The rotary kiln is a cylindrical vessel, inclined slightly to the horizontal. The material is fed into the upper end of the cylinder. As the kiln rotates, the materials gradually move towards the lower end, and they may go a certain amount of stirring and mixing. Hot gases go through the rotary kiln, in the same direction as the process material, but usually in the opposite direction. The hot gases may be generated in an external furnace, or may be generated by a flame inside the kiln. They are repetitively scattered in the air so that they are in full contact with the hot air, the material is dried gradually