Selenium recovery methods

Abstract

To provide a method for recovering selenium from selenium compounds that have become difficult to reduce (difficult-to-reduce selenium compounds) generated by the addition of ketones in the wet treatment of electrolytic lees.SOLUTION: A method for recovering selenium in which an acidic solution added with selenic acid is heated to 60°C or above, and an oxidizing agent is added thereto, followed by the addition of a reducing agent to obtain precipitate. The resulting precipitate is subjected to solid-liquid separation, and selenium contained in the liquid after the solid-liquid separation is recovered.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for recovering selenium, and particularly to a method for recovering selenium from an acidic solution containing selenous acid to which ketones are added.

Background Art

[0002] In copper pyrometallurgy, copper concentrate is melted, and after obtaining crude copper with a purity of 99% or more in a converter and a refining furnace, electrolytic copper with a purity of, for example, 99.99% or more is produced in an electrolytic refining process. Valuable substances other than copper precipitate as slime during electrolytic refining.

[0003] These slimes also simultaneously concentrate noble metals, rare metals, selenium, and tellurium contained in copper concentrate. These elements are separately separated and recovered as by-products of copper smelting.

[0004] In many cases, a hydrometallurgical method is applied to the treatment of this slime. For example, in Patent Document 1, a method is disclosed in which slime is treated with hydrochloric acid - hydrogen peroxide to recover silver, dissolved gold is recovered by solvent extraction, and then other valuable substances are sequentially reduced and recovered with sulfur dioxide. Patent Document 2 discloses a method in which gold and silver are recovered in the same manner, then valuable substances are reduced and precipitated with sulfur dioxide, and only selenium is removed by distillation to concentrate noble metals.

[0005] In particular, the method for recovering the precipitate produced by sulfur dioxide shown in Patent Document 1 has many advantages in terms of cost and production scale. The advantage of using sulfur dioxide is mainly due to the smelting gas by-produced in metal smelting.

[0006] However, metal smelting equipment needs to be repaired and maintained regularly, and during that time, the operation is stopped for several weeks to several months. In addition, the operation may be stopped due to sudden accidents or failures. During this stop period, since the supply of sulfur dioxide stops, another reducing agent must be used. Or, sulfur dioxide may be purchased as a commercially available industrial gas and used, but then the cost increases.

[0007] As a reducing agent to replace smelting gases, inexpensive and low-toxicity compounds are preferred. Furthermore, stable performance is desirable. Ketones are one such substance that meets these conditions. Ketones react with selenite to precipitate elemental selenium. Among ketones, acetone is ideal in terms of both price and low toxicity.

[0008] After selenite reacts with ketones, the ketones become aldehydes. Selenite is reduced by its reaction with ketones. Furthermore, it is reduced by the aldehyde, an intermediate product formed when the ketone is oxidized, and selenium precipitates in both reactions. Additionally, aldehydes and ketones have the advantage of hardly reducing tellurite, a chalcogen compound of the same genus, under acidic conditions. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2001-316735 [Patent Document 2] Japanese Patent Publication No. 2004-190134 [Patent Document 3] Japanese Patent Publication No. 2019-077902 [Overview of the project] [Problems that the invention aims to solve]

[0010] When using acetone as a reducing agent to reduce selenite, the required amount of acetone is approximately 0.8 L per 1 kg of selenium. In copper electrolytic precipitate treatment, the selenium solution is 30 g / L or more, and if all is reduced with acetone, the volume of liquid per batch is 15-18 m³. 3 Therefore, 300-400 liters of acetone will be needed.

[0011] Generally, this reaction takes place at a liquid temperature of 70°C or higher, so some of the acetone after the reaction is decomposed into carbon dioxide, while other acetone becomes harmless volatile organic compounds and is transferred to the exhaust gas treatment process. However, some acetone reacts with selenite and remains in the liquid as a poorly reducing substance (reaction product). The form of this reaction product remaining in the liquid is unknown, but it is not easily reduced by sulfur dioxide.

[0012] On the other hand, strict discharge standards are set for selenium. The selenium-reduced liquid is further treated in the wastewater treatment process, such as coagulation and sedimentation, before being discharged, but the burden related to selenium concentration becomes large. Patent document 3 discloses a method for removing selenium using activated carbon, but this is also not sufficient. Despite the need to reduce the selenium concentration in the liquid, no suitable treatment method for selenium after reaction with ketones has been known to date.

[0013] In view of these conventional circumstances, the present invention provides a method for recovering selenium from selenium compounds that have become less susceptible to reduction due to the addition of ketones (hereinafter also referred to as poorly reducible selenium compounds) in the wet treatment of electrolytic precipitates. [Means for solving the problem]

[0014] The inventors of this invention conducted extensive research to solve the above problems and discovered that, for poorly reducible selenium compounds, selenium can be precipitated and recovered by adding an oxidizing agent, heating to a predetermined temperature, and then reducing the compound again. This invention was completed based on this finding.

[0015] In other words, the present invention encompasses the following inventions. (1) A method for recovering selenium, comprising heating an acidic solution containing selenite with added ketones to 60°C or higher, adding an oxidizing agent, then adding a reducing agent, separating the precipitate formed by the solid-liquid separation, and recovering the selenium in the liquid after the solid-liquid separation. (2) The method for recovering selenium according to (1), wherein an acidic solution containing selenite with the ketones added is heated to 65-80°C and the oxidizing agent is added. (3) A method for recovering selenium according to (1) or (2), wherein after adding the oxidizing agent, the temperature of the acidic solution is maintained at 50 to 80°C and the reducing agent is added. (4) The method for recovering selenium according to any one of (1) to (3), wherein the oxidizing agent is an oxidizing agent capable of oxidizing elemental selenium to selenite in the acidic solution. (5) The method for recovering selenium according to any one of (1) to (4), wherein the oxidizing agent is one or more of hydrogen peroxide, hypochlorous acids, and nitric acid. (6) The method for recovering selenium according to any one of (1) to (5), wherein the ketones are acetone. (7) The method for recovering selenium according to any one of (1) to (6), wherein the reducing agent is one or more of sulfur dioxide, sulfites, and metals less noble than selenium. (8) The method for recovering selenium according to (7), wherein the metal less noble than selenium is one or more of iron, copper, and copper-clad iron. (9) The method for recovering selenium as described in (7), wherein the reducing agent is added to the acidic solution in a concentration of 5 g / L or more. (10) The method for recovering selenium according to any one of (1) to (9), wherein the oxidizing agent is added until the oxidation-reduction potential of the acidic solution, when silver-silver chloride is used as the reference electrode, reaches 450 mV or more. (11) The method for recovering selenium according to (10), wherein the oxidizing agent is added until the oxidation-reduction potential of the acidic solution, with silver-silver chloride as the reference electrode, reaches 550 to 850 mV. (12) The method for recovering selenium according to any one of (1) to (11), wherein the reducing agent is added after the oxidation-reduction potential of the acidic solution, which has risen due to the addition of the oxidizing agent, has begun to decline. (13) A method for recovering selenium according to any one of (1) to (12), wherein the addition of the oxidizing agent and the addition of the reducing agent after the addition of the oxidizing agent are repeated until the selenium concentration in the liquid after solid-liquid separation falls below a predetermined value. (14) The method for recovering selenium according to (13), wherein sulfur dioxide is added as a reducing agent after the oxidizing agent is added, and then a metal less noble than selenium is added as a reducing agent after the oxidizing agent is added again.

Advantages of the Invention

[0016] According to the present invention, in the wet treatment of electrolytic deposits, a method for recovering selenium from a selenium compound (selenium compound with low reducibility) that has become difficult to be reduced due to the addition of ketones can be provided.

Brief Description of the Drawings

[0017] [Figure 1] This is a graph showing the relationship between ORP and the amount of hydrogen peroxide added, obtained by extracting the conditions for measuring ORP from Experimental Example 1 and Experimental Example 2 and using them.

Embodiments for Carrying Out the Invention

[0018] Next, embodiments for carrying out the present invention will be described in detail. It should be understood that the present invention is not limited to the following embodiments, and modifications, improvements, etc. of the design can be appropriately added based on the ordinary knowledge of those skilled in the art without departing from the gist of the present invention.

[0019] Electrolytic slime generated in the electrolytic refining process of non-ferrous metal smelting, especially copper smelting, contains a large amount of chalcogen elements and precious metals. For example, it contains about 10 - 30 kg / ton of gold, 100 - 250 kg / ton of silver, 1 - 3 kg / ton of palladium, 200 - 500 g / ton of platinum, and about 5 - 15% by mass of selenium. Platinum group elements and chalcogen elements are not smelted alone but are recovered as by-products of other metals. In particular, they are often recovered from the electrolytic deposits of copper smelting and lead refining.

[0020] By adding hydrochloric acid and hydrogen peroxide to dissolve this electrolytic slime, a dissolution solution of copper electrolytic deposit is generated. However, silver forms an insoluble silver chloride precipitate with chloride ions immediately after dissolution. If it is a solution containing an oxidizing agent and chlorine, such as aqua regia or chlorine water, precious metals can be dissolved and silver can be separated as silver chloride. Since it is a chloride bath, precious metal elements, rare metal elements, selenium, and tellurium are distributed in the leaching pregnant liquor (PLS). Selenium is contained as selenoxonium in the acidic solution, but most of it is selenous acid.

[0021] Hydrogen peroxide is particularly suitable as an oxidizing agent, and hydrochloric acid is preferred as the chlorine-containing solution. This is because hydrogen peroxide decomposes into water and oxygen, so there is no contamination of the recovered material. After dissolving the electrolytic slime by adding hydrochloric acid and hydrogen peroxide, gold, platinum, or palladium can be recovered by solvent extraction or reduction. The recovered solution (platinum group metal recovery solution) contains, for example, 30-40 g / L of selenium and 0.3-1.5 g / L of tellurium.

[0022] Next, ketones are added to reduce and separate selenium from the platinum group metal recovery solution described above. The ketones used are preferably acetone, butanone, or methyl isobutyl ketone, as these are commercially available products. In particular, acetone is most preferred due to its low cost, low toxicity, and ease of handling. Furthermore, the liquid temperature during the reduction and separation of selenium is preferably 65°C or higher. Below 65°C, selenium may precipitate as red selenium or amorphous selenium. Red selenium has a high water content and is unsuitable for distillation purification in the next step. Amorphous selenium is highly viscous and may adhere firmly to the inside of the reaction vessel and piping, potentially causing blockages or malfunctions.

[0023] The liquid remaining after selenium recovery contains tellurium. Tellurium is recovered by heating to over 80°C and bubbling in sulfur dioxide to precipitate it. The liquid remaining after recovering this tellurium contains selenium. Even by bubbling in sulfur dioxide, this selenium-containing compound (a poorly reducing selenium compound) cannot be removed. Depending on the amount of acetone used, the amount of this poorly reducing selenium compound produced is approximately equivalent to 1.5 to 3% by mass of selenium in the input raw material.

[0024] Based on the selenium oxidation reaction, it is expected that poorly reducing selenium compounds will produce substances containing carbon-selenium bonds or carbon-selenium double bonds as side reactions. Generally, the double bond between carbon and selenium has soft unpaired electrons on the selenium atom that are easily oxidized. In the embodiments of the present invention, this property is utilized to oxidize and decompose the selenium in poorly reducing selenium compounds back into selenite using an oxidizing agent.

[0025] In other words, in the embodiment of the present invention, an acidic solution containing selenite, to which ketones have been added to the recovered liquid of gold, platinum, or palladium, is heated to 60°C or higher, an oxidizing agent is added, a reducing agent is added, the resulting precipitate is separated into solid and liquid, and selenium is recovered from the liquid after solid and liquid separation.

[0026] When an oxidizing agent is added to an acidic solution containing selenite with added ketones, the bond between carbon and selenium may be oxidized, changing the oxidation state of selenium from (-II) to (0), causing elemental selenium to precipitate. Therefore, the acidic solution should be preheated to 60°C or higher to prevent selenium from solidifying. From the viewpoint of heating efficiency, the upper limit of the heating temperature of the acidic solution is preferably 100°C or lower. Furthermore, the heating temperature of the acidic solution is preferably between 65 and 80°C.

[0027] The oxidizing agent added to the acidic solution containing selenite with added ketones, heated to 60°C or higher, is preferably an oxidizing agent capable of oxidizing elemental selenium to selenite in an acidic solution, and examples include one or more of hydrogen peroxide, hypochlorous acids, and nitric acid. Hydrogen peroxide is particularly suitable from the viewpoint of equipment load, cost, and wastewater standards.

[0028] It is preferable to reduce the selenite concentration as much as possible before adding an oxidizing agent such as hydrogen peroxide. If an excessive amount of selenite remains, selenium may be produced by the oxidizing agent, potentially requiring an excessive amount of reducing agent during re-reduction. Specifically, it is preferable to adjust the selenite concentration to at least 3 g / L or less before adding the oxidizing agent.

[0029] It is preferable to add the oxidizing agent until the oxidation-reduction potential of the acidic solution, with silver-silver chloride as the reference electrode, reaches 450 mV or higher. If the oxidation-reduction potential is less than 450 mV, the decomposition of the poorly reducible selenium compound may be insufficient. It is even more preferable to add the oxidizing agent until the oxidation-reduction potential of the acidic solution, with silver-silver chloride as the reference electrode, reaches 550 to 850 mV. If the oxidation-reduction potential exceeds 850 mV, the amount of reducing agent used during re-reduction may increase. It is even more preferable that the oxidation-reduction potential is 700 to 800 mV.

[0030] After adding the oxidizing agent, it is preferable to stir the mixture for a while to allow it to react thoroughly. When using hydrogen peroxide or hypochlorous acid as the oxidizing agent, bubbles will form for a while after addition. Once the formation of these bubbles subsides visually, it can be assumed that the oxidation has stopped.

[0031] After oxidation, a reducing agent is added again to precipitate selenium (cementation). The reducing agent is not particularly limited as long as it is a reagent that can reduce selenite to selenium, but examples include one or more of sulfur dioxide, sulfites, and metals less noble than selenium. Among these, cementation with metals less noble than selenium is most preferred in terms of reaction efficiency. Examples of metals less noble than selenium include one or more of iron, copper, and copper-clad iron.

[0032] As in the embodiments of the present invention, when the target of treatment is a metal electrolytic precipitate solution, it contains elements such as arsenic. To prevent the generation of hydrogen arsenide, the above-mentioned cementation is preferably carried out using iron, copper, or copper-coated iron as a reducing agent, and the reaction temperature at that time is preferably 50°C or higher. That is, after adding the oxidizing agent as described above, it is preferable to maintain the temperature of the acidic solution at 50°C or higher before adding the reducing agent. In particular, when using iron powder, it reacts and releases hydrogen when added, so it is preferable to maintain the temperature of the acidic solution at 50-80°C. Cementation may also be carried out with a base metal after reduction with sulfur dioxide. When the amount of oxidizing agent added is excessive, the amount of metal used for cementation can be reduced by reducing it with inexpensive sulfur dioxide.

[0033] It is preferable to add a metal less noble than selenium (reducing agent) in a concentration of 5 g / L or more relative to the acidic solution. If the reducing agent is less than 5 g / L relative to the acidic solution, removal may be insufficient. Furthermore, since too much of the reducing agent increases reagent costs, it is more preferable to add the reducing agent in a concentration of 5 to 15 g / L relative to the acidic solution.

[0034] It is preferable to add the reducing agent after the oxidation-reduction potential of the acidic solution, which has risen due to the addition of the oxidizing agent, has begun to decline. With this configuration, the reducing agent is added after sufficient oxidation has occurred, which is when the oxidation-reduction potential of the acidic solution, which has risen due to the addition of the oxidizing agent, begins to decline, thus improving the reduction efficiency.

[0035] It is preferable to repeat the addition of an oxidizing agent and the addition of a reducing agent after the addition of the oxidizing agent until the selenium concentration in the liquid after solid-liquid separation falls below a predetermined value. This "predetermined value" is not particularly limited, but for example, it can be 100 mg / L or less. Furthermore, it is preferable to first add sulfur dioxide as a reducing agent after adding the oxidizing agent, and then add a metal less noble than selenium as a reducing agent after adding the oxidizing agent again. With this configuration, reduction is performed first with less expensive sulfur dioxide, and then a more expensive but highly efficient reducing agent can be used when reducing again, which is preferable from a cost standpoint.

[0036] After the reduction treatment, the precipitated selenium-containing material is separated into solid and liquid phases using a filter press or similar method. Depending on the composition of the target liquid, if the recovered selenium-containing material also contains valuable metals such as ruthenium and iridium, it can be used as a raw material for smelting. Furthermore, the purity of the recovered selenium can be increased by further distillation. [Examples]

[0037] The following examples illustrate the present invention and its advantages, but the present invention is not limited to these examples.

[0038] <Preparation of the treatment solution (acidic solution containing selenite with added ketones)> After removing copper from the electrolytic slime recovered from copper smelting by treating it with sulfuric acid, concentrated hydrochloric acid and 60% hydrogen peroxide solution were added to dissolve it, and solid-liquid separation was performed to obtain leached noble liquor (PLS). Next, the PLS was cooled to 6°C to remove base metal components by precipitation, and then the DBC (dibutyl carbitol) and PLS were mixed to extract gold. Next, the PLS after gold extraction was heated to 70°C, and a mixture of sulfur dioxide and air (sulfur dioxide concentration 5-20%) was blown in to reduce the precious metals and separate the solids and liquids. Furthermore, 3 mL of acetone was added to 300 mL of the stock solution and stirred for 2 hours. Finally, a mixture of sulfur dioxide and air was blown in again for 1 hour, and the precipitated selenium was filtered off. This is referred to as the treated solution. The treated solution contained 0.45 g / L of selenium, 1.9 g / L of arsenic, and 1.2 g / L of copper. The oxidation-reduction potential (ORP) was 433 mV.

[0039] (Test Example 1) 200 mL of the above-mentioned treatment solution was taken, heated to 70°C, and a predetermined amount of sodium hypochlorite solution (5% or more available chlorine), nitric acid (60% by mass), or hydrogen peroxide solution (30% by mass) was added, followed by stirring for 30 minutes. In the system to which hydrogen peroxide solution was added, the oxidation-reduction potential (ORP, reference electrode: silver-silver chloride) was also measured. Next, a mixture of sulfur dioxide and air was blown into the liquid to be treated, and the blowing of the mixture was stopped after 60 minutes. Next, 2 mL of the treatment solution was taken, adjusted to 50 mL with dilute hydrochloric acid, and the selenium concentration was quantified using ICP-OES (SPS3100, manufactured by Seiko Instruments Inc.). The results are shown in Table 1.

[0040] [Table 1]

[0041] When sulfur dioxide alone was blown in without adding an oxidizing agent, selenium was hardly precipitated or removed. On the other hand, it was found that re-reduction with sulfur dioxide after adding an oxidizing agent was highly effective. When hydrogen peroxide was used as the oxidizing agent, the effect was observed from the system with 0.2 mL added, and there was no significant change in the final selenium concentration when 2 mL or more was added.

[0042] The required amount of hydrogen peroxide varies depending on the selenium concentration and the concentration of other reducing substances such as iron(II). The effect was observed when hydrogen peroxide was added until the ORP (Oxidation Rate) in the solution reached 450 mV or higher. The effect became even greater when the ORP exceeded 700 mV.

[0043] (Test Example 2) 200 mL of the same treatment solution as in Test Example 1 was taken, heated to 60-65°C, a predetermined amount of hydrogen peroxide solution (30% by mass) was added, and the mixture was stirred for 30 minutes. The ORP was then measured. Next, predetermined amounts of copper powder, iron powder, and iron powder coated with copper (copper content of 30% by mass and 60% by mass) were added, and the mixture was further stirred and reduced. When using copper powder, the reduction was carried out at 80°C, and when using iron powder, it was carried out at 50°C. To prepare copper-coated iron powder, copper(II) sulfate pentahydrate was dissolved in dilute sulfuric acid to create a 10 g / L copper solution. Commercially available iron powder (average particle size P80 = 200 μm) was added and the mixture was stirred. Here, "P80" refers to the particle size at which 80% of the powder passes through a sieve. Copper-coated iron powders with various copper content were prepared by manipulating the reaction time, and after solid-liquid separation, they were air-dried and used in experiments. Next, the reduction reaction was stopped after 1 hour, and solid-liquid separation was performed to quantify the various elements in the liquid. The quantification method was the same as in Test Example 1. The results are shown in Table 2.

[0044] [Table 2]

[0045] Comparing Test Example 1 and Test Example 2, it can be seen that re-reduction is more efficient with metal cementation. The metal can be either iron or copper-coated iron powder, but in the case of iron, hydrogen gas is generated when added to an acidic solution, which may cause overflow. Furthermore, selenium cementation with copper, a relatively noble metal, was also possible. The amount of metal required for cementation was 1g or more, which corresponds to 5g / L.

[0046] Figure 1 shows the ORP measurement conditions extracted from Test Example 1 and Test Example 2, plotted in graph form, with ORP and hydrogen peroxide added. Figure 1 shows a sharp increase in ORP above 550 mV, indicating that hydrogen peroxide no longer undergoes immediate reduction in the solution and has begun to exist in a consistent amount. It is considered that the poorly reducing selenium compound is decomposed when an oxidizing agent is added to raise the ORP above 550 mV.

[0047] In Experiment Example 2, there is an experiment in which ORP was not measured, but it is thought that the ORP reached approximately 710-770 mV when 2 mL of hydrogen peroxide was added.

[0048] (Test Example 3) 200 mL of the same treatment solution as in Test Example 1 was taken, heated to the specified temperature shown in Table 3, a predetermined amount of hydrogen peroxide solution (30% by mass) was added, and the mixture was stirred for 30 minutes. The ORP was then measured. Next, 2g of iron powder coated with copper (copper content 60%) was added, the temperature was adjusted to the specified level, and then the mixture was further stirred and reduced. Next, the reduction reaction was stopped after 1 hour, and after solid-liquid separation, the various elements in the liquid were quantified. The quantification method was the same as in Test Example 1. The results are shown in Table 3.

[0049] [Table 3]

[0050] The results in Table 3 show that the final selenium concentration decreases as the amount of hydrogen peroxide added increases. In particular, when the ORP during oxidation exceeded 550 mV, selenium was efficiently reduced and removed. However, the effect hardly changed above a certain ORP. In addition to poorly reducible selenium compounds, other reducible substances such as arsenite were also present, which is thought to have contributed to the consumption of hydrogen peroxide.

[0051] Higher temperatures were more effective during both oxidation and reduction. Temperatures above 60°C were sufficient for both oxidation and reduction.

[0052] Based on the results of Test Examples 2 and 3, it is expected that if 2 mL of hydrogen peroxide is added and the solution temperature is 60°C or higher, the ORP will initially exceed 800 mV. As the reaction progresses, the ORP is expected to gradually decrease due to the disappearance of hydrogen peroxide in the solution. Therefore, when the ORP, which had initially risen, begins to decline, it can be assumed that the oxidation of the poorly reducing selenium compound has progressed to a certain extent or more.

[0053] Under all conditions, the selenium concentration in the liquid decreased significantly. However, to further improve these values, repeated oxidation and reduction can reduce the selenium concentration even more. Furthermore, after oxidation, it is possible to treat the solution using the coagulation-sedimentation method, a common method for treating selenium in wastewater.

Claims

1. A method for recovering selenium, comprising heating an acidic solution containing selenite with added ketones to 60°C or higher, adding an oxidizing agent, then adding a reducing agent, separating the resulting precipitate into solid and liquid phases, and recovering the selenium in the liquid after the solid-liquid separation.

2. A method for recovering selenium according to claim 1, comprising heating an acidic solution containing selenite with the ketones added to 65 to 80°C and adding the oxidizing agent.

3. The method for recovering selenium according to claim 1, wherein after adding the oxidizing agent, the temperature of the acidic solution is maintained at 50 to 80°C and the reducing agent is added.

4. The method for recovering selenium according to claim 1, wherein the oxidizing agent is an oxidizing agent capable of oxidizing elemental selenium to selenite in the acidic solution.

5. The method for recovering selenium according to claim 1, wherein the oxidizing agent is one or more of hydrogen peroxide, hypochlorous acids, and nitric acid.

6. The method for recovering selenium according to claim 1, wherein the ketones are acetone.

7. The method for recovering selenium according to claim 1, wherein the reducing agent is one or more of sulfur dioxide, sulfites, and metals having a standard oxidation-reduction potential of less than 0.74 V.

8. The method for recovering selenium according to claim 7, wherein the metal having a standard oxidation-reduction potential of less than 0.74 V is one or more of iron, copper, and copper-clad iron.

9. The method for recovering selenium according to claim 7, wherein the reducing agent is added to the acidic solution in a concentration of 5 g / L or more.

10. The method for recovering selenium according to claim 1, wherein the oxidizing agent is added until the oxidation-reduction potential of the acidic solution, with silver-silver chloride as the reference electrode, reaches 450 mV or more.

11. The method for recovering selenium according to claim 10, wherein the oxidizing agent is added until the oxidation-reduction potential of the acidic solution, with silver-silver chloride as the reference electrode, reaches 550 to 850 mV.

12. The method for recovering selenium according to claim 1, wherein the reducing agent is added after the oxidation-reduction potential of the acidic solution, which has risen due to the addition of the oxidizing agent, has begun to decline.

13. The method for recovering selenium according to claim 1, wherein the addition of the oxidizing agent and the addition of the reducing agent after the addition of the oxidizing agent are repeated until the selenium concentration in the liquid after solid-liquid separation falls below a predetermined value.

14. A method for recovering selenium according to claim 13, wherein sulfur dioxide is added as a reducing agent after the oxidizing agent is added, and then a metal having a standard oxidation-reduction potential of less than 0.74 V is added as a reducing agent after the oxidizing agent is added again.

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