Salt solution treatment to prevent the low temperature reduction degradation of haematite pellet

Abstract

Most of the Indian haematite ore shows very high reduction degradation index (RDI) mainly due to high alumina content (>2.2%) and adverse Al₂O₃/SiO₂ ratio (2.0). Several investigators have reduced RDI mostly by suitable flux addition. However, high flux addition makes the pellet basic. In blast furnace, the charge combination of acidic pellet and basic sinter is preferred because, the burden with acidic pellet can accept highly basic sinter. In the present study, the acidic or mildly basic pellets were treated with different aqueous solutions such as, CaO, MgO, MgCl₂, CaCl₂ and NaCl of varying concentration under vacuum (45 mm Hg) to reduce RDI. MgCl₂ solution (8%) has very minor effect on reducing RDI. On the other hand, CaCl₂ and NaCl solutions show encouraging result. The treatment with 10% CaCl₂ solution reduces RDI of acidic pellet from 86% to 5.8%; while, treatment with 6% NaCl solution reduces RDI to a very low level (2.5%). However, it shows poor degradation resistance at relatively higher (>800°C) temperature. Therefore, to reduce this degradation and chloride input in pellet, basicity of the pellets were increased slightly to 0.25 and treated with 1–2% NaCl solution at room temperature under vacuum. This shows very low RDI (6–10%) with good cold crushing strength (330 kg/pellet), reducibility (75%) and swelling index (13%) with minor degradation at higher temperature unlike acidic pellet. This pellet will be useful for blast furnace operation.