Near-technical limit gold recovery from a double refractory Carlin-type ore after pretreatment by high-temperature pressure oxidation

A recent Nevada Gold Mines research program focused on the pressure oxidation (POx) pretreatment of a Carlin-type double refractory ore from the Turquoise Ridge Mine. The research team found that both arsenian pyrite and carbonaceous matter were highly oxidized within an hour at 300 °C using a bench-top autoclave reactor and that the gold recovery from the residues after cyanide/carbon-in-leach (CIL) approached the technical limit set by the amount of gold encapsulated by silicate minerals. Although the 60-min pretreatment at 300 °C yields robust performance when treating a whole ore, copper additives can catalyze the oxidation rate of carbonaceous matter and reduce the residence time needed at elevated temperatures. Certain gangue minerals are susceptible to dissolution in acidic solutions at 300 °C, and this may cause poor gold recovery. This recovery challenge is mitigated by keeping free acid in the discharge solution below a key threshold. Introduction Nevada Gold Mines and FLSmidth’s Minerals Testing and Research Center based in Salt Lake City, UT, developed an acidic POx program to oxidize more than 80 percent of carbonaceous matter in Nevada Gold Mines’ Turquoise Ridge Stockpile O ore. The goal was to achieve these oxidation levels within 20 min, the expected practical limit for high-temperature, high-pressure plug-flow reactor designs. Background Subcritical water oxidation of carbonaceous matter. Existing studies for similar ores achieved carbonaceous-matter oxidation as high as 40 percent when treating a 20-μm feed at 225 °C for 120 min. Gold recovery averaged 86 percent on the representative composite sample evaluated. Copper sulfate was also tested as a catalyst for carbonaceous-matter oxidation, which enhanced results as well [1]. Flotation concentrates have also been processed by POx at 240 °C with a residence time of less than 360 min [2]. Mass reduction by flotation was key in keeping the POx circuit capital cost within reason. Characterization, results and discussion Characterization and diagnostic leaching exemplified the refractoriness of the ore. Baseline POx-CIL tests with complete oxidation at 225 °C showed significant refractoriness, resulting in 71 percent gold recovery. Preg-robbing characterization tests then identified a preg-rob value of 84 percent, which again indicates high levels of gold loss by the carbonaceous matter. Problematic gangue minerals, such as muscovite, were also identified that were determined to play a critical role in recovery during testing. Results concluded that approximately 95 percent of the gold should be recovered if all gold associated with sulfides and carbonaceous matter are recovered. Considering carbonaceous-matter oxidization was the greatest challenge and focus of the program, detailed characterization of the carbonaceous matter was undertaken with Surface Science Western (SSW), a consulting and research laboratory. Various morphologies and compositions of carbonaceous matter were found to be present in the ore. The average submicroscopic gold concentration of all the carbonaceous matter, irrespective of morphology, was 45 ppm gold (Au) and was positively correlated with iron (Fe), arsenic (As) and sulfur (S) concentrations in the carbonaceous matter. This correlation suggests that the gold carrier was microfine arsenian pyrite encapsulated within the carbonaceous matter. Consequently, there was a need to oxidize the carbonaceous matter to liberate the encapsulated gold to achieve the highest gold recovery. Fig. 1 Comparison of the carbonaceous-matter morphologies against the Raman ratios of standard materials. Raman ratio (RR) studies were also performed to benchmark the carbonaceous matter within the ore against known RRs for graphitic and activated carbon. The results in Fig. 1 show sizable proportions of carbonaceous matter in the ore, plotting close to activated carbon. This distribution in RRs supports an overall view that the carbonaceous matter in the ore was strongly preg-robbing. Fig. 2 Carbonaceous-matter oxidation by dual-catalyst POx/CIL at 300 °C versus uncatalyzed baseline performance. Fig. 3 Carbonaceous-matter oxidation by dual-stage POx at 300 °C versus single-stage baseline performance. Developmental test work. Early testing of the ore established optimized testing conditions at elevated temperatures up to 300 °C with a 20-μm feed to enhance carbonaceousmatter liberation. Coarser feeds (70 μm) were also tested with success but reaction kinetics were slower. Batch roasting tests were conducted to compare high-temperature POx results for which similar levels of carbonaceous-matter oxidation resulted in percent CIL recoveries in the mid-80s. Initial phases of high-temperature POx testing revealed several significant findings. Gold recovery was highly variable, regardless of having successful sulfide and carbonaceous- matter oxidation, and appeared to be correlated to the extent of pre-acidulation prior to POx. Muscovite dissolution followed by alunite precipitation coincided with preacidulation trends. Alunite generation during POx was also found to be positively correlated with the final oxidationreduction potential, Eh, of POx solutions. SSW found that the residue morphology differed between acidulated and nonacidulated feeds. Acidulated feeds had elevated gold loaded on aluminum-contained hematite and gold colloids within residual carbonaceous matter, suggesting that the gold-halide effect was occurring [3]. Maintaining a POx discharge-free acid value under 10 g/L was paramount in reducing alunite formation and also the Eh to mitigate any gold-halide effect. Gold recovery approached the technical limit of 96 percent after implementing optimized test conditions from these findings, but residence time was higher than the objective. Focused test work — dual catalyzation. Reduction in residence times were then achieved by dual-catalyst testing, which added copper sulfate. Figure 2 shows the synergy of having both ferric and cupric (Cu2+) ions available to catalyze oxidation of carbonaceous matter, reducing overall residence times in half compared to uncatalyzed 300 °C testing [4]. Focused test work — dual stage. Dual-stage POx tests were evaluated as well to investigate the advantage of having higher levels of ferric ions present at time zero of the carbonaceous- matter oxidation stage [4]. The dual-stage POx concept first treats the ore in an existing 225 °C autoclave for 60 min to oxidize more than 95 percent of the sulfides, followed by a carbonaceous-matter oxidation stage at 300 °C. When compared against single-stage treatment, Fig. 3 shows that dual-stage treatment also reduced the residence time needed at 300 °C in half for equivalent amounts of carbonaceous- matter oxidation. Carbonaceous-matter oxidation by dual-catalyst/dualstage POx. Based on these encouraging results, the team investigated a coupling of dual-catalyst and dual-stage concepts to see if combining the two concepts could further reduce the residence time for carbonaceous-matter oxidation [4]. The dual-catalyst/dual-stage POx achieved more than 90 percent carbonaceous-matter oxidation after 60 min of sulfide oxidation at 225 °C and 10 min of carbonaceous-matter oxidation at 300 °C with 1 g/L Cu2+ added as a catalyst. Gold recovery was also close to the technical limit. Conclusions Nevada Gold Mines’ phase 1 program proved that acidic POx treatment of the ore at temperatures as low as 300 °C could oxidize more than 80 percent of the carbonaceous matter within 20 min and approach the ore’s technical limit of 95 percent gold recovery. Free acid content and solution Eh were found to negatively impact gold recovery when not controlled. A dual-catalyst/dual-stage flowsheet led to reduced retention times and may be required for a plug-flow reactor application. Understanding the primary mechanism further for gold loss when the ore was acidulated ahead of POx could create a more favorable environment for ferric iron, enhancing the oxidation kinetics of carbonaceous matter and potentially making copper catalyzation unnecessary. Selected references Simmons GL (1996) Pressure oxidation process development for treating carbonaceous ores at Twin Creeks, Randol Gold Forum ’96, pp 199–208 POLYMETAL International PLC (2019) POX Workshop, February, p 18 Dimov SS, Hart BR (2021) TR POx residues analysis report, SSW Analysis Report: 19521SD.NGM, pp 1–89 Dyson D et al. (2021) High-Temperature POx R&D testing – phase 1, FLSmidth Project Report, pp 1–25

Near-technical limit gold recovery from a double refractory Carlin-type ore after pretreatment by high-temperature pressure oxidation

Authored by:
Stamen Dimov, Devy Dyson, Brian Hart, John Langhans, Steve Yopps

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