Metal recovery during melting of extruded machining chips

Abstract

Many researchers have examined the welding of machining chips in an extrusion process. Their aim has been to recycle the material into a usable extruded component without melting. Industry has yet to embrace this processing technique despite over a decade of research. We instead investigate the potential to use extrusion as a pre-melting step to increase metal recovery during chip recycling. We extrude aluminum (AA6060) and zinc (Kirksite A) chips under different extrusion ratios. The oxide content of the extruded aluminum is predicted using evidence from the literature and measured using an Auger Nanoprobe. We compare the subsequent metal recovery rates during vmelting to those obtained during: (1) bulk metal recycling; (2) melting of chips by submersion in molten metal; (3) melting of chips with salt fluxes; and (4) melting of previously compacted chip briquettes.For aluminum chips, it is found that an extrusion ratio of 9 (and higher) results in a fully dense material which, when melted, results in a metal recovery equal to that of bulk metal melting and exceeding that possible by melting chip briquettes. In contrast, extrusion of zinc chips results in a poorly welded, porous rod. Melting of these rods results in a paste of zinc metal entrapped within a zinc oxide honeycomb. Negligible liquid zinc gathers at the base of the crucible and overall metal recovery is lower than during simple melting of un-compacted zinc chips.Cradle-to-gate life cycle assessments are used to compare conventional recycling of aluminum chips to the new extrusion plus melting route. The latter could result in savings of up to 60 MJprimary and 6 kgCO2eq per kilogram of chips recycled.