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WWRC 95-17
Development of a Carbon Dioxide Pressure Technique for Chemical Stabilization of Alkaline Clean Coal Technology (CCT) Ash


Clean Coal Technology (CCT) ash may contain trace elements such as arsenic (As), cadmium (Cd) and selenium (Se), some of which may become mobile and leach from a disposal facility. This study was undertaken to determine the effects of a carbon dioxide (CO2) pressure treatment on the leachability of trace elements in CCT ash. The CO2 pressure treatment was optimized using a three by five factorial design as well as a multiple regression analysis. Low, medium and high levels of moisture, reaction time, pressure, temperature and concentration of CO2 were tested. Treated and untreated CCT ash samples were subjected to X-ray diffraction (XRD) and leachability studies. A 1:4 (solid:solution) suspension was used to monitor the pH changes after each treatment. Optimum CO2 treatment conditions rapidly precipitated calcite, and thus lowered the pH of CCT ash samples. For example, a stable pH drop from 12.47 to 7.05 for CCT-1, 12.74 to 9.34 for CCT-2 and 11.50 to 9.16 for CCT-3 was obtained. An increase in percent calcium carbonate (CaCO3) content and percent CO2 uptake was observed in CO2 treated samples. Solubility studies suggested that Ca2+ concentration in CO2 treated CCT ash leachates appeared to be regulated by CaCO3. Our results show that reacting moist CCT ash samples with CO2 under pressure is effective in lowering the concentrations of leachable trace elements (e.g., Cd, Pb, Cr, As and Se), which could prevent their migration from disposal environments into soils and groundwaters.

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