Extraction of Germanium from Low-Grade Germanium-Bearing Lignite by Reductive Volatilization

Germanium (Ge) as an important strategic metal is widely used in many modern-technology fields such as optical fiber and thermal solar cells. In this study, the volatilization behavior of Ge from low-grade germanium-bearing lignite was investigated in detail through reductive volatilization. The results indicated that temperature and air flow rate in the semi-closed roasting system played a significant role in the process. The optimal volitation efficiency of Ge reached 98% at 1100 °C for 2 h with air flow rate of 0.7 L/min in a maffle furnace, respectively. Under optimal conditions, the contents of Ge lowered to 30 ppm in the roasting residue. Analysis of the enriched ash yielded 71,600 ppm for Ge. Chemical phase analysis of the Ge in the enrichment ash showed that soluble Ge accounted for 82.18% of the total Ge, which could be viewed as an excellent material for Ge extraction by chlorinated distillation.


Introduction
Germanium is a grayish-white semiconductor element with electrical properties intermediate between those of metals and insulators, as well as physicochemical properties similar to silicon and tin. Because of its unique physicochemical properties and compounds, it is used in a variety of industries and modern-technology fields [1][2][3]. Meanwhile, considering the lack of alternative materials and the risk of supply, germanium has been listed as a strategic metal in many countries including China, America, Japan and Europe [4]. Elshkaki et al. [5] expected that the average annual demand for germanium is expected to become 2130% of 2010 production levels by 2050.
Germanium is scarce in nature, with an average abundance of only 1.5 ppm in the earth's crust [6]. As a typical scattered element, it is generally distributed in lead and zinc mines (about 7000 tons) and coal mine (about 112,000 tons) deposits [7][8][9][10]. Therefore, germanium is mainly recovered as a by-product from lead and zinc mines and coal mines. Currently, approximately 50% of germanium is extracted from germanium coal in China.
At present, the main methods for extracting germanium from coal fly ash contained acidic leaching [11][12][13], biotechnologies [14] and volatile enrichment by pyrometallurgy [15]. Chen et al. [16] used acid to directly leach germanium from germanium coal. The results showed that, during direct acidic leaching, the leaching efficiency of germanium was as low as 83.62% (initial content of 17,300 ppm). It can be attributed to the fact that germanium in the coal appeared in the form of organic bonding [17]. Su et al. [18] used biotechnologies to recover germanium from critical raw materials, offering the advantages of environmental friendliness, low production costs and the ability to process complex minerals. However, biotechnologies are considered to be sensitive to inhibitors and show too-low kinetics. The Materials 2023, 16, 5374 2 of 15 above factors have limited the application of biotechnologies. Combustion volatilization is a promising and alternative approach for extracting germanium from coal fly ash. Zhang et al. [19] demonstrated the use of a vacuum reduction combustion process resulting in a 94.64% recovery ratio of germanium from coal fly ash with a germanium content of 6158 ppm.
High-grade germanium coal has been used for industrial production by some companies in Yunnan and Inner Mongolia, China, but low-grade germanium coal remains unexploited due to economic reasons [20,21]. However, with frequent mining over a long period of time, the reserves of high-quality coal have rapidly diminished [22,23]. Lignite is a typical low-quality coal. Although the grade of germanium (ranging from 30 ppm to 200 ppm) in lignite is poor, the large reserves of lignite will make it a major raw material for germanium extraction in the future [24,25]. There is an urgent need to develop better technology to utilize the lignite. In this study, the low-grade germanium-bearing lignite from Wulantuga Mine, Inner Mongolia was selected as the research object. During the combustion process, the main influential factors on the volatilization behaviors of germanium were investigated in detail. At the same time, various combustion residues and concentrate were characterized using ICP-OES, XRD and SEM-EDS.
The objective of this study was to achieve the separation of carbon and germanium while enriching soluble germanium to provide the basis for subsequent purification by chlorinated distillation.

Materials
This study was conducted using low-grade germanium-bearing lignite from Inner Mongolia (longitude 115 • 30 -116 • 35 E, latitude 43 • 45 -44 • 18 N). Particle size distribution of the lignite by Laser Scattering Particle Size Distribution Analyzer. ICP-OES, XRD and SEM-EDS were used to characterize the content, phase and morphology of the main elements in the lignite, and it was analyzed industrially according to the national standard GB/T 30732-2014.
The results of the industrial and chemical analysis are shown in Table 1. By ICP-OES analysis, the content of germanium in the lignite was 190 ppm. In addition, a relatively high content of S (above 2.87 wt.%) and Si (above 3.7 wt.%) was detected in the lignite. The particle size distribution and chemical phase analysis of the lignite was carried out as detailed in Figure 1. Since the experiment was carried out using untreated lignite, its particle size distribution was not uniform, and according to the measurements, its D50 was 130.5894 µm, as shown in Figure 1a. From Figure 1b, the main phase of the lignite was C, SiO 2 and CaSO 4 ·2H 2 O. Figure 2 shows SEM images of the germanium-containing lignite. Based on EDS results from Table 2, spot 4 and spot 5 contained an especially high content of germanium. Moreover, germanium presented a close relationship with As, Pb and S, as shown in Figure 2c. Spot 1 represented a C-rich particle containing low contents of S. Spot 2 was viewed as silicon dioxide owing to high contents of Si and O. Moreover, Spot 3 was recognized as calcium sulfate because of high contents of Ca, S and O. This is consistent with the XRD analysis.
its particle size distribution was not uniform, and according to the measurements, its D50 was 130.5894 μm, as shown in Figure 1a. From Figure 1b, the main phase of the lignite was C, SiO2 and CaSO4·2H2O. Figure 2 shows SEM images of the germanium-containing lignite. Based on EDS results from Table 2, spot 4 and spot 5 contained an especially high content of germanium. Moreover, germanium presented a close relationship with As, Pb and S, as shown in Figure 2c. Spot 1 represented a C-rich particle containing low contents of S. Spot 2 was viewed as silicon dioxide owing to high contents of Si and O. Moreover, Spot 3 was recognized as calcium sulfate because of high contents of Ca, S and O. This is consistent with the XRD analysis. Intensity (a.u.)   its particle size distribution was not uniform, and according to the measurements, its D50 was 130.5894 µm, as shown in Figure 1a. From Figure 1b, the main phase of the lignite was C, SiO2 and CaSO4·2H2O. Figure 2 shows SEM images of the germanium-containing lignite. Based on EDS results from Table 2, spot 4 and spot 5 contained an especially high content of germanium. Moreover, germanium presented a close relationship with As, Pb and S, as shown in Figure 2c. Spot 1 represented a C-rich particle containing low contents of S. Spot 2 was viewed as silicon dioxide owing to high contents of Si and O. Moreover, Spot 3 was recognized as calcium sulfate because of high contents of Ca, S and O. This is consistent with the XRD analysis.

Experiment
In order to research the volatile processes of germanium, combustion experiments were designed in a muffle furnace (KSL-1200X-M) under different conditions. The raw materials were placed in an alumina crucible and transferred to the middle of the muffle furnace. After the roasting process, the roasting residue was weighed. An acidic mixture (H 3 PO 4 : HF: HNO 3 = 1:1:2) was used to dissolve the residue, and the solution was used to analysis the content of germanium by ICP-OES. The volatilization efficiency of germanium was calculated through the determination of the content of germanium before and after roasting. The volatilization efficiency of germanium is calculated from Equation (1) [26] as follows: where η Me represent the volatilization rates of germanium; m 1 and m 2 refer to the mass of the material and the corresponding combustion residue, respectively; and wt 1 and wt 2 are the contents of the material and the corresponding combustion residue, respectively. In addition, the germanium in the concentrate was studied according to the properties of the different phases of germanium. The technological route of the analysis is illustrated in Figure 3.
were designed in a muffle furnace (KSL-1200X-M) under different c materials were placed in an alumina crucible and transferred to the m furnace. After the roasting process, the roasting residue was weighed (H3PO4: HF: HNO3 = 1:1:2) was used to dissolve the residue, and the so analysis the content of germanium by ICP-OES. The volatilization e nium was calculated through the determination of the content of germ after roasting. The volatilization efficiency of germanium is calculated [26] as follows: where ηMe represent the volatilization rates of germanium; m1 and m2 the material and the corresponding combustion residue, respectively; the contents of the material and the corresponding combustion residu In addition, the germanium in the concentrate was studied accor ties of the different phases of germanium. The technological route of t trated in Figure 3.

Characterization and Analysis
The residue and concentrate obtained from aforementioned exp gested in an acidic mixture in a heating panel (250 °C for digesting solution was filtered through a paper membrane and finally made u ultrapure water. The concentration of germanium was determined usi pled plasma optical emission spectrometry (ICP-OES, Spectro ARCO many) and based on the spectral line Ge209.426. X-ray diffraction (XR Tokyo, Japan) and scanning electronic microscopy using energy dispe (SEM-EDS FEI Quanta 650, Bruker Quantax, Billerica, MA, USA) were u the morphology of the residue and concentrate at optimum roasting c

Characterization and Analysis
The residue and concentrate obtained from aforementioned experiments were digested in an acidic mixture in a heating panel (250 • C for digesting 1 h). The digestion solution was filtered through a paper membrane and finally made up to 100 mL using ultrapure water. The concentration of germanium was determined using inductively coupled plasma optical emission spectrometry (ICP-OES, Spectro ARCOS MV, Kleve, Germany) and based on the spectral line Ge209.426. X-ray diffraction (XRD, Rigaku TTR-III, Tokyo, Japan) and scanning electronic microscopy using energy dispersive spectrometer (SEM-EDS FEI Quanta 650, Bruker Quantax, Billerica, MA, USA) were used to characterize the morphology of the residue and concentrate at optimum roasting conditions.

Chemical Reaction Equations for Ge-C-O
During the combustion process, the possible reaction for germanium is listed in Table 3. Meanwhile, the Gibbs free energy was calculated based on HSC 6.0 and the result is shown in Figure 4. Table 3. Possible reaction in the reduction process.

Reaction Equation Number
Materials 2023, 16, x FOR PEER REVIEW 5 of 15

Chemical Reaction Equations for Ge−C−O
During the combustion process, the possible reaction for germanium is listed in Table  3. Meanwhile, the Gibbs free energy was calculated based on HSC 6.0 and the result is shown in Figure 4. ΔrGm (kJ/mol)  Table 3. Possible reaction in the reduction process.

Reaction Equation Number
It was evident from Figure 4a that hexagonal germanium dioxide (GeO2(H)) had dif ficulty in decomposing into GeO(g) via Number 3-5 in the 1500 °C range without the in volvement of carbon in the reaction. In the presence of carbon, GeO2(H) was reduced to Ge and GeO(g) at 600 °C and 900 °C, respectively, and, as the reaction temperature in creased, decomposition to GeO(g) was more prone to occur when the temperature reached 1100 °C. From Figure 4b, it can be seen that diverse crystalline forms of GeO2 can be trans formed into each other in different temperature ranges. Among them, GeO2(H) can be spontaneously converted to tetragonal germanium dioxide (GeO2(T)). GeO2(H) and GeO2(T) can only be converted to glassy germanium dioxide (GeO2(G)) under high-tem perature conditions. Additionally, the carbon thermal reduction of GeO2(T) was the mos challenging among the carbon thermal reactions of the varied crystalline GeO2 types. It was evident from Figure 4a that hexagonal germanium dioxide (GeO 2 (H)) had difficulty in decomposing into GeO(g) via Number 3-5 in the 1500 • C range without the involvement of carbon in the reaction. In the presence of carbon, GeO 2 (H) was reduced to Ge and GeO(g) at 600 • C and 900 • C, respectively, and, as the reaction temperature increased, decomposition to GeO(g) was more prone to occur when the temperature reached 1100 • C. From Figure 4b, it can be seen that diverse crystalline forms of GeO 2 can be transformed into each other in different temperature ranges. Among them, GeO 2 (H) can be spontaneously converted to tetragonal germanium dioxide (GeO 2 (T)). GeO 2 (H) and GeO 2 (T) can only be converted to glassy germanium dioxide (GeO 2 (G)) under hightemperature conditions. Additionally, the carbon thermal reduction of GeO 2 (T) was the most challenging among the carbon thermal reactions of the varied crystalline GeO 2 types.

Equilibrium Compositions Diagram of Ge-C-O Systems
As shown in Figure 5, the equilibrium compositions of germanium in the lignite were plotted for the Ge-C-O systems using the "Equilibrium Composition" module in HSC 6.0 in order to clarify the equilibrium compositions of germanium at different temperatures and systems.

Equilibrium Compositions Diagram of Ge−C−O Systems
As shown in Figure 5, the equilibrium compositions of germanium in the lignite were plotted for the Ge−C−O systems using the "Equilibrium Composition" module in HSC 6.0 in order to clarify the equilibrium compositions of germanium at different temperatures and systems. By fixing the amount of carbon, the amount of oxygen was changed, which in turn created a distinct atmosphere. It has been calculated that germanium dioxide can be reduced to germanium metal at a low temperature (<900 • C) when the combustion environment is a potent reducing atmosphere [n(O 2 )/n(C) = 1]. When the combustion environment was a potent oxidizing atmosphere [n(O 2 )/n(C) = 2] and the temperature was below 1200 • C, germanium was mainly present as a crystal type of GeO 2 . Temperatures in excess of 1500 • C were required if GeO(g) crystals were to be formed. Clearly, germanium was difficult to reduce to GeO(g) for volatilization, either in a potent reducing nor oxidizing atmosphere. However, as can be seen in Figure 5a-d, in case the combustion environment was a suitable reducing atmosphere [(n(O 2 )/n(c) = 0.75), (n(O 2 )/n(c) = 1)], it was possible to achieve complete volatilization of GeO(g) crystal by adjusting the CO concentration and temperature fluctuations within a certain range. It was clear that complete volatilization of GeO(g) can be achieved at temperature of 900 • C when the CO ratio was approximately 50% (Figure 5a,b). The same result can be realized by increasing the temperature to 1100 • C in case the CO ratio was approximately 1% (Figure 5c,d). Figure 6 shows the TG-DTG curve of the lignite combustion process. The measurements were performed under mixed air atmosphere. Samples of 8.0223 mg were placed in an alumina crucible and heated from room temperature to 1200 • C with a heating rate of 10 • C/min. By fixing the amount of carbon, the amount of oxygen was changed, which in turn created a distinct atmosphere. It has been calculated that germanium dioxide can be reduced to germanium metal at a low temperature (<900 °C) when the combustion environment is a potent reducing atmosphere [n(O2)/n(C) = 1]. When the combustion environment was a potent oxidizing atmosphere [n(O2)/n(C) = 2] and the temperature was below 1200 °C, germanium was mainly present as a crystal type of GeO2. Temperatures in excess of 1500 °C were required if GeO(g) crystals were to be formed. Clearly, germanium was difficult to reduce to GeO(g) for volatilization, either in a potent reducing nor oxidizing atmosphere. However, as can be seen in Figure 5a-d, in case the combustion environment was a suitable reducing atmosphere [(n(O2)/n(c) = 0.75), (n(O2)/n(c) = 1)], it was possible to achieve complete volatilization of GeO(g) crystal by adjusting the CO concentration and temperature fluctuations within a certain range. It was clear that complete volatilization of GeO(g) can be achieved at temperature of 900 °C when the CO ratio was approximately 50% (Figure 5a,b). The same result can be realized by increasing the temperature to 1100° C in case the CO ratio was approximately 1% (Figure 5c,d).  Table 4 illustrates the TG−DTG data. According to the data, we can find that before 174.4 °C the weight loss of sample was about 23.87%, accounting for 26.23% of the total weight loss. The weight loss at this stage was close to the content of inherent moisture in the lignite (26.32%), which could be attributed to the water evaporates from the lignite. The second stage of weight loss occurred in the range of 174.4-668.6 °C. In this stage, the weight loss rate was 60.60%, accounting for 66.58% of the total weight loss. It can be concluded from Figure 6 that the weight was a sharp decline starting from about 245.3 °C. The explanation was that the combustion of carbon in the lignite and the weight loss in this stage were slightly higher than the content of fixed carbon (43.37%). With the heating temperature exceeding 668.6 °C, the weight loss of lignite was more moderate. The mass loss at this stage was only 6.55%, accounting for only 7.20% of the total weight loss, which might be due to the decomposition of small amounts of carbonate and sulfate, as well as oxide in the lignite; therefore, weightlessness was caused by the reaction of the sulfate,  Table 4 illustrates the TG-DTG data. According to the data, we can find that before 174.4 • C the weight loss of sample was about 23.87%, accounting for 26.23% of the total weight loss. The weight loss at this stage was close to the content of inherent moisture in the lignite (26.32%), which could be attributed to the water evaporates from the lignite. The second stage of weight loss occurred in the range of 174.4-668.6 • C. In this stage, the weight loss rate was 60.60%, accounting for 66.58% of the total weight loss. It can be concluded from Figure 6 that the weight was a sharp decline starting from about 245.3 • C. The explanation was that the combustion of carbon in the lignite and the weight loss in this stage were slightly higher than the content of fixed carbon (43.37%). With the heating temperature exceeding 668.6 • C, the weight loss of lignite was more moderate. The mass loss at this stage was only 6.55%, accounting for only 7.20% of the total weight loss, which might be due to the decomposition of small amounts of carbonate and sulfate, as well as oxide in the lignite; therefore, weightlessness was caused by the reaction of the sulfate, oxide and coke. The reduction reaction of the GeO 2 should occur in this stage, for which the following chemical reactions are involved:

Combustion Experiments
Equilibrium composition diagrams demonstrated that temperature and roasting system exerted a significant effect on the volatilization behavior of germanium. In this research, the effects of roasting system, air flow rate, temperature, heating time and heating rate on the volatilization efficiency of germanium were investigated in detail.

Effect of Roasting System
The relationship between the volatilization rate of germanium and roasting system is shown in Figure 7. It shows that the roasting system of lignite had a significant influence on the volatilization efficiency of germanium. The germanium content in the residue reached 517.1 ppm when the sample was burned in the muffle furnace and the furnace door was fully opened, and the volatilization efficiency was only 65.97%; when the semi-closed system was adopted (muffle furnace burning, furnace door closed, air was introduced), the germanium content was reduced to 60 ppm from 517.1 ppm in the residue, and the volatilization efficiency reached 96.89%. When a completely closed system (tube furnace combustion, air was introduced) was studied, the germanium content in the residue was reduced to 30 ppm and the volatilization efficiency jumped to 98.11% from 65.97%. This can be explained by the fact that when the system was fully open, because of the rapid exposure to enough oxygen, a part of the generated GeO turned into non-volatile GeO 2 and sank in the slag, resulting in a low germanium volatilization rate, whereas it was difficult for this to happen in semi-closed or closed systems. In order to make the experimental process simulate industrial production as closely as possible, the subsequent exploration will be carried out in a semi-closed system (muffle furnace burning, furnace door was closed, air was introduced). oxide and coke. The reduction reaction of the GeO2 should occur in this stage, for which the following chemical reactions are involved: GeO 2 + C = GeO + CO (3) 668.6-1200 6.55 7.20

Combustion Experiments
Equilibrium composition diagrams demonstrated that temperature and roasting system exerted a significant effect on the volatilization behavior of germanium. In this research, the effects of roasting system, air flow rate, temperature, heating time and heating rate on the volatilization efficiency of germanium were investigated in detail.

Effect of Roasting System
The relationship between the volatilization rate of germanium and roasting system is shown in Figure 7. It shows that the roasting system of lignite had a significant influence on the volatilization efficiency of germanium. The germanium content in the residue reached 517.1 ppm when the sample was burned in the muffle furnace and the furnace door was fully opened, and the volatilization efficiency was only 65.97%; when the semiclosed system was adopted (muffle furnace burning, furnace door closed, air was introduced), the germanium content was reduced to 60 ppm from 517.1 ppm in the residue, and the volatilization efficiency reached 96.89%. When a completely closed system (tube furnace combustion, air was introduced) was studied, the germanium content in the residue was reduced to 30 ppm and the volatilization efficiency jumped to 98.11% from 65.97%. This can be explained by the fact that when the system was fully open, because of the rapid exposure to enough oxygen, a part of the generated GeO turned into non-volatile GeO2 and sank in the slag, resulting in a low germanium volatilization rate, whereas it was difficult for this to happen in semi-closed or closed systems. In order to make the experimental process simulate industrial production as closely as possible, the subsequent exploration will be carried out in a semi-closed system (muffle furnace burning, furnace door was closed, air was introduced).

Effect of Air Flow Rate
It is possible to control the combustion atmosphere by varying air flow rate. Figure 8 shows the relationship between volatilization efficiency of germanium and air flow rate. The effect of air flow rate on the volatilization efficiency was investigated in the range from 0 to 1.0 L/min. Figure 8 showed that with the increase in air flow rate from 0 to 0.7 L/min, the volatilization efficiency of germanium increased from 88.46% to 96.46%. However, the volatilization efficiency of germanium was reduced when the air flow rate continued to increase to 1.0 L/min. It can be explained that the increased air flow rate was beneficial to the combustion of carbon in the lignite and the formation of easily volatile GeO(g) when the air flow rate was less than 0.7 L/min. However, when the air flow rate was in the range from 0.7 to 1.0 L/min, the volatilization efficiency of germanium was reduced because the crystal form of germanium changed as the air composition changed. The volatilization effect of germanium with different crystal forms was different, so the volatilization rate of germanium also changed with the air flow rate. This indicated that germanium may have formed compounds such as GeO 2 and CaGeO 3 , which were difficult to volatilize.

Effect of Air Flow Rate
It is possible to control the combustion atmosphere by varying air flow rate. Figure 8 shows the relationship between volatilization efficiency of germanium and air flow rate The effect of air flow rate on the volatilization efficiency was investigated in the range from 0 to 1.0 L/min. Figure 8 showed that with the increase in air flow rate from 0 to 0.7 L/min, the volatilization efficiency of germanium increased from 88.46% to 96.46%. How ever, the volatilization efficiency of germanium was reduced when the air flow rate con tinued to increase to 1.0 L/min. It can be explained that the increased air flow rate wa beneficial to the combustion of carbon in the lignite and the formation of easily volatile GeO(g) when the air flow rate was less than 0.7 L/min. However, when the air flow rate was in the range from 0.7 to 1.0 L/min, the volatilization efficiency of germanium wa reduced because the crystal form of germanium changed as the air composition changed The volatilization effect of germanium with different crystal forms was different, so the volatilization rate of germanium also changed with the air flow rate. This indicated tha germanium may have formed compounds such as GeO2 and CaGeO3, which were difficul to volatilize.

Effect of Temperature
With the temperature increasing from 800 to 1200 °C, the effect of temperature i shown in Figure 9. It shows that the volatilization efficiency of germanium was signifi cantly increased with the increase in temperature. The evaporation of metallic germanium and its oxides was based on their saturation vapor pressure and a sharp increase appear at 1100 °C, where the volatilization rate of germanium jumped to 99.53% from 82.14% Therefore, the optimal heating temperature for the volatilization efficiency of germanium was chosen to be 1100 °C.

Effect of Temperature
With the temperature increasing from 800 to 1200 • C, the effect of temperature is shown in Figure 9. It shows that the volatilization efficiency of germanium was significantly increased with the increase in temperature. The evaporation of metallic germanium and its oxides was based on their saturation vapor pressure and a sharp increase appears at 1100 • C, where the volatilization rate of germanium jumped to 99.53% from 82.14%. Therefore, the optimal heating temperature for the volatilization efficiency of germanium was chosen to be 1100 • C.

Effect of Heating Time
The relationship between volatilization efficiency of germanium and time is shown in Figure 10, which shows that the germanium's volatilization efficiency can reach up to

Effect of Heating Time
The relationship between volatilization efficiency of germanium and time is shown in Figure 10, which shows that the germanium's volatilization efficiency can reach up to 99.53% at 120 min. The continued extension of the heating time had no significant effect for germanium. This shows that the heating time had a small effect on the volatilization of germanium at the right temperature. Therefore, the optimal reaction time was 120 min for the volatilization efficiency of germanium substances from the lignite.

Effect of Heating Time
The relationship between volatilization efficiency of germanium and time is shown in Figure 10, which shows that the germanium's volatilization efficiency can reach up to 99.53% at 120 min. The continued extension of the heating time had no significant effect for germanium. This shows that the heating time had a small effect on the volatilization of germanium at the right temperature. Therefore, the optimal reaction time was 120 min for the volatilization efficiency of germanium substances from the lignite.

Effect of Heating Rate
With the heating rate increasing from 4 to 10 °C/min, the effect of heating rate is shown in Figure 11. This figure shows that the germanium's volatilization efficiency can reach up to 98.60% at 8 °C/min. It does not present the trend that the volatilization efficiency of germanium increased with the increasing heating rate. The result indicated that the effect of heating rate for volatilization efficiency was not significant. Therefore, the optimal heating rate was considered to be 8 °C/min for the volatilization efficiency of germanium.

Effect of Heating Rate
With the heating rate increasing from 4 to 10 • C/min, the effect of heating rate is shown in Figure 11. This figure shows that the germanium's volatilization efficiency can reach up to 98.60% at 8 • C/min. It does not present the trend that the volatilization efficiency of germanium increased with the increasing heating rate. The result indicated that the effect of heating rate for volatilization efficiency was not significant. Therefore, the optimal heating rate was considered to be 8 • C/min for the volatilization efficiency of germanium.

Analysis of Residues and Concentrate
The optimal parameters of volatilization efficiency of germanium from the lignite were obtained through a series of experiments. The optimal parameters were at a temperature of 1100 °C, heating time of 120 min, heating rate of 8 °C/min and air flow rate of 0.7 L/min. In order to collect concentrate, the experiments are performed in a closed system (in a tube furnace) according to the optimal parameters.

Characterization of the Combustion Residue
The chemical composition of the combustion residue is shown in Table 5. According to Table 5, germanium in the residue was reduced to 30 ppm, which meant that germa-

Analysis of Residues and Concentrate
The optimal parameters of volatilization efficiency of germanium from the lignite were obtained through a series of experiments. The optimal parameters were at a temperature of 1100 • C, heating time of 120 min, heating rate of 8 • C/min and air flow rate of 0.7 L/min. In order to collect concentrate, the experiments are performed in a closed system (in a tube furnace) according to the optimal parameters.

Characterization of the Combustion Residue
The chemical composition of the combustion residue is shown in Table 5. According to Table 5, germanium in the residue was reduced to 30 ppm, which meant that germanium in the lignite was well volatilized. Meanwhile, we analyzed the morphology of the residue with XRD and SEM. As can be seen from Figure 12, the combustion residue was mainly composed of iron oxide, silica dioxide, calcium aluminum silicate and other phases. According to Figure 13, the combustion residue had a relatively simple physical phase, mainly iron oxides, silicon oxides, rankinite and aluminum silicate. However, neither XRD nor SEM could find a physical phase of the germanium, again indicating that the germanium was volatilized quite thoroughly.

Analysis of Residues and Concentrate
The optimal parameters of volatilization efficiency of germanium from the lignite were obtained through a series of experiments. The optimal parameters were at a temperature of 1100 °C, heating time of 120 min, heating rate of 8 °C/min and air flow rate of 0.7 L/min. In order to collect concentrate, the experiments are performed in a closed system (in a tube furnace) according to the optimal parameters.

Characterization of the Combustion Residue
The chemical composition of the combustion residue is shown in Table 5. According to Table 5, germanium in the residue was reduced to 30 ppm, which meant that germanium in the lignite was well volatilized. Meanwhile, we analyzed the morphology of the residue with XRD and SEM. As can be seen from Figure 12, the combustion residue was mainly composed of iron oxide, silica dioxide, calcium aluminum silicate and other phases. According to Figure 13, the combustion residue had a relatively simple physical phase, mainly iron oxides, silicon oxides, rankinite and aluminum silicate. However, neither XRD nor SEM could find a physical phase of the germanium, again indicating that the germanium was volatilized quite thoroughly.  Figure 12. XRD patterns of the combustion residue.  Table 6 shows the chemical composition of the combustion concentrate. According to Table 6, germanium was enriched to 7.16% and concentrate products also contained a certain amount of As, as well as significant amounts of Pb and S. In addition, chemical phase analysis of the germanium in the concentrate was carried out and the results were shown in Table 7. The analytical results showed that the content of soluble (germanium oxide, germanate, germanium sulfide) and insoluble (germanosilicate, tetragonal germanium di-  Table 6 shows the chemical composition of the combustion concentrate. According to Table 6, germanium was enriched to 7.16% and concentrate products also contained a certain amount of As, as well as significant amounts of Pb and S. In addition, chemical phase analysis of the germanium in the concentrate was carried out and the results were shown in Table 7. The analytical results showed that the content of soluble (germanium oxide, germanate, germanium sulfide) and insoluble (germanosilicate, tetragonal germanium dioxide) germanium was 82.18% and 17.82%, respectively. Meanwhile, the morphology of the concentrate was analyzed by XRD and SEM-EDS. As can be seen from Figure 14, the main phase of germanium was lead germinate, and besides lead germinate in the concentrate, the concentrate also contained arsenic trioxide, lead sulfide, silica, etc. SEM-EDS analyses for the concentrate are shown in Figure 15. It can be seen that the obtained concentrate presented an irregular morphology. EDS results from Table 8 demonstrated that the concentrate was mainly composed of Pb, Ge, As, O and S. According to the results of EDS analysis, the germanium content in spot 1, 6 and 8 was approaching 60%, 20% and 15%, respectively. In addition, there were higher levels of As, O, and S. This indicated that germanium and arsenic did not only volatilize in a unique form of GeO and As 2 O 3 , respectively. Germanium and arsenic were also volatilized as sulfides, which was consistent with the results of the chemical phase analysis. The lead content in spot 2 exceeded 98%, which indicated that part of the lead was evaporated in the form of lead elemental. In spots 3, 4 and 7, the contents of Pb and S were high and the O content was low; this indicated that lead could be volatilized with PbS. The chemical phase analysis revealed only 0.62 wt.% germanosilicate in the combustion concentrate, which meant that the silicon might have been volatilized during the combustion process in different forms, such as silicon fluoride (spot 5).

Conclusions
In this study, the recovery process of germanium was investigated from low-grade germanium-bearing lignite in semi-closed or closed systems. The feasibility of recycling germanium from the lignite was verified by experiments and thermodynamic analysis. The optimal parameters of volatilization rates of germanium from the lignite were obtained through a series of laboratory scale experiments. The optimal parameters were at a temperature of 1100 • C, heating time of 120 min, heating rate of 8 • C/min and air flow rate of 0.7 L/min. Under the optimal parameters, the volatilization rate of germanium exceeds 98%, and the germanium content is only 30 ppm in the residue. The concentrate was analyzed and it was found that the germanium content in the concentrates was as high as 7.16%; among them, the content of soluble (germanium oxide, germanate, germanium sulfide) and insoluble (germanosilicate, tetragonal germanium dioxide) germanium was 82.18% and 17.82%, respectively. Therefore, this technological process can realize the enrichment of germanium from germanium-bearing lignite, which lays the foundation for further purification using chlorinated distillation or leaching to subsequently obtain pure germanium.  Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: The data of the study are available from the corresponding author upon reasonable request.