JP7530566B2 - How to recover thallium - Google Patents

Description

The present invention relates to a method for recovering thallium.

Zinc oxide ore, obtained by separating and recovering impurities from crude zinc oxide, etc., is widely used as a raw material for zinc ingots at 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 perspective of promoting resource recycling, it is desirable to reuse steel dust as a raw material for zinc.

On the other hand, crude zinc oxide derived from steel dust contains high proportions of halogens such as chlorine and fluorine, as well as impurities such as cadmium, in addition to its main component, zinc oxide. Of these impurities, cadmium in particular is a harmful metal, so separation and recovery of cadmium is generally carried out in zinc oxide manufacturing plants.

Now, this cadmium can be separated and recovered by wet processing, which is a combination of multiple processes, such as neutralization and zinc cementation, on the leachate obtained by leaching cadmium from crude zinc oxide in an acid. However, crude zinc oxide contains trace amounts of thallium, which is not removed from the system in the wet processing and gradually accumulates. Therefore, if thallium continues to accumulate in the wet processing system, there is a concern that the separation of thallium will be insufficient, which will have a negative effect on the quality of metallic cadmium.

As a method for discharging thallium from the wet processing system, for example, a method for discharging thallium is known in which the neutralization solution generated in the neutralization process is subjected to a sulfurization treatment to produce a smelting intermediate, and thallium is discharged from the system via this smelting intermediate. However, this smelting intermediate contains cadmium in addition to thallium, and when the smelting intermediate is discharged from the system, not only thallium but also cadmium is discharged from the system, which is not economical. If it were possible to separate cadmium from the smelting intermediate and recover thallium, it would be possible to discharge only thallium from the system, which would be economical and more advantageous.

As a method for recovering thallium from a smelting intermediate, for example, there can be mentioned the method shown in Patent Document 1. Although this method makes it possible to recover thallium from a smelting intermediate, it is necessary to carry out complicated processing steps such as sulfuric acid leaching while blowing air or nitrogen gas into the smelting intermediate, then adding zinc powder to reduce and precipitate sponge thallium, and then applying sulfuric acid leaching and neutralization treatments to this sponge thallium multiple times, which is not efficient.
Thus, there is a need for the development of a method for recovering thallium that can efficiently separate cadmium from smelting intermediates and recover thallium.

Japanese Unexamined Patent Publication No. 122722/1983

The object of the present invention is to provide a method for recovering thallium that can efficiently separate cadmium from smelting intermediates and recover thallium in order to solve this problem.

A first aspect of the present invention is a method for recovering thallium, comprising: a leaching step of adding sulfuric acid to a mixed sulfide containing thallium sulfide and cadmium sulfide or an aqueous solution containing the mixed sulfide, adjusting the pH of a leaching slurry consisting of a liquid portion of the sulfuric acid solution into which thallium is distributed and a solid portion of a leaching residue into which cadmium is distributed to a range of −0.5 to 1.5, subjecting the leaching slurry to a solid-liquid separation treatment, and obtaining the liquid portion of the sulfuric acid solution into which thallium is distributed as a leaching solution ; and a thallium recovery step of adding at least one of a chloride that liberates chloride ions or a solution containing chloride ions to the leaching solution obtained in the leaching step to form a chloride slurry containing thallium in the leaching solution as a precipitate of thallium chloride, adjusting the chloride ion concentration of the chloride slurry to a range of 0.5 g/L or more, subjecting the chloride slurry to a solid-liquid separation treatment, and recovering the precipitate of thallium chloride.

A second aspect of the present invention is a method for recovering thallium, characterized in that the chloride slurry in the thallium recovery step of the first aspect has a chloride ion concentration of 6.7 g/L.

The thallium recovery method of the present invention is of great industrial value because it can efficiently separate cadmium from smelting intermediates and recover thallium.

The thallium recovery method of the present invention is a method for separating and recovering thallium from a mixed sulfide containing thallium sulfide and cadmium sulfide as a starting material. The method is composed of a leaching step and a thallium recovery step, and by passing through the above steps, thallium can be separated and recovered as thallium chloride.
Each step will be described below.

<Leaching process>
The leaching step is a step of adding sulfuric acid to a mixed sulfide containing thallium sulfide and cadmium sulfide, or an aqueous solution containing the mixed sulfide, to form a sulfuric acid solution containing the mixed sulfide, adjusting the pH range of the sulfuric acid solution to a pH range that allows the thallium contained in the mixed sulfide to be preferentially leached into the liquid portion of the sulfuric acid solution that becomes a leachate, while distributing the cadmium contained in the mixed sulfide to the solid portion that becomes a leachate residue, forming a leachate slurry consisting of this leachate and the leachate residue, and then performing solid-liquid separation to obtain a leachate containing thallium.

The mixed sulfide used here can be produced, for example, by neutralizing waste liquid containing thallium and cadmium, which is generated in the process of producing zinc oxide ore from steel dust from steelmaking furnaces such as blast furnaces and electric furnaces in the steel industry, and then further sulfurizing the treated liquid obtained by this neutralization process. By carrying out the sulfurization process, impurities that do not form sulfides can be left in the treated liquid, and thallium and cadmium that form sulfides can be produced as thallium sulfide and cadmium sulfide.

When forming the leaching slurry as described above, the pH of the leaching slurry may be adjusted, for example, to a range of -0.5 to 1.5. This allows thallium to be preferentially leached from the thallium sulfide mixed sulfide into the sulfuric acid solution, while cadmium can be distributed to the leaching residue, i.e., cadmium sulfide. If the pH is lower than -0.5, the leaching rate of cadmium increases, and if the pH exceeds 1.5, the leaching rate of thallium decreases.

In this case, by further adjusting the pH to the range of 0.5 to 1.5, it is possible to suppress an increase in the amount of free acid contained in the treatment liquid after thallium is separated in the thallium recovery process described below. This makes it possible to suppress the amount of alkali used in the recycling plant when this treatment liquid is recycled, for example, as a raw material solution for producing metallic cadmium.

When forming the leaching slurry, the higher the temperature of the leaching slurry, the better the leaching of thallium and the better the separation from cadmium. However, as long as it is within the range of 10°C to 80°C, there is no need to adjust the temperature.

The leaching residue thus obtained is separated from the leachate containing thallium by applying a solid-liquid separation method such as static separation or filtration to the leach slurry, or industrially, a solid-liquid separation method such as pressure filtration, suction filtration, or centrifugation. The separated leach residue has a sufficiently reduced amount of thallium, so it can be recycled, for example, as a raw material for producing metallic cadmium or zinc oxide ore.

<Thallium recovery process>
The thallium recovery process is a process in which at least one of a chloride that liberates chloride ions or a solution containing chloride ions is added to the leachate containing thallium obtained in the previous leaching process to distribute thallium in a solid form as thallium chloride and form a chloride slurry in which other elements are left behind and distributed in the liquid, and then this is separated into solid and liquid to separate and recover the thallium contained in the mixed sulfide as thallium chloride.

An example of a chloride that liberates chloride ions and can be added here is sodium chloride, and an example of a solution containing chloride ions that can be added is an aqueous solution of sodium hypochlorite.

When forming the chloride slurry, the amount of the chloride that liberates chloride ions or the amount of the solution containing the chloride ions added may be adjusted so that the chloride ion concentration in the chloride slurry after the production of thallium chloride is in the range of, for example, 0.5 g/L or more.
This allows for the effective generation of thallium chloride, distributing thallium in a solid form and distributing other elements in the liquid. If the chloride ion concentration is less than 0.5 g/L, the amount of thallium chloride produced will decrease.

The thallium chloride thus obtained is separated from the liquid by applying the solid-liquid separation method described above. The treated liquid after the thallium chloride has been separated has a sufficiently reduced thallium content, so it can be recycled, for example, as a raw material solution for producing metallic cadmium or zinc oxide ore.

The present invention will now be described in detail with reference to examples.

Wastewater generated in the process of producing zinc oxide ore from steel dust from steelmaking furnaces such as blast furnaces and electric furnaces in the steel industry was neutralized, and the treatment liquid obtained by this neutralization process was subjected to a sulfurization process to produce 6 g of mixed sulfide with the composition shown in Table 1. 215 ml of pure water was added to the mixture, which was then heated to 60°C and 18 ml of 64% sulfuric acid was added while stirring to adjust the pH to 0.0. This state was maintained for 60 minutes to form a leaching slurry.

The leaching slurry was then filtered through a membrane filter paper to separate the leaching residue from the filtrate, and the leaching residue was then analyzed, the results of which are shown in Table 2.
Furthermore, the concentrations of thallium and cadmium in the filtrate were measured, and the leaching rates of thallium and cadmium were calculated from the results. The measurement results of the concentrations of thallium and cadmium are shown in Table 3, and the leaching rates of thallium and cadmium are shown in Table 4.

While the thallium content in the leaching residue was significantly reduced compared to the thallium content in the mixed sulfide before leaching, most of the cadmium remained in the leaching residue, confirming that thallium could be selectively leached.
Furthermore, as shown in Table 2, this leaching residue has a reduced thallium content, and it was confirmed that it can be recycled as a raw material for producing metallic cadmium or zinc oxide ore.

Next, 300 ml of the filtrate was prepared by repeating the above procedure, and 11 ml of 360 g/L sodium chloride solution was added while stirring, and this state was maintained at room temperature for 120 minutes to form a chloride slurry. Here, the chloride ion concentration of the chloride slurry after the production of thallium chloride was 6.7 g/L.

The chloride slurry was then filtered through a membrane filter paper to separate the precipitate from the filtrate, which was then analyzed to determine the recovery rate of thallium.
The analytical results of the filtrate are shown in Table 5, and the thallium recovery rate is shown in Table 6.

The thallium concentration in the filtrate was significantly reduced, and it was confirmed that 93% of the thallium contained in the mixed sulfide before leaching could be recovered as thallium chloride. In addition, as shown in Table 5, this filtrate had a reduced thallium content, and it was confirmed that it could be recycled as a raw material solution for producing metallic cadmium or zinc oxide ore.
In this manner, it was confirmed that the thallium recovery method of the present invention can efficiently separate cadmium from smelting intermediates and recover thallium.

Claims (2)

a leaching step of adding sulfuric acid to a mixed sulfide containing thallium sulfide and cadmium sulfide or an aqueous solution containing the mixed sulfide , adjusting the pH of a leaching slurry consisting of a liquid portion of the sulfuric acid solution in which thallium is distributed and a solid portion of a leaching residue in which cadmium is distributed to a range of −0.5 to 1.5, and subjecting the leaching slurry to a solid-liquid separation treatment to obtain the liquid portion of the sulfuric acid solution in which thallium is distributed as a leaching solution;
a thallium recovery step of adding at least one of a chloride that liberates chloride ions or a solution containing chloride ions to the leachate obtained in the leaching step to form a chloride slurry containing thallium in the leachate as a precipitate of thallium chloride, adjusting the chloride ion concentration of the chloride slurry to a range of 0.5 g/L or more, subjecting the chloride slurry to a solid-liquid separation treatment, and recovering the precipitate of thallium chloride;
A method for recovering thallium comprising the steps of: 2. The method for recovering thallium according to claim 1 , wherein the chloride slurry in the thallium recovery step has a chloride ion concentration of 6.7 g/L.