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Click to add WeChatIn the previous article, we introduced the content of gold ore dressing technology. In this article, we will continue to introduce the gold pretreatment of gold ore processing technology. Gold ore pretreatment is the process of treating gold ore by physical, chemical, biological and other methods before extracting gold. The purpose is to better separate gold minerals from other impurities, improve the leaching rate of gold, and reduce the difficulty and cost of subsequent extraction processes, and reduce the interference of impurities in the extraction process and the pollution to the environment.
Common gold ore pretreatment processes include bacterial preoxidation, roasting oxidation and chemical oxidation.
Bacterial preoxidation is a technology that uses microorganisms to pretreat difficult-to-treat gold ores, mainly relying on microorganisms such as Thiobacillus ferrooxidans and Thiobacillus thiooxidans. These microorganisms can oxidize sulfides, arsenides, etc. in ores, destroy the mineral structure of gold, expose gold, and thus significantly improve the recovery rate of gold.
During the bacterial pre-oxidation process, microorganisms adsorb on the surface of the ore and use their own metabolic activities to oxidize sulfides to sulfates and arsenides to arsenates. This process can not only remove harmful impurities in the ore, but also form a porous structure on the surface of the ore, increase the contact area between gold and the leaching agent, and improve the leaching efficiency of gold.
Bacterial pre-oxidation has the advantages of environmental protection and good selectivity. Compared with traditional chemical oxidation methods, it does not require the use of a large amount of chemical reagents, reducing environmental pollution. Microorganisms have high selectivity for sulfides and arsenides, and can effectively oxidize harmful impurities without affecting gold minerals. However, there are also some problems with bacterial pre-oxidation. The growth of microorganisms requires specific environmental conditions, such as suitable temperature, pH value and nutrients, and has high requirements for production equipment and operations. The cycle of bacterial pre-oxidation is relatively long, which will affect production efficiency.
Roasting oxidation is a pretreatment method that removes impurities in ore and improves the gold leaching rate through high-temperature roasting. Under high temperature conditions, impurities such as sulfides and arsenides in the ore are oxidized and decomposed, and the mineral structure that wraps the gold is destroyed, making it easier for the gold to contact the leaching agent.
The advantage of roasting oxidation is that it can effectively remove harmful impurities such as sulfur and arsenic in the ore and improve the leaching rate of gold. For difficult-to-treat gold ores with high sulfur and arsenic content, roasting oxidation can significantly improve the leaching ability of the ore. However, roasting oxidation also has some disadvantages. The roasting process consumes a lot of energy and is costly. A large amount of harmful gases, such as sulfur dioxide and arsenic oxide, will be produced during the roasting process, causing serious pollution to the environment, and complex tail gas treatment equipment is required.
Chemical oxidation is a pretreatment method that uses chemical reagents to oxidize sulfides in ores and improve the efficiency of gold leaching. Commonly used chemical reagents include strong oxidants such as potassium permanganate, hydrogen peroxide, and sodium hypochlorite. These reagents can oxidize sulfides in ores under certain conditions to expose gold, thereby improving the leaching effect of gold.
Chemical oxidation has the advantages of fast reaction speed and good oxidation effect. Compared with bacterial pre-oxidation, chemical oxidation is not restricted by microbial growth conditions and can complete the oxidation process in a shorter time. However, there are also some problems with chemical oxidation. The cost of chemical reagents is high, which will increase production costs. A large amount of chemical waste will be generated during the chemical oxidation process, which will cause certain pollution to the environment and need to be properly handled.
Gold mine pretreatment is an important link in improving the efficiency of gold extraction. Commonly used chemical reagents can be divided into several categories, such as oxidizing agents, adjusting agents, and leaching agents, according to their working principles and uses.
Oxygen and air: are common and low-cost oxidants, widely used in pretreatment processes such as roasting and pressurized oxidation. When roasting sulfur-containing gold ores, oxygen in the air can react with sulfides, oxidize them into sulfur dioxide, destroy the sulfide structure that wraps the gold, and expose the gold for subsequent extraction.
Hydrogen peroxide: It has strong oxidizing properties and can decompose to produce oxygen at room temperature, which can quickly oxidize certain reducing substances in gold ores. It is often used in chemical oxidation pretreatment, especially for treating some gold mines containing impurities such as arsenic and sulfur. It can oxidize arsenic into arsenate and sulfur into sulfate, thereby increasing the leaching rate of gold. Moreover, the product after the reaction is water, which is relatively friendly to the environment.
Potassium permanganate: It is a strong oxidant, and its oxidizing ability is affected by the pH of the solution. It can play an oxidizing role under acidic, neutral or alkaline conditions, but the oxidation products are different. In the pretreatment of gold mines, potassium permanganate can effectively oxidize sulfides and organic matter in gold mines and improve the leaching conditions of gold. However, its cost is relatively high, and byproducts such as manganese compounds will be produced after use.
Chlorine-based oxidants: including chlorine, sodium hypochlorite, chlorine dioxide, etc. Chlorine is a strong oxidant that can form substances such as hypochlorous acid with strong oxidizing properties in aqueous solution, which can oxidize a variety of substances in gold mines. Sodium hypochlorite has good stability, is easy to store and use, and is often used in gold mine pretreatment to oxidize and destroy the activity of carbonaceous materials, reduce their adsorption of gold, and improve the leaching effect of gold. Chlorine dioxide has high efficiency and rapid oxidation performance, and does not produce harmful substances such as organic chlorides, which is relatively friendly to the environment.
Acids: Sulfuric acid, hydrochloric acid, etc. are commonly used acid adjusters. Sulfuric acid is widely used in gold mine pretreatment. It can be used to adjust the pH of the pulp, making the pulp acidic, which is conducive to the conduct of certain oxidation reactions. In biological oxidation pretreatment, sulfuric acid can maintain the acidic environment required for microbial growth; hydrochloric acid can also be used to adjust the pH, and at the same time, it can dissolve some impurities such as metal oxides and carbonates to increase the exposure of gold.
Alkali: Sodium hydroxide, calcium hydroxide, etc. are common alkaline adjusters. Sodium hydroxide is often used to adjust the alkalinity of the pulp. Under alkaline conditions, some metal ions will form precipitation and be removed, and it is also conducive to the conduct of certain oxidation reactions. Calcium hydroxide is relatively cheap. It is often used to adjust the pH value of the pulp and precipitate heavy metal ions in gold mine pretreatment. It can also be used as a coagulant to promote the coagulation and sedimentation of particles in the pulp.
Salts: such as sodium sulfide, which can be used as a regulator and activator in gold ore pretreatment. Sodium sulfide can react with certain metal ions in gold ore to form metal sulfide precipitation, thereby removing impurities. At the same time, it can also activate some suppressed gold minerals and improve their flotation activity.
Cyanide: Sodium cyanide, potassium cyanide, etc. are traditional and widely used gold leaching agents. Under aerobic conditions, cyanide can react with gold to form a stable gold-cyanide complex, which dissolves gold into the solution. However, cyanide is highly toxic and extremely harmful to the environment and human health. Strict safety measures and environmental protection treatment are required when using it.
Thiourea: It is an important non-cyanide leaching agent that can form a stable complex with gold under acidic conditions to achieve gold dissolution. Thiourea leaching has the advantages of fast leaching speed and relatively low toxicity. It is suitable for treating certain difficult-to-select gold ores, especially gold ores containing carbonaceous materials.
Thiosulfate: such as sodium thiosulfate, can react with gold under alkaline conditions to form a gold thiosulfate complex to achieve the purpose of leaching gold. Thiosulfate leaching has the advantages of environmental protection and low corrosion to equipment, and is a promising non-cyanide leaching technology.
Gold mine pretreatment aims to improve the extraction efficiency of gold, but in actual operation, it will face a variety of difficulties, covering environmental pollution, cost control, process adaptability and other aspects. These problems restrict the effect and economic benefits of gold mine pretreatment.
During roasting pretreatment, sulfur- and arsenic-containing gold ores will release a large amount of harmful gases such as sulfur dioxide and arsenic oxides after high-temperature roasting. These gases will form acid rain, destroy the ecological balance, and endanger human health. Strong oxidants (such as potassium permanganate, hydrogen peroxide, etc.) used in chemical oxidation pretreatment will remain in tailings or wastewater if used improperly or not thoroughly treated afterwards, polluting soil and water. During the biological oxidation pretreatment process, microbial metabolism may produce acidic wastewater. If not properly treated, it will cause acidification of surrounding water bodies and soil, affecting the ecological environment.
In the grinding process of physical pretreatment, a large amount of electricity is consumed to fully dissociate the gold minerals, and the equipment wears quickly, resulting in increased production costs. The chemical reagents used in chemical pretreatment are expensive, such as cyanide in cyanide leaching method, which not only has high purchase costs, but also requires high costs for transportation and storage. The cultivation of microorganisms for biological oxidation pretreatment requires specific nutrients and a suitable environment, high equipment and maintenance costs, long processing cycles, and slow capital recovery.
Different gold mines have large differences in composition and structure, and a single pretreatment process is difficult to adapt to all gold mines. For example, a gold mine contains carbonaceous materials and sulfides, and it is difficult to solve the interference of carbon and sulfides at the same time by roasting alone. The pretreatment process has different requirements for equipment, and some processes require special equipment that is resistant to high temperature and high pressure. If the equipment is improperly selected or of poor quality, it will not only affect the pretreatment effect, but also pose a safety hazard.
This article focuses on three aspects: gold ore pretreatment technology, reagents and common problems. In actual ore dressing plants, how to determine and select gold pretreatment technology should be based on the properties of gold ore. Scientific experimental analysis should be carried out first, and a suitable gold ore pretreatment method should be designed based on actual conditions to improve the gold recovery rate.
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