Modelling pyrite oxidation in a refractory gold ore stockpile to estimate the gold recovery via direct cyanide leaching – A case study

Abstract

This paper presents a case study of the modelling of the natural oxidation of pyrite in a refractory gold ore stockpile at a mine on the island of Aniolam in Papua New Guinea to predict the gold recovery through direct cyanide leaching. The stockpile comprises low-grade material that was mined twenty years ago to access the orebody. The value of this low-grade resource could be increased if the processing cost could be reduced. The natural oxidation of the gold-bearing pyrite in the stockpile may reduce the processing cost by reducing or eliminating the need for pre-treatment of the material and enable gold recovery via direct cyanide leaching. Depending on the oxidation level and the gold recovery that could be achieved by direct cyanide leaching, reclamation of the stockpile may be more profitable if the pyrite oxidation stage could be fully or partially bypassed. In this paper, we describe the modelling of the pyrite oxidation level in the stockpile using the numerical model reported in our previous work. The characteristics of the stockpile used in the model are the stockpile geometry, the geothermal heat underneath the stockpile, and the properties of the different types of material, including the acid neutralization capacity, rock particle size distribution and pyrite grain size distribution. Limited onsite test results were used to calibrate the model and the final oxidation level over the stockpile was estimated using the calibrated model. Based on the simulation results, it was found that the accessibility to oxygen is the limiting factor for the level of pyrite oxidation when diffusion is the dominant mechanism for oxygen transport within the stockpile. However, experimental tests on samples showed that sufficient oxygen resupply should have been available in the stockpile and therefore there may be air movement mechanisms other than diffusion and thermal-induced air convection onsite. This study also demonstrates that the numerical model can be used to estimate the gold recovery via direct cyanide leaching from the simulated oxidation profile together with the gold distribution in pyrite grains and gold fractions in pyrite of different sizes.