The recycling of MSW incinerator bottom ash by sintering

In recognition of the trend toward an increased use of bottom ash as construction material, the authors have investigated the feasibility of recovering bottom ash for use as aggregates, by sintering size-fractioned MSW incinerator bottom ash (particle size less than 1.41 mm and between 4.76-1.41 mm) at 400-1,000°C for 60-240 min, and then determining the sintered material characteristics, such as the compressive strength, heavy metal leachability and principal material properties. The results indicate that the pH of the Toxicity Characteristic Leaching Procedure (TCLP) leachate produced from both fine and the coarse ash, ranged from 10.0-11.5, and from 7.5-11.3 respectively, and showed a tendency to decrease with an increasing sintering temperature. In addition, for both types of ash the compressive strength of the sintered monoliths, ranging from 50-55 MPa, decreased slightly when the sintering temperature was increased from 400 to 600°C. Deformation problems may arise from the melting of glassy substances in the ash when bottom ash is sintered at temperatures higher than 700°C. Thus, when sintered between 800 to 1,000°C, the sintered bottom ash pellets might disintegrate due to the formation of aluminium and/or calcium salts. The decomposition of calcium carbonate at 650°C which releases significant amounts of carbon dioxide, may also cause the destruction of a monolith. Based on considerations of loss on ignition, volume changes, water adsorption, soundness, bulk density and the compressive strength of the sintered ash, developed by the sintering of bottom ash between 400 to 600°C after removing its coarse impurities, the general results from the experiments suggest that the aggregates do meet the Chinese National Standards (CNS) for permeable blocks.