The understanding of the mixing process is of vital importance to accomplish a homogeneous and quality mixing in industrial processes of granular materials. This present work studies mixing process within a pseudo two-dimensional (2D) gas fluidized bed. These systems typically have a transparent wall that enables optical access to the interior of the system. Due to this, the study of two-phase fluidized systems is eased. Furthermore, its thin thickness restrict the domain to two dimensions, making the visualization of the interior representative of the whole domain. A non-intrusive experimental technique has been utilized, the Digital Image Analysis (DIA). Besides, it has been defined a mixing index that allows to characterize the mixing degree and its phases. The influence of fluidization velocity and particle size on mixing process have been studied. Thanks to the experiments performed, it has been identified four phases of the mixing process, and according to them, three mixing times. Therefore, the variation of these times due to particle size and fluidization velocity have been analyzed. The time needed to achieve a complete mixing decreases with increasing fluidization velocities for all particle sizes analyzed. It has been observed that this time decreases slower as the fluidization velocity increases. Thus, the results indicate a possible limit speed from which mixing time does not decrease.