Industrial electric heating furnaces – Resistance furnace device – With internal atmosphere control
Reexamination Certificate
2003-04-16
2004-12-21
Hoang, Tu (Department: 3742)
Industrial electric heating furnaces
Resistance furnace device
With internal atmosphere control
C373S137000
Reexamination Certificate
active
06834072
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method for rebaking and graphitizing pitch-impregnated carbon bodies in one method step. The invention also relates to a furnace shell that is suitable for the afore-mentioned method, can be sealed off in a largely gas-tight manner and is of a Castner-type lengthwise graphitization furnace for carrying out the afore-mentioned method. Furthermore, the invention relates to a furnace including the furnace shell.
The production of graphitized carbon bodies is a technique that has been mastered up to now for over one hundred years and is applied on a large scale industrially and has therefore been refined in many respects and optimized with regard to costs. One of the descriptions of this technique can be found in
Ullmann's Encyclopedia of Industrial Chemistry
, Vol. A5, published by VCH Verlagsgesellschaft mbH, Weinheim, 1986, pages 103 to 113.
A striking feature of this technique is the repeated reimpregnation and rebaking of carbon bodies. These method steps are necessary, because the graphitized carbon bodies must have a minimum density, strength and conductivity in order to meet the requirements with respect to the application of the graphitized carbon bodies. These method steps are associated with costly handling of the carbon bodies, that is, the carbon bodies must be repeatedly inserted into impregnating autoclaves, removed, inserted into carbonization furnaces and removed again.
There has not been any lack of attempts, therefore, to combine the method steps, to allow one or more of these method steps to be omitted. Typically, these attempts employ changed raw materials, changed recipes for the raw-material mixtures, or optimized techniques for “green production”.
Another attempt to manage with fewer method steps involved combining the last rebaking step with the final graphitization step. Trials with furnaces such as those sold under the tradename ACHESON from the past few decades are known. This so-called “transverse graphitization” is less economical than the “lengthwise graphitization” widespread today, in which the carbon bodies are disposed in the furnaces in such a way that the electric current flows directly through the carbon bodies parallel to their longitudinal axis. Reimpregnated carbon bodies were thus installed in transverse graphitization furnaces and then attempts were made to achieve rebaking and graphitization in one method step or in one furnace campaign or life with just one step of installing the reimpregnated carbon bodies and one of removing the then graphitized carbon bodies.
By the term “furnace life”, also when used further below, what is to be understood is as follows: reimpregnated or non-impregnated carbon bodies are inserted into a graphitization furnace and surrounded with a thermally insulating packing, preferably made of coke. By passing direct current passage through the packing and through the carbon bodies, the latter are heated, with the heating ranging from ambient temperature to the graphitization temperature (up to 3000° C.) using a predetermined temperature-time-program. The peak temperature can be maintained for a short period of time. Following this, the current is switched off and with that the whole furnace cools, something that, depending on the total mass of the carbon bodies, the packing, and the furnace components, can take up several days.
A lengthwise graphitization furnace in which the carbon electrodes are graphitized as a column without a surrounding insulating packing is described in German published, not prosecuted patent application No. DE 24 57 923. In this respect, this furnace is not a Castner-type lengthwise graphitization furnace. So that the electrodes are not attacked by the oxygen in the air during the thermal treatment, the electrode column is surrounded by water-cooled, bowl-shaped walls that are lined on the inside with graphite felt, and the hollow space between the electrode column and the walls is flushed with protective gas. The teaching of this specification does not specify that (re-)impregnated carbon bodies are inserted into the furnace; on the contrary, there is discussion of “carbon bodies”, see page 6, first paragraph, 4th and 9th line and also 3rd paragraph, 1st line, that is, of baked carbon that emits very little low-temperature carbonization gas or cracked gas during the thermal treatment. The rebaking and graphitization in one method step is not taught for this specific type of furnace.
U.S. Pat. No. 5,299,225 to Karagoz et al. provides teaching regarding a Heroult or Castner-type lengthwise graphitization furnace in which a column of carbon electrodes are surrounded by an insulating packing of coke grains during the thermal treatment. The column of carbon electrodes and the surrounding insulating packing of coke grains, according to the teaching of this specification, are located in a relatively gas-tight furnace shell made from metallic and ceramic components, see column 2, lines 40 to 45, and covered by a hood. By using this device, it is possible for low-temperature carbonization or cracked gases that develop during the thermal treatment to be collected and disposed of easily. In this specification, however, it is not taught that the rebaking and graphitization are carried out in one method step.
A method for combining the step of rebaking and graphitization is described in German Patent Application No. DE 22 24 905. According to this, the carbon bodies that are orientated transversely in relation to the current flow are installed in the furnace in a plurality of layers one above the other. The lower layers include carbon bodies that are reimpregnated with pitch and the upper layers include non-impregnated carbon bodies. In this way, on the one hand, the quantity of pitch introduced into the furnace was reduced. On the other hand, this configuration of the carbon bodies favorably influences the temperature distribution in the furnace during the furnace life. A higher temperature namely set in the upper portion of the furnace, which temperature effected further decomposition of the problematic cracked gases that developed from the pitch in the lower portion of the furnace to give less problematic gases. This method has not been executed to success in practice and in the end has failed with respect to the resultant cracked gases. The following explains the result.
Baked carbon bodies have a considerable porosity of the order of magnitude of 20 to 25% by volume after the initial baking. The pores are filled with pitch during (re-)impregnation. If (re-)impregnated carbon bodies are installed in a furnace such as those sold under the tradename ACHESON in comparatively large quantities of, for example, some ten tons, a few tons of impregnating pitch is also introduced into the furnace. When the (re-)impregnated carbon bodies are heated, the pitch first becomes soft and then liquid until it decomposes given further rising temperatures. From the impregnating pitch there develop solid carbon (coke) and volatile cracked gases which are composed of an extraordinarily broad spectrum of hydrocarbon compounds, starting with high-molecular tars and oils and ending with low-molecular compounds, such as CH
4
or CO. At low heating rates, small volumes of volatile cracked gases develop from the impregnating pitch per unit of time; at high heating rates, large volumes develop.
Very high final temperatures of, for example, 3000° C. are achieved in comparatively short periods of time of, for example, 15 to 25 hours, that means, high heating rates are achieved, in graphitization furnaces. In comparison, final temperatures of, for example, 1000° C. are achieved in comparatively long periods of time of, for example, seven days in carbonization furnaces. The high heating rate unfavorably causes development of extraordinarily large quantities of volatile cracked gases from the impregnating pitch. The graphitization system, the surrounding building, and the environment are loaded to excess by these undesirable gases; production ac
Daimer Johann
Gschwandtner Stefan
Kalchschmid Franz
Kals Franz
Lhotzky Walter
Hoang Tu
SGL Carbon GmbH & Co.
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