The stator in a flotation machine is there to shear the air bubbles and increase the surface area for improved flotation characteristics. The relationship between rotor speed and air flow are related to the shear capabilities of the stator. Some machines use fine bubble injectors to achieve the same characteristics.

From your answer I understand that if we figure out how to deliver fine bubbles into this forced air mechanical cell we could potentially eliminate the need for stators, therefore saving on operating costs?

Many years ago I was involved in the development of a stator that could be used 4 fold (left/right and top/tail). This obviously meant the blades could be reused. Since then routine inspection and recovering have been found to extend life still further.

The cost of a stator and its frequency of replacement are not high when compared to the other operating costs.

As for the generation of fine bubbles take a look at the Jameson cell, the G cell and of course column flotation cells.

I like that idea of maximizing the wear life on the stator by moving it around!

Regarding the Jameson cell, G cell and column cells, I would like to keep them aside for the moment as I would like to understand the fundamental reason for using stators as baffles and not axial wall baffles in forced air mechanical cells. I agree that the operation costs of these spares are relatively low in the operation of a beneficiation plant, however in some cases is not insignificant. Based on my experience a mid-size concentrator can spend US$0.5M to US$1M in a year just in stators not counting shipping, labour, downtime and other costs associated.

I don't have a copy of the original paper but this is taken from Wills' Mineral Processing Technology: "The machine stator does not change the bubble size, but only the flow pattern in the cell (Harris, 1976).

As added information to the discussion, there is a forced air mechanical cell design that I know that does not use stators and that is the Maxwell cell. I only know one concentrator that has experience with these Maxwell cells. Based on this site's experience, solids dispersion is very good but gas dispersion not so good. Are there any other forced air mechanical cell mechanisms with no stators? How do they perform metallurgically?

The concentrator I currently work at has issues with the stators mounted to the bottom of the cells breaking bolts and moving from their intended location directly underneath the air injection point. There is a clear difference in operation where the bubbles are not dispersed evenly throughout the whole cell. It is a bit hard to compare actual bubble size at the injection point though as they coalesce into larger bubbles due to the reduced mixing and shearing. There is obviously a negative effect on recovery.

I guess it isn’t the same as working with a float mechanism designed to be stator-less, just an insight into an uncontrolled and unintended nuisance "on/off" trial.

We are considering retrofitting mechanisms with the stator connected to the agitator shafts though as we don’t have problems with these in other float cells we use.

That is a very interesting comment.

If we think for a moment that there is no better alternative to stators, what can we do to increase their service life? As mentioned above that changing the position of the stator at regular intervals may extend the life of the stator which makes sense to me but is not something common. Perhaps there is a mechanical constraint on this?

What aboutusing more wear resistant material like silicon carbide instead of rubber or PU?

I think you need to look at Red Dog in particular rather than flotation plants that may run for 10-15 years then close. With a zinc operation like Red Dog that has now operated 25+/- years and now extended for 15 more, it may be an idea to take photos of the wear on stators and send them to the supplier, Stators are often ignored and may not be on the maintenance program but it would surprise me if PU is showing major wear- often coating is a more severe problem.

But yes as an engineer I think it a good idea that you are looking into it.

What you need to follow up on are the fluid dynamics of the cells, (I don't recall which design RD has) a couple of good papers http://is.gd/jR8yKx

http://is.gd/95FDty

Also you may want to look at gas flow distribution in the cell:

There are several good technical papers waiting to be written on this topic.

Extending lifetime of both rotor and stator has these components to consider

•Selecting correct lining material for the wear conditions - ore properties, particle size distribution (here PU, rubber and some others can be used, usually Rougher-Scavengers have much higher wear than Cleaners)

•In case a fragile lining material is selected for lining, avoid all possible bumps and cuts during changes of rotor and stator (refers pretty much to all lining materials)

•Selecting a well done rotor and stator from suppliers - lining can be applied on steel surfaces in many different ways, which can create a lifetime difference for the mechanisms between 1 month and 15 years (of course, taking points 1-2 into account) --> for Zn application with Outotec PU lined rotors I would expect lifetime of about 2-3 years, for stators about 4-5 years. More precise information on ore properties is still required

•Change of rotation direction of rotor can be practised every 1-12 months. Depends again on application mineralogy and cell duty, also gear type needs to allow it --> such practise generates more even wear pattern on both sides of rotor and stator blades

•Operate at adequate pulp density and particle size distribution (this is mainly to prevent sanding around the mixing mechanism - mixing mechanisms of different makes/ brands have usually different limits for flotation pulp densities/ %solids) Flotation result, recovery or throughput still will always overrule this item on the list. In case not suitable - return to point 1 in the list and rather than changing flotation, change mixing mechanism itself.

•Operate at adequate rotor speed: the rotor tip speed defines the shear force created on the stator in order to generate smaller bubbles. Depending on flotation feed PSD also bubble size requirements vary - smaller particles require smaller bubbles in order to be recovered and vice versa. Reducing rotor speed can yield thus both positive and negative results for the flotation recovery, but it will most certainly increase the stator lifetime/ decrease stator wear.

•Maintenance practices: preventive maintenance can increase lifetime of rotor/ stator by 10-30% depending on how extensive it is. Usual things to look for are the sanding in the cells, even positioning of the rotor within the stator, state of individual rotor/ stator blades and lining thickness. Some also look into adjusting rotation speed of rotor and performance of the upstream processes within the preventive maintenance program. Best, though, about preventive maintenance is that it reduces corrective maintenance events and thus improves the overall equipment availability.

That is really good information.

Regarding sanding in mechanical cells, what would you recommend as a diagnostic method to assess if the cell is sanding? What do you think about measuring axial conductivity profiles as the authors of the paper below did to compare conventional rotor against Float Force.

Is there a better electronic/automated method to stay away from taking manual deep samples and measuring % solids in a Marcy scale?

Doucet, Price, Barrette and Lawson, "Evaluating the effect of operational changes at vale Inco’s Clarabelle mill", Advances in Mineral Processing Science and Technology, Proceedings of the 48th Annual Conference of Metallurgists of CIM, Sudbury, Ontario, Ed. Gomez, Nesset and Rao.

Thank you for the links you shared, I found them very interesting. Could you please confirm if I understand the concept? I understood that axial impellers produce axial pumping action so the flow of slurry goes from top to bottom? Does this imply that mechanical cells with axial impellers have lower chances to sand up because there is a constant flow hitting the bottom of the tank?

Concerning sanding and wear: Coarse high SG particles are the first ones to settle in corners and behind stationary flow impediments. This invites a survey of grinding and mineralogy. Magnetite, garnet and other high Mohs minerals resist grinding over sulfides, feldspars etc. These minerals are certainly more abrasive than the ordinary gangue minerals. Random bursts of these high SG particles may be a significant part of your problem.

Regarding the question on the title of this discussion, is the answer yes? I mean, do the majority of us mineral processors believe that we do indeed need stators in forced air mechanical flotation machines?

From applied operational plant research in Peru, I would like point out the following sort out:

In order to get an optimal grade recovery G/R progressive relationship

•Perform Rougher and scavenger flotation stage by cells using stators with enough shear that prevents sanding and producing coarse bubbles mainly.

•First cleaning flotation stage after regrinding using sub-aerating cells with stators that produce different shear medium size bubbles.

•And final cleaner flotation stage with pneumatically cells without stators to float fine and colloidal particles as 10 µm particles.

Obviously is a mistake to design tank cells device for first cleaning flotation stage because you get coarse bubbles where the G/R don’t produce the best progressive relationship.

•The mechanism in the flotation unit is a low efficiency centrifugal pump. Therefore without it you will experience solid suspension problems at the same RPM.

•If you study the work of Schubert (1996 ) and Kock (2007) on net attachment rates, you will see that the highest rate occurs where you have the highest turbulence. Even in the boundary layer where the flow "tripped" into turbulence, the net attachment rate is higher. The highest attachment rate occurs between rotor tip and stator. The stator acts an additional "turbulence tripper”.

My gut feeling is that you will run into RPM and rotor diameter difficulties utilising radial baffles to achieve the same solid suspension and turbulence.

If memory serves the Maxwell cell impeller is simply an open centrifugal pump impeller, far from ideal for mixing or air dispersion, probably the worst example of a flotation cell apart from sticking an air hose in a tank.

The important part to remember is that we expect the rotor to both mix the slurry AND disperse the amount of air we want to add, we also want it to make a narrow distribution of fine +- 1mm bubbles or better. That’s a very big ask, hence the need for the stator to assist with bubble breakage. Cell designers look at achieving the best of both worlds while trying to reduce wear and maintain optimal operation for as long as possible.

Look at the Ultimate Flotation cell which is an upgrade of the Wemco star rotor but tries to separate air dispersion from slurry pumping and does a pretty good job.

I can see what direction you are trying to take and it’s a good question you raise and maybe this is the direction that float cell should take, why not an efficient mixer with baffles (no stator) and a bubble generation system that can give you the bubble size distribution ideal for your application?

Thanks everyone for your feedback, I am learning a lot from this!

Regarding the stator being used to assist in bubble breakage, I find that there are two distinct groups, one that says the stator does break air into fine bubbles by the shearing action of fluid and another group that says bubbles are actually produced from the edges of air cavities produced by the rotor.

I recently got an article that describes gas dispersion measurements in my own plant (I feel bad because it tells that I did not do my homework). In this paper there is data showing gas dispersion in OK50, OK38 mechanism retrofitted in a Maxwell MX-14 cell and Maxwell MX-14 with original impeller and gas sparger. There is one section in this paper that says,

"The Sb values were found to be around 48 1/s for all cells at the Jg value of 1 cm/s. This suggests that all cells are equally efficient in generating bubble size and therefore Sb at low rates, when the air is dispersed uniformly by the impellers".

What is your opinion on this statement rather strong?

B.K. Gorain, "Selection ofcell operating conditions to optimize performance of flotation circuits with large cells", Seventh Mill Operators Conference, Kargoorlie, WA, 12-14 October, 2000!

You might also find the following papers of interest:

Ata S, Jameson GJ, 'The formation of bubble clusters in flotation cells', International Journal of Mineral Processing, 76 123-139 (2005) [C1]

Evans GM, Manning SA, Jameson GJ, 'Cavity formation, growth, and dispersion behind rotating impeller blades', Industrial & Engineering Chemistry Research, 44 6304-6309 (2005) [C1]

Cannot reply from a chemical engineering sense but only from nearly 20 years experience working with both! In most chemical mixing tanks the axial baffles do not stop the swirling effect. It is still there to some extent and in float cell circuits this is undesirable. In fact turbulence of any description is undesirable and the stator takes the majority of that out. Cells where the stators have failed (I've seen a few) look like washing machines and give substandard performance.

Baffles prevent swirling and aid turbulence. As others have said, there are 2 aspects of the stator-rotor assembly:

•"Pumping" of the slurry to prevent settling and

•Generate bubbles

I haven't seen the concept in the chemical industry, only in flotation. But when you think about it, how else would you create fine air dispersion in slurry? One way is to use Rushton turbine and bring the air underneath the impeller eye. You can create fine bubbles, but you need to spin the turbine very fast- it would wear out quickly.

With stator-rotor mechanism, I think, you can run a lot slower, and still create bubbles fine enough to float the particles.

Only on fine gap between edge of stator against edge of rotor in movement is produced a cavitations regime of pulp flow where the insufflated or suctioned air addition causes a gradient of froth stable bubbles sizes from quite fine to coarse. Here he obvious opportunity for collector contact over the ore to be floated and recovered in the best flow turbulent regime conditions.

Thank you all for the replies. As a good engineer I am a tiny little bit stubborn.

I have to mention that I have not collected gas dispersion data myself (yet, the team I belong to is preparing to do it in the near future) but I do support the statement written in Gorain's paper (please see reference above) “all cells are equally efficient in generating bubble size and therefore Sb at low (air) rates”. I can say that visually a machine with and without stators look just the same from the top (note: flotation machine with axial baffles, cannot say of one without baffles). I have seen this on a Maxwell cell which was originally designed to work without stators and also Outotec mechanism retrofitted on Maxwell tank whose stator has worn out completely (after a normal wear expected life).

Does anyone know of a technical article (other than the one from above) showing a rigorous test work program to quantify differences between gas dispersion in a forced air mechanical flotation cell with and without stators?

My interest here is to determine with a strong technical ground if we really need this piece of equipment that wearsfrom time to time, therefore adding operating costs to always stringent budget

A measure of effect of cavitations in rotor- stator gap of volumetric pulp flow over air dispersion to produce fine bubbles is the resultant gradient of temperature achieve from the mechanical cell design.

I just wanted to update this discussion with observations made from actual gas dispersion measurements (bubble size, gas holdup and superficial gas velocity). Measurements have been performed on one particular cell that has no stator and that belongs to a bank with cells with stators. Preliminary results do not show a noticeable difference. I know it is too soon to confirm this but wouldn't it be easier/cheaper (maintenance wise) if forced-air mechanical cells had baffles instead of stators?