Zinc

Source : http://www.miningplazza.com.au/mining-information/130-zinc.html

Introduction

 

Produced jointly by the Minerals Council of Australia and AGSO

Zinc (Zn) was used in Rome and China more than 2000 years ago as a component of brass (zinc-copper alloy). Zinc metal was first smelted from zinc ore in India in about 1200 and is known to have been used in China soon after. Commercial production of zinc did not start in Europe until the middle of the 18th century and in the United States until 1860. In deposits mined today, zinc ore (rock containing economic content of zinc and/or other materials) usually occurs mixed with ores of lead, silver and commonly copper, and is extracted as a co-product of these metals.

Zinc is used for galvanising (coating) iron and steel to protect against rust in large-scale construction projects, in motor vehicle bodies and roof sheeting.

Occurrence

The main zinc mineral is sphalerite (Zn,FeS), which contains up to 67% zinc. Smithsonite (ZnCO3, 52% zinc), willemite (Zn2SiO4, 59% zinc) and hemimorphite (Zn4Si2O7(OH)2 H2O, 54% zinc) may occur in the near-surface weathered or oxidised zone of an orebody.

Deposits containing zinc form from hot (hydrothermal) fluids generated within the earth. These fluids may be trapped below the surface in cracks where sphalerite and other minerals may precipitate to make vein deposits. Where limestones occur, the fluids may fill cavities to form rich but patchy deposits. Some fluids may reach the ocean floor in areas of underwater volcanic activity to form 'volcanogenic' deposits. Some are forming today under the ocean off Papua New Guinea and Canada. Other fluids may escape to the surface through cracks or faults into small shallow lakes or seas and, if conditions are right, lead-zinc-silver deposits may form.

For thousands of millions of years, deposits have been forming in this manner and may eventually be exposed at the surface following weathering and erosion. Some are completely eroded away and may be recycled by natural processes into new deposits. Partially eroded deposits were relatively easy to discover, examples are the Broken Hill and Mt Isa deposits, found late last and early this century. These deposits formed the basis of Australia's zinc mining industry.

Exposed deposits are becoming harder to find in Australia and exploration companies are now looking beneath the surface for the deposits of the future. This is a more costly and difficult way to find orebodies but a series of successes have occurred since the late 1970s. Such discoveries include the Scuddles mine (140 metres deep) and Admiral Bay (1.5 kilometres deep) in Western Australia, the Cannington deposit (10 metres deep) in north Queensland, the Hellyer mine (90 metres deep) in Tasmania and the Wilga deposit (50 metres deep) in Victoria.

Australian resources and deposits

In 1883, Charles Rasp discovered the rich, large Broken Hill lead-zinc-silver deposit in New South Wales (NSW) deposit when he found some dark, heavy rocks which he thought may contain tin. Subsequent assays (analyses) of these rocks proved that he had located rich oxidised (weathered) silver and lead minerals. Broken Hill provided the basis for Australia's current major zinc mining industry . Over 100 years later ore is still being mined at Broken Hill and it has been the largest producer of lead-zinc-silver in Australia.

The rich lodes at Mt Isa were not discovered until 1923 and were developed despite the remote location and harsh environment. The nearby, rich Hilton deposit was discovered in the late 1940s but not developed until the mid-1980s. Production at Mt Isa continues and it has been the second biggest zinc producer in Australia. In the Mt Isa region, there are large mines at the George Fisher, Cannington and Century deposits, while the Dugald River and Lady Loretta deposits are yet to be developed. In the Northern Territory, the large McArthur River deposit is a major producer.

Zinc ore is also produced at Rosebery and Hellyer in Tasmania; Elura in NSW; and Pillara, Goongewa, Scuddles and Gossan Hill in WA . High-grade zinc silicate ore is mined intermittently from the small Beltana deposit in South Australia.

Australia in the world

Australia ranks first in the world in economic zinc resources because of the development of the large, world-class zinc-lead-silver deposits at McArthur River, Cannington and Century. This position is further supported by resources in the many other deposits of various sizes in Australia.

Australia ranks third behind China and Canada in world mine production and second to Canada in exports of zinc. Australia exports zinc as refined metal to a broad range of destinations in the Asia Pacific area, from India to the USA, but mainly to Indonesia, Hong Kong, Chinese Taipei and Malaysia. Our major customers for zinc in ores and concentrates are Japan and South Korea, and to a lesser extent Belgium, Germany and the United Kingdom.

Australia is also at the forefront of technological development in zinc mining and processing.

Mining and processing

In the early days of Broken Hill, zinc ore was rejected to the waste dumps because virtually none of the zinc could be extracted economically. Gravity and magnetic separation methods were unsuccessful. In 1901 a flotation process was devised at Broken Hill which achieved recovery of upwards of 60% of the zinc minerals from ore. After considerable experimentation, a selective flotation method that worked on a commercial scale was perfected in 1912. Improved versions of this flotation process, such as the Australian-developed Jameson flotation cell, are used world-wide today. The Jameson flotation cell is installed in many mines around Australia.

Almost all of Australia's zinc mines are underground operations and are highly mechanised. Ore is drilled and blasted in large volumes, transferred to underground rock crushers by large loaders and trucks, and then hoisted to the surface in skips or driven directly to the surface by truck via a spiral access tunnel (decline). Century is Australia's only large surface open pit zinc mine (so hoisting is not required), where ore extraction is by similar methods.

At the surface, the ore is subjected to additional crushing and fine grinding. The flotation process is then used to separate the zinc and other valuable sulphide minerals from the waste rock particles (tailings) to form a concentrate.

Ground ore, water and special chemicals are mixed and constantly agitated in banks of flotation cells. Air is blown through the mixture in each cell, and the fine zinc sulphide particles stick to the bubbles, which rise to form a froth on the surface of the flotation cell. The tailings sink and are removed from the bottom of the cell. The froth is skimmed off and the resulting zinc sulphide concentrate is dried. This process upgrades the ore, which may contain only 6% Zn, to a concentrate assaying more than 50% zinc. Up to 90% of the zinc in the ore can be recovered.

Electrolysis and smelting are the two processes used to produce zinc metal in Australia. The electrolytic process is used at the Risdon zinc refinery in Tasmania, where zinc concentrate from various Australian mines is roasted to eliminate most of the sulphur, as sulphur dioxide, and make impure zinc oxide. The roasted concentrate is then leached with sulphuric acid to form zinc sulphate solution. The zinc sulphate solution is purified by adding a small amount of zinc powder to precipitate and remove traces of copper, cadmium, cobalt and nickel. The solution is piped to electrolytic cells, where the zinc is electrochemically deposited on aluminium cathodes (electrodes). The zinc is removed from the cathodes, melted in a furnace and cast into slabs.

The smelting process is used at Cockle Creek near Newcastle in NSW, to produce zinc (and lead) metal simultaneously in a blast furnace. Zinc and lead concentrates from various mines are blended and sintered (partly melted) to combine the fine particles into lumps and remove some sulphur as sulphur dioxide. The sintered product is mixed with coke and smelted in a blast furnace to produce zinc vapour (gas), which is condensed by cooling with a spray of molten lead to form impure molten zinc metal (98.3% Zn). To remove the small amount of lead and cadmium impurities the liquid zinc is twice boiled to zinc vapour and recondensed to produce high purity zinc metal (up to 99.95% Zn).

At Port Pirie, zinc is recovered from the lead smelter slag (molten waste), which contains about 17% zinc, and residues from the Risdon zinc refinery in Tasmania. The molten slag is heated further to drive off zinc (and some lead) vapour, which is oxidised to form a zinc oxide fume and filtered out as dust in a 'bag filter'. This dust is ground and put through an electrolytic refining plant to produce high purity zinc.

Primary refined zinc is produced at three plants - Risdon (Tasmania), Cockle Creek (NSW), and Port Pirie (SA), while production at the new Townsville zinc refinery is expected in late 1999. Small production of secondary refined zinc occurs at Port Kembla (NSW). Zinc oxide and zinc dust is produced from primary and scrap zinc at West Footscray in Melbourne (Victoria) and in minor amounts in Brisbane (Queensland).

Risdon is Australia's largest zinc refinery and also one of the largest in the world. Overall zinc recovery from concentrates is about 95%. Less than half of Australia's zinc concentrates are processed domestically.

Uses

A large part of the world's zinc is used as protective galvanised coatings for iron and steel. In Australia, this use accounts for well over half of the domestic sales of zinc. The widespread use of zinc as a protective coating is mainly because of its resistance to normal weathering, and the protection given to steel by the preferential corrosion of zinc when the underlying iron or steel is exposed. This is an electro-chemical reaction known as galvanic action. The construction and appliance manufacturing industries use large amounts of zinc, mainly as coatings on steel beams, sheet steel and vehicle panels in the automotive industry.

Zinc is also used in alloy die cast products, zinc pigments, zinc salts, zinc oxide as additives to rubber and for zinc chemicals in agriculture, and for wrought or rolled zinc products.

Suggestions for further reading

Blainey, G 1993 The Rush that Never Ended: A History Of Australian Mining, 4th edition, Melbourne University Press, Melbourne.

Australian Geological Survey Organisation 1999 Australia's Identified Mineral Resources 1999, Australian Geological Survey Organisation, Canberra.

Hughes, F.E.(Ed) 1990 Geology of the Mineral Deposits of Australia and Papua New Guinea, Australasian Institute of Mining and Metallurgy, Monograph 14.

Roarty, M.J. 1989 Zinc In Australian Mineral Industry Annual Review for 1987, A. Paine (Ed) Australian Government Publishing Service, Canberra, pp266-275.

Woodcock, J.T. and Hamilton, J.K. (Eds) 1993 Australasian Mining and Metallurgy: The Sir Maurice Mawby Memorial Volume. Australasian Institute of Mining and Metallurgy, Monograph 19.