The purpose of this novel research was to evaluate the influence of the rheological parameters of Herschel-Bulkley fluids (e.g. xanthan gum solution) such as consistency index, power-law index, and yield stress, on the mixing performance of a coaxial mixer (Scaba-anchor) at distinct speed ratios in terms of the mixing time and power consumption. The mixing time of opaque xanthan gum solution was measured by electrical resistance tomography (ERT). A CFD model was developed for the 3D simulation of the fluid flow generated by the coaxial mixer. The rotation of the coaxial impellers was modelled employing the sliding mesh method. The experimental measurements of power drawn and mixing time were used to validate the CFD model. The results showed that both the mixing time and power consumption increased with increasing the consistency index and yield stress, while the mixing time and power drawn decreased with a rise in the power-law index. The design of experiment (DOE) and response surface methodology (RSM) were carried out to investigate the interaction between independent variables. The result showed that the interaction between the consistency index and yield stress was the most important one. The RSM analysis demonstrated that the consistency index and speed ratio had major influences on the mixing efficiency of the coaxial mixing system. Mixing time values for the coaxial mixer correlated well with the Reynolds number and specific power consumption.