The History of Zinc
History of Zinc
Centuries before zinc was discovered in the metallic form, its ores were used for making brass and zinc compounds for medicinal purposes. Zinc compounds were in the ores smelted certainly as early as 200 B.C. to obtain copper and which gave alloys of copper and zinc – the brass family. The Romans certainly were major users of brass. The Greeks also appeared to know zinc, even if not by name.
An ancient idol, containing 87.5% zinc, found in prehistoric ruins in Transylvania in Eastern Europe is the oldest known zinc object. Zinc filled silver bracelets dating back to 500 B.C. have been found on the island of Rhodes, and the Romans used a zinc alloy to fabricate coins.
Some credit India with developing the first knowledge of true zinc smelting while others attribute its discovery to the Chinese. The production of metallic zinc occurred much later than other common metals. Copper was smelted from its ores around 5000 B.C., lead about 4000 B.C. and iron about 2000 B.C., while zinc appears to have become available on a commercial scale in the 14th century A.D.
In Europe, zinc probably became first known through its import from India and China. Zinc was recognized in Europe as a separate metal in the 16th century when Agricola (1490 – 1555) and Paracelsus (1493 – 1541) wrote of a metal called “zincum.”
Commercial smelting of zinc began in Europe in the middle of the 18th century when the first European zinc smelter was established in Bristol in the United Kingdom using a vertical retort procedure. But the real advent of modern techniques dates from the introduction of the horizontal retort process in the early 19th century. In 1836 hot-dip galvanizing – the oldest anti-corrosion process – was introduced in France. Zinc smelting in the United States started in 1850s.
Zinc – Natural Occurrence
Zinc is a natural component of the earth’s crust and an inherent part of our environment. Zinc is present in rock, soil, air, and water. Plants, animals and humans also contain zinc.
The average natural level of zinc in the earth’s crust is 70 mg/kg (dry weight), ranging between 10 and 300 mg/kg (Malle 1992).
In some areas, zinc has been concentrated to much higher levels by natural geological and geochemical processes (5-15% or 50,000-150,000 mg/kg). Such concentrations, found at the earth’s surface and underground, are being exploited as ore bodies.
Zinc ore deposits are widely spread throughout the world. Zinc ores are extracted in more than 50 countries. Australia, Canada, China, India, Peru and Europe are the largest producers. Zinc is normally associated with lead and other metals including copper, gold and silver. There are four major types of zinc deposits:
- Volcanic hosted massive sulfides (VMS)
VHMS deposits are polymetallic and are an important economic source of copper and zinc often associated with significant concentrations of silver, gold, cadmium, bismuth or tin.
- Carbonate hosted (Mississippi Valley & Irish types)
Limestone and dolomite are the most common host rocks. The zinc lead content usually ranges from 5%-10% with zinc usually predominating over lead. Concentrations of copper, silver and barite of fluorite may also be present.
- Sediment hosted (sedex deposits)
The host rocks are mainly shale, siltstone, and sandstone. Sedex deposits represent some of the world’s largest accumulations of zinc, lead and silver. The mineral has a high silver content. The lead/zinc content ranges from 10-20%.
- Intrusion related (high sulfidation, skarn, manto, vein)
These deposits are typically found in carbonate rocks in conjunction with magmatic-hydrothermal systems and are characterized by mineral association of calcium and magnesium. Typically the ore body contains more lead than zinc and is associated with silver.
The most commonly found zinc mineral is sphalerite (ZnS) also known as zinc blende, which is found in almost all currently mined zinc deposits. The mineral crystallizes from the hydrothermal solution as pure zinc sulfide.
The mineral marmatite is a complex zinc-iron sulfide, which is commonly found but rarely exploited, as it is not easy to smelt.
Zinc deposits close to the earth’s surface are often converted to oxides and carbonates. Small quantities of zinc carbonate – the mineral calamine (smithsonite) in North America – often refer to the hydrated silicate mineral also known as hemimorphite.
Iron and lead sulfides, in the form of the minerals pyrite and galena are always associated in significant quantities while smaller quantities of other metals are commonly found.
Metamorphically formed oxide zinc ores such as franklinite or zincite are limited to only a few deposits.