Articles express the opinions of the author. This article was first published in Teknisk Ukeblad (TU) and is reproduced here with the kind permission of TU.
You may not be aware, but the laws of physics that make some garments waterproof, while other garments act much like blotting paper, also apply in the furnaces of the metal industry.
By performing experiments in an experimental furnace, here at SINTEF, we have gained new insight into how this field of physics affects the processes that occur in furnaces and, more specifically, the processes that transform the raw material (ore) into the metal alloy ferromanganese.
This look into the hitherto secret lives of molten materials could soon be helping ferromanganese producers avoid substantial revenue losses and cut CO2 emissions from their furnaces.
Invisible, but indispensable
As a product, ferromanganese is invisible in our everyday lives, yet it is just as indispensable.
The material originates from ore veins which contain manganese and iron. Once extracted, it is used as an alloying element in steel. More specifically, it is used to increase the strength and toughness of the steel and make the steel more suitable for heat treatment.
Ferromanganese is produced in electric furnaces both in Norway and elsewhere. When the furnaces are tapped, something unfortunate happens: As much as six percent of the metal is trapped in slag, a by-product.
This costs producers a lot of money.
The mysterious life of the droplets
In our search for a solution to this problem, we have studied the field of physics that concerns the phenomenon of wetting. In other words, the understanding of how a droplet behaves when it comes into contact with a surface.
Will the droplet spread out over the surface and thereby adhere to it? Or will it retain its original shape and run off, like water droplets do when they come into contact with “waterproof” clothing?
You may perhaps have seen a water droplet landing on very clean glass. If you have, you will know that the droplet will “run” across the surface of the glass. This indicates that the conditions for wetting are good. When the conditions are like this, strong forces will cause the droplet to adhere to the surface it has come into contact with, regardless of whether the surface is solid or belongs to a liquid substance.
However, if the same water droplet hits a plastic carrier bag, the droplet will remain virtually unchanged. In this case, the conditions for wetting will be poor and the adhesion between the droplet and surface weak.
When liquid substances come into contact with each other
This field of physics also applies to furnaces. In our study, we looked at the reciprocal wetting that takes place when two liquid substances come into contact with each other: molten ferromanganese and molten slag.
Through our studies, which are funded by the Research Council and SINTEF, we found answers to our questions. We observed what it takes for molten ferromanganese to run off from molten slag inside the furnace, rather than be drawn into the slag to the same extent as is the case at present.
What we found was that the conditions for wetting – and therefore also for adhesion between the two substances – deteriorated when we reduced the sulphur content of the metal and the slag and ensured that the temperature in the furnace was not between 1,277 and 1,427°C.
This led us to come up with a specific recipe for increasing the metal yield from the furnaces.
Good for revenues – and the climate
In 2023, one tonne of ferromanganese of a given quality cost EUR 1,200. In order words, over NOK 14,000 at today’s euro exchange rate. Eramet Norway, which is part of the Eramet Group, produced 300,000 tonnes of the metal last year in Norway.
This means that if we can save the smelters from metal losses of almost six per cent, which we believe we can, revenues in this industry will increase significantly.
As less raw material will then be needed for every tonne of finished metal, CO2 emissions from the furnaces will also drop noticeably.
Groundbreaking discovery at Berkeley
The mathematics describing the wetting of solid surfaces have been around for a long time. However, it became clear at an early stage that this set of equations did not encapsulate what happens when hot liquid substances adhere to surfaces that are being heated.
New light was shed on such wetting processes when researchers at Lawrence Berkeley National Laboratory in California made a groundbreaking discovery in the early 2000s. In an experimental furnace fitted with a high-speed camera, they saw droplets of molten metal create tiny ridges in solid surfaces that the droplets came into contact with.
It became apparent that these ridges could slow down, or even stop, spreading of the liquid substance on the solid surface.
An insight into many phenomena
In our own studies, we have used a similar furnace here at SINTEF. This has given us many opportunities to visually study a wide range of metallurgical phenomena at high temperatures – not just wetting processes between liquids, but also the oxidation of metals and reduction of oxides, i.e. the absorption of electrons. Positive phase changes, thermal expansion and softening/melting.
All of this gives us an opportunity to harvest knowledge that can help make the Norwegian metal industry both more competitive and more climate and environmentally friendly.
