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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.29 No.10 pp.592-602
DOI : https://doi.org/10.3740/MRSK.2019.29.10.592

Reduction Behavior of Self-Reducing Pellets of Chromite and Si Sludge with and without Carbon

Woo-Gwang Jung1, Sakib Tanvir Hossain2, Jong-Ho Kim3, Young-Chul Chang4
1School of Materials Science and Engineering, Kookmin University, Seoul 02707, Korea
2Department of Materials Science and Engineering, Graduate School of Kookmin University, Seoul 02707, Korea
3Advanced Metals Research Group, Research Institute of Industrial Science and Technology (RIST), Pohang City 37673, Korea
4Department of Mechatronics Engineering, Korea University of Technology and Education, Cheonan-si 31253, Korea
Corresponding author E-Mail : wgjung@kookmin.ac.kr (W.-G. Jung, Kookmin Univ.)
July 5, 2019 September 3, 2019 September 11, 2019

Abstract


Feasibility is investigated for reduction of chromium ore by Si sludge with mixed silicothermic and carbothermic reaction. The reduction behavior of chromium ore using Si sludge is investigated precisely to determine the effects of carbon addition, reaction time, and reaction temperature. The pellets are dropped into the furnace after temperature stabilized. As the amount of C addition increases, the amounts of CO and CO2 gas generation increase. After the dropping of the pellets, the pellets are heated and the reaction starts at about 1,573 K or higher. The pellets maintain their shape until 10 min after the drop, and then melted. As the holding time increased, the size of the reduced metal particles increased. The chromium ore is rapidly reduced by the Si sludge, and the slag penetrated into the chromium ore and reduction progressed inside. As the reduction temperature increased, the reaction initiation time is shortened and the reaction fraction of the reduction reaction increased. As the reaction temperature increased, agglomeration of reduced ferrochrome metal is promoted.



초록


    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    The expansion of the computer and photovoltaic industries has led to the steadily increasing demand for Si wafers.17) The production process of Si wafers involves a large amount of Si-containing sludge, which is generated both in the processes of ingot cutting and grinding for polishing the surface of the ingot and wafer. A high-purity Si ingot is sliced, during which approximately 30~50 % of the ingot is lost to the sludge. In 2014, almost 280,000 tons of polycrystalline Si was used for solar cells, but only 100,000 tons of Si powder is recovered annually; this amount is expected to increase by 25 % annually.8) Few steps have already been taken to recover Si from the cutting sludge, such as the use of centrifugation and the specific gravity separation process. In overseas, Si/SiC powders have been used to develop high-value-added products such as porous materials for insulating and high purity silica sand.9,10) The commercial uses of sludge recycling in Korea involve the use of sludge as a heatgenerating agent for steelmaking and as a refractory material. Appropriate recycling methods have not yet been developed yet rather than using in landfills.11-14) Therefore, the recycling of Si sludge is an urgent and important issue in terms of resource recycling and environmental sustainability.

    Ferrochrome alloy iron, which is an essential raw material for the manufacturing of stainless steel, is produced by reducing chromium ore with carbon in the arc furnace via the carbothermic reaction.15-18) This process requires a large input of electrical energy and releases a large amount of greenhouse gases, including carbon dioxide. Furthermore, it produces a high-carbon ferrochromium alloy because it uses carbon as a reducing agent.

    Previous research on the synthesis of ferrochrome with coal as the reducing agent has found inconsistencies and incompatibilities in the carbothermic reduction.19-23) Moreover, Chakrabarty et al.20) reported that the reduction of both Fe and chromite ore was mostly dominated by diffusion and a chemical reaction or reciprocal nucleation. Some researchers noted that before beginning the reduction of chromium ions, Fe ions in chromite ore were entirely reduced.21) By analyzing the fractionally reduced chromite ores, Nafzigar et al.22) and Soykan et al.23) both provided qualitative data on phase evolution as the reduction proceeded to different degrees. However, on the basis of the presence and amount of certain phases, such as carbides and olivine, clear differences have been identified. These differences may occur because of the compositional differences between the chromium ores and the variable cooling profiles of the samples.

    Few studies have analyzed the feasibility of using Si wafer sludge as a reducing agent in the production of ferrochrome alloys.24,25) In our previous reports, the reduction of chromium ore was investigated using Si wafer slicing sludge as the reducing agent.26-28) Feasibility was confirmed on the basis of the preliminary reduction of chromium ore by the Si wafer sludge.26) The optimum mixing ratio was determined for making pellets, and it was found that a temperature above 1,573 K was required for preliminary chromium ore reduction.27) A further study was concerned with the reduction of chromium ore and the effect of carbon addition for the production of ferrochrome (Si) alloy by using Si wafer slicing sludge.28) In evaluating the reduction ratio and metallization ratio, the start time of the reduction was not fixed because a horizontal furnace was used. As a result, the reduction was started before the holding time, and this type of reduction required us to develop a new experimental method. In the current study, the pellets were dropped into the furnace after the temperature was stabilized, and the reduction behavior of chromium ore using Si sludge was investigated by precisely relating it to the effects of carbon addition, reaction time, and reaction temperature.

    2. Experimental

    2.1 Raw materials

    In this study, Si sludge was recovered from the Si ingot slicing process and underwent cleaning, liquid separation, and drying, similar to the process reported in previous works.27,28) The Si sludge contained 38.2 wt% Si and 56.7 wt% SiC. The average particle size of the Si sludge was 1.573 μm and that the Si particles were small, whereas the SiC particles were large.27) Chromium ore with a particle size of 300~500 μm was used in this study (Table 1). XRD analyses have shown that the chromium ore mainly consisted of the chromite phase Cr2O3·FeO.29) Water glass (sodium silicate solution, Na2O(SiO2)x·xH2O, reagent grade) was used as the binder (Sigma-Aldrich) and was used after mixing with deionized water to obtain a ratio of 50:50. Carbon powder (reagent grade, 99.99 %, -325 mesh) was purchased from iTASCO, Korea. Slaked lime was produced by heating limestone at 1,173 K for over 2 h.

    In all experiments, the raw materials were mixed using the mixing ratio in Table 2 according to their weight percentage and by using water glass as the binder with water. When carbon was added into the pellet mixture, the weight of the added carbon was calculated according to the weight of the Si sludge. Table 3 shows the predicted compositions of pellets using the blending ratios given in Table 2, in which it was attempted to maintain the composition ratio of Si and SiC in the Si sludge. Si and SiC components were originated from Si sludge, and SiO2 component from Cr ore, respectively. Therefore, they (Si and SiC) are shown separately from the SiO2 component in Table 3.

    2.2 Experimental apparatus and procedure

    Figure 1(a) illustrates the experimental setup used for the reduction of the chromium ore with carbon addition. It shows the Kanthal Super heater equivalent to the apparatus used in previous studies.28) In this experiment, a quartz tube with pellet housing capability was installed so that the pellets can be dropped without opening the system. An alumina reaction tube (60 mm [OD] × 50 mm [ID] × 600 mm [L]; Samhwa Ceramic, Korea) with one closed end was used and was enclosed in a watercooling jacket. A crucible made of MgO (38 mm [OD] × 32 mm [ID] × 77 mm [L]; Ozark Tech Ceramics, USA) was used, and the experimental temperature ranged between 1,623 and 1,773 K.

    The raw materials were mixed according to the mixing ratios shown in Table 1. The mixed powder was ball milled using zirconia balls for more than 30 min to obtain a uniformly mixed powder. Thereafter, disk-shaped pellets with a diameter of approximately 9.6 mm were made using a mold and press. Each pellet weighed approximately 1.7 g, and 12~14 pellets were prepared for each experimental run. The pellets were dried in an oven at 353 K for more than 3~4 h before use.

    Pellets were loaded in the pellet housing tube, and an empty crucible was placed in the reaction tube. The reaction tube was evacuated and filled with argon gas. This process was repeated three times, and the argon gas was flown during the night before starting the experiments. The furnace was heated to the target temperature, and the pellets were introduced into the crucible when the temperature was stabilized. The temperature was held for 1 h after dropping the pellets. During the experiment, argon gas was allowed to flow continuously into the reaction tube at a flow rate of 0.6 L/min by using a mass flow controller. The CO and CO2 in the off-gas were analyzed using a gas analyzer (SWG 200-1, MRU Germany).

    A series of reduction time change experiments were performed in a high-frequency induction furnace (Fig. 1(b)). The reaction tube of graphite was adopted to avoid breakdown by thermal shock. The furnace was heated to the target temperature, and the pellets were dropped into the crucible when the temperature stabilized. After holding the samples for a predetermined time in the furnace, the graphite reaction tube with the samples in the crucible was removed from the furnace and was rapidly cooled. After the reaction, the pellets were recovered, and their weight changes before and after reduction were measured.

    XRD analyses for slag and metal were conducted using a D/max 2500 (Rigaku Japan, 40 kV 200 mA CuKα) XRF with Simultix 12 (Rigaku Japan). ICP-OES analysis employed a Spectro ARCOS EOP (Spectro Germany). Furthermore, a cross section of the sample was polished, its microstructure was observed using SEM (JSM 7601; JEOL Japan), and its composition was calculated using EDS analysis.

    3. Results and Discussion

    3.1 Carbon addition experiments

    To evaluate the effect of the addition of C on the reduction of chromium ore, pellets were prepared by adding 2~6 % of C powder to Si sludge in the ratio of raw materials (Table 2). The reduction experiment was then conducted at an experimental temperature of 1,723 K. Table 3 shows the components of the pellets before reduction. During reduction, the composition of the exhaust gas and the temperature at the sample zone in the furnace were measured.

    Figure 2 shows the profiles of exhaust gas and the temperature measurement results. After reaching the target temperature of 1,723 K and stabilizing the temperature, it was observed that when the pellets are dropped into the crucible, the temperature temporarily decreases by approximately 50 K before increasing again because the pellets entering into the crucible are at room temperature. The original temperature is recovered as heat is supplied from the surroundings.

    The pellets were heated to initiate the reduction reaction and were confirmed by the exhaust gas profile. In Fig. 2, two peaks appear in the exhaust gas analysis. The first peak occurs when the temperature decreases to its lowest point and then increases again. This finding can be attributed to the combustion of volatile components, gas components, or combustible materials in the pellets and is not due to the reduction of chromium ore. The second peak occurs when the temperature increases to approximately 1,573 K and is believed to occur during the reduction of chromium ore by Si and SiC in the Si sludge and added C.

    The total amount of gas generation is expected to differ depending on the amount of C added; therefore, the gas generation time is also expected to be different. Fig. 2(c) shows an example of the method by which gas generation time is determined from exhaust gas analysis. The time from when the second peak occurs to when the CO gas analysis value becomes zero is defined as the gas generation time. Fig. 3 shows the gas generation time obtained as described above with respect to the amount of C added. It can be seen that gas generation time increases with C addition. During the pellet reduction process, CO and CO2 gas emissions during the reduction reaction were calculated (Fig. 3(b)). The amount of gas generation increases with C addition. As shown in Fig. 2, the change in the CO and CO2 analysis values shows a second peak as the temperature increases again. The starting temperature of the reduction reaction is not influenced by the addition amount of C because the temperature is constant at 1,723 K; therefore, the starting temperatures of the reactions are similar.

    3.2 Temperature change experiments

    Figure 4 shows the gas analysis values and temperatures measured during the reduction experiments at various temperatures. The time at which the reaction starts depends on the experimental temperature. Figure 4(a) shows an example of how the initiation time of reaction is determined. The initiation time of reduction is defined as the time to second CO gas peak after the pellets drop. Figure 5 shows the reaction initiation time, CO, and CO2 emissions. The reaction initiation time is decreased with the increase of experimental temperature. On the other hand, the evolution volumes of CO and CO2 show similar level even though the temperature is increased (Fig. 5(b)).

    Figure 6 shows the XRD pattern of the slag in the sample when reduced at various temperatures. SiO2 is formed by the reduction reaction, and the slag forms a liquid phase at high temperatures. The cordierite (Mg2Al4Si5O18) phase is likely generated during the cooling process of the slag. In an earlier report,27) the cordierite phase was found to be formed at lower than 1,673 K in simulations using FactSage.30) The XRD patterns of the samples to which C was added showed similar patterns. However, the chromite phase and SiC remained, thus suggesting that the reduction was insufficient. Therefore, it is presumed that there are some unreacted parts because the carbon component is large and interferes with the reduction reaction.

    Zambrano et al.31,32) proposed the reaction fraction (Fr) to interpret the reduction behavior of a pellet composed of chromium ore, coke, lime, ferrosilicon, silica, and cement. To calculate the reaction fraction, the following equation was defined:

    Fr = (W i  - W t ) / (k w W t ),
    (1)

    where Fr = reaction fraction; Wi = initial weight of sample (in g); Wt = final weight of sample (in g); kw = maximum fraction.

    The maximum fraction was calculated from the maximum weight loss among three carbon addition change experiments by using the following equation:

    kw = (W i  - W t ) / (W i  - W t h ,
    (2)

    where (Wi−Wt)h = highest weight loss;

    The reaction fraction with temperature change was evaluated and shown in Fig. 7. Considering that the reaction time was fixed at 1 h for each experiment, the maximum weight loss achieved owing to the reaction was achieved when the temperature was the highest. Thus, the maximum fraction was calculated from the maximum weight loss, which played a vital role in deciding the quality of the reduction reaction. The reduction reaction is dependent on temperature.21,22,33) Figure 7 shows that the reduction reaction gradually increases with temperature change. At 1,773 K, reduction was achieved to almost double that at 1,623 K.

    To investigate the effect of temperature, the reduction experiments were performed between 1,623 and 1,773 K, and Fig. 8 shows the recovered sample. The crucible was broken during the cutting of samples of 1,723 and 1,773 K.

    Figure 8 shows that the coagulation of the metal is improved when the temperature increases. At 1,623 K, fine metal particles were distributed throughout the slag, aggregation progressed gradually at 1,673 K, and large metal particles were observed at 1,723 and 1,773 K. Two large particles were found at 1,723 K, whereas one larger particle was incorporated at 1,773 K. This finding indicates that at higher temperatures, a more active coalescence of metal particles occurs. This may be related to the viscosity of the slag and is discussed more comprehensively later in the manuscript.

    3.3 Reaction time change experiments

    Pellets were prepared from the mixture of Si sludge and chromium ore with a basic blending ratio to investigate the reduction mechanism of chromium ore. Pellets were dropped into a crucible in a graphite reaction tube and were maintained at a predefined temperature. In this study, the high-frequency induction furnace shown in Fig. 1(b) was used. When the temperature reached 1,623 K, the pellets were dropped from the top into the alumina crucible through a quartz tube. The holding time after the drop was maintained at between 2 and 40 min. After this predetermined reaction time, the sample was quenched by removing the reaction tube from the furnace. During the experiment, argon gas was injected into the reaction tube to prevent the influence of oxygen.

    Figure 9 shows the photographs of the pellets reduced for various times at a constant temperature of 1623 K, and Fig. 9 shows that the shape of the pellet was retained for 10 min before melting. Therefore, it can be presumed that the reduction reaction occurs when the temperature increases after the pellets were dropped and that the slag formed and dissolved between 10 and 20 min.

    In the samples heated for more than 20 min, shiny metal particles can be observed on the cut surfaces. Figure 10 shows the SEM images of the samples quenched after different holding times. The SEM photographs were taken in BSE (Back Scattering Electron) mode: the metal parts appear white, whereas chromium ore, SiC, and slag components appear gray. Metal particles clearly coexist with large and fine particles. In the metal, the elements C, Fe, Cr, and Si are mostly present. In the slag, C, Si, Na, Al, Mg, O, and Ca components were detected, but Fe and Cr components were not detected in the EDS analysis shown in Table 4. The EDS point No. in Table 4 corresponds to the number indicated in Fig. 10.

    In the 2 min sample shown in Fig. 10, the metal and slag are already formed even though the time is very short. Tiny metal droplets are formed around the perimeter of the chromium ore particles. In the right photograph, the slag is shown to have formed around the chromium ore and to have penetrated inside. Metal formed near the slag-penetrated section. Some metals may be partly reduced, as implied by the oxygen detection in EDS analysis. Therefore, the reduction of chromium ore is not complete.

    Although the formation of metal and slag can be confirmed in the 5 min samples shown in Fig. 10, some chromium ore has not yet been reduced. Although metal and slag were formed, the reaction time was not sufficiently long to reduce completely the chromium ore (see left photograph), and partly reduced forms of metal were identified in the EDS results (No. 6). There were also large metal particles of 100 mm or larger, which have two phases. An example of this is the EDS No. 7 in Table 5. No. 7-2 (less bright zone in Metal 7) shows the high value in Cr, low in Fe, and No. 7-3 (bright zone in Metal 7) includes Si component and has low Cr, high Fe value. No. 7-1 gives the EDS for large zone. Further, the reduced metal particles were finely distributed around the SiC. In the photograph of chromium ore, slag can be observed to penetrate the chromium ore where metal formed. This has been reported earlier by several researchers.23, 34-36) The gray area indicates the unreduced chromite, and dark gray area shows the affected zone by slag or spinel structure. Similar discussion have been reported for metal droplets in unreduced chromite particle in earlier researches.23, 34-36) Soykan et al.23) and A.B. Hazr-Yoruç34) have reported the reduction mechanism of chromite by graphite: the reduction occurs by the outward diffusion of Fe2+ and Cr3+ ions, and the inward diffusion of Cr2+, Al3+, and Mg2+ ions. Initially Fe3+ ion is reduced to Fe2+ ion and then reduced to metallic Fe. Similarly Cr3+ ion is reduced to Cr2+ ion and then reduced to metallic Cr. Alternatively Cr3+ ions can be reduced directly to metallic Cr. In the present study, similar mechanism can be applied for the reduction of chromite by silicothermal reaction.

    In Fig. 10, the fully reduced large metal particles are made of Cr–Fe–C–Si, and the metal in the chromium ore on the right is rich in Fe, which is a common characteristic in all samples. The SEM images shows that the chromium ore of larger size is surrounded by small Si or SiC particles and reduced by Si easily. It is also believed that the reduction of Fe oxide proceeds more easily than that of chromite.

    Figure 10 also confirms that chromium ore is reduced in the 10 min sample. Many chromium ore particles that have not yet been completely reduced, together with chromium ore particles in which fine metal particles are formed inside, can be found, but there are also many completely reduced metal parts. On the contrary, the slag in the peripheral part contains a considerably higher (more than 10 %) C component and Si component in the EDS analysis result. Thus, it is considered that the oxide has the ability to cause reduction.

    In the case of the 20 min sample shown in Fig. 10, the large metal particles showed a Cr–Fe–C–Si composition with a significant Cr component. The fact that C and Si components are detected at high levels in the EDS analysis of the slag part suggests that the reduction of the chromium ore continued throughout the 20 min. It is noteworthy that, in the image for the 20 min sample, large chromium ore particles are observed in the process of separation into small particles as the reduction reaction progresses.

    The reduction reaction is shown to have proceeded continuously in the 30 min sample because the C and Si components in the slag are still present after 30 min. As shown in the recovered sample (Fig. 8), the reduced fine metal particles aggregate, and the size of the particles increases. This is because the slag around the metal particles forms a liquid phase.

    The SEM images of the 40 min sample shown in Fig. 10 demonstrate that the reduction reaction of chromium ore is still underway and that the size of the metal particles increases further up to this time, i.e., the metal particles are further agglomerated. In this study, the temperature was 1,623 K, and the reduction rate is expected to be low. Therefore, the agglomeration of the metal particles also takes a longer time. Slag viscosity may be an important factor in the separation of metal and slag because it can require more time to separate slag and metal in high-viscosity slag and because metal particles are not easily moved.

    3.4 Recovery of metal

    In the production of ferrochrome, it is important to separate the slag and metal in smelting reduction processes by using arc furnaces. Figure 11 shows a schematic drawing of slag and metal separation in the smelting furnace. In the early stage, fine metal particles are formed by the reduction of chromium ore and become agglomerated at a later stage. Finally, the metal and slag are separated completely. Therefore, slag viscosity has considerable influence on the recovery of the metal.

    As discussed above, temperature has a large influence on slag viscosity. Furthermore, the viscosity of the slag is dependent on its chemical composition. In this study, the slag produced by the reduction of chromium ore using Si sludge contains a large amount of SiO2, Al2O3, MgO, together with some unreduced Cr and Fe oxides with very low CaO content. Given that this slag system is a strong acid slag with a large SiO2 component, the addition of CaO may decrease the viscosity of the slag and may be a favorable condition for the agglomeration of metal particles. The addition of lime flux also has been proposed by Weitz and Garbers-Craig in the report on the furnace operating model to lower the liquidus temperature of slag.24)

    Table 5 shows the results of slag analysis obtained in this study. On the basis of this slag composition, its viscosity was calculated for different CaO contents by using FactSage. Figure 12 shows the results. The viscosity of the slag decreases with increasing temperature and increasing CaO content. Thus, to facilitate the separation of the ferrochrome metal and the slag, we consider that it is advantageous to increase the temperature and add the slag containing the CaO component.

    4. Conclusions

    The effects of C addition, reaction time, and reduction temperature on the reduction reaction of chromium ore using Si sludge were quantitatively studied. Higher C addition led to an increase in the generation of CO and CO2 gas and an increase in the gas evolution time. When pellets were dropped, they were heated and began to react at 1,573 K or higher. The pellets were kept in shape until 10 min after being dropped and were then melted. As the holding time increased, the size of the reduced metal particles increased, i.e., the metal particles were agglomerated. Chromium ore was rapidly reduced by Si sludge, and the slag penetrated into the chromium ore and progressively reduced its interior. SiC seems to remain in the slag, thus allowing the reduction reaction to proceed continuously. As the reduction temperature increased, the reaction initiation time was reduced, and the reaction fraction of the reduction reaction increased. With increasing reaction temperature, the aggregation of reduced FeCr metal was promoted. This can be attributed to the decrease in the viscosity of the slag with increasing temperature. An effective approach was to add an appropriate amount of CaO to lower the viscosity of the slag, and this approach is advantageous in the separation of metal and slag.

    Acknowledgement

    This study was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and funded by the Ministry of Trade, Industry, and Energy (No. 20165020301180).

    Figure

    MRSK-29-10-592_F1.gif

    Schematic drawing of experimental apparatus setting. (a) Kanthal Super furnace and (b) Induction furnace.

    MRSK-29-10-592_F2.gif

    Temperature profile and CO, CO2 percent change in off-gas for different C addition. (a) 2% C add, (b) 4% C add, and (c) 6% C addition.

    MRSK-29-10-592_F3.gif

    (a) Gas evolution time, and (b) CO and CO2 volume evolved during the main reaction, with respect to the change of carbon addition.

    MRSK-29-10-592_F4.gif

    Temperature profile and CO, CO2 percent change in off-gas for different temperatures. (a) 1,673 K, (b) 1,723 K, and (c) 1,773 K

    MRSK-29-10-592_F5.gif

    (a) Reaction initiating time, and (b) CO and CO2 volume evolved during the main reaction, with respect to the change of temperature.

    MRSK-29-10-592_F6.gif

    Typical XRD patterns of recovered slags for different temperatures.

    MRSK-29-10-592_F7.gif

    Change of reaction fraction (Fr) with respect to the temperature change.

    MRSK-29-10-592_F8.gif

    Photographs of samples recovered after reaction at different temperature.

    MRSK-29-10-592_F9.gif

    Photographs of samples recovered after reaction with different holding time at 1,623 K.

    MRSK-29-10-592_F10.gif

    SEM images of samples after reaction with different holding time at 1,623K (SEI mode).

    MRSK-29-10-592_F11.gif

    Schematic drawing of slag/metal separation during production of FeCr alloy.

    MRSK-29-10-592_F12.gif

    Change of slag viscosity with respect to the temperature and CaO addition calculated by FactSage.

    Table

    Chemical composition of chromium-ore (wt%).

    Basic mixing ratio of starting materials.

    Expected composition of pellets before reaction (wt%).

    Energy dispersive X-ray spectrometry of phases in Fig. 10 (wt%).

    Chemical composition of slag (wt%).

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