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Ir I f ~' Page 10 - of WAD and total cyanide to levels In the range of 5-20 tng/I. By lowering the level Of metals and cyankle requiring downstream treatment, lower effluent levels can be reached. The Impoundment Into which the tailings are dlscftarged should be designed to enhance cyanide degradatkxf, thereby minimizing downstream. treatment requirements. If downsVeam treatment of decant water Is required prior to discharge to a surface water, conventional chemical or bidogical treatment should be 'employed as These processes are'best -suRed for this purpose. The use of chemical or bidogical treatment for slurries Is unwarranted or Impractical In most cases from both an environmental and economic standpoint. The next section details the approach taken by Cyprus Gdd to develop the 9rst full-scale cyanide recovery process for the tailings slurry at the proposed Gdden Cross Mine. Bench and Pilot Plant State Investigations A series of bench scale laboratory tests were first utilized In the study of cyanide recovery trop slurries. A schematic d the laboratory apparatus is presented on Figure 4. The slurry utilized was obtained from associated metallurgical tests. The chemistry of the tailings scurry and the experimental results are presented In Table fi. The pH d the slurry was lowered Into the proper range with sulfurk acki, utilizing a 2.0-hour ~ stripping period, and reneutrailzatlon with lime to a pH of about 9.5 - 10.0. TABIF 8 CHEMISTRY OF TAILINGS SLURRY BEFORE AND AFTER CYANIDE RECOVEIaY ~:> . -. Before After C vanide Recover~_ Parameter t't Cvanide Recovery Trial 1 Trial 2 pH (in pH units) 10.4 9.7 1(1.2 Total Alkalinity (as CaCO~ . 575 . _ - - . 170 -. _ 16.4 _ .+ Total CN- 218 - _ .. 29.4. 2a.6 -r. - . WAD CN- 213 7.4 E..B Total Hardness (as CaCO,) 307 2170 2030 Sulfate 360 2525 2350 SCN 34 37 3E• As 0.8 0.8 C .7 Ca 123 869 Bta Cd <0.01. <0.01 <C.Ot -, -. ., ' .. Cr ..... -: 0.02 .... ._ . .. <0.02 <[.02 Co 0.16 - - 0.33 C.30 Cu 4.7 - 6.0 E.1 . _ Fe 1.3 8.7 ; : E.7 - Pb <0.1 <0.1 <0.1 Mn 0.01 0.02 0.02 Hg 0.0155 0.0004 0.0001 Ag 0.15 0.04 0.04 Zn 0.64 0.01 0.06 Alr Flow (L/min) ---- 80 100 t'1 Ali values In mg/I, unless otherwise stated. These experiments completed at the Department of Science and Industrial Research (DSIR) in Wellington, New Zealand, were conducted to verify the effectiveness of the process, but not to optimize the nical usage or obtain design Information. The results Indicated that WAD cyanide levels of 218 mg/I could be reduced