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Dr LJ Erasmus June 2013 - Fossil Fuel...Dr LJ Erasmus June 2013. African Mineral Reserves Total...
Transcript of Dr LJ Erasmus June 2013 - Fossil Fuel...Dr LJ Erasmus June 2013. African Mineral Reserves Total...
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Ferroalloys
Dr LJ Erasmus
June 2013
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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 )
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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
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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
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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
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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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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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