At present, the large-scale production of needle coke in the world mainly includes the United States, Japan and other countries. The United States only produces oil-type needle coke, while Japan can produce both coal-type and oil-type needle coke. Needle coke is mainly used to produce HP and UHP graphite electrodes.
Needle coke production
Needle coke is produced by liquid phase carbonization, in which the coking raw materials are gradually formed into mesophase spherules through pyrolysis and condensation. The mesophase spherules were then fully grown, fused and oriented, and finally solidified into carbon products with fibrous structure, namely needle coke. Needle coke of oil or coal measures is produced by pretreatment, delayed coking and calcination of raw materials.
• Pretreatment of coal measure needle coke raw material
The pretreatment of coal measure needle coke is mainly to remove the quinoline incompatibility (QI) in coal pitch, which is generally called coal pitch purification. The composition of coal pitch has certain influence on mesophase formation, such as active component, quinoline insoluble substance, metal element, heterocycle compound and nitrogen, oxygen, sulfur, etc. The main component of coal tar pitch is aromatic hydrocarbon, but it contains a certain amount of QI, which includes not only amorphous carbon generated by the thermal polymerization of some polymer resin substances during coal tar distillation, but also coal powder and coke powder brought out with gas from coke oven chamber. They attach themselves to the mesophase and obstruct the growth and fusion of spherical crystals. The needle coke structure with good fiber structure can not be obtained after coking. Therefore, the coal asphalt raw material needs to be pretreated. The purpose of the pretreatment is to remove the original quinoline insoluble substance (QI) which hinders the growth of the spherules, and then to obtain the raw materials to meet the needs of needle-coke production, which is an important condition for the production of needle-coke with coal pitch.
The coal bitumen purification method mainly has the filtration method, the centrifugation method, the solvent method (the solvent method also divides into the solvent one settlement method, the solvent one centrifugation method, the solvent one filtration method, the solvent one flocculation method, the solvent one extraction method), the vacuum distillation method.
• Selection and pretreatment of raw materials for needle coke of oil series
Oil system of needle coke is the heavy oil, oil processing plant was mostly fat hydrocarbon, aromatic hydrocarbon content is very few, because of the long-term storage in the ground and in the process of processing using various catalysts, makes certain impurity elements contained in the oil, heavy oil, which are adverse to the needle coke production, so it must be the needle coke raw material choice and processing. The heavy oil with high aromatic hydrocarbon content and low sulfur content is selected first, and then impurities (such as catalyst powder) are removed. In addition, the gelatinous and asphaltene produced in petroleum processing are also among the heavy oils, which are unfavorable for needle coke production.
The relevant data in the United States is introduced, usually used for the production of coke (needle coke) raw materials are heavy oil, such as distillation of crude oil residue oil, lubricating oil extract, hydrodesulfurization oil extract and from petroleum distillation or cracking residue or hydrodesulfurization products. The best raw material is the so-called pyrolysis fuel oil or black oil, whose boiling point is higher than that of pyrolysis gasoline, that is, the residual heavy black oil whose boiling point is higher than 218℃, which is the raw material of pyrolysis liquid hydrocarbons, catalytic pyrolysis decanting oil, pyrolysis tar, lubricating oil extraction and hydrodesulfurization products. In general, these materials are low in sulfur, that is, less than 1.5% (quality), preferably less than 0.8%. Mixtures of these materials may also be used.
• Delayed coking
Delayed coking is a process that combines continuous and intermittent processes to produce gas, various refined oil products and coke. The raw material is heated in a heating furnace and then fed into a coking tower (an intermittent process), where it is vaporized and cracked while being kept warm. Thus, this process is a thermal cracking process of heavy oil. The coke produced consists of fuel coke, spongy coke, granular coke and needle coke.
The coking process (delayed coking) of oil series needle coke is similar to that of coal series needle coke, and the main difference is similar in the fractionation system (delayed coking) process flow, and the main difference is also different in the fractionation system, resulting in different gas and liquid products.
The delayed coking process of petroleum heavy oil includes distillate tower, heating furnace and coking tower, etc. The process is a continuous process in the distillation tower and heating furnace, but is a batch process in the coking tower, therefore, two coking towers corresponding to a fractionating tower and a heating furnace is necessary. Generally speaking, raw materials enter the bottom of the fration tower, blend with circulating oil, and stabilize the temperature at the bottom of the tower. Then the materials at the bottom of the fration tower are pumped into the coking furnace, heated to the coking condition rapidly, and cracked and vaporized within less than 3 minutes, and then enter the coking tower.
In the coking tower, the coke is cracked, vaporized, and condensed until the coke is formed. The liquid is vaporized and returns from the top of the coking tower to the fraction column. The focal points are continuously formed and piled into the tower until the desired amount is reached and then switched to the second tower continuously. After cooling the coking tower after stopping feeding, hydraulic coke cutting is carried out with high pressure water (17~23MPa), and preparations are made for the next cycle. The coking period is usually 24 hours, but sometimes it is adjusted to 14 to 20 hours to increase the handling capacity.
The pyrolysis gas from the coking tower goes into the fractionating tower and separates different liquids, including gasoline, kerosene, diesel and wax oil. Part of the product is used for processing power fuel oil or as chemical raw materials. Delayed coking of needle coke and common focus of delayed coking process flow and equipment are basically the same, just on the individual equipment, carbonization conditions and operation, some necessary measures were taken such as to control the rate of feed temperature, coking tower pressure, steam injection volume and adjust the recycle ratio, oil in coking tower to maintain a relatively stable state, make full use of the plastic flow of mesophase material and molecular arrangement orderliness, and gets the product produce shear force, creating a so-called "airflow jiao", and ultimately form a flow line structure of needle coke. In order to ensure the production process of needle coke is stable, individual equipment must be improved. The process parameters, operation cycle and coke yield are very different.
There are some differences between coal-measure needle-coke and oil-measure needle-coke in process parameters, operation cycle and coke yield, for example, the coke yield of coal-measure needle-coke is much higher than oil-measure needle-coke.
Needle coke produced by delayed coking contains high moisture and volatile components, which is called raw coke. It needs to be calcined at high temperature in isolation of air to be used as raw material for high power and ultra-high power graphite electrode. During the calcination process, the structure and element composition of needle coke undergo a series of changes, thus improving the physical and chemical properties of needle coke. The purpose of calcining is to remove water and volatilization from coke and to improve the carbon content, density, strength, electrical conductivity and chemical stability of coke.
Needle coke calcination is usually carried out in a rotary kiln. The raw coke enters from one end of the kiln and comes into contact with the waste gas of high temperature calcination. There is a gas or oil burner at the outlet. The temperature of the calcination belt is up to 1500℃. The rotation speed of the kiln has a decisive influence on the residence time and heating speed of coke in the kiln. To evaluate the effect of calcining, the true density of the needle coke after calcining is an important index, and the needle coke with a true density greater than 2.13g/m3 is of good quality.