Sensor-based control in eddy current separation of incinerator bottom ash

A sensor unit was placed online in the particle stream produced by an eddy current separator (ECS) to investigate its functionality in non-ferrous metals recovery. The targeted feed was the 1-6mm size fraction bottom ash from a municipal waste incinerator. The sensor unit was attached to the ECS splitter, where it counted in real-time metal and mineral particles and accurately measured the grade of the stream in the metals product. Influence of segregation (e.g. due to particle size or density) on the metals concentrate were detected and studied using the sensor data collected at different splitter distances. Tests were performed in the laboratory and in a bottom ash processing plant with two different types of ECS and two sources of bottom ash with different moisture content. The measured metal grades matched the manual analyses with errors 0%, 1.5% and 3.1% for moist, dry and very wet feed, respectively. For very wet feed the ECS metals recovery dropped, which was observed from the strongly reduced particle counts and the large changes in cumulative particle properties. The measured sample proved representative for the whole metals concentrate if it is collected at a representative position within the metals particle trajectory fan produced by the ECS. ECS-performance proved sensitively dependent on splitter distance, since a 10mm shift may result in 10% change in metal recovery and 18% change in grade. The main functionalities of the sensor unit are determined as online quality control and facilitation of automatic control over the ECS splitter distance. These functionalities translate in significant improvements in ECS metals recovery which in turn is linked to economic benefits, increased recycling rate of scrap metals and a further reduction of the ecological drawbacks of incinerator bottom ash.