Composition and structural identification of silicon particles in diamond wire saw silicon powder

Abstract In this study, the content and calculation equations of each substance in diamond wire saw silicon powder (DWSSP) were determined. Initially, the presence of amorphous SiO2 was proven by crystal transformation. Then, the SiO2 content was determined by SiO2-CaO phase diagram analysis, and the SiOx content was determined by using the oxygen conservation rule. The variation trend between the SiO2, SiOx, and Si contents and the O content was determined, according to the calculated SiO2 and SiOx content. In addition, linear fitting was performed on the SiO2, SiOx, and Si contents of five DWSSP raw materials, and a calculation equation for SiO2, SiOx, and Si was obtained through the total O content. The three-layer structure of DWSSP was revealed, the DWSSP consisting of a Si core, an intermediate SiOx layer, and a SiO2 shell. This study can accurately identify the content of SiO2, SiOx, and Si in DWSSP based on the detected oxygen content, and provide a better theoretical basis for the process selection and design for silicon recovery or high value materials preparation. Graphical Abstract


Introduction
Diamond wire saw silicon powder (DWSSP) waste is produced during the diamond wire sawing process in the preparation of crystalline silicon wafers. [1,2][5] Large amounts of DWSSP will likely be produced in the future. [6]At present, the annual output of DWSSP in China is about 140,000 tons, [7,8] most of which is high-purity silicon, resulting in a large amount of high-purity silicon losses.However, DWSSP is an environmentally hazardous waste if it is released into agricultural lands or rivers. [9]The efficient utilization of high-purity silicon in DWSSP can increase high-purity silicon production, improve its economic benefits, and avoid environmental pollution caused by DWSSP emissions. [10]Therefore, the efficient extraction of silicon is an important goal for the efficient utilization of DWSSP.
At present, the main difficulty in the recovery of silicon from DWSSP is the presence of a surface oxide layer on the silicon particles.The high melting range of the surface oxide layer on the silicon particles hinders the direct extraction of elemental silicon from DWSSP by high-temperature smelting. [11]Moreover, the presence of SiO 2 affects the efficiency of high-temperature smelting and the recovery of Si.DWSSP is a powder that contains many metallic contaminants. [12]Acid leaching can remove most metal impurities in DWSSP; however, some metal impurities trapped in the surface oxide layer on the silicon particles are difficult to remove by acid leaching, due to the growth of the surface oxide layer on the silicon particles. [12]There are still a lot of residue metal impurities in DWSSP after acid leaching, which limits the improvement of product purity in hydrometallurgy purification. [5]The presence of a surface oxide layer on the silicon particles in DWSSP is difficult to recover by conventional recovery methods.The surface oxides on the silicon particles in DWSSP limit the efficient purification of DWSSP, it is necessary to further study the surface oxides in DWSSP for silicon recovery. [13]owever, the composition and structure of the surface oxide layer on the silicon particles in DWSSP have not been well studied.In prior studies, it was generally believed that the surface oxide on silicon particles was SiO 2. [14] Through study after this, DWSSP was speculated to have a composite structure of amorphous SiO 2 and Si. [15]In the experiment of slag formation using CaO and DWSSP, it is assumed that all O is provided to SiO 2 .After the experiment, CaO remains, and it is suspected that the oxygen in the silicon wafer cutting waste is not fully provided to SiO 2. [16] The lack of a systematic understanding of silicon particles in the surface oxide layer of DWSSP hinders the utilization of silicon in DWSSP.At present, there is still no clear understanding of the composition and structure of complex DWSSP.
For the efficient utilization of DWSSP, it is necessary to clarify the composition and structure of the surface oxide layer on the silicon particles in DWSSP.In this study, the SiO 2 , SiO x , and Si contents were determined by SiO 2 -CaO phase diagram analysis and the oxygen conservation rule, and the three-layer structure of DWSSP was determined.According to the oxygen content in DWSSP, the content of the oxide layer on the surface silicon particles in DWSSP can be accurately calculated using a general formula.This allowed for the composition and structure of DWSSP was successfully determined.The results are helpful in the selection and design of a recycling process, that allows for the systematic recovery of silicon in DWSSP and the preparation of silicon materials using the unique properties of DWSSP.In addition, this study provides a theoretical basis for improving the effective utilization of silicon resources.

Crystal transformation of amorphous SiO 2 in
diamond wire saw silicon powder The XPS spectra and XRD patterns of the five raw DWSSP materials are shown in Figures 1(a) and 2(b).The Si and SiO 2 peaks are visible in XPS spectra, with the SiO 2 peak intensity increasing with the oxygen content.For the XRD patterns, only the Si diffraction peaks and none of the other phases were observed.SiO 2 exists in an amorphous form, so no diffraction peaks can be observed. [17]Therefore, it can be concluded that the amorphous SiO 2 surface layer exists in DWSSP.
To verify that SiO 2 exists in DWSSP, amorphous SiO 2 was converted into crystalline SiO 2 by SiO 2 crystal transformation.Several common crystal forms of SiO 2 , including coesite, quartz, stishovite, tridymite, and cristobalite, are shown in Figure 2(a).In the 2h range of 20-30 , the standard diffraction peaks of coesite, quartz, and stishovite were not significantly different from those of Si, while the diffraction peaks of tridymite are difficult to distinguish from Si.Therefore, amorphous SiO 2 was transformed into cristobalite, and the crystal transition temperature was set above the cristobalite formation temperature to 1400 C.
The XRD patterns of DWSSP after crystal transformation are shown in Figure 2(b).A clear cristobalite diffraction peaks can be observed in the XRD patterns, indicating that amorphous SiO 2 in the raw DWSSP material had been transformed into cristobalite.Therefore, the presence of SiO 2 in DWSSP was confirmed through the crystal transformation experiments.

Critical composition of the SiO 2 precipitates
The presence of SiO 2 in DWSSP was confirmed in Section 3.1, and the actual content of SiO 2 can be obtained by confirming the critical composition of SiO 2 precipitates in the SiO 2 -CaO phase diagram as show in Figure 3. [18] To determine the critical composition of the SiO 2 precipitates, a SiO 2 /(SiO 2 þ CaO) molar ratio range of 0.4-0.7 was considered, as shown in the blue area in Figure 3.When the holding temperature of the experiment was 1700 C, SiO 2 in the DWSSP and CaO were completely dissolved in the liquid state to form L. When the temperature was reduced to room temperature after holding, different phases in the slag with different CaO additives were observed and respectively listed below: 1.At the SiO 2 /(SiO 2 þ CaO) molar ratio of 0.5, the slag phase obtained at room temperature was CaSiO 3 (Equation 1): 2. At a SiO 2 /(SiO 2 þ CaO) molar ratio between 0.5 and 0.7, the eutectic reaction occurred and SiO 2 was continuously precipitated from L, and the slag phase obtained at room temperature was composed of SiO 2 and CaSiO 3 (Equation 2): 3. At a SiO 2 /(SiO 2 þ CaO) molar ratio between 0.4 and 0.5, the eutectic reaction could be occurred through, and the slag phase obtained at room temperature was entirely composed of CaSiO 3 and Ca 3 Si 2 O 7 (Equation 3): In this case, only one phase of CaSiO 3 is present at room temperature.This is due to the reaction rate of Equations ( 4) and ( 6) being faster during the cooling process.Equation ( 5) is slower, so the slag phase does not have the Ca Therefore, the molar ratio of the actual SiO 2 content (hereafter referred to as AC SiO 2 ) to SiO 2 /(SiO 2 þ CaO) should be exactly 0.5.The molar ratio of AS SiO 2 to added SiO 2 /(SiO 2 þ CaO) was first set as 0.5, and the added   [16] amount of CaO was gradually reduced until SiO 2 precipitated from the slag.This indicated that the addition of CaO shifted the composition point to 0.5 toward the right as shown in Figure 3.The molar ratio of SiO 2 and CaO can be determined according to whether SiO 2 was precipitated in the slag, allowing for the critical point of the DC line to be determined.

Slag results for different ratios of DWSSP and CaO
To determine the actual content of SiO 2 in DWSSP, the component point of a 0.5 (SiO 2 /SiO 2 þ CaO) molar ratio in the SiO 2 -CaO phase diagram was found based on the analysis of the slag phase.The molar ratio of AS SiO 2 to CaO gradually decreased from 1:1 to 1:0.9, 1:0.8, and finally to 1:0.1 until SiO 2 precipitated from the slag by adding a different dosage of CaO.Then, two adjacent groups with and without SiO 2 were precipitated from the analysis phase of the slag.The molar ratios of AS SiO 2 to SiO 2 þ CaO doses for both groups were averaged and used to find the component point of a 0.5 (SiO 2 /SiO 2 þ CaO) molar ratio.To more accurately determine the component points where the component point of 0.5 (SiO 2 /SiO 2 þ CaO) molar ratio (AC SiO 2 to CaO is closest to 1:1), the dichotomous method was used to narrow the range.The top and longitudinal views of slag formation are shown in Figure 4. Following high-temperature slag formation, the upper part of the slagenriched sample and the lower part of the silicon-enriched sample were observed, due to the different densities of Si and slag. [19]At the end of each experiment, slag in the red area was taken and cut into two parts along the longitudinal direction.One part was ground into powder for XRD analysis, and the other was cut into thin sections for morphological characterization via EPMA.
It is preliminarily believed that all oxygen supplies SiO 2 , and the ratio of As-SiO 2 to CaO is gradually reduced.The detection results show that SiO 2 precipitation, which proves that the addition ratio shifts to the right side of the direct flow line in the phase diagram.Then find out this group of experiments and the previous group of experiments, using the dichotomy to find that the content of AC SiO 2 and CaO is 1:1.This method is used to calculate the actual amount of CaO when the SiO 2 and CaO of 5 different oxygen contents DWSSP are 1:1, and then AC SiO 2 is derived.The XRD pattern of SiO 2 precipitate is shown in Figure 5(a-1)-(e-1).It can be seen from the XRD pattern that SiO 2 , CaSiO 3 and elemental Si in the slag phase are the main phases.According to Equation ( 1), it can be seen that under melting conditions, SiO 2 shells in DWSSP can react with added CaO.The precipitation of SiO 2 means that as the amount of CaO is reduced, the actual composition point moves to the right of the DC line, and the eutectic reaction occurs to generate SiO 2 and CaSiO 3 through Equation (2).Then, the molar ratio of AS SiO 2 and CaO was gradually reduced by  dichotomy.The molar ratio of AS SiO 2 to CaO was 1:0.5 when SiO 2 precipitated in the slag phase formed at high temperatures, as shown in Figure 5(a-1)-(d-1), but changed to 1:0.48 when the slag phase was as shown in Figure 5(e-1).
The slag and the adjacent SiO 2 precipitate were used to determine the molar ratio of SiO 2 to CaO close to the DC line.The molar ratio of AS SiO 2 to CaO in the SiO 2 precipitate of the slag phase formed at high temperatures was 1:0.55, as shown in Figure 6(a-2)-(d-2), and changed to 1:0.5, as indicated in Figure 6(e-2).Through the analysis of the phase diagrams, it was determined that the actual composition point was on the left of the DC line.
The EPMA microscopic observation of each phase of one of them as an example is shown in Figure 7.According to the EPMA mapping, the orange distribution is Si, the blue distribution is O, and the light-yellow distribution is Ca as shown in Figure 7(c-e).Among them, the dark plum-like shape is SiO 2 , the gray area of the ribbon distribution is CaSiO 3 , and the bright spherical droplets are Si as shown in Figure 7(a).CaSiO 3 could be formed with the added CaO combined with the SiO 2 in the shell (Equation 1), and the molten Si droplets exited the core to form Si spheres at high temperatures.In addition, the plume-shaped SiO 2 was precipitated by the redistribution of the slag composition through the eutectic reaction during the cooling process.
SiO 2 precipitation indicated in the XRD patterns shown in Figure 5 was verified by EPMA.The EPMA mapping of the SiO 2 precipitate is shown in Figure 8(a-1)-(e-1).The five slag phases showed that a large amount of CaSiO 3 and granular Si coexisted.The precipitated plum-like SiO 2 distribution was also relatively uniform, which proved that SiO 2 precipitated via the eutectic reaction shown in the phase diagram.The EPMA results further determined the precipitation of SiO 2 , which moved to the right of the DC line.The molar ratios of AS SiO 2 to CaO in the SiO 2 precipitate of the slag formed at high temperatures (a-1)-(e-1) were again calculated as 1:0.5, 1:0.5, 1:0.5, 1:0.5, and 1:0.48, respectively.
The XRD patterns without SiO 2 precipitation shown in Figure 6 were verified by EPMA.The EPMA mapping of the SiO 2 without precipitate is shown in Figure 9(a-2)-(e-2).According to the EPMA mapping, CaSiO 3 and Si droplets were observed in the slag phase, but there was no obvious SiO 2 precipitation.The EPMA results provided further evidence that SiO 2 was not precipitated and that the composition point shifted to the left of the DC line.The molar ratios of AS SiO 2 to CaO in the slag formed at high temperatures without the SiO 2 precipitate were again calculated as 1:0.55, 1:0.55, 1:0.55, 1:0.55, and 1:0.5, respectively.

Composition of DWSSP
Since excess SiO 2 was precipitated from the slag, the molar ratio of CaO added to AS SiO 2 was not 1:1, AC SiO 2 is inconsistent with AS SiO 2 , and the composition of DWSSP was found to fluctuate with O content.The composition of the surface oxide on the silicon particles in DWSSP is not unique; according to the conservation of oxygen, the oxide of the silicon particles in DWSSP was not entirely composed of SiO 2 .According to the results obtained in Section 3.3, the composition of DWSSP assumed by the weight of 100 g of raw material was determined as show in Table 1.To determine the mass percentage of Si, SiO x , and SiO 2 in DWSSP, the relationship between O and Si according to the mass conservation can be calculated using Equation (7): where u and v, respectively, represent the oxygen content (100%) in SiO 2 and SiO x ; w, y, and z represent the mass percentage content (100%) of SiO 2 , SiO x , and Si; and a represents the total oxygen content derived from SiO 2 and SiO x .In the calculation, 100 g of raw DWSSP material was used.u can be calculated using Equation ( 8), with b being the intermediate amount of added CaO for the experiments with and without precipitated SiO 2 .
The oxygen content of SiO x was obtained from the oxygen conservation rule with the O content of SiO 2 using Equation ( 9).The SiO 2 content was calculated through Equation (10).
The value of x in SiO x was determined using Equation (11).
The SiO x content was calculated with Equation (12).
Using the total oxygen content and subtracting the mass percentage of SiO 2 and SiO x , the mass percentage of Si can be calculated (Equations 10 and 13) Combining the above calculations with the ratios determined in the slag phase, the SiO 2 , SiO x , and Si contents can be determined, and the results are listed in Table 1.
The trends of the AC SiO 2 and AS SiO 2 ; AC Si and AS Si; and SiO x contents are, respectively, shown in Figure 10(a-c).The contents of AS SiO 2 and AC SiO 2 gradually increased with the O content.The slope of AC SiO 2 was lower than that of AS SiO 2 , with the AC SiO 2 content being less than that of AS SiO 2 at the same oxygen concentration, this is due to the oxygen is not all contributed to SiO 2 in the actual oxidation process.The content of AS Si and AC Si gradually increased with increasing O content.The slope of AC Si is smaller than AS Si, and the content of AC Si is less than that of AS Si at the same oxygen content, this fact further proves that SiO 2 is not the single oxide, but there are also another silicon oxides with intermediate valence states(SiO x ) in the DWSSP, and the SiO x content gradually increase with the increase of O content.This indicates that SiO 2 is formed in the outermost layer during the process of the oxidation layer on the surface of silicon particles, and is also accompanied by the formation of SiO x in the intermediate layer, and the relative content of SiO x shows an increasing trend with the increase of oxygen content.
After linear fitting, the general calculation formulas of SiO 2 , SiO x , and Si in DWSSP were obtained for , and Z ðSiÞ , respectively, through Equations ( 14)-( 16).The obtained equations can determine the real content of SiO 2 , SiO x , and Si according to the O content.Table 1.Composition of DWSSP assumed by the weight of 100 g of raw material.
The presence of a three-layer structure in DWSSP was determined, meaning that DWSSP did not have a two-layer structure.The study revealed the presence of SiO x and indicated that SiO 2 was found in the outermost layer of DWSSP.More notably, in this study, SiO x was identified in the intermediate layer, and the innermost layer was found to contain Si.The innermost Si layer was connected to the outer SiO 2 layer through the SiO x intermediate, rather than single having an amorphous SiO 2 .This discovery enriches the existing understanding of the structure of DWSSP, the novel discovery of the structure provides a theoretical basis for the process design and optimization for silicon recovery from DWSSP waste.

Materials
Five raw DWSSP materials with different oxygen contents were obtained from monocrystalline wafer manufacturers.
The raw material lumps were dried at 80-100 C for 240 h in a vacuum drier oven for analytical characterization and  slag formation in high-temperature experiments.The CaO additives used in the experiments is of analytical grade.Mineralogical analysis was performed by X-ray diffraction (XRD, D8ADVANCE), and the microstructure was recorded by X-ray photoelectron spectroscopy (XPS, THERMO).The total silicon content in DWSSP was detected by X-ray fluorescence spectrometry (XRF, ZKS100e), and the total oxygen content in DWSSP was detected by evolved gas analysis (EGA, ONH836).Observing the micro morphology after the experiment using the electron probe micro analyzer (EPMA, JXA8230).The total O, total Si, and assumed SiO 2 and Si contents are shown in Table 2. Based on the stoichiometry, it is first assumed that all oxygen is supplied to SiO 2 , and the assumed SiO 2 content (hereinafter referred to as AS SiO 2 ) can then be calculated.The O content in SiO 2 was then subtracted from the total O content (%) to give the assumed Si content (hereinafter referred to as AS Si).The AS SiO 2 content increased with increasing total O content, while the AS Si content and total Si content decreased with increasing total O content.

Experimental procedure
The experiment can be divided into the crystal transformation and high-temperature slag formation stages as shown in Figure 12.In the crystal transformation of amorphous SiO 2 in DWSSP, 50 g of DWSSP was added to a graphite crucible (U 60 Â 120 mm) and compacted before being placed in a box-type resistance furnace.After closing the furnace door, the air in the furnace was exhausted using a vacuum pump, and argon (99.999% purity) was slowly introduced into the furnace.This cycle was repeated three times to ensure that the air was exhausted.Argon gas was continuously injected during the experiment to ensure that the pressure in the furnace was consistent with the atmospheric pressure.The furnace was heated from room temperature to 1400 C at a rate of 5 CÁmin À1 , maintained for 3 h, and then cooled at 5 CÁmin À1 until reaching room temperature.After the experiment, the graphite crucible was removed, and the experimental sample was ground into a powder with an agate mortar.The powder was mixed until uniform for XRD detection.
Next, slag was formed in the high-temperature combination of DWSSP and CaO in a box-type resistance furnace.CaO was mixed with 150 g of DWSSP according to the required ratio (SiO 2 : CaO ¼ 1: 1) and evenly compacted.The mixture was added to a graphite crucible (U 70 Â 150 mm) and then placed in a box-type resistance furnace.After closing the furnace door, the air in the furnace was exhausted using a vacuum pump, and argon (99.999% purity) was slowly introduced into the furnace.The cycle was repeated three times to ensure that the furnace was exhausted.The furnace was heated from room temperature to 1700 C at a rate of 5 CÁmin À1 , maintained for 1.5 h, and then cooled at 5 CÁmin À1 until reaching room temperature.After the experiment was completed, the crucible was removed and the experimental sample was cut into two parts along the longitudinal direction.One side of the sample was taken, and the powder was uniformly ground with an agate mortar for XRD detection.The other side of the experimental sample was taken and cut into thin sections for EPMA detection.

Conclusion
In this study, the composition and structure of DWSSP were successfully identified.In this study, the three-layer structure of DWSSP containing SiO 2 , SiO x , and Si was determined using a SiO 2 -CaO phase diagram and the law of oxygen conservation.The actual SiO 2 , SiO x , and Si contents in DWSSP with different oxygen contents were calculated ranging from 4.33% to 13.07%.In addition, the variation trend and general calculation of SiO 2 , SiO x , and Si in DWSSP were analyzed by linear fitting, which can accurately identify the oxide layer of DWSSP raw materials.This research method can be used to prepare materials for DWSSP and purify Si in DWSSP, and promoting the efficient utilization of DWSSP.

Figure 1 .
Figure 1.(a) XPS spectra and (b) XRD patterns of the raw DWSSP materials.

Figure 4 .
Figure 4. (a) Top and (b) longitudinal views of the slag formation.

) 2 . 4 . 2 .
Structure of DWSSPBased on the systematic study, the structure of DWSSP is divided into three layers from inside to outside: the SiO 2 shell, the intermediate SiO x layer, and the Si core.The schematic diagram of the DWSSP structure is shown in Figure11.

Figure 12 .
Figure 12.Experimental flow chart for crystal transformation and high-temperature slag formation.

Table 2 .
Total O, total Si, and assumed SiO 2 and Si contents of 5 DWSSPs.