factors affecting iron quality

FACTORS AFFECTING IRON QUALITY

Quality of Hot Metal may be defined by

• Silicon Content in Hot Metal

• Sulphur Content in Hot Metal

• Temperature of Hot Metal

Silicon in hot metal

Routes identified for transfer of Si to the hot Metal:

1. Direct transfer of Si from molten slag to molten metal through the reaction

SiO2 (slag) + 2 C = Si + 2 CO (g)

2. Indirect route of Si transfer to the molten iron is through SiO formation in the combustion zone as follows:

SiO2 + C/CO = SiO (g) + CO/ CO2 (g)

SiO thus generated reacts with C in metal and gets reduced to Si, which enters the hot metal

SiO (g) + C = Si + 2 CO (g)

As per the investigation carried out by various authors - major portion of silicon transfer to hot metal takes place through this indirect route.

Sulphur in hot metalMost of the sulphur enters the furnace through coke. Coke starts giving off its sulphur in the shaft itself and depending upon the coke characteristics about 5-20 % of the sulphur in this zone. However, bulk of the coke sulphur is liberated at the tuyere level where coke burns forming SiS gas by the following reaction:

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

The SiS gas flows upwards and is absorbed by the slag & the metal droplets in the bosh as follows:

Below tuyere level, as the metal droplets passes through the slag layer, sulphur in hot metal decreases due to following slag-metal reaction:

CaO + FeS = CaS + FeO

SiS (g) + 2Fe = Fe-Si + FeS

This reaction predominantly decides the sulphur content of hot metal. Part of the FeO gets reduced to Fe & dissolves into the hot metal.

VARIATION OF METAL & SLAG SULPHUR ALONG THE FURNACE HEIGHT

Strategies for Obtaining Low Silicon Hot Metal

1. Generation of SiO in tuyere region to be kept low by:

a. Reducing Ash Input Through Reduction in Coke Rate

b. Decreasing Ash Content in Coke

2. Use of burden having higher softening-melting

temperature resulting in low dripping zone

3. Application of high top pressure

4. Decreasing the viscosity of bosh slag by

a. decreasing alumina input through ore beneficiation

b. decreasing bosh slag basicity

Strategies for Obtaining Low Sulphur Hot Metal

1. Use of low sulphur coke

2. Increase in the basicity of bosh slag Use of pre fluxed sinter

3. Decreasing Alumina content of slag by ore beneficiation

4. Stable furnace operation

5. Use of high hot blast temperature coupled with fuel injection

6. Low FeO in slag

Both, the hot metal quality and the blast furnaceperformance, are affected by:

1. Physical & chemical quality of raw materials

2. Blast furnace operating parameters

Apart from these, the most important factor which contribute for

better performance of the furnace and quality of the hot metal,

is its smooth operation.

1. Physical & chemical quality of raw materials

Granulometry (Particle size & Size range)

Physical Strength (Under normal as well as under

reducing condition of the furnace at low/high

temperature)

Softening-melting characteristics of iron bearing

materials

Chemical Quality: chemical composition, reducibility,

reactivity etc.

Consistency

2. Blast furnace operating parameters

Burden Distribution

Blast & Tuyere Parameters

Stock Level & Off-rod working

Casting Parameters

Particle Size & Size Consist

Uniformly sized ore of small diameter are favourable for enhancing blast furnace performance through:

• Enhanced Gas-Solid Contact• Increased heat & mass transfer• Increased reduction rate

But they exerts more resistance to gas flow causing higher pressure drop.

Whereas, Uniformly sized coarse particles exerts lower resistance to gas flow, which is desirable for more production rate.

Therefore, a compromise has to be made between smaller & larger particles.

Physical Strength

1. The burden materials should be resistant to abrasion and posses high crushing strength, so that its size consist suffers as little as possible during transit and handling. (TI, M-10)

2. After charging the burden materials into the furnace, they are subjected to mechanical stresses at higher temperatures in reducing atmosphere. Under such condition new pores are developed which may result in further flaws and failures. Therefore, iron bearing material & coke both must be strong enough to withstand the furnace atmosphere. (RDI, CSR)

3. A further cause of breakdown of coke and other iron bearing material is presence of alkali. Major source of alkali is coke and lime stone. Therefore, low alkali content of coke & lime stone must be selected.

Chemical Quality: Chemistry, Reducibility, Reactivity

1. Richer the ore, i.e. greater the iron content, the more economical the blast furnace smelting.

2. Low alumina/silica ratio is desirable.

3. Higher reducibility of iron bearing materials is desirable, this helps in decreasing the coke rate.

4. For coke, the prime objective is to maximize the carbon and thereby minimize the ash content.

5. Sulphur & alkali content in coke should also be low.

6. Less reactive coke is desirable, as high reactive coke lowers the furnace efficiency.

Consistency

A uniform operation and, therefore, an increased blast

intake, decreased fuel rate and decreased downtime

depend greatly upon the stability of the blast furnace,

which can be ensured by a uniform burden quality and

constant heat supply.

A uniform chemical composition of all the raw materials

including the ash and moisture content of coke assures a

uniformity of softening temperature and reduction of ores,

uniformity of melting of iron & slag formation zones,

permitting a uniformity in the combustion process and gas

through flow.

After fixing the quality of input materials to blast

furnace, following are the important steps for

improving the furnace performance:

Blast Furnace Operating Parametrs

Burden Distribution


Stock Level & Off-rod working

Casting Parameters

It is true that by reducing the range of lump size, gas/solid contact can be improved, but complete elimination of the size variation is very difficult and under that condition the gas will flow preferentially through those regions where it encounters least resistance, i.e. where

1. coke layers are thicker than the ore layer as the coke is more permeable than the iron ore.

2. through concentration of large particles within the ore layers.

So, apart from the quality, the distribution of burden materials also plays a very important role in the granular zone.

Burden Distribution

Stock Level & Off-Rod Working

If the furnace is operated at low stock level, coke rate

gets increased because of following reasons:

• Decrease in gas utilisation

• Increase in amount of FeO reaching the lower region of the furnace

• Increase in direct reduction ratio

• Decrease in hot metal temperature

• Burden profile gets affected


Blast Parameters includes:• Blowing Rate• Blast Pressure • Blast Temperature• Blast Humidity Level• Oxygen enrichment

Tuyere Parameters Includes:

• Number of Tuyeres in Operation • Tuyere Diameter

• Adequate supply of heat & reducing gases for smelting & reduction of iron bearing materials.

• Descend of charge materials.

• Gas distribution.

Above mentioned role of raceway zone mainly depends on its following physico-chemical characteristics:

• Thermal Characteristics • Chemical Characteristics • Geometrical Characteristics

The very first thing which is influenced by these blast & tuyeres parameters in a blast furnace is – Raceway Zone. This zone is very active & is responsible for :

Blast & Tuyere Parameters…

Thermal Characteristics of Raceway

Thermal Characteristics of Raceway is defined by Raceway Adiabatic Flame Temperature (RAFT). This is the theoretical temperature of the gases in the raceway under adiabatic condition and can be computed as follows:

0.9341 tHB + 8208 W - (2402 - 1.2177 tHB) + 94.76

RAFT = -----------------------------------------------------------------------

1 + W + 2

Where, RAFT = Raceway Adiabatic Flame temperature in degree C

tHB = Hot blast temperature, oC

W = Oxygen in blast, Nm3/Nm3 of dry blast

= Blast humidity, Nm3/Nm3 of dry blast


Based on this equation, we find the following:

Increase in blast temperature by 100oC

Increase in RAFT by 76-78oC

Increase in oxygen content by 1%

Increase in RAFT by 48-50oC

Increase in blast humidity by 1%

Decrease in RAFT by 40-42oC


Limits of RAFT

• Lower limit : Below which furnace becomes too cold

to sustain a liquid slag & metal phase.

• Upper limit : Above which furnace starts hanging.


Effect of RAFT on Furnace Indices

• Higher the RAFT, higher will be the thermal efficiency

• Higher RAFT will enhance the heat exchange between gases and liquids resulting a higher hot metal temperature

• Higher RAFT will lead to lower coke rate & higher productivity

• Higher RAFT may also upset the furnace movement


Controlling the RAFT

If RAFT increases beyond certain limit, it can be brought down by:

• Decreasing level of O2 enrichment

• Injecting coolants like steam, coal, natural gas, oil

etc. along with the blast

• Decreasing blast temperature


Chemical Characteristics of Raceways

Chemical characteristics of raceway can be defined

based on composition of gases leaving this zone, which

is a function of :

• Humidity in blast

• Oxygen enrichment of blast

• Auxiliary fuel injection


Humidity in blast does the following:

Helps in decreasing the flame temperature

Gives hydrogen, which is a better reducing agent Increase the amount of carbon burnt at the tuyere

thereby increases tuyere gas volume (per Nm3 of blast)

Oxygen enrichment in blast does the following:

Increases coke burning intensity & thereby increases hot metal production rate.

Decreases the amount of nitrogen in the blast & consequently decreases the volume of air required for burning 1 kg of carbon

decreases tuyere gas volume (per kg of C burnt))



Geometrical Characteristics of Raceways

The geometry (which includes dimension, shape and

location) of the raceway zone is important as it greatly

affects the movement of charge materials, the gas flow

distribution over the furnace cross section and the

occurrence of physico-chemical processes such as

combustion, reduction, melting, heat exchange etc.

This is strongly affected by blast as well as the tuyere

parameters and the availability of tuyeres.

Casting

All steps are to be taken to tap the hot metal & slag at regular

interval to maintain stable operation of blast furnace. Any

disruption in casting may lead to:

• Wind reduction

• Loss of production

• Increase of heat losses lowering of hot metal temperature

Change in Operating Parameters Change in HM Production, %

Change in Coke Rate, %

1% decrease in furnace stoppages +1.0 - 0.5

100oC increase in hot blast temp. + 2.0 - 3.3

0.1 atm increase in top gas press. + 1.1 - 0.5

1% decrease in coke ash + 2.0 - 2.0

1% decrease in M-10 + 2.5 - 2.0

1% increase in Fe in burden + 2.0 -1.8

1% decrease in burden fines (-5mm) + 1.0 - 0.8

10% decrease in not-dry working + 0.3 - 1.0

1% decrease in off-rod working + 0.1 - 0.2

10% increase in sinter in burden + 2.0 - 4.0

1% O2 enrichment + 3.0 -

10 kg decrease in lime stone rate + 0.4 - 0.35

The combined effect of all the above mentioned operating parameters together has been taken as 70% of the total change in production & coke rate, which results in increase in productivity by 12.0% & decrease in coke rate by 11.5%. .

Techno – Economics At a glance, the effect of various operating parameters on blast furnace performance indices in terms of production & coke rate are as follows:

Conclusions

1. Consistency in quality of input materials with regards

to size & chemistry improves the furnace

performance and directly influences the overall

techno-economics.

2. Thermal stability & consistency of input materials are

essential for decreasing Si in hot metal.

3. Silicon in hot metal can be controlled by decreasing

coke rate/coke ash and decreasing the slag rate.

Thank you

factors affecting iron quality

Documents

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sulphur content of hot

hot metal decreases

slagmetal reaction

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metal droplets