Cutting by use of rotating axially moving tool – Processes – Bit detachable
Reexamination Certificate
1998-10-19
2001-02-13
Tsai, Henry W. H. (Department: 3722)
Cutting by use of rotating axially moving tool
Processes
Bit detachable
C408S223000, C408S224000, C082S001400, C082S001500, C407S042000
Reexamination Certificate
active
06186706
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a method and equipment including a processing tool for the processing of carbon bodies, such as the formation of holes in carbon bodies, in particular nipple holes in anodes for use in electrolytic cells for the production of aluminum.
2. Description of Related Art
In modern electrolysis plants for the production of aluminum, anodes are used which comprise a prebaked or calcined carbon body fastened to an anode hanger. The anode hanger usually consists of an upper part, an anode rod made of aluminum and a lower part comprising a yoke having steel nipples which extend downwards.
The carbon body is fastened to the steel part of the anode hanger using a cast iron or monolithic lining material joint. More specifically, the joint between the carbon body and the steel part is formed by the nipples in the steel part that are first arranged so that they extend downwards into holes in the carbon body and then the holes are filled with molten cast iron or having monolithic lining material pressed into them.
The holes in the carbon body are formed in the traditional manner during the forming of the carbon bodies, the so-called “green carbon”, before the calcination process (prebaking process).
This results in the following disadvantages:
The green carbon must be stacked in a special way in the calcination furnaces to avoid creep and deformation of the holes. This special stacking results in additional work and reduced calcination capacity, i.e. reduced productivity.
However, even if the precautions mentioned are taken, some holes will still be deformed and, consequently, some carbon bodies will be unusable. The proportion of rejects may be as much as 10% or more in some cases.
Another problem in connection with the calcination of carbon bodies with preformed holes is that the packing material can burn fast to the walls of the holes.
Other disadvantages are that preformed nipple holes are associated with large density gradients and many small cracks in the carbon form body around the nipple hole. The cracks will result in a high electrical contact resistance, which is not very desirable during the electrolysis process as the increase in resistance can lead to an increase in the temperature in the joint between the anode and the nipple.
Another disadvantage of preforming the nipple holes is that there are restrictions to the geometry which can be formed with this technique as they are usually formed using projections in the vibration mold, which must be provided with a certain taper so that they can be withdrawn from the body when the stamping operation has finished. If undercut or conical nipple holes are to be formed, which will gradually become more common, the hole must be processed further.
Several solutions are proposed for cleaning the nipple holes of an anode after calcination. For example, FR 2 590 911 shows a two-part cleaning tool for nipple holes in which a first, outer part is designed to clean grooves in the walls of the hole while a second part is designed to smooth the peripheral surface and base of the hole. The latter part is driven by a rotating shaft while the first part is arranged coaxially and supported freely in relation to the shaft. This equipment, which, among other things, is designed to clean grooves in the walls of the hole by means of axial motion, will be limited to cleaning preformed holes.
EP 0 764 728 A 1 shows equipment for processing three nipple holes in one anode. The equipment comprises a centering device for centering an anode with performed holes, a liftable/lowerable support for the anode and three downward-facing spindles each with its own milling head for processing the anode's holes.
In the above publication, it is suggested that it would be conceivable to form holes in anode blocks without preformed holes with the processing tool and, moreover, that this can be done with sufficient precision. However, no indication of what such a tool would look like is given.
SUMMARY OF THE INVENTION
The present invention represents a method and apparatus for the formation of holes in carbon bodies after they have been calcined, whereby the geometry of the holes can be formed with a high degree of precision and the wear on the equipment is minimal. Further, the equipment may also serve to remove excess material of the surface of the carbon bodies, such as grading and smoothing.
The present invention makes it possible to omit the preforming of nipple holes in carbon bodies in the “green state”. By omitting the preforming of the holes, the vibration time in connection with the formation of the carbon body in the “green state” is reduced and the current problems of density variations and cracks around nipple holes can be avoided.
Moreover, the present invention results in great savings during calcination as the positioning in the furnace can be simplified because the carbon bodies have no nipple holes which can creep. A reduction in the positioning time will contribute to an increase in the capacity of the furnace.
It is also possible to increase the capacity of the individual furnaces by making the firing curves steeper, i.e. the change in temperature per unit of time can be increased as the structure of the carbon bodies is more homogeneous (without nipple holes). The present invention will also mean that the cassettes in the furnace can be made fuller (increase in production) as there is no need to take into consideration how the anodes are arranged.
Moreover, the number of carbon bodies which must be rejected after calcination will be reduced as they have a simpler geometric form and are therefore less subject to deviations. Any carbon bodies which are destroyed during calcination as a result of crack formation, etc. will represent rejects at a lower cost level than previously because less work was done on them before the calcination.
Moreover, burdensome operations such as cleaning/alignment of nipple holes after calcination can be eliminated. These are operations which are burdensome when performed manually and which are also associated with the generation of fine dust and noise.
Another advantage is that a calcined carbon body can now rapidly be adapted to different designs of anode hangers so that reserve stocks of anode carbon can be reduced. Moreover, in accordance with the present invention, holes of various geometries, such as undercut (conical) or oval holes or holes with threads/tracks, may be formed simply and rapidly.
Moreover, in connection with the mechanical processing in accordance with the present invention, better (smaller) tolerance deviations between the nipple holes will be achieved than can be achieved with the current technology.
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Hagen Odd
Linga Hogne
Nesse Oddvin
Norsk Hydro ASA
Tsai Henry W. H.
Wenderoth , Lind & Ponack, L.L.P.
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