Nucleation aluminates

Bayer A process for making pure alumina hydrate from bauxite, used principally as a raw material for the manufacture of aluminum metal. The ore is digested with hot sodium hydroxide solution, yielding a solution of sodium aluminate. Insoluble impurities are separated off in the form of red mud and the solution is then nucleated with alumina hydrate from a previous batch, causing alumina trihydrate to precipitate ... 

The usual result of mixing aluminate and silicate solutions is the formation of a gel, which may later separate into a clear supernatant liquid and a gel. One may ask whether nucleation is homogeneous (in solution) or heterogeneous (in gel). A partial answer may be provided if it can be demonstrated that zeolites can grow from clear solutions. Guth et al (2) and Ueda et al ( 3, ) have shown how such solutions can be prepared. 

In contrast to more or less well defined kinetics of the crystal growth (5,6,12-16), various nucleation mechanisms have been proposed as zeolite particles forming processes. Most authors explained the formation of primary zeolite particles by nucleation in the liquid phase supersaturated with soluble silicate, aluminate and/or aluminosilicate species (1,3,5,7,16-22), with homogeneous nucleation (1,5,7,17,22), heterogeneous nucleation (5,2 1), cell walls nucleation (16) and secondary nucleation (5) as dominant processes of zeolite particles formation, but the concepts dealing with the nucleation in the gel phase are also presented in the literature (2,6,11,12,1 1,23-25). 

Conventional wisdom ascribes nucleation to microcrystalline type X which in the presence of die proper nutrients grows into a full yield of type Y with no trace of its type X initiator. The "seed" composition is essentially sodium metasilicate solution with the addition of a small amount of sodium aluminate Na2SiC>3 0.16 NaAlC>2 -21H2O. Comparatively little "seed" is necessary for a good Y synthesis as litde as 1% of die total AI2O3 may come from die seed. 

Any possible way to modify the final <001> / size ratio appeared to require that growth of crystals be conducted in an optimum range of supersaturation (corresponding to a concentration of aluminium between 2 and 4 mmol/1) without modification of the initial conditions, which determine the number of crystals nucleated. The practical way in which we realized this objective was the use of an initial synthesis medium containing only the aluminium present in the structure directing mixture. Once the nucleation occurred, an aluminate solution was injected in the autoclave in order to maintain the aluminium concentration at the desired level. The expected increase in growth surface was calculated using the above kinetic equations, and the flow rate of the injected solution was continuously adjusted in order to balance the incorporation of nutrient by the crystals. 

On comparing samples A and D, it first appears that Al(OH)3 is a more reactive Al source than Na aluminate, as nearly pure ZSM-20 is already formed after 9 days heating. Secondly, as expected from the mechanistic considerations, an increase of the initial amount of Al markedly accelerates the nucleation process of the ZSM-20 "Figure 8 . Neither the crystallinity nor the Al content of the final zeolite are affected by die initial concentration.Analysis of the final liquid phases confirmed that in all cases all the aluminium was consumed during the growth process, which means that the overall yield in crystalline ZSM-20 has increased accordingly. Indeed this increase is confirmed experimentaly "Table IB". 

Some CaA ( /or f=ssl reacts with calcium sulphate in solution. Amorphous, aluminate-rich gel forms on the surface arxj short AFt rods nucleate at edge of gel ond in solution... 

Stubby rods of AFt phase are also seen (D25 D27). They are typically some 250 nm long and 100 nm thick. Studies using wet cells show them to occur both on the surfaces of the grains, and at some distance away (S41,S68) (Fig. 7.6b). They are probably more abundant near to the surfaces of the aluminate phase, and appear to nucleate in the solution and on the outer surface of a layer of gel. On drying, this layer shrinks, and the AFt crystals fall back onto the surfaces of the cement grains. The early products thus differ in morphology and composition from the exfoliating foils or honeycombs of C-S-H that have been observed in CjS pastes. 

Calorimetric studies show that pfa retards the reaction of alite in the early stage of reaction (G70,J30,H51,W29), but with one exception (W29) studies on the middle stage show the alite reaction to be accelerated (K45,L47,H51,T44,D12). SEM shows that C-S-H and CH are deposited on the surfaces of pfa grains before these have started to react significantly (D28). The accelerating effect is probably due mainly or entirely to the provision of additional nucleation sites on the pfa, and occurs with other fine powders (K46). A QXDA study showed that the aluminate and ferrite react more rapidly in the presence of pfa, but with belite there was no detectable effect up to 28 days and a marked reduction in rate of consumption thereafter (D12). 

Good descriptions of the production of aluminum can be found in the literature (Grjotheim etal. [7], Grjotheim and Welch [8], Grjotheim and Kvande [9], Burkin [10], and Peterson and Miller [11]). Referring to Fig. 2 [12], the first step in the production of aluminum from its ore ( bauxite ) is the selective leaching of the aluminum content (present as oxides/hy dr oxides of aluminum) into hot concentrated NaOH solution to form sodium aluminate in solution. After solution purification, very pure aluminum hydroxide is precipitated from the cooled, diluted solution by addition of seed particles to nucleate the precipitation. After solid-liquid separation the alumina is dried and calcined. These operations are the heart of the Bayer process and the alumina produced is shipped to a smelter where the alumina, dissolved in a molten salt electrolyte, is electrolyt-ically reduced to liquid aluminum in Hall- Heroult cells. This liquid aluminum,... 

Alkalinity also has an important influence on the crystallization rate of zeolites. A remarkable example is the crystallization of zeolite A (LTA) from the precursor gel with a batch composition of 5Na20 Al203 2Si02 (100-200) H20.[13] Figure 3.8 shows the effect of different alkalinity (H2O/Na2O=20, 30,40) on the crystallization rate (including induction period and growth rate) and particle size of the product. Clearly, with an increase of alkalinity the crystallization process is speeded up, the particle size is decreased, and the distribution of the particle size is narrowed due to an increased nucleation rate and an increased polymerization rate between polysilicate and aluminate anions. 

The solution-mediated transport mechanism has been extensively discussed in the literature. In the middle of the 1960s, according to their studies on the crystallization of zeolite A, Kerr and Ciric proposed the solution-mediated transport mechanism. They believed that the nucleation and growth of zeolite crystals happened in solution. The initial gel was partially or completely dissolved in the solution with the formation of active silicate and aluminate ions. These active silicate and aluminate ions could further form the structural units of zeolite crystal. 

Zhdanov and colleagues for the first time discussed in detail the solution-mediated transport mechanism.[27] They believed that 1) nucleation happened in the solution or at the interface of the solution and solid gel 2) the further growth of zeolite nuclei consumed the silicate and aluminate ions in solution 3) the solution supplied the soluble structural units for the growth of zeolite crystal and 4) the consumption of the liquid component during the crystallization process resulted in the continuous dissolution of solid gel. 

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