The Possibility of Silicon Purification by Metallurgical Methods: Part I

Abstract

This silicon purification research work has two stages and results will be provided in two articles due to the large scope of work. This paper provides the results of the first stage, concerning the metallurgical silicon purification. Silicon was purified by a combined method consisting of slag refining with new slag composition and acid leaching. In the first phase, the metallurgical grade silicon produced by the classical carbothermic reduction method was investigated. In the second phase, the metallurgical purification by slag refining was completed using slags of various new compositions. The purified silicon samples obtained after the melting processes were analyzed for concentrations of impurities. Finally, acid leaching treatment of the obtained silicon was done, followed by elemental analysis of purified silicon. According to our experience we can determine that obtained silicon purity is adequate for further stages of planned study.

1. Introduction

Today, solar power is the fastest-growing branch of the energy industry. Installed capacities are in fact ahead of all development forecasts and, in addition, solar power plants successfully compete with traditional energy sources in terms of cost per unit of generated power. According to a review of the global solar energy market, a total of 756 GW of solar energy power plants were installed and connected in 2020, despite the “lockdowns” due to COVID-19, and there was no decline in the photovoltaic market in 2020 [1,2]. Rapidly growing competitiveness in the solar energy market is one of the main development factors in the energy sector during the recent years. There has been a significant 10% decrease in the solar electricity price compared to 2019 [1] due to improvements in both silicon (Si) purification technologies and the solar cell industrial fabrication process. In 2020, the price of electricity generated by solar panels became cheaper than wind energy [1,2]. Despite numerous studies and new materials used for solar cell production, silicon remains the most popular raw material in solar energy. Currently, the share of solar energy created on the basis of Crystalline-Si is about 95% against 5% created on the basis of thin-film technologies [1]. The solar cell cost significantly depends on the price of Si, since 70–75% of the solar cell cost is made up of the Si cost.

There have been numerous studies aimed at reducing solar energy prices further by various processes, such as research on improving and optimizing the solar cells production technology [3,4,5,6,7,8,9], controlling Si wafer thickness [10], and combining the different renewable energy systems in to a multigeneration system [11,12]. Research and development to obtain and purify Si, used as the raw material for the production of solar cells based on crystalline Si are also needed and therefore conducted in different ways. For example, one study provides a recycling process from waste crystalline-silicon PV Cell and Ribbon to obtain Si (as raw material) and Ag, Cu, Pb, Sn [13]. There is also a process of purification of MG-Si by using solvent refining published in the literature [14].

Metallurgical methods, or physical methods, as they are often called in the literature, completely eliminate the use of chlorosilanes. The required Si purity is achieved by using high-purity raw materials: quartz and carbon, followed by metallurgical and physical-chemical conversions. It is planned worldwide to increase the production of solar grade Si (SoG-Si) by metallurgical methods in a short period, and their development and extensive application are being given great attention. For example, Elkem (Kristiansand, Norway), Waker (Munich, Germany), PhotoSil, SolSilc (Netherlands, Norway), CaliSolar (Los Angeles, CA, USA), CPI (Santa Clara, CA, USA), and SEMCO ENGINEERING (Montpellier, France) have implemented pilot and/or industrial projects to produce SoG-Si using such methods. Researchers have presented the results of a mass-production test of Upgraded Metallurgical Grade Silicon (UMG-Si) and production of Solar Cells and Modules Made of obtained UMG-Si [15].

This article is devoted to analyzing one Si purification method without a gas phase. Namely, the slag refining method was used for purification. The Institute of Physics and Technology (Almaty, Kazakhstan) has conducted several studies on methods of metallurgical-grade silicon (MG-Si) production and purification of silicon to UMG-Si, as well as research on silane production methods [16,17,18,19,20,21,22]. As a result of previously conducted studies and works, purified silicon samples and Si crystals grown by the Czochralski method were obtained on a laboratory scale, approximately 2–3 kg. The results are summarized in [16].

Based on the results of the literature sources [23,24,25], as well as a number of our preliminary research concerning this topic [16,17,18,19,20,21,22], further development and improvement of technologies for obtaining and refining Si to solar quality is promising and of strategic importance for the Si industry of Kazakhstan.

2. Materials and Methods

Metallurgical grade Si (MG-Si), obtained by a well-known carbothermic reduction process, was used for purification. For this study, 100 kg of MG-Si of KR-0 grade was used (LLC RUSAL Silicon Urals). Si impurity composition is shown in

Table 1. Impurity composition of metallurgical Si and Si after slag refining.

Sample	Al	Fe	Ca	In	Ga	As	B	P
	ppm
MG-Si	1277	1551	1821	11	43	-	31	27
melting № 1	44	3890	708	5	111	-	12	58
melting № 2	148	980	3379	1	28	-	12	37
melting № 3	1295	2029	785	2	13	0.2	9	27
melting № 4	184	2509	3396	8	96	0.074	9.5	56
melting № 5	1802	2880	1018	14	84	0.384	11	43
melting № 6	202	2542	921	8	70	0.053	11	49
melting № 7	624	1868	1119	4	51	1.5	11	41
melting № 8	593	1862	900	1.5	46	2.5	9	34
melting № 9	30	2296	400	2.8	52	2.2	7	35

. MG-Si is mass-produced all over the world but due to insufficient purity it cannot be used directly as a raw material for solar cell production. MG-Si was chosen for the experimental cleaning work due to its greater availability. The entire mass of MG-Si was crushed to 15 mm size using Pulverisette 1 crusher. Upon crushing, Si was mixed and divided into 9 parts intended for 9 slag refining processes. We chose samples from each batch for elemental analysis to determine the initial MG-Si impurity. In order to obtain more reliable information, the elemental analysis of all samples was completed by two methods. The atomic emission spectrometry method with high-frequency inductively coupled plasma (ICP AES) with Optima 2000DV (Perkin Elmer, Waltham, MA, USA) spectrometer and X-ray fluorescent method (XRF) with RLP-21 (Aspap-Geo, Almaty, Kazakhstan) device [26] were used. MG-Si as well as purified Si was analyzed for the main doping impurities (boron, phosphorus, arsenic, gallium, and indium) and metal impurities that have a significant negative effect on the solar cell efficiency [27]. The impurity composition of both MG-Si and Si obtained after slag refining is shown in Table 1.

Based on previous studies [16,17,18], we calculated the new slag composition for 9 slag refining processes. MG-Si slag-refining method was worked out using the “Parallel” induction furnace during the experiments (Figure 1). Mixtures of CaO, SiO₂, CaF₂, MgO, BaF₂, BaO, LiF, and other oxides and fluorides were used as fluxes. One of the main effects of the slag refining is that slag can rise to the molten Si surface or come down to the bottom of the crucible while interacting with the impurities and further could be easily removed. Practice has shown that such elements as B, Al, P, S, Ga, Ge, Sr can also be effectively removed together with slag. In total, 9 slag-refining processes with different new slag composition were completed to produce purified MG-Si. Different compositions were used to determine more acceptable slag composition that would result to the best cleaning efficiency. Compositions of the 9 completed melting processes are shown in

Table 2. Slag Composition for 9 slag refining processes.

Melting Number		CaO	SiO2	CaF2	Na2CO3	Al2O3	MgO
Melting №1	%	40	50	10	-	-	-
	kg	2	2.5	0.5	-	-	-
Melting №2	%	60	30	10	-	-	-
	kg	3	1.5	0.5	-	-	-
Melting №3	%	-	60	-	40	-	-
	kg	-	3	-	2	-	-
Melting №4	%	10	50	10	30	-	-
	kg	0.5	2.5	0.5	1.5	-	-
Melting №5	%	70	30	-	-	-	-
	kg	3.5	1.5	-	-	-	-
Melting №6	%	40	40	-	-	20	-
	kg	2	2	-	-	1	-
Melting №7	%	40	40	-	-	-	20
	kg	2	2	-	-	-	1
Melting №8	%	40	40	-	-	10	10
	kg	2	2	-	-	0.5	0.5
Melting №9	%	30	40	10	-	10	10
	kg	1.5	2	0.5	-	0.5	0.5

. Samples obtained after each slag refining process were analyzed to determine the impurity quantities in the obtained purified MG-Si (Table 1).

The weight of melted MG-Si was 5 kg, and the Slag/Si ratio was 1:1 for each slag refining process. Photos of the obtained ingots are shown in Figure 2.

The final step was the acid leaching process. For this stage the purified MG-Si obtained after the three most successful slag refining processes was selected.

Various acids and parameters (temperature, duration etc.) were used for the acid purification process. The acid leaching method was well tested and described in our previous studies, and it can be found in detail in [16]. Impurities, usually as a result of crystallization and associated segregation effect, are located at the grain boundaries of Si as silicide and intermetallic compounds. However, impurities also may be included in Si grains themselves if crystallization occurs quickly enough. Therefore, thermal conditions are very important point during melting and crystallization processes. Various oxides and carbides, in turn, are located to a greater extent at the grain boundaries and to a lesser extent inside the grains. When the ingot is crushed, fractures occur at the grain boundaries. Thus, if MG-Si pieces are crushed to a particle size equivalent to the size of a polycrystalline grain, most metallic impurities will be located on the surface of the particles. Further, it is quite easy to remove impurities from the surface of the particles using the acid leaching process, in case of optimal process parameters. As known from previous studies [16,17,18] the best Si impurity removal levels by the acid leaching process were achieved for the samples crushed to 0.5–3 mm size. Therefore, the acid purification step was completed with Si crushed to the specified size, according to a proven method. It can be seen from the analysis results (

Table 3. Impurity composition of MG-Si and purified Si after slag refining and after acid leaching (a/l).

Sample	Al	Fe	Ca	In	Ga	As	B	P
	ppm
MG-Si	1277	1551	1821	11	43	-	31	27
Melting #3	1295	2029	785	2	13	0.2	9	27
Melting #3 after a/l	4.1	738	72.38	26	15	0.7	7.86	26.4
Melting #8	593	1862	900	1.5	46	2.5	9	34
Melting #8 after a/l	220	570	173	17	13	0.9	11	35
Melting #9	30	2296	400	2,8	52	2.2	7	35
Melting #9 after a/l	9	977	92	21	20	0.6	6.4	30

) that acid purification is especially effective for impurities of transition and alkaline earth metals. The slight increase in boron and phosphorus concentrations after the acid leaching process of the melting #8 sample is most likely due to the standard measurement deviation and is within the expanded measurement uncertainty for ICP-AES and XRF analysis.

As we can see from the Table 3 results and based on our experience, we can say that obtained silicon could be used in further research. Si obtained post-acid purification will be additionally purified using a vacuum purification process. Such a process will provide additional purification of Si from impurities to the level sufficient for further directional crystallization in a vacuum furnace by the Bridgeman-Stockbarger method or the Czochralski method. It is understood that impurity concentrations decrease in Si crystal during crystallization process due to the segregation effect. Values of the segregation coefficient are defined as the ratio of the equilibrium concentration of impurities in solid and liquid states of the substance. After Si crystallization various impurities are distributed differently along the obtained ingot’s height, a segregation effect. This effect is used to purify Si when the solid–liquid interface is flat; the melt has a homogeneous composition and minor diffusion occurs in the solid composition [28,29]. The obtained Si crystal will be analyzed and its main electrical data (resistance, carrier lifetime, etc.) will be studied. Moreover, wafers will be cut from obtained Si crystals, the test solar cells will be produced, and their specifications will be studied.

3. Results and Discussion

The negative effect of various elements upon the solar cell specifications have been fairly well studied since the 1980s [27]. As can be seen from the analysis results (Table 1), the initial MG-Si contains a significant number of impurities and cannot be used as a raw material for SoG- Si production. It therefore requires purification to UMG-Si quality.

Calculations of the slag composition and the process flow for the slag refining processes were made to allow implement the following tasks:

• Obtaining a homogeneous Si phase;
• Identifying the melting parameters, the amount of slag, slag/Si ratio;
• Finding the amount and optimum excess of SiO₂ for the formation of a homogeneous Si phase (basicity modulus);
• Finding the sequence of adding the compositions into the melt (technical melting procedure), for complete melting of the charge components and Si, and to obtain the maximum yield of the final product.

On average, the yield of purified Si was 4 kg, that is 80% of Si mass loaded for purification (the yield of purified Si varied from 70% up to 99%). According to the results of nine completed processes, it can be seen that the most successful slag refining processes are meltings #3, #8 and #9 (Table 3). It should be noted that the main purpose of purification was to remove boron impurity, which has a segregation coefficient of 0.8–0.82 [28]. Because of its high segregation coefficient, boron is removed significantly less than other impurities [29] during Si crystal growing process. It means that about 80% of boron transfers from the melt to the growing Si crystal [28]. The remaining transition metal impurities have segregation coefficients in the order of magnitude 10⁻⁵–10⁻⁶ and therefore could be sufficiently well removed during the Si crystallization process. The sufficiently high phosphorus concentration can be compensated for by adding the gallium impurity without causing additional negative effects upon specifications of both Si and solar cells [19].

4. Conclusions

Nine new processes of MG-Si slag purification were completed using the induction furnace to obtain optimal process parameters. Optimal slag composition for MG-Si refining process was determined.

It is shown that stable results can be obtained for production of UMG-Si ingots, with sufficient Si yield up to 99% by weight. In addition, the flux composition used for purification allows reduces the concentration of boron 3–5 times. Elemental analyses performed by two methods, XRF and AES-ICP, allowed control over the purification process at each stage and obtained reliable results.

Acid purification is very effective for transition metal impurities and less effective for doping impurities. According to our calculation and modelling of resistivity variation along the ingot’s height, fully described in [19], the purity of the Si obtained after the final stage will be enough for further experimental work and can be used as a raw material for Si crystal growing by directional crystallization to obtain SoG-Si. In the next step, it is planned to run more detailed study of Si obtained after purification by directional crystallization method, namely, the production of test solar cells and a study of their specifications.