Waste electric and electronic equipment (WEEE) melting and resulting gases processing in microwave field

Abstract

The recycling of waste electric and electronic equipment (WEEE) is of great interest due to the large amounts of such wastes and to their contents of useful and precious metals. After reaching the end of its service lifetime, WEEE is collected and recycled through various processing methods, depending on the waste type. Pyrometallurgy is currently the most used method for the recovery of the metallic fractions from WEEE such as printed circuit boards (PCBs). This type of processing raises major issues because of the generated gas emissions. The non-metallic fraction of the PCBs, which account for a large proportion (50-70 wt.%), contains brominated flame retardants, thermosetting resin, reinforced materials and other toxic and hazardous organic substances. The combustion of these chemicals during the pyrometallurgical processes may cause serious health-related and environmental problems. In this paper, the treatment in microwave field of the toxic gases resulted from the melting of crushed PCBs has been preliminarily investigated. During the waste melting in a microwave furnace, toxic compounds such as benzene (3.2-6.5 mg×m -3 ), toluene (15.3-17.8 mg×m -3 ), ethylbenzene (2.8-5.7 mg×m -3 ), styrene (21.5-24.3 mg×m -3 ), m/pXilen (1.4-2.8 mg×m -3 ), naphthalene (1.1-1.3 mg×m -3 ) and 1,3,5 trimethylbenzene (15.2-23.5 mg×m -3 ) have been detected in the effluent gases. The treatment was carried out by passing the resulting gases through a filter fabricated from a microwave susceptor granular material (SiC), placed in a microwave transparent tube (fused quartz, 5 cm diameter). For heating the filter, a number of 3 magnetrons (2.45 GHz frequency and 850 W power each) were mounted outside the fused quartz tube at an angle of 120 degrees and in different geometrical planes. The influence of the process parameters (heating response of the susceptor materials, temperature, gas flow, gas pressure) was investigated. It was observed that the temperature of the filter (i.e. the power density in the microwave susceptor material) has a major influence on the neutralization of the toxic compounds while the gas flow and pressure had a minor influence. At a temperature of 1350-1400°C (power density of 5000 W× kg -1 ), a gas flow of 150 m 3 × h -1 and a pressure of 800-1000 mbar, the content of such substances in the gases was reduced below the legal limits. These results confirm the feasibility of the process of treating the gases resulting from the melting of e-waste in a microwave furnace.