Thermal treatment of incinerator fly ash: Factors influencing the evaporation of ZnCl2

The efficiency of separation of Zn compounds from incinerator fly ash by evaporation at temperatures up to 1000°C depends on a number of factors and competing reactions. Model experiments with ZnCl2 and a number of oxide matrices of defined composition (silica, alumina, a silica-alumina-calcia glass, and heat treated fly ash) in moist air were carried out in order to identify the chemical reactions involved and to investigate the relative importance of temperature, ZnCl2 concentration, surface area and composition of the matrix oxide. Thermo-gravimetric analysis (TGA) and X-ray diffraction (XRD) were used as experimental techniques. The hydrolysis and subsequent immobilisation of ZnO in the matrix oxide dominates the process at low temperatures and low ZnCl2 concentrations. At high ZnCl2 concentrations evaporation becomes more dominant. The Zn evaporation is governed by the vapour pressure of ZnCl2 and limited by mass transfer to the purging gas stream. The development of new technologies for improved separation of Zn (and other heavy metals) from fly ash should focus on improving the mass transfer from particle grain to gas phase and avoid long residence times at temperatures below 600°C.