Primary lead production begins with sintering. Concentrated lead ore is fed into a sintering machine with iron, silica, limestone fluxes, coke, soda ash, pyrite, zinc, caustics or pollution control particulates. The mixture is blasted with hot air to burn off the sulfur and sent to the smelter.
Lead is usually smelted in a blast furnace using the carbon from the sintering machine to provide the heat source. As melting occurs, several layers form in the furnace. The molten lead layer sinks to the bottom of the furnace. A layer of the lightest elements, including arsenic and antimony, floats to the top and is referred to as the "speiss." A "matte" layer also forms from the copper and metal sulfides. Finally, a layer of blast furnace slag, which contains mostly silicates, also forms. The speiss and the matte are usually sold to copper smelters where they are refined for copper processing. The slag is stored and partially recycled, if the metal content is sufficient.
The lead from the blast furnace, called lead bullion, then undergoes the drossing process. The bullion is agitated in kettles then cooled to 700-800 degrees. This process results in molten lead and dross. Dross refers to the lead oxides, copper, antimony and other elements that float to the top of the lead. Dross is usually skimmed off and sent to a dross furnace to recover the non-lead components which are sold to other metal manufactures.
Finally, the molten lead is refined. Pyrometallurgical methods are usually used to remove the remaining non-lead components of the mixture. The non-lead metals are usually sold to other metal processing plants. The refined lead may be made into alloys or directly cast.
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