The mechanism of mass diffusion and mass transfer in agitation leaching generally involves the diffusion of a leaching agent from a solution around the ore to be immersed in the surface of the mineral (which we call the reaction zone) to the surface of the mineral to cause a chemical reaction; the ions or complexes formed by the reaction are from minerals. The surface diffuses into the solution in the reaction zone, which is known as membrane diffusion, driven by concentration. Then, by external force, convection diffusion is carried out, and the leaching agent is quickly and continuously transported from the solution body to the reaction zone to supplement the continuously consumed leaching agent, and the amount of the leaching agent in the reaction zone solution is maintained, and Meanwhile, the reaction zone was also produced in the metal ion solution was quickly transferred to the body. Thus, the ratio of the leaching agent to the metal ion in the reaction zone solution can always meet the needs of the chemical reaction until the leaching is terminated.
In the heap leaching process, the reaction zone is the same as the agitation leaching, and the driving force is driven by the concentration. The problem is what kind of driving force is used to transport the metal ions generated in the reaction zone solution to the main body of the solution, and how the continuously consumed leaching agent in the reaction zone solution can be replenished, and both must be carried out simultaneously, otherwise leaching The reaction will be interrupted. From the characteristics of the former porous media, we know that this is achieved by capillary force. In the pores, the leaching agent diffuses from the solution body to the inner reaction zone of the pore, and the metal ions generated in the reaction zone diffuse through the pores into the solution body, and can only be carried out in the solution in the pores, that is, both The diffusion in the opposite direction is only possible if all or part of the pores are present in a continuous liquid, and the amount or volume of liquid in the pores is completely controlled by the capillary force. When sprayed to a certain extent, the pores in the ore heap are completely saturated by the liquid, and when the spray is stopped, the heap discharges a certain amount of liquid until its residual saturation is reached. Part of the pores are encroached by air, and bubbles may be formed to block the liquid, and the diffusion and mass transfer of the substance are disturbed. We know that the entry and discharge of liquid in the pores is completely determined by the capillary force. Therefore, it can be assumed that during the heap leaching process, the mass transfer in the process from the solution in the reaction zone to the intersection of the pores and the surface of the ore is mainly Driven by the capillary force.
The diffusion and mass transfer of materials in heap leaching is also controlled by a third force-gravity. This occurs in the middle of the journey from the surface of the ore to the outflow of the heap. When spraying, the leachate is infiltrated into the inner part of the heap by the surface of the heap, and is affected by two kinds of forces. On the one hand, due to gravity, it moves downwards along the surface of the ore in the form of a liquid film; When it encounters pores, it penetrates into the ore inside the pore direction by the action of the capillary force. The metal ion that has diffused from the pores in the previous spray cycle and adhered to the solution film on the surface of the ore naturally diffuses into the liquid film of the immersion liquid that moves downward in the next spray cycle, and follows it. It flows to the bottom of the heap and finally flows out of the heap. At the same time, the leaching agent molecules in the immersion liquid film that moves downward also diffuse into the solution in the pores where there is a void. According to the above discussion, it can be assumed that the material diffusion and mass transfer in the heap leaching process are divided into three stages, which are subjected to three different driving forces: the driving force of the material in the reaction zone; in the pores, the capillary force; From the surface of the ore to the section of the mine, rely on gravity. Obviously, the rate control function is the capillary force.
According to this assumption, the so-called pool immersion (bubble leaching), although natural leaching, is quite different from heap leaching in terms of diffusion mass transfer. First, the gravity in the pool immersion has little effect on the mass transfer and mass transfer; secondly, the capillary force in the pool immersion is weak, and it is limited to the moment when the first immersion liquid is added to the pool. The capillary force causes the leaching solution to enter the pores of the ore. From then on, the mass transfer and mass transfer of the whole pool (tank) immersion process, that is, the molecular diffusion between the reaction zone solution and the solution body, mainly depends on the concentration driving force. Although there are several cycles of fluid exchange in the existing pool dipping operation, the time for each liquid change is very short. The purpose of the liquid change is not to achieve the leaching of the unsaturated stream, but to discharge the body of the solution which is already in the leaching equilibrium, and to add a new leaching solution to make it a new solution body to facilitate the solution between the solution body and the reaction zone solution. The diffusion of matter promotes the progress of the leaching reaction. In fact, in the pool immersion, more than 90% of the time, the ore is surrounded by the solution, or the ore is submerged in the liquid. Even during the draining time, the air occupies only the gap between the ore and the ore, and the pores inside the ore are still occupied by the solution, not by the air. From this point of view, the mass transfer between the solution in the reaction zone and the main body of the solution in the capillary tube is not related to the capillary force and gravity.
It is generally accepted that there are two different reaction zones during the heap leaching process. One is that the chemical reaction takes place in the surface area of ​​the ore; the other is that the chemical reaction takes place in the pores of the ore, often referred to as the inner surface region. Therefore, the heap leaching process undergoes two distinct kinetic stages. In the outer surface area of ​​the ore, from the perspective of diffusion mass transfer, there is concentration-driven molecular diffusion and gravity-induced convection diffusion. In the inner surface area of ​​the ore, there is concentration-driven molecular diffusion and convective diffusion caused by capillary forces. The primary and secondary of the diffusion kinetics and chemical reaction kinetics are determined by the specific conditions in the outer surface area of ​​the ore, while the primary and secondary problems in the inner surface area of ​​the ore are recognized as diffusion dominated.
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