JP2594802B2 - Electrolytic reduction method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for reducing a metal ion from a solution containing a metal ion mainly composed of a noble metal and continuously depositing the same as metal particles in a cathode chamber.

(Prior art and its problems) In order to recover metals from waste liquids containing noble metals such as plating waste liquids and automobile catalysts composed of precious metals mainly including palladium, platinum and rhodium, there are roughly three types of metals as follows. The method has been used.

i) Drug reduction method In this method, a reducing agent such as formic acid, oxalic acid, hydrazine, sodium borohydride (SBH) is added to a solution containing a metal ion to be recovered, and the metal ion is reduced to a metal. This is a method of collecting this. This reaction can be represented by the following equation.

M ^{n +} + n RH → M + n R ⁺ + n / 2 H ₂ ii) Metal substitution reduction method This method uses a solution in which a noble metal or the like is dissolved by adding a metal (for example, zinc or aluminum) having a lower oxidation-reduction potential than the noble metal. This is a method in which the base metal ions dissolved in a powdery, granular or granular state are reduced to a noble metal state. This reaction can be represented by the following equation.

Or ^{_{M 1 n + + n M 2}} → M 1 + n M 2 + iii) The method electrolytic recovery method, dissolving the metal ion is made to electrodeposition onto a cathode by electrolysis detached from said cathode finest recovered, or This is a method in which electrically conductive particles are added to a solution to deposit a metal on the particles as a three-dimensional electrode structure. This reaction can be represented by the following equation.

M ^{n +} + ne → M Of the above three methods, the chemical reduction method uses an expensive reducing agent such as formic acid and sodium borohydride, so the cost increases, and the cost of the recovered precious metal becomes higher. In order to add the reducing agent, the solution after recovery of the metal must be regenerated by performing various treatments (for example, pH adjustment, COD removal, etc.), and further requires a chemical cleaning treatment of deposits on the deposited metal. There is a disadvantage that.

In the metal substitution reduction method, the same drawbacks have been pointed out because other metals are added.

In the electrolytic recovery method, since the metal to be reduced is deposited on the cathode, it is necessary to stop the electrolysis operation to recover the metal, and to peel off the metal deposited on the cathode,
There is a disadvantage that the operation efficiency is greatly reduced. Also, depending on the type of metal, needle-like crystals may be formed and an accident such as a short circuit of the positive electrode may occur. Furthermore, in the three-dimensional electrode electrolysis method using the electrically conductive particles, it is necessary to remodel the electrolytic cell to be used into a complicated structure suitable for using the electroconductive particles, and there is a disadvantage that the remodeling cost is increased. Further, when a bipolar three-dimensional electrode is used, there is a disadvantage that the electrodeposited metal causes a short circuit between the anode and the cathode, and the electrodeposited metal is redissolved. Also, when using a monopolar three-dimensional electrode, if the electrical conductivity of the solution is high, the electrolytic reaction does not occur on the target three-dimensional electrode, the electrode reaction occurs on the surface of the current collector, and the solution is bothersomely diluted. The disadvantage is that you have to do it.

In the operation of recovering metals from waste liquids and waste catalysts, how to obtain metals easily and cheaply is one of the most important issues, and the types of chemicals used and the operating conditions are being studied. However, it is difficult to say that it is still sufficient, and at present it has not led to a reduction in the cost of the recovered metal.

(Object of the Invention) In view of the above-mentioned disadvantages of the prior art, the present invention provides a method for reducing metal ions in a solution without stopping the electrolytic reaction using an electrolytic reaction that is relatively easy to operate. The purpose is to do.

(Means for Solving the Problems) The present invention relates to a low-hydrogen solution to which a metal ion-containing solution is added to a cathode chamber of an electrolytic cell divided into an anode chamber and a cathode chamber by a diaphragm, and a catalyst is added as a cathode. A reduction electrolysis method comprising performing electrolysis using an overvoltage carbon electrode to reduce metal ions in the solution in the solution and depositing the metal ions in the catholyte as a metal. According to the present invention, the metal precipitates in the electrolyte in the form of powder, so that the metal can be continuously recovered by filtering the electrolyte.

 Hereinafter, the present invention will be described in detail.

The present invention, when reducing metal ions using an electrolytic reaction and depositing as a metal, under normal electrolytic conditions, without depositing any or almost no metal on the cathode plate, which would be deposited on the cathode, The greatest feature is that the metal particles are precipitated and precipitated in the cathode chamber.

Next, each element constituting the electrolytic cell in the electrolysis according to the present invention will be described.

As the material of the cathode used in the present invention, it is necessary to select a material having a low overvoltage for hydrogen generation. The reaction on the cathode surface in the electrolytic reduction of a solution containing metal ions is a competitive reaction between metal deposition and hydrogen generation. It is necessary to select a material that is unlikely to cause hydrogen generation (low overvoltage against hydrogen generation), and to selectively generate only hydrogen by adjusting other electrolysis conditions such as pH, temperature and concentration. Because there is.

As a normal cathode material, nickel, iron, stainless steel, etc., and a material mainly containing lead, silver, etc. are used, but the former is not suitable because both metal electrodeposition and overvoltage for both hydrogen generation are low. However, the latter is not suitable because the overvoltages for both metal deposition and hydrogen generation are high.

The carbon material such as graphite and activated carbon used in the present invention has a low overvoltage against hydrogen generation, and is effective in limiting the reaction on the cathode surface to only hydrogen generation and preventing metal deposition.

Further, when the carbon electrode contains a catalytic metal such as platinum, palladium, ruthenium and nickel, the effect becomes more remarkable. This is presumed to be because the hydrogen overvoltage approaches the electrolysis voltage.

In order to effectively inhibit metal deposition on the cathode surface, the current density is increased to increase the rate of hydrogen generation,
It is preferable to prevent the deposition of the metal to be deposited on the cathode. Preferably, the current density is at least 10 A / dm ² , more preferably at least 15 A / dm ² , most preferably at least 20 A / dm ² . Depending on other electrolysis conditions, for example, in the catholyte reduction of palladium chloride, as shown in FIG. 3, at a current density of 10 A / dm ² , 60% of the total deposition amount is powdery as metal particles in the cathode chamber. 80% at 15 A / dm ^{2 and} almost 100% at 20 A / dm ² , respectively.

On the other hand, the material of the anode is not limited at all. For example, a conventionally used dimensionally stable electrode (DSE) can be used.

To configure an electrolytic cell using the above cathode and anode,
It is necessary to divide the electrolytic cell into a cathode chamber and an anode chamber using a diaphragm. This is to prevent the gas generated at the anode from flowing into the cathode chamber and the target metal reduced at the cathode chamber from entering the anode chamber and being oxidized. The material of the diaphragm is not limited as long as it can prevent the inflow, and an ion exchange membrane, a calcined plate, an asbestos sheet and the like can be used.

The structure of the electrolytic cell is such that the box-type electrolytic cell is divided into two parts to the left and right using the above-mentioned diaphragm, and the cylindrical electrolytic cell is formed into the outer cathode chamber and the inner anode chamber using the cylindrical diaphragm. Even if it is partitioned, or a box-shaped electrolytic cell is divided into a plurality of electrolytic chambers using a plurality of diaphragms, and the electrodes for each electrolytic chamber are connected in a bipolar or monopolar manner to form a plurality of cathode chambers. The reduction may be performed.

The noble metal ion reduced according to the method of the present invention is a noble metal having a lower ionization tendency than hydrogen, for example, palladium, rhodium, platinum, gold, silver, ruthenium, and the like. .

In the present invention, not only the metal ions contained in the waste liquid, but also the solid metal salt or the metal salt supported on the catalyst carrier can be reduced to obtain metal particles. When used to reduce a solid or supported metal salt, it must be dissolved once in a suitable solvent to form a solution containing the desired metal ion. For example, to recover precious metals such as palladium supported on a catalyst carrier,
The carrier is dissolved in a solvent such as aqua regia to form a solution containing palladium ions, and then reduced according to the method of the present invention to obtain palladium metal particles.

When electrolysis of a solution containing, for example, palladium ions is performed by the method of the present invention having the above-described configuration, hydrogen ions in the catholyte are electrolytically reduced to generate hydrogen gas. The hydrogen gas is partly foamed as it is and is diffused from the cathode chamber, and the remaining gas reduces palladium ions to metal palladium according to the following formula. The palladium ion tries to approach the carbon electrode surface at Pd ²⁺ + H ₂ → Pd + 2H ^{+ at} the cathode, but it is difficult to access due to strong hydrogen gas generation. Most of them cannot be deposited due to a high overpotential against precipitation, and most of them are reduced as metal particles in the catholyte by the above-mentioned chemical reaction, and precipitate on the bottom plate of the cathode chamber as the particle size grows. During electrolysis, it is preferable to stir the catholyte,
As a result, the metal particles are dispersed and the metal particles are suspended in the catholyte so that the particle size can be grown.

Precipitated metal particles such as palladium are taken out of the electrolytic cell together with the electrolytic solution using an appropriate extraction cock provided at the bottom or side of the electrolytic cell, or after a large amount of precipitate is deposited, the electrolysis is stopped and taken out. You can also. Even if the metal particles grow, they have a relatively small particle size, and even if they are transported together with the solution, problems such as blockage and abrasion of pumps and pipes rarely occur. The electrolytic solution in which the metal has been deposited and the metal concentration has been reduced can be appropriately drained from the supernatant and reused as a new metal solution.

According to the above-described method of the present invention, metal ions in a solution can be reduced and deposited almost quantitatively. Further, when the cathode current density is adjusted, all or most of the metal particles can be deposited without being deposited on the cathode. And precipitate it.

As described above, in the present invention, it is convenient to perform the recovery of a metal by using an electrolytic reaction.However, the application of the present invention is not limited to the recovery of the metal, and the metal-containing solution is subjected to demetallization treatment and low-purity metal. It can be applied to high purification and the like.

Hereinafter, examples of the present invention will be described, but the present invention is not limited to the examples.

(Example 1) In the center of a box-shaped electrolytic cell having a length of 12 cm, a width of 15 cm and a depth of 15 cm, a cylindrical diaphragm (cation exchange membrane, Nafion 324 manufactured by DuPont) with an inner diameter of 10 cm and a depth of 15 cm having an open upper surface. And an anode chamber was formed inside the membrane, and a cathode chamber was formed outside the membrane.

10% by weight hydrochloric acid was added to the anode compartment, and the following sample solution 1 was added to the cathode compartment, and the temperature of the electrolyte was maintained at 45 ° C. Electrolysis current 15A, cathode current density of 15A / dm ^2, a cathode current density 10A / l, the liquid volume of 1.5, 10cm × 10cm × 5m as an anode
Using a dimensionally stable electrode (DSE) having a diameter of m and a graphite electrode of the same size as a cathode, electrolysis was performed while stirring with a magnetic stirrer.

After the start of electrolysis, the catholyte was sampled to measure the concentration of each metal ion in the catholyte. The change with time of each metal ion concentration is as shown in FIG. 1. The current efficiency (0 to 1 hour and 0 to 4 hours) for reduction of each metal ion and the recovery rate of each metal after 8 hours have passed. As shown in Table 2, the recovery rate after 8 hours of palladium, platinum and silver contained in the noble metal of the present invention was 100%, whereas that of copper not contained in the base metal of the present invention was 96.5%. Met.

(Example 2) The metal was recovered in the same manner as in Example 1, except that the sample solution 2 was used as the catholyte.

After the start of electrolysis, the catholyte was sampled to measure the concentration of each metal ion in the catholyte. The change with time of each metal ion concentration is as shown in FIG. 2. The current efficiency (0 to 2 hours and 0 to 8 hours) for reduction of each metal ion and the recovery rate of each metal after 16 hours have passed. As shown in Table 3, the recovery rate after 8 hours was 99.8% or more for palladium, platinum, and silver contained in the noble metal of the present invention, while 79.7% for copper not contained in the noble metal of the present invention. Met.

(Example 3) Under the same conditions as in Example 1, the cathode current density was changed to change the precipitation rate (the amount of metal particles precipitated in the cathode chamber /
(The amount of metal particles precipitated in the cathode chamber + the amount of metal deposited on the cathode) was examined.

The results are as shown in FIG. 3. At a cathode current density of less than 15 A / dm ² , the electrodeposition ratio increased with an increase in the current density and reached about 20 A / dm ² , reaching 1.0 (on the cathode). (Electrodeposition metal disappears), and it turns out that it becomes constant thereafter.

(Effects of the Invention) In the present invention, in reducing metal ions and depositing them as a metal, a carbon electrode having a low overvoltage against hydrogen generation is used as a cathode. Therefore, the metal particles reduced by electrolysis in the cathode chamber are unlikely to precipitate on the cathode, and hydrogen is actively generated at the carbon cathode to hinder the approach of the metal particles to the cathode. Therefore, the reduced metal particles hardly precipitate on the cathode plate, but precipitate and precipitate in the cathode chamber.

Further, a catalyst metal can be contained in the carbon cathode, whereby the electrolytic performance can be improved.

Therefore, unlike the conventional reductive deposition, there is no need to separate the deposited metal from the cathode, so there is no need to stop the electrolysis operation, and the operation efficiency is greatly increased. Moreover, since electrolysis is performed without adding electrically conductive particles and the like, and an electrolytic cell having a complicated structure is not required, a conventional electrolytic cell can be used as it is. Further, since almost no metal is deposited on the cathode, there is no problem of short circuit between the electrodes, and there is no adverse effect due to the electric conductivity of the solution. Further, it can be used for electrolysis as it is regardless of the concentration of the solution, and no extra work such as dilution or concentration of the solution is required.

Furthermore, in the method of the present invention, since no chemical such as a reducing agent is added, there is no change in the composition of the solution before and after the electrolysis, for example, the catalyst carrier and the like are dissolved using aqua regia, and the solution is subjected to the method of the present invention. In the case of reduction, the solution after reduction can be repeatedly used for dissolving the catalyst carrier, and a troublesome process can be omitted.

In addition, since electrolysis is used, the reduction reaction continues even when the concentration of metal ions becomes low, and metal ions can be deposited almost quantitatively as metal particles.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the electrolysis time and the metal concentration in the catholyte in Example 1, FIG. 2 is a graph showing the relationship between the electrolysis time and the metal concentration in the catholyte in Example 2, and FIG. FIG. 9 is a graph showing the relationship between the cathode current density and the electrodeposition ratio in Example 3.

Claims (4)

(57) [Claims] 1. A solution containing a noble metal ion is added to a cathode compartment of an electrolytic cell partitioned by a diaphragm into an anode compartment and a cathode compartment, and electrolysis is carried out using a carbon electrode having a low hydrogen overvoltage as a cathode. An electrolytic reduction method characterized in that the generated gas is limited to hydrogen gas, and the noble metal ions in the solution are reduced by the hydrogen and deposited as noble metal powder in a cathode chamber. 2. The method according to claim 1, wherein the electrolysis is performed at a cathode current density of 5 A / dm ² or more. 3. The method according to claim 1, wherein the carbon electrode is graphite or activated carbon. 4. The method according to claim 1, wherein a catalyst metal is added to the carbon electrode.