Formation and Inhibition Mechanism of Na8SnSi6O18 during the Soda Roasting Process for Preparing Na2SnO3

To produce Na2SnO3, which is widely used in the ceramics and electroplating industries, a novel process for the preparation of sodium stannate from cassiterite concentrates was developed successfully by the authors’ group. It was found that sodium stannosilicate (Na8SnSi6O18) was easily formed due to the main gangue of quartz in cassiterite concentrates, which was almost insoluble and decreased the quality of Na2SnO3. The formation and transitions of Na8SnSi6O18 in the SnO2–SiO2–Na2CO3 system roasted under a CO–CO2 atmosphere were determined. The results indicated that the formation of Na8SnSi6O18 could be divided into two steps: SnO2 reacted with Na2CO3 to form Na2SnO3, and then Na2SnO3 was rapidly combined with SiO2 and Na2CO3 to form low melting point Na8SnSi6O18. In addition, Na8SnSi6O18 can be decomposed into Na2SiO3 and Na2SnO3 by using excess Na2CO3.


Introduction
Na 2 SnO 3 is an important raw material to produce stannate ceramics and electroplating materials [1][2][3][4][5]. A novel soda roasting-leaching process has been developed by the authors' group using cassiterite concentrates as raw materials, by which Na 2 SnO 3 was prepared efficiently and cleanly [6]. Previous studies have showed that the solid-state reactions between SnO 2 and Na 2 CO 3 are accelerated under a CO-CO 2 atmosphere [7][8][9][10]. Then, a trihydrate sodium stannate product with high purity was obtained, which meets the requirements of an industrial first-grade product [6]. In addition, the soda roasting process has also been applied for the comprehensive utilization of tin-bearing secondary waste [11][12][13][14].
Cassiterite (SnO 2 ) is the primary source of tin. It is naturally formed by magmatichydrothermal processes and occurs in granite pegmatites, quartz veins, greisens associated with granites, highly fractionated granites, as well as placer deposits [15][16][17][18]. Nevertheless, gangue minerals, including calcite, magnetite and other oxides, cannot be perfectly separated by beneficiation combined methods. Hence, oxidizing roasting and hydrochloric acid leaching processes are applied to remove impurity elements (Fe, Ca, Mg, S, As, Pb, Zn, etc.). However, quartz is stubborn and difficult to remove during the pretreatment process, resulting in the residue of SiO 2 in tin concentrates being as high as 8 wt.% [19,20].
Cassiterite concentrate, as reported, seldom reacts with soda (Na 2 CO 3 ) under air atmospheres. However, the authors' group found in previous research that cassiterite (or SnO 2 ) could readily react with Na 2 CO 3 under an appropriate CO-CO 2 atmosphere. In the roasting process, the CO gas molecules were firstly adsorbed on the SnO 2 surface and then combined with the bridging oxygen so that some oxygen vacancies were formed. These vacancies were replenished by the active oxygen anions in Na 2 O, which was the decomposition product of Na 2 CO 3 roasted over 851 • C. These processes accelerated the formation of Na 2 SnO 3 [6][7][8][9][10].
Our previous studies have found that the quartz (SiO 2 ) in the raw material has a significant effect on the phase transformation of SnO 2 during the soda roasting process [21].
The target products of soda roasting were Na 2 SnO 3 and Na 2 SiO 3 , which are freely soluble in NaOH solution. It was found that Na 8 SnSi 6 O 18 was easily formed and was almost insoluble during the leaching process, which decreased the recovery of tin [6,21]. A series of studies have systematically revealed the reaction mechanism of SnO 2 -SiO 2 , which was investigated during the cassiterite reduction smelting process and flat glassmaking method [22][23][24]. Furthermore, those studies confirmed that SnO 2 was an acidic oxide, while it transformed into SnO, an alkali oxide, during the reduction process. However, no studies have mentioned the reactions in the SnO 2 -SiO 2 -Na 2 CO 3 system, especially under a CO-CO 2 atmosphere.
Based on our previous studies, SiO 2 in cassiterite concentrates has adverse effects on the formation and leaching of Sn during the soda roasting-leaching process. The maximum conversion rate of Sn was around 85.6% under optimal conditions. However, the formation and phase transformation mechanisms of Na 8 SnSi 6 O 18 were unknown. Hence, in order to improve the Sn conversion rate during the soda roasting process, the formation mechanism and decomposition process of Na 8 SnSi 6 O 18 in the SnO 2 -SiO 2 -Na 2 CO 3 system were investigated, using X-ray powder diffraction (XRD), scanning electron microscopy and energy dispersion spectroscopy (SEM-EDS), thermogravimetric and differential scanning calorimetry (TG-DSC), Fourier transform infrared spectroscopy (FTIR), etc.

Materials
The cassiterite concentrates (taken from Gejiu, Yunnan Province of China, Yunnan Tin Company Limited) used in this study were pretreated by oxidizing roasting and acid leaching processes to remove impurities [6,13]. As shown in Figure 1, only diffraction peaks of cassiterite (SnO 2 ) and quartz (SiO 2 ) were found in the XRD pattern of the pretreated cassiterite concentrates. In addition, the contents of Sn and Si were determined to be 62.93 wt.% and 3.66 wt.% by ICP-AES, respectively, while impurities of Ca, Fe, Mg, Al and S were not detected. Moreover, the analytical reagents of SnO 2 , SiO 2 , Na 2 CO 3 , Na 2 SiO 3 and Na 2 SnO 3 ·3H 2 O (AR, Shanghai Aladdin Bio-Chem Technology Co., Ltd., Shanghai, China) used in this study had a purity of over 99.5 wt.%, and all the samples were pre-ground to fully pass through a 0.037 mm sieve. The gases CO, CO 2 and N 2 had a purity of 99.9 vol%.

Experimental Procedures
The experimental procedures in this study mainly include the roasting and leaching  The experimental procedures in this study mainly include the roasting and leaching process, where the details of the roasting process under a 15 vol% CO/(CO and CO 2 ) atmosphere have been described in our previous study [6,21]. The leaching of Na 2 SnO 3 tests were conducted in a water bath at 40 • C with a content of 0.05 mol/L NaOH solution. Finally, the leaching solution was filtered and prepared to determine the formation efficiency of Na 2 SnO 3 . The residues were washed with distilled water to identify the phase constituents as follows: where L is the formation efficiency of Na 2 SnO 3 , M is the weight of the roasted samples (g), W is the grade of Sn in the roasted samples (%), C is the mass content of Sn in the leaching solution (mg/mL) and V is the volume of leaching solution (mL).

Instrument Techniques
The phase constituents of the samples were identified by X-ray diffraction XRD (Cutarget Bruker D8 Advance), with a step of 0.02 • at 10 min −1 ranging from 10 • to 80 • . The microscopic morphology was observed with a scanning electron microscope (QUANTA 200, FEI, Eindhoven, The Netherlands) equipped with an EDAX energy dispersive X-ray spectroscopy (EDS) detector (EDAX Inc., Mahwah, NJ, USA). Fourier transform infrared spectroscopy (FTIR: Nicolet 8700) in the range of 400-4000 cm −1 was applied to determine the chemical bands of the roasted samples in transmission mode. TG-DSC analyses of samples were performed using a thermal analyzer (Netzsch STA 449, Selb, Germany) in the temperature range of 25-1200 • C with a heating rate of 10 • C/min in an Ar atmosphere, and a platinum crucible was used with 50 mg samples for each test. The content of Sn in the solid material and the aqueous solution was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Thermo Fisher Scientific, Waltham, MA, USA, Icap7400 Radial, King of Prussia, PA, USA). For each ICP test, a certain mass or volume of samples was first dissolved in H 2 SO 4 -HF solution system, and the solution was set to a constant volume of 100 mL. Then, the solution was tested using ICP, and the Sn content in the raw solid material and the aqueous solution were calculated. The morphological evolution of the roasted samples was monitored by an in situ high temperature thermal analyzer (S/DHTT-TA-III, Chongqing University, Chongqing, China). Figure 2 shows the experimental flowsheet of soda roasting and leaching using Sibearing cassiterite concentrates. Based on a previous paper, the optimal experimental conditions were fixed at a roasting temperature of 875 • C, a CO content of 15%, roasting time of 15 min, Na 2 CO 3 /SnO 2 mole ratio of 1.5, etc. [6,21]. A Sn leaching efficiency of 85.6% was achieved; moreover, a small number of leaching residues were obtained. Then, the roasted products and leaching residues (in Figure 2) were observed by XRD and SEM-EDS analysis, and the results are shown in Figure 3.

Phase Analysis for the Products of Soda Roasting
As shown in Figure 3a, the main phases in the roasted products were Na 2 SnO 3 and Na 2 SiO 3 , which verified the high leaching efficiency of Sn and Si. In particular, the characteristic peaks of Na 8 SnSi 6 O 18 were found in the leaching residues of Figure 3b, and unreacted SnO 2 was observed as well. It is seen from Figure 3c that Na 2 SnO 3 (Spot A in Figure 3c) was found as regular hexagonal cylinder crystal grains and slice crystal grains, which matched well with the theoretical molar ratio of sodium stannate of 2:1. Moreover, melting phases can be seen in the backscattering image of Figure 3c, which were formed irregularly at the grain edge of Na 2 SnO 3 . The EDS analysis of Spot B in Figure 3c showed that the main element composition was Sn, Na and Si, which is consistent with the chemical composition of Na 8 SnSi 6 O 18 . The morphology of leaching residues is shown in Figure 3d. The results indicated that the Na 8 SnSi 6 O 18 phase (Spot C in Figure 3d) was closely wrapped in cassiterite particles. Both SnO 2 and Na 8 SnSi 6 O 18 were insoluble during the leaching process, and then they were enriched in the leaching residues. The results in Figure 3 indicate that Na 8 SnSi 6 O 18 was rapidly formed during the roasting process, and then the melt wrapped on the surface of SnO 2 and Na 2 CO 3 , which restrained the formation of Na 2 SnO 3 . It is concluded that Na 8 SnSi 6 O 18 has a negative impact on the formation of sodium stannate from cassiterite. Next, the formation mechanism of Na 8 SnSi 6 O 18 is discussed.  Figure 2 shows the experimental flowsheet of soda roasting and leaching using Sibearing cassiterite concentrates. Based on a previous paper, the optimal experimental conditions were fixed at a roasting temperature of 875 °C, a CO content of 15%, roasting time of 15 min, Na2CO3/SnO2 mole ratio of 1.5, etc. [6,21]. A Sn leaching efficiency of 85.6% was achieved; moreover, a small number of leaching residues were obtained. Then, the roasted products and leaching residues (in Figure 2) were observed by XRD and SEM-EDS analysis, and the results are shown in Figure 3.  As shown in Figure 3a, the main phases in the roasted products were Na2SnO3 and Na2SiO3, which verified the high leaching efficiency of Sn and Si. In particular, the characteristic peaks of Na8SnSi6O18 were found in the leaching residues of Figure 3b, and unreacted SnO2 was observed as well. It is seen from Figure 3c that Na2SnO3 (Spot A in Figure  3c) was found as regular hexagonal cylinder crystal grains and slice crystal grains, which matched well with the theoretical molar ratio of sodium stannate of 2:1. Moreover, melting phases can be seen in the backscattering image of Figure 3c, which were formed irregu-

Effect of Roasting Atmospheres on the Formation of Na 8 SnSi 6 O 18
To investigate the formation mechanism of Na 8 SnSi 6 O 18 in the SnO 2 -SiO 2 -Na 2 CO 3 system, AR reagents of SnO 2 , SiO 2 and Na 2 CO 3 were mixed at a mole ratio of Na 8 SnSi 6 O 18 . The XRD patterns of the samples roasted at 875 • C under a 15 vol.% CO-CO 2 atmosphere and air atmosphere are shown in Figure 4. As shown in Figure 4a, the diffraction peaks of Na8SnSi6O18 appeared remarkably, and SnO2 and SiO2 disappeared at 10 min under a CO-CO2 atmosphere. The diffraction peak intensities of Na8SnSi6O18 increased gradually as the roasting time increased from 10 min to 30 min, while those of Na2SiO3 and Na2CO3 weakened. However, the phase compositions of the roasted products in an air atmosphere were significantly different, as shown in Figure 4b, and the generation of Na8SnSi6O18 in air was much slower than that in a CO-CO2 atmosphere. Moreover, it was noteworthy that no diffraction peaks of Na2SnO3 were found in the roasted product. Our previous studies illustrated the enhancement of the CO-CO2 atmosphere on the formation of Na2SnO3, as shown in Equation (2). Hence, it can be inferred that Na2SnO3 may be an important intermediate during the formation of Na8SnSi6O18, as shown in Equation (3). Based on the above phase analysis, in a CO-CO2 atmosphere, there were stronger diffraction peaks of Na8SnSi6O18 than in an air atmosphere. The results demonstrated that the formation of Na8SnSi6O18 in the CO-CO2 atmosphere was much easier than that in the air atmosphere.
FTIR analysis was utilized to illustrate the phase transformation of SnO2, SiO2 and Na2CO3 roasted products under a 15 vol.% CO-CO2 atmosphere, as depicted in Figure 5. As shown in Figure 4a, the diffraction peaks of Na 8 SnSi 6 O 18 appeared remarkably, and SnO 2 and SiO 2 disappeared at 10 min under a CO-CO 2 atmosphere. The diffraction peak intensities of Na 8 SnSi 6 O 18 increased gradually as the roasting time increased from 10 min to 30 min, while those of Na 2 SiO 3 and Na 2 CO 3 weakened. However, the phase compositions of the roasted products in an air atmosphere were significantly different, as shown in Figure 4b, and the generation of Na 8 SnSi 6 O 18 in air was much slower than that in a CO-CO 2 atmosphere. Moreover, it was noteworthy that no diffraction peaks of Na 2 SnO 3 were found in the roasted product. Our previous studies illustrated the enhancement of the CO-CO 2 atmosphere on the formation of Na 2 SnO 3 , as shown in Equation (2). Hence, it can be inferred that Na 2 SnO 3 may be an important intermediate during the formation of Na 8 SnSi 6 O 18 , as shown in Equation (3). Based on the above phase analysis, in a CO-CO 2 atmosphere, there were stronger diffraction peaks of Na 8 SnSi 6 O 18 than in an air atmosphere. The results demonstrated that the formation of Na 8 SnSi 6 O 18 in the CO-CO 2 atmosphere was much easier than that in the air atmosphere. Na 2 CO 3 + SnO 2 = Na 2 SnO 3 + CO 2 (2) FTIR analysis was utilized to illustrate the phase transformation of SnO 2 , SiO 2 and Na 2 CO 3 roasted products under a 15 vol.% CO-CO 2 atmosphere, as depicted in Figure 5.
The peak at 1485 cm −1 was assigned to Si=O stretching vibrations, and vibrations at 1076 cm −1 and 932 cm −1 correspond to Si-O-Si bonds [25,26]. The results in Figure 5a,b indicate that the intensity of the Si=O bond decreased obviously as the roasting temperature and roasting time increased, which revealed the conversion of SiO 2 . Simultaneously, the increase in Si-O-Si bonds and Sn-O-Si/Si-O-T bonds (614 cm −1 , 545 cm −1 and 449 cm −1 ) [25][26][27][28] can also be observed in Figure 5. The results further confirmed the molecular evolution of Si-bearing materials during the roasting process, which was consistent with the XRD analysis in Figure 4. The peak at 1485 cm −1 was assigned to Si=O stretching vibrations, and vibrations at 1076 cm −1 and 932 cm −1 correspond to Si-O-Si bonds [25,26]. The results in Figure 5a,b indicate that the intensity of the Si=O bond decreased obviously as the roasting temperature and roasting time increased, which revealed the conversion of SiO2. Simultaneously, the increase in Si-O-Si bonds and Sn-O-Si/Si-O-T bonds (614 cm −1 , 545 cm −1 and 449 cm −1 ) [25][26][27][28] can also be observed in Figure 5. The results further confirmed the molecular evolution of Si-bearing materials during the roasting process, which was consistent with the XRD analysis in Figure 4.

Effect of Intermediate Products on the Formation of Na8SnSi6O18
The results in Figure 4 demonstrate that Na2SnO3 and Na2SiO3 were generated along with Na8SnSi6O18, as shown in Equations (2) and (4), then both intermediate products were taken into consideration to reveal the reaction path for the formation of Na8SnSi6O18. SnO2 or SiO2 was first mixed with Na2CO3 at a mole ratio of 1:1 and then roasted at 875 °C under a 15 vol.% CO-CO2 atmosphere for a certain period of time. The XRD analysis of the roasted products is shown in Figure 6.

Effect of Intermediate Products on the Formation of Na 8 SnSi 6 O 18
The results in Figure 4 demonstrate that Na 2 SnO 3 and Na 2 SiO 3 were generated along with Na 8 SnSi 6 O 18 , as shown in Equations (2) and (4), then both intermediate products were taken into consideration to reveal the reaction path for the formation of Na 8 SnSi 6 O 18 . SnO 2 or SiO 2 was first mixed with Na 2 CO 3 at a mole ratio of 1:1 and then roasted at 875 • C under a 15 vol.% CO-CO 2 atmosphere for a certain period of time. The XRD analysis of the roasted products is shown in Figure 6. As shown in Figure 6a, the reaction between SnO2 and Na2CO3 (Equation (2)) proceeded much more quickly; Na2SnO3 was the main phase in the roasted products, and almost no diffraction peaks of SnO2 were found after roasting for 15 min. In contrast, the formation rate of Na2SiO3 was slow in the solid-state, and the diffraction peaks were uncertain in Figure 6b after roasting for 30 min. The difference in the solid-state reaction rates of Equations (2) and (4) may cause different reaction paths for the final products; therefore, two possible reactions were proposed, as shown in Equations (5) and (6) based on conservation of mass.
Na2SnO3 + 6SiO2 + 3Na2CO3 = Na8SnSi6O18 + 3CO2 (5) 6Na2SiO3 + SnO2 = Na8SnSi6O18 + 2Na2O (6) In view of further verification, two kinds of mixed samples were prepared as follows: Na2SnO3·3H2O, Na2CO3 and SiO2 (AR reagent) with a molar ratio of 1:3:6, as shown in As shown in Figure 6a, the reaction between SnO 2 and Na 2 CO 3 (Equation (2)) proceeded much more quickly; Na 2 SnO 3 was the main phase in the roasted products, and almost no diffraction peaks of SnO 2 were found after roasting for 15 min. In contrast, the formation rate of Na 2 SiO 3 was slow in the solid-state, and the diffraction peaks were uncertain in Figure 6b after roasting for 30 min. The difference in the solid-state reaction rates of Equations (2) and (4) may cause different reaction paths for the final products; therefore, two possible reactions were proposed, as shown in Equations (5) and (6) In view of further verification, two kinds of mixed samples were prepared as follows: Na 2 SnO 3 ·3H 2 O, Na 2 CO 3 and SiO 2 (AR reagent) with a molar ratio of 1:3:6, as shown in Equation (5), and Na 2 SiO 3 ·3H 2 O and SnO 2 with a molar ratio of 6:1 as shown in Equation (6). Then, TG-DSC (Ar atmosphere,~1000 • C) and XRD analysis were used to determine the possible reactions in the two designed systems of Na 2 SnO 3 -SiO 2 and Na 2 SiO 3 -SnO 2 , and the results are displayed in Figures 7 and 8, respectively.  As shown in Figure 7a, two weak endothermic peaks at 87.5 °C and 243.8 °C were observed with a small quantity of weight loss, which was assigned to the thermal dehydration reaction of Na2SnO3·3H2O and Na2CO3 (crystal water) [29]. After that, the mass loss increased sharply to 18.3 wt.% with a significant endothermic peak at 833.6 °C in the DSC curve. The results revealed that the solid-phase reaction proceeded in the temperature range of 800-900 °C, the weight loss was possibility attributed to the reaction of Equation (5) and released CO2 gas. In addition, the XRD results in Figure 7b indicated that almost no diffraction peak of Na2SnO3 can be found, which illustrates that Na2SnO3 in the raw materials is converted to Na8SnSi6O18 completely, as shown in Equation (5). On the other hand, the results in Figure 8 show totally different outcomes. No exothermic reactions were found in the TG-DSC curve (in Figure 8a), and the roasted products were unchanged as SnO2 and Na2SiO3 (in Figure 8b), which excluded the reaction paths expressed in Equation (6).

Reactions between Na8SnSi6O18 and Na2CO3
To find a possible transition process of Na8SnSi6O18 during the roasting process, Na8SnSi6O18 was synthesized based on our previous study [21]. In this section, Na2CO3:  As shown in Figure 7a, two weak endothermic peaks at 87.5 °C and 243.8 °C were observed with a small quantity of weight loss, which was assigned to the thermal dehydration reaction of Na2SnO3·3H2O and Na2CO3 (crystal water) [29]. After that, the mass loss increased sharply to 18.3 wt.% with a significant endothermic peak at 833.6 °C in the DSC curve. The results revealed that the solid-phase reaction proceeded in the temperature range of 800-900 °C, the weight loss was possibility attributed to the reaction of Equation (5) and released CO2 gas. In addition, the XRD results in Figure 7b indicated that almost no diffraction peak of Na2SnO3 can be found, which illustrates that Na2SnO3 in the raw materials is converted to Na8SnSi6O18 completely, as shown in Equation (5). On the other hand, the results in Figure 8 show totally different outcomes. No exothermic reactions were found in the TG-DSC curve (in Figure 8a), and the roasted products were unchanged as SnO2 and Na2SiO3 (in Figure 8b), which excluded the reaction paths expressed in Equation (6).

Reactions between Na8SnSi6O18 and Na2CO3
To find a possible transition process of Na8SnSi6O18 during the roasting process, As shown in Figure 7a, two weak endothermic peaks at 87.5 • C and 243.8 • C were observed with a small quantity of weight loss, which was assigned to the thermal dehydration reaction of Na 2 SnO 3 ·3H 2 O and Na 2 CO 3 (crystal water) [29]. After that, the mass loss increased sharply to 18.3 wt.% with a significant endothermic peak at 833.6 • C in the DSC curve. The results revealed that the solid-phase reaction proceeded in the temperature range of 800-900 • C, the weight loss was possibility attributed to the reaction of Equation (5) and released CO 2 gas. In addition, the XRD results in Figure 7b indicated that almost no diffraction peak of Na 2 SnO 3 can be found, which illustrates that Na 2 SnO 3 in the raw materials is converted to Na 8 SnSi 6 O 18 completely, as shown in Equation (5). On the other hand, the results in Figure 8 show totally different outcomes. No exother-mic reactions were found in the TG-DSC curve (in Figure 8a), and the roasted products were unchanged as SnO 2 and Na 2 SiO 3 (in Figure 8b), which excluded the reaction paths expressed in Equation (6).

Reactions between Na 8 SnSi 6 O 18 and Na 2 CO 3
To find a possible transition process of Na 8 SnSi 6 O 18 during the roasting process, Na 8 SnSi 6 O 18 was synthesized based on our previous study [21]. In this section, Na 2 CO 3 : SnO 2 : SiO 2 were mixed as mole ratio of 4:1:6, with a roasting temperature of 1000 • C and roasting time of 360 min. The XRD pattern of synthetic Na 8 SnSi 6 O 18 is shown in Figure 9a. The synthesized Na 8 SnSi 6 O 18 was well matched with the PDF standard card of No. 85-0532, and there were no diffraction peaks of impurities. The TG-DSC analysis of Na 8 SnSi 6 O 18 is given in Figure 9b. Na 8 SnSi 6 O 18 was stable during the heating process, while a phase transition occurred in the temperature range of 800-850 • C with an endothermic peak at 825.6 • C in the DSC curve. A further test to determine the melting behavior of Na 8 SnSi 6 O 18 using in situ high temperature thermal analysis is shown in Figure 9b. The results showed that the structure started to change when the temperature reached 800 • C, and a small amount of liquid was formed at this moment. The sample was almost fully molten into the liquid phase as the temperature increased to 825 • C. The results verified the endothermic peak in the DCS curve and corresponded to the melting point of Na 8  0532, and there were no diffraction peaks of impurities. The TG-DSC analysis of Na8SnSi6O18 is given in Figure 9b. Na8SnSi6O18 was stable during the heating process, while a phase transition occurred in the temperature range of 800-850 °C with an endothermic peak at 825.6 °C in the DSC curve. A further test to determine the melting behavior of Na8SnSi6O18 using in situ high temperature thermal analysis is shown in Figure 9b. The results showed that the structure started to change when the temperature reached 800 °C, and a small amount of liquid was formed at this moment. The sample was almost fully molten into the liquid phase as the temperature increased to 825 °C. The results verified the endothermic peak in the DCS curve and corresponded to the melting point of Na8SnSi6O18. Based on the above results, it was found that the mole ratio of Na in Na8SnSi6O18 was much lower than that of Na2SiO3/Na2SnO3, as shown in Equations (2) and (4). The reactions between Na8SnSi6O18 and Na2CO3 were discussed in the case of excess Na2CO3 dosage. Then, Na8SnSi6O18 and Na2CO3 were mixed at a mole ratio of 1:5, and TG-DSC analysis was conducted. The TG-DSC results and the XRD patterns of the roasted products are shown in Figure 10. According to Figure 10a, obvious weight loss started from 800 °C to 950 °C, and two endothermic peaks in the DSC curve were found at 823 °C and 851 °C. The melting point of Na2CO3 was 851 °C, and it can be inferred that the mass loss was attributed to Na2CO3 Based on the above results, it was found that the mole ratio of Na in Na 8 SnSi 6 O 18 was much lower than that of Na 2 SiO 3 /Na 2 SnO 3 , as shown in Equations (2) and (4). The reactions between Na 8 SnSi 6 O 18 and Na 2 CO 3 were discussed in the case of excess Na 2 CO 3 dosage. Then, Na 8 SnSi 6 O 18 and Na 2 CO 3 were mixed at a mole ratio of 1:5, and TG-DSC analysis was conducted. The TG-DSC results and the XRD patterns of the roasted products are shown in Figure 10.
According to Figure 10a, obvious weight loss started from 800 • C to 950 • C, and two endothermic peaks in the DSC curve were found at 823 • C and 851 • C. The melting point of Na 2 CO 3 was 851 • C, and it can be inferred that the mass loss was attributed to Na 2 CO 3 decomposition in the presence of Na 8 SnSi 6 O 18 . It is noteworthy that, as found in Figure 10b, that Na 2 SiO 3 and Na 2 SnO 3 were the main phases in the final TG products, and no diffraction peak for Na 8 SnSi 6 O 18 remained, which indicated that Na 8 SnSi 6 O 18 easily reacted with excess Na 2 SnO 3 . A possible reaction was proposed, as shown in Equation (7), based on conservation of mass. Na 8 SnSi 6 O 18 + 3Na 2 CO 3 = 6Na 2 SiO 3 + Na 2 SnO 3 + 3CO 2 (7) Figure 9. Properties of synthetic Na8SnSi6O18 ((a)-XRD patterns, (b)-TG-DSC analysis).
Based on the above results, it was found that the mole ratio of Na in Na8SnSi6O18 was much lower than that of Na2SiO3/Na2SnO3, as shown in Equations (2) and (4). The reactions between Na8SnSi6O18 and Na2CO3 were discussed in the case of excess Na2CO3 dosage. Then, Na8SnSi6O18 and Na2CO3 were mixed at a mole ratio of 1:5, and TG-DSC analysis was conducted. The TG-DSC results and the XRD patterns of the roasted products are shown in Figure 10. According to Figure 10a, obvious weight loss started from 800 °C to 950 °C, and two endothermic peaks in the DSC curve were found at 823 °C and 851 °C. The melting point of Na2CO3 was 851 °C, and it can be inferred that the mass loss was attributed to Na2CO3 To verify the above analysis, the effect of roasting temperature and roasting time on the phase transformation of Na 8 SnSi 6 O 18 was investigated, and the Na 2 CO 3 /Na 8 SnSi 6 O 18 mole ratio was fixed at 3:1, as in Equation (6). Figure 11 shows the XRD patterns of the Na 2 CO 3 /Na 8 SnSi 6 O 18 mixed samples roasted in the temperature range of 800-900 • C with a time of 30-120 min.
Materials 2022, 15, x FOR PEER REVIEW 11 of 14 decomposition in the presence of Na8SnSi6O18. It is noteworthy that, as found in Figure  10b, that Na2SiO3 and Na2SnO3 were the main phases in the final TG products, and no diffraction peak for Na8SnSi6O18 remained, which indicated that Na8SnSi6O18 easily reacted with excess Na2SnO3. A possible reaction was proposed, as shown in Equation (7), based on conservation of mass.
Na8SnSi6O18 + 3Na2CO3 = 6Na2SiO3 + Na2SnO3 + 3CO2 To verify the above analysis, the effect of roasting temperature and roasting time on the phase transformation of Na8SnSi6O18 was investigated, and the Na2CO3/Na8SnSi6O18 mole ratio was fixed at 3:1, as in Equation (6). Figure 11 shows the XRD patterns of the Na2CO3/Na8SnSi6O18 mixed samples roasted in the temperature range of 800-900 °C with a time of 30-120 min. Figure 11. Effect of roasting temperature and time on the reaction between Na8SnSi6O18 and Na2CO3.
As shown in Figure 11a, the main phase in the roasted products was unchanged as Na8SnSi6O18 at 800 °C, while the diffraction peaks of Na2SiO3 and Na2SnO3 were uncertain. The structural diffraction peaks of Na8SnSi6O18 weakened and then vanished when the temperature exceeded 850 °C. Figure 11b shows that the diffraction peaks of Na2SiO3 appeared and Na8SnSi6O18 decreased at 30 min, and then the peaks of Na8SnSi6O18 gradually decreased and disappeared as the roasting time was prolonged to 90 min and 120 min. Na2SiO3 and Na2SnO3 were the final roasted products expressed as Equation (7).

Discussion on the Reaction Mechanism of the SnO2-SiO2-Na2CO3 System
During the soda-roasting process of cassiterite concentrates, the overriding aim was to synchronously promote the transformation of stubborn minerals (SnO2 and SiO2) into freely soluble materials (Na2SnO3 and Na2SiO3). However, Na8SnSi6O18 was inevitably generated during the roasting process, which was almost insoluble in the leaching process and markedly decreased the recovery of Sn. Based on the above results and our previous studies, the reaction mechanism of the SnO2-SiO2-Na2CO3 system and the formation of Na8SnSi6O18 can be summarized as follows in Figure 12. As shown in Figure 11a, the main phase in the roasted products was unchanged as Na 8 SnSi 6 O 18 at 800 • C, while the diffraction peaks of Na 2 SiO 3 and Na 2 SnO 3 were uncertain. The structural diffraction peaks of Na 8 SnSi 6 O 18 weakened and then vanished when the temperature exceeded 850 • C. Figure 11b shows that the diffraction peaks of Na 2 SiO 3 appeared and Na 8 SnSi 6 O 18 decreased at 30 min, and then the peaks of Na 8 SnSi 6 O 18 gradually decreased and disappeared as the roasting time was prolonged to 90 min and 120 min. Na 2 SiO 3 and Na 2 SnO 3 were the final roasted products expressed as Equation (7).

Discussion on the Reaction Mechanism of the SnO 2 -SiO 2 -Na 2 CO 3 System
During the soda-roasting process of cassiterite concentrates, the overriding aim was to synchronously promote the transformation of stubborn minerals (SnO 2 and SiO 2 ) into freely soluble materials (Na 2 SnO 3 and Na 2 SiO 3 ). However, Na 8 SnSi 6 O 18 was inevitably generated during the roasting process, which was almost insoluble in the leaching process and markedly decreased the recovery of Sn. Based on the above results and our previous studies, the reaction mechanism of the SnO 2 -SiO 2 -Na 2 CO 3 system and the formation of Na 8 SnSi 6 O 18 can be summarized as follows in Figure 12. With excess NaCO 3 Figure 12. Phase evolution in the SnO2-SiO2-Na2CO3 system during soda roasting process.
First, SnO2 reacted with Na2CO3 to form Na2SnO3 as shown in Equation (2). Mea while, part of SiO2 also reacted with Na2CO3 to form a small amount of Na2SiO3, see Equ tion (4). Nonetheless, the reaction rate was much lower than that of Equation (2). Th Na2SnO3 reacted immediately with Na2CO3 and SiO2 to form Na8SnSi6O18 as shown Equation (5), and Na2SnO3 was the key intermediate during the formation of Na8SnSi6O while the reaction between Na2SiO3 and SnO2 was impossible. The melting point Na8SnSi6O18 was measured at 825 °C, which was much lower than that of other materi in the SnO2-SiO2-Na2CO3 system. Thus, Na8SnSi6O18 was always formed accompanied SnO2 and invariably closely wrapped around SnO2 particles, which blocked the cont between SnO2 and Na2CO3. Therefore, the formation of Na2SnO3 was significantly inh ited once Na8SnSi6O18 was present. In addition, Na8SnSi6O18 is an unstable compound th can react with excess Na2CO3 (Equation (7)) as the roasting temperature and time increa

Conclusions
During the process of sodium stannate preparation from cassiterite concentrate u der a CO-CO2 atmosphere, the formation of Na8SnSi6O18 in the SnO2-SiO2-Na2CO3 syste affects the quality of sodium stannate products. To solve this problem, the effects Na8SnSi6O18 formation on the product quality were investigated in this study, and the f lowing conclusions were obtained: 1. The reactions between Na2CO3 and SnO2/SiO2 proceeded simultaneously during t roasting process, while the formation of Na2SnO3 was promoted under a CO-C atmosphere. Then, Na8SnSi6O18 was easily formed once Na2SnO3 appeared; noneth less, the reaction between Na2SiO3 and SnO2 was impossible; 2. The melting point of Na8SnSi6O18 is only 825 °C, which is much lower than that Na2CO3, Na2SnO3 and Na2SiO3 in the SnO2-SiO2-Na2CO3 system. Na8SnSi6O18 clos wrapped around the SnO2 particles and restrained the reaction between SnO2 a Na2CO3; 3. Na8SnSi6O18 is an unstable compound, and the reaction between Na8SnSi6O18-Na2C can proceed as Na8SnSi6O18 + 3Na2CO3 = 6Na2SiO3 + Na2SnO3 + 3CO2. The reacti was controlled by higher temperatures of above 800 °C as the roasting time was p longed. First, SnO 2 reacted with Na 2 CO 3 to form Na 2 SnO 3 as shown in Equation (2). Meanwhile, part of SiO 2 also reacted with Na 2 CO 3 to form a small amount of Na 2 SiO 3 , see Equation (4). Nonetheless, the reaction rate was much lower than that of Equation (2). Then, Na 2 SnO 3 reacted immediately with Na 2 CO 3 and SiO 2 to form Na 8 SnSi 6 O 18 as shown in Equation (5), and Na 2 SnO 3 was the key intermediate during the formation of Na 8 SnSi 6 O 18 , while the reaction between Na 2 SiO 3 and SnO 2 was impossible. The melting point of Na 8 SnSi 6 O 18 was measured at 825 • C, which was much lower than that of other materials in the SnO 2 -SiO 2 -Na 2 CO 3 system. Thus, Na 8 SnSi 6 O 18 was always formed accompanied by SnO 2 and invariably closely wrapped around SnO 2 particles, which blocked the contact between SnO 2 and Na 2 CO 3 . Therefore, the formation of Na 2 SnO 3 was significantly inhibited once Na 8 SnSi 6 O 18 was present. In addition, Na 8 SnSi 6 O 18 is an unstable compound that can react with excess Na 2 CO 3 (Equation (7)) as the roasting temperature and time increase.

Conclusions
During the process of sodium stannate preparation from cassiterite concentrate under a CO-CO 2 atmosphere, the formation of Na 8 SnSi 6 O 18 in the SnO 2 -SiO 2 -Na 2 CO 3 system affects the quality of sodium stannate products. To solve this problem, the effects of Na 8 SnSi 6 O 18 formation on the product quality were investigated in this study, and the following conclusions were obtained: 1.
The reactions between Na 2 CO 3 and SnO 2 /SiO 2 proceeded simultaneously during the roasting process, while the formation of Na 2 SnO 3 was promoted under a CO-CO 2 atmosphere. Then, Na 8 SnSi 6 O 18 was easily formed once Na 2 SnO 3 appeared; nonetheless, the reaction between Na 2 SiO 3 and SnO 2 was impossible; 2.
The melting point of Na 8 SnSi 6 O 18 is only 825 • C, which is much lower than that of Na 2 CO 3 , Na 2 SnO 3 and Na 2 SiO 3 in the SnO 2 -SiO 2 -Na 2 CO 3 system. Na 8 SnSi 6 O 18 closely wrapped around the SnO 2 particles and restrained the reaction between SnO 2 and Na 2 CO 3 ; 3. Na 8 SnSi 6 O 18 is an unstable compound, and the reaction between Na 8 SnSi 6 O 18 -Na 2 CO 3 can proceed as Na 8 SnSi 6 O 18 + 3Na 2 CO 3 = 6Na 2 SiO 3 + Na 2 SnO 3 + 3CO 2 . The reaction was controlled by higher temperatures of above 800 • C as the roasting time was prolonged.  Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.