The present invention relates to sealing the end portions of cathode collector bars used in electrolytic cells for production of aluminum. Such cells (also known as pots) typically have an outer metal shell, refractory lining/filler material defining an area for pooling of aluminum metal and electrolyte, and upper anodes exposed to the electrolyte-aluminum pool. Carbon cathode blocks are located at the bottom of the pool area below the anodes. The cathode collector bars typically are encased in the carbon cathode blocks and have opposite ends that project through holes in the shell walls for connection to external conductor buses. Representative constructions are shown in the following publications and the references cited therein:
U.S. Pat. No. 4,619,750 (Cathode Pot for an Aluminum Electrolytic Cell);
U.S. Pat. No. 6,231,745 (Cathode Collector Bar);
U.S. Pat. No. 6,387,237 (Cathode Collector Bar With Spacer for Improved Heat Balance and Method);
U.S. Patent Publication No. 2008/0308415 (Cathodes for Aluminum Electrolysis Cell With Expanded Graphite Lining);
U.S. Pat. No. 7,618,519 (Cathode Element for Use in an Electrolytic Cell Intended for Production of Aluminum);
U.S. Pat. No. 7,776,190 (Cathodes for Aluminum Electrolysis Cell With Expanded Graphite Lining);
U.S. Patent Publication No. 2010/0258434 (Composite Collector Bar).
This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
The present invention provides a seal assembly for a cathode collector bar where it extends through a window in a sidewall of an electrolytic cell for refining aluminum. Such seal assembly maintains a hermetic seal preventing ingress of air through the sidewall window while permitting longitudinal (horizontal) movement of the collector bar and also movement in a vertical plane (side to side, or up and down, or diagonally) which can be caused by changing heat conditions inside the cell. In one embodiment, the seal assembly includes a seal member that slides between frame sheets secured around the window. In another embodiment the seal assembly includes a tapered boot having one end joined to a base member or sheet secured to the cell sidewall and a remote end forming a central opening to receive the bar, with a mechanical tightening member to adjust the fit of the boot on the bar.
The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
With reference to
Commercial electrolysis cells are designed for continuous service for at least several years at high operating temperatures (such as 940° C.). Intermittent operation typically is not practical because of serious stresses caused during startup due to different temperature characteristics of the materials used. In addition, no matter how careful the design and care taken at startup, some structural damage may occur which is not immediately detected or preventable, resulting in premature pot failure. For example, operating conditions are not static because the exact composition of the electrolyte-aluminum pool changes as more electrolyte is added and aluminum metal is tapped off. Temperature gradients can develop in unpredictable manners. Corrosive compositions may percolate through some of the pot components and/or penetrate through small cracks or gaps that are undetected. Another factor is believed to be leakage of air through the sidewall openings for the cathode collector bars which can cause oxidation of the collector bar and cathode materials. Sometimes an attempt is made to lessen the likelihood of the ingress of air by a rigid connection of the collector bar to the shell wall. In other designs, a “seal” is formed by use of a high temperature mastic or “moldable” composition. Such compositions typically are rigid when set, but may allow longitudinal sliding movement of the cathode collector bar, which can be important. Unyielding connections of the cathode collector bar ends to the shell wall can induce tensile stresses in the carbon cathode blocks, such as if the collector bars flex, warp, or creep due to heat expansion and contraction.
The present invention provides a seal assembly for the area where a collector bar end portion extends through an electrolytic cell sidewall. Such assembly maintains a hermetic seal preventing ingress of air through the sidewall opening while permitting longitudinal (horizontal) movement of the collector bar and also movement in a vertical plane (side to side, or up and down, or diagonally) which can be caused by changing heat conditions inside the cell. In the embodiment shown in
With reference to
Materials for the components of the seal assembly 30 must be chosen carefully due to the extreme operating conditions to which they are exposed. Such materials necessarily are noncombustible, high temperature resistant, and refractory both in the sense of having little or no tendency to expand or contract at the high temperature operating conditions and in the sense of being resistant to chemicals of the type commonly encountered in use such as hydrofluoric gas. The sealing sheet 42 must be capable of being cut, preferably die cut, to the exact shape of the outer periphery of the cathode collector bar, but also have some degree of flexibility along its inner margin to accommodate for transverse heat expansion and contraction of the bar, while still allowing sliding movement of the bar through the opening of the sheet and maintaining the hermetic seal. In a representative embodiment, appropriate materials include materials available from Mid-Mountain Materials, Inc., of Mercer Island, Washington, as follows:
The edge portions of the sheets 32 and 36 and spacer 38 can be secured together by sewing using a high temperature thread (such as a thread formed from Mid-Mountain ARMATEX® SGT18 which is composed of twisted and plied together fiberglass fibers).
Securing of the back of the base sheet 32 to the outside of the cell sidewall 12 around the window through which the collector bar extends can be by a high temperature adhesive or cement compatible with the frame and cell wall materials. Mid-Mountain THERMOSEAL® 1000SF cement works well for securing the QF40 fabric to steel and meets the high temperature requirements while withstanding thermal expansion and contraction under potentially fluctuating heat conditions.
Other materials with similar properties could be used.
In the case of new cell construction or refurbishing of an existing cell, the assembly 30 can be installed from the inside, as represented in
Relative dimensioning of the parts is important to assure that the cathode collector bar is movable to the maximum degree permitted by the cell window without the edge of the sealing sheet 40 being exposed. Similarly, the channel between the frame sheets 32 and 36 must be of sufficient depth to allow such maximum movement without an outer edge of the sealing sheet coming into contact with the spacer. This can be illustrated with actual dimensions for a representative embodiment in which the cathode collector bar is of rectangular cross section (ignoring rounded corners) 230 mm by 115 mm. The cell window can be rectangular with length and width dimensions of 262 mm by 150 mm. Thus, from a centered position, the maximum “sideways” movement of the bar in the cell window is 16 mm in each direction, and the maximum up and down movement of the collector bar in the cell window is 17.5 mm from the centered position. For these dimensions, the outer dimensions of the sealing sheet 40 can be 310 mm by 190 mm, so if the sheet shifts the maximum amount permitted by the fit of the collector bar in the cell window, there still is a substantial lip or marginal portion of the sealing sheet covering the cell window, fitted in the channel of the seal assembly, and not engaged against the filler piece which can have a central opening of about 345 mm by 225 mm and a width along each side of 12.5 mm.
With reference to
As seen in
Regardless of the form of the invention used, a reliable hermetic seal can be achieved without injecting mastic or moldable material.
While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.
This application claims the benefit of U.S. Provisional Application No. 61/718097, filed Oct. 24, 2012, and U.S. Provisional Application No. 61/681560, filed Aug. 9, 2012, the disclosures of which are hereby expressly incorporated by reference herein.
Number | Date | Country | |
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61718097 | Oct 2012 | US | |
61681560 | Aug 2012 | US |