US2200987A - Electrogalvanizing process - Google Patents

Description

PatentedMay 14, 1940 UNITED STATES amoraoesnvamzme. raocnss John P. Hubbell, Garden City, and Louis Weisberg, New York, N. Y.

No Drawing. Application December 1, 1937, Serial No. 177,464

23 Claims.

ble anodes are employed with electrolytes'pre pared by dissolving zinemetal or ore of 'lower purity in sulfuric acid and purifying the solution; The spent electrolyte is used to dissolve to more zinc metalv or ore, the solutionthus obtained beingrepurified and used again as an electrolyte. .Since sulfuric acid is effective to leach zinc from comparatively cheap zinc-bearing ma terials, this process oifers a means of. utilizing 20 some of the less expensive forms of zinc, for electrogalvanizing without first isolating the pure metal in the form of anodes, but it requires that numerous impurities be excluded from the electrolyte before a satisfactory deposit can be ob- 25 tained and hence is not commercially feasible for the use of some raw materials.

Proposals have been made for the production of electrolytic zinc from roasted ores and like materials bydissolving such materials in solu-, 30 tions of ammonia and ammonium carbonate, or,

of ammonia and ammonium sulfate, instead of the usual sulfuric acid electrolyte. Such solu-, tions readily dissolve zinc oxide and other zinc-' bearing materials, except the metaLand can be 35 purified by simple and inexpensive means, So far as We have been able to discover, however, no one has succeeded in obtaining smooth, adherent and ductile deposits suitable for plating from such solutions under practical operating.

40 conditions. In laboratory experiments reported in the literature, current densities in excess of about 40 amperes per square fact were found to result in porous, unsatisfactory deposits. To make an electrogalvanizing process commercially attractive current densities of at least 500 and preferably 1000 amperes per. square foot or more are ,,,desirable. Furthermore, in these' experi- .me'n'tsfv arious addition agents such as gum arabicand gelatin were used the electrolyte in I order to obtainsatisfactory deposits even at very' low current densities. A h I An object of our invention is to providev a new and. improved electrogalvanizing process which meets the commercial requirements for electro- 55' galvanizing on wire or strip and produces relaof relatively pure zinc. In another type, insolu the workduring the plating operation.

tively thick zinc coatings of high quality due to their smoothness, ductility and strong adherence to the base metal.

Another object of our invention is to provide an electrogalvanizing process in which various 6 zinc-bearing materials, including materials which are low in price because of their chlorine content, may be utilized economically as the source of the zinc. In known commercial electrogalvanizingprocesses even small amounts of 10 chlorine are objectionable. Since zinc chloride is volatile at elevated temperatures, zinc-bearing materials which contain much chlorine cannot be treated effectively by ordinary metallurgical practices. The result is that if materials containing chlorine are to be used 'as a source of zinc they must first be subjected to weathering and roasting or other treatments which remove all but a trace of the chlorine. These special treatments, apart from their expense, cause the loss of considerable zinc.

Another object of our invention is to provide an improved electrogalvanizing process which enables the production of high quality galvanized. stock from solutions of low grade zinc-bearing materials without requiring complicated or 'expensive purification of the solutions or the use of colloidal addition agents.

'Another object of the invention is to estab-' lish operating conditions under which relatively heavy, smooth, adherent electrolytic coatings'of high purity zinc. may be produced from ammoniacal zinc solutions.

Stillanother object of our invention is to provide'an improved electrogalvanizing process emplaying a novel ammoniacal zinc electrolyte. I Further objects and advantages of the invention will appear from the ensuing part of this specification, which includes a detailed descrip tion of illustrative embodiments of our improved 40 electrogalvanizing process.

We have found that the present commercial electrogalvanizing process, in which a substantially pure sulfuric acid electrolyte is essential, can be substituted to advantage by a new process 5 employing an ammoniacal solution of a complex zinc ammonium salt and high current densities for electrolysis, with special control over the concentrations of ,zinc and ammonia in the solution and the movement of the solution with respect to Contrary to prior expectations, excellent galvanized stock of a quality comparable to the best commercial product now on the market may be produced by our process, and the process possesses zinc in the molecule.

economic advantages which make it distinctly superior to known practices in many respects.

The electrolyte used in this process is a solution of a complex zinc tetrammine salt containing four atoms of ammonia and one atom of Any of the ordinary zinc compounds is suitable as a source of zinc. The electrolyte may be prepared, for example, by

leaching oxidized zinc compounds with a solution vision of a distinctly new type of electrolyte for electrogalvanizing, this discovery is especially important because it makes it-possible to use zinc-bearing raw materials which contain chlorine as, the source of zinc, without preliminary treatment to-eliminate chlorine. One such material is'sal skimmings, which is a by-product' of hot galvanizing and is the cheapest form in which zinc caribe purchased at the present time. Although variable in composition, sa-l skimmings generally contain from 40 to 50% zinc, up to .35% chlorine, less than 2% iron and the balance oxygen and dirt. The use of sal skimmingsand similar materials as a source of zinc for electrogalvanizing effects a substantial reduction in the cost of the zinc-coated product.

In plating from these ammoniacal tions current densities of from 500 to .1000 amperes per square foot of cathode surface or greater are used .for optimum performance.

Galvanized wire possessing a relatively thick and f uniformly smooth, adherent and ductile coating of zinc may be obtained from such solutions under properly controlled conditions at current densities of approximately 1000 amperes'per; square foot, and coatings of like quality are produced on objects of greater surfaces area wherever similar current] densities are local- 11 effective.

encountered adjacent the work being plated, average current densities considerably lower or higher than the preferred values may be used,

as 'will be understood by persons skilled in the art. At the high current densities we employ, large tonnages of excellent galvanized stock can be obtained with a relatively small plant investment.

Another advantageous feature of our processis that the ammoniacal zinc solutions used as the electrolyte are much less sensitive toimpurities than the sulfuric 'acid solutions heretofore employed in the art. Good coatings of zinc may be obtained from an ammoniacal solution prepared simply by leaching treated and milled zinc ash,

with no purification whatever except to oxidize and 'remove the iron. A solution of this type may contain enough copper to give it a blue color, and may also contain cadmium and other im-' purities. A sulfuric acid solution made from the same raw material would require thorough removal of these impurities before equally satisfactory results-could be obtained.

In the practice of the process the ammoniacal zinc solu Depending upon the uniform ity of current distribution and the edge effects .1

zinc solution should be prepared with a high zinc content and an excess of free ammonia, and vigorous agitation of the solution should take place during electrolysis.

For satisfactory operations the zinc content of the solution should not be allowed to fall much below'100 grams per litre, and in no event below 80 grams per litre, since there is a tendency for the zinc coating to become spongy, instead of smooth and compact, at lower zinc concentrations and also an increased tendency toward tree formation at any point where the current density is even momentarily higher than the average. Best results are obtained when the zinc content is kept as high as 110 to 150 grams per litre. Zinc concentrations of the order specified can be maintained continuously by using solution from which zinc has been partially removed for leaching fresh zinc-bearing material.

When the solution contains little'or no excess free ammonia the coatings are usually rough. On increasing the free ammonia content the coatings become smoother. With enough free ammonia they are so smooth that they have a silky feel when the finger is run over them. The free ammonia content should not be allowed to drop below about 8 grams per litre for good results. Increasing the ammonia content beyond this point does no harm and indeed may be slightly beneficial as far as the coating is concerned. The only objectiont'o maintaining a larger excess of free ammoniathan is necessary comes from the fact that with more free ammonia the tendency of the solutions to lose ammonia is increased.

Suitable relative motion between the solution and the work during electrolysis can be effected by agitation, either'with the use of special agitating means or bymovement of the work being plated. The only essential'is that relative motion between thework-and the solution be maintained at 'suflicient ,vigor to produce turbulent or eddymotionin the solution adjacent the work.

" In electrogalvanizing wire, for example; the wire may be. dra n through the solution rapidly enough tomaintainthe conditions necessary for 'good-deposita'j Thegcritical speed of the'work, at which zturbulentmiw of the solution is obtained,

is det'ermihed' "by'flthe value of the Reynolds riio dulusi" The;critical speed is fixed by the density *of the'solution, the viscosity, and the diameter-of'=the wire.- In some cases, it may be necessary to depend partly, and in others entirely, on agitation-, by means other than movement of the work, as' will be (understood fully by persons skilled'in the art.

, These three factors, zinc content, free ammonia content'and agitation, are interrelated to a considerabledegree. Thus, increasingthe zinc content allows less vigorous agitation, for example,

a slower rate of movement if the work is being moved. Conversely, when the zinc content of the SOIIItiOnLiS relatively low more vigorous relative motion between the work and the solution isnecessary'to, obtain a coating of highest quality. The amount'of free ammonia bears somewhat the same'relation' to the degree of agitation as the zinc content- In carrying ammonia and ammonium chloride and assuming out'this'process the selected zinc-'- j bearing raw'm'aterial, for example raw zinc'as'h, treated and milled zinc ash, zinc oxide or sal skimmings, is first leached witha solution of. I

ammonia and an ammonium salt in order to'dissolve the zinc.-The reaction which takes place in I ourpreferred embodiment, using a solutionof'.

lieved to be essentially asfollows:

ZnO+2NH4CI+2NH4OH+Zn (NHs) 4CI2+3H2O As pointed out above, the electrolyte thus obtained is prepared with at least 8 .grams,.or approximately A mol, per litre or moreof excess free ammonia and is preferably made of sufficient concentration to contain more than 100 grams of zinc per litre of electrolyte. In speaking of excess or freeammonia we mean the amount of ammonia present in excess of that required to form a zinc tetramminesalt and to combine with acid radicals present in the solution. Various complex salts of zinc'and ammonia may be present, but wefind it convenient to assume the presence of the tetrammine salt and to consider ammonia in excess of 4 moles per mole of zinc as uncombined with zinc. In; addition to the excess of free ammonia we prefer to employ an excess of an ammonium salt such as ammonium chloride or ammonium sulphate, since we find that this practice improves the conductivity of the solution and reduces energy consumption during electrolysis. 'A considerable excess of such ammonium salt has no deleterious effect on the quality of the zinccoating.

i In several embodiments of our process, wherein sal skimmings or raw zinc ash are used as the source of zinc, substantial purification of the electrolyte occurs automatically due to the fact that considerable metallic zinc is always present insal skimmings and usually in raw zinc ash.

When treated and milled zinc ash or other zincbearing raw materials including very little or no metallic zinc are used as the source of zinc, it may be desirable to precipitate certain impurities by adding zinc dust or scrap zinc to the solution or by treating it with hydrogen sulfide. It is generally desirable to remove the traces of iron which may be present in the solution by oxidation. for example with hydrogen peroxide. A small addition of oxidizing agent suffices to oxidize the little iron that is present when the usual raw material is employed.

Any preliminary purification of the electrolyte which may be found desirable, or in fact any operation with this solution, is preferably carried out in closed vessels in order to avoid ammonia losses. The electrolyte thus prepared is'then fil-, tered thoroughly to remove suspended matter and led to an electrolytic cell, where the electrogalvanizing takes place.

The anodes employed in electrolyzing consist of insoluble materials; the cathode is the work being galvanized, as in present commercial practice. When the electrolyte is an ammoniacal zinc solution which does not contain chloride, anodes of iron, and preferably of 18:8 stainless steel. may be used. When the electrolyte contains chloride we prefer gas carbonor graphite anodes, since ferrous anodes are attacked under these conditions.

During "the electrolysis a high cathode density is used, preferably as high as 500 to 1000 amperes per square foot, although the value of the process is not restricted to such preferred current densities but extends also to other commercial oper- 2,200,987 zincyoxide to be the material dissolved, is be- This reaction is typical of what occurs where solutions of ammonia and other ammonium salts It will be observed that oxygen passes off at" the anodes. To permit the escape of this oxygen and at the same time toavoid substantial loss of ammonia, the electrolysis preferably is carried out in a closed but vented vessel. In addition, when operating the process continuously it is desirable that the solution in the electrolyte cell be filtered continuously in order to exclude accumulating solids which otherwise may be carried into the deposit and cause it to become rough.

The current efficiency in electrogalvanizing according to this process is so high that hardly any hydrogen evolution is perceptible at the cathode. When the conditions are not at the optimum, as for example when the zinc content is lower than desired or the agitation is insufficient, more hydrogen is formed. Under these conditions bubbles of hydrogen may sometimes adhere'to the cathode surface and cause a spongy deposit. We have found it helpful in such circumstances to add a small quantity of a surface tension depressant to the solution. As .little as 10 to 20 milligrams per litre of sodium lauryl sulfate reduces the size of the bubbles considerably and reduces the tendency of the bubbles to adhere, thus permitting better deposits to be obtained.

The temperature of the solution while galvanizing is not critical, but it is desirable to avoid high temperatures in order to minimize ammonia losses. At the high current densities used in this work, the solution tends to increase in temperature; hence provision shou d be made for cooling the solution, as by the use of cooling means within the electrolytic cell or by circulating the solution through suitable cooling apparatus. Temperatures of about 30C. have been found to give very good results.

After the zinc has been partially removed from the ammoniacal zinc solution and the conditions for obtaining a good deposit'become more exacting, the solution is preferably enriched by using it to leach fresh zinc-bearing material. A suitable procedure for continuous operation of the process is to circulate the solution continuously back and'forth between the electrolytic cell and the leaching tanks so as to keep the electrolyte reasonably uniform in composition during electrolysis. Continuous counter-current leaching is desirable to insure efficient extraction of zinc and to maintain a high zinc content in the solution.

The importance and advantages of our process will be further understood by persons skilled in the art from the following description of iilustrative embodiments.

Example I Commercial sal skimmings were used, as a raw material for the production of high grade electrogalvanizedstock. The sal skimmings were leached with a solution of ammonia and ammonium chloride until an electrolyte showing approximately the following composition was obtained:

, agitation being essential. with a smooth, adherent and ductile coating of Grams-per litre Zinc -c 104- Chloride 159 Total ammonia 164.2

Including: I

Ammonia combined as zinc tetramine salt about 108 Ammonia combined as ammonium chloride s about; 22 Free ammonia .'about 34 A small amount of oxidizing agent, hydrogen peroxide, was added to the solution to oxidize the small amount of iron therein, resulting in a precipitate which was filtered oif before introducing the solution into the electrolytic cell. Without further purification, the solution was used to electrogalvanize 12 gauge steel wire, employing'graphite anodes and a cathode current density of 1000 amperes per square foot. The wire was made to travel'through the bath at a linear speed of at least feet per minute, no further It issued from the cell zinc, which was attractive in appearance and consisted of 1 ounceor more of zinc per square foot. The galvanized product was ibent andi' worked without evidence of ing the coating.

, EmamplelIl' Raw zinc ash was leached with va solutionu of loosening or rupturammonia and ammonium chloride untila soluv tion showing the following composition'wasob- The solution was oxidized to remove iron, filtered and electrolysed substantially as'in Example 1.

Example I II A solution having .the composition indicated below was obtained by leaching zinc ash with a solution of ammonia and ammonium sulfate:

I Grams per litre Zinc 101 Sulfate 228v Total ammonia. 146.4

Including: v I

Ammonia combined as zinc tetramine salt ,1o5.4 Ammonia combined" as ammonium sulfate 28.2

Free ammonia 12.8

After preliminary purification and filtration this I solution also was used for electrogalvanizing wire at high cathode current densities, substantially as in Example I.

When employing sal skimmings as a source of zinc in the practice of our process, the solution inthe electrolytic cell preferably is continuously withdrawn therefrom, passed to the leaching tank, enriched with-zinc byfiowing it in countercurrent relation to more sal skimmings, treated. again with an oxidizing agent, filtered, and re-' introduced into the cell. In addition, chlorideis continuously extracted from the solutiomsince chloride otherwise would keep building up to an I skimmings. Thiscan be done economically by treating part of the spent solution from the electrolytic cell with lime and heating. The ammonia in the solution comes off in the vapor and is recovered either by passing it through a sombbing tower countercurrent to cold water or by passing it directly back to, and absorbing it in, the leaching solutions. The zinc contained in the spent cell solution is precipitated by the lime treatment and can be redissolved in the leaching operation. No special care is necessary to obtain the zinc precipitate free from chloride or calcium. As an alternative method of operation, most of the zinc-bearing raw material can be substantially free of chloride, only enough sal skimmings being used to furnish the chloride .lost in the cycle. The amount of chloride in the raw material and consequently the amount of solution treated for elimination of chloride when operating continuously are purely economic questions, depending on the cost of zinc in the form of sal skimmingsandits cost in the form of chlorinefree oxidized materials.

While we do not wish 'to be bound by any theory as to the mechanism of our process, a

working hypothesis has been developed which may explain the significance of maintaining anexcess, of free ammonia andagitating the solution duringelectrolysis: Only the main part of the current is effective in depositing zinc. The balance tends toliberate hydrogen and produce an equivalent amount. of alkali at the cathode. During jelectrolysis. at the high current densities employed, eventhough the current efliciency for zinc deposition is very'high, the solution immediately adjacent the cathode becomes much more alkaline than the bulk of the solution. As a result, there is atendency to form insoluble basic zinc compounds, which tend to be carried into the deposit and to cause it to become rough.

The presence of sufiicient free ammonia is .believed' to keep these compounds in solution, thereby preventing interference with the orderly growth of the crystal lattice of the coating.

The apparently critical eifect of relative motion between the work and the solution is believed to be related to the above phenomena. When wire is moved through the solution, it drags along with it a film of .solution, the thickness of which depends on the speed of the wire. The wire being cathodic, the zinc content of the film tends to become depleted, and the alkalinity tends to become excessive, so that conditions unfavorable to the production of a goodelectrodeposit are soon establishedunless zinc and free ammonia reach the film rapidly enough by diffusion from the main body of the solution. The thickness of the'film across which diffusion must take place is therefore an important factor in determining whether the coating will be smoothand compact or-rough and non-adherent The faster the wire moves, the thinnerthe cathode fllm will be and the better the coating. The greatest change is produced when the motion of the wire is increased to such aspeed that the motion of the adjoining layer of solution changes from straight line to turbulent flow. This is the critical speed; determinedby the'value of the Reynolds modulus.

In the foregoing specification we have described our improved electrogalvanizing process and illustrative embodiments thereof in considerable detail and have presented an hypothesis objectionable extent by accretions from the sal with the spirit of the specification as limited only by the requirements of the claims.

We claim: I I

1. The process of producing smooth electrogalvanized products which comprises passing a current at high cathode current density through a concentrated ammoniacal zinc solution in which the work being plated is immersed as the cathode, said solution containing ammonia substantially in excess of the amount required to form zinc tetrammine compound with all of the zinc in solution.

2. The process of producing smooth electrogalvanized products which comprises subjecting the work to be plated to a current having a density of at least 500-1000 amperes per square foot in a concentrated ammoniacal zinc solution, said solution containing ammonia substantially in excess of the amount required to form zinc tetrammine compound with all of the zinc in solution.

3. The process of producing smooth electrogalvanized products which comprises passing a current at high cathode current density through a solution of zinc tetrammine salt containing more than 80 grams of zinc per litre and a substantial excess of ammonia, in which the work being plated is immersed as the cathode, and maintaining relative motion between the work and said solution during electrolysis.

4. The process of producing smooth electrogalvanized products which comprises passing a current at high cathode current density through a solution of a zinc tetrammine salt containing more than 80 grams of zinc per litre and at least 8 grams per litre of excess ammonia, in which the work being plated is immersed as the cathode.

5. The process of producing smooth electrogalvanized products which comprises subjecting the work to be plated to a cathodic current of high density in a solution ofa zinc tetrammine salt containing more than 80 grams of zinc per litre and at least 8 grams per litre of excess ammonia, and maintaining relative motion between the work and said solution during electrolysis.

6. The process of producing smooth electrogalvanized products which comprises subjecting the work to be plated to a cathodic current of high density in a solution of a zinc tetrammine salt containing more than 80 grams of zinc per litre and at least 8 grams per litre of excess ammonia, and maintaining turbulent flow in the solution adjacent the work during electrolysis.

7. The process of claim 6 in which the work to be plated is passed through the solution at a rate suflicient to maintain turbulent flow of solution adjacent thereto.

8. The process of producing smooth electrogalvanized products which comprises dissolving zinc from zinc-bearing raw material in a solution of ammonia and an inorganic ammonium salt to produce an electrolyte containing more than 100 grams of zinc per litre in the form of a zinc tetrammine saltand-an excess of ammonia and depositing metallic zii c onto work immersed in the electrolyte, by passing current of high cathode current density through the electrolyte while maintaining relative motion between the same and said work.

9. The process of producing smooth electrogalvanized products which comprises dissolving zinc from zinc-bearing raw material in a solution of ammonia and an inorganic ammonium salt to produce an electrolyte containing more than 100 grams of zinc per litre in the form of a zinc tetrammine salt and an excess of ammonia, treating the electrolyte to precipitate iron therein, filtering the same to remove suspended matter, and depositing metallic zinc onto work immersed in the electrolyte by passing'current of high cathode current density through the electrolyte ,while maintaining relative motion between the same and said work.

, 10. The process of electrogalvanizing which comprises dissolving zinc from zinc-bearing raw material in a solution of ammonia andan inorganic ammonium salt to produce an' electrolyte containing more than 80 grams of zinc per litre in the form of a zinc tetramine salt and an excess of ammonia, depositing metallic zinc onto work immersed in the electrolyte by passing current of high cathode current density through the electrolyte while maintaining relative motion between the same and said work, dissolving additional zinc in electrolyte from which zinc has been deposited and depositing additional zinc" therefrom onto work to be platedas aforesaid.

11. In a process of producing smooth electrogalvanizecl products, the step which comprises electrolysing an ammoniacal zinc solution containing chloride and ammonia in'amounts suflicient to form zinc tetramine chloride with the zinc, said solution also containing a substantial excess of ammonia.

12. The process of producing smooth electrogalvanized products which-comprises dissolving zinc from sal skimmings in a solution of ammonia and ammonium chloride to produce an electrolyte containing more than 80 grams per litre ,of zinc in the form of -a zinc tetramine salt and an excess of ammonia, and depositing metallic zinc onto work immersed in the electrolyte by passing current of high cathode current density through the electrolyte while maintaining relative motion between the same and said work.

13. The process of producing smooth electrogalvanized products which comprises dissolving zinc from sal skimmings in-a solution of ammonia and ammonium chloride to produce an electrolyte containing more than 100 grams per litre of zinc in the form of a zinc tetramine salt and an excess of ammonia, depositing metallic zinc onto work immersed in the electrolyte by passing current of high cathode current density through the electrolyte while maintaining relative motion between the same and said work,.leaching sal skimmings with electrolyte from which zinc has of and thereafter depositing more metallic zinc therefrom as aforesaid, and extracting chloride from said electrolyte to prevent objectionable accumulation of chloride by accretion from the sal skimmings.

14. The process of producing smooth electrogalvanized products which comprises dissolving zinc from low grade zinc-bearing raw material by leaching the same with a solution of ammonium and ammonium chloride to produce an electrolyte containing more than 80 grams per litre of zinc in the form of a zinc tetramine salt and at least 8 grams per litre of excess ammonia, treating the electrolyte with metallic zinc, treatbeen deposited to increase the zinc content therepassing current at a density at least of the order of 500 to 1000 amperes per square foot of cathode surface through said solution and wire.

16. An electrolyte for electrolysis containing in aqueous solution more than 80 grams of zinc per litre, acid radicals from the group consisting of chloride-sulphate and carbonate radicals in excess of the amount required to form zinc tetrammine compound with all of said zinc, and ammonia in excess of the amount required to form said/zinc tetrammine. compoundv and to -form' ammonium compound with the excess of said acid radicals.

' 17. An electrolyte for electrolysis comprising a concentrated solution of a zinc tetrammine compound, said solution containing at least 8 3!; grams per litre of excess ammonia.

18.. electrolyte for electrolysis comprising an aqueous ammoniacal zinc solution containing at least 100 grams of zinc per litre, ammonia and acid radicals fromthe group consisting of chlo- 5 ride, sulphateand carbonate radicals in amounts at least sufflcient to form zinc tetrammine com-' pound with all of said zinc, and at least 8 grams per litre of excess ammonia.

19. An electrolyte for electrolysis comprising an aqueous' ammoniacal zinc solution containing "ammonia and chloride in amounts at least sufllcient to form zinc tetrammine chloride with all of the zinc, said solution containing at.least 8 grams of excess ammonia per litre.

" 20."An electrolyte for electrolysis containing in aqueous solution more than 80 grams of zinc per litre, chloride in excess of the amount required to form' zinc tetrammine chloride with all of said zinc, and ammonia in excess of the amount required to form said zinc tetrammine chloride and to react with the excess of said chloride.

21. An electrolyte for'electrolysis comprising an aqueous solution containing more than 100 grams of zinc per litre, more than 108 grams of chloride per litre and ammonia in excess of the amount required to form zinc tetrammine chloride with all of the zinc and ammonium chloride with any chloride that does not enter into the zinc tetrammine chloride.

22. The process which comprises electrolyzing at high cathode current density an ammoniacal zinc solution containing more than 80 grams of zinc per litre, acid radicals from thegroup consisting of chloride, sulphate and carbonate radicals substantially in excess of the amount required to form zinc tetrammine compound with all of said zinc and ammonia substantially in excess of theamounts required to form said zinc tetrammine compound and to form ammonium compoundwith the excess of said acid radicals,

and maintaining turbulent flow in said solution adjacent, the cathode during the electrolysis.

23. The process which comprises electrolyzing at a cathode current density of at least 500 to 1000 amperes per square foot, an ammoniacal zinc chloride solution containing more than 80 grams of zinc per litre,-chloride radicals substantially in excess of the amount required to form zinc tetrammine chloride with all of said zinc and ammonia substantially in excess of the amounts required to form said zinc tetrammine chloride and to form ammonium chloride with the excess of said chloride radicals, and maintainingturbulent flow in said solution adjacent the cathode during the electrolysis. 7

v JOHN P. HUBBELL.