Phase Diagram and Cross-Linkages

A discussion of phase diagrams for pertinent phosphate systems is given in Chapter 5. Whether or not a presentation of the K2O-P2O5 phase diagram should be delayed until Chapter 5 was considered. If difficulties are encountered, hopefully, referring to Chapter 5 will be helpful. 

No potassium phosphate crystals of any kind can be formed in an anhydrous system with a K2O-P2O5 ratio less than 0.5. It was shown above that a 0.5 ratio is the minimum ratio at which [KPOaln crystals can form. The eutectic behavior is the dissolution of [KPOsIm crystals in a melt with a ratio near the eutectic composition when the system contains less than 68% P2O5. When all the amorphous phase with a P2O5 concentration greater than the eutectic composition is depleted, temperatures must be increased to dissolve the remaining [KPOsln crystals in a melt that is richer in K2O than the eutectic composition. This is an explanation of the behavior of this system as determined, but leaves unanswered questions as to 

It is concluded that cross-linking of Kurrol s salts is a random transient event that occurs because the system is not at equilibrium. Allowed sufficient time, these systems will become mixtures of linear [KP03]n molecules and an amorphous phase. A sample that was held at temperatures just below the melting point for five days lost much of the exaggerated aqueous solution properties usually associated with cross-linked Kurrol s salts. 

Although the above approach is easy and straightforward, care must be exercised when applying it to more complex systems. Growth of calcium polyphosphates in sodium ultraphosphate melts is a three-component system rather than two, even when all water is driven from the system. It is recommended that the simpler method of assuming all sodium and no calcium reported to an amorphous phase (as was done in Chapter 6, Section 6.5) be used to estimate ratios of amorphous to crystalline phases or portions of a melt that is expected to be water soluble when the melt is fully crystallized. These estimates will not be precise, but are useful in estimating potential yields. 

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