Pyrometallurgy

Chemical Metallurgy Principles and Practice. Chiranjib Kumar Gupta Copyright 2003 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30376-6 

Water is present in the materials of interest as free water or water of crystallization, or as combined water. The process of dehydration refers to the removal of the water of crystallization, while the removal of combined water is called dehydroxylation because hydroxyl groups in the material are broken down to form water vapor. The dehydroxylation process is very often alternately described as calcination. The drying process used in the present text pertains to both dehydration and dehydroxylation. In the processing of ores for metal extraction, drying essentially implies the removal by evaporation of water which a material holds in it in various forms. 

In general, at temperatures normally required for drying the following condition apply  

The equilibrium water vapor pressure over the materials in hydrate forms is greater than that over the materials in hydroxide forms. Elimination of water vapor and hence drying of the material occurs when the ambient water vapor pressure in the system is lower than the equilibrium water vapor pressures given by the above equations. To effect drying, there are two options. 

At a given ambient water vapor pressure (usually the level found in the open atmosphere), the temperature of the material is raised so that the equilibrium water vapor pressure over the hydrated material is higher than the ambient water vapour pressure. Generally, heating up to 400 °C is sufficient to remove all the water of crystallization from materials. This removal of water yields a material which may contain some more strongly bound water. To remove this water, the material requires to be heated to a higher temperature (400-600 °C) so that the equilibrium water vapour pressure exceeds the ambient water vapour pressure. For near-complete removal of the last traces of water, temperatures as high as 1000 °C may be required. In addition to the heat required to raise the temperature of the material, heat is also required for the evaporation of water, which is an endothermic process. The enthalpy of evaporation increases as the water content, and hence the equilibrium water vapor pressure, decreases. 

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Pyrometallurgy. Metallurgy involved in winning and refining metals where heat is used, as in roasting and smelting. PyrometaHurgy is the oldest extractive process and is probably the most important. 

C. B. Alcock, Principles of Pyrometallurgy, Academic Press, London, 1976, 348 pp. 

CVD developed slowly in the next fifty years and was limited mostly to extraction and pyrometallurgy for the production of high-purity refractory metals such as tantalum, titanium, and zirconium. Several classical CVD reactionswere developedatthattimeincludingthecarbonyl cycle (the Mond process), the iodide decomposition (the de Boer-Van Arkelprocess)andthemagnesium-reduction reaction (the Kroll process). 

Thermal properties such as thermal capacity, thermal expansion, melting temperature, thermal decomposition and sublimation are all important in considering processes to which minerals may be directly subjected in a pyro way. As for example, roasting or calcination or any pyro pre-treatment of a mineral concentrate is greatly influenced by its thermal properties. The chapter on pyrometallurgy deals with these aspects. 

Pyrometallurgy, the dominant process in chemical metallurgy, uses reactor of different types and designs. In terms of the physical states of the reactants, one generally finds that the different reactions carried out in pyrometallurgy include principally, gas/liquid, liquid/... 

Apart from pyrometallurgy, the various ingredients of heterogeneous reaction kinetics are also very pertinent to several hydrometallurgical operations. This will be discussed in Chapter 5. 

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