Calcined blank graphitization
Graphitization of calcined carbon products.
After high temperature heat treatment at 2000 ~ 3000℃, the crystallite size of carbon grid is increased, the spacing between crystal layers is decreased, and the lattice constant is close to that of natural graphite, thus obtaining the physical and chemical properties required by graphite electrode. Graphitization of fired blanks is a key process in the production of graphite electrodes. The physical and chemical properties of graphite electrode depend largely on graphitization temperature. The temperature for producing high-quality graphite electrode should reach above 2800℃. Economically speaking, it takes 5000kW to produce 1t graphite electrode. H, the cost of electricity accounts for about 25% ~ 35% of the production cost of graphite electrode, and about 80% of electricity is consumed in graphitization furnace. Therefore, how to reduce power consumption on the premise of ensuring product quality is also an important task of graphitization process.
Types of graphitization furnaces At present, graphitization furnaces widely used in graphite electrode factories are all electric heaters, that is, a large amount of electric energy is used to heat the roasted products to the high temperature required for graphitization. Graphitization furnace can be divided into direct heating furnace and indirect heating furnace according to the heating mode, and can be divided into mobile graphitization furnace and fixed graphitization furnace according to the relative position of graphitization furnace and power supply device. According to the operation mode, it can be divided into batch production and continuous production, and according to the nature of electricity consumption, it can be divided into DC graphitization and AC graphitization. The direct heating furnace refers to the product after the current is directly graphitized and roasted. At this time, the roasted product loaded into the furnace is not only a conductor that generates high temperature by current, but also an object that is heated to high temperature. Direct heating graphitization furnace is mainly used to produce graphite electrodes. There are two kinds of direct heating graphitization furnaces, one is called Acheson graphitization furnace, and the other is called longitudinal graphitization furnace (LWG furnace for short). Acheson graphitization furnace has a long history and has been widely used in carbon plants. However, as an energy-saving furnace, the longitudinal graphitization furnace has been widely used in carbon plants producing large electrodes in the 1980s. With the continuous improvement of graphite electrode quality and the increase of product cross section, longitudinal graphitization furnace is constantly used. There are many patent reports on indirect heating graphitization furnace, but there are few practical applications. In the early 1970s, British Morgan Company successfully developed an indirect heating graphitization furnace which can be continuously produced, and only small-scale electric carbon products can be produced. Because DC power supply equipment is conducive to maintaining the three-phase balance and high power factor of the power grid, most graphitization furnaces in China Carbon Plant have changed AC power supply to DC power supply in 1980s, and the investment in DC power supply is large. In addition to the transformer, a rectifier with corresponding capacity is needed to rectify alternating current into DC electricity, and then input it into the graphitization furnace. Here are five different types of graphitization furnaces.
Acheson graphitization furnace is an improved furnace type based on the resistance furnace for producing silicon carbide at the end of 19. Its main feature is that the baked product and the resistive material (coke particles) in the furnace form furnace resistance, and the high temperature of 2000 ~ 3000℃ is generated after electrifying to graphitize the baked product. The structure of Acheson graphitization furnace is relatively simple: the graphitization furnace is built on the basis of reinforced concrete. It consists of furnace head masonry and furnace tail masonry with the same structure and a rectangular furnace body between them. The furnace head and the furnace tail are built with refractory bricks on the outside, carbon blocks on the inside and down, and masonry ink blocks in the middle. Several conductive electrodes (graphite electrodes) pass through the furnace head and furnace tail masonry, and the diameter and number of conductive electrodes depend on the current intensity when the graphitization furnace is electrified. (Figure 1) The temperature of the conductive electrode continues to rise when power is applied, so it is necessary to cool the conductive electrode. There are two cooling methods: direct water spraying on the surface or indirect cooling. Indirect cooling can be divided into two ways: drilling holes in the electrode to cool the electrode and installing cooling jackets on the electrode surface. Build several layers of refractory bricks at the bottom of the rectangular furnace body with roasted products, build furnace walls on both sides of the furnace body, or vertically place wallboard made of heat-resistant concrete on both sides of the furnace body. The small Acheson graphitization furnace is 10 ~ 12m long and about 2rn wide, and can load about 20t of roasted products at a time. The length of the medium-sized Acheson graphitization furnace is 14- 16m, and the furnace width is about 3m. It can load 40 ~ 60 tons of baked products at a time. The length of the large Acheson graphitization furnace is18 ~ 20in, the furnace width is about 4.0m, and it can load about l00t of roasted products at a time. An ultra-long Acheson furnace with a length of 25 ~ 30m has a narrow shaft and a furnace capacity of only about 100t. Acheson graphitization furnace is simple in structure and easy to maintain, and it is convenient to load and unload semi-finished products by crane. However, it has the following disadvantages: (1) long power-on time, large heat loss and low energy utilization rate. When heating with electricity, a lot of electric energy is used to heat the resistive materials, the furnace head and the furnace tail masonry, and the heat loss of the external insulation materials to the surrounding space is also a lot. Therefore, only about 35% of electricity is used for heating and baking. (2) The space filled with roasted products and resistive materials in graphitization furnace is called furnace core, and the temperature of Acheson graphitization furnace core changes greatly. The temperature difference between the central part of the furnace core and the insulation materials on both sides of the furnace core can reach several hundred degrees, and the temperature difference between the upper part and the lower part of the furnace core can also reach several hundred degrees, so the resistivity of the same furnace product is different to some extent, and the uneven heating temperature in the furnace is also the main reason for the cracking of some products. (3) There is a lot of dust when loading and unloading the furnace, and a lot of harmful gases are emitted when heating by electricity, which pollutes the environment; (4) Long production cycle. The production cycle of graphitization furnace is as long as L2 14 days, from cleaning to charging, heating, cooling and discharging, in which heating only takes 2 ~ 3 days. Although there are 6 ~ 8 graphitization furnaces in each furnace group, each graphitization furnace can only rotate 1 month. (5) A large number of metallurgical coke particles must be used as resistance materials, and the consumption per ton of baked products is about 300k9.
Acheson graphitization furnace, a variant of U-shaped graphitization furnace, is beneficial to a long furnace body in order to improve the thermal efficiency and save electric energy within the allowable range of output voltage and current of power supply transformer. However, the length of the furnace body is sometimes limited by the span of the workshop, so a longitudinal Acheson graphitization furnace is designed, which is bent 180 from the middle. It becomes a new furnace type with the same side of the furnace head and the furnace tail (Figure 2). Only a partition wall is built in the middle of the furnace body to divide the furnace body into two parallel charging areas, and graphite blocks are laid on the inner side of the furnace wall opposite to the masonry of the furnace head and the furnace tail, so that the current is led from one charging area to the other through the graphite block masonry. The advantage of this U-shaped graphitization furnace is that the length of the furnace body is relatively increased, and more roasted products can be loaded, and the power transmission buses are all at one end of the furnace. The movable buses on both sides of the furnace are omitted, and the power consumption per ton of semi-finished products is slightly lower than that of the general Acheson graphitization furnace, but there is a disadvantage that the current distribution in the furnace is uneven (resulting in uneven temperature in the furnace), so the graphite electrodes installed in different parts have different qualities, and the middle partition wall is easy to burn out. The production process of this graphitization furnace is the same as that of Acheson furnace.
The mobile graphitization furnace graphitization furnace is not built on the basis of reinforced concrete, but on the rail car with steel structure. The rail car can move left and right along the track laid on the ground (Figure 3), but the position of the power supply device is fixed. Acheson graphitization furnace or longitudinal graphitization furnace can be designed as a movable furnace, and 6-8 graphitization furnaces form a furnace group, so that the power supply transformer can supply power to each graphitization furnace in turn. According to the invention, products can be loaded, unloaded and electrified in two workshops respectively, so that the length of the power transmission bus connecting the graphitization furnace is greatly shortened. Corresponding to the mobile graphitization furnace, the mobile transformer is combined with the graphitization furnace built in a fixed position, that is, the power supply transformer is installed on the rail car. When any graphitization furnace needs to be electrified, the rail car with transformer moves to the furnace head and is electrified after being connected with the transmission bus.
See "Series Graphitization" for the characteristics and production process of longitudinal graphitization furnace.
Indirect heating continuous production graphitization furnace is a horizontal double-channel continuous production graphitization furnace specially used for producing small electric carbon products (Figure 4). This double-channel continuous production graphitization furnace has a fixed high temperature zone, which is made of graphite plates. The total length of the channel is 7m, of which 5m is buried in carbon black insulating material, and both ends are respectively set.
Outside the thermal insulation material, the carbon black thermal insulation material is a shell welded with double steel plates, which can be cooled by water. The power supply equipment is an l40kV? The output voltage of A's single-phase AC transformer is l2v, and a pair of conductive clamps are clamped in the fixed position of graphite double channels. The continuous production graphitization furnace adopts bidirectional countercurrent feeding, and the mechanical propulsion device propels 25 tons of nitrogen per minute, and the total time of passing through the graphite channel is about 4.5 hours. The diameter of the products produced is less than 50 microns, the length is less than 65438±0 ITL, and the temperature in the high temperature zone in the channel can reach 2500℃, with an hourly output. H, because the channel can maintain a small amount of positive pressure by itself, there is no need to introduce inert gas for protection.
The production operation of each graphitization furnace in the graphitization furnace group consists of six contents, such as furnace cleaning, minor repair, furnace loading, power on, cooling and furnace unloading, which are carried out circularly. The production cycle of Acheson graphitization furnace is generally 12- 14 days, in which it only takes 2-3 days to be electrified. In order to make full use of the capacity of power supply transformers, each set of power supply equipment can be configured with 6-8 sets. Only at the end of a furnace power-on, sometimes the power bus contacts are switched left and right to check the power failure time of power supply equipment. A set of power supply equipment and 6-8 graphitization furnaces form a graphitization furnace group, and one graphitization furnace in the graphitization furnace group is always energized, while the other furnaces are in the process of charging, cooling, discharging, cleaning and minor repair respectively. The production of each furnace group is carried out according to the pre-arranged operation plan.
The linear size of the graphitized expansion-contraction roasting product is in an expansion state from the time of power on to about 1200℃, and then continues to rise to about l600℃ after entering the raw meal calcination temperature (1200 ~ 1450℃), and then shrinks, and the temperature further rises to1600 ~ 2000. Therefore, during this period, the linear expansion degree of baked semi-finished products is related to the content of sulfur and nitrogen in raw materials and the heating speed. When the temperature rises above 2000℃, the linear dimension of semi-finished products changes from expansion to contraction, and the contraction is greater than expansion in the whole graphitization process, so the diameter and length of calcined products decrease after graphitization, and the final shrinkage is affected by many factors. Such as raw material properties, particle size composition, binder dosage, kneading molding method, pressure, final temperature of roasting and graphitization, etc., the shrinkage of common power graphite electrode produced with 75% petroleum coke and 25% pitch coke was determined.