Guide
How rare earths are separated: solvent extraction, RapidSX and ion exchange
Rare earth separation is the chemical process that splits a mixed concentrate into single oxides such as NdPr, dysprosium and terbium. It is hard because neighboring rare earths behave almost identically in chemistry, so a plant repeats the separation many times, usually with solvent extraction.
Two US sites separate rare earths commercially, Mountain Pass and White Mesa Mill, with about 3,600 t/yr of NdPr oxide capacity between them (OSTI). Ucore and ReElement are building plants that use RapidSX and chromatography, and Aclara plans a heavy rare earth plant in Louisiana.
Verified through Oct 1, 2026
At a glance
- Standard method
- Solvent extraction in mixer-settlers
- Newer methods
- RapidSX (column-based) and continuous ion exchange chromatography
- Product
- Oxides, carbonates and chlorides; oxalates calcined to more than 99% oxide
- Largest cost
- Reagents: about 73% of operating cost in White Mesa's pre-feasibility study (OSTI)
- Next guide
- From oxide to metal and alloy
- Commercial separation sites
- 2Mountain Pass and White Mesa Mill
- Separation factor
- 1.1 to 2.6Between neighboring rare earths, common extractant (OSTI)
- Extraction stages in a circuit
- Up to 1,500From fewer than 100 to about 1,500 (OSTI)
- US NdPr oxide capacity
- 3,600 t/yrMountain Pass and White Mesa (OSTI)
How are rare earths separated?
Rare earths are separated in five stages: crack and leach the concentrate, remove impurities such as iron, thorium, uranium and radium, separate element from element by solvent extraction or chromatography, precipitate the product, and calcine it to oxide. The third stage does the difficult part.
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Crack and leach the concentrate
Break the mineral open and dissolve the rare earths. Mountain Pass roasts its flotation concentrate at about 600 °C and leaches it in cold hydrochloric acid, which leaves most of the cerium behind. White Mesa Mill cracks monazite with caustic soda and leaches it with hydrochloric acid.
Output A rare earth solution, and solid residues
In the US
- Producing: MP Materials Mountain Pass
- Plant operating: White Mesa Mill
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Purify the solution
Remove iron, aluminum, uranium, thorium and radium. Mountain Pass strips iron and uranium by ion exchange and precipitates aluminum. White Mesa precipitates radium as barium-radium sulfate and recovers the uranium it finds as U3O8.
Output A clean mixed rare earth solution, and radioactive residues
In the US
- Producing: MP Materials
- Plant operating: White Mesa Mill
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Separate element from element
The core stage. Solvent extraction passes the solution through banks of mixer-settlers where an organic liquid takes up one rare earth more readily than its neighbor. RapidSX and chromatography do the same job with different equipment.
Output Streams of single rare earths or small groups, such as NdPr, samarium-europium-gadolinium and heavies
In the US
- Producing: MP Materials solvent extraction
- Plant operating: Energy Fuels solvent extraction
- Plant building: Ucore Rare Metals RapidSX
- Plant building: ReElement Technologies chromatography
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Precipitate the product
Add oxalic acid so that NdPr and other products drop out as oxalates. Mountain Pass also makes lanthanum carbonate and cerium chloride.
Output Oxalates, carbonates and chlorides
In the US
- Producing: MP Materials
- Plant operating: Energy Fuels
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Calcine to oxide
Heat the oxalates to about 800 °C to give oxides of more than 99% purity, the form metal makers buy.
Output NdPr oxide, dysprosium oxide, terbium oxide and others
In the US
- Producing: MP Materials
- Plant operating: Energy Fuels
Marker: ProducingPlant operatingPlant building
Sources: the OSTI review for the flowsheets at Mountain Pass and White Mesa Mill, and MP Materials' 10-K for 2025 for the five processing steps at Mountain Pass.
Why is separating rare earths so hard?
Neighboring rare earths have almost the same size and chemistry, so one pass of solvent extraction splits them only slightly, with separation factors of about 1.1 to 2.6 for a common extractant. A circuit therefore needs from fewer than 100 to about 1,500 stages, which makes plants large, costly and hard to run.
- Chemistry Nearly identical ions Most rare earths form +3 ions whose radii shrink only slightly along the series, a pattern called the lanthanide contraction, so they behave alike in water (OSTI).
- Scale Hundreds of stages A simulated nine-product circuit for Mountain Pass-type feed needed about 536 stages. Solvay's plant at La Rochelle, France has more than 1,100 mixer-settlers (OSTI).
- Cost Reagents and capital Reagents were about 73% of operating cost in an Energy Fuels pre-feasibility study for White Mesa (OSTI). Aclara's planned Louisiana plant is budgeted at $277M.
- Know-how Experience counts Energy Fuels built a 1,000 t/yr NdPr circuit for about $20M in 2024 by using an existing licensed mill and more than 40 years of solvent extraction experience (OSTI).
- Safety Radioactive feed Thorium, uranium and radium follow the rare earths and must be removed and handled under radioactive materials licenses.
How does solvent extraction work?
Solvent extraction mixes the rare earth solution with an organic liquid that carries an extractant, a chemical that binds one rare earth more strongly than its neighbor. The liquids settle apart, the loaded organic liquid is washed and stripped, and the cycle repeats in a long series of mixer-settler stages.
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Extract
The water-based solution and the organic liquid are mixed. The extractant, a phosphonic acid, a phosphinic acid or tributyl phosphate dissolved in a kerosene-type diluent, pulls certain rare earth ions into the organic liquid.
Output A loaded organic liquid
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Settle
The two liquids separate by gravity in a settler. The water-based liquid left behind, the raffinate, has lost the extracted rare earths.
Output Raffinate for the next stage
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Scrub
The loaded organic liquid is washed to remove impurities that came along with the rare earths.
Output A cleaner loaded organic liquid
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Strip
An acid solution takes the rare earths back into water, which also regenerates the organic liquid so it can return to the start of the circuit.
Output A solution of the separated rare earth, and recycled solvent
Extractants make up about 30% to 50% of the organic liquid in commercial use (OSTI). A plant chains many cycles, because each one gains only a small separation.
How do RapidSX and ion-exchange chromatography differ from solvent extraction?
RapidSX uses the same chemistry as solvent extraction in a column instead of mixer-settlers, to cut time and footprint. Chromatography passes the solution through resin columns that hold rare earths in different places. Ucore uses RapidSX, ReElement and USA Rare Earth use chromatography, and MP Materials and Energy Fuels use conventional solvent extraction.
| Technology | How it separates | US users and status | What is proven | What to watch |
|---|---|---|---|---|
| Solvent extraction in mixer-settlers | Many stages of liquid-liquid contact, each gaining a small separation. | MP Materials (Mountain Pass) and Energy Fuels (White Mesa), both operating. Rare Element Resources and American Rare Earths plan similar circuits. | The industry standard. It replaced ion exchange in the 1960s. | Big circuits, many stages, heavy solvent and acid use. |
| RapidSX | The same chemistry in a column-based design, meant to cut mixing and settling time. | Ucore Rare Metals: a 52-stage demonstration plant in Kingston, Ontario; a demonstration plant in Alexandria, Louisiana began commissioning in Apr 2026; the first full machine is due in H1 2027. | Pilot scale only. Commercial scale is unproven. | Company-reported: 5.4 times less mixing time, 1.8 times less settling time, up to 60% smaller footprint and 34% lower capital cost (Ucore, 2026). OSTI notes the stage count is set by chemistry, not reactor design. |
| Continuous ion exchange and chromatography | Resin columns hold different rare earths with different strength, so they come off one after another. | ReElement Technologies: a qualification facility in Noblesville and a campus being built in Marion, Indiana. USA Rare Earth: a Round Top flowsheet built on continuous ion chromatography, tested at a demonstration plant in Wheat Ridge, Colorado. | Oak Ridge and Ames Laboratory made 99.99% pure rare earths this way in the 1940s. Batch operation let solvent extraction win in the 1960s, and continuous systems are used now. | Throughput at scale. ReElement states purities of 99.5% to 99.999% (company-reported). |
| Emerging methods | Protein-based ligands (Altatech), ionic liquids (Rivalia) and chromatography that avoids solvent extraction (Rare Earth Salts). | Early-stage companies without profile pages; Rare Earth Salts runs a demonstration plant in Nebraska. | Not at commercial scale. | Thin evidence; OSTI lists them as research and demonstration projects. |
Technology facts are from the OSTI review, the Ucore and ReElement profiles and the companies' own statements; company figures are labeled company-reported.
Which US plants separate rare earths today, and which are planned?
Two US plants separate rare earths commercially: MP Materials at Mountain Pass and Energy Fuels at White Mesa Mill. Ucore and ReElement are building plants, and Aclara, Lynas USA and ElementUSA have plants planned. The table gives the method, feed, stated capacity and status for each.
| Plant | Method and feed | Stated capacity or output | Status |
|---|---|---|---|
| Mountain Pass separation plantCalifornia | Solvent extraction; its own bastnaesite concentrate. | 2,599 t of NdPr oxide in 2025. A first heavy rare earth circuit was mechanically complete in May 2026. | Plant operating |
| White Mesa MillUtah, Energy Fuels | Solvent extraction; monazite concentrate from Chemours and others. | About 38 t of NdPr product from the 2024 commissioning run. Phase 1: up to 1,000 t/yr of NdPr oxide (company). Phase 2 design: 6,000 t/yr NdPr, 240 t/yr dysprosium and 66 t/yr terbium for $410M. | Plant operating |
| Louisiana Strategic Metals ComplexAlexandria, Louisiana | RapidSX; heavy mixed rare earth oxide feed, which is being sourced. | Machine A about 600 t/yr; about 9,600 t/yr of TREO feed at full build. A demonstration plant began commissioning in Apr 2026; Machine A is due in H1 2027. | Plant building |
| Marion campusIndiana | Chromatography; mined concentrates, recycled magnets and other feeds. | Stated targets differ between releases (up to 9,000 t/yr, over 10,000 t/yr, over 16,000 t/yr of products). No output has been published. | Plant building |
| Port of Vinton facilityLouisiana | Method not stated in the sources checked; mixed carbonate from ionic clays in Brazil and Chile. | A $277M plant; capacity not stated. Construction-ready by the end of 2026; completion targeted for the end of 2027. | Plant planned |
| Seadrift heavy rare earth plantTexas | Method not stated; Lynas says it is uncertain the plant will be built. | Not finalized. | Plant planned |
| Gramercy bauxite residue projectLouisiana | A proprietary process for bauxite residue. | A plant of 150 to 1,000 t/yr; DOE selected the project for award negotiation in Jun 2026. | Plant planned |
| Wheat Ridge demonstration plantColorado | Heap leach and chromatography flowsheet for Round Top material. | A run of at least 2,000 hours; dysprosium and NdPr oxide samples in Jul 2026. | Plant operating |
| Upton demonstration plantWyoming | Solvent extraction circuits for Bear Lodge concentrate. | Meant to make about 10 t of separated NdPr oxide over 10 months (OSTI). Started Mar 24, 2026; delays reported in Jun 2026. | Plant operating |
Capacities are what each company states. A plant under construction or planned has no output. Statuses follow the directory's methodology.
What does a rare earth separation plant leave behind?
A separation plant leaves radioactive residues such as radium, thorium and uranium, plus spent reagents. White Mesa Mill precipitates radium and recovers uranium, and it moved quickly because it already held permits to handle uranium- and thorium-bearing material. A new site must obtain that licensing first.
- Residues Radium, thorium and uranium The concentrate carries naturally radioactive elements. White Mesa precipitates radium as a barium-radium sulfate and sends uranium-bearing solids to its uranium circuit (OSTI).
- Reagents Acids and solvents Plants use large volumes of acids and specialty organic reagents, and reagents are the largest operating cost in the White Mesa study (OSTI).
- Licenses Radioactive materials rules White Mesa operates under Utah Radioactive Materials License UT1900479. Energy Fuels applied in Nov 2025 to amend it to expand rare earth processing, and Utah asked for more information in Apr 2026.
Which US companies separate or plan to separate rare earths?
Each company page carries the plant, the method, the stated capacity and the sources behind this guide.
- US producers by elementWho separates each element, and who is building
- MP MaterialsSeparation at Mountain Pass, with a heavy rare earth circuit
- Energy FuelsMonazite separation at White Mesa Mill
- USA Rare EarthChromatography flowsheet for Round Top material
- Rare Element ResourcesA demonstration plant for Bear Lodge concentrate
- Ucore Rare MetalsRapidSX in Louisiana
- Aclara ResourcesA planned heavy rare earth plant at the Port of Vinton
- ReElement TechnologiesChromatography at Marion, Indiana
- Lynas USAA Texas plant whose construction is uncertain
- ElementUSARecovery from bauxite residue in Louisiana
- Separation and refining companiesEvery company at this step, with its status
- US producers of dysprosium and terbiumWho could supply heavy rare earths, and when
Questions about separating rare earths
What is the difference between rare earth separation and refining?
Separation splits a mixed concentrate into single rare earth oxides. People use refining loosely, sometimes for the whole step after the mine. This directory calls the oxide step separation and refining, and the step that turns oxides into metal metal and alloy making.
Does the US separate heavy rare earths?
Not at commercial scale yet. Energy Fuels has made dysprosium and terbium oxide at about 99.9% purity at pilot scale, and MP's first heavy rare earth circuit was mechanically complete in May 2026. Commercial output is targeted for late 2026 at MP, and Energy Fuels' dysprosium and terbium circuits are due by the end of 2027, both company targets.
What is NdPr oxide?
A mixed oxide of neodymium and praseodymium and the main feed for magnet metal. Magnet makers use the two together as didymium, so plants often separate them as a pair rather than from each other.
How pure are separated rare earth oxides?
Calcining the oxalates gives oxides of more than 99% purity, according to OSTI. Rare Element Resources expects 99.5% NdPr oxide from Bear Lodge, and ReElement states 99.5% to 99.999% for its chromatography (company-reported).
Why did solvent extraction replace ion exchange?
Ion exchange separated rare earths at Oak Ridge and Ames Laboratory in the 1940s, but it ran in batches. Solvent extraction replaced it in the 1960s because it runs continuously. Continuous ion exchange and chromatography are now used again.
What does RapidSX do differently?
It uses the same chemistry as solvent extraction in a column-based design, to speed mixing and settling and shrink the plant. Ucore reports large gains at pilot scale; OSTI notes the number of theoretical stages is set by chemistry, not reactor design.
Sources and verification
Each profile lists its sources, a confidence grade and a verification date. Order of authority: official records (SEC filings, agency documents), company documents and technical reports, research papers, then independent press. Syndicated copies and reference works are labeled and are never the only source for a number. Company-reported figures are labeled as such, and non-binding deals are labeled "letter of intent" or "conditional".
- OSTI review, Minerals Engineering 249 (Kashyap et al., 2026) Research paper
- USGS: The Principal Rare Earth Elements Deposits of the United States (Scientific Investigations Report 2010-5220) Official record
- MP Materials 10-K for 2025 (SEC) Official record
- Energy Fuels 10-K for 2025 (SEC) Official record
- Department of Energy: Rare Earth Permanent Magnets, Supply Chain Deep Dive Assessment (Feb 2022) Official record
- Ucore: engineering report for the Louisiana Strategic Metals Complex (May 2026) Company document
- American Resources press release on the Nov 2025 partnership (SEC exhibit 99.1) Official record
- Utah DEQ: Radioactive Materials License UT1900479, White Mesa Mill Official record
- Opportunity Louisiana: Aclara invests $277M in Southwest Louisiana Official record
- Mining Weekly: Lynas flags uncertainty over Texas rare earths plant (Aug 2025) Independent press
Tags show what kind of source each link is. How sources are ranked
Guides
Which other guides are there?
Ten guides explain each step from mine to magnet, why the US is behind and how to read the technical reports behind each project in this directory.
- The chain Mine to magnet supply chain The five steps from ore to magnet, with every profiled US company at its step.
- The chain How rare earths are mined Open pits, underground mines, coal and mineral sands, and how ore becomes a concentrate.
- The chain From oxide to metal and alloy Molten-salt electrolysis and metallothermic reduction, and who makes metal in the US.
- The chain How rare earth magnets are made Strip casting, pressing and sintering, and the US magnet plants with their stated capacity.
- The chain Rare earth recycling in the US Hydrogen processing, mechanical recovery and chemistry, and why volumes are still small.
- Why the US is behind Why the US cannot process rare earths Capacity against demand, and five obstacles with the evidence for each.
- Reading the reports Types of US rare earth deposits Carbonatite, granite, clay, coal and sands, with every US project mapped to a type.
- Reading the reports How to read a project A stage ladder from exploration to production, with what each study proves and what it does not.
- Reading the reports How to read resource estimates TREO, grade, JORC, NI 43-101 and S-K 1300, with a calculator and a worked example.
- Hub All rare earth guides The five steps from mine to magnet and a reading order for the ten guides.