JP2025026261A - How to Collect Iridium - Google Patents

Abstract

The present invention provides a method for effectively separating and recovering iridium from a hydrochloric acid solution containing iridium, sulfur, and one or more of copper, antimony, lead, and arsenic.
[Solution] A method for recovering iridium, which comprises subjecting a hydrochloric acid solution containing iridium, sulfur, and one or more of copper, antimony, lead, and arsenic to the following treatment steps (1) to (3) in this order.
(1) a precipitation recovery step of heating a hydrochloric acid solution to 40° C. or higher and adding a thiosulfate salt or a solution containing thiosulfate ions to precipitate and recover iridium;
(2) A step of introducing the precipitate recovered in the step (1) into an acidic sulfuric acid or hydrochloric acid solution having a concentration of 0.5 N or more to dissolve and separate metal components;
(3) A step of introducing the precipitate obtained after separating the metal components in the step (2) into an alkaline solution containing sulfites and heating it to 20° C. or higher to dissolve and separate the sulfur.
[Selected Figure] Figure 1

Description

The present invention relates to a method for recovering iridium, and in particular to a method for recovering iridium from a hydrochloric acid solution containing iridium, sulfur, and one or more of copper, antimony, lead, and arsenic.

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 scraps containing precious metals from electronic parts have been fed into the converter as recycled raw materials, and 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 the precious metals have been 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 shown in Patent Document 1 for recovering precipitates produced by sulfur dioxide has many advantages in terms of cost and production scale. In addition, since each element precipitates sequentially, it is also effective for separation and purification.

In the method of recovering valuables using sulfur dioxide, valuables can be recovered by successive reduction after dissolution. Platinum and palladium precipitate first. Selenium is then reduced. Iridium and ruthenium have a relatively low redox potential and are therefore less susceptible to reduction, remaining in the solution until the end. As for iridium, as described in Patent Document 3, a widely known method is to separate and concentrate it by solvent extraction, then calcinate it to recover it. Patent Document 4 also discloses a method in which sodium thiosulfate salt is added to an aqueous solution containing iridium to precipitate the iridium.

JP 2001-316735 A JP 2016-160479 A JP 2004-332041 A JP 2022-135956 A

The precipitate obtained from thiosulfate or a solution containing thiosulfate ions contains many impurities. Most of the impurities are elemental sulfur produced by the decomposition of thiosulfate ions, and metal sulfides that have been sulfurized by thiosulfate ions. These generally contain only about 0.5 to 3 mass% iridium.

One method for refining iridium of this level of quality is to use solvent extraction after remelting, as shown in Patent Document 3, but this consumes a large amount of acid, increasing costs. In addition, because elemental sulfur does not dissolve, there are concerns that this could result in a decrease in the iridium recovery rate and an increase in the cost of processing elemental sulfur.

It would be ideal if it could be put into the existing iridium refining process, but there is a problem with removing elemental sulfur, which is generally achieved by the roasting method. However, the roasting method poses the problem of how to deal with sulfurous acid gas derived from sulfur. Furthermore, elemental sulfur is classified as a hazardous material and is difficult to handle due to the risk of dust explosions.

In view of the above-mentioned conventional circumstances, an embodiment of the present invention provides a method for effectively separating and recovering iridium from a hydrochloric acid solution containing iridium, sulfur, and one or more of copper, antimony, lead, and arsenic. The embodiment of the present invention is particularly suitable for separating and recovering iridium from a hydrochloric acid solution obtained by oxidizing and dissolving electrolytic sediment generated in the electrolytic refining process in copper smelting.

The above problems can be solved by the inventions specified below.
[1] A method for recovering iridium, comprising subjecting a hydrochloric acid solution containing iridium, sulfur, and one or more of copper, antimony, lead, and arsenic to the following treatment steps (1) to (3) in this order:
(1) a precipitation recovery step of heating the hydrochloric acid solution to 40° C. or higher and adding a thiosulfate salt or a solution containing thiosulfate ions to precipitate and recover iridium;
(2) A step of introducing the precipitate recovered in the step (1) into an acidic sulfuric acid or hydrochloric acid solution having a concentration of 0.5 N or more to dissolve and separate metal components;
(3) A step of introducing the precipitate obtained after separating the metal components in the step (2) into an alkaline solution containing sulfites and heating the solution to 20° C. or higher to dissolve and separate the sulfur.
[2] The method for recovering iridium according to [1] above, wherein in the step (1), a thiosulfate salt or a solution containing thiosulfate ions is added so that the concentration, calculated as sodium thiosulfate pentahydrate, is 10 g/L or more based on thiosulfate ions, thereby precipitating iridium.
[3] The method for recovering iridium according to [1] above, wherein in the step (1), the hydrochloric acid solution contains ruthenium, and the ruthenium is precipitated and recovered simultaneously with iridium.
[4] The method for recovering iridium according to [1] above, wherein in the step (2), the sulfuric acid or hydrochloric acid solution contains iron (III) ions in an amount of 0.05 times or more by mass of the iridium-containing precipitate.
[5] The method for recovering iridium according to [1] above, wherein in the step (2), the precipitate recovered in the step (1) is added to the sulfuric acid or hydrochloric acid solution so that the slurry concentration is 150 g/L or less.
[6] The method for recovering iridium according to [1] above, wherein in the step (3), the content of sulfites in the alkaline solution is equivalent to 1.5 or more times by mass, in terms of sodium sulfite, of the mass of the precipitate obtained in the step (1).
[7] The method for recovering iridium according to [1] above, wherein in the step (3), a liquid obtained by dissolving and separating the sulfur is used as the thiosulfate ion-containing solution in the step (1) after adjusting a thiosulfate ion concentration.

According to an embodiment of the present invention, a method for effectively separating and recovering iridium from a hydrochloric acid solution containing iridium, sulfur, and one or more of copper, antimony, lead, and arsenic can be provided.

1 is a flowchart showing an example of a method for recovering iridium according to an embodiment of the present invention. 1 is a graph showing the relationship between the amount of sodium sulfite added and the final residue mass in Experimental Example 3.

The following describes an embodiment of the method for recovering iridium according to the present invention, but the present invention should not be interpreted as being limited thereto, and various changes, modifications, and improvements may be made based on the knowledge of those skilled in the art without departing from the scope of the present invention.

<Iridium Precipitation Recovery>
The method for recovering iridium according to the embodiment of the present invention includes a process for precipitating and recovering iridium, a process for dissolving and separating metal components, and a process for dissolving and separating sulfur. Fig. 1 is a flow chart showing an example of the method for recovering iridium according to the embodiment of the present invention. The flow chart in Fig. 1 shows an example, and the method for recovering iridium according to the embodiment of the present invention is not limited thereto.

In the iridium recovery method according to an embodiment of the present invention, the hydrochloric acid solution containing iridium to be treated may be obtained through any process, but in particular, a hydrochloric acid solution obtained by oxidizing and dissolving electrolytic precipitate generated in the electrolytic refining process in copper smelting is suitable as the hydrochloric acid solution containing iridium to be treated in the iridium recovery method according to an embodiment of the present invention.

The hydrochloric acid solution to be treated in the iridium recovery method according to the embodiment of the present invention contains iridium (Ir), sulfur (S), and one or more of copper (Cu), antimony (Sb), lead (Pb), and arsenic (As). The hydrochloric acid solution to be treated may also contain alkali metals, alkaline earth metals, antimony (Sb), bismuth (Bi), etc. These do not react with the thiosulfate ions described below, so no special treatment is required. Furthermore, the hydrochloric acid solution to be treated may contain selenium (Se), ruthenium (Ru), tellurium (Te), etc., but since these are reduced by the thiosulfate ions, it is preferable to reduce the concentrations of these metals in advance, as described in detail below, in order to improve the treatment efficiency.

Furthermore, iron (Fe) may be contained in the hydrochloric acid solution to be treated since it does not react with thiosulfate ions if it is divalent. On the other hand, trivalent iron (Fe) may also be contained in the hydrochloric acid solution to be treated, but since it will react with thiosulfate ions, it is preferable to reduce it to divalent iron before adding thiosulfate ions.

As an example, we will show how to make a hydrochloric acid solution containing iridium from electrolytic sediment from the copper electrolytic refining process of copper smelting. First, copper is dissolved and removed from the electrolytic sediment from the copper electrolytic refining process of copper smelting using sulfuric acid. Next, concentrated hydrochloric acid and hydrogen peroxide are added to dissolve the material, and the resulting solution is separated into solid and liquid to obtain the pregnant leach solution (PLS). Platinum group elements, rare metal elements, chalcogen elements, arsenic, antimony, etc. are distributed in the pregnant leach solution (PLS), which is a chloride bath.

The pregnant leach solution (PLS) is cooled once, and chlorides of base metals such as lead and antimony are precipitated and separated. Gold is then separated into an organic phase by solvent extraction. Dibutyl carbitol (DBC) is widely used as an extractant for gold. Sulfur dioxide is then blown into the extract to reduce and remove precious metals such as platinum and palladium, as well as selenium and tellurium, and a hydrochloric acid solution containing iridium is produced by solid-liquid separation.

Since the iridium concentration in this iridium-containing hydrochloric acid solution is about 10 to 100 mg/L, it is necessary to further concentrate it. As described in Patent Document 4, precipitation separation using thiosulfate is a known concentration method. The iridium-containing hydrochloric acid solution may also contain copper, antimony, lead, arsenic, ruthenium, etc. that were not completely reduced by sulfur dioxide. Arsenic and copper can be selectively precipitated and separated by sulfurization treatment using hydrogen sulfide, sodium hydrosulfide, or sodium sulfide before precipitating and recovering iridium. In this sulfurization treatment, trace amounts of selenium, tellurium, etc. may also precipitate. In addition, ruthenium is often mixed in the raw material for refining iridium, and a method for separating them from each other has been established, so iridium and ruthenium can be simultaneously recovered by adding thiosulfate. In particular, a method of distillation using sodium bromate after dissolving in acid is known.

(Iridium Precipitation Recovery Process)
In the iridium precipitation recovery process, the hydrochloric acid solution is heated to 40° C. or higher, and a thiosulfate or thiosulfate ion-containing solution is added to precipitate and recover iridium. The precipitation of iridium (also called iridium-containing precipitate) is promoted by adding a thiosulfate or thiosulfate ion-containing solution to the hydrochloric acid solution heated to 40° C. or higher. The hydrochloric acid solution is preferably heated to 60° C. or higher, and more preferably heated to 80° C. or higher. Examples of the thiosulfate salt to be added include sodium thiosulfate and its hydrate, and ammonium thiosulfate. Examples of the thiosulfate ion-containing solution to be added include a solution prepared by dissolving elemental sulfur in sodium sulfite solution, a solution in which elemental sulfur is suspended in sodium hydroxide solution and sulfur dioxide is blown in, and the like.

In the iridium precipitation recovery process, it is preferable to precipitate iridium by adding thiosulfate or a solution containing thiosulfate ions so that the amount is 10 g/L or more, calculated as sodium thiosulfate pentahydrate, based on thiosulfate ions. This configuration can promote the precipitation of iridium by thiosulfate ions.

(Metal component dissolution and separation process)
In the dissolution and separation process of metal components, the iridium-containing precipitate recovered in the above-mentioned iridium precipitation and recovery process is put into a sulfuric acid or hydrochloric acid solution of 0.5N or more to dissolve and separate the metal components. Here, since the iridium-containing precipitate contains metal sulfides, the metal components can be dissolved and separated by decomposing and removing them with an acid. As the acid to be used, sulfuric acid or hydrochloric acid having an oxidizing power that does not re-dissolve the precipitated iridium or ruthenium is preferable. In addition, the iridium-containing precipitate recovered in the above-mentioned iridium precipitation and recovery process is in the form of a slurry, and it is preferable to add the slurry to a sulfuric acid or hydrochloric acid solution so that the concentration of the slurry is 150 g/L or less, since this promotes the dissolution and separation of the metal components.

When sulfuric acid is used as the acid, the oxidizing power may be insufficient. Therefore, it is preferable to add iron (III) ions to the sulfuric acid, since this will allow the dissolution of the metal sulfide to proceed more efficiently. In terms of specific amounts, it is preferable that the sulfuric acid or hydrochloric acid solution contains iron (III) ions in an amount 0.05 times or more by mass of the iridium-containing precipitate. As the iron (III) ion-containing liquid, a liquid obtained by oxidizing an iron (II) ion-containing liquid, ferric sulfate, etc. are suitable.

There are no particular temperature restrictions for the acid decomposition in the metal component dissolution and separation process, but a higher temperature is preferable because it shortens the processing time. From this perspective, the acid decomposition is preferably performed at 40°C or higher, more preferably at 60°C or higher, and even more preferably at 80°C or higher. In addition, the higher the concentration of the acid used in the acid decomposition, the greater the decomposition effect, so it is preferable. From this perspective, the acid concentration is preferably 0.5N or higher, more preferably 0.8N or higher, and even more preferably 1.0N or higher.

After acid decomposition in the metal component dissolution and separation process, the metal components are separated by solid-liquid separation, and the residue contains iridium, ruthenium, elemental sulfur, etc.

(Sulfur dissolution and separation process)
In the sulfur dissolution and separation step, the precipitate (iridium-containing precipitate) remaining after separation of the metal components in the above-mentioned metal component dissolution and separation step is introduced into an alkaline solution such as an aqueous sodium hydroxide solution or aqueous ammonia containing sulfites, and heated to 20° C. or higher to dissolve the sulfur (elementary sulfur), which is then separated by solid-liquid separation. At this time, when the elemental sulfur is dissolved and removed as thiosulfate ions according to the following chemical formula (1), the iridium concentration of the iridium-containing precipitate, which is the residue after solid-liquid separation, becomes high.

The sulfite used here is preferably sodium sulfite, sodium hydrogen sulfite, or water into which sulfur dioxide has been blown and absorbed. However, the thiosulfate ion generated according to the above chemical formula (1) is unstable and decomposes under acidic conditions, so the reaction must be carried out while maintaining an alkaline state.

Furthermore, the iridium-containing precipitate after dissolution and separation of the above-mentioned metal components may contain arsenic sulfide. Whether the arsenic is trivalent or pentavalent, arsenic sulfide is decomposed into sulfur and arsenic oxoanions by highly oxidizing nitric acid or hydrogen peroxide, but is stable in mineral acids. For this reason, it is preferable to prepare a solution containing sulfurous acid as an alkaline solution with a pH of 10 or higher.

In the treatment with sulfites, the temperature of the reaction solution is preferably 20 to 70°C. If the temperature of the reaction solution is 20°C or higher, the reaction rate increases, and if the temperature of the reaction solution is 70°C or lower, the decomposition of sulfites can be suppressed. The temperature of the reaction solution is more preferably 30 to 50°C.

The amount of sulfurous acid required to be added depends on the content of elemental sulfur and metal sulfides (referred to as non-sulfuric sulfur) in the iridium-containing precipitate. For example, when 37% by mass of the iridium-containing precipitate is non-sulfuric sulfur, the content of sulfurous acid in the alkaline solution is preferably 1.5 times or more by mass, calculated as sodium sulfite, relative to the mass of the iridium-containing precipitate recovered in the iridium precipitation recovery process. Since the iridium-containing precipitate is expected to contain 30 to 40% by mass of non-sulfuric sulfur, it is preferable to add sulfurous acid at least to the same extent. In other words, it is preferable to add sulfurous acid at a level equivalent to or more than 1.5 times by mass, calculated as sodium sulfite, relative to the mass of the iridium-containing precipitate, and more preferably to add sulfurous acid at a level equivalent to or more than 2 times by mass. When the content of non-sulfuric sulfur in the iridium-containing precipitate can be known in advance, it is preferable to add sulfurous acid so that the amount of sulfite ions is equal to or more than the amount of non-sulfuric sulfur.

In addition, since the reaction follows the above chemical formula (1), if non-sulfuric sulfur can be quantified, sulfites are required in an amount four times or more by mass relative to the non-sulfuric sulfur, and it is more preferable to add five times or more by mass of sulfites.

The method for quantifying non-sulfate sulfur is not specified. For example, there is a method for quantifying non-sulfate sulfur by subtracting the sulfur content in sulfuric acid dissolved with dilute hydrochloric acid of pH 1-2 from the total sulfur content in the iridium-containing precipitate. Another method is to add nitric acid to the iridium-containing precipitate to decompose the metal sulfide, then measure the mass of the residue after filtering, and then contact the residue with a solution containing sulfite ions to calculate the amount of non-sulfate sulfur from the weight loss.

As shown in the above chemical formula (1), elemental sulfur dissolves as thiosulfate ions. Therefore, the liquid obtained by dissolving and separating sulfur can be reused as a thiosulfate ion-containing solution in the above-mentioned iridium precipitation and recovery process after adjusting the thiosulfate ion concentration. If the thiosulfate ion concentration is insufficient, it can be reused by adding a thiosulfate salt such as sodium thiosulfate and adjusting the pH. Alternatively, the liquid obtained by dissolving and separating sulfur can be cooled, and the precipitated sodium thiosulfate pentahydrate can be recovered and used in the above-mentioned iridium precipitation and recovery process.

The present invention will be explained in more detail below with reference to examples. However, the present invention is not limited to these examples.

(Experimental Example 1)
Copper was dissolved and removed from the electrolytic sediment from the copper electrorefining process of copper smelting using 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 acid concentration was adjusted to 2N or more and DBC (dibutyl carbitol) was added to extract the gold.
Next, the PLS after gold extraction was heated to 70° C., and sulfur dioxide was blown in to reduce and remove the precious metals, selenium, and tellurium. This was then subjected to solid-liquid separation to obtain a hydrochloric acid solution containing iridium.
Next, the hydrochloric acid solution containing iridium was heated to 80°C, and sodium thiosulfate pentahydrate was added to 10 g/L to obtain a slurry of iridium-containing precipitate. The components of the slurry of the iridium-containing precipitate are shown in Table 1. Analysis of non-sulfate sulfur (S) was performed by heating 1 g of the iridium-containing precipitate in nitric acid, measuring the mass of the undissolved portion, and adding 2 g/100 mL of sodium sulfite to calculate the difference in the mass of the residue after dissolution. The composition of "S" in Table 1 (48 mass%) indicates the total amount of sulfate S (11 mass%) and non-sulfate S (37 mass%).

Next, 2 g of the slurry of the iridium-containing precipitate was taken, and 200 mL of each of the acidic dissolving solutions shown in Table 2 was poured into the sample.
Next, 2 g of ferric sulfate n-hydrate (iron content 20% by mass) was added as a source of iron (III) ions, and the mixture was heated to 60° C. and stirred for 90 minutes. After the reaction, the mixture was filtered and diluted 25 times with dilute hydrochloric acid, and the element concentration in the solution was measured by ICP-OES (Seiko SPS3100). The residue was washed with water and rinsed with alcohol, air-dried overnight, and the mass was measured.
4 g of sodium sulfite was added to the residue, 100 mL of water was poured, and the mixture was heated and stirred at 60° C. for 1 hour. After the reaction, the residue was filtered, washed with water and rinsed with alcohol, air-dried overnight, and the mass was measured. The difference between the mass of the residue after acid decomposition and the mass of the residue after sodium sulfite treatment (final residue) was taken as the non-sulfated sulfur (S) content. If this value is significantly lower than expected (37%), it is considered that the decomposition of the metal sulfide is insufficient.
All reagents used were of special grade manufactured by Wako Pure Chemical Industries, Ltd. The element concentrations in the solutions were quantified by taking 2 mL of the solution, adjusting the volume to 50 mL, and then quantifying the concentrations using ICP-OES. The acid leaching rate (%) was calculated from the obtained concentrations. The evaluation results are shown in Table 2.

The results in Table 2 show that with dissolving solutions other than nitric acid, it was possible to dissolve impurity elements while suppressing the loss of iridium and ruthenium. Furthermore, it can be seen that in systems in which iron (III) ions were added to hydrochloric acid and sulfuric acid, copper dissolution was also highly effective, and even cupric sulfide was able to be dissolved.

The results in Table 2 also show that there is little dependence on the acid concentration for both hydrochloric acid and sulfuric acid. However, when the hydrochloric acid concentration reached 6N, a significant loss of iridium was observed. It is questionable whether cupric sulfide could be dissolved using 0.6N hydrochloric acid, and this problem can be solved by removing the copper through a sulfurization treatment before obtaining the iridium-containing precipitate.

(Experimental Example 2)
2 g of the iridium-containing precipitate slurry obtained in Experimental Example 1 was weighed out, 200 mL of 6.9 N sulfuric acid was poured into it, and 0.5 g to 6 g of ferric sulfate n-hydrate (iron content: 20% by mass) was added as a source of iron (III) ions.
Next, acid decomposition was performed at the same temperature and time as in Experimental Example 1, and solid-liquid separation was performed. The element concentrations in the solution after separation were quantified, and the acid leaching rate of each element was calculated, as in Experimental Example 1. The experimental conditions and evaluation results are shown in Table 3.

The results in Table 3 show that adding 0.25 times or more by mass of iron (III) sulfate n-hydrate (iron content 20% by mass) to the iridium-containing precipitate, i.e., adding 0.5 g or more of iron (III) sulfate n-hydrate to 2 g of raw material, dissolves and removes the majority of impurities such as copper, arsenic, and tellurium. This amount, in terms of iron (III) ions, is 0.05 times the mass of the raw material precipitate.

More preferably, the amount of final residue is also greatly reduced by adding more than 2 g of iron (III) sulfate n-hydrate. This shows that although copper sulfide is relatively susceptible to oxidation by iron (III) ions, in order to oxidize and decompose other sulfides, such as lead sulfide, more than 2 g of iron (III) sulfate n-hydrate is required for every 2 g of raw material.

The non-sulfate sulfur content of the iridium-containing precipitate is 37% by mass, and iron (III) ions act on metal sulfides. The elemental sulfur in the iridium-containing precipitate varies depending on the lot, but as a rough guide, it is found that it is sufficient to add at least 0.6 times the mass of the non-sulfate sulfur, which is iron (III) sulfate n-hydrate (iron content 20% by mass) under the 0.5 g addition condition in Table 3, and at least 0.13 times the mass of the non-sulfate sulfur as iron (III) ions.

(Experimental Example 3)
2 g of the iridium-containing precipitate slurry obtained in Experimental Example 1 was weighed out, 200 mL of 3.2 N sulfuric acid was poured into it, 2 g of ferric sulfate n-hydrate (iron content 20 mass%) was added as a source of iron (III) ions, and the mixture was heated to 60° C. and stirred for 90 minutes. The ORP (oxidation-reduction electrode potential, reference electrode Ag/AgCl) of the solution after the reaction was 530±5 mV. The reaction was carried out by the same acid decomposition procedure as in Experimental Example 1. The mass of the obtained residue was 0.83 g for 2 g of raw material.
Next, 1 to 5 g of sodium sulfite was added, 100 mL of water was added, and the mixture was heated at 50° C. for 1 hour.
Next, elemental sulfur was dissolved and separated by the same operation as in Experimental Example 1. A graph showing the relationship between the amount of sodium sulfite added and the final residue mass is shown in Figure 2. According to the graph in Figure 2, as the amount of sodium sulfite added increases, the amount of residue decreases, and once the amount reaches 3 g, the linearity decreases. Therefore, if only elemental sulfur is to be removed, it is sufficient to add sodium sulfite in an amount 1.5 times or more by mass relative to the raw material iridium-containing precipitate.

Adding sodium sulfite also causes no problems, as the reverse reaction of the reaction shown in chemical formula (1) above is suppressed, and the amount of residue gradually decreases.

(Experimental Example 4)
As the sulfuric acid decomposition-sulfurous acid treatment, 5 g to 30 g of the slurry of the iridium-containing precipitate obtained in Experimental Example 1 was weighed out, 200 mL of 3.2 N sulfuric acid was poured, and 4 g of ferric sulfate n-hydrate (iron content 20 mass%) was added. The mixture was heated to 60°C and stirred for 90 minutes. Next, the mass of the obtained residue was measured, and sodium sulfite was added in an amount twice the mass of the iridium-containing precipitate as the raw material, and 100 mL of water was added, followed by heating at 50°C for 1 hour. Furthermore, in one condition, a test was performed by adding 8 g of ferric sulfate n-hydrate (iron content 20 mass%).
In addition, as a hydrochloric acid decomposition-sulfurous acid treatment, 5 g to 30 g of the slurry of the iridium-containing precipitate obtained in Experimental Example 1 was weighed out, 200 mL of 2.8 N hydrochloric acid was poured into it, and the mixture was heated to 60° C. and stirred for 90 minutes. Next, the mass of the obtained residue was measured, and sodium sulfite was added in an amount twice the mass of the iridium-containing precipitate as the raw material, and 100 mL of water was added, followed by heating at 50° C. for 1 hour. Furthermore, in one condition, a test was performed by adding 4 g of ferric sulfate n-hydrate (iron content 20 mass%).
Next, elemental sulfur was dissolved and separated in the same manner as in Experimental Example 1. The experimental conditions and evaluation results of the sulfuric acid decomposition-sulfurous acid treatment are shown in Table 4. The experimental conditions and evaluation results of the hydrochloric acid decomposition-sulfurous acid treatment are shown in Table 5.
In Table 4, the amount of the residue from the 25 g/L slurry of iridium-containing precipitate was too small to be quantitatively analyzed.

Tables 4 and 5 show that the slurry concentration of the iridium-containing precipitate has a significant effect on the leaching rate of copper and sulfur. If the slurry concentration of the iridium-containing precipitate is 150 g/L or less, more than half of the copper and sulfur can be expected to be leached out. Of course, it is not necessary to limit the acid decomposition to one time, and it is also possible to increase the slurry concentration and treat with acid multiple times.

It can be seen that even when the slurry concentration of the iridium-containing precipitate is high, leaching with sulfuric acid results in less loss of iridium and ruthenium than leaching with hydrochloric acid. However, since the leaching rate of sulfur is low, leaching with hydrochloric acid may be used when the sulfur content is high.

Claims (7)

A method for recovering iridium, comprising subjecting a hydrochloric acid solution containing iridium, sulfur, and one or more of copper, antimony, lead, and arsenic to the following treatment steps (1) to (3) in this order:
(1) a precipitation recovery step of heating the hydrochloric acid solution to 40° C. or higher and adding a thiosulfate salt or a solution containing thiosulfate ions to precipitate and recover iridium;
(2) A step of introducing the precipitate recovered in the step (1) into an acidic sulfuric acid or hydrochloric acid solution having a concentration of 0.5 N or more to dissolve and separate metal components;
(3) A step of introducing the precipitate obtained after separating the metal components in the step (2) into an alkaline solution containing sulfites and heating the solution to 20° C. or higher to dissolve and separate the sulfur. The method for recovering iridium according to claim 1, wherein in step (1), thiosulfate or a solution containing thiosulfate ions is added so that the amount of thiosulfate ions is 10 g/L or more calculated as sodium thiosulfate pentahydrate, to precipitate iridium. The method for recovering iridium according to claim 1, wherein in step (1), the hydrochloric acid solution contains ruthenium, and the ruthenium is precipitated and recovered simultaneously with iridium. The method for recovering iridium according to claim 1, wherein in step (2), the sulfuric acid or hydrochloric acid solution contains iron (III) ions in an amount 0.05 times or more by mass of the iridium-containing precipitate. The method for recovering iridium according to claim 1, wherein in step (2), the precipitate recovered in step (1) is added to the sulfuric acid or hydrochloric acid solution so that the slurry concentration is 150 g/L or less. The method for recovering iridium according to claim 1, wherein in the step (3), the content of sulfites in the alkaline solution is equivalent to 1.5 times or more by mass, calculated as sodium sulfite, relative to the mass of the precipitate obtained in the step (1). The method for recovering iridium according to claim 1, wherein the liquid obtained by dissolving and separating the sulfur in the step (3) is used as the thiosulfate ion-containing solution in the step (1) after adjusting the concentration of the thiosulfate ions.