Drying kinetics and energy efficiency of low-grade zinc oxide powders under microwave heating

• Kinetic and efficiency of zinc oxide powders under microwave drying were studied. • Modified Page model can accurately describe the microwave drying process. • Microwave heating substantially reduces the dehydration energy barrier. • Theoretical foundation and actionable guidelines for drying were proposed. Low-grade zinc oxide powder produced by calcination in an industrial rotary kiln mainly contains zinc oxide, zinc hypoxide, and recoverable trace metals. Excessive moisture can cause powder to agglomerate, undergo property changes, or even lead to explosions during recovery. Therefore, microwave drying is required to maintain its physical and chemical properties and eliminate potential safety hazards in subsequent recycling processes. Existing studies mostly focus on single-component materials, without considering the impact of trace metals on moisture distribution and drying efficiency This study focuses on the characteristic of containing trace valuable metals and investigates the effects of sample weight, water content, and microwave power. Under the optimal experimental conditions, the Page, Wang and Singh, Quadratic, and Simplified Fick's diffusion models are used to fit the nonlinear curves of moisture ratio changes. FT-IR spectra show that the peak value of the -OH absorption peak of the dried sample is significantly reduced. Calculations using Fick's second law indicate that the diffusion coefficients exhibit regular changes. The microwave drying activation energy of the powder is measured to be -4.4209 W/g. This study provides targeted references for dehydration treatment of metallurgical waste containing trace valuable metals and promotes application of microwave drying in complex systems. Schematic diagram of the microwave drying mechanism for the low-grade zinc oxide powder.