US2529237A - Electro-recovery of metals - Google Patents

Description

Nov. 7, 1950 J. TURNER ETAL ELECTRO-RECOVERY 0F METALS Filed Jan. 15, 1945 V IN VEN TORS Joseph L. Tuvncv mmA HwakV. Akssinironi (MQ A {W TH E 0 R ATTORNEYS Patented Nov. 7

ELECTED-RECOVERY OF METALS Joseph L. Turner, Fair Haven, and Hugh V. Alessandroni, Rumson, N. J., assignors to National Lead Company, New York, N. Y., a corporation of New Jersey Application January 13, 1945, Serial No. 572,658

Our invention concerns the electro-recovery of metals by the electrolysis of solutions containing compounds of the metals. More particularly, it relates to an improved method of preparing metallic products, which includes the step of electrolytically depositing a relatively large amount of metal upon a special type of cathode. It also embraces the novel cathodes employed in this method, as well as the heavy, compact and substantially homogeneous electrodeposits that are formed. In its preferred embodiment it deals with the production of iron powder by a process that includes electrodeposition of iron upon such a cathode.

The electro-recovery of metals by the electrolysis of solutions of metal compounds, employing an electrolytic cell in which the metal is deposited upon a cathode, has long been known and is widely practiced. The term electro-recovery includes electro-winning, electro-refining and electro-forming. The object in every case is to recover the deposited metal or to produce an article composed essentially thereof, rather than merely to apply a very thin coating of the deposited metal to an article made of some other metal, as is done in electro-plating.

It is desirable in electro-recoveryprocesses to deposit a large amount of metal upon the oathode within a short time. Usually, and especially in electro-refining, one wishes to recover the deposited metal in a very pure form. Consequently, it is the practice in such cases either to construct the cathode of the same metal as that to be recovered, the metal employed being likewise of high purity, or else to strip the deposit of pure metal from the cathode after the deposition has been completed. The cathodes used in electrorecovery processes generally consist of a metal sheet which is often specially treated to facilitate stripping off of the deposited metal.

The various electrolytes and types of electrolytic cells used in the electro-recovery of metals are known to those skilled in the art. The same is true of the operating conditions of the electrolysis, such as concentration of electrolyte, temperature, current density and the like. The metals most commonly recovered by electrolysis are gold, silver, copper, manganese, iron, nickel, cobalt, zinc, cadmium, tin and lead. Our improved method and special tytpe of cathode may be employed for the electro-recovery of any of these metals. They are particularly suited for the electro-recovery of base metals of the group consisting of copper, manganese, iron, nickel, cobalt, zinc, cadmium, tin and lead. The greatest ad- 3 Claims. (Cl. 204-) vantages accrue from their use in the electrorecovery of iron. Next in importance is the application of our invention to the electro-recovery of manganese.

The nature of the metal deposit formed upon the cathode is of great importance for the practicability of the electro-recovery processes. Great difliculty is encountered when seeking to obtain heavy deposits quickly. The deposits exhibit a marked tendency to develop cracks, followed by foliation and chipping off of metal particles into the electrolyte. Nodules and trees are formed resulting in uneven deposits which in turn may cause warping or buckling of the cathode. Any such protrusions, if substantial enough, will result in short-circuiting the cell or in damage to adjacent diaphragms or anode bags, and will interfere with the easy removal of the deposit from the cell. These undesirable tendencies may be reduced by lowering the current density, particu larly during the early stages of the deposition, but the use of such low current densities so retards the rate of deposition as to render the process uneconomical in many cases.

It is also difiicult to obtain the proper degree of adherence between the deposit and the cathode. Usually, only thin deposits can be readily stripped from the cathodes, but it is uneconomical to frequently interrupt the electrolysis for this purpose. Insufiicient adherence will prevent the formation of the desired heavy deposits. Many attempts have been made to regulate the degree of adherence by pretreatment of the cathode surface. It has generally not been possible to obtain a deposit that was sufiiciently adherent to avoid peeling throughout the cathode area during deposition and yet adequately weakened at the juncture between deposit and cathode to allow ready stripping and removal of the deposit. This has made it necessary in many cases to construct the cathode of the same metal and of the same purity as the deposit and thus to render unnecessary the subsequent removal of the deposit from the oathode. a

It is the object of our invention to overcome or at least substantially reduce the aforesaid difilculties, and to provide a practical and economical method of preparing metallic products, which includes the step of electrolytically depositing a large amount of metal within a relatively short time. We have found that this object can be accomplished by the use of the herein described, special type of cathode in place of the sheet metal cathodes heretofore employed. By the use of this special type of cathode one can obtain of choice and convenience.

.;tern of iron when recovering iron.

heavy, compact and substantially homogeneous electrodeposits possessing new and useful properties. The precise nature of the cathode and the many advantages resulting from its use in the electro-recovery of metals, will become apparent from the following detailed description.

The outstanding feature of the special type of cathode employed according to our invention, is that the greaterpart, if not all, of its effective area is composed of a thin, openwork pattern'of electrically conductive material. It'differs from the usual sheet metal cathode by reason of the numerous openings in the-pattermwhich permit the electrolyte to circulate freely, i. e., to pass through the cathode from one side to the other, during the critical, early stages of the electrodeposition. lhis produces a moreuniform and better adhering deposit at the start than isobtainable with the prior art, sheet-metal cathodes. The metal to be recovered is first deposited within these openings and around the electrically conductive material. Thus a firm basis for the deposition of additional metal is obtained and the tendency of the deposit to peel, particularly duringthe early stages of the deposition, is greatly reduced. The openings also, serve to provide maximum initial surface area with a minimum amount of .material, while retaining sufficient mechanical strength. One may, and generally does, continue the deposition until the openings are completely filled, and aheavy, compactand substantially homogeneous slab of deposited metal, having the open work pattern embedded therein, is formed. l"he metal deposited Within the open- .ingsserves to firmly unite the portions of the deposit that lie on opposite sides of the cathode, thus serving to effectively anchor the heavy, metal deposits to the cathode.

The thin, open-work pattern preferably consists of a woven wire fabric, such as screen or gauze.

It may also be composed of a highlyperforated sheet or of parallel strips or strands. The par .ticular design of the pattern is largely a matter We have found that patterns may be employed in which the total of the projected areas of the openings amount to from about to about 90%, preferably frcm about 40% to about 80%, of the projected area of the pattern. In the case of a woven wire screen, this corresponds to from about 2 to about 40 mesh, preferably from about 12 to about 24 mesh, employing wireoffrom 0.10 to about 0.001 inch, preferably 0.05 to 0.008 inch, in diameter. Since it isgenerally desiredthat the deposit completely fill the openings inthe p-attermandone does not wish to make the cathode unnecessarily large, the

'width of the openings should not exceed 0.5 inch and, preferably, should be less than 0.10 inch; It will be found, when employing such open-work ;patterns, that the ratio of their actual current density to their apparent current density (calculated as a continuous sheet of equal dimensions) is from about 4:1 to about 1:2, preferably, from about 1:1 to about 1:2.

The open-work pattern must, of course, be composed-of an-electrically conductive material that canbe-fabricated in the desired form. In most cases one will employ a metal as the cathode ma terial, and often the same metal as is to be dcpcsited thereon. Thus we prefer to make th pat- However, ;when recovering manganese, we prefer to use copper, s nce copper can readily be fabricated into the desired form and will alloy with manganese to form a commercially desirable product.

. 4 It is also possible for special applications to make the open-work pattern of a plastic substance, such as rubber or a synthetic resin which has been impregnated or coated with a conductive material, such as graphite or a metal powder.

The open-work pattern must possess sufficient ;mechanical strength to carry the desired, heavy deposit of metal :which,.however, to alarge extent becomes self-supporting as it accumulates. OW- ing to its many openings the pattern will be flexible and must, therefore, be supported within a rigid frame. This frame, which is also made of electrically conductive material, serves the further purpose of .aiding in the distribution of the current throughout the entire effective area of the cathode. Generally, and particularly in the case of a woven fabric, the open-work pattern will possess a'certain amount of resiliency and elasticity, which properties contribute to the desired result, as will be pointed out hereinafter. Ordinarily the cathode will beofa stationarytype and will consist of the open-work patternheld tautly within a rectangular frame. However, .the cathode may be cylindricalor disk-shaped, particularly if it is of the rotating type.

Our new cathodes are employed in the known electro-recoveryprocesses and apparatus in place of the usual sheet-metal cathodes. The difliculties heretofore encountered when attemptinggto obtain large metal deposits within a short time are either greatly reduced or entirely eliminated due to the special characteristics of the openwork pattern. No special controlof currentidensities or pretreatment of the cathode to insure the proper degree of adherence of the deposit are necessary. It is possible without difficulty and while applying relatively high current densities (as compared with prior practices) 'to obtain compact, substantially homogeneous deposits that are at least 0.03 inch thick. Many of the metals subject to electro-recovery may be deposited upon our new cathodes until the thickness of the layer produced exceeds 0.1 inch, and some of them, especially iron, will form deposits considerably over 0.3 inch thick. In'most cases the electrodeposition will be continued until thedeposited metal has entirely filled the openings in the pattern and has formed athick slab having the. pattern embedded therein. However, in somecases, particulariy where the pattern possesses relatively large openings, it may be desirable without sacrificing the advantages to .be derived from our in- ;venton, to terminate the ielectrodeposition before the metal deposit has entirely filled the openings.

After completion of the electrolysis the cathodes are removed from the cells and the openwork pattern, together with the metal deposited thereon, is removed from the supporting frame, which may then be re-used. The breaking away of the open-Work pattern and metal deposits thereon from the frame can be facilitated in various ways. For example, if the open-work pattern is folded'over the frame, very-little metal will be deposited on the frame and this can easily be chipped away. Also one can minimize deposition of metal on the frame by coating it with'a nonconductive material, which can be done after the pattern has been attached thereto .by welding or the like. In most cases the electrodeposits will have been built up to the form of fairly thick, continuous slabs in which the pattern isombedded about midway between the faces, the

. ping procedure.

structure and the nature and amount of impuri- -ties therein will clearly indicate its electrolytic origin.

The heavy, electrodeposits obtained in accordance with our invention will generally be processed without prior removal of the embedded cathode material. Such processing may consist in pulverization, rolling, forging, melting, or the like. For this reason, it is often preferred 'to make the open-work pattern of the same metal as is to be deposited thereon. However, the metal need not be of the same purity as that of the deposit, because the weight of the pattern usually represents but a small fraction of the weight of the deposit, so that the amount of impurity introduced by way of the pattern is insignificant for the whole. In some cases, even though the pattern and the deposit are composed of different metals, they can simply be molten together to produce a valuable alloy. The open-work pattern, particularly when it is in a conventional form, such as a woven wire screen, and is composed of metal having no unusually high degree of purity, is generally quite inexpensive and can, therefore, economically be replaced within the supporting frame of the cathode, which may then be used for further depositions.

It is not possible to remove the deposited metal from the open-work pattern by the usual strip- In those cases in which it may be desired to separate the cathode material from the deposited metal, this must be accomplished by other means. Such cases will only occur where the material of which the pattern is composed differs substantially from the deposited metal. Separation can then be effected by taking advantage of the difference in properties between the two materials. For instance, the deposit with the cathode material embedded therein may be pulverized and the particles of the respective materials separated by a magnetic process. One may also utilize a wide difierence in melting points. For example, one can melt an electrodeposit of lead off from an open-work pattern composed of stainless steel. In this case it is not necessary first to remove the pattern and deposit from the frame. Both the frame and the open-work pattern may be used again after the deposit has been molten off.

All of the reasons why our special type. of cathode possesses the considerable advantages previously mentioned, as compared with the usual sheet-metal cathodes, are not fully understood.

- We, therefore, do not wish to limit ourselves to any particular theory of operation. However, we believe that the many openings in the open-work pattern bring about a better circulation of the electrolyte during the early stages of the deposition and that this results in the more uniformly and strongly adhering deposit adjacent to the cathode. There appears to be less danger that f local depletion of.the electrolyte will result in excessive formation of hydrogen. Furthermore, it is probable that the flexibility and resiliency of the pattern allows it to absorb and equalize the stresses that are set up during the early stages. Undoubtedly the open-worl; pattern provides a far better anchorage for the deposit and thus serves to minimize foliation.

Our invention is particularly applicable to the electro-winning of iron. We shall, therefore, describe it in detail with reference to the production of metallic iron products, specifically iron powder. Its application to the production of other metal products will then be obvious. In the? electro-winning of iron one generally employs an iron chloride solution, preferably ferrous chloride, as the electrolyte. The apparatus consists of an electrolytic cell or rather a plurality of such cells of any suitable design. Preferably the cells are so constructed as to provide compartments separated by a diaphragm for the segregation of catholyte liquor from anolyte liquor. In this apparatus the iron is deposited on the cathodes and chlorine gas is evolved and liberated at the anodes. The latter are usually composed of an electrically conductive, inert material such as carbon.

Ihe special type of cathode to be employed in the aforesaid apparatus according to our invention, is illustrated in the accompanying drawings. These drawings show one form of such a cathode and it is obvious that a great number of variations and modifications thereof are possible without departing from the scope of our invention.

In the drawings:

Fig. I is a vertical view of such a cathode assembly.

Fig. II is a cross section along the line AA of the cathode assembly shown in Fig. I.

Fig. III is a greatly enlarged cross section of a portion of the screen shown in Figs. I and II embedded Within the final, heavy electrodeposit of iron.

In the cathode assembly shown in Figs. I and II the reference numeral l represents an ordinary iron wire screen that forms practically the entire effective area of the cathode. This screen is 16 mesh, i. e., has 256 openings per square inch, and is composed of iron wire of 0.01 inch in diameter. It is supported within a rigid frame 2 made of a heavy gauge, iron rod to which it is secured by folding it over the rod and holding the double thickness together by means of staples 3. The frame 2 is connected to a copper bus bar 4 by means of sheet-iron loops 5. These loops serve to conduct and distribute electricity to the screen and the frame and are firmly attached thereto by means of rivets 6. When in use, the cathode assembly'is hung in the electrolytic cell, the ends of the copper bus bar 4 resting upon opposite sides of the cell. The level of the electrolyte is indicated in Fig. I by the line 3-3.

In Fig. III reference numeral 1 represents a cross section of the iron wire of which the screen I is composed. 8 represents the final, heavy, iron deposit. The crystal structure of the iron is indicated. It radiates from the wires of the screen. The deposit tends to vary slightly at different stages of the deposition and thus becomes striated in cross section. One can visualize from this striation how the deposit was first built up about the wires, filling up the openings and finally merging to form a continuous slab.

' The iron wire screen used in the cathode illus- F as FeClz grams per liter to Fe as FeCls do-- 1 to 2 NaCl do 30 to 80 pH 0.5 to 2.0

At the start of the electrolysis this solution will form both the catholyte and the anolyte, but

7 durin he-e rl t ge o t elec oly is d; pr o to tl 1e,liberation of chlorine gas at the anode, ferric chloride will build up in the anolyte. Either brittle or;ductile.ircn may be produced, depending primarily upon the temperature at which the v,electroiyte is maintained during the electrolysis. When-the temperature is held below about 60 C. -brittle iron is produced, while at higher temperatures ductile iron is obtained. Generally speaking, the lower the temperature the more brittle will be the iron and conversely the higher the temperature the more ductile it will be. The electrolysis can be satisfactorily conducted with our special type of cathode within a wide range :of current densities on the cathode. If brittle iron is desired thecurrent densitymay be from about to about-50amperes per square foot of apparent cathode area (calculated as a continuous sheet of equal dimensions). Preferably the apparent current density ranges from about to about amperes per square foot. Higher current densi ies may be employed when producing ductile iron and in that case the apparent current density may range from about 5 to'about 225 amperes per square foot, preferably from about to about 100 amperes per square foot.

As the electrolysisproceeds, iron is deposited upon the surfaces of the open-work pattern within the supportingframe of-the cathode assembly. Gradually the deposit grows and completely fills the interstices of the pattern forming a substantially homogeneous mass of iron transfixing the patternand, to a certain extent, still evidencing its design. As additional iron is deposited, the mass gradually assumes the shape of a thick slab whose surfaces no longer evidence the design of the pattern. In the electro-recovery of iron We prefer to continue the deposition at least up to this point in order to realize in full the benefits of our invention. After the desired amount of iron has been deposited on the cathode, the oathode assemblies are removed from the cells and washed. The electrolytically deposited mass of iron having the open-work pattern embedded therein is then detached from the supporting frame. This presents no difiiculties. In the oathode assembly shown in the drawings the frame is to a large extent protected by the iron wire screen that is-Wrappedaround it, and the deposit does not adhere strongly to the frame. It can easily be chipped or broken away and the little that adheres can be scraped off, thus preparing .the'frame for re-use. If desired, in order to facilit-ate removal of the deposit, the frame may be coated with an insulating composition, such as a varnish or lacquer. If the coating is applied after the openwvork pattern has been attached to the frame, the deposit will be confined to't'hat portion of the open-work pattern that lies within the frame, thus further simplifying the removal of the entire deposit and embedded pattern from the frame.

When the electrolysis has been controlled to yield brittle iron, the deposit with the iron screen embedded therein may be pulverized to any de.-. It ,is preferable to conduct reducedlto'thepr pe har ic esiz y eread- ;ily..rsepa ated brscme p ds d rable,

order to avoid oxidation of the iron to .-,carry out the p-ulverization in a neutral atmosphere, e. g in thepresence ofnitrogen or- COagas. In mostjindustrial applications of the present invention,- the smallamount of pulverized iron wire screen present inthe metallic iron powder will notbe objectionable. In fact, under the-conditions just described, the screen will constitute only fromabout 0.33% to about 0.5% of the weight of the deposited iron. Of this amount under normal conditions at least four-fifthsis pulverized and the remainderrremoved: byscreening. Assuming that the composition of thecathode Wire screen is 98% ;F61With 2%;impurities and that the electrolytically deposited iron is 99.99% pure (analysis of annealed deposit) ,the

purity of the ifinal iron powder containing all of the pulverizediron screen,,i. e., about 0.5% of the total, willbe 999.8%, representinga lowering of purity of only 0.01%.

Whenthe conditions of the electrolysis have been controlled to yieldduotile ir.on,:,themass of deposited iron in which the screen is embedded may be directly processed further, jforinstance, rolled or pressed out into sheets. in vthiscase the presence of the embedded screen within the deposited iron will not be objectionable nor interfere in any way with the rolling or pressing. In fact, in certain instances, depending upon the relative ductility and :tensile strength of the screen and-the ductile .iron,;the. screen may impart desirable strength to the pressed ,or rolled sheet.

purities consist mainly of occluded electrolyte salts, hydrogen gas and oxygen in the form of iron oxide. Intheensuing annealing treatment .inthe presence. of reducing gases, however, these impuritiesare eliminated and the final product ,is of extreme purity, up to from 99.98% to. 99.99%

pure, with only traces of impurities, such'as carbon, phosphorus silica manganese, sulphur,

etc. Other forms of' iron, 1. e., those obtained by normal smelting processes and which are poured, are vhigher'in these latter impurities, and exhibit-crystalline patterns clearly distinguishable from that of *the electrolytic irondeposited on our open-Work pattern cathode.

If it isdesired to make a very pure iron powder from a depositof brittle iron produced in accordance with ourinvention, one can avoid convariation.

tamination by particles of the embedded cathode. material by making the open-work pattern of anon-magneticor far less magnetic substance than the deposited iron. "In that case, after, the iron deposit andembedded pattern have been pulverized, one :can readily separate the powder from the particles .of the pattern Other :details are likeby a magnetic process. wise subject to considerable modification and Thus, one may emplo solutions of ferrous-sulphate or other ironsalts in place of ferrous chloride asithe electrolyte. The-kind of open-work pattern anid the type of cell maybe varied; so also the supporting frame and the means of insuring proper distribution of the ,current to the open-work pattern. The pattern maybe grooved or crimped to increase itsresil- 9 be held relatively taut within the supporting frame. Such variations and many more will be apparent to those skilled in the art.

We claim:

1. A method for the preparation of metallic powders which comprises electrolytically depositing a brittle metal upon a thin, resilient and elastic, open-work pattern composed of an electrically conductive metal, in which the total of the projected areas of the openings constitutes from 15% to 90% of the projected area of the pattern, said openings not exceeding 0.5 inch in width, until the metal deposited upon the pattern has completely filled the openings in the pattern and has formed a substantially homogeneous slab having the pattern embedded therein substantially midway between its major surfaces, and then pulverizing the metal deposit and pattern without prior separation.

2. A method for the preparation of iron powder which comprises electrolytically depositing brittle iron upon a thin, resilient and elastic, open-work pattern composed of iron in which the total of the projected areas of the openings constitutes from 40% to 80% of the projected area of the pattern, said openings not exceeding 0.5 inch in width, the said pattern being supported within a rigid, electrically conductive frame, continuing said deposition until the metal deposited upon the pattern has completely filled the openings in the pattern and has formed a substantially homogeneous slab having the pattern embedded therein substantially midway between its major surfaces, removing said metal deposit and pattern from the frame and then pulverizing the deposit and pattern without prior separation.

3. A method for the preparation of iron powder which comprises electrolytically depositing iron at temperatures below 60 C. upon a screen of between 2 and 40 mesh, composed of iron wire of from 0.001 to 0.10 inch diameter, the said screen being supported within a rigid, electrically conductive frame, until the iron deposited on the screen has completely filled the openings in the pattern and has formed a substantially homogeneous slab having the pattern embedded therein substantially midway between its major surfaces, removing said iron deposit and screen from said frame and then pulverizing the deposit and screen without prior separation.

JOSEPH L. TURNER. HUGH V. ALESSANDRONI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATEN'I'S Number Name Date 312,485 Makin Feb. 17, 1885 591,571 Richards et a1 Oct. 12, 1897 756,328 Christy Apr. 5, 1904 883,170 Christy Mar. 31, 1908 1,007,388 Ramage Oct. 31, 1911 1,231,967 Tainton et al July 3, 1917 1,243,654 Clark Oct. 16, 1917 1,280,908 Wales et al Oct. 8, 1918 1,414,423 Langer May 2, 1922 1,470,577 Liebknecht Oct. 9, 1923 1,567,791 Duhme Dec. 29, 1925 1,644,015 Griswold Oct. 4, 19 7 1,776,787 Ergang Sept. 30, 1930 1,788,904 Zdanski Jan. 13, 1931 1,797,375 Smith Mar. 24, 1931 2,157,699 Hardy May 9, 1939 2,275,194 Sizelove Mar. 3, 1942 2,288,762 Winkler July 17, 1942 2,296,757 Young Sept. 22, 1942 FOREIGN PATENTS Number Country Date 16,557 Great Britain of 1895 133,882 Great Britain of 1919

Claims (1)

1. A METHOD FOR THE PREPARATION OF METALLIC POWDERS WHICH COMPRISES ELECTROLYTICALLY DEPOSITING A BRITTLE METAL UPON A THIN, RESILIENT AND ELASTIC, OPEN-WORK PATTERN COMPOSED OF AN ELECTRICALLY CONDUCTIVE METAL, IN WHICH THE TOTAL OF THE PROJECTED AREAS OF THE OPENINGS CONSTITUTES FROM 15% TO 90% OF THE PROJECTED AREA OF THE PATTERN, SAID OPENINGS NOT EXCEEDING 0.5 INCH IN WIDTH, UNTIL THE METAL DEPOSITED UPON THE PATTERN HAS COMPLETELY FILLED THE OPENINGS IN THE PATTERN AND HAS FORMED A SUBSTANTIALLY HOMOGENEOUS SLAB HAVING THE PATTERN EMBEDDED THEREIN SUBSTANTIALLY MIDWAY BETWEEN ITS MAJOR SURFACES, AND THEN PULVERIZING THE METAL DEPOSIT AND PATTERN WITHOUT PRIOR SEPARATION.

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