In the prior art a variety of furnaces have been proposed for heating such expandable minerals. Usually the incoming raw ore may be dropped through suitable ports providing access interiorly of a furnace tube, whereupon raw ore will drop downwardly towards an external blast nozzle. The extreme heat generated within the furnace will expand the ore, and the combination of resultant lower product density and upward directed gas pressure will propel the expanded product out from the furnace. Subsequently, the product may be conveyed to a typical cyclone separator which cools and separates the expanded ore from hot waste gases.