Study on the kinetic characteristics and energy efficiency optimization of microwave drying for ammonia-zinc leaching residues

Abstract

Abstract

Zinc ammonia leaching residues are categorized as secondary recyclable materials containing substantial quantities of ZnAl2O4, ZnCl2, and Al2O3. However, they present significant environmental and operational hazards when introduced into downstream processing stages under moisture-laden conditions. For instance, the high-temperature vaporization of residual moisture can increase system pressure, potentially resulting in operational hazards. Microwave-selective heating of polar water molecules is utilized to enhance internal dehydration within the leaching residue. The effects of initial mass (10–30 g), moisture content (5 %-15 %), and microwave power (160–480 W) on drying kinetics were systematically investigated. The experimental results indicate that the peak drying rate of 0.00867 g/s was achieved under conditions of 20 g initial mass, 7.5 % moisture content, and 400 W microwave power, representing a substantial improvement in drying efficiency compared to conventional methods. The Page model (R2>0.9) accurately characterizes the microwave drying process, as confirmed by its fitting performance within the thin-layer drying framework. Analysis of the diffusion coefficient reveals distinct stage-specific behaviors, beginning with rapid evaporation of free water, followed by a slower phase dominated by bound water removal. The calculated activation energy (-23.1 g/W) confirms that microwave irradiation effectively lowers the energy barrier for moisture migration. Fourier-transform infrared spectroscopy further indicates that the chemical structure of the leaching residue remains unchanged during microwave drying, while the reduced intensity of water-associated peaks reflects efficient free water removal. Energy efficiency analysis reveals that increasing the initial mass and moisture content under constant microwave power enhances microwave utilization efficiency and reduces energy consumption. These findings provide a theoretical basis for optimizing microwave drying processes in the treatment of metallurgical solid waste and play a vital role in promoting the development of green metallurgical technologies.