Current Development of Pinacolous Coke

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3.1 Acicular coke: Acicular coke is a high-quality carbon material that saw significant development in the 1970s within the field of carbon materials. It is primarily used in the production of high-power (HP) and ultra-high-power (UHP) graphite electrodes. Acicular 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. 3.1.1 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. Mesophase 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. 3.1.1.1 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 method, solvent-centrifugation method, solvent-filtration method, solvent-flocculation method, and solvent-extraction method), 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 low levels of aromatic hydrocarbons. 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). In addition, the gums and asphaltenes generated during the petroleum processing process are also present in heavy oils, which is detrimental to the production of needle coke. 3.1.1.2 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 petroleum-based needle coke and coal-based needle coke follows similar flowcharts, 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 the production of conventional petroleum coke) is shown in Figure 1 3.1.1.3 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 hot exhaust gases from the calcination process. 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. 3.1.2 Introduction to the application technologies of domestic needle coke manufacturers: The production methods of needle coke mainly differ in raw material pretreatment, while the principles of coking and calcination remain the same. China has carried out extensive research and development in the pretreatment of needle coke raw materials; the main methods and results are shown in the table below. The effects of pretreatment (and selection) of needle coke raw materials are presented in Table 1. Table 1: Items, Raw Material, QI Content/%, Product Yield/%, Purification Method, Remarks – Before Purification, After Purification. Shanxi Coal Chemical Research Institute: Coal tar, 6.72, 0.74, 66.7%, Filtration method, Laboratory. Anhui University of Technology: Coal tar, 2.16, 0.2, 60–65%, Centrifugation method, Laboratory. Wuhan University of Science and Technology: Coal tar, Coal pitch, 0.4–0.6, 67–85%, Solvent flocculation method, Laboratory. Anhui University of Technology: Coal pitch, 2.16, 0.1, Solvent centrifugation method, Laboratory. Wuxi Coking Plant: Coal pitch, 10.73, 0.1, Solvent filtration method, Laboratory. Poland: Coal tar, 67%, Solvent extraction method, Pilot scale. Anshan Coastal Chemical Industry: Coal pitch, 4–5%, Trace amounts, Modification method, Industrial demonstration plant. Shandong Yankuang Kelan: Coal pitch, 5.0, 0.5, 65–70%, Solvent sedimentation method, Industrial demonstration plant. Shanxi Hongte 1: Coal pitch, 4–5%, Trace amounts, Modification method, Industrial production. Shanxi Hongte 2: Coal pitch, 4–5%, Trace amounts, 65–70%, Solvent sedimentation method, Industrial production. China Steel (Anshan): Coal pitch, 4–5%, Trace amounts, Solvent sedimentation method, Industrial production. Jinzhou Petrochemical: Petroleum-based, -----, -----, ------. Selection of raw materials, Industrial production, Shandong Yida: Petroleum-based, Solvent extraction, Industrial scale. Although many institutions in China are engaged in research on needle coke, to date, only four companies have achieved industrial production: Shanxi Hongte Chemical Co., Ltd., China Steel Group (Anshan), Baosteel, and Jinzhou Petrochemical. Shanxi Hongte Chemical Co., Ltd., Baosteel, and Sinosteel Group (Anshan) produce coal-based needle coke, while Jinzhou Petrochemical produces oil-based needle coke. Shandong Yida New Materials Co., Ltd. has collaborated with the Petroleum University to adopt a solvent extraction process for raw material pretreatment, and the facility is operating normally. 3.1.3. Analysis of the main bottlenecks faced by domestic needle coke manufacturers. Although industrial production of needle coke is already possible in China, the process technology still needs further improvement. The main technical challenges include the following: 3.1.3.1 Raw material pretreatment. The purification of coal-based needle coke raw materials presents difficulties related to methods for removing QI from coal tar, the yield of refined tar, and the stability of the properties of the refined tar ； Furthermore, at present, the pretreatment of coal-based needle coke raw materials both domestically and internationally relies on solvent methods to remove QI. During the production process, the components rich in QI have poor fluidity, which poses significant difficulties to the production process. There are also challenges regarding how to utilize this portion as a by-product in the end. The challenges in selecting and purifying oil-based needle coke raw materials lie in the removal of impurities (such as catalysts), the high aromatic content, and the low sulfur content. To diversify the sources of raw materials, China has begun researching the pretreatment of raw materials for oil-based needle coke in recent years; further work is needed to remove sulfur, resins, and asphaltenes from heavy petroleum fractions. 3.1.3.2 Delayed coking: Problems in the delayed coking process: (1) Determination of operating conditions: Determining the process parameters such as temperature, pressure, and gas flow rate for operating the coking process. (2) The optimization of process parameters is a key issue in delayed coking. There is a certain relationship between these process parameters and the properties of the raw material (refined pitch). Under the condition of maintaining stable raw material quality, determining how to optimize the process parameters for delayed coking is the main challenge when aiming to produce high-quality needle coke. (3) Uniformity issue of the quality of needle coke in each tower: During the feeding process in each coking tower, factors such as temperature, pressure, and gas velocity in the empty tower change, which results in uneven quality of the coke at the upper, middle, and lower levels of the tower after coking occurs. How to effectively address the issue of uniformity in the quality of needle coke is also one of the considerations in needle coke production. 3.1.3.3 Calcination (1) Calcination temperature: All imported needle coke is calcined coke; carbon enterprises in China only calcine petroleum coke (raw coke), with the calcination temperature ranging from 1250°C to 1350°C (using rotary kilns). The calcination temperature for needle coke needs to be above 1500°C, which is 150°C higher than that of petroleum coke; whether the refractory materials in the rotary kiln can withstand such high temperatures is an issue that must be taken into consideration during the calcination process. (2) Uniformity issue of the raw coke quality: The volatiles, moisture, and particle size distribution of the raw coke (the material used in calcination) affect the uniformity of the quality of the final needle coke. In addition to optimizing delayed coking operations, these issues require treating the raw coke before calcination (such as dehydration and homogenization).