Ferroalloys

Dr LJ Erasmus

June 2013

African Mineral Reserves

Total mineral reserves 30%

Vanadium 95%

Manganese 82%

Chromite 44%

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Special alloys of iron containing additional

metals

Si, Cr, Mn, Ni, C etc.

Major industrial applications are in stainless

steel making

Confer special qualities to the final alloy.

Ferroalloys

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Range of ferroalloys

Chromium Manganese Silicon Others

Ferrochromium

containing:

Ferromanganese

containing:Silicon metal Ferromolybdenum

More than 4%

of carbon

More than 4%

of carbonFerrosilicon Ferronickel

More than 2%

of carbon

More than 2%

of carbon

Magnesium

ferrosiliconFerrotitanium

1-2% of carbon 1-2% of carbon Ferrophosphorus

Not more than

1% of carbon

Not more than

1% of carbonFerrotungsten

Ferrovanadium

Ferrochromium

siliconSilicomanganese Ferrozirconium

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FeCr: Fundamental Reduction Equations

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( Fe²+, Mg²+) O ( Fe³+,Al³+,Cr³+)2 O3

( Fe²+, Mg²+) O ( Al³+,Cr³+)2 O3

( MgO ( Al³+,Cr³+)2 O3

+ Feº

( Cr²+, Mg²+) O ( Al³+,Cr³+)2 O3

+ Fe – Cr – C

( Cr3C2 + MgO-Al2O3 )

+ Fe – Cr – C

At saturation of Cr + C� (Fe,Cr)7 C3 Starts at 1400°C� (Fe,Cr)3 C2 Starts at 1580°C

Chromite particles - spinel

Reduction of FemOn(Fe³+ ���� Fe²+ ���� Feº )

Reduction of CrmOn(Cr³+ ���� Cr²+ ���� Crº )

Reduction zones in a FeCr furncace

1. Loose Charge

2. Fused Ore+ReductantSlag+Alloy

3. Fused Slag+Alloy

4. Coke Bed

5. Slag layer

6. Alloy

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Gas Phase in Zone 1

Boudouard reaction of C with CO2 to regenerate CO

Fe reduction with CO

Rate limited by diffusion through chromite grains

Reduction Reactions

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Liquid Phase reduction in Zone 4

Temperature ≈ 1700°C

Residence time 20 – 30 minutes

Gas-ferrying Mechanism

– Carbon dissolve in Alloy

– Gasification from Alloy: [C] + CO2 → 2CO

– Gaseous reduction of Cr2O3 in Slag

(Order of magnitude larger than direct gasification of

solid reductant)

Reduction Reactions

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1

3

carbon

slag

gas

Slag phase diffusion

Gas/slag chemical reaction

Gas phase diffusion

C - gas chemical reaction

[C] - gas chemical reaction

Carbon dissolution into Alloy

1

3

Alloy/slag

22

5 5

66

3

2

4

4

1

Alloy

carbon

Carbon/slag

Reduction

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Cross-section of 10.8% FeO slag on graphite plinth 1400°C

Iron

CO Bubble

Graphite

Bubble growth in contact with alloy

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Reductant Alloy Wettability

Wettability plays a significant role in

determining dissolution rates into

liquid systems. This becomes more

critical in defining the dissolution rates

of less ordered reductant sources,

where interfacial reaction can become

the rate-limiting step.

Characterised, or represented by a

contact angle of < 90° when a liquid

exhibits good wettability; or > 90°

when a liquid exhibits poor wettability.

Both melt S content and carbon

structure play key roles in governing

dissolution from disordered carbon

sources

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C detaches “atom-by-atom”/”unzips” into fluid

Carbon dissolution in alloy

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“Ordered”, “graphitic” C = “liquid reactive”:gaseous “inactive”

promotes liquid alloy dissolution

“Less ordered”, “glassy” C = “gaseous reactive”:– retain 2-D planar arrangements of hexagonal aromatic C clusters,

but rings far more restricted in extent.

– Greater abundance of impurity atoms — leads to bonding between planes — 3-D

oxidation & FeO slag reactive

alloy dissolution “inactive”

S is an extremely strong surfactant (surface active agent), increasing contact angle θ, resulting in a decease in liquid reactivity.

Carbon Reactivity

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Coke Bed

Definition An accumulation of reductants on top of the liquid phase. High rank reductants are more likely to reach the reductant bed.

LocationA layer trapped between semi-fused slag and liquid bath (slag & alloy) before separation.

It follow the isotherms in the furnace.

It is possible to remove part of the reductant bed from the furnace during tapping. It is a symptom of eg. low liquid reactivity, bad taphole conditions, an empty furnace etc.

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Reductant Sizing

Change resistivity around electrodes;

– Smaller size reductants increase contact area

between reductant particles.

Control available surface area;

Control reductant bed condition;

Coke Bed

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The Coke bed location

1400°

2000°

1400°

1600°

1600°

1400°

1700°2000°

Reductant with incipient fusion of particles

COKE BED

High slag matrix with

Reductant highly dissolved

Some metal with reductant trapped inside

Intense agitation of bath,

Highest superheat, reactions completed

Potential formation of Si , hence no reductant

left.

1600°

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1400° C

1200° C

1600° C

400° C

1820° C

Coal

Gas Coke

Anthracite

Coke

Reaction Zones

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SAF FeCr smelter

Furnace charge is

preheated in

stationary

shaft kiln.

Smelting furnace is

closed and sealed.

CO-gas is cleaned

and utilised in the

plant.

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Smelting Energy

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DC Furnace

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FeMn

MnO2(s)

Mn2O3(s)

Mn3O4(s)

MnO(s)

CO2(g) + C(s) � 2CO(g)

→CO

Additional CO is generated by

the

Bouduard reaction

1. Not all Carbon forms are “gas

reactive”.

2. This reaction is only required if the

ore needs pre-reduction.

3. The reactivity is determined by

the CRI of the reductant.

↑ CO2

↑ CO2

↑ CO2

→CO

→CO

Solid state reduction in the burden

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Mn furnace

Reaction

zones

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Liquid state reduction

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MnO(l) + C(s) � Mn(l) + CO(g)

Slag containing

MnO, SiO2, MgO,

Al2O3, CaO.

C

COMn(llll)

CO

��

The carbon needs to be reactive in the liquid

phase

�

MnO

Silicon

Multi-step reaction:

Intermediate phases: SiO; SiC; CO

SiO2 + C � Si + CO

Solid-solid reaction

Rate limiting

Slag-less process

Solid-gas reactions

Microsil

SiO(g) + O2 � SiO2 (amorphous)

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Silicon grades

Metallurgical grade Chemical grade

Si 98.5% 99.0%

Fe

Metallurgical grade silicon

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Reaction zones

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Reactions

1500°C

Reductant filter: SiO(g) + C � SiC + CO– Highly reactive carbon – Charcoal @ 1500°C

1500°C - 1700°C

Gas phase reaction: SiO + CO � SiC + SiO2

Condensation: SiO(g) � Si + SiO2 – Carbon deficient – Glass phase slag

>1800°C

Si Production: SiO2 + SiC � Si + SiO + CO

SiO(g) + SiC � Si + CO

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30

Furnace operation

Si & SiO formation near the tip of the electrode

– SiC acts as the reductant

SiO(g) reacts higher in the furnace with CO to form SiC

Reactive carbon reacts in the upper part of the furnace with SiO(g) to form SiC

80% - 90% of energy used to produce SiO(g)

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Shrinking core

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Reductant activities with SiO

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