The application of sandstone to reduce limestone armoring in acid mine drainage
Acid mine drainage (AMD) is a significant environmental problem in most countries that have an extensive history of mining. AMD refers to the acidic water that can be produced during hard and soft rock mining. Acidic drainage forms when sulfide minerals, primarily pyrite, are exposed to oxygenated surface waters. Passive limestone trenches are a common technique used to neutralize AMD through the dissolution of limestone and release of carbonate. The addition of carbonate to AMD causes the pH to rise due to the consumption of H+ and results in precipitation of iron from AMD. In limestone trenches, the iron precipitate can adhere to limestone surfaces, armoring them, and limiting their ability to continue to neutralize AMD. In turn, this armoring lowers the effectiveness of the passive trench as an AMD remediation tool. The goal of this research was to investigate if the presence of sandstone reduces the armoring of limestone during the AMD neutralization reaction, and, if armoring of the limestone is reduced, does the mineralogy of the sandstone play a role in this reaction. AMD neutralization batch reactions were run in which AMD was reacted with just limestone, limestone and iron oxide-cemented sandstone, and limestone and quartz-cemented sandstone. In the limestone only experiments, the AMD neutralized after ~4 days. However, in the experiments with limestone and sandstone, the AMD was neutralized in slightly less time. Additionally, light microscopy revealed that the presence of sandstone reduced the armoring of limestone grains. The experimental data suggests that the addition of sandstone did increase the rate of neutralization due to the iron precipitates adhering to the sandstone rather than the limestone. Also, quartz-cemented sandstone had a greater impact on neutralization reaction than did iron oxide-cemented sandstone.