International Journal of Minerals, Metallurgy and Materials Volume 19, Number 1, Jan 2012, Page 1 DOI: 10.1007/s12613-012-0507-4

Effects of bioleaching on the mechanical and chemical properties of waste rocks Sheng-hua Yin1,2), Ai-xiang Wu1,2), Shao-yong Wang1), and Chun-ming Ai1) 1) Key Laboratory of the Ministry of Education of China for High-Efficient Mining and Safety of Metal Mines, University of Science and Technology Beijing, Beijing 100083, China 2) School of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China (Received: 24 January 2011; revised: 26 February 2011; accepted: 16 March 2011)

Abstract: Bioleaching processes cause dramatic changes in the mechanical and chemical properties of waste rocks, and play an important role in metal recovery and dump stability. This study focused on the characteristics of waste rocks subjected to bioleaching. A series of experiments were conducted to investigate the evolution of rock properties during the bioleaching process. Mechanical behaviors of the leached waste rocks, such as failure patterns, normal stress, shear strength, and cohesion were determined through mechanical tests. The results of SEM imaging show considerable differences in the surface morphology of leached rocks located at different parts of the dump. The mineralogical content of the leached rocks reflects the extent of dissolution and precipitation during bioleaching. The dump porosity and rock size change under the effect of dissolution, precipitation, and clay transportation. The particle size of the leached rocks decreased due to the loss of rock integrity and the conversion of dry precipitation into fine particles. Keywords: bioleaching; mechanical properties; chemical properties; waste rocks

[This work was financially supported by the National Natural Science Foundation of China (Nos.50934002 and 51104011), the Program for Changjiang Scholars and Innovative Research Team in Universities (IRT0950), and China Postdoctoral Science Foundation (No.20100480200).]

1. Introduction Waste rocks, with too low metal contents to be economically recovered using pyrometallurgical methods, are usually stored in large and porous dumps. Acid mine drainage, produced by these dumps due to the oxidation of sulfide minerals, which is catalyzed by bacteria, can be the source of significant contamination of ground water and surface water [1-4]. In some cases, it is economically feasible to re-process the waste rocks containing large amounts of low grade ores. Currently, bioleaching of waste rock dumps is an important application in the recovery of copper and gold from their ores because of the low cost, operational simplicity, and environmental advantages [5-7]. Bioleaching is defined as an attack and dissolution of minerals by the direct or indirect action of different microorganisms [8]. During Corresponding author: Sheng-huaYin

this process, the acidic solution containing bacteria is applied to the top of the dump and allowed to percolate downward. The solution is collected in a drainage system and metals are recovered by solvent extraction and electrowinning. Previous studies on the thermodynamics, kinetics, and chemical reaction mechanisms in dump leaching (with or without microbial mediation) have provided much insight into the effects of limiting reactions [9-11]. This has led to various strategies to accelerate the leaching process [12-13]. However, dump leaching technology still experiences serious problems related to stability that is important from an economical and safety perspective. In the last two decades, some dumps of 400-500 m in height have been reported [14]. Long-term bioleaching of waste rocks can have a significant impact on the overall stability of waste dumps as the leach-

E-mail: [email protected]

© University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2012

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ing process is inevitably accompanied by significant mechanical and chemical changes [15-16]. Changes in rock microstructure can lead to porosity and permeability variations, and as a result, rock mechanical properties can also be affected [17]. Previous contributions have focused on chemical reactions and catalytic mechanisms of bacteria. More research is needed to fully understand the effects of chemical degradation and its effect on the mechanical behavior of waste rocks. The present research attempts to document the chemical and mechanical property changes by investigating the leached waste rocks from Dexing Copper Mine (DCM), the largest open pit copper mine in China. The purpose of this paper is: (1) to examine the mechanical behavior of leached waste rocks, (2) to quantify the morphological changes and element transfer caused by the leaching reaction and bacteria attack, (3) to determine the evolution of element content and porosity of waste rocks, and (4) to investigate the particle size evolution caused by dissolution, precipitation, and crushing.

2. Interactions between rock and solution within the bioleaching dump Fluid flow, physical, chemical and bacterial attack between solution and rock are the main phenomena occurring inside leaching dumps. Movements of solution through the waste rock pile have significant effects on the efficiency of bioleaching operations because solutes move with the water and tend to mix in the bulk of rocks by the mechanisms of dispersion. Maximizing the physical contact between rock and solution is crucial for enhancing the leaching efficiency. Unsaturated flow, saturated flow, and chemical attack are the main interactions between rock and solution within the dump. There are three forces acting on the solution under unsaturated conditions: gravity, surface tension, and atmospheric pressure. Surface tension is the molecular attraction that causes solutions to preferentially adhere to solid surfaces over air and affects the ability of a solution to infiltrate pore spaces. When water comes in contact with a solid surface, the polar attractive forces between water molecules and the solid surface will dictate the affinity of water for the solid. Majority of the solution flows through the small pores when the application rate is low. Increasing saturation makes the solution flow shift from small pores to large pores due to the decrease of the capillary force in the fine particle region. It means that flow paths can be changed by controlling the application rate.

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Unsaturated and saturated flow could coexist during the leaching of waste rock dumps due to the wide range of particle size and the complex configuration of the dump structure. The solution within the dump pore space was considered to be incompressible and the fluid viscous shear stresses were assumed to be negligible. Space averaged Navier-Stokes equations provide an adequate level of detail to address the saturated pore solution flow. Chemical attack can lead to a disintegration of metal sulfides, converting them from a crystalline state to soluble or amorphous products. The indirect mechanism requires an intermediate redox couple such as ferrous/ferric ions and is a combination of pure chemical and microbiological processes, while the direct mechanism assumes the action of bacteria oxidizing the mineral to sulfate and metal cations. Bacteria obtain energy through the oxidation of ferrous ions, elemental sulfur, and sulfur containing inorganic compounds according to the following reactions [18]. The oxidation of ferrous ions: 4FeSO 4 + 2H 2SO 4 + O 2 → 2Fe 2 (SO 4 )3 + 2H 2 O

(1)

The oxidation of sulfur: 2S0 + 3O 2 + 2H 2 O → 2H 2SO 4

(2)

The dissolution and oxidation reactions of the main copper-containing minerals in dumps: Cu 2 O + 1/ 2O 2 + 2H 2SO 4 ⎯⎯ → 2CuSO 4 + 2H 2 O

(3)

2Cu 2S + O 2 + 2H 2SO 4 → 2CuS + 2CuSO 4 + 2H 2 O

(4)

CuFeS2 + O 2 + 2H 2SO 4 → CuSO 4 + FeSO 4 + 2S0 + 2H 2 O

(5)

3. Experimental 3.1. Materials The waste rock samples were obtained from the dump of DCM. Two groups of samples were separated according to their size. The rocks with diameters larger than 80 mm were selected for the mechanical test after being leached for a period of time, while the small size rocks were used for the column bioleaching test. The mineralogical analysis showed that the main elements were Cu (0.49wt%), Fe (4.66wt%), S (2.32wt%), Mo (0.03wt%), As (0.015wt%), SiO2 (60.75wt%), Al2O3

S.H. Yin et al., Effects of bioleaching on the mechanical and chemical properties of waste rocks

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(13.80wt%), CaO (3.36wt%), and MgO (2.80wt%). The untreated samples were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) without grinding. The results showed that Si, Al, K, Fe, Cu, S, Mg, and Ca were the main elements, while silicate and aluminate comprised the uppermost matrix. 3.2. Bacterial cultivation The microorganisms used throughout the experiment, dominated by Thiobacillus ferrooxidans, Thiobacillus thiooxidans, and Leptospirillum ferrooxidans, were selected from the DCM solution pool. The microorganisms were maintained in 9 K medium containing (per liter of distilled water) 3.0 g (NH4)2SO4, 0.5 g MgSO4·7H2O, 0.10 g/L KCl, 0.5 g K2HPO4, 0.01 g Ca(NO3)2, and 44.22 g FeSO4·7H2O. The pH value was adjusted to 2.0 with H2SO4. 3.3. Experimental procedure Factors leading to the instability of rock piles include the loss of rock strength caused by the attack of the bacteria-containing solution. In order to evaluate the effect of the bacteria-containing acid solution on the mechanical properties of waste rocks, large samples were fabricated into cubes with dimensions of 50 mm×50 mm×50 mm, and were immersed in sulfuric acid at pH 2.0 for 10, 15, and 20 d, respectively. After exposure to the air, the samples were dried at 40°C. Laboratory tests were performed to observe the failure pattern, compression strength, shear strength, and cohesion of the leached rocks under dry conditions. Two groups of rock particles, coarse and fine, were loaded in two sides of the column, 20 cm in diameter and 100 cm in height. The distilled water and the cultivated microorganisms were mixed to form a 15-L solution and the pH was adjusted to 2.0 with H2SO4. The solution was irrigated with its rate adjusted by a flowmeter to approximately 30 L⋅m−2⋅h−1. The pH value increased at the end of each circulation, so H2SO4 was added to adjust it to 2.0 again before the next circulation began. The whole leaching process lasted for 63 d. 3.4. Analytical procedure Mechanical testing of the waste rocks was performed on a universal servohydraulic testing machine (Instron 1342, Fig. 1). The rock specimens were uniaxially loaded under compression by the machine. The loading rate under the load control was about 0.2 MPa/s. The copper and iron contents in leaching liquors were determined by atomic absorption spectroscopy (AAS). The

Fig. 1. 1342).

Universal servohydraulic testing machine (Instron

surface of the samples from both groups was examined using a scanning electron microscope coupled to an energy dispersive X-ray spectrometer, before and after leaching treatment. The pH value of the solution was monitored at room temperature with a pH meter and calibrated with a pH buffer. For all the experiments, chemical grade reagents and distilled water were used, with the exception of the chemical analysis in which double-distilled water was used.

4. Results and discussion 4.1. Mechanical behavior The pre-existing micro fractures became larger as the shear load started to increase. Then elastic deformation followed by fracture initiation occurred. Stable fracture propagation was observed afterwards. Ultimately, the rocks failed with fractures developed from the coalescence of several microfractures. The model of failure in a particular rock is always influenced by the intrinsic properties of the material and the mode of testing. The failure model also depends on the degree of erosion, if the rock is eroded before the test. The developed fracture patterns of the leached rocks are schematically shown in Fig. 2. The mode of failure has been found to be affected by leaching time. There appears an apparent distinction in failure patterns caused by shear loading among leached waste rocks with different erosion extents. For the waste rocks subjected to a higher extent of leaching, the shear failures took paths along the weaker planes and more zigzag failure cracks appeared on the rocks. As mentioned previously, waste rock samples obtained from the Dexing Copper Mine were randomly divided into four groups, among which one group was not subjected to leaching while others were leached for 15, 30, and 45 d, respectively. Then the tensile strength, shear strength, and cohesion were investigated for each group as Figs. 3-5 shown.

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Fig. 2. Failure patterns of the leached waste rocks under the shear strength test: (a) unleached; (b) leached for 15 d; (c) leached for 30 d; (d) leached for 45 d.

It can be seen that the mechanical strength generally decreases with the increase of leaching time due to the gradual degradation of the waste rock caused by the solution and Acidithiobacillus. During the initial stage of leaching, interreaction between the solution and waste rocks occurred primarily on the rock surface, while minimal dissolution reaction happened for the inner part of the rock. So the tensile and shear strength only decreased slightly as Figs. 3 and 4 shown. Along with leaching time, the intrusion of the solution into the inner part of the rock and its reaction with minerals resulted in a greater rate of strength reduction. At the last stage of leaching, it was difficult for the acid to ac-

Fig. 3. Plot of compression stress versus leaching time.

Fig. 4. Plot of shear strength versus leaching time.

cess the core of the rock and the leaching reaction became less extensive and more stable as indicated by the slight change in tensile and shear strengths. At the beginning of leaching, the solution penetrated into the cracks and pores and acted as a filler. This led to a slightly higher cohesion between the particles inside the rock, as Fig. 5 shows. However, the cohesion decreased gradually with increasing leaching time and rock degradation under the continuous attack of acid and Acidithiobacillus. 4.2. Surface morphology In order to study the surface morphology of the leached waste rocks, six groups of leached rocks were selected from the top, middle, and bottom part of both the fine and coarse regions. SEM analysis was performed on the selected samples. Through SEM images of the waste rock collected from the upper, middle, and bottom parts of the leaching column, significant differences in surface morphology among the rocks were observed (Fig. 6). For the unleached waste rock, surface roughness is relatively smaller compared to the leached rocks due to the fact that chemical and physical processes create voids (pore and microfracture) or deposit new minerals, especially precipitation and clay, on the leached rock surface. Moreover, the surface morphologies

Fig. 5. Plot of cohesion versus leaching time.

S.H. Yin et al., Effects of bioleaching on the mechanical and chemical properties of waste rocks

of rocks at different parts of the column evolve differently from each other [19]. Dissolution of particles and minerals and microfractures were observed among the waste rocks on the upper part which facilitated effective porosity. As a result, there were deep cracks and very rough surfaces devel-

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oped on the rocks located on the upper part of the leaching column. Also, a porous surface texture and a larger pore diameter resulted from the significant development of pores in the greatly leached upper rocks.

Fig. 6. SME images of unleached and leached fine ore surfaces: (a) unleached; (b) leached (upside of the column); (c) leached (middle of the column); (d) leached (bottom of the column).

By contrast with rocks in the top part of the leaching column, the surface of particles in the bottom part appeared to be barely eroded. Particles were covered by large amounts of secondary mineral deposits such as clay, gypsum, and jarosite precipitates. The 9K medium contained a high concentration of NH+4 ions. The jarosite precipitation is generated according to the reaction below:

and the creation of kinetic barriers due to the limited diffusion of reactants and products through the precipitation zone [20-21]. 4.3. Mineralogical observations

(6)

Changes in contents of the elements on the rock surface were quantitatively evaluated. The contents of different elements on rocks at various vertical positions are shown in Figs. 7 and 8. Seven elements show the strong evidence of variations caused by dissolution and precipitation.

The formation of Jarosite has negative effects on the bioleaching process, such as the diminishment of ferric iron,

The degree of erosion of the waste rock was reflected not only in the surface morphology but also in the surface min-

3Fe3+ + NH +4 + 2HSO −4 + 6H 2 O ⎯⎯ → NH 4 Fe3 ( SO 4 )2 ( OH )6 + 8H +

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in dump porosity due to the changes in pore size distribution, pore geometry, pore connectivity, pore infilling, and new pore formation [23]. In this regard, a CT scanner was used to determine the porosity of the packed column before and after the bioleaching process.

Fig. 7. Plots of elemental content versus column height.

As can be seen in Fig. 9, the porosity within the column before leaching was about 34% at the top and gradually decreased to about 28% at the bottom due to the densification caused by the weight of the packed bed. Analysis conducted after leaching on the column revealed that precipitation, clay, and fine rocks accumulated at the bottom of the column and the pore space on the bottom column were filled with small particles. The porosity of the leached packed waste rocks in the upper part was higher due to the effects of particle dissolution. The porosity volumes measured after the leaching process for the packed waste rocks increased proportionately with the height of the position of the rock matrix.

Fig. 8. Plots of elemental content versus column height.

eralogical properties. The major elements composing the original waste rocks were, in the order of increasing content, Si, Ca, Al, Fe, S, Mg, and Cu. As shown in Fig. 7, the contents of Al, Fe, Mg, and Cu on the rock surface generally decrease with the increase in column height. Intensive erosion occurred on the top rocks. The minerals containing Al, Fe, S, Mg, and Cu elements were removed and brought out of the column by the solution flow [22]. The contents of those elements in the bottom rocks remained relatively high due to the less chemical and bacterial attack. The precipitation of Al, Fe, and S on the rock surface also contributed to the higher contents of those elements in the rock near the bottom of the column. The decrease in contents of Si and Ca (Fig. 8) in the rocks at the bottom was due to the coating of the rocks with precipitation and clay, which reduced the exposure of Si and Ca to the surface. 4.4. Pore structure Porosity is one of the most important physical properties that govern the physical attributes of a waste rock dump such as strength, deformability, and hydraulic conductivity. It can also be used to predict the permeability of the waste rock dump. Bioleaching processes cause progressive changes

Fig. 9. Comparison of porosity along column height before and after leaching.

4.5. Particle size Waste rock can be an extremely heterogeneous material with grain size ranging from small soil particles to large boulders with diameters of a few meters [24]. The grain size distribution of the treated and untreated samples is presented in Figs. 10 and 11. The particle size distribution showed only a small difference for rocks before and after the leaching process. The grain size distribution curves of both fine and coarse samples shifted above those of the original samples after leaching. This indicates that the amount of fine particles increases while the average particle size decreases after the leaching process. Acid attack on waste rocks leads to the breakage of rocks. The minerals that precipitate turn into small particles and once dried out cause an increase in the amount of fine particles. The evolution of grain sizes can result in a change in the shear strength and permeability

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terial attack on rocks leads to the loss of rock integrity. Precipitation can turn into small particles once dried. Those are the three reasons that contribute to the higher amounts of small particles inside the leached waste dumps.

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[6] Fig. 11. Grain size distribution of treated and untreated coarse rocks.

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5. Conclusions (1) Failure patterns of the leached waste rocks under unconfined shear stress show that more cracks appear on the highly leached rocks. The mechanical strength decreases as the leaching time increases. Direct shear tests indicate that both the peak and residual shear stresses increase linearly with the increase of normal stress.

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(2) The erosion degree of the waste rock could be observed by its surface morphology and elemental content. Surface morphology analysis showed that serious erosion occurred on the top rock. The result of mineralogical analysis showed that the Al, Fe, Mg, and Cu contents of the rock surface increased with decreasing the rock depth, while Ca and Si exhibited opposite behaviors because of the coverage of precipitation and clay on the rocks in the lower part.

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