J. Cent. South Univ. (2019) 26: 3213-3224

DOI: https://doi.org/10.1007/s11771-019-4247-8

Promotion of conversion activity of flue gas desulfurization gypsum into α-hemihydrate gypsum by calcination-hydration treatment

GUAN Qing-jun(管青军)^{1, 2}, SUN Wei(孙伟)², GUAN Chang-ping(官长平)², YU Wei-jian(余伟健)¹,

ZHU Xiang-nan(朱向楠)³, Sultan Ahmed KHOSO², WANG Ping(王平)¹, PENG Wen-qing(彭文庆)¹

1. School of Resource Environment and Safety Engineering, Hunan University of Science and Technology, Xiangtan 411201, China;

2. School of Mineral Processing and Bioengineering, Central South University, Changsha 410083, China;

3. College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: The massive accumulation of flue gas desulfurization (FGD) gypsum produced in the wet limestone-gypsum flue gas desulfurization process not only encroaches on lands but also causes serious environmental pollution. The preparation of α-hemihydrate gypsum (α-HH) is an important way to achieve high-value utilization of FGD gypsum. Although the glycerol-water solution approach can be used to produce α-HH from FGD gypsum under mild conditions, the transition is kinetically unfavorable in the mixed solution. Here, an easy pretreatment was used to activate FGD gypsum by calcination and hydration to readily complete the transition. The pretreatment deteriorated the crystallinity of FGD gypsum and caused it to form small irregular flaky crystals, which dramatically increased the specific surface area. Additionally, most of the organics adsorbed onto FGD gypsum surfaces were removed after pretreatment. The poor crystallinity, increased specific surface area, and elimination of organics adsorbed onto crystal surfaces effectively improved the conversion activity of FGD gypsum, thereby promoting dihydrate gypsum (DH) dissolution and α-HH nucleation. Overall, the phase transition of FGD gypsum to α-HH is facilitated.

Key words: flue gas desulfurization gypsum; α-hemihydrate gypsum; calcination-hydration treatment; dissolution; surface characteristics

Cite this article as: GUAN Qing-jun, SUN Wei, GUAN Chang-ping, YU Wei-jian, ZHU Xiang-nan, Sultan Ahmed Khoso, WANG Ping, PENG Wen-qing. Promotion of conversion activity of FGD gypsum into α-hemihydrate gypsum by calcination-hydration treatment [J]. Journal of Central South University, 2019, 26(12): 3213-3224. DOI: https://doi.org/10.1007/s11771-019-4247-8.

1 Introduction

Flue gas desulfurization (FGD) gypsum is an industrial solid waste generated in the wet limestone-gypsum flue gas desulfurization process, and its main component is dihydrate gypsum (CaSO₄·2H₂O, DH). The massive accumulation of FGD gypsum results in occupation of large amounts of land as well as serious environmental pollution. Recently, the high-value-added comprehensive utilization of FGD gypsum is eliciting considerable attention; to date, the preparation of α-hemihydrate gypsum (α-HH) is the most important utilization method with high value for FGD gypsum [1-3].

The autoclave technology is commonly used in the industry for preparing α-HH from DH [4]. However, the technology requires high temperature and pressure, and thus is high energy-consuming. In recent years, researchers have conducted extensive experiments on the preparation of α-HH from DH under mild conditions, and found that the phase transition can be achieved in the inorganic acid solution [5], salt solution [6, 7], and glycerol-water solution [8-12] under ambient pressure. Compared with the inorganic acid solution and salt solution, the glycerol-water solution method has the advantage of better working environment and no corrosion to equipment, which makes the method become a promising approach to preparing α-HH from DH. However, in the solution the phase conversion of DH into α-HH is kinetically unfavorable [13]. Additionally, FGD gypsum is characterized by high crystallinity and low reactivity, and its surfaces inevitably adsorb organic substances in flue gas [14-16], eventually hindering DH dissolution and α-HH nucleation. These further restrain the preparation of α-HH from FGD gypsum in glycerol-water solutions.

Researchers need to recognize that the phase conversion of DH into α-HH depends sensitively on DH dissolution [17, 18]. The addition of non-lattice cations (such as Na⁺, K⁺, Cu²⁺, Zn²⁺, Mn²⁺ and Mg²⁺) can improve DH solubility and effectively promote DH transformation into α-HH in glycerol-water solutions [13, 18-20]. However, these ions are unavoidably doped in the lattice of α-HH or adsorbed onto its surfaces, and they even form their isomers that are detrimental to the mechanical properties of α-HH pastes [21, 22]. Considering the great effect of surface physicochemical properties such as particle size, morphology, and specific surface area, on the dissolution of substances [23-25], optimizing the surface physicochemical properties of FGD gypsum may improve its dissolution and ultimately accelerate DH transformation into α-HH.

In this study, aiming at FGD gypsum from steel plants, we systematically explored its structure, chemical composition, particle size distribution, and surface characteristics. Furthermore, a simple and effective pretreatment was used to improve the FGD gypsum dissolution by optimizing its surface physicochemical properties to enhance the kinetic feasibility of FGD gypsum transition into α-HH in glycerol-water solution.

2 Materials and methods

2.1 Materials and reagents

The raw FGD gypsum was acquired from Panzhihua Iron & Steel Co., Ltd., Sichuan Province, China. Its chemical composition is listed in Table 1. Glycerol (C₃H₅(OH)₃) (analytical reagent grade) was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.

2.2 Experimental procedure

1) Pretreatment of FGD gypsum. First, the gypsum was put into a drying oven and calcined at 100 °C for 5 h. After that, one part of the sample was used for analysis, and the other part was blended with sufficient deionized water and stirred at 100 r/min for 30 min under room temperature. Finally, the gypsum after hydration was filtered and dried at 60 °C to constant weight.

2) Preparation of α-HH in glycerol-water solutions. A mixed solution (240 g) of glycerol (65 wt%) and water was first charged into a 500 mL three-necked flask equipped with a reflux condenser. This solution was stirred with a magnetic stirrer at 100 r/min and preheated to 90 °C in an oil bath. Then, 60 g of the gypsum sample was added into the reactor. During the reaction, 5 mL of hot slurry was taken out at certain time intervals. The slurry was immediately filtered. After that, the filtrate after diluting 100 times was used for determining Ca²⁺ concentration, and the residue was washed thrice with boiling water and once with ethanol, and then dried at 60 °C for 2 h in an oven.

Table 1 Chemical composition of original sample, sample after calcination and sample after calcination-hydration treatment of FGD gypsum analyzed by XRF (Mass fraction, %)

2.3 Characterization

To determine the elemental compositions of the FGD gypsum, X-ray fluorescence spectroscopy (XRF) was performed using a PANalytical B.V. Axios mAX spectrometer.

The X-ray powder diffraction (XRD) patterns of samples were recorded on a Bruker D8 advanced diffractometer using Cu K_α radiation (λ=1.54178 ), with a scanning rate of 5 °/min and a scanning 2θ range of 5°-70°.

A PerkinElmer STA 8000 simultaneous thermal analyzer (TG-DSC) was used to further determine the structure of the gypsum samples. During the analysis, 10 mg of dried samples was placed in an Al₂O₃ crucible with a lid at 60-300 °C at a heating rate of 10 °C/min under nitrogen atmosphere.

To gain information about the morphology of the solid phase, a field-emission scanning electron microscope (SEM, JSM-6490LV, JEOL, Japan) was used to characterize the gypsum samples.

The average particle size distribution of samples was analyzed using a particle size analyzer (Occhio Flowcell FC200S, Quantachrome, USA).

The specific surface area of samples was characterized using a specific surface area analyzer (BET, QuadraSorb SI, Quantachrome, USA).

To investigate the chemical structures of samples and organics adsorbed onto the sample surface, infrared spectra were performed using a fourier transform infrared spectrometer (FT-IR, IRAffinity-1, Shimadzu, Japan) with a resolution of 4 cm^-1 in the region of 400-4000 cm^-1.

To determine the surface chemistry of the gypsum sample, X-ray photoelectron spectroscopy characterization was performed using a Thermo Fisher ESCALAB 250Xi system with an Al K_α photon energy of 1486.6 eV. The BE of peaks was corrected using the hydrocarbon C1s BE of 284.8 eV.

An inductively coupled plasma-atomic emission spectrometer (ICP-AES, PS-6, Baird, USA) was used to analyze Ca²⁺ concentration of glycerol-water solutions.

3 Results

3.1 Effect of pretreatment on chemical structure and composition of FGD gypsum

Figure 1 and Table 1 show the chemical structures and compositions of FGD gypsum, respectively, during the calcination-hydration treatment. The XRD peaks of CaSO₄·2H₂O (DH) (JCPDS 33-0311) were at 2θ=11.61°, 20.69°, 23.35° and 29.08°, and those of CaSO₄·0.5H₂O (HH) (JCPDS 14-0453/41-0224) were at 2θ= 14.72°, 25.64°, 29.70° and 31.86°. The original sample of FGD gypsum was mainly composed of DH (Figure 1(a)). After calcination, DH underwent a dehydration reaction as expressed by Eq. (1) [26], thereby becoming HH (Figure 1(b)).

        (1)

Then, after complete hydration, the HH reformed into DH (Figure 1(c)). Compared with that of the original sample, the diffraction peak intensity of pretreated samples increased and became more uneven, indicating that the DH content increased and its crystallinity deteriorated. From Table 1, the DH content of the original sample determined from mass loss due to crystal water was 93.74%, and after pretreatment, the DH content increased to 94.65%, which is consistent with the XRD patterns.

TG-DSC technology was used to further explore the structural change of samples during pretreatment. In typical DSC curves, DH is indicated by endothermic peaks at about 150 and 170 °C [27-29], denoting a two-step loss of first the 1.5 followed by the remaining 0.5 crystal water when heated. Furthermore, β-HH is characterized by an endothermic peak at about 200 °C, which is related to the transition to anhydrite [27]. As shown in Figure 2(a), the DSC curve of the sample shows two endothermic peaks at 150 and 163 °C, and the TG curves indicate that the crystal water content is 19.7%, further confirming that the original sample is mainly composed of DH. After calcination (Figure 2(b)), the sample presents only one endothermic peak at 206 °C, and the crystal water content declines sharply to 6.1%, demonstrating that the calcined sample is mainly comprised of β-HH. From Figure 2(c), the sample after the calcination-hydration treatment displays two endothermic peaks at 150 and 165 °C, and the crystal water content is 20.0%, indicating that the pretreated sample is basically made up of DH.

Figure 1 XRD patterns of original sample (a), calcined sample (b) and sample after calcination-hydration treatment (c) of FGD gypsum

3.2 Effect of pretreatment on surface physicochemical properties of FGD gypsum

3.2.1 Effect of pretreatment on surface morphology and particle size of FGD gypsum

Figure 3 shows SEM images and particle size distributions of samples during the pretreatment. The particle size of the original sample (Figure 3(a)) appearing as regular prismatic crystals is mainly between 10 and 70 μm. After calcination, the crystal morphology and particle size of the sample (Figure 3(b)) are similar to the original sample. However, lots of pores occur on the crystal surface, possibly resulting from the crystal water evaporation of DH. Then, the calcined sample is recrystallized by hydration, thereby forming irregularly flaky crystals, of which the particle size is basically less than 25 μm (Figure 3(c)).

3.2.2 Effect of pretreatment on specific surface area of FGD gypsum

Figure 4 shows the change in the specific surface area of FGD gypsum during pretreatment. The specific surface area of the original sample is 2.27 m²/g. After calcination, the specific surface area increased to 5.55 m²/g, attributing to the occurrence of pores on the crystal surfaces [30, 31]. The specific surface area of the sample after calcination-hydration treatment was more than five times the original sample and rose remarkably to 12.27 m²/g, probably resulting from the morphology irregularity and the reduction in crystal particle size.

Figure 2 TG and DSC curves of original sample (a), calcined sample (b) and sample after calcination- hydration treatment (c) of FGD gypsum

3.2.3 FT-IR analysis of FGD gypsum during pretreatment

FT-IR spectroscopy was used to explore changes in the structure of gypsum samples and in the content of organic matters adsorbed onto its surface during pretreatment. As depicted in Figure 5(a), the presence of the crystal water is confirmed by the broad bands at 3543 and 3395 cm^-1 corresponding to O—H stretching vibration in addition to the bands at 1687 and 1620 cm^-1 assigned to H—O—H bending; additionally, the broad band observed at 1107 cm^-1 should be indexed to υ₃ SO₄^2- stretching vibration, and the bands at 667 and 600 cm^-1 can be ascribed to υ₄ SO₄^2- stretching vibration [27, 32-34]. The characteristic transmission peaks further verified that the original sample of FGD gypsum mainly comprised DH. The organics containing n-alkyl chains are determined by the presence of the bands at 2924 and 2859 cm^-1 corresponding to the asymmetric and symmetric stretching vibrations of —CH₂— groups, respectively [15, 35]. Furthermore, the band at 876 cm^-1 indicates the out-of-plane C—C—H bending vibration of an aromatic ring [36-38]. Therefore, the original sample of FGD gypsum contained organic matters with n-alkyl chains and aromatic group, which probably inhibites the dissolution of Ca²⁺ and SO₄^2- from DH lattice and the nucleation of α-HH, thereby constraining DH transformation into α-HH.

Figure 3 SEM images and particle size distributions of original sample (a), calcined sample (b) and sample after calcination-hydration treatment (c) of FGD gypsum

Figure 4 Specific surface areas of original sample, calcined sample and sample after calcination-hydration treatment of FGD gypsum

From Figure 5(b), the bands at 3614, 3561 and 1620 cm^-1 can be associated with the crystal water; the bands at 1154 and 1114 cm^-1 should arise from υ₃ SO₄^2- stretching vibration; the band at 1008 cm^-1 should be ascribed to υ₁ SO₄^2- stretching; the bands at 660 and 602 cm^-1 can be attributed to υ₄ SO₄^2- stretching [35, 39]. The characteristic bands indicate that the calcined sample mainly comprises HH. Additionally, the bands of —CH₂— (2924 and 2859 cm^-1) and aromatic (876 cm^-1) groups decrease in intensity, implying that part of the organics is removed by calcination.

The spectrum of Figure 5(c) is extremely similar to Figure 5(a), suggesting that the sample after calcination-hydration treatment mainly consisted of DH. However, the bands at 2800-3000 and 876 cm^-1 become almost negligible, indicating that most of the organics are eliminated by calcination-hydration treatment.

3.2.4 XPS analysis of FGD gypsum during pretreatment

The composition of organics adsorbed onto the surface of FGD gypsum was further determined by XPS. The binding energies of C 1s peaks corresponding to the functional groups are as follows.

Figure 5 FT-IR spectra of original sample (a), calcined sample (b) and sample after calcination-hydration treatment (c) of FGD gypsum

The peaks of C 1s at 284.2, 284.8, 285.4,286.4, 288.2, 289.0 and 289.5 (±0.1) eV correspond to the following functional groups: C=C/C—C in aromatic rings, aliphatic carbon (C—C/C—H),C—O/C—Cl, epoxy and alkoxy (C—O), carbonyl carbon (C=O), carboxylate carbon (O—C=O), and carbonate carbon (CO₃^2-), respectively [40-46].

The C 1s spectra of FGD gypsum during pretreatment are fitted, as shown in Figure 6. As depicted in Figures 6(a) and (b), after calcination, the C 1s spectra of the sample decrease remarkably in intensity, and the C 1s peaks of organics containing aromatic rings (284.2 eV) and/or C—O/C—Cl (285.4 eV) disappear, implying that the amount of organics absorbed onto the surfaces of FGD gypsum decreases significantly. This decrement probably results from the volatility of organics when heated. After calcination-hydration treatment (Figure 6(c)), the C 1s spectrum of the sample further lowers in intensity, and the organic substance containing C=O (288.2 eV) and/or O—C=O (289.0 eV) is almost eliminated. This decrease in the content of surface organics is mainly because the gypsum is recrystallized to form new crystal surfaces during the hydration process.

Furthermore, three C 1s peaks still exist in the XPS spectrum of the gypsum after pretreatment. The peak at 284.8 eV should be ascribed to carbon contamination or non-volatile aliphatic carbon (C—C/C—H). The organics containing epoxy or alkoxy groups at 286.4 eV are difficult to remove, possibly because the functional groups with good reactivity reacted with other organic molecules to form the non-volatile organics or reacted with calcium to form calcium alkoxides [47-51]. Moreover, the peak at 289.5 cm^-1 should be attributed to the limestone existing in the gypsum, which is difficult to be removed merely by calcination and hydration.

3.3 Effect of pretreatment on dissolution of FGD gypsum in glycerol-water solution

Figure 7 shows the change of Ca²⁺ concentration over time in glycerol (65 wt%)-water mixtures at 90 °C. Compared with the original sample, the pretreated sample is easier to be dissolved. As depicted in Figure 7, for the pretreatment sample, Ca²⁺ concentration increases within 0-6 h, reaches the maximum value of 262 mg/L at 6 h, and then declines. Finally, it reaches an equilibrium state at 9 h. Based on the dissolution-crystallization mechanism, the stage before the peak of Ca²⁺ concentration should be the dissolution process of FGD gypsum. After the peak concentration, the stage of decline in Ca²⁺ concentration should be the α-HH crystal nucleation and growth process. Finally, when Ca²⁺ concentration is maintained nearly the same, the α-HH equilibrium is attained. However, the Ca²⁺ concentration of the original sample slightly changes and remains at about 200 mg/L within 17 h; thereafter, the concentration increases slightly.

Figure 6 C 1s spectra of original sample (a), calcined sample (b) and sample after calcination-hydration treatment (c) of FGD gypsum

3.4 Effect of pretreatment on phase transition of FGD gypsum in glycerol-water solution

Figures 8 and 9 show the SEM images and XRD patterns of products prepared by the original sample and the sample after the calcination- hydration treatment in glycerol (65 wt%)-water solution at 90 °C. From Figure 8, the morphology of the original sample slightly changes, except for the circular edges, and prismatic crystals remain after reacting in the solution for 30 h. However,after 9 h, the pretreated sample is completely transformed into needle-like crystals. The XRD patterns determine that the products of the original sample still consist mainly of DH after 30 h of reaction, whereas the pretreated sample is completely converted into HH only after 9 h of reaction.

Figure 7 Variation of Ca²⁺ concentration of original sample and sample after pretreatment of FGD gypsum over time in glycerol (65 wt%)-water solution at 90 °C

Figure 8 SEM images of reaction products of original sample after 30 h reaction (a) and products of pretreated sample after 9 h reaction (b) in glycerol (65 wt%)-water solution at 90 °C

Figure 9 XRD patterns of reaction products of original sample after 30 h reaction (a) and products of pretreated sample after 9 hours reaction (b) in glycerol (65 wt%)- water solution at 90 °C

The crystal structure of the products is further confirmed using the TG-DSC technology. As shown in Figure 10(a), the TG and DSC curves of the original sample products are similar to those in Figure 2(a). Therefore, the products still consist mainly of DH. In typical DSC curves, α-HH is characterized by an endothermic peak at about 165 °C and a subsequent exothermic peak at about 180 °C during dehydration [27, 52, 53]. From Figure 10(b), the products of the pretreated sample possessed an exothermic peak at 180 °C immediately following an endothermic peak at 162 °C in the DSC curve, and the crystal water content is 6.2% according to the TG curve, indicating that the products are α-HH.

4 Discussion

XRD, XRF and TG-DSC analyses confirm that the calcination-hydration treatment first dehydrates FGD gypsum (DH) into β-HH and then hydrates to reform DH. After the process, the crystallinity of the reformed DH deteriorates, facilitating the transition of DH into α-HH. Additionally, recrystallized DH crystals become small and irregular, significantly increasing the crystals specific surface area and greatly improving the reactivity. Meanwhile, FT-IR and XPS analyses suggest that the pretreatment could effectively remove the organic substances adsorbed onto the crystal surfaces of DH, thereby remarkably reducing the effect of these organics on DH dissolution and α-HH nucleation and eventually promoting the conversion of DH into α-HH.

Figure 10 TG and DSC curves of reaction products of original sample after 30 h reaction (a) and products of pretreated sample after 9 h reaction (b) in glycerol (65 wt%)-water solution at 90 °C

The poor crystallinity, increased specific surface area, and elimination of organics adsorbed onto crystal surfaces dramatically improve the dissolution of the pretreated sample (Figure 7) and further provide sufficient supersaturation for α-HH nucleation. This ultimately results in the pretreated sample being more readily converted to α-HH in glycerol-water mixture than the original sample.

As discussed above, the calcination-hydration treatment could effectively improve the conversion activity of FGD gypsum and promote its conversion into α-HH.

5 Conclusions

The calcination-hydration treatment deteriorates the crystallinity of FGD gypsum and causes it to form small irregular flaky crystals, thereby dramatically increasing the specific surface area. Furthermore, the pretreatment removed most of the organics adsorbed onto the surfaced of FGD gypsum. The poor crystallinity, increases specific surface area, and elimination of the organics adsorbed onto crystal surfaces effectively improve the conversion activity of FGD gypsum, thereby promoting DH dissolution and α-HH nucleation. Consequently, the pretreated sample becomes easier to transform into α-HH in glycerol-water mixture.

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(Edited by FANG Jing-hua)

中文导读

煅烧-水化处理提高脱硫石膏转化为α-半水石膏的相变活性

摘要：湿法石灰石-石膏烟气脱硫过程中产生的脱硫石膏的大量堆积不仅会浪费土地而且会对周边环境造成严重污染。制备α-半水石膏是高附加值利用脱硫石膏的重要途径。目前，常压甘油-水溶液法被认为是一种制备α-半水石膏的优良方法，但是在这种溶液体系中二水石膏转化效率不高。用煅烧-水化预处理方法活化脱硫石膏使其能够比较容易地完成相变转化。预处理后脱硫石膏的结晶性变差并形成细碎的无规则薄片状晶体，这大幅度地提高了脱硫石膏的比表面积。另外，预处理还将大部分吸附在脱硫石膏表面的有机物去除。结晶性变差、比表面积提高以及表面有机物的消除大幅度地提高了脱硫石膏的相变反应活性，有利于二水石膏的溶解和α-半水石膏的成核结晶，最终促进脱硫石膏向α-半水石膏的转化。

关键词：脱硫石膏；α-半水石膏；煅烧-水化预处理；溶解；表面特性

Foundation item: Projects(51904104, 51974117, 51804114) supported by the National Natural Science Foundation of China; Projects(2018YFC1901601, 2018YFC1901602, 2018YFC1901605) supported by the National Key Scientific Research Project of China; Project(2015CX005) supported by the Innovation Driven Plan of Central South University, China; Project(18B226) supported by the Excellent Youth Project of Hunan Education Department, China

Received date: 2018-11-19; Accepted date: 2019-04-19

Corresponding author: SUN Wei, PhD, Professor; Tel: +86-731-88876697; E-mail: sunmenghu@csu.edu.cn; ORCID: 0000-0002- 9173-2682; YU Wei-jian, PhD, Professor; Tel: +86-731-58290181; E-mail: ywjlah@163.com; ORCID: 0000- 0003-1497-1759