Leaching Kinetics of Hemimorphite in Ammonium Chloride Solution

Abstract

The leaching kinetics of hemimorphite (Zn₄Si₂O₇(OH)₂·H₂O) in ammonium chloride solution was presented in detail. Effects of stirring speed (150–350 rpm), leaching temperature (75–108 °C), particle size of hemimorphite (45–150 μm), and the concentration of ammonium chloride (3.5–5.5 mol/L) on the zinc extraction rate were studied. The zinc extraction rate enhanced slightly with the increase in stirring speed, but increased significantly with an increase in the leaching temperature and ammonium chloride concentration. Zinc extraction was enhanced significantly in the first 60 min with decreasing particle size, but had little effect on the leaching process after 60 min. Scanning electron microscopy (SEM) analysis showed that some silica gel formed in the leaching process was not separated from the hemimorphite surface, but covered some of the active particle surface. The Elovich equation successfully described the leaching kinetics of hemimorphite in ammonium chloride solution with an apparent activation energy of 405.14 kJ/mol at temperatures of 75–90 °C and 239.61 kJ/mol at temperatures of 95–108 °C, which is characteristic for a chemically-controlled process. Silica gel is generated at temperatures of 75–90 °C and decomposed into silica at temperatures of 95–108 °C.

1. Introduction

Zinc oxide ores are difficult to use, as these ores are difficult to separate by flotation. With the consumption of zinc sulfide resources and environmental protection requirements, studies are paying more and more attention to zinc oxide ores. Zinc oxide ores are generally found in the form of carbonates and silicates, such as smithsonite, hydrozincite, zincite, willemite, and hemimorphite, etc. [1,2]. High-silicon zinc ore (willemite and hemimorphite) resources are very difficult to use because silicon is hard to dispose of in the current process [3]. To treat high-silicon zinc ores, a large amount of research has been carried out in recent years in pyrometallurgical and hydrometallurgical methods. The hydrometallurgical process is more economically attractive, as the pyrometallurgical method suffers from high energy consumption and capital investment. Acids [4,5], sodium hydroxide [6,7], and ammoniacal solutions are often used as leaching agents [8,9].

Hemimorphite (one kind of high silicon zinc ore) is blended with smithsonite, sphalerite, and zincite in the oxidized zone. Hemimorphite has a high commercial value for zinc recovery from zinc oxidized ore, as zinc sulfide is consumed in large quantities.

The dissolution kinetics of high-silicon zinc ores have been researched in acid, alkali, and ammoniacal leaching systems in recent years. He et al. studied the dissolution kinetics of zinc silicate in acidic systems [10]. The research shows that the pressure leaching kinetics were controlled by diffusion, and the apparent activation energy was 16.39 kJ/mol. Santos et al. studied the dissolution kinetics of zinc silicate ores in sodium hydroxide solutions. The research showed that the leaching kinetics were controlled by chemical reaction, and the apparent activation energy was 67.8 ± 9 kJ/mol [11]. Yang et al. studied the dissolution kinetics of zinc silicate in NH₄Cl solution. They found that the leaching kinetics were controlled by porous diffusion, and the apparent activation energy was 161.26 kJ/mol [12].

In recent years, the extraction of zinc from zinc oxide ores with ammoniacal solution is a hotspot in the zinc recovery process. The extraction process in ammonia—with or without ammonium—has been researched a great deal [13,14,15,16]. However, the leaching kinetics of hemimorphite in ammonium chloride have never been studied. Ammonium chloride solution is a nearly neutral solution, which makes the calcium, magnesium, silicon, iron, and other minor elements (such as As, Sb, and Bi) contained in the minerals remain in the residues in the leaching process [17]. The working environment is good, with little ammonia released to the air. To develop this efficient process, it is necessary to have a detailed knowledge of the leaching kinetics of hemimorphite in ammonium chloride. The effects of stirring speed, leaching temperature, particle size, and the concentration of NH₄Cl on zinc extraction were investigated in detail, and the kinetic model was discussed. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies were also conducted.

2. Material and Methods

2.1. Materials

The hemimorphite, from Yunnan Province in China, was ground and wet-sieved to yield a particle size distribution between 45 and 150 μm. The mineralogical composition of hemimorphite was analyzed by powder X-ray diffraction (XRD) (Figure 1) and chemical methods. The mineralogical composition is presented in

Table 1. Mineralogical analysis of hemimorphite.

Phase	ZnSO4	Zn4Si2O7(OH)2·H2O	ZnS	ZnFe2O4	Zn (total)
Content (%) 1	0.08	41.90	0.20	0.06	42.24
Occupancy (%) 2	0.19	99.20	0.47	0.14	100

¹ $\mathrm{Content}:\frac{\mathrm{the}\mathrm{mass}\mathrm{of}\mathrm{zinc}\mathrm{in}\mathrm{each}\mathrm{phase}}{\mathrm{the}\mathrm{mass}\mathrm{of}\mathrm{ore}}\times 100\%;$ ² $\mathrm{Occupancy}:\frac{\mathrm{the}\mathrm{mass}\mathrm{of}\mathrm{zinc}\mathrm{in}\mathrm{each}\mathrm{phase}}{\mathrm{the}\mathrm{mass}\mathrm{of}\mathrm{total}\mathrm{zinc}}\times 100\%$.

. Analytical-grade ammonium chloride and deionized water were used in the experiments.

2.2. Leaching Methods

The leaching experiments were carried out in a three-necked flask (2000 mL total volume) which was equipped with a mercury thermometer and a condenser device. The leaching temperature changed within ±0.5 degrees. A magnetic stirrer (DF-101S, Gongyi, China) was used to confuse the ore particles and the solution. The particle size distribution in the experiments was 45–61 μm, except for the particle size experiments. The leaching temperatures in the experiments were 75, 80, 85, 90, 95, 100, 104, and 108 °C, and the stirring speeds were 150, 200, 250, 300, and 350 rpm. The concentrations of NH₄Cl were 3.5, 4.0, 4.5, 5.0, and 5.5 mol/L. Typically, 5 g of solid was added into 1000 mL NH₄Cl solution. Five milliliters of slurry was withdrawn by the sampling port at a preset time and filtered quickly. Atomic absorption spectrometry (TAS-990, Persee, Peking, China) was used to analyze the concentration of zinc in solution.

The extraction percentage of zinc, α, is defined by:

$$\alpha =\frac{\mathit{CV}}{\mathit{WM}}\times 100\%$$

(1)

where C, V, W, and M are the zinc concentration in filtration, the volume (we assumed it to be constant), weight, and zinc content of the added material, respectively.

The hemimorphite and leaching residues were analyzed by XRD (TTRIII, Rigaku, Tokyo, Japan), SEM (JSM-6360LV, Jeol, Tokyo, Japan), and back-scattered electron micrographs (BEM, JSM-6360LV, Jeol, Tokyo, Japan).

3. Results

3.1. Effect of Leaching Temperature

The effect of the leaching temperature from 75 to 108 °C on zinc extraction is presented in Figure 2. This shows that elevated temperature had a noticeable enhanced effect on the percentage of extracted zinc. The increasing rates of zinc extraction at temperatures of 95–108 °C were quicker than at temperatures of 75–90 °C over 30 min. The leaching process changed obviously when the temperature was higher than 90 °C. Figure 3 shows the XRD patterns of the leaching residues obtained at different leaching temperatures. As the leaching temperature increased, the hemimorphite constantly dissolved and Si precipitated as SiO₂.

3.2. Effect of Stirring Speed

Experiments were carried out using four stirring speeds of 150, 250, 300, and 350 rpm, respectively. The mass transfer can be enhanced by increasing the stirring speed. The results, as presented in Figure 4, show that the stirring speed had a small effect on zinc extraction. Therefore, mass transfer is not the main reason affecting the leaching process when the stirring speed is higher than 350 rpm. The stirring speed was kept at 350 rpm in other experiments, unless otherwise stated in this paper.

3.3. Effect of Ammonium Chloride Concentration

The effect of ammonium chloride concentration on zinc extraction is presented in Figure 5. This shows that the rate of zinc extraction increased significantly with the increase in the ammonium chloride concentration. The concentration of Zn²⁺ was very small, as Zn(NH₃)_i²⁺ is the main species in the solution. This is one reason why zinc extraction was significantly affected by the ammonium chloride concentration. About 92.95% of zinc was extracted in 5.0 mol/L of ammonium chloride solution after 150 min, but only 47.28% of zinc was extracted in 3.5 mol/L of ammonium chloride solution.

3.4. Effect of Particle Size

The effect of particle size on zinc extraction is presented in Figure 6. It shows that the decrease in particle size had a noticeably enhanced effect on the percentage of extracted zinc in 60 min. This is because decreasing the particle size improves the reaction area and surface activity. However, the particle size had a small effect on zinc extraction after 60 min.

3.5. Kinetic Discussion

The dissolution of hemimorphite in ammonium chloride solution may include vapor–fluid–solid three-phase reaction, as free NH₃ is an important reaction species. The main chemical reactions can expressed in Equations (2) and (3):

NH₄⁺ ⇋ NH₃ + H⁺

(2)

Zn₄Si₂O₇(OH)₂·H₂O + 4iNH₃ + 8H⁺ = 4Zn(NH₃)_i²⁺ + 2SiO₂·jH₂O + 2(2 − j)H₂O

(3)

The ammoniacal system shrinking core model is the most used mathematical model for ammonical leaching kinetics [18,19,20], but it cannot explain the leaching progress in this work. Figure 2, Figure 3, Figure 5, and Figure 6 show that the zinc extraction rate is more than 50% after a few minutes, so a logarithmic equation is needed to describe the kinetics.

The empirical Elovich equation is used in this work. It is commonly used in chemisorption, but it also can describe leaching progress [21]. The Elovich equation can be expressed as:

dα/dt = δexp(−βα)

(4)

where t is time; β and δ are constant; α is zinc extraction.

After integration and simplification, Equation (4) becomes:

α = (1/β)ln(βδ) + (1/β)lnt

(5)

Thus, it predicts a linear dependence of α with lnt. The experimental data were linearly fitted with Equation (5), and the results are shown in Figure 7, Figure 8 and Figure 9. The values of β and δ can be calculated by the slope and intercept of the lines in Figure 7, Figure 8 and Figure 9, which are used to predict α using Equation (5). Figure 10 presents the predicted α (line) against the experimental α (points) at different temperatures, where they fit very well with each other. Similar plots were also calculated for other variables (Figure 7, Figure 8 and Figure 9), and they also fit equally well, which strongly supports the application of the Elovich equation to describe the dissolution kinetics of hemimorphite in ammonium chloride solution.

The Arrhenius equation is used to calculate the apparent activation energy to explain the effect of temperature on the leaching process. The Arrhenius equation is as follows:

δ = Cθexp(−E_a/RT)

(6)

where E_a is the initial apparent activation energy; C and θ are constants relating to the surface area and the number of active sites per unit area; R is the gas constant; and T is the absolute temperature.

The value of (–E_a/R) is the slope of curve of lnδ against 1/T, as shown in Figure 11. After calculation, the value of E_a was 405.14 kJ/mol at a temperature of 75–90 °C and 239.61 kJ/mol at a temperature of 95–108 °C. Chemical control appears to represent the mechanism of hemimorphite leaching kinetics with ammonium chloride.

3.6. Characterization of the Leaching Residues

Figure 12a,b shows the SEM images of hemimorphite and the leaching residue. Figure 12b shows that the silica—one kind of product in the leaching process—was adsorbed on the certain surface of the hemimorphite particle, blocking the surface-active sites and reducing the reactivity. Figure 12c,d are the BEM of hemimorphite and the leaching residue (polished section), which show smooth and flat profiles. The reticulate structure did not appear, and no reaction product layer occurred on the particle surface.

Figure 13 shows the Fourier transform infrared (FT-IR) spectra of the hemimorphite and the leaching residues at different temperatures. Farmer and Wen et al. have shown that the IR spectra of quartz, tridymite, cristobalite, silica glass, and opal in the 1250 to 400 cm⁻¹ range are similar [22,23]. Aside from the four-coordinated typical characteristic bands of SiO₂ at 1094, 800, and 470 cm⁻¹, the Si–O stretching vibration peak from the Si–OH groups of the newly-precipitated silica also appeared at 950 cm⁻¹, as shown in Figure 13b,c. However, increasing the temperature made the Si–OH groups polymerize and form Si–O–Si, and the characteristic bands disappear at 950 cm⁻¹, as shown in Figure 13d,e.

Based on the above analysis, silica gel was generated at temperatures of 75–90 °C and decomposed into silica at temperatures of 95–108 °C. The silica gel is produced and adsorbed on the surface-active sites of the hemimorphite particles. This is the reason for the high apparent activation energy (405.14 kJ/mol) at temperatures of 75–90 °C. However, with the decomposition of silica gel at temperatures of 95–108 °C, the apparent activation energy reduced to 239.61 kJ/mol.

4. Conclusions

The main parameters for ammonical dissolution for hemimorphite are determined as stirring speed, temperature, particle size, and ammonium chloride concentration. The zinc extraction rate enhanced slightly with the increase in stirring speed, decrease in particle size, raising leaching temperature, and ammonium chloride concentration. The Elovich equation successfully describes the leaching kinetics of hemimorphite in ammonium chloride solution with an apparent activation energy of 405.14 kJ/mol at temperatures of 75–90 °C and 239.61 kJ/mol at temperatures of 95–108 °C, which is characteristic for a chemically-controlled process.