Iron Making by Blast Furnace

Presentation at EDRC - Kolkata on 14th Feb 2007 by N. R. Sudheer

Steel Making Value Chain

Steel making flow lineSteel making flow line

Steel making flow line - FinishingSteel making flow line - Finishing

History and evolurtion

What is Iron making?

• Winning of Iron metal from its Ores.

• Smelting reduction of Oxide ores with Carbon as a reducing agent.

Reduction of Iron• Reduced from Ore in two ways.

• In Solid State- Mixed with slag and impurities such as Silica and Alumina and residual carbon (Puddle Iron with Charcoal as reducing agent / Sponge Iron with Coke breeze as reducing agent) – generally classified as Direct reduction and

• In Liquid State- at High temperatures above the melting point of Fe and at high pressures with a combination of reduction mechanisms- directly by Carbon and Indirectly by Carbon Monoxide. (Predominantly by Indirect means)

Tonnage Steelmaking in Integrated Steel Plants

• Done in Two Stages.

• First Stage- Iron Smelting / Iron making – To produce Liquid Iron that is impure and saturated with Carbon and other contaminant elements.

• Second Stage- Steelmaking / Refining of Liquid Iron by Oxidising the Impurities in Basic Oxygen Furnaces to produce Steel of required qualities.

Modern Ironmaking Basics

Iron making basics• Making iron is the first step in producing steel• Iron should be produced of consistent quality.

• C= 4.5 -5 %, Si =0.0.4 –1..0%, Mn = 0.2-0.3%, S= 0.015- 0.040% , P=0.080-0.090%, Rest Fe.

• Blast furnace is a reactor to produce iron. Ore reaches molten state and gets separated physically due to density from gangue and impurities.

• Presence of excess carbon reduces the melting point of iron to 1150oC.

• Iron purity is controlled by controlling the composition of the slag.

Liquid Iron – Pig Iron – Hot metal

• Pig Iron – allusion to suckling Pigs in Sand molds of a cast floor.

When cast into standard molds for merchant foundries- known as Pigs on solidification.

• Integrated Steel Plants refer to Liquid Iron as Hot metal. An Intermediate product which feeds the Refining stage of Steelmaking.

Bulk of the Hot metal is produced by Blast Furnaces

• Blast Furnace – a Counter current Gas Solid / Liquid reactor operating at high temperatures and pressures.

• Solids charged from the top. Reducing gases generated at the bottom by combustion of Carbon either as Charcoal or Coke

Blast Furnace• A Refractory lined Vertical Steel

shaft with a closure at the top to admit solid reactants (such as Iron ores, Fluxing Limestone and Coke)

• Ports below to admit Hot air to burn Carbon for generating reducing gases.

• The solid raw material above gradually descends to replace the voids created by gasifying carbon.

• The solids soften and melt along the way as they get heated up and are fully liquefied by the time they reach the bottom.

• Difference in Densities of Liquid metal and Slag makes it easy to be drained out separately.

Why the Shape of Blast Furnace?

Throat – 180 to 300 deg C -Drying

Stack – 300 to 650 deg C – Swelling and Mild reduction, decrepitation

Parallel Bosh – 650 to 950 deg C- Intensive reduction

Lower Bosh – 950 to 1800 deg C –melting and dripping zone

Hearth – 1400 to 1350 deg C – Pool, slag metal separation

Blast Furnace Profile and Logic

Role of Coke

• Supply Heat needed for the process.-- Drying- Calcination of Fluxes- Reduction of Iron Oxides- Slagging of Gangue with Fluxes- Compensate losses in cooling, radiation and sensible losses.

• Act as Spacer and Support the Burden.

Coke Rate• Coke Rate or Carbon Rate is a function of

– Raw Material,Flux and Coke chemistry and level of impuruties.

– Higher the impurities more the Carbon rate.– Mass and Heat Balance determines the CR– Typical Iron / Carbon Ratios 1.3 in India to 2.2 in

European countries.

– Typical Indian Coke has 20% to 23 % Ash– Typical Ash composition is 55% SiO2 and 30 % Al2

O3

Material Bins

Ore Fluxes Coke Return Fines

Lime

Mixing and Nodulizing Drum

Sinter Machine

Sinter Cooler

Screening-Station

Waste Gas Fan andGas Cleaning

Water

Hearth Layer

Return FinesProduct Sinter

Sintering Process

Pelletisation

Iron Balance

• Fe Input from =Ore (~ 63% ) +

Sinter ( ~ 47 – 54% ) + Pellets ( ~ 60- 63% )+ Coke (~ 2% )

• Fe Output in Hot Metal ( ~93 – 94 % )+

Slag as FeO (~ 1.0% )+ Flue Dust as Fe2O3 + Fe in Runner scrap

Iron Ore required per tonAssume • Iron content in Ore at 65%• Hot Metal has 94% Fe• Loss in Flue dust and Slag at 1%• Loss in Runner at 1%

Iron ore per ton of HM = 940/(0.9 *0.9*0.64) kg = 1498 kg

Lime Stone required per Ton

Typical AnalysisCaO - 45 %; SiO2 – 4%; Al2O3 – 1.3%; MgO – 5%Assume Slag Basicity = CaO / SiO2 = 0.95

Fluxing Efficiency = 100 / ( CaO – 0.95* SiO2) = 2.43

Every Kg of SiO2 added to BF needs 2.3 Kg of Limestone to flux and generate slag of required Basicity.

Carbon Balance

• C input from =Coke ( ~ 77 – 82 % ) +

Injected Coal, if any +

• C Output in Hot Metal ( ~ 94% ) +

in Gas as CO and CO2 +

Oxygen Balance

• O2 input from =Air +

Injected O2 + From Ore One Kg of Carbon in hearth needs 3 Nm3 of Air to

burn.

O2 Output in BF Gas as CO and

CO2

Nitrogen Balance

• N2 Input from Air =

(79% typical )

Ratio of Top Gas Volume to Blast Volume

• N2 Output in Top Gas ( 56% typical )

= 79 / 56 = 1.41

Blast Furnace Reactions

• C + O2 CO2

• CO2 + C 2 CO

• Fe2O3 + 3 CO 2 Fe + 3 CO2

Hearth Area

• Carbon Burning Intensity ( CBI )

– Carbon burnt in Kg per hour per sq. m

Normal Top Pressure - 750 kg/hr/m2

High Top Pressure - 1000 kg/hr/m2

Blast Volume

= (Carbon burnt / hr ) x ( 3 Nm3 per kg )

Hearth Area.

= Hotmetal -tpd* Coke rate * FC in Coke / 24 hrs/ CBI

Ex: Vizag BF ~ 6650 tpd / 550 kgpthm / FC –84% = 130m2 area = 13m hearth dia

Productivity

• Tons of HM per m3 of Useful Volume.

Typically 1.0 for Carbon rates of 750 kgpthm For Carbon Rates of 400 kgpthm ~ 700/400

For European CR of 280 – 300 kgpthm ~ 2.5 to 2.3

Furnace Volume

= Tons of Production per day / productivity

No of Tuyeres

= Perimeter of Tuyere tip circle in mm / 1100 mm

Tuyere tip circle dia = Hearth Dia in mm – 500 mm

No of tuyeres are in even numbers

Tuyere sizes

• Tuyere Area on basis of 220 – 250 mps of air velocity.

BF Cooling and Lining

• Water cooling- Protect Shell– Recirculating– Evaporative– Plate / Stave

• Lining - Protect Shell – Fire clay– Silica / Alumina /Mullite– Silicon Carbide– Graphite/ Carbon

Top equipment• Blast furnace can have either a Bell top (Double bell

system with or without movable throat armour) or a Bell less top.

• Double bell system has a small and a big bell/ small bell arranged one below the other.

• Hoppers present at the bells store the raw materials charged. Either one of the bells are kept closed at a time to prevent the escape of gas.

• Modern furnaces use BLT (Bell less top system)

Bell less top arrangement

Rotary Charging System

Stock house• Raw materials of different types are brought from

respective plants say coke from coke oven plant, sinter from sinter plant via conveyor belts and stored here.

• It receives, stores, screens, weighs and charges the raw material.

• Typical Stock house Assembly:• Conveyor 1: Feeding iron ore & additives from ore

handling plant• Conveyor 2: Feeding two fraction of coke (34-60 mm & 60-80 mm) from coke ovens.• Conveyor 3: Feeding Sinter from Sinter plant.

Typical stock house assembly

Stock house• The fines are separated while screening coke (-8 mm),

sinter (-5 mm), iron ore (-8 mm) and carried away by a separate conveyor to Sinter plant base mix yard.

• Bunker outlets will have Vibro-feeders and Vibro-screens fro smooth flow of burden material.

• Design specifications for 8000 t/d blast furnace:

Storage HoursCoke 8-10

Sinter (two sizes 5-15 mm & 15-40 mm) 20

Sized iron ore 16-20

Additives 24

Fines (sinter -5mm, iron ore -8mm, coke breeze -8mm) 8

Gas Cleaning Plant

Scrubber Construction

Recycle Water Treatment

Hot Blast System

• Recuperative

• Regenerative

Hot Blast Stoves

• Internal Combustion Chamber

• External Combustion Chamber

General Arrangement of internal Combustion Chamber Hot blast Stove

Thank You