US2833703A - Removal of chromium from plating baths and electroplating process - Google Patents

Description

mte States patent REMOVAL OF COMIUM FROM PLATING EATHS AND ELECTROPLATING PROCESS Erich Laue, Watertown, Conn, assignor to MacDcrmid Incorporated, Waterbury, Comm, a corporation of Connecticnt No Drawing. Application November Serial No. 696,058

16 Claims. (Cl. 204-44 purities present a frequent problem since industrial platers generally employ the same'racks for plating articles to produce successive deposits of copper, nickel and chromium. Thus, chromium is introduced from the racks to the copper plating bath, for example. Whileit might be supposed that the minute quantities of the metal which are transferred from the racks to the bath would have little effect upon the elficiency of the copper plating operation, it has been found that the presence of chromium in concentrations of only lto 10 p. p. m. CrO causes low current density dullness, etched patterns and staining. At 10 to 20 p. p. m. CrO blistering and skip plate results, and at still higher concentrations copper plating is in many instances completely inhibited. The exact chromium level at which these effects occur depends in part upon the presence in the bath of addition agents which are often employed, such as brighteners, secondary complexers or chelating agents'and their concentration inthe bath.

In many alkaline plating baths salts of organic'hydroxy acids, a class of compounds whichl term secondary complexers are used as addition agents. Many copper cyanide baths, for instance, contain Rochelle salts. These compounds have a beneficial efiect on the operation of the anodes. In other applications they are required to keep metallic addition agents in solution. agents such as ethylene-diamine tetracetate may be used in a similar manner.

In alkaline plating baths which contain no secondary complexers or chelating agents, chromium impurities can be removed by reduction to the trivalent state followed by precipitation and filtration as .Cr( H If secondary complexers or chelating agents are present, as is very often the case, trivalent chromium is tied up and kept in solution and the aforementioned simple precipitation is no longer possible. Since the deleterious effect of chromium upon the electroplating process is caused by chromium in the hexavalent state, simple reduction to 'Cr+ eliminates chromium trouble temporarily. Unfortunately, the benefit is short lived since in the'course of the plating operation trivalent chromium is re-oxidized and must be reduced anew. It can be seen that this method of handling the chromium impurities requires frequent corrective action, and by-products from the reducing agentemployed tend to accumulate in the bath to a detrimental level.

Effects of chromium contamination can be alleviated to some extent through addition of secondary complexersor chelating agents. However, asmore-chromium enters theg Patented May 6,1958

bath from the racks holding the work, more chelating agent must be added, and eventually these additions interfere with plating and ultimately the bath becomes inoperable.

Accordingly, it is an object of the present invention to completely overcome chromium contamination problems in alkaline plating baths containing secondary complexers or chelating agents. It is a further object to permanently remove chromium impurities from the plating bath by a process which will not interfere with the main constituents of the bath. It is a still further object of the present invention to employ reagents which do not interfere with plating and which can easily be removed from the bath.

In accordance with the present invention, any hexa- I valent chromium, present in the bath is first adjusted to the trivalent state. While it is possible that all or a large part of the chromiummay be present in the trivalent state,I preferably initially add to the bath an agent capable of reducing. hexavalent chromium to the trivalent state in suflicient quantity to reduce all the chromium even if all of it were present as hexavalent chromium. Next I add to the bath, a substituted 'anthraquinone, namely: l,2-dihydroxy anthraquinone (alizarin), a ring substituted 1,2-dihydroxy anthraquinone capable of forming a chromium lake in the plating bath, or mixtures of these 1,2-dihydroxy anthraquinones. Following formation of the chromium lake, suflicient activated carbon is added to the bath to adsorb any colloidal lake and substantially all of the excess substituted anthraquinone added. The carbon together with the chromium lake is then separated from the bath as by filtration. The thus treated bath is then substantially free from chromium.

Surprisingly, chromium lake formation is possible in the bath with only a few of the substituted anthraquinones. For example, if the hydroxy grouping is other I than 1,2, the compounds are ineffective. This leaves a Chelating relatively small class of compounds which includes 1,2- dihydroxy anthraquinone (alizarin) and the ring substituted 1,2-dihydroxy anthraquinones. Among ring substituents which may be contained in these compounds are OH, S0 H, etc. The following which are commercially available perform well: the sodium salt of ali'zarin 3-sulfonic acid- (alizarin red), 1,2,3-trihydroxy anthraquinone (anthraga-llol), 1,2,3,5,6,7-hexahydroxy anthraquinone (rufigallol), as well as a mixture of anthragallol and rufigallol (alizarin brown). Alizarin is preferred be cause of its more attractive price.

Chromium is easily removed with relatively small quantitles of the substituted anthraquinone. For example, a 10 fold molecular excess is often entirely satisfactory. However, in the presence of chelating agents or secondary complexers which hold the chromium more tightly larger excesses are required. Of course, unusually high chro-, mium concentrations or the presence" of large quantities of the secondary complexers or chelating agents necessitates larger substituted anthraquinone additions. Generally, the upper limit of application will be dictated by cost only. Chromium contaminations normally occurring in the industry can usually be handled with quantities rang-,

ing from 5 to 30'ounces of'solid substituted anthraquinone 4 solution.

Chromium has thus been successfully removed from 3 cyanide baths containing a variety of secondarycomplexers including tartrates and gluconates, and from baths containing chelating agents, among which were ethylene-.

diarnine tetracetate, triethylene tetramine and Yersene T. l

- in a high degree of complexity. For example, "chromium The'process of the present invention is applicable to i all alkaline baths which contain the metals to be plated was eliminated from a" silver cyanide bath containing potassium tartrate, as well as from a brass plating solution containing Rochelle salts. n the other hand, the presentprocess was ineffective for removing chromium from a'copper pyrophosphatebath, because, it is believed, inthis' bath considerablequantities of uncomplexed'copper were available and consumedthe1,2-dihydroxy'arithraquinone employed leaving none for the chromium impurity. Thus, it is seen that the'plating metal or other metal desirably present must be in a high degree of complexity so that his incapable of forming a complex with the substituted anthraquinonefl h As noted earlier, where allor part of the chromium contamination is presentin thehexavalent state, it must be; reduced before or simultaneously with'addition'of the substituted anthraquinone. Excellent results 'were' obtainedwith the use; of a premixed reagent containing both -thefi substituted -anthraquinone and the reducing.

agent in concentrated aqueous solution. Whileseveral' agents are available for-reducing chromium in a plating bath, one often employed is sodiumdithionite, the hydrosul fite of commerce, which performs exceedingly well. Stannous compounds such as stannous sulfate-may also be used. However,- stannous tin will interfere with copper cyanide plating processes,-'and accordingly when a stannous salt is used as the chromium reducing agent, followingremoval of chromium lake from the bath, a small amount of hydrogenperoxide is addedto oxidize anyexcess stannous compound to stannic tin, which does not-affect plating. I Y

Following formation of the chromium lake, sufiicient' activated carbon is added to the bath to pick up colloidal chromium lake and adsorb excess substituted anthraquinone. For the sake of economy several small carbon additions may be made. However, removal of excess substituted anthraquinone is not absolutely necessary. It does not interfere with the plating process. I The carbon and lake are removed by filtration. I

The chromiumremoval process of the present invention is. highlyeflicient, and requires no adjustment of the pH. of1the -bath,which1is usually well above a pH of 9 and in thevicinity of 13. Furthermore, the present process can be carried out over a wide range of temperature,

although elevated temperatures give best results since they favor; the colloidal reactions involved in the overall chromium removal,v The preferred operating temperature is about 160 to 212 F.

The following non-limiting example. illustrates the process of ,the present invention as applied to ,a copper cyanide plating bath.

In addition tothe alkali and cyanideconstituents, a plating solution contained 4.2 ounces 'per gallonof copper. metal and 6 ounces per gallon of potassiumttartrate as a secondary complexer. The bath had a. pH of about 13. Plating in this bath produced etched patterns. and skip plate inplaces. An analysis showed a chromium contamination of 20p. p. m. CrO .1. In order. toremovechromium 100 gallons of the above solution were first heated to 200 F. Areagent was prepared as follows; to; 3 gallons ofcold water containing 7 ouncesof solidcaus tic potash, 14 ounces of alizarin were. added, and the mixture was then agitated until, the components completely dissolved. In another vessel 3 ounces, of stannous sulfate were dissolved in 1 gallon of water. Tofthis solution were added 12 ounces of solid caustic potash [and thelmixture was stirred to complete dissolution. The reducing agent solution was stirred. into the alizarin solution and the'resulting solution was I added to theghot plating solution. The reaction mixture was kept mildly agitated at 200 FL for 15 minutes. Then' flui was. 95% hY IQEQ ml de. Q llw l wi h:

water were then added to the bath to oxidize excess stannous tin therein.

The resulting purified copper cyanide plating bath produced excellent plate completely free of all chromium contamination eifect. Analysis showed that now less chromium was present than could be detected by the analytical method, namely, less than 0.5 p. p. m. CrO

What is claimed is:

1. A process for removing trivalent chromium contaminants from a metal cyanide plating bath, which comprises adding thereto a substituted anthraquinone capable of forming a chromium lake therein selected from the group consisting of 1,2-dihydroxy anthraquinone, ring substituted 1,2-dil1ydroxy anthraquinones and mixtures thereof, contacting the thus treated bath with activated carbon and then. separating the carbon, chromium lake and adsorbed substituted anthraquinone from the bath.

2. A process for removing chromium contaminants from a metal plating bath containing a stable metal cyanide complex, which comprises adjusting any chromium of higher valence present in the bath to the trivalent state, adding a substituted anthraquinone capable of forming a chromium lake therein selected from the group consistingof 1,2-dihydroxy anthraquinone, ring substituted 1,2-dihydroxy anthraquinones and mixtures thereof, contacting the thus treated bath with activated carbon and then separating carbon, chromium lake and adsorbed substituted anthraquinone from the bath.

3. A process according to claim 2 whereinthe plating metal is selected from the group consisting of copper, silver and brass. Y

4. A process according to claim 2 wherein the substituted anthraquinone is 1,2-dihydroxy anthraquinone.

5. The process according to claim 2 wherein the substituted anthraquinone is 1,2,3-trihydroxy anthraquinone.

6. A process according to claim 2 wherein the substitued anthraquinone is l,2,3,5,6,7-hexahydroxy anthraquinone.

7. A process according to claim 2 wherein the substituted anthraquinone is a mixture of 1,2,3-trihydroxythraquinone and l,2,3,5,6,7-hexahydroxy anthraquinone.

, 8. A process according to claim 2 wherein the substituted anthraquinone is the sodium salt of alizarin 3-sulfonic acid.

9. A process for removing chromium contaminants from a metal cyanide plating bath containing an addition agent of the group consisting of secondary complexers and chelating agents, which comprises reducing any hexavalent chromium present to the trivalent state, adding to the bath a substituted anthraquinone capable of forming a chromium lake therein selected from the group consisting of 1,2-dihydroxy anthraquinone, ring substituted 1,2-dihydroxyanthraquinones and mixtures thereof, contacting the thus treated bath with activated carbon and then sep arating thecarbon, chromium lake and adsorbed sub stituted anthraquinone from the bath.

10. A process according to claim 9 whereinthere is employed a stannous compound as reducing agent.

11. A process according to claim 10 wherein following separation of the carbon and chromium lake. any excess stannous compound is oxidized.

12. A process according to claim 11 wherein following removal of carbon and chromium lake hydrogen peroxide is added to the bathin amount sutncient to oxidize excess stannous compound.

.' l3.'A process for removing chromium contaminants from a. cyanide plating bath containing metal selected from the group consisting of copper, 'silver and brass and containing an addition agent of the group consisting of secondary complexers, and chelating agents, which comprises reducingany hexavalent chromium present to the trivalentstate, adding to the bath a substituted anthraquinone capable of forming a chromium lake therein se lected from the group consisting of 1,2-dihydroxy anthraquinone, ring substituted 1,2-dihydroxy anthraquinones and mixtures, thereof, contacting. the .thus. treated .bath:

with activated carbon and then separating the carbon, chromium lake and adsorbed substituted anthraquinone from the bath.

14. A method of electroplating metal from a cyanide plating bath containing chromium impurities to produce an electrodeposited plate free from surface stain, etched patterns and blisters caused by said chromium impurities, which comprises adjusting any chromium of higher valence present in the bath to the trivalent state, adding to the bath a substituted anthraquinone capable of forming a chromium lake selected from the group consisting of 1,2-dihydroxy anthraquinone, ring substituted 1,2-dihydroxy anthraquinones and mixtures thereof, contacting the thus treated bath with activated carbon, then separating carbon, chromium lake and adsorbed substituted anthraquinone from the bath, and electroplating said metal from the resulting substantiallychromium-free bath.

15. A method as set forth in claim 14 wherein the metal is selected from the group consisting of copper, silver and brass.

16. A method of electroplating metal from a cyanide .6 plating bath containing chromium impurities and an' addition agent of the group consisting of secondary complexers and chelating agents to produce an electrodeposited plate free from surface stain, etched patterns and blisters caused by said chromium impurities, which comprises adjusting any chromium of higher valence present in the bath to the trivalent state, adding to the bath a substituted anthraquinone capable of forming a chromium lake selected from the group consisting of 1,2-dihydroxy anthraquinone, ring substituted 1,2-dihydroxy anthraquinones and mixtures thereof, contacting the thus treated bath with activated carbon, then separating carbon, chromium lake and absorbed substitued anthraquinone from the bath, and electroplating said metal from the resulting substantially chromium-free bath.

References Cited in the file of this patent Beckwith: Proceedings American Electroplaters Society, vol. 28 (1941), pages 543-549.

Claims (1)

1. A PROCESS FOR REMOVING TRIVALENT CHROMIUM CONTAMINANTS FROM A METAL CYANIDE PLATING BATH, WHICH COMPRISES ADDING THERETO A SUBSTITUTED ANTHRAQUINONE CAPABLE OF FORMING A CHROMIUM LAKE THEREIN SELECTED FROM THE GROUP CONSISTING OF 1,2-DIHYDROXY ANTHRAQUINONE, RING SUBSTITUTED 1,2-DIHYDROXY ANTHRAQUINONES AND MIXTURES THEREOF, CONTACTING THE THUS TREATED BATH WITH ACTIVATED CARBON AND THEN SEPARATING THE CARBON, CHROMIUM LAKE AND ADSORBED SUBSTITUTED ANTHRAQUINONE FROM THE BATH.