Amount of air

In roasting, air is used for oxidation of sulphides as well as for combustion of coal. Calculation of amount of air is important.

Stoichiometric amount of air (Also termed theoretical air or air for complete conversion of sulphide into oxide or for complete combustion) can be calculated by considering the products of combustion.
Consider the reaction

                                                                                                       (1)

                                                                                                   (2)

                                                                                    (3)

                                                                                    (4)

The equations 1 to 4 are chemically balanced equations. In equation1, we require 1 mole of oxygen to produce 1 moleof  and in equation 3 or 4 we require 1.5 mol of . It is known that 1 mole of oxygen is obtained from 4.76 moles of air.

Let us calculate stoichiometric amount of air for combustion of solid fuel of composition  moisture and  ash, per Kg of coal. Following the stoichiometry of combustion, the amount of air would be







Note








In the above example if actual amount of air is 0.5 mols then



We can also call-that  theoretical air is used for combustion.

Similarly stoichiometric amount of air can be calculated for reaction 3 and 4. If reactions 3 and 4 occur simultaneously then stoichiometric amount of oxygen is 3 mols and stoichiometric air is 14.28 mols.

Types of roasting

In dead roasting all sulphides are converted in to oxide. This is done to extract Zn from its sulphide ore.

In sulphatising roasting, sulphide is converted into sulphate as sulphatecan be dissolved easily into an aqueous solution. Typically used for hydro metallurgical extraction of lead sulphide ores.

Conclusions

This lecture discusses roasting. Roasting is mainly used in production of copper, lead, zinc etc. In roasting sometime fuel is used. The calculation procedure for calorific value of solid fuel and gaseous fuel  is illustrated through an example. Type of roasting presented here is in brief. Details can be seen in the references given below.

Reference:

Resenquist : principles of extractive metallurgy

H S Ray, R Sridhar and K.P Abraham : extraction of non ferrous metals.