Reactions During Heating of HBI

摘要

The chemical reactive phases in HBI briquettes were iron carbide and iron oxide mainly wustite dispersed in a matrix of iron grains. During heating of a briquette in inert gas, reaction of carbon with iron oxide started at about 550℃ and transformation of α-iron + carbide to γ-iron + carbide occurred from 730℃. The rate profile showed a maximum around 700℃ and a second maximum at temperatures higher than 740℃. The carbon content of HBI had little effect on the maximum rate. The reaction rate and the reaction completion time were strongly dependent on the heating rate and temperature and independent of the sample size, physical form and the inert atmosphere or porosity. From the generated gas composition and different structure analyses it was found that the solid-solid reaction between oxide and carbide took place although the extent of the reaction was limited. At temperatures higher than 680℃, dissolution of carbon in CO_2 occurred and reaction continued with gaseous route. At higher temperatures, where Boudouard reaction could be active, the reaction proceeded through the gaseous route between the grains containing different reactants. It was found that at temperatures up to 600℃ the reaction was possibly controlled by a chemical step such as decomposition of carbide. At temperatures of about 680-750℃ a chemical step such as iron oxide reduction was controlling. At higher temperatures, the rate depends upon the available interfacial reaction areas. Heating of HBI changes the physical structure of HBI. The changes were due to the reaction, phase change, adhesion and recrystallization of the iron particles. Open porosity of the samples increased slightly during carbon reaction and decreased at temperatures above 1000℃. At temperatures higher than 1150℃, porosity reduction was significant.