Decarbonation of Limestone-Treated Mine Drainage

Crushed limestone is used in various processes to neutralize acid mine drainage in a 2.5 mgd (9500 m³/day) facility near Hazelton, Pennsylvania. A 0.5 mgd (1900 m³/day) stream was treated by two limestone packed tumbling drums in series to increase pH from 3.7 to 5.4, followed by a lagoon for carbon dioxide removal that further increased pH to 6.1, to meet the pH 6 effluent quality target. Decarbonation process theory and design procedures are proposed. Graphs show pH increase versus carbon dioxide removal and carbon dioxide removal versus time of treatment and the overall film transfer coefficient. Nomographs also show how carbon dioxide removal depends on air-to-water ratio, air-water contact time, bubble or droplet diameter (for diffused air and spray decarbonation processes respectively), and the turbulence-dependent film transfer coefficient. Use of these nomographs is illustrated by calculation of the film transfer coefficient from experimental data.