The effect of softening recycled water on coal flotation

Abstract

Mineral processing plants usually operate with water in a closed circuit mode. For this large quantities of water have to be recycled from various unit operations continuously. To compensate natural losses small quantity of fresh make up water is added into the circuit. As a result of recycling, a gradual build up of residual concentrations of flotation reagents (frothers, collectors, coagulates) takes place in the water and the concentration levels of the contaminants increases. The recycled water contains higher amounts of dissolved components and hardness than the fresh water, due to the recirculation. The use of recycle water in flotation has significant effect on the selectivity of the flotation process since the dissolved compounds, both inorganic as well as organics alter the chemistry of the system radically. It has been observed that flotation of coal has resulted into nine to thirteen units drop in yield compared to DM water. This may be due to the many of these ions are acting as depressants. The presence of certain alkali earth metal ions (e.g., Ca ++, Mg ++) could adversely affect flotation as they precipitate sparingly soluble hydroxyl complexes on the coal surface and change its surface charge. The effectiveness of the flotation process can be increased by reducing the dissolved ions in the recycled water through softening process. By softening process the total hardness of the recycle water can be reduced from 743 ppm to 8 ppm. In softening process the concentrations of alkali earth metals were also replaced by sodium ions. It has been observed that the flotation yield increases by more than five units by treating the recycled water to the softening process. If this softening process is included in the coal preparation plant there could be significant improvement in the flotation yield and conservation of the natural resources.