Precipitation of Amorphous Aluminosilicates From Solutions Containing Monomeric Silica and Aluminium Ions

Summary Monomeric silica solutions were mixed with solutions containing AlCl 3 , or Al(OH) 2‐55 Cl 0‐45 to give a SiO 2 /Al 2 O 3 , ratio varying from 8 to 8.0 and were kept at pH 6–0, 7.0, or 8.0 over a period of about I year. All precipitates which formed from solutions with SiO 2 /Al 2 O 3 ratios of 1–0 or higher were X‐ray amorphous. The SiO 2 /Al 2 O 2 , ratio of the precipitate varied in the range from 1.0 to 3.0. It increased with the Si concentration and the SiO 2 /Al 2 O 3 ratio of the parent solution and with reaction time. The pH did not affect the SiO 2 /Al 2 O 3 ratio of the precipitate but did affect its dispersion and flocculation. Al 3+ and Al(OH) 2.55 +0.45 reacted differently with silica and yielded different reaction products. These differences were interpreted in terms of the stability of hydroxy‐Al polymer units in the reaction. The amount of NaOH per Al atom which was required to reach and maintain the same pH, increased with the increasing SiO 2 /A1 2 O ratio of the precipitate, but the value was generally lower than that predicted from ‘one in four’ substitution of Si by Al in four‐fold co‐ordination. A ‘neutralization’ of the positive charge on the hydroxy‐Al cations by silica was pointed out also to be a cause of the extra acidity. Implications of these observations on synthetic amorphous aluminosilicates to the genesis, structure, and properties of their natural counterparts were discussed.