Sensors for flotation

Electical conductivity

Water has a certain electrical conductivity. Introducing bubbles reduces the conductivity and increases the electrical resistance of the bulk fluid. This dependence can be exploited to determine the gas volume fraction by measuring the electrical resistance or capacity between electrode pairs. Depending on the electrode position and geometry, different regions of the flow can be investigated.

Global	Local	Wire-Mesh
Embedding ring-electrodes in pipe walls averages the gas volume fraction over the whole pipe volume between the electrodes.	An open cylinder with ringelectrodes can be placed at different positions in a flotation cell and measure the gas fraction locally. A second syphon cylinder acts as a reference without bubbles. [1]	Developed by the Department of Experimental Thermal Fluid Dynamics, a grid of wires can measure the gas fraction at the crossing points of the wires and map a 2D region of a flow.

Local air flux sensor

The air flux, also known as superficial gas velocity, is an important performance paramter in flotation. By placing a vertical tube in a flotation tank, bubbles are captured and the gas volume flow rate is measured. This gives a local value of the air flux at the tube entrance and allows for spacially resolved characterisation of a flotation cell. [1]

Thin film flapping velocimetry

Fast and turbulent flows are important for flotation, as their sporadic fluctuations and multiscale vortices facilitate particle-bubble collisions and attachment. Thin film flapping velocimetry analyses these the velocity fluctuations by placing small paddles into the fluid, that get deformed by the forces of the flow. These deformations are measured in different ways.

Piezoelectric vibrational sensor (PVS): A piezoelectric crystal generates a voltage when it is deformed. This voltage can be easily measured and gives the change of the fluid motion in time. [2]
Fibre-Bragg grating (FBG): A periodically changing refractive index (grating) in a glass fibre reflects certain wave lengths of light according to Bragg’s law. Deforming the fibre changes the reflected wave length which gives a very accurate measure of the thin film’s absolute deflection.

High speed imaging

At low gas/solid fractions, cameras can still visualise the inner workings of the flow. Shadowgraphy can records sharp images of bubbles whose size and position can then be analyzed with image processing and neural networks. For the latter, the Department of Computational Fluid Dynamcs has implemented reliable algorithms for bubble detection and tracking. [3]

PIV/PTV

On of the most ubiquitous measurement method in fluid dynamics is the particle image or particle tracking velocimetry (PIV/PTV). Using multiple high-speed cameras and high-intensity lasers, we can resolve the three-dimensional motion of liquid, bubbles and particles simultaneously over time, named 4D particle tracking velocimetry. [3]

Bubble size measurement

To still be able to see inside opaque fluids, the cameras have to be placed inside the flow. For this, we employ different endoscopic imaging devices. Process microscopes can be placed into the fluid alongside a light source to record images of the particle and bubble motion. An evolution of this concept is the VI Probe Pa by SOPAT that combines the lighting and imaging technologies into one device. This, for instance, makes it possible to measure the bubble sizes a different locations within a system even at 30% solid fraction. [1]

X-ray / neutron radiography

While visible light is blocked by bubbles and particles, X-ray and neutron radiation has the ability to penetrate through certain materials and reveal, for instance, the motion of bubbles through fluidized beds of particles.

Publications

[1] H. Pervez, A. Hassan, A.-E. Sommer, T. Zürner, L. Pereira, M. Rudolph, S. Maaß, J. Bowden, and K. Eckert, A multi-sensor approach to measuring hydrodynamic parameters in a pyrite-quartz flotation system, Miner. Eng. 216, 108877 (2024), DOI: 10.1016/j.mineng.2024.108877.

[2] H. Pervez, A.-E. Sommer, T. Zürner, J. Bowden, and K. Eckert, An enhanced method to measure multiphase turbulence in multiphase systems using the piezoelectric effect, In preparation.

[3] A.-E. Sommer, S. Heitkam, K. Eckert, Wake effect on bubble–particle collision: An experimental study using 4D particle tracking velocimetry, Int. J. Multiphase Flow 179, 104903 (2024), DOI: 10.1016/j.ijmultiphaseflow.2024.104903.