Magnesium 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 per cent, 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 per cent magnesium by weight.
Magnesite (MgCO3) is an ore for magnesium production and also the source of a range of industrial minerals. When pure, magnesite contains 47.8 per cent magnesium oxide and 52.2 per cent 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 per cent (equivalent to 21.7 per cent MgO) and CaCO3 54.35 per cent.
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: (1) Cryptocrystalline or amorphous magnesite and (2) 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 and 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 resultant 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. China is ranked first, followed by the Russia, and North Korea.
In the Kunwarara deposit, 60 kilometres northwest of Rockhampton, Queensland, low iron nodules of cryptocrystalline magnesite cover an area of some 63 square kilometres, 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 caused 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 per cent 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 kilometres northwest of Condobolin, consists of nodules of massive magnesite which occurs as pockets or veins in decomposed ultramafic rock. Other occurrences of magnesite in the State 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, 50 kilometres south of Burnie. The magnesite ore contains more than 40 per cent 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, 80 kilometres 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 30 kilometres 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 because its overburden averages only 4 metres in thickness the strip ratio is low. 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 700o-1000oC, 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 and is therefore useful in water treatment.
When calcined magnesia is heated to between 1530o-2300oC, 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 as a refractory material because of its inertness and high melting point.
When calcined or dead-burned magnesia is heated in excess of 2800oC 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 950oC.
Magnesia is also produced from the processing of sea water and magnesium-rich brines. This 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 now 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. This 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 345000 tonnes of beneficiated magnesite which, in turn, was used to produce 102000 tonnes of dead-burned magnesia, 27300 tonnes of electrofused magnesia and 17500 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 1500 tonnes per year demonstration plant at Gladstone, Queensland. It plans to establish a commercial magnesium metal plant at Gladstone using the Australian Magnesium Process. It is expected to be in operation by 2002.
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 for the production of dead-burned magnesia, for refractory brick use in lining furnaces in the steel industry and non-ferrous metal processing units, and cement kilns. The second is for processing to caustic calcined magnesia which is used principally as an food supplement in agribusiness, in fertilisers, and as 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 per cent of the total magnesium metal consumption. The addition of magnesium to aluminium produces high-strength, corrosion-resistant alloys. About 20 per cent is used in castings and wrought products (machinery, tools and other consumer products such as mag wheels for cars).
