Preparation of High-Quality Silicon with Silicon Cutting Waste by a Carbothermal Reduction Method

Silicon cutting waste (SCW) mainly consists of Si (80 ~ 85 wt%), SiO2 (13 ~ 16 wt%) and other impurities (2 ~ 4 wt%). Nowadays, the Si in SCW is commercially recycled to produce Si ingots by a slag refining method, but the SiO2 in SCW is melted into silicon slag and discarded as waste. In this paper, a carbothermal reduction method has been proposed for recycling Si resources from both Si and SiO2 in SCW to prepare high-quality silicon in a submerged arc furnace. Petroleum coke was selected as the carbonaceous reducing agent. Firstly, the effects of carbon content on the equilibrium compositions of the Si-SiO2-C system were simulated. Secondly, SCW was mixed with petroleum coke under the guidance of thermodynamic analysis results. Finally, the mixtures were charged into furnace and smelted. Thermodynamic equilibrium analysis results showed that the value of n(Si):n(C):n(SiO2) should be controlled as 2.62:0.22:0.44 theoretically. Experimental results revealed that the recovery ratio of SCW was 50% and the purity of Si products was 99.40%. This proposed method provides an effective and industrialized applicable approach for recycling SCW.


Introduction
Nowadays, the diamond-wire sawing method has been the mainstream method for slicing crystalline silicon ingots into wafers [1].Since the thickness of silicon wafers is nearly equal to the diameter of the cutting wires, approximately 30 ~ 35wt% of the crystalline silicon is wasted as silicon cutting waste (SCW) during the silicon wafer producing process [2,3].In the year 2021, the consumption of crystalline silicon was about 54,000 tons, and it can be calculated that more than 21,600 tons of silicon were wasted as SCW.With the rapid development of the photovoltaic industry, large amounts of SCW will be produced in the future.Due to the ultrafine particle size (0.6 ~ 0.8 μm) of SCW, the generation of SCW will cause serious environmental pollution if without proper management.SCW mainly consists of Si (80 ~ 85 wt%), SiO 2 (13 ~ 16 wt%) and other impurities (2 ~ 4 wt%) [4].The high content of Si in SCW inspires researchers to recycle it to produce Si ingots or Si-containing materials.The reported methods include non-transfer arc-assisted vacuum smelting [5], thermal plasma process [6], vacuum refining [7], vacuum carbothermal reduction [8], slag refining [9], induction smelting [10], beads milling [11], nitriding [12], Al-Si alloying [13], silicon-based anodes [14,15].Besides, many researchers focused on purifying SCW by using methods such as acid leaching [16][17][18], hydrobromination [19], aerosol reactor methodology [20].Although these reported methods are quite promising, the Si resource in SiO 2 was not recycled by these methods.At present, the Si in SCW is commercially recycled to produce Si ingots by a slag refining process in an induction furnace and the recovery ratio of SCW is about 60% [3].However, the SiO 2 in SCW is not recycled and is finally discarded as silicon slag.
In this paper, a novel method was proposed for recycling SCW to produce high-quality silicon by a carbothermal reduction method.SCW is mixed with petroleum coke and the mixtures were smelted in a submerged arc furnace.Two purposes can be realized by this method.On one hand, the Si in SCW can be melted into molten Si during the hightemperature smelting process.On the other hand, the SiO 2 in SCW can be reduced into Si according to the overall reaction (1).Thus all the Si resources from Si and SiO 2 in SCW can be recycled by this method.

Raw Material and Charge Calculation
Silicon cutting waste (SCW) is supplied by Yicheng Co., Ltd. in China, and it is composed of 83.99 wt% of Si, 13.50 wt% of SiO 2 , 1.01 wt% of C and 1.50 wt% of metallic impurities.Petroleum coke is composed of 87.51 wt% of fixed carbon, 0.40 wt% of ash component, 10.78 wt% of volatile component and 1.31 wt% of moisture.According to Eq. ( 1), the ideal mole ratio between SiO 2 and C is 1:2 for preparing Si by the carbothermal reduction method.It is known that 1 kg of SCW contains about 135 g of SiO 2 , thus the needed amount of C for reducing SiO 2 is 54 g.The mass fraction of fixed carbon is 87.51 wt% in petroleum coke, hence the required amount of petroleum coke is 61.7 g to reduce 1 kg of SCW. (1)

Carbothermal Reduction Process
As shown in Fig. 1, the carbothermal reduction process of SCW mainly involves five procedures: (i) SCW was mixed with petroleum coke powder evenly; (ii) the submerged arc furnace was baked by electric arc for about 1 h; (iii) the mixed raw materials were smelted in the furnace; (iv) Si products were released from a tap hole into a silicon ladle; (v) Si products were poured from the ladle to a mould and followed by cooling in atmosphere.The recovery ratio of SCW can be calculated by Eq. (2).
where m Si is the mass of Si products, g; m w is the mass of charged SCW, g; X is the recovery ratio of SCW, %.

Thermodynamic Equilibrium Analysis of the Si-SiO 2 -C System
During the carbothermal reduction process of SCW, the main reactants are C, Si and SiO 2 and these substances can comprise the Si-SiO 2 -C system.To provide a thermodynamic guidance Fig. 1 The flow chart of recycling SCW by the carbothermal reduction method in a submerged arc furnace for the reduction process, the thermodynamic equilibrium analysis of the Si-SiO 2 -C system was conducted by using Factsage 8.0 software.Equilibrium components represent the most stable chemical components in a system under specific conditions, which can reflect the chemical reactions that may occur in the system [21][22][23].It is known that the mass ratio of Si to SiO 2 in SCW is m (Si) :m (SiO2) = 83.99:13.50,thus the mole ratio of Si to SiO 2 is n (Si) :n (SiO2) = 2.62:0.22.According to the overall reaction (1), the ideal mole ratio of C to SiO 2 is n (C) :n (SiO2) = 2:1 for producing Si. Figure 2 shows the variation of equilibrium compositions of 2.62Si + 0.22SiO 2 + 0.22 C, 2.62Si + 0.22SiO 2 + 0.44 C, 2.62Si + 0.22SiO 2 + 0.66 C and 2.62Si + 0.22SiO 2 + 2.2 C system as a function of temperature at 1 bar.Species with a mole fraction less than 10 − 5 were omitted from the figure.Figure 2a shows that Si element primarily existed as Si (l) and SiO (g) in the final products of 2.62Si + 0.22SiO 2 + 0.22 C system. Figure 2b displays that Si element mainly existed as Si (l) in the final products of 2.62Si + 0.22SiO 2 + 0.44 C system. Figure 2c and d exhibit that Si element primarily existed as Si (l) and SiC (s) in the final products of 2.62Si + 0.22SiO 2 + 0.66 C and 2.62Si + 0.22SiO 2 + 2.2 C systems.These results reveal that operating at a suitable mole ratio between C, SiO 2 and Si is important for producing Si by reducing SCW and it is better to control the value of n (Si) :n (C) :n (SiO2) as 2.62:0.22:0.44 in this paper.As shown in Fig. 2b, the SiC (s) can be formed by the reaction between C (s) and Si (s) according to reaction (3) when the temperature is below 1500 K.When the temperature increases to the melting point of Si (1693 K), Si (s) will convert into Si (l) .With the increase of temperature to about 1993 K (melting point of SiO 2 ), the amount of SiO 2(s) begins to decrease and the amount of SiO 2(l) increases.The amounts of Si (l) , SiC (s) and SiO 2(l) remain unchanged in the range of 1993 ~ 2100 K.When the temperature reaches about 2100 K, the amounts of SiC (s) and SiO 2(l) decrease sharply along with the rapid formation of Si (l) , SiO (g) and CO (g) , which implies that reactions between SiC (s) and SiO 2(l) occur according to the Eqs.( 4) and ( 5).After the complete consumption of SiO 2(l) , the amount of SiC (s) continues to decrease and the amount of SiO (g) starts to decrease, while the amounts of Si (l) and CO (g) continue to increase.This phenomenon reveals that SiC (s) reacts with SiO (g) to produce Si (l) and CO (g) through reaction ( 6) at about 2110 K.The amount of Si (l) starts to decrease at about 2890 K and the amount of Si (g) increases with the increasing of temperature, which may be due to the evaporation of Si (l) by the effect of high temperature. (3)

Characterization of Si Products
As is shown in Fig. 1f, Si ingots were obtained after the molten Si cooling in the mould, and it was calculated that the recovery ratio of SCW is 50%.The mass fractions of impurities in Si products are shown in Table 1, and it can be noticed that the purity of Si products is 99.40%.The removal ratio of Al and Ca impurities are 76.89% and 87.35%, respectively.This is because part of Al and Ca impurities evaporate from the molten Si by the effect of plasma arc as a result of their higher saturated vapor pressure [24,25].However, the mass fraction of Fe, Ti, B and P impurities in Si products did not decrease but increased, probably because the raw petroleum coke and used electrode contain a certain of oxides impurities.During the carbothermal reduction process of SCW, part of the oxides impurities were reduced and finally entered into the Si products.Since the saturated vapor pressure of Fe, Ti and B impurities is low at the smelting temperature, the plasma arc almost had no effect on removing these impurities from Si products.
Figure 3 shows the microstructure and EDS mapping analysis result of impurities in Si products.It can be seen

Conclusion
In this paper, Si resources in silicon cutting waste (SCW) were successfully recycled to prepare high-quality silicon via a carbothermal reduction method in submerged arc furnace.During the smelting process, the Si in SCW was melted into liquid Si and the SiO 2 was reduced into Si.Thermodynamic equilibrium analysis results revealed that the SiO 2 can be reduced into Si by the reactions of 2SiC (s) + SiO 2(l) = 3Si (l) + 2CO (g) and SiC (s) + SiO (g) = 2Si (l) + CO (g) .Experimental results showed that the recovery ratio of SCW was 50% and the purity of Si products was 99.40%.The Si products are suitable for preparing alloys, silicon steel and silicon nitride.In the future, further research is needed for revealing the carbothermal reduction mechanism of SCW and increasing the recovery ratio of SCW.

Table 1
Mass fractions of the impurities in waste and silicon product Microstructure and EDS mapping analysis result of impurities in Si products from Fig.3athat the impurity phase contains many different phases, and some small impurity phases are dispersed inside the main impurity phase.EDS mapping analysis results of the impurity phase are shown in Fig.3b ~ e, which exhibit the distribution of Fe, Al, Ca and Ni impurities.EDS point analysis results of the impurity phase are shown in Table2, and it can be seen from the table that the main impurity phases in Si products are Al-Si-Fe and Al-Si-Ca phases, and a small amount of impurity phases are Al-Si-Fe-Ti intermetallic compounds.

Table 2
EDS point analysis results of different regions in Si products