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<br />Options for Residual Cyanide Degradation <br />Upon cessation of leaching activities, the leached materials <br />contain a measureible residue of cyanide compounds. The cyanide <br />exists in the form of moisture adhering to ore particles and/or <br />confined inside voids and fractures within the larger rocks or <br />agglomerated pellets of are fines. A variety of methods to <br />reduce the residual concentration either have been used or are <br />being considered for leached material. A few of these methods <br />are discussed below, in increasing order of probable <br />effectiveness. the mast effective reclamation method is highly <br />site specific, depending on factors such as ore type, climate, <br />leach solution composition, etc., and must be selected after <br />field tests. <br />Washing or Natural Purging <br />As noted above, the residual cyanide solution occurs as a <br />water content held in the rock by the capillary attraction of the <br />ore. The amount of such fluid varies with ore type, but is <br />frequently in the range of 15 to 20 percent by weight. This <br />"antecedent moisture" will be plug-flow displaced by recharge of <br />either natural prey=ipitation infiltration or artificial washing, <br />and will collect a:~ a saturated layer on the pad liner. The rate <br />of time required for such flushing depends on the use of either <br />natural infiltration or artificial raising, but frequently will <br />require 40 times or more the antecedent moisture content, based <br />on limited tests. The cyanide content of the initial displaced <br />water will be equivalent to the leach solution, and will remain <br />high with sutsequE~nt raises as a result of diffusion of cyanide <br />from the interior of ore particles. <br />Chemical Treatment <br />Same operators have attempted to reduce the amount of cyanide <br />displaced from a spent heap by raising with chemical agents such <br />as sodium hypachlorite, to react with the cyanide. Such chemical <br />agents are effetive in destroying "free" cyanide on the particle <br />surface, but will not degrade cyanide complexes such as <br />ferrocyanide, nor will they necessarily contact cyanide confined <br />inside the ore particles. Experience has shown an increase in <br />cyanide concentration of the leachate subsequent to initial <br />cyanide "kills", as the cyanide diffuses out of the are <br />fragments. <br />