How does Apatite II work?

The process consists of two steps, a dissolution reaction of Apatite II followed by a precipitation reaction of the pyromorphite nucleated on the pre-existing Apatite II surface. Although the solubility product for Apatite II is very low, Ksp 10-20, the solubility product for pyromorphite is extremely low, Ksp = 10-79, providing the driving force for the overall reaction.

The dissolution reaction of Apatite II buffers the pH to about neutral and is especially favored under acidic conditions.