Zinc

Description

Zinc is a mineral that is essential for a healthy immune system, production of certain hormones, wound healing, bone formation, and clear skin. It is required in very small amounts, and is thus known as a trace mineral. Despite the low requirement, zinc is found in nearly every cell of the body and is a key to the proper function of more than 300 enzymes, including superoxide dismutase. Normal growth and development cannot occur without it.

General use

The U.S. Recommended Dietary Allowance (RDA) for zinc is 5 milligrams (mg) for children under one year of age, 10 mg for children aged one to 10 years old, 15 mg for males 11 years or older, 12 mg for females 11 years or older, 15 mg for women who are pregnant, and 16-19 mg for women who are lactating.

Zinc has become a popular remedy for the common cold . Evidence shows that it is unlikely to prevent upper respiratory infections , but beginning a supplement promptly when symptoms occur can significantly shorten the duration of the illness. The only form of zinc proven effective for this purpose is the zinc gluconate or zinc acetate lozenge. Formulations of 13-23 mg or more appear to be most effective, and need to be dissolved in the mouth in order to exert antiviral properties. Swallowing or sucking on oral zinc tablets will not work. The lozenges can be used every two hours for up to a week or two at most.

People who are deficient in zinc are prone to getting more frequent and longer lasting infections of various types. Zinc acts as an immune booster, in part due to stimulation of the thymus gland. This gland tends to shrink with age, and consequently produces less of the hormones that boost the production of infection-fighting white blood cells. Supplemental zinc, at one to two times RDA amounts, can reverse this tendency and improve immune function.

In another immune stimulant capacity, zinc can offer some relief from chronic infections with Candida albicans, or yeast. Most women will experience a vaginal yeast infection at some time, and are particularly prone to them during the childbearing years. Some individuals appear to be more susceptible than others. One study showed yeast-fighting benefits for zinc even for those who were not deficient in the mineral to begin with. Other supplements that will complement zinc in combating yeast problems are vitamin A, vitamin C , and vitamin E . Another measure that can help to limit problems with Candida is eating yogurt, which is an excellent source of Lactobacillus, a friendly bacteria that competes with yeast. Limiting sweets in the diet and eating garlic or odor-free garlic supplements may also prove helpful.

People who are going to have surgery are well advised to make sure they are getting the RDA of zinc, vitamin A, and vitamin C in order to optimize wound healing. A deficiency of any of these nutrients can significantly lengthen the time it takes to heal. Adequate levels of these vitamins and minerals for at least a few weeks before and after surgery can speed healing. The same nutrients are important to minimize the healing time of bedsores, burns , and other skin lesions.

There are two male health problems that can potentially benefit from zinc supplementation. Testosterone is one of the hormones that requires zinc for production. Men with infertility as a result of low testosterone levels may experience improvement from taking a zinc supplement. Another common condition that zinc can be helpful for is benign prostatic hypertrophy, a common cause of abnormally frequent urination in older men. Taking an extra 50 mg a day for three to six months offers symptomatic relief for some men.

Teenagers are often low in zinc, and also tend to experience more acne than the general population. The doses used in studies have been in the high range, requiring medical supervision, but increasing dietary zinc or taking a modest supplement in order to get the RDA amount is low risk and may prove helpful for those suffering from acne. People should consult a knowledgeable health care provider before taking large doses of any supplement.

There is some evidence that zinc supplementation may slightly relieve the symptoms of rheumatoid arthritis , but the studies are not yet conclusive. It's possible that those who initially had low zinc levels benefited the most.

In 2002, new research showed certain concentrations of zinc improved the effect of a therapy called interferon for some patients with chronic hepatitis C. Although the trial was preliminary, it showed promise for further research into zinc's effects in enhancing interferon therapy.

Zinc is sometimes promoted as an aid for memory. This may be true to the extent that vitamin B₆ and neurotransmitters are not properly utilized without it. However, in the case of people with Alzheimer's disease , zinc can cause more harm than good. Some experiments indicate that zinc actually decreases intellectual function of people with this disease. Under these circumstances, it is probably best to stick to the RDA of 15 mg as a maximum daily amount of zinc.

The frequency of sickle-cell crisis in patients with sickle-cell anemia may be decreased by zinc supplementation. The decrease was significant in one study, although the severity of the attacks that occurred was not affected. Use of zinc supplementation or other treatment for sicklecell anemia, a serious condition, should not be undertaken without the supervision of a health care provider.

Both the retina of the eye, and the cochlea in the inner ear contain large amounts of zinc, which they appear to need in order to function properly. Dr. George E. Shambaugh, Jr., M.D., is a professor emeritus of otolaryngology and head and neck surgery at Northwestern University Medical School in Chicago. In Prevention's Healing with Vitamins, he "estimates that about 25% of the people he sees with severe tinnitus are zinc-deficient." He adds that they sometimes have other symptoms of zinc deficiency. Large doses may be used in order to provide relief for this problem. Medical supervision and monitoring are necessary to undertake this course of treatment.

Topical zinc can be useful for some conditions, including cold sores. It is also available in a combination formula with the antibiotic erythromycin for the treatment of acne. Zinc oxide is a commonly used ingredient in the strongest sun block preparations and some creams for the treatment of diaper rash and superficial skin injuries. Men can use topical zinc oxide to speed the healing of genital herpes lesions, but it is too drying for women to use in the vaginal area.

There is still not enough information on some of the claims that are made for zinc. A few that may have merit are the prevention or slowing of macular degeneration , and relieving psoriasis . One should consult a health care provider for these uses.

Deficiency

It is not uncommon to have mild to moderately low levels of zinc, although serious deficiency is rare. Symptoms can include an increased susceptibility to infection, rashes, hair loss , poor growth in children, delayed healing of wounds , rashes, acne, male infertility, poor appetite, decreased sense of taste and smell, and possible swelling of the mouth, tongue, and eyelids.

A more serious, chronic deficiency can cause severe growth problems, including dwarfism and poor bone maturation. The spleen and liver may become enlarged. Testicular size and function both tend to decrease. Cataracts may form in the eyes, the optic nerve can become swollen, and color vision is sometimes affected by a profound lack of zinc. Hearing is sometimes affected as well.

Since meats are the best sources of zinc, strict vegetarians and vegans are among the groups more likely to be deficient. The absorption of zinc is inhibited by high fiber foods, so people who have diets that are very high in whole grain and fiber need to take supplements separately from the fiber. Zinc is needed in larger amounts for women who are pregnant or breastfeeding. Deficiency during pregnancy may lower fetal birthweight, as well as increase maternal risk of toxemia. A good prenatal vitamin is likely to contain an adequate amount. People over age 50 don't absorb zinc as well, nor do they generally have adequate intake, and may require a supplement. Alcoholics generally have poor nutritional status to begin with, and alcohol also depletes stored zinc.

There is an increased need for most vitamins and minerals for people who are chronically under high stress . Those who have had surgery, severe burns, wasting illnesses, or poor nutrition may require larger amounts of zinc than average.

Some diseases increase the risk of zinc deficiency. Sickle-cell anemia, diabetes, and kidney disease can all affect zinc metabolism. People with Crohn's disease , sprue, chronic diarrhea , or babies with acrodermatitis enteropathica also have an increased need for zinc. Consult a health care provider for appropriate supplementation instructions.

Preparations

Natural sources

Oysters are tremendously high in zinc. Some sources, such as whole grains, beans, and nuts, have good zinc content but the fiber in these foods prevents it from being absorbed well. Foods with better utilized zinc include beef, chicken, turkey, milk, cheese, and yogurt. Pure maple syrup also is a good source of zinc.

Supplemental sources

Zinc supplements are available as oral tablets in various forms, as well as lozenges. Zinc gluconate is the type most commonly used in lozenge form to kill upper respiratory viruses. One should select brands that do not use citric acid or tartaric acid for flavoring, as these appear to impair the effectiveness. The best-absorbed oral types of zinc may include zinc citrate, zinc acetate, or zinc picolinate. Zinc sulfate is the most likely to cause stomach irritation. Topical formulations are used for acne and skin injuries. Oral zinc should not be taken with foods that will reduce its absorption, such as coffee, bran, protein, phytates, calcium , or phosphorus . Supplements should be stored in a cool, dry location, away from direct light, and out of the reach of children.

Precautions

Toxicity can occur with excessively large doses of zinc supplements, and produce symptoms, including fever, cough , abdominal pain, nausea, vomiting , diarrhea, drowsiness, restlessness, and gait abnormalities. If doses greater than 100 mg per day are taken chronically, it can result in anemia, immune insufficiency, heart problems, and copper deficiency. High doses of zinc can also cause a decrease in high density lipoprotein (HDL), or good, cholesterol .

People who have hemochromatosis, are allergic to zinc, or are infected with HIV should not take supplemental zinc. Ulcers in the stomach or duodenum may be aggravated by supplements as well. Those with glaucoma should use caution if using eye drops containing zinc. Overuse of supplemental zinc during pregnancy can increase the risk of premature birth and stillbirth, particularly if the supplement is taken in the third trimester. This increase in adverse outcomes has been documented with zinc dosages of 100 mg taken three times daily.

Side effects

Zinc may cause irritation of the stomach, and is best taken with food in order to avoid nausea. The lozenge form used to treat colds has a strong taste, and can alter the sense of taste and smell for up to a few days.

KEY TERMS

Sickle-cell anemia

—A genetic malformation of red blood cells that can cause periodic crises in sufferers.

Interactions

The absorption of vitamin A is improved by zinc supplements, but they may interfere with the absorption of other minerals taken at the same time, including calcium, magnesium, iron , and copper. Supplements of calcium, magnesium, and copper should be taken at different times than the zinc. Iron should only be taken if a known deficiency exists. Thiazide and loop diuretic medications, sometimes used for people with high blood pressure, congestive heart failure, or liver disease, increase the loss of zinc. Levels are also lowered by oral contraceptives. Zinc can decrease the absorption of tetracycline and quinolone class antibiotics, antacids, soy, or manganese , and should not be taken at the same time of day. Drinking coffee at the same time as taking zinc can reduce the absorption by as much as half. Even moderate amounts of alcohol impair zinc metabolism and increase its excretion. Chelation with EDTA can deplete zinc, so patients undergoing chelation need to supplement with zinc, according to the instructions of the health care provider.

Resources

BOOKS

Bratman, Steven and David Kroll. Natural Health Bible. California: Prima Publishing, 1999.

Feinstein, Alice. Prevention's Healing with Vitamins. Pennsylvania: Rodale Press, 1996.

Griffith, H. Winter. Vitamins, Herbs, Minerals & supplements: the complete guide. Arizona: Fisher Books, 1998.

Jellin, Jeff, Forrest Batz, and Kathy Hitchens. Pharmacist's letter/Prescriber's Letter Natural Medicines Comprehensive Database. California: Therapeutic Research Faculty, 1999.

Pressman, Alan H. and Sheila Buff. The Complete Idiot's Guide to Vitamins and Minerals. New York: alpha books, 1997.

PERIODICALS

Vernarec, Emil. "Zinc May Enhance the Efficacy of Interferon." RN (May 2002): 28.

Judith Turner

Teresa G. Odle

ZINC (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Zinc is a transition metal that occurs in the center of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. The space between Groups 2 and 13 is occupied by the transition metals. These metals share many physical and chemical properties in common.

Alloys and compounds of zinc have been known since at least 500 b.c. But zinc metal was not known or used until much later. The reason is that zinc boils away or vaporizes easily when heated. Any effort to release zinc from its compounds also causes the metal to evaporate into the air.

Zinc was probably known in Asia before it was discovered in Europe. Ancient books from both India and China refer to zinc products. Such products were imported to Europe from Asia before they were made in Europe.

SYMBOL
Zn

ATOMIC NUMBER
30

ATOMIC MASS
65.38

FAMILY
Group 12 (IIB)
Transition metal

PRONUNCIATION
ZINK

The most important use of zinc today is in galvanizing other metals. Galvanizing is the process of laying down a thin layer of zinc on the surface of a second metal. Zinc does not corrode (rust) as easily as iron and other metals. So the thin layer of zinc protects iron and other metals from corrosion.

Discovery and naming

Some metals can be obtained from their ores easily. In a few cases, all that is needed is to heat the ore. Heating an ore of zinc releases the free metal. But with zinc, there is an additional problem. Zinc metal sublimates very easily. Sublimation is the process by which a solid changes directly to a gas when heated, without first changing to a liquid. Anyone who wanted to make zinc from its ore would lose the zinc almost immediately by sublimation.

Of course, early people did not understand this process. They may very well have made zinc by heating its ores. But any zinc they made would have floated away immediately. Still, a process for extracting zinc from its ores was apparently invented in India by the 13th century. The process involves heating the zinc ore in a closed container. When zinc vapor forms, it condenses inside the container. It can then be scraped off and used. That method seems to have been passed to China and then, later, to Europe.

In the meantime, ancient people were familiar with compounds and alloys of zinc. For example, there are brass objects from Palestine dating to 1300 b.c. Brass is an alloy of copper and zinc. The alloy may have been made by humans or found naturally in the earth. No one knows the origin of the brass in these objects.

The first European to describe zinc was probably Swiss physician Paracelsus (1493-1541). Paracelsus' real name was Theophrastus Bombastus von Hohenheim. Early in life, he took the name Paracelsus, meaning "greater than Celsus." Celsus was one of the great Roman physicians. Paracelsus wanted the world to know that he was even "greater than Celsus."

Paracelsus was also an alchemist. Alchemy was a kind of pre-science that existed from about 500 b.c. to near the end of the 16th century. People who studied alchemy—alchemists—wanted to find a way to change lead, iron, and other metals into gold. They were also looking for the "secret to eternal life." Alchemy contained too much magic and mysticism to be a real science. But it developed a number of techniques and produced many new materials that were later found to be useful in modem chemistry.

Paracelsus first wrote about zinc in the early 1500s. He described some properties of the metal. But he said he did not know what the metal was made of. Because of his report on the metal, Paracelsus is sometimes called the discoverer of zinc.

The name zinc was first used in 1651. It comes from the German name for the element, Zink. What meaning that word originally had is not known.

Physical properties

Zinc is a bluish-white metal with a shiny surface. It is neither ductile nor malleable at room temperature. Ductile means capable of being drawn into thin wires. Malleable means capable of being hammered into thin sheets. At temperatures above 100°C (212°F), however, zinc becomes somewhat malleable.

Zinc's melting point is 419.5°C (787.1°F) and its boiling point is 908°C (1,670°F). Its density is 7.14 grams per cubic centimeter. Zinc is a fairly soft metal. Its hardness is 2.5 on the Mohs scale. The Mohs scale is a way of expressing the hardness of a material. It runs from 0 (for talc) to 10 (for diamond).

Chemical properties

Zinc is a fairly active element. It dissolves in both acids and alkalis. An alkali is a chemical with properties opposite those of an acid. Sodium hydroxide ("common lye") and limewater are examples of alkalis. Zinc does not react with oxygen in dry air. In moist air, however, it reacts to form zinc carbonate. The zinc carbonate forms a thin white crust on the surface which prevents further reaction. Zinc burns in air with a bluish flame.

Occurrence in nature

The abundance of zinc in the Earth's crust is estimated to be about 0.02 percent. That places the element about number 24 on the list of the elements in terms of their abundance.

A process for extracting zinc from its ores was apparently invented in India by the 13th century.

Zinc never occurs as a free element in the earth. Some of its most important ores are smithsonite, or zinc spar or zinc carbonate (ZnCO₃); sphalerite, or zinc blende or zinc sulfide (ZnS); zincite, or zinc oxide (ZnO); willemite, or zinc silicate (ZnSiO₃); and franklinite [(Zn,Mn,Fe)O (Fe,Mn₂)O₃].

The largest producer of zinc ore in the world today is Canada. Other important producing nations are Australia, China, Peru, the United States, and Mexico. In the United States, more than half of the zinc produced comes from Alaska. Other important producing states are Tennessee, Missouri, Montana, and New York.

Isotopes

Five naturally occurring isotopes of zinc are known. They are zinc-64, zinc-66, zinc-67, zinc-68, and zinc-70. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

About eight radioactive isotopes of zinc are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

One radioactive isotope of zinc, zinc-65, has some practical importance. Zinc-65 is used as a tracer to study physical and biological events. A tracer is an isotope whose presence in a system can easily be detected. The isotope is injected into the system at some point. Inside the system, the isotope gives off radiation. That radiation can be followed by means of detectors placed around the system.

Brass is an alloy of copper and zinc.

For example, zinc-65 is used to study how alloys wear out. An alloy can be made using zinc metal. But the zinc used is zinc-65 instead of ordinary zinc. Changes in radiation given off by the radioactive isotope can be followed to find patterns in the way the alloy wears out. Zinc-65 can also be used to study the role of zinc in the human body. A person can be fed food that contains a small amount of zinc-65. The movement of the isotope through the body can be followed with a detector. A researcher can see where the isotope goes and what roles it plays in the body.

Extraction

As with many metals, pure zinc can be prepared from an ore by one of two methods. First, the ore can be roasted (heated in air). Roasting converts the ore to a compound of zinc and oxygen, zinc oxide (ZnO). The compound can then be heated with charcoal (pure carbon ). The carbon takes the oxygen away from the zinc, leaving the pure metal behind:

The other method is to pass an electric current through a compound of zinc. The electric current causes the compound to break apart. Pure zinc metal is produced.

Zinc burns in air with a bluish flame.

Uses

The annual cost of corrosion (rusting) in the United States is estimated to be about $300 billion. This is money lost when metals become corroded and break apart. Buildings and bridges are weakened, cars and trucks rust, farm equipment breaks down, and metal used in many other applications is destroyed. It is hardly surprising that protecting metal from corrosion is an important objective in American industry. One of the most effective ways of providing protection is through galvanizing. Today, about half of all the zinc produced in the United States is used to galvanize other metals. The largest consumers of galvanized metal are the construction and automotive industries.

The second largest use of zinc is in making alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals. Two of the most common alloys of zinc are brass and bronze. Brass is an alloy of zinc and copper. Bronze is an alloy of copper and tin that may also contain a small amount of zinc. Alloys of zinc are used in a great variety of products, including automobile parts, roofing, gutters, batteries, organ pipes, electrical fuses, type metal, household utensils, and building materials.

Compounds

A number of zinc compounds have important uses. Some examples are the following:

Zinc deficiency can interfere with a plant's ability to reproduce.

zinc acetate (Zn(C₂H₃O₂)₂): wood preservative; dye for textiles; additive for animal feed; glazing for ceramics

zinc arsenate (Zn₃(AsO₄)₂): wood preservative; insecticide

zinc borate (ZnB₄O₇): fireproofing of textiles; prevents the growth of fungus and mildew

zinc chloride (ZnCl₂): solder (for welding metals); fireproofing; food preservative; additive in antiseptics and deodorants; treatment of textiles; adhesives; dental cement; petroleum refining; and embalming and taxidermy products

zinc fluorosilicate (ZnSiF₆): mothproofing agent; hardener for concrete

zinc hydrosulfite (ZnS₂O₄): bleaching agent for textiles, straw, vegetable oils, and other products; brightening agent for paper and beet and cane sugar juice

zinc oxide (ZnO): used in rubber production; white pigment in paint; prevents growth of molds on paints; manufacturer of glass; photocopy machines; production of many kinds of glass, ceramics, tile, and plastics

zinc phosphide (Zn₃P₂): rodenticide (rat killer)

zinc sulfate (ZnSO₄): manufacture of rayon; supplement in animal feeds; dyeing of textiles; and wood preservative

Health effects

Zinc is an essential micronutrient for plants, humans, and animals. Zinc deficiency has relatively little effect on the health of a plant, but it interferes with reproduction. Pea plants deprived of zinc, for example, will form flowers. But the flowers will not turn to seeds.

In humans, zinc deficiencies are more serious. Zinc is used to build molecules of DNA. DNA is the chemical in our body that tells cells what chemicals they should make. It directs the reproduction of humans also. Fetuses (babies that have not yet been born) deprived of zinc may grow up to have mental or physical problems. Young children who do not get enough zinc in their diet may experience loss of hair and skin lesions. They may also experience retarded growth called dwarfism. Chemists have now found that zinc plays an essential role in the manufacture of many important chemicals in the human body.

Zinc is an essential micronutrient for humans. But too much or too little can cause health problems.

On the other hand, an excess of zinc can cause health problems, too. Breathing zinc dust may cause dryness in the throat, coughing, general weakness and aching, chills, fever, nausea, and vomiting. One sign of zinc poisoning is a sweet taste in the mouth that cannot be associated with eating sweet foods. Certain compounds of zinc can be harmful to health also. Zinc chloride (ZnCl₂), for example, can cause skin rashes and sore throat.

Zinc

Background

Zinc is an elemental metal. It is listed on the Periodic Table as "Zn," with an atomic number of 30 and an atomic weight of 65.37, and it melts at 788°F (420°C). Zinc is usually a gray metallic color, but it can be polished to a shiny silver luster. In nature, it is only found as a chemical compound, not as pure zinc, and can be used as a raw material for castings and coatings.

During the era of the Roman Empire, people used zinc to alloy copper into brass for weapons. In this crude process, the zinc was captured by the copper during the heating of the ores, though little was realized at the time about the importance of zinc in metallurgy. The name zinc may be derived from the German word "zinn," which means tin. The scientific discovery of zinc is credited to Nadreas Sigismund Marggraf, a German chemist who isolated pure zinc in 1746. The first production facility, or smelter, was founded in Bristol, England by William Champion shortly thereafter.

Only about 5% of the world's zinc supply is mined in the United States, with the balance coming primarily from India, Mexico, and Canada. Approximately 6.7 million metric tons of zinc ore are produced worldwide. Roughly two thirds of the zinc used in the United States is imported.

Applications

Zinc is primarily used for galvanizing steel against corrosion, die casting of intricate machine parts, and in batteries and other electrical applications. Zinc is also alloyed with copper to form brass.

Galvanizing steel involves applying a thin coating of zinc to all exposed surfaces of the steel to guard against corrosion. Zinc offers excellent corrosion resistance because it is more easily oxidized by the atmosphere. Oxidation occurs when metal is exposed to air or water, and electrons from the metal transfer to the oxygen. When zinc is tightly bonded to steel, the zinc frees up its electrons more readily than the steel, leaving the stronger metal beneath intact. The application of the zinc coating is accomplished by dipping the steel into molten zinc or by electrolytic plating of the steel with zinc, much like chrome plating.

Die-casting alloys typically contain 96% zinc and 4% aluminum. The die-casting process uses a two-piece steel die and a casting press to hold the die halves together during injection of the molten metal. Inside the steel die is a cavity that has the negative image of the part to be cast. The molten metal is injected into the cavity under pressure, accurately filling the entire void. The metal cools, and the press opens the die halves, revealing the formed part. The zinc cast parts are very close to the desired shape, requiring little machining before they are placed into an assembly. Typical applications include copier, aircraft, and medical instrument parts. Automobile makers use zinc die castings for emblems, moldings, door handles, and brackets. Zinc die castings are easily chrome plated for durability and appearance.

One unique application of zinc takes particular advantage of its ability to transfer its corrosion resistance properties by electrical contact. This application is called a "sacrificial anode." The anodes, made of almost pure zinc, are bolted to aluminum marine engines. During operation in water, especially salt water, the oxidation forms a weak electrical current, which may corrode the hull and engine parts. Since zinc is easily oxidized in the presence of this electrical current, it "sacrifices" itself by corroding quickly, consuming all of the electrical imbalance in the ship. The remaining aluminum hull and engine are not corroded as a result. As it is consumed, the anode must be replaced to assure continued protection.

In an application similar to the sacrificial anode, zinc is used as a component in battery production. The dry cell battery creates a chemical reaction with zinc in a metal housing (or "can") that results in a voltage potential between two connections. An electrical device, such as a flashlight or portable radio, can be connected to the battery and powered by the electricity produced. Thus connected, the reaction maintains the electrical current for the duration of the available chemical reactants.

Zinc as a compound is used in pharmaceuticals, rubber, cosmetics, paint, and ceramic glaze. Other compounds use zinc in cathode-ray tubes, soldering flux, and wood preservatives.

The Manufacturing
Process

Mining

• 1 Zinc ores are dug from underground mines using conventional blasting, drilling, and hauling techniques. The ores occur as zinc sulfide (also called sphalerite), zinc carbonate (smithsonite), zinc silicate (calimine), and in compounds of manganese and iron (franklinite). Zinc ore is sometimes mined in conjunction with silver or lead ores. In addition to the ore itself, oil and sulfuric acid are required for the breakdown of the ores; and electricity, coke, or natural gas are needed to provide the heat energy for smelting.

Froth flotation

• 2 Zinc can be produced by a process called froth flotation, which is also used for reduction of copper and lead ores. This process involves grinding the zinc ore to a fine powder, mixing it with water, pine oil, and flotation chemicals, and then agitating the mixture to "float" the zinc to the surface. A variety of chemicals are used to coat the important zinc particles and prevent them from becoming wetted by the water. Then air is injected, and the coated minerals attach themselves to the bubbles. The operation is performed inside a vat and agitated with an impeller. The rotating impeller draws the air down the standpipe that surrounds the impeller shaft and dissipates it throughout the mixture or "pulp." The zinc rises to the top and the residue stays in the bottom of the pulp, since it cannot adhere to the bubbles. Automatic scrapers remove the mineral-laden froth containing the zinc.

Filtering

• 3 The froth is filtered to remove the water and liquid oils. The paste-like remainder is mixed with lime and sent to a furnace. The furnace roasts the mixture at 2500°F (1371°C), which fuses the minerals into solid chunks called sinter. At this point, the material has been completely converted to zinc oxide.

Smelting

• 4 The next reduction process uses a blast furnace to melt the prepared ore into its elemental components. The blast furnace is fueled by electricity, coke, or natural gas, which generate temperatures of up to 2200°F (1204°C). This, however, also generates carbon dioxide, which recombines with the zinc as it cools to re-form zinc oxide. To reduce this reformation, the zinc is sprayed with molten lead while it is still hot. The lead, at 1022°F (550°C), dissolves the zinc and carries it to another chamber, where it is cooled to 824°F (440°C). At this temperature, the lighter zinc separates out of the lead and is drained off the top. The lead is reheated and returned to the blast furnace.

Refining

• 5 Further metal improvement can be made by keeping the zinc molten and undisturbed for several hours. In this state, iron and other contaminants settle to the bottom, allowing the almost pure zinc to be carefully drawn off the top and cast into ingots.

Alloying

• 6 Most zinc is alloyed with other metals before use to improve its properties. Alloying involves remelting and mixing the zinc with other metals in precise proportions. For example, approximately 4% aluminum is added to improve casting quality and die life in the die-casting process. Other added alloys are small amounts of titanium, copper, and magnesium. After alloying, the molten metal is poured into sow molds and ingot molds. Sows can weigh several thousand pounds, while ingots weigh about 45 pounds (20 kg).

Quality Control

Metal alloys are inspected by a process called spectrographic analysis. The metal is burned under a protective cover using an electrical arc. The light emitted by the burning metal is passed though an apparatus much like a prism, which breaks the light into all of its individual colors. Every element has a different set of colors, or spectrum, which is like a fingerprint. Any foreign material will alter the spectrum, and in doing so show its unique color spectrum, identifying it. The computer in the spectrograph uses sensors to pick up these colors. The computer program then produces a printout that identifies each element in the spectrum and the concentration within the metal. Elements can be reduced or increased to alter the composition.

The Future

Because of the strength to weight ratio of zinc, its use by the automotive industry as a die casting has been diminishing in the past few years. Magnesium, aluminum, and plastics have taken over many zinc applications. The use of zinc to galvanize automobile body parts has been increasing, however. Many vehicles today are protected by zinc galvanizing which allows the manufacturer to offer extended warranties for body rust problems with new cars.

Where To Learn More

Periodicals

Queneau, Paul B. and Jerome P. Downey. "Secondary Zinc Production Minimizes Waste." Pollution Engineering, November 1994, pp. 42-44.

Yates, Edward M. "Zinc: Major Mine Production Cuts in 1993." Engineering and Mining Journal, March 1994, pp. 19-21.

—Douglas E. Betts

zinc metallic chemical element; symbol Zn; at. no. 30; at. wt. 65.38; m.p. 419.58°C; b.p. 907°C; sp. gr. 7.133 at 25°C; valence +2. Zinc is a lustrous bluish-white metal. It is found in Group 12 of the periodic table . It is brittle and crystalline at ordinary temperatures, but when heated to between 110°C and 150°C it becomes ductile and malleable; it can then be rolled into sheets. It is a fairly reactive metal. Although it is not abundant in nature, it is of great commercial importance. It is used principally for galvanizing iron, but is also important in the preparation of certain alloys, e.g., Babbitt metal , brass , German silver , and sometimes bronze . It is used for the negative plates in certain electric batteries and for roofing and gutters in building construction. Since the metal reacts with dilute mineral acid to liberate hydrogen, it is often used for this purpose in the laboratory. Zinc compounds are numerous and are widely used. Perhaps most important is zinc oxide , or zinc white, a versatile compound with many uses. Other zinc compounds include zinc chloride, used as a wood preservative, in soldering fluxes, as a mordant in dyeing textiles, and in adhesives and cements; and zinc sulfide, used in making lithopone as well as television screens and X-ray apparatus. The chromate, zinc yellow, serves as a pigment; sodium zincate, as a water softener and as a flocculating agent in water purification. The crystalline sulfate is known commonly as white vitriol. Zinc is essential to the growth of many kinds of organisms, both plant and animal. It is a constituent of insulin, which is used in the treatment of diabetes. Chief sources of zinc are the sulfide ore, zinc blende, or sphalerite (called also blende or "black Jack" ); zincite, an oxide; calamine, a silicate; and smithsonite, the zinc carbonate. Zinc ores are widely and abundantly distributed throughout the world. The United States is the leading producer. The metallurgy of zinc depends upon the ore used. The sulfide ore is roasted to the oxide, then mixed with coal and heated to 1,200°C. The zinc vaporizes and is condensed outside the reaction chamber and cast into blocks called spelter. In another method the ore is processed by flotation, filtering, roasting, and leaching; the resulting solution is filtered and the zinc removed by electrolysis.

Zinc

melting point: 419°C
boiling point: 907°C
density: 7.14 g/cm ³
most common ions: Zn ²⁺

Like many transition metals , zinc has been known in impure form since ancient times. Brass (copper and zinc) coins were used by Egyptians and Palestinians as early as 1400 b.c.e. The first purification of zinc probably occurred during India in the thirteenth centuryc.e. Although the origin of the name is unknown, it has been suggested that it derives from the German word Zincke, meaning "spike" or "tooth."

Zinc is a trace element (with an abundance of 0.0076%) in Earth's crust. Like the other elements in its family, zinc is found predominantly as a sulfide compound (ZnS). Pure zinc is a silver-white solid at room temperature. Like other metals , zinc conducts electricity and can be formed into wires or sheets. Some properties of zinc are quite different from those of the other transition metals—namely, its relatively low melting point, boiling point, and density. These different properties are attributed to zinc's full outermost subshell of electrons, which also causes it to be relatively unreactive.

Due to the low reactivity of zinc, its most common use is in anticorrosion coatings. Zinc is also often used to form alloys , including brass and commercial bronze. Pennies minted after 1983 are made of a core of zinc surrounded by copper. Historically, zinc was used by Alessandro Volta in 1800 to produce the first battery. Zinc ions, due to their low reactivity, are important biologically. In animals zinc is the most abundant metallic cofactor ; it is used by insulin in the regulation of glucose consumption and by hydrolytic enzymes. An adult human body contains 2 to 3 grams (0.071–0.106 ounces) of zinc.

see also Cofactors; Volta, Alessandro.

Thomas B. Rauchfuss

Amanda Lawrence

Bibliography

Greenwood, N. N., and Earnshaw, A. (1997). Chemistry of the Elements, 2nd edition. Boston: Butterworth-Heinemann.

Nechaev, I., and Jenkins, G. W. (1994). Chemical Elements: The Exciting Story of Their Discovery and of the Great Scientists Who Found Them. Jersey City, NJ: Parkwest.

Internet Resources

WebElements—Periodic Table. Available from <http://www.webelements.com/>.

zinc An essential mineral which forms the prosthetic group of a large number of enzymes, and also the receptor proteins for steroid and thyroid hormones and vitamin A and vitamin D. Deficiency results in hypogonadism and delayed puberty, small stature, and mild anaemia; it occurs mainly in subtropical regions where a great deal of zinc is lost in sweat, and the diet is largely based on unleavened wholemeal bread, in which much of the zinc is unavailable because of the high content of phytate.

Meat, fish (especially shellfish), legumes, and (leavened) wholegrain cereals are rich sources. Synergistic zinc is a trade name for zinc supplement that also contains copper and vitamin A, which are claimed to aid its absorption.

429. Zinc

See also 270. METALS .

photozincography

Obsolete, a type of photoengraving using a sensitized zinc plate.

sherardisology

the coating of steel and iron with a thin cladding of zinc. —sherardize , v.

zincography

1. a lithographic or offset process using zinc plates.

2. a letter press printing process using engraved or photoengraved zinc plates. —zincographer , n. —zincographic, zincographical , adj.

zinc / zingk/ • n. the chemical element of atomic number 30, a silvery-white metal that is a constituent of brass and is used for coating (galvanizing) iron and steel to protect against corrosion. (Symbol: Zn) ∎  [usu. as adj.] galvanized iron or steel, esp. as the material of domestic utensils or corrugated roofs: a zinc roof. • v. [tr.] [usu. as adj.] (zinced ) coat (iron) with zinc or a zinc compound.

zinc(Zn) An element that is required by plants. It is found bound to a variety of enzymes, stabilizing them and also being involved in catalysis. Deficiency in plants prevents the expansion of leaves and internodes, giving a rosette style of plant. It is a growth factor in some rodents and a constituent of certain mammalian enzymes.

zinc (zink) n. a trace element that is a cofactor of many enzymes. Deficiency is rare with a balanced diet but may occur in alcoholics and those with kidney disease; symptoms include lesions of the skin, oesophagus, and cornea and (in children) retarded growth. Symbol: Zn.

zinc (Zn) An element that is required by plants. It is found bound to a variety of enzymes, stabilizing them and also being involved in catalysis. Deficiency in plants prevents the expansion of leaves and internodes, giving a rosette style of plant.

zinc Symbol Zn. A blue-white metallic element that is a trace element (see essential element) required by living organisms. It functions as the prosthetic group of a number of enzymes.

zinc hard bluish-white metal. XVII. — G., of unkn. orig. Comb. form zinco- as in zincography.

Zn • symb. the chemical element zinc.

Zn symbol for the element zinc .

Zn See zinc.

Zn See ZINC.

zinc •ankh, bank, blank, clank, crank, dank, drank, embank, flank, franc, frank, hank, lank, outflank, outrank, Planck, plank, point-blank, prank, rank, sank, shank, shrank, spank, stank, swank, tank, thank, wank, yank •sandbank • piggy bank • mountebank •fog bank • mudbank • Bundesbank •databank • riverbank • Burbank •greenshank • sheepshank •scrimshank • Cruikshank •think tank • Franck • Eysenck •bethink, blink, brink, chink, cinque, clink, dink, drink, fink, Frink, gink, ink, interlink, jink, kink, link, mink, pink, plink, prink, rink, shrink, sink, skink, slink, stink, sync, think, wink, zinc •rinky-dink • Humperdinck • iceblink •cufflink • bobolink • Maeterlinck •lip-sync • countersink • doublethink •kiddiewink •tiddlywink (US tiddledywink) •hoodwink

ZN airline flight code for Eagle Airlines
• Irish vehicle registration for Meath
• international civil aircraft marking for New Zealand

Zn Chem., symbol for zinc