Solubility of alumina

O-SiAlON is another crystalline phase of interest. There is a limited solubility of alumina in silicon oxynitride structure to give O-SiAlONs, represented by the formula Si2 A-Aly01+A.N2 where ovaries from zero to 0.2. Formation of O-SiAION occurs in the same mechanism as P-SiAlON i.e. Si + N is replaced by Al + O. The lattice parameters of O-SiAION, Si2 AI/)i+aN2 a, increase in a very typical way with the x value.37... 

Figure 17 Influence of additives on the solubility of alumina [153,154] in cryolite melts at 1010°C.

Aluminum is the second most abundant metal on earth s crust. It is a common metal in tropical soils called laterites (red soils). It is extracted from bauxite that is a rich laterite by Bayer process that involves dissolution and separation of the oxide in caustic soda solution between 150 and 250°C and 20 atm of pressure. Though abundant and inexpensive, alumina based CBPCs are difficult to form because even in an acid solution the solubility of alumina is very low. This solubility, however, can be enhanced by a mUd thermal treatment and suitable CBPCs can be formed. Alumina is available commercially as calcined alumina called corundum, or as its hydrated forms such as aluminum hydroxide (Al(OH)3), as bohmite, (A1203-3H20), gibbsite (AI2O3 H2O) or in impure forms as in kaolin clay. These mineral forms and their use in ceramic formation are discussed in Chapter 11. 

Unlike divalent oxides, the solubility of alumina is low and hence some warm temperature treatment is required. In addition, rather than using lower solubility phosphate solutions such as ammonium and potassium phosphate solutions, phosphoric acid solution is directly used. Wagh et al. [ 17] employed a thermodynamic analysis to study the elfect of the temperature on the solubility of individual phases of alumina on the formation of its phosphate phases during heat treatment where solubility of hydrated aluminum oxide, viz., hydrargillite (A1203-3H20) is enhanced, and that contributes further to the formation of berlinite phase. They confirmed this by differential thermal analysis (DTA) and X-ray diffraction (XRD) analysis on samples heated beyond 118°C. 

Because of this neutralization, the solubility of alumina decreases till the solution attains the neutral pH, in which as noted by Pourbaix [22], there is abundance of A1203-3H20 which has a very low solubility. A1203-3H20 forms a passivation layer on the surface of individual particles, and consequently, further solubilization is hindered by this coating and formation of the ceramic is obstmcted. At this point, it is necessary to enhance the solubility of A1203-3H20 by other means. 

From Eq. 11.32, one may also calculate the amount of AIPO4 formed as a bonding phase. It gives us 1.87 X 10 g for each gram of alumina. This is a very small fraction that forms a thin layer on each particle of alumina and holds adjacent particles together. Thus, conversion of alumina needed to form a berlinite bonded alumina ceramic is very small, and therefore, berlinite bonding is possible even when the solubility of alumina is very small. 

Similar results have been reported by Mattern et al. (2002), using more recent equations of state for lower-mantle minerals and incorporating the solubility of alumina in silicate perovskite. They also used a three-layered slab model (midocean ridge basalt (MORE) over harzburgite over pyrolite), but with a MORE composition (Si/(Mg - - Fe) = 2.29) intermediate between our extreme end-members of the Helffrich et al. (1989) eclogite (1.65) and the Helffrich and Stein (1993) gabbro (2.58). 

Kvande (1980, 1986) suggested that Al20F is the most abundant species in cryolite melts with low alumina contents. He further claimed that the solubility of alumina in NaF-AlFs melts attains a maximum at the composition of cryolite. Based on this, he argued that it is reasonable to assume that alumina, when dissolving, reacts predominantly with A1F anions according to the following reaction scheme... 

Alumina is amphoteric, which means that it dissolves in acidic and basic solutions, but not in neutral aqueous solutions. The solubility of alumina in solutions of pH from about 4-9 is low at 25°C it is less than 10 7mol 1 1 at pH 6 [1], Alternatively, alumina dissolves readily in strong acids (HC1, HN03, H2S04) and strong bases (NaOH, KOH) at temperatures well above ambient (e.g., 90°C). 

Certain impurities such as aluminum in minute amounts not only reduce the rate of dissolution of silica, but by chemisorption on the surface of silica, even in amounts less than a monomolecular layer, reduce the solubility of silica at equilibrium. Jeph-cott and Johnston (179a) have shown that the apparent solubility of amorphous finely divided silica in water, which they find to be 0.017% at 37 C, is reduced to 0.003-0.0097% when aluminum oxide is added to the system and to less than 0.0001% when powdered aluminum is present. Since it was also shown that the addition of silica to a suspension of alumina depressed the solubility of alumina, it seems likely that a surface containing both SiOj and AljOa is formed on both the silica and alumina phases and has a lower solubility than either oxide. Earlier Denny. Robson, and Irwin (179b) had shown similar effects of iron and aluminum. 

The effect of soluble silica on the solubility of alumina was only slight, since the alu-... 

The products of the reactions of cryolite components with alumina silica refractories are albite (sodium feldspar) and nepheUne, which may coexist with cryolite. The solubility of alumina and silica in cryolite and sodium fluoride lowers as the acidity of the system decreases, and the freezing point of the system decreases as the silica content increases (from nepheline to albite). 

The stability of natural oxide film, which governs the corrosion resistance of aluminium, depends on pH (Figure B.1.18). Most natural, untreated and unpolluted surface waters have a pH between 6.5 and 7.5. Since the solubility of alumina is minute and is practically constant in this pH range (Figure D.1.14), pH is not an important factor for the corrosivity of natural waters. 

Alumina is incorporated as a sohd solution of the iron aluminate spinel, hercynite, in the crystal lattice. The alumina concentration should be less than the solubility of alumina in magnetite. This corresponds to a maximiun content of about 3% alumina. Any excess of alumina does not go into solid solution, and leads to a reduction in catalytic activity, particitlarly when using catalysts promoted with alumina. The presence of alumina as a structural promoter also leads to the formation of wustite and stabihzes the reduced catalyst. Small amounts of magnesia can also dissolve into magnetite and act as a promoter. The calcium component exists in the form of ferrites or alirminates by neutrahzing acidic components—such as silica—and protects the potash that activates the catalyst. 

I992H0I] Holcomb, G.R., Pierre, G.R.St., The Solubility of Alumina in Liquid Iron , Metall. Trans. B,... 

---