Decontamination and granulation of filter dusts in a microwave field

The closure of material cycles is becoming ever more important in the processing of metals, especially from an economical and environmental point of view. Although the recycling rate of scrap metal has traditionally been very high, there is still a need to develop technical measures to increase the recovery of working materials, remnants and waste materials, in order to meet the demands of modern material flow management.

Using the example of the reconditioning of filter dusts from the secondary aluminum industry, it was shown that it was possible to decontaminate chlorinated, dibenzodioxine- and dibenzofurane-polluted filter dusts by using a microwave treatment, which also allowed the raw materials to be directly reintroduced.

Aluminium is usually recovered by melting the soiled scrap metal inside a rotary furnace filled with a salt bath. Due to the high temperature (800-900°C), the salt bath evaporates to some degree, leading to flue dust which exits the furnace together with the burner exhaust gases. Aluminium particles, aluminum oxide and hydrochloric acid are also components of the exhaust gas. Afterwards, these dusts are filtered, and contain a high amount of materials suitable for recycling. For this reason, more research and development has been performed in recent years in order to find reasonable ways to process this hazardous waste.

With InVerTec's new process, these extremely hazardous filter dusts are heated with microwave radiation leading to an almost complete conversion of the PCDD/F components. Analogous to a conventional rotary kiln procedure the filter dusts are heated up to 600°C. This temperature provides the conditions for nearly total decontamination of the filter dusts. A back reaction, however, can be prevented because the materials are cooled down rapidly when they exit the microwave field.

The PCDD/F components may also be sufficiently reduced with a conventional heated kiln, but the reactor wall has to be much hotter than the reactor contents. With time the filter dust becomes a sticky compound which deposits on the reactor wall and has to be removed by hand. Using microwave technologies, a direct heating of the filter dust is possible. The reactor contents are much hotter than the reactor wall and no unwanted deposition occurs. Heating the filter dust with microwaves has other major advantages, too. By selectively overheating individual dust particles, larger agglomerates are formed, which can thus be heated more effectively. The rotary motion of the kiln leads to a snowball effect in the growth of the agglomerates.

Not only is the process suitable as a decontamination method, but it also benefits from integrated granulation which is very important for subsequent recycling steps.

At an industrial scale, these granules can be directly reintroduced into the secondary aluminum cycle and don't have to be deposited.