Formation of Gibbsite from Aging Hydroxy‐Aluminum Solutions

Hydroxy‐aluminum solutions of NaOH/AlCl 3 molar ratio = 0.3 to 2.7 and concentration of Al = 0.02 m were prepared and aged for 2 years. The solutions of NaOH/AlCl 3 = 1.5 and below remained unchanged during the entire period of study. Those solutions of NaOH/AlCl 2 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/AlCl 3 = 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 0.12 m , the solution became only slightly turbid at the end of 1 year. At 0.6 and 1.2 m 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/AlCl 3 = 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.