Long-term leaching mechanisms of ashes and slags; combining laboratory experiments with computer simulations

The leaching properties of slags and ashes have been studied on four waste materials: municipal solid waste incineration, MSWI, bottom ash, wood ash, electric arc furnace slag and blast furnace slag. The aim of the study has been to investigate properties that influence the long-term leaching of these materials. The results of the laboratory experiments have been used in computer simulations using CHEMFRONTS. CHEMFRONTS is a coupled geochemical and transport computer program developed by the author within this work. The main variable influencing the leaching of metal from solid waste is the final pH of the solution. This can be controlled by several factors, but if the contact time between the water and the waste material is sufficient, the waste itself often controls the pH by buffering reactions. The buffering capacity of a material is, therefore, one of the most important factors to investigate when the long-term leaching behaviour is to be predicted. A pH titration curve has been made for each material within this work, using a pH batch experimental series. The change in pH with time has also been studied. The materials have been studied by X-ray diffraction analysis and Scanning Electron Microscopy / Energy Dispersive Spectroscopy to assess the mineralogy and morphology of the materials. The mineralogy is important for computer simulations. A number of leaching experiments have been performed. A batch flow-through method has been developed which is a serial batch experiment, in which the contact time can be controlled. This technique makes it possible to have oxygen access in all the columns, to simulate partly saturated conditions. Sampling is also possible in the columns. The influence of particle size has been studied, and surprisingly no influence of the particle size could be noted after 24 hours of leaching. The substitution of sand in concrete with steel slag has been studied by both physical and chemical tests. The steel slag concrete had physical properties similar to those of a medium strength concrete, but had a higher density. The leaching properties of the steel slag concrete and the reference concrete were the same except for chromium, where the leaching from the steel slag concrete was much higher. The release rate was, however, very low, so there should be no environmental problems associated with the use of this concrete. Several computer simulations have been performed using CHEMFRONTS. The input data are based on experimental results. Serial batch experiments have been simulated. The simulation conditions have also been changed to simulate different conditions with the same material 62 refs, 30 figs, 10 tabs