The pellets are formed by rolling. The water-wet concentrate concentrate becomes spherical by mechanical force and capillary action during rolling, and the capillary has a certain strength by capillary force, molecular attraction and friction between the fine particles. The particle size, moisture, mechanical strength and thermal stability of the raw ball all affect the next calcination consolidation process and are related to the quality of the finished pellets.
(1) Surface properties of finely ground materials
The raw materials for the production of pellets are finely ground materials. For example, large ore mined from mines must undergo a series of processes such as crushing, fine grinding, beneficiation and filtration to obtain iron concentrates suitable for pelleting. The particle size of a finely ground material can be measured in microns or mesh, but is dictated by specific surface area. The specific surface area refers to the total area of ​​the material particles per unit volume (cm 3 ) or unit mass (g). The specific surface area of ​​the general pellet material is usually in the range of 1500-2000 cm 2 /g. The smaller the particle size of the material, the larger the specific surface area. For example, a cube with a length of 1 cm on each side is evenly divided into small cubes with a side length of 1 μm. The number of particles is changed from 1 to 10 21 and the specific surface area is changed from 6 cm 2 /cm to 6 × 10 7 cm. 2 / cm 3 . It can be seen that the greater the dispersion of the material, the larger its specific surface area.
The molecules in the surface layer of the solid particles are in an unbalanced state. Because the materials on both sides of the surface layer are different, their densities are different, so the surface layer has higher energy. For molecules of the same surface layer, they are not neatly arranged in a line. Since the surface of the solid particles is uneven, the energy of the molecules at the peak end is higher than the energy of the molecules in the valley. This part with higher energy is called the activated part. According to the principle of lowest energy, objects always have a tendency to reduce their surface energy. For solids, the density of the other side of the surface can only be increased by adsorbing foreign matter to reduce its surface energy. In addition, since the surface of the solid particles has excess energy, when it comes into contact with the surrounding medium, the surface of the particles shows an electric charge, and an electric field is formed in the surrounding space. Therefore, the finely ground materials are in contact with the surrounding medium to adsorb them.
(b) Surface properties of the liquid A layer of liquid having a thickness below the liquid surface is called a surface layer. The molecules in the surface layer are subjected to the forces of the molecules inside the liquid on the one hand, and the forces of the external gas molecules on the other hand. The gas density is much smaller than the liquid density, and the force of the gas molecules can generally be neglected. Thus the molecules in the surface layer are in an unbalanced force field. If a molecule inside the liquid is moved into the surface layer, it must overcome this imbalance to do work to increase the potential energy of the molecule. This potential energy is the surface energy.
When a system is in a stable state, it should have the lowest potential energy. Therefore, the molecules on the surface of the liquid have a tendency to squeeze into the interior of the liquid, that is, the liquid has a tendency to shrink its surface as much as possible. This force that causes the surface to shrink along the surface of the liquid is called the surface tension of the liquid. Surface tension can be seen at any time. For example, water, mercury and other droplets become spherical, which is the result of surface tension; because the liquid has a spherical shape under a certain volume, it can have the smallest surface area.
Imagine that there is a dividing line A-B on a liquid surface, dividing the liquid surface into two parts. See Figure 1. Æ’ i is the surface tension of the liquid surface ABCD to the other side of the liquid surface ABEF. Æ’ 2 and Æ’ 1 are equal in opposite directions and can be expressed by the following formula:
Æ’=Æ’ 1 =Æ’ 2 =aL (1)
In the formula ƒ———surface tension, Dyne;
a———the surface tension coefficient;
L———the length of the AB line, in centimeters.
As can be seen from equation (1), the surface tension coefficient $ is the force acting along the liquid surface on the unit length of the boundary line. It can be expressed by the following formula:
If the numerator and the denominator of the formula (2) are multiplied by 1 cm, the unit of the surface tension coefficient a will become: erg/cm 2 . Therefore, the surface tension coefficient a of a liquid can also be defined as the work done by increasing the unit surface area.
Experiments have shown that the surface tension coefficient of liquid is related to temperature. The higher the temperature, the smaller the surface tension coefficient, and the purity of the liquid. The impurity in the liquid dissolves the surface tension coefficient. This property has a certain influence on the concentrate powder into a ball.
On the horizontal liquid surface, the surface tension is consistent with the liquid surface. As shown in Fig. 2(a), if the liquid surface is a curved surface, the surface tension tends to level the liquid surface, as shown in Fig. 2(b) and Fig. 2(c). The liquid level produces an additional pressure on the liquid. For convex surfaces, the pressure is set to a positive value and the concave surface is a negative value. The additional pressure is proportional to the surface tension coefficient and inversely proportional to the radius of curvature of the liquid surface. The additional pressure is the basis of the capillary phenomenon, which is closely related to the strength of the concentrate powder and the green ball.
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