Why should it be heated to 80~90 degrees in the process of iron removal by oxidation?

Because it can remove rust from iron.

In addition to iron, commonly used zinc oxide such as hydrogen peroxide, pyrolusite, potassium permanganate, bleaching powder, etc. are used for ferrous oxide, and then iron is removed by water. After oxidation, neutralizer is needed to improve the ph value of the solution to hydrolyze trivalent iron. Commonly used neutralizers are zinc oxide and lime milk. Please discuss what kind of neutralizer is best based on your own experience.