In hydrometallurgical and mineral processing operations, slurries are often prepared at very small grind sizes and high solids concentrations. The presence of yield stress or flow limits results in flow anomalies as the slurry tends to stagnate close to vessel bottom, walls, and at any internal surface. The preparation of high-concentration slurries preferably in low shear rate ranges consuming less power is an integral part of high-concentration slurry transportation systems. Therefore, during mixing of such concentrated slurries, the selection of appropriate agitation equipment for preparation of a homogeneous suspension requires a thorough investigation. The rheological behavior of ground manganese nodule samples in solids concentration range of 55-65% by weight using both distilled water and sea water as mediums have been evaluated and the discrepancy in yield stress and plastic viscosity values affected by solids concentration and medium has been discussed and presented in the paper. The performance characteristics of a helical ribbon mixer influenced by fluid rheology, medium, and agitator rotational speed were evaluated. The appropriateness of the agitator for preparation of high-concentration manganese nodule slurry was further substantiated by comparing the experimental power consumption data with the theoretical power consumption values computed using Bakker and Gates correlations.