Evaluation of kinetic and diffusion phenomena in cyanide leaching of crushed and run-of-mine gold ores

Abstract

This paper compares the rates of pore diffusion and cyanide gold dissolution in coarse, porous gold oxide ore particles. Several size fractions of a gold oxide ore from a Nevada heap leaching operation were leached individually in a well mixed solution of constant pH (10.5, 11, or 12), constant cyanide concentration (50, 200, or 500 ppm), and constant temperature (5, 22, or 40 °C). Particles were coarse, ranging from − 1.7 mm (10 mesh) to 152 mm (6 in.). Leaching of − 1.7 mm (10 mesh) particles was complete in 24 h. The leaching kinetics was not influenced by the pH or the dissolved gold concentration. A model, which does not include the pore diffusion, as the latter was found to be extremely rapid in such small particles, predicts the conversion X as a function of time, cyanide concentration, and temperature: X = X max [ 1 − ( 1 + 1 , 140 , 000 exp ⁡ ( − 36 , 600 R T ) C NaCN 0.86 t ) − 2.1 ] . According to the data and the model, increasing temperatures and increasing cyanide concentrations were found to increase the leaching kinetics. Leaching of larger particles was much slower than − 1.7 mm (10 mesh) fines, requiring as many as 85 days to reach near complete gold extraction from particles 102 mm (4 in.) to 152 mm (6 in.) in size. In a static cyanide heap leach operation, this time period is very comparable to the time of irrigation of the uppermost lift. According to the diffusion/reaction model developed for the coarse particles, the advance of cyanide and oxygen into the pores of these particles was found to be slower than in the − 1.7 mm (10 mesh) particles, but still was much faster than the gold dissolution reaction itself. The outward diffusion of dissolved gold was also found to be very rapid. The precise measurements of the gold dissolution kinetics of coarse particles made in this study dispels, in part, the well-accepted hypothesis that solute diffusion rates control gold heap leaching of crushed and run-of-mine ores. The kinetic model that we have developed for coarse ore leaching in well mixed environments could also predict accurately the gold extraction in large columns. This evidence attests to the slowness of the dissolution of gold in coarse particles, unfortunately leaving crushing alone as the most effective option to shorten the leach cycle.