10. Suspension Rheology

The importance of rheology in the mining industry derives from the fact that all materials being processed are suspensions, that is, mixtures of solid particles and fluids, usually in water. In mineral processing plants, water is mixed with ground ore to form a pulp that constitutes the mill feed. The mill overflow is mixed again with water to adjust the solid content for classification in hydrocyclones. Pulp characteristics are essential in the transport of products to their final destination. A suspension, like all types of materials, must obey the laws of mechanics under the application of forces. The flow patterns of suspensions in tubes depend on their concentrations and transport velocities. In diluted suspensions at low velocities particles will settle. The suspension is termed a settling suspension and the flow regime is considered heterogeneous. At a velocity beyond a value at which all particles are suspended gives a non-settling suspension and the flow regime is homogeneous with Newtonian behavior. Concentrated suspensions are usually homogenous with non-Newtonian behavior. The variables and field equations for all types of fluids are presented and constitutive equations differentiate between Newtonian and non-Newtonian behavior. Empirical models of non-Newtonian behavior are presented, including pseudo-plastic and dilatant behavior with Cross and Carreau and Power-law models, and yield-stress models with Bingham and Hershel-Bulkley models. The study of the operational effect on viscosity includes variable such as solid particle size and concentration, temperature, pressure, time and pH. Rheometry provides experimental methods to determine rheological parameters such as viscosity and yield stress.