Contamination by silicon

However, a smoother reaction which gives a purer product, less contaminated by silicon tetrafluoride, is the following ... 

In fireclay and other alumina silica refractories, the interaction with Al proceeds from the first hours of the service. The practical proof for this is the increased silicon content in Al during the first 2-3 days [2, 3] if the metallurgical service is lined with fireclay or alumina silica refractories. After that, a layer of so-called corundum - a mixture of partly oxidized aluminium and partly reduced silicon -covers the lining and prevents Al melt from contamination by silicon (but doesn t stop the infiltration of Al inside the refractory). 

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