After a quick search, there were two methods of industrial production of phosphoric acid, Thermal Phosphoric Acid Method, and Wet Phosphoric Acid Method.
Here are some details on the two methods:
Thermal phosphoric acid: This very pure phosphoric acid is obtained by burning elemental phosphorus to produce phosphorus pentoxide and dissolving the product in dilute phosphoric acid. This is the cleanest way of producing phosphoric acid, since most impurities present in the rock have been removed when extracting Phosphorus from the rock in a furnace. The end result is food grade, thermal phosphoric acid; however, for critical applications additional processing to remove arsenic compounds may be needed.
Wet phosphoric acid: Green phosphoric acid is prepared by adding sulphuric acid to calcium phosphate rock, or slurry. The reaction for calcium phosphate slurry is:
3H2SO4(aq) + Ca3(PO4)2(aq) + 6H2O(l) ↔ 2H3PO4(aq) + 3CaSO4(aq)+ 6H2O(l)
Through modern filtering techniques the wet process acid can be cleaned up significantly but still isn't as pure as thermal phosphoric acid; as it may contain other acidic species such as hydrofluoric acid.
If the phosphoric acid is originally created by the Thermal Method (for food grade quality), you may want to try a cation exchanger. I am not sure if this would work given large volumes of phosphoric acid, or high levels of iron contamination, but it may be worth a try to determine the economics from a laboratory simulation-size sample.
I wish you the best.