The raw ball is mainly linked by the capillary force of water and has a certain strength. As the drying process progresses, the capillary water decreases and the capillary shrinks. The capillary force is strengthened to increase the compressive strength of the green ball. When most of the capillary water evaporates, a small amount of contact-like capillary water remains, which still maintains the strength of the green ball. The water is further evaporated, the capillary water disappears, and the strength of the green ball should be reduced. However, since the shrinkage causes the particles to close together, the molecular force and frictional resistance between the particles are increased, so the green ball still has a certain strength.
The strength of the green ball after drying has a great relationship with the properties of the raw material and the particle size composition. In particular, the raw material contains colloidal particles or a binding substance, which can make the green ball after drying have a relatively high compressive strength. Figure 1 shows the results of on-site sampling tests in China. Figure 2 is a laboratory study showing the adhesion of bentonite. It must be pointed out that although the compressive strength of the green ball is significantly improved, the impact strength is lowered. The raw ball becomes brittle because it loses the capillary water that acts as a buffer between the particles. [next]
Shrinkage occurs during the drying of the green ball, although it is beneficial to increase its strength, but uneven shrinkage can cause stress in the ball. Since the outer layer of the green ball dries faster and the temperature is higher, the shrinkage rate is large and the center is opposite. Therefore, the outer layer is subjected to tensile stress and internal compressive stress, thereby causing cracks in the green ball. Experiments have shown that the cracked raw ball, although fired at high temperature, the crack cannot be healed, and the strength of the finished ball will be affected, as shown in Figure 3.
When the green ball enters the slow-drying stage, the drying speed is controlled by the internal vapor diffusion rate. At this time, the flow rate of the drying medium cannot affect the drying speed. In order to increase the drying speed, only the medium temperature is raised, and the excessive temperature causes the water inside the raw ball to evaporate violently. If the outward diffusion does not occur, the internal vapor pressure will rise, and once it exceeds the strength limit of the surface layer, it will burst. Burst not only loses the pellets, but also deteriorates the permeability of the bed, which is not conducive to the next firing process.
There are several measures to prevent the ball from bursting:
(1) Strictly control the temperature of the drying medium below the green ball bursting temperature. Although this measure is effective, if the green ball burst temperature is too low, it will affect the drying speed, thus reducing the productivity.
(2) Adding bentonite is a measure to effectively increase the bursting temperature of the green ball. Figure 4 is a place of iron ore concentrate with addition of bentonite results. The mechanism by which bentonite raises the bursting temperature of the green ball is under study. The preliminary explanation is that bentonite can reduce the evaporation rate of water in the raw ball, so that the water is slowly released. Figure 5 reduces the vapor pressure inside the raw ball. In addition, bentonite increases the strength of the green ball as shown in Fig. 2, thus improving the anti-burst ability of the green ball. [next]
(3) In addition to the above two measures, if the density of the green ball can be controlled to have appropriate pores to facilitate the diffusion of the vapor inside the ball to the outside, and control the density of the pellet, in addition to controlling the particle size composition of the raw material. In addition, the most effective way is to control the time of making the ball. Experiments have shown that shortening the pelletizing time can increase the bursting temperature of the green ball (Fig. 6), but it contradicts the strength of the green ball. Therefore, if such measures are taken, both the strength and the burst temperature must be considered.
(4) The bursting temperature of the raw ball is related to its water content. The higher the water, the lower the burst temperature and vice versa. Therefore, a stepwise drying method can be employed to speed up the drying process.
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Application:
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