Aging of Partially Neutralized Aluminum Solutions of Sodium Hydroxide/Aluminum Molar Ratio = 2.2
1985
Tsai, Ping Ping | Hsu, Pa Ho
Aluminum hydroxide developed in all solutions in this study (0.02 M in AlCl₃, 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 Na₀.₀₈Al(OH)₂.₄₆(SO₄)₀.₃₁(H₂O)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.
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