Issues regarding the quality of metallurgical coke and its compatibility with blast furnace capacity

A blast furnace is a vertical iron-smelting furnace with a circular cross-section. A steel plate is used as the furnace shell, with a refractory brick lining inside it. The blast furnace itself is divided, from top to bottom, into five sections: the furnace throat, the furnace body, the furnace waist, the furnace belly, and the furnace hearth. Due to the favorable technical and economic indicators of blast furnace iron production, as well as its simplicity, high output volume, high labor productivity, and low energy consumption, iron produced by this method accounts for the vast majority of the world’s total iron production. During blast furnace production, iron ore, coke, and a slag-forming flux (limestone) are fed in from the top of the furnace, while preheated air is blown in through tuyeres located around the perimeter of the furnace at its lower part. At high temperatures, the carbon in coke (and in some blast furnaces, auxiliary fuels such as coal powder, heavy oil, and natural gas are also injected) burns with the oxygen in the blown-in air to produce carbon monoxide and hydrogen; as these gases rise within the furnace, they remove the oxygen from the iron ore, thereby reducing it to obtain iron. The molten iron produced is discharged from the taphole. The unreduced impurities in the iron ore combine with fluxes such as limestone to form slag, which is discharged through the slag outlet. The generated gas is discharged from the top of the furnace; after dust removal, it is used as fuel for hot blast stoves, heating furnaces, coke ovens, boilers, etc. The main product of blast furnace smelting is pig iron, with blast furnace slag and blast furnace gas as by-products. Raw materials for blast furnace smelting The raw materials used in blast furnace smelting mainly consist of iron ore, fuel (coke), and flux (limestone). 　　Typically, 1.5–2.0 tons of iron ore, 0.4–0.6 tons of coke, and 0.2–0.4 tons of flux are required to produce 1 ton of pig iron, for a total of 2–3 tons of raw materials. To ensure the continuity of blast furnace production, an adequate supply of raw materials is required. 　　Therefore, the task of purchasing raw materials is particularly important for both pig iron manufacturers and steel mills. 　　Blast furnace production is continuous. A generation of blast furnaces (one generation spanning from commissioning to shutdown for major repairs) can operate continuously for several years to over a decade. During production, iron ore, coke, and flux are continuously fed in from the top of the furnace (the furnace top generally consists of a material feed system and hoppers; modern blast furnaces use bell-type or bellless furnace tops), while hot air at temperatures of 1000–1300 degrees Celsius is blown in through tuyeres at the bottom of the furnace, along with fuels such as oil, coal, or natural gas. The iron ore fed into blast furnaces is mainly a compound of iron and oxygen. At high temperatures, the carbon in the coke and the injection materials, along with carbon monoxide produced by the combustion of carbon, remove the oxygen from the iron ore to produce iron; this process is called reduction. Iron ore is reduced to pig iron through a reduction reaction, and the molten iron is discharged from the taphole. The gangue in iron ore, the ash in coke and injection materials, combine with fluxes such as limestone added to the furnace to form slag, which is discharged separately through the tap hole and the slag outlet. The gas is extracted from the top of the furnace; after dust removal, it is used as industrial gas. Modernized blast furnaces can also utilize the high pressure at the top of the furnace to generate electricity using some of the extracted gas. 　　Cast iron is a product of blast furnaces (referring to cast iron produced through blast furnace smelting), but the products of blast furnaces are not limited to cast iron; there are also products such as ferromanganese, which belong to the category of ferroalloys. Manganese iron blast furnaces are not included in the calculation of various indicators for iron-making blast furnaces. The blast furnace iron-making process also produces by-products such as water slag, slag wool, and blast furnace gas. 　　Characteristics of blast furnace iron production: It operates on a large scale. Both in other parts of the world and in China, the volume of blast furnaces continues to increase. For example, Baosteel’s blast furnace in China has a capacity of 4,063 cubic meters; it produces over 10,000 tons of iron per day, along with more than 4,000 tons of slag, and consumes over 4,000 tons of coke per day. Smelting process Blast furnace smelting is a continuous production process for reducing iron ore into pig iron. Solid raw materials such as iron ore, coke, and fluxes are fed into the blast furnace in batches via the top charging device according to specified mixing ratios, thereby maintaining a certain level of material at the furnace throat. Coke and ore form an alternating layered structure inside the furnace. As it descends, the ore is gradually reduced and melted into iron and slag, accumulating in the furnace bowl and being discharged periodically through the iron tap and slag tap. 　　The cold air supplied by the blower is heated to 800–1350°C in the hot air furnace, and then enters the furnace chamber continuously and steadily through the tuyeres. The hot air causes the coke in front of the tuyeres to burn, producing highly hot reducing gas at temperatures above 2000°C. The rising stream of hot coal gas heats the iron ore and flux, turning them into a liquid state ； This causes the iron ore to undergo a series of physicochemical changes, while the coal gas stream gradually cools down. It is a process of intense heat, mass, and momentum transfer between the descending material column and the ascending coal gas stream. 　　The capillary moisture in the descending charge evaporates when heated to 100–200°C, while the crystalline water in limonite and certain gangues decomposes and evaporates only at 500–800°C. The main fluxes, limestone and dolomite, as well as other carbonates and sulfates, also decompose upon heating in the furnace. The decomposition temperatures of CaCO3 in limestone and MgCO3 in dolomite are 900–1000°C and 740–900°C, respectively. Iron ore begins to be reduced in a blast furnace at 400°C or slightly lower temperatures. Some iron oxide melts first in the lower high-temperature zone within the slag, and then iron is reduced from the slag. 　　Coke does not melt in the blast furnace; it only burns and vaporizes near the tuyeres, with a small portion of the coke vaporizing into CO while reducing oxides. The ore begins to soften when it is partially reduced and heated to 1000–1100°C ； It melts completely at 1350–1400°C ； It drips at temperatures above 1400°C. During the descent, coke and ore maintained an alternating layered structure. Due to the counterflow heat exchange in the blast furnace, several regions with different temperature distributions are formed. Zone ① is the dry zone where ore and coke are separated in layers; it is known as the lumpy zone, and there is no liquid in this zone ； ②The zone is a softening zone composed of a softening layer and a coke interlayer, where the ore begins to soften and eventually melt completely ； ③The zone is the area where liquid slag and iron droplet; within this zone only coke remains solid ； ④In front of the tuyeres, there is a bag-shaped coke swirling zone where the coke swirls and burns intensely; this area is the main source of heat and gaseous reducing agents within the furnace.

Blast furnace ironmaking is the main method of modern iron production and an important stage in steel manufacturing. This method evolved and was improved from ancient vertical furnace iron smelting. Although many new iron smelting methods have been developed around the world, due to the favorable economic indicators of blast furnace iron smelting, its simple process, high production volume, high labor productivity, and low energy consumption, iron produced by this method still accounts for over 95% of the world’s total iron production. During blast furnace operation, iron ore, coke, and a slag-forming flux (limestone) are fed from the top of the furnace, while preheated air is blown in through tuyeres located around the perimeter of the furnace at its lower part. At high temperatures, the carbon in coke (and in some blast furnaces, auxiliary fuels such as coal powder, heavy oil, and natural gas are also injected) burns with the oxygen in the blown-in air to produce carbon monoxide and hydrogen; as these gases rise within the furnace, they remove the oxygen from the iron ore, thereby reducing it to obtain iron. The molten iron produced is discharged from the taphole. The non-reducible impurities in iron ore combine with fluxes such as limestone to form slag, which is discharged through the slag outlet. The generated gas is led out from the top of the furnace; after dust removal, it is used as fuel for hot blast stoves, heating furnaces, coke ovens, boilers, etc. Brief history and recent developments: Early blast furnaces used charcoal or coal as fuel; in the 18th century, coke was adopted, and in the mid-19th century, cold air was replaced by hot air (see the history of metallurgy). At the beginning of the 20th century, the use of gas internal combustion engine and steam turbine blowers in blast furnaces led to rapid development in blast furnace iron production. At the beginning of the 20th century, large blast furnaces in the United States could produce 450 tons of iron per day, with a coke consumption of around 1,000 kilograms per ton of pig iron. In the early 1970s, Japan built blast furnaces with a height of 4,197 meters, capable of producing over 10,000 tons of iron per day, with a fuel consumption of less than 500 kilograms per ton of pig iron. China began to develop a modern steel industry at the end of the Qing Dynasty. Construction of the Hanyang Iron Works began in 1890, and Blast Furnace No. 1 (248 meters tall, with a daily iron production capacity of 100 tons) came online in May 1894. In 1908, the Hanyeping Company was established, which included the Daye Iron Mine and the Pingxiang Coal Mine. In 1980, the total volume of blast furnaces in China was approximately 80,000 cubic meters, of which 26 had a volume of over 1,000 cubic meters. In 1980, the country produced 38.02 million tons of iron, ranking fourth in the world. By the end of the 1970s, there were over 120 blast furnaces with a height of more than 2,000 meters worldwide, of which Japan accounted for one-third, and China had four such furnaces. There are over 20 blast furnaces in the world with a height of more than 4,000 meters; 15 of them are in Japan, and one is under construction in China. Since the 1950s, China’s steel industry has developed rapidly, and blast furnace ironmaking technology has also seen significant progress. This is evident in the comprehensive use of new technologies aimed at enhancing the smelting process and reducing energy consumption, such as the use of high-quality raw materials, adjustments to the upper and lower sections of the furnace, high-pressure furnace tops, high wind temperatures, oxygen-enriched blast air, and the injection of supplementary fuels (such as coal powder and heavy oil). China is particularly innovative in the use of coal powder injection. In 1980, the average utilization coefficient of blast furnaces in key enterprises in China was 1.56 tons/(meter·day), with a coke ratio of 539 kilograms per ton of pig iron ； ②Breakthroughs have been achieved in the comprehensive utilization of vanadium-titanium-containing iron ores, and significant progress has also been made in the utilization of rare earth-containing iron ores. The main technical and economic indicators for blast furnace smelting are described as follows: The blast furnace utilization factor refers to the tons of pig iron produced per cubic meter of the blast furnace’s effective volume in one day and night; it is an indicator used to measure the efficiency of blast furnace operation. For example, in a 1000-meter-high blast furnace that produces 2000 tons of pig iron per day, the utilization factor is 2 tons/(meter•day). Coke ratio: The amount of coke consumed per ton of pig iron, expressed in kilograms per ton of pig iron. In the early 1980s, the coke ratio in blast furnaces was generally 450–550 kilograms per ton of pig iron, while in more advanced furnaces it was 380–400 kilograms per ton of pig iron. Coke is expensive, and reducing the coke ratio can lower the cost of pig iron. Fuel ratio: The total amount of fuel consumed per ton of pig iron produced, taking into account the use of coal powder, heavy oil, or natural gas in the blast furnace. The amount of coal and oil injected per ton of pig iron are respectively called the coal ratio and the oil ratio. At this point, the fuel ratio equals the coke ratio plus the coal ratio plus the oil ratio. Depending on the replacement ratio of coal to oil in the injection, it is converted into coke (in kilograms), and this value is then added to the coke ratio to obtain the comprehensive coke ratio. The fuel ratio and comprehensive coking ratio are important indicators for determining the total fuel consumption required to produce one ton of pig iron. Smelting intensity is the ratio of the amount of coke burned in a blast furnace per day and night to the volume of the blast furnace; it is an indicator of the degree of intensification of blast furnace operation, with the unit being tons/(meter•day). Downtime rate: The percentage of downtime compared to the total calendar time of the year. Reducing the downtime rate is an important way to increase the output of blast furnaces; generally, the downtime rate for blast furnaces should be below 2%. The qualified rate of pig iron refers to the percentage of pig iron whose chemical composition meets the specified requirements, out of the total pig iron produced; it is a key indicator for evaluating high-quality production in blast furnaces. The cost of pig iron is an indicator for measuring blast furnace operations from an economic perspective. I. The principle of iron smelting (how to produce iron from iron ore) A reducing agent is used to reduce the iron oxides in iron ore to metallic iron. Iron oxides (Fe2O3, Fe3O4, FeO) + reducing agents (C, CO, H2) → Iron (Fe) II. Methods of iron production: (1) Direct reduction method (non-blast furnace method) (2) Blast furnace method (the main method) III. Raw materials used in blast furnace iron production and their functions: (1) Iron ore: (sinter, pellet) – provides iron elements.

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 volume of pores in coke relative to its total volume), which affects the reactivity and strength of the coke. Coke for different applications has varying requirements regarding porosity; 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, 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 crushing strength of coke refers to its ability to resist 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 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 value M10. There are many methods for determining the M40 and M10 values; in China, the German Migon drum test method is commonly used. Evaluation of coke quality 1. Sulfur content in coke: Sulfur is one of the harmful impurities in the smelting of pig iron; it reduces the quality of pig iron. In steel-making pig iron, a sulfur content of more than 0.07% renders it scrap. 11% of the sulfur brought into the furnace from the blast furnace charge comes from the ore ； 3.5% from limestone ； 82.5% comes from coke, so coke is the main source of sulfur in the furnace 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 required increases by 1.8%, the amount of limestone added increases by 3.7%, the amount of ore added increases by 0.3%, and the blast furnace’s production decreases by 1.5–2.0%. The specified sulfur content limit for metallurgical coke is no more than 1%; the sulfur content 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 significant impact on blast furnace smelting. An increase of 1% in coke ash content results in an increase of 2–2.5% 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 matter content. If the volatiles content is greater than 1.5%, it indicates green coke ； If the volatile content is less than 0.5–0.7%, it indicates over-cooking; generally, the volatile content of mature metallurgical coke is around 1%. 5. Moisture in coke: Fluctuations in moisture can lead to inaccurate measurement of coke, which in turn causes fluctuations in furnace operation. Furthermore, an increase in coke moisture causes M04 to be on the high side and M10 to be on the low side, leading to 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 coke ovens (1300–2000 square meters), the coke particle size had to be greater than 40 millimeters ； The coke particle size in medium and small blast furnaces is greater than 25 millimeters. However, tests conducted by some steel mills at present show that a coke particle size of 40–25 millimeters is optimal. Coke with a size greater than 80 millimeters should be in whole particles 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. Uses of coke: We sell casting coke, metallurgical coke, and high-sulfur coke of various specifications from Shanxi Province, which can be transported by truck or rail. The specific parameters are as follows: fixed carbon content of over 83%; sulfur content of less than 0.5%; volatile matter content of less than 1.5%; ash content of around 15%. e: Mechanically produced coke: for metallurgical use ； Tested in steel mills – coke particle size: 8 cm–150 cm: molten iron ； Used for casting motor housings, radiators, and mechanical counterweights; heavy coal coke: molten iron ； Used for casting motor housings, radiators, and mechanical counterweights. Improved lump coke: used for ordinary casting ； Rough components such as mechanical parts are suitable for furnaces with a capacity of 2–3.5 tons; the size of the shaped coke is 25 cm: used for ordinary casting as well as for casting products that require higher standards ； Items such as water pump fittings and fire-fighting fasteners: fixed carbon content of over 85%; volatile matter of 1.5%; ash content of 13.5%; sulfur content of less than 0.5%. The particle size of the packed coke should be over 8 cm; it is used for ordinary casting processes. The particle size of the shaped coke is 25 cm; it is used for precision casting and ductile iron casting. Modified coke: (produced from primary coking coal) – suitable for precision casting and ductile iron casting. Fat coal coke: used in ferroalloy production ； Used for casting motor housings, radiators, and mechanical counterweights. Fixed carbon content of 86% or more; sulfur content of 0.5% or less; ash content of 12% or less; volatile matter content of 1.5% or less. The particle size of crushed coke is 4 cm–8 cm or 10 cm or more. Used for ordinary casting; the particle size of shaped coke is 25 cm. Suitable for precision casting and ductile iron casting, and for furnace types with a capacity of 3 tons or more. Fixed carbon content of 88% or more; sulfur content of 0.5% or less; ash content of 10% or less; volatile matter content of 1.5% or less. For casting coke, the particle size of shaped coke is 25 cm; suitable for all types of high-precision ductile iron casting as well as for products intended for export that meet international standards. Crushed coke with a particle size of 8 cm or more is used for ordinary casting and for producing high-quality casting products ； Items such as water pump fittings and firefastening components. Coals with a fixed carbon content of 78, 83, 85 or higher – high-sulfur coals: used in copper and aluminum processing, plastic manufacturing, and various chemical applications. Bituminous coals with a fixed carbon content of 83 or higher: used for the production of carbon monoxide, as they are rich in gas-like substances. Coal powders with a fixed carbon content of around 78: used for annealing casting parts, as well as in civilian and chemical applications. Coal slurry with a fixed carbon content of around 65: used for civilian purposes, annealing casting parts, and in chemical applications. Metallurgical coals refer to blast furnace coals, casting coals, ferroalloy coals, and coals used in the processing of non-ferrous metals. Since over 90% of metallurgical coke is used in blast furnace ironmaking, blast furnace coke is often referred to as metallurgical coke. The quality standard for metallurgical coke established in China (GB/T1996-94) is the quality standard for blast furnaces. - Technology of metallurgical coke ->40 >25 25-40 Ash content A d/ % I II III Not more than 12.00 12.01-13.50 13.51-15.00 Sulfur content S t,d/% I II III More than 0.060 0.061-0.80 0.81-1.00 Mechanical strength – Crushing strength M 25 /% I II III More than 92.0 92.0-88.1 88.0-83.0 As agreed upon by the supplier and buyer Wear resistance strength M 10 /% I II III Not more than 7.0 8.5 10.5 Volatiles content V daf /% Not more than 1.9 Moisture content M t/ % 4.0 ± 1.0 5.0 ± 2.0 Not more than 12.0 Amount of coke fines /% Not more than 4.0 5.0 12.0 Note: Moisture is only used as a parameter for controlling production processes; it is not used as a basis for quality assessment. Foundry coke is coke specifically designed for melting iron in cupola furnaces. Casting coke is the main fuel for melting iron in blast furnaces. Its function is to melt the charge and superheat the molten iron, as well as to support the material column and maintain its good air permeability. Therefore, foundry coke should have large lump size, low reactivity, low porosity, sufficient impact resistance, as well as low ash and sulfur content. Grade levels: Premium, Grade 1, Grade 2. Block size:.mm – >80, 80–60, >60. Moisture content:.% – not more than 5.0. Ash content:.% – ≤8.00, 8.01–10.00, 10.01–12.00. Volatiles – not more than 1.50. Sulfur content:.% – not more than 0.60, 0.80, 0.80. Drum strength:.% – not less than 85.0, 81.0, 77.0. Drop strength:.% – not less than 92.0, 88.0, 84.0. Apparent porosity:.% – not more than 40, 45, 45. Coking rate (

Furnace capacity: m3; M40%: M10%. Strength after reaction (CSR) in %; Reactivity index (CRI) in %. 1000: ≥76, ≤8.5, ≥58, ≤28; 2000: ≥78, ≤8.0, ≥60, ≤26; 3000: ≥80, ≤7.5, ≥62, ≤25; 4000: ≥84, ≤7, ≥64, ≤25; 5000: ≥85, ≤6.5, ≥66, ≤25. Ash content in %: ≤13, ≤13, ≤12.5, ≤12, ≤12. Sulfur content in %: ≤0.7, ≤0.7, ≤0.65, ≤0.6, ≤0.6. Particle size in mm: 20–75, 20–75, 20–75, 20–75, 20–75. Reference: http://www.chem17.com/st99813/Article_18116.html. Conditions for blast furnace iron production. Last edited by vvvvvbbbbb on 2009-3-30 at 19:29

The larger the volume of a blast furnace, the higher the requirements for the quality of coke – this is a qualitative rule. As for the quantitative aspects, it’s difficult to clarify; in general, blast furnaces require coke of as high a quality as possible, but it’s hard to say whether this is truly necessary in practice.