Rainbow and Mosaic Start Prefeasibility Work at Uberaba

London-listed Rainbow Rare Earths has begun a prefeasibility study on the Uberaba rare earth project in Brazil with partner Mosaic Company, following a positive economic assessment completed in March that confirmed Uberaba as Rainbow's second major development opportunity.

Rainbow Rare Earths has begun a prefeasibility study on the Uberaba rare earth project in Brazil alongside its partner, Mosaic Company (NYSE: MOS), advancing a project the two companies say already clears an early economic hurdle.

The decision follows an economic assessment completed in March, whose positive outcome confirmed Uberaba as a second major development opportunity for the London-listed developer. A prefeasibility study, or PFS, is the stage at which a mining project's engineering, processing route, capital cost and operating cost estimates are tightened to the point where a board can credibly compare it against alternatives — and where lenders and offtake partners start paying attention. It sits between the scoping-level work that came before and the definitive feasibility study that typically precedes a construction decision.

The move was reported by Mining Weekly.

Why a fertiliser producer is in the rare earths business

Mosaic is one of the world's large phosphate and potash producers, and its interest in Uberaba is not a diversification whim. Phosphate processing generates enormous volumes of phosphogypsum, a by-product that accumulates in engineered stacks at fertiliser plants and carries no revenue of its own. Those stacks can contain rare earth elements inherited from the original phosphate rock — in effect, a feedstock that has already been mined, crushed and chemically attacked once, and that sits on ground the operator already controls.

That is the strategic logic that makes a project like Uberaba different from a conventional hard-rock rare earth mine. A greenfield mine has to fund pit development, waste handling, comminution circuits and, usually, a new tailings facility before it produces a kilogram of separated oxide. A by-product route starts further down the value chain. Whether that translates into the capital and operating advantages the concept promises is precisely the question a prefeasibility study is designed to answer, and it is the number investors will be waiting on.

For Rainbow, the appeal is optionality. The company has framed Uberaba as its second major development opportunity, which implies a pipeline rather than a single-asset story — a meaningful distinction for a junior developer, because single-asset companies are valued as binary outcomes while two-asset companies get credit for sequencing.

Where the shares stand

Mosaic closed its most recent session at 25.85, up 1.41% from a previous close of 25.49, having traded between 24.77 and 26.04 during the day, according to licensed market data as of 20:00 GMT on Friday, 4 September 2026. The market was closed at the time of writing, so that is the last traded price rather than a live quote.

The gain came against a softer broad tape. The S&P 500 tracker (NYSEARCA: SPY) closed at $770.19, down 0.39%, and the Dow 30 proxy (NYSEARCA: DIA) closed at $534.08, down 0.53%, while the Nasdaq 100 fund (NASDAQ: QQQ) closed at $718.96, up 0.18%. In other words, Mosaic outperformed both large-cap benchmarks that fell that session, though a single day's move in a fertiliser producer is driven far more by crop nutrient pricing than by a prefeasibility study on a rare earths side project.

That proportionality matters for how investors should read the announcement. Uberaba is material to Rainbow's equity story in a way it cannot yet be to Mosaic's, whose earnings are set by phosphate and potash markets. For Mosaic, the project is an option on turning a liability-shaped waste stream into a revenue line, at a cost of study spending that is small relative to its core business.

Western supply and the separation bottleneck

Rare earth elements — the neodymium and praseodymium used in permanent magnets, and the heavier dysprosium and terbium that keep those magnets working at high temperatures — are not geologically scarce. The bottleneck is chemical separation, which is concentrated in a small number of facilities and jurisdictions. That concentration is why governments in the United States, Europe, Japan and Australia have spent the past several years underwriting projects that would otherwise struggle to clear a conventional cost of capital.

Projects that draw feedstock from existing industrial residues occupy a particular niche in that push. They tend to have shorter permitting paths than new open pits, because the material is already on an operating industrial site, and they can be built in stages. What they must prove is grade, recovery and, critically, the ability to produce a separated or saleable mixed product rather than a concentrate that still needs someone else's refinery.

The milestones that will define the next phase

Several specific outputs from the Uberaba PFS will determine whether the project keeps moving:

• Capital cost and staging — how much has to be spent before first production, and whether the flowsheet can be modular.
• Recoveries by element — the split between magnet rare earths and the lower-value light elements, which determines revenue per tonne of feed.
• Product specification — whether Uberaba would ship a mixed carbonate to a third-party separator or aim for separated oxides.
• The commercial structure with Mosaic — how feedstock supply, site access, capital contributions and product marketing are divided between the partners.
• Water, reagent and residue handling — the environmental terms that will shape Brazilian permitting.

None of those figures exist publicly yet, and Rainbow has not put a completion date on the study in the material disclosed. Until the PFS lands, Uberaba is a concept with a positive early economic assessment behind it — a necessary step, but a long way short of a funded build. What has changed is that both a junior developer and a major fertiliser producer have now committed study capital to finding out.

Key facts

• Project: Uberaba rare earth project, Brazil
• Stage: Prefeasibility study initiated; economic assessment completed positively in March
• Partners: Rainbow Rare Earths (London-listed) and Mosaic Company (NYSE: MOS)
• MOS last close: 25.85, +1.41%, as of 20:00 GMT, 4 Sept 2026

Frequently asked questions

What is the Uberaba project?

Uberaba is a rare earth project in Brazil being advanced by London-listed Rainbow Rare Earths together with Mosaic Company. An economic assessment completed in March returned a positive outcome and confirmed Uberaba as Rainbow's second major development opportunity, prompting the two partners to begin a prefeasibility study on the project.

What does a prefeasibility study actually involve?

A prefeasibility study, or PFS, refines a mining project's processing flowsheet, capital cost, operating cost and production schedule to a level of accuracy that allows a board to compare development options. It sits between early scoping-level economics and a definitive feasibility study, which is usually the last technical step before a construction decision.

Why is Mosaic, a fertiliser company, involved in rare earths?

Mosaic is a major phosphate and potash producer. Phosphate processing generates large volumes of phosphogypsum, a by-product that can contain rare earth elements carried over from the original phosphate rock. Recovering those elements would turn material that currently produces no revenue into a potential product stream at sites Mosaic already operates.

Where does Mosaic stock stand?

Mosaic Company shares last traded at 25.85, a gain of 1.41% from the prior close of 25.49, with a day range of 24.77 to 26.04, as of 20:00 GMT on 4 September 2026. The market was closed at the time of writing, so that figure is the most recent close rather than a live quote.

Has a construction decision been made at Uberaba?

No. Beginning a prefeasibility study is a study commitment, not a build commitment. A construction decision would normally follow a definitive feasibility study, permitting, financing and, for most rare earth projects, offtake arrangements. No completion date for the Uberaba PFS was disclosed in the announcement.

Why does Western rare earth supply matter here?

Rare earth elements are used in the permanent magnets found in electric vehicle motors, wind turbines and defence systems. The main constraint is not ore availability but chemical separation capacity, which is concentrated in a handful of facilities. That has driven policy support for projects outside the dominant supply chain, including those using industrial residues as feedstock.

Sources

• Rainbow, Mosaic initiate PFS for Brazil rare earth project — Mining Weekly

Photo: Michal Pech · Pexels Licence — source