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-` 1 '( 1 1 1, :. ,. ~... -.. . ..~ .. . Page 10 d WAD and total cyanide to ferveis in the ranged 5-20 mg/I. By lowering the level of metals sand cyankfe requiring downstream treatment, lower effluent levels can be reached. . The Impoundment Into which the fallings are discharged should he designed to enhance cyanke degradation, thereby minimizing downstream treatment requirements. If downstream treatment of decant water is required prior to discharge to a surface water, conventional chemical or bidoglcal treatment should be 'employed as these processes are'best •suRed for this purpose. The use of chemical or bldoglcal treatment for slurries is unwarranted or imp~actlcal Inmost cases from both an environmental a fid economic standpoint. The next section details the approach taken by Cyprus Gdd to develop the first full-scale cyankfe recovery process for the tailings slurry at the proposed Gdden Cross Mine. Bench and Pilot Plant Scale Investigations A series d bench scale laboratory tests were first utilized In the study of cyankle recovery from slurries. A schematic d the laboratory apparatus Is presented on Figure 4. The slurry utilized was olptained from associated metallurgical tests. The chemistry of the fallings slurry and the expedmental results a je presented In Table 6. The pH of the slurry was lowered Into the proper range wfth sulfuric acid, utilizing a 2.0-hour stripping period, arxi reneutrallzatlon wfth Ifine to a pH of about 9.5 - t0.0. TABLE B CHEMISTRY OF TAILINGS SLURRY BEFORE AND AFTER CYANIDE RECOVEfgY Belore After C anfd Recove Parameter t't Cyanide Recovery Trial 1 Tr al 2 pH (In pH units) 10.4 9.7 1 d.2 Total Alkalinity (as CaCO,) 575 170 169 Total CN- 218 29.4 24.6 WAD CN- 213 7.4 8.8 Total Hardness (as CaCO,) 307 2170 203q Sulfate 360 2525 2350 SCN 34 37 38 A5 0.8 0.6 0.7 Ca 123 869 814 Cd <0.01 <0.01 <q.01 cr o.02 <0.02 <aoz Co 0.16 0.33 a30 CU 4.7 6.0 6t1 Fe 1.3 8.7 6.7 Pb <0.1 <0.1 <a1 Mn 0.01 0.02 0102 Hg 0.0155 0.0004 010001 Ag 0.15 0.04 0104 Zn 0.64 0.01 0106 Alr Flow (L/min) ------ 80 100 t'~ All values In mg/I, unless otherwise stated. These experiments completed al the Department of Science and Industrial Research (DSIR) Irv Wellington, New Zealand, were conducted to verify the ef(ectNeness of the process, but not to optimize chemical usage or obtain design Information. The results indicated that WAD cyanide levels of 218 mg/I could be reduced