A feed charging device, comprising: a holding vessel having an interior chamber for holding a reserve of a solid particulate feed material in a fluidized state, wherein the feed material is held in said fluidized state in a lower zone of the interior chamber; at least one inlet opening through which the feed material is supplied to the interior chamber; at least one outlet opening through which the feed material is discharged from the interior chamber, wherein said at least one outlet opening is in flow communication with the lower zone of the interior chamber and wherein an area of the at least one outlet opening is adjustable; gas supply means for supplying a fluidizing gas to the lower zone of the interior chamber; and an outlet conduit in flow communication with the at least one outlet opening for receiving said feed material discharged from the interior chamber, wherein the outlet conduit has a conduit wall in which said at least one outlet opening is formed; wherein the outlet conduit extends substantially vertically through said interior chamber, wherein said gas supply means are radially dispersed around the outlet conduit; and wherein the at least one outlet opening is arranged to receive said feed material from a plurality of radial directions, wherein the outlet conduit includes a slidable or rotatable cover member adapted to be moved steplessly or in discrete steps from a first position in which the area of the at least one outlet opening is at a maximum, to a second position in which the area of the at least one outlet opening is at a minimum; or wherein the at least one outlet opening comprises a horizontal slit extending throughout substantially the entire outer perimeter of the conduit wall, and wherein the cover member comprises a sleeve which is slidable longitudinally along a surface of the outlet conduit between said first position and said second position, and wherein the horizontal slit has a greater height with the sleeve in the first position than in the second position; and wherein the feed charging device further comprises an actuation mechanism for controlling the movement of the cover member between said first position and said second position. a holding vessel having an interior chamber for holding a reserve of a solid particulate feed material in a fluidized state, wherein the feed material is held in said fluidized state in a lower zone of the interior chamber; at least one inlet opening through which the feed material is supplied to the interior chamber; at least one outlet opening through which the feed material is discharged from the interior chamber, wherein said at least one outlet opening is in flow communication with the lower zone of the interior chamber and wherein an area of the at least one outlet opening is adjustable; gas supply means for supplying a fluidizing gas to the lower zone of the interior chamber; and an outlet conduit in flow communication with the at least one outlet opening for receiving said feed material discharged from the interior chamber, wherein the outlet conduit has a conduit wall in which said at least one outlet opening is formed; wherein the outlet conduit extends substantially vertically through said interior chamber, wherein said gas supply means are radially dispersed around the outlet conduit; and wherein the at least one outlet opening is arranged to receive said feed material from a plurality of radial directions, wherein the outlet conduit includes a slidable or rotatable cover member adapted to be moved steplessly or in discrete steps from a first position in which the area of the at least one outlet opening is at a maximum, to a second position in which the area of the at least one outlet opening is at a minimum; or wherein the at least one outlet opening comprises a horizontal slit extending throughout substantially the entire outer perimeter of the conduit wall, and wherein the cover member comprises a sleeve which is slidable longitudinally along a surface of the outlet conduit between said first position and said second position, and wherein the horizontal slit has a greater height with the sleeve in the first position than in the second position; and wherein the feed charging device further comprises an actuation mechanism for controlling the movement of the cover member between said first position and said second position.