Lignin-based slurry fuels are a potential alternative to fossil fuels in kraft pulp mills. Lignogels - mixtures of lignin, fuel oil, water and surfactant - are non-Newtonian fluids, with shear-thinning and thixotropic behaviour. Their mixing was investigated in tanks with volumes of 3 and 30 L. An A310 hydrofoil impeller was used in all experiments. Results were compared with measurements in Newtonian fluids, used to characterize the impeller over a broad range of Reynolds numbers (1-500 000). An aqueous CMC solution was also used for characterization of the impeller and estimation of the Metzner-Otto constant. Results in the transition region were corrected by introduction of two empirical parameters.