It is well known that surface roughness of mineral particles has a significant influence on their flotation behaviors. In this investigation, magnetite was ground in rod and ball mills to generate particles with similar dimension and shape but varying degree of surface roughness, which was quantified using Atomic Force Microscopy (AFM). The influence of surface roughness on the floatability of magnetite particles was performed by flotation tests using an XFG flotation machine. Flotation tests indicated magnetite particles possessed higher surface roughness had higher flotation recovery and larger flotation rate constant. The aggregation behaviors of different rough magnetite particles were compared for the first time via an optical microscopy. Results of the optical microscopic tests revealed that there were a large number of aggregations in the system of particles with higher surface roughness. A proposed model was deduced with the parameters carefully calculated (not arbitrarily selected) to analyze the bubble-particle interaction energy using an Extended DVLO (DerjaguinLandauVerweyOverbeek) theory. The theoretical interaction energy points to lowering energy barrier when magnetite particles are covered with 28.03nm asperities as compared to 9.47nm asperities. The effect of surface roughness on mineral flotation was investigated from the view of both particle aggregations and energy barrier of bubble-particle attachment for the first time, which are the primary causes for differences in flotation performance.
Magnetite (Fe3O4) and Ferrosilicon (FeSi) are the most widely used media. Magnetite is used up to a specific gravity of 2.2; a mixture of FeSi and Fe3O4 is used for the range 2.2 to 2.9; above 2.9 FeSi is used alone. Using FeSi, separations up to specific gravity of 3.4 have been made commercially.
There are two types of FeSi available on the market. The ground FeSi, manufactured in the United States, and the atomized FeSi, manufactured in West Germany. The particle geometry tends towards a sperical shape with atomized FeSi and toward a more angular shape with ground FeSi.
The particle size of the medium is important and varies with the type of equipment used. Quasi-Static vessels like Drum, O.C.C. Vessels, etc., usually tolerate a coarser medium, (-48 or -65 mesh) while static cone separators require medium as fine as -100 mesh. Dynamic separators like DSM cyclones and DWP separators require fine size (-200 to -325 mesh). In general, sharp separation is obtained with finer medium. Figure 8 illustrates the advantage of using fine medium.
Stability of the suspension is very important, otherwise the medium will settle. To provide improved stability, clay or slime are sometimes deliberately added to the suspension. Where slimes are deterimental, water soluble hetro-polysacharides are used.
In spite of higher cost, European operators prefer atomized FeSi beacuse it provides a lower viscosity of suspension which is required for sharp separation as well as high resistance to corrosion and degradation. In addition, the low adhesiveness of the medium to the surface of the minerals makes it easier to wash the media from the sink and float products. The end result is higher yield and lower media consumption.
In practice, using atomized FeSi does not necessarily mean higher media cost. It might be possible to mix a larger proportion of Fe3O4 with atomized FeSi and obtain a suspension equal or better in physical properties to a FeSi suspension. European operators use a mix of as much as 60% Fe3O4 and 40% atomized FeSi and the media loss is very low. The decision as to which media is most suitable for a given operation, among other things, should be based on economics.
The media is recovered from the sink and float fractions by the drain and wash screens. DSM screens, sometimes referred to in the coal industry as sieve bends, are preferred as drain screens as these are very efficient in removing the fine media. Where blinding is a problem, polyurethane screens are used.
The media recovered from the drain screens is returned to the media sump for recirculation in the media circuit. The medium removed by washing is returned to the dilute media sump as it is too dilute and, in many instances, is contamined with fine impurities. Therefore, prior to return to the media sump for reuse, this should be cleaned and thickened. Wet magnetic separators, with permanent magnets are normally used for cleaning and recovering media.
Single drum magnetic separators are common, however, when the medium is dilute and the feed volume is large, multiple drum separators should be used. For optimum results, the magnetic separators are fed at 30% to 35% solids by weight.
The efficiency of the magnetic separator will depend on the magnetic susceptibility of the media and the rate at which the unit is fed. Satisfactory results are obtained when the separators are fed at the proper feed rate.
Magnetic separators are available in 762 mm, 914 mm, and 1219 mm diameter with magnet widths to 3 m. The installed cost (in 1980 dollars) of a single permanent drum separator per foot of magnet width is 56,580 for 762 mm, $8,000 for a 914 mm and $10,500 for 1219 mm. The cost of a multiple drum separator is usually proportional to the number of drums.
The importance of designing an adequate media recovery circuit cannot be overemphasized. Attention should be given to details so that media loss can be kept at a minimum. Keeping the increasing media cost in mind, designers have come up with more and more sophisticated recovery circuits. Some of the recovery circuits in use today are shown in Figures 9A, B, C and D.
Media losses vary considerably. In general, higher losses are associated with the finer sizes of material processed and the fineness of the media itself. Losses on the static type separators are usually low. On cyclones or DWP plants, the losses are generally higher. Table 3 shows media losses experienced in heavy media operations.
Dilution of the medium by water coming from gland seal water presents a problem. Compression type packing used in centrifugal pumps wears constantly and there is no way of insuring a tight seal. To compensate for the dilution, in plant practice, more water is removed from the medium ahead of the media circulating pump. Using pumps with mechanical seals eliminates this problem.
Mixing tanks need some sort of mechanism to avoid mixing ineffective-swirling motion. In my mind a flotation machine is a mixing tank that introduces bubbles into the mixture to achieve separation. Most of the textbooks I used when I was studying chemical engineering mention the use of axial wall baffles to achieve good mixing, however in mineral processing the preferred baffle mechanisms are stators to act as internal baffles in forced air mechanical flotation machines.
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.
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.
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?
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.
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)
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.
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.
Doucet, Price, Barrette and Lawson, "Evaluating the effect of operational changes at vale Incos 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?
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.
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. Thats 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.
I can see what direction you are trying to take and its 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?
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".
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.
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.
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.
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?
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?
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Flotation machine is for processing minerals by means of froth flotation, which is a process for separating minerals from gangue by taking advantage of differences in their hydrophobicity. Hydrophobicity differences between valuable minerals and waste gangue are increased through the use of surfactants and wetting agents. The selective separation of the minerals makes processing complex ores economically feasible. The flotation process is used for the separation of a large range of sulfides, carbonates and oxides prior to further refinement. Phosphates and coal are also upgraded by flotation technology.
Flotation is a selective process and can be used to achieve specific separations from complex ores such as lead-zinc, copper-zinc, etc. Initially developed to treat the sulphides of copper, lead, and zinc, the field of flotation has now expended to include platinum, nickel, and gold-hosting sulphides, and oxides, such as hematite and cassiterite, oxidised minerals, such as malachite and cerussite, and non-metallic ores, such as fluorite, phosphates, and fine coal.
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