Synchrotron Radiation XRD Investigation of the Fine Phase Transformation during Synthetic Chalcocite Acidic Ferric Sulfate Leaching

Abstract

The fine phase transformation process of chalcocite (Cu₂S) leaching in acidic ferric sulfate solution was studied by leaching experiments and synchrotron radiation X-ray diffraction (SRXRD) tests. The results showed that the dissolution process of chalcocite was divided into two stages. In the first stage, Cu₂S was firstly transformed to Cu₅FeS₄ and Cu_2−xS, then the galvanic effect between Cu₅FeS₄ and Cu_2−xS accelerated the dissolution process of Cu_1.8S → Cu_1.6S → CuS, and finally Cu₅FeS₄ was also transformed to CuS. While in the second stage, CuS was transformed to elemental sulfur, which formed the passivation layer and inhibited the leaching of chalcocite. Specifically, Cu₅FeS₄ was detected during the chalcocite leaching process by SRXRD for the first time. This research is helpful for revealing the detailed leaching process of chalcocite.

1. Introduction

Flotation is one of the most important methods for primary enrichment of minerals and it has significantly developed in recent years [1,2,3,4]. However, hydrometallurgy, particularly bioleaching, has the advantages of being simple, low cost, and eco-friendly, which has been successfully applied in the processing of low grade minerals all over the world [5,6,7,8,9,10,11]. However, there are still some problems and challenges in the leaching of secondary copper sulfide minerals. The phase transformation mechanism and dissolution kinetics of chalcocite are not clear, which is considered as the main reason for low leaching efficiency and an unsatisfactory leaching rate [12,13,14,15]. Therefore, it is necessary to reveal the detailed dissolution mechanism and promote the copper leaching rate.

The crystal structure of chalcocite is more complex than its simple composition. High chalcocite belongs to the P_6-3/mmc space group, in which sulfur atoms are arranged in hexagonal close-packing order, whilst three varieties of copper atoms with four-fold, three-fold, and two-fold coordination are randomly packed in the interstices of sulfur atoms [16]. The structures of low chalcocite and djurleite are derived from the high chalcocite structure and both are monoclinic [17]. It was reported that the structure of low chalcocite was either disordered or belonged to the Pc space group, with four of the 96 copper atoms taking on two-fold coordination in the monoclinic unit. The structure of djurleite is in space group P_2-1/n and with a unit cell containing 8 Cu₃₁S₁₆, it is in general similar to low chalcocite. However, there are 62 different copper atoms, where 52 are in three-fold and triangular coordination with sulfur, 9 form a distorted tetrahedral group, and 1 is in linear coordination [18]. In addition, compared with structural data of other copper-rich sulfides, the tetrahedral, triangular, and linear Cu-S bond of low chalcocite and djurleite are of large distortions and wide variations in bond lengths [19].

Many publications show that the dissolved process of chalcocite acidic ferric sulfate leaching can be divided into two stages, with the production of secondary covellite as an intermediate (Equations (1) and (2)) [13,20,21,22,23,24,25,26], whilst ferric ions were regenerated by Equation (3).

Cu₂S + 2Fe³⁺ → Cu²⁺ + 2Fe²⁺ + CuS

(1)

CuS + 2Fe³⁺ → Cu²⁺ + 2Fe²⁺ + S⁰

(2)

4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O

(3)

The leaching rate of the first stage of chalcocite ferric leaching is very fast compared with the second stage. However, there exists several phase transformation processes in the first stage of leaching. It is reported that chalcocite would be transformed to djurleite (Cu_1.97S), digenite (Cu_1.8S), anilite (Cu_1.75S), geerite (Cu_1.6S), spionkopite (Cu_1.4S), yarrowite (Cu_1.12S), and covellite (CuS) in that order [27,28,29]. According to the stoichiometry, when copper extraction reached 10%, chalcocite would disappear and transform to digenite, whilst all the intermediates were transformed to covellite when copper extraction achieved 50%.

In essence, the second stage is the oxidative dissolution of the secondary covellite, in which the elemental sulfur and Cu²⁺ are produced. It is reported that the secondary covellite has more chemical activity than the primary covellite, which may be attributed to the increasing surface area formed by the first stage of chalcocite leaching [24,30].

The dissolution behavior of the sulfur element in chalcocite ferric leaching is an important output of this mechanism. Chalcocite and covellite can be attacked by Fe³⁺, O₂, and H⁺ attributed to its acid-soluble properties. However, different from the four dissolution pathways (metal-deficient sulfides, polysulfide, element sulfur, and jarosites) of chalcopyrite [31,32,33,34,35,36,37,38], the dissolution of sulfur element in the second stage of chalcocite ferric leaching follows the polysulfide pathway and elemental sulfur is formed [24].

Synchrotron radiation (SR) has the advantages of high-brilliance and flux, wide energy spectrum, very short pulses, and high purity, and many scientific problems could be resolved by SR-based technologies. There are three synchrotron radiation facilities in China to date, Beijing Synchrotron Radiation Facility (BSRF), Shanghai Synchrotron Radiation Facility (SSRF), and the National Synchrotron Radiation Laboratory (NSRL). In the mineral processing and hydrometallurgy research field, SR-based technologies such as SRXRD, SRXPS, and XANES were mainly used to study the fine phase transformation and variation of chemical substances on the mineral surface [39,40,41,42,43,44,45]. In this paper, synthetic chalcocite was chosen as the research object, and SRXRD was employed to explore the fine phase transformation mechanism in chalcocite acidic ferric sulfate leaching. The results would be helpful for explaining the key chemical reactions and key intermediates in chalcocite leaching, and finally to improve copper extraction and resource utilization of secondary copper sulfide minerals.

2. Materials and Methods

2.1. Mineral

The chalcocite sample used in experiments was bought from Sinopharm Chemical Reagent Co., Shanghai, China. The purchased chalcocite was ground using a laboratory mill and sieved to the size of ~0.074 mm for leaching experiments and SRXRD tests.

Chalcocite was confirmed by chemical composition analysis and an SRXRD test. The SRXRD test was completed in beamline 4B9A at the Beijing Synchrotron Radiation Facility (BSRF, Beijing, China). Figure 1 shows that the SRXRD peaks of the sample were fitted with the peaks of chalcocite.

Table 1. Chemical compositions analysis results of chalcocite (%).

Sample	Cu	S	Fe	Ca	Si
Chalcocite	78.61	18.95	0.018	0.017	0.053

shows that the contents of copper element and sulfur element were 78.61% and 18.95%, respectively. The results indicated that chalcocite sample used in this paper was of high purity.

2.2. Leaching Experiments

One gram of chalcocite with a size of ~0.074 mm was put into a 250 mL shake flask containing 100 mL of sterilized distilled water, and then two grams of ferric sulfate hydrate (Fe₂(SO₄)₃·xH₂O) was added as the source of ferric ions. The pH value was adjusted to 1.7 by diluting sulfuric acid and then the shake flasks were placed into an orbital shaker at a speed of 170 rpm and temperature of 45 °C. The shake flask was sealed with a breathable sealing film to prevent rapid evaporation of water, and the atmosphere inside the orbital shaker was standard atmospheric air and the oxygen in the air could enter the shake flask.

In the leaching process, the pH value was adjusted back to 1.7 when it was above 2.0, and water lost by evaporation was supplemented periodically by distilled water regularly. Copper ion concentrations and iron ion concentrations were measured with an inductively coupled plasma-atomic emission spectrometer (ICP-AES) (PS-6, Baird Co., Deford, MA, USA). The redox potentials of the leaching solution were determined by a Pt electrode with reference to an Ag/AgCl electrode (3.0 mol/L KCl) (BPH-221, Dalian BELL Co., Dalian, China), whilst the pH values were measured with a pH meter (PHSJ-4A, Shanghai LEICI Co., Shanghai, China).

The leaching experiments were set as multiple parallel samples, and the samples were removed according to the time set in advance. The removed sample was filtered with a neutral filter paper and cleaned with distilled water three times, afterwards, the samples were dried in a vacuum oven for 24 h for the tests of SRXRD.

2.3. Synchrotron Radiation X-ray Diffraction

The SRXRD spectra were obtained from beamline BL14B1 of the Shanghai Synchrotron Radiation Facility (SSRF). The amount of mixed sample that needed to be added for each test was 50 mg. The wavelength of incident synchrotron radiation X-ray was 0.6895 nm, and the SRXRD spectra ranged from 4° to 38°. The background removal and phase comparison of the spectra were completed using the software Jade 6.

3. Results and Discussion

3.1. Leaching Behaviors

The copper extraction of chalcocite leaching at different leaching times is shown in Figure 2a. It indicated that the leaching of chalcocite was divided into two stages according to the leaching rate. The first 21 h of leaching can be considered as the first stage, in which copper extraction increased rapidly as a straight line. The stage after 21 h of leaching can be regarded as the second stage, in which copper extraction increased much slower than the first stage. It was reported that the first stage was controlled by the diffusion of oxidant and the activation energy was 4–25 KJ/mol, whilst the activation energy of the second stage was 55–105 KJ/mol [22,30], which was considered as the main reason of the difference in copper extraction.

The variation of iron ion concentration in the first 15 h of chalcocite leaching is shown in Figure 2b. It indicated that the concentration of iron ions had a sudden decrease at the first 1 h of leaching, and then it increased slowly until leaching for 10 h, and thereafter remained stable. It has been reported that ferric ions have a stronger oxidizing ability than dissolved oxygen [46], hence the direct oxidant in chalcocite acidic ferric sulfate leaching was ferric ions. Although iron ions may hydrolyze to form a precipitate due to pH changes during leaching, the amount was small near the pH value of 2.0. Accordingly, it can be speculated that ferric ions in solution may have directly reacted with chalcocite and generated some iron-containing compounds in the first 1 h of leaching, and after 10 h of leaching, the generated iron-containing compounds were dissolved again.

Figure 2c,d shows the variations of pH value and redox potential, respectively. It revealed that pH value had a significant increase, whilst redox potential had a sharp decrease at the beginning of the leaching. This can be attribute to the rapid consumption of sulfuric acid shown by Equation (3), and the rapid consumption of ferric ions as follows in Equations (1) and (2).

3.2. Synchrotron Radiation X-ray Diffraction Tests

3.2.1. SRXRD Results of the First Stage of Leaching

To further investigate the phase transformation process of the first stage of chalcocite leaching, SRXRD was used to analyze the intermediate species during chalcocite leaching. Figure 3 and

Table 2. The minerals detected in different leaching time.

Time/h	Cu2S	Cu5FeS4	Cu1.8S	Cu7.2S4	Cu8S5	CuS	S
0	√
0.5		√	√	√	√
1		√	√	√	√
2		√		√	√	√
4		√			√	√
6		√				√
10						√	√
15						√	√

show that chalcocite disappeared just after half an hour of leaching, while Cu_7.2S₄, Cu_1.8S, Cu₈S₅, and Cu₅FeS₄ were detected by SRXRD. It indicated that chalcocite was quickly oxidized in the initial stage of ferric leaching and some intermediates were generated. Afterwards, no new phase appeared and the peak intensity of Cu_7.2S₄ decreased when leached for 1 h, which indicated part dissolution of Cu_7.2S₄.

However, CuS appeared as an intermediate when chalcocite was leached for 2 h, whilst the peak intensity of Cu_7.2S₄ and Cu₅FeS₄ experienced a decrease. Then, the peak intensity of Cu_7.2S₄ disappeared when leached for 4 h, and the peak intensity of Cu₈S₅ was when leached for 6 h. Moreover, the peak intensity of Cu₅FeS₄ was increasingly weaker, whilst the peak intensity of CuS became increasingly stronger as chalcocite was leached for 2 h to 6 h.

After leaching for 10 and 15 h, Cu₅FeS₄ disappeared and elemental sulfur was generated, which indicated that the generated intermediate Cu₅FeS₄ had dissolved after 10 h of leaching, and that part of CuS had oxidized to elemental sulfur.

According to Figure 3, it can be concluded that the first stage of chalcocite leaching was not a simple process where Cu²⁺ dissolved into solution and chalcocite directly transformed to CuS. Instead, it was a complex and fast process in which several chemical reactions occurred simultaneously. The products of these reactions were several similar minerals containing Cu, S, or Fe elements, which are close to each other in diffraction peak. In particular, Cu₅FeS₄ was detected for the first time by SRXRD, which was in accordance with the results of Figure 2b. It will be helpful for explaining the phase transformation mechanism in chalcocite leaching.

3.2.2. SRXRD Results of the Second Stage of Chalcocite Leaching

The results shown in Figure 4 suggest that no new phase appeared in the second stage of chalcocite leaching, where the peak intensity of CuS was increasingly weaker, whilst the peak intensity of elemental sulfur was increasingly stronger as the leaching continued. After leaching for 933 h, the main phase was elemental sulfur and only a few CuS existed.

Equation (2) shows that the second stage of chalcocite leaching was the redox reaction between covellite and ferric ions. In this process, ferric ions acted as an oxidant reduced to ferrous ions, whilst CuS was oxidized to elemental sulfur. Meanwhile, it should be noted that only a small amount of elemental sulfur would be continuously oxidized to sulfate under low redox potential, as shown in Figure 2d [30].

Hence, it can be concluded that in the second stage of chalcocite acidic ferric sulfate leaching, CuS was transformed to elemental sulfur and no new intermediate was generated. However, it should be noted that the kinetics of the second stage of chalcocite leaching were much lower than the first stage. The reason may be that the generated elemental sulfur formed a passivation layer and inhibited the dissolution of chalcocite.

3.3. Thermodynamic Calculation of the Generation and Dissolution of Cu₅FeS₄ in Chalcocite Leaching Process

Whether a reaction can start spontaneously depends on the variation of Gibbs free energy of the reaction. If the variation of Gibbs free energy of the reaction was negative, the reaction can be spontaneously started. On the contrary, the reaction cannot be carried out spontaneously.

According to the redox properties of various chemical components in the leaching process, it can be inferred that Cu₅FeS₄ detected by SRXRD might be generated and dissolved, as shown in Equations (4) and (5).

4Cu₂S + 4Fe³⁺ → Cu₅FeS₄+3Cu²⁺+3Fe²⁺

(4)

Cu₅FeS₄ + 4Fe³⁺ → 4CuS+Cu²⁺+5Fe²⁺

(5)

Some thermodynamic data are listed in

Table 3. Some standard thermodynamic data (298.15 K) [47,48].

Components	Enthalpy (KJ/mol)	Entropy (J/mol/K)	Gibbs Free Energy (KJ/mol) (298.15 K)
Cu2S	−79.496	120.918	−115.548
Cu5FeS4	−380.326	362.334	−488.356
CuS	−48.530	66.530	−68.366
Cu2+ (aq)	64.768	−99.579	94.458
Fe3+ (aq)	−48.534	−315.892	45.649
Fe2+ (aq)	−89.119	−137.654	−48.078

. The variations of Gibbs free energy can be calculated from Equation (6):

$${\Delta }_r{G}_m={\Delta }_r{G}_m^{\mathsf{\Theta }}+RTlnK$$

(6)

where K was the chemical reaction equilibrium constant.

According to the thermodynamic data in Table 3, it can be calculated that the variations of Gibbs free energy of Equations (4) and (5) were −69.62 KJ/mol and −113.63 KJ/mol under standard state, respectively. It indicated that Equations (4) and (5) can be carried out spontaneously on thermodynamics by Gibbs free energy criterion.

3.4. Disscussion of the Fine Phase Transformation of Chalcocite Acidic Ferric Sulfate Leaching

In the leaching process in acidic ferric sulfate solution, chalcocite was firstly reacted with ferric ions and some intermediates Cu_1.8S, Cu_7.2S₄, Cu₈S₅, and Cu₅FeS₄ were generated according to Equations (7) and (4). Then, Cu_1.8S, Cu_7.2S₄, and Cu₈S₅ were oxidized to CuS in that order, according to Equation (8). Afterwards, Cu₅FeS₄ was also oxidized to CuS, following Equation (5). Finally, CuS was oxidized to elemental sulfur, following Equation (2). Meanwhile, ferric ions played the role of oxidants during the entire leaching process and were regenerated in accordance with Equation (3).

Cu₂S + 2xFe³⁺ → Cu_2−xS + xCu²⁺ + 2xFe²⁺, (0 < x <1)

(7)

Cu_2−xS + 2(1 − x)Fe³⁺ → CuS + (1 − x)Cu²⁺ + 2(1 − x)Fe²⁺, (0 < x < 1)

(8)

It should be noting that the disappearance of those intermediates was in the order of Cu_1.8S → Cu_7.2S₄ → Cu₈S₅ → Cu₅FeS₄ → CuS, according to Figure 3 and Figure 4. In addition, it has been reported by many researchers that there exists a galvanic effect between different semiconductor minerals when they are leached [49,50,51]. In the first stage of chalcocite leaching, several copper sulfide minerals appeared; the other potentials of these minerals were different and there existed a galvanic effect between them. The other potentials of these minerals were in the following order: Cu₂S < Cu_1.8S < Cu_1.6S < Cu₅FeS₄ < CuS. Hence, it can be speculated that the galvanic effect between Cu₅FeS₄ and Cu_2−xS accelerated the dissolution of Cu₂S and Cu_2−xS, and when Cu_2−xS was completely transformed to CuS, a galvanic effect between Cu₅FeS₄ and CuS accelerated the dissolution of Cu₅FeS₄, then all the intermediates were transformed to CuS.

Therefore, it can be concluded that the probable phase transformation mechanism of chalcocite leaching in acidic ferric solution is as shown in Figure 5.

4. Conclusions

In summary, the acidic ferric sulfate leaching of synthetic chalcocite was divided into two stages. Chalcocite was transformed to covellite in the first stage, then covellite was transformed to elemental sulfur in the second stage. Meanwhile, the kinetic of the second stage was much lower than the first stage.

However, the fine phase transformation process of synthetic chalcocite acidic ferric sulfate leaching was a complicated process involving Cu, Fe, and S elements:

(1)

Firstly, chalcocite reacted with Fe³⁺ and was quickly transformed to Cu_7.2S₄, Cu_1.8S, Cu₈S₅, and Cu₅FeS₄.

(2)

Then, the galvanic effect between Cu₅FeS₄ and Cu_2-xS accelerated the dissolution of Cu_2−xS, and the phase transformation order of Cu_2−xS was Cu_1.8S → Cu_1.6S → CuS.

(3)

When Cu_2−xS was completely transformed to CuS, Cu₅FeS₄ was transformed to CuS thereafter.

(4)

In the second stage, the generated CuS was oxidized to elemental sulfur, which should be the passivation layer inhibiting the efficient leaching of chalcocite.