CA1075635A

CA1075635A - Metals

Info

Publication number: CA1075635A
Application number: CA246,350A
Authority: CA
Inventors: Frank S. Holland
Original assignee: National Research Development Corp UK
Current assignee: National Research Development Corp UK
Priority date: 1975-02-25
Filing date: 1976-02-23
Publication date: 1980-04-15
Also published as: JPS5919994B2; IT1056188B; FR2302353B1; NL7601940A; AU501058B2; IL49092A; SE7602227L; BE838915A; JPS51110471A; FR2302353A1; DE2607512C2; DE2607512A1; AU1139776A; GB1505736A; ZA761052B; IL49092A0

Abstract

"IMPROVEMENTS IN OR RELATING TO METALS"

ABSTRACT OF THE DISCLOSURE

A metal powder e.g. copper or zinc, is produced from a dilute aqueous solution of the metal by subjecting the dilute aqueous solution to electrolysis in a cell having a rotating cylinder cathode in accordance with the equation:-I = KCVx where I is the current density, K is a constant for a given cell, C is the concentration of metal ion in aqueous solution, V is the peripheral velocity of the rotating cylinder cathode and x = 0.7 to 1Ø A diaphragm cell is preferably used.
Dilute aqueous solutions treated include mining liquors and viscose rayon plant effluent. A plurality of cells in series may be used or the cathode compartment of the cell may be divided into sub-compartments.

Description

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This invention relates to metals and is particularly concerned with the recovery or extraction of metals in powder form by electrolytic means.
The production or manufacture of metal powders by electrolytic means is well known, for example see Electro-chemical Engineering by C. L. Mantel 4th Edition, published by McGraw Hill sook Company in 1960. In practicer cathode current densities for powder production are higher than those ¦of refining of metals to give massive cathodes (600 amps/m ~
compared with lOQ amps/m2) and the concentration of the metal in powder production is lower than in refining (5 grams/litre compared with approximately 40 grams/litre). The metal deposits as discrete particles at the cathode and is collected at the bottom of the cell, or as a loosely adherent deposit which may be lifted from the cell and washed off the cathodes.

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~L33i7~i635 Certain of the processes and apparatus involve the deposition of metallic powder on a movable or continuous cathode. United States Patent No. 1,736,857, for example, discloses and claims apparatus involving an endless cathode in the form of a band which continuously passes between anodes through a trough containing electrolyte. United States Patent No. 2,810,682 discloses and claims a process whereby silver powder is produced from a soluble silver anode. The anode dissolves in the electrolyte and powder is formed on a disc-shaped cathode rotating slowly through the electrolyte. Deposited powder is removed as the rotating cathode surfaces pass between a pair of metallic doctor blades. The powder settles to the bottom of the electrolyte bath and is periodically recovered by filtering the electrolyte. United States Patent No. 1,959,376 discloses a process and United States Patent No. 2,053,222 discloses an apparatus ~or producing copper powder. Accor-dlng to these patents, a series of disc-shaped copper cathodes is mounted in an electrolytic cell -tank such that each cathode is partially immersed in the electrolyte bath contained therein. Soluble copper anodes are suspended in the electrolyte bath on each side of each cathode. The cathodes are rotated as a current is applied across the _ 3 _ 635 ~ ~

electrodes. Copper deposited on the rotating cathode surfaces is removed as powder by doctor blales mounted above the electrolyte surface. United States Patent No. 3,616,277 discloses a process and an apparatus for producing copper powder. ~letallic powder, e.g. copper powder, is deposited on a series of disc-shaped cathodes as they turn through an electrolytic solution oE the metal. The cathodes, prefer-ably of titanium, are partially immersed in a bath of elec-trolyte contained in an electrolytic cell tank. Insoluble anodes, preferably of platinized titanium are disposed in the tank in an interleaved arrangement with the cathodes.
Powder is continuously deposited on the cathodes and con-tinuously removed by -the doctor blades, preferably of plas-tic, mounted adjacent to the cathodes above the elec-trolyte level of the cell.
An electrolytic cell, using a rotating cylinder electrode, is a well known and well studied device; for example see a review by D. R. Gabe in the Journal of Applied Electrochemistry, 1974, Volume 4, page 91, and the referen-ces therein. The rotating cylinder electrode cell has beenused and studied extensively Eor the deposition of metals.
The studies of many investigators have confirmed that the current density attainable on a rotating cylinder electrode s con~r~lled by the following equation:

' ~ 7563~i I = 0.0791 n.F.C.V. (Vd)P (1)-0 644 where I = current density, Amps/cm n = valency change F = Faraday (96,500 coulombs) C = Concentration in moles per cm of the metal ion V = peripheral velocity of the cylinder electrode in cm/sec d = diameter of the cylinder electrode in cm u = kinematic viscosity of the solution at the operating temperature in cm /sec D = diffusion coefficient of the metal ion at the operating temperature of the solution in cm2/sec p = an exponent This equation can be simplified to I = KCVX, where K is a constant, and x = 1 + p; ln the previous work referred to, x has been found to be approximately 0.66.
It will be appreciated that the simplified expression may also be written as I = K CV
where lo is the current in amps actually used in producing metal powder in the cell~ :

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Io is in fact the total current in the cell multiplied by the current efficiency to metal deposition.
Although no prior proposal, as far as we are a~are, has been made to use the rotating cylinder electrode cell for metal powder production, we attempted this as it would be convenient in that the process would be continuous in the same fashion as the process described in United S-tates Patent No.
3,616,277. However, we were surprised to find that instead of the limiting curren-t density for powder produc-tion in the cell being defined by the equation I = KC~0-66, a similar relationship holds b~t the value of x is higher, sometimes approaching unity. This means that the limitin~ current density is more nearly di.rectly proportlonal to the peri-pheral velocity of the rotating electrode. This also means ~
that the throughput of such a device is considerably increased, ~ -20 or that the required concen-tration of metal in solution to ~. .
produce the metal powder at a given current density is much reduced. Thus, the cell described in Example I below has a total limiting current controlled by the equation Io = 4.38 x 10 3 C VX. When the peripheral velocity 'V' is 1000 cm/sec.
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75~;35 - and the concentration 'C' of copper is 200 ppm, the total limiting current 'Io' would be expected to be 9.7 amps taking 'x' to be 0.66, whereas in the present invention 'x' for Example 1 equals 0.92 and the total limiting current 'Io' is 504 amps, i.e. an increase of 52 times. To sustain the higher current of 504 amps the process based upon the previously known characteristics of the rotating cyllnder would need to operate at 10400 ppm. of copper, i.e. 52 times the concentration, or rotate at a peripheral velocity of 398100 cm/sec., i.e. nearly 398 times as fast as in the presen-t invention. Under the conditions described in the process based upon the previously known characteristics of the rotating cylinder it would be expected that the cell could produce 11.5 grams/hour whereas 600 grams per hour can be expected to result from use of the i,nvention. Similarly, using the cell as described in Example 1, if the concentration 'C' of copper remains at 200 ppm. and the peripheral velocity 'V' is reduced to 500 cm/sec. the following applies:- in a process based upon the previously known characteristics of the rota-ting cylinder, where 'x' equals 0.66l a limiting metal deposition current 'Io' of 6 amps would be expected to result in a production of 7 grams/hour of copper powder, whereas using the pre~ent inventlon, where 'x' equals .92, '~:
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- 5635, the cell produces a current of 266 amps giving 316 grams/hour of copper powder. This is 43 times as much as could be expected Erom a process based upon the prev:iously known characteristics of the rotating cylinder. For the process based upon the previously known characteristics of the rotating cylinder to sustain such a high current and a high production rate the concentration of copper would need to he increased to 8682 ppm., or the peripheral velocity increased to 151400 cm/sec., i.e.
about 303 times AS fast as the present invention~
We do not fully understand how the invention operates.
Particles of metal are being deposited on the rotating cylinder and many of tllese particles are immediately dislodged. There-fore the surface is not steady and reproducible with time, a new surface being formed continuously. The surface is rather rough and the surface area is greater than the super-ficial area of the rotating cylinder. Therefore the actual size, the actual roughness, and the actual surface area, of the rotating cylinder on which powdered metal is being deposited cannot be defined. The fact that they cannot be defined may explain why the accepted law did not apply. We have also found lt ` ~7S635 advantageous to roughen the surface of the cathode before deposition of metal on it, for example by etching, as this pre-roughening enhances the mass transfer in the cell and therefore raises the value of x.
According to the present invention therefore, a method of producing a metal or metalloid powder from a dilute aqueous solution of the metal comprises operating a rotating cylinder cathode cell at current densities proportional to vX where V is the peripheral velocity of the rotating cylinder cathode and x is from 0.7 to 1.0 whereby the metal is deposited as particles on said cathode. The particles may be allowed to fall from or caused to be removed from, said cathode, preferably as it rotates.
The invention thus provides an electrolytic process for producing metal powders whereby a dilute aqueous solution of the metal ion is subjected to electrolysis in a cell having a rotating cylinder cathode. The peripheral speed of the rotating ,~
cylinder cathode and the current density on the cathode may be chosen in relation to the concentration of the metal ion such -~
that a powder deposit is produced and the metal powder may be dislodged from the cathode whilst rotating and discharged from the cell contlnuously.

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Dilute aqueous solutions electrolysed accordiny to the invention may contain from 2 to 10,000 parts per mill~.on of me-tal ion.
In general, the method or process of the invention mav be carried ou-t in accordance with the equation I = KCV
where x = 0.7 to 1.0, preferably 0.8 to 0.95;~I, K, C and V
have the meanings given previously above and mav have the following operable and preferred ranges of values:
10 Concentration, C ~ ppm - 10,000 ppm operable ~ppm - 1,000 ppm preferred Current Density, I rl mA/cm - 10 Amps/cm operable ~1 mA/cm2 - 1 Amp/cm2 preferred :

Peripheral Velocity V ~1 cm/sec - 10,000 cm/sec operable of Rotating Cylinder Electrode ~ 0 cm/sec - 2,000 cm/sec preferred K is a constant and refers to a particular cell and therefore cannot be defined as having operable and preferred ranges of value~ It will be appreciated from the term (Vd)P in equation (1) above that the expression I = KCVX is an over-simplifica- ~.
tion and that K cannot be an absolute constant but is dependent to a certain extent on the value of p and therefore on the value oE x. ~he value of K depends on the metal being - ~- 1 0 -~75635 deposited as well as -temperature and cell geometry, and could lie in the range 5 x 10 8 -to 5 x 10 6; Ko will vary accordingly.
In organic electrochemistry, organic material may be decomposed a-t the counter electrode, e.g. a cathodic reduction may give a product at -the cathode which is oxidised and degraded at the anode. In this case it is common (see M. J. Allen, Organic Electrochemistry, Chapman & Hall, 1954) to use a partition between the electrodes, thus defining a cathode compartment, and an anode compartment. Various materials have been used as partitions, including parchment, asbestos cloth, other cloths and ion exchange membranes which allows the electricity -to pass through but retain organic materials in whichever compartment is required.
The ion exchange membranes are membranes including ion exchange material, such as the ion exchange membranes commonly used in electrodialysis.
The invention therefore includes the further features that the method is carried out in a cell which ; -~
includes a diaphragm, such as an ion exchange diaphragm, positioned between the rotating cathode and -the anode or anodes of the cell.

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~7~;35 { The invention further includes a cell which comprises a rotating cylinder cathode in a confined compartment with substantially concentric anodes and a means of supplying liquor to and removing liquor from the confined compartment.
The confined cathode compartment may be formed by a more or less concentric anode or anode compartments. Typically, the area of the rotating cylinder cathode may be in the range

2~0 cm2 to 5,900 cm2, but could be larger.
The invention also includes the further feature of 10 separating the metal powder produced from the other materials contained in the ~electrolytic cell. This separation may be effected by fairly simple physical means such as settling, hydrocyclone separation or o-ther simple liquid/solid separation.
Chemical means may also be used, forlexample, eluting or dis-\
solving the metal with a suitable s~lvent, such as a mineral ~ acid or an alkali, to form a concentrated solution of the metal and such means may be used to remove deposited metal still adhering to the cathode. Electrochemical means may also be used to redissolve the metal deposited on the cathode, for 20 example, anodic dissolution.

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The metals which may be recovered by means of the invention are for example chromium, manganese, iron, cobalt, nickel, copper, zinc, ruthenium, rhodium, palladium, silver, cadmium, indium, tin, iridium platinum, gold, lead, uranium and the rare ear-th metals. Mixtures of me-tals may be co-deposited on the cell cathode or metals may be co-deposited ~ith metalloids such as arsenic and antimony. The metalloids themselves may be deposited alone or in admixture.
An important feature of the process of the invention is that it produces the metal in the form of a powder, which is easy to remove from the electrolytic cell.
The method of the invention may be carried out in a plurali.ty of cells in series. Alternatively, the method may be carried out in a cell having a cathode compartment divided into a plurality of sub-compartments in series and during electrolysis, the aqueous solution of the metal flows through the sub-compartments, the concentration of metal ion in the a~ueous solution becoming progressively reduced during passage of the solution through the series of sub-compartments. Pre-ferably there are from 6 to 10 sub-compartments. This type of cell may itself be used as one of the cells in a series oE cells to reduce still further the metal ion concentration in the outflow from a previous cell.
The invention also includes the concept of control-ling tho working elecLrode potential and this may be done by .. .. .

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techniques known in themselves. This is important for best results in zinc deposition but is not so cri-tical Eor copper deposition. The preferred means for contro]ling the cell voltage or the elec-trode potential can be a voltage regulator or a potentiostat, the voltage re~ulator (if used) controlling the cell voltage, and the potentios-tat ~if used) cont.olling the electrode potential.
Control of the pH of the electrolyte is also desirable in the case of zlnc as described more fully below, a suitable range being pH 4 to 7.
The cell can be supplied with any form of electric current e.g. direct current, alternating current, pulsed direct current or mixtures thereof, and the cell voltage can be controlled or the electrode potential controlled !
accordingly e.g. by using a reference electrode. The cell is preferably operabIe in the range 2 to 20 volts but higher voltages up to 250 or lower voltages may be used.
The period of time for which the cell is operated does not appear to be critical. However, the operating temperature of the cell is significant if optimum yields are to be obtained. Increasing the temperature of the aqueous electrolyte increases the total mass transfer to the cathode. While in general the operating temperature may be in between 0C and 100C, it is preferred to use an operating temperature in the range 20C to 80C. A suitable operating temperature is about 60C.

~C)75635 The elec-trolyte used in the method of the inven-tion may be any wa-ter-soluble electrically-conduc-tiny salt of the metal being produced. In the case of copper or zinc, a pre-ferred salt is -the sulpha-te. Other electrolytes may also be present.
The rotating cylinder cathode of the cell used in the method of the invention may in ~eneral be made of any suitable metal but for reasons of economy it is preferred to use a cathode made of steel which is sui-tably coated, for example with a layer of the metal to be deposited. Thus a steel cathode coated with copper may be used for copper deposition and a steel cathode coated with zinc for zinc .. . ..
deposition. The anode or each anode of the cell is prefer-ably made of a relatively corrosion-resistant metal, for example a noble metal such as platinum, but less expensive metals such as lead may also be used. An alternative form of anode which may be used is one made of valve metal coated wi.th the noble metal. Suitable valve metals are titanium, zlrconium, tantalum and hafnium and any of these may be coated with platinum.
The invention provides a cheap, continuous electrolytic plant and process, able to recover metals from dilute solutions.
Dilute solutions of metals (ca 2 - ca 10,000 ppm.) can be treated efficiently, the economics depending on the concentra-t-on.

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-~LC175G3~;i Stronger solutions can also be treated by dilution with -the cell effluent, so that the concentration in the cell is within a convenient range (e.g. 200-300 ppm.). Because the cell can be diaphragmed, very "dirty" solutions can be treated, e.g. those containing organics and other anode destroying components.
In general, the metal obtained from the process is of high purity. In particular, the metal ob-tained by the electrodeposition of copper onto a rotating cylinder elec-trode is of much higher purity than metal obtained by the deposition of copper from dilute solutions by the cementation process i.e. by the reduction of the solution with iron.
The invention is -therefore particularly advantageous in its application to mining liquors.
The following are known to con-tain fairly low concentrations of metals:
a) Copper phthalocyanine (C.P.C.) plant effluents b) Viscose plant effluents c) ~ining liquors, such as dump leach liquors and other mine waters.
dJ Tank house bleed streams from normal electrowinning .

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~Cl 75i635 e) Electropla-ting rinse waters, such as electrogalvanising rinse waters, par-ticularly strip steel and wire.
f) Pickling solutions in copper and brass wire manufacture g) Sewage sludge The following waste may be treated to produce dilute solutions at various strengths for use in the present invention.
1. Chemical Wastes:
. . _ These include:
a) _pper wastes:
i) etchants ii) catalysts in chemical manufacture iii) pickle liquors b) Chromium wastes:
i) plating liquors ii) plating shop sludges iii) sludges and solutions from dichromate oxidations c) l~ickel wastes:
i~ plating liquors and plating shop sludges . ii) electrochemical machining sludges d) Tin wastes:
plating liquors and plating shop sludges .

~Q75~3~

e) Zinc,_etc.
Zinc wastes fro~ organic chemicals manufacture 2. Solid Wastes:
These wastes occur as.
i) drosses - zinc, brass, tin, etc., ii) swarfs iii) scrap printed circuit boards - copper and precious metals, and so on.
The electrolytic process of the invention is parti-cularly applicable to the recovery of metallic zinc from viscose rayon plant effluents.
Viscose Rayon is manufactured by spinning viscose (cellulose xanthate ln caustic soda) into sulphuric acid, containing zinc salts and other metallic sulphates (see F. D. Lewis, The Chemistry and Technology of Rayon rlanufacture, 1961).
The use of zinc salts in the manufacture of viscose rayon is well known. Such salts are used throughout the world in acid spinning baths and stretching baths. Regenerated cellulose rayon made in this way contains large quantities of zinc which are removed by washlng and the washings constitute one source of effluent. Acid spinning bath rejects and stretch-i~g bath -ejects are other sources of effluent.

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~75635 These e~fluents can contain:
sulphuric acid sodium sulphate magnesium sulphate carbohydrates, such as glucose and other sugars, cellulose degradation products etc., sulphides xanthates surfactants, such as quaternary ammonium salts, e.g. cetylpyridinium bromide zinc sulphate.
The treatmen-t of -these effluents is commonly carried out in two ways.
a) recovery of zinc sulphate solutions from acid spin-ning bath and stretching bath effluents by recrys-tal-lising out the excess sodium sulphate and returning the liquor for re-use.
However, there are still zinc losses and liquor rejects because other impurities build up.

~ILCJ 75635 b) the dilute washings liquor and the above mentioned rejects are chemically treated with ferrous sulphate which precipitate the sulphides and with lime which neutralises the effluent and precipitates the zinc asla basic carbonate. The sludges from this treat-ment are considered to be harmless and are dumped on land. Thus there is no recovery of zinc.
We have found that the electrolytic process of the present inve~tion as described above is effective in the 0 recovery of zinc from viscose rayon plant effluents.
Improved yields of zinc are obtained if the acidity of the viscose e~fluent electrolysed is low e.g. be-tween pH 4 and pH 7. Viscose effluents normally have a hiyh acidity e.g.
pH 1 and the pH of the effluent may therefore be adjusted to lower acid values to avoid a poor yield of zinc. The pH may be adjusted before electrolysis but tends to fall during elec-trolysis in a non-diaphragm cell. Further adjustment during electrolysis in a non-diaphragm cell is desirable but if a diaphragm cell is used, only a major initial adjustment of pH
is needed for low acidity throughout the electrolysis process because pH control is achieved virtually automatically by the transfer of ions through the diaphragm, and only minor additional . ~

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1~7563~ , adjustmen-t of p~l is required.
The pH of the effluent may be adjusted by addition of an alkali, preferably caus-tic soda, but other alkalies,such as sodium earbonate or ammonia, may also be used; or the effluent may be buEfered e.g. by the addition of sodium acetate.
The concentration of zinc in viscose effluent is commonly 0.1 to 1.0~ i.e. 10 to 100 times more dilute than electrowinning zinc sulphate liquor. Further, the organie compounds present are of the type that can damage the common anode materials (e.g. platinum, leacl, lead dioxide).
Thus the electrolytic recovery of the zinc meta] from viscose rayon plant effluents preferably uses a cell having a) a diaphragm to prevent anode corrosion b) a rotating cylincler electrode to give economically acceptable current densities.
In one oE its preferred forms, the present invention eombines a) the teehniques of electrowinning zinc, b) the techniques of organic electrochemistry in terms of diaphragms, . .

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~75~35 c) ro-tating cylinder electrode techniques, and d) control of -the pH of the viscose effluent.
Any of the electrode materials normally used in electrowinning may he used in the electrolysis of the viscose effluent but aluminium is preferred as -the cathode material.
In the electrolysis of viscose rayon plant effluent using an anion exchange membrane, the overall process is the removal of zinc from the catholy-te and the formation of sul-phuric acid at the anolyte, i.e. both zinc and sulphuric acid are recovered by this electrolysis.
That is one advantage. A further advantage is that the recovered zinc can be dissolved in the recovered sulphuric acid to give strong solutions (e.g. ~) of zinc sulphate which can then be used in the rayon production process.
The invention will now be further described by way of example with reference to the accompanying drawings, in which:- .
Figs. 1 and 2 are sectional views of a rotating cylinder electrode diaphragm cell, Fig. 1 being a section ~
on the line BB of Fig.2 and Fig. 2 being a section on the line AA of Fig. 1, Fig. 3 shows the general arrangemen-t of a cell of the type shown in Figs. 1 and 2, .:.

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Fk~. ~ shows a labora-tory cell, Fig. 5 is a diagrammatic vertical sec-tion of a larger rotatable electrocle cell without a diaphragm, Fig. 6 is a horizontal section on the section line shown in Fig. 5, Fig. 7 is a horizontal section, corresponding to Fig. 2, of a diaphragm cell, Fig. 8 is a flow diayram illustrating a metal recovery process, Fiy. 9 is a plan view of a horizontally operated cell complete with shaft and drive motor, Fig. 10 is a sectional elevation oE the cell only, and Fig. 11 is a sectional end elevation of the cell only.

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In Fiys. 1 and 2, a rotating cylinder or drum cathode 10 is separated from approximately concentric anodes 11 by a membrane or diaphragm 12 defining anode and cathode compartments.
The membrane may be a cation exchange membrane, for example Du Pont ~lafion (Trade Mark), where the metal being recovered is copper or an anion exchange membrane, for example Ionac MA
3472 (Trade Mark~, where the metal being recovered is zinc.
Anolyte may be introduced to -the cell through the ports 13 and withdrawn through the ports 14. Catholyte (electrolyte) may be introduced to the cell through the inle~ hole 15 in the base of the cell and withdrawn through outlet hole 19 in the top oE the cell. Alternatively, use may be made of -tubes 16 .~ .
which communicate with the cathode compartment through holes 17 in the cell casing. Thus the catholy-te may be introduced to the cell through one of the tubes 16 and withdrawn through the other tube 16, the introduction and withdrawal being at both ends of each tube if desired. The drum 10 is provided with a scraper 18 mounted for removing metal adhering to the cathode as the cathode rotates. As shown the scraper extends over the full length of -the cathode but alternatively a reciprocating scraper extending over part only of the length of the cathode may be provided.

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~75~35 ~` .
In Fig. 3, a motor 30 is connected by a belt drive 31 to a shaft 32 rotatably mounted in bearings 33 (without locking ring) and 34 (with locking ring). The shaft carries a slip ring assembly 35 for the supply of curren-t to the rota-table drum cathode 36 mounted on the shaft within the electro-lytic cell 37. A water cooling housing 38 is provided above the cell and where the shaft passes through the housing it is provided with seals 39, 40. The anode 41 of the cell is concentric with the drum cathode and is provided with a fixed electrical anode terminal 42. The cell has an inlet 43 and outlets 44 for catholyte and inlet 45 and outlet 46 for anolyte.
In Fig. 4, a rotatable cylinder cathode 60 is mounted on a shaft rotatable in bearings 61 in a support frame 62 and is driven by means of a motor 63 connected to the shaft by a belt drive 64. The frame carries a scraper 65 for the cathode. An anode 66, concentric with the cathode, is disposed }n an anolyte compartment 67 having cooling coils 68, the cathode and anolyte compartment being housed in a polypropylene drum 69. Part of the wall of the anolyte compartment between the anode and the cathode is formed from an ion-exchange membrane 70. Electrical connections 71, 72 are provided for the anode and cathode respectively and the drum 69 is provided with a heater 73 and a thermometer 74.

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~ 5635 In Figs. 5 and 6 a main shaEt 105 is rotatably mounted in bearings 106. At its upper end, the shaft is driven from a variable speed motor 107 by means of a driving belt 108. At its lower end the shaft passes through a seal 109 into a cell 110 which is provided with a liquor inlet 111, a liquor outle-t 112, and a drain valve 113. A
cylindrical electrode 114 is mounted on the lower end of the shaft 105 so as to be rotatable with the shaft. Counter electrodes 115 are mounted within the cell and uniformly 10 disposed around the rotatable electrode 114. The liquor -outlet 112 determines the level of liquid in the cell and ensures immersion of the electrodes in the liquor. Current may be supplied to the rotatable electrode 114 via a slip ring assembly 116 on shaft 105.
In Fig. 7, membranes 117 separate the counter electrodes 115 from the rotatable electrode 114 so as to form anode and cathode compartments.
The rotary cathode electrolytic cell shown in Fig. 8 has a membrane defining separate anode and cathode compart-20 ments and for this reason the anolyte is circulated in a ~-separate circuit, being cooled on i.ts return to storage.
Flectrolyte from the cathode compartment (containing metal powder and hydrogen gas produced by the electrolytic process) ' :
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passes to the gas separa-tor where -the hydrogen is separated from the electrolyte and metal par-ticles. In the hydrocyclone separator ( a device well known in the art), -the hydrocyclones separate the metal powder from mos-t of the electrolyte. The thickener is a cone thickener in which the metal powder in the form of a slurry with residual electrolyte from the separator is concentrated to give a very thick slurry which can be automatically removed from the bottom of the cone and treated further as necessary. Electrolyte separated in the thickener is returned to the hydrocyclone separator. The bulk of the elec-trolyte from the separator passes to storage from which it is recirculated to the cathode compartment oE the rotary cell. The circulation of anolyte and electrolyte is eEfected by means of pumps P.
In Fig. 9 a rotatable cylinder cathode 130 is mounted on a shaft rotatable in bearings 131 and driven by means of a motor 132 connected to the shaft by a bel-t drive 133. The cell is sealed at both ends by means of a shaft seal 134. At one end of the shaft there is a slip ring and brush block assembly 135. Fig. 10 shows the sectional eleva-tion of the cell only - this comprises the main cell ` - 27 -~al7~;iG35 frame 136 which is spli.t into -ten compartments by baffles 137. The cathode compartmen-t in which the cylinder electrode 130 rotates is enclosed by a lid 144 and membranes 139 (Fig. 11) at either side. The cell lid 144 is sealed to the main cell compartment by means o-E a rubber gasket 148. The membranes are sealed against rubber seals 140 and supported by membrane supports 142. The cathode compartment is fitted with an electrolvte inlet 145 and an electrolyte outlet 146.
The compartment is also fitted with a product sump 149 and product outlets 143. Thus there are ten enclosed compartments separated from the anolyte and anode compartments. ~
Catholyte is fed into the lnlet 145 and i5 trans- :
ferred from the flrst compartment lnto the second compartment through a clearance gap around the shaft ln the baffle. Thus the electrolyte path through the cell ls down through the first compartment into the second, lnto the third and so on until the final compartment is reached and the electrolyte leaves `.
the cell througn the outlet 146. The clearance gaps in the baffles are such that there is the minimum amount of back- ~' mixing.

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1~75~35 The anolyte compartments are located between the membranes 139 and the anolyte compartment sides 138. The anolyte compartment sides are sealed to the main cell frame 136 by means of rubber gaskets 147. In the anode compar-tment there are two anodes 141. Electrical connections are made to the anodes 141 and -to -the slip ring and brush block assembly 135.
The inven-tion is further illustrated in the follow-ing Examples. The current efficiency referred to in the ExaMples is defined by F`araday's Laws and is well understood by persons skilled in the art. It differs frorn the yield oE metal powder in that the yield, can mean the overall efficiency of the process including (~or instance~ mechanical losses from the system.
In the Examples 1 to 21, the symbols used in the equations have the following meanings: -Io is the current in Amps actually used in producing metal powder in -the cell, (Io is the total current in the cell multiplied by the current efficiency to metal deposition), C is the concentration in pprn. in solu-tion of the metal ion being deposited, V is the peripheral velocity in cm. per second of the rotating cylinder electrode.

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~)7563S

Example 1 A cell as shown in Figs. 1 and 2, and cell assembly as in Fig. 3 with the cylinder electrode having an area of 1687 cm2 was used. The cylinder was rotated a-t 810 r.p.m.
giving a peripheral velocity of 1000 cm per second. Copper sulphate in sulphuric acid solution was pumped into the cell at a rate of 1 litre per second, at a temperature of 60C;
the inlet concentration was 350 ppm. copper, and this was diluted in the cell to 200 ppm. copper so that -the outlet concentration was 200 ppm. A current of 700 Amps was passed for 4 hours during which time the current efficiency to copper deposi-tion was 72% and the cell produced 600 gms. of copper powder per hour at a current densi-ty of 300 mA/cm2.
This process can be descrlbed by Io = 4.38.10 3 CV0 9 .
Example 2 The cell described in Example 1 was used; the flow rate and temperature were the same. The cylinder was rotated at 320 rpm. giving a peripheral velocity of 393 cm/sec. The inlet concentration of copper was 900 ppm. and this was dilu-ted in the cell to 680 ppm. so that the outlet concentrationwas 680 ppm. A current of 700 Amps was passed for 2 hours, during which time the current eEficiency -to copper deposition was 93% and the cell produced 770 gms. of copper powder per hour at a current density of 386 mA/cm2. This process can be described by Io = 3.93.10 3 CV0 92.

. ' ,, . . . . ,,.; , ~ :, , 1~75635 E ~mple 3 The cell described in Example 1 was used; the flow rate and temperature were the same. The cylinder was rota-ted at 440 rpm. giving a peripheral velocity of 541 cm/sec. The inlet concentration of copper was 571 ppm. and this was diluted ln the cell to 385 ppm. so that the outlet concentration was 385 ppm. A current of 700 Amps was passed for 4.25 hours, during which time the current efficiency to copper deposition was 80% and the cell produced 664 gms. of copper powder per I0 hour at a curren-t density of 332 mA/cm2. This process can be desribed by Io = 4.45.10 3 CV0 9 .
Example 4 The cell described in Example 1 was used; the flow rate and temperature were the same. The cylinder was rotated at 1380 rpm. giving a peripheral velocity of 1698 cm/sec. The inlet concentration of copper was 200 ppm. and this was diluted in the cell to 98 ppm. so that the outlet concentration was 98 ppm. A current of 550 Amps was passed for 4 hours, during which time the current efficiency to copper deposition was 70~ and the cell produced 456 gms. of copper powder per hour at a current density of 228 mA/cm2. This process can be described by Io = 4.2.10 3 CV0 9 .

-- . :

5~35 :.
_ample 5 The cell described in Example 1 was used; the flow rate and temperature were the same. The cylinder was rotated at 810 rpm. giving a peripheral velocity of 1000 cm/sec. The inlet concentration of copper was 81 ppm. and this was diluted in the cell- to 50 ppm.so that the outlet concentration was 50 ppm. A current of 285 Amps was passed for 3 hours, during which time the current efficiency to copper deposition was 37% and the cell produced 125 gms. of copper powder per hour at a current density of 62.5 mA/cm2. This process can be described by Io = 3.67.10 CV
Example 6 The cell described in Example 1 was used; the flo~
rate and temperature were the same. The cylinder was rotated at 810 rpm. giving a peripheral velocity of 1000 cm/sec. The inlet concentration of copper was 330 ppm. and this was diluted in the cell to 190 ppm. so that the outlet concentration was 190 ppm. A current of 600 Amps was passed for 2.5 hours, during which time the current efficiency to copper deposition was 86% and the cell produced 612 gms. of copper per hour at a current density of 308 mA/cm2. This process can be described by Io = 4.72.10 CV0 ...... ,. ,, . ... . .. . . . .. . - , . : : i . : : -~75635 E a ple_7 The cell described in Example 1 was used; the flow ra-te and temperature were the same. The cylinder was rotated at 810 rpm. giving a peripheral velocity of 100 cm/sec. The inlet concentration of copper was 368 ppm. and this was diluted in the cell to 193 ppm. so that the outlet concentration was 193 ppm. A current of 1000 Amps was passed for 1.5 hours, during which time the current efficiency to copper deposition was 50% and the cell produced 593 gms. of copper po~der per hour at a current density of 296 mA/cm2. This process can be described by Io = 4.17.10 3 CV0 93.
xample 8 A cell as shown in Fig. 4 wi-th a cylinder electrode having an area of 200 cm2, was used. The electrolyte was sodium sulphate (10 Kgs. anhydrous in 46 litres of solution) at pH 4 and a temperature of 60C. The cyclinder was rotated at 1800 rpm. giving a peripheral velocity of 719 cm per second; zinc sulphate solution was added continuously to the electrolyte to maintain a zinc concen-tration at 400 ppm.
Sulphuric acid was added to maintain the pH at 4. A current of 50 Amps was passed Eor 1 hour 10 mins., during which time the current efficiency to zinc powder deposition was 46~ and and the cell produced 28 gm. of zinc powder per hour at a current density of 115 mA/cm .

~563S

This process can be described by Io = 2.5.10 4 CV ' Example 9 The cell described in Example 8 was used. The electrolyte and temperature were the same. The cylinder was rotated at 1800 rpm. cJiving a peripheral velocity of 719 cm/sec;
zinc sulphate solution was added continuously to the electrolyte to maintain a zinc concen-tration of 431 ppm. Sulphuric acid was added to maintain the pH at 4. A current of 50 Amps was passed for 45 mins., during which time the current efficiency to zinc powder deposition was 37.4% and the cell produced 22.8 gm. of zinc powder per hour a-t a current density of 93.5 mA/cm2. This process can be described by Io = 2.2.10 4 CV0.806 E mple 10 The cell described in Example 8 was used. The electrolyte and temperature were the same. The cylinder was rotated at 1800 rpm. giving a peripheral velocity of 719 cm/sec;
zinc sulphate solution was added continuously to the electrolyte to maintain a zinc concentration of 458 ppm. Sulphuric acid was adde~ to maintain the pH at 4. A current oE 50 Amps was passed for 2 hours, during which time the current effieiency to zine powder deposition was 58% and the eell produced 35 gm.
of zinc powder per hour at a current density of 144 mA/cm .
This process can be described by Io = 2.7.10 4 CV0 ~32.

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, .

~S635 Example ll The cell described in Example 8 was used. The electrolyte was viscose rayon plant effluent at pH 4 and a temperature of 60C. The cylinder was rotated at 1800 rpm.
givin~ a peripheral velocity of 719 cm/sec; zinc sulphate solution was added continuously to the electrolyte to maintain a zinc concentration of 418 ppm. Sulphuric acid was added to maintain the pII at 4. A current of 50 Amps was passed for 1 hour, during which time the current efficiency to zinc powder deposition was 40.5~ and the cell produced 24.7 gm. of zinc powder per hour at a current density of lOl mA/cm2.
This process can be described by Io = 2.3.10 4 CV0 ~13.
Example 12 A cell smaller but otherwise similar to that described in Example l was used, with the cylinder electrode having an area of 200 cm2. The electrode was zinc plated stainless steel.
The electrolyte was 1 Molar sodium sulphate solution at pH 4 and a temperature of 60C. The cylinder was rotated at 800 rpm.
giving a peripheral velocity of 319 cm per second, zinc sulphate solution was added to maintain a zinc concentration of the input feed of 450 ppm. The electrolyte was pumped in to the cell at a rate of 4 litres per minute; the inlet concentration was 450 ppm. zinc and this was diluted in the cell to 350 ppm.
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~7S63S

zinc so that the outlet concentration was 350 ppm. zincO A
potential difference between the rotating cylinder electrode and a nearby mercury-mercurous sulphate reference electrode of 1.86 volts was maintained such that the rotating cylinder electrode was cathodic to the reference electrode. This resulted in a current of 28 Amps, which was maintained for 4 hours, during which time the current efficiency to zinc powder deposition was 71% and the cell produced 24.4 gm of zinc powder per hour at a current density of 100 mA/cm2.
This process can be described by Io = 3.58.10 4 CV0'83.
Example 13 The cell described in Example 12 was used; the electrolvte and temperature were the same. The cylinder was rotated at 1200 rpm. giving a peripheral velocity of 479 cm/sec; zinc sulphate solution was added to maintain a zinc concentration of the input feed of 430 ppm. The electrolyte was pumped in to the cell at a rate of 4 liters per minute; the inlet concentration was 430 ppm. zinc and this was diluted in the cell to 350 ppm. zinc so tl~at the outlet concentration was 350 ppm. zinc. A potential difference between the rotating cylinder electrode and a nearby mercury-mercurous sulphate reference electrode of 1.7 volts was maintained such that the rotating cylinder electrode was cathodic to the reference electrode.

: ' :

:- ~" - ~ : , . ............................................ . :
. ; .. , :

- ~
~6175~35 This resulted in a current of 16 Amps, which was maintained for 3 hours, during which time the current efficiency to zinc powder deposition was 100% and the cell produced 19.5 gm.
of zinc powder per hour at a current density of 80 mA/cm2.
This process can be described by Io = 2.67.10 4 CV0'333.
Example 14 A cell similar to -that described in Example 8 was used; the electrolyte was an effluent from the production of copper p~thalocyanine and contained sulphuric acid, sodium chloride, urea and other organic chemicals. The temperature was 60C. The cylinder elec-trode was made from titanium which had been pre-roughened bv etching in concentrated hydrochloric acid; the superficial area of the cylinder elec-trode was 200 ci~2. The cylinder was rota-ted at 645 r.p.m.
giving a peripheral velocity of 257 cm per second. A potential difference between the rotating cylinder electrode and a nearby saturated calomel reference electrode of 0.4 volts was maintained such that the rotating cylinder electrode was cathodic to the reference electrode. The s-tarting concentration of copper in the electrolyte was 95 pprn. and an initial current o 9.25 Amps resulted. The concentration of copper and the cell current decayed exponentially over a period "' ~.

- 37 _ ~7~i63~

of 90 minutes to 2.5 ppm. and 0.2 Amps respectively. This process can be described by Io = 5.59.10 4 CV0 93.
_ample 15 The cell described in Example 14 was used; the -electrolyte, electrode, temperature, and rotational speed were the same. The potential of the rotating cylinder cathode was held at -0.5 volts with respect to a nearby sat-~rated calomel electrode. The starting concentration of copper was 15 ppm. giving an initial curren-t of 2.8 Ainps.
The concentration of copper and the cell current decayed exponentiallv over a period of 50 minutes to 1 ppm. and 0.9 Amps respectivelv. This process can be described by I = 9.3.10 4 CV0 955.

Example 16 The cell described in Example 14 was used; the electrode was the same. The electrolyte was a solution of 0.5 Normal hydrochloric acid containing sodium and ammonium chlorides and zinc at 3,500 ppm., arsenic at 250 ppm., platinum at 20 ppm., palladium at 120 ppn., rhodium at 120 ppm., ruthenium at 45 ppm., iridium at 25 ppm., and some sliver and gold. The temperature was 60C. The potential of the rotating cylinder cathode was held at -0.2 volts with respect to a nearby saturated calomel electrode, the cylinder lL~7563~

electrodes was rotated at 400 r.p.m. giving a peripheral veloci-ty of 160 cm per second. There was an initial current of 16 Amps and this decayed over 200 minutes to 2 Amps. The powder me-tal produc-t contained zinc, arsenic, platinum, palla-dium, rhodium, ruthenium, iridium, silver and gold. This process can be described by I = 2.46.10 4 CV0 93.
Example 17 The cell described in Example 8 was used. The eleetrode was smoo-th stainless steel and was rotated at 1,250 r.p.m. giving a peripheral velocity of 500 cm per - second. The electrolyte was a solution oE ammonium sulphate tl Kym. in 46 litres) containing nickel at S00 ppm. and iron at 500 ppm. at pll8. The temperature was 35C. The po-ten-tial of the rotating cylinder electrode was held at -1.5 volts with respect to a nearby sa-turated calomel re-Eer-ence electrode. This produced a current of 30 Amps and a strong solution of nickel (115 gm. per litre) and iron (50 gm.
per litre) was added during 3 hours to maintain the nickel concentration. Nickel powder was produced at a current efficiency of 28%. The nickel powder analysed at 99.1% nickel, 0.25% iron. The eylinder electrode had a very fine deposit of nie]cel powder on i-t and was substantially smooth. This ~75~;3~i process can be described by Io = 1.3.10 4 CVn 79.
Example 18 The cell described in Example 8 was used. The cylinder was rotated at 1,250 r.p.m. giving a peripheral velocity of 500 cm. per second. The electrolyte was a strong sulphuric acid solution (150 gm. per litre) con-tain-ing nickel (8 gm per litre), arsenic (2 gm per litre) and copper at 230 ppm. Additional electrolyte con-taining sulphuric acid (150 gm. per litre) nickel (39 gm. per litre), arsenie (5 gm. per litre) and copper ~36 gm. per litre) was added to maintain the copper eoncentration. A current of 50 Amps was passed for 8 hours during whieh time the eurrent effieiency to eopper po~7der deposition was 78% and the eell produeed 46 gm. of eopper powder per hour at a eurrent density of 196 mA/em . The recovered copper powder analysed at 95% copper, 0.2% nickel and 3% arsenic. The eylinder had a very coarse deposit of copper powder on it and was substantially rough. This process can be described b~ Io - 4.5.10 CV0.953.
Example 19 A eell smaller but otherwise similar to tha-t des-eribed in Example 1 was used, with a cylinder elee-trode having an area of 500 em2. The electrode was zinc plated aluminium.

.

-The electrolyte was the effluent from the production of viscose rayon and contained a high concentration of sodium sulphate and 62 ppm. of iron. The electrolyte temperature was maintained a-t 60C and p~-I 4.5. The cylinder was rotated at 800 r.p.m. giving a peripheral velocity of 372 cm. per second. Vi,scose rayon efflue~ (zinc concentration 2,500 ppm.) was added to maintain a zinc concentration of the ~, cell input feed of 420 ppm. zinc. The electroly-te was pumped - , into the cell at a rate of 4 litres per minute; the inlet concentration was 420 ppm. zinc and this was diluted in the cell to 340 ppm. zinc so that the outlet concentration was , 340 ppm. zinc. A potential difference between the rotating ' cylinder electrode and a nearby mercury-mercurous sulphate ' reference electrode of 1.7 volts was maintained such that the rotating cylinder electrode was cathodic to the reference electrode. This resulted in a current of 23 Amps which was maintained for 5.5 hours during which time the current effi-ciency to zinc powder deposition was 66% and the cell pro-duced 18.5 gm. o'f zinc powder per hour a-t a current density of 31 mA/cm2. The recovered zinc powder, which had partially oxidized, analysed at 55% zinc and 0.6% iron. This process , can be described by Io = 4.5.10 4 CV0'78.

'' ' ' ,,, - 41 -~L~7~635 Example 20 A cell larger, but otherwise similar to that described in Example 1, was used with the c~llinder electrocle having an area of 5,900 cm2. The electrode was smooth titanium. The cylinder was rotated at 460 r.p.m. ~iving a peripheral velo-city of 1,112 cam per seeond. Copper sulphate in sulphurie aeid was pumped into the cell at a rate of 2 li-tres per second, at a temperature of 60C the inlet concentration was 362 ppm.
copper and this was diluted in the cell to 234 ppm. so that the outlet concentration was 234 ppm. A current of 1,000 Amps was passed Eor 14 hours during which time -the current effi-eieney to eopper deposition was 78% and the eell produced 925 gms. of eopper powder per hour at a eurrent density of 170 mA/em . At -the end of the run the titanlum eylinder had very little copper remaining on it and was substantially smooth. This proeess can be described by Io = 7O2.10 CV0.875 Example 21 The cell deseribed in Example 20 was used. the rotating eylinder eleetrode was hard plated eopper (0.4 mm.
thiek) on titanium. The eylinder was rota-ted at 460 r.p.m.
yiving a peripheral veloeity of 1,112 em. per seeond. Copper sulphate in sodiu.-n sulphate solution at pH3 was pumped into ':

.

~5635 the cell at a rate of 2 litres per second, a-t a temperature of 60C; the inlet concentration was 350 ppm. copper and this was dilu-ted in the cell to 150 ppm. so that the outlet concentration was 150 ppm. A current of 2,000 Amps was passed for 36 hours during which -time -the current efficiency to copper deposi-tion was 62~ and -the cell produced 1,470 gms. of copper powder per hour at a current density of 210 mA/cm2. At the end of the run the copper cylinder had a coating of copper powder on it, and the surface was rough. This process can be described by Io = 13.10 CV0.92 Example 22 The cell as shown in Fiys. 9, 10 and 11 was used.
The -total cylinder lenyth was 100 cm. and the efEectlve length of each part of the rotating cylinder in each compartment was 9 cm. The diameter of the cvlinder was 7.62 cm. and the cylinder was rotated at 2,000 rpm. giving a peripheral velo-city of 800 cm/sec. An electrolyte, which was an effluent from the production of copper phthalocyanine containiny sulphuric acid, sodium chloridej urea and other organic chemicals, was pumped into the cell at a flowrate of 6 litres/
min. and a temperature of 60C. The electrolyte in the -~ 43 _ .

. .

` ~ ~75~;35 ~.

anode compartments was IN caustic soda and the anodes were nic~el. A current of 40 ~mps at 4 volts was applied to -the cell and this was maintained for a period of 4 hours. During this time the inlet concentration and the concentrations of copper in each compartmen-t were sampled and analysed as follo~s:-_ - _ _ _ Compartment Sample Point Inlet __ _ I --- -~ ~ I -2 ~ 4 5 6 ~7 ~ ~ lO

Copper Concentration 102 9 56 36 21 ll 5 l 3 2 1.5 1.0 ppm. L __ _ _ __ i_ ~ l L L L _ ¦ ;~
The copper powder formed duriny this electrolysis remained in each compartment either retained on the cylinder or elsewhere within the compartmen-t. This process can be described by:-= 4.55.10 CV where Io is the current in ~mps in the individual compartment producing the powder deposit C is the concentration of the copper in that cell compartment in ppm.
V is the peripheral velo-city of the rotating cylinder in cmJsec.
: .

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.
.
, - . : . ~ :

~ 7563S

After this period of electrolysis -the cell was drained and a solution of 20 litres of water and 5 litres of 70~ nitric acid was pumped slowly -through the cell with the cylinder electrode rotatin~ at 2,000 r.p.m. The copper powder on the cylinder electrode and the copper powder in the cell dissolved completely over 30-60 minutes to produce a solution containing 19.5 gm. per litre of copper.
To facilitate continuous extrac-tion of copper or other metal, two of the cells of Figs. 9 to 11 may be used in parallel, one being used for electrolysis while the metal is being dissolved out of the otherO Such cells may be preceded by a cell according -to Figs. 1 and 2, -to which the metal solution may be recirculated.
The elution process illustrated in Example 22 can use, for example, HNO3, H2O2-H2SO4, HCL, NaOH, NH40H or NaCN-NaOH as the chemical solvent depending on the metal to be dissolved. Alternatively or in addition, the rotating cathode electrode can be made anodic so that anodic dissolution of the metal occurs.

;

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

~75~3~ ~:

In general the advantages of the presen-t invention may be sum~arised as follows:
Metals can be continuously and efiiciently extracted from industrial ef:Eluents and other dilute solutions by means oE the inventlon.
The mass transfer of metal in the rotating cylinder cathode cell of the invention may be up to 1000 times that of conventional plate-in-tan~c cells.
Production of~the metal as a powder facilitates recovery of the metal from the cell.
Dirty effluents can be treated because.the use of a diaphragm cell avoids or reduces anode corrosion by impurities.
Apart from its use in metal production, the invention can be used in pollution control. Effluents from which metals have been removed by the invention can be treated by biological means to remove organic impurities prior to discharge.

~ 46 -

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of producing a metal or metalloid powder from a dilute aqueous solution of the metal which comprises subjecting the dilute aqueous solution of the metal to electrolysis in a cell having a rotating cylinder cathode, the current density on the cathode being proportional to Vx where V is the peripheral velocity of the rotating cylinder cathode and x is from 0.7 to 1.0, whereby the metal is deposited as particles on the cathode.

2. A method as claimed in claim 1 in which the dilute aqueous solution contains from 2 to 10,000 parts per million of metal ion.

3. A method as claimed in claim 1 in which the electro-lysis is carried out in accordance with the equation:
I = KCVx where I is the current density, K is a constant, C is the concentration of metal ion in aqueous solution, V is the peripheral velocity of the rotating cylinder cathode and = 0.7 to 1Ø

4. A method as claimed in claim 3 in which C = 2 to 10,000 parts per million, I = 1 mA/cm2 to 10 Amps/cm2 and V = 1 cm/sec to 10,000 cms/sec.

5. A method as claimed in claim 4 in which C = 2 to 1,000 parts per million, I = 1 mA/cm2 to 1Amp/cm2 and V = 10 cms/sec to 2,000 cms/sec and x = 0.8 to 0.95.

6. A method as claimed in claim 1 in which the elec-trolysis is carried out in a cell having a diaphragm separating the cathode from the anode or anodes.

7. A method as claimed in claim 6 in which the diaphragm of the cell is an ion exchange membrane.

8. A method as claimed in claim 1, claim 2 or claim 3 in which the metal powder produced is separated by physical means from the other materials contained in the cell.

9. A method as claimed in claim 1 in which metal is recovered from the cathode by chemical or electrochemical means.

10. A method as claimed in claim 9 in which the chemical means comprise dissolving the metal in a mineral acid.

11. A method as claimed in claim 9 in which the electro-chemical means comprise anodic dissolution of the metal.

12. A method as claimed in claim 1 in which the cell voltage is from 2 to 250 volts.

13. A method as claimed in claim 12 in which the cell voltage is from 2 to 20 volts

14. A method as claimed in any one of claims 1 to 3 in which the temperature of the electrolyte is from 20°C
to 80°C.

15. A method as claimed in any one of claims 1 to 3 in which the dilute aqueous solution is a mining liquor.

16. A method as claimed in any one of claims 1 to 3 in which the dilute aqueous solution is a solution of at least one metal selected from the group consisting of copper, zinc, chromium, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, silver, cadmium, indium, tin, iridium, platinum, gold, lead/ uranium and rare earth metals.

17. A method as claimed in claim 1 in which the dilute aqueous solution is viscose rayon plant effluent obtained by spinning viscose into sulphuric acid containing zinc salts.

18. A method as claimed in claim 17 in which the electrolysis is carried out using an aluminium cathode.

19. A method as claimed in claim 17 in which the pH of the effluent during electrolysis is between 4 and 7.

20. A method as claimed in claim 19 in which the pH of the effluent is adjusted by the addition of alkali.

21. A method as claimed in claim 20 in which the alkali is caustic soda.

22. A method as claimed in claim 19 in which the effluent is buffered.

23. A method as claimed in claim 22 in which the effluent is buffered by the addition of sodium acetate.

24. A method as claimed in claim 17 in which sulphuric acid is also recovered from the effluent.

25. A method as claimed in claim 24 in which the recovered zinc is dissolved in the recovered sulphuric acid to form a solution for use in spinning viscose.

26. A method as claimed in any one of claims 1 to 3 in which the cathode is made of steel coated with the metal to be produced.

27. A method as claimed in any one of claims 1 to 3 in which the cathode is roughened prior to deposition of the metal.

28. A method as claimed in any one of claims 1 to 3 which is carried out in a plurality of cells in series.

29. A method as claimed in claim 1 in which the cell has a cathode compartment divided into a plurality of sub-compartments in series and during electrolysis, the aqueous solution of the metal flows through the sub-compartments, the concentration of metal ion in the aqueous solution becoming progressively reduced during passage of the solution through the series of sub compartments.

30. A method as claimed in claim 29 in which there are from 6 to 10 sub-compartments.