Zinc (Zn)

Where Found

Zinc is widely distributed in the Earth’s crust, with an average crustal abundance of 70 parts per million (0.007 percent). It has been concentrated into several types of ore deposits from which it is mined as the principal metal or as a by-product. The principal ore mineral is sphalerite (ZnS), also known as zinc blende or marmatite. China, Japan, Australia, and Canada are the world’s largest suppliers of zinc.

Primary Uses

Zinc is a widely used metal, but its presence is generally not obvious to the public. The largest single use is in galvanizing, a process in which iron or carbon steel is covered with a thin coating of zinc to prevent rusting. Zinc is also widely used in brass alloys and other compounds ranging from pharmaceuticals to rubber tires to paints.

Technical Definition

Zinc (symbol Zn), atomic number 30, belongs to Group IIB of the periodic table of the elements and exhibits some chemical and physical similarities to cadmium. It has five naturally occurring stable isotopes, with masses of 64, 66, 67, 68, and 70, and it has an average atomic weight of 65.38. Pure zinc is a malleable, bluish-white metal that crystallizes in a hexagonal structure and that has a density of 7.13 grams per cubic centimeter. It has a melting point of 419.6° Celsius and a boiling point of 907° Celsius.

Description, Distribution, and Forms

Zinc is one of the most widely used metals; however, throughout much of its history it was not recognized as a distinct metal. Zinc is nearly ubiquitous in trace amounts and is essential for the normal growth and development of plants and animals. It occurs in more than twenty metalloenzymes and promotes healing of wounds and burns; furthermore, zinc deficiency has been shown to have severe effects on reproduction and tissue growth in laboratory animals. Zinc deficiency in soils leads to reduced productivity but can be remedied by the application of trace amounts of zinc in fertilizers.

Numerous studies of zinc in the environment have demonstrated that severe zinc pollution is extremely rare and that zinc contamination rarely becomes a problem for plants or animals. In general, the only time that zinc concentrations can rise to harmful levels is if the pH is very low and the sources of zinc are very large. Zinc is readily adsorbed onto clays or precipitates from solutions at neutral and high pH values. Zinc and its compounds are relatively nontoxic to humans when taken in normal dosages. Very large dosages can cause gastroenteritis; however, reports of such poisoning are limited to a few rare cases of the consumption of acidic beverages having been kept in galvanized containers. Workplace poisoning has been only rarely reported as the result of inhaling zinc dust or fumes; the human body is quite efficient in the elimination of excess zinc.

Zinc has been mobilized by fluids in the Earth’s crust and precipitated into many types of ore deposits. Although there are many zinc-bearing minerals, the only mineral that serves as an economic source of zinc in mining is sphalerite (ZnS). The principal types of ores are carbonate-hosted lead-zinc ores (Mississippi Valley-type ores, so called because of their abundance in parts of the Mississippi Valley drainage system, especially Missouri), hydrothermalvein deposits, and polymetallic massive sulfide ores formed where hydrothermal fluids have deposited complex ores in sedimentary basins or in masses of volcanic rocks. The Mississippi Valley-type ores occur in limestone and dolomite beds where there has been infiltration of relatively low-temperature (75° to 150° Celsius) fluids that have already leached zinc (and commonly lead) from thick sequences of sedimentary rocks. The zinc is deposited as the mineral sphalerite, most often as light yellow coatings on, and as fracture fillings in, the carbonate rocks. This sphalerite typically contains small amounts (0.5 percent) of cadmium and iron and trace amounts (less than 0.1 percent) of germanium, gallium, and indium in solid solution. Consequently, the zinc ores serve as the world’s major sources of these rare elements, which are recovered during refining.

Zinc-bearing hydrothermal deposits commonly occur as veins from centimeters to meters in thickness associated with granitic to intermediate igneous rocks. The sphalerite in these ores is typically very dark brown to black and contains 5 to 10 percent iron substituting for the zinc; it has commonly been called “blackjack” or marmatite. The zinc ores of the volcanic and sedimentary deposits are similar mineralogically to those of the vein deposits with the sphalerite intermixed with pyrite (FeS₂), chalcopyrite (CuFeS₂), and galena (PbS). These deposits occur where fractures and faults have allowed deeply circulating groundwaters, containing small amounts of dissolved metals, to discharge and precipitate complex masses of sulfides. In recent years modern examples of these types of deposit have been observed forming at oceanic spreading centers such as the East Pacific Rise. At these sites, hydrothermal fluids issuing from the seafloor fractures are depositing polymetallic sulfide mounds that are rich in zinc. Although all the zinc in ore deposits occurs as sphalerite, much of the zinc forming in modern seafloor deposits is wurtzite, another form of ZnS; the wurtzite gradually is converted into sphalerite after deposition.

In 2008, world zinc production was about 11 million metric tons. World reserves are thought to be about 180 million metric tons. The world reserve base, which includes zinc that will likely become economic to mine, is about 480 million metric tons. United States production is dominated by Alaska, Tennessee, New York, Missouri, and Colorado and has been in the range of 800,000 metric tons per year; total U.S. reserves are about 14 million metric tons, and the U.S. reserve base is about 90 million metric tons. Accordingly, U.S. mines will be able to continue to produce zinc at about the same rates for several decades.

History

Zinc has been found in some bronze and brass artifacts made five thousand years ago, and a few early examples of nearly pure zinc metal date back to about 500 b.c.e. Significant zinc production appears to have begun in China in the sixth century c.e. Subsequent large-scale production is known from India about 1000 c.e., but zinc appears in European usage only around the sixteenth century. The technology of zinc smelting is thought to have been developed in China and brought to Europe about 1730.

The use of zinc in the United States began in 1835, when it was desired to produce alloys for the manufacture of the U.S. standard units of weights and measures. Mining in the United States began about 1850 in rich ores in New Jersey and Pennsylvania. A smelter was built in 1859. By 1900 mines had been developed in a number of other states, especially along the Mississippi Valley.

Obtaining Zinc

Zinc ores are mined by underground and surface mining methods, depending upon the depth of the deposit below the surface. Generally ores must contain 2.5 to 3.0 percent zinc or about 4 to 5 percent sphalerite to be economic; this percentage is equivalent to a concentration factor of about four hundred times the average abundance of zinc in the Earth’s crust. In order to recover the zinc, the ores are crushed finely enough that the individual mineral grains may be separated by the froth flotation process. This process selectively removes the sphalerite grains as they attach themselves to small bubbles and float off the surface of a suspension that contains many kinds of minerals. The separation yields a concentrate that contains about 55 to 65 percent zinc, depending upon the purity of the sphalerite. Other valuable metals—such as cadmium, germanium, gallium, and indium—that are present in small amounts in the sphalerite are separated during selective smelting and refining. In most smelters, the zinc sulfide is roasted to remove the sulfur and to produce zinc oxide, which is then leached with sulfuric acid to form a zinc sulfate solution. After the solution is purified, the zinc is removed in electrolytic cells and precipitates on large aluminum cathodes. Further refining is accomplished by distilling the zinc in a vapor form, which is then recondensed. Zinc recycling accounts for a relatively insignificant percentage of demand because so much of the zinc is used in forms for which recycling is difficult or inefficient.

Uses of Zinc

Zinc metal and zinc compounds have broad uses, but many applications are not easily visible or known to those who benefit from them. About 90 percent of zinc is used in the metallic form; the remainder is used as zinc oxide and a variety of other compounds. More that half of metallic zinc usage is for the galvanizing of iron and steel for construction, transportation, electrical, and machinery purposes. The application of the zinc, either by dipping the iron and steel into molten zinc or by electrolytic plating, provides a coating that greatly reduces the rusting of iron and steel. Much of the use of such galvanized materials is on the body parts of motor vehicles. Many construction materials, from nails to bridge parts, are also galvanized to reduce corrosion.

Die-cast zinc materials are also widely used in handles, grills, gauges, housings, and assorted hardware, much of it in vehicles. The modern American automobile can contain as much as 18 kilograms of zinc. Zinc is also used as a sacrificial anode on ships, oil rigs, and other structures exposed to seawater. These anodes corrode as the result of natural electrical cells that develop, and in the process they prevent the corrosion of other metals. Beginning in 1983, zinc was used in the minting of United States one-cent coins; the penny is composed of 95 percent zinc.

Zinc is a basic component of the copper-based alloy brass, which has served humankind for thousands of years. Brass is widely used in shell casings for ammunition, in tubing, in motors, in refrigeration equipment, and in communication and electronic devices. Zinc dust and zinc oxide have been used in corrosion-inhibiting primer paints for application on metals. Zinc dust is also used in the manufacture of alkaline dry-cell batteries. Zinc oxide and other compounds such as chlorides, sulfates, and sulfides are widely used in chemical catalysts, welding and soldering fluxes, paints, fungicides and pharmaceuticals, and phosphors for cathode tubes and radar scopes; they are also used as additives in lubricating oils and greases and in agricultural products.

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