Separation of Rare Earths – Art vs. Science (Part IV)
Separation of Rare Earths – Art vs Science (IV) will not follow the recent trend in reality TV shows. That is, every time you have a break, you come back, revise where you have been, present a little more of the story and then go to a break again. Repeat ad nauseum. And I mean ad nauseum! My series of articles will flow in a series and for the most part you need to read them all; and in sequence. So Separation of Rare Earths – Art vs. Science (I), (II) and (III) are accessible below and the previous articles will continue to be available at the end of each week until Separation of Rare Earths – Art vs. Science (?) is concluded.
OK. Separation of TREO into the individual REOs. The science is still out there. Well to a degree. The way you tackle this opportunity is dependent on the spectrum of the TREO across HREO, MREO and LREO and importantly the impurities. I will introduce you to a scientific method that I don’t think has a name, so I’ll call it “The Gate Method”; as in going through gates. The reason behind this approach is because you cannot get 100% recovery of the target valuable with 100% rejection of the non-valuable. Simply, the higher the recovery you go for, the lower the grade (read quality) you will get. Similarly, the higher the grade you go for, there will be a drop in recovery to get that grade. Now believe me, you don’t want to lose recovery here. You have spent probably 80% of your capital and operating costs to get here, you don’t want to lose TREO. A loss here comes straight off the IRR of the project. A 20% IRR project can go to <15% real quick if you don’t get this separation right. So how can you maximize recovery and maximize grade? I’ll use my “The Gate Method” with an example to lead you in.
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Picture a room with 1,000 people. Gender irrelevant. All of the people are between 6 foot 3 inches and 5 foot 9 inches tall. They do not know their own heights. You want to split the people according to their height. You want to separate those who are precisely 6 foot tall from those that are below 6 foot. Without elaborate measuring systems on every one of the 1,000 individuals, how can you do this? The Gate Method. Have them walk in single file through a gate with a height limitation of say, 5 foot 11 inches. Some will clearly pass through. Exit stage left. Some have to duck. Exit stage right. Some aren’t sure, exit right since you have allowed for the tolerance. So gate 1 has clearly separated out those who are less than 5 foot 11 inches. Now for gate 2. Set at 6 foot 1 inch. Same process. If you clear, go right, if you don’t or are close, go left. So after 2 stages of gating you have three groups. Those that are clearly taller than 6 foot, because they passed the 6 foot 1 inch gate; those that are clearly shorter than 6 foot, because they failed the 5 foot 11 inch gate; and the third group, that is between 5 foot 11 inches and 6 foot 1 inch tall. The scientific method at Gate 1 and Gate 2 was quick and easy, therefore cheap. You have removed a lot of the original 1,000 so you can focus on group three, the close group, where a more detailed method can be utilized to sort out those greater than 6 foot and those less. They then report to the tall group or the short group. A similar scientific method is available in REO separation.
Picture the suite of rare earths as a spectrum (just like the heights of the 1,000 people), starting with lanthanum (Atomic Number 57), then cerium (58), praseodymium (59), neodymium (60), promethium (61), samarium (62) and so on to lutetium (71). Each one progressing in atomic number in increments of one. This progressive change in the atomic structure gives progressive change to the chemical behavior. Before I discuss the gating method, it is not just the atomic number that influences the outcome. The relative proportions can also have an impact. In the people height sorting gates, imagine if 99% of the people are less than 5 foot 11 inches. Imagine if it’s the NFL (or NBL or NHL, or for us Aussies the AFL or NRL) Grand Final and seating is arranged by height. Imagine if those “short” people want the “tall” seats! Sheer weight of numbers can force the result. The same in the chemistry of REO separation. Simply put, at the first gate, you want to separate the combined HREO + MREO from the LREO. The reason you do this is to take account of the fact that promethium (61) does not occur naturally. So there is a gap in the spectrum, with LREO to the left and MREO and HREO to the right, and we, engineers utilize that gap as much as possible. However, remember this circuit is a typical LREO circuit, so there is probably 45% of the TREO as cerium, 20+% as La, and 25%+ as Nd + Pr. There is a maximum 10-15% combined HREO and MREO. So the LREO wants to dominate. For the more scientifically minded, the HREO and MREO due to their higher atomic number have a greater affinity for the extracting organic solvent and “load” quickly and preferentially. But the concentration effect of the LREO has a tendency to want to crowd out the already extracted HREO and MREO. So the first gate takes all of the HREO and MREO and some of the LREO. This gate is going for high recovery of the HREO and MREO. Think of these gates horizontally, I’ll present a full diagram later. The second gate in this horizontal progression takes out the HREO and some of the MREO. The third gate focuses on MREO discarding the “leaked” through LREO which can be recycled back to the start of gate 1. I am trying to simplify the scientific method in a way you can understand. Go for recovery of the HREO and MREO and allow a little leakage of LREO, then take that recovered material, go for grade of HREO and MREO and recycle back the “leakage” of LREO. Why is there leakage of LREO when the promethium (61) gives a break to the spectrum of the TREO? It’s due the much higher concentration of the LREO compared to the HREO and MREO.
This sequence of gates progresses horizontally. Separate the HREO from the combined HREO + MREO stream, going for recovery of HREO and “leakage” of MREO. Clean up the HREO and recycle back the “leaked” MREO. You now have a clean solution of HREO, a clean solution of MREO, and the LREO that has passed through. Time (and your patience) does not allow me to go through the entire process of getting to the individual HREO or MREO here, but follow “The Gate Method” logic. MREO is Samarium (62), Europium (63) and Gadolinium (64), SEG. Design a gate to maximize the recovery of G, allow leakage of E, then design a gate for grade of G and recycle back the E. Repeat until you have separated the individual S and E. The same theory for HREO.
A point to note. As you progress along this horizontal sequence of chemical gates to separate the MREO and HREO, and progressively on to the individuals, that the solutions become more dilute. Dilute solutions do not have the “chemical drive” to combat other influences. So it is opportune (and sometimes essential) along the way to precipitate out and re-dissolve at higher concentrations to get that “chemical drive” back. Again note. These precipitated solids are potentially saleable and can halt the horizontal progression on your site. This simplifies the circuit on your site, reduces capital cost, reduces operating cost, but reduces saleable value.
Now, all separation plants don’t go all the way through to this level of sophistication or do they need to. There is a market for combined MREO+HREO; just as there is a market for the mixed MREO or HREO products; just as there is a market for most combinations in between. It is proposed that tolling plants be built. Whatever the resultant business model looks like, the processing model could follow the above progressive “Gate Method”. The geographic location of where each gate is, is a business plan that awaits you and your unique spectrum of TREO. Next article, takes the “raffinate” from gate 1 (the original dissolved TREO minus the MREO and HREO) and looks at how we separate the individual LREO.
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>