JP7664873B2 - How to recover selenium - Google Patents

Description

The present invention relates to a method for recovering selenium, and in particular, to a method for recovering selenium from an acidic solution containing selenious acid.

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. Valuable materials other than copper are precipitated as slime during electrolytic refining.

This slime also contains concentrated 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.

This slime is often treated by hydrometallurgy. For example, Patent Document 1 discloses a method in which silver is recovered from the slime using hydrochloric acid-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.

The solution left after 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 produced by the use of a reducing agent, and mixing the solution with copper concentrate, drying it in a dryer, and then feeding it back into the smelting furnace.

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 the level of its redox potential.

Dry smelting exhaust gas is generally used as the sulfur dioxide used to reduce selenium. This is because it has a significant cost advantage, and after the reaction it becomes sulfate ions, which do not adversely affect wastewater.

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 liquefied 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 increased reagent costs.

Ketones are an inexpensive and efficient alternative reducing agent to sulfur dioxide (Patent Document 3). Acetone in particular is inexpensive, has low toxicity, and has a high reduction efficiency per unit amount.

JP 2001-316735 A JP 2004-190134 A JP 2018-109207 A JP 2019-77902 A

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 case, "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.

However, there are regulations regarding the concentration of selenium in wastewater, and although it is only a small amount of a by-product, difficult-to-reduc selenium compounds place a significant burden on wastewater treatment processes. As a solution to this problem, a method of adsorption and removal using activated carbon is known, as shown in Patent Document 4, but this is not effective enough. 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 how to dispose of the activated carbon after use.

Hardly reducible selenium compounds are produced when a small portion of selenious acid reacts 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 selenious acid as selenium while suppressing the production of hard-to-reducible selenium compounds.

In view of the above-mentioned conventional circumstances, the present invention provides a method for recovering selenium by suppressing the formation 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.

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

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) The method for recovering selenium described in (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) 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 decreased in the addition of acetone to the acidic solution.
(5) The method for recovering selenium described in (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 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 (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 liquid is 0.03 volume times or more the amount of acetone added at one time.

The present invention provides a method for recovering selenium while suppressing the formation 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.

Next, a detailed description of the embodiment of the present invention will be given. The present invention is not limited to the following embodiment, and it should be understood that appropriate design changes, improvements, etc. may be made based on the ordinary knowledge of a person skilled in the art without departing from the spirit of the present invention.

Electrolytic slime, which is produced 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 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 5-15 mass% of selenium.

By adding hydrochloric acid and hydrogen peroxide to dissolve this electrolytic slime, a solution of copper electrolytic deposits is produced, in which silver forms an insoluble silver chloride precipitate with chloride ions immediately after dissolution. 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.

Precious metals are recovered using solvent extraction or redox potential difference. Selenium is then recovered by reduction, and the most common method is to use sulfur dioxide for this. If sulfur dioxide is not available in the selenium recovery process, selenite can be reduced to selenium using acetone.

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

After the selenium oxidation reaction, acetone 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, and these compounds after the reaction have room to undergo further reduction reactions.

It is likely 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.

On the other hand, when pyruvic acid is added to an acidic solution containing selenious acid and reduced with acetone, the selenious acid is no longer 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, the intermediate product ketocarboxylic acid can be produced and simultaneously oxidized and decomposed, which leads to the suppression of the production of difficult-to-reduc selenium compounds.

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, the 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 less active in oxidation reactions, the effect of the added oxidizing agent on the reduction of selenious acid by acetone is small.

If an excessive amount of oxidizing agent is added, it may re-dissolve the precipitated selenium. Therefore, the oxidizing agent is preferably an oxidizing agent that oxidizes ketocarboxylic acids, such as one or more of hydrogen peroxide, oxygen, hypochlorous acid, and ozone. Among these, hydrogen peroxide is the most suitable due to its reactivity, non-toxicity, and ease of handling.

It is preferable to heat the acidic liquid to a temperature of 70°C or higher. This configuration makes it easier to recover selenium as black selenium. It is more preferable to heat the acidic liquid to a temperature of 75°C or higher. From the viewpoint of heating efficiency, the temperature of the acidic liquid may be 100°C or lower.

It is preferable to heat the acidic solution to 70°C or higher, and add acetone at intervals of 1 minute or more in an amount of 0.4 mL/L or less per 1 g/L selenium concentration in the acidic solution. With 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 better suppressed. Therefore, the generation of difficult-to-reduc selenium compounds can be well suppressed. By suppressing the amount of acetone added, the interval between acetone additions can be shortened. Furthermore, if the amount of acetone added is large, it is preferable to make the interval between acetone additions longer. The longer the interval between acetone additions, 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 acetone additions may be 5 minutes or more, 10 minutes or more, or even 30 minutes or more.

It is also preferable to add the above-mentioned oxidizing agent after each addition of acetone. In this way, by adding the oxidizing agent after each addition of acetone, the production of difficult-to-reduc selenium compounds can be more effectively suppressed.

In the addition of acetone to the acidic solution, it is preferable to gradually reduce 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 and gradually reducing the addition of acetone to the acidic solution, the amount of addition can be reduced in the later stage of the reaction when selenious acid is reduced, and the pyruvic acid 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 perspective, 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.

When hydrogen peroxide is used as the oxidizing agent, it is generally handled as a 20-60% aqueous solution, but the concentration is not specifically specified. The amount of hydrogen peroxide added is preferably at least 0.03 volume times the amount of acetone added at one time.

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, which can cause the precipitated selenium to form large spheres. It is preferable to handle the precipitated selenium as a slurry, and hydrogen peroxide also has the effect of oxidizing and decomposing the acetylaldehyde and ketocarboxylic acids, turning the precipitated elemental selenium into a slurry.

Selenium is obtained by solid-liquid separation of the precipitate that has formed in the acidic solution using a filter press or similar. The purity of the selenium recovered by solid-liquid separation can be increased by further distillation. In addition, sulfur dioxide can be blown into the acidic solution after the selenium has been reduced and precipitated to break down the excess oxidizing agent, and the remaining selenium can be recovered by precipitation.

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

<Preparation of treatment target 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 mixture, and the mixture was subjected to solid-liquid separation to obtain PLS (pure 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 liquid. The selenium concentration in the treatment liquid was 36 g/L.

(Test Example 1)
300 mL of the 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 during the reaction 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. After one hour, the reaction was stopped and solid-liquid separation was performed. At this stage, the selenium remaining in the solution was treated as a difficult-to-reduc selenium compound. The amount of water lost during the selenium reduction reaction was replenished with pure water.

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.

Table 1 shows that the production 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.

(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 procedures 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.

According to Table 2, the amount of selenium reduced and precipitated by 2 mL of acetone/300 mL of the target solution was 12 g/L, and it was found that the amount of hardly-reducible selenium produced was smaller the longer the interval between additions of acetone.
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 clear that the production of difficult-to-reducible selenium can be inhibited by intermittently adding acetone, and that the time interval is preferably 30 minutes or more.

(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 selenious acid. 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 no hydrogen peroxide was added each time.
The subsequent procedures were the same as in Test Example 1. The selenium concentration was quantified using ICP-OES (Seiko SPS3100).
The test conditions and the 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)."

The results of Examples 5 to 7 show that adding hydrogen peroxide during the reduction reaction of selenious acid and then decreasing the amount of acetone added as the reaction progresses can better suppress the production of difficult-to-reduc selenium.

It is believed that difficult-to-reduc selenium is produced 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 association between the intermediate product and acetone, demonstrating its effectiveness. Furthermore, a comparison of Comparative Example 1 and Comparative Example 2 shows that even gradually reducing the amount of acetone added is effective.

The reaction temperatures in Example 3 and Examples 5 to 7 are different, but this shows that 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 difficult-to-reduc selenium produced is half, despite the lower temperature.

Claims (10)

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. 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 added intermittently 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. The method for recovering selenium according to claim 2, wherein the acetone is added intermittently at intervals of 30 minutes or more in an amount of 0.4 mL/L or less. The method for recovering selenium according to any one of claims 1 to 3, wherein the amount of acetone added at one time is gradually reduced when adding acetone to the acidic solution. 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 as a 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. 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 the acetone. The method for recovering selenium according to any one of claims 1 to 6, in which sulfur dioxide is blown into the acidic liquid after the selenium has been reduced and precipitated to decompose the excess oxidizing agent and recover the remaining selenium by precipitation. 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. 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. 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 the volume of the amount of acetone added at one time.