Apparatus for discharging molten matter from cold crucible induction melting furnace

Abstract: An apparatus for discharging molten matter from a cold crucible induction melting furnace provided with a furnace body disposed within a high-frequency coil for heating the furnace body. The molten matter discharging apparatus comprises a metallic discharging nozzle (14) extending downward from an inner bottom portion of the furnace body (10) and disposed in an electrically insulated state from the furnace body, a high-frequency coil (15) for heating the discharging nozzle (14) disposed around the nozzle, and an electric circuit (17) for removing a high-frequency noise generated from the high-frequency coil for heating the furnace body and disposed in the high-frequency coil (15) for heating the nozzle. This discharging apparatus has high reliability and high controllability, without inviting electric short-circuit between the furnace body and the nozzle and noise interference between the high-frequency coils.

Description: BACKGROUND OF THE INVENTION This invention relates to a molten matter discharging apparatus for efficiently and reliably discharging molten matter formed inside a furnace when substances to be melted such as metals are induction-heated by using a cold crucible induction melting furnace. A cold crucible induction melting furnace has a construction in which a slit-divided water-cooled type metallic cold crucible is disposed inside a water-cooled type high-frequency coil. When substances to be melted such. as metals are charged into this melting furnace and a high-frequency current is supplied to the high-frequency coil, the metals are induction-heated and are converted to a molten matter. In this instance, a floating force acts on the molten matter itself due to the operation of an electromagnetic field and the molten matter does not come into direct contact with a furnace body of the melting furnace. Therefore, this induction melting furnace has the features that materials having high melting points can be melted and erosion of the furnace body by the molten matter scarely occurs. Furthermore, because the furnace body itself is cooled with water, high temperature melting of the substances to be melted can be made without being limited by the heat-resistant temperature of the furnace body. For these reasons, the cold crucible induction melting furnace has been utilized at present for melting special metals in the iron and steel industry. On the other hand, a method for collectively and conveniently melting radioactive miscellaneous solid wastes including a variety of substances such as combustibles, metals, glass and other non-combustibles generated from nuclear facilities and the like by using such a cold crucible induction melting furnace has been proposed by the applicant of the present invention. (See U. S. Patent No. 5457264 corresponding to Japanese Patent Laid-open No. 7-63895/1995; hereinafter referred to as "prior art method".) In this prior art method, when the radioactive miscellaneous solid wastes are charged into the cold crucible induction melting furnace and a high-frequency current is supplied to the high-frequency coil, conductive substances contained in the miscellaneous solid wastes such as metals are first induction-heated and are melted. Due to the heat generated at this time, the remaining miscellaneous solid wastes having a low conductivity surrounding the metals are indirectly heated, too. In other words, the metals function as a starting source of heating and the miscellaneous solid wastes are entirely melted. By the prior art method, the molten metal does not come into direct contact with the furnace body because the floating force acts on the molten matter due to the operation of the electromagnetic field as described above. Also in the case of glass melting, the contact surface of the molten glass with the furnace body is cooled and is converted to a solid layer (skull layer), so that the direct contact of the high temperature molten glass with the furnace body does not occur. Thus, high temperature erosion of the furnace body does not occur, and high temperature melting of the substances to be melted becomes possible. In order to carry out a continuous melting operation by using the cold crucible induction melting furnace described above, the high temperature molten matter must be discharged from the furnace. Conventional methods of discharging the molten matter include a system which allows the molten matter to overflow from the furnace top by tilting the melting furnace itself, a system which allows the molten matter to flow down from an outflow port at the furnace bottom portion by pressurizing the inside of the furnace, and the like. However, the former system requires a moving structure for tilting the furnace body and the latter requires a gas-tight structure of the furnace body. On the other hand, a nozzle heating system (a freeze valve system) has been employed in the past for a glass melting furnace used in a vitrification technology of high-level radioactive wastes. This system has a construction wherein heating means is disposed around a discharging nozzle extending downward from the furnace bottom portion. Since the molten glass inside the nozzle is solidified under the state where the discharging nozzle is not heated, the molten glass inside the furnace does not flow down. To discharge the molten glass inside the furnace, the discharging nozzle is heated so as to melt the solidified glass inside the nozzle and allow it to flow down by the gravitational force, and at the same time, the molten glass inside the furnace can be discharged. As nozzle heating means in the nozzle heating system, there has been proposed high-frequency heating means wherein a high-frequency coil is disposed around a metallic discharging nozzle and a high-frequency current is supplied to this coil to heat the nozzle. However, when this nozzle heating system is adopted as a molten matter discharging apparatus in a cold crucible induction melting furnace and the nozzle is heated by high-frequency heating, there remain the problems that the metallic furnace body and the metallic discharging nozzle are electrically short-circuited and furthermore, noise interference occurs between the high-frequency heating system for heating the furnace body and the high-frequency heating system for heating the discharging nozzle. GB 2,279,543 A discloses a cold crucible induction melting furnace having at least two induction coils. One coil surrounds the discharging nozzle and another coil surrounds the central base section of the crucible. The nozzle is made as a unitary structure with the furnace and has insulating slits. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a molten matter discharging apparatus used in a cold crucible induction melting furnace which employs a nozzle heating system as molten matter discharging means, prevents electric short-circuit between a metallic furnace body and a metallic discharging nozzle when the nozzle is heated by high-frequency heating, eliminates the occurrence of noise interference between a high-frequency heating system for heating the furnace body and a high-frequency heating system for heating the discharging nozzle, and has high reliability and high controllability. According to the present invention, there is provided an apparatus according to claim 1 for discharging molten matter from a water-cooled type cold crucible induction melting furnace provided with a furnace body disposed within a high-frequency coil for heating the furnace body. The apparatus comprises a furnace body; a discharging nozzle made of a metal and extending downward from an inner bottom portion of the furnace body, the discharging nozzle being disposed in an "electrically insulated state" from the furnace body; a high-frequency coil for heating the discharging nozzle disposed around the nozzle; and an electric circuit for removing a high-frequency noise generated from the high-frequency coil for heating the furnace body, the electric circuit being disposed between the high-frequency coil for heating the nozzle and a power source. According to the present invention, the "electric insulation" between the furnace body and the discharging nozzle can be secured and therefore their electric short-circuit can be reliably prevented. Furthermore, noise interference applied to the high-frequency heating system for heating the discharging nozzle from the high-frequency heating system for heating the furnace body can be effectively prevented. As a result, the discharge/stop operations of the molten matter from the discharging nozzle can be reliably controlled without being affected by the high-frequency heating system for heating the furnace body. In the case where the discharging nozzle is formed as a unitary structure with the furnace body, the "electric insulation state" between the furnace body and the discharging nozzle can be established by forming insulating slits in the discharging nozzle portion. In the case where. the discharging nozzle is formed separately from the furnace body, their "electric insulation state" can be established by fixing the discharging nozzle to the furnace body through an electrically insulating material. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an explanatory view showing an embodiment of the present invention. Fig. 2 is a partial sectional perspective view showing an example of the construction for establishing the "electric insulation" between the discharging nozzle and the cold crucible furnace body when they are formed as a unitary structure. Figs. 3A and 3B are perspective views showing the furnace body and the discharging nozzle, respectively, when the discharging nozzle used in the present invention is formed separately from the cold crucible furnace body. Fig. 4 is an explanatory view showing the state where the furnace body shown in Fig. 3A and the discharging nozzle shown in Fig. 3B are assembled. Fig. 5 is a circuit diagram showing an example of the noise removing circuit used in the present invention. PREFERRED EMBODIMENTS OF THE INVENTION Fig. 1 shows a schematic structure of an apparatus for discharging molten matter from a cold crucible induction melting furnace according to an embodiment of the present invention. A slit-divided water-cooled type cold crucible 10 made of copper is disposed inside a water-cooled high-frequency coil 11 for heating a furnace body of the cold crucible in the same manner as in a conventional cold crucible induction melting furnace. The cold crucible 10 and the high-frequency coil 11 are cooled by circulating cooling water 12 inside them, and a high-frequency current having a predetermined frequency is supplied to the high-frequency coil 11 from a high-frequency power source 13 for heating the furnace body. The molten matter discharging apparatus according to the present Invention Includes a metallic discharging nozzle 14 extending downward from an inner bottom portion of a furnace body of the cold crucible 10, a high-frequency coil 15 for heating the nozzle which is disposed around the discharging nozzle, and a high-frequency power source 16 for heating the nozzle which supplies a high-frequency current having a predetermined frequency to the high-frequency coil 15. The present invention is specifically constituted so that 'the furnace body of the cold crucible 10 and the discharging nozzle 14 establish an "electric insulation state", and a noise removing circuit 17 for removing the high-frequency noise generated from the high-frequency coil 11 for heating the furnace body is interposed between the high-frequency coil 15 for heating the nozzle and the high-frequency power source 16 for heating the nozzle. A method of melting radioactive miscellaneous solid wastes and a method of discharging molten matter by using the cold crucible induction melting furnace equipped with the molten matter discharging apparatus described above will be explained hereinbelow. After radioactive miscellaneous solid wastes 18 are charged into the cold crucible 10, the high-frequency current is supplied from the high-frequency power source 13 for heating the furnace body to the high-frequency coil 11 for heating the furnace body. Thus, electrically conductive substances such as metals contained in the miscellaneous solid wastes are first heated and melted by induction heating. The remaining miscellaneous solid wastes having a low conductivity such as glass are also heated indirectly, so that the wastes change to molten matter 19 as a whole. During this melting operation, the high-frequency current is not supplied to the high-frequency coil 15 for heating the nozzle and the discharging nozzle 14 is not heated. Consequently, the molten matter remaining inside the nozzle becomes a solidified state and clog the nozzle, so that the molten matter 19 inside the furnace do not flow down. In order to discharge the molten matter 19, the high-frequency current is supplied from the high-frequency power source 16 for heating the nozzle to the high-frequency coil 15 for heating the nozzle and the nozzle 14 is heated by high-frequency heating. Since the furnace body of the cold crucible 10 and the discharging nozzle 14 are electrically insulated from each other, they are not electrically short-circuited and can be heated by high-frequency heating separately from each other. Further, noise interference from the high-frequency heating system for heating the furnace body to the high-frequency heating system for heating the nozzle can be effectively prevented by the noise removing circuit 17. When the discharging nozzle 14 is heated by high-frequency heating, the molten matter in the solidified state which remains inside the nozzle is melted and is brought into the fluidized state, and then flows down from the nozzle due to the gravitational force. With this flow-down of the molten matter inside the nozzle, the high temperature molten matter inside the furnace also flows down. The molten matter 19 is poured into a canister or a stainless steel vessel (not shown) so as to be cooled and solidified to become a solidified waste matter 20. The supply of the high-frequency current from the high-frequency power source 16 for heating the nozzle to the high-frequency coil 15 is stopped when discharging of the molten natter 19 inside the cold crucible 10 is completed, and high-frequency heating of the discharging nozzle is stopped. As a result, the discharging nozzle is cooled gradually, and the molten matter remaining inside the nozzle is solidified and close the nozzle. In the case where it is desired to quickly cool the nozzle and to quickly accomplish closing of the nozzle after the stop of high-frequency heating of the nozzle; an air cooling pipe (not shown) for positively cooling the nozzle is preferably wound around the outside of the high-frequency coil 15 for heating the nozzle. Figs. 2, 3 and 4 show structural examples for securing the "electric insulation state" between the furnace body of the cold crucible 10 and the discharging nozzle 14. Fig. 2 shows an example of the case where the furnace body of the cold crucible 10 and the discharging nozzle 14 are formed into a unitary structure. The furnace body of the cold crucible 10 is divided by a large number of insulating slits 21 and cooling water is allowed to flow inside each divided structure so as to cool the furnace body. The portion which is to serve as the discharging nozzle 14 extends downward from the furnace inner bottom portion in the integral form with the furnace body, and the slits 21 are also formed in the discharging nozzle portion 14 so as to communicate with the slits in the furnace body portion. An insulating material such as silicon nitride (Si 3 N 4 ) is ordinarily inserted into these insulating slits 21. However, since the current supplied to the high-frequency coil 15 for heating the nozzle has a frequency lower than that of the current supplied to the high-frequency coil 11 for heating the furnace body and thus the insulating slits 21 at the portion of the discharging nozzle may be air insulation, the insulating material need not always be inserted. The high-frequency coil 11 for heating the furnace body and the high-frequency coil 15 for heating the nozzle are wound around the outer periphery of the furnace body of the cold crucible 10 and the discharging nozzle 14 so formed, respectively. Fig. 3 shows a structural example for securing the "electric insulation" between the furnace body of the cold crucible 10 and the discharging nozzle 14 when they are formed as separate members. Fig. 3A shows the furnace body of the cold crucible 10. This furnace body is divided by a large number of insulating slits 21, an insulating material is inserted into each of the insulating slits 21, and cooling water is allowed to flow inside each divided structure so as to cool the furnace body. This construction is the same as the construction shown in Fig. 2 but is different from the latter in that a portion 22 protruding inward is formed in the vicinity of the position serving as the bottom portion of the furnace body. Fig. 3B shows the discharging nozzle members. A flange portion 14a having an increased diameter is formed at the upper end of the cylindrical discharging nozzle 14 and a ring-shaped insulating material 23 is fixed to the upper portion of the discharging nozzle 14 inclusive of the flange portion. The lower periphery of the ring-shaped insulating material 23 is chamfered to define a taper surface 23a. Fig. 4 shows an assembled state of the furnace body shown in Fig. 3A and the discharging nozzle members shown in Fig. 3B. The taper surface 23a is supported by the taper surface 22a of the inward protruding portion 22 of the furnace body and the top face of the ring-shaped insulating material 23 serves as the inner surface of the furnace bottom. Because the ring-shaped insulating material comes into direct contact with the high temperature molten matter, silicon. nitride having excellent high temperature erosion resistance can be used preferably. The noise removing circuit 17 will be explained hereinbelow. The phenomenon in which a high-frequency magnetic field affects other electric circuits and imparts troubles to the circuits and components is referred to as "noise trouble", and the influences of the noise trouble generally become greater with a higher output and a higher frequency. In the present invention, the influences of the furnace body heating system having a high-frequency of 4 MHz, for example, on the nozzle heating system having a relatively low frequency of about 20 kHz, for example, must be removed. Therefore, the present invention interposes an ordinary LC circuit comprising the combination of suitable inductances L 1 and L 2 and capacitances C, C 1 and C 2 between the high-frequency coil 15 for heating the nozzle and the high-frequency power source 16 for heating the nozzle as shown in Fig. 5 so as to suppress the noise trouble from the furnace body heating system. Incidentally, the capacitances of L and C in the circuit can be set appropriately in consideration of the frequency of the high-frequency, the electric resistances of the substances to be melted, and the like. Although the embodiments given above have been explained about the example using the radioactive miscellaneous solid wastes as the substances to be melted, any materials or substances can be used as an objective article to be melted so long as they can be melted by high-frequency heating such as metals, glass and the like. Hereinbelow, the present invention will be explained with reference to an experimental example thereof. The cold crucible induction melting furnace used had the same construction as the apparatus shown in Fig. 1. A furnace body made of copper and having an inner diameter of 100 mm and a depth of 150 mm was divided into ten segments. A high-frequency coil for the furnace body had an outer diameter of 170 mm, a height of 100 mm and the number of turns of seven. A discharging nozzle was formed separately from the furnace body as shown in Figs. 3A and 3B and they were assembled when used, as shown in Fig. 4. The discharging nozzle was made of a nickel alloy (Inconel 690) and had a pore diameter of 25 mm and a length of 260 mm. A high-frequency coil for nozzle had an inner diameter of 45 mm, a length of 280 mm and the number of turns of 15. An air cooling pipe for cooling the discharging nozzle, made of copper, was wound around the outside of the high-frequency coil for nozzle. An LC circuit was disposed as a noise removing circuit upstream of a high-frequency power source for the nozzle, as shown in Fig. 5, wherein both of L 1 and L 2 were 2.2 H while both of C 1 and C 2 were 103 pF. About 1,000 g of borosilicate glass beads were charged as a substance to be melted into the furnace and were melted by supplying a high-frequency current having an output of 50 kW and a frequency of 4 MHz from a high-frequency power source for the furnace body to the high-frequency coil for the furnace body. The melting temperature was about 1,300°C. To discharge molten glass inside the furnace, a high-frequency current having an output of 10 kW and a frequency of 20 kHz was supplied to the high-frequency coil for nozzle from the high-frequency power source for nozzle and the discharging nozzle was heated to about 1,000°C. In consequence, the full amount of about 1,000 g of the glass molten matter inside the furnace could be allowed to flow down within about 1.5 minutes. The noise from the high-frequency heating system for the furnace body could be suppressed effectively without inviting noise troubles, such as abnormal oscillation or troubles in the oscillation circuit, in the high-frequency heating system for the nozzle. As can be understood from the foregoing, the molten matter discharging apparatus according to the present invention includes the discharging nozzle extending downward from the inner bottom of the cold crucible induction melting furnace, and heats this discharging nozzle by high-frequency heating in order to cause the molten matter in the furnace to flow down or to stop its flow-down. Therefore, the present invention eliminates the necessity for a moving structure which is required for the furnace body tilting system in the conventional molten matter discharging apparatus and the gas-tight structure required for the furnace pressurization system, and can efficiently discharge the high temperature molten matter and can therefore attain the continuous melting operation. Even when the metallic furnace body of the cold crucible and the metallic discharging nozzle are heated by high-frequency heating using the high-frequency currents having mutually different frequencies, the present invention can prevent their short-circuit by securing the "electric insulation" between the furnace body and the nozzle. Furthermore, because the present invention can effectively suppress noise interference from the high-frequency heating system for the furnace body to the high-frequency heating system for the nozzle by using the noise removing circuit, it becomes possible to discharge the molten matter with high reliability and high controllability.

Claims: An apparatus for discharging molten matter from a water-cooled type cold crucible induction melting furnace (10) provided with a furnace body disposed within a high-frequency coil (11) for heating said furnace body, said apparatus comprising a furnace body (10); a discharging nozzle (14) made of a metal and extending downward from an inner bottom portion of said furnace body (10), said discharging nozzle either (a) being formed as a unitary structure with the furnace body and having insulating slits or (b) being formed separately from the furnace body and being fixed to the furnace body through an electrically insulating material; a high frequency coil (15) for heating said discharging nozzle disposed around said nozzle; and an electric circuit (17) for removing a high-frequency noise generated from said high-frequency coil (11) for heating said furnace body, said electric circuit being disposed between said high-frequency coil (15) for heating said nozzle and a power source (16). The apparatus for discharging molten matter according to claim 1, wherein said discharging nozzle (14) is formed as a unitary structure with said furnace body, and the electric insulation state from said furnace body is established by forming insulating slits (21) in said discharging nozzle portion (14). The apparatus for discharging molten matter according to claim 1, wherein said discharging nozzle is formed separately from said furnace body, and the electric insulation state from said furnace body is established by providing an electrically insulating material and fixing said discharging nozzle (14) to said furnace body (10) throug said electrically insulating material.

Inventors: KOBAYASHI HIROAKI, KOBAYASHI, HIROAKI
Applicants: JAPAN NUCLEAR CYCLE DEV INST, JAPAN NUCLEAR CYCLE DEVELOPMENT INSTITUTE
Application year: 1997
Main classification: F27B14
IPC codes: F27B14, H05B6, G21F9