The significance of green ball drying, the mechanism of green ball drying

First, the significance of the dry ball
The green ball contains a large amount of water, which must be dried before the high temperature is calcined to remove most of the water.
(1) Before the raw ball enters the high temperature roasting, it must be preheated. Preheating is generally carried out in a flow of from 900 ° C to 1100 ° C. If the raw ball is not dry, take a lot of water into the preheating zone, and the water in the ball will evaporate fiercely, which will cause the green ball to split and even burst.
(2) The raw ball that has not been sufficiently dried directly enters the high temperature zone for roasting, even if it does not burst, but because the water in the ball is very high, the evaporation of water absorbs a large amount of heat energy, and the pellets cannot rise quickly to the roasting designation. The temperature is bound to prolong the firing time, reduce productivity, and increase fuel consumption.
(3) Iron ore concentrate magnetically or high sulfur ore pellet production, sufficiently dried particularly necessary. The raw ball that has not been sufficiently dried enters the high-temperature baking zone with a large amount of water, and the water evaporates, which affects the oxidation of Fe ₃ O _{4 and} the oxidation of S. Low-cost iron oxide reacts with gangue at high temperature to form a low-melting melt, preventing further oxidation of Fe ²⁺ to Fe ⁺³ , preventing desulfurization, increasing the amount of FeO in the pellet, and reducing the desulfurization rate. Even excessive molten liquid is produced and formed into large pieces.
Therefore, dry ball drying is a necessary process to prevent cracking of the green ball, increase the yield of pellets, and improve the quality of the pellets. Moreover, in the production, the dry ball is used to cool the residual heat of the pellet or the waste heat of the calcined pellet.
Second, the mechanism of the dry ball
The raw ball with very high moisture content meets the drying medium-hot air flow. The water vapor pressure on the surface of the green ball is greater than the partial pressure of water vapor in the drying medium. The water evaporates on the surface of the green ball and passes through the boundary layer as the drying medium zone. go. As the surface moisture evaporates, along the radius of the green ball, there is a difference in humidity between the inside and the outside, the water in the ball diffuses to the surface layer, and then evaporates on the surface until the humidity balance is reached. That is, the temperature and humidity of the drying medium are unchanged, the moisture of the raw ball is also reached, and the drying process of the raw ball is terminated.
It can be seen that the drying process of the green ball consists of two parts: surface evaporation and internal water diffusion. Drying speed is controlled by slower links.

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The green ball drying characteristic curve is shown in Figure 1 and Figure 2. The green ball begins to dry if the temperature, flow rate and humidity of the drying medium do not change. The water on the surface of the green ball evaporates at a constant rate. When the surface water evaporates, a temperature difference occurs in the radial direction inside and outside the pellet. Due to the effect of the wetting phenomenon, water diffuses from the inner surface of the ball. In the early stage of drying, the speed of water spreading out in the sphere can ensure the evaporation of water on the surface of the sphere. At this time, the surface of the pellet is kept moist, the temperature is not high, the drying speed is controlled by surface vaporization, and the evaporation rate of water per unit surface area is The drying speed can be expressed by the following formula:

Where F—the evaporation surface area;
A—the heat transfer coefficient between the drying medium and the surface of the sphere;
γ _t ———the latent heat of evaporation of water at the temperature t on the surface of the green ball;
T———drying medium temperature:
t———the surface temperature of the raw ball;
K _p ———vaporization coefficient, K _p =k(ω•γ) ^0.8 , W: medium flow rate, γ: medium gravity;
P _H — the vapor pressure of the surface water of the ball;
P _N — the partial pressure of water in the drying medium;
K _c ———vaporization coefficient (water diffusion mass transfer coefficient from the surface of the green ball through the boundary layer);
C _H ———the saturated humidity in the surface gas of the raw ball;
C _N — The humidity of the drying medium.
In the surface vaporization control stage, the drying speed is only affected by the temperature of the drying medium, the flow rate and its humidity.
As shown in Figure 1, when the raw water drops to point B, the drying process shifts from constant speed to deceleration, corresponding to the B-C and C-D segments in Figure 2. As the water evaporates, the capillary shrinks and the water migration resistance increases within the capillary, and the connected cellular capillary water gradually becomes an isolated contact state capillary water. At the same time, the humidity gradient decreases along the radius of the green ball, and due to the dry surface of the green ball, the temperature rises, and the thermal conduction phenomenon occurs along the radius of the ball, that is, the water diffuses from the high temperature region to the low temperature region, and the influence of these factors is combined. The drying speed is reduced. The water is further evaporated, and only the adsorbed water and the film water remain in the raw ball. They are firmly bonded to the ore particles and cannot move freely. Only when they become steam can they leave. So in the late stage of drying, the surface of the green ball is dry, the temperature rises, the heat is conducted into the ball, and the vaporization of water proceeds in the ball until the equilibrium humidity is reached.
The slow down drying stage is a relatively complicated process, and the drying speed can be approximated by the following formula:

Where F—the surface area of ​​the green ball;
K _c ——— coefficient;
c———the humidity of the raw ball at t;
c _E ———balance the humidity.
Experiments have shown that the water content of the raw ball is generally 60-90% evaporated in the constant-speed drying stage, and the rest is removed in the slow-drying stage.

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