Analysis and Prediction of Corrosion of Refractory Materials by Sodium Salts during Waste Liquid Incineration—Thermodynamic Study

Abstract

Incineration of high-content sodium salt organic waste liquid will corrode the refractory material in the incinerator, causing the refractory material to peel off and be damaged. A thermodynamics method was used to study the thermodynamic properties of three common sodium salts (NaCl, Na₂CO₃ and Na₂SO₄) on the corrosion of refractory materials (MgO·Cr₂O₃, MgO·Al₂O₃, Al₂O₃, MgO and Cr₂O₃). The results determined that MgO has the best corrosion resistance and is not corroded by the three sodium salts. On this basis, the thermodynamic corrosion experiments of NaCl corrosion of magnesium oxide at three temperatures of 600, 1000 and 1200 °C were carried out. Analysis of the corrosion product by X-ray diffraction (XRD) showed no corrosion product formation. Studies have shown that thermodynamic calculation can accurately predict the thermodynamic mechanism of alkali metal corrosion to refractory materials, and MgO is a good anti-alkali metal corrosion refractory material.

1. Introduction

The rapid development of the petrochemical industry has led to an increase in emissions of high-concentration organic waste liquids. This type of wastewater is not only difficult to be decomposed by microorganisms, but also has the potential to induce genetic mutations and toxicity to humans and animals. Incineration is a commonly used method for treating organic waste liquids in industry, especially for some waste liquids with high concentration, complex composition and high heat value. However, the organic waste liquid contains alkali metal salts such as Na₂SO₄, KCl, NaCl, etc., and the acidic waste water is usually neutralized by adding alkaline substances (such as KOH, NaOH) before incineration to reduce the corrosion of the pump and the pipeline by the acidic wastewater, bringing more alkali metal salts to the organic waste liquid. Most alkali metal salts have a low melting point, causing severe corrosion to the refractory layer of the incinerator, affecting the service life of the refractory, and causing great safety hazards and economic losses to the operation of the incinerator [1].

With the advancement of society and the development of science and technology, the performance requirements of refractory materials are getting higher and higher. Many researchers have studied and improved the corrosion resistance of refractories [2,3,4,5,6], thermal shock resistance [7,8,9], and anti-wear properties [10]. M. Yoshikawa et al. [11] studied the erosion resistance of MgO-based spinel and Al₂O₃-based spinel refractories and found that these two refractories can replace Al₂O₃-Cr₂O₃ refractories with low chromium content and are more suitable for alkali furnaces. P. Prigent et al. [1] studied the sodium corrosion resistance of three types of silica-alumina refractories (andalusite, mullite, and refractory clay) and compared the microstructure of the refractory surface before and after NaF vapour corrosion. J. Moda et al. [12] added MgO and NiO to an alumina refractory to realize Al₂O₃-MgO-NiO refractories and found that a (Mg, Ni)O solid solution was formed in the material and that its ability to resist slag erosion was improved. R. D. Fan et al. [13] found that alumina chrome slag can replace alumina to prepare Al₂O₃-SiC-C troughe castables, which remarkably improved oxidation resistance of the sample, but reduced its thermo-mechanical properties and anti-slag corrosion performance. Some researchers have paid attention to the effect of additives on the corrosion resistance of refractory materials. K. Igabo et al. [14] added ZrO₂ to MgO-Al₂O₃ refractories and studied the effect of the addition on the performance of the refractories. P. Gehre et al. [15] reported that adding spinel materials, spinel-rich cements, and calcium aluminate cement to the spinel-containing alumina lining of steel ladles could significantly improve the slag corrosion resistance.

There are many studies concerning the performance of refractories used in the steelmaking process [16,17,18,19], but the thermodynamic research on sodium salt corrosion of refractory materials is limited. D. Gregurek et al. [20] used Factsage software to carry out thermodynamic simulation on the corrosion of refractory lining caused by slag and CuO. The research results can provide a theoretical basis for the wear of refractories in copper anode furnaces. J. Stjernberg et al. [21] used XRD, electron microscopy and spectroscopy (Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN) and Scanning Electron Microscopy (SEM)) characterization and thermodynamic kinetics to study the erosion of mullite based refractory bricks by alkali metals. R. Huang et al. [22] calculated the erosion of four refractory materials (i.e., magnesia carbon brick, burned magnesite brick, SiC castable, and corundum castable) by titanium slag using FactSage software. The results found that silicon carbide castable has the best corrosion resistance among the four refractories, because it can interact with Ti slag to form TiC with a high melting point, which can prevent Ti slag from penetrating more deeply into refractories.

Based on the thermodynamic study of refractory corrosion, this paper compares the corrosion resistance of five refractories under different sodium salts, providing a theoretical basis and data support for further research on alkali metal corrosion resistance of refractories.

2. Thermodynamic Calculations

Thermodynamic simulation is based on the version 7.2 of FactSage^TM software for thermodynamic equilibrium calculations. In this paper, the components of refractory materials such as MgO·Cr₂O₃, MgO·Al₂O₃, Al₂O₃, MgO and Cr₂O₃ are selected as research objects. The corrosion mechanism of Na₂CO₃, Na₂SO₄ and NaCl on refractory materials at different temperatures (600 °C–1200 °C with a temperature step of 100 °C) was investigated. The refractory materials components of MgO·Cr₂O₃, MgO·Al₂O₃, Al₂O₃, MgO and Cr₂O₃ and 3-sodium salts of 1mol were input in the table of the FactSage^TM software for the calculation. In this paper, the Equilib module of FactSage^TM 7.2 software was used, which is suitable for calculating the concentration of various species when the reaction of a given element or compound reaches chemical equilibrium.

3. Thermodynamic Experimental Procedure

Similar to our previous work [23], the experiment was conducted in a high-temperature tube furnace (Figure 1). A 1:1 molar ratio of NaCl and MgO was put into a corundum boat and full mechanical mixing was carried out. The rest of experimental and data analysis procedures were all the same as previously reported [23].

4. Results and Discussion

4.1. Corrosion Effect of Na₂CO₃ on Refractory Materials

Figure 2 illustrates that the Gibbs free energy values for the reactions of Na₂CO₃ with five refractory materials are all negative. As the temperature increases, the Gibbs free energy value decreases, indicating that with the increase in temperature, the corrosion tendency of refractories by Na₂CO₃ becomes more obvious. It can also be obtained from the figure that the order of the tendency of the five refractories to be corroded by Na₂CO₃ is MgO·Al₂O₃ > MgO·Cr₂O₃ > Al₂O₃ > Cr₂O₃ > MgO.

In general, MgO-based refractory materials are basically free from corrosion reaction in the temperature range below 1200 °C, because MgO ions carry two charges and the radii of oxygen ions and magnesium ions are relatively small, which causes magnesium oxide to have large lattice energy, a high melting point and stable properties [24].

The corrosion products of refractory materials corroded by sodium carbonate and the corrosion reaction equations deduced therefrom are shown in

Table 1. Predicted corrosion products and reaction equations of Na₂CO₃ on refractory materials at 600–1200 °C.

Species	Temperature (°C)
	600	700	800	900–1200
Al2O3	0.16667 mol Na2Al12O19	2 mol NaAlO2
	Equation (1)	Equation (2)
MgO	-
Cr2O3	1 mol Na2CrO4
	Equation (4)
MgO·Cr2O3	1 mol Na2CrO4, 0.5mol MgO
	Equation (5)
MgO·Al2O3	0.00962 mol NaAlO2, 0.00481 mol MgO	0.08292 mol NaAlO2, 0.04146 mol MgO	0.53843 mol NaAlO2, 0.26922 mol MgO	2 mol NaAlO2, 1 mol MgO
	Equation (3)	Equation (3)	Equation (3)	Equation (3)

“-”: The amount of corrosion products is less than 10⁻⁴ mol.

shows that, in addition to MgO, other refractories are corroded by sodium carbonate at 600 °C.

The reaction products and reaction degree of Al₂O₃ and MgO·Al₂O₃ corroded by Na₂CO₃ are related to temperature. Al₂O₃ is more easily corroded by Na₂CO₃, and has been completely corroded at 600 °C to generate 0.16667 mol Na₂Al₁₂O₁₉, and 2 mol of NaAlO₂ at 700–1200 °C. At 600–1100 °C, only a small amount of MgO·Al₂O₃ is corroded to form NaAlO₂ and MgO; when the temperature reaches 1200 °C, MgO·Al₂O₃ is completely corroded. The reaction of Al₂O₃ and MgO·Al₂O₃ by Na₂CO₃ corrosion is as follows:

Na₂CO₃ + 6Al₂O₃ = Na₂Al₁₂O₁₉ + CO₂

(1)

Na₂CO₃ + Al₂O₃ = 2NaAlO₂ + CO₂

(2)

Na₂CO₃ + MgO·Al₂O₃ = 2NaAlO₂ +MgO + CO₂

(3)

In the reaction of Cr₂O₃ and MgO·Cr₂O₃ by Na₂CO₃, Cr³⁺ is oxidized to Cr⁶⁺ due to the presence of O₂, and the corrosion of Cr₂O₃ and MgO·Cr₂O₃ at 600–1200 °C by Na₂CO₃ is:

Na₂CO₃ + 1/2 Cr₂O₃ + 3/4 O₂ = Na₂CrO₄ +CO₂

(4)

Na₂CO₃ + 1/2 MgO·Cr₂O₃ + 3/4 O₂ =1/2 MgO + Na₂CrO₄ + CO₂

(5)

In addition, it can be seen from the table that no corrosion product of Na₂CO₃ corroding MgO is found. However, Na₂CO₃ will decompose at 600–1200 °C, so the Gibbs free energy of the MgO-Na₂CO₃ reaction is negative. To sum up, MgO is hard to be eroded by Na₂CO₃.

4.2. Corrosion Reaction of Na₂SO₄ and Refractory Materials

As shown in Figure 3 and

Table 2. Predicted corrosion products and reaction equations of Na₂SO₄ on refractory materials at 600–1200 °C.

Species	Temperature (°C)
	600–900	1000	1100	1200
Al2O3	-	0.0003 mol NaAl9O14	0.0033mol NaAl9O14	0.031672 mol NaAl9O14
		Equation (6)	Equation (6)	Equation (6)
MgO	-
Cr2O3	-	0.0002 mol Na2CrO4	0.0013 mol Na 2CrO4	0.0073 mol Na 2CrO4
		Equation (7)	Equation (7)	Equation (7)
MgO·Cr2O3	-		0.0001 mol Na2CrO4	0.0008 mol Na2CrO4, 0.0004mol MgO
			Equation (8)	Equation (8)
MgO·Al2O3	-

“-”: The amount of corrosion products is less than 10⁻⁴ mol.

, under 600–1200 °C, neither MgO nor MgO·Al₂O₃ will be corroded by sodium sulfate. A small amount of Al₂O₃ (0.00135 mol) and Cr₂O₃ (0.0001 mol) will be corroded by sodium sulfate at 1000 °C. As the temperature rises to 1200 °C, more Al₂O₃ (0.1425 mol) and Cr₂O₃ (0.00365 mol) will be corroded by sodium sulfate. The reaction of Al₂O₃ and Cr₂O₃ corroded by sodium sulfate is:

Na₂SO₄ + 9Al₂O₃ = 2NaAl₉O₁₄ + SO₃

(6)

Na₂SO₄ + 1/2 Cr₂O₃ + 3/4 O₂ = Na₂CrO₄ + SO₃

(7)

Similarly, a very small amount of MgO·Cr₂O₃ (0.00005 mol) will be corroded by sodium sulfate to form Na₂CrO₄ at 1100 °C With the temperature rises, even if the temperature rises to 1200 °C, as long as 0.0004 mol of MgO·Cr₂O₃ is corroded by sodium sulfate to form Na₂CrO₄, the reaction of MgO·Cr₂O₃ being corroded by sodium sulfate is:

Na₂SO₄ + 1/2 MgO·Cr₂O₃ + 3/4O₂ = Na₂CrO₄ + 1/2MgO + SO₃

(8)

From the above results, it can be seen that the order of corrosion degree of five refractories by Na₂SO₄ from strong to weak is as follows: Cr₂O₃ and Al₂O₃ > MgO·Cr₂O₃ > MgO·Al₂O₃ and MgO.

4.3. Corrosion Reaction of NaCl and Refractory Materials

The results in Figure 4 show that the Gibbs free energy of the refractory material corroded by sodium chloride has a tendency to change with that of the refractory material with sodium carbonate and sodium sulfate at 600–1200 °C. I t can be seen from

Table 3. Predicted corrosion products and reaction equations of NaCl on refractory materials at 600–1200 °C.

Species	Temperature (°C)
	600–800	900	1000	1100	1200
Al2O3	-	0.0001 mol NaAl9O14	0.0003 mol NaAl9O14	0.0008 mol NaAl9O14	0.0019 mol NaAl9O14
		Equation (9)	Equation (9)	Equation (9)	Equation (9)
MgO		-
Cr2O3	-	0.0001 mol Na2CrO4	0.0002 mol Na2CrO4	0.0003 mol Na2CrO4	0.0006 mol Na2CrO4
		Equation (10)	Equation (10)	Equation (10)	Equation (10)
MgO·Cr2O3		-			0.0001 mol Na2CrO4
					Equation (11)
MgO·Al2O3	-

“-”: The amount of corrosion products is less than 10⁻⁴ mol.

that NaCl is less corrosive to refractory materials. Even at 1200 °C, MgO and MgO·Al₂O₃ are not corroded by sodium chloride; only a small amount of Al₂O₃ (0.00855 mol), Cr₂O₃ (0.0003 mol) and MgO·Cr₂O₃ (0.000005 mol) is corroded by sodium chloride, and the reaction equation is:

2NaCl + 9Al₂O₃ + 1/2 O₂ = 2NaAl₉O₁₄ + Cl₂

(9)

4NaCl + Cr₂O₃ + 5/2 O₂ = 2Na₂CrO₄ + 2Cl₂

(10)

4NaCl + MgO·Cr₂O₃ + 5/2 O₂ = 2Na₂CrO₄ + MgO +2Cl₂

(11)

It is worth noting that Figure 2, Figure 3 and Figure 4 shows that the Gibbs free energy of the reaction of MgO·Cr₂O₃ and MgO·Al₂O₃ corroded by sodium carbonate, sodium sulfate and sodium chloride is smaller than that of Al₂O₃ and Cr₂O₃ corroded by sodium carbonate, sodium sulfate and sodium chloride, but, as can be seen from Table 1, Table 2 and Table 3, MgO·Cr₂O₃ and MgO·Al₂O₃ are less susceptible to corrosion by sodium carbonate, sodium sulfate and sodium chloride than Al₂O₃ and Cr₂O₃. This is because MgO·Cr₂O₃ and MgO·Al₂O₃ will undergo decomposition reaction at high temperature to generate MgO and Cr₂O₃, MgO and Al₂O₃, respectively. This also shows that the trend of corrosion resistance of refractory materials cannot be judged solely by the Gibbs free energy of the reaction, but must be judged comprehensively by combining reaction products.

Therefore, Al₂O₃ and Cr₂O₃ are most easily corroded by sodium carbonate, sodium sulfate and sodium chloride, followed by MgO·Cr₂O₃, and MgO·Al₂O₃ and MgO, which are the most difficult to be corroded by sodium salts.

4.4. Thermodynamic Experiment of NaCl Corrosion of MgO

As can be seen from Figure 5, XRD patterns show that only MgO and NaCl are present in the samples at temperatures of 600, 1000 and 1200 °C, indicating that MgO has not been corroded by NaCl, and it has good corrosion resistance, which is consistent with thermodynamic calculation results.

Figure 5 shows the XRD spectra of samples after NaCl and MgO high temperature corrosion tests at 600, 1000 and 1200 °C. The results present that only MgO and NaCl were detected in the sample, and no new substance was formed. The experimental results and simulation results by FactSage^TM 7.2 software (see Table 3) indicated that MgO is an excellent refractory, which can avoid NaCl corrosion. According to our previous thermodynamic experimental research [23], the thermodynamic model of the software can accurately predict the thermodynamic mechanism of alkali metal corrosion to refractory materials.

5. Conclusions

In this paper, different refractory components (MgO·Al₂O₃, MgO·Cr₂O₃, Al₂O₃, Cr₂O₃ and MgO) are corroded by different sodium salts (NaCl, Na₂CO₃ and Na₂SO₄) at different temperatures (600–1200 °C). Thermodynamics studies have obtained the effect of sodium salt type and temperature on the corrosion of refractory materials. By comparison and analysis, the refractory materials with good corrosion resistance are obtained.

• The temperature has a great influence on the corrosion of refractory materials by sodium salt at 600–1200 °C. Besides MgO and MgO·Al₂O₃, the higher the temperature, the stronger the corrosion of refractory materials by sodium sulfate and sodium chloride.
• Among the five refractory materials, MgO has the best resistance to sodium salt corrosion, followed by MgO·Cr₂O₃ and MgO·Al₂O₃; Cr₂O₃ and Al₂O₃ have the worst resistance to sodium salt corrosion. Due to the presence of O₂, Cr³⁺ is oxidized to Cr⁶⁺ during corrosion.
• The accuracy of the thermodynamic calculation of FactSage^TM software was verified for the MgO-NaCl system by analyzing the results of thermodynamic experiments. Combined with the author’s previous research work, high-content MgO refractories can solve the corrosion problem of refractories caused by NaCl and KCl.