| dc.description.abstract | Filtration of liquid aluminium is widely used in the industry for the removal of inclusions, and Ceramic Foam Filters (CFFs) are often the filtration media of choice. Following an incident with generation of PH3 (g) (phosphine gas) after cutting used alumina based (Al2O3-based) CFFs, the need for an improved understanding for the thermal stability and chemical reactivity was realised. This was especially in regards to the stability of the phosphate-based binders used in ceramic refractories as limited information is available in the literature.
In the present work, three CFF substrates, i.e. Substrate 1-3, with varying AlPO4 (aluminium phosphate) content were investigated under controlled conditions. Samples of the substrates, as produced and in contact with 5N (99.999 % pure) aluminium and the aluminium alloys 3103, 5182 and 6060, were heat-treated in a vacuum induction furnace, and in a Differential Scanning Calorimetric (DSC) unit equipped with a Thermogravimetric Analyser (TG), at 850 °C in an inert atmosphere with a holding time of 2 hours.
Chemical changes were identified for all substrates when heated in contact with metal due to exothermic reactions, resulting in a colour change from white to orange/brown at the interface between the substrates and the metal. The decreasing AlPO4 content in the CFF substrates was concluded to be inverse proportional with the thickness of the coloured boundary layer, independent of chemical composition of the aluminium in contact with the substrates. Light Optic Microscopy (LOM) was used to identify the coloured boundary layers.
When Substrate 1 and 2 were heated in contact with the aluminium alloys 5182 and 6060, it was confirmed by Electron Probe Micro Analyses (EPMA) and DSC-TG that magnesium phosphide (Mg3P2) was generated. In the case of Substrate 3, Mg3P2 was generated in contact with the aluminium alloy 5182. Furthermore, when Mg3P2 came in contact with moisture/water, PH3 (g) was generated at concentration levels varying from 0.1 ppm to over 10 ppm PH3 (g) (detected by Dräger tubes). Based on the present results, the thermal and chemical stability of Substrate 1-3 can be questioned. | en |
