laterally and adjacently to the clean sugarcane lower outlet 19 and superiorly communicating with the first chamber portion 15 in the region of the forced air flow inlet 18 to collect part of the mineral and vegetable impurities separated from the sugarcane flow the first collecting compartment 21 is inferiorly provided with an impurity outlet 21 a from which the impurities are discharged to an impurity conveyor device 24 the separating device d also comprises a second chamber portion 22 superiorly communicating with the first chamber portion 15 by means of an admission opening 16 c defined between a pair of the additional deflectors 16 a and 16 b arranged upstream and in a level above the first collecting compartment 21 thus the second chamber portion 22 can receive part of the forced air flow which passes through the sugarcane flow or curtain in a gravitational displacement and part of the vegetable and mineral impurities collaborating to promote a pre decompression of the volume of air admitted in the interior of the separating device d the second chamber portion 22 is constructed so as to define in a lower region a second collecting compartment 23 which on its turn is provided with an impurity outlet 23 a from which the collected impurities are discharged to the impurity conveyor device 24 the separating device d further comprises a third chamber portion 27 superiorly communicating with the first chamber portion 15 in a place located downstream the first and second deflectors 16 17 and above the additional deflector 16 a disposed immediately above.