Market and product

Compiled by Bao Hien

Phosphoric acid (H3PO4) made by the wet process, in which concentrated sulfuric acid digests phosphate rock, is the second most produced acid in the world after sulfuric acid. Most of it goes into fertilizers, but a share is used in food, detergents and pharmaceuticals, where purity requirements are far stricter. Several research groups are now working on removing impurities from the crude acid along three different lines: new extractants, closed-loop processes that recover metals, and nanofiltration membranes.

Why wet-process acid needs purification

Acid obtained directly from the rock carries impurities already present in the raw material: iron, aluminum, copper, zinc, magnesium, cadmium, rare earth elements, fluorine, arsenic and sulfate ions. These impurities limit the use of the acid in food and pharmaceuticals. For food-grade acid, the research groups cite very low limits as examples: arsenic below 3 ppm, iron below 20 ppm, fluorine below 10 ppm, magnesium below 20 ppm and chromium below 2 ppm.

Conventional methods include precipitation with ammonium or sodium compounds, low-temperature crystallization, solvent extraction with tributyl phosphate mixtures, and ion exchange with sulfonic resins. Each has drawbacks: precipitation generates large amounts of sludge that must be separated further, crystallization is hard to apply at industrial scale, solvent extraction requires chemical regeneration, and ion exchange is limited in selectivity. This is why the newer approaches focus on higher selectivity, fewer auxiliary chemicals, and the ability to recover valuable metals.

Approach one: designing an extractant that survives strong acid

A group at the University of Jinan (China) started from the extractant molecule itself. They synthesized a new multidentate extractant from the phosphonic acid family, with a rigid benzene backbone linked through octylimine bridges, designed to remain stable in strongly acidic media, conditions in which conventional extractants tend to degrade.

According to the paper in Chemical Engineering Journal, the extractant removes a wide range of impurity ions from wet-process acid, including iron, aluminum, chromium, titanium, uranium, vanadium, zirconium and 14 rare earth elements. The authors report that its extraction ability and acid stability exceed those of common commercial extractants (P204, P507 and Cyanex 272). Mechanistic analysis shows that iron(III) forms a trinuclear complex with the extractant through a combination of coordination and ionic bonding, exploiting the high charge density around the oxygen atoms of the phosphonic groups.

Note that the published abstract does not give extraction efficiencies as percentages, so the claimed superiority over commercial extractants is currently the authors' assessment and cannot be checked against numbers in this article.

Approach two: a closed loop that purifies and recovers metals

A group at Mohammed VI Polytechnic University (Morocco), a country with a major phosphate industry, takes a different view: impurities are a resource. They built a two-stage solvent extraction process that uses D2EHPA to recover rare earths and Cyanex 301 to separate heavy metals, then regenerates the solvents and precipitates the products.

Figures published in Desalination show extraction of 90% of yttrium, 80% of holmium and 99% of ytterbium, along with removal of 89% of zinc, 99% of copper and 99% of cadmium. After extraction, the rare earths are stripped with sulfuric acid and precipitated with oxalic acid, while the heavy metals are stripped with hydrochloric acid and precipitated with sodium carbonate. This allows the solvent loop to be reused (closed loop) and turns an impurity stream into separate metal products.

Approach three: nanofiltration membranes instead of chemicals

The third approach almost eliminates solvents. A study in Separations used a spiral-wound nanofiltration membrane (molecular weight cut-off 1,000 g/mol) modified with polyethylenimine to improve retention of metal ions. The crude acid is pretreated to reduce sulfate, fluorine and arsenic, then passed through the membrane at 25% P2O5 and 60 °C, at 30 bar in the laboratory and up to 40 bar at pilot scale.

The membrane retained 99.63% of iron, 99.30% of aluminum, 96.67% of rare earths and 94.81% of cadmium. The filtered acid was then concentrated by evaporation to 54% P2O5, which the authors describe as meeting food-grade quality.

Comparison: each approach solves a different problem

The three studies used different starting acids and test conditions, so the figures above only illustrate how effective each approach is and cannot be ranked directly against each other. Even so, they point to two common trends: purifying the acid is no longer only about meeting a quality standard but increasingly about recovering rare earths and metals from the acid stream itself, and these technologies remain mostly at laboratory or pilot stage, with no data yet on operating costs at plant scale.