Precipitation from supersaturated aluminate solutions. II. Role of temperature

Abstract

The effect of temperature on the precipitation of aluminum hydroxide from dilute potassium aluminate solutions (C~a<OH~, = 4 X 10 -3 M) was studied in acid titration and pH-stat experiments. The precipitation sequence is largely dictated by the supersaturation (II) and follows the Ostwald rule of stages: amorphous --* pseudoboehmite --~ bayerite --~ gibbsite. The precipitation boundaries, -log aAltOH~/aoH-, of the amorphous phase (-1.9) and pseudoboehmite (-1.2) evaluated by interpretations of relaxation time curves are temperature insensitive. The interpretation of relaxation curves in the supersaturation region where pure bayerite forms is based on a surface nucleation mechanism and allows estimates of the free energy of formation of the critical nucleus (~20 El/mole), interfacial tension and two activation energies (-30 El/mole, +95 El/mole). The preferred formation of pseudoboehmite at high pOH is shown to be the result of a lower interfacial energy and the retarding effect it exerts at all temperatures on the competing growth of bayerite crystals. The growth rate of bayerite is shown to be proportional to the available surface and the square of the supersaturation and is characterized by an activation energy of about 60-80 El/mole.

The Influence of Alkali Metal Ions on Homogeneous Nucleation of Al(OH)3 Crystals from Supersaturated Caustic Aluminate Solutions

“…E a values of 160 ± 20 and 158 ± 20 kJ mol −1 were calculated for nucleation from sodium and potassium aluminate solutions, respectively. These values are higher than the 95 ± 10 kJ mol −1 reported for the nucleation of pure bayerite (41), which exhibits faster crystallization kinetics than gibbsite. The present value is also higher than 55 kJ mol −1 reported for the growth of Al(OH) 3 bayerite and gibbsite (23,41).…”

“…The value obtained in the present work is comparable to 45 ± 6 mJ m −2 reported by (11), from gibbsite nucleation studies in which a rather low β value of 2.2 (<β = 16.8 for sphere) was used. γ of 67 ± 20 and 25 mJ m −2 have been respectively estimated for bayerite and pseudoboehmite crystallization (40,41), while 140 mJ m −2 was determined from apparent solubility of gibbsite in saturated aqueous solutions (25,42).…”

Hidratación del cemento de aluminato de calcio en condiciones de muy elevada alcalinidad

“…This would appear to be related to aluminium solubility at high pH. Earlier studies (24,25) showed that in supersaturated aluminium solutions, the sequence of aluminium hydroxide precipitation is "amorphous-pseudoboehmite-bayerite". In other words, rapid precipitation in alkaline aluminate solutions generates bayerite.…”

“…De nuevo este comportamiento parece estar relacionado con la solubilidad del aluminio a elevados pH. En trabajos previos (24,25) se ha demostrado que en disoluciones sobresaturadas de aluminio a temperatura ambiente la secuencia de precipitación del hidróxido de aluminio es "amorfo-pseudoboehmite-bayerita". Es decir, la precipitación rápida de las disoluciones de aluminatos alcalinos produce bayerita.…”

Uticaj Dodatka Glukoze Na Specifičnu Površinu I Teksturalne Osobine Nano Kristalne Mezoporozne Alumine

“…bajerit, nordstrandit, slabo kristalni bemit [1,2,4]. Kristalizacija dobro kristalnog bajerita uglavnom preovlađuje iz razrijeđenih rastvora aluminata (≤ 1M NaOH) [8], dok kod koncentrovanih rastvora (≥2,5 M NaOH) [7,8] preovlađuje gibsit. Rastvori srednjih koncentracija (1,0-2,5 M NaOH) daju smješu gibsita i bajerita.…”