Effect of Dissolved CO2 in Acid Mine Drainage on Sludge Composition

Among the variables affecting the volume and composition sludge generated in the semi-active mine drainage treatment facility operating in Korea, the effect of CO₂ dissolved in the mine drainage on the sludge composition was studied.

The water quality change was analyzed by neutralizing 1m³ of mine drainage from the Ilkwang abandoned mine. The neutralization with lime slurry resulted in removal of Fe, Al, Cu and SiO₂, and an increase in Ca content in neutralized water. The dissolved CO₂ in raw water was 56 mg/L but decreased to <1 mg/L after neutralization. The dissolved CO₂ in the mine drainage can be consumed to form calcite or be removed by degassing before neutralization. There is also a possibility that gypsum may be produced depending on pH due to the high concentration of Ca and SO₄ enriched.

The amount of sludge generated through the neutralization test was insufficient for analysis. Therefore, chemical analysis and mineral assessment were performed on the effluent, sludge and cakes generated in the settling pond in the Ilkwang abandoned mine. XRD analysis showed that calcite (CaCO₃) and Bassanite (CaSO₄ · H₂O) were commonly found in red and white sludge. CO₂ in the mine drainage reacted with CaO to affect the sludge composition. As a result of the XRF, white sludge contained 35.57% of CaO, indicating that a large amount of slaked lime was included. Red sludge and cakes had a Fe₂O₃ content of more than 35%, so iron was the main ingredient. In addition, the CaO content was analyzed as 15.28%, the second major component. As a result of measuring the paste pH of the dehydrated cake, it can be seen that the pH is in the range of 9 to 10 due to CaO. The sludge shows strong alkali characteristics, indicating that slaked lime remains in the sludge. The remaining slaked lime will increase the sludge volume and increase the sludge management costs.