The internal flow characteristics of a kaolin suspension were examined via magnetic resonance imaging (MRI) in a flow regime for which the material behaves as an almost pure plastic material. The paste also showed a significant tendency toward liquid phase migration under squeezing tests. The MRI measurements produced a 2D slice of the 3D velocity field near the entrance of the cylindrical extrusion die, as well as the relative liquid–solid ratio within the cylindrical extrusion cartridge. Finite element simulations were also carried out using a simple yield stress fluid model, i.e. Herschel–Bulkley model, which was fit to rheometrical data obtained from independent experiments. A relatively good agreement between numerical simulations and the measured velocity field was obtained. In addition, the clay concentration of the paste within the extrusion column was imaged before and after extrusion experiments displaying a migration of water from the dead zones toward the entrance of the die. It is remarkable, however, that this phenomenon does not significantly affect the velocity distribution.