Aging of Partially Neutralized Aluminum Solutions of Sodium Hydroxide/Aluminum Molar Ratio = 2.2

Abstract

Abstract Aluminum hydroxide developed in all solutions in this study (0.02 M in AlCl 3 , NaOH/Al molar ratio in preparation = 2.2) after prolonged aging. The induction period and the rate of aluminum hydroxide [Al(OH) 3 ] crystallization varied greatly among solutions, however. The Al ions in the freshly prepared solutions were predominantly OH‐Al polymers that reacted with ferron rapidly and yielded tetrahedral crystals of basic aluminum sulfate, of composition Na 0.08 Al(OH) 2.46 (SO 4 ) 0.31 (H 2 O) x , upon the addition of sodium sulfate. During aging, the initial OH‐Al polymers gradually converted to either more stable polymers of similar basicity or Al(OH) 3 precipitate. The conversion of metastable to stable polymers probably proceeded through internal structural rearrangement, but the development of Al(OH) 3 proceeded through redissolution and crystallization. The variation in the induction period was attributed to the initial number of nuclei present as a result of localized high alkalinity during solution preparation. The variation in the rate of Al(OH) 3 crystallization was attributed to the distribution of metastable and stable OH‐Al polymers at the time of crystallization. The crystallization of Al(OH) 3 was faster from metastable polymers than from stable ones.