# Reducing Iron Oxide Content in Phosphoric Acid Using Polyacrylates and Phosphonic Acids

## TL;DR

This study explores methods to reduce iron oxide contamination in phosphoric acid to improve fertilizer quality and efficiency.

## Contribution

The study evaluates the effectiveness of polyacrylates and DTPMP in removing iron oxides from phosphoric acid.

## Key findings

- DTPMP solution reduced Fe2O3 content by 40% and increased the P2O5/Fe2O3 ratio by 59%.
- Optimal conditions for DTPMP were 55°C, 14 hours, and 5.57% reagent by mass.
- DTPMP removed 0.58% Fe2O3, comparable to other studies.

## Abstract

Phosphoric acid is
the second most produced and consumed inorganic
acid in the world and the most important intermediary in the production
of phosphate fertilizers. The quality and agronomic utilization of
phosphate fertilizers are highly dependent on the levels of contaminants
present on the phosphoric acid, especially iron oxides. The presence
of iron oxides in phosphate fertilizers results in the formation of
iron phosphates possessing lower agronomic efficiency and a reduced
commercial value. The wet-process phosphoric acid uses phosphate rocks
or concentrates as a source of P2O5. Most phosphate
rocks in Brazil originate from igneous mines, with high concentrations
of iron oxides, as opposed to most mines in operation in the world,
with sedimentary origins and lower contaminant contents. Iron content
is a limiting factor in the economic and technical feasibility of
phosphate mines, and the complete removal of iron oxides during the
industrial concentration of phosphate rock is unfeasible due to phosphate
losses. To improve the quality of the phosphoric acid produced from
such ores and to enable the usage of phosphate rock concentrates with
higher levels of contaminants, this study aimed to evaluate the effectiveness
of two compounds for removing the iron oxides from the acid: (i) a
mixture of polyacrylates associated with phosphonic acids and (ii)
a solution with 60% of diethylenetriamine penta (phosphonic methylene)
(60% DTPMP). The mixture of polyacrylates associated with phosphonic
acids showed limited effectiveness in removing contaminants from the
phosphoric acid. The DTPMP solution, however, proved effective as
an iron chelating agent in phosphoric acid, whereas the Fe2O3 content on a dry basis was reduced by 40% and the P2O5/Fe2O3 ratio was increased
by 59%. The optimal reactional conditions were T =
55 °C, 14 h of precipitation time, and 5.57% of chelating reagent
in relation to the initial phosphoric acid mass (w/w). A total of
0.58% of Fe2O3 was removed from the acid, a
yield comparable to the 0.55% observed in other benchmark studies.