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Metal Casting Technologies : September 2006
65 FURNACES A variety of furnace types are used for melting aluminium prior to casting. Some have externally heated crucibles to contain the melt - known generically as 'crucible furnaces'. Others employ refractory lined chambers heated from above by burners or electric elements, these are known as 'reverberatory furnaces'. Electric induction furnaces are not widely used in aluminium foundries; those used are mainly 'bulk' melting for on-site production of specification alloys using internal scrap and bought-in scrap as feedstock. Furnace selection depends upon many operational considerations such as output, number and type of alloys needed in a foundry, casting process, availability of fuels and their cost, space available, environmental constraints, capital cost, maintenance requirements, and ease of operation. However, furnace efficiency in terms of both energy and metal yield (net output after oxidation losses) is often of secondary importance in the selection process to most of the criteria listed. METAL LOSS Improving melt quality is of paramount importance for better casting quality and reducing scrap and the associated waste of material and energy. Figure 1 shows typical material flows during casting production. The specific material wastes from the system are trimmings from as-cast components, (sprues, runners and risers), and metal oxides in the form of dross and slag. Dross skimmed from the surface of molten aluminium prior to pouring varies in its composition; the amount of metal entrapped in dross may range upwards from 30% by weight. The metallic content of the dross depends upon several factors including the alloy composition of the charge, the melting procedure followed, and the care with which the dross is removed. The dross can often represent a substantial Figure 1. Flow diagram for the production of unmachined castings WHO'S WHO OF METALS -- ANNUAL 2006/7