Separation of Rare Earths – Art vs. Science (Part III)
Separation plants for the processing of rare earths typically start with a solid mixed rare earths carbonate, or oxide, and dissolve that material in an acid, generally hydrochloric or nitric, so that further processing can commence. The feed material, let’s say the carbonate, is not pure. It has impurities that have been brought forward from the upstream extraction circuit and co-precipitated as the carbonate was produced. There are many specifications for these feed materials, typically based around the impurities. That is because the specific separation plant has been designed to manage those impurities so they do not pass through into the products. For example, a common impurity is aluminium. It is a potential problem since it occurs with a similar valency state (charge in solution) as the rare earths, when in solution, and separation by solvent extraction can be problematic. The dissolution step, where the rare earth carbonate is reacted with the acid is where the aluminium can start to be managed. This step is also a possible opportunity to manage some of the radioactive elements that may have leaked through. As the radiation issue is also quite topical at the moment, I will expand.
It is a geological fact or coincidence, that rare earth minerals are associated with uranium and thorium containing minerals. Uranium and thorium are naturally radioactive and need to be managed as such. Some deposits have high uranium, some high thorium, others quite high or low in either, but they all need management. I need to introduce some science here but I’ll keep it as simple as I can. For those who want more understanding search for “uranium decay chain” and all levels of explanation are available. I’ll go for simple. Uranium and thorium are radioactive, they decay and emit radiation. With each decay a molecule of uranium transforms into another molecule. This new molecule is called a daughter product. It is also radioactive and also decays. There is a chain of these decays until the eventual molecule is a stable form of lead. The same process occurs with thorium, although different daughters are on its decay chain. Another issue to be noted is that over 99% of the uranium is present as the isotope Uranium-238 (the internet search can fill in an explanation). The remaining uranium is a different isotope called uranium-235. It is also radioactive and has its own decay chain ending in stable lead. So there are three decay chains occurring, each with their own sequence of daughters decaying and being replenished from the parent. The important point to note now is the chemistry. The individual daughters do not behave chemically like the parent since they are a different element. The daughters may be radium, radon, protactinium, and, actinium as the major ones to be concerned about. There are others but they have short half-lives, that is, they don’t hang around long enough to be a quality issue. They quickly decay (sometimes in micro-seconds). So the key long half-life daughters and the parent uranium and thorium need to managed within the process flow sheet (obviously as well as for EHS reasons).
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From a processing point of view, nature is a little on our side. Radon is a gas and is released to the atmosphere, and so is not a process issue. It is however an EHS issue, particularly in underground mines. Processing to remove uranium and thorium is well known and fits into the processes prior to separation quite easily. There are some isotopes of lead that occur as daughters in the decay chains but most circuits utilise sulphuric acid and this makes lead sulphate which is insoluble and departs with the bake residues, for example. The remainder of the daughters have to be identified and tracked through the circuit. There are obviously quality issues here but also EHS. All of which are manageable, all of which have internationally acknowledged standards, but they must be tracked.
So back to separation. The specification for the incoming rare earth carbonate has well defined limits for uranium and thorium. It has been identified, though, that there can be leakage of some of the daughters into the rare earth carbonate. That is, they have not been effectively removed in those processing stages prior to the precipitation of the rare earth carbonate. The dissolution step (and pre-solvent extraction satges) is an opportunity to start to manage any minor leakage. Obviously, those mines with low levels of uranium or thorium should be less concerned here, but you still need to know. You should prove to yourself that you do not have a problem. It will certainly help with any PR concerns later on. Those elements most likely to leak through the process are protactinium and actinium, both of which are daughters in the Uranium-235 decay chain. So this is not so much a thorium (read monazite) issue. So if you have high uranium in the mined ore, you are lucky in that it may be high enough to be a valuable by-product, but you will need to be more cautionary in the management of the decay products. The knowledge of the chemistry of these two daughters is not well defined but recently published research seems to indicate that the protactinium is generally tracking praseodymium and actinium is generally tracking lanthanum. It must be stressed here that these levels are every low, and there are known process solutions. It just adds an extra level of process complication. And with process complication comes the possibility higher cost and increased probability of losses.
I have previously stated that separation circuits are specifically designed for the feed material that the plant will receive. The first stage, the dissolution of the rare earth carbonate, is an important stage to start the process of getting rid of any unwanted impurities that can result in a higher probability of losses or reductions in final product quality. Next week, I will take the rare earth solution and present a simplified circuit configuration that will show you how the HREO, MREO and LREO can separated.
Click here to access Separation of Rare Earths – Art vs. Science Part I or Part II
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>