Trans. Nonferrous Met. Soc. China 22(2012) 3108-3112

Effect of Na₂O on formation of calcium aluminates in CaO-Al₂O₃-SiO₂ system

YU Hai-yan¹, PAN Xiao-lin¹, WANG Bo^{1, 2}, ZHANG Wu¹, SUN Hui-lan², BI Shi-wen¹

1. School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China;

2. School of Materials Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China

Received 12 December 2011; accepted 21 May 2012

Abstract: The formation characteristics of calcium aluminates in the CaO-Al₂O₃-SiO₂ system with sodium oxide was investigated by XRD, SEM-EDS and DSC-TG technologies. The main phases in the clinker after sintering at 1350 °C are 12CaO·7Al₂O₃, 2CaO·Al₂O₃·SiO₂ and 2CaO·SiO₂ when the mass ratio of Al₂O₃ to SiO₂ is 3.0 and the molar ratio of CaO to Al₂O₃ is 1.0. The proportion of 12CaO·7Al₂O₃ increases with the increase of Na₂O addition when the molar ratio of Na₂O to Al₂O₃ is from 0 to 0.4, while the proportion of 2CaO·Al₂O₃·SiO₂ decreases with the increase of Na₂O addition. Na₂O forms solid solution in 12CaO·7Al₂O₃, which increases the volume of elementary cell of 12CaO·7Al₂O₃. The formation temperature of 12CaO·7Al₂O₃ is decreased by 30 °C when the molar ratio of Na₂O to Al₂O₃ increases from 0 to 0.4 determined by DSC. The alumina leaching property of clinker increases obviously with the increase of Na₂O addition.

Key words: CaO-Al₂O₃-SiO₂ system; Na₂O; Al₂O₃; 12CaO·7Al₂O₃; sintering; leaching

1 Introduction

CaO-Al₂O₃-SiO₂ (C-A-S) ternary system is of considerable importance in oxide ceramics, cement chemistry, metallurgical slags and geochemistry [1-3]. Meanwhile, the C-A-S system plays an important role in the production of alumina industry by the sintering process in China [4]. The phase diagram of the C-A-S system can be seen elsewhere, and the thermodynamic analyses of the ternary system have been performed by PELTON et al [5] using a quasichemical model, by WANG et al [6] using an ionic sublattice model and by FABRICHNAYA and NERAD [7] using a molecular model.

Meanwhile, the formation characteristics of calcium aluminates has been widely investigated [8,9], such as 3CaO·Al₂O₃ (C₃A) [10], 12CaO·7Al₂O₃ (C₁₂A₇) [11], CaO·Al₂O₃ (CA) [12], CaO·2Al₂O₃ (CA₂) [13] and CaO·6Al₂O₃ (CA₆) [14]. GRZESZCZYK [15] and OSTROWSKI and FLAZNY [16] studied the solid solutions of calcium aluminates with sodium ion formed at high temperature in CaO-Al₂O₃ binary system, and sodium ions can be built into the calcium aluminate system. Furthermore, SUN et al [17] found that addition of Na₂O during the sintering process can promote the alumina leaching property of calcium aluminate slag, but the mechanism was not proposed.

In the C-A-S system, the phases in the sintered clinker referred to alumina production consist of calcium aluminates, 2CaO·SiO₂ (C₂S) and 2CaO·Al₂O₃·SiO₂ (C₂AS). C₂S comprises several different types, such as α′-C₂S, β-C₂S, γ-C₂S. Previous studies [18] indicated that C₁₂A₇ has better alumina leaching properties than other calcium aluminates in sodium carbonate solution, while C₂AS is difficult to be extracted to solution. C₂S is very stable in sodium carbonate solution. For the great role of sodium oxide during the sintering process, it is necessary to reveal the reaction mechanisms in the C-A-S system in the presence of sodium ions. Therefore, the aim of this work is to study the effect of sodium oxide on the sintering characteristics and leaching property of calcium aluminates in the C-A-S system with low mass ratios of Al₂O₃ to SiO₂ and CaO to Al₂O₃.

2 Experimental

Analytically pure reactants were used in the present work and the calculated oxide ratios for sintering process are listed in Table 1. CaO and Na₂O were added in the forms of CaCO₃ and Na₂CO₃, respectively. The mass ratio of Al₂O₃ to SiO₂ (A/S) is 3.0, and the molar ratio of CaO to Al₂O₃ (C/A) is 1.0 (the residual CaO subtracted the composition to form 2CaO·SiO₂). The molar ratio of Na₂O to Al₂O₃ (N/A) ranges from 0 to 0.4 as presented in Table 1. The mixtures were milled in a ball mill for 3 h, and then sintered at 1350 °C for 1 h in a MoSi₂ resistance furnace followed by cooling in the furnace.

Table 1 Oxide ratios of samples for sintering

The sintered clinkers were leached at 75 °C for 30 min in sodium carbonate solution. The concentration of sodium carbonate solution (in the form of Na₂O) is 80 g/L. The liquid-to-solid ratio of sodium carbonate solution to clinker for leaching is 10. The leached slurry was filtrated using a Buchner funnel. The concentration of caustic alkali (N_K), total alkali (N_T) and Al₂O₃ (AO) in the filter liquor were determined by the volumetric method, while the filter residue was washed carefully and dried for chemical analysis. The alumina leaching rate is calculated according to the following formula:

                 (1)

where (A/S)_residue and (A/S)_clinker are the mass ratios of Al₂O₃ to SiO₂ in the leached residue and sintered clinker, respectively.

The contents of Al₂O₃, SiO₂ and Na₂O in samples and filtrate were analyzed by X-ray fluorescence (XRF, ZSX100e). Phase components of the clinker were identified by X-ray diffraction (PANalytical PW3040/60). SEM (SHIMADZU SSX-550) and EDS (DX-4) were used for microstructure and component analysis. Simultaneously recorded studies of differential scanning calorimetry (DSC) and thermogravimetric analysis were carried out using a NETZSCH STA409C/CD simultaneous thermal analyzer in a dynamic Ar atmosphere. The samples with the N/A ratios of 0 and 0.4 in Table 1 were selected. The samples were heated up to 1500 °C at a rate of 10 °C/min.

3 Results and discussion

3.1 Phase composition characteristics

The XRD patterns of sintered clinkers at different N/A ratios are shown in Fig. 1. When N/A=0.1, the clinker contains C₂AS, C₁₂A₇, β-C₂S, γ-C₂S as well as some CA and CA₂. As the N/A ratio increases to 0.2, the phase composition and content of clinker are different. The content of C₁₂A₇ increases, while the content of C₂AS decreases. Meanwhile, CA₂ disappears, and β-C₂S is formed during the sintering process. When N/A=0.4, most of the clinker is C₁₂A₇, and the content of C₂AS is very low. Furthermore, both CA and CA₂ do not exist, while β-C₂S and γ-C₂S coexist in the clinker. It can be concluded that as the molar ratio of Na₂O to Al₂O₃ increases from 0 to 0.4, the proportion of C₁₂A₇ in the clinker increases with the increase of N/A ratio when A/S=3.0 and C/A=1.0.

Fig. 1 XRD patterns of sintered clinkers at different N/A ratios

The crystal structure of C₁₂A₇ also changes as the N/A ratio increases. The interplanar spacing corresponding to the strongest characteristic peak (2θ=18.109°) of C₁₂A₇ and the parameter of cubic cell as well as its volume at different N/A were calculated, as listed in Table 2. Both the interplanar spacing and the volume of elementary cell of C₁₂A₇ increase with the increase of N/A, indicating that Na₂O forms solid solution in C₁₂A₇, which is consistent with Ostrowski’s results [16]. Therefore, the solid solution of Na₂O in C₁₂A₇ is beneficial to the formation of C₁₂A₇.

Table 2 Effect of N/A ratio on lattice parameters of C₁₂A₇

When N/A=0.4, the representative microstructure of sintered clinker is shown in Fig. 2. The morphology of particles is massive, and the particles can be divided to two kinds by the size. Most of the particles have a larger size, and are usually several micrometers, even larger than 10 μm. The other particles are relatively small, and are usually smaller than 1 μm. These particles are adsorbed on the surface of the larger particles.

Fig. 2 SEM images of sintered clinker when N/A=0.4

The compositions of larger particles (points 1 and 2 in Fig. 2(b)) and smaller particles (point 3) determined by EDS analysis are listed in the left of Table 3, and the corresponding compositions of oxides are calculated in the right of Table 3. No SiO₂ is discerned in the large particles, and the compositions are similar to of C₁₂A₇. Therefore, the larger particles are C₁₂A₇. The composition ratio of CaO to SiO₂ in the smaller particles is close to 2, indicating that the smaller particles are C₂S.

Furthermore, as presented in Table 3, the composition of Na₂O in the larger particles is about 4%, while the composition of Na₂O in the smaller particles is below 1%. Because no compound containing Na₂O was found by XRD analysis, Na₂O must be solid dissolved in the crystal lattice of C₁₂A₇, which is consistent with the calculated results of lattice parameters of C₁₂A₇, as presented in Table 2.

Table 3 Phase compositions of clinker when N/A=0.4 corresponding to Fig. 2(b)

3.2 Thermal analysis

The DSC heating and TG curves of both samples with the N/A ratios of 0 and 0.4 are shown in Fig. 3. Two strong endothermic peaks with a large gravity decrease at 798 °C exist in both curves, indicating that they relate to the decomposition of CaCO₃. A small endothermic peak with a small gravity decrease at 864 °C in Fig. 3(b) indicates that it relates to the melting of Na₂CO₃. C₂S is formed below 1300 °C [4], occurring at the endothermic peaks between 1000 °C and 1300 °C for both samples. As shown in Fig. 1, C₁₂A₇ is the main phase in the clinker when the N/A ratio is 0.4, and therefore, the large endothermic peak at 1360 °C in Fig. 3(b) relates to the formation of C₁₂A₇. As an intermediate phase, C₂AS is formed before C₁₂A₇ in the CaO-Al₂O₃-SiO₂ system [4]. Therefore, the endothermic peak at 1342 °C should be associated with the formation of C₂AS, and the endothermic peak at 1390 °C represents the formation of C₁₂A₇. Addition of Na₂O can not only promote the formation of C₁₂A₇ and inhibit the formation of C₂AS, but also decrease the formation temperature of C₁₂A₇ by 30 °C.

Fig. 3 DSC-TG curves of different mixtures during heating process

3.3 Alumina leaching property

The leaching results of clinkers at different N/A ratios in sodium carbonate solution are listed in Table 4, and alumina leaching rates are calculated by formula (1) as shown in Fig. 4. The reaction of calcium aluminates with sodium carbonate solution is shown in formula (2).

xCaO·yAl₂O₃+xNa₂CO₃+(x+3y)H₂O→xCaCO₃↓+2yNaAl(OH)₄+(2x-2y)NaOH     (2)

The alumina leaching rate increases obviously with the increase of N/A ratio. It is 86.91% when N/A=0.4, which is much higher than that when N/A=0 by 43.52%. Therefore, the addition of Na₂O can greatly improve the leaching properties of sintered clinker.

Table 4 Leaching results of clinkers at different N/A ratios in sodium carbonate solution

Fig. 4 Effect of N/A ratio on alumina leaching rate of clinkers

Fig. 5 XRD patterns of leached residues at different N/A ratios

The XRD patterns of leached residues are shown in Fig. 5. The main phases are CaCO₃, C₂S and C₂AS. No C₁₂A₇ is found in the leached residues, indicating that all Al₂O₃ in C₁₂A₇ is extracted into the solution during the leaching process.

4 Conclusions

1) Na₂O promotes the formation of C₁₂A₇ and inhibits the formation of C₂AS sintered at 1350 °C when A/S=3.0 and C/A=1.0 in the CaO-Al₂O₃-SiO₂ system.

2) Na₂O forms solid solution in C₁₂A₇ which increases the volume of elementary cell of C₁₂A₇.

3) The formation temperature of C₁₂A₇ is decreased by 30 °C when the molar ratio of Na₂O to Al₂O₃ increases from 0 to 0.4, and the leaching properties of the sintered clinker are greatly increased.

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氧化钠对CaO-Al₂O₃-SiO₂三元系铝酸钙形成规律的影响

于海燕¹，潘晓林¹，王 波^{1, 2}，张 武¹，孙会兰²，毕诗文¹

1. 东北大学 材料与冶金学院，沈阳 110004；

2. 河北科技大学 材料科学与工程学院，石家庄 050018

摘  要：利用XRD、SEM-EDS和DSC-TG技术研究了添加Na₂O的CaO-Al₂O₃-SiO₂体系中铝酸钙的形成规律。结果表明，当Al₂O₃与SiO₂的质量比为3.0、CaO与Al₂O₃的摩尔比为1.0时，在1350 °C烧结后的熟料主要由12CaO·7Al₂O₃、2CaO·Al₂O₃·SiO₂和2CaO·SiO₂组成。熟料中12CaO·7Al₂O₃的含量随着Na₂O的增加而增加，2CaO·Al₂O₃·SiO₂的含量随着Na₂O的增加而降低。Na₂O在12CaO·7Al₂O₃中形成固溶体，增加了其单位晶胞体积。DSC分析表明，Na₂O不仅促进了12CaO·7Al₂O₃的形成，而且使C₁₂A₇的形成温度降低了30 °C。烧结熟料中的氧化铝溶出性能随着Na₂O的增加而大幅度提高。

关键词：CaO-Al₂O₃-SiO₂系；Na₂O；Al₂O₃；12CaO·7Al₂O₃；烧结；溶出

(Edited by YANG Hua)

Foundation item: Projects (51174054, 51104041) supported by the National Natural Science Foundation of China

Corresponding author: YU Hai-yan; Tel: +86-24-83686460; E-mail: yuhy@smm.neu.edu.cn

DOI: 10.1016/S1003-6326(11)61578-1