Solidification And Stabilization Of Heavy Metal Waste Using Reject Fly Ash Activated By FGD

Stabilization/solidification (S/S) has been routinely used for the final treatment of hazardous wastes prior to land disposal. These processes involve adding one or more solidifying reagents into the waste to transform it into a monolithic solid with improved structural integrity. Cement based systems with partial replacement by pulverized fuel ash (PFA) have been widely used to minimize leaching of contaminants from hazardous wastes.The finer fraction of PFA (<45-m, fine fly ash, fFA), produced by passing the raw ash through a classifying process is commonly used in building and S/S processes. Low-grade fly ash is rejected (rFA) from the ash classifying process and is largely unused due to high carbon content and large particle size but represents a significant proportion of PFA.Flue Gas Desulphurisation (FGD) sludge is a by-product of the air pollution control process in coal-fired power plants. Its chemical composition is mainly gypsum. As the reactivity of fly ash can be activated by gypsum, it was of interest to conduct research on the possible activation of rFA using FGD and using these materials in S/S process.This thesis includes two research parts, one focuses on the research on the pozzolanic property of rFA, the other focuses on the research on the feasibility of using rFA and FGD in the cement-based S/S of a synthetic heavy metal waste. The results on the pozzolanic property of rFA show that r-FA generally has a lower rate of hydration than f-FA particularly at the early stage of hydration. Adding Ca(OH)2 alone almost had no effect on accelerating the hydration of r-FA. But adding Na2SO4 together with Ca(OH)2 can significantly accelerate the strength development both at 28 and 90 days. Adding K2SO4 only shows effective at 28 days and the FGD takes obvious effect at 90 days. The effect of adding CaCl2 on the reaction of rFA is less than Na2SO4 or K2SO4. Pretreatment of steam curing can slightly increase the strength at 28 days but decrease the strength of samples at 90 days. The grinding of r-FA can significantly increase the strength at 90 days. Adding chemical activator in ground r-FA-Ca(OH)2 system shows the most effective both at 28 and 90 days.The results on the S/S effect using rFA and FGD in the cement-based S/S process show that the strength of cement-based waste forms with rFA replacement are suitable for disposal at landfill. And the results of the Toxicity Characteristics Leaching Procedure (TCLP) and Dynamic Leach Test (DLT) show that rFA can be used in cement-based S/S wastes without compromising performance of the product. The radial leaching test show that contaminants transfer from the core of the sample to the solid-liquid interface during leaching process. And these contaminants can accumulate in the sample .which is far from the centre of the sample. The accumulation degree of contaminant is related with the species of contaminant and the quantity of Ca(OH)2 content. The zinc can retard the hydration of CSS completely and can also significantly slow down the reaction of C3A. The lead and copper can retard the reaction of C3A considerably but have little effect on the hydration of C3S. All these three heavy metals have little effect on the reaction of sulphoaluminate.