Experimental assessment of aluminium recovery from MSWI bottom ash

Despite the separate and multi-material collection of aluminium is well established in Italy and in the EU Countries due to the implementation of nationwide aluminium packaging recycling schemes, a significant amount still ends up in the residual waste generally routed to incineration for energy recovery. The direct extraction of aluminium from municipal solid waste incineration (MSWI) bottom ash represents an important breakthrough in the aluminium recycling industry. However, during the combustion process, aluminium scraps contained in the waste undergo volatilization and oxidation mechanisms that are little known but certainly determine a loss of recoverable mass. The research project described in this thesis has the purpose of assessing the potential recovery of aluminium from the bottom ashes produced in a full-scale waste-to-energy (WTE) plant, in an attempt to provide useful indications for a better understanding of those parameters that affect the overall yield of the process. Two experimental campaigns were conducted on the same WTE plant in order to investigate the behaviour of four different aluminium packaging items inside the incineration furnace: during the first campaign, three types of aluminium rigid packaging (beverage cans, trays and spray cans) were tested, whereas the second campaign was devoted to aluminium flexible packaging (mix of aluminium and polylaminated thin foils). Each predetermined amount of packaging items was mixed with the residual waste ready for incineration in order to increase by four times its standard aluminium concentration. The doped waste was fed into the furnace and the solid incineration residues (i.e. bottom ashes and fly ashes) were sampled at regular intervals, before and after the effect of doping. After the simulation of an advanced treatment aimed at recovering aluminium from bottom ashes (which included the melting process in a crucible simulating the conditions of secondary aluminium production), all the samples were analyzed in laboratory for the measurement of their aluminium content. The results indicate that the structure of the packaging has a significant impact both on the recovery yield and on the average oxidation of aluminium. Aluminium recovery yield is directly related to packaging thickness and ranges between 27% for aluminium and polylaminated foils and 80% for beverage cans. In contrast, aluminium average oxidation is inversely related to packaging thickness and ranges between 9% for beverage cans and 59% for aluminium and polylaminated foils. The evaluation of the results also highlighted all the problems and limitations relevant to the planning of future experimental campaigns.