Reducibility Study of Briquettes Made from Iron Bearing Metallurgical Wastes for use in Iron Blast Furnace

Abstract

Various by-products of iron and steel making industries viz. Iron ore fines, coke fines, blast furnace dust and basic oxygen furnace sludges, etc. are generated at various units of ore to steel processing. Although, these by-products consist of valuable minerals, such are being considered as waste in context to cost economy and environmental well-being. Recycling of these wastes is essential to recover the valuable minerals for the preservation of natural recourses and to reduce the disposal cost for the fulfillment of the environmental protection regulations. Even though these iron, carbon, and calcium contents wastes are recycled through several conventional agglomeration methodologies; all types of integrated steel plant wastes cannot be used precisely due to cost effectiveness and metallurgical process constraints.

Therefore, in the present research work, an attempt has been made to utilize the aforementioned wastes through agglomeration, i.e. pelletization and briquetting of that iron ore fines, BF dust, mill scales, BOF sludges, coke fines and reduction behavior of these were studied. Further, agglomerates containing iron ore fines and aforementioned metallurgical wastes were subjected to reduction under identical conditions for obtaining comparative results.Basicity plays a vital role in determining the physiochemical behavior of agglomerates and a very few research works have been available on the reducibility of high basicity agglomerates at elevated temperatures. Hence, investigations were taken into consideration in the present study for the same.

Initially, plain iron ore pellets in the basicity range of 0.9 to 2.4 were undergone a carbothermic reduction in the temperature range of 1100 to 1400°C. Reduction test results showed that the highest degrees of reduction and metallization of about 95.1 and 94.3% respectively were achieved for pellets having C/S ratio 1.6 and reduced at 1400°C for 60 minutes. Further increase in C/S ratio resulted in lowering the reduction rates due to the formation of calcium-rich ferrite phases, as confirmed by X-Ray analysis and microscopic investigations. However, softening-melting characteristics study of slags (C/S above 1.6) revealed that the slag having C/S ratio of 2.0 can be considered as primary slag. On the contrary,the higher reduction rate has been achieved for mixed pellets (made from iron ore fines, aforementioned metallurgical wastes and coke fines) having basicity ratios of 1.6, 1.8, and 2.0 reduced at similar temperatures, in comparison to plain iron ore pellets. Despite lower viireducibility of FeO at lower temperatures as compared to that of other higher iron oxides, higher reducibilities of these mixed pellets were achieved due to the presence of extra carbon (as BF dust and coke fines) in the pellets. The presence of carbon in the mixed pellets, in turn, gets close contact with the iron oxides and hence enriches the gasification rate. The dominance of FeO gets diminished at higher temperatures which resulted in highest reduction rate.

Stiff extrusion briquetting was implemented in the preparation of cold briquettes from iron ore fines, BF dust, BOF sludges, mill scale and coke fines having different basicities (in the range of 1.6 to 2.0). The reducibility tests of the briquettes were carried out in stimulated blast furnace conditions and physical properties of the briquettes were determined by using appropriate characterization techniques. The highest degrees of reduction and metallization of about 95.84 and 95.22% were obtained for the mixed briquettes (made from iron ore fines,aforementioned metallurgical wastes and coke fines) having basicity of 1.6. The resence/formation of different ferrites at higher basicity enhanced the reduction potential of the briquette. Comparatively, mixed briquettes showed higher reducibility than plain iron ore briquettes because of close contact of carbon (present in BF dust and coke fines) with iron oxide in mixed briquettes. The industrial trial of cold charging extruded briquettes (after curing in normal temperature and atmosphere) in a 38 m 3 mini blast furnace showed encouraging results.