Calcination is a process in which a physical change and a chemical change of a target mineral in a raw material are carried out under a suitable atmosphere (sometimes with some chemical agents added) and at a temperature lower than the melting point of the mineral raw material. It can be used as an independent chemical beneficiation or as a preparation to transform the target mineral into an easy-to-select or easily dip form.
Calcination is a multiphase chemical reaction in which a dry-solid-gas interface occurs. The change in free energy of the reaction can be expressed as:
△G=AG°+RTlnQ
=-RTlnK+RTlnQ
=RT(lnQ-lnK) (1)
Where ΔG——the free energy change of the reaction process, J/mol;
△G°——the standard free energy change of the reaction process, J/mol;
Q—the activity quotient of each component under specified conditions;
K——reaction equilibrium constant;
T - absolute temperature. K;
R - ideal gas constant, R = 8.3143 J / k · mol.
According to formula (1), the direction in which the reaction proceeds can be determined; when Q<K, △G<0, the positive reaction can be automatically performed; when Q>K, ΔG>0, the reverse reaction can be automatically performed; when Q=K , △ G = 0, the reaction reached equilibrium. Therefore, although the ΔG° value of a certain reaction is a fixed value, the direction in which the reaction proceeds can be changed by changing the activity of the reactant or the product and the reaction temperature.
△G is a function of reaction temperature and activity quotient, and ΔG° is a standard free energy variable in the standard state, which is a function of the reaction temperature. It is the freedom of reaction when the substance is in a standard state at a specified temperature (usually 25 ° C). Can change. Therefore, the ΔG° value can be used to compare the ability of different materials to react automatically under the same conditions. Thermodynamic data of many stable simple substances and compounds were determined by experiments, and they were sorted into various thermodynamic data tables or drawn into different coordinate graphs to show the functional relationship between them. The ΔG°-T curve is one of them. . From the position of FIG curve can be seen visually different metals in the stability of the compound under the same conditions, and can identify and estimate various metals, a compound which acts in the course of the reaction. It must be pointed out that under the constant temperature and constant pressure conditions, the true criterion for judging whether the process can be carried out automatically is △G, instead of △G°, which can provide the most basic conditions for us to predict whether the reaction can be automatically carried out.
The overall reaction rate of the mineral calcination reaction is controlled by the slowest reaction step. The whole multiphase chemical reaction process can be roughly divided into two steps of gas diffusion and adsorption-chemical reaction. The relationship between the corresponding reaction rate constants K, K D , K K and temperature is shown in the above figure. At low temperatures, the chemical reaction rate is much smaller than the diffusion rate (ie, K K <<K D ), where the total reaction rate depends on the chemical reaction rate of the interface, and regardless of the gas flow rate, its relationship with temperature can be used in Arrhenia. The Uz formula says:
Where K is the total reaction rate constant;
K K - chemical reaction rate constant;
A - constant;
E - activation energy.
At low temperatures, the reaction proceeds in the kinetic zone. As the temperature increases, the gradient of the chemical reaction rate increases more than the gradient of the diffusion rate. When the chemical reaction rate is much larger than the diffusion rate (ie, K K >>K D ), the total reaction rate is determined by the diffusion rate. The value has a small relationship with temperature. This region of the process is called the diffusion region, and the transition temperature from the kinetic region into the diffusion region varies with the reaction. When other conditions are the same, diffusion is often a control step for high temperature reactions.
The diffusion process is diffused and diffused. After the reaction has been carried out for a certain period of time, it is usually internal diffusion that plays a decisive role. The internal diffusion is inversely proportional to the thickness of the solid product layer on the surface of the ore. However, in the initial stage of the reaction, the reaction rate of the process is mainly related to the external diffusion. The rate of outflow depends mainly on the kinematics of the airflow - laminar or flocculated. When the gas moves in a laminar flow, the gas molecules move in a direction parallel to the surface of the solid reaction product, and the fractional velocity perpendicular to the reaction interface is equal to zero. At this time, the diffusion rate of gas molecules can be expressed by Fick's law:
Where V D - the diffusion rate of gas molecules, mol / s;
D——the diffusion coefficient, the value is Diffusion rate per unit area, cm 2 /s
Δ——the thickness of the gas film layer, cm;
c, c s - the concentration of gas on the bulk of the gas stream and the solid surface, respectively, mol / cm 3 ;
A - reaction surface area, cm 2 .
When the gas acts as a flocculation, the diffusion of gas molecules is greatly accelerated, but at this time, a layer of gas film is still maintained on the surface of the solid particles, and gas molecules slowly diffuse through the laminar gas film layer, and finally limit the rate of out-diffusion.
In the solid-gas multiphase chemical reaction, the particle size of the ore particles has a great influence on the diffusion process, and the reaction rate generally increases with the decrease of the grain size of the ore.
According to the atmospheric conditions at the time of firing and the main chemical changes occurring in the target component, the calcination process can be roughly classified into the following categories: oxidative calcination and sulfation roasting, reduction roasting, chlorination roasting, calcination, and sintering.
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