JP2749503B2 - How to remove chromium from galvanizing bath - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing trivalent chromium dissolved in a zincate zinc plating bath from the plating bath.

[0002]

2. Description of the Related Art Although a dense and glossy plated surface is obtained by zinc plating, a chromate treatment is usually performed after zinc plating for the purpose of further imparting corrosion resistance. The chromate treatment is a treatment in which a plated component is immersed in an aqueous solution containing chromium. Therefore, when the galvanizing bath and the chromate treatment bath are in the same working process, it is difficult to prevent chromium from being mixed into the galvanizing bath because the basket housing the plated parts is immersed in the two baths and used. Therefore, 0.1 to 5 ppm of chromium is usually present in the galvanizing bath. Chromium is brought into the plating bath in the form of hexavalent chromium or trivalent chromium,
It causes the appearance of zinc plating to be defective. When 0.1 ppm of hexavalent chromium is mixed in the zinc plating bath, the plating appearance becomes insufficient in gloss, and when the hexavalent chromium concentration exceeds 1 ppm, the appearance becomes brown and the commercial value is remarkably reduced. On the other hand, trivalent chromium itself has little effect on the plating appearance, but a part of the trivalent chromium changes to hexavalent chromium due to the anodic oxidation action during the electrolysis of the plating bath, which causes a poor plating appearance. Under normal plating operation conditions, when the trivalent chromium concentration is 1.0 ppm or less, such a phenomenon does not occur because the generated hexavalent chromium concentration is low.

[0003] However, when the trivalent chromium concentration is 2.0 ppm or more, many defects in plating appearance are observed.
As a method of solving the poor plating appearance caused by hexavalent chromium, there is a method of adding hexagonal chromium to trivalent chromium by adding a reducing agent such as zinc dust or hydrosulfite. However, these methods merely change hexavalent chromium into trivalent chromium and cannot reduce the chromium concentration in the galvanizing bath. Therefore, chromium accumulates in the galvanizing bath under plating operation conditions where chromium is frequently carried in. Therefore, when performing a plating operation using a galvanizing bath in which chromium has accumulated, it is necessary to frequently add the above-mentioned reducing agent, and the labor and cost required for the addition are considerable. Therefore, the plating solution is eventually discarded. Therefore, as a conventional technique for removing trivalent chromium from a plating bath, for example, there is a method of adding an aqueous solution of calcium chloride to the plating bath (Electroplating Technology Guidebook, p.2181975). However, this method is not generally used because a large amount of sludge is generated and the chromium removing effect is poor.

[0004]

Means for Solving the Problems Thus, the present inventors,
When the zinc plating bath and the chromate treatment bath are on the same working step, there are many methods for removing trivalent chromium present in the plating bath and providing a method for avoiding poor appearance of the material to be plated due to chromium. As a result of the research,
The inventors have found that the object can be achieved by adding a magnesium compound to the galvanizing bath, and have reached the present invention.

Accordingly, the present invention is characterized in that a magnesium compound is added to a zincate type zinc plating bath containing zinc and alkali hydroxide and further containing trivalent chromium dissolved therein to remove the trivalent chromium. A method for removing chromium in a galvanizing bath is provided.

The present invention will be described in detail below. As described above, the present invention is characterized by adding a magnesium compound to a zincate type zinc plating bath. Generally, there are an acidic bath and an alkaline bath in a zinc plating bath, but an alkaline bath is preferably used industrially,
In the present invention, an alkaline zincate zinc plating bath is also used. The zincate type zinc plating bath usually contains 1 to 50 g / l of zinc and 10 to 200 g / l of alkali hydroxide and usually sodium hydroxide. The main component of the bath is usually Na ₂ ZnO ₂ , but it has a gloss A small amount of an additive such as an agent can be contained. A reducing agent such as zinc dust or hydrosulfite is added to reduce all hexavalent chromium in the dissolved chromium to trivalent chromium.

[0007] A magnesium compound is added to such a galvanizing bath. As the magnesium compound, various types of inorganic magnesium compounds and organic magnesium compounds can be used. As the inorganic magnesium compound, for example, magnesium chloride, magnesium sulfate, magnesium hydroxide, magnesium oxide and the like can be used. Since magnesium chloride and magnesium sulfate are water-soluble, they can be used as they are in the bath.However, since magnesium hydroxide and magnesium oxide are not water-soluble, they are dissolved in aqueous solutions of inorganic acids such as nitric acid and hydrochloric acid. Used. As described above, even when an acid is used, the change in the pH of the alkaline bath is small and can be used well. As described above, not only water-soluble magnesium compounds but also non-water-soluble magnesium compounds can be used as long as they are soluble in water by some means. As the organic magnesium compound, aliphatic and aromatic compounds such as magnesium acetate, magnesium phenolsulfonate, magnesium benzoate, magnesium glycerophosphate, magnesium formate, and magnesium acetylsalicylate can be used. However, magnesium compounds of fatty acids and aromatic acids having a large number of carbon atoms can hardly be used because they are almost insoluble in water.

Such a magnesium compound is used as magnesium in an amount of 1 mg to 5 g per liter of plating solution. If the amount is less than the above range, chromium cannot be effectively removed. It is preferably from 10 to 100 mg, more preferably from 20 to 50 mg. After adding the magnesium compound in such an amount and stirring for 2 to 4 hours, chromium in the system is removed by filtering to remove the precipitate outside the system. It is necessary to remove at least 50% of chromium. For example, when the chromium removal rate in one operation is about 20%, the chromium is repeatedly removed several times.
% Can be removed.

In the plating bath, hexavalent chromium exists as chromate ions, and trivalent chromium exists as chromate ions. It is believed that the magnesium ion forms an insoluble precipitate with the chromite ion. Therefore, in order to efficiently remove chromium from the plating bath, it is necessary to first add a reducing agent to the plating bath to convert chromium into trivalent chromium as described above. Thereafter, a magnesium compound is added, and the precipitate is taken out of the plating bath system by filtration. By performing these treatments about once every several months, the chromium concentration can be kept low, and troubles due to poor plating appearance can be avoided. As described above, according to the present invention, it is possible to return a plating solution, which has conventionally caused a problem due to chromium accumulation, to a normal plating solution.

[0010]

The present invention will be further described below with reference to examples and comparative examples. However, the present invention is not limited to this example. Example-1 One liter of zincate type zinc plating solution was used as chromium in one liter.
3 ml of a chromic anhydride solution having a concentration of 000 mg / l is added to prepare a plating solution having a chromium concentration of 3 mg / l. Next, 2 g / L of zinc dust is added to reduce all hexavalent chromium to trivalent chromium. An 8.4 wt% aqueous solution of magnesium chloride hexahydrate (10 g / L as magnesium) was added to the plating solution.
Is added at 1, 2, and 5 ml / L. That is, 10, 20, 5 per 1 liter of plating solution as magnesium
Add 0 mg. After stirring for several hours, filtration is performed, and the chromium concentration of the filtrate is determined. Plating bath composition Zn 8 g / L NaOH 100 g / L Mel zinc NR-52A 4 ml / L Mel zinc NR-52B 1 ml / L Mel zinc NR-52A and Mel zinc NR-52B are brightening agents for zinc plating manufactured by Meltex Corporation.

EXAMPLE 2 A chromic anhydride solution is added to a zincate zinc plating solution to prepare a plating solution having a chromium concentration of 3 mg / L. Next, 2 g / L of zinc dust is added to reduce all hexavalent chromium to trivalent chromium. An aqueous solution of magnesium sulfate heptahydrate 10.1 wt% (10 g / L as magnesium)
5 ml / L. After stirring for several hours, filtration is performed, and the chromium concentration of the filtrate is determined. The plating bath composition was the same as in Example-1. Example 3 A chromic anhydride solution was added to a zincate zinc plating solution to prepare a plating solution having a chromium concentration of 3 mg / L. Next, 2 g / L of zinc dust is added to reduce all hexavalent chromium to trivalent chromium. This plating solution contains magnesium hydroxide 2.5w
5% aqueous solution (containing 10 g / L as magnesium)
Add ml / L. After stirring for several hours, filtration is performed, and the chromium concentration of the filtrate is determined. The plating bath composition was the same as in Example-1. (Note) Magnesium hydroxide was dissolved in a 5.0 wt% nitric acid solution to form an aqueous solution.

EXAMPLE 4 A chromic anhydride solution is added to a zincate zinc plating solution to prepare a plating solution having a chromium concentration of 3 mg / L. Next, 2 g / L of zinc dust is added to reduce all hexavalent chromium to trivalent chromium. 5.9 wt of magnesium acetate was added to this plating solution.
5% aqueous solution (containing 10 g / L as magnesium)
Add 1 / L. After stirring for several hours, filtration is performed, and the chromium concentration of the filtrate is determined. The plating bath composition was the same as in Example-1. Comparative Example 1 A chromic anhydride solution was added to a zincate zinc plating solution to prepare a plating solution having a chromium concentration of 3 mg / L. Next, 2 g / L of zinc dust is added to reduce all hexavalent chromium to trivalent chromium. Calcium chloride dihydrate 1
1, 10, and 20 ml / L of an 8.3 wt% aqueous solution (50 g / L as calcium) are added. After stirring for several hours, filtration is performed, and the chromium concentration of the filtrate is determined. The plating bath composition was the same as in Example-1.

The conditions and results of the examples and comparative examples are summarized in the table. This indicates that the present invention has an excellent chromium removing effect as compared with the conventional technology.

_{Table Magnesium compound Calcium salt} aqueous solution _{Chromium removal rate} Aqueous solution addition amount (ml / L) Liquid addition amount (ml / L) (%) Example-1 1.0 23 2.0 48 5.0 72 Example-2 5.0 82 Example-3 5.080 Example-4 5.075 Comparative Example-1 1.000 0 17 20.0 28 The same effect was obtained in a zincate-type zinc plating bath using a commercially available brightener for zinc plating, including the examples and comparative examples.

[0015]

According to the present invention, trivalent chromium can be efficiently removed from a plating bath by adding a small amount of a magnesium compound to a zincate zinc plating bath and filtering the zinc compound. And the problem of poor appearance of the object to be plated caused by such chromium can be satisfactorily solved.

Claims (3)

(57) [Claims] 1. A zinc plating bath characterized by adding a magnesium compound to a zincate type zinc plating bath containing zinc and alkali hydroxide and further containing trivalent chromium dissolved therein to remove said trivalent chromium. How to remove chromium inside. 2. The magnesium compound is an inorganic magnesium compound selected from the group consisting of magnesium chloride, magnesium sulfate, magnesium hydroxide and magnesium oxide, or magnesium acetate, magnesium phenolsulfonate, magnesium benzoate, magnesium glycerophosphate and magnesium formate. The method according to claim 1, which is an organomagnesium compound selected from the group consisting of magnesium acetylsalicylate compound. 3. The amount of the magnesium compound to be added is 1 mg to 5 g, preferably 10 to 100 mg, more preferably 20 to 50 mg per liter of plating solution as magnesium.
3. The method according to any of claims 1-2, in the range of mg.