Lead (Pb)
Lead (Pb) is a soft, malleable metal with an atomic number of 82 and a density of 11.35 grams per cubic centimeter. It is found in the Earth's crust at a concentration of approximately 0.0013%, making it more common than silver or gold but less so than copper or zinc. Historically, lead has been utilized for various purposes, including plumbing, roofing, and as an additive in paints and cosmetics. However, due to its toxicity—known since ancient times—many of its uses have been phased out or heavily regulated, especially in the last few decades. The primary modern use of lead is in lead-acid batteries, particularly for automotive applications.
Lead is primarily extracted from ore deposits, with the most significant being galena (lead sulfide). Environmental concerns arise from lead mining and processing, which can contaminate air, water, and soil, posing health risks, especially to children. Exposure to lead can lead to severe health issues, including neurological damage and kidney dysfunction. Given the diminishing availability of rich lead deposits, recycling has become increasingly important, with secondary lead production surpassing that from primary mining. The ongoing challenge is balancing the metal's industrial utility against its environmental and health impacts.
Lead (Pb)
Where Found
Lead is widely distributed in the Earth’s crust; it has an estimated percentage of the crustal weight of 0.0013, making it more common than silver or gold but less common then copper or zinc; these are the four minerals with which lead is most commonly found in ore deposits. All five may occur together in a deposit, or only two or three may occur in concentrations sufficiently rich to be economically attractive to miners.
Primary Uses
The major use of lead in the United States is in the lead-acid batteries used in automotive vehicles. Because lead is so toxic, a fact that has been known since ancient times, many of its former uses have been curtailed or discontinued. While it is still used in cables, ammunition, solders, shielding of radiation, and electrical parts, its use as an antiknock additive in gasoline was phased out during the 1970’s and 1980’s. Nevertheless, lead production has been maintained at about the same level as before the phase out. Should a suitable substitute ever be developed for lead-acid batteries, the use of lead will decline to very low levels.
Technical Definition
Lead (abbreviated Pb), atomic number 82, belongs to Group IV of the periodic table of the elements. It is a mixture of four stable isotopes and has twenty-seven other isotopes, all radioactive, resulting from the fact that lead is the end product of three series of radioactive elements: the uranium series, actinium series, and thorium series. It has an average atomic weight of 207.2 and a density of 11.35 grams per cubic centimeter; it has a melting point of 327.5° Celsius and a boiling point of 1,740° Celsius.
Description, Distribution, and Forms
Lead is soft, malleable, and ductile, and is second only to tin in possessing the lowest melting point among the common metals. It may well have been the first metal smelted by humans, although it was probably not the first metal used—an honor claimed by gold, silver, or copper, which occur naturally in their metallic states. The fact that the principal ore of lead, galena (lead sulfide), frequently resembles the metal itself in its gray-black metallic color probably encouraged early humans to experiment with crude smelting. Inorganic lead also occurs as a carbonate (cerrusite), sulfate (anglesite), and oxides. Organic compounds of lead exist; these were used for many years in automobile gasoline as antiknock additives (tetraethyl and tetramethyl lead). Lead is widely distributed in the environment, but except in bedrock, concentrations are largely a consequence of human activity. Clair Patterson demonstrated that dramatic human-related increases in lead concentrations exist in the oceans, in polar ice sheets, and in the atmosphere. Before the human use of lead, the global flux into the oceans was only one-tenth to one-hundredth what it is today; lead in the atmosphere has increased a hundredfold globally and a thousandfold in urban areas.
Considering that only an estimated 0.0013 percent of the Earth’s crust is lead, it is surprisingly widely distributed in the environment. Lead is found in both crystalline (igneous and metamorphic) and sedimentary rocks. Because it is the stable end product of radioactive disintegration of minerals that form in igneous rocks (it is the rate of this disintegration that is employed to determine the age of the rock), virtually all older crystalline rocks contain at least tiny amounts of lead. As sedimentary rocks are derived from the weathering, erosion, and sedimentation of fragments from existing rocks, it follows that lead compounds will be among those that are sedimented. The higher concentrations of lead—those that pose toxicity problems or are valuable to miners—depend upon quite different processes. Some toxic concentrations of lead are transported by water and then sedimented or absorbed by rock particles, depending on the salinity or acidity levels of the solution. Most toxic concentrations of lead, however, are transported as dust by the atmosphere.
Deposits of lead ore exist at far higher concentrations than those levels that pose problems in water, dust, or soil. They are the result of natural geologic processes, including igneous intrusions, mountain building, and the flow of hot and cold solutions through bedrock over millions of years. The richest lead ores may contain 20 to 25 percent lead, usually with substantial fractions of zinc and minor quantities of silver. Copper and gold are also frequently associated with lead deposits, or vice versa (minor amounts of lead are usually found in copper ore).
Lead affects the environment in two major ways: through mining and processing, and because many of its uses, particularly in the past, have exposed the general public to its toxicity. Lead mining has environmental impacts similar to those of the mining of any mineral. Surface mining destroys the local ecosystem and disrupts the use of land for other purposes; reclamation rarely prepares the land for as valuable a use as it enjoyed before mining. The majority of lead is mined underground, where surface disruption is not as great unless subsidence over the mined areas is a problem. In both surface and underground mining, water is generally contaminated, mine wastes must be stored (waste dumps frequently occupy more space than the mine itself), and the transportation of mine products and waste serves as a source of dust, noise, and disruption to the surrounding population. The milling, smelting, and refining of lead pose further problems. First, lead itself escapes and pollutes the atmosphere with toxic substances. Second, most lead is derived from sulfides, which upon heating in the smelting and refining processes form sulfur dioxide. Sulfur dioxide combines with water in the atmosphere to create sulfuric acid, which devastates and denudes the vegetation cover in the immediate vicinity and contributes to acid rain fallout generally.
Humans may come into contact with lead and its toxic effects in the air, dust, and water, and by direct contamination of food, drink, or cosmetics. The effects of lead on human health are diverse and severe, with their greatest impact on children. The effects are exacerbated by the fact that lead accumulates in the body, and damage is often irreversible—especially damage to the brain. Lead damages blood biochemistry, the renal and endocrine system, liver functions, and the central nervous system, and it contributes to osteoporosis, high blood pressure, and reproductive abnormalities. The Environmental Protection Agency and the Occupational Safety and Health Administration set standards of acceptable levels of lead in air, dust, soil, and water; the standards are updated frequently based on new research, and they are quite complex, depending on the duration and nature of exposure.
History
While lead apparently was not the first or second metal to attract early humans, because it did not occur in a metallic state, it was exploited relatively early and may have been smelted in Anatolia (modern day Turkey) as early as 7000-6500 b.c.e. The softness and malleability of lead proved to be both attractive and undesirable to people in antiquity. Most early lead mining was carried on to recover the associated silver, and the lead remaining from the process was piled in waste heaps. Lead may be strengthened by alloying with other metals, but this process was carried out only to a limited degree in lead’s earliest usage.
While lead may not have proved attractive for uses requiring strength and hardness, its malleability caused the Romans, in particular, to put it to widespread use in piping, roofing, and vessels. In addition, lead compounds were used in paints, cosmetics, and as additives to wine and food. Lead poisoning was therefore widespread. The problem was recognized possibly as early as 370 b.c.e. by Hippocrates and certainly was known by Nikander in the second century b.c.e. The Romans nevertheless continued to press lead into a variety of services until the fall of their empire. Some authorities believe that lead poisoning was central to this fall, and many more believe that it at least contributed (especially to the disorganization of Roman leaders). Others maintain that the critical lead-related factor in the decline of Rome was the exhaustion of the richer silver-bearing ores. Exhaustion of mines or ores at any period in history is usually a function of the technology and economics of the time; many of these ores were particularly rich by modern standards. Silver was critical to maintenance of the Roman financial system, and the decline in its availability brought economic chaos.
Medieval production of lead declined dramatically in Europe following the fall of the Roman Empire, although recurring cases of lead poisoning during this period serve as a reminder that lead was still utilized widely in storage vessels. The Industrial Revolution, beginning with its earliest stages, revived the production level of lead, both for itself and as a by-product of silver mining. The expansion of European exploration into the Western Hemisphere and of European colonization worldwide from the fifteenth century onward undoubtedly contributed to the rise in lead production. Gold and silver were sought avidly in these expansions of domain, and lead mining frequently serves as the final use or “mop-up” stage in the life history of a mining district. Also, industrial uses and mining technology became increasingly sophisticated, leading to a new demand for lead and zinc, its frequent associate, especially beginning in the nineteenth century. The production curve of lead and zinc goes exponentially upward through history, with far greater production today than in earlier centuries.
Obtaining Lead
The largest lead deposits in the United States and Europe are of the Mississippi Valley type: lead sulfide (galena) deposits of uncertain origin in limestone or dolomite rocks. Many large mines throughout the world are found in crystalline rocks, where they are usually associated with igneous intrusions. Some lead is recovered as a by-product of the mining of copper or other associated minerals from large open-pit mines developed in low-grade ores, called porphyries. This type of recovery is a triumph of modern technology and engineering, because the ores frequently contain less than 0.5 percent copper, with even smaller fractions of lead. Most lead is recovered from underground mines that are exploiting much smaller concentrations in veins or disseminated beds of lead-zinc, zinc-lead, or lead-silver ores.
From 2003 to 2007, the average U.S. primary lead production (lead from mines) was 162,000 metric tons per year, while production of secondary lead (recycled from scrap, chiefly automotive batteries) during the same time period was 1.2 million metric tons per year. World mine production was somewhat less than lead from secondary sources: about 3.5 million metric tons from mines compared to 3.8 million metric tons from secondary sources. Recycling should prove even more important in the future as the richest deposits—those in which the lead content of the ore ranges between 5 and 10 percent—are depleted. This type of “exhaustion” of a deposit is a function of the prevailing technology and economics. In the first half of the twentieth century, the tristate lead-zinc mining district of Missouri, Oklahoma, and Kansas was the world’s greatest. Production there essentially ceased in the 1950’s, not because the lead and zinc were literally exhausted but because the concentrations available dropped below the level at which mining could be done profitably.
Technology is continuously improving, however, and the history of mining is filled with examples (particularly concerning the five associated metals gold, silver, copper, lead, and zinc) in which improvements in technology, combined with changing economic conditions, have made it possible to reopen or rework older and less attractive deposits. Some mine tailings or waste dumps have been reworked several times under these circumstances.
Uses of Lead
More than most metals, the uses to which lead and lead compounds have been put have changed considerably throughout history. One reason is that new opportunities have presented themselves, such as automotive lead-acid batteries, the shielding of dangerous radiation, and antiknock additives for gasoline—all twentieth century phenomena. Largely, however, this has occurred because people have become increasingly cognizant of the dangers posed by lead’s toxicity. While the dangers of exposure to lead have been known since Greek and Roman times, in few cases has this led to regulation of uses. Not until the 1960’s, 1970’s, and 1980’s were specific controls or regulations imposed restricting the use of lead in paint pigments, as an additive to gasoline, and in construction. Lead piping is still found in structures built in the 1970’s; the use of lead in storage vessels for food or drink has been regulated even more recently. Lead foil was used in capping wine bottles into the early 1990’s, and many people are still unaware that storage of wine or other liquids in fine leaded-glass decanters permits leaching of the lead content into the fluid over time.
The post-World War II era saw the elimination or substantial reduction of the following uses of lead: water pipes, solder in food cans, paint pigments, gasoline additives, and fishing sinkers. The major remaining uses include storage batteries, ammunition, paint pigments (for nonresidential use), glass and ceramics, sheet lead (largely for shielding against radiation), cable coverings, and solder.
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