Plants for the production of lead are generally referred to as lead smelters. Primary lead production[clarification needed] 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. Smelting uses suitable reducing substances that will combine with those oxidizing elements to free the metal. Reduction is the final, high-temperature step in smelting. It is here that the oxide becomes the elemental metal. A reducing environment (often provided by carbon monoxide in an air-starved furnace) pulls the final oxygen atoms from the raw metal.
Lead is usually smelted in a blast furnace, using the lead sinter produced in the sintering process and coke to provide the heat source. As melting occurs, several layers form in the furnace. A combination of molten lead and slag sinks to the bottom of the furnace, with a layer of the lightest elements referred to as speiss, including arsenic and antimony floating to the top of the molten material. The crude bullion and lead slag layers flow out of the 'furnace front' and into the 'forehearth', where the two streams are separated. The lead slag stream, containing most of the 'fluxing' elements added to the sintering machine (predominantly silica, limestone, iron and zinc) can either be discarded or further processed to recover the contained zinc.
The crude lead bullion, containing significant quantities of copper will then undergo 'copper drossing'. In this step elemental sulphur, usually in solid form is added to the molten crude lead bullion to react with the contained copper. A "matte" layer forms in this step, containing most of the copper originating from the crude lead bullion and some other impurities as metal sulfides. The speiss and the matte are usually sold to copper smelters where they are refined for copper processing.
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 manufacturers. The Parkes process is used to separate silver or gold from lead.
Finally, the molten lead is refined. Pyrometallurgical methods are usually used to remove the remaining non-lead components of the mixture, for example the Betterton-Kroll process and the Betts electrolytic process. The non-lead metals are usually sold to other metal processing plants. The refined lead may be made into alloys or directly cast.[1]
People who operate or work in such plants are also referred to as smelters.
Lead ores
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Evolution of the extracted lead ores grade in different countries (Canada, Australia).Galena, the most common mineral of lead, is primarily lead sulfide (PbS). The sulfide is oxidized to a sulfite (PbSO3) which thermally decomposes into lead oxide and sulfur dioxide gas. (PbO and SO2) The sulfur dioxide (like the carbon dioxide in the example above) is expelled, and the lead oxide is reduced. Anglesite, Cerussite, Pyromorphite, Mimetite and Wulfenite are other lead ores.
Other elements frequently present with lead ores include zinc and silver.[2]
Secondary lead processing
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Most of the lead produced comes from secondary sources. Lead scrap includes lead-acid 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.[3][4]
Lead exposure
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Humans have been smelting lead for thousands of years, poisoning themselves in the process. Although lead poisoning is one of the oldest known work and environmental hazards, the modern understanding of the small amount of lead necessary to cause harm did not come about until the latter half of the 20th century. No safe threshold for lead exposure has been discovered—that is, there is no known amount of lead that is too small to cause the body harm.
The US Centers for Disease Control and Prevention and the World Health Organization state that a blood lead level of 10 μg/dL or above is a cause for concern; however, lead may impair development and have harmful health effects even at lower levels, and there is no known safe exposure level.[5] Authorities such as the American Academy of Pediatrics define lead poisoning as blood lead levels higher than 10 μg/dL.
Lead smelters with little pollution controls contribute to several environmental problems, especially raised blood lead levels in the surrounding population. The problem is particularly significant in many children who have grown up in the proximity to a lead smelter.[6]
History
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The earliest known cast lead beads were thought found in the Çatalhöyük site in Anatolia (Turkey), and dated from about 6500 BC. They were later identified in fact to be made from cerussite and galena, minerals rich in, but distinct from, lead.[7] Ancient smelting was done using loads of lead ore and charcoal in outdoor hearths and furnaces.
Although lead is a common metal, its discovery had relatively little impact in the ancient world. It is too soft to be used for weapons (except possibly as sling projectiles) or for structural elements. However, being easy to cast and shape, it came to be extensively used in the classical world of Ancient Greece and Ancient Rome for piping and storage of water. It was also used as a mortar in stone buildings, and as a writing material. Smeltmills were water-powered mills used to smelt lead or other metals. The Roman lead smelting has led to evidence of global pollution. Greenland ice cores from 500 BCE to 300 CE show measurably elevated lead content in the atmosphere.[8] Researchers studying an ice core from Colle Gnifetti, in the Swiss part of the Monte Rosa massif, have found that higher historical European airborne lead pollution levels are associated with changes in the monetary system from gold to silver from the year 640 CE, with the principal source likely to be the Melle mines in France. Later airborne pollution, between the years 1170 and 1216 CE, correlates even more strongly with contemporaneous records of lead and silver production from mines in the Peak District of England, at levels similar to those seen in the Industrial Revolution.[9][10][11]
Georgius Agricola (1494–1555) presented details of lead smelting methods and facilities current in Europe in the first half of the 16th century in Book IX of his treatise on mining and metallurgy, De Re Metallica. Methods ranged from primitive open-hearth arrangements (essentially bonfires on which lead ore was piled) to blast furnaces capable of continuous operation.[12]
In the USA there are 400 forgotten lead smelting firms that operated in the 1930s to 1960s and may have deposited dangerous levels of lead contamination in nearby soil.[13]
Historic mining and smelting sites
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Asia
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Australia
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Europe
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North America
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South America
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Active lead mines and smelters
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Doe Run lead smelter in Herculaneum, MissouriAfrica
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See also
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References
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What does Gopher do? Gopher Resource recycles lead-acid batteries. The plant is called a secondary lead smelter because it doesn’t actually produce new lead from ore; it recycles lead that already exists. (There are no more primary lead smelters in the United States.) The recycling process works like this: used batteries are broken open, the lead is extracted and then melted in furnaces and purified with chemicals in the refinery. It’s then poured into molds and sold as new blocks. Those blocks go to companies that include battery and ammunition manufacturers.
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Learn about donatingWhat are the dangers? During the battery breaking, lead smelting and refining processes, poisonous lead dust and fumes are released into the air, exposing workers. The dust then coats the pipes and floor inside the plant. Molten lead is also a danger, sometimes splashing workers during chemical explosions. Workers also can be exposed to other toxic substances, including cadmium, arsenic and sulfur dioxide.
What is a baghouse? At Gopher, this is a multi-story building that plays one of the most important roles in protecting the community from pollution. It is designed to receive and filter the dangerous dust created in other areas of the plant. The dust travels to the baghouse through a system of pipes and vents and is routed to small, cell-like rooms. Inside are cloth bags. At regular intervals throughout the day, the bags dump the dust into a hopper after becoming too full. The dust then travels by way of a conveyor belt system to a large mixing machine, where it’s treated with water and chemicals and ultimately discharged into the city of Tampa’s wastewater system.
What’s the danger? Workers emerged covered in dust that was laden with lead and cancer-causing cadmium. Sometimes they passed out because of the heat and the fumes. Often their respirators were overwhelmed. Their skin developed rashes. Some workers saw the lead shoot up in their blood. Others were whisked to the hospital after their bodies seized up. Multiple baghouse veterans report lingering body pains, heart and breathing problems.
Under the best circumstances, an automatic device empties the bags. But because of design and engineering flaws, the automatic function didn’t work for years. Workers were forced to enter the cells and manually shake the bags, amid soaring temperatures and gas levels.
The assembly line-like system that transports the dust jammed often; workers got blanketed in poisons trying to fix the machines. Workers also have been overwhelmed by the dust swarms that overtook the lower level of the baghouse.
Why did workers shake the dust bags? Gopher’s lead-making process is delicate and interconnected. A dust-clogged baghouse can shut down the entire furnace department and bring the production of lead to a halt. The company estimates it loses $500 every minute the furnaces aren’t operating.
What are furnace exhaust hoods? Furnace exhaust hoods are designed to capture gases, dust and fumes that are released during the lead-smelting process. The hoods are supposed to vent the poisonous substances out of the work area and into ducts that travel to the baghouse.
What are refining kettles? Refining kettles are huge circular containers into which molten lead is poured and chemicals are added to rid the metal of impurities. Workers add a variety of different substances to the kettles, including calcium and arsenic.
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