Recycling of Si3n4 and Zro2-Y2o3 Ceramics by Hydrothermal Treatment

The possibility of recycling of silicon nitride (Si3N4) ceramics with sintering aids, Mg, Sr and Ce oxides, and yttria partially stabilized zirconia (YSZ) ceramics was investigated by hydrothermal treatment in order to reduce the emission of energy and pollutants in their recycling processes. Si3N4 ceramics was disintegrated into the powdery particles by hydrothermal treatment in HF aqueous solution at 100-150degreesC for 0-120h and subsequent sonication treatment. However, insoluble fluorides such as CeF3 and MgF2 were remained in the recovered Si3N4 powder as residues. As a result of rinsing the recovered powder with HNO3 solution, these fluorides were completely removed and only needle-like particles arising from beta-Si3N4 were recovered.YSZ ceramics were treated in pure water at 150-350degreesC for 24h under hydrothermal conditions. Low-temperature transformation from tetragonal to monoclinic crystal system occurred with the results of YSZ being disintegrated into fragments or powdery particles. Dissolution of yttria used as stabilizer into water was negligible. Quantitative analysis of monoclinic; volume fraction (V-m) by XRD revealed that V. was about 80% in the whole temperature range investigated. The degree of disintegration of YSZ was found to increase with an increase in treatment temperature. It was concluded that the recovery of new structural ceramics as re-sinterable ceramic powder would be possible by hydrothermal treatment technique.