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This article in SSSAJ

  1. Vol. 30 No. 2, p. 173-176
    Received: Aug 16, 1965



Formation of Gibbsite from Aging Hydroxy-Aluminum Solutions1

  1. Pa Ho Hsu2



Hydroxy-aluminum solutions of NaOH/AlCl3 molar ratio = 0.3 to 2.7 and concentration of Al = 0.02m were prepared and aged for 2 years. The solutions of NaOH/AlCl3 = 1.5 and below remained unchanged during the entire period of study. Those solutions of NaOH/AlCl2 molar ratio = 1.8 to 2.7 were stable for about 6 months to 1 year, then the solutions gradually became turbid, and eventually gibbsite formed in association with a decrease in pH. When a solution of NaOH/AlCl3 = 2.7 was either diluted ten-fold with water or dialyzed to remove the Na+ and Cl-, the same reaction took place in 2 weeks. When NaCl was added at a concentration of 0.12m, the solution became only slightly turbid at the end of 1 year. At 0.6 and 1.2m NaCl, amorphous Al precipitates formed immediately after preparation and remained amorphous even after 2 years.

It is proposed that the formation of gibbsite from aging hydroxy-aluminum solutions follows the same mechanism as the rapid precipitation of bayerite at NaOH/AlCl3 = 3 in an alkaline medium, the latter being discussed previously by Hsu and Bates in 1964. The difference between these two processes is the source of OH- for the completion of neutralization. The OH- added as NaOH is taken up by hydroxy-aluminum polymers almost instantaneously, whereas the spontaneous hydrolysis of these polymers is slow, particularly in the presence of high salt concentration.

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