Removal of impurities from waste dental zirconia ceramic powder using an alkali dissolution method

Background　Dental zirconia ceramics are widely used in the field of oral restoration. During the CNC machining stage of dental zirconia restorations, over 80% of the volume of the zirconia pre-sintered blocks is removed as waste powder, resulting in significant resource waste. Therefore, the recycling of waste zirconia powder is imperative. Objective　Impurity removal is a crucial step in the recycling process of waste zirconia powder, yet there is limited research reported. This paper analyzes the types of impurities in waste zirconia powder and proposes a feasible method for impurity removal.Methods　The waste zirconia powder used was ST zirconia produced by Upcera, collected from Beijing Yingguan Technology Co., Ltd. The CNC equipment utilized diamond-coated tungsten carbide tools. The microscopic morphology and aggregation state of the waste zirconia powder were observed using scanning electron microscopy (SEM), and its chemical composition was characterized by energy-dispersive Xray spectroscopy (EDS). Thermogravimetric analysis (TG-DSC) was employed to preliminarily determine the oxidation temperatures of tungsten carbide and diamond. The oxidation process of tungsten carbide was quantitatively analyzed at different temperatures, and X-ray, diffraction (XRD) was used for phase analysis of tungsten carbide treated at various temperatures, and the oxidation protocol was established based on the phase composition after oxidation. The dissolution test of tungsten oxide was conducted in a hydrothermal reactor. Tungsten carbide powder after high-temperature treatment at 700°C was reacted with ammonia solution at different temperatures and durations to form soluble ammonium tungstate. The removal rate of tungsten oxide was measured through suction filtration, washing, and drying to identify the optimal process conditions. The waste zirconia powder was treated at 700°C for 2 hours, then reacted with 0.3N ammonia solution in a hydrothermal reactor at 120°C for 6 hours. The impurityremoved powder was obtained via suction filtration, washing, and drying. The elemental compositions of the purified powder and the original powder were analyzed using X-ray fluorescence (XRF) to evaluate the impurity removal effect. Results　SEM observation, EDS elemental mapping of selected areas, and XRF analysis confirmed that, in addition to the inherent elements of the original zirconia blanks (Zr, Y, Hf), the waste zirconia powder contained W, attributed to wear from the tungsten carbide tools used in CNC machining. To remove the tungsten carbide and diamond impurities, a combined process of heat treatment and alkaline leaching was adopted. TG-DSC curves indicated that diamond oxidizes prior to tungsten carbide at high temperatures, and the complete oxidation temperature of diamond is lower than that of tungsten carbide. Therefore, adopting the oxidation protocol for ,tungsten carbide ensures complete oxidation of diamond. Quantitative analysis and phase testing of the tungsten carbide oxidation process demonstrated that holding at 700°C for 2 hours is a reasonable high-temperature oxidation treatment. The tungsten oxide generated after oxidation was removed by hydrothermal alkali dissolution. The removal rate of tungsten oxide increased with higher reaction temperature. At a reaction time of 8 hours, a 100% removal rate was achieved at 120℃. To reduce reaction time for energy savings and lower processing costs, the hydrothermal reaction time was optimized. At 120℃, a 100% removal rate was attained after 6 hours of reaction. Thus, the optimal alkali dissolution conditions for tungsten oxide were determined to be 120℃ for 6 hours.After treating the waste zirconia powder at 700°C for 2 hours followed by reaction with 0.3N ammonia solution at 120°C for 6 hours in a reactor, the purified zirconia powder showed complete removal of tungsten carbide, while the content of intrinsic components in the zirconia powder remained unchanged. Conclusion　For the waste powder generated during the manufacturing process of dental zirconia restorations, this paper proposes a combined method of heat treatment and alkaline leaching for impurity removal. The results indicate that the suitable heat treatment condition is holding at 700° C for 2 hours, and the appropriate alkaline leaching condition is reacting in a reactor at 120°C for 6 hours. This method can effectively remove impurities from waste zirconia powder, providing a reference for the recycling of waste zirconia powder.