Immobilization and encapsulation during vitrification of incineration ashes in a coke bed furnace

A real-scale coke bed furnace system has been successfully applied to vitrify the incineration ashes into glassy slags. The object of this research was to evaluate the effect of the system on the immobilization of metal species in the slag. Ashes and slag specimens were tested to identify their metal phase distribution following a sequential extraction procedure. The mobility of Al, Ca, Cd, Cr, Mg, Mn and Pb was noticeably reduced by vitrification. An important implication is the reduction of Cr6+ to Cr3+ along with its immobilization with this coke bed furnace. The Ni and Zn contents were relatively low in slag, indicating that their availability of mobile phases in ashes was reduced during vitrification. The XRD analyses identified the major crystalline phase in slag as åkermanite, which is inert and helpful in metal immobilization. The phase distribution analysis also contributed to verify that the incorporation of Ca and Mg enhanced the encapsulation in the slag matrix. The low oxygen content in slag made the structure of silicate resistant to the proton-promoted attack and also enhanced the mobility reduction of matrix elements (Ca, Mg and Al) in the moderately reducible phase and also contributed to the immobilization of other metal species. However, the properties of slag also deserve further studies to ensure its long-term stability and safety.