Water extraction with CO2 bubbling as pretreatment of melting-furnace fly ash for metal recovery

In Japan, melting-furnace fly ash (MFA) generated from ash melting and gasification/melting plants is considered an “urban mine” due to its high metal content. This study aimed to develop a novel approach to pretreating MFA for metal recovery. Water extraction with CO₂ bubbling was investigated because MFA mainly consists of water-soluble salts containing elements such as Cl, Ca, Na, and K. Instead of acid addition, CO₂ bubbling was applied to maintain the optimal pH for minimizing the release of target metal elements and maximizing the removal of undesirable elements during water extraction. The results revealed that CO₂ bubbling effectively decreased the release of Pb, Zn, and Cd into the treatment water. This was mainly due to coprecipitation with CaCO₃, which was primarily formed by the reaction of Ca²⁺ from the MFA with CO₃ ²⁻ from the CO₂ gas. The bubbling process also helped accelerate the removal of Cl from MFA. Furthermore, the study showed that it is possible to lower the water-to-solid ratio to 5 with only a slight reduction in water extraction effect. Finally, approximately four times the concentration of target metals (rare metals and Cu, Pb, and Zn) was achieved by removing 90% of Cl, 70%–90% of Na and K, and 30%–40% of Ca through water extraction with CO₂ bubbling, resulting in a concentration of target metals that was nearly equal to that of ore.