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

Abstract

To produce Na₂SnO_3, 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.

1. Introduction

Na₂SnO₃ 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 Na₂SnO₃ was prepared efficiently and cleanly [6]. Previous studies have showed that the solid-state reactions between SnO₂ and Na₂CO₃ are accelerated under a CO–CO₂ 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 (SnO₂) is the primary source of tin. It is naturally formed by magmatic-hydrothermal 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 SiO₂ in tin concentrates being as high as 8 wt.% [19,20].

Cassiterite concentrate, as reported, seldom reacts with soda (Na₂CO₃) under air atmospheres. However, the authors’ group found in previous research that cassiterite (or SnO₂) could readily react with Na₂CO₃ under an appropriate CO–CO₂ atmosphere. In the roasting process, the CO gas molecules were firstly adsorbed on the SnO₂ 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 Na₂O, which was the decomposition product of Na₂CO₃ roasted over 851 °C. These processes accelerated the formation of Na₂SnO₃ [6,7,8,9,10].

Our previous studies have found that the quartz (SiO₂) in the raw material has a significant effect on the phase transformation of SnO₂ during the soda roasting process [21]. The target products of soda roasting were Na₂SnO₃ and Na₂SiO₃, which are freely soluble in NaOH solution. It was found that Na₈SnSi₆O₁₈ 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 SnO₂–SiO₂, which was investigated during the cassiterite reduction smelting process and flat glassmaking method [22,23,24]. Furthermore, those studies confirmed that SnO₂ 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 SnO₂–SiO₂–Na₂CO₃ system, especially under a CO–CO₂ atmosphere.

Based on our previous studies, SiO₂ 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 Na₈SnSi₆O₁₈ were unknown. Hence, in order to improve the Sn conversion rate during the soda roasting process, the formation mechanism and decomposition process of Na₈SnSi₆O₁₈ in the SnO₂–SiO₂–Na₂CO₃ 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.

2. Experimental

2.1. 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 (SnO₂) and quartz (SiO₂) 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 SnO₂, SiO₂, Na₂CO₃, Na₂SiO₃ and Na₂SnO₃·3H₂O (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, CO₂ and N₂ had a purity of 99.9 vol%.

2.2. Methods

2.2.1. Experimental Procedures

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 CO₂) atmosphere have been described in our previous study [6,21]. The leaching of Na₂SnO₃ 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 Na₂SnO₃. The residues were washed with distilled water to identify the phase constituents as follows:

$$L=\frac{1000\mathrm{CV}}{MW}\times 100\%$$

(1)

where L is the formation efficiency of Na₂SnO₃, 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).

2.2.2. Instrument Techniques

The phase constituents of the samples were identified by X-ray diffraction XRD (Cu-target 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 H₂SO₄–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).

3. Results and Discussion

3.1. Phase Analysis for the Products of Soda Roasting

Figure 2 shows the experimental flowsheet of soda roasting and leaching using Si-bearing 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 irregularly at the grain edge of Na₂SnO₃. 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 Na₈SnSi₆O₁₈. The morphology of leaching residues is shown in Figure 3d. The results indicated that the Na₈SnSi₆O₁₈ phase (Spot C in Figure 3d) was closely wrapped in cassiterite particles. Both SnO₂ and Na₈SnSi₆O₁₈ were insoluble during the leaching process, and then they were enriched in the leaching residues. The results in Figure 3 indicate that Na₈SnSi₆O₁₈ was rapidly formed during the roasting process, and then the melt wrapped on the surface of SnO₂ and Na₂CO₃, which restrained the formation of Na₂SnO₃. It is concluded that Na₈SnSi₆O₁₈ has a negative impact on the formation of sodium stannate from cassiterite. Next, the formation mechanism of Na₈SnSi₆O₁₈ is discussed.

3.2. Effect of Roasting Atmospheres on the Formation of Na₈SnSi₆O₁₈

To investigate the formation mechanism of Na₈SnSi₆O₁₈ in the SnO₂–SiO₂–Na₂CO₃ system, AR reagents of SnO₂, SiO₂ and Na₂CO₃ were mixed at a mole ratio of Na₈SnSi₆O₁₈. The XRD patterns of the samples roasted at 875 °C under a 15 vol.% CO–CO₂ 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.

Na₂CO₃ + SnO₂ = Na₂SnO₃ + CO₂

(2)

SnO₂ + 6SiO₂ + 4Na₂CO₃ = Na₈SnSi₆O₁₈ + 4CO₂

(3)

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.

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.

3.3. 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.

Na₂CO₃ + SiO₂ = Na₂SiO₃ + CO₂

(4)

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 Equation (5), and Na₂SiO₃·3H₂O and SnO₂ 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 Na₂SnO₃–SiO₂ and Na₂SiO₃–SnO₂, and the results are displayed in Figure 7 and Figure 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).

3.4. 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: SnO₂: SiO₂ 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 Na₈SnSi₆O₁₈ is shown in Figure 9a. The synthesized Na₈SnSi₆O₁₈ 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 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 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

(7)

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.

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).

3.5. 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.

First, SnO₂ reacted with Na₂CO₃ to form Na₂SnO₃ as shown in Equation (2). Meanwhile, part of SiO₂ also reacted with Na₂CO₃ to form a small amount of Na₂SiO₃, see Equation (4). Nonetheless, the reaction rate was much lower than that of Equation (2). Then, Na₂SnO₃ reacted immediately with Na₂CO₃ and SiO₂ to form Na₈SnSi₆O₁₈ as shown in Equation (5), and Na₂SnO₃ was the key intermediate during the formation of Na₈SnSi₆O₁₈, while the reaction between Na₂SiO₃ and SnO₂ was impossible. The melting point of Na₈SnSi₆O₁₈ was measured at 825 °C, which was much lower than that of other materials in the SnO₂–SiO₂–Na₂CO₃ system. Thus, Na₈SnSi₆O₁₈ was always formed accompanied by SnO₂ and invariably closely wrapped around SnO₂ particles, which blocked the contact between SnO₂ and Na₂CO₃. Therefore, the formation of Na₂SnO₃ was significantly inhibited once Na₈SnSi₆O₁₈ was present. In addition, Na₈SnSi₆O₁₈ is an unstable compound that can react with excess Na₂CO₃ (Equation (7)) as the roasting temperature and time increase.

4. Conclusions

During the process of sodium stannate preparation from cassiterite concentrate under a CO–CO₂ atmosphere, the formation of Na₈SnSi₆O₁₈ in the SnO₂–SiO₂–Na₂CO₃ system affects the quality of sodium stannate products. To solve this problem, the effects of Na₈SnSi₆O₁₈ formation on the product quality were investigated in this study, and the following conclusions were obtained:

• The reactions between Na₂CO₃ and SnO₂/SiO₂ proceeded simultaneously during the roasting process, while the formation of Na₂SnO₃ was promoted under a CO–CO₂ atmosphere. Then, Na₈SnSi₆O₁₈ was easily formed once Na₂SnO₃ appeared; nonetheless, the reaction between Na₂SiO₃ and SnO₂ was impossible;
• The melting point of Na₈SnSi₆O₁₈ is only 825 °C, which is much lower than that of Na₂CO₃, Na₂SnO₃ and Na₂SiO₃ in the SnO₂–SiO₂–Na₂CO₃ system. Na₈SnSi₆O₁₈ closely wrapped around the SnO₂ particles and restrained the reaction between SnO₂ and Na₂CO₃;
• Na₈SnSi₆O₁₈ is an unstable compound, and the reaction between Na₈SnSi₆O₁₈–Na₂CO₃ can proceed as Na₈SnSi₆O₁₈ + 3Na₂CO₃ = 6Na₂SiO₃ + Na₂SnO₃ + 3CO₂. The reaction was controlled by higher temperatures of above 800 °C as the roasting time was prolonged.