Kinetics of natural degradation of cyanide from gold mill effluents

Abstract

<p>Most Canadian gold mining facilities utilize the cyanidation process in which cyanide is added to sequester gold from the ore. After zinc addition, gold is precipitated from a gold-cyanide complex. Waste streams from the process generally contain a sufficiently high concentration of cyanide and heavy metals that treatment is essential.</p> <p>The oldest treatment method practiced by Canadian gold mines for cyanide destruction is "natural degradation".</p> <p>Based on the literature review of the previous studies, the most important mechanism in the natural degradation of cyanide was recognized as being volatilization. Among the variables affecting the volatilization process the three most important were selected for this study, namely: temperature, UV light and aeration rate. The experimental pH chosen was 7.0. In order to evaluate the relative importance of these variables a full 2³ factorial design was employed and appropriate experiments conducted over a period of one year.</p> <p>The synthetic solutions examined were simple cyanide (NaCN), four single metallo-cyanide complexes (Cu, Zn, Ni and Fe) and two mixtures - a "low mix" containing a low concentration of metals relative to the total cyanide concentration and a "high mix", saturated with metals.</p> <p>Analysis of the results of the experimental design led to the conclusion that temperature had the largest effect upon reaction kinetics. The rate of aeration together with temperature had a significant effect upon the volatilization rate within the first 48 to 72 hours. Ultraviolet irradiation only had a significant effect upon the decay rate of the iron cyanide complex and low mix.</p> <p>A mathematical model for the degradation of a single metallo-cyanide complex solution was postulated for a batch reactor taking into consideration the law of conservation of mass, and assuming that chemical equilibrimn existed between hydrocyanic acid (HCN) and cyanide ion (CN-). The rate of volatilization of HCN and metal decay rate were the mechanisms assumed to be controlling the cyanide degradation process.</p> <p>The best estimates of the metal decay coefficients from single metallo-cyanide solutions used to simulate the experimental data of the mixed metallo-cyanide solutions are as follows:</p> <p>[Figure Removed]</p> <p>The model fits the data at the 95% confidence level for five of the eight test conditions; however, for the other three it is poorer.</p> <p>The model, calibrated using coefficients from the synthetic solutions, has been applied to actual gold mill effluents. Results show that the model can be used for the basic estimate of the cyanide degradation rate. For a more precise result, a recalibration of the model is necessary as a function of the particular gold mill effluent's metal content.</p>