ISSN 1070-4272, Russian Journal of Applied Chemistry, 2016, Vol. 89, No. 11, pp. 1763−1768. © Pleiades Publishing, Ltd., 2016. Original Russian Text © M.V. Batygina, N.M. Dobrynkin, A.S. Noskov, 2016, published in Zhurnal Prikladnoi Khimii, 2016, Vol. 89, No. 11, pp. 1392−1397.

INORGANIC SYNTHESIS AND INDUSTRIAL INORGANIC CHEMISTRY

Synthesis of Boehmite and Hematite by Joint Hydrolysis of Carbamide, Aluminum Chloride, and Iron(III) Chloride under Hydrothermal Conditions M. V. Batygina, N. M. Dobrynkin*, and A. S. Noskov Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, pr. Akademika Lavrentʼeva 5, Novosibirsk, 630090 Russia * e-mail: [email protected]; [email protected] Received November 8, 2016

Abstract—The formation of boehmite and hematite in dependence of the conditions of joint hydrothermal hydrolysis of carbamide and a mixture of aluminum and iron(III) chlorides in the presence of K, Na, Ca, and Mg chlorides at T = 160–200°C and P = 0.6–1.6 MPa was studied. It was shown that the amount of boehmite and hematite being formed in hydrolysis of Al and Fe chlorides strongly depends on pressure, temperature, hydrolysis duration, and composition of the model mixture of Al, Fe, Mg, Ca, K, and Na chlorides. It was found that a complete hydrolysis of AlCl3 and FeCl3 with 99% yield of boehmite and hematite occurs at the stoichiometric ratio between carbamide and aluminum and iron chlorides in the starting solution, whereas mostly iron oxyhydroxide [goethite FeO(OH)] and aluminum oxychloride [Al17O16(OH)16Cl3] are formed at nonstoichiometric ratios. DOI: 10.1134/S1070427216110057

Synthesis of highly dispersed iron and aluminum oxides is of particular importance for such application fields as manufacture of sorbents, catalysts, and catalyst supports, production of various ceramic and abrasive materials, pigments, fire-retardant additives, etc. Nanosize Al2O3 and Fe2O3 particles are most frequently produced by thermal decomposition [1–3], laser pyrolysis [4], precipitation and coprecipitation [5–7], microemulsion methods [8, 9], sonolysis [10], sol-gel synthesis [11, 12], hydrothermal [13, 14], and other methods [15–18]. It is known that the hydrolysis of aluminochloride solutions that are based on aluminum chloride and contain iron, magnesium, calcium, sodium, and potassium chlorides occurs in accordance with the following equations [19]: → Al(OH)3 +3 HCl↑, AlCl3 + 3H2O ←

(1)

FeCl3 + 3H2O → ← Fe(OH)3 +3 HCl↑,

(2)

MgCl2+ 2H2O → ← Mg(OH)2 +2 HCl↑.

(3)

Under normal conditions, potassium, sodium, and calcium chlorides are not subject to hydrolysis because of being formed by a strong base and a strong acid. The hydrolysis of aluminum (AlCl3) and iron chlorides (FeCl3) occurs by the successive stage-by-stage mechanism, with the rate and degree of hydrolysis growing with increasing temperature and decreasing salt concentration in the aqueous solution [20]. The following are important specific features of the hydrolysis of aluminum chlorides under the ordinary conditions: incomplete occurrence of the second stage of hydrolysis and nearly full impossibility of its third stage, formation of soluble polynuclear complexes of aluminum oxychlorides forming colloid solutions, and strong dependence of the extent of hydrolysis on the acidity of the medium and on the presence of additional components in solution [19]. As also in the case of hydrolysis of aluminum chloride, the hydrolysis of iron chloride is characterized by the incomplete occurrence of the second and third stages of hydrolysis and formation of polynuclear complexes, such as Fe2(OH)24+ dimers, which is the most important at pH < 3 [21, 22].

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BATYGINA et al. Evaporator unit

Air

Analytical unit

Argon Autoclave Pressure control unit

Flow-through reactor

Ventilation Temperature control unit

High-pressure reactor unit Reagent delivery unit

Pressure control unit

Fig. 1. Schematic of the laboratory installation. The following units are shown: reagent delivery, high-pressure reactors, reactor-evaporator of superheated solutions, recovery of reaction products, monitoring and control of temperature modes, monitoring and adjustment of pressures, analytical.

Four phases are observed under hydrothermal conditions at temperatures of 100°C < T < 500°C and pressures of 1 MPa < P < 100 MPa [23]: gibbsite Al(OH)3, boehmite γ-AlOOH, diaspore α-AlOOH, and γ-Al2O3. At higher temperatures in the hydrothermal conditions, boehmite γ-AlOOH is converted directly to α-Al2O3 at pressures below 15 MPa and temperature of about 380°C or to an intermediate phase diaspore α-AlOOH at higher pressures and temperatures beginning at 210°C, and then, beginning at 360°C, to γ-Al2O3 without formation of intermediate aluminum oxides in all cases. According to published data [23], the only Al–O–H product that can be obtained at a temperature of 160–200°C and pressure of 0.6–1.6 MPa is boehmite (or its modification, pseudoboehmite). It was also noted in [13] that lowering the pressure and temperature of the hydrothermal process leads to an increase in the share of nanosize (<100 nm) α-Al2O3 particles. The hydrothermal hydrolysis of iron chloride to obtain iron oxide (mostly as hematite α-Fe2O3) has been studied in various temperature and pressure ranges in sufficient detail [24–28]. It was found that, in the case of iron chloride hydrolysis in the absence of other additives, the process in which α-Fe2O3 is formed occurs is exceedingly slow even at high temperatures, whereas upon introduction of additives, e.g., 1 wt % polyvinyl alcohol [28], it takes seven days and more at T = 110°C and up to one day at T = 160–200°C.

The goal of our study was to examine the possibility of obtaining boehmite and hematite in joint hydrolysis of carbamide and aluminum and iron chlorides in the presence of K, Na, Ca, and Mg chlorides under hydrothermal conditions at T = 160–200°C and P = 0.6–1.6 MPa. This is of interest for development of an integrated technology for recovery of aluminum and iron from natural high-silica aluminum-containing raw materials and wastes from metallurgical industries. EXPERIMENTAL The hydrolysis was performed under hydrothermal conditions with model mixtures of aqueous solutions, whose composition and substance concentrations corresponded to the conditions of its possible application under real industrial conditions (Table 1). As an additive that accelerated the hydrolysis served carbamide of analytically pure grade with concentration of 8.5– 20.2 wt %. The solution volume was 30–60 mL. Experiments were performed at a temperature of 160–200°C and pressure of 0.6–1.6 MPa in a 200-mL autoclave reactor, which was a part of the laboratory experimental installation shown schematically in Fig. 1. The characteristics of the installation enabled experiments on hydrothermal hydrolysis at temperatures of 170–250°C and pressures of 0.1–5.0 MPa, with the

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SYNTHESIS OF BOEHMITE AND HEMATITE

measurement error and parameter adjustment accuracy of no worse than 3% under the conditions of an inert gas flowing through the reactor. The pressure in the system was provided by the flow of argon delivered to the reactor inlet and varied with upstream and downstream highpressure gas controllers, with the pressure corresponding to the saturated water vapor pressure at the prescribed temperature of experiment [29]. The reactor outlet was connected to a trap filled with distilled water to catch gaseous products carried away by the flow of argon. The argon flow rate was maintained constant and equal to 0.00125 m3 h–1. The experiment duration was 1 to 5 h. The resulting insoluble hydrolysis products were washed several times with distilled water, with the subsequent centrifugation of the sediment. The centrifugation was performed in the course of 8 min at a rotor speed of 2600 rpm. Further, the washed samples were dried at T = 80°C for 5 h. The composition and content of the insoluble precipitate was determined by weighing, X-ray fluorescence method, and X-ray diffraction (XRD) analyses. RESULTS AND DISCUSSION Table 2 lists the results of a hydrolysis of a mixture of carbamide and Al, Fe, Mg, Na, Ca, and K chlorides at T = 200°C and P = 1.6 MPa for 5 h at a varied content of carbamide in the starting solution. Presumably, the joint-hydrolysis process occurs in a mixture of carbamide and aluminum and iron chlorides in water under hydrothermal conditions in accordance with the equations 2AlCl3 + 3(NH2)2CO + 7H2O = 2AlO(OH)↓ + 6NH4Cl + 3CO2↑,

(4)

2FeCl3 + 3(NH2)2CO + 6H2O = Fe2O3↓ + 6NH4Cl + 3CO2↑.

(5)

It follows from the data obtained that boehmite and hematite are only formed at the stoichiometric ratio between carbamide, aluminum chloride, and iron chloride (at a carbamide content of 17.9%). The X-ray fluorescence method by Quantas software on an ARL ADVANT’X instrument with Rh anode of the X-ray tube was used to determine the chemical composition of the insoluble hydrolysis products. The sample obtained in a hydrolysis of carbamide (17.9%) and a model mixture of metal chlorides at T = 180°C, P = 10 MPa, and τ = 3 h contained 64.87 Al2O3, 32.95% Fe2O3, 1.29% Cl, and 0.73% MgO.

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Table 1. Composition of the model mixture of solutions Reagent grade

Concentration, wt %

Aluminum chloride hexahydrate

Analytically pure

18–20

Iron chloride hexahydrate

Analytically pure

5–7

Magnesium chloride

Analytically pure

1.9–2.4

Anhydrous granulated calcium chloride

Pure

0.4–0.6

Potassium chloride

Chemically pure

0.3–0.4

Sodium chloride

Chemically pure

0.2–0.3

Component

Table 2. Hydrolysis of AlCl3and FeCl3in the presence of carbamide Т = 200°С; P = 1.6 MPa; reaction duration 5 h; с(AlCl3) = 20, c(FeCl3) = 6, с(MgCl2) = 2, с(CaCl2) = 0.5, с(KCl) = 0.4, с(NaCl) = 0.3% Amount of (NH2)2CO, wt %

Yield, % hematite

boehmite

8.5

0.2

2.5

13.3

3.5

28.1

17.9

99.5

99.3

20.2

97.3

99.0

The presence of an insignificant amount of chlorine in the sample is presumably due to the incomplete washing of the sample to remove ammonium chloride. The XRD method was used to find that the main insoluble products formed in hydrolysis of carbamide and a model mixture of metal chlorides at T = 20°C and P = 1.6 MPa at a nonstoichiometric ratio between carbamide and aluminum and iron chlorides are goethite FeO(OH) and aluminum oxychloride Al17O16(OH)16Cl3(Fig. 2). The X-ray diffraction patterns were obtained with a monochromatized cobalt radiation (λKα = 1.7902 nm) on a HZG-4 instrument. The scanning was performed at angles in the range 2θ = 5–85° at a step of 0.1°. The X-ray diffraction patterns of samples produced by joint hydrolysis of carbamide and the model mixture of metal chlorides at varied hydrolysis duration (1, 3,

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2θ, deg Fig. 2. XRD patterns of samples produced in joint hydrolysis of carbamide and a model mixture of metal chlorides. с(AlCl3) = 20, с(FeCl3) = 6, с(MgCl2) = 2, с(CaCl2) = 0.5, с(KCl) = 0.4, с(NaCl) = 0.3, с[(NH2)2CO] = 13.3%; Т = 200°С; Р = 1.6 MPa. (2θ) Bragg angle; the same for Figs. 3–5. (I) Goethite, (II) oxychloride phase Al16O17(OH)17Cl3, (III) ammonium chloride not removed by washing. Hydrolysis duration (h): (1) 1, (2) 3, (3) 5.

and 5 h) show diffraction peaks (10, 32.8, 36.1, 39, 41.4, 58.3, and 76.7°) corresponding to the oxychloride phase Al45O45(OH)46Cl. To the goethite phase correspond the peaks at 24.6, 31.1, 46.2, 47, and 62.8°. Also, all the diffraction patterns have peaks (26.6, 38.05, 43.1, 55.0, and 68.80°) associated with ammonium chloride not removed by washing. Figures 3 and 4 show diffraction patterns of samples obtained at the stoichiometric ratio between carbamide and aluminum and iron chlorides at various hydrolysis temperatures (Fig. 3) and various hydrolysis durations at T = 180°C and P = 1.0 MPa (Fig. 4). It can be seen in the diffraction patterns that the insoluble hydrolysis products are boehmite (16.7, 32.5, 44.6, 57.3, and 63.8°) and hematite (28.1, 38.7, 41.7, and 48°) and the content of Table 3. Composition of model mixtures Component

Content of a component, wt % mixture no. 1 mixture no. 2 mixture no. 3

AlCl3

18

20

20

FeCl3

5

6

7

MgCl2

1.9

2

2.4

KCl

0.3

0.4

0.4

CaCl2

0.4

0.5

0.6

NaCl

0.2

0.3

0.3

2θ, deg

Fig. 3. XRD patterns of samples produced in joint hydrolysis of carbamide and a model mixture of metal chlorides at various temperatures. с(AlCl3) = 20, с(FeCl3) = 6, с(MgCl2) = 2, с(CaCl2) = 0.5, с(KCl) = 0.4, с(NaCl) = 0.3, с[(NH2)2CO] = 17.9%; τ = 3 h. (1) Boehmite, (2) hematite, (3) ammonium chloride not removed by washing; the same for Figs. 4 and 5.

these grows with increasing temperature and hydrolysis duration. The boehmite and hematite formed at 180 and 200°C are finely dispersed phases. The complete hydrolysis of iron and aluminum chlorides occurs in 3 h. Raising the hydrolysis duration further, to 4 h, does not lead to any increase in the intensity of the diffraction peaks. The diffraction patterns of the samples obtained at 180 and 200°C correspond to boehmite with particle size D of 4.8 nm (according to the line at 16.7°) and hematite with particle size D of 11.0 nm at 180°C and 20 nm at 200°C (according to the line at 41.7°). To determine how the composition of the model mixture affects the formation of boehmite and hematite in the joint hydrolysis, we prepared solutions with varied content of metal chlorides (Table 3). The diffraction patterns of the samples obtained in joint hydrolysis of carbamide and these model solutions at T = 180°C, P = 1.0 MPa, c[(NH2)2CO] = 17.9% and synthesis duration of 3 h are shown in Fig. 5. It turned out that boehmite and hematite are formed when all the mixtures of the specified compositions are subjected to hydrolysis at T = 180°C, with the composition of the model mixture being used strongly affecting the completeness of the process in which γ-AlOOH and α-Fe2O3 are formed. For example, it was found that the maximum yield of boehmite (94.2%

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4h Mixture no. 3 3h 2h

Mixture no. 2

1h Mixture no. 1

2θ, deg Fig. 4. XRD patterns of samples produced in joint hydrolysis of carbamide and a model mixture of metal chlorides at various hydrolysis durations. с(AlCl3) = 20, с(FeCl3) = 6, с(MgCl2) = 2, с(CaCl2) = 0.5, с(KCl) = 0.4, с(NaCl) = 0.3, с[(NH2)2CO] = 17.9%; Т = 180°С; Р = 1.0 MPa.

relative to the calculated value) and hematite (93.3%) is observed with mixture no. 2, whereas for composition nos. 1 and 3, the yield of boehmite was 74.2 and 68.3%, and that of hematite, 65.2 and 61.4%, respectively. These dependences can be understood on the basis of the mechanism suggested in [30] for describing the formation of aluminum hydroxides in a hydrolysis of nitrate salts in the presence of carbamide at elevated pressures under atmospheric pressure. In accordance with this mechanism, intermediate formation of a large number of monomeric and polymeric complexes of Al3+ is possible, with the stability and transformation rate of these complexes strongly affected by the reaction conditions. CONCLUSIONS (1) A new method was suggested for simultaneous synthesis of boehmite and hematite via joint hydrolysis of carbamide and a mixture of aluminum and iron(III) chlorides in the presence of K, Na, Ca, and Mg chlorides under hydrothermal conditions at temperatures in the range 160–200°C. This method is of interest for developing an integrated technology for extraction of aluminum and iron from natural high-silica aluminumcontaining raw materials and wastes from metallurgical industries. (2) It was found that the formation of boehmite and hematite is strongly affected by the process conditions (pressure, temperature, hydrolysis duration) and also by the composition of the model mixture of metal chlorides. The complete hydrolysis of AlCl 3 and FeCl 3 with

2θ, deg Fig. 5. XRD patterns of samples produced in joint hydrolysis of carbamide and a model mixture of metal chlorides at c[(NH2)2CO] = 17.9%, T = 180°C, P = 1.0 MPa.

quantitative (>99%) yield of boehmite and hematite occurs at the stoichiometric content of carbamide in the starting solution containing a mixture of carbamide and Al, Fe, Mg, Na, Ca, and K chlorides at T = 200°C, P = 1.6 MPa in the course of 5 h. (3) It was shown that the hydrolysis of AlCl3 and FeCl3 under hydrothermal conditions yields nanosize particles of boehmite (D = 4.8 nm) and hematite (D = 11.0–20 nm). ACKNOWLEDGMENTS The authors are grateful to V.A. Ushakov and I.L. Kraevskaya for the assistance in carrying out the study. The study was financially supported by the Ministry of Education and Science of the Russian Federation, unique project identifier RFMEFI60715X0142. REFERENCES 1. Diab, M. and Mokari, T., Inorg. Chem., 2014, vol. 53, no. 4, pp. 2304–2309. 2. Vakhshouri, A.R., Azizov, A., Aliyeva, R., and Bagirova, S., J. Appl. Polym. Sci., 2012, vol. 124, no. 6, pp. 5106–5112. 3. Suzdalev I. P., Suzdalev P. I., Russ. Chem. Rev., 2001, vol. 70, no. 3, pp. 177–210. 4. Li, Ch., Zhang, L., Li, Ya., and Wang, X., Phys. Lett. A, 2016, vol. 380, nos. 5–6, pp. 753–763. 5. Nagarjuna, R., Challagulla, S., Ganesan, R., and Roy, S., Chem. Eng. J., 2017, vol. 308, pp. 59–66. 6. Liu, R., Gao, N., Zhen, F., et al., Chem. Eng. J., 2013,

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