The consolidation mechanism of pellets is essentially different from that of sinter. The consolidation of the sinter mainly relies on the production of a large amount of liquid phase at high temperatures, the precipitation of crystals from the liquid phase upon cooling, or the bonding of a portion of the unmelted ore particles by the liquid phase. Consolidation of sinter is also referred to as liquid phase consolidation. If observed under a microscope, it can be found that the proportion of the liquid phase in the sinter is generally above 25%, otherwise it is not sufficient to maintain its strength. In order to obtain a sufficient amount of liquid phase, a certain amount of SiO 2 is required in the raw material, thereby limiting the further reduction in the amount of blast furnace slag.
Consolidation of pellets mainly depends on solid phase reaction. The solid phase diffusion and consolidation of the unit particles are included at a high temperature, and the multicomponent system forms a compound or a solid solution by solid diffusion. These processes generally occur below their melting temperature, without the formation of a liquid phase, which allows the pellets to be consolidated and have sufficient strength. Of course, it is not completely excluded from the liquid phase in the pellet. However, the liquid phase is rare, and from the principle of consolidation, the pellets can be consolidated without a liquid phase. Under the microscope, the liquid phase in the pellets generally does not exceed 5%, and the liquid phase in the self-fluxing pellets may be more. For this reason, pellets do not require a certain amount of SiO 2 in the raw materials, so the Swedish slag slag slag amount is only 170 kg / ton of iron , which is the result of the use of pellets. Under current conditions, blast furnaces using sinter are not possible, which is one of the advantages of pellets.
1. Solid phase diffusion in the process of roasting pellets The particles in the solid matter are constantly moving. The higher the temperature, the more intense the movement, and the reaction between the particles. According to the results of many studies, the temperature at which solid matter begins to react is much lower than their melting temperature.
During the roasting process, as the temperature increases, the particles in the mineral lattice, the molecular or ion motion, are brushed. Once the necessary activation energy is obtained, they can overcome the force of the surrounding particles, not only in the The internal migration of the lattice, the so-called internal diffusion, can also diffuse to the surface of the crystal lattice, or even diffuse into the crystal lattice of other crystals adjacent to it, and carry out a chemical reaction. ADDING forsterite hematite pellets, at 1250 deg.] C calcined consolidated microscopic image. It can be clearly seen that at the junction of the two minerals, iron ions diffuse into the forsterite and magnesium ions diffuse into the hematite.
There are many factors affecting the solid phase reaction. In addition to the temperature and the residence time at high temperature, such as increasing the pulverization degree of the material, the highly dispersed crystal powder has a large surface energy and is in an activated state, and the reactants are physically and chemically oxidized. Changes, such as removal of crystal water, crystallisation, oxidation, reduction, formation of a solid solution, etc., can activate the crystal lattice of the reactants and help accelerate the solid phase reaction.
There are three types of solid phase diffusion reactions that occur during the calcination and consolidation of pellets:
The solid phase diffusion of hematite (Fe 2 O 3 ) is the main form of consolidation of pellets. Under the action of high temperature, hematite particles form a hematite crystal bond through solid phase diffusion, which connects the particles. To make the pellets have a certain strength. In order to confirm this consolidation mechanism, a sample of pure hematite pellets with a diameter of 100 μm was taken and calcined in air at 1300 ° C for 30 minutes. After cooling, the fracture was observed to observe the fracture. The ball has been firmly joined by the crystal bond, and the bonding strength is so high that the ball breaks along the center without breaking the crystal bond. [next]
If the raw material for pelletizing is magnetite, the lattice structure change when oxidized to hematite will be more favorable for solid phase diffusion and hematite crystal bond formation. However, the crystal orientation of adjacent particles is difficult to achieve exactly the same, as shown in Figure 1. The newly formed crystal bond becomes a transition zone of two particles with different crystal orientations. The lattice structure of this part is imperfect, and it needs to be recrystallized at a higher temperature to complete the crystal lattice, so that the pellets can be obtained higher. Strength of.
When a self-fluxing pellet or a magnesia-containing pellet is produced, a CaO-Fe 2 O 3 , MgO-Fe 2 O 3 binary system appears in the pellet. CaO and Fe 2 O 3 undergo solid phase diffusion reaction between 500 ° and 675 ° C, and firstly CaO•Fe 2 O 3 is formed . The relationship between the reaction rate and temperature is shown in Fig. 2, the temperature increases, and the reaction speed increases.
If there is excess CaO, 2CaO•Fe 2 O 3 can be formed by the following reaction.
CaO + CaO•Fe 2 O 3 =2CaO•Fe 2 O 3 (1)
When CaO is low in the ingredients, it is difficult to form ferrite.
The chemical affinity of SiO 2 for CaO is larger than that for Fe 2 O 3 , but at lower temperatures, CaO•Fe 2 O 3 is preferentially formed. However, the compounds in this system and the solid solution formed between them have a lower melting point. After the liquid phase appears, SiO 2 reacts with CaO of the calcium ferrite system to decompose calcium ferrite to form calcium silicate and free Fe 2 O 2 .
MgO reacts with Fe 2 O 3 to form MgO•Fe 2 O 3 , and solid phase reaction begins at 600 °C. The relationship between the amount of formation and temperature is shown in Fig. 2. In fact, there is always more or less FeO solid solution. MgO•Fe 2 O 3 promotes the form of [(1-X)Mg, xFe]O•Fe 2 O 3 and is called mafic ore. Its amount depends on the temperature, the partial pressure of oxygen in the medium, and the amount of MgO. The higher the temperature, the lower the partial pressure of oxygen, and the more MgO, the more magnesite. This also explains the reason for the increase in the FeO content of the pellets containing MgO. [next]
The gangue in the iron concentrate powder is often quartz- SiO 2 . When producing self-fluxing pellets, SiO 2 acts with the flux CaO to form a calcium silicate system compound. They are first formed by solid phase reaction. Regardless of the amount of CaO, the first is 2CaO•SiO 2 . The calcium silicate system has several compounds, which are 3CaO•SiO 2 , 2CaO•SiO 2 , 3CaO•2SiO 2 and CaO•SiO 2 . If the solid phase reaction is carried out with an excess of CaO and SiO 2 , although the first product is 2CaO•SiO 2 , the final product will be 3CaO•SiO 2 and the remaining CaO; conversely, if an excess of SiO 2 is used In reaction with CaO, the first product is also 2CaO•SiO 2 , and the final product will be CaO•SiO 2 and excess SiO 2 . Fig. 3 is a mixture of CaO and SiO 2 in a molar ratio, and the product of the solid phase reaction is changed at 1200 °C. The first is 2CaO•SiO 2 , followed by 3CaO•2SiO 2 , followed by CaO•SiO 2 . After six hours, 2CaO•SiO 2 disappeared, and 3CaO•2SiO 2 almost disappeared, eventually only CaO•SiO 2 . It should be noted that laboratory studies can take a long time to bring the reaction to near equilibrium conditions, while in production, the reaction time is short and the reaction often does not reach equilibrium.
Second, the liquid phase consolidation Although the consolidation of pellets mainly depends on the solid phase reaction, but roasting at high temperatures, it is inevitable that more or less liquid phase will be produced. There are two sources of liquid phase. One for iron ore gangue contained in a low melting point, such as potassium feldspar, can be melted at about 1100 ℃; pelletizing additive bentonite melting temperature is lower, about 1000 ℃ began to melt. The second is the low melting point compound or co-melt produced during the calcination process, especially when producing self-fluxing pellets, more liquid phase may be produced.
In the process of roasting of pellets, although the liquid phase is not much, it plays an important role in the consolidation of pellets. First, the liquid phase wets the surface of the solid particles and causes the particles to approach, tension, and realign by surface tension. Therefore, in the case of liquid phase production, the pellets have a high shrinkage rate after compaction and a relatively dense structure. Second, some solid phase particles can be dissolved in the formed liquid phase, especially fine particles, and the surface lattice defects are more , more soluble. As the temperature changes, the solubility changes, the dissolved material recrystallizes and the lattice defects are eliminated. Third, since the liquid phase can rearrange the crystal grains and dissolve some soluble substances to recrystallize, it contributes to the growth of mineral grains inside the pellets. In summary, the liquid phase contributes to the consolidation of the pellets. However, the liquid phase should not be too much, too much will produce large round pores, and the structure of the pellets will become brittle, reducing the strength and reduction of the pellets.
Comparing the two photomicrographs, it is more intuitive to see the role of the liquid phase in the consolidation process of the pellets. (a) The figure shows an acidic pellet, which is rare in gangue and is quartz (SiO 2 ), and has almost no liquid phase. The pellets are consolidated by the formation of crystal bonds between the hematite particles and the recrystallization and grain growth of the hematite. Irregular hematite grains and pores can be clearly seen from the photograph. (b) The picture shows a self-fluxing pellet, which produces a certain amount of liquid phase during calcination. Hematite crystallizes in the liquid phase and is completely intact, and the condensed liquid phase is filled in the gap. As a result of the surface tension of the liquid phase, the pores are large and mostly round. [next]
3. Magnetite Consolidation Mechanism Magnetite concentrate is the main raw material for the production of pellets. The consolidation mechanism of pellets made with it is as follows:
(1) The magnetite is oxidized to form hematite microcrystalline bonds and the crystallites grow and recrystallize. Magnetite begins to oxidize at 200 °C. From the surface layer of the particles, Fe 3 O 4 is converted to Fe 2 O 3 to form Fe 2 O 3 crystallites. At the contact of adjacent particles, the new Fe 2 O 3 crystallites have a high mobility, which causes them to bond together to form a microcrystalline bond, also known as a “crystal bridgeâ€, see Figure 4(a). When the temperature rises to At 600 ° C, as long as the oxidizing atmosphere is sufficient, the new crystal bond has a certain strength, so that the pellets form a hard shell. The temperature continues to rise and remains at a high temperature for a period of time. Oxidation advances from the surface of the pellet to the inside. For one particle, the oxidation expands toward the center until it is completely oxidized to hematite. At high temperatures, the hematite crystallites grow and recrystallize, allowing the particles to coalesce into a solid whole, see Figure 4(b).
(2) Magnetite recrystallization. In the absence of oxygen, when the temperature reaches a certain level, the magnetite particles can also be bonded by diffusion to form Fe 2 O 4 crystal bonds, and then at a higher temperature, recrystallization of Fe 3 O 4 and grain growth occur. To combine the magnetite particles into a whole, as shown in Figure 4.
(3) Liquid phase bonding. As shown in Fig. 4(d), the two ore particles are bonded by the liquid phase. If an acidic pellet is produced, in a strong oxidizing atmosphere, the liquid phase which may be produced is a low melting point gangue mineral or an additive bentonite. When calcined in a neutral or weak reducing atmosphere, the magnetite reacts with SiO 2 in the gangue to produce a 2FeO•SiO 2 liquid phase. The experimental conditions are:
The SiO 2 6% magnetite concentrate is made into a pellet sample, and calcined at 1250 ° C for 30 minutes in a nitrogen gas. The magnetite is recrystallized in the 2FeO•SiO 2 liquid phase, and the development is very good. It has a round shape and is bonded in the liquid phase and has a certain strength. This sample was further calcined in air at 1200 ° C for 30 minutes. Since the liquid phase encloses the magnetite particles and deteriorates the oxidation conditions, only part of the magnetite is oxidized to hematite.
When the self-fluxing pellets are fired in an oxidizing atmosphere, the main liquid phase is the calcium ferrite system. If the oxidation is insufficient, a liquid phase of the calcium fayalite system may be formed. In summary, the production of self-fluxing pellets, the production of liquid phase is difficult to avoid.
In combination with the three consolidated forms of the above magnetite pellets, the first one is most desirable. It not only makes the pellets have high strength, but also releases thermal energy when the magnetite is oxidized; the second type of consolidation, although it can also make the pellets have a certain strength, but the energy consumption is high, it is more difficult to avoid the difficulty of reduction. The liquid phase, such as iron silicate, calcium olivine, etc.; the third form of consolidation, is difficult to avoid when producing self-fluxing pellets. If the production of acidic pellets, it is necessary to avoid liquid phase bonding, because it is not necessarily beneficial to improve the strength of the pellets, but will reduce the reduction of pellets.
Fourth, hematite consolidation mechanism
The consolidation mechanism of hematite pellets is very different from that of magnetite. Calcination and consolidation under oxidizing atmosphere, hematite will not re-oxidize, and there is no crystal transformation, so the consolidation is more difficult, requiring higher temperatures, almost double the energy consumption of roasting magnetite pellets. .
As early as the 1950s, it was proved that the pellets made of hematite were calcined to 1270 °C and still could not be consolidated. The compressive strength was almost the same as that of the green ball, but the temperature was raised to 1290 ° C, and its compressive strength. Proliferated to 500 kgf/ball (4903.325 N/ball). The liquid phase was not observed from the microscope. From this, it is judged that the consolidation mechanism of hematite pellets is a simple process of grain growth and recrystallization at high temperatures. Therefore, in the subsequent industrial production, the calcination temperature of the hematite pellets is about 1300 °C.
High temperatures decompose hematite into magnetite, and their decomposition temperature in air is 1383 °C. As the partial pressure of oxygen in the calcining medium decreases, the decomposition temperature of hematite decreases. According to this principle, another way to consolidate the hematite pellets is to first decompose the hematite with high temperature and low oxygen to form magnetite, and then lower the calcination temperature, increase the partial pressure of oxygen in the medium, and make the pellets The ore is consolidated by magnetite consolidation.
The hematite pellet is calcined in a reducing atmosphere, which can be reduced to magnetite or even FeO, and then calcined and consolidated as magnetite. In this way, if the hematite contains quartz gangue, Fe 3 O 4 and FeO react with SiO 2 at about 1000 ° C to form a low melting point of iron silicate, which is very disadvantageous for the reducibility of the pellet.
When hematite concentrate is added with limestone powder to produce self-fluxing pellets, during the roasting process, calcium ferrite and calcium silicate system compounds are formed, which melt at high temperature, improving the conditions of hematite recrystallization. And hematite particles are bonded together.
Although the above four hematite pellet consolidation methods can consolidate hematite, the second and third methods are complicated in process and cannot reduce energy consumption, and the practical value is not large. At present, the first and fourth consolidation methods are practical in production.
In recent years, when producing self-fluxing pellets from hematite concentrate powder, some factories have added a small amount of coke breeze. Canadian experimental research found that in the preheating stage, about 900 ~ 950 °C. The coke powder can quickly reduce the hematite to magnetite, and then the coke powder is burned out, and the pellets are consolidated by magnetite. Although this method does not reduce energy consumption, it replaces expensive gas or liquid fuel with solid fuel, and improves the consolidation conditions of pellets, which deserves further study.
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