Graphitization is one of the main processes of heat treatment in the production of carbon graphite products. Acheson graphitization furnace is the main furnace type for the graphitization production of carbon graphite products. It is a special resistance furnace that directly heats the products and resistance materials loaded into the furnace as the "internal heat source" and works intermittently. The space for loading products and resistive materials in the furnace is called the furnace core. The cross-sectional area of the furnace core is usually 3-6M2. A strong current is passed into the graphitization furnace to convert electrical energy into heat energy, so that the product reaches the highest temperature of graphitization, completes the graphitization process, and follows the Joule-Lenz law.
It can be seen that the temperature at different points in the graphitization furnace core is different, and at the same point, the temperature at different times is also different. It can be seen that the temperature of the graphitized furnace core is not only a function of space but also a function of time. Therefore, the temperature distribution of each part in the furnace core is unbalanced.
After the Acheson graphitization furnace is energized, the product is heated by the heat generated by the resistance material, so that the temperature of the furnace core gradually increases. The heating of the furnace core is very uneven, and the temperature distribution varies greatly. The temperature difference between the central part of the graphitization furnace core and the two sides of the furnace core close to the insulation material can reach hundreds of degrees Celsius, and the temperature difference between the upper part and the lower part of the furnace core can reach hundreds of degrees Celsius. Therefore, the uneven heating temperature distribution in the core of the same graphitization furnace is the main reason for the cracks in the core products.
Based on many years of graphitization production experience, I briefly summarized and analyzed the reasons for the pyrolysis waste products of carbon graphite products in the graphitization production process; here, I will discuss with carbon engineering and technical personnel how to reduce the content of carbon graphite products. Crack waste products in the graphitization process, increase the yield of the graphitization process, reduce the production cost of the graphitization process, and improve economic benefits.
Causes of graphitization cracks in products
In the graphitization process, the internal factors of product cracking are low product quality and poor heat resistance; the external factors are that the temperature of the furnace core rises too fast during the graphitization process, and the temperature difference between the upper and lower parts of the product is too small. As it increases, the thermal stress increases accordingly, which is the main cause of cracks in the product.
1. The graphitization process system is unreasonable
Furnace loading method
Acheson graphitization furnace products are usually installed in a vertical installation method, and the vertical installation method has two forms: normal installation and wrong installation. When installing furnace core products, there is only one high-density current heating belt for any product. The wider the heating belt, the more uniform the product is heated, otherwise the heating is very uneven. When installed incorrectly, each product has two high-density current heating belts, and the product is heated more evenly than normal products. Therefore, if the charging method of the graphitization furnace product is not selected properly, the temperature rise rate around the product will vary greatly during the process of graphitization and power transmission, and the thermal stress generated by the product exceeds the thermal stress borne by the body. It can be tolerated, which can easily lead to cracks in the product.
Unreasonable power system
The temperature change curve of the Acheson graphitization furnace core is controlled by the power curve of constant power distribution. If the power supply system of the graphitization furnace is unreasonable, the determined power curve of the graphitization furnace will start with too much power and the power will rise too fast. The internal and external temperature gradient of the product is too large during the power-on process, and the thermal stress generated greatly exceeds the resistance of the product, resulting in cracks. Especially when the furnace temperature is 1300-1800 degrees, to strictly control the furnace temperature heating stage, the physical structure and chemical composition of the product begin to change greatly at this stage, and the graphitization of amorphous carbon has not yet started. The main reason is that the hydrogen, oxygen, nitrogen, sulfur, and other elements combined in the amorphous carbon microcrystalline structure are continuously released. The result of the escape is that the impurity elements at the edge of the amorphous carbon microcrystalline structure are continuously reduced, and some lattice defects still exist. , while promoting the relative concentration of thermal stress, it is easy to cause cracks in the product.
resistance of resistive material
The resistance of the graphitized furnace core is formed by the resistance of the product and the resistance of the resistance material in series. When the graphitization furnace is energized, the resistance of the resistance material accounts for about 99% of the resistance of the furnace core. About 97%, it can be seen that in the whole graphitization process, the current mainly heats the product through the heat generated by the resistance material. The heat is much greater than the product itself, resulting in an excessive temperature difference between the inside and outside of the product, which will cause excessive thermal stress, causing cracks and scrapping of the product.
2. The quality of graphitization operation is not high
The furnace is not high quality
The loading operation of the graphitization furnace does not meet the technical standard requirements. When the furnace is installed, the arrangement of the furnace core products is not neat, the spacing between product groups is inconsistent, the filling of the resistance material is uneven, and even the phenomenon of "expansion" of the resistance material will appear in the graphitization furnace. During the power transmission process, the current distribution of the furnace core is very uneven, resulting in very uneven heating and temperature rise speed of the product. The temperature difference inside the product is too large, and the thermal stress generated will cause cracks and waste products in the product.
Resistor material quality varies
When the graphitization furnace uses mixed coke as the resistive material, since the resistivity of metallurgical coke is 5-8 times larger than that of graphitized coke, if the metallurgical coke and graphitized coke are not mixed uniformly, the resistance distribution of the furnace core is very uneven, resulting in energized Finally, the heating rate of each part of the furnace core is very uneven, the temperature difference between the upper and lower sides of the product and the surrounding area is too large, and the thermal stress also increases accordingly, which is easy to cause a large amount of pyrolysis waste products.
Graphitization Furnace Core Bias Current
According to Acheson's graphitization furnace electrothermal law, the temperature distribution in the graphitization furnace core is not only related to the resistance of the furnace core but also closely related to the magnitude of the current passing through the furnace core. When the furnace core of the Acheson graphitization furnace is deflected due to various reasons, the current passing through the furnace core is very different, and the temperature distribution of the furnace core is also very different. When the current distribution of the furnace core has a large difference, the part with a large current generates more heat, and the product in this area heats up quickly, while the part with a small current generates less heat, and the temperature of the product in this area rises slowly, so the temperature distribution of the furnace core varies greatly, so the product The internal temperature difference is also large, and the thermal stress generated is correspondingly increased, resulting in cracks and waste products.
3. The quality of fried products
Internal Cracks in Baked Goods
It is understood that the temperature ranges of 350-500 degrees and 700 degrees and above are the most dangerous, and carbon materials may be damaged during the roasting process. When the outer surface temperature of the product is 800 degrees and the maximum radial temperature difference is 10.7 degrees, the area with a radius of 50-65 mm determines the strength of the material, and the area with a radius of 50-65 mm forms a dangerous tensile stress zone. The center of the blank is 65 mm. When the temperature reaches 700 degrees or higher, the stress in this area is far greater than the limit of the material's fracture strength, which is why the product produces longitudinal flat cracks, which generally do not extend to the outer surface of the product, that is, cracks appear inside the product.
product homogeneity
The uniformity of density distribution of carbon-graphite products during graphitization heat treatment, the uniformity of radial density, and the axial density distribution of products are closely related to the quality of products. Where the density distribution of the product is uneven, during the graphitization heat treatment process, due to the effect of thermal stress, the product is prone to internal stress, and the corresponding internal stress distribution of the product is also uneven. This uneven internal stress is likely to cause cracks in the product. As a result, cracked waste products appear during graphitization.
High product density
The bulk density of carbon-graphite products mainly varies with the production of raw materials and process conditions. As the bulk density increases, the flexural strength, elastic modulus and thermal conductivity of the product also increase. When the bulk density is high, the elastic modulus of the product increases and the brittleness increases, resulting in poor thermal shock resistance of the product. During the graphitization heat treatment process, the thermal stress generated by high temperature greatly exceeds the stress that the product itself can bear, and the internal and external stresses are very different, resulting in cracks and waste products.
Unstable production in the previous process
Since graphitization is the last heat treatment process in the production of carbon-graphite products, it is also the heat treatment process with the highest temperature. It is generally believed that when the current production process is unstable or the quality fluctuates, it will be exposed intensively during the graphitization process. If the temperature of calcined material is low, the softening point of asphalt is unqualified, the roasting temperature is low, and the weight gain rate of impregnation is unqualified, etc., the product will cause secondary shrinkage or uneven shrinkage graphitization during high-temperature treatment, and it is easy to produce pyrolysis waste products.
Flatulence
The product will undergo a certain degree of irreversible volume expansion during the graphitization process. The main reason is that this product is caused by the rapid escape of sulfur concentration during graphitization. The degree of this irreversible expansion increases with the increase of sulfur content, and this irreversible expansion behavior is called the "swelling phenomenon".
As we all know, the content of non-carbon elements such as hydrogen, oxygen, and nitrogen in petroleum coke calcined at 1350°C is generally less than 0.1%; when the temperature is above 1400°C, the bonds begin to break, and sulfur and sulfur-carbon compounds are formed; at higher temperatures, mainly at 1500°C At -1800 degrees, the generated sulfur and sulfur-carbon compounds are released from the product in the form of gas. When the sulfur content reaches a certain level, it often leads to cracks in the product during graphitization.
4. Prevent graphitization cracks in products
a.The graphitization process should be reasonable
The choice of furnace loading method
In the production process of the Acheson graphitization furnace, a reasonable furnace loading method is the guarantee of product graphitization. Whether the product is installed vertically or horizontally, whether the installation is normal or incorrect, should be determined according to the product variety, specification, quality standard, and process parameters of the equipment, to ensure that the product is relatively evenly heated. The furnace core reduces thermal stress and reduces product cracks during graphitization. For large-scale products, the use of the dislocation 1/2D furnace charging method can reduce product cracks, and the graphitization effect is good. For products with high scrapping rates due to graphitization cracks and unstable quality, furnace core flow equalization measures can also be taken.
Determining the Right Electrification System
The graphitization furnace core temperature is controlled by a constant power distribution power curve. Correctly and rationally formulating and using the graphitization furnace power supply system is of great significance for improving the yield, saving energy, and shortening the graphitization cycle of products. significance. The determination of the power supply system of the graphitization furnace should not only consider factors such as furnace body structure, product specifications, quality information, resistance materials, heat preservation effect, and power distribution system parameters, but also the temperature rise rate at different stages.
A reasonable electrification system of a graphitization furnace should be a "fast-slow-fast" three-stage power curve to meet the different requirements of the product in the three stages of the heating process. The furnace core should have a faster heating rate, which not only reduces the heat loss of the graphitization furnace but also does not cause the temperature gradient of the furnace core to be too large, resulting in cracks in the product. For products with unstable graphitization quality, it is necessary to strictly control the heating rate of the graphitization furnace and the furnace core during the heating stage to avoid product cracks caused by excessive heating. At this time, the rising power of the force transmission curve should be adjusted appropriately to form a "fast-slow" The four-stage force transmission curve-slow and fast".
Determining the Appropriate Resistive Material
Acheson graphitization furnace mainly heats the product through the heat generated by the resistance material. The resistance material is closely related to the temperature change of the furnace core. From the perspective of increasing the temperature of the graphitization furnace core, the resistance value of the resistance material is required to be greater. Especially in the late stage of power transmission, the secondary output current of the transformer has reached the maximum value. At this time, the resistance of the furnace core is large, which can maintain high electrical efficiency; but the resistance value of the resistance material is too large, which is unreasonable. Therefore, when determining the resistance material, not only the performance of the equipment must be considered, but also the product specification and power transmission curve, so that the resistance value of the product and the resistance value of the resistance material cannot be too different. For small and medium-sized products, metallurgical Coke can be used as the resistance material. Even with higher initial power and faster-rising power, the product will generally not produce cracks; for large products, mixed coke or graphitized coke can be used as the resistance material. It is more suitable, so that the resistance difference between the product and the resistance material is small, and the temperature difference between the inside and outside of the product is also reduced. Even if the power rises faster, it will not cause cracks in the product.
b. The quality of work must meet the standards
During the graphitization production operation, the furnace loading operation is the key. Since the product loaded into the graphitization furnace is both a heating resistor and a heated object, it is combined with a suitable resistance material to form a furnace core resistance. Core resistance is a necessary condition for the graphitization of products. First of all, the condition of the furnace body of the graphitization furnace, the short circuit of the busbar, and the equipment of the power supply system must be in good condition. The products should be arranged horizontally and vertically in the furnace core, the group distance of the products should be consistent, the resistance material should be filled to avoid the phenomenon of suspension, and ensure the temperature distribution of the furnace core is balanced during use. The power transmission process of the graphitization furnace. Secondly, the ratio of resistance materials must meet the technical standards of the production process, and the quality must be stable and uniform, to avoid uneven temperature distribution of the furnace core during the graphitization process. furnace. Third, the graphitization furnace should transmit power according to a given power transmission curve, and the power fluctuation should be controlled within the normal range to avoid abnormal fluctuations in power transmission power to ensure a balanced rise in the furnace core temperature.
c.Master the quality information of the previous process
It is necessary to grasp the production situation and quality information of the previous process promptly, aiming at the product production stability and quality technical indicators of the previous process, combined with the actual production of this process, formulate a feasible production process of graphitization technical conditions to prevent the product from being damaged during the graphitization process. damaged. Generate pyrolysis waste to ensure stable graphitization quality. When installing the graphitization furnace, the appearance and body quality of the product should be checked one by one. For products that do not meet the technical conditions of the graphitization process, they must not be put into the graphitization furnace for graphitization treatment, and the materials should be returned to the upstream process in time.
d. Add an appropriate amount of anti-flatulence agent to the ingredients
The irreversible expansion and cracking caused by the presence of sulfur during graphitization cannot be eliminated but must be controlled. At present, the more effective method is to control the escape rate of sulfur during the graphitization process of the product. The most practical method is to add an appropriate amount of flattening inhibitors during the batching, generally using 1%-2% powder.
Regarding the mechanism of adding a gas expansion inhibitor, the main reason is that the inhibitor can capture sulfur in the temperature range of product graphitization and gas expansion, form sulfur compounds, and release it in the form of gas in a higher temperature range, thereby expanding the sulfur content. The escape temperature range prevents the product from cracking due to excessive internal stress caused by rapidly escaping gas concentrations. The most commonly used flatulence inhibitor is powder, and its mechanism of action is that at a high temperature above 1000 degrees, the powder is easily reduced to form iron or carbon-iron compounds, and the carbon-iron compounds will be further decomposed into iron and carbon at higher temperatures. The iron formed in this process chemically reacts with the sulfur released by the decomposition of the product and is slowly released in the form of iron sulfide, thereby slowing down the escape rate of sulfur in the product and inhibiting sulfur. The relevant chemical reactions are:
Because the powder not only has a high chemical affinity with the sulfur in the product, but also has a good sulfur suppression effect, is rich in resources and low in price, and will not have any adverse effects on steelmaking during electric furnace processing. In addition, the powder also has a strong catalytic effect on the graphitization process of the product and is an excellent graphitization catalyst. It can be seen that for petroleum coke with high sulfur content, adding an appropriate amount of flattening agent powder can play multiple roles in the production of carbon graphite products.
in conclusion
In short, there are many reasons for the pyrolysis waste produced by carbon graphite products during graphitization heat treatment, and they are also relatively complicated. To prevent carbon-graphite products from producing cracked waste products during the graphitization heat treatment process, various technical improvement measures need to be taken, and the samples should be paid equal attention to. The most important thing is that the quality of the product body should be high, the heat resistance should be good, and homogeneous production should be achieved. The quality and technical indicators of the previous process should meet the technical standards of the production process, and the quality fluctuation should be controlled within the normal range. In addition, during the graphitization heat treatment process, the temperature rise rate of the products in the Acheson graphitization furnace core should be strictly controlled to avoid an excessively fast temperature rise of the furnace core and large temperature differences inside the furnace core. The product increases, resulting in a corresponding increase in thermal stress. Large, resulting in cracked waste.