A thermal drying, batch-type process, in which the action of high-pressure steam on a lump of lignite produces the following effects. The lump is heated inside and out to an approx. uniform temperature by its envelope of condensing steam. As the temperature rises and the pressure increases part of the colloidal water is expelled from the lump as a liquid. The lump shrinks as water leaves and the cells collapse, and when the pressure is lowered, more water leaves by evaporation caused by the sensible heat stored in the lump. When the pressure is lowered further by vacuum, additional moisture is evaporated, which cools the lump.