Ore preparation and cyanidation before gold extraction
The first step in gold extraction is to properly pre-treat and cyanide the gold-bearing ore, which is a prerequisite for the subsequent coconut shell activated carbon to effectively adsorb gold. The process selection and treatment quality at this stage directly affect the efficiency and recovery rate of the entire refining process.Ore crushing
Ore crushing and grinding are the basic processes for gold extraction. Depending on the subsequent processing technology, there are different requirements for the ore particle size: for heap leaching, the ore usually needs to be crushed to a particle size range of 10-50 mm; for the carbon slurry method, the ore is required to be ground to a finer 300 mesh (about 0.05 mm) to increase the contact area between gold and cyanide solution. Modern gold mines often use multi-stage crushing (coarse crushing, medium crushing, fine crushing) and ball mill grinding process to ensure that the ore reaches the ideal particle size distribution.Cyanidation
Cyanidation is a key step in dissolving gold from the ore. In this process, gold reacts with cyanide to form a soluble cyanide gold complex, and its recognized chemical reaction formula is: 4Au + 8CN⁻ + O₂ + 2H₂O → 4[Au(CN)₂]⁻ + 4OH⁻. This reaction needs to be carried out under a strictly controlled pH environment, and the optimal pH value is about 10.3. If the pH value is too low, cyanide will volatilize in the form of highly toxic hydrocyanic acid, which will not only cause safety hazards but also reduce the dissolution efficiency of gold; if the pH value is too high, it may affect the subsequent adsorption performance of activated carbon.Requirements of different gold refining processes on ore particle size
| Process Type | Applicable Ore Size | Suitable Ore Characteristics | Process Features |
|---|---|---|---|
| Heap Leaching | 10–50 mm | Low-grade oxidized ore (typically <2 g/t) | Low investment, suitable for large-scale low-grade ores |
| Carbon-in-Pulp/Carbon-in-Leach (CIP/CIL) | ~300 mesh (0.05 mm) | All grades, especially applicable to muddy ores | High recovery rate (up to 93–94%), saves cyanide costs |
| Conventional Cyanidation and Zinc Replacement | ~200 mesh (0.075 mm) | Medium-grade concentrate | Mature process, but high requirements for solid-liquid separation |
The reagents used in the cyanidation process mainly include:
- Sodium cyanide (NaCN): the dosage is generally 0.4-0.5 kg/ton of ore
- Lime (CaO): used to adjust pH, the dosage is 0.25-3 kg/ton of ore
- Sodium hydroxide (NaOH): auxiliary pH adjustment, the dosage is 0.1-0.2 kg/ton of ore
The cyanidation time varies according to the nature of the ore and the process, and usually takes 12-48 hours. At this stage, in addition to gold, precious metals such as silver will also preferentially dissolve to form corresponding cyanide complexes. It is worth noting that due to the toxicity and environmental risks of cyanide, modern gold smelters must be equipped with complete environmental protection facilities, strictly treat wastewater and waste gas, and gradually explore cyanide-free or low-cyanide gold extraction technology as an alternative.
For different types of ores, the cyanidation treatment methods are also different. Oxidized ores are usually easier to handle and can be directly cyanided; while difficult-to-handle ores such as sulfide ores may require pre-oxidation (such as roasting, biological oxidation or high-pressure oxidation) to effectively cyanidate. In addition, for ores with high clay content, due to the difficulty in solid-liquid separation, the conventional cyanidation-zinc replacement process is not effective, and it is more suitable to use the carbon-in-slurry method to directly treat the slurry.
After the cyanidation treatment, the gold-containing solution (for heap leaching) or slurry (for carbon-in-slurry) is ready for the adsorption stage of coconut shell activated carbon. The gold is now present in the liquid phase as a [Au(CN)₂]⁻ complex, which is the key form for activated carbon to effectively adsorb.
The process of activated carbon for gold refining
In the gold refining industry, there are two main processes for coconut shell activated carbon to adsorb gold: heap leaching and carbon-in-slurry. Although both methods use the adsorption characteristics of activated carbon to recover gold, there are significant differences in process flow, equipment configuration and applicable conditions. The choice of which method mainly depends on factors such as ore properties, gold grade, investment budget and environmental conditions.
Heap leaching process
Heap leaching is a relatively simple, low-investment gold extraction process that is particularly suitable for processing low-grade oxidized ores (usually gold grades below 2 g/t). The core of this method is to use cyanide solution to leach the crushed ore pile to dissolve and enrich the gold, and then use coconut shell activated carbon to absorb the gold from the cyanide solution.The specific operation steps of heap leaching are as follows:
1. Ore preparation: The raw ore is crushed to a particle size of 10-50 mm and sometimes agglomerated to improve permeability. The crushed ore is piled on an impermeable liner to form a pile of ore with a height of up to 5-15 meters.
2. Cyanide leaching: A certain concentration of NaCN solution (usually 0.03-0.1%) is used to evenly elute the ore pile from top to bottom through a spray system. This process lasts for weeks to months until the gold content in the eluent reaches an economically recoverable concentration (usually 1-10ppm).
3. Activated carbon adsorption: The gold-rich cyanide solution (called precious solution) is collected and passed through a series of adsorption columns (usually 4-6) filled with coconut shell activated carbon. The cyanide gold complex [Au(CN)₂]⁻ is selectively adsorbed in the microporous structure of the activated carbon. The adsorption columns are usually arranged in a countercurrent manner, with fresh activated carbon first contacting the solution with a lower gold concentration, while the gold-loaded carbon contacts the solution with the highest gold concentration to maximize the gold recovery rate.
4. Lean solution treatment and circulation: The "lean solution" after activated carbon adsorption (gold content reduced to about 0.01ppm) can be supplemented with cyanide and lime and returned to the spray system to achieve reagent recycling, reducing costs and environmental burden.
The advantages of heap leaching are its simplicity and low investment cost, which makes it particularly suitable for small or low-grade deposits in remote areas. However, the method also has some limitations: the gold recovery rate is generally low (60-80%), it is greatly affected by climatic conditions (it may not be able to operate in cold areas in winter), and it occupies a large area. In addition, heap leaching is selective for ore type and is more suitable for permeable oxidized ores, while it is not effective for ores with high clay content or high sulfide content.
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Carbon pulp process flow
Carbon-in-Pulp (CIP) is a more advanced and efficient process in modern gold extraction. It is particularly suitable for processing medium to high grade ores (usually >1 g/t) and ores with high mud content, which are difficult to separate solid from liquid using traditional cyanide-zinc exchange process.The specific operating steps of the carbon-in-pulp process are as follows:
1. Ore preparation: The raw ore is crushed and finely ground, usually to a fineness of about 300 mesh (0.05 mm), to form a pulp. The grinding fineness is crucial to the gold recovery rate and needs to ensure that the gold particles are fully dissociated.
2. Cyanide leaching: The ground slurry enters a series of (usually 5-8) stirring tanks, where sodium cyanide and lime are added and cyanide leaching is carried out at pH 10-11. In the CIL process, activated carbon is added directly to the leaching tank; in the CIP process, carbon adsorption is carried out after cyanide leaching is completed.
3. Carbon adsorption: The slurry passes through a series of (usually 4-6) adsorption tanks, each of which is equipped with coconut shell activated carbon and a stirring device. The activated carbon flows in the opposite direction to the slurry - fresh activated carbon is added to the last adsorption tank (the lowest gold concentration), while the gold-loaded carbon is taken out from the first adsorption tank (the highest gold concentration). This configuration maximizes the gold recovery rate.
4. Separation of carbon and slurry: Each adsorption tank is equipped with a screen (usually about 28 mesh) to prevent the activated carbon from being lost with the slurry. The screen needs to be cleaned regularly with compressed air to prevent clogging.
5. Tailings treatment: The tailings slurry after adsorption is washed and treated with cyanide destruction (such as using hydrogen peroxide or SO₂/air method) and then discharged into the tailings pond, which meets environmental protection standards.
The gold recovery rate of carbon-in-slurry method is usually high, up to 90-95%. For example, Henan Linghu Gold Mine has achieved a total recovery rate of 93-94% by using carbon-in-slurry method. The main advantage of this method is that it eliminates the expensive solid-liquid separation process, and is particularly suitable for processing ores with high clay content and difficult to filter. In addition, the carbon-in-slurry method has good adsorption selectivity for gold and can effectively reduce the co-adsorption of impurity metals.
In actual industrial production, the adsorption performance of coconut shell activated carbon is crucial to the efficiency of both processes. High-quality coconut shell activated carbon should have a developed microporous structure (iodine value ≥ 900-1200mg/g), high mechanical strength (ball milling strength ≥ 98%) and appropriate particle size (usually 6-12 mesh or 8-16 mesh). These characteristics ensure the long-term stability of activated carbon in high cyanide and strong alkaline environments, as well as the ability to resist slurry abrasion, thereby reducing gold losses.
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