Getting to know /


Remember the old arcade games from the 80s? No fancy hyper-realistic graphics, no third dimension. Nonetheless, in just a few pixels, you could live the most amazing adventures: Rescue abducted princesses, save Earth from alien invasions, flying spaceships through asteroids and… eat a lot of fruit while staying away from indigestible ghosts.

Yes, that was Pacman’s thing. The yellow pal was on a mission to seek and collect all the pieces of fruit that were disseminated within a labyrinth, avoiding contact with the ghosts.

What does that have to do with solvent extraction? Regrettably, there are no arcades at CHROMIC facilities, yet Pacman’s lifestyle comes in handy to describe the object of this Getting to Know, as we’ll see in a moment.

After the “tea preparing” stage (i.e. leaching stage) of the metal recovery process, we were left with our valuable metals (Cr, Mo, V, Nb) dissolved in a complex solution containing solvents and some impurities: The final step of the recovery process is then that of separating the metals from the solution, and possibly enrich them. That can be achieved very efficiently by cunningly combining different chemical techniques, one of which is solvent extraction.


CHROMIC Podcast – Episode 6 is about solvent extraction. Click here to listen to the podcast


This method is used to separate compounds based on their solubility in two different immiscible liquids, which in most cases are an aqueous solution containing the target element(s) and an organic compound. To do that, the two liquids (or “phases”) are mixed, so that the solutes can distribute between them until equilibrium is established, and the two liquids are separated again. The transfer of species from one phase to the other is driven by the chemical potential, which by the end of the reaction brings the whole system to a more stable energetic configuration. The liquid containing the desired solute is called the “extract” and what is left behind in the other solution is called the “raffinate”.

Going back to the analogy we started from, we can conveniently link the elements at play with the characters and dynamics of the beloved arcade game. The organic compound is represented by Pacman itself, the fruits are the valuable metals, and the ghosts are the impurities which Pacman does not want to get into contact. The solutions are immiscible, but they can be mixed through stirring, so Pacman comes into contact with the fruits, transferring them into the organic phase. When Pacman has explored the whole labyrinth, the mixing stops and the two solutions separate, leaving the ghosts and impurities in the aqueous leachate, whereas Pacman and the fruits are located in the organic phase. In a second step another solution can be added, which encourages Pacman to release the fruits into the aqueous phase, so to finally get a pure solution of our valuable metals.

Aside from this simplified image, the process is indeed in principle pretty straightforward, but in practice very challenging. First off, in case of the reactive extraction of metal ions selecting the right extractants, modifiers and solventsfor the task is far from being trivial. A number of features have to be taken into account: The ability of the extractant to bind to the target metal to a much larger extent than the rest of the components in the mixture; the irreversibility of the reaction must be guaranteed, in order for the dissolved components not to go back to their previous form; the compound formed after the reaction has taken place must be easily recoverable. Other factors that affect selection for the composition of the organic phase are its solubility in the aqueous phase and its long term stability; for industrial applications other factors like low toxicity are important, too. . Furthermore, the conditions under which the extractive reaction takes place greatly impact on the final result, and need to be fine-tuned. For instance, it is very important to maintain  a stablepH and a constant temperature of the compound during the extraction process, as well as to find the right residence time (the time in which the two solutions are in contact) and the suitable phase ratio so that the reaction is optimized.

Solvent extraction is widely used both on small – chemical laboratories – and industrial scale, due to its cost-effectiveness and capacity of separating the required components without altering their properties. It is for instance applied in the production of fine organic compounds, the production of vegetable oils and biodiesel, the processing of perfumes. It is also employed in the petrochemical refining industries, where extraction allowsthe procurement of pure petroleum from the impurity-filled crude oil. From a hydrometallurgical standpoint, the ability to selectively separate out even very similar metals makes solvent extraction the way to go for separation and purification of elements like uranium and plutonium, cobalt and nickel, as well as rare earth elements.

Depending on the application, different devices and apparatus can be used to perform solvent extraction. Those commonly include so called separatory funnels (at lab scale), and machines that bring the two liquids into contact with each other, like extraction columns  and mixer-settlers.

In CHROMIC the research on solvent extraction to recover Cr, V, Mo and Nb is brought forward by HZDR, while other methods which are to be combined with solvent extraction, like selective precipitation and sorbent materials are investigated respectively by BFI and FehS, and VITO. Lastly, TUK focuses on the final processing.

No arcade game characters are being harmed during the process.

(Photo: bdyczewskiPixabay )


Think of a Magnetic Resonance Imaging (MRI) machine, the most powerful particle accelerator ever built and the fastest train on the globe. They all are very remarkable pieces of technology and human ingenuity, but apart from that, what do they have in common? The answer is niobium. This element is a superconductor, a material in which – below a certain critical temperature –   electricity flows with zero resistance, while the magnetic field is expelled out in a peculiar way. That is why the strong magnetic fields required by MRI devices, as well as the ones used to steer particles at the Large Hadron Collider and those which allow to keep a levitating train (the SCMAGLEV in Japan) locked to its binary at 600 kmph, are provided by superconductive magnets made of niobium alloys.

Shouldn’t that be enough to make it interesting, there are much more properties and applications to talk about in getting to know niobium.


What does a tasty cup of tea and a metal recovery plant have in common? Well in practice nothing, but in principle much more than one can expect. Fortunately, it’s not about the ingredients. Instead, the comparison has to do with the process through which one is able to extract a substance from a solid material that has come into contact with a liquid. The process is called leaching.

Indeed, as every morning billions of people unwittingly use this extraction technique to enjoy the organoleptic properties of tea and coffee, at the same time many industries greatly benefit from the vast number of leaching applications. For example, leaching is widely used in the biological and food processing industries for the separation of sugar from sugar beets with hot water, or for the extraction of oil from peanuts, soybeans and sunflower seeds. Likewise, many pharmaceutical products are obtained by leaching plant roots, leaves and stems.


There may be a responsible for an almost 2 billion years delay in the evolution of Earth’s living organisms: Molybdenum. Or more precisely, the lack of it.

Until recently, it was hard to explain why from 2.5 billion to about 500 million years ago not much happened in terms of life development. Then, by studying deep oceanic sediments, scientists found that levels of molybdenum in the ocean in that timespan were very low, which is quite a big deal: Without molybdenum bacteria could not convert atmospheric nitrogen to essential metabolic compounds, so that more complex forms of life – our multicellular ancestors – could not develop and thrive. Of course, other factors may have contributed to that huge lag, but the lesson is clear… Never run short on molybdenum!

Evolution aside, this element has outstanding characteristics, such as one of the highest melting points of all pure elements and an impressive resistance to corrosion, which, as we’ll see, make its metals and alloys the first choice in many demanding specialized industrial applications.

Molybdenum (Mo) is the second element of Group 6 (the sixth column) of the Periodic Table, a transition metal with atomic number 42, an atomic weight of 95.95 and density of 10.28 grams per cubic centimeter. It is a silvery-white, ductile metal, named after the Greek word “molybdos”, meaning lead.


The secret to prepare perfect slags? Heat them up properly in a microwave, before serving.

Of course that’s a (lazy) joke, based on the fact that it is quite natural to associate the words “microwave heating” to the act of preparing food at home. No kitchen oven is part of the story here, but as surprising as it may sound, what claimed above is actually a good recipe for some metallurgical processes, especially the ones CHROMIC is interested in.

Indeed, we should start by pointing out a not very well known fact: microwave energy is not only used to warm up meals, instead, it has a wide range of industrial applications. For instance, it is used in the pharmaceutical industry to remove moisture from powder substances and in the materials supply industry to permit the addition of different types of coatings to plastic and rubber materials. It is also exploited in agriculture for drying of grain and as a thermal process to eliminate bacteria from commercial food. The list is far from being exhaustive.

Within the CHROMIC project, microwave energy is used to crack in a specific, smart, way the slags before they enter the comminution step – the phase in which the slags undergo various crushing and grinding procedures, in order to free fractions of valuable materials from them.

Pre-treating the slags with microwaves makes comminution much more efficient and less energy consuming. Let’s see how.

Microwaves are electromagnetic radiations, with wavelengths ranging from one meter to one millimeter (thus frequencies ranging from 300Hz to 300GHz).

When some material is irradiated by microwaves, it may happen that the electric field carried by the radiation sets in motion the molecules and/or ions within the material. That occurs in so-called dielectric materials, which are characterized by a polar structure of their molecules/atoms.

In a dielectric, molecules start to rotate continuously trying to align with the oscillating electric field provided by the microwaves. Rotating molecules then collide with each other distributing their kinetic energy in the material. As temperature is related to motion (it is actually a measure of the average kinetic energy of the molecules and atoms in a material), the dielectric heats up.

As mentioned above, not all materials are dielectric. Some, as many plastics, are transparent to microwave radiation, meaning that microwaves pass through the material without being absorbed; others, like metals, do instead reflect microwaves. This different behavior means that when some material is a mixture of different substances, part of it will be heated up by the microwaves and part won’t. That is the effect – called “selective heating” – exploited in CHROMIC.

As the slags to be treated in the project are composed of various substances with different microwave absorption properties, the application of an intense microwave radiation generates a very fast temperature gradient within them, which forms strong compressive forces resulting in microscopic cracks at the boundary layer of the different substances. Slags treated with this procedure are fractured in a way such that it is easier to reach the valuable metals inside them in the next stages.

Especially for comminution, that allows to significantly reduce the power needed for grinding, which otherwise would be a very energy consuming process.

Allowing to save energy and improve efficiency in subsequent steps is not the only merit of microwave heating: In fact, this is an extremely economical and environment friendly procedure on its own. This is mostly due to the fact that microwave absorption acts as an internal heat source (the material itself), which means no exhaust gases are produced, no heat is dispersed in the environment, yields are generally higher and the use of electrical energy is very efficient.

The microwave heating process takes place in machines that may operate at the same 2.45GHz frequency as the microwave ovens used at home to warm up a cup of milk. The main difference is that these apparatuses are much more powerful and significantly larger. As a minimal setup, they are composed of electromagnetic-shielded chambers where the material is subjected to microwave irradiation, and one or more wave sources called magnetrons.

Within CHROMIC, such (and much more sophisticated) equipments are operated by MEAM (Microwave Energy Application Management), a partner of the project and a long experience provider in the microwave technology industrial sector.

Hopefully, by now the link between slags and microwave heating should make much sense. In light of that, we can rephrase the opening sentence in a more appropriate way: A very smart way to pre-treat slags? Crack them through selective heating by means of microwave energy transfer, before handing them to the comminution stage of metal recovery.

(Image credits: MEAM official website)


“Vanadium steel, the strongest, toughest and most enduring steel ever manufactured, is used throughout the entire car”. What car? A very special one: the quote is from a 1908 advertisement for the Ford Model T, the first car ever to be produced in an assembly line. Also, the first of a series of many remarkable industrial uses of vanadium.

The extraordinary reinforcing properties of this element were already well known more than a century ago, and through time this and other characteristics have been exploited for many important applications. From spacecrafts to batteries, though little known, vanadium is ubiquitous. So let’s take a closer look at this precious element.


Vanadium (V) is the first element of Group 5 (the fifth column) of the Periodic Table, a transition metal with atomic number 23, an atomic weight of 50,942 and density of 6,1 grams per cubic centimeter. It is a steel-gray, shimmering, ductile and malleable metal, named after the Scandinavian goddess “Vanadis” because of its beautiful multicolored compounds.

Vanadium is rather common in the Earth’s crust, being the 20th most abundant among the many elements the latter is composed of; it is not found free in nature, but it occurs in many (ca. 65) minerals, like vanadinite, carnotite and patronite; it is also present in phosphate, bauxite and iron ores. The world’s largest mines of vanadium are from titaniferous magnetite (an iron ore) and are located in the Ural Mountains, in South Africa, and in China. However, vanadium is primarily obtained as a by-product during the production of steel. Its two industrially most valuable forms – pure vanadium powder and ferrovanadium – are produced through reduction of vanadium pentoxide (V2O5), which in turn is obtained from ores as a result of a variety of smelting, leaching and roasting processes.

By far, the largest share of the vanadium oxides production (about 80%) is used as ferrovanadium – which is an alloy of iron containing 35 to 80 percent of vanadium – or as a HSLA (High-Strength Low-Alloy) additive. Vanadium itself is soft, but when alloyed with other metals such as iron, it can  harden and strengthen them enormously: just adding 0,2% of vanadium to steel makes the latter up to 100% stronger and, also importantly, significantly lighter for many applications. Moreover, the addition of vanadium in alloys increases their resistance to corrosion. For these reasons, alloys of this element are extensively used for tool steel (e.g. axles and crankshafts), for tubes and pipes manufacturing, and in the automotive industry to make components such as hoods, door panels and piston rods. Furthermore, mixed with aluminum, vanadium is used to strengthen and promote thermal stability in titanium alloys, which are largely utilized in the aviation sector for the production of jet engines, airframes and spacecrafts. Vanadium alloys are also used in nuclear reactors because of the element’s low neutron absorption abilities and resistance to high temperature stress. When used in chemical compounds, vanadium also finds applications in dye manufacturing, in glass and ceramics production and as a catalyst in manufacturing sulfuric acid. Last but not (at all) least, vanadium has recently emerged as a useful mineral for electricity storage and battery production sectors, whose importance in a world slowly transitioning away from fossil fuels is self-evident.

The wide range of industrial applications highlight the relevance of vanadium within the EU economy. However, at present there is no extraction of this metal in the EU, which is fully reliant on the imports of its ores and concentrates.

It is the element that makes cool motorcycle engines and wheel rims shiny and bright, the one that  gives ruby its distinctive red color (and most American school buses their distinctive yellow one); it’s a metal with unique magnetic properties, it helps metabolize sugars and lipids and in small amounts it is essential for animal’s and plant’s  health (beware not to exceed the adequate intake, though!).

But most of all – economically speaking –  it’s the element that makes stainless steel “stainless”.

It’s chromium, not to be confused with its toxic hexavalent form – which has nothing to do with the metal used in the steel industry.

Chromium (Cr) is the first element of Group 6 (the sixth column) of the Periodic Table, a transition metal with atomic number 24, an atomic weight of 51,996 and density of 7,19 grams per cubic centimeter. It is a hard, lustrous, steel-gray metal, whose multicolored compounds – like emerald and the above mentioned ruby –  earned its name (from the Greek “chromos”, which means “color”).

Chromium is quite abundant in the Earth’s crust – ranking 22nd among the many elements the latter is composed of – and it also occurs in many minerals, but despite of that, its free metal form is almost never found in nature. Instead, chromium is mostly obtained by mining chromite, a black oxide mineral of chromium and iron (FeCr2O4). This ore and its concentrates are mostly produced in South Africa and Kazakhstan (about two fifths and one third of the total world’s production, respectively), with India, Russia and Turkey being substantial producers as well; in Europe only Finland has chromite mines.

In the second episode of the CHROMIC podcast you can find out more about the chromium value chain 

Chromite is usually dispersed in natural deposits with a high contamination of elements like oxygen, magnesium and aluminum, or minerals like silica. That lowers the content of chromium in the ore, which commonly varies between 42 and 56 percent. Once extracted, chromite needs to be processed in order to obtain the two most commercially valuable forms of chromium: its pure metal state, or an alloy called ferrochromium. The former is obtained by first converting – through a series of passages – the chromite’s chromium content to a specific oxide (Cr2O7), and then by thermally reducing this oxide with aluminum; the latter, which is an alloy of chromium with 30 to 50 percent of iron, is produced in electric furnaces through reduction with carbon.

The industrial applications of chromium are multiple. Among those, as far as usage volume is concerned, the ones that fall within metallurgy are by far the most important.

The leading role in this field is played by ferrochromium, which is mainly used in stainless steel production. This is due to the fact that stainless steel acquires its characteristic resistance to oxidation and atmospheric corrosion through the presence of 10-26% of  chromium, which is provided by the addition of ferrochromium to the steel. On the other hand, because of the high resistance to ordinary corrosive reagents, pure chromium is extensively used as a protective coating for other metals, like cobalt and nickel, by electroplating   (i.e. layer deposition by means of an electric current).

On the whole, metal alloys account for more than 85% of the usage of chromium. The remainder is split between the use of its chemical compounds and the applications of chromite. Regarding the former, different chromium compounds are used for coloring pigments, leather tanning, cosmetics, drilling muds, catalysts or wood preservatives. As for chromite, it is used for manufacturing bricks and various devices in the refractory industry.

The many applications of chromium – especially in stainless steel manufacturing – make it a vital resource for the European economy. Currently, the EU import reliance on this metal is 75%.

This project received funding from the European Union’s Horizon 2020 Research and Innovation program under Grant Agreement n° 730471

effiCient mineral processing and Hydrometallurgical RecOvery of by-product Metals from low-grade metal contaIning seCondary raw materials