Study On Soil Particle Separation And Suspension Flocculation In The Process Of Soil Washing

This paper studied the suitable diameter for separating the fine particles before washing, the flocculation and precipitation efficiency of ferrous sulfate, aluminum sulfate. PAC and CaO in the suspension containing fine soil particles, the difference in the removal effects for heavy metals (Cd, Pb, Zn, Cu) from soil by HCl solution with and without fine particle separation, and the washing procedure for removing residual Cl-in the soil. Based on the above studies, a washing procedure for removing Cd, Pb, Zn and Cu from contaminated soils was established. The main study results were as follows:1. The contents of heavy metals varied greatly in different soil particles. The contents of Cu and Cd were gradually reduced as particle size augmented with the higher contents appearing in the particles of <0.001mm followed by those of0.001-0.002mm and the lowest being in those of0.05-0.25mm. From finer to coarser particles, the Zn content increased first and then decreased rapidly with the highest content being in the particles of0.001-0.002mm followed by the ones of0.001mm and the lowest content being in the0.25-2mm particles. The Pb content was highest in the0.001-0.002mm particles of Pb-contaminated soils but lowest in the0.05-0.25mm particles in most other soils.2. The accumulative curves of Cu, Cd, Pb and Zn in different soils were similar. When the particle size was<0.05mm, the metals accumulated apparently with the increase in particle size. However, the accumulative curves tended to be gentle when the particle size further augmented. The particles<0.005mm could be separated from the soil before washing.3. The best flocculation dosage of aluminum sulfate, PAC, ferrous sulfate and CaO for the flocculation and precipitation of the particles in the suspension were0.25g/L,0.04g/L,1.0g/L and0.3g/L, respectively. The costs of the four flocculants for treating same volume suspension were ferrous sulfate> aluminum sulfate> PAC> CaO. Therefore, CaO was the best flocculants for treating the suspension containing fine particles. The corresponding optimum coagulation pH range was10～11.4. The best concentration of HCl was0.1mol/L. The removal rate of Cu, Cd, Pb and Zn for the soils after removal of fine particles was11.66%,22.25%,28.52%and13.77%higher than that for the soils without pre-separating the fine particles from the soil, respectively.5. CaO was selected as to remove the heavy metals in the waste solution after washing. The pH of the waste solution should be adjusted between8～11. The leachate after CaO treatment can be released into the field. The soil after HC1washing contained lots of Cl-. Twice washing with tap water can remove most of the Cl-and the washed soil can be refilled into the fields.