Na⁺ Ion-Exchange of Montmorillonite by Autoclave Treatment

Abstract

The autoclave treatment was adopted for Na⁺ ion-exchange of the montmorillonite previously deironation-treated in order to prepare the Na-montmorillonite for use in the ceramic manufacturing as the material giving high plasticity. Osudo Ca, Mgmontmorillonite with a small amount of iron in the clay structure was refined by elutriation under 2μm and oxidation of the iron sulfide impurity with ozone at pH 2.5, then it was reacted with sodium hydroxide and sodium metasillicate respectively under the hydrothermal condition of temperature at 135-150°C (-3-5kg/cm²) for 24 hrs.
In each reaction, the silica components dissolving in the clay slurry acted fixing Ca²⁺ and Mg²⁺ ions which occupied about 25% of the total base exchangeable cations of the clay by forming the silicate compounds with low solubility, resulting the promotion of Na⁺ ion-exchange. By the treatment with sodium hydroxide, a nearly whole amount of H⁺ ions which were the main interlayer cation and a large part of Mg²⁺ ions were displaced by Na⁺ ions. Sodium hydroxide acted not only neutralizing the H⁺ ions on tne clay surface but also increasing the dissolution of the amorphous silica in the clay. In the reaction with sodium metasilicate, some conditioning reagent for decreasing the solubility of the silicate ions was necessary so that Na⁺ ions were displaced effectively as the counter-ions on the clay surface. When the p H of the slurry was lowered to 7 with hydrochloric acid, Na⁺ ion displacement improved with the decrease of the exchangeable Ca²⁺ and Mg²⁺ ions to a great extent. Na-montmorillonite with high quality was obtained without p H control by the treatment with sodium metasilicate plus silica sol, which presumably acted as the nuclei for polimerization of the silicate ions.