Article pubs.acs.org/IECR
Transforming Waste Plastic into Reductants for Synthesis of Ferrosilicon Alloy Rifat Farzana,* Ravindra Rajarao,† and Veena Sahajwalla‡ Centre for Sustainable Materials Research and Technology (SMaRT@UNSW), School of Materials Science and Engineering, UNSW Australia, Sydney, New South Wales 2052, Australia ABSTRACT: A novel process to transform waste plastic as a reducing agent and therefore as an alternative carbonaceous source in the production of silicon bearing alloys is investigated. Bakelite, a thermoset plastic that is difficult to recycle, is used in this study. Heat treatment of Bakelite was carried out at 1550 °C in argon atmosphere to investigate volatiles generation, presence of ash impurities, carbon structure, and properties as a result of transformations occurring during heat treatment. Simultaneous influence of these properties on ferrosilicon alloy synthesis is established. Initial volatiles generation from Bakelite accelerated the rate of silica reduction, particularly the formation of silicon carbide (SiC) through gas phase reduction reactions of silica. Calcium carbonate (CaCO3) impurity in Bakelite acts as a fluxing agent and facilitates the separation of metal alloy from slag. Bakelite derived carbon obtained during heat treatment showed enhanced crystallinity coupled with lower levels of porosity in structure with increasing time. Carbon obtained through transformation of Bakelite could serve as a reducing agent to produce ferrosilicon. This scientific study of using Bakelite as a reductant will create new opportunities to re-form waste plastics as raw materials in ferrosilicon alloy synthesis.
1. INTRODUCTION The use of plastics in the modern world is increasing at significant rates because of their numerous applications. Plastics pose a potential environmental, economic threat as waste because of landfill and increasing trend of landfilling cost. Conventional lower temperature plastics recycling techniques and waste to energy concept mostly offer solutions for thermoplastics;1,2 however, thermoset plastics are mainly consigned to landfills, as they are considered as infusible.3 Bakelite (C6H4OHCH2)n was the first man-made thermoset plastic by Dr. Leo Hendrik Baekeland in around 1907, from two synthetic components: phenol and formaldehyde. Since then, Bakelite is used in various applications like kitchenwares, domestic plugs, switches jewelry, toys, telephones, radios, washing machines impeller, auto parts, etc.4,5 Bakelite is extensively used in electrical and heavy duty automotive parts because of its high hardness, strength, electrical, and thermal insulating properties.3,5 CaCO3 is added to the commercially grade Bakelite as a filler to improve its properties and to reduce the cost of production.6 Since Bakelite cannot be remolded, it is difficult to recycle and hence generally landfilled. Studies on pyrolysis of phenol formaldehyde (Bakelite) were carried out at temperatures of ∼1000 °C.7−9 In chemical recycling, Bakelite polymer chains are broken down and reused as monomers or oligomers. Primarily, Bakelite material is simply burned to get resultant energy as heat a source.10 Alternative solutions to recycle and transform waste Bakelite into materials resources are necessary. A number of studies have investigated the effect of coke and coal properties in the production of high silicon alloys focusing on the SiO reactivity and petrographic tests.11−13 Reduction material represents the major cost in the ferrosilicon and silicon production.13 The cost of reduction material as percentage of energy and materials cost is around 40% for ferrosilicon.14 Therefore, in the present context it is crucial to investigate the © 2014 American Chemical Society
influence of waste plastic as an alternative reduction material in ferrosilicon alloy synthesis. In our group, we have studied the use of Bakelite−coke blend as a carbon source in steel making particularly for recarburization.6 Recently we have reported the synthesis of ferrosilicon alloy using Bakelite and waste automotive glass as resources.15 Optimization of silicon recovery from waste glass during ferrosilicon synthesis was also studied.16 In this paper, fundamental study on Bakelite transformation into carbon is reported. Bakelite derived carbon showed more ordered and less porous carbon structure that can be used as reductant. Additionally off gases generated from Bakelite degradation could also be a source of reducing gas. Novel synthesis of ferrosilicon alloy using Bakelite as a reductant is also established, and influence of Bakelite as a reductant was investigated and compared with synthetic graphite results. This study has led to a sustainable alternative solution by transforming waste plastic into a reducing material for ferrosilicon synthesis.
2. EXPERIMENTAL SECTION Bakelite (125 μm) was used for this study, and its chemical analysis is shown in Tables 1 and 2. Other reagent grade materials used for this study were iron oxide (Fe2O3) powder (
