Recovery of heavy metals in electrostatic precipitator ash by carbon dioxide as a pre-extraction reagent

This study investigated the efficiency of heavy metal separation and recovery in electrostatic precipitator (ESP) ash from a municipal solid waste incinerator (MSWI) when carbon dioxide is used as the pre-extraction agent. The experiments were conducted at 120°C with pH values of 3-6 and liquid-to-solid (L/S) ratios varying from 10:1 to 40:1. The results indicate that an increase in the L/S ratio enhances the separation and recovery efficiency of heavy metals in the MSWI ash. The separation efficiency of Cd, Cr, and Zn increased significantly with the increased L/S ratio. The pH level was the prominent factor affecting the extraction of heavy metals from ESP ash: at pH 3-4, maximum amounts of Cd and Cr were extracted. However, due to the increased release of ions with increased CO₃^2-, Zn was relatively soluble at high pH values. In cases with an L/S ratio of 40:1 and pH 3, the Cd, Cr, and Cu extraction amounts were 86.4%, 71.2%, and 34.9%, respectively. Pb and its compounds remained in a stable state under experimental conditions. The above results point toward the feasibility of heavy metal recovery and fly ash reutilization when carbon dioxide is used as the pre-extraction agent. Both greenhouse gas (CO₂) emissions and acid extraction operating costs are reduced with this process.