Cyanide process

Strictly speaking, cyanidation does not belong to the category of dressing, but should be classified as a hydrometallurgical extraction of gold using cyanide is the world's leading method, the principle is to produce complex use of cyanide ions and free gold The effect is to generate gold cyanide, so as to achieve the purpose of dissolving gold. We know that gold in nature is in a free state. Even if it is wrapped in other minerals, it is still elemental gold after opening, so we say that the ability of cyanidation to raise gold (Recovery) In a sense, it depends on the opening of the parcel.
Generally, the cyanide gold extraction method we define as the beneficiation category is the carbon slurry method we often say. In fact, the history of carbon extraction by the carbon slurry method is not long. The first carbon slurry factory designed by our country was built in 1985. However, it is 30 years old. The "char" and "pulp" in the carbon slurry method refer to activated carbon and pulp, respectively, rather than the suspension of carbon in our concept. According to the different intervention time of activated carbon, we will call the first leaching and re-adsorption as the carbon slurry method (CIP), and the leaching and adsorption simultaneously called the carbon leaching method (CIL), which can be more effectively utilized due to the carbon leaching method. Equipment, so more and more cyanide plants use this approach.

Cyanide operation requires strict grinding fineness

For a typical gold-bearing oxide ore vein quartz, gold-bearing minerals because of poor floatability, generally by flotation beneficiation process is difficult to obtain a good index, the use of CIP process, under the premise of less harmful impurities It can basically reach the index of 93% or more of the total recovery rate.

But we said earlier that the ability to cyanide depends on the ability to open, so in the preparation of carbon slurry plant, grinding is particularly important. In the common carbon slurry plant in China, it is suitable for the fineness of grinding in cyanide operation. Generally, it reaches -0.074mm, accounting for 80-95%. Some mines with disseminated distribution even require grinding fineness of -0.037mm95% or more. It can be seen that it is difficult to achieve fine grinding in a carbon slurry plant. The degree required, generally requires two sections of grinding or even three sections of grinding.

It is very important to prevent the hydrolysis of cyanide

Since our commonly used cyanide (potassium cyanide, sodium cyanide, calcium hydride) is a strong base weak acid salt, it is prone to hydrolysis in water, which generates hydrogen cyanide volatilization and affects the concentration of cyanide ions in the slurry, so it is in leaching. In the process, it is also very important to prevent the hydrolysis of cyanide. The most effective way is to increase the concentration of hydroxide ions, which is what we usually call increasing the pH value. The most economical pH adjuster in the industry is lime. However, lime adjusts the pH value and is prone to flocculation, so we will add it to the grinding operation to make it fully dispersed.

Under normal circumstances, the pH value of the carbon slurry operation is better at 10-11.

Controlling pulp concentration is one of the keys to cyanidation

Whether it is gold and cyanide or gold cyanide and activated carbon, only good contact can achieve good beneficiation index, which puts higher requirements on the concentration of pulp. The concentration is too high, it is easy to precipitate on the surface of activated carbon, and the concentration is too low. It is easy to settle, and in order to maintain a suitable pH value and cyanide concentration, it is necessary to increase the amount of the agent. After many years of production practice, it is considered that the concentration of gold extracted by the carbon slurry method is 40-45%, and the concentration of cyanide is 300-500 ppm.

However, we said that grinding requires two to three stages of operation, and the final product concentration is generally below 20%. Therefore, before entering the leaching operation, the slurry needs to be densely operated.

Basic mechanism of cyanide leaching

From the above formula, we can find that the cyanidation process is an aerobic process, because the oxygen in the production can accelerate the leaching speed. Of course, we can also add oxidizing agents such as hydrogen peroxide, but the excess of oxidant will cause The cyanide formation of cyanide is also not conducive to leaching production.

From the formula, we can also find that 1 mol of gold only needs 2 mol of cyanide for complexation, which is converted into mass. About 1 g of gold requires 0.5 g of cyanide as the leaching agent, but in actual production, due to the influence of other minerals, for example Silver , copper , lead , zinc, etc., can also form a complexation reaction with cyanide. Therefore, the amount of the drug can not be limited to the calculation, more should be adjusted according to the final leaching rate, and the ore property should be tracked and adjusted when the ore property changes. . Usually 200-500 times higher than the calculated value is a reasonable range. All in all, ensuring the cyanide concentration in the slurry is a necessary condition for obtaining good indicators.

In order to ensure the stability of continuous operation, we usually use multi-stage leaching to obtain a smoother cyanide concentration, and the leaching time is the primary basis for determining the volume of the leaching tank, but the leaching time does not have a detailed calculation rule, so any charcoal The design of the plant must be based on experimental data. Blindly relying on experience will inevitably lead to production failure.

When the gold adsorbed on the gold-loaded carbon reaches 3000g/t or more, we believe that the adsorption process of the entire carbon slurry has been completed. Of course, some ore with high copper and silver impurities will also affect the adsorption capacity of activated carbon. The grade of gold charcoal does not reach our target. When activated carbon no longer has adsorption capacity, we think it is saturated. For saturated activated carbon, we need analytical and electrolytic energy to obtain gold. However, the post-operations are more complicated and the investment is larger. For areas with more concentrated gold production, we can sell gold-bearing carbon to obtain profits, or a simpler method can obtain the quality of gold through combustion.

After a preliminary understanding of the various processes of gold selection, we will conduct some in-depth analysis of gold ore dressing through specific examples, and welcome peers to provide case studies and joint research.

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