PRECIPITATION OF HEAVY METALS WITH SODIUM SULFIDE: BENCH-SCALE AND FULL-SCALE EXPERIMENTAL RESULTS.

The results of an extensive investigation involving both laboratory-scale and full-scale sulfide precipitation behavior of heavy metals and arsenic are presented. The feasibility of a combination of hydroxide-sulfide precipitation (at pH 8-9) process, and a process involving sulfide precipitation (at pH 3-5) followed by lime precipitation, is established to achieve a high degree of separation of heavy metals and arsenic from smelter wastewaters. For precipitation of arsenic and zinc sulfide at pH less than 5, the side reaction between dissolved sulfur dioxide (if present in wastewater) and sulfide must be considered in process design. Hydrogen sulfide gas formation is never found to be a problem during precipitation. With the combination hydroxide-sulfide process highly effective separation of heavy metals and arsenic is obtained even at Na//2S dosage less than the theoretical demand. This work is pertinent to metal finishing.