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Metal Casting Technologies : Dec 2009
26 www.metals.rala.com.au We are also collaborating with the AutoCRC to implement a project based on LCA to assess various types of gaseous fuels, e.g., liquefied versus compressed natural gas," said Dr Paul Koltun. CAST is also keen to encourage the use of LCA in areas outside of its regular network. This will feed back new data to allow researchers to compare the "cradle to grave" environmental sustainability of different materials and process technologies. Life Cycle Analysis -- the devil in the detail CAST has applied detailed LCA to five major metal manufacturing activities, as follows: ● Ingot casting within a smelter environment; ● The die casting of components for automotive applications; ● The comparison of different materials in automotive applications over the life of a vehicle; ● The environmental benefits of cover gas technology in magnesium melting operations; and ● The effect of rare earth additions on the environmental impact of magnesium alloys. LCA study of rare earth elements for magnesium alloy applications Rare earths are a group of chemically similar elements that make up the lanthanide series of the periodic table from lanthanum (atomic number 57) through to lutetium (atomic number 71). Two lighter elements, scandium (atomic number 21) and yttrium (atomic number 39) are often included in the definition. Some other better known rare earths are cerium, neodymium and ytterbium. Although rare, these elements can give amazing properties to materials. Neodymium, for example, is the "magic" Even in small quantities, they can vastly improve the properties of metal alloys - properties such as corrosion resistance, high temperature creep resistance and formability. Why magnesium? Magnesium is a logical lightweight alternative to traditional materials, especially for automotive applications. Magnesium is 33 per cent lighter than aluminium and 75 per cent lighter than cast iron, has an excellent strength to weight ratio, high shock and dent resistance and will dampen noise and vibrations significantly more than either aluminium or steel. The properties of magnesium alloys can be significantly altered by the addition of rare earths. For example, magnesium alloys which contain cerium, yttrium and neodymium have good high temperature properties for use in automotive powertrains. It is in components such as the engine block that lightweight alloys of magnesium can make a significant difference - lighter cars result in fewer greenhouse gas emissions and hence a reduced environmental impact. However, the benefits of reduced weight need to be offset by the environmental impact of producing the magnesium alloys and manufacturing the engine block compared to the alternatives such as iron and aluminium. While CAST and others have done a lot of work in the life cycle analysis of producing magnesium engine components, there has previously been no analysis of the rare earth additions to the alloys. To date, cost has been the major concern in using rare earths as alloying elements, but knowledge of their overall environmental impact is also required - knowledge that comes from analysing the whole "life cycle" from mining the ore through to recycling and disposal of the manufactured products. More than 90 per cent of the world supply of rare earths comes from iron- rich ores mined in Bayan Obo, Inner Mongolia. Bastnasite and monazite are concentrated from the ores using gravity separation, magnetic separation and flotation. This is followed by cracking and purification to separate the rare earth oxides from the mineral concentrates. These oxides are subsequently reduced into the rare earth metals. LCA of rare earth alloying elements The scope of this LCA is illustrated in Figure One. Since the production of the alloys and their subsequent conversion to manufactured products has already been extensively studied, the present analysis is not a complete lifecycle but rather a "cradle-to-gate" assessment of the environmental impacts of providing the elements into the alloying plant (or any other user of rare earth elements). The factors considered included ore tonnage and grade; recovery rates; transport (from mine to ore processing plant to export market to end-user/manufacturer); energy consumption and consumables. Major assumptions in analysis are: 45,000 tonnes of rare earth oxides are produced per annum from Bayan Obo mineral deposit with 75 per cent from bastnasite and 25 per cent from monazite. The rare earth oxide concentration in both minerals is 60 per cent with the average grade in the mining ore of 6 per cent and final oxide recovery rate of 10 per cent. An economic (price) allocation is used to share the environmental burden of mining and ore processing among co--products. The distance between mining pit and beneficiation plant is 15 kilometres and distance between beneficiation and oxide separation plant is 30 kilometres. "We would like to have web-based access to this tool to increase adoption. W l llb ti ith th ingredient that turns iron boride into a super-strong permanent magnet. E i ll titi th tl Where do the rare earths come from?
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