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'~ i - 1 THE RECOVERY OF CYANIDE FROM SLURRIES _ by ' T.I. Mudder and A.J. Goldstone INTRODUCTION . ' The use of cyanide for extraction of precious metals has been practiced for nearly a century, since h was first employed In New Zealand In 1898. Cyanldatlon Is an eHlclent process capable of extracting minute amounts of gold from complex chemical and mineral matrices. In theory, a sdution of 100 mg/I of free ' cyanide can provide the maximum rate and extent of gdd dissdution. In practice, concentra(ions of 200- 800 mg/I of total cyanide are often required for gdd and sliver recovery. The metal extraction process may Invdve eRher vat or heap leach, with recovery being agcomplished ' through zinc precipitation or activated carbon adsorption. The milling, extraction, and recovery processes produce waste streams Including barren solutions, tailings slurries, and excess decant water. qualitatively, the chemistry of these waste streams are similar containing a variety of constituents Including cyanide ' complexes, metals, and ln-organics (I.e.ammonia, thlocyanate, cyanate, sulfate, and nitrates). OUantitatively, the waste streams vary substantially exhibiting concentrations ranging over several orders oft magnitude. A summary of the chemical composftlon of various barren solutions and slurries are presentedi In Tabie 1. TABLE 1 ' CHEMICAL COMPOSITION OF BARREN SOLUTIONS AND TAILINGS SLURRIES ~- Parametef'r Range of Concentrations Arsenic <0.02 - 10.0 Cadmium <0.005 - 0.10 Chromium <0.02 - 0.10 Copper 0.10 -400.0 Iron 0.50 -40.0 Lead ~ <0.01 - 0.10 Manganese 0.10 - 20.0 Mercury <0.0001 - 0.05 Nickel 0.02 - 10.0 Selenium <0.02 - 6.0 Silver <0.005 - 2.0 Zinc 0.05 - 100.0 Total Cyanide 0.50 - 1000.0 WAD Cyanide 0.50 -650.0 Free Cyanide <0.01 - 200.0 Ammonia-N <0.10 - 50.0 Thiocyanate <1.0 - 2000.0 Hardness (as CaCO,) 200 - 1500 Sulfate 5 - 10000 Temperature (in °C) 0 - 35 Flow (In gpm) 1 -6000 r'r All values In mg/I, unless otherwise stated. ~I