Production of cast coke using rammed coke ovens

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Production of cast coke using rammed coke ovens 1 Selection of binders: Asphalt has the best binding properties, resin is next best, while oils have poorer properties. After modification, petroleum asphalt is generally used as a binder; an appropriate amount of it can modify and thicken the coal material, thereby having a positive effect on the distribution of pores and strengthening the pore walls, and it also creates favorable conditions for the addition of inert substances. Coal tar is used as a binding additive; it is the asphaltic portion that provides bonding properties, while the other components, due to their low boiling points and low molecular weights, volatilize easily when heated. Therefore, tar is generally not used as a binder on its own, but is mixed with cohesive coal in inert components or used together with asphalt as a binder to lower the softening point of the asphalt. When tar is used as a binder alone, it should be subjected to blow-air oxidation treatment. The volatile matter content of petroleum delayed coking oil is 15%–25%, and it contains bonding substances similar to those in fat coal, making it suitable as an additive for bonding components. If the volatile content is less than 15%, the cohesion will decrease as the volatile content falls; it is considered a semi-inert material, but it can combine well with coal. 2 Inert additives: The addition of inert additives helps to produce high-quality cast coke and increases its bulk density. Inert additives include weakly caking coals with a high degree of degradation, anthracite, petroleum delayed coking, and coke dust. The first three are semi-inert substances, while coke dust is inert. To reduce the stress that causes cracking, it is best to add both types of inert additives simultaneously. Inert additives play a role in reducing porosity, increasing the average pore thickness, and decreasing the average pore diameter. 3 Addition of retarding agents: High-volatility, high-fluidity coal materials are mixed with retarding agents. These agents can adsorb some of the liquid substances produced during coal pyrolysis, thereby reducing its fluidity and expansibility. This makes it easier for gaseous products to be released, improves cohesion, thickens the pore walls, and slows down the contraction rate during coking. As a result, cracks in the coke are reduced (hence they are also known as crack-resistant agents), which in turn increases the strength and density of the coke. However, the fluidity and swelling capacity of colloids can only be reduced to a certain extent; otherwise, the adhesiveness decreases, the wear resistance of the coke improves, which leads to a rough surface of the coke as well as poor bonding at the interfaces. The selection of retarding agents to be added to blended coal should follow these principles: (1) When both the volatile matter and fluidity of the blended coal are high, it is generally better to use coke dust rather than lean coal; the coke dust should be finely ground, with a dosage of around 5% being optimal. (2) When the volatiles and fluidity in the coal blend are moderate and good wear resistance is required, anthracite powder or semi-coke powder with about 15% volatiles can be used. Delayed coking coke can be used when it is necessary to reduce the porosity of coke, increase its bulk density and strength, and lower its ash content and reactivity. The selection of thinning agents requires comprehensive consideration; they can be used in combination or mixed with an appropriate amount of binder. Generally, strongly caking coal should be roughly crushed to maintain its caking property. Weakly bonded coal is finely ground to facilitate dispersion. That is, the thinning agents should all be finely ground separately to prevent uneven mixing from creating crack centers in the cast coke. The fineness of the blending coal should be over 90% when it is <3mm. 4 Production Process: For the production of cast coke, it is advisable to use wider carbonization chambers, lower furnace temperatures, and longer coking times. We use a furnace temperature of around 1050°C (≯ 1150°C) in the production of cast coke. The coking time increased by more than 2/3 compared to that for producing metallurgical coke, while the coking rate remained at around 7–8 mm/h. To prevent excessively high temperatures in the carbonization chamber during coal loading and rapid coking, the moisture content of the coal fed into the furnace is kept at 11%–12%. To increase the bulk density of the coal charged into the furnace, measures were taken to appropriately increase the number of coal distribution layers and the number of ramming times. If the lean components in the coal blend are abundant, the coking temperature can be increased appropriately to boost coke strength and enhance its plastic flowability. The red-hot coke pushed out from the carbonization chamber is prone to cracking due to the sudden temperature changes during wet quenching, which results in an increase in cracks and a reduction in the size of the coke pieces. To this end, a brick wall about 2m high was built around the quenching tank to maintain the temperature of the quenching water at above 50–60°C. After implementing the above measures, we produced cast coke that met the users’ requirements; in 2007 we turned a loss into a profit of nearly 10 million yuan.