Basics of Rare Earth Processing Methods - MSE Supplies
Cell phones, electric cars, sensors – just some of the many devices containing Rare Earths, that we’ve come to rely on in the modern world. While there’s a growing awareness of the importance of Rare Earths (REs) in these new technologies, the same can’t be said for the illusive question of just how Rare Earth Elements (REEs) end up in these products.
Mining REs is relatively simple – but producing individual elements from the ore is tremendously difficult. Rare Earth processing often requires dozens of procedures – each resulting in minute changes in the complex RE stream. Separating and extracting a single Rare Earth Element– especially one of the Heavy Rare Earths – takes a great deal of time, effort and expertise. Not to mention money – processing facilities cost hundreds of millions of dollars to build.
It’s something to think about, next time you text a friend or take your Prius out for a spin. But wrapping one’s head around the vast array of separation and extraction techniques for REEs, is far easier said than done. It’s terribly complex, and there just isn’t much information out there. For your benefit, here’s a basic primer on RE processing.
After rocks containing REEs are removed from the ground, they go to a facility where the valuable mineral material in the ore is separated from impurities. This process is known as milling or beneficiation. Here’s how it works:
The mined ore is crushed into gravel, which in turn is ground up into progressively smaller particles. These particles are sifted and sorted by such means as flotation and electromagnetic separation to extract usable material and set the waste products – called tailings – aside. This milling process is usually carried at or near the mine site, with the tailings stored in special facilities built to rigorous engineering and environmental standards.
For scarce resources like Heavy Rare Earths, this beneficiation process could be considered critical, “because it takes advantage of every scrap of material available. This practice can also make a marginal mining facility more practical than it might otherwise be, and may in fact be used to extract ore from a facility previously believed to be exhausted.
This milling method uses magnetic principals to separate Rare Earth bearing minerals from other materials in the mined ore. Monazite – along with bastnaesite, the primary commercial source of REs mined around the world – is highly magnetic, meaning it can be separated from non-magnetic impurities in the ore through repeated electromagnetic separation.
This technique uses a magnetic separator device that consists of a belt moving on two rollers, one of which contains strong magnets. When powdered ore is dropped onto the belt, magnetic and non-magnetic particles within the ore will fall away differently from the magnetic roller as this diagram illustrates.
This is another beneficiation method that’s used to separate bastnaesite from other minerals. First, the ore is ground into a fine powder and added to liquids in flotation tanks. Chemicals are added to cause impurities to settle out, and air is pumped in to create air bubbles. The finer bastnaesite particles stick to the bubbles, which rise to the top and form a froth that is then skimmed off.2 The following diagram shows how this works:
Although they are commonly used in the gold industry, devices called Falcon Concentrators are also used in Rare Earth extraction at the milling stage. These concentrators contain rotating cones or bowls that are spun at high speed to generate a gravitational or centrifugal force, which acts to separate small particles by exploiting minute differences in density and specific gravity between the valuable minerals and waste products. Compared to other beneficiation technologies, gravitational separation offers lower installed and operating costs. It also tends to also have less environmental impact as gravity concentration does not require the use of chemicals.
All of these milling processes produce mineral concentrates that contain a substantially higher proportion of REs. But there’s still much work to be done to separate the concentrate into its constituent REEs, and this is why things start to get really tricky.
As the generations of scientists who have tackled the problem can attest, isolating REs safely and effectively is not only a very long and costly exercise, but extremely complicated. The complex separation and extraction techniques in use today, like ion exchange and solvent extraction, are rooted in of a branch of geologic science known as hydrometallurgy.
In hydrometallurgy, mineral concentrates are separated into usable oxides and metals through liquid processes, including leaching, extraction, and precipitation. By these means, the elements are dissolved and purified into leach solutions. The RE metal or one of its pure compounds (such as an oxide) is then precipitated from the leach solution by chemical or electrolytic means.
Although hydrometallurgy originated in the 16th century, its principal development took place in the 20th century. The development of ion exchange, solvent extraction, and other processes now permits more than 70 metallic elements to be produced by hydro metallurgy, including the REEs.4 Here is a run-down on some of these techniques.
Devised by British chemist Charles James in the early 1900s, fractional crystallization is based on differences in solubility. In this process, a mixture of two or more substances in solution is allowed to crystallize, either through evaporation or by a changing the temperature of the solution. This precipitate will contain more of the least soluble substance. The process is repeated until purer forms of the desired substance are eventually attained.
Like all early extraction techniques, fractional crystallization is very slow and tedious. James found that an enormous number of stages of crystallization were required to get the high purity of individual REEs. For example, RE bromates had to be crystallized for four years daily to obtain good quality Holmium. And the fractional crystallization process had to be repeated a staggering 15,000 times to get decent quality Thulium (which even then, still contained traces of other REEs).
Despite these constraints, James’ methods were widely adopted by other chemists. Fractional crystallization continued to be considered be the best technique of separating REEs until the discovery of ion exchange technology in the 1940s.
The ion exchange method was first used during Second World War as a way to separate fission products obtained from nuclear reactors. In this process, a solution containing a RE mixture is filtered through minerals called zeolites, or through synthetic resins that act as zeolites. Zeolites exchange ions (or atoms carrying an electrical charge); in the ion exchange process, zeolite ions are added to the solution and RE ions bind tightly to the zeolites.6
Various solutions are then used to wash out elements one at a time. Each is then mixed with acid to create an oxalate compound, and then heated to form the usable oxide.7
These oxides – which are a mixture of various REEs and oxygen – are now ready to be broken down into their constituent elements… albeit through yet more complicated processing steps that we`ll talk more about later.
Ion exchange – like fractional crystallization – is a long and laborious process. But it was better than any other method that had come before, and it was capable of producing high purity REEs. As a result, ion exchange remained the most the most widely used separation technique until the arrival of a new method – solvent extraction – in the 1970s.
The process of solvent extraction uses chemical agents to break down the components within a substance. Those materials which more soluble or react more readily to a particular acid or base get separated from the rest. The separated materials are then removed, and the process begins all over again with the introduction of more chemicals to leach out more components. When it comes to Rare Earths, these steps need to be repeated again, again and again… sometimes hundreds of times, depending on which REE you’re trying to produce.
The solvent extraction method used today to separate REEs relies on the slightly different solubilities of rare earth compounds between two liquids that do not dissolve in each other (in essence, oil and water). For example, one process has bastnaesite repeatedly treated with hot sulphuric acid to create water-soluble sulphates. More chemicals are added to neutralize acids and remove various elements like thorium. The mineral solution is treated with ammonium to convert the REEs into insoluble oxides.
Another chemical technique for separating monazite into RE compounds is called alkaline opening. This process uses a hot sodium hydroxide solution that makes thorium precipitate out as a phosphate. The remaining mixture of thorium and lanthanides (REEs) is further broken down when treated with a hydrochloric acid that creates a liquid solution of lanthanide chlorides, and a sludge made up of thorium hydroxide.
Because the Rare Earths are all so close to each other in terms of atomic weight, chemical separation methods require multiple stages to complete the extraction process. One stream in this process often takes hundreds of steps, involving a cascade of dozens of different tanks and machines for mixing, settling, filtering and evaporating all the various solutions. One advantage solvent extraction has over ion exchange is that it can be continuous – a counter-current system can be employed in which the many, many extraction steps are carried out in a continuous stream, progressively increasing the degree of separation until the substance in one phase in nearly pure.
The time required for solvent extraction can vary widely; it can be very lengthy in some cases where materials need to be allowed to mix and sit for a time for the components to separate out. Complicating things even further, is that many of the chemicals used as well as some by-products of solvent extraction are extremely hazardous and must be handled (and disposed of) with great care.
These methods produce compounds like RE oxides, which have a growing number of useful applications today and as such can be considered end-products in the Rare Earth supply chain. However, demand is also growing for RE metals – which means even more refining in the long hydro metallurgic process.
As is the case with every preceding step, it’s not easy turning chemical compounds into a single metal. Several techniques have evolved to meet the tremendous challenges associated with distilling Rare Earths down to their purest form.
The primary types of metal recovery processes are electrolysis, gaseous reduction, and precipitation. A common technique for REEs is metallothermic reduction, which uses heat and chemicals to yield metal from RE oxides. In this process, the oxides are dispersed in a molten, calcium chloride bath along with sodium metal. The sodium reacts with the calcium chloride to produce calcium metal, which reduces the oxides to RE metals.
Calcination is an extraction technique that also employs thermal principles. In this instance, ovens and other devices like induction furnaces and arc furnaces are used to heat up substances to the point where volatile, chemically combined components like carbon dioxide are driven off.
Another extraction technique is sorption, in which one substance takes up or holds another. It is actually a combination of the two processes – absorption, in which a substance diffuses into a liquid or solid to form a solution, and adsorption, in which a gas or liquid accumulates on the surface of another substance to form a molecular or atomic film.
RE extraction technology also includes methods like vacuum distillation and mercury amalgamate oxidation-reduction. And we can list a few others– like high-performance centrifugal partition chromatoagraphy and Sl-octyl phenyloxy acetic acid treatment – that have impressive sounding names, but require a few doctorate degrees to decipher.
Costs can be prohibitive
It is clear that while mining material containing REEs isn’t all that complicated, processing the stuff is about as far from simple as you can get. This is particularly true with Heavy Rare Earths such as dysprosium, terbium and yttrium. The complex metallurgical technologies have taken decades to evolve, and each RE deposit presents its own unique challenges for separating and extracting. As a result, it can take many years for scientists to crack the geological code and design appropriate metallurgic processes for each RE stream.
All this means processing Rare Earths is not cheap – far from it. Because of the complex technologies involved and other issues, such as the disposal of radioactive waste, it can cost hundreds of millions of dollars to build a processing plant from scratch. And there are other costs to consider when going into the RE business, such as the considerable expense of ensuring adequate infrastructure and transportation systems are in place to support the mining and processing facilities and for transporting products out to market. There are also costs involved in building the necessary expertise and training up a labour force to the very high standards required for running a RE processing facility.
Investors, therefore, should take heed: while there’s no shortage of junior Rare Earths competing for your dollars these days, the real value lies with companies that have existing processing capacity and everything –the facilities, infrastructure, expertise, and proven hydrometallurgy – already in place. Without that, your REs are just a bunch of rocks.