Historical Articles

October, 1952 issue of Plating

 

Increasing Copper Content in White Brass and Copper Cyanide Solutions by Electrolytic Regeneration

Walter R. Binai
Chemical Engineer, Indianapolis, IN.

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WITH THE VANISHING of nickel and nickel chemicals from the civilian goods electroplating market, decorative plating consequently is being more and more replaced with chromium on bright copper as well as chromium on bright white brass. Both white brass, as well as cyanide copper solutions, require frequent additions of copper cyanide in order to maintain the copper concentration. Due to the scarcity of copper cyanide, copper concentrate made its appearance and became quite a popular substitute and indeed a life saver. The object of this paper is to illustrate a new method of introducing copper to the plating solution without the addition of copper cyanide or copper cyanide concentrate. It is accomplished by an electrolytic regeneration cell which will be described in detail later in this paper.

White Brass Plating
White brass is an alloy of zinc and copper plated from a cyanide solution containing zinc cyanide and copper cyanide and addition agents, and has a deposit composition of approximately 82 per cent zinc and 18 per cent copper. The white brass electrolyte is quite similar in composition to that of a cyanide zinc solution with the exception of the added copper cyanide and addition agents. One of the features of such a plated alloy, unlike that of bright zinc, is that it can be plated directly with chromium to produce a finish comparable to that of chromium over bright nickel.

In a white brass solution where only zinc anodes are employed, it becomes necessary to make additions of copper cyanide during the plating operation in order to maintain the alloy composition. Another difficulty experienced is that fluffy and spongey copper is cemented out on the zinc anodes which not only causes roughness but reduces the copper content in the plating solution.

Cyanide Copper Plating
Most proprietary bright copper solutions operate with a cathode efficiency near 100 per cent in order to produce a bright plate. Normally the anode efficiency of such a solution is also 100 per cent. When operating under such ideal conditions, the rate of metal going into the solution will be equal to that of metal plating out and the decrease in metal in the solution will be due to drag out only. Copper in a cyanide plating solution exists in a monovalent state. The rate of deposition or the rate of solution, however, is governed by the state of valency of the copper ions at the anode as well as the cathode film. Quite frequently the copper anodes become polarized, either due to insufficient area or too tight bagging or poor circulation of the solution. Under such conditions a copper hydroxide anode film is formed. When the anode thus becomes polarized with a bi-valent film, its efficiency is reduced well below that of the cathode. The copper concentration is rapidly decreased and the free cyanide increased along with an increase of carbonates due to cyanogen decomposition. A sharp rise in carbonate content is a good indication of anode polarization and a warning of potential solution decomposition.

Decomposition Products
When the rate of metal going into the solution lags behind that of metal plating out, then the free cyanide content will increase with the liberation of free cyanogen. With the presence of free alkali the cyanogen serves to form cyanides as well as cyanates. Howerer, after the free alkali becomes depleted the cyanogen will gradually polymerize into compounds that are highly unstable with an ultimate liberation of ammonia and carbon dioxide. These unstable organic compounds have a bad influence upon the addition agents in the solution, and impair brightness and throwing power. Such organic contamination derived from cyanogen decomposition will foul the solution beyond use in a short time.

Electrolytic Regeneration Cell
The use of insoluble anodes in cyanide solutions is a commonly accepted practice to prevent metal build up in solutions where the cathode efficiency is low. Where insoluble anodes are used altogether, provisions must be made to introduce metal ions to the solution, not intermittently but constantly, at the same rate that metal is plating out. The copper content of white brass or cyanide copper baths can be maintained by the use of an electrolytic regeneration system. The electrolytic concentration cell is an apparatus which will introduce copper ions to the electrolyte at a rate that can be controlled to fit individual requirements.

The use of diaphragm cells for producing metal in a plating solution is not at all new. For many years diaphragm cells have been employed in making gold and silver solutions. Ceramic pots such as alundum and aluminum silicate vessels have been used, but are fragile, hare poor porosity and hence, poor conductivity, and tend to result in the build up of’ nuggets which when broken off damage the cell. The silicate containers may foul the solution with objectionable silica which not only causes roughness of deposit but tends to polarize the steel anodes. An ideal diaphragm is one which has the following properties:

(a) Must have a high porosity per unit area
(b) The porosity must be of such a nature as not to permit metal ions to pass through.
(c) Must wet readily, i. e., have high capillary attraction.
(d) Must be mechanically strong.
(e) Must be impervious to the chemical action of the electrolyte.
(f) Must have a high electrical conductivity per - unit area.
(g) Must have the least voltage drop. Preferably below 3 volts when operating at approximately 75 amperes per square foot of cell area.

The writer has developed a cell which is more suitable add more economical for production use than the ceramic pots. This cell is rigidly constructed of rubber coated steel and is an oblong box, with approximate dimensions of 30 inches x 30 inches x 4 inches wide. It is provided with four windows on each side. Each window has an approximate opening of 100 square inches and these windows are so constructed that they will receive diaphragm panels and make a perfect water seal. Cypress was found to be a very satisfactory diaphragm medium. A cypress panel 100 square inches and 1/8 inch in thickness after being properly deresinized will conduct approximately 75 amperes. One cell has an ampere capacity of approximately 50000 amperes.

Deresinized Wood Diaphragm
The wood panels are soaked in a solution of 10-15 per cent potassium hydroxide solution at room temperature for a-period of 2-3 days. During this operation the panels are weighed down to prevent curling and warping. After this period the solution is drained ‘out and fresh water is allowed to circulate until it is clear of brown resins. The panels are stored under water until ready for use.

Effect of Temperature on Wood Panels
For silver solutions as well as white brass where the operating temperature is not in excess of 110° F, cypress has a reasonably long life. In cyanide copper where the solution temperature reaches as high as 180° F, cypress will deteriorate in a short time. In a copper solution, however, where the required rate of copper addition is very small, the regenerating tank required will also be quite small, perhaps less than 10 per cent of its total volume. It would be well to provide means of cooling the electrolyte in the regenerating tank. The solution temperature in the regenerating tank should not exceed 120° F.

Intermittent Regeneration of Metal in a Cyanide Copper Solution
After tho cell has been properly fitted with deresinized wood panels, an electrolyte of 2 oz/gal potassium hydroxide and 2 oz/gal potassium cyanide is placed in the cell. It is important at this point to examine the cell to be sure that no solution leaks out. It must be solution tight. The cell is then placed on the cathode rod of the plating tank. The wood diaphragm serves as a screen, when the current is turned on. Although metal is going into solution at the anode, metal will not pass through the wood panel and consequently remains in the plating solution. Hydrogen gas is evolved at the steel cathode within the cell. In order to raise the metal content to a given value, it would be necessary to calculate the total metal required in terms of Faradays. 1,000 ampere hours will electrochemically produce 7.37 lbs. of copper cyanide. Since the cell has an ampere capacity of 500 amperes, each cell will impart to the solution 3.68 lbs. of copper cyanide per hour or approximately 30 lbs of copper cyanide per eight-hour day. Assume that it is required to add 28 lbs of copper cyanide to a white brass solution during the shut-down period of four P. M. and seven A. M. the next morning; 28 lbs of copper cyanide represents 3800 ampere hours. Dividing 15 hours into 3800 ampere hours yields a figure of 254 amperes which is the required amount for the 28 lbs. needed. A single regeneration cell will be sufficient in capacity to do this.

Continuous Regeneration of Copper Cyanide
Either in a cyanide copper solution or in a white brass solution where it is necessary to replenish copper cyanide, it is more desirable to resort to continuous regeneration.: For continuous regeneration an auxiliary tank approximately one-tenth in size to that of the plating tank is provided. This auxiliary tank is connected with the main plating tank by means of an overflow weir and a suitable circulating pump or filter. The tank is equipped with copper anodes and a regeneration cell. It is also provided with a suitable rheostat. For example—a 3,000-gallon white brass solution with a current capacity of 3,000 amperes will require only one cell and the total current required for regeneration will not exceed 300 amperes. Electrochemically, if 90 per cent of the amperes were used .for zinc and 10 per cent used for copper, the ratio of metal entering the solution would be 82 per cent zinc and 18 per cent copper. This is true because the atomic weights of both metals are almost the same. Zinc is bi-valent and copper monovalent in the same solution. In actual practice,: however, one must take into consideration that the cathode efficiency of white brass may be as low as 65 per cent and the zinc anode efficiency is above 100 per cent. Sufficient insoluble anodes must be used to compensate for the low cathode efficiency. The anode area can also be manipulated so that the anode can be polarized sufficiently to balance the poor cathode efficiency. With the aid of the regenerating tank; the copper may be raised or lowered as required and the amount required can be calculated accurately. It assures solution stability and ;eliminates the periodic addition of copper cyanide. Fig. 1 illustrates the auxiliary regenerating tank completely: equipped with concentration cell, copper anodes, rheostat and circulating pump.

Fig. 1. Electrolytic Concentration System

 

SUMMARY
1. In a cyanide copper or a white brass bath, copper cyanide can be introduced by means of electrolytic regeneration. This can be done intermittently or continuously.
2. In alloy plating, one or more metals may be introduced to the electrolyte by means of electrolytic regeneration. Such additions may be controlled to equal rate of deposition.
3. To prevent cyanogen accumulation, cyanogen’ polymerization, carbonate increase, organic contamination, addition agent break down, it is desirable to replenish copper by means of electrolytic regeneration rather than by periodic copper cyanide addition.
4. In order to maintain a stable cyanide plating solution, it is necessary that the rate of metal plating out at the cathode be as near equal to the rate of metal entering the solution at the anode. This will minimize the free cyanogen concentration.
5. When wood diaphragms are employed, it is desirable that the solution temperature in the regenerating tank does not exceed 120° F. Wood, although an excellent diaphragm material, does not withstand high temperature. In cyanide copper plating where the operating temperature is around 160 to 180° F, it is necessary to provide cooling for the regenerating tank solution.

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