US2861927A - Process for adjusting the components in aqueous alkali cyanide electrolytes - Google Patents

Description

United States Patent Ofiice 2,861,927 Patented Nov. 25, 1958 2,861,927 PROCESS FOR ADJUSTING THE COMPONENTS IN AQUEOUS ALKALI CYANIDE ELECTROLYTES Myron Ceresa and James R. Crain, Penn Township, Allegheny County, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application April 25, 1956 Serial No. 580,458

4 Claims. (Cl. 204-44) The present invention relates to the treatment of electrolytes. and has particular reference to the treatment of aqueous alkali cyanide electrolytes to purify them and to adjust the components thereof to achieve improved a great excess which may amount to from 20 to 25 ounces of alkali metal carbonate per gallon of electrolyte. This is far above the desired carbonate content which usually varies from 4 to 8 ounces per gallon, and preferably not greater than 10 ounces per gallon.

Increase of the carbonate content in alkali cyanide electrolytes has been particularly severe in plating installations wherein the rinse water applied to the plated members to wash off the dragout cyanide solution is returned to the plating tank. Previously, it was a common practice to discard to a drain the rinse water containing the electrolyte dragout. Thus, in the operation of a conveyorized plating line, as previously practiced, the dragout losses were sufficient to equal a complete turnover of the electroplating electrolyte every 3 to 6 months on the average. Such dragout tended to keep the carbonate content of a cyanide electrolyte at a low level. Consequently, there was no problem of excessive carbonate build-up as there is today when the electrolyte dragout is returned to the plating tank.

Also, due to present day high production demands the operating conditions and components of the electrolyte baths have been changed to meet desired increased plating rates. Shorter plating times are used today than formerly and this necessitates operating the electrolyte at a higher plating current density. Higher-plating current densities usually require increasing the electrolyte temperature, applying more agitation to the electrolyte by air bubbling or by mechanical stirring, and increasing the concentration of the dissolved components in the electrolyte. These changes in operating conditions and compositions have been important factors in the increased rate at which the carbonate concentration is built up in cyanide electroplating baths. In a cyanide bath operated with air or mechanical agitation the oxygen and carbon dioxide in the air appreciably increase the rate at which the carbonates build up in the bath. Higher operating temperatures and higher concentrations of the components of the bath, especially the alkali metal cyanide and alkali metal hydroxide, also increase the rates at which these components are converted to carbonates in the cyanide plating bath.

Thus, the present day practice of returning the rinse water to the electroplating tank and the changes in operating conditions to meet higher production demands has resulted in a great increase in the carbonate build-up in cyanide electroplating baths.

It is well known that organic impurities tend to accumulate in plating tanks. Such organic impurities have ,been found in many cases to affect detrimentally'the quality of the electro deposited metal. Such organic contaminants may comprise material leached out of the tank lining and rack coatings. Air, contaminated with oil and organic dust particles, also contributes a substantial amount of organic impurities to the electrolyte, particularly where air agitation of the electrolyte is employed. In some cases organic matter may be added inadvertently or in some cases it may be the result of the decomposition of organic addition agents present in the electrolyte.

In many cases, the work being plated in the electrolyte drags in some amounts of organic and other impurities such as chromates and sulfates. The chromates and sulfates are generally undesirable components and should be kept at a very low concentration in the alkali cyanide plating bath.

The following reactions have been found to occur in the electrolyte and result in depletion of certain constitutents of the plating bath while simultaneously increasing the carbonate content.

H2O +CO2'9H2CO3 The above equations relate to the reactions of the respective sodium compounds but similar reactions apply to the potassium or lithium compounds. It will be observed that in Equation 1 sodium cyanide is depleted, while in Equation 2 sodium hydroxide is depleted. In Equations 3 and 4, sodium cyanide and sodium hydroxide are destroyed by the carbonate forming reaction.

It is well known that there is a specified range of proportions within which the components of an electrolyte must be maintained in order to produce the best plating results. If any of the components of an alkaline cyanide electrolyte such as the alkali metal hydroxide, alkali metal cyanide, or alkali metal carbonate depart from specification, then the plating operation will be less satisfactory and the quality and speed of plating will suffer in proportion to such departure from the desired proportions. Inasmuch as most of the carbonate forming reactions deplete the alkali metal cyanide and alkali metal hydroxide, there not only results an undesirable excess of carbonate but also a reduction of the free alkali metal cyanide present in the electrolyte and an undesirable change in the alkalinity of the electrolyte.

While it has been proposed in the art to add some one ingredient, such as gypsum (calcium sulfate), magnesium carbonate, barium hydroxide, or calcium hydroxide to an electrolyte in order to precipitate magnesium, barium or calcium carbonates therefrom, the treated electrolyte is not in proper balance or adjustment with respect to free cyanide or the hydroxide content. In some cases the addition. of such carbonate removing agents has required a plurality of succeeding adjusting treatments of the electrolyte in order to bring it into reasonable operating balance.

The addition of calcium hydroxide alone, for example, to a plating electrolyte has been found to result in a number of unsatisfactory features. In many cases, the calcium carbonate formed thereby is in the form of a colloidal or slimy precipitate that settles out of the electrolyte extremely slowly and is' dilficult to remove therefrom by filtration.

In copending application Serial No. 477,424 to Myron Ceresa et al., which application is assigned to the same assignee as the present application, there is disclosed a (place a portion-of the'calcium compounds. -to an aqueous electrolyte which is out of specification,

process for treating aqueous alkali cyanide electrolytes which are not in desired operating specification because *That application discloses a process for treating such electrolytes --to return them to desired operating specification. Broadly, that process comprises treating the aqueous electrolyte with a single composition containing calcium cyanide, calcium oxide, and an alkali metal cyanide. In some cases barium compounds may re- Whenadded that composition brings aboutthe precipitation of the excess carbonates -an d.. adds free alkali metal cyanide and alkali metal hydroxide whereby the electrolyte is restored -to a desired operating specification.

'The process disclosed in the said copending application is particularly suitable for use in treating electrolyte plating baths wherein some or all the rinse waters containing the electrolyte dragout are discarded to a drain. In those plating installations wherein the electrolyte dragout is' reclaimed and-returned to theplating tank there are problems of attaining a. complete balance of composition using the treating agents of that application. The reason for-thisis that in installations of the latter type there is only a moderate or minor loss of free alkali metal cyanide from the plating bath since substantially all the electrolyte dragout containing alkali cyanide is returned to the tank. When the composition of the copending application containing calcium cyanide is added to such an electrolyte, the lkali metal cyanide present therein often increases the free alkali metal cyanide in the electrolyte above a desired level. As a result, the electrolyte must be subjected to additional treating steps or to a modified treatment to reduce the free alkali metal cyanide to the desired level before best plating can be obtained.

The object of the present invention is to provide a process for treating aqueous alkali cyanide electrolyte plating baths wherein the cyanide dragout is reclaimed during use and returned to the bath, the process comprising adding a composition to the bath to remove excessive carbonates therefrom and return the bath to within desired operating specification.

Still another object 'of this invention is to provide a process for treating aqueous alkali cyanide electrolyte plating baths wherein the cyanide dragout is reclaimed during use and returned to the bath, the process comprising adding a composition to the bath consisting essentially of calcium cyanide and calcium oxide to remove excessive carbonates therefrom and return the bath to within desired operating specification.

A further object of this invention is to provide a process for treatingaqueous alkali cyanide electrolyte plating baths wherein at least a substantial proportion of the cyanide electrolyte dragged out of the bath during plating is reclaimed and returned to the bath, the process comprising treating the bath with predetermined quantities of calcium cyanide and calcium oxide to precipitate excessive carbonates and organic impurities fromthe bath.

Other and further objects of the invention will, in part, be obvious, and will, in part, appear hereinafter.

In the attainment of the foregoing objects and in .accordance with the present invention, a process is provided for treating aqueous alkali cyanide electrolytes with a calcium cyanide and calcium oxide composition to remove excess carbonates therefrom and increase the free alkali metal cyanide and free alkali metal hydroxide content thereof to a desired predetermined level whereby excellent electroplating results. The treatment can be applied to alkali cyanide baths employed for plating gold, silver, copper, brass, zi nc andcadmium. These electrolyteslwill contain at least 0.5 ounce per. gallon f a kal qxi More specifically, the process of this invention comprises adding a composition consisting essentially offrom 9% to 91% by Weight of calcium cyanide and from 91% to 9% by weight of calcium oxide to an aqueous alkali cyanide electrolyte. The composition is added in an amount to provide sutticient Ca'(CN) and CaO (1)10 react with substantially all of theexcess carbonate to causethe same to precipitate from the electrolyte solution as calcium carbonate leaving only the specified amount of carbonate to meet specification, and (2) increase the free alkali metal cyanide and alkali metal hydroxide content of the electrolyte to a desired predetermined level.

The process of the present invention is adapted to the treatment or purification of aqueous alkalicyanideelectroplating baths which, through use, are out of satisfactory operating specification. More particularly, the process of this invention is adapted to the treatment of electrolyte plating baths in which a substantial proportion, for example, at least 5% by weight of the cyanide dragout is re-claimed and returned to the plating tank.

It has been determined that organic impurities are removed from aqueous electrolytes treated in accordance with this invention without the need of adding an additional treating agent. It has been found that the organic impurities are removed, apparently by being occluded by the calcium carbonate precipitate and thereby removed from the electrolyte.

Aqueous alkali cyanide electrolyte plating baths sometimes are contaminated with relatively small amounts of chromates and sulfates. It has been determined that these contaminants may be removed from the plating bath by incorporating a relatively small amount, for example, from 0.01% to 5% by weight of barium cyanide with'the composition consisting essentially of calcium cyanide and calcium oxide and adding the combined composition ,to the bath. The chromates and sulfates will precipitateas barium salts with the carbonate precipitate and may be removed from the bath in accordance with conventional procedures.

Thus, the invention provides a process for treating aqueous alkali cyanide electrolytes, which electrolytes have at least 5% by weight of the cyanide dragout .returned thereto, to remove therefrom excess carhonate, organic impurities, chromates and sulfates, and provides a method for increasing the free alkali metal cyanide and alkali metal hydroxide content thereof to a desired predetermined level. Such a process rapidly and effectively returns the electrolyte to within operating specification by adding thereto a composition consisting essentially of calcium cyanide and calcium oxide together with a minor amount of barium cyanide. It will be understood that calcium hydroxide may be used in place of all ,or a portion of the calcium oxide.

The two essential ingredients forming the electrolyte treating composition of this invention may be introduced into the electrolyte either jointly or separately, in the form of solids or completely or partially dissolved in water, preferably deionized water. While the use of chemically pure ingredients is preferred, it has been determined that the presence of minute amounts ofcarbon, metal oxides, halogen salts and the like may be present in the compositions.

in applying the compositions of this invention to aqueous alkali cyanide electrolytes, it is desirable that the electrolyte be at a temperature of not over 200 F., and preferably about to F.

In order to indicate even more fully the advantages and capabilities of the present invention, the following examples are set forth. The parts indicated are by weight unless otherwise indicated. In each of these examples the Plat n s l n were m lo e n u h m nn r her t l a t 5% by we h f th iIQ Y -F I?S9ut wast claimedand returned to the tank.

EXAMPLE I Chemical analysis-0z./gal.

Time, Hours Free KOH K2003 Copper KCN The cyanide copper plating solution, operated in the above manner, consumed per week 0.44 oz./ gal. of free KCN and 0.65 oz./ gal. of KOH and formed 0.95 02/ gal. Of K2CO3.

After one week of operation, the free KCN and KOH in the plating solution were approaching the lower limits and the K CO the upper limit of satisfactory operating specification. In order to continue to obtain the optimum plating results, the plating solution in the tank was treated by adding thereto 2 pounds 10 ounces of a dry mixture consisting of 54 percent calcium oxide and 46 percent calcium cyanide. The dry mixture was added to the plating solution at l40160 F. About one half pound activated carbon was also added to the solution. After stirring the solution for 2 hours, the carbonate precipitate was permitted to settle and was removed from the solution. The treated cyanide copper plating solution then Was found to contain:

Oz./gal. Free KCN 1.48 KOH 5.20 K CO 8.86 Copper 6.72

The cyanide copper plating solution had been returned to a better operating condition, within specification, by this treatment.

Satisfactory results also are obtained by adding the calcium cyanide portion of the treating composition first and then adding the calcium oxide. The two need not be added simultaneously.

EXAMPLE II Periodically throughout a twelve week period, a production installation of 16,500 gallons of cyanide copper plating solution was analyzed to determine what change had occurred in the quantities of the various components in the solution. It was determined that 0.37 oz./gal. of free KCN and 0.62 oz./ gal. of KOH were consumed per week and that carbonate build-up per week amounted to 0.95 oz./gal. To maintain this cyanide copper plating solution Within desired operating specification, it was necessary totreat a 1000 gallon quantity of the solution once a week with 800 pounds of a composition consisting of 60 percent by weight of calcium oxide and 40 percent by weight of calcium cyanide. After vigorously stirring the mixture for 3 hours, the precipitate which formed was allowed to settle and the 1000 gallons of treated solution was filtered back into the plating tank. Treatment of the 1000 gal. portion of plating solution in this manner lowcred the carbonate content of the entire solution by 1.04 oz./gal./week and increased the free KCN and KOH content by 0.37 oz./gal./week and 0.61 oZ./gal./week respectively, whereby the entire 16,500 gal. solution was returned to and maintained within desired specification.

After 10 weeks of further operation with this production cyanide copper solution, it was determined that the rate at which the components were being consumed and carbonates increased had changed again. Thus, it was determined that each week 0.19 oz./ gal. of free KCN and 0.74 oz./gal. of KOH were being consumed and that the carbonate was building up at a rate of 1.17 oz./gal. To maintain the solution in desired operating specification, it was necessary to treat 1000 gallon quantities once each Week with 700 pounds of a mixture consisting of 76 per cent calcium oxide and 24 percent calcium cyanide. After each weeks treatment, Whether the calcium cyanide and calcium oxide were both added at the same time or successively, it was found that the carbonate content was reduced by 1.09 oz./gal. and that the free KCN content was increased by 0.22 oz./gal. and the KOH by 0.79 oz./gal. The solution was maintained in. desired operating specification by this weekly treatment.

EXAMPLE III Time, Hours Free KOH K 003 Copper KCN Thus, this cyanide copper plating solution consumed per week 0.64 oz./gal. of free KCN and 0.31 oz./gal. of KOH. The carbonate content increased at a rate of 0.52 oz./gaI./week.

The plating solution in the tank was returned to a desired operating range by adding thereto 2 pounds of a mixture of a composition containing 71% calcium cyanide and 29% calcium oxide. This treatment resulted in the plating solution having the following composition:

Oz./gal.

ree KCN 1.98 KOH 5.00 K CO 6.75 Copper 7.20

It will be understood that this solution will deviate from desired specification during each week of use. As indicated above, it may be returned to desired specification simply by the weekly treatment with two pound quan tities of the described additive composition.

EXAMPLE IV Fifty gallons of cyanide copper plating solution was maintained at F. in a covered plating tank for 113 hours. The solution was not agitated during this period nor was it used to plate any articles. The solution was merely maintained at 180 F. to illustrate the effect of heat on such a solution with respect to carbonate buildup and the reduction in free cyanide and hydroxide.

Chemical analysis-ozJ gal.

Time, Hours Free KOH K 003 Copper Based. on these data, at the end of one week during which the solution is maintained at 180 Ftand not operated, the solution was losing free KCN and KOH at the rate of 0.04 oz./gal./week and 0.45 oz./gal./week, respectively, and carbonate was building up at the rate of 0.40 oz./ gaL/ week. This means that during down time of the plating tank, when the plating solution is only being maintained at operating temperature, proper treatment with a composition consisting of 90 percent calcium oxide and percent calcium cyanide would reestablish the solution to within operating specification. Thus, a treatment similar to the one in Example I, viz., a once a Week treatment with one pound of a mixture consisting of 90 percent calcium oxide and 10 percent calcium cyanide would add 0.04 02/ gal. of free KCN and 0.44 oz./ gal. of KOH andremove 0.49 oz./gal. of K CO EXAMPLE V Fifty gallons of a cyanide copper plating solution in a covered tank maintained at 180 F. was agitated by passing nitrogen therethrough at a rate of 0.011 cubic foot per gallon per minute. This solution was operated for 200 hours according to the manner described in Example I. It was determined that 0.64 oz./ gal. of free KCN and 0.10 02/ gal. of KOH were consumed after one week of operation. The carbonate content increased 0.59 oz./ gal. at the end of one week. It was observed that agitation of the solution with nitrogen gas brought about a relatively large consumption of free KCN with a relatively small consumption of KOH. It also was evident that the cyanide was not completely converted to carbonate by the reactions taking place in the cyanide copper plating solution. Rather, it is believed that the nitrogen accelerated the hydrolysis of cyanide to oxalates and. formates according to the reactions illustrated in Equation 3 hereinabove. In order to control a solution operated under these conditions, it is necessary to treat the solution, twice each week, with one pound of a composition consisting of 91% calcium cyanide and 9% calcium oxide. With-a total treatment of two pounds each week of the additive composition, the free KCN and KOH willbe increased 0.66 oz./gal. and 0.07 oz./gal., respectively, and the carbonate will be reduced 0.79 oz./gal.

EXAMPLE VI A 9000 gallon production cyanide zinc plating solution was analyzed periodically during operation as a still bath. The analytical data indicated that 0.25 oz./ gal. of sodium hydroxide and 0.18 oz./ gal. of free sodium cyanide were being consumed with each week of operation. it was also determined that the sodium carbonate was building up at the rate of 0.40 oz./gal. per week. To maintain this cyanide zinc plating solution within operating specification, it was necessary to treat 1000 gallons of the plating solution each week with 270 pounds of a mixture consisting of 60% calcium oxide and 40% calcium cyanide. This treatment removed, eachweek, 0.52 oz./gal. of sodium carbonate and added 0.25 oz. /gal. of sodium hydroxide and 0.18 oz./ gal. of sodium cyanide in the entire solution. This treatment maintained the solution within operating specification.

While thepresent invention has been described with particularreference to the preferred embodiments thereof, it will be understood that changes, substitutions, modia fications and the likemay be made therein without departing from its true scope.

We claim as our invention:

1. In the process of plating articles in an aqueous alkali cyanide electrolyte plating bath containing at least 0.5 ounce per gallon of alkali hydroxide in which excess carbonate builds up and in which deficiencies of free alkali metal cyanide and alkali metal hydroxide develop during use, which process includes the step of returning the electrolyte dragout to the bath, the improvement which comprises adding to the aqueous electrolyte a composition consisting essentially of from 9% to 91% by weight of calcium cyanide and from 91% to 9% by weight of calcium dioxide, said composition being added in an amount sufficient to precipitate the excess carbonate, there remain ing at least about 4 ounces per gallon of carbonate in the electrolyte, and increase the freealkali metal cyanide and alkali metal hydroxide to a desired predetermined level, said composition maintaining the pH of the electrolyte at a level of at least 0.5 ounce per gallon of alkali hydroxide.

2. The process as set forth in claim 1 in which the calcium cyanidecomponent and calcium oxide component of said composition are added to the electrolyte separately.

3. The process as set forth in claim 1 in which the calcium cyanide component and calcium oxide component of said composition are added to the electrolyte simul taneously.

4. In the process of plating articles in an aqueous alkali cyanide electrolyte plating bath containing at least 0.5 ounce per gallon of alkali hydroxide in which excess carbonate builds up and in which deficiencies of free alkali metal cyanide and alkali metal hydroxide develop during use, which process includesthe step of returning at least 5% by weight of the electrolyte dragout to the bath, the improvement which comprises adding to the aqueous electrolyte bath a composition consisting essentially of from 9% to 91% by weight of calcium cyanide and from 91% to 9% by weight of calcium oxide, said composition being added in an amount (1) to provide suflicient calcium cyanide to react with substantially all the excess carbonate to form a precipitate of calcium carbonate, there remaining at least about 4 ounces per gallon of carbonate in the electrolyte, and (2) to increase the free alkali metal cyanide and alkali metal hydroxide to a desired predetermined level, said composition maintaining the pH of the electrolyte at a level of at least 0.5 ounce per gallon of alkali hydroxide, agitating the electrolyte and the added composition until the precipitate forming reaction is complete, and then separating the precipitate from. the treated electrolyte.

References Cited in the file of this patent UNITED STATES 'PATENTS OTHER REFERENCES Transactions Electrochem. Soc., vol. (1941), pp. 358-359.

row 1,.

Claims (1)

1. IN THE PROCESS OF PLATING ARTICLES IN AN AQUEOUS ALKALI CYANIDE ELECTROLYTE PLATING BATH CONTAINING AT LEAST 0.5 OUNCE PER GALLON OF ALKALI HYDROXIDE IN WHICH EXCESS CARBONATE BUILDS UP AND IN WHICH DEFICIENCES OF FREE ALKALI METAL CYANIDE AND ALKALI METAL HYDROXIDE DEVELOP DURING USE, WHICH PROCESS INCLUDES THE STEP OF RETURNING THE ELECTROLYTE DRAGOUT TO THE BATH, THE IMPROVEMENT WHICH COMPRISES ADDING TO THE AQUEOUS ELECTROLYTE A COMPOSITION CONSISTING ESSENTIALLY OF FROM 9% TO 91% BY WEIGHT OF CALCIUM CYANIDE AND FROM 91% TO 9% BY WEIGHT OF CALCIUM DIOXIDE, SAID COMPOSITION BEING ADDED IN AN AMOUNT SUFFICIENT TO PRECIPITATE THE EXCESS CARBONATE, THERE REMAINING AT LEAST ABOUT 4 OUNCES PER GALLON OF CARBONATE IN THE ELECTROLYTE, AND INCREASE THE FREE ALKALI METAL CYANIDE AND ALKALI METAL HYDROXIDE TO A DESIRED PREDETERMINED LEVEL, SAID COMPOSITION MAINTAINING THE PH OF THE ELECTROLYTE AT A LEVEL OF AT LEAST 0.5 OUNCE PER GALLON OF ALKALI HYDROXIDE.