Study on the process for producing soda ash via double-displacement reaction using mirabilite and ammonium bicarbonate

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This post was last edited by mkp369 on 2017-2-15 at 14:09. Research on the Process of Producing Soda Ash via Double Displacement Reaction Using Glauber’s Salt and Ammonium Bicarbonate 1. Principle of Production 1.1 Phase diagram analysis of the Na+, NH4+/SO42-, HCO3- system Using glauber’s salt and ammonium bicarbonate as raw materials, soda ash is produced through a double displacement reaction; the reaction equations are as follows: Na2SO4 + 2NH4HCO3 → 2NaHCO3 + (NH4)2SO4 2NaHCO3 → (upon calcination) Na2CO3 + H2O + CO2↑ A phase diagram of the Na+, NH4+/SO42-, HCO3- system is drawn based on solubility equilibrium data, as shown in Figure 1 (omitted). As can be seen from Figure 1, the system contains 6 crystalline regions, namely the (NH4)2SO4, NH4HCO3, NaHCO3, Na2SO4·(NH4)2SO4·4H2O, Na2SO4·10H2O, and Na2SO4 crystalline regions, as well as 4 triple-salt co-saturation points. The crystalline region of NaHCO3 accounts for a considerable proportion, indicating that it is easy to obtain NaHCO3 crystals in such a system, as its solubility in the system is the lowest. The raw materials for producing NaHCO3 are Na2SO4 and NH4HCO3, and the ratio between these two materials directly affects the amount of NaHCO3 crystals that are formed. Phase diagrams can be used to help determine the appropriate conditions for obtaining the maximum amount of NaHCO3. As can be seen from the phase diagram, the crystallization line for sodium bicarbonate is CPl. When the composition point lies at the intersection K of BD and CP1, the maximum precipitation rate of sodium bicarbonate is given by: nNaHCO3 = n_total × KPl/CP1, where n_total is the total amount of salts in the solution, and nNaHCO3 is the amount of NaHCO3 ； When the ingredient selection is point B or M, the amount of sodium bicarbonate precipitated is clearly less than that mentioned above. Because, as can be seen from its crystallization line CP1, the segment KP1, which is proportional to the amount of crystallization, is the longest, while the segment CK, which is proportional to the liquid phase, is the shortest. Therefore, under these conditions, the amount of sodium bicarbonate precipitated is the highest, and the amount of mother liquor is the lowest. In experiments, it is necessary to focus not only on the quality of soda ash but also on the utilization rates of sodium and ammonia; achieving a higher sodium utilization rate is beneficial for production. As can be seen from the phase diagram ； As the mother liquor composition moves along the co-saturation line IP1 between NaHCO3 and NH4HCO3 toward P1, the sodium utilization rate increases accordingly ； As movement proceeds along the P2P1 line toward P2, the sodium utilization rate decreases accordingly ； When the mother liquor composition is P1, the sodium utilization rate is highest; when the mother liquor composition falls at point P2, the ammonia utilization rate is highest. Overall, although the utilization rate of single-cycle sodium is not very high, since the process is cyclic, the sulfur that is not converted into sodium bicarbonate is not discarded; therefore, the overall sodium utilization rate can approach 100%. ‘Therefore, in the production of soda from mirabilite, it is not necessary to aim for a single cycle; instead, the goal should be to maximize the yield of sodium bicarbonate per unit weight of the mother liquor. To meet this requirement, the solution composition must fall at point P1. The above is the theoretically optimal mixing ratio; in actual production, the liquidus point must not fall on the eutectic line or eutectic point, otherwise eutectic phenomena of other salts will occur. In actual production, regardless of the mixing ratio, the liquidus point must remain at an appropriate distance from the co-saturation line or co-saturation point; this way, although the amount of NaHCO3 crystals formed decreases, the purity of the product is maintained. 1.2 Analysis of the (NH4)2SO4-Na2SO4-H2O ternary system: The thermal separation of Na2SO4 and (NH4)2SO4 from heavy-alkali mother liquors is based on the principle that the solubility of Na2SO4 decreases at high temperatures, while at room temperature the (NH4)2SO4-Na2SO4-H2O system allows the formation of the double salt Na2SO4·(NH4)2SO4·4H2O. Therefore, the heavy-alkali mother liquor from ammonia evaporation can be evaporated at higher temperatures; the resulting Na2SO4 is returned to the soda production process, while the double salt that crystallizes upon cooling the mother liquor is recycled within the system. Na2SO4 is separated through high-temperature evaporation; after the (NH4)2SO4-Na2SO4-H2O system crystallizes to form a double salt at room temperature, the concentration of (NH4)2SO4 in the solution increases relatively. Subsequently, evaporation at high temperature results in the precipitation of (NH4)2SO4. After the separation of the heavy alkalis, the mother liquor contains sodium sulfate, ammonium bicarbonate, and sulfuric acid. If the mother liquor is sent directly to evaporation, the ammonium bicarbonate will decompose and be consumed; therefore, sulfuric acid is used for neutralization. After neutralization, the solution becomes a (NH4)2SO4-Na2SO4-H2O ternary system. Since a double salt is formed in the system at temperatures below 60°C, evaporation is carried out at 80–100°C; evaporation is stopped once the co-saturation line is reached, to prevent the co-precipitation of sodium sulfate and sulfuric anhydride from affecting the quality of sulfuric anhydride. The phase diagram of the (NH4)2SO4-Na2SO4-H2O ternary system at 80 and 100°C is shown in Figure 2 (omitted). As can be seen from Figure 2 (omitted), the phase regions at 80°C and 100°C are very similar; therefore, for energy-saving reasons, evaporation at 80°C is chosen. 2 Process flow: At around 35°C, mirabilite is made into a saturated solution. A small amount of Na2CO3 and NaOH is added to remove Ca2+ and Mg2+ from the solution, thereby producing a purified mirabilite solution. In the reactor, solid NH4HCO3 is added to the mirabilite solution to undergo a reaction, producing a NaHCO3 slurry. After slurry filtration, the filter cake is washed with deionized water to yield pure NaHCO3 crystals, which are then calcined in a muffle furnace to produce pure soda ash ； The filtrate is subjected to vacuum evaporation once; after the evaporation is complete, the resulting liquid is filtered at a temperature above 60°C. The by-product Na2SO4 obtained is returned to the previous reactor to participate in the reaction ； The filtrate is cooled to around 25°C and then filtered; the resulting double salt Na2SO4·(NH4)2SO4·4H2O is returned to the primary evaporation process of the Na2SO4 solution ； The filtrate from the removal of double salts was subjected to secondary vacuum evaporation, and after filtration at a temperature above 60°C, the final product (NH4)2SO4 was obtained ； The filtrate free of sulfuric acid an is mixed with the filtrate from the first evaporation step, and fed into the next cycle. In this process, the heavy alkali filtrate can be mixed with the sulfuric acid an filtrate in a mixing tank to recover the unconverted Na2SO4, which can then be used for alkali production. In this way, the utilization rate of mirabilite can be **increased**. This is the advantage of the mirabilite method for alkali production over the sodium chloride-calcium chloride method. 3 Experimental Section 3.1 Raw Materials Mirabilite: ω(Na2SO4)=42.98% ； ω(Ca2++Mg2+)=0.25% ； ω(Cl-)=0.17%. Ammonium bicarbonate (agricultural fertilizer): ω(NH4HCO3)=96%. Sulfuric acid (industrial grade): ω(H2SO4)=98%. 3.2 Analysis and testing methods: Raw materials and products are tested in accordance with the methods specified in the **standards. The components of the solution are detected by the following methods: Ca2+ and Mg2+ are detected using the EDTA method ； Detection of NH+ by the formalin method ； SO42- is detected by the barium chloride method. 4 Experimental Results and Discussion 4.1 Product Quality (see Tables 1 and 2) Table 1 Comparison of the quality of sulfuric acid AN with the **standard GB 535-1983 Indicator Name ω(N)/% (on a dry basis) ω(H2O)/% ω(H2SO4)/% Sulfuric acid AN product 21.0–21.6 0.5 0.08 **Standard: First-class grade ≥21.0 ≤0.5 ≤0.08; Second-class grade ≥20.8 ≤1.0 ≤0.20 Table 2 Comparison of the quality of soda ash with the **standard GB 210.1–2004 (Class II) Indicator Name ω(Na2CO3)/% ω(NaCl)/% ω(Fe)/% ω(Water-insoluble substances)/% Soda ash product ≥99.0–99.4 0.5–0.6 ≤0.004 ≤0.04 Standard: Top-grade ≥99.2 ≤0.7 ≤0.004 ≤0.04; First-class grade ≥98.8 ≤0.9 ≤0.006 ≤0.10 Indicator Name ω(Loss on ignition)/% Apparent density/(g·cm-3) ω(Particles ≥180μm)/% Soda ash product 0.5–0.6 ≥0.9268–73 Standard: Top-grade ≤0.8 ≥0.90 ≥70; First-class grade ≤1.0 ≥0.90 ≥65 4.2 Factors Affecting Product Quality 4.2.1 Influence of the ratio of reactants The ratio of Na2SO4 to NH4HCO3 is determined through theoretical calculations based on phase diagrams; the theoretical mass ratio is 1:1.1, with a Yek index of x(2Na+):21.2% and x(SO42--)=87.6%. Considering the easy decomposition of ammonium bicarbonate, several sets of experiments were conducted with mass ratios of 1:1.1, 1:1.2, and 1:1.3 between the two substances respectively; the analysis results are shown in Table 3. Based on the data in Table 3, the corresponding mother liquor point was identified on the phase diagram; it was found that a mass ratio of Na2SO4 to NH4HCO3 of 1:1.2 yields an ideal mother liquor point. Table 3 Yek index at the mother liquor point for Na2SO4 and NH4HCO3 at different ratios. m(Na2SO4):m(NH4HCO3) Yek index x(2Na+)/% x(SO2-)/% 1:1.13 2.94 32.68 82.408 2.50 1:1.22 3.56 24.788 6.638 3.96 1:1.32 8.44 27.318 2.598 1.69 4.2.2 Effect of reaction temperature Once the ratio of reactants is determined, the reaction temperature is set based on the utilization rate of ammonium bicarbonate. The experimental results are shown in Table 4. As shown in Table 4, the utilization rate of ammonium bicarbonate is high at low temperatures; above 40°C, its utilization rate drops significantly. The reason for this is that above 40°C, ammonium bicarbonate decomposes to release CO2 and NH3 gases, reducing the efficiency of raw material utilization ； At low temperatures, the decomposition rate of ammonium bicarbonate decreases ； When the temperature is below 32.4°C, sodium sulfate may precipitate as Na2SO4·10H2O, increasing the contents of sodium sulfate and ammonium bicarbonate in the resulting heavy alkali and thereby increasing the difficulty of washing. Therefore, 35-40°C is chosen. Table 4: Utilization rate of ammonium bicarbonate at different temperatures. Temperature/°C, Utilization rate of ammonium bicarbonate/%: 35–40: 91.17, 92.48, 91.87; 40–50: 87.45, 86.07, 87.48. 4.2.3 Effect of reaction time: Table 5 shows the effect of reaction time on the quality of soda ash, with a reaction temperature of 30–40°C. As can be seen from Table 5, when the reaction time is too short, a large amount of sodium sulfate is adsorbed or enclosed within the precipitated sodium bicarbonate. During washing with water, the residual mother liquor on the surface of the sodium bicarbonate can be removed, but the sodium sulfate enclosed within the crystals cannot be removed, which affects the quality of soda ash. Therefore, the reaction time is set at 60 minutes. Table 5 Effect of reaction time on the quality of soda ash. Reaction time/min: ω(Na2CO3)/% = 20, 97.14; 30, 97.79; 30, 97.99; 40, 98.86; 40, 98.89; 60, 99.33; 60, 99.25; 70, 99.01. 4.2.4 Effect of stirring speed: The stirring speed has a significant impact on the crystallization of NaHCO3. The experimental results are shown in Table 6. As shown in Table 6, a stirring speed of 50 r/min is appropriate. Table 6 Effect of stirring speed on the quality of soda ash. Stirring speed/(r·min-1) ω(Na2CO3)/% 30 97.15 96.53 97.42 50 98.51 99.25 99.01 70 96.86 97.26 98.11 4.2.5 Effect of calcination time on product quality. Table 7 shows the effect of calcination time on product quality (calcination temperature: 220–240°C). As shown in Table 7, after 80 minutes of calcination, the decomposition rate of sodium bicarbonate approaches 100%. Table 7 Effect of calcination time on product quality. Calcination time/min: ω(Na2CO3)/% = 40, 97.87; 60, 98.30; 60, 98.62; 80, 99.16; 80, 99.28; 100, 99.33. 5 Conclusions: 1) The quality of the soda ash and sulfuric acid produced meets the national standard requirements. 2) The optimal conditions for preparing soda ash using the mirabilite method are: a mass ratio of sodium sulfate to ammonium bicarbonate of 1:1.2, a reaction time of 60 minutes, a reaction temperature of 35–40°C, a calcination temperature of 220–240°C for wet ammonium bicarbonate, and a calcination time of 80 minutes. 3) Using mirabilite and ammonium bicarbonate as raw materials to produce soda ash and sulfuric acid an offers advantages such as inexpensive and readily available raw materials, the ability to recycle the mother liquor, high product yields, a simple process, low investment costs, high profits, and virtually no waste emissions ； 4) The market prospects for soda ash and sulfuric acid AN are promising; in particular, the important role of sulfuric acid AN products in agriculture makes this process even more worthy of development.