How does low phosphoric acid become high phosphoric acid?

The preparation of high-concentration phosphoric acid from low-concentration phosphoric acid is generally made from thermal phosphoric acid with low impurity content by dehydration and concentration at a certain temperature. Some of them are dehydrated and concentrated from industrial or food-grade phosphoric acid produced by wet method. For example, there is an absorption method in which phosphorus pentoxide is obtained by burning yellow phosphorus, which is cooled and absorbed by precooled phosphoric acid to produce polyphosphoric acid and high-concentration phosphoric acid products. Fertilizer-grade phosphoric acid prepared by wet method is clarified and filtered for a certain period of time, and then pretreated, defluorinated and concentrated to prepare high-concentration phosphoric acid.