Hydrothermal hydrolysis of aluminum(3+) and the precipitation of boehmite from aqueous solution

Abstract

Pubiication costs assisted by Atomic Energy of Canada Ltd.The hydrothermal hydrolysis of A13+ in aqueous KC1 solutions and the precipitation of boehmite (7-A100H) have been studied by in situ acidity measurements at temperatures to 200". The effect of varying the initial solution pH and the total aluminum concentration was investigated. The hydrothermal hydrolysis behavior was found to be consistent with the equilibria 2A13+ + 2H20 = A12(OH)24+ + 2H+ and 14A13+ + 34H20 = A114(OH)34~+ + 34H+ followed by the irreversible formation of boehmite from the polymeric species AIl4( OH)348+. IntroductionIt has long been recognized that oxides and hydroxides can be precipitated hydrothermally from solutions containing dissolved metal ions. This phenomenon is of considerable scientific and practical interest since it is believed to occur in processes such as the formation of terrestrial ore deposits28 and the growth of oxide layers on corroding metal surfaces.2b Helgesonza has recently reviewed previous work on the hydrothermal deposition of solid products from ionic solutions. These studies show that the deposition process is largely determined by the effect of temperature on the position of equilibria between metal ions and complexing species in solution. The equilibria of interest in the present study involve the hydrolysis of metal ions to form progressively higher hydrolyzed species and ultimately the solid oxide or hydroxide. While it is known that these equilibria shift in the general direction of the solid product with increasing temperature, few details of this process have been determined. For instance, almost no data are available on the identity of the hydrolyzed species in the system prior to precipitation or as to how temperature affects the pH of the solution and the average number of hydroxide ions bound per metal atom in the system. These data are required before a quantitative description of hydrothermal precipitation is possible.In this study we have used recently developed potentiometric techniques334 to measure the acidity of aluminum chloride-potassium chloride solutions at temperatures to 200". The hydrolysis and precipitation phenomena have been studied as a function of initial solution pH, total aluminum concentration, and time of storage of the solution at 25". Detailed X-ray diffraction studies of the precipitated phase were made to supplement the aciditytemperature measurements outlined above.

Stabilized Ceramic Membrane Electrodes for the Measurement of pH at Elevated Temperatures

“…[5] into Eq. [4] therefore yields the deviation function as DEV --RT in (]l/f2)/2F [6] where ]1 and f2 are the fugacities of water for the internal and test solutions, respectively. Precise values for fl [0.1m KC1 q-0.01m B(OH)3 -t-0.01m KOH] and S2 [0.01m B(OH)3 -b 0.01m KOH] are not available.…”

“…The importance of pH measurements for defining the properties of aqueous systems at elevated temperatures is well recognized (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13). Previously, both palladium hydride (1,(9)(10)(11)(12)(13) and platinized (or palladized) platinum hydrogen electrodes (2-7) have been employed for this purpose.…”

Hydrolysis-precipitation studies of aluminum (III) solutions. I. Titration of acidified aluminum nitrate solutions

“…Potentiometric techniques are featured prominently in experiments designed to measure precisely the hydrolysis equilibria in aluminum salt solutions (1)(2)(3)(4). These and other studies suggest that at very low total aluminum concentrations (about 10 -5 M) the mono-nuclear complexes AI(OH) ~+ and AI(OH)2 +, are present in significant amounts.…”

“…The best quantitative evidence for this conclusion is furnished by the precise measurements of Kubota (5) and Nazarenko and Nevskaya (6). At increasing A1 concentrations the formation of polynuclear hydroxo-complexes are called upon to explain the experimental data (2)(3)(4)(7)(8)(9). Unfortunately, no general consensus of opinion exists as to the exact identity and number of such polymeric species in solution.…”

Measurement of pH and Redox Potential in Boric Acid/Lithium Hydroxide Buffer Solutions at Elevated Temperatures

“…A major obstacle to the measurement of pH at elevated temperatures is the lack of a reliable and generally applicable method for measuring the activity of hydrogen ion in these hostile environments. The precise pH measurements using concentration cells, which have been described by Mesmer et al (2)(3)(4)(5) and Macdonald and co-workers (6)(7)(8), and the use of single hydrogen electrodes, require the presence of molecular hydrogen in the system. Accordingly, these techniques are applicable only to those systems that are stable to reduction by molecular hydrogen.…”