Rainbow Rare Earths and Mosaic have moved the Uberaba rare earths project in Brazil into prefeasibility, a step that matters because it is the first formal engineering and economic filter after an earlier assessment showed the project could be commercially attractive. The new study does not de-risk the asset on its own, but it does signal that both partners see enough technical merit to spend more time and money on the next stage. For investors, the key question is whether the project’s paper economics can survive the harder test of plant design, metallurgy, capital intensity and operating complexity.
Rainbow says the decision to proceed followed its March economic assessment, which it described as a positive outcome and as confirmation that Uberaba is a second major development opportunity. That earlier assessment is the main reason the project has now advanced. In other words, this is not a greenfield concept being tested for the first time; it is a project that has already cleared an initial commercial screen. Still, readers should treat an economic assessment as a preliminary document, not as a commitment to build. A prefeasibility study is meant to convert encouraging assumptions into a more realistic project model.
The company has also appointed SRK Consulting to complete a mineral resource estimate, which Rainbow says is the first of several technical consultants expected to be brought into the study. That is a normal next step, but it is also a reminder that the project is still building its technical foundation. A resource estimate is important because it underpins mine planning, plant throughput, and long-term economics. Until that work is complete, any valuation attached to Uberaba should be considered provisional.
Uberaba is in Minas Gerais state in Brazil, where Mosaic is already mining phosphate rock and producing phosphoric acid for the fertiliser industry. That location matters because the rare earth opportunity is tied to an existing industrial stream rather than a standalone mining development. The PFS is focused on building a processing plant on site to treat phosphogypsum waste residue, with a yearly throughput of about 2.7-million tonnes over an initial project life of 30 years. Rainbow also says the project has an originally modelled head grade of about 5 100 parts per million total rare earth oxides.
That modelled feed profile is central to the investment case. Rare earth projects often rise or fall on the combination of grade, volume, recovery, and processing cost. Here, the appeal is not just the grade figure but the fact that the project is linked to an existing phosphate operation with a long-life feedstock base. Rainbow says the underlying phosphate feedstock at Uberaba could extend the life of the project beyond the initial plan. That potential is important, but it remains a potential until the PFS and later studies confirm it.
Rainbow says it has leveraged proprietary intellectual property developed for its Phalaborwa rare earths project to establish a flowsheet for Uberaba. The company adds that further testwork with Mosaic has demonstrated the potential to replicate and apply the Phalaborwa process at other phosphogypsum projects. That is one of the more important strategic points in the announcement. If the processing route can be copied across similar feedstocks, the value may lie not only in Uberaba itself but also in the broader application of Rainbow’s technical work.
There are still practical questions to answer. Mosaic is shipping a further about 6 t of phosphogypsum feedstock to Rainbow’s laboratory and pilot plant in South Africa for more testwork and piloting. That suggests the companies are still verifying metallurgy and recoveries before the study can be completed. The planned product suite is also notable: separated neodymium and praseodymium oxide, plus a samarium, europium and gadolinium product with mixed rare earth carbonate rich in medium and heavy rare earth elements, each at 99.5% purity. Those are useful products in the supply chain, but the real issue is whether they can be produced consistently and at acceptable cost.
Rainbow’s March assessment gave Uberaba a post-tax net present value of $916-million, a post-tax internal rate of return of 45%, average EBITDA of $217-million a year over a 30-year life-of-mine, and a payback of 1.7 years. Those are strong headline figures, especially for a rare earth project. They explain why management chose to advance the work. But investors should keep two caveats in mind. First, the numbers were based on spot rare earth pricing reported by Argus Media at the time. Second, these are assessment-stage figures, not reserves-backed development metrics.
That distinction matters because rare earth pricing can move sharply, and project economics built on spot assumptions can weaken if market conditions change. In addition, early-stage studies often rely on simplified cost and operating assumptions that get tested as engineering advances. A 45% IRR and a 1.7-year payback are impressive on paper, but they do not guarantee financing, permitting success, or smooth construction. The next study phase will need to show that the project can hold up under more conservative assumptions and detailed process design.
Mosaic is not just a passive partner here. The company operates in Brazil and the project is being developed through Mosaic’s Brazilian operations together with Rainbow’s rare earth expertise. Mosaic’s own description was that the Uberaba project brings together Mosaic’s Brazilian operations with Rainbow’s rare earth expertise. That matters because existing infrastructure and operating familiarity can reduce execution risk relative to a standalone site. It also suggests that the partners may be trying to turn an industrial waste stream into a secondary value source, rather than create a new mine from scratch.
For Rainbow, the partnership also provides validation. The company’s broader thesis is that the process developed at Phalaborwa can be adapted to phosphogypsum projects elsewhere. Uberaba is the real-world test of that claim. If the PFS confirms the economics and the metallurgical route, it would support the idea that Rainbow’s intellectual property has broader application. If it does not, then the company may still have a valuable asset, but the scalability story would look weaker.
The immediate milestone is completion of the PFS. Mosaic says that is the next expected study step, and any subsequent definitive feasibility study would depend on the PFS outcome. That sequence is standard, but it tells investors where the risk still sits: resource definition, metallurgical performance, plant design, capital cost, and operating cost. The current announcement is constructive because it confirms forward movement, but it is not a final investment case. The study will need to show that the project can deliver the rare earth products at a cost and scale that match the early headline economics.
Brazil is also relevant from a strategic standpoint because Rainbow says the country is a crucial hub for rare earths supply chain independence and a focus of extensive US government support for the industry. That is Rainbow’s framing, and it fits the broader policy backdrop, but support for a supply chain does not automatically translate into project finance or development approval. For now, the story is still about technical progress. The next update that matters will be the PFS results, because that is where the market can start to judge whether Uberaba is a promising concept or a project that looked better in an initial assessment than in detailed engineering.
