Studies in low temperature self-reduction of by-products from integrated iron and steelmaking

Sammanfattning: Within the last decade, the landfill tax in Sweden has increased over 70% with comparable rates in the UK and elsewhere in EU. This development, among others, highlights the incentive for increased recycling efforts of fine particulate solid by-products in the iron and steel making industry in order to avoid depositing costs and to recover valuable metal fractions. As pre-treatment is necessary for recycling fine particulate material, cold bonded agglomeration is considered to be a well-suited alternative for recycling of steel industry by-product dust and sludge. The major objective of this work has been to provide insight to help further the development of cold bonded agglomeration technology in order to increase the recycling of iron and steelmaking by-products. Laboratory pelletizing tests were conducted based on a factorial design test plan to help identify those variables that most influence the cold strength and capacity of product cold bonded pellets (CBPs). The design variables were the individual components of the by-product pellet mix; i.e blast furnace (BF) flue dust, oily mill scale sludge, both fine and coarse basic oxygen furnace (BOF) dust and Portland cement as binder. A related statistical procedure was used to plan reduction experiments in inert gas over a temperature range of 20-1200¢XC. The results from cold strength and reduction tests have been evaluated using multivariate statistical analysis to model the experimental variables with chosen responses in order to help identify those variables that have most significance. For agglomerate dimensions used in this study, pellet blends with large fractions of particles in the size range of 10-40 ƒÝm promote good self-reduction while maintaining good cold strength. The fundamental reactions occurring during the heat treatment of CBPs have also been studied. BF flue dust, which contains fractions of coal and coke particles, has been included in the CBP blend as a source of solid reductant. Thermal analysis was performed on samples in inert atmosphere at a heating rate of 10¢XC/min in order to observe their high temperature properties, specifically, the mechanism(s) of self-reduction. The gases generated during thermal analysis were analyzed with a mass spectrometer. Furthermore, raw and heat treated pellet samples were analyzed using X-ray diffraction and scanning electron microscopy. Results demonstrate that the decomposition of hydrates and carbonates in CBP samples contribute, as gaseous intermediates, to the prereduction of contained iron oxides. The gaseous intermediates are responsible for an initial gasification of carbon contained in BF flue dust leading to low temperature iron oxide reduction. The step-wise reduction of iron oxides in the pellets at the given conditions begins at roughly 500¢XC and is nearly completed at 1200¢XC. Detailed studies of the system Ca(OH)2-C-Fe2O3 have also been conducted because of the inherent importance of this system in the self-reduction of by-product agglomerates. Experiments have confirmed that carbon gasification does occur during the dehydration of lime and kinetic results highly correlate to behavior in CBPs. Hematite reduction in the simplified chemical system is controlled by either carbon gasification or availability of direct contact with graphite depending on hematite grain size. These results have less correlation with CBP behavior. After dehydration of Ca(OH) 2, calcium ferrite formation begins at temperatures below 600¢XC. Furthermore, the varying composition of this system has been studied in order to optimize self-reduction and low temperature bonding characteristics. The metallurgical characteristics of CBPs have been tested in additional laboratory tests; i.e. isothermal reduction tests, BF simulation and softening and melting tests. These test results indicate that CBPs; can disintegrate during reduction at high temperature, are self-reducing to a high extent and, as a supplement to the normal ferrous burden, show good softening and melting properties. Large-scale trials with CBPs have been conducted in a commercial BOF and a pilot scale BF. Results from BOF testing show that charging of CBPs is feasible at levels up to 2.2 wt% of the total charge. At these levels, CBP addition resulted in no adverse disturbances to steel and slag chemistry and to the process in general. Results from pilot scale BF tests show that operation was very stable during testing with 150 kg CBPs/tHM but that the burden descent rate and burden gas distribution were disturbed during periods with more CBP burden. The rate of reducing agents was significantly decreased and slag amount was increased when CBPs were charged.