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Influence of recycled wastes on ferrosilicon production in steel making applications: A short review

The potential transforming the waste materials into an alternative source was found in iron and steelmaking application that also would solved the world’s most problematic waste stream. Generally, converting the waste materials into auxiliary source only is accessible to certain wastes industries due to its difficulty to recycle hence generally landfilled. Recycling waste materials used in ferrosilicon production as carbon and silica source to control the reduction reaction with iron oxide. The present paper reviews the phase transformation and morphology in the production of ferrosilicon at temperature 1550°C on graphite and plastic waste (bakelite) as the potential carbon materials and silica powder and glass waste (automotive glass - windshield, window glass) as alternatives silica source in production of ferrosilicon. The utilization of carbon material from plastic waste and silica source from glass waste (automotive glass) can be used for ferrosilicon synthesis and CO gas concentration comparable with conventional carbon source (graphite) typical silica source (silica powder). The utilization of recycled wasted is efficient due to enhancement in the reaction with iron oxide thus potentially replacing the conventional materials in ferrosilicon synthesis as well as minimizing the landfill wastes.


Introduction
Ferrosilicon (FeSi) production process is a carbothermic reduction process that used carbon material as reducing agent [1,2]. In steelmaking industry, the type of carbon and silica source used in the ferrosilicon production have a significantly effect on the cost, energy consumption and stability of the operation [3]. An innovative approach based on recycling waste glass, plastic waste and agricultural waste has been reported by several researchers with aim to maximize use of waste resources for production of ferrosilicon [1,[3][4][5][6][7][8].  reported that plastic waste from bakelite mainly used in kitchenwares, toys, electronic devices such telephones and radios, washing machines and etc. However, these wastes are being abundances due to its characteristic that cannot be remoulded thus problematic to be recycled. The bekelite going through the pyrolysis in order to develop resultant energy associated with less porous and uniformly carbon structure that can be used as carbon source in steelmaking [7].
Other flexible and resourceful material that can be used in multifaceted application is waste glass. Waste glass such windshield and window glass from automotive sector destined to landfill due to difficulty and costly processes, Waste glass can be used in ceramic production [9], as construction material [10] and for silicon production [11]. This provides an opportunity for industry to become a competitive price by converting the automotive waste glass as an auxiliary silica source.
In ferrosilicon production, the reduction of iron oxide followed the reaction in Eq. 1 and the Boudouard reaction in Eq. 2 occurred. The CO (carbon monoxide) released from iron oxide reduction process (Eq 1) facilitates the gas  3 and 4). At higher temperature, the reduction of silica takes place by carbon. The silica was completely volatilized and gets vaporized at high temperature reaction [3,5]. The reaction in silica and carbon materials initiates the contact point between SiO2 and C. Once both SiO and CO are formed (Eq. 2 and 3), the reaction proceeds at the surface of carbon and silica particles through gas phase. Some amount of SiO will be used in the system and the remains will escape in the gas phase. Some amount of reduced silica in form of silicon (Eq. 4) dissolved into molten iron to form ferrosilicon while the remains were reduced to SiC (Eq. 5).

Raw Materials
In general, the raw materials used to produce ferrosilicon were graphite (conventional material, C: 99%) and bakelite (plastic waste, C: 53.4%) as carbon materials while silica powder (typical silica, SiO2: 99%) and automotive glass (glass waste -windshield, SiO2: 71% and window glass, SiO2: 69.21%) as silica source [5,6,8]. The elemental analysis of the graphite and bakelite conducted using X-ray diffraction as shown in Table 1. It has been reported that XRD pattern of graphite showed mainly carbon with small amount of silica impurity pattern [8]. While, for the bakelite showed in Table 1 was dominated by the calcium carbonate with small amount of silica impurities were detected in the XRD pattern [6]. The reactions undergo between iron oxide, Fe2O3 (99%) with two different carbon materials and two types of silica source.

Ferrosilicon Synthesis
In ferrosilicon synthesis, the waste glass and carbon materials are usually ground to powder. The waste glass, carbon material and iron oxide then placed in a glass jar and mixed and form into spherical shape. The reduction reactions done by previous researchers were conducted at 1550ºC in horizontal tube furnace under Argon gas atmosphere with flowrate 1 L/min within 2, 4, 8 and 15 minutes reaction time. The schematic of the experimental arrangement presented in Figure 1. In order to measure the gas evolved during the reduction, an IR gas analyser was coupled to the furnace [4]. The reduced ferrosilicon samples were analysed by X-ray diffractometer.

Phase Transformation
The phase transformation of the reduced pellets of silica/ graphite, silica/ bakelite, automotive glass/ bakelite and automotive glass/ graphite/ bakelite at 1550ºC as reported by several researchers are shown in Table 2. XRD of the overall reduced pellets showed the formation of ferrosilicon (15 min). In silica/ graphite reduced pellet, it was reported that the formation of SiC, FeSi and FeSi2 were predominantly after 15 min reduction time [4]. The appearance of SiC may cause by the solid phase reaction between high content of silica in silica powder (99%) and high content of carbon in graphite (99%) during the reduction process [6]. According to previous research stated that the formation of iron silicon (FeSi and FeSi2) in reduced pellet was due to the gas phase reaction of silica with the iron oxide [13]. For the silica/ bakelite reduced pellet, peak of Fe5Si3, CaSiO3/Ca2SiO4 and FeSi were majorly detected in XRD pattern after 15 min of reduction time [4]. Boonyaratchinda and Kongkarat (2020) reported that the appearance CaSiO3/Ca2SiO4 was probably occurred during the reaction between CaCO3/CaO and silica. During the smelting process, CaSiO3/Ca2SiO4 covers the liquid molten steel surface avoiding the heat loss [14]. Compared to silica/ graphite, in within 8 -15 min, the formation of ferrosilicon by graphite (FeSi, FeSi2) is in a greater level of silicon compared to bakelite (Fe5Si3, FeSi) [1,6]. High crystalline graphite drives the rapid carbon dissolution in iron and lead to increase the rate of ferrosilicon formation [7].
While, the XRD analysis of automotive glass/ bakelite reduced pellet at beginning of reaction (2 min) it was reported that the iron phase was detected along with solute carbon and unreacted silica. The Fe3Si phase starts to appear at 4 min reaction time and become sharper until 15 min reduction time due to the distinguished from the slag [1]. Compared to XRD analysis of automotive/ graphite/ bakelite reduced pellet, various phases detected at the end of the reduction and predominantly by Fe5Si3 and SiC phase as presented in Table 2. Utilization of graphite/ bakelite as carbon material elucidating that the reduction of iron oxide is much easier than the reduction of silica according to the formation of Fe3Si within 2 min [5,15]. As the reduction proceed, the reduction of silica into silicon become favoured and the structural composition of ferrosilicon changes from Fe3Si to Fe5Si3 indicating the increasing of silicon content in ferrosilicon [6].

CO Gas Concentrations
The gas released during the reduction process also play a major role in silica/ graphite, silica/ bakelite, automotive glass/ bakelite and automotive/ graphite/ bakelite in silica reduction. Table 3 shows the CO gas concentration released during the reduction reported by several researchers [1,4,6]. The CO gas released of the overall reduced pellets was dominated by CO gas that was produced during the reduction reaction (Eq. 1, 4 and 5) for ferrosilicon synthesis. Based on the Table 3, the CO gas released by silica/ bakelite (0.0012 moles at 1 min) was comparatively high compare to others followed by silica/graphite (0.0011 moles at 1.5 mins), automotive glass/ bakelite (0.0010 at 1.3 mins) and automotive glass/ graphite/ bakelite (0.0007 moles at 1 min). However, the utilization of graphite as carbon materials [4,6] was found that the CO gas release over a longer time compared to bakelite [1,4] as the steady period was achieved at 18 and 12 mins for silica/graphite and automotive glass/graphite/Bakelite respectively while steady period was achieved at 9 mins for sailica/ bakelite and automotive glass/ bakelite.
At initial stage of reduction, gases from volatilization and degradation of bakelite enhance the reduction reaction therefore higher CO generated was observed. As the process proceed, the carbon content of bakelite was used up and become lower than graphite that contained high carbon content, the reactions of bakelite reached to the almost completion before the graphite one. XRD analysis also showed the production of ferrosilicon at the beginning of reduction (2 -4 min) due to the fast rate of reduction reaction according to higher CO gas released when bakelite was used as carbon materials (Table 3). For the automotive glass/ graphite/ bakelite, the carbon materials of bakelite contribute at the beginning of the reduction while graphite reacted steadily until the end of the reduction since the CO concentration started to increase again at 3 mins reaction time then slowly decrease at 5 min reaction time [6]. The formation and the structural composition changes of ferrosilicon can be observed with longer reduction time.

Conclusions
In conclusion, the ferrosilicon phases can be developed during the reduction reaction at 1550ºC within 15 min reaction time using graphite (conventional material) and bakelite (plastic waste) as carbon materials and silica powder (typical silica) and automotive glass (glass waste) as silica sources. The formation of ferrosilicon at early reduction time indicates high CO gas can be released by volatilization and degradation processes of bakelite at initial stage that resulted to faster reduction reaction. However, high carbon content in graphite leads to the CO gas releases over a longer period drives to steady reduction reaction. Moreover, the utilization of carbon material from plastic waste and silica source from glass waste (automotive glass) has the potential to be used for ferrosilicon synthesis. Thus, a sustainable ferrosilicon production process with new carbon source and silica source is more preferable.