Mineralogy and pore water chemistry of a boiler ash from a MSW fluidized-bed incinerator.

This paper presents an investigation of the mineralogy and pore water chemistry of a boiler ash sampled from a municipal solid waste fluidized-bed incinerator, subject to 18 months of dynamic leaching in a large percolation column experiment. A particular focus is on the redox behaviour of Cr(VI) in relation to metal aluminium Al(0), as chromium may represent an environmental or health hazard. The leaching behaviour and interaction between Cr(VI) and Al(0) are interpreted on the basis of mineralogical evolutions observed over the 18-month period and of saturation indices calculated with the geochemical code PhreeqC and reviewed thermodynamic data. Results of mineralogical analyses show in particular the alteration of mineral phases during leaching (e.g. quartz and metal aluminium grains), while geochemical calculations suggest equilibria of percolating fluids with respect to specific mineral phases (e.g. monohydrocalcite and aluminium hydroxide). The combination of leaching data on a large scale and mineralogical analyses document the coupled leaching behaviour of aluminium and chromium, with chromium appearing in the pore fluids in its hexavalent and mobile state once metal aluminium is no longer available for chromium reduction.

Characterization of a mineral waste resulting from the melting treatment of air pollution control residues.

Air pollution control (APC) residues which are generated by municipal solid waste (MSW) incineration show a high-level of pollution potential. In order to stabilize such APC residues, the French power supply company (EDF) is developing a thermal treatment process which leads to the production of a vitrified material. A structural characterization of the vitrified product was carried out by applying complementary investigation methods: XRD, SEM, Raman spectroscopy, EPMA, and data interpretation methods such as mineralogical analysis and principal component analysis (PCA). The major phase of the material was a solid solution of melilite type composed of five end-members: gehlenite (44%), åkermanite (25%), ferri-gehlenite (5%), sodamelilite (14%) and hardystonite (11%). The minor phases identified were spinels and pyroxenes. An ANC leaching test was performed in order to observe the treatment effect on pollutant release. The natural pH was close to 10, and the major element release was less than in the case of untreated APC. This was a consequence of melilite formation. The effect of pH was fundamental for heavy metals release: lower solubilization occurs at pH 10 than at APC's natural pH (11-12).

Multiple-scale dynamic leaching of a municipal solid waste incineration ash.

Predicting the impact on the subsurface and groundwater of a pollutant source, such as municipal solid waste (MSW) incineration ash, requires a knowledge of the so-called "source term". The source term describes the manner in which concentrations in dissolved elements in water percolating through waste evolve over time, for a given percolation scenario (infiltration rate, waste source dimensions, etc.). If the source term is known, it can be coupled with a model that simulates the fate and transport of dissolved constituents in the environment of the waste (in particular in groundwater), in order to calculate potential exposures or impacts. The standardized laboratory upward-flow percolation test is generally considered a relevant test for helping to define the source term for granular waste. The LIMULE project (Multiple-Scale Leaching) examined to what extent this test, performed in very specific conditions, could help predict the behaviour of waste at other scales and for other conditions of percolation. Three distinct scales of percolation were tested: a laboratory upward-flow percolation column (30 cm), lysimeter cells (1-2 m) and a large column (5 m) instrumented at different depths. Comparison of concentration data collected from the different experiments suggests that for some non-reactive constituents (Cl, Na, K, etc.), the liquid versus solid ratio (L/S) provides a reasonable means of extrapolating from one scale to another; if concentration data are plotted versus this ratio, the curves coincide quite well. On the other hand, for reactive elements such as chromium and aluminium, which are linked by redox reactions, the L/S ratio does not provide a means of extrapolation, due in particular to kinetic control on reactions. Hence extrapolation with the help of coupled chemistry-transport modelling is proposed.

Monitoring the stabilization of municipal solid waste incineration fly ash by phosphation: mineralogical and balance approach.

The application of a micro-characterization protocol coupled with a balance approach has allowed the relevant monitoring of a phosphation process for fly ash produced by municipal solid waste incineration. The three main steps of this process consist in removing the salts (chlorides, sulfates) by dissolution at basic pH, phosphation of the residue to trap metals, and its calcination to destroy dioxin-like compounds. The chemical and mineralogical balances compiled on the samples after each step of the process validate these main objectives and highlight the wide phosphorus distribution throughout the sample during the phosphation process, as well as the formation of apatite-type crystallized phosphates. During calcination, the increase in the proportion of crystallized phosphates apatite and whitlockite is largely attributable to the presence of an available calcium source, corresponding to the calcite formed during washing. The metals Pb and Zn, initially distributed in the silicate and carbonate phases, are broadly redistributed in the phosphate neoformations after carbonate dissolution, thus guaranteeing a more permanent stabilization.

Chemical changes and leachate mass balance of municipal solid waste bottom ash submitted to weathering.

A study on the chemical stability of municipal solid waste (MSW) bottom ash submitted to weathering was carried out in order to identify and quantify the physico-chemical maturation mechanisms in a large heap (375 tonnes) over a period of about 18 months. The mineralogy and chemical composition of MSW bottom ash were analysed on fresh and maturated material. Calcite is the predominant newly formed mineral during bottom ash maturation, combined with aluminium hydroxides and various sulphates. Lead and zinc are trapped primarily by newly formed carbonates. Monitoring of the pore water and the outlet leachates revealed a marked contrast in the physico-chemical conditions within the heap and at the outlet. The salinity of the fluids peaked at around 16 g/l within the first few weeks and then progressively decreased to fluctuate between 5 and 8 g/l. Due to the high pH of the pore water, the average concentrations of heavy metals in the heap are high: 42.7, 9.6 and 0.8 mg/l for Cu, Pb and Zn. At the heap outlet, however, the leachates are buffered by carbonate precipitation due to equilibration with atmospheric CO2. Copper complexed as a chloride at the outlet remains at a relatively high concentration (10.2 mg/l), whereas Pb and Zn concentrations are below the limit of detection (<25 microg/l). A mass balance carried out over the 18 months of monitoring indicated that 86% of remobilized material within the heap is evacuated from the system. Within the heap, carbonation trapped 43 and 54% of the calcium and bicarbonate flux. The copper, lead and zinc flux at the heap outlet represent only 34, 18 and 19% of the actual remobilized mass of heavy metals.

Weathering of a MSW bottom ash heap: a modelling approach.

Establishing plausible predictive scenarios represents a challenge for the long-term evolution of waste such as municipal solid waste bottom ash. These systems are characterized by complex and sometimes poorly understood physico-chemical mechanisms. The long term prediction of the evolution of such systems must be based on a dynamic approach involving their study in space and time. A preliminary outline of a model integrating chemistry and mass transfer is currently being tested by BRGM on the results obtained from a 16-month monitoring survey of a pilot bottom ash heap subjected to meteoric weathering. The model is based on a simplified coupled chemistry-transport approach using mass action laws and Kinetics chemical model (the Networks of Chemical Reactors approach). This modelling approach is used to monitor the evolution in chemical composition of a column of meteoric water percolating through the pilot bottom ash heap. The system is divided into representative elementary volumes (chemical reactors) on the basis of the major chemical and mineralogical zonations identified in the system.