Abstract:
Oxides on the surfaces of metal are reduced by directing reducing gases at them in a forceful and turbulent manner. In a preferred version, the gas is passed through at least two reducing zones designed to maintain a higher concentration of reducing gas in at least one of them than would be the case in a single reducing zone. The oxide-bearing surface is heated at the beginning of the process..

Description:
RELATED APPLICATION  
       [0001]    This is a divisional application of Application Ser. No. 09/584,931 that was filed on Jun. 1, 2000, which is a Continuation-in-Part Application of Application Ser. No. 09/144,003 filed on Aug. 31, 1998. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to the reduction and removal of oxides from the surface of metal. The metal containing surface oxides is passed into or through an enclosure, continuously, intermittently, or batchwise, in which it is heated and contacted with reducing gas.  
         BACKGROUND OF THE INVENTION  
         [0003]    Newly formed metal strip, rod, and the like tends to develop oxides on its surface which must be removed before further processing. In the steel industry, this oxide layer is called mill scale. Mill scale is almost universally removed by acid pickling. Copper and other metals also require treatment for removal of oxide scale.  
           [0004]    Hydrogen and other reducing gases such as carbon monoxide have been used for the reduction of oxides in ores, where they are substantially consumed within a reducing furnace or vessel. Hydrogen is readily burned and can cause explosions under certain circumstances, and carbon monoxide is poisonous and generally considered dangerous unless confined and reacted in a vessel of the type generally contemplated in ore reduction. Moreover, steel strip and many other metal products made continuously move at a rapid pace, increasing the difficulty of conducting the oxide removal process with gases within the time constraints normally imposed. Thus, while the elementary chemical principles of oxide removal and/or reduction by reducing gases are known, an acceptable continuous surface oxide reduction system employing reducing gases has not been forthcoming in the art.  
         SUMMARY OF THE INVENTION  
         [0005]    Our process and apparatus provide for three stages or zones for the processing of the moving metal, which may be any metal having oxide on its surface, in any commercially common shape, such as strip or rod. The three basic stages are heating, reducing, and cooling. All three steps take place within an enclosure of the type to be described in more detail below, and under the conditions to be described in more detail below. Heating in the heating zone is accomplished by a combination of a heating element or device to be described below and post-combustion of unreacted reducing gas. Reduction of the oxide scale in the reduction zone is accomplished by assuring a turbulent and/or vigorous application of reducing gas to the surface of the metal, preferably in the presence of elemental carbon; cooling of the metal in the cooling zone prior to its exit from the enclosure is accomplished by the introduction of inert gas along with the unheated reducing gas to contact the reduced surface of the metal just prior to its exit from the enclosure. The metal surface should preferably be cooled to a temperature at which reoxidation is unlikely to occur; in the case of steel strip, this is 500° F. or lower. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a more or less diagrammatic side sectional view of a preferred configuration of the enclosure including all three zones included in our invention, as applied to steel strip.  
         [0007]    [0007]FIG. 2 is an overhead view from within the same enclosure.  
         [0008]    [0008]FIG. 3 shows a preferred device for distributing carbon on the strip surface.  
         [0009]    [0009]FIG. 4 is a more or less diagrammatic side view of a further preferred separate enclosure for the reducing zone.  
         [0010]    [0010]FIG. 5 is an overhead view of the reducing zone enclosure of FIG. 4.  
         [0011]    [0011]FIG. 6 is a side sectional view of a preferred exhaust trap for waste gases. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    It is known that the oxide layer on steel strip may contain Fe 2 O 3 , Fe 3 O 4 , and/or FeO, or various ratios of the three oxide forms depending on the conditions in which the product is made and conducted to the next processing stage. Fe 3 O 4  may pass through the Fe 2 O 3  stage before it is further reduced to FeO and then completely reduced to iron. Where hydrogen is the reducing agent, water is produced; where carbon is the reducing agent, carbon monoxide is first produced, and where carbon monoxide is the reducing agent, carbon dioxide results. Our invention contemplates the use of either hydrogen or carbon monoxide, or any other commercially feasible reducing gas, in the absence of or together with elementary carbon as a supplementary reductant.  
         [0013]    Further, the hydrogen may be manufactured within the enclosure or in its immediate vicinity. Examples of the manufacture of hydrogen include known processes for accomplishing the dissociation of methane, and the combustion of methane or other hydrocarbons in such a way as to produce excess hydrogen.  
         [0014]    [0014]FIG. 1 illustrates the invention applied to steel strip  1  from which mill scale, or a layer of oxide, must be removed. Steel strip  1  is caused to pass into enclosure  2  in the direction, as depicted, from left to right. It may be held in enclosure  2  for a period of time or moving at a speed up to as fast as 2000 feet per minute. The strip  1  may be preheated before entering enclosure  2 , but is heated within enclosure  2  by heating elements  3 , preferably radiant heaters, to ensure that the temperature of its surfaces is at least 752° F. (400° C.) by the time it leaves the heating zone, which is designated by the numeral 4. In the case of copper and other metals, the oxide surface should be heated to at least 300° F.  
         [0015]    At the entrance of the strip  1  to the enclosure  2  is a flame  13  and a flue  9  for conducting exhaust gases out of the system. The heating of strip  1  is assisted by the post-combustion of the unconsumed reducing gases by air optionally introduced through inlets  14  in the heating zone  4 . Introduction of the air through inlets  14  will cause immediate combustion of whatever reducing gas, usually hydrogen, remains in the atmosphere moving from right to left, as depicted. Preferably the flow of air will be directed at the strip so as to ensure the most efficient use of the thermal energy generated by the combustion, that is, to heat the strip. The action of the flame  13  creates a draft continuously moving gases from right to left, as depicted—from the enclosure strip exit  15  to the strip entrance  16 , thus providing a constant countercurrent contact of gas to the strip.  
         [0016]    The strip  1 , supported by rolls  5  and  6 , is then passed into reducing zone  7 . Rolls  5  and  6  may be replaced by any suitable support, and also may be replaced by graphite or carbon blocks of a consistency so that a thin film of elemental carbon is deposited or rubbed onto the strip surface, preferably both the top side and the under side. Reducing gas  11 , usually hydrogen, is continuously introduced through small apertures  7  (see FIG. 2) in manifolds  10 , and directed, preferably at a slight angle of 5-30 degrees, in the direction of the oncoming strip  1  at a velocity to create turbulence on impact with the strip  1 . Where carbon is deposited on the strip, the deposition preferably occurs in the upstrean half of the reducing zone  7 , so there will be time for it to react with the oxides on the surface of strip  1 . This zone is called the reducing zone because a large part of the reduction of the oxides occurs in this zone, but it should be understood that some oxide may be reduced in the heating zone  4  due to the continued presence there of at least some reducing gas, and in the cooling zone  8  in part because of the continued presence of reducing gas carried into the cooling zone  8  by strip  1 . In the reducing zone  7 , the temperature of the surfaces of the strip is maintained at the temperature necessary for the reducing reaction to take place. In the case of steel strip, this is at least 400° C. (752° F.). In the case of copper and other metals, the surface should have a temperature of at least 300° F.  
         [0017]    Moving on, the strip I passes into the cooling zone  8 . In cooling zone  8 , the strip  1  is caused to cool by the introduction of new reducing gases through manifolds  10 . Tile reducing gases introduced separately through manifolds  10  may be mixed with inert gases introduced through separate inlets  21  or premixed with the reducing gases. Introduction of inert gases here will minimize the possibility of mixing air with the reducing gases. When used, inert gases may be mixed with the reducing gas in volume ratios of from 1:99.9 to 99.9:1. The strip then passes out of enclosure  2  through fabric curtain  12  and may be coiled or further processed in a hot or cold rolling mill, a slitting station, a galvanizing line, or it may be oiled, otherwise processed, or simply coiled. Brushing may be beneficial during or after cooling. In the case of steel strip, the mill scale will typically have been reduced to a sponge iron layer, perhaps still containing some oxides, but readily removable by brushing. In the case of copper and other metals, brushing will assure a clean appearance.  
         [0018]    [0018]FIG. 2 illustrates the parts of enclosure  2  from above heating elements  3  and manifolds  10 . Strip  1  is underneath heating elements  3  and manifolds  10 . Manifolds  10  are seen to have a plurality of gas apertures  17  for releasing gas. These are on the underside of the manifolds  10  and aimed so the reducing gas may be directed with force toward the strip  1 , preferably in the direction from which the strip  1  is traveling. Heating elements  3  have electrical connections  16 . Note that divider  18  appears only on the top side of strip  1  (see FIG. 1); dividers  19  and  20  are above and below the strip  1 . Preferably the reducing gas manifolds  10  have one or two lengths  28  within enclosure  2  before releasing gas through apertures  17 , so the gas can be partially preheated before being released.  
         [0019]    [0019]FIG. 3 is an optional device for depositing elemental carbon on both sides of strip  1 . The device includes carbon blocks  23  and  24  secured to bases  25  and  26 , which in turn are connected to pneumatic cylinder  27  made to urge the carbon blocks  23  and  24  toward strip  1 . The carbon blocks  23  and  24  may be made of graphite, anode pitch, or any other convenient composition substantially of carbon which will deposit a thin film of carbon on the strip as it passes between the blocks  23  and  24 . Alternatively, only one block may be used; in either case the carbon blocks may to some extent replace or supplement the supporting function of rolls  5  and  6  (FIG. 1).  
         [0020]    The following guidelines may be used for the treatment of steel strip by our invention, although it should be understood that our invention is applicable to other metals, such as copper, having oxides on their surfaces.  
         [0021]    Typically, steel strip will have an oxide layer about 0.009 inch thick, commonly from 0.005 to 0.015 inch, and contain about 1 mole to about 1400 moles of oxygen per square meter of surface. Thus, about 1.1 moles to about 1400 moles of hydrogen, will be required for complete reduction of the oxides, The oxide layer on copper is generally from about 0.0005 to 0.025 inch. It is known that the microstructure of the scale on the surface of steel shows numerous small crevices between adherent particles of iron oxide, and a significant portion of the oxide is effectively undermined and loosened by the effect of the reducing fluid. This is true also of copper and other metals. Our invention therefore requires that the reducing gas is contacted with the oxide layer in a vigorous, turbulent manner to assure the continuous replenishment of reactants to the metal/oxide surface and continuous convection of the reaction products, i.e. especially water, away from the gas/solid interface. This vigorous, turbulent contacting to enhance the gas phase mass transfer is preferably accomplished by introducing the gas through ports directed toward the surface from which the oxide is to be removed. Because of the undermining and loosening effects mentioned above, it is not necessary for every atom of oxygen to react with a reducing gas; as a significant portion of the oxide will be sufficiently loosened and/or undermined that it can be easily removed mechanically, such as by brushing; in addition, the turbulent action of blowing the reducing gas on the surface of the strip in the strip cooling zone  8  will loosen and remove some of the oxide particles.  
         [0022]    To further enhance the reducing reaction in the reducing zone, reducing gas may be introduced directly to the reducing zone after first being preheated. Because gas in the cooling zone is employed partly to cool the strip, the gas introduced there is not to be preheated. Preheating of gas for introduction to the reducing zone may desirably be to a temperature of 900 to 2000° F., and can be accomplished at least partially by directing the fresh reducing gas through extra lengths  28  of manifolds  10  within enclosure  2 , where it will pick up heat energy from the environment. Prior to passing into such pipes within the enclosure, the gas may be partially preheated by any suitable means.  
         [0023]    Only the surface need be heated to the desirable reduction reaction temperature. Suitable devices for heating are radiant tubes, induction coils, and gas burners. By heating of the surface, we mean the oxide layer, which may be from 0.005 inch thick to 0.01 inch thick, on steel strip, and seldom more than 0.015 inch. Thus, temperatures of 752° F. (for steel) need not extend to a depth of more than 0.017 inch and, in most cases, 0.015 inch will be sufficient. Because the oxide layer on copper is less thermodynamically stable, the heating to 300° F. need not extend below the oxide layer.  
         [0024]    In addition to the heating methods and means mentioned above, heating of the reducing gas may be accomplished by passing it through passages in heated carbon blocks.  
         [0025]    It will be noted that our invention contemplates a use of the reducing gases to a such degree of efficiency that no recycling is necessary. Recycling of the exhausted reducing gas stream would require removal of the chief reduction product, water, from the gas to be recycled, which is very difficult to do to the extent necessary. Likewise, it would mean cooling the recycled reducing gas, thus setting up a continuous process of heating and cooling of the reducing gas. Rather, our invention contemplates the efficient use of the reducing gas in enclosure  2  by inducing turbulence and direction of the gas onto the surface of the metal to assure continuing contact and replacement of gas and reduction products on the surface. Preferably at least 5%, more preferably at least 50%, and most preferably at least 90%, of the reducing gas introduced to the enclosure is consumed in the reduction reaction, and the rest is consumed in flame curtain  13 .  
         [0026]    Referring now to FIG. 4, a further preferred reducing system is illustrated. In this variation, hot rolled steel strip  30  is seen moving from right to left. The steel strip  30  emerges from a furnace  34  where it was heated to at least 400° C. (about 750° F.). Although there is no need to heat it above 750° C. (about 1380° F.), our process will accommodate temperatures up to about 2400° F. The heated strip  30  first passes through sealing rolls  31  to enter initial reducing compartment  32 . The sealing rolls  31  are configured and installed to seal off the front end  33  of reducing compartment  32 , to minimize the escape or leakage of hydrogen and other gases into furnace  34  or into the atmosphere. There may be an overall enclosure  61 . Radiant tubes  35  may be used to further heat the strip  30  or maintain it at a desired temperature.  
         [0027]    The reducing zone may comprise one reducing compartment but preferably comprises at least two reducing compartments  32  and  37 , positioned in tandem so the strip  30  will pass directly from one to the other. The compartments  32  and  37  are each sealed enclosures except for the provisions for entrance and exit of the strip  30  and reducing gas to be explained below. Compartments  32  and  37  may have a common wall.  
         [0028]    Reducing gas, preferably hydrogen, is introduced near the strip exit  44 , in this case into the second reducing compartment  37 . As illustrated, it is preferably introduced to reducing compartment  37  prior to the point where the strip  30  leaves reducing compartment  37 . An inert gas, preferably nitrogen, may be introduced to provide a positive pressure in a chamber  29  also near the strip exit. Radiant heaters  45  may be employed for the strip because the hydrogen is normally not heated. The reducing gas flows generally from left to right, as depicted, countercurrently to the strip  30 , through reducing compartment  37  where it continually contacts strip  30 , through passage  43  to reducing compartment  32 , where it again continually contacts strip  30  moving countercurrently, and proceeds to exhaust trap  46 , to be explained in detail in FIG. 6. Throughout its passage from near strip exit  44  to passage  43  and further to exhaust trap  46 , the gas is contacted with strip  30  and reacts with the mill scale on the strip  30 , manufacturing water (where hydrogen is the reducing gas) from the combination of hydrogen and oxygen from the mill scale. As indicated above, not all of the oxygen needs necessarily to combine with hydrogen to effect removal of the mill scale, since some of the reaction will undermine the scale, weakening its adhesion to the strip  30  so it can be readily removed by physical contact such as brushing. Ideally, the reducing gas will be entirely consumed by the time it reaches the safety trap  46 , but in practice as little as five percent is consumed, and provisions must be made for assuring that no hydrogen or other reducing gas escapes to the atmosphere, where it could cause a fire or explosion. These will be explained with reference to FIG. 6. Sealing rolls  36 ,  43 , and  31  substantially compartmentalize the gas.  
         [0029]    Persons skilled in the art will realize that the composition of the reducing gas in reducing compartment  32  is somewhat different from that of reducing compartment  37 . The reducing gas in reducing compartment  37 , particularly near strip exit  44 , may be relatively pure, or at least have a high concentration, while that in reducing compartment  32  has a lower concentration, having already reduced a large portion of the mill scale on strip  30 . Thus the strength of the reducing gas in reducing compartment  37  is substantially greater than that of reducing compartment  32 . As the rate of the reducing reaction is greater with higher purity compared to lower concentrations of active reducing gas, the compartmentalization illustrated by the use of reducing compartments  37  and  32  is utilized to maintain the concentration of hydrogen or other reducing gas at a higher level in compartment  37  than it would otherwise be throughout an equivalent length of strip  30  if the reducing zone were not compartmentalized. We prefer that, even near sealing rolls  47 , where strip  30  enters reducing compartment  37 , the concentration of hydrogen or other reducing gas be maintained at least at 2% in the gaseous atmosphere of reducing compartment  37 , preferably at least 25%, and at least 2%, preferably at least 10%, in compartment  32 . Generally, the gas entering exhaust trap  46  will contain at least 0.001% water vapor and, where hydrogen is used as the reducing gas, no more than 99% hydrogen. The balance of the gas entering exhaust trap  46  may include carbon monoxide, nitrogen, and methane. Passage  43 , which contributes to the maintenance of the higher concentration of reducing gas in reducing compartment  37  than in compartment  32 , may include a small diameter pipe connecting compartments  37  and  32 .  
         [0030]    Fans  48  are placed in both reducing compartments  32  and  37  to provide turbulence for assuring good contact of the reducing gas with the strip  30 , and to mix the reducing atmosphere so there will be no pockets of very low active reducing gas concentration in the atmosphere contacting the strip  30 . Intakes for fans  48  are within the reducing compartments  32  and  37 . Fans  48  are placed to assure turbulence in the reducing atmosphere both above and below the strip  30 . As seen in FIG. 5, an overhead view of a preferred configuration of reducing zone  36 , the fans  48  are preferably placed so the strip  30  is exposed to alternating gas flow in alternate directions as it proceeds through the reducing compartment  37 . A similar configuration of fans is in reducing compartment  32 . The fans  48  are deployed also so they are riot directly across from each other, and so they will provide a more or less continuous circulation of the gas while providing turbulent contact with the strip  30 . Dotted lines  49  depict a preferred projection of the gas flow from fans  48 , avoiding energy-consuming conflict with other fans  48 . This separation of the gal flows may be assisted by baffles, not shown.  
         [0031]    In FIG. 6, details of exhaust trap  46  (see FIG. 4) are shown. Exhaust trap  46  comprises a hood  50  having a gas exit  51 . Gas exit  51  leads to a duct  52  of a higher elevation than hood  50 , which leads to a downcomer  53  terminating at an elbow  54  positioned lower than hood  50 . Elbow  54  leads in turn to damper  55  and thence to chamber  56  which contains at least one constantly lit burner  57 . Combustion in burner  57  may be assisted by air introduced through inlet  59 . Chamber  56  has a flue  58  to atmosphere. In operation, used or substantially exhausted reducing gas from reducing compartment  32  is led, by a negative pressure from flue  58 , through gas exit  51 , to hood  50 . Pressure differences will cause the exhaust gas to descend into elbow  54  from downcomer  53 , and immediately into chamber  56  where it is burned at burner  57 . The configuration of the duct  52  and downcomer  53  tends to stabilize the flow of gas. Damper  55  may be adjusted either automatically as a function of flow, or manually as conditions may dictate. Damper  55  may also be used to shut off the flow of gas and/or to prevent the backflow of air into downcomer  53 . Heat generated by burner  57  and/or the combustion of the used or substantially exhausted reducing gas may be conserved and used in any known manner to assist in the heating of strip  30 , either in furnace  34  or elsewhere.  
         [0032]    Thus our invention may utilize a single reducing compartment but includes a variation in which the reducing zone comprises at least two sealed reducing compartments in tandem. The reducing gas, preferably hydrogen, is passed turbulently and countercurrently to the moving steel strip containing mill scale, or other oxied-covered metal, at a first relatively high concentration in a first reducing compartment and at a second, lower, concentration in a second reducing compartment. An exit is provided for the exhaust reducing gas, wherein any remaining combustible component is combusted after passing through an inverted U shaped duct to minimize surges and the risk of explosion.  
         [0033]    After emerging from sealing rolls  36 , strip  30  is typically still covered by a sponge layer—that is, a thin layer substantially of iron (in the case of steel), the remains of the mill scale. The strip or other metal is then led to a cooling zone, not shown, where it may be cooled by any effective means to a temperature preferably no higher than 150° C., preferably within an inert or slightly reducing atmosphere during the cooling process, to minimize reoxidation after the process is finished. A preferred method of cooling is to spray or otherwise contact the surface of the strip or other metal with cooling water containing a corrosion inhibitor such as sodium nitrite. To preserve the neutral or slightly reducing atmosphere, the entrance and exit to the cooling zone should be sealed with fabric or any other other effective sealing means. The cooling zone is preferably followed by scrubbing by a brush scrubber or other effective scrubber means for mechanically removing foreign matter and/or the reduced mill scale or sponge layer, and/or polishing or retexturing the surface. We intend to include high pressure sprays, as well as brushes, as means for cleaning, scribbing, or mechanically removing foreign matter.