The removal of metallic impurities from off-grade copper concentrate was investigated in alkaline solution with H 2 O 2 . The analysis results of XRD and SEM-EDS revealed the oxidative mechanisms of all sulfides. The influence of various parameters of alkaline leaching were investigated including concentrations of sodium hydroxide and hydrogen peroxide, liquid to solid ratio, leaching time and temperature. The results showed that the removal rate of Mo, As and Zn were increased with increasing leaching time and H 2 O 2 concentration, and that the removal rate of Mo, As and Zn were firstly increased and then slightly decreased with increasing liquid to solid ratio, temperature and NaOH concentration, respectively. More than 95% Mo, 94% As, and 94 % Zn are removed from the off-grade concentrate under the optimum conditions, while only 1.7% Cu is dissolved. These optimum conditions were sodium hydroxide 1.5 mol/L, hydrogen peroxide 1.0 mol/L, temperature 50°C, liquid to solid ratio 5/1 mL/g and leaching time 5 h.
In China, There are many porphyry copper deposits bearing molybdenum, arsenic, zinc and aluminum et al. [
The copper concentrates with the content of arsenic less than 0.5% are generally processed by smelting/converting technology. Whilst the copper concentrates contain high amount of arsenic, the smelting/converting technology has a potential risk of environmental pollution with toxic arsenic compounds. In addition, the presence of arsenic in copper concentrates will result in the deterioration of the quality of the final copper cathodes.
There are an off-grade copper concentrate with high content of arsenic (2.17%) and low content molybdenum (2.62%), which is collected from Tibet region in China. Owing to high content of arsenic and low content of molybdenum, this copper concentrate is low economic value and is difficult to sell out. Therefore the removal impurities from this off-grade concentrate make a focus.
The removal methods of arsenic such as alkaline leaching and acid leaching have been investigated from copper concentrate, for example, enargite leaching with Na2S-NaOH, NaHS-NaOH and NaOH-H2O2 media [
Although there many reports on dealing with molybdenite and As-sulfides, respectively, there is little information concerning the treatment of copper concentrate associated with arsenic, molybdenum and zinc. To upgrade the copper concentrate, a new method is investigated to remove the impurities in the copper concentrate in alkaline solution using H2O2 as an oxidant.
An off-grade copper concentrate with chalcopyrite, tennantite and molybdenum was selected as the raw material in this work to better understand the leaching behavior of metals in the alkaline leaching process. The concentrate was from Huatailong Mining Development Co. Ltd., Mozhugongka region in Tibet province, P.R. China. It was first dried overnight in an oven to remove moisture, and then sieve to collect particles of less than 0.075 mm. All chemicals like sodium hydroxide and hydrogen peroxide were of analytical grade, supplied by Shanghai Sinopharm Chemical Co. Ltd., P.R. China.
All leaching experiments were conducted in a conical flask using a shaking machine. The copper concentrate sample was used in 20 to 100 g scale. The liquid to solid ratio varied from 2/1 to 10/1 mL/g. The temperature variation studies were performed from 20˚C to elevated temperature 70˚C. The concentrations of NaOH and H2O2 were changed from 0.5 to 2.5 mol/L and 0.4 to 1.4 mol/L, respectively. The leaching time also was ranged from 1 h to 7 h. After a certain leaching treatment, the slurry was withdrawn from the reactor and vacuum filtered was washed twice with 400 mL distilled water. The volume of the filtered pregnant solution was measured by a measuring cylinder and 5 mL of liquor sample was collected to assay molybdenum, copper, arsenic and zinc by an inductive coupled plasma-atomic emission spectrometer (ICP-AES), and then the residues were dried for 8 h at 105˚C. The dried residues were used to be analyzed.
S content in solid samples was determined using a high frequency IR carbon and sulfur analyzer (CS-3000, Beijing Nake Analysis Instrument Co. Ltd). The other elements in solid samples were all analyzed using acid or alkaline digestion and atomic absorption spectrometer (PE-AA700, PerkinElmer Company, USA). Element concentrations in solution were all measured by an inductive coupled plasma-atomic emission spectrometer (Optima 2100DV, PerkinElmer Company, USA). Mineral compositions of solid samples were performed by an X-ray diffractometer (D/Max 2500, Rigaku). The constituents and morphology of the copper concentrates before and after leaching were examined by Scanning Electron Microscope-Energy Dispersive X-Ray spectroscopy analyses (SEM-EDS, Jeol JSM 6010LA) at 10-20 KeV and high vacuum conditions.
The removal of metallic impurities from the copper concentrate is related to the oxidation and dissolution of sulfides including Cu-, As-, Mo- and Zn- sulfides in the copper concentrate. The leaching behaviors of metal phases in the copper concentrate under alkaline leaching conditions can be predicted from thermodynamic diagrams.
Constituent | Cu | Fe | S | Ca | Mo | As | SiO2 | Zn |
---|---|---|---|---|---|---|---|---|
Wt (%) | 11.40 | 10.51 | 13.70 | 5.98 | 2.62 | 2.17 | 39.36 | 0.42 |
Fe2+ exist only in acid solution (pH < 6) and Fe3+ exist only in acid solution (pH < 3). A potential increase leads to the sequential alternation CuFeS2®Fe2O3 and CuS®Cu2S® Cu2+ or Cu+. In alkaline solution, as potential increases CuFeS2®CuS® Cu2S is possible and its further increase leads to the formation of insoluble CuO and Fe2O3.
At the same time the oxidation of CuFeS2 during oxidative alkaline leaching is difficult and complicated owing to the multilevel translation (CuFeS2®CuS®Cu2S). From thermodynamic considering, the oxidative reaction of chalcopyrite may be written as follows:
4 CuFeS 2 + 17 O 2 + 16 OH − = 4 CuO + 16 SO 4 2 − + 2 Fe 2 O 3 + 8 H 2 O (1)
Copper oxide is expected to solubilize in alkaline solution via the following reaction by Wagman et al. [
CuO + ( 2 − n ) H + = Cu ( OH ) n 2 − n + ( 1 − n ) H 2 O (2)
where n refers to the ionic state of hydrolysis and may present 0, 1, 2, 3 or 4. As shown in Equation (2), the species of CuO dissolved in alkaline solution may be Cu(OH)2, Cu ( OH ) 3 − or Cu ( OH ) 4 2 − . But Cu(OH)2 is a dominant aqueous species and is easily transformed into CuO in high alkaline solution.
The Eh-pH diagram for the behavior of Cu-As-S-H2O system is presented in
The Eh-pH diagram for the behavior of Mo-S-H2O system [
The Eh-pH diagram for the behavior of Zn-S-H2O [
H2O2 is thermolabile, and higher temperatures are generally favorable for leaching from the perspective of leaching kinetics, therefore, the temperature was researched in the range of 20˚C to 70˚C under the other constant conditions of NaOH 1.5 mol/L, H2O2 1.0 mol/L, leaching time 5 h, and liquid to solid ratio 5/1 mL/g. The experimental results are presented in
The extractions of Mo, As and Zn increase from 49.2% to 95.6%, 38.9% to 93.4%, 45.3% to 94.4%, respectively, when the temperature increases from
20˚C to 50˚C. The extractions of Mo, As and Zn plateau 95.6%, 93.4%, and 94.4%, respectively, at 60˚C, and then decrease to 86.9%, 84.9% and 88.2%, respectively, when the temperature was increased to 70˚C. These results indicate the optimum effect of H2O2 for the oxidation of CuFeS2, MoS2 and ZnS is
obtained at 50˚C, whereas at temperatures higher than 60˚C and peculiarly at 70˚C, H2O2 was resolved too early due to the high temperature and did not act a role in the oxidation [
The influence of the leaching time on Cu, Mo, As and Zn extraction was investigated at NaOH 1.5 mol/L, H2O2 1.0 mol/L temperature 50˚C, and liquid to solid ratio 5/1 mL/g. The result is given in
In order to study the effects of NaOH concentration on the extraction of Mo, As and Zn, and the dissolution of copper. NaOH concentration was varied from 0.5 to 2.5 mol/L, while liquid to solid ratio 5/1 mL/g, H2O2 1.0 mol/L, leaching time 5 h and temperature 50˚C. The results are shown in
The influence of H2O2 concentration on the removal of metallic impurities was investigated at NaOH 1.5 mol/L, liquid/solid ratio 5/1 mL/g, leaching time 5 h and temperature 50˚C. As presented in
The standard reduction potential of H2O2 ( E H 2 O 2 / OH - θ = 0.87 V) is higher than that of O2 ( E O 2 / OH - θ = 0.401 V), therefore using H2O2 as an oxidant instead of oxygen, the oxidation reactions of CuFeS2, Cu12As4S13, MoS2 and ZnS can take place as follows:
4 CuFeS 2 + 34 H 2 O 2 + 16 OH − = 4 CuO + 16 SO 4 2 − + 2 Fe 2 O 3 + 42 H 2 O (6)
S 13 + 68 H 2 O 2 + 24 OH − = 12 CuO + 4 AsO 4 3 − + 80 H 2 O + 13 SO 4 2 − (7)
MoS 2 + 6 OH − + 9 H 2 O 2 = MoO 4 2 − + 2 SO 4 2 − + 12 H 2 O (8)
ZnS + 4 H 2 O 2 + 4 OH − = ZnO 2 2 − + SO 4 2 − + 6 H 2 O (9)
The extraction of Mo, As and Zn increase from 30.8% to 95.7%, 48.4% to 93.4% and 51.3% to 94.4%, respectively, when hydrogen peroxide concentration increases from 0.4 mol/L to 1.0 mol/L. However there is no significant change on the extraction of Mo, As and Zn when the hydrogen peroxide concentration is above 1.0 mol/L. Although the Cu extraction increases with the rise of H2O2 concentration, the maximum Cu extraction is only 2.0% at H2O2 1.3 mol/L. Considering the economic factor, the optimum H2O2 concentration is fixed at 1.0 mol/L for the subsequent experiments.
The effect of liquid to solid ratio on the removal of Mo, As, Zn and Cu was investigated under the conditions of sodium hydroxide 1.5 mol/L, hydrogen peroxide 1.0 mol/L, leaching time 5 h and temperature 50˚C. The results are given in
According to the above results, the optimum conditions for selective extraction of Mo, As and Zn from off-grade copper concentrate are listed as follows: NaOH concentration of 1.5 mol/L, H2O2 concentration of 1.0 mol/L, temperature 50˚C, liquid to solid ratio of 5/1 mL/g, leaching time 5 h. Under these conditions, the Mo, As and Zn extractions are up to about 95.8%, 93.4% and 94.4%, respectively, and only 1.74 % Cu was dissolved into the solution.
CuFeS 2 + 7 H 2 O 2 + 4 NaOH − = CuFeO 2 + 2 Na 2 SO 4 + 9 H 2 O (10)
In order to further understand the oxidation mechanism of chalcopyrite, the microscopic structure of the residue and the chemical compositions of the points analyzed by SEM coupled with EDS, are presented in
The studies have been carried out to up-grade the copper concentrate. These impurities of Mo, As and Zn were leached mainly in alkaline solution containing hydrogen peroxide.
The oxidative mechanisms of chalcopyrite, tennantite and molybdenite in the copper concentrate were demonstrated by methods of XRD and SEM-EDS analysis, especially the chalcopyrite oxidation simultaneously takes place as Equations (6) and (10):
4 CuFeS 2 + 34 H 2 O 2 + 16 OH − = 4 CuO + 16 SO 4 2 − + 2 Fe 2 O 3 + 42 H 2 O (6)
CuFeS 2 + 7 H 2 O 2 + 4 NaOH − = CuFeO 2 + 2 Na 2 SO 4 + 9 H 2 O (10)
About 95.80% Mo, 93.4% As, and 94.4% Zn can be removed under the optional conditions: sodium hydroxide 1.5 mol/L, hydrogen peroxide 1.0 mol/L, temperature 50˚C, liquid to solid ratio 5/1 mL/g and leaching time 5 h.
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51364009), Key Research Projects of Education Department of Hunan Province (Grant No. 15A151), Natural Science Foundation of Hunan Province (Grant No. 2015JJ2115), Teaching Reform Projects of Education Department of Hunan Province (Grant No. 20150228) and Teaching Reform Projects of Jishou University (Grant No. 2015003).
Liu, Z.-X., Sun, L., Tang, L.-D., Hu, J., Xiao, Y., Chen, Y.-G. and Yin, Z.-L. (2018) Removal of Metallic Impurities from Off-Grade Copper Concentrate in Alkaline Solution. International Journal of Nonferrous Metallurgy, 7, 9-23. https://doi.org/10.4236/ijnm.2018.72002