Lead and Zinc Smelting

Pollution Prevention and Abatement Handbook WORLD BANK GROUP Effective July 1998 Lead and Zinc Smelting Industry Description and Practices blast furnace slag (primarily silicates), and lead bullion (98% by weight). All layers are then Lead and zinc can be produced pyrometal- drained off. The speiss and matte are sold to cop- lurgically or hydrometallurgically, depending on per smelters for recovery of copper and precious the type of ore used as a charge. In the pyromet- metals. The blast furnace slag, which contains allurgical process, ore concentrate containing zinc, iron, silica, and lime, is stored in piles and lead, zinc, or both is fed, in some cases after sin- is partially recycled. Sulfur oxide emissions are tering, into a primary smelter. Lead concentra- generated in blast furnaces from small quanti- tions can be 50–70%, and the sulfur content of ties of residual lead sulfide and lead sulfates in sulfidic ores is in the range of 15–20%. Zinc con- the sinter feed. centration is in the range of 40–60%, with sulfur Rough lead bullion from the blast furnace usu- content in sulfidic ores in the range of 26–34%. ally requires preliminary treatment in kettles be- Ores with a mixture of lead and zinc concentrate fore undergoing refining operations. During usually have lower respective metal concentra- drossing, the bullion is agitated in a drossing tions. During sintering, a blast of hot air or oxy- kettle and cooled to just above its freezing point, gen is used to oxidize the sulfur present in the 370°–425°C (700°–800°F). A dross composed of feed to sulfur dioxide (SO2). Blast furnaces are lead oxide, along with copper, antimony, and used in conventional processes for reduction and other elements, floats to the top and solidifies refining of lead compounds to produce lead. above the molten lead. The dross is removed and Modern direct smelting processes include QSL, is fed into a dross furnace for recovery of the Kivcet, AUSMELT, and TBRC. nonlead mineral values. The lead bullion is refined using pyrometal- Primary Lead Processing lurgical methods to remove any remaining nonlead materials (e.g., gold, silver, bismuth, The conventional pyrometallurgical primary lead zinc, and metal oxides such as oxides of anti- production process consists of four steps: sinter- mony, arsenic, tin, and copper). The lead is re- ing, smelting, drossing, and refining. A feedstock fined in a cast-iron kettle in five stages. First, made up mainly of lead concentrate is fed into a antimony, tin, and arsenic are removed. Next, sintering machine. Other raw materials may be gold and silver are removed by adding zinc. The added, including iron, silica, limestone flux, coke, lead is then refined by vacuum removal of zinc. soda, ash, pyrite, zinc, caustic, and particulates Refining continues with the addition of calcium gathered from pollution control devices. The sin- and magnesium, which combine with bismuth tering feed, along with coke, is fed into a blast to form an insoluble compound that is skimmed furnace for reducing, where the carbon also acts from the kettle. In the final step, caustic soda, as a fuel and smelts the lead-containing materi- nitrates, or both may be added to remove any als. The molten lead flows to the bottom of the remaining traces of metal impurities. The refined furnace, where four layers form: “speiss” (the lead will have a purity of 99.90–99.99%. It may lightest material, basically arsenic and antimony), be mixed with other metals to form alloys, or it “matte” (copper sulfide and other metal sulfides), may be directly cast into shapes. 332 Lead and Zinc Smelting 333 Secondary Lead Processing sulfuric acid to extract the lead/zinc. These processes can operate at atmospheric pressure or as The secondary production of lead begins with the pressure leach circuits. Lead/zinc is recovered recovery of old scrap from worn-out, damaged, from solution by electrowinning, a process simi- or obsolete products and with new scrap. The lar to electrolytic refining. The process most com- chief source of old scrap is lead-acid batteries; monly used for low-grade deposits is heap other sources include cable coverings, pipe, sheet, leaching. Imperial smelting is also used for zinc and other lead-bearing metals. Solder, a tin-based ores. alloy, may be recovered from the processing of circuit boards for use as lead charge. Waste Characteristics Prior to smelting, batteries are usually broken up and sorted into their constituent products. The principal air pollutants emitted from the pro- Fractions of cleaned plastic (such as polypropy- cesses are particulate matter and sulfur dioxide lene) case are recycled into battery cases or other (SO2). Fugitive emissions occur at furnace open- products. The dilute sulfuric acid is either neu- ings and from launders, casting molds, and ladles tralized for disposal or recycled to the local acid carrying molten materials, which release sulfur market. One of the three main smelting processes dioxide and volatile substances into the work- is then used to reduce the lead fractions and pro- ing environment. Additional fugitive particulate duce lead bullion. emissions occur from materials handling and Most domestic battery scrap is processed in transport of ores and concentrates. Some vapors blast furnaces, rotary furnaces, or reverberatory are produced in hydrometallurgy and in various furnaces. A reverberatory furnace is more suit- refining processes. The principal constituents of able for processing fine particles and may be op- the particulate matter are lead/zinc and iron ox- erated in conjunction with a blast furnace. ides, but oxides of metals such as arsenic, anti- Blast furnaces produce hard lead from charges mony, cadmium, copper, and mercury are also containing siliceous slag from previous runs present, along with metallic sulfates. Dust from (about 4.5% of the charge), scrap iron (about raw materials handling contains metals, mainly 4.5%), limestone (about 3%), and coke (about in sulfidic form, although chlorides, fluorides, 5.5%). The remaining 82.5% of the charge is made and metals in other chemical forms may be up of oxides, pot furnace refining drosses, and present. Off-gases contain fine dust particles and reverberatory slag. The proportions of rerun slags, volatile impurities such as arsenic, fluorine, and limestone, and coke vary but can run as high as mercury. Air emissions for processes with few 8% for slags, 10% for limestone, and 8% for coke. controls may be of the order of 30 kilograms lead The processing capacity of the blast furnace or zinc per metric ton (kg/t) of lead or zinc pro- ranges from 20 to 80 metric tons per day (tpd). duced. The presence of metals in vapor form is Newer secondary recovery plants use lead dependent on temperature. Leaching processes paste desulfurization to reduce sulfur dioxide will generate acid vapors, while refining pro- emissions and generation of waste sludge dur- cesses result in products of incomplete combus- ing smelting. Battery paste containing lead sul- tion (PICs). Emissions of arsine, chlorine, and fate and lead oxide is desulfurized with soda ash, hydrogen chloride vapors and acid mists are as- yielding market-grade sodium sulfate as a by- sociated with electrorefining. product. The desulfurized paste is processed in Wastewaters are generated by wet air scrub- a reverberatory furnace, and the lead carbonate bers and cooling water. Scrubber effluents may product may then be treated in a short rotary contain lead/zinc, arsenic, and other metals. In furnace. The battery grids and posts are pro- the electrolytic refining process, by-products such cessed separately in a rotary smelter. as gold and silver are collected as slimes and are subsequently recovered. Sources of waste- Zinc Manufacturing water include spent electrolytic baths, slimes recovery, spent acid from hydrometallurgy pro- In the most common hydrometallurgical process cesses, cooling water, air scrubbers, washdowns, for zinc manufacturing, the ore is leached with and stormwater. Pollutants include dissolved 334 PROJECT GUIDELINES: INDUSTRY SECTOR GUIDELINES and suspended solids, metals, and oil and • Recover acid, plastics, and other materials grease. when handling battery scrap in secondary lead The larger proportion of the solid waste is dis- production. carded slag from the smelter. Discard slag may • Recycle condensates, rainwater, and excess contain 0.5–0.7% lead/zinc and is frequently used process water for washing, for dust control, for as fill or for sandblasting. Slags with higher lead/ gas scrubbing, and for other process applica- zinc) content—say, 15% zinc—can be sent for tions where water quality is not of particular metals recovery. Leaching processes produce resi- concern. dues, while effluent treatment results in sludges • Give preference to natural gas over heavy fuel that require appropriate disposal. The smelting oil for use as fuel and to coke with lower sul- process typically produces less than 3 tons of fur content. solid waste per ton of lead/zinc produced. • Use low-NOx burners. • Use suspension or fluidized bed roasters, Pollution Prevention and Control where appropriate, to achieve high SO2 concentrations when roasting zinc sulfides. The most effective pollution prevention option • Recover and reuse iron-bearing residues from is to choose a process that entails lower energy zinc production for use in the steel or construc- usage and lower emissions. Modern flash-smelt- tion industries. ing processes save energy, compared with the • Give preference to fabric filters over wet scrub- conventional sintering and blast furnace process. bers or wet electrostatic precipitators (ESPs) Process gas streams containing over 5% sulfur for dust control. dioxide are usually used to manufacture sulfu- Good housekeeping practices are key to mini- ric acid. The smelting furnace will generate gas mizing losses and preventing fugitive emissions. streams with SO2 concentrations ranging from Losses and emissions are minimized by enclosed 0.5% to 10%, depending on the method used.

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