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Smarajit SarkarDepartment of Metallurgical and Materials Engineering

NIT Rourkela

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Blast furnace productivity depends upon an optimum gas

through flow as well as smooth and rapid burden

descent.

The character of the gas and stock movements is

intimately associated with the furnace lines.

The solid materials expand due to heating as they

descend and their volume contracts when they begin to

soften and ultimately melt at high temperatures in the

lower furnace.

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A further volume contraction occurs when the solid coke burns

before the tuyeres.

An enormous volume of the combustion gas has to bubble

through the coke grid irrigated with a mass of liquid metal and

slag.

An optimum furnace profile should cater to the physical and

chemical requirements of counter flow of the descending solid,

viscous pasty or liquid stock and the ascending gases at allplaces from the hearth to the top

cont

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Only then, an optimum utilization of the

chemical and thermal energies of thegases as well as a smooth, uniform and

maximum iron production with minimumcoke rate will be realized.

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o In an integrated steel works the capacity of the

Blast Furnace depends upon

The capacity of the works.

The process of steelmaking adopted.

The ratio of hot metal and steel scrap in thecharge.

Consumption of foundry iron in the works.

Losses of iron in the ladle and the casting

machine. The number of furnaces to be installed

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It is the volume of Blast Furnace occupied by the charge

materials and the products , i.e. the volume of furnace

from the stock line to the tap hole.

Useful volume = the furnace capacity C.U.U.V.

C.U.U.V = coefficient of utilization of useful volume.

The value of C.U.U.V. varies in a wide range from 0.48-

1.50 m3/ton of pig iron

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V =k D2H

V=Useful volume

H=Total height

D=Diameter at the bottom of the shaft

K=A coefficient usually lies with in the range of 0.47

to 0.53. High value is for slim profile.

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Total height = useful height +distance betweenstock line and the charging platform (it isgoverned by the construction of gas off-take and

charging platform, this dimensions varies from 3to 4m.)

Useful height= height from the tapping hole tothe stock line.

The height of the blast furnace is mainlygoverned by the strength of the raw materials,particularly that of coke.

cont

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The strength of the coke charged to the

furnace should be sufficient to withstand the

load of raw materials without gettingcrushed. Coke provides permeability(in thedry as well as wet zones )and also

mechanical support to the large chargecolumn, permitting the gases to ascendthrough the voids.

Total height (H)= 5.55V0.24

Useful height (H0) =0.88H

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Diameter:The belly /bosh parallel is the cylinder that

connects the tapers of the shaft and the bosh.Its diameter, dbll, and the ratio of this diameter to

the useful or inner height of the furnace as wellas to the diameter of the hearth play animportant role in the operation of the furnace.The correct descent of the stock, ascent of the

gas and efficient utilization of the chemical andthermal energies of the gas depend greatly uponthese ratios.

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The importance of an adequate belly diameter lies in the

fact that softening and melting of the gangue and

formation of the slag occurs in this region.

An increase in the diameter facilitates gas passage

through the sticky mass and also slows down stock

movement, thus increasing the residence time for indirect

reduction.

However, the belly diameter cannot be increased

arbitrarily as it is directly related to bosh angle, bosh

height, hearth and throat diameters and useful height.

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The belly height depends upon the softenability of the

ferrous burden and also on the shaft angle desired.

If the slag fusion occurs at higher temperatures and in a

narrow temperature range as in the case of pre-fluxed

burden, the hydraulic resistance decreases in the

vertical cross-section and the belly height can be

correspondingly reduced.

dbelly =0.59 (V)0.38

HbelIy = 0.07H

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The hearth is designed such that its volumebetween the iron notch and tuyeres is sufficientto hold the molten metal and the slag.

The dia of hearth depends upon:

The intensity of coke consumption.

The quality of burden.

The type of iron being produced.

D hearth =0.32 V0.45

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A very approximate relationship between

the coke burning rate and hearth diameteris given by the following equation:D = c Q 0.5D = hearth diameter, m

Q = coke throughput, tonnes/24hc = throughput coefficient which variesbetween 0.2-0.3 depending upon burdenpreparation.

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For highly prepared burden, the value of

c = 0.2 has been achieved in modern largefurnaces .

Therefore, for a furnace planned to produce10,000 THM per day with a coke rate of 500kg/THM, i.e., a coke throughput of 5,000tonnes per day, the hearth diameter shouldbe about 14.1 m.

The value will be 21.2 m if the value ofc=0.3.

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With increasing diameter of the hearth,the gas penetration must be ensured

by providing adequate bedpermeability with the use ofmechanically strong, rich, pre-fluxed

burden of uniform size and low slagbulk as well as strong lumpy coke.

The Hearth height should be 10% of thetotal height of the furnace

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The shaft height must be sufficient to allow theheating, preparation and reduction of ore beforethe burden reaches the bosh. In the upperregions of the shaft , volume changes due toincrease in temperature and carbon deposition.These demand an outward batter for smoothflow of materials. In the lower region of the shaft

, the material starts fusing and tends to stick tothe furnace wall. So to counteract the wall dragan outward butter is necessary.

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Stack height Hstack

= 0.63 H- 3.2 m

Stack angle

The stack angle usually ranges from 850to870(i) 850for weak and powdery ores;

(ii) 860for mixture of strong and weak, lumpy or

fine ores;(iii) 870for strong, lumpy ore and coke.

Th i i i h l

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The variations in the angles are necessaryfor obtaining an adequate peripheral flowwhich is an essential pre-requisite for

forcing of the blast furnace.

Since the ore hump is located in theintermediate zone and it moves almost

vertically downwards pushing the lightercoke towards the wall and the axis.

A smaller shaft angle in the case of weak

and powdery ore helps to loosen theperiphery.

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Stack angle can be calculated from the formula

Stack angle ()= Cot-1(D-d1/2xStack Height)

Where, D= Bosh parallel Diameter

d1= Throat Diameter

Bosh angle can be calculated from the formula

Bosh angle ()= Cot-1(D-d/2xBosh Height)

Where, D= Bosh parallel Diameter

d= Hearth Diameter

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When the raw materials are charged into the

blast furnace, little volume change takesplace for a few meters of their descent andhence the walls of the throat are generallyparallel

Throat diameter can not be too small as ithas to allow the enormous volume of the gasto pass through at a reasonably low velocity

to maintain adequate solid gas contact andto decrease the dust emission, throathanging and channeling.

Cont..

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Throat diameter can not be too wide as itmay compact the charge. A certainvelocity and lifting power of gas is

necessary for losening the charge at top.

Throat Diameter d throat =0.59 V0.35

Where, V= useful volume

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A considerable amount of slag and iron

descends to the hearth through the inter-tuyere

zones. If they do so without having beenadequately heated, the thermal state of the

hearth may be disturbed with attendant high

sulphur in iron, sluggish slag movement, erratic

metal analysis, frequent tuyere burning, etc.

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The distance between the adjacent tuyeres

around the hearth circumference should be such

as to obtain, as far as possible, a merging of theindividual combustion zones of each tuyere into

a continuous ring.

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The number of tuyeres mainly depend upon thediameter of the hearth. The diameter of thetuyeres depend upon the blast volume.

The following formulae can be used to determinethe number of tuyeres

Pavlov: n = 2d +1

Rice: n = 2.6d-0.3

Tikhomirov et al : n = 3d-8Where n= Number of tuyeres,

d=hearth diameter

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Capacity

(THM/Day)

Parameter

2000 3000 5000

Useful Volume (m3) 1700 2550 4250

Total Height (m) 33.08 36.46 41.22

Useful Height (m) 29.11 32.08 36.27

Bosh Parallel Dia (m) 9.96 11.62 14.11

Bosh Parallel Height (m) 2.32 2.55 2.89

Bosh Height (m) 4.37 4.81 5.44

Hearth Dia (m) 9.1 10.92 13.74

Hearth Area (m2) 65.04 93.66 148.27

Hearth Height (m) 3.308 3.646 4.122

Stack/Shaft Height (m) 17.64 19.77 22.77

Throat Dia (m) 6.87 7.85 9.29

Bosh Angle (0) 84.32 85.84 88.05

Stack Angle (0) 85 84.55 83.96

Nos. of Tuyeres 20 25 34

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NIT Rourkela

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Burden distribution is one of the key operating

parameters influencing blast furnace

performance, particularly the productivity and

the coke rate.

The proper distribution of burden materials

improves bed permeability, wind acceptance,

and efficiency of gas utilisation.

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In a typical Indian blast furnace equipped with a bell-

less (Paul Wurth) distribution system, the decrease

in coke rate that is due exclusively to burden

distribution was found to be 1012 kg/thm.

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Design of the blast furnace

and its charging device

(effect of these factors is

constant).

Angle and size of the big bell.

Additional mechanical

device(s) used for obtaining

better distribution.

Speed of lowering of large

bell.

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Inconsistency inphysical properties ofcharge materials(deficiencies caused bythis should be

eliminated by improvingquality of the burden.

Size range of the various

charge materials

Angle of repose of raw

materials and other

physical characteristics of

the charge.

Density of charge

materials.

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Level, system andsequence ofcharging, programmeof revolving thedistributor (conditionsdetermining majormeans of blast

furnace processcontrol from top).

Distribution of chargeon the big bell

Height of the big bell

from the stock-line i.e.charge level in thefurnace throat.

Order and proportion

of charging of variousraw materials.

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The density of three important raw materials viz. the ore, the

coke and the limestone are quite different.

The heaviest is iron ore with around 5-6 glcc, the lightest is

coke with density of around 15 glccand the limestone is

intermediate with-a value of density around 30-35 glcc.

It means that the rolling tendency of coke particles is maxi-

mum and that of the ore is minimum. Since the density values

cannot be altered, the sizes may be so chosen that their

differential rolling tendencies are offset to some extent.

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The problem of very dense ores is serious

from the point of view of their sluggish

reduction rates rather than their tendency

towards segregation. Such ores are therefore

invariably crushed and sintered to obtain re

porous agglomerates before charging these

in the furnaces.

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When a multi-particle material is allowed to

gently fall on a horizontal plane it tends to form

a conical heap. The base angle of this cone is

known as angle of reposeof that material.

This angle depends upon the particle size, its

surface characteristics, moisture content, shape,

size distribution, etc.

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For an iron ore of 10-30 mm size, with an

average mean size of 18 mm, the angle of

repose is around 33-35. For coke of 27-75 mm

size, with an average size of 45 mm, the same is

around 35-38. Similarly the angle of repose for

sinter is in the range of 31-34 and for pellets it

is around 26-28.

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The higher is the angle of repose the more it has the tendency to

form ridges on charging in a blast furnace.

The more dried is the ore and the more it is free from fines the

less pronounced is the angle of repose and thus less is the

tendency towards segregation.

The clayey ores tend to form ridges because of their high angle

of repose. The effective way to reduce the angle of repose of any

iron ore is to eliminate the fines, dry the ore if wet and to wash

off clay, if any, adhering the ore.

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On dumping, as the materials fall on the stock

surface, they take a parabolic path and mainly

two different profiles of the accumulated mass

emerge depending upon whether the particles

hit the in-wall directly(V- shape) or the stock

surface (M-shape)

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The M-profile itself is generally obtained if the material

strikes the stock surface. This happens when the

bell/throat diameter ratio is small (larger bell-inwall

distance) or the charging distance is small . It is clear

that the peak of the M-contour approaches the inwall

(hence the peripheral permeability decreases) as the

charging distance increases and ultimately the M

changes to V profile.

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NIT Rourkela

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Right at the top of the furnace is the granular zonethat contains

the coke and the iron bearing materials charged, sometimes

along with small quantities of limestone and other fluxes. The

iron-bearing oxides charged get reduced to wustite and metallic

iron towards the lower end of the granular zone.

As the burden descends further, and its temperature rises on

account of contact with the ascending hot gases, softening and

melting of the iron-bearing solids takes place in the so-calledcohesive zone(mushy zone).

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Further down the furnace, impure liquid iron and liquid slag are

formed. The absorption of carbon lowers the melting point of iron

drastically. For example, an iron alloy containing 4 wt. % carbon

melts at only 1185C..

In the cohesive zone and below it, coke is the source of carbon forcarburisation of liquid iron. However, carbon directly does not

dissolve in liquid iron at this stage. The possible mechanism of

carburisation of iron entails the formation of CO by gasification of

carbon, followed by the absorption of carbon by the reaction:

2CO(g) = [C]in Fe+ CO2(g)

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Coke is the only material of the blast furnace charge which descends to

the tuyere level in the solid state. It burns with air in front of the tuyeres

in a 1-2 m deep raceway around the hearth periphery.

Beyond the raceway there is a closely packed bed of coke, the central

coke column or dead man's zone.

The continuous consumption of coke and the consequent creation of an

empty space permit the downward flow of the charge materials.

The combustion zone is in the form of a pear shape, called 'raceway'in

which the hot gases rotate at high speeds carrying a small amount of

burning coke in suspension.

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The raceway is a vital part of the blast furnace since it is the heat

source in a gigantic reactor and at the same time a source of re-

ducing gas.

The salient features of Combustion zone are summarized below:

The force of the blast forms a cavity the roof of which is formed of

loosely packed or suspended coke lumps and the wall more closely

packed.

The CO2 concentration tends to increase gradually from the centre

and reaches a maximum value just before the raceway boundary

where most of the combustion of coke occurs according to:

C+O2 (air) =CO2+94450 cal

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The temperature of the gas rises as the coke

consumption proceeds and reaches a maximum just

before the raceway boundary. Thereafter, it falls sharply

as the endothermal reduction of CO2 by C proceeds;

CO2 +C =2CO-41000 cal

The concentration of CO2 fall; rapidly from the raceway

boundary and the gasification is completed within 200-

400 mm from the starting point of the reaction.

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The primary slag of relatively low melting point which forms in the lower

part of the stack or in the belly consists of FeO-containing silicate and

aluminates with varying amounts of lime which has become incorporated

depending upon the degree of calcination undergone .

As the slag descends, ferrous oxide is rapidly reduced by carbon as well

as by CO. As the lime is continually absorbed, the original

FeO-Si02-AI203system rapidly changes to the CaO-Si02-AI203system

with some minor impurities accompanying the burden. The dissolution

of lime and the approach to the CaO-Si02-Al203 system is morepronounced,

.

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As the liquid primary slag runs down the bosh and loses its fluxing

constituent FeO, the liquidus temperature also increases. If,

therefore, the slag has to remain liquid it must move down to hotter

parts of the furnace as rapidly as its melting point is raised. As the

reduction of FeO is almost complete above the tuyeres the resulting

bosh slag, composed mainly of CaO-Si02-AI203

The hearth slag is formed on dissolution of the lime which was not

incorporated in the bosh and on absorption of the coke ash released

during combustion. The formation is more or less complete in the

combustion zone.

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This slag runs along with the molten iron into thehearth and accumulates there and forms a poolwith the molten metal underneath. During thepassage of iron droplets through the slag layer,

the slag reacts with the metal and a transferenceof mainly Si, Mn and S occurs from or to themetal, tending to attain equilibrium betweenthemselves as far as possible.

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Below 600C :

Pre-heating and pre-reduction

600 -950C:

Indirect reduction of iron oxides by CO and H2

9500C to softening temperature:

Direct reduction; gasification of carbon (solution loss

reactions) by CO2 and H2 becomes prominent.

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The formation o{ cohesive layers or partiallyreduced and partially molten iron oxide takesplace.

The coke slits provide passage for gaseous flow.

Dripping or Dropping Zone Semi fluidized region in which liquids drip and

fragments of cohesive layers drop.

Zone through which liquids trickle down to thehearth. It is the final stage of iron oxide reduction

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Blast, injectants and coke are converted to hot reducing gas. This

gas reduces the ore as it moves counter currently towards the top of

the furnace.

Hearth

It is a container for liquids and coke where slag/metal! coke/gas

reactions take place. Metal droplets pass through the slag/coke

layer. Liquid metal/coke layer in which chemical reactions take

place only to a small extent.

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fluidization of small particles when the local gas

velocity is excessive;

diminution of void age due to swelling andsoftening-melting;

flooding of slag in the bosh zone when the slag

volume and gas velocity are excessive.

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The charge in the blast furnace descends under gravity against the

frictional forces of solids and buoyancy of gas. With increasing gas

velocity, the pressure drop increases approximately quadratically

until the upward thrust of the gas and downward thrust of the solids

are held in balance. When this critical velocity is exceeded (the point of incipient

fluidization), the packing in the bed becomes loose, the finer

particles begin to teeterand the pressure drop ceases to increase,

i.e., the resistance to gas flow drops (due to increase in void age at

places where the fines become suspended).

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The mechanism of the softening-melting phenomenais schematically illustrated in previous Figure. It isevident that with the onset of softening, the voidage inthe bed decreases and the bed becomes more

compact (origin of the terminology cohesive). As aconsequence, further indirect reduction of iron oxideby gases becomes increasingly difficult. Upon melting,dripping of molten FeO-containing slag through the

coke layers increases the flow resistance through thecoke slits and the active (i.e. dripping) coke zonebecause of loss of permeability.

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The cohesive zone has the lowest permeability.

Hence, for proper gas flow:

Tsshould be as high as possible

The thickness of the cohesive zone should be

as small as possible. This thickness depends on

the difference between Ts

and Tm

(Tm

- Ts

), and

therefore, the difference should be as low as

possible.

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Gas flow through Granular zone:

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For resistance to gas flow, more important than theparticle diameter is the relative size of the materials inthe bed.In a mixed bed of widely varying particle size, thesmall particles land in the interstices of the large onesand decrease the void age .Starting with large uniform spheres, the void agedecreases as the small ones are introduced and the bedbecomes more and more compact as the proportion ofthe latter increases.The bed is most dense, i.e., the voidage is minimumwhen 60-70 percent of the total volume of theparticles consists of the large ones for about all thecases.

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The m increases on either side of theminimum, i.e., with increasing or decreasingvolume fraction of the small particles(approaching more uniformity of the sizedistribution).The voidage decreases greatly as the ratio ds/d1 decreases.This shows that for a good and uniformpermeability and low resistance to gas flow in amixed bed, the size fractions should be asnarrow as possible.One can easily visualize the adverse effects ofmulti-granular bed of particles of varyingdiameter on the voidage.

A narrow size distribution has the following advantages:

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charge permeability increases and the gas distribution is

more uniform with better utilization of the chemical andthermal energies of the gases;

more even material distribution at the stock level and less

material segregation in the shaft during descent;

gas flow is not impeded if the size ratio is within limits but

at the same time gives rise to a tortuous flow of gases with

continuous changing of flow directions, providing a larger

gas/solid contact time.

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The fraction of iron bearing material below the limiting size is

therefore termed as 'fines'by the blast furnace technologists and

is invariably eliminated by screening at every possible stage.

From the point of view of reduction the maximum top size of

an iron bearing material should be as low as possible, since the

rate of reduction decreases, perhaps exponentially, with

increasing size.

The size range of materials charged in the blast furnace

represents a compromise to give both good stack permeability

and adequate bulk reducibility.

Gas flow in wet zone:

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Wet zones consist of the coke beds in the bosh andbelly regions, i.e. inactive coke zone, active coke zone,and the coke slits in the cohesive zone. Here molten iron and molten slag flow downwardsthrough the bed of coke. This reduces the free crosssection available for gas flow, thus offering greaterresistance, thereby increasing the pressure drop.An extreme situation arises when, at high gasvelocity, the gas prevents the downward flow of liquid.This is known as loading. With further increase in gasvelocity, the liquid gets carried upwards mechanically,causing flooding.

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Scientists have tried to estimate pressure

drop in blast furnace. However, they are

approximate. Moreover, they are only for the

granular zone and coke zones.

The situation in the cohesive zone is very

complex, and reliable theoretical estimates

are extremely difficult to come by.

Therefore for practical applications in blast

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Therefore, for practical applications in blast

furnaces, an empirical parameter, calledFlow

Resistance Coefficient(FRC) has become

popular. The FRC for a bed is given as

where the gas flow rate is for unit cross section

of the bed, i.e. either mass flow velocity or

volumetric flow velocity .

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FRC=1/ bed permeability

The FRC for a furnace can be empirically determined

from measurements of pressure drop and gas flow rate.

Since it is possible to measure pressures at various

heights within a furnace, the values of FRC for individual

zones can also be determined.

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These measurements have indicated that

FRCs for the granular, cohesive, coke +

tuyere zones are approximately 20%, 50%

and 30% of the overall furnace FRC.

This means that the cohesive zone is

responsible for the maximum flow resistance

and pressure drop, to a very large extent.

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Decreasing the extent of SiO formation by:o Lowering ash in coke, and the coke rate

o Lowering RAFT

o Lowering the activity of Si02 in coke ash by lime

injection through the tuyeres.

Decreasing Si absorption by liquid iron in the boshby enhancing the absorption of Si02 by the bosh

slag. This can be achieved by:o Increasing the bosh slag basicity.

o Lowering the bosh slag viscosity..

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Removal of Si from metal by slag-metal reaction atthe hearth by:

o Lowering the hearth temperature

o Producing a slag of optimum basicity and fluidity.

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Desulphurisation of metal droplets through slag-

metal reaction in the furnace hearth :

(CaO) + [S] + [C ]= (CaS) + CO (g)

Desulphurisation through the coupled reaction:

(CaO) +[S] +[ Mn] = (CaS) + (MnO)

(CaO) + [S] + [ Si] = (CaS) + 1/2 (SiOz)

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Sulphur pick-up through the vapour-phase reaction:

CaS( in coke ash) + SiO (g) = SiS(g) +

CaOFeS( in coke ash) + SiO (g) = SiS(g) +CO(g) +[Fe]

In the bosh and belly regions, SiSdecomposes as

SiS(g) = [Si] + [S]

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Reducing slag i.e. FeO content should be low

High basicity

High temperature, since desulphurisation is an

endothermic reaction

Kinetic factor

Contact surface of metal and slag ( by agitation)

Fluidity of slag( by adding MgO , MnO)

Time of desulphurisation

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NIT Rourkela

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P=Q/K

Where,

P= Productivity, THM per day

Q= Coke burned, tonnes per day

K= Coke consumed, tonnes per day

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0.8-0.9t0.5-0.6t1.7-1.8t

2500m3

0.6t1t

FuelReducing agent supplyPermeable bed (spacer)

3200m3

+80kg dust

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The efficiency of operation of a blast furnace may be

measured in terms of coke rate which should of course

be as low as possible. The achievement of a satisfactory

coke rate depends on optimising the extent to which the

carbon deposition reaction proceeds. If the top gas is

high in C02 sensible heat is carried from the furnace as a

result of the exothermic reaction.

2CO=CO2+C

If on the other hand the top gas is high in CO, chemical

heat leaves the furnace.

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I d C ib i %

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CO2emission

Industry Contribution %Power 51Transport 16Steel 10other 23

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The purpose of HTP is to introduce more

oxygen to burn more carbon by blowing moreair and at the same time maintaining thelinear gas velocity (and pressure drop)identical to that in the conventional practice

without any formation of channels,maldistribution of gas, increase in coke rateor flue dust emission

Advantages:

For the same volume flow rate, a greater mass of air(hence, oxygen) can be blown with HTP; higheroutput;

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A major benefit that is so obvious is increased

production rate because of increased time of contact of

gas and solid as a result of reduced velocity of gases

through the furnace. Increased pressure also increases

the reduction rate of oxide;

Suppression of Boudouard reaction (C02 + C= 2CO) and

hence savings in fuel;

More uniform distribution of gas velocity and reduction

across furnace cross-section; smoother furnace

operation due to increased permeability;

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less flue dust losses, less variation of coke input, better

maintenance of the thermal state of the hearth, more

uniform iron analysis;

More uniform operation with lower and more consistent

hot metal silicon content have been claimed to be the

benefit of high top pressure;

Bhilai Steel Plant (operative), RSP yet to implement

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SiO2 +C ={SiO} +{CO}

From above equation it can be seen that partial

pressure of SiO can be brought down by increasing

the partial pressure of CO; in other words the SiO2

reduction reaction can be discouraged by application

of top pressure which enables a higher blast pressure

and hence an increase in partial pressure of CO.

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'raceway adiabatic flame temperature

This is the highest temperature available inside thefurnace. There is temperature gradient in vertical

direction on either side of this zone. This temperature

is critically related to the hearth temperature known as

operating temperature of the furnace. It is equally

related to the top gas temperature such that the hot

raceway gasses have to impart their heat to the

descending burden to the extent expected and leave

the furnace as off-gases at the desired temperature.

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The primary purpose of using injectants with the

blast is profitability which depends upon the

relative price of coke and injectants and the

amount of coke that can be saved per unit of the

latter, i.e., upon the replacement ratio:

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H20 + C = CO + H2(1)

HO (1200C) = + 2700 kcal/kg C

Presence of moisture in the blast generates double the

volume of reducing gas per mole of carbon burnt. As per

Eq.1 for every carbon burnt one mole of CO and an

additional mole of hydrogen will be available as product

of burning of coke for reduction in bosh and stack.

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The more the moisture the more will be this additional

hydrogen available. Kinetically hydrogen reduction of iron oxide is faster than

that by CO because of its small size. Presence of

moisture helps to burn coke at a faster rate with its

attendant favorable effects.

Some of the endothermic heat of moisture disintegration

is compensated by way of exothermic reduction of iron

oxide by hydrogen.

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higher gasifying power which intensifies coke

consumption In the raceway;

smoothens the temperature gradient and facilitates stock

descent ;

enlarges the combustion zone and accelerates stock

descent; heats up the axial zone; maintains thermal

state of the hearth;

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even with incomplete temperature compensation, the coke

rate may not rise because of higher reducing power and

higher heat transfer coefficient of hydrogen;

decreases pressure loss due to lower density and viscosity ofhydrogen.

The blast pressure may drop even by0.1-0.2 atm. which

means the furnace can be blown at a higher blast rate.

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It has been estimated that for an increase of 20 g/Nm3

moisture in the blast the endothermicity can be

compensated by a rise of 200C in the blast preheat.

By increasing moisture and compensating it by

additional rise of preheat means that cheaper heat

energy can be used to feed the furnace and thereby

decrease the coke consumption and economise the

operation.

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Oxygen enrichment of the blast and moisture enrichment

have quite opposite thermal effects. The two can be saddled

together to obtain better inputs.

Hot blast temperature, extent of oxygen enrichment and

humidification of blast have to be adjusted as interrelated

parameters simultaneously to obtain optimum conditions of

operation for maximum benefits such as minimum coke rate,

higher productivity and so on.

Th f th i j ti f l h b

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The reasons for the injection of coal have beeneconomic as well as operational flexibility and include

the following: After the steep rise in oil prices following the oil

crisis, iron makers were compelled to abandonheavy oil injection and were looking for a less

expensive auxiliary fuel. PCI accommodate shortages of coking capacity, by

replacing coke by coal in the blast furnaces. After athorough investment analysis, it has been found that

a reliable coal injection system requires much lowercapital cost and involves operating cost than theextension of coking capacity.

.

C l l d ti i fl

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Coal causes a lower reduction in flametemperature per unit injection than oil or natural

gas. It, therefore, allows more scope for blasttemperature adjustment/oxygen enrichment forincreased rates of injection and consequently, lesscoke consumption.

The PCI system design is capable ofinjecting coal on a continuous and stable basis and

ensure accurate and uniform distribution

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The coke savings from fluxed burden emanate from the following

causes :

better reducibility and enhanced indirect reduction (6-7 kg.C saved

from every 1 percent increase in indirect reduction);

use of higher blast temperatures because the thermal load is

smaller and the slag is pre-made; the primary slag melts at higher

temperatures and does so within a vertically narrow softening zone;

avoidance of carbon dioxide generated from limestone in the stack

which adversely affects indirect reduction;

transference of heat of calcination from the furnace to the

agglomerating plant.

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This is a unique design in which

large bell is replaced by a distributor

chute with 2 hoppers

A rotating chute is provided inside

the furnace top cone

Advantages: Greater charge distribution

flexibility

more operational safety and

easy control over varyingcharging particles

Less wearing parts: easy

maintenance

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NIT Rourkela

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Oxidation of carbon:Bottom blowing increases sharply the

intensity of bath stirring and increases the area of gas-metal

boundaries (10-20 times the values typical of top blowing) .

Since the hydrocarbons supplied into the bath together

with oxygen dissociate into H2, H2O and CO2gas

bubbles in the bath have a lower partial pressure of

carbon monoxide (Pco )

All these factors facilitate substantially the formation

and evolution of carbon monoxide, which leads to a

higher rate of decarburization in bottom blowing

The degree of oxidation of metal and slag

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The degree of oxidation of metal and slag

Removal of phosphorous: Since the slag ofthe bottom-blown converter process have alow degree of oxidation almost during thewhole operation, the conditions existing

during these periods are unfavorable forphosphorus removal

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A small amount of inert gas, about 3% of the volume of oxygen

blown from top, introduced from bottom, agitates the bath so

effectively that slopping is almost eliminated.

However for obtaining near equilibrium state of the system

inside the vessel a substantial amount of gas has to be

introduced from the bottom.

If 20-30% of the total oxygen, if blown from bottom, can cause

adequate stirring for the system to achieve near equilibrium

conditions. The increase beyond 30% therefore contributes

negligible addition of benefits.

However at 30% oxygen blowing from bottom leads to formation of

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However at 30% oxygen blowing from bottom leads to formation of

very dry slag and possibility of its ejection during refining unless it is

accompanied by lime also.

The more the oxygen fraction blown from bottom the less is the post

combustion of CO gas and consequently less is the scrap ons of

processing.Blowing of inert gas from bottom has a chilling effect on bath and

hence should be minimum. On the contrary the more is the gas blown

the more is the stirring effect and resultant better metallurgical results.

A optimum choice therefore has to be made judiciously.

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The more the oxygen fraction blown from bottom the

less is the post combustion of CO gas andconsequently less is the scrap consumption in the

charge under identical conditions of processing.

Blowing of inert gas from bottom has a chilling effect

on bath and hence should be minimum. On the

contrary the more is the gas blown the more is the

stirring effect and resultant better metallurgical results.

A optimum choice therefore has to be made

judiciously.

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Better mixing and homogeneity in the bath offer the following

advantages:

Less slopping, since non-homogeneity causes formation of

regions with high supersaturation and consequent violentreactions and ejections.

Better mixing and mass transfer in the metal bath with closer

approach to equilibrium for [C]-[O]-CO reaction, andconsequently, lower bath oxygen content at the same carbon

content

Better slag-metal mixing and mass transfer andconsequently closer approach to slag metal equilibrium

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consequently, closer approach to slag- metal equilibrium,leading to:

o lower FeO in slag and hence higher Fe yieldo transfer of more phosphorus from the metal to the slag

(i.e. better bath dephosphorisation)

o transfer of more Mn from the slag to the metal, and

thus better Mn recoveryo lower nitrogen and hydrogen contents of the bath.

More reliable temperature measurement and samplingof metal and slag, and thus better process control

Faster dissolution of the scrap added into the metal bath

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As compared to top blowing, the hybrid blowingeliminates the temperature and concentration

gradients and effects improved blowing control, less

slopping and higher blowing rates. It also reduces overoxidation and improves the yield. It leads the process

to near equilibrium with resultant effective

dephosphorisation and desulphurisation and ability tomake very low carbon steels.

What is blown from the bottom, inert gas or oxygen?

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How much inert gas is blown from the bottom?

At what stage of the blow the inert gas is blown,although the blow, at the end of the blow, after theblow ends and so on?

What inert gas is blown, argon, nitrogen or theircombination?

How the inert gas is blown, permeable plug, tuyere,etc.?

What oxidising media is blown from bottom, oxygen orair?

If oxygen is blown from bottom as well then how muchof the total oxygen is blown from bottom ?

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The processes have been developed to obtain the combined advantages of

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both LD and OBM to the extent possible. Therefore the metallurgical

performance of a hybrid process has to be evaluated in relation to these

two extremes, namely the LD and the OBM. The parameters on which this

can be done are :

Iron content of the slag as a function of carbon content of bath

Oxidation levels in slag and metal

Manganese content of the bath at the turndown

Desulphurisation efficiency in terms of partition coefficient

Dephosphorisation efficiency in terms of partition coefficient

Hydrogen and nitrogen contents of the bath at turndown

Yield of liquid steel

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The advantages of continuous casting (over ingotcasting) are:

It is directly possible to cast blooms, slabs andbillets, thus eliminating blooming, slabbing mills

completely, and billet mills to a large extent. Better quality of the cast product.

Higher crude-to-finished steel yield (about 10 to20% more than ingot casting).

Higher extent of automation and process control.

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Solidification must be completed before the withdrawal

rolls.

The liquid core should be bowl-shapedas shown in theFigure and not pointed at the bottom (as indicated by thedotted lines), since the latter increases the tendency forundesirable centerline (i.e. axial) macro-segregation andporosity

The solidified shell of metal should be strong enough at

the exit region of the mould so that it does not crack orbreakoutunder pressure of the liquid.

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The surface area-to-volume ratio per unit length ofcontinuously cast ingot is larger than that for ingotcasting. As a consequence, the linear rate ofsolidification (dx/dt) is an order of magnitude

higher than that in ingot casting.

The dendrite arm spacing in continuously castproducts is smaller compared with that in ingot

casting.

Macro segregation is less and is restricted to the

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Macro-segregation is less, and is restricted to the

centreline zone only. Endogenous inclusions are smaller in size, since they

get less time to grow. For the same reason, the blow

holes are, on an average, smaller in size. Inclusions get less time to float-up. Therefore, any

non-metallic particle coming into the melt at the later

stages tends to remain entrapped in the cast product.

In addition to more rapid freezing, continuous castingdiffers from ingot casting in several ways. These arenoted below.

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Mathematically speaking, continuously cast ingot isinfinitely long. Hence, the heat flow is essentially in the

transverse direction, and there is no end-effect as is the

case in ingot casting (e.g. bottom cone of negative

segregation, pipe at the top, etc.).

The depth of the liquid metal pool is several metres long.

Hence, the ferrostatic pressure of the liquid is high

during the latter stages of solidification, resulting in

significant difficulties of blow-hole formation.

Since the ingot is withdrawn continuously from the mould, the frozen

layer of steel is subjected to stresses. This is aggravated by the

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stresses arising out of thermal expansion/ contraction and phase

transformations.

Such stresses are the highest at the surface. Moreover, when the

ingot comes out of the mould, the thickness of the frozen steel shell

is not very appreciable. Furthermore, it is at around 1l00-1200C, andis therefore, weak. All these factors tend to cause cracks at the

surface of the ingot leading to rejections.

Use of a tundish between the ladle and the mould results in extratemperature loss. Therefore, better refractory lining in the ladles,

tundish, etc. are required in order to minimise corrosion and erosion

by molten metal.

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NIT Rourkela

P i l ki i i d f l i

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Primary steelmaking is aimed at fast melting

and rapid refining. It is capable of refining ata macro level to arrive at broad steelspecifications, but is not designed to meetthe stringent demands on steel quality, and

consistency of composition and temperaturethat is required for very sophisticated gradesof steel. In order to achieve suchrequirements, liquid steel from primary

steelmaking units has to be further refined inthe ladle after tapping. This is known asSecondary Steelmaking.

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improvement in quality improvement in production rate

decrease in energy consumption

use of relatively cheaper grade oralternative raw materials

use of alternate sources of energy

higher recovery of alloying elements.

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Lower impurity contents . Better cleanliness. (i.e. lower inclusion

contents)

Stringent quality control. (i.e. less variationfrom heat-to-heat)

Microalloying to impart superior properties.

Better surface quality and homogeneity inthe cast product.

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The term clean steelshould mean a steel

free of inclusions. However, no steel canbe free from all inclusions.

Macro-inclusions are the primary harmfulones. Hence, a clean steel means acleanersteel, i.e., one containing a much

lower level of harmful macro-inclusions.)

I i i i di id

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In practice, it is customary to divide

inclusions by size into macro inclusionsandmicro inclusions. Macro inclusions ought tobe eliminated because of their harmfuleffects. However, the presence of microinclusions can be tolerated, since they donot necessarily have a harmful effect on theproperties of steel and can even bebeneficial. They can, for example, restrictgrain growth, increase yield strength and

hardness, and act as nuclei for theprecipitation of carbides, nitrides, etc.

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The critical inclusion size is not fixed butdepends on many factors, including servicerequirements.

Broadly speaking, it is in the range of 5 to 500

m (5 X 10-3 to 0.5 mm). It decreases with anincrease in yield stress. In high-strength steels,its size will be very small.

Scientists advocated the use of fracturemechanics concepts for theoretical estimation ofthe critical size for a specific situation.

Precipitation due to reaction from molten steel or during

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Precipitation due to reaction from molten steel or duringfreezing because of reaction between dissolved oxygen

and the deoxidisers, with consequent formation of oxides(also reaction with dissolved sulphur as well). These areknown as endogenous inclusions.

Mechanical and chemical erosion of the refractory lining

Entrapment of slag particles in steel Oxygen pick up from the atmosphere, especially during

teeming, and consequent oxide formation.

Inclusions originating from contact with external sourcesas listed in items 2 to 4 above, are called exogenousinclusions.

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With a lower wettability (higher value of Me inc),

an inclusion can be retained in contact with the

metal by lower forces, and therefore, can break

off more easily and float up in the metal. On the

contrary, inclusion which are wetted readily by the

metal, cannot break off from it as easily.

Carryover slag from the furnace into the ladle

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Carryover slag from the furnace into the ladleshould be minimised, since it contains high

percentage of FeO + MnO and makes efficientdeoxidation fairly difficult.

Deoxidation products should be chemically

stable. Otherwise, they would tend todecompose and transfer oxygen back into liquidsteel. Si02 and Al203 are preferred to MnO.Moreover the products should preferably beliquid for faster growth by agglomeration and

hence faster removal by floatation. Complexdeoxidation gives this advantage.

Stirring of the melt in the ladle by argon flowing throughbottom tuyeres is a must for mixing and homogenisation,

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faster growth, and floatation of the deoxidation products.

However, very high gas flow rates are not desirable fromthe cleanliness point of view, since it has the followingadverse effects:

o Too vigorous stirring of the metal can cause

disintegration of earlier formed inclusion conglomerates.o Re-entrainment of slag particles into molten steel.

o Increased erosion of refractories and consequent

generation of exogenous inclusions.

o More ejection of metal droplets into the atmosphere with

consequent oxide formation.

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The varieties of secondary steelmakingprocesses that have proved to be ofcommercial value can broadly be categorisedas under:

Stirring treatments Synthetic slag refining with stirring Vacuum treatments

Decarburisation techniques Injection metallurgy

Plunging techniques

Post-solidification treatments.

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Ladle degassing processes (VD, VOD, VAD)

Stream degassing processes

Circulation degassing processes (DH and RH).

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Why RH-OB Process?

To meet increasing demand for cold-rolled steel sheets with improved

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mechanical properties, and to cope with the change from batch-type to

continuous annealing, the production of ULC steel (C < 20 ppm) is

increasing.

A major problem in the conventional RH process is that the time

required to achieve such low carbon is so long that carbon content at

BOF tapping should be lowered. However, this is accompanied by

excessive oxidation of molten steel and loss of iron oxide in the slag.

It adversely affects surface the quality of sheet as well.

Hence, decarburization in RH degasser is to bespeeded up. This is achieved by some oxygenblowing (OB) during degassing.

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The RH-OBprocess, which uses an oxygenblowing facility during degassing, was originallydeveloped for decarburization of stainless steel byNippon Steel Corp., Japan, in 1972.

Subsequently, it was employed for the manufactureof ULC steels.

The present thrust is to decrease carbon contentfrom something like 300 ppm to 10 or 20 ppmwithin 10 min. Cont

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Simplified by Hiltey and Kaveney

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