Colloidal magnesium aluminum hydroxide and heterocoagulation with a clay mineral. I. Properties of colloidal magnesium aluminum hydroxide

Magnesium aluminum hydroxide, the most important member of layered double hydroxides, was peptized by intense washing. The particle diameter, 70-130 nm, depended on the temperature of aging the parent material. The electrophoretic mobility of the particles decreased with increasing pH, from 3.7 x 10(-8) m(2)/Vs at pH 5 to 0.5 x 10(-8) m(2)/Vs at pH 12.3. An isoelectric point at pH similar to 7 was reached with the addition of 87 mmol/l NaSCN, 3 mmol/l Na2SO4 and Na2CO3, and 0.7 mmol/l Na2HPO4. The critical coagulation concentration for the 2% (w/w) dispersion was 88 mmol/l NaCl, 1.8 mmol/l Na2CO3, 1.4 mmol/l Na2SO4, and 1.2 mmol/l Na2HPO4 at pH similar to 7. The 2% dispersion at pH similar to 7 showed an almost Newtonian flow behavior. Yield values were developed after salt addition. The 2% dispersion reached a yield value of 2 Pa at 100 mmol/l NaCl, 3 Pa at 100 mmol/l Na2SO4, and 5 Pa at 100 mmol/l Na2CO3. Sodium phosphate in comparison with the other salts showed a liquefying effect. The yield value increased to 3 Pa at 1-10 mmol/l Na2HPO4 and decreased to 0.5 Pa at 100 mmol/l Na2HPO.