Historical Articles

September, 1953 issue of Plating

 

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STABILITY OF ION EXCHANGE RESINS TOWARD CHROMIC ACID PLATING SOLUTIONS*

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C. F. PAULSON AND G. H. SAUNDERS

*Presented at the Fortieth Annual Convention of the American Electroplaters’ Society June 16,1953

INTRODUCTION
Treatment of chromic acid solution by cation and anion exchange is now an accepted process in plating plants. Costa first published¹ the results of his preliminary experiments on cation exchange in February, 1950. It was soon thereafter that the authors’ company installed the first commercial scale unit of this type at Grumman Aircraft Engineering Co., Bethpage, N. Y. In the short span of three years, the process has become widely recognized and appreciated. Now, more than a dozen plants are using this process on a commercial scale and numerous others are evaluating it for their purposes.

The cation exchange process was discussed² at the 39th Annual American Electroplaters’ Society meeting and some of its strong and weak points described. Essentially, a cation exchanger in the hydrogen cycle is used to remove metallic cations from chromic acid solutions, substituting hydrogen cations and reconstituting the chromic acid from its salts. When the cation exchanger becomes exhausted, it is regenerated by passing a strong acid solution through it.

Discussion
Costa reported¹, upon his tests with a commercially available sulfonated polystyrene cation exchanger. He found that in solutions containing 100 g/l of CrO₃ or less, removal of metallic cations was essentially complete. In solutions containing above 100 g/l of CrO₃, the effective capacity and degree of completion of metal cation removal were both reduced. In solutions containing more than 150 g/l of CrO₃, he found attack upon the resin, as evidenced by formation of trivalent chromium. He tested anodizing baths, copper stripping baths and chromium plating baths, both with and without dilution.

Use in Anodizing Plants
This basic process is most applicable in anodizing plants where solutions containing less than 100 g/l CrO3 are customarily used. Dilution is therefore not necessary. The acceptance of the process has been excellent and it will probably not be long before all anodizers of appreciable size will employ cation exchange. At frequent intervals, a portion of the bath is withdrawn and passed through a cation exchange unit and then back to the anodizing bath. The cation exchanger is operated beyond breakthrough to exhaustion as complete removal of the cations from the anodizing bath is not desirable. The chromic acid remaining in the cation exchange bed when the bed becomes exhausted is displaced by water and returned to the anodizing tank. The small amount of dilution which results can generally be tolerated since it makes up for evaporation and dragout losses.

Use in Electroplating Plants
Electroplating operations, on the other hand, have necessitated a more complex treatment method due to the inability of the cation exchangers to withstand 400 g/l CrO₃ solutions. In 1952 the recovery of a hard chromium plating solution was described² at the technical sessions of the 39th Annual Meeting of the American Electroplaters’ Society. In that report the authors stated that the plating solution was diluted, passed through the cation exchanger, and then all of the resulting solution containing chromate was concentrated back to plating strength in a glass-lined evaporator.

The need for the evaporator, unless it is also necessary to concentrate the save rinse solution, places a very heavy capital load upon the process. The result has been that many decorative chromium plating’ plants have felt that it was better to tolerate the high metal cation content in their baths than to try to take it out. This is done in spite of the greatly increased electrical efficiency of the treated bath. Hard-chromium platers, however, find the process economically feasible because they can tolerate less impurities in the bath without an adverse effect on throwing power.

EXPERIMENTAL EVIDENCE
The desirability of treating a plating-strength bath directly by cation exchange was obvious, so the authors undertook to investigate the possibility of developing a practical process. First tests were run with a sulfonated polystyrene cation exchanger (Permutit Q). It was found that reduction of the metal content in 400 g/l CrO₃ solutions was possible but that the capacity was low; above 200 g/l, there was some resin degradation as evidenced by the reduction of CrO₃ to Cr⁺⁺⁺, and bleaching and swelling of the exchanger.

Modified Resin Used
The authors, having established that this resin was not sufficiently stable, developed several cation exchangers for handling 40 percent CrO₃ solutions which gave promise of greater oxidation resistance. One of these identified as a modified sulfonated polystyrene (Permutit QC) performed well when tested. Its capacity for metals in chromic acid was almost identical with that of the resin first used, but no physical or chemical breakdown was encountered using 400 g/l CrO₃ solutions.

The solution used in these tests contained approximately 10 g/l of copper. In typical runs, during which the breakthrough was considerably overrun, the copper concentration was reduced by 200 per cent. No Cr⁺⁺⁺ was encountered in the effluent. Up to 14 cycles were run on individual resin samples and there did not appear to be any reduction in capacity.

Method # 1
Tests were run in two different manners. In the first, several volumes of strong CrO₃ solution were allowed to percolate through a bed of the modified exchanger. The water initially in the bed was displaced to waste, then a quantity of solution equivalent to that introduced was collected by draining the bed. This resulted in the loss of some CrO₃ held up in the modified resin.

Method #2
The second method was essentially the same except that after introduction of the strong CrO₃ solution, water was introduced and allowed to percolate down through the bed displacing the CrO₃. This resulted in the overall dilution of the influent but recovery of all the chromium. Two parallel tests are shown in Table I.

The mechanism of ion exchange depends upon diffusion of a portion of each ionic constituent into the ion exchange particle. Initially a regenerated particle is filled, much like a tiny sponge, with water. When the chromic acid is introduced, part of the CrO₃ enters the particle and displaces some of the water. Thus the exhausted resin particle contains some CrO₃. Introduction of water reverses the equilibrium and the CrO₃ is removed from the particles.

For Method 1, the apparent holdup amounted to 140 grams CrO₃ per liter of resin. It required an additional 1.2 liters of water per liter of the modified resin to recover this CrO₃. To recover 811 of the chromium in a typical experiment would result in 25 per cent dilution of a 400 g/l solution. Otherwise, losses would approximate 10 per cent of the CrO₃. The losses are in direct proportion to the degree of contamination of the CrO₃ solution, since the volume treated is in inverse proportion to the contamination

Regenerant Tests
Tests were made with the original and the modified sulfonated polystyrene cation exchangers to determine the effect of regenerant upon capacity. Sufficient confirming data were developed upon the special resin to indicate that results were almost identical with those obtained for the conventional material. Increasing regenerant dosage increased capacity, although not proportionately. Increasing regenerant concentration from 10 per cent to 30 per cent decreased capacity. Hydrochloric acid was found to be a more efficient regenerant than sulfuric acid in terms of pounds, although in terms of dollars such may not be the case. One major advantage of hydrochloric acid is that a higher capacity is achieved. A higher resin capacity results in less dilution or chromate loss, depending upon which operating method is resorted to. Hydrochloric acid is warranted where a low initial cost is preferred to a minimum operating cost.

Fig. 1 Removal of metals from chromic acid plating solutions. Effect of dilution upon operating cost.

The cost of the hydrochloric acid regenerant is approximately 1/3 of the value of the recovered CrO₃. Thus the treatment is economically very attractive, particularly in locations where there are waste disposal problems.

Operating Costs Data
The operating cost of treatment of a strong chromic acid solution at any concentration is the sum of the costs of regenerant chemicals, and steam for evaporation back to the initial concentration, plus amortization of ion exchange unit and evaporator. The authors surveyed the effect of the concentration to which an initial 400 g/l solution as given in Table II is diluted upon the operating cost of the overall treatment.
The minimum dilution possible is 25 percent so the figure covers only lower concentrations. Thus in operation the water used to displace the CrO₃ from the modified resin at the completion of one cycle would be used to dilute the plating solution prior to the next cycle. More water could obviously be added to give greater dilution. Any concentration below 300 g/l can be treated.

Fig. 1, based on the data shown in Table II, indicates that treatment should be carried out at as high concentration as possible. Some items were neglected (such as labor, electrical power and cooling water) but these are either minor or not changed appreciably with concentration. If one assumes the bath must be dumped when it reaches the concentrations of cations listed above (because, for instance, of its poor throwing power) the yearly replacement cost of chromic acid without ion exchange would be $19,850. Thus the treatment cost at 300 g/l of $7,700 would be rapidly repaid.

Anion Exchanger Stability
There has been considerable discussion on the stability of anion exchangers when treating dilute chromic acid rinse solutions. The use of ion exchange for recovery of chromate from rinse waters and purification of these rinse waters has much to recommend it. Compared with conventional waste disposal processes, it is the only method which pays its own way in terms of the chromic acid recovered. Its application automatically results in the formation of demineralized water which is excellent for rinsing purposes.

The authors have discussed previously³ the accelerated laboratory and plant scale tests that were made to determine the stability of a highly basic anion exchanger (Permatit S), when contacting chromic acid. Table III shows the relative stability of that basic anion exchanger in acid and alkaline solution. At 1000 ppm CrO₃ content and below, the resin maintains its operating capacity exceptionally well over long periods of time either in acid or alkaline solutions.

Fig. 2. Stability of a highly basic anion exchanger. Life test on contact with chromic acid.

Leakage
Long term tube tests-also have been made at various CrO₃ levels to confirm these findings. Fig. 2 shows the operating capacity of the basic anion exchanger when treating a solution of 500 ppm CrO₃, 5 ppm SO₄ and 10 ppm Cr⁺⁺⁺ expressed as per cent change in original capacity. It was found that leakage of CrO₃ through the resin increased slightly as the test proceeded. However, it was always below 10 ppm. Since the water was recirculated in a closed cycle, the leakage did not result in a loss in CrO₃ nor did it constitute a waste problem. This concentration should be satisfactory in a final rinse bath. A rinse water of 500 ppm is higher than would occur in most operations thus the leakage found in the test is greater than actually would be encountered. The leakage is a function of the influent concentration so that in lower concentrations the leakage would be negligible.

Plant Scale Experience
Plant scale experience in many locations generally has confirmed these laboratory results. It has also been found that in several locations where the treatment was carried out at elevated temperatures there was little or no harm to the resin.

Fig. 3. Chromate recovery flow diagram.

The authors conclude from the laboratory and numerous plant scale installation tests, that the method of operation shown in Fig. 3 is most satisfactory. The rinse-water is withdrawn and passed directly to the highly basic anion exchanger where the chromate is removed. No neutralization prior to anion exchange treatment seems necessary. The effluent from the anion exchanger goes directly to the sulfonated polystyrene cation exchanger where traces of cations are removed and then the demineralized water is returned to the rinse tank.
When the anion exchanger becomes exhausted, caustic soda solution is sent in series through it and the cation exchanger. The effluent is H₂CrO₄ which can be used as produced, or concentrated for addition to the bath.
The recovery process is of value to the plater and metal finisher in several important respects. It eliminates a serious waste; it recovers a valuable chemical; it improves the quality of work..

LITERATURE CITED
1. R. L. Costa, Ind. Eng. Chem. 42, 308 (1950).
2. L. Giert, W. S. Morrison and F. H. Eahler, ”Use of Ion Exchange Resins in Purification of Chromic Acid Solutions”, Proc. Am. Electroplaters’ Soc. 39, 31(1952).
3. Purdue Univ. Eng. Bull., Extension Ser. Vol. 36, No. 6, 209, November, 1952.

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DISCUSSION

MR. GILBERT (Rock Island Arsenal, Rock Island, Ill.): I note from Table I of your paper that no mention was made of trivalent chromium ion. That is, by far, the foremost contaminant of most hard-chromium plating solutions. I wonder if you could give us some idea of the capacity of the resins in terms of equivalents.

MR. PAULSON: The capacity of the resins for trivalent chromium is comparable to their capacity for copper and other cations. In treatment at high concentrations, incomplete removal of cations is always achieved, so there is no differentiation. In the operations where complete removal is desired, it is more difficult to get complete removal of trivalent chromium.

MR. GILBERT: Then, in other words, there is bed leakage of trivalent chromium through the system.

MR. PAULSON: Yes, in all cases, in these concentrated solutions, there is leakage of every cation.

MR. GILBERT: Could you give us some idea of the total capacity of this new resin in terms of equivalents?

MR. PAULSON: As I said, it is comparable to the previously used resins in dilute concentrations and falls off in more concentrated solutions.^{1 2 3}

MR. GILBERT: In other words, around 2 normal?

MR. PAULSON: We have found that normality bears a much smaller effect than the pH of the influent solution. The pH of the influent solution is a major criterion. Above ,pH approximately 0.22-0.25 the capacity is rather high; whenever you get below that pH, whether in concentrated solution or dilute solution which contain a small amount of contaminants, the capacity falls off.

MR. GILBERT: Under those circumstances, would it not be true that the efficiency of the recycling of the solution would drop with the increasing purity of solution, In other words, the law of diminishing return would prevail.

MR. PAULSON: The capacity falls as the concentration rises. However, as I pointed out, the overall treatment becomes more attractive as the concentration rises.

MR. GILBERT: I note that you recommend the use of hydrochloric acid as a regenerant. Does this not seem to be a hazardous procedure in view of the fact that while leakage or inadvertent introduction of sulfuric acid to the bath can be removed, the chloride ion is much more difficult to remove from the solution?

MR. PAULSON: We think that is a function of equipment design. With proper design, no chloride will leak into the bath. Also, as shown in the text, normally we do use sulfuric acid; it is only in the treatment of small baths where high CrO3 concentrations are desirable that we recommend the use of hydrochloric acid for its higher resulting capacity.

MR. GILBERT: What are the concentrations of the regenerants, sulfuric and hydrochloric acids?

MR. PAULSON: The optimum for both is between 10 and 25 per cent.

MR. H. A. FUDEMAN (Trico Products Corp., Buffalo, N. Y.): Have you studied the effect upon your resins of the fluoride type catalyst baths that are sometimes used in chromium plating?

MR. PAULSON: We have studied proprietary baths containing large amounts of fluoride such as are used in aluminum surface finishing and found there was no particular effect. A considerable amount of work is being carried out on that problem, and as far as we .know, no deleterious effect traceable to the fluoride ion has been found.

MR. FUDEMAN: What becomes of these catalysts in the process?

MR. PAULSON: Sulfate, chromate, and fluoride anions are all recovered by the anion exchangers; they are all unaffected by the cation exchangers. There is evidence to indicate that some: sulfate is held by the cation exchanger in a complex with trivalent chromium, but it is a rather minor effect as far as operating conditions go. The same holds true for high speed self-regulating baths which contain cations that have catalytic properties; those cations will be removed along with copper and other cations.

DR. LOUIS WEISBERG (New York, N. Y. ): Mr. Paulson, would you care to say how Permutit Q and QC compare in temperature resistance?

MR. PAULSON: Theoretically, QC should be more resistant. Since both are resistant to temperatures of 250° F, the question is an academic one when treatment of chromic acid solutions is concerned.

MR. J. M. ANDRUS (Croname, Inc., Chicago, Ill): Does the ion exchange system remove strontium which is used widely in chromium plating for self-regulating purposes?

MR. PAULSON: It would remove some of the strontium.

MR. GEORGE E. BEST (Mutual Chemical Company of America, Baltimore, Md.): As far as the two resins are concerned, Q and QC, what is the cost on a purely relative basis?

MR. PAULSON: The Permutit QC is about 20 per cent higher.

MR. BEST: With reference again to trivalent chromium, have you explored the chemical pre oxidation of trivalent chromium prior to cation exchange treatment?

MR. PAULSON: We have not explored that problem. However, since you must take the solution out of the plating tank for treatment in other equipment, the conditions are good for any oxidation process. Where the trivalent chromium is a relatively unimportant constituent, it is probably more economical to forget about oxidation. Where it is the main constituent, oxidation effects should be considered along with some simple method for avoiding them.

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