Device for producing expanded granulated material

A device for producing an expanded granulated material from an unexpanded mineral material in the form of grains of sand with an expanding agent, the device comprising a furnace with a substantially vertically standing furnace shaft having an upper end and a lower end, wherein a conveying section extends between the two ends, wherein further at least one feeding means is provided which is adapted to charge at least the unexpanded mineral, material at one of the two ends of the furnace shaft into the furnace shaft in, the direction of the other of the two ends of the furnace shaft in order to expand the unexpanded mineral material, as seen in a conveying direction, in the last half of the conveying section, wherein at least one rotatable shaft insert is provided, which is arranged at least in sections in the furnace shaft and has at least one scraper blade, which forms with an inner wall of the furnace shaft at least one gap having a gap width and which is designed, during rotation of the at least one shaft insert in an operating state of the device, to remove caking on the inner wall in sections if a thickness of the caking is greater than the respective gap width, wherein the at least one shaft insert is rotatable about at least one axis of rotation which extends parallel to a longitudinal axis of the furnace shaft, wherein the conveying section leads through a plurality of heating zones which are arranged separately from one another in a conveying direction, wherein the heating zones each comprise at least one heating element which can be controlled independently of one another in order to heat the unexpanded mineral material at least to a critical temperature and to expand the sand grains, and wherein the at least one shaft insert each comprises a base body from which the at least one scraper blade projects with a directional portion parallel to a radial direction, wherein the radial direction lies in a plane normal to the axis of rotation of the respective shaft insert and, starting from the respective axis of rotation, faces away therefrom, wherein the respective base body, as viewed at least in the radial direction, is substantially closed.