The influence of mineralogy and surface chemistry on flotation of Cortez complex carbonaceous double refractory gold ore

Abstract

The recovery of gold from complex refractory gold ores is challenging. The refractory nature of these ores is due to the encapsulation of gold as colloidal gold particles or as gold in solid solution in a mineral matrix which is unreactive and impervious during the cyanide leaching process. The complexity of processing increases when both the presence of disseminated gold in the mineral matrix as well as the interference phases are present in the ore. The ore investigated in this research, from Barrick’s Cortez mine in Nevada, USA, is a notable example of this type of complex refractory ore which is described as double-refractory due to the presence of gold in a sulfide mineral matrix as well as in carbonaceous matter in the ore.Double-refractory ores give low recoveries of gold during the cyanide leaching process, and a pre-concentration stage is required to achieve acceptable gold recoveries. Typically, flotation is the pre-concentration method employed to recover the gold-bearing mineral phases in the ore. Although the flotation response of Cortez double refractory gold ore has been assessed at a number of commercial laboratories, gold recoveries remain lower than expected, and the reasons for this are not well understood. This thesis examines the influence of mineralogy and surface chemistry on the flotation recovery of gold in the Cortez double refractory gold ore. A comprehensive, novel experimental approach that combines solution analysis, optical microscopy, SEM-based automated mineral analysis (MLA/QEMSCAN), Raman spectroscopy, Dynamic secondary ion mass spectrometry, ToF-SIMS and X-ray photoelectron spectroscopy analysis on flotation feed, concentrates (both carbonaceous matter (TCM) and sulfides concentrates) and tailings has been applied to identify the causes of low gold recovery in flotation.The results of the research have shown that contrary to the expected deportment of gold in Carlin-type deposits as invisible gold associated with sulfide minerals, the gold in this Cortez ore is predominantly visible gold, present as native gold grains ranging in size from 2 to 33 μm with an average of 10μm. This is the main cause of the low gold recovery observed for this ore when treated with the standard Cortez ore flotation reagent regime which is designed to recover gold-bearing pyrite. The xanthate collector used in the standard Cortez reagent regime would not selectively recover grains of native gold. Exploratory flotation tests with a collector that specifically targets native gold (mercaptobenzothiazole and dialkyl dithiophosphate (MBT+DTP)) showed improved gold recovery. A mixture of mercaptobenzothiazole and dialkyl dithiophosphate (MBT+DTP) and PAX could potentially improve gold recoveries.The gold loss to the tailings stream is 35% of the total gold in the flotation feed and the majority of this unrecovered gold in the tail stream is present as visible grains of native gold (87%) with only 13% present as invisible gold in the pyrite. The visible gold is mainly liberated (69%) and locked with goethite/silicates/carbonates (26%) with only 5% of the visible gold locked with pyrite and therefore potentially recoverable in the flotation process which targets sulfide mineral recovery.The ore characterisation performed in this research has shown that the main reason for the low recovery of gold from this Cortez ore is the presence of the majority of the gold as visible grains of native gold. The Cortez gold deportment is significantly different to that typically observed in other Carlin-type ores where the majority of the gold is present as invisible gold associated with pyrite, and it is this unexpectedly different gold deportment that results in its poor recovery in a flotation process which specifically targets the recovery of sulfide minerals. Improved recovery is likely if the flotation conditions (including types and dosages of flotation reagents) are optimised to recover liberated gold grains.