Leaching is a multiphase chemical reaction process carried out at the solid-liquid interface, which is similar to the calcination process carried out at the solid-gas interface, and generally comprises five steps of diffusion→adsorption→chemical reaction→desorption→diffusion.
Since the solid-liquid heterogeneous reaction kinetics involve a wide range of kinetics, when discussing the non-catalytic general heterogeneous reaction kinetics, the reaction rate is mainly determined by the two reaction steps of diffusion and chemical reaction due to the relatively fast adsorption rate at the phase interface.
When the solid is in contact with the liquid, a layer of liquid is adhered to the solid surface, which is called the Nernst boundary layer. The mass transfer process in the layer is carried out only by diffusion. At this time, the diffusion rate of the leaching agent from the solution body to the surface of the ore particle It can be expressed by Fick's law:

Where V _{D is} the concentration reduction caused by the migration of solvent to the surface of the ore particles per unit time, called diffusion rate, mol/s;
C—the concentration of the leaching agent in the solution, mol/mL;
c _s ——— the concentration of the leaching agent on the surface of the ore particles, mol/mL;
A—the phase boundary area of ​​the solution in contact with the ore particles, cm ² ;
Δ—the thickness of the diffusion layer, cm;
D——— diffusion coefficient, cm2/s;
K _D ——— diffusion rate constant,
The rate of chemical reaction on the surface of the ore particles can be expressed as follows:

Where V _K --- the concentration of the reagent caused by the chemical reaction on the surface of the ore particles per unit time is called chemical reaction speed, mol / s;
K _K ———Chemical reaction rate constant, cm/s;
n———the number of reaction stages, generally n=1;
A, c _s --- are the phase boundary area and the concentration of the reagent on the surface of the ore.
After leaching for a certain period of time, the equilibrium is reached. In the steady state, the two speeds are equal:

It can be seen from the above formula that (1) when K _K <<K _D , V=K _K A _c , the leaching process is controlled by chemical reaction, and the process is carried out in the kinetic zone;
(2) When K _K >>K _D , Ï…=K _D A _c , diffusion control during the leaching process, the process is carried out in the diffusion zone;
(3) When K _K ≈K _D , the above formula cannot be simplified, and the process is performed in the mixed zone or transition zone. [next]
According to the theory of activation complexation, the relationship between the reaction rate constant and temperature follows the Arrhenius equation:

Therefore, the logarithm of the reaction rate constant is linear with 1/T (below). Since E is a positive value, the slope A is a negative value. It can be seen from the curve in the figure that the slope is large at low temperature and the E value is large, so the reaction is in the kinetic region; the slope is small at high temperature, the E value is small, and the reaction is in the extended region, which is a general rule. It is generally believed that the diffusion activation energy is less than 13 kJ/m, and the chemical reaction activation energy is as high as 42 kJ/m. Therefore, the speed control step can be judged by the activation energy value and the temperature coefficient (ratio of the increase in the speed constant corresponding to the temperature increase of 10 degrees). Knowing the speed control step of the leaching process, the characteristics can be used to increase the reaction speed and strengthen the leaching process. If the process is controlled by the chemical reaction step, the temperature coefficient is large, and the leaching temperature is effective to increase the leaching speed. When the process is controlled by diffusion. When the temperature coefficient is small, the effect of increasing the temperature is small, but increasing the stirring speed and the fineness of the grinding can effectively increase the leaching speed.

From the above analysis, the main parameters affecting the leaching speed are leaching temperature, grinding fineness, reagent concentration, stirring strength, slurry solid ratio and leaching time. Within a certain range, the leaching rate of the target components is as listed above. Each value increases as the value increases, but has a suitable value. Due to the complexity of the leaching process itself, the optimum process parameters for leaching are generally determined by experimentation.

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