injected into the bath through orifices located between the blades the ratio of the outside diameter of the rotor to the diameter of its central hub being between 1 5 and 4 in which complete blades with a given contact surface area with the bath are alternated with small blades with a contact surface area with the bath 10 to 30 less than that of the contact surface of complete blades at the bottom end of the drive shaft there is a threaded piece or part on which the rotor will be attached the rotor itself comprises a central hub and a threaded tube that is used to fix the rotor onto the threaded piece or part of the drive shaft blades are fitted onto this central hub laid out like spokes the number of these blades may be variable and may be even or odd if the number of blades is too small the agitation and therefore the efficiency of the treatment may be inadequate if the number of blades is too large the assembly will be more difficult to manufacture and therefore more expensive the choice is made individually for each case depending on the volume of metal to be treated within a given time the size of the ladle which may consist of one or several compartments etc between six and eight blades is a good compromise under normal aluminum treatment conditions the blades are usually rectangular but trapezoidal blades can also be used in which the height of the blade is less at the external end than it is at its connection to the central hub or triangular blades can be used in which the height of.