Thermodynamic Analysis of Ca-Mg-Al-based Refractory Resistance to Na₂CO₃ Corrosion

Papermaking black liquor contains Na₂CO₃, which can corrode refractory materials and cause economic losses. It is considered to introduce CaO and MgO alkaline oxides into Al₂O₃ to prepare calcium magnesium aluminum composite oxides as a substitute for Al₂O₃ as corrosion shell materials. Using the FactSage material balance module, the optimal ratio of CaO-MgO-Al₂O₃ was calculated, and it was found that the Gibbs free energy of C₂M₂A₁₄ reacting with Na₂CO₃ at 800-1200 ℃ was positive. C₂M₂A₁₄ was selected as the optimal ratio of calcium-magnesium aluminum composite oxide to resist Na₂CO₃.

The alkali recovery method for treating papermaking black liquor can recycle Na₂CO₃, realizing the reuse of resources [1-4]. When burning in the furnace, sodium salts will adhere to the surface of the refractory materials, and undergo cladding and penetration, causing corrosion and spalling of the refractory materials [5-7]. Al₂O₃ has stable performance and has high hardness and strength. SiO₂ has strong resistance to molten slag erosion, and MgO has the characteristic of resistance to alkali molten slag erosion [8-10]. FactSage can be used for the simulation and calculation of corrosion reactions of refractory materials and molten salts [11,12]. This paper focuses on the research of the corrosion resistance of Ca-Mg-Al-based materials against Na₂CO₃.

The corrosion and penetration mechanism of Na₂CO₃

The vapor of Na₂CO₃ in the alkali recovery furnace at 1200 °C penetrates the pores of the refractory material, resulting in the loss and thinning. The vapor of Na₂CO₃ reacts with the refractory material to form new products, and the thermal expansion and contraction properties of these new products are different from those of the original refractory material. This leads to changes in the internal stress, causing cracks to appear [13,14], as shown in Figure 1.

Figure 1: Schematic diagram of the corrosion principle of refractory materials.

Thermodynamic Simulation of the Reaction between Na₂CO₃ and Al₂O₃/SiO₂/ MgO/CaO

As shown in Figure 2, the Gibbs free energy of the reactions between Al₂O₃, SiO₂, and Na₂CO₃ is negative. In contrast, the Gibbs free energy of the reactions between MgO, CaO, and Na₂CO₃ is positive. Use the FactSage 7.3 software, select the databases of Fact PS, FToxid, and FTsalt, and perform the calculation assuming a pressure of one atmosphere, it is found that for the reaction between Al₂O₃ and Na₂CO₃, the reaction intensifies as the temperature rises. At 800 °C, approximately 99.9% of Al₂O₃ is converted into NaAlO₂. At 1200 °C, there is a tendency for approximately 98.6% of NaAlO₂ to transform into Na₂Al₁₂O₁₉. Under the same reaction conditions, approximately 99.9% of the SiO₂ is converted into Na₂SiO₃.

Figure 2: Gibbs free energy of the reaction between Na₂CO₃ and four kinds of refractory materials.

Simulation of calcium-magnesium-aluminum composite oxide

Simulation and screening of composite oxides: In Figure 3, the substances generated by different ratios of MgO/Al₂O₃/CaO at different temperatures are Ca₃MgAl₄O₁₀, CaMg₂Al₁₆O₂₇, and Ca₂Mg₂Al₂₈O₄₆, all of which are derived from the magnetoplumbite crystal structure of CaAl₁₂O₁₉ [14]. Ca₂Mg₂Al₂₈O₄₆ is a high-melting-point compound in the sub-solid phase of the Al₂O₃-rich part of the Al₂O₃-MgO-CaO ternary system. The solid solution effect of Mg²⁺ promotes the growth of the crystal along the c-axis direction, and the crystals exhibit hexagonal plate-like and hexagonal prismatic morphologies respectively. Its reaction products with Na₂CO₃ under air and water vapor conditions are mainly CO₂, Na₂Al₁₂O₁₉(s), Na₂Ca₃Al₁₆O₂₈(s), NaAlO₂(s) and NaOH(g).

Figure 3: High-temperature Ternary Phase Diagram of MgO-Al₂O₃-CaO.

The influence of oxide additives on the properties of refractory materials: As shown in Figure 4, at a temperature range of 800-1200 °C, when Ca₂Mg₂Al₂₈O₄₆ is added with CuO and reacts with Na₂CO₃, the Gibbs free energy is positive. This is because after the addition of CuO, the Gibbs free energy required for the reaction between Ca₂Mg₂Al₂₈O₄₆ and Na₂CO₃ to form other products increases. When Ca₂Mg₂Al₂₈O₄₆ is added with Fe₂O₃, MnO₂, TiO₂ and reacts with Na₂CO₃, the Gibbs free energy is negative. Therefore, when choosing additives for Ca₂Mg₂Al₂₈O₄₆, CuO can be selected.

Figure 4: Effect of Additives on the Gibbs free energy of the Reaction between Ca₂Mg₂Al₂₈O₄₆ and Na₂CO₃.

Conclusion

According to the simulation calculation, the calcium-magnesium-aluminum composite oxide Ca₂Mg₂Al₂₈O₄₆ has the best resistance effect against Na₂CO₃. When the additive CuO is added, the Gibbs free energy of the reaction with Na₂CO₃ is positive. Therefore, CuO can be selected as the additive for Ca₂Mg₂Al₂₈O₄₆ to enhance the performance of the material.

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