Studies on the Mixed Gels of Silica, Alumina and Ferric Oxide

Abstract

AbstractSol‐gel transformation of mixed gels of silica, alumina and ferric oxide has been studied by determining the variations in pH with time of dialysis. For silica‐alumina mixtures a pH of 2.9 is invariably reached irrespective of the alumina content of the mixtures. The amount of alumina present in the mixture influences the time of setting of the gel to a very great extent and it decreases with increase of the alumina content of the mixtures. These results have been explained in light of chemical interaction between the two gels to give aluminosilicates as well as in terms of controlled coagulation phenomenon with alumina behaving as a foreign electrolyte for bringing about the gelation of silicic acid.The results of silicic acid, ferric oxide gel forming mixtures are altogether different. In this case the time of gelation increases with the increase in the amount of ferric oxide in the gelation mixture, while a pH of 4.2 is reached irrespective of the amount of ferric oxide present. No evidence for the formation of ferric‐silicate is obtained nor the gelation of these mixed sols can be explained in terms of controlled coagulation.The colloidal silica appears to act as protective colloid for the highly acedic ferric oxide sol. Viscometric and pH metric studies on the addition of alumina sol to mixtures of ferric oxide and silica sols indicate that alumina competes with ferric oxide in the formation of mixed gels. Here, too a pH of 2.9 is invariably reached irrespective of the alumina content in the mixed gels.