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

Sep 29, 2026

Phosphoric acid is a key intermediary for producing phosphate fertilizers, and its quality strongly depends on contaminants, particularly iron oxides. In Brazil, many phosphate rocks come from igneous mines rich in iron oxides, while industrial removal of these oxides during wet-process concentration is limited by phosphate losses. This study evaluates two chelating approaches: polyacrylates with phosphonic acids and a 60% DTPMP solution. DTPMP effectively chelates iron, reducing Fe2O3 by 40% and raising the P2O5/Fe2O3 ratio by 59% under optimized conditions. Reaction settings were T=55 °C, 14 h precipitation, and 5.57% reagent (w/w).

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages13

Reading Time33 min

AccessFree to download

Abstract

Introduction

Materials and Approach

Results and Optimization

This article is licensed under CC-BY 4.0 [http://pubs.acs.org/journal/acsodf](http://pubs.acs.org/journal/acsodf) Article Reducing Iron Oxide Content in Phosphoric Acid Using Polyacrylatesand Phosphonic Acids Gustavo Paiva Ribeiro, Sandra Cristina Dantas, Eloízio J́ulio Ribeiro, and Carla Eponina Hori* Cite This: ACS Omega 2025, 10, 58317−58329 Read Online | | | | | | --- | --- | --- | --- | | ACCESS | Metrics & More | | Article Recommendations | 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. 1. INTRODUCTION One of the Sustainable Development Goals set by the United Nations is to ensure food security while shifting toward sustainable agricultural practices.1 Brazil is a country with a great vocation for agribusiness, and in recent years, more than 20% of its Gross Domestic Product has come from this sector.2 However, Brazilian soils have generally low concentrations of nutrients essential to plant growth, among them phosphates (P2O5), which must be supplemented by soil fertilization.3,4 There are several phosphate-based fertilizers available on the market, and basically all high-grade options use phosphoric acid as an intermediate product or ingredient.5−8 Approximately 90% of the phosphoric acid produced in the world uses a production route denominated “Wet Process Phosphoric Acid” (WPA), which consists of the reaction of phosphate rocks or phosphate concentrates with sulfuric acid and water, yielding phosphoric acid and calcium sulfate as main products.9 However, phosphoric acids produced by the wet route retain most of the impurities present in the phosphate concentrates or rocks used in their production.10 Therefore, the phosphate rock or concentrate used affects the composition, characteristics, and potential use of the produced phosphoric acid. For all economical and practical purposes, the phosphate rock is the main source of P2O5 for most phosphates’ fertilizers and phosphor

FAQs

Why are iron oxides a problem in phosphoric acid and phosphate fertilizers?

Iron oxides lead to the formation of iron phosphates with lower agronomic efficiency and reduced commercial value. They can also cause insolubilization or dilution issues that affect fertilizer specifications.

What chelating agents were evaluated to remove iron oxides from phosphoric acid?

The study evaluated (i) a mixture of polyacrylates associated with phosphonic acids and (ii) a 60% diethylenetriamine penta (phosphonic methylene) solution (60% DTPMP).

What optimization conditions produced the best performance with DTPMP?

The optimal conditions were T=55 °C, 14 h precipitation time, and 5.57% chelating reagent relative to the initial phosphoric acid mass (w/w).

How much Fe2O3 was removed, and how did it compare with benchmark studies?

Under optimal conditions, 0.58% of Fe2O3 was removed from the acid. This yield is comparable to 0.55% reported in other benchmark studies.

Sep 29, 2026

Upload by Jordan Avery