Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ S8333 :`
i Th~ invention relates to the electrodepos~tion
¦ of metals and more particularly but not exclusively, i5concerned with the electrowinning of cobalt and nickel.
Most electrowon cobalt is presently produced in `
S Za;~re and Zambia from mixed copper cobalt sulphide or
I oxide orss. In the case of sulphide ores, the normal
~ - practice ls to "sulphate" roast the ore prior to leachlng
.J: . whilst in the case o~ oxide ores the ore may be leached
directly in an acld solution. A leach solution containing
copper, cobalt and sulphate ions is then separated from
the leach pulp by normal solids/liquids separation
practices. The leach residue is washed before being
discarded. Ater clarification of the leach solution,
copper is remo~ed ~rom the leach solution by electro-
winning. The leach solution may be sub~ected to a "partial
~ron precipitat~on and washing" stage prior to
~¦ clarification and copper-electrowinning. Where there is
l~ an acid deflciency in the leach liquor, for example when
I the oxlde ore is being leached, a portion o~ the-spent
electrolyte from the copper-electrowinning operation may
l~ be returned to the leach 11quor. The remainder o~ the
spent electrolyte ~which contains the cobalt but only a
low cQncentration o~ copper) is then purified ~or removal
of impurities such as iron, copper and sometimes z~nc
z~h~
before completely precipitat~ng or crystalli~ 4 the
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I - cobalt out of solution. The cobalt solids thus obtained
are separated from the rema1ni~g solution, known as the
liquor, which is discarded. In one process the cobalt
solids are then washed and redissolved in spent
.
electrolyte from a subsequent, cobalt-electrowinning stage.
The cobalt-rich solution from this cobalt re-solution
stages ~hen passes ~ usually a~er clarification, to the
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' cobalt-electrowinning stage in which cobalt is electro-
fj deposited. In some cases, additional purification stages
are included prior to the cobalt electrowinning stage.
~i In another process the cobalt solids are only partially
S redissolved at the re-solut~on stage, and the result~ng
i pulp containing dissolved cobalt anld undissolved coba}t
-j solids is electrowon in air agitateld cells; this is done
¦ so that as ac~d is formed during the electrowlnning
it is consumed by the undlssolved cobalt so1ids ln the
¦ 10 pulp. The temperature of the electrolyte during the
electrowinning of the cobalt has a significant affect on ;-
,l the cobalt electrowinning process, particularly on the
cathode potential and the internal stress and hydrogen
content of the cobalt deposit. The common practice is
to carry out the cobalt-electrowinning stage with the
~ ; temperature of the electrolyte at approximately 60C.
¦ In conventional cobalt electrowinning practice
the cobal~ is usually deposited on stainless steel
~ -:
"blank" cathodes~ After the deposit has grown to an
.,.
acceptable thickness it is stripped from these bl~nks.
, The removal of this deposit from the blan~ can be an
arduous process because in some cases the cobalt metal
`l , adheres strongly to ~he stainless steel blank. The
- 25 remoYal of cobalt from stainless steel blanks is usually
carrled out manually using hammers or chisels. In the
process the blanks themselves often become damaged and
;~ the eventual replacement of these blanks is costly. The
pieces of cobalt metal that have been removed from the
blank are then reduced in size by crushing or by other
~1 suitable means and then, ~efore the cobalt metal is
: `1
I marketed, are usually vacuum degassed for rem~val of
entrained hydrogen.
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Since the electrowon cobalt must be of acceptable
purity and, if lead anodes are used the lead content of
the cobalt deposit is often unacceptably high, cobalt
- silicon manganese alloy anodes are usually used. These
; - 5 anodes are expensive and are liable to breakage because
of their brittle nature~ The cost of these alloy anodes
is a significant proportion o~ ths total cost of
conventional cobalt electrowinning processes,
The relative positions of cobalt and hydrogen in
the electrochemical series can result in simultaneous
discharge of these elements. This is significant ~nter
1 alia because it can have a deleterious e~fect on the
¦ quality and physical properties o~ the electrodeposited
;¦ metal. In common with other electrowlnning processes,
~ 15 when cobalt is electrowon it is desirable to achieve aj high current efficiency as well as to produce a cobaltdeposit with acceptable physical and chemical properties.
Unfortunately the conditions required in the known
~ processes described above for depositing cobalt at high
-'1
current efficiency are not conducive to producing a
cobalt deposit which has acceptable physlcal properties.
Thus, when operating a conventional cell at a pH above
~¦ 2.5 and at about 60C. the internal stress within the
¦ deposit is likely ~o be high and the deposit itself isoften brittle. Ths internal stress is in fact sometimes
so high that the deposlt tends to peel away from the
"blank~' cathode and this can cause short circuiting
1 between the cathode and the a~ode. In order to produce;¦ on a planar cathode a deposit which has acceptable
physical properties the electrolysis should be carried
out at pH le~els ~cwer than pH 2.0 and at about 60C~
In conventional cobalt electrowinnlng practice the
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concentration of acid in the cell electrolyte is a~
high as 15 gpl sulphuric acid and the current e~iciency
may be only about 80%. Furthermore as the acidity of
' J' the electrolyte increases, so the current ef~iciency
decreases due to hydrogen evolution at the ~athode, and,
~ therefore, in conventional practice the acidity of the
.: electrolyte is usually limited to be withln the range of
~i from 8 to 15 gp} sulphuric acid. Thisg in turn, limits
`l the amount of cobalt that can be deposited per unit volume
. 10 of cobalt catholyte~ Thus the cobalt depletion per unit .
¦ volume o~ catholyte is only about 7 gpl from a solutlon
containing about 40 gp} of cobalt.
j The maxlmum practical cathodic current densi~y for
¦. electrowlnning cobalt is limited by the rate of dl~usion
¦ 15 of cobalt ions from the bulk of the electrolyte to the
¦ ; cathode surface, and in practice w~:th the known processes
described above this means that the current density is
limited to approximately 400 A/m , and it ~s sometimes as
¦ low as 300 A/m2
1 20 Many of the disad~antages of the above-descrlbed
! cobalt electrowinning processes are also to be found in
.. . . . .
processes for the electrowinning of such metals as nickel
~ and zinc. ~ ~
I ~ccording to the f~rst aspect of the present ;
: ! 25 invention there is pro~ided an electrolytic process, for
~¦ the electrodeposition of a metal from an aqueous solution
~ of a sa}t of said metal~ which process comprises the steps ~ :
:~ of disposlng between the anode and sathode of an electro-
~z ~o s
. 7f;J chemical cell a separator whlch inc~P~or~c~ an anion
:' ~:
.~ 30 exchange membrane which is substantially impermeable to
¦ cations, so as to form separate anode and cathode
. compartments within said cell, establishing within said
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~055883 :
cathode compartment a mobile bed particulate cathode, flowing
said aqueous solution into said cathode compartment, imposing
a potential difference across the anode and cathode o~ said
cell sufficient to electrodeposit metal from said aqueous
solution of a salt of said metal on to the particulate cathode,
and allowing passage of anions through said anion exchange
membrane.
According to a second aspect of the present invention
there is provided an electrochemical cell, suitable for use -
10 in the electrodeposition of metal from an aqueous solution of ;
`~ a salt of said metal, wherein the cell is provided with a
separator which is disposed between the cathode and anode of
the electrochemical cell so as to form separate anode and
;~ cathode compartments within said cell and which incorporates
an anion exchange membrane, and wherein the cathode compart-
ment contains a mobile bed particulate cathode.
The anion exchange membrane may consist of any
, material which is anion permeable but which is cation imper-
meable. Membranes that have been used in the process and
elect~ochemical cell of the invention and have been found to
give satisfactory results are an anion exchange membr~ne
manufactured by Asahi Glass Co. Ltd. in Japan and identified
by them as AMV anion exchange membrane and anion exchange
membranes manufactured by Ionac Chemical Co. and identified
: ~ ,
by them as MA 3148 and MA 3475 anion exchange membranes.
The mobile bed particulate cathode can be, for
~, example, of the type disclosed in British Patent Specifica-
tion No. 1,194,181 but preferably is of the type disclosed
' ",I : .
in Belgian Patent Specification No. 818,453, corresponding
to German application number 2,437,273 published on
., ,
February 20, 1975.
The present invention has particular application to
,~ ' .
~()5~83
the electrowinning of cobalt from electrolytes contalning
sulphate ions or chloride ions or mixtures of thPse two
spec~es of ions, and will be further described with
re~erence to the electrowinning of cobalt from a cobalt
: -i
s~lphate electrolyte. For this purpose the cathode and
anode compartments of an electrochemical cell according to
the invention were separated by an anion exchange membrane
and separate catholyte and anolyte electrolytes were passed
through the respective cathode and anode compartments.
During electrolysis, co~alt was deposited at the cathoda
which was a particulate electrode of the type disclosed in,
,
l for example, Belgian Patent Speclfication No. 818,453
¦ and the sulphate ions migrated across the anion exchange
¦ membrane to the anode compartment. Since the ma~or portion
¦ 15 of the sulphate ions generated ln the cathode compartment
as the cobalt was deposited, were removed from the cathode
compartment through the anion-exchange me~brane, very much
less acid was retained within the cathode compartment, and
..
it was therefore possible to electrowin more cobalt mçtal ~;
per unit volume of catholyte, while maintaining a high
curren~ efficiency, than could be achieved in conventional
co~alt electrowinning cells.
;~ At any given cobalt concentration in the catholyte
; and at any given cathodic current density, the pH in the
cathode compartment has been found to be dependent on the
concentration of hydrogen ions in the anode compartment.
I By selecting a certain hydrogen ion concentration In the
anolyte and by maintaining this concentration by bleeding
acid from the anolyte and replaclng this acid with water,
; 30 it is possible to operate the cathodic reaction at a
preferred pH range. As cobalt is deposited from the
catholyte its concentration in`the catholyte falls and in
¦ some cases an acceptable pH range for this deposition can
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j ~ be maintained by keeping the acld ConCentratiQn in the
anolyte at a predetermlned ~ixed concentration.
The fact that the anolyte is separated from the
catholyte means that alternative anode materials can be
used, ra~her than the expensive Co-Si-Mn alloy anodes~
A dimensionally stable anode, such ias one consisting of a
~j t~tanium sheet coated with platinum, is preferably used,
,
~' especlally when high current densities are used.
~i I When cobalt is deposited onto the largely spher~cal
. . .,, - .
-l 10 - particles of a particulate cathode the adverse effect of
~ ! higher stressed deposits or brittle deposits is not so:.
marked. This means that when a particulate electrode is
used for the cathode a wider pH range in the catholyte can
; be tolerated. For cobalt, the catholyte pH will generally
; 'I ,
vary from 1.2 to 2.50. An additional advantage of uslng a
; particulate cathode is that a large cathodic surface area
is contained in a small cell volume and therefore high
1 current densities with respect to active membrane area or
¦ with respect to cell volume can be used~ Thus, it is
possible to work at current densities of up to 10~000 A~m2
with respect to the active membrane area, although
I generally the optimum current density will lie ~n the
;~ range 1500 to 5000 A/m . ~inally the size and shape of. ~ . .
the product from a particulate cathode requires no
~- 25 ~urther size reduction since it is already of a suitable
.:
- size for marketing.
` For a better understanding of the invention and to
show more clearly how the same may be carried into
~,1
-~ effect~ reference will now be made, by way of example, to
~¦ 30 the accompanying drawings, in which:
igure 1 shows a vertical section o~ an electro-
chemical cell according to thè invention.
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Figure 1 shows an electrical cell having a cathode
compartment 30 which abuts an anode compartment 32 between
which is disposed a separator 33 which incorporates an AMV
anlon exchange membrane made by Asahi Glass Company.
,- 5 Within the cathode compartment is a current feeder'34 and a
' - bed 36 o~ copper particles constituting a particula e
cathode which was of the type disclosed in Belgian Patent
' Specification No. 818,453. Within the anode compartment
', is an anode 38 formed from a sheet of tltanium part of which
' 10 is coated with platinum 39. An anolyte inlet 40, an anolyte
I outlet 42, a catholyte inlet 44 and a catholyte ~utlet 46
i are also provided. The anolyte composition is con~rolled
by an anolyte bleed 48 and a water input 50. Power is
~ supplied by a D.C. rectifier 52.
,, 15 In operation, a stream o catholyte $s passed through
,~ , the cathode compartment of the cell and an anolyte of
1, different composition is passed through the anode compartment.'~ A potential difference is applied across the electrodes of
,¦ the cell and this allows electrodeposition o~ metal onto
,,~ 20 the cathode and passage of anions through the anion~permeable
;l ' membrane.
~ he electrolytic process of the invention is
`i1, , , ' illustrated by t,he following Examples~
l Example I
,,`~ 25 Four runs using the cell shown in Figure 1 were
, carried out a~ a current of about 30 amps (equivalent
, to a current density with resp,ect to the active area of
the membrane of about 3000 A/m2)~ 15 litres o~ catholyte
~' was made up from technical grade CoS04.7H20 crystals and
the solution was acidified with H2S04. 8 litres o~
~' anolyte ~100 g/l o H2S04) were prepared.
,,~ ~'~" The two solutions were passed through the respective ' .,
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~OS5883
co~partmen~ of the cell while a potential difference was
imposed across the elec~rodes. Electrodeposition of cobalt ~
onto the copper particles of the cathode took place the ~ ;
particles being in the size range o~ from 4~0 to 1400 ~m.
The progress of the four runs i5 sho~m in Tables I~
IV below.
Table I
Run l
... Time ~emp Current Cell pH Co Concn.
(Mins) (C) (a)Volts (g/l)
. 0 21 30 ~.5 2.9 30.0
:, 30 28 30 5.8 2.4 29.5
:~ 60 40 30 5.8 2~2 30.0 .
;`' g7 43 30 5.7 2.0 30.0
,, ! 127 45 30 505 1.9 28.6 : ~:
lSg 47 30 5.3 1.8 28.5
i~ 18~ 49 30 5~3 1.8 26.8
.. 218 50 30 5.2 1~8 26.0
:~, 24~3 52 30 5.2 1~8 25.5 .
: 278 52l 30 5.2 1.7 24.8
308 52 30 5.2 1.7 24.8
340 52 30 504 1.7 2
368 52 3~ _ 5.4 i.8 2~.5
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; Table II
Run 2
_ _ _ ~ _ _ ,
Time Tem~ Current Cell pH Co Concn.
! (M~ns) (oc) tA) Volt~ ~g/l)
' .___ _ . . _ __ _. ___ '
0 ~8 30 5.0 ;2.8 ~0~
. 30 50 30 S.0 ,2.4 38.9
.; 60 51 30 4.9 2.5 ~800
O 51 30 5.1 2.1 37.g
135 S1 30 . 5.0 200 36.9
:` 180 52 30 5.0 2.0 35.0
210 52 30 5.0 1.7 35.1
240 52 30 5.2 1.8 35.~
300 52 30 5.2 1.7 34s8 ,
ll 330 52 30 5.1 1.7 35.0
.1 . 360 52 30 5.0 1.8 35.1
. 390 52 30 5.0 108 33.8
'~ ' _ . . __~,
Table III
, Run 3
. . . _ ~ , _,_ _ _ _ .
.. , Time Temp Curr~nt Cell pH Co Concn.
~Mlns) (C) SA) Volts Sg/l)
., _ _ _ ~ . _......... _
0 60 30 4.6 6.5 39.6
1 45 65 30 4.7 2.6 38.4 ~ '
.l 90 62 30 4.7 1.9 38.4 .
:. . 135 63 30 . 4.6 107 37~7
. 192 64 30 ~.6 1~7 36.4
. 237 65 30 4;6 1.7 35.3
. 282 65 30 4~6 1.7 34.7
::l 348 65 30 4~6 1.7 :~4.0
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! Table IV
Run 4
-- - - 1 ----~ ~
i Time Temp Current Cell pH Co Concn.
~ tMins) (C) ~A~ Volts tg~l~
;; ~ . , ____ __
O 65 30 405 400 4101
, 45 65 ~0 4.5 2~6 40.0
1 90 65 30 4~5 2.3 3~.8 .
-l 135 65 3~ 4.4 2.1 38.9
~80 65 30 ~.3 200 37.9 ~:
~ 10 225 64 30 4~5 1.9 37.0
:-l 273 _ j 6~ 30 1 4.5 1.8 36.'l
'l
~t . ExamPle II
1 A similar experiment ~o that described ln Example I
Il was undertaken but at a higher catholyte pH, viz pH 3.
1 15 In this case, the composit~on of the anolyte was
i maintained at 30 g/l of H2S04. Over a period of 12 hours
. the concentratlon of cobalt in the catholyte dropped from : : .
; 50 to 35 g~l, indicating an overall current efficiency
; of about 80%. Up to 80% o~ the acid theoretically
produced in the cathode compartment may be recovered from
l the anolyte.
:l . . Example III
.j Four runs using the cell shown ln Figure 1 were
~.~ carried out at a current density with respect to the
: 25 actlve membrane area of about 4000 A/m2~ hnolytes and
. catholytes of different initial acid and cobalt
concentration, respectively, were prepared and were passed
. respectively through the anode and cathode compartments.
The conditions obtaining during the process and the
~ 30 results obtained are summarised in Table V below.
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The application o~ the invention to the electrowinning
of nickel is illustrated in the following Example IV.
Again with an imposed current density of about 3000
, 5 ~m2~ a catholyte of nickel sulphate was tr~ated over a
period o~ six days during which t~me the concentration of
nickel therein fell from 80 g/l to 30 g/10 Anolyte
composltion was maintained at 30 g/l of H2S04 result~ng ~n
a catholyte pH of about 3. Current efficiency varied
between 100% at the beginning of the run to about 80% at
the end. The cell voltage was rather variable
t7.5V ~ 0.6V).
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