Phase equilibria of AlCl3 + FeCl3 + H2O, AlCl3 + CaCl2 + H2O, and

Apr 21, 2016 - In the recovery of aluminum from coal fly ash, the phase equilibria of solid–liquid systems are important for purifying the aluminum ...
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Phase equilibria of AlCl3 + FeCl3 + H2O, AlCl3 + CaCl2 + H2O, and FeCl3 + CaCl2 + H2O at 298.15 K Mengxia Yuan,† Xiuchen Qiao,*,† and Jianguo Yu‡ †

School of Resource and Environmental Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ‡ State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ABSTRACT: In the recovery of aluminum from coal fly ash, the phase equilibria of solid−liquid systems are important for purifying the aluminum salts. This research investigated phase equilibria of the ternary systems AlCl3 + CaCl2 + H2O, AlCl3 + FeCl3 + H2O, and FeCl3 + CaCl2 + H2O at 298.15 K using isothermal dissolution method. The corresponding phase diagrams and density diagrams were plotted. Both AlCl3 + CaCl2 + H2O and AlCl3 + FeCl3 + H2O systems had one invariant point, two univariate curves, two crystallization regions, and no double salts. The FeCl3 + CaCl2 + H2O system had two invariant points, three crystallization regions and a double salt (CaCl2·2FeCl3·8H2O) that formed at 298.15 K. There is no solid solution in all the three systems.

the phase diagram of the solid−liquid system. Wang et al.16 and Li et al.17 reported the solubility of the AlCl3 + CaCl2 + H2O system. However, few studies have shown the solubility of the AlCl3 + FeCl3 + H2O and CaCl2 + FeCl3 + H2O systems. In this study, the solubility data of all these three ternary systems were measured at 298.15 K and the corresponding phase diagrams were plotted.

1. INTRODUCTION Aluminum chloride is widely used in the production of flocculants, catalysts, aluminum hydroxide, aluminum oxide, and other products.1−3 Recovering aluminum from coal fly ash (CFA) was proposed in the 1950s by the Polish scientist Jerzy Grzymek4 and was further developed by global researchers during the next half century. The two largest coal-producing provinces in China, Inner Mongolia and Shanxi, generate considerable amounts of high-aluminum-content CFA (aluminum equivalent: 40 wt %−50 wt %).5−7 Hence, CFA is a potential alternative to bauxite for extracting aluminum. In China, decreasing the amount of CFA is as important as recycling CFA. The acid leaching process for recovering aluminum from CFA has received more attention recently due to its contribution to reducing the amount of CFA. The Shenhua Group Corp. of China has constructed a hydrochloric acid leaching plant with an annual production capacity of one million tons of alumina. Calcite is an additive which sintered with CFA to increase the leaching ratio of Al. Therefore, CaCl2 is one of the major impurities in the leachate of sintered clinker.8,9 Chinese CFA generally contains 5%−7% Fe 2O3; hence, the leachate also contains FeCl3 as an impurity. Solvent extraction is one of the main methods separating Fe3+ from Al3+ ions in the acid liquor.10−13 Crystalline AlCl3· 6H2O is generally produced through evaporation crystallization after solvent extraction. However, the extraction stripping process is difficult because it requires a concentrated acid. The recycling of the extraction solvent is also unsatisfactory. Crystallization is considered to be a more useful method for purifying aluminum salts.14,15 The design and optimization of the crystallization process rely on the solubility of components, that is, © XXXX American Chemical Society

2. EXPERIMENTAL SECTION Reagents. Analytical grade aluminum chloride hexahydrate (≥97.0%) and ferric chloride hexahydrate (≥98.0%) from Sinopharm Chemical Reagents Co., Ltd., and calcium chloride (≥96.0%) from Shanghai Ling Feng Chemical Reagent Co., Ltd. were used in this study (see Table 1). The ultrapure-grade water with an electrical conductivity of 0.055 μS/cm was used in the experiments. The total amounts of impurities in each reagent are all lower than 0.01%, which are measured by inductively coupled plasma (ICP). Therefore, the reagents used in this study have a small amount of free water. Experimental Methods. The solid−liquid equilibria of the ternary systems AlCl3 + CaCl2 + H2O, AlCl3 + FeCl3 + H2O, and FeCl3 + CaCl2 + H2O were measured using the isothermal dissolution method18,19 at the temperature (T) of 298.15 K. The solubility was measured in a thermostatic oil bath with a standard uncertainty of 0.02 K (F32-ME, Julabo) under atmospheric pressure (P). To determine the solid−liquid equilibrium, the required amount of salts and ultrapure-grade water were placed in a 500 mL glass reactor, which ensured that Received: November 3, 2015 Accepted: April 8, 2016

A

DOI: 10.1021/acs.jced.5b00932 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 1. Chemical Sample Descriptions

a

chemical name

sourcea

initial mass fraction purity

purification method

final mass fraction purity

analysis method

AlCl3·6H2O CaCl2·6H2O FeCl3·6H2O

S1 S2 S1

0.97 0.96 0.98

none none none

0.97 0.96 0.98

titration titration spectrophotometry

S1: Sinopharm Chemical Reagents Co., Ltd. S2: Shanghai Ling Feng Chemical Reagent Co., Ltd.

Table 2. Solubility and Density Data of the Ternary System AlCl3 + CaCl2 + H2O Tested at 298.15 K and 0.1 MPab,c composition of liquid phase, w(B)c·102 number

AlCl3

CaCl2

H2 O

1D1 2 3 4 5 6 7 8 9 10 11E1 12 13F1

31.08 29.93 28.33 28.30 22.24 18.17 13.28 12.30 7.26 4.36 2.16 1.79 0.00

0.00 2.14 3.61 4.11 12.77 17.94 25.24 26.34 33.92 40.20 42.65 43.01 46.47

68.92 67.93 68.06 67.59 64.99 63.89 61.48 61.36 58.82 55.44 55.19 55.20 53.53

density ρc

composition of wet residual, w(B)·102 AlCl3

CaCl2

H2O

47.55 47.26 54.21 52.39 53.78 53.24 50.50 53.22 53.38

0.62 1.00 0.35 1.17 0.78 1.61 2.76 1.59 1.92

51.83 51.74 45.44 46.44 45.44 45.15 46.74 45.19 44.70

1.50

48.14

50.36

equilibrium solid phase

g·cm−3

A A A A A A A A A A A+C C C

1.3146 1.3234 1.3257 1.3265 1.3478 1.3619 1.3799 1.3798 1.4037 1.4299 1.4436 1.4431 1.4561

a

A, AlCl3·6H2O; C, CaCl2·6H2O. bThe standard uncertainties u(T) = 0.02 K, u(P) = 0.5 kPa, u(AlCl3) = 0.003, u(CaCl2) = 0.003, u(ρ) = 0.001 g·cm−3. T is the temperature, P is the pressure, w(B) is the mass fraction component B, ρ is the density.

a c

the solid phase never completely dissolved and was always present during the entire equilibrium process. Then, the reactor was sealed with plugs and placed in the thermostatic oil bath at 298.15 K. The samples were stirred by a stirring paddle at a speed of 250 rpm. The ion concentration of the liquid phase was examined every 2−3 h, and the sample was allowed to settle for 4 h prior to analysis. When the difference in the ion concentration over four sequential measurements was less than 1%, the solid−liquid equilibrium was considered to be achieved. The solid−liquid equilibrium in this study was guaranteed by 48 h of stirring and 24 h of settling. After equilibration, a sample tube heated at 298.15 K for 4 h was used to collect a certain quantity of the liquid phase for chemical analysis. The clarified solution was diluted in a 100 mL volumetric flask. Wet residue was separated from the liquid phase by vacuum filtration using a sintered glass filter crucible. One part of wet residue was dissolved in water and analyzed using chemical methods. The other part was dried for X-ray diffraction (XRD) analysis (Rigaku D/MAX 2550 VB with a Cu target, operated with a 2θ step size of 0.02°). The density (ρ) of the liquid phase was measured using a previously weighed 10 mL density bottle, and the standard uncertainty was 0.001 g·cm−3. The density bottle containing liquid phase was maintained at 298.15 K with a standard uncertainty of 0.1 K for 4 h. Analytical Methods. There are many methods for the concentration measurement of ions in the liquid phase and wet residue, such as inductively coupled plasma (ICP),16,17 titration,20−25 ion chromatography (IC),26 and gravimetrical method.27 In this research, a 905 Titrando (Metrohm) was used to measure the concentrations of chlorine and calcium ions. The chlorine ion (Cl−) concentration was measured by titration with silver nitrate.20,21 The calcium ion (Ca2+) concentration was determined by titration with an EDTA standard solution in the presence of alkali and a Ca-sensitive indicator.22,23 The ferric ion (Fe3+) concentration was measured by spectrophotometry (UV-2802, UNICO).24 The

Figure 1. Solubility diagram of the ternary system AlCl3 + CaCl2 + H2O at 298.15 K: ●, liquid phase; ○, wet solid phase; □, solubility data from Wang et al.;16 E1, invariant point of the ternary system; A1 and C1, composition points of pure AlCl3·6H2O and CaCl2·6H2O salts, respectively; A1D1E1A1 and C1F1E1C1, crystallization fields of AlCl3·6H2O and CaCl2·6H2O, respectively.

aluminum ion (Al3+) concentration was determined by EDTA complexometric titration with xylenol orange as the indicator.25 Three parallel samples of each equilibrated liquid phase were analyzed three times each, and the average value of the three measurements was considered as the reported result. The standard uncertainties of the Cl−, Ca2+, Fe3+, and Al3+ were 0.001, 0.003, 0.005, and 0.003, respectively.

3. RESULTS AND DISCUSSION AlCl3 + CaCl2 + H2O System. The solubility and density data of the ternary system AlCl3 + CaCl2 + H2O at 298.15 K are listed in Table 2. The ion concentrations in the liquid phase and wet residue are expressed as mass fractions w(B). A phase diagram based on the solubility is presented in Figure 1. B

DOI: 10.1021/acs.jced.5b00932 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 2. XRD pattern of phases A (AlCl3·6H2O) and C (CaCl2·6H2O) in the ternary system AlCl3 + CaCl2 + H2O at 298.15 K.

Figure 3. XRD pattern of the invariant point E1 in the ternary system AlCl3 + CaCl2 + H2O at 298.15 K.

The solid dots in Figure 1 were the liquid phase points, and the hollow circular dots were the wet residue points. Points D1 and F1 were the solubilities of AlCl3 and CaCl2 in pure water at 298.15 K. The solubility data agree well with the results reported by Wang et al.16 shown in Figure 1. The measured solubility of AlCl3, that is, point D1 in Figure 1, was 31.08, which has little difference from the reported data in literatures such as 31.08,16 31.10,28 and 31.09.29 The measured solubility of CaCl2, that is, point F1 in Figure 1, was 46.47 and the data from literatures were 45.30,16,28 46.08,30 and 44.33.23 The deviation of solubility data of CaCl2 was mainly attributed to the analysis method; for example, titration was used for the measurement of solubility in this study, whereas ICP was used in the work of Wang et al.16 Nevertheless, the deviations and the data were believed to be acceptable for industrial design and optimization. Point E1 was the invariant point of the AlCl3 + CaCl2 + H2O system saturated with AlCl3 and CaCl2. The phase diagram had two univariant curves (D1E1 and E1F1). Along the curve D1E1, AlCl3·6H2O had priority to precipita; however, CaCl2·6H2O had priority along the curve E1F1.

Figure 4. Solubility diagram of the ternary system AlCl3 + FeCl3 + H2O at 298.15 K: ●, liquid phase; ○, wet solid phase; E2, invariant point of the ternary system; A2 and C2, composition points of pure AlCl3·6H2O and FeCl3·6H2O salts, respectively; A2D2E2A2 and C2F2E2C2, crystallization fields of AlCl3·6H2O and FeCl3·6H2O, respectively. C

DOI: 10.1021/acs.jced.5b00932 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 3. Solubility and Density Data of the Ternary System AlCl3 + FeCl3 + H2O Tested at 298.15 K and 0.1 MPab,c composition of liquid phase, w(B)c·102

density ρc

composition of wet residual, w(B)·102

number

AlCl3

FeCl3

H2 O

AlCl3

FeCl3

H2 O

equilibrium solid phase

1D2 2 3 4 5 6 7 8 9 10 E2 11 12 13F2

31.08 29.87 26.15 23.81 20.53 18.76 16.98 14.09 12.88 11.60 7.84 2.23 0.00

0.00 2.61 9.49 14.82 19.65 23.35 27.24 33.17 35.99 37.03 40.84 45.98 50.69

68.92 67.52 64.36 61.37 59.82 57.89 55.78 52.74 51.13 51.37 51.32 51.79 49.31

53.95 53.89 53.81 52.66 51.83 51.2 50.52 0.52 1.96 1.58 0.25

0.18 0.60 0.87 1.35 2.19 2.43 3.36 58.99 56.13 57.06 58.69

45.87 45.51 45.32 45.99 45.98 46.37 46.12 40.49 41.91 41.36 41.06

A A A A A A A A A A+F F F F

a

g·cm−3 1.3146 1.3312 1.3691 1.4116 1.4311 1.4494 1.4745 1.5119 1.5339 1.5420 1.5337 1.5405 1.5215

A, AlCl3·6H2O; F, FeCl3·6H2O. bThe standard uncertainties u(T) = 0.02 K, u(P) = 0.5 kPa, u(AlCl3) = 0.003, u(FeCl3) = 0.005, u(ρ) = 0.001 g·cm−3. T is the temperature, P is the pressure, w(B) is the mass fraction component B, ρ is the density.

a c

Figure 5. XRD pattern of the invariant point E2 in the ternary system AlCl3 + FeCl3 + H2O at 298.15 K.

Figure 6. XRD pattern of phase F (FeCl3·6H2O) in the ternary system AlCl3 + FeCl3 + H2O at 298.15 K.

solid phase AlCl3·6H2O was therefore plotted on D1E1, and the equilibrium solid phase CaCl2·6H2O was plotted on F1E1. There were two crystallization fields corresponding to AlCl3·

In a phase diagram, the point at which the lines extending from the saturated liquid points to corresponding wet residue points intersect is the pure solid composition. The equilibrium D

DOI: 10.1021/acs.jced.5b00932 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 4. Solubility and Density Data of the Ternary System FeCl3 + CaCl2 + H2O Tested at 298.15 K and 0.1 MPab,c composition of liquid phase, w(B)c·102 number

FeCl3

CaCl2

H2O

1D3 2 3 4 5 E3 6 7 8 9 10 11E4 12 13 14 15F3

50.69 47.58 43.69 41.96 47.99 41.40 32.94 26.87 23.52 20.72 18.48 15.11 6.43 4.50 0.00

0.00 2.97 7.01 11.42 15.61 19.94 24.83 29.24 31.72 34.34 36.86 38.78 41.75 43.12 46.47

49.31 49.45 49.30 46.62 36.40 38.66 42.23 43.89 44.76 44.94 44.66 46.11 51.82 52.38 53.53

density ρc

composition of wet residual, w(B)·102 FeCl3

CaCl2

H2O

57.19 51.37 55.97 55.87 55.63 55.90 55.81 54.05 0.00 0.34

1.38 5.13 3.52 19.90 19.75 19.71 19.75 20.20 50.66 50.84

41.43 43.50 40.51 24.23 24.62 24.39 24.44 25.75 49.34 48.82

0.42

50.3

49.28

equilibrium solid phase F F F F Db Db Db Db Db Db C C C C C

a

g·cm−3 1.5215 1.5510 1.5595 1.5719 1.6284 1.6199 1.5973 1.5863 1.5806 1.5770 1.5745 1.5584 1.4825 1.4705 1.4561

F, FeCl3·6H2O; C, CaCl2·6H2O; Db, CaCl2·2FeCl3·8H2O. bThe standard uncertainties u(T) = 0.02 K, u(P) = 0.5 kPa, u(FeCl3) = 0.005, u(CaCl2) = 0.003, u(ρ) = 0.001 g·cm−3. cT is the temperature, P is the pressure, w(B) is the mass fraction component B, ρ is the density.

a

Similar to the AlCl3 + CaCl2 + H2O system, there was no double salt or solid solution in the AlCl3 + FeCl3 + H2O system at 298.15 K (Figure 4). Points D2 and F2 were the solubilities of AlCl3 and FeCl3 in pure water at 298.15 K, respectively. The measured solubility of FeCl3 in this study was 50.69, which is higher than 49.57 reported in literature.31 This deviation was mainly ascribed to the different analysis methods and it was considered to be acceptable in this study. The point E2 was the invariant point of the AlCl3 + FeCl3 + H2O system, which was proved by the XRD result in Figure 5. The XRD pattern in Figure 6 showed that phase F was FeCl3·6H2O. It is known that AlCl3 and FeCl3 are easy to hydrolyze in the solution; however, hydrate deposits were not detected in wet residue subjected to XRD test. The hydrolysis of AlCl3 and FeCl3 at 298.15 K, hence, is not obvious and its effect on the solubility is negligible. There were two univariant curves (D2E2 and E2F2) in Figure 4. Along the curve D2E2, AlCl3·6H2O had priority to precipitate; however, FeCl3·6H2O had priority along the curve E2F2. There were two crystallization fields corresponding to AlCl3·6H2O (A2D2E2A2) and FeCl3·6H2O (C2F2E2C2) in Figure 4. The crystallization field of FeCl3·6H2O was smaller than that of AlCl3·6H2O, however, it was larger than that of CaCl2·6H2O in Figure 1. Therefore, CaCl2 showed more obvious salting out effect than FeCl3 on the solution of AlCl3. And separating FeCl3 from the solution of AlCl3 by crystallization process was more difficult than separating CaCl2 from AlCl3. FeCl3 + CaCl2 + H2O System. The solubility and density data of the FeCl3 + CaCl2 + H2O system at 298.15 K are listed in Table 4 and the corresponding phase diagram is shown in Figure 7. The points D3 and F3 are the solubilities of FeCl3 and CaCl2 in pure water at 298.15 K, respectively. There were two invariant points (E3, E4) and a double salt in the FeCl3 + CaCl2 + H2O system (Figure 7). The point E3 was the equilibrium of FeCl3·6H2O + CaCl2·2FeCl3·8H2O, and the point E4 was the equilibrium of CaCl2·6H2O + CaCl2·2FeCl3·8H2O. There were three univariant curves (D3E3, E3E4, and E4F3) in Figure 7. The point Db was a double salt CaCl2·2FeCl3·8H2O. Although there is no PDF file of CaCl2·2FeCl3·8H2O from JCPDS, it has to declare that the characteristic diffraction peaks in Figure 8 belong to neither FeCl3·nH2O nor CaCl2·nH2O. The intensities

Figure 7. Solubility diagram of the ternary system FeCl3 + CaCl2 + H2O at 298.15 K: ●, liquid phase; ○, wet solid phase; E3 and E4, invariant points of the ternary system; A3 and C3, composition points of pure FeCl3·6H2O and CaCl2·6H2O salts, respectively; A3D3E3A3, E3DbE4E3, and E4C3F3E4, crystallization fields of FeCl3·6H2O, CaCl2· 6H2O, and CaCl2·2FeCl3·8H2O, respectively).

6H2O (A1D1E1A1) and CaCl2·6H2O (C1F1E1C1). The crystallization field of CaCl2·6H2O was considerably smaller than that of AlCl3·6H2O. The larger crystallization field indicated that the solubility of AlCl3 in the AlCl3 + CaCl2 + H2O system was low and AlCl3 would salt out of the solution easily with an increase of CaCl2 concentration. There was no double salt or solid solution in this system at 298.15 K. The components of the equilibrium solid phase were also verified by XRD. The XRD pattern of phases A (AlCl3·6H2O) and C (CaCl2·6H2O) is shown in Figure 2. The XRD pattern in Figure 3 showed that the point E1 is the invariant point of AlCl3·6H2O and CaCl2· 6H2O. AlCl3 + FeCl3 + H2O System. The phase diagram of the ternary system AlCl3 + FeCl3 + H2O at 298.15 K is plotted in Figure 4 based on the data in Table 3. The ion concentrations in the liquid phase and wet residue are also expressed in mass fractions w(B). E

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Figure 8. XRD pattern of phases C (CaCl2·6H2O), F (FeCl3·6H2O), and Db (CaCl2·2FeCl3·8H2O) in the ternary system FeCl3 + CaCl2 + H2O at 298.15 K.

Figure 9. Density versus composition diagrams of the ternary systems AlCl3 + CaCl2 + H2O (a), AlCl3 + FeCl3 + H2O (b), and FeCl3 + CaCl2 + H2O (c) at 298.15 K.

of these characteristic peaks were not very high and the diffraction pattern showed the characters of amorphous matters. Therefore, the salt CaCl2·2FeCl3·8H2O may have an instable crystal form. The XRD results in Figure 8 showed that the phase C was CaCl2·6H2O and the phase F was FeCl3·6H2O in the ternary system FeCl3 + CaCl2 + H2O. There were three crystallization fields corresponding to FeCl3·6H2O (A3D3E3A3), CaCl2·6H2O (E3DbE4E3), and CaCl2·2FeCl3·8H2O (E4C3F3E4) in Figure 7. The crystallization fields of FeCl3·6H2O and CaCl2·6H2O were smaller than that of CaCl2·2FeCl3·8H2O. The combined effect of CaCl2 and FeCl3 on the solubility of AlCl3 was a new phenomenon and requires further investigation in the quaternary system. Density of the Solutions. The relationship between the solution density and the mass fraction of either CaCl2 or FeCl3 (w(B)) is plotted in Figure 9. Figure 9a showed that the solution density of the AlCl3 + CaCl2 + H2O system increased regularly with the increase in the concentration of CaCl2. The solution density of the AlCl3 + FeCl3 + H2O system (Figure 9b) also changed with the concentration of FeCl3 and reached a maximum value of 1.5420 g·cm−3 at the cosaturation point E2. The solution density of the FeCl3 + CaCl2 + H2O system first increased with the increase in the concentration of CaCl2 and reached a maximum value of 1.6284 g·cm−3 at point E3

(Figure 9c), gradually decreased to point E4, and then sharply decreased.

4. CONCLUSION Three ternary diagrams of AlCl3 + CaCl2 + H2O, AlCl3 + FeCl3 + H2O, and FeCl3 + CaCl2 + H2O that serve the recovering of aluminum from coal fly ash were measured at 298.15 K in this study. CaCl2 was more effective than FeCl3 in salting out of AlCl3 from the solution. A double salt of CaCl2·2FeCl3·8H2O formed in the FeCl3 + CaCl2 + H2O system when the concentration of CaCl2 ranged from 15.61% to 36.86%. There was no solid solution in any of the three systems. AUTHOR INFORMATION

■

Corresponding Author

*E-mail: [email protected]. Tel.: 86-21-64252171. Fax: 86-21-64252826. Notes

The authors declare no competing financial interest. Funding

The work described in this paper was funded by the National High Technology Research and Development Program (863 Program) (2011AA06A102), the National Natural Science Foundation of China (21176082) and the Specialized Research Fund for the Doctoral Program of Higher Education (20110074110002). F

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DOI: 10.1021/acs.jced.5b00932 J. Chem. Eng. Data XXXX, XXX, XXX−XXX