(1) Mechanism of concentrate powder into balls
Finely pulverized concentrate powder, which is wetted to a suitable extent by water, will aggregate into a certain size ball under the action of external force. The ball formation process can be roughly divided into three steps: the nucleation of concentrate powder is the first step in the formation of the ball. The mineral powder particles are wetted by water, first forming a film water on the surface thereof, as shown in Fig. 1(a); if further wetted, and the wetted particles have a chance to contact, capillary water is formed at the contact, by capillary The force that connects two or more particles to form a small ball, as shown in Figures 1(b) and (c), continues to increase the water, and under the action of mechanical force, the particles inside the ball are rearranged. Further dense, forming a relatively stable and stable ball, as shown in Figure 1 (d), generally referred to as the cue ball. The formation process of the cue ball, that is, the nucleation process of the concentrate powder. The cue ball is still porous, and it contains three phases of solid, liquid and gas. Its stability depends on the particle size and particle size composition of the ore powder, as well as the shape and hydrophilicity of the particles.
Growing up is the second step in becoming a ball. During the rolling process, the cue ball collides with each other, causing the shape of the capillary between the internal particles to change, the particles are densely packed, the capillary shrinks, the capillary capillary water becomes saturated capillary water, and a part of the water is squeezed onto the surface of the cue ball. The ball can grow up in three mechanisms. The mother ball has a high moisture content and good plasticity. They are combined with each other to make the green ball grow rapidly, as shown in Figure 2(a). It is called the coalescence mechanism; if a large batch of wet material is poured into the pelletizer in industrial production, or the concentrate powder is extremely fine, the hydrophilicity is extremely strong, and the mother ball is mostly grown by the coalescence mechanism. The wetting material is evenly added to the pelletizer, and the surface of the ball is high in water. When the mineral powder is encountered in the rolling, the ore powder is stuck on the surface layer, the balls collide with each other, and the newly adhered layer is wet. The mineral powder is compacted, the water in the capillary is squeezed onto the surface, and a new layer of mineral powder can be bonded. If the water is insufficient, water can be sprayed onto the surface of the small ball, so that the mother ball grows up, as shown in the figure. 2 (b), is called the layer mechanism; in addition, the ball moves in the pelletizer, there are always a few balls due to insufficient strength, low moisture, etc., damage and cracking, the resulting debris, adhered to another On the ball, see Figure 2(c), it is called the mechanism of grinding and stripping. In short, from the fine grain concentrate to the generation of the cue ball, to the growth ball with a certain size, the growth mechanism, but the above three. As for which mechanism is dominant, it depends on the nature of the raw materials and the conditions of the pelletizing process.
When the cue ball grows to the required size, it should stop adding water and wetting, so that the raw ball rolls in the pelletizer for a certain period of time. As a result of the collision, the inner particles of the green ball are arranged more closely, which is the first ball. Three steps. The action of mechanical force during the rolling process of the green ball causes the internal particles to be selectively arranged according to the maximum contact surface, the particles are close to each other, the capillary diameter is reduced, and even the water layer of the film surface can be connected to each other. In this case, the combination of molecular force, capillary force and frictional resistance between the particles gives the green ball a high mechanical strength. The above three steps of growing the ball are actually occurring in the same pelletizer at the same time in production. [next]
(2) Factors affecting the concentrate into balls
There are many factors affecting the concentrate into a ball. In summary, it can be divided into two categories, one is the natural nature of the raw materials, and the other is the conditions of the ball making process.
(1) The natural properties of the raw materials. Among the natural properties of the pelletizing material, the hydrophilicity and particle shape of the particle surface have the greatest influence on the sphericity. The higher the hydrophilicity of the surface of the particles, the smaller the contact angle between the solid phase and the liquid phase, the particles are easily wetted by water, the film water and capillary water content are high, and the migration speed of capillary water is also high, so that the sphericity is good. The result of the measurement, the maximum content of water molecules usual additives and pelletizing iron ore and capillary water.
The sphericity of the finely ground material can be expressed by the sphericity index, see formula (1)
Where W Æ’ --- maximum molecular water content, %;
W m --- Capillary water contains the most, %.
K=0.20～0.35 The material is weakly spherical,
K=0.35～0.60 The material is globular,
K = 0.60 ~ 0.80 The material is good,
K>0.80 The material is excellent in sphericity.
The spheroidality of iron ore fines is best with limonite and magnetite is the worst. In addition to their different hydrophilicity, the shape of the particles also has a relationship. For example, the limonite particles are needle-like, flaky, have a large specific surface area, and are porous and porous, so that the wet capacity is large and the sphericity is good.
(2) Particle size and particle size composition of the raw materials. The particle size and particle size composition of the raw materials have a great influence on the spheronization. The particle size is small, the specific surface area is large, and the sphericity is good. The raw material has a suitable particle size composition, the particles can be arranged closely, the average diameter of the capillary is reduced, and the bonding force between the particles is increased. All kinds of raw materials have suitable granule size, for example, magnetite for pelletizing, the upper limit of particle size should not exceed 0.2 mm, and the size of -200 mesh should account for more than 80%. Some pellets and mines abroad have re-ground iron concentrates in order to meet the requirements of the particle size of raw materials.
The content of the fine fraction (-0.01 mm) in the raw material has an important influence on the sphericity, and it is filled in the void between the larger particles to narrow the capillary diameter between the particles. Moreover, the frictional resistance of the particles is increased. Of course, the finer the finer the finer the better, because the grinding consumes a lot of electrical energy, and too thin will lead to an increase in production costs. Moreover, the finer the particle size, the smaller the capillary diameter, and the lower the migration speed of water between the particles, thereby lowering the ball forming speed.
(3) The moisture of the raw materials. The amount of water in the raw material has a great influence on the ball. For different raw materials, the raw balls have different suitable moisture. For example, the raw ball caused by magnetite concentrate generally has a moisture content of 8 to 10%, and at this time, the pelletizing rate of the raw ball is high and the strength is good. Under normal production conditions, it is often necessary to maintain the moisture content of the raw material slightly lower than the suitable moisture of the green ball, leaving room for additional water during the ball making.
If the water content of the raw material is too low, although the water can be replenished during the ball making, the ball forming speed is slow, the productivity is lowered, and the raw ball is often fragile due to uneven watering.
The water content of the raw material is too high, which brings great difficulty to the pelleting, makes the green balls have uneven particle size, and bonds to each other to form a large block. In this case, the raw material must be pre-dried to reduce the moisture therein.
When making a ball, the range of suitable moisture content of the raw material varies depending on the raw material. For example, magnetite concentrate pelletizing is most sensitive to fluctuations in moisture, so for different raw materials, the appropriate moisture should be determined experimentally. [next]
(4) The influence of additives. Adding certain additives to the pelletizing material can improve the spheronization of the material. Commonly used additives are bentonite, hydrated lime, limestone and the like. Their hydrophilicity and sphericity index are superior to iron ore fines.
Bentonite is a commonly used additive for pelletizing. It can improve the spheronization of concentrate powder, increase the strength of the ball, and more importantly, it can increase the burst temperature of the ball. Generally, 0.6 to 1.2% bentonite is added to the pellets and minerals, which has an obvious effect.
Bentonite , also known as bentonite , its main mineral is montmorillonite. Its chemical structure is: Al 2 (Si 4 O 10 )(OH) 2 , containing 28.3% of Al 2 O 3 and 66.7% of SiO 2 . the H 2 O5%. montmorillonite aluminosilicate form is a layered structure, the silica tetrahedron and the aluminum octahedron parallel links, the composition unit cell, Figure 3 vertically stacked, laminar Structure.
Isomorphous substitution of unequal cations often occurs inside montmorillonite crystals. In the silicon tetrahedron, Si + 4 can be replaced by Al + 3 , and in the aluminum octahedron, Al + 3 can be replaced by Fe + 2 and Mg + 2 , thereby causing the structure to have a negative charge.
Montmorillonite is often negatively charged, it can adsorb cations, and it is often adsorbed by Ca +2 , Mg +2 , Na + and K + in nature. The adsorption of Ca + 2 is mainly called calcium-based bentonite, and the adsorption of Na + is called sodium bentonite. These cations adsorbed by montmorillonite can be exchanged according to the following principles.
a cation having a high concentration in the medium can exchange cations having a low concentration;
When the medium concentration is the same, the high-priced cation can exchange low-cost cations;
When the medium concentration and the cation price are the same, the ionic radius is large, and the exchange radius is small.
Based on the above principles, in actual production, bentonite can be modified as needed. For example, calcium bentonite can be changed to sodium bentonite.
Montmorillonite has a strong ability to absorb water. In addition to adsorbing water molecules like the surface of a solid solid mineral, there are a large number of intercalated inner surfaces that adsorb water. With the increase of water absorption, the intercalation interval of the calcium-based bentonite is enlarged, but it can not be increased after reaching 21.4A o . The sodium bentonite can continue to absorb water and expand, even in a separated state, so the role of sodium bentonite in pelleting is more obvious. .
Slaked lime is a commonly used additive for the production of flux pellets, and its chemical formula is Ca(OH) 2 . It is produced by the digestion of quicklime (CaO-based) with water and has a large specific surface area. The surface of the hydrated lime particles is negatively charged, while the water molecules are evenly polar, so it can adsorb water molecules and remain negatively charged. It has a strong hydrophilicity and natural adhesion to improve the spheronization of the material. However, the specific gravity of slaked lime is small, and the amount of addition should not be too much. Otherwise, it accounts for an excessive proportion of the material in terms of volume, so that the migration speed of capillary water is lowered, which affects the speed of ball formation. In addition, in large-scale industrial production, it is difficult to digest the lime and at the same time keep its water stable without forming a large block, so the limestone powder is often used instead.
The main component of limestone powder is CaCO 3 . Although the hydrophilicity and cohesive force of finely ground limestone powder is not as good as that of slaked lime, its surface is rough and hydrophilic is better than that of magnetite powder. Therefore, finely ground limestone powder is added to the ingredients, which has improved pelvic properties. help.
In recent years, countries around the world have begun to study organic additives to replace bentonite. Because bentonite can effectively improve the sphericity of the material, but the SiO 2 content of up to 60%, will reduce the iron content of the pellets, increase the amount of slag during smelting, in addition, the bentonite also brings the most undesirable blast furnace the alkali metal. The organic additive currently used in industrial production is PERIDUR XC-3 manufactured by the Dutch company, and the effect can be exhibited by adding 0.5%. The economic effect is similar to that of bentonite, but it does not bring SiO 2 , which is very important for the production of pellets for direct reduction. [next]
(5) The impact of the pelleting process. The impact of the ball making process on the ball can be summarized as both equipment and operation.
In terms of pelletizing equipment, including the speed of the pelletizer, the angle of inclination, the height of the edge of the pelletizing disc, and the like. Disc pelletizers are commonly used in pellet trains in Western Europe and China. The diameters of the discs vary in size, but the angle of inclination is generally between 45 and 50 degrees. When the inclination angle is fixed, the speed of the spheroidal disk can be adjusted within a certain range, and is always maintained at 1.0 to 2.0 m/sec in terms of the peripheral tangential speed of the spheroidal disk. When the peripheral speed is too small, the material rises less than the upper part of the disc rhyme. On the one hand, the area of ​​the spheroidal disc is not fully utilized. On the other hand, the position of the raw ball rolling in the disc is low, so the kinetic energy is small when rolling. The mechanical force that collides with the ball is small, so that the ball is slow and the strength of the ball is low. If the peripheral speed is too large, due to the centrifugal force, the material is thrown to the edge, following the rotation of the pelletizing disc, and there is no material zone in the center, and the effect of rolling into a ball is destroyed, and even the ball cannot be formed. The angle of inclination of the pelletizing disc is relatively large, requiring a higher peripheral speed, which increases the number of rolling of the material in the disc, which is advantageous for increasing the yield of the raw ball and increasing its strength.
The side height of the pelletizing disc is related to its diameter. The large pelletizing disc with a diameter of 5.5 meters is 600-650 mm high. The height of the ball affects the filling rate of the pelletizing disc. The edge of the pelletizing machine is high and the inclination angle is small. Under the conditions, the material stays in the pelletizing disk for a long time, which is beneficial to improve the strength of the green ball.
The position of the scraper is also important. It scrapes off the material sticking to the pelletizing disc, maintaining the proper thickness of the primer, avoiding excessive sticking and increasing the load on the drive motor. In addition, the squeegee also acts as a diversion stream to separate the nucleation zone from the growth zone in order to control the growth of the green ball.
In terms of process operation, the factors affecting the ball are: the method of adding water and feeding, and controlling the time of pelleting. Under normal circumstances, the water content of the pelletizing material should be controlled to be slightly lower than the water suitable for pelletizing, and a small amount of water should be added during the ball to control the formation of the cue ball and grow the ball. Most of the additional water is added to the nucleation zone in drops to form the cue ball, and a small part is sprayed in the growth zone of the green ball to help the cue ball grow up rapidly.
The method of feeding must also take into account the generation of the cue ball and the cue ball to prevent the formation of too many cue balls. Under the premise of ensuring that the raw ball reaches the required size, the generation speed of the cue ball should be balanced with the growth speed of the raw ball.
The time of rolling into a ball is related to the requirements for the particle size of the pellets and the difficulty of forming the ball into the ball. The size of the pellets is large, and the pelletizing time is longer; the raw materials are poorly formed, and the pelletizing time is also prolonged. The general rule is: prolonging the ball making time is conducive to improving the strength of the ball, especially for raw materials with very fine grain size, and longer ball making time, in order to make the ball have higher strength.
(3) Control of raw ball quality The raw ball is not the final product, but its quality determines to a large extent whether the next calcination process can proceed smoothly and the quality of the finished pellets. The basic requirements for the quality of the raw ball are: the particle size is suitable and uniform, and the mechanical strength is high. It should not be broken before entering the next step, and the thermal stability is good.
The particle size of the raw ball directly determines the size of the finished pellet, and the particle size of the finished pellet is constrained by the blast furnace smelting process. In the past, the particle size of the pellets was relatively large. In recent years, in order to improve the reduction process in the blast furnace, the particle size of the pellets is mostly in the range of 9 to 12 mm. During the firing of the green ball, volume shrinkage occurs, but the particle size of the green ball cannot be too large. In addition, the smaller the particle size of the green ball, the higher the productivity of the pelletizer.
The raw ball comes out of the pelletizer and passes through the belt conveyor to the roasting equipment. The pellets are piled into a bed of a certain thickness in the roasting apparatus. The ball should have sufficient compressive strength and strength against falling impact. Must pass the compression and drop test.
Determination of compressive strength: usually take 10 to 20 green balls, use spring scale or balance to measure the number of kilograms of fracturing, and take the average value and standard deviation.
Determination of impact strength: Take 10 raw balls, free fall on the steel plate or rubber plate from 0.5 meters high, and fall back until crack or collapse. Accumulate the number of times each ball is not broken, take the average and standard deviation.
The temperature at which the pellet begins to burst indicates the thermal stability of the pellet. Generally should not be lower than 300 °C. Because the raw water has a very high moisture content, it must be dried before roasting. If it bursts during drying, it not only loses the pellets, but also affects the smooth progress of the next roasting process. There are two methods for determining the bursting temperature of the green ball. The so-called static, that is, measured without hot air flow. Dynamic is the flow of hot water at a specified temperature through a green ball at a certain flow rate, depending on the temperature at which it begins to burst. Obviously the latter is closer to reality, but the measured results are generally lower than the former.
The high burst temperature of the green ball indicates that the green ball can be dried with a hot air stream at a higher temperature, thereby enabling the device to achieve higher productivity.
Determination of moisture content of raw balls: Generally, a certain number of raw ball samples are taken, and the moisture is measured by a drying method. The suitability and stability of moisture represent the level of ball making operation, and only the moisture is suitable and stable, and the quality of the ball is guaranteed.
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