JPH1046399A - Method for controlling concentration of insoluble-anode electrogalvanizing solution - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain the Zn concn. and H2 SO4 concn. (or pH) in the range suitable to plating with high precision even when the plating soln. is leaked to the outside of a plating equipment. SOLUTION: A plating soln. is circulated between the main body 10 of an electrogalvanizing equipment using an insoluble anode and a device 12 for bringing an acidic plating soln. into contact with metallic Zn to dissolve the Zn, hence Zn ion is replenished to the main body 10, and the concn. of the plating soln. is controlled. In this case, the plating soln. is circulated by the dissolving device 12 so that the Zn concn. and H2 SO4 concn. (or pH) are maintained in a specified range based on both concns. measured by a soln. analyzer 14 with respect to the plating soln. in the main body 10, the metallic Zn contained in a dissolving tank 16 is dissolved to replenish Zn ion to the main body 10, and the deficiency of sulfuric acid is supplied to the dissolving tank 16 by opening a feed valve 18.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling the concentration of an insoluble anode zinc electroplating solution, and more particularly to a method for controlling the concentration of a zinc ion in a plating solution for a sulfuric acid acid zinc electroplating using an insoluble anode, using zinc as a source of zinc ions. The present invention relates to a method for controlling the concentration of an insoluble anodic zinc electroplating solution, which is preferably applied when controlling both the concentration and the sulfuric acid concentration (or pH) within a predetermined range.

[0002]

2. Description of the Related Art Electroplating is performed using an insoluble anode such as platinum while continuously transporting a strip-shaped steel sheet. Among such electroplating, in the case of alloy plating, the concentration accuracy of the plating solution is strictly required in controlling the concentration of the plating solution. Many methods have been reported for predicting the drag-out rate, which is the rate at which components are reduced by taking out a plating solution, and controlling the drag-out rate as strictly as possible. However, regarding the concentration of a single plating solution used for plating one kind of metal such as pure zinc electroplating, since the required accuracy is relatively low as compared with alloy plating, the following control method is known. I have.

For example, JP-A-51-97543 and JP-B-63-50439 disclose zinc (Z) using an insoluble anode.
As a method for controlling the concentration of single electroplating such as n) or nickel (Ni), by detecting only the pH value and dissolving the metal agent based on the pH value, the metal concentration and p
A method for controlling H is disclosed.

That is, in this case, theoretically, the metal Ni is added according to the detected pH value by the balance between the electrodeposition reaction of the following formulas (A) and (B) and the dissolution reaction of the formula (C). By dissolving, both Ni concentration and pH can be controlled.

[0005] (electrodeposition ^{reaction) Ni 2+ + 2e - → Ni} ↓ ... (A) H2 O + SO4 2- → H2 SO4 + 1 / 2O2 ↑ + 2e - ... (B) ( dissolution reaction) Ni + H2 SO4 → NiSO4 + H2 ↑ ... (C )

Japanese Patent Publication No. 60-48598 discloses a method for controlling the concentration of a Zn electroplating solution using an insoluble anode by detecting the Zn concentration and, based on the detected value, a zinc agent (metal Zn, Zn oxide, water). A method of controlling the Zn concentration and pH by supplying a powder of Zn oxide) is disclosed.

That is, in this case as well, theoretically, according to the Zn concentration detected by the balance between the electrodeposition reaction of the following formulas (D) and (E) and the dissolution reaction of the formula (F), By injecting the powder of the Zn agent, both the Zn concentration and the pH can be controlled.

[0008] (electrodeposition ^{reaction) Zn 2+ + 2e - → Zn} ↓ ... (D) H2 O + SO4 2- → H2 SO4 + 1 / 2O2 ↑ + 2e - ... (E) ( dissolution reaction) Zn + H2 SO4 → ZnSO4 + H2 ↑ ... (F )

[0009]

However, in the control method disclosed in the above-mentioned JP-B-51-97543 and JP-B-63-50439, in an actual plating facility, the plating solution leaks (is conveyed) out of the system. (Including removal by steel plates, leakage of seals, etc.)
Is controlled to a target value, the Ni concentration gradually decreases, and as a result, there is a problem that both cannot be controlled to an appropriate concentration.

In the control method disclosed in Japanese Patent Publication No. 60-48598, the plating solution similarly leaks out of the system in actual plating equipment. However, there is a problem that the H2 SO4 concentration gradually decreases (the pH gradually increases).

Further, the Zn concentration, the H2 SO4 concentration (or p
In the case where both of H) are to be appropriately controlled, the following problem arises.

That is, assuming that there is no leakage of the plating solution in pure Zn electroplating, the Zn concentration of the plating solution and H 2 SO 4
4 For the sake of convenience, as shown schematically in FIG. 2B where a part of the concentration diagram of FIG. 2A is extracted for convenience, when the dissolution of metal Zn is started, the dissolution reaction of the formula (F) is started. According to Zn
Move to the lower right to increase ions and decrease H2SO4,
When the dissolution is stopped, Zn ions decrease and H2 SO4 increases according to the electrodeposition reactions of the above formulas (D) and (E), so that the concentration rises to the upper left.

Therefore, in order to control the component concentration by dissolving the metal Zn, for example, as shown by the thick broken line in FIG. 5, the H2SO4 concentration supply target value is maintained so as to be constant regardless of the Zn concentration. When the metal Zn is dissolved, the concentration of H2 SO4 decreases when the metal Zn is dissolved. Therefore, it is necessary to additionally supply H2 SO4 to compensate for the decrease. However, when the dissolution is stopped and the dissolved Zn is consumed in the electrodeposition reaction, the above (D), (E)
Since the concentration of H2 SO4 is increased by the reaction of the formula, the amount of H2 SO4 additionally supplied becomes excessive. Therefore, when control is performed to maintain only the H2 SO4 concentration at the target value without considering the Zn concentration, the H2 SO4 concentration is gradually increased by repeating this.
An excessive amount of SO4 accumulates and eventually becomes uncontrollable.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems.
SO4) Metal Zn based on the measured value of concentration (or pH)
By controlling both the amount of dissolution and the supply of free acid,
It is an object of the present invention to provide a technique capable of maintaining a Zn concentration and a free acid concentration (or pH) within a range suitable for plating with high accuracy even when a plating solution leaks out of the system.

[0015]

SUMMARY OF THE INVENTION The present invention relates to a method for manufacturing a zinc plating apparatus using an insoluble anode, and a dissolving apparatus for dissolving the zinc by bringing an acidic plating solution into contact with metallic zinc. In the method of controlling the concentration of the insoluble anodic zinc electroplating solution, which controls the concentration of the plating solution by circulating the plating solution and replenishing the plating equipment with zinc ions, the zinc solution actually measured for the plating solution in the plating equipment body was used. Based on the concentration and the free acid concentration (or pH),
The object has been achieved by dissolving metallic zinc by the dissolving device to replenish zinc ions and replenishing free acid so that both are maintained in predetermined ranges.

That is, in the present invention, the free acid is H 2 S
The case of O4 will be described. Based on the measured values of the Zn ion concentration and the H2 SO4 concentration (or pH), the metal Zn is added according to the above formula (F) so that both are maintained within a predetermined allowable range. In addition to dissolving and replenishing Zn ions, the amount of H2 SO4 replenishment can be determined and supplied in consideration of the influence of by-produced SO4 ions, so that the plating solution leaks out of the system. For
The Zn concentration and the H2 SO4 concentration (or pH) can be maintained within a range suitable for plating with high precision.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the principle of a method for supplying H2 SO4 in a preferred embodiment of the present invention will be described below.

The metal Zn is circulated between the plating equipment main body and the melting device, and the metal Zn is repeatedly turned on to contact the metal Zn and dissolve the metal plating and to stop the melting by releasing the contact. When the on / off control of
As shown in FIG. 2A, as a method of supplying 2 SO4, as shown in FIG. 2A, the H2 SO4 concentration is set to a higher value, for example, the target upper limit concentration B2, in the region where the target lower limit Zn concentration A1 at which the dissolution of metal Zn is started. When the dissolution operation of the metal Zn is started, the H2SO4 concentration naturally decreases due to the dissolution reaction of the formula (F) (dissolution reaction direction shown in FIG. 2B). Further, in the region above the target upper limit Zn concentration A2 at which the dissolution operation of the metal Zn is stopped, the H2 SO4 concentration is set to a lower value, for example, the target lower limit concentration B1, and the dissolution of the metal Zn is stopped. E) spontaneously H2 SO4 by the electrodeposition reaction of the formula
The concentration increases (the direction of the electrodeposition reaction in FIG. 2B).

In this embodiment, by controlling the dissolution amount of metallic Zn while taking the H2 SO4 concentration into consideration, the Zn concentration and the Zn concentration can be reduced without excessively supplying H2 SO4.
Both the H2SO4 concentration (or pH) can be controlled within an appropriate concentration range, that is, in FIG. 2A, the Zn concentration is within the allowable concentration range of A1 to A2, and the H2 SO4 is B1 to B
It was made based on the finding that each can be controlled within the allowable concentration range of 2.

In a preferred embodiment of the present invention, when the component concentration exceeds the allowable range, the H 2 SO 4 concentration (or pH) is maintained within the allowable range by giving priority to the Zn concentration. To control.

That is, the measured Zn concentration and H 2 SO 4
Starting and stopping the melting operation of the metal Zn based on the concentration is defined as a stop region even when both the Zn concentration and the H2 SO4 concentration are high as shown in FIG. 6, and the melting operation of the metal Zn is performed even in this state. Therefore, only the electrodeposition reaction proceeds, and the H2 SO4 concentration further increases.

As described above, when the concentration of H 2 SO 4 becomes higher than necessary (or when the pH becomes lower than necessary), the efficiency of the electrodeposition reaction deteriorates (the power consumption decreases) and the electrode deteriorates. As a result, pure Zn
It has been found that in controlling the concentration of the plating solution, preferentially controlling the H2 SO4 concentration is the best in terms of quality and cost.

Hereinafter, a more specific embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an outline of an entire plating facility applied to one embodiment of the present invention, which is a plating facility body 10 for performing zinc electroplating using an insoluble anode.
And a dissolving device 12 for dissolving the metal Zn by bringing a sulfuric acid plating solution into contact with the metal Zn.

In the plating equipment body 10, the liquid analyzer 1
4, the analysis of the plating solution component is performed. Further, the metal Zn dissolving apparatus 12 contains a metal Zn, and a dissolving tank 16 for dissolving the Zn by contacting a plating solution with the Zn.
The dissolution tank 16 is replenished with metallic Zn, and the H2 SO4 chemical is supplied through its supply valve 18.

The plating equipment body 10 and the metal Zn dissolving device 12 are connected by piping, and the plating solution is circulated between the two by pumps 20 and 22 installed on the respective outlet sides. In addition, by opening and closing the receiving valve 24 and the bypass valve 26, on / off control of whether or not the circulating plating solution is introduced into the dissolving tank 16 to dissolve the metal Zn is performed. . Incidentally, reference numeral 2 in the figure
Reference numerals 8 and 30 denote a plating equipment main body 10 and a melting tank 16, respectively.
It is a level meter attached to.

In the present embodiment, the plating equipment body 1
On the basis of the Zn ion concentration and the H2 SO4 concentration actually measured by the solution analyzer 14 for the plating solution No. 0, these two are respectively within predetermined ranges, that is, A1 to A2 and B1 to B2 shown in FIG. The dissolving device 12 is operated to circulate the plating solution to dissolve the metallic zinc contained in the dissolving tank 16 and replenish the Zn ions to the main body 10 so that the concentration is maintained within the range of Control for replenishing the insufficient H2SO4 is performed.

More specifically, the control method of the present embodiment will be described.
The operation of supplying (on) / stopping (off) Zn ions from 6 to the equipment body 10 can be performed as follows.

As a result of the actual measurement by the liquid analyzer 14, when the measured Zn concentration and the measured H2 SO4 concentration are in the operation region shown in FIG. 2A, the receiving valve 24 is opened and the bypass valve 26 is closed. The dissolving device 12 is brought into an operating state. In this way, the plating solution is received from the plating equipment body 10 into the dissolving device 12, and after dissolving the metal Zn, the plating solution is sent back to the plating equipment body 10 for circulation.

Thereafter, since the reaction of the formula (F) proceeded, the measured Zn concentration and the H 2 value measured by the liquid analyzer 14 were measured.
When the measured SO4 concentration reaches the stop region shown in FIG. 2A, the dissolution of the metallic Zn is stopped by closing the receiving valve 24 and opening the bypass valve 26.

Thereafter, the plating is continued with this stopped state, and when the measured result by the liquid analyzer 14 reaches the operating region again, the dissolution of the metal Zn is restarted. In this way, the on / off control of the dissolution of metallic Zn by operating / stopping the dissolving device 12 controls both the Zn concentration and the H2 SO4 concentration within the allowable range.

When such control is performed,
When H2 SO4 becomes insufficient due to leakage of the plating solution or the like, replenishment is performed.

When replenishing H 2 SO 4, a target H 2 SO 4 supply concentration curve which is a target when replenishing H 2 SO 4 is prepared in advance for a predetermined range of Zn concentration, and the actually measured Zn concentration is used as the target concentration. By applying the curve to the curve, a target supply concentration of H2SO4 at the time of actual measurement is determined, and H2OS4 is determined based on a deviation between the determined target supply concentration of H2SO4 and the actual concentration (or pH) of H2SO4 actually measured. Is determined, and H2 SO4 is supplied.

FIG. 2 shows the target supply density curve.
What is shown by a thick broken line in (A) can be used. The target supply concentration curve will be described below.

FIG. 3 shows the above-mentioned FIG. 2 (B) more specifically. The line segment AB shown in FIG. 3 corresponds to a graph obtained by plotting the relationship between the plating reaction of the formulas (D) and (E) and the dissolution reaction of the formula (F).

Accordingly, the Zn concentration and H
If the 2 SO4 concentration can be controlled, it can be expected that the respective concentrations approach the target values (● in the figure) by plating and dissolution of Zn. Therefore, this line segment AB will be referred to as an appropriate fluctuation line. That is, the appropriate fluctuation line is
This shows an appropriate change in H2SO4 concentration with respect to a change in Zn concentration when there is no balance between sulfuric acid and the outside.

Since the allowable range of the Zn concentration and the H2 SO4 concentration of the pure Zn plating solution is relatively wider than that of the Zn-Ni alloy plating solution, the target value set at the time of control is set at a point on the appropriate fluctuation line. I will.

The above-mentioned appropriate fluctuation line is a Zn concentration target value CZC.
sp (or CZsp) [g / l] and H2 SO4 concentration target value CA
Since the line segment passes through sp [g / l] and has a gradient of the gram equivalent ratio of Zn and H2 SO4, (CZ-CZCsp) / Mz + (CAC-CAsp) / Ma = 0 (1) is satisfied. Thus, the corrected H2 SO4 concentration CAC [g / l], which is a value on the appropriate fluctuation line, can be obtained by the following equation.

CAC = CAsp− (Ma / Mz) (CZ−CZCsp) [g / l] (2) where CZ: measured value of Zn concentration Mz: atomic weight of Zn Ma: molecular weight of H 2 SO 4

Then, the correction H2 set at the time of actual control
The SO4 concentration target value CACsp (corresponding to the H2 SO4 concentration supply target value shown by the thick broken line in FIG. 2A) is expressed by the H2 SO4 concentration target value on the appropriate fluctuation line of the second term on the right side of the above equation (2). The determination is made within the range of the lower limit CAL of the concentration of CAsp and H2 SO4. Therefore, the CACsp can be set by the following equation (3). In other words, the smaller one of CAsp and the corrected density CAC obtained in the above (2) and the larger one between the CAL obtained by the following equation (4) are set as the corrected target value CACsp.
When CAC is calculated by the above equation (2), Zn
As the measured concentration value CZ [g / l], the latest value is used.

CACsp = MAX (MIN (CAsp, CAC), CAL) [g / l] (3) CAL = CAsp−ΔCAL [g / l] (4) where ΔCAL: H 2 SO 4 concentration lower limit deviation [g / l]

The modified H 2 SO 4 given by the above equation (3)
The concentration target value CACsp is conceptually shown as a thick solid line in FIG. 4, and FIG. 4 corresponds to FIG. 2A.

Therefore, A1 in FIG. 2A is Zn
In the target concentration CZsp, B1 is the H2 SO4 concentration lower limit value CA.
L and B2 correspond to the H2 SO4 concentration target value CAsp, respectively, and A2 is the upper limit of the Zn concentration allowed at that time.

In FIG. 4, the capacity link "ON" means that the target value of the Zn concentration is corrected and the concentration control is performed so that the H2 SO4 concentration is kept constant even when the bath amount fluctuates. This means that the bath amount link mode is selected.

That is, in this concentration control, (1) H2SO4
A bath amount link for stably controlling the concentration and (2) the appropriate fluctuation line for increasing the degree of freedom of the metal supply amount can be adopted. The calculation cycle for the concentration control is a reception cycle (bath volume measurement cycle) of the bath volume measurement value from the instrumentation, for example, 30 seconds.

When the bath amount link mode is selected, the target value of the H2 SO4 concentration is kept constant while the total amount of SO4 ^2- in the plating solution is constant.
Correct the target value of the Zn concentration. The corrected Zn concentration target value CZCsp [g / l] can be obtained by the following equation (6) derived based on the following equation (5).

The target total SO 4 ²⁻ quantity is as shown on the left side of the following equation. The modified Zn concentration CZCsp is set so as not to affect the metal concentration in the incidental equipment separated from the circulation system and the entire H 2 SO 4 concentration. If you think about it, it looks like the right side.

(CZsp / Mz) Vsp + (CAsp / Ma) Vsp = (CZCsp / Mz) Vrot + (CZsp / Mz) (Vall−Vrot) + (CAsp / Ma) Vall [kmol] (5) CZsp: target Zn concentration [g / l] CAsp: target H2 SO4 concentration [g / l] Vsp: total bath target [m ³ ] Vall: total bath actual value [m ³ ] Vrot: circulating bath Actual value [m ³ ] Mz: Zn atomic weight Ma: H2 SO4 molecular weight Therefore, CZCsp = CZsp (Vrot + Vsp-Vall) / Vrot + CAsp (Mz / Ma) (Vsp-Vall) / Vrot [g / l] (6)

Thereafter, when the bath amount link mode is selected, CZsp
Is replaced with CZCsp. That is, since the solution is diluted when the bath volume is increased according to the above equation (6), as shown by * 1 in FIG.
As the concentration decreases when the bath volume decreases, as indicated by * 2,
These are corrected in the high density direction and control is performed.

As a specific method of supplying the H2SO4 chemical in the present embodiment, the measured value of the H2SO4 concentration by the liquid analyzer 14 becomes equal to or less than the target value of the H2SO4 concentration supply shown in FIG. At this time, the supply valve 18 is kept open for the th time, so that the H2SO4 chemical supply amount calculated by the following equation (7) according to the deviation from the target value: H2SO4 chemical of Sh Can be adopted.

Sh = g × (CAsp−CA) × Vall [m ³ ] (7) where CAsp: H2SO4 concentration supply target value [g / l] CA: H2SO4 concentration measurement value [g / l] Vall : Actual value of plating bath total bath amount [m ³ ] g: Adjustment coefficient H2 SO4 chemical supply valve opening time: th th = 3600 × Sh / HS [sec] where HS: H2 SO4 chemical supply capacity [m ³ / h]

The supply amount of H 2 SO 4 obtained by the above equation (7)
Makes it possible to always control the H2 SO4 concentration to the target concentration set according to the Zn concentration.

The above Vall is a level meter 28, 30
The total amount of the plating solution including the plating equipment main body 10 can be calculated from the measurement values obtained by the above method.

According to the present embodiment described in detail above, H 2 S
As shown in FIG. 2 (A), the O4 concentration supply target value is
In the operating range of Zn concentration at which the dissolution of metallic Zn starts, H2
By keeping the SO4 concentration high, the H2 SO4 concentration is naturally reduced by the subsequent dissolution reaction, and in the stop region of the Zn concentration where the dissolution operation of the metal Zn is stopped, the H2 concentration is reduced.
By keeping the SO4 concentration low, the H2SO4 concentration can be spontaneously increased by the electrodeposition reaction during the process. Therefore, it is necessary to control both the Zn concentration and the H2SO4 concentration to a concentration range suitable for plating where both are allowed. Becomes possible.

By performing such control, the following effects are also obtained.

(1) The solubility of metal Zn increases as the H2 SO4 concentration increases. Therefore, when starting the dissolution operation of the metal Zn, the concentration can be quickly returned to the predetermined concentration by increasing the H2 SO4 concentration.

(2) As described above, as shown in FIG. 5, when H2 SO4 is supplied in order to maintain a constant H2 SO4 concentration supply target value, when the melting operation is performed, H
Since the 2 SO4 concentration decreases, H2 SO4 is additionally supplied. However, when the dissolution is stopped, the H2SO4 concentration increases due to the electrodeposition reaction, so that the additional supply of H2SO4 becomes excessive. Accordingly, the H2 SO4 concentration gradually increases and the Zn concentration also increases by this repetition, but such a phenomenon can be prevented from occurring.

(3) By using inexpensive metal Zn, it is possible to automatically control both the Zn concentration and the H 2 SO 4 concentration with relatively high accuracy.

Therefore, according to the present embodiment, in the pure Zn electroplating, the inexpensive equipment and the inexpensive Zn chemical are used, and the Zn concentration and the H2SO4 concentration of the plating solution are both set within a permissible range. Automatic control with high accuracy was realized. The realization of this automatic control makes it possible to omit the manual analysis, so that the load on the operator can be reduced and the personnel can be rationalized (however,
When the reduction of the equipment cost is emphasized, the analysis operation by the liquid analyzer 14 can be handled by manual input.)

According to the present embodiment, Zn and H 2 S
By being able to stably maintain O4 within a predetermined concentration range that is both acceptable, plating efficiency has become highly stable, and it has become possible to provide a product of stable quality while reducing power consumption.

Although the present invention has been specifically described above, the present invention is not limited to the above-described embodiment, and can be variously modified without departing from the gist thereof.

For example, in the above embodiment, the case where the concentration of H 2 SO 4 is used as the measured value of the free acid has been described.
A pH value may be used.

Further, the case where the on / off control of the dissolution of the metal Zn has been described. However, the present invention is not limited to this case.
May be controlled.

[0064]

As described above, according to the present invention,
By controlling both the dissolution amount of metal Zn and the supply amount of H2 SO4 based on the actually measured values of the Zn concentration and the H2 SO4 concentration (or pH), the leakage of the plating solution to the outside of the system can be controlled with high accuracy. Zn concentration and H2 SO4 concentration (or p
H) can be maintained in a range suitable for plating.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an outline of an entire plating facility applied to an embodiment according to the present invention.

FIG. 2 is an explanatory diagram for clarifying the effect of the present embodiment.

FIG. 3 is a diagram showing an appropriate fluctuation line of the sulfuric acid concentration.

FIG. 4 is a diagram conceptually showing a corrected H2SO4 concentration target value;

FIG. 5 is an explanatory diagram for pointing out a conventional problem.

FIG. 6 is an explanatory diagram for pointing out another conventional problem.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Plating equipment main body 12 ... Metal Zn dissolution apparatus 14 ... Liquid analyzer 16 ... Dissolution tank 18 ... H2 SO4 chemical supply valve 20,22 ... Pump 24 ... Reception valve 26 ... Bypass valve 28,30 ... Level meter

Claims (4)

[Claims] A plating solution is circulated between a plating equipment body for performing zinc electroplating using an insoluble anode and a dissolving device for dissolving the zinc by bringing an acidic plating solution into contact with metallic zinc. In a method for controlling the concentration of an insoluble anodic zinc electroplating solution, which controls the concentration of a plating solution by replenishing zinc ions into a body, the zinc concentration and the free acid concentration (or pH) measured for the plating solution of the plating equipment body. ), The dissolving device dissolves zinc metal and replenishes zinc ions, and replenishes free acid so that both are maintained within predetermined ranges. Concentration control method. 2. The method according to claim 1, wherein when the measured zinc concentration is high and the free acid concentration is high (or pH is low), the free acid concentration is higher than the target upper limit concentration (or pH is lower than the target lower limit). A method of controlling the concentration of an insoluble anodic zinc electroplating solution, comprising dissolving metallic zinc so as not to cause dissolution and giving priority to control of the free acid concentration (or pH). 3. A target free acid supply target concentration (or pH) curve for replenishing free acid is prepared in advance for a predetermined range of zinc concentration, and an actually measured zinc concentration is determined. Applying to the target supply concentration curve,
Determine the supply target concentration (or pH) of the free acid at the time of actual measurement, and, based on the deviation between the determined supply target concentration (or pH) of the free acid and the actually measured actual concentration (or pH) of the free acid. A method for controlling the concentration of an insoluble anodic zinc electroplating solution, comprising determining a supply amount of a free acid and replenishing the free acid. 4. The insoluble anode zinc electroplating according to claim 1, wherein replenishment of zinc ions by said melting device is performed by on / off control of whether or not a plating solution to be circulated to metal zinc is brought into contact. Liquid concentration control method.