JP2014203762A - Microwave heating device and microwave heating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a microwave heating device and a microwave heating method, capable of heating a desired product to be processed while reducing thermally caused damage.SOLUTION: A microwave heating device according to the present invention includes: a drum for holding a product to be processed; a microwave irradiation device irradiating a microwave into the drum; a rotation device rotating the drum; an agitation fin provided in the drum; and a cooling device cooling the drum.

Description

  The present invention relates to a microwave heating apparatus and a microwave heating method.

Microwave heating technology has been developed as a technology for warming food. The characteristics of microwave heating are as compared with general heating by infrared rays, hot air, etc.-Materials that easily absorb microwaves can be selectively heated.
・ High heating efficiency and short heating time.
It has the features such as.

  At present, from the above characteristics, it is industrially used in a wide range of fields such as drying of wood and the like, sterilization, sintering of ceramics, and synthesis of polymer materials. Among them, the microwave heating apparatus will be described using an example used for waste incineration. (For example, refer to Patent Document 1).

  FIG. 5 is an explanatory diagram showing the configuration of the microwave heating apparatus described in Patent Document 1. As shown in FIG. In the casing 2, a rotatable drum body 10 and a support roller 11 are provided. A plurality of stirring blades 19 are installed inside the drum body 10, and the waste 40 thrown into the drum body 10 is stirred. An intake means 31 is connected to the lower part of the casing 2, and an exhaust gas purification means 35 is connected to the upper part of the casing 2. The microwave radiated from the magnetron 21 is irradiated to the waste 40 in the drum body 10. Thereby, the waste 40 can be heated.

JP 2004-183989 A

  By the way, when the object to be processed is a mixture of a material that hardly absorbs microwaves and a material that easily absorbs microwaves, it absorbs microwaves without damaging the materials that hardly absorb microwaves by heat. There is a demand to burn only easy-to-use materials.

  However, in the conventional apparatus, heat from a material that easily absorbs microwaves is conducted to a material that hardly absorbs microwaves due to heat conduction, and the material that hardly absorbs microwaves is thermally damaged. Had.

  The present invention solves the above-described problems, and an object of the present invention is to provide a microwave heating apparatus and a microwave heating method that reduce thermal damage to a material that hardly absorbs microwaves.

  In order to achieve the above object, a microwave heating apparatus of the present invention includes a drum for holding an object to be processed, a microwave irradiation apparatus for irradiating microwaves in the drum, and a rotating apparatus for rotating the drum. And a stirring fin provided in the drum, and a cooling device for cooling the drum.

  Further, the microwave heating method of the present invention is a microwave heating method in which the object to be processed being stirred in the drum is irradiated with microwaves to heat the object to be processed, and the drum is simultaneously with the microwave irradiation. It is characterized by cooling.

  As described above, according to the present invention, it is possible to suppress the temperature rise of the object to be processed and reduce the occurrence of thermal damage to the object to be processed.

Process diagram showing the silicon recycling system Sectional schematic diagram of the microwave heating apparatus according to the first embodiment Schematic diagram of drum of microwave heating apparatus according to Embodiment 1 Sectional drawing of the microwave heating apparatus which concerns on the other example of Embodiment 1. FIG. Explanatory drawing which shows the structure of the microwave heating apparatus of patent document 1

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
Silicon sludge, which is a mixture of silicon, water, and carbon, is generated as a byproduct of the manufacturing process of the silicon wafer for solar cells. Silicon is a material that hardly absorbs microwaves, and water and carbon are materials that easily absorb microwaves. For this reason, if removal of moisture and selective combustion of carbon can be realized without causing thermal damage to the silicon in the silicon sludge, high quality silicon for recycling can be obtained.

  When silicon sludge is irradiated with microwaves, silicon that hardly absorbs microwaves is not easily heated by the microwave itself, but the temperature of silicon rises due to heat conduction from water and carbon that easily absorbs microwaves. Sometimes. In contrast, the microwave heating apparatus according to the present embodiment can selectively heat only a material that easily absorbs microwaves while suppressing thermal damage to the material that hardly absorbs microwaves. Is possible. For this reason, this microwave heating device is useful for obtaining silicon for recycling from silicon sludge. Hereinafter, the microwave heating apparatus will be described using an example applied to a process of obtaining silicon for recycling from silicon sludge.

  First, the silicon wafer manufacturing process for solar cells will be described with reference to FIG. First, initial silicon with few impurities is prepared in a silicon material preparation step (S110), and then an ingot is produced in an ingot production step (S120). The ingot is produced by the Czochralski (CZ) method or the like. The produced ingot is processed by a silicon ingot processing / bonding step (S130), and is bonded and fixed to the beam material. The ingot bonded and fixed to the beam material is sliced using a wire saw in the slicing step (S140). Then, a silicon wafer for solar cells is manufactured through a wafer cleaning process (S150).

  Next, the silicon recycling process (S200) will be described. The silicon recycling process (S200) means that silicon sludge produced as a by-product in the slicing process (S140) is separated and recovered in the separation / recovery process (S210), and the carbon / moisture removal process (S220), This is a series of steps for producing the silicon for recycling through the solidification step (S230) and returning to the ingot production step (S120).

  In the slicing step (S140), silicon chips generated when processing the ingot are mixed with the beam material to which the ingot is bonded and water containing a coolant (organic system) and discharged in large amounts as silicon sludge. In this silicon sludge separation / recovery step (S210), the solid component and the liquid component are separated / recovered by, for example, a filter press. The solid component of the separated silicon sludge is recovered as powdered silicon sludge. This powdery silicon sludge contains an organic carbon component as a residual coolant component, graphite as a material of a beam material, and inorganic carbon components as residues such as abrasive grains of a diamond wire saw. The organic / inorganic carbon component contained is about 7 at% of the entire powdery silicon sludge. The moisture content of the powdered silicon sludge is about 50% to 60% of the whole.

  The collected powdery silicon sludge is introduced into the microwave heating apparatus according to this embodiment as an object to be processed, and the carbon / water removal step (S220) is performed by performing heat treatment for several minutes to several tens of minutes. To do. By removing the carbon component and moisture while suppressing oxidation of silicon in this step, it is possible to obtain silicon for recycling with high purity and low thermal damage.

  Here, the detail of the microwave heating apparatus 100 which concerns on this Embodiment is demonstrated using FIG.

  The microwave heating apparatus 100 includes a microwave generator 300, a tuner 310, a waveguide 320, a metal chamber 330, a drum 350 for holding an object to be processed (silicon sludge 360), and a rotating mechanism that is a rotating device for the drum 350. 340, a cooling fin 370 having a stirring fin 370 for stirring the silicon sludge 360, and a water cooling pipe 390 for cooling the drum 350. Further, the microwave heating apparatus 100 includes an intake port 400 for taking air or gas into the metal chamber 330, an exhaust port 410 for exhausting, and a thermometer 420 for measuring temperature.

  The metal chamber 330 is installed with the central axis inclined by 30 ° from the horizontal direction so that the workpiece can be easily stirred. Inside the metal chamber 330, a drum 350 made of a heat-resistant metal that is rotated in the depth direction of the drawing sheet by the rotation mechanism 340 is installed.

  The microwave generated by the microwave generator 300 is irradiated to the silicon sludge 360 held in the drum 350 through the waveguide 320. Here, as an example of the microwave irradiation apparatus, an apparatus in which the microwave generator 300, the tuner 310, and the waveguide 320 are combined is given.

  An intake port 400 for introducing air from the atmosphere is provided on the lower surface of the metal chamber 330. The upper surface of the metal chamber 330 is provided with an exhaust port 410 that discharges gas generated when water vapor or carbon generated when the silicon sludge 360 is heated is combusted. The intake port 400 and the exhaust port 410 allow the flow rate adjustment of the intake air amount and the exhaust air amount.

  Here, details of the drum 350 will be described. FIG. 3A is a schematic cross-sectional view of the drum 350 as viewed from the side, and FIG. 3B is an enlarged schematic view of the opening of the drum 350. On the inner surface of the drum 350, a stirring fin 370 is installed in parallel with the rotation axis of the drum 350, and the workpiece can be stirred by rotating the drum 350. The stirring fin 370 is made of a heat-resistant metal, and a stainless steel plate is an example. Here, the depth (depth) of the stirring fin 370 is about 3/4 times the depth of the drum 350, and the height of the stirring fin 370 (height from the inner surface of the drum 350) is the diameter of the drum 350. The figure shows a state in which four sheets are installed on the inner surface of the drum 350 so as to be positioned in the cross direction, approximately 1/8 times. This arrangement is useful for performing uniform processing because a shadow portion where the object is not irradiated with microwaves can be reduced. Since the metal stirring fin 370 cuts off the microwave and easily generates a shadow portion, the shadow portion is made as small as possible in the above arrangement.

  The stirring fins 370 can efficiently stir the object to be processed. Even if the powdery substance having a different particle diameter is the object to be processed, not only the rotation direction of the drum 350 (the depth direction in the figure) but also the opening and the back direction. It can also be stirred (in the horizontal direction in the figure).

  The stirring fin 370 is preferably made of a material that has heat resistance such as quartz and transmits microwaves. This is because if the stirring fin 370 is made of a material that transmits microwaves, a shadow portion where the microwave is not irradiated on the object to be processed is not formed, and the processing efficiency is improved.

  A water cooling tube 390 (water cooling mechanism) covered with metal is provided on the inner wall of the drum 350 and the inside of the stirring fin 370. That is, the cooling device 380 (FIG. 2) has a water cooling mechanism that cools the stirring fins 370 with the coolant. The inside of the drum 350 and the stirring fins 370 can be cooled by flowing the cooling liquid through the water cooling tube 390. Thereby, the vicinity of the center of the drum 350 having the highest temperature can be effectively cooled, and the cooling efficiency of the workpiece is increased. Further, since the stirring fin 370 has many opportunities to come into contact with the object to be processed, the object to be processed can be cooled more efficiently. In addition, water, fats and oils, etc. are applicable as a cooling fluid.

  In the present embodiment, the silicon sludge 360 is processed into a granular material of 5 mm or less by a crusher to suppress abnormal discharge and partial heating during microwave heating of the silicon sludge 360 of FIG. Several kilograms of this silicon sludge 360 is introduced into the drum 350, and the drum 350 is rotated at 3 to 20 rpm by the rotating mechanism 340.

  The microwave heating apparatus 100 is provided with a control unit 430. The control unit 430 is a computer. The control unit 430 performs drive control of the rotation mechanism 340. Further, the control unit 430 controls the cooling device 380 so as to cool the drum 350 even during microwave irradiation. The control unit 430 is also connected to the microwave generator 300 and the (radiation) thermometer 420, and the cooling device 380 and the microwave generator so that the surface temperature of the silicon sludge 360 being processed is 600 ° C. or less. 300 is controlled. The surface temperature of the silicon sludge 360 is measured with a thermometer 420. This surface temperature is different from the temperature of the portion (center portion) where the silicon sludge 360 is most heated by microwave heating. When the temperature at the highest temperature rises to about 900 ° C., the oxidation reaction of silicon proceeds and is not suitable for silicon recycling. As a result of the experiment, the inventors have found that the oxidation of silicon in the silicon sludge 360 can be suppressed if the surface temperature is kept at 600 ° C. or lower.

  Here, the carbon / hydrogen removal step (S220) of FIG. 1 will be described in detail.

In the carbon / hydrogen removal step (S220), the silicon sludge 360 is irradiated with, for example, a microwave of about 3 to 9 kW simultaneously with the cooling of the inside of the drum 350 in FIG. Then, the organic / inorganic carbon components and moisture in the silicon sludge 360 are heated by absorbing the microwave, and first, water vapor is generated, and the generated water vapor is discharged from the exhaust port 410 to the outside of the metal chamber 330. Thereafter, the organic / inorganic carbon components that have reached a higher temperature evaporate or undergo a combustion reaction with oxygen in the atmosphere to generate organic gases and CO ₂ . The generated organic gas and CO ₂ are also discharged from the exhaust port 410. Thus, by continuing to irradiate microwaves, the moisture concentration and carbon concentration in the silicon sludge 360 are lowered. By performing microwave irradiation for several minutes, the water concentration in the silicon sludge 360 can be reduced to, for example, 1.5% or less, and the carbon concentration, for example, to about 0.1 at%.

  Originally, since silicon powder is a material that hardly absorbs microwaves, it is difficult to be directly heated by microwaves. However, there is a concern that the temperature of silicon may increase due to heat conduction from carbon at a high temperature. When heat is applied to silicon, alteration (increased oxygen concentration due to oxidation) and damage are accumulated, and the quality of the silicon for recycling may deteriorate. Therefore, in the present embodiment, moisture and carbon in the silicon sludge 360 are burned and removed while cooling the inside of the drum 350 or the stirring fins 370 in order to prevent the temperature of the silicon from rising. Thereby, the temperature rise of silicon | silicone can be suppressed and the quality change and thermal damage of silicon | silicone can be reduced.

It is desirable that the outer side of the drum 350 has a lower thermal conductivity than the inner side. More specifically, a material having high heat resistance and high thermal conductivity (for example, a metal material such as stainless steel, or a thermal conductivity of 15 W · m ⁻¹ · K ⁻¹ or more and 500 W · m ⁻ is provided inside the drum 350. ^It is desirable to adopt a double structure using a material ( ¹ · K ⁻¹ or less) and a material that hardly absorbs microwaves and has low thermal conductivity (eg, quartz, zirconia, heat-resistant board). In particular, it is desirable that the thermal conductivity inside the drum 350 is not less than 2 times and not more than 100 times the outside. By increasing the difference in thermal conductivity between the inner side and the outer side of the drum 350 by two times or more, the temperature rise of the metal chamber 330 surrounding the drum 350 can be suppressed. When the temperature rises, the metal may easily absorb the microwave, but by suppressing the temperature rise of the metal chamber 330, the energy of the microwave absorbed in the metal chamber 330 can be reduced. Further, since different materials are used for the inner side and the outer side of the drum 350, the process of embedding the water-cooled tube 390 between them (side surface of the drum 350) becomes easy, and the drum 350 can be easily manufactured. By embedding the cooling device 380 in the side surface of the drum 350, cooling can be performed efficiently.

  The microwave heat treatment may be performed under reduced pressure. Further, a mixed gas of oxygen gas and inert gas (such as nitrogen or argon) or a mixed gas of hydrogen gas and inert gas may be supplied into the metal chamber 330. By these, the oxidation of silicon can be suppressed.

  Note that this microwave heating apparatus can also be applied to sludge generated when a substrate such as a sapphire substrate, a gallium nitride (GaN) substrate, or a gallium arsenide (GaAs) substrate is manufactured. These sludges also contain materials that easily absorb microwaves and materials that do not absorb microwaves easily. Microwave heat treatment is performed while preventing thermal damage to materials that do not absorb microwaves. it can.

Note that the material (first substance) that easily absorbs microwaves is a polar substance, a substance having a dielectric loss angle (Tan δ) of 0.01 or more, or a substance having a conductivity of 10 ⁻² or more and 10 ⁴ or less. Any of the substances is shown. In addition, the material that hardly absorbs microwaves (second substance) is any of a polar substance, a substance having a dielectric loss angle Tan δ of 0.01 or more, or a substance having a conductivity of 10 ⁻² or more and 10 ⁴ or less. Indicates different substances. An object to be processed including both the first substance and the second substance described above is suitable for treatment by the microwave heating apparatus 100 according to the present embodiment. In this case, the first substance can be heated without causing thermal damage to the second substance.

  Here, a microwave heating apparatus 101 according to another example of the present embodiment will be described with reference to FIG. 4, the same components as those in FIG. 2 are denoted by the same reference numerals, and description thereof is omitted. The difference from FIG. 2 is that the cooling device 380 has an air cooling mechanism 391 that injects cooling gas onto the drum 350. The air cooling mechanism 391 has a structure in which the cooled cooling gas is introduced from the air inlet 400 and applied to the outer surface of the drum 350 to cool the drum 350 during microwave irradiation. Note that the water-cooled pipe 390 that is a mechanism for cooling the drum 350 with the coolant described in FIG. 2 may be referred to as a water-cooled mechanism. As the cooling gas, air, an inert gas, an oxygen gas that promotes combustion of carbon, or a mixed gas thereof is used.

  Since the water cooling mechanism has high cooling efficiency and temperature control is possible in detail, it is preferable to use it when the material to be heated has a high temperature (600 ° C or higher) or when it is not desired to raise the temperature of the material as much as possible. is there. However, if the water cooling mechanism is provided in the drum 350, the structure becomes complicated and the manufacturing cost increases. On the other hand, the cooling efficiency of the air cooling mechanism 391 is lower than that of the water cooling mechanism, but the structure of the drum 350 can be simplified and the manufacturing cost can be kept low. Therefore, the air cooling mechanism 391 is suitable when a substance is to be processed at a relatively low temperature of 600 ° C. or lower.

  However, it is preferable to arrange the water cooling mechanism together with the air cooling mechanism 391. That is, it is desirable that the cooling device 380 further includes an air cooling mechanism 391 in addition to the water cooling mechanism (water cooling pipe 390). Thereby, since cooling efficiency can be raised, the temperature rise of the material which cannot absorb a microwave can be suppressed as much as possible. In addition, since the temperature of the entire metal chamber 330 can be lowered, the amount of microwaves absorbed by the metal chamber 330 can be reduced, and the workpiece can be efficiently heated.

  Furthermore, rapid cooling can be realized by controlling the cooling mechanism of the two systems (water cooling mechanism and air cooling mechanism 391) provided in the cooling device 380 with the control unit 430. Specifically, the silicon sludge 360 is irradiated with the microwave from the microwave generator 300, and at the same time, either the water cooling mechanism or the air cooling mechanism 391 is cooled. In this case, the control unit 430 controls the microwave generator 300 or the cooling device 380 so that the surface temperature of the silicon sludge 360 is 600 ° C. or less. Until the moisture in the silicon sludge 360 evaporates, heat is taken away by the evaporation of the moisture, so that the temperature of the silicon sludge 360 hardly increases and it is easy to keep it at 600 ° C. or less. However, the temperature of the silicon sludge 360 increases rapidly when the moisture has completely evaporated. At this time, the cooling of only one system of the water cooling mechanism or the air cooling mechanism 391 cannot cope with the rapid temperature rise, and there is a risk that the temperature in the silicon sludge 360 is increased. Therefore, at the same time as the temperature of the silicon sludge 360 is rapidly increased, the water cooling mechanism and the air cooling mechanism 391 are used in combination to perform rapid cooling. By carrying out rapid cooling using both of them, the temperature of the object to be processed can be maintained at a desired temperature even when a sudden temperature change occurs. In this case, the control unit 430 causes both the water cooling mechanism (water cooling pipe 390) and the air cooling mechanism 391 to perform cooling when the temperature in the drum 350 (temperature of the silicon sludge 360) reaches a preset temperature. The preset temperature is, for example, 500 degrees when the object to be processed is silicon sludge, and is stored in the control unit 430 in advance.

  The present invention can be used in heat treatment processes in various industrial fields, such as a material recycling process for solar cells and semiconductors, and a synthesis of functional materials and polymer materials.

DESCRIPTION OF SYMBOLS 100,101 Microwave heating apparatus 300 Microwave generator 310 Tuner 320 Waveguide 330 Metal chamber 340 Rotation mechanism 350 Drum 360 Silicon sludge 370 Stirring fin 380 Cooling device 390 Water cooling pipe 391 Air cooling mechanism 400 Intake port 410 Exhaust port 420 Thermometer 430 control unit

Claims (16)

A drum for holding the workpiece;
A microwave irradiation device for irradiating the drum with microwaves;
A rotating device for rotating the drum;
Stirring fins provided in the drum;
A cooling device for cooling the drum;
A microwave heating apparatus comprising:   The microwave heating device according to claim 1, wherein the cooling device includes a water cooling mechanism that cools the drum with a coolant. The microwave heating apparatus according to claim 2, wherein the water cooling mechanism is embedded in a side surface of the drum.   The microwave heating device according to claim 2 or 3, wherein the water cooling mechanism is configured by providing a tube through which the coolant passes in the stirring fin.   The microwave heating device according to any one of claims 2 to 4, wherein the cooling device further includes an air cooling mechanism for injecting a cooling gas to the drum. A thermometer for measuring the temperature in the drum;
The microwave irradiation device is irradiated with microwaves, and at the same time, either the water cooling mechanism or the air cooling mechanism is cooled, and the water cooling mechanism and the air cooling are performed when the temperature in the drum reaches a preset temperature. The microwave heating device according to claim 5 provided with a control part which makes both mechanisms cool.   The microwave heating device according to any one of claims 1 to 6, wherein the drum is configured such that an outer side thermal conductivity is lower than an inner side.   The microwave heating device according to any one of claims 1 to 7, further comprising a control unit that causes the microwave irradiation device to irradiate the microwave and simultaneously causes the cooling device to perform cooling. The microwave heating apparatus according to claim 1, wherein the stirring fin is made of a material that transmits microwaves. The object to be processed includes a first substance and a second substance,
The first material is a polar material, a material having a dielectric loss angle Tan δ of 0.01 or more, or a material having a conductivity of 10 ⁻² or more and 10 ⁴ or less,
10. The second material according to claim 1, wherein the second material is different from any of a polar material, a material having a dielectric loss angle Tan δ of 0.01 or more, or a material having a conductivity of 10 ⁻² or more and 10 ⁴ or less. The microwave heating apparatus as described. A microwave heating method for heating a workpiece by irradiating the workpiece to be stirred in a drum with microwaves,
A microwave heating method, wherein the drum is cooled simultaneously with microwave irradiation.   The microwave heating method according to claim 11, wherein the cooling of the drum is performed by cooling a stirring fin provided on the drum.   The microwave heating method according to claim 11 or 12, wherein the cooling of the drum is performed using a coolant.   The microwave heating method according to claim 13, wherein the cooling of the drum is further performed using a cooling gas.   The cooling of the drum is performed using either the cooling liquid or the cooling gas simultaneously with the microwave irradiation, and when the temperature in the drum reaches a preset temperature, the cooling liquid and the cooling gas The microwave heating method of Claim 14 performed using both. The object to be processed includes a first substance and a second substance,
The first material is a polar material, a material having a dielectric loss angle Tan δ of 0.01 or more, or a material having a conductivity of 10 ⁻² or more and 10 ⁴ or less,
The second material is different from any one of a polar material, a material having a dielectric loss angle Tan δ of 0.01 or more, or a material having a conductivity of 10 ⁻² or more and 10 ⁴ or less. The microwave heating method as described.

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