Research on the Behavior of Germanium in the Leaching Process of Germanium-Bearing Zinc Oxide by Sulfuric Acid

Abstract

Germanium is widely used in many fields, and the production of germanium is very important. The effects of initial sulfuric acid concentration, liquid to solid ratio and other parameters on the leaching rate of germanium and zinc from germanium -bearing zinc oxide has been studied. The occurrence and form of germanium and other elements in the leach feed and the leach residue were studied by EPMA, XRD and chemical analyses. The experiments showed that the leaching rate of germanium and zinc was 91% and 88%, respectively under optimal conditions. Wurtzite was the main limiting factor for the leaching rate of zinc while the leaching rate of germanium was limited by insoluble germanium compounds. Furthermore, it was found that some germanium was bound up in galena in the leach residue. These results may be helpful for further improving the leaching rate of germanium in germanium -bearing zinc oxide .

Introduction

Germanium is a rare metal that is widely applied in various fields such as fiber optics communication, infrared optics, catalysts, photovoltaic cells and so on. Many countries have listed germanium as an important strategic resource [1]. According to data from the US Geological Survey, China is the largest refined germanium producer in the world [2].

There are very few primary deposits of germanium . Therefore, germanium is usually extracted from other germanium -bearing metal ores, germanium -bearing coal or other germanium secondary resources [3, 4]. There is abundant germanium in many of the lead -zinc deposits in China [5]. In the treatment of germanium -bearing lead and zinc concentrate, zinc leach residue and lead smelting slag are processed by reduction and volatilization processes with the production of zinc oxide [6]. It is worth noting that germanium is enriched in this process.

At the current time, the methods for recovering germanium from aqueous solutions are tannin precipitation , zinc powder cementation, solvent extraction , germanium -iron coprecipitation and so on [7–10]. In order to dissolve zinc and germanium , sulfuric acid leaching is usually the first step when processing germanium -bearing zinc oxide . Thereafter, germanium and zinc are separated by various methods. Zhuzhou Smelter, Huize Plant of Yunnan Chihong Zinc and Germanium Company Limited, Guiyang Smelter, and Shaoguan Smelter have used the sulfuric acid leaching process for years. Although the sulfuric acid leaching process has the advantages of simple operation and high compatibility with the zinc smelting process, the low leaching rate of germanium (60–80%) is unsatisfactory [11, 12].

In order to improve the comprehensive utilization of resources, researchers have carried out various studies. It was found that the germanium -bearing zinc oxide form Hezhang Zhazi Plant contains tetragonal GeO₂ accounting for 9.88% of the total germanium . This form of germanium could not be leached by sulfuric acid and the overall leaching rate of germanium was 87.62% [13]. In another study, it was shown that part of the germanium existed as Fe₄Ge₃O₁₂ in the germanium -bearing zinc oxide of a smelter in Yunnan Province. This form of germanium was insoluble and limited the overall leaching rate of germanium . Although the leaching rate of germanium can be increased to 85–90% by means of microwave roasting or other methods [13–15], it was found that there was still some germanium (about 15%) that was difficult to leach . Because the content of this form of germanium is very low, it is very difficult to find out the occurrence and its distribution . Therefore, how to further improve the germanium leaching rate and industrialization remains a question to be studied and answered urgently.

This study began with the process mineralogical analysis of germanium -bearing zinc oxide . The occurrence and distribution of germanium was studied by means of electron probe microanalyzer (EPMA) combined with chemical analysis and other methods. Then the effects of temperature and other conditions on the leaching rate of germanium and the behavior of germanium and the other main elements during the leaching process were investigated. Thereafter, the occurrence of germanium and main elements in leaching residue were analyzed. Finally, the key limiting factors for the low leaching rate of germanium were determined. Based on the study, reliable improvement measures have been proposed that that will theoretically improve the comprehensive recovery of germanium -bearing zinc oxide resources.

Experimental

Reagents and Materials

Germanium -bearing secondary zinc oxide supplied by a lead and zinc smelting plant in China was used as the raw material in this study. Before analysis, the raw material had been dried for 24 h at 65 °C in atmospheric conditions. All chemical reagents were analytical grade and supplied by Sinopharm Chemical Reagent Co ., Ltd. The water used in this study was deionized water.

Experimental Procedure

Leaching tests were carried out in a 250-mL flask in a thermostatically-controlled water bath kept at a constant temperature during the leaching process. A certain volume of sulfuric acid solution at a defined concentration was added into the flask and heated to the predetermined temperature . Then, a certain quantity of secondary zinc oxide , determined by liquid to solid ratio, was added to the reactor while the contents of the flask were being stirred at a certain speed and the timer started. After the reaction time was reached, the leached slurry was immediately separated by vacuum filtration , and the residue was washed three times with deionized water (30 mL each time). Then the volume of the mixed wash solution and filtrate was measured, sampled, and assayed for zinc , germanium and other elements. The residue was dried for 24 h in atmospheric conditions, then was milled and sampled.

Characterization and Analysis

The mineral phases in the secondary zinc oxide sample were mainly analyzed by chemical phase analysis that a quantitative phase analysis method based primarily on the differences of the solubility and dissolution velocity of various minerals in chemical solvents such as a selective solvent, a chelate, resin or other isolating agents. X-ray diffraction (XRD) using a Rigaku TTR III XRD (Cu target, K α 1, λ = 1.5406 Å) was used as an auxiliary method.

XRD was the major method for identifying the phases in the residues. EPMA was applied to detect the distribution of elements in solid sample. This work was implemented using a JEOL JXA-8530F instrument operated with 15 kV accelerating voltage and 10 nA beam current.

The observation of sample morphologies was carried out using a JEOL JSM-IT300LA scanning electron microscope (SEM ) with an energy dispersive X-ray spectrometer (EDS ).

Concentrations of arsenic , silicon , iron , and cadmium in aqueous samples were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES), and solid samples were analyzed by ICP-OES after chemical dissolution.

In order to minimize interference from other elements, the concentrations of germanium were determined by hydride generation inductively coupled plasma optical emission spectrometry (HG-ICP-OES) method using a Thermo Electron IRIS Intrepid II XSP instrument. This method had been calibrated by germanium single element standard solution produced by General Research Institute for Nonferrous Metals. Zinc concentrations were determined by the method of EDTA -2Na titration. Sulfuric acid concentrations were analyzed by sodium hydroxide titration using a methyl red-methylene blue ethanol solution as the indicator.

Results and Discussion

Process Mineralogy Analysis

Element Contents and Mineral Phase Analyses

The chemical analysis result of the secondary zinc oxide sample is shown in Table 1. It indicates that the raw material is mainly composed of zinc, lead , sulfur , arsenic , and silicon , and it contains 0.0593 wt% germanium , 48.36 wt% zinc, and 24.08 wt% lead . The silicon content is 0.75 wt%, which could partially limit the leaching of germanium .

Table 1

Chemical composition of secondary zinc oxide sample

Element	Zn	Pb	S	As	Si	Cd	Fe	Ge
Content, wt%	48.36	24.08	4.82	1.40	0.75	0.44	0.44	0.0593

The result of chemical phase analysis of germanium in the secondary zinc oxide sample is shown in Table 2. It indicates that more than 89% of germanium exists as the form of germanate, and 4.88% of germanium is insoluble germanium , this is one of the limiting factors for germanium leaching .

Table 2

Chemical phase analysis result of germanium in secondary zinc oxide sample

Chemical phase	Germanate	GeS	GeO₂ + GeS₂	Insoluble Ge	Total
Content, wt%	0.0528	0.0022	0.0014	0.0029	0.0593
Distribution , %	89.02	3.66	2.44	4.88	100

The XRD pattern of the secondary zinc oxide sample is shown in Fig. 1a, it shows diffraction peaks for zincite (ZnO), galena (PbS), anglesite (PbSO₄), wurtzite (ZnS), and lead (Pb). Diffraction peaks for germanium compounds were not detected because of their low contents. It is important to note that there is a considerable amount of lead in the raw material, and it came from the reduction of lead oxide in the fuming furnace [16].

../images/468727_1_En_210_Chapter/468727_1_En_210_Fig1_HTML.gif — Fig. 1
XRD (a) pattern and SEM image (b) of secondary zinc oxide sample

Morphology Analysis and Particle Size Distribution

Figure 1b is the secondary electron images of germanium -bearing zinc oxide . It can be seen from the 1 µm scale bar that the particles are small, inhomogeneous, of various shapes and slightly agglomerated. Table 3 shows the results of the wet screen analysis of the raw material. It indicates that more than 90% of the particles are less than 38 µm on the average size. This may affect the consumption time of leaching to a considerable extent. The morphology of secondary zinc oxide in different particle size had been observed. Morphology images are shown in Fig. 2. From image (1)–(5), the particle size increases, and there are some morphology differences. As the results show, particles in Fig. 2(1) have smooth surfaces and are very small, under 5 µm. However, in Fig. 2(5), the particles are larger rough agglomerates.

Table 3

Particle size distribution of secondary zinc oxide samples by wet screen analysis

Particle size, μm	<38(1)	38–48(2)	48–75(3)	75–150(4)	>150(5)
Distribution , %	91.40	0.74	1.34	5.25	1.27

../images/468727_1_En_210_Chapter/468727_1_En_210_Fig2_HTML.gif — Fig. 2
SEM images of secondary zinc oxide samples in different particle size

Analysis of Elemental Distribution by EPMA

Figure 3 is the distribution maps of lead , sulfur , germanium , zinc, oxygen, and silicon in the secondary zinc oxide sample. As observed, the distribution of germanium is diffuse and has a correlation with the distributions of zinc and oxygen. Furthermore, the comparison between the distribution maps of lead , sulfur , and germanium shows that a portion of germanium coexists with galena . In particular, the areas highlighted with a red rhombus illustrate this point clearly. However, the correlation of germanium and silicon is not obvious in this figure. Therefore, it can be speculated that germanium is closely related to zincite and galena . The main occurrence state of germanium is germanate associated with zincite.

../images/468727_1_En_210_Chapter/468727_1_En_210_Fig3_HTML.gif — Fig. 3
Elements distribution maps of the secondary zinc oxide sample

Leaching Process

The outcomes of the leaching tests are shown in Figs. 4a, b,5a, 6a, b, 8a, b. The leach conditions are provided in the corresponding figure caption and a discussion of the details follows.

../images/468727_1_En_210_Chapter/468727_1_En_210_Fig4_HTML.gif — Fig. 4
Effect of stirring speed (a) and initial sulfuric acid concentration (b) on the leaching of Zn, Ge, As, Fe, Si, Cd and final H₂SO₄ concentration (a: T = 35 °C; L/S = 6/1 mL g^-1; [H₂SO₄]_Initial = 160 g L^-1; t = 60 min; b: T = 35 °C; L/S = 6/1 mL g^-1; V_stirring = 180 r min^-1; t = 60 min. [H₂SO₄]_Initial is initial sulfuric acid concentration. V_stirring is stirring speed. The meanings of these symbols will be the same in the following figures)

../images/468727_1_En_210_Chapter/468727_1_En_210_Fig5_HTML.gif — Fig. 5
Relationship of GeO₂ solubility and sulfuric acid concentration (a) in H₂SO₄–H₂O system at 25 °C, and effects of liquid to solid ratio (b) on the leaching of Zn, Ge, As, Fe, Si, Cd and final H₂SO₄ concentration (b: T = 35 °C; V_stirring = 180 r·min^-1;[H₂SO₄]_Initial = 120 g L^-1; t = 60 min.)

../images/468727_1_En_210_Chapter/468727_1_En_210_Fig6_HTML.gif — Fig. 6
Effects of leaching temperature (a) and leaching time (b) on the leaching of Zn, Ge, As, Fe, Si, Cd and final H₂SO₄ concentration (a: L/S = 10/1 mL g^-1; V_stirring = 180 r min^-1; [H₂SO₄]_Initial = 120 g L^-1; t = 60 min; b: L/S = 10/1 mL g^-1; V_stirring = 180 r min^-1; [H₂SO₄]_Initial = 120 g L^-1; T = 95 °C)

Effect of Stirring Speed

Figure 4a reveals that the leaching rate of all investigated parameters increases when increasing the stirring speed until 180 r·min^-1 is reached. Final sulfuric acid concentration decreases from nearly 120 g·L^-1 to about 40 g·L^-1 at the same time. After that, increasing the stirring speed has very little effect on the leaching rate and final sulfuric acid concentration. Under current test conditions, a stirring speed of 180–240 r·min^-1 was enough for the mass transfer of the leaching process in this research.

Effect of Initial Sulfuric Acid Concentration

Figure 4b shows that the leaching rates of germanium and zinc increase from about 35% to 90% and 88%, respectively, as the initial sulfuric acid concentration increases from 40 to 120 g·L^-1. Further increasing initial acidity does not obviously improve the leaching rate. On the contrary, the leaching rate of germanium is decreased slightly when the initial sulfuric acid concentration increases to 200 g·L^-1. As Fig. 5a shows [17], the sulfuric acid concentration is not the limiting factor for germanium dissolution, because more than 2 g·L^-1 GeO₂ (Point A, containing 1.5 g·L^-1 germanium ) can be dissolved when the sulfuric acid concentration is lower than 200 g·L^-1, and the germanium concentration is lower than 100 mg·L^-1 in the test. The possible cause of this phenomenon is that large amounts of anglesite are generated under a high sulfuric acid concentration, and a part of germanium was coated by or co -precipitated with anglesite .

Effect of Liquid to Solid Ratio

As Fig. 5b signifies, the leaching rates of zinc and germanium increase to 90% and 87%, respectively, as the liquid to solid ratio increases from 4/1 to 6/1 mL·g^-1. However, increasing the liquid to solid ratio even to 12/1 mL·g^-1 has very little effect on the leaching rate of all investigated elements excluding silicon and iron . As observed, the leaching rate of silicon and iron decrease by about 10% points when the liquid to solid ratio is increases to 12/1 mL·g^-1. Just like the influence of initial sulfuric acid concentration, silica gel generation tends to occur under high sulfuric acid concentrations. The increasing of final sulfuric acid concentration is reasonable, while the acid consumption for a certain quantity of zinc oxide is fixed. Therefore, the optimal liquid solid ratio is 10/1 mL·g^-1.

Effect of Leaching Temperature

As Fig. 6a shows, that the leaching rates of germanium , zinc, and arsenic increase a little with temperature between 25 and 35 °C, but there is nearly no change for leaching rates of silicon , iron , and cadmium . Further increasing temperature contributes very little to the leaching rate of the studied elements except for arsenic , whose leaching rate increases by about 15% points. It is generally known that arsenic is a harmful element in the hydrometallurgical processing of zinc , toxic AsH3 gas will be produced in the process of removing impurities for leaching solution and the ‘burned holes’ phenomenon will appear in the zinc electrowinning process when the concentration of arsenic in the electrolyte exceeds 0.1 mg/L [18]. Considering germanium only, the optimal temperature is 95 °C.

Effect of Leaching Time

Figure 6b shows that the leaching reaction is very fast because leaching rates of all studied elements are close to the highest level after just 5 min. As the leaching time increases to 60 min, the leaching rates of all elements increases only by about 5% points. This phenomenon can be explained by the fact that the particle size of the dust is small and so there is good contact of solid and liquid. As mentioned in Table 3, more than 90% of dust is smaller than 38 µm. Thus, the rate of the leaching reaction is fast. Considering germanium only, the optimal leaching time is 120 min.

Test Under Optimal Conditions

The results of this test are shown in Fig. 7a; the leaching rates of zinc and germanium were 91% and 88%, respectively. Figure 7b shows that the main components of the residue were anglesite , galena , and wurtzite.

../images/468727_1_En_210_Chapter/468727_1_En_210_Fig7_HTML.gif — Fig. 7
Results of experiment (a) and XRD pattern (b) of the leaching residue under optimum conditions (T = 95 °C; L/S = 10/1 mL·g^-1; V_stirring = 180 r·min^-1; [H₂SO₄]_Initial = 120 g·L^-1; t = 120 min)

Behavior of Germanium

(1) Germanium in secondary zinc oxide . As Table 2 mentioned, the major phase of germanium is a germanate, maybe zinc germanate (Zn₂GeO₄) [19]. The formation process of zinc germanate is similar to the formation of zinc silicate (Zn₂SiO₄). In addition, there may be trace amounts of other types of germanate. Figure 3 clearly shows that germanium is closely related to zincite, and part of germanium is enriched in galena .

(2) Behavior of germanium in leaching process. In the leaching process, germanate and zincite are dissolved and zinc and germanium enter solution. The results of the experiments described in Sects. 3.2.1, 3.2.2, 3.2.3, 3.2.4, 3.2.5 and 3.2.6 show that the leaching rates of zinc and germanium have a close relationship with each other. This is consistent with the deduction presented in Sect. 3.2.7(1).

(3) Germanium in leaching residue. Table 4 shows that residues have a lead content of about 60 wt% and a zinc content of about 13 wt%, but germanium cannot be detected because of its low content. Figure 8, presents the EPMA distribution image for leaching resides and shows anglesite (marked in orange), wurtzite (marked in white), and galena (marked in red) can be found. It can be seen from the image of germanium that the content of germanium in galena is higher than in other phases. Therefore, it can be speculated that germanium continues to coexist with galena . Under the tests conditions, galena was not dissolved. Therefore, germanium in galena and insoluble germanium are the fundamental limiting factors of germanium leaching rate in the leaching process of germanium bearing secondary zinc oxide .

Table 4

XRF analysis results of 3 leaching residue samples under optimal conditions (wt%)

Number	Pb	Zn	S	O	As	Fe	Si
1	59.36	13.06	12.14	10.70	0.57	0.53	0.39
2	59.10	12.82	13.31	10.68	0.97	0.46	0.47
3	59.46	13.42	12.32	10.91	1.00	0.28	0.54

../images/468727_1_En_210_Chapter/468727_1_En_210_Fig8_HTML.gif — Fig. 8
Elemental distribution maps of the leach residue (Wz: wurtzite; Gn: galena ; At: anglesite )

Conclusion

The main phases in the germanium -bearing secondary zinc oxide were zincite, galena , and anglesite . The main germanium phase was a germanate dispersed in zincite. Some germanium co -existed with galena in other forms. Under the optimal conditions, the leaching extent of Ge and Zn were 91% and 88% respectively when leached with an initial 120 g/L sulfuric acid at 95 °C and a L/S ratio of 10/1. The main phases in the leached residue were anglesite , galena , and wurtzite. Germanate and soluble germanium dioxide were dissolved. Germanium in residue mainly existed in the form of insoluble germanium compounds and as germanium in galena . These are the limiting factors for further increasing the extent of germanium leaching .

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