Abstract: Apparatus (1) for grinding a suspension of a particulate solid material. The apparatus (1) comprises a grinding chamber (2), means (7, 8) for introducing a suspension of the particulate solid material into the grinding chamber (2) and an impeller (6) which is rotated and thereby agitates the particulate grinding medium and the suspension of particulate solid material in the grinding chamber (2). The impeller (6) has at least one blade (12), the said blade (12) having a leading face (14) in the direction of rotation (R) of the impeller (6), which leading face (14) is offset relative to a plane of rotation of the impeller (6). In a preferred embodiment, an upper edge of the said leading face (14) is the leading edge and a lower edge of the said leading face is the trailing edge of each blade (12), so that the impeller (6) acts to drive the material in the grinding chamber (2) downwards towards a base of the grinding chamber (2), thereby compresing the material in the grinding chamber (2).

Abstract: An apparatus and a process for drying a suspension of a finely divided particulate solid material to produce the solid material in a substantially dry and unagglomerated form. The apparatus consists of a grinding chamber containing a particulate grinding medium such as silica having a diameter of 1-5 mm, a conduit for introducing the suspension into the grinding chamber and an impeller which rotates in the grinding chamber. The grinding chamber has a perforated base through which a heated gas is introduced to provide an upward flow of gas through the grinding chamber and through the grinding medium. As the particulate material is dried, fine particles of the dried material are carried upwards in the upflow of gas through the grinding chamber. A filter such as one or more cyclones and/or a bag filter assembly is located downstream of the grinding chamber in order to separate the particles of dried material from the gas.