REMOVAL OF HEAVY METALS, ARSENIC, AND FLUORIDE FROM SMELTER EFFLUENTS BY SULFIDE-LIME PRECIPITATION.

The paper reports on a study of precipitation of heavY metals, arsenic, and fluoride present in wastewaters generated in the non-ferrous metal production operations. Results of an extensive investigation involving both laboratory-scale and full-scale precipitation behavior of metals are presented. The decision to select a process to remove heavy metals and arsenic by Na//2S precipitation at low pH and post-precipitation of fluoride by lime at high pH is based on program objectives to precipitate As//2S//3, CuS, ZnS, HgS, CdS, etc. , without precipitating fluoride, making the compounds suitable for metal recovery; and to precipitate fluoride as CaF//2 (without heavy metal precipitates) for land-fill discharge. The precipitation of arsenic and Zn is found to be dependent on reaction pH whereas Hg, Cu, Cd, and Pb may be completely precipitated even at pH 3. Sulfide dosage control techniques and influent arsenic variation problems are also discussed.