Note: Descriptions are shown in the official language in which they were submitted.
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PC-1155
The present invention relates to an improved
process for electrowinning nickel.
In electrowinning nickel from aqueous solutions
insoluble anodes are needed that exhibit relatively low
overvoltages and do not introduce into the electrolyte
any impurities which might contaminate the electrowon
product. While precious metal anodes are excellent for
this purpose, their cost militates against their use com-
- mercially, and as an alternative valve metal substrates
coated with precious metal, for example platinum-coated
titanium, have been suggested. However in order for the
precious metal doping to be effective in preventing the
passivation of the valve metal a relatively thick and hence
costly coating is needed; moreover the coating is prone
to slow dissolution which limits the anode life.
An anode consisting of a titanium substrate,
coated with a thin layer of platinum on which i9 super-
imposed a lead dioxide layer is described in U.S. Patent
No. 3,616,302. However that patent criticizes the rather
high overvoltage of such an electrode and advocates the
use of a platinized titanium substrate coated with manganese
; dioxide. An anode coated with manganese dioxide is pre- -
ferable to a lead dioxide-coated anode for conventional
nickel electrowinning practice because of the tendency of the
latter to contaminate the electrolyte, and hence the electro-
won nickel,with lead. Nevertheless the useful life of
; manganese dioxide of the type specified in the above-mentioned
patent is limited.
More recently it has been proposed in the copending
application, Serial No. 212,965 of 4th November, 1974,
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assigned in common with the present application, to use
anodes which comprise a valve metal substrate coated with
a precious metal on which are superimposed in ~urn a
layer of lea~ dioxide and further layer of manganese dioxide.
7Jse of such a "double-dioxide" coated electrode as an anode
in conventional nickel electrowinning processes represents
a significant advance in that excellent characteristics
of the anode are achieved, while the outer coating of
manganese dioxide prevents the contamination of the elec-
trolyte with lead from the lead dioxide layer. The main
drawback of such a process is the need for an elaborate
multistage procedure for preparing the anodes. It has
now been discovered that all the advantages of a nickel
electrowinning process using the "double dioxide" coated
anodes can be realized in a modified process which does
not involve the incovenience of preparing a "double-dioxide'
coated anode.
It is thus an object of the invention to provide
a process of electrowinning nickel wherein an anode having
an exposed surface of lead-dioxide can be used without sub-
stantial contamination of the electrolyte by lead from the
anode surface.
It is a further object of the invention to provide
a nickel electrowinning process wherein contamination of
the electrowon nickel by small amounts of lead present in
the electrolyte is minimized.
The above objects are realized in a process for
electrowinning nickel from an electrolyte in a cell having
an insoluble anode and an insoluble cathode wherein the
improvement comprises:
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i) using as the insoluhle anode a substrate
formed of titanium, zirconium, tantalum or
an alloy thereof, the substrate having a
- flash-coating of a platinum-group metal or
an alloy of platinum-group metals and the
flash-coating having a lead dioxid^ coating
thereon;
ii) using as the electrolyte an essentially chloride-
free aqueous solution which contains, in
1~ addition to nickel, a dissolved manganese
compound in an amount corresponding to at
least about 0.1 grams per liter of manganese,and
;; iii) passing between the anode and cathode an elec-
trical current corresponding to at least about
; 300 amperes per square meter of anode surface
, area;
whereby nickel is deposited on the cathode while manganese
dioxide is formed at the anode.
In the process of the invention, the presence of
manganese in the electrolyte serves a dual purpose. Firstly
it leads to some deposition of manganese dioxide onto the
lead dioxide surface of the anode, and such deposition
inhibits dissolution of lead from the anode. ~owever an
additional benefit results from the fact that the process is
~` conducted in such a way that some manganese dioxide formed
at the anode passes into the bulk of the electrolyte and
its presence therein serves to impede the contamination of
the nickel product by lead which may have been present in
a small amount in the electrolyte fed into the cell.
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The electrolyte from which nickel is to be
recovered will generally be of the type commonly referred
to as "all sulfate" electrolytes, in that substantially
all of the nickel in solution is present as nickel sulfate.
While the electrolyte may also contain small amounts of
such anions as acetate and sulfamate, it must be essentially
free of chloride ions. This is ~ecause the presence of
chloride ions results in chlorine evolution at the anode
which would increase the tendency for lead dissolution at
the anode.
The manganese ions which must be present therein
to practise the process of the invention will conveniently
be present in the form of manganese sulfate. This is
preferred from the viewpoint of avoiding unnecessary foreign
anions in the electrolyte; however it i9 by no means essen-
tial that the manganese be added in the form of manganese
sulfate and various other manganese salts, the anions of
which are known not to affect the electrowinning adversely,
may be resorted to. Such salts include manganese acetate,
propionate, butyrate, citrate, succinate, sulfamate and
perchlorate. Whatever salt is relied on it is essential
to ensure that the electrolyte contains at least n. 1 qrams
per liter (g/l) of manganese in solution in order to afford
the necessary protection against lead contamination of both
the electrolyte itself and the nickel product electrowon
therefrom. Providing the necessary minimum is exceeded,
' the man~anese ion concentration has relatively little
influence on manganese dioxide formation, which is es-
sentially dictated by the conditions of electrowinning. In
view of this it will be appreciated that there is little
or nothing to be gained by adding large amounts of the
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manganese salt to the electrolyte. Moreover we have
found that very high manganese contents may lead to man-
ganese contamination of the electrowon nickel. It is
therefore undesirable to have a manganese content in the
electrolyte in excess of about 5 g/l, and preferably the
electrolyte contains from 0.5 to 2 g/l of dissolved
manganese.
The anode used in the present process is of a
type well known per se and methods of producing it are
described in more detail in the art. The anode may be of
any one of various forms, such as a sheet or a rod, and
will be selected in accordance with the overall cell
geometry. The anode is produced by selecting the appro-
priate form of substrate, which may be foraminous, e.g.,
by use of a mesh, and applying the necessary coatings
thereto. The substrate, of titanium, zirconium, or tan-
talum or an alloy of one of these valve metals, is first
coated with a platinum-group metal. The latter may be
platinum, palladium, ruthenium, or rhodium, iridium or an
alloy thereof and its application to the substrate can be
achieved by a rapid electrodeposition. For economy
purposes, as thin a layer of precious metal is deposited
as is capable of effectively protecting the valve metal
substrate from passivation. For this purpose, a flash
coating of 0.01 to 0.2 micron thickness is adequate. An
example of an electrolytic process for producing the flash
coating involves using a bath of sulfato-dinitro-platinous
acid, H2Pt(NO2)2 SO4, dissolved in sulfuric acid, the bath
containing 5 g/l of platinum and having a pH up to 2.
Using a current density of the order of 50 amperes per
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square meter (amp/m2), the valve metal substrate to be
coated is used as cathode in the bath, and a suitable
platinum coating can be deposited thereon in two to three
minutes.
Following application of the platinum-group metal
coating thereto, the valve metal is washed and a lead dioxide
layer which may range in thickness from about 50 to about
2000 microns is applied, for example, electrolytically
using a lead nitrate bath. A bath containing between about
100 and 300 g/l of lead nitrate and between about 30 and
300 g/l of nitric acid is suitable for this purpose.
As mentioned above, the amount of manganese
dioxide formed at the anode during the electrowinning and
the proportion thereof which deposits onto the anode are
influenced by the conditions of the electrowinning and
most significantly by the current density employed. For
this reason, the anodic current density used must not be
less than about 300 amp/m2, and preferably it is about
500-1000 amp/m2. Under such electrowinning conditions, it
has been found that the majority of the manganese dioxide
which forms passes into the electrolyte where it
inhibits lead deposition onto the cathode. As will
be readily appreciated, the cathodic current density is not
dictated by the value of the current density at the anode,
and it can be chosen to be higher or lower as desired by
suitable choice of the relative surface areas of the anode
and cathode.
The invention and its advantages will be more
readily appreciated from the following more detailed
description of examples thereof.
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EXAMPLR 1
A pair of identical anodes were prepared using
titanium blanks, each blank being in the form of a 15 cm
long rod of 0.45 cm diameter. Each blank was sandblasted,
cleaned and degreased, and given a flash coating of
platinum, 0.05 micron thick, by electrolysis for 2 to
3 minutes in a bath consistinq of 5 q/l of platinum as
sulfato-dinitro-platinous acid in a sulfuric acid solution
at pH up to 2 and at a temperature of 25 to 70C using
a current density of 50 amp/m2. The platinized blanks
were then coated with a 300 micron lead dioxide layer by
electrodeposition for 24 hours in a bath consisting of
300 g/l of lead nitrate, 100 ml/l of nitric acid, as well
as 10 ml/l of a commercial wetting agent comprising
polypropylene - glycol methyl ether, at a temperature of
65C, using a current density of 50 amp/m .
These anodes were then tested by using them
respectively in a pair of diaphragm-free nickel electro-
winning cells run in parallel to one another. The cathodes
used in both cells were of sandblasted titanium. The first
cell (A), in accordance with the invention,was fed with
a manganese-containing electrolyte. The electrol~te con-
sisted of a nickel sulfate solution (70 g/l of nickel)
with 75 g/l of sodium sulfate, 10 g/l of boric acid and
1 g/l of manganese as manganese sulfate. Prior to its
use, it was purified of lead by trèating it with 1 g/l of
barium carbonate and filtering off the precipitated barium
sulfate. The other cell (B) was fed with an electrolyte
which was not in accordance with the invention in that it
contained no manganese, but was otherwise of the same
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composition as the first electrolyte and was also treated
for lead removal.
Electrowinning of nickel was carried out in these
parallel-connected cells A and B for 40 hours, at a pH
of about 2.5 ! using an anodic current density of 1000
amp/m2 and a cathodic current density of 500 amp/m2, the
electrolyte being maintained at 85C. Cathode assays
showed that the average lead level in the nickel from the
B cell was 9 parts per million (ppm), while the ~ cell
produced nickel with only 4 ppm of lead.
EXAMPLE 2
A pair of anodes of platinized titanium coated
with lead dioxide were prepared in an identical manner to
that described in Example 1. A pair of cells C and D were
loaded with electrolytes of the same composition as those
used in cells A and B respectively.
~lectrowinnin~ was carried out with the cells in
parallel in the manner described in Example 1, except that
various equal anodic and cathodic current densities,
between 300 and 1400 amp/m2 were used. After 1000 hours
of electrolysis, the current flow was discontinued, but
the electrodes were left on open circuit in the respective
electrolyte for 7 hours. Electrowinning was then resumed
at 300 amp/m for 40 hours and the nickel deposited
during the resumed electrowinning was analyzed for lead.
It was found that nickel containing over 500 ppm of lead
was deposited in cell D where the electrolyte contained
no manganese ions, whereas nickel electrodeposited in cell C
from the manganese-containing electrolyte had only 12 ppm
of lead. This result indicates that the coating formed in
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situ on the lead dioxide surface effectively seals off
the lead dioxide from the electrolyte when the cell is
on open circuit as well as during electrowinning.
EXAMPLE 3
To illustrate the advantage of the process of
the invention in coping with lead-contaminated electrolytes,
a pair of cells (E and F) were supplied with electrolytes
which were identical respectively to the manganese-contain-
ing and manganese-free electrolytes 1lsed in either of the
preceding examples except that each electrolyte was
deliberately contaminated with l mg/l of lead.
17sing anodic and cathodic current densities of
300 amp/m2, the cells were operated in parallel and the
electrodeposited nickel in cell F was found to contain
46 ppm of lead, whereas cell E produced nickel with only
4 ppm of lead. It is clear therefore that the presence of
a small amount of dissolved manganese in the electrolyte
serves not only to seal-off the lead dioxide anode surface
from the electrolyte, but also inhibits lead contamination
of the electrodeposit when the electrolyte is itself
slightly contaminated with lead.
While the present invention has been described
with reference to preferred embodiments thereof, various
modifications may be made to these embodiments without
departing from the scope of the invention which is defined
by the appended claims.
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