Design and scale-up of a heavy metals recovery process from municipal solid waste incinerator residues.

摘要

The high level of leachable salts and toxic metals characteristic of municipal solid waste incinerator residues may necessitate pretreatment of the residues prior to disposal or utilization for other applications. Bottom ash, fly ash or fly ash and scrubber residue, and combined ash was obtained from three facilities and characterized for chemical composition and buffering capacities. Equilibrium extractions utilizing an acidic, inorganic salt solution investigated the leaching dependency on pH and provided insight to the structure of the ash. Kinetics studies were carried out at the optimum equilibrium pH and the unsteady state behavior of the lead extraction process observed. Sequential batch extractions were carried out to increase metallic and salt species concentrations by adding ash to recycled extract and to observe the solubility limitations and interaction of these species upon full recycle of an extract stream.; A model was derived for the lead extraction dependency on equilibrium pH for three fly ash residuals. The extraction patterns were compared to the solubilization and chloride complexation of several complex lead minerals. The empirical rate of neutralization was also determined. Evaluation of the rate of lead extraction indicated mass transfer limitations for two of the residuals, while a third residual exhibited an irreversible, surface oriented reaction.; Bottom and combined ash studies were carried out to evaluate separation and subsequent treatment processes. Another alternative process, soluble salts recovery from fly ash or fly ash and scrubber residue, also was investigated.; A continuous laboratory scale pilot plant was designed and operated to treat ashes at a rate of 1 kilogram/hour. The pilot plant was operated in a single pass mode and the residence time distribution of the reactor was evaluated. A secondary investigation observed the effect of recycle streams on the extraction efficiency.; Three conceptual processes were designed for heavy metals recovery from combustion residuals, separation and subsequent treatment of bottom or combined ash, and the soluble salts recovery from fly ash or fly ash scrubber residuals at the pilot scale of 20 tons per day. The primary economic analysis of all three processes are presented.