Characterisation of residues : Release of contaminants from slags and ashes
Abstract: Residues from industrial processes as well as from energy production utilities have in many cases properties similar to geological materials. They can be used as filling and construction materials if the net environmental impact and the technical properties are acceptable. Generally, the impact of the leachates has been the major environmental concern. This thesis focus on leaching from slags (blast furnace and steel slag) and ashes (municipal solid waste incineration bottom ash and wood ash) and specifically on the aspects of leaching processes in laboratory tests, differences in leaching behaviour between laboratory and field conditions, and possible interfaces between leaching under different specified conditions.Solubility controlled leaching was predicted already after short exposure times in batch tests (less than 6 hours). It is proposed that solubility controlled concentrations should be used as one end point in batch leaching tests. Leaching by diffusion from granular material was also demonstrated in batch test. Both these leaching mechanisms can be studied in a factorial design of batch tests with the parameters grain size, LIS ratio and leaching time. In addition, a diffusion model for spherical particles considering the actual grain size distribution is needed. The potential leachability, determined by the proposed availability test, appeared to correspond to a depletion of the matrix of the substances released at an effective diffusivity (De) greater orequal to 10-15 m2/s.There was a correspondence between leached amounts in laboratory (column) and lysimeter tests for many substances. Concentrations of other substances reflected the change in pH, oxidation or complexing capacity and did not correlate with laboratory tests results. Field test leachates generally held a pH at a level around 8.4 (representative of a CaCO3 bufferedsystem), irrespectively of the pH found in laboratory tests. Exceptions were found due to the amounts of fines, which reflected the degree of water saturation in the field situation, and also due to oxidation of sulphides. The pH dependency of the leaching process was different for the various materials and the lysimeter leachate concentrations did not fully reflect the pH-dependent solubility identified in the laboratory.Solubility controlled concentrations was predicted for barium and chromium in the leachates from steel slag, both in laboratory and field tests. It was anticipated that secondary solids may form and control the leachate concentrations from these residues.The thesis consists of a framework and seven appended papers addressing the topics above.
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