Experimental results were obtained with various industrial suspensions with densities of 1100 to 1600 kg/m3 using a 0.15 by 0.1 m four-vane centrifugal slurry pump with an impeller diameter of 0.63 m operating at 650-850 rpm. The non-Newtonian behaviour for a fixated scrubber sludge and a red mud product was characterised in pipeline loops with diameters of 0.075-0.2 m. The lowering of the pump water head and efficiency at the best efficiency flow rate region was at a maximum about 10 and 20%, respectively. With the scrubber sludge, a sharp reduction in head occurred at flow rates below 40% of the best efficiency value, resulting in an unstable head curve. This result is used to demonstrate the risk of unstable operation if a Newtonian correction procedure with a high viscosity is applied giving very large reductions in efficiency and a shift of the best efficiency point to smaller flow rates