The influence of micro-cracks on copper extraction by bioleaching

Journal Pre-proof The influence of micro-cracks on copper extraction by bioleaching

Jinghe Chen, Ding Tang, Shuiping Zhong, Wen Zhong, Baozhu Li PII:

S0304-386X(19)30527-4

DOI:

https://doi.org/10.1016/j.hydromet.2019.105243

Reference:

HYDROM 105243

To appear in:

Hydrometallurgy

Received date:

13 June 2019

Revised date:

7 December 2019

Accepted date:

21 December 2019

Please cite this article as: J. Chen, D. Tang, S. Zhong, et al., The influence of micro-cracks on copper extraction by bioleaching, Hydrometallurgy(2019), https://doi.org/10.1016/ j.hydromet.2019.105243

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The Influence of Micro-cracks on Copper Extraction by Bioleaching Jinghe Chena,b,c, Ding Tanga,b,c, Shuiping Zhonga,b,c, , Wen Zhonga, Baozhu Lia a

College of Zijin Mining, Fuzhou University, Fuzhou, 350108, China b

c

Zijin Mining Group Co. Ltd, Shanghang, Fujian 364200, China

State Key Laboratory of Comprehensive Utilization of Low Grade Refractory Gold Ores,

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Shanghang, Fujian 364200, China

Jinghe Chen: Supervision, Writing - Review & Editing

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Ding Tang: Investigation, Writing - Original Draft

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administration, Funding acquisition

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Wen Zhong: Investigation Baozhu Li: Investigation

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Shuiping Zhong: Conceptualization, Writing - Review & Editing, Project

 Corresponding author: Shuiping Zhong; E-mail address: [email protected]

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Abstract: Micro-cracks produced in the crushing stage could improve the permeability and diffusion of leaching solution within the ore particles, which would favor for the leaching process. However, its influence on the extraction of copper by bioleaching has rarely been studied. In this work, we reported the enhancement effect of micro-cracks on the bioleaching of low-grade copper sulphide ore. Jaw crusher and

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high-pressure grinding rolls were adopted to prepare ore samples of the same size fraction but with different number of micro-cracks in the ore particles. Statistical data,

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scanning electron microscope (SEM) and stereomicroscopy (SM) images clearly

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showed that high-pressure grinding rolls could generate more and larger micro-cracks

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than the same produced by jaw crusher. Subsamples from the -0.9 mm and 3-6 mm size fractions were utilized for shake flask and column bioleaching experiments,

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respectively. The earlier change in pH and higher oxidation-reduction potential (Eh) indicated that more micro-cracks were beneficial for the growth of bacteria, which

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then enhanced the bioleaching efficiency of copper. In the shake flask experiments, the copper extraction efficiency was increased from 87.4% to 94.3% on the 30th day at 40 °C and 150 rpm. Similarly, in the column experiment with the ore sample prepared by high-pressure grinding rolls, an approximately 12.2% improvement of bioleaching efficiency was observed as compared to the system with the ore sample prepared by jaw crusher. These results demonstrated that creating micro-cracks could be crucial for heap bioleaching of low-grade copper sulphide ores.

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Keywords : Low-grade copper sulphide ore; Bioleaching; Micro-cracks; Enhanced

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leaching; High-pressure grinding rolls

1. Introduction

As an environment- friendly technology, heap bioleaching of the low- grade copper ore (e.g. secondary copper sulphide minerals) has been successfully implemented in numerous commercial copper plants worldwide since the 1990s (Domic, 2007; Panda et al., 2012). Nowadays, all these plants produce around 25% of world's annual yield of copper (Kang et al., 2014). Heap bioleaching involves a complicated biochemical process, in which various leaching conditions need to be optimized for fast and

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efficient extraction of copper. Normally, it takes several months or even more than one year to reach a copper extraction of >80% (Domic, 2007). During the past three decades, numerous biological, chemical and physical factors have been considered and investigated with the purpose to enhance the bioleaching process. For example, Konishi et al. (2001) reported that the acidophilic thermophile bacteria Acidianus

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brierleyi isolated from the German Collection of Microorganisms and Cell Cultures (DSMZ) could solubilise chalcopyrite much faster at 65 °C than the mesophile

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bacteria Thiobacillus ferrooxidans at 30 °C. Moreover, it was found that the copper

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extraction of chalcopyrite by a mixed culture was higher than that of a pure culture

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(Qiu et al., 2005; Yang et al., 2014; Feng et al., 2015; Feng et al., 2016). Additionally, several studies showed that the addition of surfactant or catalyst could lead to an

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enhanced bioleaching performance (Peng et al., 2012; Zhang et al., 2016). Furthermore, Murray et al. (2017) utilised solar thermal energy to raise the

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temperature in the heap and increase the copper extraction efficiency. Apart from above factors, particle size of minerals also exhibited the vital influence on the bioleaching rate (Nemati et al., 2000; Acevedo et al., 1989). It can be expected that small particles could have more opportunity to expose the sulphide minerals so that the rate of bioleaching would be faster (Ehrlich et al., 1967; Kodali et al., 2011). However, it should be pointed out that very small particles would require extra energy during the crushing process. In the meantime, too much fine particles could bring

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circulation problem of air and leaching solution which is unfavorable for bioleaching. (Acevedo et al., 1989; Yang et al., 2019). In the process of crushing, micro-cracks could be generated in the ore particles (Petersen, 2016; Ghorbani et al., 2013). These micro-cracks probably connected mineral grains to particle surface, by which the leaching solution could permeate and

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diffuse within the ore particles and then solubilize the minerals. Similar to exposing minerals of interest by reducing particle size, micro-cracks to some extent also could

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increase the contact probability of minerals and reagents, which would enhance the

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leaching process. Tang et al. (2016) found that low-grade gold ore crushed by

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high-pressure grinding rolls contained more micro-cracks on the surface of particles and had higher saturated water content than the same produced by jaw crusher. The

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results of column leaching experiments showed that the leaching efficiency of gold could be improved by 3.5%~6.8% and the consumption of leaching solution was

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decreased by 3.2%~11.3% (Tang et al., 2016). Ghorbani et al. (2012) discovered that the higher sessile cell population density was obtained when more cracks formed on the surface of sphalerite ore particles, which resulted in the increase of zinc extraction by bioleaching. Moreover, Ghorbani et al. (2013) employed X-ray Computed Tomography (CT) to investigate the mineral conversion in selected individual particles from zinc sulphide ore and concluded that the leaching zone covered both the particle surface and a relatively deep subsurface zone with the existence of micro-cracks, while outer surface of the particle was the main reaction zone for those

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without micro-cracks. In addition, the microwave treatment was employed to increase the internal cracking in ore particles, and thereafter an increased recovery of zinc was achieved (Charikinya et al., 2017). Interestingly, Kodali et al. (2011) reported that copper recovery was independent of the crushing method for copper sulphide ore despite that the damage and exposure of minerals was different. This unusual

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behavior was attributed to the high head grade and strong leaching solution. However, they expected that the effect of damage and exposure would be obvious for copper

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sulphides when a less aggressive leaching solution was used (Kodali et al., 2011).

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To the best of our knowledge, the influence of micro-cracks on the extraction of

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copper from low- grade copper ore by bioleaching has not been extensively studied. Herein, the Zijinshan low-grade copper ore was crushed by jaw crusher and

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high-pressure grinding rolls, and the ore samples of the same size fractions were collected to analysis the number of micro-cracks in the ore particles. The shake flask

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and column bioleaching experiments were conducted to investigate the effect of micro-cracks on the extraction of copper. Based on the experimental results, an improved approach for enhancing the heap bioleaching of low-grade copper sulphide ores was presented. 2. Materials and methods 2.1 Mineral characteristics Low- grade copper ore was collected from the Zijinshan Gold & Copper Mine in Fujian Province, China. Table 1 shows the main chemical composition of the

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Zijinshan copper ore. Table 2 displays the mineral composition of the Zijinshan copper ore. The copper grade is only 0.23%, and copper mainly exists in the form of secondary copper sulphides (e.g. digenite, covellite, enargite). These make the Zijinshan copper ore suitable for heap bioleaching (Ruan et al., 2013). Additionally, a small amount of soluble copper and copper oxide minerals was also detected in the

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Zijinshan copper ore. Furthermore, it can be noted that the content of pyrite is high (6.32%), which would be oxidized to generate heat, iron and sulfuric acid during the

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bioleaching process.

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Table 1 Main chemical composition of the Zijinshan copper ore (mass fraction %). * *  Au Ag Cu Fe TS As K2O CaO MgO Al2O3 SiO2 0.003 0.05 0.23 2.30 4.53 0.021 2.01 0.11 0.03 11.17 70.74 *: Unit is g/t; : Total sulfur

Table 2 Mineral composition of the Zijinshan copper ore (mass fraction %). Digeni Covelli Enargi Pyrit Limoni Aluni Dicki Quart Othe te te te e te te te z rs 0.21 0.13 0.11 6.32 0.34 7.34 8.26 73.82 3.47

2.2 Sample preparation First, the primary jaw crusher (PEX-150×250 mm) and secondary jaw crusher (XPC-60×100 mm) were sequentially applied to comminute the Zijinshan copper ore. The crushed copper ore passed through a sieve of 20 mm was evenly mixed and

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divided into two parts for further crushing by a jaw crusher (PEF-60×100 mm) and a high-pressure grinding rolls (CLF-25-10, with an applied pressure of 5.5 N/mm2 ), respectively. Both the width of discharge port of jaw crusher and the distance between the two rollers of high-pressure grinding rolls were set to 9.0 mm. The ore samples obtained by two crushing methods were sieved into four different size fractions (-0.9

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mm，0.9-3 mm，3-6 mm，6-9 mm). Table 3 lists the yield and copper grade (determined by using chemical element analysis) of various size fractions for two

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crushing methods. It can be seen that high-pressure grinding rolls could produce

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higher proportion of fine fractions than the jaw crusher (Tang et al., 2016). Although

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the yield of the same size fraction shows a significant difference, the copper grade remains the same.

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Table 3 The yield and copper grade of various size fractions. Size fraction /mm

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Yield /%

Copper grade /%

High-pressure grinding rolls

Jaw crusher

High-pressure grinding rolls

27.81

9.86

0.22

0.24

32.45

13.21

0.23

0.21

0.9-3

28.91

47.55

0.21

0.23

-0.9

10.83

29.38

0.25

0.23

6-9 3-6

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Jaw crusher

2.3 Micro-cracks analysis

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Micro-cracks were observed and counted by using a scanning electron microscopy (MLA 650) and a stereomicroscopy (Leica S8 APO). Scanning electron microscope (SEM) was employed to observe the micro-cracks for ore particles less than 0.9 mm, and stereomicroscope (SM) was used for the other size fractions. Different visual fields and at least 300 particles of the crushed ores were chosen

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randomly. Everywhere in each individual particle was checked to make sure that all

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the micro-cracks were counted. The observation of each field was done twice to

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reduce the accidental error, and the final result was the average of two independent

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2.4 Bacteria and culture conditions

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values.

Mixed culture of bacteria including Acidithiobacillus spp., Leptospirillum spp.,

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Sulfobacillus spp., and Ferroplasma spp. was isolated from the acid mine drainage in the Zijinshan Gold & Copper Mine (Liu et al., 2010). Bacteria were cultured in 9K

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culture medium, which consisted of 0.5 g/L MgSO 4 ·7H2 O, 3.0 g/L NH4 SO 4 , 0.1 g/L KCl, 0.01 g/L Ca(NO 3 )2 ·H2 O and 0.5 g/L K2 HPO 4 . pH of the culture medium was adjusted to 2.0 using dilute sulfuric acid. A constant-temperature shaker with the temperature of 40 °C and rotating speed of 150 rpm was employed to culture bacteria. 2.5 Bioleaching experiments 200 ml of 9K culture medium was used in shake flask bioleaching experiments with 10% (wt/wt) suspension of -0.9 mm size fraction copper ore. The initial pH of ore pulp was adjusted to 2.0. The cultures were incubated at 40 °C for 30 days. pH

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and oxidation-reduction potential (Eh) were monitored by a pH- meter (PH-HJ90B) and a HI3131B electrode (AHS STARTER 3100) every day. Copper dissolution was determined by analysis of the culture solution using atomic absorption spectroscopy. 1000 g of 3-6 mm size fraction ore particles was fed into the cylindrical column with a diameter of 50 mm and a height of 510 mm. Columns were irrigated from the

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top using bacteria inoculated 9K culture medium, and the irrigation was performed in

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a closed circuit. The flow rate of solution passed through the column was set to 10

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L/m2 ·h. The initial pH of lixivium was adjusted to 2.0, and the temperature was

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controlled at 40 °C. pH and oxidation-reduction potential (Eh) were monitored by a

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pH-meter (PH-HJ90B) and a HI3131B electrode (AHS STARTER 3100). The pregnant leaching solution (5 mL) was collected at the bottom of the column for

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analysis every five days. The column bioleaching experiments were run for 60 days. 3. Results and discussion

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3.1 Comparison of micro-cracks

The statistical results of ore particles with/without micro-cracks prepared by two crushing methods are summarized in Table 4. Clearly, high-pressure grinding rolls was able to produce higher proportion of particles with micro-cracks than the jaw crusher for the same size fraction. For instance, the percentage of particles with micro-cracks is found to be around 2-fold higher in the 3-6 mm and 6-9 mm size fractions. In addition, the greater proportions occur in the -0.9 mm and 3-6 mm size fractions for both crushing methods.

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Table 4 Statistical results of the ore particles with/without micro-cracks. Jaw crusher

High-pressure grinding rolls

Number of

%

Number of

Number of

%

particles

particles

particles

particles

particles

particles

with

without

with

with

without

with

cracks

cracks

cracks

cracks

cracks

cracks

6-9

10

301

3.32

21

314

6.27

3-6

16

306

4.97

39

345

10.16

0.9-3

9

367

2.39

13

389

3.23

-0.9

21

302

6.50

32

335

8.72

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Number of

Size fraction/ mm

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Fig. 1 shows the typical SEM images of the ore particles from the -0.9 mm size

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fractions, in which the micro-cracks are marked by yellow arrows. It can be noted that

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the product of high-pressure grinding rolls contains more and larger micro-cracks on

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the particle surfaces compared with the ore particles crushed by jaw crusher. The

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stereomicroscopy images of the ore particles from the 3-6 mm size fractions are displayed in Fig. 2. Abundant and large cracks can be found in the ore particles crushed by high-pressure grinding rolls, while only a few and tiny cracks are formed when crushed by jaw crusher. These observations have been indirectly reflected in Table 4.

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b

c

d

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a

Fig. 1 SEM images of ore particles from the -0.9 mm size fractions prepared by (a, b) jaw crusher

500 μm

d

c

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and (c, d) high-pressure grinding rolls.

500 μm

500 μm

500 μm

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500 μm

500 μm

Fig. 2 SM images of ore particles from the 3-6 mm size fractions prepared by (a, b, c) jaw crusher and (d, e, f) high-pressure grinding rolls.

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It is well known that solution can flow in narrow spaces without the assistance of external forces because of the capillary effect (Roy et al., 1999; Ilankoon et al., 2016). Therefore, micro-cracks formed in the ore particles should facilitate mass transfer of dissolved species and diffusion of leaching solution within the ore particles, which should be able to speed-up the bioleaching process.

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3.2 Shake flask bioleaching The changes of pH and Eh over the time are shown in Fig. 3. As shown in Fig.

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3(a), acid is consumed at the initial stage, and then acid is produced. A small amount

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of alkaline gangue existing in ore samples would consume acid, which results in the

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increase in pH value at the beginning. pH in the system with ore particles prepared by high-pressure grinding rolls starts to decline since 10th day, after which pH

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continuously deceases. Differently, the variation of pH occurs at first several days in the system with ore particles prepared by jaw crusher probably due to the incomplete

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contact between ore particles and leaching solution. After the 20th day, pH begins to decrease constantly. It is about 10 more days later compared with that of the system with the ore particles prepared by high-pressure grinding rolls. The declining pH is attributed to the acid production associated with the bio-oxidation of reduced sulfur compounds (Bailey et al., 1993). Therefore, it can be inferred that the oxidation activities of bacteria increase slowly in the system with ore particles prepared by jaw crusher. The variations of Eh in both systems are plotted in Fig. 3(b). After a short adaptation period, Eh in the system with ore particles prepared by high-pressure

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grinding rolls increase sharply to around 650 mV vs. SHE (standard hydrogen electrode), then reaching a relatively stable state. As shown, it takes a longer time to

a

b

achieve the steady Eh value in the system with the ore particles prepared by jaw crusher. This result also reveals that bacteria have better oxidation activities in the

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system with the ore particles prepared by high-pressure grinding rolls.

Fig. 3 The change of (a) pH and (b) Eh during the shake flask bioleaching of copper sulphide ore

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with particle size of -0.9 mm.

Fig. 4 shows the time course of copper concentration in the leaching solution. At

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early stage, the concentration of copper raise rapidly in both systems, which should be resulted in from the dissolution of acid-soluble copper minerals (e.g. soluble copper, copper oxides). After the 9th day, the copper concentration in the system with the ore particles prepared by high-pressure grinding rolls is consistently higher than that of system with the ore particles prepared by jaw crusher. This can be explained by the changes of pH and Eh in these two systems, which manifest the better oxidation activities of bacteria in the system with the ore particles prepared by high-pressure grinding rolls. By analyzing the copper grade in the leaching residues, the extraction

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of copper on the 30th day was determined to be 87.4% and 94.3% for the samples prepared by jaw crusher and high-pressure grinding rolls. It showed that copper extraction is improved by 6.9% through creating more micro-cracks in the ore

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particles.

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Fig. 4 The plot of copper concentration in the leaching solution.

3.3 Column bioleaching

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Fig. 5 illustrates the variations of pH and Eh in the tested columns during the experiment period. Both pHs increase slightly to about 2.4 at the early stage, which

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could be related to the proton consumption by alkaline gangue. It can be seen that pH begins to decrease since 10th day in the system with the ore particles prepared by high-pressure grinding rolls, while pH declines since 15th day in the system with the ore particles prepared by jaw crusher. Again, the former system reaches the stable pH value (~1.4) about 10 days earlier than the later system. The reduced sulfur compounds would be bio-oxidized by bacteria during the bioleaching process, in which acid (sulphuric acid) is produced simultaneously. An earlier change in pH indicates that the bacteria are more active in the system with the ore particles prepared

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by high-pressure grinding rolls. Fig. 5(b) shows that Eh in the system with the ore particles prepared by high-pressure grinding rolls at the stable stage is higher than that of system with the ore particles prepared by jaw crusher. High Eh has been considered as indicators of the presence of high population of bacteria (Wong et al., 2002). These results reveal that more micro-cracks are beneficial for the growth of bacteria, which

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can promote the bioleaching of copper. The bacterial populations at the end of

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a experiments were about 35×106 /mL andb10×106 /mL in the systems with the ore

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particles prepared by high-pressure grinding rolls and jaw crusher, respectively.

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Fig. 5 The change of (a) pH and (b) Eh during the column bioleaching of copper sulphide ore with particle size of 3-6 mm.

Fig. 6 shows the copper extraction curves during the whole test period (60 days). For both systems, the copper extraction increases fast in the initial 30 days and then gradually slows down. The copper extraction in the system with the ore particles prepared by high-pressure grinding rolls is continuously higher. The final copper extractions were 51.2% and 45.6%, respectively. In other words, the copper extraction

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is improved by 12.2% through creating more micro-cracks in the ore particles by

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high-pressure grinding rolls.

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Fig. 6 Copper leaching curves in the column bioleaching of copper sulphide ore with particle size

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of 3-6 mm.

Most recently, an ideal leaching kinetic model was established to reveal the

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effect of micro-cracks on the recovery of gold (Yin et al., 2017): 2

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[1 − (1 − 𝜀)3 ] =

𝑘 𝐶 𝑛𝑤𝑙 2𝜋𝜌𝑟1 𝑟22

𝑡 = 𝐾𝑡

(1)

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where ε is the extraction of gold, k is the apparent reaction constant, C is the concentration of leaching agent, n is the reaction order, w is the width of micro-crack, l is the length of micro-crack, ρ is the density of gold, r1 and r2 are the radii of ore particle and gold grain, respectively, and t is the leaching time. It can be known from Equation (1) that the properties of micro-crack (e.g. the width w and length l) have direct influence on the extraction of metal. More and larger micro-cracks could result in improved metal extraction.

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Considering that the cyanide leaching of gold and bioleaching of copper sulphides are essentially the same chemical dissolution process, Equation (1) is assumed to be valid for this system. Fig. 7 plots the function [1- (1- ε)2/3 ] against t adopting the data displayed in Fig. 6. Two good linear relationships can be obtained, verifying by that the resultant R-square values were greater than 0.98. However, note

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that the fitting curves do not pass through the coordinate origin, which should be due to the quick dissolution of acid-soluble copper minerals (e.g. soluble copper, copper

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oxides) in the first few days (as seen in Fig. 4). According to the fitting equations, the

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bioleaching time for achieving 90% of copper extraction can be calculated: 73.3 days

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and 83.9 days for high-pressure grinding rolls and jaw crusher, respectively. This result reveals that creating more and larger micro-cracks in the ore particles could

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shorten the duration of bioleaching process.

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Fig. 7 The plot [1- (1- ε) ] against t by adopting the equation (1) (Yin et al., 2017). The data was gathered from the curves of copper extraction in the column bioleaching of copper sulphide ore with particle size of 3-6 mm (as shown in Fig. 6).

3.4 Discussion

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In the bioleaching process, not only the exposure degree of mineral grain but also the actively flowing path of leaching solution could significantly affect the extraction efficiency of metal (Petersen, 2016; Acevedo et al., 1989; Ilankoon et al., 2016). For the low-grade Zijinshan copper ore, a large amount of mineral composition is quartz (73.82%). In this case, the formation of micro-cracks on the ore particles would

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increase the possibility of mineral grain exposure. Additionally, the leaching solution could penetrate through the micro-cracks to reach the mineral grains because of the

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capillary effect. Furthermore, the bio-oxidation activities of bacteria are increased by

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the enhanced exposure of mineral grains at the same time. Therefore, an improved

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bioleaching process was achieved through creating more and larger micro-cracks in

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4. Conclusions

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the ore particles by employing high-pressure grinding rolls in this study.

Heap bioleaching of the low-grade copper ores has been successfully

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industrialized in many copper plants. However, the approaches to enhance the bioleaching process are still highly desirable. In this work, the enhancement effect of micro-cracks on the bioleaching of low- grade Zijinshan copper ore was reported. Ore particles of the same size fraction but with different number of micro-cracks were prepared by jaw crusher and high-pressure grinding rolls. It was found that the more and larger micro-cracks could be produced by high-pressure grinding rolls compared with jaw crusher based on the statistical data obtained by scanning electron microscope (SEM) and stereomicroscopy (SM) measurements. Both in the shake flask

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and column bioleaching experiments, the variation tendency of pH and Eh suggested that more micro-cracks were beneficial for the growth of bacteria, which then enhanced the bioleaching efficiency of copper. To be specific, the extraction of copper was increased from 87.4% to 94.3% in the shake flask bioleaching of ore samples with particle size of -0.9 mm. Similarly, the copper extraction was improved

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from 45.6% to 51.2% in the column bioleaching of ore samples with particle size of 3-6 mm. This study demonstrated an improved approach to promote bioleaching of

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low-grade copper sulphide ores by creating micro-cracks in the ore particles.

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Acknowledgements

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This work is supported by the National Natural Science Foundation of China

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under the grant 51874101 and 51704153.

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The authors declare no competing interests.

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Minerals Engineering 111, 140-147.

Journal Pre-proof Highlights 

Ore particles of the same size fraction but with different number of micro-cracks were prepared.

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The influence of micro-cracks on the activities of bacteria was investigated.

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An improved approach to extract copper from low-grade copper ores by

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bioleaching was reported.