Accelerated co-precipitation of lead, zinc and copper by carbon dioxide bubbling in alkaline municipal solid waste incinerator (MSWI) fly ash wash water

Abstract

MSWI fly ash is a potential substitute for some virgin materials, but the soluble salts and hazardous trace elements in the ash can limit this potential. This study investigated the use of a water-based washing process to remove the soluble salts from MSWI fly ash. The removal of trace elements by bubbling CO₂ through the resulting wastewater was also evaluated and compared to the use of a Na₂CO₃ solution. Washing was accomplished at liquid-to-solid ratios (L/S) (L kg⁻¹) ranging from 3 to 20, and at durations from 5 min to 1 h. The optimum washing condition was identified by an orthogonal test and an L/S ratio of 10 for 10 min. The extraction of chlorides by washing ranged from 62% to 95%, while the extraction of sulfate was less than 50% because the solubility of these salts was strongly influenced by the L/S ratio. Critical trace elements (lead, zinc and copper) were also leached in high concentrations (63.7 mg L⁻¹, 4.53 mg L⁻¹ and 0.40 mg L⁻¹, respectively) at the optimum washing condition. These elements were effectively removed in the CaCO₃ or ferrum/aluminum-hydroxides that precipitated when CO₂ was bubbled into the wastewater. Various analyses showed that the precipitate was primarily CaCO₃ which formed into spheres. The concentration of trace elements incorporated into the precipitate varied across the radius of the sphere. A geochemical model was used to help explain the mechanism of trace element precipitation. The accelerated carbonation of the alkaline MSWI fly ash wash water was effective in removing trace elements (Pb, Zn and Cu).