Method for melting incineration residue and apparatus therefor

Abstract: A method for melting an incineration residue comprises the steps of: charging an incineration residue containing salts into a melting furnace; heating the charged incineration residue in the melting furnace to produce a melt comprising a molten slag and a molten salt; controlling a temperature of the molten salt; discharging the molten slag and the molten salt from the melting furnace. An apparatus comprises: a melting furnace main body; charge means for charging the incineration residue in the melting furnace main body; heating means for melting the charged incineration residue to produce a melt comprising a molten slag and a molten salt; control means for controlling a temperature of the molten salt; slag discharge means; and salt discharge means.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for melting incineration residues containing salts, such as an incineration residue from municipal refuse, and an apparatus therefor. 2. Description of the Related Arts A major portion of incineration residue which forms during incineration from municipal refuse and industrial waste has been disposed in landfills, and volume reduction of the residue has been required as reclaimed grounds can be secured only with great difficulties. Since ash (fly ash) in the incineration residue, which is scattered with exhaust gas after combustion and collected by a dust collector, contains heavy metals, such as lead and cadmium, and is designated as general waste under special control, it must be subjected to innocuous disposal before dumping. Therefore, both volume reduction of the incineration residue in the incinerator and innocuous disposal of scattered ash are required. Disposal methods for melting and solidifying the incineration residue have been developed as a technology for simultaneously achieving volume reduction of the incineration residue and innocuous disposal of heavy metals by insolubilization in order to overcome the above-mentioned problems. JP-A-59-132931 discloses a method for melting and solidifying including continuous feeding and melting of the incineration residue while holding the melt of the incineration residue. This method is capable of separating the melt components held in the furnace into a molten slag layer and a molten salt layer by the differences in specific gravity, and thus the melt can be fractionally exhausted as molten slag and molten salt. Those features of the present invention which are disclosed in JP-A-59-132931 are indicated in the preambles of independent claims 1 and 16. In the above-mentioned method, the molten salt is always present in the furnace, and large amounts of low-boiling-point materials, i.e. alkaline chlorides, such as NaCl and KCl, which are contained in the molten salt, are volatilized. The volatilized alkaline metal salts condense in the components of the exhausting system, result in clogging the exhausting system, corroding the components, and inhibit stable operation of the melting furnace. A melting-disposal method for solving such problems is disclosed in Japanese Unexamined Patent Publication No. 7-42,924 in which a melting furnace suppressing volatilization of low-boiling-point material is used. This method relates to an electrical-resistance heating-type melting furnace, in which electrodes immersed into the melt of the incineration residue are energized to heat the melt by the electric resistance heat and melt the incineration residue which is continuously fed. Figure 15 is a longitudinal cross-sectional view of such an electrical-resistance heating-type melting furnace. Reference numeral 30 represents a furnace main body, reference numeral 31 represents electrodes, reference numeral 50 represents a fed incineration residue, reference numeral 52 represents a molten slag layer formed by melting the incineration residue, and reference numeral 51 represents a molten salt layer formed by the melting of salts in the incineration residue. Also, reference numeral 33 represents an outlet of the exhaust gas and reference numeral 34 is an outlet of the melt. In this melting furnace, the molten salt layer 51 inside a partition wall 35 provided between the roof and the melting zone in the furnace main body is covered by the fed incineration residue 50, and the molten salt layer 51 outside the partition wall 35 is covered with a powdery material having a lower specific gravity than that of the molten salt, such as carbon, to suppress volatilization of the low-boiling-point materials from the molten salt layer. The incineration residue containing salts is a mixture of oxide components having melting points ranging from 1,400 °C to 1,600 °C and salt components having melting points ranging from 700 to 800 °C, whereas the incineration residue is heated to a high temperature region higher than the melting points of the oxide components during the furnace operation, such that all the components are melted. Therefore, salts having low melting points are heated to an excessively high temperature. Because the melting points of these salts range from 1,400 °C to 1,600 °C and their vapor pressures markedly increase at near 1,100 °C, the low-boiling-point materials are extremely volatile in the furnace. In this regard, the prior art method cannot suppress the vapor pressure of the molten salt as a source of the low-boiling-point materials regardless of the use of a remedy in which the molten salt and the incineration residue are covered with a powdery material such as carbon, because such a remedy has merely an intention of collecting the molten salt with the incineration residue and carbon. Therefore, such a method is not effective against suppression of volatilization of the low-boiling-point materials. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for melting incineration residues containing salts and an apparatus therefor which permits a drastic reduction of the volatilization of low-boiling-point materials. To attain the object, the present invention provides a method for melting an incineration residue according to claim 1. The heating of the charged incineration residue is preferably carried out by the following methods: (a) heating the molten slag by applying a voltage between a plurality of electrodes which are submerged in the molten slag having an electric resistance to generate a heat by the electric resistance of the molten slag, (b) heating the molten slag by applying a high frequency electric current to an induction-heating coil which is arranged on the periphery of the melting furnace, or (c) heating the molten slag by applying a voltage between a movable electrode which is immersed in the molten slag and a bottom electrode which is arranged at the bottom of the melting furnace. In the step of controlling the temperature of the molten salt, it is preferable to control the temperature within a range of 700 to 1000 °C. When the temperature of the molten salt is over 1000 °C, the molten salt volatilizes. When the temperature of the molten salt is below 700 °C, the melt may be solidified. Since the molten salt is heated by the heat transferred from the molten slag, it is desirable to lower the temperature of the molten salt. Whereas, since the surface of the molten salt layer is easily cooled, it is preferable to heat the surface of the molten salt layer. The following methods are preferable to lower the temperature of the molten salt: (A) decreasing a heat transfer from the molten slag to the molten salt, (B) increasing heat radiation from the molten salt, or (C) forcedly cooling the molten salt. There are two manners in the method (A) for decreasing the heat transfer. In the first manner, the incineration residue is charged into the molten salt layer to form a mixed layer of the incineration residue and the molten salt in the bottom of the molten salt layer. In the second manner, a cross-sectional area of a molten salt hold-up section is reduced to be smaller than that of a molten slag hold-up section. Further, the present invention provides an apparatus for melting an incineration residue according to claim 16. The heating means for melting the charged incineration residue is preferably constituted according to one of the following three manners: (a) The heating means comprises a plurality electrodes which are submerged in the molten slag, a voltage being applied between the plurality of electrodes; (b) the heating means comprises an induction-heating coil which is arranged on the periphery of the molten slag hold-up section, the molten slag being heated by applying a high frequency electric current to the induction-heating coil, or (c) the heating means comprises a movable electrode which is submerged in the molten slag and a bottom electrode which is arranged at the bottom of the melting furnace main body, the molten slag being heated by applying a voltage between the movable electrode and the bottom electrode. The control means preferably controls the temperature of the molten salt within a range of 700 to 1000 °C. Since the molten salt is heated by the heat transferred from the molten slag, it is desirable to lower the temperature of the molten salt. Since the surface of the molten salt layer is easily cooled, it is preferable to heat the surface of the molten salt layer. The following means are preferable to lower the temperature of the molten salt: (A) means for decreasing a heat transfer from the molten slag to the molten salt, (B) means for increasing heat radiation from the molten salt, or (C) means for forcedly cooling the molten salt. There are two manners in said case (A) for decreasing the heat transfer. In the first manner, the incineration residue is charged into the molten salt layer to form a mixed layer of the incineration residue and the molten salt in the bottom of the molten salt layer. In the second manner, a cross-sectional area of a molten salt hold-up section is reduced to be smaller than that of a molten slag hold-up section. The charge means comprises an incineration residue feeding pipe arranged at the upper part of the melting furnace main body. The lower end of the incineration residue feeding pipe is positioned below the surface of the molten salt. In the present invention, the incineration residue containing salts refers to both dust-collection ash (fly ash) formed during incineration from municipal refuse and a mixture of several types of dust-collection ashes. Further preferred embodiments of the invention are indicated in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view illustrating an embodiment of an apparatus for melting an incineration residue in accordance with the present invention. FIG. 2 is a cross-sectional view taken from the line A-A in FIG. 1. FIG.3 is a plan view illustrating another embodiment of an apparatus in accordance with the present invention. FIG. 4 is a cross-sectional view taken from the line B-B in FIG. 3. FIG. 5 is a plan view illustrating an embodiment of an apparatus in accordance with the present invention. FIG. 6 is a cross-sectional view taken from the line A-A in FIG. 5. FIG. 7 is a plan view illustrating another embodiment of an apparatus in accordance with the present invention. FIG. 8 is a cross-sectional view taken from the line B-B in FIG. 7. FIG. 9 is a cross-sectional view illustrating a different interface level between a molten salt layer and a molten slag layer. FIG. 10 is a plan view illustrating an embodiment of an apparatus in accordance with the present invention. FIG. 11 is a cross-sectional view taken from the line A-A in FIG. 10. FIG. 12 is a plan view illustrating another embodiment of an apparatus in accordance with the present invention. FIG. 13 is a cross-sectional view taken from the line B-B in FIG. 12. FIG. 14 is a cross-sectional view illustrating a different interface level between a molten salt layer and a molten slag layer. FIG. 15 is an outlined longitudinal cross-sectional view of a conventional electric-resistance-heating-type melting furnace. DESCRIPTION OF THE EMBODIMENT An essential feature in the present invention is to reduce the vapor pressure of the low-boiling-point materials by controlling and thereby decreasing the temperature of the molten salt layer and to suppress volatilization of the low-boiling-point materials thereby. In the melting-disposal method in accordance with the present invention, the temperature of the molten salt layer is controlled to be within a specified range. Because the low-boiling-point materials contained in the incineration residue essentially consist of alkaline metal chlorides, such as NaCl and KCl, and vigorously volatilize at a temperature above near 1,100 °C as described above, the temperature of the molten salt layer must be controlled to be 1,000 °C or less to suppress volatilization. However, the melt may be solidified at a temperature of the molten salt layer of 700 °C or less. Therefore, it is preferable that the temperature of the molten salt layer be within a range of 700 °C to 1,000 °C. The molten salt layer formed by melting the incineration residue is heated by means of heat transfer from the molten slag layer thereunder. Methods of control for preventing a rise in the temperature of the molten salt layer include decreasing heat transfer from the molten slag layer, increasing heat dissipation from the molten salt layer, or forced cooling of the molten salt layer. Therefore, the melting furnace for disposing the incineration residue containing salts has a structure which can control the molten salt layer within an adequate temperature range. One means for decreasing the temperature of the molten salt layer in the present invention is a molten salt layer cooling means for forcibly cooling the molten salt layer which is provided on the periphery of the salt holdup section. There are some methods for decreasing heat transfer from the molten slag layer to the molten salt layer, and one of these is feeding of the incineration residue into the molten salt layer. Therefore, in the melting furnace in accordance with the present invention, the bottom end of the incineration residue feeding pipe is located below the upper surface of the molten salt layer so that the lower portion of the incineration residue feeding pipe is immersed into the molten salt layer. When the incineration residue is fed in such a state it gradually settles and is dispersed into the molten salt layer, a layer is formed, in which both the incineration residue and the molten salt are present, in the bottom section of the molten salt layer (hereinafter referred to as a mixed layer). Since the mixed layer is a suspension or slurry of incineration residue particles and contains a solid component having a low thermal conductivity, the thermal conductivity of the mixed layer is lower than that of the single molten salt material. Thus, the mixed layer inhibits heat transfer and can suppress an excessive temperature rise of the molten salt layer. Another method for decreasing the heat transfer from the molten slag is an area reduction of the interface between the molten salt layer and the molten slag layer. In this regard, the upper section of the furnace in accordance with the present invention is narrowed to hold the molten salt, whereas the molten slag is held in the lower section of the furnace which is not narrowed. Thus, the area of the interface between both layers is reduced, heat transfer from the molten slag layer to the molten salt layer is reduced, and an excessive temperature rise of the molten salt layer can be suppressed. Since the furnace wall has a large area relative to the volume of the molten salt held in the upper narrow section, heat dissipation from the molten salt layer is promoted to suppress an excessive temperature rise of the molten salt layer. Since the periphery of the furnace has a large area relative to the volume of the molten salt held in the upper narrow section and a small amount of molten salt forms a thick molten salt layer, the molten salt layer can be readily cooled by a forced cooling means provided on the periphery of the furnace. FIG. 1 is a plan view illustrating an embodiment of a melting furnace in accordance with the present invention, and FIG. 2 is a cross-sectional view taken from the line A-A in FIG. 1. The melting furnace is of an electric resistance heating type and includes a melting furnace main body 10, a plurality of electrodes 13, and a plurality of incineration residue feeding pipes 15. In the drawings, reference numeral 50 represents an incineration residue containing salts, reference numeral 51 represents a molten salt layer, and reference numeral 52 represents a molten slag layer. The melting furnace main body 10 is divided into a section for holding the molten slag and a section for holding the molten salt, the lower section of the melting furnace main body 10 is a molten slag holding section 11, and the upper section is a molten salt holding section 12. Also the melting furnace main body 10 is provided with a plurality of electrode immersion sections 14 having feeding ports of electrodes therein. The slag holding section 11 holds the molten slag, heats the molten slag layer 52 through the electrodes 13 immersed therein, and melts the incineration residue fed from the incineration residue feeding pipes 15 by heat of the molten slag layer 52 maintained at a high temperature. The salt holding section 12 is provided with three incineration residue feeding pipes 15 which pass through the furnace lid of the salt holding section 12 and are immersed into the molten salt layer 51 such that these lower ends are located below the upper surface of the molten salt layer 51. Therefore, the incineration residue is fed into the molten salt layer 51. The shape of the incineration residue feeding pipe 15 is not limited to be cylindrical, and may be prismatic or have some other shape as long as the incineration residue is fed into the molten salt layer 51. The salt holding section 12 is provided with a cooling unit 16 on its periphery. The cooling unit 16 forcedly cools the molten salt layer 51 and has a structure in which a water-cooling pipe is embedded in a refractory material. The water-cooling pipe is provided over a region from near the interface between the molten salt layer 51 and the molten slag layer 52 to near the surface of the molten salt layer 51. Since the surface of the molten salt layer 51 is easily cooled, the molten salt may not be readily discharged due to decreased flowability as a result of excessively decreased temperature. Therefore, it is preferable that a heating unit such as an induction-heating coil be provided on the periphery of the salt discharge port 17 to heat the surface area according to demand. The operation of the melting furnace for melting the incineration residue containing salt is as follows: A weighed incineration residue is fed into the incineration residue feeding pipes 15, which are immersed into the molten salt layer 51, and packed in the lower section of the feeding pipes. The incineration residue gradually sinks by its own weight in the incineration residue feeding pipes 15 while being preheated and is fed into the molten salt layer 51 formed in the salt holdup section 12. The incineration residue 50 in the molten salt layer 51 is gradually settled and dispersed to form a mixed layer 51a in which the incineration residue is suspended. As described above, the mixed layer 51a inhibits heat transfer from the molten slag layer 52 to the molten salt layer 51 and suppresses an excessive temperature rise of the molten salt layer 51. A part of the incineration residue 50 fed into the molten salt layer 51 melts in the molten salt layer 51, and the rest of it sinks to the interface with the molten slag layer 51 maintained at a high temperature and melts there. The melt of the incineration residue is separated into three layers, i.e., the molten salt layer 51, the molten slag layer 52 and the molten metal layer 53, because of specific gravity difference. The molten slag layer 52 is heated in the slag holding section 11 by means of electric resistance heat of the molten slag itself formed by energizing between electrodes 13. The temperature of the molten slag layer 52 is controlled to a range of 1,400 °C to 1,600 °C by adjusting the input power. On the other hand, the temperature of the molten salt layer 51 formed in the salt holding section 12 is controlled within an adequate range with the cooling unit 16 based on the observed temperature with a thermometer 22 provided in the central section of the molten salt layer 51. The flow rate of the cooling water in the cooling unit 16 is adjusted so that the temperature of the central section of the molten salt layer 51 measured by the thermometer 22 is in a range of 700 °C to 1,000 °C in order to prevent volatilization of the low-boiling-point materials. These melts are separately discharged from the furnace. The molten salt is discharged through the salt discharge port 17 provided on the upper side wall of the salt holding section 12 (above the bottom end of the incineration residue feeding pipes 15). The molten salt does not substantially contain the solid component such as the unmelted incineration residue and exhibits high flowability. Therefore, the molten salt can be discharged at a low flow rate from the salt discharge port 17, and an overflow system can be applied to the discharge. The reason that the molten salt is discharged in a desirable state is as follows. Mixing of the unmelted incineration residue into the molten salt which is discharged from the salt discharge port 17 does not occur because the incineration residue feeding pipes 15 are immersed into the molten salt layer 51, and the salt discharge port 17 is placed below the bottom end of the incineration residue feeding pipes 15 and above the mixed layer 51a containing the suspended incineration residue. If the incineration residue feeding pipes 15 are not immersed into the molten salt layer 51 and the incineration residue is fed above the molten salt layer 51, the incineration residue is suspended in the entire molten salt layer and the molten salt containing the incineration residue is discharged. Thus, the molten salt has significantly high viscosity and thus low flowability, resulting in difficulty in discharge of the molten metal itself. The molten slag is continuously or intermittently discharged through a slag discharge port 18 provided on the upper side wall of the slag holding section 11. Since the furnace main body 10 is divided into the slag holding section 11 and the salt holding section 12, the level of the interface between the molten salt layer 51 and the slag layer 52 must be located below the salt holding section 12, i.e., near the bottom of the cooling unit 16, and must be maintained constant in order to achieve successful discharge. Therefore, the molten slag must be discharged while monitoring the level of the interface between the molten salt layer 51 and the molten slag layer 52. Since the temperature along the vertical direction of the molten salt layer 51 increases toward the interface with the molten slag layer 52, the level of the interface can be monitored by continuously observing the temperature near the interface. The discharge volume of the molten slag is controlled so that the temperature observed with the thermometer 22 provided at the lower portion of the molten salt layer 51 is maintained at a given value. The interface between the molten salt layer 51 and the molten slag layer 52 can, therefore, be maintained to a given level. The molten metal is intermittently discharged through a metal discharge port 19 provided on the lower side wall of the slag holdup section 11. Gas formed during melting is discharged through an exhaust gas discharge port 20 and disposed. Figure 3 is a plan view illustrating another embodiment of a melting furnace in accordance with the present invention, and Figure 4 is a cross-sectional view taken from the line B-B in Figure 3. In Figures 3 and 4, parts having the same function as in Figures 1 and 2 are referred to with the same reference signs without further description. In this embodiment, the salt holding section 12 has a significantly smaller inner diameter (a smaller cross-section area of the furnace) compared to the inner diameter of the slag holding section 11 and is located on the central portion of the slag holding section 11. From the side view, the salt holding section 12 is protrudent relative to the slag holding section 11. The electrode immersion sections 14 is provided near the salt holding section. Therefore, the salt holding section 12 and the electrode immersion sections 14 are protrudent on the slag holding section 11. The incineration residue feeding pipes 15 are provided in the center of the salt holding section 12. During the operation of this melting furnace, the interface between the molten salt layer 51 and the molten slag layer 52 is maintained in the salt holding section 12 having a decreased inner diameter. The decreased diameter of the salt holding section 12 results in further suppression of temperature rise in the molten salt layer due to reduced heat transfer from the molten slag layer 52 to the molten salt layer 51 and increased heat radiation through the furnace wall. Therefore, cooling energy for the molten salt layer and the load of the cooling unit 16 can be significantly decreased. In some small furnaces, no cooling unit 16 is required. According to the present invention, since the temperature of the molten salt layer is controlled to be within a given range, the vapor pressure of the low-boiling-point materials can be decreased by lowering the temperature of the molten salt layer, and thus the volatilization of the low-boiling-point materials can be suppressed. Further, since the incineration residue is fed into the molten salt layer during the melting disposal, a mixed layer composed of the molten salt and the incineration residue is formed in the bottom section of the molten salt layer, and heat transfer from the molten slag layer to the molten salt layer can be prevented. Therefore, temperature rise of the molten salt layer and volatilization of the low-boiling-point materials can be suppressed. FIG. 5 is a plan view illustrating an embodiment of a melting furnace in accordance with the present invention, and FIG. 6 is a cross-sectional view taken from the line A-A in FIG. 5. The melting furnace is of the electromagnetic induction heating type and includes a melting furnace main body 110, an induction-heating coil 113, and an incineration residue feeding pipe 114. In the drawings, reference numeral 150 represents an incineration residue containing salts, reference numeral 151 represents a molten salt layer, and reference numeral 152 represents a molten slag layer. The melting furnace main body 110 has a double layer structure consisting of a graphite container 110a covered with a refractory material 110b. Also, the melting furnace main body 110 is divided into a section for holding the molten slag and a section for holding the molten salt, the lower section of the melting furnace main body 110 is a molten slag holding section 111, and the upper section is a molten salt holding section 112. The slag holding section 111 is provided with an induction-heating coil 113, heats the molten slag by applying a high frequency current flow to the induction-heating coil, and melts the incineration residue fed from the incineration residue feeding pipe 114 by heat of the molten slag layer 152 maintained at high temperature. The incineration residue feeding pipe 114 attached to the furnace top main body 110 passes through the furnace wall of the salt holding section 112 and is immersed into the molten salt layer 151 such that its bottom end is located below the upper surface of the molten salt layer 151. Therefore, the incineration residue is fed into the molten salt layer 151. The shape of the incineration residue feeding pipe 114 is not limited to be cylindrical, and may be prismatic or have some other shape as long as the incineration residue is fed into the molten salt layer 151. The salt holding section 112 is provided with a cooling unit 115 on its periphery. The cooling unit 115 forcedly cools the molten salt layer 151 and has a structure in which a water-cooling pipe is embedded in a refractory material. The water-cooling pipe is provided over a region from near the interface between the molten salt layer 151 and the molten slag layer 152 to near the surface of the molten salt layer 151. Since the surface of the molten salt layer 151 is easily cooled, the molten salt may not be readily discharged due to decreased flowability as a result of excessively decreased temperature. Therefore, it is preferable that a heating unit having an induction-heating coil be provided on the periphery of the salt discharge port 116 to heat the surface area according to demand. The operation of the melting furnace for melting the incineration residue containing salt is as follows: A weighed incineration residue is fed into the incineration residue feeding pipe 114, which is inserted into the molten salt layer 151, and packed in the lower section of the feeding pipes. The incineration residue gradually sinks by its own weight in the incineration residue feeding pipe 14 while being preheated and is fed into the molten salt layer 151 formed in the salt holding section 112. The incineration residue 150 in the molten salt layer 151 is gradually settled and dispersed to form a mixed layer 151a in which the incineration residue is suspended. As described above, the mixed layer 151a inhibits heat transfer from the molten slag layer 152 to the molten salt layer 151 and suppresses an excessive temperature rise of the molten salt layer 151. A part of the incineration residue 150 fed into the molten salt layer 151 melts in the molten salt layer 151, and the rest of it sinks to the interface with the molten slag layer 151 maintained at high temperature and melts there. The melt of the incineration residue is separated into three layers, i.e., the molten salt layer 151, the molten slag layer 152 and the molten metal layer 153, because of specific gravity difference. The slag holding section 111 is provided with the induction-heating coil 113 and the molten slag is heated by the heat generation of the container 110a under a high frequency current flow applied to the induction-heating coil 113. The temperature of the molten slag layer 152 is controlled to be within a range of 1,400 °C to 1,600 °C by adjusting the input power. On the other hand, the temperature of the molten salt layer 151 formed in the salt holding section 112 is controlled to be within an adequate range with the cooling unit 115 based on the temperature observed with a thermometer 121 provided in the central section of the molten salt layer 151. The flow rate of the cooling water in the cooling unit 115 is adjusted so that the temperature of the central section of the molten salt layer 151 observed by the thermometer 121 is in a range of 700 °C to 1,000 °C in order to prevent volatilization of the low-boiling-point materials. These melts are separately discharged from the furnace. The molten salt is discharged through the salt discharge port 116 provided on the upper side wall of the salt holding section 112 (above the bottom end of the incineration residue feeding pipe 114). The molten salt does not substantially contain a solid component such as the unmelted incineration residue and exhibits high flowability. Therefore, the molten salt can be discharged at a low flow rate from the salt discharge port 116, and an overflow system can be applied to the discharge. The reason that the molten salt is discharged in a desirable state is as follows. Mixing of the unmelted incineration residue into the molten salt which is discharged from the salt discharge port 116 does not occur because the incineration residue feeding pipe 114 is immersed into the molten salt layer 151, and the salt discharge port 116 is placed below the bottom end of the incineration residue feeding pipe 114 and above the mixed layer 151a containing the suspended incineration residue. If the incineration residue feeding pipe 114 is not immersed into the molten salt layer 151 and the incineration residue is fed above the molten salt layer 151, the incineration residue is suspended in the entire molten salt layer and the molten salt containing the incineration residue is discharged. Thus, the molten salt has significantly high viscosity and thus low flowability, resulting in difficulty in discharge of the molten metal. The molten slag is continuously or intermittently discharged through a slag discharge port 117 provided on the upper side wall of the slag holding section 111. Since the furnace main body 110 is divided into the slag holding section 111 and the salt holding section 112, the level of the interface between the molten salt layer 151 and the slag layer 152 must be located below the salt holding section 112, and must be maintained constant in order to achieve successful discharge. Therefore, the molten slag must be discharged while monitoring the level of the interface between the molten salt layer 151 and the molten slag layer 152. Since the temperature along the vertical direction of the molten salt layer 151 increases toward the interface with the molten slag layer 152, the level of the interface can be monitored by continuously observing the temperature near the interface. The discharge volume of the molten slag is controlled so that the temperature observed with the thermometer 122 provided at the lower portion of the molten salt layer 151 is maintained at a given value. The interface between the molten salt layer 151 and the molten slag layer 152 can, therefore, be maintained to a given level. The molten metal is intermittently discharged through a metal discharge port 118 provided on the lower side wall of the slag holding section 111. Gas formed during melting is discharged through an exhaust gas discharge port 119 and disposed. FIG. 7 is a plan view illustrating another embodiment of a melting furnace in accordance with the present invention, and FIG. 8 is a cross-sectional view taken from the line B-B in FIG. 7. In FIG. 7 and FIG. 8, parts having the same function as in FIG. 5 and FIG. 6 are referred to with the same reference signs without further description. In this embodiment, the salt holding section 112 has a significantly smaller inner diameter (a smaller cross-section area of the furnace) compared to the inner diameter of the slag holding section 111 and is located on the central portion of the slag holding section 111. From the side view, the salt holding section 112 is protrudent relative to the slag holding section 111. The salt holding section 112 is provided with an incineration residue feeding pipe 114 which is immersed into the molten salt layer 151 so that the bottom end of the incineration residue feeding pipe 114 is located under the surface level of the molten salt layer 151. During the operation of this melting furnace, the interface between the molten salt layer 151 and the molten slag layer 152 is adjustable to various levels having different effects. When the interface between the molten salt layer 151 and the molten slag layer 152 is formed in the narrowed portion having a small inner diameter in the furnace as shown in FIG. 8, heat transfer from the molten slag layer 152 to the molten salt layer 151 is reduced due to the reduction in the interface area and heat dissipation from the furnace wall is also reduced. Therefore, the temperature rise of the molten salt layer can be more effectively suppressed. Accordingly, this operation mode can achieve noticeable energy reduction in cooling the molten salt layer and decrease the load of the cooling unit 115. In the case of a small size of the melting furnace, the cooling unit 115 may be not necessary. When the interface between the molten salt layer 151 and the molten slag layer 152 is formed in the thickening and thickened portions having larger inner diameters compared to the narrowed portion, the interface area increases compared to that shown in FIG. 8, heat transfer from the molten slag layer 152 to the molten salt layer 151 is promoted and the cooling unit 115 is forced to be operated under a high load. This mode, however, causes an increase in disposal capacity of the incineration residue because the incineration residue is mainly melted near the interface. In such an operation mode, a melting furnace as shown in FIG. 9 is used, wherein no induction-heating coil 13 is provided above the level of the interface. In the present invention, since the furnace main body is divided into a slag holding section for holding and heating the molten slag and a molten salt holding section provided thereon for holding the molten salt, and a temperature-controlling means for the molten salt layer is provided on the periphery of the salt holding section, the temperature of the molten salt layer can be decreased, the vapor pressure of the low-boiling-point materials can be decreased by lowering the temperature of the molten salt layer, and thus the volatilization of the low-boiling-point materials can be suppressed. When the salt holding section has a smaller cross-section area than that of the slag holding section, the interface area between the molten salt layer and the molten slag layer can be decreased. Therefore, heat transfer from the molten slag layer to the molten salt layer decreases and heat dissipation from the furnace wall increases at the same time. As a result, the temperature rise of the molten salt layer and volatilization of the low-boiling-point materials can be more effectively suppressed. When the bottom end of the incineration residue feeding pipe, which is provided in the salt holding section, is inserted into a position below the upper surface level of the molten salt layer, the incineration residue is fed into the molten salt layer, and a mixed layer composed of the molten salt and the incineration residue is formed in the bottom section of the molten salt layer. Therefore, heat transfer from the molten slag layer to the molten salt layer can be prevented. As a result, a temperature rise of the molten salt layer and volatilization of the low-boiling-point materials can be suppressed. FIG. 10 is a plan view illustrating an embodiment of a melting furnace in accordance with the present invention, and FIG. 11 is a cross-sectional view taken from the line A-A in FIG. 10. The melting furnace is a direct current electrical resistance heating type and includes a melting furnace main body 210, a movable electrode 213, a furnace bottom electrode 214, and a plurality of incineration residue feeding pipes 215. In the drawings, reference numeral 250 represents an incineration residue containing salts, reference numeral 251 represents a molten salt layer, and reference numeral 252 represents a molten slag layer. The melting furnace main body 210 is divided into a section for holding the molten slag and a section for holding the molten salt, the lower section of the melting furnace main body 210 is a molten slag holding section 211, and the upper section is a molten salt holding section 212. The slag holding section 211 holds the molten slag, heats the molten slag layer 252 by the introduction of electric current between the movable electrode 213 and the furnace bottom electrode 214, and melts the incineration residue fed from the incineration residue feeding pipes 215 by heat of the molten slag layer 252 maintained at a high temperature. The salt holding section 212 is provided with three incineration residue feeding pipes 215 which pass through the furnace cover of the salt holding section 212 and are immersed into the molten salt layer 251 such that these lower ends are located below the upper surface of the molten salt layer 251. Therefore, the incineration residue is fed into the molten salt layer 251. The shape of the incineration residue feeding pipe 214 is not limited to be cylindrical, and may be prismatic or have some other shape as long as the incineration residue is fed into the molten salt layer 251. The salt holding section 212 is provided with a cooling unit 216 on its periphery. The cooling unit 216 forcedly cools the molten salt layer 251 and has a structure in which a water-cooling pipe is embedded in a refractory material. The water-cooling pipe is provided over a region from near the interface between the molten salt layer 251 and the molten slag layer 252 to near the surface of the molten salt layer 251. Since the surface of the molten salt layer 251 is easily cooled, the molten salt may not be readily discharged due to decreased flowability as a result of excessively decreased temperature. Therefore, it is preferable that a heating unit having an induction-heating coil be provided on the periphery of the salt discharge port 217 to heat the surface area according to demand. The operation of the melting furnace for melting the incineration residue containing salt is as follows: A weighed incineration residue is fed into the incineration residue feeding pipes 215, which are inserted into the molten salt layer 251, and packed in the lower section of the feeding pipes. The incineration residue gradually sinks by its own weight in the incineration residue feeding pipes 215 while being preheated and is fed into the molten salt layer 251 formed in the salt holding section 212. The incineration residue 250 in the molten salt layer 251 is gradually settled and dispersed to form a mixed layer 251a in which the incineration residue is suspended. As described above, the mixed layer 251a inhibits heat transfer from the molten slag layer 252 to the molten salt layer 251 and suppresses an excessive temperature rise of the molten salt layer 251. A part of the incineration residue 250 fed into the molten salt layer 251 melts in the molten salt layer 251, and the rest of it sinks to the interface with the molten slag layer 251 maintained at high temperature and melts there. The melt of the incineration residue is separated into three layers, i.e., the molten salt layer 251, the molten slag layer 252 and the molten metal layer 253, because of specific gravity difference. The molten slag layer 252 in the slag holding section 211 is heated by the electrical resistance heat formed in the molten metal slag layer 252 itself due to a current flow between the electrodes. The temperature of the molten slag layer 252 is controlled to be within a range of 1,400 °C to 1,600 °C by adjusting the input power. On the other hand, the temperature of the molten salt layer 251 formed in the salt holding section 212 is controlled to be within an adequate range with the cooling unit 216, based on the temperature observed with a thermometer 222 provided in the central section of the molten salt layer 251. The flow rate of the cooling water in the cooling unit 216 is adjusted so that the temperature of the central section of the molten salt layer 251 observed by the thermometer 222 is in a range of 700 °C to 1,000 °C in order to prevent volatilization of the low-boiling-point materials. These melts are separately discharged from the furnace. The molten salt is discharged through the salt discharge port 217 provided on the upper side wall of the salt holding section 212 (above the bottom end of the incineration residue feeding pipe 215). The molten salt does not substantially contain the solid component such as the unmelted incineration residue and exhibits high flowability. Therefore, the molten salt can be discharged at a low flow rate from the salt discharge port 217, and an overflow system can be applied to the discharge. The reason for that the molten salt is discharged in a desirable state is as follows. Mixing of the unmelted incineration residue into the molten salt which is discharged from the salt discharge port 217 does not occur because the incineration residue feeding pipe 215 is immersed into the molten salt layer 251, and the salt discharge port 217 is placed below the bottom end of the incineration residue feeding pipe 215 and above the mixed layer 251a containing the suspended incineration residue. If the incineration residue feeding pipe 215 is not immersed into the molten salt layer 251 and the incineration residue is fed above the molten salt layer 251, the incineration residue is suspended in the entire molten salt layer and the molten salt containing the incineration residue is discharged. Thus, the molten salt has significantly high viscosity and thus low flowability, resulting in difficulty in discharge of the molten metal. The molten slag is continuously or intermittently discharged through a slag discharge port 218 provided on the upper side wall of the slag holding section 211. Since the furnace main body 210 is divided into the slag holding section 211 and the salt holding section 212, the level of the interface between the molten salt layer 251 and the slag layer 252 must be located below the salt holding section 212, and must be maintained constant in order to achieve successful discharge. Therefore, the molten slag must be discharged while monitoring the level of the interface between the molten salt layer 251 and the molten slag layer 252. Since the temperature along the vertical direction of the molten salt layer 251 increases toward the interface with the molten slag layer 252, the level of the interface can be monitored by continuously observing the temperature near the interface. The discharge volume of the molten slag is controlled so that the temperature observed with the thermometer 223 provided at the lower portion of the molten salt layer 251 is maintained at a given value. The interface between the molten salt layer 251 and the molten slag layer 252 can, therefore, be maintained on a given level. The molten metal is intermittently discharged through a metal discharge port 219 provided on the lower side wall of the slag holding section 211. Gas formed during melting is discharged through an exhaust gas discharge port 220 and disposed. FIG. 12 is a plan view illustrating another embodiment of a melting furnace in accordance with the present invention, and FIG. 13 is a cross-sectional view taken from the line B-B in FIG. 12. In FIG. 12 and FIG. 13, parts having the same function as in FIG. 10 and FIG. 11 are referred to with the same reference signs without further description. In this embodiment, the salt holding section 212 has a significantly smaller inner diameter (a smaller cross-section area of the furnace) compared to the inner diameter of the slag holding section 11 and is located on the central portion of the slag holding section 211. From the side view, the salt holding section 212 is protrudent relative to the slag holding section 211. The salt holding section 212 is provided with incineration residue feeding pipes 215 which are immersed into the molten salt layer 251 so that the bottom end of each incineration residue feeding pipe 215 is located under the surface level of the molten salt layer 251. During the operation of this melting furnace, the interface between the molten salt layer 251 and the molten slag layer 252 is adjustable to various levels having different effects. When the interface between the molten salt layer 251 and the molten slag layer 252 is formed in the narrowed portion having a small inner diameter in the furnace as shown in FIG. 13, heat transfer from the molten slag layer 252 to the molten salt layer 251 is reduced due to the reduction of the interface area and heat dissipation from the furnace wall is also reduced. Therefore, the temperature rise of the molten salt layer can be more effectively suppressed. Accordingly, this operation mode can achieve noticeable energy reduction in cooling the molten salt layer and decrease the load of the cooling unit 216. In case of a small size of melting furnace, the cooling unit 216 may be not necessary. When the interface between the molten salt layer 251 and the molten slag layer 252 is formed in the thickening and thickened portions having larger inner diameters compared to the narrowed portion as shown in FIG. 14, the interface area increases compared to that shown in FIG. 13, heat transfer from the molten slag layer 252 to the molten salt layer 251 is promoted and the cooling unit 216 is forced to be operated under a high load. This mode, however, causes an increase in disposal capacity of the incineration residue because the incineration residue is mainly melted near the interface. In the present invention, since the furnace main body is divided into a slag holding section for holding and heating the molten slag and a molten salt holding section provided thereon for holding the molten salt, and a temperature-controlling means for the molten salt layer is provided on the periphery of the salt holding section, the temperature of the molten salt layer can be decreased, the vapor pressure of the low-boiling-point materials can be decreased by lowering the temperature of the molten salt layer, and thus the volatilization of the low-boiling-point materials can be suppressed. When the salt holding section has a smaller cross-section area than that of the slag holding section, the interface area between the molten salt layer and the molten slag layer can be decreased. Therefore, heat transfer from the molten slag layer to the molten salt layer decreases and heat dissipation from the furnace wall increases at the same time. As a result, the temperature rise of the molten salt layer and volatilization of the low-boiling-point materials can be more effectively suppressed. When the bottom end of the incineration residue feeding pipe, which is provided in the salt holding section, is inserted into a position below the upper surface level of the molten salt layer, the incineration residue is fed into the molten salt layer, and a mixed layer composed of the molten salt and the incineration residue is formed in the bottom section of the molten salt layer. Therefore, heat transfer from the molten slag layer to the molten salt layer can be prevented. As a result, temperature rise of the molten salt layer and volatilization of the low-boiling-point materials can be suppressed.

Claims: A method for melting an incineration residue (50) comprising the steps of: charging an incineration residue (50) containing salts into a melting furnace; heating the charged incineration residue (50) in the melting furnace to produce a melt comprising a molten slag and a molten salt, said molten slag forming a molten slag layer (52) and said molten salt forming a molten salt layer (51) on the molten slag layer (52); discharging the molten slag from the melting furnace; and discharging the molten salt from the melting furnace; characterized by : controlling the temperature of the molten salt in the molten salt layer (51) to suppress volatilization thereof. The method of claim 1, wherein said step of heating comprises: heating the molten slag by applying a voltage between a plurality of electrodes (13) which are immersed in the molten slag having an electric resistance to generate heat by the electric resistance of the molten slag. The method of claim 1, wherein said step of heating comprises: heating the molten slag by applying a high frequency electric current to an induction-heating coil (113) which is arranged on the periphery of the melting furnace. The method of claim 1, wherein said step of heating comprises: heating the molten slag by applying a voltage between a movable electrode (213) which is submerged in the molten slag and a bottom electrode (214) which is arranged at the bottom of the melting furnace. The method of claim 1, wherein the temperature of the molten salt is controlled to be in the range of 700 to 1000 °C. The method of claim 1, wherein the step of controlling the temperature of the molten salt comprises lowering the temperature of the molten salt. The method of claim 6, wherein the lowering of the temperature comprises decreasing the heat transfer from the molten slag to the molten salt. The method of claim 7, wherein the decreasing of the heat transfer comprises charging the incineration residue (50) into the molten salt layer (51) to form a mixed layer (51a) of the incineration residue (50) and the molten salt in the bottom of the molten salt layer (51). The method of claim 7, wherein the decreasing of the heat transfer comprises reducing the cross-sectional area of a molten salt holding section (12) such that it is smaller than that of a molten slag holding section (11) in the melting furnace having the molten slag holding section (11) and the molten salt holding section (12). The method of claim 6, wherein the lowering of the temperature comprises increasing the heat radiation from the molten salt. The method of claim 6, wherein the lowering of the temperature comprises cooling the molten salt. The method of claim 1, wherein said charging of the incineration residue comprises charging the incineration residue into the molten salt layer (51). The method of claim 1, wherein the step of controlling the temperature of the molten salt comprises heating the surface of the molten salt. The method of claim 13, wherein the heating of the surface is carried out by induction heating. The method of claim 1, further comprising the step of controlling the temperature of the molten slag within the range of 1400 to 1600 °C. An apparatus for melting an incineration residue (50) comprising: a melting furnace main body (10) into which an incineration residue (50) containing salts is charged; the melting furnace main body (10) including a molten slag holding section (11) and a molten salt holding section (12) on the molten slag holding section (11); charge means (15) for charging the incineration residue (50) in the melting furnace main body (10) ; heating means (13) for melting the charged incineration residue (50) to produce a melt comprising a molten slag and a molten salt, said molten slag being in the molten slag holding section (11) and said molten salt being in the molten salt holding section (12); slag discharge means (18) for discharging the molten slag from the molten slag holding section (11); and salt discharge means (17) for discharging the molten salt form the molten salt holding section (12); characterized by : control means (22) for controlling the temperature of the molten salt in the molten salt holding section (12) to suppress volatilization of the molten salt; The apparatus of claim 16, wherein said heating means comprises a plurality of electrodes (13) which are immersed in the molten slag, a voltage being applied between the plurality of electrodes (13). The apparatus of claim 16, wherein said heating means comprises an induction-heating coil (113) which is arranged on the periphery of the molten slag holding section (111), the molten slag being heated by applying a high frequency electric current to the induction-heating coil (113). The apparatus of claim 16, wherein said heating means comprises a movable electrode (213) which is immersed in the molten slag and a bottom electrode (214) which is arranged at the bottom of the melting furnace main body (210), the molten slag being heated by applying a voltage between the movable electrode (213) and the bottom electrode (214). The apparatus of claim 16, wherein said control means (115) is arranged on the periphery of the molten salt holding section (112). The apparatus of claim 16, wherein said control means comprises means (115) for lowering the temperature of the molten salt in the molten salt holding section (112). The apparatus of claim 21, wherein said means for lowering the temperature of the molten salt comprises means (111,112) for decreasing the heat transfer from the molten slag to the molten salt. The apparatus of claim 22, wherein said means for decreasing the heat transfer comprises the molten salt holding section (112) having a cross-sectional area which is smaller than that of the molten slag holding section (111). The apparatus of claim 23, further comprising means (115) for lowering the temperature of the molten salt in the molten salt holding section (112). The apparatus of claim 21, wherein said means for lowering the temperature of the molten salt comprises means for increasing heat radiation from the molten salt. The apparatus of claim 21, wherein said means for lowering the temperature of the molten salt comprises means (115) for cooling the molten salt arranged on the periphery of the molten salt holding section (112). The apparatus of claim 16, wherein said charge means comprises an incineration residue feeding pipe (15) arranged at the upper part of the melting furnace main body (10), the lower end of the incineration residue feeding pipe (15) being positioned below the surface of the molten salt. The apparatus of claim 16, wherein said control means comprises means (113) for heating the molten salt on the surface of the molten salt holding section (112).

Inventors: FUJISAWA YOSHINARI, FUKUSHIMA TSUTOMU, NAKAHARA KEISUKE, NAKAO TSUYOSHI, SUDOU MASAHIRO, FUJISAWA, YOSHINARI, FUKUSHIMA, TSUTOMU, NAKAHARA, KEISUKE, NAKAO, TSUYOSHI, SUDOU, MASAHIRO, Fujisawa, Yoshinari, c/o NKK Corporation, Fukushima, Tsutomu, c/o NKK Corporation, Nakahara, Keisuke, c/o NKK Corporation, Nakao, Tsuyoshi, c/o NKK Corporation, Sudou, Masahiro, c/o NKK Corporation
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1997
Main classification: C03B5
IPC codes: C03B5