Often pyrometallurgical processes are autogenous, and so the Energy required to heat the minerals comes from the Exothermic reaction of the minerals in the process and no further energy is required.
As an example, roasting is used to remove Sulfur from sulfide ores. The ore is heated in an oxidizing atmosphere such as air. Sulfides are oxidized by the Oxygen in the air, with the sulfur liberated as Sulfur dioxide gas, leaving an oxidized mineral. The metal oxides can then be purified by heating with coke or Charcoal (forms of Carbon), a Reducing agent that liberates the oxygen as Carbon dioxide leaving a refined mineral. Carbonate ores are also smelted with charcoal. Other materials are often added as flux, aiding the melting of the oxide ores.
The sulfur dioxide and carbon dioxide produced in the process are significant air pollutants.