Rheological Properties of Aqueous Suspensions of Palygorskite

Abstract

The rheology of aqueous suspensions of six palygorskites with different particle morphologies has been investigated as a function of clay concentration, adsorbed ion, pH, and electrolyte concentration, using a rotary viscometer. Rheological parameters (plastic viscosity, Bingham yield value, and apparent viscosity) increase with the length/width ratio of individual palygorskite fibers. The plastic viscosity increases linearly with clay concentration within the range investigated (up to 5% w/v). The rheological parameters are higher for clays saturated with divalent ions than those for clays saturated with monovalent ions. For ions of the same valency the rheological parameters tend to increase with decreasing radius of the hydrated ion. The flow of suspensions is pseudoplastic regardless of the nature of the adsorbed ion. Measurements of electrophoretic mobility in very dilute suspensions suggest that the point of zero charge of the palygorskite surface is at pH 4 to 4.5. Scanning electron microscopy indicates significant differences in fiber arrangement between low and high pH values. Face‐to‐face particle association occurs at low pH values, giving rise to close‐packed domains of fibers, while at high pH values the fibers adopt a random orientation. The flow of suspensions is pseudoplastic at pH ≤ 7 and changes to Newtonian at pH ≥ 9. At pH ≤ 7 the rheological parameters remain relatively constant even at high electrolyte concentrations, but at pH ≥ 9 they are influenced significantly by electrolyte addition. Electrolyte‐free palygorskite suspensions at pH 7 are antithixotropic. When the pH falls below 7 or when electrolyte is added, the suspensions coagulate and become thixotropic. Some rheological properties of palygorskite are similar to those of platy clay minerals such as kaolinite and montmorillonite, while other rheological properties deviate considerably. The models developed to explain the rheological behavior of platy clay minerals do not always account for the behavior of palygorskite, because of differences in particle morphology and surface structure.