Leaded Gas and Other Uses for the Heavy Metal
Since ancient times, humans have known about lead. Lead was originally a novelty with little use or value. Its first use was for artwork. But entrepreneurial Romans used lead extensively and took advantage of its malleability and resistance to corrosion. They made lead pipes to carry water and to drain sewage water. They also used lead to line containers that stored water. The English word "plumbing" and the chemical symbol for lead (Pb) come from the Latin word plumbum, which just means "lead." Lead plumbing from Roman times survives today in Roman baths and structures.
Aside from its role in plumbing, lead has been added to cosmetics, colorful paints and pigments, glass, pewter jewelry and tableware, munitions and pottery. In the 20th century, lead could also be found in household paints, plumbing, cable sheathing and additives to gasoline (tetraethyl lead). However, as health officials began to recognize the metal's toxic effects on human and environmental health, the use of lead in these areas has been greatly reduced, if not eliminated.
Today, you predominantly find lead in lead acid batteries for electrical power, like your car battery. According to the International Lead Association, 70 percent of lead-acid batteries are recycled and used for secondary lead production.
Besides batteries, you can find lead in roofing products and radiation shielding -- the element's high density makes it ideal for absorbing gamma radiation and X-rays. Most glass cathode ray tubes (like those in computer monitors) have leaded glass in them to shield the viewer from radiation made inside. You'll also find lead added to glass for decorative crystal. Lastly, lead solder is good for making electrical connections, and the element is also handy for ceramic devices used in the electronics industry.
Where does all that lead come from? It's hidden in the Earth's crust, primarily as a sulfide (PbS) in the mineral galena. Currently, 75 percent of the world's lead comes from China, the United States, Australia, Canada, Mexico and Peru [source: International Lead Association]. More lead is produced secondarily by recycling lead scrap.
Before lead can exist in its finished form, you have to process and refine the lead ore. The Romans used a process called cupellation to separate silver from lead. Today, the lead industry extracts the element using the processes of roasting and smelting, which are similar to the methods the Romans used.
1. Roasting: Galena is heated in air to convert lead sulfide (PbS) to lead oxide (PbO), plus sulfur dioxide.
2PbS + 3O2 --> 2PbO + 2SO2
2. Smelting: Coke (carbon -- C) is added to lead oxide (PbO) and mixed with air in a high-temperature blast furnace to obtain lead metal. In the shaft of the furnace, the carbon displaces the lead from lead oxide to form carbon dioxide gas (CO2) and molten lead (Pb).
2PbO + C --> 2Pb + CO2
The molten lead sinks to the bottom of the furnace, is siphoned off and cools to form lead bricks or pigs (big, oblong masses). Slag, a nonmetallic by-product of metal smelting, separates from molten lead, gets siphoned off, cooled and dumped as waste. Molten lead extracted through smelting often contains other metal impurities, such as zinc, arsenic, copper, silver, gold and bismuth. Electrowinning removes these impurities with the help of an electrical current.
Besides extracting lead from ore, the heavy metal can also be produced from secondary sources such as used batteries and scrap lead. Secondary extraction also involves processing the raw materials, smelting (or turning reclaimed lead back into alloys or its elemental form), cooling and casting. For a virtual tour of a secondary lead smelter and its environmental controls, see OSHA's Secondary Lead Smelter eTool.
Next, learn how lead gets into our bodies.