Ultimate Guide for Gold Cyanidation Process

2019-03-29 20:33:33XinHai

Cyanide method has a history of more than 200 years since it was used in extracting gold and silver in 1887. With the high recovery rate, strong ore adaptability, fast extracting speed, cyanide method is still one of the main gold extraction methods.

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Gold cyanidation process refers to a kind of gold extraction process that adopting cyanide as the leaching liquid to extract gold, which is the main modern method of extracting gold from ore or concentrate.

Gold cyanidation process mainly include cyanide leaching, washing and filtration of leaching pulp, gold extraction from cyanide liquid or cyanide pulp and smelting of finished products.

Types of gold cyanidation process

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Gold cyanidation process can be divided into agitating cyanidation process and leaching cyanidation process.

Agitating cyanidation process is applied to treating the tailings after gravity separation, amalgamative tailings and flotation gold-bearing concentrate, or applied to full slime cyanidation. 

Leaching cyanidation process is used to treat flotation tailings and low-graded gold ore.

Among them, agitating cyanidation process mainly includes two types of gold extraction process.

One is cyanide-zinc replacement process (CCD method and CCF method), which uses zinc powder (silk) replace and recover gold after the continuous counter-current washing. 

Another is the all-slime cyanidation (CIP process and CIL process), which adopts activated carbon to absorb and recover gold from cyanide pulp directly without filtration and washing. 

Leaching cyanidation process consists of pool leaching process and heap leaching process, which infiltrates cyanide leaching solution first, then absorbs by activated carbon or replace by zinc powder (silk).


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Cyanide-zinc replacement Process (CCD method and CCF method)

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The working principle of cyanide-zinc replacement process is to use zinc powder (silk) as the reducing agent to replace gold and silver from the leaching liquid, which mainly includes four operations: washing (solid-liquid separation), purification, deoxidation,replacement.

1. Washing operation: Using thickener, multi-layer thickener or press filter to realize multi-stage counter-current washing, and separating the gold-loaded solution (pregnant solution) from the solid.

2. Purification operation: Adopting plate and frame filter, tubular filter for vacuum filtration and pressure filtration,which can remove the suspended matter in the pregnant solution, then make the pregnant solution of replacement operation become clear.

3. Deoxygenation operation: Applying the deoxygenation tower for vacuum deoxygenation, which can remove the dissolved cyanide in the pregnant solution.

4. Zinc addition operation: Using zinc disc feeder to add zinc and lead nitrate to the pregnant solution.

5. Replacement operation: Replacing the precious metal in the pregnant solution with zinc, then forming the gold(silver) mud.

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Carbon in pulp (CIP process)

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1. Pretreatment: 

Screening the coarse granular materials (such as sand) and wood chips before the adsorption, which is beneficial to the adsorption, separation of gold-bearing activated carbon and gold removal slurry, slow down the accelerated wear and the regeneration of activated carbon. Before sent to the adsorption tank, the activated carbon should also be pre-ground to remove sharp corners and edges. If not pre-ground, these fragments will be mixed in the demineralized slurry then causes gold loss.

2. Adsorption: 

Adding activated carbon to the fully leaching pulp, and the activated carbon will absorb the gold in the cyanide pulp then become the gold-bearing carbon. The whole adsorption operations carried out in the adsorption tank (carbon slurry tank). There are various types of adsorption tanks. For the slurry with fine mud, the low speed and centering adsorption tank should be adopted; For the coarser grained pulp, air-agitated tank should be used. Besides, the adsorption tank is often used in series in the production. After the adsorption is completed, the gold-bearing activated carbon and gold removal slurry will be separated thorough the screen installed in the carbon slurry tank.

3. Desorption: 

Desorption refers to make gold removal with the gold-bearing carbon that separated from the gold ore slurry. The Common desorption methods are atmospheric desorption and pressure desorption. The desorption operation is often finished in the desorption column.Adding the clean gold-bearing carbon into the desorption column, then using the solution (4% NaCN and 2% NaOH) immerse the carbon layer. Then, heating to 90 ℃~ 95 ℃ under the normal pressure or pressurizing with 2 ~ 4 hours, washing the gold with water. The desorption time is 12 ~ 24 hours. After desorption, the gold-rich desorption liquid and carbon are obtained.

4. Gold precipitation: 

Recovering the gold from gold-rich desorption liquid with electrodeposition method.

5. Regeneration and activation of desorption carbon: 

After the desorption, the lean carbon will be mixed into the new activated carbon in proportion and reused in the process.

At present, the common activated carbon used in gold production is mostly coconut shell carbon and apricot kernel carbon. When choosing the activated carbon, please consider the strength of activated carbon (that is, wear resistance), adsorption capacity, desorption and regeneration performance, selectivity and price, etc., of which the strength is the most important factor.

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Carbon in leach (CIL process)

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1. Pretreatment: 

Screening the coarse granular materials (such as sand) and wood chips before the adsorption, which is beneficial to the adsorption, separation of gold-bearing activated carbon and gold removal slurry, slow down the accelerated wear and the regeneration of activated carbon.

2. Preparation for pulp leaching: 

Adding the pulp into nine high-efficiency cyanide leaching tanks that arranged insteps. Cyaniding the gold pulp in the first two leaching tanks and carrying on counter-current adsorption in the last six-seven leaching tanks. (The number of leaching tanks depends on the processing capacity of the mineral processing plant).

3. Leaching and adsorption: 

Pumping the slurry into the high-efficiency cyanide leaching tanks that arranged in steps (usually five to eight tanks in series). Adding cyanide into the first tanks, and adding the activated carbon into the last five tanks. The pulp and activated carbon are counter-current adsorption, and the activated carbon becomes the gold-bearing carbon after adsorbing the gold in the pulp. When the adsorption is completed, gold-bearing pulp is sent to carbon screen through the air lifter, and the carbon screen separates the activated carbon and pulp.

4. Desorption electrolysis: 

Usually, the desorption of gold-bearing carbon and the electrolytic deposition of liquid gold are completed in a closed system. Under the high temperature and high pressure, the gold-bearing carbon can be rapidly desorbed into gold mud and lean carbon. The main equipment includes desorption column, pickling tank,heater, filter, desorption tank, electrolytic cell and so on.

5. Smelting: 

The gold ingots can be directly smelted as the gold sludge after simple pickling and impurity removal.By the hydrometallurgy, the purity of gold ingot can reach 99.99%.

6. Regeneration of activated carbon: 

After the desorption, the activated carbon firstly needs pickling to remove carbonate and other aggregates. Then, heating it for restoring the adsorption activity of carbon, which can be recycling. The main equipment includes activated carbon regeneration kiln, water quenching groove, fine carbon separation screen.

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Differences between CIP process and CIL process

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1. CIP process is longer than the CIL process, but the CIL process is featured with a large amount of carbon and low carbon concentration, so the amount of slurry transporting is several times (generally 4 times) of CIP process;

2. The amount of overstocked metal in the CIP process is relatively large, and the distribution of the amount of overstocked metal in the two processes is different. The distribution of overstocked metal on activated carbon is basically the same as that in solution in CIP process. But in CIL process, the metal mainly accumulates on the activated carbon, and the amount of gold in the solution also varies.

3. CIL process has a higher gold content than CIP process, which is determined by the process structure. The CIL process means absorb and absorb simultaneously. The new dissolved gold is continuously added into the solution, so the gold in the solution is constantly supplemented and the gold content in the solution is relatively high. The CIP process is a single adsorption, the adding amount of dissolved gold in the solution is very small, so the grade of gold is lower.

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Pool leaching process

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The main percolation leaching equipment is leachate tank, which includes tank, iron tank or cement tank. The bottom of the pool is horizontal and turn as slightly sloping, round, rectangular or square. The tank is fitted with a false bottom made of perforated acid-proof board, which is covered with a filter cloth. And the grid that made of wood or corrosion-resistant metal strips covers on the filter cloth. When leaching, loading the ore in the pool, and adding leaching agent at the top of the pool, so the leaching liquid flows out from the false bottom part, where is used for filtering and supporting the ore.

The leaching time of leachate tank is related to the dissolution rate of the solvent to mineral, and the penetration rate of the solvent in the material layer. The infiltration velocity mainly depends on the loading height, material porosity, silt content, the viscosity of leaching agent and material characteristics.

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Heap leaching process

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Heap leaching process means transfer the raw ore to the prepared yard, or directly stock on the waste rock or low-grade ore,then use cyanide leaching liquid to spray or infiltrate, and the ore reacts with the cyanide leaching liquid. After several recycling of leaching liquid and repeat spray, the leaching liquid can be collected. Then, treating leaching liquid by the activated carbon or metal zinc method and the barren solution returns to heap leaching operation for reuse.

Generally, heap leaching site is the gentle slope hills (natural grade 5-15 ˚) that nears the mining site. Removing weeds and lose material firstly, and then consolidate it as the foundation with 5 ˚ gradient.The sides are high and the middle is low, which makes leach liquid flow into the stationary tank easily. The yard is covered with geomembrane to prevent leakage. Around the storage yard, a 0.4-meter ridge of soil shall be built as aditch to prevent the rainwater from flowing into the yard. Besides, build a 0.3-meter-thick of lean ore before stacking the ore.

Heap leaching process has low production cost and it can be put into production quickly. In addition, the scale of heap leaching is more flexible, even more than tens of thousands of tons. Heap leaching or granulated heap leaching can be adopted after the ore is crushed to a certain size.

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Factors affecting gold leaching rate of gold cyanidation process

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1. Concentration of cyanide and oxygen

When leaching, gold (silver) dissolving capacity of cyanide (cyanide reagent) is: ammonium cyanide, calcium cyanide, sodium cyanide, potassium cyanide. Potassium cyanide is the most expensive cyanide reagent. Most gold cyanidation plants adopt sodium cyanide as the cyanide reagent. The cyanide consumption depends on material property and operating factor (often 20 to 200 times of the theoretical amount, concentration is generally 0.03% to 0.08%), the dissolution rate of gold increases linearly to the maximum with the cyanide concentration, then increases slowly. When the cyanide concentration reaches 0.15%, the dissolution rate of gold has nothing to do with the cyanide concentration, or even decreases (due to cyanide hydrolysis).Besides, the dissolution rate of gold increases with the increase of oxygen concentration, so the gold dissolution can be enhanced by oxygen-enriched solution or high-pressure aeration cyanidation.

2. Pulp pH value

When the pulp pH is less than 9.4, the hydrocyanicacid gas is resoled from the sodium cyanide. Usually lime is added to make the pH value stays 10-11 (protective base). When the pulp pH is more than 11.5, it is not good for gold leaching. Calcium oxide concentration in pulp is 0.01-0.05% and time content is 1-2 kg/t.

3. Raw material properties of gold mineral

Although gold cyanidation process is the main gold extraction method at present, there are still some raw gold minerals that are not suitable to be treated by gold cyanidation process. If the content of copper, arsenic, antimony, tin, sulfur, phosphorus, magnetite, pyrite and other components in the ore is high, the consumption of cyanide and the consuming of oxygen in the ore slurry will be increased greatly and the leaching rate of gold will be reduced. When the carbon content in the ore is high, the carbon will absorb the dissolved gold and lose with the tailings. Of course, the effect of harmful impurities can be removed by preoxidized roasting or flotation process.

4. Pulp temperature

Higher temperature, faster leaching speed, and the leaching speed reaches the maximum when the temperature is 85 ℃. But under the high-temperature, the consumption of cyanide is also large. Therefore, the temperature shall be larger than 15 ℃.

5. Size of gold grains

The size of gold grains dissolution mainly affects the cyanide time. If the gold grains is larger than 74 micron, the dissolution rate of gold will be slowed down. Therefore, it is reasonable to recover the coarse gold by amalgamation, gravity separation or flotation before cyanidation process. In the grinding process, make sure fine particle realize monomer separation fully is still an important factor to improve the gold leaching rate.

6. Slime content and pulp concentration

Slime content and pulp concentration directly affect the diffusion velocity of the components. In the zinc powder replacement process, the leaching concentration shall be 30-33%, the carbon paste shall be 40%. If the slime is too thick, the slurry concentration shall be less than 22~25%. But the concentration should be too low, otherwise the consumption of cyanide will be increased.

7. Condition of cyanide solution

Due to recycling, the ion accumulates, and the ability to dissolve gold is decreased.

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