Alumina dissolution rate

Finally, scheme D is probably more realistic than either of the previous ones. It is hard to determine without further study which effect will predominate in this case, but in any case tungstate adsorption may be expected to exert an influence on alumina dissolution rates. 

Indeed, the oxide/water contact time is drastically reduced if one freezes the impregnated catalyst with liquid nitrogen a few seconds after impregnation, and then freeze-dries the solvent in vacuum [48]. The consequence of this procedure is to limit the extent of M0O3 formation after calcination (Figure 4.6) alumina dissolution and [AlMoe] precipitation have been inhibited, whereas conventional drying at about 100-120°C probably speeds up the alumina dissolution rate, especially since liquid water is initially present. 

Experimental method and apparatus for alumina dissolution rate measurement... 

Specific adsorption of ligands can enhance or inhibit dissolution rates by altering the strength and lability of Me-0 lattice bonds. Salicylate, oxalate, and citrate promote the dissolution of alumina (40). In the presence of ligand (L) the dissolution rate becomes (7 ) , ... 

A prior model was described by Marshall et. al. (36), but this did not Include the hemlhydrate population balance. A version of the present model was given by Steemson et. al. (37), centred around a simulation package developed for alumina plant modelling. This version Included hemlhydrate dissolution but used an assumed correlation for the dissolution rate. 

The dissolution rate of alumina depends on its properties specific surface, a-alumina content, particle size, the content of volatile components, the way of addition, etc. Corundum, or a-alumina, is the crystal modification of alumina which crystallizes in a compact lattice and it is the stable phase at high temperature. 

Bagshaw et al. [155] noticed that a low dissolution time is achieved by using alumina which release volatiles in the bath. Generally as the contact surface of alumina-electrolyte increases, the dissolution time of alumina decreases [156], but also when the a-alumina content increases the dissolution rate of alumina decreases [157-160],... 

II. The dissolution rate is controlled by interdiffusion of hydrogen or hydronium ions and species contained in lattice sites within the interior of the silicate phase. This process results in a leached layer consisting mainly of silica and alumina. Such a layer may retain the original silicate structure (W, 11) or may represent a collapsed or hydrated layer (12, 13). 

However, a higher KOH concentration leads to a higher removal rate. In Figure 5.18, the point with highest removal rate in the curve for KOH corresponds to 2 wt% of KOH concentration. KOH probably enhances dissolution rate of oxide. This explanation is supported by the experimental fact that 2 wt% of KOH solution without any abrasive gives a removal rate of about 20 nm/min (which is about half of the CMP rate when 2 wt. % of alumina ab-... 

There are also reports of opening the barrier layer by anodic dissolution [13]. An applied voltage less than the anodic oxidation voltage ( 10-20 V) can lead to dissolution of the barrier oxide. For this to happen the dissolution rate has to be more than the anodic oxidation rate. This method is particularly suitable for cases where the alumina is being grown as a template on a substrate and the membrane is thin and supported by a substrate. 

In this paper, the influence of alumina LOI (loss on ignition) on the dissolution behavior of alumina in cryolite-based melts was investigated. Besides bath temperature and bath composition, the dissolution rate of alumina depends on its physicochemical properties such as particle size distribution, density, specific surface area, crystal structure, and LOI. 

As shown in the Figure 1.8.4, the higher the LOI of the alumina, the faster it dissolves. This is because of the high activity of low-temperature calcined alumina such as y-Al203 (800°C,lh). With the increase in calcination temperature, the crystal form changes from y phase to a phase, causing the activity of the alumina to decrease gradually. The stmcture of the sample prepared from calcination at 1100 °C is mainly a phase, therefore this sample showed the lowest dissolution rate. 

Influence of specific surface area on dissolution rate of alumina... 

Under low alumina concentration, the dissolved alumina has little effect on the dissolution rate of newly added alumina. The time interval between each addition is almost the same, indictated in Figure 1.8.6a. Ignoring the effects on newly added alumina caused by dissolved alumina, the average dissolution rate of the alumina samples, which is the linear fitting slopes of the curves in Figure 1.8.6a, is shown in Figure 1.8.6b. 

Figure 1.8.6 (a, b) Average dissolution rates of alumina samples with different LOI... 

Aluminas prepared at different calcination temperatures have a great difference in BET surface area and LOI. The difference directly leads to different dissolution rates of alumina. The alumina dissolves faster with higher LOI, which means that alumina with high LOI is a more suitable raw feeding material for low temperature aluminum electrolysis. 

The dissolution rate of alumina depends on the pH value, as shown in Figure B.1.18. It is higher at acidic and alkaline pH values, which reflects the amphoteric properties of aluminium oxide. 

Figure B.1.18. Dissolution rate of alumina in aqueous media as a function of pH [33].

Aluminium and alumina are amphoteric, i.e. can be attacked both by acids, mainly hydracids such as HF and HCl, and by bases such as sodium hydroxide (NaOH), potassium hydroxide (KOH) and sodium carbonate (Na2C03). For this reason, the dissolution rate... 

At the O/S interface, for each molecule of alumina formed inside the oxide layer, i.e., three O2- ions transferred across the O/S interface, six hydrogen ions are formed. Thus, the acidity at the interface tends to rise to an extent which depends on the rate removal of these ions by some mechanism. In view of Eqs. (13) to (15), this should lead to oxide dissolution and a further decrease... 

With such a model, the rate of increase in oxide thickness is determined by the difference between alumina formation, strictly following Faraday s law, and its dissolution, the rate of which should be some function of hydrogen ion concentration at the interface, i.e.,... 

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