REDUCTION MECHANISM OF RECYCLED POLYPROPYLENE (PP) IRON OXIDE COMPOSITE PELLET FOR STEELMAKING PROCESSES Nur Farhana M. Yunos, Muhammad Asri Idris, Sri Raj Rajeswari Munusamy, Khairunnisa Amanzuria and Nur Hazira Najmi School of Materials Engineering, Universiti Malaysia Perlis, Jejawi, Perlis, Malaysia E-Mail: farhanadiyana@unimap.edu.my ABSTRACT The use of polymeric material has increased progressively in recent years with small percentage of plastics are currently recycled for further use. In this present study, carbon from polymeric material such as recycled Polypropylene (PP) is utilized as a reducing agent which has not been extensively investigated especially in steelmaking processes. The reduction reaction through composite pellet approach was conducted in a horizontal tube furnace at low temperatures of 9 C, 95 C and 1 C. The reduced sample was examined with Scanning Electron Microscope (SEM) and X-ray Diffraction (XRD) to understand the reduction mechanism of composite pellets. XRD pattern confirmed that hematite was the main phase present in the sample while the phase s transformation was occurred from iron oxide to metallic iron in tandem of temperatures were rising up. The influence of high temperatures revealed a clear structural evolutions of the samples confirms that the occurrence of three successive of reduction steps, through magnetite and wustite to metallic iron. This scientific study of using PP as a reductant will create new opportunities to transform waste plastics as carbon materials in steelmaking processes. Keywords: polypropylene (PP), iron oxide, reduction. INTRODUCTION The plastic industries of Malaysia is a highly diversified sector producing an array of products including automotive components, electrical and electronics parts, components for the telecommunication industry, construction materials, housewares products and packaging materials. In the last 5 years, with almost 8 thousand tonnes of polymeric waste created annually in Malaysia alone in 211, which only.7% is recycled (Ecosystem, 211). Disposal of waste plastics in landfill requires a large area of favourable geology, low-cost transportation to the site and must meet environmental regulations, thus the use of waste polymeric materials within metallurgical industries is a relatively new concept in order to reduce these wastes. In the field of electric arc furnace (EAF) steelmaking, the utilisation of polymeric materials as a partial replacement for carbon injection has been conducted where the foaming and reduction behavior were improved with different types of plastics used (Dankwah, et al., 211), (Yunos, 212), (Sahajwalla, 26), (Sahajwalla, et al., 215), (Zaharia, 21). The global demand for steel today is making the iron and steel industry growth as one of the largest energy extensive industries in producing the material. To meet those demands, solutions to enhance the industry process is essential. As carbon and hydrogen are the major constituents of waste plastics, these source clearly have the potential to be used as an alternative to replace coke in electric arc furnace steelmaking (Kongkarat, 211), (Murakami and Kasai, 211), (Sahajwalla, et al., 213). Although a number of research works were conducted on reduction using plastics waste, the fundamental research on the utilisation of waste plastics blends with iron oxide as reducing agent in iron and steel making is still scarce. Thus, this present study focuses on the potential of producing metallic iron from reagent grade iron oxide using PP as reducing agent under different temperatures to understand the reduction mechanism. By recycling these plastics waste from PP as a reducing agent in iron reduction can be a promising solution to the alarming environmental problem. METHODOLOGY Sample preparations This research is focusing on reduction behavior of iron oxide with carbon from PP as reductant agent at different temperatures. Recycled PP was supplied by SLT Plastic Sdn Bhd in Penang. The elemental analysis of PP with major constituents of carbon is shown in Table-1. PP was selected due to high carbon content that almost similar with metallurgical coke (Yunos, et al., 212). Table-1. Ultimate analysis of PP. The PP used as carbon material in this research was first crushed into particles of 63 μm sizes. For preparation of composite pellet, the molar amount of reducable oxygen in iron oxide is equal to the molar amount of total carbon in PP. Iron oxide powder with 96 wt% purity was mixed with recycled PP and the composite pellets were compacted in a die to produce cylindrical pellets (~1 mm diameter), by applying a load of 7 tonnes for 2 minutes in a hydraulic press. The elemental analysis of iron oxide was examined by X-ray Fluorescence (XRF) and presented in Table-2. 9765
Table-2. Chemical composition of iron oxide (wt %). Reduction Reaction and Samples Characterization The reduction reaction was conducted in a horizontal tube furnace set to different temperatures of 9 C, 95 C and 1 C with heating rate of 1 C/min under inert argon gas flow. Composite pellets were placed in the alumina crucible and the reaction took place for 2 minutes (Dankwah, 213). Once the samples were cooled down until the room temperature, the reduced samples were taken out for further analysis by using X-ray Diffraction (XRD) for qualitative analysis. The operating parameters was set up under 2 ma current with scanning ranges from 2 to 7, step size of.2. Scanning Electron Microscopy (SEM) was applied for further understanding on the structural evolutions during the reduction process under different temperatures of reaction. Original and reduced samples were examined by JOEL JSM-64LA operating at an accelerating voltage of 1 kv. RESULTS AND DISCUSSION Influence of Different Temperatures on Phase Transformations The phase transformation of composite pellets prior to and after reduction reaction were analyzed by XRD and presented in Figure-1. The main phase detected was hematite for initial reagent iron oxide (Figure-1 (a)). After reduction conducted at temperature of 9 C, carbon from PP as reactant agent reduced hematite into magnetite and wustite. were detected at angles of 3 and 46 and there was no metallic iron peak can be observed due to the uncompleted reduction process (Figure-1 (b)). The reaction time and temperature influenced the reduction process where lower temperature reaction slowing down the reactions (Murakami and Kasai, 211). In the present study, iron oxide is the only source of oxygen in the composite pellet. The first step of reduction took place where the iron oxide in the composite was reduced by H 2 and hydrocarbon gases generated from PP. However, the reaction process is very complicated because there are many possible elementary reactions. It is impossible to elucidate the elementary reduction reactions from the results obtained in this study alone. The overall reaction can be expressed by Eq. (1). 27Fe 2O 3+(C 3H 6) n=18fe 3O 4+3CO 2+3H 2O+(C 3H 6) n-1 (1) The second step of reduction was originally come from the carbon originated from the PP remained in the composite. Baker reported that not only metallic iron but also magnetite and hematite acted as catalysts of carbon decomposition from C 3H 6 and CH 3COCH 3 at 973 K (Baker, et al., 1982). It is predicted that these oxides would act as a catalyst for other hydrocarbon gases. It is therefore suggested that carbon remained in the composite because the presence of iron oxide enabled decomposition of PP-derived hydrocarbons as shown in Equation. (2). CH n = C + n/2h 2 (2) Counts 2 1 2 1 2 1 6 4 2 (d)1 C (c)95 C (b)9 C (a) Fe2O3 2 3 4 5 6 7 Position (2θ) Hematite Iron Figure-1. XRD patterns of (a) initial hematite and composite pellets after reduction reaction at temperatures of (b) 9 C, (c) 95 C and (d) 1 C. 9766
Thus, as the temperatures were rising up, metallic iron s peak increases as the wustite peaks shift a little (Figure-1 (c) and (d)). The movement of these peaks could come from the evolution of the stoichiometry from hematite to magnetite to wustite. The transformation from hematite to magnetite is very fast and complete. appears as magnetite is consumed but wustite begins to be reduced while magnetite remains until the end. This late presence of magnetite contradicts the X-ray diffraction results and can be attributed to a lower sensitivity of the X-ray analysis. The reduction of wustite is the longest step of the whole reduction process. The present results were in agreement with similar experiments performed by previous researchers (Damien Wagner, 28), (Kumar, et al., 29), (Sah and Dutta, 21). These authors found that many chemical reactions contribute to reduction of iron ore coal pellets. In general, the reactions for carbon-based direct reduction actually takes place in three steps by gaseous intermediate (carbon monoxide, carbon dioxide) through gas solid reduction. These intermediates are hematite to magnetite, magnetite to wustite, and wustite to metallic iron. Thermodynamically, carbon can react with oxygen to form carbon dioxide. Meanwhile, the CO produced by the latter reaction will be the primary reducing gas. As reduction takes place, CO 2 will be the by products. To regenerate the CO, the Boudouard reaction will allow the gasification of carbon by CO 2 (Dankwah, et al., 211), (Fruehan, 1977). Influence of Different Temperatures on Structural Transformations Figure-2 represents SEM micrographs of reduced composite pellets at temperatures of 9 C, 95 C and 1 C. The initial hematite (Figure-2 (a)) is composed of small aggregates of approximately 1 μm, which are themselves made of smaller grains. At a higher magnification, these grains appear to be dense and of irregular shapes and the porosity between the particles was observed high. When PP was applied as reducing agent at 9 C, the transformation of hematite into magnetite was observed with a little morphological changes. One can only notice where more continuous structure of solids with smaller constituents grains less visible. The iron phase is quite continuous is expected to be wustite with development of porous structure (Figure-2 (b)). There were different structures formed during the reduction process. As the temperature raised up, the grains appear to be more dense and transformed into more fingerlike shape than spheroid shape. The smaller grains become invisible and correspond to the transformation in the morphological changes in the irregular shapes (Damien Wagner, 28). More structure of solid appear and bridge growth toward the exterior of the grains as in Figure-2(c). The small bright regions (like spheroid) are expected to be wustite. It is indicated that the hematite reduced into magnetite in the intermediate stages to begin the wustite formation (Adam, et al., 1989a). Pores (a) Hematite (b) 9 C Meta llic iron Pores (c) 95 C (d) 1 C Figure-2. Micrographs of (a) initial hematite and composite pellets after reduction reaction at temperatures of (b) 9 C, (c) 95 C and (d) 1 C. 9767
In Figure-2(d), the hematite was reduced as the reduction temperature went up to 1 C. The tranformed microstructure is the evidence for the reduction of hematite where metallic iron occurred caused by a reaction chemically controlled. The iron phase is seen quite continuous, however with porous structures and the grains size approximately 3 μm. The small bright points at the surface are the last wustite grains that were reduced. This kind of growth was observed and explained by many authors (Adam, et al., 1989b), (Sasaki and Soma, 1977), (Srinivasan and Lahiri, 1975). It is resulted from a competition between the chemical reaction at the surface of the particles, which increases the iron concentration and activity at this place. Therefore, the utilization of carbonaceous materials from recycled plastic (PP) is important for the promotion of reduction reactions at high temperature since the reduction reactions can proceed where the hematite reduced into metallic iron with PP as reducing agent. CONCLUSIONS From this study, reduction of iron oxide with recycled PP could be effectively used as an alternative carbon source and reducing agent in steelmaking. As the temperature increases from 9 C to 1 C, reduction reactions of composite pellets also accelerates with phase transformations from hematite into metallic iron. Significantly, the phase transformation from hematite into metallic iron occurred when PP used as reducing agent at high reduction temperature of 1 C. Structural alterations associated with phase transformation from hematite to metallic iron clearly proven by the presence of hematite peaks in the unreacted composite pellets and the existence of hematite, magnetite, wustite and iron peaks in the reduced pellets of 1 C. Accordingly, the morphology changes from finger like shape (magnetite) into more spherical grains (wustite) and finally the iron phase grow outwards. Overall, reduction reactions showed that PP can be used as an effective reducing agent in steelmaking for its high carbon content aside from reducing the dumping of these waste plastics into the landfills. ACKNOWLEDGEMENTS This present work is supported by the Ministry of Education Malaysia under grant of Fundamental Research Grant Scheme (FRGS 1, 213) and is gratefully acknowledged. REFERENCES [1] Adam, F., Dupre, B., and Gleitzer, C., (1989a), Cracking of Hematite Crystals during Their Low Temperature Reduction Into, Solid State Ionics, 32 33, Part 1 (a), pp. 33--333. [2] Adam, F., Dupre, B., and Gleitzer, C., (1989b), Cracking of Hematite Crystals during Their Low- Temperature Reduction Into, Solid State Ionics, 32 33, Part 2 (b), pp. 33--333. [3] Baker, R. T. K., Alonzo, J. R., Dumesic, J. A., Yates, D. J. C., (1982), Effect of the Surface State of Iron on Filamentous Carbon Formation, Journal of Catalysis, 77 (1), pp. 74--84. [4] Damien Wagner, O. D., Fabrice Patisson, Denis Ablitzer, (28), A Laboratory Study of The Reduction of Iron Oxides by Hydrogen, Proceedings of TMS, 111-12, pp. 111--12. [5] Dankwah, J. R., (213), Recycling Blends of Waste Plastics and Biomass as Reducing Agent for The Production of Metallic Iron from Iron Oxide, International Journal of Engineering Science and Technology (IJEST), 5 (12), pp. 1967--1976. [6] Dankwah, J. R., Koshy, P., Saha-Chaudhury, N. M., O'Kane, P., Skidmore, C., Knights, D., Sahajwalla, V., (211), Reduction of FeO in EAF Steelmaking Slag by Metallurgical Coke and Waste Plastics Blends, ISIJ International, 51 (3), pp. 498--57. [7] Ecosystem, G., (211), A Study on Plastic Management in Peninsular Malaysia, National Solid Waste Management Department MIinisry of Housing and Local Government Malaysia, pp. 1--282. [8] Fruehan, R., (1977), The Rate of Reduction of Iron Oxides by Carbon, Metallurgical and Materials Transactions B, 8 (1), pp. 279--286. [9] Kongkarat, S., (211), Recycling of Waste Polymers in Electric Arc Furnace Steelmaking: Slag/Carbon and Steel/Carbon Interactions. Ph.D Thesis, Materials Science Engineering, Faculty of Science UNSW, Australia, pp. 1--267. [1] Kumar, M., Mohapatra, P., and Patel, S. K., (29), Studies on The Reduction Kinetics of Hematite Iron Ore Pellet with Noncoking Coals for Sponge Iron Plants, Mineral Processing and Extractive Metallurgy Review, 3 (4), pp. 372--392. [11] Murakami, T., and Kasai, E., (211), Reduction Mechanism of Iron Oxide - Carbon Composite with Polyethylene at Lower Temperature, ISIJ International, 51 (1), pp. 9--13. [12] Sah, R., and Dutta, S. K., (21), Effects of Binder on the Properties of Iron Ore-Coal Composite Pellets, Mineral Processing and Extractive Metallurgy Review, 31 (2), pp. 73--85. [13] Sahajwalla, V., (26), Recycling of Waste Plastics for Slag Foaming in EAF Steelmaking AISTech - Iron 9768
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