Reactions of waste rubber tyres and polypropylene plastics with gases and electric arc furnace steelmaking slags

摘要

Steelmaking using an Electric Arc Furnace (EAF) has gained more interest in recent years, due to the low capital and operating cost; and its flexibility in the use of raw materials (i.e. scrap). Depending on the cost and availability, anthracite and metallurgical coke are among the conventionally injected materials used for foaming in EAF steelmaking. Considering the energy and green house gas emissions requirements, alternative carbon sources are put on the spot to replace, at least partially, the conventional materials, i.e.. waste materials such as Rubber and polypropylene (PP) plastics may react with gas and slag phases resulting in devolatilization, combustion and iron oxide reduction reactions. In the present study, the conventional material investigated was metallurgical coke which was blended with different proportions of rubber and PP. Metallurgical coke, Rubber-coke blends and PP –coke blends were combusted in a drop tube furnace (DTF) at 1200 ºC under 20%O2 and 80% N2 gas mixture. Subsequently, the residual materials were put in contact with two EAF iron oxide rich slags and their interfacial reactions and phenomena have been studied at 1450-1550ºC in a horizontal tube furnace under inert atmosphere (1l/min Ar) with off gases measured using an IR analyser. The initial devolatilization and the subsequent step of combustion of these samples are considered in a Drop Tube Furnace (DTF) and in a Thermogravimetric Analyser (TGA), respectively, while the sessile drop approach investigated the interfacial reactions taking place in the slag/carbon region. The gas phase studies showed a reactive rubber-coke residue with an improved surface area resulted from volatile matter removal. The rate of devolatilization appears to improve the coke-rubber burnout as well as its foaming behavior when put in contact with an iron oxide rich slag. However, a higher rate of devolatilization, as seen in PP-coke blends shows a fragmented particle which affects the combustion while less volatiles are available to sustain subsequent reactions in the slag phase.Off gas analyses following the carbon/slag interfacial reactions have been measured for all the carbonaceous materials and significantly different gas concentrations have been observed. The CO and CO2 emissions from metallurgical coke showed lower concentrations in comparison to that from the coke-rubber and coke-PP blends. At 1550º C, both slags were non-wetting in the initial stage of reaction dictated by the chemical reaction and gas evolution. A slower rate of chemical reaction is seen when coke reacted with the EAF slag, while the rubber blends showed a faster reduction leading to a further drop in the contact angle θ. Independent of the carbon material used as a substrate, the final stage of reaction reveals comparable contact angles due to similar extents of reduction and Fe deposition at the interface. Extensive FeO reduction followed when the EAF slag reacted with the carbonaceous materials. Coke and its low activation energy value correlates with less gas and thus less stirring in the slag and mass transfer in the liquid becomes an important rate limiting step. With increasing proportion of rubber in the blend, moving the process towards mixed controlled in which chemical control becomes important as seen in the trends in the activation energy values. Therefore a mixed controlled process limits the FeO reduction reaction for coke and coke-rubber blends. Higher gas entrapment in the slag was observed when rubber partially replaced coke. The lower devolatilization rates seen in rubber blends compared to PP blends allows the formation of a highly porous particle, promoting gasification and allowing more gases to be trapped in the slag phase compared to PP blends. Optimization between the two phenomena, reduction and foaming is required for improved EAF process performance. Industrial trials using rubber as partial carbon source in EAF steelmaking resulted in decreased specific energy consumption compared to coke alone. This study highlights significant differences in the carbon/slag interactions of coke/rubber blends with EAF slag, compared to interaction of coke with the same slag and enhances the possibility of utilizing polymeric wastes in blends with conventional materials, such as coke, in EAF steelmaking.