Analysis of Hydrated Lime Consumption in Circumneutral Underground Coal Mine Drainage Treatment

Abstract

Treatment evaluations were performed at four Pennsylvania hydrated lime treatment sites and one quicklime treatment site designed to remove ferrous iron from underground coal mine drainage. Two of the sites included a pretreatment decarbonation step to exsolve CO2(aq) and reduce hydrate consumption due to hydroxylation and calcite formation. Decarbonation reduced the daily hydrated lime dose by 22 % at one site and 28 % at the other. Field-measured CO2 mass transfer coefficients were determined for both decarbonation systems. CO2 mass transfer modeling predicted that Ca(OH)2 use would be reduced by an additional 19 and 28 % at these sites if the decarbonation systems were optimized. Hydroxylation of CO2 species and calcite formation consume between 40 and 90 % of the Ca(OH)2 dose. In terms of cost, more money is being spent on consumption due to hydroxylation and calcite formation than on removing the targeted parameter, ferrous iron. These processes can be minimized by improving decarbonation and oversizing ferrous reactor tanks to lower treatment pH.