Removal of inclusions in liquid metals by flotation
Motivation and background
In metallurgy, the control of inclusion populations plays a key role to improve and ensure cleanliness of structural materials, e.g. aluminium alloys or steels. From the material-scientific point of view, non-metallic inclusions having a means diameter of around 10 µm act as defects in the metallic microstructure and therefore impair the material’s properties as well as the final product quality. Commonly used industrial processes, like ladle refining in steel production, are applied to reduce the inclusion size and concentration within the bulk material. Heated up above the melting temperature, the liquid metal alloy is stirred by gas bubbles which are generated due to chemical reactions inside the metal bath or are injected into the ladle additionally. As a consequence, the probability of collisions between solid inclusions is increased and bigger inclusion aggregates are formed. Inclusions attached to bubbles are floated up towards the free surface of the liquid metal bath, get absorbed by the slag and can finally be removed.
Within the FLOTINC project, founded by the French (ANR) and German (DFG) research agencies, the research topic is focussed on inclusions in gas-liquid experiments. Liquid metal is employed as the experimental fluid, because of significantly higher surface tension compared to water. “Cold” gallium-based metal alloys which are already liquid at room temperature (~ 25 °C) are used to perform model experiments without additional heating. In our labs at HZDR we are able to apply different X-ray imaging setups. Moreover, neutron imaging studies have been started at the Swiss Spallation Neutron Source (SINQ) of the Paul Scherrer Institute (PSI), Switzerland. The experimental investigations aim to image gas bubbles as well as solid particles, and to study their dynamics within the opaque liquid metal.
Experimental investigations by X-ray and neutron transmission imaging
The measurement principle for both X-ray and neutron imaging is based on the material-dependent attenuation of the transmitted X-ray respectively neutron beam intensity. The output of all imaging experiments are grey-scale image sequences showing the two-dimensional (and hence depth-limited) projection of rising bubbles and moving particles within the liquid metal volume. The lateral and temporal resolution for transmission imaging by means of X-ray or neutron radiation strongly depends on the imaging setup as well as the design of the flow experiment.
Measurement principle based on Beer-Lambert law
Experimental setup for X-ray or neutron transmission imaging
X-ray imaging can provide quantitative analysis of two-dimensional bubble flows in liquid metal, including data about bubble trajectories, diameter and velocity. The material selection for suitable particles is limited, since the attenuation characteristics for X-ray imaging are closely related to the particles’ mass density which strongly affects their flow behaviour, too.
X-ray imaging of bubble chain in liquid gallium alloy; quantitative analysis by fitting of bubble projections as elliptical shape (Video).
X-ray imaging of spherical hard metal particles in liquid gallium alloy during gas injection.
Neutron imaging enables visualizing and tracking of submillimetre-sized particles made of gadolinium. This element exhibits outstanding attenuation characteristics for neutron radiation, plus the mass density is similar to the density of the liquid gallium alloy. Hence, compared to X-ray imaging, the required particles tend to be smaller. This allows adjusting the flow conditions depending on the particle size and shape.
Neutron imaging of different submillimetre-sized gadolinium particle fractions, in contrast to liquid gallium alloy.
Zhang, L. F., Thomas, B. G. (2003). State of the art in evaluation and control of steel cleanliness. ISIJ International, 43(3), 271-291. doi:DOI 10.2355/isijinternational.43.27