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.
Most importantly, leaching is applied in the metals processing industry to remove the desired metals from their ores, which are usually formed by many unwanted components. Gold, for instance, is leached from its ore using an aqueous cyanide solution. Chromium (Cr), vanadium (V), molybdenum (Mo) and niobium (Nb) – the valuable metals for the CHROMIC project – need to be extracted by leaching too.
To see how, we should begin by saying that in a typical leaching operation there is a solid mixture (called “matrix”) from which the metal of interest (the “solute”) has to be dissolved into some liquid (the “solvent”). Now, to understand how the process works, let’s go back to our cup of tea. In this simple but sound example the teabag is the solid matrix, the tea is the solute and the hot water the solvent. Everybody knows what happens when the teabag is plunged into the hot water: The latter penetrates into the teabag, the color and flavor come out of the tea leaves and dilute into the solution.
The basic procedure for industrial extraction of metals is the same. It goes without saying that in practice the process is far more complex. One major difference is that the solid matrix (pre-treated ores in this case) may contain a very low percentage of the desired metal, dispersed in a mixture of many other elements, none of which should be dissolved in the final solution, that is alongside the target metal. Also, different target metals have different chemical properties. That means there is no “universal solvent” and finding the most suitable one for a given purpose is a challenge. Solvents can be acidic or basic, and may vary in terms of pH, redox potential, selectivity of dissolution and many other sensible characteristics, all of which affecting the final output of the process.
The aim is then to identify the best liquid solvent (the one which ideally reacts the most with the target metal but preserves the matrix), find the right amount of it and the most appropriate physical conditions for the process to happen, to finally get to the optimal rate extraction efficiency with minimal resource consumption (i.e. energy, materials and time).
There are various leaching methods available, each one has its own strengths and drawbacks, depending on the situation. The most traditional ones being in-situ leaching, vat leaching and heap leaching. In the first one, leaching holes are drilled directly into the deposit (in-situ means in-place) and the solvent is pumped through them, reaching the ore. In vat leaching the contact between matrix and solvent is facilitated by putting the ore in large vats or tanks which can be stirred. Finally, heap leaching is successfully used to extract metals like gold, silver, copper, nickel and uranium and consists in piling up crushed, agglomerated ores on pads and irrigate the heap from the top with the solvent.
In CHROMIC vat and heap leaching will be used, together with new, innovative techniques that are being explored and developed within the project. Such as, for example, microwave and radiowave assisted leaching of stainless steel and ferrocromium steel slags, which are under investigation for the first time. These are two instances of dielectric heating, which through selective heating of the slags allow for an intensification of the leaching reaction, increasing its efficiency and reducing the process time. Ultrasonic waves are also being employed as a novel method to remove inert top layers from the slags, enhancing the dissolution rates. Furthermore, ozonation leaching is being extensively tested, primarily because of the ozone’s potential to lower the needed reaction temperatures.
These and other innovative procedures will contribute to make the industrial process of leaching more efficient by increasing yields and reducing costs and energy consumption. They are currently being investigated and improved by (in alphabetical order): BFI, FehS, MEAM, ORBIX, TUKE and VITO.
Of course, at the end of all these procedures Cr, V, Mo and Nb are dissolved in a solution with solvents and a small, unavoidable, percentage of unwanted elements from the matrices: They are yet to be retrieved. This will happen in the last step of the metal recovery process.