5 heat Transfer - How are pyrometallurgical reactors...

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1 Heat Transfer How are pyrometallurgical reactors heated? • Combustion of fuel (solid or liquid or gas) • Combustion of sulphide ore • Electrical (arc or slag resistance) • Gas preheating • Indirect heating Modes of heating Conduction Convection Radiation Conduction • Transfer of vibrational energy from one molecule to another, in direct contact • Fourier equation Q = - kAdT / dx • k = ? W/mK for various materials • Composite conduction • Steady-state (as opposed to unsteady state) • Radial conduction in a cylindrical body Thermal conductivity, W/mK (Typical values given by Rosenqvist) Ag 427 Cu 400 Al 236 Steel 50 Stainless steel 15 Brick 0.5 – 1.0 Sand 0.6 Asbestos 0.1 Stagnant air 0.025 Thermal conductivity Solids k, W/mK Liquids k, W/mK Al, 300ºC 230 Water, 30ºC 0.62 Cu, 100ºC 377 Water, 60ºC 0.66 Fe (wrought), 18ºC 61 Mercury, 28ºC 8.36 Steel (1% C), 18ºC 45 Pb, 100ºC 33 Gases k, W/mK Ni, 100ºC 57 Hydrogen, 0ºC 0.17 Stainless steel, 20ºC 16 Nitrogen, 0ºC 0.024 Asbestos, 100ºC 0.19 Oxygen, 0ºC 0.024 Alumina brick, 430ºC 1.8 Air, 0ºC 0.024 Building brick, 20ºC 0.69 Air, 100ºC 0.031 Magnesite, 200ºC 3.8 Water vapour, 100ºC 0.025 Glass, 30ºC 1.09 CO 2 , 0ºC 0.015 Magnesia (85%) 0.07 Graphite, 0ºC 151
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2 Thermal conductivity … • Metals have very high thermal conductivities, non-metallic solids lower values, non-metallic liquids low values, and gases very low values • Stainless steel has a low value amongst metals • Water has a high value for liquids (due to partial ionization) • Hydrogen has a high value for gases (due to the high mobility of the molecules) Conduction of energy through a plane wall T 1 T 2 x Fourier equation: = dx dT A k Q () k x A T T Q 2 1 = x / k is known as the thermal resistance k Conduction of energy through a composite wall x 3 x 2 x 1 k 3 k 2 k 1 T 3 T 2 T 1 T 4 Q A k x T T 1 1 2 1 = Q A k x T T 2 2 3 2 = Q A k x T T 3 3 4 3 = By addition: Q A k x A k x A k x T T + + = 3 3 2 2 1 1 4 1 ) ( area) / resistance (thermal Total force driving Total = Σ
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5 heat Transfer - How are pyrometallurgical reactors...

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