| Summary: | The recycling of a foundry ceramic waste from investment casting has been investigated. The waste was reduced in size by fly pressing and disc milling to d50 < 20 μm and cleaned by magnetic separation and acid leaching. The powder contained zircon, alumina and amorphous silica with 37, 38 and 24 wt. % (ZrSiO4: Al2O3: SiO2) respectively. Two products were targeted: zirconia toughened mullite (ZTM) ceramics produced with an addition of alumina and zircon based pigments developed by the removal of alumina and reaction with colourant ions. With an addition of 23.5 wt. % Al2O3, a ZTM containing 30 wt. % zirconia and 70 wt. % mullite exhibiting strength, hardness, thermal shock resistance and toughness commensurate with data reported in the literature were developed. Milling in isopropanol, dry pressing and sintering at 1600 ℃ for two hours optimised the properties. The transition to ZTM appeared to be through an intermediate glassy phase and limited by the dissociation of ZrSiO4. It was estimated that 70 % of the ZrO2 was transformable tetragonal without the addition of Y2O3. With Y2O3 non-transformable tetragonal ZrO2 was produced. It was shown that a clean zircon powder free of Al2O3 was generated by reaction with K2S2O7. Dissociation-synthesis and direct-synthesis routes were used to produce pigment. It was found that higher reaction temperature and the introduction of flux can significantly increase yellowness. The yellow produced from waste materials performed as well as those from commercial grade feeds.
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