The smelting of Zinc Oxide (obtained from roasted ZnS) happens
Zinc boils at 9200C and the smelting happens at a significantly higher 13000C. This is because of the following side reaction during smelting:
CO2(g)+C → 2CO(g)△H>0 endothermic
This reaction lowers the temperature thus impeding the vapourization (of Zinc)