Zinc is a bluish-white metal with a hexagonal close-packed crystal structure. It is a little less dense than iron but not especially strong when pure. High-strength casting alloys are obtained by alloying with aluminum, copper, and magnesium. Zinc melts at 788°F (420°C) and boils at the comparatively low temperature of 1665°F (907°C).
Zinc is not found in the natural state, and the earliest metallic form would almost certainly have been in brass, an alloy of zinc and copper. Primitive metallurgists had learned to make brass by heating copper with zinc ores, but they did not understand the nature of the process, and if it happened that any free zinc metal was formed and not absorbed by the copper, it would have vaporized and been lost. It was not until the 17th century that substantial quantities of the metal were brought to Europe from China. It was about this time that the name zinc was specifically applied; the term had been used in Europe for over a hundred years to describe a variety of metals and ores. It is possible that the name is derived from the German word Zinn, which means "tin." The first zinc smelter in the Western world was set up near Bristol in the United Kingdom in the early 1740s by William Champion.
Production
The most important ore of zinc is its sulfide, called zinc blende or sphalerite, which is the source of more than 90 percent of the world’s zinc. Zinc carbonate, (ZnCO3) is another common zinc ore. The ores seldom occur in the pure form and are most often associated with lead or copper ores. The zinc concentrate (the ore after flotation) is usually roasted in air to convert the zinc sulfide or carbonate to the oxide:
At atmospheric pressure, the reduction to zinc can occur only above the boiling point of the metal. For this reason, the reaction was traditionally carried out in horizontal retorts, and the resulting zinc vapor was condensed in clay vessels and iron pipes.
About 80 percent of the world’s zinc production is by the electrolytic process. The roasted oxide is leached with sulfuric acid to produce an impure zinc sulfate solution that is then purified to a high degree to remove such elements as cadmium, copper, cobalt, and iron. The purified zinc sulfate solution is electrolyzed in vast cell houses, with zinc of 99.99 percent purity being deposited at the cathodes, which are then mechanically stripped. This process, which was introduced in the 1960s, has since been further developed, and plants producing 200,000 tons (180,000 tonnes) per year are now common.
The other frequently used process, which currently produces approximately 12 percent of the world’s zinc, is the Imperial Smelting Process (ISP) using metallurgical coke as the reductant.
The ISP is a zinc blast furnace with the principal reaction being
Unfortunately, when burning carbon in air, both CO and CO2 are generated and the reaction
also takes place. In the ISP, this problem of reversion is overcome by absorbing the zinc vapor onto lead droplets in the condenser. The molten lead containing approximately 2.5 percent zinc is then externally cooled from 1040°F (560°C) to 806°F (430°C), when a portion of the zinc comes out of solution and—being the less dense of the two metals—floats and is collected separately. The grade is lower than that produced by the electrolytic process, containing 98.75 percent zinc.
The ISP will treat a mixed concentrate of zinc and lead, with a lead bullion (containing most of the copper, silver, and gold) being tapped off the furnace bottom together with the slag, the two phases being separated in a forehearth.
Zinc can be purified by distillation, and many plants still operate by taking advantage of the different boiling points of cadmium (1407°F, 764°C), zinc (1663°F, 906°C), and lead (3191°F, 1755°C) to effect a separation—with 99.99 percent cadmium and 99.99 percent zinc quality as end products.
