Processing of nickel laterite ores is complicated by the slurry's non-Newtonian rheological properties, which can range from shear-thinning to shear-thickening and from thixotropic to rheopectic. Rheological studies on materials from the Ramu deposit support findings that laterite suspensions exhibit rheopectic properties. Adding a dispersing agent or by lowering the pH was found to reduce the apparent viscosity and the degree of rheopexy. Since both of these actions would increase the particles' electrostatic repulsion and dispersion, it is proposed that the rheopectic response is due to shear induced (secondary minimum) coagulation. In practice, these results could be exploited to increase the maximum solid content for pressure acid leaching. (C) 2002 Elsevier Science Ltd. All rights reserved.