Lead
by Chris Woodford. Last updated: August 4, 2011.
As heavy as lead—it's probably something you say all the time and, if you've ever tried to lift a car battery, you'll know it's true. But have you ever stopped to think what makes this famously "heavy metal" weigh so much? Lead was one of the very first metals people discovered; in ancient times, the Romans used it for making elaborate networks of water pipes, and their word for lead—plumbum—gives us both our modern word for "plumbing" and the chemical symbol for lead (Pb). But this dull gray metal has an ambivalent history. Known to cause serious health problems, it's been slowly and systematically removed from everyday things such as gasoline and paint for a number of years; on the plus side, it's still just about the most effective substance for protecting us from radiation. More to lead than you thought? Let's take a closer look!
Photo: A sample of galena (lead sulfide), by far the most important lead ore. Photo by courtesy of US Geological Survey.
Where does lead come from?
Lead is reasonably widespread (about the 36th most common element in Earth's rocky crust) but, like most other metals, hardly ever found in its pure form. Most of it comes from an ore (raw mineral) called galena (lead sulfide, PbS), though smaller amounts are present in other ores such as cerussite (lead carbonate, PbCO3, also called white lead ore) and anglesite (lead sulfate, PbSO4).
Photo: The Old Wheatly lead mine, near Phoenixville. Chester County, Pennsylvania. Photo by Geological Society of America courtesy of US Geological Survey.
Galena is turned into pure lead metal through a multi-stage process. First it is crushed and washed to remove dirt and impurities. Then the lead is extracted either by smelting (heating in a furnace with coke) or in a two-stage process involving first roasting to convert lead sulfide into lead oxide, then smelting to remove the oxygen. Other precious metals are usually mixed in with the galena ore (including gold, silver, copper, and zinc) and are extracted during the process. Roughly half of all the lead now consumed is produced by recycling unwanted waste materials such as old car batteries.
Many countries mine lead, with leading producers including China (responsible for over 40 percent of mining), Australia, Peru, Mexico, and Bolivia. Although the United States once mined about a third of all the world's lead, by 2010 it was mining only around 10 percent. Over 90 percent of US mined lead comes from Alaska and Missouri, with the rest from Idaho, Montana, and Washington. Over 75 percent of US lead is produced from "secondary" (recycled) sources.
Who produces the world's lead? Chart shows world mine production for 2010. Source: U.S. Geological Survey, Mineral Commodity Summaries, January 2011.
What is lead like?
Lead is a soft, heavy, blueish gray metal that lives in group 14 (formerly IVa) of the Periodic Table of chemical elements.
Physical properties
On the face of it, when it comes to metallic properties, lead doesn't put up much of a showing. It's soft, weak, a poor conductor of electricity, heavy, dense, and has a low melting point. On the positive side, its softness means it's very malleable (easy to shape and work) and ductile (easy to pull into wires). Pure lead metal oxidizes rapidly in air to form a protective coating of lead oxide, and is resistant to corrosion by both acids (sulfuric and hydrochloric) and water.
Chemical properties
Lead has a valency (combining power) of either +2 or +4, joining with a variety of other elements to make useful lead (II) and lead (IV) compounds, including oxides, sulfates, and carbonates. Industrially, the most important lead compound is a yellow powder called litharge (lead (II) oxide or lead monoxide), which is a vital ingredient in all kinds of glass, as well as oils and insecticides. Another useful oxide, trilead tetroxide (Pb3O4), makes up the familiar pigment "red lead"—once widely used in rust-resistant paints to protect iron structures such as the famous Forth railway bridge. Lead carbonate, (PbCO3)2·Pb(OH)2, is also a pigment (better known as "white lead"), used for at least 2000 years, while lead chromate ("chrome yellow") is used both by itself as a yellow pigment and to make various other lead-based pigments. Until the 1980s, one of the most important lead compounds was tetraethyl lead (lead with four C2H5 ethyl groups bonded to it, with the formula (CH3CH2)4Pb), an additive that improved performance in internal combustion engines. Following widespread concerns about air pollution, most cars are now built to run on "unleaded" gasoline (petrol) that doesn't contain this substance. Other important lead compounds include lead (II) acetate or "sugar of lead" (used in making paints, varnishes, and dyes) and lead azide (an explosive used in such things as airbags).
What do we use lead for?
Photo: Health and safety concerns mean lead is no longer so widely used in paints. Photo by Brian M. Brooks courtesy of US Navy and Defense Imagery.
Consider lead's useful properties and you can more or less predict what people will
use it for. It's very dense and heavy, for example, and that's
why it's so useful as a material for screening people against harmful
radiation (X ray technicians, known as radiographers, usually stand
behind lead screens or wear lead aprons). Lead's brightly colored,
highly durable compounds were a natural choice for pigments and dyes,
though health and safety concerns have seen lead removed from many
paints (especially those used on children's toys). When people
realized that lead resisted corrosion, they recognized it as an
excellent material for such things as roofing and water pipes.
(Again, health and safety concerns mean many lead water pipes have been
removed and replaced with far safer plastics.)
Lead's weightiness made it useful in bullets and shot though, once
more, these uses are dwindling with concerns about health effects and
pollution.
Although lead doesn't conduct electricity well, it can be
used with sulfuric acid to store and release electrical energy
through chemical reactions—and that's how
car batteries work (the
ones that kick-start cold car engines). With other uses of lead in
rapid decline, batteries are now the biggest single use of lead.
Important lead alloys include pewter (used for making tableware),
corrosion-resistant coverings for electrical cables, acid-resistant
linings for chemical tanks, and solders
with relatively low melting points.
Photo: Much of the world's lead finds its way into car batteries. In the United States, some 87 percent of all lead ends up in batteries like this (2010 figures).
