Abstract:
A continuous galvanizing line uses a coating pot containing a molten zinc bath having bottom dross and further comprises a pump. The pump agitates the bottom dross so the bottom dross interacts with aluminum and converts to top dross, which can be removed without needing to stop the galvanizing line. A reaction vessel may also be used to provide a higher concentration of aluminum to react with the bottom dross.

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 60/911,347, filed Apr. 12, 2007. 
    
    
     BACKGROUND 
     The present disclosure relates to apparatuses and methods for reducing the buildup of bottom dross in a zinc bath and reducing the transition time between two bath states. 
     Galvanizing (GI) and galvannealing (GA) are two known processes. Galvanization is a chemical process that is used to coat steel or iron with zinc in order to reduce corrosion (specifically, rusting). In galvannealing, steel or iron that has been coated with zinc is then heated (annealed) to improve fabrication and corrosion resistance characteristics. 
     Continuous galvanizing or galvannealing is typically done by running a steel or iron sheet through a molten zinc bath contained in a coating pot. The zinc bath contains zinc (Zn), aluminum (Al), and iron (Fe) and usually has a temperature of 450-480° C. (840-890° F.). Zinc is the overwhelming component of the zinc bath. The aluminum content of the zinc bath ranges from 0.10 weight percent (wt %) to 0.4 weight percent. In GI, the aluminum content of the zinc bath is greater than 0.13 wt %. In GA, the aluminum content of the zinc bath is less than 0.13 wt %. In another related process called galvalume, the zinc bath contains 55 wt % Al and 45 wt % Zn. The iron content is usually very low (less than 0.1 wt %) and generally comes from the steel sheet itself. 
     The zinc-rich field of the Zn—Fe—Al phase diagram is helpful for understanding the chemical processes that occurs during GI and GA. In particular, the phase field changes around 0.13 wt % Al at these temperatures and different impurities (i.e. intermetallic compounds) occur in different phase fields. GA is usually operated within the δ+L phase field, wherein the impurity is FeZn 7 (δ). This impurity is denser than the zinc bath itself and collects on the bottom of the coating pot; thus, it is also known as bottom dross. GI operates within the η+L phase field, wherein the impurity is Fe 2 Al 5 (η). This impurity is less dense than the zinc bath itself and collects on the surface of the molten zinc bath in the coating pot; thus, it is also known as top dross. These impurities generally form because the solubility limit of Fe is reached in a local region. Dross particles can thus nucleate and grow. 
     The bottom dross and top dross are undesired. Whereas the top dross can be continuously removed by skimming the top of the zinc bath, the bottom dross cannot. Continued operation of the GA process thus builds up bottom dross, which can solidify. In addition, the bottom dross (FeZn 7 ) consumes the desired zinc reactant in the zinc bath. This aspect is also undesired. 
     Bottom dross can be removed. If the bottom dross has solidified, it can be mechanically removed by jack-hammering; however, this usually results in a week of downtime. Bottom dross can also be removed using scoops before it solidifies, but this method is dangerous, tedious and still results in downtime. 
     Bottom dross can be removed chemically by exploiting the differences between GA and GI. Aluminum is added to the zinc bath as solid ingots to change the phase field from δ+L to η+L. This allows the bottom dross (FeZn 7 ) to convert to top dross (Fe 2 Al 5 ), which can then be skimmed off. However, this method of removing bottom dross has its own disadvantages. Typically, the transition time during which the bottom dross converts to top dross is 24-30 hours. During this transition time, the continuous galvanizing line produces only products having significantly lower value. 
     Galvannealed steel is widely used in the automobile, appliance, and construction industries because of its comparatively superior corrosion resistance properties. Thus, it would be desirable to continually run a galvannealing process or, at a minimum, reduce the transition time between the GA to GI processes. 
     BRIEF DESCRIPTION 
     The present disclosure is directed to apparatuses and methods for reducing the buildup of bottom dross in a zinc bath and reducing the transition time between two bath states. Generally, the apparatuses comprise a coating pot and a pump. The pump accelerates the conversion of bottom dross to top dross by intimately mixing the zinc-rich bottom dross with aluminum. This reduces the transition time between the two bath states (GA: low Al content to GI: higher Al content). If run continually, bottom dross buildup can also be reduced or prevented. Either result occurs in a more profitable continuous production line. 
     The pump has one of an inlet or an outlet located near the bottom of the coating pot (where bottom dross will build up). The other of the inlet and the outlet can be located in the molten zinc bath. 
     The apparatuses may further comprise a separate reaction vessel, located within or outside the coating pot. Aluminum may be added to the reaction vessel, increasing the Al content of the portion of the zinc bath inside the reaction vessel. The bottom dross is then mixed with this Al-enriched zinc bath via flow provided by the pump. Again, this accelerates the conversion of bottom dross to top dross. 
     The methods comprise mixing the bottom dross with an Al-enriched zinc bath. The method may further comprise providing a second Al-enriched zinc bath separate from the coating pot and contacting the second bath with the bottom dross, either in the coating pot or in the reaction vessel. The second zinc bath may or may not be derived from the zinc bath in the coating pot. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following is a brief description of the drawings, which are presented for the purposes of illustrating the exemplary embodiments disclosed herein and not for the purposes of limiting the same. 
         FIG. 1  is a schematic view of an exemplary embodiment of an apparatus of the present disclosure. 
         FIG. 2  is a schematic view of a second exemplary embodiment of an apparatus of the present disclosure. 
         FIG. 3  is a schematic view of a third exemplary embodiment of an apparatus of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A more complete understanding of the components, processes and apparatuses disclosed herein can be obtained by reference to the accompanying drawings. These figures are merely schematic representations based on convenience and the ease of demonstrating the present disclosure, and are, therefore, size and dimensions of the devices or components thereof and/or to define or limit the scope of the present disclosure. 
       FIG. 1  is a cross-sectional view of an exemplary embodiment of an apparatus of the present disclosure. The apparatus  10  comprises a coating pot  20  defined by a sidewall  30  and a base  40 . As shown here, the sidewall  30  and base  40  are an integral unit. The sidewall  30  and base  40  may contain passages used for various purposes, such as the entrance, exit, or circulation of the molten zinc bath. As shown here, the coating pot  20  has a flat base (flat base is shown, but pot may have a sloped base) and vertical sidewalls; however, the coating pot  20  may be of any shape. Contained within the coating pot  20  is a primary molten zinc bath  50 . The molten zinc bath contains Zn, Fe, Al, and may contain other trace elements as well. A continuous galvanizing line (CGL) comprises a continuous steel sheet  60  that enters the primary zinc bath  50  from a snout  70  and is kept in tension by a sink roll  80  located within the coating pot  20 . The steel sheet  60  then travels out of the primary zinc bath  50  and, typically, past a correcting roll  90  and a stabilizer roll  100  (sometimes a stabilizer roll is not used) which are on opposite sides of the steel sheet  60 . If galvannealing is desired, the steel sheet  60  may then enter a galvannealing furnace (not shown) which further heats the steel sheet  60 . 
     Located at the bottom  25  of the coating pot  20  is bottom dross  110 . The bottom  25  of the coating pot  20  may be considered to be a lowest point in the coating pot  20 , where dross particles will accumulate as they sink. Depending on the architecture of the base  40 , there may be more than one such bottom  25 . The bottom dross  110  may be in either a solid or viscous state and is approximately FeZn 7 , particles. Located within the coating pot  20  is an impeller  122  of a circulation pump  120 . An example of a circulation pump is an L-series Molten Metal Circulation Pump available from Metaullics Systems of Solon, Ohio. In this embodiment, the impeller  122  of the circulation pump  120  is located near a bottom  25  of the coating pot  20 . As shown here, an inlet pipe  125 , which is in communication with the impeller housing  124 , is within the bottom dross  110 . An impeller housing outlet  126 , which is in fluid communication with the inlet pipe  125 , is located in primary zinc bath  50 , preferably in a zone having a relatively high Al concentration compared to the bottom dross  110 . 
     The pump  120  operates by promoting the conversion of bottom dross to top dross. This conversion occurs during the transition from a GA process to a GI process. Aluminum is added to the primary molten zinc bath  50 , which increases its Al concentration relative to that of the bottom dross  110 . The recirculation pump  120  stirs up the bottom dross, either by sucking bottom dross  110  up through the inlet pipe  125  and expelling it into the primary zinc bath  50  at the outlet  126 , or by impinging the primary zinc bath  50  collected from the outlet  126  into the bottom dross  110  through the inlet  125  in this example the inlet would be acting as an outlet and the outlet would be acting as an inlet). Either way, the flow created by the pump action promotes intimate interaction between the bottom dross  110  and the aluminum added to the primary zinc bath  50 . This intimate interaction promotes the conversion of FeZn 7  to Fe 2 Al 5  in a shorter transition time. The circulation pump may be run continuously to suspend the dross particles (and thus prevent their solidification) or intermittently to agitate the dross particles and force interaction during a GA to GI transition. 
       FIG. 2  is a cross-sectional view of a second exemplary embodiment of an apparatus of the present disclosure. Here, the coating pot  20  comprises a reaction apparatus  200 . In particular, the reaction apparatus  200  may be similar to the submergence apparatuses described in WO 2005/054521, including U.S. Pat. Nos. 6,217,823; 6,036,745; and 4,286,985 each of which are incorporated herein in their entirety. That apparatus is shaped so that incoming molten zinc creates a vortex wherein low-density aluminum is rapidly submerged and melted. Solid aluminum has a density of about 2.7 grams per cubic centimeter (g/cc) and liquid zinc has a density of about 6.6 g/cc. Accordingly, the reaction apparatus  200  is properly designed to promote the submergence of the solid aluminum into the liquid zinc, which is described in more detail in WO 2005/054521. 
     The reaction apparatus  200  is also defined by a sidewall  210  and base  220 . The reaction apparatus  200  further comprises an entry port  230  and an exit port  240 . As shown here, the entry port  230  is in the sidewall  210  and the exit port  240  is in the base  220 . A pipe  250  is connected to the exit port  240  and the output end  260  of the pipe  250  is located near a bottom  25  of the coating pot  20 . Of course, the reaction apparatus  200  and pipe  250  may be an integral unit (i.e. unitary). In this embodiment, the impeller housing outlet  126  of the pump  120  is connected to and in communication with the entry port  230  of the reaction apparatus  200  such that the interior of the reaction apparatus  200  can be filled from the primary molten zinc bath  50  in the coating pot  20 . Molten zinc is drawn into the inlet pipe  125  through the impeller housing  124  and into the reaction apparatus  200  through the pipe  230 . Of course, the impeller housing  124  and pipe  230  may be an integral unit as well. When used, aluminum, either in the form of Al ingots, Zn—Al ingots or granular pellets, is added to the reaction apparatus  200  which results in the aluminum melting in the zinc bath. This increases the Al concentration in the molten zinc inside the reaction apparatus  200 . That molten zinc and Al combination is then discharged through the output end  260  onto or into the bottom dross  110 . Again, this forces intermingling of the bottom dross  110  with the added aluminum. 
       FIG. 3  is a cross-sectional view of a third exemplary embodiment of an apparatus of the present disclosure. This embodiment differs from that of  FIG. 2  by including a reaction vessel  300  which contains a second molten zinc bath  310 . The primary molten zinc bath  50  of the coating pot  20  can be separated from the second molten zinc bath  310  of the reaction vessel  300 . The pump  120  collects bottom dross  110  through the inlet pipe  125  and transfers the bottom dross  110  through the impeller housing  124  to the second molten zinc bath  310 . The second molten zinc bath  310  has a higher Al content than the primary molten zinc bath  50  of the coating pot  20 . This higher Al content can be achieved by operating the reaction vessel  300  as a GI process or adding aluminum to the second molten zinc bath  310 . Regardless, the bottom dross  110  converts to top dross in the reaction vessel  300 , where it can be skimmed off. In this embodiment, there is no need to change the Al content of the primary molten zinc bath  50 . Thus, the coating pot  20  can continuously run as a GA process without needing to transition to GI at all. 
     As shown here, the reaction vessel  300  is outside the coating pot  20 . Of course, their relative location is not important. For example, the reaction vessel  300  could be located inside the coating pot  20 . The key is that the interior of the reaction vessel  300  (i.e. the second molten zinc bath  310 ) can be separated from the primary zinc bath  50  so that the local Al concentration in the reaction vessel  300  can be increased relative to that of the primary zinc bath  50 . If desired, the reaction vessel  300  may be configured so that the second molten zinc bath  310  can be replenished from the primary molten zinc bath  50 . For example, as mentioned above, the iron content in the primary zinc bath  50  is very low and generally comes from the steel sheet  60  itself. 
     Generally, all three embodiments move the bottom dross so that it can interact with aluminum and form top dross. In  FIGS. 2 and 3 , a molten zinc bath having a relatively high concentration of Al is formed and the bottom dross is interacted with that higher-concentration zinc bath. As a result, the transition time from GA to GI processes is reduced. In the embodiment of  FIG. 3 , the coating pot may not need to be transitioned to GI at all. 
     The present disclosure has been described with reference to certain exemplary embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.