Recent loop testing performed at the GIW Hydraulic Lab([1,2]) has provided pump performance data for two highly non-Newtonian slurries with significantly different characteristics: a high clay content slurry with minimal coarse solids; and a typical, low clay content, two-component tailings slurry. The importance of air removal in the sump and pipe loop was demonstrated using a simple, yet novel de-aeration system. In addition to the measurement of performance losses, determination of the upper limit of pumpability for these slurries relative to their concentration and associated yield stress was investigated. However, once the slurry was de-aerated, no limits could be found, other than those dictated by suction side losses (NPSHA) or excessive pipeline friction gradients, indicating that the only true limit in practice is one of system economics, i.e. pump operating and capital cost. Experimentally measured pump head and efficiency were compared against corresponding predictions from two different models: the Walker and Goulas technique([3]) and the Graham et al. technique,([4]) with special focus given to the dependence of the losses on pump rotary speed.