How to extract gold from oxidized gold ore?

Oxidized gold ore is a specific type of oxidized ore, in which the gold element mainly exists in the ore in an oxidized state. In oxidized gold ore, in addition to gold oxidized minerals, it may also contain oxidized minerals of other metals and some gangue minerals. As an important type of gold resource, oxidized gold ore accounts for about 30%-40% of the global gold reserves. Effective development is of strategic significance to alleviating the contradiction between gold supply and demand. This article will focus on the characteristics of oxidized gold ore, the difficulties in refining gold, and the technology of refining gold from oxidized gold ore.

01Characteristics of oxidized gold ore

Oxidized ore refers to a type of ore formed by oxidation of metal minerals in ore. According to the main metal oxide components, it can be divided into iron oxide type, manganese oxide type, aluminosilicate type, etc. Among them, iron oxide type oxidized gold ore is the most common, often accompanied by minerals such as hematite and limonite. Compared with non-oxidized ores, metal minerals in oxidized ores mostly exist in the form of high-valent oxides or hydroxides, and the bonding force between mineral particles is weak, resulting in different physical and chemical properties during the beneficiation process. According to the main metal oxide composition, oxidized ores can be divided into iron oxide type, manganese oxide type, aluminosilicate type, etc., among which iron oxide type oxidized gold ores are very common. Moreover, the occurrence state of gold in oxidized ores is complex and diverse, mainly including:

Free gold: exists in the form of natural gold particles, with particle sizes ranging from a few microns to hundreds of microns, usually accounting for 10%-30%;

Adsorbed gold: adsorbed on the surface of clay minerals (such as kaolinite, montmorillonite) or iron-manganese oxides in the form of ions;

Encapsulated gold: tightly wrapped by gangue minerals such as iron oxide and quartz, with fine particle size, often less than 10μm;

Chemically bound gold: forms solid solutions or compounds with other elements, such as gold-silver alloys and gold-tellurides.

02Difficulties in extracting gold from oxidized gold ores

The difficulty of extracting gold from oxidized ores stems from its complex mineral composition and gold occurrence state. On the one hand, a large number of clay minerals in the oxide ore are easily muddied during the grinding process, forming fine mud covering the surface of the gold particles, which hinders the contact between the leaching agent and the gold; on the other hand, impurities such as iron oxide and manganese oxide will consume the leaching agent and may adsorb gold-cyanide complexes, reducing the gold recovery rate. In addition, the organic carbonaceous matter contained in some oxide ores will adsorb the gold in the leaching solution, resulting in the "gold robbery" phenomenon. These factors make the gold extraction of oxide ores face technical difficulties such as low gold recovery rate, high reagent consumption, and complex process flow.

03Technology for gold extraction from oxide gold

The current technology for gold extraction from oxide gold ores is mainly divided into two categories: cyanidation and non-cyanidation. In addition, emerging technologies such as chlorination, bromination, and supercritical fluid extraction are also being explored and improved.

1. Cyanide gold extraction technology

Cyanide gold extraction is based on the oxidation and dissolution characteristics of gold in alkaline cyanide solution. In an aerobic environment, gold and cyanide react with each other, mainly gold generates a stable monovalent gold-cyanide complex under the combined action of oxygen and cyanide ions. This reaction needs to be carried out under alkaline conditions to prevent cyanide from hydrolyzing and producing toxic hydrogen cyanide gas. Common methods of gold extraction by cyanide include heap leaching, tank leaching, and stirring cyanide extraction.

Heap leaching gold extraction: suitable for processing low-grade (<1g/t) and large-scale oxidized gold ores. The process is to crush the mined ore to a suitable particle size (usually 10-50mm), pile up the ore on an impermeable cushion (such as a high-density polyethylene film), and pile up 3-10 meters high; then spray sodium cyanide solution on the surface of the ore pile through a spray system, and the concentration is controlled at 0.03%-0.08%. During the leaching process, the solution slowly penetrates through the ore pile by gravity and reacts with gold. The leaching cycle is usually 30-90 days. In order to improve the leaching efficiency, lime is often added to adjust the pH value of the ore pile to prevent cyanide decomposition. This method has a long leaching cycle, occupies a large area, and has a relatively low gold recovery rate.

Tannel leaching gold extraction: It is mostly used for medium-grade (1-3g/t) oxidized gold ores, and a stirred tank is used for leaching operations. In the stirred tank, the slurry and sodium cyanide solution are fully mixed through a mechanical stirring device, and air is introduced into the tank to provide oxygen for the oxidation and dissolution of gold. Usually the slurry concentration is controlled at 30%-40%, the cyanide concentration is 0.05%-0.1%, and the leaching time is 24-48 hours. Compared with the heap leaching method, the tank leaching method can better control the leaching conditions, and the gold recovery rate can reach 80%-90%, but the equipment investment and operating costs are also relatively high.

Stirring cyanide gold extraction: It is based on the tank leaching method and improves the dissolution rate of gold by optimizing key parameters. The stirring intensity needs to ensure that the slurry is fully mixed and has no precipitation, but too high agitation speed will increase energy consumption and equipment wear; the aeration volume needs to ensure that the dissolved oxygen content in the slurry is sufficient, generally maintained at 6-8 mg/L; the slurry temperature is controlled at 25-35°C. In addition, adding lime to adjust the pH value to 10-11 can not only prevent cyanide hydrolysis, but also inhibit the dissolution of other metal impurities and reduce reagent consumption.

2. Non-cyanide gold extraction technology

Non-cyanide gold extraction technology is a technology that avoids the toxicity risk of cyanide and achieves gold extraction through different chemical, biological or physical mechanisms, including thiosulfate method, thiourea method, biological oxidation method, halogenation method and lime sulfur method.

Thiosulfate method: Under alkaline conditions (pH 9-11), thiosulfate reacts with gold to form a stable complex. This reaction process requires copper ammonia complex as a catalyst. Copper ions play an electron transfer role in the reaction, accelerating the oxidation and complexation of gold, thereby realizing the transfer of gold from ore to solution. Generally, the concentration of thiosulfate is 0.1-0.5mol/L, the concentration of copper sulfate is 0.01-0.05mol/L, the concentration of ammonia water is 1-3mol/L, and the reaction temperature is 30-50℃.

Thiourea method: In an acidic sulfuric acid medium (pH1-2), thiourea forms a complex with gold. This process requires the participation of an oxidant (such as manganese dioxide, hydrogen peroxide). The oxidant first oxidizes the gold into an ionic state, and then the thiourea quickly combines with the gold ions to form a stable complex, so that the gold dissolves into the solution. This system has fast leaching speed, good selectivity, non-toxic and environmentally friendly, and can effectively leach gold wrapped in iron oxide. However, it is easily oxidized and decomposed under acidic conditions, and the corrosion resistance of the equipment is required to be high.

Biological oxidation method: Using mesophilic and acidophilic microorganisms such as ferrothiobacillus and thiothiobacillus oxidans, these microorganisms can oxidize sulfides (such as pyrite) and iron minerals in ores, and produce sulfuric acid and iron ions while obtaining energy. Sulfuric acid lowers the pH value of the system and creates an acidic environment; iron ions act as oxidants to oxidize low-valent iron, sulfur and other elements in the ore, while causing the sulfide structure that wraps the gold to be destroyed, exposing the gold and facilitating subsequent extraction by other methods.

Halogenation method: Under neutral or weakly acidic conditions, bromine in the sodium bromide-bromine system acts as a strong oxidant to oxidize gold into gold ions, which then combine with bromide ions to form a stable complex to achieve gold dissolution. This method has a fast reaction rate, and bromine and its compounds are relatively environmentally friendly, making it a potential non-cyanide gold extraction technology.

Lime sulfur method: Lime sulfur (a mixture of calcium polysulfide and calcium thiosulfate) will dissociate into polysulfide ions and thiosulfate ions in aqueous solution, which react chemically with gold to form a stable sulfur-containing complex, thereby leaching gold from the ore. This method has the characteristics of low raw material cost and environmental friendliness.

The above is an introduction to the technology of gold oxide refining. In actual ore dressing plants, the key to gold oxide ore refining lies in selecting appropriate pretreatment methods according to the characteristics of the ore to improve the gold leaching rate; optimizing the reagent concentration, temperature, pH value and other parameters in the leaching process; selecting efficient solid-liquid separation and gold recovery technology; and paying attention to environmental protection treatment to achieve green and sustainable development of the process.