An Integrated Biotechnological Approach to Gold Processing 

    This paper reviews the state of the art in processing and extraction of gold. The ore bodies which were considered uneconomical at one time are becoming economical due to new and advanced methods of extraction. The paper discusses the gold treatment methods on free milling ores with conventional cyanidation and refractory ores with direct and pretreatment techniques for the recovery of high gold values.

    In the extraction aspect, the paper discloses two different extraction schemes on treating refractory ores, namely pretreatment followed by gold leaching and direct leaching. Pretreatment process involving roasting chemical oxidation and bio-oxidation have been discussed. Direct leaching of gold ore processing such as heap leaching, carbon in pulp (CIP), carbon in leach (CIL) and resin in pulp (RIP) are summarized. This paper also dicloses in a detailed manner the research approach on the development of alternative leach reagents which could improve environmental concerns as compared to the use of cyanide.

    Special emphasis of the review is focussed on the technical and economic guidelines for developing a small gold mine on the basis of capital and operating cost analysis.

    Gold miners are facing a reserves crisis, and what is left in the ground is becoming more and more challenging to process. Refractory gold reserves, which require more sophisticated treatment methods in order to achieve oxide-ore recovery rates, correspond to 24 percent of current gold reserves and 22 percent of gold resources worldwide (Exhibit 1). Despite offering a higher grade, these ores can only be processed using specific pretreatment 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 sulfide 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 MineSpans analysis, 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). It is important to note 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 percent higher than those of nonrefractory-type deposits (2.25 grams per metric ton on average, versus 1.21 grams per metric ton for nonrefractory ores).

    Our analysis shows that, in the near future, production from refractory-type deposits is expected to grow at a higher rate than production from nonrefractory ores (Exhibit 2). This production growth for refractory ores can be explained by analyzing two main factors: costs and grade.

    Our analysis of recently developed and planned gold projects for refractory and nonrefractory ores found that:

    Capital costs per metric ton 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 percent higher investments compared to plants with regular tank-leaching processes (Exhibit 3). Recently constructed POX facilities in Russia and Turkey had a price tag of nearly $1 billion, and the construction of other facilities, which are expected to cost more than $2.5 billion, are still in the pipeline.

    Operational costs per metric ton of processed ore are higher on average. Operational costs vary depending on the mining method and is notably 50 percent 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 (Exhibit 4).

    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 percent higher; as a consequence, the operational costs per ounce of gold produced are approximately 19 percent lower in the case of refractory gold mines.

    Thus, according to MineSpans data, 54 percent of gold production from refractory deposits comes from mines situated in the bottom half of the cost curve, while only 18 percent sits in the fourth quartile (Exhibit 5). This high-grade effect is expected to remain in place at least until 2023, but grade erosion should dampen it over time.

    In order to generate the most value from refractory gold ore cip extraction plant and prevent longer-term distress due to grade erosion, we see three main areas for action that miners should 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. A suboptimal plant design for a specific ore can quickly erode the benefits of higher-grade refractory ores through reduced recovery or the inability to approach nominal capacity. 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 percent throughput increase and a 2 percent 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 and Polymetal International. 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. Another benefit of having a refractory-ore processing plant is the transfer of know-how and experience between different operations.

    Miners can tap into the opportunity for lower costs per ounce offered by higher-grade refractory gold reserves. However, when considering this potential, they need to factor in higher capital and operational costs. The winners will be those that pay close attention to optimizing mine design, maximizing plant performance via digital technology, and asset-portfolio planning.

    The role of biotechnology in different facets of gold extraction metallurgy is illustrated with respect to biogenesis of gold ore processing plant, biobenefication, bioliberation and bioenvironmental control. The use and commerical potential of this technology are discussed with reference to the mining and processing of Hutti gold ores. A microbial survey of the Hutti gold mines revealed the ubiquitous presence of various autotrophic and heterotrophic bacteria, fungi and yeast of relevance in gold processing. The possible roles played by different indigenous microorganisms in the formation, conversion and transport of gold along with various associated minerals have been brought out. Similarly, the role of Thiobacillus ferrooxidans in the flotation beneficiation of gold-bearing sulphides and in enhancing gold recovery from refractory sulphide ores and concentrates has been demonstrated. Direct gold solubilization could be achieved by Bacillus spp. Various fungi and yeast were found to be useful in the biosorption of gold and other base metals from cyanide effluents. It could thus show that biotechnology could be beneficially utilized in different stages of precious metal processing spanning from mining to waste disposal.

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    Open AccessReview

    A Review of the Cyanidation Treatment of Copper ore Separation and Concentrates

    by Diego Medina * andCorby G. Anderson

    Department of Mining Engineering, Kroll Institute for Extractive Metallurgy Colorado School of Mines 1500 Illinois St., Hill Hall 337, Golden, CO 80401, USA

    *

    Author to whom correspondence should be addressed.

    Metals 2020, 10(7), 897; https://doi.org/10.3390/met10070897

    Received: 21 May 2020 / Revised: 13 June 2020 / Accepted: 23 June 2020 / Published: 5 July 2020

    (This article belongs to the Special Issue Advances in Mineral Processing and Hydrometallurgy)

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    Abstract

    Globally, copper, silver, and gold orebody grades have been dropping, and the mineralogy surrounding them has become more diversified and complex. The cyanidation process for gold production has remained dominant for over 130 years because of its selectivity and feasibility in the mining industry. For this reason, the industry has been adjusting its methods for the extraction of gold, by utilizing more efficient processes and technologies. Often, gold may be found in conjunction with copper and silver in ores and concentrates. Hence, the application of cyanide to these types of ores can present some difficulty, as the diversity of minerals found within these ores can cause the application of cyanidation to become more complicated. This paper outlines the practices, processes, and reagents proposed for the effective treatment of these ores. The primary purpose of this review paper is to present the hydrometallurgical processes that currently exist in the mining industry for the treatment of silver, copper, and tantalum ore separation, as well as concentrate treatments. In addition, this paper aims to present the most important challenges that the industry currently faces, so that future processes that are both more efficient and feasible may be established.

    The history of modern hydrometallurgy started with the discovery of how to obtain gold and silver from ores, on 19 October 1887, by John Steward MacArthur, who was recognized for having established the application of the cyanidation process. Gold production around the world readily doubled as a consequence of cyanidation’s initial application within the mining industry. Following the first application of cyanidation in the recovery of gold, the hydrometallurgical industry has developed and grown according to the needs of the process and the mineral complexity of the ore deposits.

    Hydrometallurgical processes can be defined as the leaching of a desired metal into a solution, followed by the concentration and purification of the pregnant solution, and finally, the recovery of the metal or its compounds. The processing of gold and silver ore by leaching is one of the most prominent examples of early hydrometallurgy-based processes.

    Most of the gold extraction from ore is accomplished by the implementation of an alkaline cyanide leaching process. The chemical recovery of gold can be defined by two different operations: the oxidative dissolution of gold and the reductive precipitation of metallic gold from the solution. Cyanide is one of the most attractive lixiviants in the current industrial gold leaching process. During gold cyanidation, silver and copper are commonly present within the solution, which causes their metal ions to react with the cyanide (CN?), thus forming complexes [1].

    Cyanide is considered to be a hazardous compound because of its toxicity; there is currently environmental pressure by different groups around the world to ban the industrial use of cyanide. Research on replacing cyanide as a lixiviant has been ongoing over the years, and has found that there are other potentially workable compounds, such as thiosulfate, thiourea, halides, various sulfide systems, ammonia, bacteria, natural acids, thiocyanate, nitriles, and combinations of cyanide with other compounds [1]. Many of these alternative gold processes are still in the early development stages. A key factor for the commercial success of these alternative lixiviants relates to the overall stability of the lixiviant and the gold complex in solution.

    Currently, the mining industry faces the problem of separating these complex valuable minerals from the ore in which they reside. This paper outlines various options that hydrometallurgical processes offer for the treatment of these complex minerals, containing precious metals such as Cu, Ag, and Au.


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