ABSTRACT
Recovery of trace elements from waste is drawing increasing attention of engineers in order to abate environmental impact of different emission sources, such as energy boilers, solid waste incinerators, sea vessel engines etc. Recently, in order to achieve this goal, an interest was turned to magnetic separation, a method suggested in 1970s (Watson 1973, Sakata et al. 1976, Ariman et al. 1979, Gooding & Drehmel 1979, Mendrela et al. 1985). Two types of devices utilizing magnetic forces to separate fine magnetic particles are used: wet and dry separators. Wet separators remove the particles
1 INTRODUCTION
Power industry is the largest anthropogenic emitter of PM10 and gaseous contaminants. Electrostatic precipitators and bag filters are the most frequently used devices for the removal of fly ash particles produced in coal fired boilers. Although mass collection efficiency of those devices is higher than 99.9%, in the flue gases there still remain particles smaller than 2.5 μm, which are classified as PM2.5. Main components of fly ash leaving coal fired boiler are SiO2 (roughly 1/2), Al2O3 (1/4) and Fe2O3 (1/8). Other compounds found in fly ash in amounts larger than 1 mg/g each, are CaO, Na2O, MgO, K2O, TiO2 (Coles et al. 1979, Smith et al. 1979, Ylätalo & Hautanen 1998, Hower & Robertson 2004). The percentage of those compounds can vary depending on the coal mine and the combustion conditions. Because of the presence of iron containing particles, some of them can be separated from the non-magnetic fractions. The percentage of toxic elements (As, Cd, Hg, Ni, Zn, Pb, Cr, Sr, Be, V, U) is significantly higher in submicron particles (PM1) than in larger ones (Shu et al. 2001). It was found that trace elements abundant in nanometer particles adhere to the larger particles or are fused with them. Up to 20% of PM2.5 particles are of magnetic properties. The magnetic fractions are mainly composed
from a liquid conveying them. This method is used for enrichment of materials from ores or for water purification. Dry magnetic filters or separators are used for the collection of fine para-or ferromagnetic particles from fluidized beds or from aerosol phase (Sakata et al. 1976, Rossier et al. 2012, Zyryanov et al. 2011).
