Mineral Transition and Chemical Reactivity Evolution of a Low-Lime Calcium Aluminate Clinker with MgO and Na2SO4 Codopants

The mineral formation–transition mechanism, microstructure evolution, crystal structure, pulverization property, and chemical reactivity of the CaO–Al2O3–SiO2 clinker with MgO and Na2SO4 dopants during the sintering process at 1300 °C for 2.0 h were systematically studied using CaO, Al2O3, SiO2, MgO, and Na2SO4 as raw materials when the molar ratio of CaO to Al2O3 is 1.4, the mass ratio of Al2O3 to SiO2 is 3.0, and the mass percentage of MgO and Na2SO4 is 2%. The MgO dopant could result in 12CaO·7Al2O3 and γ-2CaO·SiO2, transform into 20CaO·13Al2O3·3MgO·3SiO2, restrain the crystal transformation of 2CaO·SiO2 from β to γ, and then deteriorate the pulverization and alumina leaching property corresponding to parts of Al, Si, and Mg atoms occupying the same lattice positions of the crystal structure. MgO and Na2SO4 codoped could promote transformation of 20CaO·13Al2O3·3MgO·3SiO2 into 3CaO·3Al2O3·CaSO4 as well as some 2CaO·Al2O3·SiO2, while 3CaO·3Al2O3·CaSO4 has good alumina leaching property in the Na2CO3–NaOH solution. The ultrasonic assistant mainly could promote the diffusion of reactive samples, enhance the separation of agglomeration, and then accelerate the chemical reaction of the sintered clinker with Na2CO3–NaOH.


INTRODUCTION
Calcium aluminate-based materials have become popular composites for scientific technological progress, which have wide applications in concrete (low density and hardness), 1,2 biomaterials (physical, mechanical, bioactive, and biocompatible), 3 functional ceramic, and refractory (high-temperature stability). Meanwhile, they also play an important role in the metallurgical industry recovering alumina from low-grade aluminum-containing resources with the lime sinter process, 4,5 such as high-iron bauxite, Bayer red mud, kaolinite, and highalumina fly ash. Obviously, they show incomparable advantages of dry sintering, self-pulverization, and being environmentally friendly and free from secondary pollution as compared with the soda sinter process, acid process, and acid-soda combined process. CaO·2Al 2 O 3 , CaO·Al 2 O 3 , and 12CaO·7Al 2 O 3 have good chemical reactivity in the Na 2 CO 3 −NaOH leaching process, 5−7 while the β → γ crystal transformation of 2CaO·SiO 2 is responsible for the pulverization process. Based on the crystal structure, CaO·Al 2 O 3 contains double sheets of edge-sharing AlO 6 octahedra or two crystallography-independent AlO 6 octahedra forming edge-sharing double chains. 8,9 12CaO· 7Al 2 O 3 contains vertex-shared AlO 4 tetrahedra with some of the corners linked by bridging Ca atoms, in which O atoms are trapped in the Ca−Al−O cage. 10−12 In the mineral transition and physical−chemical property, previous literature shows that the MgO dopant would deteriorate the chemical reactivity and pulverization property of the CaO−Al 2 O 3 clinker sharply, 13 but the effect mechanism has not been reported distinctly and systematically, especially on the mineral formation−transition process and crystal structure evolution. Actually, the negative effect of MgO is difficult to eliminate completely. Because the raw materials such as alumina-containing resources and lime usually contain trace amounts of MgO; it also could reduce the solubility of sulfur-containing compounds in calcium silicate, therefore compensating for the negative effects of SO 3 . 14 MgO seems to favor exsolution of the sulfate to form 2CaO·SiO 2 , and the inhibition of conversion from 2CaO·SiO 2 to 3CaO· SiO 2 is more readily overcome. 15 SO 3 could promote the formation of 2CaO·SiO 2 rims, while MgO could enhance 3CaO·SiO 2 in the SO 3 -bearing clinker. 16 Furthermore, the diffusion of the elements Na, Mg, Si, S, V, and Fe into the calcium aluminate and silicate compounds also has been observed, which mainly leads to the formation of spinels, vanadate sulfates, and silicates. 17 Moreover, calcium sulfoaluminate phases can partially accommodate metals (Cu, Cr, Cd, Pb, Zn, and Fe) in the crystal lattice, giving rise to chemical entrapment. It is also observed that calcium sulfoaluminate (3CaO·3Al 2 O 3 ·CaSO 4 ) would decompose into CaO·Al 2 O 3 , CaO, and SO 3 when the sintering temperature exceeds 1300°C. 18 The aim of this paper is to study the "structure−activity" relationship of the crystal structure characteristics and mineral formation−transition mechanism with the physicochemical property (self-pulverization property and chemical reactivity in the Na 2 CO 3 −NaOH solution) in the CaO−Al 2 O 3 −SiO 2 system with the MgO and Na 2 SO 4 dopant. The influence mechanism of MgO on the mineral transition process and chemical reactivity will be studied comprehensively, while the eliminate mechanism of Na 2 SO 4 on the MgO negative interaction will also be reported. Then, it would be a breakthrough application and development in the lime sinter process recovering alumina and other valuable elements from low-grade alumina-containing resources.

RESULTS AND DISCUSSION
2.1. Characterization of Sintered Clinker with MgO− Na 2 SO 4 Dopant. The X-ray diffraction (XRD) patterns of the CaO−Al 2 O 3 −SiO 2 clinker with MgO and Na 2 SO 4 dopants are shown in Figure 1. As shown in Figure 1  The Visualizer module of Materials Studio is used to establish the three-dimensional models of 20CaO·13Al 2 O 3 · 3MgO·3SiO 2 and 3CaO·3Al 2 O 3 ·CaSO 4 . The unit cell models data, corresponding to the space group, lattice parameters, and atom distribution position, are calculated according to the XRD results ( Figure 1). 20CaO·13Al 2 O 3 ·3MgO·3SiO 2 belongs to the orthorhombic crystal system, Pmmn(59) space group, and lattice parameters are as follows: a = 27.15 Å, b = 10.63 Å, c = 5.09 Å, and α = β = γ = 90°. 3CaO·3Al 2 O 3 ·CaSO 4 belongs to the orthorhombic crystal system, Pcc2(27) space group, and lattice parameters are as follows: a = 12.97 Å, b = 13.03 Å, c = 9.16 Å, and α = β = γ = 90°. The atom distribution position corresponds to the ICSD data, which is shown in Tables 1 and 2. In addition, the unit cell models could provide the foundational models for the subsequent calculation and explanation, which are shown in Figure 2.
According to Figure 2a and Table 1, the unit cell of 20CaO·   shows a regular and porous morphology, and the magnified characteristic region (B) also shows a porous morphology, corresponding to the agglomeration of well-organized and cuboid molecules, which mainly attributes to 20CaO·13Al 2 O 3 · 3MgO·3SiO 2 . Further, Figure 3c shows that the regular morphology (in Figure 3b) turns into a dense porous-layered morphology when Na 2 SO 4 is added. The sintered clinker melts completely in the sintering process with amounts of pores, mainly because of Na 2 SO 4 decomposition. It not only could decrease the melting point of the sintered materials but also could generate 3CaO·3Al 2 O 3 ·CaSO 4 with low melting point. The magnified characteristic region (C) in Figure 3c mainly corresponds to the coexisting phases of 12CaO·7Al 2 O 3 and γ-2CaO·SiO 2 as well as some 3CaO·3Al 2 O 3 ·CaSO 4 , which is in accordance with the XRD results ( Figure 1) and EDS analysis (Figure 3d). Figure 4 shows the particle size distribution of the CaO− Al 2 O 3 −SiO 2 clinker with MgO and Na 2 SO 4 dopants, and its particle size parameters are also shown in Table 3 to form 20CaO· 13Al 2 O 3 ·3MgO·3SiO 2 but also restrain the transformation process of 2CaO·SiO 2 from β to γ. Because 2CaO·SiO 2 is comprised of several polymorphs, such as α, α L ′, α H ′ , α m ′ , β, and γ, the polycrystalline type transformation from β to γ is responsible for the pulverization process at 12 vol % expansion. 19 Therefore, the decrease of the 2CaO·SiO 2 content and the inhibition of the transformation process from β-2CaO·SiO 2 to γ-2CaO·SiO 2 are the main reasons that the MgO dopant could deteriorate the pulverization property obviously. The CaO−Al 2 O 3 −SiO 2 clinker with the MgO dopant also shows a bimodal distribution with the size ranging from 0.85 to 768.55 μm. The two peaks are centered at 0.75 and 60.05 μm. The D 50 increases, while the specific surface area decreases slightly, which are 5.84 μm and 0.68 m 2 ·g −1 , respectively. It is in accordance with microstructural characteristics obtained by the SEM analysis ( Figure 3) that the sintered clinker agglomerate easily with amounts of small particles in the surface of the major structure. MgO and Na 2 SO 4 codopants could not only restrain the formation of 20CaO· 13Al 2 O 3 ·3MgO·3SiO 2 but also result in the formation of

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Article are shown in eqs 1−3. Furthermore, additional sodium hydrate could restrain the hydrolytic process of NaAl(OH) 4 , as also shown in eq 4.      As shown in Figure 7a−c, increasing leaching temperature from 50 to 70°C could reinforce the chemical reactivity of the sintered clinker in the Na 2 CO 3 −NaOH solution significantly, and the alumina leaching property increases circa 12%. The process of the sintered clinker with Na 2 CO 3 −NaOH solution has a fast reaction rate. Improving the sodium carbonate concentration from 80 to 120 g·L −1 also has an obvious effect on the alumina leaching property circa 9%. Increasing leaching temperature higher than 70°C, improving the sodium carbonate concentration higher than 120 g·L −1 , and prolonging leaching duration higher than 15 min also has no obvious effect on the alumina leaching property. The ultrasonic assistant could also improve the alumina leaching property, and the reinforcement decreases gradually with the increase of leaching temperature and sodium carbonate concentration. It has a remarkable reinforcement circa 6%, especially when the leaching temperature is lower than 70°C or the sodium carbonate content is less than 110 g·L −1 . The reinforcement mechanism mainly attributes to the chemical effect and heat effect, and then the ultrasonic assistant could promote the diffusion process of the sintered clinker and accelerate the chemical reaction.
The morphology characteristics and particle size distribution of the CaO−Al 2 O 3 −SiO 2 (2% MgO−2% Na 2 SO 4 ) clinker (preball-milled) and the leached residue are shown in Figure  10, while the particle size parameters (D 10 , D 50 , D 90 , and specific surface area) are shown in Table 4.
According to Figure 10a,d and Table 4, the sintered clinker (ball-milled) shows a uniform particle size distribution with the size ranging from 0.26 to 179.74 μm. The bimodal peaks are centered at 0.76 and 40.15 μm, while the D 50 and specific surface area are 36.38 μm and 0.17 m 2 ·g −1 . According to Figure  10b,d and Table 4, the leached residue from the CaO−Al 2 O 3 − SiO 2 clinker without ultrasonic shows a complete irregular particle size distribution with the size ranging from 0.23 to 200.05 μm. Most parts of small particles agglomerate into larger particles, while it has a rough surface. The D 50 and specific surface area are 39.14 μm and 0.16 m 2 ·g −1 , respectively. The leached residue with the ultrasonic assistant shows a relative regular particle size distribution when compared with the leached residue without ultrasonic. It also shifts toward much smaller particle size, and the D 50 and specific surface area are 28.20 μm and 0.22 m 2 ·g −1 , respectively.    to produce a cylindrical sample with 10 mm in diameter and 30 mm in length using polyvinylidene fluoride as the binder. Then, the samples were sintered at 1300°C for 2.0 h in the MoSi 2 resistance furnace (KLS-1700x) followed by cooling in the furnace. The temperature system in the heating and cooling process is shown in Figure 11. The sintered clinker varying with the oxide dopant is leached at 80°C for 0.5 h in sodium carbonate with sodium hydrate (Na 2 CO 3 −NaOH) solution in a three-necked flask. The concentration of sodium carbonate (in the form of Na 2 O C ) and sodium hydrate (in the form of Na 2 O K ) in the solution is 80 and 10 g·L −1 . The liquid−solid ratio of leaching solution to the sintered clinker is 10. The alumina concentration (C Al 2 O 3 ) in the filter liquor is analyzed by the volumetric method, and the alumina leaching property is calculated by eq 7.   Figure 11. Sintering temperature system in the heating−cooling process.

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