Lead
Lead
Lead—a metal that archaeological discoveries prove was used at least 5,000 years ago—is the heaviest element in Group 14 of the periodic table, a group often known as the carbon family. It is a metallic element with atomic number 82, with symbol Pb, atomic weight 207.19, specific gravity 11.35, melting point 621.32°F (327.4°C), and boiling point 3,191°F (1,755°C).
Lead is in column IVA of the periodic table. It has four naturally occurring stable isotopes, lead-204, lead-206, lead-207, and lead-208. The last three of these are all end products of one or another radioactive family.
Lead is one of the first elements known to human societies. It is described in some of the oldest books of the Old Testament and was widely used by some early civilizations. Examples of objects containing lead from fifth millennium Egyptian cultures have been found. The Greeks and Romans also used lead for the manufacture of a variety of tools and containers. Some experts claim that one reason for the decline of the Roman civilization was the extensive use of lead in the empire’s water supply system. Lead is now known to have a variety of serious effects on the human nervous system, including diminished mental capacity.
Lead was originally known by its Latin name of plumbum, from which its modern chemical symbol (Pb) is derived. The Latin name is still preserved also in the common names for lead compounds, as in plumbic and plumbous chlorides.
General properties
Lead is a heavy, ductile, soft, grayish solid. It can be cast and fabricated easily and has the unusual ability to absorb sound and other forms of vibration. Although it is dissolved by dilute nitric acid, it tends to be resistant to other forms of acid solutions.
By far the most important use of lead is in storage batteries, with the production of antiknock gasolines, pigments, ammunition, solder, and plumbing materials following. In the last half century of the twentieth century, evidence about the toxic effects of lead has accumulated to the point where its presence in the environment, now in the twenty-first century, is considered to be a serious hazard for humans, especially for young children.
Where it comes from
Lead is thought to be the thirty-sixth most abundant element in the Earth’s crust, with a concentration of about 13 parts per million. This makes the element more common than other heavy metals such as thallium or uranium, but much less abundant than less well known elements such as niobium, neodymium, lanthanum, and gallium.
The most important ore of lead is galena, lead sulfide (PbS). Anglesite (lead sulfate; PbSO4) and cerussite (PbCO3) are also economically important. Both are formed by the weathering of galena.
Over half of the lead produced in the world comes from just four regions and nations: the United States, Russia and other members of the former Soviet Union, Australia, and Canada. In the United States, about 90% of all lead comes from seven mines in Missouri, with the rest originating from mines in Colorado, Idaho, and Utah.
How the metal is obtained
The raw material from which lead metal is produced is either a naturally occurring ore or, more commonly today, lead products returned for recycling. In the United States, more than half of all lead produced comes from recycled materials, especially recycled storage batteries. After initial treatment, the major steps by which lead is obtained from either ore or recycled material are very similar.
In the case of a naturally occurring ore of lead, the first step is usually to concentrate the ore and separate it from other metallic ores. This step often involves the froth flotation process in which the mixture of ores is finely ground and then added to a water mixture that contains one or more other materials, such as hydrocarbons, sodium cyanide, copper sulfate, or pine oil. Air is then pumped through the ore/water/secondary material mixture, producing a frothy mixture containing many small bubbles.
The added material causes secondary ores such as ores of copper or zinc either to adhere or not adhere to the bubbles in the froth, allowing their separation from lead ores that respond in the opposite manner to the secondary material. The use of copper sulfate in the flotation process, for example, aids in the separation of zinc ores from lead ores.
After separation, lead sulfide is heated in a limited supply of air to convert it to lead oxide. The lead oxide is then mixed with coke (carbon) and a flux such as limestone in a blast furnace. Within the blast furnace, coke burns to form carbon monoxide which, in turn, reacts with lead oxide to form metallic lead and carbon dioxide.
In a variation of this procedure, the lead oxide can be mixed with lead sulfide and heated. The reaction that takes place results in the formation of lead metal and sulfur dioxide gas.
The lead produced by either of these methods is still impure, containing small amounts of copper, tin, arsenic, antimony, and other metals. Each metallic impurity is then removed by some additional step. In the case of copper, for example, the impure lead is heated to a temperature just above its melting point. At this temperature, copper is still a solid. Any copper mixed with the lead floats on top of the lead, and can be scraped off.
How it is used
Metallic lead is sometimes used in a pure or nearly pure form, usually because of its high density and ability to be bent and shaped. The metal is an efficient absorber of radiation and, for that reason, is commonly used as a shield for x rays, nuclear radiation, and other forms of radiation.
Far more commonly, however, lead is alloyed with one or more other elements to produce a product with special properties of interest for some specific application. More than half of all the lead used in the United States, for example, goes to the production of lead storage batteries. The positive plates, made of lead(IV) oxide, and the negative plates, made of spongy lead, are both made from an alloy containing 91% lead and 9% antimony. Over 80% of this lead is now recovered and recycled as a source of lead metal.
At one time, very large amounts of lead were used in the production of tetraethyl lead, a compound that reduces the amount of knocking in an internal combustion engine. The problem with tetraethyl lead, however, is that it tends to break down within an engine, releasing free lead to the environment. Because of the health hazards that lead poses for humans and other animals, tetraethyl lead has been banned for use as a gasoline additive.
Because of its chemical inertness, lead has also been popular as a covering for underground cables, such as buried cables that carry telephone messages, and for pipes through which liquids are transported. For many years, lead was the material of choice in the construction of water pipes since it was inert to most chemicals occurring in nature and easily shaped. With the recognition of lead’s threat to humans, however, many of these applications have been discontinued.
Alloys of lead are also popular for the manufacture of solders. Ordinary plumber’s solder, for example, contains about two parts of lead to one part of tin. This alloy has a melting point of about 527°F (275°C).
Lead compounds were once widely used also for paints. They were in great demand because they covered surfaces well and were available in a number of vivid colors. Among these were lead chromate (yellow), lead molybdate (reddish-orange), lead(II) oxide (canary yellow), red lead oxide (Pb3O4; red), and white lead, a complex lead carbonate/lead hydroxide mixture. As with other lead compounds, however, the potential health hazards of the element have greatly reduced the availability of lead-based paints.
Chemistry and compounds
Lead is a reactive metal, but its reactivity is somewhat inhibited by the formation of an outer skin of protective compounds. For example, when a freshly cut piece of lead metal is exposed to the air, it quickly reacts with oxygen to form a thin outer layer of lead oxide. This outer layer then prevents further reaction between the metal and oxygen and other constituents of the air. A similar phenomenon occurs when lead metal is placed into water. Compounds present in water react with lead to form an outer skin of lead carbonate, lead silicate, or similar compounds that protect the metal from further attack. This property helps to explain the long popularity of lead for the lining of pipes designed to carry many different kinds of liquids.
From its position in a table of electrode potentials, one would expect lead to replace hydrogen from acids. However, the difference in electrode potentials between the two elements is so small (0.126 volts), that lead reacts with most acids only very slowly indeed. The element does tend to react with oxygen-containing acids more readily, but only because of oxidation that may take place at the same time.
One application that takes advantage of this property is the use of lead to line containers that hold concentrated sulfuric acid. As long as those containers are kept at temperatures below 140°F (60°C), there is essentially no reaction between the acid and metal lining.
When lead does take part in a chemical reaction, it demonstrates one of two oxidation states, 4+ and 2+. Compounds of the former class are known as lead(IV) or plumbic compounds, while those of the latter class are lead(II) or plumbous compounds. Like aluminum, lead is amphoteric and will react with strong bases. The products of such reactions are known as plum-bates and plumbites.
Biological effects
Throughout most of human history, lead was used for a wide variety of applications with little or no appreciation of the serious health hazards it poses. Today, physiologists understand that the human body is able to excrete about 2 milligrams of lead efficiently each day, but that quantities in excess of that can cause serious health problems.
Children are especially at risk for lead poisoning. Their bodies do not metabolize lead as quickly as do those of adults, so a given concentration of lead in the blood will have more serious consequences for a child than for an adult.
At relatively low concentrations, lead produces relatively modest or short-term effects, including elevation of blood pressure, reduction in the synthesis of hemoglobin, and decreased ability to utilize vitamin D and calcium. With increased blood concentrations
KEY TERMS
Ductile —Capable of being drawn or stretched into a thin wire.
Flux —In the conversion of ores to metals, a substance used to remove impurities from a blast furnace or other kind of processing system.
Froth flotation —A system for separating a desired ore from other kinds of ores by pumping air through a mixture of water and one or more other substances.
Hemoglobin —An iron-containing, protein complex carried in red blood cells that binds oxygen for transport to other areas of the body.
Isotopes —Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties.
of lead, however, these problems become more severe. Impairment of the central nervous system can occur, with decreased mental functioning and hearing damage as two possible results. At very high lead concentrations, a person can fall into a coma and, eventually, die.
With the recognition of these problems, governmental agencies have continually restricted the number of applications in which lead can be used. Unfortunately, its widespread use in previous years means that many children (especially) and adults are still at risk for lead poisoning. As an example, children sometimes pick off and then eat chips of paint from the walls of old buildings. Since many of these paints were made with compounds of lead, those children are then exposed to the harmful effects of the element. In addition, people who work in factories where lead is used may inhale lead fumes and accumulate increasingly large amounts of the metal in their bodies.
See also Element, chemical; Metallurgy.
Resources
BOOKS
Emsley, John. Nature’s Building Blocks: An A-Z Guide to the
Elements. Oxford: Oxford University Press, 2003. Siekierski, Slawomir. Concise Chemistry of the Elements.
Chichester, UK: Horwood Publishing, 2002.
Wiley-Interscience, eds. Kirk-Othmer Encyclopedia of Chemical Technology. Hoboken, NJ: Wiley-Interscience, 2004.
David E. Newton
Lead (revised)
LEAD (REVISED)
Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
Overview
Lead is the heaviest member of the carbon family. The carbon family consists of the five elements in Group 14 (IVA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. Although a member of the carbon family, lead looks and behaves very differently from carbon.
Lead is one of only a few elements known to ancient peoples. One of the oldest examples of lead is a small statue found in Egypt. It was made during the First Dynasty, in about 3400 B.C. Mention of lead and lead objects can also be found in very old writing from India. And the Bible mentions lead in a number of passages.
SYMBOL
Pb
ATOMIC NUMBER
82
ATOMIC MASS
207.2
FAMILY
Group 14 (IVA)
Carbon
PRONUNCIATION
LED
Throughout history, Lead has been used to make water and sewer pipes; roofing; cable coverings; type metal and other alloys; paints; wrappings for food, tobacco, and other products; and as an additive in gasoline. Since the 1960s, however, there has been a growing concern about the health effects of lead. For instance, scientists have found that lead can cause mental and physical problems in growing children. As a result, many common lead products are now being phased out.
Discovery and naming
Lead has been around for thousands of years. It is impossible to say when humans first discovered the element. It does not occur as an element in the earth very often. But one of its ores, lead sulfide (PbS), is fairly common. It is not difficult to obtain pure lead metal from lead sulfide. Humans probably discovered methods for doing so thousands of years ago.
By Roman times, lead metal was widely used. The far-reaching system that brought water to Rome contained many lead pipes. Sheets of lead were used as writing tablets and some Roman coins were also made of lead. Perhaps of greatest interest was the use of lead in making pots and pans. Modern scientists believe many Romans may have become ill and died because of this practice. Cooking liquids in lead utensils tends to make the lead dissolve. It got into the food being cooked. People who ate those foods got more and more lead into their bodies. Eventually, the effects of lead poisoning must have begun to appear.
Of course, the Romans had little understanding of the connection between lead and disease. They probably never realized that they were poisoning themselves by using lead pots and pans.
No one is quite sure how lead got its name. The word has been traced to manuscripts that date to before the 12th century. Romans called the metal plumbum. It is from this name that the element's chemical symbol comes: Pb. Compounds of lead are sometimes called by this old name, such as plumbous chloride.
Physical properties
Lead is a heavy, soft, gray solid. It is both ductile and malleable. Ductile means capable of being drawn into thin wires. Malleable means capable of being hammered into thin sheets. It has a shiny surface when first cut, but it slowly tarnishes (rusts) and becomes dull. Lead is easily worked. "Working" a metal means bending, cutting, shaping, pulling, and otherwise changing the shape of the metal.
The melting point of lead is 327.4°C (621.3°F), and its boiling point is 1,750 to 1,755°C (3,180 to 3,190°F). Its density is 11.34 grams per cubic centimeter. Lead does not conduct an electric current, sound, or vibrations very well.
Chemical properties
Lead is a moderately active metal. It dissolves slowly in water and in most cold acids. It reacts more rapidly with hot acids. It does not react with oxygen in the air readily and does not burn.
Occurrence in nature
The abundance of lead in the Earth's crust is estimated to be between 13 and 20 parts per million. It ranks in the upper third among the elements in terms of its abundance.
Lead rarely occurs as a pure element in the earth. Its most common ore is galena, or lead sulfide (PbS). Other ores of Lead are anglesite, or lead sulfate (PbSO4); cerussite, or lead carbonate (PbCO3); and mimetite (PbCL2 ○ Pb3(AsO4)2).
The largest producers of lead ore in the world are Australia, China, the United States, Peru, Canada, Mexico, and Sweden. In the United States, more than 93 percent of all the lead produced comes from Missouri. Other lead-producing states are Montana, Colorado, Idaho, Illinois, New York, and Tennessee. In 1996, 426,000 metric tons of lead were produced in the United States.
Isotopes
Four naturally occurring isotopes of lead occur. They are lead-204, lead-206, lead-207, and lead-208. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
About sixteen radioactive isotopes of lead are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
Romans routinely ate food cooked in lead pots and pans. The connection between lead and disease was not known then, so many people became ill and died of lead poisoning.
One radioactive isotope of lead, lead-210, is sometimes used in medicine. This isotope gives off radiation that can kill cancer cells. It is also used to treat non-cancerous eye disorders.
Extraction
Lead is obtained from its ores by a method used with many metals. First, the ore is roasted (heated in air). Roasting, also called smelting, converts the ore to a compound of lead and oxygen, lead oxide (PbO2). Lead oxide is then heated with charcoal (pure carbon). The carbon takes oxygen away from the lead oxide. It leaves pure lead behind:
Lead obtained in this way is not very pure. It can be purified electrolytically. Electrolytic refining involves passing an electric current through a compound. Very pure lead is collected at one side of the container in which the reaction is carried out.
A major source of lead is recycled car batteries.
Lead is also recovered in recycling programs. Recycling is the process by which a material is retrieved from a product that is no longer used. For example, old car batteries were once just thrown away. Now they are sent to recycling plants where lead can be extracted and used over and over again. It is not necessary to get all the lead that industry needs from new sources, such as ores.
Uses
The lead industry is undergoing dramatic change. Many products once made with lead no longer use the element. The purpose of this change is to reduce the amount of lead that gets into the environment. Examples of such products include ammunition, such as shot and bullets; sheet lead used in building construction; solder; water and sewer pipes; ball bearings; radiation shielding; and gasoline. These changes are possible because manufacturers are finding safer elements to use in place of lead.
The price of a gallon of gas
F or many years, lead was regarded as a miracle chemical by the automotive industry. The power to run a car comes from the burning of gasoline in the engine. However, burning gasoline is not a simple process. Many things happen inside an engine when gasoline burns in the carburetor.
For example, an engine can "knock" if the gasoline does not burn properly. "Knocking" is a "bang-bang" sound from the engine. It occurs when low-grade gasoline is used.
One way to prevent knocking is to use high-grade gasoline. Another way is to add chemicals to the gasoline. The best gasoline additive discovered was a compound called tetraethyl lead (Pb(C2H5)4). Tetraethyl lead was usually called "lead" by the automotive industry, the consumer, and advertisers. When someone bought "leaded" gasoline, it contained not lead metal, but tetraethyl lead.
Leaded gasoline was a great discovery. It could be made fairly cheaply and it prevented car engines from knocking. No wonder people thought it was a miracle chemical.
What people didn't realize was that tetraethyl lead breaks down in a car engine because of the high temperature at which engines operate. When tetraethyl lead breaks down, elemental lead (Pb) is formed:
The result—with millions of cars being driven every day—was more and more lead getting into the air. And more and more people inhaled that lead. Eventually, doctors began to see more people with lead-related diseases.
The federal government finally decided that tetraethyl lead was too dangerous to use in gasoline. By 1990, the use of this compound had been banned by all governments in North America.
Other uses of lead have not declined. The best example is lead storage batteries. A lead storage battery is a device for converting chemical energy into electrical energy. Almost every car and truck has at least one lead storage battery. But no satisfactory substitute for it has been found. About 87 percent of all lead produced in the United States now goes to the manufacture of lead storage batteries. In addition to cars and trucks, these batteries are used for communication networks and emergency power supplies in hospitals, and in forklifts, airline ground equipment, and mining vehicles.
Compounds
A small percentage of lead is used to make lead compounds. Although the amount of lead is small, the variety of uses for these compounds is large. Some examples of important lead compounds are:
lead acetate (Pb(C2H3O2)2): insecticides; waterproofing; varnishes; dyeing of cloth; production of gold; hair dye
lead antimonate (Pb3(SbO4)2): staining of glass, porcelain and other ceramics
lead azide (Pb(N3)2): used as a "primer" for high explosives
lead chromate ("chrome yellow"; PbCrO4): industrial paints (use restricted by law)
lead fluoride (PbF2): used to make lasers; specialized optical glasses
lead iodide (PbI2): photography; cloud seeding to produce rain
lead naphthenate (Pb(C7H12O2)): wood preservative; insecticide; additive for lubricating oil; paint and varnish drier
lead phosphite (2PbO ○ PbHPO3): used to screen out ultraviolet radiation in plastics and paints
lead stearate (Pb(C18H35O2)2): used to make soaps, greases, waxes, and paints; lubricant; drier for paints and varnishes
lead telluride (PbTe): used to make semiconductors, photoconductors, and other electronic equipment
Health effects
The health effects of lead have become much better understood since the middle of the 20th century. At one time, the metal was regarded as quite safe to use for most applications. Now lead is known to cause both immediate and long-term health problems, especially with children. It is toxic when swallowed, eaten, or inhaled.
Young children are most at risk from lead poisoning. Some children have a condition known as pica. They have an abnormal desire to eat materials like dirt, paper, and chalk. Children with pica sometimes eat paint chips off walls. At one time, many interior house paints were made with lead compounds. Thus, crawling babies or children with pica ran the risk of eating large amounts of lead and being poisoned.
Some symptoms of lead poisoning include nausea, vomiting, extreme tiredness, high blood pressure, and convulsions (spasms). Over a long period of time, these children often suffer brain damage. They lose the ability to carry out normal mental functions.
Other forms of lead poisoning can also occur. For example, people who work in factories where lead is used can inhale lead fumes. The amount of fumes inhaled at any one time may be small. But over months or years, the lead in a person's body can build up. This kind of lead poisoning can lead to nerve damage and problems with the gastrointestinal system (stomach and intestines).
Lead causes both immediate and long-term health problems, especially with children. It is toxic when swallowed, eaten, or inhaled.
Today, there is an effort to reduce the use of lead in consumer products. For instance, older homes are often tested for lead paint before they are resold. Lead paint has also been removed from older school buildings.
Lead
Lead
Background
Lead is a dense, soft, low-melting metal. It is an important component of batteries, and about 75% of the world's lead production is consumed by the battery industry. Lead is the densest common metal except for gold, and this quality makes it effective in sound barriers and as a shield against X-rays. Lead resists corrosion by water, so it has long been used in 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, interfering with 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 now mostly used on outdoor steel structures such as bridges, to improve their weatherability. A lead compound called tetraethyl lead was added to gasoline as early as 1921 because it prevented the "knocking" problem of high-compression automobile engines. However, most gasoline now contains no lead, because lead from car exhaust was a major source of air pollution.
Lead is also commonly used in glass and enamel. In television picture tubes and computer video display terminals, lead helps block radiation, and the inner, though not the outer, portion of the common light bulb is made of leaded glass. Lead also increases the strength and brilliance of crystal glassware. Lead is used to make bearings and solder, and it is important in rubber production and oil refining.
Lead production dates back at least 8,000 years. Lead was used in Egypt as 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. A white lead paint was also used in ancient Egypt, Greece, and Rome. Ancient Rome used lead pipes for its extensive water works. Some of the toxic effects of lead were also noted as early as the Roman era, though lead was also thought to have positive medical qualities. In the 15th and 16th centuries, builders used lead as a roofing material for cathedrals, and lead was also used to hold together the different panels of colored glass 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 being commercially produced.
Modern lead mines produce about 3 million metric tons of lead annually. This is only about half the lead used worldwide; the remainder is obtained by recycling. The top producer of lead is Australia, followed by the United States, China, and Canada. Other countries with major lead deposits are Mexico, Peru, Russia, and Kazakhstan.
Raw Materials
Lead is extracted from ores dug from under-ground mines. More than 60 minerals contain some form of lead, but only three are usually mined for lead production. The most common is called galena. The pure form of galena contains only lead and sulfur, but it is usually found with traces of other metals in it, including silver, copper, zinc, cadmium, and antimony as well as arsenic. Two other minerals commercially mined for lead are cerussite and anglesite. Over 95% of all lead mined is derived from one of these three minerals. However, most deposits of these ores are not found alone but mixed with other minerals such as pyrite, marcasite, and zinc blende. Therefore much lead ore is obtained as a byproduct of other metal mining, usually zinc or silver. Only half of all lead used yearly derives from mining, as half is recovered through recycling, mostly of automobile batteries.
Besides the ore itself, only a few raw materials are necessary for the refining of lead. The ore concentrating process requires pine oil, alum, lime, and xanthate. Limestone or iron ore is added to the lead ore during the roasting process. Coke, a coal distillate, is used to further heat the ore.
The Manufacturing
Process
Mining the ore
- 1 The first step in retrieving lead-bearing ore is to mine it underground. Workers using heavy machinery drill the rock from deep tunnels with heavy machinery or blast it with dynamite, leaving the ore in pieces. Then they shovel the ore onto loaders and trucks, and haul it to a shaft. The shaft 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 to the surface.
Concentrating the ore
- 2 After the ore is removed from the mine, it is treated at a concentrating mill. Concentrating means to remove the waste rock from the lead. To begin, the ore must be crushed into very small pieces. The ore is ground at 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 is something like granulated sugar.
Flotation
- 3 The principal lead ore, galena, is properly known as lead sulfide, and sulfur makes up a substantial portion of the mineral. The flotation process collects the sulfur-bearing portions of the ore, which also contains the valuable metal. First, the finely crushed ore is diluted with water and then poured into a tank called a flotation cell. The ground ore and water mixture is called slurry. One percent pine oil or a similar chemical is then added to the slurry in the tank. The tank then agitates, shaking the mixture violently. The pine oil attracts the sulfide particles. Then air is bubbled through the mixture. This causes the sulfide particles to form an oily 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-ray analyzers. A flotation monitor in the control room can check the metal content of the slurry using the X-ray analysis. Then, with the aid of a computer, the monitor may adjust the proportion of the chemical additive to optimize recovery of the metal. Other chemicals are also added to the flotation cell to help concentrate the minerals. Alum and lime aggregate the metal, or make the particles larger. Xanthate is also added to the slurry, in order to help the metal particles float to the surface. At the end of the flotation process, the lead has been separated from the rock, and other minerals too, such as zinc and copper, have been separated out.
Filtering
- 4 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 point contains from 40-80% lead, with large amounts of other impurities, mostly sulfur and zinc. It is ready at this stage to be shipped to the smelter. The gangue, or rock that was not mineral-bearing, must be pumped out of the flotation 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
- 5 The lead concentrate fresh from the filter needs to be further refined to remove the sulfur. After the concentrate is unloaded at what is called the sinter plant, it is mixed with other lead-bearing materials and with sand and limestone. Then the mixture is spread on a moving grate. Air which has been heated to 2,550°F (1,400°C) 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 the lead refining process. It is captured at a separate acid plant and converted to sulfuric acid, which has many uses. After the ore has been roasted in this way, it fuses into a brittle material called sinter. The sinter is mostly lead oxide, but it can also contain oxides of zinc, iron, and silicon, some lime, and sulfur. As the sinter passes off the moving grate, it is broken into lumps. The lumps are then loaded into the blast furnace.
Blasting
- 6 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 the coke. The burning coke generates a temperature of about 2,200°F (1,200°C) and produces carbon monoxide. The carbon monoxide reacts with the lead and other metal oxides, producing molten lead, nonmetallic waste slag, and carbon dioxide. Then the molten metal is drawn off into drossing kettles or molds.
Refining
- 7 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 to remove 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 626°F (330°C). At this temperature, any copper left in the bullion rises to the top of the kettle and forms a scum or dross which can be skimmed off. Gold and silver can be removed from the bullion by adding to it a small quantity of zinc. The gold and silver dissolves more easily in zinc than in lead, and when the bullion is cooled slightly, a zinc dross rises to the top, bringing the other metals with it.
Costing
- 8 When the lead has been sufficiently refined, it is cooled and cast into blocks 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 case metals are added to the molten lead in precise proportions to produce a lead material for specific industrial uses. For example the lead commonly used in car batteries, and also for pipe, sheet, cable sheathing, and ammunition, is alloyed with antimony because this increases the metal's strength.
Byproducts/Waste
Lead refining produces several byproducts. The gangue, or waste rock, accumulates as the ore is concentrated. Most of the minerals have been removed from the rock, so this waste is not considered by the industry to be an environmental hazard. It can be pumped into a disposal pond, which resembles a natural lake. Sulfuric acid is the major byproduct of the smelting process. Sulfur dioxide gas is released when the ore is roasted at the sinter plant. To protect the atmosphere, fumes and smoke are captured, and the air released by the plant is first cleaned. The sulfur dioxide is collected at a separate acid plant, and converted to sulfuric acid. The refinery can sell this acid as well as its primary product, the lead itself.
Air pollution can result from lead processing as well. The smelter requires a "bag house," that is, a separate facility to filter and vacuum the fumes so that lead is not released into the atmosphere. Nevertheless, lead particles do reach the atmosphere, and in the United States, federal regulations attempt to control how much is allowable. Most of the solid waste product produced by the smelting process is a dense, glassy substance called slag. This contains traces of lead as well as zinc and copper. The slag is more toxic than the gangue, and it must be stored securely and monitored so that it does not escape into the environment or come in contact with populations.
The Future
New developments in the lead industry seem aimed less at improvements in the manufacturing process than towards finding new uses for the lead itself. Since a large proportion of the lead mined and recycled is sold to the automotive industry for batteries, lead producers are quite dependent on the health of the auto industry. But lead producers are interested in finding new applications for lead to give them more market stability.
One recent new application for lead is a lead-fiberglass laminate. Lead sheeting can be laminated between gypsum and fiberglass, forming a superior duct material that helps isolate noise. If this is used in an air conditioning unit, for example, it effectively dampens the din of the machine. Another prospective market for lead is in nuclear waste containment. Safely storing radioactive material is a growing concern around the world. The lead industry is researching canisters made of titanium with an inner layer of lead or lead and plastic, contending that a one-inch layer of lead could add 880 years to the life of a properly buried container. 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.
Where To Learn More
Periodicals
Goodwin, Frank E. and Dodd S. Carr. "Brilliant Performer." Natural Science, July 1989, pp. 317-23.
King, Angela. "Producers Hunt New Lead Uses." American Metal Market. April 11, 1988, pp. 10-13.
Knights, Mikell. "Higher Auto Output Boosts Lead, Zinc Use." American Metal Market, August 6, 1993, pp. 5-12.
Schmitt, Bill. "Lead, Zinc Vie for Place in Future Electric Cars." American Metal Market, August 6, 1993, p.6.
—Angela Woodward
lead
lead1 / lēd/ • v. (past and past part. led / led/ ) [tr.] 1. cause (a person or animal) to go with one by holding them by the hand, a halter, a rope, etc., while moving forward: she emerged leading a bay horse. ∎ show (someone or something) the way to a destination by going in front of or beside them: she stood up and led her friend to the door. ∎ be a reason or motive for (someone): nothing that I have read about the case leads me to the conclusion that anything untoward happened a fascination for art led him to start a collection of paintings. ∎ [intr.] be a route or means of access to a particular place or in a particular direction: a door leading to a better-lit corridor. ∎ [intr.] (lead to) culminate in (a particular event): closing the plant will lead to the loss of 300 jobs. ∎ [intr.] (lead on to) form a stage in a process that leads probably or inevitably to (a particular end): his work on digestion led on to study of proteins and fats. ∎ (lead something through) cause a liquid or easily moving matter to pass through (a channel).2. [tr.] be in charge or command of: a military delegation was led by the Chief of Staff. ∎ organize and direct: the conference included sessions led by people with personal knowledge of the area. ∎ set (a process) in motion: they are waiting for an expansion of world trade to lead a recovery. ∎ be the principal player of (a group of musicians): since the forties he has led his own big bands. ∎ [intr.] (lead with) assign the most important position to (a particular news item): the news on the radio led with the murder.3. be superior to (competitors or colleagues): there will be specific areas or skills in which other nations lead the world. ∎ have the first place in (a competition); be ahead of (competitors): the veteran jockey was leading the field. ∎ [intr.] have the advantage in a race or game: Dallas was fortunate to lead 85-72. 4. have or experience (a particular way of life): she's led a completely sheltered life.5. initiate (action in a game or contest), in particular: ∎ (in card games) play (the first card) in a trick or round of play. ∎ [intr.] (lead with) Boxing make an attack with (a particular punch or fist): Adam led with a left. ∎ [intr.] Baseball (of a base runner) advance one or more steps from the base one occupies while the pitcher has the ball: the runner leads from first.• n. / lēd/ 1. the initiative in an action; an example for others to follow: The U.S. is now taking the environmental lead. ∎ a clue to be followed in the resolution of a problem: detectives investigating the murder are chasing new leads. ∎ (in card games) an act or right of playing first in a trick or round of play: it's your lead. ∎ the card played first in a trick or round.2. (the lead) a position of advantage in a contest; first place: they were beaten 5-3 after twice being in the lead. ∎ an amount by which a competitor is ahead of the others: the team held a slender one-goal lead. ∎ Baseball an advance of one or more steps taken by a base runner from the base they occupy while the pitcher has the ball.3. the chief part in a play or film: she had the lead in a new film [as adj.] the lead role. ∎ the person playing the chief part: he still looked like a romantic lead. ∎ [usu. as adj.] the chief performer or instrument of a specified type: that girl will be your lead dancer. ∎ [often as adj.] the item of news given the greatest prominence in a newspaper or magazine: the lead story.4. a leash for a dog or other animal.5. a wire that conveys electric current from a source to an appliance, or that connects two points of a circuit together.6. the distance advanced by a screw in one turn.7. a channel, in particular: ∎ an artificial watercourse leading to a mill. ∎ a channel of water in an ice field.PHRASES: lead someone astray cause someone to act or think foolishly or wrongly.lead someone by the nose inf. control someone totally, esp. by deceiving them.lead someone a dancesee dance.lead from the front take an active role in what one is urging and directing others to do.lead someone up (or down) the garden path inf. give someone misleading clues or signals.lead the waysee way.lead with one's chin inf. (of a boxer) leave one's chin unprotected. ∎ fig. behave or speak incautiously.PHRASAL VERBS: lead off1. start: the newsletter leads off with a report on tax bills. ∎ Baseball bat first in a game or inning.2. (of a door, room, or path) provide access away from a central space: a farm track led off to the left.lead someone on mislead or deceive someone, esp. into believing that one is in love with or attracted to them.lead up to immediately precede: the weeks leading up to the elections. ∎ result in: fashioning a policy appropriate to the situation entails understanding the forces that led up to it.lead2 / led/ • n. 1. a heavy, bluish-gray, soft, ductile metal, the chemical element of atomic number 82. It has been used in roofing, plumbing, ammunition, storage batteries, radiation shields, etc., and its compounds have been used in crystal glass, as an antiknock agent in gasoline, and (formerly) in paints. (Symbol: Pb) 2. an item or implement made of lead, in particular: ∎ Naut. a lead casting suspended on a line to determine the depth of water. ∎ bullets.3. graphite used as the part of a pencil that makes a mark.4. Printing a blank space between lines of print. [ORIGIN: originally with reference to the metal strip used to create this space.]PHRASES: get the lead out inf. move or work more quickly.
Lead
Lead
A metallic element with atomic number 82. Symbol Pb, atomic weight 207.19, specific gravity 11.35, melting point 621.32°F (327.4°C), boiling point 3,191°F (1,755°C).
Lead is in column IVA of the periodic table . It has four naturally occurring stable isotopes, lead-204, lead-206, lead-207, and lead-208. The last three of these are all end products of one or another radioactive family.
Lead is one of the first elements known to human societies. It is described in some of the oldest books of the Old Testament and was widely used by some early civilizations. Examples of objects containing lead from fifth millennium Egyptian cultures have been found. The Greeks and Romans also used lead for the manufacture of a variety of tools and containers. Some experts claim that one reason for the decline of the Roman civilization was the extensive use of lead in the empire's water supply system. Lead is now known to have a variety of serious effects on the human nervous system , including diminished mental capacity.
Lead was originally known by its Latin name of plumbum, from which its modern chemical symbol (Pb) is derived. The Latin name is still preserved also in the common names for lead compounds, as in plumbic and plumbous chlorides.
General properties
Lead is a heavy, ductile, soft, grayish solid. It can be cast and fabricated easily and has the unusual ability to absorb sound and other forms of vibration. Although it is dissolved by dilute nitric acid , it tends to be resistant to other forms of acid solutions.
By far the most important use of lead is in storage batteries, with the production of antiknock gasolines, pigments, ammunition, solder, and plumbing materials following. In the last half century, evidence about the toxic effects of lead has accumulated to the point where its presence in the environment is considered to be a serious hazard for humans, especially for young children.
Where it comes from
Lead is thought to be the thirty-sixth most abundant element in Earth's crust, with a concentration of about 13 parts per million. This makes the element more common than other heavy metals such as thallium or uranium , but much less abundant than less well known elements such as niobium, neodymium, lanthanum, and gallium.
The most important ore of lead is galena, lead sulfide (PbS). Anglesite (lead sulfate; PbSO4) and cerussite (PbCO3) are also economically important. Both are formed by the weathering of galena.
Over half of the lead produced in the world comes from just four regions and nations: the United States, Russia, other members of the former Soviet Union, Australia , and Canada. In the United States, about 90% of all lead comes from seven mines in Missouri, with the rest originating from mines in Colorado, Idaho, and Utah.
How the metal is obtained
The raw material from which lead metal is produced is either a naturally occurring ore or, more commonly today, lead products returned for recycling . In the United States, more than half of all lead produced comes from recycled materials, especially recycled storage batteries. After initial treatment, the major steps by which lead is obtained from either ore or recycled material are very similar.
In the case of a naturally occurring ore of lead, the first step is usually to concentrate the ore and separate it from other metallic ores. This step often involves the froth flotation process in which the mixture of ores is finely ground and then added to a water mixture that contains one or more other materials, such as hydrocarbons, sodium cyanide, copper sulfate, or pine oil. Air is then pumped through the ore/water/secondary material mixture, producing a frothy mixture containing many small bubbles.
The added material causes secondary ores such as ores of copper or zinc either to adhere or not adhere to the bubbles in the froth, allowing their separation from lead ores which respond in the opposite manner to the secondary material. The use of copper sulfate in the flotation process, for example, aids in the separation of zinc ores from lead ores.
After separation, lead sulfide is heated in a limited supply of air to convert it to lead oxide. The lead oxide is then mixed with coke (carbon ) and a flux such as limestone in a blast furnace. Within the blastfurnace, coke burns to form carbon monoxide which, in turn, reacts with lead oxide to form metallic lead and carbon dioxide .
In a variation of this procedure, the lead oxide can be mixed with lead sulfide and heated. The reaction that takes place results in the formation of lead metal and sulfur dioxide gas.
The lead produced by either of these methods is still impure, containing small amounts of copper, tin, arsenic, antimony, and other metals. Each metallic impurity is then removed by some additional step. In the case of copper, for example, the impure lead is heated to a temperature just above its melting point. At this temperature, copper is still a solid. Any copper mixed with the lead floats on top of the lead, and can be scraped off.
How we use it
Metallic lead is sometimes used in a pure or nearly pure form, usually because of its high density and ability to be bent and shaped. The metal is an efficient absorber of radiation and, for that reason, is commonly used as a shield for x rays , nuclear radiation, and other forms of radiation.
Far more commonly, however, lead is alloyed with one or more other elements to produce a product with special properties of interest for some specific application. More than half of all the lead used in the United States, for example, goes to the production of lead storage batteries. The positive plates, made of lead(IV) oxide, and the negative plates, made of spongy lead, are both made from an alloy containing 91% lead and 9% antimony. Over 80% of this lead is now recovered and recycled as a source of lead metal.
At one time, very large amounts of lead were used in the production of tetraethyl lead, a compound that reduces the amount of knocking in an internal combustion engine . The problem with tetraethyl lead, however, is that it tends to break down within an engine, releasing free lead to the environment. Because of the health hazards that lead poses for humans and other animals, tetraethyl lead has been banned for use as a gasoline additive.
Because of its chemical inertness, lead has also been popular as a covering for underground cables, such as buried cables that carry telephone messages, and for pipes through which liquids are transported. For many years, lead was the material of choice in the construction of water pipes since it was inert to most chemicals occurring in nature and easily shaped. With the recognition of lead's threat to humans, however, many of these applications have been discontinued.
Alloys of lead are also popular for the manufacture of solders. Ordinary plumber's solder, for example, contains about two parts of lead to one part of tin. This alloy has a melting point of about 527°F (275°C).
Lead compounds were once widely used also for paints. They were in great demand because they covered surfaces well and were available in a number of vivid colors. Among these were lead chromate (yellow), lead molybdate (reddish-orange), lead(II) oxide (canary yellow), red lead oxide (Pb3O4; red), and white lead, a complex lead carbonate/lead hydroxide mixture. As with other lead compounds, however, the potential health hazards of the element have greatly reduced the availability of lead-based paints.
Chemistry and compounds
Lead is a reactive metal, but its reactivity is somewhat inhibited by the formation of an outer skin of protective compounds. For example, when a freshly cut piece of lead metal is exposed to the air, it quickly reacts with oxygen to form a thin outer layer of lead oxide. This outer layer then prevents further reaction between the metal and oxygen and other constituents of the air. A similar phenomenon occurs when lead metal is placed into water. Compounds present in water react with lead to form an outer skin of lead carbonate, lead silicate, or similar compounds that protect the metal from further attack. This property helps to explain the long popularity of lead for the lining of pipes designed to carry many different kinds of liquids.
From its position in a table of electrode potentials, one would expect lead to replace hydrogen from acids. But the difference in electrode potentials between the two elements is so small (0.126 volts), that lead reacts with most acids only very slowly indeed. The element does tend to react with oxygen-containing acids more readily, but only because of oxidation that may take place at the same time.
One application that takes advantage of this property is the use of lead to line containers that hold concentrated sulfuric acid . As long as those containers are kept at temperatures below 140°F (60°C), there is essentially no reaction between the acid and metal lining.
When lead does take part in a chemical reaction, it demonstrates one of two oxidation states, 4+ and 2+. Compounds of the former class are known as lead(IV) or plumbic compounds, while those of the latter class are lead(II) or plumbous compounds. Like aluminum , lead is amphoteric and will react with strong bases. The products of such reactions are known as plumbates and plumbites.
Biological effects
Throughout most of human history, lead was used for a wide variety of applications with little or no appreciation of the serious health hazards it poses. Today, physiologists understand that the human body is able to excrete about 2 milligrams of lead efficiently each day, but that quantities in excess of that can cause serious health problems.
Children are especially at risk for lead poisoning. Their bodies do not metabolize lead as quickly as do those of adults, so a given concentration of lead in the blood will have more serious consequences for a child than for an adult.
At relatively low concentrations, lead produces relatively modest or short-term effects, including elevation of blood pressure , reduction in the synthesis of hemoglobin, and decreased ability to utilize vitamin D and calcium . With increased blood concentrations of lead, however, these problems become more severe. Impairment of the central nervous system can occur, with decreased mental functioning and hearing damage as two possible results. At very high lead concentrations, a person can fall into a coma and, eventually, die.
With the recognition of these problems, governmental agencies have continually restricted the number of applications in which lead can be used. Unfortunately, its widespread use in previous years means that many children (especially) and adults are still at risk for lead poisoning. As an example, children not uncommonly pick off and then eat chips of paint from the walls of old buildings. Since many of these paints were made with compounds of lead, those children are then exposed to the harmful effects of the element.
See also Element, chemical; Metallurgy.
Resources
books
Emsley, John. Nature's Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press, 2002.
Greenwood, N.N., and A. Earnshaw. Chemistry of the Elements. 2nd ed. Oxford: Butterworth-Heinneman Press, 1997.
Hawley, Gessner G., ed. The Condensed Chemical Dictionary. 9th edition. New York: Van Nostrand Reinhold 1977.
Joesten, Melvin D., David O. Johnston, John T. Netterville, and James L. Wood. World of Chemistry. Philadelphia: Saunders, 1991.
"Lead." Kirk-Othmer Encyclopedia of Chemical Technology. 4th ed. Suppl. New York: John Wiley & Sons, 1998.
David E. Newton
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Ductile
—Capable of being drawn or stretched into a thin wire.
- Flux
—In the conversion of ores to metals, a substance used to remove impurities from a blast furnace or other kind of processing system.
- Froth flotation
—A system for separating a desired ore from other kinds of ores by pumping air through a mixture of water and one or more other substances.
- Hemoglobin
—An iron-containing, protein complex carried in red blood cells that binds oxygen for transport to other areas of the body.
- Isotopes
—Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties.
Lead
Lead
Lead (symbol Pb, atomic number 82) is a soft, dense, bluish-gray metal that melts at the relatively low temperature of 328°C (662°F). It has many beneficial uses in compounds as well as in its metallic form, but is toxic at almost any level in the body. Mentioned in the Bible, lead was one of the first known metals. Its toxicity was also recognized long ago; Greek physicians made the first clinical description of lead poisoning in the first century b.c.e., and lead is arguably the earliest known industrial pollutant.
Lead taken internally in any of its forms is highly toxic. At higher body levels, the symptoms of lead poisoning are anemia, weakness, constipation, colic, palsy, and often a paralysis of the wrists and ankles. At low levels, there may be no symptoms. Young children are especially at risk from lead, even at levels once thought safe. Low-level lead poisoning can reduce intelligence, delay motor development, impair memory, and cause hearing problems and troubles in balance.
Higher levels of lead poisoning are reduced with the use of chelating agents that help the body to excrete the lead in urine. Although this may address the physical symptoms mentioned above, there is no cure for the loss of IQ and other neurological effects that lead poisoning has on young children.
Lead was used by the Romans to make water pipes and create elaborate urban water systems. The word plumbing comes from the Latin word for lead, plumbum. Lead was, and still remains, a natural choice for plumbing, Widely available, it is durable and easily malleable , and it does not rust. Water is still delivered to homes in many U.S. cities via lead supply pipes. Alloys of lead are also used in solder and in brass faucets and fixtures. Drinking water can leach lead out of a plumbing system, and this may be one source of lead exposure.
The most common uses of lead today are in lead-acid storage batteries and to shield against radiation. Computer screens are made of leaded glass to contain the electromagnetic radiation within, and as a consequence, two U.S. states have banned the disposal of CRT monitors in landfills and incinerators.
Lead is useful in many compounds. Lead carbonate, called white lead, has been used for over 2,000 years as a white pigment in paint and ceramic glazes, and other lead compounds have been used as pigments and driers . Lead-based paint was first identified as the source of deadly childhood poisoning in Australia in 1904.
Subsequently, lead-based paint was banned in Australia and much of Europe in the 1920s, but the United States did not prohibit its residential use for another fifty years. By 1971 it was determined that two hundred children a year died annually in this country as a result of lead poisoning. That year Congress passed the Lead-Based Poisoning Prevention Act, but delayed implementation of its official ban until 1977.
The lead-based paint applied to homes during the first two-thirds of the twentieth century continues to be the primary cause of childhood lead poisoning. Children who eat flakes of peeling and chipping paint in older, unmaintained housing are at serious risk. The National Survey of Lead and Allergens in Housing for 1998 to 2000 found that some 38 million housing units contain lead-based paint. Some 25 million of these units have "significant" lead-based paint hazards.
Even lead-based paint that is in good condition can pose a risk as the dust created by the friction of opening and closing windows may cause low-level lead poisoning. The renovation of an older home, when done improperly, can poison adults and children as well as pets living in that residence. Residential lead-based paint should never be sanded or burned off.
Lead poisoning is an important health problem, affecting an estimated 890,000 preschoolers, according to the U.S. Centers for Disease Control and Prevention. That means that about 4.4 percent of children aged one to five have unacceptably high levels of lead in their bodies. Although lead poisoning crosses all socioeconomic, geographic, and racial boundaries, the burden of this disease falls disproportionately on low-income families and those of color. In the United States, children from poor families are eight times more likely to be poisoned by lead than those from higher-income families.
Another compound, tetraethyl lead, was once routinely added to gasoline to prevent knocking or premature detonation in internal combustion engines. The lead survived the combustion process and became a significant contributor to air pollution. Leaded gasoline was phased out in the United States starting in 1976. All gasoline-powered cars and trucks now sold in this country must burn unleaded gasoline.
Leaded gasoline nevertheless remains a problem in many other countries. In 1995 fewer than thirty countries worldwide had banned leaded gasoline. In 1996 the World Bank called for the international phasing out of leaded gasoline, claiming that most of the 1.7 billion urban dwellers in developing countries were at risk from lead poisoning. The United Nations Commission on Human Settlements—known as Habitat—approved a resolution in 1999 that committed member nations to begin phasing out leaded gas. By 2001 forty-five nations worldwide had banned its use.
Because lead is an element; it does not biodegrade . Lead pollution from the dawn of civilization remains in the environment. Ice-core researchers in North Greenland have found layers of glacial ice contaminated with lead from ancient Rome's smelters. The lead pollution emitted by smelters can reach staggering levels. In Herculaneum, Missouri, where the nation's largest lead smelter has been in operation for more than one hundred years, health officials documented that almost 28 percent of children under seven have elevated levels of lead in their bloodstream; close to the facility that figure rose to 45 percent. Dust samples along the roads used by trucks serving the smelter contained extremely high concentrations of lead (up to 300,000 parts per million), and the site has been declared an urgent public health hazard.
Enforcement actions by the U.S. Environmental Protection Agency (EPA) and the Missouri Department of Natural Resources led to an agreement by the Doe Run Company, the smelter's owner, to install new controls on air emissions, remediate lead contamination in residential yards, and stabilize a contaminated slag pile located in the Mississippi River flood plain.
Although lead is a persistent and widespread contaminant in both natural and man-made environments, lead poisoning is an entirely preventable disease. The key to prevention is the elimination of sources and pathways . The positive results of bans on leaded gasoline, lead in paints and glazes, lead solder, and lead plumbing can be seen in the reduction in the number of lead-poisoning cases as well as the decreased levels of lead found in the general population.
Bibliography
Stapleton, Richard. (1994). Lead Is a Silent Hazard. New York: Walker and Company.
Warren, Christian. (2001). Brush with Death; A Social History of Lead Poisoning. Baltimore: Johns Hopkins University Press, 2001.
internet resources
Alliance to End Childhood Lead Poisoning. Available at http://www.aeclp.org.
CDC Childhood Lead Poisoning Prevention Program. Available at http://www.cdc. gov/nceh/lead/lead.htm.
Richard M. Stapleton
Lead
LEAD
Lead (Pb) is a soft, corrosion-resistant gray metal that is a common environmental contaminant in air, food, paint, and water. Lead is recovered from mined sulfide ores, and has been used to fashion items such as statues and tools since at least 6500 b.c.e. The Romans used lead to fashion potable water piping. The relationship between plumbing and lead has become a permanent part of the English language—the word "plumbing" derives from the Latin word for lead, plumbum. Besides plumbing, lead has been used to manufacture items such as ceramics, cosmetics, lead batteries, leaded paint, and leaded gasoline. Common chemical species of lead used commercially include lead acetate, lead carbonate, lead chloride, and lead oxide.
Figure 1
The Agency for Toxic Substances and Disease Registry (ATSDR) estimates that more than one million workers in one hundred occupations are exposed to lead, such as in the lead-battery recycling and lead-smelting industries. Equally important, almost all persons are exposed to lead in residential settings from sources such as paint chips, food, water, cigarettes, and clothing that has been worn in lead-contaminated work environments.
Adverse health effects from lead exposure have been recognized since the time of the Romans. The National Research Council (NRC) traces society's more recent interest in lead poisoning to an 1839 publication by Tanquerel des Planches, who described lead colic in 1,207 occupationally exposed workers. It is now recognized that even low levels of lead exposure are associated with adverse health effects. The U.S. Centers for Disease Control and Prevention (CDC) has identified a lead concentration of ten micrograms per deciliter (µg/dL) of blood as the level of concern above which significant health risks occur. Acute exposure to lead compounds may cause brain damage, kidney damage, and gastrointestinal distress. Chronic exposure to lead results in effects on the blood, the central nervous system, blood pressure, the kidneys, the male reproductive system, and vitamin D metabolism. Children, particularly impoverished children living in homes with lead paint, are particularly at risk from the toxic effects of lead, and may exhibit slowed cognitive development and decreased intelligence after chronic exposure. Figure 1 identifies the health effects of lead at different blood level concentrations.
Because lead does not biodegrade, the approximately 300 million metric tons of lead produced to date remains in the environment. This suggests that humans will continue to be exposed to lead despite the phasing out of lead in consumer products such as gasoline and paint. In the early 1970s, the federal government recognized that steps had to be taken to reduce human exposure to lead, and banned residential leaded paint (1978), and phased out leaded gasoline between 1975 and 1995. The removal of lead from gasoline has proceeded more slowly in the rest of the world. In some countries leaded gasoline remains a significant source of exposure.
The CDC estimates that children's blood lead levels have declined over eighty percent since the mid-1970s. The Lead Contamination Act of 1988 authorized the CDC to initiate programs to eliminate childhood lead poisoning in the United States. The Lead Contamination Act of 1988 authorized the CDC to make grants to state and local agencies for comprehensive programs designed to screen infants and children for elevated blood lead levels, ensure referral for medical and environmental intervention for lead-poisoned infants and children, and provide education about childhood lead poisoning. Despite this impressive decrease in blood lead levels, more than one million children in the United States have blood lead levels above 10 µg/dL, and are at risk of permanent neurological impairment.
Margaret H. Whitaker
Bruce A. Fowler
(see also: Blood Lead; Environmental Determinants of Health; Heavy Metals; Occupational Disease; Occupational Safety and Health; Regulations Affecting Housing )
Bibliography
Agency for Toxic Substances and Disease Registry (ATSDR) (1993). Toxicological Profile for Lead. Washington, DC: U.S. Department of Health and Human Services.
—— (2000). Case Studies in Environmental Medicine: Lead Toxicity. Available at http://www.atsdr.cdc.gov/HEC/caselead.html.
Centers for Disease Control and Prevention (1997). "Update: Blood Lead Levels—United States 1991–1994." Morbidity and Mortality Weekly Report 46(7):141–146 and erratum in 46(26):607.
Lewis, J. (1985). "Lead Poisoning: A Historical Perspective." EPA Journal. Available at http://www.epa.gov/history/topics/perspect/lead.htm.
National Research Council, Commission on Life Sciences (1993). Measuring Lead Exposure in Infants, Children, and Other Sensitive Populations. Available at http://stills.nap.edu/books/030904927X/html/.
President's Task Force on Environmental Health Risks and Safety Risks to Children (2000). Eliminating Childhood Lead Poisoning: A Federal Strategy Targeting Lead Paint Hazards. Available at http://www.epa.gov/children/whatwe/leadhaz.pdf.
U.S. Environmental Protection Agency (U.S. EPA) (1998). Lead in Your Home: A Parent's Reference Guide. Office of Prevention, Pesticides, and Toxic Substances. Available at http://www.epa.gov/lead/leadrev.pdf.
Lead
Lead
Lead (Pb) is a relatively common element in Earth crustal materials. Lead is a heavy, soft metal that is a solid at normal atmospheric and crustal pressures. Lead is reactive with oxygen and tarnishes and dulls when in contact with oxygen. Lead is not a good conductor of electricity , heat, sound, or other pressure vibrations.
Lead is found in Earth's crust at an abundance of about 13–20 parts per million. It rarely occurs as a free element, and is found most commonly as a compound in the form of galena (lead sulfide; PbS), anglesite (lead sulfate; PbSO4), cerussite (lead carbonate; PbCO3), and mimetite. Geochemically, lead is a moderately active metal that dissolves very slowly in water.
Lead is both ductile and malleable. These properties allow lead to be easily bent, cut, pulled, or otherwise worked to produce specific shapes. The melting point of lead is 621.3°F (327.4°C), its boiling point is about 3,180°F (1,749°C), and its density is 11.34 grams per cubic centimeter.
The largest producers of lead in the world are Australia , China, the United States, Peru, Canada, Mexico, and Sweden. In the United States, more than 90% of all the lead produced comes from a single state, Missouri. Lead is extracted from its ores by first converting the ore to lead oxide and then heating the oxide with charcoal (pure carbon ). The lead produced by this process is usually not very pure and can be further refined electrolytically.
Over the past decades, evidence has mounted indicating lead as a significant environmental hazard. Low levels of lead in products (e.g., paint) can accumulate in tissues over time. As a result, many manufactured items (e.g., batteries) now have or seek lead substitutes or provide for contained disposal.
See also Chemical elements; Minerals
Lead
Lead
melting point: 327.5°C
boiling point: 1,740.0°C
density: 11.34 g/cm 3
most common ions: Pb 2+, Pb 4+
Lead makes up only about 0.0013 percent of Earth's crust but was well known in the ancient world and was even mentioned in the Book of Exodus. The word "lead" is derived from the Anglo-Saxon word laedan. Lead's symbol, Pb, comes from the Latin word for lead, plumbum. Because of lead's long use in piping, the word "plumber" comes from that same root. Lead is an extremely dense but malleable metal that is very resistant to corrosion.
Lead is sometimes found free in nature but is usually obtained from ores such as galena (PbS) or cerussite (PbCO3), from which it is easily mined and refined. Most lead is obtained by simply roasting galena in hot air. About one-third of the lead used in the United States is obtained through recycling efforts.
Lead has seen many uses over the ages. As a constituent of pewter (an alloy of tin and lead), lead was a component of Roman eating and drinking utensils. It has been suggested that the decline of the Roman Empire may have been tied to this use, since acidic foodstuffs extract small amounts of lead, a cumulative human poison. Lead's use as a pottery glaze has been banned for the same reason—the danger of lead ingestion via the extraction of the lead by food and drink. During the twentieth century, a volatile form of lead—tetraethyl lead [Pb(CH2CH3)4]—was developed and widely used to improve the octane level of gasoline. That use has also been banned for health and environmental reasons.
Lead remains in wide use in electrical cable sheathing, automobile batteries, lead crystal, radiation protection, and some solders.
see also Inorganic Chemistry; Radioactivity.
George H. Wahl Jr.
Bibliography
Internet Resources
Jefferson Lab. "It's Elemental: The Element Lead." Available from <http://education.jlab.org/itselemental/ele082.html>.
Lead
Lead
One of the oldest metals known to humans, lead compounds were used by Egyptians to glaze pottery as far back as 7000 b.c. The toxic effects of lead also have been known for many centuries. In fact, the Romans limited the amount of time slaves could work in lead mines because of the element's harmful effects. Some consequences of lead poisoning are anemia , headaches, convulsions, and damage to the kidneys and central nervous system. The widespread use of lead in plumbing, gasoline , and lead-acid batteries, for example, has made it a serious environmental health problem. Bans on the use of lead in motor fuels and paints attempt to deal with this problem.