Abstract: A calcinating device, or calcinator, is provided with a rotatable drum, the axis of which is slightly inclined with respect to the horizontal, and is adapted to be mounted inside a furnace and to be used for mixing and heating a mixture of sintered glass pieces and of radioactive wastes. The calcinator has, at the extremities thereof, two flanges to which are respectively fixed an upper sleeve on a level with which open an inlet conduit for the radioactive liquids, a conduit connected to a hopper for feeding sintered glass and a gas exhaust conduit, and a lower sleeve, the extension of which away from said drum is provided with an opening for expelling the calcinate obtained and transferring it to a smelting furnace. The calcinator is further provided with means for rotating and sealing same.