Note: Descriptions are shown in the official language in which they were submitted.
1 ~55790
This invention relates to a method and apparatus for
continuously monitoring the electrodeposition of metals other
than zinc and, more particularly, to a method for measuring the
activation overpotential of metal deposition and controlling the
electrolyte purification and electrodeposition processes in
response to deviations of recorded values of the overpotential
from the desired values, and an apparatus to carry out the
method.
~n processes for electrodeposition of metals such as
electrowinning, electro-refining and electro-plating, electrolytes
are used which contain impurities which, when present above
certain critical concentrations, can electrodeposit with the
metal and thereby contaminate or cause re-solution of the
deposit with a corresponding decrease in the efficiency of the
metal deposition process. To reduce the concentration of
impurities in the electrolyte purification procedures may be
employed prior to electrolysis. In addition to the purific-
ation, one or more polarizing additives may be added to the
electrolyte to assist in providing smooth and level deposits,
as well as to reduce the effects of remaining impurities.
These polarizing additives act to change polarization.
Polarization can be changed by increasing, OT decreasing, the
concentration of a polarization-causing agent. Polarization
also can be changed by decreasing or increasing the concent-
ration of a de-polarization-causing agent. These agents, both
polarizing and de-polarizing, can be present in the electrolyte
as it comes to the cells from the purification plant, or may be
-- 1 --
1155790
substances, such as animal glue, that are added to the electrolyte
to effect control of the electrodeposition process. It is thus
to be clearly understood that the term "polarization affecting
agents" includes agents of both the polarization-causing and de-
polarization-causing types. It is also pertinent to note that a
given substance will not always act in the same manner in
different processes. Thus a substance may act as a polariz-
ation agent in one process, as de-polarization agent in another
process involving a different metal, or may be inactive in a
third process. Similarly it is not unknown for impurities to
"catalyse" the effects of polarization affecting agents. Thus
it is well known that some experimentation may be needed in
order to decide which polarization affecting agents are suitable
for any given process.
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 addition of suitable polarization
affecting agents are based simply on maintaining a constant con-
centration of agents in the electrolyte despite variations in
the quality of the electrolyte. These procedures result in vari-
11~5790
ations in the quality of the deposited metal and the efficiencyof the electrodeposition process.
The prior art contains a number of references related
to methods for determining the effects of impurities, and
addition agents on electrodeposition processes for metals and
for determining the purity of electrolyte 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 determin-
ing 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 to Canadian Patent 988,879,
C. J. Krauss et al, May ll, 1976, there is disclosed a method
for determining and controlling the cathode polarization voltage
in relation to current density of a lead refinery electrolyte,
wherein the slope of the polarization voltage-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
1155790
impurities. Polarization curves related to hydrogen evolution
were shown to be sensitive to metallic impurities which affect the
cathode surface thereby altering the hydrogen overvoltage. H. S.
Jennings et al (Metallurgical Transactions, 4, 921-926, April 1973)
describe a method for measuring cathodic polarization curves of
copper sulfate solutions containing varying amounts of addition
agents by varying an applied voltage and recording the relationship
between voltage and current density. O. Vennesland et al (Acta
Chem. Scand., 27, 3, 846-850, 1973) studied the effects of anti-
mony, cobalt, and beta-naphthol concentrations in zinc sulfate
electrolyte on the current-potential curve by changing the cathode
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 and copper sulfate electrolytes. Cyclic voltammograms, which
include the cathodic deposition as well as the anodic dissolution
portions of the current-potential relationships, and polarization
curves, are recorded as a means for approximating the quantities
of impurities and addition agents in zinc and copper sulfate
electrolytes.
The first group of references discloses methods wherein
metal is deposited on an electrode and wherein current,
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1155790
or current density-potential, curves represent cathode polariz-
ation potentials in relation to varying currents and/or current
densities.
T. R. Ingraham et al ~Can. Met. Quarterly, 11, 2,
451-454, 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
efficiency 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 4,013,412, Satoshi Mukae,
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, 196~) describe
methods for detection of impurities and measuring the purity of
zinc sulfate solutions by depositing zinc and then dissolving
deposited zinc electrolytically and relating calculated current
efficiency to impurity content.
This second group of references relates to methods and
apparatus for determining electrolyte purity wherein electrolysis
of solutions is used to determine current efficiency which is
subsequently 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
1155790
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 sub-
stantially zero current, measuring the decreasing potential,
terminating the decreasing of the potential at a value correspond-
ing to the point at which zinc starts to deposit, determining
the activation overpotential, relating the activation over-
potential to the concentration of impurities and adjusting the
process to obtain optimum recovery of zinc.
Although the method according to this co-pending
application overcomes the necessity for electrolyzing solution
to determine current efficiencies or for measuring polarization
potentials in relation to varying currents 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 sample 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 further 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 control-
led, low current, and that the purification and electrodeposition
processes can be controlled by correcting the amounts of reagents
in response to deviations of recorded values of the overpotential
1155790
from desired values in order to return to the desired optimum
values.
The method and apparatus of the present invention
apply to electrodeposition processes of metals other than zinc
employing electrolytes which are prepared for electroplating of
metals; which are used in processes for the recovery of metals
by electrowinning; or which are used in processes for the
electro-refining of metals. In many cases, the electrodeposition
processes include a purification process. The method and
apparatus of the invention may be used in the electrodeposition
processes for metals other than zinc such as copper, lead, iron,
cobalt, nickel, manganese, chromium, tin, cadmium, bismuth,
indium, silver, gold, rhodium and platinum.
When metal-ion containing solution, or electrolyte,
is subjected to a controlled low current applied to electrodes
placed in a test cell containing electrolyte, and the current
has a value which is sufficient to cause inchoate deposition of
metal on a suitable cathode made of a material other than the
metal being deposited, the value of the resulting potential
represents the activation overpotential for the metal deposition
onto the suitable cathode. When the cathode is a moving cathode,
values of the activation overpotential can be measured and
recorded and the electrodeposition process of the metal can be
controlled in response to the measured and recorded values of
the activation overpotential. Inchoate deposition is defined,
for the purpose of this invention, as the deposition of metal
which has just begun and is limited to partial covering of the
1 155790
moving cathode surface by metal. The measured values of the
activation overpotential can be used as a direct measure of the
impurity concentration, ~i.e. the effectiveness of a purification
process), of the polarization affecting agent concentration, and
of the polarization affecting agent concentration relative to the
impurity concentration in the electrolyte in a process for the
electrodeposition of metal which includes a purification process.
In response to measured values of the activation overpotential,
several changes are possible, either alone, or in combination: the
purification process can be adjusted; the concentration of polar-
ization affecting agent in the electrolyte can be adjusted, or the
concentration of polarization affecting agent in the electrolyte
can be adjusted relative to the impurity concentration; and the
impurity concentration can be adjusted, so that optimum efficiency
and level metal deposits are obtained in the electrodeposition
process.
Accordingly, there is provided a method for controlling
a process for the electrodeposition of a metal other than zinc
using electrolyte containing concentrations of impurities, said
method comprising the steps of establishing a test circuit com-
prising a test cell, a sample of electrolyte, a moving cathode,
made of an electrically conductive material other than the metal
being deposited 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; apply-
ing a low current to the electrodes in said test cell, said
current being sufficient to cause inchoate deposition of said metal
on said cathode; measuring the activation overpotential at the point
, -
~ ~ - 8 -
1 155790
of inchoate deposition of said metal; relating said activation
overpotential to the concentration of impurities in said sample;
and adjusting the process for the deposition of metal to obtain op-
timum deposition of metal, wherein inchoate deposition is defined
as the deposition of metal which has just begun and is limited to
a partial covering of the moving cathode surface by deposited metal.
In a second embodiment, the method includes controlling
a process for the electro deposition of a metal other than zinc
using electrolyte containing concentrations of at least one polar-
ization affecting agent, 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 the metal being deposited 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 elec-
trodes in said test cell, said current being sufficient to cause
inchoate deposition of said metal other than zinc on said cathode;
measuring the activation overpotential at the point of inchoate
deposition of said metal other than zinc/ relating the measured
activation overpotential to the concentration of said at least one
polarization affecting agent in said sample; and adjusting the con-
centration of said polarization affecting agent in the electrolyte
of the process to obtain optimum efficiency and level metal depos-
its in the electro deposition process, wherein inchoate deposition
is defined as the deposition of metal which has just begun and is
limited to a partial covering of the moving cathode surface by
~,
~;, _ g _
1155790
deposited metal.
In a third embodiment, the method includes controlling
a process for the electro deposition of a metal other than zinc
using electrolyte containing concentrations of impurities and at
least one polarization affecting agent, 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 the metal being
deposited which moving cathode has an area exposed to said electro-
lyte, 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 said metal other than
zinc on said cathode; measuring the activation overpotential at
the point of inchoate deposition of said metal;relating the measur-
ed activation overpotential to the concentration ratio between
impurities and said at least one polarization affecting agent in
said sample; and adjusting the concentration ratio in the electro-
lyte of the process to obtain optimum efficiency and level metal
deposits in the electro deposition process, wherein inchoate
deposition is defined as the deposition of metal which has just
begun and is limited to a partial covering of the moving cathode
surface by deposited metal.
The invention will now be described in detail. The
apparatus used in the method for measuring the activation over-
potential of a metal consists of a test circuit which comprises a
test cell, a sample of electrolyte, a moving cathode, an anode, a
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1 155~90
reference electrode, means to supply a constant current and means
for measuring 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
corrosion by electrolyte and large enough to hold a suitable sample
of electrolyte. If desired, the cell may be ventilated to remove
evolved gas. 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 elec-
trolyte sample and are removably positioned in the cell at constant
distances from each other.
The moving cathode is made of an electrically conductivematerial other than the metal being deposited, which material is
suitable for using in the electrodeposition process of the metal
being deposited and which is compatible with the electrolyte used
in the process. The material of the moving cathode may be made of
the same material as is used for the cathode(s) in the electro-
deposition process, provided that the
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1 155790
material is different from the metal being deposited. For
example, in the controllîng of electrodeposition processes using
sulfate-based electrolyte such as in processes for deposition of
copper, manganese, nickel, cobalt, iron and cadmium, the moving
cathode is preferably made of aluminum or a suitable aluminum
alloy. If desired, a moving cathode of other suitable materials
such as e.g. titanium, iron, steel, stainless steel, nickel,
lead, copper and the like may be used in an electro-deposition
process. A moving flexible plastic electrode of a suitable
electroconductive material could also be used.
The moving cathode has a constant area of its sur-
face in contact with electrolyte in the cell. I have determined
that a surface area in contact with the cell electrolyte in the
range of about 0.1 to 1 cm2 gives excellent results. The moving
cathode is preferably a 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 con-
tact with electrolyte. Means are provided to move the moving
cathode intermittently or continuously through the cathode
holder and, consequently in contact with electrolyte. Prefer-
ably, these means include provisions to move the cathode con-
tinuously at a constant rate. The use of a moving cathode made
of wire or strip has a number of advantages. No special pre-
paration of the cathode surface is usually necessary, wire or
strip is readily available at low cost and test results are
reproducible. The moving cathode does not have to be replaced
and thus allows intermittent or continuous operation. Most
1 15579~
commercially available suitable cathode materials in the form of
wire and strip are useful as long as they have sufficiently
smooth and clean surfaces, possess good corrosion resistant
properties in the electrolyte, are sufficiently ductile to be
moved through the cathode holder and have electrochemical
characteristics that produce reproducible test results.
The anode is made of a suitably inert material that
allows gas evolution. For example, in sulfate electrolyte a
suitable anode material may be a lead-silver alloy. I have
found that, for sulfate electrolytes, anodes made of lead-silver
alloy containing 0.75~ silver are satisfactory and that for
chloride-containing electrolytes, other than those containing
concentrated hydrochloric acid, platihum is satisfactory. Other
suitably inert materials for the anode include car~on and
graphite. The reference electrode can be any one of a number
of suitable reference electrodes such as, for example, a
standard calomel electrode (SCE).
The three electrodes are electrically connected to a
source of constant current and to measuring means for the
activation overpotential. The source of constant current is con-
nected 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 alter-
natively may be recorded in the ~orm of a line or trace as a
function of time. The electrodes are removably positioned in
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1155790
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. If desired, a diaphragm or semi-permeable membrane
may be positioned in the test cell between the anode and the
cathode to provide separate anodic and cathodic compartments; the
reference electrode is then placed in the cathodic compartment.
Suitable means may be provided to maintain the
electrolyte in the cell at a suitable constant temperature.
Such means may comprise controlled heating/cooling means for
electrolyte prior to electrolyte entering the cell or a control-
led heating/cooling coil placed in the test cell, or a constant
temperature bath or the like.
In the method of the invention, a sample of electrolyte,
which may be basic, neutral or acidic and which may contain
addition agents, and which may be obtained from either an
electrolyte purification process or a metal electrodeposition
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 con-
tinuously passing a small flow of electrolyte through the test
cell. According to this preferred embodiment, a sample of
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1 15~790
electrolyte is continuously withdrawn from the purification or
electrodeposition process, passed through the test cell and sub-
sequently returned to the respective process. Electrolyte added
to the cell may be adjusted to certain concentrations of the
components of the electrolyte, such as, for example metal and
acid content in order to reduce to a minimum any variation in
the test method that may be caused by variations in component
concentrations in the electrolyte. When using a continuous
sample flow, excess electrolyte is discharged from the cell, for
example, 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 metal. 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 metal and the weight of deposited metal. The rate is
also somewhat dependent on the degree of motion of electrolyte
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
metal onto that metal and will not be the activation over-
potential of metal onto the moving cathode. At rates several
orders of magnitude higher than the minimum rate, such as for
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1155790
example, one thousand times higher, the amount of ~etal de-
posited may be insufficient to obtain reliable and reproducible
values for the activation overpotential. Preferably, the rate
at which the cathode is moved is about tento five hundred times
the minimum rate. The minimum rate may be calculated by using
the following formula:
Rmin= abcd, cm/h,
wherein:
a represents the cathode length to surface area ratio,
cm/cm ;
b represents current density, A/cm2;
c represents cathode exposed area, cm2;
d represents the electrochemical equivalent for the
metal being deposited, g/Ah;
x represents the fraction of the exposed cathode area
covered with metal; and
y represents the weight of deposited metal per unit
of cathode surface area 9 g/cm2 .
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 low current is applied to the anode and the moving
cathode to cause inchoate deposition of metal onto the cathode.
Preferably, the low current is controlled at a constant value.
The current should be limited to low values, which only cause
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1155790
inchoate deposition. This is necessary to a~oid the deposition
of metal onto previously deposited metal whereby values of the
overpotential are obtained of metal deposition onto that metal
rather than values of the desired activation overpotential of
metal deposition onto the moving cathode material. Thus, the
deposition of too much metal should be avoided. Preferably, not
more than about 10 to 30~ of the cathode surface in contact with
electrolyte should become covered with deposited metal at any
time. Currents,[expressed as current density),that cause inchoate
deposition on the cathode moving at rates defined above should
be at least 0.01 mA/cm2. Current densities in respect of the
moving cathode as hereafter discussed are to be taken as refer-
ring to the exposed area of the moving cathode. Below a value
of the current, expressed as current density, of 0.01 mA/cm2,
special precautions may be required to obtain reliable measure-
ments. For practical purposes, the current, expressed as
current density, should be in the range of 0.01 to 4.0 mA/cm2.
Values of the current higher than the equivalent current density
of 4.0 mA/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 mA/cm2. For example, for inchoate
copper deposition at a current density of 0.4 mA/cm2 on a moving
aluminum cathode, which has a ratio of length to ,surface area
of 2.4, an exposed area of 0.25 cm2 of which 20~ is
covered with deposited copper and on ~hich 7 x lO 3g copperlcm2
is depositcd the minimum ratc as calculated from the above formula
is about 0.20 cm/h. Using the same values for the parametcrs,
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~ 155790
the minimum rate for lead is about O.G5 cm/h, and that for
manganese is about 0.17 cm/h.
The temperature of the electrolyte being measured may
be maintained constant. Changes in temperature affect the
measured activation overpotential, e.g., a decreasing temper-
ature increases the measured overpotential. If desired, the
cell and its contents are adjusted to and maintained at a suit-
able, controlled, constant temperature, which may be between 0
and 100C, preferably between 20`~and 75nC. If desired, the
constant temperature may be approximately the same as the
temperature of the electrolyte in the electrodeposition process
and/or purification process, whichever is applicable. If the
temperature is not maintained constant, the value of the activ-
ation 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 metal onto the moving cathode. For practical
application of the method of this invention, the activation
overpotential is expressed as the value of the measured over-
potential at a current corresponding to a current density in the
range of about 0.01 to 4.0 mA/cm of exposed cathode area,
preferably in the range of about 0.1 to 0.4 mA/cm2. The activ-
ation overpotential is recorded on a suitable read-out instru-
ment, or, alternatively, recorded as a function of time. The
recorded values of the overpotential are maintained in a pre-
ferred range. This is accomplished by making adjustments to
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115~790
the purification and electrodeposition processes when values
of the overpotential deviate from the preferred range of values.
The activation overpotential has specific values for
each metal dependent on the composition of the electrolyte. As
every electrolyte composition can be purified to an optimum
degree, has an optimum range of polarization affecting agent(s)
contents and has an optimum range of polarization affecting
agent(s) 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 number of suitable polarization affecting agents
known in the art may be used in the electrodeposition
process of each metal. One of the most commonly used polarizing
affecting agents is animal glue. I have determined that
increasing concentrations of impurities may cause a decrease
in activation overpotential, while increasing polarizing
agent concentrations increase the overpotential, and increasing
de-polarization agent concentrations decrease the overpotential.
If the value of the measured activation overpotential
in the purification of electrolyte is too low, the impurity
concentration is too high for optimum metal recovery in the
electrodeposition process. Thus, dependent on the composition
of 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
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~155790
is lowered. Insufficiently 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 electrodeposition process
is too low, the concentration of polarization affecting
agent(s) in the electrolyte is insufficient to control cathodic
metal deposition adequately, or the impurity concentration
is too high relative to the concentration of polarization
affecting agent(s). On the other hand, if the value is too
high, the concentration of polarization affecting agent(s)
is too high, and a resultant loss in efficiency and a rougher
metal deposit occur. Thus, depending on the composition of
the electrolyte, the activation overpotential is an indicator
of the efficiency of the electrodeposition process and
deviations from optimum operation can be corrected by changing
the concentration or character of the polarization affecting
agent(s) or by changing the concentration ratio between
polarization affecting agent(s) and impurities in the electrolyte,
as required in relation to values of the activation overpotential.
Change in the concentration of polarization affecting agent(s)
may be accomplished in a suitable manner such as by increasing
or decreasing the rate of addition of polarization affecting
agent(s) to the electrolyte. A decrease in the impurity
concentration may be achieved by more effective purification
of the electrolyte prior to the electrodeposition process.
In the case of the presence of an excess concentration
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1155790
of polarization affecting agent~s~ corrective action may
also be taken b~ adding an agent to the electrolyte of
opposite polarization affecting characteristics in a controlled
fashion to bring the ratio of concentrations of impurities
and polarization affecting agent~s) to the correct value.
Adding a de-polarizing agent when required may be done
conveniently by controlled addition of a metal salt, which
acts as a de-polarizing agent, which addition results in
correcting the impurity to polarizing agent concentration
ratio.
The method of the invention has a number of applic-
ations in processes for the electrodeposition of metals. Thus,
the method can be used before, during and after purification of
electrolyte and before, during and after the electrodeposition
of metal using electrolyte. In the electrodeposition process,
the method can be advantageously used to determine the required
amount of polarization affecting agent(s) alone and in relation
to impurity concentration, the required amount of impurities
in relation to concentration of polarization affecting agent(s),
the need for adjustments to the electrolyte feed, or to
electrolyte in process and the quality of recycled electrolyte.
The invention will now be described 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
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1 155790
moving cathode consisting of a wire contained in and advanced
through a stationary cathode holder fixedly positioned in the
cell allowing 0.25 cm2 of the cathode to be exposed to
electrolyte, an anode and a SCE positioned between cathode and
anode. The surface of the exposed area of the moving cathode
was 4 cm away from the surface of the anode and the tip of the
SCE 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 value. The anode and
the moving cathode were connected to a source of constant
current and the SCE and the cathode were connected to the
measuring means for the activation overpotential using a
voltmeter with digital read-out and a recorder. A constant
current was applied to cause inchoate deposition of metal on
the moving cathode and values of the activation overpotential
were measured and recorded. The fraction of the exposed
cathode surface area covered with metal was 0.1.
Example 1
This example illustrates how the activation over-
potential measurements can be used to control the electro-
deposition of copper. Values of the activation overpotential
were measured using a current corresponding to a current density
of 1.0 mA/cm2. The cathode was a 1.19 mm diameter wire of
No. 1100 aluminum alloy with a length to surface area ratio of
2.51 cm/cm2. The minimum rate of movement of the cathode
through the electrolyte was calculated to be 1.09 cm/h, The
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1155790
cathode was advanced through the cathode holder at 160 cm/h.
The anode was a lead - 0.75~ silver anode. The electrolyte temp-
erature was 50C and the electrolyte flow rate was 660 ml/min.
~alues of the measured activation overpotential were recorded on
a recorder. The copper sulfate electrolyte contained 20 g/L
copper and 150 g/L sulfuric acid, as well as varying added
amounts of glue, chloride and antimony.
The electrodeposition process was carried out at a
current density of 400 A/m2 for a period of 24 hours. After
the 24 hour deposition time, the deposit was analyzed for
its surface quality and its ductility. The ductility was
assessed by the number of times the deposit could be bent
180 before it showed cracking. The amounts of additives to
the electrolyte, the average of the recorded values of the
activation overpotential for each electrolyte composition
and the qualities of the electro deposited copper are given
in Table 1.
1155790
TABLE 1
Test Additions to Electrolyte Activation Ductoility Nature
No. glue Cl- Sb ions Overpotential 180 bends of
mg/L mg/L mg/L mV No. Deposit
1 0 - - 118 10 rough
2 2.5 - - 122 15 rough
3 5.0 - - 132 14 smooth
4 7.5 - - 140 20 smooth
- - 142 15 rough
6 20 - - 150 16 rough
7 0 1 - 129 29 rough
8 0 10 - 140 25 rough
9 0 30 - 145 10 rough
1 - 140 6 smooth
11 7.5 1 - 143 7 rough
12 5 10 . - 180 7 rough
13 5 30 - 182 10 rough
14 0 - 300 118 12 rough
2.5 - 300 125 7 smooth
16 5 - 300 128 22 smooth
17 10 - 300 153 18 rough
18 20 - 300 164 7 rough
19 0 10 300 140 25 rough
10 300 178 23 smooth
21 30 10 300 203 11 rough
22 5 1 300 136 11 smooth
27` 5 30 300 181 13 smooth
24 5 30 20 180 16 smooth
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1 155790
The results of tests 1-6 show that values for the
activation overpotential increase with increasing amounts of
glue in the electrolyte. In order to obtain a smooth deposit
the activation overpotential should be controlled in the range
of about 130 to 140 mV and the amount of glue in the electrolyte
should be adjusted when the measured value of the activation
overpotential is outside this range.
The results of tests 7-13 show that the presence of
chloride in the electrolyte results in rough deposits, which in
the additional presence of glue become also less ductile.
The results of tests 14-18 show that, when an electro-
lyte contains antimony and increasing amounts of glue, the
values of the overpotential increase but that the results of
the ductility test of the deposit become erratic. In order to
correct this erratic behaviour, chloride should be present also.
As can be seen from the results of tests 19-24, the presence
of about 10 mg/L chloride as well as 5 mg/L glue give smooth
and ductile deposits in the presence of antimony. In such a
system, the values of the activation overpotential are about
180 mV and the system can be controlled at about this value
by making judicious adjustments to the concentrations of
either glue, or chloride or both.
Example 2
This example illustrates how the activation over-
potential measurements can be used to control the electro-
deposition of manganese. Values of the activation over-
potential were measured using a current corresponding to a
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1155790
current density of 1.0 mA/cm2. The cathode was a 1.19 mm
diameter wire of No. 1100 aluminum alloy with a length to
surface area ratio of 2.51 cm/cm2. The minimum rate of
movement of the cathode through the electrolyte was
calculated to be 0.92 cm/h. The cathode was advanced at a rate
of 160 cm/h. The anode was a lead - 0.75% silver anode. The
electrolyte temperature was 50C and the electrolyte flow rate
was 660 mL/min. Values of the measured activation over-
potential were recorded on a recorder. The manganese sulfate
electrolyte contained 45 g/L manganese sulfate, 135 g/L ammonium
sulfate and 0.1 g/L sulfur dioxide, as well as varying addedamounts of glue, copper, nickel, zinc and antimony. The pH of
the electrolyte was 7Ø Electrodeposition of manganese at a
current density of 400 A/m2 requires the presence of 8 to 16 mg
glue per litre of purified electrolyte to attain optimum current
efficiency and smooth deposits. The values of the activation
overpotential for electrolyte with specific additions are given
in Table II.
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7 9 0
TABLE II
Additions to electrolyte in mg/L Activation Over-
Cu Ni Zn Sb potential in mV
o - - - ~ 166
4 - - - - 172
8 - - - - 174
12 - - - - 179
16 - - - - 183
- - - - 190
24 - - - - 196
0 5 - - - 155
0 10 - - - 151
16 10 - - - 151
0 - 1 - - 168
0 - 2 - - 166
16 - 2 - - 183
16 10 2 - - 158
0 - - 0.5 - 168
0 - - 1 - 164
16 - - 1 - 178
0 - - - 1 166
0 - - - 10 160
0 - - - 25 137
8 - - - 25 139
- - - 25 139
- - - 25 143
16 - - 1 1 172
16 - - 1 10 164
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It can be seen from the tabulated results that glue
acts as a polarizing agent, that copper, zinc and antimony
act as de-polarizing agents and that nickel is essentially
inactive. Thus, increasing amounts of glue have the effect of
increasing the values of the activation overpotential while
copper, zinc and antimony have the opposite effect. The results
indicate also that the optimum glue levels in purified
electrolyte of 8 to 16 mg/L result in values of the activation
overpotential in the range of from 174 to 183 mV and that the
amount of glue must be increased when de-polarizing agents
(impurities) are present in small concentrations to return the
values of the activation overpotential to within the desired
range of 174 to 183. When concentrations of impurities higher
than a few mg/L are present, the process for the purification
of electrolyte should be enhanced. In this context it also
follows from the test results that the effectiveness of the
purification process can also be advantageously monitored by
measuring the activation overpotential of the electrolyte.
Example 3
This example illustrates how the activation over-
potential measurements can be used to control the electro-
deposition of nickel. ~alues of the activation overpotential
were measured at 55C using a current corresponding to a
current density of 1 mA/cm2. The cathode was a 1.19 mm wire of
copper with a length to surface area ratio of 2.51 cm/cm2. The
minimum rate of movement of the cathode through the electrolyte
was calculated to be 0.98 cm~h. The cathode was advanced
- 27 -
1 1 55790
through the cathode holder at a rate of 160 cm/h. The anode
was a lead -0.75~ silver anode.
The electrolyte temperature was 55C and the electrolyte
flow rate was 660 mL/min. Values of the measured activation
overpotential were recorded on a recorder. The electrolyte
contained 50 g/L nickel as nickel sulfate, 40 g/L sulfuric
acid, 90 g/L sodium chloride and 16 g/L boric acid, as well as
varying amounts of glue or sodium lauryl sulfate. Sodium
lauryl sulfate is an addition agent in the electrowinning of
nickel to prevent pitting of the deposited nickel and glue is
added in electroplating of nickel as a levelling agent. The
results are given in Table III.
TABLE III
Additions to Electrolyte in mg/L Activation Over-
Sodium Glue potential in mV
lauryl sulfate
0 - 70
- 50
- 44
- 0 70
- 5 84
93
It can be seen from the results that sodium lauryl sulfate acts
as a depolarizing agent and, when present in increasing amounts,
lowers the values of the activation overpotential, and that glue
acts as a polarizing agent and, when present in increasing
amounts, increases the values of the activation overpotential.
- 2~ -
1155790
Values of activation overpotential can be maintained at desired
values by adjusting the concentration of the addition agent
that changes polarization.
Example 4
This example illustrates how the activation over-
potential measurements can be used to control the electro-
deposition of lead. Values of the activation overpotential
were measured using a current corresponding to a current
density of 4.0 mA/cm2. The cathode was a 1.19 mm diameter wire
of copper with a length to surface area ratio of 2.51 cm/cm2.
The minimum rate of movement of the cathode through the
electrolyte was calculated to be 13.8 cm/h. The cathode was
advanced through the cathode holder at a rate of 160 cm/h. The
anode was a lead - 0.75% silver anode.
The temperature of the electrolyte was 40~ and the
electrolyte flow rate was 660 mL/min. Values of the measured
activation overpotential were recorded on a recorder. The lead
fluosilicate electrolyte contained 75 g/L lead as lead
fluosilicate, 90 g/L fluosilicic acid, as well as varying amounts
of ECA (Aloes extract), lignin sulfonate and sodium thiosulfate.
Electrolysis was carried out for 24 hours at 200 A/m2 and at 4GC
using lead bullion electrodes to obtain a lead deposit. The
quality of the lead deposit was determined.
The additions to the electrolyte, the activation
overpotential and the nature of the lead deposit for each test
are given in Table IV.
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1 15~790
TABLE IV
Addition to Electrolyte Activation Nature of
ECA lignin Sodium Overpotential Deposit
mg/L Sulfonate thiosulfate
mg/L mg/L mV
0 0 0 16 rough
4 0 35 rough
0.5 4 0 40 rough
1.0 4 0 43 rough
1.5 4 0 46 smooth
2.0 4 0 52 smooth
3.0 4 0 58 rough
3.0 4 50 18 rough
From the results in Table IV can be seen that both ECA and
lignin sulfonate act as polarizing agents and sodium thiosulfate
acts as a de-polarizing agent. In electrodeposition of lead the
amount of lignin sulfonate is usually maintained substantially
constant, while the amount of ECA is ad~usted to maintain the
activation overpotential in the desired range of 45 to 55 mV in
order to obtain a smooth and level lead deposit. When the
desired values of the activation overpotential are exceeded,
sodium thiosulfate can be added to return the value of the
activation overpotential to within the desired range.
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