6 Lead Processing
The US is the third largest producer of primary lead, with most coming from Missouri. The primary end users of lead are batteries and ammunition. Consumers of lead include TV glass, computer glass, construction (including radiation shields), and protective coatings.
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.
Primary lead production produces air emissions, process wastes and solid wastes. Air emissions consist primarily of sulfur oxides and particulates. Sintering plant air emissions include sulfur and particulates. These emissions are usually burned in the blast furnace and eliminated. Particulate emissions from blast furnaces include lead oxides, quartz, limestone, iron pyrites, iron-limestone-silicate slag, arsenic and other metallic compounds. The exact type and amount of particulates depends largely on the input material. Emissions control equipment, usually a baghouse, is most often used to control particulates.
Blast furnaces also produce slag. The slag consists primarily of iron and silicon oxides, aluminum and calcium oxides, and other metals, depending on inputs. The slag can either be reused in the process to capture more metals or disposed of on site. If the slag is not reused, it may be treated to recover the metals for sale.
Liquid wastes from primary lead production include wastewater and slurries. Acid plant blowout from sulfuric acid production plants, slag granulation water from slag disposal, and plant wash down water from housekeeping are the primary types of liquid wastes. The water is considered RCRA K065 hazardous waste due to the lead content.
Most of the lead produced comes from secondary sources. Lead scrap includes lead acetate batteries, cable coverings, pipes, sheets and lead coated, or terne bearing, metals. Solder, product waste and dross may also be recovered for its small lead content. Most secondary lead is used in batteries.
To recover lead from a battery, the battery is broken and the components are classified. The lead containing components are processed in blast furnaces for hard lead or rotary reverberatory furnaces for fine particles. The blast furnace is similar in structure to a cupola furnace used in iron foundries. The furnace is charged with slag, scrap iron, limestone, coke, oxides, dross, and reverberatory slag. The coke is used to melt and reduce the lead. Limestone reacts with impurities and floats to the top. This process also keeps the lead from oxidizing. The molten lead flows from the blast furnace into holding pots. Lead may be mixed with alloys, including antimony, tin, arsenic, copper and nickel. It is then cast into ingots.
Air emissions and solid waste are the primary pollution concerns for secondary lead producers. Reverberatory or blast furnaces produce most of the lead waste. Products can include sulfur oxides, nitrogen oxides, antimony, arsenic, copper, or tin, depending on inputs. Lead paste desulfurization is a process used by new lead processing plants to reduce the sulfur dioxide emissions and waste sludge. The air emissions in older plants are usually controlled with settling and cooling chambers and with baghouse emissions control equipment.
The battery breaking process contributes sulfuric acid, dust, battery cases and lead compounds. The solid emissions include emissions control dust and slag from smelting. They are considered K069 hazardous wastes.
USEPA, Profile of The Nonferrous Metals Industry. EPA 310-R-95-010. This document discusses uses, processes and pollution prevention opportunities associated with lead production.