J. Cent. South Univ. (2014) 21: 884−890 DOI: 10.1007/s11771-014-2014-4

Dissolution mechanism and solubility of hemimorphite in NH3-(NH4)2SO4-H2O system at 298.15 K

LI Qin-xiang(李琴香), CHEN Qi-yuan(陈启元), HU Hui-ping(胡慧萍)

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

© Central South University Press and Springer-Verlag Berlin Heidelberg 2014

Abstract: The dissolution mechanism of hemimorphite in NH3-(NH4)2SO4-H2O system at 298.15 K was investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis. The results show that hemimorphite is soluble in NH3-(NH4)2SO4-H2O system and its residue exists in the form of an amorphous SiO2 layer on the hemimorphite surface. The XPS data also indicate that the Si 2p3/2 and O 1s spectra of the hemimorphite are broadened and shift to higher binding energies and their binding energies are closer to silica with an increase of total ammonia and time. Solubility of hemimorphite in NH3-(NH4)2SO4-H2O system was measured by means of isothermal solution method at 298.15 K based on the study of the dissolution mechanism of hemimorphite. The results show that the solubility of zinc in solution increases firstly and then decreases with the increase of cT(NH3) (total ammonia concentration) at different NH3/NH4

+ ratios. The solubility of silicon in solution decreases from 0.0334 mol/kg in cT(NH3)=4.1245 mol/kg NH3-(NH4)2SO4-H2O solution to 0.0046 mol/kg in cT(NH3)=7.6035 mol/kg NH3-(NH4)2SO4-H2O solution. Key words: hemimorphite; ammoniacal solution; dissolution mechanism; solubility

1 Introduction

Zinc is an important base metal in galvanizing and battery manufacturing industries. Zn is primarily produced from zinc sulfide ores because sulfides are easily separated from gangue and concentrated by conventional flotation techniques. However, with the depletion of nature zinc sulfide ores, it is difficult to find the new sphalerite mines. Hence, oxidized zinc ores are becoming the main source of zinc metal, which are generally found in silicate or carbonate forms around the world. As a secondary mineral typically found in the oxidized zone of zinc-bearing mineral deposits, hemimorphite is hardly separated from the zinc-oxidized ores in various carbonates and silicates. Therefore, extensive work has been carried out to treat the zinc silicate ores by pyrometallurgical and hydrometallurgical methods.

Nowadays, the pyrometallurgical process is hard to be accepted because of the heavy pollution, high consumption and high capital investment. Mineralogical studies show that hemimorphite is relatively refractory to leach by inorganic acid because of the high silica content [1−3]. The high silica content results in the introduction of silica gel with Zn(Ⅱ) into the solution in the

hydrometallurgical process. The formation of silica gel adds difficulties to the filtration. Considerable effort has been made to precipitate silica such as pH control and the addition of Al3+ or lime [4−6]. However, the increase of production cost and the severe operating conditions of the process can not be ignored. In order to avoid the appearance of the silica gel, CHEN et al [7] developed a process for extracting zinc from hemimorphite by leaching in sodium hydroxide solution, which requires high temperature, pressure and alkalinity. Based on that work, ZHAO et al [8] used a mechano-chemical method to avoid the extreme operating conditions and inhibit the leaching of silica by adding calcium oxide.

Currently, hydrometallurgy in ammonia together with chloride or sulfate has been considered as a prospective medium for the leaching of complex zinc ores of both oxide and sulfide types. In our previous work [9−10], the dissolution behavior and the kinetic of hemimorphite in the ammonia-ammonium chloride were studied at normal temperature and pressure. However, because the chloride ions can interact with most metal ions, the metal ion impurities would be introduced into the leaching solution, increasing the difficulty in purification process. LIU et al [11] studied the leaching and its mechanism of hemimorphite in NH3-(NH4)2SO4- H2O system at 333.15 K using the total ammonia

Foundation item: Projects(511340071) supported by the National Natural Science Foundation of China Received date: 2012−11−19; Accepted date: 2013−05−07 Corresponding author: CHEN Qi-yuan, Professor, PhD; Tel: +86−731−88877364; Email: cqy@csu.edu.cn

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concentration cT(NH3)=7 mol/L. Based on the studies of previous work, the dissolution mechanism of hemimorphite in NH3-(NH4)2SO4-H2O system at 298.15 K was investigated by XRD, SEM, FTIR and XPS analyses. The solubility of hemimorphite in NH3- (NH4)2SO4-H2O system was also measured at 298.15 K. This work will provide the theoretical foundation for studying the thermodynamic of hemimorphite in NH3- (NH4)2SO4-H2O system. 2 Experimental 2.1 Materials and apparatus

Natural hemimorphite was provided from Yunnan Province in China in this work. The mineralogical analysis result of hemimorphite was presented in Table 1. The hemimorphite sample was crushed and grounded to be a powder with size less than 0.075 mm by using an agate mortar and a pestle. The dissolution mechanism experiments were conducted with 2 g hemimorphite in a 100 mL NH3-(NH4)2SO4-H2O solution in a glass flask (total volume, 150 mL). The solubility experiments of hemimorphite were carried out batch-wise with 10 g solids in a 50 mL NH3-(NH4)2SO4-H2O solution in a glass flask (total volume, 150 mL), and then the glass flask was immersed in a thermostatic water bath agitated for 3 m by a magnetic stirrer. This thermostatic water bath was set at (298.15±0.05) K. The concentration of zinc and silica in solution was 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 Sample characterization 2.2.1 X-ray powder diffraction (XRD)

X-ray powder diffraction (XRD) measurements of the samples were determined by a Rigaku D/max 2550VB+18 kW powder diffractometer with a Cu/Kα

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. 2.2.2 Fourier transform infrared spectroscopy (FTIR)

Fourier transform infrared spectra (FTIR) 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. 2.2.3 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. 2.2.4 X-ray photoelectron spectroscopy (XPS)

The samples were examined by XPS before and after NH3-(NH4)2SO4-H2O system treated. Photoelectron spectra were recorded on the vacuum generator K-Alpha1063 using Al Kα micro gathered monochromator X-ray source with the 180 ° double focusing hemisphere analyzer-128 channels detector. The Si 2p3/2 spectrum was measured as a silicon photoelectron line in this work. Based on the XPS wide scan spectrum and narrow spectrum of Zn 2p3/2, O 2s and Si 2p3/2, the atomic composition relative to silicon was calculated from the area intensities of the photoelectron electron spectra on the basis of relative sensitivity of each line. 3 Results and discussion 3.1 Characterization of hemimorphite

The XRD pattern of the natural ore is shown in Fig. 1(a). From the XRD analysis, the major minerals in the ore are hemimorphite [Zn4Si2O7(OH)2·H2O], exhibiting sharp diffraction peaks; no other material diffraction peaks were observed, indicating a high level of purity for hemimorphite. The NH3-(NH4)2SO4-H2O solution (c(NH3)=2 mol/L, c((NH4)2SO4)=1.5 mol/L) was used to dissolve the hemimorphite. It took 1 month to dissolve completely. Although a wide and weak peak appears (Fig. 1, curves (b), (c), (d)) after dissolution for 4, 5 and 21 d, the relative intensity of hemimorphite was very low. After being treated for 1 month, the species with a faint peak (Fig. 1, curve (e)) was thought to be the amorphous silica (SiO2·nH2O), which might be the result of slow coagulation and crystallization as the solution got close to neutral and the time extended.

Figure 2 displays the infrared spectra of hemimorphite samples in NH3-(NH4)2SO4-H2O system. The infrared spectrum of natural hemimorphite (Fig. 2, curve (a)) is in agreement with the spectrum in Ref. [12]. Very strong band around 3453.77 cm−1

Table 1 Mineralogical composition of hemimorphite (mass fraction, %)

Phase Hemimorphite Franklinite Zinc carbonate Zinc sulfide Zinc sulfate TZn

Zinc

content 53.80 0.02 0.10 Trace amount Trace amount 53.93

Occupancy 99.76 0.04 0.19 — — 100

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Fig. 1 X–ray power diffraction patterns of hemimorphite

samples: (a) Natural ore; (b) In NH3-(NH4)2SO4-H2O system

(cT(NH3)=5 mol/L, c(NH3)=2 mol/L)) for 4 d; (c) In

NH3-(NH4)2SO4-H2O system (cT(NH3)=5 mol/L, c(NH3)=

2 mol/L) for 5 d; (d) In NH3-(NH4)2SO4-H2O system

(c(NH3)=5 mol/L, c(NH3)=2 mol/L) for 21 d; (e) In

NH3-(NH4)2SO4-H2O system (c(NH3)=5 mol/L, cT(NH3)=

2 mol/L) for 1 month; (f) In NH3-(NH4)2SO4-H2O system

(m(NH3)=4.1245 mol/kg, m(NH3)=2.9567 mol/kg) for 3 month;

(g) In NH3-(NH4)2SO4-H2O system (m(NH3)=4.8057 mol/kg,

m(NH3)=2.6115 mol/kg) for 3 month; (h) In NH3-(NH4)2SO4-

H2O system (mT(NH3)=5.6982 mol/kg, m(NH3)=1.8136 mol/kg)

for 3 month; (i) In NH3-(NH4)2SO4-H2O system (mT(NH3)=

6.3238 mol/kg, m(NH3)=0.9490 mol/kg) for 3 month; (j) In

NH3-(NH4)2SO4-H2O system (m(NH3)=7.6035 mol/kg,

m(NH3)=0.6401 mol/kg) for 3 month

Fig. 2 FT-IR spectra of hemimorphite samples: (a) Natural ore;

(b) In NH3-(NH4)2SO4-H2O system (cT(NH3)=5 mol/L,

c(NH3)=2 mol/L) for 1 month; (c) In NH3-(NH4)2SO4-H2O

system (m(NH3)=4.1245 mol/kg, m(NH3)=2.9567 mol/kg) for 3

month; (d) In NH3-(NH4)2SO4-H2O system (mT(NH3)=

4.8057 mol/kg, m(NH3)=2.6115 mol/kg) for 3 month; (e) In

NH3-(NH4)2SO4-H2O system (m(NH3)=5.6982 mol/kg,

m(NH3)=1.8136 mol/kg) for 3 month; (f) In NH3-(NH4)2SO4-

H2O system (m(NH3)=6.3238 mol/kg, m(NH3)=0.9490 mol/kg)

for 3 month; (g) In NH3-(NH4)2SO4-H2O system (mT(NH3)=

7.6035 mol/kg, m(NH3)=0.6401 mol/kg) for 3 month

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 band at 1086.53 cm−1 could be assigned as asymmetric stretching vibration (νas(Si—Ob—Si)). The medium intensity band is observed at 676.70 cm−1, which was assigned to an O—Si—O bending vibration. The remaining stretching vibrations of the SiO3 groups were manifested by the strong peak at 933.35 and 864.75 cm−1. The number of the bands of the lower-frequencies at 603.88, 557.20 and 448.63 cm−1 should be attributed to the Si—Ob—Si bendings.

A number of infrared bands of hemimorphite after dissolved in cT(NH3)=2 mol/L, c((NH4)2SO4)=1.5 mol/L solution at (298.15 ± 0.05) K for 1 month are observed in Fig. 2 (curve (b)) at 470.63, 793.94, 1078.04, 1633.41 and 3369.04 cm−1 which obviously coincided with the infrared spectrum of SiO2·nH2O [13]. But FARMER [14] believed that a medium intensity band at 950 cm−1 of silica with just sedimentation should be observed, besides the three typical quadridentate characteristic peaks which are attributed to νas(Si—Ob—Si) of the SiOH group. However, the band could be weak or absent because of Si—OH group polymerization to form the infrared inactive linear Si—O—Si band. It is more reliable to judge the existence of SiO2·nH2O using the band at 793.94 cm−1.

Figures 3(a) and (b) show the morphology of natural hemimorphite and hemimorphite after being dissolved in NH3-(NH4)2SO4-H2O system at (298.15 ± 0.05) K for 1 month. It can be seen that the section of natural ore appears regular, blocky and smooth (Fig. 3(a)). After dissolution, the residue turns to be honeycomb-like with tiny particles and spherical shape (Fig. 3(b)).

3.2 Dissolution mechanism of hemimorphite According to the local corrosion theory, the

wide-scan X-ray photoelectron spectrum was used to seek the dissolution mechanism of hemimorphite in NH3-(NH4)2SO4-H2O system by analyzing the relative abundances of surface composition and the binding energy of hemimorphite samples, which were dissolved in different concentrations of NH3-(NH4)2SO4-H2O systems for a certain time. Figures 4 (a)−(e) reveals the relative intensities of electron emission line of hemimorphite in NH3-(NH4)2SO4-H2O system. It is obvious that the intensity of zinc on the surface of hemimorphite decreases with the increase of cT(NH3) and time. In contrast, the intensity of silicon on the surface of hemimorphite increases with the increase of cT(NH3) and time.

The surface composition of hemimorphite is given in Table 2, which reveals the compositional differences between the natural ore and the natural ore dissolved in

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Fig. 3 Scanning electron micrographs of hemimorphite samples: (a) Natural ore; (b) In NH3-(NH4)2SO4-H2O system (cT(NH3)=5 mol/L, m(NH3)=2 mol/L) for 1 month; (c) In NH3- (NH4)2SO4-H2O system (m(NH3)=4.1245 mol/kg, m(NH3)=2.9567 mol/kg) for 3 month; (d) In NH3- (NH4)2SO4-H2O system (m(NH3)=4.8057 mol/kg, m(NH3)=2.6115 mol/kg) for 3 month; (e) In NH3- (NH4)2SO4-H2O system m(NH3)=5.6982 mol/kg, m(NH3)=1.8136 mol/kg) for 3 month; (f) In NH3- (NH4)2SO4-H2O system (m(NH3)=6.3238 mol/kg, m(NH3)=0.9490 mol/kg) for 3 month; (g) In NH3- (NH4)2SO4-H2O system (m(NH3)=7.6035 mol/kg, m(NH3)=0.6401 mol/ kg) for 3 month

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Fig. 4 Wide–scan X–ray photoelectron spectra of

hemimorphite: (a) Natural ore; (b) In NH3-(NH4)2SO4-H2O

system (cT(NH3)=1.5 mol/L) for 7 d; (c) In NH3-(NH4)2SO4-

H2O system (cT(NH3)=3 mol/L) for 7 d; (d) In NH3-(NH4)2SO4-

H2O system (cT(NH3)=5 mol/L) for 7 d; (e) In NH3-(NH4)2SO4-

H2O system (cT(NH3)=5 mol/L) for 1 month

NH3-(NH4)2SO4-H2O system. The abundances of the zinc and oxygen relative to Si 2p3/2 decrease with the increase of cT(NH3), which means that a layer rich in silicon is formed at the sample surface in dissolution process. And the Si/O ratio is close to 2, indicating that the dissolved SiO2·nH2O is coagulated and absorbed back onto the hemimorphite.

Table 3 lists the chemical shifts of electron energies of samples. The electron energies of Zn 2p3/2 almost remain unchanged during dissolution process. In contrast, the Si 2p3/2 and the O 1s spectra of the hemimorphite dissolved in ammoniacal solutions are broadened and shifted to higher binding energies. The full widths at half maximum (FWHM) of the Si 2p3/2 and O 1s lines of the

hemimorphite treated in ammoniacal solutions (FWHM of Si 2p3/2=2.14 eV and FWHM of O 1s=2.15 eV) are larger than those for the natural hemimorphite (FWHM of Si 2p3/2=1.8 eV and FWHM of O 1s=1.75 eV). In some cases [10, 15], the broad characteristics of the spectra can be ascribed to the existence of two phases. Similar broadening is also observed in the Si 2p3/2 of hemimorphite which is treated in ammonia-ammonium chloride solution, the un-dissolved hemimorphite and the dissolved surface layer. It is assumed that both the Si 2p3/2 and O 1s spectra of hemimorphite after ammoniacal dissolution could be de-convoluted into the contributions of un-dissolved and the dissolved phases. The components of the un-dissolved phase are subtracted from the measured spectra. The intensities of the un-dissolved components are adjusted to obtain residual spectra with reasonable line shapes. The results of de-convolution are shown in Fig. 5, which indicates that the Si 2p3/2 and O 1s binding energies of the dissolved phase are higher than that of the un-dissolved phase.

LIN and SHEN [16] pointed out that a protonation process of the bonds is involved between the divalent metals and oxygens. Bonds between silicon and oxygen are relatively un-reactive and the silicate anion is probably released from the mineral surface as silicic acid. According to Table 2, both of the relative abundances of zinc and oxygen are increased with the time extended at cT(NH3)=5 mol/L. These differences in surface composition could be resulted from the products adsorbed back onto the hemimorphite surface with the time extended during the dissolution process. All the evidences of XRD, SEM, FTIR show that the residues are characteristic of SiO2·nH2O. Based on the previous studies, the dissolution mechanism of hemimorphite in NH3-(NH4)2SO4-H2O system is expressed as follows:

Table 2 Surface composition of hemimorphite samples determined by XPSa

Element Natural

ore

cT(NH3)=1.5 mol/L, c((NH4)2SO4)=0.5 mol/L

cT(NH3)=3 mol/L, c((NH4)2SO4)=1 mol/L

cT(NH3)=5 mol/L, c((NH4)2SO4)=1.5mol/L

5 h 1 d 7 d 5 h 1 d 7 d 5 h 1 d 7 d

Si 2p3/2 1 1 1 1 1 1 1 1 1 1

Zn 2p3/2 2.04 0.49 0.54 0.49 0.28 0.13 0.19 0.08 0.11 0.17

O 1s 5.07 1.99 2.22 2.27 2.03 1.84 1.98 1.77 1.80 1.96 avalues are atomic composition relative to Si (atomic ratio)

Table 3 Photoelectron energies (eV) of hemimorphite samples

Element Natural

ore

cT(NH3)=1.5 mol/L, c(NH3)=0.5 mol/L

cT(NH3)=3 mol/L, c(NH3)=1 mol/L

cT(NH3)=5 mol/L, c(NH3)=1.5mol/L

5 h 1 d 7 d 5 h 1 d 7 d 5 h 1 d 7 d

Si 2p3/2 102.1 103.1 103.1 103.0 103.2 103.4 103.1 103.2 103.3 103.1

Zn 2p3/2 1022.2 1022.5 1022.5 1022.5 1022.8 1022.8 1022.8 1022.6 1022.7 1022.5

O 1s 531.5 532.6 532.5 532.5 532.6 532.9 532.5 532.6 532.7 532.6

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Fig. 5 O 1s (a) and Si 2p3/2 (b) spectra of hemimorphite after

ammonia–ammonium sulfate solution for 7 d

Ionization equilibrium of ammonia:

NH3+H2O⇌NH4++OH−

Ionization equilibrium of ammonium sulfate:

(NH4)2SO4⇌2 NH4++SO4

2− Hydrolytic equilibrium of NH4

+:

NH4++H2O⇌NH3+H3O

+ Dissolution reaction of hemimorphite:

Zn4Si2O7(OH)2.H2O(s)+8NH4++4(i−2)NH3⇌4Zn(NH3)i

2++

2Si(OH)4(aq)+ 2H2O (i=1, 2, 3, 4) The amorphous silicon dioxide or hydrated silicon

dioxide is precipitated from the solution as follows:

Si(OH)4(aq)=SiO2(amorphous)+2H2O

Si(OH)4(aq)=SiO2.nH2O(s)+(4−n)H2O In summary, the final reaction equation for

hemimorphite in NH3-(NH4)2SO4-H2O system is expressed as

Zn4Si2O7(OH)2·H2O(s)+8 NH4

++4(i−2)NH3⇌ 4Zn(NH3)i

2++Si(OH)4(aq)+SiO2·nH2O(s)+(4−n)H2O (i=1, 2, 3, 4; n=0.5, 1, 1.5, 2)

3.3 Solubility of hemimorphite

After dissolved in NH3-(NH4)2SO4-H2O system for 3 months, solubility of hemimorphite in NH3-(NH4)2SO4- H2O system was measured at (298.15±0.05) K. The results are listed in Table 4. It is obvious that the molality of Si(OH)4 decreases with the increase of cT(NH3), which shows decreasing from

4)OH(Sim =0.0334 mol/kg in m(NH3)=4.1245 mol/kg (n(NH3)/n(NH4)

+=2.5) NH3- (NH4)2SO4-H2O solution to

4)OH(Sim =0.0046 mol/kg (n(NH3)/n(NH4

+)=0.01) in m(NH3)=7.6035 mol/kg NH3-(NH4)2SO4-H2O solution. It is interpreted that the silicic acid released from the mineral surface mainly depends on the concentration of ammonia in dissolution process. However, the solubility of zinc in solution depends on both cT(NH3) and the ratio of n(NH3)/ n(NH4

+). It is explained that the ammonia by interaction between NH3 and NH4

+ extracted zinc from the hemimorphite to form stable metal ammine complexes, leading to higher solubility. The pH of hemimorphite in NH3-(NH4)2SO4-H2O system is lower than that of the solution of NH3-(NH4)2SO4-H2O except sample number f. The probable reason is that the OH− is gradually getting depleted as the Si(OH)4 being produced and polymerized to SiO2·nH2O. The XRD patterns of residues in NH3-(NH4)2SO4-H2O systems at different total ammonia

Table 4 Solubility of hemimorphite in NH3-(NH4)2SO4-H2O systems at (298.15±0.05) K

Sample number

Composition solution pH

Solid phase 424 so)NH(m /

(mol·kg−1) 3NHm /

(mol·kg−1)

2Znm /

(mol·kg−1)4Si(OH)m /

(mol·kg−1)A B

f 0.5839 2.9567 0.3718 0.0334 10.16 10.34 Zn4Si2O7(OH)2·H2O+ SiO2

g 1.0971 2.6115 0.5738 0.0231 9.87 9.73 Zn4Si2O7(OH)2·H2O+ SiO2

h 1.9423 1.8136 0.5532 0.0172 9.67 9.25 Zn4Si2O7(OH)2·H2O+ SiO2

i 2.6874 0.9490 0.5031 0.0051 9.47 8.64 Zn4Si2O7(OH)2·H2O+ SiO2

j 3.4817 0.6401 0.2898 0.0046 9.12 8.17 Zn4Si2O7(OH)2·H2O+SiO2

A represents the NH3-(NH4)2SO4-H2O system; B represents the NH3-(NH4)2SO4-H2O system after hemimorphite dissolved.

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concentrations are shown in Figs. 1 (f)−(j). The dissolved residues exist in two phases (hemimorphite and hydrated silicon dioxide). The peak of hydrated silicon dioxide is very weak, exhibiting low content. Figures 2 (c)−(g) show the FTIR results of the residue samples obtained by dissolving in NH3-(NH4)2SO4-H2O systems. The IR spectra of the dissolved residues display the characteristic peak of hemimorphite at 3435, 1634, 1086, 933, 864, 676, 603, 557 and 448 cm−1. The characteristic peak of OH in structural water (1634 cm−1) becomes two shoulder peaks (curve (b)). Figures 3 (c)−(g) show the morphology of the residues obtained from the hemimorphite-NH3-(NH4)2SO4-H2O system. It can be seen that the section of hemimorphite is corroded and appears a few tiny particles with spherical shape. 4 Conclusions

1) XRD, FTIR, SEM methods are used to analyze the dissolution behavior of hemimorphite in NH3-(NH4)2SO4-H2O system. The results indicate that the hemimorphite is soluble in NH3-(NH4)2SO4-H2O system. There is no evidence that phase transformation takes place except SiO2·nH2O during dissolution process.

2) The local corrosion theory was applied to study the dissolution mechanism of hemimorphite in NH3-(NH4)2SO4-H2O system at (298.15±0.05) K. XPS data show that the Si 2p3/2 and O 1s spectra of the dissolved hemimorphite are broadened and shifted to higher binding energies in NH3-(NH4)2SO4-H2O system dissolution process and that their binding energies are closer to SiO2 with an increase of total ammonia concentration and time.

3) The solubility of hemimorphite in NH3- (NH4)2SO4-H2O system is measured at (298.15±0.05) K based on the study of its dissolution mechanism. The results indicate that the solubility of silicon in NH3-(NH4)2SO4-H2O system decreases with the increase of total ammonia concentration, the solubility of zinc in NH3-(NH4)2SO4-H2O system depends on the total ammonia concentration and the ratio of n(NH3)/n(NH4

+). The pH of hemimorphite-NH3-(NH4)2SO4-H2O system is lower than that of NH3-(NH4)2SO4-H2O system. The dissolution residues are identified as hemimorphite and hydrated silicon dioxide.

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(Edited by HE Yun-bin)