Critical issues in material selection for phosphoric acid plants

摘要

In comparison with stronger mineral acids such as sulphuric and hydrochloric acids,pure phosphoric acid is not very corrosive.But these days industry produces very little of the very pure "thermal"phosphoric acid,which was obtained by producing elemental phosphorus from phosphate ore,then burning it and combining the oxidation products with water,because of the high capital and running costs of the operation.Almost all industrially produced phosphoric acid is now generated directly from the ore(phosphate rock)by the "wet process"(acidulation with sulphuric acid),in the course of which much of the impurity content of the ore goes into solution in the product acid.This impure phosphoric acid product is much more corrosive than the pure substance,though to what extent depends on the composition and impurity content of the phosphate rock,which varies according to its provenance.The problem can only get worse as the better-quality phosphate deposits become progressively worked out and the industry is obliged to turn to ever lower-grade deposits.