Note: Descriptions are shown in the official language in which they were submitted.
_ l 1333664
METHOD FOR THE SOFTENING OF LEAD BULLION
This invention relates to the refining of lead and, more
particularly to a method for the softening of lead bullion
with pure oxygen.
BACKGROUND OF THE INVENTION
In the production of lead from minerals and concentrates, a
lead bullion obtained from a lead smelting process is
normally subjected to a number of refining steps. In one of
those steps, the bullion is subjected to oxidation or
softening to remove impurity metals such as antimony,
arsenic and tin in a slag. The softening is usually
carried out after the bullion has been decopperized.
In order to soften lead bullion, a relatively high
temperature such as a temperature in the range of 590 to
750C is required, but the lead bullion after decopperizing
is typically at a temperature in the range of 400 to 450C.
Before the softening of the lead bullion is carried out the
temperature of the lead bullion must therefore be raised
about 200C to reach the usually required softening
temperatures.
BRIEF DESCRIPTION OF PRIOR ART
The softening process is well-documented and generally
comprises the blowing of air or oxygen-enriched air into a
bath of lead bullion at temperatures of from 590 to 750C,
with or without the addition of an alkali metal, usually
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sodium, or an alkaline chemical such as caustic or caustic
soda.
Recent descriptions of the softening process can be found in
U.S. Patents 4 194 904, 4 308 058 and 4 425 160, German
Publications 30 48 860 Al and 33 32 796 Cl, and in a paper
by P.J. Dugdale presented in 1977 at the 16th Annual
Conference of Metallurgists in Vancouver, B.C.
The prior art processes all use air or oxygen-enriched air.
The use of pure oxygen is considered impractical as it
causes considerable damage to the refining vessel or kettle.
The injection of oxygen into molten metal during refining is
possible as is disclosed in Canadian Patent 1 141 168, but
the oxygen stream must then be surrounded by a protective
fluid using specially designed lances.
SUMMARY OF THE INVENTION
We have now found that the softening of lead can be carried
out with pure oxygen. We have furthermore found that the
softening process can be carried out without heating all of
the lead bullion to the high temperature required for the
softening process. More particularly, we have found that by
diverting a small portion of a charge of lead bullion to a
small softening furnace, the softening can be carried out
with pure oxygen at about 650C, and the major portion of
the lead bullion does not have to be heated to the high
temperature required for softening. The small portion of
bullion is diverted from the main charge of bullion to the
softening furnace, heated and softened in the small
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softening furnace, separated from the slag and the softened
bullion is either returned to the main charge or fed to a
separate vessel. In this manner, the whole bullion charge
can be softened. Alternatively, the softening process can
be carried out semi-continuously or continuously.
Accordingly, it is an object of the present invention to
provide a method for softening lead bullion with pure
oxygen. It is another object to provide a method for
refining lead bullion whereby the major portion of the lead
bullion is kept at a relatively low temperature. These and
other objects of the present invention can be achieved by
providing, in its broadest aspect, a method for the
softening of lead bullion comprising the steps of
maintaining a charge of lead bullion comprising antimony,
arsenic and tin, removing a minor portion of bullion from
said charge, feeding said minor portion to a softening
furnace, maintaining a temperature in said furnace in the
range of about 590 to 650C, softening said minor portion at
said temperature in said furnace with pure oxygen to form
softened lead bullion with a predetermined content of
antimony and a slag comprising lead and at least a portion
of said antimony, arsenic and tin, and separately removing
softened lead bullion and slag from said furnace.
DETAILED DESCRIPTION
The method of the invention will now be described in detail.
Lead bullion is one of the intermediates, or the end
product, formed in a lead smelting process wherein lead
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1333664
concentrates or sinter products are treated for the recovery
of lead and other contained values. The lead bullion is
usually subjected to one or more refining steps for the
removal of at least a portion of the impurity metals that
include antimony, arsenic, tin, copper, bismuth and silver,
and for the recovery of refined lead. One of the refining
steps is the softening of lead bullion, usually carried out
after the bullion has been decopperized. The softening
process is carried out until the antimony, arsenic and tin
contents in the lead bullion have been reduced either to
very low levels, or to predetermined levels; the latter
being necessary when the softened bullion is to be cast into
electrodes for electrolytic refining.
The method of the present invention is particularly useful
for the partial removal of arsenic, antimony and tin from
bullion containing relatively high contents of these metals
and for the removal to very low levels when the bullion
contains relatively low contents of these metals.
Lead bullion, usually after decopperizing, contains
antimony, arsenic and tin as the main impurity metals.
Other impurity metals that may be present comprise bismuth,
copper and silver. The lead bullion is maintained as a
charge in a suitable vessel, such as a refining pot or
kettle. The temperature of the bullion charge in the kettle
is usually in the range of about 400 to 450C. The charge
of the bullion in the kettle is softened and the softened
bullion is removed for a subsequent refining treatment or
for casting into electrodes. A portion of the lead bullion
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is removed from the charge in the kettle and is fed to a
softening furnace wherein the portion is heated and treated
with pure oxgyen. Preferably, a minor portion of the charge
is intermittently or continuously pumped or flowed from the
kettle to the softening furnace. The softening furnace may
be a relatively small vessel of suitable shape, preferably
brick-lined and equipped with one or more oxygen lances.
The antimony, arsenic and tin are oxidized in part and,
together with any oxidized lead, form a slag which separates
to the top of the furnace leaving softened lead bullion in
the bottom portion of the furnace.
After softening is completed, the minor portion may be
returned to the bullion kettle or to a second kettle from
which it is discharged for further treatment. The softening
of a small portion of the charge is repeated until the whole
charge has been softened. In a preferred embodiment, the
softening is carried out with consecutive minor portions,
softened bullion of each portion is returned to the charge
in the bullion kettle and softening is continued until the
charge has the desired composition. The softened charge is
then transferred to a second kettle from which it is
subsequently discharged for casting into anodes for
electrorefining. It is also possible to carry out the
softening continuously, in which case the volume of the
contents of the furnace is maintained substantially constant
by continuously displacing softened lead bullion from the
furnace with a corresponding volume of lead bullion from the
lead bullion charge in the kettle, while allowing for the
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6 1333664
volume of slag formed. In a further preferred embodiment,
the softening is carried out continuously by continuously
feeding oxygen into the furnace to effect softening,
intermittently pumping bullion from the kettle to the
furnace and returning a substantially equal volume of
softened bullion from the furnace to the kettle such that
the temperature in the furnace is maintained between about
625C and about 650C. The pumping is started when the
furnace charge has a temperature of about 650C and is
stopped when the furnace temperature has decreased to about
625C. The furnace charge is then reheated to 650C after
which the pumping is started. This cycle of pumping,
cooling and heating is continuously repeated. The flow of
bullion to the furnace is sufficient to remove the heat
generated by the oxidation of metals in the furnace.
The slag accumulated in the top portion of the furnace is
periodically or continuously discharged from the softening
furnace and recovered for further treatment. The amount of
oxygen fed into the furnace contents through the one or more
oxygen lances determines the temperature of the furnace
contents as well as, together with residence time, the
amount of impurity metals removed from the bullion into the
slag. The amount of oxygen lanced into the bullion in the
furnace should be sufficient, firstly, to raise the
temperature to the required softening temperature of about
650C, and, secondly, to reduce the content of the impurity
metals to the desired level, i.e. to oxidize the desired
amount of impurity metals, as well as lead which is co-
oxidized.
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The amount of oxygen is determined from relationships thatexist between the amount of lead bullion to be treated, its
arsenic content and the amount of antimony that is to be
removed in order to obtain softened bullion with a
predetermined antimony content. For example, for partial
softening, if 5 t/h of bullion with an As content of 0.6~ is
to be softened with removal of 0.7% Sb, 21 Nm3/h (normal
cubic metre per hour) of oxygen must be lanced into the
bullion in the softening furnace. Similarly, 15 t/h bullion
with 0.3% As and 0.3% Sb removal requires 31 Nm3/h oxygen.
Similarly, 20 t/h bullion, containing 0.9% As and requiring
1.0% Sb removal, requires 122 Nm3/h oxygen. In practice, an
excess of 5% of the required amounts of oxygen is used.
The temperature in the softening furnace is preferably
maintained in the range of about 590 to 650C and most
preferably in the range of about 625 to 650C. Below about
590C, slag begins to solidify while above 650C, especially
above 670C, the oxygen lance(s) deteriorate(s) rapidly.
The temperature in the softening furnace is controlled by
varying the rate at which bullion is added to the furnace.
When softened bullion is to be cast into anodes for
subsequent electrolytic refining, the desired antimony
content is in the range of about 0.9 to 1.2%. The total
amount of antimony and arsenic should be about 1.5%. In
other cases, the content of impurity metals, such as
antimony arsenic and tin, can be reduced to very low levels
in the softened bullion, such as 0.03% or less. It is noted
that impurity elements more noble than lead will not oxidize
8 133366~
before lead and are not removed by softening. The more
noble impurities will remain in the lead. The residence
time of the bullion in the furnace depends on the time
required to attain the desired degree of softening.
5 The invention will now be illustrated with the following
non-limitative example, wherein the most preferred operation
of the method according to the invention is described.
According to this example, lead bullion is partially
softened in a continuous operation.
10 In steady-state, continuous operation, decopperized lead
bullion containing 1.7% antimony and having an assay as
given in Table I was charged at a temperature of 410C and
at a rate of 16.4 t/h to a bullion kettle having a capacity
of 200 t and containing a charge of partially softened
15 bullion having an antimony content of 1.2%.
Bullion was pumped from the kettle to a softening furnace
having a capacity of 40 t and containing bullion at a
temperature of 625C. Oxygen was continuously supplied at a
rate of 88 Nm3/h to the softening furnace via four lances.
20 The temperature of the bullion in the furnace rose from 625
to 650C due to the heat generated by the softening
reactions. Bullion at 410C was then pumped from the kettle
to the furnace until the temperature in the furnace
decreased to 625C. The fresh bullion displaced partially
25 softened bullion from the furnace which was returned to the
kettle and slag which was tapped and collected for further
processing.
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The furnace temperature was then allowed to rise again to650C, and bullion was again pumped into the furnace from
the kettle. This cycle of pumping, cooling and heating was
continuously repeated. The frequency of this cycle was four
per hour.
While the cyclic softening was in progress, partially
softened bullion was flowed from the bullion kettle into a
second kettle in an intermittent fashion. The second kettle
already contained a charge of partially softened lead.
Flowing softened lead from the bullion kettle in the charge
of softened lead in the second kettle caused homogenization
of the charge of partially softened lead.
Partially softened lead was discharged from the second
kettle for casting into anodes for the subsequent
electrorefining of lead.
The various flows of bullion were sampled at 3 h intervals
and the samples were analyzed. The analysis results and the
mass balance for steady state continuous operation are given
in Tables I and II, respectively.
Table r
Allaly8i8 in 0
Stream N~r~ Pb Sb ~a ~G Ç~ ~i Aa
feed to kettle 39397.25 1.70 0.570.04 0.14 0.10 0.159
feed to furnace 36797.98 1.29 0.32O.OZ 0.10 0.10 0.164
partially softened
Pb to kettle 35798.80 0.76 0.030.02 0.10 0.10 0.168
slag fro~ furnace 1264.00 17.2 9.300.30 0.05 0 0
feed to 2nd kettle
(and to ca~tlng~ 38198.22 1.18 0.290.02 0.09 0.10 0.16
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Table II
Mass t/day
Stream Name Pb Sb A8 Sn Cu Pi Aa
feed to kettle3826.682.24 0.16 0.55 0.39 0.62
feed to furnace 3604.731.17 0.07 0.37 0.37 0.60
partial1y softened
Pb to kettle 3532.710.11 0.07 0.36 0.35 0.60
slag from furnace 8 2.061.12 0.03 0.01 0 0
feed to 2nd kettle
0 (and to casting) 3744.501.10 0.08 0.34 0.38 0.62
any discrepancy in data is caused by small unaccounted
losses.
It can be seen from this example that lead bullion can be
effectively and continuously softened using pure oxygen, and
that the main charge of lead bullion does not have to be
heated to the softening temperature of 650C.
It is understood that modifications can be made in the
embodiments of the method according to the present invention
without departing from the spirit and scope of the appended
claims.
