This paper reports and discusses results from pumping experiments conducted with heterogeneous slurries containing coarse particles of apatite and hematite industrial concentrates and quartz, clustered in two ranges of particles size (d): Class 1 (0.210 mm < d < 0.297 mm), and Class 2 (0.105 mm < d < 0.149 mm). Using two recirculating pipeline testing loops of pipe diameter (D) of 25 mm and 50 mm and mineral slurries with volumetric concentrations ranging from 8% to 27%, it was possible to vary bulk flow velocity (V) and measure the horizontal pipe pressure gradient (Delta P/L). The results were depicted by curves Delta P/L versus V. The limit of particle deposition in the pipe was addressed as a critical velocity (V-C) of slurry flow at which a moving bed of particles started to form on the bottom of the pipe. Based on the experimental results, a model was proposed to predict V-C using process variables, yielding a deviation of less than 10% from the predicted versus observed data. The sensitivity of the input variables can be evaluated from the magnitude of the powers term of any process variable. (C) 2014 Elsevier Ltd. All rights reserved.