26. May 26. 2014 from Steve Mackowski
The separation process for the treatment of rare earths usually begins with a mixture of rare earth carbonate or oxide which is dissolved by an acid, generally hydrochloric acid or nitric acid, so that further processing can begin. The starting material, say the carbonate in this case, has impurities caused by the upstream extraction cycle when the carbonate was produced.
There are many regulations for these starting materials, which are usually based on the impurities. Specific separation systems have been developed to handle these contaminants so that they do not get into the products. One of the usual contamination is, for example, aluminum. This poses a potential problem because it occurs with a similar valence state in the solution process as rare earths. The separation by solvent extraction can be problematic here.
The solution process where the rare earth carbonate reacts with the acid is the time to take care of the aluminum. This step also provides the opportunity to take care of some of the radioactive elements that may have leaked out. Since the radiation problem is currently a current topic, I will discuss it in more detail below.
It is a geological fact that rare earths are associated with uranium and thorium-containing minerals. Uranium and thorium are radioactive and must be treated accordingly. Some deposits contain high uranium or thorium deposits, others contain small deposits. All of them require careful management. At this point, I have to involve science. I will, however, keep it as simple as possible. For those who want to learn more: Look for "uranium decay chain".
Uranium and thorium are radioactive, they decay and give off radiation. With each decay, one uranium molecule turns into another molecule. This new molecule is called decay product. This is also radioactive and disintegrates. A decay chain takes place until the last molecule adopts the stable form of lead. The same process occurs in thorium, although different daughter products are produced within the decay chain.
Another problem to note is that over 99% of uranium is present as the isotope uranium 238 (the internet search can provide a detailed explanation here). The remaining uranium is another isotope called Uranium 235. It is also radioactive and has its own decay chain, which also ends in stable lead. So there are three decay chains. Each with their own sequence of decay products.
The individual decay products behave chemically differently than their predecessors because they are a different element. The elements to be most concerned about are the decay products radium, radon, protactinium, and actinium. There are others that have short half-lives, which means that they don't really exist long enough to pose a quality problem. They disintegrate quickly (sometimes in microseconds). Accordingly, both the long-lived decay products and the parents uranium and thorium must be taken care of during the process (obviously also for EHS - Environmental Health and Safety reasons).
From a processing point of view, nature is a little on our side. Radon is a gas released into the atmosphere and therefore does not pose a process problem. However, it is an EHS (Environmental Health and Safety) issue, especially in underground mines.
The process of removing uranium and thorium is well known and fits into the processes before separation. There are some lead isotopes that occur as decomposition products in the decay chains, but most circuits use sulfuric acid and this gives lead sulfate, which is insoluble and, for example, peels off with the residues.
The remaining decay products must be identified and tracked by the cycle. Here are obvious quality problems but also EHS. However, all these are managed and fall under internationally recognized standards, yet they must be located.
Back to the breakup. Rare earth carbonate has defined limits for uranium and thorium. However, it has been found that some decay products can occur in the rare earth carbonate. This occurs when they have not been effectively removed in the processing stages prior to the deposition of the rare earth carbonate.
The dissolution process (and the pre-solvent extraction stages) provides the opportunity to bring small leaks under control. Of course, mines containing small amounts of uranium or thorium are less affected, but they should still be aware of the eventuality. The elements that are most likely to leak during the process are protactinium and actinium - both are decay products in the uranium 235 decay series.
So this is not so much a thorium problem. If uranium is extracted from ore, then you are lucky if it is high-quality uranium and thus a valuable by-product. However, you must take precautions in handling these decay products.
The knowledge of the chemical properties of both of the above-mentioned products is not clearly defined, but a recent research report seems to indicate that protactinium generally follows praseodymium and actinium generally follows lanthanum. Here it must be emphasized that these levels are very low and there are known process solutions. It merely represents another level of process complications. With process complication, the possibility of further costs and an increased likelihood of losses increases.
As I have already mentioned, the separation processes are specially designed for the raw material that is in the plants. The first stage, the dissolution of the rare earth carbonate, is an important step in starting the process to eliminate unwanted impurities that can lead to a higher likelihood of losses or reductions in product quality. Next week I will present you a simplified circuit configuration that will show you how to separate HREO, MREO and LREO.
Source: http://investorintel.com/rare-earth-intel/separation-rare-earths-art-vs-science-3/
