Boehmite precipitation

Finally it is also possible to obtain a crystalline boehmite precipitate directly, by simultaneous addition of a basic and an acidic solution to a reactor to obtain precipitation in the pH range between 6 and 8. 

Panias, D.. Role of boehmite/solution interface in boehmite precipitation from supersaturated sodium aluminate solutions. Hydrometallurgy, 74, 203, 2004. 

As shown, A1(0H)3 precipitates at moderate temperatures (60°C), but as the temparature is raised to 368 K (95°C) Boehmite (formally AIO(OH)) begins to precipitate from solutions rich in Na20 (dissolved NaOH) and AI2O3 (dissolved Al(OH)3). At 423 K (150°C) only Boehmite precipitates, but at 623 K (350°C) both Boehmite and alumina may precipitate. Depending on the reaction conditions, the Al(OH)3 precipitates as Gibbsite or Bayerite, respectively. In Figure 8.22 the compositions of the industrial extraction and precipitation of Gibbsite are illustrated. ... 

E.H. (2007) The Use of Multimodal Tomography in the Study of Boehmite Precipitation. Proceedings 5th World Congress on Industrial Process Tomography, Bergen. 

FIG. 17 A1 NMR spectra at 130.3 MHz of boehmite sols (4% A1203) after peptization with [HN03l/[Al] =0.11 of boehmite precipitates derived from inorganic route (see Tables 3 and 4). (From Ref 65.)... 

The alkali process uses sodium hydroxide and is well known as Bayer s process. It involves relatively simple inorganic and physical chemistry and the entire flowsheet can be divided into caustic digestion, clarification, precipitation and calcination. Although mineral assemblage in bauxites is extensive, processing conditions are primarily influenced by the relative proportions of alumina minerals (gibbsite and boehmite), the iron minerals (goethite and hematite), and the silica minerals (quartz and clays-usually as kaolinite). 

A number of reagents containing oxide components are used in zeolite manufacture [19]. Silica is provided by addihon of sodium or other alkali silicate solutions, precipitated, colloidal, or fumed silica, or tetraalkylorthosihcate (alkyl = methyl, ethyl) and certain mineral silicates such as clays and kaolin. Alumina is provided as sodium aluminate, aluminum sulfate soluhon, hydrous aluminum oxides such as pseudo boehmite, aluminum nitrate, or aluminum alkoxides. Additional alkali is added as hydroxide or as halide salts, while organic amines and/or... 

Aluminum is present in many primary minerals. The weathering of these primary minerals over time results in the deposition of sedimentary clay minerals, such as the aluminosilicates kaolinite and montmorillonite. The weathering of soil results in the more rapid release of silicon, and aluminum precipitates as hydrated aluminum oxides such as gibbsite and boehmite, which are constituents of bauxites and laterites (Bodek et al. 1988). Aluminum is found in the soil complexed with other electron rich species such as fluoride, sulfate, and phosphate. 

Standard-State chemical potentials for aqueous and solid A1(III) species are discussed carefiilly in the context of dissolution-precipitation reactions by B. S. Hemingway, R. A. Robie, and J. A. Apps, Revised values for the thermodynamic properties of boehmite, A10(OH), and related species and phases in the system Al-H-O, Am. Mineralog. 76 445 (1991). 

In the aluminum oxide system the precipitation pH is one of the variables which controls the nature of the phase eventually obtained. However, aging conditions of the initially formed amorphous precipitate are at least equally important. In general, it can be stated that precipitation above pH = 8 leads to the formation of bayerite, while precipitation under more acidic conditions favors the subsequent formation of boehmite. Hydrargillite is formed as the product of the Bayer process by seeding a supersaturated alkali containing aluminum solution. The formation of bayerite is strongly facilitated by the presence of alkali cations which stabilize the structure. 

In addition to gibbsite there are other routes to manufacture Al(OH>3 and the consecutive transition oxides. One is the precipitation of Al(OH)3 from aluminum salts by adjusting the pH between 7 and 12 by adding bases. Precipitation at elevated temperatures and high pH leads to formation of bayerite, whereas at lower pH pseudoboehmite and subsequently boehmite are formed. By heating, these materials can be converted to the active transition aluminas. 

Starting from an aqueous acidic Al3+ solution (for example an aluminium sulphate solution) precipitation occurs if the pH of the solution is increased above about pH = 3 by addition of a base. The first precipitate is a gel-like substance in which minute crystals of boehmite (A10(0H)) are present. If this is filtered without aging and then calcined at temperatures up to 600°C an X-ray amorphous material is obtained. The material remains amorphous until after firing to temperatures greater than 1100°C. (X-AI2O3 is formed at higher temperatures. 

There are several important hydrated forms of alumina corresponding to the stoichiometries A1(0)0H and Al(OH)3. Addition of ammonia to a boiling solution of an aluminum salt produces a form of A1(0)0H known as boehmite, which may be prepared in other ways also. A second form of A1(0)0H occurs in Nature as the mineral diaspore. The true hydroxide Al(OH)3 is obtained as a crystalline white precipitate when carbon dioxide is passed into alkaline aluminate solutions. It occurs in Nature as the mineral gibbsite. Materials sometimes referred to as /1-aluminas have other ions such as Na+ and Mg2+ present. They possess the idealized composition Na2011Al203. They can act as ion exchangers, have high electrical conductivity, and are potential solid state electrolytes for batteries. 

FIGURE 121 Pseudo-boehmite gel produced by precipitation of alumina by acid-base neutralization. Versal a Kaiser Aluminum Corp. product. Photo courteay Ron Rigge. 

Diaspore, o -AlO(OH), occurs in some types of clay and bauxite and can be synthesized by hydrothermal treatment of boehmite, y-AlO(OH), in 0.4% aqueous NaOH at 380 °C and 500 atm. Crystalline boehmite is easily obtained upon warming the amorphous, gelatinous precipitate that forms when cold solutions of aluminum salts are treated with ammonia. 

Tchoubar, C., and Oberlin, A. Alteration de I albite par action d eau. Etude en microscopie et microdiffraction electroniques de la precipitation et de 1 evolution des fibres de boehmite formee, Jour. Microsc. 2, 415-432 (1963). 

We will next consider the case of a lew silica content co-gel. A 5% silica-content silica-alumina was prepared by precipitation of aluminum isdsutoxide and tetraethoxv-silane as described for the silica-free gel. After gelation water was added just sufficient to fill the pore voids of the gel. The added water led to formation of a boehmite-rich hase during recrystallization. After drying at 120 and calcination at 500 0 for 16 hours, a transitional alumina hase is formed with a surface area of 410 m /g and a pore volume of 1.9 oc/g. This silica-alumina had an average pore diameter of 18 nm, similar to the silica-free material discussed previously. Steam treatment of this 18 nm pore diameter silica-alumina at 870°C (1600 ) in 90% H20-10% N2 for 16 hours resulted in a material with surface area of 196 m /g. This surface area is much hi er than expected for an amori ous gel and is consistent with silica enrichment of the outer surface during the recrystallization step vhere water was added to the pores of the amoridious gel. Silica stabilization of bodunite alumina by formation of a surface Aiase complex has been reported in recent work (9). ESCA analysis also indicates silica surface enrichment vhen compared to the amori ous gel. 

The sequence of phase transformations shown in Figure 2 is an approximationp largely because process variables such as time, atmosphere and properties of precursor hydroxides are not included. Thus, for example, bayerite and glbbslte may be converted to boehmite and thence to y-alumina during calcination if the particle size Is large and the precipitate Is moist [18]. 

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