J. Cent. South Univ. (2012) 19: 77-84

DOI: 10.1007/s11771-012-0975-8

Leaching kinetics of low-grade copper ore with high-alkality gangues in ammonia-ammonium sulphate solution

LIU Zhi-xiong(刘志雄)^{1, 2}, YIN Zhou-lan(尹周澜)¹, HU Hui-ping(胡慧萍)¹, CHEN Qi-yuan(陈启元)¹

1. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China;

2. College of Chemistry and Chemical Engineering, Jishou University, Jishou 416000, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: The leaching kinetics of low-grade copper ore with high-alkality gangues was studied in ammonia-ammonium sulphate solution. The main parameters, such as ammonia and ammonium sulphate concentrations, particle size, solid-to-liquid ratio and reaction temperature, were chosen in the experiments. The results show that the increase of temperature, concentrations of ammonia and ammonium sulphate is propitious to the leaching rate of copper ore. The leaching rate increases with the decrease of particle size and solid-to-liquid ratio. The leaching rate is controlled by the diffusion through the ash layer and the activation energy is determined to be 25.54 kJ/mol. A semi-empirical equation was proposed to describe the leaching kinetics.

Key words: leaching kinetics; ammonia-ammonium sulphate solution; low-grade copper ore; high-alkality gangues

1 Introduction

With the high speed of economic improvement, the demand of copper in the rapid rush is becoming more and more urgent, but the reserves of copper mineral resources are facing serious challenge nowadays. Therefore, recovery of copper from low-grade ores, waste rocks, tailings, etc, which were abandoned before has been focused due to the depletion of higher grade copper sulfides [1-4].

Hydrometallurgical methods are used to extract metallic values from low-grade ores [5-7]. Sulfuric acid is the most common lixiviant used for copper leaching [8-9]. However, acid leaching for the preponderance of carbonates in some ore deposits is not economical due to the excessive acid consumption. Some impurities dissolved in the acid solution bring about difficulties in the further treatment of the leaching solution. Therefore, more selective reagents are necessary for such deposits. Ammonia-ammonium sulphate leaching for copper ores has some advantages over other leaching reagents. One of the important characteristics of leaching in ammonia- ammonium sulphate solution is that the carbonates are not dissolved, which greatly leads to its ease of handling and reduces eventually the cost. Ammonia-ammonium sulphate provides ammonium ions that can form stable complexes with copper ions like Cu²⁺ and Cu⁺, and as a result, the leaching of copper ores can be enhanced. At the same time, Fe³⁺ ions are precipitated during the leaching process as the pH value in the solution is adequately high.

The leaching kinetics of oxidized copper ores containing malachite and chrysocolla in ammonium sulfate solution was investigated by BRYDEN [10]. The results indicated that the leaching rate was controlled by the surface reaction and that the calculated activation energy was 48.5 kJ/mol. The leaching kinetics of malachite in ammonium carbonate solution was performed by OUDENNE and OLSON [11]. It was discovered that the whole leaching process was divided into two stages. The equations of leaching kinetics were defined as 1-(1-x)^1/3=k₁t for stage I and 1-(1-x)^1/2=k₂t for stage ΙI (where x is the reaction fraction of the solid, k₁ and k₂ are rate constants, and t is the leaching time). The activation energies were 64 kJ/mol for stage I and    75 kJ/mol for stage II. And the reactions at two stages were both controlled by heterogeneous reaction. A study by EKMEKYAPAR and OYA [12] demonstrated that the dissolution of malachite in ammonium chloride solution was controlled by mixed kinetics. The mathematical model which was applied to describing the kinetics was 1-2(1-x)^1/3+(1-x)^2/3=kt. And the activation energy was calculated to be 71 kJ/mol.

The leaching kinetics of malachite in ammonia solution was examined by K?NK?L et al [13]. It was found that the activation energy was 22.338 kJ/mol and the leaching progress was controlled by diffusion through the ash film. BING?L et al [14] investigated the dissolution kinetics of malachite in NH₃-(NH₄)₂CO₃ solution. They declared that the leaching rate was controlled by interface transfer and diffusion across the product and the activation energy was 15 kJ/mol.

Tangdan oxidized copper ore, which contains high-grade calcium magnesium carbonate gangues and has poor floatable characteristics, is the largest copper mine in China with a reserve of about 115×10⁴ t. Its average content of copper is 0.75% (mass fraction) and the phase composition of copper is quite complex. The leaching of the ore in the ammoniacal systems is often carried out as the only doable and economical choice. Ammonia-ammonium sulphate solution was used as the lixiviant in the present work and the leaching kinetics was investigated in detail.

2 Experimental

2.1 Materials

The oxidized copper ore was obtained from Tangdan, Yunnan Province, China. The chemical composition and the X-ray diffraction pattern of the sample are shown in Table 1 and Fig. 1, respectively. The copper ore is composed of a majority of oxides and a small amount of sulfides, and its phase composition is given in Table 2. The content of copper in the ore is too low to be detected by the X-ray diffraction method. And the phases of copper were identified by the polarizing microscope method to be mainly malachite, chrysocolla and bornite, corresponding to free copper, copper silicate and secondary copper sulfide, respectively. Gangue contains high-grade calcium magnesium carbonate and silica, such as muscovite, dolomite, calcite and quartz.

Table 1 Chemical analysis of oxidized copper ore (mass fraction, %)

Fig. 1 X-ray diffraction pattern of copper ore

Table 2 Phase composition of copper ore

2.2 Methods

A 500 mL split flask was used as a leaching reactor with four necked tops for taking samples from the leaching solution, mechanical stirrer, mercury thermometer and cooler to avoid evaporation loss of solution and ammonia. The desired temperature of the flask contents within ±0.5 °C was adjusted by a thermostatically controlled electric heating mantle. All experiments were controlled at temperatures between 288 K and 328 K. Agitation was provided by a mechanical stirrer. Solution of 200 mL containing specific concentration of NH₄OH/(NH₄)₂SO₄ was added into the leaching reactor. After the desired stirring speed and reaction temperature were obtained, the solid sample was added to the solution in reactor. At specific time intervals, 5 mL solution was withdrawn from the reactor and filtered for analyzing the concentration of copper in the solution by an atomic absorbance spectrophotometer (AAS) and 5 mL fresh lixiviant was added into the reactor immediately to keep the volume of the solution constant. Filtration was made after each leaching experiment. The leached residue was weighed after being dried for 8 h at 110 °C.

According to the preliminary experimental results, the agitation rate in excess of 400 r/min was sufficient to eliminate the effect of agitation on the leaching rate. All experiments were performed at the stirring speed of  450 r/min. The influences of ammonia and ammonium sulphate concentration, particle size, solid-to-liquid ratio, and reaction temperature on the leaching rate were investigated.

3 Results and discussion

3.1 Dissolution reaction

The ammonia leaching reactions for malachite and chrysocolla, which are the main phases of copper in the ore, take place according to Eq. (1) and Eq. (2), respectively:

Cu₂(OH)₂CO₃+6NH₄OH+2(NH₄)₂SO₄→

8H₂O   (1)

CuSiO₃×H₂O+2NH₄OH+(NH₄)₂SO₄→

SiO₂↓+4H₂O                    (2)

The solubility of bronite in ammonia-ammonium sulphate is negligible when oxidant is not added.

3.2 Effect of operation parameters on leaching rate of copper

The effect of lixiviant concentration, particle size, solid-to-liquid ratio, reaction temperature on the leaching rate of copper was studied. The fixed conditions were selected as follows: ammonia concentration 0.5 mol/L, ammonium sulphate concentration 1.0 mol/L, particle size 0.075-0.109 mm, solid-to-liquid ratio 4/100 g/mL, and reaction temperature 298 K.

3.2.1 Effect of ammonia and ammonium concentrations

The leaching experiments for the oxidized copper ore were performed to investigate the effect of various ammonia and ammonium concentrations. The results of copper recoveries by NH₄OH/(NH₄)₂SO₄ solution    are given in Fig. 2. The leaching rate increases with the increase of ammonia concentration (Fig. 2(a)). With the increase of ammonia concentration, more reaction molecules can attack the solid. Without ammonia, the extremely low leaching rate could be attributed to the extremely low concentration of free ammonia which could be formed from the hydrolysate of ammonium. The leaching rate increases as the concentration of ammonium sulphate increases (Fig. 2(b)). This may be attributed to two reasons. One is that ammonium sulphate forms a buffer solution with ammonia and pH of the solution is kept at a certain value; the other is that ammonium ion is hydrolyzed to form ammonia, which will subsequently increase the concentration of ammonia reactant.

Fig. 2 Effect of ammoniacal concentration in ammoniac solution on copper recovery: (a) Ammonia; (b) Ammonium sulphate

3.2.2 Effect of particle size

The effect of particle size on the leaching rate of the oxidized copper ore was investigated by using 0.50-0.70, 0.25-0.38, 0.12-0.15, 0.075-0.109, 0.048-0.058 mm fractions at 25 °C. As can be seen in Fig. 3, the leaching rate increases as the particle size decreases. When the particle size decreases, a corresponding increase in the surface area causes better exposure of copper ores to the solution.

Fig. 3 Effect of particle size on copper recovery

3.2.3 Effect of solid-to-liquid ratio

The effect of solid-to-liquid ratio was studied from 1/100 to 8/100 g/mL. All the experiments were performed with the fraction of 0.075-0.109 mm at 25 °C. The results are given in Fig. 4. It can be seen that the leaching rate decreases as the solid-to-liquid ratio increases. This can be explained by the fact that the amount of fluid reactant per unit surface of the solid decreases with the increase of solid-to-liquid ratio.

3.2.4 Effect of reaction temperature

To investigate the effect of temperature, samples with the fraction of 0.075-0.109 mm were used. All the experiments were performed at 288-328 K. As shown in Fig. 5, the higher the temperature is, the higher the leaching rate is.

Fig. 4 Effect of solid-to-liquid ratio on copper recovery

3.3 Phase and morphology transformation

The SEM images of the particles of oxidized copper ore and the SEM-EDS of the leached residue are shown in Fig. 6 and Fig. 7, respectively. The copper minerals which exist on the surface and edge of particles can be easily observed by the image before leaching, while scarcely observed by the image of the leached residue. The copper minerals existing in the inner of the ore particles and the gangue which contains high content of silicon are difficult to be leached out. This means that the leached residue is likely to contain copper silicates. The phase compositions of particles with different diameters before and after leaching are given in Table 3 and Table 4, respectively. As can be seen in Table 3 and Table 4, free copper oxide is easily extracted, and almost all the free copper oxides are leached out when the particle size is in the range from 0.048 mm to 0.058 mm. Copper silicates are difficult to be extracted. The SEM-EDS analysis result (Fig. 7) also indicates that the copper phase of the leached residue is copper-bearing silicate such as chrysocolla.

Fig. 5 Effect of reaction temperature on copper recovery

Because copper atoms in the phase of chrysocolla are always embedded in the crystal lattice of the gangue, it is difficult for the lixiviant to infiltrate through the lattice and consequently the leaching rate will decrease. The copper sulfides can be extracted partly, especially those with the smaller particle size. This phenomenon could be explained by the fact that the copper sulfides could be oxidized by the oxygen dissolved in the solution [15].

Fig. 6 SEM images of particles with diameter of 0.075-0.109 mm (White parts: copper containing minerals): (a) Copper minerals emerging on surface of particle; (b) Copper minerals emerging on edge of particle; (c) Copper minerals embedded inside particle;  (d) Copper minerals emerging inside and on surface of particle

Fig. 7 SEM (a) and EDS (b) of leached residue (White parts: copper-containing minerals)

Table 3 Phase composition of particles with different diameters before leaching

Table 4 Phase composition of particles with different diameters after leaching

3.4 Kinetics analysis

Fluid-solid reactions are numerous and of great industrial importance. It has been assumed that the solid particles in the leaching process of ores with lixiviant solution do not change their size during the heterogeneous reaction. In addition, they contain reactive material uniformly embedded in an inert matrix. In this kind of reaction, there are some physical steps such as diffusion through the fluid film and/or inert solid layer of the reactant or product, and chemical reaction, which can affect the reaction rate.

The most important models for fluid-solid reaction are the shrinking core model and the progressive conversion model. According to the shrinking core model, it is thought that the reaction takes place on the outer surface of the solid and this surface shrinks toward the center of the solid as the reaction proceeds, leaving behind an inert solid layer called “ash layer”, around the unreacted shrinking core [16].

Considering a solid particle B immersed in a fluid A, and the reaction is

A(fluid)+bB(solid)?Product

If the reaction rate is controlled by diffusion of the fluid A through ash layer, the equation of reaction rate for the spherical solid can be written as

                               (3)

where x is the fractional conversion of copper; t is the leaching time; D is the diffusivity of ions through the ash layer, m²/s; c₀ is the concentration of fluid outside the particle, mol/L; c_B is the apparent concentration of the solid reactant, mol/L; r₀ is the initial outside radius of the particles, m.

If the reaction rate is controlled by chemical reaction, the integrated rate equation is expressed by

                                              (4)

where k_d is the rate constant of the chemical reaction.

Dissolution kinetics of Tangdan copper oxide ores in ammonia-ammonium sulphate solution can be described by the shrinking core model. The leaching process could be controlled by the diffusion through the ash layer around the shrinking unreacted core, as given in Fig. 8. Therefore, the kinetics modes were tested, and it was found that the following equation could suitably well represent the dissolution kinetics:

                                         (5)

where k is the apparent rate constant.

In order to determine the effect of experimental variables, such as temperature, particle size, solid-to-liquid ratio (S/L), ammonia concentration and ammonium sulphate concentration on the dissolution kinetics, the following semi-empirical model is established:

=

    (6)

where T is the thermodynamic temperature, K; C(NH₃·H₂O) is the ammonia concentration; C((NH₄)₂SO₄) is the ammonium sulphate concentration; k₀ is the apparent reaction rate coefficient, min^-1; S/L is the solid- to-liquid ratio, g/mL; d_p is the particle diameter, mm.

Fig. 8 Plots of 1-3(1-x)^2/3+2(1-x) vs t at different temperatures

For different ammonia concentrations, when the other parameters are kept constant, Eq. (6) could be rewritten as

               (7)

           (8)

The value of d[1-3(1-x)^2/3+2(1-x)]/dt is the slope of the straight line corresponding to different ammonia concentrations in Fig. 9(a). The value of ln{d[1-3(1- x)^2/3+2(1-x)]/dt} versus lnC(NH₃·H₂O) is plotted to get a straight line in Fig. 10(a), and the slope is calculated to be 0.386 3. Similarly, the values for Figs. 10(b), (c) and (d) could be estimated to be 0.449 4, -0.325 2, -0.885 1, respectively.

Fig. 9 Plots of 1-3(1-x)^2/3+2(1-x) versus t at different operation parameters: (a) Ammonia concentration; (b) Ammonium sulphate concentration; (c) Solid-to-liquid ratio; (d) Particle size

Fig. 10 Plots of ln{d[1-3(1-x)^2/3+2(1-x)]/dt} versus lnC(NH₃·H₂O) (a), lnC((NH₄)₂SO₄) (b), ln(S/L) (c) and ln(d_p) (d)

When the other parameters are fixed with temperature as the variable factor, the semi-empirical equation may be written as

                   (9)

                   (10)

In order to determine the activation energy of the leaching reaction, ln{d[1-3(1-x)^2/3+2(1-x)]/dt} versus 1/T was plotted (Fig. 11). The slope of the line gives the E/R value. From Fig. 11, the activation energy is calculated to be 25.54 kJ/mol. The activation energy controlled by diffusion is about 20 kJ/mol [17]. The low activation energy in this work indicates that the leaching process is controlled by the diffusion through the ash layer around the shrinking unreacted core.

Substituting the value of Figs. 10(a), (b), (c), (d) and E into Eq. (6), the value of k₀ is calculated to be about 4.95×10^-3min^-1 when the equation fits different straight lines in Fig. 9, and consequently the leaching kinetics of low-grade copper ore can be expressed by the following equation:

=

          (11)

Fig. 11 Plots of ln{d[1-3(1-x)^2/3+2(1-x)]/dt} versus 1/T

4 Conclusions

1) The dissolution rate of copper increases with the increase of the concentrations of ammonia and ammonium sulphate and reaction temperature, and with the decrease of solid-to-liquid ratio and particle size.

2) A shrinking unreacted-core model is applied to describing the dissolution rate. The leaching process is controlled by the diffusion through the ash layer and the activation energy is calculated to be 25.54 kJ/mol.

3) The leaching kinetics can be expressed by the following equation:

=

4) Free copper oxide can be leached out easily and copper silicates are difficult to be extracted. The sulfides ore of copper can be dissolved partly.

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(Edited by YANG Bing)

Foundation item: Project(2007CB613601) supported by the National Basic Research Program of China; Project(10C1095) supported by the Foundation of Hunan Educational Committee, China

Received date: 2011-03-04; Accepted date: 2011-05-16

Corresponding author: YIN Zhou-lan, Professor, PhD; Tel: +86-731-88877364; E-mail: yzllxh@gmail.com