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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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Coal types

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Metamorphosis of Coal

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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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Fe

Cr

Al

Ca

Si, TiO2

M + O = MO

Carbon

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Is Coal Dead ?

“The report of my death

was an exaggeration”

- Mark Twain

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