Research on microwave hydrothermal alkaline leaching defluorination of spent cathode carbon from aluminum electrolysis

Abstract

Soluble fluoride present in spent cathode carbon (SCC) is a serious concern for both the environment and human health. To address this issue, the microwave hydrothermal alkaline leaching (MHAL) was used to removed the soluble fluoride from SCC. The structural characteristics and elemental distribution of SCC before and after MHAL treatment were analyzed by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). The effect of the primary parameters, i.e., temperature, time, and alkali concentration in the MHAL process, on the soluble fluoride removal rate was assessed using the Box-Behnken design (BBD) in response surface methodology (RSM), and subsequently optimized. The optimization results illustrate that the soluble fluoride removal rate of SCC could reach up to 98.06% under the optimum reaction conditions of 80 °C, 1.16 mol/L alkali concentration, and 8.4 min of reaction time. Under this condition, the experimental data showed that the average soluble fluoride removal rate was 97.11% and were in good agreement with the model predicted values. Furthermore, the MHAL process reduced the treatment process time by 99.4% and increased the soluble fluoride removal rate by 5.8% compared to the traditional hydrothermal treatment process. The MHAL process ensures rapid and efficient hazardous treatment of SCC while minimizing environmental pollution.