In the steel industry, our company has made strict requirement on the nature and working condition of the exhaust fume from blast furnace, smelting silicon iron furnace, steel making arc furnace as well as on the dust pelletizing system and the use of dedusting cloth (filter) bag and filter material.
1. The exhaust gas as discharged from the iron-making blast furnace is coal gas containing dust, which is referred to as raw gas. When the blast furnace is operating normally, the temperature of the exhaust gas as discharged from the blast furnace is from 150℃to 250℃. When there is collapse of materials in the hearth, the temperature of the instantaneous exhaust gas can be as high as 400℃, and even 600℃. When the furnace has opened and the forced draught has stopped, the exhaust gas temperature can reduce to below 100℃. Therefore, the filter material for the dedusting cloth (filter) bag must have temperature resistance from 250℃to 300℃, prevent paste bag and resist hydrolysis. The blast-furnace gas has such proprieties as high pressure and being poisonous, combustible, and explosive. Besides, the filter material for the dedusting cloth (filter) bag shall be preventive of static, flame, and explosion. The pulse valve must adopt the blowing by inert gases. When the blast furnace is smelting normally, the discharge amount of exhaust gas each ton shall be 1,400 to 2,000 NM, the pressure ranges from 0.05 to 0.25 Mpa, the temperature is from 150℃to 300℃, CO accounts for 20% to 30%, H accounts for 2% to 5%, and the heat value ranges from 3,000 to 3,800 KJ/NM. It contains plenty of physical and chemical energy and high recovery value. The filter material for the dedusting cloth (filter) bag must have dense porosity, surface filtration property and outstanding filtering quality. The best velocity of filtering is 0.5M/min, and the maximum is no higher than 0.8M/min, so as to make sure the emission concentration is less than 5mg/NM. The filter material needs to have comprehensive mechanical strength to raise its service life.
2、The ferrosilicon arc furnace adopts semi-sealed cowling mostly (have hanging curtain for activity). The cool air coming from outside the cowling mixes with the CO emitted from smelting for combustion. Adjusting the hanging curtain for activity may adjust the air coming into the furnace and the flue-gas temperature to strengthen the combustion. The flue-gas temperature shall be controlled at 400℃and the temperature of anti-burnt bag and exhaust gas shall be controlled above 140℃. The dust with grain size of less than 1um shall account for over 80% of the smoke. The thinner the dust is, the more viscous it will be. Dust is higher than resistance, therefore, bag filter must be adopted. During operation, the filter bag has large quantity of electric charge and high filtration resistance. Moreover, the filter bag shall be anti-static.
3、During smelting, the steelmaking arc furnace is divided into such three stages as melting, oxidation and passing original. The oxidation stag is to strengthen decarburization, that is, to blow a large amount of oxygen or add mineral to the furnace. The smoke volume as generated in the oxidizing period is the highest. And the dust content and flue-gas temperature are the maximum. Thus, the purification system of the steelmaking arc furnace gas is designed as per the oxidizing period. In the oxygen blowing period, the average grain size of smoke ranges from 0.01 to 0.1um. Approx. 85% of the smoke is dust particles of less than 4um. When ejecting smoke in the furnace, the smoke temperature is on the high side. Therefore, cooling measure shall be available. Generally ejecting smoke outside the furnace shall be adopted. During general smelting, the exhaust gas temperature is 100℃; when blowing oxygen, the temperature is from 100 to 200℃. The designed temperature is 150 ℃. Given this, the filter material for the dedusting cloth (filter) bag must be preventive of burnt bag, and resistant to oxidation and hydrolysis.
4、Steelmaking converter is to smelt the liquid pig iron in the air blast furnace. China has already made steel by oxygen-blown converter. During oxygen blowing, the furnace temperature is 1400 ℃. The content of CO gas is as high as 90% (CO gas and air combust at once.), thus the concentration of smoke dust is very high. 75% of the smoke has grain size of below 1um. The dust pelletizing system shall consider the utilization of CO and the cooling measure for flue gas.