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Historical Articles

October, 1953 issue of Plating


Evaluation of Carbonate Removal Methods

Presented at the 40th Annual Meeting of the American Electroplaters Society, June 16, 1953.

R. Scott Modjeska, Scientific Control Laboratories, Chicago 7, Ill.

At approximately fifteen-year intervals the subject of the removal of carbonates from cyanide solutions is brought before the members of the American Electroplaters’ Society. The last series of papers were presented in 19371 and 19382 by R. O. Hull. In his introduction in 1937, which sums up the problem neatly he said, ”One of the results of the operation of cyanide plating solutions is the inevitable tendency toward accumulation or ‘building-up’ of carbonate in the solution”.

Also listed by that author were the principal reactions which produced carbonates within the bath and although these reactions have been printed and reprinted, they are still worthy of repetition. They are:

2NaOH + CO2 —> Na2CO3 + H2O
2NaCN + 2H2O + 2NaOH + O2—> 2Na2CO3 + 2NH3

NaCN + 2H2O —> HCOONa + NH3
2HCOONa + O2—> Na2CO3 + CO2 + H2O

2NaCN + H2O + CO2—> Na2CO3 + 2HCN
2HCN + 4H2O —> 2HCOOH + 2NH3 etc,

Hull’s presentation covered also the precipitation of carbonates through the use of calcium sulfate. The quantitative precipitation rates are recorded1 in the 1937 proceedings of the Society and need not be repeated here. However, as a part of the preparation for this presentation, some of Hull’s work was spot checked. As expected, no disagreement was found. In his presentation he mentioned also some of the objections to other materials used as precipitates in plating solutions.

At the presentation of the subject paper in 1937, Dr. Blum asked questions as to the relative undesirability of sulfate ions or carbonate ions in cyanide plating solutions. The following year, in discussions during the presentation of papers, Dr. Blum indicated that it took Hull a year to find an answer. Hull replied, ”It didn’t take a year to have the answer, but it took a year to prepare a paper for answer”. And, as recorded in the 1938 proceedings of the A. E. S., he presented2 a paper on the effects of carbonates and sulfates on the plating characteristics of cyanide solutions.

Subsequent to the presentation of these two papers, DuPont, Udylite, MacDermid, and Lea, in the writing of operating manuals for their proprietary processes, have discussed carbonate removal methods.

The question may be asked, ”If there has been so much written about carbonate removal, why then, a paper at this time;” Two factors have influenced the decision to review the situation with respect to carbonates. First and foremost is the increased usage of potassium salt solutions. Second, the fifteen-year cycle seems to parallel the rise from foreman plater to plant superintendent, and that leaves the foreman plater receptive to the information with which his predecessor has already been presented.

Carbonate Additions in Laboratory Tests
In the laboratory work preceding the writing of this paper, carbonate increases in cyanide solutions were accomplished by the following methods:

A. Direct addition of the carbonate (either the potassium or sodium salt, depending upon the particular bath)
B. Prolonged aeration
C. Prolonged aeration plus heating
D. Electrolysis
E. Acid additions, to simulate the drag-in caused by inadequate rinsing

Panel Plating Tests
Panels were plated both in Hull Cells and in experimental tank set-ups, using solutions treated by the above methods. Since certain of the methods resulted in a change in concentration of the cyanide, this variable was maintained at as nearly a constant concentration as possible, by frequent analyses.

In all of the solutions, save those with acid additions, the resulting panels were comparable. In other words: the means through which the carbonate increment was achieved had no bearing upon the quality of the deposit obtained. For example: in some instances the solution was evaporated almost to dryness, re-diluted, cyanide concentration was adjusted and a satisfactory plate was still obtained.

In acid drag-in tests, sulfuric acid had little if any effect upon the deposits. However, phosphoric acid, nitric acid or hydrochloric acid exhibited the effects that these ions characteristically produce as contaminating impurities in cyanide plating solutions.

Table I. Range of Carbonate Concentration Limits*
Lower Limit
Recommended Upper Limit
Objectionable Limit


*Values given in oz/gal

Carbonate Concentration Limits
In considering the removal of carbonates one must first determine what is an objectionable concentration. Mather reported3 on the advisability of having carbonate present in the electrodeposition of copper from cyanide solutions. Watts, informally, referred4 to carbonates in the electrodeposition of silver in 1920. Graham spoke5 on the advisability of carbonates in Rochelle salt copper solutions in 1937. Various reporters have listed6 different upper and lower limits for carbonate concentration while observations at this laboratory have indicated still different tolerances.

These figures differ somewhat from the concentrations listed7 by other workers. However, based upon frequent observations of more than a hundred different plating operations, these values may be considered as representative. The values listed as objectionable are for concentrations at which lower cathode efficiency, anode polarization, poor deposit characteristics demand attention. The values listed as upper limits are the concentrations which should be maintained in controlled production.

The effects of high carbonates are well known, but they may be listed as follows:

1. They increase anode polarization
2. Reduce cathode efficiency
3. Reduce conductivity (increase resistivity)
4. Reduce bright current density range
5. Increase tendency to spotting-out
6. Cause spongy deposits (particularly from gold, silver and indium solutions), and
7. Increase the viscosity of the solution thus causing greater drag-out losses

Curves showing the relationship of carbonates to anode polarization, and carbonate concentration—specific resistivity relationship have been published8 and are included as a part of operating instructions for certain proprietary solutions.

While acknowledging the necessity for some carbonates plus listing the limits of concentration as well as the effects of high concentrations, one wonders about methods for the prevention of carbonate increase. In the main, the prevention methods that have been presented are of sufficiently little value that one is justified in passing on to methods for carbonate removal.

Carbonate Removal Methods
Methods for the removal of carbonates may be divided into the following groups:

1. Freezing-Out
2. Chemical Precipitation A. Calcium Sulfate B. Calcium Hydrate C. Barium Hydrate
3. Acid Treatment
4. Dilution

” Freezing-out” is probably the oldest method known but is not applicable to potassium salt formulations due to both the greater solubility of the potassium salts, and to the fact that the potassium compound with its high water of crystallization content can be formed only under very special conditions. Theoretically only the sodium carbonate and some complex iron cyanides will separate out from the solution through freezing.

Various researchers report differing temperatures for freezing-out. Some workers suggest9 40° F, others suggest10 32° F; however, the majority suggest a temperature of 26° F. Observations on the precipitation of carbonates from brass and copper solutions indicate that 40° F freezes out approximately two ounces of carbonates per gallon of solution when this lower temperature is maintained for eight hours. Though the removal of only two ounces of carbonates generally is insufficient, the amounts of metal and cyanide retained in the crystalline mass at this temperature seem to be at a minimum. Freezing can reduce the carbonate concentration to as little as five to six ounces per gallon and some workers report11 reduction to as low as three ounces per gallon, but in the author’s experience it has not been possible to reach this low figure.

In practice the amount of metal and cyanide lost by the freezing method makes the process highly comparable to dilution. Solutions treated by freezing methods were found to yield satisfactory deposits when diluted back to the original volume and electrolyzed though, as indicated, some loss in metal was observed. Freezing, followed by immediate decantation, resulted in losses of metal as high as 80 per cent. However, observations in tests by the author indicate that this same method in industrial practice has not resulted in losses this great but has averaged approximately 10 percent. The freezing technique may be used with all sodium formulation solutions though with precious metals the carbonate crystals should be treated for precious metal recovery rather than being discarded as are the common or base metal sludges.

Precipitation with Calcium Sulfate
Precipitation with calcium sulfate has been very well covered by Hull, but since this paper is intended to evaluate, it must be noted that: while calcium carbonate precipitates quite readily and in approximately equivalent concentrations, the addition of the sulfate ion is, in many cases, objectionable. The sulfate ion is objectionable particularly if the solution is contained in a steel tank. In Rochelle salt or tartrate formulated solutions the presence of the sulfate ion appears to have a beneficial action. Small concentrations of the ion were found not detrimental to the deposit in any of the solutions examined.

The removal of the sodium sulfate from solution is accomplished through refrigeration of the solution to 40° F. It has been claimed12 that sodium sulfate is removed sufficiently well over a temperature range of from 40° to 50° F but practice indicates that the lower temperature is more desirable. Less metal and cyanide were lost in the process of freezing-out the sulfate ion than in the carbonate ion separation. In sodium solutions some carbonates were removed by the freezing out of sulfates but no repetitive results were obtained.

Opponents to the use of calcium sulfate as a precipitant will argue that: due to the necessity of alkali sulfate removal it is, in effect, a two step ”freezing-out” method. Of course, they are correct theoretically but incorrect practically since the freezing-out will not remove the carbonates in a potassium bath. Again the separation of the sodium sulfate is easier than is the separation of potassium sulfate due to the lower solubility of the sodium salt.

Precipitation with Barium or Calcium Hydrate
The majority of suppliers of proprietary processes list either barium or calcium hydrate as the corrective material for their particular baths. Precipitation of the carbonate with either barium hydrate or calcium hydrate is especially desirable-since they introduce no foreign anion into the cyanide solutions. The carbonate is precipitated quantitatively and, since, the majority of solutions to be treated operate with a free caustic content, the additional hydroxyl ion is not objectionable. However, in the event the pH of the solution is raised too high or the concentration of free caustic exceeds the operating limits of the bath (this seldom happens), electrolysis with insoluble anodes or additions of carboxy acid salts may be used to reduce the alkalinity.

In a comparison of costs, barium hydrate is approximately twelve times the cost, per pound, of chemical lime, and, due to the difference in molecular weight of barium and the waters of crystallization in barium hydrate, the per pound cost of carbonate removed by barium hydrate is approximately 48 times the cost of carbonate removed through the use of calcium hydrate.

Experimental evidence supports the recommendations that frequent small reductions of carbonate (2 to 3 ounces per gallon) are advisable since they are more easily and more efficiently accomplished. In general, precipitation methods result in little or a negligible loss in cyanide and metal.

In brass or copper solutions which frequently have the tartrate ion as a part of their make-up, a high carbonate concentration can be reduced through the addition of tartaric acid or potassium acid tartrate and the resultant di-alkali tartrate will be beneficial. ADEQUATE VENTILATION must be observed in acid treatment and, naturally, there are justifiable limits to the tartrate concentration.

Dilution of any plating bath is usually the last resort and though the existence of the method is acknowledged, it is preferred not to consider dilution as a production tool except where the losses encountered can be made up by gains in production.

As a result of experimental data and observations of plant operations, it is the author’s opinion that calcium hydrate is the most efficient and, thanks to advances in filtration techniques, the simplest method for the removal of excessive carbonate concentrations from either potassium or sodium formulations.

The removal of carbonates by the use of calcium hydrate proceeds according to the reaction:

M2CO3 + Ca(OH)2 2MOH + CaCO3

Where M is either sodium or potassium. Wherein: 0.70 ounce of calcium hydroxide Ca(OH)2

Removes: 1.0 ounce of sodium carbonate Na2CO

Liberating: 0.75 ounce of metal hydroxide MOH

The procedure for precipitating the carbonate is as follows:

1. The quantity of carbonate to be removed and the amount of lime required for its removal are determined analytically.
2. Solutions are heated to 170°-190° F, with the optimum desirable at 175° F; then slowly a slurry of the lime is added to the plating solution.
3. The bath is agitated for at least two hours.
4. Then the precipitated calcium carbonate is filtered out.

Investigation of methods for the reduction of carbonate in alkaline tin solutions resulted in no completely satisfactory procedure as all treatments reduced the tin-concentration by an objectionable amount.

Appreciation is expressed to John Pawlak, head of the analytical laboratory at Scientific Control Laboratories for his cooperation in obtaining the corroborating data necessary to this presentation.

1. R. O. Hull, Proc. Am. Electroplaters’ Soc. 25,164-172 (1937).
2. R. O. Hull, Proc. Am. Electroplaters’ Soc. 26, 190-206 (1938).
3. F. C. Mather, Trans. Electrochen. Soc. 33 147 (1918).
4. O. P. Watts, Reference to year made erbaily by Dr. Watts in Milwaukee Wis., April, 1937.
5. A. K. Graham, Metal Industry, p. 559, November, 1937.
6. D. Hartshorn, et al, Proc. A’n. Electroplaters’ Soc. 26, 102113, (1938).
7. U(lylite Corp., Operating Manual for Cadmium.
8. A. I-lirsch, et al, Proc. Am. Electroplaters’ Soc. 25, 70-77 (1937).
9. G. B. Hogaboom, Metal Industry Vol. 12, No. 1, 1914.
10. G B. Hogaboom, Metal Indlstry Vol. 35, No. 2, 1937.
11. E. L. Gann, iTerbal Discussion at Chicago Branch A. E. S. Meeting, 1935.
12. R. O. Hull, Proc. Am. Electroplaters’Soc. 26, 190-206 (1938).


MR. E. V. COLLINS (Chromium Corporation of America, Waterbury, Conn.): What method do you recommend for removal of carbonate from silver baths?

MR. MODJESKA: The use of calcium hydroxide has been most satisfactory. Most of the new formulations are operated at high pH, particularly potassium, depending upon the formulation.

MR. J. V. McCANDLISH (Bar-Rusto Plating Corporation, Kansas City, Mo.): Recently, in the literature, a process was described for using carbide for the removal of carbonates. Can you give us any information on that?

MR. MODJESKA: I saw the article—in fact, several articles; in this investigation we did not consider the use of carbide.

DR. LANCY (Ellwood City, Pa.): I would like to answer the previous question and say that calcium carbide may be the very best of the precipitating agents for carbonate. Naturally, with the calcium carbide addition, acetylene gas is generated and an explosion hazard is on hand, thus the additions have to be made judiciously. Just as with calcium hydroxide additions, normally we do not precipitate 2 to 3 oz/gal of carbonate at once, but do it in small increments from day to day. The acetylene gas has to be mixed up with the surrounding air in such a way that we do not generate an explosive mixture. Practical experience indicates that by limiting the additions to 1 lb of calcium carbide to each 5 square feet of tank surface area the danger of explosion can be avoided constantly. The additions should be made when operations are shut down for at least eight hours.

MR. MODJESKA: The producers of several of the proprietary baths have made definite statements against the use of carbide because as the acetylene is given off, it does give the possibility of side reaction with the addition agents.

DR. LANCY: Some carbide has a sulphide impurity present. I would say that no danger did I ever find with its use in the proprietary zinc, or copper plating baths. With silver, gold or brass solutions I would not want to venture to say what would happen.

MR. MODJESKA: In the proprietary high speed coppers particularly, I am thinking of Lea’s—I do not want to give any one producer a plug, but I do know that if an operator of a Ronal solution should use some calcium carbide, I am sure the supplier would disclaim any credit or blame for the solution not working.

MR. JULIUS TERES (U. S. Air Force, Wright-Patterson Air Base, Dayton, Ohio): What grade of lime do you use?

MR. MODJESKA: There are one dozen manufacturers to choose from. Any good chemical lime will suffice so long as it is up to chemical specification.

MR. NATHANIEL HALL (Metal Finishing, New York, N. Y.): The subject of precipitation of carbonates, of course, is of interest to practically every plater using a cyanide solution. We have discussed many methods of precipitating carbonate, but I think what the average plater needs is a description of a good method to get calcium carbonate out of the solution.
You speak of precipitating a couple of ounces per gallon at a time in order to prevent an increase in hydroxide concentration, which is very good. But, even the matter of three or four ounces of carbonate, from a practical standpoint, involves so much of a labor charge in connection with cleaning filters, cleaning the bottom of tanks, loss of-plating solution, which is practically unavoidable; when you get all through, I think you can question whether it pays to precipitate carbonates or whether it pays to dump some solution and reduce the carbonates that way.

MR. MODJESKA: In the paper proper, we refer to the use of dilution as method of carbonate removal. However, the reason that the cost of filtration was not considered as a factor is that, as all of us know, the frequency of filtration is much higher now. In plants across the country, an ever-increasing number employ continual filtration. We have found that where the filtration is done either on a fixed schedule or is done continuously, the removal of the calcium carbonate is not considered as objectionable. Under the old technique where the facilities were left in the corner until needed, and when something required filtering, 467 pounds of nickel had to be moved to get out the filter, then filtration was a problem. Tow, I would say that at least 50 per cent of the plating departments, counting in the job plater as well as manufacturing firms with plating departments, have some filtration going on all the time. I think we can accept the advances in filtration techniques to offset the costs of removing the calcium carbonate.

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