The chemical reaction rate v k and the diffusion mass transfer rate v D vary according to their own laws. For heap leaching, after a period of leaching, it is possible to achieve v k and v D equal, then the rate v of the heap leaching process can be expressed as follows:
Or k D (C 0 -C i )=k k C" j (2)
Most of the chemical reactions in the leaching are first-order reactions, that is, n=1, and then the formula (2) becomes
k D C 0 -k' D C t =k k C i (3)
Finishing (3), you have to
Therefore, the leaching rate v is equal to v k , so
When the heap leaching process is determined by the chemical reaction rate, then k k "k D , ie v = k k C 0
When the heap leaching process is controlled by the diffusion mass transfer rate, then k D "k k " , ie v = k D C 0
In the above two extreme cases, the difference between the chemical reaction rate and the diffusion mass transfer rate of most heap leaching processes is not so great, but the entire leaching process is still controlled by the slow step. Since chemical reactions often occur at the interface, it is often referred to as the chemical reaction kinetics zone, referred to as the dynamic zone; while the mass transfer and mass transfer needs to be carried out in a homogeneous liquid phase, so the solution body for diffusion mass transfer is usually used. It is called the diffusion dynamic zone. Referred to as the diffusion zone.
In the reaction zone, the chemical reaction rate is controlled by the main factors such as the type, concentration, contact area, mineral composition and valence state, and reaction temperature of the leaching agent. In the diffusion zone, the main factors affecting the mass transfer rate of mass diffusion are: the concentration gradient causing molecular diffusion, the external force causing convective diffusion - mechanical force, gravity, capillary force, etc., the diameter or mass of the molecule or ion, the viscosity of the medium solution, etc. . Temperature also has an effect on the diffusion rate, but the effect of temperature on the chemical reaction is particularly important. The degree of influence of temperature on the apparent rate of the leaching reaction is often used as an important basis for distinguishing whether the leaching process is controlled by chemical reaction or by diffusion dynamics control. If the change in temperature can cause a large change in the leaching rate, it is often considered that the leaching reaction is a chemical reaction kinetics control; conversely, if the humidity change does not cause a large change in the leaching rate, it is generally considered that the leaching process is controlled by diffusion, such as As mentioned above, the cyanidation gold leaching process is generally not heated because it is considered that the cyanidation gold leaching process is a typical diffusion kinetic process. The alkaline immersion uranium process generally requires higher temperatures because it is dominated by chemical reaction kinetics.
Many leaching processes are very complicated. Therefore, in a broader sense, whether chemical reaction control or diffusion control has only relative meaning, it should not be absolute. It can only be combined with specific actual conditions to carry out detailed analysis and research in order to truly depict the leaching process. . In the case of heap leaching, in general, uranium, gold, and copper are mainly controlled by the diffusion process. However, under certain conditions, such as heap leaching of certain primary ores, the rate of chemical reaction is slow and becomes the rate determining step of the leaching process. For the leaching process, it is usually assumed that the following five stages are experienced:
1. The leaching agent diffuses from the solution body to the surface of the ore, including the action of gravity and surface tension from the body of the solution to the surface of the ore and molecular diffusion-external diffusion through the liquid film.
Second, the leaching agent diffuses from the outer surface of the ore to the crack of the ore and the in-diffusion of the capillary.
Third, the leaching agent reacts with minerals, including chemical changes and phase changes.
4. The product diffuses from the inner surface of the ore through the crack and capillary to the outer surface.
5. The product diffuses from the outer surface of the ore to the main body of the solution - external diffusion.
Only the third step of these five steps is related to the rate of chemical reaction, and most of them are primary reactions, while the other four steps are related to diffusion.
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