How to recover selenium

Abstract

To provide a method for recovering selenium through suppressing the formation of poorly reducible selenium compounds when selenium is precipitated and separated from an acidic solution containing selenious acid with ketone used as a reducing agent.SOLUTION: A method for recovering selenium by reducing and precipitating selenium from an acidic solution containing selenious acid is provided. In the method, acetone is continuously or intermittently added to the acidic solution while an oxidizing agent is used to oxidize organic compounds produced by the reaction of acetone with selenious acid.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 solution containing selenious acid. [Background technology]

[0002] In copper pyrometallurgy, copper concentrate is melted, and then processed into crude copper of 99% purity or more in a converter and a refining furnace. After that, electrolytic refining process produces electrolytic copper with a purity of 99.99% or more. 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] This slime is often treated by hydrometallurgy. For example, Patent Document 1 discloses a method in which silver is recovered from slime using hydrochloric acid-hydrogen peroxide, dissolved gold is recovered by solvent extraction, and other valuables are subsequently 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 valuables are reduced and precipitated using sulfur dioxide, and only selenium is removed by distillation to concentrate precious metals.

[0005] The solution after the precious metals are recovered contains rare metal ions, tellurium, and selenium, and it is necessary to further recover these valuable materials. Known recovery methods include recovering the precipitate generated by the reduction agent, mixing the solution with copper concentrate, drying it with 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 in separation and purification, since each element precipitates in order according to the height of its redox potential.

[0007] Dry smelting exhaust gas is generally used as the sulfur dioxide used to reduce selenium, because it has a large cost advantage and turns into sulfate ions after the reaction, which does not adversely affect wastewater.

[0008] However, when using dry smelting exhaust gas, there are cases where sulfur dioxide cannot be supplied due to regular furnace maintenance or sudden accidents. In such cases, high-purity liquid sulfur dioxide can be used instead, but this increases the cost of the reagent. Another possible substitute is sulfite, but this has problems such as a decrease in the acid concentration in the liquid, the precipitation of sparingly soluble inorganic salts when the sodium concentration increases, and an increase in the reagent cost.

[0009] Ketones are examples of inexpensive and efficient alternative reducing agents for sulfur dioxide (Patent Document 3). In particular, acetone is inexpensive, has low toxicity, and has a high reduction efficiency per unit amount. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2001-316735 A [Patent Document 2] JP 2004-190134 A [Patent Document 3] JP 2018-109207 A [Patent Document 4] JP 2019-77902 A Summary of the Invention [Problem to be solved by the invention]

[0011] When selenious acid is reduced using ketones as a reducing agent, most of the selenious acid is reduced to selenium, but some of it produces difficult-to-reduc selenium compounds. In this application, the term "difficult-to-reduc selenium compounds" refers to selenium compounds that are difficult to reduce to elemental selenium using sulfur dioxide. After selenium is recovered, valuable materials such as tellurium and platinum group elements are further reduced and recovered using sulfur dioxide, so the difficult-to-reduc selenium compounds in the solution are carried over to the wastewater treatment process.

[0012] However, there are regulations on the concentration of selenium in wastewater, and although it is a small amount of a by-product, difficult-to-reduc selenium compounds place a significant burden on the wastewater treatment process. As a countermeasure to this problem, a method of adsorption and removal using activated carbon is known, as shown in Patent Document 4, but the effect is not sufficient. Removal using activated carbon alone requires a large amount of activated carbon, and it is necessary to consider not only the cost of the activated carbon reagent, but also the treatment of the activated carbon after use.

[0013] The hard-to-reduc selenium compounds are produced as a result of a small portion of selenious acid reacting with acetone, and are not the main reaction. The detailed mechanism of their production is not known, and there is no known method for recovering selenium from selenious acid while suppressing the production of hard-to-reduc selenium compounds.

[0014] In view of the above-mentioned conventional circumstances, the present invention provides a method for recovering selenium while suppressing the production of difficult-to-reduc selenium compounds when precipitating and separating selenium from an acidic solution containing selenious acid using a ketone as a reducing agent. [Means for solving the problem]

[0015] As a result of intensive research conducted by the present inventors to solve the above problems, they found that the production of difficult-to-reduc selenium compounds can be suppressed by continuously or intermittently adding acetone to an acidic solution and oxidizing the organic compounds produced by the reaction between acetone and selenious acid with an oxidizing agent. The present invention was completed based on this finding.

[0016] That is, the present invention includes the following inventions. (1) A method for recovering selenium from an acidic solution containing selenious acid by reduction and precipitation, A method for recovering selenium, comprising the steps of: continuously or intermittently adding acetone to the acidic solution; and oxidizing an organic compound produced by a reaction between the acetone and the selenious acid with an oxidizing agent. (2) A method for recovering selenium according to (1), comprising heating the acidic solution to a temperature of 70°C or higher, and intermittently adding acetone in an amount of 0.4 mL / L or less per 1 g / L of selenium concentration in the acidic solution at intervals of 1 minute or more. (3) The method for recovering selenium according to (2), wherein the acetone is added intermittently in an amount of 0.4 mL / L or less at intervals of 30 minutes or more. (4) The method for recovering selenium according to any one of (1) to (3), wherein the amount of acetone added at one time is gradually reduced in the addition of acetone to the acidic liquid. (5) A method for recovering selenium according to (4), wherein after the concentration of selenious acid in the acidic solution reaches 25 g / L or less in terms of selenium concentration, acetone is intermittently added in an amount of 0.04 mL / L or less per 1 g / L of selenium concentration in the acidic solution. (6) The method for recovering selenium according to any one of (1) to (5), wherein acetone is intermittently added to the acidic liquid, and the oxidizing agent is added after each addition of the acetone. (7) The method for recovering selenium according to any one of (1) to (6), further comprising blowing sulfur dioxide into the acidic liquid after the reduction precipitation of the selenium to decompose excess oxidizing agent and recover remaining selenium by precipitation. (8) The method for recovering selenium according to any one of (1) to (7), wherein the oxidizing agent is an oxidizing agent that oxidizes a ketocarboxylic acid. (9) The method for recovering selenium according to (8), wherein the oxidizing agent for oxidizing the ketocarboxylic acid is at least one of hydrogen peroxide, oxygen, hypochlorous acid, and ozone. (10) The method for recovering selenium according to (9), wherein the amount of hydrogen peroxide added to the acidic solution is 0.03 volume times or more the amount of acetone added at one time. Effect of the Invention

[0017] According to the present invention, it is possible to provide a method for recovering selenium while suppressing the production of difficult-to-reduc selenium compounds when precipitating and separating selenium from an acidic solution containing selenious acid using a ketone as a reducing agent. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] 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 changes and improvements in the design may be made based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention.

[0019] Electrolytic slime, which is generated during the electrolytic refining process of non-ferrous metal smelting, especially copper smelting, contains a lot of chalcogen elements and precious metals. For example, it contains 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 5-15 mass% of selenium.

[0020] By adding hydrochloric acid and hydrogen peroxide to dissolve this electrolytic slime, a solution of copper electrolytic deposits is produced, in which silver immediately dissolves and 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, the precious metals dissolve and the 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 solution as selenium oxonium, but most of it is in the form of selenite.

[0021] Precious metals are recovered using solvent extraction or redox potential difference. Selenium is then reduced and recovered, usually using sulfur dioxide. If sulfur dioxide is not available in the selenium recovery process, selenite can be reduced to selenium using acetone.

[0022] It is known that selenious acid or selenium dioxide undergoes a reaction called selenization of aryl carbon-hydrogen bonds. Acetone reacts but 2-propanol does not, so it is believed that the enol produced by keto-enol tautomerization in acetone reacts. This is not limited to acetone, but all ketones with aryl carbon-hydrogen bonds undergo a similar reaction, reducing selenious acid to selenium.

[0023] Acetone that has undergone the selenium oxidation reaction becomes hydroxyacetone, and selenious acid is reduced and precipitates as elemental selenium. The precipitated elemental selenium becomes red or black selenium depending on the temperature. On the other hand, the reaction of ketones such as acetone proceeds until they become acetylaldehyde and ketocarboxylic acids, leaving these compounds open to further reduction reactions.

[0024] It is believed that the efficiency of selenite reduction per unit amount of acetone is good because the acetylaldehyde and ketocarboxylic acid (pyruvic acid when acetone is used) also act as reducing agents.

[0025] On the other hand, when pyruvic acid is added to an acidic solution containing selenious acid and reduced with acetone, the selenious acid is not reduced to elemental selenium by sulfur dioxide. In other words, difficult-to-reduc selenium compounds are produced. Naturally, some pyruvic acid also reduces selenious acid, but when acetone, selenious acid, and pyruvic acid react, difficult-to-reduc selenium compounds are likely to be produced. In response to this, if the intermediate product ketocarboxylic acid is produced and then oxidized and decomposed at the same time, the production of difficult-to-reduc selenium compounds is suppressed.

[0026] In an embodiment of the present invention, acetone is continuously or intermittently added to an acidic solution containing selenious acid, and an organic compound that promotes the production of difficult-to-reduc selenium compounds such as pyruvic acid produced by the reaction of acetone with selenious acid is oxidized using an oxidizing agent, thereby reducing and precipitating and recovering selenium. According to this configuration, an organic compound that promotes the production of difficult-to-reduc selenium compounds such as pyruvic acid produced by adding acetone to an acidic solution containing selenious acid can be decomposed by oxidation using an oxidizing agent. Therefore, the production of difficult-to-reduc selenium compounds can be suppressed. Regarding the effect of the addition of an oxidizing agent on acetone, since ketones are originally low in activity in oxidation reactions, the effect of the added oxidizing agent on the reduction of selenious acid by acetone is small.

[0027] If an excessive amount of the oxidizing agent is added, the precipitated selenium may be dissolved again. Therefore, the oxidizing agent is preferably an oxidizing agent that oxidizes ketocarboxylic acid, such as one or more of hydrogen peroxide, oxygen, hypochlorous acid, and ozone. Among them, hydrogen peroxide is most preferable because of its reactivity, non-toxicity, and ease of handling.

[0028] The temperature of the acidic solution is preferably heated to 70° C. or higher. With this configuration, selenium can be easily recovered as black selenium. The temperature of the acidic solution is more preferably heated to 75° C. or higher. From the viewpoint of heating efficiency, the temperature of the acidic solution may be 100° C. or lower.

[0029] It is preferable to heat the acidic solution to 70° C. or higher, and add acetone at an amount of 0.4 mL / L or less per 1 g / L selenium concentration in the acidic solution at intervals of 1 minute or more. According to this configuration, acetone can be added intermittently in small amounts rather than at once, so that the concentration of ketocarboxylic acid in the acidic solution can be more effectively suppressed. Therefore, the generation of difficult-to-reduc selenium compounds can be effectively suppressed. By suppressing the amount of acetone added, the interval between addition of acetone can be shortened. Moreover, when the amount of acetone added is large, it is preferable to lengthen the interval between addition of acetone. The longer the interval between addition of acetone, the more time can be provided for acetone to react as a reducing agent. Therefore, depending on the amount of acetone added, the interval between addition of acetone may be 5 minutes or more, 10 minutes or more, or even 30 minutes or more.

[0030] It is also preferable to add the oxidizing agent after each addition of acetone, which can more effectively suppress the generation of difficult-to-reduc selenium compounds.

[0031] In the addition of acetone to the acidic solution, it is preferable to gradually decrease the amount of acetone added at one time. In the acidic solution, acetone and selenious acid react first to produce an organic selenium intermediate. This intermediate becomes an aldehyde or pyruvic acid after rearrangement. Of these, pyruvic acid is considered to be the cause of the production of difficult-to-reduc selenium compounds. Pyruvic acid itself is thermally decomposed or oxidized to selenious acid, but the reaction rate is not fast. By utilizing this property, the amount of acetone added to the acidic solution can be decreased in the later stage of the reaction when the amount of selenious acid is reduced, and the amount of pyruvic acid added in the reaction system is adjusted so that it does not become excessive. In this way, the concentration of ketocarboxylic acid in the acidic solution can be further suppressed. From this viewpoint, it is preferable to intermittently add acetone in an amount of 0.04 mL / L or less per 1 g / L of selenium concentration in the acidic solution after the concentration of selenious acid in the acidic solution reaches 25 g / L or less in terms of selenium concentration.

[0032] When hydrogen peroxide is used as an oxidizing agent, it is generally handled as a 20 to 60% aqueous solution, but the concentration is not particularly specified. The amount of hydrogen peroxide added is preferably 0.03 volume times or more the amount of acetone added at one time.

[0033] When acetylaldehyde and ketocarboxylic acids accumulate in the acidic solution, the viscosity of the solution increases, and the rate at which selenious acid is reduced increases, causing the precipitated selenium to form large spheres. The precipitated selenium is preferably treated as a slurry, and hydrogen peroxide has the effect of oxidizing and decomposing the acetylaldehyde and ketocarboxylic acids to turn the precipitated elemental selenium into a slurry.

[0034] The precipitate formed in the acidic solution is subjected to solid-liquid separation using a filter press or the like to obtain selenium. The purity of the selenium recovered by solid-liquid separation can be increased by further distillation. Alternatively, sulfur dioxide may be blown into the acidic solution after the reduction and precipitation of selenium to decompose the excess oxidizing agent, and the remaining selenium may be recovered by precipitation. EXAMPLES

[0035] The following examples are provided to better understand the present invention and its advantages, but the present invention is not limited to these examples.

[0036] <Preparation of the treatment liquid (acidic liquid containing selenious acid)> Copper was removed from the electrolytic precipitate recovered from the copper electrorefining process of copper smelting by treating it with sulfuric acid. Next, concentrated hydrochloric acid and 60% hydrogen peroxide were added to dissolve the material, and the material was subjected to solid-liquid separation to obtain PLS (pregnant leach liquor). Next, the PLS was cooled to 6°C to precipitate and remove the base metals, and then the PLS was mixed with DBC (dibutyl carbitol) whose acid concentration had been adjusted to 2N or more to extract the gold. The PLS after gold extraction was used as the treatment solution. The selenium concentration of the treatment solution was 36 g / L.

[0037] (Test Example 1) 300 mL of the above liquid to be treated was measured out and heated to 80 to 85°C. Next, 1 mL of acetone was added to start the reaction in Example 1. 1 mL of acetone was added every 60 minutes. Here, 0.1 mL of hydrogen peroxide solution (30% by volume) was added 40 minutes before each addition of acetone from the second time onwards. In Example 2, 1 mL of acetone was added to start the reaction. 1 mL of acetone was added every 60 minutes. In Example 2, hydrogen peroxide solution was not added, and the reaction was aerated from the addition of acetone until the reaction was stopped. For the aeration, a tube was inserted into the reaction solution, and air was supplied at 200 mL / min using a pump. In both cases of Examples 1 and 2, after the total amount of acetone added reached 5 mL, stirring was continued for an additional 30 minutes. After the addition of acetone was completed, 2 mL of hydrogen peroxide was added again and stirred for 30 minutes in order to decompose the remaining aldehyde and pyruvic acid and suppress the generation of difficult-to-reduc selenium during the subsequent reduction with sulfur dioxide. Next, the temperature of the reaction solution was adjusted to 75°C, and a mixture of sulfur dioxide and air was blown in. The reaction was stopped after one hour, and solid-liquid separation was performed. The selenium remaining in the solution at this stage was treated as a difficult-to-reduc selenium compound. The amount of water lost during the selenium reduction reaction was replenished with pure water.

[0038] Before and after the blowing of the mixture of sulfur dioxide and air, sample solutions of Examples 1 and 2 were collected. 2 mL was taken from the sample solution and adjusted to 50 mL. The selenium concentration in the solution was quantified using ICP-OES (Seiko SPS3100). The test conditions and evaluation results are shown in Table 1.

[0039] [Table 1]

[0040] Table 1 shows that the generation of difficult-to-reduc selenium was suppressed by supplying an oxidizing agent during the reduction of selenious acid. In terms of the amount of hydrogen peroxide added, the effect was seen with 0.1 mL of hydrogen peroxide (30% by volume) per 1 mL of acetone. In other words, the amount of hydrogen peroxide added was 0.03 times the volume of acetone.

[0041] (Test Example 2) 300 mL of the same liquid to be treated as in Test Example 1 was measured out and heated to 80 to 85°C. Next, 0.5 mL of acetone was added to start the reaction. 0.5 mL of acetone was added every 30 minutes (Example 3) or every 60 minutes (Example 4). 15 minutes before the addition of acetone, 0.1 mL of hydrogen peroxide solution was added each time. After the total amount of acetone added reached 2 mL, the mixture was stirred for 30 minutes and a mixture of sulfur dioxide and air was blown in. In Comparative Example 1, 2 mL of acetone was initially added, and after stirring for 120 minutes, a mixture of sulfur dioxide and air was blown in to reduce selenious acid. In both the Example and Comparative Example, hydrogen peroxide solution was not added before blowing in the mixture of sulfur dioxide and air. The subsequent operations were the same as in Test Example 1. The selenium concentration was quantified using ICP-OES (Seiko SPS3100). The test conditions and evaluation results are shown in Table 2.

[0042] [Table 2]

[0043] According to Table 2, the amount of selenium reduced and precipitated by 2 mL of acetone / 300 mL of target liquid was 12 g / L, and it was found that the amount of hardly-reducible selenium produced was smaller the longer the interval between acetone additions. The reduction rate of selenious acid by acetone is fast, and the inhibition of the production of difficult-to-reducible selenium is affected by the reduction rate of selenious acid. However, it is found that the production of difficult-to-reducible selenium can be inhibited by intermittently adding acetone, and it is more preferable that the time interval be 30 minutes or more.

[0044] (Test Example 3) 300 mL of the same liquid to be treated as in Test Example 1 was measured out and heated to 75 to 80°C. Next, 1 mL of acetone was added to start the reaction. Acetone was added every 60 minutes. The amount of acetone added was gradually reduced. 0.1 mL (Examples 5 and 7) or 0.2 mL (Example 6) of hydrogen peroxide solution was added each time 15 minutes before the addition of acetone. After the total amount of acetone added reached 2 mL, the mixture was stirred for 30 minutes and then a mixture of sulfur dioxide and air was blown in to reduce the remaining selenite. No hydrogen peroxide was added before blowing in the mixture of sulfur dioxide and air. As Comparative Example 2, a reduction was also carried out in which hydrogen peroxide was not added each time. The subsequent operations were the same as in Test Example 1. The selenium concentration was quantified using ICP-OES (Seiko SPS3100). The test conditions and evaluation results are shown in Table 3. The selenium concentration during the reaction is shown in Table 4. "Amount of acetone added / Se" in Table 4 indicates "amount of acetone added per one addition (ml / L) ÷ selenium concentration in the acidic solution (g / L)."

[0045] [Table 3]

[0046] [Table 4]

[0047] The results of Examples 5 to 7 show that the production of difficult-to-reduc selenium can be more effectively suppressed by adding hydrogen peroxide during the reduction reaction of selenious acid and then decreasing the amount of acetone added as the reaction progresses.

[0048] It is believed that the difficult-to-reduc selenium is generated when selenious acid reacts with acetone before it reaches the final reduction product, elemental selenium. Therefore, the intermediate product was oxidized and decomposed with hydrogen peroxide, and acetone was added to minimize the association of the intermediate product with acetone, which proved effective. Furthermore, comparing Comparative Example 1 and Comparative Example 2, it is clear that even gradually reducing the amount of acetone added is effective.

[0049] Although the reaction temperature is different between Example 3 and Examples 5 to 7, it is possible to overcome the effect of temperature by adjusting the amount of acetone added. In particular, compared to Example 7, the amount of hardly reducible selenium produced is half that of Example 7, despite the lower temperature.

Claims

1. This is a method for recovering selenium from an acidic solution containing selenious acid by reduction and precipitation, A method for recovering selenium, comprising the steps of: continuously or intermittently adding acetone to the acidic solution; and oxidizing an organic compound produced by a reaction between the acetone and the selenious acid with an oxidizing agent.

2. 2. The method for recovering selenium according to claim 1, wherein the temperature of the acidic solution is raised to 70° C. or higher, and acetone is intermittently added at intervals of 1 minute or more in an amount of 0.4 mL / L or less per 1 g / L of selenium concentration in the acidic solution.

3. 3. The method for recovering selenium according to claim 2, wherein the acetone is added intermittently in an amount of 0.4 mL / L or less at intervals of 30 minutes or more.

4. The method for recovering selenium according to any one of claims 1 to 3, wherein the amount of acetone added at one time is reduced stepwise in the addition of acetone to the acidic solution.

5. 5. The method for recovering selenium according to claim 4, wherein after the concentration of selenious acid in the acidic solution reaches 25 g / L or less in terms of selenium concentration, acetone is intermittently added in an amount of 0.04 mL / L or less per 1 g / L of selenium concentration in the acidic solution.

6. The method for recovering selenium according to any one of claims 1 to 5, wherein acetone is intermittently added to the acidic solution, and the oxidizing agent is added after each addition of acetone.

7. 7. The method for recovering selenium according to claim 1, further comprising blowing sulfur dioxide into the acidic solution after the selenium has been reduced and precipitated to decompose excess oxidizing agent and recover remaining selenium by precipitation.

8. The method for recovering selenium according to any one of claims 1 to 7, wherein the oxidizing agent is an oxidizing agent that oxidizes a ketocarboxylic acid.

9. 9. The method for recovering selenium according to claim 8, wherein the oxidizing agent for oxidizing the ketocarboxylic acid is at least one of hydrogen peroxide, oxygen, hypochlorous acid, and ozone.

10. 10. The method for recovering selenium according to claim 9, wherein the amount of hydrogen peroxide added to the acidic solution is at least 0.03 times by volume the amount of acetone added at one time.

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