Degradation of mullite based materials by alkali containing slags

Iron is one of the most important resources that can be found in the lithosphere; 90 % of all metal ores extracted are iron ore. Many steps are included in the extraction from iron ore to metallic iron, where the processes vary between different producers. Iron ore pellets, are a prepared burden material for ironmaking in the blast furnace. Such pellets are commonly sintered in a grate-kiln furnace system, where the kiln usually is insulated with mullite containing bricks. Different mechanisms wear these bricks and they need to be replaced regularly and this causes production stops. The slag present in the kiln consists of ~95 % hematite, alkali-, alkaline earth- and other oxides, mainly from pellets that have disintegrated and adheres in chunks on the bricks. This study is focusing on the interaction between refractories and slags that occurs in kilns during the sintering process in the iron ore pellet production. Results are shown from lab scale experiments, and from samples collected in industrial furnaces, commonly called rotary kilns. Slag/brick compatibility tests were performed in a laboratory furnace at various temperatures, holding times and atmospheres. Slag collected from a production kiln and three commercial bricks, in powder or solid form, were used. Deliberate additions of alkali species were included in order to evaluate their influence. XRD, DSC, TG and in-situ mass spectrometry confirm that addition of alkali dissolves the mullite in the bricks, and forms the phase nepheline (Na2O•Al2O3•2SiO2), which disintegrate to an amorphous phase at elevated temperature. QEMSCAN were used to view mineralogical mappings of different chemical phases by field image scans. It was found that when alkali penetrates the surface of the brick, besides formation of nepheline, phases as kalsilite (K2O•Al2O3•2SiO2), leucite (K2O•Al2O3•4SiO2) and potassium â-alumina (K2O•11Al2O3) are formed. Also seen is that potassium penetrates deeper, and in larger amounts than sodium in the lining material. Formations of alkali containing phases as the feldspathoid minerals kalsilite and nepheline are coupled to an expansion in the lining material, observed by dilatometry, causing structural spalling observed as cracks in some of the slag/brick compatibility tests. Grains of hematite with sizes between 50- 100 ìm stay on the original surface of the brick, while micrometer sized hematite migrates through the dissolved brick by capillary infiltration and diffusion, and nucleates in needle formations deeper in the lining material. We propose a wear mechanism of the bricks in an iron ore pellet producing kiln that involves these chemical reactions in combination with erosion by the continuously flowing slag.Godkänd; 2008; 20081120 (ysko