5. Economic Aspects of Solid-Liquid Separation

Some aspects of this have already been mentioned. These are typically rather slow processes. Capital costs are therefore high. Rates of removal of water tend to decrease rapidly as the water content decreases (law of diminishing returns again). Going from 1% to 0.1% water may take as long as going from 50% to 1% did. Likewise the same idea applies to removing suspended solids from a solution. The more clear the liquid needs to be, the greater the time and effort required. Sedimentation processes have moderate electricity requirements. Flocculating reagents may be quite costly, but they tend to be added at low to moderate concentrations. Drying costs may be quite high since high amounts of energy are required to evaporate water. This is not commonly employed in hydrometallurgy.

The highest solid-liquid separation costs are encountered just after leaching of fine solids in a reactor (tank or autoclave etc.). After initial separation to obtain a leach solution, the solids must be washed and then dewatered again to recover valuable metals in solution. The leach solution itself must be clarified to a high degree (very low suspended solids content) for it to be suitable for metal recovery. This may involve considerable capital cost. Good lab testing is always required. Generally the cost of solid-liquid separation increases with decreasing particle size. It is preferable to avoid having to conduct solid-liquid separations on very fine particles.