Note: Descriptions are shown in the official language in which they were submitted.
1174~9~
This invention relates to improvements in the
process for the bipolar refining of lead and, more particularly,
to a method for improving the efficiency of the process.
In bipolar refining of lead, a number of lead
bullion electrodes are immersed in an electrolytic cell
containing a lead fluosilicate- fluosilicic acid electrolyte.
Only the first and last electrodes in the cell are connected to
a source of direct electrical current, the remainder of the
electrodes being left unconnected to the current source. The
current causes lead to dissolve from the lead bullion electrodes
leaving a layer of slimes containing impurities such as, for
example, bismuth, arsenic and antimony, adhering to the
anodic side of the electrodes, and causes dissolved lead to
deposit as refined lead on the cathodic side of the electrodes.
Vpon completion of the refining cycle, electrodes are removed
from the cell and slimes and refined lead are stripped from the
electrodes. The efficiency of this process is high and is much
improved over that of the conventional Betts Process. Supply
of electrical power to cell and electrodes is vastly simplified,
current densities can be much higher and mechanization is
possible to a much greater degree than with the Betts Process.
The process for the bipolar refining of lead is described in
detail in United States Patent 4,177,117, which issued
December 4, 1979.
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Although the bipolar refining process has many
advantages over the Betts Process, control of the process
has been found to be difficult when the process is operated at
high current densities. Maintaining the desired low impurity
content of the refined lead becomes more difficult with increasing
current densities, in spite of operating at the optimum current-
voltage relationship to prevent the anode overvoltage from
exceeding the voltage at which impurities begin to dissolve from
the lead bullion. In addition, at high current densities the
layer of slimes which remains adhering to the anodic side of the
bipolar electrodes becomes less stable. Detachment of the
slimes from the anodic side of the bipolar electrodes results in
an increasing amount of slimes in the electrolyte and of
impurities in the refined lead. The lead deposited at high
current densities tends to become coarser, less dense and more
brittle which results in difficulties when the refined lead
is to be stripped from the electrodes.
I have now discovered that the control of the
bipolar refining process can be improved when a number of
interdependent process parameters are carefully regulated. More
specifically, I have discovered that, when operating at high
current densities, the impurity content of the refined lead
and the stability of the slimes layer can be considerably
improved by adjusting the composition of the electrolyte in
conjunction with operating the process with a programmed
current within defined limits.
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The use of programmed current has been disclosed
in the above named United States Patent 4,177,117 and is
carried out according to a procedure described in more
detail, in the context of the conventional Betts Process,
in Canadian Patent 1,020,491 issued November 8, 1977.
In accordance with this procedure, the anode
overvoltage may be established at the beginning of the refin-
ing process at a value just below the critical value at which
impurities dissolve and the current is increased to its
maximum value allowable in relation to the cell resistance.
The current is gradually decreased from its initial maximum
allowable value to allow, at all times, for the effects of
the increasing thickness, and hence increasing resistance,
of the slimes layer, thereby to ensure that the critical
value for the anode overvoltage at which impurities dissolve
is not exceeded. The process may be operated at a constant
value for the anode overvoltage of about but not exceeding
the value of the voltage at which impurities, especially
bismuth, dissolve by controlling the current which passes
through the cells at maximum allowable decreasing values.
This results in a reduction of the duration of the refining
process to its minimum value. The process may also be
operated with a cell potential giving anode overvoltage
values further below the critical value, allowing the anode
overvoltage to increase to its critical value during
electrolysis and with currents at values below the maximum
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values allowable. This results in a proportional increase in
the duration of the refining process. Thus, while the
number of Ampere-hours remains constant for the deposition of
a given amount of lead, the duration of the refining process
varies correspondingly to the electrical current applied to
the cell.
Although the use of electrolyte containing varying
concentrations of lead as lead fluosilicate and fluosilicic
acid is known, the prior art is silent on the necessity of
using low acid concentrations when high lead concentrations
are used in the electrolyte. It has, moreover, not been
appreciated that high lead concentrations in the electrolyte
are necessary when the refining process is operated at high
current densities.
The present invention seeks to operate the bipolar
process for the refining of lead at high current densities
with current supplied to the process in a programmed fashion.
The present invention further seeks to operate the
bipolar process for the refining of lead at high current
densities and whilst maintaining a stable layer of slimes
adhering to the anodic surfaces of the electrodes.
In a further aspect, this invention seeks to
produce strong, coherent and easily strippable lead deposits on
the electrodes.
Accordingly, there is provided a process for
controlling the bipolar refining of lead in an electrolytic
cell containing impure lead bullion electrodes, and an
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electrolyte containing lead fluosilicate, and fluosilicic acid, which process
comprises in combination the steps of:
(a) feeding electrolyte containing at least about 85 g/L lead as lead fluosili-
cate and not more than about 85 g/L free fluosilicic acid to the cell, so
that the amount of lead as lead fluosilicate exceeds the amount of free
fluosilicic acid in g/L;
(b) applying a current across the end electrodes at the beginning of the
refining cycle at a value, expressed as current density, in the range of
about 240 to about 450 A/m2;
(c) maintaining the anode overvoltage at a value not exceeding the voltage in
which impurities dissolve from the anodic slimes and maintaining the
electrical current at the maximum value possible related to the internal
resistance of the cell which will not cause the anode overvoltage to rise
above the voltage at which impurities dissolve, whereby the slimes remain
adhering to the electrodes; and
(d) recovering the refined lead.
Preferably, the electrolyte contains lead in the range of about 85 to
120 g/L and free fluosilicic acid in the range of about 50 to 85 g/L, most
preferably about 60 to 7n g/L. Preferably, the initial current expressed as
current density at the electrodes is in the range of about 260 to 400 A/m2.
Preferably, the value of the anode overvoltage is about 200 mV, and is below the
value at which impurities dissolve. Preferably, the current is applied for a
period of time in the range of about 72 to 130 hours, most preferably in the
range of about 84 to 120 hours.
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For obtaining the highest productivity, the refining
process should be operated at the highest possible current
density and shortest possible refining cycle, while maintaining
the highest possible current efficiency and obtaining a high
quality refined lead. When operating the bipolar refining
process, the critical value of the anode overvoltage, i.e.,
the value at which impurities, especially bismuth, dissolve
from the electrodes, must not be exceeded. When the critical
value is exceeded, even for a short period, not only do
impurities dissolve, but the layer of slimes remaining on the
electrodes becomes unstable and slimes separate. Separated
slimes contaminate the electrolyte, form a basis for the
occurrence of electrical shorting, and complicate any elec-
trolyte purification procedure.
When current is applied to the electrolytic cell
in a programmed manner, the length of the refining cycle can
be decreased. The values of the current, or current density,
during the refining cycle are at the maximum allowable decreas-
ing values related to the change of the internal resistance
of the cell. The anode overvoltage is at a value close to
but not exceeding the critical value. However, because
higher inter-electrode voltages result from increased initial
values of the current, the power consumption per tonne of
lead and, therefore, the operating costs of the process
increase with the higher initial currents. Consequently, there
exist a set of optimum values for the current that is initi~lly
applied to the electrodes and for the length of the refining
cycle.
I have found that values for the current initially
applied to the electrodes at the beginning of the refining
cycle, expressed as current density at the electrodes, are
in the range of about 240 to 450 A/m2, preferably in the range
of about 260 to 400 A/m2. Corresponding values for the
duration of the refining cycle are in the range of about
72 to 130 hours, preferably, in the range of about 84 to 120
hours. Above an initial current, expressed as current density,
of about 450 A/m the gain in productivity does not warrant
the additional requirements to make it possible to increase
the current. During the refining cycle, the current is
automatically reduced by use of a programmer. The programmer
maintains the current at maximum allowable values, maintains
the value of the anode overvoltage at about but not exceeding
its critical value and reduces the current to the electrodes
in response to the increasing resistance of the slimes layer.
At the end of the refining cycle the current, expressed as
current density at the electrodes, generally has values in
the range of about 200 to 220 A/m2. Using the programmed
current, the stability of the slimes is excellent and the
impurity content of the refined lead is low.
Using an electrolyte with the conventionally
used composition of about 60 g/L lead as lead fluosilicate
and about 90 g/L free fluosilicic acid gave unsatisfactory
lead deposits when operating at current densities over 240
A/m . The lead deposits were brittle, of low ductility and
of relatively low density. This resulted in difficulties
117~
during the stripping of the deposits from the residual
electrodes.
I have found quite unexpeetedly that in the bipolar
refining process the quality of the lead deposit is related to
the composition of the electrolyte. Thus, I have discovered
that when the bipolar refining process is operated at high
current densities, the lead content of the electrolyte must
be increased and the free acid content decreased in order to
produce dense and strong lead deposits which ean be readily
stripped. Dense and strong lead deposits are obtained when
the eleetrolyte eontains at least about 85 g/L lead as lead
fluosilieate and not more than about 85 g/L free fluosilieie
aeid. Preferably, the lead eoneentration is maintained in
the range of about 85 to 120 g/L lead and the acid concentration
in the range of about 50 to 85 g/L. Above about 120 g/L lead,
significant reductions in the current supplied to the electrodes
are necessary to avoid exceeding the critical value of the
anode overvoltage. Below about 50 g/L free fluosilicic acid,
the conductivity of the electrolyte becomes too low, resulting
in high energy losses. The most preferred range of the acid
concentration is about 60 to 70 g/L.
The advantages of the process according to the
invention are many. The use of an electrolyte with an
increased lead concentration and decreased free acid coneen-
tration make it possible to produee a dense, strong, easily
strippable lead deposit and to operate with high eurrent
densities to increase productivity. The use of programmed
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117'~198
current makes it also possible to operate at the desirable
high average current densities with high initial currents.
The refining cycle can be shortened and productivity increased.
The layer of slimes is stable and impurity content of refined
lead is low.
The invention will now be illustrated by means of the
following non-limitative examples.
Example 1
In a series of tests, lead bullion electrodes containing
such impurities as bismuth, silver, arsenic and antimony were
subjected to bipolar refining in a small cell using electrolyte
containing varying amounts of lead as lead fluosilicate and
fluosilicic acid. An initial current giving an electrode
current density of 390 A/m2 was applied to the electrodes.
The anodic overvoltage was maintained constant at a value
just below 200 mV. The initial current was decreased at
maximum allowable values during the refining cycle to account
for the increasing resistance, such that the value of the
anodic overvoltage did not exceed 200 mV at any time during
the refining cycle. After 96 hours the refining cycle was
completed, the electrodes were removed from the cell and the
lead deposits separated from the remaining lead bullion. The
average ductility of the refined lead was determined by
bending each lead deposit and noting the degrees bending
at which the deposit cracked. Lead deposits with a ductility
of less than about 20 degrees are generally too brittle
for satisfactory stripping. The results are given in Table
I.
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TABLE I
Electrolyte Composition Average Ductility
Pb H2SiF6
in g/L in g/L in Degrees
115 10
110 50 20
115 80 20
135 55 180
135 70 180
210 55 180
The figures shown in Table I indicate that electrolyte containing
85 g/L lead or more and 50 to 85 g/L fluosilicic acid gave
satisfactory deposits.
Example 2
The tests described in Example 1 were repeated in a
commercial size cell using different current densities.
The first test was run at a constant, conventional
current density of 220 A/m2, without the current being pro-
grammed. The refining cycle was terminated after 184 hours
when the anode overvoltage reached 0.2V. In the other tests,
the current was automatically programmed from current densities
of 390 and 500 A/m2 at the beginning of the tests to 220 A/m2
at the end of the tests. The length of each refining cycle was
recorded. The average ductility of the lead deposits in
each of the tests was determined as in Example 1. The
results are given in Table II.
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TABLE II
CurrentElectrolyte Average Length of
DensityCompositionDuctility refining
cycle
in2 Pb H2SiF6 in in
A/m (g/L) (g/L~ degrees hours
220* 70 85 180 184
390 90 80 22 96
390 95 75 27 97
390 100 80 69 96
390 100 70 125 95
390 120 70 158 93
500 55 95 5 110
500 70 85 2 110
500 85 85 25 96
500 170 60 165 110
*conventional
The results in Table II clearly show that the
refining process can be operated at high current densities
with a 4 to 4 1/2 day refining cycle. Ductile, dense and
level lead deposits, which can be easily stripped, are
obtained when the electrolyte contains 85 g/L lead fluo-
silicate or more and 85 g/L fluosilicic acid or less. The
best lead deposits were obtained when the acid concentrations
were from 60 to 70 g/L.
