GRATE - KILN - COOLER. WHERE TO OXIDIZE AND WHY?

摘要

The Swedish iron ore company LKAB has iron ore mines in Kiruna and Malmberget, ore processing plants in Kiruna, Malmberget and Svappavaara and shipping ports in Lulea and Narvik (Norway). The main products are sintering fines and pellets. Pellets accounted for nearly 78% of the total ore sales in 2000. Three types of pellets are produced today: olivine fluxed pellets (KPBO) and olivine-quartzite fluxed pellets (KPBA) for ironmaking in blast furnaces and dolomite fluxed pellets (MPRD) for ironmaking in direct reduction furnaces. LKAB has four pelletizing plants in operation. Two plants in Kiruna and one in Svappavaara are of the grate-kiln type, the plant in Malmberget is the straight-grate type. The total pelletizing capacity is about 16 Mt/year. The predominant iron mineral in the ore is magnetite (Fe~3O_4) which is oxidized to hematite (Fe_2O_3) during the sintering process. This reaction is strongly exothermic and a lot of energy is liberated. The oxidation also influences the phase composition and microstructure in the pellet and therefore also the mechanical and metallurgical properties of the final product. Therefore it is not unimportant where in the process the oxidation takes place. It is also important when this happens in relation to other reactions like melt formation, calcination of carbonates and breakdown of olivine. The purpose of this paper is to describe how the microstructure in the pellet varies depending on where in the process the oxidation takes place and what it means for the process and the product quality. All studied pellets are olivine fluxed pellets (KPBO) from LKAB's pelletizing plants of the grate-kiln type. They were investigated in a polarizing microscope in reflected light and in a scaning electron microscope (SEM).