Use of ultrasound for tar removal from tar-contaminated sand

Abstract

A countercurrent continuous washing apparatus for tar removal under ultrasonic irradiation has been developed. Tar was dissolved in dimethylformamide, DMF and sand was soaked into the resulting tar solution to prepare samples of tar-contaminated sand. Tar contents in DMF were determined by a UV-spectrophotometer from absorbance at 336.5 nm. The removal rate of tar content from this tar-contaminated sand was measured in two different conditions, one under the condition of mechanical stirring and the other with ultrasonically induced agitation. The removal rate was described in terms of a first order reaction equation, which enables us to calculate the residue fraction in continuous washing at a steady state. Comparison of tar-removal with mechanical stirring and ultrasonically induced agitation has demonstrated that the ultrasound is more effective than the simple mechanical stirring. The basic mechanism of tar removal is, for both removal procedures, peeling of a tar-covered layer on the sand surface and the particles produced under the ultrasonic field are much finer than for the case of mechanical agitation.