Factors affecting heap leaching rate

There are many factors affecting the heap leaching rate, which can be generally divided into two categories, one is the factor affecting the chemical reaction rate, and the other is the factor affecting the mass transfer rate of the material. Many of these factors affect both the rate of chemical reactions and the rate of mass transfer and mass transfer. For example, the temperature of the medium has an influence on the diffusion rate of the substance and on the rate constant of the chemical reaction. Another factor, such as ore particle size, affects both chemical reactions in heap leaching and diffusion and mass transfer of materials. For the same ore, the smaller the particle size, the more fully the mineral dissociation, the larger the area of ​​the chemical reaction will promote the chemical reaction rate; on the other hand, for the same ore, the smaller the particle size, the shorter the pore path in the ore. Obviously it also contributes to the mass transfer and mass transfer.

I. Oxidizer

From the thermodynamic discussion of uranium , gold and copper , it is known that oxidants have a great influence on the leaching reaction rate of uranium, gold and copper ore. When selecting an oxidant, in addition to considering the reduction potential of the oxidant to meet the leaching conditions, it is also necessary to seriously consider whether the introduction of the oxidant will affect the subsequent treatment process and whether there is pollution to the environment. In actual production and application, the physical and chemical properties of the oxidant itself, storage and transportation conditions, especially the price and other factors need to be demonstrated in detail. Therefore, in the experimental study, it is necessary to compare several oxidants to have practical conclusions and appropriate choices.

In the actual production of heap leaching uranium, gold, copper ore, the most commonly used oxidant is air (essentially using oxygen in the air). Only when conditions require, the only additional add other oxidants, such as high iron (Fe ^{3 +)} salts, chlorate, soft manganese ore.

Experimental studies have shown that the gold cyanide leaching reaction is an electrochemical process, or electrochemical corrosion process. This includes the oxidation of gold on the anode in the presence of CN ^{- to} form cyanoic acid complex ions:

Au+2CN ^- →Au(CN) ₂ ^- +e

On the cathode, oxygen acquires electrons and water to form hydroxide ions:

O ₂ +2H ₂ O+4e→4OH ^-

In the above two reaction processes, due to the polarization, the cathode potential becomes negative, the anode potential becomes positive, and finally the potential difference is zero. Assuming that under the conditions of diffusion limit, and the cyanide concentration in the solution is sufficiently high, the anode dissolution rate of gold is v=2DSC/δ, where D is the diffusion coefficient of oxygen, S is the anode area, and δ is the thickness of the diffusion layer. C is the oxygen concentration in the immersion liquid.

Under the above conditions, the dissolution leaching rate of gold depends on the oxygen concentration in the leaching solution. Many studies have shown that the higher the oxygen content in the leaching solution, the greater the leaching rate of gold, and vice versa; when the oxygen concentration is very low, increasing the concentration of cyanide is ineffective. Therefore, it is an important measure to accelerate the leaching by measuring the dissolved oxygen content in the solution at any time during the heap leaching to ensure the oxidation conditions required for the reaction.

It is well known that dilute sulfuric acid or carbonate cannot dissolve and leach UO ₂ if no oxidant is present. In uranium heap leaching, the importance of oxidizing conditions is second only to the concentration factor of leaching agent. The most important oxidant for acid uranium heap leaching is Fe ^{3 +} , generally without adding other oxidants, by leaching the iron produced by itself. The oxygen in the air maintains the proper ratio of Fe ^{3 +} /Fe ^{2 +} in the leachate to oxidize UO ₂ in the ore.

The effect of the ratio of ferric iron to divalent iron on the dissolution rate of uranium dioxide is shown in the table below.

Effect of Fe ^{3 +} /Fe ^{2 +} ratio on dilute uranium leaching

Leaching conditions: temperature 28 ° C, time 18 h. The sulfuric acid concentration was 4 g/L.

It is generally believed that the ratio of Fe ^{3 +} /Fe ^{2 +} reflects the potential of the Fe ^{3 +} /Fe ^{2 +} electrode in the leaching solution, and the ratio of the ore that is difficult to treat is reduced, and the leaching effect on uranium is particularly significant. Figure 1 shows the effect of Fe ^{3 +} on the dissolution rate of crystalline uranium ore.

Fig.1 Effect of Fe ^{3 +} concentration on dissolution rate of crystalline uranium ore

As can be seen from the figure, increasing the amount of Fe ^{3 +} is advantageous for the dissolution rate of crystalline uranium ore. This increased rate of dissolution of the reaction described is a surface leaching, adsorption on the mineral surface of the Fe ^{3 +} amount determines the rate of dissolution of the leaching reaction. Therefore, increasing the amount of Fe ^{3 +} in the leaching solution is an important condition for uranium ore heap leaching.

But also it suggested that, when the same amount of Fe ^{3 +,} Fe ^{2 +} increase the amount of uranium leaching rate cause a reduced not only further reduce the potential problem, in that there is on a solid surface of Fe ³ UO ₂ ^+, Competition of Fe ^{2 +} and other cations, Fe ^{2 +} ions occupy more active sites, causing a decrease in the oxidation rate of UO ₂ .

Second, the ore particle size

In heap leaching, for most hard and dense ores, the metal minerals that are leached are small and scattered, and are mostly inclusions. Heap leaching of such ores, particle size often becomes the main factor affecting the leaching rate. It can be seen from the formula (1) that the larger the contact area of ​​the leaching agent with the mineral, the larger the amount of the substance to be transferred, and vice versa. Reducing the particle size of the immersed ore and fully dissociating the immersed mineral from the wrapped gangue to increase the contact area between the leaching agent and the mineral is an important measure to accelerate the diffusion rate of the substance.

C' _i =C ₀ (1-e ) (1)

Third, the spray intensity

The size of the spray intensity is also an important factor affecting the heap leaching rate. The greater the spray intensity, the faster the mass transfer rate of matter.

Fourth, the concentration of leaching agent

The chemical reaction in heap leaching, like hydrometallurgy, is the driving force, usually the concentration gradient. In general, increasing the concentration of the leaching agent can accelerate the reaction rate more or less. However, low concentration leaching agents are usually used in heap leaching to reduce leaching of impurities and prevent scaling. Once fouled, the rate of heap leaching will drop quickly. The concentration of the desired leaching agent must be determined by column immersion experiments.

5. Ore structure and mineral embedding state

The state of ore structure and mineral embedding is one of the fundamental factors affecting the rate of heap leaching. The degree of development of ore fissures, the size of minerals, the state of mineral inclusion in ores is the state of adsorption, or inclusions, whether they exist as compounds, or in the presence of elemental metals, etc. These factors affect the rate of chemical reactions. Affect the rate of diffusion mass transfer.

6. Harmful elements or compounds associated with ore

Ore often contains associated minerals or elements that interfere with the leaching of major metal elements. Their influence on the heap leaching rate, especially the chemical reaction rate, firstly appears to compete with the main leaching metal elements to consume the leaching agent, so that the concentration of the leaching agent is lowered, thereby reducing the leaching rate of the main metal elements, such as cyanide immersion gold. , associated copper, zinc and so on. Secondly, some substances formed during the leaching process of the associated minerals will deposit or adsorb on the mineral surface of the main metal, or passivate or occupy its active position, thereby reducing the reaction rate. When heaped with sulfur-bearing gold ore, the sulfur ions generated by the reaction of sulfide with sodium cyanide solution tend to deposit on the surface of natural gold, which inactivates gold. For example, when acid leaching uranium ore, if there is a large amount of Fe ^{2 +} , Fe ^{2 +} may occupy the active position on the surface of crystalline uranium ore, which hinders the oxidation of crystalline uranium ore. Immersing uranium or copper ore with dilute sulfuric acid solution. If the ore contains higher minerals such as fluorite or calcium carbonate, it will not only consume the leachant, reduce its concentration, reduce the reaction rate, but also dissolve during the leaching process. The calcium will precipitate with the sulfate ions to form insoluble calcium sulfate, which will deposit in the pores of the ore, which will reduce the porosity of the ore and hinder the diffusion and mass transfer of the material. These deposits also deposit on the surface of the mineral, preventing the contact of the leaching agent with the mineral, thereby affecting the progress of the chemical reaction.

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