JPH0372005B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method of manufacturing a graphite electrode by improving graphitization conditions during the manufacture of an artificial graphite electrode. (Prior art) Graphite electrodes used as electrodes in electric furnaces for steelmaking are made from easily graphitizable coke, a binder, and an impregnating agent such as pitch. Nonetsuka (3) Molding (4) Primary firing
It is generally manufactured through the steps of (5) impregnation with an impregnating agent, (6) secondary firing, (7) graphitization, and (8) cutting. In the graphitization process, the secondary fired electrode is heated in an electric furnace with nitrogen,
This process converts coke into graphite by heating it to approximately 3000℃ in an inert gas atmosphere such as argon, or in a state where the air is cut off using a compressor. There has been a change from graphitization to a method using a rapid graphite furnace.
When a rapid graphitization furnace is used, graphitization is performed in, for example, several tens of hours, so the temperature rise rate during graphitization is fast. On the other hand, when producing graphite electrodes, it is known that irreversible expansion, so-called puffing, occurs during graphitization. If the degree of puffing is large, the bulk specific gravity of the electrode becomes small and the strength becomes weak, which is not preferable. This puffing becomes more pronounced as the rate of temperature rise during graphitization becomes faster, and therefore becomes a particular problem when graphitizing in a rapid graphitization furnace. Conventionally, coke made from petroleum-based heavy oil has been used as a raw material for graphite electrodes, but this petroleum-based coke contains about 0.3 to 2.0% sulfur, so it is difficult to use this coke. The cause of puffing is when sulfur begins to form a graphite structure.
It is said that this occurs due to rapid volatilization at 1700 to 2000℃ (for example, E.Fitzer et al., High
Temperatures−High Pressures, vol9, 243~
250 pages, 1977). Also, when puffing occurs, approximately
It is also known that the number of micropores below 1000〓 increases (MPWhittaker et al., Carbon vol7,
pp. 615-621, 1969). Therefore, as a method for preventing puffing to date, anti-puffing agents have been proposed that combine with sulfur to form sulfides and prevent volatilization as sulfur. As specific anti-puffing agents, iron oxide, calcium fluoride, titanium oxide, and various other compounds have been proposed (U.S. Patents 3338993, 3563705,
4140623, 4312745, 4334980). It has also been proposed to reduce puffing by heating the coke and binder to 1600-2200° C. for at least 0.1 hour in an air-free atmosphere before mixing the coke and binder (US Pat. No. 4,061,600). (Object of the invention) The present invention relates to the prevention of puffing during the production of graphite electrodes using easily graphitizable coke based on coal tar pitch. Although the sulfur content is low, the nitrogen content is high, and the nitrogen content in coal tar-based pitch coke is 0.3~
It is around 0.6%. The present inventors studied the rate of reduction of sulfur and nitrogen contained in coke during heating, and as a result, completed an effective method for reducing puffing when producing graphite electrodes from coal tar-based coke. (Structure of the Invention) As a result of various studies on the causes of puffing of coal tar-based coke, the present inventors found that it is partially due to nitrogen volatilization at 1700 to 1800°C, where graphite crystal growth begins rapidly. I discovered something big. Of course, coal tar-based coke also contains a small amount of sulfur, so a small amount of puffing due to sulfur is thought to occur at the same time, but its contribution is small. As described above, the main cause of puffing in coal tar-based coke is nitrogen rather than sulfur, so conventional anti-puffing agents such as iron oxide are less effective than in the case of petroleum-based coke. In the case of coal tar-based coke, nitrogen is the main cause of puffing, but even in this case, puffing causes
Since the number of micropores of 1000 Å or less increases, the degree of puffing can also be determined by measuring the amount of increase in the number of micropores of 1000 Å or less, and the smaller the micropores after heating, the less puffing occurs. The sulfur content in coke decreases even below 2000℃, but the extent of the decrease is small, whereas nitrogen easily decreases just by heating to a temperature below 2000℃, especially at 1400 to 1700℃ for a long time. The rate of decrease is large when heated for a long time. Therefore, when producing graphite electrodes using coal tar-based coke as a raw material, the temperature is maintained at a desired temperature below the temperature at which puffing occurs during the graphitization process for a certain period of time, and then the temperature is raised without cooling to produce graphitization. This will remove nitrogen, which is the main cause of puffing.
Puffing can be reduced. The temperature to be maintained is preferably in the range of 1400 to 1700°C. If the temperature is less than 1400°C, the rate of nitrogen reduction is slow, so it is necessary to increase the holding time, which is not practical in an industrial sense. Moreover, it is meaningless if it exceeds 1700°C because this is the temperature at which puffing occurs. The holding time varies depending on the temperature, the dimensions of the electrode, the particle size of the coke that constitutes it, the structure of the graphitization furnace, etc.
More than 1 hour is required, preferably 1 to 5 hours. If the holding time is short, nitrogen will not be reduced substantially, and if the holding time is long, there is no point in using a rapid graphitization furnace, and productivity will decrease. On the other hand, when petroleum coke is used as raw material, puffing is mainly caused by the sulfur content, so even if the temperature is kept within the 1400-1700℃ range,
There is no reduction in sulfur content and puffing cannot be prevented with this method. However, since coal tar-based coke also contains a small amount of sulfur, it is more effective to use this method after adding a small amount of a puffing inhibitor such as iron oxide. the above
Temperatures outside the range of 1400 to 1700℃, room temperature to 1400℃,
Between 1700°C and 3000°C, the temperature may be increased at a normal rate. Example 1 Coal tar-based coke containing 0.58% nitrogen and 0.22% sulfur was used, and this coke was pulverized.
A 10 g sample with a particle size of 2.83 to 5.66 mm was taken and placed in a Tammann furnace and heated under an argon atmosphere. The temperature was raised from room temperature to the specified temperature at a heating rate of 20°C/min, held at the specified temperature listed in Table 1 for 1 hour, and then raised again to 2200°C at a rate of 20°C/min. The temperature was lowered at a rate of °C/min. Note that the comparative example was not maintained at a constant temperature. The pore volume of the sample treated by the above method was measured using a mercury porosimeter, and the results shown in Table 1 were obtained.

[Table] As can be seen from the table, by holding the sample at a predetermined temperature of 1430 to 1680° C. for 1 hour, micropores of 1000 Å or less are reduced and puffing is reduced. Example 2 A test piece was prepared using coal tar coke containing 0.41% nitrogen and 0.26% sulfur, and puffing was measured. The conditions for making the test piece are as follows. Particle size: 16-60 mesh: 20%, 60-200 mesh: 45%, 200 mesh or less: 35% Binder blend ratio: 35% Molding method: Mold test piece shape: 20 mmφ x 100 mm The test piece made under the above conditions After firing at 900°C, it was placed in a graphitization furnace and the degree of expansion was measured. In an argon gas atmosphere, the temperature was rapidly raised to 1000°C, and then at a rate of 10°C/min to 2600°C. FIG. 1 shows a pattern of temperature rise and temperature fall, and the solid line in the figure is an example of the present invention, and the broken line is a comparative example corresponding to the conventional method. FIG. 2 shows the puffing curve obtained, and it can be seen that puffing was reduced when the sample was held at 1600° C. for 3 hours (solid line) compared to the comparative example (broken line). The expansion rate indicates the heating expansion amount ΔL/the electrode length L before heating.

[Brief explanation of drawings]

FIG. 1 is a diagram of the temperature increase pattern in the graphitization process, and FIG. 2 is a diagram of the puffing curve in the process.

Claims (1)

[Claims] 1 In a method of manufacturing an artificial graphite electrode by blending easily graphitizable coke produced from coal tar pitch, kneading it with a binder and pitch, molding it, firing it, and graphitizing it, the molded and fired electrode is A method for producing a graphite electrode, which comprises maintaining a desired temperature within a temperature range of 1400°C or more and 1700°C or less for 0.5 hours or more, and then increasing the temperature without cooling to graphitize it.