How to recover selenium

Abstract

To provide a method for recovering selenium from an acidic aqueous solution containing selenous acid and ruthenium by suitably suppressing the incorporation of ruthenium in the precipitation recovery of selenium.SOLUTION: A method for recovering selenium comprises adding dimethyl sulfoxide to an acidic aqueous solution containing selenous acid and ruthenium, heating the acidic aqueous solution to a liquid temperature of 65°C or higher, adding a reducing agent to precipitate selenium, stopping the reduction, and then recovering the precipitated selenium.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for recovering selenium, and more particularly to a method for recovering selenium from an acidic aqueous solution containing selenium and ruthenium. [Background technology]

[0002] Selenium belongs to the chalcogen group and is used in various glass industries and optical devices. Ruthenium is a metal used as a catalyst and a trace additive in various alloys. Both elements are mostly produced as by-products of copper pyrometallurgy or from its smelting intermediates.

[0003] In copper pyrometallurgy, copper concentrate is melted and processed into crude copper with a purity of 99% or more in a converter and a refining furnace, after which electrolytic refining produces refined copper with a purity of 99.99% or more. Valuable materials other than copper are precipitated as slime during electrolytic refining.

[0004] This slime is enriched with rare metals such as gold, silver, platinum, and palladium, as well as ruthenium, rhodium, and iridium, as well as selenium and tellurium, which are contained in copper concentrate. These elements are separated and recovered individually as by-products of copper smelting.

[0005] This slime is often treated by hydrometallurgy. For example, in Patent Document 1, silver is recovered by treating the slime with hydrochloric acid and hydrogen peroxide. The dissolved gold is recovered by solvent extraction, and then other valuables are sequentially reduced and recovered with sulfur dioxide.

[0006] Patent Document 2 also discloses a method of concentrating precious metals by recovering gold and silver using a similar method, reducing and precipitating the valuables with sulfur dioxide, and removing only the selenium by distillation.

[0007] The solution obtained after the precious metals have been recovered contains valuable materials such as tellurium and selenium, and it is necessary to recover these valuable materials. A known method for recovering these valuable materials is to recover the precipitates formed by adding a reducing agent.

[0008] In particular, as disclosed in Patent Document 1, the method of recovering precipitates produced by sulfur dioxide has many advantages in terms of cost and production scale. In addition, since each element is precipitated sequentially, it is also effective for separation and purification. [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 Summary of the Invention [Problem to be solved by the invention]

[0010] Selenium is recovered as a by-product of copper smelting, and other elements are mixed in as impurities during reduction and recovery using sulfur dioxide or sulfite. Typical impurities are ruthenium and tellurium. In the process of reducing selenium and recovering the precipitate, the raw material solution contains approximately 100 to 500 mg / L of ruthenium.

[0011] Generally, in the selenium recovery process, sulfur dioxide is blown in for a long time, during which time some of the ruthenium precipitates along with the selenium. The precipitated selenium is then distilled to further increase its purity. The ruthenium that is brought into the distillation pot together with the selenium does not vaporize and remains in the distillation pot.

[0012] Although the ruthenium content in the precipitate from the selenium recovery process is minute compared to selenium, it is repeatedly charged into the distillation process, and ruthenium gradually accumulates as distillation residue in the distillation still. As the amount of residual ruthenium in the distillation still increases, it reduces the throughput per unit operation and reduces the thermal conductivity of the distillation still. Therefore, the interior of the selenium distillation still must be cleaned periodically, which means that selenium distillation stops during cleaning, reducing production efficiency. Furthermore, the selenium distillation residue recovered by cleaning the distillation still contains concentrated ruthenium, which must be separately processed and refined into a valuable resource. Therefore, if ruthenium contamination can be suppressed during selenium precipitation recovery and distributed to the post-selenium recovery liquid for processing in the appropriate ruthenium recovery process, selenium production efficiency can be improved.

[0013] In view of the above-mentioned conventional circumstances, the present invention provides a method for recovering selenium from an acidic aqueous solution containing selenious acid and ruthenium, while effectively suppressing the incorporation of ruthenium during the precipitation and recovery of selenium. [Means for solving the problem]

[0014] As a result of extensive research aimed at solving the above problems, the present inventors have found that by adding dimethyl sulfoxide to an acidic aqueous solution containing selenious acid and ruthenium, heating the acidic aqueous solution to a liquid temperature of 65°C or higher, and adding a reducing agent to precipitate selenium, it is possible to recover selenium while effectively suppressing the incorporation of ruthenium during selenium precipitation recovery. The present invention was made based on this finding.

[0015] That is, the present invention includes the following inventions. (1) A method for recovering selenium, comprising adding dimethyl sulfoxide to an acidic aqueous solution containing selenious acid and ruthenium, heating the acidic aqueous solution to a liquid temperature of 65°C or higher, adding a reducing agent to precipitate selenium, stopping the reduction, and recovering the precipitated selenium. (2) The method for recovering selenium according to (1), wherein the reduction is stopped before the selenium concentration in the acidic aqueous solution reaches 0.002 g / L or less, and the precipitated selenium is recovered. (3) The method for recovering selenium according to (1), wherein the reduction is stopped before the selenium concentration in the acidic aqueous solution reaches 0.5 g / L or less, and the precipitated selenium is recovered. (4) The method for recovering selenium according to any one of (1) to (3), wherein the mass concentration ratio of ruthenium / selenium in the acidic aqueous solution is 70 or less when the reduction is stopped. (5) The method for recovering selenium according to any one of (1) to (4), wherein 1 to 20 mL of the dimethyl sulfoxide is added per 1 L of the acidic aqueous solution. (6) The method for recovering selenium according to any one of (1) to (5), wherein the acidic aqueous solution before the addition of dimethyl sulfoxide contains 100 to 500 mg / L of ruthenium. (7) The method for recovering selenium according to any one of (1) to (6), wherein the reducing agent is sulfur dioxide. (8) The method for recovering selenium according to any one of (1) to (6), wherein the reducing agent is thiosulfate ion, thiourea, acetone, or a metal less noble than selenium. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a method for recovering selenium from an acidic aqueous solution containing selenious acid and ruthenium, while effectively suppressing the incorporation of ruthenium during precipitation and recovery of selenium. DETAILED DESCRIPTION OF THE INVENTION

[0017] Next, the 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 that appropriate design changes, improvements, etc. may be made based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention.

[0018] Electrolytic slime, which is produced during the electrolytic refining process of non-ferrous metal smelting, especially copper smelting, contains large amounts of chalcogen elements and precious metals. For example, the electrolytic slime contains approximately 10 to 30 kg / t of gold, 100 to 250 kg / t of silver, 1 to 3 kg / t of palladium, 800 to 3,000 g / t of ruthenium, and 5 to 15 mass% of selenium.

[0019] When this electrolytic slime is dissolved by adding hydrochloric acid and hydrogen peroxide, it becomes a solution of copper electrolytic deposits, but immediately after dissolution, silver forms an insoluble silver chloride precipitate with chloride ions. 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, precious metal elements, rare metal elements, selenium, and tellurium are distributed in the pregnant leach solution (PLS). Selenium is contained in the acidic aqueous solution as selenium oxonium, but most of it is in the form of selenite.

[0020] Precious metals are recovered using solvent extraction or oxidation-reduction potential difference. The acidic aqueous solution after removing the precious metals contains, for example, 25 to 40 g / L of selenium and 100 to 500 mg / L of ruthenium. A reducing agent is added to this acidic aqueous solution to recover selenium. Sulfur dioxide can be used as the reducing agent. Sulfur dioxide can be obtained from smelting exhaust gas and is suitable as a reducing agent. In addition to sulfur dioxide, other strong reducing agents, as described below, may also be used as reducing agents.

[0021] Depending on the size of the reactor and the amount of sulfur dioxide supplied, selenium reduction takes 15 m 3 This process takes approximately 8 to 10 hours for the raw material solution. Since selenium is present in an overwhelmingly higher concentration than other elements, more than 98 mass% of the precipitate is selenium. However, at this stage, 30 to 50 mass% of the ruthenium in the solution precipitates along with the selenium. The ruthenium content in the precipitate from this process is equivalent to approximately 0.2 mass%. Therefore, the amount of ruthenium brought into the selenium still, where the precipitate is distilled, cannot be ignored.

[0022] Dimethyl sulfoxide (DMSO) is added to suppress the amount of ruthenium co-precipitating with selenium. DMSO interacts with ruthenium(II) through the unpaired electron of the sulfur atom or the highly negatively polarized oxygen atom, resulting in a stronger coordination bond than sulfite. Furthermore, when DMSO coordinates with ruthenium, steric hindrance in the first coordination sphere increases, hindering the access of the reducing agent, sulfite ion. If sulfite ion access to ruthenium(II) is hindered, the reducing agent, sulfite, is more likely to react with the excessive selenite rather than ruthenium. While sulfur dioxide or sulfite ion are most affected by steric hindrance, steric hindrance also effectively suppresses the reduction and precipitation of ruthenium when selenium is reduced with other reducing agents, preventing ruthenium from contaminating the precipitate. This method is particularly effective when the ruthenium concentration in the acidic aqueous solution before adding DMSO is low. When the ruthenium concentration is high, it is possible to use other conventional ruthenium recovery techniques, such as adding sodium bromate and distilling it as is, but when the ruthenium concentration is low, there is no cost-effective method using the conventional methods. In the case of the present invention, even when the ruthenium concentration is as low as 100 to 500 mg / L, reduction and precipitation of ruthenium can be effectively suppressed.

[0023] While the steric hindrance described above is the primary factor in the successful suppression of ruthenium reductive precipitation, the indirect stabilization of ruthenium by DMSO is also a factor. DMSO is reduced to dimethyl sulfide by a reducing agent. DMSO, located in the first coordination sphere of the ruthenium ion, is more susceptible to reducing agents and electrons than ruthenium, and is therefore sacrificially reduced before ruthenium. After reduction, DMSO becomes dimethyl sulfide and evaporates, preventing selenium contamination. This indirect stabilization of ruthenium by DMSO is a key factor in the suppression of ruthenium reductive precipitation, particularly when using a strong reducing agent such as acetone. These strong reducing agents are substances that are effective in the reduction of selenious acid and also partially reduce ruthenium, such as thiosulfate ion, thiourea, water-soluble aldehydes, water-soluble ketones such as acetone, and metals less noble than selenium. Among these, the use of metals less noble than selenium as reducing agents is most preferable in terms of reduction reaction efficiency. Examples of metals less noble than selenium include one or more of iron, copper, and copper-coated iron.

[0024] After adding dimethyl sulfoxide to the acidic aqueous solution, the solution is heated to a temperature of 65°C or higher and the reducing agent is added. It is known that selenium precipitates as red selenium or sticky amorphous selenium when the temperature of the acidic aqueous solution is lower than 65°C. Selenium in these forms must be dehydrated or adhered to peripheral equipment before distillation, and then peeled off. In contrast, by heating the acidic aqueous solution to a temperature of 65°C or higher and adding the reducing agent, the selenium can be recovered as black selenium.

[0025] As the reduction of selenium progresses, the concentration of selenious acid in the solution decreases. This increases the likelihood that DMSO-coordinated ruthenium complexes will encounter the reducing agent, increasing the probability of reduction. To prevent ruthenium from contaminating the precipitate after reduction, it is preferable for a certain amount of selenious acid to be present in the solution. From this perspective, it is preferable to stop the reduction before the selenium concentration in the acidic aqueous solution reaches 0.002 g / L or less and recover the precipitated selenium. Furthermore, when reducing selenious acid to selenium, if the selenious acid in the solution is 0.05 g / L or less in terms of selenium, the reduction rate, i.e., the rate at which the reducing agent is oxidized, decreases. To reduce the probability of ruthenium reduction, it is more preferable to stop the reduction before the selenium concentration in the acidic aqueous solution reaches 0.5 g / L or less, and even more preferable to stop the reduction before it reaches 3 to 5 g / L or less.

[0026] The mass concentration ratio (mg / mg) of ruthenium / selenium in the acidic aqueous solution when the reduction is stopped is preferably 70 or less. If the mass concentration ratio of ruthenium / selenium in the acidic aqueous solution when the reduction is stopped exceeds 70, reduction of ruthenium may begin to occur. The mass concentration ratio of ruthenium / selenium in the acidic aqueous solution when the reduction is stopped is more preferably 55 or less, and even more preferably 35 or less.

[0027] The amount of DMSO added is preferably 1 to 20 mL per 1 L of acidic aqueous solution. If the amount of DMSO added exceeds 20 mL per 1 L of acidic aqueous solution, problems such as an increase in COD and the odor of dimethyl sulfide, a decomposition product of DMSO, may become significant. If the amount of DMSO added is less than 1 mL per 1 L of acidic aqueous solution, the effect of preventing the reduction of ruthenium may decrease. The amount of DMSO added is more preferably 5 to 10 mL per 1 L of acidic aqueous solution.

[0028] As described above, a reducing agent is added to precipitate selenium, and after the reduction is stopped, the precipitated selenium is recovered. At this time, the selenium-containing material that precipitates after the reduction treatment is subjected to solid-liquid separation using a filter press or the like. Depending on the composition of the target liquid, the recovered selenium-containing material may be used as a smelting raw material if it also contains valuable metals such as iridium. In addition, the purity of the recovered selenium can be increased by further distillation. [Example]

[0029] 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.

[0030] <Preparation of the treatment solution (acidic aqueous solution containing selenious acid and ruthenium)> Electrolytic slime recovered from copper smelting was leached with sulfuric acid to remove copper. Concentrated hydrochloric acid and 60% by mass hydrogen peroxide were added to dissolve the slime, followed by solid-liquid separation to obtain pregnant leached solution (PLS). After removing gold by solvent extraction, the PLS was heated with stirring. When the temperature reached 70°C, a mixture of sulfur dioxide and air (sulfur dioxide concentration 5-20% by volume) was blown in at 0.1 L / min. After confirming that the concentrations of platinum and palladium were both 5 mg / L or less, the resulting precipitate was separated from the acidic aqueous solution. The acidic aqueous solution (liquid to be treated) from which the precipitate was separated contained selenious acid and ruthenium, and the selenium concentration in the acidic aqueous solution was 34 g / L and the ruthenium concentration was 140 mg / L.

[0031] (Test Example 1) A 300 mL aliquot of the above-mentioned solution to be treated was taken, its temperature adjusted to 70-75°C, 0-5 mL of DMSO was added, and the mixture was stirred. A mixture of sulfur dioxide and air was then blown in for reduction. Samples were then taken at regular intervals (120 minutes, 150 minutes). Pure water was added as needed to replace the water lost through evaporation. After 210 minutes, the mixture was filtered to remove the precipitate. The filtrate was reheated to 75°C, and 2 g of copper-coated iron powder (copper content 70% by mass) was added. The mixture was stirred for 30 minutes and then subjected to cementation (metal replacement). After cementation, the collected sample was subjected to solid-liquid separation using 5C filter paper. The filtrate was diluted 25 times with dilute hydrochloric acid, and the concentrations of various components were measured using ICP-OES (Seiko SPS-3100) with yttrium as an internal standard. The ruthenium concentration was corrected by the copper concentration to eliminate the influence of increases or decreases in the solution. The initial copper concentration was 0.64 g / L. Since copper does not precipitate, the concentration rate was calculated from the copper concentration and the measured value was multiplied by this rate to correct the ruthenium concentration. The results are shown in Table 1. In Table 1, "ND" indicates a concentration below the detection limit. "-" indicates that the selenium concentration was zero and therefore undefined.

[0032] [Table 1]

[0033] The results in Table 1 show that the ruthenium concentration after cementation increased when the amount of DMSO added was 0.5 mL or more. Since there is a positive correlation with the amount of DMSO added, it is clear that the reduction of ruthenium is suppressed when DMSO is added in amounts of 1.5 mL / L or more.

[0034] Furthermore, looking at the ruthenium concentration after 210 minutes, it can be seen that the addition of 1 mL or more of DMSO had a greater effect. After 210 minutes, the ruthenium concentration in the system with 0.5 mL of DMSO added was lower than in the system without DMSO, but this is due to the longer sulfur dioxide injection time after selenium disappeared. Furthermore, looking at the results after 150 minutes when 0.5 mL of DMSO was added, it can be seen that selenium had been reduced to 0.002 g / L, while ruthenium was barely reduced at 120 mg / L. This shows that selenium has an inhibitory effect on ruthenium reduction in acidic aqueous solutions up to a selenium concentration of 0.002 g / L.

[0035] Ruthenium begins to be reduced when the selenium concentration in the acidic aqueous solution drops significantly. Although the amount of DMSO added also affects this, it is expected that reduction of ruthenium may occur when the ruthenium / selenium weight ratio in the acidic aqueous solution is 70 or more.

[0036] (Test Example 2) A 200 mL aliquot of the same liquid to be treated as in Test Example 1 was heated to 70-75°C. The amount of DMSO added was 2 mL, 1 mL, or no DMSO. Next, 2 mL of a solution prepared by diluting acetone 5 times with water was added, and the liquid temperature was maintained at 70-75°C while stirring. Subsequently, 2 mL of 5-times diluted acetone was added every 30 minutes. After the amount of 5-times diluted acetone reached 16 mL (3.5 hours), stirring was continued for another 30 minutes. Next, heating was stopped, and the precipitated selenium was filtered off. The filtrate was again heated to 60-65°C, and 3 mL of hydrogen peroxide solution (30% by mass) was added and stirred for 30 minutes. Next, 2 g of iron powder was added, and the mixture was stirred for 30 minutes, followed by filtration. The ruthenium concentration in the filtrate was quantified. The concentrations of various components in the sample solution were quantified using the same procedures as in Test Example 1. The results are shown in Table 2. The ruthenium concentration after acetone reduction is shown as a value corrected for the copper concentration, as in Test Example 1.

[0037] [Table 2]

[0038] It can be seen that the addition of DMSO makes ruthenium less susceptible to reduction, even when the reducing agent is changed to acetone. Although acetone has almost no reducing effect to begin with, it protects ruthenium from aldehydes and pyruvic acid, which are produced when acetone reacts with selenious acid. It can also be seen that ruthenium exhibits an inhibitory effect on the reduction of iron substitution.

[0039] DMSO also exhibited an inhibitory effect on selenite in acetone reduction. Because it does not coordinate with selenite, this is thought to be due to the oxidation of pyruvate, a reducing agent produced by the reaction of selenite with acetone. The pyruvate oxidized by DMSO cannot contribute to the reduction of selenite. Therefore, if excessive amounts of DMSO are used, the amount of acetone added must be increased during acetone reduction. From this perspective, it is preferable to add 20 mL or less of DMSO per 1 L of acidic aqueous solution.

Claims

1. A method for recovering selenium, comprising adding dimethyl sulfoxide to an acidic aqueous solution containing selenious acid and ruthenium, heating the acidic aqueous solution to a liquid temperature of 65°C or higher, adding a reducing agent to precipitate selenium, stopping the reduction, and recovering the precipitated selenium.

2. 2. The method for recovering selenium according to claim 1, wherein the reduction is stopped before the selenium concentration in the acidic aqueous solution reaches 0.002 g / L or less, and the precipitated selenium is recovered.

3. 2. The method for recovering selenium according to claim 1, wherein the reduction is stopped before the selenium concentration in the acidic aqueous solution reaches 0.5 g / L or less, and the precipitated selenium is recovered.

4. 2. The method for recovering selenium according to claim 1, wherein the mass concentration ratio of ruthenium to selenium in the acidic aqueous solution when the reduction is stopped is 70 or less.

5. The method for recovering selenium according to claim 1, wherein 1 to 20 mL of dimethyl sulfoxide is added per 1 L of the acidic aqueous solution.

6. 2. The method for recovering selenium according to claim 1, wherein the acidic aqueous solution before the addition of dimethyl sulfoxide contains 100 to 500 mg / L of ruthenium.

7. 2. The method for recovering selenium according to claim 1, wherein the reducing agent is sulfur dioxide.

8. 2. The method for recovering selenium according to claim 1, wherein the reducing agent is thiosulfate ion, thiourea, acetone, or a metal less noble than selenium.

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