LD Vessel Hood

● The area where oxygen jet impinges on the metal bath during refining, an intense hot zone is developed due to the exothermic reaction. The hot zone temperature is estimated to be over 2500℃ and as a result nearly a percent of the metal charge is blown in the gases as vapours. When the gas containing iron vapours comes in contact with oxygen from the atmosphere iron vapour from red brown and dark brown solid oxide practices. About 80% of the practicles have size less than 0.8 microns and at least 20% are in the range of 0.1 to 0.3 microns.

● The gases in which fine dust like this is suspended appear as dense brown fumes. In old plants these gases are led through a hood in to the stack and then let off in the atmosphere. The hood and the stack are plate shells which are lined from inside with a course of fire bricks and then plastered by clay. The hood is water cooled by a complex cooling system. A water cooled sleeve is provided for inserting and guiding the lance in the vessel through the hood.
Gas cleaning system
LD Vessel Hood | Gas cleaning system

● One or two water cooled chutes are fitted in the hood for charging solids when the vessel is in vertical position. Access doors are provided for cleaning the hood. The hood is sealed at the bottom by two water cooled removable dampers to prevent infiltration of air, while the LD converter is not being blown. The hood is supported clearly off the vessels in such a way that enough of atmospheric air is available for burning the carbon monoxide at the mouth of the vessel. Explosion may occur if CO is not entriely burnt in the lower part of the hood.

● The dense dark brown fumes are let off in open atmosphere in the early designs, although these are quite hazardous for public health. The problem of fume formation did worry the originators of the process in spite of all the advantages that accrued from oxygen blowing from top for refining iron. The problem has been solved to a satisfaction of all concerned. Necessary gas cleaning systems have been developed to remove over 95% of the dust in the gases. A million tonne steel plant may blow and then recover nearly 10,0000 tonne of iron oxide dust which could be recycled in to the iron making process.

Waste Gas Treatment | Gas Cleaning Plant

● Many plants have installed waste gas heat Boiler but it has been observed that they add to problems. The intermittent nature of blowing necessitates either synchronising of blowing or installation of secondary burner to raise steam during the idle period between two blows. Boiler cleaning is a problem because of the dust laden gases. The whole economy of waste heat recovery depends on how best the steam that is generated is utilised. The gas cleaning consists of preliminary gas cooling by water sprays and dust collection by wet or dry method.
Gas cleaning ventury scrubber
Vantury Scrubber

● Ventury scrubber is quite a popular wet dust collecting system. The dust laden gases are accelerated through a ventury throat and water jets are directly on the gas at right angles. The dust particles get wet and become heavy and separated by cyclonic or centrifugal action in a separator. There is no danger of explosion and the cost of installation can be much less if wet slurry can be disposed off. The ventury system required less ground space but substantial water.

● Electrostatics precipitated can clean the gas in dry or wet condition. The gas is is cooled by water sprays to 300℃ in flues and in sprak boxes to eliminate large dust particle. The  gases entries the electro static precipitor at 130 to 150℃. Due to the corona discharge in the tubes solid particle get charged and move to the neutral electrode. The dust collected on neutral electrode is periodically scrapped and collected for disposal. The gas velocity during precipitation is very low.  Wet electrostatic precipitators are also in use.

Wet gas cleaning plant
Gas cleaning system

● Flowing water film over the neutral electrode washes down the dust in the form of a slurry. The operating cost of electrostatic precipitator is low and cleaning efficiency is high. The dust being rich in iron oxide can be readily pelletized and charged in a blast furnace.

References :
1. Modern Steel Making :  Dr R.H. Tupkary and V.R. Tupkary.
2. Ironmaking and Steelmaking Theory an d Practice : A. Ghosh and A. Chatterjee
3. Steel Making : A.K.Chakravorty


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