The heap leaching process is a liquid-solid reaction which is not only related to the reaction rate inherent in the chemical reaction, but also follows the multiphase reaction law. The actual reaction rate of heap leaching often does not depend on the chemical reaction, but on the diffusion and mass transfer of the material. The rate of some chemical reactions (such as the reaction between ions in an aqueous solution) is very fast. If the rate of diffusion of the reactants or products at this time is very slow, the diffusion and mass transfer rate become the determinants of the rate of the leaching process. The actual or apparent rate of leaching at this point is controlled by diffusion, i.e., diffusion becomes the rate determining step of the leaching process.
According to Fick's law, the formula for the diffusion rate of the leaching agent to the unit area of ​​the mineral is expressed as follows:
v D =- 
= (C 0 -C i )=kD(C0-Ci) (1)
Wherein the VD-diffusion rate is a value indicating a concentration decrease of the leaching agent due to migration to a surface area of ​​the mineral unit per unit time;
C, C0, Ci-solution concentration, initial concentration, instantaneous concentration of the solution;
The thickness of the δ-diffusion layer;
D-diffusion coefficient;
kD-diffusion or mass transfer rate constant, kD=D/δ;
T-leaching time.
In time t, the mass mass of the diffused material at the entire phase interface (or the amount of leaching lost due to diffusion) is equal to the product of the diffusion rate and the phase interface area S time t, ie
m=Sv D t=Sk D â–³Ct=k D S(C 0 -C i )t (2)
It can be seen from the formula (2) that the interphase mass transfer m is proportional to the parameters such as the diffusion rate constant, the phase interface area, the concentration difference of the leaching agent, and the mass transfer time. The diffusion rate constant is a reflection of the intrinsic properties of both mass transfer materials and mass transfer media, and is affected by temperature.
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