Note: Descriptions are shown in the official language in which they were submitted.
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PURIFICATION OF NICKEL ELECTROLYTE
TECHNICAL FIELD
The present invention relates to an electrolytic system and an
electrochemical process for removing impurities îrom nickel-containing solu-
tions. More particularly, it relates to an improved electrochemical process for
removing contaminants such as cobalt, iron, arsenie and lead from a nickel
refinery electrolyte.
BACKGROUND OF THE IMVENTION
In the electrorefining of nickel, crude nickel flnodes are corroded
electrolytically, resulting in the dissoluffon of nickel and some OI the impurities
in the electrolyte. The electrolyte is purified and then nickel is deposited
cathodic&lly from the purified electrolyte. In nickeI refinery practice, the
electrolyte is usually a slightly acidic aqueous solution containing nickel, sulfate,
sodium and chloride ions, and boric acid, plus impurities. The impurities vary but
may include one or more of the metal values cobalt, copper, iron, arsenic, and
lead. To prevent or lower deposition of such impurities with nickel at the
c&thode, it is necessary to remove them ~rom the electrolyte or to reduce lthem
to an acceptable lcvel. The extent of removal will affect the purity oî the
nickel deposited, and the degree of purity desired will depend on such factors as
ultimate use of the nickel, cost of purification, diîficulty of removal OI the
particular contaminant, and the vfllue of the impurity. For example, cobQlt is
not considered a harmful contamillant in nickel for many uses of nickel because
of the similarity of properties of nickel and cobalt. With the increased value of
cobalt, it is now desir&ble to separate and recover the cobalt.
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To remove contaminants from the electrolyte, th0 imp~lre electrolyte
is usually pumped from the anode dissolution cell and the impurities are removedby chemical, physical or electrical techniques or fl combination thereof. Copper,
for example, can be removed from solution to very low levels by cementation
with metallic nickel or by precipitation with H2S. Iron is removed by aeration
and hydrolysis. Cobalt, arsenic and lead are removed by addition OI chlorine gas.
Nickel carbonate is used to neutralize the acidifying effect that occurs as cob~lt
and other impurities are preeipitated fronn solution. The purified electrolyte is
then returned to the cell for plating of niclcel at the cathode.
An electrolyffc process for removing cobalt, iron, arsenic and/or lead
contaminants from impure, decopperized nickel refinery electrolytes is disclosedin U.~. Patent 3,983,018. In such process nickel hydroxide and hydrogen form at
the cathode of an electrolytic cell, and elemental chlorine, which is the agent to
remove cobalt, iron, arsenic and lead impurities, is generated in-situ at the
anode. Cobalt, iron, arsenic and lefld precipitates begin to form in the electr~lytic tank in time-dependent reactions, and precipitation is generally completedin a retention tank following treatment of the electrolyte in ths electrolytic
tank. The precipitates are removed from the electrolyte by a conventional
technique, e.g. filtration, and the purified electrolyte can be used for deposit of
high purity nickel. The oxidative electrolytic process disclosed in the afore-
mentioned patent is an improvement over the art in that iron, lead, arsenic and
cobalt can be removed concurrently and effectively, without the need for
complex stages required by chemical methods for the removal of such contam-
inants. Furthermore, the in-situ generation of C12 permits easy handling and
control of a difficult reagent. No chlorine gas handling equipment is required,
and the cost of in-situ generated chlorine is lower than the cost of liquid chlorine
currently used for chemical purification. With the proper controls, i.e. adjust-ment of anodic current density to generate only the chlorine required to oxidizethe undesired impurities, pollution due to excessive chlorine can be prevented
and reagent economy can be insured. In addition, no chloride ion imbalance in
the electrolyte i~ created because the in-situ generated chlorine will be
completely reconverted to chloride ion. The present process, which also involvesthe advRntageous in-situ generaffon of chlorine, offers a further improvement
over the aforementioned electrolytic purification process in that the precipita-tion CRn be achieved with lower energy requirements and with more efficient
nickel recovery. 'rhe cathode can be operated at low current density, resulting
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3 PC~ 0
in low cell voltage and low power consumption. Also, the cells can be operated
with essentiallg no nickel platinF on the cathodes, re~ultin~ in efficient H2
evolution and NaOH nroduction in the cathocle compartments.
BRIEF l)ESCRIPTION OF DRAWINaS
Fi,~ure 1 i8 a schematic version o~ a diaphragm electrolytic cell in
accordance with the present inventinn. The cell ia ~hown in lsometric ~A) nnd
front (B) views.
Fi~ure 2 i8 a simplified flowsheet illustrating a nickel refinerv
purification process in accordance with the present invention.
ln T~IE INVRNTION
In an electrolytic process for purifvin~ a nickel-containinF ~olution bv
removal of at leaYt one impuritv selected from the group con~istlng of cobalt,
iron, araenic (if iron i~ present3, and lead, which proce~s comprises forming anelectrolvtic s~stem comprised of at least one electrolytic cell containinR~ at least
one anode, at least one cathode, and an aqueous acidic electrolyte, said electro-
lvte beinF substantially copper-free and containinF in solution nickel, alkali
metal, and chloride ions and at least one of sflid impurities, the impro~ement
comprisin~: establishlng in the electrolytic cell an anode compartment and
cathode compartment aeparated by a chlorine-resistant diaphragm through which
electrolvtic contact is established between the anode and cathode compartment~;
flowing the electrolvte through the anode compartment; maintainin~ a
non-cir~!ulatinF catholvte in the cathode compartment, ~aid catholvte being an
aqueous alkaline solution containing su~ffcient h~droxyl ions such that under
operatin~ conditions nickel ions are prevented from migrating to the cathode
compartment and substantiallv onlv the decomposition of water and evolution of
hvdro~en can occur in the cathode compartment; impressin~ an electrical current
in the cell to generate chlorine at the anode; said chlorine in the aqueous elec-
trolvte reacting with at least one impurity present in the electrolyte to provide a
precipitflte containin~ the impuritv; and separatin~ the precipitate from the
electrolvte. The process can be used to recover cobalt with reduced power
c- naumption from a nickel refinery electrolyte.
Operation of' the proces~ depends on the in-~itu generation of chlorine,
which is derived from chloride in the electrolyte. The chloride ion may be
present in the re~inery electrolyte a~ a result of previous process steps or it
mav be added, e.g. a~ an alkali metal chloride or nickel chloride. A typical
decopperized nickel refinery electrolyte may contain the followin~ components in~rams per liter (~pl~.
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Ni++ = 40-80 Co++ = 0.05-5
Na~ = 10-50 As~ = 0.001-2 (or AS~++)
Cl- = 15-90 Pb++ = 0.0001-0.01
SO4= = 2-150 Cu++ = 0.~01-0.01
H3BO3 = 5-20 Fe++ = 0.01-1
Typically, the pH of the impure electrolyte! will vary from about 2.5 to about 5.2.
The composition of the electrolyte will, of course, vary with the ore and the
various processes for treating said ores. The electrolyte is the anolyte
component of the electrolytie cell, thus the components of the electrolyte, apart
from the chloride ion and nickel refining electrolyte are selected to satisfy the
electrical as well as chemical requirements of the system. The desired
concentration of the essential components in the electrolyte is dependent on thefunction of each. The pH of the impure electrolyte feed should be about 3.8 to
5.2 and can be adjusted, if required, with nickel carbonate or another suitable
base. The nickel ion concentration should not exceed its maximum solubility in
the electrolyte. The alkali metal level should be sufficient to ensure conduc-
tivity of the electrolyte. The chloride ion must be present, and it should be
present in sufficient quantity so that C12 rather than 2 is generated at the
anode. The pararneters of the system can be designed so that the amount of
chlorine generated is substantially not in excess over that amount reguired for
the oxidation. Typically, at least S gpl of chloride and preferably greater than 15
or 20 gpl of chloride ions should be present in the electrolyte. The chloride ions
may be provided by an aL"ali metal chloride such as sodium or potassium
chloride. Sodium chloride is preferred because it is less expensive. The other
metal values present may also be present as chloride salts. The use of other
halides other than chloride are within the contemplation of the invention;
however, they are not practical. Therefore, the description of the invention is
given herein in terms of the generation in-situ of chlorine from chloride ions in
the electrolyte.
The impure nickel-containing solution (e.g. nickel refining
electrolyte) fed to the anode compartment constitutes the anolyte. It is
characterlstic of tlle cell of this invention that the catholyte can be maintained
essentially separate from the anolyte and in essentially a steady state. In
operation of the c~ell, hydrogen ions and alkali metal ions migrate from the
anolyte into the caltholyte and are available to react with hydroxyl ions to form,
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respectively, water and alkali metal hydroxide. The water decomposes
cathodically and E12 is discharged. The ~lydroxyl ions remaining from the
decomposition are available for the migrating hydrogen and alkali metal ions.
At the cathode, as indicated above, 3I2 is discharged. The catholyte
is maintained separate from the anolyle and as an aqueous ~lkaline solution
advantageously containing at least 0.5 moles of hydroxyl ions. For example, the
sodium hydroxide level is maintained at about 20 or 4~ to 200 gpl, e.g. about lS0
gpl. Under the alkaline conditions in the cathode compartment, essentially no
nickel migrates into the cathode compartment. To prevent a build-up of alkali
hydroxide in the catholyte, a hydrostatic head is maintained at the catholyte,
with water flow into the catholyte provided at a pressure designed to keep a
positive flow through the diaphragm from the catholyte into the anolyte. In thisway, the catholyte can be maintained essentially in a steady state with H2
discharge at the cathode, and essentially no nickel plating at the cathode and
essentially no precipitate of nickel in the alkaline catholyte.
The electrodes must be made of materials which are good conductors
and resistant to their respective environments. They may take any appropriate
form for the ce~ design, e.g., they can be sheets, rods, tubes, metal mesh,
expanded metal, etc. The substrate materials for the electrodes can be surface
materials on more conductive cores. The anodes are insoluble and any anodes
appropriate for Cla production are useful. For example, graphite anodes or
dimensionally stable anodes may be used. It is well known, for example, to use
platinum group metal anodes or platinum group metals and/or platinum group
metal oxide-coated valve metals. The cathodes must be of an alkali-resistant
material, e.g., steel, stainless steel, or nickel. The spacing of the anodes andcathodes is not considered critical, however they should be placed such that
there is no short circuiting between the electrode and the diaphragm.
The diaphragm separating the electrolyte and catholyte must be of a
material which is resistant to the reactants and products on both sides of the cell
and which has low electrical resistance. Since chloride is present and chlorine is
generated in the cell, it is particularly important that the diaphragm be resistant
to chloride and to chlorine, and this property is referred to herein as chlorine-
resistant material. Additional features of the diaphragm are permeability and
physical strength. In general, diaphragms of the type used in cells for chlorinemanu~acture would be suitable. Examples of suitable materials are asbestos and
a fluorinated Ihydrocarbon such as polytetrafluoroethylene ~modified for
~l97~
-6- PC-l l 90
wettability). An otherwise suitable material may be made chlorine-resistant by
coating it with a suitable polymeric material. Suitable materials are
commercially available, e.g., under the names KANEKALON* (a product of
Kanegafuchi Chemical lndustry Co.), and DYNEL* (a product of Union Carbide~.
The cell is operated with an anode current density, typically, OI about
200 Amperes per square meter (ASM), and varied between about l00 to 500 ASM.
The temperature in the cell is maintained bletween 50~ and 70.
The flow of electrolyte through the anolyte compartment is adjusted
so that the desired reaction takes place at a suitable îixed current and in a
desired number of cells. The numbers of cells is dictated in part by the system
designed. A major advantage in the present process is that the electrolytic
purification of a nickel electrolyte can be carried out with a lower power
consumption, e.g. with a decrease of at least 30% over that required for
diaphragmless cells. Another advantage is that the present process can be
operated at a cell voltage below 5 V, e.~. about 3V. In addition to the lower cost
of purification of the electrolyte, recovery of cobalt from the electrolyte can
likewise be effected at a considerable saving in power. As indicated previously,the aforementioned diaphragmless electrode process of U.S. Patent No.
3,983,018 is an improvement over prior art non-electrolytic methods, and the
present process provides a further improvement in the electrolytic system.
The process of the present invention is illustrated by reference to the
accompanying drawings in which Figure 1 is a schematic cell and Figure 2 is a
simplified flowsheet in accordance with the present invention. The flowsheet of
~igure 2 shows electrolytic cell l0 with a chlorine-resistant diaphragm ll
separating the anode compartment 14 irom the cathode compartment 15. The
schematic cell represented in Figure l shows anode 13, which is suitable for C12generation made of, e.g., graphite, and two cathodes 12 made of, e.g., stainlesssteel. Referring to Figure 2:
An impure electrolyte derived, for example, from the dissolution of
crude nickel anodes in an electrolytic process (not shown) and decopperized, e.g.,
by cementation with metallic nickel (not shown) is fed to the anode compartmenb
l4 of electrolytic cell l0. The composition of the electrolyte depen~; on the
composition of the the ore and preceding treatments. However, in the present
process -in addition to nickel and contaminants such as cobalt, arsenic, iron and
~Trademark
~ ~74~9~Ç
q PC-1190
lead - the electrolvt/? will contnin alkali metal ion~ and chlor~de ion~. Pref-
erably, the chloride ion content will be sufffcient to Five high Cl~ e~ficiency
at the anode. Sulfate ions are often pre3ent frnm prevlous proces~ ~tep~ and
boric acid i~ a well-known additive to refinery electrolytes. To illustrate the
invention an impure electrolyte i8 uaed which contain~ in solution, appro~lmately,
60 E~pl nickel, 95 gpl sulfate, 35 gpl ~odium, 16 ~pl boric acld, 35 ~pl chloride
and as impurities n.1 ~pl cobalt, 0.05 gpl iron, 0.005 gpl ar~enic, and 0.002 gpl
lead. The p~ of the impure electrolyte i8 about 2.0 to 5.2. Under an impressed
current elemental chlcrine ~ormed &t anode 13 beFlns to react in the anode
lQ compartment 14 with divalent cobalt ions, and similarly with the ions of iron,
ar~enic and lead in the electrolvte. Preferabl~, air ig 6par~ed into the anolvte(not shown) to improve contact of cell-g~enerated C12 and of ba~e with impurities
to be oxidized and to redissolve nickel hvdroxide precipitated on the diaphragm.The o7c{dation reactiona converting the contaminants cobalt, ~r~enic,` iron and lead
16 to in~oluble hvdroxides are time-dependent, and the reacting anolvte i8 tranaferred
to a retention tank 16 for the oxidation reaction~ and h~ydroly~i~ to occur. TheoxidizinF power of the anolyte i~ governed by the current density at the anode,
emciencv of the chlorine reaction, tank size and re3idence time for reacti~rity.In the cathode compartment 15, the catholyte, which i~ an aqueouc
~0 medium containin~ aodium h~rdroxide i~ a non-circulating electrolyte. The initial
NaOTI content can be about 80 ~pl and sodium chloride about 0 to 40 gpl. A
hvdrostatic head ia ~aintalned in the cathode compartment by water addition to
eatablish a bleed rate from the catholyte into the anol~te through the diaphragm80 that the ~:odium hydroxide level in the catholyte i8 maintalned at about 8û to
~5 ~00 ~pl.
enerallv, in the retention tank, hydrolysi~ take~ about 20 minute~ to
about 1 hour, typicallv about 4n minutes. To maintain the pll at the de~ired
value, depending on the aelectivitv desired, a small amount of alkaline material,
e.F. NiCO3, i~ added to the electrolvte (i.e., the anolYte fed from the anode
compartment) in retention tank 16. The oxidized electrolyte i8 pumped or ~ravit~r
fed to filter 17 and the cobalt, iron, ar~enic and lead precipitates formed are
removed from the liquid. PuriIied electrolyte may be returned to the reffning
circuit or, alternati~rel~, sent through one or more addiffonal purification
sequence~ ~r further reduction of impurity content, if necessary.
TJnder the influence of applied current chlorine is generated at anode
74gl~
-8- PC-l 1 90
13 and H2 is generated at the cathodes 12. In general, the impurities are oxidized
and precipitated by hydrolysis. The overall simplified reaction with respect to
Co++ in the electrolyte and retention tanks can be represented as îollows:
Ci2 + 2Co~+ + 3NiCO3 + 3H20~ 2Co~OH).~ + 3Ni++ ~ 2Cl- t 3C02
Other reactions, e.g. NiC03 oxidation to Ni+++, are di~ficL~t to prevent. Iron
behaves similar to cobalt and arsenic and lead precipitate out with the iron.
By careful control of the oxidation potential and pH of the
electrolyte9 it is possible to selectively precipitate unwanted elements. In this
way, lor example, a purer cobalt precipitate can be obtained. The selective
process, which can be effected advantageously in a retention tank, involves two
stages. In the first stage, the electrolyte is partially oxidized, adjusted to a pH
slightly below that required for cobalt precipitation, but high enough for
precipitation of iron, arsenic and lead, e.g. a pH of about 3.5 to 4Ø After the
iron, arsenic and lead precipitates are removed by filtration, in a second stagethe electrolyte is fully oxidized in an electrolytic tank and the pH in the
retention tank is increased sufficiently, e.g. between about 4.3 and 4.8, to
precipitate cobalt. Alternatively, or in combinatîon with retention tanks, it ispossible to adjust the number of cells and r~te of anolyte flow so that at a given
current density, the electrolyte (anolyte) will emerge from the electrolytic
system at a desired pH.
In order to give those skil1ed in the art a better understanding of the
invention, the following illustrative example is given.
EXAMPLE
In the tests of this Example, a diaphragm-equipped Clg generating
cell is compared with a diaphragm-free cell used to obtain the data of l~xamples1, 2 and 3 of the aforementioned U.S. Patent No. 3,983,081. In the tests shown in
the patent, the impure electrolyte used is decopperized tank house electrolyte
from a nickel refinery containing about 40-80 gpl nickel, about 33-45 gpl sodium,
about 46-56 gpl chloride, about 100-150 gpl sulfate, 13-16 gpl H3BO3, less than
0.01 gpl of dissolved copper, and having a pH of 2.9. The cell contains a stainless
steel wire cathode 2.4 mm diameter by 6.4 cm long and graphite anodes 1.3 cm x
6.4 cm x 9 cm. The temperature of the electrolytes and retention tanks are
controlled between 54C and 60C. Upon leaving the electrolytic tank, the
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oxidizing power of the solution is equivalent to 0.290 gpl chlorine.
The comperative tests run according to the present process were
essentially the same as described above ex~ept that a KANEKALON diaphragm
separates th0 anolyte (impure electrolyte feed~ from the catholyte. The
S catholyte compartment contains 20 gpl NaOH. Water is pumped into a cathode
bag to maintain a head of l cm.
The accompanying TABLE: shows the general conditions under which
the comparative experiments are carried out. It also gives the analysis of the
impurities in the feed and the effluent.
The comparative results in the TABLE show that for the purific~tion
of similar electrorefining electrolytes, the power consumption required in the
cell of the present invention is only about one third to one half that needed with
a diaphragm-free C12 generating cell.
Although the present invention has been described in conjunction with
preferred embodiments, it is to be understood that modificaffons and variations
may be resorted to without departing from the spirit and scope of the invention,as those skilled in the art will readily understand. Such modifications and
variations are considered to be within the pUrYieW and scope of the in~ention and
appended claims.
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