High alkalinity sinter and low temperature sintering--Experimental study on calcium ferrite

I. Overview
High alkalinity sinter appeared in the 1960s, with its high alkalinity, excellent metallurgical properties different from the self-fluxing sinter. Low-temperature sintering technology is the production of high-quality and high alkalinity sinter basic measures to reduce the energy consumption of sintering, which appeared in the 1970s, the core of the low-temperature sintering technology is to create a suitable temperature, atmosphere and material composition conditions, the formation of large needle-like calcium ferrite (SF-CA) makes it the main binder phase of sinter. High alkalinity sinter and low temperature sintering techniques have been widely used in production practice.
Second, the basic characteristics of high alkalinity sinter
The high alkalinity sinter has the characteristics of low FeO, good reducibility and high strength. The main reason is that the main binder phase is calcium ferrite (SFCA).
The alkalinity value (m(CaO)/m(SiO 2 )) of the high-quality and high-alkalinity sinter is generally in the range of 1.8 to 2.2, and the calcium ferrite is mainly present in the form of needles. Sintered ore has the best metallurgical properties and low energy consumption.
The alkalinity value is lower than 1.8, the liquid phase of the iron-containing silicate in the sinter increases, the alkalinity value is higher than 2.2, the calcium ferrite is excessively formed, and a considerable amount of ferric acid-calcium or even ferric acid dicalcium appears. , are not conducive to the strength and reduction of the sinter.
Third, the experimental study on calcium ferrite
In view of the fact that calcium ferrite, especially acicular calcium ferrite, has a decisive influence on the metallurgical properties of sinter, iron and steel sintering workers at home and abroad have conducted a large number of experimental studies on calcium ferrite.
(1) Chemical composition of calcium ferrite A large number of experimental studies have confirmed that the calcium ferrite component in the sintered ore contains a certain amount of SiO 2 and Al 2 O 3 as Fe 2 except for mainly Fe 2 O 3 and CaO. O 3 -CaO-SiO 2 -Al 2 O 3 quaternary composite calcium ferrite, its chemical formula is 5CaO•2SiO 2 •9(Fe,Al) 2 O 3 , abbreviated as SFCA, and often contains some components such as MgO and FeO. . Scanning electron microscopy-energy spectroscopy was used to analyze more than ten sinter mines such as Anshan Iron and Steel, Baosteel and Shougang in China. The results show that although their iron content, alkalinity and calcium ferrite are different, the sinter's iron-containing raw materials are different. However, the calcium ferrite formed is a composite compound of iron, calcium, silicon and aluminum quaternary compounds, and its chemical formula is SFCA. The ratio of the amount of Fe 2 O 3 to the content of CaO is about 2, which belongs to ferric acid hemi-calcium. The SiO 2 content and the Al 2 O 3 content in the calcium ferrite vary from 5% to 10% and from 1% to 3%, respectively. In the sinter of Baosteel, there is also a high-iron ferric calcium ferrite with a ratio of the amount of Fe 2 O 3 to CaO. The X-ray diffraction and chemical analysis prove that the iron in the calcium ferrite is mainly Fe 2 O 3 . Form exists, the FeO content is only about 1%.
(II) About the strength and reducibility of calcium ferrite Through the strength measurement of the main minerals of the sinter, it is found that the strength of hematite is the highest, followed by calcium ferrite, and the magnetite is again, various silicate minerals. Especially the glass phase has the lowest strength. See Figure 1


Experimental studies have shown that the reduction of calcium ferrite (SFCA) is similar to that of hematite, which is significantly better than magnetite. The higher the ratio of m(Fe 2 O 3 )/m(CaO) in calcium ferrite, the better the reducing property, and the order is: ferric acid hemi-calcium-ferric acid-calcium-ferric acid dicalcium. Acicular calcium ferrite belongs to the ferric acid hemi-calcium type, which has the best reduction, see Figure 2.

When the sinter steel 23m 3 test blast furnace was used for anatomical test in 1979, the sintered ore sample was observed under the microscope. It was found that the metal iron preferentially appeared in the Fe/O periphery formed by the reduction of hematite and calcium ferrite, and the reduction of calcium ferrite was confirmed. In magnetite. The reduction of the sinter present in the acicular calcium ferrite is significantly better than that of the sinter present in the form of flaky, columnar calcium ferrite.
In high alkalinity sinter, the content of calcium ferrite is generally 30% to 50%, and the content of Fe 2 O 3 is more than 70%, so acicular calcium ferrite is not only a good binder phase, but also a red Iron ore and magnetite are equally important iron minerals, and their reductivity is excellent. SiO 2 and Al 2 O 3 in high alkalinity sinter are intensively entered into calcium ferrite to make iron silicate liquid residue Greatly reduced, this is also the reason for the high alkalinity sinter strength and reductive properties. Acicular calcium ferrite is a very low binder phase containing Fe 2+ , so the strength of high alkalinity sinter is not directly related to the content of FeO, thus breaking the traditional concept of FeO as a sinter strength index. The use of acicular calcium ferrite instead of silicate as a binder for sinter makes it possible to reduce SiO 2 and increase the iron content of the sinter. It was previously thought that the SiO 2 content of the sinter should not be less than 6%, otherwise the strength would be affected. At present, the SiO 2 content of high-quality and high-alkalinity sinter has been reduced to 4% to 5%, and still has sufficient strength.
(III) Formation mechanism of acicular calcium ferrite The author used micro-sintering experiments, interrupting the sintering cup experimental process, dissecting the sinter column, etc., to make the mechanism, process conditions and influencing factors of acicular calcium ferrite formation more detailed. In-depth research, the following is the research results.
1. The formation of acicular calcium ferrite The interruption of the sintering cup experiment and the dissection of the sintered column are ideal methods for the study of the formation of acicular calcium ferrite. Hematite was used as a raw material for sintering, and the alkalinity value (m(CaO)/m(SiO 2 )) was 2, the sintering process was interrupted, the samples were dissected, and their mineral compositions were analyzed under a microscope.
When hematite is sintered, in the pre-tropical zone, in addition to the limestone decomposition reaction, more calcium-calcium ferrite (including ferric-monocalcium ferrite and dicalcium ferrite) is formed. A large amount of acicular calcium ferrite (SF-CA) is rapidly formed in the combustion zone, and at the same time, more hematite is reduced to magnetite. In the high temperature oxidation zone (referring to the cooling zone with a temperature above 1100 °C), some of the magnetite is reoxidized, and the acicular calcium ferrite is further increased. The calcium ferrite forms an interlaced structure or forms an interwoven and aerobic structure with magnetite. The original and regenerated hematite is bonded together.
When sintered with magnetite (the alkalinity value is also 2), the pre-tropical zone is mainly the decomposition reaction of the flux, and the amount of calcium ferrite is extremely small. In the combustion zone, iron oxide is still mainly composed of magnetite, and only a few flakes of high calcium type calcium ferrite are formed. CaO is dissolved in magnetite in a large amount, and silicic acid is formed with SiO 2 and Al 2 O 3 . Calcium and silicate liquid phase. Under the temperature and atmosphere of the high temperature oxidation zone, a large amount of magnetite is oxidized, and a large amount of acicular calcium ferrite is formed in a large amount of newly formed hematite strontium and dicalcium silicate. The chemical reaction can be expressed as follows:
9(Fe,Al) 2 O 3 +2(2CaO•SiO 2 )+CaO solid solution →5CaO•2SiO 2 •9(Fe,Al) 2 O 3
Studies have shown that magnetite can be used as a raw material to form high-quality high-alkalinity sinter with acicular calcium ferrite as the main binder phase, but the amount of acicular calcium ferrite is less than that of hematite. , FeO content is more.
It can be seen from the above that the formation of acicular calcium ferrite in the high-temperature oxidation zone (the cooling zone above 1100 °C) located in the upper part of the combustion zone is very important for magnetite or hematite sintering, especially for magnetite. sintering.
2. Effect of temperature on the formation of acicular calcium ferrite The magnetite is prepared into a sintered sample of alkalinity (m(CaO)/m(SiO 2 )) 2.0, which is pressed into a cake (Ф 8 mm × 4 mm). In the air medium, calcination at 1260 ° C can only produce a small amount of flaky calcium ferrite. Calcium ferrite starts to form at 1210 ° C and is converted to acicular calcium ferrite. At 1250 ° C, the acicular calcium ferrite content in the sample reached 75% to 80%. When the temperature is higher than 1260 °C, the acicular calcium ferrite is obviously decomposed and converted into hematite, dicalcium silicate and silicate liquid phase, and the calcium ferrite content drops sharply. Experiments show that for magnetite, the optimum temperature for the formation of acicular calcium ferrite is 1230 ~ 1250 ° C, and hematite is 1250 ~ 1270 ° C.
The above-mentioned cake experiment was verified by sintering cup sintering. The magnetite mixture with a basicity value of 2.0 decreased the fuel ratio by 4.3%, 4%, 3.8%, 3.6%, and 3.2%. As the fuel ratio decreased, the calcium ferrite content in the sintered ore increased from 30% to 50% ~ 55%, the morphology changed from multi-melting sheet to needle-like, FeO content decreased from 10.6% to 5.62%.
The above experiments show that the formation of acicular calcium ferrite is sensitive to temperature and requires a lower sintering temperature. This is the fundamental reason for the production of high alkalinity sinter with acicular calcium ferrite as the main binder phase and allows low temperature sintering, indicating that the high quality and high alkalinity sinter production technology is a combination of high quality and energy saving.

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