JPS582596B2 - How to supply zinc ions to the plating bath - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a method for replenishing zinc ions into a plating bath in order to maintain an appropriate zinc concentration in the plating bath when galvanizing an object to be plated using a galvanizing bath. be.

Galvanizing baths can be roughly divided into cyanide galvanizing baths,
It consists of an alkaline galvanizing bath that does not contain cyanide compounds (hereinafter referred to as alkaline non-cyanide bath) and an acidic galvanizing bath, which have recently been used to avoid pollution problems.

Conventional methods for supplying zinc ions to these galvanizing baths will now be described.

First, from the perspective of pollution prevention, the use of alkaline non-cyanide baths that do not use cyanide compounds has increased dramatically in recent years, but the methods for supplying zinc ions in this case include using a zinc anode and using zinc oxide (ZnO). There are cases.

Using a zinc anode, place the object to be plated on the cathode and apply electricity.
In the method of replenishing zinc ions by dissolving anode zinc,
A black-gray coating is formed on the surface of the anode, which often falls off due to interruptions in current flow, resulting in variations in the plating surface.

Furthermore, as the anode progresses to melt and its surface area decreases, the current density increases, and a similar phenomenon occurs at this time as well.

That is, in any case, there is a drawback that the anode tends to become passivated while plating continues, and there is also a time loss when replacing the zinc anode due to the trouble of casting and the anode wear.

Therefore, in order to solve these problems, an insoluble anode such as iron or carbon may be used instead of a zinc anode.
In this case, replenishment of zinc ions to the plating bath becomes a problem.

One possible solution is to add ZnO to the plating bath, but normal alkaline non-cyanide baths contain Zn2+4.
-13g/l, NaOH70-150g/l, organic additives 2-30mg/l, temperature about 20-30℃, Z
Normally, nO does not dissolve and forms an emulsion at most, and Z
In order to dissolve nO, it is necessary to use a highly concentrated sodium hydroxide solution at high temperatures, such as heating a 40% NaOH solution and adding ZnO to obtain a 10% zinc solution. becomes extremely complicated.

Furthermore, if a concentrated zincate solution is prepared by dissolving ZnO and concentrated NaOH in a separate tank and replenished into the plating bath, the NaOH content in the plating bath will gradually increase, causing plating defects.

Therefore, in order to solve this problem, it is desirable to replenish only the zinc ions that have been reduced from the solution by electrodepositing on the object to be plated.

As a solution to this problem, for example, according to JP-A-52-7828, a battery is formed between metal zinc and a counter electrode whose unipolar potential is nobler than that of zinc and whose overvoltage in the plating solution is lower. A method is described in which at least zinc is dissolved and this is supplied to the plating bath, but this method requires electrochemical zinc dissolving equipment in addition to the plating bath, and also requires electricity other than plating power to dissolve the zinc. This method requires electricity, and has the disadvantage that the zinc deposited on the anticathode in the zinc dissolving tank becomes a useless component.

Next, in conventionally used zinc cyanide plating baths, for example, in high concentration cyanide baths, the temperature is 20 to 30°C.
Sodium cyanide 40g/l, caustic soda 80g/l
, has a composition of about 60 g/l of zinc cyanide, and its manufacturing method is to first prepare a concentrated mixture of sodium cyanide and sodium hydroxide, then dissolve zinc oxide or zinc cyanide in this, and then completely dissolve it. After dissolving, water is added to make the appropriate amount.

As a method for replenishing zinc ions during continued plating, a metal zinc cast anode is used, and zinc plating is performed by dissolving the anode, using the cathode as the object to be plated.

This method is widely used because, unlike the alkaline non-cyanide bath, the phenomenon of anodic passivation does not occur, but the properties of the plating vary considerably depending on the concentration of nam cyanide in the solution. For example, if the concentration is low, the anode zinc will not dissolve well and will turn white, and the current will gradually become difficult to flow, resulting in good current efficiency but poor plating gloss.

In addition, when the concentration is too high, a foul odor of cyanic acid is generated at the cathode, and although the anode shines and melts quickly, the plating efficiency is poor and the plating becomes rough.

Furthermore, anode zinc often dissolves during periods when plating operations are stopped, such as at night, so it must be recovered from the plating bath after operations are completed, but this recovery process also releases toxic cyanide solution. This is quite a complicated task.

In general, the cathode current efficiency is 60-85%, whereas the anodic dissolution efficiency is as high as 100-105%, and the zinc content in the bath tends to be high, and zinc anodes tend to produce zinc anode mud. This causes the plating to become rough, so an anode bag made of Saran or the like is used to prevent this.

Therefore, although zinc anodes are widely used, careful attention must be paid to bath management.

Note that it is inappropriate to use zinc oxide as a means for supplying zinc ions to the zinc cyanide bath because it does not dissolve in the plating bath.

Also, if you use zinc cyanide, it will dissolve in the plating bath, but
This is unfavorable in terms of operation since the cyanide concentration in the bath gradually increases.

Recently, low cyanide zinc plating has been used from the viewpoint of pollution prevention, but the liquid composition of the bath is 6 to 11 g/l of Zn and Na.
The temperature is 20 to 35°C with 60 to 90 g/l of OH and 5 to 10 g/l of NaCN, and zinc ions are replenished by dissolving the zinc anode, similar to the high concentration cyanide zinc plating bath, and there are the same problems as above. .

Next, the acidic zinc plating bath has a pH of 3 to 5 and ZnSO4
240-410g/l, NH4Cl 0-30g/l,
Al2(SO4)3O~30g/l, AlCl30~2
0g/l, Na2SO40-75g/l, temperature 20-3
A bath liquid of about 0° C. is used, and the bath liquid is mainly composed of ZnSO4, to which a neutral salt is added to improve conductivity, and an aluminum compound is further added as a pH adjuster.

In this case as well, zinc ions are replenished by dissolving the anode zinc, but careful management is required to maintain the balance between the amount of dissolved zinc and the amount of precipitated zinc, including treatment of the anode mud and replacement of the anode. This creates complicated work.

Therefore, a method of dissolving zinc oxide in the plating bath instead of the anode zinc has been considered, but it has not been put to practical use at present because the dissolution rate of zinc oxide is slow.

The method of the present invention uses epsilon zinc hydroxide [ε
-Zn(OH)2] was found to dissolve easily in any of the above-mentioned galvanizing baths even at low temperatures, and devised the use of this as a zinc ion replenisher.

In the first place, crystalline zinc hydroxide is generally expensive and difficult to obtain, and could only be synthesized under special conditions. However, the present inventors have developed an economical method for producing it and are researching its various uses. As a result, it was found that there are various advantages when used in galvanizing baths.

That is, SO42-, Cl- or (NO3-
) A zinc salt aqueous solution consisting of one or a combination of 2 and NaOH
, NH4OH or NH3, or a combination thereof, are continuously fed in a constant quantity so that the pH inside the tank is maintained at 10.5 or more. OH)2 precipitates.

For example, 400 ml was maintained at a pH of 11.5 and a temperature of 35°C.
A zinc sulfate aqueous solution with a zinc concentration of 80 g/l is fed into a reactor having a volume of 1 at a quantitative rate of 4001/Hr, and at the same time a 48% concentration caustic soda solution is fed at a rate of 80 l/Hr.

When continuous neutralization is carried out while maintaining the above pH and temperature, 55.5 kg/Hr of highly pure crystalline ε-Zn(OH)2 is produced.

This was identified by X-ray diffraction and confirmed by chemical analysis.

In addition, the impurity quality is Na30ppm, S100ppm,
It is crystalline ε-Zn(OH)2 with extremely high purity, containing 20 ppm of Ca, 10 ppm of Mg, and 10 ppm or less of others.

This highly crystalline ε-Zn(OH)2 has good filterability and is easy to clean, so unlike fine particles like conventional Zn(OH)2, which are difficult to filter, cations such as Na+ and NH4+ There is no adhesion of ions or anions such as SO4- and Cl-, and the solubility in alkali is good.

For example, when 20 g ε-Zn(OH)2 is added to 1 liter of 10% NaOH solution and stirred at 30°C, 10
Taking advantage of the phenomenon that it dissolves in 5 minutes at 40°C, a separate tank is placed in the alkaline non-cyanide bath system, the liquid is continuously circulated and extracted, and ε-Zn(OH)2 is added to this tank. By adding and dissolving the calculated amount, continuous plating work became possible.

That is, according to this method, the gradual decrease in zinc concentration in the bath is analytically grasped, a calculated amount of ε-Zn(OH)2 is added to the dissolution bath, and after staying there for 30 minutes, the process is repeated to the plating bath. This has the advantage that there is no need to worry about the accumulation of NaOH in the solution as with ZnO, and the moisture generated by dissolution is commensurate with the evaporated moisture in the plating bath, so it is sufficient to manage the liquid level of the plating bath.

In addition, as mentioned above, zinc anodes are used exclusively to replenish zinc ions in high-concentration cyan plating baths, and zinc oxide cannot be used; however, crystalline ε-Zn(OH
) 2 is suitable because it easily dissolves in the plating bath.

Furthermore, crystalline ε-Zn (
By adding OH)2 to the bath solution, zinc ions can be easily replenished, which greatly simplifies work management.

Example I Zn 10g/l, NaOH 100g/l and brightener 10
Zinc plating was carried out using 100 g of an alkaline non-cyanide bath consisting of cc/l, with an iron plate as an anode and a copper plate to be plated as a cathode.

The amount of zinc deposited is 2000 g per day, and a part of the plating bath is continuously drawn out at 10 l/hr into a 10 l dissolving tank to replenish zinc ions, and 134 g of ε-Zn(OH)2 is continuously added to this. /Hr was added, stirred and dissolved, and circulated to the plating bath. As a result, the zinc concentration in the plating bath could be easily maintained at a constant level, and a good plating work could be performed.

Example 2 NaCN40g/l, NaOH80g/l, Zn(CN
) 260 g/l and a temperature of 25° C. When 210 g of ε-Zn(OH) was added to 1 liter of the bath solution of a high concentration zinc cyanide plating bath and stirred,
Dissolved in 5 minutes.

Claims (1)

[Claims] 1 Crystalline ε-Zn (
A method for supplying zinc ions to a plating bath, characterized by adding OH)2.