Effect of an organic binder and copper slag on pelletization of hematite ore eliminating bentonite

Abstract

SYNOPSIS Ammasi. A Traditionally, bentonite is used as the most common binder in iron ore pelletization from the very beginning. It gives good bonding property to green and dry pellets at ambient as well as elevated temperature. However, bentonite contains around 60% silica and 18 % alumina that proportionally increase the alumina and silica content in pellet and increases slag volume and energy consumption in downstream process of blast furnace iron making. An organic binder may be used for replacement of bentonite as it hardly contain alumina and silica and does not leave any significant residue after burning at high temperature during induration of pellet. Many researchers have studied on it and found that several organic binders such as dextrin, starch, molasses, cellulose, peridur, polyacrylamide etc. provide sufficient green and dry compressive strength, drop strength. However, most of these are failed to provide strength at high temperature due to the poor thermal stability (burning or evaporation) during induration of iron ore pellets. Furthermore, Fe3O4 in magnetite pellet is oxidized during induration and provides exothermic heat to the pellet. This reaction helps in forming diffusion bond and supplying internal heat to the pellet for bond formation at 1050-1250 ºC. But, the hematite pellet does not contain lower iron oxides due to which it requires higher induration temperature (1300ºC) and consumes higher energy than magnetite pellet. The required induration temperature can be lowered if the diffusion bond formation is enhanced or some low melting compound is added in pellet for slag bond formation. In present study, the feasibility of a suitable organic material for bond formation in green and dry pellet has been investigated. The Na-Lignosulphonate (NLS), a byproduct of paper mill has been used for this purpose. Since organic binder does not provide strength at high temperature (700-900°C), the low melting (1200ºC) Cu-smelting slag has been used to form bond at relatively lower temperature, where organic binder loses its strength. The Cu-smelting slag enhances slag bonding at relatively lower temperature and can reduce the induration temperature as it contains a significant amount of FeO with some silica (fayalite) and magnetite. Moreover, FeO in copper slag is oxidized at elevated temperature during heating in induration strand. This oxidation enhances diffusion bonding and recrystallization of secondary hematite. Thus, during induration, NLS can provide cold bonding at the low temperature region and Cu-smelting slag may enhance diffusion bonding and slag bonding at the higher temperature region, when NLS evaporates / burns and loses its bonding property. The combined effects of these two materials may help eliminating bentonite completely in developing blast furnace (BF) quality pellets. Therefore, the objectives of the present study are; • Feasibility study for use of NLS as binder and Cu-smelting slag to enhance the strength at elevated temperature and lowering the induration temperature of hematite pellet without bentonite. • Development of good quality hematite pellet in combination of Na-lignosulphonate and Cu-smelting slag replacing bentonite and reducing induration temperature. The study was carried out in laboratory scale taking Noamundi iron ore fines with varying amount of NLS (0.3, 0.5, 0.7 and 1 wt%) as a binder and constant amount of copper slag (1.0 wt %) and basicity (0.3) to optimize the binder content. The characterizations of green balls were done to measure the green pellet property such as green compressive strength (GCS), dry compressive strength (DCS) and drop numbers. The Green balls were indurated at 1200,1225,1250,1275 and 1300°C for 15min. Then, characterizations of indurated pellets were performed to measure their property such as cold compressive strength (CCS), reducibility index (RI), reduction degradation index (RDI), swelling index (SI), apparent porosity (AP) and phase formation (used X-ray diffraction (XRD) and optical microscope). In order to optimize the copper slag content, the pellets were prepared with varying amount of copper slag (0.5, 1.0, 1.5, 2.0& 3.0 wt %) and constant amount of sodium lignosulfonate (0.5%) as optimized. After studying green properties, the pellets were indurated and characterized for CCS, RI, RDI, SI, AP and phase identification as above. The properties of the developed pellet with optimum amount of NLS and copper smelting slag was compared with usual bentonite pellets of identical basicity. The heat of exothermic reaction of FeO and Fe2O3 with copper slag in pellet has been calculated by thermodynamics software ‘Factsage6.4’. The possible phase formation with the flux composition used in pellet at varying temperature near induration has been estimated by the equilibrium module of ‘Factsage6.4’ and it was compared with the phases identified by X-ray diffraction techniques. The microstructures of indurated pellets were analysed using optical microscope. It has been observed that only 0.5% NLS can provide very good green properties (1.8 kg GCS, 4.5kg DCS and 22 drop numbers) to the pellets which are beyond the acceptable limit in plant (1.2 kg, 2.2 kg and 6 Nos, respectively). At 0.3% level of NLS, drop number deteriorates drastically and also with increasing to 0.7% NLS, the GCS deteriorates slightly. Therefore, 0.5% NLS may be considered as optimum percentage. From the properties of indurated pellet, it has been found that 0.5% of only NLS bonded pellet shows comparable strength properties with 0.5% bentonite bonded pellet. However, only NLS bonded pellet shows very high RDI (27%). The evaporation or burning of NLS may be the primary reason behind this. Thus, use of only NLS would not be acceptable. The varying amount of copper smelting slag has been added with 0.5% NLS and found to improve CCS to a great extent. RI has been improved and RDI and swelling has been decreased. 1.0% copper smelting slag addition with 0.5% NLS has been found to be optimum. The developed pellet with this composition shows around 300 kg/pellet strength at induration temperature of only 1250°C. However, the pellet of similar basicity with 0.5% bentonite shows 300 kg/ pellet CCS at induration temperature of 1300°C; i.e. the induration temperature requirement for the developed pellet is 50 °C lower than the usual bentonite bonded pellet. The developed pellet (0.5%NLS+1% Cu-slag added) also shows better RI (80%), almost similar RDI (18%) and swelling (10%) to the usual bentonite pellet. The above properties may be well acceptable in plants. Phase identification of both developed pellets and bentonite bonded pellet through XRD shows mainly Fe2O3, CaAl2Si2O8 phases which are in agreement with the estimation through Factsage. Thus, the present work developed a process of pellet preparation wherein, CCS and reducibility of pellet is superior than bentonite bonded pellet and other properties are also comparable. The bentonite has been eliminated completely by using NLS and copper smelting slag and induration temperature has been lowered by 50°C that may provide a considerable energy and cost saving.