METHOD FOR VACUUM/REDUCED-PRESSURE REFINING AND FACILITY FOR VACUUM/REDUCED-PRESSURE REFINING

Abstract: (Problem) To conduct dust collection in vacuum/reduced pressure refining treatment, using a low cost filter without causing the filter to damage, burn, etc. (Solving Means) Vacuum/reduced pressure refining vessel 1 is connected to dry type dust collector 3 using filter 2 by upstream duct 5, and dry type duct collector 3 is connected to reduced pressure evacuating apparatus 4 by downstream duct 6 and block valve 7 is provided in upstream duct 5. At the start of vacuum/reduced pressure refining treatment, a non-oxidizing gas is injected into the upstream duct upstream of block valve 7 to substantially replace the oxygen in the upstream duct and then hermetically close the upstream duct. After the hermetically closed state has been established in the upstream duct, block valve 7 is opened and dust collector 3 is started to operate. At the end of vacuum/reduced pressure refining treatment, block valve 7 is closed and only the non-oxidizing gas is injected into the upstream duct to return the pressure and then the upstream duct is made open to the atmospheric air, and preferably an open port siding with dust collector 3 is closed in a waiting period from the end of vacuum/reduced pressure refining treatment to the start of next vacuum/reduced pressure refining treatment.

Description: The present invention relates to a vacuum/reduced pressure refining process and a vacuum/reduced pressure refining facility for use in metal refining, i.e. refining of alloys such as steel, etc., e.g. molten metal, etc. in a vacuum/reduced pressure converter, a degassing apparatus for a vacuum ladle, etc. In continuous operation of vacuum/reduced pressure refining treatment under a subatmospheric pressure, the following steps are repeatedly carried out in series to exchange the treated molten metal in the refining vessel with an untreated molten metal. That is, in case of a vacuum/reduced pressure refining apparatus of such a mode as shown in Fig. 11; at first molten metal is charged into a refining vessel, followed by closing the vessel with a lid and reducing the inside pressure of the refining vessel to vacuum/reduced pressure. In case of a vacuum/reduced pressure refining apparatus of such a mode as shown in Fig. 12, a ladle containing the molten metal is placed in a refining vessel, followed by closing the vessel with a lid and reducing the inside pressure of the refining vessel to vacuum/reduced pressure. In case of a vacuum/reduced pressure refining apparatus of such a mode as shown in Fig. 13, a ladle containing the molten metal is positioned under a refining vessel, followed by dipping the lower end of the refining vessel into the molten metal and reducing the inside pressure of the refining vessel to vacuum/reduced pressure. After the vacuum/reduced pressure treatment, the inside pressure of the vacuum/reduced pressure refining vessel is returned to the atmospheric pressure, followed by removing the lid from the refining vessel to make the vessel open or releasing the lower end of the refining vessel from the molten metal in the ladle. Then, the treated molten metal is discharged from the refining vessel, or the ladle is taken out. The period from the end of these operations to the start of next treatment is a waiting period. Use of a filter-type dust collector in a vacuum evacuating apparatus is known, for example, from JP-A-6-17115. In such a system the dust collector must be connected to a vacuum/reduced pressure refining vessel and used in a hermetically closed state during the vacuum/reduced pressure refining treatment, and thus there is no suction of excess air therein during the treatment. When dusts in an unoxidized metallic state are generated in the vacuum/reduced pressure refining vessel, the dusts reach the dust collector, while keeping the unoxidized state. As a result, when air invades into the dust collector for some reasons such as pressure returning to the atmospheric pressure by air, etc., the metal dusts deposited on the filter react with the air, thereby causing problems of oxidation/heat generation. As a result, in case of filter cloth as used for a filter, the filter cloth is damaged by heat or completely burnt in serious cases. In case of ceramics as used for a filter, the filter itself undergoes no direct damage by heat, but the collected dusts are sintered to make clogging in the filter meshes or impair the filterability of sound filter due to solidification on the filter. To solve these problems, JP-A-8-3627 discloses that, in the case that combustible substances are contained in dusts, the dust collector must be subjected to pressure returning or back washing with an argon gas or a nitrogen gas to prevent damaging of filter by the air introduced at the time of pressure returning after the vacuum degassing treatment of treated molten metal. DE-B-1225679 discloses a process and an apparatus for inactivating spontaneously inflammable metal dust accruing during vacuum steel degassing 1) The problem of filter damage at the time of pressure returning to the atmospheric pressure just after the vacuum/reduced pressure treatment can be solved by the above-mentioned measures, but any measures have not been so far taken to the start of next treatment, etc. That is, even if back washing with an argon gas, a nitrogen gas or the like is carried out after the treatment, all the dusts captured on the filter cannot be separated and cannot fall down, and a portion of dusts may be still kept to remain as attached to the filter till the start of next treatment. In the case that the remaining dusts contain unoxidized fine powders of metal of high oxygen affinity such as magnesium, etc., we have another problem of filter damaging at the start of next treatment, even if the pressure returning with an argongas, a nitrogen gas or the like is carried out. Specifically, filter damaging as a result of suction of a large amount of air from the open connection port at upstream side (refining vessel) of the dust collector into the dust collector at the start of vacuum/reduced pressure refining treatment, for example, from the open port of an expansion joint unconnected to the refining vessel, the open port of the refining vessel out of lid engagement, the lower ends of RH dipping pipes, etc. occurs, for example, when reduced pressure evacuating apparatus 4 is to be started before expansion joint 9 is connected, as in case of a vacuum/reduced pressure refining facility having expansion joint 9 between vaccum/reduced pressure refining vessel 1 and dust collector 3 in upstream duct 5, as shown in Fig. 4, or when reduced pressure evacuating apparatus 4 is to be started before vacuum lid 14 is completely engaged in a vacuum/reduced pressure refining facility as shown in Fig. 9 , or when reduced pressure evacuating apparatus 4 is to be started before suction pipes 19 are dipped into molten metal 13 by elevating ladle 17 in a vacuum/reduced pressure refining facility as shown in Fig. 10.Furthermore, when the refined molten metal is to be exchanged with untreated molten metal during the operation of vacuum/reduced pressure treatment, pressure returning to the atmospheric pressure is carried out, and then the lid is released from the refining vessel or the bottom end of the. vessel is released from the molten metal to exchange molten metals. During the exchangement or the waiting period between one treatment and another, the atmospheric air invades into the refining vessel and duct 5 connecting the refining vessel to the dust collector from the port open to the atmophere. Simplified duct is shown in Fig. 4, etc., but actually a gas cooler, a cyclone separator, etc. (not shown in the drawings) are provided in the duct. That is, the actual duct often has a large net capacity. Thus, at the initial stage of the treatment, not only the air introduced from the outside by suction but also the air remaining in the duct extending from the dust collector to the refining vessel is led to the dust collector to oxidize the remaining dusts on the filter to generate heat and damage the filter in some cases. 2) No measures have been so far taken to prevent filter cloth from damaging due to oxidation of dusts deposited on the filter cloth or to prevent ceramic filter from clogging, caused by suction of atmospheric air from the dust discharge port during the vacuum/reduced pressure treatment, or to prevent apparatus from damaging and dust discharge trouble due to oxidation and sintering of dusts accumulated at the lower part of the dust collector through separation and falling down. That is, vacuum sealing is carried out by providing a valve, a lid or the like for vacuum sealing at the dust discharge port, but its sealability is liable to deteriorate by the dusts due to the function to pass the dusts, and leakage is more liable to occur than at other positions of the vacuum/reduced pressure refining facility. When the degree of leakage is considerably high, the filter is damaged by the oxygen in the introduced air by suction during the vacuum/reduced pressure treatment. Even if the degree of leakage is not so high to directly damage the filter, the dusts accumulatd at the lower part of the dust collector through separation and falling down are oxidized by the oxygen to generate heat and cause such problems as vacuum seal damage and discharge troubles due to dust sintering, etc. 3) Any industrial process for stably discharging dusts in an unoxidized metallic state of high reactivity with oxygen, as mentioned above, from the dust discharge outlet during the off period from the vacuum/reduced pressure treatment has not been known yet. That is, even if the pressure returning is carried out with a non-oxidizing gas after the vacuum/reduced pressure treatment, and once the atmospheric air is introduced into the dust collector from the dust discharge outlet during the successive period of discharging the dusts once collected on the filter and then separated to fall down from the filter to the outside of the dust collector, the remaining dusts deposited on the filter are oxidized to give heat damages when filter cloth is used as a filter, and cause dust sintering and clogging when ceramics are used as a filter, thereby giving troubles to the functions of the dust collector. Furthermore, oxidation and heat generation by the atmospheric air of dusts, which are near the dust discharge outlet or in the course of discharging, cause heat damaging of devices near the outlet such as vacuum seal packings, etc. or discharge troubles due to dust sintering and solidification. 4) No measures have been so far taken to suction of atmospheric air, etc. during the period from the pressure returning to the start of next treatment. That is, unless air invasion into the dust collector by leakage, etc. is prevented after the pressure returning, the filterability of the filter is deteriorated by the remaining dusts, or the remaining dusts undergo reaction and sintering, causing such problems as troubles to the next discharging. Thus, the object of the present invention can be achieved by the features defined in the claims. The invention is described in more detail in connection with the drawings, in which; Fig. 1 is a view showing one embodiment of the present vacuum/reduced pressure refining facility, Fig. 2 is a view showing another embodiment of the present vacuum/reduced pressure refining facility, Fig. 3 is a view showing an embodiment of an expansion joint provided with a seal lid at the open port of duct sided with the dust collector in the present vacuum/reduced pressure refining facility, Fig. 4 is a view showing a further embodiment of the present vacuum/reduced pressure refining facility, which is an embodiment of a vacuum/reduced pressure refining facility for carring out the present vacuum/reduced pressure refining process C, Fig. 5 is a view showing a still further embodiment of the present vacuum/reduced pressure refining facility, which is an embodiment of a vacuum/reduced pressure refining facility for carring out the present vacuum/reduced pressure refining process B, Fig. 6 is a view showing a further embodiment of a vacuum/reduced pressure refining facility for carrying out the present vacuum/reduced pressure refining process A, Fig. 7 is a view showing a still further embodiment of a vacuum/reduced pressure refining facility for carrying out the present vacuum/reduced pressure refining process B, Fig. 8 is a view showing a still further embodiment of a vacuum/reduced pressure refining facility, Fig. 9 is a view showing a still further embodiment of a vacuum/reduced pressure refining facility, Fig. 10 is a view showing a still further embodiment of a vacuum/reduced refining facility, Fig. 11 is a view showing an embodiment of the structure of a dust discharge port, and Fig. 12 is a view showing another embodiment of the structure of a dust discharge port. At first, description will be made below referring to the first embodiment of the present invention . For evacuating apparatus 4, as shown, for example, in Fig. 1, any type or any structure of evacuating apparatuses, for example, and ejector or a mechanical pump, can be used, so long as it can attain pressure reduction in a refining furnace and a dry type dust collector. For filter 2 in a dry type dust collector, filters of any material, for example, filter cloth or ceramics, can be used. The filter is used against all such dusts that can cause heat damage or clogging, on which the present invention has an effect. The term "connection port" herein used means a part of shield enclosure wall for forming a hermetically closed space during the vacuum/reduced pressure refining period, the part being made open for some reasons during the off-period from the vacuum/reduced pressure refining period. Specifically, in such a vacuum/reduced pressure refining facility as shown in Fig. 1, the connection port refers to open port 24a of expansion joint 9, etc., which takes part in engaging or disengaging of vacuum lid 14 with or from refining vessel 1. In such a vacuum/reduced pressure refining facility as shown in Fig. 2, the connection port refers to open port 24b of vacuum/reduced pressure refining vessel 1, which takes part in engaging or disengaging of vacuum lid 14 with or from the refining vessel 1. In such a vacuum/reduced pressure refining facility as shown in Fig.10, the connection port refers to open port 24c at the lower end of suction pipe 19. Closing of the connection port means, for example, connection of open port 24a of the expansion joint 9 to the open part of vacuum/reduced pressure refining vessel 1 to hermetically close the latter as shown in Figs. 1 and 8; engaging of vacuum lid 14 with the open part of the refining vessel 1 to hermetically close the latter, as shown in Figs. 2 and 9; and dipping of open port 24c at the lower end of suction pipe 19 into molten metal as shown in Fig. 10 to hermetically close the latter. Needless to say, all other routes to the atmosphere, for example, leak valve 15, etc. must be closed beforehand. The term "non-oxidizing gas" herein used means a gas incapable of causing oxidation (combustion) reaction with unoxidized metallic dusts (fine powders), specifically an inert gas such as a nitrogen gas or an argon gas. In a strict sense, it does not mean only chemically inert elements, but a gas substantially incapable of causing oxidation (combustion) reaction with unoxidized metallic dusts (fine powders). When the filter of the dust collector is made of a non-combustible material, for example, ceramics, it can include even a CO gas. Why the term "substantially" is herein used is because the necessary upper limit oxygen concentration for preventing a filter damage depends on the species, concentration, etc. of unoxidized metal elements contained in the dusts and thus cannot be simply defined. For example, even in case that at least 10% of fine powder dusts of metallic magnesium, metallic manganese, etc. is contained, the filter is not damaged at all, so long as the oxygen is replaced by the non-oxidizing as to an oxygen concentration of not more than about 2 - 3%. Opening of the connection port means releasing of the hermetically closed state established by closing the connection port as mentioned above, thereby exposing the connection port to the atmosphere. Pressure returning is to return the pressure of the facility atmosphere, which is once reduced to less than the ambient pressure, again to substantially the ambient atmospheric pressure, thereby establishing a facility pressure almost to such a degree as not to make suction of the ambient atmospheric air through clearances of the facility. For example, a difference of about 20 to 50 Torr will not make suction of the ambient atmospheric air and operations of opening the vacuum lid and the expansion joint can be fully carried out, when the atmosphere of the reduced pressure in the facility is maintained in such a manner that connection, valve, ect. have the ordinary vacuum seal function. Furthermore, the term "open port siding with the refining vessel and existing upstream of block valve" herein used means a cross-sectional open port of a duct, etc. when the connection port is opened. To prevent filter damages, all the connection ports to the ambient atmosphere, which are upstream of the dust collector, must be closed to establish a hermetically closed state in the duct from the furnace to upstream block valve 7, and then block valve 7 is to be opened to operate dust collector 3. Specifically, block valve 7 in upstream duct 5 is to be opened, after expansion joint 9 is connected to connection port of vacuum lid 14 in case of Fig. 1, or after vacuum lid 14 is allowed to descend and is engaged with vacuum/reduced pressure refining vessel 1 in case of Fig. 2, or after ladle 17 is allowed to ascend and suction pipe 19 is dipped into molten metal 13 in case of Fig. 10. Closing of the connection ports to establish the hermetically closed state includes, needless to say, closing leak valve 15, etc., as opened to return the facility pressure, if any, besides the above-mentioned expansion joint, the vacuum lid, etc. In brief, it is essential to bring dust collector 3 into operation after the completely hermetically closed state has been established. The dust collector must be brought into operation by starting evacuating apparatus 4 and opening downstream block valve 8 when or before upstream block valve 7 is opened. That is, evacuating apparatus 4 must be operated before block valve 7 is opened, thereby establishing a hermetically closed state, and suction and filtration of loaded gas are carried out by opening upstream block valve 7 to bring dust collector 3 into operation. Even if dust collector 3 is brought into operation after the hermetically closed state has been established, as mentioned above, and if the inside volume of duct 5, etc. from upstream block valve 7 in duct 5 to vacuum/reduced pressure refining vessel 1 is large, atmospheric oxygen remaining in duct 5 etc. can give considerable damages to filter 2 at the initial stage of dust collector operation. For example, an oxygen concentration of nearly 20% sometimes prevails within first one minute at the initial stage of dust collector operation. To prevent such a state, a piping 10 for introducing a non-oxidizing gas and an opening/closing valve 11 are provided upstream of block valve 7 in duct 5, as shown in Fig. 1, to inject the non-oxidizing gas into duct 5 upstream of block valve 7 and substantially replace the residual oxygen in duct 5, etc. with the non-oxidizing gas, and then the connection port to the ambient atmosphere is closed. For the position of injecting the non-oxidizing gas, a position with a good replacement efficiency must be selected in view of the entire structure and arrangement of the vacuum/reduced pressure refining facility. Generally, a position far from the connection port, for example, a position near block valve 7 in upstream duct 5, as shown in Fig. 1, is desirable. In case of large open port area, as shown in Fig. 2, it is efficient to make the injection through a plurality of pipings 10 just before vacuum lid 14 is brought into a hermetically closed state. The piping for introducing the non-oxidizing gas into the upstream duct sided with the furnance and upstream of the block valve in the upsteream duct must be provided with an opening/closing valve capable of starting or interrupting the gas flow to inject the necessary non-oxidizing gas for substantial replacement, depending on the structure and arrangement of the vacuum/reduced pressure refining facility. The procedure for injecting the non-oxidizing gas is limited not only to use of special piping as mentioned above. For example, a bottom injection type non-oxidizing gas for refining in the vacuum/reduced pressure refining vessel may be used. In the case as shown in Fig. 1, gas replacement for the duct extending from expansion joint 9 to vacuum/reduced pressure vessel 1 can be efficiently carried out with the bottom-injected non-oxidizing gas in the furnace, and can take likewise other facility arrangements. Another procedure for reducing the oxygen concentration in the air remaining in the duct, etc. upstream of block valve 7 to prevent the filter damages at the initial stage of the dust collector operation is previously to replace the remaining air with a non-oxidizing gas such as a nitrogen gas, an argon gas, etc. before starting the vacuum/reduced pressure refining treatment. Most efficient timing for replacing the remaining air with the non-oxidizing gas beforehand is to utilize the pressure returning at the end of the last refining treatment. That is, before opening leak valve 15 and disengaging expansion joint 9 at the end of vacuum/reduced pressure refining period, pressure should be returned in the duct upstream of the block valve with the non-oxidizing gas by closing block valve 7 at upstream duct 5, where it is efficient to utilize piping 10 and opening/closing valve 11 for introducing the non-oxidizing gas to the duct upstream of the block valve, though not limited only to such a piping. That is, the bottom-injected stirring gas to the furnace, ladle, etc. can be used together with or as a substitute for the above-mentioned non-oxidizing gas, so long as it is also a non-oxidizing gas. It is most efficient to utilize the timining of pressure returning, as mentioned above, for the replacement of the remaining air in the duct, etc. on the upstream side with the non-oxidizing gas, but when the waiting period for the next treatment is long, air gradually leaks into the duct through open port 24a of expansion joint 9, etc. to increase the oxygen concentration in the duct. To prevent the leakage, detachable seal lid 21 to the open port sided with the dust collector is provided at the joint such as expansion joint 9, etc., as shown in Fig. 3, and seal lid 21 is closed during the waiting period from the end of pressure returning and gas replacement to the start of the next vacuum/reduced pressure refining treatment, thereby closing the open port of the duct 5 sided with the duct collector. Seal lid 21 as shown in Fig. 3 is directed to expansion joint 9 and comprises a seal lid 21 proper, a seal lid elevating/descending means 22 and a cylinder 23 for hermetically closing with the seal lid. After exapansion joint 9 is retreated and disengaged from the connection, seal lid 21 descends from the overhead to be faced to the duct open port of expansion joint 9 sided with the dust collector, and then closely positioned to the open port by cylinder 23 to effect hermetical closing of the seal lid. Seal lid 21 is not always limited to this structure and can take other structures and arrangements, so long as it has a function to close the open port during the waiting period out of refining without giving any adverse effect on establishing a tightly closed evacuating system during the period of vacuum/reduced pressure refining treatment. Seal lid 21 can be provided at any position that enables substantial prevention of air invasion into duct 5, etc. with the non-oxidizing gas introduced by replacement at the time of the pressure returning, as mentioned above. For example, in case of a vacuum/reduced pressure refining facility as shown in Fig. 2, such an appropriate position is an open port at the upper end of refining vessel 1. As a best alternative, the seal lid can be provided at the connection part of refining vessel 1 to upstream duct 5 with a partial effect. In case of a vacuum/reduced pressure refining facility as shown in Fig. 10, the seal lid is provided at the lower end of suction pipes 19. Next, description will be made below, referring to the second embodiment which is not the subject matter of the present invention. "Substantially shutting off the atmospheric air" can be attained by controlling the oxygen concentration of the atmosphere in sealing enclosure 54 in Fig. 4 to a few percent by means of a gas injected through piping 47 without any necessity of strictly closed space in sealing enclosure 54 in contrast to the vacuum evacuating system. "Non-oxydizing gas" has the same meaning as defined before. "Vacuum evacuating period" means a period during which the inside of the dry type dust collector is maintained under reduced pressure below the ambient atmospheric pressure and the atmospheric air is introduced into the dry type dust collector from dust discharge outlet 39 by suction. Dust discharge outlet 39 is independent on type and structure, so long as it can be vacuum sealed during the period of vacuum/reduced pressure refining treatment and can discharge dusts, when required, during the off period out of the vacuum treatment. Structural examples of dust discharge outlet 39 are shown by reference numeral 39 Figs 5, 11 and 12, respectively. Basic idea is to prevent the inside dusts from oxidation and heat generation, even if there is any leakage, by replacing the atmospheric air at the outside of the ready-to-leak positions with a non-oxidizing gas, since it is technically difficult to completely prevent any leakage. "Leakage" means unintended suction of atmospheric air from the outside, which occurs at joints of a vessel, ducts, etc. for forming a vacuum space and at valves, etc. to the outside. Sealing of particularly the dust discharge port in preference to other positions and valves of dry type dust collector such as leak valves, etc. during the period of vacuum/reduced pressure refining is carried out to take measures against a vacuum seal failure to lead easily to leakage for the following two reasons. That is, the first reason is a liability to a hermetic seal failure, etc. due to dust intrusion into seal parts and the second reason is a degradation of seal parts due to wear caused by high attrition of dusts. When the sealability is deteriorated, dusts falling down from the filter are liable to accumulate around the dust discharge port, and thus the apparatus parts are easily damaged by oxidation and heat generation of dusts, such as heat deterioration of seal O-rings. Furthermore, dusts per se, once sintered to agglomerate by oxidation and heat generation of dusts, will be a trouble in dust discharging after the vacuum/reduced pressure refining treatment. For these reasons, it is particulary necessary to seal the immediate outside of the dust discharge port with a non-oxidizing gas during the period of vacuum/reduced pressure refining treatment. This embodiment will be described in detail below, referring to drawings. Figs. 11 and 12 show examples of a vacuum seal valve and a vacuum seal lid. Vacuum seal valve 30 may be any of ordinary vacuum ball valves, butterfly valves, etc. and vacuum seal lid 44 is independent on type and structure, so long as it can attain vacuum sealing. According to the prior art, there is the atmospheric air at the outside (bottom side) of vacuum seal valve 30 and vacuum seal lid 44, and once leakage takes place at the vacuum seal part, oxygen-containing air leaks in by suction. On the other hand, sealing enclosure 54, as shown in Fig. 4, is provided to shut the outside (bottom side) of vacuum seal valve 30 and vacuum seal lid off the atmospheric air. Function of dust discharge outlet 39 to bring the dusts into the off-line from dry type dust collector 3 requires door 53 capable of be freely opened or closed for discharging dusts from dust discharge outlet 39 to the outside of sealing enclosure 54. To bring the inside of sealing enclosure 54 into a non-oxidizing gas atmosphere during the vacuum evacuating period, piping 47 for introducing the non-oxidizing gas therein is also required, and opening/closing valve 48 is required to interrupt the introduction of the non-oxidizing gas when any sealing is required during the off-period out of the treatment or dust discharging period through open door. Without the interruption of the introduction of the non-oxidizing gas, the object can be attained, but the above-mentioned procedures are industrially essential from the viewpoint of cost. Fig. 5 shows an example of providing rotary valve 46 as an auxiliary apparatus for discharging dusts. In a wide sense "auxiliary appratus for discharging dusts" means an auxiliary apparatus for discharging dusts, including a screw conveyer, etc. besides the rotary valve. That is, the auxiliary apparatus for discharging dusts generally refers to apparatuses without any vacuum seal, as provided to adjust a discharge rate to an appropriate one for successive dust transport by pneumatic conveying, etc. Heretofore, there has been neither means nor apparatus for introducing the non-oxidizing gas in a space between vacuum seal valve 30, etc. and the auxiliary apparatus for discharging dusts. In this embodiment a space between vaccum seal valve 30, etc. and the auxiliary apparatus for discharging dusts is utilized to be functioned as a substitute for the sealing enclosure, and the non-oxidizing gas is likewise introduced therein to replace or maintain the outside atmosphere around vacuum seal valve 30, etc. with or in a non-oxidizing gas during the vacuum evacuating period. It is the present vacuum/reduced pressure refining process to seal the outside of the vacuum seal valve or vacuum seal lid at the dust discharge outlet at the bottom of dry type dust collector with a non-oxidizing gas during the vacuum evacuating period for operating the dry type dust collector by means of the present vacuum/reduced pressure refining facility. Next, description will be made below, referring to the third embodiment which is not the subject matter of the present invention. The present vacuum/reduced pressure refining facility, which comprises at least a vacuum/reduced pressure refining furnace, a dry type dust collector with a filter and a freely opening/closing dust discharge outlet provided at the bottom, and an evacuating means, is characterized in that a transport piping for pneumatically transporting discharged dusts is hermetically connected to the outside of the dust discharge outlet, a supply piping for a non-oxidizing gas for the pneumatic transport is provided at the transport piping, and an apparatus in a heat-resistant structure, or a cooling structure or an apparatus in a dust-cooling structure is provided at a destination point of pneumatic transport by the transport piping. When dusts are discharged from the dry type dust collector, the atmospheric air invades by at least a substitution or displacement volume corresponding to the volume of discharged dusts into the dry type dust collector by suction. One idea to prevent such a suction is to introduce a non-oxidizing gas by at least an equivalent volume into the dry type dust collector separately. When the open port area is large, the atmospheric air invades into the dry type dust collector by natural convection. To prevent such invasion, it is necessary-to increase the amount of non-oxidizing gas to be introduced and maintain the dischrging of non-oxidizing gas from the open port. The present vacuum/reduced pressure refining process A based on this idea is shown, for example, in Fig. 6. According to an alternative, the invading atmospheric air can be replaced with non-oxidizing gas for the air. Specifically, a non-oxidizing gas atmosphere is made dominant at the outisde of dust discharge outlet. The present vacuum/reduced pressure refining process B is based on such an alternative. A suitable facility for carrying out this vacuum/reduced pressure refining process is the present vacuum/reduced pressure refining facility. It is an object to prevent air invasion into the dry type dust collector during the dust discharging period and also to prevent oxidation of dusts by air, and thus it is a premise that the non-oxidizing gas atmosphere is dominant in the dry type dust collector before the start to discharge dusts. For example, filter 2 and evacuating apparatus 4 of dry type duct collector shown in Fig. 6 have the same meanings as defined before. Dust discharge outlet 69 has the same meaning as that of dust discharge outlet 39 as mentioned before. Non-oxidizing gas has the same meaning as defined before. Example of a method for introducing the non-oxidizing gas is shown in Fig. 6. Special piping 64 for introducing the non-oxidizing gas during the dust discharging period may be used, or gas introduction piping 63 for returning the pressure or pipings for other purposes may be used. Since such non-oxidizing gas should not be introduced during the period of vacuum/reduced pressure refining treatment, it is essential to provide opening/closing valve 65 in special piping 64 for introducing the non-oxidizing gas. To adjust a proper amount of the injection gas to satisfy both function and cost at the same time, it is preferable to provide flow rate control valve 66 in specific piping 64 for introducing the non-oxidizing gas. Flow rate of the non-oxidizing gas to be introduced depends on the structure of dust discharge outlet 69, properties and amount of dusts and overall size and structure of dry type duct collector 3 and thus cannot be determined by single meaning. To substantially prevent air suction or convection invasion through dust discharge outlet 69, the non-oxidizing gas must be introduced into dry type dust collector 3 to such a degree as to allow the non-oxidizing gas to flow out of dust discharge outlet 69. Specifically, the flow rate must be determined upon adjustment based on trial run, etc. Most preferable period for introducing the non-oxidizing gas into the dry type dust collector is a period from the start to introduce the gas just before starting to open the dust discharge outlet to discharge dusts to the end to close the dust discharge outlet after the completion of dust discharging. The gas introduction can be started/discontinued at the same time as opening/closing the dust discharge outlet, depending on such conditions as small size of dust discharge outlet, rapid opening/closing speed, etc. According to the present vacuum/reduced pressure refining process B, the non-oxidizing gas atmosphere must be kept dominant at the outside of the dust discharge outlet. Degree of the gas atmosphere to be kept dominant is enough, if the oxygen concentration can be controlled to a few percent or less. Thus, the degree of sealing the apparatus for maintaining the non-oxidizing gas atmosphere is not necessary for attaining strict sealing as in the vacuum sealing, and is enough, if the non-oxidizing gas atmosphere can be substantially maintained. A coverage of the non-oxidizing gas atmosphere may be such that the above-mentioned oxygen concentration can be maintained just outside the dust discharge outlet so as not to allow the air to leak into the dust collector by suction through the dust discharge outlet. The period of maintaining the non-oxidizing gas atmosphere is the same period as for introducing the non-oxidizing gas into the dry type dust collector according to the above-mentioned vacuum/reduced pressure refining process A. For example, the present vacuum/reduced pressure refining process C as shown in Fig. 4 is a process for carrying out the present vacuum/reduced pressure refining processes A and B at the same time. Example of the present vacuum/reduced pressure refining facility suitable for carrying out the present vacuum/reduced pressure refining process B is shown in Fig. 7. First of all, transport piping 75 for pneumatically transporting discharged dusts is hermetically connected to the outside of dust discharge outlet 69. If not hermetically connected, air invasion takes place and the non-oxidizing gas atmosphere cannot be kept dominant at the outside of dust discharge outlet 69, with a failure to prevent contact of dusts with air, and resulting heat generation, air suction into the dry type dust collector and resulting troubles. So long as the hermetic connection can be attained, a discharge auxiliary apparatus such as rotary valve 76 etc. can be provided between dust discharge outlet 69 and transport piping 75. Transport piping 75 is provided with supply piping 77 for introducing the non-oxidizing gas for pneumatic transport. By introducing the non-oxidizing gas through supply piping 77, dusts can be pneumatically transported, while keeping the non-oxidizing gas atmosphere dominant at the outside of dust discharge outlet 69. When an oxidizing gas such as air, etc. is used as a gas for pneumatic transport, air invades into dry type dust collector 3 through dust discharge outlet 69 to damage filter 2, cause heat damages/deteriorations of devices such as packings, etc. near dust discharge outlet 69 or cause discharge troubles due to sintering/solidification of dusts. Furthermore, pneumatic transport troubles, due to piping damages/deteriorations caused by dust heat generation in transport piping 75 and plugging due to sintering/solidification of dusts, take place. An apparatus in a heat-resistant structure or cooling structure or an apparatus in a dust-cooling structure must be provided at a destination point of pneumatic transport by transport piping 75. The dusts, when pneumatically transported by the non-oxidizing gas after returning the pressure by the non-oxidizing gas, are released from transport piping 75 finally at the destination point of pneumatic transport and brought into contact with oxygen in the air. When the dusts contain fine metal powders in an unoxidized metallic state such as Mg, Mn, etc., heat generation takes place there. Thus, it is essential that the apparatus at the destination point of pneumatic transport is in such a structure as not to cause damages of the apparatus even if the dusts undergo strong heat generation. In other words, when the apparatus at the destination point of pneumatic transport is a secondary dust collector using filter cloth, the filter cloth may be burnt due to heat generation of dusts in some cases. Specific examples of the apparatuses at the destination point of pneumatic transport include apparatuses in a heat-resistant structure, such as a refractory-lined dust pot, a refractory-lined dust-collecting duct, etc., apparatuses in a cooling structure such as a water-cooled dust-collecting duct, a gas cooler, a water-cooled cyclone separator, etc., and apparatuses in a structure enabling direct cooling of dusts per se such as a water tank, a dust-collecting duct through which passes an ordinary temperature gas having a heat capacity large enough to take up the quantity of heat generation of pneumatically transported dusts, etc. Furthermore, it is not desirable from the viewpoint of cost not to pass the non-oxidizing gas except the period of pneumatically transporting the dusts, and thus it is desirable to provide opening/closing valve 78 in the supply piping for introducing the non-oxidizing gas for pneumatic transport. Furthermore, to establish suitable conditions for pneumatically transporting the dusts, it is desirable to provide a pressure control device and flow rate control device 79 in supply line 77 for introducing the non-oxidizing gas for pneumatic transport. Examples of other facility capable of carrying out the present vacuum/reduced pressure refining process B than the present vacuum/reduced pressure refining facility as shown in Figs. 6 and 7 include those shown in Figs. 4 and 5, where the flow rate of the non-oxidizing gas, for example, Ar, is not the same. Next, description will be made below, referring to the fourth embodiment which is not the subject matter of the present invention. The period from the end of pressure returning to the start of next treatment is the so called waiting period, during which the atmospheric air invades in case of a negative pressure (pressure lower than the atmospheric pressure), even if the dry type dust collector is not in operation, and the oxygen in the atmospheric air may react with metals remaining in the system, as attached, to ignite the metals, resulting in damaging the filter and other devices near the dust-remaining positions, for example, the vacuum valve, vacuum seal packings, etc. at the port for discharging dusts. Remaining dusts are liable to cause a failure in hermetically closing the seal part of block valve due to the presence of dusts and deteriorate the vacuum seal more often than usual due to the attrition of seal members by dusts. Even if the connection ports of the upstream/downstream block valve, dust-transporting devices or the like to the atmospheric air are fully closed, it is industrially difficult to maintain the completely hermetically closed state. Furthermore, the temperature of the dust collector and internal structures decreases in the course of the treatment period to the treatment end, and the volume of the filled non-oxidizing gas shrinks at the time of pressure returning. To compensate for it, it is necessary to continuously or intermittently inject the non-oxydizing gas such as a nitrogen gas, an argon gas, etc. into the dust collector to suppress an increase in oxygen concentration due to the leakage through the valves. The flow rate of the gas to be injected is enough, if it can maintain the inside of the dust collector under a superatmospheric pressure, i.e. the so called positive pressure, and should be determined in view of the structure and capacity of the individual apparatuses and devices and leakage through the valves. A higher flow rate has no problem to the object, so long as it can provide a positive pressure, but is useless from the view point of cost. Specifically, as shown in Fig. 6, the non-oxidizing gas is injected into dry type dust collector 3 during the waiting period following the pressure returning to provide a positive pressure, using non-oxidizing gas injection piping 64 for injecting a non-oxidizing gas such as a nitrogen gas, an argon gas, etc. to dry type dust collector 3, opening/closing valve 65 and manual or automatic flow rate control valve 66 for adjusting the flow rate to necessary one. It is preferable to use other piping and opening/closing valve than those for pressure returning, as shown in Fig. 6, but gas-introducing piping 63 for use in injection of a non-oxidizing gas such as a nitrogen gas, and argon gas, etc. during the pressure-returning period may be used to this effect. For the other piping, it is preferable that dry type dust collector 3 is provided with a non-oxidizing gas injection piping 64 with opening/closing valve 65 having a non-electrically/non-pneumatically opening function to open/close the valve freely and flow rate control valve 66, and also with safety valve 61 capable of opening when the inside of dry type dust collector 3 is brought into a superatmospheric pressure, besides gas introduction piping 63 for pressure returning by closing both freely opening/closing block valve 7 provided in upstream duct 5 for connecting vacuum/reduced pressure refining vessel 1 to dry type dust collector 3 and freely opening/closing block valve 8 provided in downstreatm duct 6 for connecting dry type dust collector 3 to reduced pressure evacuating apparatus 4. First reason for the preference for providing the other piping is to design non-oxidizing gas injection piping 64 in a control system effectively capable of automatic opening at an uncontrollable time, i.e. so called non-electrically/non-pneumatically opened state, whereas opening/closing valve 59 for pressure returning is usually designed in such a control circuit so as to attain automatic closing, i.e. the so called non-electrically/non-pneumatically closed state at an uncontrollable time such as an electric power failure, interruption of driving compressed air supply, etc. to prevent troubles such as excessively high returned pressure, etc. The term "non-electrically/non-pneumatically opened" herein used broadly means an "emergency opening" design to open the valve by a spring force, etc. at any uncontrollable time caused not always by interruption electric power/compressed air supply. Second reason is to use a low flow rate such as not more than 1 Nm 3 /min. at maximum for providing a positive pressure during the waiting period, whereas in case of pressure returning, a high flow rate, such as a several tens Nm 3 /min, must be used because the pressure returning must be carried out usually for a short time such as less than a few minutes. Thus, when one and same piping is used, it is inevitable to set two flow rates in the flow rate control valve, etc., and it is usually difficult to obtain a flow rate control valve capable of controlling the flow rate in a broad ratio ranging from 1 to more than several tens. Furthermore, in the embodiment of Fig. 6, the inside of dry type dust collector 3 is kept under a positive pressure in a nitrogen gas atmosphere by providing safety valve 61 set to a little higher discharge pressure than the atmospheric pressure on dry type dust collector 3 and always continuously injecting a non-oxidizing gas therein at a flow rate to a little excessive degree during the waiting period. Positive pressure can be maintained by manipulating opening/closing valve 65 in non-oxidizing gas injection piping 64 as interlocked with values indicated by a pressure-detecting device in dry type duct collector 3 and intermittently injecting the gas so as to keep the inside of dry type dust collector 3 neither in a negative pressure nor in an excessive positive pressure, but to this effect it is desirable to provide a backup device capable of continuously maintaining a function to keep a positive pressure even at an electric power failure, etc. (Examples) First of all, description will be made below, referring to Examples according to the first embodiment of the present invention. A specific example will be given by operating results of oxidation/reduction refining of molten steel containing slags in vacuum/reduced pressure refining vessel 1 having a 60-ton capacity shown in Fig. 1. For filter 2 a Tetron-made filter having an ordinary heat-resistant temperature of 130°C was used. Filter damages were checked not by visual observation after every vacuum/reduced pressure refining treatment, but by judging the soundness of the filter by way of filter pressure loss measured at positions before and after the filter, concentration, PH, etc. of effluent water from a condenser (not shown in the drawing) of downstream reduced pressure evacuating apparatus 4, and filter 2, when determined abnormal thereby, was directly checked. (Example 1) After connection of expansion joint 9 was completed at the satrt of vacuum/reduced pressure refining treatment, block valve 7 in upstream duct 5 was opened. Before opening block valve 7, reduced pressure evacuating apparatus 4 was operated and downstream block valve 8 was opened. As a result, the filter was found sound for the plain steel, but damaged at the next vacuum/reduced pressure refining treatment of high Mn steel. (Example 2) At the start of vacuum/reduced refining treatment, a nitrogen gas was introduced from piping 10 for 60 seconds and then expansion joint 9 was connected. After the end of connection, block valve 7 in upstream duct 5 was opened. Before opening block valve 7, reduced pressure evacuating apparatus 4 was operated and downstream block valve 8 was opened. As a result, the filter was not damaged at all. (Example 3) At the time of ending the last vacuum/reduced pressure refining treatment, the pressure of the side upstream of block valve 7 in upstream duct 5 was returned with a nitrogen gas by utilizing piping 10 and furnace bottom injection. At the start of the present vacuum/reduced pressure refining treatment, operation was carried out in the same manner as in Example 1. As a result, the filter was not damaged during the period of continuous treatment, but damaged during the period of treatment following the waiting period for 2 hours. (Example 4) In Example 3, the open port of expansion joint 9 at the dust collector side was closed with a seal lid throughout the waiting period. As a result, the filter was not damaged, irrespective of the duration of the waiting period. (Example 5) In Example 3, a nitrogen gas was injected from piping 10 for 30 seconds at the start of the present vacuum/reduced pressure refining treatment. As a result, the filter was not damaged during the period of continuous treatment, but damaged during the period of treatment following the waiting period for 8 hours. (Example 6) In Example 4, a nitrogen gas was injected from piping 10 for 20 seconds at the start of the present vacuum/reduced pressure refining treatment. As a result, the filter was not damaged, irrespective of the duration of the waiting period, including the case of high Mn steel. (Comparative Example 1) At the start of vacuum/reduced pressure refining treatment, before the end of connecting expansion joint 9, reduced pressure evacuating apparatus 4 was operated, and then block valve 7 was opened to introduce the gas into dust collector 3. As a result, the filter was damaged by burning during the period of 6th treatment. Next, description will be made below referring to Example according to the second embodiment. (Example 7) The embodiment was applied to oxidation/reduction refining to molten steel containing slags in vacuum/reduced pressure refining furnace 1 having a capacity of 60 tons as shown in Fig. 4. Dry type duct collector 3 used filter cloth of Tetron having an ordinary heat-resistant temperature of 130°C as filter 2. At port 39 for discharging dusts from dry type dust collector 3 was used a pneumatically driven vacuum ball valve as vacuum seal valve 30. At every time after end of pressure returning after the vacuum/reduced pressure refining, vacuum seal ball valve 30 was opened to discharge the dusts. Initially, the bottom side of vacuum seal valve 30 was open to the atmospheric air and only dust receiving box 42 was provided below the valve, as shown in Fig. 11 as a comparative example. As a result, heat was generated in conical region 55 at the lower part of dry type dust collector 3 during the vacuum evacuating period, and dusts were sintered in conical region 55 in three runs of total 20 runs. Dusts were impossible to discharge after the treatment and holes of small bean size were made even through the filter cloth. Then, seal enclosure 54 was provided below vacuum seal valve 30 as shown in Fig. 4, and the inside of seal enclosure 54 was flushed with a nitrogen gas. Then, the vacuum/reduced pressure refining was carried out. Oxygen concentration in seal enclosure 54 was measured by an oxygen concentration meter and the nitrogen gas flow rate was so set as to keep the oxygen concentration not more than about 2%. As a result, neither heat generation in conical region 55 during the vacuum evacuating period nor failure to discharge dusts after the treatment was found in total 50 runs. Furthermore, rotary valve 46 was provided below vacuum seal valve 30 and piping 47 for supplying a nitrogen gas was provided in short piping 39 connecting the vacuum seal valve to the rotary valve, so shown in Fig. 5. Nitrogen gas was passed from piping 47 at a flow rate of 0.3 Nm 3 /min during the vacuum evavuating period. As a result, neither heat generation in conical region 55 during the vacuum evacuating period nor failure to discharge dusts after the treatment was found in total 103 runs. Next, description will be made below, referring to Examples according to the third embodiment. (Examples 8 to 11) The embodiment invention was applied to oxidation/reduction refining of molten steel containing slags in a vacuum/reduced pressure refining furnace having a capacity of 60 tons. Filter cloth of Tetron having an ordinary heat-resistant temperature of 130°C was used as a filter. Damages of filter cloth were open checked after the operation for a specific duration of time. Dust discharging was carried out at every time after end of pressure returning after vacuum/reduced pressure refining. (Example 8) The present vacuum/reduced pressure refining process A was carried out in a vacuum/reduced pressure refining facility, as shown in Fig. 6 by injecting a nitrogen gas at a flow rate of 2 Nm 3 /min into dry type dust collector 3 during the dust discharging period. As a result, heat generation occurred in conical region 85 at the lower part of dry type dust collector 3 only in 3 runs of total 50 runs during the dust discharging period, but nor dust remaining nor failure to open/close ball valve 60 for discharging the dusts, etc. took place, and the filter cloth was also found sound. (Example 9) The present vacuum/reduced pressure refining process B was carried out in a vacuum/reduced pressured refining facility as shown in Fig. 4 by sealing the direct outside of dust discharge outlet 39 during the dust discharging period with a nitrogen gas, thereby making an oxygen concentration ≒ 1.5%. As a result, heat generation occurred in conical region 55 at the lower part of dry type dust collector 3 only in one run of total 63 runs during the dust discharging period, but nor dust remaining nor a failure to open or close ball valve 30 for discharging the dusts, etc. took place, and the filter cloth was also found sound. (Example 10) The present vacuum/reduced pressure refining process C was carried out in a vacuum/reduced pressure refining facility as shown in Fig. by providing the same seal enclosure 54 and non-oxidizing gas supply piping 47 at the lower part of dust discharge outlet 39 of the vacuum/reduced pressure refining facility as shown in Fig. 4, the process carrying out nitrogen gas injection into dry type dust collector 3 under the same conditions as in Example 8 and nitrogen gas sealing at the direct outside of dust discharge outlet 39 under the same conditions as in Example 9 at the same time. As a result, no heat generation, dust remaining, failure to open or close ball valve 30 for dishcarging the dusts, etc. took place at all, and the filter cloth was also found sound. (Comparative Example 2) In the vacuum/reduced pressure refining facility as shown in Fig. 6, neither nitrogen gas injection into dry type dust collector 3 during the dust discharging period nor maintenance of non-oxidizing gas atmosphere at the direct outside of dust discharge outlet 69 was carried out. As a result, heat generation, occured in conical region 85 at the lower part of dry type dust collector 3, occurred in 13 runs of total 20 runs during the dust discharging period, in two runs of which ball valve 60 for discharging the dusts underwent seizure and a failure to close the valve took place. Dust remaining due to sintering and solidification partly took place and holes of small bean size were made through the filter cloth after 20 runs of heat treatment. (Example 11) In the present vacuum/reduced pressure refining facility as shown in Fig. 7, dusts were pneumatically transported with a nitrogen gas supplied from non-oxidizing gas holder 80. As a result, no heat generation occurred at all in conical region 85 and transport piping 75 and no failure to open or close ball valve 60 for discharging the dusts was observed, either. (Comperative Example 3) A compressor was connected to supply piping 77 of the present vacuum/reduced pressure refining facility as shown in Fig. 7 to pneumatically transport dusts by air pressure. As a result, heat generation occurred in treansport piping 75 in 4 runs of total 10 runs, in two runs of which a failure to catch the dusts by rotary valve 76 and a failure to discharge the dusts from rotary valve 76 were observed. Next, description will be made below, referring to Example according to the fourth embodiment. (Example 12) Specific example will be given below, referring to operating results of oxidation/reduction refining of molten metal containing slags in vacuum/reduced pressure refining vessel 1 having a capacity of 60 tons, as shown in Fig. 6. Filter cloth of Tetron having an ordinary heat-resistant temperature of 130° was used as a filter. Damages of filter were open checked after operation for a specific period. Results of oxygen concentration measurement in dry type dust collector 3 in Example based on nitrogen gas injection during the waiting time according to the embodiment and in Comparative Example without the injection are shown in Table 1. In Table 2, filter damage after the oepration and the dust discharging state during the operating period are shown. It is evident therefrom that Example shows neither filter damages nor failure to discharge the dusts and is distinguished. Just after pressure returning 1hr thereafter 6hr thereafter 24hr thereafter Remarks Example 0.4% 0.5% 0.6%, 0.5% Nitrogen injection rate: 0.5Nm 3 /min Comp. Ex. 0.4% 1.2% 4.5%, 12.3% Nitrogen injection rate: 0 Nm 3 /min Filter damage Dust discharging state Example Filter was sound after 120 runs of heat treatment No dust bridging ( ) was found at the lower part of dust collector and ball valve ( ) Comp. Ex. Many holes of small bean sizes were made after 103 runs of heat treatments Heat generation and failure to discharge the dust occurred at the lower part of dust collector and ball valve in 14 runs By application of the above-mentioned 4 embodiments in combination, a stable technique of using vacuum dry type dust collection with a filter throughout all the phases of treatment, dust scraping and transport and protection against atmospheric air invasion can be established. Effects according to the first embodiment of the Invention According to the present invention, combustible filters such as filter cloth, etc., when used in a dust collector, will be no more damaged, or burnt, unnecesitating use of expensive high temperature-enduring filters, ceramic filters, etc. of highly restricted use conditions. Low costs, non-ceramic (combustible) filters can be used. Even if non-combustible filters such as high temperature-enduring filters or ceramic filters are used, problems of dust sintering on the filter surface can be eliminated to prevent lowering of filterability (gas permeation) of filters due to cloggings. Effects according to the second embodiment According to the embodiment, inconveniences due to burning and hole penetration in case of filters of filter cloth type, cloggings in case of ceramic filters, heat generation and damages of devices relating to the dust discharge port at the lower part of dry type dust collector, and air oxidation of dusts such as dust sintering and failure to discharge the dusts in the dry type dust collector, can be prevented to enable safe use of a dry type duct collector with a filter in the vacuum/reduced pressure refining. Effects according to the third embodiment According to the embodiment, a dry type dust collector with a filter can be used in the vacuum/reduced pressure refining without any inconveniences such as filter damage during the period of discharging dusts from the dry type dust collector, damages of devices near the dust discharge outlet, heat generation damages and plugging of transport piping and heat damages of apparatuses at the destination position of pneumatic transport of dusts. Effects according to the fourth embodiment According to the embodiment, combustible filters such as filter cloth, etc. even when used in a dry type dust collector, never undergo damaging, burning, etc., unnecessitating use of expensive high temperature-enduring filter cloth, ceramic filters, etc. of restricted use conditions, but enabling use of low-cost, non-ceramic (combustible) filters. Even if non-combustible filters such as high temperature-enduring filter cloth or ceramic filters are used, any decrease in filterability due to cloggings of filter surfaces caused by dust sintering can be prevented, and also dust discharge troubles at the dust discharge outlet due to dust sintering can be prevented. (Notes on reference numerals in the drawings) 1 Refining vessel (refining furnace) 2 Filter 3 Dust collector 4 Evacuating apparatus 5 Upstream duct 6 Downstream duct 7 Block valve 8 Block valve 9 Expansion joint 10 Piping 11 Opening/closing valve 12 Non-oxydizing gas holder 13 Molten metal 14 Vacuum lid 15 Leak valve 16 Vent stack 17 Ladle 18 Ladle elevating/descending device 19 Suction pipes 20 Cylinder for releasing expansion joint connection 21 Seal lid 22 Seal lid elevating/descending device 23 Cylinder for hermetically closing seal lid 24a Open port 24b Open port 24c Open port 30 Dust discharge ball valve (vacuum seal valve) 39 Dust discharge outlet 41 Actuator 42 Dust receiving box 43 Opening/closing cylinder 44 Vacuum seal lid 46 Rotary valve 47 Piping 48 Opening/closing valve 50 Non-oxidizing gas holder 53 Door (discharge outlet of dust receiving box) 54 Sealing enclosure 55 Conical region 59 Opening/closing valve for pressure returning 60 Dust discharge ball valve 61 Safety valve 62 N 2 holder 63 Gas-introducing piping for pressure returning 64 Special piping for introducing non-oxidizing gas 65 Opening/closing valve 66 Flow rate control valve 69 Dust discharge outlet 71 Actuator 72 Dust-receiving box 75 Transport piping 76 Rotary valve 77 Supply piping 78 Opening/closing valve 79 Flow rate control device 80 Non-oxidizing gas holder 81 Refractory-lined dust pot 82 Dust spreading-preventing cover 83 Door 85 Conical region

Claims: A vaccum/reduced pressure refining process using a vacuum/reduced pressure refining facility comprising a vacuum/reduced pressure refining vessel, a dry type dust collector using a filter, a reduced pressure evacuating apparatus and ducts connecting said apparatuses successively, wherein a freely opening/closing block valve is provided in an upstream duct for connecting the vaccum/reduced pressure refining vessel to the dust collector and a connection port is provided in the duct further upstream of the block valve in the upstream duct or in a hermetically-to-be closed space including the refining vessel, comprising the steps of injecting a non-oxidizing gas into the upstream duct at the nearer side to the vacuum/reduced pressure refining vessel than the block valve provided in the upstream duct at the start of vaccum/reduced pressure refining treatment and thereby conducting substantial reduction to zero of the oxygen concentration in the upstream duct; and then closing the connection port and opening the block valve upstream of the dust collector after the atmosphere is brought into a hermetically closed state in the upstream duct from the vacuum/reduced pressure refining vessel to the block valve provided in the upstream duct at the nearer side to the vacuum/reduced pressure refining vessel to operate the dust collector. A vaccum/reduced pressure refining process using a vacuum/reduced pressure refining facility comprising a vacuum/reduced pressure refining vessel, a dry type dust collector using a filter, a reduced pressure evacuating apparatus and ducts connecting said apparatuses successively, wherein a freely opening/closing block valve is provided in an upstream duct for connecting the vacuum/reduced pressure refining vessel to the dust collector and a connection port is provided in the duct further upstream of the block valve in the upstream duct or in a hermetically-to-be closed space including the refining vessel, comprising the steps of closing the connection port at the start of vacuum/reduced pressure refining treatment and opening the block valve upstream of the dust collector after the atmosphere is brought into a hermetically closed state in the upstream duct from the vacuum/reduced pressure refining vessel to the block valve provided in the upstream duct at the nearer side to the vaccum/reduced pressure refining vessel to operate the dust collector; and closing the block valve provided in the upstream duct before opening the connection port provided in the upstream duct at the end of vacuum/reduced pressure refining treatment and returning the pressure of the atmosphere in the upstream duct at the nearer side to the vacuum/reduced pressure refining vessel than the block valve only by injection of the non-oxidizing gas therein. A vacuum/reduced pressure refining process according to claim 1, characterized by closing the block valve provided in the upstream duct before opening the connection port provided in the upstream duct at the end of vacuum/reduced pressure refining treatment and returning the pressure of the atmosphere in the upstream duct at the nearer side to the vacuum/reduced pressure refining vessel than the block valve only by injection of the non-oxidizing gas therein. A vacuum/reduced pressure refining process according to claim 2 or 3, characterized by closing an open port of connection apparatus connected to the upstream duct at the nearer side to the vacuum/reduced pressure refining vessel during a waiting period from the end of vacuum/reduced pressure refining treatment to the start of next treatment. A vaccum/reduce pressure refining facility, which comprises a vacuum/reduced pressure refining vessel, a dry type dust collector using a filter, a reduced pressure evacuating apparatus and ducts connecting said apparatuses successively, where a freely opening/closing block valve is provided in an upstream duct connecting the vacuum/reduced pressure refining vessel to the dust collector, characterized by providing with a piping and its opening/closing valve at an upstream duct at the side of the vaccum/reduced pressure refining vessel and upstream of the block valve provided at the upstream side near the vacuum/reduced pressure refining vessel to introduce a non-oxidizing gas therein. A vacuum/reduced pressure refining apparatus according to claim 5, characterized by providing a detachable seal lid for dust collector-sided duct open port at an open port siding with the refining vessel and existing upstream of the block valve in the upstream duct.

Inventors: ABE KENJI, IWASAKI HIROSHI, MAKINO NOBUYUKI, MORISHIGE HIROAKI, OGAWA GAKU, OKIMORI MAYUMI, SADACHIKA MASARU, SHIMOMURA KENSUKE, TAKANO HIRONORI, TANAKA TOMOAKI, ABE, KENJI, IWASAKI, HIROSHI, MAKINO, NOBUYUKI, MORISHIGE, HIROAKI, OGAWA, GAKU, OKIMORI, MAYUMI, SADACHIKA, MASARU, SHIMOMURA, KENSUKE, TAKANO, HIRONORI, TANAKA, TOMOAKI, ABE, Kenji, c/o Nippon Steel Corporation, IWASAKI, HIROSHI, C/O NIPPON STEEL CORPORATION, MAKINO, Nobuyuki, c/o Nippon Steel Corporation, MORISHIGE, Hiroaki, c/o Nippon Steel Corporation, OGAWA, Gaku, c/o Nippon Steel Corporation, OKIMORI, Mayumi, c/o Nippon Steel Corporation, SADACHIKA, Masaru, c/o Nippon Steel Corporation, SHIMOMURA, Kensuke, Nippon Steel Corporation, TAKANO, Hironori, c/o Nippon Steel Corporation, TANAKA, Tomoaki, c/o Nippon Steel Corporation
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1997
Main classification: C21C7
IPC codes: C21C5, C21C7