Separation of Rare Earths – Art vs. Science (Part V)
Separation of Rare Earths – Art vs. Science (Part IV) had us finalizing the removal of MREO and HREO from the dissolved mixed REO carbonate in an acidic solution by solvent extraction using “The Gate Method” as a means of explaining how separation works. What we are now left with is the “raffinate”. A generic chemical term meaning the liquor waste stream after the extraction stage. This typical LREO based example has the raffinate containing all the LREO (lanthanum, cerium, neodymium and praseodymium), any residual impurities and the acidic host liquor. We are now to take the next stage of REO separation.
For those who have not read the preceding articles, the following pre-reads are recommended to follow the entire story.
Separation of Rare Earths – Art vs. Science (Part 1)
Separation of Rare Earths – Art vs. Science (Part 2)
Separation of Rare Earths – Art vs. Science (Part 3)
Separation of Rare Earths – Art vs. Science (Part 4)
The next step is the Nd/Pr gate. This gate is next as the Nd/Pr have a higher atomic number than the La and Ce and tend to be more responsive in solvent extraction circuits. Just as with the previous gates, working on maximum recovery followed by a cleaning stage, with leakage recycle, the Nd/Pr gate works the same. Design the process to take maximum Nd/Pr from the liquor. By necessity, some of the other LREO will “leak” through. Design the next gate to extract Pr with Nd leakage and recycle the LREO. And so on until clean Pr, clean Nd and the other LREO is totally recycled. Easy. As explained before, there are opportunities to sell products before they get to the fully separated stage. So a pure mixed Nd/Pr is saleable. This is didydium (a marketing term – not scientific). Just as pure Nd or pure Pr are obviously saleable. Again, similar advantages of lower capex and lower opex, but having lower value.
So is the REO separation an art or a science? There are components of both. So firstly, what is the science? All of what I have presented around “The Gate Method” is predictable if you have knowledge of the REO spectrum in the feed mixed REO carbonate. It is possible to model these separations. It is known (or able to be determined mathematically), depending on that spectrum, how many extraction stages are needed at each gate; how many stripping stages are needed at each gate; and what are the organic to aqueous ratios in these stages. These models are available for separating MREO + HREO from TREO, for separating MREO from a combined MREO + HREO, for separating MREO into its components (samarium, europium and gadolinium), and for separating Nd/Pr from the previous gate stage, etc etc. Caveat emptor! The models that are available rely on a certain consistency of the spectrums of TREO. So a normal bastnasite-feed TREO carbonate is predictable, as is a monazite-feed TREO carbonate, as is a HREO predominate TREO carbonate, but the model gets less reliable as the experience and knowledge around each new spectrum is researched. I was once amazed at a very prominent Chinese REO expert telling me that he didn’t need a pilot plant to design the separation plant. He just needed to know the spectrum of TREO! This is based on the science. So why is separation acclaimed to be so difficult? Well for a start you don’t have to start at the start as the above models can help. But there are pitfalls that can take time and effort.
Designing an REO separation plant is not just about the science. You may have a different ore assemblage of REO minerals from the models. This changes the TREO spectrum and introduces uncertainty into the science. Different mineralogy certainly introduces different impurities and they influence the flow sheet and the performances at each gate. Variation of the ore body laterally or with depth can also change the separation performance. This is where the concept of art comes in. This is where you have to “learn and adapt” the science to take into account all of the differences and uncertainties. And where ever possible, this should be done before the finalization of the design so that the ability to manage this variance can be available to the process operator.
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There are other parts of the art.
- How do you ensure that the process starts up and is maintained at the right phase continuity? (refer earlier articles)
- How do you respond to total power failure?
- How do you respond to power blips when only sections of the plant are effected?
All of these “art” questions can be asked and partially answered during pilot plant and design. Some of the art is awaiting at commissioning and ramp-up to be experienced, learnt from and incorporated into the art of processing.
Other parts that are more art than science are in the form of the product. It’s physical form as opposed to it’s chemical form. This form of product discussion is very pertinent to the story of cerium and lanthanum and so I will leave that part until the next article.
Mr Mackowski is a qualified engineer in mineral processing with over 30 years technical and operational experience in rare earths, uranium, industrial minerals, nickel, kaolin ... <Read more about Steve Mackowski>