Reactivity of alkaline lignite fly ashes towards CO2 in water

The reaction kinetics between alkaline lignite fly ashes and CO2 (pCO(2) = 0.01 -0.03 MPa) were studied in a laboratory CO2 flow through reactor at 25-75 degrees C. The reaction is characterized by three phases that can be separated according to the predominating buffering systems and the rates Of CO2 uptake. Phase I (pH > 12, < 30 min) is characterized by the dissolution of lime, the onset of calcite precipitation and a maximum uptake, the rate of which seems to be limited by dissolution Of CO2 Phase II (pH < 10.5,10-60 min) is dominated by the carbonation reaction. CO2 uptake in phase III (pH < 8.3) is controlled by the dissolution of periclase (MgO) leading to the formation of dissolved magnesium-bicarbonate. Phase I could be significantly extended by increasing the solid-liquid ratios and temperature, respectively. At 75 degrees C the rate of calcite precipitation was doubled leading to the neutralization of approximately 0.23 kg CO2 per kg fly ash within 4.5 h, which corresponds to nearly 90% of the total acid neutralizing capacity.