A two-step chemical precipitation process involving hydroxide precipitation and
sulfide precipitation combined with "field separation" technology such as magnetic
separation, dissolved air flotation, vortex separation or expanded plastics flotation,
effectively removes chelated and non-chelated heavy metal precipitates and other
fine particles from water. In the first-step, the non-chelated heavy metals are
precipitated as hydroxides and removed from the water by a conventional liquid/solids
separator such as an inclined plate clarifier to remove a large percentage of the
dissolved heavy metals. The cleaned water is then treated in a second precipitation
step to remove the residual heavy metals to meet discharge limits. In the second
precipitation step, any metal precipitant more effective than hydroxide for metal
precipitation can be used. The invention improves metal removal, lowers cost because
fewer chemicals are used, produces less sludge, and reduces the discharge of toxic
metals and metal precipitants to the environment.
