Effect of Na₂CO₃ on Production of NaHCO₃ Using Desulfurized Na₂SO₄ Waste

Abstract

CO₂ is the primary greenhouse gas emitted through human activities which cause global warming. Many countries and international regulations aim to reduce CO₂ and encourage development of a net-zero society by 2050, thus, CCUS (carbon capture, utilization, and storage) can be considered as a key technology for achieving this goal. Among the many industrial processes, NaHCO₃ production is a representative CCUS technology which satisfies both CO₂ reduction and economic incentive. Recently, Na₂SO₄ waste, containing 15% impurities (i.e., various heavy metals), produced as a by-product generated from desulfurized processes in steel industry has received a great deal of attention since landfilling has been the only way to treat it. Na₂SO₄ could be a suitable source of NaHCO₃ production as CCUS; however, there are no commercial processes for production of NaHCO₃ using Na₂SO₄ as the raw material, since it has low production efficiency compared to other Na⁺ sources. This study evaluates the regeneration of NaHCO₃ using Na₂SO₄ waste and our results show that a combination of Na₂SO₄ waste and the addition of 20% Na₂CO₃ can sharply increase the yield of NaHCO₃ with minimizing the consumption of expensive NH₃ since alkaline properties of Na₂CO₃.

Graphical Abstract

Introduction

Currently, CO₂ capture, utilization, and storage (CCUS) technologies are receiving a great deal of attention in the efforts to combat global warming [1, 2]. The international community has also started to constructively take action in reducing carbon levels and has established a goal of being a net-zero society by 2050 [3, 4]. However, among CCUS, few technologies can satisfy both CO₂ reduction and economic incentives. The production of NaHCO₃ is a representative CO₂ capture and utilization technology which achieves both objectives [2, 5,6,7].

Traditional NaHCO₃ production is based on the Solvay process developed in 1861 [8]. The major goal of this process is the production of CaCl₂ and Na₂CO₃ using CaCO₃ and NaCl extracted from sea water as raw materials, which produces NaHCO₃ as a by-product. Except for NaCl, there are other Na⁺ sources also able to be used for production of NaHCO₃, such as, NaOH, Na₂SO₄, and Na₂CO₃ [9,10,11,12,13]. Among them, recently, Na₂SO₄ has received a great deal of attention due to excess production as by-product from steel industry [14,15,16].

Recent tightening of regulations governing CO₂ emissions as well as other environmental issues are at the forefront of the world’s governments’ efforts to rein in runaway global warming and allay the public’s concerns. The regulation of SO₂ emissions, which mainly use NaHCO₃ as the SO₂ adsorbent, is receiving greater attention since it causes respiratory diseases in humans and acid rain [17]. The regulations governing SO₂ emissions are mainly applied at steel industry. After SO₂ reacts with NaHCO₃, it converts to Na₂SO₄ waste containing impurities (K, Ca, Fe, and Si), and related chemical equations are shown in Eqs. (1)–(2) [18,19,20].

$$ 2NaHCO_{3} \left( s \right) \, + {\text{ heat}} \to Na_{2} CO_{3} \left( s \right) \, + \, CO_{2} \left( g \right) \, + \, H_{2} O \, \left( g \right) $$

(1)

$$ Na_{2} CO_{3} \left( s \right) \, + \, 1/2O_{2} \left( g \right) \, + \, SO_{2} \left( g \right) \to Na_{2} SO_{4} \left( s \right) \, + \, H_{2} O \, \left( g \right) \, + \, CO_{2} \left( g \right) $$

(2)

Currently, there is no clear solution to treat Na₂SO₄ waste, thus, the only choice is direct landfill which can cause secondary pollution. In current situation in Republic of Korea, the import cost of NaHCO₃ and Na₂CO₃ are 315 and 345 $/ton, respectively. In the case of desulfurization waste, it is treated as “special waste” and its landfill cost is higher (180 $/ton) than common waste (90 $/ton) due to various heavy metals. Therefore, company have to pay for 495 $/ton-NaHCO₃ to removal SO₂, which is large burden for steel company [21]. With stricter environment regulation every year, more Na₂SO₄ waste will be produced. As such, there is an urgent need for its treatment method.

In this paper, we studied NaHCO₃ production using Na₂SO₄ waste and suggested feasible industrial processes. For comparison, pure Na₂SO₄ was employed. Various factors, such as the effect of H₂O:Na₂SO₄, NH₃:Na⁺, reaction temperature, pressure, and especially Na₂SO₄:Na₂CO₃ ratio, were explored to optimize NaHCO₃ production. We should note here that one of the reasons for selecting Na₂CO₃ as the Na⁺ source is that it can be easily obtained by simple heating of NaHCO₃. Reaction rate of different concentration of Na₂SO₄ and Na₂CO₃ solution also be explored to compare conversion rate to NaHCO₃. Finally, SO₂ removal efficiency of regenerated NaHCO₃ was estimated and compared with commercial NaHCO₃. The MINEQL + software, chemical equilibrium modeling system, was used to clarify the effect of impurities in Na₂SO₄ waste when produce NaHCO₃.

The major regeneration process consisted of five steps: (i) separation of insoluble components from Na₂SO₄ waste to increase the purity of NaHCO₃, (ii) production of a reaction solution with the mixture of Na₂SO₄, ammonia, and Na₂CO₃, (iii) carbonation reaction with CO₂, (iv) a part of regenerated NaHCO₃ is converted to Na₂CO₃ by heating, and (v) collection of the remaining NaHCO₃. The purity of regenerated NaHCO₃ was evaluated using X-ray diffraction (XRD) and X-ray fluorescence (XRF). The schematic for the suggested regeneration process of NaHCO₃ is shown in Fig. 1 and related chemical reactions are shown in Eqs. 3–5.

$$ Na_{2} SO_{4} \left( s \right) \, + \, 2H_{2} O \, \left( l \right) \, + \, 2NH_{3} \left( g \right) \, + \, 2CO_{2} \left( g \right) \to 2NaHCO_{3} \left( s \right) \, + \, \left( {NH_{4} } \right)_{2} SO_{4} \left( {aq} \right) $$

(3)

$$ Na_{2} SO_{4} \left( s \right) \, + \, 2H_{2} O \, \left( l \right) \, + \, 2CO_{2} \left( g \right) \to 2NaHCO_{3} \left( s \right) \, + \, H_{2} SO_{4} \left( {aq} \right) $$

(4)

$$ 2NaHCO_{3} \left( s \right) \, + {\text{ heat}} \to \, Na_{2} CO_{3} \left( s \right) + \, H_{2} O\left( g \right) + \, CO_{2} \left( g \right) $$

(5)

Fig. 1

The schematic for the regeneration of NaHCO3 using desulfurization waste with Na2CO3

The desulfurized Na₂SO₄ waste with Na₂CO₃ show higher NaHCO₃ yield and purity than that of pure Na₂SO₄ due to its higher starting pH (> 10). When comparing conversion rate to NaHCO₃ between Na₂SO₄ and Na₂CO₃, the desulfurized form show higher Na₂SO₄ conversion rate than Na₂CO₃, indicating that it can replace expensive NH₃ sources. Furthermore, regenerated NaHCO₃ show higher SO₂ removal efficiency than commercial NaHCO₃ because of its smaller particle size, i.e., greater surface area. The overall results suggest that the addition of more than 20% of Na₂CO₃ can significantly enhance the NaHCO₃ yield, which minimizes the need for expensive NH₃ sources. Typically, steel manufacturing companies struggle to meet carbon emission restrictions. We expect that this eco-friendly zero-waste process from industrial desulfurized Na₂SO₄ waste will help to meet the 2050 Carbon Neutral Declaration.

Experimental

Materials

Na₂SO₄ waste was obtained from the steel industry after the deSOx process. Reagents including sodium sulfate (Na₂SO₄, 99%), sodium carbonate (Na₂CO₃, 99%), ammonia solution (NH₄OH, 25–30%), calcium oxide (CaO, 98%) were purchased from SAMCHUN Chemicals, South Korea. Carbon dioxide (CO₂, 99.995%) was purchased from Linde, South Korea.

Characterization

Power X-ray diffraction (XRD) patterns were collected on PANalytical X’Pert diffractometer (Cu Ka radiation) with an X’Celerator detector. The purity of NaHCO₃ was determined by comparing the area of X-ray peaks around 2θ = 30.3°, 31.5°, and 16.4°, which correspond to the main peak of NaHCO₃, Na₂SO₄, and ammonia salt reflections, respectively. Crystal morphology was determined by a JEOL JSM-6510 scanning electron microscope (SEM). The solid-phase X-ray fluorescence (XRF) spectroscopy was obtained on RIGAKU ZSX and used in obtaining chemical composition data. The ICS-6000 ion chromatography measurements determined the Na⁺ and NH₄⁺ ion concentrations in solution. Particle size distribution was determined using a SYMPATEC-001 particle size analyzer. The pH of the sample before and after the reaction was measured using an Orion STAR A211 pH meter.

NaHCO ₃ Production

The suggested process of production of NaHCO₃ using Na₂SO₄ waste is shown in Fig. 1. To achieve Na⁺ solution, Na₂SO₄ waste was dissolved in purified water with different H₂O:Na₂SO₄ waste ratios from 1.0 to 10 under 300 rpm for 30 min. To separate insoluble solids, filtering was conducted. Next, the Na₂SO₄ solution and ammonia solution were mixed with different NH₃:Na⁺ ratios from 0.8 to 1.5 together with different amounts of Na₂CO₃. Here, we have used ammonia solution as ammonia source since a sublimable ammonium carbonate type complex can be produced when use NH₃ gas and easy to block the pipeline in the case of batch-type reactor [13]. During these processes, stirring and filtering temperatures were above 40 °C to avoid crystallization. The extracted solution was placed in a high-pressure reactor. CO₂ gas was then injected to increase the pressure ranging from 1.0 to 7.0 bar and the temperature varied from 5 to 40 °C for 8 h. The stirring speed was set as 300 rpm. After finishing the carbonation process, the NaHCO₃ slurry was filtered. The product was then washed, dried, and crushed. When conducting dry processes, the temperature remained below 50 °C to avoid the formation of Na₂CO₃. Collected NaHCO₃ was re-used for SO₂ removal. When necessary, temperatures exceeding 150 °C for 2 h were used to obtain Na₂CO₃.

NaHCO ₃ Yield and Purity Calculation

The NaHCO₃ yield and purity were calculated by the following equation:

$$ NaHCO_{3} yield\left( \% \right) = \frac{{mol\;of\;Na^{ + } \;in\;final\;product}}{{mol\;of\;Na^{ + } in\;Na_{2} SO_{4} \;waste}} \times 100\% . $$

The molar concentration of Na⁺ was measured by ion chromatography and/or XRF.

$$ NaHCO_{3} \;purity\left( \% \right) = \frac{{PA(30.3^{o} )}}{{PA\left( {31.5^{o} } \right) + PA(30.3^{o} )}} \times 100\% . $$

PA is the peak area of the XRD pattern. The 2θ values of 31.5 and 30.3 are major peaks of Na₂SO₄ and NaHCO₃, respectively. If there are other major crystalline impurities, corresponding major PA should be added to the denominator in the equation. If there were amorphous phases of impurities in a solid product, NaHCO₃ purity will be calculated using a combination of XRD and XRF analyses.

Simulation of Metal Speciation

Solubility domains were calculated using the chemical equilibrium modeling system MINEQL + (version 5.0) [22, 23]. This program can be used to compute equilibrium quantities of aqueous and solid species of each component using the appropriate thermodynamic stability constants. The calculations were performed at a constant temperature of 25 °C and the number of iteration cycles was constrained to 100. Calculations were made for all systems over the pH range from 1 to 20. The detailed simulation conditions of MINEQL + are shown in Table S1 in supporting information.

SO ₂ Removal

A schematic of the experimental setup is shown in Fig. S1. The system is composed of a gas control system, gas mixer, solid feeder, heater, filter-bag type reactor, induced draft fan, and gas analysis system. The desired amount of sulfur dioxide (SO₂), i.e., 200 ppm, as the inlet gas was prepared by mixing commercially available SO₂ with air. The gas flow rates are metered by rotameters and gas flow rate was set as 200 L/min. The gas samples at the reactor’s inlet and outlet were measured using gas analyzer (MRU analyzer VARIOluxx), based on NDIR (nondispersive infrared radiation) method. The reactor consists of a gas pulsing system and five cylindrical filters. The cylindrical filters set up in a bag house along the vertical direction. A needle punched polyester bag filter with a thickness of 2.0 mm was used in our experiment. Clean air was supplied to a bag house at pressure 2 bar to remove accumulated powder on filters. The pulsing interval was set as 1 min. The height of the bag filters were 50 cm. The feeding speed of NaHCO₃ varied from 0.5 to 10 g/min and temperature of whole system was set as 160 °C.

Results and Discussion

Figure 2 shows the powder XRD pattern, SEM image, and particle size distribution of Na₂SO₄ waste, i.e., NaHCO₃ after SO₂ removal, used as a raw material Na⁺ source in this study. For comparison, we have added reference XRD data, international centre for diffraction data (ICDD), of Na₂SO₄ (ICDD 00-037-1465), NaHCO₃ (ICDD 00-–015-070), NaCl (ICDD 01-077-2064), respectively. It can be seen in the amorphous phase with particle size approximately ~ 20 um. In its XRD pattern, except for the main peak of Na₂SO₄ at 31.5°, it also detected unreacted NaHCO₃ during SO₂ removal process and NaCl peaks at 45.3° and 30.1°, respectively. We have thought that NaCl is originated from HCl component in the flue gas during the desulfurization process. Table 1 shows the chemical composition data of Na₂SO₄ waste measured using the XRF method. As expected, major components of Na₂SO₄ waste were Na₂O and SO₃, which were determined to have 46.9 and 39.3 wt%, respectively. Besides that, Cl (7.15), CaO (2.14), K₂O (1.26), and Fe₂O₃ (1.04 wt%) were observed to be the major impurities. As known, calcium (Ca) and/or iron (Fe) salts are only slightly soluble in water, indicating that the insoluble components were required to be removed to increase purity of NaHCO₃.

Fig. 2

a XRD pattern of Na2SO4 waste (green) and reference data from international centre for diffraction data (ICDD) of Na2SO4 (blue), NaHCO3 (red), NaCl (black), and SEM image of Na2SO4 waste and (b) particle size distribution of Na2SO4 waste

Table 1 Chemical compositions data

The first step of regeneration NaHCO₃ is to extract the Na₂SO₄ solution. As mentioned above, insoluble phase is required to be removed. Here, we also need to point out the unusual solubility curve of Na₂SO₄, which sharply increased up to 40 °C, and decreased slightly thereafter at higher temperatures (shown in Fig. S2). It indicates that the temperature for dissolving Na₂SO₄ should be higher than 40 °C to gain the maximum recovery rate of Na⁺ ions. Figure 3a shows the amount of remaining solids after dissolving Na₂SO₄ waste at 40 °C with different H₂O:Na₂SO₄ waste ratios. It shows that the amount of insoluble solid significantly increased in H₂O:Na₂SO₄ waste ratios lower than 2.0. When this ratio increased, no significant changes were observed and maintained the value < 5 wt% of initial mass of Na₂SO₄. The chemical composition data of insoluble Na₂SO₄ waste (H₂O:Na₂SO₄ waste ratio 2.0) after solid/liquid separation are also shown in Table 1. As expected, the results indicate that its major components are calcium, iron, and sodium salts. Consistently, as shown in Fig. 3b, the Na⁺ recovery rate also followed the same tendency, i.e., the Na⁺ recovery rate increased with increasing H₂O/Na₂SO₄ waste ratio (> 90%). Considering only these two factors, higher H₂O/Na₂SO₄ waste ratios are more attractive due to the lower amount of waste and higher Na⁺ recovery rate.

Fig. 3

a Amount of insoluble solid and (b) Na + recovery rate after solid/liquid separation with different H2O:Na2SO4 waste ratios at 40 ℃ and atmospheric pressure

To produce NaHCO₃, the Na⁺ ions in solution need to react with CO₂. Most of the HCO₃⁻ ions which are part of the negative ions of NaHCO₃ salt are formed when the pH ranges from 7 to 9, suggesting that maintaining the pH between 7 and 9 is a key factor to maximize the yield of NaHCO₃ (Fig. S3). However, pH will get out of this range and decease below 7 when excess amount of CO₂ dissolve in Na₂SO₄ solution. Therefore, the addition of ammonia solution (NH₄OH) is necessary since it can serve as a pH buffer.

To check the effect of the concentration of Na⁺, we synthesized NaHCO₃ using extracted Na⁺ solutions from different H₂O:Na₂SO₄ waste ratios ranging from 1.0 to 3.5 at 40 °C and atmospheric pressure (Fig. 4a). As shown, NaHCO₃ purity had no significant change with different Na⁺ concentrations. On the other hand, solid yields (> 70%) showed an optimized range from 1.5 to 2.0. Both lower (< 1.5) and higher (> 2.0) range showed poor solid yields (< 40%) which can be attributed to the generation of a large amount of insoluble Na₂SO₄ waste and low concentration of Na⁺ solution, respectively.

Fig. 4

NaHCO3 yield (navy) and purity (green) with different (a) H2O:Na2SO4 waste ratio, (b) NH3:Na + ratio, (c) pressure, and (d) temperature

We also have changed the NH₃:Na⁺ ratio using Na⁺ solution with an H₂O:Na₂SO₄ ratio of 2.0 at 40 °C and 7 bar to check the effect of NH₃ (Fig. 4b). As mentioned, the pH range was maintained between 7 and 9 due to the formation of HCO₃⁻ ions. Thus, solid yield increased from 72 to 80% with increasing NH₃:Na⁺ ratios, i.e., increasing the pH and dissolving more CO₂ gas to solution. On the other hand, NaHCO₃ purity suddenly decreased from 90 to 70% when the NH₃:Na⁺ ratio reached 1.5 due to the formation of ammonia carbonate salt. The corresponding XRD patterns are shown in Fig. S4. If required, the purity of NaHCO₃ can be further increased (> 95%) through washing with H₂O. However, ~ 15% NaHCO₃ will be lost since it will dissolve in water when the (w/w) ratio of H₂O:NaHCO₃ is 1.0. Figure 4c, d exhibits the effect of pressure and temperature, respectively. In the case of pressure, NaHCO₃ yields are increased with increasing pressure since more CO₂ gas can be dissolved in solution. The sharp increase can be observed in pressure until 2 bar (at 40 °C) with constant NaHCO₃ purity (~ 90%). On the other hand, both yield and purity of NaHCO₃ underwent significant changes with increasing temperature (at 7 bar). The NaHCO₃ yield decreased while the purity of NaHCO₃ increased with increasing temperatures due to the increased solubility of Na₂SO₄ with higher temperatures.

The Na₂CO₃ can be easily obtained by simple heating of NaHCO₃. Figure 5 shows a series of XRD patterns of NaHCO₃ products after drying at different temperatures. We can observe NaHCO₃ peaks at 32 ° with a small amount of Na₂SO₄ peaks when the temperature is lower than 50 °C. On the other hand, major peaks of Na₂CO₃ gradually increased with increasing temperature to 150 °C, indicating that NaHCO₃ was being converted to Na₂CO₃. The produced Na₂CO₃ can recycle to carbonation process to production of NaHCO₃.

Fig. 5

A series of XRD patterns of NaHCO3 after drying at (a) 35, (b) 50, (c) 75, (d) 100, and (e) 150 °C. The XRD pattern of pure NaHCO3 and Na2CO3 are shown on the bottom lines for comparison

To further increase the yield and purity of NaHCO₃, we added different amounts of Na₂CO₃ source in the Na₂SO₄ waste solution and maintained the same concentration of Na⁺ and NH₃. Interestingly, solid yield significantly increased from 72 to 90% with an increasing amount of Na₂CO₃ from 0 to 50 wt%, as shown in Fig. 6a. When the Na₂CO₃/(Na₂CO₃ + Na₂SO₄) ratio was increased to over 20 wt%, the solid yield increased only negligibly, indicating that the optimized point had been reached.

Fig. 6

a NaHCO3 yield (navy) and purity (green) with different Na2CO3/(Na2CO3 + Na2SO4) ratio of Na2SO4 waste solution. The pH changes with different amount of NH4OH added for different Na2CO3/(Na2CO3 + Na2SO4) ratio on (b) Na2SO4 waste and (c) pure Na2SO4 solution, and (d) MINEQL simulation of chemical equilibria of various ions in aqueous systems with different pH

To clarify why the addition of a certain amount of Na₂CO₃ significantly enhanced the yield of NaHCO₃, we measured the pH changes with different amounts of NH₄OH added (0–30 g/100 g solution) on Na₂SO₄ waste and pure Na₂SO₄ solution, as shown in Fig. 6b and c, respectively. The pH of both solutions increased with increased amounts of NH₄OH and Na₂CO₃ due to their basic properties. It can also explain why the addition of Na₂CO₃ can increase the NaHCO₃ yield. The starting pH of Na₂SO₄ waste solution is higher than that of pure Na₂SO₄ solution (10 vs 7).

Initially, a rapid increase of pH value on pure Na₂SO₄ solution can be observed in whole range with the addition of 5% of Na₂CO₃ (Fig. 6c). And then, pH value is gradually increased with increasing Na₂CO₃ amount but converges when the Na₂CO₃ content exceeds 50%, as can be expected from traditional acid–base titration curves. Whereas, the overlapped pH curves during the addition of 10–20 wt% Na₂CO₃ can be observed on Na₂SO₄ waste solution, indicating that a buffering effect may occur. To understand this phenomenon, we carried out chemical equilibrium simulation using MINEQL + software based on experimental composition. As shown in Fig. 6d, NaHCO₃ occupies the major concentration in the solution when the pH ranges from 5 to 12. It can be attributable to a higher concentration of HCO₃⁻ ions in this pH range according to the carbonate system (Fig. S2). Except for that, ~ 10 wt% of CaCO₃ and FeCO₃ are formed when the pH is higher than 7 due to their strong ionic strength. According to chemical composition data of Na₂SO₄ waste, Table S2 lists possible OH⁻, CO₃²⁻, and SO₄²⁻ salts which can be precipitated during the reaction. We expect that not only calcium and iron but also other insoluble salts can serve as a pH buffer. To compare with experimental data, we collected precipitated solids before the carbonation reaction. The chemical composition result shows that it is mainly composed of a mixture of calcium and iron salts. Furthermore, the XRD pattern also can be assigned as CaCO₃, FeCO₃, and MgFeCO₃, which can further support our simulation result, as shown in Table S3 and Fig. S5, respectively.

In the practical application, NH₃ consumption is critical factor to determine operating cost of production of NaHCO₃. To minimize NH₃ consumption, we have compared amount of NH₃ (NH₃:Na₂SO₄ waste ratio) with different Na₂CO₃/(Na₂CO₃ + Na₂SO₄) ratio under the same NaHCO₃ yield, as shown in Fig. 7. In the case of a pure Na₂SO₄ solution, NH₃ consumption is linearly decreased with increasing Na₂CO₃/(Na₂CO₃ + Na₂SO₄) ratios. Whereas, a gentle decreasing curve is observed until a Na₂CO₃/(Na₂CO₃ + Na₂SO₄) ratio of 20 in the case of Na₂SO₄ waste solution. After that, the NH₃:Na⁺ ratio is suddenly diminished and follows the same tendency with that of pure Na₂SO₄ solution. This result also can be explained by the pH buffer effect caused by impurities. NH₃ consumption can be reduced by more than 50% when Na₂CO₃/(Na₂CO₃ + Na₂SO₄) ratios are higher than 20, and 20% lower NH₃ usage can be achieved on Na₂SO₄ waste solution compared to pure Na₂SO₄ solution. After production of NaHCO₃, in waste water treatment, NH₃ recovery process should be included to minimize operating cost and effluent standard since ammonia is the most expensive raw material in this process, i.e., 525 $/ton [21]. The ammonia recovery rate of commercial plant is generally higher than 80%. If applied to our process, it can further decrease operating cost.

Fig. 7

NaHCO3 yield (navy), purity (green), and NH3:Na + ratio (pink) changes with different Na2CO3 / (Na2CO3 + Na2SO4) ratio on (a) Na2SO4 waste and (b) pure Na2SO4 solution

To compare Na⁺ conversion rate between Na₂SO₄ and Na₂CO₃, we have measured NaHCO₃ yield, i.e., Na⁺ conversion, in different Na⁺ concentration of Na₂SO₄ and Na₂CO₃ solution, respectively. Figure S6 shows both Na⁺ conversion rate between Na₂SO₄ and Na₂CO₃ are increased with increasing their concentration. In the case of their pure form, the Na₂CO₃ show higher yield (78% vs 60%) and rapid conversion rate (max. slope 7.7 vs 5.2) than that of Na₂SO₄. Interestingly, significantly enhanced Na⁺ yield (81% vs 60%) and conversion rate (8.1 vs 5.2) can be observed in the case of Na₂SO₄ waste when compared to pure Na₂SO₄. It can be attributed to relative high pH (> 10) of Na₂SO₄ waste. No significant changes can be observed on Na⁺ yield (78% vs 77%) and conversion rate (7.8 vs 7.7) in the case of Na₂CO₃ waste and its pure form. This result shows that Na₂SO₄ waste exhibit higher Na⁺ conversion rate of Na₂SO₄ waste than that of Na₂CO₃ in the range of Na₂CO₃/(Na₂CO₃ + Na₂SO₄) from 0 to 30%, which can further support our results.

Finally, we measured the SO₂ removal efficiency of regenerated NaHCO₃ and compared it with commercial NaHCO₃ using a simulated gas mixture containing 200 ppm of SO₂ gas, as shown in Fig. 8. Interestingly, SO₂ removal efficiency of regenerated NaHCO₃ is higher than that of commercial NaHCO₃ at a feed input speed of 1.0 g/min (54 vs 47%), and regenerated NaHCO₃ reached the 95% level of SO₂ removal efficiency at 2.0 g/min. To understand why regenerated NaHCO₃ had a better performance for SO₂ removal, we measured the particle size distribution. Figure 8b indicates that regenerated NaHCO₃ has a smaller particle (~ 40 um) size than commercial NaHCO₃ (~ 80 μm). This result suggests that physisorption of SO₂ plays an important role due to its higher surface area, which can be obtained by using smaller particle sizes. In addition, the lower impurity of regenerated NaHCO₃ than commercial NaHCO₃ (99% vs. 90%) further supports this claim. Overall results concluded that regenerated NaHCO₃ adsorbent was also able to be used for SO₂ removal.

Fig. 8

a SO2 removal efficiency and (b) particle size distribution of regenerated NaHCO3 (navy) and commercial NaHCO3 (green)

Conclusions

We were able to regenerate NaHCO₃ using Na₂SO₄ waste with Na₂CO₃ by the suggested NaHCO₃ production processes. Interestingly, the yield of NaHCO₃ is significantly increased (~ 90%) with the addition of 20 wt% of Na₂CO₃, and its purity is higher than 95% after washing. To clarify this phenomenon, we also checked pH changes during the production of NaHCO₃. Our results clearly show that enhanced NaHCO₃ yield by the addition of Na₂CO₃ is caused by the higher pH of Na₂CO₃ (11) compared to Na₂SO₄ waste (9.5) or pure Na₂SO₄ (7). They also indicate that the addition of Na₂CO₃ can reduce the consumption of expensive NH₃. We have found that addition of 20% of Na₂CO₃ in Na₂SO₄ waste solution can save 50% of NH₃ consumption. This value is 20% lower than pure Na₂SO₄ solution. The regenerated NaHCO₃ also shows excellent SO₂ removal efficiency compared to commercial NaHCO₃ due to the smaller particle size distribution. The overall experimental results suggest that the addition of Na₂CO₃ can further enhance NaHCO₃ production efficiency when using Na₂SO₄ waste as the Na⁺ source.