Thus, additional coal can be fed into the ore reduction kiln, that is to say, coal in an amount in excess of that required for the reduction of the Na2 SO4, together with enough air to burn the additional coal to yield the necessary heat for the reduction of the Na2 SO4 ; or, if a gas such as CH4 is fed into the kiln to reduce the Na2 SO4, enough gas can be used, together with excess air, to furni