(A) study on the crystallization behavior of Fe2O3 base molten oxide with cooling rate

다국어 초록 (Multilingual Abstract)

Iron-ore sinter constitutes the major component of the iron-bearing burden in the blast furnace in most countries in the Asia-Pacific region. Therefore, its quality and consistency have a significant impact on blast furnace performance. It is generally accepted that the quality of iron-ore sinter is governed by its microstructure and phase distribution, which occurs during the sintering process, as well as the properties of individual mineral phases and the size, shape, and distribution of their grains, and mutual interaction among the mineral phases.
In modern highly basic iron ore sinter-making, complex calcium ferrites consisting of predominantly Fe, Ca, Al, Si, (Mg) and O are formed from high temperature reactions between the iron ore fines, coke breeze and flux. The calcium ferrite phases are considered the dominant bonding phases in the sinter product and are usually grouped together under the acronym ‘SFCA’ (Silico-ferrite of calcium and aluminum) phases. The properties and behavior of the ‘SFCA’ phases are of significant importance to iron makers as they impact the physical properties of the sinter product and subsequently the reducibility.
Therefore, in this study, the crystallization behavior, microstructure, and reduction of the Fe2O3 based molten oxide system was investigated under the non-equilibrium state at different cooling rates and chemical compositions. The results of this work are expected to further improve the understanding of the phase formation in Fe2O3 based molten oxide system. Furthermore, the information on the SFCA phases is expected to improve the understanding of phase formation as well as provide guidance on the formation of the bonding-phase in sinter through changing the composition of the sintering raw materials and sinter plant operating conditions. The following main points have been addressed within the present work.
In Chapter 4, the crystallization behavior of the binary CaO-Fe2O3 system in the non-equilibrium state with different chemical compositions and cooling rates were investigated. The continuous cooling and transformation (CCT) and quasi-equilibrium phase diagrams of the CaO-Fe2O3 system were determined at various cooling rates. As the cooling rate increased, the non-equilibrium state of the CaO-Fe2O3 system moved in a left-downward direction as a whole. In addition, Darken’s excess stability function was used to understand the changes in the phase diagram. As the cooling rate increased in the 70Fe2O3-30CaO samples, the phase fraction of C2F increased whereas the phase fraction of CF decreased. In the 85Fe2O3-15CaO samples, the CF phase fraction increased as the cooling rate increased, but the hematite phase fraction decreased significantly.
In Chapter 5, the crystallization behavior of the ternary Fe2O3-CaO-SiO2 system in a non-equilibrium state with various chemical compositions and cooling rates were investigated by the confocal laser-scanning. Molten samples were cooled at a rate of 1 K/min to simulate a quasi-equilibrium condition. The effect of SiO2 on the phase distribution was investigated using the Image Analyzer, and the changes in the phase ratios were analyzed using the activity values of Fe2O3 estimated from FactSage 7.1. The CCT diagrams of the Fe2O3-CaO-SiO2 system were obtained for various cooling rates. The temperatures at which the primary phase formed for various SiO2 contents and cooling rates differed from the equilibrium temperatures, and the phase-formation temperature ranges varied with the cooling rate. As the cooling rate increased to 100 K/min, the primary-phase formation temperature of each sample was lower than the equilibrium temperature by 8-53 K. The continuous cooling temperature (CCT) diagram illustrates the primary-phase formation temperature for various cooling rates and SiO2 contents in the Fe2O3-CaO-SiO2 system. The reduction degree increased with increasing SiO2 content and showed the highest reduction ratio of about 5 wt % SiO2. Subsequently, the reduction degree decreased with increasing SiO2 content. Therefore, the reduction ratio in the Fe2O3-CaO-SiO2 system has the greatest effect on the bonding-phase morphology of SiO2.
In Chapter 6, the experiment was conducted to investigate the crystallization behaviors of the ternary Fe2O3-CaO-Al2O3 system according to Al2O3 contents and cooling rates. As Al2O3 content increases, the amount of hematite phase decreases and the phase becomes very fine. Additionally, C2A and CFA phases are formed. Fe2O3 activity gradually decreases, but C2A and CFA activity increases. As Al2O3 content increases at the same cooling rate, the temperature at which the primary phase forms decreases. The temperature for primary phase formation at the calculated equilibrium state decreases with increasing Al2O3 content. As the cooling rate increases to 100 K/min, the primary phase formation temperature for each sample becomes lower than the equilibrium temperature by at least 8–24 K. The reduction rate decreased with increasing Al2O3 content in quasi-equilibrium samples. But as the cooling rate was increased, the reduction rate increases.
In Chapter 7, the experiment was conducted to investigate the crystallization behaviors of the ternary Fe2O3-CaO-SiO2-Al2O3 system according to cooling rates. As Al2O3 content increases at the same cooling rate, the temperature at which the primary phase forms decreases. One can see that the temperature at which the primary phase forms differs from the equilibrium state temperature based on the SiO2 and Al2O3 content and cooling rate. The 1 and 3 wt % Al2O3 at 3 wt % SiO2 sample, the hematite phase, which was present in the quasi-equilibrium state, was no longer present. As the cooling rate increased, an irregularly shaped phase in the equilibrium state was transformed into noodle-type SFCA, CAF and CF phases, and the gap became narrower. The reduction degree was the highest in the 3 wt % SiO2 sample, and the other samples were similar.

국문 초록 (Abstract)

고로 조업을 하는 대부분의 아시아 지역 국가에서 소결광은 고로 장입물 중 철성분을 포함하는 원료 중 가장 중요한 구성요소이다. 따라서 그 품질과 일관성이 고로 조업에 중요한 영향을 미친다. 소결광의 품질은 일반적으로 소결 과정에서 형성되는 미세 조직과 기공 분포에 따라 좌우 된다. 미세조직은 미용융 광석, 부분 용융 산화 광석, 결합조직 그리고 기공으로 구성된다. 결합조직은 칼슘페라이트(CaO-Fe2O3), SFCA(Silco-ferrite of calcium aluminum) 그리고 Slag로 구성되고, 구성비는 소결 과정에서 가열 및 냉각 패턴에 따라 결정 된다.
SFCA는 소결광에서 중요한 결합조직이며, 기계적 특성, 환원성 및 환원분화 특성에 중요한 역할을 하기 때문에 그것의 생성 메커니즘에 대한 연구가 광범위하게 진행되었다. 최근에 개발된 분석 방법들을 이용하여 SFCA의 생성 메커니즘에 대한 연구가 활발히 진행되었으며, SFCA의 형성은 주로 Fe2O3, CaO, SiO2 와 Al2O3 산화물에 의해 지배된다고 알려져 있다. 그러나 대부분의 연구는 평성 상태에서의 반응 방식에 초점을 두었기 때문에 실제 소결공정에서 일어나는 비평형 상태의 높은 가열온도, 속도 및 냉각 중 반응 및 결정화에 대한 정보는 거의 없었다. 따라서 본 연구에서는 빠른 가열 속도와 용융물의 다양한 냉각 조건을 시뮬레이션 할 수 있고, 결정화되는 과정을 실시간으로 관찰 할 수 있는 컴포콜 레이저 현미경을 사용하여 용융 이후 냉각 과정에서 결정화 되는 과정을 조사하였다.
또한, 소결광의 사용비율이 높은 고로조업에서는 소결광의 환원성은 고로의 효율에 직접적인 영향을 미치는 중요한 인자이다. 기존 연구에서는 소결광의 기공과 환원 영향성에 연구가 진행되었다. 그러나 기공의 생성과 형상을 제어하기는 매우 어렵다. 이러한 부분을 극복하기 위해서 본 연구에서는 냉각속도를 변경 함으로써 미세조직 제어에 초점을 두었다. 그 결과 미세조직과 환원성의 상관성을 고찰하였다.
SiO2와 Al2O3함량과 냉각속도에 변화에 따라 조직의 생성온도와 형태가 다양하게 변화하는 것을 알 수 있고, 환원성에 직접적인 영향을 미친다. 이러한 결과들은 소결광 조직 생성에 매우 중요한 정보를 제공한다. CCT diagram과 미세 조직 생성을 기초로 하여 빠른 냉각속도와 화학성분이 변화하는 실제 소결 공정에서 다양한 조업 조건 변화를 통해 소결광의 최종 미세조직과 환원 특성을 조절 할 수 있는 기초 자료가 될 수 있다.