Liquid phase formation and crystallization process (2)

(4) Fe 3 O 4 -2FeO•SiO 2 system (Fig. 5). This system has a low melting point eutectic mixture with a composition of 83% 2FeO•SiO 2 and 17% Fe 3 O 4 , which has a melting point of only 1142. ℃, lower than the melting point of fayalite 1205 ℃, so that it first forms a liquid phase during sintering. As the Fe3O4 in the material gradually dissolves into the liquid phase, the melting point of the liquid phase gradually rises as indicated by the arrows in the figure. When the liquid phase reaches the shaded portion, most of it: Fe 3 O 4 (about 70%) is melted into the liquid phase. Therefore, when the acid sinter is sintered, the solid material is heated to a very high temperature before being dissolved in the liquid phase, which consumes a high heat of fusion, so that a high carbon content is required.

When the high-temperature liquid phase is cooled, the magnetite crystals are first precipitated in the liquid phase in the shaded portion, and the composition of the olivine in the liquid phase is continuously increased. When the temperature continues to decrease, the liquid phase in which the olivine is the main component is finally solidified in the magnetic phase. Formation of granular structure around iron ore crystals (III) Metallogenic process of self-fluxing sintered ore
The main feature of the self-fluxing sinter is that a part of the flux CaO is added to the sinter. The main components of the sinter are Fe 2 O 3 , Fe 3 O 4 , CaO, SiO 2 , and FeO is formed during the sintering process. It is a CaO-FeO-SiO 2 , a ternary diagram, and a binary diagram of CaO-SiO 2 , 2CaO•SiO 2 -2FeO•SiO 2 , CaO•SiO 2 -FeO•SiO 2 .
(1) CaO-FeO-SiO 2 system state diagram (Figure 6)

Compounds CS, C 3 S 2 , C 2 S, C 3 S appear in the CaO-S 1 O 2 binary system, in which the melting point of CS is lower. The low melting point compound 2FeO•SiO 2 appears in the FeO-SiO 2 system, and its melting point It is 1205 ° C.
The area near the CaO, FeO, and SiO 2 is a crystalline region of each species. It is worth noting that when FeO-SiO 2 is added with CaO, 2FeO•SiO 2 moves toward the center, and a low melting point region, a calcium olivine crystallization region, has a melting point below 1200 °C. When CaO=10% and FeO/SiO 2 is close to 1, the melting point of the lowest eutectic mixture in the C 2 S-2FeO•SiO 2 system is 1105 ° C. Continue to increase CaO, and CS, C 2 S, C 3 S appear. The crystallization zone. [next]
When the sinter alkalinity is 0.5-0.6, the calcium fayalite in the sinter replaces the fayalite, and a low-melting eutectic mixture of the calcium olivine and FeO(Fe 3 O 4 ) is found. When the basicity is from 0.5 to 1.0, calcium silicate exists as a separate phase between the olivine. Continue to increase CaO due to the saturation of calcium olivine to form a considerable amount of calcium silicate. When the alkalinity is increased to about 2.0, the composition of calcium ferrite is very prominent. Another condition for the formation of calcium ferrite is the presence of more Fe 2 O 3 in the feedstock, as illustrated by the CaO-Fe 2 O 3 binary phase diagram.
(2) CaO-SiO 2 system (Figure 7).

Both CaO and SiO 2 have a high melting point, CaO has a melting point of 2570 ° C, and SiO 2 is about 1723 ° C, but when they form a compound, the melting point thereof is remarkably lowered.
There are four compounds in the CaO-SiO 2 system: CS (CaO•SiO 2 ), C 3 S 2 (3CaO•2SiO 2 ), C 2 S(2CaO•SiO 2 )C 3 S(3CaO•SiO 2 ), among which CS, C 2 S is an isomer melting compound whose compound is stable, and C 3 S 2 and C 3 S are hetero-melting compounds which are unstable compounds. [next]
1) CS is wollastonite , there are two variants, a and β. A is a high temperature type, β low temperature type. The melting point of CS is 1544 ° C, and its crystal transition temperature is 1125 ° C. CS and SiO 2 and C 3 S 2 can constitute a CS-SiO 2 and C 3 S 2 -CS eutectic mixture, and its melting point is lowered to 1436 ° C to 1460 ° C, which can be reached during the sintering process with higher carbon content.
2) C 2 S is calcium orthosilicate. It is a compound having a melting point of 2130 ° C. However, it can form a low melting point eutectic mixture with 2FeO•SiO 2 , as shown in Fig. 9. When the temperature drops, it precipitates from the eutectic mixture.

C 2 S is a phase change material having four variants aa'-β-γ, presumably also having a β' phase. C 2 S 8 is a schematic diagram of polymorphic transformation .C 2 S crystal transition sequence for heating γ-a'-a, cooling transformation order of a → a '→ β → γ.β phase is metastable, having γC 2 The singularity of S. Due to the rearrangement of the crystal lattice during the crystal transformation, the density changes, from a'-γ, the volume expands by 12%, and the volume expands by 10% from β→γ, which is the self-fluxing of quartz gangue concentrate. The main reason for sinter crushing and pulverization.
3) C 3 S is tricalcium silicate, which is an unstable compound. It maintains a stable solid state at 1250 ° C ~ 2070 ° C, begins to melt at 2070 ° C, and decomposes into CaO and C 2 S below 1250 ° C. In sintered ore. When the alkalinity exceeds 2.0, C 3 S appears and replaces C 2 S. Since the sintering process has a fast cooling rate, it appears in the high alkalinity sintered ore. It has no polycrystalline transformation, so it is advantageous for the strength of the sinter instead of C 2 S.
(3) State diagram of 2CaO•SiO 2 -2FeO•SiO 2 system (Fig. 9). It is a cut surface of CaO-FeO-SiO 2 .

1) C 2 S-2FeO•SiO 2 can form a stable compound, calcium olivine CFS (CaO•FeO•SiO 2 ), which has a melting point of 1280 ° C and additionally constitutes three solid solutions. The first one is that CFS is dissolved in C 2 S, the second solid solution is that C 2 S is dissolved in CFS, and the third solid solution is a continuous solid solution of CFS and F 2 S (2FeO•SiO 2 ), which is the lowest. The melting point is 1170 ° C.
2) Since the high melting point C 2 S and F 2 S can form a low melting point solid solution, it can become a liquid phase at the sintering temperature, and when cooled, the C 2 S solid solution is precipitated from the liquid phase.

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