Magnesium Fact Sheet
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Magnesium (Mg) is the lightest of all metals, being about two-thirds lighter than aluminium. Magnesium is non-toxic, non-magnetic, has high-impact strength and is resistant to denting.
Magnesium is too reactive to occur in nature as an element, but its compounds are common. At 2.5%, magnesium is the eighth most abundant element in the earth's crust. It is the third most abundant element in sea water which averages about 0.13% magnesium by weight.
Magnesite (MgCO3) is an ore for magnesium production and the source of a range of industrial minerals. When pure, magnesite contains 47.8% magnesium oxide and 52.2% carbon dioxide. Natural magnesite almost always contains some calcium carbonate as the mineral calcite and iron carbonate as the mineral siderite. Magnesium also occurs in dolomite, which has the formula CaMg(CO3)2 and in which MgCO3 constitutes 45.65% (equivalent to 21.7% MgO) and CaCO3 54.35%.
Magnesite colour varies from white when pure to yellowish or grey white and brown. Hardness is 3.5 to 4.5 and the specific gravity varies from 3 to 3.2. A vitreous lustre and very slow reaction with cold acids distinguishes magnesite from other carbonates.
Magnesite, dolomite, sea water and lake brines are used as sources of magnesium metal with the most common source being lake brines and sea water.
Magnesite occurs in two physical forms: Cryptocrystalline or amorphous magnesite and Macrocrystalline magnesite. It occurs in five different ways: a replacement mineral in carbonate rocks; an alteration product in ultramafic rocks (igneous rocks composed mainly of one or more dark coloured ferromagnesian minerals); a vein-filling material; a sedimentary rock; as nodules formed in a lacustrine (lake) environment.
Replacement-type magnesite deposits involve magnesium-rich fluids entering limestone via openings to produce both magnesite and dolomite.
The alteration-type deposits are formed by the action of carbon dioxide-rich waters on magnesium-rich serpentinite a rock which has been formed from the alteration of magnesium and iron silicate minerals. The resulting magnesite may be very pure.
Sedimentary deposits usually occur as thin layers of variable magnesite quality. Lacustrine magnesite deposits consist of nodules of cryptocrystalline magnesite formed in a lake environment. Both vein filling and sedimentary magnesite occurrences are rarely mined on a large scale.
Australia's economic demonstrated resources (i.e. those that could be economically extracted at current prices with existing technology) are 202 million tonnes of magnesite.
The estimated world economic resources of magnesite are about 8600 million tonnes of MgCO3 with China having the most followed by the Russia and North Korea.
In the Kunwarara deposit, 60km northwest of Rockhampton in Queensland, low iron nodules of cryptocrystalline magnesite cover an area of about 63 square km which is entirely overlain by black clay up to 12 metres thick. The deposit is thought to have formed by the deposition in lakes of magnesium bicarbonate derived from the alteration of serpentinite rock. Evaporation caused hydrated magnesium carbonate to precipitate. Deposition of mud over the magnesite resulted in further evaporation and the formation of hard nodules of dehydrated magnesite. Mining of this deposit commenced in 1989. Similar magnesite deposits occur at Yaamba and Triple Four, also in the Rockhampton area. Magnesite occurs near Gunnawarra southwest of Cairns, and in southern Queensland near Kilkivan and at Upper Widgee.
In New South Wales, magnesite at Thuddungra, northwest of Young, occurs as veins and nodules formed by the alteration of mafic rocks by magnesium-rich fluids. The magnesite ore contains 95 to 99% MgCO3 and varies in thickness from 2 to 10 metres. The Thuddungra mine has been in operation since 1935.
A former magnesite mine near Fifield about 30 km northwest of Condobolin consists of nodules of massive magnesite nodules occurring as pockets or veins in decomposed ultramafic rock. Other occurrences of magnesite in New South Wales are at Lake Cargellico, Cobar, Nyngan, between Attunga and Warialda.
In Tasmania, fine-grained, massive magnesite, formed by the replacement of limestone and dolomite, occurs at Arthur River and in the Lyons River area 50km south west of Burnie. The magnesite ore contains more than 40% magnesium oxide. Another deposit of magnesite also formed from the alteration of limestone is situated south of Arthur River at Main Creek.
In South Australia, beds of magnesite ranging from 5 centimetres to 9 metres in thickness occur at Witchelina, 80km north west of Leigh Creek, Copley, and Myrtle Springs. Small occurrences are at Balcanoona and near Robertstown.
In Western Australia, hard magnesite nodules in dark clayey material crop out 30km east of Ravensthorpe. Magnesite also occurs in the Kalgoorlie region.
Magnesite formed by the replacement of dolomite occurs at Huandot near Woodcutters in the Northern Territory. The deposit has an average width of 40 metres and an average overburden depth of 11 metres.
In Australia, all magnesite deposits are mined by open-cut methods. During mining the strip ratio, that is, the quantity of magnesite ore to waste material, may be high. An advantage of the Kunwarara deposit is that the strip ration is low because the overburden averages only 4 metres thick. A second advantage is that only 5 to 10 tonnes of ore has to be mined and beneficiated to produce one tonne of high grade magnesite. The processing of magnesite ore begins with crushing, screening and washing.
When crude magnesite is heated to between 700°-1000°C, carbon dioxide is driven off to produce caustic-calcined magnesia (caustic magnesia). Caustic magnesia is able to absorb liquids and to absorb heavy metals and ions from liquid streams making it useful in water treatment.
When calcined magnesia is heated to between 1530°-2300°C, the product produced is non-reactive and exhibits exceptional stability and strength at high temperatures. This product known as 'dead-burned' or 'sintered' magnesia is mainly used mainly as a refractory material because of its inertness and high melting point.
When calcined or dead-burned magnesia is heated in excess of 2800°C in an electric arc furnace, electrofused magnesia is produced. It has higher strength, resistance to abrasion and chemical stability than dead-burned magnesia. It is used in the manufacture of premium grade refractory bricks used in the high wear hot spots of Basic Oxygen Furnaces, electric arc or similar furnaces where temperatures can approach 950°C.
Magnesia is produced also from the processing of sea water and magnesium-rich brines but it is a very complex process using much larger amounts of energy than the process of heating of natural magnesite.
Magnesium metal can be produced by one of three processes. The electrolytic process uses magnesium chloride produced from either magnesite, seawater or brines rich in magnesium chloride. Magnesite is favoured as the source of magnesium because chlorine is recycled within the process rather than being disposed of as a waste or a by-product. The silicothermic process mixes calcined dolomite or magnesite with ferrosilicon (a combination of iron and silicon metal) to produce a magnesium vapour which is then condensed in cooling vessels to form magnesium metal. Both processes are energy intensive and require low-cost electricity to be competitive.
The Australian Magnesium Process developed in Australia involves dissolving pure magnesite ore in hydrochloric acid to produce magnesium chloride. The magnesium chloride is then purified, dehydrated to a dry feed and electrolysed in an Alcan cell. The molten magnesium is tapped from the cell and cast into ingots. The chlorine gas released is recycled and combined with hydrogen, from natural gas to produce hydrochloric acid for use in the process.
In 1998, Queensland Metals Corporation Limited mined 2.44 million tonnes to produce 345,000 tonnes of beneficiated magnesite which, in turn was used to produce 102,000 tonnes of dead-burned magnesia, 27,300 tonnes of electrofused magnesia and 17,500 tonnes of calcined magnesia. A small quantity of calcined magnesium oxide and magnesium carbonate was produced from stockpiled magnesite ore from Thuddungra.
Currently, Australia does not produce magnesium metal. However in 1998, Australian Magnesium Corporation commenced operating a 1,500 tonne per year demonstration plant at Gladstone in Queensland. It plans to establish a commercial magnesium metal plant at Gladstone using the Australian Magnesium Process.
Other companies including Crest Resources Australia NL, Pima Mining NL, and Golden Triangle Resources NL are investigating the possibility of building magnesium metal plants based on magnesite resources in located mainly in Tasmania and South Australia.
There are two main uses for magnesite. The first is as feedstock in the production of dead-burned magnesia and for refractory brick use in lining furnaces in the steel industry and non-ferrous metal processing units and cement kilns. The second use is for processing to caustic calcined magnesia which is used principally as a food supplement in agribusiness and in fertilisers as well for fillers in paints, paper and plastics. Raw magnesite is used for surface coatings, landscaping, ceramics and as a fire retardant.
The largest single use for magnesium metal is in aluminium alloying, accounting for about 50% of the total magnesium metal consumption. The addition of magnesium to aluminium produces high-strength, corrosion-resistant alloys. About 20% is used in castings and wrought products including machinery, tools and other consumer products such as mag wheels for cars.