Numerical simulations were used to study mobilization and mixing of a bottom layer of Bingham liquid by agitating a Newtonian liquid above the Bingham layer. The agitation is done by a pitched-blade impeller at a Reynolds number of 6000. The Bingham liquid and the Newtonian liquid are miscible. The parameter space of the simulations has a yield stress number and a Richardson number as dimensionless variables. The yield stress number quantifies the importance of the yield stress relative to inertial stresses, the Richardson number the role of the density difference between the two liquids. The simulation procedure is based on the lattice-Boltzmann method for the flow dynamics, and a finite volume scheme to solve for the local and time dependent composition of the liquid mixture. Flow dynamics and liquid composition are intimately coupled. The moderate Reynolds number tentatively allows us to directly simulate the transitional flow, without a need for a turbulence closure model. The results quantify the increase of mixing time with increasing yield stress and (to a weaker extent) density difference. (C) 2012 Elsevier B.V. All rights reserved.