RF thermal plasma treatment of dredged sediments: vitrification and silicon extraction

In the present work, dredged polluted sediments with high level of hydrocarbons and heavy metals have been treated by means of a laboratory scale radiofrequency (RF) thermal plasma source inside a graphite crucible. Two different experimental approaches have been utilized (1) to fully decontaminate and vitrify this material, and (2) to study the technical feasibility of metallurgical-grade silicon (MGS) smelting by carbothermal-reduction reactions of carbon with silica (SiO2) content in these dredged sediments. A two-dimensional model of a commercial inductively coupled RF plasma torch has been used to investigate the effects of plasma flow and temperature distributions of the plasma discharge interacting with the material inside the crucible. Samples of both vitrification and carbothermal reduction processes were characterized by scanning electron microscopy analysis (SEM), energy dispersion spectroscopy (EDS), X-ray fluorescence (XRF) and leaching tests. Results obtained showed a fully decontamination of the dredged sediments, with release values of heavy metals in leachates well below the law limits. Moreover, SEM/EDS analyses suggested that separation of MGS by carbothermal-reduction process is possible.