J. Cent. South Univ. (2016) 23: 1569-1575

DOI: 10.1007/s11771-016-3210-1

Influence of silicate anions structure on desilication in silicate-bearing sodium aluminate solution

LIU Gui-hua(刘桂华), ZHANG Wen(张闻), QI Tian-gui(齐天贵), PENG Zhi-hong(彭志宏),

ZHOU Qiu-sheng(周秋生), LI Xiao-bin(李小斌)

School of Metallurgy and Environment, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: The structural changes of silicate anions in the desilication process with the addition of calcium hydrate alumino-carbonate were studied by measuring Raman spectra, infrared spectra and corresponding second derivative spectra. The results show that the desilication ratio in the solution prepared by the addition of sodium silicate (solution-SS) is much greater than that in the solution by the addition of green liquor (solution-GL), and low alumina concentration in the sodium aluminate solutions facilitates the desilication process. It is also shown that alumino-silicate anions in the solution-GL, and Q³ polymeric silicate anions in solution-SS are predominant, respectively. In addition, increasing the concentration of silica favors respectively the formation of the alumino-silicate or the Q³ silicate anions in the solution-GL or the solution-SS. Therefore, it can be inferred that the low desilication ratio in the silicate-bearing aluminate solution is mainly attributed to the existence of alumino-silicate anions.

Key words: desilication; silicate anion; structure; sodium aluminate solution; second derivative spectrum of IR

1 Introduction

It is still difficult to remove silica from sodium aluminate solution in alumina production by either Bayer or sintering process. Most of the silica in bauxite (or clinker containing sodium aluminate) enters into the red mud in the form of sodium hydrate alumino-silicate (DSP) and hydrogarnet (or β dicalcium silicate). But there remains 1-8 g/L SiO₂ in the sodium aluminate solutions, leading to much scale on the surfaces of equipments, long induction period in gibbsite precipitation, poor quality of products and so on. Thus, much attention has been paid on the desilication of the silicate-bearing sodium aluminate solution [1-6].

The structure of silicate anions in the sodium aluminate solutions highly affects the composition of the desilication residue, desilication ratio and rate. Numerous investigations about the structure of the silicate anions have been undertaken by measuring Fourier Transform Infrared spectrum (FTIR), UV spectrum, electrical conductivity, surface tension and so on. Conclusions can be summarized as follows: 1), or polymeric silicate anions exist in sodium silicate solutions, depending on the mass ratio of SiO₂/Na₂O, the concentration of caustic soda and the concentration of SiO₂ [7-8]; 2) Besides anions, there are alumino-silicate anions, which may be in the form of or other complicated forms in sodium aluminate solution [7, 9-10]. In addition, different desilication models are set up when various SiO₂ sources are adopted in the preparation of silicate- bearing solutions [4]. All facts imply that the structure of silicate anions is still uncertain in sodium aluminate solution. Moreover, there are few publications on the relationship between silicate structure and the desilication ratio in sodium aluminate solutions (>100 g/L, >100 g/L).

In contrast, much work has been done on the structure of silicate anions in sodium silicate solutions by measurement of IR spectra, NMR and XPS [9, 11-15]. It is found that silicate anions exist predominantly in the forms of monomer (Q¹), dimer (Q²), tripolymer (Q³), tetramer (Q⁴) (superscripts 1-4 stand for the number of neighboring Si atoms connected to a [SiO₄]^4- tetrahedron) or more complicated silicate anions in silicate solutions, and the Q⁰, Q¹, Q², Q³ and Q⁴ in IR spectrum are located at 850-875 cm^-1, 815-1005 cm^-1, 938-1050 cm^-1, 1030-1099 cm^-1 and 1140-1220 cm^-1, respectively [15-19]. Although there is a significant difference in Al₂O₃ and Na₂O concentration between sodium silicate solutions and silicate-bearing sodium aluminate solutions,these results will benefit the understanding of the silicate structure in silicate-bearing sodium aluminate solution.

In this work, the variation of desilication ratio was investigated when the desilication process was carried out in the silicate-bearing sodium aluminate solutions. And the structure of the silicate anions was then studied by measuring Raman spectra, FTIR spectra and the corresponding second derivative spectra. Finally, the relationship of the desilication ratio and the silicate anions’ structure was discussed.

2 Experimental

2.1 Chemicals and materials preparation

1) Sodium aluminate solution was made by dissolving aluminum hydroxide (CHALCO) into sodium hydroxide solution. 2) Two kinds of silicate-bearing solution were prepared. The sodium silicate solution ( of 250.10 g/L,of 28.01 g/L) was prepared by dissolving sodium metasilicate monohydrate (XILONG Chemical Industry Co., Ltd, China, AR) into sodium hydroxide (XILONG Chemical Industry Co., Ltd, AR) solution. And the green liquorof 205.87 g/L,of 260.33 g/L, of 13.35 g/L) was obtained by leaching clinker (CHALCO) in sodium aluminate solution at 60 °C for 2 h. 3) Solution-SS was obtained by slowly adding sodium silicate solution into sodium aluminate solution at stirring speed of 400 r/min, while solution-GL was prepared by injecting the green liquor into sodium aluminate solution at stirring speed of 400 r/min. Furthermore, all solution was held for more than 2 h at room temperature. 4) CaO was obtained by roasting calcium hydroxide (Tianjin Kemiou Chemical Reagent Co., Ltd, China, AR) at 850 °C for 2 h, the fresh lime slurry (solid concentration of 250 g/L) was conducted by CaO reacting with distilled water at room temperature. 5) Calcium hydrate alumino-carbonate (CHAC) was prepared according to the following procedures. The fresh lime slurry was reacted with the diluted sodium aluminate solution (of 95.42 g/L, of 99.67 g/L,of 49.75 g/L) according to the volume ratio of 1:3 at 65 °C for 50 min. The slurry was then filtered and washed with water. Lastly, wet CHAC was used for desilication. 6) Silicate-bearing aluminate solution was obtained for measurement of Raman and FTIR spectrum as follows: sodium aluminate solution was prepared by dissolving aluminum hydroxide (Tianjin Kemiou Chemical Reagent Co., Ltd, China, AR) into sodium hydroxide (XILONG Chemical Industry Co., Ltd, China, AR) solution. And then the silicate-bearing sodium aluminate solutions were obtained by mixing the sodium aluminate solution with sodium silicate solution or the green liquor, respectively.

2.2 Desilication procedure

CHAC and 100 mL silicate-bearing sodium aluminate solution were added into a 150 mL steel bomb. The bomb, bathed in glycerin (±1 °C), was then heated to a given temperature. And the desilication lasted for 2-4 h at 90-100 °C, after which the slurry was filtered. The filtrate liquors were used for detecting concentration of alumina (Al₂O₃), silica and caustic soda (Na₂O) after diluting to 1000 mL.

2.3 Analysis and characterization

The concentration of Al₂O₃ and Na₂O was determined by the titration method. SiO₂ was measured with UV-Vis spectrophotometer (SP-752, Spectrum Shanghai China) at 625 nm.

Raman spectra was scanned to collect by Raman spectroscopy (Lab RamHR800, Horiba, JobinYvon, France) in 325 nm laser excitation at 20 °C. And the Raman spectra were processed by Labspec5 software.

The IR spectra of thin films for solution on KBr plates were collected on a FT-IR 6700 spectrometer with 4 cm^-1 resolution (manufactured by Nicolet Co., USA) [20]. Based on the above FTIR spectra, the corresponding second derivative spectra were then obtained by OMNIC software. Finally, two kinds of spectra were used for distinguishing structures of silicate anions in the sodium aluminate solution.

3 Results and discussion

3.1 Influence of preparation for silicate-bearing aluminate solution on desilication

Sintering process is employed to deal with bauxite with high SiO₂ content, but SiO₂ concentration in the green liquor remains about 3-8 g/L due to β-2CaO·SiO₂ in clinker reacting with sodium aluminate solution in leaching process. Thus, two-stage desilication process is adopted. The A/S ratio (concentration ratio of alumina to silica) varies over the range of 200-300 after DSP is formed in the first desilication stage, and the A/S ratio of solution must reach over 500 by addition of calcium-bearing compounds in the second desilication stage. CHAC was added to the second desilication stage in some Chinese alumina refineries because CHAC is more effective than fresh lime slurry. In addition, we find that the desilication ratio in laboratory is much greater than that in alumina refineries under almost of the same condition only due to the different solution. Therefore, two kinds of solutions are adopted, namely, solution-SS as many works were done in laboratory [4, 21-23], and solution-GL which is similar to the alumina refineries’ liquor. The desilication results <110 g/L) are shown in Table1.

Table 1 shows that the A/S ratio in solution-SS and solution-GL exceeds 600 after desilication at 90 °C for 1 h. Extending desilication time significantly facilitates the desilication process. In addition, the A/S ratio in solution-SS reaches 7000 (99% desilication ratio) at 2 h, which is much greater than A/S of 1268 (83% desilication ratio) in solution-GL.

Table 1 Influence of silicate-bearing sodium aluminate solution on desilication with different duration time

The desilication process in concentrated solutions >130 g/L) is then carried out. The results are shown in Fig. 1.

Fig. 1 Influence of CHAC addition on desilication (solution composition: of 144.72 g/L, of 166.49 g/L, of 0.64 g/L, A/S of 260; desilication condition: CHAC added according to CaO stoichiometry, temperature of 90 °C, time of 4 h)

As shown in Fig. 1, the A/S ratio increases with an increase of CHAC dosage in silicate-bearing sodium aluminate solution, and the desilication ratio in solution-SS reaches 49% for 3 h, which is also much greater than that (16%) in solution-GL. Moreover, the bigger the CHAC dosage, the greater the difference of A/S ratio between solution-GL and solution-SS. Results also indicate that increasing Al₂O₃ concentration hinders the desilication process as compared with the results in Table 1, mainly due to the equilibrium concentration of SiO₂ increasing with the rise of Al₂O₃ concentration.

Therefore, there is a significant difference of A/S ratio after desilication between solution-SS and solution-GL under the same desilication conditions, which may be due to the different structure of silicate in solution. Silicate structure in silicate-bearing sodium aluminate solution is then investigated by detecting Raman spectra and FTIR spectra.

3.2 Raman spectra of silicate-bearing aluminate solution

The Raman spectra of solution-GL and solution-SS are shown in Fig. 2.

Fig. 2 Raman spectra of silicate-bearing aluminate solution:(solution composition: α_k of 1.50, of 160 g/L, of 175 g/L)

As seen in Fig. 2, the strong peak at 625 cm^-1 can be observed due to the symmetric Al—O stretch in the weak peaks at 540 cm^-1 may be the consequence of dimer aluminate ion (AlO(OH)₆^2-), and the weak peak at 700 cm^-1 also can be found with increasing SiO₂ concentration [24-26]. Results in Fig. 2(b) is in good agreement with the reported results [8, 24-26] when solution is prepared by adding sodium silicate solution similar to solution-SS. Compared with Fig. 2(b), some new peaks appear in Fig. 2(a). Peaks at 850 cm^-1 and 940 cm^-1 are assigned to SiO₄^4- bonding to four and three Al atoms, respectively. Furthermore, peak at 980 cm^-1 can be found in solution-GL of of 6 g/L corresponding to SiO₄^4- bonding to two Al atoms [27-28]. This suggests that alumino-silicate anion can be found in solution-GL rather than in solution-SS, possibly resulting in different desilication ratio in silicate-bearing sodium aluminate solution.

3.3 FTIR spectra of silicate-bearing aluminate solution

The infrared spectra of the solution-GL in the range of 500-1000 cm^-1 are shown in Fig. 3.

Fig. 3 Effect of silica concentration on FTIR spectra of solution-GL: (of 0, 0.5, 1, 2 and 3 g/L from bottom to top)

There are three bands of FTIR for solution-GL, centering at 540, 630 and 720 cm^-1, respectively. The shoulder peak at 880 cm^-1 only appears in solution of α_k= 1.5, where α_k is the molar ratio of Na₂O and Al₂O₃ in sodium aluminate solution, of 160 g/L, resulting from Al—O bond vibration of AlO₆ rather than Al—O bond vibration of Al(OH)₄^- [8, 12, 20, 24, 29-30]. Moreover, there appears a broad weak peak at about 540 cm^-1, possibly due to Al—O—Al bond vibration. With increasing SiO₂ concentration, peaks at 720 cm^-1 and 630 cm^-1 remain almost unchanged, however, it seems that the intensity of the peak at 880 cm^{-1 in Fig. 3(a) becomes weak, and the peak at 540 cm-1} (Figs. 3(a) and (b)) becomes broad.

The influence of SiO₂ concentration on the spectra of solution-SS is presented in Fig. 4.

Similarly, there are three separate well- characterized bands at 535, 630 and 720 cm^-1 indifferent solutions 160 g/L, 220 g/L) in Fig. 4, and the shoulder peak also centers at 880 cm^-1 for solution of α_k 1.5. In addition, with the increase of SiO₂ concentration, the shapes of the FTIR absorption peaks at 630 and 720 cm^-1 bands vary very slightly, but the peak at 535 cm^-1 appears to become broader.

Fig. 4 Effect of silica concentration on FTIR spectra of solution-SS:(of 0, 0.5, 1, 2 and 3 g/L from bottom to top)

Therefore, the FTIR absorption peaks at 720 cm^-1, 630 cm^-1 and 880cm^-1 are assigned to Al—O bond of aluminate anion [10]. No strong peaks corresponding to Si—O or Al—O—Si structure can be easily observed due to low concentration of SiO₂.

3.4 IR second derivative spectra of silicate-bearing sodium aluminate solutions

FTIR second derivative spectra can identify the exact positions of all absorption and shoulder peaks from original IR spectra. Based on Fig. 3, the second derivative spectra of the silicate-bearing sodium aluminate solutions are shown in Figs. 5 and 6.

Compared with the peak number in Fig. 3, new peaks can be found over the range of 400-1100 cm^{-1 in Figs. 5-6. For silicate-free solution, more new weak} peaks (466 cm^-1, 492 cm^-1, 526 cm^-1 and 555 cm^-1) overthe range of 460-590 cm^-1 can be observed, confirming the overlap of the peaks centering at about 535 cm ^-1. For silicate-bearing solution of 225 g/L, α_k=2.8, peaks at 883, 832, 721 and 625 cm^-1 are always observed (in Fig. 5), while peaks at 533 cm^-1, 789 cm^-1 and 855 cm^-1 disappear. Meanwhile, a new peak at 793-807 cm^-1 appears when SiO₂ concentration increases. For silicate- bearing solution of =160 g/L, α_k=1.4, peaks at 891, 829, 721 and 625 cm^-1 can be found while the peaks at 530 cm^-1 and 788 cm^-1 disappear (in Fig. 6). However, the weak peak at 803 cm^-1, as well as at 543 and 575 cm^-1, can be observed.

Fig. 5 IR second derivative spectra of solution-GL at α_k of 2.80 (solution composition: =225 g/L, α_k=2.80, of 0, 0.5, 1 and 3 g/L from bottom to top)

Fig. 6 IR second derivative spectra of solution-GL at α_k of 1.40 (solution composition: =160 g/L, α_k=1.40, of 0, 0.5, 2 and 3 g/L from bottom to top)

Except for the Al—O or Al—OH bond vibration occurring at about 880, 721, 635 cm^-1, the new peak at about 800 cm^-1 is assigned to Al—O—Si bond of alumino- silicate anion, which is due to Si—O in SiO₄ substituted by Al—O [29-30]. The new peaks at 543 and 575 cm^-1 are also attributed to the Si—O—Si or Al—O—Si bond [29-30], owing to the larger dipole moment of Al—O bond as compared with the Si-O bond vibration at 535 cm^-1.

Similarly, the second derivative spectra of solution-SS, based on Fig. 4, are presented in Figs. 7 and 8.

As shown in Figs. 7 and 8, a remarkable difference in structures can be observed in comparison with Fig. 4. For silicate-bearing solutions of α_k of 1.4, of 160 g/L, peaks at 1064, 883, 824, 717, 629 cm^-1 are also found, but the weak peak at 534 cm^-1 disappears and more weak peaks appear at 559-565 cm^-1. For silicate- bearing solutions of α_k of 2.8, of 220 g/L, peaks at 1064, 883, 829, 721, 629 and 543 cm^-1 are always observed at different SiO₂ concentrations, and the weak peaks at 859 cm^-1 and 803 cm^-1 also appear at the SiO₂ concentration of 3 g/L.

Fig. 7 IR second derivative spectrum of solution-SS at α_k of 1.50 (solution composition: =160 g/L, α_k=1.50, of 0, 0.5, 1, 2 and 3 g/L from bottom to top)

Fig. 8 IR second derivative spectrum of solution-SS at α_k of 2.80 (solution composition:g/L, α_k=2.80, of 0, 0.5, 1, 2 and 3 g/L from bottom to top)

The new peak at 1064 cm^-1 is assigned to Q³ polymeric silicate anions, and the weak peak at 803 cm^-1 also emerges at of 3 g/L. These results suggest that Q³ polymeric silicate anion occurs in solution-SS, and increasing concentration of SiO₂ also benefits the formation of the alumino-silicate anions.

Therefore, alumino-silicate anion in solution-GL and Q³ polymeric silicate anion in solution-SS occurs predominantly, being in good agreement with the results of Raman spectra. In addition, there is a overlap of peaks at about 535 cm^{-1 in sodium aluminate solution}

3.5 Relationship of desilication ratio and silicate structure

Al(OH)₄^- anion is predominant in sodium aluminate solution [1, 7, 26], and its concentration is much greater than SiO₂ concentration owing to high A/S of solution. Thus, the concentration of Al(OH)₄^- can be approximately considered as a constant concentration for a given solution. Meanwhile, the ratio of peak areas of the silicate anions to the peak areas of the characteristic peak of Al(OH)₄^- is employed to semi-quantitatively present the concentration variation of the silicate anions on basis of the strong peak of Al(OH)₄^- at about 720 cm^-1. The influence of SiO₂ concentration on the area ratio of the peak of 800 cm^-1 to 720 cm^-1 could be seen for solution-GL in Fig. 9, whilst area ratio of the peak of 1064 cm^-1 to the peak of 720 cm^-1 for solution-SS is presented in Fig. 10.

Figure 9 shows that the peak area increases with increasing SiO₂ concentration for solution-GL, and that it increases remarkably for solution α_k of 1.4, of 160 g/L in comparison with solution α_k of 2.8, of 200 g/L. This suggests that the alumino-silicate anion is easily formed in solution-GL. Moreover, increasing SiO₂ concentration significantly promotes the formation of alumino-silicate anion.

In contrast, the peak area ratio also goes up remarkably and then rises slowly with increasing concentration of SiO₂ for solution-SS (Fig. 10). Furthermore, the area ratio increases more significantly for solution of α_k of 2.8, of 200 g/L, suggesting that Q³ polymeric silicate anion occurs in the solution-SS, especially in solution with low concentration of alumina.In addition, increasing SiO₂ concentration also benefits the formation of alumino-silicate anions in the solution-SS owing to peaks at 803 cm^-1 being also observed. For example, peak area ratio of 803 cm^-1 to 721 cm^-1 is 0.024 for solution with 2 g/L SiO₂ and that for 3 g/L SiO₂ is 0.027, respectively.

Fig. 9 Dependence of SiO₂ concentration on peak area ratio in solution-GL (silicate-bearing aluminate solution: of 160 g/L, peak area ratio represents area ratio of peak at 800 cm^-1 to peak at 720 cm^-1)

Fig. 10 Dependence of SiO₂ concentration on peak area ratio in solution-SS (silicate-bearing aluminate solution: of 160 g/L, peak area ratio represents peak at 1064 cm^-1 Q² or Q³ to peak at 720 cm^-1)

Results from Raman spectra and the second derivative spectra of FTIR indicate that alumino-silicate anion occurs in solution-GL while Q³ polymeric silicate anion in the solution-SS. Since desilication ratio in solution-GL is much less than that in solution-SS as mentioned above, it is inferred that the alumino-silicate anion accounts for the low desilication ratio of solution-GL, while polymeric silicate anion will benefit the desilication process.

4 Conclusions

1) Desilication ratio in the solution-SS is much greater than that in the solution-GL with adding CHAC, and increase of alumina concentration decreases the desilication ratio.

2) Based on the Raman spectra and the second deviation of FTIR, much alumino-silicate anions, Q³ polymeric silicate anions can be found in solution-GL and solution-SS, respectively. Increasing SiO₂ concentration also favors respectively the formation of alumino-silicate anions or Q³ polymeric silicate anion. In addition, alumino-silicate anions can be formed at SiO₂ concentration over 2 g/L in solution-SS. Therefore, low desilication ratio in solution-GL is attributed to the existence of alumino-silicate anion.

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(Edited by FANG Jing-hua)

Foundation item: Project(51274242) supported by the National Natural Science Foundation of China; Project(2015CX001) supported by the Innovation-driven Plan in Central South University, China

Received date: 2015-04-07; Accepted date: 2015-09-28

Corresponding author: QI Tian-gui, PhD; Tel: +86-731-88830453; E-mail: qitiangui@csu.edu.cn