Degradation Behaviour of a High CSR Coke in an Experimental Blast Furnace: Effect of Carbon Structure and Alkali Reactions

Abstract

A high CSR coke was tested in the LKAB's Experimental Blast Furnace (EBF) at Luleå. The evolution of physical and chemical properties of the centre-line coke samples were analysed by Light Optical Microscopy (LOM), BET N 2 absorption and SEM/XRF/XRD. Alkali distribution in the EBF cokes was examined by XRF/SEM and EDS. Thermo Gravimetric Analysis (TGA) was used to measure isothermal and nonisothermal CO 2 reactivity of the cokes. The crystalline order of carbon and the concentration of alkalis were found to increase as the coke descended through thermal reserve zone to the cohesive zone of the EBF. The crystallite height (L c ) of EBF coke carbon displayed a linear correlation with the measured EBF temperatures demonstrating the strong effect of temperature on carbon structure of coke in the EBF. Alkali concentration of the coke was increased as it descended into the EBF, and was uniformly distributed throughout the coke matrix. The CO 2 reactivity of lower zone cokes was found to increase when compared to the reactivity of the upper zones cokes, and was related to the catalytic effect of increased alkalis concentration. The deterioration of coke quality particularly coke strength and abrasion propensity were related to coke graphitisation, alkalization and reactivity. Coke graphitisation is shown to have a strong influence on the coke degradation behaviour in the EBF.KEY WORDS: coke; CSR; abrasion; graphitisation, XRD; gasification; TGA reactivity; alkali.experimental blast furnace. Even though the EBF tests are time consuming, tedious and highly expensive, the information generated is of great value in terms of their reliability suitability due to simulation of more realistic conditions of a blast furnace process.Coke degradation in a blast furnace occurs due to chemical, mechanical and thermal effects. At higher flame temperature, cracking of coke is mainly attributed to thermal stress while at lower temperatures, the degradation behaviour is influenced by coke reactivity which is dependent on other coke properties. The coke reactivity can be influenced by its three major properties namely porosity, carbon structure and constituent minerals. Coke pore structure is modified by growth and/or coalescence of pores, which is often related to fluidity and swelling characteristics of parent coals, 5) and modifies the available carbon surface area for gas reactions. Coke displays graphitisation behaviour particularly at temperatures exceeding 1 200°C, which is influenced by the catalytic effect of minerals such as iron, and is believed to weaken the abrasion resistance.6) Iron in coke is also believed to catalyze gasification reactions 7) which could have different implications on coke behavior in the EBF. The influence of other minerals particularly those containing alkali on the coke degradation is less certain. The main aim of this study is to investigate the effect of alkalis on coke behaviour in the experimental blast furnace. Therefore, it is imperative to discuss further various aspects of alkalis influence on...

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