Phosphogypsum Tailings

Phosphogypsum is the solid waste left when phosphate ore is processed into phosphoric acid for fertilizer (U.S. EPA). Most of the rare earths in the ore follow the gypsum, so phosphogypsum rare earths are a possible supply that has already been mined and now sits in stacks. Recovering them at a profit is unproven. A 2025 review concluded that building a commercial rare earth plant on phosphogypsum is not currently economically feasible (Khalil et al., Sustainability, 2025).

What phosphogypsum is and how much of it exists

Phosphogypsum is the main byproduct when phosphoric acid is made by digesting phosphate rock with sulfuric acid. Each ton of product acid, measured as P₂O₅, leaves 4 to 5 tons of it (Xie et al., Molecules, 2023). In the U.S., more than 95% of mined phosphate rock goes into wet-process phosphoric acid and superphosphoric acid (USGS Mineral Commodity Summaries 2026).

Global generation is approaching 300 million tons a year, and about 3.7 billion tons produced since 1994 sits in stacks. Assuming a rare earth content of 0.1%, the 2025 review's authors estimate those stacks may hold more than 3.7 million tons of rare earths (Khalil et al.).

Which rare earths end up in phosphogypsum

Two reviews put the share of the rock's rare earths that moves into phosphogypsum during acid digestion at more than 60% (Khalil et al.) and more than 70% (Xie et al.). In a study the 2025 review cites, light rare earths such as lanthanum, cerium, praseodymium, neodymium and samarium were strongly enriched in the gypsum under both acid processes tested, while scandium went into the phosphoric acid (Khalil et al.).

Reported values vary greatly between regions, with some below 0.01 weight percent (Xie et al.). The 2025 review says the concentration and mix of rare earths and impurities depend greatly on where the phosphate rock came from (Khalil et al.).

Why recovering phosphogypsum rare earths is hard

In most cases the rare earths are locked inside the gypsum crystals, which keeps leaching efficiency low. Researchers commonly leach with mineral acids or some organic acids, then purify the liquor by solvent extraction and precipitation. Ion exchange resins can capture the released rare earths (Xie et al.).

Radioactivity adds a second problem. Phosphate ore and phosphogypsum contain radium, which decays into radon, a radioactive gas that can cause lung cancer (EPA). Most uranium leaves with the acid, typically more than 80% to 90% of it. Radium and thorium in phosphogypsum can in some cases reach 10 to 100 times the levels in concrete or natural gypsum (Khalil et al.).

As of September 2026, EPA rules require phosphogypsum in the U.S. to be managed in engineered stacks. Removal is allowed, under conditions, for outdoor agricultural use and indoor research and development. Any other use needs prior EPA approval (EPA).

Phosphogypsum rare earth projects (as of October 2026)

Phalaborwa, South Africa. Rainbow Rare Earths reports a resource of 35.0 million metric tons of phosphogypsum at Phalaborwa, in stacks left by historic phosphate mining (Rainbow). During acid production there, the rare earths broke down and re-precipitated in the gypsum as calcium-aluminum-rare earth fluoride minerals (resource report, December 2025). Rainbow says most of them are in a "cracked" chemical form suited to direct acid leaching (Rainbow).

Rainbow's July 2026 flowsheet reclaims the gypsum mechanically and leaches it, then uses continuous ion exchange and impurity reduction ahead of solvent extraction (Rainbow, July 2026). As of October 21, 2026, Rainbow targets a pre-feasibility study in the fourth quarter of 2026 and expects the definitive feasibility study in the first half of 2027 (Rainbow results, October 2026; REEx coverage).

Uberaba, Brazil. On March 11, 2026, Mosaic Fertilizantes P&K and Rainbow entered a joint project development agreement for an onsite plant in Minas Gerais to treat about 2.7 million metric tons of phosphogypsum a year. Initial production is targeted for 2030, subject to further studies, regulatory approvals and financing (Mosaic). Rainbow said the Uberaba pre-feasibility study began in September 2026 (Rainbow results). REEx noted in March 2026 that the economic assessment behind the project is not a JORC-compliant resource study, and called the project's economics promising but far from bankable (REEx).

Morocco. In August 2022, Rainbow entered into a master agreement with OCP to develop a way to extract rare earths from the phosphogypsum produced by OCP's phosphoric acid plants. Rainbow describes the material as sedimentary-sourced and says sedimentary ores are typically lower grade than hardrock carbonatites. Rainbow expects the test program to take longer than Phalaborwa's (Rainbow). Rainbow's October 2026 results do not mention OCP (Rainbow results).

OCP holds a U.S. patent, granted in November 2024, on a process to purify and concentrate rare earths from phosphogypsum (U.S. Patent 12,139,775). In May 2026, the board of the International Finance Corporation (IFC) approved a debt facility for an inland phosphogypsum storage site at OCP's Jorf Lasfar fertilizer complex. The site is meant to store the material instead of discharging it into the ocean (IFC).

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