Why are calcination and roasting processes carried out at a temperature just below the ore's fusion temperature?
The purpose of roasting a/o calcination is oxidation a/o decomposition of original ore to form metal oxides.
It is desirable to keep the intermediate product solid to obtain porous product. This allows easy access for further reduction by carbon monoxide by coke or charcoal reduction process. If oxides were in form of a solidified melt, the reduction arrangement would be problematic.
Additionally, some ores or oxides have too high melting point to be practical or easily possible to melt them.