The processing mineralogy of refractory gold ores reveals that the reasons behind the hindrance of cyanidation of gold are primarily due to the state of gold occurrence and the mineral composition. These reasons can be categorized into two major types: physical encapsulation and chemical interference.
Physical encapsulation refers to gold being finely disseminated or encapsulated in other primary minerals, making it highly dispersed and difficult to extract. The main host minerals encapsulating gold are pyrite and arsenopyrite, followed by sulfides of copper, lead, and zinc. While encapsulated gold is less commonly found in quartz and sulfates, its recovery from quartz and silicates remains economically unviable.
This type of refractory gold ore is the most significant and well-studied, with considerable research focused on finding effective solutions. Notably, the main host minerals such as pyrite and arsenopyrite, which encapsulate gold, are also key factors in causing chemical interference.
Chemical interference occurs when substances in the ore consume cyanide and oxygen or adsorb gold, thereby hindering the cyanidation process. Specific types of chemical interference include:
1. Sulphide Minerals: Various sulfide minerals in gold ores consume cyanide.
2. Oxygen-Consuming Minerals: Minerals that consume oxygen during decomposition.
3. Carbonaceous Materials: Substances that adsorb dissolved gold complexes, causing "preg-robbing" phenomena similar to activated carbon.
4. Protective Films: Minerals like arsenic, antimony, and lead that dissolve to form compounds or colloids, creating protective films on gold particles, hindering extraction.
5. Insoluble Compounds: Gold present in insoluble compounds or forms.
6. Passivation: Gold dissolution is passivated when in contact with other conductive minerals.
Among these, high-arsenic, high-sulfur, and high-carbon sulfide ores are the most common and challenging globally.
To enhance the treatment of refractory gold ores, several methods can be employed:
1. Mechanical Methods: Breaking down encapsulating materials to liberate gold.
2. Pretreatment Before Cyanidation: Oxidizing and decomposing primary minerals to release encapsulated gold and remove interfering components. Techniques include oxidative roasting, pressure oxidation, and bacterial oxidation.
3. Non-Cyanide Leaching Methods: Avoiding the adverse effects of interfering substances by using alternatives such as thiosulfate or thiourea leaching.
4. Enhanced Cyanidation: Improving the cyanidation process through methods like pressure cyanidation, addition of oxidants, or using chemicals to neutralize harmful components.
In recent years, the number of gold mines adopting these treatment technologies has increased rapidly. However, oxidative roasting, pressure oxidation, and bacterial pre-oxidation remain the most common methods in research and practical applications.
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