How do rabble blades move material through a multiple-hearth furnace?

In a multiple-hearth furnace used for molybdenum processing, material is moved through the system by rabble blades attached to air-cooled arms. These arms are rotated by an air-cooled shaft located within the furnace. As the arms rotate, the rabble blades rake the material toward either the outside or the center of each hearth, where it then drops down to the next hearth level.1