LEAD (PB) HAS been used since ancient times. It is commonly used in modern petroleum engineering, atomic reactors, and many organic (synthetic) compounds. Lead is a soft metal that has very little strength by itself. To add strength, it is usually combined with other metals to form alloys. Tin and antimony are often used to make lead alloys that have greater strength. Called plumbum by the Romans, pure lead is almost never found in nature; rather, it is found bonded with other compounds. Its most common form as an ore is galena, which is lead sulfide, a gray metallic ore.
Deposits of galena have been found all over the world. Numerous deposits have been mined in southwestern Wisconsin, Iowa, and Missouri, and in Cornwall, England. Joplin, Missouri, was a major galena-mining center for many years. Its galena deposits were worked as open-pit mines or deep shaft mines. Tailings of rock that contained the ore still cover large areas. Galena deposits frequently include copper, gold, and silver. Joplin’s deposits also included zinc.
Galena ore is extracted from dirt, rocks, and other debris by crushing and by flotation. The process of flotation concentrates the ore in a tank containing a chemical such as copper sulfate or an oil of some type. The ore floats in the flotation bath, and the tailings sink to the bottom. Then the ore is skimmed from the flotation bath into a concentrated mass.
Concentrated galena is roasted to break the bonding with the lead. The roasting causes the sulfur to react with oxygen, forming sulfur dioxide (SO2), which has many industrial uses, especially in the paper industry. Lead oxide (PbO2) is also created in particulate form. Then the substance is sintered into pellets that can be reduced to pure lead by burning them with coke. The carbon in the coke reacts with the oxygen in the lead oxide to form carbon dioxide and a mass of metal. The resulting metal is rarely pure lead; instead, it is mixed with gold, silver, zinc, copper, and slag, which are separated in turn to produce pure lead.
Lead has a relatively low melting point of 327.5 degrees C (621.50 degrees F). It has numerous uses—as a covering to prevent leaks, for example. Its density makes it an excellent shield against X-rays in medical procedures. Lead is extensively used in atomic reactors to safeguard against radiation; it is also used as shielding on shipping containers for radioactive wastes and materials.
Chemical companies use lead for pipes because it resists corrosion. Lead is also used for shipping and storing many chemicals. Usually, the lead in these containers is an alloy of some type that adds strength to the lead’s anticorrosive properties. Many electrical and electronic products use lead alloys to protect equipment from water corrosion. Power companies apply lead alloys to areas of power equipment that need protection. Lead is a component of solder, a tin–lead alloy that is used to join metal surfaces. The use of solder is common in the automotive and electronics industries.
The products that consume the greatest quantities of lead are lead–acid storage batteries, gasoline additives, and paints and dyes. In addition, large quantities are consumed in the manufacture of corrosion-resistant red and white lead paints, as well as explosives, insecticides, and some rubber products.
Lead–acid batteries, used in gasoline and electrical vehicles, consume most of all the lead currently produced. The dumping of these batteries has virtually ceased, and almost 100 percent of them are recycled globally, so lead–acid batteries are now insignificant sources of lead pollution.
Gasoline additives made with lead are used to reduce or prevent engine knocking. Tetraethyl lead was for a long time the second-largest consumer of lead. Leaded gasoline was standard for decades until its chemical-pollution drawbacks were identified. The use of tetraethyl lead as an antiknock chemical was discovered in 1922. DuPont Chemical Co. and General Motors created Ethyl Corp. in 1924 to manufacture and distribute tetraethyl lead. Almost immediately, the health risks were identified, and action to prevent its use was considered. The gasoline industry was able to block opposition and to use tetraethyl lead for decades, however. At the plants where tetraethyl lead was produced, some workers who handled it suffered illness or death from lead poisoning—one result of careless handling of tetraethyl lead or inadequate safety measures.
Beginning in the 1970s the U.S. Environmental Protection Agency took steps to ban lead as a gasoline additive. The sale of leaded gasoline in the United States has been illegal since 1995. Leaded gasoline is still manufactured, however, and is still used in farm machinery, race cars, and aviation.
Lead is used in paint to achieve many goals. It is used as a pigment; it is used to add durability, to keep the paint looking fresh for a longer period; and it is used to speed drying time and resist moisture that could cause corrosion. White and red lead paints are usually used to coat steel beams or steel plating. Lead paint is also used extensively on military equipment and to paint lines on roadways and parking lots. Since 1978 paint with lead content of more than .006 percent has been banned in residential housing (Code of Federal Regulations 1303), due to the danger that children will eat flaking lead paint. Lead is very harmful to children under the age of 6 because it can stunt growth, harm mental development, damage the nervous system, and destroy hearing. Some states seek to force paint companies to pay for the removal of lead paint from older housing. In 2006 several state cases involving issues related to lead paint reached the U.S. Supreme Court; decisions were expected in 2007.
Exposure to lead is usually through ingestion or inhalation. In addition, lead can be absorbed through the skin with long exposure. Inhaling lead particles or absorbing them through the skin poses is a danger to health, especially if exposure has been extensive and over a long period. The toxicity of lead arises from its similarity to other metals used by the human body, such as calcium, iron, and zinc. The proteins that combine with metals used by the body react with enzymes to perform necessary biological functions. Lead’s reactivity displaces ions of calcium, iron, zinc, and other metals to create a metabolic disorder. There is no known function for lead in the human body. Lead accumulates in the body, especially in the skeleton.
Lead poisoning is also called saturnism, plumbism, or painter’s colic. It is a serious health condition that can, in severe cases, cause seizure, coma, and death. It disrupts the production of red blood cells and may damage the kidneys, liver, brain, and other organs.
Symptoms of chronic lead poisoning include neurological problems, nausea, stomach cramps, fatigue, insomnia, hyperactivity, headaches, and loss of mental capacities. Gastrointestinal symptoms include constipation, diarrhea, vomiting, and changes in appetite.
To date, studies have found that lead is a carcinogenic agent in animals, but the evidence for human carcinogenetic reactions to lead is still inadequate. Studies strongly suggest that exposure to lead can be a secondary factor in the development of cancer.
The National Toxicology Program has reported through its Carcinogens Review Committee that lead and lead compounds should be considered to be possible human carcinogens. Four epidemiological studies have been made of workers in lead–acid battery plants and lead smelters. Those with obvious lead poisoning were examined, but only slightly higher rates of cancer were found.
The studies conducted to date did not report on smoking among the subjects of the study. The results were inconsistent in the events, locations, kinds of cancers, and other data points. The evidence is believed to be inadequate to determine whether lead is a human carcinogen.
Studies of laboratory animals produced tumors in rats and mice after exposure to soluble lead salts (phosphates and acetates). The studies found that lead affected gene expression. The studies examined different lead compounds but did not examine the carcinogenicity of lead in a systematic way. In the 10 studies of rats, for example, the strain of rats used was not reported. By contrast, hamsters given lead salts did not develop tumors.
Studies have indicated that lead compounds can create chromosomal changes. Lead has been shown to affect DNA structure and the molecular processes associated with gene expression. Lead exposure is a variable, but studies have not been able to demonstrate the effects of age, health, exposure duration, nutritional state, and other factors that may be connected with cancer events.
Battery Acid; Dyes and Pigments; Gasoline; Genetics; Insecticides; Paint.
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