JP7627436B2 - How to manufacture cadmium metal - Google Patents

Description

The present invention relates to a method for producing metallic cadmium with reduced impurities using only wet processing.

Zinc oxide ore obtained by separating and recovering impurities from crude zinc oxide, etc., is widely used as a raw material for zinc ingots in zinc smelters. This crude zinc oxide can be obtained, for example, from steel dust generated in steelmaking furnaces such as blast furnaces and electric furnaces in the steel industry through a reduction roasting process, and from the viewpoint of promoting resource recycling, it is desirable to reuse steel dust as a raw material for zinc.
Such crude zinc oxide derived from steel dust contains, in addition to zinc oxide which is its main component, halogen components such as chlorine and fluorine, and impurities such as cadmium at high rates.
Of these impurities, cadmium in particular has the properties of a harmful metal, and so in zinc oxide manufacturing plants, a process for separating and recovering cadmium is essential.

On the other hand, cadmium is one of the important metals used in electronics, such as for use as the anode material in nickel-cadmium batteries. In this context, there is a need for technology to separate and recover cadmium at high purity from the above-mentioned crude zinc oxide and to utilize it effectively as an electronic material.
A commonly used method for separating and recovering cadmium is, for example, to roughly separate impurities by a wet process and then to refine the same by a dry process.

However, dry processing has the problem of high environmental and energy burden due to the use of fossil fuels and electricity.
On the other hand, when steel dust leachate contains a large amount of impurities, when wet processing is used alone, even if the cementation method using zinc, the ion exchange method using ion exchange resin, or the electrowinning method using electrolysis are performed, the impurities cannot be sufficiently separated, and only a rough separation of the impurities is achieved, as mentioned above.

For example, the method described in Patent Document 1 is a wet method only, but it can only increase the cadmium content to about 90%.
If impurities can be separated by wet processing, the problems of fossil fuels and energy burdens that occur in dry processing can be resolved.
Therefore, there has been a demand for the development of a method for producing high-purity metallic cadmium with reduced impurity concentrations by using only wet processing.

Japanese Patent Application Publication No. 11-12667

The object of the present invention is to solve these problems by providing a method for producing high-purity metallic cadmium with a high cadmium concentration and reduced impurity concentration using only wet processing.

As a result of extensive research into achieving the above object, the inventors discovered that the problems associated with the present invention could be solved by going through seven steps, namely, the first neutralization step, the second neutralization step, the leaching step, the metal extraction step, the acid washing step, the alkali washing step, and the water washing step, and thus completed the present invention.

That is, the first invention of the present invention includes a first neutralization step of adding a pH adjuster to a cadmium-containing solution to adjust the pH of the cadmium-containing solution to a range of 7.0 to 9.0 to form a first neutralized slurry in which zinc hydroxide and lead hydroxide are distributed in a solid phase portion , subjecting the first neutralized slurry to a solid-liquid separation treatment to remove the zinc hydroxide and the lead hydroxide from the system, and obtaining a filtrate containing cadmium;
a second neutralization step of adding a pH adjuster to the cadmium-containing filtrate obtained in the first neutralization step to adjust the pH of the filtrate to a value higher than the pH adjusted in the first neutralization step and within a range of 8.5 to 11.0, thereby forming a second neutralized slurry in which cadmium hydroxide is distributed in a solid phase portion , and subjecting the second neutralized slurry to a solid-liquid separation treatment to obtain a cadmium precipitate containing the cadmium hydroxide;
a leaching step of adding an acid to the cadmium precipitate obtained in the second neutralization step to adjust the pH to a range of 2.0 to 6.5 to form a leaching slurry containing cadmium, and then subjecting the leaching slurry containing cadmium to a solid-liquid separation treatment to obtain a cadmium leachate;
a metal extraction step of reducing cadmium in the cadmium leaching solution obtained in the leaching step by electrolysis to obtain metallic cadmium ;
an acid washing step of adding an acid to the metallic cadmium obtained in the metal extraction step to adjust the pH to a range of 0 to 4.0 to form an acid-washed slurry, and subjecting the acid-washed slurry to a solid-liquid separation treatment to obtain acid-washed metallic cadmium having reduced zinc and sulfur ;
an alkali washing step of adding an alkali to the acid-washed metallic cadmium obtained in the acid washing step to adjust the pH to a range of 10 to 14 to form an alkali-washed slurry, and subjecting the alkali-washed slurry to a solid-liquid separation treatment to obtain alkali-washed metallic cadmium having reduced sulfur ;
and a water washing step of washing the alkali-washed metallic cadmium obtained in the alkali washing step with water to obtain metallic cadmium from which alkali metals and alkaline earth metals have been removed .

A second aspect of the present invention is a method for producing metallic cadmium, wherein the current density in the metal extracting step of the first aspect is 50 A/m ² to 2000 A/m ² , and the cadmium concentration at the end point of electrolysis is 1.0 g/L to 3.0 g/L.

The third invention of the present invention is a method for producing metallic cadmium according to the first invention, in which the composition of the cadmium-containing solution is Cd≧0.05g/L, 0<Zn≦3g/L, 0<Pb≦0.1g/L, Cu≦0.01g/L, Tl≦0.05g/L, Ni≦0.005g/L, Mn≦0.1g/L, Ca≦1.0g/L, Mg≦0.1g/L, and Si≦0.1g/L.

The fourth invention of the present invention is a method for producing metallic cadmium according to the first invention, in which the starting solution used in the electrolysis method is a cadmium solution obtained by adding zinc powder to the cadmium leaching solution obtained in the leaching step in an amount of 0.2 g/L or more and 5 g/L or less, forming a slurry containing lead and copper as precipitates, and then subjecting the slurry to solid-liquid separation.

The fifth invention of the present invention is a method for producing metallic cadmium according to the first invention, in which in the leaching step, an oxidizing agent is further added to the leaching slurry to obtain an oxidation-reduction potential of 1000 mV or more (vs. Ag/AgCl), and the oxidation slurry containing manganese oxide precipitates is subjected to solid-liquid separation to obtain a cadmium leachate.

The method for producing cadmium metal of the present invention is of great industrial value because it can produce high-quality, i.e., high-purity, cadmium metal with reduced impurity concentrations using only wet processing.

The method for producing metallic cadmium of the present invention is a method for producing metallic cadmium, which uses a cadmium-containing solution containing cadmium as a starting material to obtain high-purity cadmium metal. The method is composed of seven steps, a first neutralization step, a second neutralization step, a leaching step, a metal extraction step, an acid washing step, an alkali washing step, and a water washing step. By going through the above steps, it is possible to produce high-purity cadmium that contains cadmium with a purity of 99.9% or more and has reduced impurities such as zinc, lead, copper, thallium, nickel, manganese, calcium, magnesium, silicon, sulfur, and sodium. Here, the cadmium-containing solution of the present invention means a cadmium solution with a cadmium concentration of 0.05 g/L or more. Each step will be explained below.

<First neutralization step>
The first neutralization step is a step in which a cadmium-containing solution is used as a raw material solution, a pH adjuster is added to this cadmium-containing solution, and the pH of the cadmium-containing solution is adjusted to a range in which zinc and lead form hydroxides, thereby forming a first neutralized slurry in which zinc hydroxide and lead hydroxide are distributed in the solid phase portion, and then the first neutralized slurry is subjected to a solid-liquid separation process to remove the zinc hydroxide and lead hydroxide from the system, thereby obtaining a filtrate containing cadmium.

Here, the cadmium-containing solution serving as the raw material solution is preferably a cadmium-containing solution having the following composition: Cd≧0.05 g/L, 0<Zn≦3 g/L, 0<Pb≦0.1 g/L, Cu≦0.01 g/L, Tl≦0.05 g/L, Ni≦0.005 g/L, Mn≦0.1 g/L, Ca≦1.0 g/L, Mg≦0.1 g/L, Si≦0.1 g/L. By using a cadmium-containing solution satisfying this composition as the raw material solution, the subsequent steps can be carried out effectively.
The pH adjuster used for adjusting the pH may be any alkali that is industrially usable. For example, not only calcium hydroxide but also sodium hydroxide, magnesium hydroxide, potassium hydroxide, etc. may be used.

In this pH adjustment, the adjustment range is not particularly limited as long as the pH is adjusted to a range in which zinc and lead form hydroxides. For example, the pH can be adjusted to a range of 7.0 or more and 9.0 or less. This makes it possible to preferentially precipitate and separate zinc and lead.
If the pH is adjusted to less than 7.0, the amount of zinc and lead precipitated will be insufficient. On the other hand, if the pH is adjusted to more than 9.0, the amount of zinc and lead precipitated will hardly increase, but the amount of cadmium precipitated will increase. This increase in the amount of cadmium precipitate indicates an increase in cadmium loss.
Even in this pH range of 7.0 to 9.0, thallium does not form a hydroxide and remains in the filtrate together with cadmium.

<Second neutralization step>
The second neutralization step is a step in which a pH adjuster is further added to the filtrate obtained in the previous first neutralization step to adjust the pH of the filtrate to a value higher than the pH adjusted in the first neutralization step and to a value within a range in which the cadmium contained in the filtrate forms cadmium hydroxide, thereby forming a second neutralized slurry in which the cadmium hydroxide is distributed in the solid phase portion, and then the second neutralized slurry is subjected to a solid-liquid separation treatment to obtain a cadmium precipitate containing the cadmium hydroxide.

As in the previous step, the pH adjuster used to adjust the pH can be any alkali that is industrially usable. For example, not only calcium hydroxide, but also sodium hydroxide, magnesium hydroxide, potassium hydroxide, etc. may be used.

In this pH adjustment, the second neutralization step adjusts the pH to a range higher than the pH of the filtrate adjusted in the first neutralization step and within which the cadmium contained in the filtrate forms cadmium hydroxide.
In this case, it is preferable to adjust the pH to 8.5 or more. This allows effective precipitation and separation. In addition, by further adjusting the pH to 11.0 or less, even if the filtrate contains thallium, the thallium can be effectively distributed in the liquid.

If the pH is adjusted to less than 8.5, the amount of cadmium precipitated will be insufficient, which indicates an increase in cadmium loss.
On the other hand, if the pH is adjusted to be greater than 11.0, there is a concern that a precipitate of thallium may be generated. Note that, since almost the entire amount of thallium contained in the raw material solution is distributed to the filtrate obtained through the second neutralization step, the effect of reducing thallium is large.

In this second neutralization step, instead of the filtrate obtained in the first neutralization step, the filtrate obtained in the previous step may be brought into contact with a strongly basic anion exchange resin to adsorb cadmium, and then the strongly basic anion exchange resin with adsorbed cadmium may be brought into contact with pure water to obtain a cadmium eluate, to which a pH adjuster may be added to adjust the pH.
In this case, by adjusting the pH of the cadmium eluent to a range in which the cadmium contained in the cadmium eluent forms cadmium hydroxide, a second neutralized slurry in which cadmium hydroxide is distributed in the solid phase can be formed. By subjecting this second neutralized slurry to solid-liquid separation, a cadmium precipitate containing cadmium hydroxide can be obtained.

The strongly basic anion exchange resin used adsorbs cadmium via a chloro complex, making it possible to separate impurities that do not form chloro complexes to a high degree. In addition, since metal elements that do not form chloro complexes are not distributed in the cadmium eluent, there is no need to perform operations such as limiting the upper limit of pH to prevent elements that do not form chloro complexes, such as thallium, from precipitating.

<Leaching process>
The leaching step is a step in which an acid is added to an aqueous solution or slurry prepared by adding water to the cadmium precipitate containing cadmium hydroxide obtained in the previous neutralization step, to preferentially leach cadmium, and components other than cadmium are partitioned as impurities into solid forms such as hydroxides and sulfates to form a leached slurry from which cadmium has been leached. This leached slurry is then subjected to solid-liquid separation to obtain a cadmium leachate from which cadmium has been leached.

The acid used in this step is not particularly limited, and for example, sulfuric acid can be used. The pH of the cadmium leaching solution is not particularly limited as long as it is a pH at which cadmium is preferentially leached.
For example, the pH can be adjusted to a range of 2.0 or more and 6.5 or less. In particular, in the low pH region, the leaching rates of zinc, nickel, and silicon approach that of cadmium, making it difficult to separate cadmium sufficiently. However, even such a cadmium leachate can be used as the starting solution for the electrolysis method, which is a method for reducing cadmium that is excellent in terms of separation from zinc, nickel, and silicon, when the electrolysis method is adopted in the metal recovery step described below.

Furthermore, by adjusting the pH of the cadmium leaching solution to a range of 5.5 or more and 6.5 or less, it is possible to effectively separate the elements zinc, nickel, silicon, and manganese from cadmium. The cadmium leaching solution obtained in this manner has a relatively reduced content of zinc, lead, nickel, silicon, and manganese compared to cadmium, so it can be used as the starting solution for the electrolysis method, and since it has excellent separation from silicon in particular, it can be used as the starting solution for the zinc cementation method when this method is used in the metal extraction process described below.

By the way, an oxidizing agent may be added to the leaching slurry to adjust the redox potential to a range in which the manganese contained in the leaching slurry is converted into manganese oxide, and an oxidation slurry containing the manganese oxide as a precipitate may be formed. After that, a cadmium leaching solution may be obtained by subjecting the oxidation slurry to a solid-liquid separation process.

The cadmium leachate thus obtained is a cadmium leachate in which cadmium has been leached and manganese contained as manganese oxide has been removed, resulting in a cadmium leachate with a reduced manganese content.
When adding such an oxidizing agent, the oxidizing agent to be used is not particularly limited, and for example, sodium hypochlorite can be used.
The amount of the oxidizing agent added is not particularly limited as long as it is within a range in which the redox potential is such that manganese precipitates as manganese oxide. For example, it can be adjusted to a range in which the redox potential is 1000 mV or more (vs. Ag/AgCl).

<Metal extraction process>
The metal recovery process is a process for reducing cadmium in the cadmium leachate obtained in the previous leaching process. This reduction can be carried out, for example, by the "zinc cementation method" or the "electrolysis method."

(Zinc Cementation Method)
The zinc cementation method is a method for reducing cadmium, in which zinc powder is added to the cadmium leachate obtained in the previous leaching step, and the cadmium concentration in the cadmium solution at the end of zinc cementation (hereinafter also referred to as the cadmium concentration at the end point of zinc cementation) is adjusted according to the nickel and cadmium concentrations in the leachate, thereby leaving most of the nickel in the cadmium solution and obtaining metallic cadmium having pores in which nickel precipitation from the solution is suppressed (hereinafter also referred to as sponge cadmium).

In this zinc cementation method, the amount of nickel precipitation increases as the cadmium concentration at the end point of zinc cementation decreases, so that the cadmium concentration at the end point of zinc cementation is adjusted according to the nickel and cadmium concentrations of the leachate so that the nickel quality of the resulting sponge cadmium satisfies the desired nickel quality. In this adjustment, the cadmium concentration at the end point of zinc cementation is preferably adjusted to a range of 1.0 to 10.0 g/L.
This suppresses the precipitation of nickel and also suppresses the amount of cadmium remaining in the solution, making it possible to efficiently precipitate sponge cadmium.

In order to suppress the nickel content of sponge cadmium, it is preferable that the nickel concentration of the leachate is low. However, even if the nickel concentration of the leachate is high, if the cadmium concentration of the leachate is also high, the amount of nickel that precipitates increases while the amount of cadmium that precipitates also increases relatively. Therefore, by appropriately selecting the cadmium concentration at the end point of zinc cementation, it is possible to suppress the nickel content of sponge cadmium.

For example, if the cadmium concentration of the leachate is 20 g/L and the nickel concentration is 0.02 g/L, sponge cadmium with a nickel content of 200 ppm or less can be obtained by adjusting the cadmium concentration at the end point of zinc cementation to 1.0 g/L. Also, if the cadmium concentration of the leachate is 70 g/L and the nickel concentration is 0.05 g/L, sponge cadmium with a nickel content of 100 ppm or less can be obtained by adjusting the cadmium concentration at the end point of zinc cementation to 9.7 g/L.

(Electrolysis method)
The reduction of cadmium in the leachate can also be carried out by electrolysis instead of the zinc cementation method described above.
The electrolysis method is a method for reducing cadmium, in which an anode and a cathode are immersed in an electrolytic cell, and cadmium is precipitated on the cathode while adjusting the current density and the cadmium concentration in the electrolyte at the end of the electrolysis (hereinafter also referred to as the cadmium concentration at the end point of electrolysis) to a range in which the precipitation of impurities is suppressed, thereby obtaining metallic cadmium.

The current density in this electrolysis method and the cadmium concentration at the end of electrolysis can be adjusted, for example, to a current density in the range of 50 A/m2 to 2000 A/m2 and a cadmium concentration at the end of electrolysis in the range of 1.0 g/L to 3.0 g/L.
By adjusting the content within this range, precipitation of zinc, nickel, magnesium, calcium, silicon, and thallium can be effectively suppressed.

Here, for example, a SUS plate or an Al plate, which has excellent electrical conductivity and cost, can be used as the cathode. Also, for example, an oxygen generating electrode can be used as the anode. By using an oxygen generating electrode, it is possible to effectively suppress the generation of chlorine at the anode. If chlorine is generated at the anode, it is inefficient because the metallic cadmium deposited on the cathode is re-oxidized, i.e., re-dissolved.

As the starting solution used in this electrolysis method, instead of the high-concentration cadmium leaching solution obtained in the leaching step, zinc powder may be added to the leaching solution, lead and copper are precipitated as metals, a slurry containing lead and copper as precipitates is formed, and then the slurry is subjected to solid-liquid separation to obtain a cadmium solution. The amount of zinc powder added is not particularly limited, but can be adjusted to, for example, a range of 0.2 g/L or more and 5 g/L or less (preferably a range of 0.7 g/L or more and 1.5 g/L or less).
The cadmium solution thus obtained is a cadmium solution in which lead and copper have been removed and at least 50% of the cadmium contained in the cadmium leachate remains. By using this cadmium solution as a starting solution for the electrolysis method, it is possible to further reduce the lead and copper content of metallic cadmium.

<Acid washing process>
In the acid washing process, acid is added to the metallic cadmium obtained in the previous metal extraction process to adjust the pH to a range in which zinc and sulfur are discharged into the liquid.
As a result, a post-acid-washed slurry is formed, which is composed of a post-acid-washed liquid (liquid phase) in which zinc and sulfur have been discharged into the liquid, and post-acid-washed metallic cadmium (solid phase). By subjecting this slurry to solid-liquid separation, post-acid-washed metallic cadmium with reduced zinc and sulfur can be obtained.

The acid used to adjust the pH in this step is preferably an acid with a relatively low oxidizing power. If an acid with a high oxidizing power is used, the metallic cadmium will dissolve.
For example, if the acid used is sulfuric acid or hydrochloric acid, dissolution of metallic cadmium can be suppressed, which is preferable.
In this case, in order to obtain post-acid-washing metallic cadmium with reduced zinc and sulfur, it is preferable to adjust the pH of the post-acid-washing solution to a pH range of 0 to 4.

<Alkaline cleaning process>
In the alkaline washing step, an alkali is added to the acid-washed metallic cadmium obtained in the previous acid washing step, and the pH is adjusted to a range in which sulfur is discharged into the liquid.
As a result, a post-alkali-washed slurry is formed, which is composed of a post-alkali-washed liquid (liquid phase) in which sulfur has been discharged into the liquid, and post-alkali-washed metallic cadmium (solid phase). By subjecting this slurry to solid-liquid separation, post-alkali-washed metallic cadmium with reduced sulfur can be obtained.

The alkali used to adjust the pH in this step is not particularly limited.
For example, industrially available alkalis such as sodium hydroxide, calcium hydroxide, magnesium hydroxide, potassium hydroxide, sodium carbonate, calcium carbonate, etc. may be used.
At this time, in order to obtain metallic cadmium after alkaline washing with reduced zinc, it is preferable to adjust the pH after alkaline washing to be in the pH range of 10 to 14.

<Water washing process>
In the water washing step, water is added to the alkaline-washed metallic cadmium obtained in the previous alkaline washing step to form a slurry after water washing, and the slurry is subjected to solid-liquid separation, so that the alkali metals and alkaline earth metals contained in the alkaline-washed metallic cadmium can be discharged into the liquid.

A cadmium-containing solution with the composition shown in Table 1 was prepared by adding sulfuric acid to the electric furnace dust and adjusting the pH to 6.5.

Next, a calcium hydroxide slurry was added to this cadmium-containing solution to adjust the pH to 8.0, and the solution was stirred for 60 minutes. The precipitate that formed was filtered through membrane filter paper, and the composition of the filtrate was analyzed. Table 2 shows the results. A filtrate containing cadmium with reduced amounts of zinc, lead, copper, and manganese was obtained.

Next, a slurry of calcium hydroxide was added to the cadmium-containing filtrate from which the zinc, lead, copper, and manganese had been reduced, to adjust the pH to 11.0, and the mixture was held with stirring for 60 minutes. The precipitate that formed was then filtered through a membrane filter paper to obtain cadmium hydroxide.
The compositions of the filtrate and the cadmium hydroxide were then analyzed. Table 3 shows the results of analyzing the composition of the filtrate, and Table 4 shows the results of analyzing the composition of the cadmium hydroxide. The thallium concentration shown in Table 3 did not change compared to the thallium concentration shown in Table 2, and it can be determined that cadmium hydroxide in which thallium pickup was suppressed was obtained.

Next, 40 g of the cadmium hydroxide was dispensed into a 200 ml beaker, 65 ml of pure water was added to the cadmium hydroxide, and while stirring at room temperature of 25°C, 64% by mass sulfuric acid was added so that the pH became 6.0. After the pH stabilized, the mixture was stirred for 60 minutes, filtered through a membrane filter paper, and the composition of the filtrate (leachate) was analyzed. Table 5 shows the results. A high-concentration cadmium leachate was obtained in which the leaching of zinc, lead, nickel, manganese, and silicon was suppressed.

Next, the high concentration cadmium leachate was divided into two, one of which was reduced at a current density of 1000 A/m2, and the other was reduced by zinc cementation to obtain metallic cadmium. In this case, the cadmium concentration at the end of electrolysis was 1.0 g/L, and the cadmium concentration at the end of zinc cementation was 1.0 g/L. Then, the composition of impurities contained in these metallic cadmiums was analyzed. Table 6 shows the results.

Next, the metallic cadmium obtained by the electrolysis method and the metallic cadmium obtained by the zinc cementation method were separated and immersed in pure water so that the slurry concentration was 100 g/L to prepare a metal slurry. 64 wt % sulfuric acid was added to the metallic slurry, and the pH of the solution after acid washing was adjusted to 1.5 and stirred for 5 minutes. Then, the solid-liquid separation was performed to obtain metallic cadmium after acid washing.
Next, the above-mentioned acid-washed metallic cadmium was added to an 80 g/L aqueous sodium hydroxide solution so that the slurry concentration was 100 g/L, and the pH of the alkali-washed liquid was adjusted to 11, followed by stirring for 5 minutes. Thereafter, solid-liquid separation was performed to obtain alkali-washed metallic cadmium.

The above-mentioned alkaline washed metallic cadmium was then added so that the slurry concentration was 100 g/L, and the mixture was stirred for 10 minutes. After that, the mixture was separated into solid and liquid to obtain metallic cadmium after washing with water. The composition of this metallic cadmium after washing with water was then analyzed. Table 7 shows the results.

By going through each step of the present invention, metallic cadmium was obtained that contains 99.9% or more cadmium and the impurities zinc, lead, copper, thallium, nickel, manganese, calcium, magnesium, silicon, sulfur, and sodium are all less than 100 ppm.

Claims (5)

a first neutralization step of adding a pH adjuster to a cadmium-containing solution to adjust the pH of the cadmium-containing solution to a range of 7.0 to 9.0 to form a first neutralized slurry in which zinc hydroxide and lead hydroxide are distributed in a solid phase portion , subjecting the first neutralized slurry to a solid-liquid separation treatment to remove the zinc hydroxide and the lead hydroxide from the system, and obtaining a filtrate containing cadmium;
a second neutralization step of adding a pH adjuster to the cadmium-containing filtrate obtained in the first neutralization step to adjust the pH of the filtrate to a value higher than the pH adjusted in the first neutralization step and within a range of 8.5 to 11.0, thereby forming a second neutralized slurry in which cadmium hydroxide is distributed in a solid phase portion , and subjecting the second neutralized slurry to a solid-liquid separation treatment to obtain a cadmium precipitate containing the cadmium hydroxide;
a leaching step of adding an acid to the cadmium precipitate obtained in the second neutralization step to adjust the pH to a range of 2.0 to 6.5 to form a leaching slurry containing cadmium, and then subjecting the leaching slurry containing cadmium to a solid-liquid separation treatment to obtain a cadmium leachate;
a metal extraction step of reducing cadmium in the cadmium leaching solution obtained in the leaching step by electrolysis to obtain metallic cadmium ;
an acid washing step of adding an acid to the metallic cadmium obtained in the metal extraction step to adjust the pH to a range of 0 to 4.0 to form an acid-washed slurry, and subjecting the acid-washed slurry to a solid-liquid separation treatment to obtain acid-washed metallic cadmium having reduced zinc and sulfur ;
an alkali washing step of adding an alkali to the acid-washed metallic cadmium obtained in the acid washing step to adjust the pH to a range of 10 to 14 to form an alkali-washed slurry, and subjecting the alkali-washed slurry to a solid-liquid separation treatment to obtain alkali-washed metallic cadmium having reduced sulfur ;
and a water washing step of washing the alkali-washed metallic cadmium obtained in the alkali washing step with water to obtain metallic cadmium from which alkali metals and alkaline earth metals have been removed . 2. The method for producing metallic cadmium according to claim 1, wherein the current density in said metal extracting step is 50 A/m ² to 2000 A/m ² , and the cadmium concentration at the end point of electrolysis is 1.0 g/l to 3.0 g/L. The method for producing metallic cadmium according to claim 1, wherein the composition of the cadmium-containing solution is Cd≧0.05g/L, 0<Zn≦3g/L, 0<Pb≦0.1g/L, Cu≦0.01g/L, Tl≦0.05g/L, Ni≦0.005g/L, Mn≦0.1g/L, Ca≦1.0g/L, Mg≦0.1g/L, and Si≦0.1g/L. The method for producing metallic cadmium according to claim 1, wherein the starting solution used in the electrolysis method is a cadmium solution obtained by adding zinc powder to the cadmium leaching solution obtained in the leaching step in an amount of 0.2 g/L or more and 5 g/L or less to form a slurry containing lead and copper as precipitates, and then subjecting the slurry to solid-liquid separation. The method for producing metallic cadmium according to claim 1, wherein in the leaching step, an oxidizing agent is further added to the leaching slurry to obtain an oxidation-reduction potential of 1000 mV or more (vs. Ag/AgCl), and the oxidation slurry containing manganese oxide precipitates is subjected to solid-liquid separation to obtain a cadmium leachate.