How to recover selenium

Abstract

To provide a method for recovering as sandy black color selenium, wherein, when selenium is precipitated and recovered from a hydrochloric acid acidic solution containing selenious acid, sulfur dioxide is not used as a reducing agent, or, even when the sulfur dioxide is used as a reducing agent, its use amount is suppressed.SOLUTION: There is provided a method of recovering selenium by reducing and precipitating from a hydrochloric acid acidic solution containing selenious acid, wherein the hydrochloric acid acidic solution is heated to a liquid temperature of 70°C or more such that an average reducible rate of selenious acid is 120 mg / L or less per minute as selenium, and the reducing agent excluding the sulfur dioxide is added.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for recovering selenium, and more particularly to a method for recovering selenium from a hydrochloric acid solution containing selenious acid. [Background technology]

[0002] In copper pyrometallurgy, copper concentrate is melted and processed into crude copper of 99% purity or more in a converter and a refining furnace, after which electrolytic refining produces electrolytic copper with a purity of, for example, 99.99% or more. In recent years, metal scrap containing precious metals from electronic components has been added to converters as recycled raw materials, and valuable materials other than copper are precipitated as slime during electrolytic refining.

[0003] This slime also contains precious metals, rare metals, and the selenium and tellurium contained in copper concentrate. These elements are separated and recovered individually as by-products of copper smelting.

[0004] Hydrometallurgy is often used to treat this slime. For example, Patent Document 1 discloses a method in which silver is recovered from slime using hydrochloric acid and hydrogen peroxide, the dissolved gold is recovered by solvent extraction, and then other valuables are sequentially reduced and recovered using sulfur dioxide. Patent Document 2 discloses a method in which gold and silver are recovered using a similar method, and then the valuables are reduced and precipitated using sulfur dioxide, and only the selenium is removed by distillation to concentrate the precious metals.

[0005] The solution left after the precious metals have been recovered contains rare metal ions, tellurium, and selenium, and these valuable materials must be further recovered. Known recovery methods include recovering the precipitate formed by the reduction agent, or mixing the solution with copper concentrate, drying it in a dryer, and then returning it to the smelting furnace.

[0006] In particular, the method of recovering precipitates produced by sulfur dioxide, as shown in Patent Document 1, has many advantages in terms of cost and production scale. In addition, it is also effective for separation and purification, as each element precipitates sequentially according to its redox potential.

[0007] The sulfur dioxide that reduces selenious acid, which is produced when slime is treated using hydrometallurgy, is generally produced using exhaust gas from dry smelting. This is because it has a significant cost advantage and becomes sulfate ions after the reaction, which does not adversely affect the wastewater.

[0008] As described in Non-Patent Document 1, selenium takes on various forms when reduced in an aqueous solution. Immediately after reduction, it precipitates as red selenium, but changes to various forms depending on the temperature. Among the forms of selenium, red selenium is easy to obtain but has poor filterability and a high moisture content, black rubbery selenium is sticky, and black coke selenium is a fine powder with a low moisture content. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-316735 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-190134 [Non-patent literature]

[0010] [Non-Patent Document 1] Kunio Usuki, Hiroshi Kinoshita, Denichi Imamura Journal of the Mining Industry of Japan 88 309 (1972) Summary of the Invention [Problem to be solved by the invention]

[0011] When producing selenium, the precipitate obtained after reducing selenious acid must be distilled to increase its purity. During distillation, it is desirable for the target raw material to have a low moisture content, and it is preferable to recover it as black selenium. Among black selenium, rubbery selenium is sticky and sometimes forms large ball-like lumps with diameters of 3 cm or more, which can clog the extraction pipes and transfer pipes used to extract it from the reactor. Selenium can be easily handled if it is converted into a coke-like sandy precipitate and recovered as a slurry.

[0012] Non-ferrous smelters often use sulfur dioxide gas, which is contained in dry smelting exhaust gas, as a reducing agent to reduce selenite. However, during periods when operations are suspended for long periods to perform repairs, sulfur dioxide gas must be purchased because no smelting exhaust gas is generated. However, in recent years, it has become difficult to purchase sulfur dioxide gas domestically, which has led to the problem of having to rely on imported gas, which is expensive and has a long delivery time.

[0013] There are no known methods for precipitating selenium as black coke selenium using substances other than sulfur dioxide as a reducing agent. Sulfur dioxide is supplied as a gas and functions as a reducing agent after dissolving in an aqueous solution, resulting in a slow reduction rate. When using a more efficient solid or solution reducing agent such as acetone, a large amount of rubbery selenium is temporarily generated from the red selenium, and this sticky rubbery selenium aggregates upon stirring. If the particle size of the precipitated aggregated selenium becomes too large, it becomes impossible to handle as a slurry.

[0014] In view of the above-mentioned conventional circumstances, the present invention provides a method for recovering selenium as sandy black selenium by precipitating it from a hydrochloric acid solution containing selenious acid without using sulfur dioxide as a reducing agent, or, if sulfur dioxide is used as a reducing agent, by reducing the amount of sulfur dioxide used. [Means for solving the problem]

[0015] As a result of extensive research aimed at solving the above problems, the present inventors have found that by raising the temperature of a hydrochloric acid solution containing selenious acid to 70°C or higher and adding a reducing agent so that the average reduction rate of selenious acid is 120 mg / L or less per minute (calculated as selenium), selenium can be recovered as sandy black coke without using sulfur dioxide as a reducing agent, or by reducing the amount of sulfur dioxide used if sulfur dioxide is used as a reducing agent. The present invention was made based on this finding.

[0016] That is, the present invention includes the following inventions. (1) A method for recovering selenium by reduction and precipitation from a hydrochloric acid solution containing selenious acid, in which the temperature of the hydrochloric acid solution is raised to 70°C or higher and a reducing agent other than sulfur dioxide is added so that the average reduction rate of selenious acid is 120 mg / L or less per minute in terms of selenium. (2) The method for recovering selenium according to (1), wherein the average reduction rate of the selenious acid is the reduction rate until the selenium concentration in the hydrochloric acid solution reaches half or less of its initial concentration. (3) The method for recovering selenium according to (1) or (2), wherein the reducing agent is a solid or a liquid. (4) The method for recovering selenium according to any one of (1) to (3), wherein the reducing agent is a ketone. (5) The method for recovering selenium according to any one of (1) to (4), wherein acetone is used as the reducing agent, and when the selenium concentration in the hydrochloric acid solution reaches 5 g / L or less, the supply of acetone is stopped, and selenium is reduced with sulfur dioxide or air containing sulfur dioxide. (6) The method for recovering selenium according to any one of (1) to (5), wherein acetone is used as the reducing agent and the total amount of acetone added is 0.6 mL / L or less per 1 g / L of selenium. (7) The method for recovering selenium according to (6), wherein the acetone is added intermittently, the amount of acetone added is 3 mL or less per 1 L of the hydrochloric acid solution, and the interval between additions is 30 minutes or more. (8) The method for recovering selenium according to any one of (1) to (7), wherein the average reduction rate of the selenious acid is the reduction rate until the selenium concentration in the hydrochloric acid solution reaches 5 g / L or less. (9) The method for recovering selenium according to any one of (1) to (8), wherein the proportion of the mass of the recovered selenium particles passing through a sieve with a mesh size of 4.75 mm is 70% or more. [Effects of the Invention]

[0017] According to the present invention, a method for recovering selenium as sandy black selenium can be provided, in which selenium is precipitated and recovered from a hydrochloric acid solution containing selenious acid without using sulfur dioxide as a reducing agent, or, if sulfur dioxide is used as a reducing agent, the amount of sulfur dioxide used can be reduced. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a graph showing the change in selenium concentration over time in Test Example 2. [Figure 2] 1 is a graph showing the change in selenium concentration over time and a linear regression line in Test Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0019] Next, embodiments of the present invention will be described in detail with reference to the drawings. It should be understood that the present invention is not limited to the following embodiments, and that appropriate design changes and improvements may be made based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention.

[0020] Conventionally, when producing selenium, it is necessary to distill the precipitate obtained after reducing selenious acid to increase its purity. During distillation, it is desirable for the water content of the raw material to be treated to be low, and recovery as black selenium is preferable. Among black selenium, rubber-like selenium is sticky and difficult to extract from the reactor. If selenium can be converted into a coke-like sandy precipitate and recovered as a slurry, handling becomes easier. However, even with the same coke selenium, if the particle size is too large, it may clog the pipes used to extract and discharge the slurry, such as the extraction pipes and transfer piping, or damage the filter cloth.

[0021] When using smelting exhaust gas as a reducing agent, if the solution temperature is heated to 70°C or higher, a sandy black coke containing selenium will precipitate. However, if a supply of sulfur dioxide gas is unavailable due to repairs or breakdowns of the smelting equipment, acetone can be used as an inexpensive reducing agent.

[0022] For example, when acetone is used as a reducing agent, selenium tends to precipitate in clumps, making it difficult to handle as a slurry and making it difficult to remove from the reactor. If the selenium precipitate contains a high content of amorphous rubbery selenium, it becomes sticky, making it even more difficult to handle.

[0023] In response to these conventional problems, the method of the present disclosure allows selenium to be recovered as sandy black selenium by precipitation from a hydrochloric acid solution containing selenious acid without using sulfur dioxide as a reducing agent, or, if sulfur dioxide is used as a reducing agent, by using a reduced amount of sulfur dioxide. In this specification, "sandy" refers to a powder state in which 70% or more of the particle mass passes through a sieve with a mesh size of 4.75 mm.

[0024] The electrolytic precipitate produced during the electrolytic refining process of non-ferrous metal smelting, particularly copper smelting, is enriched with platinum group elements, heavy metals, and various valuable materials. Platinum group elements and various valuable materials are not smelted separately, but are recovered as by-products of other metals or separated from recycled raw materials such as spent catalysts. Therefore, the selenium recovery method of the present disclosure can also be applied to recycling from waste materials.

[0025] Electrolytic precipitates can be dissolved using an acidic solution containing hydrochloric acid and hydrogen peroxide, but silver immediately forms an insoluble silver chloride precipitate with chloride ions after dissolution. In a solution containing an oxidizing agent and chlorine, such as aqua regia or chlorine water, precious metals dissolve and silver can be separated as silver chloride. Because it is a chloride bath, platinum group elements, rare metal elements, selenium, and tellurium are distributed in the pregnant leach solution (PLS).

[0026] The pregnant leach solution (PLS) is cooled, and the chlorides of base metals such as lead and antimony are precipitated and separated. The gold is then separated into an organic phase by solvent extraction. Dibutyl carbitol (DBC) is the most commonly used gold extractant.

[0027] Valuables can be precipitated and recovered by reducing PLS, an acidic hydrochloric acid solution containing selenious acid after gold extraction, but because the oxidation-reduction potential of each element is different, the order of precipitation is naturally determined. First, gold, platinum, and palladium precipitate, followed by chalcogens such as selenium and tellurium, and then inert precious metals such as ruthenium and iridium.

[0028] Various reducing agents can be used in the hydrochloric acid solution containing selenious acid. In the present invention, there are no particular limitations on the reducing agent, as long as it is excluding sulfur dioxide, and reducing agents that are solid, liquid, or gaseous at room temperature can be used. Among these, solid or liquid reducing agents are preferred because they dissolve in the hydrochloric acid solution more easily than gaseous reducing agents, generate ions, have good reactivity, and increase the reaction rate. Furthermore, solid or liquid reducing agents improve production efficiency because the entire amount added functions as a reducing agent.

[0029] Examples of the solid reducing agent include ferrous sulfate, thiourea, sodium thiosulfate, etc. Examples of the liquid reducing agent include liquid ketones such as acetone, which will be described later, an aqueous sodium sulfite solution, and hydrazine.

[0030] Ketones can also be used as reducing agents in hydrochloric acid solutions containing selenious acid. Ketones are organic compounds that have a ketone (a carbonyl group to which two hydrocarbon groups are bonded) in the molecule, and examples of ketones suitable as reducing agents in the present invention include acetone, methyl ethyl ketone, and methyl isobutyl ketone.

[0031] When selenious acid is reduced to selenium, a large amount of red selenium is generated immediately after adding the reducing agent. Then, when heated, the selenium changes form into black rubbery selenium and black coke selenium. When the solution is stirred in this rubbery state, it aggregates to form large spherical black selenium particles.

[0032] To avoid the formation of spherical black selenium, it is necessary to control the rate of red selenium generation and convert the morphology of the black rubbery selenium produced by heating the red selenium into sandy black coke selenium without agglomeration. Once the morphology of the black rubbery selenium is formed, it does not revert to rubbery selenium, and the selenium can be handled as a slurry.

[0033] The selenium concentration of the red selenium slurry is determined by the reduction rate of selenious acid and the rate of morphological change of the red selenium. If the reduction rate is too fast, the red selenium generated in a short period of time simultaneously converts into a considerable amount of rubbery selenium, and the adhesiveness of the rubbery selenium results in the formation of spherical black selenium. In the present invention, the rate of red selenium generation is controlled by adding a reducing agent other than sulfur dioxide so that the average reduction rate of selenious acid is 120 mg / L or less per minute in terms of selenium. This allows the morphology of the black rubbery selenium generated by heating the red selenium to be converted into sandy black coke selenium without aggregation. Furthermore, the reducing agent is preferably added so that the average reduction rate of selenious acid is 100 mg / L or less per minute in terms of selenium, more preferably 70 mg / L or less per minute.

[0034] In the present invention, by adding a reducing agent excluding sulfur dioxide so that the average reduction rate of selenious acid is 120 mg / L or less per minute in terms of selenium, sandy black selenium can be recovered without using sulfur dioxide as a reducing agent. Furthermore, as described below, even if the reducing agent is switched to sulfur dioxide when predetermined conditions are reached after adding the reducing agent excluding sulfur dioxide, the amount of sulfur dioxide used can be reduced to recover sandy black selenium because the reducing agent excluding sulfur dioxide is added first.

[0035] The morphological change of red selenium occurs at temperatures above 60°C. However, in order for the black rubbery selenium that red selenium has transformed into to further transform into black coke selenium, the temperature of the hydrochloric acid solution must be controlled to above 70°C. The higher the temperature, the more rapidly this morphological change occurs, so the temperature of the hydrochloric acid solution is preferably above 75°C, and more preferably above 80°C. There is no particular upper limit to the temperature of the hydrochloric acid solution, but from the viewpoint of energy conservation, it is preferably below 90°C.

[0036] Even if the reducing agent is not added continuously but only intermittently, it is sufficient as long as the average reduction rate of selenious acid is 120 mg / L per minute or less. If the reduction rate is slow, the instantaneous concentration of red selenium is suppressed and the problem of aggregation does not occur. However, if the reduction rate is too slow, production efficiency will decrease.

[0037] When reducing by intermittent addition of a reducing agent, if the reducing agent is acetone, it is recommended that the amount of acetone added be 3 mL or less per liter of hydrochloric acid solution to be treated, and that the interval between additions be 30 minutes or more. If a large amount of acetone is added per unit time, a large amount of red selenium will be present in an instant, which is likely to produce black rubbery selenium. As a result, selenium will settle in the tank as spherical aggregates.

[0038] The average reduction rate of selenious acid may be the reduction rate until the selenium concentration in the hydrochloric acid solution is reduced by half or less from the initial concentration. Furthermore, once the selenium concentration in the hydrochloric acid solution reaches 5 g / L or less, the amount of reducing agent added must be increased to maintain the reduction rate of selenious acid. Furthermore, if the reaction product, such as acetone, exhibits reducibility and undergoes a chain reaction of multiple reduction reactions, excessive reduction agent must be added. Adding excessive reducing agent increases the chemical oxygen demand (COD) in the wastewater, which may result in the COD being mixed with the recovered sandy black coke selenium as an organic impurity. Therefore, the average reduction rate of selenious acid may be the reduction rate until the selenium concentration in the hydrochloric acid solution reaches 5 g / L or less.

[0039] In particular, when using organic reagents such as acetone as a reducing agent, the viscosity of the solution increases toward the end of the reaction, reducing the stirring efficiency and increasing the particle size of the precipitated selenium particles. Therefore, switching to sulfur dioxide as a reducing agent when the selenium concentration in the hydrochloric acid solution reaches 5 g / L or less makes it easier to obtain sandy black selenium. Sulfur dioxide can also be a gas mixture with air.

[0040] When using acetone as a reducing agent, it is preferable to add acetone at a total concentration of 0.6 mL / L or less per 1 g / L of selenium. Selenious acid receives four electrons when reduced to selenium, but acetone undergoes many side reactions, and it is unknown how many electrons it will donate. However, adding too much acetone increases the burden on wastewater treatment. Furthermore, when selenious acid is reduced with acetone, a small portion becomes a difficult-to-reduc selenium compound, which is difficult to treat, so it is best to limit the amount of acetone used. [Example]

[0041] The following examples are provided to provide a better understanding of the present invention and its advantages, but the present invention is not limited to these examples.

[0042] <Preparation of the solution to be treated (hydrochloric acid solution containing selenious acid)> Copper was removed from the electrolytic sediment recovered from the copper electrorefining process of copper smelting by treating it with sulfuric acid. Next, concentrated hydrochloric acid and 60% hydrogen peroxide solution were added to dissolve the material, and the material was subjected to solid-liquid separation to obtain PLS (pregnant leach solution). Next, the PLS was cooled to 6°C to precipitate and remove base metals, and then the PLS was mixed with DBC (dibutyl carbitol) whose acid concentration had been adjusted to 2N or higher 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% by volume) was blown in to reduce the precious metals and perform solid-liquid separation. The selenium concentration in the liquid after precious metal separation was 36 g / L.

[0043] (Test Example 1) 300 mL of the liquid after the precious metal separation was measured out and heated to 80 to 85°C. Next, a predetermined amount of acetone was added intermittently at the addition intervals shown in Table 1. The amount of acetone added per addition was also varied in some cases, as in condition 2 in Table 1. Under condition 2 in Table 1, the amount of acetone added was varied each time, for a total of four additions. The acetone was diluted appropriately before addition depending on the amount. Note that the amount of acetone added in Table 1 is not the volume of the diluted solution, but the amount of acetone contained in the diluted solution.

[0044] When the total amount of acetone added reached the total amount of acetone listed in Table 1, the mixture was stirred for 30 minutes, after which a mixed gas of air and sulfur dioxide (sulfur dioxide concentration 10-20% by volume) was blown in while stirring. After 60 minutes, the supply of the mixed gas of air and sulfur dioxide was stopped and solid-liquid separation was carried out. The precipitate was washed with water, the water was removed with alcohol, and then air-dried overnight. The proportion of selenium particles passing through a sieve with 4.75 mm openings (particle size distribution) was measured. Powdered selenium with this proportion of 70% or more was designated as sandy black selenium.

[0045] Test samples were collected before each addition of acetone. 2 mL of the sample solution was taken and adjusted to 50 mL. The selenium concentration in the solution was quantified using an ICP-OES (Seiko SPS3100). The average reduction rate of selenite was calculated by linear regression of the selenium concentration before the start of sulfur dioxide reduction and the elapsed time. Note that the selenium concentration before the start of sulfur dioxide reduction was below 5 g / L under all conditions. The test conditions and evaluation results are shown in Table 1.

[0046] [Table 1]

[0047] Table 1 shows that the higher the reduction rate of selenious acid, the lower the proportion of sandy black selenium. The longer the interval between acetone additions, the lower the reduction rate, but the amount of acetone added each time also plays a role. If the proportion of sandy black selenium is low, it is difficult to handle the selenium precipitate as a slurry.

[0048] (Test Example 2) 300 mL of the same liquid after precious metal separation as in Test Example 1 was measured and heated to 75 to 80°C. Next, a predetermined amount of acetone was added intermittently at the addition intervals shown in Table 2. The acetone was diluted appropriately depending on the amount added. Note that the numerical values ​​for the amount of acetone added in Table 2 are not the volume of the diluted solution, but the amount of acetone contained in the diluted solution.

[0049] When the total amount of acetone added reached the total amount of acetone listed in Table 2, the mixture was stirred for 30 minutes, after which a mixed gas of air and sulfur dioxide (sulfur dioxide concentration: 10-20% by volume) was blown in while stirring. After 60 minutes, the supply of the mixed gas of air and sulfur dioxide was stopped and solid-liquid separation was carried out. The precipitate was washed with water, the water was removed with alcohol, and then air-dried overnight. The proportion of selenium particles passing through a sieve with 4.75 mm openings (particle size distribution) was measured. Powdered selenium with this proportion of 70% or more was designated as sandy black selenium.

[0050] Test samples were taken before and after each acetone addition. Selenium concentrations were monitored up to 120 minutes in condition 5 and up to 30 minutes after the end of acetone addition in the other conditions. The test sample was prepared by taking 2 mL of the solution and adjusting the volume to 50 mL. The selenium concentration was quantified using ICP-OES (Seiko SPS3100). The lost water volume was replenished with pure water. Figure 1 shows the change in selenium concentration over time. The change in selenium concentration over time was regressed using a linear equation, and the slope of the regression line was taken as the average reduction rate. However, it was clear that the regression line for condition 5 was not linear, so the rate was taken as the ratio of change between the point at reaction time 0 and the point after 30 minutes had elapsed. The test conditions and evaluation results are shown in Table 2.

[0051] [Table 2]

[0052] Table 2 shows that the rate of reduction of selenious acid increases, while the proportion of sandy black selenium decreases. Comparing Conditions 5 and 6, even if the total amount of acetone added is the same, adding it all at once significantly increases the rate of reduction of selenious acid and increases the particle size of the precipitated selenium. Therefore, it is better to add ketones as reducing agents intermittently.

[0053] As shown in Tables 1 and 2, when the total amount of acetone added exceeded 3 mL, the average reduction rate of selenious acid increased and the proportion of sandy black selenium decreased. Even when the total amount of acetone added was greater than 3 mL, the reduction rate did not increase significantly up to an amount of 1 mL added per acetone addition. This amount corresponds to 3.3 mL / L of acetone per 1 L of the liquid to be treated. It is expected that the proportion of sandy black selenium can be increased by further extending the addition interval to slow the reduction rate.

[0054] (Test Example 3) 300 mL of the same liquid after precious metal separation as in Test Example 1 was measured and heated to 75 to 80°C. Next, 0.5 mL of acetone was added every 15 minutes, and samples for selenium concentration analysis were taken every 30 minutes. After the total amount of acetone added reached 5 mL, the mixture was stirred for 30 minutes. The selenium concentration was determined in accordance with Test Example 1. Figure 2 shows the time course of selenium concentration and the linear regression line.

[0055] The results in Figure 2 show that the selenium concentration decreases linearly, and the linear regression line obtained by regression is y = -0.66X + 37.3. The amount of acetone required for all selenium reductions can be calculated based on this regression line to be 5.6 mL, or 19 mL / L. Since the selenium concentration is 36 g / L, the amount of acetone required for 1 g / L of selenium is 0.53 mL / L.

[0056] In other words, the amount of acetone required to reduce and recover all the selenium in the solution with acetone is approximately 0.6 mL / L per 1 g / L of selenium. This indicates that the amount of acetone added should be set to 0.6 mL / L or less per 1 g / L of selenium in order to avoid adding too much acetone and unnecessarily increasing the reduction rate of selenious acid.

Claims

1. A method for recovering selenium by reducing and precipitating selenious acid from a hydrochloric acid solution containing 10 g / L or more of selenious acid in terms of selenium, comprising heating the hydrochloric acid solution to 70°C or higher and adding a reducing agent other than sulfur dioxide so that the average reduction rate of selenious acid is 120 mg / L or less per minute in terms of selenium; The method for recovering selenium, wherein the ratio of the total amount of the reducing agent added to the selenium concentration in the hydrochloric acid solution is greater than 3 (mmol) / 4 (g / L).

2. 2. The method for recovering selenium according to claim 1, wherein the average reduction rate of the selenious acid is the reduction rate until the selenium concentration in the hydrochloric acid solution decreases by half or less from the initial concentration.

3. 3. The method for recovering selenium according to claim 1, wherein the reducing agent is a solid or a liquid.

4. The method for recovering selenium according to any one of claims 1 to 3, wherein the reducing agent is a ketone.

5. 5. The method for recovering selenium according to claim 1, wherein acetone is used as the reducing agent, the supply of acetone is stopped when the selenium concentration in the hydrochloric acid solution reaches 5 g / L or less, and selenious acid is reduced with sulfur dioxide or air containing sulfur dioxide.

6. 6. The method for recovering selenium according to claim 1, wherein acetone is used as the reducing agent, and the total amount of acetone added is 0.6 mL / L or less per 1 g / L of selenium.

7. 7. The method for recovering selenium according to claim 6, wherein the acetone is added intermittently, the amount of acetone added is 3 mL or less per 1 L of the hydrochloric acid solution, and the interval between additions is 30 minutes or more.

8. 8. The method for recovering selenium according to claim 1, wherein the average reduction rate of the selenious acid is the reduction rate until the selenium concentration in the hydrochloric acid solution reaches 5 g / L or less.

9. 9. The method for recovering selenium according to claim 1, wherein the proportion of the mass of the recovered selenium particles passing through a sieve with a mesh size of 4.75 mm is 70% or more.

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