Experimental Study on SiC Composition in Refractory Products

Sample decomposition method selection is crucial for accurate determination of silicon carbide (SiC). According to the properties of SiC, it is stable and insoluble in hydrofluoric acid, sulfuric acid, and other acids, but can be dissolved in molten alkali. In this study, two different melting reagents were used: Na₂O + KOH and KNO₃ + KOH, to decompose the sample. After leaching with hot water and acidification using hydrochloric and nitric acid, an insoluble residue was observed, indicating that these reagents were ineffective for complete decomposition of SiC. Therefore, the potassium fluorosilicate volumetric method could not be applied. Based on the decomposition capability of the mixed solvents, a high-temperature melting approach was selected. The decomposition temperature was set at 1050°C and 1100°C, and both gave consistent results. This paper uses 1050°C. During the process, samples were observed every 10 minutes. It was found that after 30 minutes, the sample became clear, indicating full decomposition. Thus, 30 minutes was chosen as the optimal time.

Regarding leaching acidity, silicic acid tends to polymerize easily, so the leaching acidity should be kept below 2 mol/L. In this study, the leaching procedure adopted from the SiO₂ determination method in ores was used—leaching with 100 mL of (1+6) hydrochloric acid. For the chromogenic reaction of the silicon molybdenum heteropoly acid, the acidity is typically maintained at C(H⁺) = 0.1–0.6 mol/L. To ensure consistency, a reagent blank solution was used to regulate and control the acidity during color development.

Interference from other elements was also considered. Besides SiC, refractory products containing SiC mainly consist of SiO₂, Al₂O₃, CaO, MgO, Fe₂O₃, TiO₂, MnO, Cr₂O₃, P₂O₅, and free carbon. SiO₂ can be removed by volatilization with hydrofluoric acid, while free carbon can be burned off in a high-temperature furnace. Non-ferrous metal oxides like Cr₂O₃ can be eliminated through reference preparation. P₂O₅ and Fe₂O₃ can be masked using oxalic acid. However, this masking technique is not suitable for phosphate-bonded refractories. Other components did not interfere significantly with the analysis.

The reliability of the results was assessed without the use of standard samples. Pure SiC results only reflect recovery rates, and it is difficult to synthesize samples that exactly match real samples. Therefore, a gravimetric method was employed, which does not require a standard sample. The sample was decomposed using the method described in this paper, and the results were compared with those obtained using the perchloric acid dehydration method. The proposed method is applicable for determining SiC in SiC-based materials and has a wide range of applications. It can accurately analyze SiC content in refractory products containing 1% or more of SiC. For samples with SiC content above 30%, the allowable difference can be controlled below 0.3%. When using the differential photometric method for SiC contents greater than 30%, the sample weight and fractionation rate remain unaffected, and the allowable difference can be kept within 0.5%. The method is simple, efficient, and less prone to interference. Additionally, the sample solution prepared using this method can also be used for the determination of Al₂O₃, CaO, MgO, and Fe₂O₃, making it a versatile analytical approach.

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