Improved method for prediction of heavy metal recoveries from soil using high intensity magnetic separation (HIMS)

Abstract

In recent years the soil cleaning industry has developed a number of cleaning strategies mostly based on classification techniques. Sometimes, however, those strategies fail for a number of reasons and as a result cleaning targets are not met. Therefore, some soil cleaning facilities have experimented with HIMS as an alternative process step. This paper describes a prediction method based on magnetic separation techniques for use of HIMS separators in soil cleaning plants. Two types of HIMS separators were used. First a (dry) Frantz isodynamic separator (FIS) and secondly a (wet) Jones high gradient magnetic separator (model P40 KHD and model DP 90 Humboldt Wedag). The FIS was used to separate heavy metals from soil and for observation of heavy metal behaviour in magnetic fields. With the Jones P40, pilot-scale experiments were performed to confirm the FIS results. In total six soil samples uncleanable by soil washing plants and three soil samples prepared by laboratory classification were used in the experiments. Furthermore, three soils were treated on full plant scale. FIS experiments indicated that soils can be distinguished by their different magnetic properties and that each soil shows its own characteristic magnetic susceptibility fingerprint. A similarity, however, was observed between the magnetic properties of soils and the magnetic properties of hematite which is a strong indication for the importance of iron in removing heavy metals from contaminated soil. The Jones P40 experiments showed a similar removal efficiency as the FIS for iron but not for the heavy metals. Soils treated with wet HIMS, however, showed considerably lower heavy metal contents than soils only treated with conventional washing techniques, indicating the usefulness of HIMS in cleaning heavy-metal-contaminated soils. Plant-scale treatment of three polluted soils showed that the Jones separator is a feasible technique for soil washing and treatment of heavy metals.