In this interview, AZoMining speaks with Koch Modular Process Systems about how modular solvent extraction (SX) enables domestic separation of rare earth concentrates into high-purity products. This addresses a critical gap in the rare earth supply chain. The discussion explores the principles of SX, the flexibility of modular column technology, and the benefits of modular plants.
In simple terms, what part of the US rare earth supply chain is missing today, and how would modular solvent extraction fill that gap between the mine and manufacturers?
One missing part of today’s US rare earth supply chain is the large-scale chemical-processing step that separates mixed rare earth concentrates into the individual, high-purity elements that manufacturers need.
The US can mine rare earths for manufacturing, but the critical separation stage is performed almost entirely in China, creating economic and national security vulnerabilities.
Modular solvent extraction (SX) fills this gap by providing a fast and scalable method for building domestic separation capacity.
Instead of relying on large mixer-settler systems that require hundreds of tanks and years of construction, modular SCHEIBEL® column technology performs the same work in a compact vertical column with a smaller footprint, substantially less solvent, and fewer rare earths retained in the process.
Modular systems are built and tested off-site and then quickly installed on-site, meaning a full processing facility can be operational in roughly a year and a half.
In simple terms, modular solvent extraction helps fill the US processing gap, bridging the mine and the manufacturer with a rapid, efficient, and domestically controlled separation solution.
How does solvent extraction actually separate rare earths from a mixed solution: what’s the “working principle”?
The lanthanides all have very similar chemical properties, which makes it difficult to easily achieve a thorough and complete separation. The best method discovered thus far is ion-exchange solvent extraction, in which an oily liquid is mixed with a specially designed oil-soluble acid and then treated with a basic solution like sodium hydroxide.
When this oil-based acid is brought into contact with an aqueous solution of rare earth metals, the positively charged metals move into the oily liquid, replacing the positively charged sodium (Na) cations. They, in turn, move into the aqueous solution to maintain overall charge balance.
The positional swapping of the two cations is why this method is called ion-exchange solvent extraction. This shift is entirely controlled by the acidity of the mixture: under very acidic conditions, the rare earth metals will prefer the aqueous phase, and under neutral conditions, they prefer the oil phase.
Between these two extremes, if the pH of the aqueous phase is controlled, we can selectively extract rare earth elements one at a time, starting with the heaviest (Lu) and progressing to the lightest (La).
If a miner is advancing a project, where would a modular SX plant physically sit: at the mine site, near a concentrator, or at a regional hub? What drives this choice?
The ore beneficiation plant needs to be co-located at the mine so the rock and sand do not need to be transported with the rare earths.
However, the SX plant does not need to be co-located with the mine because the rare earth concentrate (which consists of over 70% rare earth mineral) that leaves the beneficiation plant can easily be transported at a cost of just 10 to 20 cents per ton per mile. If the concentrate were transported 200 miles, this would only add approximately two to four cents per kilogram of rare earths in costs.
Given that rare earth concentrate is valued at five to 10 dollars per kilogram, this transport cost is on the order of just half a percent. This opens up the possibility of a regional hub SX plant.
Different deposits produce different feeds (bastnäsite, monazite, ionic clays, recycled magnets). How much does the flowsheet change across these, and what pre-treatment is typically needed before SX?
Different feed sources require different front-end processing, but the goal is always the same: deliver a clean, stable rare earth solution to the SX circuit. Hard-rock ores typically require flotation and chemical digestion to break down the mineral structure and remove impurities, such as iron (Fe), aluminum (Al), and fluorides (F-), while ionic clays rely on mild leaching and clarification to strip out co-leached metals.
Some deposits contain mild levels of radioactive elements, which are removed as early in the process as possible. Recycling streams introduce transition metals such as Iron (Fe), which must be removed through selective precipitation to prevent interference in SX.
Despite these variations, the SX flowsheet itself changes very little; once the feed is properly conditioned, the same extraction and separation steps can deliver high-purity rare earths across all feed types. Koch Modular’s modular SX systems are designed to plug into any of these upstream approaches, ensuring consistent performance regardless of the deposit or source.
From first samples to product, what are the practical steps a mining company should plan for, such as lab testing, pilot work, and a first modular train? What realistic timelines and decision gates go with each?
The first step is bench-scale equilibrium testing, which takes two to three months, and confirms the effectiveness of the chosen extractant while also generating separation factors. This data enables testing in pilot SCHEIBEL Columns. This takes three to four months, but can vary based on the number of rare earth separations involved.
Pilot testing results are used to design a modular system (three to six months) with process performance, project schedule, and firm price. This allows for a fully informed decision.
When the decision is made to invest in completing the project, the final step would be detailed engineering and modular construction, which takes 12 to 24 months, depending upon the scale of the project and the number of separations.
What are the biggest operational risks miners face when adding SX, and what design features in a modular setup help keep plants stable and easy to run?
The main risk with a SCHEIBEL column-based rare earth separation plant is its vulnerability to power interruptions, because even a brief loss of power causes the delicate concentration profile in each column to collapse, requiring the column to be restarted and causing weeks of delay.
This paradigm prevented wide-scale adoption of SCHEIBEL columns for 50 years, but over the last decade, there have been significant advances and cost reductions in stationary battery packs, such as the Tesla Megapack.
Today, for about 5% of the cost of the plant, a stationary battery can be added that is large enough to provide power through up to a five-day interruption, eliminating the biggest operational risk.
Many projects must manage thorium/uranium traces and process water. How do modular SX systems address environmental compliance, waste streams, and monitoring, enabling sites to permit and operate responsibly?
Removing radioactive elements as early as possible in a rare earth purification process is critical for complying with environmental standards and maintaining the purity of the rare earth product streams.
Solvent extraction has been proven effective in removing contaminants such as uranium (U) and thorium (Th) from leachate. Modular SX systems can reduce the amount of process water required due to the extraction efficiency achieved in SCHEIBEL columns.
Furthermore, all waste streams would be considered in the design of the rare earths separations process, with all of them being identified (for permitting) and with the potential for some of them to be recycled, reducing waste.
What does the business case look like for a miner: how do modular plants affect upfront capital, unit costs, and schedule compared with a stick-built facility, and when does “numbering up” beat “scaling up”?
Modular SX plants offer advantages in cost and schedule over stick-built construction because they can be fabricated horizontally indoors. This method is faster, ensures access for the necessary personnel, and avoids unforeseen weather-related delays. The modules can then be delivered to remote areas where access to the necessary personnel would be uncertain or impractical.
In addition to SCHEIBEL Columns, modular SX systems include all associated process components, such as piping, heat exchangers, pumps, insulation, controls, and lighting. These are all pre-installed to minimize the time between the modules' delivery to the site and successful start-up.
Regarding “numbering up” vs “scaling up”, increasing equipment sizes is typically a more cost-efficient way to increase capacity than increasing the number of pieces of equipment.
If ore grades or throughput change over time, how flexible are modular SX trains? Can you add or remove skids, switch products (e.g., NdPr vs. mixed REE), or adapt to a new leach chemistry without major rework?
Historical experience has shown that SCHEIBEL columns outperform their design capacity by 10 to 20%. They also offer turndown flexibility, typically at a four-to-one ratio, achieved by increasing the agitation speed to compensate for the effects of lower flow rates. For these reasons, they readily handle routine fluctuations in throughput.
A modular SX plant could be designed to handle multiple different leach chemistries. Each chemistry would be pilot-tested, and the chemistry that defines the SCHEIBEL column dimensions would be identified. This ensures the required flexibility needs will be met during plant operation.
Furthermore, extra agitated stages could be added to select SCHEIBEL columns, allowing for greater operational flexibility across varying leach chemistries.
Can you share a concrete example where modular SX shortened the path from pilot to sellable rare earth product, and the lessons that mining teams should apply to their next project?
While Koch Modular has not yet commercialized a rare earth SX plant, we are actively working with several companies in the early stages of these types of projects. The results so far, which include pilot testing in three-inch-diameter SCHEIBEL Columns, are very promising.
The SCHEIBEL extraction technology was developed in the 1950s and 60s and has been deployed at an industrial scale since then.
Over the past 25 years, Koch Modular has designed and built over 100 commercial-scale SCHEIBEL and KARR extraction columns for various solvent extraction processes domestically and internationally.
About the Interviewees
Brendan Cross is a Principal Extraction Engineer at Koch Modular in the Extraction Technology Group. He is responsible for solvent extraction application evaluation, extraction process development, pilot-test design, and the design of commercial-scale extraction columns and processes.
He has been with Koch Modular for nearly 20 years and also has substantial experience in distillation and process design. Cross holds a BS in chemical engineering from Columbia University.
Chris Rentsch is the Process Development Manager at Koch Modular Process Systems. He has spent the last 20 years in various production and process engineering roles in many industry sectors, including oil and gas, herbicides, lithium-ion batteries, and biomass-to-renewable chemicals. Rentsch holds a B.S.Ch.E. from the University of Michigan.
Kyle W. Parkinson is a Senior Extraction Engineer at Koch Modular. He has over 13 years of process design experience in the chemical and petrochemical industry, with a recent focus on solvent extraction. Parkinson holds a BS in chemical engineering from Bucknell University.
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