Solution: Basis 1000 kg ore concentrate

Roasting reactions are (considering roast product given in the problem)

                               (1)

                   (2)

Weight of roasted product  ,



  Ans (a)

110% excess air is used in the process for roasting and combustion.

Combustion reactions are (assuming complete combustion)
,                                                  (3)
                                          (4)

From reactions 1, 2,3 and 4 one can calculate mols of oxygen theoretically (or stoichiometrically) required for roasting and combustion. According to problem all sulphure passes to

 Total moles of  required (theoretically) =17.42 moles.

Theoretic air: 1mole of oxygen is equivalent to 4.76 moles of air and 1kg mol of any gas occupies

.

Theoretical air .

Actual amount of air  Ans b

The flue gas. Consists

Amount of flue gas

Amount of water .

Water/ flue gas

Note the following:

• Roasting produces roast product which is approximately  of the ore concentrate
  
  
• Roasting always employs excess amount of air since the roasting reactions are carried out in the solid state at high temperature. Excess air is necessary.
  
  
• Material balance shows that flue gas contains , which amounts to  kg concentrate. Flue gas exist at temperature at least equal to roasting temperature. This means large amount of heat is carried out by  of flue gas.
  
  
• Use of excess air brings  in flue gas and so  also carries heat.
  
  
• Excess air optimization would make roasting energy efficient