High solids density gypsum production through an improved neutralization process for zinc plant effluent

A common wastewater treatment process practiced by zinc production facilities is the single-stage mixing of acidic wastewaters with slaked lime, inducing the reactive precipitation of fine (∼1 mum) gypsum (CaSO4.2H 2O) and other solids with a solids density less than 10%. These solids report to a tailings pond for containment.Tailings pond life would be increased if the solids density of the precipitated solids was improved. Previous work at McGill University suggested that a staged neutralization process with solids recycle and seeded with gypsum would produce large-sized gypsum crystals with a high solids density. A continuous lab-scale process run with synthetic zinc plant effluent produced large (∼100 mum) gypsum crystals with a solids density of 50 +/- 3%.Meissner's method of calculating mean activity coefficients allowed for the calculation of gypsum solubility in mixed, strong sulphate electrolyte solutions