Trans. Nonferrous Met. Soc. China 25(2015) 1334-1340

Secondary reaction mechanism of leaching process of calcium aluminate slag

Hui-lan SUN^1,2, Bo WANG^1,2, Jian-xin ZHANG¹, Shu-feng ZONG¹, Jia-jia LIU¹

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

2. Hebei Key Laboratory of Material Near-net Forming Technology, Hebei University of Science and Technology, Shijiazhuang 050018, China

Received 12 March 2014; accepted 26 January 2015

Abstract: SiO₂ in calcium aluminate slag exists in the form of γ-2CaO·SiO₂ which is more stable than β-2CaO·SiO₂. However, it is decomposed by sodium carbonate solution during leaching process, leading to the secondary reaction. The extent of secondary reaction and reaction mechanism of calcium aluminate slag were studied using XRD. The results show that the decomposition rate of γ-2CaO·SiO₂ increases with the increase in leaching time and sodium carbonate concentration. The main products of secondary reaction are the mixture of hydrogarnet and sodium hydrate alumina-silicate. SiO₂ concentration rises firstly and then drops with the increase of leaching temperature. XRD results indicate that the stable product of secondary reaction at low temperature is hydrogarnet. But hydrogarnet is transformed into sodium hydrate alumina-silicate at high temperature.

Key words: calcium aluminate slag; secondary reaction; alumina; leaching

1 Introduction

With the significantly increasing alumina production capability in recent years, the discrepancy between the supply and the demand of the materials of bauxite in China is increasingly serious [1]. One of the methods to solve this discrepancy is to explore and utilize new alumina-containing materials [2-4]. Calcium aluminate process (lime sintering process) is a method to leach alumina from iron-bearing bauxite, fly ash and alumina-containing slags [5-7]. In this process, iron- bearing bauxite is sintered with limestone at 1500 °C. The main phases of the sinter are 12CaO·7Al₂O₃ and γ-2CaO·SiO₂. The alumina of 12CaO·7Al₂O₃ is easily leached into sodium carbonate solution.

The crystal form of 2CaO·SiO₂ of calcium aluminate slag is γ type [8], but that of soda-lime sinter is β type [9]. The different crystal structures of dicalcium silicate have different influences on alumina leaching of sinters. β-2CaO·SiO₂ of soda-lime sinter will react with NaOH, Na₂CO₃ and NaAl(OH)₄. Then, sodium hydrate alumino-silicate and hydrogarnet are formed [10-13]. In this way, Al(OH)₄^- and Na⁺ are leached out from sinter and then precipitated into the red mud again. Thus, alumina leaching rate of sinter decreased. This is called the secondary reaction. Secondary reaction will cause the alumina loss of the leaching process. And the decomposed silicon dioxide will come into the crude liquor and complicate the desilication system.

It has been reported that γ-2CaO·SiO₂ is inert in sodium aluminate solution, and it normally does not react with other phases [14]. But our former research indicates that partial γ-2CaO·SiO₂ will be decomposed and cause secondary reaction in the solution with high sodium carbonate concentration [15]. The secondary reaction of γ-2CaO·SiO₂ is seldom reported. Therefore, in this work, the effects of sodium carbonate concentration, leaching time and leaching temperature on the decomposition of γ-2CaO·SiO₂ were studied and the secondary reaction mechanism was discussed.

2 Experimental

2.1 Materials

CaCO₃, Na₂CO₃, NaOH and SiO₂ used in the experimental studies are analytical pure reagents. Al(OH)₃ used in the experiment is an industrial pure reagent.

2.2 Equipments

Muffle furnace, gas shielded MoSi₂ furnace, magnetic stirring constant temperature water bath, X-ray diffraction analyzer (PANalytical PW3040/60) and spectrophotometer (722S) were used.

2.3 Smelting of calcium aluminate slag

The smelting experiments were carried out in a gas shielded MoSi₂ furnace and the vessel was a graphite crucible. The smelting temperature was 1500 °C and the holding time was 1 h. After smelting, the melt was cooled at the speed of 5 °C/min, and it was taken out at the temperature of 400 °C. After grinding, the particle size of the slag is less than 74 μm.

The calcium aluminate slag is synthesized by chemical reagents. Its m(Al₂O₃)/m(SiO₂) (the mass ratio of Al₂O₃ to SiO₂) and n(CaO)/n(Al₂O₃) (the mole ratio of CaO to Al₂O₃, excluding the CaO of 2CaO·SiO₂) are 1.3 and 1.7, respectively. Table 1 shows the chemical composition of the slag and Fig. 1 shows the XRD results of the slag. XRD result indicates that the main phases in the slag are 12CaO·7Al₂O₃ and γ-2CaO·SiO₂, whose contents are 46.57% and 52.96%, respectively according to Rietveld analysis. Pure 12CaO·7Al₂O₃ and γ-2CaO·SiO₂ are synthesized at 1500 °C by chemical reagents separately.

Table 1 Chemical composition of slag (mass fraction, %)

Fig. 1 XRD pattern of calcium aluminate slag

2.4 Leaching of calcium aluminate slag

The sodium aluminate solution obtained from the slag was treated according to the carbonization precipitation process. Circulating mother liquid was used to leach new calcium aluminate slag. The conditions of the leaching solution are: caustic alkali concentration of 7 g/L, α_k=1.6 (mole ratio of Na₂O to Al₂O₃) and liquid- solid ratio of 4.5 mL/g.

2.5 Methods of analysis

The contents of Al₂O₃, SiO₂, CaO and Na₂O in samples and filtrate were analyzed by XRF. Alumina leaching rate is calculated according to Eq. (1).

               (1)

where η_A is alumina leaching rate; m(Al₂O₃) is the mass of Al₂O₃ in slag or residue; m(SiO₂) is the mass of SiO₂ in slag or residue.

Phase components of calcium aluminate slag and leaching residues were identified by X-ray diffraction (PANalytical PW3040/60). The concentration of SiO₂ in leaching solution was determined with silico- molybdenum blue spectrophotometer. Thus, the decomposition property of γ-2CaO·SiO₂ (Δρ(SiO₂)) during alumina leaching process is defined as the concentration change of SiO₂ after alumina leaching.

3 Results and discussion

3.1 Effect of sodium carbonate concentration on decomposition of γ-2CaO·SiO₂

The reaction between 12CaO·7Al₂O₃ and Na₂CO₃ (Eq. (2)) can realize the recovery of alumina from calcium aluminate slag. Meanwhile, γ-2CaO·SiO₂ will also react with Na₂CO₃, leading to the decomposition of dicalcium silicate (Eq. (3)) and the loss of alumina extracted into solution (Eq. (4)-(6)). Therefore, the effect of sodium carbonate concentration (abbr., Na₂O_c) on decomposition characteristics of γ-2CaO·SiO₂ was studied. The results are shown in Fig. 2.

12CaO·7Al₂O₃+12Na₂CO₃+33H₂O=14NaAl(OH)₄+12CaCO₃+10NaOH     (2)

2CaO·SiO₂+2Na₂CO₃+H₂O=2CaCO₃+Na₂SiO₃+2NaOH                 (3)

1.7Na₂SiO₃+2NaAl(OH)₄+(n-2.3) H₂O=Na₂O·Al₂O₃·1.7SiO₂·nH₂O+3.4NaOH         (4)

3Ca(OH)₂+2NaAl(OH)₄=3CaO·Al₂O₃·6H₂O+2NaOH                 (5)

3CaO·Al₂O₃·6H₂O+nNa₂SiO₃=3CaO·Al₂O₃·nSiO₂·(6-2n)H₂O+2nNaOH+nH₂O (6)

The results indicate that the Δρ(SiO₂) concentration increases obviously when Na₂O_c concentration increases from 40 g/L to 80 g/L. The increasing rate of Δρ(SiO₂) decreases as the Na₂O_c concentration continuously increases.

Fig. 2 Effect of Na₂O_c on leaching rate of slag and decomposability of γ-2CaO·SiO₂

The values of Δρ(SiO₂) vary from 0.4 g/L to 1.7 g/L, indicating that the decomposition of γ-2CaO·SiO₂ is serious when Na₂O_c concentration is up to 80 g/L.

The change trend of alumina leaching rate under different Na₂O_c concentrations is similar to that of Δρ(SiO₂). When liquid-solid ratio is 4.5 mL/g, calcium aluminate slag will consume Na₂O_c at approximately 50-60 g/L according to Eq. (2). Therefore, the alumina leaching rate increases from 77.59% to 90.49% under different Na₂O_c concentrations. Further extension of Na₂O_c has little effect on the improvement of alumina leaching ratio.

3.2 Effect of Na₂O_k concentration and α_k on decomposition of γ-2CaO·SiO₂

In order to avoid the effect of sodium carbonate on decomposition of γ-2CaO·SiO₂, its concentration is fixed at 7 g/L in this section. The effects of Na₂O_k concentration and α_k on the alumina leaching rate of calcium aluminate slag and the decomposition of γ-2CaO·SiO₂ are shown in Fig. 3 and Fig. 4. It can be seen that the leaching rates of samples are less than 21% under these conditions. This is because the Na₂O_c is not enough to react with 12CaO·7Al₂O₃ according to Eq. (2). The Δρ(SiO₂) concentration increases when Na₂O_k concentration (Fig. 3) and α_k (Fig. 4) increase. The maximum values of Δρ(SiO₂) concentration are 0.75 g/L and 0.47 g/L when Na₂O_k concentration is 50 g/L (Fig. 3) and α_k is 11 (Fig. 4). Both of the two values are lower than 1.69 g/L which is obtianed when Na₂O_c concentration is 120 g/L (Fig. 2). These results indicate that Na₂O_k concentration and α_k have important effect on the decomposability of γ-2CaO·SiO₂, but they are not the main factors compared to Na₂O_c.

Fig. 3 Effect of Na₂O_k concentration on leaching rate of slag and decomposability of γ-2CaO·SiO₂

Fig. 4 Effect of α_k on leaching rate of slag and decomposability of γ-2CaO·SiO₂

3.3 Effect of leaching time on decomposition of γ-2CaO·SiO₂

The effect of leaching time on the alumina leaching rate of calcium aluminate slag and the decomposition of γ-2CaO·SiO₂ is shown in Fig. 5. It can be seen that the leaching rate and the Δρ(SiO₂) concentration increase with increasing leaching time. When leaching time is up to 80 min, its effect becomes little. The inflection point of leaching ratio does not appear under this condition because the reaction extent of secondary reaction is lower than that of leaching reaction.

3.4 Effect of leaching temperature on decomposition of γ-2CaO·SiO₂

Figure 6 shows alumina leaching rate and decomposability under different leaching temperatures. The Δρ(SiO₂) concentration increases to 1.73 g/L when leaching temperature is 80 °C, and then decreases subsequently. It has been reported that reaction temperature rise will enhance the equilibrium solubility of silica solution. It indicates that the solubilised silica is transferred into the residue when leaching temperature is higher than 80 °C. There are two transferred forms of silica in residue: sodium hydrate alumino-silicate and hydrogarnet. Both will cause the loss of alumina leached into solution.

Fig. 5 Effect of leaching time on leaching rate of slag and decomposability of γ-2CaO·SiO₂

Fig. 6 Effect of leaching temperature on leaching rate of slag and decomposability of γ-2CaO·SiO₂

The inverted V trend of alumina leaching rate also proves the existence of the secondary reaction. The maximum value of leaching rate is 93.69% at the inflection point (75 °C). The results of different leaching temperatures are shown in Table 2. The leaching experiments of pure 12CaO·7Al₂O₃ and γ-2CaO·SiO₂ are also carried out separately under the same conditions. The molecular formulas of sodium hydrate alumino- silicate and hydrogarnet are Na₂O·Al₂O₃·1.7SiO₂·nH₂O and 3CaO·Al₂O₃·nSiO₂·(6-2n)H₂O (n≈0.1), respectively. Thus, the mass ratios of Al₂O₃ to SiO₂ in sodium hydrate alumino-silicate and hydrogarnet are 1.0 and 17, respectively.

From the ratio of Δρ(Al₂O₃) and Δρ′(SiO₂), it can be concluded that the products of secondary reaction are the mixture of sodium hydrate alumino-silicate and hydrogarnet. With the increase of leaching temperature, hydrogarnet tends to be transformed into sodium hydrate alumino-silicate. The amount of secondary reaction becomes large and then alumina leaching rate decreases obviously when leaching temperature is up to 85°C. The further study of secondary reaction mechanism is discussed in the next section.

3.5 Mechanism of secondary reaction of calcium aluminate slag

3.5.1 Principle of secondary reaction

About 30% of sinter produced during soda-lime sintering process is β-2CaO·SiO₂, which will be decomposed and cause the loss of alumina. The phenomenon mentioned above is called “secondary reaction” [16]. There are three main phases as NaOH, Na₂CO₃ and NaAl(OH)₄ in sodium aluminate solution. But the dominant factors of the decomposition of dicalcium silicate have not been determined at present. These opinions may be described as follows [17-20].

Table 2 Liquid results of leaching solution at different temperatures

1) The decomposition of dicalcium silicate is mainly caused by NaOH. And the decomposition reaction is shown in Eq. (7).

β-2CaO·SiO₂+2OH^-+2H₂O=2Ca(OH)₂+H₂SiO₄^2- (7)

2) The decomposition of dicalcium silicate is mainly caused by Na₂CO₃. And the decomposition reaction is shown in Eq. (8):

β-2CaO·SiO₂+2CO₃^2-+2H₂O=2CaCO₃+H₂SiO₄^2-+2OH^-    (8)

3) The decomposition of dicalcium silicate is mainly caused by NaAl(OH)₄. And the decomposition reaction is shown in Eq. (9):

3(2CaO·SiO₂)+4Al(OH)₄^-+6H₂O+2OH^-=2(3CaO·Al₂O₃·6H₂O)+3H₂SiO₄^2-    (9)

Therefore, β type dicalcium silicate can be decomposed by the phases mentioned above under proper conditions. CHEN et al [17] has studied the thermomechanics of the decomposition reaction with NaOH, Na₂CO₃ and NaAl(OH)₄. The results indicate that the stability of dicalcium silicate in the solution is decreased in the following sequence: NaOH, NaAl(OH)₄, Na₂CO₃. That is to say, Na₂CO₃ has the strongest decomposability on β-2CaO·SiO₂. Furthermore, BI [14] believed that dicalcium silicate could be decomposed thoroughly under pure Na₂CO₃ solution.

Our research is focused on lime sintering process which has two differences compared with soda-lime sintering process. Firstly, the crystal structure of dicalcium silicate of the former process is γ type with low activity; but that of the latter process is β type with high activity. Secondly, the sodium carbonate concentration in the former process is greater than that in the latter process, but the concentrations of caustic alkali and sodium aluminate show the contrary trend. Interestingly, γ-2CaO·SiO₂ is more stable, but Na₂CO₃ has the stronger decomposability in lime sintering process. Therefore, the mechanism of the secondary reaction between lime sintering process and soda-lime sintering process is different.

3.5.2 Mechanism of secondary reaction of calcium aluminate slag

XRD analysis of leaching residues formed under different conditions is used to study the mechanism of the secondary reaction. The XRD patterns are shown in Figs. 7 and 9.

The XRD results (Figs. 7-9) show that there are large amounts of CaCO₃ and γ-2CaO·SiO₂ in leaching residues under different conditions. Small quantity of 12CaO·7Al₂O₃ remains in the residues. The product of secondary reaction is only sodium hydrate alumino- silicate when sodium carbonate concentrations are 50 g/L and 100 g/L, respectively (Fig. 7). Hydrogarnet is formed when leaching time is 20 min, but it will be transformed into sodium hydrate alumino-silicate with leaching time increasing (Fig. 8, Eq. (10)). Sodium hydrate alumino- silicate and hydrogarnet are formed at 55 °C, but the latter disappears when the leaching temperature is up to 80 °C (Fig. 9).

3CaO·Al₂O₃·xSiO₂·(6-2x)H₂O+3Na₂CO₃+xH₂O=3CaCO₃+2NaAl(OH)₄+xNa₂SiO₃+2(2-x)NaOH  (10)

According to Eq. (10), NaAl(OH)₄ will be formed in this reaction especially when silica saturation of hydrogarnet is low. And the macro phenomenon of the reaction is the increase in alumina leaching rate. This is another reason for the increase of leaching rate with increasing leaching time (Figs. 5 and 8).

Fig. 7 XRD patterns of leaching residue with different Na₂O_c concentrations

Fig. 8 XRD patterns of leaching residue with different leaching time

Fig. 9 XRD patterns of leaching residue at different temperatures

High reaction temperature could promote not only Eq. (10) but also Eq. (4). NaAl(OH)₄ formed by Eq. (10) is less than that consumed by Eq. (4). Thus, alumina leaching ratio decreases when leaching temperature is higher than 75 °C (Fig. 6).

4 Conclusions

1) γ-2CaO·SiO₂ is decomposed during alumina leaching process of calcium aluminate slag. But it is not stable in sodium carbonate solution. The occurrence of secondary reaction causes the loss of alumina from solution.

2) The dominant factor of the decomposition of dicalcium silicate is Na₂CO₃. The concentration of decomposed SiO₂ increases to 1.71 g/L when the Na₂CO₃ concentration increases to 120 g/L under the studied conditions.

3) The products of secondary reaction are sodium hydrate alumino-silicate and hydrogarnet when leaching time is 20 min and leaching temperature is 55 °C. Hydrogarnet is transformed into sodium hydrate alumino-silicate with increasing leaching time and rising leaching temperature.

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铝酸钙炉渣浸出过程的二次反应机理

孙会兰^1,2，王 波^1,2，张建新¹，宗书凤¹，刘佳佳¹

1. 河北科技大学 材料科学与工程学院，石家庄 050018；

2. 河北科技大学 河北省材料近净成形技术重点实验室，石家庄 050018

摘  要：SiO₂以γ-2CaO·SiO₂的形式存在于铝酸钙炉渣中，γ-2CaO·SiO₂比β-2CaO·SiO₂稳定，但是在氧化铝溶出过程中它仍然可以被碳酸钠溶液分解，并引起二次反应。利用XRD研究铝酸钙炉渣二次反应的程度和机理。结果表明，γ-2CaO·SiO₂的分解率随着浸出时间和碳酸钠浓度的增加而上升，主要二次反应产物为水化石榴石和钠硅渣的混合物。溶液中SiO₂的浓度随着溶出温度的上升先增加而后降低。XRD分析表明，低温下二次反应的产物是水化石榴石，而高温下水化石榴石则会转变为钠硅渣。

关键词：铝酸钙炉渣；二次反应；氧化铝；浸出

(Edited by Yun-bin HE)

Foundation item: Project (51104053) supported by the National Natural Science Foundation of China; Project (E2012208047) supported by the Natural Science Foundation of Hebei Province, China

Corresponding author: Bo WANG; Tel: +86-311-81668705; E-mail: wangbo1996@gmail.com

DOI: 10.1016/S1003-6326(15)63732-3