1.

Introduction to iron making

Ironmaking

Ironmaking is the extraction of metallic Iron from the ores by preheating, reducing and finally
smelting them in a furnace (Blast furnace) in the presence of a Fuel (metallurgical coke) and a Flux
(Lime or Limestone).

All the impurities present in the ore are oxidized and joins the slag. All the reduced metal joins the
melt. The metal called Pig iron consists of nearly 92-95%Fe and balance is impurities like C, Si, Mn, P
and S.

List of various types of iron ores, their chemical formulae, composition, colour and specific gravity

Iron ores Colour Formula Chemical composition Specific gravity

Magnetite Black Fe3O4 70 to 75% Fe, Balance - gangue 7.4
Hematite Red Fe2O3 65 to 70% Fe 5.2
Balance – gangue
Limonite Brown 2Fe2O33H2O 55 to 60% Fe 3.9
Balance – gangue
Goethite Brown Fe2O3H2 60 to 65% Fe 4.2
Balance – gangue
Siderite Pale yellow FeCO3 45 to 50% Fe 3.8
Balance – gangue
Pyrites Greenish FeS2 40 to 46% Fe 5.0
Balance – gangue
Ilmenite Black FeOTiO2 30 to 40% Fe,20 to 27% TiO2 4.8
Balance – gangue

Classification of Iron ores

Iron ores can be classified based on colour, gangue and impurity elements present
• Based on color

Black ores - Magnetite

Red ores - Hematite
Brown ores - Limonite
Green ores – Pyrites

• Based on gangue present in the ore

Siliceous ores - Main impurity is silica (SiO2)

Calcareous ores - The gangue is lime (CaO)
Aluminous ores - Main impurity is alumina (Al2O3)
Argillaceous ores - The impurity is clay
Bituminous ores - Contains largely coaly matter

• Based on the impurity elements present

Phosphoric ore – contains Phosphorus (P)

 Sulphurous ore – contains Sulphur (S)
Arsenical ore – contains arsenic (As)
Titaniferous ore – contains Titanium (Ti)

Evaluation of ores
It is an estimation of the worth of ores by taking into certain factors. Evaluation guides us in
making a judicious selection of ores for the more economical extraction of metals.

Importance of evaluation

Evaluation determines
 whether mining of ore is profitable or not
 The Fuel and Flux requirements basing on the % of Fe
 Composition of slag made
 The burden preparation

Factors in valuation

The factors involved in valuation of iron ores are

• Richness
• Location
• Composition of gangue
• Treatment and preparation required

Richness

Richness is the percentage of metallic Iron (% Fe) the ore contains. As richness increases, the
valuation also increases.
The higher the % Fe present in the ore, the lesser will be the ore required to produce one ton of
metal to produce one ton of metal,
example: 3 tons of U.K. ore (35%Fe),
2 tons of Indian ore (60% Fe) or
1 ton of Australian ore(70%Fe) is required.
For rich ores (high % Fe) the Fuel and Flux requirements will be lesser. For lean ores (low % Fe) the
consumption will be more

Location

Both the Geological and Geographical location is an important factor. Geological location means
whether the ore deposit (mine) is underground or open cast i.e. in the form of a hill. Geographical
location is the distance; the ore deposit is situated from the smelter. It is also related to the method
of transportation to bring the ore to the smelter.
The ore mine has more value if it is in the form of a hill and situated near the smelter

Composition of the gangue

The impurities present in the ore may enhance or reduce the value of an ore. The entire amount of
phosphorus present in the ore is reduced and joins the metal.
The alumina in the ore joins the slag and increases its melting point, which result more fuel
consumption.
The alkalis in the ore tend to eat away the furnace lining and causes difficulties in the furnace
operation

The lime and magnesia in the ore act as flux and reduces the consumption of flux Silica, MnO and
sulphur in the ore, partially reduce during smelting and joins the metal.

The value of an iron ore is reduced by the presence of alkali, phosphorus and alumina. The value of
an ore is enhanced by the presence of lime and magnesia

Treatment and preparation

The treatment and preparation of the ore needs before it is charged in to the furnace determines
the value of the ore.

 Dense ores need crushing and breaking.

 Fine ores will have to be agglomerated
 Carbonate ores needs calcinations
If preparation needed increases the value of the ore decreases

Estimated Reserves of Iron Ores in India, in M.T

Noamundi & Joda Jharkhand 977

Bolani Bihar 610

Barsua & Banspani Orissa 350

Dalli & Rajhara Madhya Pradesh 1250

Kiriburu Bihar 480

Kemmenjundi Karnataka 300

Bailadila Chattisgarh 830

Goa Goa 550

Kudremukh Karnataka 250

Bellary & Tumkur Karnataka 180

The Integrated Steel Plants in India

Steel plant state Iron ore

Tata iron and steel plant(TISCO) Jharkhand Noamundi

Indian iron and steel plant(IISCO) Burnpur W. B Manoharpur (Rajasthan)

Visweswarayya iron and steel Ltd(VISL) Karnataka Kemengundi (Karnataka)

Durgapur steel plant(DSP) W.B. Bolani (Orissa)

Rourkela steel plant(RSP) Orissa Barsua (Orissa)

Bhilai steel plant(BSP) M.P. Dallirajhara (Chattisgarh)

Bokaro steel plant(BSL) Bihar Kiriburu (Jharkhand)

Visakhapatnam steel plant(VSP) A.P. Bailadila

Salem steel plant(SSP) Tamilnadu Salem, Tiruchi

Need of Ore Preparation
The ore from the mines cannot be charged directly into the blast furnace since the ores may
 Contain moisture and combined water,
 Lumpy in size,
 Contain harmful elements like S, P, As, etc.
 Be dense and less porous
Therefore, the first step in the extraction of any metal from the ore is ore dressing or ore
preparation. Ore preparation is a process of mechanically or chemically separating the grains of
gangue minerals from the ore to produce an enriched ore concentrate and making the ore physically
and chemically suitable for the extraction of metal

Objectives of ore preparation

 To get uniform size of ore particles
 To enrich the % of metal in the ore and to separate the impurities from the ore
 To make the dense ores more permeable
 To utilize the fine ores generated
 To obtain good reducibility of the ore

Different methods of preparation of iron ores

Depending on the quality, characteristics, and properties of the ore deposit a number of methods
are used for preparing the ore for charging in to the furnace. Some important ore preparation
methods are
• Drying
• Weathering
• Comminution: 1. crushing
2.grinding
3.milling
• Concentration:
1.Gravity concentration
2.Magnetic concentration
3.Flotation
• Calcination
• Roasting
• Agglomeration

Drying To remove moisture & combined water

Weathering For removal of Sulphur from the ore
Crushing It is a Preliminary treatment and used to get
uniform size of ore
Magnetic concentration To separate non-magnetic particles from the ore
Gravity concentration To remove impurities with different densities
Ex: Jigging, Tabling.
Flotation To remove impurities with different surface
properties
Calcination To remove carbon di-oxide and combined water
To make dense ores more permeable
 Roasting To convert the ore chemically
To remove Sulphur, oxygen from the ore
Agglomeration It is a process of clustering fine ore particles into
small lumps or balls ultimately making the ore
particles as a suitable feed for blast furnace

Agglomeration
• Agglomeration may be defined as a process of converting fine ore particles into a
porous mass with or without the use of a binder or heat.

Purpose of agglomeration

 To utilize the fines of iron ore since they have a very good percentage of iron, they
cannot be thrown as waste
 To convert them into a porous mass
 To prepare a suitable feed for furnace

Sources of Iron ore fines

Lot of iron ore fines are obtained from

• screenings of good grade ores
• beneficiation of lean ores
• collection of flue dust
• pyrite residue
• mill scale

Methods of agglomeration

• Briquetting
• Nodulising
• Sintering
• Pelletizing
Briquetting
• Pressing of ore fines into briquettes by using organic or inorganic binders.
• Hydraulic or mechanical presses are used for pressing

Briquetting press Iron Briquettes

Nodulising
 • The fine ores along with some carbonaceous material is passed through a kiln to convert
them into nodules.
• The temperature inside the kiln is just sufficient to soften the ore

Both Briquetting and Nodulising are not popular and not used extensively, sintering and Pelletizing
are widely used and are in commercial practice

Sintering

Sintering is the agglomeration of fine ore particles into a porous mass by incipient fusion (i.e.
temperature little below melting point) caused by heat produced by combustion within the mass
itself

The material to be sintered, together with Sufficient combustible matter is placed on a grated
bed through which air can be sucked. The material is ignited and sintering of the mass proceeds
automatically

Raw materials for sintering

• Iron bearing fines

• Coke breeze
• Lime powder
• Sinter returns

Principle of sintering

In sintering, a shallow bed of fine particles is agglomerated by heat exchange and partial fusion of
the ore fines. The bonding is effected by a combination of fusion, grain growth and slag liquidation

process
• Sintering is carried out by putting a mixture of iron bearing fines mixed with solid fuel on a
permeable grate
• The top layer of this bed is heated to sintering temperature 1200 – 1300 0C by gas or oil
burner
• Air is drawn downwards, through the permeable grate, with the help of exhaust blowers
• Combustion takes place between the carbon of coke breeze and oxygen of air
• The narrow combustion zone developed initially at the top layer, travels downwards layer by
layer
Cross-section of Sinter Bed After Ignition

• The cold air drawn through the bed cools the top sinter layer and gets itself heated
• The hot blast is used in drying, preheating and burning the lower layers of the bed
 • In the combustion zone, bonding takes place between the ore particles and a strong, porous
aggregate is formed
• The sintering process is finished when the combustion zone has reached the lowest layer of
the bed
• The sinter cake is broken, screened and cooled
• This process is a down draught sintering

Factors affecting sintering

• The porosity of the charge

• The suction applied to draw air through the mass
• Time for sintering
• Proper ignition

Porosity of the charge

The porosity depends on

• Moisture - Addition of little moisture improves porosity

• Returned fines - These returns fuse more readily than the original charge
• Coarse material - Mixing fine ores with mill scale, flue dust improves the porosity

The suction applied to draw air through the mass

• Since sintering is an oxidizing process, the volume of air passing through the charge and
rapidly to produce the necessary heat is a very important factor, which can be ensured by a
proper suction

Time of sintering

• The time taken for the sintering zone to travel through the charge should be 10-18 minutes

Proper Ignition

• Method of igniting the charge should be quick and uniform. The time for ignition is 30
seconds maximum

Types of sintering machines

• The Green-Walt sintering machine

• The Dwight-Lloyd sintering machine
Sintering of iron ore fines is universally carried out by Dwight-Lloyd machines. The production
capacity of these machines vary from 4000 to 24000 t/day.

Dwight-Lloyd sintering machine

Construction

The important parts of the machine are

• Storage bins, mixer and feeder
 • Pallets
• Ignition hood
• Discharge screens
• Wind boxes

Dwight-Lloyd sintering machine

Storage bins, mixer and feeder

• The raw materials like iron ore fines, coke breeze, reclaimed flue dust, lime powder, sinter
returns, etc. are stored in these bins.
• The charge materials in weighed quantities are mixed with sufficient amount of water in a
mixer drum.
• This mix is laid evenly across the whole width of the pallet through a vibrating feeder

Pallets

• Pallets are rectangular boxes with permeable grate

• They are 2 mt. length, 1 mt. width and 600mm depth
• They are charged up to a depth of 350 – 450mm
• The pallets have to support the mix during its travel from feed end to discharge ends
• They must withstand the high temperatures of sintering
• The grate bars should provide maximum openings without allowing the fines into wind
boxes

Ignition hood

• The productivity and quality of sinter depends on proper ignition of the top layer of the
charge
• Gas or Oil fired burners are provided for igniting
• The method of igniting the charge should be quick and uniform.
• The temperature in the hood is maintained at around 12500 C

Wind boxes
• Wind boxes are provided for uniform suction of air across the width of the machine
 • Depending on the thickness of the bed, quality of feed, ignition temperature, amount of fuel
in the charge mix, etc. an exhaust blower of suitable size is provided

Operation of the Dwight-Lloyd sintering machine

• It is an endless band of pallets moving over rails, stretched over two huge pulleys driven by
motor
• The pallets travel continuously between charging hoppers to discharge screen through the
ignition hood

• The charge, mixed with sufficient amount of water is fed into the pallets at the charge end.
• The top layer of the charge is ignited as it comes under a fixed ignition hood
• Because of the ignition temperature, the fine ore particles are fused together to form a
sinter cake in the top layer
• As the pallets move over a series of stationary wind boxes connected to an exhaust blower,
the combustion zone moves downwards and sinter cake is formed down to the grate
• Sintering is completed by the time the pallets reaches the discharge end
• The sinter cake is discharged into a crusher
• The sinter cake is broken, screened and the over size is cooled
• The under size i.e. – 9mm, is returned to the storage bins for re-sintering
• From the discharge end the empty pallets return to the feed end

Types of sinter

Based on the addition of flux in the charge mix, the different types of sinter are

• Acid sinter
• Fluxed sinter
 • Super fluxed sinter

Acid sinter

• No flux in the sinter mix. The basicity of the mix is less than one
• Required amount of flux is added in the furnace
• The production of this type sinter is declining

Fluxed sinter

• The amount of flux added in the sinter mix is just equal to the amount of gangue present in
the sinter mix
• The basicity of the mix is one
• This type of sinter is used when the furnace is charged with 100% sinter
• If ore is charged into the furnace, separate flux is required

Super fluxed sinter

• The amount of flux, added in the sinter mix, will take care of the gangue in the sinter mix as
well as the gangue in the ore
• The basicity of the mix is more than one
• When this type of sinter is charged in the furnace, no additional flux is required

Pelletisation

Pelletisation is the formation of green balls by rolling fine iron bearing material with critical amount
of water and additives

• The green balls of 5 – 20mm size are then dried, preheated and fired at 1250 – 1350 0 C
• At such high temperatures bonding is developed between the fine ore particles
• Surface tension of water in contact with the particles plays a dominant role in binding the
particles together
• Additives like Bentonite, lime or dolomite up to 0.5 -1.0% may be added for improved
bonding

Steps in Pelletisation

• Feed preparation
• Green ball production and sizing
• Drying, preheating and firing of green balls
• Cooling of hardened pellets

Feed size

• The feed size should be -100 mesh, with more than 60% as -350mesh
• The size and shape of feed particles leads to maximum mechanical interlocking and surface
tension to obtain strong green pellets

Green ball production

Two important methods of green ball production

 • Disc Pelletisation
• Drum Pelletisation

Disc pelletiser

• It contains a disc with an outwardly sloping and rotates around its own center
• They are 3.6 – 5.6 m in diameter and inclined at45o to the horizontal
• The charge is directly fed onto the disc and moisture level is made up with the water sprays
• Seeds of green balls are immediately formed with the addition of water
• These seeds grow in size due to the centrifugal force of the disc
• The green balls rise on the inclined surface of the pelletiser in the direction of rotation and
fall down against the toe section of the disc

Disc pelletiser

• Pelletisation can be carried continuously on a disc by balancing the discharge of green balls
with an equivalent addition of feed
• The production rate of balls depends on
Diameter of the disc
Angle of inclination
Speed of rotation
Height of the peripheral wall
Rate of moisture addition

Drum pelletiser

• The drum pelletiser is a steel drum, open at both ends

• It is 2- 3 m in dia and 6 – 9 m in length
• It is inclined at 2 – 10 0 to the horizontal and rotates at 10 – 15 rpm
• The charge is fed at the higher end, where water sprays are located
• The material rolls over the surface and slides downwards due to inclination
• The motion of the balls is called cascading and the size range is 10 – 16 mm
• The production rate of balls depends on
 Speed of rotation
Angle of inclination
Diameter of drum
Rate of feed
Moisture content

Formation of Green Balls in The Drum

Drying and Firing of Green Balls

 • The green balls produced in either Disc or Drum are dried by passing hot air through the bed
of balls laid on a grate

• During drying the water is removed from the balls and they got strength due to mechanical
interlocking

• The dried balls are further hardened by firing them at a temperature of 1250 – 1350 0C

Iron pellets

Sintering Vs Pelletising

S.No. Sintering Pelletisation

1 Sintering is preferred for ore fines of – 10 mm to Pelletising is preferred for ultra-fines of -

+100 mesh size 325 mesh size

2 The porosity of sinter is 10 – 18% The porosity of pellets is 20 – 30%

3 The shape of sinter is irregular Pellets are spherical in shape

4 The bond formed in sintering is either diffusion In Pelletisation the bond is due to
bond or slag bond oxidation and mechanical interlocking
of ore particles

5 Self-fluxed sinter is a common practice Self-fluxed pellets are not yet in

practice

6 The coke breeze in the sinter charge may contain Using pellets in the blast furnace
high ash and effects the coke rate and burden, results increased productivity
productivity of the furnace and lower coke rate

7 Sintering process is not lengthy and the Pelletisation process is comparatively

installation costs are comparatively low lengthy and the installation cost of
equipment are more

Applications of sinter

• Sinter is used as a raw material in blast furnace.

• However, the percentage of sinter in the blast furnace burden may vary from 50 – 100%,
depending on the composition of coke breeze and percentage of flux

• If the coke breeze has low ash content,100% sinter can be charged in the furnace

• The Indian cokes have 25% ash, therefore with increasing percentage of sinter in the burden
will adversely effects and reduces the furnace productivity