JPH09165698A - Method for electrolytic reduction of ferric ion in iron-based plating solution - Google Patents

Abstract

(57) Abstract: To suppress generation of hydrogen and further improve the reduction efficiency of ferric ion. SOLUTION: Since a proportional relationship is established between the ferric iron ion concentration and the limiting current density, the limiting current value can be obtained by measuring the ferric ion concentration in the plating solution, and the limiting current density can be obtained. By controlling the energization so that the current value is obtained, it is possible to efficiently suppress the generation of hydrogen and efficiently perform electrolytic reduction at a high ferric ion reduction rate.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an electrolytic reduction method for reducing ferric iron ions produced in an iron-based plating solution to ferrous iron ions by air oxidation or the like, and more specifically, a soluble electrode is used at least as an anode. And a method for electrolytically reducing ferric ion.

[0002]

2. Description of the Related Art When electroplating is performed using an iron-based plating bath containing ferrous iron, ferrous iron (Fe ²⁺ ) in the plating bath is expressed by the following equations (1) and (2) ), It is easily oxidized by anodic oxidation reaction or oxygen in the air or oxygen generated at the anode to generate ferric ion (Fe
³⁺ ). Fe ²⁺ + 1 / 4O ₂ + 1 / 2H ₂ O → Fe ³⁺ + OH- (1) Fe ²⁺ → Fe ³⁺ + e (2) That is, with the passage of time, ferrous iron in the plating bath The concentration decreases and the ferric ion concentration increases. However, some of the ferric ions are reduced to ferrous ions or iron at the cathode, which requires extra electric power, resulting in a problem that the cathode current efficiency decreases. Further, when the ferric ion concentration increases, the gloss of the plating decreases and the plating quality deteriorates. There is also a problem that some ferric iron ions are deposited as iron hydroxide or hydrous iron oxide, which is taken into the plating film and deteriorates the quality of the plating film, although it depends on the pH of the plating bath. . Therefore, it is necessary to prevent an increase in ferric ion concentration in the plating bath.

Therefore, there is known a method of utilizing the electrolytic reduction method to reduce ferric ions in the plating solution to ferrous ions. As a typical electrolytic reduction method, a method using an ion exchange membrane is known, as described in, for example, Japanese Patent Application Laid-Open No. 1-263291. This method
For example, the cathode chamber and the anode chamber which is filled with a conductive solution such as sulfuric acid aqueous solution and has an insoluble anode such as platinum are separated by an anion exchange membrane, and the plating solution to be reduced is supplied to the cathode chamber. The anion exchange membrane prevents oxidation of ferrous ions in the anode chamber and reduces ferric ions in the cathode chamber.

However, in the electrolytic reduction method using the above-mentioned ion exchange membrane, since the ion exchange membrane is a resistor, a high voltage is required and power consumption becomes large. Further, when the ferric ion in the plating solution is reduced during the treatment, iron is electrodeposited to reduce the reduction efficiency, making it difficult to carry out electrolytic reduction in the low concentration region of ferric ion, and efficiently reducing ferric ion. It is difficult to carry out electrolytic reduction.

Therefore, Japanese Patent Laid-Open No. 63-259089 discloses an electrolytic reduction method which solves the above problems by using a soluble anode without using an ion exchange membrane. According to this electrolytic reduction method, the production of ferrous ions can be preferentially caused by using iron as the soluble anode. Therefore, even if the cathode chamber and the anode chamber are not separated, oxidation of ferrous iron in the anode is prevented, and an expensive insoluble electrode such as platinum is not required.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In the case of electrolytic reduction by the method disclosed in Japanese Patent No. 259089, even if the current density is increased to increase the reduction efficiency, the reduction rate of ferric ion does not increase so much, but on the other hand,
It was found that the rate of hydrogen generation increased. Generation of hydrogen raises the pH of the plating solution and eventually deteriorates the quality of the plating film, which is not preferable. However, if the current density is lowered in order to suppress the generation of hydrogen, the reduction rate of ferric ions is reduced and the reduction efficiency is reduced.

Therefore, it is difficult to increase the current density above the limiting current density by this electrolytic reduction method,
There was a limit to the reduction efficiency. The present invention has been made in view of such circumstances, and an object thereof is to suppress the generation of hydrogen and further improve the reduction efficiency of ferric ion.

[0008]

Means for Solving the Problems A first method for solving the above problems is described below.
The feature of the method for electrolytic reduction of ferric iron ions in the iron-based plating solution of the invention is that, when electrolytically reducing ferric iron ions in the iron-based plating solution, the actual ferric iron ion concentration in the iron-based plating solution is To determine the limiting current density at the actual ferric ion concentration from the previously determined ferric ion concentration-limiting current density relationship, and control the actual current density to be the limiting current density. It is in.

A second aspect of the present invention is that the soluble electrode is used as at least the anode in the first invention, and the area of the soluble electrode is controlled so that the anode current density becomes a predetermined value. Further, the feature of the invention according to claim 3 is that in the first invention, a soluble electrode is used as at least the anode, and the flow rate of the iron-based plating solution is controlled so that the amounts of ferrous ions and ferric ions are constant. To do so.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION There is a relationship shown in FIG. 11 between the cathode current density and the ferric ion reduction rate and the hydrogen generation rate. That is, when the cathode current density exceeds a specific value, the hydrogen generation rate rapidly increases, which is not preferable. Further, when the specific current density is exceeded, the reduction rate of ferric ion hardly increases even if the current density is increased. Therefore, it is most desirable to carry out electrolytic reduction at this specific current density, and this specific current density is tentatively called the limiting current density. The present inventors, as a result of diligent research on the relationship between the composition in the iron-based plating solution at the time of electrolytic reduction and the above-mentioned limiting current density,
The present invention has been completed by discovering that the proportional relationship shown in FIG. 3 is established between the ferric ion concentration and the limiting current density. That is, the proportional relationship between the ferric ion concentration and the limiting current density is obtained in advance, and the target limiting current value is calculated from the proportional relationship by measuring the actual ferric ion concentration in the plating solution. Then, by controlling the energization so that the actual current density becomes the limiting current value, the electrolytic reduction can be efficiently performed at a high ferric ion reduction rate while always suppressing the generation of hydrogen.

In the case of the first aspect of the invention, both an insoluble anode and a soluble anode can be used as the anode. Examples of the insoluble anode include platinum and platinum-iridium alloys, and the soluble anode is iron. , Iron alloys, etc. are exemplified. In particular, if a soluble anode is used, the ferrous ion can be preferentially produced by the reaction of the following formula (3) at the anode rather than the oxidation of the ferrous ion by the reaction of the formula (2).

Fe → Fe ²⁺ + 2e (3) By the way, when a soluble anode is used, there is a problem due to the generation of ferrous ions by the formula (3). The amount of ferrous iron ions in the plating solution is preferably in a state in which the amount of ferrous ions produced by the electrolytic reduction reaction and the amount of ferrous ion consumed by the plating reaction are in equilibrium. When the reaction occurs, the concentration of ferrous ions in the plating solution becomes unstable, and there is a problem in that the plating quality varies and deteriorates.

However, according to the first aspect of the present invention, the reduction current value necessary for the reduction is necessary and sufficiently controlled,
The reaction of the formula (3) at the anode can be suppressed to the minimum. On the other hand, it became clear that there is a relationship as shown in FIG. 6 between the anode current density and the amount of dissolved anode per unit area of the soluble anode. That is, above a certain anode current density, the amount of dissolved anode per unit area of the anode is almost proportional to the anode current density due to the reaction of equation (3) due to electrolysis, but below that, In addition to the reaction, a reaction with the hydrogen ions in the plating solution represented by the following formula (4) occurs, and the amount of anodic dissolution becomes almost constant regardless of the anode current density.

Fe + 2H ⁺ → Fe ²⁺ + H ₂ (4) Therefore, when the reduction current value is controlled by the ferric ion concentration according to the above invention, the ferric ion concentration in the plating solution becomes low. When the reduction current value, that is, the anode current density, becomes smaller, the amount of anodic dissolution becomes higher than the amount of electrolysis of the soluble anode due to the reaction of the formula (4), and the ferrous ion concentration becomes high. According to the first invention, there is a problem that the effect of suppressing the reaction of the formula (3) to the minimum disappears.

Therefore, in the invention of claim 2 which solves such a problem, in the invention of claim 1, a soluble anode is used as at least the anode, and the anode area can be varied. Then, the minimum anode current density, which is the lowest anode current density at which the reaction of the formula (4) does not occur, is obtained in advance, and when electrolytic reduction is performed according to the reduction current value determined based on the ferric ion concentration, The area of the soluble anode is controlled so that the anode current density is equal to or higher than this minimum anode current density.

Therefore, according to the second aspect of the present invention, when electrolytic reduction proceeds and the concentration of ferric ion in the plating solution decreases, and the reduction current value, that is, the anode current density decreases, the area of the soluble anode increases and the anode current increases. The density is controlled to be higher than the minimum anode current density. As a result, the reaction of formula (4) does not occur, and the ferrous ion concentration can be reduced.

The area of the anode may be varied by varying the area of the anode immersed in the plating solution. For example, an anode plate is arranged so as to be movable up and down, and a portion immersed in the plating solution and a portion not immersed in the plating solution. This can be done by changing the area ratio of Alternatively, the anode area may be varied by forming an anode structure in which a plurality of thin plates are laminated and shifting each thin plate in the plating solution.

The reduction rate of ferric ion in the plating solution is
As shown in FIG. 8, it is proportional to the reduction current value and also proportional to the diffusion rate of ferric ions on the cathode surface. Further, the diffusion rate of ferric iron ions on the cathode surface increases as the liquid flow velocity in the electrolytic reduction apparatus increases. That is, the higher the liquid flow rate, the higher the reduction rate of ferric ion. On the other hand, the dissolution rate of the soluble anode, that is, the production rate of ferrous ions due to anodic dissolution is almost proportional to the reduction current value as shown in FIG. 9, but is constant regardless of the liquid flow rate in the electrolytic reduction apparatus. It became clear that there is.

The invention according to claim 3 based on this finding is effective when the plating solution is circulated between the electrolytic reduction apparatus and the plating apparatus, and electrolytic reduction and plating are carried out simultaneously. That is, the production rate of ferric ions due to the plating reaction is obtained in advance, and the liquid flow rate in the electrolytic reduction apparatus is controlled so that the reduction rate of ferric ions is approximately equal to that. Thereby, the production rate of ferric ion and the reduction rate are balanced, and the ferric ion concentration can be made constant, so that the quality of plating is stabilized.

Further, since the amount of ferrous ions produced by the dissolution of the soluble anode represented by the equation (3) is constant regardless of the liquid flow rate, the amount of ferrous ion produced is controlled even if the liquid flow rate is controlled. Can be constant. Therefore, the ferrous ion concentration and the ferric ion concentration can be made constant, and the plating quality becomes more stable. In addition to controlling the liquid flow rate, the consumption rate of ferrous ions due to the plating reaction is obtained in advance,
If the reduction current value is determined so that the amount of dissolved anode is equal to the consumption amount of ferrous ions, the ferrous ion concentration and ferric ion concentration can be made more stable and constant.

When the consumption rate of ferrous ions and the production rate of ferric ions vary due to the plating reaction,
It is also possible to measure the rate of change in the plating solution for each, and thereby continuously control the reduction current value and the solution flow rate. By doing so, the composition of the plating solution can be maintained more stable and constant.

[0022]

The present invention will be described below in detail with reference to examples. (Embodiment 1) FIG. 1 shows an electrolytic reduction apparatus used in the electrolytic reduction method of this embodiment embodying the invention of claim 1. This electrolytic reduction apparatus comprises an electrolytic reduction tank 1 having an anode 10 and a cathode 11, a plating solution storage tank 2, a rectifier 3, an analyzer 4, and a controller 5.

The electrolytic reduction tank 1 is connected to the plating solution storage tank 2 via a pump 20 and a communication pipe 21, and the plating solution containing ferric ions is pumped to the electrolytic reduction tank 1 and the plating solution storage tank 21 by the communication pipe 21. It is configured to be able to circulate 2. The anode 10 is a soluble anode made of an iron-based alloy, the cathode 11 is an insoluble cathode made of an alloy, and the plating solution 6 in the electrolytic reduction tank 1 faces each other.
Is soaked in.

The anode 10 and the cathode 11 are connected to the rectifier 3 by a cable 12, and the rectifier 3 is constructed so that a direct current can be passed through the anode 10 and the cathode 11. Analyzer 4
Collects the plating solution 6 in the electrolytic reduction tank 1 via the sampling tube 40, analyzes the ferric ion concentration in the plating solution 6 and converts it into an electric signal, and controls the electric signal via the cable 41. It is configured to input to the device 5.

The control device 5 calculates the limiting current density according to the electric signal input from the analyzer 4, and outputs the control signal to the rectifier 3 via the cable 50. Hereinafter, the processing contents of the control device 5 will be described with reference to the flowchart shown in FIG. When a signal based on the actual ferric ion concentration is input from the analyzer 4 in step 100, the table showing the relationship between the ferric ion concentration and the limiting current density created in advance is referred to in step 101. As shown in FIG. 3, since there is a proportional relationship between the ferric ion concentration and the limiting current density, in step 102, the limiting current density corresponding to the actual ferric ion concentration is calculated and determined from the table. . Then, a signal is output to the rectifier 3 so that the limiting current density determined in step 103 is reached, and the rectifier 3
Is the anode 10 and the cathode 11 so that the limiting current density is reached.
Turn on electricity.

Then, from step 100 to step 103
By repeatedly performing the above process, the plating solution 6 in the electrolytic reduction tank 1 is energized so that the limiting current density according to the actual ferric ion concentration is reached, and electrolytic reduction is performed. As a result, electrolytic reduction can be efficiently performed at the maximum reduction rate, hydrogen generation can be suppressed to suppress pH to the minimum, and deterioration of plating quality can be prevented.

(Embodiment 2) FIG. 4 shows an electrolytic reduction apparatus used in the electrolytic reduction method of this embodiment embodying the invention of claim 2. This electrolytic reduction device has the same configuration as that of the first embodiment except that the anode 10 is connected to the lifting device 13. The elevating device 13 includes a stepping motor 13a having a pinion fixed to a rotating shaft and a rack 13b meshing with the pinion, and the rack 13b is configured to elevate and lower by the rotation of the stepping motor 13a. Then, the anode 10 is fixed to the rack 13b, and the anode 10 moves up and down as the rack 13b moves up and down.
The immersion depth in the plating solution 6 inside is variable.

The processing contents of the control device 5 of the present embodiment will be described below with reference to the flow chart shown in FIG. When a signal based on the actual ferric ion concentration is input from the analyzer 4 in step 200, a table showing the relationship between the ferric ion concentration and the limiting current density created in advance in step 201 is referred to. As shown in FIG. 3, since there is a proportional relationship between the ferric ion concentration and the limiting current density, the limiting current density corresponding to the actual ferric ion concentration is calculated and determined from the table in step 202. .

There is a relationship shown in FIG. 6 between the anodic current density and the amount of anodic dissolution per unit area. When the anodic current density is lower than the minimum anodic current density, the reaction of the formula (4) occurs, so that the anodic dissolution amount. Is almost constant. Therefore, the anode current density needs to be higher than the minimum anode current density. Therefore, the control device 5 stores the value of the minimum anode current density measured in advance in this electrolytic reduction device and the amount of anode dissolution per unit area at that time, and in step 203, the limiting current density determined in step 202 is stored. The required anode area is calculated from the value of.

Then, in step 204, a signal is output to the rectifier 3 so as to reach the determined limiting current density, and the rectifier 3 energizes the anode 10 and the cathode 11 so as to attain the limiting current density. Further, in step 205, the elevating device 13 is controlled so as to have the determined anode area, and the stepping motor 13a elevates and lowers the anode 10 so that the immersion area of the anode 10 in the plating solution 6 becomes the anode area.
That is, when the electrolytic reduction proceeds and the anode current density decreases, the area of the soluble anode 10 is enlarged so that the anode current density becomes equal to or higher than the minimum anode current density.

Then, from step 200 to step 205
By repeatedly performing the above process, the plating solution 6 in the reduction electrolysis tank 1 is energized so that the limiting current density according to the actual ferric ion concentration is reached, and reduction electrolysis is performed. As a result, reduction electrolysis can be efficiently performed at the maximum reduction rate, hydrogen generation can be suppressed and pH can be suppressed to a minimum, and deterioration of plating quality can be prevented.

Also, when electrolytic reduction progresses and the ferric ion concentration in the plating solution becomes low, and the limiting current density determined becomes low, the anode area is increased and the anode current density becomes the minimum anode current density. Since the current density is equal to or higher than the above, the reaction of formula (4) does not occur and the ferrous iron ion concentration can be reduced, and the variation or deterioration of the plating quality due to the instability of the ferrous iron ion concentration is prevented. It

(Embodiment 3) FIG. 7 shows an electrolytic reduction apparatus used in the electrolytic reduction method of this embodiment embodying the invention of claim 3. In this electrolytic reduction apparatus, the electrolytic reduction tank 1 is provided with a stirrer 7 controlled by the controller 4 to stir the plating solution 6, and the analyzer 4 is provided with a sampling pipe 41 in the plating solution storage tank 2. The configuration is the same as that of the example 1 except that the plating solution 6 is collected.

The stirring device 7 comprises a motor 70 and a stirring blade 71 fixed to the rotating shaft of the motor 70.
Controls the rotation speed of the stirring blade 71 by controlling the rotation speed of the motor 70 via the cable 72.
The flow rate of the plating solution 6 inside is controllable. The Maya analyzer 4 is configured to be able to analyze the ferrous ion concentration and ferric ion concentration in the plating solution 6 in the plating solution storage tank 2 and their changing rates.

As shown in FIG. 8, the reduction rate of ferric iron ions is proportional to the reduction current value and also proportional to the diffusion rate of ferric iron ions on the cathode surface. Further, the diffusion rate of ferric iron ions on the cathode surface increases as the liquid flow velocity in the electrolytic reduction apparatus increases. On the other hand, the dissolution rate of the soluble anode, that is, the production rate of ferrous ions by anodic dissolution is almost proportional to the reduction current value as shown in FIG. 9, but does not depend on the diffusion rate, that is, the liquid flow rate in the electrolytic reduction apparatus. .

The control unit 5 includes a relationship between the reduction current value and the anodic dissolution rate measured in advance in the electrolytic reduction apparatus having this structure, a relationship between the liquid flow rate and the reduction current value due to stirring by the stirrer 7, and the reduction current. The relationship between the value and the reduction rate is entered. Then, the control device 5 performs the electrolysis necessary for keeping the ferrous iron concentration and the ferric ion concentration constant based on the changing rates of the ferrous ion concentration and the ferric ion concentration input from the analyzer 4. The anodic dissolution rate and reduction rate in the reduction tank 1 are calculated, and further, the reduction current value and stirring conditions necessary to obtain them are calculated.

Then, the rectifier 3 is controlled based on the obtained reduction current value, and the stirring device 7 is controlled based on the obtained stirring condition. Therefore, in the present embodiment, in addition to the control of the first embodiment, the production rate of ferric iron due to the plating reaction in the plating solution storage tank 2 is obtained in advance, and the control device 5 determines that the reduction rate of ferric ion is The stirring device 7 is further controlled so that the liquid flow rates are substantially equal to each other. Thereby, the ferrous ion concentration and the ferric ion concentration can be made constant.

In the present embodiment, the liquid flow velocity is controlled by controlling the stirring device 7, but the control device 5 controls the pump 2
The flow rate of the plating solution 6 in the electrolytic reduction tank 1 may be controlled by controlling the output of 0. Also, as shown in FIG.
The flow velocity can be controlled by rotating the cathode 11. In FIG. 10, the inlet 14 and the outlet 15 of the plating solution 6
In the cylindrical reduction electrolysis cell 1 having the above, the perforated disk-shaped anode 10 is provided so as to project radially from the inner peripheral wall surface. Further, a rotating shaft 17 rotatably driven by a motor 16 is rotatably arranged in the center, and a disk-shaped cathode 11 is separated from the anode 10 on the rotating shaft 17 and is opposed to the anode 10 in parallel with each other. It is provided in. The rotation of the motor 16 is controlled by the control device 5, and the cathode 11
Is driven to rotate together with the rotating shaft 17 by the rotation of the motor 16.

Since the reduction rate depends on the diffusion control of ferric ions in the plating solution near the cathode, the flow rate of the solution near the cathode greatly affects the reduction rate. Therefore, it is the most preferable method to rotate the cathode 11 as in the above-mentioned embodiment.

[0040]

That is, according to the invention of claim 1,
Since the maximum reduction rate can be obtained, ferric ion can be efficiently reduced, and hydrogen generation can be suppressed and the increase in pH can be suppressed to the minimum. Further, according to the invention of claim 2, in addition to the effect of the invention of claim 1, the reaction of the formula (4) can be prevented when the reduction current value becomes low, and the ferrous ion Since the increase can be prevented, it is possible to prevent the concentration of ferrous iron in the plating solution from becoming unstable and the variation or deterioration of the plating quality to occur.

Further, according to the invention of claim 3, in addition to the effect of the invention of claim 1 or 2,
It is possible to further stabilize the ferrous iron ion concentration in the plating solution, and thus to prevent variations and deterioration of plating quality.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an electrolytic reduction apparatus used in an example of the present invention.

FIG. 2 is a flowchart showing the control contents of the control device in the embodiment of the present invention.

FIG. 3 is a graph showing the relationship between ferric ion concentration and limiting current density.

FIG. 4 is a block diagram showing a configuration of an electrolytic reduction apparatus used in a second embodiment of the present invention.

FIG. 5 is a flowchart showing the control contents of the control device in the second embodiment of the present invention.

FIG. 6 is a graph showing the relationship between the anode current density and the amount of dissolved anode.

FIG. 7 is a block diagram showing the configuration of an electrolytic reduction apparatus used in a third embodiment of the present invention.

FIG. 8 is a graph showing the relationship between the diffusion rate of ferric ions to the cathode and the reduction rate.

FIG. 9 is a graph showing the relationship between the diffusion rate of ferric ions into the cathode and the anodic dissolution rate.

FIG. 10 is a schematic cross-sectional view showing another aspect of the electrolytic reduction apparatus used in the third embodiment of the present invention.

FIG. 11 is a graph showing the relationship between the cathode current density and the ferric ion reduction rate and hydrogen generation rate.

[Explanation of symbols]

1: Electrolytic reduction tank 2: Plating solution storage tank 3: Rectifier 4: Analyzer 5: Control device 6: Plating solution 10: Anode 11: Cathode 13: Lifting device

Claims (3)

[Claims] 1. When the ferric iron ion in the iron-based plating solution is electrolytically reduced, the actual ferric iron ion concentration in the iron-based plating solution is measured, and the ferric iron ion concentration-limit determined in advance A ferric ion in an iron-based plating solution, characterized in that the limiting current density at the actual ferric ion concentration is determined from the relationship of the current density, and the actual current density is controlled to be the limiting current density. Electrolytic reduction method. 2. The ferric iron in the iron-based plating solution according to claim 1, wherein a soluble electrode is used as at least the anode, and the area of the soluble electrode is controlled so that the anode current density has a predetermined value. Method of electrolytic reduction of ions. 3. The method according to claim 1, wherein a soluble electrode is used at least as an anode and the flow rate of the iron-based plating solution is controlled to make the amounts of ferrous and ferric ions constant. 2. The method for electrolytic reduction of ferric ion in an iron-based plating solution according to 2.