AlF3 reactive Al2O3 foam filter for the removal of dissolved impurities from molten aluminum: Preliminary results

Filters coated with AlF3 can be used to filter molten aluminum to simultaneously remove nonmetallic inclusions and dissolved alkali and alkaline earth metal impurities. Coating experiments were carried out in which anhydrous HF gas was generated from reactions involving NaF or CaF2 and concentrated H2SO4, and used in a reaction with Al2O3 ceramic foam filter to produce a layer of AlF3 coating on the surface of the Al2O3 filter. Samples from these experiments were studied by X-ray diffraction, scanning electron microscopy and electron probe microanalysis. Preliminary results of the coating experiments showed that it is possible to coat Al2O3 filters with AlF3 by this method. Increasing the pressure of HF gas increased the yield of AlF3 in the filter. Theoretical evaluation of the removal efficiency of dissolve impurity elements showed that dissolved calcium can be removed up to 99.8% within 30 s of contact time between the filter material and the molten aluminum.