The pyrometallurgical Lead refining process is one of the most important procedures of lead battery recycling. This process is also known as “Softening of Lead” because the metal is softened after the removal of some impurities which are present in the raw lead coming from lead battery recycling. It would be more correct to call only the process of antimony, arsenic and tin removing “softening”. The full process is carried out by a series of operations at different temperatures using chemical agents.
The most important impurities, normally present after the lead battery recycling, are removed in the following order: copper, tin, arsenic, antimony and bismuth.
Silver removing needs a special process called Parkes process, which is not very common in lead battery recycling. At the beginning of the lead refining process the bullion is put into lead refining kettles where the lead is heated and continuously stirred. These kettles can normally contain between 60 and 120 tons of lead, and are heated using fuel oil, gas or in some cases electricity.
- Copper (Cu): If the bullion has a content of copper above 0.2% copper removing is suggested to be done in two steps: in the first step a limit of solubility of copper in lead is used to separate copper. As soon as the bullion cools down to 480°C, a solid alloy of lead and copper separates and floats to the surface, forming what is called “dross”. In this way great part of the copper is removed. During this operation fine pieces of wood are added to the molten metal in order to protect lead from oxidation in the surface. Then the temperature of the bullion is cooled down to 340-350°C. At this temperature the residual copper is removed as dross. Since at this temperature the viscosity of molten lead is high, a lot of lead is removed together with the dross. This secondary dross must be recycled in the next batch. At the end of this operation the content of copper in molten lead should be about 0,1%. In the second step, In order to remove last amounts of copper, fine milled wood and elemental sulphur (S) in powder is added to the molten lead at a temperature of ca. 400°C. Copper is removed as copper sulphide (Cu2S or CuS) which floats to the surface leaving the bullion free of copper (less than 0.005%). The residual from this operation, containing Lead and Copper sulfide is called Matte. Normally matte contains 95% of lead, 3% of Cu and 2% of S. During the copper removing with sulphur, also nickel (Ni) and Cobalt (Co), if present, tend to follow the copper.
- Tin (Sn): For the removal of tin (Sn) an old process it is still used in some places. In this process the molten lead is heated up to ca. 600°C. At this temperature ammonium chloride is added. This operation of the lead refining process must absolutely be carried out in a kettle equipped with an extremely efficient aspiration and vapour treatment system, because the effluent vapour contains metal-chloride compounds which are dangerous for human health and the environment. STC suggests the Harris Process for Tin, arsenic and antimony removing. In the Harris process copper is pumped into a reverberatory furnace at a temperature of 700°C. Air enriched in oxygen is blown through a pipe in the bottom of the kettle. Tin is the first to be oxided as lead stannate, PbSnO3. When the tin concentration becomes low, arsenic and some antimony are oxidized. The Harris process uses also some reagents as sodium hydroxide and sodium nitrate.
Other elements can be removed during the lead refining process by adding other metals to the molten lead. For the removal of silver (Ag) through the Parkes process, for example, zinc is added at high temperatures. Silver has a high affinity for zinc forming intermetallic compounds. The zinc removal process is carried out by cooling the molten lead down to a temperature lower than the zinc fusion temperature. At this temperature metallic zinc floats to the top and can be easily skimmed off. For the recovery of silver from zinc, vacuum distillation is applied.
This process is not normally used in lead battery recycling but is more important in primary lead refining.