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Text 14

 

Lead is a chemical element in the carbon group with symbol Pb (from Latin: plumbum) and atomic number 82. Lead is a soft and malleable metal, which is regarded as a heavy metal and poor metal. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed to air. Lead has a shiny chrome-silver luster when it is melted into a liquid.

Lead is used in building construction, lead-acid batteries, bullets and shot, weights, as part of solders, pewters, fusible alloys, and as a radiation shield. Lead has the highest atomic number of all of the stable elements, although the next higher element, bismuth, has a half-life that is so long (much longer than the age of the universe) that it can be considered stable. Its four stable isotopes have 82 protons, a magic number in the nuclear shell model of atomic nuclei.

Lead, at certain contact degrees, is a poisonous substance to animals, including humans. It damages the nervous system and causes brain disorders. Excessive lead also causes blood disorders in mammals. Like the element mercury, another heavy metal, lead is a neurotoxin that accumulates both in soft tissues and the bones. Lead poisoning has been documented from ancient Rome, ancient Greece, and ancient China.

Lead is a bright and silvery metal with a very slight shade of blue in a dry atmosphere. Upon contact with air, it begins to tarnish by forming a complex mixture of compounds depending on the conditions. It has a few characteristic properties: high density, softness, ductility and malleability, poor electrical conductivity compared to other metals, high resistance to corrosion, and ability to react with organic chemicals.

Various traces of other metals change its properties significantly: the addition of small amounts of antimony or copper increases hardness and improves the corrosion reflection from sulfuric acid for lead. A few other metals also improve only hardness and fight metal fatigue, such as cadmium, tin, or tellurium; metals like sodium or calcium also have this ability, but they weaken the chemical stability. Finally, zinc and bismuth simply impair the corrosion resistance (0.1% bismuth content is the industrial usage threshold). In return, lead impurities mostly worsen the quality of industrial materials.

Lead has been commonly used for thousands of years because it is widespread, easy to extract and easy to work with. It is highly malleable as well as easy to smelt. Metallic lead beads dating back to 6400 BCE have been found in Çatalhöyük in modern-day Turkey. In the early Bronze Age, lead was used with antimony and arsenic.

The largest preindustrial producer of lead was the Roman economy, with an estimated annual output of 80,000 tones, which was typically won as a by-product of extensive silver smelting. Roman mining activities occurred in Central Europe, Roman Britain, the Balkans, Greece, Asia Minor and Hispania which alone accounted for 40% of world production.

Roman lead pipes often bore the insignia of Roman emperors (see Roman lead pipe inscriptions). Lead plumbing in the Latin West may have been continued beyond the age of Theoderic the Great into the medieval period. Many Roman "pigs" (ingots) of lead figure in Derbyshire lead mining history and in the history of the industry in other English centers. The Romans also used lead in molten form to secure iron pins that held together large limestone blocks in certain monumental buildings. In alchemy, lead was thought to be the oldest metal and was associated with the planet Saturn. Alchemists accordingly used Saturn's symbol (the scythe, ♄) to refer to lead.

Up to the 17th century, tin was often not distinguished from lead: lead was called plumbum nigrum (literally, "black lead"), while tin was called plumbum candidum (literally, "bright lead"). Their inherence through history can also be seen in other languages: the word "olovo" means lead in Czech, but in Russian it ("олово") means tin. Lead's symbol Pb is an abbreviation of its Latin name plumbum for soft metals; the English words "plumbing", "plumber", "plumb", and "plumb-bob" also derive from this Latin root.

Lead production in the US commenced as early as the late 1600s by Indians in the The Southeast Missouri Lead District, commonly called the Lead Belt, is a lead mining district in the southeastern part of Missouri. Significant among Missouri's lead mining concerns in the district was the Desloge Family and Desloge Consolidated Lead Company in Desloge, Missouri and Bonne Terre – having been active in lead trading, mining and lead smelting from 1823 in Potosi to 1929.

Metallic lead does occur in nature, but it is rare. Lead is usually found in ore with zinc, silver and (most abundantly) copper, and is extracted together with these metals. The main lead mineral is galena (PbS), which contains 86.6% lead by weight. Other common varieties are cerussite (PbCO3) and anglesite (PbSO4).

Most ores contain less than 10% lead, and ores containing as little as 3% lead can be economically exploited. Ores are crushed and concentrated by froth flotation typically to 70% or more. Sulfide ores are roasted, producing primarily lead oxide and a mixture of sulfates and silicates of lead and other metals contained in the ore. Lead oxide from the roasting process is reduced in a coke-fired blast furnace to the metal. Additional layers separate in the process and float to the top of the metallic lead. These are slag (silicates containing 1.5% lead), matte (sulfides containing 15% lead), and speiss (arsenides of iron and copper). These wastes contain concentrations of copper, zinc, cadmium, and bismuth that can be recovered economically, as can their content of unreduced lead.

Metallic lead that results from the roasting and blast furnace processes still contains significant contaminants of arsenic, antimony, bismuth, zinc, copper, silver, and gold. The melt is treated in a reverberatory furnace with air, steam, and sulfur, which oxidizes the contaminants except silver, gold, and bismuth. The oxidized contaminants are removed by drossing, where they float to the top and are skimmed off. Since lead ores contain significant concentrations of silver, the smelted metal also is commonly contaminated with silver. Metallic silver as well as gold is removed and recovered economically by means of the Parkes process.

Desilvered lead is freed of bismuth according to the Betterton-Kroll process by treating it with metallic calcium and magnesium, which forms a bismuth dross that can be skimmed off. Very pure lead can be obtained by processing smelted lead electrolytically by means of the Betts process. The process uses anodes of impure lead and cathodes of pure lead in an electrolyte of silica fluoride.

Production and consumption of lead is increasing worldwide. Total annual production is about 8 million tonnes; about half is produced from recycled scrap. The top lead producing countries, as of 2008, are Australia, China, USA, Peru, Canada, Mexico, Sweden, Morocco, South Africa and North Korea. Australia, China and the United States account for more than half of primary production. In 2010, 9.6 million tonnes of lead were produced, of which 4.1 million tonnes came from mining.

At current use rates, the supply of lead is estimated to run out in 42 years. Environmental analyst Lester Brown has suggested lead could run out within 18 years based on an extrapolation of 2% growth per year. This may need to be reviewed to take account of renewed interest in recycling, and rapid progress in fuel cell technology. According to the International Resource Panel's Metal Stocks in Society report, the global per capita stock of lead in use in society is 8 kg. Much of this is in more-developed countries (20–150 kg per capita) rather than less-developed countries (1–4 kg per capita).

Lead is a highly poisonous metal (regardless if inhaled or swallowed), affecting almost every organ and system in the body. The main target for lead toxicity is the nervous system, both in adults and children. Long-term exposure of adults can result in decreased performance in some tests that measure functions of the nervous system. Long-term exposure to lead or its salts (especially soluble salts or the strong oxidant PbO2) can cause nephropathy, and colic-like abdominal pains. It may also cause weakness in fingers, wrists, or ankles. Lead exposure also causes small increases in blood pressure, particularly in middle-aged and older people and can cause anemia. Exposure to high lead levels can severely damage the brain and kidneys in adults or children and ultimately cause death. In pregnant women, high levels of exposure to lead may cause miscarriage. Chronic, high-level exposure has shown to reduce fertility in males.

Lead also damages nervous connections (especially in young children) and cause blood and brain disorders. Lead poisoning typically results from ingestion of food or water contaminated with lead; but may also occur after accidental ingestion of contaminated soil, dust, or lead-based paint. It is rapidly absorbed into the bloodstream and is believed to have adverse effects on the central nervous system, the cardiovascular system, kidneys, and the immune system. The component limit of lead (1.0 μg/g) is a test benchmark for pharmaceuticals, representing the maximum daily intake an individual should have. However, even at this low level, a prolonged intake can be hazardous to human beings. The treatment for lead poisoning consists of dimercaprol and succimer.

 




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