Download Complex Sulphide Ores - Materials and Heat Balance in Metallurgical Processes - Lecture Notes and more Study notes Materials Physics in PDF only on Docsity! Lecture 15: Thermodynamics of roasting Contents Preamble Phase rule Predominance area diagram Method of construction Utility of predominance –area diagram Roasting of complex sulphide ores Technology of roasting Conclusions Reference Key words: roasting, dead roasting, Predominance area diagram Preamble Roasting is gas/solid reaction in which sulphide is converted to oxide or sulphate or even to metal. Whether roast product is oxide or sulphate or partially sulphide would depend on temperature and partial pressures. The purpose of this lecture is to determine thermodynamic conditions for roasting. Phase rule Gibbs phase rule is P F C 2 F 3 P O SO SO O is called predominance‐ area diagram. Predominance area diagram P is the number of phases and C is the minimum number of chemical components requiresconstituting all the phases in the system. F is the number of degrees of freedom in the system also referred to as the variance of the system). The integer in the Gibbs phase rule is related to the number of intensive parameters such as temperature and pressure that are being considered. In roasting we have 3 components, that is metal, sulphur and oxygen. Also pressure has no effect on condensed phases. Mostly roasting is carried out at a constant pressure. The phase rule as applied to a 3‐ component system at constant temperature and pressure reduces to . For a given temperature the composition of the gas mixture is defined by the partial pressure of gaseous components, p and p . Thus the phase relations in the ternary system as constant temperature may be described in two dimensional diagram where p and p are the two coordinates. Such a diagram Figure 15.1 shows predominance area diagram for Ni S Osystem, at constant temperature. The phases are shown in the figure. igure 15.1: predominance area diagram for Ni S O system at constant temperature. the figure at points B C and D, three condensed phases area at equilibrium for a particular value of pO and pSO . Degree of freedom is zero. For example at point B Ni S /Ni/NiO can co‐exist at fixedpO the nts The lines describe the equilibrium between any two condensed phases. Along the lines degree of ry either pO orpSO to obtain the phases. For example line EB is s AB en and equilibrium le in the area A le phase. In the area degree of freedom is 2 which means both pSO and pO can be varied to obtain a phase within the area. nan a d In a two dimensional diagram, temperature is fixed. These are the equilibrium diagrams and hence we have to consider all the a systems. F In andpSO , at point C . Thus se poi are called invariant point. freedom F 1,which means we can va equilibrium between and Ni, where along line BC equilibrium exists between . Along line and GD equilibrium exists betwe Ni NiO, and . This shows that NiO/Ni or NiS/ is independent of pSO . The figure also shows predominance areas for a single phase, for examp BCDHNiO is a stable phase, whereas in the area FCDG, NiS is a stab Method of construction The predomi ce are iagram depends on the system and temperature. phases which can form in Consider Ni S O system in which Ni, NiO, Ni S , Ni SO and NiS phase can form. Let us write chemical equation representing equilibrium between any two condensed phases
