Liquid phase formation and crystallization process (1)

(1) Formation of liquid phase and formation of liquid phase during crystallization and sintering are the basis of consolidation of sinter. The nature and quantity of liquid phase largely determine the reduction and strength of sinter, so the liquid phase is studied. The conditions of generation and their nature are of great significance.
As pointed out in the previous section, in the sintering process, since the composition of the sinter is large, and the particles are fine and in close contact with each other, when heated to a certain temperature, a solid phase reaction begins between the components, generating a new one. Compound. A eutectic point is formed between each of the nascent compounds, between the constituents of the raw sinter, and between the nascent compound and the original component, so that at a lower temperature, a liquid phase is formed and melting begins. Figure 1 shows the solid phase reaction flow in a self-fluxing sinter. From the figure we can see the solid phase reaction product - the novel compounds of iron, calcium, fayalite after melting and decomposition temperature increases into the melt.

Low melting point compounds and eutectic mixtures may be formed in both self-fluxing or acid frit. They form a liquid phase at temperatures which are achievable at the sintering temperature (1250 ° C to 1450 ° C). For example, 2CaO•SiO ₂ has a melting point of 2130 ° C. However, it can form a eutectic mixture with FeO, and its melting point is 1280 ° C, and thus it is melted into the liquid phase. Other solid phase fractions either remain as a solid phase, become a residue in the sinter structure, or are digested by the molten liquid phase and dissolved in the liquid phase. 3 This effect is due to the liquid phase in its unmelted The result of the expansion of the contact interface of the substance to the latter. Due to the complexity of the mineral composition in the raw materials, the particle size and distribution of the fuel are not uniform, the combustion zone is very fast, and the formation of the liquid phase in the sintered material is also very uneven. Nevertheless, the liquid phase is the basis for the consolidation of sinter.
As the combustion zone moves, the temperature of the material being melted drops, and the liquid phase releases energy and crystallizes or becomes glass. If almost all of the excess energy is released during crystallization, the result is that all of the crystals are precipitated, and the crystal is in the most stable state. However, in the case of a sinter melt, in many cases, it is often impossible to precipitate all of the crystalline material. A lot of silicates become vitreous bodies, and there is considerable potential in them. It is not released in the inside. With the degree of cooling, the number of vitreous bodies is different, so the glass is in a thermodynamically unstable state (stable state). It always has a tendency to crystallize as long as it is given certain conditions, such as heat treatment. [next]
The conditions for the formation of the liquid phase in the sintering process of various sinter materials and the crystallization properties during the cooling process can be studied by means of the equilibrium state diagram of the constituent systems of the bismuth sinter.
(1) Fe ₂ O ₃ -FeO-SiO ₂ system. The main chemical components in the acid sinter are Fe ₂ O ₃ , Fe ₃ O ₄ and SiO ₂ , and the cao is very small. Fe ₂ O ₃ is reduced and decomposed into Fe ₃ O ₄ during the sintering process, thus Fe ₂ O ₃ The ternary system state diagram of -FeO-SiO ₂ composition can illustrate the process of liquid phase formation and crystallization, and FIG. 2 is a Fe ₂ O ₃ -FeO-SiO ₂ state diagram.

1) Distribution of various phases. The region close to FeO, Fe ₂ O ₃ and SiO ₂ is the crystalline region of the phase, in which the lower melting point of the region is the olivine crystal region, and the melting point of the lower and lower floating body regions is also lower.
2) Low-melting compounds and eutectic mixtures exist in both FeO-SiO ₂ , FeO-Fe ₃ O ₄ and Fe ₃ O ₄ -2FeO•SiO ₂ systems. Studying the three system states Figure 3 is important for the liquid phase formation of acidic sinter and the crystallization of the cooling process.

There is a stable low melting point compound olivine with a melting point of 1205 ° C, a composition of FeO72%, SiO ₂ 28%. There are two low melting point eutectic mixtures on both sides, namely 2FeO•SiO ₂ -FeO and SiO ₂ - A mixture of FeO•SiO ₂ , the latter of which is FeO 62%, SiO ₂ 38%, melting point 1178 ° C, the former is FeO76%, SiO ₂ 24%, melting point is 1177 ° C.
When heated, quartz becomes tridymite and cristobalite and even quartz glass. The conversion is very slow. To maintain a certain high temperature, it takes a long time to reach because of crystal structure changes. The phase transition between scaly quartz and cristobalite a, β, γ is very easy to carry out. Above 1700 ° C there are two liquid zones, in which two solutions do not melt each other.
The formation of a low melting point of fayalite under sintering conditions requires a higher temperature and a reducing atmosphere, so that Fe ₄ O _{4 is} reduced to FeO, and thus the amount of the liquid phase formed by the fayalite is determined by the amount of FeO and SiO ₂ . In acid sinter, the amount of the liquid phase of the fayalite is necessary to ensure the strength of the sinter, but the too high sinter strength has a tendency to become brittle.
In the case of normal carbon blending, 80% of the quartz in the acid sinter is digested by liquid phase into the olivine. In the self-fluxing sinter, it is almost 100% digested, so there is not much residual quartz. The bulk expansion produced by the polycrystalline transformation did not have a significant effect on the strength of the sintered ore.
A portion above the figure indicates that FeO will oxidize to form Fe ₂ O ₃ (3FeO + Fe ₂ O ₃ + Fe) when the amount of FeO increases, and the curve indicates the amount of Fe ₂ O ₃ that may exist at each corresponding point. [next]
When the composition of the liquid phase is at the right aa' of the figure, the liquid crystal cooling process first crystallizes the higher melting point of the floss (Fe _x O), the concentration of the floating body in the liquid phase decreases, and the concentration of the fayalite increase. As the temperature drops again, a low melting point eutectic mixture of fayalite and floc is condensed around the crystal of the float.
(3) FeO-Fe ₃ O ₄ system. This system is part of the Fe-O system, see Figure 4.

In this system, as the oxygen content in the melt increases, there are two compounds and a solid solution having a lower melting point. Fe ₂ O ₃ is one of the compounds, containing 30.06% of oxygen, which decomposes into Fe ₃ O ₄ and O ₂ at 1457 ° C, and is an unstable compound, or an isomeric melting point compound. The other one is Fe ₃ O ₄ , which has an oxygen content of 27.64% and a melting point of 1597 ° C, which is a stable compound or an iso-melting point compound. The other one is Fe _x O, its composition is between pure FeO and Fe ₃ O ₄ , and can be regarded as a solid solution of FeO-Fe ₃ O ₄ (actually no FeO), and its maximum oxygen content is equivalent to FeO as Fe. ₃ O ₄ is saturated, and the lowest value is slightly higher than oxygen (22.28%) in FeO. Its melting point is between 1371 ° C and 1423 ° C, which is much lower than the melting point of Fe ₃ O ₄ of 1597 ° C. The amount of oxygen in the solid solution varies with temperature. The chemical formula can be written as Fe _x O, x=0.933～0.953 (570°C～1300°C). The structure of Fe _x O is a salt-type cubic system, iron The node is empty, so it is a lack of solid solution. The character constant is 4.302A ^o ~ 4.272A ^o .
The appearance of Fe _x O is of great significance for sintering pure magnetite concentrate. Since the melting point of Fe _x O is low, a liquid phase of Fe _x O can be produced, thereby consolidating the magnetite concentrate ore. The continuous crystal structure of Fe _x O is found in this sintering, which is one of the reasons why acid sintered ore can maintain a certain drum strength.
It can be seen from the figure that decomposition of Fe _x O to below 570 ° C occurs:
FeO===Fe ₃ O ₄ +Fe
However, under rapid cooling conditions, Fe _x O is not retained as it decomposes, which is one reason why Fe _x O remains in the normal or high carbon content in the sintered ore structure.
The above three kinds of iron oxides have different lattice structures with different oxygen contents, and their lattice constants are also different, aFe2.907A ^o , Fe _x O4.302A ^o ~4.372A ^o , Fe ₃ O ₄ 8.41A ^o , γFe ₂ O ₃ 8.32A ^o , volume expansion occurs during oxidation, which is also a cause of poor sinter strength during reoxidation of sinter. Its volume expansion % is as follows:

In the case of higher carbon content, Fe ₃ O ₄ is reduced to Fe _x O. At this time, FeO in the sinter liquid phase accounts for a large amount, for example, a relatively dry aa' component, then the first precipitation in the cooling process is Fe _x O, in the sinter iron-containing minerals, mainly floating crystals. If the carbon content is slightly lower, the liquid phase composition is first crystallized in the bb' liquid phase is Fe ₃ O ₄ . If very slow cooling is achieved When the reaction is balanced, the eutectic structure of Fe _x O and Fe ₃ O ₄ can be seen.

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