Gas dispersion plays important role in flotation – Part two

By Rob Coleman*

To read the first part of this report click here

3 Superficial gas velocity (Jg)
Superficial gas velocity is the bubble’s upward velocity relative to the cell cross-sectional area.
Superficial gas velocity is proportional to the air addition rate and can indicate local flow patterns and gas short-circuiting. Excessive air addition increases bubble size, as the mechanism is unable to disperse the air, and is therefore detrimental to flotation performance. Controlling the air rate within an optimal range is very important.
The average rise velocity of bubbles in the flotation cell can be measured in combination with the bubble size measurements from the Bubble Sizer. A cylinder connected above the viewing chamber is filled with water before the bubble sizing takes place. During the bubble size measurement, the water in the chamber is displaced by the rising air bubbles and the water level drops. The time taken (t) for the water level to fall a known distance, L, is measured and the superficial gas velocity calculated from the following equation:

Jg = L / t

Adjustments are then made to account for the pressure difference between the location of the sampling valve and where the measurement is made in the cylinder. Typical superficial gas velocities in Outotec flotation cells are between 0.5 cm/s and 1.5 cm/s. As the air rises into the froth zone, the superficial gas velocity increases with decreasing surface area in the froth zone.
Superficial gas velocity measurements performed radially across a flotation cell can provide information on the gas dispersion efficiency. It is common for the superficial gas velocity to be slightly higher in the middle of the cell due to the air addition there. As the air rate increases the bubbles rise faster in the cell centre as the mechanism becomes less efficient at air dispersion, until the air cannot be dispersed and “boiling” occurs. Measurements of superficial gas velocity can also provide information on mechanism wear. If there is, for example, an uneven distribution across the cell the stator could be worn out on one side.
Another important application of superficial gas velocity is the measurement of the velocity profile down a bank of flotation cells, such as a rougher circuit, or a cleaner circuit. The velocity profiles down a bank can either be increasing (low at the front of the bank, high at the end), decreasing (high at the front, low at the end), balanced (equal across the bank) or unbalanced (increasing and decreasing from cell to cell down the bank). As the air addition to most flotation cells is by visual inspection of how the concentrate flows over the lip, it is common to find an unbalanced profile down most banks of “unmonitored” flotation cells.
Recent test work by various researchers at concentrators around the world underlines the significant benefits from varying the Jg profile across the bank. On the cleaner circuit at one concentrator, for example, the “as-measured” profile was unbalanced. After changing the profile to increasing, a recovery improvement of over 30 per cent was achieved, at the same concentrate grade.
These down-the-bank superficial gas velocity measurements were performed in conjunction with metallurgical surveys to determine the grade-recovery curve at the different conditions. From this the optimum profile for the best performance can be quickly determined. Off-the-shelf on-line superficial gas velocity probes are also now available that can be used to monitor and control the profiles automatically.

4. Bubble surface area flux
Bubble surface area flux is the amount of bubble surface area rising up a flotation cell per cross sectional area per unit time. It depends directly on the bubble size and superficial gas velocity and at shallow froth depths is linearly proportional to the first order flotation rate constant. So generally, the greater the bubble surface area flux, the higher the recovery rate in the pulp zone of a cell. However if excessive air is added the recovery rate in the pulp zone can decrease due to “boiling”.
A significant amount of test work has been performed on bubble surface area flux over the past fifteen years, particularly in the AMIRA P9 Project, where Outotec is a participant. The relationship between bubble surface area flux and the first order flotation rate constant has been successfully validated and holds for cells of all sizes, from 60 litres to 300 m3. It is essentially a direct measure of pulp zone flotation efficiency.
The bubble surface area flux can be measured directly using the following equation:

Sb = 6.Jg
       d32

Where:
      Sb = Bubble surface area flux (cm2/cm2s)
      d32 = Sauter mean bubble diameter (cm)
      Jg = Superficial gas velocity (cm/s)

Typically in Outotec flotation cells the bubble surface area flux ranges between 30 s -1 and 60 s -1and can be varied directly by changing the air addition rate. This is another distinct advantage of forced air flotation cells.

The gas phase of any flotation cell is critical for optimum cell performance. Understanding and being able to vary the four key parameters in the gas phase can bring real results – with more than 30 per cent recovery improvement at the same grade, in one particular case. The key is using the right flotation partner to provide the tools and expertise to benchmark the gas dispersion in your current floatation cells.  With this valuable information it becomes far easier to get the most out of your operation.

* Dr Rob Coleman is currently Technology Leader – Flotation for Outotec in Australia. He has a Chemical Engineering degree from the University of Witwatersrand in Johannesburg and a Doctorate in Minerals Processing from the JK Minerals Research Centre - University of Queensland. He has more than 14 years experience in the operation, design, modelling, simulation and optimisation of flotation circuits and has published and presented papers at many international conferences.

For more information contact tel: +61 (0)2 9984 2500 or email: laura.white@outotec.com

To read the first part of this report click here