The production of graphitized carbon has been practiced since the end of the 19th century and marks the early stages of the industrial revolution. The production of graphite from so-called carbon electrodes had traditionally been carried out in the Acheson furnace in which the electrodes are typically placed in a transverse orientation to the flow of the electrical current, and surrounded by a resistor medium, thereby causing the current to pass alternately through tiers of electrodes and resistor media, the latter being typically metallurgical or petroleum coke. The Acheson process is of such ancient vintage and so well known as not to require any further description. However, it is worth noting that the process is a batch process, not particularly energy efficient and relatively labor intensive.
The LWG process, although very old, is less well known and has been practiced on a commercial scale only recently. This process is carried out by arranging the carbon electrodes in a continuous column with an electrical connection at each end of the column. See U.S. Pat. No. 1,029,121 issued to Heroult and U.S. Pat. No. 4,015,068, issued to Vohler. In the LWG process, the electrodes themselves form the dominant path for the heating current, with one or both of the ends of the column subjected to a mechanical or hydrostatic pressure source in order to keep the connection tight under expansion or contraction of the column during the heating cycle. A packing medium of granular coke is used for insulation, however, Vohler does not use a packing medium, but discloses a metal shell with a felt liner as insulation.
Carbon electrodes consist of the essentially amorphous carbon from petroleum coke which has been calcined, ground, classified by size, mixed with a binder, and bound in a matrix of amorphous carbon derived from the binder after baking at temperatures of approximately 700.degree. C. to 1100.degree. C. in a baking furnace. Graphite electrodes are produced from the carbon forms by placing them in an Acheson furnace and in recent years in a Lengthwise Graphitization (LWG) type furnace and heating them to a temperature between 2500.degree. C. to 3000.degree. C., which converts the amorphous form of carbon to the crystalline graphite, and vaporizes most of the impurities present in the original carbon, including most metals and sulfur compounds. Graphite, compared to amorphous carbon has much higher electrical and thermal conductivity, lower coefficient of thermal expansion (CTE), superior ductility and vastly superior thermal shock resistance at the operating temperatures of the electric arc steel furnace. These physical properties are uniquely valuable in the electric furnace with its need for high electrical currents, and the need to resist the mechanical and thermal shock suffered by the electrodes from the falling scrap, fluctuations in metal and electrode level, and generally high thermal stresses. Consequently, graphite is universally used as an electrode in the electric arc melting of steel.
The LWG process has many advantages over the Acheson process. The energy efficiency is much higher, as the material is heated directly instead of indirectly, and the cycle time for the process is much faster taking typically less than 12 hours as compared to 60 to 120 hours for the Acheson process. U.S. Pat. No. 4,394,766 issued to Karagoz describes an LWG furnace where “ . . . the current is applied, heating the column of electrodes rapidly by the Joule effect to the required graphitization temperature, usually from 2400.degree.-2800.degree. C., sometimes as high as 3000.degree. C., taking approximately 4 to 12 hours, until the graphitization process is completed. The power is turned off, the furnace moved to a cooling station and the electrodes allowed to cool. When the electrodes have reached approximately 1500.degree. C.-1700.degree. C., the furnace is moved to the dump and re-load station and the transporter is replaced by a chute car with ducts leading from the dumping gates to the hoppers below. The electrodes are unloaded by a grab (stock extractor), the insulation medium is dumped at a weighted average temperature of from 700.degree. to 1100.degree. C. into the hoppers, and the furnace loaded with another electrode string and insulation charge. The chute car is removed and the furnace is transported back to the firing station.” There is no heat exchange between the carbon electrodes and the graphite electrodes in this process resulting in significant energy inefficiency.
U.S. Pat. No. 5,229,225 issued to Karagoz, et al., also shows an improved LWG furnace of modular design comprising roughly semi-circular shell coupled by expansion joints in which a pressure seal is held in place by deadweights. The shell is formed of corrugated steel panels with cast-able ceramic lining. The panel design allows more freedom for thermal expansion in the transverse direction while accommodating longitudinal expansion by freedom to slide over its support cradle. The corrugated panel design also enhances faster cooling of the furnace after off-fire by improved heat transfer in comparison to a simple steel plate structure. The corrugated structure has a higher surface area than a simple plate, which gives more radiative cooling and turbulent air movement giving more convective cooling. Again however, there is no energy exchange between starting materials and finished product resulting in significant energy loss.