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Vol. 9. Issue 1.
Pages 263-270 (January - February 2020)
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Vol. 9. Issue 1.
Pages 263-270 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.10.054
Open Access
Microwave improving copper extraction from chalcopyrite through modifying the surface structure
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Tong Wena,b, Yunliang Zhaoa,b,
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zyl286@whut.edu.cn

Corresponding author.
, Qiulin Maa,b, Qihang Xiaoa,b, Tingting Zhanga,b, Jianxin Chena,b, Shaoxian Songa,b
a Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China
b School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China
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Table 1. Chemical analysis of handpicked chalcopyrite.
Abstract

Microwave-assisted leaching as a green and efficient method for leaching chalcopyrite has attracted increasing attention. Researchers have actively explored the reasons why microwaves have a positive impact on copper extraction. In this work, X-ray diffraction (XRD), an optical microscope, electron probe micro-analyzer (EPMA) and field emission scanning electron microscope with energy dispersive spectrometer (SEM-EDS) were employed to investigate the surface structure modification of chalcopyrite by microwave during leaching. The modified chalcopyrite had a positive effect on better mineral leachability, higher effective surface for leaching, and the fewer passivation layer. The superior mineral leachability was related to the surplus intermediate products of covellite, while microwave promoted the conversion of chalcopyrite to covellite. In addition, microwave is capable of increasing active sites upon the removal of the passivation layer and the enlargement of the effective reaction surface. This work summarized the relationship between the high copper recovery and the surface structure modification of chalcopyrite systematically.

Keywords:
Microwave-assisted leaching
Chalcopyrite
Modification
Surface structure
Passivation layer
Full Text
1Introduction

Chalcopyrite is a significant resource of copper as accounting for around 70% of the entire Earth’s copper [1,2]. Valuable copper is extracted mainly by pyrometallurgy throughout the current tech [3]. However, a particular focus is given to hydrometallurgy due to its low air pollution and wide adaptability of the different grade of ore [4,5]. Whereas, the limited leaching rate of chalcopyrite is an obvious defect of hydrometallurgy [6], due to the generation of passivation layer [7,8]. Nakazawa [9] mentioned that the passivation prevented the transfer of Cu2+ from the chalcopyrite. Therefore, many techniques have been explored to weaken the effect of the passivation layer and increase the copper recovery from chalcopyrite, such as bioleaching and catalyst addition [10–14]. From Panda et al.’s review of bioleaching on heap-leach technology [15], heap bioleaching of chalcopyrite is an effective method to conquer passivation problems and increase the leaching rate. In addition, pyrite and silver have been found to be effective catalysts for enhancing copper recovery [16].

Microwave-assisted heating is a new processing technology and has been attempted to recover copper from chalcopyrite by leaching [17–19]. It enhances copper extraction with a simple condition of operating [20]. According to the study of Onol and Saridede [21], microwave-assisted leaching of chalcopyrite has a higher leaching rate than the conventional leaching. Our previous research has also proved that it has positive effects and works well [22,23]. The copper recovery during microwave-assisted leaching is higher than the conventional leaching at boiling temperature, however, the mechanisms of positive effects remain controversial.

At present, the studies on microwave-assisted leaching of chalcopyrite still rely on external influence like kinetics and the optimal process condition of leaching [24]. Unfortunately, microwave seems invalid on promoting leaching of chalcopyrite dynamics [25]. The activation energy calculated for microwave leaching is 76.5kJ/mole, closing to 79.5kJ/mole for conventional leaching. However, few studies seem to notice the changes in chalcopyrite properties at microwave condition.

The objective of this work was to investigate the surface structure modification of chalcopyrite by microwave during leaching. The modifications of mineral leachability, effective leaching surface, and passivation layer on the surface of chalcopyrite were studied and discussed in detail to reveal the mechanism of modification by microwave.

2Experimental2.1Materials

The chalcopyrite used in this work was natural crystals obtained from Daye, Hubei, China. The crystals contained some impurities, such as quartz, bornite, etc., however, chalcopyrite was still the main component. In order to reject impurities as much as possible, chalcopyrite was obtained carefully from these crystals by hand picking after crushing with hammer. Then handpicked chalcopyrite used for leaching was milled until a suitable particle size by a three-head mill (RK/XPM-Φ120×3, Wuhan Locke Mill equipment Manufacturing Co., Ltd, China). The curve in Fig. 1 shows the size distribution of chalcopyrite and the median diameter d50 is around 21μm. In addition, cubic chalcopyrite was prepared to observe subtle changes in the surface of chalcopyrite. The sample was kept in the refrigerator avoiding oxidation.

Fig. 1.

Particle size distribution of handpicked chalcopyrite.

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The main elements of handpicked chalcopyrite, as presented in Table 1, are 35.2% Cu, 25.4% Fe, 32.3% S, and 5.8% Ca. The components of handpicked chalcopyrite are chalcopyrite, bornite, covellite, and quartz. Chalcopyrite accounts for about 83.35%, bornite about 0.31%, and covellite about 9.24% of the mineralogical species of copper ore. The Cu content of experimental chalcopyrite higher than that of theoretical is affected by the presence of bornite and covellite.

Table 1.

Chemical analysis of handpicked chalcopyrite.

Item  Al  Mg  Cu  Fe  Ca  Si 
Content (%)  0.8  0.2  35.2  25.4  5.8  32.3  0.2 

All the chemicals of the experiment were of the analytically pure grade from Sinopharm chemical reagent Co., Ltd.

2.2Procedure

The chalcopyrite was added to the configured leaching solution in a special three-necked flask. The leaching condition was determined on the basis of our previous study [22]. The compositions of the configured leaching solution were 0.6mol/L sulphuric acid and 0.06mol/L ferric sulfate. In order to avoid the adhesion loss of chalcopyrite in reactor caused by high pulp density, a low pulp density (1:150g/mL) is selected, which does not cause errors in the conclusion of this article. A magnetic stirrer was adjusted to 400r/min for keeping chalcopyrite suspended in the leaching solution. The design temperature was controlled above the boiling point of leaching solution to keep the equipment running. The equipment of conventional and microwave-assisted heating was SZCL-2A homoiothermal heater and MASIIPlus2 microwave chemical reactor of 500W, respectively. Both types of equipment could as much as possible to avoid the evaporation losses of leaching solution by an effective condensation. After leaching, the residues were collected from suspension by filtering and then freeze-dried for analysing. The change of surface morphology was studied by replacing powder chalcopyrite with cubic chalcopyrite. In-situ surface morphologies of chalcopyrite were obtained from the cubic chalcopyrite with 10min leaching time for avoiding the test obstacles caused by a violent chemical reaction. Surfaces used for observation were repeatedly cleaned with deionized water, then dried and tested.

2.3Characterizations

The particle size of chalcopyrite was analyzed by Malvern Mastersizer (APA2000, Malvern, England). The copper concentration in leachate was tested using spectrophotometry (V-1100D, Mapada, China) for calculation of copper recovery. X-ray diffraction (XRD: D/MAX-RB, Rigaku, Japan) and inductively coupled plasma-optical emission spectr (ICP: Prodigy 7, Leemanlabs, America) were used to analyze the phases and chemical composition of chalcopyrite, respectively. The morphology of sediment and the distribution of elements were analyzed using field emission scanning electron microscope (FESEM: Zeiss Ultra Plus, Zeiss, Germany) with energy dispersive spectrometer (EDS: X-max 50, Oxford, England) and electron probe micro-analyzer (EPMA: JXA-8230, JEOL, Japan) with EDS detector (Inca X-Act, Oxford, England). The surface of the cubic chalcopyrite was observed from an optical microscope (CX40P, Ningbo sunny instruments Co. Ltd, China).

3Results and discussion3.1Effect of microwave on copper leaching recovery of chalcopyrite at the boiling condition

Fig. 2 presents the fraction of copper releases versus time at boiling condition under the conventional and microwave-assisted heating system. Different from our previous study [23] where data is used only to illustrate the macroscopic relationship between temperature and copper recovery, this work it is further analyzed about the key role of microwave in copper extraction. Experimental results show that copper recovery is highly leaching time dependent. The data constantly increases over leaching time in both conditions. However, the copper recovery of microwave system is found to be greater than that of conventional under similar experimental conditions. Only 31.0% of copper is recovered after 10h conventional leaching, whereas 60.4% is extracted with microwave-assisted after same leaching time. Hence, microwave-assisted heating obviously results in a high copper recovery of chalcopyrite at the boiling condition, which can be increased by about 30% under the experimental conditions. It hints that microwave has a potential role in enhancing the extraction of copper from chalcopyrite during leaching.

Fig. 2.

Effect of microwave-assisted and conventional heating on copper recovery at different leaching time at the boiling condition.

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3.2Surface structure modification of chalcopyrite by microwave

The XRD pattern shown in Fig. 3 reviews that the components of fresh chalcopyrite are chalcopyrite, bornite, covellite, and quartz. Quartz is not a leaching participant because its characteristic peaks can still be detected in the leaching residue. Meanwhile, the bornite and covellite have a significantly better leachability compared to chalcopyrite [26,27]. Bornite dissolves easily and is hard to find from chalcopyrite after leaching on the basis of the disappearance of its characteristic peaks. As observed for bornite, covellite dissolves preferentially based on the XRD pattern of chalcopyrite and residues of conventional leaching. Surprisingly, covellite still can be detected in the residues of microwave-assisted leaching, which is inconsistent with its better leachability. High copper recovery compared to conventional heating is likely to theoretically point out the nearly complete dissolution of covellite at microwave condition. Hence, the sensible reason is speculated that the covellite is an intermediate product of chalcopyrite during microwave-assisted leaching. Moreover, the reaction supply of covellite exceeds the consumption in the leaching solution, whose tendency is obviously superior to that of conventional heating.

Fig. 3.

XRD pattern of chalcopyrite (a), residues of conventional leaching (b) and microwave-assisted leaching (c) for 8h.

(0.16MB).

Related research has reported that an intermediate product generates during the leaching of chalcopyrite. Chalcopyrite leaches with ferric ion through the intermediate product of covellite, as reviewed by Córdoba et al. [28]. Although the intermediate is hard to identify, Sasaki et al. [29] speculate the formation of covellite during bioleaching based on the results of XPS-spectrum and Raman spectra. According to the study by He et al. [30], the sulfur K-edge spectra of chalcopyrite also indicate the formation of covellite during bioleaching. Moreover, Vuković et al. have summarized the reaction pathway of chalcopyrite (CuFeS2→ Cu2S→ CuS→ Cu2+) by voltammetric of anodic dissolution of natural chalcopyrite [31]. Anyhow, the formation of leaching intermediate product (covellite) is an appropriate conclusion based on the many-sided demonstration, which is consistent with the XRD pattern in this work. Hence, the microwave-assisted leaching technique has potentially positive effects on the conversion of chalcopyrite into covellite. The more covellite that chalcopyrite has, the better leachability it will be. Microwave enhances copper extraction from chalcopyrite through modifying the surface structure.

The surface morphologies of cubic chalcopyrite during leaching were showed in Fig. 4. In order to accurately reveal the leaching behavior, the microphotographs are obtained on in-situ observation of fresh and reactive chalcopyrite. According to the Fig. 4a and b, obvious defects appear on the surface and corners of cubic chalcopyrite during conventional leaching, located on the position of impurity or pointedness rather than the complete surface only composed of chalcopyrite. Contrast to the complete surface, the pointedness of chalcopyrite is easier to leach because of the more opportunities for contacting leachate. However, Fig. 4e and f show that the defects are inconspicuous during microwave-assisted leaching, which is inconsistent with the expected result. Generally speaking, high copper recovery corresponds to a rougher surface of chalcopyrite. In other words, microwave condition should make a rougher surface in contrast to the conventional condition. The abnormal phenomenon is explained by comparing the microphotographs, and it is found that the surface of chalcopyrite is smooth and changes little based on Fig. 4c and d under conventional condition. Whereas, there are a lot of pits under microwave condition according to Fig. 4g and h. Hence, the conclusion can be assumed that microwave irradiation has a special influence on the reactive behavior of chalcopyrite.

Fig. 4.

In-situ surface morphologies of chalcopyrite before (a and c) and after (b and d) conventional leaching; in-situ surface morphologies of chalcopyrite before (e and g) and after (f and h) microwave-assisted leaching; electron probe microanalysis (i) of chalcopyrite (f).

(0.76MB).

In Fig. 4i, the cubic chalcopyrite under microwave condition is tested in the smooth (point 3), pits (points 1 and 2), and impurities (points 4) parts using the electron probe microanalysis. Data from the table in Fig. 4i clearly shows that the elemental type (Cu, Fe, and S) of the smooth part is similar to chalcopyrite. However, the elements of the pit portion are S, Fe, Cu and O. Chalcopyrite dissolved into Cu2+ with the participation of dissolved oxygen in the oxidative leaching solution. Hence, the presence of oxygen qualitatively proves the leaching reaction of chalcopyrite in pitting positions. Combined with the results in Fig. 4g and h, it is concluded that the leaching reaction under microwave-assisted heating takes place on the whole surface of chalcopyrite uniformly. Nevertheless, the chalcopyrite dissolves preferentially from where it is deficient under conventional leaching. Comparing to the limited position in defects, the whole surface of chalcopyrite has more abundant sites to extract copper. In other words, there is a bigger and effective reaction surface of chalcopyrite under microwave-assisted leaching, which can be described as a surface modification of chalcopyrite by microwave.

As we know, the microwave is an electromagnetic wave with a frequency of 0.3–300GHz, which penetrates materials and delivers energy directly [32,33]. The ions and molecules convert electromagnetic energy into heat through changing the directions and rubbing against each other. The heat transfer of microwave is different from the heat conduction of conventional heating, making a rapid and unified heating behavior. Hence, the whole surface of chalcopyrite synchronously reaches the threshold of leaching reaction, which enlarges effective leaching surface.

The formation of passivation layer is a serious factor hindering chalcopyrite leaching [9]. However, microwave weakens this factor during modifying the surface structure of chalcopyrite in this study. The results in Fig. 5 show the morphologies of chalcopyrite. The surface of fresh chalcopyrite is smooth, nevertheless, it is rugged after both conventional and microwave-assisted leaching. Chalcopyrite is destroyed during leaching, especially at the microwave condition. Simultaneously, the more obvious destruction under microwave condition matches the higher copper recovery compared to conventional. However, it seems hard to judge the existing form of the passivation layer depending on the surface morphologies of chalcopyrite.

Fig. 5.

SEM images of fresh chalcopyrite (a), residues of conventional leaching (b) and microwave-assisted leaching (c).

(0.51MB).

The element distribution of the chalcopyrite is presented in Fig. 6 after microwave-assisted leaching. The elements of S, Fe, and Cu distribute in the region of leaching residue, respectively represented by red, green, and purple. However, some unique regions only composed of S appear to be different from the distribution of Fe and Cu. The unique regions stay alone indicated by the arrow. The isolated sulfur is not the composing component of chalcopyrite, that is, the isolated composition of S is a new crystal. This may be a guide to understanding the high copper recovery of chalcopyrite under microwave-assisted heating. It has been widely reported that the passivation layer generates on the surface of chalcopyrite during leaching. This passivation layer hinders the transfer of ions from chalcopyrite, resulting in low copper recovery [9]. Assuming that it is removed from the surface of chalcopyrite, there still be sufficient surface for leaching, which will weaken the passivation problem. Combined with the existence of isolated sulfur and the high copper recovery, it can be conjectured that S is the major passivation layer and removed from chalcopyrite during microwave-assisted leaching. At present, the study of passivation layer is still incomplete and controversial [34]. Klauber [35] summarizes that elemental sulfur and jarosite are the suggested candidates to compose the passivation layer. Recently, it is confirmed that sulfur is the passivation layer during leaching of chalcopyrite by the results of SEM/EDX [36]. Hence, it is logically concluded that S, as a passivation layer, generates and covers the chalcopyrite during the leaching, and is tripped from chalcopyrite due to the effect of microwave-assisted heating. Then, the fresh chalcopyrite exposes so that the copper recovery increases during microwave-assisted leaching. The positive effect can be attributed to the surface structure modification of chalcopyrite by microwave.

Fig. 6.

Elemental EDS maps of S (b), Fe (c), and Cu (d) of residues after microwave-assisted leaching.

(0.6MB).

The removal phenomenon of passivation layer is an unusual discovery and depends on temperature based on this research. Fig. 7 presents the research on the boiling point of leaching media containing different sulphuric acid concentration. The boiling point of the leaching media with 0.5mol/L sulphuric acid (114.5°C) is higher than that with 0mol/L sulphuric acid (109°C) under microwave-assisted heating. And it is just 104°C with 0.5mol/L sulphuric acid under conventional heating. Experimental results show that the sulphuric acid changes the boiling point of leaching media. On the other hand, the leaching media under microwave-assisted heating has a higher boiling point, even is higher than the thawing point of S (112°C).

Fig. 7.

The higher boiling point of microwave-assisted heating compared to conventional heating.

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The microwave is selective heating, volumetric heating, and non-thermal effect [24] and will selectively heat the material with a higher loss component due to energy transfer at a molecular level. Hence, the temperature of chalcopyrite is higher than the boiling point of the solution under microwave-assisted heating because the electrical conductivity of chalcopyrite is 1000S/m [25]. Our previous study [23] has illustrated the phenomenon and found that the interface temperature of leaching increases with the positive effect about the high boiling point of leaching solution and selective heating of microwave. Therefore, the temperature between chalcopyrite and passivation layer rises further beyond the thawing point of S (112°C). However, previous analyses have not taken this view into account. The surface of passivation layer contacted with chalcopyrite would dissolve, and be tripped from it with a driving effect of thermal convection. Hence, copper is ulteriorly extracted from fresh chalcopyrite due to the surface structure modified by microwave.

4Conclusions

Microwave-assisted heating is a valid method to increase copper recovery of chalcopyrite. The high recovery benefits from the surface structure modification of chalcopyrite caused by microwave. The results show that microwave potentially promotes the conversion of surface chalcopyrite into covellite. Covellite, compared to chalcopyrite, has a better leachability and enhances the extraction of copper. In addition, microwave makes the whole surface of chalcopyrite synchronously reaching the threshold of leaching reaction. There are a lot of pits on the surface of chalcopyrite at microwave condition based on the results of the optical microscope. Hence, chalcopyrite dissolves uniformly from the whole surface during microwave-assisted leaching rather than firstly from the defect locations during conventional leaching. The unique reaction behavior of chalcopyrite improves copper recovery by enlarging the effective leaching surface on the effect of microwave. Moreover, microwave makes the temperature of chalcopyrite and the boiling point of leaching solution higher than conventional heating due to its selective heating, volumetric heating, and non-thermal effect. Therefore, the temperature on the submerged interface of chalcopyrite is further higher than of the melting point of S (112°C). The passivation layer composed of S dissolved and stripped from the chalcopyrite suggests that microwave modified the surface structure of chalcopyrite. The fresh chalcopyrite on the surface provides power for copper extraction.

Conflict of interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Microwave improving copper extraction from chalcopyrite through modifying the surface structure”.

Acknowledgments

The financial supports for this work from the National Natural Science Foundation of China (projects Nos. 51874220, 51904215 and 51674183) and Natural Science Foundation of Hubei Province of China (project No. 2018CFB468) are gratefully acknowledged.

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