[0002] Graphite electrodes are used in electric arc furnaces which are utilized in a wide range of industrial processes from steel making to waste treatment. Graphite electrodes are made of calcined petroleum needle coke, which is ground to size to form an aggregate. The aggregate is heated and mixed with coal tar pitch and other resin binders and is formed through pressing or extrusion into the green electrodes.

[0003] Once cooled, the green electrodes are carefully baked in an oven to convert them into hard amorphous carbon electrodes. Prior to this baking step, either a single green electrode or multiple green electrodes are placed in a container, commonly known as a sagger, which is typically made of stainless steel. The green electrodes are supported inside of the sagger by a packing material, which is typically a flowable material such as petroleum coke, metallurgical coke or silica sand. The sagger and the electrodes contained therein are then baked for a period of generally between 8 and 40 days to create the finished electrodes.

[0004] The baking carbonizes the electrodes. However, during the baking step, carbon also forms on the bottom of the sagger and may also adhere to the electrodes. The carbon formation is often difficult to remove from the sagger and the removal of this deposit increases the maintenance cost for the sagger, as well as increasing the unloading cycle time. The electrodes must also be cleaned of any carbon deposits formed thereon.

[0005] Accordingly, it is the principal object of the present invention to provide a method for baking green graphite electrodes and the like in which the sagger may be easily and quickly cleaned after use.

[0006] It is a related object to provide a packing material for use in the baking of green graphite electrodes and the like in which the packing material does not leave a difficult-to-remove deposit on the sagger or the electrodes.

SUMMARY OF THE INVENTION [0007] These objects, as well as others which will become apparent upon reference to the following detailed description and accompanying drawings, are provided for in a method for baking graphite electrodes in which at least one green electrode is supported in a sagger can with graphite particles. In one method, amorphous graphite is used to line the bottom of the sagger underneath the electrode (s), while the remainder of the sagger is filled with standard sized petroleum coke. In a second method, the green electrode (s) are completely supported within the sagger with amorphous graphite. In the preferred method, the graphite is sized between 30 and 6 mesh (U. S.) and is generally greater than 70% carbon.

[0008] The amorphous graphite preferred for use in the present method has a lower surface area and a more planar structure than the calcined petroleum coke which is typically used for the packing of saggers. This lower surface area makes amorphous graphite much less likely to adhere to the surrounding objects.

[0009] The method is equally advantageous for baking graphite bodies other than electrodes. Further, while the method is described as using amorphous graphite, natural crystalline, synthetic and/or purified graphites could also be used.

BRIEF DESCRIPTION OF THE DRAWING [0010] Fig. 1 is a cross-sectional view of a sagger containing an electrode or other carbon body and containing a packing material according to the method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED METHODS [0011] In practicing the method of the present invention, a sagger 10 is provided. This typically comprises a large carbon or stainless steel can with an open top, with either a single or multiple electrodes loaded into each sagger. Typically, a guide or magazine (not shown) is first loaded into the sagger in its upright position, and either a single electrode or a plurality of electrodes 12, generally of a cylindrical shape, are supported in the magazine.

[0012] Graphite media 14 is poured across the bottom of the sagger to form a layer of preferably from approximately four to six inches of graphite in order to assist in the elimination of residue buildup that sticks to the bottom of the can. Then, the bottom layer of graphite having been loaded, the guide is slightly raised and the sagger filled to a halfway point with packing media 16 of either graphite or petroleum coke. Next, the guide is completely removed and the sagger is filled the rest of the way with similar packing media. When the guide is removed, the packing media holds the electrode (s) separate from each other, and the sagger is filled to a point at which all of the electrode tops are completely covered by the packing media.

The sagger is then vibrated to settle all of the material around the electrode (s). Then, the sagger is typically transported to a station where gravel is placed on the top in order to level its upper end. The gravel holds the packing media in place in the oven while the air is being circulated, the saggers are then positioned on a car-bottom oven car and fired on a cycle normally varying from 8 to 40 days.

[0013] After baking the sagger is unloaded, generally by using a lift mechanism to invert the sagger and dump the electrode (s). The electrode (s) are then separated from the packing media. The media is recycled, screened, and. stored for reuse. It has been found that, in addition to making the saggers easier to clean, the use of graphite as the packing material also makes the electrodes easier to clean.

This is due to the low wetability and porosity of graphite, as compared to petroleum coke, as well as its inherent lubricity. Further, a more uniform baking of the electrodes results due to the graphite having better thermal conductivity than petroleum coke, thus reducing hot or cold spots within the sagger.

EXAMPLES [0014] Four tests were conducted using two sizes of the amorphous graphite. Each type of graphite was used in a first method where the amorphous graphite was the sole packing material used in the sagger can. In the second method, the amorphous graphite was used to form a bottom layer of packing material in the sagger, with the remainder of the sagger filled with standard size petroleum coke packing material. The first type of amorphous graphite used was a-30 mesh (US), which is generally greater than 70% carbon, and is designated by the Superior Graphite Co. of Chicago, Illinois, U. S. A., as product number 3216. The second size of the amorphous graphite was between-6 mesh and +30 mesh (US), and. is also generally greater than 70% carbon. This latter product is Superior Graphite Co. product number 3212.

[0015] The use of the 3216 amorphous graphite as the sole packing material resulted in electrode surfaces that were, after removal, smooth and pit free, with no apparent cracks or surface defects noted. Normal build-up in the bottom of the sagger was easily removed, unlike what happens with the petroleum coke packing that is typically used. The unloading of the sagger did however result in an undesirable dust level.

[0016] When the 3212 amorphous graphite was used as the sole packing material, the packing material stuck to the outside of the electrode surfaces, but was easily removed.

The electrode surfaces after removal were rough, with pitting and embedded particles, but no apparent cracks or surface defects were noted. Again, normal build-up in the sagger bottom was easily removed. Importantly, the dust generated by unloading the sagger was minimal in comparison to that generated when unloading the sagger packed with 3216 amorphous graphite.

[0017] In a third test when the 3216 amorphous graphite was used only to coat the bottom layer of the sagger, with petroleum coke being used as the remainder of the packing material. The packing material stuck to the outside of the electrode in a manner typical with when coke is the sole packing material. However, the normal build-up in the sagger bottom was easily removed. Again, however, the dust level was undesirable during the unloading.

[0018] When the 3212 amorphous graphite was used for the bottom layer, with the remainder of the packing material comprising petroleum coke the normal build-up in the sagger bottom was easily removed. However, the dust was minimal in comparison to that generated during unloading of the sagger having a bottom layer of 3216 amorphous graphite.

[0019] Accordingly, a method for firing graphite bodies in a sagger has been provided that meets all the objects of the present invention. While the invention has been described in terms of certain preferred methods, there is no intent to limit to the same. Instead, the invention is intended to be covered by the following claims.