Degradation mechanisms in refractory lining materials of rotary kilns for iron ore pellet production

Sammanfattning: Iron is one of the most important resources in the lithosphere; 90% of all metal ores extracted are based on iron. Many steps are included in the extraction of iron from ore, with extraction processes varying among different producers. Iron ore pellets are a prepared burden material for ironmaking in the blast furnace or by direct reduction. Such pellets can be sintered in a grate-kiln furnace system, in which the kiln is usually lined with mullite-containing bricks. These bricks degrade by various mechanisms, and need to be replaced regularly, which causes expensive production stops. Materials deposit in chunks on the lining in the kiln. These deposits comprise ~95% hematite with oxides of alkali metals, alkaline earth metals and others, which originate mainly from pellets that have disintegrated, but also from fly-ash from the burner fuel used to heat the kiln. This study investigated the interaction of deposit materials with refractory lining bricks in kilns during the sintering process in iron ore pelletproduction. Results are obtained from laboratory scale experiments, and from samples collected from industrial kilns, both in-situ and during production stops. Refractory/deposit compatibility tests were performed in a laboratory furnace at various temperatures, for different holding times, and in a number of atmospheres. Deposit materials collected from three commercial lining bricks in production kilns were analysed in both powder and solid forms. Deliberate additions of alkali species (carbonates of potassium and sodium) were made in order to evaluate their influence on degradation mechanisms. Analysis using scanning electron microscopy, QEMSCAN (quantitative evaluation of minerals by scanning electron microscopy), x-ray diffraction, differential scanning calorimetry, thermogravimetry and in-situ mass spectrometry confirmed that alkali additions in the deposit materials dissolve mullite in the liner bricks, which accelerates degradation. Phases such as nepheline (Na2O·Al2O3·2SiO2), kalsilite (K2O·Al2O3·2SiO2), leucite (K2O·Al2O3·4SiO2) and potassium β-alumina (K2O·11Al2O3) were formed. Moreover, it was observed that potassium penetrates deeper into the lining material, and in larger amounts, than sodium, both on the laboratory scale and in industrial furnaces. Formations of alkalicontaining phases such as the feldspathoid minerals kalsilite and nepheline are coupled with an expansion in the lining material, observed by dilatometry, which causes structural spalling that appears as cracks in some of the refractory/deposit compatibility tests. Grains of hematite with sizes between 50-100 μm remain on the original surface of the brick, whereas micrometer-scale hematite migrates through capillary infiltration (in pores, brick joints and cracks) and diffusion, and appears in finer grains deeper in the lining material. The degradation mechanisms of the bricks in an iron ore pellet producing kiln are shown to involve these chemical reactions in combination with thermomechanical stresses. Recommendations are given regarding the choice of materials, the design of refractory liners in the kilns to extend the time between production stops necessary for repair.