Magnesium Alloys - Magnesium Alloys Produced from Fly Ash, Technology and Applications

Topics Covered

Background

Magnesium From Power Station Fly Ash

Raw Materials and Plant Location

Advantages of the Location

Advantages of Using Fly Ash As The Raw Material

How Does the Process Compare with Existing Magnesium Smelting Processes

The Alcan Dehydration Process

Timelines for Magnesium Production

Comparing Production Costs

Global Magnesium Demand

Factors Affecting Magnesium Prices

Future Applications for Magnesium Alloys

Summary

Background

In nineteenth century London, as described by Charles Dickens in ‘Our Mutual Friend’, waste was collected from the streets by an army of private contractors, and stacked into enormous ‘dust piles’, in which people searched for accidentally discarded money, jewellery and valuable materials. Occasionally, the piles yielded rich pickings, but more often than not, they were simply sold on as raw materials for road building and other processes. Skip forward a century and a half, and Australian minerals scientists are looking to update Dickens with an ambitious project that could yield both huge quantities of valuable magnesium plus other useful raw materials from ‘piles’ of waste fly ash, produced by coal-fired power stations.

Magnesium From Power Station Fly Ash

The Latrobe Magnesium Project could generate as much as 100,000 tonnes per annum of the metal and its alloys, if all goes according to plan over the next six years. When you consider that the current worldwide supply of magnesium is approximately 400,000 tonnes per annum, the potential impact of the project becomes obvious. But as well as providing a massive injection of material into a rapidly growing market, the Latrobe project will produce magnesium more cheaply and in a much more eco-efficient way than conventional processing plants, claims the team behind the scheme. For example, by re-mining waste fly ash material, the process will not produce any of the direct carbon dioxide emissions that are usually associated with extracting magnesium from its ore. There are a number of other environmental benefits promised by the project too. As CEO Chris Sylvester puts it, ‘This has been a green project from the outset.’

Raw Materials and Plant Location

The Latrobe project is to be sited in the eponymous valley in Victoria state, to the east of Melbourne, exploiting the fly ash from the power stations burning the region’s brown coal. In fact, it couldn’t be sited anywhere else. ‘The Latrobe coal is almost unique in having a high enough magnesium content to make it recoverable,’ explains Sylvester. ‘It is doubtful that there is another site in the world offering these advantages.’ And despite the coal seams having been exploited for more than 80 years already, producing the huge quantities of fly ash that will provide feedstock for the process for at least 20 years, there is still plenty more where it came from - Sylvester estimates that no more than 5% of the existing coal has been extracted. If the power companies continue to burn the coal, the ‘raw material’ for the Latrobe Magnesium Project will just keep on coming.

Advantages of the Location

Both the location and the chemical makeup of the raw material are important to the project’s environmental credentials. Not only will construction of the magnesium extraction plant take place on an existing brownfield site, but the process will to a certain extent aid the remediation of the land currently used for fly ash ponds. On top of that, the plant will be sited next door to the power stations producing the ‘raw material’, and so no transportation will be needed. ‘The plant is located within the grounds of the Hazelwood power station, owned by international Power plc of the UK,’ says Sylvester. ‘This provides us with all necessary services, including power directly from the power station, avoiding distribution and transmission losses and charges.’ Geography also lends a hand - the site is adjacent to a port, for exports to the rest of the world, while the state of Victoria is Australia’s leading location for the automotive industry, a key future market for magnesium alloys.

Advantages of Using Fly Ash As The Raw Material

The magnesium in the fly ash is in the form of its oxide, MgO, in contrast to magnesite, the mineral from which magnesium is usually extracted, which is magnesium carbonate, MgCO3. As a result, carbon dioxide emissions from the direct processing of magnesium in the Latrobe project will be minimal compared to those from standard extraction plants. ‘When MgO is dissolved in the leach plant, there is no carbon dioxide available to be released,’ explains Sylvester. ‘This compares with using a magnesite feed, which releases carbon dioxide when it is dissolved.’ At least 1.8 tonnes of CO2 is released for every tonne of magnesium metal produced using magnesite, he says. Meanwhile, thanks to manufacturing a by-product of precipitated calcium carbonate, the Latrobe project will actually consume carbon dioxide. ‘The calcium carbonate product is made by adding carbon dioxide from the power station off-gas to the calcium rich ash slurry, forming the precipitated compound, which is currently imported into Australia,’ says Sylvester.

The physical state of the fly ash is an important factor in the Latrobe project, too. The material is in the form of a very fine powder, with an average particle size of less than 40 microns. ‘This eliminates the costly requirements to mine it, crush it and grind it,’ says Sylvester. Costs are also saved because the material can be fed to the plant as a slurry, via a pipeline, rather than having to be transported by truck or rail. And once it gets to the plant, the material is easier to process thanks to the fine particle size. ‘This permits quicker leaching, resulting in a smaller leach plant with lower capital and operating costs compared with a standard magnesium plant using magnesium carbonate as feed,’ says Sylvester.

How Does the Process Compare with Existing Magnesium Smelting Processes

The feedstock material is clearly the most important difference for the Latrobe Magnesium Project compared to standard magnesium processing operations, a fact that is reinforced by looking at the other technological aspects of the Latrobe process. Most of the process is based on proven technology, something that is being highlighted as one of the advantages of the project. The material will go through the standard leaching, purification, electrolysis, alloying and casting stages, with the only novel step being the new Alcan dehydration process, which is being commercialised by the Latrobe team in partnership with Alcan.

The Alcan Dehydration Process

In the new process, magnesium chloride, which is produced by leaching the feedstock in hydrochloric acid, is extracted from the aqueous solution and transferred to an ammoniated methyl alcohol solution, which is water-free. The magnesium chloride is then precipitated as an ammonia salt, the ammonia is driven off by calcination and reused in the process, and the anhydrous magnesium chloride is sent for electrolysis. ‘This is a simpler process than the alternative Nalco dehydration process,’ says Sylvester, ‘using considerably less energy for lower capital and operating costs.’ However, he says, if any problems crop up in trying to introduce the new process, the Nalco process remains an alternative.

The next stage in the development of the Latrobe project will involve scaling up the process to prove that the new Alcan technology will work properly with large quantities of material. It has already proved a success at the laboratory scale, with a sample of the actual ash to be used in the project, but the pilot plant stage is necessary for the project team to be confident of using the technology in the full-scale plant.

Timelines for Magnesium Production

The piloting of the technology will form part of the Bankable Feasibility Study (BFS), which is the next stage in the development of the process, following a positive pre-feasibility study and consultant’s report from Worley Engineers, produced earlier this year. This report was very well received internationally, says Sylvester, and has paved the way for the BFS, which will take two years and will cost A$20 million. ‘The BFS will incorporate sufficient engineering that the capital and operating costs can be calculated to an accuracy acceptable to international banks for the purpose of financing the A$974 million plant,’ he says. Assuming the BFS goes according to plan, construction of the full-scale plant would begin in early 2005, and take another two years to complete. ‘The start up date for magnesium production is mid 2007,’ says Sylvester, ‘with full production achieved before the end of 2008.’

Comparing Production Costs

When the plant reaches its expected 100,000 tonnes per annum production capacity, both the volume of magnesium and its alloys being produced, as well as the price at which they are being produced, could have a significant effect on the global magnesium market. The report from Worley Engineers estimates that the cost of production for material from the Latrobe Magnesium Project will be A$1.55 per kilogram, or A$0.705 per pound, as these figures are normally quoted. According to Sylvester, this compares very well with costs of production at other plants, which include A$095 per pound from AMC and A$1.06 per pound from SAMAG, both based in Australia, plus A$097 per pound from the Noranda plant in Quebec, Canada. The production costs of Chinese producers are less well known, says Sylvester, but he believes that many have operating costs above those achievable with the Latrobe project.

Global Magnesium Demand

Of course, the Latrobe project is not the only one scheduled to increase the global supply of magnesium. Several major producers are bringing new projects online over the next few years, which Sylvester hopes will stimulate a long-term increase in usage and applications for the metal and its alloys. ‘Projections of future demand vary considerably but all are way in excess of current supply’ he says. “The MD of another Australian project spoke recently of a demand of five million tonnes. I anticipate a demand of more than one million tonnes by 2010.’ If correct, that figure would represent more than a doubling of the current world supply.

Factors Affecting Magnesium Prices

Sylvester believes this upsurge in demand has come about as a result of numerous small Chinese producers driving the costs of the materials down in recent years to much more affordable levels. ‘This has greatly stimulated the R&D efforts of the European users of the metal,’ he says. ‘In particular, the German Government, research institutions and automobile producers have been investing tens of millions of euros in researching alloys and products for lighter automobiles and transport vehicles in order to save weight, reduce emissions and improve recyclability.’

Future Applications for Magnesium Alloys

With an eye on future developments in the automotive industry and others, what materials will be produced by the Latrobe project? The site will do away with the currently common step of ingot re-melting for alloying, and instead concentrate on producing alloys directly to reflect the products in demand at the time. ‘We will be producing alloy directly on site, and producing directly useable alloy products such as billet, bar stock and sheets, as well as alloy ingot,’ says Sylvester, who foresees magnesium engine blocks and alloy sheeting for body panels as two future applications. ‘Increased usage will be found in truck, rail, ship, aviation and aerospace applications too,’ he thinks.

Summary

So, 10-20 years from now, magnesium alloys could be much more common materials in the new cars rolling off the production lines around the world. The buyers will certainly realise the benefits of reduced fuel consumption and reduced emissions thanks to the lighter weight of the vehicle, and also of the ease of recycling the vehicle at the end of its life. What they may not realise is that parts of their beloved vehicle started life in an enormous pile of fly ash in southern Australia.

 

Source: Materials World, Vol, 10, no. 12, pp. 10-12, December 2002.

 

For more information on this source please visit The Institute of Materials, Minerals and Mining.

 

Tell Us What You Think

Do you have a review, update or anything you would like to add to this article?

Leave your feedback
Submit