Chapter 18. The Effects of Alkali Metal Cations on Kaolin Rheology

Summary

The effects of alkali metal cations on kaolin rheology were studied across a wide concentration range, focusing on millimolar levels. Kaolin slurries partially dispersed with sodium polyacrylic acid were subjected to addition of alkali metal chloride salts. The slurries were approximately 29% vollvol solids. The critical coagulation concentration ofM was determined for Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺. The larger metals demonstrated significant specific adsorption on the surface of the clay, modifying the viscoelastic properties of the flocculated system. Comparison of NaCI and Na₂B₄O₇ additions demonstrates that cation effects are important when weakly coordinating anions (Cl^-) are present; however, anions that oligomerize (borates, silicates, phosphates) can help disperse the clay system by specific adsorption to the clay surface. Dispersion stability is eventually overcome by cation effects as the cation concentration increases. Rheology of high solids content dispersed slurries is sensitive to combined alkali concentrations at the tens of millimolar level, which is occasionally a concern for whiteware producers. The impact of alkaline earth cations tends to dominate the effects observed in production.