TABLE OF CONTENTS Recovery of metal values and other potentially useful products from metallurgical wastes often requires chemical treatment. While physical separation (Chapter 3) is still the most desirable route as it requires no chemical input and, therefore could be easier for environmental control, as explained in Chapter 3, the application of those methods is limited to material where the components are liberated from each other and show significant differences between them in at least one of the physical properties exploited for the separation. Such conditions do not exist with many metallurgical waste, where the solids are very fine and do not differ significantly in physical properties. Chemical treatment or thermal treatment has to be used for such materials. The chemical treatment route is called hydrometallurgical processing, which will be discussed in this chapter. Thermal treatment route is called pyrometallurgical processing, which will be discussed in Chapter 6.
Precipitation of dissolved metal ions in water by the addition of a suitable precipitant, which form a sparingly soluble compound has been known and applied in many areas for several centuries. Most commonly, the precipitant could be an alkaline compound like sodium or calcium hydroxide to precipitate metallic hydroxides, a soluble sulfide like sodium sulfide. Other precipitants are a soluble sulfate to precipitate those metals (e.g., lead), which form sparingly soluble metal sulfates or a soluble chloride, which precipitates those metals which form sparingly soluble chlorides (e.g., silver and mercury as mercurous). Specific conditions have to be met to achieve selective precipitation...
