Response surface study on continuous supercritical hydrothermal synthesis of nano-zirconia: Scale-up from laboratory to industrialization

Commercial interest in nanomaterials and their applications lead to the development of many new methods for mass production. Among them, supercritical hydrothermal synthesis (SCHS) is a highly efficient and environmental -friendly technology. In this work, an advanced industrialized device for SCHS was used to verify the synthesis of nano-zirconia particles. The influencing laws and mechanisms of different material parameters (precursor type, precursor concentration, and pH) and process parameters (reaction temperature, reaction pressure, residence time, and Reynolds number) of nano-zirconia synthesis were studied. For the first time, the state changes of zirconium precursors and the precipitation behavior of nano-zirconia particles were in -situ observed by a high-speed camera. Central composite design (CCD) of response surface methodology (RSM) was adopted to investigate interactive effects of reaction temperature (200 - 420 degree celsius), reactant concentration (0.01 - 0.6 mol L-1), alkali ratio (0 - 5) and flow rate (3 - 15 mL min (-1)), with particle size (APS), crystal form ratio (k) and reactant conversion rate ( alpha) serving as response values. The influence of key process parameters follows an order of reaction concentration > reaction temperature > system flow > alkali ratio for APS, alkali ratio > system flow > reaction temperature > reaction concentration for k, and reaction temperature > alkali ratio > system flow > reaction concentration for alpha, respectively. The optimal process parameters were then obtained and further verified at lab and industrial scale. Zirconia powders with particle sizes of 4.98 nm and 8.74 nm were synthesized, respectively, indicating five hundred times magnification of continuous SCHS of nano zirconia from laboratory to industrialization.