Blast Furnace Plant
The blast furnace iron making typically implies a reduction and smelting in a single reactor. It is operated on counter current principle. The charge descent in the downward direction and the blast (hot air) ascends from tuyeres. Therefore the residence time of the reducing gas inside the furnace is less.
A modern blast furnace plant mainly consists of:-
Blast furnace proper
Hot blast supply equipments
Gas cleaning system and gas storage
Raw material storage and handling
Liquid products disposal
Process control equipments
Blast Furnace Proper
The blast furnace is a tall shaft-type furnace with a vertical stack superimposed over a crucible-like hearth. Iron-bearing materials (iron ore, sinter, pellets etc.), coke and flux (limestone and dolomite) are charged into the top of the shaft. A blast of heated air and also, in most instances, a gaseous, liquid or powdered fuel are introduced through openings at the bottom of the shaft just above the hearth crucible. The heated air burns the injected fuel and much of the coke charged in from the top to produce the heat required by the process and to provide reducing gas (CO) that removes oxygen from the ore. The reduced iron melts and runs down to the bottom of the hearth. The flux combines with the impurities in the ore to produce a slag which also melts and accumulates on top of the liquid iron in the hearth. The iron and slag are drained and separated out due to their density difference through tap holes.
The structure essentially consists of the following parts:.
It is a massive steel reinforced concrete mass partially embedded below the ground level. It should be sufficiently strong to withstand the loaded furnace weight. It may be about 15m in diameter and 6-8m thick and lined with fireclay bricks.
It is a receptacle to collect the molten metal and slag. It is lined with carbon blocks. It essentially consists of a tap hole for Iron extraction and slag notch for slag removal. But modern practices involve a common tap hole for both Iron and slag. These holes are closed with clay by means of a Mud-gun when not in use. It is about 3-3.5m in depth.
3. Bosh :
It is the zone of intense heat constructed with a steel reinforcement. The top of the bosh has the maximum diameter of the furnace to accommodate the volume expansion of the charge materials. Tuyeres are present at the bottom level of the bosh, which are inclined at some angle in the upward direction. It is about 3-4m high.
4. Stack :
It is a frustum of a huge cone mounted on the mantle and extends to the furnace top i.e., the bell, the charging system, the gas off-takes, etc. It is lined with Fireclay and is about 18- 20m in height. The top 2-3m, known as Stock-line, is protected from abrasion caused by the falling charge by providing armour plates on the inner surface of the lining.
5. Tuyere and bustle pipe :
Immediately above the hearth are located the tuyeres through which hot air blast is blown for fuel combustion. Usually, the tuyeres are even numbered(between 10- 20) and uniformly spaced.
Air from the hot blast stove is supplied through a huge circular pipe, known as the bustle pipe, which by virtue of its enormous size, equalizes the pressure of the blast at the tuyeres.
6. Bell and hopper :
It is a cup and cone arrangement used for charging the solid charge. But, modern furnaces prefer bell less charging system. Some furnaces use a double bell arrangement to ensure that the charging continues without the leakage of exhaust gas out of the furnace.
7. Off-takes :
These are four exhaust pipes connected to the furnace top evenly at four points. These rise vertically up above the furnace top and then join to a bigger single pipe known as the down-comer which delivers the gas to the gas cleaning system i.e, dust catcher.
HOT BLAST STOVE:-
It is a regenerative furnace where the air blast is preheated to a temperature of 700- 1300 ⁰C before entering into the blast furnace. Generally, three to four hot blast stoves are provided for each blast furnace. It is about 6- 9m in diameter and 30- 35m in height. During working one stove is „on-blast‟, where the pressurized air is inserted into the stove, while the remaining two or three are „on-gas‟, that is getting themselves heated by burning the cleaned blast furnace gas. Each stove consists of two chambers- chequers chamber and combustion chamber. The combustion chamber involves heating of the cleaned B.F. gas which results in absorption of heat by the chequer bricks. Consequently, in the next cycle, the pressurised air supply is then forced into the chequers chamber which in turn absorbs the heat from the chequer bricks. This heated air blast is then supplied into the B.F. proper.
The B.F. gas is cleaned thoroughly before being used as a fuel. The down-comer opens up in a dust-catcher, where the coarse particles present in the gas settle down due to change in the direction of flow. This gas then passes through wet and dry scrubbers and an electrostatic precipitator. Nearly 25% of the B.F. gas is consumed in the stove for producing the hot blast. The remaining gas is used in other processes in the plant. A gas holder is used for storing the surplus gas.