Reactivity of coke and its strength after reaction

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The reactivity of coke and its strength after reaction: Coke reactivity refers to its ability to undergo chemical reactions with substances such as carbon dioxide, oxygen, and water vapor. The strength of coke after reaction denotes its capacity to resist fragmentation and wear under mechanical and thermal stresses. In blast furnace ironmaking, foundry ironmaking, and fixed-bed gasification, coke undergoes chemical reactions with carbon dioxide, oxygen, and water vapor. Since the reaction of coke with oxygen and water vapor follows patterns similar to those of its reaction with carbon dioxide, most cases **use the reaction properties of coke with carbon dioxide to assess its reactivity. The standard (GB/T 4000–1996) specifies the test methods for coke reactivity and strength after reaction. The method involves reacting coke with carbon dioxide at high temperatures, and then measuring the weight loss of the coke after the reaction as well as its mechanical strength; these values represent the reactivity of the coke, denoted as CRI, and its strength after reaction, denoted as CSK. The test results for coke reactivity and strength after reaction are both the arithmetic averages of the results from parallel tests. 6. Quality indicators of coke: Coke is a solid product obtained through high-temperature carbonization; its main component is carbon, and it has a cracked and irregular pore structure (or a porous structure with pores). The number of cracks directly affects the strength and crush resistance of coke, with this property generally being measured by the crack density (referring to the length of cracks per unit volume of coke). The indicator for measuring the pore structure is primarily the porosity (the percentage of the coke’s pore volume relative to its total volume), which affects the reactivity and strength of the coke. Coke intended for different purposes has varying requirements regarding the porosity index; generally, metallurgical coke requires a porosity of 40%–45%, casting coke requires 35%–40%, while coke intended for export needs a porosity of around 30%. The degree of cracking and porosity in coke are directly related to the type of coal used in coking; for example, coke produced from bituminous coal tends to have many cracks, a high porosity level, and low strength ； Coke produced from coking coal as the base coal has fewer cracks, a lower porosity, and higher strength. Coke strength is usually expressed by two indicators: crush resistance and wear resistance. The crush resistance of coke refers to its ability to withstand external impacts without breaking along the cracks or defects in its structure, and it is expressed by the M40 value ； The wear resistance of coke refers to its ability to resist external friction forces without the formation of surface glass debris or powder, and it is expressed by the M10 value. The crackness of coke affects its crushing strength value M40, while the pore structure of coke influences its wear resistance strength value M10. There are many methods for determining the M40 and M10 values; in China, the German Migon drum test method is commonly used. 7. Evaluation of coke quality (1) Sulfur content in coke: Sulfur is one of the harmful impurities in the production of pig iron, as it reduces the quality of pig iron. In steel-making pig iron, a sulfur content of more than 0.07% renders it scrap. Of the sulfur brought into the furnace from the blast furnace charge, 11% comes from the ore, 3.5% from limestone, and 82.5% from coke; therefore, coke is the main source of sulfur in the charge. The sulfur content in coke has a direct impact on blast furnace iron production. When the sulfur content in coke is greater than 1.6%, for every 0.1% increase in sulfur content, the amount of coke used increases by 1.8%, the amount of limestone added increases by 3.7%, and the amount of ore added increases by 0.3%; meanwhile, the blast furnace production decreases by 1.5% to 2.0%. The sulfur content in metallurgical coke is specified to be no more than 1%, while that in metallurgical coke used in large and medium-sized blast furnaces is less than 0.4%–0.7%. (2) Phosphorus content in coke: The phosphorus content in metallurgical coke used for iron production should be below 0.02%–0.03%. (3) Ash content in coke: The ash content in coke has a very significant impact on blast furnace smelting. An increase of 1% in coke ash content leads to a 2%–2.5% increase in the amount of coke required. Therefore, it is very necessary to reduce coke ash content. (4) Volatiles in coke: The maturity of coke can be determined based on its volatile content; if the volatility is greater than 1.5%, it indicates that the coke is unripe ； A volatil content of less than 0.5%–0.7% indicates over-cooking; generally, the volatil content of mature metallurgical coke is around 1%. (5) Moisture in coke: Fluctuations in moisture can lead to inaccurate measurement of coke, thereby causing fluctuations in furnace operation. Furthermore, an increase in coke moisture leads to higher M40 values and lower M10 values, causing errors in the drum test results. (6) Screening composition of coke: The particle size of coke is also very important in blast furnace smelting. In the past, China’s requirements regarding coke particle size were as follows: for large blast furnaces (1300–2000 m³), the coke particle size had to be greater than 40 mm ； The coke particle size in medium and small blast furnaces is larger than 25 mm ； However, tests conducted by some steel mills at present show that a coke particle size of 40–25 mm is optimal. Coke larger than 80 mm must be kept in whole pieces to maintain a relatively constant particle size range. In this way, the coke has uniform size, large pores, low resistance, and the furnace operates well. In recent years, with the enlargement of blast furnaces, new requirements have been placed on the quality of coke. It is necessary for us to understand the properties of coke, accurately evaluate its quality, guide the operation of blast furnaces, reduce the coke ratio appropriately, lower production costs, and thereby bring economic benefits to steel companies.