Zinc
Brass, which is an alloy of copper and zinc, has been used since at least the 10th century BC. Impure zinc metal was not produced in large scale until the 13th century in India, while the metal was unknown to Europe until the end of the 16th century. Alchemists burned zinc in air to form what they called "philosopher's wool" or "white snow."
The element was probably named by the alchemist Paracelsus after the German word Zinke. German chemist Andreas Sigismund Marggraf is normally given credit for discovering pure metallic zinc in 1746. Work by Luigi Galvani and Alessandro Volta uncovered the electrochemical properties of zinc by 1800. Corrosion-resistant zinc plating of iron (hot-dip galvanizing) is the major application for zinc. Other applications are in batteries, small non-structural castings, and alloys, such as brass. A variety of zinc compounds are commonly used, such as zinc carbonate and zinc gluconate (as dietary supplements), zinc chloride (in deodorants), zinc pyrithione (anti-dandruff shampoos), zinc sulfide (in luminescent paints), and zinc methyl or zinc diethyl in the organic laboratory.
Zinc is an essential mineral of "exceptional biologic and public health importance". Zinc deficiency affects about two billion people in the developing world and is associated with many diseases. In children it causes growth retardation, delayed sexual maturation, infection susceptibility, and diarrhea, contributing to the death of about 800,000 children worldwide per year. Enzymes with a zinc atom in the reactive center are widespread in biochemistry, such as alcohol dehydrogenase in humans. Consumption of excess zinc can cause ataxia, lethargy and copper deficiency.
Production
Mining and processing
Zinc is the fourth most common metal in use, trailing only iron, aluminium, and copper with an annual production of about 12 million tonnes. The world's largest zinc producer is Nyrstar, a merger of the Australian OZ Minerals and the Belgian Umicore. About 70% of the world's zinc originates from mining, while the remaining 30% comes from recycling secondary zinc. Commercially pure zinc is known as Special High Grade, often abbreviated SHG, and is 99.995% pure.
Worldwide, 95% of the zinc is mined from sulfidic ore deposits, in which sphalerite ZnS is nearly always mixed with the sulfides of copper, lead and iron. There are zinc mines throughout the world, with the main mining areas being China, Australia and Peru. China produced 29% of the global zinc output in 2010.
Zinc metal is produced using extractive metallurgy. After grinding the ore, froth flotation, which selectively separates minerals from gangue by taking advantage of differences in their hydrophobicity, is used to get an ore concentrate. A final concentration of zinc of about 50% is reached by this process with the remainder of the concentrate being sulfur (32%), iron (13%), and SiO2 (5%).
Roasting converts the zinc sulfide concentrate produced during processing to zinc oxide:
2 ZnS + 3 O2 → 2 ZnO + 2 SO2
The sulfur dioxide is used for the production of sulfuric acid, which is necessary for the leaching process. If deposits of zinc carbonate, zinc silicate or zinc spinel, like the Skorpion Deposit in Namibia are used for zinc production the roasting can be omitted.
For further processing two basic methods are used: pyrometallurgy or electrowinning. Pyrometallurgy processing reduces zinc oxide with carbon or carbon monoxide at 950 °C (1,740 °F) into the metal, which is distilled as zinc vapor. The zinc vapor is collected in a condenser. The below set of equations demonstrate this process:
2 ZnO + C → 2 Zn + CO2
ZnO + CO → Zn + CO2
Electrowinning processing leaches zinc from the ore concentrate by sulfuric acid:
ZnO + H2SO4 → ZnSO4 + H2O
After this step electrolysis is used to produce zinc metal.
2 ZnSO4 + 2 H2O → 2 Zn + 2 H2SO4 + O2
The sulfuric acid regenerated is recycled to the leaching step.
Environmental impact
The production for sulfidic zinc ores produces large amounts of sulfur dioxide and cadmium vapor. Smelter slag and other residues of process also contain significant amounts of heavy metals. About 1.1 million tonnes of metallic zinc and 130 thousand tonnes of lead were mined and smelted in the Belgian towns of La Calamine and Plombières between 1806 and 1882. The dumps of the past mining operations leach significant amounts of zinc and cadmium, and, as a result, the sediments of the Geul River contain significant amounts of heavy metals. About two thousand years ago emissions of zinc from mining and smelting totaled 10 thousand tonnes a year. After increasing 10-fold from 1850, zinc emissions peaked at 3.4 million tonnes per year in the 1980s and declined to 2.7 million tonnes in the 1990s, although a 2005 study of the Arctic troposphere found that the concentrations there did not reflect the decline. Anthropogenic and natural emissions occur at a ratio of 20 to 1.
Levels of zinc in rivers flowing through industrial or mining areas can be as high as 20 ppm. Effective sewage treatment greatly reduces this; treatment along the Rhine, for example, has decreased zinc levels to 50 ppb. Concentrations of zinc as low as 2 ppm adversely affects the amount of oxygen that fish can carry in their blood.
Soils contaminated with zinc through the mining of zinc-containing ores, refining, or where zinc-containing sludge is used as fertilizer, can contain several grams of zinc per kilogram of dry soil. Levels of zinc in excess of 500 ppm in soil interfere with the ability of plants to absorb other essential metals, such as iron and manganese. Zinc levels of 2000 ppm to 180,000 ppm (18%) have been recorded in some soil samples.
| Symbol | Zn | |
| Atomic Number | 30 | |
| Atomic Weight | 65.39 | |
| Oxidation States | +2 | |
| Electronegativity, Pauling | 1.65 | |
| State at RT | Solid, Metal | |
| Melting Point, K | 692.73 | |
| Boiling Point, K | 1180 |
Appearance and Characteristics
Harmful effects:
Zinc is not considered to be particularly toxic.
Zinc is essential for good health, but excess zinc is toxic and can cause nausea.
Characteristics:
- Zinc is a bluish-silver, lustrous metal that tarnishes in moist air, producing a layer of carbonate. It is somewhat brittle at room temperature but malleable above 100 oC.
- Zinc reacts with both acids and alkalis.
- It is a fair conductor of electricity, and burns in air with a bright bluish-green flame producing white clouds of the oxide.
Uses of Zinc
- Zinc is used to galvanize iron to inhibit corrosion.
- Zinc forms many alloys with other metals such as brass, German silver, nickel silver, typewriter metal and solders. Zinc alloys are used in die-castings for the car industry.
- The metal acts as both the container and as an electrode in zinc-carbon batteries. The drawback of this dual use is that the electrode is consumed when the battery is in use, getting thinner until eventually the battery starts leaking.
- Zinc is an essential trace element for animals and plants.
- Zinc oxide, a white powder, is a versatile compound that has many uses.
- It is used in sun block, make-up and in ointments such as calamine lotion.
- It is also used in the rubber industry, concrete manufacturing and in paints.
Reaction of zinc with iodine.
Zinc reacts very vigorously with sulfur. Here’s a test firing of a sulfur/zinc rocket.
