Mechanically agitated reactors find extensive application in the chemical, biochemical and mineral processing industries because of the improved solids suspension and mixing they provide. Optimized design and operation of such reactors require the understanding of the solids dynamics and flow field in the reactor. Although few studies exist for hydrodynamics in dilute suspensions, information at relatively dense conditions are not available in the open literature. The Computer Automated Radioactive Particle Tracking (CARPT) technique is used in this work to study the solids dynamics in a stirred tank at solids loading of 2.5 to 19%. CARPT is a non-invasive technique that provides Lagrangian description of the solids flow field, which subsequently provides the time-averaged velocity field and turbulent quantities. The Lagrangian information is also used to calculate the solids sojourn time distributions in the tank, which clearly show that the popular Zwietering's correlation over-predicts the impeller speed for incipient solid suspension. This has serious consequence on the operation of slurry reactors, since the power required scales as the cube of the impeller speed. The experimental data obtained is used to evaluate the Euler-Euler model (Fluent 6.2) in predicting the complex solid-liquid flow in stirred tank.