Introduction to the needle coke process

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Introduction to needle coke process: Needle coke is a high-quality carbon material that emerged as carbon materials saw significant development in the 1970s. It is primarily used in the production of high-power (HP) and ultra-high-power (UHP) graphite electrodes. Needle coke is also an ideal material for use as a negative electrode in lithium-ion batteries. With the rapid development of electric arc furnace steelmaking technology and the increasing demand for lithium-ion anode materials, the requirements for needle coke are growing ever greater. The development and production of needle coke are continuously adapting to meet these evolving demands. Depending on the raw material route, needle coke is divided into oil-based and coal-based types, and the production methods for these two types of needle coke also differ to some extent. 1. Production of needle coke: Needle coke is prepared using liquid-phase carbonization technology, during which the carbonization feedstock gradually undergoes pyrolysis and polycondensation to form mesophase spherules. Intermediate-phase spherules, after sufficient growth, merging, orientation, and final solidification into fibrous carbon structures, yield needle coke. Whether it is oil-based needle coke or coal-based needle coke, the production process consists of three stages: raw material pretreatment, delayed coking, and calcination. 1.1 Raw material pretreatment (1) Purification of coal tar pitch (pretreatment of coal-based acicular coke raw materials) The raw material for coal-based acicular coke is coal tar pitch. However, coal tar pitch cannot be used directly as a raw material for producing needle coke. The pretreatment of coal-based needle coke raw materials mainly involves removing the native quinoline impurities (QI) from the coal tar pitch, a process commonly referred to as coal tar pitch purification. The main purification methods for coal tar pitch include filtration, centrifugal separation, solvent methods (which are further divided into solvent-sedimentation, solvent-centrifugation, solvent-filtration, solvent-flocculation, and solvent extraction), and vacuum distillation. (2) Selection and pretreatment of oil-based needle coke raw materials The raw material for oil-based needle coke is heavy oil from petroleum refineries, whose main components are aliphatic hydrocarbons with very little aromatic content. Due to long-term storage underground and the use of various catalysts during processing, heavy oil contains certain impurity elements, all of which are detrimental to the production of needle coke. Therefore, it is necessary to select and treat the raw materials used for needle coke production. First, select heavy oil with a high content of aromatic hydrocarbons and low sulfur content, and then remove impurities from it (such as catalyst powder). Furthermore, the gums and asphaltenes produced during the petroleum refining process are also present in heavy oils, which is detrimental to the production of needle coke. 1.2 Delayed coking: Delayed coking is a process that combines continuous and batch operations to produce gas, various grades of oil, and coke. The raw material is heated in a heating furnace, then fed into a coking tower (a batch process), where it vaporizes and cracks while being kept at a constant temperature inside the tower. Therefore, this process is a type of thermal cracking of heavy oil. The different types of coke produced include fuel coke, sponge coke, granular coke, and needle coke. The coking process (delayed coking) for oil-based needle coke and coal-based needle coke follows similar procedures, but there are significant differences in the selection of process parameters. In the distillation system, the resulting gas and liquid products are also different. The process flow for petroleum delayed coking (for producing conventional petroleum coke) is shown in Figure 1. 1.3 Calcination: The calcination of needle coke is usually carried out in a rotary kiln. Green coke enters from one end of the kiln; as it moves to the other end, it comes into contact with the exhaust gases from high-temperature calcination. At the exit end, there are gas or fuel burners, and the temperature in the calcination zone can reach up to 1500°C. The rotation speed of the kiln body has a decisive impact on the residence time of coke inside the kiln and its heating rate. The quality of calcination is indicated by the specific gravity of the needle coke after calcination; a specific gravity greater than 2.130–2.136 g/cm3 is considered good.