Historical Articles, September, 1949


by GORDON C. HARROLD, Consultant, Industrial Health, Hygiene and Safely, Service, Detroit. Michigan (published September, 1949)

There are principally two sources of pollution resulting from electroplating operations, namely (1) solutions and (2) mists, gases and vapors.

The methods of air dilution advocated by some industrial hygienists are not always applicable. Considerable investigation of various methods of trapping chemical substances from the air has shown that the ideal methods of collection for most substances are wet methods.

Some ways of using wet methods are cheaper and far superior to any other means. By the use of combinations of impingement, atomization and scrubbing, we have been able to solve problems involving gases, as well as particles in the size range of 0.1 micron, with efficiencies ranging from 90 to 95 percent and at costs approximately half of those involved in various commercially available pieces of equipment. This is in part because of the fact that commercial equipment is designed for all jobs and a premium for inefficiency is the result. In addition, some chemical substances are not easily collectible and then special equipment has to be designed for the specific job under consideration. For plating, we have used high-pressure sprays in various combinations and have found that the pressures required, as well as the amounts of water or other liquids consumed, can be kept within economical limits.

By the use of wet methods for mists, gases and vapors, there remain to be disposed only solutions. Generally speaking the problems that the electroplaters face are quite minor as compared to those of many other industries. There are a number of reasons for this: (1) the volume of chemical discharge is relatively small compared to that in such operations as pickling of steel; (2) the concentration of the solutions is usually quite low; (3) the chemicals to be disposed of can be handled fairly readily; and, (4) the diversity of operations in many electroplating shops are such as to provide a balance between alkaline and acid substances which, properly scheduled and disposed of, reduce the problems involved to considerable extent.

The usage of acid by platers is quite low and can in no way be compared with that found in steel pickling operations which involve huge volumes of low-percentage sulfuric acid. The latter operations do require reduction of acidity, and perhaps both acidity and ferrous salts content because of the extremely high immediate oxygen demand placed on the stream load by ferrous salts.

It should be pointed out that where treatment is required, the bringing together of various kinds of waste, properly chosen, can ameliorate conditions satisfactorily. For instance, pickling wastes require, in one type of treatment, the use of time and simple aeration. If the pickling people would look around, they would find various places where they could have lime for merely hauling it away. Thus lime is a waste product in many acetylene manufacturing plants.

This example is important because it emphasizes a fundamental point which should always be considered in the electroplaters disposal of waste: that the proper chemical balance of waste products can result in an acceptable waste. And one does not always have to go to different industries to collect a suitable wastes mixture that will arrive at sewage treatment points as a relatively easily handled material.

The complete finishing of metals by electroplating may include (1) polishing of the base metals, (2) cleaning by solvents and alkaline solutions, (3) pickling by acids, (4) metal disposition, (5) buffing of the deposits, and (6) lacquering. All know that the polishing and buffing operations, the lacquering operation, as well as any grinding operation that may have to be done, all produce solids which can be carried to dumps for disposal. That is certainly a reasonable means for the foreseeable future as long as atomic wastes are being disposed by that method.

In passing, practically any substance has value if that value can be found; in a properly balanced use of materials, there should be no waste of any sort to bc removed.

At any rate, the problems remaining to the electroplater concern the disposal of (1) solvents and alkaline solutions used in cleaning, (2) acids used in pickling, and (3) chemicals used in the plating baths.


In shops which use alkaline cleaning solution, the balance between the acids from the pickling operations (sulfuric, hydrochloric, phosphoric, hydrofluoric and other acids) and the alkali cleaners (containing caustic soda, soda ash, trisodium phosphate and other phosphates, as well as smaller amounts of other sodium salts, soaps, and wetting agents) may be made such as to produce a substantially neutral effluent. This may require proper and judicious scheduling, under rigorous control, of the different waste solutions. It may even be better to provide a waste tank for alkaline and acid liquids which are released to the sewer system only at such times as neutrality has been achieved.

It should be emphasized that we are not here discussing the plating baths, some of which contain cyanides; the plating baths should be considered in a separate category, with one or two exceptions. The outstanding exception is that where alkaline waste exceeds the acid waste and where some oxidation may be demanded, the chromic acid waste may be used to supply such deficiencies. An instance would be where there are alkaline tin solutions. These are strongly alkaline and need an excess of oxidant to prevent the appearance of sodium stannite which, in an effluent, would create high immediate oxygen demand. The example does point up the fact that the various wastes in any given electroplating shop may be used to provide an acceptable effluent without resort to large treatment plants.

Certain other materials such as hydrofluoric acid, if used in large quantities not common to the usual electroplating operations, can be disposed by lime treatment because the calcium fluoride formed is extremely insoluble; if proper arrangements are made, the material can be deposited on waste land with complete safety. This special situation is being brought up only to point out the ease with which such situations can usually be handled. In fact, the lime requirement in the instance just mentioned could be largely supplied by the waste from the acetylene manufacturing plant.

It should also be mentioned that some plating plants anodize or otherwise surface treat magnesium or aluminum and that salts of these metals are regularly used in the flocculation and purification of water supplies.


Before proceeding with a discussion of the chemicals from the plating baths themselves, it might be well to dispose of the lacquers, paints, etc., that are used in some plating shops. Where dry booths are used, there is no water pollution problem. However, in many instances it has been found advantageous to recover the pigment which would otherwise be thrown away is waste. The most successful method has been the use of water-wash, spray booths. The water carries the pigments which have not been deposited on the surface of the object to central sump tanks where they are allowed to settle. As a rule, most high boiling oils, as well as phosphates and other substances used as plasticizers, have very low solubility in water, have a tendency to stay close to the pigment in agglomerate particles, and are removed with it by filtration for reprocessing.


The one waste product from cleaning which should not, under any circumstance, be delivered to the sewer systems is the sludge from halogenated hydrocarbon degreasing operations. This has been well recognized by file electroplating industry. The sludge may be redistilled, and this in many instances is so economically mandatory that stills are sold as part of the original equipment.

However, one finally reaches a point where sludge consisting of from a few to many percent of halogenated hydrocarbon, grease, oil, acid, and a multitude of other products must be removed. This material may be disposed of by dumping or burning in a suitable area; it may also be burned in an incinerator.


The variety of chemicals used in electroplating is quite impressive and, on the surface, would seem make the problem of keeping waste water clean very difficult, particularly in view of the large amounts of wash water used. However, these very large quantities of water, much of it relatively free from chemicals other than those found in the normal supply, often provide a safety factor which in many instances reduces contamination to very safe levels, But this may not always be the case. For this reason, the more common types of plating baths will be mentioned with some remarks directed toward principles which have been, or can be, used in eliminating difficulties. In a few instances, at least, one waste may be balanced by another.

Among the acid solutions are (1) acid copper baths containing copper sulfate, sulfuric acid, and small amounts of addition agent; (2) acid zinc baths containing zinc sulfate, aluminum or ammonium chloride, and small amounts of organic agent; (3) nickel baths with various nickel salts such as the sulfate and the chloride, boric acid, and addition agents; and (4) the acid lead baths which may contain sulfamate, fluoborate, or possibly fluosilicate. In practically none of these plating solutions will be found really high acidities, and in a few instances, the materials could be dumped directly and meet current regulations without any dilution as far as pH is concerned.

The chromium plating bath holds a special position among acid baths. The high oxidizing power of chromic acid, its main constituent, may be extremely useful, particularly where discharged into open streams is reasonable quantities, because it relieves the immediate oxygen demand of other chemical wastes. Thus, the controlled delivery of chromic acid waste into other plating wastes should be a positive benefit rather than a detriment. This, of coarse, does not result if a bath which is no longer useful is dumped without consideration being given to scheduling of disposal with other acid and alkaline wastes.

There are special instances in which the chromic acid concentrations cannot be balanced or reduced by dilutions. Then special devices must be employed. In one instance chromic hydroxide, Cr(OH)2, was formed and precipitated; in another where lead wastes were available from other manufacturing, lead chromate was precipitated, eliminating two difficulties at once. (Both lead chromate and trivalent chromium salts are nontoxic according to our studies.)

It should be noted that chromates and phosphates, which are present in plating shop waste, under specified conditions form quite insoluble salts with several of the cations used in plating including load and zinc. Most other plating cations, such as cadmium, arsenic, mercury, silver, gold, etc., are found in such small quantities that, as a rule, they need not be considered in liquid-waste disposal. Should, for any reach, dilution not be sufficient, there are in most plating shops numerous other wastes which can simply and quickly reduce their concentrations to very safe levels.

Among the alkaline plating baths we have already mentioned the alkaline tin solution, and propose now to consider the various cyanide baths which are also alkaline. Thus, copper, zinc, gold, silver and cadmium are plated from alkali metal cyanide complexes of the metal, usually in the presence of free alkali and free cyanide. Some part of the wash waters may contain some 50-100 ppm of cyanide and, of course, the cyanide plating solution itself has a cyanide content of up to 25,000 ppm.

While dilution under carefully controlled conditions can be used, in some circumstances, for the disposal of cyanide wastes, it is not recommended as there is always the possibility of the material reacting with acids liberating hydrogen cyanide and, further, even under controlled dumping conditions the concentrations may not meet with regulatory approval. Neither do we favor use of the method advocated in some quarters in Michigan which involves the collection of all cyanide wastes, their treatment with sulfuric or other acid for liberation of hydrogen cyanide, and the delivery of this gas to the atmosphere through high stacks.

As mentioned earlier, it is our conviction that the waste inside the workroom should be collected and made innocuous if need be and not delivered to the outside air or to the outside water supply. There are a number of other ways of treating these wastes which are more satisfactory. Space does not permit a detailed discussion, but some of these ways can be indicated: (1) the cyanide wash may be delivered to lagoons if sufficient ground is available, (2) it may be treated with various sulfur compounds, or wastes containing sulfur compounds, perhaps with the addition of waste lime, and heated to form thioicyanate which may be deposited in lagoons or in porous ground areas, (3) it may be treated and oxidized to carbonate, (4) it may be oxidized with waste oxidizing chemicals and then delivered to the sewer system or, in special cases where the insoluble load from the plant or operation is too high, be precipitated by other wastes, and (5) it may be burned. Should there be no immediately available free ground space, the same principles can be applied in settling and filtering tanks within the premises. Should oxidation be resorted to within the plant, conditions of neutrality should be observed and the operation should he conducted in a closed, ventilated place which workmen are prohibited from entering as a precaution against accident.


The previous discussion may make one believe that the problems are difficult. Actually, a careful analysis will show that the only problem which might, in some circumstances, involve not a particular difficulty but an expenditure of money is the disposal of cyanide wastes. It will also show that this expenditure, in most instances, will not be large because existing extra tanks and ventilation facilities can be changed and adapted to cheap oxidation of the wastes. That has been done successfully in the past, perhaps with more expensive chemicals than are needed but still at very reasonable cost. In case of doubt one should analyze the residues in the stacks where cyanides have been heat-treated. It is difficult to find any cyanide; the white powder, in all the cases examined by the writer, turned out to be largely carbonate.

The progressiveness of the plating industry as exemplified by the early installation of ventilation and other safeguards before the public was aware that airborne or water-pollution problems might exist, and its alertness in meeting any new problem, will, without doubt, make it relatively easy to meet new reasonable demands.



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