2013/06/13 The Death of Coal 1957 R. P. Wolensky et al., THE KNOX MINE DISASTER, PHMC, 1999 Evan McColl and Peggy Seeger, The Ballad of Spring Hill. Dr LJ Erasmus June 2013 Contents Reductants Pyrometallurgy Industrial Processes Electrodes 1
Carbonaceous Reductants Naturally occurring reductants: Coal Anthracite Graphite Processed reductants: Char; Gas coke; Coke Charcoal Graphite SiC Reductants 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 2
Reductant Properties Strength and Friability Coke Strength after Reaction (CSR) Electrical conductivity Control power distribution the furnace. Coke bed Furnace stability Reductant Properties Gaseous Reactivity of areductantwith CO 2 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 3
Coal types Metamorphosis of Coal 4
Coal Pyrolysis Temperature band Reactions Products <120 C Evaporation of water 100-350 C Low T pyrolysis 350-750 C Medium T pyrolysis 750-900 High T pyrolysis 900-1100 Coking 1100-1300 Plasma pyrolysis >1650 C Evaporation of volatile organics Primary degradation Secondary reactions including thermal destruction and repolymerization (T=800 C) Secondary reactions Softning of vitrinite binder phase Gas, tar and liquor Gas, tar, liquor and additional hydrogen, char Gas, tar, liquor and additional hydrogen, char Gas, tar, liquor and additional hydrogen, Coke Acetylene, carbon black (Uneconomic) Char and Gas Coke 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. 5
Coke Heating coking coal blend in the absence of oxygen to above 1100 C. Quality and properties Coal rank Maceraland Mineral matter composition as well as Processing conditions. Coke 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 6
Anthracite Naturally heat and pressure modified coal. Most of the carbon is in aromatic structures. Can be transformed into graphite Graphite Highly ordered form of carbonaceous materials (Synthetic and natural graphites) Limited application as a reductant High cost Limited availability 7
Charcoal 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 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 CO 2 and SO 2 emissions 8
Graphite The overheating of a carborundum (SiC) furnace led to the discovery that by suitable decomposition of a carbide, graphite is left behind. SiO 2 + 3 C SiC + 2 CO SiC Si + C (graphite) 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 CO 2 reactivity 60 40 45 9
Processed Carbonaceous Reductants Type Gas coke / Char Mittal Nut Coke Wankie Coke Proximate Analysis, Air dry % Imported Coke 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 CO 2 reactivity 50 20 10
Discovery Accidentally produced in 1891 He passed a strong electric current from a carbon electrode through a mixture of clay and coke Edward G Acheson He founded the Carborundum Company in September 1891, and filed application for a patent on May 10, 1892. Production Process 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 SiO 2 reacts with Carbon SiO 2 +3C = SiC+2CO 11
Production Process Coke Coal Silica Furnace Crushing Screening Met Grade SiC 85% 15% Crystalline Grade SiC Sublime Technologies 12
Conclusions 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 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 13
Pyrometallurgy Thermodynamics Required reaction conditions To what extent? Energy requirement? Is it possible Reaction Kinetics How long will it take Reaction rate Transport Phenomena How to make it Reactor selection Economics Will it pay 27 Thermodynamics 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 28 14
Reaction Stability Thermodynamics G = H - T S G Gibbs free energy H Enthalpy S Entropy T Temperature H(T) = Cp.dT S(T) = C p /T.dT C p = a + bt+ ct n 29 Extractive Metallurgy 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 30 15
Carbon Fe Cr Si, TiO 2 Al Ca M + O = MO 31 Is Coal Dead? The report of my death was an exaggeration - Mark Twain 16