Major leaching processes of combustion residues : characterisation, modelling and experimental investigation

Characterising leaching behaviour provides ample evidence toidentify the majorleaching processes of combustion residues.Neutralisation and chemical weatheringgovern the leachingreactions and control the release of major, minor and traceelementsfrom combustion residues, and are thus considered to bethe major leaching processes.Several geochemical models are used to describe theneutralisation and chemicalweathering processes based onleaching kinetics and the features of leaching systems.Areaction path model is used to evaluate the neutralisingprocesses in a batch system. Theneutralising processes in aflow-through system are assessed using an equilibriumreactivetransport model which accounts for most neutralising reactionsunder the experimental conditions. A kineticreactive transportmodel taking full kineticconsideration for heterogeneousreactions is used to simulate long-term chemicalweathering.According to experimental investigations and geochemicalsimulations, the leachingkinetics of buffering materials arekey issues for the understanding of the neutralisingprocesses.The acid neutralising capacity (ANC) at different pH levelsdepends mainly onthe mineralogy of the combustion residues. Thetime-dependent and pH-dependent neutralising behaviours aredetermined by the matrix phases of a solid waste. Incombustionresidues, the dissolution of glass phases is expected to playan important rolein a long-term neutralising process. Theneutralising process in a flow system issignificantly differentfrom that in a batch system. In general, the informationobtainedfrom batch experiments cannot directly to be used in aflow system. The neutralising ability of a combustion residuemay be strongly affected by solute transport and carbonationreactions in a natural leaching environment.The chemical weathering mainly involves the matrix ofcombustion residues consistingmostly of glass phases. Thedissolution kinetics of waste glass and other possibleprocesses involved in the chemical weathering have beeninvestigated and incorporatedinto a kinetic reactive transportmodel. Most important processes in the chemical weathering canbe simulated simultaneously using this model. The results showthat thereis a complicated relationship between the factorscontrolling the long-term chemicalweathering. The dissolutionof the waste matrix is strongly affected by itsdissolutionkinetics and weathering environment. Theenvironmental impact of the glass dissolutioncannot beneglected. Although the glass dissolution provides considerablebufferingcapacity in long-term weathering, the carbonate isusually a dominant buffering mineralin actual weatheringprocesses. The transformation of carbonate should be consideredasan important process in the chemical weathering. Theformation of secondary minerals,clay-like minerals (e.g.illite) and amorphous silica, may considerably alter themineralogy of the waste, and thus change the leachingbehaviours of the combustion residue duringlong-term chemicalweathering.Keywords:Leaching; neutralisation; chemical weathering;solid waste; combustionresidues; long term; geochemicalmodelling; reaction path model; reactive transportmodel