Preparation of anhydrite from eggshell via pyrolysis

2.1 Materials

Duck eggshell was collected from the cafeteria at Kasetsart University, Bangkok, Thailand. The duck eggshells were cleaned with tap water, dried in the air for 2 days and ground with high-speed mill for 120 min.

Hydrosulfuric acid (H₂SO₄) has a high purity of 98% and was purchased from Arsom Co., Ltd., Bangkok, Thailand. Hydrosulfuric acid is colorless and odorless, with the melting temperature of 104°C at 1.0 atm.

2.2 Instruments

Muffle furnace (Nabertherm, Ceramotherm, Bangkok, Thailand) with a thermocouple of type K, NiCr-Ni was used to calcine the duck eggshells at 900°C for 2 h with a heating rate of 5°C/min. The muffle furnace was used to calcine the precipitated calcium sulfate powders, prepared from the reaction between duck eggshell and concentrated sulfuric acid at room temperature, at firing temperatures of 700, 800 and 900°C for 2 h with a heating rate of 10°C/min.

High-speed mill (Compound Clay, model RM 1105, Bangkok, Thailand) with a speed 500 rpm was used to grind calcium sulfate powders.

Particle size analyzer (Malvern, Mastersizer S long, Bangkok, Thailand) was used to analyze the particle size distribution and cumulative mass percent finer. The samples were dispersed in a water medium and vibrated in an ultrasonic cleaner for 20 min.

XRD (Bruker, AXS D8 Discover, Bangkok, Thailand) with a VANTEC-1 Detector was used to obtain X-ray diffraction patterns. Samples were analyzed using a double-crystal wide-angle goniometry. Scans were measured from 5°–80° 2θ at a scan speed of 5° 2θ/min in 0.05° or 0.03° 2θ increments using CuK_α radiation (λ=0.15406 nm). Peak positions were consistent with those of the International Center for Diffraction Data Standard (JCPDS) patterns to identify crystalline phases.

XRF (Phillips, PW 2400, Bangkok, Thailand) was used to determine the chemical compositions using a tube current of 1000 mA and an acquisition lifetime of 30 s.

SEM (JEOL, 5200, Bangkok, Thailand) was used to obtain SEM images. The samples were mounted on stubs using a carbon paste and were sputter-coated to ~0.1 μm of gold to improve conductivity. The acceleration voltages of 11 and 13 kV with magnifications of 5000 times were used.

FTIR (PerkinElmer, Spectrum One, Bangkok, Thailand) was used to measure FTIR spectra at a spectral resolution of 4 cm⁻¹ in the range 400–4000 cm⁻¹ using ATR zinc selenide, ZnSe, as a reference.

TGA (Perkin-Elmer, TGA 7, Bangkok, Thailand) was used to investigate thermal properties of samples in terms of weight loss or residue mass with a heating rate of 10°C/min.

DTA (Perkin Elmer, DTA 7, Bangkok, Thailand) was used to study the thermal properties of the raw duck eggshell powder and the precipitated calcium sulfate prepared at room temperature (25°C).

An AUTOSORB-1 (Quantachrome, Bangkok, Thailand) was used to characterize the specific surface area, adsorption and/or desorption isotherms and pore size distribution. These material characteristics were evaluated by measuring of the quantity of gas adsorbed onto or desorbed from the solid surface at equilibrium vapor pressure by the static volumetric method. The volume-pressure data were interpreted by the AUTOSORB-1 software into the BET surface area (single and/or multipoint), the Langmuir surface area, adsorption and/or desorption isotherms and micro-pore volume. The determination of the surface area of the solid materials involved the use of the BET equation:

where W is the weight of the gas adsorbed at the relative pressure, P/P_o, W_m is the weight of the adsorbate constituting a monolayer of the surface coverage; and C is the constant related to the energy of adsorption in the first adsorbed layer. The specific surface area, S, of the solid was calculated from the total surface area and the sample weight, according to Eqs. (2) and (3):

where S is the specific surface area of the solid, S_t is the total surface area, W is the sample weight, N is Avogadro’s number (6.023×10²³ molecules/mol), M is the molecular weight of the adsorbate and A_cs is the area occupied by one adsorbate molecule (16.2×10⁻²⁰ m² for N₂ and 19.5×10⁻²⁰ m² for Kr). The characteristics of the gas adsorption can be categorized to three porosity classifications: (i) pores with openings exceeding 500 Å in diameter (called macropores); (ii) micropores, or pores with diameters not exceeding 20 Å; and (iii) pores of intermediate size or mesopores [28].

True density of the raw material (duck eggshells) and calcium sulfate powder was measured by a Gas Pycnometer (Quantachrome, Ultra pycnometer 1000) following Eq. (4):

where ρ is the true density (g cm⁻³), D is the weight of dry sample (g) and the true volume is the volume of the solid component only (cm³). It was determined by crushing the sample into a powder form so that all pores were destroyed and then using a gas pycnometer method.

2.3 Calcium sulfate (CaSO₄·2H₂O, CaSO₄·0.5H₂O and CaSO₄) powder preparation

The duck eggshell was ground for 120 min, as a calcium carbonate source (CaCO₃) and then allowed to react with sulfuric acid at room temperature (25°C) according to the chemical reactions in Eqs. (5) and (6):

When the chemical reaction between CaCO₃ and H₂SO₄ occurred, the precipitated calcium sulfate or calcium sulfate dihydrate (CaSO₄·2H₂O) powder was filtered, rinsed with tap water two to three and then dried in the oven at 110°C for 24 h. Calcium sulfate dihydrate (CaSO₄·2H₂O) was transformed into calcium sulfate hemihydrate (CaSO₄·0.5H₂O) according to Eq. (7). Then, the dried calcium sulfate hemihydrate powder was calcined at 700, 800 and 900°C for 2 h. Calcium sulfate hemihydrate was converted to anhydrite or anhydrous calcium sulfate (CaSO₄) according to Eq. (8). The obtained calcined powder was then used to investigate the physical properties and characterized by SEM, FTIR, TGA, DTA, XRD, particle size distribution, BET and pycnometer measurement.

3.1 Characteristics of raw material and calcium sulfate powder

The chemical compositions of the duck eggshell and calcined duck eggshell were measured by XRF and data are tabulated in Table 1. The raw duck eggshell is composed of calcium carbonate (CaCO₃) of 98.101 wt% and other oxide compounds of 1.899 wt%, while the calcined duck eggshell is composed of calcium oxide (CaO) of 97.805 wt% and other oxide compounds such as MgO, Na₂O, K₂O and SiO₂ of 2.195 wt%. The raw duck eggshell and calcined duck eggshell reacted with sulfuric acid at room temperature (25°C) according to Eqs. (5) and (6) to obtain calcium sulfate dihydrate or gypsum. When calcium sulfate dihydrate was heated, it was transformed into calcium hemihydrate or plaster of Paris and anhydrite according to Eqs. (7) and (8), respectively.

3.2 Physical properties and microstructures of calcium sulfate powder

The physical properties (particle size, true density, specific surface area and pore diameter) of the raw duck eggshell and anhydrous calcium sulfate calcined at 800 and 900°C were measured and the data are tabulated in Table 2. The particle size of the raw duck eggshell is 32.33 μm. The true density, specific surface area and pore diameter of the raw duck eggshell are 2.25 g/cm³, 7.79 m²/g and 196.90 Å, respectively, while the particle size of the raw duck eggshell reacted with sulfuric acid and calcined at 900°C for 2 h is equal to 3.99 μm. The true density, specific surface area and pore diameter of the raw duck eggshell reacted with sulfuric acid and calcined at 900°C for 2 h are 2.95 g/cm³, 3.57 m²/g and 96.98 Å, respectively. The porosity of the anhydrous calcium sulfate calcined at 900°C for 2 h is in the range of meso-porous structure (20–500 Å). The adsorption-desorption isotherm of the anhydrous calcium sulfate or anhydrite is consistent with the isotherm of type IV according to the Kelvin equation having the hysteresis loop at a lower relative pressure, causing a lower free energy state and the thermodynamic equilibrium. The physical properties of the raw duck eggshell and calcium sulfate samples, namely, particle size and shape, specific surface area, true density and solubility, are important factors for calcium sulfate formation and applications [6], [9], [29].

Figure 1 shows the DTA measurement of the dried precipitate calcium sulfate hemihydrate powder prepared from the chemical reaction between the duck eggshell and sulfuric acid from 25 to 1000°C. There are two sharp peaks in the exothermic reaction occurring at 163.70°C to form calcium sulfate hemihydrate or plaster of Paris (CaSO₄·0.5H₂O). The exothermic reaction still shows a metastable calcium sulfate up to 820.08°C, after which the calcium sulfate hemihydrate was converted to the calcium sulfate anhydrite. Kontrec et al. [12] reported the loss of water molecules from the calcium sulfate dihydrate or gypsum at about 160°C where the rest of H₂O remained in the calcium sulfate hemihydrate (CaSO₄·0.5H₂O) which was finally converted to the anhydrite or anhydrous calcium sulfate (CaSO₄) at high temperature.

Figure 1:

DTA measurement of the calcium sulfate dihydrate powder as prepared from the chemical reaction between duck eggshell and sulfuric acid from room temperature (25°C) to 1000°C.

TGA measurement of the calcium sulfate dihydrate powder prepared from the raw duck eggshell reacting with sulfuric acid is shown in Figure 2. The weight loss in the range of 25–1000°C is equal to 22.68 wt% or with a residue mass of 78.32 wt%. Water was released by about 6.0 wt% from the calcium sulfate dihydrate (CaSO₄·2H₂O) to yield the calcium sulfate hemihydrate (CaSO₄·0.5H₂O) at 163.70°C, consistent with the obtained DTA result. The obtained water release of 6.0 wt% is close to the theoretical water content of 6.30 wt% of the exothermic reaction at 160°C [21].

Figure 2:

TGA measurement of the dried calcium sulfate dihydrate as prepared from the chemical reaction between duck eggshell and sulfuric acid measured in the range of 25–1000°C.

The chemical compositions of the calcium sulfate before and after firing at 900°C for 2 h were measured by XRF and the data are tabulated in Table 3. The main compositions of the calcium sulfate dihydrate before firing at 900°C for 2 h are close to the chemical compositions after firing. The chemical composition of calcium sulfate dihydrate is CaO at 87.946 wt% and SO₃ at 12.054 wt%, while the anhydrous calcium sulfate or anhydrite is composed of calcium oxide (CaO) of 86.017 wt% and sulfur trioxide (SO₃) of 13.983 wt%. The SO₂ and SO₃ can react with CaCO₃ to yield CaSO₄ and CaSO₄·2H₂O as in Eq. (9):

The FTIR spectra of the obtained samples prepared from the chemical reaction between the raw duck eggshell reacted with sulfuric acid (H₂SO₄), before and after calcination at 700, 800 and 900°C, for 2 h are shown in Figure 3. The FTIR spectrum of the dried precipitate calcium sulfate powder before firing is consistent with the results obtained by Greish [1] and Kontrec et al. [12]. They showed that the hemihydrate calcium sulfate spectrum consists of the ester peak S-O at 659.13 cm⁻¹, the C-H bending at 874.98 cm⁻¹, the S=O stretching at 1007.99, a sharp peak at 1088.99 cm⁻¹, the S=O stretching at 1112.32 and 1142.56 cm⁻¹ and the O-H stretching at 3556.02 and 3604.69 cm⁻¹. When calcium sulfate hemihydrate samples were calcined at 700, 800 and 900°C, for 2 h, the corresponding FTIR spectra contain the anhydrite calcium sulfate peaks at 613.01 and 672.64 cm⁻¹ (S-O ester), at 876.09 cm¹ (C-H bending), at 1095.88 cm⁻¹ (S=O stretching) and at 3642 cm⁻¹ (O-H stretching), consistent with the result reported by Kontrec et al. [12]. From the obtained FTIR results, the SO₂ or SO₃ from the sulfuric acid reacted with CaCO₃ to yield CaSO₄·2H₂O, CaSO₄·0.5H₂O and CaSO₄.

Figure 3:

FTIR spectra of the obtained calcium sulfate samples as prepared from the chemical reaction between the raw duck eggshell and sulfuric acid (H₂SO₄) before and after calcinations at 700, 800 and 900°C, for 2 h, respectively.

The XRD peak patterns of the raw duck eggshell before and after firing at 900°C for 2 h and the dried calcium sulfate powders obtained from the raw duck eggshell reacting with sulfuric acid and calcined at 700, 800 and 900°C, for 2 h, are shown in Figure 4. The XRD peak pattern of the raw duck eggshell shows the crystalline phase formation of rhombohedral or calcite consistent with the JCPDS file no. 01-086-2339 at the (hkl): (104) 29.364°, (012) 23.058° and (113) 39.424°. The XRD peak pattern of the calcined raw duck eggshell shows the crystalline phase formation of lime or calcia consistent with the JCPDS file no. 00-037-1497 at the (hkl): (200) 37.347°, (220) 53.856° and (111) 32.204°. The main XRD peak pattern of the dried precipitated calcium sulfate powder before firing shows the rhombohedral structure of the calcium sulfate hemihydrate (CaSO₄·0.5H₂O), consistent with the obtained DTA and TGA results, the FTIR results and the JCPDS file no. 01-070-0909 at the (hkl): (020) 25.432°, (104) 29.364°, (012) 31.366°, (022) 38.648°, (212) 40.820° and (032) 48.696°. The dried precipitated sample, or the calcium sulfate hemihydrate (CaSO₄·0.5H₂O) powder, was calcined at 700 and 800°C for 2 h. The XRD peak patterns show the anhydrous calcium sulfate (CaSO₄) metastable phase belonging to the hexagonal structure consistent with the JCPDS file nos. 01-089-1458 and 01-070-0909 at the (hkl): (100) 14.665°, (200) 29.577°, (102) 32.011° and (020) 25.432°, (012) 31.366° and (212) 40.820°. The (CaSO₄), present with a small amount of the calcium hydroxide or portlandite (Ca(OH)₂), belongs to the orthorhombic structure consistent with the JCPDS file no. 00-004-0733 at the (hkl): (101) 34.089°, (102) 47.124° and (110) 50.795°. Furthermore, the dried precipitated calcium sulfate dihydrate sample calcined at 900°C for 2 h shows an XRD pattern consisting of a stable crystalline phase formation of a hexagonal structure, namely, the anhydrite structure (CaSO₄), consistent with the JCPDS file no. 01-089-1458 at the (hkl): (020) 25.432°, (012) 31.366° and (022) 38.648, the anhydrite crystal structure [3]. Therefore, the obtained XRD patterns indicate that the calcium sulfate dihydrate (CaSO₄·2H₂O) can transform into the calcium sulfate hemihydrate or plaster of Paris (CaSO₄·0.5H₂O) and then to the anhydrite (CaSO₄) crystal structure depending on processing temperature and time, consistent with the dehydration-rehydration of calcium sulfates flowchart [30].

Figure 4:

XRD peak patterns of the duck eggshell before and after calcination at 900°C for 2 h and calcium sulfate hemihydrate powders calcined at 700, 800 and 900°C, for 2 h, respectively.

The SEM micrographs of the duck eggshell powder, calcium sulfate hemihydrate powder before firing and dried precipitated calcium sulfate hemihydrate powder calcined at 700, 800 and 900°C at the magnifications of 1000 and 5000 times are shown in Figure 5. The SEM micrographs of the ground raw duck eggshell powder show the agglomerate particles and non-uniform size as shown in Figure 5A. The SEM micrographs of the precipitated calcium sulfate hemihydrate powder dried at 110°C without firing show a uniform needle-shaped crystal (Figure 5B) [4], [13], [31]. The agglomeration of the needle shape to form a plate-like shape or a disk-like shape is due to the phase transformation at the firing temperature of 700°C as shown in Figure 5C consistent with the SEM result obtained by Gartner [31]. The microstructure changes to a more plate-like shape at the firing temperature of 800°C, as shown in Figure 5D. Furthermore, the microstructure crystallizes completely and changes to form the small rod-like shape at the calcination temperature of 900°C as shown in Figure 5E consistent with the SEM results obtained by Azimi et al. [30].

Figure 5:

SEM micrographs of samples at magnifications of 5000 times: (A) duck eggshell powder; (B) calcium sulfate hemihydrate (CaSO₄·0.5H₂O) powder before firing; (C) calcium sulfate hemihydrate powder calcined at 700°C; (D) calcium sulfate hemihydrate powder calcined at 800°C; and (E) calcium sulfate hemihydrate powder calcined at 900°C.