신제선 원료로써 고 Al2O3 광의 효과적인 활용 방안

Abstract

Two pyro-metallurgical processes are considered for the effective use of ultra high Al2O3 (>~5mass%) as an ironmaking resource. In the aspect of high temperature (>1673K) smelting reduction of CCA, CaO-Al2O3 based slag system is considered. To understand slag-metal separation in CaO-Al2O3 slag system, In-situ observations of metal-slag separation behaviors between CaO-Al2O3 based slag, iron and graphite powder mixed pellets by a confocal laser-scanning microscope (CLSM) is carried out. Since a particular type of high Al2O3 iron ore such as laterite contains small amount of NiO and Cr2O3, the effect of NiO and Cr2O3 on the metal-slag separation behaviors are also studied. The observed metal-slag separation behaviors are analyzed based on the equilibrium phase fractions calculated by FactSage and carbon diffusion simulation in an iron sphere particle with a spot carbon source condition. Based on these in-situ observations and carbon diffusion results, starting temperature of metal-slag separation is found to correspond to the eutectic temperature of CaO-Al2O3 based slag. The iron carburization is initiated by slag melting and most of iron particles are melted within about 20s after slag melting. The rapid carburization after slag melting is introduced by the good wettability between solid iron and molten slag. NiO and Cr2O3 additions do not change the fundamental behaviors of metal-slag separation at least up to about 3 mass%. Based on these results, it is confirmed that high Al2O3 content iron ore can be used in CCA reduction process by using CaO-Al2O3 based slag. To understand the effect of CaO-Al2O3 based slag system on CCA reduction, the effect of slag composition on the isothermal reaction kinetics of CCA is investigated at 1273K ~ 1573K. Reduction and gasification reaction are separately measured by quadruple mass spectrometry (QMS) gas analysis. For better understanding on reaction kinetics, conventional unimolecular reaction model is modified to get two individual rate equations for reduction and gasification by including reaction driving force. Based on gas composition data and modified kinetic model, rate controlling step of CCA at various temperature and reaction stage are determined. When gasification controls the overall reaction, CaO-Al2O3 slag increases rate of reaction by accelerating gasification while CaO-SiO2 slag decreases rate of reaction by retarding gasification. When reduction control on over reaction becomes dominant, rate of reaction is adversely affected by addition of slag component. Finally, activation energy of the reaction is calculated by conventional unimolecular reaction model and compares with modified model in this study. It is found that gas composition during reaction and modified model are helpful to interpret reaction kinetics when controlling reaction changes depending on the temperature and reaction time. Lab scale test of smelting reduction of CaO-Al2O3 based CCA using high Al2O3 laterite and low ash petro-coke confirms that use of CaO-Al2O3 based slag is highly feasible to produce low sulfur and low phosphorus iron nugget. Finally, in the aspect of DRI process where slag does not melt, effort is made to decrease Ni content in DRI by introducing oxidation roasting. The effect of oxidation roasting of limonitic laterite ores on NiO reduction is investigated with the goal of producing Ni-free DRI and Ni-bearing slag. Oxidation roasting makes NiO inert under H2 reduction at 1173K by forming Ni-Olivine. Optimum roasting temperature is proposed by examining phase transformation of limonitic laterite ores during heating and by FactSage calculation of the equilibrium Ni fraction in Ni-bearing phases. Furthermore, the effect of Mg-silicate forming additives on the control of NiO reducibility is clarified to maximize the suppression of NiO reduction. Among various additives such as MgSiO3, Mg2SiO4 and Fe-Ni smelting slag, Ni-free olivine-typed flux is found to be the most effective form of Ni-olivine because Ni-Mg ion exchange between Ni-bearing phase and Ni free olivine occurs more readily than other Ni-olivine formation schemes. Finally, the mechanism of Ni-olivine formation during roasting is studied using a diffusion couple test. Calculated diffusivity values of Ni in Mg2SiO4 indicates that the two major routes of Ni-olivine formation while roasting limonitic laterite ore are (1) Ni partitioning within Mg-Ni silicate before crystallization and (2) Ni diffusion from spinel to Ni free olivine after crystallization.