A novel approach to rapidly purify acid mine drainage through chemically forming schwertmannite followed by lime neutralization.

Acid mine drainage (AMD) has been recognized as a major challenge to the global mining industry due to its environmental consequences. Lime neutralization has been a traditional treatment for AMD, but the abundance of iron and sulfate in AMD usually renders it ineffective by forming iron hydroxide and gypsum precipitate coating on the surface of lime. In light of ubiquitous biological mineralization phenomena present at AMD sites, a rapid chemical mineralization was developed to reduce iron and sulfate and recovery of iron from AMD prior to lime neutralization. Through a cyclic reduction-oxidation process by the addition of iron powder and hydrogen peroxide (H2O2), over 90% Fe3+ and 40% SO42- could be removed beforehand by forming schwertmannite. The pretreated AMD drastically reduced both the required lime slurry in the subsequent lime neutralization process and the amount of neutralized sludge. The resultant schwertmannite had a specific surface area of 3.9 m2/g and a formula of Fe8O8(OH)4·5(SO4)1.75. This technique could reduce lime consumption, increase metal level in neutralized sludge, and harvest environmentally friendly materials. These results demonstrate that the integrated chemical mineralization and lime neutralization treatment is expected to be widely used in practical application.