How to recover germanium

Abstract

To provide a germanium recovery method that can recover germanium from exhaust in the production of an optical fiber efficiently and at low cost.SOLUTION: A method for recovering germanium includes a first step for separating germanium-containing dust from exhaust, a second step for extracting germanium from the germanium-containing dust separated in the first step into liquid to give a germanium extract, a third step for separating the germanium extract resulting from the second step from solid residues, a fourth step for precipitating a germanium-containing compound from the germanium extract separated in the third step, and a fifth step for separating the germanium-containing compound precipitated in the fourth step from the liquid.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a method for recovering germanium. [Background technology]

[0002] Germanium (Ge) is used as a raw material for various industrial products, such as semiconductor materials for diodes and the like, and as an additive for the core of glass optical fibers. The price of Ge has been rising sharply in recent years. For example, Patent Document 1 and Patent Document 2 disclose techniques for recovering Ge. Patent Document 1 discloses a technique for recovering Ge using a bag filter and reusing it in the optical fiber manufacturing process. Patent Document 2 discloses a method and device for treating wastewater from optical fiber manufacturing, which recovers Ge contained in the wastewater from optical fiber manufacturing by adsorbing it onto a chelating fiber. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2004-2088 A [Patent Document 2] JP 2017-140571 A Summary of the Invention [Problem to be solved by the invention]

[0004] During the manufacture of optical fibers, a portion of germanium, which is an additive to the core of the optical fiber, is not used in the production of the core and is discharged as exhaust gas. The amount of germanium contained in the exhaust gas is very small, and the particles containing germanium are very fine, so it has not been easy to recover germanium efficiently and inexpensively. Even with conventional methods, the recovery efficiency of germanium is not necessarily high. In addition, the technology using chelating fibers shown in Patent Document 2 requires expensive equipment.

[0005] An object of the present disclosure is to provide a germanium recovery method capable of efficiently and inexpensively recovering germanium from exhaust gas during the manufacture of optical fibers. [Means for solving the problem]

[0006] The method for recovering germanium according to the present disclosure comprises the steps of: A method for recovering germanium from exhaust gas discharged during the manufacture of optical fiber, comprising the steps of: a first step of separating germanium-containing dust from said exhaust gas; a second step of extracting germanium from the germanium-containing dust separated in the first step into a liquid to obtain a germanium extract; a third step of separating the germanium extract obtained in the second step from a solid residue; a fourth step of precipitating a germanium-containing compound from the germanium extract separated in the third step; a fifth step of separating the germanium-containing compound precipitated in the fourth step from a liquid; Includes. Effect of the Invention

[0007] According to the present disclosure, it is possible to provide a germanium recovery method capable of efficiently and inexpensively recovering germanium from exhaust gas during the manufacture of optical fibers. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing a flow of a germanium recovery method according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a diagram illustrating the second and third steps according to an embodiment of the present disclosure. [Diagram 3] FIG. 3 is a diagram illustrating the fourth and fifth steps according to one aspect of the embodiment of the present disclosure. [Figure 4] FIG. 4 is a diagram illustrating a fourth step and a fifth step according to another aspect of the embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be listed and described. The method for recovering germanium according to the embodiment includes the steps of: (1) A method for recovering germanium from exhaust gas discharged during the manufacture of optical fiber, comprising the steps of: a first step of separating germanium-containing dust from said exhaust gas; a second step of extracting germanium from the germanium-containing dust separated in the first step into a liquid to obtain a germanium extract; a third step of separating the germanium extract obtained in the second step from a solid residue; a fourth step of precipitating a germanium-containing compound from the germanium extract separated in the third step; a fifth step of separating the germanium-containing compound precipitated in the fourth step from a liquid; Includes.

[0010] According to the above-described germanium recovery method, germanium is extracted from the germanium-containing dust into a liquid, and a germanium-containing compound is precipitated from the resulting extract, without relying on a method for separating the germanium-containing dust from the exhaust gas. This makes it possible to efficiently and inexpensively recover germanium contained in the exhaust gas.

[0011] (2) The method for recovering germanium described above in (1) is as follows: In the first step, the germanium-containing dust is collected from the exhaust gas using a wet dust collector or a dry dust collector; In the second step, the pH or temperature of the slurry containing the germanium-containing dust is adjusted to extract germanium from the germanium-containing dust into a liquid; In the fourth step, the germanium extract may be concentrated or a metal salt may be added to the germanium extract to precipitate the germanium-containing compound from the germanium extract.

[0012] In the above recovery method, either a wet dust collector or a dry dust collector may be used in the first step. By adopting a method of adjusting the pH or temperature of the slurry in the second step, germanium can be efficiently extracted into the liquid even with an inexpensive method. Then, in the fourth step, the germanium extract is concentrated or a metal salt is added to the germanium extract to cause coprecipitation, thereby efficiently precipitating a germanium-containing compound with an inexpensive means. By following this procedure, germanium contained in the exhaust gas can be efficiently and inexpensively recovered.

[0013] (3) The method for recovering germanium described above in (2) is as follows: In the second step, the pH of the slurry may be adjusted to 8.5 or more and 11 or less to extract germanium into the liquid.

[0014] By making the slurry alkaline, the dissolution of germanium-containing dust into the liquid is promoted, and the amount of germanium eluted into the liquid increases, allowing for efficient recovery of germanium.

[0015] (4) The method for recovering germanium described above in (2) is as follows: In the second step, the temperature of the slurry may be adjusted to 30° C. or more and 95° C. or less to extract germanium into the liquid.

[0016] By increasing the temperature of the slurry, the germanium-containing dust can be suitably dispersed in the slurry, and the amount of germanium eluted increases with the dispersion, allowing germanium to be efficiently recovered.

[0017] (5) Any of the above methods for recovering germanium (2) to (4) above, In the first step, the germanium-containing dust is collected from the exhaust gas using a dry dust collector; In the second step, the germanium-containing dust collected by the dry dust collector may be added to the germanium extract separated in the third step to extract germanium into the liquid.

[0018] In the method using a dry dust collector, a step of forming a slurry of the germanium-containing dust is added. The liquid for forming the slurry may be the germanium extract separated in the third step.

[0019] (6) Any of the above methods for recovering germanium (2) to (5) above, In the fourth step, a soluble metal salt that forms a composite oxide with germanium may be added to precipitate the germanium-containing compound from the germanium extract.

[0020] By adding a soluble metal salt that forms a complex oxide with germanium, germanium dissolved in the germanium extract can be recovered in high yield as a complex oxide with the added metal.

[0021] (7) Any of the above methods for recovering germanium (2) to (5) above, In the fourth step, a sparingly soluble metal salt capable of adsorbing germanium may be added to precipitate the germanium-containing compound from the germanium extract.

[0022] By adding a sparingly soluble metal salt that adsorbs germanium, germanium can be collected on the surface of the sparingly soluble metal salt to precipitate a germanium-containing compound, and the germanium dissolved in the germanium extract can be recovered in high yield.

[0023] (8) Any of the above methods for recovering germanium (2) to (7) above, In the fourth step, an alkaline earth metal salt may be added to precipitate the germanium-containing compound from the germanium extract.

[0024] By adding an alkaline earth metal salt, the germanium-containing compound can be favorably precipitated, and the germanium dissolved in the germanium extract can be recovered in a high yield.

[0025] (9) The method for recovering germanium described above in (8) is as follows: The amount of the alkaline earth metal salt added in the fourth step may be 0.8 equivalents or more and 3.0 equivalents or less relative to the germanium equivalent in the liquid.

[0026] By adding the alkaline earth metal salt in the above-mentioned amount, it is possible to induce the production of not only a composite oxide containing germanium and an alkaline earth metal in an equivalent ratio of 1:1, but also a composite oxide containing a higher equivalent ratio of alkaline earth metal, and the germanium dissolved in the germanium extract can be recovered in a high yield.

[0027] (10) Any of the above methods for recovering germanium (2) to (9) comprises the steps of: In the fifth step, the germanium-containing compound may be subjected to a classification treatment for separating it into a coarse particle fraction and a fine particle fraction, the coarse particle fraction may be recovered, and the fine particle fraction may be returned to the fourth step for recycling.

[0028] In the above-mentioned configuration, the fine particles of the classified germanium-containing compound are returned to the fourth step, whereby the fine particles function as seed crystals when precipitating the germanium-containing compound, thereby inducing the precipitation of the germanium compound and allowing the germanium dissolved in the germanium extract to be recovered in a high yield.

[0029] [Details of the embodiment of the present disclosure] Hereinafter, the details of the embodiment of the germanium recovery method of the present disclosure will be described with reference to the drawings. FIG. 1 is a diagram showing the flow of the germanium recovery method according to this embodiment. The germanium recovery method is a method for recovering germanium from exhaust gas discharged when manufacturing optical fiber (optical fiber manufacturing S0). This method includes a first step S1 of separating germanium-containing dust from exhaust gas discharged in optical fiber manufacturing S0, a second step S2 of extracting germanium from the germanium-containing dust separated in the first step S1 into a liquid to obtain a germanium extract, a third step S3 of separating the germanium extract obtained in the second step S2 from a solid residue, a fourth step S4 of precipitating a germanium-containing compound from the germanium extract separated in the third step S3, and a fifth step S5 of separating the germanium-containing compound precipitated in the fourth step S4 from the liquid.

[0030] Optical fiber manufacturing S0 refers to the process of manufacturing a glass body that will be the base material of an optical fiber by the MCVD method, OVD method, VAD method, etc. When germanium is added as a raw material for the glass body in optical fiber manufacturing S0, germanium-containing particles that are not layered on the glass body are discharged as exhaust gas together with other undeposited particles and gases.

[0031] The first step S1 is a step of separating germanium-containing dust including germanium-containing particles and other undeposited particles from the exhaust gas discharged from the optical fiber manufacturing S0. In the first step S1, the germanium-containing dust may be collected from the exhaust gas using a wet dust collector or a dry dust collector. As the wet dust collector, a scrubber, a wet electric dust collector, etc. may be adopted. As the dry dust collector, a cyclone, a dry electric dust collector, a bag filter, a ceramic filter, etc. may be adopted. The wet dust collector may increase the amount of slurry when collecting the germanium-containing dust, so there is a risk of making the equipment large. The dry dust collector may be smaller than the wet dust collector because there is no concern of increasing the amount of slurry. The dry dust collector may be a facility with excellent heat resistance according to the temperature and amount of the exhaust gas. The exhaust gas from which the germanium-containing dust has been collected is detoxified by the exhaust gas treatment AS1. The exhaust gas treatment AS1 may adopt a known means.

[0032] The second step S2 is a step of extracting germanium from the germanium-containing dust separated in the first step S1 into a liquid to obtain a germanium extract. When dry dust collection is used in the first step S1, the collected germanium-containing dust is mixed with a liquid such as industrial water to form a slurry. When wet dust collection is used in the first step S1, the slurry of the collected germanium-containing dust may be used as is. The pH of the slurry obtained by this method is usually 3 or less. By forming a slurry, germanium can be extracted into the liquid to obtain a germanium extract.

[0033] In the second step S2, germanium may be further extracted from the germanium-containing dust into the liquid by adjusting the pH or temperature of the slurry containing the germanium-containing dust. By adopting a method of adjusting the pH or temperature of the slurry in the second step S2, germanium can be efficiently extracted into the liquid even by an inexpensive method.

[0034] When adjusting the pH of the slurry, the pH of the slurry may be adjusted to an alkaline range, for example, the pH may be adjusted to 8.5 or more and 11 or less. The pH may also be adjusted to 9.5 or more and 11 or less. By making the slurry alkaline, a part of the silica containing a trace amount of germanium can be dissolved, and germanium can be further extracted into the liquid. The pH of the slurry may also be adjusted to a strong alkaline range (for example, pH=10) and then to an acidic or neutral range again.

[0035] When adjusting the temperature of the slurry, the temperature of the slurry may be adjusted to 30° C. or more and 100° C. or less. The temperature of the slurry may also be adjusted to 30° C. or more and 95° C. or less, 50° C. or more and 95° C. or less, or even 60° C. or more and 95° C. or less. By increasing the temperature of the slurry, the germanium-containing dust can be suitably dispersed in the slurry, and germanium can be further extracted into the liquid.

[0036] The third step S3 is a step of separating the germanium extract obtained in the second step S2 from the solid residue. The solid residue is a solid mainly composed of silica. As a method of separation, methods such as sedimentation, vacuum filtration, and pressure filtration can be adopted. Pressure filtration such as a filter press may be adopted considering that the silica particles are fine particles. When vacuum filtration or pressure filtration is adopted, the recovery rate of germanium can be improved by washing the cake after solid-liquid separation. In addition, by washing the cake, the silica particles of the solid residue can be effectively used as high-purity silica. The separated solid residue is collected as sludge (sludge collection AS3).

[0037] The germanium extract solution separated in the third step S3 may be immediately transported to the next step (fourth step S4) for processing, or may be stored in a stock tank. In addition, when germanium-containing dust is collected from the exhaust gas using a dry dust collector in the first step S1, the germanium extract solution separated in the third step S3 may be used to form a germanium-containing dust slurry in the second step S2. In the slurry, germanium can be extracted into the liquid. For example, when the fourth step S4, which is the next step of the third step S3, is not continuous and the germanium extract solution separated in the third step S3 is stored in a stock tank, the amount of waste liquid finally generated can be reduced by performing germanium extraction using the stored extract solution.

[0038] Here, one embodiment of the second step S2 and the third step S3 will be described with reference to FIG. 2. FIG. 2 is a diagram showing the second step S2 and the third step S3 according to the embodiment. In FIG. 2, an extraction tank 20 for carrying out the second step S2, a filter press 30 for carrying out the third step S3, a germanium extract stock tank 40 for storing the germanium extract 42 separated in the third step S3, and a precipitation tank 50 for carrying out the fourth step S4 at the latter stage are shown. The extraction tank 20, the filter press 30, the germanium extract stock tank 40, and the precipitation tank 50 are connected by a liquid delivery pipe as shown in FIG. 2.

[0039] The extraction tank 20 includes an agitating blade 22, a dust inlet 24, and a reagent inlet 26. In the second step S2, the germanium-containing dust 10 collected by the dry dust collector in the first step S1 is introduced into the extraction tank 20 through the dust inlet 24. Then, industrial water is introduced into the extraction tank 20 through an inlet (not shown) and stirred with the agitating blade 22 to prepare a slurry 28. An alkaline reagent such as sodium hydroxide is introduced into the slurry 28 through the reagent inlet 26, thereby allowing germanium to be further dissolved into the liquid.

[0040] In the third step S3, the slurry 28 prepared in the extraction tank 20 is pressure filtered by the filter press 30 to separate the germanium extract 42 and the solid residue 32. The separated germanium extract 42 may be transported to the germanium extract stock tank 40, or may be directly transported to the subsequent precipitation tank 50. In addition, the germanium extract 42 may be transported from the germanium extract stock tank 40 to the extraction tank 20, and the germanium extract 42 may be used to adjust the slurry 28.

[0041] Returning to FIG. 1, the fourth step S4 and the fifth step S5 will be described. The fourth step S4 is a step of precipitating a germanium-containing compound from the germanium extract separated in the third step S3. In the fourth step S4, the germanium extract may be concentrated or a metal salt may be added to the germanium extract to precipitate the germanium-containing compound from the germanium extract. In the fourth step S4, the germanium extract may be concentrated or a metal salt may be added to the germanium extract to cause coprecipitation, thereby efficiently precipitating the germanium-containing compound by inexpensive means.

[0042] When a metal salt is added in the fourth step S4, a soluble metal salt that forms a composite oxide with germanium or a sparingly soluble metal salt that adsorbs germanium may be added. When a soluble metal salt is added, germanium dissolved in the germanium extract can be recovered in high yield as a composite oxide with the added metal. When a sparingly soluble metal salt is added, germanium can be collected on the surface of the sparingly soluble metal salt to precipitate a germanium-containing compound, and germanium dissolved in the germanium extract can be recovered in high yield.

[0043] When a metal salt is added in the fourth step S4, an alkaline earth metal salt may be added. By adding an alkaline earth metal salt, a germanium-containing compound can be favorably precipitated, and germanium dissolved in the germanium extract can be recovered with a high yield. Examples of alkaline earth metal salts include magnesium chloride, magnesium sulfate, and calcium chloride. The amount of alkaline earth metal added may be 0.8 equivalents or more and 3.0 equivalents or less relative to the germanium equivalent in the liquid. By adding an alkaline earth metal salt in the above amount, it is possible to induce the production of not only a composite oxide containing germanium and an alkaline earth metal in an equivalent ratio of 1:1, but also a composite oxide containing a large amount of alkaline earth metal in the equivalent ratio, and it is possible to recover germanium dissolved in the germanium extract with a high yield.

[0044] The fifth step S5 is a step of separating the germanium-containing compound precipitated in the fourth step S4 from the liquid. As a method of separation, the methods mentioned in the third step S3 can be adopted. The separated germanium-containing compound is recovered (germanium recovery BS5), and the separated liquid is treated as waste liquid as necessary (waste liquid treatment AS5).

[0045] In the fifth step S5, the germanium-containing compound may be subjected to a classification process to separate the germanium-containing compound into a coarse particle fraction and a fine particle fraction. For the classification process, a thickener, a liquid cyclone, a centrifuge, etc. may be used. After the coarse particle fraction and the fine particle fraction are separated, solid-liquid separation of the coarse particle fraction may be performed by a method such as settling separation, reduced pressure filtration, or pressure filtration to recover the coarse particle fraction of the germanium-containing compound (germanium recovery BS5). The liquid containing the fine particle fraction of the germanium-containing compound may be returned to the fourth step S4 and mixed with the germanium extract for recycling. By returning the classified fine particle fraction of the germanium-containing compound to the fourth step S4, the fine particle fraction functions as a seed crystal when precipitating the germanium-containing compound. This can induce the precipitation of the germanium compound, and the germanium dissolved in the germanium extract can be recovered at a high yield.

[0046] Here, one aspect of the fourth step S4 and the fifth step S5 will be described with reference to FIG. 3. FIG. 3 is a diagram showing the fourth step S4 and the fifth step S5 according to one aspect of the embodiment. In FIG. 3, a precipitation tank 50 for carrying out the fourth step S4, a filter press 60 for carrying out the fifth step S5, a fine particle-containing liquid stock tank 70 for storing a fine particle-containing liquid 72, which is a liquid containing fine particles of the classified germanium-containing compound, and a waste liquid treatment facility 80 for carrying out the subsequent waste liquid treatment AS5 are shown. The fine particle-containing liquid stock tank 70, the precipitation tank 50, the filter press 60 and the waste liquid treatment facility 80 are connected by a liquid delivery pipe as shown in FIG. 3.

[0047] The precipitation tank 50 is equipped with an agitator blade 52, a germanium extract inlet (not shown), and a reagent inlet 56. In the fourth step S4, the germanium extract separated in the third step S3 is introduced into the precipitation tank 50 from the germanium extract inlet. Then, a metal salt such as magnesium chloride is introduced into the germanium extract via the reagent inlet 56, whereby a germanium-containing compound 62 is precipitated, and a slurry 58 containing the germanium-containing compound 62 is prepared.

[0048] In the fifth step S5, the slurry 58 prepared in the precipitation tank 50 is filtered under pressure by the filter press 60, so that the germanium-containing compound 62 and the liquid are separated, and the germanium-containing compound 62 can be recovered. The separated liquid is transported to a waste liquid treatment facility 80 as necessary. Note that before the slurry is transported from the precipitation tank 50 to the filter press 60, a classification process may be performed to separate the germanium-containing compound into coarse particles and fine particles. The liquid containing the coarse particles of the germanium-containing compound 62 may be transported to the filter press 60 and filtered under pressure, and the liquid containing the fine particles of the germanium-containing compound 62 (fine particle-containing liquid 72) may be stored in a fine particle-containing liquid stock tank 70. Then, in the fourth step, the fine particle-containing liquid 72 may be supplied from the fine particle-containing liquid stock tank 70 to the precipitation tank 50, and the fine particles may be used as seed crystals of the germanium-containing compound.

[0049] Next, another aspect of the fourth step S4 and the fifth step S5 will be described with reference to Fig. 4. Fig. 4 is a diagram showing the fourth step S4 and the fifth step S5 according to another aspect of the embodiment. In Fig. 4, a germanium extract stock tank 140 for storing the germanium extract 142 separated in the third step S3, an evaporator 150 for carrying out the fourth step S4, and a filter press 160 for carrying out the fifth step S5 are shown. The germanium extract stock tank 140, the evaporator 150, and the filter press 160 are connected by a liquid delivery pipe as shown in Fig. 4.

[0050] The evaporator 150 receives the germanium extract 142 from the germanium extract stock tank 140, evaporates the liquid to prepare a concentrated liquid 152, and precipitates the germanium-containing compound 162 (fourth step S4). The liquid containing the precipitated germanium-containing compound 162 is transported to a filter press 160, and the germanium-containing compound 162 is separated from the liquid, so that the germanium-containing compound 162 can be recovered (fifth step S5). The liquid separated in the fifth step S5 may be returned to the germanium extract stock tank 140.

[0051] According to the germanium recovery method described above, germanium is extracted from the germanium-containing dust into a liquid, and a germanium-containing compound is precipitated from the obtained extract, without relying on a method for separating the germanium-containing dust from the exhaust gas, thereby making it possible to efficiently and inexpensively recover germanium contained in the exhaust gas. EXAMPLES

[0052] Specific examples (study examples) according to the present disclosure will be described below, but the present disclosure is not limited to these examples. Example 1 (Study of the second process S2) The difference in the dissolution rate of germanium into liquid depending on the temperature of water when preparing a slurry of germanium-containing dust was evaluated. 10 g of soot (germanium-containing dust) discharged during the manufacture of an optical fiber preform was dispersed in 50 mL of water (20°C or 95°C) to form a slurry. The analytical value of the germanium concentration of the soot was 0.39 mass%. The concentration of germanium contained in the slurry liquid was measured, and the ratio (%) of the germanium concentration when the germanium contained in the soot was completely dissolved was calculated as the dissolution rate. The dissolution rate of germanium was 88% when water at 20°C was used, while the dissolution rate of germanium was 94% when water at 95°C was used. The pH of the slurry was 2.4 in both cases. It was shown that the dissolution of germanium is promoted by forming a slurry of soot using hot water. In addition, the dispersion of soot was better and it was easier to form a slurry when hot water was used.

[0053] Example 2 (Study of the second process S2) The difference in the elution rate of germanium into liquid due to pH when preparing a slurry of germanium-containing dust was evaluated. The soot and elution rate used in the evaluation were the same as those explained in Example 1. 10 g of soot (germanium-containing dust) discharged during the manufacture of an optical fiber preform was dispersed in 50 mL of water (20°C) to form a slurry. The pH of the slurry was then adjusted. The concentration of germanium in the slurry liquid after pH adjustment was measured, and the elution rate was calculated. The results are shown below.

[0054] Adjusted pH Elution rate (%) Example 2-1 1.0 88 Example 2-2 2.4 88 Example 2-3 3.6 86 Example 2-4 5.0 87 Example 2-5 8.2 88 Example 2-6 10.0 94

[0055] It was shown that the dissolution of germanium was promoted in a slurry with a pH of 10.0.

[0056] Example 3 (Study of the second process S2) After adjusting the pH of the germanium-containing dust slurry to 10, the change in the elution rate of germanium into the liquid when the pH was lowered was evaluated. The pH of the slurry of Example 2-6 was further adjusted, and the concentration of germanium contained in the slurry liquid after the pH adjustment was measured, and the elution rate was calculated. The results are shown below.

[0057] Final pH elution rate (%) Example 3-1 1.0 94 Example 3-2 3.0 93 Example 3-3 4.8 91

[0058] It was shown that the germanium concentration in the solution was maintained even when the pH of the slurry was adjusted to 10, etching was performed, and the final pH was adjusted to an acidic value. In addition, a comparison of cases where etching was performed (Examples 3-1 to 3-3) with cases where no etching was performed (Examples 2-1 to 2-4) showed that etching improved the elution rate of germanium.

[0059] Example 4 (Study of the fourth step S4) A method for precipitating a germanium-containing compound by concentrating a germanium extract was studied. 1.5 kg of soot (germanium-containing dust) identical to that used in Example 1 was dispersed in 7.5 L of pure water to prepare a slurry. The slurry was filtered, and 4.0 L of the filtrate was concentrated to 750 mL to prepare a germanium extract as a sample for evaluation. The pH of the germanium extract was 2.1, and the analytical value of the germanium concentration was 3.5 g / L. When this sample for evaluation was further concentrated four times, a solid precipitated. The pH of the four-fold concentrated solution was 1.33, and the analytical value of the germanium concentration of the four-fold concentrated solution was 10.4 g / L. This indicated that the germanium in the sample precipitated as a solid.

[0060] Example 5 (Study of the fourth step S4) A method for precipitating germanium-containing compounds by adding metal salts to a germanium extract was investigated. The same sample as the evaluation sample prepared in Example 4 was prepared. When 3.16 g of magnesium chloride hexahydrate (MgCl2·6H2O) was added to 200 mL of this sample (germanium content 0.50 g, converted to germanium oxide amount 1.02 g), a solid precipitated. The final pH of the liquid was adjusted to 10.0. The analytical value of the germanium concentration in the adjusted liquid was 88.3 mg / L. This indicated that the germanium in the sample precipitated as a solid. It was also shown that the amount of germanium precipitated as a solid was greater in the method of adding metal salts than in the method of concentration.

[0061] The present disclosure has been described above based on specific embodiments and examples, but the present invention is not limited to these examples, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0062] 10: germanium-containing dust, 20: extraction tank, 22, 52: stirring blade, 24: dust inlet, 26, 56: reagent inlet, 28, 58: slurry, 30, 60, 160: filter press, 32: solid residue, 40, 140: germanium extract stock tank, 42, 142: germanium extract, 50: precipitation tank, 62, 162: germanium-containing compound, 70: fine particle-containing liquid stock tank, 72: fine particle-containing liquid, 80: waste liquid treatment equipment, 150: evaporator, 152: concentrated liquid

Claims

1. A method for recovering germanium from exhaust gas discharged during the manufacture of optical fiber, comprising the steps of: a first step of separating germanium-containing dust, including germanium-containing particles and other undeposited particles, from said exhaust gas; a second step of extracting germanium from the germanium-containing dust separated in the first step into a liquid to obtain a germanium extract; a third step of separating the germanium extract obtained in the second step from a solid residue; a fourth step of precipitating a germanium-containing compound from the germanium extract separated in the third step; a fifth step of separating the germanium-containing compound precipitated in the fourth step from a liquid.

2. In the first step, the germanium-containing dust is collected from the exhaust gas using a wet dust collector or a dry dust collector; In the second step, the pH of the slurry containing the germanium-containing dust is adjusted to extract germanium from the germanium-containing dust into a liquid; In the fourth step, the germanium extract is concentrated or a metal salt is added to the germanium extract to precipitate the germanium-containing compound from the germanium extract. The method for recovering germanium according to claim 1.

3. In the second step, the pH of the slurry is adjusted to 8.5 or more and 11 or less to extract germanium into the liquid. The method for recovering germanium according to claim 2.

4. The germanium extract separated by the third step is stored in a stock tank, Then, in the first step, the germanium-containing dust is collected from the exhaust gas using a dry dust collector; In the second step, the germanium-containing dust collected by the dry dust collector is added to the germanium extract stored in the stock tank to extract germanium into the liquid. The method for recovering germanium according to claim 2 or 3.

5. In the fourth step, a soluble metal salt that forms a composite oxide with germanium is added to precipitate the germanium-containing compound from the germanium extract. The method for recovering germanium according to any one of claims 2 to 4.

6. In the fourth step, an alkaline earth metal salt is added to precipitate the germanium-containing compound from the germanium extract. The method for recovering germanium according to any one of claims 2 to 4.

7. The amount of the alkaline earth metal salt added in the fourth step is 0.8 equivalents or more and 3.0 equivalents or less with respect to the germanium equivalent in the liquid. The method for recovering germanium according to claim 6.

8. In the fifth step, the germanium-containing compound is subjected to a classification treatment to separate it into a coarse particle fraction and a fine particle fraction, the coarse particle fraction is recovered, and the fine particle fraction is returned to the fourth step for recycling. The method for recovering germanium according to any one of claims 2 to 7.

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