摘要:

The Fwaday Societj is not respoizsible f o r opinions expressed befme it by Authors or Sp8akel.s. Gransact ions OF FOUNDED 1903. To PROMOTE THE STUDY OF ELECTROCHEMISTRY, ELECTROMETALLURGY CHEMICAL PHYSICS, METALLOGRAPHY, AND KINDRED SUBJECTS. _--PAP_______________ - - ____ - p-______---- _______ VOL. XVI. JULY, 1 9 2 1 . PART 3. ELECTBODEPOSITION AND ELECTROPLATING. A SYMPOSIUM AND GEiVERAL DISCUSSION. An Ordinary Meeting of the Faraday Society was held jointly with the Sheffield Section of the Institute of Metals on Friday, November Igth, 1920, in the iflappin Hall of the Department of Applied Science of the University of Sheffield. A series of Papers on “ ELECTRODEPOSITION AND ELECTROPLATXNG ” were presented and discussed. Professor C. H. Desch, D.Sc., Ph.D., Dean of the Faculty of Metallurgy, Vice- President of the Faraday Society, presided over the afternoon session in the unavoidable absence of the President, Sir Robert Hadfield, who had been expected to take the Chair and from whom a letter was read.PAPERS AND DISCUSSION ON GENERAL ASPECTS OF THE ELECTRODEPOSITION OF METALS. The Chairman in opening the meeting stated that the afternoon session would be devoted to papers and discussion dealing with certain general aspects of the electrolytic deposition of metals and the evening session to papers dealing more especially with the local industry of silver plating. He called upon Mr. W. R. Barclay to present the introductory paper on “ELECTROPLATING FOR THE PREVENTION OF COK- ROSION,” by Dr. Leslie Aitchison.472 A SYMPOSIUM AND GENERAL DISCUSSION Mr. W. R. Barclay in reading the paper in the absence of the author said that he was reading it because to some extent he was familiar with the author’s point of view, and with the view-point? really not only of Dr. Aitchison himself, but of the Air Board officials with whom he had done a considerable amount of work. The speaker was, during the greater part of the war and still was, consulting metallurgist to the Air Board and in that capacity had examined a number of specimens of work sent in which had been plated with metals purely and simply for their use as preventives of corrosion.

ISSN:0014-7672    

DOI:10.1039/TF9211600471

出版商:RSC    

年代:1921

数据来源: RSC

摘要:

ELECTRO-PLATING FOR THE PREVENTION OF CORROSION. BY LESLIE AITCHISON, D.MET., B.Sc., A.I.C. In almost all the industrial applications of iron and its alloys (i.e. steel) it is of great importance that steps shall be taken to ensure the permanence of the metal, or in other words that efficient protection against corrosion shall be provided. Very much the same problems arise in connection with the employment of aluminium and its light alloys. The conditions con- nected with iron and steel are more familiar than those with aluminium and its alloys and may most profitably be discussed here. Stainless steel is not to be considered, and therefore it may be assumed at once that all iron and all steel is affected similarly. The usual method of protecting iron and steel against corrosion is to cover its surface with a layer of some material which will perform two functions.The first function is that the layer must in itself possess per- manence in a high degree, and must not suffer or undergo disintegration or decay or be destroyed by corrosive actions, and the second function is that the layer must be one which will prevent any access to the metal beneath it of the corrosive agents which are usually destructive. These two conditions really mean that the layer must itself be able to resist at- tack by oxygen and moisture, weak acids, and solutions of electrolytes, and it also must be non-permeable to these agents and also have no interstices which might allow of the passage inwards of these destructive agents. The actual reagents which have been and still are employed for the purpose of protecting iron and steel may be conveniently divided into two classes, organic and inorganic.In the former class come the paints, var- nishes, and enamels, and in the second come the various kinds of metallic coatings. The second class-which is the class of interest-includes metallic coatings applied in various ways. In general the important ways of apply- ing the metal coatings may be divided into two classesl: ( I ) those in which the metal is put on hot, i.e. when molten, and (2) those in which the metal is put on cold, i.e. from a solution. The former includes such well-known processes as dip galvanising, tinning, and metal spraying. The latter in- cludes the processes of zinc and nickel-plating by electro-deposition. The cold deposition of metals as a prevention of corrosion has advanced very considerably in recent -years, but probably the proportion of iron and steel parts which are protected by this method is not very high, as will be readily appreciated when the magnitude of the tin plate and galvanised sheet (corrugated iron) output is realised.In general it may be assumed that the use of the plating methods is more or less confined to articles which require special conditions of coatings, or whose nature makes them such that they can only be plated effectively-i.e. without spoiling the base article-in the cold. The two general advantages which can be claimed for the plating processes as regards the coatings are (I) regulated weight, and (2) superior appearance.The second advantage is by no means small and would appear to account for the large use of nickel-plated steel articles 473474 ELECTRO-PLATING FOR PREVENTION OF CORROSION in bicycles, motor cycles, and automobiles. The former advantage is not very apparent in general industry, but comes quite considerably into play in aircraft-of whatever type. The weight of the coating is of course pro- portional to its thickness, and it is found that a coating of zinc of about three-quarters of a thousandth thick will provide a very satisfactory resist- ance to corrosion under aircraft conditions. It is quite impossible with a coating such as is provided by hot-dip galvanising to have a covering which is anything like so thin as this. Similarly of course the plated deposit has the notable advantage of uniformity, a property which is markedly absent from the coating produced by hot dipping.It seems probable, therefore, that the metal coatings deposited in the cold have such advantages as will result in their continued use. The general advantage of the cold coatings with respect to the base metal is the fact that the steel or iron need not be heated at all in order to effect the deposition. For such articles as tin plate or hot dip gal- vanised sheet, the heating is of no importance whatever, but for such parts as aircraft steels which have been put into a definite condition of heat treatment-either by cold working or by hardening and tempering-the heating for the deposition of the metal may be dangerous and may destroy the heat treatment condition completely. If, for instance, a hard rolled steel sheet was heated for galvanising it would be partially if not wholly softened, Also a nickel chromium steel sheet, hardened and tempered, might be af- fected in strength by such a heating, and would almost certainly be affected in its toughness, since it would be heated to a temperature at which temper brittleness could be induced.The metals which are most generally used for the protection of iron and steel by cold plating are zinc, nickel, tin, and copper. Writing from the point of view of the user of protective coatings and not that of the pro- ducer of the coatings, the most important metals are zinc and nickel. Whatever metal is used for the protective coatings it must possess certain properties.These properties appear to be as follows :- I. Uniformity. 2. Permanence. 3. Good appearance. 4. Freedom from porosity. 5. Freedom from pinholes. 6. Good adhesion to base metal. 7. Reasonable ductility. 8. Freedom from scaling or flaking. 9. Penetration into all parts of a complicated surface. 10. Minimum tendency to promote corrosion of the base metal if Most of these properties will be recognised at once as belonging to the sphere of the plater rather than to anybody else, and therefore it is not proposed to refer further to them here. The two properties which are of special interest are 7 and 10. The need for ductility in a protective coating is not always recognised and in a great number of parts the ductility is never called into play. Certain forms of material require, however, to be shaped after they have been galvanised, and it is of the utmost importance that in such circumstances the ductility of the coating shall not be inferior to that of the metal below.It is frequently necessary to bend such material over a radius equal to its thickness, and the coating must be capable of doing the same bend-that is when on the base metal, the coating is pierced.ELECTRO-PLATING FOR PREVENTION OF CORROSION 475 the actual severity of the bend for the coating being really very much less than that endured by the base metal because of the thinness of the former. Condition 10 is a more peculiar one, and is less easy to define or to present quite accurately. By the ordinary laws of electro-chemistry certain metals (owing to their position in the electro-chemical scale) will, when joined in a galvanic circuit to iron, be anodic and others will be kathodic.Theoretically, those which are kathodic should cause the iron to go into solution, i.e. to corrode, whilst those which are anodic should tend to in- hibit this action. Copper and zinc are two typical metals which can be considered. In theory copper should cause the iron to corrode, but zinc should protect it. The practical result which is obtained when, for in- stance, the protective coating on the iron or steel is destroyed or pierced at some point is that the iron corrodes whether copper or zinc is the covering layer, but that it corrodes more when copper is present than when it is covered with zinc. With a copper coating the attack upon the iron is very marked, but with zinc it is very much less noticeable, but it occurs all the same-apparentiy to approximately the same amount as it would if the zinc coating were absent.The corrosive action is not inhibited by the presence of the zinc, but on the other hand appears not to be accelerated as it is by copper. In connection with the protection of iron and steel by metallic coatings the preparation of the surface which is to receive the coating is of some importance. Obviously the surface must be clean and free from rust or scale. The usual way of removing the objectionable materials from the surface is by pickling in an acid solution. Normally this is quite a reasonable thing to do, but under certain conditions and with various kinds of steel, the action of the pickling bath upon the steel is to make the metal very brittle and consequently to render it quite unfit for use in structures or for bending or shaping. I t is generally thought that the brittleness is due to the absorption of hydrogen by the steel, but it appears from investigations now going on that the explanation of the action is not quite so simple. Practically the trouble can be overcome in a great degree by cleaning the articles electrolytically (i.e. by making them the anode in a galvanic cell). Preferably the articles may be sand blasted, but this is not always practical.

ISSN:0014-7672    

DOI:10.1039/TF9211600473

出版商:RSC    

年代:1921

数据来源: RSC

摘要:

ELECTRO-PLATING FOR PREVENTION OF CORROSION 475 DISCUSSION. The Chairman: I think it would be best if we discussed this paper now, since the ground is not quite covered by any of the subsequent papers. Mr. Stainer Hutchins : In reference to the statement that the hardness of the metal is not wholly due to hydrogen, I think the problem is not so simple and I should like to know what the alternative is, if it is possible to explain it. Formerly it was supposed that the hardness or brittleness of the deposited metal was due to hydrogen, but the author explains at the end of his paper that the solution is not so simple as that. Will he explain some of the difficulties? Mr. S . Field: This paper interests me very much, not because I am directly interested in the deposition of zinc for the prevention of corrosion but rather in the production of pure zinc. It appears to me that the present method of galvanising zinc by the hot process is open to much improvement.In the first place every student of chemistry knows the difference in the behaviour of pure and impure zinc against sulphuric acid-taken as an example of corrosive media.476 ELECTRO-PLATING FOR PREVENTION OF CORROSION I imagine that in electroplating processes where zinc is deposited coId the zinc must be infinitely purer. By the hot process the zinc deposit frequently con- tains anything up to and even more than 2 per cent. of iron. The ‘‘ protective ” metal is one which is essentially susceptible to corrosion, and it seems to me that, without any experience of the cold process, there is the advantage that it must put on a very much purer metal.In the cold method it would be advan- tageous for those concerned to use in every case the purest metal for their anode, and to maintain the solutions in a high degree of purity. Pure zinc will stand up to any sulphuric acid. I was glad to hear of the rigid tests with regard to ductility now being made. The Americans have always been very particular on that point and the brittleness of the zinc deposit is attributed to the presence of small quantities of cadmium. I have no personal experience of these galvanised deposits, but it seems to favour the suggestion that only the purest metal should be used for galvanising by the cold process. The Chairman: Are there any members present who have practical ex- perience of preparing galvanised coatings by electrodeposition ? I know that in the shipyards the small parts used in shipbuilding are very commonly protected in this way.Mr. S. Field: I should like if I may to supplement my remarks. I have for some time urged the use of purer zinc for hot galvanising in place of the ordinary impure spelter which contains anything up to 2 per cent. of impurities. Small scale experiments apparently yielded an inferior looking product. Pre- sumably appearance counted for everything. On a larger scale, it was then admitted that pure zinc gave an improved appearance to the galvanised product but no attempt appears to have been made to ascertain if there is any improve- ment in the durability of the deposit. I feel that if such a problem were tackled by those in a position to do it we should get a good deal of light upon this question of the protection of iron and steel against corrosion by means of zinc.Dr. W. H. Hatfield: It seems to me that the paper hardly lends itself to discussion; it is broad and covers much ground. I think if the author had restricted himself to some particular problems of interest to us in this locality we might have had a very much more intimate technical discussion. Speaking in the general way that one can, I may say I recently had the pleasure of going through several of the works in South Wales where the zinc galvanising process is in operation and also through some of the tin plating works, and I really think that if those gentlemen in this district who are interested in this question of non-ferrous coating would visit some of those works they would be surprised not only at the excellent technical manner in which the processes are conducted but they would be really surprised at the refined state in which the the tin is employed.I cannot remember now the weight of tin per pound of steel but it is very low and I under- stand that the coating is perfectly adherent. There is one point the author of the paper might have very thoroughly dealt with and that is the surface of contact of the non-ferrous with the ferrous metal. A good deal of work is called for concerning the nature of the adherence of one metal to the other. Electroplating methods are peculiar to the old industries in this town, and I think this meeting will serve a very valuable purpose in focus- ing the attention of the silver and allied industries to what science can do for them.We have now in Dr. Desch one who is interested not only in iron andELECTRO-PLATING FOR PREVENTION OF CORROSION 4 7 7 steel. It is necessary that other industries besides iron and steel should focus attention upon the scientific problems that affect their industry, and I hope this meeting will assist in that direction. Mr. W. R. Barclay replied to the discussion as follows : With regard to Mr. Hutchins’ question as to what other factors in addition to hydrogen may be held accountable for the brittleness of steel sheets when cleaned in acid pickl- ing solutions I think I know Dr. Aitchison’s mind a little in that direction but I ought to say in fairness to him that what he had in mind was to guard against committing himself to the view that hydrogen was the sole and only reason for such brittleness.There are as a matter of fact investigations going on in the direction of attempting to find a solution for the problem and he is aware of them and I think those who have studied the subject at all, that is from the empirical or practical point, feel that hydrogen is not the only factor that is accountable for this brittleness which is experienced. Mr. Field has compared the electrolytic deposition of zinc and hot galvanis- ing and one gathers from his remarks that he is strongly in favour of the cold electrolytic deposition of zinc. I think from a scientific point of view the advant- age is unquestionable and one would like to call attention to the very admirable work done by Burgess in that direction in which he examined the merits of the processes.The work was only published in Germany but it is a most valuable work. He showed very clearly by a long series of experiments that, even when you get the same thickness of electrolytic zinc and the same thickness of galvanised zinc, the difference is very great. Indeed very much thinner coatings of electrolytic zinc will give much more efficient protection, at least against dilute sulphuric acid, than will a coating of four or five times the thickness when made by the ordinary hot galvanising process. The Chairman : With reference to the last point that Mr. Barclay mentioned, there is a third method which often finds acceptance-the sheradising hot pro- cess, carried on at a very much lower temperature than the hot galvanising. You can apply a very thin layer and obtain quite efficient protection. Dr.L. Aitchison (communicded re$&) : In reply to the point raised by Mr. Stainer Hutchins regarding the cause of the brittleness of steel or iron which has been pickled, I state in the paper that this is not due solely to hydrogen. As to elucidating this statement, it is scarcely possible to go fully into the matter, as the experimental work in connection with it is not yet complete. It appears, however, to have been proved quite definitely that the brittleness resulting from pickling is only found in metals of certain types of constitution. An ordinary normal structure in plain carbon steel makes the metal in which it occurs almost immune from pickling brittleness. It is only under the most drastic conditions that such material becomes brittle. On the other hand, cold worked material is very susceptible to the effect of the acid, whilst hardened and tempered steel is not much less susceptible. The methods of removing the brittleness and the temperatures employed vary notably with the structure of the steel.

ISSN:0014-7672    

DOI:10.1039/TF9211600475

出版商:RSC    

年代:1921

数据来源: RSC

摘要:

SOME APPLICATIONS OF ELECTRO-DEPOSITION IN AERO- NAUTICAL ENGINEERING. BY W. A. THAIN, A.M.I.C.E., LATE CAPT., R.A.F. For the purposes of this short paper the applications of electro-deposi- tion in aeronautical engineering may be divided into three classes :- I. Original processes involved in engine construction. 2 . Processes designed to act as a protection against corrosion. 3. Salvage processes such as iron depositing. Copper-depositing processes coming under Class I are the only ones (a) Copper deposition as a protection against carburisation. (b) Copper deposition as a means of improving heat conductivity. (c) Copper deposition as a means of building up a definite construc- These three cases are considered below from the point of view of considered in this paper, and three special cases are discussed, otz.:- tional detail. practical application only. (a) COPPER DEPOSITION AS A PROTECTION AGAINST CARBURISATION. In some aero-engine details, such as cam-shafts, valve-gear rocking levers, etc., it is necessary that certain parts shall be case-hardened, i.e., the carbon content of that part is raised to about 0.9 to 1-0 per cent., so that on quenching from a suitable temperature, say 750' C., it shall give an extremely hard surface, and one offering great resistance to the external abrasive forces applied. The conditions of the design require that the carbon content of the remaining portion of the article shall not be increased, so that in the final condition these portions shall be comparatively soft and ductile. These conditions are obtained by two methods :- I .A sufficient machining allowance is made, so that where necessary 2. The portions not to be carburised are covered with a protective Of the many protective methods tried in actual practice, the only one which has given satisfaction, is the method of copper depositing. In order to illustrate the efficiency of good copper depositing as a protection against carbon penetration, the specimen, a section through which is shown in Fig. I , was prepared. On a steel of very lo* carbon content the protective coat of copper was first deposited, this is the intermediate layer and is clearly shown in the figure. The specimen was next carburised by the usual method of heating in a carburising medium at about gooo C. I t will be seen that the deposited iron is practically saturated with carbon, but that no carbon has passed through the pro- tective coating of copper.478 the carburised material may be machined off. coating impervious to carbon penetration. On this layer, iron was next deposited.APPLICATIONS OF ELECTRO-DEPOSITION 479 Another point of some interest is that the efficiency of the coating ap- pears to be independent of the composition of the steel, within the limits of composition laid down for case-hardening steels. FIG. I.-Illustrating the Efficiency of the Protective Coating of Copper. The specification requirements as regards composition are given in the following table :- TABLE I. CHEMICAL COMPOSITION OF CASE HARDENING STEELS. Specification. Carbon. . . . Silicon . . . . Manganese .. . Sulphur. . . . Phosphorus . . . Nickel . . . . Chromium . . . S. 14. Not more than 0 ' 2 0 per cent. Not more than 0'30 per cent. Between 0.40 and 1.00 per cent. Not more than Not more than 0.070 per cent. 0.070 per cent. - S. 15. Not more than 0'15 per cent. Not niore than 0.30 per cent. Between 0.20 and 0.60 per cent. Not more than 0.050 per cent. Not more than 0.050 per cent. Between 2.5 and 3'5 per cent. Not more than 0'30 per cent. s. 17. Not more than 0.15 per cent. Not more than 0'30 per cent. Not more than 0'40 per cent. Not more than 0.050 per cent. Not more than 0'050 per cent. Between 4.5 and 6.0 per cent. Not more than 0'30 per cent. VOL. XVl-T24480 SOME APPLICATIONS OF ELECTRO-DEPOSITION I t will be seen that the steels are of a fairly wide range of composition, varying from a plain carbon steel to a 6 per cent.nickel steel, although in all cases they are of low carbon content. Fig. 2 shows the main stages of manufacture of the cam-shaft, from the original bar to the finished shaft. The parts marked A, B, C, etc., are those requiring the protective copper coating. FIG. 2. Fig. 3 shows an example of a valve rocker lever for a rotary engine on which an efficient protective coating is particularly necessary. The only portions of this lever which are carburised, are the internal surface of the bore, and the external faces of the boss. The lever rocking on a case- hardened knife-edged pin. The The top photograph on this slide shows a side view of the lever. FIti. 3. lower photograph illustrates the particular necessity for efficient protection in this example.I t will be seen that the valve push rod is connected to the lever by means of a spherical-headed adjusting screw. The spherical head, working in the spherical cup at the end of the lever arm. The cup is made sufficiently deep so that connectional restraint may be obtained by riveting over the lower portion of the cup. This is clearly shown in the lower photograph. I t will be evident that inefficient protection wouldI N AERONAUTICAL ENGINEERING 481 mean brittle material, and would render the cup and ball method of con- nection impossible. Coypm'ag.-The methods employed for copper plating these parts have been of the usual standard types, with slight variations in detail, necessitated by the particular problem.The procedure consisting of- I. The general cleaning operations. 2. The striking in the cyanide bath. 3. The final deposition in the copper sulphate bath. The general principles of these processes are well known, and need not here be considered in detail. Scrupulous care, however, must be paid to the details of the processes at all stages, and in particular to cleanliness. The article to be plated must be perfectly clean both chemically and mechanically, all grease, rust, dirt, etc., must be absolutely removed. Where possible the surface of the article to be plated should be first sand- blasted with fine Calais sand, as this gives a uniform matt surface, and re- sults in an improved adherence of the copper. Anodes.-The anodes should be of the highest grade electrolytic copper, of say 99-7 per cent.purity, free from arsenic, antimony, and bismuth. The anode surface area should be slightly greater than the surface area of the articles to be plated (the cathode area). I n all aeronautical work a close fine-grained deposit is required and this means working with a comparatively low current density. In order to ensure uniformity of deposit, it is also necessary to arrange for the efficient circulation of the solution or for the rotation or rocking of the work to be plated. The following is a brief outline of the process of copper plating cam- s hafts and similar articles. I . The shafts are carefully brushed in a caustic-soda solution of half-a-pound of caustic soda to I gallon of water at a temperature of not less than 80" C.2 . Brush thoroughly in cold water with pumice powder. 3. Swill in hot water and dry. 4. Cover parts not to be case-hardened with stopping-off varnish, to prevent A mixture of yellow chrome and copal varnish mixed to the copper adhering. the consistency of treacle may be used. 5. Stove at a temperature of about 180" C., till the varnish is baked hard. 6. Parts to be plated, again scoured in cold water with pumice powder to 7. Swill in cold water. 8. Immerse in copper cyanide bath, a striking time of about three to five minutes being allowed, The parts are then copper tinted. This cyanide bath is necessary in the case of steel, as without this initial copper tinting, adherence of the copper in the copper sulphate bath cannot be obtained.The solution may be 8 02s. of cyanide of copper to I gallon of water, and should be kept at about 16" C. A warmed and agitated bath with free cyanide is essential in order that the anodes may be kept perfectly clean. 9. Articles removed from striking bath and well swilled in cold water. 10. Place in sulphate of copper bath made up in the proportion of 34 02s. A suitable thickness of deposit for case-hardening work, is about four-thou- The bath should be agitated and free acid should be present. Both in the cyanide and sulphate baths the work is arranged between two rows of copper anodes, and the distance between the work being plated and the anodes may be from 4 to 6 inches. ensure that no varnish is adhering to the parts to be plated. of copper sulphate, 5 ozs.of sulphuric acid, and I gallon of water. sandths of an inch.482 SOME APPLICATIONS OF ELECTRO-DEPOSITION With regard to the use of a stopping-off varnish, it may be noted that the following alternative method may be adopted. The article is plated all over, and the copper deposit finally removed by means of emery from those parts of the article which have to be case-hardened. In many cases this simplifies and sho tens the process, as in any case, the stopping-off varnish has to be removed before the case-hardening operation can be commenced. Fig. 4 shows a section through a copper-deposited coating, and shows clearly the possible uniformity of deposition obtainable. The surface of the steel (the lower section of the figure) had been sand-blasted. I n order to polish the section accurately it was embedded in white metal, which is shown at the top.(b) PARTS PLATED TO IMPROVE THEIR HEAT CONDUCTIVITY. An example of the problem in which the part is plated to improve its Whether the advantages gained heat conductivity may also be considered. FIG. 4. ( x 300.) are sufficient to warrant the additional work and expense involved is stili a debatable question and does not concern us here. The example con- sidered is the cylinder of a radial engine. The cylinder is of the air-cooled type with the usual fins integral with the main cylinder body. The cylinder is manufactured in a plain carbon steel, and is machined from the solid The surfaces of the fins and cylinder are copper plated in order to improve the heat-conducting properties of the mass.I n this particular example the great difficulty is to obtain anything approaching uniformity of deposit on the surface of the fins. The deposit from the heat-conducting point of view should have its maximum thickness at the root of the fin, and if anything diminish in thickness to the point. Fig. 5 shows the actual form of the deposit at the tip of the fin, and Fig 6 shows the actual form of the deposit at the base.I N AERONAUTICAL ENGINEERING 483 The principal causes of this inequality in the thickness of the deposit I. The diminution of current density owing to the opening out of the are the following :- current flow lines between the fins. FIG. 5. ( x 30,) FIG. 6. ( x 30.) 2. The difficulty of obtaining uniform flow of the electrolyte in the 3. Working at too high a current density.spaces between the fins.484 SOME APPLICATIONS OF ELECTRO-DEPOSITION in. -0025 '0035 -0032 '0035 The trouble was to some extent alleviated by careful circulation of the eiectrolyte, or by rotation of the cylinder in the bath, and principally by an extension of the time of deposition, from forty to fifty hours to a period of about sixty to eighty hours, i.e., by diminishing the current density. A series of measurements on sectioned cylinders gave the following results :- TABLE 11. in. '002 '002 -0025 -003 5th Space. Cylinder. A. in. -0025 moo4 '0035 '0035 I 2 3 4 in. .0015 '002 '0025 '0025 in. '003 '0035 '003 '004 15th Space. in. '0025 '003 ,0025 '003 A. 25th Space. A. The above results were considered satisfactory, and batches of cylinders were passed into service if the test cylinders gave results of the above order.Other methods of dealing with the problem will suggest themselves, and some were tried, but the results obtained are not sufficiently consistent and convincing to warrant discussion at the present stage. (4. The third case to be considered is that in which copper-depositing is used as a building-up process. The example selected as representative of this process is the copper- deposited water jacket. of the Beardmore aero-engine cylinder. The cylinder is of cast iron, and it is obvious that if the scheme of using copper- deposited water jackets can be successfully carried out, a very consider- able saving in weight will result. Many hundreds of cylinders have been water jacketed by this means, and have stood the severe test of active.service. The method has, therefore, been thoroughly proved and must be considered a sound practical proposition. The general method of procedure is briefly as follows : The cast iron cylinder is first rough bored, and the skirt turned. I t is then tested for strength and porosity under a water pressure of 450 lbs. per sq. in. If this test is satisfactory it is next finished, machined as far as possible, and the thread cut at the skirt to take the steel flange which secures the cylinder to the crankcase. To prepare the tapered thread on the skirt and the flange, they are thoroughly cleaned, then dipped for about ten minutes in a solution of copper cyanide, a thin coating of copper being deposited.This gives a good surface for the effective adhesion of the solder when the flange is sweated on. The sweating is accomplished by heating up the cylinder and flange so as to maintain the solder in a molten condition whilst the flange is screwed hard up. The flange is next machined up, and the casting dressed and cleaned. The cylinder is next put into a mould of cast iron, the shape of the mould This flange is screwed up and sweated on.I N AERONAUTICAL ENGINEERING 48s ;being that of the inside of the jacket. is placed in position in the mould. alloy corresponding to the inside of the water jacket. alloy used, may be either " Woods Metal," having a composition of- The outlet nipple made of copper A low-fusion alloy is now cast around the cylinder, the outside of this The low-fusion Lead , .25 per cent. Bismuth . - 5 0 9 , Tin . * 12.5 7, Cadmium . 12.5 ,) a r a special low-fusion alloy having a composition of- Tin . . 33*1/3rd per cent. Bismuth . 9 , 9 , Cadmium . 7 9 1 , l h e mould is removed and the surface of the alloy dressed, finished smooth and polished, corners and sharp fillets being avoided. The sur- face must be free from cracks and blowholes as the copper will not deposit over such irregularities The whole cylinder is now thoroughly scoured with pumice powder, etc., and every effort made to get it thoroughly clean, and free from grease and dirt. This is an absolutely critical operation, and the trouble and time spent on this operation is well repaid. The inside of the cylinder is now coated with paraffin wax to protect it against attack by the acid.Plugs or stoppers are fitted to the exhaust and inlet ports, and to the bore of the cylinder, and the flanges covered with beeswax where not to be copper deposited. It is now suspended in the vat by means of brass stirrups, the suspension being such that the cylinder is free to rotate on a belt-driven vertical spindle. The spindle is revolved at about 30 r.p.m. to ensure even distribution of the deposit. The copper anode plates are placed alongside and between each cjlinder and must be of sufficient depth to extend below the lowest part of the cylinder. The cyanide vat solution consists of- Cyanide of Potassium . . 4 ozs. Copper Sulphate . . 8 02s. Water . . I gallon. Sulphuric acid is added according to the working of the vats.When fully covered with the cyanide deposit, the cylinder is removed, cleaned and transferred to the copper sulphate vat. The vats were of sufficient .dimensions to deal with five cylinders and required an e.m.f. of I to 2 volts and a current of IOO amps. for five cylinders, or about 20 amps. per cylinder. Length of time in vat eighty hours for the first deposition. This gives a thickness of about 1-5 mm. and a weight of 3-5 lbs. of deposited copper. The cylinders are removed twice a day, scrubbed, and the surface made smooth. The shape of the cylinder is such that no eddies are set up in the solution, and an easy flow of the liquid is maintained when the cylinder is revolving, otherwise unequal deposition would take place. After the first coat the cylinder is washed and dried, and is then im- mersed in a bath of hot oil at 230' C.This temperature is sufficiently high to melt the " Woods Metal," which then runs out at the water inlet and outlet ports. I f the special fusible alloy is used, it can be melted out in boiling486 SOME APPLICATIONS OF ELECTRO-DEPOSITION water, and this, of course, considerably simplifies the succeeding cleaning operations. The collars of bushes and nipples are soldered round, to fill in the sharp corners and to prepare for the second coat of copper. The inlet water connections, which are of copper and made separately, are then soldered in place, The cylinder is now again thoroughly cleaned, and the usual pre- cautions taken to prevent the acid attacking the steel parts, rubber tubing being placed over the studs and round the thread of the exhaust nipple. The second coat of copper is then deposited, the time occupied being ten hours and the weight of deposited copper 3 lb. This second coating adheres firmly to the first coating, provided the conditions of cleanliness The bushes, nipples and studs are next screwed into place. FIG. 7, have been rigidly observed. The final thickness of the jacket wall being about I -7 mm. The cylinder is again thoroughly cleaned and all stopping- off material removed. Machining operations are now completed and the bore finally ground. The jacket is subjected to a water-pressure test of 2 5 to 3 0 lbs. per sq. in., and if satisfactory is passed out to service. As will be seen from Fig. 7, corrugations are made in the sides of the jacket, and also around the sparking plug bosses in order to take up the expansion occurring under working conditions. Fig. 7 shows a photograph of a sectioned finished cylinder and water jacket. The copper-deposited jacket is clearly shown in white in the photograph. The above examples indicate the general lines on which the application of copper-depositing has developed in aero-engine practice.I N AERONAUTICAL ENGINEERING 487 I n conclusion I desire to express my thanks to Brig.-General R. K. Bagnall-Wild, the Director of Aeronautical Inspection, for permission to publish much of the information contained in this paper. VOL. XVI--T25

ISSN:0014-7672    

DOI:10.1039/TF9211600478

出版商:RSC    

年代:1921

数据来源: RSC

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I N AERONAUTICAL ENGINEERING 487 DZS c USSZON. Mr. D. J. Macnaughton asked what other metals besides copper were tried to prevent carbon penetration. It is generally recognised that such deposits as electrolytic iron will not be satisfactory as wearing surfaces unless they can sub- sequently be case-hardened, and in France where they made use of the process it appears that at some of the centres they first copper-plated and then deposited electrolytic iron on the copper. This would prevent any subsequent case- hardening, and the process was tried in some of the centres whereby electrolytic iron could be deposited direct on the steel. It would be interesting to learn what other materials were used which would prevent carbon penetration, as this would indicate any material which must be avoided as preliminary to the deposi- tion of electrolytic iron. The Chairman : I am sorry there has been no further discussion.I take it that the applications are rather too novel and we have not had direct experience of the work, but it struck me that the last application is one likely to be extended very widely because it provides a quite new method of building up elaborate parts. I should think that this is the most elaborate thing of the kind that has been done in copper deposition and it seems exceedingly interesting. The Chairman : I have special pleasure in calling on Mr. Carr because he has been working in this University for a very long time. H e was a practical and experienced metallurgist I suppose before most of us in this room were born and he still sets a most excellent example to the younger workers by his keenness in research. We are greatly indebted to him for always placing at our disposal his great store of experience in electrolytic deposition and kindred problems in non-ferrous metallurgy. VOL. XVI--T25

ISSN:0014-7672    

DOI:10.1039/TF9211600487

出版商:RSC    

年代:1921

数据来源: RSC

摘要:

THE ELECTRODEPOSITION OF COBALT. BY BYRON CARR. The following tests were carried out more than two years ago to check the results of Dr. H. T. Kalmus referred to in a paper by Mr. W. S. Burrows to the American Electro-Platers’ Association. The cobalt bath used was that numbered XIIIb which is made up as follows :- Five 02s. boric acid are dissolved in 59 pints of boiling water. In this z+ 02s. of sodium chloride are dissolved, and then 4+ lbs. of crystalline cobalt sulphate. The bath was used hot at 34” C. in these experiments, though temperatures as high as 100’ F. have been recommended when working with very high current density reaching 780 amps. per. square foot with stationary electrodes, and even 1000 amps. per square foot where the solution is agitated. To obtain satisfactory results the cathode must be perfectly clean and smooth, all imperfections being still clearly seen after the deposition of the cobalt.The bath should be clear and excess of boric acid, if un- dissolved, should be filtered off. One set of experiments made with a muddy bath, possibly due to an impure sample of the cobalt salt, gave deposits which, though appearing at first sight sound and adherent, left the basis plate at once on the slightest bending. “Quicking” in a solution of mercuric cyanide was not found to be appreciably better or worse. The deposit, which is very hard and will take on a very fine polish, is remarkably resistant to atmospheric corrosion. One polished deposit which has been lying in a chemical laboratory for over two years is still practically perfect.I t will be noted from Table I. that only the thinner deposits are adherent, and that as they thicken they begin to blister and peel off spontaneously or do so under very small bending stresses. The cobalt foil made in this way, i.e. by allowing a thick non-adherent deposit to peel off, may have commercial value. This is quite in accord with the results of Dr. Kalmus, who recommends a current density of about 150 amps. per square foot, and a period of im- mersion of two minutes. Some experiments were carried out to determine the influence of the composition and surface of the basis plate. The results are collected in Table 11. The deposits polish well, and though thin, have no tendency to expose the basis metal beneath. It will also be noted that in comparison, for instance, with nickel plating, these cobalt deposits are obtained very quickly indeed. 468 The electrodes on this work were stationary.THE ELECTRODEPOSITION OF COBALT 2 6 3 5+ 5 TABLE I. DEPOSITS IN BUFFED SURFACE OF BRASS. Do. do. do. Do. do. do. Do. do. do, Deposit peeled off cathode. Deposit distinctly blistered and beginning to peel off. 489 Nickel . . . Polished brass , . Do. do. . . Brass plate coppered in alkaline copper bath Copper plate buffed . Copper plate as received No. 48 48 81.6 50'5 81.0 71.5 I 10 I IU 2 3 4 5 6 7b Current density, amps. per sq. ft. -1 I- 22'8 22'8 49 ' 2 49'2 72 72 72 72 Ti.me, mins. Deposit. TABLE 11. Basis Plate. Current density, amps. per sq ft. Ti.me, mins. 3 6 5 2 Deposit. Deposit peeled off. Very good adherent deposit. Polishes well. The lower C.D. gave slightly better deposit. Good deposit. Blistered : not adherent. Good but rough deposit. Very smooth deposit,

ISSN:0014-7672    

DOI:10.1039/TF9211600488

出版商:RSC    

年代:1921

数据来源: RSC

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THE ELECTRODEPOSITION OF COBALT 489 Mr. W. R. Barciay : I am very glad to have the pleasure of opening this discussion in order first of all to pay a tribute to Mr. Carr for veryvaluable help at a critical time. It happened that, as Mr. Carr has said, these experiments of Dr. Kalmus and his assistants aroused a good deal of our interest, but the more valuable and perhaps the more practical result came later on. I had the responsibility at one period of the war of advising some of the Army authorities in France as to the use of electrodeposits for building up worn parts, and a very interesting discussion occurred at one stage as to whether it would be better to make permanent the plants which had already been started for the deposition of iron for such a purpose, or to adopt nickel or some alternative metal.Experi- ments were put in hand for nickel and then Mr. Carr’s work, following that of Dr. Kalmus, suggested the possibilities of cobalt. Mr. Carr’s experiments have shown that in a very considerable degree at any rate, the original results of Kalmus and his collaborators were confirmed. But I think it must be pointed out-and I think Mr. Carr will agree-that we were not satisfied at all as to the possibilities of cobalt when such a purpose was in view as the building up of worn parts to the extent of possibly from 4-thousanths of an inch to 10, I I or even 12-thousanths. We obtained, as Mr. Carr has explained, some very good re- sults, owing largely, if I may say so, to his extraordinary patience and mani- pulative skill in depositing, but I think the real difficulty with the process is the difficulty of getting these very thick deposits at high current densities to be perfectly adherent. Incidentally there is another difficulty that of course arises490 THE ELECTRODEPOSITION OF COBALT from the comparatively little knowledge we have of the metallurgy of cobalt.The trouble is to obtain anodes of a very high degree of purity, and during the last eighteen months I have had to face the thing from a purely metallurgical point of view. It was only when I had the task of producing a small quantity of rolled cobalt anodes that I realised what difficulty there was in the operation of producing rolled cobalt anodes. There again one gets a very distinct and definite lesson as to the difference between cobalt and nickel.Malleable nickel for rolling is readily produced by addition of magnesium and one would naturally conclude that cobalt could be treated similarly. An attempt to do this, however, resulted disastrously and other methods had to be sought. Mr. S. Field : It seems to me that this subject of cobalt-plating must have a very big future on account of the wonderful properties of the deposit which are obtained and the rapidity with which work can be produced. At the Northamp- ton Polytechnic Institute we have installed a cobalt bath in connection with the stereotyping class, and I understand that one of the large London printing houses is putting in a cobalt solution to replace nickel for facing stereos. One understands the intense hardness of the cobalt deposit and quite a few minutes immersion should suffice to give a durable deposit to stereos.I am not aware of that being done to any large extent but certainly the method is being introduced. Mr. E. A. Smith : We are much indebted to Mr. Can for these experiments which confirm the work of Dr. Kalmus and add valuable facts based on his long experience in the electrodeposition of metals. I should like to endorse what the last speaker has said with regard to the future. I think there is a promising future for the electrodeposition of cobalt for special purposes. The ordinary hardness of cobalt in the Brine11 test is about 124 while that of nickel is only about 76.4, so that the surface of deposited cobalt should have a much greater resistance to wear than nickel.I should like to ask Mr. Carr whether he has made any attempts to determine the hardness of the cobalt deposit as compared with nickel deposits ? I should also be glad to know whether he has tried to deposit cobalt on cast iron, as it is not usually easy to get a good deposit of other metals on such material. A problem connected with cast iron has recently been referred to me and it seems to me that cobalt, if it could be satisfactorily deposited on cast iron, may possibly solve this problem. I join with previous speakers in adding my thanks to Mr. Carr for his very interesting paper. Mr. Stainer Hutchins : As there seems to be such a great difficulty in getting pure cobalt for the anode, can a satisfactory deposit be obtained with an impure anode ? Mr.Byron Carr replying to the discussion said : In regard to Mr. Smith's suggestion, I have not measured the hardness because it would be necessary to deposit the cobalt on a much stronger base metal than we have used up to the present, but I believe Dr. Kalmus does give the hardness. The hardness might be easily found when a man comes to polish the deposit ; he cannot buff it like a nickel surface. I do not think there would be any difficulty in plating cast iron with cobalt. With regard to anodes, these deposits have been obtained with cast anodes which are readily obtained. They do contain some impurity, but the im- purity does not amount to above 1 . 5 per cent. I have never attempted to de- termine it, but you can readily get cast anodes of cobalt, and they have given deposits which are fairly satisfactory.THE ELECTRODEPOSITION OF COBALT 491 The Chairman : The figures given by Kalmus are only for the cast metal ; he gives the Brine11 hardness of cast cobalt at 124 and cast nickel as 76.4.Of course it does not follow that there would be the same difference between the electrodeposits, but I think your experiments in polishing showed a very much better hardness. Mr. Carr : Yes, and in future I will deposit some on some stronger base and perhaps Dr. Thompson will determine its hardness. The Chairman : I am sure those who look at the specimens will be struck at the beautiful character of the deposits obtained with cobalt, the very fine surface and the remarkable way in which it stands atmospheric corrosion. I was sur- prised to hear that it stood corrosion by fruit juice so badly ; that seems a little bit unexpected.; it stands quite remarkably well not only in ordinary air but in the laboratory air ; I do not know the comparative acidity of gooseberry juice and the atmosphere of the Sheffield metallurgical laboratory, but these specimens have been exposed here for at least three years.

ISSN:0014-7672    

DOI:10.1039/TF9211600489

出版商:RSC    

年代:1921

数据来源: RSC

摘要:

THE COMMERCIAL ELECTROLYSIS OF ZINC SULPHATE SOLUTIONS. BY SAMUEL FIELD, A.R.C.Sc. (LoND.). liztroductiofi.-Some slight extension of the usual definition of electro- plating is necessary to justify the inclusion of this subject in a symposium on electroplating. The problems of the deposition of compact, yet readily detachable, sheets of pure zinc from acid zinc sulphate solutions derived by the leaching of zinc ores with sulphuric acid, are of wider scope than those usually associated with the deposition of the more negative metals from either acid neutral or alkaline solutions. Partly upon the solution of these problems depends the economic success of the electrolytic extrac- tion of zinc. This industry has, in some countries, attained considerable vogue. At Anaconda a 150 ton-a-day plant is in operation.Smaller plants are running in other parts of America while in Tasmania a plant yielding 10 tons of pure zinc per day is being extended to IOO tons per day. These are results which have been achieved only after long periods of experiment, and success or failure in one or other stage of the opera- tion depends upon the most scientific control of mere details. One of the earliest difficulties was that of the production of sound deposits of zinc. Later troubles have, however, centred round the production of adequately pure solutions of zinc sulphate from the relatively impure mzterials from which it was and is sought to extract zinc. I t is now pro- posed to illustrate some of these difficulties, more particularly in relation to the deposition processes.Preliminary Hydro-metaZZurgicaZ Processes.-Normally, sulphide ores of zinc which may be very complex by reason of admixture or combination with many other metals, notably, lead, copper, cadmium, iron, manganese, nickel, cobalt, arsenic and antimony, are after metallurgical concentration, calcined with a view to the elimination of the bulk of the sulphur as sulphur dioxide, and the conversion of the remainder to zinc sulphate in predetermined amount. The essential zinc compounds-viz. zinc oxide and sulphate are extracted from the calcine by leaching with acid zinc sulphate liquors outflowing from the electrolytic cells. These liquors may have an approximate composition of z to 3 per cent. zinc and 10.5 to 12 per cent. sulphuric acid. A judicious mixture of acid liquor and calcine economically reduces the acidity to *I per cent. after which complete neutralisation is effected by the addition of whiting or available zinc hydrate or carbonate.Passing over the purely chemical problems as- sociated with this operation, a neutral solution of zinc sulphate, containing small but appreciable quantities of many metallic impurities is obtained. The solution contains practically 10 per cent. of zinc. The data given in this paper serves to emphasise the importance attaching to the processes of purifying this zinc liquor to a degree at which it becomes almost im- possible to detect, by the most searching analytical tests, some of the more 492ELECTROLYSIS OF ZINC SULPHATE SOLUTIONS 493 deleterious of these impurities.So pronounced are their evil effects in the subsequent electro-deposition, that it has been claimed that the zinc deposition cell is much more sensitive than chemical tests ordinarily applied. With this statement, however, we find it impossible to agree. Taking nickel as one of the common impurities in these liquors-and a highly detrimental one too-we are persuaded that the dimethyl-glyoxime test will discover one part of nickel in twenty million parts of liquor, while the maximum nickel ordinarily allowable in the liquors for electrolysis would be I to 2 parts per million, while having regard to commercial economy, recently devised methods of purification bring down the nickel content to one part in five million of liquor. Subsequent to adequate purification the liquors are acidified (to impart conductance) with a proportion of the f NCOTS FIG, I.outflowing cell liquor to give a composition of 7 to 8 per cent. of zinc and 4.5 to 3 per cent. of acid. This liquor flows through suitable cascades of electrolytic cells provided with chemical lead anodes and aluminium cathodes. Steps are taken to prevent the deposition of the zinc on the edges of the cathodes so that when deposits of 24 to 48 hours run are obtained they are readily removed from, and without injury to, the cathodes. These detached deposits are then run down to ingots which, when due care has been exercised in all stages of the process, will assay 99-95 per cent. of zinc with not more than -02 per cent. cadmium, -02 per cent. lead, and '01 per cent.of copper and iron together. The simple scheme of these operations is indicated in Fig. I which will be readily followed.494 THE COMMERCIAL ELECTROLYSIS OF Normally a daily output of IOO tons of zinc cathodes involves stripping about 8000 cathodes yielding 16,000 plates of zinc each weighing about 15 lb. and -07 in. thick. The total cathode area exposed to the cur- rent amounts to 150,000 sq. ft. The plant of the Metals Extraction Corporation at Llansamlet upon which it was our privilege to carry out many tests which are here referred to, had a capacity of 14 tons of metal per day and involved a daily stripping of 2 2 0 cathodes yielding 440 sheets each weighing 7 '5 lb. and approximately & in. thick. Conditions Inzuencing Economic Deposition. -In this connection it must be borne in mind that success in any attempt to establish an electrolytic zinc industry in this country will be largely determined by the greatest economy in comparatively costly electrical energy-the comparison being made with the cheap hydro-electric power available in other countries, The conditions influencing energy consumption are therefore of paramount importance.At the same time economy of electrical power will have to be set against the cost of producing and handling the volumes of zinc liquors in which such economic energy consumption is to be realised, and also the capital outlay on the hydro- and electro-metallurgical plant required in these operations. Economy will therefore be determinable only when all the local conditions of cost of current and plant are known.We pro- pose, however, to touch upon only a few of these conditions and these mainly associated with the presence of impurities. Energy Consumption will be controlled by the average cell P.D. and current-efficiency :- I ton zinc f 840000 ampere hours (approx.). 840000 x P.D. IOO :. I ton zinc x - c. ER. K.W.H. 1000 P.D. x 84000 K.W.H. - - C. Efficiency Minimum P.D. and maximum current efficiency tend to the greatest economy in energy consumption. Taking quite normal figures an average P.D. of 3-35 volts with a current efficiency of go per cent. represents 3125 K.W.H. per ton of zinc cathodes. The importance of current efficiency will be realised also from the fact that a loss of I per cent. in current efficiency represents, on a IOO ton plant, a ton of zinc short per day.Again the composition and temperature of the starting and outflowing liquors has an important bearing on energy consumption. A neutral 10 per cent. zinc solution would be impracticable on account of its high re- sistance, and the slight sacrifice in current efficiency which results from reducing the zinc to 8 per cent. and adding 3 per cent. of acid is much more than compensated for by the considerably lower P.D. necessitated. The composition of the outflowing liquor is of even greater importance. A very marked falling off of current efficiency is likely to occur when the zinc content is down to less than 4 per cent. Under these circumstances it might be thought desirable not to proceed with the extraction of zinc beyond this point.Another consideration however comes into play. Assume that by some sacrifice of current efficiency with increased K.W.H. consumption that the zinc content could be reduced to z per cent. JVith a neutral IO per cent. zinc liquor delivered from the purification plant, the two cases can be compared as follows :- Case L-Outflowing liquor with 4 per cent. zinc, the larger part of which goes back to the leaching vats.ZINC SULPHATE SCLUTIONS 49 5 Extraction of zinc = 10 - 4 = 6 per cent. I 0 0 Tons purified zinc liquor per ton zinc cathodes = - = 16.6 tons. 6 Further, a quantity of zinc equal to or 66 per cent. of the output of cathodes is returned to the leaching tanks, there to be again contaminated with impurities and cyclically requiring purification with unduly high costs both of plant and operation.Case IL-Outflowing liquor with 2 per cent. zinc, the larger part of which goes back to the leaching vats. Extraction of zinc = 10 - 2 = 8 per cent. Tons purified zinc liquor per ton of zinc cathodes = - = I 2 ' 5 tons, with a quantity of zinc now only = 25 per cent. of the output of cathodes going back to the leaching vats and necessitating cyclical purification. The two cases clearly indicate the desirability of the maximum extrac- tion of zinc from the liquor in the cells and the attainment of just those conditions which make this high extraction economically possible. In$uence of Impurities.--The presence of impurities-in some cases even in most minute traces-exercises a most important bearing on this economy.Of the impurities of which mention has already been made, the briefest statement of behaviour in the electrolytic cells must here suffice. Co&w becomes deposited with the zinc and is found in the final ingots. In the cells, however, it sets up, with the increase of acidity and temperature, a vigorous corrosion resulting in a marked re-solution of the zinc, corresponding to a much reduced current efficiency. The presence of copper may cause the entire re-solution of comparatively thick zinc cathodes. Copper must be eliminated. Cadmium becomes deposited with the zinc and occurs in the final ingots. Being of like electro-chemical properties with zinc, it produces no such corrosion as is occasioned with copper, but assuming that the whole of the cadmium is deposited with the zinc, and this usually occurs, and that the allowable quantity in the ingot is '02 per cent., a simple calculation will show the permissible quantity in the liquor.When present in the liquors, it depre- ciates current efficiency by anodic oxidation to ferric and cathodic reduction to ferrous. Manganese becomes oxidised to permanganic compounds without sub- sequent sensible reduction of current efficiency. Arsenic and Antimony seriously interfere with the deposition process and an increase of antimony content from -3 to I part per million of liquor is said to convert a good cell operation to one which is economically im- practicable. Nickel' and CohzZf in minute quantities similarly exert disastrous results in the electrolytic operation. Curiously, they behave in totally diverse manners.Each of these metals has occasioned considerable trouble in electrolytic zinc processes, the dangerous traces of the metals not being readily removed except by special purification processes. Their insidious effects are, moreover, cumulative-as the metals are not deposited in the zinc-unless special methods are employed to effect their removal in the cycle of operations. Their effects are so marked and so different that they might well provide means of identification for the small amounts con- cerned. The effect of nickel is observed in deep sharply cut and clean holes in the zinc deposit. That of cobalt is a general pinhole corrosion I 0 0 8 Iron is not sensibly deposited. VOL. SVI--T2j*496 THE COMMERCIAL ELECTROLYSIS OF yielding a deposit having a structure similar to a fine lace, with dark patches on the back of the deposit.These effects taken in conjunction with kilowatt hour consumption per ton of zinc, conclusively prove that the chief problem in electrolytic zinc is that of obtaining a sufficiently pure liquor for electrolysis. The methods of purification which have been worked out do not come up for description here, but it may suffice to say that methods-economic methods-are available for the removal of the whole of these impurities, giving the electrolytic operation immunity from complications which in some industrial attempts seemed to have proved insuperable. Experimental Results.-Concurrently with the production of zinc on the commercially experimental unit of I+ tons per day, a series of labora- tory tests, to investigate problems arising, was carried out.These experi- ments were effected in a series of 12 cells in cascade, reproducing approxi- mately, though not entirely, the conditions obtaining on the larger plant. Single experiments usually extended over a period of 24 hours. A typical series of figures obtained from a commercially pure solution are embodied in Table I. from which the curves in Fig. 2 have been plotted. The high current efficiency obtained even in a liquor containing 9 per cent. of acid is indicative of what should be obtained on a commercial plant. A study of the K.W.H. curve indicates the possible advantage accruing from further acidification prior to electrolysis, this dilution in no way affecting the volumes of liquor in the metallurgical section of the plant.In any case the curves in Fig. z may be taken as typical and provide some standard for comparison.ZINC SULPHATE SOLUTIONS 497 TABLE No. I. 12 cells in cascade Starting liquor Zinc 8.0 per cent. Acid 3'0 per eent. Outflowing liquor ,, 4-06 per cent. ,, 8-91 per cent. CD = 28 amperes per square €oot. Solution pure. ~~ P.D. Average Composition of Liquor. K.W.H. Per Ton Zinc. 3991 3195 3139 2935 2814 28 10 2794 2799 2810 2845 2922 2959 ---- Current Efficiency. Zinc Deposit, Grams. 171.7 171'2 171.9 171'7 170.7 169.2 168.6 167.8 165.1 160.5 152.6 150.8 Cell No. I 2 3 4 5 6 7 8 9 I0 I1 I2 Per Cent. Acid. 3-26 3'78 4 '3 4-82 5 '34 5-84 6.85 7'34 7'83 8-30 8.76 6.3 5 Per Cent. Zinc. 4'63 3-59 3'64 3-40 3-24 3-2 I 3-18 3-17 3'13 3-08 3 '04 3.01 7'83 7'48 7'13 6'79 6'44 6.10 5'77 5'43 5-10 4'47 4-16 4-78 96-8 96'5 96'9 96.8 96'3 95'5 95'1 94'7 93'1 90.5 87-0 85.0 ----- 3'37 Table No.z sufficiently illustrates the trend with added impurities. In this case -02 per cent. of cobalt is quite an unusually large amount. Th2 very rapid falling off in current efficiencyis to be noted. Little note, however, need be taken of the possibly inaccurate P.D. figure in cell No. I. TABLE No. 2. Conditions same as in Table No. I, but '02 per cent. Cobalt added. Average Composition of Liquor. K.W.H. Per Ton Zinc. Zinc Deposit, Grams. 134'9 132.8 128.5 127.5 127.4 105-0 103.2 94'6 82'2 80.1 62.3 59'05 Current Efficiency. P.D. Cell No. I 2 3 4 5 6 7 8 9 I0 I1 I2 Per Cent.Zinc. Per Cent. Acid. 5.28 3 '4 3'25 3 '23 320 3-20 3'17 3'15 3-15 3'13 3'09 3 '09 --- 4620 3022 2985 2962 2983 3597 3603 3955 4523 4612 5855 61 77 _----- 1 4074 95'5 94'0 91'0 90'3 90.2 74'4 73'1 67'0 58.2 56'7 44'1 41.8 ---- 7'79 7'38 6-97 6.58 6-19 5'83 5-50 5.20 4'93 4-68 4'46 4'27 3'31 3 '93 4'54 5-13 5'72 6-26 6'74 7-19 7-60 7'98 8-31 8-59 Averages 73'0 1 3-36 By way of comparison the current efficiency curve from this table is inserted in Fig. z in a dotted line, the P.D. and K.W.H. curves being omitted.498 THE COMMERCIAL ELECTROLYSIS OF Zinc Deposit, Grams. -- 175'3 174'5 174.2 172.8 172-1 172.1 146.6 '37'5 126.9 126.7 122.4 I 14.2 Coming down, however, to a more normal quantity of cobalt which may occur in commercial zinc sulphate solutions, Table No.3 shows the results obtained with a liquor containing 10 parts of cobalt per million of liquor. TABLE No. 3. Effect of -001 per cent. Cobalt. l- Cell No. I 2 3 4 5 6 7 8 9 I0 KI I2 I 2 3 4 5 6 7 8 g 11 12 10 185.2 185 184.7 184.5 184.4 182.7 181'4 181.1 151.2 143'9 140-9 128.3 Current Efficiency. 96-8 96.4 96-2 95'45 95'1 95'1 81.0 75'95 70-1 70.0 67'7 63'1 -- Averages . 1 83.5 P.D. 3.81 3'72 3-51 3'32 3'23 3.16 3-08 3'05 3-00 3-00 3.00 3-00 3'24 --- ~- Average Composition of Liquor. Per Cent. Zinc. 7-8 t 7'43 7 '05 6.68 6.3 I 5'94 5 -59 5-28 4'99 4.71 4'44 4-17 Per Cent. Acid. 3-28 3-86 4'43 5'53 6.09 6.6 I 7-08 7-51 7'93 8'34 8'74 4'98 K.W.H. Per Ton Zinc. 3246 3226 30.50 2908 2839 2777 3179 3357 3578 3583 3 709 3975 3255 -- From these figures, for which curves are not necessary, a striking ob- servation and one which has been repeatedly made is the sudden decrease in the current efficiency figures rather than a gradual change which might be anticipated from the slowly changing conditions.This point is under close investigation. It may, however, be said that in the large scale opera- tion the cobalt would be reduced to about one part per million of liquor. The effect of nickel is comparable with, though perhaps worse than, that of cobalt. TABLE No4. Effect of -0005 per cent. Nickel. Zinc Deposit. Grams. Current Efficiency. 96-96 96-85 96'7 96.6 96'5 95'6 95 94'8 79'2 75'3 73'8 67'2 88.7 - P.D. 4'25 3-50 3'46 3'30 3.28 3-21 3-13 3.02 3-01 3-01 3-00 2-95 3 '26 Average Composition of Liquor. Per Cent. Zinc. 7-82 7'46 7.10 6'74 6.38 6-a3 5'68 5'33 5-01 4'72 4'44 4*=7 --- Per Cent. Acid.3'27 3-81 4'35 4-89 5'43 5'96 6.48 7 '00 7'49 7'92 8-34 8 '75 K.W.H. Per Ton Zinc. 3664 3021 2992 2856 2841 2807 2754 2663 3179 3342 3398 3670 3099ZINC SULPHATE SOLUTIONS 499 I Table 4 shows the effect of the addition of *ooo5 per cent. (5 parts per million of liquor) of nickel. I t must be pointed out that this quantity is far in excess of the amount remaining in our liquors by normal purification on the commercial scale, the methods employed reducing the nickel con- tent to one part in five million. I t must be remembered, however, that whereas in these experiments impurities were introduced singly, commercial liquors will contain traces of a number of impurities. The beneficial effect of colloidal materials on electro-deposits is well known and Table No.5 illustrates this in the case of the deposition of zinc from acid solutions. The solution is again that used in Table 4 with the addition of 5 parts of dried size per million of liquor. TABLE No. 5. Effect of addition of '0005 per cent. Size to liquor containing '0005 per cent. Nickel. Cell No. I 2 3 4 5 6 7 8 9 I 0 I1 I2 Zinc Deposit. Grams. 191'1 191-6 191.5 191.9 190.7 190.6 191.5 189.3 187.2 180.9 174'4 171.2 Averages Current Efficiency. P.D. 96 '2 96'6 96 '0 96.0 96'4 95'3 94'2 91-04 87'8 86.2 96.4 96'4 4-07 3 -66 3'45 3'3 3'32 3'26 3-10 3-02 3-02 3-00 3 '00 2'93 94 Average Composition of Liquor. Per Cent. Zinc. 7-83 7 '49 7-15 6-81 6.4 7 6-13 5'79 5'45 5-1 2 4-80 4'48 4.18 Per Cent. Acid.3'25 3 -76 4'27 4'79 5-29 5-81 6-3 I 6.83 7'32 7.80 8-28 8-73 K.W.H. Per Ton Zinc. 3538 3173 2991 2856 2892 2840 2688 2650 2679 2754 2859 2842 2898 --- The beneficial effect is marked but only of a temporary character. The maintenance of the improved results necessitates the frequent further addition of colloid. While the addition of a colloid might be made merely as a temporary expedient, it must be borne in mind that it in no way re- moves the cause of trouble which recurs in an aggravated form in the next cycle of operations. Indeed the only way to maintain satisfactory deposition is a high degree of purification by methods which admit of commercial application. ConcZusion.-It will be seen that a large subject has only been lightly touched upon. There is an accumulation of experience in electro-deposi- tion for which no adequate explanation is at present forthcoming. The influence of traces of impurities (if the term is permissible) is sometimes beneficial and frequently the reverse. Without suggesting that the con- di tions uilder which zinc is electrolytically extracted are exactly comparable with those obtaining in the general processes of electro-deposition, it must be agreed that any possible light on the causes of the diseases to which deposits are susceptible may be of value in designing processes to impart immunity to suoh operations. At least the lesson will be impressed that more care should be exercised over substances admitted to depositing solutions than usually obtains,500 THE COMMERCIAL ELECTROLYSIS OF and that no constituent should be present without a definitely known object and then in only the proportion which will yield the best result. On this score, electroplaters doubtless have a large accumulation of ex- perience extending over many years, and a meeting of this character achieves much in offering facilities for bringing such experience to light with a view to collation, explanation, and wider application. The author acknowledges with thanks and appreciation the painstaking assistance of Mr. W. H. Hawkes to whom he is indebted for carrying out much of the work of which that embodied in this paper is but a very small part. Electro- chemica I Laboratory, Northamp ton Po0 technic Institute, hndon, E.C. I .

ISSN:0014-7672    

DOI:10.1039/TF9211600492

出版商:RSC    

年代:1921

数据来源: RSC

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THE COMMERCIAL ELECTROLYSIS OF Mr. W. R. Barclay : Mr. Field has brought forward some very interesting data. I am sorry that I have not hadmore time to think the questionover. One thing that must be taken into consideration with 'regard to this and similar papers is that the problems of electrolytic refining and the problems of electroplating are essentially different and I should be rather sorry if anyone went away from this meeting with the idea that the lessons learned from electrolytic refining are necessarily to be taken as as applying to electroplating. The point of the purity of the solutions, for example, by no means relates, so far as I know, to any par- ticular process of electroplating of metals, unless it is to a very small extent, and it is important that we should always differentiate between the two.In electro- lytic refining what is required is a perfectly pure metal which has to be re- melted. On the other hand for electroplating almost invariably we have to secure : first of all a perfectly adherent coating; secondly, a coating of good appearance and a coating which can, very often at any rate, be highly polished. None of these three factors has any relation to electrolytic refining. Mr. E. A. Smith : I think we should welcome this paper because Mr. Field has touched on a branch of electrolytic deposition of metals which is sometimes neglected by those who are more directly concerned with the electroplating in- dustry. He has dealt with a branch which has made enormous strides within recent years, possibly greater strides than in connection with the deposition of any other metal.During the last fifty years a very large number of processes have been introduced for dealing with low-grade zinc ores so that the zinc might be profitably extracted. One of the great difficulties met with in these processes was to get the zinc out in the form of metal. In most of them they only recovered the zinc in the form of oxide or in the form of a compound which was eventually converted into oxide, and it was only when the electrolytic deposition of zinc was successfully accomplished as the result of a great deal of research that it was possible to make these processes for the treatment of low-grade zinc ores a commercial success. As Mr. Field has reminded us there are large plants being erected in Tasmania and in other parts of the British Empire to deal with low-grade zinc ores, and I think that in the future the amount of zinc produced by electrolytic deposition processes will be a very formidable competitor to the present smelting processes.I have been very much interested in the paper andZINC SULPHATE SOLUTIONS 50= the authcr has pointed out some of the difficulties which have to be over- come in making the electrodeposition of metals for extraction purposes on a large scale a commercial success. Mr. Turner : I should like to ask Mr. Field whether he can tell us why such a remarkable improvement takes place on the addition of *0005 per cent. of size to the solution ? Was it a question of increased or decreased conductivity or has Mr.Field any theory which he cares to expound? The Chairman: I should like to ask one question. Within recent years there has been a considerable quantity of high-grade zinc produced in this country by electrolytic processes, but I believe it is a fact that all the firms who have been manufacturing zinc by that process have recently closed down their plant. Can Mr. Field throw any light upon this practical failure of the process within recent years? There is also a scientific point which struck me as of consider- able interest ; the difference in the behaviour of nickel and cobalt. From our studies of chemistry we have always been led to understand that these two metals were so closely alike as to be almost indistinguishable. But in iron and steel metallurgy one finds a very considerable difference in the behaviour of the two, and here it comes out again.It would seem that the similarity of these two closely allied metals was not so great as one has been led to expect. Mr. Macnaughton : I would supplement that previous question by asking if the effect is so pronounced in the case of nickel as cobalt and whether the effect is permanent or whether additions have to be made from time to time ? Mr. S. Field, in reply to the discussion said : This subject is not one of elec- trolytic refining but of extraction. In the early days of this zinc metallurgy the problem was to get a pretty good deposit, and the next to get out the zinc in a pure form and to get it out efficiently. I am not able to offer any explana- tion of the wonderful effect of those colloids. Many of us are awaiting a simple yet adequate explanation of apparently very simple effects which every electro- plater knows.Dr. Desch referred to the apparently wonderful difference between cobalt and nickel. I am at a total loss to account why these two seemingly similar substances should act so differently. In reply to Mr. Macnaughton, I may say that the use of colloids is permissible only as a temporary measure. What is wanted is to get out the maximum of zinc from solutions highly purified ; it is not a matter of covering up impurities. With regard to Dr. Desch’s ques- tion it should be remembered that no large scale on commercial plant has yet been put down in this country. Certain experiments carried out in South Wales by one of the zinc firms produced between IOO and 200 tons of the pure metal. Then the experiments closed down with a view to building upon a more con- venient site and on a very much larger scale. You could not produce electro- lytic zinc commercially in this country with a plant of I & tons a day capacity ; the nearer it gets to 2 0 or even IOO tons a day the better the prospect will be.

ISSN:0014-7672    

DOI:10.1039/TF9211600500

出版商:RSC    

年代:1921

数据来源: RSC

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THE DEPOSITION OF GOLD-SILVER ALLOYS. BY SAMUEL FIELD, A.R.C.Sc. (LoND.). Introduction . The composition of electro-deposited alloys has not claimed wide attention, though such deposition has long been established practice. In two previous papers alloys of copper and zinc and silver and copper have been dealt with. The investigation has now been carried a step further in a study of the conditions influencing the simultaneous deposition of silver and gold. Both of these highly electronegative metals are deposited singly with ease. Further, it is a well-known fact, and one which it is now proposed to follow quantitatively, that the colour of gold deposits is considerably influenced by the presence of impurities such as silver and copper. This fact is utilised in the production of coloured deposits, a gilding solution containing 10 to 20 per cent. of its metal content of copper yielding a pleasing " red gold," while a smaller addition of silver cyanide produces the familiar " green gold ".These cases provide a marked contrast with that of silver and copper, for, in a previous paper,' it has been shown that considerable difficulty is experienced in the simultaneous deposition of these metals, and analytical methods of separation are even possible. With gold, however, copper and silver are readily precipitated electrolytically from the cyanide solution, and from this it would appear that gold occupies an intermediate position being positive to silver. and negative to copper. In spite of the use of these coloured deposits for so long no attempt seems to have been made to ascertain by systematic experiment the influence of conditions upon the composition of the resulting alloy.This we have essayed to do, supplementing the results of laboratory experiments by the analysis of a series of deposits obtained on prepared electrodes under commercial conditions in a London workshop. The P.D.'s of the three metals in cyanide solution are given by Christie as follows :- N . KCN. Copper '81 Gold 1 '37 Silver '3 3 !! KCN. KCN. KCN, I0 '62 '3 7 - -05 -16 - -38 - -36 If then copper and silver present difficulties in simultaneous deposition, and this has been previously confirmed, so also it might be anticipated that lTrans. Faraday Society, vol. vi., p. I. a Amer. Inst. Miniiig Eng., Sept., 1899. 502THE DEPOSITION OF GOLD-SILVER ALLOYS 503 .I difficulty should be experienced in the deposition of alloys of copper and gold.Method-Solutions of the double cyanides being invariably used in the deposition of silver and gold, these have been exclusively examined. Pre- liminary experiments were made with the quantitative deposition of gold befme dealing with the alloy solution. The results obtained show that a good cathode efficiency is onIy obtained with solutions much stronger than those ordinarilj employed in the gilding process, and for the greater part of the work a solution containing 19.7 grams. of gold per litre [= 2.9 02s. troy per gallon] was used. The cathode efficiency was ob- tained by reference to the copper deposited in a coulomb meter in series and the following figures show the order of the efficiencies obtained, being preceded by the equivalents of the metals :- This matter is reserved for treatment in a further paper. Copper Silver.Gold from from from sulphate. cyanide. cyanide. oras . . . . . I 2 3'396 6.195 - Chemical equivalent . . . 31'8 63.6 108 I97 Grams per ampere-hour . . . 1.182 2.364 4-024 7'35 TABLE I. I (CURRENT EFFICIENCIES IN WARM MOVING GILDING SOLUTION USING PLATINUM ANODE.) '0522 ,0692 '4493 '4858 '5232 -6608 I Experiment. I 2 3 4 5 6 Current (amperes). Copper from Coulomb Meter. '25 '50 *60 '75 '75 '75 '1436 'I432 01856 "478 '1424 i Gold. I Cathode Deposit* 1 Efficiency. 5.88 7-80 39'0 53'2 59'3 70.0 It was thus early learned that good efficiencies are best obtainable with moderately high current densities, and a number of other important points soon came to light.Thus with low current densities very little deposition occurs even in relatively strong solutions. Further the deposit of gold is susceptible to solution in the cyanide liquid causing a depreciation in the efficiency. Still further, some difficulty was encountered in selecting a cathode material which would be most suitable for the subsequent deposi- tion of the alloys. Ultimately for the reasons given later, assay lead foil was selected. The quantitative deposition of silver offers no difficulty given fairly strong solutions of the metal. Thus, using a semi-normal solution of the double cyanide (approximately 8 02s. metal per gallon) the following results were obtained :- Experiment.I 1 I 2 l 3 98.8 1 ,?T8 1 z:': 1 95'8 Cathode efficiency . . . Anode ,, . . .504 THE DEPOSITION OF GOLD-SILVER ALLOYS Weight of Gold Deposit. Some loss, however, was persistently recorded in subsequently cupel- ling the deposits made on lead cathodes. Such differences were :- Weight after Cupellation. i Weight of Silver after Cupellation. I Weight of Silver Deposit. Loss. Percentage Loss. I i i -3660 '5725 '5408 '3617 '5678 '5336 '0043 '0047 '0072 1.17 '82 1-33 These losses, however, include a small loss by corrosion of the lead cathodes, but in view of the method subsequently adopted in the deter- mination of gold would be negligible. Similar, though much smaller, errors for gold were observed as follows :- Loss. Percentage Loss. 1.3022 '6544 '4865 1*3010 '6506 '4848 '00 I 2 -0038 *oor7 '09 -58 '35 but as these figures also include whatever corrosion of the lead cathode had occurred they were disregarded.DEPOSITION OF GOLD-SILVER ALLOYS. ( I ) The soZuzction first used in this work was obtained by mixing calculated volumes of decinormal gold and seminormal silver double cyanide solutions, each containing a minimum of free cyanide. The mixed solution contained gold and silver in the proportion of 2 : I , 2 I *6 grams of the mixed metals per litre [ = 3-18 02s. troy per gallon]. I t should be noted that green gold solutions vary very widely in their composition, one ordinary recipe being :- Gold ~ d w t . . . , . 2-74 grams. Potassium cyanide I 02. . . . ' } Silver + ,, Water I pint . , . . . I litre. . . . .. corresponding *46 ,, to 50 9 , The generally low metal-con tent is noticeable. ( 2 ) Choice of a Cathode.-The subsequent analysis of the deposit in- fluences this choice. Deposits readily attacked by a single acid are con- veniently made on platinum gauze. The use of platinum, however, is precluded in ihis case. I n view of a simple method of analysis of deposits by cupellation, assay lead offered great advantages. Chemical cleaning and drying prior to weighing did not prove satisfactory, the deposits invariably stripping owing to the difficulty of preventing oxidation during drying. Coppered lead, and silvered lead cathodes were tried, but proved impracticable owing to excessive corrosion in the cyanide liquid, while plain lead was almost unaffected. Cathodes were therefore prepared by carefully scraping off the surface of the sheet, in preference to chemical cleaning and scouring with pumice. Adherent deposits were thus obtained though on drying, a decided increase in temperature was likely to effect separation of lead and deposit owing to the considerably differing coefficientsTHE DEPOSITION OF GOLD-SILVER ALLOYS 505 of superficial expansion, via.: gold, .oooo2g4 ; silver, .0000382 ; lead, *oooo572 ; or, as I : 1'3 : 2. Flaking away of the deposit from the lead in the early stage of heating during cupellation, was avoided by wrapping the dried and weighed deposit in more lead foil previous to rolling up for cupellation. The separation of lead and deposit could in some cases be distinctly discerned as a cracking noise.The corrosion of lead in the cyanide solution amounted in several trial experiments to only decimal parts ('5 or '6) of a milligram, and this was, in view of deposits of a metal with so high an equivalent as that of either silver or gold, regarded as negligible. Silvered lead cathodes, previously referred to, seemed to offer considerable advantages : (I) I n the greater adherence of the deposit, and ( 2 ) in that sufficient silver could thus be used to produce a parting mixture with the subsequent deposited alloy after cupellation. Excessive corrosion, however, amounting to 2 to 3 centigrams on the small cathodes in use, militated against their use. (3) Choice ofAnode.-The use of gold and silver anodes is in this case impossible owing to (a) their more than efficient electrolytic corrosion, leading to an increase of the metal content of the solution, and (8) the impossibility of thus arranging for a constant ratio of gold to silver in the bath.Platinum anodes were therefore employed, and the loss of metal from the solution was made good before a succeeding experiment by the addition of approximately computed volumes of standard gold and silver cyanide solutions. In some cases the analysis of a deposit was completed before the next experiment was arranged, and even where this was not possible, a rough idea of the silver and gold withdrawn from the solution could easily be gained and made good. Slight changes in composition in the relatively large volume of solution are negligible in single experiments, but errors are prevented from accumulating by correcting the amounts as the analytical results were obtained.Thus in a particular case noted the following figures show the approximately computed quantities added, and the correct quantities calculated from the analysis. In successive experiments :- Volumes of Standard Solutions (c.cs.). Added by Rough Computation. Gold. I Silver. Required by Calculation from Analysis. Gold. I Silver. I5 I5 15 Totals 45 5 7'5 5 17'5 -- 4'9 6-8 6.0 17'7 The solution was then corrected after the three experiments by the addition of '9 C.C. gold solution, and '2 C.C. silver solution to the 600 C.C. of solution in use. Slight increase of concentration by slow evaporation at 50' C. at which some of the experiments were made was practically compensated by these additions of solutions.(4) Anaosis of Deposits.-After drying and weighing, these were wrapped in a further quantity of lead foil and sufficient silver added toTHE DEPOSITION OF GOLD-SILVER ALLOYS give a parting mixture. After cupellation and treatment with acid the residual gold was washed on to a small filter, and after roughly drying, this was again enclosed in a minimum of lead foil and cupelled. ( 5 ) Cumjosition of Deposits.-Following the method referred to in previous papers the composition of the alloy may, under exceptional cir- cumstances, be obtained by a comparison of its weight with that of the copper deposited in the coulomb meter. Thus if deposition is quantitative, that is, strictly proportional to the chemical equivalents, the relation of the weight of the extreme alloy containing only gold to that of '' sulphate," copper will be as 197 : 31.8 or as 6.195 : I , while with the oppositely ex- treme alloy free from gold, the proportion will become 108 : 31.8 or as 3.396 : I .Between these extremes each intermediate proportion corre- sponds to a definite composition of alloy as may be seen from the following table :- TABLE 11. Composition of Alloy. ~~ Per Cent. Gold. 1 - Per Cent. Silver. I00 90 80 70 60 50 40 3 0 20 1 0 0 = Weight Deposited Alloy Copper from Meter ' 0 I0 20 30 40 50 60 70 80 90 I00 6.195 5'72 5'32 4'97 4-66 4'39 4*=4 3'93 3'736 3'557 3'396 These values are plotted in the accompanying curve (Fig. I ) and from the curve or figures the following expressions are obtained for calculating the percentages of gold or silver for any given value of R.Per cent. gold in 221 deposit = - (R - 3,396). R 121.5 R Per cent. silver in deposit = - (6.195 - R). These calculated values, however, can only be true when hydrogen is not deposited, but in the case of simultaneous evolution of the gas they represent limits which, in the case of gold are minima, and with silver, cannot be exceeded. EFFECT OF CURRENT DENSITY. For these experiments the solution contained- Gold . . 14.4 grams per litre Free cyanide . . negligible. Silver . - 7.2. ?, 3 ) Solution at room temperature and stirred. The results are embodied in Table 111. and the variation of gold per cent. with current density is clearly seen in curve A Fig. 2. The anticipated increase in the propor-TABLE 111.SEE FIG. 2, CURVE A. 18-02 15.93 40.01 47'35 57'90 SOLUTION 10.35 9 '1 26.0 31'4 34'5 Temperature of solution = 20' C. Solution stirred. I Gold, 14.4 grms. per litre, Silver, 7'2 f f 9 9 ,, P*D* Copper from Coulomb meter. Per Cent. Percentage 1 Current Depositing. Weight Alloy Deposited. Copper from Coulomb meter. Remarks on Deposits. C. P.D. R. Experiment. 1 Found. 1 Gold. Silver. ~. 87'5 86.1 71.2 63 -6 58'3 Calc. 10'15 10.5 34'7 38.3 38.6 I----- I ~ _ _ ~ White. No trace of yellow. Faint tinge of yellow. Yellow deposit slightly burned. Darker. Rough dark deposit. '4363 '4335 ,6138 '503 5 '65 3 7 '3 I '41 '515 -62 -81 2.15 2.40 3'15 3 -80 4 '0 -1232 ,1216 '1524 '1226 '1589 3'549 3'565 4.028 4.107 4.114 TABLE IV. SEE FIG. 2, CURVE B. SOLUTION As in Table 111.Temperature of solution = 50' C. Solution stirred. I Percentage Gold. Per Cent. Current Depositing. Weight Alloy Deposited. Remarks on Deposits. R. Found. Gold. 9'15 11'0 24% 30'7 31.2 31.8 Silver. 85'5 86.3 72.0 66.8 68.8 61.0 Calc. 5'2 13'5 32'4 41'7 45'3 35'2 White. No trace of yellow. Good deposit ; trace of yellqw. Uniform light yellow deposit. Light yellow deposit. Dark yellow deposit. Dark yellow and rough. 16-31 I 8.86 3 8 9 45'67 45'20 48.75 I '52 '62 '65 '706 -802 r o z 2'20 2-40 2'75 2'95 3'15 3-40 -1023 '1233 7282 '1389 -1580 -1603 '3556 '4459 '5104 '5796 '6752 '6476508 THE DEPOSITION OF GOLD-SILVER ALLOYS tion of the more positive gold is forthcoming, though the rate of deposition is far in excess of that which would ordinarily be employed in the gilding process, amounting, in experiment 5, to as much as 40 amperes per square foot.EFFECT OF TEMPERATURE. The temperature of the solution was raised to 50' C. and the stirring continued. A similar series of experiments was made and the results are given in Table IV. Here the P.D.'s required to maintain the same current densities are markedly less and the percentage of gold is thereby considerably diminished, this being very obvious when comparing gold-silver alloy copper from sulphate to composition of alloy. FIG. 1.-Relation of ratio curves A (cold) and B (warm) in Fig. 2. On the whole the conditions are favourable to quantitative deposition of the metal, this being seen from the addition of the percentages in the last two columns, in one case (experiment 5) no hydrogen appearing at all.I t may perhaps be explained here that the percentage of current depositing silver is calculated from the difference in the weight of the deposit and that of the gold which it is found to contain. EFFECT OF CONCENTRATION. A portion of the solution was then diluted with its own volume of water and a further series of deposits made under otherwise similar conditions,THE DEPOSITION OF GOLD-SILVER ALLOYS 509 PEI CEN COLl 5( 4a 3c 20 10 - 0 FIG. 2.--ERect of current density and concentration. I I I t I 1 FIG. 3.-Effect of free cyanide.510 THE DEPOSlTION OF GOLD-SILVER ALLOYS using the solution both cold and warm. The results are embodied in curves C and D in Fig. 2 which indicate an increased proportion of the more positive gold in both cold and warm solutions. EFFECT OF FREE CYANIDE.It now remained to find the influence of excess of potassium cyanide on the composition of the resulting alloy. For this purpose successive ad- ditions of the salt were made to the solution previously in use to bring up the content to 10, 20, 30, and 40 grams per litre. Experiments were then conducted with both cold and warm solutions, the results being indicated in curves E and F in Fig. 3. In these experiments a constant current of '3 ampere was aimed at corresponding to *08865 gram of copper in the coulomb meter. Variations from this figure were small and except in one case did not exceed 1-3 per cent. being more usually below I per cent. In both cold and warm solu- tions free cyanide raises the proportion of gold in the deposit, the effect being apparently more pronounced in the warm solution.Summary of ResuZts.-Thus while gold is in many of its compounds decidedly more negative than silver, the cyanide solutions provide an exception. The figures for the single potentials of gold and silver given on p. 502 in potassium cyanide solutions are confirmed by :- I . The increase in the proportion of gold with increased current density. 2. The decrease in the proportion of gold with increased temperature. 3. The larger proportion of gold obtained from more dilute solutions. 4. When however we trace the effect of free cyanide it is found that- (a) There is a quantitative improvement of the deposit with increasing cyanide, but (8) In spite of the lowering of P.D.with additions of free cyanide the proportion of gold increases, indicative of a smaller difference between the two metals as shown by the increase of the percentage of gold without change of other conditions. In the case of copper zinc alloys the reverse occurs, the addition of cyanide inducing a greater difference between the two metals and thus facilitating the easier separation of the copper. Szlpplenten fa I. In order to supplement the work an attempt has been made to obtain a number of similar deposits during the course of the routine work of one whose experience of the deposition of coloured gold deposits is far wider than that of the author. A series of three weighed lead cathodes was supplied, and from the ordinary solution in constant use deposits were made with varying current densities.The plates were returned for analysis and yielded the following results :- Solution used :- . . . . . . 2-74 gms. Silver 14 . . . . . . . . .. 50 * t Gold 8 dwts. KCN. 8 02s. to Water I gallon . . . . . . . . . . . I litre Gold. I 77'9 No. of Weight of Weight of Per cent. Plate. C. Deposit. Gold. 2 5 amp. '3420 -2664 2 '75 9 9 '5300 '4 I 20 77'7 3 1.25, $ 9 '3614 '2774 76'7THE DEPOSlTION OF GOLD-SILVER ALLOYS 5 1 I Comparing these results with those obtained on the laboratory scale xhe higher percentage of gold is readily accounted for by- I . the larger proportion of gold to silver, viz., 6 : I , 2. the more dilute solution, 3, the relatively large proportion of free cyanide. The author of these experiments, reports without any idea of the com- position of his deposits that he finds very little variation in the shade of the deposit with varying current density. Variations in shade arise mainly owing to the loss of silver from the bath. A small silver anode is used side by side with a larger gold plate but no control over the corrosion of the two plates and consequently of the composition of the solution can be effected apart from a knowledge of the respective amounts of gold and silver passing out into the deposits. With the preponderance of free cyanide common to ordinary plating solutions the conditions of deposition become much more uniform and thus capable of more easy control. The author acknowledges with gratitude the able assistance rendered by Mr. T. Johnson in carrying out the assays of the many deposits which have been made. EZeciro -chemica Z Labora tory, London, E. C. I . Noriha mp to n Po Zy iech n ic Iristitu k ,

ISSN:0014-7672    

DOI:10.1039/TF9211600502

出版商:RSC    

年代:1921

数据来源: RSC