Stabilization of mercury containing sludge by a combined process of two-stage pretreatment and solidification

Abstract

The stabilization of a mercury-bearing sludge, which is a typical hazardous waste of the chlor-alkali industry in the southern of Taiwan, has been performed by using a cement-fly ash solidification method. A two-stage pretreatment procedure consisting of using sodium sulfide and ferrous sulfate is employed to stabilize the solid end-product. Both the concentrations of the organic mercury in the extraction leachate ( C o) of the solids matrix, which had not been previously paid much attention to, and of the total mercury ( C t) have been examined. The results indicate that the two-stage pretreatment greatly enhances the stabilization efficiency of the solid matrix. The value of C t can be reduced to a value lower than 1 ppb, which is well below 5 ppb (a Japanese safety regulation on Hg for the in-land sanitary landfill). No organic mercury in the extraction leachate has been detected for the combined process of the two- stage pretreatment and the solidification. Furthermore, within the ranges of experimental conditions of this work, the tendency of C t to increase with curing time in the solidification by the process without two-stage pretreatment is greatly prevented by the two-stage pretreatment process. In addition, the 28-day compressive strength of the solid end-product can reach a value larger than 33 kg/cm 2, which is well above 10 kg/cm 2 (another Japanese regulation)