Ways to Improve the Efficiency of Soil Washing

Abstract

28.1 Introduction The physical treatment with respect to soil washing is an established technique for cleaning contaminated soils. Its basic principle is the removal of the fine particle fraction from the soil material, in which the contaminants are predominantly con-centrated. The goal of the separation process of the fine fraction is to achieve a lowly contaminated coarse fraction that is economically reusable at a yield which should be as high as possible. Industrial soil washing plants basically consist of a wet liberation step and a classification unit. The wet liberation step, which is car-ried out by application of mechanical energy through impact, is used to transfer the contaminants from the coarse into the fine particle fraction, and to concentrate the contaminant in the fine part of the solids input. The highly contaminated fines are removed in the classification step by means of screens and hydrocyclones, for example. The cleaning principle of collecting the contaminants in the fme particle frac-tion does not work efficiently for silty or loamy soils, because the yield of cleaned solids mass is too low. Further problems arise for contaminants that are not dis-placeable to such an extent that the concentration of the contaminant in the coarse particle fraction meets the required threshold limits (Tittel et al. 1995; NeeBe et al. 1991). In these cases, additional process steps are required to achieve both a lower contaminant concentration in the cleaned fraction, and a higher yield of cleaned soil, by concentrating the contaminants in a very narrow particle fraction. These further treatment steps require either a sharper separation at low particle sizes (e.g. cut sizes below 20 IJ.m), or a process which separates according to particle proper-ties other than size. Furthermore, the liberation step should be as intense as neces-sary to enhance the change of the contaminant distribution. In the research work carried out at TUHH's Chemical Engineering Depart-ment I in the 12 years of SFB, several treatment methods were investigated to improve the efficiency of soil washing. Attrition and high pressure water jet lib-eration was investigated to improve the change in the contaminant distribution. Flotation as a method that separates particles according to different surface R. Stegmann et al. (eds.), Treatment of Contaminated Soil © Springer-Verlag Berlin Heidelberg 2001 436 Physical Treatment properties was tested for the hydrocyclone underflow to improve the mass yield of the soil washing process. More recently, the hydrocyclone overflow was processed with two different classification methods which work in the particle range below 20 jlm, namely the hydro deflector wheel classifier and the centrifugal fluidised bed counter-current classifier. 28.2 Wet Liberation The wet liberation step is carried out to remove contaminants and highly contami-nated fines from large particles and in this way to concentrate the contamination in the fines fraction. Three mechanisms predominantly cause the shifting process into the fine fraction, namely abrasion, which means that contaminants and fine particles are washed off from larger particles; destruction of agglomerates and disintegration of particles. Different methods can be applied to realise the libera-tion step technically: hydraulic transport for dredged sludges, impact stress in high pressure water jets, attrition in rotating drums and attrition with agitators for soils. An attrition drum is used on the industrial scale in the LURGI-DECONTERRA ® process (Hankel et al. 1992) to concentrate the contaminants in the fine particle fraction. This attrition drum consists of a horizontal tube with special lining and inserts. The contaminant distributions of soils polluted with mineral oils were altered effectively by treating the soil according to this process. After the treatment, the contaminants were found to be concentrated in the fines fraction below 80 jlm. In the authors' group, wet liberation was carried out in an attrition cell, and with a high pressure jet liberation unit for soils contaminated with heavy metals. It was investigated whether the shifting of the contaminant distribution exceeds the shifting of the size distribution ofthe solids material. section 8-B: