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Trans. Nonferrous Met. Soc. China 262016 835−841 Effect of ore size and heap porosity on capillary process inside leaching heap Sheng-hua YIN1, Lei-ming WANG1, Xun CHEN1, Ai-xiang WU2 1. Key Laboratory for High-Efficient Mining and Safety of Metal Mines, Ministry of Education, University of Science and Technology Beijing, Beijing 100083, China; 2. School of Civil and Environment Engineering, University of Science and Technology Beijing, Beijing 100083, China Received 27 April 2015; accepted 4 December 2015 Abstract The capillary process coexists with gravity flow within leaching heap due to the dual-porosity structure. Capillary rise is responsible for the mineral dissolution in fine particle zones and interior coarse rock. The effect of particle size and heap porosity on the capillary process was investigated through a series of column tests. Macropore of the ore heap was identified, and its capillary rise theory analysis was put forward. Two groups of ore particles, mono-size and non-uni, were selected for the capillary rise test. The result shows that particle size has an inverse effect on the capillary ultimate height, and smaller particles exhibit higher capillary rise. Meanwhile, the poorly graded group exhibits small rise height and velocity, while the capillary rise in the well-graded particles is much greater. The relationship between porosity and fitting parameters of capillary rise was obtained. Low porosity and high surface tension lead to higher capillary height of the fine gradation. Moisture content increases with the capillary rise level going up, the relationship between capillary height and moisture content was obtained. Key words heap leaching; capillary process; ore size; heap porosity 1 1 I Introductionntroduction Heap leaching involves stacking of metal-bearing ore into a heap on an impermeable pad, irrigating the ore for an extended period of time weeks, months, or years with a chemical solution to dissolve the valuable metals, and collecting the leachant as it percolates out from the base of the heap. The process has become a widely used of mining low-grade gold, silver, copper, uranium, and nickel laterite ores [1−4]. Recovery of the target metal can range from approximately 30 for some difficult to leach sulfide copper ores to over 90 for easier oxide gold ores [5]. The solution flow characteristic is a very important aspect of the leaching process, influencing both the overall recovery and apparent kinetics of the system [6], but these flow behaviors in ore heaps are poorly understood. The present understanding of solution flow in ore heaps is derived primarily from soil mechanics, hydrogeology and chemical engineering theory coupled with experimental ination. However, the investigation of solution flow in ore heap has made significant progress in recent years. To analyze, design, control and optimize the heap leaching process, some optimal flow rates were computed by using analytical models based on the first order kinetic equation [7,8]. The optimum magnetic resonance imaging was established to quantify the water distribution in both saturated and unsaturated ore packings [9]. Tracer tests using sodium bromide and sodium chloride were conducted to characterize the hydrodynamic behavior of heap leach systems [10]. The computational fluid dynamics simulation was used to estimate the flow and ferric iron concentration profiles around a single cell or pairs of cells of A. ferrooxidans, immobilized on the surface of a sulfide crystal [11]. A bed of non-overlapping spherical particles in cylindrical geometry were employed, and a computational model was developed to analyze the flow of fluid through the cylindrical bed of ore particles and the transport of liquid within the heap [12]. Error estimates were described for a finite element approximation to partial differential systems describing two-phase immiscible flows in Foundation item Project 51374035 supported by the National Natural Science Foundation of China; Project 201351 supported by the Foundation for the Author of National Excellent Doctoral Dissertation of China; Project NCET-13-0669 supported by Program for New Century Excellent Talents in University, China Corresponding author Sheng-hua YIN; Tel 86-10-62334680; E-mail csuysh DOI 10.1016/S1003-63261664174-2 Sheng-hua YIN, et al/Trans. Nonferrous Met. Soc. China 262016 835−841 836 porous media, with applications to heap leaching of copper ores [13]. A hydrodynamic study pered in a bench scale column, where the axial dispersion and the liquid holdup, were presented as a function of the liquid flow rate, is estimated using residence time distribution experiments [14]. The above research focuses on the solution flow driven by gravity, and the ore heap is frequently assumed to be rigidity and homogeneity. In practice, layers of coarse and fine textured ore inevitably develop within heap and dump leach piles as natural processes segregate coarse and fine material during material placement. During leaching process, pore sizes and flow paths may change over time due to chemical precipitation and dissolution of minerals, as well as weathering of rock surfaces and subsequent transport of fine material [15]. Under such conditions, gravity flows preferentially in the coarser and more conductive layer [16]. Portions of the heap, where solution does not enter laterally into the finer zones or inside the rock fractures, do not receive sufficient contact with the solution and remain unleached. Usually, leach piles are unsaturated systems, therefore solution can enter the micropores by capillary action. The capillary penetration process is of vital importance for the mineral extraction. But few attention has been focused on this process. This work attempts to reveal the capillary theory for heap leaching, and investigates factors such as porosity and ore size influencing the capillary process. 2 Capillary theory inside leaching heap For the purpose of solution flow discussion, it is important to distinguish between macropore and micropore in the stacked heap [17]. The er refers to the inter-rock large-size void within the coarse zone. And the latter often refers to the small size pore inside the fine zone and intra-rock fracture. A schematic diagram of stacked ore heap is shown in Fig. 1, and there are two zones of different flow regimes. 1 Gravity flow, which refers to the solution percolation primary driven by gravity. As the solution application starts and application rate increases, macropores tend to conduct solution rapidly. Very large vertical flow velocities can be attained with only a small increase in solution application, which result in short circuiting of water in pores. 2 Capillary flow, which refers to the diffusion of solution into small-size pores of fine zones and rock fractures. There are three forces acting on the solution in an ore heap undergoing percolation leaching gravity, surface tension and atmospheric pressure. Axial dispersion of water occurs in the lateral direction because the surface tension is greater than atmospheric pressure [18]. Besides the vertical gravity flow, lateral dispersion also occurs because of capillary action drawing water sideways. An unsaturated porous heap is often conceptualized as a bundle of capillary tubes. The solution/rock interface is of lower surface energy than the air/rock interface, causing capillary penetration of the micropores. Fine pores with higher suction dominate the capillary flow. The driving force is a vector sum of gravity and capillary forces or capillary pressure or suction [19]. As shown in Fig. 2, in the balance condition, the upward force caused by interfacial tension in capillaries equals the gravity gh r p⋅∆⋅ρ θσcos2 c 1 where r is the capillary radius, ∆ρ is the density difference of the media, g is the gravitational acceleration, σ is the interface tension, θ is the contact angle, and the h is the height of fluid column. This expression explains for the cause and calculation of capillary pressure. In fact, most of the parameters are not easy for experimental measurement in specific condition, let alone the standardization. Actually, capillary rise is a dynamic process, during which h is variable. It has been experimentally proved that for the glass-bead-filled columns, early stage data are well fitted by the Washburn equation t r h η θσ 2 cos 2 2 Fig. 1 Dual-porosity structure and solution flow inside heap Sheng-hua YIN, et al/Trans. Nonferrous Met. Soc. China 262016 835−841 837 Fig. 2 Capillary rise in porous media where t is the time for solution to rise, η is the viscosity of the wetting liquid. The ores own wetability, and their absorbability to water makes the situation more complicated. On the basis of Eq. 1, excluding permeability, connection among capillary rise height, matric suction, capillary pressure and moisture content can be derived for steady state hgSp r p r ppρφ θσγ ∆ ∑∑MM 12 CM 2 cos2 2 * 2 3 where pM is the matric suction, pC is the capillary pressure, γ12 is interfacial tension; r is radius of curvature, φ is the solution content, S is the cross section area of the packed bed. The capillary rise flow is an important phenomenon that would occur in every unsaturated ore heap. The flow rate at which solution flows through a saturated ore/sand bed can be described by Darcy’s law and is proportional to the hydraulic head gradient [20]. However, the flow rate of solution through unsaturated soil is equal to the hydraulic conductivity, which is not constant but is related to ore size, porosity and saturation degree. The capillary rise has seldom been measured or calculated in precedent works. This manuscript tries to reveal the relationship between capillary rise and ore size, porosity and saturation degree. 3 Experimental The capillary rise is used to investigate the capillary flow between micropores. The capillary process was measured in a series of columns loaded with ore particles with different sizes. The experimental s and procedure are described below. 3.1 Ore samples Low grade copper ore was obtained from Wushan Mine in Inner Mongolia, China. The chemical composition of the ore samples is mainly covellite, chalcocite, chalcopyrite, molybdenite, and gangue minerals are comprised of quartz, dickite, and sericite. The original ore was crushed, screened and grouped into seven mono-size gradations shown in Table 1. Five non-uni gradations were obtained by mixing several mono-size gradations shown in Table 2. These two groups of samples have been respectively prepared to find influencing factors of capillary rise. Table 1 Porosity and average diameter of mono-size gradations Gradation Particle diameter/mm Average diameter/mm Porosity/ A 10−15 12.5 61.2 B 7−10 8.5 58.9 C 5−7 6 58.1 D 3−5 4 57.1 E 1−2 1.5 54.7 F 0.5−1 0.75 53.1 G 0.1−0.5 0.3 52.5 Table 2 Porosity and average diameter of non-uni gradations Gradation Combination Average diameter/mm Porosity/ H BG 0.38 51.4 I CF 0.94 53.6 J DE 2.93 56.3 K BDG 0.43 51.7 L BDEG 0.47 51.9 3.2 Apparatus The capillary rise measurement apparatus is shown in Fig. 3. Ore sample was dried and loaded into a column with height of 500 mm and diameter of 100 mm, which is 6 times larger than the largest diameter ore particle to minimize wall effects. A perforated plastic sheet and support base were used at the bottom of the column to hold the ore particles. A barrel with an over flow port was used to keep the solution at a constant level. 3.3 Firstly, the column was loaded with the ore sample supported by perforated sheet. The column, screen, and support base were placed in the barrel. After the column as assembled, solution was added into the barrels. An outlet was set to maintain the solution level approximately 1 cm above the perforated sheet. For columns with different ore gradations, the time to achieve the final wet interfaces was different. The capillary rise height of each ore sample was measured at Sheng-hua YIN, et al/Trans. Nonferrous Met. Soc. China 262016 835−841 838 an interval time of one hour until the final wet interface was obtained. Due to relatively large porosity of ores, the direct observation was used in this study instead of some measurement instrument, and there are scales on the outlet surface of pipe. Fig. 3 Schematic diagram of capillary rise measurement apparatus 4 Results 4.1 Effect of mono-size particle diameter on capillary rise Particle size and surface area are very important parameters that are closely related to the capillary rise. The importance of this relationship led to an uation of the effect of diameter on capillary rise. Seven groups of particle with different diameters were used to uate the capillary rise capacity. The suction of solution into column containing mono-size particles was measured by a rule for a period of 600 h. The results from these tests are shown in Fig. 4. Fig. 4 Capillary rise height of mono-size particles The data in Fig. 4 show the expected result that the capillary rise velocity and height are closely related with the particle size. At the beginning stage, the solution rises rapidly. The wetting front level goes up slowly after about 100 h. Meanwhile, the smaller particles exhibit higher capillary rise. The capillary height of Gradation G with particle diameter smaller than 0.5 mm reaches almost as high as the column height. But the capillary rise of coarse particles like Gradation A is less than 150 mm. Thus, from an ore crushing and dumping perspective, it is expected that fine particles are required to facilitate the capillary process. This test result is consistent with the previous theoretical analysis. The pore radius between the fine particles is small. According to Eq. 1, the capillary pressure has an inverse relationship with pore radius. Therefore, the capillary rise within the fine particle is higher than that within the coarse particles. Taking logarithm of the height, the relationship between the capillary height and time can be expressed by yaxb 4 where y is lgh, h is the capillary rise height, x is the time. The capillary rise process could be expressed by the fitting equations as shown in Table 3. Table 3 Fitting equations for mono-size gradations Gradation a b Fitting equation A 1.384 0.075 y1.384x0.075 B 1.465 0.070 y1.465x0.07 C 1.527 0.066 y1.527x0.066 D 1.630 0.057 y1.63x0.057 E 1.795 0.051 y1.795x0.051 F 1.959 0.042 y1.959x0.042 G 2.005 0.051 y2.005x0.051 4.2 Effect of non-uni particle size on capillary rise Non-uni ores are usually used in the p
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