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Insight into Influence of Glycerol on Preparing #CaSO4•½H2O from Flue Gas Desulfurization Gypsum in Glycerol–Water Solutions with Succinic Acid and NaCl Qingjun Guan, Honghu Tang, Wei Sun, Yuehua Hu, and zhigang yin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02067 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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Industrial & Engineering Chemistry Research

Insight into Influence of Glycerol on Preparing α-CaSO4·½H2O from Flue Gas Desulfurization Gypsum in Glycerol–Water Solutions with Succinic Acid and NaCl Qingjun Guan, Honghu Tang, Wei Sun*, Yuehua Hu, and Zhigang Yin School of Mineral Processing and Bioengineering, Central South University, Changsha 410083, China

ABSTRACT The conversion of CaSO4·2H2O (DH) to CaSO4·½H2O (HH) was facilitated by increasing glycerol content from 55% to 85% in glycerol-water mixtures, the average length and diameter of produced α-HH crystals decreased from 60.97 to 7.21 µm and from 15.21 to 5.17 µm, respectively, resulting in a decrease in the average aspect ratio from 4.06 to 1.41. The phase transformation was reduced dramatically from 30 to 3 hours. As glycerol content increased in the mixtures, an increase in α-HH maximal relative supersaturation and decrease of ion diffusion rate were hypothesized as explanations for the morphology change in the alpha-hemihydrate (α-HH) produced. The kinematic feasibility of FGD gypsum transition to α-HH in the mixed solutions with the increase of glycerol should also be attributed to the enhancement of maximal relative supersaturation for α-HH precipitation.

1. INTRODUCTION The production of calcium sulfate hemihydrate (HH) from flue gas desulfurization gypsum and phosphogypsum, which are mainly composed of CaSO4·2H2O (DH), is the primary use of the chemical gypsum1, 2. Two forms of CaSO4·½H2O (HH) are recognized: an alpha form (α-HH) which has a reduced fluidization water demand and higher compressive strength, and a beta form (β-HH) with a comparatively increased fluidization water demand and lower compressive strength3, 4.

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The reported production methods of α-HH from DH include commercialized autoclave technology5 and salts solution process2, 6-8. The former involves heating of the DH in an autoclave at elevated temperature (140 ~ 150 °C) and pressure in an atmosphere of saturated vapor, which is an energy intensive procedure. While the latter requires high concentration salt solutions that can result in serious corrosion to equipment. Therefore, alternative approaches for the preparation of α-HH in alcohol-water solutions with trace amount of inorganic salts as phase transformation accelerators have been suggested in recent years9, 10. According to Guan`s research11, the water activity of the glycerol-water solution can be reduced by increasing glycerol content and temperature, which accounts for the feasibility of DH transition to α-HH in the glycerol-water solution. However, the transition is kinetically unfavorable in glycerol-water solution. Addition of small amount of non-lattice cations can significantly shorten the transition time, of which Na+ has the best effect and can reduce the time from 510 min to 35 min12. Meanwhile, in order to prepare high strength gypsum (α-HH), the crystal modifiers are usually used to regulate the crystal morphology in the transformation process. And carboxylic acids or carboxylates as the modifiers are highly effective2,

10, 13

. For example, succinic acid14 or sodium

succinate15 can effectively suppress crystals` growth along the c axis and α-HH particles with a low aspect ratio or spherical shape can be prepared, which possesses better injectability and mechanic strength. However, in the solution system, except for the reduction of water activity, the influence of alcohol on other aspects, for example the supersaturation for α-HH precipitation, dissociation properties of modifiers and effect of alcohol on products` morphology, has been rarely studied. In the process of preparing high strength gypsum (α-HH), α-HH supersaturation and dissociation properties of carboxylic acids or carboxylate as the modifiers were very important factors to influence the phase transformation rate and α-HH crystal morphology. Therefore, the research into influence of alcohol on these factors has important theoretical significance in guiding the preparation of high strength gypsum with certain aspect ratios. In the paper, aiming at the preparation of α-HH from FGD gypsum regulated by


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succinic acid (C4H6O4) and NaCl in glycerol-water solution, we investigated systematically the influence of glycerol on α-HH crystal morphology and the phase transformation time and provided theoretical guidance for the production practice.

2. EXPERIMENTAL 2.1 Materials The FGD gypsum, of which the chemical compositions were given in Table 1, was received from Panzhihua Iron & Steel Co., Ltd., Sichuan Province, China. Analytical reagent-grade glycerol (C3H5(OH)3), sodium chloride (NaCl), succinic acid ((CH2)2(COOH)2) and guaranteed reagent-grade sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. pH buffer reagents (potassium hydrogen phthalate, sodium tetraborate) used to prepare pH buffer solutions were purchased from Kemiou Chemical Reagent Co., Ltd., Tianjin, China. 1.0 mol/L HCl standard volumetric solution was provided by Anze Technology Co., Ltd., Shenzhen, China. Table 1. Chemical composition of the FGD gypsum analyzed by XRF (wt%) CaO

SO3

H2O

SiO2

Fe2O3

Al2O3

MgO

PbO

K2O

Others

36.21

42.30

19.62

0.65

0.32

0.25

0.10

0.06

0.03

0.46

2.2 Experimental Procedure Pretreatment of FGD gypsum: FGD gypsum was calcinated at 100 °C for 5h in a drying oven, and then was mixed with enough deionized water and stirred (100rpm) at room temperature for half hours. After complete hydration, the gypsum was filtrated and dried at 60 °C for 2 hours. Preparation of α-HH in glycerol-water mixtures: Mixed solutions composed of a certain amount of glycerol and water with 0.2 mol·kg-1 NaCl and 7.42 ×10-4 mol·kg-1 succinic acid (based on the glycerol-water mixtures) were firstly added into a 500 mL three-necked flask equipped with a reflux condenser on top of it, and the solution was



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stirred with a magnetic stirrer at a constant rate of 150 rpm and preheated to 90°C in an oil bath. Then 60g FGD gypsum after pretreated per 240 g of the mixed solution was added into the reactor. During the reaction, 10 mL slurry was withdrawn at certain time intervals. After that the sample was filtrated immediately, washed three times with boiling water and then rinsed once with ethanol with the vacuum suction pump equipped with 1000 mL suction bottle before it was dried at 60 °C for 2 hours in an oven. The experimental setup is shown in Fig.1.

Fig.1 A photograph of the experimental setup

Determination of dissociation constants of succinic acid in glycerol-water mixtures containing different glycerol content: the solution of sodium hydroxide (NaOH) (1.0 M) were prepared in deionized water. And an electrical balance (AX124ZH, OHAUS, China) with an accuracy of ±0.1mg was used for the solution preparation. The sodium hydroxide solution were standardized by titrating with 1.0 mol/L HCl standard volumetric solution. An automatic potentiometric titrator (Titroline 6000, SI Analytics, Germany) equipped with a pH combination electrode (1L-pHT-A120MF-DIN-N, SI Analytics, Germany) was employed for pH measurements. The pH meter was calibrated before and after each titration against two buffers, one in the acid range of potassium hydrogen phthalate, pH=4.00 (25°C)


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and the other in the alkaline range of sodium tetraborate, pH=9.18 (25°C). Each titration was carried out using 40.0 mL glycerol-water mixtures containing 0.05 mol/L succinic acid. Determination of Ca2+ concentration over reaction time in glycerol-water mixtures: the reaction system and experimental setup are the same as those used in the preparation of α-HH in glycerol-water mixtures, except that there is no modifiers in the reaction system. During the reaction, 10 mL slurry withdrawn at certain time intervals was immediately filtered by a syringe filter with a 0.45 µm cellulose membrane, and then the filtrate after diluting 100 times was used for ICP-AES. 2.3 Characterization The chemical compositions of FGD gypsum were investigated by using X-ray fluorescence spectroscopy (XRF, Axios mAX, PANalytical B.V., Netherlands). The structures of the samples were determined by X-ray diffraction (XRD D8 Advanced, Bruker, Germany) using Cu Kα radiation (λ=1.54178Å), with a scanning rate of 5° min-1 and a scanning 2θ range of 5° to 70°. The morphology of the samples were characterized with the field-emission scanning electron microscopy (SEM, JSM-6490LV, JEOL, Japan). The surfaces of the crystal products were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher, USA) with a Al Kα photon energy of 1486.6 eV. C 1s peak at 284.8 eV, which is related to the carbon adsorbed on the surface during the exposure of the samples to the ambient atmosphere, was used as reference for all spectra. The interaction between the crystal modifier and the crystal surfaces was analyzed by Fourier Translation Infrared Spectroscopy (FT-IR, IRAffinity-1, Shimadzu, Japan) with a resolution of 4 cm−1 over the frequency range of 400−4000 cm−1. For thermogravimetry and differential scanning calorimetry (TG-DSC, STA 8000, PerkinElmer, USA), 10 mg sample after dried was put into an Al2O3 crucible with a lid under temperature programming from 60°C to 400°C at a heating rate of 10°C /



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min under nitrogen gas atmosphere. An automatic potentiometric titrator (Titroline 6000, SI Analytics, Germany) equipped with a pH combination electrode (1L-pHT-A120MF-DIN-N, SI Analytics, Germany) was used to determine stability constants of succinic acid in glycerol-water mixtures with Bjerrum`s half-integer formation function method. The concentration of Ca2+ in the filtrate was determined by the inductively coupled plasma-atomic emission spectrometry (ICP-AES, PS-6, Baird, USA). The average lengths, diameters and average aspect ratios of α-HH products for each sample were counted up about 100 complete α-HH particles from SEM images. The particles were from nine positions evenly distributed at the sample. And 10~12 complete particles at each position, at which SEM photos were taken at a magnification of 500~1000, were used for the length, diameter and aspect ratio measurement.

3. RESULTS AND DISCUSSIONS 3.1 Influence of Glycerol on Morphology of the Crystal Products

a

b


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c

d

Fig.2 Influence of glycerol on the morphology of α-HH products prepared with 7.42 ×10-4 mol·kg-1 succinic acid and 0.2 mol·kg-1 NaCl at 90°C in glycerol -water solutions (a: 55% (w/w); b: 65% (w/w); c: 75% (w/w); d: 85% (w/w))

Fig.3 Average lengths, diameters and aspect ratios of α-HH products prepared in the presence of different glycerol concentration (wt%) with 7.42 ×10-4 mol·kg-1 succinic acid and 0.2 mol·kg-1 NaCl at 90°C in glycerol -water solutions



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Fig.4 XRD patterns of the crystal products prepared in the presence of different glycerol concentration (wt%) with 7.42 ×10-4 mol·kg-1 succinic acid and 0.2 mol·kg-1 NaCl at 90°C in glycerol -water solutions (a: 55% (w/w); b: 65% (w/w); c: 75% (w/w); d: 85% (w/w))

Fig.2 and Fig.3 show the morphological images and crystal size distribution of crystal products prepared with different glycerol content. Fig.4 exhibits XRD patterns of the crystal products. The diffraction peaks of α-HH are at 2θ = 14.72°, 25.64°, 29.70°, 31.86°, and the peaks appearing in the crystal products as shown in Fig.4 could be well indexed to α-HH. As shown in Fig.2a, the rod-like crystals with an average length of 60.97 µm, an average diameter of 15.21 µm and an average aspect ratio of 4.06 were formed in the presence of 55% weight fraction glycerol. With an increase in glycerol content to 65%, the average length decreased remarkably to 36.91 µm, and the average diameter of the crystals declined slightly to 13.86 µm, leading to a decrease in the average aspect ratio to 2.73. When the glycerol content went up to 75%, the average length and diameter of α-HH crystal products descended continually to 16.92 µm and to 9.36 µm, respectively, which resulted in a decline of the average aspect ratio to 1.83. From Fig.2b and Fig.2c, we can see that the α-HH products were transformed gradually from rod-like crystals to fat hexagonal prismatic crystals. Further increasing the glycerol content to 85% (Fig.2d), the crystal products appeared shorter column with a size of 7.21 µm in average length, 5.17 µm in average diameter and 1.41 in average aspect ratio. As shown in Fig.2, Fig.3 and the above discussion,


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the change of the glycerol content in mixed solutions of glycerol and water could effectively tune the size and morphology of α-HH crystals. 3.2 Influence of Glycerol on the Time of Phase Transformation from FGD Gypsum to α-HH

Fig.5 Influence of glycerol on the time of phase transformation from FGD gypsum to α-HH in glycerol -water solutions with 7.42 ×10-4 mol·kg-1 succinic acid and 0.2 mol·kg-1 NaCl at 90°C

The water content of samples withdrawn at certain time intervals was cited to indicate whether FGD gypsum (20.93 wt%) has been completely transformed into α-HH (6.21 wt%). Fig.5 shows the water content change of samples prepared in glycerol-water mixtures with different glycerol content and reaction time. As depicted in Fig.5, in the mixtures containing 55 wt% glycerol, the transformation starts at 16h, and the crystal water content generally decreased over time and reached 6.21 wt% at 30h, suggesting a complete transformation. When increasing glycerol content to 65 wt%, the FGD gypsum began to dehydrate at 6h and the complete transformation needed 14h. Further increasing glycerol content to 75 wt%, the phase transformation starts earlier and the crystal water content dropped dramatically with time. And in 9 hours the FGD gypsum was completely transformed into α-HH products. As glycerol content went up to 85 wt%, the transformation began within 30 min and completed in



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3 hours. Thus it can be seen that the increase of glycerol benefited the transition of FGD gypsum to α-HH in glycerol-water mixtures. 3.3 Influence of Glycerol on α-HH Crystallization Regulated by Succinic Acid

Fig.6 FT-IR spectrum of α-HH prepared in glycerol (55%) (w/w)-water solutions with 0.2 mol·kg-1 NaCl at 90°C


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Fig.7 FT-IR spectra of α-HH formed in the presence of different glycerol concentration (wt%) with 7.42 ×10-4 mol·kg-1 succinic acid and 0.2 mol·kg-1 NaCl at 90°C in glycerol -water solutions (a: 55% (w/w); b: 85% (w/w))

In order to explore the influence of glycerol content change on the interactions between the crystal modifier and the crystal surfaces of α-HH, FT-IR spectra were explored as shown in Fig.6 and Fig.7. As shown in Fig.6, the peaks at 3612, 3564 and 1620 cm-1 could be assigned to the O-H vibration of the crystal water molecule in α-HH crystals. The triple peaks at 1155, 1115, and 1097 cm-1 could be assigned to the asymmetric stretching vibration of υ3 SO42-, and the peak at 1007 cm-1 should be assigned to the distorted symmetric stretching vibration of υ1 SO42-. And the peaks at 660 and 602 cm-1 should be indexed to the υ4 SO42-stretching.16, 17 As depicted in Fig.7 (line a and b), the peaks of 2972 and 2951 cm-1 should be attributed to the asymmetric



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and symmetric stretching vibrations of methylene (-CH2-), and the 1558 and 1435 cm-1 peaks could be derived from the asymmetric and symmetric stretching vibrations of ester group (–COO-), which all demonstrated the adsorption of succinic acid on the α-HH crystal surfaces18. And with the increase of glycerol content, the vibration intensity of methylene (-CH2-) and ester group (–COO-) both generally decreased, which might resulted from a decrease in the amount of adsorbed modifiers on the surface of α-HH. In order to further illustrate the change in the amount of adsorbed modifiers on the surface of α-HH, X-ray photoelectron spectroscopy (XPS) was used to characterize the surface composition of α-HH formed in mixed solutions of glycerol and water with different glycerol content.

Fig.8 C 1s XPS spectra of α-HH formed in the presence of different glycerol concentration


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(wt%) without modifiers (a) or with 7.42 ×10-4 mol·kg-1 succinic acid (b and c) and 0.2 mol·kg-1 NaCl at 90°C in glycerol -water solutions (a: 55% (w/w); b: 55% (w/w); c: 85% (w/w))

Fig.9 Ca 2p2/3 XPS spectra of α-HH formed in the presence of different glycerol concentration (wt%) without modifiers (a) or with 7.42 ×10-4 mol·kg-1 succinic acid (b and c) and 0.2 mol·kg-1 NaCl at 90°C in glycerol -water solutions (a: 55% (w/w); b: 55% (w/w); c: 85% (w/w))

Fig.8 and Fig.9 depicts C 1s and Ca 2p3/2 XPS spectra of the crystal products formed in the presence of different glycerol content with a certain amount of succinic acid. As shown in C 1s spectra, the most intense C 1s peak observed on all samples was at 284.8 eV, which could be associated with the carbon contamination19 or the C-(C, H) species in the crystal modifier20, 21. Under the condition of no modifiers and 55% weight fraction glycerol (Fig.8a), C 1s spectra of α-HH crystals prepared could be fitted with three components: Besides the major component at 284.8 eV, the component located at 286.5 eV should be associated with C-O in the glycerol19, 20, 22, 23

, implying the adsorption of trace amount of glycerol on the crystal surfaces24-26.

The high binding energy component of C1s at 289.4 should be derived from the carbonate species (CO32-) of the limestone existed in FGD gypsum27, 28. As depicted in



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Fig.8b and 8c, when a certain amount of the crystal modifier was added into the crystallization reaction system, another C 1s component appeared at 288.8 eV, which should be assigned to the carboxylic or carboxylate group (COO−)29, 30. And with an increase in the glycerol content at the same amount of the modifier, the intensity of the peaks at 288.8 eV declined sharply, further demonstrating that the amount of adsorbed modifiers on the surface of α-HH decreased. As shown in Fig.9a, in the absence of the crystal modifier with 55% weight fraction glycerol, Ca 2p3/2 spectra of α-HH could be fitted with two components. One component at 347.2 eV should be attributed to the calcium of limestone existed in FGD gypsum, and the other could be derived from the calcium of α-HH. When the crystal modifier was added into the reaction system, the third peak at 347.7 eV, which should be associated with the calcium chelated by carboxyl group of the crystal modifier, occurred as shown in Fig.9b and c.29 However, with an increase of glycerol concentration, the intensity of the peak at 347.7 eV decreased on the contrary, which also implied a decrease in the amount of adsorbed modifiers on the surface of α-HH. According to previous researches31-35, the increase of alcohol content tended to suppress the dissociation of carboxylic acid in alcohol-water mixtures, which resulted in the decline of carboxylate ions in the mixtures. This might cause a decrease of the modifiers adsorbed on the surface of α-HH. In order to illuminate the influence of glycerol on dissociation constants of the modifier and find out the reason for the decrease in the amount of adsorbed modifiers with the increase of glycerol in the mixtures, we determined dissociation constants of succinic acid in glycerol-water mixtures containing different glycerol content at 25°C.


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Fig.10 Variation of pH* against volume of NaOH for succinic acid in glycerol-water mixtures at 25°C

+

Fig.11 The relationship between half-integer formation function n H and –lg[H ] at 25°C

Table 2 The dissociation constants of succinic acid in glycerol-water mixtures containing different



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glycerol content at 25°C. Glycerol concentration (wt%)

0%

45%

55%

65%

75%

pK1 pK2

3.90 5.36

4.46 5.88

4.53 6.01

4.78 6.25

5.01 6.51

Stability constants of succinic acid in glycerol-water mixtures were obtained using Bjerrum`s half-integer formation function method36-38. The basic principle of this method is as follows:

nH = 2 −

VNaOH × cNaOH V0 + VNaOH K + × ( W+ − [ H + ]) V0 × c V0 × c [H ]

(1)

where, n H is the formation function; V0 and c are the initial volume and acid concentration of the mixtures; V

NaOH

and c

NaOH are

the volume and concentration of

sodium hydroxide solution added by titration method; [H+] is the hydrogen ion concentration and Kw is the autoprotolysis constant of water. When n H is equal to half integer j-0.5 (j=1, 2), hydrogen ion concentration [H+] and the stability constant

KH2A,j have following relationship: log10 KH2 A, j = − log10[H + ]

(2)

where, j represents the order of stability constants. pH values obtained by calibration of the pH meter assembly with aqueous buffers were converted to pH* values for the glycerol media by introducing a correction factor, δ, as follows: pH*=pH-δ

(3)

where δ accounts for the residual liquid junction potential and the medium effects. A previous reported value of δ for glycerol-water mixtures39 was used for this calculation. Variation of pH* against volume of NaOH for succinic acid in glycerol-water mixtures containing different content of glycerol and the relationship between n H and –log10[H+] (pH*) at 25°C are depicted in Fig.10 and Fig.11, respectively. According to the relationship between dissociation constants and stability constants, the


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dissociation constants of succinic acid in glycerol-water mixtures at 25°C are shown in Table 2. From Table 2, we can see that as an increase of glycerol content in the mixtures, pK1 and pK2 generally increased, which led to the decrease of carboxylate ions dissociated by succinic acid in the mixtures. This might be the ultimate reason for the decrease in the amount of the crystal modifier on α-HH surfaces as the glycerol content in glycerol-water mixtures rose. From the above discussion, we could make it clear that with the increase of glycerol in glycerol-water mixtures, the adsorption of the modifier has not been the dominant reason to modulate α-HH crystal morphology. 3.4 Influence of Glycerol on Supersaturation for α-HH In order to explore the main reason of the crystal morphology change with glycerol increase in the mixtures of glycerol and water, we investigated the influence of glycerol on supersaturation for α-HH precipitation.



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Fig.12 Variation of Ca2+ concentration over reaction time in glycerol-water mixtures with different glycerol content without crystal modifiers in the presence of 0.2 mol·kg-1 NaCl at 90°C

From Fig.12, we can see that with the increase of reaction time, Ca2+ concentration increased at first and then decreased, and finally reached the equilibrium states. Before Ca2+ concentration reached the top point, the stage should be the dissolution process of FGD gypsum. And after the peak concentration, Ca2+ concentration dropped sharply, which might be the crystal nucleation and growth stage of α-HH. And after that Ca2+ concentration kept nearly the same. When the concentration was constant, we assumed that the α-HH equilibrium was attained. Therefore, the maximal relative supersaturation of α-HH could be obtained by equation (4) and the results were shown in Table 3.


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S max = In this equation S

max

cα − HH , Max − cα − HH , Equilibrium

(4)

cα − HH , Equilibrium

is the maximal relative supersaturation, cα − HH , Max is the top

concentration of α-HH calculated by the top concentration of Ca2+, and

cα − HH , Equilibrium is the equilibrium concentration of α-HH estimated from the last point of concentration. Table 3 α-HH maximal relative supersaturation in glycerol-water mixtures with different glycerol content without crystal modifiers in the presence of 0.2 mol·kg-1 NaCl at 90°C Glycerol content (wt%) in the mixtures

cCa 2+ , Max

c C a 2 + , E quilibrium

mg/L

mg/L

mg/L

mg/L

55% 65% 75% 85%

1032.2 800.0 850.0 1000.0

969.5 716.9 734.4 768.4

3741.7 2900.0 3081.3 3625.0

3514.4 2598.8 2662.2 2785.5

As shown in Table 3, S

max

cα − HH , Max

cα − HH , Equilibrium

S max 0.06 0.12 0.16 0.30

of α-HH enhanced with the increase of glycerol in the

mixtures, and S max in the mixtures with 85% glycerol was four times larger than that in the mixtures with 55%. Meanwhile, as glycerol in the mixtures increased, carboxylate ions in the solution decreased and Ca2+ ions chelated by carboxylate ions reduced

correspondingly,

which

further

improved

the

maximal

relative

supersaturation for α-HH in the mixtures with higher glycerol content. And with the increase of maximal relative supersaturation, the rapid nucleation rate boosted a large number of nuclei, resulting in the reduction of phase transformation time and the formation of massive small α-HH particles24, 40, 41. At the same time, the kinematic viscosity and density of the glycerol-water mixtures rose with glycerol increase, which resulted in the reduction of ion (Ca2+ and SO42-) diffusion rate and the evolution of small crystal particles42, 43. From what has been discussed above, we can see that with the increase of glycerol content in glycerol-water mixtures, the increase of α-HH maximal relative supersaturation and decrease of ion diffusion rate, which were superior to the adsorption of the crystal modifier, were hypothesized as explanations for the change



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of α-HH crystal morphology. And the increase of the maximal relative supersaturation which determined the nucleation rate40, 44 was one of the reasons for the kinematic feasibility of FGD gypsum (DH) transition to α-HH.

4. CONCLUSIONS The influence of glycerol on the phase transformation time and α-HH crystal morphology was explored systematically, aiming at the preparation of α-HH tuned by succinic acid and NaCl in glycerol-water mixtures. With an increase in glycerol content from 55% to 85%, the average length and diameter of α-HH decreased from 60.97 to 7.21 µm and from 15.21 to 5.17 µm, respectively, which resulted in a decrease in average aspect ratio from 4.06 to 1.41. The phase transformation time was reduced dramatically from 30 to 3 hours. As glycerol increased from 55% to 85% in glycerol-water mixtures, the maximal relative supersaturation for α-HH precipitation enhanced significantly from 0.06 to 0.30. Meanwhile, as the glycerol content increased, the dissociation constants of succinic acid generally rose, which resulted in the decrease of carboxylate ions in the solution and a corresponding reduction of Ca2+ ions chelated by carboxylate ions. This further improved the maximal relative supersaturation of α-HH in the mixtures with higher glycerol content. At the same time, the enhancement of glycerol increased the kinematic viscosity and density of the mixed solutions, which led to the reduction of ion diffusion rate. Therefore, the increase of α-HH maximal relative supersaturation and decrease of ion diffusion rate, which were superior to the adsorption of the crystal modifier, were hypothesized as explanations for the change of α-HH crystal morphology in the mixtures with high glycerol. The enhancement of α-HH maximal relative supersaturation was the explanation for the reduction of phase transformation time with the increase of glycerol in glycerol-water mixtures.


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ASSOCIATED CONTENT Supporting Information S1. Pretreatment of FGD gypsum; S2. Effect of glycerol content on the formation of α-HH; S3. Images of α-HH prepared without succinic acid and without succinic acid and NaCl in glycerol-water solution; S4. The evolution of the mass loss during the transformation from DH to α-HH; S5. Density and viscosity of the glycerol-water solution containing different glycerol content.

AUTHOR INFORMATION Corresponding Author *W.

Sun.

Tel:

+86-731-88876697.

Fax:

+86-731-88710804.

E-mail:

[email protected], [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is financially supported by Innovation Driven Plan of Central South University (No. 2015CX005), the National 111 Project (No. B14034), Collaborative Innovation Center for Clean and Efficient Utilization of Strategic Metal Mineral Resources, and the project of Sublimation Scholar`s Distinguished Professor of Central South University. The Fundamental Research Funds for the Central Universities of Central South University (2016zzts104) also supported the research financially.



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