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Article

Preparation of Gypsum–Urea with Enhanced Sustainability from Flue Gas Desulfurization Gypsum in Saturated Urea Solution

by
Caiyun Jia
1,2,3,*,
Jiang Zhao
1,2,3,
Xiaoxia Fang
1,2,3,
Pujun Wang
1,2,3,
Anni Xiao
1,2,3 and
Haijun Zhang
1,2,3,*
1
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, China
2
State Key Laboratory of Coking Coal Resources Green Exploitation, China University of Mining and Technology, Xuzhou 221116, China
3
National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou 221116, China
*
Authors to whom correspondence should be addressed.
Sustainability 2024, 16(14), 6208; https://doi.org/10.3390/su16146208
Submission received: 8 June 2024 / Revised: 2 July 2024 / Accepted: 12 July 2024 / Published: 20 July 2024
(This article belongs to the Topic Advances in Sustainable Materials and Products)
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Versions Notes

Abstract

:
Gypsum–urea is a kind of urea product with substantially reduced aqueous solubility and lower hygroscopicity that increases the soil retention time of urea and thus enhance its environmental sustainability. Here, gypsum–urea was prepared using bulk industrial solid waste flue gas desulfurization (FGD) gypsum as a raw material in a saturated urea solution via immobilizing urea molecules into the crystal lattice. The preparation process was achieved through a dissolution–recrystallization mechanism during which FGD gypsum dissolved into Ca2+ and SO42−, which then recrystallized with CO(NH2)2 to form gypsum–urea. The preparation process was almost completed within 10 min, and the formed gypsum–urea presented a uniform size distribution of 30–90 μm and a much lower hygroscopicity and nitrogen release efficiency than that of urea. With a high efficiency of synthesis, and sustainable features, and the recyclability of the saturated mother urea solution, the dissolution–recrystallization-based urea immobilization approach is highly promising regarding the preparation of gypsum–urea with the desired environmental sustainability and contributes to the realization of the sustainable reutilization of FGD gypsum.

1. Introduction

Urea is a widely used fertilizer to provide the main source of nitrogen for plant production. However, urea has a high solubility in water, with 105 g per 100 mL water at 20 °C, thus resulting in a loss of nitrogen that could exceed 50% of the nitrogen used [1] and yielding watershed and air contaminants (N2O, a potent greenhouse gas [2,3,4], or NO3− ions, a primary source of N-related eutrophication [2,5,6]). Improving nitrogen management for greater agricultural output while minimizing unintended environmental consequences is critical in the sustainable feeding of a growing population amid climate change [7]. Some technologies have been developed to control the release of nitrogen into the environment via better matching of the plant’s nutrient needs using physical coating materials to hinder the contact between the urea and the soil, thus resulting in a gradual release of nitrogen [8,9,10]. This could also be achieved through the addition of biological inhibitors to conventional urea fertilizer, mainly including urease inhibitors and nitrification inhibitors, to delay the hydrolysis of urea or prevent the microbial conversion of ammonium (NH4+) to nitrate (NO3−), thereby reducing the potential nitrogen losses and improving the nitrogen utilization efficiency [11,12,13]. However, these physical or biological technologies often suffer from complex product processing and irregular nutrient release and therefore are not panaceas for resolving the nitrogen problem [7].
Currently, countries around the world are acknowledging the significance of green chemical technology as a pivotal strategy for attaining sustainable development in the 21st century. Green chemical technology represents a fundamental aspect of sustainable development technology, which strives to maximize efficiency and economy and minimize issues of high pollution in the chemical industry to alleviate the shortage of resources and energy [14]. With regard to the development of green chemical technologies of urea, recent efforts have been made to the modification of the intrinsic properties of urea-based compound fertilizers, including their solubility or propensity to react in moist environments [15,16,17,18,19,20]. In particular, the formation of urea cocrystals with substantially reduced aqueous solubility and lower hygroscopicity has been shown to be a promising strategy to increase the soil retention time of urea and thus enhance its environmental sustainability [21,22,23,24,25]. Among these urea cocrystals, gypsum–urea (CaSO4·4CO(NH2)2) is an excellent urea fertilizer candidate [26] serving as a source of nitrogen, calcium, and sulfur, all of which are needed for plant growth, and being thought to be more stable than urea in moist environments [27,28]. Gypsum–urea can be prepared using industrial byproducts of gypsum, such as drywall gypsum [18] and phosphogypsum [28], as raw materials, and their current preparation strategies include mechanochemistry [18,29] or the saturated urea solution method [30]. For mechanochemistry, a mixture of gypsum and urea with corresponding molar ratios is loaded into a jar together with individual steel balls and is grounded in a mill under a controlled temperature using different amounts of water. Consequently, sometimes a complete conversion to gypsum–urea is not always achieved, even with very long grinding times [27], and therefore, the procedure is time consuming and energy intensive in some cases [18]. For the saturated urea solution method, a mixture of gypsum and urea in a molar ratio of 1:4 is added to the saturated urea solution and then reacts spontaneously at room temperature. The preparation process is completed within a certain timeframe, and the saturated mother urea solution can be recycled, which presents a great potential for sustainable production from a green chemistry perspective.
Here, gypsum–urea is prepared in a saturated urea solution at room temperature using flue gas desulfurization (FGD) gypsum as a raw material (Figure 1). FGD gypsum is a typical bulk industrial solid waste formed during the flue gas desulfurization process and is mainly composed of calcium sulfate dihydrate (CaSO4·2H2O) [31,32]. FGD gypsum is mostly utilized as a soil amendment in agriculture to supply Ca and S for plant nutrition, thus improving soil physicochemical properties, controlling soil and nutrient loss, and increasing crop yield [33,34,35,36]. Therefore, the preparation of gypsum–urea using FGD gypsum as a raw material is promising.
When mixing urea with FGD gypsum in saturated urea solution, urea will react with calcium sulfate dihydrate to form gypsum–urea through the following reaction [27]:
CaSO4·2H2O + 4CO(NH2)2 → CaSO4·4CO(NH2)2 + 2H2O
Such a preparation process can immobilize urea molecules into the crystal lattice, and the formed gypsum–urea is separated from the reaction solution to realize the recycling of the mother saturated urea solution. Therefore, this precipitation process is capable of continuously preparing gypsum–urea without the concern of handling capacity. Our results showed that the preparation process could be completed within 10 min in the temperature range of 25 to 35 °C. The formed gypsum–urea presented a reduced hygroscopicity and nitrogen release efficiency than that of urea.

2. Experimental Section

2.1. Materials

Analytical reagent grade urea and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. FGD gypsum was received from the Yuanyanghu Power Plant, Yinchuan, Ningxia Hui Autonomous Region, China. The initial FGD gypsum was dried in an oven at 60 °C for 24 h, and then ball-milled to powders with a size distribution of 80–120 μm. The chemical composition analysis (Table 1) indicated that the initial FGD gypsum was composed of 86.99% calcium sulfate. Deionized water was used in all experiments.

2.2. Preparation Methods

The preparation process was performed in a 500 mL glass reactor with constant magnetic stirring at the temperatures of 25 °C, 30 °C, and 35 °C (with a deviation of ± 0.5 °C). A typical procedure was as follows: a saturated urea solution was prepared by dissolving certain amounts of urea into 200 mL of deionized water, and heated to 25 °C, 30 °C, and 35 °C (with a deviation of ± 0.5 °C), respectively. Then, 20.87 g of FGD gypsum and 29.13 g of urea were added into the saturated urea solution. During the reaction, 20 mL of reaction solution was withdrawn at selected time intervals, vacuum filtered immediately, washed three times with ethanol, and dried in an oven at 60 °C for 4 h. As the reaction was finished, the formed gypsum–urea was separated from solution via vacuum filtration, and then a certain amount of urea was added into the segregated urea solution to maintain its saturation. The saturated urea solution was used in the next preparation process.
To test the nitrogen release efficiency, gypsum–urea and urea crystals were mixed with starch paste and then molded to pellets, respectively. The starch paste was formed by mixing starch and water in a mass ratio of 1:3, heated to 80 °C for 30 min, and then cooled down. Next, 10 g of gypsum–urea or urea crystals was mixed with 0.6 g of starch paste to form a mixture. Then, 1.7 g of mixture was added into a mold to form a cylindrical pellet in a height of 13 mm and a diameter of 7 mm under a load of 10 MPa.

2.3. Characterization

The dried gypsum–urea samples were collected for morphological and chemical analyses. A metallographic microscope (XJP-6A, Chongqing Optical and Electrical Instrument Co., Ltd., Chongqing, China) and scanning electron microscopy (SEM, HITACHI S-4800, Tokyo, Japan) were used to obtain the crystal morphology and analyze the reaction process. The X-ray fluorescence (XRF, S8 Tiger, Bruker, Saarbrucken, Germany) was used to obtain the chemical composition of FGD gypsum. X-ray diffraction (XRD, D/Max-2550PC, Rigaku Inc., Tokyo, Japan) analyses were conducted using Cu Kα radiation at a scanning rate of 5°/min in the 2 θ range from 5° to 85°. For thermogravimetry and differential scanning calorimetry (TG-DSC, STA 409PC NETZSCH, Selb, Germany) analysis, 20 mg of dried sample was put into an Al2O3 crucible with a lid under temperature programming from 35 to 600 °C at a heating rate of 10 °C/min under a nitrogen gas atmosphere. The Fourier transform infrared (FTIR) spectra were tested on a spectrometer (IRAffiinity-1S, Shimadzu, Shimane, Japan) with a resolution of 4 cm−1 over a frequency range of 400–4000 cm−1. To qualitatively assess the hygroscopicity of the formed gypsum–urea, the synthesized gypsum–urea and urea crystals were exposed to moist air at 23 °C and 84% relative humidity in a closed static environment and their physical state was monitored for 15 days. The nitrogen release efficiency of gypsum–urea and urea crystals pellets was tested according to GB/T 23348-2009: one pellet of gypsum–urea or urea was put into a 250 mL flask and immersed into 200 mL of deionized water. Then, the flask was sealed and put into an electro-thermostatic water cabinet at 25 °C. The concentration of nitrogen in solution was tested at the 430 nm wavelength using an ultraviolet and visible spectrophotometer (UV-2600i, Shimadzu Co., Ltd., Shimane, Japan). Firstly, a series of urea standard solutions were tested at the 430 nm wavelength and the standard curve of absorbance vs. urea concentration was drawn. Then, the absorbance of nitrogen release solution was tested on the same condition as the urea standard solution. The corresponding nitrogen concentration was calculated from the standard curve.

3. Results and Discussion

3.1. Analysis of Initial FGD Gypsum

The initial FGD gypsum was a classic bulk industrial solid waste generated from a coal-fired thermal power plant in the Ningxia Hui Autonomous Region, China. The initial wet FGD gypsum samples were dried and crushed before use. SEM images (Figure 2a) showed the initial FGD gypsum particles presented a blocky shape in the size of 80–120 μm, and there were also many fragments of particles formed by the crush process. What is more, the initial FGD gypsum particles were riddled with cracks on all of the crystal surfaces (Figure 2b). The cracked morphology was consistent with the metallographic microscope image in Figure 2c where the initial FGD gypsum particles presented a dark instead of transparent image because of the cracked surfaces. Further XRD analysis showed the initial FGD gypsum presented sharp diffraction peaks located at 11.6°, 20.7°, 23.4°, and 29.1° (Figure 2d), which were typical for the crystalline gypsum (calcium sulfate dihydrate, CaSO4·2H2O) phase, and no diffraction peaks corresponding to bassanite (calcium sulfate hemihydrate, CaSO4·0.5H2O) and anhydrite (calcium sulfate anhydrate, CaSO4) were detected. Altogether, it was obvious that the initial FGD gypsum was composed of calcium sulfate dihydrate that presented great potential to serve as a raw material for the preparation of gypsum–urea.

3.2. Preparation of Gypsum–Urea

The preparation of gypsum–urea was conducted in a saturated urea solution at room temperature (30 °C) under atmospheric pressure. Metallographic microscope images (Figure 3a–e) showed that the preparation process was spontaneously proceeded via a dissolution–recrystallization mechanism. Within the first minute, a mixture of undissolved FGD gypsum and urea particles was captured in the solution (Figure 3a). Subsequently, FGD gypsum and urea particles dissolved to form short prismatic gypsum–urea particles with dimensions of 20–50 μm and no FGD gypsum or urea particles were observed within 4 min (Figure 3b). Thereafter, the gypsum–urea particles grew up to 50–90 μm and kept stable in solution in the following 20 min (Figure 3c–f). What is more, the formed gypsum–urea particles presented good transparency under the metallographic microscope, which indicated the gypsum–urea particles had a fine crystallinity and a smooth surface, as confirmed by the SEM image (Figure 3f) where the crystal plane surfaces developed well without cracks.
To have a quantitative evaluation of the preparation process, TG-DSC analysis was applied. As shown in Figure 4, urea had a weight loss of 100 wt% under a nitrogen atmosphere and presented four endothermic peaks at 141 °C, 230 °C, 249 °C, and 352 °C on the DSC curve, which represented to the melting of urea, melting of biuret in an eutectic mixture, urea decomposition and melting of biuret, and sublimation of cyanuric acid, respectively [18]. The weight loss at 1 min was 90 wt% owing to a mixture of a large amount of undissolved urea and a small amount of FGD gypsum crystals as observed in Figure 4a; simultaneously, the DSC curve presented a typical profile of urea with a negative-shift of the endothermic peaks from 249 to 242 °C and from 352 to 340 °C. While as the preparation process proceeded to 4 min, the weight loss reduced to 70 wt%, and in the DSC curve, a typical profile of gypsum–urea with characteristic endothermic peak at 201 °C appeared with the disappearance of the endothermic peaks for urea at 230 °C, 242 °C, and 340 °C, both of which indicated that the majority of the urea had dissolved and reacted with Ca2+ and SO42− ions to form gypsum–urea. The preparation process was almost completed within 10 min, during which the weight loss reached 64 wt%, consistent with the theoretical value of gypsum–urea [CaSO4·4CO(NH2)2]. The residual endothermic peak in the DSC curve at 141 °C was maybe due to the microprecipitation of urea from the saturated urea solution during the vacuum filtration process.
Furthermore, the XRD (Figure 5a) and FTIR (Figure 5b) analyses were performed to analyze the preparation process. The XRD pattern at 1 min showed a characteristic diffraction pattern of urea, while the corresponding FTIR spectrum presented a typical profile of urea with characteristic δs(NH2) vibration at 1680 cm−1, ν(CO) vibration at 1598 cm−1, νas(CN) vibration at 1465 cm−1, and π(CO) vibration at 789 cm−1. Typically, the adsorption peaks at 1152, 1065, and 1003 cm−1 corresponded to 70% ρs(NH2) + 28% ν(CO) vibrations, 81% ρs(NH2) + 19% νas(CN) vibrations, and 84% νas(CN) vibration, respectively. Both of the analyses confirmed that the samples obtained at 1 min were mainly composed of undissolved urea crystals. The preparation process was indicated to have largely proceeded at 4 min, as the characteristic diffraction peaks located at 11.9°, 12.1°, 16.5°, 17.5°, 26.3°, and 27.8° for gypsum–urea emerged in the XRD pattern along with the disappearance of those located at 22.3° for urea; while in the FTIR spectrum, the typical adsorption peaks for gypsum–urea at 1094, 645, and 611 cm−1 corresponding to γ3(SO42−) and γ4(SO42−) stretching appeared with the disappearance of the adsorption peaks for urea at 1152, 1065, and 1003 cm−1. What is more, the adsorption peaks at 1680 and 1598 cm−1 presented a negative-shift to 1673 and a positive-shift to 1604 cm−1, respectively. The preparation process was almost completed within 10 min, during which the XRD pattern showed the typical diffraction peaks for gypsum–urea [18] and the FTIR spectrum exhibited a typical profile of gypsum–urea with the π(CO) vibration at 789 cm−1 shifted to 796 cm−1 and the νas(CN) vibration at 1465 cm−1 being enhanced in intensity.
The above results confirmed that the FGD gypsum particles with cracked surfaces were completely transformed to gypsum–urea in the saturated urea solution at 30 °C. The preparation process was almost completed within 10 min and the prepared gypsum–urea particles presented a uniform size distribution of about 50–90 μm with fine crystallinity. In summary, FGD gypsum could serve as a great resource to achieve the phase-transition of urea to gypsum–urea, which presented an unusual reactivity and strong potential for sustainable production.
Based on the above analysis, a “dissolution–recrystallization” process could be illustrated as in Figure 6. When FGD gypsum and urea particles were added into the saturated urea solution, FGD gypsum dissolved to form Ca2+ and SO42− in solution. When the concentration of Ca2+, SO42−, and CO(NH2)2 had reached the supersaturation of CaSO4·4CO(NH2)2, the crystallization of gypsum–urea was proceeded spontaneously which immobilized CO(NH2)2 molecules into the crystal lattice of gypsum–urea, during which urea dissolved to CO(NH2)2 to keep a saturated urea state. Along with the preparation process, the gypsum–urea particles grew larger and settled to the bottom of the container. The formed gypsum–urea particles were then separated from the reaction solution and the mother saturated urea reaction solution was recycled.
Figure 7 showed the corresponding crystal structural transition of gypsum to gypsum–urea with a view in the c-direction. Gypsum and gypsum–urea had the same structural motif of (–Ca–SO4–Ca–SO4–) chains but were differentiated in how these chains were assembled and where H2O molecules and CO(NH2)2 molecules laid. In gypsum, the chains aligned to form perfect layers parallel to the (010) plane and the H2O molecules laid between every two layers, which formed a sandwich-like structure. In gypsum–urea, the chains aligned along the c-direction, and because of ion–dipole attractions, each calcium ion laid roughly at the center between the oxygen atoms of four urea molecules that were interconnected through hydrogen bonds [37]. The alignment transition of the (–Ca–SO4–Ca–SO4–) chains provided a large volume capacity to hold CO(NH2)2 molecules, which should be responsible for the capability of immobilization of CO(NH2)2 into the crystal lattice of gypsum–urea. Compared with common urea crystals, the immobilization of CO(NH2)2 into the crystal lattice could reduce the hygroscopicity and enhance the stability of CO(NH2)2 and therefore realize the goal of sustained release of nitrogen.

3.3. Effect of Temperature on Preparation Process and Morphology Evolution

To tune the preparation process of gypsum–urea in saturated urea solution, temperature was the top priority considering its high efficiency and contribution to energy conservation. The evolution of XRD patterns for samples prepared at 25 °C and 35 °C is shown in Figure 8a,b. The preparation process was found to have mostly proceeded within 4 min and almost completed within 10 min, and the prepared gypsum–urea had very fine crystallinity with sharp diffraction peaks, which were very similar to that prepared at 30 °C, as shown in Figure 8a.
The dependence of the size distribution and shape of gypsum–urea particles on the temperature range of 25 °C to 35 °C is presented in Figure 9 with a similar size distribution. In the condition of 25 °C, gypsum–urea particles presented a short prismatic shape with a length of 30–90 μm and a width of 10–40 μm (Figure 9a,d). As the reaction temperature was increased to 30 °C, gypsum–urea particles remained as short and prismatic with a length of 50–90 μm and a width of 10–40 μm (Figure 9b,e). Further increasing the reaction temperature to 35 °C generated a length to 30–70 μm and a width to 20–40 μm (Figure 9c,f). What is more, the aspect ratio of the above gypsum–urea products kept a close value of around 1.5–2.6.
The above XRD and morphology evolution analyses indicated that the phase-transition process was not temperature–dependent in the temperature range of 25 to 35 °C. The prepared gypsum–urea particles presented a short prismatic shape in a length of 30–90 μm and a width of 10–40 μm with fine crystallinity. It is indicated that the dissolution–recrystallization based process keeps a high phase-transition efficiency to spontaneously prepare gypsum–urea with fine crystallinity at room temperature.

3.4. Hygroscopicity and Nitrogen Release Efficiency of Gypsum–Urea

Hygroscopicity is an important factor in evaluating the stability of urea-based fertilizers. Previous work had shown that the dry solid Ca(H2PO4)2·4CO(NH2)2 was non-hygroscopic at a relative humidity below 60% [38], Ca(NO3)2·4CO(NH2)2 was less hygroscopic than Ca(NO3)2·4H2O because of non–crystalline water [39], CaSO4·4CO(NH2)2 was dry and free-flowing at relative humidity below 75%, and urea became wet at around 72% relative humidity [21]. Here, the hygroscopicity test was performed that urea and gypsum–urea particles prepared at 25 °C, 30 °C, and 35 °C were exposed to 84% relative humidity at 23 °C for 15 days. The time-resolved images are shown in Table 2. It appeared that urea had a much higher hygroscopicity than that of gypsum–urea. On Day 1, all the samples were in a free-flowing state. Later, on Day 6, part of urea particles had deliquesced and absorbed moisture to form liquids in the edge of the plate, while some small caking appeared in the surface of the three gypsum–urea samples. Furthermore, more urea particles had deliquesced and the bottom of the plate was a spread of liquids (Day 6). Still, the gypsum–urea samples maintained a minor amount of moisture formation, and no liquid droplets were seen even after 15 days of moisture exposure. This result suggested that the prepared gypsum–urea particles had a much higher stability than that of pure urea, thus stabilizing the immobilized urea molecules from hydrolysis and potentially minimizing nitrogen losses.
To further have a quantitative evaluation of the nitrogen release efficiency of the prepared gypsum–urea particles, nitrogen release experiments were performed and the nitrogen release curve is shown in Figure 10. It appeared that the nitrogen release efficiency of urea was much faster than that of gypsum–urea. For urea, the cumulative nitrogen released reached almost 100% within 30 min; while gypsum–urea exhibited a much slower nitrogen release efficiency with only 22.5% of nitrogen released after 30 min. Thereafter, the cumulative nitrogen released reached 82.6% after 180 min and the entire amount of nitrogen was released after about 480 min, which indicated an enhanced stability of urea within the gypsum–urea crystal lattice.

4. Conclusions

This work prepared gypsum–urea using bulk industrial solid waste FGD gypsum as a raw material in a saturated urea solution in a sustainable production pathway at room temperature for the enhanced environmental sustainability of nitrogen. The preparation process was almost completed within 10 min through a dissolution–recrystallization mechanism, and the prepared gypsum–urea particles had a uniform size distribution of 30–90 μm with fine crystallinity. What is more, the preparation process kept a similar crystallization rate in the temperature range of 25 to 35 °C. The prepared gypsum–urea particles had a much lower hygroscopicity and nitrogen release efficiency than that of urea. Altogether, the dissolution–recrystallization based approach to immobilize CO(NH2)2 molecules into the crystal lattice of gypsum–urea spontaneously prepared gypsum–urea with fine crystallinity, reduced hygroscopicity, and enhanced stability of urea, which contributed to an environmentally sustained release of nitrogen and sustainable reutilization of FGD gypsum.

Author Contributions

C.J.: Conceptualization, methodology, investigation, formal analysis, funding acquisition, writing—original draft. J.Z.: Investigation, formal analysis. X.F.: Investigation, formal analysis. P.W.: Investigation. A.X.: Investigation. H.Z.: Conceptualization, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (52204290), Science and Technology Planning Project of Jiangsu Province (BK20221129), and Funds for Jiangsu Provincial Double-Innovation Doctor (140923028).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of financial interest.

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Figure 1. Utilization of FGD gypsum and urea to prepare gypsum–urea as sustained release fertilizer. (Reprinted with permission from Xiaoxia Fang. Copyright 2024).
Figure 1. Utilization of FGD gypsum and urea to prepare gypsum–urea as sustained release fertilizer. (Reprinted with permission from Xiaoxia Fang. Copyright 2024).
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Figure 2. Characterization of initial FGD gypsum. (a,b) SEM images of initial FGD gypsum. (c) Metallographic microscope image of initial FGD gypsum. (d) XRD analysis of initial FGD gypsum in comparison with crystalline calcium sulfate phases: gypsum (calcium sulfate dihydrate, CaSO4·2H2O), bassanite (calcium sulfate hemihydrate, CaSO4·0.5H2O), and anhydrite (calcium sulfate anhydrate, CaSO4).
Figure 2. Characterization of initial FGD gypsum. (a,b) SEM images of initial FGD gypsum. (c) Metallographic microscope image of initial FGD gypsum. (d) XRD analysis of initial FGD gypsum in comparison with crystalline calcium sulfate phases: gypsum (calcium sulfate dihydrate, CaSO4·2H2O), bassanite (calcium sulfate hemihydrate, CaSO4·0.5H2O), and anhydrite (calcium sulfate anhydrate, CaSO4).
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Figure 3. (a–e) Morphological evolution of the solid phase along the synthesis of gypsum–urea in saturated urea solution at 30 °C as observed by metallographic microscope. (f) SEM image of gypsum–urea obtained at 20 min showing a smooth surface with fine crystallinity.
Figure 3. (a–e) Morphological evolution of the solid phase along the synthesis of gypsum–urea in saturated urea solution at 30 °C as observed by metallographic microscope. (f) SEM image of gypsum–urea obtained at 20 min showing a smooth surface with fine crystallinity.
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Figure 4. (a) TG and (b) DSC curves; evolution of solid phase along the preparation of gypsum–urea in saturated urea solution at 30 °C. *—residual micro-precipitated urea.
Figure 4. (a) TG and (b) DSC curves; evolution of solid phase along the preparation of gypsum–urea in saturated urea solution at 30 °C. *—residual micro-precipitated urea.
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Figure 5. (a) XRD and (b) FTIR patterns’ evolution of the solid phase along the preparation of gypsum–urea in saturated urea solution at 30 °C. *—residual micro-precipitated urea.
Figure 5. (a) XRD and (b) FTIR patterns’ evolution of the solid phase along the preparation of gypsum–urea in saturated urea solution at 30 °C. *—residual micro-precipitated urea.
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Figure 6. Schematic illustration of the preparation of gypsum–urea in a saturated urea solution at room temperature.
Figure 6. Schematic illustration of the preparation of gypsum–urea in a saturated urea solution at room temperature.
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Figure 7. Crystal structural transition from gypsum to gypsum–urea with a view in the c-direction [37].
Figure 7. Crystal structural transition from gypsum to gypsum–urea with a view in the c-direction [37].
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Figure 8. XRD patterns evolution of solid phase along the preparation of gypsum–urea in saturated urea solution at 25 °C (a) and 35 °C (b). *—residual micro-precipitated urea.
Figure 8. XRD patterns evolution of solid phase along the preparation of gypsum–urea in saturated urea solution at 25 °C (a) and 35 °C (b). *—residual micro-precipitated urea.
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Figure 9. Morphology of gypsum–urea synthesized in saturated urea solution under different temperatures: (a,d) 25 °C, (b,e) 30 °C, (c,f) 35 °C.
Figure 9. Morphology of gypsum–urea synthesized in saturated urea solution under different temperatures: (a,d) 25 °C, (b,e) 30 °C, (c,f) 35 °C.
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Figure 10. Comparison of cumulative nitrogen release efficiency between urea and gypsum–urea prepared at 30 °C.
Figure 10. Comparison of cumulative nitrogen release efficiency between urea and gypsum–urea prepared at 30 °C.
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Table 1. Chemical composition of initial FGD gypsum (wt%).
Table 1. Chemical composition of initial FGD gypsum (wt%).
CaOSO3Fe2O3Al2O3SiO2MgOK2ONa2OFClTiO2CuOZnOSrOBaOTotal
40.6646.330.295.513.642.480.190.180.140.320.050.020.010.050.13100.00
Table 2. Comparison of hygroscopicity between urea and gypsum–urea prepared at 25 °C, 30 °C, and 35 °C. (Reprinted with permission from Xiaoxia Fang. Copyright 2024).
Table 2. Comparison of hygroscopicity between urea and gypsum–urea prepared at 25 °C, 30 °C, and 35 °C. (Reprinted with permission from Xiaoxia Fang. Copyright 2024).
UreaGypsum–Urea
25 °C30 °C35 °C
Day 1Sustainability 16 06208 i001Sustainability 16 06208 i002Sustainability 16 06208 i003Sustainability 16 06208 i004
Day 6Sustainability 16 06208 i005Sustainability 16 06208 i006Sustainability 16 06208 i007Sustainability 16 06208 i008
Day 10Sustainability 16 06208 i009Sustainability 16 06208 i010Sustainability 16 06208 i011Sustainability 16 06208 i012
Day 15Sustainability 16 06208 i013Sustainability 16 06208 i014Sustainability 16 06208 i015Sustainability 16 06208 i016
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Jia, C.; Zhao, J.; Fang, X.; Wang, P.; Xiao, A.; Zhang, H. Preparation of Gypsum–Urea with Enhanced Sustainability from Flue Gas Desulfurization Gypsum in Saturated Urea Solution. Sustainability 2024, 16, 6208. https://doi.org/10.3390/su16146208

AMA Style

Jia C, Zhao J, Fang X, Wang P, Xiao A, Zhang H. Preparation of Gypsum–Urea with Enhanced Sustainability from Flue Gas Desulfurization Gypsum in Saturated Urea Solution. Sustainability. 2024; 16(14):6208. https://doi.org/10.3390/su16146208

Chicago/Turabian Style

Jia, Caiyun, Jiang Zhao, Xiaoxia Fang, Pujun Wang, Anni Xiao, and Haijun Zhang. 2024. "Preparation of Gypsum–Urea with Enhanced Sustainability from Flue Gas Desulfurization Gypsum in Saturated Urea Solution" Sustainability 16, no. 14: 6208. https://doi.org/10.3390/su16146208

APA Style

Jia, C., Zhao, J., Fang, X., Wang, P., Xiao, A., & Zhang, H. (2024). Preparation of Gypsum–Urea with Enhanced Sustainability from Flue Gas Desulfurization Gypsum in Saturated Urea Solution. Sustainability, 16(14), 6208. https://doi.org/10.3390/su16146208

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