with Composite Adsorbents, (III)

Transcription

1 Journal of NUCLEBR SCIENCE and TECHNOLOGY, 17[3], pp. 204~212 (March 1980). Uranium Extraction from Sea Water with Composite Adsorbents, (III) Magnetic Properties of Composite Hydrous Titanium (IV)-Iron (II) Oxides and their Magnetic Separation Yoshihiro OZAWA, Toshifumi MURATAt, Energy Research Laboratory, Hitachi Ltd.* Hisao YAMASHITA and Fumito NAKAJIMA Hitachi Research Laboratory, Hitachi Ltd.** Received May 14, 1979 Revised August 31, 1979 Measurements were made of the extent of magnetization shown by composite hydrous titanium (IV)-iron (II) oxide adsorbents for use in uranium extraction from sea water. The possibility of removing the adsorbent by means of high-gradient magnetic separation was demonstrated by calculations based on a force balance model. Removal experiments were also carried out, which demonstrated that the composite hydrous oxide can be trapped effectively : A composite hydrous oxide with 1 : 1 Ti-to-Fe mole ratio, of 400~625 mesh particle size, proved to be removed to 99.9% by a magnetic field of 2.5 koe, with the slurry flowing at 20 cm/s through a filter matrix 50 cm long packed to 90% void with 100 pm diameter nickel wire stuffing. Together with the evaluations made of the electric power consumed by the highgradient magnetic separators, the experimental results indicated the possibility of economically extractng uranium from sea water using these separators in combination with magnetic adsorbents. KEYWORDS: uranium, sea water, extraction, adsorbents, magnetic adsorbents, magnetic separation, electric power consumption, composite hydrous titanium iron oxide, magnetization I. INTRODUCTION The technology of magnetic separation has markedly progressed(1)(2) in recent years, f ollowing the introduction of high-gradient magnetic separation devices, which permit effective separation of even weakly paramagnetic particles. These separators can also be applied to ferromagnetic particles of size below 100 mm, which is beyond the range of conventional magnetic separators. Studies have been published on magnetic adsorbents for uranium extraction from sea water and the present authors have reported(3) on work covering a composite hydrous titanium (IV)-iron(II) oxide, mainly comprising anatase and magnetite particles, which * Moriyama -cho, Hitachi-shi 316. ** Kuji-machi, Hitachi-shi 317. Present address : Nippon 311 t Nuclear Fuel Development Corp., Narita-machi, O-arai-cho, lbaraki-ken

2 Vol. 17, No. 3 (Mar. 1980) 205 has a relatively high uranium adsorption capacity comparable to that of hydrous titanium (V) oxide. Such magnetic adsorbents could possibly be applied to the extraction of uranium from sea water, with adsorption on slurry followed by separation of the adsorbents from sea water by means of magnetic separation. The present paper describes the magnetic p-roperties of the composite hydrous oxide and the results of removal experiments carried out using a high-gradient magnetic separator. The possibility is then discussed of applying of high-gradient magnetic separators to uranium extraction. II. PHYSICS OF MAGNETIC SEPARATIONG) The magnetic force acting in a given direction x on a small particle of ferromagnetic material located in a magnetic field is given by where V is the volume of the particle, M, and Mf are the magnetization of the particle and the medium respectively, and B(r) is the magnetic induction at a point r. If the particle is of paramagnetic material, the magnetization M(r) is equal to the product, XH(r) of susceptibility and magnetic field intensity, so that where X, and Xf are the susceptibility of the particle and the medium respectively, and H(r) the magnetic field intensity at point r. If the magnetic field is uniform, a paramagnetic spherical particle of radius b will be drawn toward a ferromagnetic wire of radius a with a magnetic force ( 1) ( 2 ) (3) where r is the distance between the center of the particle and the center of the wire. On a ferromagnetic particle, the corresponding magnetic force The term Fm in Eq. (3) is known to acquire its maximum value when the radius of the wire is equal to 2.69 times the particle radius(1). If a=3b, Eq. (3) can be rewritten in the form and Eq. (4) in the form The gravitational force is given by Fm=1.18(Mp-Mf)H0b2. (6) (4) (5) (7) where p, and pf are the densities of the particle and the fluid medium, respectively, and g is the gravitational constant. The hydrodynamic drap force Fd=-6pbe(Vp Vf), (8) 39

3 206 J. Nucl. Sci. Technol., where is the viscocity of the fluid medium, Vp and Vf are the velocities of the particle and fluid medium, respectively. This relation holds in the Stokes region, that is in the case where the product of the relative velocity and the particle diameter is smaller than about 0.01 cm2/s. The force balance model, in which these three forces Fm, Fg and Fe are compared, suffices to show the possibility of obtaining adsorbent removal with a device for high-gradient magnetic separation. III. EXPERIMENTAL 1. Preparation of Adsorbent The composite hydrous titanium (IV)-iron ( II) oxide was prepared as described previously(3), with the following modifications. The reaction was carried out at 50-C and sodium hydroxide solution was added to adjust the solution to ph 9. The mole ratio between titanium and iron in the adsorbent was controlled by regulation of the corresponding mole ratio in the mixed solution. Elemental analysis indicated that the Ti/Fe mole ratio was almost the same between the composite hydrous oxide and the mixed solution. The dependence of the adsorbent magnetization on the magnetic field was measured by a vibrating sample magnetometer. 2. Apparatus and Procedures for Removal Experiments Using High-gradient Magnetic Separation The apparatus used in the adsorbent removal experiments is shown schematically in Fig. 1. The high-gradient magnetic separator comprises an iron yoke with magnet coil and a packed filter placed in the gap of the yoke. The packing consists of a nickel mesh filter at packing densities of 5~25%. The slurry was prepared by mixing with 3 l of water a known amount of the adsorbent particles sieved between 400 and 625 mesh to obtain slurry concentration ranging between 60 and 170 ppm. Then, the slurries were passed through the packed filter matrix (4 cm length of 1 cm2 sectional area) at flow rates from 20 to 70 cm/s. Thereafter, the packed filter matrix was detached, and the trapped adsorbent particles washed out with water. The slurry was then again passed through the matrix. This sequence was repeated at least 5 times in each case. Ten milliliters of the slurry were sampled each time and iron or titanium content was measured by Fig. Apparatus for adsorbent recovery experiments 40

4 Vol. 17, No. 3 (Mar. 1980) 207 either atomic absorption spectrophotometry or colorimetry. The results of preliminary experiments showed that the mole ratio between titanium and iron was the same before and after the slurry was passed through the packed filter matrix. This indicates the absence of any separation occurring between the anatase or magnetite particles and the composite hydrous oxide in the course of the magnetic separation procedure. IV. RESULTS AND DISCUSSION 1. Magnetic Properties of Adsorbents Figure 2 shows the dependence on magnetic field intensity shown by the magnetization of the adsorbents at room temperature. The composite hydrous titanium (IV)- iron ( II) oxide (Ti-to-Fe mole ratios of 0.4, 1 and 6) and hydrous iron (II) oxide were both found to be ferromagnetic, their magnetization reaching saturation at 2 koe. The saturation value a, was 29 emu/g at 20dc for the hydrous iron (II) oxide, which is about one third of magnetite. The composite hydrous oxide was found to decrease its magnetization with lowering iron content. At a Ti-to-Fe mole ratio of 10 : 1, it became almost paramagnetic in property, showing a magnetization of only emu/g at 3 koe. 2. Comparison of Three Forces The gravitational, hydrodynamic drag and magnetic forces were calculated in reference to particle diameter, taking as example the composite hydrous titanium (IV)-iron (II) oxide 1 : 1 (Ti-to-Fe mole ratio) in a magnetic field gradient produced by ferromagnetic wire in a 3 koe magnetic field. The slurry velocity was fixed at 50 cm/s, and the diameter chosen for the ferromagnetic wire was three times that of the particle. The results of calculation are shown in Fig. 3. The magnetic force is seen to be stronger than either the gravitational or the hydrodynamic force in the range of particle diameter between 10 and 10' pm. This means that the particles in this range of diameter can be trapped by magnetic separator. The magnetic and hydrodynamic forces become predominant Fig. 2 Magnetization of composite hydrous titanium (N)-iron (If) ) oxide plotted against field intensity, at room temperature Fig. 3 Magnetic and competing gravitational and hydrodynamic drag forces vs. particle diameter, plotted in log-log scale, for composite hydrous oxide (Ti/Fe=-1) 41

5 208 J. Nucl. Sci. Technol., when the diameter of the particles is below several hundred micrometers. The calculated limiting diameter of composite hydrous oxide particles that should be trapped is given in Fig. 4 as function of the slurry velocity and with the Ti-to-Fe mole ratio as parameter for the case of a magnetic field gradient created by ferromagnetic wire in a 3 koe magnetic field. Again, the diameter adopted for the ferromagnetic wire was 3 times that of the particles. It is indicated that, with 1 : 1 Ti/Fe ratio, the composite hydrous oxide would be trapped at a slurry velocity of 20 cm/s when the diameter of the particle is over 2 mm, but when the Ti/Fe ratio is raised to 10 :1, no trapping can be expected to take place even at a slurry velocity of 5 cm/s unless the particle size exceeds 10 mm. 3. Recovery of Adsorbents Fig. 4 Limiting diameter of particles trapped plotted against slurry velocity, for composite hydrous oxides Examination of the three forces just mentioned indicates that the essential independent factors to be considered in magnetic separation are : (a) the magnetic property of the adsorbents, (b) particle size, (c) slurry velocity, (d) field intensity, and (e) wire diameter. Thus a simple model representing only the balance between the three forces should not suffice for determining the removal efficiency of the adsorbents by magnetic separator. Of the five factors cited above, the particle size was fixed in the present experiment between 400 and 625 mesh, and the remaining four were varied to examine their effect on recovery, taking account of differences in the length of the packed filter matrix. ( 1 ) Effect of Differences in Magnetic Properties of Adsorbents Degree of recovery is expressed by ( 9 ) where C(O) is the initial concentration particles in the slurry, and C(L) the corresponding value after the slurry has passed through the packed filter matrix of length L. In Fig. 5, the recovery of hydrous oxides of different Ti/Fe mole ratios are plotted against filter matrix length. It is seen that, even at the quite high slurry velocity of 20 cm/s, the composite hydrous of Fig. 5 Removal of composite hydrous oxides of mesh particle size at 20 cm/s slurry velocity, 2.5 koe magnetic field strength, and with 100 mesh filter matrix (75% void) 42

6 Vol. 17, No. 3 (Mar. 1980) 209 oxide was effectively removed when the Ti/Fe mole ratio was 0.4 : 1 or 1 :1, but that when the ratio was increased to 6 : 1, it became difficult to realize the better than 99.5% recovery, obtained at the lower mole ratios. ( 2 ) Effect of Change in Magnetic Field The effect brought on recovery by changing the magnetic field intensity is shown in Fig. 6. It is indicated that, for a given Ti/Fe ratio, the length of the packed filter matrix yielding the same percentage of hydrous oxide recovery shortened as the magnetic field was increased from 0.5, 1.5 and 2.5 koe. It is also revealed that, with 1.5 koe magnetic field and 24 cm long filter matrix, a removal performance exceeding 99% was effectively obtained. A field of 0.5 koe, however, is seen to have been insufficient for effective removal : This field intensity was too weak to magnetise both the composite hydrous oxide particles and the ferromagnetic wire. ( 3 ) Effect of Change in Slurry Velocity Figure 7 represents the effect on recovery brought by changing the slurry velocity. It is revealed that a slurry velocity of 70 cm/s was too high for obtaining effective recovery above 99%. Even at 40 cm/s, the packed filter matrix had to be at least 24 cm long to obtain a recovery exceeding 99.5%. Fig. 6 Effect of differences in magnetic field on recovery of composite hydrous oxide (Ti/Fe=1) of 400~625 mesh particle size, 40 cm/s slurry velocity, and with 100 mesh filter matrix (90% void) Fig. 7 Effect of differences in slurry velocity on recovery of composite hydrous oxide (Ti/Fe =1) of 400~625 mesh particle size, 2.5 koe magnetic field, and with 100 mesh filter matrix (90% void) Fig. 8 Effect of differences in filter matrix mesh wire diameter on recovery of composite hydrous oxide (Ti/Fe= 1) of 400~625 mesh particle size at 20 cm/s slurry velocity, 2.5 koe magnetic field, and with matrix of 90% void 43

7 210 J. Nucl. Sci. Technol., ( 4 ) Effect of Changing Wire Diameter of Packed Filter Matrix Mesh The effect of changing the diameter of the mesh wire packed into the filter matrix was examined, with the result presented in Fig. 8. It is seen that, between the three thickness of wire that were tested by packing the filter to the same 90% void, the smaller diameter material more effectively removed the composite hydrous oxide particles, of size between 400 and 625 mesh (20~40mm diameter). With the thickness 370 pm wire filled in a filter matrix 50 cm long, the recovery of slurry flowing at 20 cm/s did not reach 99%, whereas it was more than 99.9% with filter matrixes only 20 or 48 cm long when the mesh wire was 100 or 140 pm thick. These wire diameters already represented nearly 3 times the particle size, and thus a wire diameter of 370 pm must be considered too large in reference to the size of the particles to be filtered. 4. Application of High-gradient Magnetic Separation Using Magnetic Adsorbents to Uranium Extraction from Sea Water The foregoing experiment has shown that magnetic adsorbents like composite hydrous titanium (IV)-iron (II) oxide can be removed effectively with high-gradient magnetic separators under the conditions such as described in the previous section. Application of magnetic adsorbent to uranium extraction from sea water might be realized with a concept that would consist of : (a) adding magnetic adsorbent particles to the sea water to adsorb uranium ; (b) separating the particles from the sea water with high-gradient magnetic separators ; (c) eluting the uranium adsorbed on the particles ; and (d) reutilizing the recovered particles. Electric power consumption in the process acquires particular importance when consideration is given to energy gain(4), which is the ratio of energy between that contained in the product and that consumed in producing the fuel. It may be feared that the high-gradient magnetic separators used in the process should contribute markedly to raising the electric power consumption. The total electric power consumption including that of the magnetic separators can only be estimated with the adsorption of certain extrapolations and assumptions. The conditions assumed for the magnetic separators are : diameter of the packed filter mesh wire= 200 mm ; filter void=95%, which, with 0.5 m length of packing in filter matrix, should effectively remove particles of about 70 pm diameter, in a magnetic field of 2 koe. Electric power consumption of the high-gradient magnetic separators can be expected to decrease with increasing water treatment capacity. Based on the reported value(5) of 0.01 kwh/t water for the electric power consumed by the magnet coil with a flow velocity the filter matrix of m/s and magnetic field of 2 koe, it is assumed to be reduced to kwh/t sea water when the velocity is raised to 0.35 m/s. The large quantities of sea water that required to be treated also contribute a significant portion of the electric power consumption to drive the pumps. This portion of the power consumption is governed by the pressure drop through the packed filter matrix, expressed by the equation(6) (10) where and L is the length of the packed filter matrix (m), D1 the diameter of the packed filter 44

8 Vol. 17, No. 3 (Mar. 1980) 211 matrix, g the density of fluid (kg/me), u is the linear velocity of the fluid (m/s), and Re the Reynolds number, which is smaller than 100. From the value of H, thus determined, were obtain the power required to drive the pumps(7) : where V is the flow rate of sea water (m3/s), H the pressure drop expressed this time in meters of water head (m), and ri the pump efficiency. Assuming values of 50% for the uranium adsorption efficiency from sea water, 26 ms/s of sea water are needed(7) to obtain 1 tu/yr. Figure 9 presents, as function of sea water throughput, the total electric power consumption, as well as the portions taken up by the magnetic separators and the pumps assuming 80% pump efficiency. It is seen that the high-gradient magnetic separators consume less electric power than the pumps, and should further come to demand only a negligibly small fraction of the power consumption at the higher ranges of sea water throughput. The total electric power consumption is seen to rise quite sharply beyond 0.25 m/s throughput, where pumps come to take the dominant share of electric power consumption. With the pump column method, a pump head of 2.2 m would be normal(8), which corresponds to 5.3x 106 kwh of electric power consumption per ton of uranium with the present extraction method, and hence, this method can be considered competitive with the pump column method from the viewpoint of electric power consumption, once it is applied in industrial scale. Fig. 9 Estimated electric power consump (11) tion vs. linear velocity of sez water, including that of high gradient magnetic separators Moreover, the present system retains room for further reducing the electric power consumption, through improvements that could be brought to the adsorbent, by increasing the uranium adsorption capacity to enhance the adsorption efficiency, and by improving the magnetic properties to reduce the pressure drop through the separators, which latter effect could also be brought about by better performance characteristics obtained on the packed filter matrix. V. CONCLUSIONS Composite hydrous titanium (IV)-iron (II) oxide mixed in Ti-to-Fe mole ratios of 1 :0.4, 1 :1 and 1 : 6 manifested ferromagnetic properties. Its magnetization was found to saturate at a magnetic field strength of about 2 koe. From force balance model, it was estimated that a composite hydrous oxide of 45

9 212 J. Nucl. Sci. Technol., particle diameter exceeding 2 pm, mixed to 1 :1 Ti-to-Fe mole ratio, and flowing at 20 cm/s should be trapped by a magnetic field of 3 koe. Removal experiments conducted on composite hydrous oxide particles of 400~625 mesh, mixed to 1 : 1 Ti-to-Fe mole ratio and flowing at 20 cm/s, proved obtainable with 2.5 koe magnetic field intensity, using 100 mm diameter nickel wire packed to 90% void through a length of 50 cm in the filter matrix. It was proved from an evaluation of the electric power consumption incurred with the use of high-gradient magnetic separators along with magnetic adsorbents for the purpose of uranium extraction that the power required by the magnetic coils would be far smaller than that of the pumps to move sea water, when the process is applied in industrial scale. Hence, this method should be competitive with the pump column method from the viewpoint of electric power consumption. ACKNOWLEDGMENTS The authors wish to express their thanks to Prof. M. Kanno, Department of Nuclear Engineering, University of Tokyo, for his valuable suggestions on this work. (Text edited grammatically by Mr. M. Yoshida.) - REFERENCES- (1) OBERTEUFFER, J. A. : IEEE Trans. Magnetics, MAG-10[2], 223 (1974). (2) OKAMOTO, S., SEKIZAWA, H.: Oyo Buturi, (in Japanese), 43, 183 (1974). (3) OZAWA, Y., et al.: J. Nucl. Sci. Technol., 16[9], 671 (1979). (4) HAIGH, C. P.: CEGB Rep., R/M/N 787, (1976). (5) ODER, R. R., HORST, B.I.: Filtr. Sep., July/August, 363 (1976). (6) KIMURA, N., IINOYA, K.: Kogaku Kogaku, (in Japanese), 23, 792 (1962). (7) OGATA, N.: Genshiryoku Kogyo, (in Japanese), 24, 27 (1978). (8) KANNO, M.: J. At. Energy Soc. Japan, (in Japanese), 19[9], 586 (1977). 46