Separation of Rare Earths – Art vs. Science
When Tracy asked me to be a regular contributor to InvestorIntel, I thought long and hard about what I could add to an already outstanding list of contributors. I more importantly, thought about what I would not add or present. Comparisons between exploration companies to see who is the “best” investment is somewhere I will not go. The politics of countries and the drivers in their decision making is something best left to in-country experts. So I agreed to present my thoughts of REO (rare earth oxides) in a generalist way. In a logical, scientific and hopefully readable manner to allow you to become more aware of the surrounding issues, better equipped to comprehend some of the science, and, with a now more knowledgeable REO understanding be able to operate in the REO space in a manner that suits your needs.
One of the most topical issues in the REO business at the moment is the issue of separation. Now there’s separation and there’s separation! Just as there was light REO (LREO – for the record cerium, lanthanum, neodymium and praseodymium), medium REO (samarium, europium and gadolinium) and heavy REO (the rest plus yttrium), and we would all remember the confusion around definitions for the last 10 years about that. Lucky it has finally been resolved with the “new” definitions (at least for today) of REO as Critical (CREO) or recently magnet feed REO (MFREO). Won’t be going there today, but I will get to discussing separation and what it is and what it is not. And I will clarify what the issues of separation are and how they impact on project development, CAPEX and OPEX costs, process efficiency and product quality. But first a little history, presented as a development of my understanding of separation.
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Back in 1977, I first met Gordon Ritcey. He was presenting a solvent extraction workshop in Adelaide, and, I was a youngish engineer working on uranium production based at Mary Kathleen in North West Queensland. Gordon presented separation as a very logical science based on the ability to take a liquid (generally water (aqueous) based) and to remove a valuable component (uranium) leaving behind the non-valuable components. The valuable component was transferred into an organic (solvent) phase (think kerosene with special chemical additives). With this extraction stage over the now relatively pure valuable phase was transferred into an aqueous phase for precipitation as a solid product. This stage is called stripping. So the system comes in two parts, extraction and stripping. What are the important parameters to address?
Well according to Gordon (who since became a good friend and mentor) there are many parameters in play. Sorry about the science but there is an important point being developed here. Obviously the composition of the starting liquid is important. What are the valuables (uranium in this example), what are the major impurities (silica, iron, phosphate, sulphate), what are the minor impurities that can cause you grief? Gordon placed science behind them all and described how all of the composition variables were influencing the final efficiency of separation. This matched my experiences at Mary Kathleen where I had seen first hand the process impacts of pH (acidity), Eh (redox potential), too much silica, etc. What I was really interested in was how to control the circuits? How do you keep them stable? What reactive things can you do when things go wry? Well as a start, you have those things that are fixed. Simply the number of extraction stages (maybe 4) and the number of stripping stages (maybe 4). They will have been configured based upon laboratory experiments so as to give optimum extraction and stripping (high recovery) and optimum selectivity (high grade of product). What were the variables I could play with?
Again sorry for the science but there is a point coming.
When you mix the aqueous with the organic, mass transfer of the valuable phase occurs. Trust me it matters if you have droplets of aqueous in a carrier fluid of organic (called organic continuous) or if you have the opposite, ie droplets of organic in a carrier fluid of aqueous (aqueous continuous). Think here a water emulsion or an oily emulsion. It also matters as to the relative amounts of the aqueous compared to the organic (the phase ratio or O/A ratio). Considerable differences in performance can therefore be obtained by varying some critical parameters. Can you imagine having 5 parts organic to one part aqueous as a mixture and it is still aqueous continuous? And also the opposite. Try it at home using water and canola oil. The point is that there were many variables. Variables I might add that are unknown for the most part as the plant is running real time but analysis is 24 hours later. Control options that may or may not improve things, but again the subtle effects of which are not readily apparent. How do you operate a single valuable component separation circuit (uranium)? What is your control system? Answer is we learnt how to manage the variations in chemistry of the feed liquor, understood the control opportunities and their impacts, and also understood our limitations.
So when we designed, built and operated an REO separation plant on the waste stream from the uranium plant, life was easy? Although the chemistry was different, looking to extract REO using a different organic additive, although the phase ratios were different, although the mixing times were different, the plant was simple. Extraction and stripping. But, and here is the big but, back in 1978 REO was about misch metal. In today’s terms a rough chemical concentrate of predominately LREO. No-one was interested in MREO or HREO. Uses had not been invented. So separation philosophy was simple. Extraction of the valuable, strip and precipitate. I witnessed the same philosophy at Ranger, Nabalek, Olympic Dam and all the other uranium plants that were coming on stream. The point! At last you may say. I was salivating in 2005, when I had my first opportunity to visit a Chinese REO separation plant. Producing multiple separated products, not just LREO. They produced cerium oxide, lanthanum oxide, neodymium etc. They were producing 3, 4 and 5 nines quality and were certified to ISO standards. I could hardly wait to discover the secret and to use that knowledge to assist the REO development program I was on at time. What did I see? What did I learn? Just as I had to wait many years for the secret, you will have to wait for the next issue of InvestorIntel.
Steve Mackowski signing off.
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>