Abstract: The effect of pH and electrolytes on stability of aluminium magnesium hydroxide positive sol was studied. It was found that aluminium magnesium hydroxide positive sol is stable in the range of pH 5～10; the acidic dissolution of the sol occurs at pH<5, and the surface hydroxys of sol particles lose hydrions to lose stability at pH>10.5. The higher the valence number of anions, the stronger the coagulation ability for the positive sol. Decreasing the pH, the coagulation abilities of monovalent anions (Cl^-, NO₃^-) and CO₃^2-, PO₄^3- decrcase, but the coagulation ability of SO₄^2- is almost not varied. The higher the valence number of cations, the stronger the stability for positive sol. The studies of coagulation kinetics showed that the coagulation process of monovalent anions may be divided into rapid and slow coagulation. In slow coagulation region, the stability ratio are of good linear relationship with concentration of electrolytes in agreement with classical slow coagulation theory. In addition,the static method for measuring coagulation ability has been improved.

Key words: aluminium magnesium hydroxide, positive sol, coagulation, dispersion