CA1063061A - Electrowinning cell with reduced anodic surfaces - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Describes an electrochemical process operated at relatively low cathodic current density and high anodic current density, particularly for electrowinning and electro-refining processes using dimensionally stable metal anodes with reduced anodic electrocatalytic surfaces to allow full exploitation of the advantages of insoluble metal anodes, such as lower overvoltages, high current density capability, and purer deposits, together with an economical employment of expensive anode materials. Catbodic/anodic current den-sity ratios down to 1/20 are used without materially decreas-ing overall process efficiency or product morphology.
Choice of geometric parameters, practical aspects, typical embodiments and examples are disclosed.

Description

-` 1063061 Tilis invention rcl~tes to a process for electro-winning of metals from acid solutions containing the metal to be recovered by the use of dimensionally stable non~cor-rodible valve metal anodes made from titanium, zirçonium, hafnium, vanadium, niobium, tantalum, molybdenum and tung-sten, or alloys thereof, provided with a non~passivating exterior surface or coating having electrocatalytic proper-ties with reference to the release of oxygen and chlorine and in which the surface area of the anode facing the cath-ode is from 1.5 to 20 times smaller than the surface area ofthe cathode, whereby the anodic current density can be from 1.5 to 20 times higher than the cathode current without exceeding the cathode current density limit. The anodes preferably have an open framework structure which permits free circulation of the electrolyte through the anodes to provide a more uniform electrolyte composition and are spaced from the cathodes, so that the lines of current reach the cathode in a parallel uniformly distributed configuration and produce better deposits of the metal to be recovered on the ?Q cathode.
Valve metals (sometimes also referred to as film forming metals~ such as titanium, zirconium, hafnium, va-nadium, niobium, tantalum, molybdenum and tungsten, or alloys thereof, have the capacity to conduct current in the anodic direction and to resist the passage of current from the cathode direction and are sufficiently resistant to the electrolytes and the -~
conditions in an electrowinning cell that su~stantially no corrosion takes place even during periods of current interrup-tion or cell shut down. The valve metals, however, will not .
3~ conduct current to the electrolyte unless provided with a non- -passivating electrocatalytic sur~ace or coating æuch as a coating containing a platinum group metal oxide or a self-m~p/ -1-.
: . . :
- . . : .. ..

.. .. .

gencratiny alloy coating which is electrocatalytic in the elec~rolyte with reference to the release of oxygen or chlorine without excessive overvoltage.
In many of the processes of electrowinning metals from solutions containing the said metals, in order to pro-duce satisfactory metal deposits on the cathodes, it is necessary to operate the process at relatively low cathodic current density. If the cathode current density for a - specific metal, the specific electrolyte composition, the cell geometry, etc., exceeds that at which s~ooth adherent deposits are ~ormed on the cathode, the cathode current density limit . .
is exceeded and ro~ghened cathode deposits are produced ~hich can occlude electrolytes which contaminate the deposited metal. If the cathodic deposit is of a dendritic nature, short circuits may be formed in the cell. If lead or lead alloy anodes are used, a 510w dissolution of both the lead and its alloying elements occurs and the corrosion products are deposited on the cathode and contaminate the metal being won. At higher cathode current densities, these conditions are aggravated and in addition the potential of the electro-winning cell rises and hydrogen is liberated at the cathode causing further complications. "Cathode current density"-at which hydrogen is liberated at the cathode is defined as the limiting cathode current density and the current density at which the cathode deposits become rough and striated is defined as the cathode current density limit. It is usually lower than the limiting cathode current density. There is, therefore, a well defined limited cathodic current density which may be used. -High production rates are, however, necessary in order to produce high yields of pure easily recoverable metal with a minimum expenditure of electrical energy.

m~p/ -2--` 1063061 B~CKGROVND OF THIS INVI~NTION
.. ..
So called "insoluble lead or lead alloy" anodes have been used in electrowinning for many years. However, these anodes are slightly soluble and contaminate the electrodeposited metal which is being won. For example, the 84% lead, 14.5% antimony, 0.6% silver alloy anodes used in the Chuquicamata, Chile electrowinning plant, which last from six to eight years in service, have a corrosion rate of 0.6 to 2.5 kilos per ton of copper recovered. At the end of the useful life of these anodes, only 14% of the original weight remains and is recovered as scrap. Most of the lead going into solution in the electrolyte is deposited with the recovered copper. The use of rod type lead anodes and high-er anode vs. cathode current density, as described, for ex-ample, in the.Carosella United States Patent No. 2,766,168 - reduces the corrosion rate of the lead based anodes but does not eliminate it.
Likewise when titanium base anodes coated with platinum or platinum group metals are used in electrowinning, the platinum group metals are dissolved and, in addition to the expense of these anodes, the dissolved metals contaminate :
the metal being deposited on the cathodes.
The use of non-corrodible, dimensionally stable ~-valve metal anode bases, such as titanium, zirconium, hafnium, - vanadium, niobium, tantalum and molybdenum having conductive non-passivating surfaces and electrocatalytic properties for ~ -oxygen or chlorine discharge at low overvoltages, gives substantial advantages over the graphite, lead, lead alloy, silicon, iron and platinum anodes heretofore used in electro-winning, but to economically use these valve metal anodes for electrowinning, they must be operated at higher current densities than the permissible cathode current densities. : -m~ _3_ : .
i' .. .. ,......... . , ' .
:. , ~, : . . .
.

106306~
-OBJ~:CTS OF Tll:L8 lNVl~NT~ON
One of the objects of thisi invention is to provide methods for electrowinning va~ious metals in which non~
corrodible dimen~ionally stakle a~lodes are used and in which a higher anodic current density relative to the cathodic current density is used to more economically produce relatively pure metal deposits of the desired crystalline structure and purity on the cathodes.
Another object is to provide apparatus by which the 1~ above defined methods can be economically carried out.
Another object of the invention is to provide elec-trolysis cells for use in electrowinning in which high current densities can be applied at the anodes and lower current densi-ties at the cathodes, so that relatively pure, smooth metal deposits can be produced on the cathodes with minimum expendi-ture of electrical energy.
Another object is to provide an electrowinniny cell and method of operating said cell, in which spaced rod or strip non-corrodible anodes are used having a surface area of - from 1.5 to 20 times smaller than the active surface area of the cathodes, whereby a greater current density can be applied to the anodes without exceeding the current density at which the cathodes produced good metal deposits and whereby the electrolyte can circulate freely through the rods or holes of the anodes to provide more uniform electrolyte compositions.
Another object is to provide dimensionally stable non-corrosive valve metal anodes for electrowinning which are substantially free from warpage or distortion in service and , on which the electrocatalytic conductive surface does not 3Q contaminate the electrolyte or the metal electrodeposited on the cathode.
Various other objects and advantages of this invention m~ 4-.:,, .. . . . . , ~

will ~ppear as this dcscription proceeds.
In most electrowinning processes, copper, zinc, nickel, cobalt, manganese, iron, lead, tin, cadmium, etc.
are electrodeposited on the cathode from sulfate solutions of the metal to be won, but in some cases chloride solutions, for example, nickel chloride solutions, may be used and in other cases, a combination of sulfate and chloride solutions may be used. The acids usually used to dissolve the metals - are sulfuric acid or hydrochloric acid or sometimes a combi-nation of sulfuric and hydrochloric acid. When the solvent acid is H2SO4, oxygen is liberated at the anodes, the metal is deposited on the cathode and the metal depleted acid is recycled to dissolve more metal from the ore, mine, dump or other source. When a chloride solution is used, chlorine is liberated at the anode.
The metals may be dissolved from ore, calcined ore, residue slag, scrap, underground deposits or other sources.
If the solution contains contaminating metals which would deposit on the cathode, with the metal to be won, the con-taminates are removed by the use of precipitants, sequestra-tion agents or in other ~-ays now used in electrowinning processes.
The non-corrodible dimensionally stable anodes used in our process are preferably titanium or other valve metals or alloys thereof provided with a conducting-non-passivating electrocatalytic surface which may be an oxide -or a mixture of oxides of the platinum group metals, i.e., ~platinum, iridium, palladium, ruthenium and osmium, applied a~ a coating to the surface of the valve metal, as described for example, in British Patents No. 1,147,442 and No. 1,195,871, or the electrocatalytic material may be incorporated as an alloy layer of bronze oxides in the surface of the valve _ `` 1063061 metal anode as descri~ed in Canadian ~atent No. 989,773 (May 25, 1976). If portions of these coatings are dislodged or dissolved into the electrolyte, their characteristics are such that they are mucll less likely to deposit upon th~ -cathode or contaminate the electrowon metal than the lead anodes of the prior art.
The valve metal anodes may be in the form of solid or reticulated sheets of, for example, titanium, or in the form of rods, bars or other arrangements, but in all instances the anode area facing the cathode should be 1.5 to 20 times smaller than the opposed cathode surface area and should be operated at a current density of 1.5 to 20 times higher than the cathode current density. Thus, when carrying out the pro-cess according to the present invention, the cathode current density can be in excess of 22 Ajm2 greater than that tolerable when lead anodes are used. The s~rface or coating of the anodes should have a high electrocatalytic activity for oxygen evolution in sulfate electrolytes or for chlorine evolution in chloride electrolytes. ~
The electrocatalytic coated dimensionally stable ~ -valve metal anodes of this invention are still active at 30 KA/sq. mt, after 2 to 3.5 years of anodic operation in-chloride solutions with an anode potential ranging from 1.35 to 1.50 V(NHE) and in 10~ (by weight) H2SO4 solutions, the ~; bronze oxide coatlngs are still active at 0.6 to 1.2 KA/sq.mt;
after more than one year of anodic operation with an anode potential ranging between 1.5 to 1.6 V(NHE), whereas lead and lead alloy anodes at the same working conditions show anode potentials greater than 1.98 V(NHE). If these dimensionally 30 stable anodes become passivated, they can be cleaned and recoated at a small expense~
The anodes of this invention under the operating m~p( -6-. . ~ . , ,,, ~

--~ 1063061 conditions hereinafter described produce cathode deposi~s free of contamination, whereas lead, lead alloy a~d titanium coated with noble metals produce cathode contamination in sulfate solution electrolyses. Owing to the low over-potential of these anodes, oxidation of Ni+2 and Co+2 to Ni+3 and Co+3 does not occur in either sulfate or chloride solutions.
The use of dimensionally stable metal anodes with - reduced anodic surface allows a more economical employment of anode materials for all those electrolytic processes for which a limit is imposed on the current density by other con-siderations than the anodic current density capability, in particular where the cathodic current density limit value is largely below the maximum current density economically acceptable by the dimensionally stable metal anodes.
The anodic structures with reduced surface, repre-sented by this invention are the result of an extensive theoretical and experimental research carried out to determine the most efficient geometric configurations for anodes that

2~ allow the maximum saving in anodic material while maintaining substantially unchanged the overall efficiency of the electro-lytic process.
In many electrolytic processes, and particularly in electrowinning and electrorefining of metals, to obtain a cathodic deposit of uniform thickness, the electric current must be uniformly distributed over the cathodic surface. Thus uniformity is achieved when the "lines of current", emanating normally from the surface of the anodic structure, assume a parallel and uniformly disposed configuration before reaching the plane cathodic surface at a normal direction of incidence.
It is obvious that the interelectrodic distance or electrode gap ha8 an essential beaxing on the process and it is one of .

m~p/ -7-, ., , : . - ~ - - . .. .. , , - .: -:.. ,. - . ~:
: ................ . . . . . .
, ,'~'h.,, . .- . . ' . ' . .' - :, ' , ., . ' the geometric parameters of the invention. The gcornetric parameters, con~rolling the achievement of the optimum con-ditions required by thc process and allowing the maximum saving of anodic materials, are a combination of the geometric configuration of the anodic structure, the interelectrodic distance and the shape and cross section of the members which ~ -constitute the reduced surface anode structure. In general, the electrode gap should be between 20 and 150 mm depending on the composition of the electrolyte, the anode configuration, 1~ the current density, the ohmic voltage drop in the electrolyte and in the anode structure, and other factors prevailing in a specific electrowinning plant.
By careful choice of each of the controlling para-meters, it is possible to operate an electrometallurgical process with anodes having an anodic surface down to 20 times smaller than the cathodic surface and therefore working at a current density 20 times higher than the cathodic current densit~.
- Special care must be exercised to distribute the electric current to the anodic structure so as to minimize the ohmic drop along the members of the anodic structure which have reduced cross sectional areas. Current is, hence, suppiied at several points on the structure in relation to the geometric and dimensional characteristics of the anode.
In the accompanying illustrative drawings, one embodi-ment of an apparatus and process for practicing this invention is illustrated, but it will be clear to persons skilled in the electrowinning of metals that the process can be carried out in other types of electrolysis cells and by other equivalent methods.
Fig. 1 is a diagrammatic perspective view of a typical electrowinning cell;

m~ 8 , ~
- , -, . .. : .: - .

., : . : . . .

Flg. 2 is a cros~ sectional view along the line 2 - 2 of Fig. 1, in whi~h the anodes are in the form illus- -trated in Fig. 6;
Figs. 3, 4, 5 and ~ are diayrammatic perspective views of various forms of dimensionally stable anode con-structions which may be used in the cells of Figs. 1 and 2 or in other electrowinning cells; and Figs. 7 and 8 are illustrations of one set o~ cell conditions designed to produce the best form of cathode deposit for the specific anode configuration diagrammed in these figures.
The illustrative cells of Figs. 1 and 2 comprise ~anks lQ which may be m~de of concrete or any other suitable material, lined with rubber or other suitable inert material 11. Tanks 10 are provided with positive bus bars 12 and negative bus bars 13 through which direct electric current from a source not shown, is supplied to anodes 14 and cathodes lS via contact bars 16 and 17, respectively, and anode and cathode straps 18. Electrolyte 20 containing a solution of the metal to be won is continually pumped into and out of the tanks 10 as the electrolyte is depleted in metal values by electrodeposition on cathodes 15. Tanks 10 are arranged in p~rallel and the electrolyte flow may be as desired.
Electrolytic cells of this type are most frequently connected in series with each cell containing one more anode than cathodes, so that each cathode is faced by an anode at each side of the cathode. Series arrangement of the electro-lytic cells is effected by placing tanks 10 side by side, so ;~ that, for example, current flowing through bus bar 12 is con- -ducted through anode contact bars 16 and anodes 14 and in each direction from the anodes through electrolyte 20 to cathodes 15 and through cathode straps 18, contact bars 11 and bus bars ~.
. . ., ~ . . :
9_ , ~::

- . . . - . . - , . .
- . . ...... : :.... :i . ... - - . .. ~ ..

" 106306~
13 to anodc co~tact hars 16 for thc immediately adjacent electrolytic cell. Contact bars 16 and 17 are supported at opposite ends ~y non-conducting supports 21 and 22, respect-ively, to insure that the electrodes are in a level poaition and to insure the desired parallel circuits for the current.
Anodes 14 shown in Figs. 1 and 2, have a spaced bar form, similar to that shown in Fig. 6 and described in greater detail below, which is advantageous for the practice of the present invention. Although the present invention is not limited to any particular anode dimensions or specific anode shape, a typical example given solely for the purpose of illustrating the invention is an anode structure having spaced rods, bars or strips illustrated and described below, having an active surface area from 1.5 to 20 times smaller than the opposed active surface area of the cathodes and operating at a current density of from 1.5 to 20 times higher than the current density on the opposed active cathodes surface.
Fig. 3 shows a substantially planar anodic structure, with reduced anodic surface, made from a series of parallel xods 23, vertically disposed and widely separated from one another, connected to a horizontal bar 24 which sustains-them and distributes the electric current to the same. The hori-zontal bar 24 is electrically connected to the positive supply bus bar 25 or to anode contact bars 16 of Figs. 1 and 2 which are connected to the electric current source. A horizontal ~ -bottom bar 26 joins the lower ends of the rods 23 to maintain a fixed separation pitch and to equalize eventual potential differences between the anodic rods 23. Bars 24 may be pro-vided with straps similar to straps 18 for connection to anode contact bars 16 of Fig. 1. ~ -Fig. 4 illustrates another example of substantially m~p~ -10-:

^-~ 1063061 plan~r anodic ~ruc~ures made according to the invention.
In this embodimcnt, tlle elec~rical supply bar 27 is in a central position with respect to the length of anodic rods 28. In this way the averaye path of the electric current in the structure is halved and a lower ohmic drop is achieved.
Conductors 29 can be made of different material, such as copper or aluminum and may be sealed off from the electrolyte by a protective sheath 30 resistant to the corrosive environ-- ment founa inside the cell. Conductors 29 distribute the electric current at several points along the supply bar 27 to minimize the ohmic drop.
Fig. 5 represents an anodic structure comprising a series of parallel anodic rods 31 horizontally disposed and connected to a central vertical supply bar 32 which consists of a round bar of highl~ electroconductive material such as copper or aluminum to which are connected the ends of the anodîc rods 31. The supply bar 32 is sealed from the electro-lyte by a protective sheath 33.
Fig. 6 shows anodic rods 34 horizontally disposed and connected at both ends to vertical supply bars 35, which extend vertically inside the electrolytic bath and are con-nected by straps 18 to anode contact bars 16 tFig. 1) or to a cable connected to the positive terminal of an electric source. In this embodiment, the path of electric current along the anodic rods 34 is halved with respect to their -length. In Figs. 3 to 6, the anodic rods may be round, square, rectangular with convex sides, oval or any other shape, or they may be flat strips of titanium or other valve metal in reticu-lated mesh or scxeen form provided with a conducting electro- ; ~ -catalytîc coating. However, the preferred shape of the rods - is the round section. The anode rods may be unevenly spaaed along the width of the anodes, such a~ more widely spaced -m~p~

:: ,: . . .. . . ..

. . - ~ , . . :~ :.

- - .: . . ' ' ' ,, :' . I . i. . ~ .

`" 1063061 toward the center and more closely spaced towards the ends of the allodes, to counteract the edge effect of the cathode deposits, and th~y may be diagonally sloped instead of hori-zontal or vertical, as illustrated in Figs. 3 and 6. The anode configuration which gives the best and most uniform deposits on the cathodes under the conditions of a specific electrowinning operation should be used. The values of geo-metrical parameters a and b in Figs. 3 to 6, namely, the spacing and diameter of the rods, may vary within wide limits depending on the interelectrodic distance and the maximum anodic-cathodic current density ratio which is to be used.
The rods 14-23-28, 31 and 34 are formed of a valve metal which in an electrolyte conducts electric current in one direction and does not conduct current in the opposite direction. Metals such as titanium, zirconium, hafnium, vanadium, niobium, tantalum, etc;, may be used, but we prefer to use titanium. The rods may be copper cored titanium rods as shown in Fig. 6, which have a high conductivity and may be detachably screwed into the horizontal or vertical bars 24-27 or 35 and detachably connected to the horizontal rods 26.
The rods 14-23, etc., may be flat strips of metal, strips of reticulated expanded metal or bars of any suitable shape;
The valve metal rods 23-28-31-34 are coated with a -conductive electrocatalytic coating containing at least one metal of the group consisting of gold, platinum, silver, palla-dium, iridium, ruthenium, osmium, rhodium, iron (magnetite), nickel, chromium in the form of oxides, nitrides, carbides `~ and sulfides of said metals.
The conductive electrocatal~tic coating on rods 23-28-31 preferably contains at least one platinum group metal oxide and at least one oxide from the group consisting of titanium, tantalum, zirconium, hafnium, vanadium, niobium, ~ `

~`

m~p/ -12- . -, a, ~, , ,, , , ,, . ~ ~ , .

.,, .'. . , ~' . ' " ' ' ' ' ' ' ~ ' ~

, ~, iron, nickel, tin, aluminum, cobalt and chromium.
The conductiv~ electrocatalytic surface on the anode rods 23, 28, 31, 34, etc., is pre~erably composed of a major port;on of titanium dioxide (~iO2) or tantalum pent-oY.ide (Ta2G,) together with a minor portion of an oxide of a platinum group metal capable of catalysing chlorine or oxygen discharge at the surfaces of the anode rods. Other electrocatalytic active surfaces may be used. We have found surfaces containing tantalum pentoxide and iridium oxide in the ratio of 1:1 to 0.34:1 of tantalum to iridium, calculated as metal,-with or without other oxides which improve the semi-conductivity of the surface, to be particularly effective.
Instead of rods or bars 23, 28, 31, etc., perforated flat titanium strips or other shapes having a small surface area with reference to the cathode surface area may be used and the rods, bars or strips may be tapered or of reduced cross section toward their lower end to produce different current density toward the bottom portion of the cathodes.
For each electrowinning installation, the anode configuration which will give the most uniform and best ca~thode deposit should be used. The best anode configuration fQr a specific set of cell operating conditions can be readily determined by persons skilled in the electrowinning art by a few simple electrodeposition experiments with the electrolyte and cell conditions to be used in the specific electrowinning plant in which the process and apparatus of this invention are to be used.
Fiqs. 7 and 8 show diagrammatically how these con- ;
ditions can be determined for one specific set of operating conditions.
ILLVSTRATION A

The ratio of the distance a between the anode rods , m~p~ - -13_ j ., .~ ~ ,..:
.... . .

and the interelectrodic gap d to maintain a good degree of uniformity of the deposit on a flat cathode has been summar-ized or di~ferent sets of operation conditions in the diagram of Fig. 7. This diagram illustrates how the ratio, expressed in percent, of deposit thickness between two repre-sentati~e points, near the middle of the cathode width, namely, between a point P on the cathode directly opposite an anode ~od and an adjacent point on the cathode directly underneath the midpoint between adjacent anode rods P' and P", varies with varying ratios of a/d.
In all cases, the distance d was kept constant at lQQ mm and the spacing of the rods _ along the anode was çhanged, the diameter b of the rods was small with respect to _ (i.e., b ~ 0.2d~, and the influence of the rod diameters on the deposit thickness uniformity was negligible.
The curve x te~ds to become steeper when anode structures having overall width smaller than about 10 times the interelec~rode distance were used and to flatten slightly when anodic structures having increasingly larger overall widths are used, which fact is explained by intervening edge effects.
The results indicate that an optimum ratio of ajd of about 1.5 or 2.0 is quite satisfactory, especially for wide anodic structures.
The edge effect of the anode to the cathode is summarized in the diagram of Fig. 8. The results illustrated -are for the specific case of an anodic structure having an overall width of 800 mm and constituted by a series of spaced round anode rods 5 mm in diameter.
The interelectrodic gap was-100 mm in the case of Structures I, II and III. Structure I had a uniform distance a between centerpoints of the rods of 100 mm. The profile of ~.
m~p~ -14- `

. . . . . .

the deposit thickncss of thc cath~dc's sur~ace is represented by cuxve I. As can be seen, neair the edge of the cathode plate, the thickness was down to about 0.7 times the thick-ness at the centerpoint which made the cathode deposit unsatis-factory.
Structure III had an e~enly decreasing spacing between the anode rods towards the edge of the structure, n~mely, 100 mm at the center of the anode structure and 90 mm, ~0 mm, 70 mm and 60 mm, progressive spacings of the rods from the center rods toward each outer edge of the anode. This Produced a deposit profile represented by curve III.
The optimum condition was approached with Structure II, characterized by ten anode rods spaced from edge to edge --as follows: -60-mm, 90 mm, 100 mm, 100 mm, 100 mm, 100 mm, 100 mm, 90 mm, 60 mm.
The profile of the cathodic dèposit thickness obtained is illus-trated by curve II and it is effectively comparable with the profile obtained using an integral plate anode.
The illustration of diagram 8 is true of only one set of ratios of anode rod spacing a, and the interelectrodic ~ap d. For each variation of these ratios, different variations in the thickness of the cathodic deposit may be secured. The optimum ratio for a given set of operating conditions can be xeadil~ determined by persons skilled in the electrowinning ~' axt and should be determined in advance for the conditions pre-vailing for each electrowinning in which the principles of this inYention are to be used.
`; The most satisfactory value of the distance a is bet~een 10 and 100 mm; the diameter b of the anodic rods is -- ~. . - .
between 2 and 16 mm. The interelectrodic gap may be from 20 mm to 150 mm.

m~p~ -lS- . -13Xl~Mr'LE 1 Nickel was electrodeposited from an electrolyte solution containing NiCl2 and using an anode structure similar to the one illustrated in E~ig. 3. The uniform center spacin~ a w~s 60 mm, reduced to 50 mm and 45 mm for the two extreme end rods of the anode. The diameter of the anode rods b w~s 6 mm.
The operational test conditions were the following:
~lectrolyte: NiCl 2 . 80 gpl as metallic Ni Cu~rent density (cathodic) 300 A/m2 Current density (anodic) 2,000 A/m2 Cat~ode dimensions 600 mm x 400 mm Overall dimensions of anodic 550 mm x 350 mm Structure Immersed surface area of anode 31,000 mm2 Cathodic material Stainless Steel AISI 316 ~node base material - Titanium, Interelectrodic distance 80 mm Te,mperature 60 to 80~ C

Buffering agent H3BO~; 20 to 40 gpl hn asbestos diaphragm 1.5 mm thick was used to separate anolyte from catholyte compartments. The cathode was between two anodes. 6 mm of nickel were deposited over both faces of the ~lat cathode operating at high efficiency. The quality of the deposit obtained was very good; the metallic deposit was ~ree from dendrites, of uniform thickness and mechanically stable.

Co~per was electrodeposited from an electrolyte solution containing CuSO~, using an anode structure similar to the one illustrated in Fig. 4. The uniform center spacing '~ of the rods 25 was 35 mm, reduced to 30 mm and 25 mm for m~p/ -16-7 ~ -, . ' ,' ' ' ' .~ ' thc two extr~me rodi~ of thc struct~re, and thc diameter of the anode xods b was 5 ~n.
The operation conditions were the following:
Elcctrolyte: CuSO4 40 gpl as metallic Cu H2SOh 150 to 200 gpl Current density (cathodic) 300 A/m2 Cu~rent density (.anodic) 1,300 A/m2 Cathode dimensions 700 mm x 400 mm , Oyerall dlmensions of anodic 670 mm x 390 mm Structure Immersed surface area of anode 64,500 mm2 ~nt~relectrodic distance 90 mm ~emperature 60 to 80 C
6 mm of copper were deposited Qver both faces of ~ -the flat cathode at a high overall efficiency. The quality of the deposit was very good; the deposit was substantially ~
free from dendrites and had good mechanical stability. The ~.
thickness was substantially uniform ove~ the entire cathodic ~ ~ .
surface. ' ~ ~' Zinc w,as electrodeposited from an electrolyte solu-tion containing ZnSO4, using an anode structure similar to the , one illustrated in ~ig. 5. The uniform center spacing a was .
30 mm, reduced to 28 mm and 22 mm for the two extreme rods at ' ' the top and bottom of the structure, and the diameter of the ' '' -anode rods b was 6 mm.
Th~ operational test conditions were the following:
Electrolyte: ZnSO4 60 gpl as metallic Zn H~SO4 150 to 200 gpl Current density (cathodic) 300 A/m2 Current density (anodic~ 1,000 A/m2 Cathode dimensions 600 mm x 400 mm '.

.~ m~ -17-Overall dimensions ~f ano~ic 580 mm x 390 mm Structur~
Immersed surface area of anode 72,000 mm2 Intcrclectrodic distance 80 mm Tempexature 30 to 35 C

3 mm of zinc were deposited over both faces of the ~l~t c~thode ~t hiyh overall efficiency. The quality of the deposit was very good; the deposit was substantially free from dendrites and had good mechanical stability. The thick-.I0 ness was substantially uniform over the entire cathodic surface.

Cob~lt was electrodeposited from an electrolytesolution containing CoS04, using an anode structure similar to the one illustrated in Fig. 5. The uniform center spacing _ was 35 mm, reduced to 30 mm and 25 mm for the two extreme rods of the structure, and the d1ameter of the anode rods b ~as S mm.
The operational conditions were the following:
Electrolyte: CoS04 8Q gpl as metallic Co H2 S04 To a pH of 2 Current density (cathode) 300 A/m2 Current density (anodic~ 1,300 A/m2 Cathode dimensions 700 mm x 400 mm Overall dimensions of anodic 670 mm x 390 mm Structure Immersed surface area of anode 64,500 mm2 Interelectrodic distance 90 mm Temperature 60 to 80 C

The cathodes were between two anodes. 6 mm of cobalt were deposited over both faces of the flat cathode at a higher overall efficiency. The quality of the deposit was very good;
the deposit was substantially free from dendrites and had good m~p/ -18-.. . . . .

--:`" 1063061 mech~nical stability. ~rhe thickness was substantially uniform over the entire cathodic surfacc.

~ ickel was electrodeposit2d from an electrolyte solution containing NiSO4, using an anode structure similar to the one illustrated in Fig. 5. The uni~orm center spacing a was 35 mm, reduced to 30 mm and 25 mm for the two extreme rods of the structure, and the diameter of the anode rods b was 5 mm.
1~ The operational conditions were the following:
Electrolyte: NiSO4 80 gpl as metallic Ni ~2 SO4 To a pH of 2 Current density (cathodic) 300 A/m Current density (anodic~ 1,300 A/m2 Cathode dimensions 700 mm x 400 mm Overall dimensions of anodic 670 mm x 390 mm Stxucture Immersed surface area of anode 64,500 mm2 Interelectrod~c distance 90 mm . . .
Temperature 60 to 80 C

6 mm of nickel were deposited over both faces of the flat cathode at a high overall efficiency. The quality of the deposit was very good; the deposit was substantially free from dendrites and had good mechanical stability. The thickness was substantially uniform over the entire cathodlc surface.
The examples describe the applications of our inven-tion to electrorefining or electrowinning. Anodes with reduced anodic sur~ace combine the well-known advantages of dimension-ally stable metal anodes, such as the lower energy consumption,high purity of the metallic deposit, freedom from warpage and operational long life with a reduced investment cost!

`.

~ ~ . '., m~p/ -19-., ; , ~. .
. . . . .. . . . . ..

Anodes with reduced anodi.c surface may be used in other electrolytic processes where conditions exist such that the cathodic current density is limited by other con-siderations than the current density capability of the anode and where there is a valuable possibility of economizing on the cost of thc rather expensive materials necessary to fabricate dimensionally stable metal anodes, such as organic electro-oxidation and electroreduction processes in electro-f"lotation processes, etc.

The terms "spaced rods" or "spaced conducting e.lectrocatalytic surfaces" as used in the following claims are intended to include spaced rods, bars, flat strips of m~etalt strips of xeticulated expanded metal, bars of any ~u;table shape, perforated flat titanium strips or other shapes ! having a small conductive surface area with reference to the cathode surface area. - -m~p~ -20_ -

Claims (25)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An electrowinning cell comprising a tank; a metal-containing aqueous acid electrolyte in said tank; at least one cathode substantially vertically immersed in said electrolyte;

at least one anode insoluble in said electrolyte substantially vertically immersed in said electrolyte and spaced from the cathode; said anode comprising a series of spaced bars of a valve metal having an electrically conductive electrocatalytic surface; said cathode having an area substantially greater than said anode; and a direct current source for supplying current to the anode and the cathode at an anodic current density of from 1.5 to 20 times higher than the cathodic current density;

said bars being spaced at unequal distances along said anode with the widest spacing, at the mid-portion of the anode, being approximately equal to the spacing between the anode and the cathode and the narrowest spacing, near the edges of the anode, being less than the spacing between the anode and the cathode, and said anode conducting surface having an area between 1.5 and 20 times smaller than the area of the conducting surface of the cathode.

2. The eloctrowinning cell as described in claim 1, wherein said anode comprises a series of bars of a conductive valve metal alloy resistant to the corrosive cell conditions and having a surface which is catalytic with reference to the anodic process and the current source supplies electric current to the anode and the cathode at an anodic current density of from 3 to 20 times higher than the cathodic current density.

3. The cell of claim 1, in which the portion of said anode bars exposed to the electrolyte is formed of a valve metal and said valve metal has an electrically conducting electro-catalytic surface thereon and the current source supplies electric current to the anode and the cathode at an anodic current density of from 4 to 20 times higher than the cathodic current density and at a cathode current density in excess of 22 A/m2 greater than tolerable with lead anodes.

4. The cell of claim 1, in which the electrocatalytic surface is a coating which contains an oxide of tantalum and an oxide of iridium.

5. The cell of claim 4, in which the electrocatalytic surcace is a coating which in addition to at least one platinum group metal oxide contains at least one oxide from the group consisting of titanium, tantalum, zirconium, hafnium, vanadium, niobium, iron, nickel, tin, aluminum, cobalt and chromium.

6. An electrowinning cell according to claim 1, in which the axes of said bars are spaced between 10 mm and 100 mm apart, the bars have a diameter between 2 and 16 mm and are spaced at unequal distances from one outer edge to the other outer edge of said anodes, with the widest spacing opposite the mid-portion of the cathode and the narrowest spacing opposite the edges of the cathode, and the distance between said bars and the cathode is between 20 and 150 mm.

7. The electrowinning cell of claim 1, in which the ratio of the distance between said bars and the distance of said bars from the cathode is between 0.3 to 1 and 2 to 1.

8. The cell of claim 1 in which the axes of said bars are spaced between 10 mm to 100 mm apart and the distance between said bars and the cathode is between 20 and 150 mm.

9. The cell of claim 1, in which the anode bars are unequally spaced along the anode.

10. The cell of claim 1, in which the conducting electro-catalytic surface contains an oxide of at least one metal from the group consisting of platinum, palladium, iridium, ruthenium and osmium.

11. An electrowinning method which comprises establishing a bath of an aqueous acid electrolyte having dissolved therein at least one electrodepositable metal; immersing a substantially solid cathode and an insoluble non-corrodible valve metal anode with spaced conducting electrocatalytic surfaces containing a platinum group metal oxide, of slightly less length and width than the length and width of the cathode in the electrolyte in a generally vertica] direction to form an electrolytic cell, said anode conducting surfaces being spaced equal distances apart in correspondence to the mid-portion of the cathode and closer together toward the edges of the cathode, to produce substantially uniform thickness deposits on the mid-portion of the cathode and on the edges of the cathode, and said anode conducting surfaces having an area between 1.5 and 20 times smaller than the area of the conducting surface of the cathode, within the reduced length and width of the anode; and using an anode current density between 1.5 and 20 times greater than the current density on the cathode to produce smooth adherent deposits of substantially uniform thickness of said metal on the cathode.

12. The method of recovering metal from an aqueous acid solution containing the metal to be recovered, which comprises electrolyzing said solution in an electrolytic cell between an insoluble metal anode formed of spaced rods having a non-corrodible valve metal base and conducting electrocatalytic surfaces, containing a platinum group metal oxide and a solid cathode onto which the metal is deposited, in which the active conducting surface area of the anode facing the cathode with reference to the active cathode conducting surface area is 1.5 to 20 times smaller than the active area of the cathode, the anode current density with reference to the cathode current density is 1.5 to 20 times greater than the cathode current density, the anode rods with reference to the width of the interelectrodic gap are spaced uniformly in the mid-portion of the anode structure to produce substantially uniform thickness deposits of the metal to be won on the mid-portion of the cathode, and the anode rods are spaced closer together at the extremities of the anode structure to reduce the edge effect at the edges of the cathode to produce substantially uniform deposits on the edges of the cathode.

13. The method of claim 12, in which the spacing of the anode rods with reference to the width of the interelectrodic gap is in an optimum ratio of 1.5 to 2.0 and the conducting electrocatalytic surfaces contain an oxide of at least one platinum group metal.

14. The method of claim 13, in which the conducting electrocatalytic surfaces contain at least one oxide of a platinum group metal and at least one oxide of a non-precious metal.

15. The method of claim 11, in which the spaced conducting electrocatalytic surfaces of the anode are unequally spaced along the anode to produce deposits on the cathode comparable in uniformity of thickness to the cathode deposits produced with a flat planar integral anode plate.

16. The method of claim 12, in which the anode rods are spaced unequal distances along the anode and are diagonal with reference to the vertical direction of the cathode.

17. The method of recovering metal from an aqueous acid solution containing the metal to be recovered, which comprises electrolyzing said solution in an electrolytic cell between an insoluble metal anode having a non-corrodible valve metal base with spaced active conducting electrocatalytic surface areas containing a platinum group metaloxide, and a cathode onto which the metal is deposited, in which the spaced active con-ducting surface areas of the anode facing the cathode with reference to the active conducting cathode surface area are at least 1.5 times smaller than the active conducting surface area of the cathode, the anode current density with reference to the cathode current density is at least 1.5 times greater than the cathode current density, the active conducting surface areas of the anode are spaced unequal distances apart to produce metal deposits of substantially uniform thickness on the cathode, with the center to center distance between adjacent active conductive surface areas of the anode facing the cathode being approximately the width of the interelectrodic gap and the conducting surface areas of the anode toward the edges of the cathode being closer together.

18. The method of claim 17, in which the conducting electrocatalytic surfaces contain an oxide of at least one platinum group metal, and the ratio of the center spacing to the interelectrodic gap at the mid-portion of the cathode is 1.1 and toward the edges of the cathode from 0.6 to 1.

19. The method of claim 18, in which the conducting electrocatalytic surfaces contain at least one oxide of a platinum group metal and at least one oxide of a valve metal.

20. The method of claim 17, in which the ratio of active anode surface area to active cathode surface area is equal to or smaller than 1/3.

21. The method of claim 17, in which the cathode current density is in excess of 22 A/m greater than that tolerable when lead anodes are used.

22. The method of claim 17, in which anodes in the form of spaced rods are used, the rods are between 2 and 16 mm in diameter, the center spacing of the rods is between 10 and 100 mm and is unequal and the interelectrodic gap is 20 to 150 mm.

23. The method of electrowinning which comprises providing a tank; a metal-containing aqueous acid electrolyte in said tank;
at least one cathode substantially vertically immersed in said electrolyte; at least one anode insoluble in said electrolyte substantially vertically immersed in said electrolyte and spaced from the cathode, said anode comprising a series of spaced bars of a non-corrodible valve metal having an electrically conductive electrocatalytic surface containing a platinum group metal oxide, said cathode having a surface area substantially greater than the anode surface area; a direct current source of supplying current to the anode and the cathode at an anodic current density of from 1.5 to 20 times higher than the cathodic current density;

spacing the bars at unequal distances, with the widest spacing at the mid-portion of the anode being approximately equal to the width of the interelectrodic gap and the narrowest spacing near the edges of the anode being less than the width of the interelectrodic gap, to electrodeposit the metal in said electrolyte on the cathode as a smooth, relatively pure metal deposit of relatively uniform thickness from edge to edge of the cathode and substantially free of contamination from the metals of said anode.

24. The electrowinning method of claim 23 wherein said anode comprises a series of bars of a conductive valve metal alloy resistant to the corrosive cell conditions and having a surface which is catalytic with reference to the anodic process, and the current source supplies electric current to the anode and the cathode at an anodic current density of from 3 to 20 times higher than the cathodic current density.

25. The method of claim 23 in which the current source supplies electric current to the anode and the cathode at an anodic current density of from 4 to 20 times higher than the cathodic current density and at a cathode current density in excess of 22 A/m greater than tolerable with lead anodes.