Abstract:
An improved scrubbing dust collector and exhaust gas treatment installation are described. The collector ( 2 ) includes a tank ( 21 ) containing a washing liquid, a scrubbing tower ( 22 ) on the upper portion of the tank into which an exhaust gas is introduced, a jetting nozzle ( 25 ) on the upper portion of the tower, and at least one netting member ( 42 ) located in a lower portion of the tower. The netting may be installed in a frame ( 40 ) which has slits therein. A plurality of netting members may be employed and a lower member may have a smaller mesh size than an upper one. The installation includes the scrubbing dust collector ( 2 ), a combustor ( 1 ) and a separator tower ( 23 ). As washing liquid is jetted from the nozzle ( 25 ), it passes downward in the scrubbing tower and forms a membrane-like layer on the netting member thereby providing a larger contact area between the exhaust gas and the washing liquid.

Description:
This application claims priority under 35 U.S.C. §§119 and/or 365 to 86 385/1999 filed in Japan on Mar. 29, 1999; the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a scrubbing dust collector for subjecting an exhaust gas from discharged from a semiconductor-manufacturing unit or the like to scrubbing and dust-collection, and relates to an exhaust gas treatment installation comprising this scrubbing dust collector. 
     2. Description of the Related Art 
     In the manufacture of semiconductors, an exhaust gas containing poisonous gases such as silane gas is produced. Therefore, it is obliged in accordance with a low (High pressure gas control low) to install, in a place where such an exhaust gas is discharged, an exhaust gas treatment installation, and to discharge the exhaust gas after the poisonous gases are made harmless. 
     As an exhaust gas treatment installation of the prior art, there has been used, for instance, one comprising a combustor for burning an exhaust gas and a jet type scrubbing dust collector for subjecting the exhaust gas burned in this combustor to scrubbing and dust-collection. In a case where a removal treatment of, for example, silane gas (SiH 4 ) contained in an exhaust gas is carried out in such an exhaust gas treatment installation, said exhaust gas is first reacted with oxygen in air by the combustor and a combustion exhaust gas containing the resulting particles (SiO 2 ) is mixed with cooling air so as to be cooled down to about 80° C. By contacting the exhaust gas with a washing liquid (in general, water) by use of the scrubbing dust collector, SiO 2  is thereafter caught in the washing liquid, and namely SiO 2  is removed from the exhaust gas. 
     In the aforementioned prior art, however, there has been such a problem that it is very difficult to catch fine dust particulates whose diameter is as small as 0.1˜0.5 micron, of dust particulates contained in an exhaust gas, by a circulating washing liquid so that the same fine dust particulates are permitted to be directly released to the atmosphere. 
     It is an object of the present invention to provide a scrubbing dust collector and an exhaust gas treatment installation capable of catching fine dust particulates in an exhaust gas by a washing liquid, thereby decreasing dust particulates released to the atmosphere. 
     SUMMARY OF THE INVENTION 
     In order to achieve the aforementioned purpose, the invention of the first aspect provides a scrubbing dust collector comprising a washing liquid tank containing a washing liquid stored therein, and a scrubbing tower provided on the upper portion of said washing liquid tank and having a washing liquid jetting means for jetting said washing liquid, where an exhaust gas is introduced into said scrubbing tower so as to be subjected to scrubbing and dust-collection, characterized in that a netting member is provided below said washing liquid jetting means in said scrubbing tower. 
     By virtue of the provision of the netting member, as mentioned above, a washing liquid jetted from the washing liquid jetting means collects temporarily in a section of the netting member so that a liquid membrane is formed. When the washing liquid passes through the meshes of the netting member, the same washing liquid falls down as it packs the exhaust gas, and hence the contact area of the exhaust gas containing fine dust particulates with the washing liquid becomes larger, whereby the fine dust particulates are easily caught in the washing liquid. Accordingly, the fine dust particulates removed from the exhaust gas increase, and as a result, dust particulates released to the atmosphere decrease. 
     In the aforementioned scrubbing dust collector, a plurality of the netting members are preferably disposed vertically. Even if fine dust particulates contained in the exhaust gas are not caught in the washing liquid in a section of the netting member on the upper side, owing to this construction, they will be caught in the washing liquid in a section of the netting member on the lower side. Namely, fine dust particulates are surely removed from the exhaust gas and dust particulates released to the atmosphere further decrease. 
     In this case, the mesh size of the netting member on the lower side is preferably made smaller than that of the netting member on the upper side. Owing to this construction, dust particulates of a so-called group of cluster particles formed by combining a plurality of fine dust particulates whose diameter is for example as small as 0.1 micron, of dust particulates contained in an exhaust gas, are caught by the washing Liquid in a section of the netting member on the upper side, and fine dust particulates as simple substances will be caught by the washing liquid in a section of the netting member on the lower side. Accordingly, fine dust particulates are removed from the exhaust gas, with an excellent efficiency. 
     Further, a frame body having slits formed therein is preferably provided below said washing liquid jetting means in said scrubbing tower, and said netting members are preferably attached below said slits in said frame body. If the quantity of the washing liquid collecting in a section of the netting members becomes larger, owing to this construction, the washing liquid will be discharged out of the frame body through said slits. 
     By attaching an ultrasonic vibrator on said frame body, furthermore, the clogging of the netting members can be prevented, and at the same time the fine dust particulates-collecting efficiency can be improved. 
     In order to achieve the aforementioned purpose, the invention of the second aspect provides an exhaust gas treatment installation comprising a combustor for burning an exhaust gas and the aforementioned scrubbing dust collector for subjecting the exhaust gas burned in this combustor to scrubbing and dust-collection. Owing to this construction, fine dust particulates removed from an exhaust gas increase and hence dust particulates released to the atmosphere decrease, as mentioned above. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING 
     FIG. 1 is a block flow diagram showing an embodiment of the exhaust gas treatment installation including the scrubbing dust collector according to the present invention; 
     FIG. 2 is a perspective view showing the circulating water hold member illustrated in FIG. 1; and 
     FIG. 3 is a block flow diagram showing another embodiment of the exhaust gas treatment installation including the scrubbing dust collector according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the accompanying drawings, a preferred embodiment of the present invention will be described in detail. 
     FIG. 1 is a block flow diagram showing one embodiment of the exhaust gas treatment installation according to the present invention. In the same drawing, the exhaust gas treatment installation of this embodiment comprises a combustor  1  equipped with a flame tube for burning an exhaust gas discharged from a semiconductor-manufacturing unit  5  such as CVD unit or etching unit and containing monosilane gas (SiH 4 ), a jet type scrubbing dust collector  2  for contacting the exhaust gas discharged from said combustor  1  with a circulating washing liquid, this is circulating water, to collect SiO 2  which is powders contained in the exhaust gas, and a filter press  3  for separating the powders from the circulating water. 
     The combustor  1  is constructed such that the exhaust gas from the semiconductor-manufacturing unit  5  and combustion air are introduced therein, and in the flame tube, SiH 4  contained in the exhaust gas is reacted with oxygen in the air at a high temperature to produce SiO 2 . Since SiH 4  contained in the exhaust gas has a relatively higher concentration, in addition, it is designed to introduce the exhaust gas into the combustor  1  after it is optionally diluted with nitrogen gas. In the combustor  1 , cooling air is further introduced therein to cool down the exhaust gas which has got a high temperature in the flame tube, and the cooled exhaust gas is then discharged. 
     The scrubbing dust collector  2  comprises a water tank  21  containing circulating water stored therein, a scrubbing tower  22  provided on the upper portion of said water tank  21  and having at its top portion an exhaust gas inlet port  22   a  connected with the combustor  1 , and a separator tower  23  provided adjacently to the scrubbing tower  22  on the upper portion of said water tank  21  and having at its top portion an exhaust port  23   a  connected with an exhaust blower  8 . 
     In the upper portion of the scrubbing tower  22  is installed a nozzle  25  as the washing liquid jetting means. This nozzle  25  is connected with the water tank  21  by way of a circulation pump  26 , where the circulating water sucked up from the water tank  21  by the circulation pump  26  is radially jetted downwards. Further, in the lower portion of the scrubbing tower  22  is installed a circulating water hold member  40  having a wire mesh plate  42  which is a stainless steel-made netting member for interrupting the circulating water jetted from the nozzle  25 . The structure of this circulating water hold member  40  is illustrated in FIG.  2 . 
     In the same drawing, the circulating water hold member  40  comprises a square frame body  41  having a flange part  41   a  provided at its upper end. Four walls which form this frame body  41  each has a slit  43  formed as an escape hole for the circulating water. In the lower portion of the frame body  41 , the said wire mesh plate  42  is detachably attached. This wire mesh plate  42  is shaped in the form of a lattice, and the mesh size thereof is made to be such a size that the circulating water released from the nozzle  25  can remain only in a sufficient amount for collecting SiO 2  in the exhaust gas, for example 30˜80 meshes. The term “mesh” used here is a unit represented by the number of meshes (the common ratio=2 1/2 ) contained in 1 in 2 . In the circulating water hold member  40  having such a structure as mentioned above, the flange part  41   a  of the frame body  41  is fixed on a plurality of attachment frames (not shown) provided on the inner surface of the wall part of the scrubbing tower  22  by means of bolts or the likes. 
     Now returning to FIG. 1, the separator tower  23  has Raschig ring trays  31  disposed in plural stages (in three stages in the drawing) and a separator  32  disposed above these Raschig ring trays  31 . The Raschig ring trays  31  each is designed to further remove dust particulates in a small amount remaining in a rising gas by the circulating water jetted from a nozzles  33 , and the separator  32  is also designed to eliminate water drops contained in the gas freed of dust particulates. 
     The filter press  3  is connected with the water tank  21  by way of a suction pump  29 , where the circulating water containing dust particulates in the water tank  21  is introduced therein by the suction pump  29  and the same circulating water is freed of dust particulates and then returned to the water tank  21 . 
     In the next place, a treatment of monosilane gas (SiH 4 ) using the exhaust gas treatment installation of this embodiment constructed as mentioned above will be described here. 
     By actuating the exhaust pump  6  at first, an exhaust gas containing at a high concentration SiH 4  which has been discharged from the semiconductor-manufacturing unit  5  is diluted with nitrogen gas and then introduced into the flame tube of the combustor  1 . Into this flame tube, combustion air is also introduced. In the flame tube, SiH 4  contained in the exhaust gas is reacted with oxygen in the air and as a result, SiO 2  is produced. A combustion exhaust gas containing this SiO 2  is then cooled down to about 80° C. by cooling air. Thereafter, the exhaust gas containing SiO 2  is sent from the combustor  1  to the scrubbing tower  22  of the scrubbing dust collector  2  by means of the exhaust blower  8 . 
     The exhaust gas introduced in the scrubbing tower  22  is subjected to scrubbing and dust-collection by contacting SiO 2  contained therein with the circulating water jetted from the nozzle  25 . SiO 2  powders include fine dust particulates whose diameter is as small as about 0.1 micron and dust particulates of a group of cluster particles formed by combining (combining to cluster) a plurality of these fine dust particulates. As to fine dust particulates as simple substances and dust particles of a group of cluster particles whose diameter is small (i.e. less than 0.5 micron in diameter), both of which will be hereinafter called fine dust particulates en bloc, of these dust powders, they seldom collide with the circulating water while falling down because the contact area of them with the circulating water jetted from the nozzle  25  is small, and it is namely difficult to catch fine dust particulates only by the circulating water as falling down. 
     Since the wire mesh plate  42  is provided in the lower portion of the scrubbing tower  22 , in this embodiment, the circulating water jetted from the nozzle  25  spreads in a section of the wire mesh plate  42  so that a liquid membrane is formed. The contact area of the exhaust gas containing SiO 2  with the circulating water therefore becomes larger, whereby SiO 2  is easily caught in the circulating water. When the circulating water which has temporarily collected in a section of the wire mesh plate  42  falls down through the meshes of the wire mesh plate  42 , the same circulating water falls down as it packs the exhaust gas, and hence the contact area of the exhaust gas with the circulating water becomes larger, whereby SiO 2  in the exhaust gas is easily caught in the circulating water. 
     The circulating water which has caught SiO 2  is sent to the filter press  3  by way of the suction pump  29 , where SiO 2  is removed therefrom by the filter press  3 . The gas freed of SiO 2  is then sent to the separator tower  23  through the upper portion of the water tank  21  by means of the exhaust blower  8 . After a small amount of dust particulates remaining in the rising gas are removed by the circulating water jetted from the nozzles  33  in a section of the Raschig ring trays  31  and water drops contained in the gas are removed by the separator  32 , the same gas will be released to the atmosphere by way of the exhaust blower  8 . 
     According to this embodiment, as mentioned above, the circulating water hold member  40  having the wire mesh plate  42  has been installed in the scrubbing tower  22  so that the exhaust gas containing dust particulates and the circulating water are sufficiently contacted with each other, and hence even fine dust particulates in the exhaust gas can be surely caught in the circulating water, whereby the dust particulates can be prevented from being released to the atmosphere. 
     In this embodiment, further, the slits  43  have been formed in the frame body  41  of the circulating water hold member  40 , and hence when the quantity of the circulating water which collects in a section of the wire mesh plate  42  becomes much more than as needed, the circulating water will be discharged outside of the frame body  41  through the slits  43 . 
     Although one wire mesh plate  42  has been provided in the scrubbing tower  22 , moreover in this embodiment, the number of the wire mesh plates  42  may be plural. Referring to FIG. 3, another embodiment using a plurality of the wire mesh plates will be described. In this drawing, members which are the same as or equal to those in the aforementioned embodiment of FIG. 1 will be given the same numerals and their explanation will be omitted. 
     In the embodiment depicted in FIG. 3, four wire mesh plates are provided in a scrubbing tower  22 A in a scrubbing dust collector  2 A. Describing further in detail, in the lower portion of the scrubbing tower  22 A are provided two circulating water hold members  52 , each having the wire mesh plate  50 , and above these circulating water hold members  52  are provided two circulating water hold members  56 , each having the wire mesh plate  54  whose mesh is more rough than that of the wire mesh plates  50 . These circulating water hold members  52 ,  56  each is constructed similarly to the circulating water hold member  40  shown in FIG. 2 such that the wire mesh plates  50 ,  54  are detachably attached on the bottom surface portion of a square frame body. The mesh size of the wire mesh plates  50  is made to be an optimum size for catching dust particulates having a particle unit, for example 50˜80 meshes, and the mesh size of the wire mesh plates  54  is made to be an optimum size for catching dust particulates of a cluster particle group, for example 30˜50 meshes. The other constructions are the same as those illustrated in FIG.  1 . 
     In the scrubbing dust collector  2 A having such a construction as mentioned above, dust particulates of a group of cluster particles, of SiO 2  contained in an exhaust gas, are caught in the circulating water in a section of the wire mesh plates  54 , and fine dust particulates which have been not caught in this section are caught by the circulating water in a section of the wire mesh plates  50 . Accordingly, a scrubbing dust collection excellent in efficiency is carried out. 
     Although it has been constructed that the frame body is installed on the wall portion of the scrubbing tower and the wire mesh plates are detachably attached on this frame body, in these two aforementioned embodiments, it may be constructed that the wire mesh plates are detachably attached on the wall portion of the scrubbing tower, without using such a frame body. 
     Although it has been constructed that the wire mesh plates are installed as the netting members in the scrubbing tower, plastic net plates or others may be used as the netting members, not limited to the wire mesh plates. 
     Although the removal treatment of SiO 2  produced by burning an exhaust gas containing monosilane gas (SiH 4 ) has been described above, the present invention is applicable to a removal treatment of other dust particulates contained in an exhaust gas, not limited to SiO 2 . Also in this case, it is necessary to set the mesh size of the netting members in proportion to dust particulates to be removed. 
     According to the present invention, the netting members are provided in the scrubbing tower, and hence an exhaust gas containing fine dust particulates and a circulating washing liquid are sufficiently contacted with each other, whereby fine dust particulates in the exhaust gas are surely caught in the washing liquid and dust particulates can be prevented from being released to the atmosphere.