A Study on Physical Separation Techniques for Recovery of Metals from Municipal Solid Waste Incineration (MSWI) Bottom Ash

In this thesis suitable physical separation methods have been studied and experimentshave been carried out for metals recovery from MSWI bottom ash. The literature studyincluded an overview of the composition and variations of MSWI bottom ash, the factorsinfluencing the bottom ash quality and the usual processing practices.In the experimental part a bottom ash sample was first characterized. Characterization wasimportant with regard to the mechanical processing of bottom ash, because it gave a goodoverview of bulk properties and the distribution of mechanically separable metals indifferent particle size classes. The experimental process flowsheet designed was based onthe literature study and the bottom ash characterization.The process developed included wet screening of the bottom ash into three particle sizeclasses: +6.25mm (Coarse), 0.18-6.25mm (Medium) and sludge of 0-0.18mm (Fines).Wet screening proved an efficient way for washing the small fraction to the underflow.The Fines product was 32w-% of the total dry output. It was delivered for treatebility teststo Salvor Oy.The Coarse and Medium products were processed with magnetic and eddy currentseparators and the performance of the processes were assessed. The Coarse and Mediummagnetic products were both 7w-% of total dry output. Recovery of iron from bottom ashwas possible with good recovery. From the Coarse fraction 87% of iron was recovered ata grade of 25% and from the Medium fraction 71% of iron was recovered at a grade of20%. In addition, magnetic separation step remover environmentally harmful elementsfrom bottom ash, such as Sn, Ni, Zn and Cu, possibly improving bottom ash’senvironmental quality at the same time. The amount of coarse nonmagnetic metal productwas 3w-% of total dry output. For particles larger than 6.25mm also the recovery ofnonferrous metals is possible by traditional eddy current separation. The recovery wasgood (85% for grey nonmagnetic metals and 90% for red nonferrous metals) with metalgrade of around 55%. The tested nonferrous metals separation steps, eddy currentseparator and pneumatic shaking table for Medium fraction did not work.The results showed that for many environmentally harmful elements the concentration inFines higher than for the produced Granulate products. The process results indicated thatthe environmental quality of MSWI bottom ash can be already improved by removal ofthe finest fraction by wet screening