An effective way to treat the iron-rich acid mine drainage from coal mining in Guizhou’s mountainous areas

Abstract

Acid Mine Drainage (AMD) from coal mining is a serious environmental issue which affects water quality, ecology, and the overall landscape of the basin. A large number of coal mine tailings in the mountainous regions of Guizhou Province, China were unattended and iron-rich AMD was directly discharged to the rivers. This discharge leaves the river “yellow” and heavily polluted. This study tries to find an efficient and economical method for treating iron-rich AMD. We sampled AMD water in two sites: Yangliujie town of Duyun city (hereafter, called Yangliujie), and Xinglong Coal Mine, Duliu town of Guiding county (hereafter, called Xinglong). We performed iron removal laboratory experiment with Cement-Bentonite Agent (CBA, 80% cement and 20% bentonite) in 500 mL AMD water from Yangliujie, scale-up experiment in 15 L AMD water from both Yangliujie and Xinglong, and engineering application in Xinglong respectively. Laboratory experiment results showed the iron removal rate can reach 99.8% and the removal rate depends on the CBA dosage and the treatment time. In the scale-up experiment, we found that Fe concentration could be reduced from 587.0 to 0.2 mg/L when adding 20 g/L CBA to the AMD water and aerating for 3 hours. As sampled water in Xinglong has a very high Fe concentration (Fe 1019.8 mg/L) and the concentration varies with seasons, it is not economical to add the CBA directly to the AMD water. Considering the abundant and cheap limestone resources in Guizhou, we used a twostep treatment method, first we added CaCO3 to raise the pH, and then we took the supernatant liquor and added CBA to the liquor. It was shown that 15 g/L of CBA was a good dosage for iron removal with Fe concentration being reduced from 1019.8 to 0.3 mg/L when CaCO3 was used to raise the pH. The best treatment realized over 99.9% iron removal, 99.2% NH3-N removal, 98.9% CODMn removal, and heavy metals in the treated water were reduced to under the limit stipulated in the “Environmental Quality Standards of Surface Water in China”. Thus in the engineering application, we used this two-step treatment method. After the treatment, the pH of the iron-rich AMD (pH 2.86, Fe 2624.6 mg/L) increased to 8.53, the concentration of Fe was reduced to 59.5 mg/L, NH3-N decreased from 16.15 to less than 0.05 mg/L, CODMn decreased from 323.33 to 24.57 mg/L, heavy metals except Fe and Mn were reduced to under the limit of surface water. In conclusion, the use of CBA can effectively remove Fe and other heavy metals from the iron-rich AMD and adjust the pH value to the range of a natural water body.