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 is the number of phases and C is the minimum number of chemical components requires constituting 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 and . Thus the phase relations in the ternary system as constant temperature may be described in two dimensional diagram where  and  are the two coordinates. Such a diagram is called predominance area diagram.

Predominance area diagram

Figure 15.1 shows predominance area diagram for system, at constant temperature. The phases are shown in the figure.

Figure 15.1:

predominance area diagram for  system at constant temperature.

In the figure at points B, C and D; three condensed phases area at equilibrium for a particular value of  and . Degree of freedom is zero. For example at point B  can co-exist at fixed and, at point C . Thus these points are called invariant point.

The lines describe the equilibrium between any two condensed phases. Along the lines degree of freedom ,which means  we can vary either or to obtain the phases.  For example line EB is equilibrium between and Ni, where along line BC equilibrium exists between  Along lines AB and GD equilibrium exists between Ni and NiO, and . This shows that NiO/Ni or NiS/ equilibrium is independent of .

The figure also shows predominance areas for a single phase, for example in the area ABCDH  is a stable phase, whereas in the area FCDG,  is a stable phase. In the area degree of freedom is 2 which means both  and  can be varied to obtain a phase within the area.