Ind. Eng. Chem. Res. 2004, 43, 161-165
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SEPARATIONS Effect of Operating Parameters on Magnetic Filtration Processes Asım Ku 1 nku 1 l,*,† Ahmet Ekmekyapar,† Canan Akmil,† and Teymuraz Abbasov‡ Departments of Chemical Engineering and of Electrics-Electronics Engineering, Faculty of Engineering, Inonu University, 44069 Malatya, Turkey
The effect of operating parameters and the effect of the pH of the liquids to be cleaned on the magnetic filter performance in the filtration processes were investigated. The pH of aqueous suspensions containing ferromagnetic particulates was changed in the range of 3-10. The suspensions were prepared in our laboratory. The effect of the size of filter matrix elements (diameter of the ferromagnetic spheres) and filtration velocity on the filter performance was determined. The filter performance of the filter matrix composed of a mixture of different size spheres was also investigated. A magnetic filter performance above 60% was obtained with a filtration velocity of 0.07 m/s. It was observed that if the size of particles and the fraction of ferromagnetic particles in the suspension are varied in a short range, the effect of the initial concentration of corrosion products on the magnetic filter performance is negligible. The experimental results were compared with theoretical calculations and the data in the literature. Suggestions are given about choosing optimum values of the operating parameters for effective filtration of industrial fluids in magnetic filters. Introduction The cleaning of chemical and other industrial process fluids from corrosion products is one of the important problems encountered in the industry. In general, the corrosion products have very low concentrations and are micron-sized. The difficulty of removing micron-sized low-concentration corrosion products from suspensions with classical filtration methods has led to the utilization of new alternative filtration methods such as magnetic filtration, which has also been used in various branches of industry.1-5 A magnetic filter matrix is composed of ferromagnetic materials of various shapes such as spheres, rods, wires, plates, and metal scraps. A high-gradient magnetic field may be formed around ferromagnetic materials, which can easily be magnetized on the external homogeneous magnetic field. A high-gradient magnetic field results in particle capture zones in the filter matrix, during the flow of suspensions through the magnetic filter. Magnetic corrosion components in the suspension are held and deposited in magnetic capture zones. Because the filter matrix composed of ferromagnetic materials is more resistant to physical and chemical factors, the fluids at high temperature can be cleaned by the use of a magnetic filtration technique. Magnetic filtration is more advantageous because the filtration rate in magnetic filtration is 3-10 times faster than that in classical filtration techniques.5 The cleaning of liquids and gases used in industry from the pollutant elements by magnetic filters has been investigated by many researchers.5-12 These researchers * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ Department of Electrics-Electronics Engineering.
cleaned out industrial fluids by adding a certain amount of magnetite fine particles. The results of these studies showed that magnetic filtration is effective for cleaning industrial fluids containing phosphate and aluminum salts, ferrous hydroxide flocculants, bacteria, microorganisms, and various heavy-metal ions such as Cd, Pb, Zn, and Cr. Parameters (magnetic field intensity, filtration flow rate, pH, geometrical dimension, etc.) affecting the performance of the cleaning systems have been evaluated in these studies. Basically, these studies were conducted in magnetic separators, in which magnetic filter matrixes were composed of ferromagnetic wires. In some cases, it may be advantageous to use ball matrixes.5,13-17 However, the porosity of the magnetic filter matrix of packed beds (ball matrix) is smaller than that of the separator. For this reason, compared to magnetic separators, ball matrix filters have different properties such as the flow profile in the pores, magnetization, and holding characteristics of the matrix. Such properties of the filtration mechanism should be taken into account in the course of determining process parameters of magnetic filtration. The effects of magnetic, hydrodynamic, rheologic, and geometric parameters on the filtration processes have been investigated in a ferromagnetic sphere and metal scraps containing magnetic filters.5,13,15-17 However, these studies mainly were limited to relatively low magnetic field strength (100 kA/m), and the matrix elements used were composed of equal-size homogeneous spheres. The results reported were inconsistent because the effect of rheologic properties, hydrodynamic flow regimes, and the pH of the fluids were not considered and synthetic test suspensions contained coarse disperse magnetite scraps.5,13,14 Furthermore, the effect of the principal parameters of magnetic filtration systems on the filtration and separation processes is still
10.1021/ie030444e CCC: $27.50 © 2004 American Chemical Society Published on Web 12/10/2003
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Figure 1. X-ray diffraction of corrosion products. Table 1. Particle Size Distribution of the Corrosion Product Sample size (µm)
vol. under (%)
size (µm)
vol. under (%)
0.091 0.138 0.275 0.479 0.832 1.259 1.660 1.905 2.188 2.884
0.07 0.31 0.95 3.55 10.64 19.04 25.62 29.07 32.55 39.44
3.802 4.365 5.012 5.754 8.710 13.183 19.953 26.303 34.674 45.709
46.01 49.14 52.20 55.22 64.57 75.26 86.45 92.71 97.06 100.00
a disputed problem among the researchers.18-20 The establishment of an accurate relationship that explains the effect of the operating parameters on the filtration process is very important in practical applications. In this study, the magnetic filter performance was investigated experimentally by varying the pH of the fluid to be cleaned, the initial concentration of the particles, the flow rate, and the size of the filter matrix elements (spheres) at laboratory and relatively high magnetic field intensity conditions. The experimental results were compared to theoretical and literature values. Experimental Section Rust material containing iron and steel used in various industrial processes was collected, dried, and ground to obtain fine particles. The size distribution analysis of the particles was made using a Malvern Mastersizer 2000 laser diffraction particle size analyzer, and the results are given Table 1. The average particle size was about 4.5 µm. The structure of rust products was determined by the X-ray diffraction method (Figure 1). The results showed that the composition of the rust product consisted of ferromagnetic (Fe3O4), antiferromagnetic (Fe2O3), and paramagnetic (FeOOH) particulates. After multiple passages of the samples through a permanent magnetic system, 85% of the corrosion products exhibited good magnetic properties.
Corrosive material containing a suspension was used as the base filtration medium. The suspension used for the experiments had a solid/liquid ratio of 0.012 g/L. Experiments were carried out at room temperature range. The suspension was stirred thoroughly prior to the experiments. Scraps in the suspension samples were visually controlled to check whether coagulation or sedimentation took place. A hydrochloric acid and NaOH solution was used to adjust the pH of the suspension, and pH values were measured by a pH meter (WTW). An external magnetic field was formed by using a LHWL-type laboratory high-intensity wet magnetic separator (Boxmag-Rapid Ltd., Birmingham, U.K.). The distance between the poles of the magnetic separator was 3.2 × 10-2 m. The magnetic field intensity was in the range of 0-1.5 T. Experiments were conducted at B ) 0.25 T (H ) 200 kA/m) external magnetic field. The magnetic filter external dimensions were 0.03 × 0.04 × 0.09 m. Three different size (d1 ) 6.3 × 10-3, d2 ) 7.9 × 10-3, and d3 ) 11.9 × 10-3 m) stainless steel ferromagnetic spheres with magnetization properties of µ ) 9.5 at H ) 200 kA/m were used as packing material for the filter matrix. The porosity () of the filter matrix consisting of homogeneous spheres was about 0.4. The matrix elements were carefully washed and dried at the end of each experiment. To investigate the effect of hydrodynamics of the fluid to be cleaned on the filter performance, the suspension flow rate was kept between 0.032 and 0.14 m/s. The effects of the filter matrix size and flow rate on the filter performance were investigated. Experiments were carried out at 19 °C temperature, a pH value of 7.29, a magnetic field intensity of B ) 0.25, and a suspension concentration of 0.012 g/L solid-liquid ratio. The magnetic filter performance is determined by measuring the mass concentration of the inlet and outlet iron. The outlet iron concentration was determined by titration.
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Theory The theories of magnetic filtration and magnetic separators are similar.1-5,11,16-20 However, magnetic filtration operations with granular and packed beds have some differences. In magnetic filtration, the filter matrix porosity and the concentration of the cleaned mixture are low.5,13 The magnetic filter and separators performance for cleaning micron and submicron particulates containing suspensions can be estimated by the following equation:
Ψ ) λ[1 - exp(-ξ)]
(1)
where Ψ is the filter performance (or filtration efficiency, difference between the inlet and outlet concentrations divided by the inlet concentration), λ is the fraction of ferromagnetic particles in the mixture, and ξ is the logarithmic filter efficiency coefficient. The effect of the magnetic filtration system parameters such as the filtration rate, magnetic field intensity, and particle size, etc., is included in the logarithmic filter efficiency coefficient given in eq 1. The logarithmic filter efficiency coefficient term is evaluated by using the particle capture radius in the capture zone. The particle capture radius is determined by the balance of magnetic and drag forces1-5,16-18 or the moments of these forces13 on the accumulating particles in the capture zone. The fluid flow rate profile through porous ferromagnetic spheres may be estimated approximately by21
u ) 11.5(1 + 1.3γ5/6)Vf
(ar)
2
(2)
Hence, the magnetic filter performance may be derived with operations given in ref 13 as follows:
Ψ ) λ
[
1 - exp -0.524 × 10-5
[
kµ1.38H 2(1 - φ)δ
] ]
Fd(1 + 1.3γ5/3)2Vf2
0.6
Ld (3)
where k is the effective magnetic susceptibility, k ) kp - kf, kp and kf are the magnetic susceptibilities of the particle and fluid, respectively, H is the magnetic field intensity (A/m), φ is the volumetric concentration of the particles collected in the pores, δ is the particle diameter (m), Vf is the filtration velocity of the suspension (superficial velocity, m/s), F is the density of the suspension (kg/m3), d is the diameter of the matrix spheres (m), γ is the packed bed density, Ld ) L/d is the dimensionless length of filter, µ is the magnetic permeability of the matrix spheres, r is the distance from touching points of spheres to the local position of captured particles, and a is the radius of the spheres. Because of low concentration of rust particles in the samples used for the experiments of the present study, as is the case in industrial liquids, the effect of these particles on the magnetization properties of the filter matrix and the hydrodynamic properties of the transportation medium can be neglected.5,13 On the other hand, theoretical calculations and experimental results show that the volumetric concentration of particles collected in pores is smaller than 0.1.5 For this reason, the volumetric concentration of particles is taken as 0.1 in eq 3. Equation 3 allows the study of the effect of base operating parameters of filtration (magnetic field in-
tensity, geometry of matrix, flow rate of suspension, length of filter, etc.) on the filter performance. It is seen that the main parameter affecting the filter performance is the magnetic field intensity. However, variation of this parameter in a wide range (especially H >200-300 kA/m) is not advantageous because saturation in the magnetic circuit of the magnetic filter matrix element has occurred in this range. Saturation causes loss of energy in the magnetic filter, increases cost of magnetic filters, and as a result decreases the magnetic filter performance. Another parameter affecting the filter performance in eq 3 is the filter matrix porosity () or packed bed density (γ). In general, as the filter matrix porosity is decreased, the filter performance should increase. The porosity of beds randomly packed with magnetic spheres is approximately 0.4. The porosity of the bed may be changed by packing the bed with different diameter spheres. Thus, the ratio of sphere diameters is important. Investigations have showed that a sphere diameter ratio greater than 0.5 results in a very small change in the packed bed density.22 If the ratio of sphere diameters is greater than 0.5, the packed bed density can only increase by 1.03-1.06-fold. Hence, the packed bed density stays about the same as that of the randomly placed equal size spheres (γ ) 0.6). In our experiments, sphere diameter ratios were d1/d3 ) 0.529 and d2/d3 ) 0.664 and the packed bed densities were 0.63 and 0.62, respectively.5,22 This value is very close to 0.6, and the value of (γ) was taken as 0.6 in eq 3. It should be noted here that, to increase the packed bed density to 0.70.75, the ratio of sphere diameters should be below 0.20.3. Thus, the small spheres could fill in the spaces between large ones and effectively increase the packed bed density. However, a high packed bed density is not preferred because settling of spheres becomes harder, the pressure drop increases sharply, and magnetization of the capture zone weakens. One other main parameter affecting the filter performance is the size of the filter matrix element (diameter of spheres). In general, the diameter of the filter elements used in industrial applications is between 2 × 10-3 and 8 × 10-3 m. It is reported that for H < 100 kA/m and a sphere diameter between 5 × 10-3 and 6 × 10-3 m a good performance for the capture of micron and submicron particles is obtained in the filter.5 On the other hand, to capture particles of a few microns with a good filter performance if the magnetic field intensity (H) is above 150 kA/m, the sphere diameters can be chosen to be as high as (12-14) × 10-3 m. In this case, filters may be operated at high volumetric flow rates. One of the most important parameters affecting the filtration processes is the velocity of the fluid to be cleaned through the filter. It is believed that the optimum velocity for filters used in industry is below 0.1 m/s for H < 100 kA/m and particles of less than 1 µm diameter size.5 If the magnetic field intensity is increased to above 150 kA/m and the size of captured particles is high, the optimal filtration velocity may be expected to increase. Thus, in this work, the magnetic field intensity was chosen as 200 kA/m to investigate the effect of the diameter of the filter matrix elements and the variation of the filtration rate on the filter performance.
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Figure 2. Variation of the magnetic filter performance with the pH of the cleaning suspension.
Figure 3. Variation of the magnetic filter performance with the diameter of matrix spheres. Theoretical results: H ) 200 kA/m, µ ) 9.5, k ) kp ) 0.17, γ ) 0.6, φ ) 0.1, λ ) 0.85, L ) 9 cm, δ ) 4.5 µm, Vf ) 0.05 m/s, and F ) 103 kg/m3.
Results and Discussion Figure 2 shows the effect of changing the pH of the suspension on the magnetic filter performance. The filter performance was determined by changing the pH values between 4 and 10. Figure 2 shows that the effect of changing the pH of the suspension even a wide range on the magnetic filter performance is negligible. Thus, magnetic filters with ferromagnetic packed beds are used to sufficiently clean fluids having various physicochemical properties. The effect of changing the diameter of the filter matrix elements on the filter performance is presented in Figure 3. Figure 3 shows that, because small-diameter spheres are used in the filter matrix, the magnetic filter performance increases. It is seen in Figure 3 that a magnetic filter having spheres of 6.3 × 10-3 m diameter has a performance of about 0.7. Under identical conditions, by increasing the sphere diameter by 2-fold, the filter performance will still be above 0.5. To investigate the effect of the packed bed density on the magnetic filter performance, different diameter ferromagnetic spheres were used. For this purpose, five experiments were conducted with the filter matrix composed of only d1 spheres, d2 spheres, d3 spheres, 50% d1 + 50% d2 spheres, and 50% d2 + 50% d3 spheres. The average diameter of the spheres given in Figure 3 was taken as d1a ) 0.5(d1 + d2) and d2a ) 0.5(d2 + d3). As the filter matrix density is increased, the bed pressure also increases. As seen in the figure with the mixture of spheres, the variation in the filter performance is not as important as it was thought to be. Hence, the use of identical spheres is more advantageous.
Figure 4. Variation of the magnetic filter performance with the filtration velocity of the suspension. (b) Experimental results. (]) Theoretical results (solid line): H ) 200 kA/m, µ ) 9.5, k ) kp ) 0.17, γ ) 0.6, φ ) 0.1, λ ) 0.85, L ) 9 cm, d ) 6.3 mm, δ ) 4.5 µm, and F ) 103 kg/m3. (∆) Magnetite (Fe3O4) suspension:5 H ) 30 kA/m, λ ) 1, L ) 4.2 cm, d ) 5.7 mm, and δ ) 3-5 µm. (*) Cleaning of an ammonia solution:5 H ) 60-80 kA/m, λ ) 0.85, L ) 20 cm, d ) 2.4 mm, and δ ) 1 µm. (O) Feedwater in a hydropower plant:5 H ) 90 kA/m, λ ) 0.85, L ) 100 cm, d ) 5 mm, and δ ) 1-2 µm.5
Figure 5. Variation of the magnetic filter performance with the inlet concentration of corrosion products. δ ) 4.5 µm, L ) 9.0 cm, and Vf ) 0.05 m/s.
The dependence of the magnetic filter performance on the filtration velocity is presented in Figure 4. As seen from the figure, the magnetic filter performance decreases with increasing filtration velocity. Some experimental results from the literature are also given in Figure 4 for comparison. To obtain a performance above 0.6 with filters operating below 100 kA/m magnetic field intensity, the optimum filtration velocity should be kept below 0.1 m/s when the fluid to be cleaned contains 1-2 µm particulates.5,13 As seen in Figure 4, for effective capture of 4-5 µm particles even at high magnetic field intensity (H ) 200 kA/m), the optimum filtration velocity is 0.07 m/s, which is less than 0.1 m/s. Thus, to increase the filtration velocity at relatively high magnetic field intensities, the particulate diameters should be larger than 10 µm. Figure 5 shows that the initial concentration of corrosion products in the suspension has essentially no effect on the magnetic filter performance. This result is also obtained with the filtration of liquids in different industrial plants.5 This result is valid for the case where the diameter of the particles and the fraction of the ferromagnetic particles in the suspension is changing in a short range. If these properties change widely, the filter performance will be sensitive to variations. Conclusions The effect of the change of the operating parameters on the filter performance in the filtration operations was investigated. Results obtained from this study may be itemized as shown below.
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(1) Changing the pH of the suspension to be cleaned had no effect on the filter performance. Hence, suspensions with a wide range of pH may be filtered effectively in magnetic filters. (2) To increase the effectiveness of magnetic filtration operation, the diameter of the filter matrix elements must be decreased or the filter matrix density must be increased. However, the diameter of the filter matrix elements may not be reduced drastically because the flow regime may be affected. Hence, the diameter of the filter matrix elements may be chosen to be between 5 × 10-3 and 10 × 10-3 m for cleaning industrial fluids. To increase the performance of filters having high diameter spheres, the magnetic field intensity should be increased to 150-200 kA/m. Using packed beds composed of homogeneous spheres as magnetic filter matrix elements is more advantageous. (3) One of most important parameters affecting the performance in the magnetic filtration operations is the filtration velocity (Vf) of the liquids to be cleaned. The magnetic filter performance decreased with increased filtration velocity. To capture particles of 4-5 µm diameters (λ g 0.8) at H e 200 kA/m in magnetic filters with 60-70% performance, the optimum filtration velocity is below 0.1 m/s. However, if higher particles are used, then the optimum filtration velocity will also be higher. (4) The external magnetic field intensity (H) is one of the basic operating parameters affecting the magnetic filtration, but increasing the magnetic field intensity is not practical because saturation of the magnetic system has occurred at H > 200-300 kA/m. In this case, energy losses in the magnetic system increase and magnetic filtration operation becomes more expensive. (5) If the size of the particles and the fraction of the ferromagnetic particles in the suspension are not varied in a wide range, the effect of the initial concentration of corrosion products on the magnetic filter performance is negligible. Literature Cited (1) Watson, J. H. P. Magnetic filtration. J. Appl. Phys. 1973, 44, 4209. (2) Liberman, S. J.; Kelland, D. Magnetic filtration aqueous suspensions of submicron reactor corrosion products. IEEE Trans. Magn. 1984, MAG 20, 1195. (3) ) Gerber, R.; Lawson, P. Magnetic cage filter. IEEE Trans. Magn. 1994, 30, 4653.
(4) Svoboda J. Magnetic methods for the treatment of minerals; Elsevier: New York, 1987. (5) Sandulyak, A. V. Magnetic filtration of liquids and gases; Ximiya: Moscow, 1988 (in Russian). (6) De Latour, D. Magnetic separation in water pollution control. IEEE Trans. Magn. 1973, MAG-9 (3), 314. (7) Harding, K.; Baxter, W. Application of high gradient magnetic separation to ferric hydroxide filtration. IEEE Trans. Magn. 1981, MAG-17, 2795. (8) Anand, P.; Etzel, J. E.; Friedlaender, F. J. Heavy metals removal by high gradient magnetic separations. IEEE Trans. Magn. 1985, MAG-21 (5), 2062. (9) Terashima, Y.; Ozaki, H.; Sekine, M. Removal of dissolved heavy metals by chemical coagulation, magnetic seeding and high gradient magnetic filtration. Water Res. 1986, 20 (5), 537. (10) Liu, Q.; Friedlaender, F. J. Selective Collection of nonMagnetic Rutile and Quartz by Means of a Magnetic Reagent by HGMS. IEEE Trans. Magn. 1994, 30 (6), 4668. (11) Franz, M.; Franzreb, M. Determination of the capture radii of magnetite bearing hydroxide flocs in magnetic filtration. IEEE Trans. Magn. 1998, 34 (6), 3902. (12) Bahaj, A. S.; Ellwood, D. C.; Watson, J. H. P. Extraction of heavy metals using microorganisms and high gradient magnetic separation. IEEE Trans. Magn. 1991, 27 (6), 5371. (13) Abbasov, T.; Ko¨ksal, M.; Herdem, S. Theory of high gradient magnetic filter performance. IEEE Trans. Magn. 1999, 35, 2128. (14) Karapinar, N. Magnetic separation of ferrihydrite from wastewater by magnetic seeding and high-gradient magnetic separation. Int. J. Miner. Process. 2003, 71, 45. (15) Heitmann, H. G. Iron oxides in boiler water removed magnetically. Ind. Water Eng. 1969, 12, 31. (16) Watson, J. H. P.; Watson, S. J. P. The ball matrix magnetic separator. IEEE Trans. Magn. 1983, MAG-19 (6), 2698. (17) Friedlaender, F. J.; Takayasu, M. A steady of the mechanisms of particle buildup on single ferromagnetic wires and spheres. IEEE Trans. Magn. 1982, MAG-18, 817. (18) Ebner, A. D.; Ritter, J. A. New correlation for the capture cross section in high gradient magnetic separation. AIChE J. 2001, 47 (2), 303. (19) Svoboda, J. The effect of the magnetic field strength on the efficiency of magnetic separation. Miner. Eng. 1994, 7, 747. (20) Svoboda, J. A realistic description of the process of high gradient magnetic separation. Miner. Eng. 2001, 14 (11), 1493. (21) Abbasov, T.; Altunbas, A. Determination of the particle capture radius in magnetic filters with velocity distribution profile in pores. Sep. Sci. Technol. 2002, 37 (9), 2037. (22) Aerov, M. E.; Todes, O. M.; Narinskiy, D. A. Apparatus with stationary granular packed beds; Ximiya; Leningrad, 1979 (in Russian).
Received for review May 23, 2003 Revised manuscript received October 21, 2003 Accepted November 5, 2003 IE030444E
