Leaching kinetics of the process is to diffuse heterogeneous reaction of zinc-containing solid materials (such as zinc oxide calcine, or zinc, etc.) with a solution (e.g., sulfuric acid) produced. The characteristic is that the chemical reaction between the solution and the zinc material is carried out at the interface between the phase and the phase, and a thin layer of saturated solution (diffusion layer) is formed on the surface of the dissolved solid, so that the solid material is saturated with the solution layer. After being surrounded, the ions are diffused to the inside through the saturated layer and the ions in the saturated layer solution are diffused outward, so that the dissolution process can continue. Therefore, the mechanism and steps of the leaching process can be understood as:
(1) adsorption of dilute sulfuric acid on the surface of the solid (raw material) (including pores and capillaries);
(2) on the surface contacted by the two, the dilute sulfuric acid is chemically reacted with the solid (zinc raw material) to form a sulfate and dissolved in the solution;
(3) The solution layer on the solid surface is continuously enriched with sulfate, and a thin layer of sulfate saturated liquid (generally called diffusion layer) is formed on the solid surface;
(4) a saturated layer of sulfate, which hinders the contact of the calcine with dilute sulfuric acid;
(5) The diffusion reaction of the raw material is continued by the diffusion of the saturated solution from the interface to the solution and the diffusion of the sulfuric acid into the saturated solution layer.
It can be seen, the leaching of zinc material is composed of two stages, i.e. by the chemical reaction stage dilute sulfuric acid and the gold compound and a zinc metal material resulting metal sulfate dissolved diffusion phase solution composition of human hand. [next]
The rate of leaching during leaching is also determined by the rate of chemical reaction and the rate of diffusion.
A Diffusion rate According to Fick's law, the diffusion rate of solvent to the surface area of ​​the material surface can be expressed by the following formula:
Dc D
ξ _D = - —— = — (cc _s ) = k _D (cc _s ) (4)
Dt δ

Where ξ _D —— diffusion rate (for the total surface area of ​​the material);
c - the concentration of sulfuric acid in the solution;
c _s - the concentration of sulfuric acid on the surface of the mineral;
δ - the thickness of the diffusion layer;
D - diffusion coefficient (concentration gradient equal to 1 diffusion rate);
k _D - diffusion or mass transfer rate constant k _D = D / δ ;
t - leaching time.
B Chemical reaction rate During the leaching process, when a chemical reaction occurs on the surface of the zinc material, the reaction rate is determined by the law of mass action, which can be expressed as follows:
Dc
ξ _k = —— k _K ·C _s ⁿ (5)
Dt

Where ξ _k —— the concentration of the solvent caused by the chemical reaction on the total surface of the mineral per unit time is the chemical reaction rate;
k _K - the rate constant of the chemical kinetic phase;
n - the number of reaction stages. [next]
The leaching process establishes a steady state after a period of time, in which case the chemical reaction rate is equal to the diffusion rate, ie the macroscopic rate of the process is:
Dc
ξ = ξ _k = ξ _D = - —— (6)
Dt

Therefore, according to formula (4) and formula (5), we can get:

k _D (cc _s )=k _K c _s ⁿ (7)

The calcined ore is leached in sulfuric acid, and when the reaction rate is subjected to the first-order reaction, i=1.
From the above formula:
kD
c _s = ————·c (8)
k _D +k _K

Substituting equation (8) into equation (7) gives dc k _K ·k _D
ξ = -—— = ————·c = kc (9)
Dt k _D + k _K

Combining all of the above factors that increase the chemical reaction rate constant or increase the diffusion rate constant can cause the process to enter different rate control regions. Generally, for the acid-soluble reaction of the leaching process, the temperature increase can increase the reaction rate, and it can be considered that the influence of the temperature on the reaction rate mainly affects the reaction rate constant k value. The k value increases as the temperature increases. According to the principle of Fan Hezhen, the reaction rate increases by about 2 to 4 times for every 10 °C increase in reaction temperature.
When calculating the activation energy of the acid-soluble reaction of zinc ferrite Zn0·Fe ₂ 0 ₃ , it is known that the acid solution temperature is raised from 40 ° C to 50 ° C or from 60 ° C to 70 ° C, and the temperature coefficient of the reaction rate constant is 2.01 and 1.84, the calculated results show that the decomposition rate of ZnO·Fe ₂ 0 ₃ will increase exponentially at elevated temperature, and the elevated temperature is very favorable for the dissolution of zinc ferrite.
references:
1 Mei Guanggui, Wang Derun, Zhou Jingyuan, Wang Hui, and wet smelting zinc. Changsha: Central South University of Technology Press, 2001
2 Peng Rongqiu, ed. Handbook of Metallurgy Extraction of Non-ferrous Metals (Zinc, Cadmium , Lead , Bismuth ). Beijing: Metallurgical Industry Press, 1992
3 Hu Xigeng, editor. Non-ferrous metal sulfide ore dressing . Beijing: Metallurgical Industry Press, 1987
7 China Metallurgy Encyclopedia (Non-ferrous Metallurgy Volume). Beijing: Metallurgical Industry Press, 1999
8 Xu Caidong, Lin Rong, Wang Dacheng. Zinc Metallurgy Physical Chemistry. Shanghai: Shanghai Science and Technology Press, 1979

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