The mixing of dense (5-40 wt %) binary mixtures of glass particles in water has been studied in a stirred vessel at the "just-suspended" speed and at speeds above it, using a Eulerian-Eulerian computational fluid dynamics (CFD) model. For each phase component, numerical predictions are compared to 3-dimensional distributions of local velocity components and solid concentration measured by an accurate technique of positron emission particle tracking. For the first time, it has been possible to conduct such a detailed "pointwise" validation of a CFD model within opaque, dense multicomponent slurries of this type. Predictions of flow number and mean velocity profiles of all phase components are generally excellent. The spatial solid distribution is well-predicted except near the base of the vessel and underneath the agitator where it is largely overestimated; however, predictions improve significantly with increasing solid concentration. Other phenomena and parameters such as particle slip velocities and homogeneity of suspension are analyzed.