3.1 Crystal transformation of amorphous SiO2 in diamond wire saw silicon powder
The XPS spectra and XRD patterns of the five raw DWSSP materials are shown in Figs. 2a and 2b. The Si and SiO2 peaks are visible in XPS spectra, with the SiO2 peak intensity increasing with the oxygen content. For the XRD patterns, only the Si diffraction peaks and none of the other phases were observed. SiO2 exists in an amorphous form, so no diffraction peaks can be observed [17]. Therefore, it can be concluded that the amorphous SiO2 surface layer exists in DWSSP.
To verify that SiO2 exists in DWSSP, amorphous SiO2 was converted into crystalline SiO2 by SiO2 crystal transformation. Several common crystal forms of SiO2, including coesite, quartz, stishovite, tridymite, and cristobalite, are shown in Fig. 3a. In the 2θ range of 20–30°, the standard diffraction peaks of coesite, quart, and stishovite were not significantly different from those of Si, while the diffraction peaks of tridymite are difficult to distinguish from Si. Therefore, amorphous SiO2 was transformed into cristobalite, and the crystal transition temperature was set above the cristobalite formation temperature to 1400 ℃
The XRD patterns of DWSSP after crystal transformation are shown in Fig. 3b. Clear cristobalite diffraction peaks can be observed in the XRD patterns, indicating that amorphous SiO2 in the raw DWSSP material had been transformed into cristobalite. Therefore, the presence of SiO2 in DWSSP was confirmed through the crystal transformation experiments.
3.2 Critical composition of the SiO2 precipitates
The presence of SiO2 in DWSSP was confirmed in section 3.1, and the actual content of SiO2 can be obtained by confirming the critical composition of SiO2 precipitates in the SiO2-CaO phase diagram (Fig. 4)[18]. To determine the critical composition of the SiO2 precipitates, a SiO2/(SiO2 + CaO) molar ratio range of 0.4 to 0.7 was considered, as shown in the blue area in Fig. 4. When the holding temperature of the experiment was 1700 ℃, SiO2 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 SiO2/(SiO2 + CaO) molar ratio of 0.5, the slag phase obtained at room temperature was CaSiO3 (Eq. [1]):
(2) At a SiO2/(SiO2 + CaO) molar ratio between 0.5 and 0.7, the eutectic reaction occurred and SiO2 was continuously precipitated from L, and the slag phase obtained at room temperature was composed of SiO2 and CaSiO3 (Eq. [2]):
(3) At a SiO2/(SiO2 + 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 CaSiO3 and Ca3Si2O7 (Eq. [3]):
L ⇋ CaSiO3 + Ca3Si2O7
|
(3)
|
In this case, only one phase of CaSiO3 is present at room temperature. This is due to the reaction rate of Eqs. (4) and (6) being faster during the cooling process. Eq. (5) is slower, so the slag phase does not have the Ca3Si2O7 precipitate.
2CaO + SiO2 ⇋ Ca2SiO4
|
(4) |
3Ca2SiO4 + SiO2 ⇋ 2Ca3Si2O7
|
(5)
|
Ca3Si2O7 + SiO2 ⇋ 3CaSiO3
|
(6)
|
Therefore, the molar ratio of the actual SiO2 content (hereafter referred to as AC SiO2) to SiO2/(SiO2 + CaO) should be exactly 0.5. The molar ratio of AS SiO2 to added SiO2/(SiO2 + CaO) was first set as 0.5, and the added amount of CaO was gradually reduced until SiO2 precipitated from the slag. This indicated that the addition of CaO shifted the composition point to 0.5 toward the right (Fig. 4). The molar ratio of SiO2 and CaO can be determined according to whether SiO2 was precipitated in the slag, allowing for the critical point of the DC line to be determined.
3.3 Slag results for different ratios of DWSSP and CaO
To determine the actual content of SiO2 in DWSSP, the component point of a 0.5 (SiO2/SiO2 + CaO) molar ratio in the SiO2-CaO phase diagram was found based on the analysis of the slag phase. The molar ratio of AS SiO2 to CaO gradually decreased from 1:1 to 1:0.9, 1:0.8, and finally to 1:0.1 until SiO2 precipitated from the slag by adding a different dosage of CaO. Then, two adjacent groups with and without SiO2 were precipitated from the analysis phase of the slag. The molar ratios of AS SiO2 to SiO2 + CaO doses for both groups were averaged and used to find the component point of a 0.5 (SiO2/SiO2 + CaO) molar ratio. To more accurately determine the component points where the component point of 0.5 (SiO2/SiO2 + CaO) molar ratio (AC SiO2 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 Fig. 5. Following high-temperature slag formation, the upper part of the slag-enriched 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.
The XRD patterns of the SiO2 precipitate are shown in Figs. 6(a-1)-(e-1). From the XRD patterns, it was found that SiO2, CaSiO3, and elemental Si were the main phases in the slag phase, which indicated that the reaction between the SiO2 shell in the DWSSP and the added CaO could occur under the smelting conditions through Eq. (1) [17]. The precipitation of SiO2 meant that the actual composition point moved to the right of the DC line with the reduction of the added dosage of CaO, and eutectic reactions occur to form SiO2 and CaSiO3 through Eq. (2). Then, the molar ratio of AS SiO2 and CaO was gradually reduced by dichotomy. The molar ratio of AS SiO2 to CaO was 1:0.5 when SiO2 precipitated in the slag phase formed at high temperatures, as shown in Figs. 6(a-1)-(d-1), but changed to 1:0.48 when the slag phase was as shown in Fig. 6(e-1).
The slag and the adjacent SiO2 precipitate were used to determine the molar ratio of SiO2 to CaO close to the DC line. The molar ratio of AS SiO2 to CaO in the SiO2 precipitate of the slag phase formed at high temperatures was 1:0.55, as shown in Figs. 7(a-2)-(d-2), and changed to 1:0.5, as indicated in Fig. 7(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 Fig. 8. According to the EPMA mapping, the orange distribution is Si, the blue distribution is O, and the light-yellow distribution is Ca (Fig. 8c-8e). Among them, the dark plum-like shape is SiO2, the gray area of the ribbon distribution is CaSiO3, and the bright spherical droplets are Si (Fig. 8a, 8f-8h). CaSiO3 could be formed with the added CaO combined with the SiO2 in the shell (Eq. [1]), and the molten Si droplets exited the core to form Si spheres at high temperatures. In addition, the plume-shaped SiO2 was precipitated by the redistribution of the slag composition through the eutectic reaction during the cooling process.
SiO2 precipitation indicated in the XRD patterns shown in Fig. 6 was verified by EPMA. The EPMA mapping of the SiO2 precipitate is shown in Figs. 9(a-1)-(e-1). The five slag phases showed that a large amount of CaSiO3 and granular Si coexisted. The precipitated plum-like SiO2 distribution was also relatively uniform, which proved that SiO2 precipitated via the eutectic reaction shown in the phase diagram. The EPMA results further determined the precipitation of SiO2, which moved to the right of the DC line. The molar ratios of AS SiO2 to CaO in the SiO2 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 SiO2 precipitation shown in Fig. 7 were verified by EPMA. The EPMA mapping of the SiO2 without precipitate is shown in Figs. 9(a-2)-(e-2). According to the EPMA mapping, CaSiO3 and Si droplets were observed in the slag phase, but there was no obvious SiO2 precipitation. The EPMA results provided further evidence that SiO2 was not precipitated and that the composition point shifted to the left of the DC line. The molar ratios of AS SiO2 to CaO in the slag formed at high temperatures without the SiO2 precipitate were again calculated as 1:0.55, 1:0.55, 1:0.55, 1:0.55, and 1:0.5, respectively.
In the experiments (a-1)-(e-1) and (a-2)-(e-2) the intermediate values of the molar ratios of AS SiO2 and CaO are the component points of AC SiO2 and CaO with a molar ratio of 1:1, and the molar ratios of AS SiO2 and CaO added were 1:0.53, 1:0.53, 1:0.53, 1:0.53 and 1:0.49, respectively.
3.4 Composition and structure of DWSSP
3.4.1 Composition of DWSSP
Since excess SiO2 was precipitated from the slag, the molar ratio of CaO added to AS SiO2 was not 1:1, AC SiO2 is inconsistent with AS SiO2, 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 SiO2 and SiOx (0 < x < 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 (Table 2).
Table 2
Composition of DWSSP assumed by the weight of 100 g of raw material.
DWSSP
|
Precipitation SiO2 ratio
|
Without
precipitation SiO2 ratio
|
Middle rate
|
CaO
added
amount
(g)
|
AC
SiO2
amount
(g)
|
AS
SiO2
Amount
(g)
|
AC
Si
amount
(g)
|
AS
Si
amount
(g)
|
SiOx
amount
(g)
|
1
|
1 : 0.50
|
1 : 0.55
|
1 : 0.53
|
4.02
|
4.30
|
8.12
|
67.12
|
91.88
|
28.58
|
2
|
1 : 0.50
|
1 : 0.55
|
1 : 0.53
|
5.05
|
5.41
|
10.20
|
65.86
|
89.80
|
28.73
|
3
|
1 : 0.50
|
1 : 0.55
|
1 : 0.53
|
5.86
|
7.74
|
14.60
|
63.20
|
85.39
|
29.06
|
4
|
1 : 0.50
|
1 : 0.55
|
1 : 0.53
|
10.00
|
10.71
|
20.2
|
59.80
|
79.79
|
29.49
|
5
|
1 : 0.48
|
1 : 0.50
|
1 : 0.49
|
11.21
|
12.00
|
24.50
|
58.00
|
75.49
|
30.00
|
\(\left\{\begin{array}{c}\text{u}\text{+}\text{v}\text{=}\text{a}\text{%}\\ \text{w}\text{+}\text{y}\text{+}\text{z}\text{=}\text{100}\text{%}\text{ }\end{array}\right.\)(7)
|
|
where u and v respectively represent the oxygen content (100%) in SiO2 and SiOx; w, y, and z represent the mass percentage content (100%) of SiO2, SiOx, and Si; and a represents the total oxygen content derived from SiO2 and SiOx. In the calculation, 100 g of raw DWSSP material was used. u can be calculated using Eq. (8), with b being the intermediate amount of added CaO for the experiments with and without precipitated SiO2.
\(\text{u = }\left(\text{ b}\text{/56}\text{× }\text{60}\text{ ×}\text{32/60} \right)\text{ × }\text{100}\text{% }\)(8)
|
The oxygen content of SiOx was obtained from the oxygen conservation rule with the O content of SiO2 using Eq. (9).
$$\text{ v = }{\text{O}}_{\text{(T) }}-{\text{O}}_{\text{(}\text{Si}\text{) }}\text{ }$$
9
The SiO2 content was calculated through Eq. (10).
$$\text{ }\text{w}\text{ = }\left(\text{ }\text{u}\text{ ×60/32 }\right)\text{ × 100%}$$
10
The value of x in SiOx was determined using Eq. (11).
$$\text{ }\text{v}\text{ = 16}\text{x}\text{/(16}\text{x}\text{ + 28)}$$
11
The SiOx content was calculated with Eq. (12).
$$\text{y }\text{= ( }\text{28 + 16}\text{x}\text{ ) }\text{×}\text{ 100}\text{%}$$
12
Using the total oxygen content and subtracting the mass percentage of SiO2 and SiOx, the mass percentage of Si can be calculated (Eq. [10] and Eq. [13]).
$$\text{ }\text{z}\text{ =}\text{ }\text{100}\text{%}\text{-}\text{ }\text{w -}\text{ }\text{y}$$
13
Combining the above calculations with the ratios determined in the slag phase, the SiO2, SiOx, and Si contents can be determined, and the results are listed in Table 2.
The trends of the AC SiO2 and AS SiO2; AC Si and AS Si; and SiOx contents are respectively shown in Figs. 11a-11c. The contents of AS SiO2 and AC SiO2 gradually increased with the O content. The slope of AC SiO2 was lower than that of AS SiO2, with the AC SiO2 content being less than that of AS SiO2 at the same oxygen concentration, this is due to the oxygen is not all contributed to SiO2 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 SiO2 is not the single oxide, but there are also another silicon oxides with intermediate valence states(SiOx) in the DWSSP, and the SiOx content gradually increase with the increase of O content. This indicates that SiO2 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 SiOx in the intermediate layer, and the relative content of SiOx shows an increasing trend with the increase of oxygen content.
After linear fitting, the general calculation formulas of SiO2, SiOx, and Si in DWSSP were obtained for \({W}_{\left(\text{S}\text{i}{\text{O}}_{2}\right)}\), \({Y}_{\left(\text{S}\text{i}{\text{O}}_{x}\right)}\), and \({Z}_{\left(\text{S}\text{i}\right)}\), respectively through Eq. (14)-(16). The obtained equations can determine the real content of SiO2, SiOx, and Si according to the O content.
\({W}_{\left(\text{S}\text{i}{\text{O}}_{2}\right)}\)= (0.54 × O(T)) + 0.90
|
(14)
|
\({Y}_{\left(\text{S}\text{i}{\text{O}}_{x}\right)}\) = (− 1.06 × O(T)) + 71.60
|
(15)
|
\({Z}_{\left(\text{S}\text{i}\right)}\)= (0.16 × O(T)) + 27.86
|
(16)
|
3.4.2 Structure of DWSSP
Based on the systematic study, the structure of DWSSP is divided into three layers from inside to outside: the SiO2 shell, the intermediate SiOx layer, and the Si core. The schematic diagram of the DWSSP structure is shown in Fig. 12. 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 SiOx and indicated that SiO2 was found in the outermost layer of DWSSP. More notably, in this study, SiOx was identified in the intermediate layer, and the innermost layer was found to contain Si. The innermost Si layer was connected to the outer SiO2 layer through the SiOx intermediate, rather than single having an amorphous SiO2 and Si structure. 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.