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 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.
80% of zinc mines are located underground, 8% are of the open pit type, with the remainder a combination of both. In terms of production volume, underground mines account for as much as 64% of overall zinc production, combined underground/open pit mines provide 21%, and the remaining 15% is sourced from open pit mines.
Rarely is the ore, as mined, rich enough to be used directly by smelters; it needs to be concentrated. Zinc ores contain 5-15% zinc. To concentrate the ore it is first crushed and then ground to enable optimal separation from the other minerals. Typically, a zinc concentrate contains about 55% of zinc with some copper, lead and iron. Zinc concentration is usually done at the mine site to keep transport costs to smelters as low as possible.
Roasting & Sintering
Over 95% of the world’s zinc is produced from zinc blende (ZnS). Apart from zinc, the concentrate contains some 25-30% or more sulfur as well as different amounts of iron, lead and silver and other minerals. Before metallic zinc can be recovered, by using either hydrometallurgical or pyrometallurgical techniques, sulfur in the concentrate must be removed, which is done by roasting or sintering. The concentrate is brought to a temperature of more than 900°C where zinc sulfide (ZnS) converts into the more active zinc oxide (ZnO). At the same time sulfur reacts with oxygen giving out sulfur dioxide which subsequently is converted to sulfuric acid – an important commercial by-product.
The Hydrometallurgical Process
In a leaching stage the zinc oxide is separated from the other calcines. sulfuric acid is used to do this. The zinc content dissolves whereas iron precipitates and lead and silver remain undissolved. However, the dissolved solution contains some impurities which need to be eliminated in order to obtain a high-purity zinc product at the end of the production process. Purification is mainly done by adding zinc dust to the solution. As all the elements to be removed lie below zinc in the electrochemical series they can be precipitated by cementation. The purified solution then passes an electrolytic process where it is electrolyzed between lead alloy anodes and aluminium cathodes. An electrical current is circulated through the electrolyte by applying an electrical difference of 3.3-3.5 volts between the anode and cathode causing the zinc to deposit on the aluminum cathodes in high purity. The deposited zinc is stripped off, dried, melted and cast into ingots. The zinc ingots may have different grades: High Grade (HG) 99.95% and Special High Grade (SHG) 99.99% of zinc. Today over 90% of zinc is produced hydrometallurgically in electrolytic plants.
The Pyrometallurgical Process
The Imperial Smelting process is based on the reduction of zinc and lead into metal with carbon in a specially designed Imperial Smelting furnace. The IS process is an energy-intensive process and thus became very expensive following the rise of energy prices. Today, Imperial Smelting furnaces are only in operation in China, India, Japan and Poland.
More than 13 million tons of zinc was produced in 2013 worldwide. Sixty percent of this is used for galvanizing to protect steel from corrosion thus prolonging the service life of steel products significantly. Approximately 15% goes into the production of zinc base alloys, mainly to supply the die casting industry, 14% goes into the production of brass and bronze and 8% into the production of compounds including zinc oxide and zinc sulfate. The remainder is zinc alloys, mainly rolled, utilized in semi-manufactured applications including coinage and architectural applications..
These first-use suppliers then convert zinc into in a broad range of products. By far the largest application area is construction with 45% of all first-use zinc products used in this area. The transportation sector consumes 25% of global zinc consumption and consumer goods – including electrical and electronic appliances – accounts for 23%. The remaining 7% is used for the manufacture of industrial machinery.