SECONDARY LEAD PRODUCTION
Almost 50% of the 1.6 million tonnes of lead produced in Europe each
year has been recycled and is known as secondary lead. In the United
Kingdom, the figure is nearer 60%. Much of the secondary lead comes
from lead batteries with the remainder coming from other sources
such as lead pipe and sheet.
Lead scrap from pipes and sheet is "clean" and can be melted and
refined without the need for a smelting stage.
With batteries, the lead can only be obtained by breaking open the
case. This is commonly done using a battery breaking machine which,
in addition to crushing the case, separates out the different
components of the battery and collects them in hoppers. Thus, the
pastes (oxide and sulphate), grids, separators and fragmented cases
are all separated from one another. The battery acid is drained,
neutralised and disposed of carefully.
The case material is separated by the battery breaker into hard
rubber and polypropylene fractions, the latter being the more common
today. The hard rubber fraction is either washed and discarded or
can be utilised as a reductant in the smelting process. The
polypropylene is subjected to a cleaning and reprocessing operation
to make a good quality recycled material. In view of the diverse
range of colours found in battery case materials, the polypropylene
is normally reprocessed to black or other dark shades. Typical
applications for the reprocessed plastic are new battery cases,
water tanks, video cassette boxes or flower pots.
The sorted materials are collected in bays to await make-up of the
feed to the smelting furnace.
The workhorse of the secondary industry used to be the blast
furnace. However in Europe this has largely gone out of favour
because of the high price of metallurgical coke and the relative
difficulty of preventing the escape of dust and fume. The blast
furnace was used to provide a low grade antimonial lead, which would
be softened - either in a refining kettle or a reverberatory
furnace. The high antimony slags would be accumulated for a
subsequent blast furnace campaign to produce a high antimony bullion
for blending into alloys of the desired composition.
Most companies now use rotary furnaces which can be either oil or
gas fired. The charge can either be tailored to give a lead of
approximately the desired composition or a two stage smelting
procedure can be employed which can yield crude soft lead and crude
antimonial lead. In stage one, the furnace conditions are held
oxidising for antimony but neutral to lead, thus forming antimony
oxides which are insoluble in molten lead. In the second stage,
conditions reducing to both lead and antimony are used which reduces
any metallic oxides to the metal and liberates carbon monoxide and
carbon dioxide. For example:
Stage One Battery plates are charged using little or no reducing
agent and crude soft lead is tapped off after a few hours with the
antimonial slag and lead oxide and sulphate being retained in the
furnace. Further plates are charged and more soft lead withdrawn
until sufficient slag has accumulated for the slag reduction stage.
Stage Two Coke or anthracite fines and soda ash are now charged,
both lead and antimony oxides and lead sulphate are reduced and the
cycle ends with the furnace being emptied of antimonial lead and of
slag for discarding.
Some companies make use of hard rubber battery cases as co-reductants
because of their high carbon content and because of the high cost of
metallurgical coke. Iron may be added to the charge in moderate
amounts to matte any sulphides produced from the reduction of
sulphates and to prevent any sulphurous fumes from leaving the
As with primary smelting, large volumes of gas are produced,
carrying with them substantial quantities of dust. On leaving the
smelter, the gas is cooled from about 900oC to about 100oC using air
and/or water cooling. The gases pass into a baghouse which contains
hundreds of woven cloth bags. The gases pass through the bags and
the dust remains on the surface. Periodically, there is a negative
back pressure and the flow to a particular bag is cut. The dust-cake
cracks and the dust tails to the bottom of the bag chamber. It is
collected, agglomerated and fed back into the smelter. The gases
pass out of the stack and into the atmosphere, dust free. In the
course of processing one tonne of lead, as much as 100 tonnes of air
have to be cleaned in this way.
The lsasmelt process
The lsasmelt furnace for secondary lead production works on a
semi-continuous basis with a total cycle time of about 40 hours. The
furnace is fed with lead carbonate paste containing 1% sulphur. This
is obtained as a result of the battery paste having gone through a
desulphurising process after battery breaking.
At the start of the cycle the empty furnace is charged and the paste
melted by the introduction of the lance to form a liquid bath. Over
the next 36 hours wet paste and coal as a reluctant are continuously
fed to the furnace. The soft lead produced is tapped every 3 hours
and has a composition of 99.9% lead. During this part of the process
the lead content of the slag falls as the impurities accumulate,
consequently the lance maintains the slag fluidity by raising the
temperature from 900OC to 1000OC at the end. At the end of 36 hours
the paste feed is stopped and the slag reduction step commences, to
produce antimonial lead alloy. This takes approximately two hours
during which fluxes are added and the furnace temperature is raised
to 1200OC. On completion of reduction the lance is withdrawn and the
metal bath settled, followed by tapping of the antimonial lead alloy
(Pb + Sb >98.5%) and the discard slag (Pb <0.5%).
Off gases from the furnace are first cooled and then passed to a
baghouse for fume and dust control. The collected dust is recycled
to the furnace feed as a slurry. The battery grid metal is melted
separately in a grid melting facility.
The Isasmelt process is claimed to address environmental concerns
and give reduced operating costs. Its main advantages over a
traditional rotary furnace operation are stated to be:
high thermal efficiency and low operating cost;
the elimination of soda fluxes;
direct production of both soft lead and antimonial lead alloy
giving blending flexibility;
ability to produce low lead discard slags facilitating ease of
environmentally acceptable disposal;
good process hygiene due to semi-continuous nature of process.
Secondary lead refining
Once smelting is complete, the molten lead is removed from the
smelting furnace and can be cast into large blocks (called pigs)
weighing 1.5 to 2.5 tonnes. These are transferred to the refining
kettles which are top-access pots sunk into the refinery floor.
Alternatively, in more modern plants, the molten lead is pumped
directly from the smelting furnace to the refinery pots thus saving
on time and energy in remelting.
The principal impurities which are removed in secondary lead
refining are copper, tin, antimony and arsenic. Copper can be
removed in a similar fashion to that outlined for primary lead. Some
companies use iron pyrites and sulphur which works at a higher
temperature and can also remove any nickel present. The other
elements are removed by a modified Harris process. Bismuth and
silver levels tend to be slightly higher than in primary lead but
are rarely removed.
When scrap lead is supplied in clean metallic form e.g. sheet or
pipe, this may be remelted in the refinery kettle without having to
undergo smelting. The common impurities in scrap lead are copper,
tin and antimony, whilst occasionally zinc, iron or arsenic may be
present. The material is often contaminated with dirt and other
materials. These impurities are removed using the same basic
techniques as have been previously described.