I think by ‘settlement’ you are referring to sanding of the flotation cell (where particles come out of suspension and settle in tank). To answer your question I think it’s best to consider the anatomy of forced air vs. induced air (e.g. WEMCO) flotation cells. The induced air flotation cell has the rotor position at the top of the cell as it needs to be close to the surface to pull in air; hence the majority of the mixing energy is dissipated in the top of the cell. True these cells are designed for pumping and circulating pulp throughout but as the high energy zone is generally adjacent to the rotor you would have to put in even more energy to ensure that particles entering the cell at the bottom remain suspended (especially when coarse). Conversely the forced air designs of cells have rotors at the bottom so the high energy zone of the mixing is where the particles are most likely to settle. If you think about this which arrangement is less likely to have coarse and high SG particles sanding?

In addition for a forced air cell if settling occurs you can stop the air, thus maximize the energy used for mixing/suspension, and get the material moving again (prior to turning the air back on). The mixing and air dispersion actions on an induced air cell cannot be decoupled so this is more difficult. Also it is quiet difficult for a rotor located at the top to get sanded material in the bottom of the cell moving.

There are also additional important considerations when dealing with coarse and heavy minerals; motor size and valve arrangement. Typical float cells are designed with a particle SG or around 2.8-3.0 SG if you have a majority magnetite or hematite feed then you are looking at particle SGs approaching 5.0 for the overall feed. This needs to be taken into account and an appropriate motor/mechanism selected. The valve arrangement is also important. For this type of application internal downflow dart valves are likely the best as this minimizes the chances of particles sanding within the ducting and transfer boxes. I suggest you find a manufacturer who has references in dealing with this type of material as you really can’t afford to get this wrong and have the cells sanding up.

I mean sanding area. The problem during operation is that self aeration system could not suspend all particles in a cell, especially when particles are large (e.g. > 150 micron) with high specific gravity. So the materials choke in the cell and you have to stop the operation. What is the advantage of the rotor new position in WEMCO cells in comparison with others while we faced such problem in working with this cell?

In forced air cells we can control the turbulence zone by changing the air flow but in induced air cells we have to stable the froth zone manipulating the rotor speed or other mechanical parameters. Therefore, we reduce the required energy for particles suspension and sanding will emerge as a result.

Control of the valves in this operation is another troublesome issue since dart valves move to close the path is difficult when pulp solidity increases and in most cases this valves is broken.

Good helpful comments above.

May I ask what type of WEMCO cells you are referring to Tank Cells, or Trough Cells? From my experience with these cells the Tank Cells are more prone to sanding than the trough cells. As you say the high SG Iron minerals will tend to settle especially in a dilute pulp density, have you tried floating at 50-55wt% solids to see if the settling stops?

What is your particle size?

I propose to do some settling tests in laboratory with difference solid content, and then use lowest settling rate for flotation, but please note that in high concentrate pulps we will have some problems such as:

•Big amounts of conc. material will reduce the flotation cell surface area,

•Flotation will not be good at high concentrate due to particle interactions.