TNO (the Netherlands Organisation for Applied Scientific Research) and Corus RD&T are working on the modification of the TNO-Thijssen process for fractional crystallisation so that it can be applied to the purification of molten aluminium scrap. This process comprises a suspension crystalliser, which produces relatively pure aluminium crystals suspended in molten metal, and a hydraulic wash column, which subsequently separates these crystals from the relatively impure liquid. The thermodynamic background of the crystalliser was studied to establish the applicability of the process. To that end, the thermochemical programs ThermoCalc, and ChemSage were used to calculate the solidification paths for the alloy systems studied. DSC experiments were performed to check their results. The upshot of the calculations and the measurements is that fractional crystallisation can purify these alloy systems. Whether this process will meet the requirements for the purification of a specific scrap type depends on the behaviour of alloying elements or impurities present. Other important factors are the requirements for the product, the residue, which is in fact a by-product, and the desired yield of the process.
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