Mechanism And Morphology Of Calcium Ferrite Crystallization In Liquid Binding-phase Of High-basicity Sinter

Calcium ferrite is a main binding phase of high-basicity sinter,and its type,composition,and morphology directly affect the quality of sinter.However,due to the short crystallization time of the liquid phase during the sintering process and the rapid variety of crystalline phases,it is difficult to determine the crystallization temperature,order,and morphology of the crystalline phases,resulting in the very limited study of the liquid phase crystallization stage.Therefore,in this thesis,the melt structure and precipitable minerals,the composition and morphology of calcium ferrite in the melt,the crystallization order of calcium ferrite in the quaternary melt,and the mechanism of the effect of MgO on the crystallization of calcium ferrite were investigated around the liquid phase composition of high-basicity sinter,and the following results were obtained.The relationship between CaO-SiO2-Al2O3 melt crystallization and molecular structure was investigated by preparing CaO-SiO2-Al2O3 glass phases with different basicity and Al2O3 contents to maintain the liquid phase structure at high temperature,using glass phase annealing treatment at constant temperature,and with the aid of Raman spectroscopy.The results showed that increasing the basicity and Al2O3 content led to the crystallization growth rate of CaO-SiO2-Al2O3 melt slows down,which was consistent with the increase of[Si2O5]2-and[SiO3]2-polymerization,respectively.Then,by adding Fe2O3 to the CaO-SiO2-Al2O3 melt,the change pattern of the primary crystalline phase and calcium ferrite during the transition from CaO-SiO2-Al2O3 liquid phase to Fe2O3-CaO-SiO2-Al2O3 liquid phase was revealed,and the crystallization of SFCA was more favorable when the Fe2O3 content was 60.0%.The effects of Fe2O3/CaO molar ratio,SiO2 or Al2O3 content,and cooling rate on the composition and morphology of calcium ferrite in Fe2O3-CaO-SiO2-Al2O3 melt were investigated using an in-house designed crystallization device to establish the link between calcium ferrite species,morphology and generation conditions.It was found that increasing the Fe2O3/CaO molar ratio,SiO2 or Al2O3 content was favorable to promote the transformation of CF and C4F7 to SFCA-I and SFCA;however,the effects on the generation of β-C2S were different,with an inhibitory effect by increasing the Fe2O3/CaO molar ratio and a promotional effect by increasing the SiO2 and Al2O3 content.The crystalline morphology of calcium ferrite is mainly influenced by the crystallization environment.Rational regulation of chemical composition and cooling rate can help to obtain a suitable sinter structure,which can provide theoretical guidance for the production of high-quality high-basicity sinter.The crystallization order of calcium ferrite in Fe2O3-CaO-SiO2-Al2O3 melt(C4F7→SFCA-I→CF→SFCA)was clarified by changing the crystallization temperature and high-temperature duration.Meanwhile,the generation energy of crystallized minerals was calculated using the first principles to explain the rationality of experimental phenomena.And two production pathways of complex calcium ferrite in the melt crystallization process(CF+Si4++Al3++O2-→SFCA or C4F7+Si4++Al3++O2-→SFCA-I,SFCA-I+Si4++Al3++O2-→SFCA)are given to reveal the crystallization mechanism of calcium ferrite in the melt during the sintering process.The changes in the crystalline mineral composition and microstructure of Fe2O3-CaO-SiO2-Al2O3-MgO melt were investigated by changing the MgO content and cooling rate,and it was found that increasing the MgO content promoted the crystallization of MF and γ-C2S of magnesium-containing magnetite,which led to the decrease of Fe content in the melt and was unfavorable to the calcium ferrite(CF,C4F7,SFCA-I and SFCA)crystallization;increasing the cooling rate promotes the crystallization of SFCA-I,MF and Fe2O3,while inhibiting the crystallization of CF.The results show the changes brought by MgO to the crystallization process of sintered ore,and provide a theoretical basis for an in-depth understanding of the mechanism of the effect of MgO on calcium ferrite crystallization.