The possibility of recycling silicon nitride ceramic by hydrothermal treatment was investigated. A Si
3N
4 sintered compact with 0.9 mass% SrCO
3, 3.6 mass% MgO and 4.5 mass% CeO
2 as sintering aids was used as a specimen for treatment. Hydrothermal treatment was performed using aqueous HF solution at 70–150°C for 0–120 h. The dense sintered compact was easily disintegrated into powdery particles and a core fragment by the combination of hydrothermal treatment and subsequent sonication treatment. The treated powder was identified by X-ray diffraction (XRD) analysis as β-Si
3N
4 containing CeF
3 and MgF
2, which were formed in aqueous HF solution during hydrothermal treatment. These fluorides were completely removed by rinsing in HNO
3 solution. Then, a powder consisting only of needle like β-Si
3N
4 crystals was recovered by a series of these treatments. X-ray photoelectron spectroscopy (XPS) revealed that the surface of the recovered Si
3N
4 powder was oxidized very slightly. Hydrothermal treatment at 120°C for 24 h gave approximately 30% yield of powdery β-Si
3N
4 particles, excluding the amount of sintering aid components. In the case of hydrothermal treatment for shorter terms or at lower temperatures, the corrosion behavior of the Si
3N
4 ceramic is linearly dependent on treatment time and is controlled by surface chemical reaction. The activation energies for the dissolution of the Si
3N
4 ceramic, and the grain boundary components of strontium, magnesium and cerium were estimated as 31.9±0.7, 54.4±3.6, 74.3±1.4 and 51.7±3.9 kJ/mol, respectively.
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