Trans. Nonferrous Met. Soc. China 23(2013) 2160-2165

Solubility of hemimorphite in ammonium sulfate solution at 25 °C

Qin-xiang LI, Hui-ping HU, De-wen ZENG, Qi-yuan CHEN

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

Received 7 September 2012; accepted 20 November 2012

Abstract: The solubility of natural hemimorphite in ammonium sulfate solution was measured by isothermal solution method at 25 °C and the dissolved residue of hemimorphite was investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) methods. The results show that zinc and silica in hemimorphite simultaneously dissolve in ammonium sulfate solution. The solubility of zinc in solution increases rapidly from 4.5381 mmol/kg in 0.5469 mol/kg ammonium sulfate solution to 11.5083 mmol/kg in 3.7038 mol/kg ammonium sulfate solution. The solubility of silica in solution increases slowly from 2.5509 mmol/kg in 0.5469 mol/kg ammonium sulfate solution to 7.2891 mmol/kg in 3.7038 mol/kg ammonium sulfate solution. The dissolved residue is the characteristic of hemimorphite Zn₄Si₂O₇(OH)₂·H₂O based on the results of the XRD, SEM and FTIR. Thus, no phase transition occurs in the dissolution process of hemimorphite in ammonium sulfate solution.

Key words: hemimorphite; ammonium sulfate; silicic acid; solubility

1 Introduction

As the primary source of zinc, oxidized zinc ores are the main source of zinc metal after nature sulfide ores are steadily getting depleted. Oxidized zinc ores are generally found in the form of silicates or carbonates such as smithsonite (ZnCO₃), hydrozincite (2ZnCO₃·3Zn(OH)₂), zincite (ZnO), willemite (Zn₂SiO₄) and hemimorphite (Zn₄Si₂O₇(OH)₂·H₂O) in different regions of the world. As zinc silicate, hemimorphite is becoming increasingly attractive as one of most spread minerals of the oxidation zone of zinc-bearing ores.

Extensive work has been carried out on the treatment of zinc silicate ores by hydrometallurgical and pyrometallurgical methods. Sulfuric acid has been used to be a leaching reagent in hydrometallurgy process [1,2]. However, the high quantity of silica in zinc silicate ores results in the introduction of silica gel with Zn into the solution. The formation of silica gel adds difficulties to the filtration.

Recently, ammonia has attracted immense attention as a leaching agent for it has a number of inherent advantages due to its low cost, low toxicity, good complexes ability and easy regeneration. The first account of zinc extraction by ammoniacal leaching was published as “the Schnabel Process” in 1880, which pre-dated the better-known applications of ammoniacal leaching to copper and nickel by 30 years or more [3]. Extensive works have studied on the application to a specific feedstock, mainly non-silica minerals such as zinc dust [4], high grade zinc ores containing smithsonite [5] and sphalerite [6].

Hemimorphite is a zinc silicate hydroxide hydrate mineral. Mineralogical studies show that hemimorphite is relatively refractory to leaching with sulfuric and with caustic soda [7-9]. To inhibit the leaching of silica, some processes were developed for extracting zinc from hemimorphite in sodium hydroxide solution [10,11]. However, these processes needed the extreme operating conditions such as high temperature, pressure and alkalinity or adding calcium oxide. Based on these works, we studied the dissolution kinetics of hemimorphite in ammonia/ammonium chloride systems [12,13]. The experimental data indicate that the ammonia/ammonium chloride system is appropriate for the leaching of complex zinc ores of both oxide and sulfide types. Unexpectedly, chloride ions can interact with most metal ions, and their use would introduce metal ion impurities into the solution. Furthermore, chloride ions can corrode equipment. To solve these problems, the system of ammonia/ammonium sulfate was chose to leach hemimorphite [14]. The results show that zinc rapidly dissolved in ammonia/ ammonium sulfate system and the leaching efficiency of zinc reached 95%. Despite its extensive history and works, however, there is limited investigation about the thermodynamics of hemimorphite. In the present study, the solubility of hemimorphite in ammonium sulfate system at 25 °C was measured, and the dissolved residue was investigated in detail. The results may be used to provide the foundation data for studying the thermodynamic properties for zinc silicate ores in hydrometallurgy process.

2 Experimental

2.1 Experimental materials

Natural hemimorphite in this study was provided by Lanping Town in Yunnan Province in China. The hemimorphite sample was ground with an agate mortar and a pestle into 39 μm and then 6 g of hemimorphite was put into 150 mL ammonium sulfate solution with different concentrations from 0.5469 to 3.7038 mol/kg. The flask was put into a temperature-controlled water bath set at (25±0.05) °C and stirred for three months. Analytically pure ammonium sulfate was supplied by Sinopharm Chemical Reagent Company Limited. The concentrations of zinc and silica in solution were measured respectively by zinc-complexometric titration and silica-molybdenum blue methods. After dissolution, the residue was filtered and dried in an oven at 60 °C.

2.2 X-ray powder diffraction (XRD)

X-ray powder diffraction (XRD) measurement of the samples was determined by a Rigaku D/max 2550VB⁺18 kW powder diffractometer with a Cu/K_α X-ray source at 40 kV and 300 mA. The XRD results were recorded with a scan rate of 0.075 (°)/s and sampling interval of 0.02° and compared with the PDF-4/Mineral powder diffraction file.

2.3 Fourier transform infrared spectroscopy (FTIR)

IR spectra analyses were obtained using an American Nicolet 6700 spectrometer with a smart endurance single-bounce diamond ATR cell. The samples were prepared using the KBr pellet method. The spectra over the 4000 cm^-1 to -400 cm^-1 range were obtained with a resolution of 4 cm^-1and mirror velocity of 0.6329 cm/s.

2.4 Scanning electron microscopy (SEM)

The surface morphology of crude and dissolved residues was examined by SEM using a Japan Jeol JSM-6360 instrument at 20 kV. The grains mounted on a stub were coated with Au/Pd for SEM observation.

3 Results and discussion

3.1 Characterization of natural hemimorphite

The mineralogical data obtained from chemical phase are presented in Table 1. The mineralogical analysis shows that the sample contained hemimorphite (99.76 %) as the major oxide mineral, with only small amounts of zinc carbonate (0.19 %), franklinite (0.04 %) and trace amount of zinc sulfide and zinc sulfate. From the XRD analysis, the major minerals present in the ore are hemimorphite (Zn₄Si₂O₇(OH)₂·H₂O), exhibiting sharp diffraction peaks; no other material diffraction peaks are observed, indicating a high level of purity for hemimorphite.

Table 1 Mineralogical composition of hemimorphite

The IR spectrum of hemimorphite is characterized in agreement with the spectrum in Ref. [15], very strong band around 3453.77 cm^-1 frequency side, together with an overtone of the bending H₂O mode around 1634 cm^-1, which are main characteristic of —OH stretching region of hemimorphite. The bands at 1086.53 and 676.70 cm^-1 could be assigned as ν_as(Si—O_b—Si) modes (O_b denotes bridging oxygen). The remaining stretching vibrations of the SiO₃ groups, manifested by the strong peaks at 933.35 and 864.75 cm^-1 are also positioned lower than ν_as(Si—O_b—Si) mode. The number of the bands of the lower-frequencies at 603.88, 557.20 and 448.63 cm^-1 should be attributed to the δ(Si—O_b—Si) modes.

The surface morphology of hemimorphite shows that the section of natural ore appears regular, blocky and smooth.

3.2 Solubility of hemimorphite in ammonium sulfate solution

Solubilities of hemimorphite in ammonium sulfate solution at (25±0.05) °C are listed in Table 2. Figure 1 and Table 2 show that the solubility of zinc in the hemimorphite - ammonium sulfate - water system increases with the increase of ammonium sulfate concentration, which shows monotonically increasing from m(Zn)=4.5381 mmol/kg in 0.5469 mol/kg ammonium sulfate solution to m(Zn)=11.5083 mmol/kg in 3.7038 mol/kg ammonium sulfate solution. It is interpreted that the ammonia by ionization extracted zinc from the hemimorphite to form stable metal ammine complexes, leading to higher solubility.

Table 2 Solubility of hemimorphite in ammonium sulfate solution at (25±0.05) °C

Fig. 1 Solubility of zinc in hemimorphite-ammonium sulfate- water system at (25±0.05) °C

Figure 2 displays the solubility of silicon in hemimorphite-ammonium sulfate-water system at (25±0.05) °C. It is easily shown that solubility of silicon in solution slowly increases with the ammonium sulfate concentration up to 3.7038 mol/kg. According to Ref. [16], the probable reason is that a protonation process of the bonds is involved between the divalent metals and oxygen, bonds between silicon and oxygen are relatively un-reactive and the silicate anion is probablely released intact from the mineral surface as silicic acid.

Fig. 2 Solubility of Si in hemimorphite-ammonium sulfate- water system at (25±0.05) °C

Fig. 3 Comparison of pH of ammonium sulfate solution and system of hemimorphite-ammonium sulfate-water at (25±0.05) °C

Figure 3 reveals the pH of ammonium sulfate solution (curve (b)) and the pH of hemimorphite- ammonium sulfate-water system (curve (a)). It can be seen that the pH of hemimorphite-ammonium sulfate- water system is higher than the pH of ammonium sulfate solution. It may be interpreted that protonation processes were also involved in silicic acid, which always were in the dynamic balance depending on the pH. The polymerization of silicic acid to form a gel process is explained by two different mechanisms [17]: in alkaline and slightly acid solution, it is the condensation of H₄SiO₄ and ; in the acid side, it is the polymerization of H₄SiO₄ and . In this study, the range of pH is 5-8, in which the forms of silica in solution are mainly H₄SiO₄ and . The polymerization mechanism in solution expressed by the following reaction [18]:

+H⁺H₄SiO₄SiO₂+H₂O

Combining Fig. 3 with the above reaction, the reason that pH value of hemimorphite-ammonium sulfate-water system is greater than the ammonium sulfate solution could be explained.

3.3 Characterizations of dissolved residues

The dissolved residue of hemimorphite in ammonium sulfate solutions was characterized via XRD, FTIR and SEM. The XRD, FTIR and SEM results of dissolved residues are shown in Figs. 4-6, respectively.

After hemimorphite dissolved in ammonium sulfate solution for 3 months, the intensity of the characteristic peak weakens and its peak height decreases as shown in Fig. 4. All the residues have the characteristic of Zn₄Si₂O₇(OH)₂·H₂O.

A number of infrared bands of dissolved residues of hemimorphite in ammonium sulfate solutions are shown in Fig. 5. The characteristic bands appear at 449, 559, 603, 677, 864, 933, 1087, 1635 and 3445 cm^-1, which obviously coincide with the infrared spectra of hemimorphite [15]. According to Fig. 5, it can be seen that the characteristic O—H structural band in water at 3453 cm^-1 appears red shift, which means the O—H group in water of hemimorphite tends to be unstable after hemimorphite dissolved in ammonium sulfate solutions. In contrast, the stretching vibrations of the —SiO₃ group manifested as strong peak at 931 cm^-1 appears blue shift, which means that the —SiO₃ group of hemimorphite becomes more stable after hemimorphite was dissolved in ammonium sulfate solutions.

Fig. 4 XTD patterns of hemimorphite samples

Figure 6 shows the morphologies of hemimorphite in several ammonium sulfate concentrations. It can be seen that the residue is corroded to be tiny and irregular particles after dissolving in ammonium sulfate.

Fig. 5 IR spectra of hemimorphite samples

Fig. 6 SEM images of hemimorphite samples

4 Conclusions

The solubility of hemimorphite in ammonium sulfate solution was measured by isothermal solution method at (25±0.05) °C. The result reveals that the solubility of zinc increases with the increase of the ammonium sulfate concentration. It can be interpreted that the zinc was extracted from hemimorphite by ammonia to form stable zinc ammine complexes. The solubility of silicon also increases with the increase of ammonium sulfate concentration and the silicon in solution is mainly in forms of  and H₄SiO₄. Data on the dissolution residue of hemimorphite in ammonium sulfate at (25±0.05) °C was obtained by XRD, FTIR and SEM analysis. The results indicate that the residue is characteristic of Zn₄Si₂O₇(OH)₂·H₂O.

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25 °C时异极矿在硫酸铵溶液中的溶解度

李琴香, 胡慧萍，曾德文，陈启元

中南大学 化学化工学院， 长沙 410083

摘  要：采用等温法测定了25 °C时异极矿在硫酸铵溶液中的溶解度，并利用X射线衍射、傅里叶转换红外光谱及扫描电子显微镜分析了溶解湿渣的性质。结果表明：异极矿中的锌、硅同时溶解在硫酸铵溶液中且随着硫酸铵浓度的增加，溶液中锌的浓度从4.5381 mmol/kg迅速增加到11.5083 mmol/kg，硅的浓度从2.5509 mmol/kg缓慢增加到7.2891 mmol/kg。结合X射线衍射、傅里叶转换红外光谱以及扫描电子显微镜的结果，证实了在溶解过程中无相变发生，湿渣的组成仍为异极矿。

关键词：异极矿；硫酸铵；硅酸；溶解度

(Edited by Hua YANG)

Foundation item: Project (2007CB613601) supported by the National Basic Research and Development Program of China; Project (511340071) supported by the National Natural Science Foundation of China

Corresponding author: Qi-yuan CHEN; Tel: +86-731-88877364; E-mail: cqy@csu.edu.cn

DOI: 10.1016/S1003-6326(13)62712-0