Abstract
The existing α-hemihydrate gypsum preparation process has low production efficiency and high energy consumption. In this paper, α-type hemihydrate gypsum was prepared by microwave irradiation using phosphogypsum as the raw material, calcium chloride solution as the reaction medium, and succinic acid and aluminum sulfate as crystal-transforming agents. Both aluminum sulfate and succinic acid were studied to determine the mechanism influencing the effect on the growth of α-type hemihydrate gypsum crystals. This study found that, without added succinic acid or aluminum ions, the crystal transformation rate of α-calcium sulfate hemihydrate reached 96% with the average length-diameter ratio reaching 21 after 1 h; when the dosage of succinic acid was 0.02%, the crystal transformation rate of α-calcium sulfate hemihydrate reached 96% with the average length-diameter ratio reaching 1.5 after 1.5 h; and when the aluminum ion dosage was 5 mM, the crystal transformation rate of α-calcium sulfate hemihydrate reached 97% with the average length-diameter ratio reaching 12.3 after 1 h. In addition, it was discovered that the reaction time was significantly shortened under microwave irradiation, and with an increase in succinic acid content, the regulation of the microscopic morphology of the calcium sulfate hemihydrate crystals was continuously enhanced and the aspect ratio of the crystals was continuously reduced. The EDS and Fourier transform infrared spectroscopy (FTIR) analysis showed that succinic acid did not adsorb onto the hemihydrate gypsum crystal during the reaction under microwave irradiation. The X-ray photoelectron spectroscopy (XPS) analysis revealed that aluminum ions affected crystal growth by incorporating into calcium sulfate hemihydrate crystals after reacting with sulfate radicals.
1. Introduction
Phosphogypsum (PG) is a solid waste product that is discharged during wet phosphoric acid production processes. One ton of phosphoric acid can produce 4∼6 tons of PG. Currently, the annual global production of PG is approximately 170 million tons, and the comprehensive utilization rate is approximately 5% [1]. The massive accumulation of PG has created many environmental problems, such as encroachment on land, soil pollution, and destruction of ecology, which have aroused widespread social attention. Resourceful remediation of these issues has become one of the main obstacles restricting the sustainable development of phosphorus chemical enterprises. The main component of PG is calcium sulfate dihydrate, which usually accounts for more than 85% of the total mass of the ideal raw material for the preparation of α-calcium sulfate hemihydrate. Since dihydrate gypsum forms after the hydration of α-calcium sulfate hemihydrate and is coarser and has much shorter columns, the degree of interlacement and overlap between crystals is better than that of flaky dihydrate gypsum; thus, its strength is higher. Domestic and foreign scholars have carried out extensive research on this topic. Fruitful research results have been achieved using the atmospheric salt solution method [2], the semiliquid phase method [3], recrystallization of β-calcium sulfate hemihydrate to obtain α-calcium sulfate hemihydrate [4], and the hydrothermal autoclave method [5]. α-Calcium sulfate hemihydrate crystals with short length-diameter ratios were prepared from gypsum. However, the aforementioned preparation methods often use conventional heat sources, such as electric furnaces, autoclaves, or ovens, which rely on heat conduction from the outside to the inside and thus face problems such as a slow heating rate, large heat loss, and high energy consumption.
Microwave heating produces heat by means of wave absorption resonance of molecules in the material, simultaneously heating the inside and outside the material. In addition, microwave irradiation has the advantages of improving dehydration efficiency, promoting uniform nucleation of crystals, and uniform distribution of crystal nuclei [6–10]. In addition to ensuring certain crystal morphology and conversion effects, the reaction time can be greatly shortened [11–15].
In this paper, α-calcium sulfate hemihydrate was prepared by microwave irradiation instead of by the traditional heating method, with PG as the raw material and calcium chloride solution as the reaction medium. Using scanning electron microscopy (SEM), X-ray diffraction (XRD), EDS, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS), the effects and mechanisms of reaction time and conversion rate, the crystal morphology, and the image composition of the reaction system were analyzed, which provided important basic data and technical guidance for industrial production of α-calcium sulfate hemihydrate under microwave irradiation of PG. The results provide a data-based foundation for studying the effect of crystallizer on the growth of α-calcium sulfate hemihydrate crystals under microwave irradiation.
2. Materials and Methods
2.1. Materials
In the experiment, PG was obtained from Yunnan Sanhuan Chemical Fertilizer Co., Ltd., whose chemical composition after pretreatment is shown in Table 1. Succinic acid was produced by Tianjin Kemiou Chemical Reagent Co., Ltd. Aluminum sulfate was produced by Tianjin Sailboat Chemical Reagent Technology Co., Ltd. Calcium chloride was produced by Tianjin Shentai Chemical Reagent Co., Ltd. Anhydrous ethanol and aluminum sulfate were both produced by Tianjin Zhiyuan Chemical Reagent Co., Ltd. All of these materials were of pure analytical grade. Deionized water was used when preparing the solutions.
2.2. Methods
2.2.1. Raw Material Pretreatment and Preparation of α-Calcium Sulfate Hemihydrate Powder
The test method for preparation of hemihydrate gypsum under microwave irradiation was as follows: PG was mixed with water at a ratio of 1 : 1, stirred, and cleaned, and the floating impurities remaining on the surface after standing were removed. The sample was cleaned four times and then dried for use. PG was pretreated by mixing it with a calcium chloride solution (mass fraction = 10%) and a crystal modifier, maintaining the solid-liquid ratio. After undergoing microwave irradiation, the solution was stirred evenly at a temperature of 100°C in an external condensing unit for 0.5 h, 1 h, 1.5 h, and 2 h. The samples were filtered and quenched using an anhydrous ethanol termination transformation process. The solid-phase product was placed in a drying oven at 55°C and dried until a constant weight was reached. The samples were reserved for SEM and XRD characterization tests.
2.2.2. Experimental Characterization and Equipment
The microwave radiation device is shown in Figure 1. The microwave frequency of the device was 2.45 GHz, the magnetron model was 1.5 kW. 2M463 K, and the rated power percentage was 70%. X-ray fluorescence spectroscopy (XRF, Axios Max, NLD) was used to analyze the elemental composition of the treated PG. An ultrasharp end-window RH target X-ray tube (4 kW) and a thermogravimetric analyzer (Mettler TGA/DSC HT 1600) were used to study the composition and phase transformation of phosphogypsum with temperature changes. The heating rate was 10.0 K/min, the heating range was 25∼1000, and the heating atmosphere was nitrogen. SEM (JSM-6510 LV, Japan) and polarized light microscopy (Zeiss, Axio Imager A2M, Germany) were used to observe the crystal morphology changes with the change in parameters and to calculate the aspect ratio of the short columnar crystal. XRD (Panalytical, XPERT3POWDER, NLD) was used to analyze the image composition of the samples. The tube voltage and current of the CuKα radiation were 40 kV and 40 mA, respectively. The carbon, sulfur, oxygen, and calcium moieties on the surface of the hemihydrate gypsum crystals were analyzed by EDS (Vega 3 SBH). XPS (Thermo Fisher Scientific K-Alpha+, USA) was used to determine the binding energy of the samples. FTIR (Tensor 27, Bruker Optics, Germany) was used to analyze the group types in the hemihydrate gypsum products. The spectra ranged from 4000 to 400 cm−1, each sample was scanned 10 times, and the spectral resolution was 4 cm−1. The content of crystal water in the dehydrated PG products was determined by GB/T 5484-2012 “Methods for Chemical Analysis of Gypsum.”
3. Results
3.1. The Growth of Calcium Sulfate Hemihydrate Crystals under Microwave Irradiation
Under microwave irradiation, PG was used as the raw material to prepare calcium sulfate hemihydrate in calcium chloride solution. When the reaction lasted for 0.5 h, 1 h, 1.5 h, and 2 h, the crystal microstructure of the solid phase was obtained; the microstructure is shown in Figure 2. When the reaction lasted for 1 h, the morphology of the solid-phase crystal changed from a sheet shape to a long columnar crystal with a length-to-diameter ratio of approximately 21. The crystal water content was 6.8% and the conversion rate reached 96%. When the reaction lasted for 2 h, the long columnar crystals in the solid phase disappeared. Under microwave irradiation, PG was prepared as a raw material in calcium chloride solution to form hemihydrate gypsum crystals, and after the initial increase, the calcium sulfate hemihydrate conversion rate showed a decreasing trend over time. At 1 h, the conversion rate was 96%, and as the reaction time progressed, the calcium sulfate hemihydrate crystal reverted to dihydrate calcium sulfate.
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3.2. Effect of a Single Dose of Aluminum Sulfate on the Growth of Calcium Sulfate Hemihydrate Crystals
3.2.1. Laws Governing Effects of Aluminum Ion Dosage on α-Calcium Sulfate Hemihydrate
The SEM image and XRD pattern presented in Figure 3 show that when the aluminum ion content and the reaction time were 1 mM for 2 h, 3 mM for 1.5 h, and 5 mM for 1 h, the three samples all contained long columnar α-calcium sulfate hemihydrate and flakes of calcium sulfate dihydrate. The diagram in Figure 4 shows the change in crystal water content and the average length-to-diameter ratios of the columnar crystals. Figure 4 shows that, with the increase in dosage of aluminum ions from 11 to 12.3, the average length-to-diameter ratio increased from 6.38 μm m 9.7 μm. In conclusion, the addition of aluminum ions promoted an increase in the dehydration rate and the growth of hemihydrate gypsum crystals, and when the higher the number of aluminum ions was within a certain range, the promoting effect was more apparent.
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3.3. Effect of 5 mM Aluminum Ions on the Crystal Morphology and Dehydration Reaction of α-Calcium Sulfate Hemihydrate
The effect of time on the growth of hemihydrate gypsum was studied by selecting the sample with a 5 mM aluminum ion content and the best reaction rate and conversion rate. Combined with the crystal morphology and product composition in Figures 5-6, it was apparent that when the aluminum sulfate crystallizer was mixed with 5 mM aluminum ions, columnar α-type hemihydrate gypsum was formed after 0.5 h, the crystal water content decreased to 6.47% after 1 h of reaction time, and the conversion rate reached 97%. After the addition of aluminum ions and as the dehydration reaction proceeded over time, the quantity of flake-shaped dihydrate gypsum crystals decreased and the quantity of columnar hemihydrate gypsum crystals increased. With increasing time, the amount of hemihydrate gypsum gradually decreased, and the amount of dihydrate gypsum increased.
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3.3.1. Mechanism of the Effect of a Single Doping Ratio of 5 mM Aluminum Ions on the Growth of α-Calcium Sulfate Hemihydrate Crystals
The EDS energy spectra shown in Figures 7-8 show that, in addition to oxygen, sulfur, calcium, and silicon on the α-calcium sulfate hemihydrate crystals, aluminum also has a relatively uniform distribution on the crystal surface; XPS analysis revealed that the aluminum 2p peak in the calcium sulfate hemihydrate crystal was located at a binding energy of 74.75 eV, which suggested the presence of aluminum ions in the sample [11]. It was found that s2p can be fitted to two peaks, and the two peaks are located at binding energies of 169.4 eV [14] and 170.45 eV. The binding energy of the first peak at 169.4 eV is attributed to calcium and sulfate ions, and the binding energy of the second peak at 170.45 eV is attributed to aluminum and sulfate ions. The second peak shifted by 0.52 eV within a reasonable error range [11], which preliminarily shows that sulfate ions can change the growth rate of crystals by adsorbing α-calcium sulfate hemihydrate crystals. Figure 9 shows that the XRD pattern of the three main peaks of hemihydrate gypsum shifted to the left after a reaction time of 1 h with an aluminum ion content of 5 mM. The calculated unit cell parameters were a = 12.0124, B = 7.3778, c = 8.5478, α = 90.0°, β = 97.23°, and γ = 90.0°, and the particle size was 79.92 nm. These data were compared with the data in the standard calcium sulfate hemihydrate crystal PDF card. The data in the standard calcium sulfate hemihydrate crystal PDF card are a = 12.028, b = 6.932, c = 12.691, α = 90.0°, β = 90.138°, and γ = 90.0°. In comparison, it was determined that the β angle of α-hemihydrate gypsum changed, and the c-axis length became shorter as the b-axis side length increased. The doping of aluminum ions led to a change in cell parameters of the hemihydrate gypsum. The change in hemihydrate gypsum unit cell parameters reflects the change in the crystal growth environment [16, 17]. Due to the nonthermal effect of microwaves [18, 19], aluminum and sulfate ions are more easily combined than they are during normal heating; therefore, aluminum ions are doped during the growth process. The mechanism of aluminum ion crystal transformation under microwave irradiation involves the combination of aluminum ions and sulfate radicals changing the crystal morphology of calcium sulfate hemihydrate, which is different from the crystal transformation mechanism under conventional heating [15].
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3.4. Effect of Succinic Acid on the Growth of Calcium Sulfate Hemihydrate Crystals under Microwave Irradiation
3.4.1. Effect of Succinic Acid Content on α-Calcium Sulfate Hemihydrate
Under microwave irradiation, PG was used as the raw material and mixed with a succinic acid conversion agent (dosing amounts of 0.01%, 0.02%, and 0.03%), and hemihydrate gypsum crystals were prepared in calcium chloride solution. The samples were analyzed when the reaction proceeded to 1.5 h, and the results are shown in the following figures: Figure 10 shows the microscopic morphology of the solid phase, Figure 11(a) shows the change in the solid-phase crystal water content, and Figure 11(b) shows the XRD analysis results. When the content of succinic acid was 0.01% and 0.02%, the microscopic morphology of the solid-phase crystal obtained in 1.5 h was basically short hexagonal prism crystals (see Figures 10(a) and 10(b)), and the measured water content of the crystals was 7.23% and 6.89%, respectively. The conversion rates of calcium sulfate dihydrate crystals to calcium sulfate hemihydrate were 94% and 96%, respectively. When the content of succinic acid was 0.03%, the plate-like crystals in the solid phase obtained over 1.5 h increased (see Figure 10(c)), the measured water content of crystallization was 9.44%, and the conversion rate of calcium sulfate dihydrate crystals to calcium sulfate hemihydrate decreased to 81%, indicating that some of the calcium sulfate hemihydrate crystals were converted back to calcium sulfate dihydrate. The effect of different succinic acid contents on the crystal size of calcium sulfate hemihydrate is shown in Figure 11(c). It can be seen from the figure that, with increasing succinic acid content, the aspect ratio of the prepared calcium sulfate hemihydrate crystals decreased from 2.3 to 1.3, but the average size of the crystals first increased and then decreased. The abovementioned experiments prove that when the crystal conversion agent contains only small levels of dopant, with increasing succinic acid content, the control effect on the microscopic morphology of the calcium sulfate hemihydrate crystals is continuously strengthened, and the aspect ratio of the crystals is continuously reduced.
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3.4.2. Effect of Time on the Crystal Morphology and Dehydration Reaction of α-Calcium Sulfate Hemihydrate under a Single Doping Ratio of 0.02% Succinic Acid
Figures 12–13 show that when the reaction of 0.02% succinic acid is carried out for 1.5 h, the plate-shaped calcium sulfate dihydrate crystals have basically been transformed into short hexagonal prism-shaped α-type hemihydrates with an aspect ratio of approximately 1.5. Hydrated gypsum crystals have a conversion rate of approximately 96%; as the reaction progresses to 2 h, the main component in the product becomes calcium sulfate dihydrate flakes. Compared with the nontransforming agent, the addition of 0.02% succinic acid has better control over the crystal morphology of calcium sulfate hemihydrate, and the aspect ratio of calcium sulfate hemihydrate crystals is reduced from 21 to 1.5, but it is extended. At the same time, the incorporation of the crystal conversion agent did not affect the conversion rate law of calcium sulfate dihydrate to calcium sulfate hemihydrate, the conversion rate first increased, and then decreased with time, and it had no effect on the maximum conversion rate. This was the result of microwaves being strong electromagnetic waves, and the generated microwave plasma usually contains high energy atoms, molecules, and ions that are not involved in thermodynamic methods, thereby reducing the activation energy of the reaction and accelerating the reaction.
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3.4.3. Mechanism of the Effect of a Single Doping Ratio of 0.02% Succinic Acid on the Growth of α-Calcium Sulfate Hemihydrate Crystals
The α-type calcium sulfate hemihydrate crystal grows faster on the (111) crystal plane than other crystal planes without the intervention of the crystal conversion agent; therefore, it will grow into needle-like crystals. This crystal morphology forms because there are Ca2+ and SO42- ions in the direction of the crystal c-axis. For the two free end valence bonds of the ions [20], the growth of crystal faces is faster than that of other crystal faces. The reason why organic acids control the change in crystal shape under other heating methods is that the complexation of carboxyl groups and calcium ions forms a cyclic complex, which slows the growth rate of these crystal faces and changes the crystal shape [5]. As shown in Figure 14 and Table 2, the end surface perpendicular to the c-axis of α-calcium sulfate hemihydrate has approximately the same strength, weight, and atomic content as the carbon parallel to the c-axis side, which indicates that there is no selective succinic acid doping or adsorption.
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Figure 15 shows that the two absorption peaks at 3165.29 and 3560.01 cm−1 are the two absorption peaks of crystal water in α-calcium sulfate hemihydrate, while the absorption peaks at 598.25 cm−1 and 1152.22 cm−1 are caused by the flexural vibration and asymmetric stretching of sulfate radicals [21]. However, the FTIR does not include the free carboxyl COO- absorption peak in the range of 1750–1770 cm−1, the liquid or solid carboxyl absorption peak in the range of 1670–1725 cm−1, or the carboxylate absorption peak in the 1550–1650 cm−1 range. This test shows that the succinic acid crystal conversion agent inhibits the growth rate of hemihydrate gypsum crystals on the c-axis crystal surface under microwave irradiation and controls the morphology of hemihydrate gypsum crystals, but it is not adsorbed onto the hemihydrate gypsum crystals. Succinic acid remained on top or was mixed into the hemihydrate gypsum crystals and was eliminated from the hemihydrate gypsum system with the filtrate or absolute ethanol in the filtration stage.
4. Conclusions
The α-type calcium sulfate hemihydrate was prepared by the salt solution method under microwave irradiation, and the effect of two crystal conversion agents, succinic acid, and aluminum sulfate, on the growth of hemihydrate gypsum crystals was studied. The following conclusions were drawn from the analysis, and the results of this paper are summarized as follows:(1)In the microwave irradiation method in which salt solutions were used, pretreated PG was used as the raw material, the average aspect ratio of the hemihydrate gypsum crystals without the crystal conversion agent was 21 after reaction for 1 h, and the conversion rate reached 96%; single-doped succinic acid was converted to the crystalline form at 0.02%, the length-to-diameter ratio of hemihydrate gypsum crystals was approximately 1.5 after 1.5 h, and the conversion rate reached 96%. When the aluminum sulfate conversion agent was doped at only 5 mM, it reacted for 1 h. The aspect ratio of gypsum crystals was approximately 12.3, and the conversion rate reached 97%.(2)Under microwave irradiation, when PG was used as a raw material and calcium chloride solution was not mixed with a crystal conversion agent, mixed with aluminum sulfate alone, or mixed with succinic acid alone, the conversion law of calcium sulfate dihydrate crystals to calcium sulfate hemihydrate was uniform. Therefore, the conversion rate first increased and then decreased with time. Under microwave irradiation with salt solutions, the time required for the dehydration reaction of calcium sulfate dihydrate was significantly shortened; an appropriate amount of aluminum sulfate conversion agent continued to accelerate the dehydration reaction process, and an appropriate amount of succinic acid conversion agent affected the crystal form of hemihydrate gypsum. The appearance was well controlled.(3)Under microwave irradiation, succinic acid did not enter or adsorb onto the hemihydrate gypsum crystals during the reaction but appeared in the crystal growth stage, and one-half of it was eliminated from the reaction with the filtrate after the crystal transformation was completed. The crystalline system incorporated aluminum ions into hemihydrate gypsum crystals when they grew.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This research was funded by the National Natural Science Foundation of China (11562010) and Kunming University of Science and Technology Analysis and Testing Fund (2020M20172110016).