Assessment of the multi-scale leaching behaviour of compacted coal fly ash.
Peer experimental-modelling tools were developed and applied in the case of coal fly ashes with the aim to assess the leaching behaviour of ash compacted layers in a use scenario. Laboratory-scale (dissolution kinetics, ANC test, column percolation) and field pilot experimental studies (release monitoring during 18 month, hydrodynamic study, ANC on 44 month leached waste) were performed in order to identify and quantify the main transport phenomena and chemical processes. A quantitative geochemical model was developed taking into account equilibrium chemical reactions as well as kinetic processes for silicate phases like albite, K-feldspar and Ca-olivine. Phases like BaHAsO(4) and a solid solution Ba(x)Sr(1-x)(SO(4))(y)(CrO(4))(1-y) were proposed to explain the complex leaching behaviour of As, Cr, Ba, S; the soluble CaMoO(4) seems to control the Mo concentration. At neutral and acid pH, the model of surface complexation on ferric hydroxides was added for describing the behaviour of As, Cr, and Mo. At each scale the dynamic processes were identified and quantified by modelling. During the first contact with water an equilibration time of about 10 days was identified and then considered in all other laboratory experiments (ANC, column percolation). The hydrodynamic properties of compacted fly ashes were identified: a high water retention capacity (97% of the pores are still filled after draining under normal pressure), a flow regime close to plug type, a low fraction of stagnant zones (<0.03%). The scenario factors like carbonation and rainfall play an important role on the leaching behaviour at field scale. The carbonation diminishes the leachate pH from 11 to 8.5. The alternation of rain periods determines an apparent batch behaviour which slows down the outflow of the initial soluble fraction in pore water, if compared with the laboratory percolation column. The coupled geochemical-transport model was validated by comparison of the simulation results on ANC data obtained on the waste after 44 months of leaching under natural conditions.
Tiruta-Barna
L
L
LIPE, National Institute of Applied Sciences of Toulouse, 135 av. de Rangueil, F-31077 Toulouse Cedex, France. Ligia.barna@insa-toulouse.fr
Rakotoarisoa
Z
Z
Méhu
J
J
D000143:Acids / Q000737:chemistry; D002244:Carbon / Q000737:chemistry; D003031:Coal; D060729:Coal Ash; D003198:Computer Simulation; D004058:Diffusion; D006863:Hydrogen-Ion Concentration; D007220:Industrial Waste; D007700:Kinetics; D008956:Models, Chemical; D052638:Particulate Matter / Q000737:chemistry; D012996:Solutions; D014867:Water
D000143:Acids; D003031:Coal; D060729:Coal Ash; D007220:Industrial Waste; D052638:Particulate Matter; D012996:Solutions; D014867:Water; D002244:Carbon