US2507475A - Electrodeposition of chromium - Google Patents

Description

Patented May 9, 1950 ELECTRODEPOSITION CHROMIUM Rex R. Lloyd, Boulder City, Nev., assignor, by

mesne assignments, to Crimora Research & Development Corporation, New York, N. Y., a corporation of Virginia No Drawing. Application February 4, 1946, Serial No. 645,473

1 Claim.

The present invention relates to the electrodeposition of chromium and, more particularly, to the electrowinning of chromium from chromium salt solutions. This application is a continuation in part of applications Ser. Nos. 578,549 and 578,551, filed February 17, 1945, now abandoned.

Heretofore, it has not been .possible to recover chromium economically from chromium salt baths by electrolytic methods. For example, the use of prior methods generally has resulted in thin, usually oxide contaminated deposits at low current efficiency.

In accordance with the present invention, it is possible to plate out thick deposits of chromium of good quality which can be readily stripped from the cathode.

The present invention also makes it possible to deposit chromium uniformly and satisfactorily at a high current efficiency.

An important aim of the invention is to provide a method of electrowinning chromium which may be carried out continuously over longperiods of time without materially decreasing the quality of the deposit, while maintaining a high current efficiency.

Other aims and advantages of the invention will be apparent from the following description of the invention and the appended claim. i In general, the present invention is applicable to the electrowinning of chromium from a solution containing ammonium, sodium, sulfate, chromous and chromic ions. The invention resides in the discovery that the new and improved results referred to above may be obtained by controlling the conditions of electrolysis within the ranges or limits defined more particularly hereinafter. Such conditions include, for example, the composition, purity, temperature and pH of the bath, and the current density used.

In the practice of the present invention, it is preferred, but not essential, to use an electrolytic cell divided into catholyte and anolyte compartments by means of an ion-permeable diaphragm, which type of apparatus is well known in the art. With respect to the electrodes, it is preferred to use lead or lead-alloy anodes while the cathodes preferably are of bronze containing about 88% copper and the ibalance tin. The particular composition of the electrodes is not critical. With respect to the anodes, for instance, it is essential only that they will not contaminate the bath during electrolysis. With respect to the cathodes, there is less danger of contamination-and a Wide variety of materials or combina- 2 tion of materials may be used such as graphite, lead, copper, nickel, brass, aluminum, tin and various stainless steels.

In accordance with the invention the electro-' lyte contains the following elements in an amount within the ranges shown:

Grams per liter Sodium 20 to 40 Ammonium 30 to Chromium 18 to Sulfate ion is also present in an amount at least stoichiometrically equivalent to the ammonia, sodium and. chromium.

It has also been found that when the amount of chromium present exceeds about 30 grams per liter, the concentration of the sodium must be less than that of the chromium in order to obtain satisfactory results.

As a typical example of an electrolyte prepared in accordance with the invention, which is given merely by way of illustration, satisfactory results have been obtained by utilizing an electrolyte containing approximately 32 grams of chromium per liter, with sulfate ion present in the necessary stoichiometric amount.

The method of forming the electrolyte is not critical except in so far as it is necessary that the solution be substantially free of lead, aluminum and magnesium. The aqueous solution can be made conveniently, for example, by dissolving the necessary amounts of sodium sulfate, ammonium sulfate and chromium sulfate. In general, it is preferred to use chromium. ammonium alum, which is readily available in pure form; although it has been found that potassium alum may be substituted for or used in combination with the ammonium alum. The preparation of this alum in a form suitable for use in the present invention, is disclosed in my copending application Ser. No. 645,474, filed February 4, 1946. The solution is stabilized in the green modification by heating for a short period at a temperature above C.; but not substantially above the boiling point at atmospheric pressure.

Substitution of some of the sulfates in the electrolyte may be made with chlorides; provided enoughsulfate is present to form a complex of the alum type.

At the beginning of electrolysis the chromium present will be substantially all in the trivalent state (Cr+++). As electrolysis proceeds, however, divalent chromium (Cr++) is formed. Ingeneral, the chromous ion will not accumulate to an extent beyond about20 grams per liter, when the total chromium is about 35 grams per liter. Satisfactory results are obtained when the chromous ion has been formed by electrolysis to a substantial extent of say, about 45% of the total chromium present. If desired, the formation of chromousion may be expedited. by;omitting the ammonium-.sulfate from the bath during 'the initial period of electrolysis. Sufficient chromium should be present, however, so that the amount of chromous ion will not exceed 95% of the total chromium.

It has been found that the pI-Ibfithe electro lyte is another determiningafactor inathesuccessful carrying out of the presentinvention'. It:

is preferred to adjust the pH of the bath to within the range of about 1.8 to 2.2.. This maybe accomplished without the addition"ofanynew:

radicals, for example, by the addition of sulfuric acid. pH of the electrolyte shbuld be within the range Of) 1.0;23

'Ihe:.-temperature= of'the batiris also. a very important factor in accomplishing; the aimsofthepresent invention. At temperatures; below 27-" 0.; if maintainedfor any substantial period of time, there isartendency to form insoluble; violet chromium salts. At temperatures above about-45"'C., there is a decrease in current effici'ency andthe nature of theplate is also adversely affected;

The-current density utilized in thecell also has-a. controlling. influence -upon. the; successful operation of thermethodofthe present-inven-.- ti'on. In general, a-. cathodic current density. in. therange of. about.3 l to 16.0: amperes per squarev foot .is necessary to obtainagood. plate. Ifl-thepH isf-rom 2.51 to 2.8,. the preferred. current density. is 30lt'o1 60- amperes. per square foot; from pH 2.3 to 1.8, current density, 50. to.100 amperespersquare foot; and fromlpH. 1.0Lto 1.8, current density. 100' to- 160 amperes. per square foot..

Dhringz the. operation. of the .cell, it hasv been found, in accordance withthe present invention..that a.substantiallincreasein current efiis ciency and agreatly: improved. structure. of the depositisbbtained. by. adding a. reduced sulfur compound suclias. sulfur. dioxide, .or water.- sol'uble sulfites,. h-yposulfites, sulfamites Xanthates, thionates, .thiosulfates, sulfidesandother sulphuncompound's-in. which the-sulfur has :a valence of less than .6. The amount required to efiectlsuch improvement. is .very small and pref-e erably about l2 1ilri mols: per..liter. of; any of these: compounds.

An excess of reduced sulfur. compounds is; to be: avoided. Since; the; reduced: sulfur com-.- pounds-are. destroyed during, electrolysis,-. it is necessary to replenish themairnount ofthese compounds in: the catholytez This:-is-preferably done continuously,- by add-ing,.forexample, 0.00002? 0.0002-.-mols.-per liter, per hour; of'sul-fite ionto the. catholyte Even: when added: ina small amount; it has-beerrfound that after. several hours of.- cyclic operation; the accumulationof sulfur= compoundsaham an adverse. effect. on the current-.efiiciency and structure of the deposit. It is an-- advantage- 0f theapresent invention, however, that such excess may be effectively removedwithout adversely afiecting; the electrolyte: In accordance: with. theinvention, this; is a-cconlplisl'iedi periodically by heating the electrolyte-fwith ammonium persulfate:orihydrogen peroxideeat. a. convenient point irr thecircuit,

In general, for satisfactory results; the:

required is small. For example, with the preferred addition agent ammonium persulfate, only about 2 grams per liter is required to remove excess reduced sulfur compound. After such treatment, the electrolyte is suitable for re-use in the cell without further treatment.

It will beapparent that the present invention may be practiced, if desired, in the usual batch type of operation. For example, in plating, the bath could be used with an insoluble anode to plate out Cr within the concentration range 18-60. grams per liter. A soluble chromium anodecouldals'o. be used to maintain the chromium concentration. However, it is preferred, and-an advantage-of the invention, to operate the process continuously. This may be accomplished' by continuously or intermittently replacing part or all of the spent electrolyte with freshsor: replenished electrolyte prepared as set forth above, or by replacing the chromium, whiohcwas =removed byelectroplating, inthe electrolyte, for example, in accordance. with...the.

processesset forth in my copending applications- Ser. No. 645,474, filed February 4,. 1946,. and Ser. No. 578,549, filed. February 17, 1945,.the latter now: abandoned.v

The following specific examples are given..of-. the carrying out of. the present. invention, merelyby way of illustration,. to. assist in.. the. understanding of theinvention-and are .notto: be construed as limiting-the scopeohthe inyen: tionz.

As an example of the practiceof theinvention, reference ismade to.- a test involving. five. weeks of. continuous electrolysisusing. one cathode. at aitime; Each cathode remained: in. the (38112148 hours. The original electrclytecontained. about 36 grams of chromium per; liter, 33 gramsiof sodium per liter and. 42 grams: of. ammonium. per liter. It was heated to abouteiiil C. for about. thirty minutes and cooled. The solutionlwas circulated through the cathode compartment of: a cell and the chromium was replenished:-.-.to maintain the chromium content about'35'grams per liter. About: 0.01 ofagram of. ammonium sulphite per liter was added per hour.- The-av,- erage-pl-F of; theeatholyte was:'1-.85 and the =anolyte contained about250 grams of sulphuric acid' per liter." Afterza few hours of operation; the ratioof chromousto chromic: ions was: about 1 to 1. ThBT-SOhltiOni contained not more than milligrams per liter-oft.magnesium'and: alumie num and substantially no'otherimpurities. The

average catholyte temperature Was 29.5 C.:.ancl

its density was about 1.26: The current density was .maintained' at about amperes per. square foot;v On the; average, three pounds-of. dense and satisfactory chromium was obtainedaeach 24 hours at a" greatly increased current efficiency with anexpenditure' of about 8 kilowatt hours per pound of metal. This depositedchromium was readily stripped from the cathode plateby hammering and passing 'a thin steel blade under theideposit'.

m a similar run, in which no. ammoniumsul phite was added, the deposits werehard and bright. They peeled. readily from the cathode; thus causing. somedifiiculty by prematuresepe aration. Then, 0.3 to 0.4 of a gram of ammonium sulphite. was added to 30 liters. of'catholyte... The currentefiiciency was. increased. by the-hourly addition of this." amount ofa-mmoniumsulphite and'was. maintained substantially; constant.- at

outside-thewcell..- Theamount ofiadditionr agent diificultypwas, experienced. from prematureepeele ing, and the current efficiency increased substantially.

It will be appreciated that the invention Will be susceptible of many variations and modifications by one skilled in the art without departing from the spirit of the invention, and all such variations and modifications are intended to be included within the scope of the following claim.

What is claimed is:

.A method of electrowinning chromium in a compartment cell comprising forming an electrolyte having a pH range of 1.8 to 2.3 and consisting of 18-60 grams of chromium per liter, 30-50 grams of ammonium per liter, 20-40 grams of sodium per liter, the amount of said sodium not exceeding the amount of said chromium when more than grams of chromium per liter are present, and sulfate ion in an amount at least the stoichiometric equivalent of the chromium, sodium and ammonium, heating the electrolyte to a temperature of approximately C. to stabilize the chromium sulfate in the green modirfication, and electrolyzing said electrolyte at a cathodic current density of about 50 to amperes per square foot while maintaining the electrolyte at a temperature in the range of approximately 27 C. to 45 C., and during the elec- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 526,114 Placet et a1. Sept. 18, 1894 FOREIGN PATENTS Number Country Date 19,344 Great Britain Sept. 26, 1890 22,855 Great Britain Nov. 19, 1891 18,743 Great Britain Aug. 5, 1899 OTHER REFERENCES Rogers et al., Transactions of the Electrochemical Society, vol. 64 (1933), pages 299-304.