This section is from the book "An Elementary Outline Of Mechanical Processes", by G. W. Danforth. Also available from Amazon: An elementary outline of mechanical processes.
Another method of reducing the quantity of impurity before the ore is transported from the mine is by calcination, which consists of heating the ores to a point short of fusion. This drives away moisture and some combined sulphur, and breaks up carbonates, changing them to oxides by driving off Co2. Calcination is done crudely by making alternate layers of fuel and ore on the ground and lighting the fuel, but more effective methods are to confine the ore and fuel by walls, or still better by kilns, in which the heat is more intense and more uniformly distributed.
After calcining, the ore is ready for transportation to the smelter, which is usually located away from the mines for the convenience of obtaining fuel and labor.