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.