SECONDARY LEAD PRODUCTION

Introduction

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.

Secondary Smelting

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 furnace.

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.