Abstract
Limestone (calcite) is the most common acid-neutralizing and alkalinity-generating material used in passive mine water treatment systems. Calcite’s solubility and dissolution kinetics can limit its utility in passive systems, especially for mine waters containing high concentrations of Fe2+ and Mn2+. Dissolved carbon dioxide is an important controller of calcite dissolution in natural and mine water treatment systems. The carbonation of mine water should increase the generation of alkalinity in limestone-based passive treatment systems. This project investigated the effects of mine water carbonation on alkalinity generation by passive limestone systems.
Mine waters were carbonated with varying amounts of CO2 and incubated in limestone systems for varying amounts of time. Carbonation was provided with in-line nozzles commonly used in conventional environmental applications and a membrane system used in industrial bottling operations. Carbonated waters were exposed to limestone aggregate in pilot units containing 10 – 25 tonne aggregate and in operational passive systems containing 300 – 1000 tonne limestone.
Carbonation of mine water increased both the extent and rate of alkalinity generation. At the Orcutt site, mine water containing 103 mg/L Fe and 40 mg/L Mn was directed through experimental containers containing 25 tonnes limestone aggregate. Treatment without carbonation resulted in 250 mg/L alkalinity after 8-12 hours of theoretical retention time (TRT). Carbonation with a nozzle system resulted in 250 mg/L alkalinity after 3-4 hours of TRT and 350 mg/L alkalinity after 9 hours of TRT. Carbonation of operational limestone systems produced similar results. The Howe Bridge anoxic limestone drain (300 ton aggregate) received 75 L/min of water containing 130 mg/L Fe and 21 mg/L Mn. Without carbonation the limestone bed discharged water with 184 mg/L alkalinity and net acidity of 85 mg/L. Nozzle carbonation of the influent resulted in discharges from the bed containing on average 276 mg/L alkalinity and net acidity of -25 mg/L. Testing was also done at the Orcutt site using a membrane-based carbonator and a 10 tonne experimental system. The membrane carbonator produced 525 mg/L alkalinity after only 2 hour TRT. The membrane system also provided more efficient transfer of CO2 than the nozzle-based system.
The research shows that calcite solubility limitations can be modified through the carbonation of mine water using existing carbonation technologies. Mine water carbonation can expand the potential applications for limestone-based mine water treatment systems while substantially decreasing the sizing of the limestone beds.