A further problem with prior art devices is that certain mineral impurities and heavy, unexpanded mineral elements may not expand properly during the process, and they will thus drop downwardly through the furnace tube. Resultant, potentially dangerous formations of waste material near the blast nozzle area may effect efficiency. Therefore it is desirable to provide a system wherein open flame injection may be employed at the bottom of a blast furnace tube which permits unobstructed gas flow, but results in improved control of unwanted unexpanded mineral waste.