MODERN BLAST FURNACE PLANT & PROCESS

Modern blast furnace plant consists of the following importants sections :

1. Blast furnace proper

2. Hot blast supply equipments

3. Gas cleaning system and gas storage

4. Raw material storage and handling

5. Liquid products disposal

6. Process control equipments.

The schematic arrangement of the various sections in a modern blast furnace.

A typical lay-out of the blast furnace plant is

MODERN BLAST FURNACE

modern blast furnace is nearly a 30 m (about 100 feet) tallw welded plate construction with a circular cross section of varying size a different levels. The cross section area increase from the top or throat downwards, a maximum being at the bosh level and it decreases thereafter. The structure essentially consists of the foundationt the hearth, the bosh, the mantle and columns, the stack the rawm material hauling and charging facilities and the top A typical set of dimension of a furnace producing 2000t of pig iron

per day and with an effective inner volume in the range of around 2000m3

FURNACE DETAILS

Foundations. It is a massive steel reinforced concrete mass partially embedded below the ground level. It should be sufficiently strong to stand the loaded furnace weight, which may be as much as 10,000 t for a 2000 t/day furnace. It may be about 15 m in dia and 6-8m thick upon which is placed the furnace bottom consisting of about 4-5 m thick of fire bricks.

Hearth. It is a receptacle to collect the liquid slag and metal and is also referred to as a crucible. The old practice of using fire bricksf for hearth construction is almost universally replaced by carbon blocks. Watercooled ct copper or steel plates are laid in the side walls to portec the lining. The carbon wall may be more than a met thickness or a stadium-lype construction. In the hearth wall are located a tap hole for iron, 12-15 cm in dia and about 0.3-0.6 m above

BOSH. The bosh is the hottest part of the furnace because of its close proximity to the reaction between air and coke. Molten iron accumulates in the hearth, which has a taphole to draw off the molten iron and, higher up, a slag hole to remove the mixture of impurities and flux. The hearth and bosh are thick-walled structures lined with carbon-type refractory blocks, while the stack is lined with high-quality fireclay brick to protect the furnace shell. To keep these refractory materials from burning out, plates, staves, or sprays for circulating cool water are built into them.

STACK. The stack is kept full with alternating layers of coke, ore, and limestone admitted at the top during continuous operation. Coke is ignited at the bottom and burned rapidly with the forced air from the tuyeres. The iron oxides in the ore are chemically reduced to molten iron by carbon and carbon monoxide from the coke. The slag formed consists of the limestone flux, ash from the coke, and substances formed by the reaction of impurities in the ore with the flux; it floats in

Tuyere and Bustle Pipe. Immediately above the hearth are located the tuyeres through which hot air blast ís blown for fuel combustion see. The number of tyeres vary ith the size the furnace. Usually it is any even number between 10-20 and are uniformly spaced all over the periphery. Airfrom hot blast stoves is supplied to a huge circular pipe: eicir cling the furnace at the bosh level., This is called the bustle pipe. The individual tuyeres are connected to the bustle pípe whích, by virtue of its enormous size, equalises the pressure of the blast at alI the tuyeres.

Off-takes. There are four exhaust pipes which are connected to the furnace top evenly at four points. These rise vertically up above the furnace top and thenjoin toa bigger single pipe known as the downcomer which delivers the gas to the gas cleaning systen i.e. dust catcher.

Hot Blast Stove

Air blast is preheated to a temperature of 700-1300°C in Cowper regenerator stoves. A set of three or four regenerators is provided for each furnace. The stove is about 6-9 m in dia and 30-35 m in height. Special thin walled bricks are used to construct the checker work in the stove. During working one stove is 'on-blast', heating it, while the remaining two or three are 'on-gas' that is getting themselves heated by burning the cleaned blast furnace gas. The earlier stove had nearly 5000 m2 as the checker surface area for heat exchange but the modern one may have about five times this much. The thermal efficiency of such a stove is around 80-90%. Several valves are provided on the stove assembly to carry out the changeover from gas to air and vice versa smoothly.

Gas Cleaning

The blast furnace gas is cleaned thoroughly before being used as a fuel. The downcomer opens up in a dust catcher where coarse parti-cles contained in the gas settle down by virtue of the change in their direction of flow. The coarse cleaned gas then passes through wet ordry scrubbers and electrastatic precipitators. Nearly 25% of the gas obtained from the furnace is consumed in the stove for producing hot preheated blast. The remaining nearly 75% of the gas is consumed eleswhere in the plant. This is known as BF gas. A gas holder may he provided to store the surplus gas.

Blast Furnace Operation and Control

A blast fiurnace once started in expected to run for not less than five years before it is due for complete relining and major repairs. During the campaign charge materials are regularly put in the furnace from the top and the liquid metal and slag are tapped from the bottomn

periodically.

The charge materials are drawn out from the storage bins in required amounts and in desired sequence and are weighted as accu-rately as is feasible without impairing the rate of feed. The weighed materials are put in the charging skips which raise them to the furnace top and discharge them on the bell. The ore, coke and limestone are never premixed. On the contrary, these are charged separately, one skipful at a time, and depending upon the requirements that many skips of the required material are charged

The blast furnace operation is controlled from the top by trying to distribute the solid charge more uniformly in the furnace. It is simultaneously controlled from the bottom by the temperature and driving rate (rate of blast feed) of the blast. In order to exercise this control a series of instruments are provided at various levels in the blast furnace complex to measure and control various parameters. Computers are provided in order to do this job more efficiently.

Essential of the Blast Furnace Process

In the blast furnace solid charge materials like iron ore (or agglomerated ore), coke and limestone are charged in the vertical shaft of the furnace at the top and hot air blast is blown through tuyeres located at the bottom level. The oxygen from the preheated blast combines with the carbon of coke and generates heat and carbon monoxide. The gas phase containing mainly nitrogen and carbon monoxide as-cend upwards through the charge which reacts with and extracts heat from the gas phase. Eventually the charge melts and metal and slag thus produced get stratified and are separated to obtain the metal. Fundamentally therefore the blast furnaçe is a counter-current reactor..

MODERN IROON MAKING;

The main chemienl reaction the reduction of iron oxide

iron nnd the burning of coke, The iron oxicde may get reduced ns

2(Fooz) 3{co} 3{co,}+4(Fe)

(FeO)+ a{C} -s{C:o}+2(Fe)

the actual mechanism shall be discussed later. Since the iron oxide is reduced directly by carbon it has tradition ally been referred to as 'direct reduction' ofiron oxide. The CO generated by burning of the fuel at the tuyeres or by dircet reduction of iron oxide as in reduces iron oxide also as in which is referred to as indirect reduction'. The CO required for indirect reduction is essentially generated at the tuyer level where carbon of coke burns as

2(c)+{o%}=2{Co}

As the gases rise and carry out the reduction the CO2 content ofthe gas increases and a reaction of the type

CO}+(C)=2{co}

may take place thereby regenerating the reducing gas. This is known as 'solution loss reaction'.

Limestone and dolomite, if added as a flux in the charge get

calcinedas:

(CaCO2)(Ca0)+{coa

(MgCOg)-(Mg0)+{Coa}

Reduction of oxides ofiron is practically over while it is in solid state. This reduced iron shall be impure because of associated ganguue constituents of the ore. Separation of iron from the associated gangue descends can only be brought about by melting the whole charge. As the charge melted in the furnace its temperature is increased and finally it is

one in the bosh region. Two liquid phases form as a result of melting oxides which the iron phase and the other molten oxides called 'slag'. Those are not reduced, in spite of melting, and those which join the slag phase

The slag are reduced to elemental state join the metal phase. thus contains mainly silica, alumina, alkali-alkaline oxides, etc. with some minor earth

SiO, Mn0, P205, etc.other oxides and sulphides. A part of the

by Si, P, Mn,get reduced and hence the metal is contaminatedof its contact S, with etc.coke The metal also gets saturated with carbon because

iron melt may contain inside the furnace. On the whole therefore the anything upto 8 wt% of the total impurity

The gas composition and the temperature vary smoothly over the vertical as well as the horizontal corss section of the furnace. The chemical processes are therefore continuous from bottom to top and throughout the whole volume of the furnace. The furnace, however, can conveniently be divided into four main zones with respect to the physical processes occurring in the furnace.

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