JPS6158560B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Broadly speaking, the present invention relates to a trivalent chromium plating bath;
In particular, nickel, zinc, and
Refining and improving the performance of plating baths that have become incapable of producing chrome plating of commercially acceptable quality due to increased concentrations of contaminant metal ions such as iron, steel, and lead. Concerning methods for recovery. When one or the combined amount of such metal ion impurities begins to adversely affect the performance of the plating solution, black streaks may appear on the resulting plating film.
Cloudiness or haze occurs, and sometimes a spongy film appears, which is accompanied by a reduction in the coating ability of the plating solution, resulting in industrially unfavorable results.

In order to overcome the disadvantages caused by contamination by such metal ions in a trivalent chromium plating bath, a method is disclosed in US Pat. No. 4,038,160 in which a small amount of ferrocyanide is added to the solution to precipitate such contaminating metal ions. proposed in No. Although this purification method has been found to be effective in many cases, the residual ferrocyanide adversely affects bath performance and does not produce commercially acceptable films. There are drawbacks. If ferrocyanide is present in excess in the bath, additional steps are required to intentionally add and remove contaminating metal ions to precipitate this excess ferrocyanide. For this reason, the method described in the above-mentioned U.S. patent requires precise analysis of contaminant ions in the bath and precise addition of precipitants, which is laborious and time-consuming, and requires a lot of skill. It takes. Moreover, the ferrocyanide precipitate is toxic and requires special treatment when disposed of.

A method has also been proposed in which the contaminated trivalent chromium electrolyte is electrolytically purified by electrolyzing the bath for a long time to eutectoid the contaminating metal ions on the cathode. Such electrolytic refining methods are somewhat effective in reducing contaminating steel ions, but are not very effective in removing nickel and zinc ions, and are only partially effective in removing iron.

A method of precipitating heavy metals from the plating washing solution by using dithiocarbamates to make the pH level alkaline has also been proposed. Surprisingly, according to the discoveries that constitute the present invention, chromium ions present in the bath at concentrations of about 50 times or more than the contaminant metal ions can be removed without substantially removing them. Dithiocarbamate compounds selected for use in the purification of aqueous acidic trivalent chromium plating baths to preferentially react with contaminant metal ions contained in the chromium chromium plating solution to form a precipitate. was discovered. Moreover, such selective dithiocarbamate additives often contain metals such as vanadium as preferred components to control or reduce the formation of hexavalent chromium ions in such trivalent chromium plating solutions. It is surprising that vanadium is not substantially removed even with the use of Also, 3
Chrome-valent plating solutions are typically in the acidic PH range of about 1.5 to about 5, which is significantly lower than the alkaline PH range conventionally proposed for treatment of plating rinse water.

An unexpected result of the present invention is further evidenced by the fact that certain dithiocarbamates, such as dibutyldithiocarbamate, do not give satisfactory results. Similarly, dimethylglyoxin, a compound known as a precipitant for nickel ions, is not effective in sufficiently precipitating contaminated nickel ions from a trivalent chromium bath, and furthermore, the precipitate formed has a gelatinous appearance that can cause damage to storage tank walls and work items. They stick to surfaces and form undesirable films that contaminate the equipment. Similarly, when sulfide-containing compounds such as sodium thiocarbonate (Na ₂ CS ₃ ) and sodium thiocyanide (NaSCN) are added to the trivalent chromium bath, they can significantly degrade the chromium film and cause the low current density recesses to turn dark blue or black. , which significantly impairs the coating ability of the bath.

The benefits and advantages of the present invention can be achieved by using dimethyldithiocarbamate and diethyldithiocarbamate and mixtures thereof as purifying agents for trivalent chromium plating solutions, in which case undesirable contamination is avoided. Since it selectively and preferentially reacts with ions to form a crystalline precipitate, it can be easily separated using a conventional filtration device. Since dimethyldithiocarbamic acid and diethyl-dithiocarbamic acid are unstable, it is economical and preferable to add them to the bath as bath-soluble alkali metal and ammonium salts such as sodium salts. This additive is preferably introduced in the form of an aqueous concentrate solution and dispersed in the plating solution. The amount of additive added during the purification process depends on the amount of contaminant metal ions in the liquid. Therefore, the concentration of contaminant ions is reduced to a level at which the performance of the plating solution is restored by using an addition amount calculated to remove at least a sufficient amount of such contaminant metal ions. A typical concentration of precipitant to contaminant metal ion concentration is about 8:1 g/.

Other advantages and inventive steps of the invention will become apparent from the description of preferred embodiments given in conjunction with the examples.

The purification method of the present invention comprises trivalent chromium ions at a concentration of about 0.2 to about 0.8 molar and a complexing agent for maintaining the chromium ions in a liquid, the molar ratio of complexing agent to chromium ions being about 1. :1 to about 3:1 of formate ions, acetate ions, or mixtures thereof, and a conductive salt such as an alkali metal salt or alkaline earth metal salt, and about It is particularly effective for purifying trivalent chromium plating solutions containing strong acids such as hydrochloric acid and sulfuric acid in an amount of 300 g or less. Such conductive salts include hydroborofluoric acid and its alkali metal, alkaline earth metal and bath soluble ammonium salts. Optionally, the trivalent chromium plating solution further comprises a total ammonium ion to chromium ion molar ratio of about 2:1 to about 11:
ammonium ions and halide ions to chromium ions in an amount of about 0.8:1 to about 1.
It is advantageous to contain halide ions, including chloride and bromide ions, in amounts such that the molar ratio is 10:1. Vanadium ion, a metal reducing agent of the type disclosed in U.S. Patent Application No. 205,406 (filed November 10, 1980), also
Advantageously, it is present in a concentration of 0.015 to about 6.3 g/.

Additionally, it has been found to be particularly preferred to optionally include boric acid as a buffer, from about 0.15 moles up to the limit of solubility in the bath. Also about
Wetting agents of the type commonly used in nickel and hexavalent chromium plating solutions in concentrations ranging from 0.05 to about 1 g/g/g are also preferably used. The trivalent chromium plating solution also contains hydrogen ions to make the pH more acidic, from about 2.5 to about 5.5.

U.S. Patent No. 3954574; U.S. Patent No. 4107004: U.S. Patent No.
The benefits of the present invention can also be achieved using this refining agent for trivalent chromium plating solutions of the type generally and specifically described in Nos. 4,169,022 and 4,196,063.

During normal plating work using such trivalent chromium plating solution, there may be cases where it is brought in, melted from the surface of the container or tank containing the plating solution, melted the metallic surface of the article to be plated, melted the easily exposed parts, and Contamination from replenishment and make-up water and chemicals can significantly contaminate the plating solution. the result,
Concentrations of undesirable contaminating metal ions such as nickel ions, zinc ions, iron ions, copper ions, and lead ions have increased significantly, and experiments have shown that nickel ions of about 150 ppm or more are harmful and chromium ions are harmful. It begins to damage the membrane. Iron ions up to about 500 ppm are preferred as they tend to promote chromium film precipitation;
Above 1000ppm (1g/), it generally harms the chromium film. Similarly, copper ions above about 15 ppm and zinc ions above about 10 ppm are also harmful. When multiple such metal ions are present, the harmful effects of each ion become synergistic,
Low concentrations of these become damaging to the chromium coating, a fact evidenced by the formation of black streaks, haze and haze.

In the present invention, a sufficient amount of dimethyldithiocarbamate and/or diethyldithiocarbamate is added as an aqueous concentrated solution of an alkali metal and an ammonium salt, preferably a sodium salt, to the contaminated trivalent chromium plating solution to remove at least one of the contaminated metal ions. Some of the metal ions are precipitated to reduce the concentration of contaminant metal ions to a level where satisfactory performance of the fluid is restored. In contrast to the hazardous precipitates produced by ferrocyanides, this non-toxic precipitate can be easily disposed of using conventional waste disposal techniques. Unlike the case of ferrocyanide precipitants, excessive use of the refining agent of the present invention does not affect the quality of the chromium-plated film;
Also, any unreacted refining agent remaining in the solution after use is progressively decomposed and/or removed during electrolysis of the plating solution during normal plating operations. Preferably, the refining agent is not in the form of a dry soluble powder, but rather in the form of a bath-soluble concentrated aqueous solution, which is added to the bath with stirring to ensure uniform dispersion. This liquid concentrate is usually about 30% by weight.
It contains a purifying agent and has a pH of about 8, preferably about 9 or more to improve storage stability. When using industrial grade dimethyldithiocarbamate and diethyldithiocarbamate, it is usually best to purify them using carbon filters or other methods, which may have a sub-effect on the performance of the plating bath after purification. Possible undesirable side reaction impurities are removed.

The purification agents of the present invention are generally effective over a wide PH range. Generally, trivalent chromium plating solutions operate at a pH of about 2 to about 5.5, typically about 3 to about 4. The purification agents of this invention can be used effectively within these PH ranges as well as at PH's as low as about 1.5.

Next, the present invention will be explained in detail with reference to Examples. These are merely for illustrative purposes and the invention is not limited thereto.

Example 1 An acidic trivalent chromium aqueous test plating solution was prepared with the following composition: Component concentration Cr ⁺³ 24.2g/ NH ₄ COOH 44.0g/ NaBF ₄ 55.0g/ NH ₄ Cl 150.0g/H ₃ BO ₃ 57.1g/ VOSO ₄ 1.0g/ Wetting agent ^* 2.0cc/ ^* Wetting agent consists of 0.1344g/ sodium sulfosuccinic acid dihexyl ester and 0.244g/ sodium sulfate derivative of 2-ethyl-1-hexanol .

iron, copper and nickel ions by adding the corresponding sulfates to give an iron ion concentration of about 0.312 g/, a copper ion concentration of about 0.032 g/ and a nickel ion concentration of about 0.110 g/. A contaminated breeding solution was prepared. 2.5 g of sodium diethyldithiocarbamate trihydrate was added to the plating solution 1 containing contaminant ions, and the solution was stirred at a pH of approximately 3.2 at 75°C (23°C) for approximately 10 minutes, then for 2 hours. , left undisturbed. The liquid was then separated through a carbon filter and analyzed for metal contaminants. The analysis result is iron ion concentration 0.168g/
The copper ion concentration was 0.012 g/, and the nickel ion concentration was 0.042 g/. Therefore, the iron ion's approx.
It was shown that 46.2%; about 62.5% of copper ions; and about 62% of nickel ions were removed.

Example 2 Trivalent chromium test plating solution described in Example 1,
Add contaminant metal salts to 400ml to reduce iron ion concentration to approx.
0.312g/, Nickel ion concentration approximately 0.120g/,
The copper ion concentration was approximately 0.080 g/. To this plating solution containing contaminated metal ions, add 3.5 g of sodium diethyldithiocarbamate trihydrate,
The solution was stirred at a pH of about 3 and 120°C (49°C) for about an hour and a half. The mixture was then allowed to stand at 75°C (23°C) for about 2 hours, and then separated through a carbon filter. According to the analysis of the liquid, the iron ion concentration is approximately 0.072g/
, the nickel ion concentration was reduced to about 0.010 g/, and the copper ion concentration was negligible.
Therefore, approximately 77% of iron ions and 91.7% of nickel ions are removed, and virtually 100% of copper ions are removed.
% removed.

Example 3 In order to evaluate the influence of PH on the effectiveness of the refining agent according to the invention, the PH of the same test plating solution as described in Example 2, containing the same level of contaminant metals, was changed to ammonium hydroxide. I added it to about 4. The solution was then treated in the same manner as in Example 2.
The analysis results of the liquid showed that iron ions were reduced to about 0.012g/, and nickel ions and copper ions were negligible. Therefore, approximately 96.2% of iron ions are removed, and virtually 100% of nickel and copper ions are removed.
had been removed. The results of Example 3 show that this additive is more effective at PH4 compared to approximately PH2 used in Example 2.

Example 4 The trivalent chromium ion concentration was 21.7 g/, the ammonium formate concentration was about 51.0 g/, the boric acid concentration was about 50.8 g/, and the remainder were the same components as those described in Example 1. A trivalent chromium test plating solution was prepared having a composition similar to that described in Example 1, except for the following. The pH of this plating solution was adjusted to approximately 3.5, but the contaminant ion concentration was approximately 0.298 iron ions.
g/; nickel ions were approximately 0.188 g/; zinc ions were approximately 0.047 g/.

Current density approximately 45ASF (4.8A/Dm ² ), bath temperature 145〓
The specimens, which had been plated with bright nickel to a thickness of approximately 0.3 mil (0.0762 mm) by plating for 10 minutes at (63°C) using a conventional Watt-type bright nickel plating bath, were rinsed with water and then rinsed with water. Cathode current density approximately 100ASF in contaminated trivalent chromium plating solution
(10.7A/Dm ² ) and a bath temperature of 75°C (23°C) for 3 minutes. The chromium film exhibited a rainbow color in the intermediate current density region and black streaks in the high current density region. This chromium coating was considered unacceptable from a commercial standpoint due to the large amount of contaminant metals contained within it.

Next, while continuously stirring the contaminated trivalent chromium plating solution, 3.8 g of sodium diethyldithiocarbamate was added and treated at 75°C (23°C) for 1 hour. After the solution was allowed to stand still, it was passed through a carbon filter and the remaining metal contaminant ions in the solution were analyzed. Iron ion is approximately 0.164g/, Nickel ion is negligible, and zinc ion concentration is 0.0004
It was found that g/. Therefore, about 45% of iron ions, about 100% of nickel ions, and about 99% of zinc ions were removed.

Nickel-plated specimens of the type described above were plated in the treated solution under the same conditions as applied to the untreated solution. A photosensitive chromium film was formed over the entire surface and had a uniform appearance.
This plating film was of commercially acceptable quality. This plating test further indicated that the presence of excess sodium diethyldithiocarbamate in the solution after processing had no adverse effect on the performance of the trivalent chromium plating solution.
This example demonstrates that the present invention is effective in regenerating a trivalent chromium plating bath contaminated with metal ions and having commercially unacceptable performance, and provides a satisfactory chromium coating. This clearly shows that the contaminated bath can be regenerated.

Example 5 A trial treatment of 4000 gallons (15 Kl) of an industrial trivalent chromium plating solution having the same composition as shown in Example 1 was conducted. The performance of this solution was degraded by the accumulation of iron and nickel contaminating ions during normal plating operations. The samples before and after treatment were analyzed for iron ions, nickel ions, and vanadium ions present as a reducing agent.

Before treatment (g/) After treatment (g/) Fe 1.310 1.000 Ni 0.501 0.351 V 0.238 0.238 The purification treatment was carried out by adding 30 gallons (113) of a solution containing about 350 g/ of sodium diethyldithiocarbamate. The additive solution was added while the electrolysis was stopped. A dark precipitate formed immediately upon addition, but surprisingly this precipitate did not increase but rather decrease the filter backpressure. Therefore, removal of this precipitate was so easy that within about one hour after treatment, the liquor performance was restored and a commercially acceptable film was produced. Analysis of the precipitate recovered by filtration revealed that it contained approximately 66.7 mol% iron diethyldithiocarbamate and 33.3 mol% nickel diethyldithiocarbamate. This additive was found to selectively extract only contaminating metal ions, as no vanadium and chromium were extracted by this treatment. Similar tests using ferrocyanide additives have been found to remove significant amounts of both chromium and vanadium, which are useful components in trivalent chromium plating solutions.

Since it is clear that widely different embodiments may be constructed without departing from the spirit and scope of the invention, the invention is not limited to the particular embodiments thereof, except as limited in the appended claims.

Claims (1)

[Scope of Claims] 1. A method for purifying an aqueous trivalent chromium plating solution containing a lethal amount of contaminating metal ions, wherein dimethyldithiocarbamate ions, diethyldithiocarbamate ions, and mixtures thereof are present in the contaminated plating solution. A sufficient amount of a soluble, compatible refining agent selected from the group consisting of: is added to uniformly disperse the contaminating metal ions to precipitate at least a portion of the contaminant metal ions to a level that restores satisfactory performance of the plating solution. A purification method comprising the steps of reducing the concentration of reactive metal ions and then removing precipitates from the plating solution. 2. The method according to claim 1, characterized in that dimethyldithiocarbamate ions and diethyldithiocarbamate ions are added in the form of their alkali metal salts and ammonium salts which are soluble and compatible with the plating solution. . 3. Process according to claim 1, characterized in that dimethyldithiocarbamate ions and diethyldithiocarbamate ions are added as their sodium salts. 4. Process according to claim 1, characterized in that dimethyldithiocarbamate ions and diethyldithiocarbamate ions are added in the form of an aqueous solution. 5. The method according to claim 1, characterized in that the removal of the precipitate is carried out by filtration. 6. The method of claim 1, wherein the refining agent is added at a weight ratio of approximately 8:1 refining agent to contaminant metal ions.