Wet physical separation of MSWI bottom ash

Abstract

Bottom ash (BA) from municipal solid waste incineration (MSWI) has high potential for the recovery of valuable secondary materials. For example, the MSWI bottom ash produced by the incinerator at Amsterdam contains materials such as non-ferrous metals (2.3%), ferrous metals (8-13%), gold (0.4 ppm), silver (10 ppm), stainless steel (0.1%) and minerals that can be converted into building products such as aggregates utilized for concrete, asphalt, etc. Since the composition of BA varies from country to country, the first step in valorizing it should be characterization. Therefore, this thesis proposes a new system for analysing BA, which concentrates on properties of fractions related to their ease of separation and material value. A common practice in BA treatment in Western Europe is to separate coarse metal particles from the rest of the material by dry physical separation and to use the residue as a low-value construction material (road fillers) in controlled applications. Unrestricted re-use of this residue as a mineral resource is not possible in most EU countries because, generally, some of the leaching values for salts and metals exceed regulatory limits. In the Netherlands, the quality of building materials must be measured according to the Soil Quality Decree (SQD), which mandates strict environmental limits. Therefore, a new separation solution, based on wet physical treatment, was developed and tested to recover more metals and at the same time produce environmentally clean products that are suitable for various building applications. Wet physical separation processes are preferable to dry methods of mineral processing due to the fact that they allow for more efficient and accurate classifications of size and density. The wet separation system can recover more metals of all size fractions down to 0.3 mm. Wet separation provides an important advantage through the wet screening process, as fine particles and organics are washed out from the granulate fractions, thereby improving the environmental quality of building products. The high recovery of metals provides financial benefits, and in combination with the washing and removal of organics it also has positive environmental effects. This thesis describes a new separation system, based on wet treatment, which was developed and implemented in a pilot plant in Amsterdam between 2003 and 2006. Furthermore, in addition to the treatments applied in the pilot plant, techniques to separate precious metals, separate stainless steel and improve the quality of the recovered sand were developed, all of which, when implemented, should increase the value of bottom ash. Wet treatment is also able to decrease leaching values to close to, legislative limits in order to produce building materials from BA of a category for which no extra isolation is needed. Nevertheless, leaching values of some contaminants such as copper, molybdenum, antimony, sulphate and chloride still occasionally exceeded the limits. Therefore, this research focused on the correlation between contaminants (salts and metals) and separation settings, composition and input materials, in aim to find the factors which can minimise leaching values. Another objective was to develop a convenient quality control system for building products generated from BA capable of rapidly providing accurate results based on measurements obtained using the column test. A model based on physical parameters such as diffusion, contaminant levels and dissolving factors was developed and demonstrated that it is possible to decrease the time consumed by the column test while still obtaining accurate predictions. The quality of building materials can determine their future utilization, therefore this study also examined the chemical, geometrical and mechanical properties of building materials such as sand and aggregates. Initial tests in which those aggregates and sand were utilized in asphalt, concrete and lime-sand stone were then conducted in attempts to identify the optimal utilization route for such materials. The last part of this thesis presents a financial overview of the process, products and environmental impacts of BA treatment. The wet separation system was compared in these terms with the existing, commonly used dry separation system, using Life Cycle Assessment (LCA). This analysis showed that the wet process has less adverse environmental impacts, of diverse kinds (e.g. global warming and human toxicity potential), than the dry system. The investment and process costs of the pilot system in Amsterdam were also calculated under several scenarios, with various modelled separation capacities, to identify the most financially beneficial scheme for treating (and utilizing) BA.Materials & ScienceCivil Engineering and Geoscience