Purification of metals through filtration and electromagnetic separation

The applicability of a high frequency electromagnetic field to the removal of nonmetallic inclusions from silicon and aluminum, and the mechanism of depth mode filtration during aluminum purification were investigated. Electromagnetic separation experiments at frequencies of 63 - 120 kHz and aluminum filtration experiments using both conventional Al₂O₃ filters and AlF₃ coated Al₂O₃ filters were carried out by flowing molten aluminum through, and partly solidified in the filter bed followed by analysis of the metal and filter material. Materials were characterized with an optical microscope and macroscope, scanning electron microscope (SEM/EDX), x-ray diffraction (XRD), electron probe microanalysis (EPMA) and glow discharge mass spectrometry (GDMS). 3D FLUENT CFD simulation was made in support of the filtration experiments. In the high frequency electromagnetic field work, the induced fluid flow significantly enhanced particle segregation either at the wall or at the bottom or deposition close to the top. The electromagnetic particle separation efficiency was significantly improved by higher coil current and longer separation time. Higher frequency also improved particle separation efficiency but was less significant than current or separation time. Filtration results showed that depth filtration of aluminum involves the contribution of three important mechanisms which are (1) collision and interception effect which involves particles transport from the melt and attachment to filter wall, (2) effect of inclusion bridges and (3) interfacial energy between collided inclusions --Abstract, page iv