Abstract:
A device for surface treatment of continuously moving strip, particularly for pickling rolled metal strip, includes a trough-shaped tank containing a treating liquid bath. The trough includes a panel mounted therein and submerged in the liquid, and aligned approximately parallel to the strip. The panel divides the treatment bath into a treatment area underneath the panel through which the strip being treated passes and a return flow sector for the liquid above the panel. The treating liquid is collected in the return flow sector at a discharge end of the trough and is directed along the return flow sector to a receiving end of the trough.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process and device for contacting the surface treatment of a strip material with a liquid. More particularly, the invention is directed to a trough for treating a rolled metal strip with a pickling solution. 
     BACKGROUND OF THE INVENTION 
     According to the state of the art, strip processing plants, such as those used for metal pickling plants, using liquid baths are designed with each individual bath measuring approximately 100 to 200 mm deep and normally 10 to approximately 40 m long. The reaction speed of the treatment medium on the surface of the strip material being processed is improved by reducing the liquid boundary layer on the strip surface. To reduce the liquid boundary layer, the current trend is now towards shallower liquid baths. This leads to greater bath turbulence due to the higher Reynolds number and a reduction in the liquid boundary layer due to the increased shearing effect. 
     The lower limit of the average depth of the bath is dependent on the speed of the strip material passing through the bath. Due to the dragging effect of the moving strip, the liquid treatment medium is carried along in the treatment tank in the same direction as the advancing direction of the strip. This results in a slant or incline of the liquid surface level with a corresponding increase in the depth of the bath at the strip exit point and the equivalent reduction in the depth of the bath at the strip entry point. At higher speeds (e.g., above a strip speed of approximately 200 m/min with 200 mm mean depth and 20 m length of the bath), the liquid is carried away by suction from the strip entry point into the treatment bath such that the distance over which the strip is submerged in the treatment medium is shorted accordingly. This results in the length of the treatment bath and the tank not being fully utilized. As a result, the mean liquid level of the bath must be raised to avoid the strip entry area being emptied by the suction from the advancing strip. The optimum efficiency of a treatment bath with an open top surface is thus a compromise between the shortened bath length as a result of the draft effect and the reduced pickling effect because of the greater mean depth of the treating liquid bath. 
     One example of a bath having a structure to maintain a small mean bath depth at high speeds is by the use of a closed treatment channel. The depth of the bath is thus limited to the height of the treatment channel. In order to prevent the liquid from being drawn or sucked out of the treatment channel, the strip exit point from the treatment channel must be sealed hydrodynamically against the back-up pressure of the dragging effect. An example of this type of structure and process is disclosed in, for example, EP 0 655 519 A1. The hydrodynamic seal requires a much higher energy input compared to the treatment baths with an open top surface. Furthermore, the high velocity and head pressure caused by being sealing off at the strip exit point of the treatment channel makes it more difficult to provide the required supply of fresh treatment medium. The closed channel requires more energy to feed the fresh liquid treatment medium into the channel. Formation of a closed channel reduces evaporation of the liquid treatment medium and thus reduces the amount of energy needed to maintain the treatment temperature. This, however, it not an advantage, particularly in pickling plants which recover the acid by heat decomposition using additional equipment. The quantity of liquid that is not evaporated in the processing tank must be removed by other processes thereby increasing the energy consumption for recovery of the acid. 
     Accordingly, there is a continuing need in the industry for an improved process and apparatus for treating a moving strip in a liquid bath. 
     SUMMARY OF THE INVENTION 
     A primary object of this invention is to improve the efficiency of a treatment bath with an open top surface thereby avoiding the disadvantages of a closed treatment channel. 
     A further object of the invention is to provide a panel mounted above a strip being treated and submerged in the liquid in a treatment bath which divides the treatment bath into a treatment area underneath the panel and a return flow sector for the liquid above the panel. 
     Another object of the invention is to provide a process and apparatus for returning the liquid from the discharge end of a treatment bath tank to a strip material receiving end of the tank. 
     A further aspect of the invention is to provide a trough containing a liquid for treating a sheet material where the trough has a small bath depth. 
     Another aspect of the invention is to provide a trough containing a liquid treating bath for treating a sheet material where the movement of the sheet material through the treating path produces mixing of the bath and circulates the bath from the one end of the other. 
     A further aspect of the invention is to provide a trough containing a liquid for treating a sheet material having a divider defining a treating zone and a liquid return zone, and having openings in the divider to provide liquid communication between the treating and return zones. 
     Another aspect of the invention is to provide a trough containing a liquid for treating a sheet material having a divider forming a treating zone and a liquid return zone where the treating and return zones are in fluid communication at a receiving end and a discharge end of the trough. 
     Another object of the invention is to provide a trough for contacting a continuously moving sheet material through a liquid bath using a small bath depth. 
     A further object of the invention is to provide a trough for contacting a continuously moving sheet material with a liquid bath in a manner to maximize the usable length of the trough. 
     The objects of the invention are basically attained by providing an apparatus for immersing a strip material in a liquid bath, the apparatus comprising: a trough having a longitudinal dimension with a bottom wall, first and second side walls, a strip material receiving end and a strip material discharge end, the trough being dimensioned to receive a treating liquid and contain a liquid bath; and at least one panel mounted within the trough and submerged in the liquid bath, the at least one panel having a bottom surface spaced from the bottom wall of the trough for defining a contact zone in the trough, the at least one panel further having a top surface defining a return zone for receiving liquid from the discharge end of the trough and directing the liquid along the top surface from the discharge end to the receiving end of the trough. 
     The objects of the invention are also attained by providing an apparatus for immersing a strip material in a liquid bath, the apparatus comprising: a trough containing a liquid bath, the trough having a longitudinal dimension, first and second side walls, a bottom wall, and a first end wall at a strip material receiving end and a second end wall at a discharge end for the strip material; a divider positioned in the trough defining a contact zone and a liquid return zone for directing liquid from the discharge end to the receiving end of the trough; and a feed device for feeding strip material from the receiving end to the discharge end through the contact zone. 
     The objects of the invention are further attained by providing a process for contacting a strip material with a liquid bath, comprising the steps of: feeding a strip material to a receiving end to a discharge end of a trough, wherein the trough contains a liquid bath between the receiving and discharge ends, the trough including a divider defining a strip material contact zone in a bottom section of the trough and a liquid return zone in a top section of the trough; conveying the strip material through the contact zone and creating a current of the liquid through the contacting zone from the receiving end to the discharge end; directing the current of liquid at the discharge end to the return zone and returning the liquid to the receiving end; and discharging the material strip from the discharge end of the trough. 
     These and other objects of the invention will become apparent from the detailed description of the invention which taken in conjunction with the annexed drawings disclose preferred and various embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the drawings which form part of this original disclosure, in which: 
     FIG. 1 is a cross-sectional side view of the apparatus in a first embodiment of the invention showing the liquid treatment tank and divider; 
     FIG. 2 is a cross-sectional end elevational view of the apparatus of FIG. 1; 
     FIG. 3 is a schematic side view of the apparatus showing the liquid flow around the divider in the tank; 
     FIG. 4 is a cross-sectional end view showing an alternative embodiment of the divider; 
     FIG. 5 is a top plan view of the apparatus showing a further alternative embodiment of the divider; and 
     FIG. 6 is a top plan view of the apparatus showing another alternative embodiment of the divider. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to a process and apparatus for contacting a sheet material in a liquid bath using a minimum amount of liquid in the bath. The invention is particularly directed to an apparatus for treating a continuously moving strip of sheet material through a liquid bath in a manner to maximize contact time of the sheet material with the liquid bath. The apparatus produces continuous movement of the liquid bath to provide a continuous supply of fresh treating liquid on the surface of the sheet material being treated. 
     Referring to FIGS. 1 and 2, the apparatus  10  is a liquid treatment tank which includes a trough  12  and a divider  14 . Trough  12  is generally a rectangular shape having a longitudinal dimension. Trough  12  has a bottom wall  16 , a pair of opposing side walls  18 ,  20 , a first front end wall  22  at a receiving end  24  and a second rear end wall  26  at a discharge end  28 . An overflow block  30 , which is typically made of stone or other chemically resistant material, is positioned at the receiving end  24  on the front wall  22  for directing liquid overflow into an overflow collecting trough  32 . A similar overflow block  34  is positioned at the discharge end  28  of trough  12  to direct liquid from trough  12  into an overflow trough  36 . Overflow blocks  30  and  34  define the upper limit of the depth of the liquid bath  38  in trough  12  and are dimensioned to maintain a desired liquid bath depth. 
     A pipe  40  is connected to overflow collection trough  36  to carry the overflow liquid to a collection and circulating tank  42 . A similar pipe  44  is connected to overflow collection trough  32  to carry overflow liquid to circulating tank  42 . A pump  46  circulates the liquid from circulation tank  42  through a pipe  48  to a heat exchanger  50  and through pipe  52  to injection nozzles  54 . 
     In the embodiment of FIG. 1, several injection nozzles  54  are provided to inject the liquid through the bottom wall  16  of trough  12  in a substantially upward direction into liquid bath  38 . In preferred embodiments, injection nozzles  54  direct the injected liquid at an angle toward the receiving end  24  of trough  12 . Nozzles  54  preferably inject the treating liquid with sufficient force to provide mixing and turbulence to liquid bath  38  and to direct fresh treating liquid directly into contact with the sheet material being treated. 
     Trough  12  is particularly suitable for contacting an elongated sheet or strip material  56  by continuously conveying the strip material  56  through the liquid bath  38  from the receiving end  24  to the discharge end  28  of trough  12 . In the embodiment of FIG. 1, strip material  56  is fed from a suitable supply (not shown) between a pair of pinch rollers  58  into liquid bath  38  at the receiving end  24  and is directed below divider  14 . Strip material  56  is carried through liquid bath  38  under divider  14  to discharge end  28  and to pinch rollers  60  where the strip material  56  exits trough  12 . Excess treating liquid on strip material  57  is removed by pinch rollers  60  so that the liquid falls into overflow trough  36 . 
     Divider  14  generally has a substantially flat panel shape and is positioned below the surface  62  of liquid bath  38 . Divider  14  defines a liquid contact and treatment zone  64  between divider  14  and bottom wall  16 . Above divider  14  is formed liquid return zone  66  as discussed hereinafter in greater detail. In the embodiment of FIG. 1, divider  14  is a single continuous panel extending from receiving end  24  to discharge end  28  of trough  12 . Divider  14  has a front edge  68  spaced from front wall  22  defining a strip material inlet  70 . A rear edge  72  of divider  14  is spaced from rear wall  26  of trough  12  to define a strip material outlet  74 . 
     The apparatus  10  of the invention is suitable for contacting a strip material with a liquid to treat or coat the strip material. In preferred embodiments, the process and apparatus are for contacting a metal sheet or strip with a pickling bath to treat or clean the surface of the strip material with a pickling solution. The pickling solution can be a conventional acid or alkaline solution as known in the art. 
     The apparatus  10  is operated by feeding the strip material  56  into inlet area  70  of receiving end  24  and into the liquid bath  38 . The strip material continues through liquid contact zone  64  below divider  14  to outlet  74 . The advancing speed of strip material  56  in the direction of arrow  76  creates a drag and current on the liquid and draws the liquid bath  38  through the contact zone  64  toward outlet  74 . This induced liquid current produces a reduced bath depth at the receiving end  24  and an increased depth at discharge end  28 . The increased bath depth at the strip material outlet  74  causes the liquid to flow over the rear edge  72  of divider  14  as indicated by arrow  77  into liquid return zone  66 . As shown in FIG. 1, divider  14  is positioned below the mean or average depth of liquid bath  38  to form return zone  66  separated from contact zone  64 . The increased depth of the liquid carried through contact zone  64  at discharge end  28  readily flows through return zone  66  back to the receiving end  24  to sheet material inlet  70  where the average bath depth is generally lower to maintain a more uniform liquid bath depth through the length of trough  12 . 
     In a preferred embodiment shown in FIGS. 1 and 2, apparatus  10  includes a cover  78  to enclosed trough  12 . As shown in FIG. 2, cover  78  has a top wall  80  and side walls  82  which are supported on side walls  18  and  20  of trough  12 . End walls  84  of cover  78  are dimensioned to provide an opening  86  at the receiving  24  and an opening  88  and the discharge end  28  of trough  12 . 
     Hanger members  90  are coupled to top wall  80  of cover  78  and extend downwardly toward trough  12 . As shown in FIG. 2, hanger members  90  have a substantially L-shape. Complementing L-shaped hanger members  92  extend upwardly from divider  14  to removably couple divider  14  to cover  78 . In this manner, divider  14  can be lifted simultaneously with cover  78 . 
     In preferred embodiments, hanger members  90  and  92  permit some linear up and down movement and longitudinal movement of divider  14  with respect to cover  78  and with respect to trough  12 . In the embodiment shown, divider  14  hangs from cover  78  whereby an upward force on divider  14 , such as that created by the strip material  56  contacting divider  14 , allows divider  14  to lift upwardly and prevent damage to strip material  56  and divider  14 . Cover can be suspended and supported directly by cover  78  or rest on blocks  97  in trough  14  as shown in FIG. 2 adjacent injection nozzles  94  extending through side walls  18  and  20 . 
     Liquid injection nozzles  94  are also connected to pipe  52  for injecting liquid into contact zone  64  below divider  14 . Preferably, injection nozzles  94  inject the liquid at an angle against the advancing direction of strip material  56 . This reduces, the dragging effect caused by the advancing strip material, increases mixing of the liquid and reduces the depth of the bath at the discharge end  28 . 
     In preferred embodiments, divider  14  includes a plurality of bars or fins  96  extending substantially perpendicular from a bottom surface  98  thereof. Fins  96  extend substantially side to side and transverse to the advancing direction of strip material  56  and the induced current of liquid through contact zone  64 . As shown in FIG. 3, fins  96  are spaced apart to create turbulence as indicated by arrows  99  in the flow of liquid and decrease the speed of the liquid current passing through contact zone  64 . The treating liquid is carried to the rear end  72  of divider  14  and into return zone  66  as shown by arrows  77  and  101 . The treating liquid then flows back into the receiving end  70  as indicated by arrow  103 . 
     Separation of the contact zone  64  and a return flow zone  66  is advantageous in obtaining a carefully directed supply of fresh treatment liquid by feeding the liquid through inlet nozzles  94  at the sides of the tank and under the divider by the force of the injection nozzles  54 . Fresh treating liquid is fed through nozzles  54 ,  94 , preferably at the strip inlet area, so that the freshly supplied treatment liquid first runs along-side the strip first before circulating back in the trough  12  through the return flow zone  66  formed by the divider  14 . 
     Positioning divider  14  in the treatment tank creates improved return flow of the treatment liquid dammed up, from the strip outlet area  74  back to the strip inlet area  70 , so that the liquid depth at the strip outlet area  74  is lowered. This enables the mean bath depth to be reduced which increased efficiency and reduces operating costs. A further advantage of the divider  14  is that it forms the contact zone  64  as a channel with a small bath depth, thus keeping the surface of the liquid bath open and uses the dragging effect of the strip for circulation and mixing of the liquid bath in the processing trough. 
     In one preferred embodiment, the divider  14  contacts side walls  18 ,  20  and extends almost the entire length of the trough  12  and the only fluid communication areas between the return zone  66  and the contact zone  64  through which the treating liquid can flow are located at the beginning and the end of the trough. In this way, all of the treatment liquid is directed from the strip outlet area  74  back to the strip entry area  70  through the return zone  66 . 
     In an alternative embodiment shown in FIG. 4, apparatus  10  includes a divider  100  having a substantially upwardly directed chevron or inverted V-shape. A plurality of fins  102  extend downwardly from the divider  100 . The V-shape of divider  100  has an apex  104  which extends in the longitudinal direction of trough  12 . The V-shape of divider  100  improves drainage of the liquid in the liquid return zone above the divider  100 . In addition, the V-shape reduces the contact area of the strip material with the divider  100  and the fins  102 . The remaining components of the apparatus are as shown in FIGS. 1 and 2 and are identified by the same reference numbers. 
     A further embodiment is illustrated in FIG. 5 where the treating tank  106  is substantially the same as in the embodiment of FIG. 1, but divider  108  is made up of a plurality of smaller individual panels  110 . The identical structural elements in the embodiment of FIG. 5 are identified by the same reference number with the addition of a prime. Panels  110  are supported in the liquid bath substantially parallel to the bottom  107  of tank  106 . Preferably, panels  110  are disposed in the same plane and spaced apart a distance to form gaps  112  between adjacent panels  110 . Gaps  112  allow the liquid to pass freely between the return zone above the panels  110  and the contact zone below the panels in the tank  106 . Sheet material  56 ′ is fed into trough  12 ′ through the contact zone  64 ′ below the panels  110 . The treating liquid drawn into the contact zone  64 ′ creates an increase in fluid pressure toward the rear end  72 ′ of each panel  110  which causes the treating liquid to flow from the contact zone  64 ′ through the gaps  112  into the return zone  66 ′ above the panels  110  where the liquid then flows back toward the front edge  68 ′ of panels  110  and to the sheet material inlet  70 ′. The direction of flow of the treating liquid through the gaps  112  depends on the pressure difference between the contact zone  64 ′ and the return zone  66 ′. 
     In a further embodiment shown in FIG. 6, apparatus  114  is similar to the embodiment of FIG. 1 except that divider  116  in the form of a panel has a plurality of holes  118  spaced across the width and along the length of the divider  116 . The various elements identical to those of the embodiment of FIG. 1 are indicated by the same reference numbers with the addition of a double prime. The holes  118  in divider allow liquid to pass between the return zone  66 ″ and the contact zone  64 ″. The holes  118  also provide circulation of the liquid and assist in drainage of the tank. The treating liquid flows through the holes  118  in the same manner as through gaps  112  of the embodiment of FIG.  5 . 
     In the embodiment illustrated, the divider extends substantially the width of the trough and contacts the side walls. This substantially prevents the passage of liquid around the sides of the divider and directs the liquid along the length of the divider toward the receiving end of the trough. In preferred embodiments, the divider is movable with respect to the trough, although in alternative embodiments the divider can be attached to the side walls of the trough. In alternative embodiments, openings or spaces can be provided along the side edges of the divider to provide liquid circulation and passage between the liquid return zone and the contact zone. 
     Although several embodiments have been chosen to illustrate the invention, it will be readily understood by those skilled in the art that various modifications can be made without departing from the scope of the invention as set forth in the following claims.