When I was studying engineering I did a Vacation Employment at ACF & Shirleys fertilizer plant on the Brisbane River. They imported phosphate rock from Naru and had a Sulphuric Acid Plant, an Ammonia Plant and a Phosphoric Acid plant. I worked in all of them, and prepared a report for assessment.
They produced Superphosphate, which is SOP, by digesting the rock phosphate with the sulphuric acid. Double Super by further reaction with phosphoric acid, and Triple Super using even more phosphoric acid. The ammonia added nitrogen to various mixes.
It was a long time ago and I have forgotten most of what I learned, but that might give you some idea of what is involved in the quest to make the phosphate soluble and get rid of the harmful chloride from MOP (KCl, potassium chloride) which will render most Australian soil barren by salt build up.
Amazing that GPH just pumps the good stuff up ready to use.
Those who wish to do a bit of extra research here might also like to look at the price of Phosphate Fertilizers on Google or Amazon etc. Retail of course but SOP >>> MOP