Patent Application
20070022842
The present invention generally relates to the recycling of bottom dross formed in molten metal baths, such as during hot dipped galvanizing processes, and particularly to a device and process for removing bottom dross from a kettle of molten metal.
In the hot dipped galvanizing art, practitioners have long combated the formations of dross at the bottom of the galvanizing kettles. Bottom dross is a slushy mixture of pure zinc and iron/zinc crystals. The bottom dross is frequently removed for processing to prevent fouling of the surface of the galvanized metal products and to recover the zinc retained in the dross.
Even most contemporary procedures for removing bottom dross in hot dipped galvanizing kettles are relatively crude, generally labor intensive and dangerous for the operators using the removal equipment. The current removal processes generally consists of one of four methods.
The first method involves the use of a drag bucket in which one operator operates an overhead crane to pull the drag bucket across the bottom of the kettle while two other operators guide the drag bucket by holding on to a bar attached to both sides of the bucket. Once the drag bucket is emerged from the kettle, an operator hammers on the sides of the bucket to try and extract as much of the zinc as possible. The crane operator then has to strategically place the bucket over a 55 gallon (about 200 l) drum and manipulate the crane to precisely drop the dross into the drum. The drag bucket is then turned 180 degrees for another drag in the opposite direction.
In a second method, one operator handles a long dross spoon attached to an overhead pulley system where another operator raises or lowers the spoon as needed. Once the spoon emerges from the bath, another operator chops at the dross with a blade to help extract the entrapped zinc. The spoon is then positioned over a catch pan and turned over to release the dross. Another operator again chops at the dross to extract additional zinc. This process is repeated several times across the kettle. The process is lengthy due to the fact that the spoon is only several inches wide and the kettle is several feet long.
The third process involves a clamshell design operated with two air cylinders to move each half of the clamshell and an overhead hoist to raise and lower the unit into the kettle. The operator lowers the opened clamshell into the bath, activates the cylinders to close the clamshell and then raises the clamshell. Two operators, then, chop at the dross in the closed clamshell with a long handled chop plate to help extract zinc from the dross. The dross is then deposited into a container. This process is repeated along the entire length of the kettle.
A fourth method uses an apparatus as describes U.S. Pat. No. 5,961,285 for removing bottom dross from molten zinc contained in a coating pot including a pump, a traversing carriage for positioning the inlet of the pump in a desired horizontal location and a slide mechanism for positioning the inlet of the pump in a desired vertical location. The pump is moved in a horizontal and vertical direction to remove the accumulated bottom dross. The bottom dross contained in molten zinc is pumped into a V-shaped channel and allowed to flow into either a removable basket or into a pre-melt pot. The actual separation step of the dross from the molten metal is not described. The positioning mechanism does not allow to reach the entire surface of the bottom of the pot and no mention is made about how this mechanism is operated.
A device for removing dross from the surface of the molten metal, the so-called top dross, and for extracting usable elemental metal from the dross is described in U.S. Pat. No. 6,235,234. The device comprises a container in which the dross is collected, a means for lifting the container out of the molten bath of metal and an agitating means for chopping the dross to separate the elemental metal from the particles of metal oxides contained in the dross. Preferably, the container has perforations therein such that the elemental metal may drain through the perforations and back into the molten bath. The remaining dross is then dumped in a recycle bin by the device. Although the teaching of this document gives no indication on how to collect the top dross into the basket, the dross needs to be skimmed into the basket by some means, e.g. manually. This device does not enable the removal of bottom dross and the document is not concerned with problems arising from the higher density of the bottom dross, such as the higher initial content of molten metal in the bottom dross or the contamination of the bath with sinking particles of dross, the resultant risk of surface fouling or even of extended interruptions of the normal galvanizing operation needed for the settlement of such resuspended particles. Furthermore, the agitation action of the dross is limited to an up and down reciprocating motion (chopping) due to the movement of the chopping blades alone or to a combined reciprocal movement of the blades and the basket. Finally, dumping problems and perforation obstruction may arise when the dross chills out too fast in the basket.
Accordingly, a number of problems and disadvantages remain in the removal of bottom dross from molten metal baths, such as an insufficient separation of the dross from the molten metal, insufficient reliability and safety, compulsory interruptions of the galvanizing process, product quality problems due to surface fouling, solidification and obstruction problems, etc.
It is an object of the present invention to provide an apparatus allowing a safe and assisted removal of bottom dross from molten metal baths, thereby greatly enhancing the separation yield of the molten metal from the dross as compared to known solutions. Further objects and advantages of the invention will become apparent by the description below.
This object is achieved by a device as claimed in claim 1.
A further object of the invention to provide a process for reducing the content of molten metal in a mixture of dross and molten metal.
In order to overcome at least some of the above-mentioned problems, the present invention proposes a device for removing dross from the bottom of a molten metal bath, comprising
Hence, the invention not only provides a device for assisting the operator in the removal process of bottom dross, but it further results in a dross having less usable molten elemental metal, such as zinc, therein, less likelihood of bodily injury and a savings of labor and material costs.
The invention eliminates the most dangerous and difficult parts of the removal process of manhandling in known forms of removal equipment. This is accomplished by providing a device which mechanically pumps a mixture of bottom dross and molten metal into a perforated basket assembly, wherein the excess (free) molten metal flows back to the bath through the perforations or, as it is less dense than the dross, over the top of the perforated basket assembly. When needed, the basket assembly is lifted out of the bath in a separation position wherein a double action paddle assembly effects an efficient coalescence of the molten metal, i.e. with a high separation yield, the coalesced metal returning back to the metal bath through the perforations. Thereafter the basket assembly containing the bottom dross is rotated in a disposal position, such a disposal position of the perforated basket assembly being preferably located outside said molten metal bath above a recycle bin, wherein the dross is deposited. The so recycled low elemental metal containing dross may then be sold for scrap.
As stated above, it is desirable and an object of the invention to remove as much of the usable elemental metal from the dross as possible before depositing the dross into the recycle bin. This can be achieved, to a high degree, by effecting on the dross an up/down reciprocating cutting action therethrough with a number of reciprocating paddles in one position followed by an up/down reciprocating cutting action in an adjacent position, thus alternating the contact area on the dross, which in turn will separate the useable elemental metal from the waste products in the dross. By alternating the entry of the paddles on the peaks of the dross made from the previous stroke of the paddles greatly enhances the elemental metal release from the dross. Furthermore, this method reduces the amount of dross that is forced or extruded through the openings by eliminating the constant force of the blades in a repeated position on the dross causing extrusion of the dross through the openings. More then two alternating positions may be used to further enhance the separation yield. Finally, the thus separated useable metal will then return to the molten bath for reuse.
Accordingly, a further advantage of an integrated device according to the invention is the fact that the time spent outside of the bath for the separation of the dross from the elemental metal may be significantly reduced. This is an important aspect not only for economical reasons, but especially because it reduces the risk of an untimely, even partial solidification of the molten metal, which would render any separation impossible and jeopardize the actual galvanizing process.
In a further, preferred embodiment of the invention, the device further comprises an outer non-perforated bucket assembly in which the perforated basket assembly is located when in its pumping position. The main advantage of such a configuration is that the dross removal process may be accomplished at any time, even during galvanizing operation, in a continuous or batch-like manner when needed, because particles of bottom dross too fine to be retained in the perforated inner basket do not directly and permanently return to the metal bath, thereby preventing the fouling of the surface of the galvanized metal products processed in the metal bath.
In a still further embodiment, the device further comprises lifting and/or rotating means for moving the outer non-perforated bucket assembly in a controlled dumping position above said molten metal bath. Thus, the fine bottom dross may not only be collected when the inner perforated basket assembly is in the pumping or the separation position and advantageously be returned to the bath at a specific moment in time, but also at a specific location of the metal bath without disturbing the normal operation and conditions of the galvanizing process.
In a preferred embodiment of the device, the lifting and/or rotating means for moving the outer non-perforated bucket assembly or the lifting and/or rotating means for moving the perforated basket assembly, or both, comprise rotary actuator(s). In an especially preferred embodiment, said basket or bucket assemblies are supported by four bar linkage arrangement(s) operatively attached to rotary actuator(s). The use of such four bar linkage arrangements is particularly advantageous in maintaining a proper orientation of the basket or bucket assemblies at any time. However, other arrangements allowing the lifting and/or rotating are known to the person skilled in the art, such as a lift and swing versus lift and rotate of the basket with a slide bottom to release the dross, and may be used within the context of the present invention.
In a further embodiment of the invention, said outer non-perforated bucket assembly and/or said perforated basket assembly comprise(s) each a stationary and a movable part, wherein the movement of the movable part relative to said respective stationary part effects the emptying of said assembly. Such an arrangement has three major advantages: first it enables a straight and hence more controlled drop of material (dross) out of the basket or bucket assembly, second, it allows to evacuate the material by effecting a scraping action thereon due to the relative movement of both parts, preferably in combination with a retention or back wall, and third, contrary to known devices, no heating element will generally be necessary to prevent solidification of the molten metal, because the basket assembly is mostly immerged in the bath, the separation and disposal steps are short and the scraping action of both perforated two-part basket and non-perforated two-part bucket assemblies allow the emptying thereof even in the case of partial re-solidification of the residual elemental metal. Nevertheless, such heating elements may be of use in certain circumstances and may therefore be provided if desirable.
The shape of the basket assembly, respectively of the bucket assembly, as well as their size are not critical as long as they are appropriate for the particular galvanizing unit. Furthermore, the size, number, spacing and location of the perforations or holes in the perforated basket assembly are adapted to the particular conditions of the galvanizing unit, such as the particle size of the dross, the expected throughput, and the prevention of extrusion of the dross based on paddle size and spacing. In practice, the size and spacing of the perforations will easily be determined by the person of ordinary skill in the art to effectively allow the maximum amount of elemental zinc to return to the bath and the maximum amount of dross to remain in the basket.
The separation means comprise a paddle assembly, which preferably consists of a plurality of parallel paddles whose shape, size and number are adapted to fit into the perforated basket assembly when the perforated basket assembly is in the separation position. Generally, the shape and size of the paddles will be determined by the cross section of the perforated basket assembly and their number (or spacing) by parameters such as the particle size of the dross, the size of the perforations and/or the size of the basket assembly itself.
In a further embodiment, the device further comprises a carriage assembly for the pumping means allowing to move the pumping means across the width and the length of the molten metal bath. The object of such a carriage assembly is to allow the pumping means to reach every place at the bottom of metal bath where dross is accumulating. An example of carriage assembly comprises a main frame carriage assembly transporting, along the length of the metal bath, a pump carriage assembly, the latter allowing the movement of the pump across the width of the bath.
A further advantage of such a carriage assembly is that the entire process of dross removal may be automated. Hence, in a further preferred embodiment, the carriage assembly further comprises actuators, such as air/hydraulic actuators, air/hydraulic motors, linear actuator, gear tooth rail, etc., controlled by a control unit.
The pumping means are not critical as long as their specifications meet the requirements of the medium used, i.e. temperature resistance, viscosity, capacity, etc. It has been found that commercial type slurry pumps that coincidently can handle the temperature and viscosity of the material, such as zinc dross, are suitable. Accordingly, the evacuation of the bottom dross is preferably accomplished by an industrial grade single stage centrifugal slurry pump.
The pumping means therefore generally comprise a slurry pump with an inlet near the bottom of the molten bath, an outlet located above the inner basket assembly at least when the latter is in the pumping position and a piping or hose assembly for connecting said pump to said outlet. The inlet is preferably conceived to allow the removal of substantially all the dross accumulated at the bottom of the metal bath, i.e. it has a substantially peripheral opening or multiple openings in different directions.
Another important aspect of the invention is a process for reducing the content of molten metal in a mixture of dross and molten metal comprising the steps of
The process for reducing the content of molten metal in a mixture of dross and molten metal preferably uses a device as described herein and comprises the steps as taught above or in the detailed description of a preferred embodiment below.
A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which:
Further details and advantages of the present invention will be apparent from the following detailed description of a preferred not limiting embodiment with reference to the attached drawings.
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The dumping step described in relation with
Although a preferred embodiment has specifically been described above, it is to be understood that a variety of configurations may be utilized by a person skilled in the art that are still within the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 91 186 | Jul 2005 | LU | national |
