Ind. Eng. Chem. Res. 1998, 37, 2687-2690
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Physical and Chemical Properties of a Highly Viscous Aluminum Sulfate Melt Dogan Ornek,* Turker Gurkan, and Cevdet Oztin Department of Chemical Engineering, Middle East Technical University, Ankara, 06531 Turkey
The physical and chemical properties of the commercial aluminum sulfate melt that do not exist in the literature were measured in this study. The aluminum content of commercial aluminum sulfate hydrate varies between 16 and 17% Al2O3. The viscosity of this product is in the range 160-260 cP, density is in the range 1.705-1.735 g/cm3, and the surface tension is predicted to be 105 dyn/cm. These properties change sharply with temperature and concentration. 1. Introduction Commercial aluminum sulfate hydrate is produced according to the reaction
2Al(OH)3 + 3H2SO4 + 12H2O f A2(SO4)3‚18H2O The reaction is highly exothermic and the product that is commonly called aluminum sulfate melt is at about 110 °C. It is a dense, highly viscous fluid and is extremely corrosive. The solid commercial hydrate, generally written as the 18 hydrate, is typically dehydrated to 16.0-17.0% Al2O3. This dehydrated form is called dry alum. It is either in the form of lumps or is ground to finer solid forms. It appears as slightly efflorescent, odorless, colorless, and monoclinic crystals. Aluminum sulfate solutions are typically 7.5-8.5% Al2O3 and are known as liquid alum (Kirk and Othmer, 1978). Solid aluminum sulfate hydrate is primarily used in the paper industry and wastewater treatment. In addition, aluminum sulfate with a very low iron content is used in the food industry and in the manufacture of metal alums, deodorants, astringents, and catalysts (McKetta, 1976). The physical and chemical properties of the commercial aluminum sulfate melt were measured in this study. In the literature, data have been reported for the physical properties of aluminum sulfate solutions at relatively low Al2O3 concentrations. However, data on the physical properties of aluminum sulfate melt containing 16.0-17.0%Al2O3 is not available with few exceptions (Gronvold and Meisingset, 1982; Hashmi et al., 1992; Komives et al., 1984; Sacks et al., 1984). 2. Experimental Procedure For the determination of physical and chemical properties of aluminum sulfate hydrate, molten aluminum sulfate is required. The solid product of the industrial product was dissolved in water, and then an equivalent amount of water was evaporated. Thus, the desired melt was obtained. The behavior of the commercial aluminum sulfate melt on cooling was determined by allowing it to cool * To whom correspondence should be addressed. Current address: Cornell University, Riley Robb Hall, Ithaca, NY, 14853. E-mail:
[email protected]. Tel.: (607) 255-2249. Fax: (607) 255-4080.
under ambient conditions and the temperature was measured by an immersed thermometer. The melt was stirred until it visibly started to solidify. Temperature readings were taken every 30 s. Then, the plot of temperature against time was obtained. Viscosity of the aluminum sulfate solutions was measured with the Cannon Fenske viscometer which is a modified Ostwald type. The equipment consisted of a capillary glass and an oil bath with thermostatic regulation for immersion of the viscometer and the thermometer. Density of aluminum sulfate solutions at 110 °C was determined by using the Baume hydrometer. Surface tension of the aluminum sulfate melt was obtained with the Harkins-Brown drop volume technique. First, the volume of a drop detached from the nozzle was measured experimentally. Five different sizes of nozzles with a length of 1.0 cm were used. Then, the Harkins-Brown correction factor, ψ, was determined from eq 1 which is developed by Mori (1990):
[ ( )( ) ] Do ψ 1.4 V
ψ ) 0.6 + 0.4 1 -
1/3 2.2
(1)
With these established, surface tension, σ, was calculated from eq 2:
( )
V ) ψVp ) ψ
πDoσ Fg
(2)
Changes in the water content of the aluminum sulfate hydrate on contact with air with different relative humidities were measured gravimetrically at room temperature. Forty grams of samples were placed in desiccators with different relative humidities (RH) at 25 °C, and the mass changes in the samples were determined as a function of time. Relative humidities of 60 and 79% were maintained by a saturated aqueous solution of glycerol and NH4Cl, respectively, and data were taken every 30 days. The degree of crystallinity of the commercial product was detected by means of X-ray diffraction measurements. The behavior of Al2(SO4)3‚16H2O on heating was determined by thermogravimetric analysis (TGA) measurements. The TGA module monitored the weight of a material and its rate of change continuously. The purpose of this study was to identify intermediate compounds and to determine the temperature range
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2688 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
Figure 1. Saturation temperature of aluminum sulfate solutions. Table 1. Physical and Chemical Properties of Aluminum Sulfate property
commercial product
% Al2O3 % Al2(SO4)3 % Fe2O3 % free Al2O3 % insolubles % H2O pH of 1% solution
16.0-16.5 53.0-55.0 0.010-0.017 0.15-0.65 0.3 44-45 3.3-3.5
over which each compound is stable. All experiments were run on powdered samples with particle sizes between 60 and 80 µm. A nitrogen flow of 53 mL/min was maintained through the gas space over the sample to drive off the gaseous products of the reaction. Information on the stable temperature ranges of the intermediate compounds were obtained with a preliminary decomposition run at heating rates of 10 °C/min. After these preliminary tests, TGA for the same materials were conducted at the slower heating rate of 2 °C/ min over the temperature range of interest.
Figure 2. Cooling behavior of aluminum sulfate melt (55.25% Al2(SO4)3).
Figure 3. Variation of viscosity of concentrated aluminum sulfate solutions with temperature and composition. % Al2(SO4)3: [, 47.85; 9, 49.73; 2, 50.51; ×, 51.57; f, 53.17; b, 55.25.
3. Results and Discussions The melt simulating the industrial product is obtained at 113 °C at 685 bar pressure (Figure 1). The chemical properties of the melt so obtained were measured. The results are shown in Table 1 (Ornek, 1993). 3.1. Behavior on Cooling. The temperature of hydrated aluminum sulfate melt falls until solid Al2(SO4)3‚16H2O begins to form at 90 °C. The resultant cooling behavior is shown in Figure 2. The temperature then remains constant until solidification is complete. The dip below the freezing point is due to supercooling. The solution supercools before solidification of aluminum sulfate hydrate takes place. After the sample is completely solidified, the temperature drops as solid aluminum sulfate is cooled. 3.2. Viscosity. Viscosity measurements were carried out for the range 99-110 °C. The relationships between the viscosity, concentration, and temperature of the melt are plotted on the viscosity temperature chart shown in Figure 3. Viscosity of the aluminum sulfate melt is 235 cP at 110 °C. As it is seen in Figure 3, the viscosity of the melt is affected significantly by the temperature. However, when the aluminum content of the aluminum sulfate solution is decreased, the viscosity of the aluminum sulfate solution does not change much with temperature, but it sharply changes with concentration.
Figure 4. Variations of the density of aluminum sulfate melt with temperature and composition. % Al2(SO4)3: [, 49.73; 9, 52.49; 2, 55.25.
3.3. Density. Data on the density of the aluminum sulfate melt covering the concentration range 50-55% Al2(SO4)3 are presented in Figure 4. It is seen that the liquid density changes sharply with concentration. However, the temperature does not significantly affect the density of the aluminum sulfate melt. At 110 °C the density of the aluminum sulfate melt is 1.735 g/cm3, and it is 1.705 when Al2(SO4)3 content is 53.17%. 3.4. Surface Tension. The average value of the surface tension of the aluminum sulfate melt is 105 dyn/ cm (Table 2).
Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2689
Figure 5. Changes in the water content of the solid aluminum sulfate kept in air with different moisture content, RH: 2, 60%; ×, 79%.
Figure 7. Dehydration and decomposition of industrial aluminum sulfate hydrate in the temperature interval 27-827 °C.
Figure 8. Dehydration and decomposition of industrial aluminum sulfate hydrate in the temperature interval 27-377 °C.
Figure 6. X-ray diffraction spectra of solid product: Cu/30 kV/ 24 mA/4 f 40/0.05/20/2 × 103/1.0/0.2. Table 2. Prediction of the Surface Tension of Saturated Aluminum Sulfate Solutions by the Harkins-Brown Correlation Do, mm
ψ
Vp, cm3
σ, dyn/cm
0.9 1.0 1.3 1.5 1.6
0.815 0.804 0.774 0.765 0.749
0.0142 0.0158 0.0198 0.0223 0.0239
103.4 104.9 105.1 103.8 106.5
3.5. Changes in the Water Content of Solid Aluminum Sulfate. The changes in the water content of solid aluminum sulfate hydrate with time at different relative humidity conditions are given in Figure 5. At 60 and 70% relative humidity conditions the samples took up a large amount of water in 30 days. After that, the mass of the samples reached a constant level. Thus, this experiment showed that the water content of aluminum sulfate hydrate strongly depends on humidity. 3.6. Degree of Crystallinity. The X-ray diffraction pattern of the industrial plant product is illustrated in Figure 6. The plant product has a high degree of crystallinity. 3.7. Thermal Decomposition of Al2(SO4)3‚16H2O. Thermogravimetric analysis (TGA) up to 817 °C was
carried out on the commercial sample. The results for the dehydration and decomposition of industrial aluminum sulfate hydrate are presented in Figure 7. Aluminum sulfate hydrate contains a significant amount of hydration water. In the received condition, the aluminum sulfate is labeled as a hexadecahydrate; however because Al2(SO4)3‚16H2O is unstable at room temperature, it is already partially dehydrated to Al2(SO4)3‚14H2O before the run. As the temperature is increased, Al2(SO4)3‚14H2O dehydrates further to Al2(SO4)3‚8H2O. The reaction reaches completion at 333 °C. The anhydrous sulfate is stable up to 727 °C. It then begins to be converted to Al2O3; the process completes at 817 °C. The losses are associated with dehydration, dehydroxylization, and desulfurization. Samples at temperatures above 817 °C loses about 82.9% of its weight. After these preliminary tests, TGA for the same material was conducted at the slower heating rate of 2 °C/min over the temperature range of 20-400 °C to identify any possible intermediates. The results are presented in Figure 8. This is almost the same as Figure 7; no new intermediates were identified. Acknowledgment We are grateful to Etibank(Ankara) for the financial support to complete this work. Our thanks are also due to the engineers of Seydisehir Aluminum Sulfate Factory, Turkey, for their cooperations and helpful suggestions.
2690 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
Nomenclature Do ) orifice diameter, cm g ) acceleration due to gravity, 981 cm/s2 V ) drop volume detached from orifice, cm3 Vp ) drop volume calculated by eq 2, cm3 F ) density, g/cm3 σ ) interfacial surface tension, dyn/cm ψ ) Harkins-Brown factor
Literature Cited Gronvold, F.; Meisingset, K. K. Thermodynamic Properties and Phase Transitions of Salt Hydrates Between 270 and 400 °K. J. Chem. Thermodyn. 1982, 14, 1083. Hashmi, S. A.; Rai, K.; Chandra, S. Protonic Conduction in Al2(SO4)3‚16H2O: Coulometry, Transient ionic Current, Infrared and Electrical Conductivity Studies. J. Mater. Sci. 1992, 27, 175. Kirk, R. E.; Othmer, D. F. Encyclopedia of Chemical Technology; John Wiley & Sons: New York, 1978.
Komives, J.; Tomer, K.; Sztatisz, J.; Lassu, L.; Gal, S. Thermoanalytical Studies on the Preparation of Industrial Crystalline Aluminum Sulfate. J. Therm. Anal. 1984, 29, 1083. McKetta, J. J. Encyclopedia of Chemical Processing and Design; Marcel Dekker: New York, 1976. Mori, Y. H. Harkins-Brown Correction Factor for Drop Formation. AIChE J. 1990, 36, 1272. Ornek, D. Developing and Designing Conditions for the Prilling of Molten Aluminum Sulfate Hydrate. M.S. Thesis, Middle East Technical University, Ankara, Turkey, 1993. Sacks, M. D.; Tseng, T.; Lee, S. Y. Thermal Decomposition of Spherical Hydrated Basic Aluminum Sulfate. Ceram. Bull. 1984, 63, 301.
Received for review January 23, 1998 Revised manuscript received April 17, 1998 Accepted April 29, 1998 IE9800445
