Effect of CO₂-water washing on Na/Ca removal performance of Zhundong high-alkali coal

In China, coal is still a dominant energy resource, and the Zhundong coalfield, as the largest coalfield, is of great potential for exploitation. However, the high content of alkali and alkaline earth metals(AAEMs) such as Na and Ca in the Zhundong coal has led to some serious slag staining problems in industrial production. Therefore, to investigate the specific effect of CO₂-water washing treatment on the dealkalization of the Zhundong high-alkali coal and the distribution characteristics of anions such as Cl^-/NO^-₃ in the solution during the washing process, the water washing/CO₂-water washing experiments of the Zhundong coal were carried out. By varying different washing conditions such as washing temperature of 20,40,60 ℃ and washing time of 2,5,8 h, a comparative analysis of the effect of Na/Ca and Cl^-/NO^-₃ ions removal from the Zhundong coal by water washing/CO₂-water washing was carried out. And the feasibility of Na/Ca removal from water washing under other gas atmospheres was also explored. The experimental results showed that with the increase of washing time, washing temperature and liquid-solid ratio, the removal efficiencies of Na and Ca increased, while the concentrations of Cl^- and NO^-₃ ions in the washing filtrate also increased. The Na and Ca removal efficiencies could reach a high level when the washing time was 5 h, the washing temperature was at 60 ℃,the liquid-solid ratio was 50∶1,and the CO₂ flow rate was 100 mL/min, with a removal efficiency of 73.8% for Na and 15.0% for Ca. The removal efficiencies of Na/Ca are basically positively correlated with the CO₂ flow rate and the volume ratio of CO₂ in the CO₂/O₂ mixture. The flow rate of CO₂ also had a certain inhibitory and promotional effect on the leaching of Cl^-/NO^-₃ ions, where the concentrations of Cl^-/NO^-₃ ions fluctuated with the CO₂ flow rate. Apart from the physical bubbling effect, there might be other chemical reaction mechanisms. Water washing on the Zhundong coal under other gas atmospheres, such as N₂,also resulted in some enhancement of the Na/Ca removal. However, the dealkalization effect of the Zhundong coal by N₂-water washing is worse than that obtained by CO₂-water washing. Therefore, the application of CO₂ to water washing the Zhundong coal for dealkalization is feasible and promising for development and application, which is expected to be promoted on a certain scale in industry through some reasonable modifications of industrial processes.