Effect of temperatures on formation and transformation of hydrolytic aluminum in aqueous solutions

Abstract

The effect of temperature and aging on the formation and transformation of hydrolytic Al species were investigated. Partially neutralized Al solutions with OH/Al ranging from 0.5 to 2.6 were prepared at 25 and 70 °C, and were then aged at 25 °C for up to 180 days. The Al speciation in these solutions was determined using 27Al nuclear magnetic resonance (NMR) spectroscopy, ferron colorimetric and sulfate precipitation methods. Synthesis at high temperature results in a lower degree of hydrolysis. Specifically, the formation of Al13 was lower at OH/Al<1.8 and higher at OH/Al>1.8 when the synthesis temperature was high. Upon aging, a significant difference in the formation of gibbsite was observed for the samples synthesized at different temperatures. After aging for 180 days, gibbsite precipitates were seen in the samples prepared at 25 °C and OH/Al>1.0, while no gibbsite precipitates were collected for the sample prepared at 70 °C except the one of OH/Al=2.6. The polymeric Al fractions decreased in the samples where gibbsite was formed. The comparison of the Al speciation of the samples synthesized at different temperature shows no direct relationship between the Al13 content and the production of gibbsite. The conversion of Al13 into precipitates and the eventual formation of gibbsite may be through the decomposition of Al13 into monomeric and small polymeric an Al, which serves as Al pool for gibbsite formation.