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Home , Flash smelting, Leaching (metallurgy), Lead, Ore concentrate, Refining (metallurgy), Roasting (metallurgy)

Prevention and Abatement Handbook WORLD BANK Effective July 1998

Lead and

Industry Description and Practices (primarily silicates), and 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 and precious the type of used as a charge. In the pyromet- . The blast furnace slag, which contains allurgical process, containing zinc, , silica, and , is stored in piles and lead, zinc, or both is fed, in some cases after sin- is partially recycled. 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 and lead sulfates in sulfidic 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 operations. During usually have lower respective concentra- drossing, the bullion is agitated in a drossing tions. During sintering, a blast of hot air or oxy- 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 composed of feed to (SO2). Blast furnaces are lead oxide, along with copper, , and used in conventional processes for reduction and other elements, floats to the top and solidifies refining of 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 values. The lead bullion is refined using pyrometal- Primary Lead Processing lurgical methods to remove any remaining nonlead materials (e.g., , , , The conventional pyrometallurgical primary lead zinc, and metal such as oxides of anti- production process consists of four steps: sinter- mony, , , 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, , , lead is then refined by vacuum removal of zinc. soda, ash, , zinc, caustic, and Refining continues with the addition of gathered from pollution control devices. The sin- and , 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 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 ), may be directly cast into shapes.

332 Lead and 333

Secondary Lead Processing sulfuric to extract the lead/zinc. These pro- cesses 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 from worn-out, damaged, from solution by , 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, , sheet, . Imperial smelting is also used for zinc and other lead- metals. , a tin-based ores. , may be recovered from the processing of circuit boards for use as lead charge. Characteristics Prior to smelting, batteries are usually broken up and sorted into their constituent products. The principal air 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 is either neu- ings and from launders, 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 of ores and concentrates. Some vapors blast furnaces, rotary furnaces, or reverberatory are produced in and in various furnaces. A 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, , copper, and 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 , and present. Off-gases contain fine dust particles and reverberatory slag. The proportions of rerun , volatile impurities such as arsenic, , 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, , and fate and lead oxide is desulfurized with soda ash, chloride vapors and acid mists are as- yielding market-grade 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 bers and cooling . 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 , metals, and oil and • Recover acid, plastics, and other materials grease. when handling battery scrap in secondary lead The larger proportion of the 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 is not of particular metals recovery. Leaching processes produce resi- concern. dues, while effluent treatment results in sludges • Give preference to 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, and Control where appropriate, to achieve high SO2 con- centrations when zinc sulfides. The most effective pollution prevention option • Recover and iron-bearing residues from is to choose a process that entails lower energy zinc production for use in the 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. It buildings, covered conveyors and transfer points, is important, therefore, to select a process that and dust collection equipment. Yards should be uses -enriched air or pure oxygen. The aim paved and runoff water routed to settling . is to save energy and raise the SO2 content of the process gas stream by reducing the total volume Pollution Reduction Targets of the stream, thus permitting efficient fixation of sulfur dioxide. Processes should be operated Implementation of processes to maximize the concentration of the sulfur di- and pollution prevention measures can yield both oxide An added benefit is the reduction (or elimi- . economic and environmental benefits. The fol- nation) of oxides (NO ). x lowing production-related targets can be • Use doghouse enclosures where appropriate; achieved by measures such as those described use hoods to collect fugitive emissions. above. The figures relate to the production pro- • Mix strong acidic gases with weak ones to fa- cesses before the addition of pollution control cilitate production of sulfuric acid from sulfur measures. oxides, thereby avoiding the release of weak The target load for lead and zinc acidic gases. smelting operations for particulate matter is 0.5 • Maximize the recovery of sulfur by operating kg/t of concentrated ore processed. ESPs are used

the furnaces to increase the SO2 content of the to recover dust. Pollutant load factors for lead in flue gas and by providing efficient sulfur con- air emissions are 0.08 kg/t from roasting, 0.08 kg/ version. Use a double-contact, double-absorp- t from smelting, and 0.13 kg/t from refining. tion process. A double-contact, double-absorption plant • Desulfurize paste with caustic soda or soda ash should emit no more than 2 kg of sulfur dioxide

to reduce SO2 emissions. per ton of sulfuric acid produced, based on a con- • Use energy-efficient measures such as waste version efficiency of 99.7%. Sulfur dioxide should heat recovery from process gases to reduce fuel be recovered to produce sulfuric acid, thus yield-

usage and associated emissions. ing a marketable product and reducing SO2 emis- Lead and Zinc Smelting 335 sions. Fugitive emissions are controlled by using ceptable. All of the maximum levels should be enclosed conveyors. achieved for at least 95% of the time that the plant or unit is operating, to be calculated as a propor- Treatment tion of annual operating hours.

ESPs and baghouses are used for product recov- Air Emissions ery and for the control of particulate emissions. Dust that is captured but not recycled will need The air emissions levels presented in Table 1 to be disposed of in a secure or in an- should be achieved. other acceptable manner. The environmental assessment should address or pentoxide is in vapor form the buildup of from particulate fall- because of the high gas temperatures and must out in the vicinity of the plant over its projected life. be condensed by gas cooling so that it can be re- moved in fabric filters. Liquid Effluents Collection and treatment of vent gases by al- kali scrubbing may be required when sulfur di- The effluent emissions levels presented in Table oxide is not being recovered in an acid plant. 2 should be achieved. Effluent treatment of process bleed streams, filter backwash , boiler blowdown, and Table 1. Emissions from Lead/Zinc Smelting other streams is required to reduce suspended (milligrams per normal cubic meter) and dissolved solids and heavy metals and to adjust pH. Residues that result from treatment Parameter Maximum value are recycled to other industries such as the con- Sulfur dioxide 400 struction industry, sent to settling ponds (pro- Arsenic 0.1 vided that groundwater and surface water Cadmium 0.05 contamination is not a concern), or disposed of Copper 0.5 in a secure landfill. Lead 0.5 Slag should be either landfilled or granulated Mercury 0.05 and sold for use in building materials. Zinc 1.0 Particulates 20 Emissions Guidelines

Emissions levels for the design and operation of Table 2. Effluents from Lead/Zinc Smelting each project must be established through the envi- (milligrams per liter, except for pH and temperature) ronmental assessment (EA) process on the basis of Parameter Maximum value country legislation and the Pollution Prevention and Abatement Handbook, as applied to local conditions. pH 6–9 The emissions levels selected must be justified in TSS 20 Arsenic 0.1 the EA and acceptable to the World Bank Group. Cadmium 0.1 The guidelines given below present emissions Copper 0.5 levels normally acceptable to the World Bank Iron 3.5 Group in making decisions regarding provision Lead 0.1 of World Bank Group assistance. Any deviations Mercury 0.01 from these levels must be described in the World Zinc 2.0 a Bank Group project documentation. The emis- Total metals 5 Temperature increase ≤ 3oCb sions levels given here can be consistently achieved by -designed, well-operated, and a. Includes arsenic, , cadmium , gold, lead, well-maintained pollution control systems. mercury, , , silver, , and . The guidelines are expressed as concentrations b. The effluent should result in a temperature increase of no more than 3° C at the edge of the zone where initial mixing and to facilitate . Dilution of air emissions dilution take place. Where the zone is not defined, use 100 or effluents to achieve these guidelines is unac- meters from the point of discharge. 336 PROJECT GUIDELINES: INDUSTRY SECTOR GUIDELINES

Ambient Noise the operating standards so that any necessary corrective actions can be taken. Records of moni- Noise abatement measures should achieve either toring results should be kept in an acceptable the levels given below or a maximum increase in format. The results should be reported to the background levels of 3 decibels (measured on the responsible authorities and relevant parties, as A scale) [dB(A)]. Measurements are to be taken required. at noise receptors located outside the project property boundary. Key Issues

Maximum allowable log The key production and control practices that will equivalent (hourly lead to compliance with emissions requirements measurements), in dB(A) can be summarized as follows: Day Night Receptor (07:00–22:00) (22:00–07:00) • Give preference to the flash-smelting process where appropriate. Residential, • Choose oxygen enrichment processes that al- institutional, low higher SO2 concentrations in smelter gases educational 55 45 to assist in sulfur recovery; use the double-con- Industrial, tact, double-absorption process. commercial 70 70 • Improve energy efficiency to reduce fuel us-

age and associated emissions; use low-NOx Monitoring and Reporting burners; give preference to natural gas as fuel. • Reduce air emissions of toxic metals to accept- Frequent sampling may be required during start- able levels. up and upset conditions. Once a record of con- • Maximize the recovery of dust and minimize sistent performance has been established, fugitive emissions; use hoods and doghouse sampling for the parameters listed in this docu- enclosures. ment should be as described below. • Reduce effluent discharge by maximizing Air emissions should be monitored continu- wastewater . ously for sulfur dioxide and particulate matter. • Avoid contamination of groundwater and sur- Other air emissions parameters should be moni- face waters by leaching of toxic metals from tored monthly. Fugitive emissions should be , process residues, slag, and other monitored annually. . Liquid effluents should be monitored daily for pH and total suspended solids and at least Sources weekly for all other parameters. All solid waste, tailings, and should Bounicore, Anthony J., and Wayne T. Davis, eds. 1992. be monitored for toxic metals. Contamination of Engineering Manual. New York: Van groundwater and surface waters should be Nostrand Reinhold. avoided. Environment . 1980. “A Study of Sulphur Con- Monitoring data should be analyzed and re- tainment in the Non-Ferrous Metallur- viewed at regular intervals and compared with gical Industries.” Report EPS 3-AP-79-8. Ottawa.