Method and device for producing an expanded granulate

A method for producing an expanded granulate (29) made of a sand grain-shaped mineral material (1) using a propellant, e.g. an expanded granulate of perlite (1) or obsidian sand;

• wherein the material (1) is fed to a substantially upright furnace (2);
• wherein the material (1) is conveyed along a conveying path (4) through a plurality of vertically separated heating zones (5) in a furnace shaft (3) of the furnace (2), wherein each heating zone (5) can be heated by at least one independently controllable heating element (6);
• wherein the material (1) is heated to a critical temperature at which the surfaces of the sand grains (1) become plastic and the sand grains (1) are expanded through the propellant:
• wherein the expanded material (29) is discharged from the furnace (2), characterized in that the material (1) is fed together with an amount of air from below, wherein the material (1) is conveyed from bottom to top along the conveying path (4) by means of the amount of air which flows from bottom to top within the furnace shaft (3) and forms an air flow (14), and wherein the expanding of the sand grains (1) occurs in the upper half, preferably in the uppermost third, of the conveying path (4).