Coke Metallurgy
Coke is a substance with very high carbon and few impurities that is used as a fuel. Coke is a carbonated solid substance that is obtained from low-ash and low-sulfur bituminous coals. The coke that is obtained from coal has a gray color and is also hard and It is porous.
Of course, coke also exists naturally, but the most commonly used coke is the coke made from coal by humans. Special coals are used to produce metallurgical coke, and the properties of coal determine the properties of the production coke. The inorganic materials of the coal are called sterile and remain in the coke as ash and affect the performance of the blast furnace. Ash in coke increases the consumption of coke and limestone in the blast furnace and reduces the production capacity of coking batteries.
Coke Metallurgy
Metallurgical coke is a porous material that is used in the blast furnace and is one of the main uses of coke in the metallurgical industry. Coke in the blast furnace plays various roles in providing energy and regenerating iron ores and creating porosity for the passage of regenerating gases. and it has cast iron carburization. Coke is also used as fuel and regenerating agent in Kopel furnace. It should be noted that coke is also used in casting industries.
Coke has a percentage of sulfur and phosphorus. As a result, coke is the most important factor in the introduction of sulfur and phosphorus into the steel production process. Sulfur and phosphorus are harmful elements in steel.
Getting to know the coking process:
For the reduction of iron, an agent is needed that separates oxygen from iron oxides. This substance is called a reducing agent. All blast furnace reactions are carried out at high temperatures, which are the result of burning coke. For reduction and melting in the furnace Coke obtained from coal is used for a long time. Natural gas and fuel oil are also used in small amounts instead of coke.
Ability to revive
The regeneration ability of natural gas hydrogen is much higher than that of coke carbon. Therefore, it is tried to use natural gas injection in the blast furnace to produce hydrogen gas in the blast furnace by burning it. In today’s modern furnaces, the method Coal powder injection is also used, which greatly reduces coke consumption.
Because coal has low strength and high impurity (approximately 35%), it cannot have all the necessary conditions as a blast furnace fuel, so it turns into coke until the necessary conditions (sufficient strength and low percentage of impurities and high calorific value) ) to charge in the blast furnace.
The changes that result from converting coal to coke are:
1. Increasing the percentage of carbon and thus increasing the calorific value.
2. Increasing resistance against mechanical factors such as impact, pressure, falling and wear.
3. Reduction of volatile impurities resulting from the emission of coal gases and volatile substances
4. Increasing useful surface area for more burning provided by coke porosity.
In addition to providing the necessary thermal energy in the blast furnace, coke also performs the reduction of iron oxides in iron ore.
Regeneration can be done directly by carbon (in front of blast furnace forms) or CO2, H2 gas.
The effect of coke on the furnace
The calorific value of coke should be maximum and its moisture content should be minimum. (The moisture content of the coke depends on the duration of the stay in the quenching towers and the amount of water absorption in the coke.) If the moisture content of the coke is high, it has a negative effect on the work of the blast furnace, because it prevents the creation of a suitable thermal condition in the blast furnace. It prevents and the efficiency of the furnace decreases.
Currently, in Isfahan iron smelting, the wet method or spraying cold water on the hot coke is used to extinguish the coke, and the dry method (extinguishing with an inert gas, i.e. nitrogen) is not used. It is possible to reduce the moisture content of coke and increase its strength, and of course, the energy of the gas blown into the coke can also be used.
According to the mentioned cases, the necessary conditions for long-term coke are:
1. Its calorific value should not be less than 4000 kcal/kg.
2. The percentage of sulfur should not be more than 1%.
3. Coke must have sufficient strength.
4. Its ash percentage should not be less than 14%.
5. The amount of moisture in coke should be less than 9% and the size of coke grains should be 5-80 mm. Usually, 450-500 kg of coke is needed to melt one ton of raw iron, and the price of this amount of coke is 50-30%. The price of one ton of pig iron.
6. Volatile substances should be less than 1%.
Iran’s coal mines are:
1. Kerman Coal Company
2. Central Alborz Coal Company (Underwater)
3. Eastern Alborz Coal Company (Shahrood)
4. Western Alborz Coal Company (Sangroud)
5. Tabas coal mine Coke strength test:
To check the strength of the coke produced from the coking unit, the barban test is performed. In this test, 50 kg of coke with a grain size of more than 60 mm is poured into the barban cylinder and it is rotated for 4 minutes at a speed of 25 revolutions per minute. Then the fleas are removed from the barb and passed through 10 and 40 mm needles (the needles have circular holes with a diameter of 40 mm or 10 mm).
Weigh the particles of coke that have passed through the 10th screen and calculate its percentage in 50 kg and report it as M10 and the percentage of coke particles that remain on the 40th screen as M40. Strong coke It is suitable that its M10 is less than 6.8 and its M40 is above 80.
The coke and chemicals section includes the following three main sections:
1. Coal sector 2. Coke department 3. Chemicals Department: This department has the following five sub-parts: 4. Material recovery section 5. Acid part 6. Refinement of Benzol 7. Department of Biochemistry Energy 8. Coal and coke research unit
• Coal sector:
The task of the coal department is to prepare and store coal for charging in coking cells. Coal is transported from mines to the factory by rail and truck. These coals are supplied from internal mines. Also, about 40% Coal charged in coking cells is imported from Australia.
First
The coal section of the turning wagon is located. The wagons containing coal enter the turning wagon one by one and are overturned by it. As a result, the coal is poured into the coal bunkers located under the turning wagon device.
Second
Then the coal goes uniformly to the magnetic separator by means of the conveyor belts that are under the bunkers. In the magnetic separator, the metal pieces that are probably mixed with the coal are separated.
Third
After that, the coal passes through a mill and goes to the top of the coal silos, which are located in two rows of thirteen and two rows of seven. Each coal silo has a capacity of approximately 2500 tons of coal. Storage temperature Coals in silos should not be more than 60 degrees Celsius.
The coals of each mine are charged in separate silos. Then a certain amount of coal is taken out from under the silos and falls on the conveyors that are located below and along the 4 rows of silos. In this way, different types of coal with certain weight percentages are removed. These coals are crushed in two mills to a size of 0 to 3 mm, then on the way to the batteries, they enter the mixer and at the same time, water is sprinkled on it to prevent the creation of dust in addition to mixing more. and the necessary humidity for coal is also provided.
The technical conditions of charging coal are selected as follows:
1. Ash up to 8.10%.
2. Maximum humidity of 9%. 3. Volatile substances 25 to 28%.
4. Sulfur maximum 1%.
5. Particles 0 to 3 mm 75 to 77%.
Here, the task of the coal department ends and the ready coal, which has sufficient moisture, appropriate dimensions and appropriate percentages of all types of coal, is sent to the coal tower. Coke department: In this section, the ready coal in the coal tower is charged by the charging wagon in the coking cells, and during a certain period of time, which is called the coking period, in the conditions of suitable temperature (1100-1000 degrees Celsius) and the absence of air and during It turns into coke in five stages. These stages include:
1. Evaporation of coal moisture at a temperature of 100 degrees Celsius.
2. Hydrocarbons release stage at 350-300 degrees Celsius.
3. Turning the coal mass into a paste at a temperature of 350-480 degrees Celsius.
4. The stage of converting the plastic mass into semi-coke at a temperature of 480-550 degrees Celsius. 5. The stage of converting the mass of semi-coke into a solid state of coke at a temperature of 480-1000 degrees Celsius.
Coking battery
The coking battery is in the shape of a rectangular cube, which is divided into smaller units called cells by transverse walls. In these cells, coal is charged. Each cell is purified on both sides by thermal cells of coke gas and burns in them. and it is heated and the necessary heat is provided to convert coal into coke. Charging and unloading of coal in each cell is done by three charging and pushing (discharging) machines and door openers.
Regulator
The cells in which the coal is charged are called coke cells and the cells in which the refined coke gas burns with air are called thermal cells. The temperature of the thermal cells must always be controlled. To use the heat of the exhaust smoke and prevent energy loss. A heating system of gases and incoming air is used, which is located under the coking battery. This system is called a regulator.
The exit path of the smoke in the combustion cells is changed alternately with the path of the incoming air by the central path switching device and in the valves, and as a result, the incoming air is preheated for combustion.
After the end of the coking period, the two side doors of the cell are opened by the door opener and pusher machines, and after the coordination between the pusher machine and the molten locomotive, the arm of the pusher machine enters the coke cell from outside the cell. and pushes the molten coke forward, as a result, the coke is removed from the front of the cell and poured into the extinguishing wagon.
Classification of coke in terms of dimensions
The locomotive of the extinguishing wagon takes the molten coke under the extinguishing tower, and by spraying phenol water on it, the coke is extinguished in about 90 to 100 seconds, and then the extinguishing wagon goes out from under the extinguishing tower and the coke is It falls evenly on the coke platform. After about 20 minutes, the coke goes to the coke granulation section through a series of conveyor belts, where after being cooled, it is granulated into three sizes:
1. Coke grains smaller than 25 mm which are used as fuel in agglomeration and other parts.
2. Coke grains of 25-80 mm that are sent directly to the blast furnace.
3. Coke grains larger than 80 mm are removed from the granulation department and sent to the coke open warehouse by truck, and re-entered the coke granulation department and are filtered and used. Cokes can also be used in Kopel type casting furnaces.
If the blast furnace is not needed, the surplus produced coke is transferred to open storages.
Chemical recovery and refining department:
After leaving the coking cell, the coke gas is sent to the chemical recovery section. In this section, different chemicals are separated from the coke gas with special methods. The chemistry of this gas.
One cubic meter of raw coke gas with an average density of 650 grams per cubic meter has the following chemicals:
Steam: 250-450 grams.
Tar steam: 80-130 grams. Ammonia: 82-120 grams. Benzoyl hydrocarbons: 32-34 grams.
Hydrogen sulfide: 2.8-5.6 grams.
Naphthalene: 22-24 grams.
Hydrocyanic acid: 1-8%.
Raw coke gas produced after leaving the coke section:
The raw coke gas produced from the coking process, after leaving the coke section, first, its temperature reaches from 700 to about 80 degrees Celsius by directly spraying ammonia water on it. As a result of this temperature reduction, most of the tar and water Ammonia and naphthalene are separated along with fine particles of coke. Then, the low-temperature gas goes to the separator (separator) by two suction machines through the regulators of the coke section, and the heavy materials that contain ammonia water and the remaining tar are separated from it. In the next stage, it goes to 8 cooling towers and its temperature reaches 30 degrees Celsius.
Ammonium sulfate part
Next, this gas reaches 60 degrees Celsius by indirect water steam and then enters the ammonium sulfate part. In this part, when the gas passes over the surface of sulfuric acid, ammonium sulfate is deposited as a deposit. These deposits after Drying is packaged and it is used as fertilizer in agriculture. The exhaust gas from the ammonium sulfate acid part is sent to the benzol absorption part.
Spraying diesel on benzene gas
By spraying diesel on benzene gas, diesel is absorbed. The diesel that absorbs benzene is sent to the benzene refining section. The products of this refinery are benzene, toluene, xylene, and solvent. Sulfuric acid is sent. In these compounds, the sulfur in the gas is separated and then burned to convert into SO2.
By injecting water vapor and vanadium pentoxide catalyst, SO3 is converted into H2SO4. The resulting sulfuric acid is sent to the ammonium sulfate part. The gas output from the sulfuric acid part enters the gas network of the factory and is used to heat the coking or production cells. Steam can be used in power plants or preheated rolling furnaces or agglomeration baking machines.
Chemical composition of coke:
The organic matter of coke contains 5.97-5.96% carbon, 5-8% hydrogen, 3-4% oxygen and a small amount of sulfur and nitrogen. When the coke is heated to 85 degrees Celsius or more, the volatile substances are removed from it. The removal of volatile substances in Secondary heating is due to the breakdown of hydrocarbons in the coke. The more uniform the heating flow in the coking oven and the higher the temperature of the coke before the coking exit, the lower the percentage of volatile substances in it. Metallurgical coke usually contains 0.9 to 1.1% volatile matter. The reason for the moisture in the coke after quenching The most important reason for the presence of moisture in coke after turning it off is the entry of water into the coke cavities. When coke is stored in open space, coke quickly loses its moisture and its percentage decreases to 2.3 to 2.7 Therefore, the moisture percentage of coke depends on the method of quenching and also its surface per unit weight. The higher the ratio of surface to weight, the higher the moisture percentage. (If all other conditions are the same.) Therefore, the moisture percentage of fine coke It is 3-4 times more than coke with large seeds
.
The characteristics of metallurgical coke are divided into two categories of characteristics, which include physical characteristics and chemical characteristics. If we want to talk about the appearance of this product, we should mention its appearance. In fact, the appearance of metallurgical coke is like coal; But with the difference that its pieces are harder and more resistant. On the other hand, in many cases, their color is similar to crystalline and transparent black. Another thing that can be mentioned about the appearance of this product is the absence of moisture in it. So that when you see its appearance, you will see that this product is too dry. In fact, this product is pure carbon, which has a very small percentage and amount of moisture. Its chemical characteristics also include some impurities such as sulfur, phosphorus, and ash in this product. Of course, high-quality metallurgical coke has a very low percentage of these impurities, to the point where the percentage of sulfur in it has reached below one, and the percentage of phosphorus is also very small. But the percentage of ash and moisture can be measured at around 10-15%. The carbon in metallurgical coke is one of its chemical characteristics. As we have already said, the carbon in this product should be at least 80%. Carbon not only generates a lot of heat to melt metals in the blast furnace, but also plays a very important role in separating oxygen from iron and purifying iron ore. You can inquire about the price of metallurgical coke and to know how to buy it, you can contact dynamic iron sales experts through the communication links to guide you fully in this field.
In addition to providing the necessary thermal energy in the blast furnace, coke also performs the regeneration of iron oxides in the iron ore. The regeneration can be done directly by carbon (in front of the blast furnace forms) or CO2, H2 gas.
The calorific value of coke should be maximum and its humidity should be minimum.
If the amount of moisture in the coke is high, it has a negative effect on the operation of the blast furnace because it prevents the creation of a suitable thermal condition in the blast furnace and the efficiency of the blast furnace decreases.
Currently, in Isfahan iron smelting, the wet method or spraying cold water on the hot coke is used to extinguish the coke, and the dry method (extinguishing with inert gas, i.e. nitrogen) is not used.
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