Carbonation of Mine Water to Increase Limestone Dissolution and Alkalinity Generation

Conventional methods of treating contaminated mine waters with lime (Ca(OH)2 and CaO) and caustic (NaOH) are expensive to construct and operate. Limestone (CaCO3) is 1/8 to 1/40 the cost of lime and caustic, respectively, and is commonly used in passive mine water treatment systems. The generation of alkalinity from limestone is limited by calcite solubility. Where mine waters are contaminated with >100 mg/L divalent metals (Fe, Mn, Zn, Cu), limestone is typically not able to generate sufficient alkalinity for complete treatment. This project seeks to use carbonation in increase alkalinity generation from limestone dissolution and expand the applicability of limestone treatment. Hedin Environmental demonstrated the technical feasibility of this technology in 2017 with a carbonation experiment at the Elizabeth Mine Superfund site. Carbonation quadrupled alkalinity generation from a pilot scale limestone bed and produced net alkaline water that could meet discharge limits with only aeration and solids settling. This SBIR Phase I project builds on this experiment to identify low-cost methods of carbonation and develop alkalinity generation kinetics and treatment cost estimates in a series of bench tests followed by 5,000 gallon field tests. CO2-enhanced limestone dissolution technology is applicable to thousands of coal and metal mines in the US where water is contaminated by high concentrations of metals such as Fe, Mn, Zn and Cu. This technology would benefit any organization that treats polluted mine water including federal and state agencies, non-profit organizations, and private companies. In the next decade, reclamation agencies in the eastern US plan to spend more than $100 million on new mine water treatment plants at coal mine sites. Initial calculations suggest that CO2 enhanced limestone dissolution is less expensive than lime and caustic technologies. This project seeks to develop the reliability, sizing, and costs of this disruptive water treatment technology. Using cost effective CO2-enhanced limestone dissolution will benefit the Nation's environment by increasing the amount of mine water that can be cost-effectively treated. There will be carbon benefits because limestone has a much lower carbon footprint than lime and caustic. There will be aquatic ecosystem benefits because bicarbonate alkalinity produced from a limestone system is environmentally benign compared to the hydroxyl alkalinity produced by lime and caustic systems. In sum, the technology could be a cost-effective alternative to conventional treatment projects while also providing ecological and environmental benefits.