect of temperature on leaching rates of Cu and Zn under condition of grinding time of 20 min, initial H₂SO₄ concentration of 150 g/L, oxygen partial pressure of 1.4 MPa and leaching time of 2 h

Fig. 5 Effect of oxygen partial pressure on leaching rates of Cu and Zn under condition of grinding time of 20 min, initial H₂SO₄ concentration of 150 g/L, 150 °C and leaching time of  2 h

Fig. 6 Effect of leaching time on leaching rates of Cu and Zn under condition of grinding time of 20 min, initial H₂SO₄ concentration of 150 g/L, 150 °C and oxygen partial pressure of 1.2 MPa

3.1.3 Pilot scale test

The pilot scale test was implemented under the optimum process parameters established in 3.1.2, and the result is given in Table 5. The chemical phase analysis result of Ag in the leaching residue is given in Table 6.

Table 5 Result of pilot scale test

Table 6 Chemical phase analysis result of Ag in leaching residue

It can be seen from Table 5 that 97.10% of Cu and 89.83% of Zn were dissolved during the leaching whilst iron and arsenic as impurities were also dissolved in part. Lead and silver were transformed into sulfate and sulfide separately and stayed in leaching residue. As shown in Table 6, the primary silver phase in the residue was silver sulfide and there were also double salts such as antimonite and ferrite. The reason of the generation of abundant silver sulfide might be that silver ions produced by the dissolution of silver-bearing tetrahedrite reacted with sulfureted hydrogen in the solution. The sulfureted hydrogen was the intermediate product of the reaction between the sulfides in the concentrate and sulfuric acid in autoclave, as shown in reactions (10) and (11). The Me in the reaction (10) stands for Pb, Zn, Cu, etc. The double salts were obtained because part of the silver ions also reacted with H₃SbO₄, H₂FeO₄, etc in the solution:

MeS+H₂SO₄→MeSO₄+H₂S↑                                    (10)

2H₂S+O₂→2H₂O+2S⁰                                               (11)

3.2 Reclaiming of Cu and Zn from lixivium

The copper in the lixivium of the pilot test was reclaimed by the method of extraction-electrowinning. After the lixivium was neutralized, the copper was extracted with Acorga M5640. Volume ratio of extractant to 260^# kerosene was 1:4, and the process of eleven-stage countercurrent extraction, one-stage washing and five-stage stripping was adopted in this test. The operating parameters of extraction were established as follows: temperature of 25 °C, mixing time of 3 min, phase separation time of 2 min and phase ratios (O/A) of 4. Extracted phase was washed adequately with deionized water before stripping. A sulfuric acid solution was used to strip copper under the condition of temperature of 25 °C, H₂SO₄ of 250 g/L, mixing time of 2 min and phase ratio (O/A) of 1. The extraction and stripping recovery of copper were as high as 96% and 95%, respectively, in the above processes while the extraction of zinc was less than 0.1%. High pure cathode copper was obtained by electrowinning from stripping solution. After the copper raffinate was neutralized and purified, the zinc can be reclaimed effectively by electrowinning from the solution.

3.3 Reclaiming of Pb and Ag from residue

Lead and silver in the leaching residue were reclaimed by chloride leaching and thiourea leaching separately. At first, the lead in the primary form of sulfate in the residue was leached with sodium chloride in the medium of hydrochloric acid. As shown in the reaction (12), the lead was transferred into lixivium in the form of lead dichloride during the leaching, and the lead dichloride solid product was obtained by condensation and crystallization from the lixivium. The silver in the primary form of sulfide in the lead leach residue was again leached with thiourea, and equation (13) shows the leaching chemical reaction. The thiourea is comparatively stable in acidic solution, and the complexity constant of the complex compound [Ag(SC(NH₂)₂)₃]⁺ between thiourea and silver ion is as high as 13.6. Silver product can be obtained by zinc dust precipitation from the thiourea leaching solution:

PbSO₄+2NaCl→PbCl₂+Na₂SO₄                                (12)

Ag₂S+6SC(NH₂)₂→2[Ag(SC(NH₂)₂)₃]⁺+S^2-            (13)

In the test, after sulfur was reclaimed by flotation, the leaching residue was first leached with chloride under the condition of NaCl concentration of 280 g/L, HCl concentration of 12.75 g/L, temperature of 90 °C, liquid-to-solid ratio of 18 and leaching time of 8 h, and then the chloride leaching residue was again leached with thiourea under the process parameters of SC(NH₂)₂ concentration of 60 g/L, Fe³⁺ concentration of 10 g/L, temperature of 70 °C, liquid-to-solid ratios of 10, pH of 1.5-2.0 and leaching time of 180 min. The leaching rates of lead and silver achieved 90% and 95%, respectively.

4 Conclusions

1) An oxygen pressure acid leaching process was developed in order to recover valuable metals from the Gacun complex copper concentrate. Copper and zinc were leached efficiently while lead and silver were transformed into sulfate and sulfide respectively and stayed in leaching residue. After lixivium was neutralized, copper was selectively extracted well with Acorga M5640, and then zinc in raffinate could be recovered effectively by purification-electrowinning. After sulfur was reclaimed by flotation, lead and silver in the residue were extracted availably by chloride leaching and thiourea leaching, respectively.

2) The valuable elements as Cu, Pb, Zn and Ag in Gacun complex Cu concentrate can be reclaimed synthetically by the oxygen pressure acid leaching process. This process is not only environmentally friendly because it does not release sulfur dioxide, but also is energy-saving because it utilizes the heat generated by the oxidation of sulfide sulfur in the concentrate. However, there are still some disadvantages such as high concentration of spent acid in lixivium, substantive dissolution of iron, etc, and further researches need to be done.

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(Edited by YANG Bing)

Foundation item: Project(2007BAB22B01) supported by the National Science and Technology Pillar Program during the 11th Five-year Plan Period of China

Received date: 2011-02-22; Accepted date: 2011-04-22

Corresponding author: JIANG Tao, Professor, PhD; Tel: +86-731-88877656; E-mail: jiangtao@csu.edu.cn