JPH0210875B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention provides a method for efficiently electrowinning metals from an electrolyte aqueous solution containing a low concentration of metals whose standard electrode potential is nobler than zinc, using an electrolytic cell with a fixed-bed bed electrode. Regarding. [Prior art] Methods for electrolytically extracting metals from aqueous electrolyte solutions containing metals at low concentrations can be roughly divided into methods using bed electrodes and methods using rotating electrodes. , trickle bed, slurry, etc., but the present invention relates to a fixed bed method. The fixed-bed electrolytic cell used in the method of the present invention has been tested extensively in the past, and in some cases has been able to electrolytically win metals at a relatively high yield, but when the target aqueous solution changes, the yield drops significantly. There were problems such as a drop in the performance. In view of the above problems, improved methods include (1) a method of forming a bed electrode by mixing a non-conductive material such as plastic or ceramic into an electrically conductive material (Japanese Patent Laid-Open No. 130292/1983); (2) ) Methods of increasing the cathode surface area as much as possible have been proposed. However, the method (1) above is not practical because the current efficiency decreases significantly depending on the electrolyte. Also, (2)
According to the study results of the present inventors, the method of
It was observed that the current efficiency was significantly lowered and the electrolyte partially passed through, and no increase in electrodeposit was realized. [Problems to be Solved by the Invention] An object of the present invention is to provide an electrolytic method for electrowinning metals from dilute aqueous solutions that does not have the above-mentioned drawbacks. [Means for solving the problem] In order to achieve this objective, the inventors of the present invention have made the discovery as a result of intensive research, and the method of the present invention is shown in an explanatory diagram (cross-sectional view of an electrolytic cell) in Fig. 1. ), an aqueous solution containing a dilute concentration of a metal whose standard electrode potential is more noble than zinc is made by flowing the SV of the aqueous solution with an electrically conductive substance in the cathode chamber. SV = 40 or less, preferably 20 or less, and the current density to the cathode plate is
0.01A/dm ² to 1.5A/dm ² , the spacing between the cathode plate 2 and the diaphragm 3 is adjusted according to the electrical conductivity of the target aqueous solution.
For example, if the electrical conductivity is 10 mS/cm, the interplanar spacing should be 0.5 cm or more and 8.5 cm or less, and if the electrical conductivity is smaller than this, the interplanar spacing should be 0.5 cm or more and 8.0 cm or less, preferably 5 cm or less. In this way, electrolysis is carried out with a predetermined spacing between the surfaces. [Function] The electrolytic cell used in the present invention is a fixed-bed unipolar bed electrode-equipped electrolytic cell 1 as shown in FIG.
It consists of an anode plate 2, a cathode plate 3, a diaphragm 4, a bed cathode 5, an electrolyte aqueous solution inlet 6, and an overflow port 7 for electrolyzed aqueous solution. Although not specified as the above electrode, anode plate 2
is a plate made of graphite, lead, stainless steel, platinum-plated titanium, etc., which is insoluble in the target electrolyte aqueous solution, and the cathode plate 3 is made of graphite, lead, stainless steel, or the like.
A plate made of titanium or the like, the diaphragm 4 is made of filter cloth, unglazed porcelain, porous synthetic resin, anion exchange membrane, etc., and the bed cathode 5 is made of the same material used for the cathodes and anodes, activated carbon, particulate metal, pellets, rings, etc. can be used. The diaphragm 4 partitions the anode plate 2 and the cathode plate 3 and forms a cathode chamber. The cathode chamber is filled with particulate metal etc. to form a bed cathode, and an electrolyte aqueous solution (hereinafter referred to as The electrolytic solution (abbreviated as "electrolytic solution") enters from the electrolytic solution inlet 6, passes through the bed electrode 5, and directly overflows from the overflow port 7 to the outside. In other words, electrolysis is performed by a so-called one-time process in which electrolysis is normally performed in one pass. . Of course, it is also possible to repeatedly electrolyze the overflow of the electrolytic cell, but in the case of an electrolytic solution containing a metal at a very dilute concentration, this repeated electrolysis method is not preferred. Next, regarding the width of the electrolytic cell, if the upper side is a and the lower side is b, there is a case where a>b is satisfied when a or b is equal, but any of these is possible in the present invention. In the method of the present invention, the reason why the SV of the electrolytic solution is set to 40 or less, preferably 20 or less, is that if the SV is too large, the bed cathode will flow, or even if it does not flow, the current efficiency will decrease. However, the upper limit of SV varies depending on the type of metal; for example, in the case of nickel, even when SV=30, the metal yield deteriorates significantly (see Table 1 of Example 3), so care must be taken. The reason for setting the current density to the cathode plate in the range of 0.01 to 1.5 A/dm ² is that below 0.01 A/dm ² the metal recovery rate is extremely poor due to the reduction of dissolved oxygen; When it is ² or more, the amount of metal electrodeposited increases, but hydrogen generation is involved, resulting in a decrease in current efficiency and an increase in cost. Next, depending on the electrical conductivity of the target electrolyte,
The reason why the spacing between the cathode plate and the diaphragm is set to a predetermined value is as follows.
When the electrical conductivity is a normal value of around 10 mS/cm, the surface spacing is in the range of 0.5 to 8.5 cm, and when the electrical conductivity is smaller, such as 5 mS/cm, the lower limit of the surface spacing is Although it is the same as 0.5cm,
The upper limit is 8 cm or less, preferably 5 cm. If this spacing is narrower than 0.5 cm, the passage resistance of the electrolyte becomes large, and the area of the cathode plate needs to be increased, and in addition, distortion of the cathode plate is unavoidable, resulting in problems other than those mentioned above. The upper limit of this spacing is not specified and varies depending on the electrical conductivity of the electrolyte, i.e., the type and concentration of metals contained, the concentration of coexisting salts, etc., but if the spacing is too large, the current efficiency will drop significantly, making it impractical. There isn't. In order to understand the method of the present invention, the potential-distance diagram mainly between the surfaces of the cathode and the diaphragm when electrolyzing an electrolytic solution using a fixed-bed unipolar bed electrode electrolytic cell will be explained below. An example of electrowinning of copper from a dilute aqueous sulfuric acid solution will now be described. In an electrolytic cell with width a = b = 15 cm in Fig. 1, 100 mg/free sulfate radical (hereinafter abbreviated as fSO ₄ ^-2 ) as Cu is added.
A copper sulfate aqueous solution of 150 mg/, PH 0.8, electrical conductivity 9 mS/cm was placed at room temperature, SV = 1, and the current was 1 dm ² on the cathode plate.
When the potential of the electrolyte tank and the bed cathode are measured with respect to the cathode plate when a current of 0.5A is applied, the relationship between the potential and the distance diagram between the cathode plate and the diaphragm surface as shown in Figure 3 is obtained. can get. Since the bed cathode conducts electrons as a cathode plate, the potential difference between AC is usually 10 mV or less, which can be ignored in normal cases, and the cathode plate and bed cathode can be considered to have approximately the same potential. On the other hand, the potential of the aqueous phase of the electrolyte relative to the cathode plate can be measured by inserting a Luggin tube into the particles of the bed cathode, but a potential gradient is generated due to liquid resistance, resulting in a curve as shown in Figure 3. The negative potential is on the cathode plate side, and the noble potential is on the side closer to the diaphragm between the anode chamber and the anode chamber. Now, Cu ⁺² 100mg/
Calculating the value of the equilibrium potential for the reaction Cu ⁺² +2e = Cu ⁰ in the case of E = +0.254 (V vs. standard (1 normal) hydrogen electrode (NHE)) = +0.01 (V vs. It becomes a saturated SCE electrode (SCE), like the dotted line in the figure, and if the potential is more base than this line (below in the figure), theoretically Cu ⁺²
can be reduced to Cu ⁰ . The bed cathode particles in the range AB in the figure have a more noble potential than the dotted line, and Cu
cannot be electrodeposited. In reality, there is a difference in the concentration of Cu between the top and bottom of the tank, so the length of AB is slightly different between the top and bottom of the tank, but it is not that big and can be ignored. There is no ability to deposit and recover Cu, and in this area Cu passes through without being electrodeposited at its original concentration (or even if it is electrodeposited, it is redissolved). Note that the electrochemical reaction occurring to the left of the vicinity of point B is mainly a reduction reaction of dissolved oxygen. Next, in the same tank, with a similar solution but with Cu ⁺² 100
mg/, f.SO ₄ ^-2 14000mg/, electrical conductivity 7m
When electrolysis is carried out under the same conditions with a liquid of S/cm, the potential of the liquid phase relative to the cathode plate and the potential of the bed cathode are measured as shown in FIG. 4. In this case, since there is a large amount of f.SO ₄ ^-2 , the electrical conductivity of the aqueous solution is high and the potential gradient of the aqueous solution phase is gentle.
At this time, the particles are Cu ⁺² +2e=
Since the potential is less noble than the equilibrium potential of Cu ⁰ over the entire range, the entire area effectively acts as a cathode, allowing Cu to be electrodeposited, and there is no part through which the aqueous solution passes. As shown in this example, even if the aqueous solution has the same Cu concentration, if the f.SO ₄ ^-2 concentration differs, the state of Cu deposited into the bed cathode will differ. It is generally said that an aqueous solution exhibiting a potential gradient as shown in Figure 3 is an aqueous solution with low electrical conductivity, and an aqueous solution exhibiting a potential gradient as shown in Figure 4 is an aqueous solution with high electrical conductivity. The difficulty of electrowinning metals has a large relationship with the electrical conductivity of the aqueous solution. Furthermore, a factor that affects the electric refraction in the case of electrowinning of metals is the magnitude of polarization at the time of electrorefraction. Since the spacing is far apart, the range of electrolysis becomes wider, but since electrowinning is usually performed from an aqueous solution with a low metal concentration, concentration polarization is large in any aqueous solution, so the main factor that comes into play is the electrical conductivity of the electrolyte. . In addition, in Figures 3 and 4, the equilibrium potential of Cu ⁺² +2e-Cu ⁰ was calculated to determine whether Cu electrolytic extraction is possible, but in actual electrolysis of aqueous solutions, H ₂ due to water reduction Gas generation occurs as a side reaction, and especially in the case of a dilute aqueous solution of metal, the concentration polarization is large, and the potential at which the metal precipitates deviates from the equilibrium potential, reaching the equilibrium potential for H ₂ gas generation and accompanied by H ₂ gas. There are many things that happen. The generation of H ₂ gas itself does not occur at the equilibrium potential because there is a hydrogen overvoltage depending on the material and surface condition of the cathode plate and bed cathode particles, but for the sake of calculation, if we take the equilibrium potential as a reference, the potential is 25
At °C, E = −0.059PH (V vs. NHE) = −0.059PH−
Calculated at 0.24 (V vs. SCE). The equilibrium potential for hydrogen generation in the example of Figure 3 is shown by the broken line in the figure (bottom), but it is actually better to consider that Cu is electrodeposited on the part of the cathode particle that is less base than this potential. close. Similarly, FIG. 4 also shows the equilibrium potential for H ₂ generation. The explanation has been given using the electrowinning of Cu as an example, but electrowinning from NiSO ₄ aqueous solution, KAu
(CN) ₂ Electrowinning of Au from aqueous solution, KAg(CN) ₂
The situation is the same when it comes to electrolytic extraction of Ag from an aqueous solution. If you think about it, you can get an idea of the range of cathode particles that can efficiently electrowinning metals. The potential-distance diagram of the aqueous solution tank between the cathode and the diaphragm is SV, that is, when the flow rate is increased,
Since the concentration polarization becomes smaller, it shifts to the base potential side.
Also, when the current is increased, the potential shifts toward the noble side, but in this case, water is generated, so it does not shift indefinitely and does not become nobler than a certain line. As a result of actually experimenting with these things, we found that in the case of an aqueous solution with an electrical conductivity of 10 mS/cm, as shown in Figure 3.
It was found that the length of B'C is 4 to 8.5 cm, about 2 to 5 cm in the case of an aqueous solution of 5 mS/cm, and about 0 to 3 cm in the case of an aqueous solution of 1 mS/cm or less. Therefore, for example, in the case of an aqueous solution with an electrical conductivity of 10 mS/cm, the length of B'C must be limited to 8.5 cm at most, and the length of B'C with a smaller electrical conductivity value is 5 cm.
In the case of an aqueous solution of mS/cm, it is necessary to limit the length of B'C to 5 cm at the maximum, and in the case of an aqueous solution of 1 mS/cm or less, it is necessary to limit the length of B'C to 3 cm at the maximum. For aqueous solutions with electrical conductivity greater than 10 mS/cm, the length of B'C may be greater than 8.5 cm, but aqueous solutions that require electrowinning are often factory effluents;
Since electrical conductivity is often not constant, it is safer to assume that the electrical conductivity will be around 10 mS/cm at the lower limit. In addition, if the width of B'C is too narrow, the resistance of the flow path will increase, and a cathode plate with a fairly large area is required for electrowinning, so as mentioned above, the lower limit is set to 0.5 cm. . Although I have explained this in detail above, it can also be stated qualitatively as follows. For example, when placing an anode in front of a radiator with fins and using the radiator pipe as the cathode to electrodeposit metal onto the radiator in an aqueous metal solution, if the liquid has high electrical conductivity, the entire fin and the wick will It is possible to electrodeposit up to the pipe, but if the liquid has low electrical conductivity, the metal will only be electrodeposited on the side near the anode, and the core near the pipe will not be electrodeposited, so the width of the fin will must be made shorter, but if we consider the pipe as a cathode plate and the fins as bed cathode particles, the situation is roughly the same. (However, in this case, not only the electrical conductivity but also the magnitude of polarization comes into play, so it would be more accurate to describe the magnitude of uniform electrodeposition instead of the magnitude of electrical conductivity.) As explained above, The method of the present invention uses an aqueous solution containing a dilute concentration of a metal whose standard electrode potential is nobler than zinc, while maintaining an appropriate SV and current density to the cathode plate, and keeping the spacing between the cathode plate and the diaphragm within a suitable range. This method performs electrolysis, and although it varies depending on the type of metal, it is possible to efficiently recover metals with a recovery rate of about 90% even if it is low in one-off treatment, but usually more than 95%. be able to. [Example] Examples will be described below. Example 1 In an electrolytic cell made of vinyl chloride with a width of 14 cm, a depth of 10 cm, and a depth of 11 cm (inside dimensions), the distance between the anode plate and the diaphragm plate was 5 cm, and the distance between the cathode plate and the diaphragm plate was 7.5 cm.
cm, as shown in the perspective view of Figure 2, and prepare an anode chamber and a cathode chamber. Co., Ltd.) was filled to a height of 10 cm. The anode plate and the cathode plate were each 10 x 12 x 0.5 cm graphite plates (manufactured by Toyo Carbon Co., Ltd.), and the diaphragm plate was an anion exchange membrane, trade name IDNAC, MA3475 (manufactured by Muromachi Chemical Industry Co., Ltd.). . The electrolytic solution contains 100 mg of Cu and 1.6 g of free sulfuric acid, and has an electrical conductivity of 9.4×10 ^-5 s/m.
An aqueous solution is supplied to the cathode chamber from the lower inlet 6 at a SV of 5.0, and is discharged to the outside from the upper overflow port 7 to increase the current density to the cathode plate using a transient treatment method.
Electrolysis was carried out at 0.1 A/dm ² and 25° C. for 24 hours. During this time, the anode chamber was first filled with an aqueous solution containing 1.8 g of sulfuric acid. As a result, 8.38 g of electrolytic copper was obtained, and the copper recovery rate was 99.8%. For comparison, the same electrolytic solution was electrolyzed in the same manner as in Example 1, except that the distance between the cathode plate and the diaphragm was 9.5 cm using the same electrolytic cell, and the copper recovery rate was
It remained at 68.5%. As is clear from these results, it was found that when the distance between the surfaces of the cathode plate and the diaphragm was outside the range of the present invention, the current efficiency decreased significantly and a sufficient recovery rate could not be obtained by the one-time treatment method. Example 2 As an electrolyte, a commercially available gold-plated aqueous solution containing 4 mg/Au, PH 4.0, electrical conductivity 3.1 mS/cm, and approximately 0.5 mol of citric acid and phosphoric acid, respectively, was used.
Example 1 except that the electrolytic cell used in Example 1 was used, the distance between the planes of the cathode plate and the diaphragm was 3 cm, the SV was 4.0, and the current density to the cathode plate was 0.05 A/dm ²
When electrolyzed in the same manner as above, Au at the outlet of the electrolytic tank was
The concentration was 0.08 mg/gold, and the gold recovery rate was 98.0%. As a comparative example, the distance between the cathode plate and the diaphragm is 6.0.
When electrolysis was carried out in the same manner as in Example 2 except that the setting was cm, the Au concentration at the outlet of the electrolytic cell remained at 1.20 mg/cm, and when the electrical conductivity was 3.1 mS/cm, the above surface spacing was 5.0 cm or less. I realized that I needed to. Example 3 Two aqueous solutions each containing 100 mg of Ni and 80 mg of Zn as electrolytes and 1.6 g of each free sulfuric acid and having an electrical conductivity of 10 mS/cm were prepared, and SV
Electrolysis was carried out in the same manner as in Example 1, except that the current density to the cathode plate was set to a predetermined value and the distance between the planes of the cathode plate and the diaphragm was constant at 8.3 cm. The results are shown in Table 1.

〔Effect of the invention〕

The present invention can be used as long as the electrolytic solution has uniform electrodeposition even in a dilute aqueous solution with a metal concentration of several ppm, is efficiently electrolyzed even in a transient treatment, and contains a metal whose standard electrode potential is nobler than zinc. The method allows high recoveries to be obtained. It is suitable for application to the treatment of factory wastewater, plating wastewater, etc. that contain low concentrations of harmful metals.

[Brief explanation of drawings]

Figure 1 is a longitudinal cross-sectional view of the electrolytic cell, Figure 2 is a perspective view of its appearance, and Figures 3 and 4 are potentials showing the relationship between the distance between the surfaces of the cathode plate and the surface of the diaphragm and the potential, respectively. - It is a distance diagram, where the vertical axis is the potential and the horizontal axis is the distance between the surfaces. 1... Electrolytic cell, 2... Anode plate, 3... Cathode plate,
4...Diaphragm, 5...Bed cathode, 6...Aqueous electrolyte solution inlet, 7...Overflow port.

Claims (1)

[Scope of Claims] 1. An electrolytic diaphragm that separates an electrolytic cell into an anode chamber and a cathode chamber, an anode plate and a cathode plate arranged in the anode chamber and the cathode chamber, respectively, and a space between the diaphragm and the cathode plate. When electrolyzing an aqueous solution containing a metal at a dilute concentration using a fixed-bed unipolar bed electrode electrolyzer consisting of an electrically conductive material filled with an electrically conductive material, the metal whose standard electrode potential is more noble than zinc is used at a dilute concentration. The electrolyte is an aqueous solution contained in the cathode chamber, and its space velocity (SV) is limited to such an extent that the electrically conductive material in the cathode chamber does not flow, and the current density to the cathode plate is
0.01 to 1.5 A/dm ² , and the spacing between the cathode plate and the diaphragm is set to a predetermined value depending on the electrical conductivity of the target aqueous solution. How to collect. 2. A method for electrowinning a metal from an aqueous solution containing a dilute concentration of the metal according to claim 1, wherein the SV is 20 or less. 3. An aqueous solution containing a dilute concentration of the metal according to claim 1 or 2, wherein the spacing between the cathode plate and the diaphragm is 0.5 to 8.5 cm when the electrical conductivity of the target aqueous solution is 10 mS/cm or more. A method of electrowinning metals from. 4 When the electrical conductivity of the target aqueous solution is less than 10 mS/cm, the spacing between the cathode plate and the diaphragm is 0.5 to 8.0.
3. A method for electrowinning a metal from an aqueous solution containing the metal according to claim 1 or 2 at a dilute concentration.