Determination of silicon content in metallurgical-grade silicon re�ned slag using two-step dissolution chemical analysis method

: Under the demand of global green and low - carbon development, it is 12 urgent to recycle silicon from metallurgical - grade silicon refined slag (MGSRS). 13 Content of elemental silicon (Si) in MGSRS is the key of silicon recovery, and 14 directly affects the selection and economic value of the silicon recovery technology. 15 In this study, a two - step dissolution chemical analysis method is proposed to separate 16 and determine the silicon and other silicon phases content of MGSRS. First, the 17 silicate in the MGSRS was dissolved by hydrochloric acid to obtain the silicate 18 content, and mixed particles containing silicon and silicon carbide (SiC) were 19 separated. Second, silicon was dissolved by a mixture of nitric and hydrofluoric acid 20 (HF/HNO 3 ), and the SiC particles were separated, then the Si and SiC content were 21 determined respectively. During the two steps, the dissolution effect and repeatability 22 of silicate and silicon with different acid was studied. Finally, The silicon content in 23 MGSRS measured by the two - step chemical method was 21.84 ± 0.53%, under the 24 condition of volume ratio of HNO 3 :HF is 1:1 and reaction time is 1h. This method 25 breaks the situation that the content of elemental silicon in MGSRS is estimated only 26 by experience. It has a positive impact on the further development of silicon recovery 27 from MGSRS and can reduce environmental pollution


Introduction
As raw material of silicon alloys, organosilicon, and solar-grade silicon, metallurgical-grade silicon (MG-Si) is widely used in photovoltaic field, electronics, renewable energy, and other industries [1][2][3].To improve the purity of MG-Si, a large amount of MG-Si refined slag (MGSRS) is produced in the MG-Si secondary ladle refining process [4].During this step, oxides form a slag layer floating on the surface of the MG-Si melt that is then separated from the silicon melt and removed [5].On the order of 3 million tons of MG-Si are produced annually worldwide [6], generating approximately 500,000 tons of MGSRS, which contains over 60,000 tons MG-Si.Due to the lack of proper recovery technology, the waste of silicon resources is serious.
MG-Si smelting is a high energy-consuming industry, which consumes about 28.8 billion kW of electricity in a year.The recovery of silicon from MGSRS can save energy and reduce consumption, which is helpful for the green and sustainable development of silicon enterprises.Therefore, it is urgently necessary to develop methods for the recovering and reusing of MGSRS [7].
In recent years, silicon recycling from MGSRS [8] and other silicon waste [9] has gradually attracted increasing attention of industry and academic researchers.In the investigations of MGSRS, complex constituents, mainly silicate, Si, and SiC have been identified [10].Unfortunately, the content of Si in the MGSRS is not known precisely, which is the key factor of recycling technology, and it also directly affects the selection and economic value of silicon recovery technology.To date, analysis of the silicon content in the MGSRS has relied mainly on methods such as X-ray fluorescence (XRF) and inductively coupled plasma atomic emission spectroscopy (ICP-AES).Wang et al. [11] used X-ray diffraction (XRD) to identify the slag phases and study mass transfer during the MG-Si purification process.With the ongoing improvement of the analytical methods and software technology, it is possible to perform a quantitative analysis of the slag using XRD with the Rietveld method [12].
However, the presence of the amorphous phase in MGSRS may affect the accuracy of the XRD analysis [9].The chemical composition of MGSRS was investigated by XRF and ICP-AES, and determined the contents of every element in the slag [13].However, XRF and ICP-AES can only measure the total content of a given element such as silicon, and cannot determine the contributions of silicon and silicon compounds to the slag.Moreover, ICP-AES is widely used to measure the content of a microelement or trace metal [14][15], whereas the total silicon content in MGSRS is known to be approximately 30%.In addition, the potassium fluosilicate volumetric method [16] can be used to detect a high content of silicon and was used by to determine the silicon content in the steel and slag rapidly and accurately.However, this method must be carried out by an experienced operator, and only the total amount of silicon dioxide or silicon element can be obtained.Comprehensive, existing analytical methods are based on the content of impurity elements or surface analysis, and only the total silicon or silicon dioxide content can be measured.
Analysis method of the silicon in the MGSRS with containing complex phases has not been carried out to date.This directly affects the selection of the technological route for the silicon recovery from MGSRS.In this work, a two-step dissolution chemical method for silicon content in MGSRS was proposed and carried out.Based on the difference of acid solubility of the phases in the MGSRS, dissolution and separation of the phases from the MGSRS can be achieved gradually.The content of silicon and other silicon phases in the MGSRS can be determined.This method can also be used to analyze the content of silicon phase in recovery technologies of other slags containing silicon.The determination of silicon content will be beneficial to the development of recycling technology of silicon from MGSRS, promoting the green and sustainable development of silicon industry.

Raw materials and characterization
MG-Si refined slag (MGSRS) used in this research is from Yunnan Yongchang Silicon Industry Co., Ltd., which is a metallurgical grade silicon manufacturer.
Deionized water was used in all experiments, and HCl, HNO3, and HF were guaranteed reagent used in dissolution steps.Hydrochloric acid(HCl), nitric acid(HNO3) and hydrofluoric acid(HF) using in the analysis method test are superior pure.Phases of the sample were identified by X-ray diffraction analysis (XRD, D8Advance, Bruker, Germany) with Cu Kα, and a 2θ range between 10 and 90 degree was scanned, scanning speed is 8 degrees per minute.The chemical composition of the MGSRS was quantitatively analyzed by the XRD semi-quantitative analysis method, X-ray fluorescence (XRF, AxiosmAX WDXPS, Holland), and chemical methods.Oxygen content of MGSRS was detected by ONH content analyzer 105(ONH836), and carbon content was detected by infrared carbon sulfur analyzer (HCS140, China).

Experimental Procedure
MGSRS were first crushed and ground into a powder with a particle size of 187µm-48µm.Two-step dissolution experiments were conducted using a DF-101S apparatus.The experimental procedure was as follows: (1) Dissolution of the silicates with hydrochloric acid: weigh the MGSRS (10 g) and pour into water (100 ml).Then, 1-3 ml/ (g slag) hydrochloric acid (GR) was gradually added into the slurry with stirring at for 2 h.After filtration and vacuum drying at 60 ℃, the mass of the solid particles containing only silicon and silicon carbide that are obtained after HCl acid leaching was determined by weighing.The weight loss after acid leaching was the mass of the silicate in the MGSRS.According to the loss weight, the content of silicate can be calculated.(2) Dissolution of silicon with HNO3/HF mixed acid after hydrochloric acid leaching: Si and SiC mixed particles were added to a 250 ml polytetrafluoroethylene beaker.HNO3 acid and HF acid were added into the beaker sequentially at room temperature.The volume ratio of HNO3:HF was 0.5:1-3:1, and the stirring time was 0.5-3h.Filtration and vacuum drying were also carried out at 60 ℃, and the mass of SiC particles was obtained.The weight loss after the silicon dissolution process was the mass of the silicon in the MGSRS.Finally the mass content of the silicon and silicon carbon in MGSRS can be calculated.(3) Accuracy analysis of this method: Repeat the silicate dissolution and silicon dissolution experiments under the optimal conditions.And then calculate the standard deviation (SD) and relative standard deviation (RSD) of the analysis method.The experimental procedure is illustrated in Fig. 1.After the acid leaching-dissolving Si reaction, the phases in the samples were identified by XRD to explore the extent of the reactions.The contents of silicon, silicon carbide, and silicate can be calculated according to the following equations ( 1) -(4): .% 100% X wt m m Where mraw is the mass of MGSRS [g], m1 is the mass of the solid particles (silicon and silicon carbide) [g], m2 is the mass of silicate [g], m3 is the mass of silicon carbide [g], and m4 is the mass of silicon [g].X is silicate, Y is SiC, Z is silicon.

Calculation of SD and RSD
Standard deviation (SD) and relative standard deviation (RSD) are commonly used in analyzing measurement, which are necessary for an accurate data evaluation.SD measures the degree to which individual numbers tend to spread about their average or value.RSD is generally used to evaluate the precision and repeatability of the method in the verification of analytical methods.The smaller the RSD value, the higher the precision and the better the repeatability.The conventional calculation of SD and RSD is represented by the equations ( 5) and ( 6) [17][18] Where Σ is the summation operator, and its index i indicates the measurement number from 1 to n.Where i x is the analyze data for each sample, x is the average of analyze data for all samples.
3 Results and discussion

XRD semi-quantitative analysis
The Semi-quantitative analysis results of phase identification and semi-quantitative analysis of MGSRS by XRD are presented in Fig. 2. Silicate, Si and SiC were detected in MGSRS.Silicate contains calcium silicate (CaSiO3), and the Ca2Al2SiO7 complex oxide.Semi-quantitative analysis of MGSRS was performed and showed that the contents of metal Si, SiC and silicate were 16.4%, 7.3%, and 76.3%.

XRF analysis method
XRF was usually used to analysis the content of substance.To further increase the accuracy of the XRF results, carbon and oxygen elements were analyzed using a carbon and sulfur analyzer and an oxygen and nitrogen analyzer, with the results of the measurements shown in Table 1: Main elements and content of MGSRS.
An examination of the results presented in Table 1 shows that the total content of the silicon element in MGSRS is 33.1%.These results indicate that XRF analysis can only obtain the total content of the silicon element, but not the content of the silicon.
The measured total content of the silicon element is near the value of 33.28% calculated by the XRD semi-quantitative method.

Chemical analysis methods
The elements mainly contributing to the content in the MGSRS are listed in Table 2.Chemical methods were used to analyze the composition and content of the MGSRS.The total Si element content was measured using the potassium fluosilicate volumetric method.The EDTA volumetric method was used to analyze the Ca content and the titrimetric method was used to analyze the Al content.The total content of the Si element in the MGSRS was approximately 26.93%, but the content of the silicon could not be obtained.In summary, the above methods cannot accurately obtain the content of silicon in MGSRS.It is necessary to find an accurate analysis method to determine the content of silicon and other silicon phases in the MGSRS.

Thermodynamic analysis of MGSRS acid dissolution separation
Acid leaching has been widely used in the purification of MG-Si [12,19] and diamond wire saw silicon powder [20][21].In  [22].The effect of hydrochloric acid on the gelation of sodium silicate was also studied [23].The results of this study indicated that the separation of silicate from the other silicon phases in MGSRS can be realized by hydrochloric acid leaching.Liu et al. [24] recovered silicon powder from silicon-containing materials using nitric acid and hydrofluoric acid for the treatment of the sample powder.The contents of silicon and silicon carbide can be calculated because the filtered solid powder was the SiC powder after reaction.At last separation of silicon and silicon carbide can be realized by leaching with a nitric and hydrofluoric acid mixture.
In the present work, MSGRS was treated by silicate dissolution with hydrochloric acid and silicon dissolution with HF/HNO3 mixed acid.The separation of silicon, silicon carbide and silicate in MGSRS was realized.The content of silicon and other phases in MGSRS was calculated by the differential weight method.Finally, the content of silicon and other phases in the MGSRS was obtained.
To predict the reactions that occur during the acid leaching process, it is   For the first step, the reaction conditions for the silicate dissolution of MGSRS are illustrated in Figure 3.At 273-773 K, the Gibbs free energy of the reaction of Ca2Al2SiO7 or CaSiO3 with hydrochloric acid is less than zero.The results indicated that the reactions ( 7) and ( 8) can occur at room temperature (298 K).After the reaction, only silicon and silicon carbide particles still remain in the solid form.In the second step, silicon dissolution by the HF/HNO3 mixtures follows a chemical process with two basic reaction stages.In the first stage, silicon is formally oxidized by HNO3 and formed SiO2 (Eq.9).Then, SiO2 was dissolved by HF (Eq.10).Finally, the overall reaction is expressed by Eq. ( 11) [24].During this second step, silicon particles were dissolved, accompanied by NO gas, but silicon carbide particles did not react with mixed acid due to its high stability.After the reaction was completed, silicon carbide particles still remained.These results show that the acid leaching reactions can occur at room temperature.The thermodynamic analysis clearly indicates that the phase separation of silicon, silicon carbide and silicate in MGSRS can be achieved.

Silicate Dissolution of MGSRS
The particle size of the MGSRS has a significant influence on the effectiveness of the hydrochloric acid in the silicate dissolution process.The results of the examination of the particle size effect are shown in Fig. 4. than -74µm, the amount of silicate removal decreased relative to that for slag particle size of -74µm, and SiO2 exists in the remaining particles.However, it was observed that in the case of the slag particle size of -48µm, silicate removal is still reduced relative to that at -74µm, and there is little SiO2 in the remaining particles.Based on above results, it can be concluded that the optimal silicate removal can be obtained for the particle size of the MGSRS of -74µm.
The amount of hydrochloric acid also plays an important role in silicate removal.
The results obtained using different amounts of hydrochloric acid for silicate removal are shown in Fig. 5: were separated.Fig. 5 also shows the poor effectiveness of the silicates' removal with 10-18 ml of HCl.For these amounts of HCl, the obtained solid particles still contain silicate, indicating that silicate removal is insufficient.Silicic acid is contained in the reaction product of silicate and hydrochloric acid.The leaching was performed at a quite stable pH varying between 1.0 and 1.4 and silicic acid is dissolved [29], but fresh solutions are suitable for this method as long as the pulp density is below 12%.
Otherwise, silicic acid can auto-condense to eventually precipitate as silica [30], which cannot achieve the removal of silicate.For the HCl amounts of more than 20 ml, the excess silicic acid precipitates in a gelatinous state, affecting silicate separation.
Consequently, the best silicate removal is achieved using 20 ml of HCl.
The above results demonstrate that the silicate in the MGSRS can be removed by acid leaching with HCl, which is beneficial for the separation of Si and SiC in the second step.The optimum reaction conditions were the MGSRS particle size of 200 mesh and 20 ml of hydrochloric acid.
In order to further prove the reaction of silicate with hydrochloric acid, in-situ corrosion test was carried out on a bulk sample of MGSRS.The results are shown in Fig. 6: showed the reaction of silicate with hydrochloric acid was obvious, hydrochloric acid eroded from cracks and the silicon/silicate interface.The surface of the corroded MGSRS had obvious gullies and pits.The corrosion was intense at the interface, where the Si and SiC were surrounded by distinct grooves.The surface of silicate also has obvious white honeycomb stripes.The reason may be that hydrochloric acid acted and eroded on the surface of silicate because of the short acting time and no deep trench was formed.At the same time the shape of silicon was not changed and the surface was not corroded.It was shown that silicon in MGSRS did not react with hydrochloric acid.While because silicate around silicon particles was corroded seriously, the silicon was lifted up like an island and stands on the flat ground.
According to the optimal conditions of silicate dissolution obtained above, repeat the experiments, and the obtained samples were prepared to the next-step silicon dissolution.The results of repeated experiments are shown in Table 3: It can be seen from the results in Table 3 that the maximum difference of silicate content measured by method of silicate dissolution used hydrochloric acid is 1.42%.
This result indicates that the deviation of silicate content from average value is not more than 1.42%.The average content of silicate was 69.50%, the standard deviation (SD) was 0.60%, the relative standard deviation (RSD) was 0.86%, the SD and RSD values were less than 1%.From the above results, it can be seen that this method has high precision and good repeatability, and the accuracy of the analytical method is guaranteed.The content of silicate in the sample is 69.50 ± 0.60% by the method of silicate dissolution in hydrochloric acid under this condition.The method has high repeatability and precision.The obtained sample will be used for the second steps of silicon dissolution.

Silicon dissolution of MGSRS
In this step, to understand the effect of the mixed acid solution on the dissolution of silicon, the volume ratio of nitric acid to hydrofluoric acid is an important factor.
The effect of different volume ratios of nitric acid to hydrofluoric acid on silicon dissolution is shown in Fig. 7.It is observed from Fig. 7(a) that when the HNO3: HF ratio of 0.5:1, the amount of SiC remaining in the sample after silicon dissolution process is 9.69%.This value is higher than that under other ratio conditions (all is above 8%).Moreover, Fig. 7 (b) show that SiO2 characteristic peak is observed for the solid particles remaining after a dissolution reaction carried out at the HNO3:HF ratio of 0.5:1.The reaction was showed as following: First, Si was oxidized to SiO2 due to the addition of HNO3.
Subsequently, SiO2 reacted with the added HF.Finally, the silicon in the slag was dissolved and removed.With a short reaction time, Si was oxidized to SiO2 without being completely removed.The results implied that silicon cannot be completely dissolved under this condition.While when the HNO3:HF ratio of 1:1, only the characteristic peak of SiC was found.It can be concluded that HNO3/HF mixed acid is effective for silicon dissolution, and silicon can be completely dissolved under the HNO3:HF ratio of 1:1.
As shown in Fig. 8, reaction time is another important factor for silicon dissolution.The above results demonstrated that silicon can be dissolved and the separation of Si and SiC can be realized using HF/HNO3 acid mixture leaching.The optimal reaction conditions of this process are the HNO3/HF volume ratio of 1:1 and the reaction time of 1 h.
According to the optimal conditions of silicon dissolution obtained above, the experiments were repeated.The results of repeated experiments are shown in Table 4: From the results of Table 4, it can be seen that the maximum difference of silicon and silicon carbide content is 1.14% measured by silicon dissolution method.This indicates that the deviation of Si and SiC from average value is not more than 1.14%.
The average content of silicon was 21.84%, the standard deviation (SD) was 0.53%, and the relative standard deviation (RSD) was 2.45%.The results show that the precision and accuracy of the method are high and the content of silicon in the sample is 21.84±0.53%.The average content of SiC was 8.66%, the standard deviation (SD) was 0.53%, and the relative standard deviation (RSD) was 6.17%, the value of RSD less than 10%.That means this method has high precision and the content of SiC in the sample is 8.66±0.53%.
Compare with existing analytical testing methods, this method can overcome the problem that the content of silicon cannot be determined under the existence of many kinds of silicon.The method can be used to quantify the content of silicon in MGSRS, with high repeatability and reliable analysis results.This provides a guarantee for the research of MGSRS recycling technology.Guo et al. [7] indicated that separating and purifying silicon from MG-Si refined slag can save the production cost of MG-Si, and reduce environmental problem.The recycling of silicon has a positive impact on the sustainable development of PV industry and can bring huge economic value [31].This method can be used to exact determine the amount of silicon loss and economic value of enterprise loss.This is helpful to choose a more economical, effective and feasible recovery process for silicon recovery.
In this study, MGSRS was a batch of materials randomly selected from MG-Si producer.Although the silicon content of MGSRS obtained by each producer is different, this process is still applicable.According to the mechanism of this process, this process can be used to analyze the silicon content in different batches of MGSRS of different producers.In addition, this method can also be used to analyze the content of silicon of the recycled silicon products, which is not involved in this study.Further research can verify the extensive applicability and accuracy of this method.

Conclusions
(1) MGSRS mainly contains silicate, silicon and silicon carbide.The content of silicon and other silicon phases in MGSRS cannot be measured by traditional methods.
The technological route of silicon content in MGSRS was proposed and determined for the first time.
(2) The silicon and other silicon phases in MGSRS were separated and the contents were determined by two-step dissolution method.During the first step, silicates were completely dissolved by hydrochloric acid.The second step, silicon particles were completely dissolved by HF/HNO3 mixed acid, and silicon carbide was separated.
(3) The contents of silicon in MGSRS examined using two-step method were obtained as 21.84%, the SD is 0.53%.The accuracy of the analysis method is high.This method has significant practical implications to promote the research of silicon recycling technology and the sustainable development of MG-Si smelting industry.

Fig. 4 .
Fig. 4. Effect of the removal of silicate in MGSRS of different sizes Fig.4(a) shows that with the increase in the particle size of the MGSRS, the amount of the silicate removed gradually increased.Moreover, when the particle size of the MGSRS is -74µm, the largest mass fraction of the removed silicate was obtained.The XRD patterns in Fig.4(b) shows that the filtrated indissoluble particles can be attributed to the Si and SiC powder.The best silicate removal effect is obtained when the particle size of MGSRS is -74µm.Fig.4 also shows that for the MGSRS less

Fig. 5 .
Fig. 5. Results for the removal of silicate in MGSRS with different amounts of added hydrochloric acid.Fig.5(a) shows that with increasing amount of hydrochloric acid, the amount of removed silicates first increases and then decreases, with the highest silicate removal obtained for 20 ml of HCl.In this case, Si and SiC powder (as shown in Fig. 5(b))

Fig. 6 .
Fig. 6.EPMA analysis of in-situ corrosion of MGSRS by hydrochloric acid: (a) MGSRS before corrosion; (b)-(d): MGSRS after corrosion From Fig. 6(a), it can be seen that the light gray part of the original MGSRS was silicate, the spherical gray part with obvious boundary was Si, the dark black part was SiC, and the boundaries of phases distribution in the MGSRS was clear.Fig. 6(b)-(d)

Fig. 8 .Fig. 9
Fig. 8. Silicon dissolution results for the reaction time of MGSRS with mixed acid According to the silicon dissolution results show in Fig. 8(a), silicon was almost completely dissolved for the reaction time of 0.5h.With the increasing of reaction time, the dissolving effect of silicon was approach.The obtained silicon carbide accounts after silicon dissolution was between 7%-8% of the MGSRS, and the content of silicon is 22%-23%.In Fig. 8(b) the results revealed that only the SiC peaks in the

Table 2
Main elements comprising the MGSRS.

Table 3
Accuracy analysis of silicate dissolution method

Table 4
Accuracy analysis of silicon dissolution method