How is zinc produced?
Nearly all zinc refined today (over 95 percent) is extracted from sphalerite (ZnS), otherwise known as zinc blende.
Sphalerite ores generally contain between 5 and 15 percent zinc. To get this to a pure state that can be used by consumers, the metal must be separated, concentrated and refined into a pure metallic form. Either hydro-metallurgical or pyro-metallurgical processes can do this, however, over 90 percent of current production now originates from hydrometallurgical plants.
Once mined, zinc ore is crushed into powder and separated from lead via froth flotation. Lead and other metals, such as silver, that may be present in the ore in recoverable quantities will be extracted and treated separately.
The remaining concentrate will consist of between 1 to 15 percent zinc, but retain a high sulphur content (around 30 percent) that must be removed. This can be done by roasting the crushed ore at temperatures over 1652°F (900°C), producing zinc oxide (ZnO) and sulphur dioxide.
If hydro-metallurgical techniques are employed, zinc oxide is then leached from the other calcines using sulphuric acid. While zinc dissolves in the acid, lead and silver are undisolved and iron precipitates.
Zinc dust is added to help purify the solution via a cementation process before it is electrolysed.
Using lead alloy anodes and aluminum cathodes, the zinc-rich solution is electrolysed causing zinc to deposit on the aluminum. This is then stripped off, melted and cast into ingots with purities of 99.95 to 99.995 percent or directly alloyed with copper, aluminum or magnesium.
Physical zinc traded on the London Metal Exchange (LME) must meet the special high-grade zinc specification of 99.995 percent.
While pyro-metallurgical techniques are still employed in a number of plants, the process is extremely energy intensive and expensive.
The Imperial Smelting Corporation developed a technique during the 1940s and 1950s that reduced zinc and lead metal using carbon within a specially designed blast furnace.
The process, referred to as the Imperial Smelting process, involves roasting zinc and lead sulphides, which naturally occur together, so as to produce oxides. The zinc and lead oxides are then charged in a blast furnace with coking coal.
As heated air enters the furnace, carbon monoxide and dioxide vapours containing zinc rise up the shaft and are cooled in a lead splash condenser. The zinc is absorbed by the molten lead, which is then cooled, allowing the zinc to float to the surface.
The zinc layer is poured off and cast while the lead is sent back to the condenser.
According to the International Zinc Association, Imperial Smelting furnaces are only presently operational in China, India, Japan, and Poland.
Aside from extracting zinc from ore (primary production), the metal can also be recycled from existing sources (secondary production). Roughly one quarter of all zinc consumed now comes from secondary sources.
Electric arc furnaces, which are used for process and recycling scrap steel, can also collect zinc that is present in galvanized steels.
As in Imperial Smelting furnaces, the high temperatures in electric arc furnaces vaporize zinc, causing it to rise with gases that can be collected. Enrichment in a rotary kiln (Waelz kiln) produces zinc oxide that can be then fed back into the production loop.
World zinc mine production was roughly 13.7 million tonnes in 2013, while global refined zinc metal output was approximately 13.0 million tonnes over the same period. About 3 million tonnes, or just less than one-quarter of global production, came from recycled sources.
By country, the largest producer of refined zinc is China, accounting for about 40 percent of global production, followed by Europe and Korea.
By company, the largest producers of refined zinc are Nyrstar, Glencore and Korea Zinc.
The largest sources of zinc ores are China, Australia and Peru.
International Lead & Zinc Study Group. Review of Trends in 2013: Zinc. February 17, 2014.
International Zinc Association. Zinc Recycling: Material Supply. 2011.
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