This study focuses on a new optical measurement technique for understanding the flow structure in an opaque suspension. Such opaque systems occur frequently within flotation cells. Adding fluorescent dye to the liquid phase of the suspension enhances the visibility of the particles significantly, at a solid concentration of 10% by mass. The image contrast is sufficient to derive the velocity field from Particle Image Velocimetry (PIV), based on cross-correlation of subsequent images. We studied the correlation quality as a function of the dyes molar concentration and added solid concentration of Magnetite or Quartz. At ideal dye concentration, cross-correlation coefficients in the range of 0.4 to 0.8 can be reached, allowing for reliable determination of flow fields. The technique is simple, cheap and can enable experiments to understand the complex flow conditions in flotation cells with up to at least 10% solid mass fraction.