Patent Publication Number: US-6656415-B2

Title: Process and device for precipitating compounds from zinc metal baths by means of a hollow rotary body that can be driven about an axis and is dipped into the molten zinc

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. application Ser. No. 09/402,465, filed Feb. 10, 1999, now U.S. Pat. No. 6,364,930 which is hereby incorporated by reference in its entirety and which is a National Stage application of PCT/AT99/00034, filed Feb. 10, 1999, which was not published in English under PCT Article 21(2). The present application claims priority under 35 U.S.C. §119 of Austrian Patent Application No. 247/98, filed Feb. 11, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for precipitating solid compounds from the liquid zinc or liquid zinc-based alloys of a metal bath thereof. 
     The invention further relates to a device for precipitating solid compounds from a molten mass of liquid zinc or liquid zinc-based alloys, in particular from a metal bath thereof, for example, a galvanizing bath. 
     Finally, the invention relates to the use of the process and the use of the device for precipitating compounds and oxides, particularly intermetallic iron compounds from galvanizing baths, in particular from continuous sheet galvanizing systems. 
     2. Discussion of Background Information 
     Zinc is a metal with a comparatively low melting point of 419.6° C. and has a standard electrode potential of −0.762 V, and is therefore baser than iron with −0.447 V. In addition, zinc is relatively stable towards oxygen, because a zinc oxide layer formed on a zinc surface protects the zinc from the attack by oxygen. Thus, if a steel part is coated with a zinc layer, the zinc acts as an anode, i.e., protects the iron from oxidation, with the formed zinc oxide preserving the zinc layer from further oxidation. Because long-term rust protection of iron and steel can be achieved by zinc and zinc has a melting point that is lower by more than 1100° C., galvanizing parts in a liquid zinc bath is one of the most important protective measures against atmospheric and similar corrosion of steel components. 
     Galvanizing the steel parts takes mostly place by dipping them into a bath with liquid zinc or a zinc-based alloy to form a coating. Sheet metal is passed through a zinc bath, for example, which allows to achieve a thin, uniform and perfectly smooth surface coating. 
     Good adhesion of the zinc layer is ensured due to the solubility of the zinc in the alpha (α)-mixed crystal of the iron of up to approximately 7.3 at. % at usual bath temperatures. On the other hand, iron is only slightly soluble in zinc at its melting temperature, with the three-phase equilibrium at 419.35° C. having an eutectic composition: zinc and 0.021 at. % or 0.018% by weight of iron. At higher temperatures of 530° C., for example, the solubility of iron in liquid zinc is 0.3 at. % or 0.25% by weight. 
     If iron is introduced into a galvanizing bath due to iron dust or the like adhering to the part as well as by system parts, then iron-zinc mixed crystals are formed, for example FeZn 3 , FeZn 7 , Fe 3 Zn 10 , Fe 5 Zn 21 . These FeZn mixed crystals may be heavier than pure zinc and enrich themselves at the bottom of quiescent liquid galvanizing baths as hard zinc or as so-called “dross” or “bottom dross.” 
     In order to improve the quality of the galvanizing or the zinc layer formation, in particular on metal sheet metal which passes through the zinc bath at a high speed, aluminum can be added at an order of magnitude of 0.1 to 0.2% by weight to form an alloy with the zinc bath. In the given case, the “bottom dross” interacts, in terms of reaction kinetics, with the zinc molten mass that due to subsequent alloying contains aluminum, and iron-aluminum mixed crystals with a configuration of Fe 2 Al (5-x) Zn x  and a specific weight of significantly less than 6×10 3  kg/m 3  are formed. 
     The mixed crystals, in particular the compounds Fe 2 Al (5-x) Zn x  form a pulp in the galvanizing bath, and with a continuation of the galvanizing treatments and further introduction of iron into the bath the particles become larger, reach a diameter of over 30 μm and agglomerates, so-called clusters, are generated. These coarse compound particles in the galvanizing bath, which may be in the form of lumps, can cause surface defects or can adversely affect the surface quality of the coating, especially in the galvanization of smooth sheet metal bands, such as those for the automobile industry, for example. 
     In order to purify galvanizing baths contaminated with intermetallic compounds, it is necessary to render the bath quiescent with respect to bath currents, if possible, whereafter the layer enriched on the surface with the compounds is skimmed off. Performing this kind of purification in sedimentation basins has already been proposed. 
     The previously known purification processes all have the disadvantages of low efficiency, high expenditure, reduced economic efficiency, and production safety, as well as productivity of the system. 
     The object of the invention is to eliminate the deficiencies of previous precipitation methods and to disclose a process by means of which even with large throughput quantities an insignificantly low content of solid compounds can be maintained in galvanizing baths. 
     An additional object of the invention is to provide a device for the optionally continuous precipitation of solid compounds from galvanizing and zinc baths, which device does not impede production. 
     SUMMARY OF THE INVENTION 
     The object is attained in a process of the type recited at the outset by subjecting partial amounts of the liquid metal phase containing the compound(s) and being above the melting temperature of zinc to an acceleration higher than the acceleration due to gravity and causing an at least partial demixing into fractions containing heavier and/or lighter components, whereafter the molten mass depleted of solid compounds is returned to the metal bath or made available in purified form for such a bath, and the part of the molten mass enriched with the desired compounds is discharged and/or supplied to a further separation process. 
     The additional object of the invention is attained with a device according to the species in that a hollow rotary body that can be driven about an axis and has conveyor means, such as conveyor impellers or pump impellers, projecting into the cavity in the feed or lower portion is introduced at least partially into the molten mass, which hollow rotary body is provided in its discharge or upper portion with at least one discharge opening for the depleted molten mass, which opening is eccentrically arranged in the wall of said body and, subsequent thereto with respect to the discharge side, with at least one further centrally and/or eccentrically arranged discharge opening for the liquid metal enriched with compounds wherein at least one of the upper molten mass discharge openings in the hollow rotary body opens into a discharge area of a housing which at least partially surrounds the hollow rotary body, which housing area has at least one discharge channel for the enriched molten mass and, optionally, a further one for the depleted molten mass designed to return to the metal bath. 
     The advantages of purifying a galvanizing bath of intermetallic compounds achieved in such a way can essentially be seen in that with a high acceleration of the metal contaminated with compounds of different densities a high degree of demixing can be achieved and utilized. A rapid and highly effective separation or accumulation of solid suspended particles, in particular Fe 2 Al (5-x) Zn x , in the center of a rotating molten mass, was surprising for the person skilled in the art because on the one hand the difference in the specific weights is to be considered as rather small and, on the other hand, the low particle size and especially the phase boundary tensions between the compound and the liquid metal counteract demixing. The further advantage according to the invention is that, as was found, larger intermetallic particles and particularly compound agglomerates, which can cause an especially large reduction in quality of the coating, can be concentrated and discharged fully and in a highly effective manner by high acceleration. 
     If in so doing the molten mass containing the compounds is subjected to a centrifugal acceleration of at least 1.2 times, preferably of at least 2.1 times, particularly of more than 10.1 times the acceleration due to gravity, an efficient purification effect can be achieved. 
     To achieve high productivity, it is advantageous for the precipitation of compounds to be conducted essentially continuously with continuous removal of partial amounts of contaminated molten mass from a zinc (alloy) bath of an iron galvanizing device, in particular of a steel sheet galvanizing system, with return of the depleted liquid metal to the bath. 
     The purification effect of the process can be increased further if before or during the demixing treatment by applying an increased acceleration on the metal phase containing the compounds said mixture is adjusted or heated or cooled to a treatment temperature to between 420° C. and 480° C., preferably to between 429° C. and 450° C. This results in establishing a low solubility of the compounds in liquid metal and thus a precipitation or growth of nuclei or particles of the intermetallic phase. 
     The advantages of a device according to the invention essentially consist in that by the conveyor means projecting into the cavity of the hollow rotary body through the feed side a throughput and an increase in pressure of the rotating molten mass therein can be achieved. Both the amount fed through and the pressure in the liquid metal are, as was found, important for precipitation kinetics and the concentration behavior of the solid compounds in the liquid metal when centrifugal acceleration is applied. Both the flow rate through the hollow rotary body as well as the pressure built up therein can be adjusted synergistically with respect to optimal precipitation criteria of intermetallic particles by the design of the conveyor means and by the size of the discharge openings. 
     If the device is put out of operation by turning off the drive and is lifted, the entire interior space runs dry so that it is available for re-use without further cleaning work. 
     The rotation of the contaminated metal molten mass induced by the conveyor means can be supported effectively, whereby the hollow rotary body can also be reduced in terms of its length, if the interior wall of the hollow rotary body comprises carrier or guide means promoting the rotational movement of the molten mass therein. 
     In this connection, a separate molten mass discharge is improved if the hollow rotary body has centrifugal sealing elements or similar sealing elements cooperating with the housing parts. 
     It is advantageous for the input speed of the rotating shaft of the hollow rotary body to be adjustable in order to adjust the optimal flow rate through the hollow rotary body and the pressure inside the hollow rotary body, which are important particularly for varying temperatures of the molten mass. 
     It has proven to be favorable for both reducing the heat removal and for draining the device of liquid metal if at least parts of the housing have heat insulation. 
     In one aspect, the present invention relates to a process for removing solid compounds from a molten metal bath, the molten metal selected from liquid zinc and liquid, zinc-based alloys. The process comprises subjecting the molten metal containing the solid compounds to an acceleration higher than the acceleration due to gravity, whereby at least partial demixing of the molten metal into fractions comprising at least one of heavier and lighter components is effected. A fraction enriched with the solid compounds is separated from a fraction depleted of the solid compounds and the molten metal depleted of the solid compounds is returned to the metal bath or used in a new bath. The molten metal enriched with the solid compounds is discharged and/or supplied to a further separation process. 
     In one embodiment the solid compounds comprise intermetallic compounds, metal oxides or both. The intermetallic compounds may comprise iron and zinc, or iron, zinc and aluminum. 
     According to another embodiment of the above process the acceleration higher than the acceleration due to gravity is centrifugal acceleration. This centrifugal acceleration may be at least 1.2 times, preferably at least 2.1 times, and more preferably more than 10.1 times the acceleration due to gravity. 
     In a further embodiment of the instant process the metal bath is a galvanizing bath; for example, the metal bath may be part of an iron galvanizing device, e.g., a steel sheet galvanizing system. 
     According to a still further embodiment the process is carried out continuously. For example, a portion of the metal bath is subjected to said acceleration and the fraction depleted of the solid compounds is returned to the metal bath. 
     In another embodiment the molten metal is subjected to said acceleration while at a temperature between 420° C. and 480° C., e.g., between 429° C. and 450° C. 
     The density of the fraction enriched with the solid compounds may be different from the density of the fraction depleted of the solid compounds. For example, the fraction enriched with the solid compounds may have a higher density than the fraction depleted of the solid compounds or the fraction enriched with the solid compounds may have a lower density than the fraction depleted of the solid compounds. 
     According to yet another embodiment the instant process is carried out with a device that includes a hollow rotary body rotatable about an axis thereof, the hollow rotary body comprising a wall, a lower feed portion and an upper discharge portion. A housing surrounds said hollow rotary body at least partially. The housing has a discharge area that includes at least one discharge channel. The device also includes a conveyor arranged at least partially into the molten metal and projecting into a cavity in the lower feed portion. Further included are at least one first discharge opening for the fraction depleted of the solid compounds, located in said upper discharge portion and arranged eccentrically in a wall of the hollow rotary body, and at least one second discharge opening for the fraction enriched with the solid compounds, the at least one second discharge opening being arranged centrally and/or eccentrically in the upper discharge portion of the hollow rotary body and after the at least one first discharge opening with respect to the direction of discharge. At least one of the first and second discharge openings opens into the discharge area of the housing. 
     In a further embodiment the fraction depleted of the solid compounds has a higher density than the fraction enriched with the solid compounds and the fraction depleted of the solid compounds is discharged through the at least one first discharge opening. According to a still further embodiment the fraction depleted of the solid compounds is returned to the molten metal via a discharge channel of the discharge area of the housing. 
     Another aspect of the present invention is a process for removing solid compounds from a molten metal derived from a metal bath and selected from liquid zinc and liquid, zinc-based alloys, wherein the molten metal containing the solid compounds is subjected to an acceleration higher than the acceleration due to gravity, thereby effecting at least partial demixing of the molten metal into a fraction enriched with the solid compounds and a fraction depleted of the solid compounds. The density of the fraction enriched with the solid compounds is different from the density of the fraction depleted of the solid compounds. The fraction enriched with the solid compounds is separated from the fraction depleted of the solid compounds, whereby molten metal depleted of the solid compounds for further use in a metal bath is obtained. 
     In a further aspect the present invention relates to a device for precipitating solid compounds from a molten mass composed of liquid zinc or liquid, zinc-based alloys. The device comprises a hollow rotary body, rotatable about an axis thereof, that includes a wall, a lower feed portion and an upper discharge portion. The device further comprises a housing which at least partially surrounds the hollow rotary body and has a discharge area that includes at least one discharge channel for molten mass depleted of solid compounds. Also included in the device is a conveyor that is arranged at least partially into the molten mass and projects into a cavity in the lower feed portion. The device further includes at least one first discharge opening for molten mass depleted of solid compounds, located in the upper discharge portion and eccentrically arranged in a wall of the hollow rotary body, and at least one second discharge opening for molten mass enriched with solid compounds, the second discharge opening being arranged centrally and/or eccentrically in the upper discharge portion of the hollow rotary body and after the first discharge opening with respect to the direction of discharge. At least one of the first and second discharge openings opens into the discharge area of the housing. 
     In one embodiment the conveyor comprises conveyor impellers or pump impellers. According to another embodiment the interior wall of the hollow rotary body is designed to promote a rotational movement of the molten metal inside said hollow rotary body. For example, the interior wall may have carrier or guide elements. 
     In further embodiments of the device the hollow rotary body additionally has centrifugal sealing elements which cooperate with the housing, and at least parts of the housing have heat insulation. 
     According to a still further embodiment the discharge area of the housing has discharge channels for both the first and second discharge openings, while in another embodiment the discharge channel for the first discharge opening is designed to return the molten mass depleted of solid compounds back to original molten mass. 
     Another embodiment of the device includes a hollow rotary body that has a rotating shaft. The input speed of the rotating shaft may be adjustable. 
     In a further embodiment the discharge area of the housing also includes at least one discharge channel adapted for discharging the molten mass enriched with solid compounds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in more detail by the following drawings which each show just one embodiment. 
     In the drawings: 
     FIG. 1 is a precipitating device having a conical hollow rotary body. 
     FIG. 2 is a precipitating device with a cylindrical hollow rotary body and two molten mass discharge channels. 
     FIG. 3 is a precipitating device with direct return of the depleted molten mass into the metal bath. 
    
    
     The list of reference signs of the systematic representations is as follows: 
       1  molten mass 
       11  molten mass contaminated with compounds 
       12  enriched molten mass 
       13  compound concentrate 
       14  depleted molten mass 
       2  hollow rotary body 
       21  conveyor means or conveyor impeller, pump impeller 
       22  carrier or guide means 
       23  discharge opening for depleted molten mass 
       24  rotary sealing means 
       25  discharge opening for enriched molten mass 
       26  rotating shaft 
       3  housing 
       31  sealing means 
       32  discharge channel for enriched molten mass 
       33  discharge channel for depleted molten mass 
       34  insulation 
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     FIG. 1 shows a schematic view of a hollow rotary body  2  partially dipped, at an angle, into a galvanizing bath  1 , which hollow rotary body is partially enclosed by a housing  3 . The hollow rotary body  1  experiences a rotational movement around its longitudinal axis when driven by means of a rotating shaft  26 , while the conveyor means  21  projecting into the cavity introduce the molten mass  11  contaminated with compounds into the cavity until it is filled. The conveyor means  21  can be designed as slanted conveyor impellers or pump impellers or similarly acting parts. The molten mass  11  contaminated with particles which is introduced into the hollow rotary body  2  is set into rotation by a coaxial movement of the conveyor means  21  and/or by any optionally provided carrier or guide means  22  so that radial acceleration is effective in the molten mass. For example, some compounds such as, e.g., oxides or oxide layers and iron-aluminum-zinc mixed crystals, have a lower density than a liquid zinc-based alloy and, as a result, are thrust in the direction of the axis due to the acceleration effect, thereby creating a concentration of solid particles, i.e., an enrichment  12  in the liquid metal, in the axis area of the cavity, whereas depleted molten mass is present in the areas of the wall of the hollow rotary body  2 . The conveyor means  21  and the optional carrier means  22  effect, with rotation of the hollow body  2 , constant introduction of the contaminated molten mass  11  into said hollow body with induction of a rotary movement of the molten mass therein, as well as a disintegration with the formation of a portion of molten mass that is enriched with compounds  12  and a portion that is depleted of compounds. The hollow body  2  has eccentric discharge openings  23  in its upper portion, through which the depleted molten mass  14  collected on the hollow body wall is discharged, returned inside the housing  3 , and can be removed by means of a discharge element  33 . The molten mass enriched with compounds  12  remains in the axis area and is pushed further upwards due to the build-up of hydrostatic pressure of the conveyor means  21 . The uppermost part of the hollow rotary body  2  has other discharge openings  25  in its wall, through which the enriched molten mass  12 , forming a compound concentrate, can be discharged into a housing part with a discharge channel  32  for said concentrate  13 . The areas formed in the housing  3  for the enriched molten mass  12 ,  13  and the depleted molten mass  14  can be sealed by means of sealing means  31  which cooperate with the rotary sealing means  24  of the hollow rotary body  2 . With respect to the maximum effectiveness of the precipitation of compounds from the metal molten mass  1 , it is advantageous to coordinate the rotational speed as well as a conicity of the hollow rotary body  2 , a design of the conveyor means  21  and the size and shape of the cross-sections of the discharge openings  23 ,  25  synergistically with one another. 
     FIG. 2 depicts another embodiment of a device according to the invention. An essentially cylindrical hollow rotary body  2  is enclosed in the upper portion by a housing  3  having an insulation  34  of the outer surface. Discharge openings  23  in the hollow body  2  for the depleted molten mass  14  are arranged on the outside front face and, on the other hand, a feed hollow is positioned centrically on the front face and the discharge openings  25  for the enriched molten mass are positioned subsequently on the surface of a partially hollow drive shaft  26 . A rotary sealing means  24  and a sealing means  31  connected with the housing  3 , which cooperate, form a separation of the respective discharge channels for the metal molten mass enriched with solid compounds  32  and the depleted metal molten mass  14 . 
     FIG. 3 shows a schematic view of a cylindrical hollow rotary body  2  submerged essentially completely in the metal molten mass, such that the depleted molten mass  14  can be returned directly to the metal bath or galvanizing bath  1  through discharge openings  23  on the jacket side. An outlet  25  arranged centrically upward as well as subsequent discharge openings  24  arranged in a partially hollow rotating shaft or drive shaft  26  enable a discharge of the enriched molten mass  12  in a housing  3  reduced to a discharge channel  32  with a sealing system  24  and an insulation  34 . 
     All devices according to the invention have the advantage that the hollow rotary body  2  and the channels  32 ,  33  can be run dry after the rotary drive has been turned off and they have been lifted out of the galvanizing bath, thereby rendering cleaning work unnecessary. However, heating devices and/or cooling devices for molten mass treatment can be provided in a favorable manner during implementation of the process and/or for complete cleaning of the device.