METHOD AND APPARATUS FOR CONTROLLING THE QUALITY OF ZINC SULFATE ELECTROLYTE

METHODE ET DISPOSITIF DE CONTROLE DE LA QUALITE D'UN ELECTROLYTE AU SULFATE DE ZINC

English Abstract

ABSTRACT OF THE DISCLOSURE
A method and apparatus for controlling a process for the
recovery of zinc from zinc sulfate electrolyte containing concentrations
of impurities and addition agents. The method and apparatus include
a test circuit comprising a test cell, a sample of electrolyte, a
moving cathode, an anode and a reference electrode immersed in said
sample, a constant current source and measuring means electrically
connected to the electrodes. A controlled low current is applied
to the electrodes in the test cell to measure the activation overpotential
between the cathode and the reference electrode. The activation overpotential
is measured at the point of inchoate deposition of zinc and is related
to the concentration of impurities and addition agents in the sample.
The processes for the purification of zinc sulfate electrolyte and
the recovery of zinc from zinc sulfate electrolyte are subsequently
adjusted in relation to the measured value of the activation overpotential
to obtain optimum zinc recovery.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method for controlling a process for the recovery of zinc from
a zinc sulfate electrolyte containing concentrations of impurities, said
method comprising the steps of establishing a test circuit comprising a test
cell, a sample of electrolyte, a moving cathode, made of an electrically
conductive material other than zinc, which moving cathode has an area
exposed to said electrolyte, an anode and a reference electrode, said
electrodes being immersed in said sample, a constant current supply and
measuring means electrically connected to said electrodes; applying a low
current to the electrodes in said test cell, said current being sufficient
to cause inchoate deposition of zinc on said cathode; measuring the
activation overpotential at the point of inchoate deposition, relating the
measured activation overpotential to the concentration of impurities in said
sample; and adjusting the concentration of impurities in the electrolyte
of the process for the recovery of zinc to obtain optimum recovery of zinc,
wherein inchoate deposition is defined as the deposition of zinc which has
just begun and is limited to a partial covering of the moving cathode
surface by zinc.
2. A method for controlling a process for the electrowinning of zinc
from zinc sulfate electrolyte containing concentrations of impurities and at
least one polarizing additive, said method comprising the steps of establishing
an electrolytic test circuit comprising a test cell, a sample of electrolyte,
a moving cathode, made of an electrically conductive material other than
zinc, which moving cathode has an area exposed to said electrolyte, an
anode and a reference electrode, said electrodes being immersed in said
sample, a constant current supply and measuring means electrically connected
to said electrodes; applying a low current to the electrodes in said test
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cell, said current being sufficient to cause inchoate deposition of zinc on
said cathode; measuring the activation overpotential at the point of inchoate
deposition; relating the measured activation overpotential to the
concentration ratio between impurities and polarizing additive in said
sample; and adjusting the concentration ratio in the process electrolyte
to obtain optimum current efficiency and level zinc deposits in the electro-
winning process, wherein inchoate deposition is defined as the deposition of
zinc which has just begun and is limited to a partial covering of the moving
cathode surface by deposited zinc.
3. A method as defined in claim 1 or 2, wherein the value of said
current corresponds to a value of corresponding current density in the range
of 0.01 to 4.0 mA/cm2, based on the exposed area of the moving cathode.
4. A method as defined in claim 1 or 2, wherein-the value of said
current corresponds to a value of corresponding current density in the
range of 0.1 to 0.4 mA/cm2, based on the exposed area of the moving cathode.
5. A method as defined in claim 1 or 2, wherein said low current
has a constant value.
6. A method as defined in claim 1 or 2, wherein the activation
overpotential is measured continuously.
7. A method as defined in claim 1 or 2, wherein the activa-
-27-

tion overpotential is measured intermittently.
8. A method as defined in claim 1 or 2, wherein said sample
of electrolyte is kept in motion.
9. A method as defined in claim 1 or 2, wherein said sample
of electrolyte is kept in motion by continuously passing a flow of
electrolyte through said test cell.
10. A method as defined in claim 1 or 2, wherein the value of
said current causing inchoate deposition of zinc is sufficient to
cause not more than 10 to 30% of the surface area of the cathode
which is exposed to electrolyte to become covered with deposited
zinc.
11. A method as defined in claim 1 or 2, wherein the moving
cathode is advanced at a substantially constant rate.
12. A method as defined in claim 1 or 2, wherein the moving
cathode is continuously advanced.
13. A method as defined in claims 1 or 2, wherein the moving
cathode is advanced in intermittent fashion.
14. A method as defined in claim 1 or 2 wherein the moving
cathode is advanced through the sample of electrolyte at a
substantially constant rate, said substantially constant rate being
at least equal to a minimum rate defined by the following formula:
<IMG>
wherein:
a represents the cathode length to surface area ratios,
- 28 -

cm/cm2;
b represents current density, A/cm2;
c represents cathode exposed area, cm2;
d represents the electrochemical equivalent for zinc
deposition, g/Ah;
x represents fraction of cathode covered with zinc;
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y represents the weight of deposited zinc per unit of cathode
surface area, g/cm2.
15. A method as defined in claim 1 or 2, wherein the moving cathode is
advanced through the sample of electrolyte at a substantially constant
rate, said rate being in the range of 10 to 500 times the minimum rate,
said minimum rate being defined by the following formula:
<IMG>,
wherein:
a represents the cathode length to surface area ratios, cm/cm2;
b represents current density, A/cm2;
c represents cathode exposed area, cm2;
d represents the electrochemical equivalent for zinc deposition,
g/Ah;
x represents fraction of cathode covered with zinc;
y represents the weight of deposited zinc per unit of cathode
surface area, g/cm2.
16. A method as defined in claim 1 or 2, wherein the electrolyte
in the test cell is kept at a substantially constant temperature and
wherein the constant temperature selected is between 20°C and 75°C.
17. A method as defined in claim l or 2, wherein the electrolyte
in the test cell is kept at a substantially constant temperature, and
wherein the constant temperature selected is between 25° and 40°C.
18. A method as defined in claim 1 or 2, wherein the moving cathode
in the test cell is a wire made of a material chosen from aluminum and
aluminum alloys.
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19. A method as defined in claim 1 or 2, wherein the moving cathode
in the test cell is a strip made of a material chosen from aluminum and
aluminum alloys.
20. A method as defined in claim 1 or 2, wherein the electrodes
are positioned in the cell in fixed relation to one another.
21. A method as defined in claim 1 or 2, wherein the measuring of
the activation overpotential is effected by recording values of said
activation overpotential as a function of time.
22. A method as defined in claim 2, wherein the adjusting of the
concentration ratio comprises adjusting the concentration of said at
least one polarizing additive relative to the impurity concentration.
23. A method as defined in claim 2, wherein the adjusting of the
concentration ratio comprises adjusting the impurity concentration.
24. A method as defined in claim 2, wherein the polarizing additive
is animal glue.
25. A method as defined in claim 2, wherein the impurities include
antimony; and wherein the adjusting of the concentration ratio comprises
adjusting the impurity concentration by adjusting the concentration of
antimony.
26. A method as defined in claim 2, wherein the polarizing additive
is animal glue, and in which the adjusting of the concentration ratio
comprises adjusting the concentration of the glue relative to the impurity
concentration.
27. A method as defined in claim 2, wherein the polarizing additive
-31-

is animal glue; wherein the concentration ratio is adjusted by adjusting
the concentration of glue to a value at which the activation overpotential
measured at a temperature of between 25°C and 40°C is in the range of 70
to 150 millivolts; and wherein the activation overpotential is measured
at a value of a low constant current which value corresponds to a value
of corresponding current density in the range of 0.01 to 4.0 mA/cm2,based
on the exposed area of the moving cathode.
28. A method as defined in claim 27, wherein the value of the low
constant current correspond to a value of corresponding current density
in the range of 0.1 to 0.4 mA/cm2, based on the exposed area of the
moving cathode.
29. A method as defined in claim 27 or 28, wherein the concentration
of glue is increased when the value of the measured activation overpotential
decreases below about 70 mV and wherein the concentration of glue is
descreased when the value of the measured activation overpotential
increases above about 150 mV, whereby the value of the measured activation
overpotential returns to within the range of 70 to 150 mV.
30. A method as defined in claim 2, wherein the polarizing additive
is animal glue; wherein the impurities include antimony; wherein the
concentration ratio is adjusted by adjusting the concentration of
antimony to a value at which the activation overpotential measured at a
temperature in the range of 25 to 40°C is in the range of 70 to 150 mV; and
wherein the activation overpotential is measured at a value of a low constant
current which corresponds to a value of corresponding current density in the
range of 0.01 to 4.0 mA/cm2, based on the exposed area of the moving cathode.
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31. A method as defined in claim 30, wherein the value of the
low constant current corresponds to a value of corresponding current
density in the range of 0.1 to 0.4 mA/cm2, based on the exposed area
of the moving cathode.
32. A method defined in claim 30 or 31, wherein the concentration
of antimony is increased when the value of the measured activation
overpotential increases above about 150 mV and wherein the concentration
of antimony is decreased when the value of the measured activation
overpotential decreases below about 70 mV, whereby the value of the
measured activation overpotential returns to within the range of 70
to 150 mV.
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Description

~l174~
This invention Telates to a method and apparatus for con-
tinuously monitoring the quality of zinc sulfate electrolyte and,
more particularly, to a method for measuring the activation overpotential
of zinc deposition onto aluminum and controlling the electrolyte purifi-
cation and electrowinning processes in response to deviations of recorded
values of the overpotential from the desired values and an apparatus
to carry out the method.
In the process for electrowinning zinc from zinc sulfate
solutions, impurities such as antimony~ germanium, copper, nickel,
cobalt, iron, cadmium and lead, when present above certain critical
concentrations, cause re-solution of deposited zinc and a corresponding
decrease in the current efficiency of the zinc deposition. To reduce
the concentration of impurities in electrolyte to the desired low
levels, thereby to reduce these effects to a minimum, a complex purification
procedure, which generally includes an iron oxide precipitation and
a zinc dust treatment, is employed prior to electrolysis. In addition
to ~he purification, polarizing additives such as glue are added to
the electrolyte to reduce the effects of the remaining impurities,
as well as to provide smooth and level deposits, and, to some extent,
to control acid mist evolution.
The procedures presently used for determining the purity
of electrolyte are based on chemical analyses and determination of
current efficiencies as a measure of impurity content, while those
for the additions of polarizing additives such as animal glue and
the like are based on maintaining a constant concentration of additive
in the electrolyte despite variations in the concentration of impurities.
l~ese procedures result in variations in the quality of the deposited
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~il1 742~
zinc and the current efficiency of the electrowinning process.
The prior art contains a number of references related to
methods for determining the effects of impurities, glue and other
addition a~ents on electrodeposition processes for metals and for
determining the purity of zinc sulfate solutions.
According to United States Patent 3,925,168, L.P. Costas,
December 9, 1975, there is disclosed a method and apparatus for determining
the content of colloidal material, glue or active roughening agent
in a copper plating bath by determining the overpotential-current
density relationships of solutions having varying known reagent content
and comparing the results with that of a solution with a known plating
behaviour and roughening agent content. According ~o Canadian Patent
988,879, C.J. Krauss et al, May 11, 1976, there is disclosed a method
for determining and controlling the cathode polarization voltage in
relation to current d~nsity of a lead refinery electrolyte, wherein
the slope of the polarization vol~age-current density curve is a measure
of the amount of addition agents and wherein the effectiveness of
addition agents is changed when the cathode polarization voltage attains
values outside the predetermined range of values.
A number of studies are reported in the published literature
which relate to similar methods. C.L. Mantell et al (Trans. Met.
Soc. of AIME, 236, 718-725, May 1966) determined the feasibility of
current-potential curves as an analytical tool for monitoring manganese
electrowinning solutions for metallic impurities. Polarization curves
related to hydrogen evolution were shown ~o be sensitive to metallic
impurities which affect the cathode surface thereby altering the hydrogen
overvoltage. H.S. Jennings et al ~Metallurgical Transactions, ~,
921-926, April 1973) describe a method for measuring cathodic polarization

~L~74~
curves of copper sulate solutions containing varying amounts of addition
agents by varying an applied voltage and recording the relationship
between voltage and current density. 0. Vennesland et al CActa Chem.
Scand., 27, 3, 846-850, 1973) studied the effects of antimony, cobalt,
and beta-naphthol concentrations in zinc sulfate electrolyte on the ''
current-poten~ial curve ~y changing the ca~hode potential at a programmed
rate, recording the curves and comparing the curves with a standard.
T.N. Anderson et al ~Metallurgical Transactions B, 7B, 333-338, September
1976~ discuss a method for measuring the concentration of glue in
copper refinery electrolyte by determining polarization scan curves,
which upon comparison provide a measure of glue concentration. According
to United States Patent 4,146,437 issued March 27, 1979, T.J. O'Keefe
there is disclosed the use of cyclic voltammetry for the evaluation of
zinc sulfate electrolytes. Cyclic voltammograms, which include the cathodic
deposition as well as the anodic dissolution portions of *he current-
potential relationships, and polarization curves, are recorded as
a means for approximating the quantities of impurities and addition
agents in zinc sulfate electrolytes.
This first group of references discloses methods wherein
metal is deposited on an electrode and wherein current, or current
density-potential, curves represent cathode polarization potentials
in relation to varying currents and/or current densities.
T.R. Ingraham et al (Can. Met. Quarterly, 11, 2, ~51-454,
1972) describe a meter for measuring the quality of zinc electrolytes
for measuring the amount of cathodic hydrogen released during electrodeposition
of zinc and indicating current ef~iciency by comparing the weight
of deposited zinc with both the amount of zinc to be expected and the
rate of hydrogen evolution. In United States Patent 4,013,~12, Satoshi Mukae,

1~174~0(~
March 22, 1977, there is disclosed a method for judging purity of
purified zinc sulfate solution by subjecting a sample of solution to
electrolysis, combusting generated gases and measuring the internal
pressure in the combustion chamber which is an indirect measure of
current efficiency. M. Maja et al (J. Electrochem. Soc., 118, 9,
1538-1540, 1971) and P. Benvenuti et al (La Metallurgia Italiana,
60, 5, 417-423, 1968) describe methods for detection of impurities
and measuring the purity of zinc sulfate solutions by depositing
zinc and then dissolving deposited zinc electrolytically and relat-
ing calculated current efficiency to impurity content.
This second group of references relates to methods andapparatus for determining electrolyte purity wherein electrolysis of
solutions is used to determine current efficiency which is subseq-
uently related to electrolyte purity.
In my co-pending Canadian Patent Application 306,805
which was filed on July 5, 1978, there is disclosed a method for
controlling a process for the recovery of zinc from a zinc sulfate
electrowinning solution which comprises decreasing a potential,
which is applied between electrodes in a test cell containing a
sample of solution, at a constant rate at substantially zero cur-
rent, measuring the decreasing potential, terminating the decreasing
of the potential at a value corresponding to the point at which zinc
starts to deposit, determining the activation overpotential,
relating the activation overpotential to the concentration of
impurities and adjusting the process to obtain optimum recovery of
zinc.
*
Issued as Canadian Patent 1,111,125
~ .t?

i~ 7~Z~
Although the method according to my co-pending application
overcomes the necessity for electrolyzing solution to determine
current ef~iciencies or for measuring polarization potentials in
relation
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~,. ..
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1~74~6~0
to varying c~rrents or current densities, several disadvantages still
exist. The method is not continuous and it is necessary to determine
the value of the activation overpotential for each sa~ple by decreasing
the applied potential each time until the value is reached at which zinc
starts to deposit and the current density increases from its substantially
zero value for a fu~ther small decrease in potential.
I have now found that it is unnecessary to decrease the applied
potential in order to determine the value of the activation overpotential.
Thus, I have found that the activation overpotential can be measured as a
function of time at a controlle~, low current, and that the purification
and electrowdnning processes can be controlled by correcting the amounts
of reagents in response to deviations of recorded values of the over-
potential from desired values in order to return to the desired optimum values.
The method and apparatus of the invention apply to zinc
sulfate solutions which are obtained in processes for the treatment of
zinc containing materials such as ores, concentrates, etc. Treatment
includes thermal treatments and hydro-metallurgical treatments such as
roasting, leaching, in situ leaching, bacterial leaching and pressure
leaching. Such solutions, which are referred to in this application as
zinc sulfate electrolyte, may be acidic or neutral solutions.
When zinc sulfate electrolyte is subjected to a co~trolled
low ourrent applied to electrodes placed in a test cell containing electrolyte
and the current has a value which is sufficient to cause inchoate deposition
of zinc on an aluminum cathode, the value of the résulting potential
represents the activation overpotential for zinc deposition onto
aluminum. ~hen the cathode is a moving cathode, values of the activation
overpotential can be measured and recorded and the
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purification process of zinc sul-fate electrolyte and the electrowinning
process of zinc from zinc sulfate electrolyte can be controlled in response
to the measured and recorded values of the activation overpotential. Inchoate
deposicion is defined, for the purpose of this invention, as the deposition
of zinc which has just begun and is limited to partial covering of the moving
cathode surface by zinc. The measured values of the activation over-potential
can be used as a direct measure of the impurity concentration, i.e. the
effectiveness of the purification process, and of the polarizing additive
concentration relative to the impurity concentration in the electrolyte in
the process for the recovery of zinc which includes the purification process
and the electrowinning process. In response to measured values of the
activation over-potential, the purification process can be adjusted, or the
concentration of polarizing additive in the electrolyte can be adjusted
relative to the impurity concentration and/or the impurity concentration can be
adjusted, so that optimum current efficiency and level zinc deposits are
obtained in the electrowinning process.
Accordingly, there is provided a method for controlling a process
for the recovery of zinc from zinc sulphate electrolyte containing concentrations
of impurities, said method comprising the steps of establishing a test circuit
comprising a test cell, a sample of electrolyte, a moving cathode, made of an
electrically conductive material other than zinc, which moving cathode has
an area exposed to said electrolyte, an anode and a reference electrode, said
electrodes being immersed in said sample, a constant current supply and
measuring means electrically connected to said electrodes; applying a low
current to the electrodes in said test cell, said current being sufficient
to cause inchoate deposition of zinc on said cathode; measuring the activation
overpotential at the point of inchoate deposition; relating said activation
over-potential to the cencentration of impurities

o
in said sample; and adjusting the concentration of impurities in the electrolyte
of the process for the recovery of zinc to obtain optimum recovery of zinc,
wherein inchoate deposition is defined as the deposition of zinc which has just
begun and is limited to a partial covering of the moving cathode surface by zinc.
In another embodiment, the method for controlling a process for
the electrowinning of zinc from zinc sulfate electrolyte containing concentra-
tions of impurities and at least one polarizing additive, said method
comprising the steps of establishing an electrolytic test circuit comprising
a test cell, a sample of electrolyte, a moving cathode, made of an electrically
conductive material other than zinc, which moving cathode has an area exposed
to said electrolyte, an anode and a reference electrode, said electrodes being
immersed in said sample, a constant current supply and measuring means electric-
ally connected to said electrodes; applying a low current to the electrodes in
said te~t cell, said current being sufficient to cause inchoate deposition of
zinc on said cathode; measuring the activation overpotential at the point of
inchoate deposition; relating the measured activation overpotential to the
concentration ratio between impurities and polarizing additive in said sample;
and adjusting the concentration ratio in the process electrolyte to obtain
optimum current efficiency and level zinc deposits in the electrowinning
process, wherein inchoate deposition is defined as the deposition of zinc which
has just begun and is limited to a partial covering of the moving cathode surface
by deposited zinc.
The invention will now be described in detail. The apparatus used
in the method for measuring the activation overpotential of zinc consists of
a test circuit which comprises a test

cell, a sample of zinc sulfate electrolyLe, a moving cathode; an
anode, a reference electrode, means to supply a constant current
and means for measuriny the activation overpotential. The test
cell is a small container of circular, square or rectangular cross-
section made of a suitable material, which is preferably resistant
to acid zinc sulfate electrolyte and large enough to hold a suitable
sample of electrolyte. Means are provided in the cell to make it
possible to continuously add electrolyte to, and to discharge
electrolyte from, the test cell. The three electrodes are immersed
in the electrolyte sample and are removably positioned in the cell
at constant distances from each other.
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The moving cathode is preferably made of aluminum or an alum mum
alloy and has a constant area of its surface in contact with electrolyte
in the cell. If desired, a moving cathode of other suitable materials
such as titanium may ke used.
I have determined that a surface area in contact with electrolyte
in the range of about 0.1 to 1 cm gives excellent results, me moving
cathode is preferably made of aluminum or aluminum alloy wire or strip
which is contained in and moves through a cathode holder. The holder
envelopes the moving cathode except for the constant surface area which is
in contact with electrolyte. Means are provided to move the moving cathode
intermittently or continuously through the cathode holder and, consequently
in contact with electrolyte. Preferably, these means include provisions
to move the cathode continuously at a constant rate. The use of a moving
cathode made of aluminum wire or strip has a number of advantages, No special
preparation of the alum mum surface is necessary, aluminum or aluminum alloy
wire or strip is readily available at low cost and test results are re-
producible. The moving cathode does not have to be replaced and thus allows
intermittent or continuous operation. I have found that m~st oommercially
available aluminum and aluminum alloys in the form of wire and strip are
suitable, as long as they have sufficiently smooth and clean surfaces and
have electrochemical characteristics that produce reproducible test results.
me anode is made of a suitable material such as, for example,
platinum or lead-silver alloy. I have found that anodes made of lead-
silver alloy containing 0.75% silver are satisfactory. me reference
electrode can be any one of a number of suitable reference electrodes such
as, for example, a standard calomel electrode (SOE).
The three electrodes are electrically connected to a source

1174;2(~)
of constant current and to measuring means for the activation overpotential.
The source of constant current is connected to the anode and the moving
cathode. The measuring means for the activation overpotential measures
the overpotential on the moving cathode relative to the reference
electrode. The measured activation overpotential may, for example,
be recorded on a meter or other suitable read-out instrument, or alternatively,
may be recorded in the form of a line or trace as a function of time.
The electrodes are removably positioned in the cell in fixed relation
to each other. I have found that good results are obtained when the
surface area of the moving cathode that is in contact with electrolyte
is kept at a fixed distance of about 4 cm from the surface of the
anode and when the reference electrode is positioned between the cathode
and the anode in such a way that the tip of the reference electrode
is rigidly located at a distance of about 1 cm from but not covering
the surface area of the moving cathode in contact with electrolyte.
Suitable means may be provided to maintain the electrolyte
in the call at a suitable constant temperature. Such means may comprise
controlled heating/cooling means for electrolyte prior to electrolyte
entering the cell or a controlled heating/cooling coil placed in the
test cell, or a constant temperature bath or the like.
In the method of the inven~ion, a sample of zinc sulfate
electrolyte, which may be neutral or acidic and which may contain
addition agents, and which may be obtained from either the purification
process or the zinc electrowinning process, is added to the test cell.
To ensure reproducible results, the sample is kept in motion such
as by agitation or circulation. Preferably, the sample is kept in
motion by continuously passing a small flow of electrolyte through
the test cel~. According to this preferred embodiment, a sample of

~t7~o
electrolyte is continuously withdralnn from the purification or electrowinning
process, passed through the test cell and subsequently returned to
the respective process. Electrolyte added to the cell may be adjusted
to a certain zinc or zinc and acid content in order to reduce to a
minimum any variation in the test method that may be caused by variations
in zinc or zinc and acid concentrations in the electrolyte. Adjustment
of zinc concentration to, for example, 150 g/L zinc, or of zinc and
acid concentrations to, for example 55 g/L zinc and 150 g/L sulfuric
acid is satisfac~ory. However, concentrations in the range of 1 to
250 g/L zinc and 0 to 250 g/L sulfuric acid are equally satisfactory.
When using a continuous sample flow, excess electTolyte is discharged
from the cell, for example5 by means of an overflow.
The moving cathode is advanced through the cathode holder
intermittently or continuously at a rate sufficient to allow inchoate
deposition of zinc. Preferably, the moving cathode is advanced continuously
at a constant rate. The rate is dependent on the ratio between cathode
length and cathode surface area, the value of the current density
for inchoate deposition, the surface area of the cathode exposed to
electrolyte, the fraction of exposed cathode area covered with deposited
zinc and the weight of deposited zinc. The rate is also somewhat
dependent on the degree of motion of elect~olyte in the cell. Thus,
the rate at which the cathode is moved through the cathode holder
and, consequently, through the electrolyte, may vary in a range of
values. At rates lower than the minimum rate, the measured overpotential
will be that of zinc onto zinc and will not b~ the activation overpotential
of zinc onto aluminum. ~t Tates several orders of magnitude high~r
than the minimum rate, such as fo~ example, one thousand times higher,
the amount of zinc deposited may be insufficient to obtain reliable
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~L~742(~0
and reproducible values for the activation overpotential. Preferably, the
rate at which the cathsde is moved is about ten to five hundred times the
minimum rate. The m mimum rate may be calculated by using the following
formula:
~in _ _ , cm/h
wherein:
a represents the cathode length to surface area ratios, cm/cm2;
b represents current density, A/cm2;
c represents cathode exposed area, cm2;
d represents the electrochemical equivalent for zinc deposition, g/Ah;
x represents fraction of cathode covered with zinc;
y represents the weight of deposited zinc per unit of cathode surfa oe
area, g/cm .
When moving the cathode intermittently, the average rate of
movement should be at least equal to the minimum rate for continuous
movement, while the periods of time when the cathode is stationary should
not exceed the period of time that will cause more than inchoate deposition.
A 1GW current is applied to the anode and the moving cathode
to cause inchoate deposition of zinc onto the cathode. Preferably, the low
current is controlled at a constant value. m e current should be limited
to low values, which only cause inchoate deposition. This is necessary
to avoid the deposition of zinc onto previously deposited zinc whereby
values of the overpotential are obtained of zinc deposition onto
zinc rather than values of the desired activation overpotential
of zinc deposition onto aluminum. Thus, the deposition of too much
,~

i~7~Z(?O
æinc should be avoided. Preferably, not more than about 10 to 30% of the cathode
surface in contact With electrolyte should b æame covered with deposited
zinc at any time. Currents, expressed as current density, that cause
inchoate deposition on the cathode moving at rates defined above should ~e
at least 0.01 m~/cm . Current densities in respect of the moving cathode
as hereafter discussed are to be taken as referring to the exposed area
of the moving cathode. Below a value of the current, expressed as current
density, of 0.01 m~/cm2, the current may be primarily associated with
hydrogen deposition. For practical purposes, the current, expressed as
current density, should be in the range of 0.01 to 4.0 mP/cm2. Values
of the current higher than the equivalent current density of 4.0 m~/cm2
will require impractically high cathode movement rates. Preferably, current
values, expressed as current density, are in the range of 0.1 to 0.4 m~/cm2.
For example, for inchoate zinc deposition at a current density of
0.4 m~/cm2 on a moving alumlnum cathcde, which has a ratio of length to
surface area of 0.6 to 0.25, an exposed area of 0.25 cm2 of which 20% is
c~vered with deposited zinc and on which 7 x 10 3g zinc/cm2 is deposited
(determined fram scanning electron microscope examination of inchoate
zinc deposits~, the minimum rate as calculated from the above formula
is about 0.21 cmJh.
The temperature of the electrolyte being measured is usually
maintained constant. Changes in temperature affect the measured activation
overpotential, e.g., a decreasing temperature increases the measured over-
potential. I'he cell and its c~ntents are adjusted to and
maintained at a suitable, controlled, constant temperature, which may be
between 0 and 100C, preferably ~etween 20 and 75C and,
more preferably, in the range of 25 to 40 &. If desired, the constant
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~74~
temperature may be approximately the same as the temperature of the
electrolyte in the electrowinning process or purification process,
whichever is applicable. If the temperature is not maintained constant,
the value of the activation overpotential should be corrected for the
temperature variations so that the results of the tests are comparable.
The activation overpotential is measured continuously or
intermittently and is recorded at the point of inchoate deposition of
zinc onto an aluminum cathode. For practical application of the method
of this invention, the activation overpotential is expressed as the
value of the measured overpotential at a current corresponding to a
current density in the range of about 0.01 to 4.0 A/cm2, of exposed
cathode area preferably in the range of about 0.1 to 0.4 m~/cm2. The
activation overpotential is recorded on a suitable read-out instr~nent,
or, alternatively, recorded as a function of time. The recorded values
of the overpotential are maintained in a preferred range. This is
accomplished by making adjustments to the purification and electrowinning
processes when values of the overpotential deviate from the preferred
range of values.
The activation overpotential has specific values dependent on
the cc~position of the electrolyte~ As every electrolyte compo~ition
can be purified to an optimum degree and as every electrolyte composition
has an optimum range of polarizing additive contents relative to its
impurity content, the activation overpotential will similarly have a
range of values that is required to yield the desired optimum results.
Any one of a wide range of suitable polarizing additives may be used;
the most ccmmonly used polarizing additive is animal glue. I have
determined that increasing concentrations of impurities such as antimDny,
cobalt, nickel, germanium and copper cause a decrease in activation
overpotential, while increasing polarizing additive concentrations
,, i~
~ 13 -

~7~30
increase the overpotential.
If the value of the measured activation overpo~ential in the
purification of electrolyte is too low, the impurity concentration is
too high for optimum z;nc recovery in the electro~inning process. Thus,
dependent on the composition o the electrolyte, the activation overpotential
is an indicator of the effectiveness of the purification process and
deviations from optimum operation can be corrected by adjusting the purification
process in relation to values of the activation overpotential, whereby
the impurity concentration is lowered. Correction of the zinc
dust purification process, may be accomplished, for example, by adjusting
the temperature of the purification process, adjusting the duration
of the purification process, increasing the amount of zinc dust,
or increasing the concentration of a zinc dust activator such as
antimony, copper, or arsenic in ionic form. Alternatively, insuf~iciently
purified electrolyte may be further purified in an additional
purification step or by recirculation in the purification process.
If the value of the activation overpotential measured for the
electrolyte in the electrowinning process is too low, the concentration
of polarizing additive in the electrolyte is too low to adequately control
cathodic zinc resolution caused by the impurities present, or the impurity
concentration is too high relative to the cor.centration of polarizing
additive. On the other hand, if the value is too high, the concentration
of polarizing additive is too high relative to the impurity concentration,
and a resultant loss in current efficiency and a rougher zinc deposit
occur. Thus, depending on the composition of the electrolyte, the
activation overpotential is an indicator of the efficiency of the
electrowinning process and deviations from optimum operation can be
corrected by changing the concentration of polarizing additive or the

~'7~QO
concentration of impuri~cies in the electrolyte as required in relation to values
of the activation overpotential. Change in the concentration of polarizing
additive may be accomplished in a suitable manner such as by increasing or
decreasing the rate of addition of glue to the electroly~e. A decrease in the
impurity concentration may be achieved by more effective purification of the
electrolyte prior to the electrowinning process.
Similarly, in the case of the presence of an excess concentration of
polarizing additive, corrective action may be taken by allowing the level of
impurities in the electrolyte to rise in a controlled fashion. Preferably this
is achieved by the controlled addition of antimony, which has the most economical
effect in correcting the impurity to polarization additive concentration ratio,
rather than by any change in the electrolyte purification procedure.
The method of the invention has a number of applications in the
process for the recovery of zinc from zinc sulfa~e electrolyte~ '~hus, the method
can be used before, during and after purification of zinc sulfate electrolyte
and before, during and after the electrowinning of zinc from zinc sulfate
electrolyte. For example, prior to the zinc dust purification process, the
method can be used to determine the degree of iron oxide removal and the degree
of removal by iron oxides of impurities such as arsenic, antimony and germanium
from zinc sulfate solutions obtained in the leaching of ores, concentrates or
calcines. During purification, the method can be used to determine the degree
of purification obtained, for example, with zinc dust, in the various steps of
the purification process. After purification, the effectiveness of the
purification can be determined, as well as the possible need for adjustments
to the purification process or to the subsequent electrowinning process. In the
electrowinning process, the method can be advantageously used to determine the
required amount of polarizing additive in relation to impurity concentration,
the required
-15-

7~Z(~O
amount of impurities, such as, for example, antimony, in relation to
concentration of polarizing additives, the need for adjustments to the
electrolyte feed, or to electrolyte in process and the quality of return
acid.
m e invention will now be descriked by means of the following
non-limitative examples.
In the following examples, values of the activation overpotential
were measured using a test cell having a sample volume of 500 ml. A
volume of electrolyte was passed through the cell. Immersed in the
electrolyte in the cell were a moving cathode consisting of a 1.19 mm
diameter wire of No. 4043 aluminum alloy ~length to surface area ratio
2.51 cm/cm2) contained in and advanced through a st~tionary cathode
holder fixedly positioned in the cell allowing 0.25 cm2 of the cathode
to be exposed to electrolyte, a lead -0.75~ silver anode and a SCE*
positioned ketween cathode and anode. m e surface of the exposed area
of the moving cathode was 4 cm away from the surface of the anode and
the tip of the S OE was 1 cm away from the cathode such that the tip was
not in direct line between the anode and the exposed area of the cathode.
The temperature of the electrolyte flowing through the test cell was
controlled at the desired valueO The anode and the maving cathode were
connected to a source of constant current and the S OE and the cathode
were connected ~o the measuring means for the activation overpotential
using a voltmeter with digital read-out and a x-y recorder. A constant
current was applied to cause inchoate deposition of zinc on the m~ving
cathode and values of the activation overpotential were measured and
recorded. m e minimum rate of movement of the cathode through the
electrolyte was calculated to ke 1.1 cm/h for a current corresponding to
a current density of 1 m~/cm2 and the fraction of the exposed cathode
- 16 -

surface area covered with zinc being 0.1.
Example I
This example illustrates the effects of variations in the
values of the applied current expressed as current density, the cathode
wire speed~ the temperature of the electrolyte and the flowrate of
the electrolyte. A quantity of plant electrolyte was analyzed and
adjusted to 55 g/L Zn and 150 g/L H2SO4. The adjusted electrolyte
also contained 0.01 mg/L Sb, 0.03 mg/L Cu, 0.1 mg/L Co, 0.1 mg/L Ni,
0.005 mg/L Ge, 0.5 mg/L Cd, 30 mg/L Cl and 2 mg/L F. Adjus*ed electrolyte
io was continuously passed through the test cell and the activation overpo~ential
measured and recorded as described. The results are tabulated in Table
I.
TABLE I
Currentmoving cathode temperature flow rate activation
densi~y speed of electro of electrooverpotential
mA/cm cm/hr lyte Clyte mL/min. mV
0.1 60 30 668 10
1.0 60 30 668 75
1.0 180 30 668 82
1.0 300 30 668 90
1.0 60 21 668 83
1.0 60 35 668 55
1.0 60 60 668 20
1.0 60 30 100 70
0.4 60 30 668 35
2.0 60 30 668 75
4.0 60 30 668 115
The results clearly indicate the efect of variables in the
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2~(~
method and the need for using standardized conditions for practical
application.
Example 2
This example illustrates the effects of varying amounts of
antimony, germanium, cobalt and glue added to adjusted electrolyte
with the composi~ion as in Example 1. The activation overpotential
was measured using a current corresponding to a current density of
1.0 mA/cm2, a cathode speed of 160 cm/hr, an electrolyte temperature
of 35C, and an electrolyte flow rate of 660 mL/min. Values of the
measured activation overpotential were recorded on an x-y recorder.
The results are tabulated in Table II. In the Table, the values of
the overpotential represent the average recorded value.
TABLE II
Additions, in mg/L. Activation overpotential
Glue Sb Ge Co m/V
0 0 0 0 80
0 0 0 98
0 0 0 110
0 0 0 12
0 0.02 0 0 58
10 0.02 ~ 0 89
20 0.02 0 0 106
30 0.02 0 0 109
0 0.04 0 0 53
10 0.04 0 0 90
30 0.04 0 0 96
5 0.06 0 0 76
0 0.08 0 0 50 ~con't)
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1~7~L2(~
TABLE II ~con't)
Additions, in mg/L. Activation overpotential
Glue Sb Ge Co m/~
0.08 0 0 78
0.08 0 0 83
0 0 0.04 0 70
0 o o~ o 116
0 0.04 0 2.0 55
0.04 0 2.0 lO~
The results indicate that increasing concentrations of glue
increase the activation overpotential and that increasing concentrations
of impurities decrease the activation overpotential of zinc.
Example 3
This example illustrates that increasing concentrations of
glue are required to give good current efficiency when increasing impurity
concentrations are present in electrolyte and that optimum ranges for
glue concentrations in relation to impurity concentrations exist to
give highest current efficiencies. Samples of adjusted plant electralyte
as used in Example 2, to which varying amounts of glue and antimony
and/or cobalt were added as potassium antimony tartrate and cobalt
sulfate, respectively, were subjected to electrolysis in a cell at
a current density of 400 A/m2 at 35C for 24 hours. The current efficiencies
for the zinc deposition were determined by determing the ratio of the
weight of the deposited zinc to the calculated weight based on the
total amount of current passed through the cell for the deposition
of zinc. The results are given in Table III.
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z~o
TABLE III
g ue added in mg/L 0 10 15 20 25 30 40 45 50
Sb added Co added current efficiencies in %
in mg/L in mg/L _ _ _ _ _ _ _ _ _
0.01 0 88 92 91 90 89 88 87 87 85
0.03 0 7~ 90 92 93 92 91 89 88 87
0.05 0 56 ~6 90 92 93 93 92 91 88
0.07 0 43 72 81 85 89 92 93 92 89
0.01 0.05 89 92 92 91 90 89 88 87 85
0.01 2 88 92 92 92 9~ 91 91 90 89
0.01 5 65 87 92 92 92 92 92 92 91
0.01 5* - 43 74 82 82 81 79 77 75
0.03 0.05 80 90 92 93 93 91 90 ~8 86
0.03 1 40 74 85 92 9~ 93 92 91 89
0.03 5 - 58 74 87 92 94 94 93 90
0.03 5* - - - 40 72 82 83 83 78
*48 hour deposit
It is evident from the tabulated results that for each antimony
concentration, a corresponding narrow range of glue concentrations
was required to give the highest possible current efficiencies. Current
efficiencies decreased for both deficient and excessive glue concentrations.
Thus, a range of op~imum glue concentrations exis~s for each antimony
concentration. Similarly, when antimony and cobalt are present, glue
additions are required to counteract the harmful effects of these impuTities
and optimum glue concentrations exist for each antimony and cobalt
concentration. The optimum glue concentrations were the same for 48
hours as for 24 hours deposits, but ~he current efficiencies had decreased.
- 20 -

~4Z~
~xample 4
Values for the activation overpotential for glue and impurities
concentrations obtained in tests as illustrated in Examples 1 and 2
and Tables I and II were combined with ranges of maximum current efficiencies
for combinations of concentrations of glue and impurities obtained in
t~sts as illustrated in Example 3 and Table III. Thus, the following
ranges of values for optimum current efficiency were obtained in relation
to ratios between glue and impurities as indicated by the values of
the activation overpotential. The ranges are tabulated in Table IV.
TABLE IV
activati~n overpotential range of current efficiency in %
in mV
75-83
79-86
83~89
86-91
88-93
100 90-94
105 90-94
110 89-93
115 87-92
120 86 89
125 83-87
It can be seen from the tabulated figures that the highes~
ranges of current efficiencies are obtained when the activation over-
potential is maintained in the range of 90 to 115 mV, measured at 35C.
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~ 79~Z~O
Example 5
This example illustrates that the activation overpotential
measurements can be used to determine whether the correct glue concentration
is present in the electrolyte relative to the impurity concentration
and what changes are required in glue concentration to optimize the
zinc electrowinnin~ process. Using the same electrolyte as used in
previous examples, tests as described in Example 2 were repeated, current
efficiencies were determined as in Example 3 and the results combined
as illustrated in Example 4. Using the results of the tests according
to this example, the required change in glue concentration in mg/L
was determined at measured values for the activation overpotential
to obtain the optimum value for the current efficiency in the electrolytic
process. Data presented in Table V show the program to control the
electrowinning process for zinc by making specified changes in glue
concentration in zinc electrolyte.
TABLE V
measured activation overpotential required change in glue concen-
in mV at 35C tration in mg/L for optimum
current efficiency
increase by 9
increase by 7
increase by 5
increase by 3
increase by 1
100 no change
105 no change
110 decrease by 1
115 decrease by 3
120 decrease by 5
125 decrease by 7
130 decrease by 9
- 22 -

00
Example 6
This example illustrates that antimony can be used in relation
to measured values of the activation overpotential to control the zinc
electrowinning process at optimum current efficiency.
In a series of electrowinning cells using an acidic zinc
sulfate electrolyte, having the adjusted composition as given in Example
1, both glue and antimony are added. Glue is added to the electrolyte
at a constant rate of 20 mg/L, while antimony is normally added at
a rate of 0.04 mg/L.
Using the electrolyte and the above mentioned additions of
glue and antimony, activation overpotentials and current efficiencies
were determined as in Example 5. Optimum values ~or current efficiencies
were attained with activation overpotentials of 100 to 105 mV. '13sing
the results of these determinations, the required changes in antimony
concentrations in the electrolyte in mg/L were determined at measured
values for the activation overpotential to obtain the optimum value
for the curr0nt efficiency in the electrolytic process. The control
program is given in Table VI.
TABLE VI
__
measured activation required change in antimony
overpotential in mV concentration in mg/L for
optimum current efficiency
decrease by 0.03
decrease by 0.02
decrease by 0.01
100 no change
105 no change
110 increase by ~.01
115 increase by 0.02
120 increase by 0.03
,,

~'79~Z~O
Example 7
This example illus~rates that the removal of impurities from
neutral zinc electrolyte by cementation with atomized zinc can be monitored
by activation overpotential measurements. Impure plant electrolyte
was subjected to purification with varying amounts of atomized zinc
added to the electrolyte containing previously added antimony as antimony
potassium tartrate in varying amounts. Cementation was carried out
at 50C with agitation and using a retention time of one hour. A flow
of purified electrolyte was filtered and cooled to 35C. The flow
was then passed through the test cell for measurement of the activation
overpotential. The purified electrolyte was analyzed to determine
impurity concentrations. The results are tabulated in Table VII.
TABLE VII
Sb added atomized activation impurities in purified electrolyte
in mg/L zinc overpotential in mg/L
added in in mV Cd Cu Co Ni Sb
g/ L - -
_
0.75 0 34 200 3.5 1.6 1.8 0.75
0,75 0.5 48 21 4.1 0.3 0.9 0.09
0.751.0 64 1~ 3.4 0.3 0.5 0.05
0.751.5 74 3.9 1.3 0.2 0.6 0.04
0.75 2.0 76 1.9 1.0 0.2 0.4 0.03
0.75 2.5 88 0.4 0.8 0.3 0~3 0.03
0.75 3.0 94 0.3 0.6 0.2 0.2 0.02
0.25 2.0 70 2.2 0.5 0.2 ~0.1 0.07
0.50 2.0 74 2.2 0.6 0.1 0.1 0.03
l.00 2.0 85 0.6 0.6 0.1 ~0.1 0.02
- 24 -
--.

~74~
Example 8
This example illustrates how the activation overpotential
measurements such as those given in Example 7 can ke used to determine
what corrections must be made to the process for controlling variables
such as zinc dust and antimony additions to optimize the zinc dust
purification of electrolyte. Data presented in Table VIII shDw the
program to control the zinc dust purification process by making specified
changes in zinc dust or antimony salt additions to the zinc electrolyte
during purification if the measured activation overpotential indicates
purification has not proceeded to completion.
TABLE VIII
measured activation required additions
overpotential in m~ of
for neutral
electrolyte zinc dust (g/L) Sb (mg/L)

0 0
0.3 0.1
0.6 0.2
0-9 0 3
1.2 0.4
1.5 0.5
1.8 0.5
~0 2.1 0.5
- 25
~,