www.ireng.org the loss of the cast product when it is machined.


The latest degassing rotor technology has been developed in silicon carbide, delivering a high-performance and cost-effective alternative to the graphite material traditionally used for this task. Graphite has previously been the most widely used material for degassing rotors but is subject to high replacement costs and frequent changeovers. Silicon carbide boasts superior wear resistance and anti-oxidation qualities when compared with graphite, meaning the new rotors can last several times as long as their graphite counterparts - one test revealed a usable life of more than 800 cycles in a heavy fluxing application, compared with an average of 300 for comparable graphite products - and are made from an isostatically pressed, single-piece design. The rotor head has been optimised to reduce bubble size and deliver optimum gas dispersal through an innovative six-vane design. In testing, the new rotors have shown significantly lower oxidation levels compared with graphite products, whose degassing performance deteriorated as head geometry became distorted, while melt densities using the silicon carbide rotors were notably higher over time than with graphite products.


Degassing technology is also widely adopted in the primary aluminium sector, with the use of compact in-line degassing rotors to process molten aluminium via rotating nozzles directly in the casting trough between the furnace and the casting pit. These products are contributing to improvements in overall metal quality, productivity, and safety, as well as reducing operation and maintenance costs by up to 60%. In particular, the need for high cost heating elements and thermocouples is removed, while there is no need to remelt aluminium or to maintain molten aluminium between casts in the degassing chamber.


Coating technology


Another potential source of contamination in aluminium casting is the crucible in which the aluminium is melted. The high operating temperatures can cause fragments from crucibles, especially older products which have


Technical Paper


already seen lengthy service, to break off or melt into the molten aluminium, impacting significantly on purity and therefore on casting quality down the line – which may not be discovered until it is too late. The composition of the crucible itself can also be a cause of pollution. Where crucibles are ‘run to failure’ or changed at timed intervals rather than on the basis of actual wear, these effects can be significant and highly deleterious.


To combat these issues, a variety of specialist coatings have been developed for all types of crucibles with different performance attributes depending on usage temperatures and desired performance. Coatings made from Al2O3, for example, play a key role in reducing dross adhesion and limiting metal contamination at temperatures of up to 1,600°C (2912°F). Other Al2O3 formulations deliver the same performance in very high purity applications. Where alloys using many fluxes are being processed, special glaze formulations can be applied to reduce flux attack on the crucible material.


These coating types are all well-established but are now being joined by a new technology which pushes performance boundaries even further. Boron nitride coatings can contribute towards superior dross adhesion reduction and limit contamination in very high purity applications (e.g. 5N and 6N Al) and can withstand temperatures of up to 1,000°C (1832°F).


The global aluminium market is set to remain buoyant for the next few years at least due to its versatility, the variety of new applications, especially high-purity ones, and the high costs of many alternatives. Most regional markets are committed to growth and are seeking to work with consumable partners able to deliver solutions which can help them marry productivity and quality with reduced energy usage and emissions. The harnessing of innovative materials technology and design, and the expansion of existing technologies, will continue to create new opportunities for those suppliers able also to deliver agile and responsive service.


For further information visit www.morganadvancedmaterials.com.


May 2016 Issue


ENGINEER THE REFRACTORIES


27


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