7 Roasting - Solid-state pyrometallurgy Metal-bearing...

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1 Roasting Solid-state pyrometallurgy Metal-bearing species remain in solid phase Gas-solid reactions only Drying – thermal removal of water from solids Sintering – partial fusion to form an agglomerate Calcining – thermal decomposition of solids for the removal of H 2 O, CO 2 , or other gases – converts hydrates and carbonates to oxides Reduction – pre-reduction partially changes metal oxides to a more reduced state – direct reduction completely reduces metal oxide to metal Roasting – converts sulphides to oxides or sulphates – converts metal compounds to their chlorides Calcination • Achieved by the application of energy • Aluminium hydroxide, Al(OH) 3 may be treated at high temperature to drive off water and produce anhydrous aluminium oxide, Al 2 O 3 (for subsequent extraction of aluminium) • Conversion of carbonates to oxides, e.g. – Magnesite (MgCO 3 ) to periclase (MgO) around 400ºC – Limestone (CaCO 3 ) to lime (CaO) around 900ºC – Dolomite (CaCO 3 .MgCO 3 ) –MeCO 3 = MeO + CO 2 • May be done in a fluidized bed or rotary kiln (depending on material or particle size) Rotary Kiln for the calcination of magnesium hydroxide Calcination of magnesium hydroxide to magnesium oxide at Martin Marietta Materials, USA (headquarters in Raleigh, North Carolina) Why roast? • Metals such as Cu, Ni, Pb, Zn occur in nature as sulphides • Oxides are more conveniently reduced to metals than are sulphides • Metals are more conveniently leached from ores if present as a sulphate, chloride, or oxide • Therefore chemical conversion to the desired species may be important • Sulphide concentrates can be heated in an oxidizing atmosphere to convert them to an oxide (oxidizing roast) or sulphate (sulphating roast) • Metal compounds may be converted to chlorides (chloridizing roast); this sometimes becomes a gaseous- state operation, as chlorides are often volatile • Product from a roasting operation is known as a calcine Sulphide roasting • Controlled oxidation of metal sulphides below their melting points, usually between 500 and 1000ºC • Many different species can co-exist, depending upon operating conditions –O 2 , SO 2 , SO 3 , S 2 –MeO x , MeS x , MeSO 4 , complex oxy-sulphides such as MeO.MeSO 4 – complex oxides such as MeO.Me 2 O 3 • Process is controlled by manipulating the temperature and composition of the gas phase (and residence time of solid particles)
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2 Thermodynamic aspects of sulphide roasting • Relationships between possible solid and
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7 Roasting - Solid-state pyrometallurgy Metal-bearing...

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