MDMW-Zinc&Lead08

8/14/2019 MDMW-Zinc&Lead08

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Background

Lead is a dense, soft, low-melting metal. It is an important component ofbatteries, and about 75% of the world's lead production is consumed bythe battery industry. Lead is the densest common metal except for gold,and this quality makes it effective in sound barriers and as a shieldagainst X-rays. Lead resists corrosion by water, so it has long been usedin the plumbing industry. It is also added to paints, and it makes a long-lasting roofing material.

Lead is a health hazard to humans if it is inhaled or ingested, interferingwith the production of red blood cells. Its use must be carefully controlled,and several formerly common uses of lead are now restricted by the U.S.government. Lead paint is found in many older buildings, but it is nowmostly used on outdoor steel structures such as bridges, to improve theirweatherability. A lead compound called tetraethyl lead was added togasoline as early as 1921 because it prevented the "knocking" problem ofhigh-compression automobile engines. However, most gasoline nowcontains no lead, because lead from car exhaust was a major source ofair pollution.

Lead is also commonly used in glass and enamel. In television picturetubes and computer video display terminals, lead helps block radiation,and the inner, though not the outer, portion of the common light bulb ismade of leaded glass. Lead also increases the strength and brilliance ofcrystal glassware. Lead is used to make bearings and solder, and it isimportant in rubber production and oil refining.

Lead production dates back at least 8,000 years. Lead was used in Egyptas early as 5,000 B.C., and in the time of the Pharaohs it was used in

pottery glazes and as solder. It was also cast into ornamental objects. Awhite lead paint was also used in ancient Egypt, Greece, and Rome.Ancient Rome used lead pipes for its extensive water works. Some of thetoxic effects of lead were also noted as early as the Roman era, thoughlead was also thought to have positive medical qualities. In the 15th and16th centuries, builders used lead as a roofing material for cathedrals,and lead was also used to hold together the different panels of coloredglass in stained glass windows. The first lead battery is credited to a

French physicist, Gaston Plante, who invented it in 1859. By 1889, so-called lead-acid storage batteries of the modern type were beingcommercially produced.


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Modern lead mines produce about 3 million metric tons of lead annually.This is only about half the lead used worldwide; the remainder is obtainedby recycling. The top producer of lead is Australia, followed by the UnitedStates, China, and Canada. Other countries with major lead deposits areMexico, Peru, Russia, and Kazakhstan.

Raw Materials

Lead is extracted from ores dug from under-ground mines. More than 60minerals contain some form of lead, but only three are usually mined forlead production. The most common is called galena. The pure form ofgalena contains only lead and sulfur, but it is usually found with traces ofother metals in it, including silver, copper, zinc, cadmium, and antimonyas well as arsenic. Two other minerals commercially mined for lead arecerussite and anglesite. Over 95% of all lead mined is derived from one ofthese three minerals. However, most deposits of these ores are not foundalone but mixed with other minerals such as pyrite, marcasite, and zincblende. Therefore much lead ore is obtained as a byproduct of othermetal mining, usually zinc or silver. Only half of all lead used yearlyderives from mining, as half is recovered through recycling, mostly ofautomobile batteries.

Besides the ore itself, only a few raw materials are necessary for therefining of lead. The ore concentrating process requires pine oil, alum,lime, and xanthate. Limestone or iron ore is added to the lead ore duringthe roasting process. Coke, a coal distillate, is used to further heat theore.


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The Manufacturing Process

Mining the ore

* The first step in retrieving lead-bearing ore is to mine it underground.Workers using heavy machinery drill the rock from deep tunnels withheavy machinery or blast it with dynamite, leaving the ore in pieces. Thenthey shovel the ore onto loaders and trucks, and haul it to a shaft. Theshaft at a large mine may be a mile or more from the drill or blast site.The miners dump the ore down the shaft, and from there it is hoisted tothe surface.

Concentrating the ore

* After the ore is removed from the mine, it is treated at a concentratingmill. Concentrating means to remove the waste rock from the lead. Tobegin, the ore must be crushed into very small pieces. The ore is groundat the mill, leaving it in particles with diameters of 0.1 millimeter or less.This means the individual granules are finer than table salt. The texture issomething like granulated sugar.

Flotation

* The principal lead ore, galena, is properly known as lead sulfide, andsulfur makes up a substantial portion of the mineral. The flotation processcollects the sulfur-bearing portions of the ore, which also contains thevaluable metal. First, the finely crushed ore is diluted with water and thenpoured into a tank called a flotation cell. The ground ore and watermixture is called slurry. One percent pine oil or a similar chemical is thenadded to the slurry in the tank. The tank then agitates, shaking the

mixture violently. The pine oil attracts the sulfide particles. Then air isbubbled through the mixture. This causes the sulfide particles to form anoily froth at the top of the tank. The waste rock, which is called gangue,sinks to the bottom. The flotation process is controlled by means of X-rayanalyzers. A flotation monitor in the control room can check the metalcontent of the slurry using the X-ray analysis. Then, with the aid of acomputer, the monitor may adjust the proportion of the chemical additiveto optimize recovery of the metal. Other chemicals are also added to the

flotation cell to help concentrate the minerals. Alum and lime aggregatethe metal, or make the particles larger. Xanthate is also added to theslurry, in order to help the metal particles float to the surface. At the end of


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the flotation process, the lead has been separated from the rock, andother minerals too, such as zinc and copper, have been separated out.

Filtering

* After the ore is concentrated in the flotation cells, it flows to a filter,which removes up to 90% of the water. The concentrate at this pointcontains from 40-80% lead, with large amounts of other impurities, mostlysulfur and zinc. It is ready at this stage to be shipped to the smelter. Thegangue, or rock that was not mineral-bearing, must be pumped out of theflotation tank. It may be dumped into a pond resembling a natural lake,and when the pond eventually fills, the land can be replanted.

Roasting the ore

* The lead concentrate fresh from the filter needs to be further refinedto remove the sulfur. After the concentrate is unloaded at what is calledthe sinter plant, it is mixed with other lead-bearing materials and withsand and limestone. Then the mixture is spread on a moving grate. Airwhich has been heated to 2,550F (1,400C) blows through the grate.Coke is added as fuel, and the sulfur in the ore concentrate combusts to

sulfur dioxide gas. This sulfur dioxide is an important byproduct of thelead refining process. It is captured at a separate acid plant andconverted to sulfuric acid, which has many uses. After the ore has beenroasted in this way, it fuses into a brittle material called sinter. The sinteris mostly lead oxide, but it can also contain oxides of zinc, iron, andsilicon, some lime, and sulfur. As the sinter passes off the moving grate, itis broken into lumps. The lumps are then loaded into the blast furnace.

Blasting

* The sinter falls into the top of the blast furnace, along with coke fuel.A blast of air comes through the lower part of the furnace, combusting thecoke. The burning coke generates a temperature of about 2,200F(1,200C) and produces carbon monoxide. The carbon monoxide reactswith the lead and other metal oxides, producing molten lead, nonmetallicwaste slag, and carbon dioxide. Then the molten metal is drawn off into

drossing kettles or molds.


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Refining

* The molten lead as it comes from the blast furnace is from 95-99%pure. It is called at this point base bullion. It must be further refined toremove impurities, because commercial lead must be from 99-99.999%pure. To refine the bullion, it is kept in the drossing kettle at a temperature

just above its melting point, about 626F (330C). At this temperature, anycopper left in the bullion rises to the top of the kettle and forms a scum ordross which can be skimmed off. Gold and silver can be removed fromthe bullion by adding to it a small quantity of zinc. The gold and silverdissolves more easily in zinc than in lead, and when the bullion is cooledslightly, a zinc dross rises to the top, bringing the other metals with it.


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Costing

* When the lead has been sufficiently refined, it is cooled and cast intoblocks which may weigh as much as a ton. This is the finished product.Lead alloys may also be produced at the smelter plant. In this casemetals are added to the molten lead in precise proportions to produce alead material for specific industrial uses. For example the lead commonlyused in car batteries, and also for pipe, sheet, cable sheathing, andammunition, is alloyed with antimony because this increases the metal'sstrength.

Byproducts/Waste

Lead refining produces several byproducts. The gangue, or waste rock,accumulates as the ore is concentrated. Most of the minerals have beenremoved from the rock, so this waste is not considered by the industry tobe an environmental hazard. It can be pumped into a disposal pond,which resembles a natural lake. Sulfuric acid is the major byproduct of thesmelting process. Sulfur dioxide gas is released when the ore is roastedat the sinter plant. To protect the atmosphere, fumes and smoke are

captured, and the air released by the plant is first cleaned. The sulfurdioxide is collected at a separate acid plant, and converted to sulfuricacid. The refinery can sell this acid as well as its primary product, the leaditself.

Air pollution can result from lead processing as well. The smelter requiresa "bag house," that is, a separate facility to filter and vacuum the fumesso that lead is not released into the atmosphere. Nevertheless, lead

particles do reach the atmosphere, and in the United States, federalregulations attempt to control how much is allowable. Most of the solidwaste product produced by the smelting process is a dense, glassysubstance called slag. This contains traces of lead as well as zinc andcopper. The slag is more toxic than the gangue, and it must be storedsecurely and monitored so that it does not escape into the environment orcome in contact with populations.


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The Future

New developments in the lead industry seem aimed less at improvementsin the manufacturing process than towards finding new uses for the leaditself. Since a large proportion of the lead mined and recycled is sold tothe automotive industry for batteries, lead producers are quite dependenton the health of the auto industry. But lead producers are interested infinding new applications for lead to give them more market stability.

One recent new application for lead is a lead-fiberglass laminate. Leadsheeting can be laminated between gypsum and fiberglass, forming asuperior duct material that helps isolate noise. If this is used in an airconditioning unit, for example, it effectively dampens the din of themachine. Another prospective market for lead is in nuclear wastecontainment. Safely storing radioactive material is a growing concernaround the world. The lead industry is researching canisters made oftitanium with an inner layer of lead or lead and plastic, contending that aone-inch layer of lead could add 880 years to the life of a properly buriedcontainer. And looking to the cars of the future, researchers in the U.S.and several other countries have been studying ways of improving lead-acid battery technology in order to power electric cars.

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MDMW-Zinc&Lead08