Thallium recovery equipment and thallium recovery method

Abstract

The present invention provides a thallium recovery system and a thallium recovery method that can reduce the amount of oxidizing agent used. [Solution] The thallium recovery equipment AA includes a dissolution tank 10 that obtains a dissolved slurry by adding hydrochloric acid to a thallium-containing raw material, a solid-liquid separation device 20 that continuously discharges thallium-containing water while washing the insoluble residue with water after solid-liquid separation of the dissolved slurry into an insoluble residue and a dissolved solution, a storage tank 31 that stores the initial thallium-containing water discharged in the initial stage of washing, and an oxidation tank 32 that generates thallium precipitate by adding an oxidizing agent to the later thallium-containing water discharged in the later stage of washing. Since the acid concentration of the later thallium-containing water is low, the amount of oxidizing agent consumed by reaction with hydrogen ions in the generation of thallium precipitate can be reduced. Therefore, the amount of oxidizing agent required for the generation of thallium precipitate can be reduced.

Description

Technical Field

[0001] The present invention relates to thallium recovery equipment and a thallium recovery method. More specifically, the present invention relates to equipment and a method for adding an oxidizing agent to thallium-containing water to recover thallium precipitate.

Background Art

[0002] Patent Document 1 discloses a method for recovering cadmium from zinc-containing slag discharged from a crude zinc oxide plant by the Waelz process. Specifically, first, the zinc-containing slag is dissolved in sulfuric acid, and insoluble solids are removed by solid-liquid separation to obtain an aqueous solution of cadmium sulfate. Next, metallic zinc powder is added to the aqueous solution of cadmium sulfate to cause a cementation reaction between cadmium ions and metallic zinc, and the precipitated metallic cadmium is recovered.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In addition to zinc and cadmium, zinc-containing slag contains lead and thallium. Since lead contained in zinc-containing slag is hardly soluble in sulfuric acid, it is removed as an insoluble residue by solid-liquid separation after sulfuric acid dissolution. Since most of the lead contained in the zinc-containing slag is removed, the lead grade of metallic cadmium can be lowered. However, since thallium is easily soluble in sulfuric acid, it mixes into the aqueous solution of cadmium sulfate. Further, thallium precipitates due to the cementation reaction between thallium ions and metallic zinc in the aqueous solution of cadmium sulfate and mixes into metallic cadmium as an impurity.

[0005] In contrast, a method of dissolving the zinc slag with hydrochloric acid can be considered. This method allows for the separation of thallium contained in the zinc slag as an insoluble residue. Therefore, the thallium grade of metallic cadmium can be reduced.

[0006] Furthermore, thallium contained in the insoluble residue can be recovered by processing the insoluble residue separately. Specifically, first, the insoluble residue is washed with water to dissolve the thallium and obtain thallium-containing water. Next, an oxidizing agent such as sodium hypochlorite is added to the thallium-containing water to obtain thallium precipitate. This reaction is represented by the following reaction equation (1). 2Tl + +2NaClO+4H2O→2Tl(OH)3↓+2H + +2NaCl (1)

[0007] Since the acid used to dissolve the zinc slag remains in the insoluble residue along with the adhering water, washing the insoluble residue with water washes away the acid as well, and it becomes mixed into the thallium-containing water. When an oxidizing agent is added to the thallium-containing water mixed with acid, the oxidizing agent is consumed by a reaction with hydrogen ions, as shown in reaction equation (2). Therefore, in order to increase the thallium recovery rate, it is necessary to add more oxidizing agent than the amount corresponding to the thallium concentration in the thallium-containing water, which increases the cost of the oxidizing agent. ClO - +2H + →Cl - +H2O (2)

[0008] In view of the above circumstances, the present invention aims to provide a thallium recovery system and a thallium recovery method that can reduce the amount of oxidizing agent used. [Means for solving the problem]

[0009] The thallium recovery equipment of the first embodiment is characterized by comprising: a dissolution tank to obtain a dissolution slurry consisting of an insoluble residue containing thallium and a dissolving solution by adding at least hydrochloric acid to a thallium-containing raw material; a solid-liquid separation device that, after solid-liquid separation of the dissolution slurry into the insoluble residue and the dissolving solution, washes the insoluble residue with water and continuously discharges the thallium-containing water as washed water; a storage tank for storing the initial thallium-containing water, which is the thallium-containing water discharged from the solid-liquid separation device during the initial washing period in which the insoluble residue is washed with water; and an oxidation tank that adds an oxidizing agent to the later thallium-containing water, which is the thallium-containing water discharged from the solid-liquid separation device during the later washing period after the initial washing period, to produce thallium precipitate. The thallium recovery equipment of the second embodiment is characterized in that, in the first embodiment, the pH of the initial thallium-containing water is below a threshold, the pH of the later thallium-containing water exceeds the threshold, and the threshold is a predetermined value within the range of pH 2 to 4. The thallium recovery equipment of the third embodiment is characterized in that, in the first or second embodiment, it includes a return channel for sending the initial thallium-containing water in the storage tank to the dissolution tank. The thallium recovery equipment of the fourth embodiment is characterized in that, in any of the first to third embodiments, hydrochloric acid and sulfuric acid are supplied to the dissolution tank. A thallium recovery method according to a fifth embodiment is characterized by comprising: a dissolution step of adding at least hydrochloric acid to a thallium-containing raw material to obtain a dissolution slurry consisting of an insoluble residue containing thallium and a dissolving solution; a solid-liquid separation step of separating the dissolution slurry into the insoluble residue and the dissolving solution; a washing step of continuously discharging thallium-containing water as washed water while washing the insoluble residue with water, and separating the initial thallium-containing water, which is the thallium-containing water discharged in the initial washing period, and the later thallium-containing water, which is the thallium-containing water discharged in the later washing period, after the initial washing period; and an oxidation step of adding an oxidizing agent to the later thallium-containing water to produce thallium precipitate. The thallium recovery method of the sixth embodiment is characterized in that, in the fifth embodiment, the pH of the thallium-containing water continuously discharged in the washing step is measured, the thallium-containing water discharged until the pH measurement value reaches a predetermined threshold is defined as the initial thallium-containing water, and the thallium-containing water discharged after the pH measurement value reaches the threshold is defined as the late thallium-containing water. The thallium recovery method of the seventh embodiment is characterized in that, in the sixth embodiment, the threshold value is a predetermined value within the range of pH 2 to 4. The thallium recovery method of the eighth embodiment is characterized in that, in any of the fifth to seventh embodiments, the initial thallium-containing water is returned to the dissolution step as part of the thallium-containing raw material. The thallium recovery method of the ninth embodiment is characterized in that, in any of the fifth to eighth embodiments, hydrochloric acid and sulfuric acid are added to the thallium-containing raw material in the dissolution step. [Effects of the Invention]

[0010] According to the first embodiment, the late-stage thallium-containing water obtained in the later stages of washing has a low acid concentration, which reduces the amount of oxidizing agent consumed by reaction with hydrogen ions during the formation of thallium precipitates. Therefore, the amount of oxidizing agent required for the formation of thallium precipitates can be reduced. According to the second embodiment, the pH of the late-stage thallium-containing water exceeds the threshold, resulting in a sufficiently low acid concentration. Therefore, the amount of oxidizing agent required for the formation of thallium precipitate can be reduced. According to the third embodiment, since the initial thallium-containing water is returned to the dissolution tank, the loss of thallium contained in the initial thallium-containing water can be suppressed. According to the fourth embodiment, if the thallium-containing raw material contains lead, a solution with a low lead concentration can be obtained by dissolving the thallium-containing raw material in a mixed acid of hydrochloric acid and sulfuric acid. According to the fifth embodiment, the late-stage thallium-containing water obtained in the later stages of washing has a low acid concentration, which reduces the amount of oxidizing agent consumed by reaction with hydrogen ions in the oxidation process. Therefore, the amount of oxidizing agent required to produce thallium precipitate can be reduced. According to the sixth aspect, since the initial thallium-containing water and the later thallium-containing water are separated based on the pH of the thallium-containing water, the acid concentration of the later thallium-containing water can be surely lowered. According to the seventh aspect, since the pH of the later thallium-containing water exceeds a predetermined threshold within the range of pH 2 to 4, the acid concentration is sufficiently low. Therefore, the amount of the oxidizing agent required for the formation of thallium precipitate can be reduced. According to the eighth aspect, since the initial thallium-containing water is returned to the dissolution step, the loss of thallium contained in the initial thallium-containing water can be suppressed. According to the ninth aspect, when the thallium-containing raw material contains lead, a solution with a low lead concentration can be obtained by dissolving the thallium-containing raw material in a mixed acid of hydrochloric acid and sulfuric acid.

Brief Description of Drawings

[0011] [Figure 1] It is an explanatory view of thallium recovery equipment according to one embodiment. [Figure 2] It is a process diagram of a thallium recovery method according to one embodiment. [Figure 3] It is a graph showing the relationship between the pH of thallium-containing water and the thallium recovery rate.

Mode for Carrying Out the Invention

[0014] In addition to hydrochloric acid, other acids may be supplied to the dissolution tank 10. For example, when using zinc-containing slag as the thallium-containing raw material, sulfuric acid may be supplied to the dissolution tank 10 in addition to hydrochloric acid. When the zinc-containing slag is dissolved only with hydrochloric acid, zinc, cadmium and lead contained in the zinc-containing slag are dissolved, and thallium can be separated as an insoluble residue. If the zinc-containing slag is dissolved with a mixed acid of hydrochloric acid and sulfuric acid, zinc and cadmium contained in the zinc-containing slag are dissolved, and lead and thallium can be separated as an insoluble residue. Thus, when the thallium-containing raw material contains lead, by dissolving the thallium-containing raw material with a mixed acid of hydrochloric acid and sulfuric acid, a dissolution solution with a low lead concentration can be obtained. As will be described later, if metallic zinc powder is added to the dissolution solution derived from zinc-containing slag, metallic cadmium can be recovered. By keeping the lead concentration of the dissolution solution low, the lead grade of metallic cadmium can be lowered. Note that hydrochloric acid and sulfuric acid may be mixed in advance and supplied to the dissolution tank 10, or hydrochloric acid and sulfuric acid may be supplied to the dissolution tank 10 separately.

[0015] The dissolution slurry in the dissolution tank 10 is supplied to the slurry supply port of the solid-liquid separation device 20. As the solid-liquid separation device 20, a batch-type solid-liquid separation device such as a filter press can be used, and among them, a filter press is preferably used. As the solid-liquid separation device 20, for example, a leaf filter or a candle filter may be used. The solid-liquid separation device 20 separates the dissolution slurry into an insoluble residue and a dissolution solution by solid-liquid separation.

[0016] The dissolved solution is discharged from the outlet of the solid-liquid separator 20. When zinc slag containing hydrochloric acid is dissolved in a mixed acid of hydrochloric acid and sulfuric acid, the dissolved solution is a mixed aqueous solution of cadmium chloride, cadmium sulfate, zinc chloride, and zinc sulfate (hereinafter sometimes referred to as "cadmium-zinc aqueous solution"). When metallic zinc powder is added to the cadmium-zinc aqueous solution, metallic cadmium precipitates due to a cementation reaction between cadmium ions and metallic zinc. In addition, the metallic zinc powder dissolves due to the cementation reaction, and a mixed aqueous solution of zinc chloride and zinc sulfate (hereinafter sometimes referred to as "zinc aqueous solution") is produced. By separating the slurry after the cementation reaction into solid and liquid components, metallic cadmium and the zinc aqueous solution can be recovered.

[0017] After solid-liquid separation of the dissolved slurry, water is supplied to the washing water supply port of the solid-liquid separator 20 to wash away any insoluble residue remaining inside the solid-liquid separator 20. When a filter press is used as the solid-liquid separator 20, a through-wash method in which water is flowed through the filter cloth to the insoluble residue is preferred. However, a direct wash method in which water is flowed directly into the insoluble residue in the filtration chamber is also acceptable.

[0018] The insoluble residue contains thallium chloride. Since thallium chloride readily dissolves in water when the chloride ion concentration in the solution decreases, the water after washing (hereinafter sometimes referred to as "washed water") contains thallium ions. Therefore, the washed water obtained after washing the insoluble residue is thallium-containing water. The insoluble residue after washing with water is referred to as "washed residue."

[0019] When zinc slag containing thallium is dissolved in a mixed acid of hydrochloric acid and sulfuric acid, the insoluble residue contains lead in addition to thallium. Even if this insoluble residue is washed with water, lead remains in the insoluble residue. Therefore, thallium and lead can be separated by washing the insoluble residue with water. The residue after washing is discharged from the solid-liquid separator 20. This allows for the recovery of the post-wash residue containing lead.

[0020] While the insoluble residue is being washed with water, the thallium-containing water is continuously discharged from the outlet of the solid-liquid separator 20. Hereinafter, the period during which the insoluble residue is washed with water will be referred to as the "washing period." The washing period will be divided into two periods: the "initial washing period" and the "later washing period" following the initial washing period. Furthermore, the thallium-containing water discharged from the solid-liquid separator 20 during the initial washing period will be referred to as the "initial thallium-containing water," and the thallium-containing water discharged from the solid-liquid separator 20 during the later washing period will be referred to as the "later thallium-containing water."

[0021] After solid-liquid separation of the dissolved slurry, the insoluble residue remaining inside the solid-liquid separator 20 contains the acid used to dissolve the thallium-containing raw material, along with the adhering water. When the insoluble residue is washed with water, the acid is also washed away and mixed into the thallium-containing water. Since much of the acid adhering to the insoluble residue is washed away in the initial stages of washing, a relatively large amount of acid is mixed into the initial thallium-containing water. On the other hand, the amount of acid mixed into the later thallium-containing water is relatively small.

[0022] This embodiment is characterized by separating thallium-containing water into initial thallium-containing water with a high acid concentration and later thallium-containing water with a low acid concentration. The initial and later thallium-containing waters are separated and stored in separate tanks. The initial thallium-containing water is stored in storage tank 31. The later thallium-containing water is stored in oxidation tank 32.

[0023] The dissolving solution, the initial thallium-containing water, and the later thallium-containing water are all discharged from the outlet of the solid-liquid separator 20, but at different times. Therefore, a discharge channel 40 is connected to the outlet of the solid-liquid separator 20, which allows switching the destination of the discharged liquids. The discharge channel 40 consists of, for example, piping and valves. By opening and closing the valve, the destination of the dissolving solution, the initial thallium-containing water, and the later thallium-containing water can be switched.

[0024] The timing for switching between the initial and final stages of cleaning can be determined by various methods. For example, the cleaning time can be used as the basis, with the initial stage defined as the period from the start of cleaning to a predetermined time, and the subsequent period as the final stage. Alternatively, the amount of thallium-containing water discharged can be used as the basis, with the initial stage defined as the period from the start of cleaning until a predetermined amount of thallium-containing water is discharged, and the subsequent period as the final stage.

[0025] However, as described later, the purpose of separating thallium-containing water into initial thallium-containing water and late thallium-containing water is to obtain late thallium-containing water with a lower acid concentration. Therefore, it is preferable to determine the timing of switching between the initial and late stages of washing based on the pH of the thallium-containing water.

[0026] Specifically, the pH of the thallium-containing water discharged from the solid-liquid separator 20 is monitored. The pH of the thallium-containing water can be measured with a pH meter. For example, thallium-containing water is sampled periodically or irregularly from the discharge channel 40, and its pH is measured with a pH meter. A pH meter may be installed in the piping that makes up the discharge channel 40. Alternatively, a buffer tank may be installed in the middle of the discharge channel 40, and a pH meter may be installed in the buffer tank. With such a configuration that includes a pH meter, the pH of the thallium-containing water can be measured continuously.

[0027] As the washing process progresses, the acid concentration in the thallium-containing water decreases, causing its pH to rise. Therefore, the pH of the thallium-containing water continuously discharged from the solid-liquid separator 20 is monitored, and the initial washing phase is defined as the period until the pH measurement reaches a predetermined threshold. During this phase, the initial thallium-containing water discharged is supplied to the storage tank 31. When the pH of the thallium-containing water reaches the threshold, the valve in the discharge channel 40 is operated to switch the supply destination of the thallium-containing water to the oxidation tank 32. This allows the later-stage thallium-containing water discharged during the later stages of washing to be stored in the oxidation tank 32.

[0028] After performing the above operations, the pH of the initial thallium-containing water will be below the threshold, and the pH of the later thallium-containing water will be above the threshold. As a result, the acid concentration of the later thallium-containing water will be sufficiently low. Here, the threshold is preferably set within the range of pH 2 to 4, and more preferably within the range of pH 2.5 to 3.5.

[0029] An oxidizing agent is supplied to the oxidation tank 32. When the oxidizing agent is added to the late-stage thallium-containing water, the thallium contained in the late-stage thallium-containing water precipitates as an oxide or hydroxide, forming a thallium precipitate. Although not particularly limited, sodium hypochlorite and potassium permanganate are preferably used as the oxidizing agent.

[0030] Late-stage thallium-containing water has a low acid concentration. Therefore, the amount of oxidizing agent consumed by reaction with hydrogen ions during thallium precipitate formation can be reduced. As a result, the amount of oxidizing agent required for thallium precipitate formation can be reduced, and the cost of oxidizing agents can be lowered.

[0031] The slurry containing thallium sediment is referred to as "thallium slurry." The thallium slurry discharged from the oxidation tank 32 may be separated into solid and liquid components using a second solid-liquid separation device (not shown) to recover the thallium sediment.

[0032] The storage tank 31 and the dissolution tank 10 are connected by a return channel 50. The return channel 50 consists of, for example, piping and a pump. It is preferable to transfer the initial thallium-containing water from the storage tank 31 to the dissolution tank 10 via the return channel 50.

[0033] The initial thallium-containing water contains thallium. By returning the initial thallium-containing water to the dissolution tank 10 instead of discharging it from the system, thallium loss can be suppressed. In addition, thallium chloride is produced by adding hydrochloric acid to the initial thallium-containing water in the dissolution tank 10. Most of the thallium chloride becomes solid and is contained in the insoluble residue.

[0034] (Thallium recovery method) Next, a thallium recovery method according to one embodiment of the present invention will be described based on Figure 2. Note that the thallium recovery method of this embodiment may be carried out using the thallium recovery equipment AA shown in Figure 1, or it may be carried out using equipment with other configurations.

[0035] First, an acid is added to the thallium-containing raw material to dissolve it, and a dissolution slurry consisting of an insoluble residue and a dissolved solution is obtained (dissolution step). As the thallium-containing raw material, for example, zinc-containing slag discharged from a crude zinc oxide plant using the Wertz process can be used.

[0036] At least hydrochloric acid is added to the thallium-containing raw material. Adding hydrochloric acid to the thallium-containing raw material yields an insoluble residue containing thallium. In addition to hydrochloric acid, other acids may be added to the thallium-containing raw material. For example, when zinc slag is used as the thallium-containing raw material, sulfuric acid may be added to the zinc slag in addition to hydrochloric acid. Dissolving the zinc slag with a mixture of hydrochloric acid and sulfuric acid dissolves the zinc and cadmium contained in the zinc slag, allowing the lead and thallium to be separated as insoluble residues. Thus, when the thallium-containing raw material contains lead, a solution with low lead and thallium concentrations can be obtained by dissolving the thallium-containing raw material with a mixture of hydrochloric acid and sulfuric acid.

[0037] Next, the dissolved slurry is separated into solid-liquid and undissolved residue (solid-liquid separation step). Solid-liquid separation equipment such as filter presses and belt filters are used for solid-liquid separation of the dissolved slurry. The dissolved liquid is discharged from the solid-liquid separation equipment, while the undissolved residue remains inside the solid-liquid separation equipment or on the filter cloth.

[0038] Next, for example, water is supplied to a solid-liquid separator to wash the insoluble residue with water (washing step). When a filter press is used as the solid-liquid separator, either a through-washing method or a direct washing method may be adopted. The insoluble residue contains thallium chloride. Since thallium chloride readily dissolves in water when the chloride ion concentration in the liquid decreases, the water after washing (thallium-containing water) contains thallium ions. The insoluble residue after washing (residue after washing) is discharged from the solid-liquid separator.

[0039] In the washing process, thallium-containing water is continuously discharged from the solid-liquid separator. The initial thallium-containing water discharged at the beginning of washing and the later thallium-containing water discharged at the end of washing are separated and recovered. The insoluble residue contains the acid used to dissolve the thallium-containing raw material along with the adhering water, and this acid is washed away by washing and mixed into the thallium-containing water. The initial thallium-containing water contains a relatively large amount of acid, while the later thallium-containing water contains a relatively small amount of acid. This embodiment is characterized by separating the thallium-containing water into initial thallium-containing water with a high acid concentration and later thallium-containing water with a low acid concentration.

[0040] The above description explains a method in which a filter press is used as a solid-liquid separation device, and water is supplied to the inside of the filter press to clean the insoluble residue remaining inside the device. However, the method is not limited to the above. For example, after the insoluble residue is discharged from the solid-liquid separation device, it may be washed again continuously with water. However, in this case, a repulping tank or an additional solid-liquid separation device would be necessary. Therefore, the method using a filter press as the solid-liquid separation device is more efficient and preferable.

[0041] The timing for switching between the initial and final stages of cleaning can be determined by various methods. The timing can be determined based on the cleaning time, or based on the amount of thallium-containing water discharged. However, from the viewpoint of obtaining final thallium-containing water with a lower acid concentration, it is preferable to determine the timing of the switch based on the pH of the thallium-containing water.

[0042] Specifically, the pH of the thallium-containing water continuously discharged during the washing process is measured. The pH measurement may be intermittent or continuous. The thallium-containing water discharged before the pH measurement reaches a predetermined threshold is defined as the initial thallium-containing water. The thallium-containing water discharged after the pH measurement reaches the threshold is defined as the later thallium-containing water. Here, the threshold is preferably set within the range of pH 2 to 4, and more preferably within the range of pH 2.5 to 3.5. By separating the initial and later thallium-containing water based on the pH of the thallium-containing water in this way, the acid concentration of the later thallium-containing water can be reliably reduced.

[0043] It is preferable to return the initial thallium-containing water to the dissolution process as part of the thallium-containing raw material. The initial thallium-containing water contains thallium. By returning the initial thallium-containing water to the dissolution process instead of discharging it outside the system, thallium loss can be suppressed.

[0044] Next, an oxidizing agent is added to the late-stage thallium-containing water to produce thallium precipitate (oxidation step). The thallium contained in the late-stage thallium-containing water precipitates as an oxide or hydroxide, producing thallium precipitate. Although not particularly limited, sodium hypochlorite and potassium permanganate are preferably used as oxidizing agents.

[0045] Because the acid concentration in late-stage thallium-containing water is low, the amount of oxidizing agent consumed by reaction with hydrogen ions during the oxidation process can be reduced. Therefore, the amount of oxidizing agent required for the formation of thallium precipitate can be reduced.

[0046] Thallium precipitates may also be recovered by solid-liquid separation of a slurry containing thallium precipitates (thallium slurry). [Examples]

[0047] (Beaker test) Three types of thallium-containing water with the same thallium concentration but different pH levels were prepared and placed in beakers. Based on the following reaction equation (1), 2 equivalents of sodium hypochlorite relative to the amount of thallium were added to each thallium-containing water and the mixture was stirred. 2Tl + +2NaClO+4H2O→2Tl(OH)3↓+2H + +2NaCl (1)

[0048] The thallium concentration of the aqueous solution after the reaction was measured, and the amount of thallium remaining in the solution was determined. Furthermore, the thallium recovery rate was calculated from the amount of thallium remaining in the aqueous solution after the reaction and the amount of thallium contained in the thallium-containing water before the reaction. Here, the thallium recovery rate refers to the proportion of thallium contained in the thallium-containing water before the reaction that is recovered as thallium precipitate.

[0049] The results are shown in the graph in Figure 3. The higher the pH of the thallium-containing water, the higher the thallium recovery rate. Specifically, the thallium recovery rate was 6% at pH 1.2, 20% at pH 1.6, and 100% at pH 3.0. From this, it was confirmed that a 100% thallium recovery rate can be achieved by setting the pH of the thallium-containing water to 3.0 or higher. From the perspective of increasing the thallium recovery rate, a pH of 2 to 4 is preferable for thallium-containing water, and 2.5 to 3.5 is more preferable.

[0050] (Actual machine testing) The thallium recovery facility AA shown in Figure 1 was operated. Zinc slag containing thallium was used as the thallium-containing raw material. A mixed acid of hydrochloric acid and sulfuric acid was used to dissolve the zinc slag containing thallium. The pH of the thallium-containing water discharged from the solid-liquid separator 20 was monitored, and water with a pH of 3.0 or less was separated as initial thallium-containing water, and water with a pH above 3.0 was separated as late thallium-containing water. The initial thallium-containing water was returned to the dissolution tank 10. Sodium hypochlorite was added to the late thallium-containing water to generate thallium precipitate. The amount of sodium hypochlorite (12% available chlorine) required to recover 1 kg of thallium contained in the late thallium-containing water as thallium precipitate was 5.9 kg. The thallium concentration of the thallium precipitate was 70-80% by weight.

[0051] When sodium hypochlorite is added to all thallium-containing water discharged from the solid-liquid separation device 20 to produce thallium precipitate, the amount of sodium hypochlorite (12% available chlorine) required to recover 1 kg of thallium contained in the thallium-containing water as thallium precipitate is 11.8 kg. It was confirmed that when thallium precipitate is produced using only late-stage thallium-containing water, the amount of sodium hypochlorite used per unit weight of thallium can be halved. [Explanation of Symbols]

[0052] AA Thallium Recovery Facility 10 Dissolution tank 20 Solid-liquid separator 31 Storage tank 32 Oxidation tank 40 Discharge channel 50 Return channel

Claims

1. A dissolution tank is used to obtain a dissolution slurry consisting of an insoluble residue containing thallium and a dissolving solution by adding at least hydrochloric acid to a thallium-containing raw material. A solid-liquid separation apparatus that separates the dissolved slurry into the insoluble residue and the dissolved liquid, and then continuously discharges thallium-containing water as post-wash water while washing the insoluble residue with water, A storage tank for storing the initial thallium-containing water, which is the thallium-containing water discharged from the solid-liquid separator during the initial washing period, which is the beginning of the washing period, in which the insoluble residue is washed with the water. The system includes an oxidation tank that adds an oxidizing agent to the late thallium-containing water, which is the thallium-containing water discharged from the solid-liquid separator during the late cleaning period after the initial cleaning period, in order to generate thallium precipitate. A thallium recovery facility characterized by the following features.

2. The pH of the initial thallium-containing water is below the threshold, and the pH of the later thallium-containing water exceeds the threshold. The aforementioned threshold value is a predetermined value within the range of pH 2 to 4. The thallium recovery equipment according to feature 1.

3. The storage tank is equipped with a return channel for sending the initial thallium-containing water from the storage tank to the dissolution tank. The thallium recovery equipment according to feature 1.

4. Hydrochloric acid and sulfuric acid are supplied to the dissolution tank. The thallium recovery equipment according to feature 1.

5. A dissolution step in which at least hydrochloric acid is added to a thallium-containing raw material to obtain a dissolution slurry consisting of an insoluble residue containing thallium and a dissolving solution, A solid-liquid separation step is performed to separate the dissolved slurry into the insoluble residue and the dissolved liquid. A washing step comprising: washing the insoluble residue with water while continuously discharging thallium-containing water as post-wash water; and separating the initial thallium-containing water, which is the thallium-containing water discharged during the initial washing period, and the later thallium-containing water, which is the thallium-containing water discharged during the later washing period, after the initial washing period; The system includes an oxidation step in which an oxidizing agent is added to the aforementioned late-stage thallium-containing water to produce thallium sediment. A method for recovering thallium characterized by the following features.

6. The pH of the thallium-containing water continuously discharged during the washing process is measured, and the thallium-containing water discharged until the pH measurement reaches a predetermined threshold is defined as the initial thallium-containing water, and the thallium-containing water discharged after the pH measurement reaches the threshold is defined as the later thallium-containing water. The thallium recovery method according to feature 5.

7. The aforementioned threshold value is a predetermined value within the range of pH 2 to 4. The thallium recovery method according to feature 6.

8. The initial thallium-containing water is returned to the dissolution process as part of the thallium-containing raw material. The thallium recovery method according to feature 5.

9. In the dissolution step, hydrochloric acid and sulfuric acid are added to the thallium-containing raw material. The thallium recovery method according to feature 5.

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