Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR THE REFINING OF LEAD
This invention relates to the refining of lead and,
particularly, to a method for the refining of lead for
reducing the amount of dross formed on remelting.
BACKGROUND OF THE INVENTION
Numerous methods are known for the refining of lead bullion
that is made by the various smelting processes for the
recovery of lead from ores and concentrates. The refining
is necessary for the removal of impurities which include
antimony, arsenic, bismuth, copper, iron, selenium, silver,
tellurium, tin, zinc, gold and other metals, and which
impurities may be present in varying amounts. A great
number of refining processes are known. These processes
include electrorefining processes, and processes wherein
metals and alloys are used. In the electrorefining
process, the lead bullion electrodes contain certain
amounts of arsenic and antimony that are necessary for
preventing slime from falling off the electrodes. Although
electrorefining removes most of the impurities from the
lead, and the electrorefined lead meets the specifications
for purity, the small amount of impurities remaining,
usually less than 0.1% total, causes undesirable formation
of dross when the electrorefined lead is remelted.
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BRIEF DESCRIPTION OF PRIOR ART
The refining of lead by treating molten lead with metals or
alloys is well documented. According to most of these
refining treatments, molten lead is contacted with suitable
metals or alloys for the formation of dross of alloys or
compounds of the added metals or alloys with the impurities
in the lead. The dross is separated, and refined lead is
recovered. According to the numerous patents on lead
refining, the metals and alloys used in refining include
calcium, barium, magnesium, zinc, sodium, lithium,
potassium, aluminum, aluminum-tin alloy, aluminum-zinc
alloy, calcium-lead alloy, and calcium and magnesium.
Residuals of the added metals and alloys are often removed
by adding a suitable drossing agent such as ammonium
chloride, sodium or potassium hydroxide, phosphorus,
sulfur, coal, a halide, or air.
The oxidation of molten lead affects its refining and the
degree of drossing. It has been determined that certain
impurities in or additives to stationary or moving melts of
lead accelerate or reduce the rate of oxidation, while some
have little effect. Lithium, sodium, calcium, magnesium,
arsenic, antimony, copper, silver and tellurium increase
the rate, aluminum and tin reduce the rate, and bismuth,
cadmium and zinc have little effect on the rate tLead
Handbook, ed. Heubner etal., Metallgesellschaft AG,
_ 3 1 337579
Frankfurt, 1983). The reduction of the oxidation of lead
with aluminum has been investigated by Krysko (Krysko W.W.,
Oxidation of molten lead, Third International Conference of
Lead, Venice, September 1968). According to Canadian
Patent 1 204 596, deoxidation of molten metal (steel) is
accomplished with a refining agent essentially consisting
of calcium and at least one of aluminum and silicon, and a
flux composed mainly of calcium and aluminum oxides, the
alloy and the flux being integrally bonded to one another.
It is disclosed that an aluminum-silicon alloy may be used
in the calcium alloy. The use of the aluminum-silicon
eutectic alloy and its oxidation inhibiting effect in the
manufacture of lead alloys for storage batteries are known
from German Patent DE 27 46 713 Al.
SUMMARY OF THE INVENTION
It has now been found that molten lead may be refined by a
treatment with refining agents consisting essentially of
calcium metal and aluminum-silicon eutectic alloy. More
specifically, we have found that when molten lead is
treated with calcium metal and aluminum-silicon eutectic
alloy and the resulting melt is treated with sodium
compounds, a dross is formed, and a refined lead is
recovered that contains less than 0.0001% of combined
calcium, aluminum and silicon (<lppm), and that reduces the
amount of dross formed upon remelting.
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It is an object of the present invention to provide a
method for the refining of lead. It is another object to
provide a method for reducing the amount of dross formed
upon remelting of lead. These and other objects of the
invention will become clear from the following detailed
description.
Accordingly, there is provided a method for the refining of
lead that forms dross upon melting which comprises the
steps of forming a bath of molten lead; heating said bath
to a temperature in the range of about 585 to 620C; adding
to the heated bath effective amounts of refining agents
consisting essentially of calcium metal and aluminum-
silicon eutectic alloy, said effective amounts being
sufficient to effectively reduce the amount of dross formed
upon remelting of the solidified lead recovered from said
method; when said refining agents are substantially
completely dissolved, allowing said bath to cool to a
temperature in the range of about 375 to 475C; adding
suitable amounts of sodium hydroxide and sodium nitrate to
said bath for the substantial removal of said refining
agents in a dross with the formation of refined lead and a
layer of dross on the bath; skimming said dross from the
bath; and recovering solidified refined lead that forms a
reduced amount of dross upon remelting.
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DETAILED DESCRIPTION
Lead that forms dross upon melting is added in molten form
to a suitable vessel to form a bath of molten lead therein.
The lead that forms dross upon melting may be, for example,
lead that has been refined by the Betts process or other
refining processes. Other lead that is subject to excess
dross formation upon melting may also be treated by the
method according to the invention. Any dross on the bath
of molten lead in the vessel is removed, and the bath is
heated to a temperature that is somewhat above the melting
temperature of the to-be-added aluminum-silicon eutectic
alloy. A convenient temperature is in the range of about
585 to 620C. The preferred temperature is about 600C,
allowing rapid dissoluton of refining agents. Effective
amounts of refining agents, consisting essentially of
calcium metal and aluminum-silicon eutectic alloy, are
added to the heated bath, and are submerged therein until
molten. The bath is agitated to ensure rapid and complete
dissolution of the refining agents. The refining agents
are preferably loaded into a perforated basket that is
subsequently submerged in the bath. Pieces, chunks, bars
or other suitable forms of calcium metal and aluminum-
silicon eutectic alloy are loaded in the basket such that
the alloy is substantially surrounded by calcium. When the
?5 loaded basket is submerged in the bath of molten lead, the
strongly exothermic reaction of calcium with lead causes
rapid melting of the calcium and the eutectic alloy. The
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eutectic alloy contains about 12.0 to 12.5% silicon, and
melts at about 575 to 580C. The required amounts of
refining agents are small, but should be effective in
reducing the amounts of dross formed upon remelting of the
solidified lead recovered according to the method of the
invention. Effective amounts are, for example, an amount
of calcium in the range of about 0.03 to 0.09% and an
amount of aluminum-silicon eutectic alloy in the range of
about 0.01 to 0.04% by weight of the amount of lead being
treated in the bath of molten lead. The preferred amounts
are about 0.06% calcium and about 0.03% aluminum-silicon
eutectic alloy. When the refining agents are substantially
completely dissolved the basket is removed from the vessel,
and the bath is allowed to cool to a temperature in the
range of about 375 to 475C. Cooling to a temperature of
about 450C is preferred, as at this temperature refined
lead may be cast conveniently. The bath is then treated
for the substantial removal of the refining agents from the
refined lead in a dross. Suitable amounts of sodium
hydroxide and sodium nitrate are added to the bath while
the bath is being agitated with the formation of refined
lead and the formation of a layer of dross on the surface
of the bath. Additions of sodium hydroxide and sodium
nitrate are continued until a skilled operator judges by
visual observations that the refined lead has the right
appearance. Agitation is then stopped, the agitator is
removed from the bath, and the dross is skimmed from the
surface of the bath. The refined lead is subsequently
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poured into moulds and solidified. The solidified refined
lead is recovered. Upon remelting, the recovered
solidified lead forms an amount of dross that is reduced,
and is considerably less than the amount formed on remelted
lead not treated according to the method of the present
invent lon .
The invention will now be illustrated by means of the
following non-limitative example.
Example
177 metric tonnes of lead from the Betts electrorefining
process were melted and added to a kettle. Dross was
skimmed, and the lead was heated to 600C. 113 kg of
calcium metal and 45 kg of aluminum-silicon eutectic alloy
were loaded as refining agents in a wire mesh basket, alloy
being substantially surrounded by calcium, and the loaded
basket was submerged in the bath. The bath was agitated,
drawing a slight vortex. Dissolution was completed in ten
minutes. The basket was withdrawn from the bath, and the
bath was allowed to cool to 450C. Portions of sodium
hydroxide and sodium nitrate were then added to the bath,
while the bath was being agitated. Additions and agitation
were continued until the operator judged visually that the
refined lead had the right appearance, the refining agents
at that time having been substantially removed from the
refined lead into a dross. The dross was skimmed, and
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refined lead was cast into moulds and solidifed. The
solidified lead was sampled and found to contain less than
one part per million of combined calcium, aluminum and
silicon. 32 metric tonnes of the solidified lead were
remelted and the dross formed on the surface of the
remelted lead was removed and weighed. The amount of dross
was 0.2% of the weight of the refined lead. In a parallel
test, it was determined that lead recovered from the Betts
electrorefining process and remelted produced 0.4% of its
weight in dross. The results show that the method
according to the invention produces a refined lead that
upon melting forms half the amount of dross of lead not so
refined, and that the treatment with sodium compounds
effectively removes the refining agents from the refined
lead.
It is understood that variations may be made in the method
according to the invention without departing from the scope
and purview of the appended claims.
