Abstract:
A distilling apparatus and method use a two step distillation and purification process for processing a waste liquid, such as an impure sulfuric acid solution, to form a highly concentrated sulfuric acid solution. First, the waste liquid is stored in a concentrating column, where it is heated. A condenser, which uses the waste liquid as a cooling medium, condenses the vapor generated by the heater. The condensed vapor is passed through a filter, which separates impurities out of the waste liquid, prior to feeding the waste liquid back into the concentrating column. Water is then removed from the waste liquid via a distilling process. The resulting concentrated liquid is then fed to a purifying column, where it is again heated, to remove residue, and condensed, resulting in a highly pure waste liquid. The liquid level and physical state of the waste liquid in both the concentrating column and the purifying column are monitored to insure that only an optimum amount of waste liquid for distillation is stored in each of the columns.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a distilling apparatus for distilling and purifying waste liquids such as sulfuric acid employed for chemical treatment in processes for producing semiconductors, etc.  
           [0002]    As semiconductor devices such as LCD (Liquid Crystal Display) are produced in increasing numbers, huge amounts of solutions, such as of sulfuric acid, are employed. To achieve distillatory purification of waste liquids employed in production processes efficiently and accurately reduces production costs and contributes to environmental conservation.  
           [0003]    In a process of producing semiconductor devices, for example, in a step of cleaning wafers, a solution prepared by adding about 2% by weight (wt %) of hydrogen peroxide to about 97 wt % of a commercially available sulfuric acid and heated to 80 to 120° C. is employed. When the waste liquid left after use of the solution is to be recovered, the added hydrogen peroxide is partly converted into water, and also, water migrates into the solution. Accordingly, the concentration of the sulfuric acid in the waste liquid drops to 80 to 90 wt %. Further, impurities such as silicon, iron, sodium and organic materials are contained in the waste liquid. The recovered waste liquid is sold or recycled by subjecting it to distillatory purification under atmospheric pressure. FIG. 1 shows an example of a prior art sulfuric acid distilling apparatus for distilling and purifying a sulfuric acid waste liquid.  
           [0004]    As shown in FIG. 1, the sulfuric acid distilling apparatus is equipped with a concentrating column  101  and a purifying column  102 . A contact type liquid level sensor  103  is disposed in the concentrating column  101 . A valve  104  is opened and closed based on detection results of the liquid level sensor  103  such that a fixed amount of sulfuric acid waste liquid may be in the concentrating column  101 . The waste liquid supplied to the concentrating column  101  is heated by a heater  105  to a predetermined temperature required to achieve distillation (e.g., 300° C. under the atmospheric pressure). The water content formed by distillation is condensed by a condenser  106 , stored in a tank  107 , and discharged by opening a valve  108 . Water is supplied as a cooling medium to the condenser  106 . The sulfuric acid thus concentrated is supplied to the purifying column  102 .  
           [0005]    The concentrated sulfuric acid is heated to a predetermined temperature (about 300° C. under atmospheric pressure) by a second heater  109  provided in the purifying column  102 , and thus a purified sulfuric acid solution is formed. The purified sulfuric acid solution is passed through a condenser  110 , stored in a tank  111  and can be discharged by opening a valve  112 . Water is supplied as a cooling medium to the condenser  110 . The residue in the purifying column  102  left, after purification, is condensed by a condenser  113  and stored in a tank  114 . The residue in the tank  114  is discharged by opening a valve  115 .  
           [0006]    It is important to control the liquid level such that the amount of sulfuric acid waste liquid in the concentrating column  101  is constant in order to carry out correct control of distillation in the concentrating column  101 . However, in the concentrating column  101 , when the solution is heated, the hydrogen peroxide contained in the solution is vaporized to form foams, and also a large amount of hydrogen peroxide is decomposed to form oxygen foams. Such foaming greatly changes the level of the sulfuric acid waste liquid. Accordingly, the liquid level sensor  103  may erroneously detect the liquid level, making it is very difficult to control the amount of solution in the concentrating column  101 .  
           [0007]    Further, since the sulfuric acid in the concentrating column  101  is heated to about 300° C. or higher, related parts in the concentrating column  101  should have sufficient resistance to high temperatures of 300° C. or higher. Further, when the concentrating column  101  is heated to a high temperature, leakage at junctions and damage of the junctions can be induced by heat shock.  
           [0008]    Japanese Unexamined Patent Publication No. 61-291407 discloses a process in which hydrogen peroxide contained in a sulfuric acid waste liquid is removed before the waste liquid is treated. According to this process, hydrogen peroxide is preliminarily removed from the sulfuric acid waste liquid by adding a sulfurous acid gas in an amount equal to or more than the equivalent amount of hydrogen peroxide to effect a chemical reaction. This process can prevent oxidation of a membrane for filtering the sulfuric acid waste liquid by hydrogen peroxide. However, the sulfurous acid does not entirely react with the hydrogen peroxide, and can form a reaction product of sulfuric acid and sulfurous acid. The reaction product is present in the sulfuric acid waste liquid as an impurity. As a result, the state of the sulfuric acid waste liquid is changed to lower the sulfuric acid purification accuracy.  
           [0009]    The present invention provides a distilling apparatus which facilitates control of the amount of waste liquid.  
         SUMMARY OF THE INVENTION  
         [0010]    Briefly stated, the present invention provides a distilling apparatus including: a distiller which stores therein a solution to be treated and distills the solution to form a distillate; a liquid level detector disposed in the distiller which detects the liquid level of the to be treated solution; a state detector for detecting a physical state of the to be treated solution contained in the distiller; and a control unit for controlling the amount of to be treated solution stored in the distiller based on detection results of the liquid level detector and the state detector.  
           [0011]    The present invention further provides a distilling apparatus for distilling a hydrogen peroxide-containing solution to be treated. The apparatus includes: a remover for removing a substantial part of the hydrogen peroxide from the to be treated solution; a distiller, connected to the remover, for storing therein the to be treated solution from which the hydrogen peroxide has been removed and distilling the solution to form a distillate; and a control unit for controlling the amount of to be treated solution stored in the distiller based on a liquid level of the to be treated solution in the distiller.  
           [0012]    The present invention provides a method for distilling a solution to be treated. The method includes the steps of: storing the to be treated solution in a distiller; detecting a liquid level of the to be treated solution stored in the distiller with a liquid level detector disposed in the distiller; detecting a physical state of the to be treated solution in the distiller with a state detector; controlling the amount of to be treated solution to be supplied to the distiller based on the liquid level and physical state detection results; and heating the to be treated solution stored in the distiller to form a distillate.  
           [0013]    The present invention further provides a method for distilling a hydrogen peroxide-containing solution to be treated. The method includes the steps of: removing a substantial part of the hydrogen peroxide from the to be treated solution; storing the to be treated solution from which hydrogen peroxide has been removed in a distiller; controlling the amount of to be treated solution supplied to the distiller based on a liquid level of the to be treated solution in the distiller; and heating the to be treated solution stored in the distiller to form a distillate.  
           [0014]    Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
         [0016]    [0016]FIG. 1 is a schematic diagram of a prior art sulfuric acid distilling apparatus;  
         [0017]    [0017]FIG. 2 is a schematic diagram of a distilling apparatus according to a first embodiment of the present invention;  
         [0018]    [0018]FIG. 3 is also a schematic diagram of the distilling apparatus according to the first embodiment of the present invention;  
         [0019]    [0019]FIG. 4A is a schematic cross-sectional side view of a concentrating column of the apparatus of FIG. 2, and FIG. 4B is a horizontal cross-sectional view of the concentrating column of the apparatus of FIG. 2;  
         [0020]    [0020]FIGS. 5A and 5B are schematic diagrams showing actions of the purifying column of FIG. 3;  
         [0021]    [0021]FIG. 6 is a schematic diagram of a distilling apparatus according to a second embodiment of the present invention;  
         [0022]    [0022]FIGS. 7A and 7B are partial schematic diagram of third distilling apparatus;  
         [0023]    [0023]FIG. 8 is a partial schematic diagram of fourth distilling apparatus;  
         [0024]    [0024]FIG. 9 is a partial schematic diagram of fifth distilling apparatus; and  
         [0025]    [0025]FIG. 10 is a partial schematic diagram of sixth distilling apparatus.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    Certain terminology is used herein for convenience and is not meant to be construed narrowly or limiting. The word “substantial” generally means greater than half. The terminology includes the word above specifically mentioned, derivatives thereof and words similar import. In the drawings, like numerals indicate like elements throughout.  
         [0027]    (First Embodiment)  
         [0028]    A distillatory purifying apparatus according to a first embodiment of the present invention will be described referring to the drawings. FIGS. 2 and 3 show schematic diagrams of the distillatory purifying apparatus. The distillatory purifying apparatus of the first embodiment, which includes a concentrating column  1  shown in FIG. 2 and a purifying column  2  shown in FIG. 3, carries out a two step distillatory purification process of a waste liquid, such as a sulfuric acid waste liquid.  
         [0029]    As shown in FIG. 2, the concentrating column  1  has a column head  1   a  containing a condenser  3 . A waste liquid, referred to herein as a sulfuric acid waste liquid, is supplied as a cooling medium (coolant) from a waste liquid tank (not shown) to the condenser  3 . The sulfuric acid waste liquid is, for example, one employed in a step of cleaning semiconductor wafers, such as a sulfuric acid solution containing about 2% by weight (wt %) of aqueous hydrogen peroxide as an oxidizing agent, or a sulfuric acid waste liquid employed in an LCD production process may be supplied to the concentrating column  1 .  
         [0030]    The sulfuric acid waste liquid serves as a cooling medium for condensing the vapor in the concentrating column  1 . Accordingly, the vapor in the concentrating column  1  is cooled by heat exchange with the sulfuric acid waste liquid, so that the waste liquid is heated to a predetermined temperature (e.g., 30 to 80° C.). When the sulfuric acid waste liquid is heated, the hydrogen peroxide contained therein is partly vaporized and partly decomposed into water and oxygen. If the condenser  3  is damaged, the cooling medium can migrate into the concentrating column  1 . However, since the cooling medium according to the present invention is the sulfuric acid waste liquid, the sulfuric acid waste liquid in the concentrating column  1  does not react with the vapor.  
         [0031]    The sulfuric acid waste liquid passed from the condenser  3  is supplied through a filter  4  and a supply valve  5  into the concentrating column  1 . The filter  4  is a filter having a hydrophobic membrane, preferably a Teflon filter. The filter  4  separates the vaporized hydrogen peroxide and decomposition product oxygen from the sulfuric acid waste liquid (gas-liquid separation). Accordingly, the vaporized hydrogen peroxide and the decomposition product oxygen preliminarily removed from the sulfuric acid waste liquid supplied to the concentrating column  1 . That is, in the first embodiment, no extra substance is added to remove hydrogen peroxide. Thus, no reaction product is formed, and the state of the sulfuric acid waste liquid remains unchanged, which maintains purification accuracy at a high level.  
         [0032]    The hydrogen peroxide separated from the sulfuric acid waste liquid may be reacted with sulfur dioxide to form sulfuric acid, and the thus formed sulfuric acid may be added to the sulfuric acid waste liquid before it is subjected to distillation. In this case, the concentration of sulfuric acid in such untreated sulfuric acid waste liquid is increased to form a purified sulfuric acid with higher purity.  
         [0033]    A water seal vacuum pump  6  is preferably connected to the concentrating column  1 . However, other vacuum pumps, such as an oil-sealed rotary vacuum pump or a reciprocating pump may be used.  
         [0034]    The vacuum pump  6  reduces the internal pressure of the concentrating column  1  to a predetermined level. In the first embodiment, the vacuum pump  6  is designed to reduce the internal pressure of the concentrating column  1  preferably to 50 Torr or less, and more preferably to about 5 to 20 Torr. A buffer  7  is interposed between the concentrating column  1  and the vacuum pump  6 .  
         [0035]    A heater  8  for heating the sulfuric acid waste liquid is disposed in the concentrating column  1 . The heater  8 , the external surface of which is preferably protected with quartz, is directly inserted laterally into the column  1 , so as to increase the efficiency of heating the sulfuric acid waste liquid. The sulfuric acid waste liquid supplied to the concentrating column  1  contains no hydrogen peroxide, since it has been removed beforehand. Accordingly, the sulfuric acid waste liquid heated by the heater  8  is controlled so as not to undergo foaming. In other words, the sulfuric acid waste liquid is heated and distilled relatively mildly in the concentrating column  1 .  
         [0036]    Further, since the internal pressure of the concentrating column  1  is reduced by the vacuum pump  6 , the boiling point of the sulfuric acid waste liquid is lower than under atmospheric pressure. For example, when the internal pressure of the concentrating column  1  is reduced to about 5 to 20 Torr, the boiling point of the sulfuric acid waste liquid is about 180 to 220° C. Accordingly, the sulfuric acid waste liquid is heated to a temperature (about 180 to 220° C.) which is lower than the temperature (300° C. or higher) to which it is heated under atmospheric pressure, which reduces the load of the heater  8 . The sulfuric acid waste liquid is heated to 30 to 80° C. by heat exchange when it passes the condenser  106  and then supplied to the concentrating column  1 . Accordingly, the heater  8  heats the preheated sulfuric acid waste liquid to 180 to 220° C., which further reduces the load of the heater  8 . Further, there is no to employ parts having high-temperature resistance in the concentrating column  1 . In addition, since rise in the temperature of the parts employed is controlled, the liability of leakage at junctions or damage of the junctions induced by heat shock is reduced.  
         [0037]    The concentrating column  1  has a liquid level sensor  9 , a temperature sensor  10   a , a concentration sensor  10   b  and a specific gravity sensor  10   c . The sensors  9  and  10   a  to  10   c  are connected to a control unit  12 . The liquid level sensor  9  is preferably a continuous capacitance sensor which detects the level of the sulfuric acid waste liquid in the column  1 . The temperature sensor  10   a , concentration sensor  10   b  and specific gravity sensor  10   c  detect the temperature, concentration and specific gravity of the sulfuric acid waste liquid in the column  1 , respectively.  
         [0038]    The control unit  12  receives signals expressing detection results from the sensors  9  and  10   a  to  10   c  respectively, and calculates an optimum volume of sulfuric acid waste liquid for carrying out distillation, based on the detection results of the sensors, and finds a target capacitance value of the liquid level sensor  9 , depending on the calculated optimum volume. The control unit  12  further controls opening and closing of the supply valve  5  so that the actual capacitance value of the liquid level sensor  9  substantially coincides with the target capacitance value when the sulfuric acid waste liquid is supplied into the concentrating column  1 . The sulfuric acid waste liquid contains no hydrogen peroxide, as previously discussed, since it has been removed before the waste liquid is supplied to the concentrating column  1 , so that the waste liquid undergoes less foaming during heating, and thus there is less induced vibration on the liquid surface. Therefore, the level of the sulfuric acid waste liquid can be detected accurately by the liquid level sensor  9 . Thus, the level of the sulfuric acid waste liquid can be controlled accurately, depending on the temperature and pressure.  
         [0039]    The concentrating column  1  contains partitions  13  for separating an inner space thereof. As shown in FIG. 4A, each partition  13  has side end faces conforming to the profile of the inner wall surface of the concentrating column  1  and an upper end face formed such that it is substantially at a maximum level of the sulfuric acid waste liquid supplied into the concentrating column  1 . In the first embodiment, four partitions  13  are connected by two connecting parts  14 , such that the partitions  13  are retained at equal intervals, as shown in FIG. 4B. The four partitions  13  divide the inner solution storing space of the concentrating column  1  into a plurality of spaces. Each partition  13  makes the sulfuric acid waste liquid present in one of the thus divided spaces difficult to flow into the other divided spaces. That is, vibration at the liquid surface in one divided space is not readily transmitted to the liquid surfaces in the other divided spaces. In other words, the partitions  13  serve as restricting plates for restricting migration of the sulfuric acid waste liquid, and thus reduce vibration at the liquid surface.  
         [0040]    As shown in FIG. 4A, each partition  13  contains a plurality of pores  15 . The pores  15  are preferably formed at a lower part of each partition  13 . The pores  15  absorb foams formed when the sulfuric acid waste liquid is boiled. The pores  15  are effective for suppressing rippling of the sulfuric acid waste liquid and thus allow accurate detection of the liquid level.  
         [0041]    As shown in FIG. 2, a cylindrical filler (rasching ring)  16  for cooling the vapor formed during distillation is disposed in the concentrating column  1 . The vapor contains sulfuric acid vapor and water vapor. Since the boiling point of sulfuric acid vapor is higher than that of water vapor, the sulfuric acid vapor is liquefied to drop when it is cooled by the rasching ring  16 . The water vapor passes the rasching ring  16  and is introduced to the condenser  3 . The water vapor is condensed by the condenser  3 , and the liquefied water content is stored in a receiving container  17 . Thus, the concentration of sulfuric acid contained in the sulfuric acid waste liquid in the concentrating column  1  is increased.  
         [0042]    The receiving container  17  contains a primary container  17   a  and a secondary container  17   b . A primary discharge valve  18  is interposed between the containers  17   a  and  17   b , and a secondary discharge valve  19  is disposed below the secondary container  17   b . The secondary container  17   b  is connected via a vacuum valve  20  to the buffer  7 . When the vacuum valve  20  is opened, the internal pressure of the secondary container  17   b  is reduced to the same predetermined level as the pressure in the concentrating column  1 . The secondary container  17   b  contains an atmospheric pressure valve  21  for opening the inner space of the container  17   b  to the atmosphere.  
         [0043]    The water content condensed by heat exchange by the condenser  3  is first stored in the primary container  17   a  of the receiving container  17  and then transferred to the secondary container  17   b  under opening and closing operation of the primary discharge valve  18 . In this step, the vacuum valve  20  is opened, which reduces the internal pressure of the secondary container  17   b . Accordingly, when the water content stored in the primary container  17   a  is transferred to the secondary container  17   b , the internal pressure of the concentrating column  1  is maintained at a fixed level. Further, the buffer  7  absorbs pressure fluctuation occurring when the water content is transferred from the primary container  17   a  to the secondary container  17   b . Accordingly, the internal pressure of the concentrating column  1  is maintained at a fixed level.  
         [0044]    The water content stored in the secondary container  17   b  is discharged in the following order. First, the primary valve  18  and the vacuum valve  20  are closed. Next, the atmospheric pressure valve  21  is opened. Finally, the secondary discharge valve  19  is opened. Since the primary valve  18  is closed when the water content is discharged, the internal pressure of the concentrating column  1  is maintained at a fixed level. Further, since the inner space of the secondary container  17   b  is open to the atmosphere, the water content stored therein is speedily discharged.  
         [0045]    As shown in FIGS. 2 and 3, the concentrating column  1  and the purifying column  2  are connected to each other via a connecting pipe  22  having a supply valve  23 . When the sulfuric acid waste liquid is subjected to distillation in the concentrating column  1 , the water content is evaporated to form a sulfuric acid solution (concentrated sulfuric acid) having a predetermined concentration (about 97 wt %). The sulfuric acid solution is transferred from the concentrating column  1  through the connecting pipe  22  to the purifying column  2  by opening the supply valve  23 , due to the difference between the specific gravity of the liquid in the concentrating column  1  and that of the liquid in the purifying column  2 .  
         [0046]    The purifying column  2  is connected to the vacuum pump  6  via the buffer  7  and is maintained at a predetermined reduced pressure by the vacuum pump  6  and the buffer  7 . The purifying column  2  has a heater  24  for heating the concentrated sulfuric acid. The heater  24 , which is preferably protected on the external surface with quartz, is directly inserted laterally into the column  2 .  
         [0047]    Since the internal pressure of the purifying column  2  is reduced, the boiling point of the concentrated sulfuric acid is lower than the boiling point under atmospheric pressure. For example, when the internal pressure of the purifying column  2  is reduced to about 5 to 20 Torr, the boiling point of the sulfuric acid is lowered to about 180 to 220° C. Accordingly, the heater  24  heats the sulfuric acid to a temperature (about 180 to 220° C.) which is lower than the temperature (300° C. or higher) required under atmospheric pressure. This reduces the load of the heater  24 .  
         [0048]    The purifying column  2  has a liquid level sensor  25 , a temperature sensor  26   a , a concentration sensor  26   b  and a specific gravity sensor  26   c , which are all connected to the control unit  12 . The liquid level sensor  25  is preferably a continuous capacitance sensor which detects the level of the sulfuric acid in the column  2  and outputs a signal expressing the liquid level of the sulfuric acid solution. The temperature sensor  26   a , concentration sensor  26   b  and specific gravity sensor  26   c  detect the temperature, concentration and specific gravity of the sulfuric acid waste liquid in the column  2  and output signals expressing related detection results, respectively.  
         [0049]    The control unit  12  receives the signals from the sensors  25  and  26   a  to  26   c  and calculates an optimum volume of the sulfuric acid for carrying out purification and determines the target capacitance value of the liquid level sensor  25 , depending on the calculated optimum volume. The control unit  12  controls opening and closing of the supply valve  23  so that the actual capacitance value of the liquid level sensor  25  substantially coincides with the target capacitance value, when the sulfuric acid is supplied into the concentrating column  2 . Thus, the level of the sulfuric acid is controlled accurately depending on the temperature and pressure.  
         [0050]    A rasching ring  28 , as a filter for cooling the vapor (sulfuric acid gas), is disposed in the purifying column  2 . The sulfuric acid vapor contains impurities, which deteriorate the quality of the purified sulfuric acid. The rasching ring  28  removes impurities from the sulfuric acid vapor by liquefying the sulfuric acid vapor partly, to drops. The greatest part of the vapor passes the rasching ring  28  and is introduced from the column head  2   a  to a condenser  29 .  
         [0051]    The sulfuric acid waste liquid which is not yet distilled is supplied as a cooling medium to the condenser  29 . The sulfuric acid vapor undergoes heat exchange with the sulfuric acid waste liquid when the former passes the condenser  29 . Thus, the sulfuric acid waste liquid is heated, while the sulfuric acid vapor is condensed to assume a liquid form. The sulfuric acid liquid is transferred to a second condenser  30  and condensed there. The untreated sulfuric acid waste liquid is also supplied as a cooling medium to the second condenser  30 . The sulfuric acid liquid condensed by the second condenser  30  the waste liquid stored as a purified sulfuric acid in a receiving container  31 .  
         [0052]    Since the untreated sulfuric acid waste liquid is employed as a cooling medium for the condensers  29  and  30 , it does not react with the purified sulfuric acid even if the condensers  29  and  30  are damaged and the waste liquid migrates into the purified acid in the receiving container  31 .  
         [0053]    The receiving container  31  includes a primary container  31   a  and a secondary container  31   b . A primary discharge valve  32  is interposed between the containers  31   a  and  31   b , and a secondary discharge valve  33  is connected to the secondary container  31   b . The secondary container  31   b  is connected to the buffer  7 . A vacuum valve  34  is interposed between the secondary container  31   b  and the buffer  7 . When the vacuum valve  34  is opened, the internal pressure of the secondary container  31   b  is reduced to the same predetermined level as the pressure in the purifying column  2 . The secondary container  31   b  contains an atmospheric pressure valve  35  for opening an inner space of the container  31   b  to the atmosphere.  
         [0054]    The purified sulfuric acid condensed by the condensers  29  and  30  is first stored in the primary container  31   a  of the receiving container  31  and then transferred to the secondary container  31   b  by opening the primary discharge valve  32 . When the purified sulfuric acid is transferred, the vacuum valve  34  is opened, so that the internal pressure of the secondary container  31   b  is reduced. Accordingly, the internal pressure of the purifying column  2  is maintained at a fixed level, when the purified sulfuric acid stored in the primary container  31   a  is transferred to the secondary container  31   b . Further, the buffer  7  absorbs pressure fluctuation occurring when the purified sulfuric acid is transferred from the primary container  31   a  to the secondary container  31   b . Thus, the internal pressure of the purifying column  2  is maintained at a fixed level.  
         [0055]    The purified sulfuric acid stored in the secondary container  31   b  is discharged in the following order. First, the primary valve  32  and the vacuum valve  34  are closed. Next, the atmospheric pressure valve  35  is opened to open the inner space of the secondary container  31   b  to the atmosphere. Finally, the secondary discharge valve  33  is opened to discharge the purified sulfuric acid. Since the primary discharge valve  32  is closed when the purified sulfuric acid is discharged, the internal pressure of the purifying column  2  is maintained at a fixed level. Further, since the inner space of the secondary container  31   b  is open to the atmosphere, the purified sulfuric acid stored therein is discharged speedily.  
         [0056]    The purifying column  2  is provided with a metering discharge mechanism  36  which adjusts the amount of residue-containing liquid (concentrated solution) discharged from the purifying column  2  so as to maintain high purity of the sulfuric acid in the purifying column  2 . The residue contains impurities. The metering discharge mechanism  36  increases the discharge amount of concentrated sulfuric acid when the specific gravity of the liquid in the purifying column  2  is high and decreases the discharge amount when the specific gravity thereof is low. Thus, the residue is discharged at a predetermined ratio relative to the purified sulfuric acid discharged from the secondary container  31   b . In the first embodiment, the residue is discharged in an amount of about 10% relative-to the purified sulfuric acid.  
         [0057]    The metering discharge mechanism  36  includes a resilient body  37  for urging the purifying column  2  upward, a discharge pipe  38  connected to the purifying column  2  and a discharge pipe  39  connected to the discharge pipe  38 . A condenser  40  for condensing the residue is interposed between the discharge pipe  38  and the discharge pipe  39 .  
         [0058]    The resilient body  37  preferably includes springs disposed under-the purifying column  2  which support the column  2 . Under the action of the springs, the purifying column  2  sinks when the concentration of the residue is increased (when the specific gravity of the liquid in the purifying column  2  is high), while it rises when the concentration of the residue is reduced (when the specific gravity thereof is low). That is, the purifying column  2  floats up and down depending on the weight of the purifying column  2  and the weight of the liquid (sulfuric acid and residue) present therein. The discharge pipe  38  also preferably floats up and down together with the purifying column  2 , and the liquid discharged from the purifying column  2  flows through the pipe  38  toward the discharge pipe  39 .  
         [0059]    The discharge pipe  39  is connected by a control mechanism  200  which causes the discharge pipe  39  to move up and down in accordance with the up and down floating of the purifying column  2 . Accordingly, the control mechanism  200  causes the discharge pipe  39  to move down when the purifying column  2  moves down and to move up when the purifying column  2  moves up, so that the portion of the liquid which is higher than the discharge pipe  39  is discharged through the discharge pipes  38 ,  39 . That is, the control mechanism  200  controls the position of the discharge pipe  39  in accordance with the up and down floating of the entire purifying column  2 . The residue is discharged at a predetermined ratio. The control mechanism  200  is implemented by applying, for example, a principle of lever. For example, when the concentration of the residue (impurities) in the liquid sulfuric acid is high, as shown in FIG. 5A, the purifying column  2  sinks (downward in FIG. 5A) against the urging force of the resilient body  37 . Then, the control mechanism  200  moves down the discharge pipe  39  with the downward movement of the purifying column  2 . Thus, the liquid level becomes higher than the fixed position (as indicated by the dash-dotted line in FIG. 5A) of the discharge pipe  39 . Consequently, a large amount of liquid is discharged through the discharge pipe  39 .  
         [0060]    Meanwhile, when the concentration of the residue in the sulfuric acid is low, as shown in FIG. 5B, the purifying column  2  moves up (upward in FIG. 5B) by the urging force of the resilient body  37 . Then, the control mechanism  200  moves up the discharge pipe  39  with the upward movement of the purifying column  2 . Thus, the liquid level becomes slightly higher than the fixed position (as indicated by the dash-dotted line in FIG. 5B) of the discharge pipe  39 . Consequently, a small amount of liquid is discharged through the discharge pipe  39 . As shown in FIGS. 5A and 5B, the discharge pipe  38  and the discharge pipe  39  are connected by a flexible pipe  41 . In FIGS. 5A and 5B, the condenser  40 , shown in FIG. 3, is omitted.  
         [0061]    As shown in FIG. 3, the residue-containing concentrated sulfuric acid discharged from the purifying column  2  is stored in a receiving container  42 . The receiving container  42  includes a primary container  42   a  and a secondary container  42   b . A primary discharge valve  43  is interposed between the containers  42   a  and  42   b  and a secondary discharge valve  44  is connected to the secondary container  42   b . The secondary container  42   b  is connected to the buffer  7 . A vacuum valve  45  is interposed between the secondary container  42   b  and the buffer  7 . When the vacuum valve  45  is opened, the internal pressure of the secondary container  42   b  is reduced to the same predetermined level as in the purifying column  2 . The secondary container  42   b  contains an atmospheric pressure valve  46  for opening the inner space of the container  42   b  to the atmosphere.  
         [0062]    The residue-containing concentrated sulfuric acid condensed by the condenser  40  is first stored in the primary container  42   a  and then transferred to the secondary container  42   b  by opening the primary discharge valve  43 . When the residue-containing sulfuric acid is transferred, the vacuum valve  45  is opened, so that the internal pressure of the purifying column  2  is maintained at a fixed level. Further, the buffer  7  of the vacuum system absorbs pressure fluctuation occurring when the residue-containing sulfuric acid is transferred from the primary container  42   a  to the secondary container  42   b.    
         [0063]    The residue-containing sulfuric acid is discharged from the secondary container  42   b  in the following order. First, the primary valve  43  and the vacuum valve  45  are closed. Next, the atmospheric pressure valve  46  is opened, to open the inner space of the secondary container  42   b  to the atmosphere. Finally, the secondary discharge valve  44  is opened to discharge the residue-containing sulfuric acid. Since the primary discharge valve  43  is closed when the residue-containing sulfuric acid is discharged, the internal pressure of the purifying column  2  is maintained at a fixed level. Further, since the inner space of the secondary container  42   b  is open to the atmosphere, the residue-containing sulfuric acid is discharged speedily.  
         [0064]    In the first embodiment, the hydrogen peroxide separated beforehand from the sulfuric acid waste liquid may be bubbled through the liquid containing water, residues, etc., stored in the secondary container  17   b ,  31   b  or  42   b . The liquid stored in the secondary container  17   b ,  31   b  or  42   b  contains a large amount of sulfur dioxide which reacts with hydrogen peroxide to form sulfuric acid. As a result, the hydrogen peroxide separated can be utilized effectively to control discharge of harmful sulfur dioxide.  
         [0065]    (Second Embodiment)  
         [0066]    Next, a distillatory purifying apparatus according to a second embodiment of the invention will be described. FIG. 6 shows a schematic diagram of a distillatory purifying apparatus for distilling and purifying a waste liquid, such as nitric acid. The distillatory purifying apparatus contains a purifying column  51 , a first cooler  52 , a second cooler  53 , an exhaust chamber  54 , a denitrating column  55 , a product cooler  56 , a waste acid cooler  57 , a receiving container  58  and a control unit  59 . The control unit  59  performs various controls in the distilling treatment of the nitric acid waste liquid.  
         [0067]    In the second embodiment, the nitric acid waste liquid is distilled and purified in the purifying column  51  to form a purified nitric acid. More specifically, the purifying column  51  has both the function of the concentrating column  1  shown in FIG. 2 and the function of the purifying column  2  shown in FIG. 3. A purified nitric acid having the desired concentration cannot sometimes be formed by the treatment in the purifying column  51  only, depending on the concentration of the nitric acid waste liquid. In this case, the concentrating column  1  shown in FIG. 2 is employed to concentrate the nitric acid waste liquid to supply the resulting concentrated nitric acid to the purifying column  51 . Thus, a purified nitric acid having the desired concentration can be obtained. Since the nitric acid waste liquid has a low boiling point under atmospheric pressure, the requirement of reducing the internal pressure of the purifying column  51  is not necessary.  
         [0068]    The nitric acid waste liquid is supplied as a cooling medium (coolant) from a waste liquid tank (not shown) to the first cooler  52 . The-nitric acid waste liquid is subjected to heat exchange with the vapor in the first cooler  52 . Thus, the vapor in the first cooler  52  is condensed, and the nitric acid waste liquid is heated to a predetermined temperature (e.g., about 30 to about 80° C.).  
         [0069]    The thus heated nitric acid waste liquid is supplied through a supply valve  61  to the purifying column  51 . A heater  62  for heating and preferably boiling the nitric acid waste liquid is disposed in the purifying column  51 . The heater  62 , the external surface of which is preferably protected, such as with quartz, is directly inserted laterally into the column  51 , so as to enhance the efficiency of heating the nitric acid waste liquid. The heater  62  may heat the nitric acid waste liquid, preheated to 30 to 80° C. by heat exchange, to about 100 to about 120° C. The preheating via heat exchange reduces the load of the heater  62 .  
         [0070]    The purifying column  51  has a liquid level sensor  63 , a temperature sensor  64   a , a concentration sensor  64   b  and a specific gravity sensor  64   c , which are connected to the control unit  59 . The liquid level sensor  63  is preferably a continuous capacitance sensor which outputs a signal expressing the level of the nitric acid waste liquid in the purifying column  51 . The temperature sensor  64   a , concentration sensor  64   b  and specific gravity sensor  64   c  output signals expressing the temperature, concentration and specific gravity of the nitric acid waste liquid in the purifying column  51 , respectively.  
         [0071]    The control unit  59  receives the sensor signals from the sensors  63  and  64   a  to  64   c  to recognize the state of the nitric acid waste liquid in the purifying column  51 . The control unit  59  calculates an optimum volume of the nitric acid waste liquid for carrying out concentration and purification based on the detection results of the sensors and determines the target value of the level of the nitric acid waste liquid in the purifying column  51 , based on the calculated volume optimum. The control unit  59  controls opening and closing of the supply valve  61  so that the liquid level in the purifying column  51  coincides with the determined target value, when the nitric acid waste liquid is supplied into the concentrating column  51 . The liquid level in the purifying column  51  can be controlled accurately by the control unit  59 , depending on the conditions, such as temperature.  
         [0072]    The purifying column  51  is connected to the control unit  59  and includes a pressure sensor  65  which outputs a signal expressing the internal pressure of the purifying column  51 . The control unit  59  receives the pressure sensor signal to monitor the internal pressure of the purifying column  51 . The control unit  59 , upon recognition of an abrupt rise in the internal pressure of the purifying column  51 , judges that some abnormality has occurred in the distillatory purifying apparatus and behaves to stop operation of the apparatus.  
         [0073]    The purifying column  51  has a second temperature sensor  66  disposed in the column head  51   a  thereof. The second temperature sensor  66  is connected to the control unit  59  and outputs a signal expressing the internal temperature of the column head  51   a . The control unit  59  receives the temperature signal from the temperature sensor  66  to monitor the temperature at the column head  51   a . The control unit  59  adjusts the flow rate of the nitric acid waste liquid supplied to the first cooler  52  such that the temperature at the column head  51   a  is generally constant.  
         [0074]    The purifying column  51  contains a plurality of partitions  13  which are essentially the same as the partitions in the first embodiment. Since the partitions  13  have already been described in the first embodiment, a detailed description herein is omitted. As before, each partition  13  contains a plurality of pores  15  formed therethrough (see FIG. 4A).  
         [0075]    The vapor (nitric acid gas) in the purifying column  51  is cooled by a filler  67  disposed therein and then introduced from the column head  51   a  to the first cooler  52 , where the nitric acid gas undergoes heat exchange with the untreated nitric acid waste liquid serving as a cooling medium. Thus, the nitric acid waste liquid is heated, and the nitric acid gas is condensed to assume a liquid form, i.e., into a purified nitric acid. The purified nitric acid is introduced through a pipe  68  to the denitrating column  55 . The pipe  68  is provided with a check valve  69  for preventing back flow of the purified nitric acid from the denitrating column  55  to the first cooler  52 .  
         [0076]    The purified nitric acid in the denitrating column  55  reacts with air supplied thereto and is converted from a brown liquid to a colorless transparent liquid. The transparent purified nitric acid thus formed is supplied to the product cooler  56 , where it is cooled by cold water and stored in a product container (not shown).  
         [0077]    The pipe  68  is provided with a temperature sensor  70  connected to the control unit  59  which outputs a signal expressing the temperature of the purified nitric acid flowing through the pipe  68 . The control unit  59  receives the signal from the temperature sensor  70  and monitors the temperature of the purified nitric acid in the denitrating column  55 . The control unit  59  adjusts the amount of air supplied to the denitrating column  55  depending on the temperature of the purified nitric acid.  
         [0078]    A gas containing NO x  (nitric acid mist) is formed in the first cooler  52 . The gas is cooled by cold water supplied to the second cooler  53  and then the gas is introduced to the exhaust chamber  54  to which the air containing NO x  (nitric acid mist) formed by the reaction between air and nitric acid in the denitrating column  55  is introduced.  
         [0079]    The exhaust chamber  54  is packed with a catalyst  71  for removing NO x . The catalyst  71  adsorbs NO x  contained in the gas introduced to the exhaust chamber  54  so that a purified gas is exhausted from the exhaust chamber  54 .  
         [0080]    The purifying column  51  is provided with the same metering discharge mechanism  36  as in the first embodiment. The metering discharge mechanism  36  adjusts the amount of residue-containing liquid such that residue may be discharged from the purifying column  51  at a predetermined ratio relative to the purified nitric acid. Since the metering discharge mechanism  36  has already been described in the first embodiment, further description is omitted. The discharge pipe  38  is provided with the waste acid cooler  57  for cooling the nitric acid waste liquid to be discharged.  
         [0081]    A temperature sensor  72  is provided in the route of discharging the nitric acid waste liquid, preferably on the discharge pipe  39 . The temperature sensor  72  is connected to the control unit  59  and outputs a signal expressing the temperature of the nitric acid waste liquid. The control unit  59  receives the signal from the temperature sensor  72  and monitors the temperature of the nitric acid waste liquid discharged from the purifying column  51 . The control unit  59  recognizes an occurrence of some abnormality in the distillatory purifying apparatus when the temperature of the waste acid exceeds a predetermined temperature and behaves to stop operation of the apparatus.  
         [0082]    The nitric acid waste liquid discharged is stored in the receiving container  58 . The receiving container  58  is connected to piping having a discharge valve  73 . The receiving container  58  is provided with a liquid level sensor  74  which is connected to the control unit  59  and outputs a signal expressing the level of the nitric acid waste liquid stored in the receiving container  58 . More specifically, the liquid level sensor  74  is a capacitive sensor and is fixed at a predetermined height. The liquid level sensor  74  is designed such that its capacity may change depending on the level of the nitric acid waste liquid. The control unit  59  receives the signal from the liquid level sensor  74  and monitors the amount of nitric acid waste liquid stored in the receiving container  58 . The control unit  59  opens the discharge valve  73  discharges the nitric acid waste liquid when the amount of nitric acid waste liquid reaches a predetermined level (e.g., the position where the liquid level sensor  74  is attached). The control unit  59  counts the frequency that the nitric acid waste liquid has reached the predetermined level in response to the signals from the liquid level sensor  74 . The amount of nitric acid waste liquid stored in the receiving container  58  is determined depending on the position where the liquid level sensor  74  is attached. The control unit  59  monitors the discharge amount of nitric acid waste liquid based on the count.  
         [0083]    In the second embodiment, the filter  4  as employed in the first embodiment is interposed between the first cooler  52  and the supply valve  61 . The filter  4  enables treatment of a nitric acid waste liquid containing hydrogen peroxide. The hydrogen peroxide contained in the nitric acid waste liquid is heated by heat exchange in the first cooler  52  and vaporized. The filter  4  separates the vaporized hydrogen peroxide from the nitric acid waste liquid.  
         [0084]    It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms.  
         [0085]    As shown in FIGS. 7A and 7B, a cylindrical container  91  containing a catalyst  92  (FIG. 7A) or  93  (FIG. 7B) may be interposed between a sulfuric acid waste liquid tank and the filter  4 . The catalyst  92  or  93  decomposes hydrogen peroxide into oxygen and water, and the filter  4  separates the oxygen from the hydrogen peroxide. As the catalyst, a metal having low reactivity with sulfuric acid, for example, gold or platinum is preferably employed. The catalyst  92  or  93  preferably has a granular form, bar-like form, etc. FIG. 7A shows an example where bars, such as of gold or platinum, are employed as the catalyst  92 . It is preferred to use as the catalyst  92  platinum which is employed as electrodes. FIG. 7B shows an example where a granular catalyst, such as gold or platinum, is employed as the catalyst  93 . The catalyst  93  is highly reactive with hydrogen peroxide to facilitate decomposition of hydrogen peroxide, advantageously.  
         [0086]    As shown in FIG. 8, a condenser  94  may be interposed between the waste liquid tank and the filter  4  to heat the sulfuric acid waste liquid and remove hydrogen peroxide, prior to distillation treatment. As a heating medium supplied to the condenser  94 , a warm water is preferably used. The sulfuric acid waste liquid may be heated, for example, utilizing the heat of reaction between the water content stored in the secondary container  17   b  or  31   b  with the purified sulfuric acid. The water content is preferably supplied by the water seal vacuum pump  6 . Such supply of water content reduces the amount of coolant employed.  
         [0087]    As shown in FIG. 9, a condenser  95  may be provided on the pipe  22  connecting the concentrating column  1  and the purifying column  2 . When a heating medium is supplied to the condenser  95 , the concentrated sulfuric acid flowing through the pipe  22  is heated. Since the temperature of the sulfuric acid is increased by the condenser  95 , the load of the heater in the concentrating column  1  can be reduced. The mixture and the purified sulfuric acid employed as the heating medium may be returned to the tank. In this case, the concentration of sulfuric acid in the untreated sulfuric acid waste liquid is increased, so that a purified sulfuric acid having a high purity is obtained.  
         [0088]    A piping may be inserted to the concentrating column  1 , purifying column  2  or purifying column  51  so that the untreated sulfuric acid waste liquid (or untreated nitric acid waste liquid) may flow therethrough. The untreated sulfuric acid (or nitric acid) waste liquid is heated when it flows through the piping, and hydrogen peroxide contained therein is removed. As shown in FIG. 10, piping  96  may be wound around the heater  8 ,  24  or  62 . The piping  96  allows the sulfuric acid waste liquid flowing therethrough to be efficiently heated by the heater  8 .  
         [0089]    The resilient body  37  may be disposed above the purifying column  2  or  51  with the column being suspended therefrom. Further or additional, resilient bodies  37  may also be disposed above and under the purifying column  2  or  51 , so that the purifying column  2  or  51  may be urged upward.  
         [0090]    The filter  4  having a hydrophobic membrane may be replaced with a separator such as a chamber.  
         [0091]    Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.