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| CSIRO | SOLVE | Issue 3 - May 05 | |
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COVER STORY
By GRAEME O'NEILL
A NEW GAS TECHNOLOGY MAY BE A MAJOR BOOST TO THE USE OF MAGNESIUM. The magnesium industry's efforts to become a major supplier to global manufacturers such as car-makers have been given a significant lift through pioneering research into a new 'cover gas' for magnesium smelting and diecasting. Called AM-cover, the new cover gas (used to stop molten magnesium from burning) could reduce global greenhouse gas (GHG) emissions by five million tonnes a year, if adopted by manufacturers. The breakthrough removes a major environmental impediment to the metal's wider use, and to Australia's ambitions to be a significant player in the magnesium industry. The US Environment Protection Agency (EPA) has certified AM-cover for use by US manufacturers, and was sufficiently impressed to give CSIRO's partners — the Cooperative Research Centre for Cast Metals Manufacturing (CAST) and Advanced Magnesium Technologies (AMT) — a joint award in December 2004 for outstanding technical contributions to its 'SF6 Emission Reduction Partnership' scheme.
The Australian breakthrough was made by scientists at CSIRO Manufacturing and Infrastructure Technology (CMIT) - the largest partner in CAST. Their journey of discovery began in 1995 when Dr Nigel Ricketts and a University of Queensland PhD student, Simon Cashion, set out to find a more environmentally benign cover gas than SF6. The emerging problem was that vehicle manufacturers were increasingly turning to lightweight magnesium alloy components for weight savings, improved fuel economy and lower CO2 emissions - but the magnesium manufacturing process using SF6 had the potential to produce more greenhouse gases than what could be saved by using the lighter metal. Dr Ricketts and Dr Cashion eventually identified the refrigerant gas HFC-134a as an ideal, third-generation cover gas for magnesium processing. Dr Ricketts says his team decided to trial HFC-134a because it is one of the cheapest forms of non-toxic fluorine, plus it meant they would not be introducing an exotic new chemical that might cause unforeseen problems later on. "It is produced on a scale that keeps costs down - half the cost of SF6, and its greenhouse gas potential is 18 times lower," he says. "When it is used as a cover gas in a special mix now marketed as AM-cover, recent US EPA and Australian Greenhouse Office sponsored trials showed that its greenhouse emissions are 99.8 per cent lower than SF6. "When SF6 is put into a magnesium furnace, only about 10 per cent of it is consumed. Most is released into the atmosphere. However, with HFC-134a, 90 per cent of the gas is destroyed in the process."
Although Australia currently has no magnesium smelters, an Adelaide magnesium diecasting company, Bridgestone TG Australia Pty Ltd (BSTG), has successfully tested AM-cover to reduce its emissions. It is one of nine companies around the world - predominantly in the US and Europe - that have either adopted or are trialling AM-cover. BSTG makes automotive components, such as steering wheels, for all four automotive manufacturers in Australia. It has a high-pressure diecast facility that produces the magnesium substrates for these assemblies. It plans to introduce the AM-cover system by the end of 2005. Researchers have calculated that widespread adoption of AM-cover would reduce greenhouse emissions from magnesium manufacture by the equivalent of about five million tonnes of carbon dioxide a year - roughly the same as taking more than a million vehicles off the road, or planting 17 million trees to absorb atmospheric carbon dioxide. It would also save magnesium manufacturers real money. About 2 per cent of magnesium is lost during melting when SF6 is used as a cover gas. With AM-cover, the loss is halved. CMIT's Dr Paul Koltun has completed a life-cycle assessment to measure how increased use of diecast magnesium components would affect greenhouse emissions. Replacing steel components with magnesium will yield fuel savings and cut greenhouse emissions, but the savings have to be balanced against the increase in greenhouse emissions from smelting and diecasting magnesium, which consumes more energy than, for example, the production of steel. The particular cover gas used also influences this equation, so Dr Koltun's research group has compared the different magnesium production processes. The use of AM-cover becomes part of a complicated 'cause and effect' juggling act that manufacturers have to manage as they strive to meet increasingly stringent greenhouse targets - lighter, cleaner vehicles on the one hand, potentially negated in terms of greenhouse increases in the manufacturing process. Dr Ricketts says that even with the weight savings achieved by using magnesium components, a vehicle would still have to run for a long time to recover this greenhouse gas 'cost' through improved fuel economy. The average Australian vehicle produces about 4.3 tonnes of CO2 per year. However, replacing SF6 with AM-cover technology in magnesium manufacture and processing could save 13 kilograms of CO2 equivalent emissions for each kilogram of magnesium produced. "So getting rid of SF6 can save a substantial proportion of the greenhouse gas emissions that are produced in the manufacturing process," says Dr Ricketts. However this is not the end of the story or problem. The issue re-emerges when the magnesium is re-melted for diecasting. "Diecasting uses a lot of SF6, particularly in North America, so the CO2 equivalent emissions rise again by 20 to 60 kg per kilogram of magnesium," explains Dr Ricketts. And the equation is further complicated when manufacturers use Chinese magnesium. China produces about 70 per cent of the world's magnesium and uses sulfur dioxide as a cover gas (discarded by US and European manufacturers in the 1970s). This process creates about 43 kg of CO2 for every kilogram of magnesium. "If, for example, you subsequently use Chinese magnesium in North American or European casting operations that use SF6 for melt protection, you make more greenhouse gases in producing that magnesium part than the greenhouse gas savings that come from making a lighter vehicle," says Dr Ricketts. About 50 per cent of the magnesium produced in the world currently goes into the manufacture of aluminium alloys, including so-called 'mag wheels', which despite the name are mainly aluminium alloyed with small amounts of magnesium. The only large volume magnesium components manufactured in Australia are magnesium steering wheels. The other use is in aluminium-magnesium alloys used for drink cans. The body of a can contains about 0.75 per cent magnesium, while the reinforced lid is about 3 to 4 per cent magnesium. This is where some of the real environmental savings currently accrue because of the high recycling rate of drink cans. "The more secondary magnesium that is made, the better the emission equation looks," says Dr Ricketts. And cover gas is needed at all steps of the process. The AM-cover technology was developed by CSIRO researchers working within the CAST CRC. CAST has granted an exclusive licence to AMT, another CAST member, to market AM-cover worldwide. The company has run evaluation trials and held training programs for customers in the US, Europe and Asia-Pacific. The CRC Association last year gave AM-cover its Innovation in Excellence Award. AMT's technology manager, Gordon Dunlop, says current annual world production of magnesium is around 370,000 tonnes, of which about 170,000 tonnes is used for diecasting. Half of the metal used in diecast components is ultimately recycled. Once a plant makes the decision to change to AM-cover technology, the changeover is relatively simple, taking just one or two days, he says. Aside from helping industry to meet greenhouse gas targets, there are also more immediate savings. Mr Dunlop explains that magnesium costs around US$1850 a tonne, and each step involved in processing, diecasting and recycling causes metal losses of between 2 and 4 per cent. Overall these add up to losses of about about 10 per cent, or $185 per tonne.
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