Patent Number: 055815898
Section: description

DETAILED DESCRIPTION OF THE INVENTION As result of the intensive studies conducted in order to attain the objects of the invention, the present inventors found that the problem of the prior art could effectively be solved by adopting the following three design features. First, the overall system was configured as shown in FIG. 1, comprising a magnetron oscillator 1, circulator A designated by 2, directional coupler A designated by 3, incident and reflected wave power meter 4, stub tuner 5, circulator B designated by 6, directional coupler B designated by 7, cavity resonator 8, spectrum analyzer 9, discharge detector/interlocking signal transmitter 10, gelling stock solution or sol supplying/small droplet forming unit 11, and gel particle receptacle 12. Components 1-3 are interconnected by waveguides 13, and components 3-8 are also interconnected by waveguides 13. It should be noted here that a commercial power source consists typically of components 1-3 or 1-5. Secondly, the resonator 8 shown in FIG. 1 was fitted with a micrometer-type drive mechanism 15 that permitted a quartz rod 14 for fine tuning of the resonant state to be inserted into or withdrawn out of the resonator 8. As in the Ledergerber process, a quartz tube 16 for protecting the resonator against fouling by the deposition of falling droplets and other contaminants was set in the center of the resonator so that the small droplets of the stock solution or sol would fall down through the center of the quartz tube 16. Thirdly, a microwave power source commercially available for use in dielectric heating which had a frequency spectrum width of several megahertz was adapted to be capable of fine tuning over 1 MHz. In addition to these three alterations, the present inventors proposed that the resonator be positioned to create a vertical electric field as shown in FIG. 2b in contrast with the layout shown in FIG. 2a adopted by Ledergerber to create a horizontal electric field. Referring to the block diagram shown in FIG. 1, circulator A designated by 2 is for protecting the magnetron oscillator 1, whereas circulator B designated by 6 is provided to insure isolation of the resonator 8 from the stub tuner 5. The stub tuner 5 is provided for varying the incident energy to be launched into the resonator 8. Directional coupler A designated by 3 is provided so that the energy supplied from the magnetron oscillator 1 and the energy reflected by the stub tuner are read on the incident and reflected wave power meter 4, thereby indicating the effective energy that will be supplied to the resonator 8, Directional coupler B designated by 7 is provided chiefly for the purpose of matching the resonator 8 by monitoring with the spectrum analyzer 9 the frequency spectrum of the microwave being reflected from the resonator 8. Matching of the resonator 8 is roughly accomplished by combining the width of the iris provided at the entrance to the resonator with the length of the resonator. However, considering that the resonant frequency of the system varies with the temperature of the resonator, the position of the quartz tube and the presence or absence of small droplets, the quartz rod 14 for fine tuning of the resonant state is moved up and down by means of the drive mechanism 15 with the spectrum of the reflected microwaves from the resonator being monitored with the spectrum analyzer, so that the resonant frequency will lie within the frequency spectrum of the incident microwaves (defined by minimum value f.sub.min and maximum value f.sub.max) as shown in FIG. 3. In addition, the center of the resonant frequency for the case where there are no small droplets in the system (f.sub.0 in FIG. 3b) differs from the value for the case where such small droplets are present in the system (f.sub.1 in FIG. 3c) and the latter is lower than the former by about 0.7 MHz. Therefore, in the warmup process of resonating the system before starting to have the small droplets fall down, the quartz rod 14 for fine tuning of the resonant state must be manipulated to insure that f.sub.0 is positioned in such a way that both f.sub.0 and f.sub.1 will lie between f.sub.min and f.sub.max. Such fine tuning of the resonant state may be obviated by using microwaves having a wide frequency spectrum. However, when microwaves having a wide spectrum are to be used, the heating efficiency will deteriorate in proportion to the spectrum width and it becomes necessary to use a microwave power source having an even higher output or frequency. In the present invention, a commercial power source was adapted for achieving variations in the frequency spectrum width with a view to achieving an improvement in heating efficiency. Discharge may occur within the resonator 8 for certain reasons such as the deviation of the path in which the small droplets will fall down, whereby the droplets are deposited on the inner surface of the quartz tube within the resonator. The discharge detector/interlocking signal transmitter 10 is provided for the purpose of detecting the occurrence of such discharge by sensing the increase in the reflected power toward directional coupler B on account of the departure from the resonant state and then transmitting a signal for interlocking the power supply unit so that the stock solution or sol will not be supplied any more. As FIG. 2a shows, the resonator which is so positioned as to create a horizontal electric field (according to the Ledergerber process) produces a field intensity distribution where the intensity is zero at top and bottom ends of the resonator and maximal in the center. The velocity of small droplets falling down through the resonator is accelerated by gravity and the time over which they pass through the Position of great field intensity is comparatively short. In contrast, the resonator shown in FIG. 2b which is one characteristic portion of the invention and which is so positioned as to create a vertical electric field produces a field intensity distribution where the intensity is maximal at any position in the central portion of the resonator through which the small droplets pass. Hence, efficient heating is insured even at the top end of the resonator where the velocity of falling small droplets is minimal and the resonator is capable of comparable heating to the type shown in FIG. 2a, although it is shorter in the vertical direction. The particles that have gelled upon dielectric heating are typically recovered into an aqueous fluid but the gel particles, which appear solid, are very low in strength and prone to deform or break. In order to avoid such deformation or breakage, the gel particles should plunge into the recovery fluid at the smallest possible rate. In this respect, the resonator used in the present invention which is shorter in the vertical direction than the Ledergerber resonator is advantageous because it reduces the chance of the deformation or breakage of gel particles, whereby the scope of its application is expanded. The following examples are provided for the purpose of further illustrating the invention but are in no way to be taken as limiting. EXAMPLE 1 A copper resonator (200 mm.sup.H .times.62 mm.sup.W .times.27 mm.sup.D) was set up in such a way as to produce a horizontal electric field. A quartz tube (o.d., 10.5 mm; i.d., 7.5 mm) was placed in the center of the resonator. A microwave with an effective power of 3 kW (readings on an incident and reflected wave power meter: 5 kW for the incident wave; 2 kW for the reflected wave) was launched into the system. After the resonant frequency of the system was stabilized to lie within the incident spectrum width, the small particles of an aqueous stock solution at 2.degree. C. (having ammonium nitrate and urea each dissolved at a concentration of 1 mol per liter) that were formed 30 mm above the top end of the resonator were allowed to fall down through a nozzle (o.d., 0.7 mm), with one drop passing per second. The small droplets that passed through the resonator were at 82.degree. C., having experienced a temperature rise of 80K. EXAMPLE 2 The procedure of Example 1 was repeated, except that the effective power of the launched microwave was 3.2 kW (5.0 kW for the incident wave and 1.8 kW for the reflected wave). The small droplets that passed through the resonator were at 87.degree. C., having experienced a temperature rise of 85K. EXAMPLE 3 A microwave with an effective power of 2.7 kW (4.6 kW for the incident wave and 1.9 kW for the reflected wave) was launched into a resonator of the same type as used in Example 1. A stock solution at 2.degree. C. for the production of UO.sub.2 particles (consisting of U, nitrate ions, urea and HMTA at respective concentrations of 1.5 mol, 2.25 mol, 1.5 mol and 1.5 mol per liter) was added dropwise to the resonator from the position 35 mm above its top end through a nozzle (o.d., 0.7 mm). The droplets were passed through the resonator and recovered into pure water the surface level of which lied 55 mm below the bottom end of the resonator. The particles thus recovered had gelled in a satisfactory way. EXAMPLE 4 A copper resonator (55 mm.sup.H .times.74 mm.sup.W .times.110 mm.sup.D) was set up in such a way as to produce a vertical electric field. A quartz tube (o.d., 10.5 mm; i.d., 7.5 mm) was placed in the center of the resonator. A microwave with an effective power of 3 kW (readings on an incident and reflected wave power meter: 4 kW for the incident wave; 1 kW for the reflected wave) was launched into the system. After the resonant frequency of the system was stabilized to lie within the incident spectrum width, the small particles of an aqueous stock solution at 2.degree. C. (having ammonium nitrate and urea each dissolved at a concentration of 1 mol per liter) that were formed 30 mm above the top end of the resonator were allowed to fall down through a nozzle (o.d., 0.7 mm), with one drop passing per second. The small droplets that passed through the resonator were at 83.degree. C., having experienced a temperature rise of 81K. EXAMPLE 5 At the last stage of the experiment in Example 1, the nozzle was vibrated so that the path in which the small droplets were falling down would shift, causing the droplets to deposit on the inner surface of the central quartz tube. As a result, discharge occurred and the high-frequency power source and the power supply to the stock solution feeding/small droplet forming unit were both interlocked. The method of gelation by dielectric heating with microwaves was first applied by Ledergerber to the production of gel particles as a precursor of particulate ceramic fuels but in that method, communication frequencies of 8.2-12.4 GHz had to be used and this made it necessary to fabricate a special power source at high cost. According to the present invention, it has been demonstrated that a dielectric heating power source that operates at an engineering frequency of 2.45 GHz needs only small design changeovers to realize a method and apparatus that enable the temperature elevation of about 80K necessary to cause internal gelation of the small particles of a stock solution or sol. The present invention also uses a cavity resonator of a layout that produces a vertical electric field and this contributes a lot to the purpose of avoiding the deformation or breakage of gel particles. Additionally, the installation of a discharge detector/interlocking signal transmitter improves the safety of the overall system, thereby reducing much of the burden on the operating personnel of the gelling apparatus of the invention.