US2532629A - Method of electrodepositing iron - Google Patents

Description

Dec. 5, 1950 E. H. KONRAD ET AL 2,532,629

METHOD OF ELECTRODEPOSITING IRON 2 Sheets-Sheet 1 Filed Nov. 7, 1947 [it U6 niars Eric/z EKmzr'awl WZZZQEu/Jfas' Patented Dec. 5, 1950 UNITED- STATES OFFICE METHOD or 'ELECTRODEPOSI-TIING IRON Erich: n. Konrad and William E LQ Eustis, stunt Strafltord, Vt, assignors toSulphideOre Process Company, Inc.,' New York,'N. Y., acoriporation of De awa Application November '7, 1947,.SerialNo. 7843686 (or..20i+4s 2 C ims.

The present invention relates tea-method of electrodepositingmetals and to an electrolytic cell suitable for carrying-out the method.

In .the electrolytic deposition of metals from aqueous solutions of their soluble compounds, such-astheir soluble salts, it is customary to pass the electric current through theelectrolyte between two opposed electrode-surfaces, of which the anode, especially when insoluble in the electrolyte,=is slightlvsmaller'in area, in order to avoid excessive current densities at the edges of the cathode and concomitant thickening of thedeposit and tree formation thereon.

There is therefore an inherent loss in efficiency in the-operation of such an electrolytic cell, for the cross-sectional area, through which the electric current-istransmitted, is reduced, and the corresponding elcctrodeposition of metal is. efiected relative to i the "total electrode surface areas. Atthesame time, withv the same amperage across the cell, the current-density per unit area of anode surface is increased and the potential or voltage across the cell is likewise increased in even greater proportion, for a given rate of electrodeposition of the metal.

The electric potential ,or voltage required is dissipated in such a cell in overcoming the-resistance of the electrolyte and of the diaphragm, when used, as over voltage at the anode and at the cathode, and in :polarization at the anode and'cathode. Of these, the voltage required to overcome the resistance in the electrolyte and the -over voltage at the electrodes -may;be reduced somewhat advantageously but *i essential as representing the work performed by the cell. in liberating and depositing "the free metal. The polarization eifectsfihowev'er, represent 'loss of electrical energy and inefiiciency of cell operation. Nevertheless such polarization effects are not readily overcome, for if they are sought to be corrected at the electrodes, the means employed for their correction may attack the electrode and if the electrolyte is altered in condition or composition a less desirable cell operationor'deposit of the metal in question may-re- 2 ide .e ro ee et n n luble g a h;- ite anode ,-whichis surrounded bya porous aphragm, and an iron or stainless steel oath.- ode. In the course of this type of operation, it has heeniound that the anode is attacked and deteriorates in use. It has been further found that when the anodic current density is 1. creased-there'is a; disproportionate increase in the cellvoltages For example, in a test run, the anode area was purposely decreased by one third. that is, from 1,0.2-square feet 't0 6.8 square feet, while the c l am ra e as k t s a t at. 500 amperesperanode. This caused an in.- crease in the anedic current density from A9 amperes'per square ioot 'to 73.5 amperes per square foot. Keepin all'other conditions eon stant, in the cell, --the voltage increased from' lfi to voltsh In other words-the increase oi -in the=-current density--while maintaining the sarneoverallcurrent o f the cell constant'at 5 ampe s pe anod -in re d th olta requirement of the cell by 28%. This voltage rise seems-tube 'due chiefiy togovercomingins d po a izati n and a siv ph nomena at the anode face.

It-isnow fou' d n; ac o da i h the s ent invention; that in the electrolytic deposition of metals between aninsoluble anode and the cathode, .a-given current may be maintained-at a lower initial potential or voltage and without appreciableincrease in potential, throughout prolongedoperation of thece l, by con a t ng the face orthe-anode, Whichopposes the oathode, with a =layer-ot'granu1ar insoluble electros conductive material. The individual granules of this-material are of cross-sectiona-l dimensions considerably less than thespace separating the electrodes ".(and preferably less than one-half of suchdimension') and=present at least approximately ten-times as great a surface area as that of"theanode-surtaceper se. The granular material may be-of uniform size andshape or-of nonuniform= sizes-or irregular shapes, but are adapted to form intimate and numerous con.- tacts with the anode surface and with each other and are of high electrical conductivity and free 'from' dust." Graphite granules or crushed graphite electrodes are especially":suitable.-

A typical instance of the practical application of invention "will'be' described with reference to theelectrodeposition or iron, using the apparatus" and obtaining rthe results which are shown in 'the accompanying drawings, in which:

Fig. 1 is a longitudinal"diagrammatic eleyation of the electrolytic cell Fig. 2 is a plan View of thesame;

F st .3 is an enc view o he 'cath edeand" a 1 Oppo d anode;

His :1 'iS- a d a ram show ineth in r ase in volta e and power consumption per pound ordeposited iron, in terms of time of service of the cell when using the anode plates of the cell only; and

Fig. is a diagram showing the increase in voltage and power consumption per pound of deposited iron, in terms of time of service of the cell, with the anode plates, the cathode-opposed surfaces of which are provided with a layer of crushed graphite electrode granules in accordance with the present invention.

Referring to the drawings, a plurality of flat anode bars I, of molded oil impregnated graphite, were suspended from copper crossbars 2, making contact with the main cell bus bar 3. Each anode was immersed in the electrolyte 4 and surrounded 'by a porous diaphragm 5, such as an asbestos bag, or other acid resistant, permeable material, the whole being contained in the tank 5.

The space between each anode surface (opposed to a cathode surface), and its porous diaphragm, or the entire free space in the bag, as

shown, was filled by a charge of crushed graphite pieces, having a granule size of uniform or closely averaged overall diameter and preferably free from smaller sizes and dust. For convenience, all -=r of the space between the diaphragm or anode bag and the anode may be filled with the granular graphite, but in those portions not directly opposed to a cathode surface, they are less eifec The cathode l was composed of a single sheet Figs. 2 and 3.

In a particular instance of operation of the cell, as thus arranged and constructed, each of the six anode bars I, as shown, was 6 wide by 1" thick and long, 32 of the length of the bar being submerged in the electrolyte s, which comprised an aqueous solution of ferrous chloride containing 150 grams of ferrous iron per liter.

The cathode was a sheet of steel thick and presented a submerged cathode surface 36 x which was equal to 12.5 square feet per face. The area of the anodes was about 11.1 square feet per face.

The space between the anode and cathode was 4" from center to center.

The anode was surrounded by broken pieces of graphite electrode, 15 varying from /8 to and averaging about which were packed firmly between the surface of the anode and the anode bag and formed a layer about 1%," thick. The surface area of each such layer of crushed graphite on the anode surface was calculated to be about 180 square feet.

The cell was put into operation and maintained with the temperature of the electrolyte averaging about 88 C.

Operation I Operation II Upon filling the anode compartments with the layer of crushed graphite as above described and shown in the drawings, the cell voltage dropped to 2.6 volts, maintaining the same current densitive, though they promote somewhat the overall conductivity of the cell.

ty of 30 a./ft. (thirty amperes per square foot).

Part of this drop in voltage was no doubt due to the effective reduction in the space between the anode and cathode faces, namely, from 3 to 2 A", or by 1%". But only 0.2 volt per inch or 0.25 volt of the drop of .5 to .8 volt is attributable to this effect. The balance of the voltage drop can be attributed directly to the larger anode area and corresponding reduction in anode current density.

In separate operations of the cell under these parallel conditions for a prolonged period of time, it was found that whereas in Operation I deterioration of the graphite anode surfaces immediately set in, upon putting the cell into service, and considerable deterioration of the anode bars occurred, extending to a depth of in 800 hours, in Operation II the anode bars were unaffected at the end of 500 hours operation.

The deterioration of the set of anode bars used without the granular graphite layer is plotted in the diagram of Fig. 4 and shows that after 800 hours of service the voltage increased from 3.1 to 4.4 volts or 42% and that there was a proportionate increase in the power consumption, per pound of electrolytic iron deposited, from 1.5 to 2.4 kilowatt hours.

Results upon a set of graphite anode bars which were run under similar conditions, except that they were surrounded by pieces of crushed graphite, as above described, showed a lower initial voltage and power consumption per pound of electrolytic iron deposited and a much slower and lower rise in both voltage and power consumption, per pound of iron, upon continued operation of the cell.

The attack upon the anodes was not only reduced in this way but such as occurred was transferred to the granules of graphite contained in the anode chamber, forming contact with the anode surface and with each other. The granules thus consumed are easily and cheaply replaced, without interrupting the operation of the cell, by introducing fresh granules into the open tops of the anode bags.

During operation of the cell, some tendency to film formation may be observed upon the granule surfaces. If this occurs, it may be readily displaced by washing the granules with hydrochloric acid, preferably in concentrated solution and if necessary at elevated temperature.

It is further observed, as a part of the present procedure, that in an electrolytic cell, in which an insoluble anode is used, per se, as the anode deteriorates in the course of continued operation the nature of the anode reaction of the electrolyte changes, leading to the formation of an increasing amount of free acid at the anode.

In the electrodeposition of iron, this increased formation of free acid at the anode leads to difiiculties, especially when the anolyte is to be used as a leach liquor for the dissolving of fresh ores such as pyrrhotite. In such cases, any excess of free acid present must be neutralized in order to return the electrolyte to the original pH of 1.5 which is desirably maintained in the feed back to the electrolytic cell.

Ferric chloride anolyte even though containing free hydrochloric acid will react preferentially with pyrrhotite concentrates to form ferrous chloride and free sulphur as follows:

2 FeClz+FeS- 3 FeClz-i-S Under the proper conditions of operation of the cell and of the leaching of the ore with the anolyte from the cell, the pH value of the resulting leach liquor after this reaction will be about 1.2 to 1.4. An excess of pyrrhotite may then be added whereupon the free acid in the electrolyte will then react therewith to liberate hydrogen sulphide from such excess of pyrrhotite, as follows:

FeS+2HCl FeClz+H2S Such liberation of hydrogen sulphide is desirable and effective to precipitate impurities dissolved from the ore and contained in solution in the leach liquor, whose sulphides are insoluble at a pH of 1.5 such as copper, silver, nickel and cobalt, and may then be filtered off, leaving a clear, purified solution to return to the catholyte of the electrolytic cell.

Thereupon the pH value of the solution will rise to about 1.5, which is a suitable condition in which to return it to the catholyte of the electrolytic cell.

But if appreciable .anode deterioration has occurred and more free acid liberated, producing a lower pH value of the anolyte accordingly: either less pyrrhotite must be added thereto, to avoid excess hydrogen sulphide formation which leaves the pH value lower than is desirable for return of the leach liquor to the electrolytic cell-or if sufficient pyrrhotite is added to neutralize the excess acid in the anolyte (after complete reduction of the ferric chloride) then an excessive amount of hydrogen sulphide gas is liberated which is undesirable in many ways which are well known and recognized.

For example, in operating a cell without the layer of graphite granules upon the anode surface, the pH value of the leach liquor, following the reaction of the anolyte with pyrrhotite as given above, was 1.2. As anode deterioration progressed, this pH of the leach liquor, at this stage of reaction upon the pyrrhotite dropped to an average of 1.0 during the second week and then dropped progressively until at the end of 800 hours of operation of the cell or 8 weeks of overall operation, it averaged 0.7-0.8.

Employing a crushed graphite granular layer as above described, but otherwise under like conditions of operation, the pH of the anolyte, after being employed as a leach liquor on pyrrhotitic ores, at the end of the first reaction, given above,

averaged 1.5-1.7. Some addition of hydrochloric acid was therefore desirable to promote the second reaction for the liberation of hydrogen sulphide. Such addition of free hydrochloric acid to the leach liquor is desirable, not only as a matter of more accurate control of the leach liquor, of hydrogen sulphide liberation, and of the subsequent feed back to the electrolytic cell-but also to replace the necessary drag out loss of electrolyte which is entailed in the removal of insoluble matter and other unavoidable losses, such as spraying from the bath and leach liquor. 7

On the other hand, the areas of granular conductive surface and anode surface may possibly and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

We claim:

1. A method for the electrodeposition of iron, comprising as steps the passing of an electric current through an aqueous electrolyte containing ferrous chloride, between a cathode surface and a graphite anode bar, in spaced, opposed position to each other in the aqueous solution, and separated by a diaphragm which is pervious to said aqueous solution, the anode bar, on its surface opposed to the cathode surface, being in contact with a layer of granules of graphite having individual cross-sectional dimensions upward from one-eighth inch, and the granules being in electrical contact with each other and presenting a surface area to the aqueous solution of ferrous chloride which is greater than the area of the graphite anode bar, and electrodepositing iron on the cathode.

2. A method for the electrodeposition of iron, comprising as steps the passing of an electric current through an aqueous electrolyte containing ferrous chloride, between a cathode surface and a graphite anode bar, in spaced, opposed position to each other in the aqueous solution, and separated by a diaphragm which is pervious to said aqueous solution, the anode bar, on its surface opposed to the cathode surface, being in contact with a layer of crushed granules of molded graphite having individual cross-sectional dimensions upward from one-eighth inch, and the granules being in electrical contact with each other and presenting a surface area to the aqueous solution of ferrous chloride which is greater than the area of the graphite anode bar, and electro-depositing iron on the cathode.

ERICH H. KONRAD. WILLIAM E. C. EUSTIS.

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

UNITED STATES PATENTS Number Name Date 155,884 Mormandeau Oct. 13, 1874 830,918 Nelson Sept. 11, 1906 1,239,443 Antisell Sept. 11, 1917 2,273,036 Heise et a1. Feb. 17, 1942 2,464,889 Pike et al. Mar. 22, 1949

Claims (1)

1. A METHOD FOR THE ELECTRODEPOSITION OF IRON COMPRISING AS STEPS THE PASSING OF AN ELECTRIC CURRENT THROUGH AN AQUEOUS ELECTROLYTE CONTAINING FERROUS CHLORIDE, BETWEEN A CATHODE SURFACE AND A GRAPHITE ANODE BAR, IN SPACED, OPPOSED POSITION TO EACH OTHER IN THE AQUEOUS SOLUTION, AND SEPARATED BY A DIAPHRAGM WHICH IS PERVIOUS TO SAID AQUEOUS SOLUTION, THE ANODE BAR, ON ITS SURFACE OPPOSED TO THE CATHODE SURFACE, BEING IN CONTACT WITH A LAYER OF GRANULES OF GRAPHITE HAVING INDIVIDUAL CROSS-SECTIONAL DIMENSIONS UPWARD FROM ONE-EIGHT INCH, AND THE GRANULES BEING IN ELECTRICAL CONTACT WITH EACH OTHER AND PRESENTING A SURFACE AREA TO THE AQUEOUS SOLUTION OF FERROUS CHLORIDE WHICH IS GREATER THAN THE AREA OF THE GRAPHITE ANODE BAR, AND ELECTRODEPOSITING IRON ON THE CATHODE.

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