Hydrometallurgical refining of metallurgical silicon

Введение

Technical (metallurgical) silicon finds widespread application globally in various fields [1]. It is utilized as an alloying element in ferrosilicon and high-silicon alloys [2 – 4], as an alloy in the aluminum industry to produce silumins [5], as a steel deoxidizer in the steel industry [6], and in the chemical industry to obtain organosilanes and other compounds [1; 4]. In the electronics industry, high-purity semiconductor silicon and silicon of “solar” quality serve as the basis for photoelectric current converters [4; 6].

Metallurgical silicon produced by melting in ore thermal furnaces [5; 7 – 9] typically has a purity of only 98.0 – 99.5 %. At present, “solar” quality silicon is manufactured by combining the costly “electronic” silicon and metallurgical silicon, followed by refining through crystallization methods. The Siemens process [10] is the traditional industrial method of producing “electronic” silicon, along with other similar methods that involve vapor phase chemical precipitation for the production of chlorosilane compounds. However, these methods have drawbacks such as high volatility, toxicity, and equipment corrosion in the presence of water. The Siemens process is also highly energy-intensive, consuming approximately 120 kWh/kg of silicon [11]. Alternatively, “solar” quality silicon can be obtained by processing metallurgical silicon, which can involve oxidative refining [5; 12; 13], hydrometallurgical purification [5; 14; 15], vacuum refining [16 – 18], and crystallization methods of purification (direct crystallization, zone melting) [19 – 22]. Of these methods, hydrometallurgical refining is the only process that does not require high temperatures (below 100 °C) or expensive equipment. This method is energy-efficient and cost-effective.

The objective of this study is to conduct experiments for hydrometallurgical purification of metallurgical silicon using various inorganic acids.

Subject of research

The aim of this study was to investigate metallurgical silicon samples obtained from Silicon JSC, RUSAL (Shelekhov, Irkutsk Region) after undergoing oxidative refining.

Technical silicon is typically produced through a continuous method in ore-thermal furnaces (OTF) using silica-containing raw materials with a minimum of 98.5 % SiO2 . Fossil quartzite is a mineral component commonly used in charge mixtures for industrial processes. The mixture typically includes a combination of carbon reducing agents such as charcoal, petroleum coke, and hard coal from various producers such as Kazakhstan and Colombia. Additionally, wood chips are often used as a charge loosener [5; 7].

The process of silicon smelting can be generally described by the reaction SiO₂ + 2C = Si + 2CO. However, this oversimplified reaction does not adequately capture the complexity of the reduction of silica that occurs in a furnace during silicon production. In an OTF, silicon production is a highly intricate, high-temperature process that involves various chemical reactions resulting in the formation of intermediate compounds such as SiO and SiC.

The raw materials used for silicon production, such as Cheremshansk Mine quartzite, exhibit heterogeneity in terms of impurity content. As a result, the resulting smelted silicon contains small amounts of iron, calcium, aluminum, and titanium, which give rise to the formation of various intermetallic inclusions within the silicon [23; 24].

At Silicon JSC, the process of silicon production involves the smelting of the charge in an OTF, followed by an oxidative refining step in which air blowing is used to remove mainly aluminum and calcium from the silicon melt [7]. However, this method does not remove iron from the silicon, which highlights the importance of strict control over the supply of iron from the charge materials (quartzite, carbonaceous reducing agents). Alternatively, other methods may be suggested to improve the quality of silicon.

Different methods were employed to study the chemical composition of technical silicon samples used in experimental works.

Different methods were employed to study the chemical composition of technical silicon samples used in experimental works. Metallographic studies of thin sections of the initial lump silicon revealed the presence of mainly impurity intermetallic inclusions (Fig. 1). This study was conducted using an Olympus GX-51 metallographic microscope (Olympus, Japan) equipped with an Altera20 digital camera. Additionally, X-ray spectral microanalysis (Fig. 2) was performed using an S4 Pioneer X-ray spectrometer (Bruker, Germany). The results indicated that iron and aluminum were the main impurity components of intermetallic inclusions present in the silicon sample, albeit in insignificant amounts.

The initial lump material was crushed using a by a ShchD-10 jaw crusher (Russia), and subsequently ground using a ShM-1408 ball mill (Russia). The grinding process employed steel balls without lining, and magnetic separation was then applied. The particle-size distribution of the resulting fine silicon was analyzed using a laser particle size analyzer, Analyzette 22 NanoTecPlus (FRITSCH, Germany). The iron impurity is present in small inclusions (Fig. 2), resulting in a uniform distribution by fractions. The results of the analysis are presented in Fig. 3 and in the Table.

Based on the particle-size analysis, it was found that the metallurgical silicon particles had a fraction size of –200 µm; with 80 % of all particles falling within the fraction size range of +12 ÷ 100 µm. Elemental chemical composition (wt. %) of the material was determined through X-ray fluorescent analysis (XRF), which resulted in the following values (wt. %): Al 0.53; Ti 0.0491; Ca 0.0628; V 0.0066; Cr 0.0024; Mn 0.0145; Fe 0.6094; Cu 0.0037; P 0.0106; Ba 0.0077; Ni 0.0071; Zn 0.0022 and Si 98.6939 (taking into account 12 impurities). The XRF analysis was conducted using an S4 Pioneer X-ray spectrometer.

The XRF spectra of the technical silicon samples after oxidative refining (Fig. 4) indicated the presence of various impurity elements such as iron, titanium, aluminum, and calcium. Iron was the most prevalent impurity, and is notoriously difficult to remove through flux-oxygen refining of silicon, primarily due to its low affinity to oxygen. As a result, iron remains almost entirely within the silicon melt and does not transfer to slag.

Experimental work on hydrometallurgical purification of silicon

The quality of technical silicon was attempted to be improved by using solutions of sulfuric, nitric, hydrochloric, and hydrofluoric acids, as well as their mixtures in different proportions as reagents for silicon processing. To assess the potential of these solvents in hydrometallurgical refining, changes in Gibbs energy (Δ\(G^{\circ }_{298}\)) were calculated as an indicator of the thermodynamic likelihood of chemical interactions between impurity inclusions (intermetallics) and various solvents.

Iron impurities in silicon can exist in various forms such as double silicides including (FeSi₂, Fe₂Si, FeSi) and in complex intermetallic compounds containing titanium and/or aluminum including (AlFeSi, AlFeSi₂, FeTiSi, FeTiSi₂, FeAl₃Si₂, FeAlTiSi) [23 – 25]. The interaction of FeSiTi intermetallic with a sulfuric acid solution can be described by the following reaction

2FeSiTi + 7H₂SO₄ + 6H₂O = Fe₂(SO₄)₃ + 2Ti(SO₄)₂ + 2H₂SiO₃ + 11H₂ 

the Δ\(G^{\circ }_{298}\) value is –2412.34 kJ/mol, indicating a spontaneous process.

A software program was developed in Microsoft Excel for the rapid calculation of the Δ\(G^{\circ }_{298}\) values of chemical reactions [26]. The program was used to determine the Δ\(G^{\circ }_{298}\) values for the interactions of various compounds, including FeSi₂, Fe₂Si, FeSi, AlFeSi, AlFeSi₂, Al₃FeSi₂, FeSi₂Ti, FeAlTiSi, TiSi₂ and Ca₂Si with solutions of different acids. The calculated Δ\(G^{\circ }_{298}\) values were found to be negative [27].

In order to conduct the experiments, silicon samples with a particle size of –200 µm were obtained. Leaching of a 40 g portion of silicon was performed in a 400 mL thermostable beaker for duration of 1 h in a sand bath using a PE6110 magnetic stirrer (100 rpm) with automatic heating. During the experiment, the solution temperature spontaneously reached 60 °C. Acid solutions of varying concentrations, expressed as weight percentages, were employed, including (wt. %): H₂SO₄ 10; HCl 10; HNO₃ 10; HF 4. These specific values of solvent concentrations were chosen based on the prior research experiences of other scholars [5; 28 – 30]. The liquid to solid ratio was maintained at 5:1 and the required volume of the reagent was calculated taking into account the density of the acid of the set concentration [31].

Results and discussion

Following acid refining of silicon, the resulting solutions were analyzed for impurity content (Fig. 5) using atomic-emission analysis (AEA) through a PDA-8000 spectrometer (Shimadzu, Japan).

The best transfer of impurity elements to the solution was achieved when hydrofluoric acid was used as the solvent, resulting in concentrations of 2380 mg/dm³ of iron, 831 mg/dm³ of aluminum, and 145 mg/dm³ of titanium. The highest concentration of calcium transfer to the leaching solution was obtained using hydrochloric acid (with a concentration of 147 mg/dm³).

These results demonstrate the potential use of the above acids for the deeper purification of metallurgical silicon obtained after oxidative refining at Silicon JSC. Experiments were conducted to leach impurities in the solution using acid mixtures. Ratios of sulfuric, hydrochloric, nitric, and hydrofluoric acids were selected as follows: 1:1, 3:1, and 1:1. The experimental conditions, including temperature, liquid-to-solid (L:S) ratio, stirring speed, and duration, remained unchanged. Fifteen experiments were conducted to leach impurities using various combinations of acid mixtures. The most effective results of impurity transfer to the solution of hydrometallurgical treatment of fine silicon are presented in Figure 6 (AEA data).

The mass concentration of titanium in the leaching solution was 2.77, 2.39 and 1.54 mg/dm³, respectively, when using three acids in different ratios (H₂SO₄:HCl = 1:3, HNO₃:HCl = 3:1, HNO₃:H₂SO₄ = 1:3).

AEA results of the solutions after silicon hydrometallurgical refining showed that a mixture of sulfuric and hydrofluoric acids in a ratio of 1:1 was the most effective in transferring impurities to the solution. When this acid mixture was used as a solvent, the mass concentration of iron, aluminum, calcium, and titanium in the solution was the highest. The use of a mixture of sulfuric and hydrochloric acids as a solvent in a 1:3 ratio resulted in relatively high mass concentrations of impurity elements in the leaching solution (compared to the use of acids separately, except HF).

Conclusion

In this study, acid treatment was carried out on fine silicon samples to purify metallurgical silicon. Reagents such as 10 % solutions of hydrochloric, sulfuric, and nitric acids, as well as 4 % hydrofluoric acid, were used. These solvents were thermodynamically capable of interacting with impurity metal-containing compounds in silicon.

The use of hydrofluoric acid as a solvent resulted in the highest transfer of iron, aluminum, and titanium to the solution. Hydrochloric acid was found to be the most effective for calcium transfer. Furthermore, a mixture of H₂SO₄ and HCl at a ratio of 1:3 produced rather high mass concentrations of impurity elements in the leaching solution. The degree of iron removal was 33.32 %, aluminum – 54.64 %, calcium – 65.77 %, and titanium – 15.64 %. When a mixture of acids was used, a 1:1 ratio of sulfuric and hydrofluoric acids was found to be the most effective for maximum transfer of impurities to the solution, with the highest mass concentration of iron, aluminum, calcium, and titanium. The degree of removal for iron was 88.37 %, aluminum – 81.85 %, calcium – 94.62 % and titanium – 92.22 %. From an industrial standpoint, it is more economically and ecologically feasible to choose a mixture of 10 % sulfuric and hydrochloric acids at a 1:3 ratio for silicon purification.