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Dr LJ Erasmus
June 2013
The Death of Coal—1957
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R. P. Wolensky et al., THE KNOX MINE DISASTER, PHMC, 1999
Evan McColl and Peggy Seeger, “The Ballad of Spring Hill”.
Reductants
Pyrometallurgy
Industrial Processes
Electrodes
Contents
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Naturally occurring reductants: Coal
Anthracite
Graphite
Processed reductants: Char; Gas coke;
Coke
Charcoal
Graphite
SiC
Carbonaceous Reductants
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Traditional selection criteria for ferroalloys production
Proximate analyses
– Fixed Carbon
– Volatile matter content,
– Ash content and chemistry
– Inherent moisture
Petrographic composition.
– Rank - the degree of metamorphism of coal.
– Maceral composition
Reductants
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Strength and FriabilityCoke Strength after Reaction (CSR)
Electrical conductivityControl power distribution the furnace.
Coke bed Furnace stability
Reductant Properties
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Reductant Properties
Gaseous Reactivity of a reductant with CO2
Coke Reactivity Index (CRI)
Liquid Reactivity of a reductant with fully molten
slag & alloy. Wettability– Most important in FeCr.
No standard procedure.
– Difficult to measure.
– Associated with high structural order
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Temperature band Reactions Products
<120°C Evaporation of water
100-350°C Evaporation of volatile organics
Low T pyrolysis
350-750°CPrimary degradation Gas, tar and liquor
Medium T pyrolysis
750-900
Secondary reactions including
thermal destruction and
repolymerization (T=800°C)
Gas, tar, liquor and additional
hydrogen, char
High T pyrolysis
900-1100Secondary reactions
Gas, tar, liquor and additional
hydrogen, char
Coking
1100 - 1300
Softning of vitrinite –
binder phase
Gas, tar, liquor and additional
hydrogen, Coke
Plasma pyrolysis
>1650°C
Acetylene, carbon black
(Uneconomic)
Coal Pyrolysis
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The first stage of coal combustion.
Pyrolysis - heated coal particles are devolatilised
yielding a carbon-rich solid residue.
Char properties
Properties of the parent coal,
Temperature and time history.
Char and Gas Coke
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Heating coking coal blend in the absence of
oxygen to above 1100 °C.
Quality and properties
Coal rank
Maceral and
Mineral matter composition as well as
Processing conditions.
Coke
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Blast Furnace operation
Chemistry, particle size, reactivity (CRI) and
strength after reaction (CSR) are considered as the
most important properties.
Electric furnace coke
Higher reactivity, lower strength and electrical
resistivity
Coke
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Naturally heat and pressure modified coal.
Most of the carbon is in aromatic structures.
Can be transformed into graphite
Anthracite
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Highly ordered form of carbonaceous materials
(Synthetic and natural graphites)
Limited application as a reductant
High cost
Limited availability
Graphite
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Plant-derived biomass material (trees).
As compared to coal
Higher fixed carbon content and reactivity
Lower sulfur and ash contents
High volatile charcoal is less friable but more
hygroscopic and easy to ignite.
Charcoal
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It is a renewable and sustainable resource but is
one of the most expensive raw material.
Applications in metallurgy are considered as
clean technology due to reduced levels of CO2
and SO2 emissions
Charcoal
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Graphite
“The overheating of a carborundum (SiC) furnace led to the discovery that by suitable decomposition of a carbide, graphite is left behind.”
SiO2 + 3 C → SiC + 2 CO
SiC → Si + C (graphite)
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Natural Carbonaceous Reductants
Type Coal Local Anthracite Imported Anthracite
Proximate Analysis, Air dry %
Inh. water 0.3 – 4.0 0.7 – 2.7 3.6 – 4.0
Ash 10 – 21 11 – 22 2 – 10
Volatiles 22 – 35 6.7 – 9.6 2.2 – 7.0
Fixed Carbon 51 – 56 69 – 81 80 – 92
Phosphorus 0.005 – 0.06 0.002 – 0.08 0.002 – 0.009
Tot.Sulphur 0.02 – 0.9 0.60 – 2.2 0.06 – 1.0
Petrographic analysis, %
Rank, Rr 0.6 – 0.75 2.2 – 3.8 3.3 – 5.7
Reactinite 45 – 75 19 – 90 90 - 95
CO2 reactivity 60 40 45
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Processed Carbonaceous Reductants
TypeGas coke /
CharMittal Nut
CokeWankie Coke
Imported Coke
Proximate Analysis, Air dry %
Inh. water 2.7 – 4.8 1.0 – 2.0 1.0 – 1.7 0.3 – 1.8
Ash 17 – 21 15 – 18 12 – 15 11 – 14
Volatiles 1.4 – 10 0.2 – 2.0 1.1 – 1.9 0.9 – 2.3
Fixed Carbon 68 - 75 80 – 84 82 – 85 82 – 88
Phosphorus 0.004 – 0.03 0.002 – 0.012 0.057 – 0.116 0.005 – 0.016
Tot.Sulphur 0.1 – 0.7 0.3 – 0.8 0.7 – 0.8 0.5 – 0.8
Petrographic analysis, %
Rank, Rr 3.9 – 7.0 7.0 – 7.5 6.9 – 7.5 7.7 – 8.6
Reactinite 30 – 70 80 - 84 57 – 68 54 – 86
CO2 reactivity 50 20
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Discovery
Accidentally produced in 1891
He passed a strong electric current from a carbon electrode through a mixture of clay and coke
He founded the CarborundumCompany in September 1891, and filed application for a patent on May 10, 1892.
Edward G Acheson
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Silicon carbide is made today in much the same way as it was in 1891
High purity quartz is mixed with a high quality coke or anhracite in large electric resistance
Reaction temperature > 2000°C SiO2 reacts with Carbon
SiO2 +3C = SiC +2CO
Production Process
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Production Process
15%85%
Furnace
SilicaCoke Coal
Crushing
Screening
Met Grade SiC Crystalline Grade SiC
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Sublime Technologies
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Coke: The best reductant but expensive
Char: Partial substitute for coke, especially in
closed furnaces (low Volatiles)
Anthracite: Partial substitute for coke and total
substitute for char in open furnaces.
A constraint is its friability.
Conclusions
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Coal : Cheap reductant.
Limit set by volatile and carbon combustion
– Furnace hot bed conditions.
– Release of unburned tar
– Practical limit < 30% mass.
New furnace technology to use only coal as reductant
Other issues:
Carbon is not only used for reduction,
but also to control bed resistance.
Graphite and SiC is too expensive for primary smelting
Conclusions
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Thermodynamics Is it possible
Required reaction conditions– To what extent?
– Energy requirement?
Reaction Kinetics How long will it take
Reaction rate
Transport Phenomena How to make it
Reactor selection
Economics Will it pay
Pyrometallurgy
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Metals in ores are generally present as oxides
Oxidising conditions
– M + O � MO;
Gibbs free energy
– ∆G < 0
Reduction
Conditions where ∆G > 0
Carbonaceous reduction
– MO + C � M + CO
Thermodynamics
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Thermodynamics
∆G = ∆H - T ∆S
– ∆G – Gibbs free energy
– ∆H – Enthalpy
– ∆S – Entropy
– T – Temperature
H(T) = ∫Cp.dT
S(T) = ∫Cp/T.dT
Cp = a + bT + cTn
Reaction Stability
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Carbonaceous reduction
– MO + C � M + CO
– ∆G < 0
∆G = ∆H - T ∆S
– ∆H > 0 (endothermic; high energy demand)
– ∆S > 0
High temperature to make the reaction possible
Extractive Metallurgy
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