The materials to be processed are blended with fluxes and recycled furnace products to make suitable charge for treatment, according to chemical analysis. In the preparation charge, in some refineries add fuel is added. Pyrite and coke are added too. The lead, initially in the form of oxides and silicates is needed as collector of the precious metals. In preparing a blend, the following aspects have to be taken in consideration,
- Borax slags must constitute less than 28% since they melt during sintering and inhibit the formation of a porous open frit
- Magnesia must not exceed approximately 2.0-2.5% to prevent formation of viscous slag
- Alumina must be less than 7.5- 9.0% for the same reason
- Lime must be added at minimum, since it may be slake
- Zinc can be added until 4-5%. An excess may result in drossing when the lead bullion is cupelled
- Copper , mostly in the form of matte must not exceed approximately 7-8%
- Arsenic must be about 0.2-0.5% to give a small speiss fall
- Chlorides must be as low as possible to prevent volatility losses
- Metallics can be added in an amount equal to one-third of total lead
- Limestone is required to produce a slag low in lead and with higher melting point than the raw materials
- If chlorides are present, fluxing with sufficient sodium carbonate is required
- A typical charge contents 8-12% Pb, 21-30% Fe, 8-11% S and 20-26% SiO2
When these aspects are considered during the blending process, a charge is made up containing an average of 40-55% raw materials, 20-25% recycled furnace products and 30-40% fluxes. A small percentage of pyrite is added to form matte in the furnace and to collect the copper. The blending equipment is usually a 10-20 ton double cone tumbler blender, fitted with a pneumatic system for operating the two doors, when fully charged, the blender is rotated for 15-20 minutes at 5-10 rpm to mix materials, and the blend is discharged into drums to await treatment.