The method of extracting gold from ores is determined by the type and properties of the ore. Generally, gold ores are categorized into two types based on their adaptability to cyanidation: easily leachable ores and difficult-to-leach ores. Difficult-to-leach gold ores are those that cannot be effectively leached using conventional cyanidation methods, even after fine grinding. Some authors define difficult-to-leach gold ores as those with a cyanide leaching recovery rate of less than 80% after fine grinding. In English, "refractory gold ores" can also be translated as "difficult-to-process gold ores," "difficult-to-leach gold ores," or "recalcitrant gold ores," but the term "difficult-to-leach gold ores" is the most accurate based on its definition.
There are multiple reasons why some gold ores are difficult to leach, encompassing physical, chemical, and mineralogical factors. These reasons can be summarized into five main categories:
Gold particles are often finely disseminated or submicroscopic within sulfide minerals (such as pyrite, arsenopyrite, and pyrrhotite) or silicate minerals (like quartz). They can also be present within the crystal lattice of sulfide minerals. Such encapsulated gold is difficult to liberate even with fine grinding, preventing contact with cyanide during the leaching process.
Ores often contain sulfide and oxide minerals of metals such as arsenic, copper, antimony, iron, manganese, lead, zinc, nickel, and cobalt. These minerals have high solubility in alkaline cyanide solutions, consuming significant amounts of cyanide and dissolved oxygen, and forming various cyanide complexes and thiocyanate (SCN-). This negatively affects the leaching process. The most important oxygen-consuming minerals are pyrrhotite, marcasite, and arsenopyrite, while the most significant cyanide-consuming minerals are arsenopyrite, chalcopyrite, bornite, stibnite, and galena.
During ore oxidation, the surface of gold particles in contact with cyanide pulp may form films such as sulfide films, peroxide films (e.g., calcium peroxide film), oxide films, and insoluble cyanide films. These films cause surface passivation of gold, significantly reducing the oxidation and leaching rates of gold particles. When sulfide minerals are present in the ore, the dissolution of gold can be hindered in various ways. One explanation is that soluble sulfides (S2- or HS-) produced by mineral dissolution can react with gold to form a sulfide film, passivating the gold surface. Another theory is that a dynamic reduction couple forms on the sulfide surface, leading to the formation of a dense cyanide complex film on the gold particles, thus passivating them.
Ores often contain carbonaceous materials (such as activated carbon, graphite, and humic acid) and clays that can adsorb gold. These materials can preferentially adsorb gold-cyanide complexes during cyanide leaching, causing a "robbing" effect, which results in gold losses in the cyanide tailings and severely impacts gold recovery.
In some ores, gold exists in the form of tellurides (such as calaverite, sylvanite, and krennerite), solid solution silver-gold minerals, and other alloys that are slow to react in cyanide solutions. Additionally, minerals such as aurostibite, black bismuthinite, and gold-humic acid complexes are also difficult to dissolve in cyanide solutions.
Y&X's popular product YX500 gold leaching reagent is an environmentally friendly alternative to the highly toxic sodium cyanide, effectively addressing nearly all of sodium cyanide's drawbacks. YX500 has already achieved industrial production and application. The developed "combined leaching" and "on-site cleaning" technologies ensure the standard discharge of tailing pond sludge while maintaining high gold leaching rates.
The main advantages of YX500 are:
1. Environmentally friendly with low toxicity, ensuring safer transportation, usage, and storage.
2. As a common chemical product, it can be transported by sea, rail, or road, significantly reducing transportation costs.
3. Can directly replace sodium cyanide without altering any existing leaching processes.
4. Offers faster leaching speed compared to sodium cyanide, reducing production cycles by 30%, saving labor, reducing costs, and conserving water.
5. Exhibits good stability and increased carbon adsorption capacity, effectively enhancing the adsorption capacity of activated carbon and increasing recovery rates.
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