Getting to know microwave heating

11 June 2018 Getting to know

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)

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