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Formation of Gibbsite from Aging Hydroxy-Aluminum Solutions¹

ABSTRACT

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.

¹ Contribution No. 65-3 from the College of Mineral Industries, The Pennsylvania State University. The author wishes to express his sincere thanks to Drs. L. J. Johnson and T. F. Bates of The Pennsylvania State University for their critical review and comments in the preparation of this report. The research is supported by a grant from the Earth Science Division of the National Science Foundation.

² Assistant Professor of Mineralogy, The Pennsylvania State University.

Received for publication August 16, 1965. Accepted for publication December 7, 1965.