JP7126185B1 - Method for recovering platinum group metals - Google Patents

Abstract

Since a titanium-iridium laminate is used as an anode for electroplating, resource recovery is desired after disposal. However, since these substances are extremely stable, they are not easy to separate and recover. In addition, when the iridium is exfoliated using a constant current, it is found that the titanium of the metal substrate dissolves at once, and it is difficult to achieve both the recovery of the metal substrate without damage and the efficient recovery of the platinum group metals. is. The present invention is a method for recovering the platinum group metal from a laminate in which a layer of a platinum group metal is formed on a metal substrate, wherein the laminate is immersed in an aqueous solution containing fluorine, and the laminate is A method for recovering platinum group metals, which includes a step of applying a positive constant voltage, can solve the above problems.

Description

TECHNICAL FIELD The present invention relates to a method for recovering a platinum group metal from a laminate in which a layer of a platinum group metal or its oxide is formed on a metal. As a more specific embodiment, the present invention relates to a method of recovering a platinum group metal from a metal electrode substrate having a platinum group metal layer formed thereon by chemical treatment or chemical electrolysis.

Salt water electrolysis, production of electrolyzed water, chemical conversion treatment of aluminum, etc., and platinum group metals such as iridium and ruthenium on hard metal substrates such as titanium, zirconium, and niobium as anodes for electrolytic plating of copper, nickel, zinc, chromium, gold, etc. , or a layer of its oxide is used. These substances have high hardness and are chemically extremely stable substances.

Here, the surface of the metal substrate is roughened to improve adhesion of the layer of platinum group metal. Then, on the surface of the platinum group metal layer, the current density generated between it and the cathode, which is the counter electrode, differs depending on the location, and cracks occur in the platinum group metal layer over time, resulting in deterioration.

Deteriorated electrodes are discarded, but since metal substrates and platinum group metals are scarce resources, their reuse is desired. However, these metals are extremely stable and cannot be separated and collected easily.

Patent Literature 1 discloses a method of cutting an electrode into small pieces, peeling off the platinum group metal layer by polishing, and then removing the platinum group metal by acid cleaning.

In addition, in Patent Document 2, an anode made of a metal substrate having a gold or platinum group metal coating and a cathode are immersed in an electrolytic solution containing a fluoride as a main component, and an electric current is passed through these electrodes for anodic electrolytic treatment. is disclosed to recover gold or platinum group metals.

Japanese Patent Application Laid-Open No. 2001-303141: Japanese Patent No. 4450945 Japanese Patent Application Laid-Open No. 7-109594: Patent No. 2630702

A layer of a platinum group metal such as iridium or ruthenium on a hard metal substrate such as titanium, zirconium, or niobium can be obtained by applying a solution containing platinum group metal ions (for example, a platinum group chlorocomplex solution) to the surface of the substrate. , obtained by firing. In order to adjust the thickness of the layer to a predetermined value, the coating and firing are repeated many times to produce the product. It is not easy to peel off the platinum group metal layer once formed on the metal substrate.

For example, when the laminate is separated in the air by mechanical force such as blasting, dust from the separated platinum group metal and metal substrate spreads in the atmosphere, and there is a risk of dust explosion. Further, scraping off the platinum group metal layer by polishing as in Patent Document 1 is a process that takes too much time and effort, and the platinum group metal cannot be efficiently recovered from a large amount of the object to be processed. In addition, there is also a problem that uniform peeling cannot be achieved by blasting or polishing on an irregularly shaped surface (for example, a mesh-like electrode structure), resulting in low recovery efficiency.

Moreover, it is desirable to reuse the metal base material after recovering the platinum group metal. The term "reuse" as used herein means recoating to form a platinum group metal layer again on a metal substrate.

Including the reuse of the metal base material, the recovery of platinum group metals by the anodic electrolysis treatment of Patent Document 2 can be said to be effective. However, when anodic electrolytic treatment is performed with a constant current, the platinum group metal is peeled off, and the metal base material suddenly dissolves near the surface of the metal base material, and the metal base material is rapidly thinned. there were.

The timing at which the platinum group metal is exfoliated and the metal base material appears can be determined by monitoring the point in time when the voltage rises sharply when the anodic electrolytic treatment is performed at a constant current. However, due to the shape of the object to be treated, which will be the anode, and variations in the thickness of the platinum group metal formed on the surface of the metal substrate, the change in voltage value when the platinum group metal peels off may be very steep. There was a problem that it was difficult to know when to stop processing.

For example, if the electrolytic treatment of the object (anode) from which the platinum group metal is stripped is insufficient, the amount of unrecovered platinum group metal increases, which is not an effective treatment.

On the other hand, if the object to be treated (anode) from which the platinum group metal is stripped is subjected to too much electrolytic treatment, the platinum group metal can be sufficiently recovered, but the metal substrate becomes thin and thin, and the metal substrate cannot be reused. Gone.

The following problems arise when attempting to chemically strip the platinum group metal using only chemicals. Although laminates have various shapes, when a platinum group metal layer is formed on a metal base material having an irregular surface such as a punching mesh, it is difficult to peel off the platinum group metal layer uniformly.

In the case of an electrode having a platinum group metal layer formed on only one side, the metal base material is exposed on the back side, and when chemical peeling is applied, the base material on the back side is significantly dissolved. As a result, the metal substrate cannot be reused (recoated), and the value of the metal substrate after peeling treatment is lost.

Next, when the platinum group metal layer is peeled off by dissolving the interface between the platinum group metal layer and the metal substrate by contacting the platinum group metal layer with a chemical solution, for example, the penetration of the chemical solution is Since surface cracks serve as starting points, the peeling time depends on the state of surface cracks. Therefore, it was necessary to set the treatment time according to the surface cracks, and it was difficult to set a uniform treatment process.

Since the peeling time of the platinum group metal layer depends on the surface crack state of the laminate, there are areas where it is easy to peel and areas where it is difficult to peel, making complete peeling difficult.

In general, there was a problem with work safety due to exposure to acidic chemicals such as sulfuric acid, hydrochloric acid, nitric acid, hydrofluoric acid, and oxalic acid.

In the case of chemical treatment using an acid, a large amount of hydrogen gas is generated by dissolving the metal substrate, so there is a problem from the viewpoint of work safety.

Fine catalyst powder that has been exfoliated is dispersed in the chemical solution, requiring complicated solid-liquid separation.

Since the recovered material after solid-liquid separation has a chemical solution attached thereto, it is necessary to remove the chemical solution component by alkali cleaning for neutralization and pure water cleaning in the latter stage.

Since the metal base material was exposed at the site where the platinum group metal layer was peeled off and dissolved and hydrogen was generated, the chemical solution became cloudy with air bubbles, and the end point of the peeling reaction completion could not be detected even visually.

Since the metal substrate is exposed and dissolved from the part where the platinum group metal layer is peeled off, the active ingredient of the chemical solution is likely to be deactivated, and the solution needs to be replaced with a new one frequently, which poses the problem of high cost. rice field.

The present invention has been made in view of the above problems, and provides a method for effectively recovering not only platinum group metals from laminates but also metal substrates.

Specifically, the method for recovering platinum group metals according to the present invention includes:
A method for recovering a platinum group metal from a laminate in which a layer of a platinum group metal is formed on a metal substrate,
a step of immersing the laminate and the conductor in an aqueous solution containing fluorine;
A first energizing step of applying a constant voltage such that the current flowing between the anode and the cathode is constant, with the laminate being the anode and the conductor being the cathode;
The method further comprises a second energizing step of applying a constant voltage at which a current smaller than the current applied in the first energizing step flows before the current applied in the first energizing step does not change .

In the platinum group metal recovery method according to the present invention, the platinum group metal layer is peeled off simply by immersing the laminate in which the platinum group metal layer is formed on the metal substrate in an aqueous solution containing fluorine, so that the hardness can be improved. A highly chemically very stable platinum group metal layer can be easily peeled off.

In addition, when the metal base material comes into contact with hydrofluoric acid, it violently generates hydrogen. Metal can be recovered.

Further, in the method for recovering platinum group metals according to the present invention, the laminate is immersed in an aqueous solution containing fluorine (hereinafter referred to as "solution for electrolysis"), and a positive voltage is applied. In this way, the platinum group metal layer can be peeled off (lifted off) from the dissolution portion of the base material and recovered.

On the other hand, the part where the platinum group metal layer is peeled off and the base material is exposed continues to apply a positive voltage to the laminate (object to be treated), and the reaction (depot) that protects the base material and melts the base material. Since the reaction (etching) occurs simultaneously, dissolution of the metal substrate after stripping the platinum group metal can be suppressed as much as possible, and the metal substrate can be recovered in a recoatable state.

Therefore, with the recovery method of the present invention, even if the film thickness of the platinum group metal varies, only the platinum group metal can be effectively recovered. , the metal substrate can be recovered in a state equivalent to the original state (roughened state).

In addition, the first half of the stripping process is processed with a high voltage (which may be a constant voltage or a constant current), and the second half is processed with a constant voltage, so that the time can be shortened compared to the case of the constant voltage only process. can be done.

Furthermore, by gradually increasing the positive voltage applied to the laminate, even if the thickness of the platinum group metal is unknown, the platinum group metal can be efficiently peeled off without excessively dissolving the base material. can.

In addition, by immersing a pair of laminates in the electrolytic solution and applying an alternating current to them, the laminates alternately become anodes, and the platinum group metal can be recovered more effectively.

It is a figure which shows the structure of the peeling apparatus which enforces the collection|recovery method of this invention. FIG. 10 is a graph showing a change in current value over time when a voltage is applied between both laminates in ammonium fluoride, with objects (laminates) to be treated being paired as electrodes. 5 is a graph showing changes in current when a constant current is applied in ammonium fluoride using the laminate as an anode. As another laminate, a graph showing changes in voltage when a constant current is applied in ammonium fluoride (a), the amount of dissolution of the metal substrate (Ti) (b), and the liquid temperature of ammonium fluoride (c). is. 5 is a graph showing change in current (a), dissolution amount of metal substrate (Ti) (b), and liquid temperature of ammonium fluoride (c) when a constant voltage is applied to the same sample as in FIG. 4. FIG. 7 is a graph showing a change in current value when processing with a constant current and processing with a constant voltage are combined. 5 is a graph showing changes in current value (a) and changes in set voltage (b) when the laminate is used as an anode and the applied voltage is increased at regular time intervals. FIG. 4 is a diagram showing the results of elemental analysis of black powdery precipitates collected by chemical stripping with different concentrations of hydrogen fluoride. FIG. 4 is a diagram showing the concentration of hydrogen generated when titanium is put into a mixed solution of hydrofluoric acid and hydrogen peroxide of different concentrations, for each concentration of hydrogen peroxide.

The method for recovering platinum group metals according to the present invention will be described below with reference to examples. In addition, the following description illustrates one embodiment of the present invention, and the present invention is not limited to the following description. The following description can be modified without departing from the spirit of the invention. In the following description, "upper" refers to the direction away from the surface of the metal substrate serving as a reference, and "lower" refers to the direction closer to the surface of the metal substrate. Also, "directly above" and "directly below" refer to configurations that do not include other layers in between. In addition, the concentration of an element of interest in the recovered material is also called the "grade" of that element. A positive voltage and a negative voltage are the same as a positive voltage and a negative voltage. Constant voltage is control in which a constant voltage is applied between the anode and cathode, and constant current is control in which constant current flows between the anode and cathode.

In the method for recovering platinum group metals according to the present invention (hereinafter referred to simply as the "recovery method according to the present invention"), a laminate in which a layer of platinum group metal is formed on a metal substrate is used as a recovery raw material. Here, as the metal base material, high-hardness metals (including alloys) containing Group 4 and Group 5 elements such as titanium (Ti), zirconium (Zr), and niobium (Nb) are used. As platinum group metals, metals such as platinum (Pt), gold (Au), iridium (Ir), and ruthenium (Ru) are used. Therefore, the laminate in the present invention is a laminate in which a platinum group metal is formed in layers or islands on these metal substrates.

Such a laminate is often used as an anode electrode for salt water electrolysis, production of electrolyzed water, chemical conversion treatment of aluminum and the like, and electrolytic plating of copper, nickel, zinc, chromium, gold and the like. Moreover, in the recovery method according to the present invention, not only the platinum group metal can be recovered from the metal substrate, but also the metal substrate can be recovered. That is, it can be said that the material constituting the laminate is separated and recovered.

Therefore, when the shape of the laminate remains in a new state, the shape of the metal substrate is left so that it can be recycled. If the new shape does not remain (if it has already been cut, pulverized, etc.), the base material and the material of the platinum group metal layer are recovered. That is, in the recovery method according to the present invention, no mechanical impact is applied to the laminate.

In the recovery method according to the present invention, the platinum group metal is recovered from the laminate in accordance with the policy described above. Therefore, the recovery method according to the present invention provides a chemical separation method and an electrolytic separation method.

<Chemical separation method>
It is known that the metal base material and the platinum group metal constituting the laminate have high hardness, are extremely chemically stable, and are not soluble in aqua regia, but are soluble in fluorine-containing aqueous solutions.

Further, when the metal base material (for example, titanium) of the laminate is exposed to an aqueous solution containing fluorine, it violently releases hydrogen to generate fluoride (fluorotitanate ion (H2TiF6)). This is a high-risk process when processing a large number of laminates. Therefore, in the recovery method according to the present invention, hydrogen peroxide is added to an aqueous solution containing fluorine. By adding hydrogen peroxide, the amount of generated hydrogen can be greatly suppressed, and great danger can be avoided even in mass processing of laminates.

<Electrolytic separation method>
The chemical separation method is an easy method because it is only necessary to immerse the laminate in a solution, but it also dissolves the metal base material (for example, titanium) and recovers the solid metal base material in its original shape. It is not possible. However, when a constant positive voltage is applied in an electrolytic solution, which is a fluorine-containing aqueous solution, hydrofluoric acid is generated near the surface of the laminate that serves as the anode, dissolving the metal substrate, and as a result, forming the platinum group metal layer. exfoliate. On the other hand, since an anodized film is formed on the surface of the metal substrate after dissolution, further dissolution can be suppressed as much as possible after the platinum group metal layer is peeled off. As a result, the peeling of the platinum group metal layer can be automatically stopped, and the metal substrate can be recovered without destroying its shape.

More specifically, for example, when the laminate is impregnated with a fluorine-containing solution having a pH of 4.5 or higher, it can be said that both the platinum group layer on the surface and the metal substrate exposed on the back and the like are chemically non-reactive. The reason for this is that the form of fluorine that can exist in this pH range is F- (fluoride ion), which is not an active species acting on metal etching.

On the other hand, when a positive voltage is applied to the laminate in this state, oxygen gas (O2) is added at the anode and protons (H+) are generated. As a result, only on the surface of the anode (surface of the platinum group catalyst layer), F- in the electrolytic solution and protons combine to generate hydrofluoric acid (HF). Since the laminate to be collected is in a deteriorated state (cracks on the surface of the platinum group catalyst layer), HF penetrates through the deteriorated portions (cracks) and dissolves the metal substrate (eg, titanium). As a result, the platinum group metal layer can be peeled off (lifted off) from the dissolution portion of the base material and recovered.

On the other hand, at the site where the platinum group metal layer is peeled off and the base material is exposed, the following two reactions occur simultaneously when a positive voltage is continuously applied to the layered body (object to be treated).
(1) Formation reaction of an oxide layer (oxide film) on the surface of a metal substrate (=depot) accompanying anodization
(2) Dissolution reaction (=etching) of the oxide film accompanying generation of HF
As described above, the reaction that protects the base material (called the "depot reaction") and the reaction that dissolves the base material (called the "etching reaction") occur simultaneously, so that the metal base material after stripping the platinum group is dissolved. can be suppressed as much as possible, and can be collected in a recoatable state.

The electrolytic solution that can be used here is a liquid containing fluorine. More specifically, an aqueous solution of a fluorine salt can be preferably used. For example, ammonium fluoride can be suitably used. Such fluorine-containing liquids can be used around pH 7 and are safer than chemical separation methods. A fluorine salt concentration of 1M to 4M can be suitably used.

In addition, since ammonium fluoride has a neutral pH, there is no reaction (the metal substrate does not dissolve) when the laminate is simply immersed, and the dissolution reaction of the metal substrate occurs only when electrolysis is applied. become (deposition reaction and etching reaction). When considering the recoating of a metal substrate, it is very important to control the dissolution reaction of the metal substrate, and NH4F is suitable in that the reaction can be controlled only by ON/OFF of electrolysis.

If a constant current source is used as the electrical energy applied here, an anodic oxide film is formed on the surface of the metal substrate, and the amount of dissolution from the surface of the metal substrate increases rapidly, causing the metal substrate to become thin. rice field.

This is because the etching reaction greatly exceeds the deposition reaction, and it is presumed that the same current density as before exposure was applied even after the platinum group layer was peeled off and the metal substrate was exposed. be. The deposition reaction is a reaction for forming an oxide film (insulator), and the liquid temperature rises greatly at a constant current density. Since the increase in liquid temperature greatly accelerates the other etching reaction, the etching reaction becomes dominant over the deposition reaction, resulting in thinning of the metal substrate. Therefore, it is preferable to have cooling means for making the liquid temperature of the electrolytic solution constant.

Furthermore, after the metal base material is exposed (after the platinum group metal layer is peeled off), it is preferable to perform an operation to reduce the current density and suppress the temperature rise of the solution. Under constant current conditions, the exposure of the metal substrate may be detected, and thereafter the current may be lowered, but most preferably, the stripping process is proceeded under constant "voltage" conditions.

Since the platinum group layer exists before the base material is exposed, current flows easily (resistance is low). On the other hand, after the base material is exposed, a deposition reaction occurs, resulting in a state in which it is difficult for current to flow (high resistance). When a series of these processes are performed under constant voltage conditions, the current density inevitably decreases as the resistance increases. can be greatly improved.

Moreover, in the recovery method according to the present invention, a pair of laminates are immersed in an electrolytic solution, and an alternating current is applied. In this way, the platinum group metal layers can be peeled off when they become anodes of each other, so that effective peeling and recovery is possible.

<Peeling device>
A peeling device for carrying out the recovery method according to the present invention will be described with reference to FIG. The stripping apparatus 1 comprises a stripping tank 10 , a negative electrode section 12 , a positive electrode section 14 and a power source 16 . A control device 18 that controls the power supply 16 may also be provided. Further, a thermometer 22 for measuring the temperature of the solvent in the stripping bath 10, a pH meter 24 for measuring the pH, and a constant temperature device 26 for keeping the temperature of the electrolytic solution constant may be arranged.

The power supply 16 is provided with a constant voltage source 16a, an ammeter 16e, and a voltmeter 16f. The voltmeter 16f measures the voltage between the negative electrode section 12 and the positive electrode section 14 here. The ammeter 16 e measures the current flowing between the negative electrode section 12 and the positive electrode section 14 .

Further, the power supply 16 may be provided with a constant current source 16b, an AC power source 16c, and a switch 16d for switching between the constant voltage source 16a, the constant current source 16b, and the AC power source 16c. Further, it is desirable that the polarities of the constant current source 16b and the constant voltage source 16a are interchangeable.

Controller 18 receives signals Sa and Sv from ammeter 16e and voltmeter 16f. The signal Sa is the value of the current flowing between the positive electrode portion 14 and the negative electrode portion 12 measured by the ammeter 16e, and the signal Sv is the voltage value between the positive electrode portion 14 and the negative electrode portion 12 measured by the voltmeter 16f. represent each.

Further, the control device 18 instructs the output voltage value to the constant voltage source 16a with the output instruction signal Ca. The output instruction signal Ca is the output polarity and output voltage value for the constant voltage source 16a, the output polarity and output current value for the constant current source 16b, and the output voltage value and output signal value for the AC power source 16c. The frequency can also be instructed. Note that the side on which the change-over switch 16d is provided is usually the negative electrode section 12 side. When there are a plurality of power supplies in this way, the control device 18 can send a switching signal Cs to the switching switch 16d. Any one of the constant voltage source 16a, the constant current source 16b, and the AC power source 16c can be selected and output by the switching signal Cs.

The controller 18 monitors the current value during constant voltage processing. Then, when the point where the current value decreases is detected, the output from the power source 16 is stopped. This means that the stripping process by constant voltage is stopped. The point of decrease is the point where the current flowing between the anode and the cathode clearly turns to decrease. As will be described later, it is relatively easy to determine the decreasing point by looking at the time change of the flowing current. However, it is not easy to determine the point of decrease while monitoring the current value that changes from moment to moment. If you wait too long for the current value to decrease, the amount of dissolution of the metal substrate will increase, resulting in unnecessary power consumption.

According to experiments by the inventors, the reduction point can be set when the current applied between the anode and the cathode becomes 70% to 90% of the current that was last applied. "Last applied voltage" refers to the voltage when the same voltage is applied from the beginning to the end of voltage application. Also, when a relatively high voltage is applied at the beginning of the voltage application, and then changed to a lower voltage, 70% to 90% of the current flowed by the last applied voltage means 70% to 90%.

The controller 18 can also receive signals St and Sph from a thermometer 22 and a pH meter 24 that measure the temperature of the electrolytic solution 10m in the stripping tank 10 . Furthermore, the control device 18 can send an instruction signal Ct to the constant temperature device 26 .

The constant temperature device 26 may be of any type as long as it can control the temperature of the electrolytic solution 10m constant. For example, the temperature of the electrolytic solution 10m may be controlled by passing a liquid or gas that can be heated and cooled to a constant temperature through a tube 26a arranged in the stripping bath 10. FIG.

When performing control to keep the temperature of the electrolytic solution 10m in the stripping tank 10 constant, the control device 18 monitors the temperature of the electrolytic solution 10m with a thermometer 22, and if the temperature exceeds the expected temperature, , an instruction signal Ct to lower the temperature is sent to the constant temperature device 26, and if it becomes lower, an instruction signal Ct to raise the temperature is sent.

Of course, the control device 18 is provided with an input section through which the user inputs instructions to the control device 18 itself, and a display section (not shown) capable of showing the current state of the peeling apparatus 1 as a whole. good.

When performing the electrolytic separation method with the stripping apparatus 1, first, the cathode plate 52 is attached to the cathode part 12 in a state in which conductivity is ensured. In addition, the laminate 50 in which the platinum group metal layer 50a is formed on the metal substrate 50b is attached to the positive electrode portion 14 in a state in which conductivity is ensured. Then, the control device 18 controls the constant voltage source 16a and the constant current source 16b to apply electric power between the electrodes to carry out the electrolytic separation method, thereby exfoliating the platinum group metal layer 50a on the laminate 50. FIG.

The recovery method according to the present invention will be described below with reference to Examples.

(Example 1)
10 g of ammonium fluoride (NH ₄ F) and 400 cc of pure water were placed in a beaker to dissolve the ammonium fluoride. This is a 0.675M ammonium fluoride aqueous solution. A mesh-shaped laminate used as an anode for electrolytic plating was used as a pair as a laminate as a recovery source. One laminate was used as an anode and the other laminate as a cathode, and a voltage of 10 V was applied between both laminates. At this time, a current of 1.75 A flowed.

FIG. 2 shows changes in the current value after the current started to flow. Although the current value was constant for about 20 minutes after the current started to flow, the current value gradually decreased thereafter. While the current value was constant, a black powder precipitated under the layered body which was used as the anode. After the current value started to decrease, an interference film was formed on the black laminate, and the laminate appeared rainbow-colored.

From this, it was considered that titanium oxide was formed on the surface of the anode after the current was applied, and the anode became non-conductive, resulting in a decrease in the current. That is, by monitoring the current value, it is possible to confirm the peeling of the platinum group metal layer. Moreover, even if the titanium substrate is left as it is, if an anodized film is formed on the surface of the titanium base material, current will automatically stop flowing, and the treatment can be automatically stopped.

Elemental analysis of the black powder that had precipitated under the anode revealed that the iridium content was 22% by mass. It turned out to be peeling off.

In addition, when the anode was replaced with a new laminate, the same experiment was performed, and the change in the current value of the second laminate was confirmed as in the first laminate (see FIG. 2). In addition, black powder was precipitated under the anode, and elemental analysis revealed that almost all the iridium was exfoliated as in the first sheet.

(Example 2)
The same sample and peeling device as in Example 1 were prepared, and the polarity of the power supply was switched between the anode and the cathode every 30 seconds. As in Example 12, the change in current value decreased after a certain period of time (see FIG. 2). A bubble generator was placed near the anode and cathode so that the ammonium fluoride aqueous solution near the electrodes was always fresh. When both electrodes were taken out after the current value became sufficiently low, peeling of the platinum group metal layer was confirmed on both electrodes.

(Example 3)
FIG. 3 shows the results of electrolytic separation of a laminate (sample) in which iridium oxide is formed on a titanium metal substrate with a constant current. 4M ammonium fluoride (NH ₄ F) was used as the electrolytic solution, and the current was kept constant at 5.34A for all samples. The laminate was connected to the positive electrode portion, the cathode plate connected to the negative electrode portion was made of Ti, and the distance between the electrodes was 5 mm.

Samples 1, 2 and 3 differ in size and iridium thickness. Referring to FIG. 3, the horizontal axis is the treatment time (min), and the vertical axis is the voltage (V) between the electrodes of the cathode and the anode. When the process was started, the voltage value seemed to be almost constant, but if you look closely, the voltage value fluctuates. During this time, iridium was peeled off from the anode sample.

When the iridium peels off from the surface of each sample, an oxide film is formed on the metal substrate and the electrical conductivity is lowered. Since the power supply is constant current, the voltage between the electrodes increased rapidly. The difference in voltage abrupt time is due to the difference in iridium thickness and shape in these samples. It seems that the processing should be stopped when the voltage value changes abruptly as shown in this graph.

FIG. 4 is a graph showing changes in processing for other samples. The cathode plate was replaced with stainless steel. Also, the applied current was set to 7A. The horizontal axis of each graph is the processing time (min). The vertical axis of FIG. 4(a) is the voltage applied between the electrodes, and the vertical axis of FIG. 4(b) is the measured amount (mg/L) of Ti, which is the metal base material. The vertical axis of FIG. 4(c) is the temperature (° C.) of the electrolytic solution.

As in the case of FIG. 3, when the iridium on the metal substrate was peeled off, the surface of the metal substrate was oxidized and the voltage between the electrodes increased. This point is indicated by point E in FIG. 4(a). The same time as the E point is also shown as the E point in FIGS. 4(b) and 4(c).

Here, referring to FIG. 4(b), dissolution of Ti from the surface of the metal substrate started 5 minutes after the start of the treatment (referred to as point B) before point E where the voltage abruptly increased. At this time, the increase in the amount of dissolution from point B to point E was proportional to time. This is called "time-proportional increase". It is represented by line L in FIG.

On the other hand, after the E point where the voltage changed abruptly, the metal substrate melted at a rate higher than the time proportional increase. In other words, after the voltage changed abruptly, the amount of dissolution of Ti increased abruptly (see the white arrow).

Since iridium continues to peel off during this time, it is not easy to determine when to stop the treatment in order to remove as much iridium (platinum group light metal) as possible while leaving as much of the metal substrate as possible. In particular, when the voltage rises rapidly, a large amount of the metal base material melts with a slight time delay, resulting in thinning and hindering reuse of the metal base material.

3 and 4A differ in how the voltage rises, and the manner in which the voltage rises differs depending on the current density applied to the laminate and the shape of the laminate. In FIG. 4(a), the voltage seems to rise relatively slowly, but if the voltage rises in the way shown in FIG. The metal substrate is in a state of melting at the above rate. Therefore, it becomes more difficult to determine when to stop processing.

FIG. 5 shows a sample having the same configuration as that of FIG. 4, and having the same size, thickness of iridium, and the like. The electrolytic solution is 4M ammonium fluoride and the cathode is stainless steel. The distance between the electrodes is 5 mm, which is the same as in FIG. A constant voltage of 6.15 V was applied between the electrodes here.

In each diagram of FIG. 5, the horizontal axis represents the processing time (min). The vertical axis of FIG. 5(a) is current (A), the vertical axis of FIG. 7(b) is the amount of dissolved Ti (mg/L), and the vertical axis of FIG. 5(c) is the amount of electrolytic solution. temperature (°C). Referring to FIG. 5(a), after the region where the current is constant, there is a point where the current drops sharply. Let this be point A. The same time is also shown as point A in FIGS. 5(b) and 5(c).

Referring to FIG. 5(a) again, the current continued to drop after point A. On the other hand, referring to FIG. 5(b), the dissolution of Ti in the metal substrate begins before point A (about 5 minutes) where the current drops sharply. Point B is the same as in FIG.

However, unlike the case of constant current in FIG. 4, the amount of dissolution of Ti did not suddenly increase, but increased at a time-proportional rate of increase, and then became lower than the time-proportional rate of increase. (Point D in FIG. 5(b)). The rate of the temperature of the electrolytic solution in FIG. 5(c) also increased proportionally with time, and the rate clearly decreased, suggesting that the flowing current itself suddenly decreased and that Joule heat was no longer generated.

This is because the constant voltage treatment does not cause a sudden change in the dissolving power of Ti, and even if the power supply is turned off while monitoring the current change, the metal base can be prevented from becoming thinner than expected. means that you can In other words, it is possible to easily determine whether to stop.

For example, in the case of FIG. 4A, the "current that flows due to the last applied voltage" is 7A, so the current value is set between 6.3A (90%) and 4.9A (70%). The power supply may be stopped when the current value becomes smaller than the value. In FIG. 4A, the time when the current value reaches 6.3 A (90%) is indicated as "t90", and the time when the current value reaches 4.9 A ( ₇₀ %) is indicated as " _t70 ". rice field. The power can be turned off at any time during this period. By doing so, it is possible to separate and recover a sufficient amount of the platinum group metal and the metal base material which does not have an unnecessary dissolution amount of Ti and is less damaged.

(Example 4)
FIG. 6 shows the results of the combination of the constant current process and the constant voltage process. Referring to FIG. 6, the horizontal axis is time (min) and the vertical axis is current value (A). In method 1 (indicated as “Met1”), a constant current (9 A) was applied for a certain period from the start of treatment (indicated as “Met1 (CA)” in FIG. 6), and after 10 minutes had passed, the constant current was kept constant. Treatment was performed with a voltage (9.7 V) (shown as “Met1 (CV) in FIG. 6). On the other hand, as a comparative example, method 2 (shown as “Met2”) was applied at a constant voltage (9.7 V) from the start of the treatment. .7V) (shown as "Met2 (CV)" in FIG. 6) is shown in FIG.

It should be noted that the section in which the constant current is applied in method 1 may be regarded as processing with a constant voltage. This is because, in the electrolytic separation method, the resistance between the anode and the cathode is constant while the platinum group metal is attached. Also, as shown in FIG. 6, when the applied voltage is applied in two stages, the "current that flows due to the last applied voltage" is about 7 A that flows at 9.7V.

If the reduction point is 6 A, which is about 86% of the current (about 7 A) due to 9.7 V, in method 1, the treatment can be stopped in about 15 minutes, but in method 2, about 21 minutes must pass. processing cannot be stopped.

That is, by applying a relatively large current at the beginning of the treatment, most of the platinum group metal is exfoliated, and then by applying a constant voltage treatment, rapid dissolution of the metal substrate can be prevented, and platinum can be removed in a short time. stripping of group metals becomes possible.

(Example 5)
It can also be said that the results of FIG. 6 were obtained by changing the voltage applied to the laminate, which is the object to be processed. In other words, the power source is operated so that a high voltage is applied at the beginning of the processing and a low voltage is applied during the latter half of the processing.

Conversely, FIG. 7 shows the results when the voltage is low at the beginning of the treatment and the applied voltage is gradually increased. FIG. 7(a) shows the relationship between the treatment time and the current value between the electrodes (A), and FIG. 7(b) shows the relationship between the voltage (V) applied between the electrodes and the treatment time. .

In FIG. 7A, the horizontal axis is (processing) time (min) and the vertical axis is current (A). In FIG. 7B, the horizontal axis is the (processing) time (min), and the vertical axis is the set voltage (V) applied between the electrodes. Referring to FIG. 7B, here, the applied voltage value was increased to 6V, 7.3V, 9V, 9.7V and 11V every 5 minutes. 11 V was applied, and the current value began to decrease after 3 minutes had passed.

When the film thickness of the platinum group metal of the object to be treated is unknown, if a high voltage is applied at once, the metal substrate may begin to dissolve earlier than the platinum group metal on the surface of the laminate (object to be treated) is peeled off. It becomes difficult to recover a metal base material with little damage.

When the film thickness of the platinum group metal on the object to be processed is unknown, the treatment is started with a low applied voltage and the applied voltage is increased over time, as in this example, so that the platinum group metal is gradually peeled off. , it is possible to reliably recover platinum group metals and to recover metal substrates without thinning.

Also in this case, the last applied voltage was 11 V, and the current that flowed at this time was 8.2 A. Therefore, the decreasing point to terminate the process is the time between 7.4A and 5.7A.
(Example 6)
As the recovered raw material (laminate), a mesh-like material used as an anode for electrolytic plating was used. (It is the same as that used in Example 1.) Hydrogen fluoride of 2% by mass, 10% by mass, and 20% by mass was added to the recovered raw material, and left for 30 minutes. In both cases hydrogen was liberated violently. After 30 minutes had elapsed, the recovered raw material was pulled up, and the material, which had been black at first, exhibited a silver luster. In addition, black powder was precipitated. From this, it was found that the platinum group metal formed on the titanium base material can be entirely removed with hydrogen fluoride of 2% by mass or more.

FIG. 8 shows the result of elemental analysis of the black powdery precipitate. Referring to FIG. 8, the horizontal axis indicates the concentration of hydrofluoric acid (% by mass), and the vertical axis indicates the content (% by mass). According to FIG. 1, aluminum, titanium, iridium, platinum, and others were detected. From this, it was found that the purity of iridium was about 25% by mass.

(Example 7)
A test tube having a volume of 50 cm ³ was charged with 20 cm ³ of 2.6% by mass hydrofluoric acid and a predetermined concentration of hydrogen peroxide solution, and a titanium plate weighing 0.25 g was immersed therein. A stirrer was also placed in the test tube, and the inside of the test tube was stirred at a rotation speed of 500 rpm. The hydrogen concentration was measured at the position of the scale of 50 cm ³ inside the test tube. The results are shown in FIG.

Referring to FIG. 9, the horizontal axis is the reaction time (min) and the vertical axis is the detected hydrogen concentration (ppm). Note that the vertical axis is logarithmic. The concentration of the hydrogen peroxide solution was changed to 0% by mass, 1.6% by mass, 3.2% by mass, and 6.3% by mass. As a result, the hydrogen concentration, which was 10,000 ppm when no hydrogen peroxide solution was contained, decreased to 100 ppm (1/100) by adding 6.3% by mass of hydrogen peroxide solution.

The recovery method according to the present invention can be suitably used as a method for recovering a platinum group metal and a metal substrate from a laminate in which a layer of a platinum group metal is formed on a metal substrate.

Reference Signs List 1 stripping device 10 stripping tank 10m electrolytic solution 12 negative electrode section 14 positive electrode section 16 power source 16a constant voltage source 16b constant current source 16c alternating current power source 16d switch 16e ammeter 16f voltmeter 18 control device 22 thermometer 24 pH meter 26 Constant temperature device 26a Tube 52 Cathode plate 50 Laminate 50a Platinum group metal layer 50b Metal substrate

Claims (5)

A method for recovering a platinum group metal from a laminate in which a layer of a platinum group metal is formed on a metal substrate,
a step of immersing the laminate and the conductor in an aqueous solution containing fluorine;
A first energizing step of applying a constant voltage such that the current flowing between the anode and the cathode is constant, with the laminate being the anode and the conductor being the cathode;
A method for recovering a platinum group metal comprising a second energizing step of applying a constant voltage at which a current smaller than the current applied in the first energizing step flows before the current applied in the first energizing step does not change. . 2. The method according to claim 1, wherein the application of the constant voltage is stopped after it is detected that the voltage applied in the second energizing step becomes 70% to 90% of the current initially flowing between the anode and the cathode. A method for recovering the platinum group metals described. 3. The method for recovering platinum group metals according to claim 2, wherein the fluorine source of said aqueous solution containing fluorine is a fluoride salt. 3. The method for recovering platinum group metals according to claim 2, wherein said metal substrate is titanium. 3. The method for recovering platinum group metals according to claim 2, further comprising the step of maintaining said fluorine-containing aqueous solution at a constant temperature.

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