not always basic pH is chosen. it generally depends on the gangue minerals as well. pH is also used to achieve the selectivity. so there is not a general rule for this. look at flotation pattern of your mineral, your gangue minerals, select a point to achieve the highest selectivity and recovery.

It can be decided on the basis of previous literature. But what is the significance of doing zeta potential then?

you can get closer with the help of literature, but not all systems behave the same. Theoretically you should have a specific pH as PZC, but the game is not that easy. every single ion in the solution can play a significant role on this. So it strongly depends on what you have in solution. Again, if you just want to do a fundamental work with a single pure mineral in ultra pure water, your results will be very close to those reported previously.

Actually in my case (natural iron ore), I am getting IEP around pH 5.9. What does it indicate ? and how will I decide the pH for direct and reverse flotation.

Depends on what you want to do. take a look at this:

Article The Surface State of Hematite and Its Wetting Characteristics

lets say you have silica and hematite, at pH 3 to 5 you have hematite positively charged and silica negatively charges. then you can use cationic surfactant to float silica or anionic surfactant to float hematite.

Abhyarthana Pattanaik Take a look at:

Froth Flotation in MMM

https://www.brainshark.com/malvern/vu?pi=706627883&text=M021507