Utilisation of end-of-life plastics/rubber tyres and their blends with metallurgical coke in EAF steelmaking: Reduction of iron oxide

摘要

The world’s crude steel production has seen a progressive growth from about 595 million tons in 1970 to 1527 million tons in 2011. This progressive growth, often attributed to the recovery of South East Asia from the economic and financial recession in 1998, has been accompanied by an equally progressive shift from the BOF to EAF as the mode of production of crude steel. The shift from the BOF to EAF has often been attributed to the latter’s low capital and operating cost coupled with its flexibility in the use of scrap. Accordingly, the demand for high quality scrap has soared, almost to the point of being scarce in most steel producing countries, a situation that calls for the development of novel technologies to produce alternative iron materials (AI) to supplement scrap in EAF steelmaking. Such novel technologies must not only be affordable from the economic point of view, they must also be environmentally benign in order to be able to meet the stiff national and international legislations on the environment. Anthracite and metallurgical coke are the conventional materials used for reduction of iron oxides and slag foaming in EAF steelmaking. In order to address the issues of cost, availability and restrictions on greenhouse gas emissions, alternative carbon sources are required to replace, at least partially, these conventional materials. Postconsumer plastics like polyethylene and end-of-life tyres (ELTs) contain both carbon and hydrogen, which are known reductants for iron oxide reduction. In the present study, the reduction of EAF slags containing FeO by metallurgical coke (Met Coke), high density polyethylene (HDPE), rubber tyre (RT), polyethylene terephthalate (PET) and different blends of coke with the three polymers was investigated. The potential feasibility of utilising these polymers as reducing agents along with the kinetics of the reaction forms the central theme.Composite pellets were formed from EAF slags (47.1% FeO) and different blends of coke-polymer carbonaceous materials and were heated rapidly under inert environment (1 l/min argon) in a horizontal resistance tube furnace at four different temperatures – 1450, 1500, 1550 and 1600 °C. The off gas produced from the reaction was analysed continuously by an infrared gas analyser attached to the system to monitor CO, CO2 and CH4 gases produced by the reduction reaction and the results were recorded in a data-logging computer. The rate and extent of reaction were calculated based on a mass balance for oxygen removal from the iron oxide and the content of reduced iron metal was determined by the following chemical analysis methods:•LECO™ C/S analysis for its C and S contents and•LECO™ Oxygen analyser for its O content.The gas phase interactions showed a reactive coke-polymer carbonaceous blend characterised by an improved generation of gaseous reducing species (H2, CO, CH4) in the furnace environment. The CO emissions from metallurgical coke showed lower concentrations in comparison to those from the Coke-polymer blends; however, the CO2 emissions were generally higher than those of HDPE, RT, Coke-HDPE and Coke-RT blends but lower than PET and Coke-PET blends. At all the temperatures considered under this investigation, an improved rate of chemical reaction is seen when the coke was blended with each of the polymers. Calculated values of activation energies showed a progressive increase from coke as it was blended with the polymers. The relatively low activation energy value observed for coke compared to the blends and the polymers is consistent with less gas evolution and thus less stirring in the slag. Accordingly, mass transfer in the liquid becomes an important rate limiting step. As the proportion of polymer in the blend increases, gas evolution increases leading to increased stirring and foaming and the process shifts towards mixed control in which chemical reaction control becomes important as seen in the trends in the activation energy values. Therefore blending of the coke with the polymers has the effect of decreasing mass transfer effects and promoting the influence of chemical control. The percent reduction, time for complete reduction, level of carburisation and desulphurisation were calculated for each carbonaceous blend and the results were compared with those obtained by reduction of the iron oxide using Met Coke as reductant under the same experimental conditions. It was revealed that the blends, generally, performed better than Met Coke. A significant decrease in direct CO2 emissions from the reduction process was observed when Met Coke was blended with two of the three polymers utilised for this investigation (HDPE and RT). However, blending of Met Coke with PET resulted in increased levels of CO2 emissions; this is attributed to the presence of a significant amount of oxygen (33.3 wt %) in the carbon-hydrogen backbone of the polymer. Consequently, based on direct CO2 emissions to the environment, HDPE, RT, coke-HDPE and coke-RT blends are better carbonaceous reductants than coke. However, PET and coke-PET blends are not suitable as reductants beyond 10 wt% of PET in the carbonaceous blend.