Study of Pellets and Lumps as Raw Materials in Silicon Production from Quartz and Silicon Carbide
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- Dal Martello, E., Tranell, G., Gaal, S. et al. Metall and Materi Trans B (2011) 42: 939. doi:10.1007/s11663-011-9529-y
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The use of high-purity carbon and quartz raw materials reduces the need for comprehensive refining steps after the silicon has been produced carbothermically in the electric reduction furnace. The current work aims at comparing the reaction mechanisms and kinetics occurring in the inner part of the reduction furnace when pellets or lumpy charge is used, as well as the effect of the raw material mix. Laboratory-scale carbothermic reduction experiments have been carried out in an induction furnace. High-purity silicon carbide and two different high-purity hydrothermal quartzes were charged as raw materials at different molar ratios. The charge was in the form of lumps (size, 2–5 mm) or as powder (size, 10–20 μm), mixed and agglomerated as pellets (size, 1–3 mm) and reacted at 2273 K (2000 °C). The thermal properties of the quartzes were measured also by heating a small piece of quartz in CO atmosphere. The investigated quartzes have different reactivity in reducing atmosphere. The carbothermal reduction experiments show differences in the reacted charge between pellets and lumps as charge material. Solid–gas reactions take place from the inside of the pellets porosity, whereas reactions in lumps occur topochemically. Silicon in pellets is produced mainly in the rim zone. Larger volumes of silicon have been found when using lumpy charge. More SiO is produced when using pellets than for lumpy SiO2 for the same molar ratio and heating conditions. The two SiC polytypes used in the carbothermal reduction experiments as carbon reductants presented different reactivity.
Metallurgical silicon is produced by the reduction of silica in an electric arc furnace by means of carbonaceous materials. As alternatives to the dominating Siemens chemical route, two main direct metallurgical routes have been established for the production of low-cost solar grade silicon in Norway: the Elkem route (developed by Elkem Solar) and the Solsilc route (developed by Fesil Sunergy). The Solsilc route uses high-purity raw materials in form of pellets. It aims at “direct” high-purity silicon production, avoiding several downstream refining steps. Elkem produces a high-grade MG-Si, which is subsequently refined using pyrometallurgical and hydrometallurgical processes.
For both routes, the raw materials play an important role for the optimization of the process. The main quartz specifications are the purity chemistry, lump size, lump mechanical and thermal strength, and softening properties. The absence of fines (particles less than 2 mm in size) and a softening temperature close to the quartz melting point are desirable to maintain a high gas permeability in the furnace burden. Common impurities in quartz, like Na, K, Al, and Fe, come from minerals like mica and feldspar. Hydrothermal quartz and pegmatite core are the purest among the silica sources.
These can be purified even more by means of mineral liberation, separations, and acid treatments.[4,5] The purification processes must be carried out with fine particles (in the range of micrometers). To charge these materials in an electric arc furnace, agglomeration (pellets or briquettes) is necessary.
SiO reactivity and chemistry are the most important distinguishing properties of carbon. High-reactivity carbon materials are desirable as reductants, because they preserve matter and energy in the process by rapidly reacting with energy-rich SiO(g). The SiO reactivity of coal based reductants depends mainly on the ranks of the coals, on its petrographic properties, on the carbon particle size and on the SiO diffusion in pores. Coal and charcoal contain both B, P, and ash minerals mainly consisting of SiO2, Fe2O3, Al2O3, TiO2, CaO, and MgO. B and P originate from the plants, whereas ash containing oxides derive from clay minerals deposited onto the plant material precursors. High-purity carbon black is a synthetic carbonaceous powder material produced from pure liquefied natural gas or methane. Low B and P content in the metal produced is necessary because these elements are difficult to remove by directional solidification in the subsequent ingot production of silicon for photovoltaic purposes.
Danes et al. carried out a thermodynamic study of the Si-C-O system in an isobaric reactor. The reactor was first filled with inert gas at 1 atm and the pressure was held constant by means of a regulating valve which allowed gas evacuation. When an initial complex of SiC + SiO2 is heated, SiO + CO pressure increases until it reaches 1 atm (105 Pa). This condition represents the invariant points where three condensed phases (Si, SiC, and SiO2) are in equilibrium at the specific temperature 2104 K (1811 °C) and gas composition. The reaction  runs at constant temperature until either SiC or SiO2 is consumed. Above 2104 K (1811 °C) depending on the progress of the reactions  and , Si, SiC + Si, or Si + SiO2 will not be consumed completely.
Hirasawa produced silicon in a two-stage reduction process. It was observed that more silicon was formed at 2273 K (2000 °C) than at 2223 K (1950 °C), and that a larger amount of Si was obtained for longer holding time at 2273 K (2000 °C). They found that the reduction of SiO2 into SiO(g) (reaction ) is the reaction rate-controlling step.
Fruehan and Ozturk and Wiik studied the rate of formation of SiO(g) by reacting CO(g) with silica (reaction ). Experimental observations strongly indicated that the rate of formation of SiO(g) is controlled by chemical kinetics on the silica surface.
SiC occurs in different polytypes. According to the JANAF Thermochemical Tables, the cubic polytype (β-SiC) is more stable than the hexagonal (α-SiC) at all temperatures, but the difference is so small that is not important in the equilibrium evaluation of the system. Filsinger and Bourrie and Presser and Nickel stated that the reactivity of silica with SiC does not depend on the crystal structure of the SiC.
The solid-state reactivity and the surface structure of quartz changes when the surface is activated mechanically.[22, 23, 24, 25] During the mechanical treatment of quartz (grinding or milling), Si-O bonds are broken and highly disordered silica layers (thickness from 20 to 500 nm) are formed on the surface of the quartz grains. These deviations from ideal lattice are metastable defects that lead to the storage of energy and increase the reactivity of the material. According to Steinike and Tkáčová, mechanically activated quartz shows an increase in the extent of adsorption and gas diffusion into the disturbed near surface layers. Quartz seems more reactive with CO2(g) than O2(g). They also observed that mechanically activated quartz (low temperature form) transforms directly into crystobalite at a temperature of 1473 K (1200 °C), which is lower than the high-temperature quartz-crystobalite transformation at approximately 1673 K (1400 °C). This was confirmed by Balek et al.
Agarwal and Pad used thermogravimetry to study the kinetics of reactions in pellets, which were made of carbon black and silica. The reaction rate increases by reducing both carbon and silica particle size, but no significant improvement occurs below a critical size of 20 μm. In general, in a powder compact, where the reaction occurs between solids through gaseous intermediates, the possible reaction controlling steps can be surface reactions or diffusion of the gases through the pores or a combination of these. When pellets made of carbon and silica mixture are heated, they react quickly to SiO(g) and CO(g). When (PCO/(PCO + PSiO)) reaches the chemical equilibrium value, the reaction retards quickly. CO(g) has to diffuse out of the pellet for subsequent reaction taking place. Agarwal and Pad state that pellets porosity leads to higher rates of diffusions of CO(g) and SiO(g) from pellets and, therefore, an increase in reaction rate for the reduction of SiO2 to SiO(g).
Jensen recorded the reactions taking place in the crater zone of a 50-kW single electrode pilot scale furnace when lumpy charge was used. From the images, the crater appears complex and dynamic. SiO2 reacts with SiC and forms Si(l), SiO(g), and CO. The cavity expands as the reaction proceeds. The expansion of the cavity is balanced by SiO2 and SiC entering slowly the crater. Melted viscous quartz enters in the crater zone slowly. SiC is still a solid phase and is covered by a thin Si layer. A Si bath is present on the bottom; SiC pieces and melted quartz float into it. When quartz enters in contact with SiC, it reacts violently starting bubbling and SiO(g) is produced according to reaction . When SiC enters in the crater zone it disappears fast according to reaction . When Si and SiO2 enter in contact, SiO2 starts bubbling and the reactants quickly disappear according to reaction .
Although there have been some theoretical suggestions[13,15] and experimental attempts[11,12,17,28] to produce silicon on small scale, no experiment has successfully reproduced the inner zone environment of the industrial furnace. There have also been investigations on the reactivity of lumpy silica[12,17,28] and pellets. However, these studies were not performed under the conditions of silicon production and did not attempt to compare the reaction mechanisms and kinetics of the same raw materials used either as lumpy or pellet charge.
To develop a small-scale experimental setup that can adequately simulate the hot zone of the industrial silicon production furnace
To study the mechanisms and kinetics of the reactions between quartz and SiC when the two compounds are present as fine powder in pellets or as lumps in different charge mixes
Materials and Methods
Lumps of size 3–5 mm and pellets of 1–2 mm diameter were used. The total weight of the charge was 25 g when pellets were used and 18 g for the case of lumpy charge. Pellets consist of a mixture of quartz and silicon carbide powder. The powder was produced by milling the quartz up to 20 μm and the SiC up to 10 μm size (the values represent the median). The size was chosen following the study by Agarwal and Pal. The two components of the mixture, SiC and SiO2, were first dry blended together and then delivered into a pelletizing plate (22 cm diameter and 24 cm rim on its outer circumference). Simultaneously with the feeding of the two-component mixture into the pelletizing plate, demineralized water was supplied to enable the formation of micro pellets (1–2 mm diameter). No binding agents have been used. Because pellets contain water, the charge was dried in a graphite crucible at 353 K (80 °C) for 10 hours prior to the experiments.
Experimental Setup and Procedure
Carbothermic reduction experiments, where mixtures of SiC and SiO2 in the form of pellets or lumps were used as charge materials, for the purpose of studying and comparing reaction mechanisms
In situ melting experiments on small silica pieces, to study their thermal properties
A reducing atmosphere was maintained during the whole experiment. The furnace was first evacuated: The calculated oxygen content after evacuation was ~1.2 × 10−6 moles O2. The furnace was subsequently filled with argon 5.0 purity at 5.8 mbar (580 Pa): The oxygen content carried by the argon filling is ~4 × 10−5 moles O2. The oxygen coming from the crucible is assumed negligible because the crucible was preheated to 373 K (100 °C) overnight. Leakages were measured in a time interval of 10 hours and were found to be negligible for the time needed by the experiment: 0.003 mbar (0.3 Pa) during the argon filling and 0.0025 mbar (0.25 Pa) during the experiment. At the beginning of each experiment, the chamber was filled to approximately 0.75 atm (75000 Pa) with argon. During heating the pressure inside the chamber increased. The total pressure in the chamber reached approximately 1 atm (105 Pa) at 2273 K (2000 °C).
Experimental Variables in the Carbothermic Reduction Experiments: Quartz Source, Charge Size, and SiO2:SiC Molar Ratio
The total weight of the charge mix was maintained constant for all the experiments; 25 g and 18 g have been used for pellets and lumpy charge, respectively. The weight difference is caused by the pelletizing procedure. The pellets were weighed once in the crucible and once dried, and it was not possible to predict the amount of water adsorbed during the pelletizing procedure. Two molar ratios have been investigated. The molar ratio of SiO2/SiC = 1/2 refers to the stoichiometry of reactions  and . A molar ratio of 1:1 refers to the stoichiometric overall reaction taking place in the inner zone proposed by Schei et al. and Müller. It was decided to start the experiment without liquid Si bath on the bottom to focus on the reactions between SiC and SiO2.
Method for Analysis
Experiments in the sessile drop furnace aimed at measuring the temperature of melting and the quartz weight losses (as SiO) in CO atmosphere. It was possible to calculate the weight losses by weighing the silica sample before and after the experiments. The reaction rate was determined in terms of SiO losses. The temperature of melting was determined by the moment the quartz became glassy. When the quartz starts melting, voids are sealed; there is no more light diffraction and the sample turns transparent and glassy. This valid method is used to compare the melting properties of different quartzes, but the melting temperature must not be considered an absolute value because it is influenced by the sample size and by the heating rate. In an industrial furnace, the large quartz lumps subjected to the highest possible heating rate do not usually soften before reaching the hot inner crater zone of the furnace.
Silicon Production Experiments
The carbothermic reduction experiments simulate a complicated system involving reactions between solid, liquid, and gaseous phases. Because the duration of the experiment is short, these reactions might not reach equilibrium but the experiments are instead intended to represent the relative rates of the predominant reactions.
Non-reacted quartz, silicon carbide and silicon phases have been found in the crucible after experiments. Figure 5 shows the product morphology of the reacted crucible for the case of lumps (on the left side) and pellets (on the right side). The cross sections of the reacted crucibles looked similar regardless of the type of quartz and molar ratio used. Reacted lumpy charge is shown in Figure 5 on the left side: the structure is loose, a cavity is present, melted quartz and droplets of silicon are found mainly on the bottom of the crucible. Microscope and XRD investigations showed that non-reacted silica was present only as an amorphous (melted) phase. A cavity and large area of single phase silicon metal were also found. Pellets have a different reaction pattern than lumps. The reacted pellet charge is compacted, porous and has low interaction with the crucible. The pellets have shrunk and Si is found mainly in the bottom of the crucible. It is difficult to recognize the spherical original shape of pellets for the experiments with molar ratio SiO2:SiC = 1:1. SiC particles decreases in size from a mean diameter of 9.26 µm to approximately 5 µm.
Average Total Weight Losses for Different SiO2:SiC Molar Ratio and Type Charge*
Thermal Properties of Silica
Difference Between Pellets and Lumps: Reaction Mechanisms
The images of the reacted charge and the calculated SiO(g) total production (Figures 6, 7, and 10) make it possible to suggest the predominant reactions when pellets or lumps of SiO2 and SiC are heated up to 2273 K (2000 °C).
At the beginning, SiO(g) is produced and reacts with the graphite crucible to form 3C:SiC. As the SiC layer on the inner surface of the crucible builds up, the reaction slows down and the SiC forming reaction stops. A slightly greater amount of 3C:SiC has been formed with pellet charge probably because of the larger production of SiO(g). Three reactions involving silica and silicon carbide take place: reaction  between SiO2(l,s) and SiC(s), reaction  between SiO(g) and SiC(s), and reaction  between SiO2(l) and Si(l).
Reaction(1) between SiO2 and SiC
The reaction is expected to be faster in presence of molten SiO2[28,33] and for the case of pellets because of the larger surface area available and closer contact between the reactants in the charge mixture. More SiO2 is expected to react with SiC in pellets for the same reason. This was confirmed by the calculated mass balance. The estimated nonreacted silica was on average 2 wt pct for the case of lumpy charge and on average only 0.18 wt pct for the case of pellets.
Reaction (2) between SiO and SiC
As the PSiO increases, the reaction between SiO(g) and SiC takes place and Si is produced. The pellets and the SiC particles inside the pellets decreased in size as the reduction proceeds. The silicon production reaction when pellets are used takes place inside the pellets and mostly in the rim zone (Figure 6(b)). The more favorable Si formation at pellets rim might be pressure related. Larger zones of silicon occur with lumpy charge. This is probably because of several Si-forming reactions that occur simultaneously in the lumpy charge. Si forms on the SiC surface in SiC pores and concave surfaces according to reaction . Once the silicon has covered the available SiC surface, instead of reaction , a coupled reaction between SiC and Si(l) and between dissolved carbon in Si(l) and SiO may take place. Si grows inward both the SiC particle and the molten silica. SiC at higher temperatures may dissolve in Si. Dissolved carbon diffuses to the surface of the Si droplets, reacts with SiO(g), and forms Si. The carbon solubility in liquid silicon at 2073 K and 2273 K (1800 °C and 2000 °C) is 500 ppmw and 2100 ppmw, respectively. The proposed reactions are described by Eqs.  and .
Reaction (3) between SiO2 and Si
When lumpy charge was used, melted quartz and silicon were found on the bottom of the crucibles. The reaction between SiO2(l) and Si(l) is expected to take place at high temperatures after some silicon has been produced. Because for the case of pellets, no quartz was left after the experiments and, as mentioned previously, SiO2 seems to react quickly to SiO(g) before any Si(l) is produced, the reaction between SiO2 and Si(l) seems to be negligible in the pellet charge.
Difference Between Pellets and Lumps: SiO Production
More SiO(g) has been produced with pellets. The particles in the pellets are in close contact so that the diffusion of CO2 from the SiO2 particle to the SiC particle and the diffusion of CO from the SiC particle to the SiO2 particles are enhanced. Moreover, because the quartz powder has been milled, the silica surface is mechanically activated, which leads to an increase in the extent of adsorption and gas diffusion into the disturbed near surface layers.[22, 23, 24, 25]
With pellets materials, there is a bigger amount of SiO(g) leaving the high-temperature zone. In industrial practice, the SiO(g) is recovered in the upper part of the furnace by reaction with carbon that is a strongly heat-consuming reaction. Then, the gas is cooled and the heat-producing condensation reaction takes place. The produced heat will raise the temperature of the charge, and again, the strongly heat consuming reaction takes place. However, the price of the increased silicon recovery is a high load of condensed SiO that may obstruct the gas flow and make stoking difficult and inefficient.
Effect of Raw Material Mixtures
The reactivity of different quartz and silicon carbide sources has been studied. The SiC charge material is a mixture of hexagonal and rhombohedral polytypes. In contrast with Filsinger and Bourrie and Presser and Nickel, the two polytypes show different reactivity with silica (Figure 12). Hexagonal silicon carbide 6H:SiC is more reactive than the rhombohedral 15R:SiC for all the experiments. All polytypes have the same distance between neighboring Si or C atoms, and same distance of the C atom to each of the Si atoms. The only difference between them is the degree of hexagonality, which is defined as the ratio of number of atoms at hexagonal sites to the total number of atoms per unit cell. 6H:SiC has degree of hexagonality 0.33, whereas 15R:SiC has a 0.4 degree of hexagonality. The degree of hexagonality seems to influence the SiC reactivity: The lower the degree of hexagonality, the better the reactivity.
The reactivity of different hydrothermal vein quartzes has been studied in two experimental setups: the sessile drop furnace and induction furnace. The reactivity of SiO2 is measured as the amount of SiO(g) produced. Silica reacts with CO(g) and it forms SiO(g) and CO2(g), both in presence of high and low PSiO. In particular, on the one hand, the experiments in the sessile drop furnace were run in CO(g) atmosphere and were significant for reaction  occurring at high PCO pressure. On the other hand, the reactions in the induction furnace take place both at high and low PCO and the total amount of SiO(g) formed is also caused by the reaction of SiC with CO2(g). Because the same SiC has been used for all the experiments, we can assume SiO2 as the only source for difference in the amount of SiO(g) produced. The experiments in the sessile drop furnace show higher reactivity for quartz A (Figure 13), which has been confirmed by the carbothermic reduction experiments (Figure 10). This study shows that the reactivity of quartz depends on the silica source used. The higher reaction rate in quartz A might be caused by the larger amount of fluid inclusions. Fluid inclusions contain gaseous and liquid compounds. When the quartz is heated, the fluid inclusions are opened and might induce cracks. The cracks increase the surface available for further reactions.
The two quartz types present different melting points. Quartz A has slightly lower melting point than quartz B. A higher melting point quartz is preferable because softened and melted silica on the upper part of the furnace obstruct the gas flow. Melted quartz A presents larger amount of bubbles than melted quartz B. Quartz A may also be more reactive because of the presence of bubbles that increase the available surface. The formation of bubbles can be correlated to the presence of fluid inclusions or to the dehydration reaction of mica minerals taking place at calcination temperatures lower than the melting point of silica, or it may be a result of the production of SiO(g). Microscopy investigation prior to melting revealed a larger amount of microcracks and fluid inclusions in quartz A. Mica was found in both quartz sources.
Experiments reproducing the inner zone of a furnace have been performed in an induction furnace. Silica and silicon carbide have been charged as lumps and pellets for different molar ratio. The reaction behavior of lumps and pellets was studied in terms of qualitative and quantitative analysis of the reacted phases.
The reactions in pellets take place inside the pellets mostly in the rim zone, whereas the reactions in lumps occur on the outside. Larger zones of single-phase silicon have been found when a lumpy charge is used. A cavity is present when lumpy charge is used, whereas for the case of pellets, the reacted charge is porous and compacted.
More SiO(g) is produced and almost no quartz is left when pellets are used as charge material for the same molar ratio. Lumps are characterized by a higher silicon yield because of less loss of SiO(g).
Different SiC polytypes have been used as charge materials. The polytype with lower degree of hexagonal SiC content has a higher reactivity in terms of reaction rate.
Two different hydrothermal vein quartzes have been used as charge materials. The quartz containing more fluid inclusions and more microcracks has lower melting point and better reactivity in terms of reaction rate with SiO(g).
The thermal properties and reactivities of the two quartzes have also been investigated in a sessile drop furnace, and the results from investigation done in the silicon production experiments were confirmed.
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