The effect of secondary precipitates on the dissolution rate of calcite in AMD solutions

摘要

Acid mine drainage (AMD) is often reacted with limestone in order to neutralize acidity and increase the net alkalinity. During treatment secondary mineral coatings form on the limestone (armoring) and slow its dissolution rate reducing the effectiveness of this treatment. Investigations indicate that high sulfate concentrations in AMD can lead to the precipitation of gypsum on calcite surfaces. Experiments that measured the coupled reaction rate for calcite dissolution in conjunction with gypsum and/or iron hydroxide precipitation on calcite surfaces show that gypsum coatings may be the key player in armoring. These experiments show that calcite dissolution rates slow with time as calcite becomes coated with gypsum. Scanning electron microscopy shows the distribution and type of the coatings. The rate and coating data provide a basis for a quantitative model that can be used to manage the armoring problem.