According to a recent article by global consulting firm McKinsey, approximately one-quarter of the gold in geological reserves and resources can be considered refractory, with gold production from refractory ores expected to grow 1.4 % in the next four years, versus 0.3 % for non-refractory ores.
It has been suggested that refractory gold reserves, which require more sophisticated treatment methods in order to achieve oxide-ore recovery rates, correspond to 24% of current gold reserves and 22% of gold resources worldwide.
But what is refractory ore? A “refractory” gold ore is an ore that has ultra-fine gold particles disseminated throughout its gold occluded minerals. These ores are naturally resistant to recovery by standard cyanidation and carbon adsorption processes. A refractory ore generally contains sulphide minerals, organic carbon, or both, generally leading to a more complex – and more costly – processing option to remove the gold.
Optimizing gold recovery
Despite offering a higher grade, these ores can only be processed using specific pre-treatment methods such as ultrafine grinding, bio oxidation, roasting, or pressure oxidation (POX). This special treatment is required for two reasons: first, to liberate gold particles encapsulated in sulphide or arsenic minerals; and second, to eliminate carbonaceous material occurring in the ore, which adsorbs dissolved gold instead of active carbon that is normally added to the leaching solution.
According to leading global testing specialist, SGS, the problems that result from refractory gold are caused by:
- Physical encapsulation of disseminated gold particles (usually less than a few micrometres in size) within a mineral that is unreactive and impervious to leaching
- Chemical interference by one or more of the ore constituents in the cyanide leaching process
SGS says it is critical to understand how the gold occurs in the rock (gold characterization analysis). Is the gold standing alone? Is it physically surrounded or locked in another mineral? Or is it in solid solution substituting for another element? Can the gold be contacted by the cyanide leach solution?
It suggests that by understanding these questions is critical to optimizing gold recovery in flowsheet development and troubleshooting low gold recovery.
SGS itself has developed a series of testing and mineralogy assessment protocols that allow us to determine the character of refractory gold and the mass balance between refractory and free-milling gold.
Once the deportment or textural associations of the gold is determined and the mass balance known, flowsheet options are developed. Gold in sulphide minerals could be liberated by oxidization using techniques such as roasting, high temperature pressure leaching, or bacterial leaching.
Also, grinding the ore to an extremely fine particle size can liberate gold encapsulated in sulphides.
Refractory production poised for growth
According to the McKinsey article, approximately one-quarter of the gold in geological reserves and resources can be considered refractory, and most is located in regions with a long history of gold exploration and mining, as well as a lower investment risk, such as North America, Oceania, and the Commonwealth of Independent States (CIS).
The article noted that the additional processing steps required for treating refractory ores generate additional costs compared with conventional plants; however, the reserve grade for these ores is on average 86% higher than those of nonrefractory-type deposits (2.25 grams per metric tonne on average, versus 1.21 grams per metric tonne for nonrefractory ores).
According to the article’s authors, Gabriel Motta, Michal Polcyn, and Elijah Saragosa, production from refractory-type deposits in the near future is expected to grow at a higher rate than production from non-refractory ores. This production growth for refractory ores can be explained by analysing two main factors: costs and grade. They suggest significantly higher grades should continue to offset higher costs per metric tonne in the near future. Their analysis of recently developed and planned gold projects for refractory and nonrefractory ores found that:
- Capital costs per metric tonne of ore capacity are higher in refractory-ore projects. Construction of processing plants with POX circuits (the technology that recently became the most popular to treat difficult ore) requires approximately 48% higher investments compared to plants with regular tank-leaching processes. Recently constructed POX facilities in Russia and Turkey had a price tag of nearly US$1B, and the construction of other facilities, which are expected to cost more than US$2.5B, are still in the pipeline.
- Operational costs per metric tonne of processed ore are higher on average. Operational costs vary depending on the mining method and is notably 50% higher for open-pit refractory-ore projects. MineSpans data analysis shows that the increase in operational costs is primarily driven by higher consumables and energy costs.
- Due to their significantly higher grades, refractory ores yield costs per ounce that are frequently lower than the average costs for conventional ores. Mill-head grades of refractory deposits can be 86% higher; as a consequence, the operational costs per ounce of gold produced are approximately 19% lower in the case of refractory gold mines.
In order to generate the most value from refractory gold ores and prevent longer-term distress due to grade erosion, the McKinsey article identified three main areas for action that miners may consider:
- Diligent mine planning and plant design are crucial to keep capital expenditures (capex) on budget and ensure that operating expenditures (opex) stay in the expected range during the production stage. In order to decrease capex overspending, miners should pay extra attention to the plant design prior to construction. Identification of bottlenecks and overcapacities is crucial due to the many recirculation systems needed at the processing plant, and miners should also be mindful of proper material selection during plant construction in order to decrease downtimes caused by, for example, extensive material wear in the highly corrosive environment associated with autoclaves
- Digital technology will help optimize throughput and yield at the plant and mitigate costs arising from grade erosion. Custom-built artificial intelligence systems that use massive data generated by operations can significantly increase plant performance, as we have seen with clients that were able to achieve up to a 10% throughput increase and a 2% increase in recovery
- Inclusion of refractory processing plants in a miner’s asset list may unlock the value of refractory-type reserves in other locations. Miners could ship high-grade refractory ore or concentrate from several mines to centralized processing facilities designed to process difficult ores – an approach already employed by Russian miners Petropavlovsk Plc (LSE: POG) and Polymetal International Plc (LSE: POLY). Spare capacity at refractory-processing hubs can be used to process ore delivered from third-party miners that don’t have their own refractory-processing circuit
According to a recent article by Hong Qin, Xueyi Guo, Qinghua Tian, Dawei Yu, and Lei Zhang of the School of Metallurgy and Environment, Central South University, China, recovering gold from sulphide refractory gold ore is an important guarantee for the sustainable development of the gold industry in the future.