Requirements of Medium Frequency Smelting Furnace for Coke

Requirements of Medium Frequency Smelting Furnace for Coke

First of all, it is necessary to understand the function of the intermediate frequency furnace in the smelting process.

The results show that in the process of medium frequency smelting furnace, the role of coke is not only the main provider of heat (accounting for more than 95%) but also the main heat transfer medium in the superheated zone of molten iron.

Most of the overheated heat is directly transferred from coke to molten iron droplets (Accounting for more than 85%).

At the same time, coke is also the source of hot metal carburization and sulfur addition.

In addition, coke is the main source of hot metal in the smelting process In order to keep the whole smelting process normal, stable and continuous, it can bear the pressure of the upper material column at high combustion temperature.

According to the role of coke in medium frequency smelting furnace, and considering the structure and operation characteristics of the cupola, the following requirements for coke are put forward.

1. During combustion, a part of sulfur in coke with low sulfur content is discharged with flue gas in the form of SO2, while most of the sulfur (mainly mineral sulfur and iron sulfide sulfur) is transferred to molten iron and slag.

The statistical results of mass production show that 60-70% of sulfur in coke is transferred to molten iron. Sulfur is a harmful element for the production of cast iron, especially for nodular iron and vermicular iron.

It is impossible to desulphurize in the cupola (during blast furnace smelting), so the sulfur content of foundry coke is required to be as low as possible.

Considering the actual conditions in China, the sulfur content in the national standard is required to be less than 0.8% (super grade coke is less than 0.6%). In order to reduce the sulfur content in coke, only high-quality raw coal and other raw materials (such as petroleum coke) are selected, which will bring adverse effects on resource allocation and production cost.

2. The content of fixed carbon is high. In order to obtain high-temperature hot metal, the highest temperature in the high-temperature zone of the furnace should be about 1800 ℃. The fixed carbon content in coke is required to be high (i.e. the ash content in corresponding coke is low).

The results show that the ash content in coke decreases by 1%, and the temperature of hot metal increases by about 10 ℃. There is even foundry coke with the fixed carbon content of 95% in foreign countries.

With the increase of fixed carbon content, the rate of hot metal recarburization in furnace increases, and the consumption of scrap steel in furnace charge also increases dramatically. For developing countries with underdeveloped industry and short scrap supply, attention must be paid to this problem.

According to the ash content of coke, the standard of foundry coke in China is divided into three grades to meet the needs of different production. Even the fixed carbon content of secondary foundry coke is more than 86% (ash content is less than 12%), which is much higher than that of local coke and metallurgical coke.

It is worth pointing out that in order to improve the fixed carbon content, the ash content of coking raw coal should be reduced and the production cost should be increased accordingly.

3. The process of low intermediate frequency smelting furnace with reaction performance is to melt the metal block into molten iron with certain temperature and composition, so the atmosphere in the furnace is usually weak oxidizing.

Blast furnace smelting is to reduce iron ore to iron, so the atmosphere in the furnace must be strong reducing.

Therefore, the reactivity of cast coke is lower than that of metallurgical coke. In the coking process, measures such as prolonging coking time are adopted to reduce porosity.

On the one hand, the range of high-temperature zone can be increased, and the surface combustion temperature of the coke can also be increased by reducing the reactivity of cast coke, which is very beneficial to hot metal overheating.

With the decrease of porosity, the reactivity of coke (i.e. the reaction between coke and CO2) and the combustion rate of coke (i.e. the reaction between coke and O2) is also reduced. If the porosity is too low, the coke combustion in the furnace can not form a relatively concentrated high-temperature zone and obtain the highest temperature value, which is very unfavorable to the overheating of molten iron.

At the same time, if the oxidation zone is too dispersed, the oxidizability in the furnace will be enhanced and the burning loss rate of elements will be increased, which is unreasonable in technology and economy.

4. Large block size and high strength In order to ensure the formation of a certain high temperature area in the coke combustion zone in the cupola, the temperature distribution of the cross section is uniform, so that the molten iron droplets can be fully overheated and high temperature molten iron can be obtained. The block size of the cast coke is required to be relatively high. Metallurgical coke is large.

In terms of technology, the degree of coke lumps is required to be ～ of the diameter of the cupola furnace. After being put into the furnace, the coke is broken by the impact of the iron lump, and the actual lump of the coke in the furnace will be reduced, so the coke strength is required to be higher.

The intermediate frequency power supply is mainly caused by the impact of the metal block, so the casting coke adopts the falling strength, and the corresponding metallurgical coke is mainly broken by friction, so the drum strength is adopted.