Determination and analysis of solid–liquid equilibria for ternary systems Na 2 SO 4 –H 2 O 2 –H 2 O and Na 2 CO 3 –H 2 O 2 –H 2 O at 5 °C

Abstract

To develop a better process for synthesizing sodium percarbonate and getting separation of the sodium salt, the equilibrium of the ternary system Na₂SO₄–H₂O₂–H₂O and Na₂CO₃–H₂O₂–H₂O at 5 °C was determined using the isothermal method, and the appropriate synthesis conditions for sodium carbonate from a theoretical perspective was discussed. The Na₂SO₄–H₂O₂–H₂O system phase diagram contains one co-saturated point of Na₂SO₄·0.5H₂O₂·H₂O and Na₂SO₄·10H₂O. The Na₂CO₃–H₂O₂–H₂O system phase diagram contains three crystal zones: Na₂CO₃·10H₂O crystallization field, 0Na₂CO₃·1.5H₂O₂·H₂O crystallization field and Na₂CO₃·2H₂O₂·H₂O crystallization field. According to the phase diagram, the suitable raw material ratio of sodium percarbonate synthesis was obtained, to get the maximum of product yield, the optimal mass ratio of Na₂CO₃ to 30% H₂O₂ is 0.609.

Introduction

In the process of producing aluminum oxide by the Bayer process, various carbonate minerals (Ca₂CO₃, Ca₂CO₃·Mg₂CO₃) exist in the raw materials (bauxite, limestone, and alkali).^1–3 They are easily decomposed during leaching, which can convert sodium hydroxide produced in the production process into sodium carbonate. Due to the effect of air agitation, sodium aluminate in the solution is acidified by CO₂ in the air to generate a large amount of sodium carbonate, which accumulates significantly, and most of the red mud that cannot be digested by the sintering method will be stacked after mixed conveying, resulting in a significant increase in alkali consumption. The presence of sulfur elements in bauxite and the use of alkali and water materials cause the concentration of chloride and sulfate ions in the solution to gradually increase. As the solution concentration increases, the solubility of sodium carbonate and sodium sulfate decreases sharply.^4–6 Therefore, during the process of mother liquor evaporation, sodium carbonate and sodium sulfate will crystallize and precipitate due to the sharp decrease in solubility, and the accumulation of a large amount of sodium salt crystals will mainly be discharged with the red mud, making it impossible for them to be reasonably utilized and separated, becoming a major common problem faced by the Bayer process for producing aluminum oxide. Each ton of aluminum oxide produced generates 1–1.8 tons of red mud, and global production of 6  $\mathrm{\times }$  10⁷ tons per year. However, the utilization rate of red mud globally is only around 15%.

Considering that there is a high content of sodium carbonate in the red mud, synthesizing and preparing sodium percarbonate has become a reasonable choice for its utilization. Sodium percarbonate, as a new type of oxygen-based bleach, consists of Na₂CO₃ and a certain concentration of H₂O₂.^7,8 It is non-toxic and does not cause any irritation under dilute conditions, making it environmentally friendly in practical applications. At the same time, sodium percarbonate in solution can easily decompose into sodium carbonate and H₂O₂. H₂O₂ can continue to react in an alkaline solution, producing water and oxygen, showing significant bleaching characteristics. In addition, sodium percarbonate is non-toxic and harmless, and can maintain the original color of fabrics. Therefore, it is extremely effective in synthetic fiber washing, tableware and food disinfection, fresh-keeping fruits, and sewage treatment, and has significant application effects.

Depending on the manufacturing method of sodium percarbonate, the product form and specifications will differ. The preparation methods are mainly divided into dry and wet methods. The dry synthesis method involves the rapid formation of sodium percarbonate crystals by mixing anhydrous sodium carbonate and high-concentration H₂O₂ together in a short period of time. A potential drawback of this method is that heat generation during the reaction can cause decomposition of H₂O₂, leading to a loss of active oxygen and reduced product purity. In addition, the product has poor flow properties, and the method involves high energy consumption and equipment pollution, so its use in industrial production is limited. The wet method involves saturating a sodium carbonate solution with a 30% H₂O₂ solution and reacting the two components at an appropriate temperature to generate sodium percarbonate. The resulting product is further processed through crystallization, separation, and drying to obtain the final product. The wet method is advantageous, in that, it is easy and simple to operate. Other wet methods that are currently widely used include solvent method, spray drying, and salt precipitation. However, it leads to certain levels of decomposition loss of H₂O₂ and sodium percarbonate at the high temperature during reaction, crystallization and drying, resulting in low product quality and stability. In addition, the high energy consumption and equipment pollution are also limiting factors for this method, which is usually used for small-scale production. To obtain the phase equilibrium data of the quaternary system, the ternary subsystems included in the system must first be conducted. In this study, the equilibrium data of the ternary system Na₂SO₄–H₂O₂–H₂O and Na₂CO₃–H₂O₂–H₂O at 5 °C was determined using the isothermal method, and the appropriate synthesis conditions for sodium carbonate from a theoretical perspective was discussed. ⁹

Results and discussion

The phase equilibrium of ternary system Na₂SO₄–H₂O₂–H₂O at 5 °C

The phase equilibrium data obtained from the experimental study of the ternary system are presented in Table 1, and a detailed diagram based on these data can be found in Figure 1. In addition, the composition of the saturated residue was analyzed using infrared (IR) spectroscopy, as shown in Figure 2.

Table 1.

Phase equilibrium data of Na₂SO₄–H₂O₂–H₂O ternary system at 5 °C.

Composition of equilibrium liquid phase (wt%)			Composition of wet slag (wt%)			Equilibrium solid phase
Na2SO4	H2O2	H2O	Na2SO4	H2O2	H2O
3.06	1.99	94.5	23.96	0.87	75.17	S10
7.99	7.39	84.2	33.42	2.47	64.11	S10
12.5	11.2	76.3	22.66	7.56	69.78	S10
14.67	13.9	71.43	34.45	4.01	61.54	S10
17.9	18.67	63.43	30.44	10.2	59.36	S10
21.38	20.29	58.33	35.55	8.68	55.77	S10
23.26	22.91	53.83	46.61	10.89	42.5	S10 + G
22.65	22.15	55.2	40.34	10.61	49.05	S10 + G
21.02	25.56	53.42	61.12	15.06	23.82	G
18.97	29.28	51.75	52	19.07	28.93	G
18.41	34.21	47.38	60.91	17.92	21.17	G

G: Na₂SO₄·0.5H₂O₂·H₂O; S₁₀: Na₂SO₄·10H₂O.

Figure 1.

Phase diagram of Na₂SO₄–H₂O₂–H₂O ternary system at 5 °C.

Figure 2.

Infrared characterization of the co-saturation point of the Na₂SO₄–H₂O₂–H₂O ternary system.

In the above figure, s10 represents the composition point of Na₂SO₄·10H₂O, G represents the composition point of Na₂SO₄·0.5H₂O₂·H₂O, and s point is the saturation solubility composition point of Na₂SO₄ at 5 °C, with a mass fraction of 5% for Na₂SO₄ obtained from literature. The ternary system has a common saturation point “e”, which is the common saturation of Na₂SO₄·10H₂O and Na₂SO₄·0.5H₂O₂·H₂O. There are two crystallization zones, s1se is the crystallization zone of Na₂SO₄·0.5H₂O₂·H₂O, and eGd is the crystallization zone of Na₂SO₄·0.5H₂O₂·H₂O.

According to the literature, the absorption peak at 2800–3600 cm^–1 in the IR spectrum coincides with the O–H absorption peak of intra-molecular hydrogen bond coupling. The change in characteristic wave numbers is related to the amount of H₂O₂ in the sample. This region is caused by the stretching vibration of H₂O₂ and H₂O. Since the measured sample is moist residue, it contains not only H₂O₂ but also a large amount of water. There is a weak absorption peak at 1390–1611 cm^–1, which is the vibration absorption peak of intermolecular hydrogen bond H–O, caused by the bending vibration of H–O–O. This indicates that the sample contains an H–O–O bond. The absorption peak of the O–O stretching vibration at 877 cm^–1 moves to a weak absorption at 981 cm^–1 in the low-frequency direction in the IR spectrum of this moist residue.

The phase equilibrium of ternary system Na₂CO₃–H₂O₂–H₂O at 5 °C

The experimental results of the phase equilibrium of the Na₂CO₃–H₂O₂–H₂O ternary system are shown in Table 2, and the phase diagram is plotted in Figure 3.

Table 2.

Phase equilibrium data of the Na₂CO₃–H₂O₂–H₂O ternary system at 5 °C.

Composition of equilibrium liquid phase (wt%)			Composition of wet slag (wt%)			Equilibrium solid phase
Na2CO3	H2O2	H2O	Na2CO3	H2O2	H2O
5.08	14.17	80.75	20.81	18.23	60.96	B
5.33	12.96	81.71	18.71	17.21	64.08	B
5.38	10.79	83.83	16.2	14.64	69.16	B
5.51	23.87	70.62	18.67	26.43	54.9	C
5.53	20.17	74.3	19.92	22.83	57.25	B + C
5.75	20.13	74.12	25.71	23.69	50.6	B + C
5.63	17.71	76.66	19.55	19.88	60.57	B
5.72	22.88	71.4	20.22	26.75	53.03	C
6.05	6.86	87.09	16.2	10.53	73.27	B
7.1	4.42	88.48	16.26	8.22	75.52	B
5.43	18.27	76.3	17.88	15.96	66.16	B
8.98	3.42	87.6	15.98	6	78.02	N10 + B
8.98	3.73	87.29	21.41	8.56	70.03	N10 + B
10.19	2.63	87.18	11.88	2.54	85.58	N10
5.43	18.27	76.3	17.88	21.04	61.08	B

B: Na₂CO₃·1.5H₂O₂·H₂O; C: Na₂CO₃·2H₂O₂·H₂O; N₁₀: Na₂CO₃·10H₂O.

Figure 3.

Phase diagram of Na₂CO₃–H₂O₂–H₂O ternary system at 5 °C.

The phase diagram of the system contains two eutectic points and three crystallization regions. Points A, B, and C represent the composition points of Na₂CO₃·10H₂O, Na₂CO₃·1.5H₂O₂·H₂O, and Na₂CO₃·2H₂O₂·H₂O, respectively. Point “s” is the saturation solubility of Na₂CO₃ at 5 °C, and the mass fraction of Na₂CO₃ at saturation dissolution was found to be 8.67% according to the literature. e1 represents the eutectic point between Na₂CO₃·10H₂O and Na₂CO₃·1.5H₂O₂·H₂O, while e2 represents the eutectic point between Na₂CO₃·1.5H₂O₂·H₂O and Na₂CO₃·2H₂O₂·H₂O. The BE1E2 region is the crystallization region of Na₂CO₃·1.5H₂O₂·H₂O, and the solid phase precipitated at any point in this region is sodium percarbonate. The CE2D region is the crystallization region of Na₂CO₃·2H₂O₂·H₂O, and the solid phase precipitated at any point in this region is Na₂CO₃·2H₂O₂·H₂O. The SE1D region is the crystallization region of Na₂CO₃·10H₂O. The larger crystallization region of Na₂CO₃·1.5H₂O₂·H₂O at this temperature indicates that sodium percarbonate is easier to be generated at this temperature.

Suitable conditions for the synthesis of sodium percarbonate based on the phase diagram of Na₂CO₃–H₂O₂–H₂O ternary system

According to the phase diagram Figure 3, the suitable conditions for synthesizing sodium percarbonate at 5°C are discussed. The calculation analysis is shown in Figure 4. Since the H₂O₂ used is mostly low-concentration aqueous solution, to increase the concentration of H₂O₂ and the yield of sodium percarbonate, only Na₂CO₃ and H₂O₂ aqueous solution are used as input materials. With 1 kg of 30% H₂O₂ as the feeding benchmark, the feed point must be on the NH line, and the NH line equation is given $y=-0.3x+30$ . At the same time, the amount of Na₂CO₃ and H₂O₂ fed satisfy the condition $x:(100-x)=\overline{Hh}:\overline{hN}$ , set up the mass of Na₂CO₃ added is r, then the ratio is r:1.

Figure 4.

Calculation and analysis of Na₂CO₃–H₂O₂–H₂O ternary system phase diagram at 5 °C.

Set up the feed point is h (x, y), that is, $x:(100-x)=r:1$ . Then, we can get x = 100r/r + 1, y = 30/r + 1 The corresponding liquid-phase point coordinates are set to (x₀, y₀) and the coordinates of point B is (60.57, 29.14). Since points B, h and g are on a straight line, then

$$\frac{60.57-x_0}{29.14-y_0}=\frac{60.57-\frac{100r}{r+1}}{29.14-\frac{30}{r+1}}$$

(1)

Assuming that the mass of produced solid sodium percarbonate is w (kg), according to the conservation of mass of sodium carbonate, there are

$$r=w\times \frac{106}{175}+(1+r-w)\times x\%$$

(2)

Yield of the product sodium percarbonate

$$\begin{array}{l}\eta =\mathrm{mass}\mathrm{of}{\mathrm{Na}}_2{\mathrm{CO}}_3\cdot 1.5{\mathrm{H}}_2{\mathrm{O}}_2\mathrm{in}\mathrm{solid}-\mathrm{phase}\\ \mathrm{product}/\mathrm{sum}\mathrm{of}\mathrm{mass}\mathrm{of}\mathrm{feed}{\mathrm{Na}}_2{\mathrm{CO}}_3\\ +{\mathrm{H}}_2{\mathrm{O}}_2\times 100\%=\mathrm{w}\times 157/175/\left(r+0.3\right)\times 100\%\end{array}$$

(3)

By solving the set of equations mentioned above, the values of r, w, and η of any liquid-phase composition point (x₀, y₀) on the solubility curve of sodium carbonate can be obtained. The results of sodium carbonate yield of different liquid-phase composition points and the corresponding feed material ratios are summarized in Table 3. It can be concluded that, when the sodium carbonate yield reaches its maximum at 5 °C, the formula ratio r is 0.609, and the sodium carbonate yield is 91.29%.

Table 3.

Different formula points and corresponding product yields at 5 °C.

(x0, y0)	5.08	5.63	8.98	5.43	7.1	6.05	5.53	5.38	5.33
	14.17	17.71	3.42	18.27	4.42	6.86	20.17	10.79	12.96
r	0.433	0.360	0.636	0.347	0.609	0.568	0.289	0.497	0.457
w (kg)	0.648	0.517	0.948	0.496	0.925	0.869	0.396	0.754	0.687
η (%)	79.40	70.23	90.86	68.81	91.29	89.81	60.32	84.93	81.38

According to the literature, ¹⁰ the feed mass ratio (Na₂CO₃: 30% H₂O₂) for the maximum yield of synthesizing sodium percarbonate at 25 °C is 0.661, with a product yield of 78.46%. It can be seen that there is an optimal formulation ratio for the reaction at a certain temperature. The optimal formulation ratio varies at different reaction temperatures, and the maximum yield obtained is also different. Lower reaction temperatures are more conducive to improving the yield of sodium percarbonate. Therefore, the reaction should be conducted at a lower temperature, and the feedstock ratio should be controlled to ensure the maximum product yield in production.

Sodium salt separation using the Na₂CO₃–Na₂SO₄–H₂O ternary phase diagram

The phase equilibrium data of the ternary subsystem Na₂CO₃–Na₂SO₄–H₂O in the quaternary system studied in this article were obtained from both literature and experiments, ¹¹ and the phase diagram was plotted as shown in Figure 5.

Figure 5.

Phase diagram of the Na₂CO₃–Na₂SO₄–H₂O ternary system at 5 °C.

In Figure 5, s₁₀ represents the point of Na₂SO₄·10H₂O substance composition. N₁₀ represents the point of Na₂CO₃·10H₂O substance composition. Points s and n are the points of saturation solubility of Na₂SO₄ and Na₂CO₃ at this temperature, respectively. Point E is the point of co-saturation of Na₂SO₄·10H₂O and Na₂CO₃·10H₂O. ess₁₀ is the crystalline zone of Na₂SO₄·10H₂O, and nN₁₀ e is the crystalline zone of Na₂CO₃·10H₂O.

The chemical composition of red mud of Bayer process from an alumina plant was analyzed to obtain the sodium salt content in the red mud, in which the Na₂CO₃ content was 55.25%. The Na₂SO₄ content was 16.23%, and the NaCl content was 0.154%. The H₂O content was 27.75%, and a small amount of filtered metal impurities were also contained. Since the content of NaCl and metal impurities in this red mud is very small. Only the separation of two salt components, Na₂CO₃ and Na₂SO₄, are considered in the phase equilibrium separation study. The analytical calculation procedure is shown in Figure 6.

Figure 6.

Phase diagram analysis of Na₂CO₃–Na₂SO₄–H₂O system at 5 and 25 °C.

Figure 6 shows the phase diagram of the ternary system Na₂CO₃–Na₂SO₄–H₂O at low temperatures of 5 and 25 °C. In Figure 6, there are Na₂SO₄·10H₂O and Na₂CO₃·10H₂O co-saturation points e₁ and e₂. No complex salts were generated at this temperature. Ss₁₀e is the crystalline zone of Na₂SO₄–10H₂O. nn₁₀e is the crystalline zone of Na₂CO₃·10H₂O, and s₁₀ N10e is the zone where the two solid phases coexist. Point M is the red mud sample composition point (55.25, 16.23), which is in the all-solid-phase zone, solid-phase dehydrated state. So, water was added to it to N to dilute the sample. Reaching the point N is close to the Na₂CO₃·10H₂O crystallization zone at 25 °C. Therefore, water is added to the sample at 25 °C, so that, the system point extends the motion of the MW line and enters the Na₂CO₃·10H₂O crystallization zone. The N → Q stage is in the co-dissolution zone of Na₂CO₃·10H₂O and Na₂SO₄·10H₂O, with the liquid phase e₂. The addition of water to the point Q dissolves all the Na₂SO₄·10H₂O in the sample. The system point arrives at the N₁₀ crystallization zone, which is in the full solid-phase zone, with solid-phase dehydration. The Na₂CO₃·10H₂O solid phase is obtained more fully. If the mother liquor e₂ at the Q point is directly cooled to 5 °C, it can be seen that the e₂ point is still in the zone of co-precipitation of Na₂CO₃·10H₂O and Na₂SO₄·10H₂O at 5 °C, which will lead to the impurity of the obtained product. Therefore, before cooling, continue to add water to the e₂ point to the Q’ point. Then, cool it to 5 °C, and it can be cooled after the cooling of the largest amount of S₁₀. The mother liquor e₁ was obtained after separation. In summary, the analysis of the 25 °C with the addition of water to dissolve the sample to get the largest amount of Na₂CO₃·10H₂O, the mother liquor diluted and cooled to 5 °C to get the largest amount of Na₂SO₄·10H₂O.

For the volume calculation of the above graph, using 1 kg of sample M as the basis for the calculation, let the amount of water added to the point Q at 25 °C be w₁ kg, to obtain N₁₀ product mass of a. The mass of the mother liquor is e₂.

Then, the total material is conserved: $1+w_1=a+e_2;$

$$\begin{array}{l}{\mathrm{Na}}_2{\mathrm{CO}}_3\mathrm{conservation}\mathrm{of}\mathrm{mass}:0.5525=a\times 106/286\\ +e_2\times 0.1828\end{array}$$

$${\mathrm{Na}}_2{\mathrm{SO}}_4\mathrm{conservation}\mathrm{of}\mathrm{mass}:0.1623=e_2\times 0.1638$$

Solve for $a$  = 1.002 kg, $w_1$  = 0.9928 kg and $e_2$  = 0.9908 kg.

Let the amount of water added to dilute the mother liquor be w₂, the mass of the product S₁₀ obtained after cooling and separation be b. The mass of the mother liquor be e₁.

Then, there is total material conservation: $e_2+w_2=e_1+b$ ;

$${\mathrm{Na}}_2{\mathrm{CO}}_3\mathrm{conservation}\mathrm{of}\mathrm{mass}:e_2\times 0.1828=e_1\times 0.0683;$$

$$\begin{array}{l}{\mathrm{Na}}_2{\mathrm{SO}}_4\mathrm{conservation}\mathrm{of}\mathrm{mass}:e_2\times 0.1638=e_1\times 0.03752\\ +b\times 142/322\end{array}$$

Solve $e_1$  = 2.71 kg, $b$  = 0.1374 kg, $w_2$  = 1.8566 kg.

$$\begin{array}{l}\mathrm{The}\mathrm{yield}\mathrm{of}{\mathrm{Na}}_2{\mathrm{CO}}_3\mathrm{is}\eta =\frac{1.002\times 106/286}{0.5525}\\ \times 100\%=67.21\%\end{array}$$

$$\begin{array}{l}\mathrm{The}\mathrm{yield}\mathrm{of}{\mathrm{Na}}_2{\mathrm{SO}}_4\mathrm{is}\eta =\frac{0.1374\times 142/322}{0.1623}\\ \times 100\%=37.33\%\end{array}$$

After cooling and separation, the mother liquor e₁ still contains Na₂CO₃ and Na₂SO₄, which should be reasonably utilized to improve the yield of the product. From the above figure, it can be seen that e₁ point is in the unsaturated zone of 25 °C phase diagram, indicating that the mother liquor still has the ability to dissolve salt at this temperature. Therefore, consider using the mother liquor e₁ instead of water to dissolve the raw materials. Through the analysis of the diagram, it can be seen that the optimal dosage point for the intersection of the line of e₁M and N₁₀e₂ of the intersection point M₁. Calculate the quality of the mother liquor e₀. $e_0:M=M{M}_1:e_1{M}_1$ can be calculated to be the quality of the mother liquor e₀ = 1.414 kg. It can be seen that 1.414 kg of the 2.71 kg of mother liquor e₁ can be used to leach the sample. There is still 1.296 kg leftover, and the loss of the two sodium salts is contained in the leftover mother liquor. If you want to make full use of this remaining mother liquor, that is, to introduce mother liquor e₁ into the crystallization zone of a certain product. Evaporating the mother liquor at 25 °C, the mother liquor extends the movement in the direction of We₁ to the point M₂, which gives Na₂CO₃·10H₂O and mothser liquor e₂. e₂ can be continued to dilute with water and then cooled to separate to get Na₂SO₄·10H₂O, which is the same as the mother liquor separation process of raw material M.

The remaining 1.296 kg of mother liquor was calculated and analyzed as follows:

• Evaporated water w₃ = 0.9116 kg

After evaporation of water, the product N₁₀ is obtained in the amount of a₁ = 0.03705 kg.

• Amount of mother liquor e₂: e₂ = 0.3474 kg

• For mother liquor e₂ dilution water addition w₄ = 0.6324 kg

• After cooling, the product S₁₀ is separated and the amount of b₁ = 0.05 kg.

• Once again the amount of residual mother liquor e₁ is 0.9298 kg

$$\begin{array}{l}{\mathrm{Na}}_2{\mathrm{CO}}_3\mathrm{product}\\ \mathrm{yield}\mathrm{increased}\mathrm{to}\eta =\frac{(1.002+0.03705)\times 106/286}{0.5525}\\ \times 100\%=69.70\%\end{array}$$

$$\begin{array}{l}{\mathrm{Na}}_2{\mathrm{SO}}_4\mathrm{product}\\ \mathrm{yield}\mathrm{increased}\mathrm{to}\eta =\frac{(0.1374+0.05)\times 142/322}{0.1623}\\ \times 100\%=50.92\%\end{array}$$

It can be seen that after the recycling of the remaining mother liquor e₁, the recovery of both sodium salts can be improved. However, the amount of diluted water added during the whole process (1.856 + 0.6324 kg) is greater than the amount of water evaporated (0.9116 kg). There must be a solution generated. So, there will always be a mother liquor e₁ in the system, the theory of the mother liquor can be evaporated several times, dilution, cooling, and separation to improve the recovery of sodium salt products as much as possible. However, taking into account the actual operation in production and the cost and other factors, the number of mother liquor circulation generally depends on the actual situation. The above analyses and calculations show that the theoretical guidance of using the ternary 5 and 25 °C phase diagram for production is feasible.

Conclusion

The phase equilibrium solubility data of the ternary systems Na₂SO₄–H₂O₂–H₂O and Na₂CO₃–H₂O₂–H₂O at 5 °C were determined by isothermal method and plotted as a phase diagram. The Na₂CO₃–H₂O₂–H₂O system exists one point of co-saturation for sodium sulfate hydrate and Na₂CO₃·2H₂O₂·H₂O co-saturation, and crystalline zones of the two solid phases were determined. The Na₂CO₃–H₂O₂–H₂O system has two points of co-saturation, which are co-saturation of sodium carbonate hydrate and sodium percarbonate, and co-saturation of sodium percarbonate and Na₂CO₃·2H₂O₂·H₂O. The crystallization zones of the three solid phases were determined and the crystallization zone of the sodium percarbonate solid phase was larger at this temperature. The optimum feed dosage mass ratio (Na₂CO₃:30% H₂O₂) for the synthesis of sodium percarbonate at this temperature was determined to be 0.609. The maximum yield of sodium percarbonate achieved at this time was 91.29%, which is greater than the maximum yield of the product at 25 °C documented in the literature. This indicates that the synthesis of sodium percarbonate is suitable to be carried out at low temperature.

According to the Na₂SO₄–Na₂CO₃–H₂O ternary experimental data at 5 °C and literature data to draw phase diagram. Combined with the analysis of the phase diagram of the system at 25 °C, it was discussed that it was appropriate to use the solid phase of sodium carbonate hydrate precipitated by dilution at high temperature for red mud samples. The solid phase of sodium carbonate hydrate was then diluted and cooled to low temperature to obtain the solid phase of sodium sulfate hydrate. The remaining mother liquor can continue to leach red mud samples and be recycled. This increases the recovery of the solid phase and improves the degree of separation of the sodium salt from the sample.

Experimental

Materials

The chemical reagents used in this experiment are shown in Table 4.

Table 4.

The chemical reagents of the experiment.

Reagent name	Molecular formula	Relative molecular weight	Quantity contained	Source	Norm
Sodium carbonate anhydrous	Na2CO3	105.99	Not less than 99.80%	Tianjin Beifang Tianyi Chemical Reagent Factory	Analytical purity
Anhydrous sodium sulfate	Na2SO4	142.04	Not less than 99%	Tianjin Yingda Rare Chemical Reagent Factory	Analytical purity
Hydrogen peroxide H2O2	H2O2	34.01	No greater than 30%	Tianjin Yongda Chemical Reagent Development Center	Analytical purity
Disodium ethylenediammonium tetraacetate	C10H14N2O8Na2·2H2O	372.24	99.0%	Tianjin Beifang Tianyi Chemical Reagent Factory	Analytical purity
Sodium oxalate	Na2C2O4	134.00	99.8%	Tianjin Huayue Chemical Reagent Factory	Analytical purity
Methyl Orange	C14H14N3NaO8S	327.35	99.5%	Tianjin Chemical Reagent Factory	Analytical purity
Barium chloride	BaCl2	244.26	Not less than 99.5%	Tianjin Beifang Tianyi Chemical Reagent Factory	Analytical purity
Oxalate	H2SO4	98.08	95% to 98%	Tianjin Chemical Reagent Third Factory	Analytical purity
Hydrochloric acid	HCl	36.46	36% to 38%	Tianjin Chemical Reagent Third Factory	Analytical purity
Potassium permanganate	KmnO4	158.04	Not less than 99.5%	Tianjin Chemical Reagent Factory	Analytical purity
Manganese sulfate	MnSO4	174.26	Not less than 99.5%	Tianjin Chemical Reagent Factory	Analytical purity

The experimental apparatus used in this experiment is shown in Table 5.

Table 5.

Experimental apparatus.

Experimental equipment	Model number	Manufacturer
Super thermostatic bath	SY-601	Tianjin Ounuo Instrumentation Co., Ltd
Electric mixer	OJ-60	Tianjin Ounuo Instrumentation Co., Ltd
Electrothermal blast drying oven	DGG101-2	Tianjin Tianyu Electromechanical Co., Ltd
Electronic balance	BS 124 S	Beijing Sartorius Instrument System Co., Ltd

Apparatus and experimental procedure

The experiments were carried out using atmospheric isothermal wet slag method to determine the phase equilibrium data of the electrolyte—non-electrolyte—water system, solubility determination experimental setup is shown in Figure 7. The glass tube containing samples was placed in a thermostatic water bath with a temperature control accuracy of ±0.1 °C, and to ensure that the reactant solution in the glass tube was completely submerged. The glass tube was 30 mL in volume and the mouth was closed with a rubber stopper with a stirring rod to avoid water evaporation during the experiment.

Figure 7.

Experimental set-up for phase equilibrium solubility determination by isothermal method: (1) constant temperature bath, (2) spot contact thermometer, (3) thermometer, (4) electric stirrer, and (5) reactor.

Determination of phase equilibrium time: the reactor will be adjusted to a temperature of 5 °C, and until the temperature is stable to 5 ± 0.1 °C. Accurately, weigh a certain amount of sodium carbonate, sodium sulfate, hydrogen peroxide added to the reaction vessel, which will be placed in a constant temperature water bath. Open the stirring device. Take out the appropriate amount of liquid phase every period of time to analyze and determine the chemical composition of the solution to be unchanged when the composition of liquid phase to prove that the system has reached Equilibrium. The time to equilibrium was measured to be 5 h.

Phase equilibrium experimental steps to ternary system as an example: the temperature of the constant temperature water bath device is controlled at 5 ± 0.1 °C, and the solubility of sodium carbonate at 5 °C as the basis for data. According to the liquid phase saturated with sodium carbonate, the electronic balance accurately weighing the calculated amount of anhydrous sodium carbonate. Add disodium ethylenediaminetetraacetate in a 5% ratio as a stabilizer, with a pipette to take the preparation of the composition of the water and hydrogen peroxide solution added to the reactor, fixed in a constant temperature water bath stirring. Stirring in a water bath, the reaction time is 5 h. Then, stop stirring, and rest for a period of time, until the solution stratification is obvious. Take out the upper layer of clear liquid and the lower layer of wet residue into the weighing dish, weighing. Afterwards, the diluted solution of clear solution and wet residue were poured into 250 mL volumetric flask and diluted to scale. Two 50 mL dilutions were taken out to determine the content of sodium carbonate and hydrogen peroxide. According to the experimental data to determine the liquid phase and wet residue composition point in the phase diagram. The amount of sodium carbonate and hydrogen peroxide was changed continuously to obtain different liquid-phase composition points, and the complete phase equilibrium curve was plotted.^12–16

Characterization

The solid-phase composition was tested by a combination of chemical analysis and IR spectroscopy and X-ray diffraction. The content of each component in the wet slag is first analyzed chemically, and the point is plotted in the dry salt map. ¹⁷ If the composition of a solid phase is close to a certain solid phase, IR spectroscopy or X-ray diffraction analysis can be used to determine the composition of the substance, and then accurately determine the composition of the solid phase.