Rheological study of kaolin clay slurries

Abstract

Concentrated kaolin clay slurries are found in a number of industrial operations including mine tailings surface disposal, underground paste backfill, and riverbed dredging. An understanding of the impact of solids concentration and addition of chemical species on slurry rheology is of importance to designers of pipeline transport and waste disposal systems. A project to determine the rheology of an idealized industrial kaolin clay slurry using a concentric cylinder viscometer and an experimental pipeline loop was undertaken. Additional laboratory test work including particle size analysis, slurry pH, calcium ion concentration in the slurry supernatant and particle electrophoretic mobility measurements were completed to aid in the understanding of their effects on the slurry rheology. The slurries were prepared in varying kaolin clay solids concentrations with reverse osmosis water. A flocculant, dihydrated calcium chloride (CaCl2 ¡¤ 2H2O), was added to the reverse osmosis water in concentrations equivalent to those found in typical industrial hard water supply. A dispersant, tetra-sodium pyrophosphate (TSPP, Na4P2O7) was used to disperse the clay particles for selected slurries. It was found that the kaolin clay slurries, in the absence of TSPP, exhibited yield stresses and could be characterized with either the two-parameter Bingham or Casson continuum flow models. Increasing the clay concentration in the slurry, while keeping the mass ratio of flocculant to kaolin constant, increased both the yield and plastic viscosity parameters. There was generally good agreement between the rheological parameters obtained in the Couette flow viscometer and that in the pipeline loop. In slurries for which it was possible to obtain turbulent flow, the transition to turbulent flow was predicted accurately by the Wilson & Thomas method for both Bingham and Casson models. It was possible to eliminate the yield stress of a slurry with the addition of the dispersing agent TSPP. The calcium ion content of the supernatant extracted from the slurries proved to be a indicator of the degree of flocculation. When exposed to extended periods of high shear conditions in the pipeline loop, slurries with clay concentrations of 17% by volume solids or greater exhibited an irreversible increase in apparent viscosity with time. An attempt was made to investigate this irreversible thickening characteristic. Laboratory tests did not reveal any appreciable differences in particle size, electrophoretic mobility, calcium ion concentration or pH with this irreversible change. The shear duration test shows the importance of using the appropriate shear environment when testing high solids concentration kaolin clay slurries