Aluminium-phosphate formation

We have noted earlier that aluminium is unusual in forming alumino-phosphate complexes in phosphoric acid solution which may be of a polymeric nature. Bearing in mind the analogies between aluminium phosphate and silica structures, it may well be that during cement formation an aluminium phosphate hydrogel is formed. Its character may be analogous to that of silica gel, where a structure is built up by the... 

The setting reaction of dental silicate cement was not understood until 1970. An early opinion, that of Steenbock (quoted by Voelker, 1916a,b), was that setting was due to the formation of calcium and aluminium phosphates. Later, Ray (1934) attributed setting to the gelation of silicic acid, and this became the received opinion (Skinner Phillips, 1960). Wilson Batchelor (1968) disagreed and concluded from a study of the acid solubility that the dental silicate cement matrix could not be composed of silica gel but instead could be a silico-phosphate gel. However, infrared spectroscopy failed to detect the presence of P-O-Si and P-O-P bonds (Wilson Mesley, 1968). 

The nature of the setting reaction was finally elucidated by Wilson et al. (1970a), who established that formation of an aluminium phosphate gel was responsible although siliceous gel was also formed it merely coated the partly reacted glass particles. 

The following account is based mainly on the studies of Wilson and coworkers, with some re-interpretation of experimental data. The composition of the cement used is given in Table 6.9. In brief, the reaction takes place in several overlapping stages extraction of ions from the glass, migration of cations into the aqueous phase, precipitation of insoluble salts as pH increases, leading to formation of an aluminium phosphate gel. 

Bjerrum, N. Dahm, C. R. (1931). Studies on aluminium phosphate. I. Complex formation in acid solution. Zeitschrift fur physikalische Chemie, Bodenstein Festband 627-37 Chemical Abstracts, 26, 666). 

Genge, J. A. R. Salmon, J. E. (1959). Aluminium phosphates. Part III. Complex formation between tervalent metals and orthophosphoric acid. Journal of the Chemical Society, 1459-63. 

The senior author first became interested in acid-base cements in 1964 when he undertook to examine the deficiencies of the dental silicate cement with a view to improving performance. At that time there was much concern by both dental surgeon and patient at the failure of this aesthetic material which was used to restore front teeth. Indeed, at the time, one correspondent commenting on this problem to a newspaper remarked that although mankind had solved the problem of nuclear energy the same could not be said of the restoration of front teeth. At the time it was supposed that the dental silicate cement was, as its name implied, a silicate cement which set by the formation of silica gel. Structural studies at the Laboratory of the Government Chemist (LGC) soon proved that this view was incorrect and that the cement set by formation of an amorphous aluminium phosphate salt. Thus we became aware of and intrigued by a class of materials that set by an acid-base reaction. It appeared that there was endless scope for the formulation of novel materials based on this concept. And so it proved. 

Iron interferes with the test and should be absent. Chromium forms a similar lake in acetate solution, but this is rapidly decomposed by the addition of the ammoniacal ammonium carbonate solution. Beryllium gives a lake similar to that formed by aluminium. Phosphates, when present in considerable quantity, prevent the formation of the lake. It is then best to precipitate the aluminium phosphate by the addition of ammonia solution the resultant precipitate is redissolved in dilute acid, and the test applied in the usual way. 

Although many soil scientists had considered the possible mechanisms which soils employ for the retention [fixation] of phosphorus, it remained for Haseman et al. (1950) to demonstrate that phosphorus could — and in experimental situations did — replace the silicon of micas and clay minerals in order to form crystalline hydrous aluminium phosphates of sodium, ammonium and potassium. Prior to experimentation by this group, associated with the laboratories of the Tennessee Valley Authority (TVA), most authors attributed the retention of phosphorus by soils to combination with calcium to produce fairly insoluble minerals to adsorptive, exchangeable combination with silicate minerals and to formation of phosphates of iron... 

The effect of water content and heat treatment on the formation of aluminium phosphates, particularly Al4(P40i2)3, has been investigated, and the changes in the properties of this material and AIPO4 on grinding have... 

Aluminium induces demineralization of previously formed bone (Frayssinet et al, 1994), which can be ascribed to formation of stable complex aluminium phosphate compounds (Ribeiro et al, 1995). Aluminium ions may be produced either as a result of dissolution of Ti-6A1-4V alloy or of corrosion of alumina coatings. In pH 4 buffer the release of aluminium ions from alumina is much more significant than at... 

The chemistry of the acid aluminium phosphates and their hydrates is complicated. Thermal dehydration of the anhydrous salts leads to condensation and the formation of polyphosphates (5.123), (5.135). Dehydration of the crystalline hydrates can give rise to various mixtures of anhydrous acid phosphates, AIPO4 and amorphous material, depending on the conditions employed. Aluminium acid phosphates have isomorphous iron analogues with which they can form solid solutions. Acid phosphates of iron and aluminium are present in soils and are very much involved in the reactions which take place between soil and fertilisers (Chapter 12.2). Aluminium hydroxide/phos-phoric acid solutions with AI2O3/P2O5 = 1.0-1.5 will give extremely viscous fluids which can be dried down to amorphous solids. Such solids can then be re-dispersed in water to form viscous solutions which are stable under acid conditions. 

Polymeric varieties of aluminium phosphates have found important applications in cements and in the bonding of refractories. The basis of phosphate bonding is the formation of polymers on dehydration, which may be poly or metaphosphates, or ultimately AIPO4 (5.135) (Chapter 12.10). The many applications of aluminium orthophosphates include dental cements, metal coatings, binders and adhesives and corrosion-inhibiting pigments (Chapter 12.8). 

Sodium aluminium phosphate, 5Na3P04 3AIPO4 (approximate), is also used for these purposes, since it also increases the cheesy flavour and prevents the formation of calcium phosphate crystals which can make the cheese gritty [28]. Only 1% or 2% of phosphate additives are usually needed. 

Walker process. The first successful industrial facility for the oxidation of hydrocarbon gases is apparently that based on the process developed by the Cities Service Oil Co Company in Tallant (Oklahoma), also known as the Walker process. The process was discovered by accident, as a result of studying the oxidation of natural gas as a probable cause of pipeline corrosion because of the formation of carboxylic acids. Partially stripped natural gas was used, into which air compressed to 21 atm was injected. The process was conducted over an aluminium phosphate catalyst without recycling at low conversion, pressure of 21 atm, and temperature of 425 °C. C e of the subsequent patents describes the oxidation of natural gas to a mixture of methanol, formaldehyde, and acetaldehyde over a mixed catalyst composed of aluminium phosphate and metal oxides. The exhaust gas was discharged into the gas supply system for sale. The main products were methanol and formaldehyde [93,260,264]. 

Filiform corrosion is characterised by the formation of a network of threadlike filaments of corrosion products on the surface of a metal coated with a transparent lacquer or a paint him, as a result of exposure to a humid atmosphere. This phenomenon first attracted attention because of its formation on lacquered steel, and for this reason it is sometimes referred to as underfilm corrosion, but although it is most readily observed under a transparent lacquer it can also occur under an opaque paint film or on a bare metal surface. Filiform corrosion has been observed on steel, zinc, magnesium and aluminium coated with lacquers and paints, and with aluminium foil coated with paper. Surface treatment of the metal by phosphating or chromating lessens the tendency for filiform corrosion to occur, but it is not completely... 

Cleaners containing silicate can cause problems. They should not be used prior to an alkaline process on aluminium, owing to the formation on the surface of alkali-insoluble aluminium silicate. Silicated cleaners can also cause problems before some surface-sensitive zinc phosphating solutions, especially the more modern low-zinc type. 

It is the formation of this material which makes the reaction have a low atom economy and, owing to the cost of disposal (usually by conversion to calcium phosphate and disposal as hazardous waste), has limited its commercial usefulness to high value products. Several methods have been developed to recycle (Ph)3PO into (Ph)3P but these have proved more complex than might be expected. Typically the oxide is converted to the chloride which is reduced by heating with aluminium. Overall this recovery is expensive and also produces significant amounts of waste. 

The actions of zinc and aluminium differ. In general, metal ions such as zinc merely serve to neutralize the acid and are present in solution as simple ions (Holroyd Salmon, 1956 O Neill et al., 1982). But aluminium has a special effect in contrast to zinc, it prevents the formation of crystallites during the cement-forming reaction in zinc phosphate cements. 

Cement formation between MgO and various acid phosphates involves both acid-base and hydration reactions. The reaction products can be either crystalline or amorphous some crystalline species are shown in Table 6.5. The presence of ammonium or aluminium ions exerts a decisive influence on the course of the cement-forming reaction. 

Cement formation with aluminium acid phosphate... 

Chemical effects include stable compound formation and ionization, both of which decrease the population of free atoms in the sample vapour and thereby lower the measured absorbance. Examples of compound formation include reactions between alkaline earth metals and oxyanions such as aluminates, silicates and phosphates, as well as the formation of stable oxides of aluminium, vanadium, boron etc. 

However, even simple antacid therapy can have unlooked-for effects. Thus, because of the formation of insoluble phosphates, prolonged use of antacids (excepting aluminium or calcium phosphate) can lead to hypophosphataemia and a compensating increase in calcium absorption or mobilisation from bone4. ... 

---