A significant improvement of scheelite recovery using recycled flotation wastewater treated by hydrometallurgical waste acid

The research demonstrates an efficient, innovative and environmentally friendly technology which uses one waste (waste acid) to treat another (wastewater) and significantly improves the scheelite roughing recovery by 5% compared with the same periods of previous years in the winter times. A series of modelling exercises and laboratory experiments are conducted to understand and explain the improvement achieved at the plant in Luanchuan. The speciation modelling shows that at high pH, calcium ion hydrolyses to form calcium hydroxide which cannot react with silicate to create calcium silicate and prevent the silicate removal, and thereby affects the scheelite flotation. The experiments of precipitation and sedimentation, FFIR and SEM have confirmed the prediction. Specifically, hydrochloric acid contained in the waste acid reduces pH and decreases the hydrolysis of calcium and formation of calcium hydroxide. The calcium chloride in the waste acid can precipitate silicate as calcium silicate which effectively absorbs fine solid particles in the wastewater and increases their settling velocity. The tailings provide the carrier for the precipitation, increase their settling velocity and compress the sediment bed. At the plant, the new process using the waste acid removes 60-65% of silicate from the recycled water from the tailings dam which is frozen in winter, leading to the inefficient removal of silicate being harmful to the scheelite flotation. (C) 2017 Published by Elsevier Ltd.