4.0 AIR SEPARATORS
Separators are utilized in a closed circuit grinding system for the purpose of separating (classifying) the fine and coarse particles of the feed. The fine particles are passed through the separator to become finished product while the coarse particles are returned to the mill as rejects for further grinding. All dry grinding applications in the cement industry use what is referred to as air separators or classifiers
The simplest form of a separator is a dropout box. By slowing the gas velocity down the coarse particles will settle out. Dropout boxes are frequently used to separate dust from gases. Since the velocities must be extremely slow to cause superfines to drop out, they in fact can be used as a classifier. Exshaw FM2 successfully uses this idea to make the mill sweep dust fine enough for finish cement. However the box is extremely large to accomplish this. As a rule of thumb for cement mills, the air velocity around the discharge trunnion and hood must be less than 4 m/s. In Exshaw’s dropout box they strive for about 2 m/s.
Evolution from a Dropout box and Cyclone
A more sophisticated approach is the cyclone, shown in figure 4.1. The outward spin forces the coarse particles out of the gas stream. Cyclones are intended to separate dust from gases – they are very poor size classifiers. WARNING: do not confuse cyclone efficiency with selection (recovery) efficiency. Cyclone efficiency refers to the effectiveness of dust removal from gas stream – not how good of a classifier it is. As a classifier we need inefficient cyclones. The performance of the cyclone is very dependent on the internal gas velocity. Higher speeds means more coarse particles are captured. Control by just velocity alone is poor.
4.2 Types of Air Separators
Beyond dropout boxes and cyclones we’ve become very ingenious is separation technology. There are basically 4 types:
Static Grit Separators
These permit the separation of large particles from the material being transported in the mill air sweep. These are common on vertical roller mills, air swept coal mills. One of the largest units in service can found on Alpena’s raw grind circuits.
First Generation Separators
These are separators with internal cyclones, e.g. Sturtevant and Raymond. They have been in use for decades. Most plants in Lafarge Corporation are equipped with these.
Second Generation Separators
These are separators with external cyclones, e.g. Wedag-Zub and O&K, (mainly in Europe). As a general rule their efficiency in terms of bypass, fractional separation, and grinding is superior to the first generation separators. Some first generation separators can reach the same performance levels with modifications. To date, none are in service with Lafarge Corporation. Therefore this class is not discussed in the Mill Grinding Reference.
High Efficiency Separators of the Third Generation
Introduced in the mid-eighties, these are characterized by external ventilation, and by the presence of a squirrel-cage (rotor cage), which allows an improved fractional separation over other separator types. Upgrades and new installations will one of these units installed as the type of choice.
4.2.1 Static (Grit) Separator
Principle of operation
The method of particle separation in a static separator is a very basic process – not unlike a cyclone. The dust laden air swept from the ball mill flows upward into the bottom of the separator cone. As the air (gas) rises, it flows between the inner and outer cone until it reaches the inlet vanes. There, it flows through the vanes, imparting a circular motion, into the upper section of the inner cone. Due to the much larger areas of the inner cone, the velocity decays and the coarse particles (grits) fall by gravity to the bottom of the inner cone; out the reject spout back to the grind circuit. In addition, the circular motion of the air (gas) improves the separation of coarse and fines by centrifuge. The air (gas) with the fine particles continues up through the thimble out of the separator to elsewhere in the process.During the process of grinding in the mill, an external fan drafts fine and coarse particles from inside the mill through the mill discharge. In process terms, the material is swept from the mill.
Static Grit separators are commonly employed on semi air swept finish mills and on air swept coal mills. Some large raw grinding vertical roller mills use them. Alpena’s roll press raw grinding circuit uses them as well.
This separator does not compare in design to the conventional separator. Its design compares to a cyclone with the exception that the internal vanes can be adjusted.
- a) Outer Cone
Can be described as the outer shell of the separator. The bottom of the cone (inlet) receives unclassified material from the mill sweep. The outer may not exist where static separators are built into a unit such as a vertical roller mill. Poor outer cone designs have led to significant performance problems.
- b) Inner Cone
Installed inside the outer cone. The bottom is piped outside the separator for rejecting coarse (grit) particles from the separator.
- c) Inlet Vanes
Located below the top of the separator, separates the inner and outer cone. The vanes are adjustable and can be moved to increase or decrease the circular motion of air as it enters the inner cone. Typically they are mechanically linked and are adjusted using a single hand wheel.
- d) Thimble
Placed through the separator top (center). Fines and (exit gas) leaves the separator through this duct. Thimble height can be adjusted to pick fines from the air flow inside the separator. However, the thimble is set up at installation and not moved as a regular procedure.
- e) Bleed-in Air Valves
This is a modification idea that originated in Demopolis. Small manual valves are mounted in front of each vane. Bleeding air in accomplishes two things. First it reduces mill sweep such that super fines are preferred. Second the air velocity inside the inner cone is kept high for best separator efficiency. Demopolis only bleeds air in when making TIII or high blaine products.
- a) Fineness Control
The product fineness can be changed by changing the volume of air swept from the mill or the separator vanes are adjusted. Keep in mind that either change will have effect on the separator efficiency.
- Air Volume
An increase in volume of air swept from the mill results in a higher percentage of coarse material going through the separator to the collector – which lowers product blaine. If the sweep also increases the internal velocities then bypass also falls. Large fans are needed to make these units effective.
- Vane Adjustment
Each static separator vane adjustment area is marked for direction of movement. A coarser adjustment (less spin) will result in the removal of fines to the collector with a higher percentage of coarse particles – which lowers product blaine.
The main advantage is that it has no moving components or power source.
Basic inspection requirement is a visual inspection of the separator and related intake and exhaust piping for leaks. Depending on the material abrasiveness the vanes and the inner cone should be checked for replacement at least every 4 to 6 years.
Outer Cone Design Problems
All static separators with double cones have this particular problem, shown on the left side of figure 4.2.1b. Inlet gas velocities have to be high enough to keep all particles in suspension. As it enters the separator, the cross sectional area increases because it goes from a round duct to an annulus that’s increasing in diameter. This causes the velocity to drop and the coarse particles to fall out. Unfortunately these particles fall down into the inlet – only to be re-entrained. A circulating load is created and this blocks flow, (sometimes referred to as a dust cloud), drastically reducing the separator performance. The same phenomena has been known to occur in kiln riser ducts where there’s an expansion in duct area going up. This blockage can be detected by extremely high differential pressures across the separator or the inlet duct (especially if its a straight duct).
This problem has occurred on Demopolis’ Fuller and KHD (1984) static separators on FM 1 & 2, and on Alpena’s Polysius static separators on FG 20 & 21 (1991). To cure this problem, both plants replaced the inlet ducts with dropout boxes. By dropping out the coarse particles, alleviated the internal circulating load.
Recently suppliers, (KHD and FCB) have modified the design to take advantage of this characteristic and improve efficiency. This is shown on the right side of figure 4.2.1b
4.2.2 Sturtevant Separators
Operation (see Figure 4.2.2a)
- Material goes down the feed spout to the intake cone and to the lower distributing plate hub.
- By centrifugal action (rotation) the particles are thrown outward through the ports of the hub and onto the lower distributing plate.
- Particles are dispersed from the plate into the separating zone. A curtain or umbrella of material is formed outside the lower distributing plate.
- Forces acting on the particles are ascending air, gravity, and centrifugal action.
- Coarse particles settle by gravity to the tailings or rejects chamber.
- Finer particles are acted upon by the upward air flow created by the main circulating fan and lifted to the selective zone where final selection takes place.
- The selector blades impart additional centrifugal force. Heavier particles are thrown outward underneath the drum cover to the rejects cone. The finer, lighter particles are drawn through the path of the selector blades to the finished product area.
- More selector blades or fewer main fan blades will result in a finer product.
- Control valves (or diaphragm) are located between selector blades and main fan blades, and move in or out to vary the size of the opening between the two – fine tuning the upward draft to control fineness.
- Return air vanes (between the fines cone and the inside drum cone) allows fines to settle, while returning air to the separating zone.
- Drying in the separator can also be achieved through balanced hot air inlet(s) and outlet(s) to a dust collector and fan.
- Cooling in the separator can also be achieved drawing in cold air.
Parts of a Sturtevant Separator are shown in figure 4.2.2c.
Sturtevant Separator Adjustment (see table & figure 4.2.2b)
The possible operating adjustments that can be made on a separator are, in order of decreasing priority:
- Diaphragm if there is one
- Number of fan or selector blades
- Main fan diameter
or by means of the feed flow rate in terms of the output and fineness.
The general rules under all circumstances are:
In the case of a separator with a diaphragm, as a general rule, it should be adjusted for the different modes of operation (selector fan, main fan) in such a manner that, during normal operation, the diaphragm will be near its maximum opening.
Shown above, the diaphragm has many names depending on the plant – diaphragm, valves, plates, selector (valves), etc. Understanding what they do is easier when you look at it in plan view. The main fan is a centrifugal fan on its side. The diaphragm then is just an iris type opening. Pushing the valves in closes them restricting fan flow. The reduced up draft causes less of the coarser particles to be drawn up and hence blaine and 325 mesh rises. However separator bypass will increase too. The diaphragm allows the operator to make small adjustments while running.
Cooling, Drying and Venting
These units are vented to a dust collector. By connecting the air intakes to a furnace allows one to dry materials. Many plants put fresh feed to the separator first – to “flash” dry the material. By bringing fresh air only cools the rejects flow and helps cool the mill circuit. Over venting is possible. As a rule of thumb the dust from the separator must as fine or finer than the finish product. Otherwise it’s over-vented. (See also figure 4.2.2d)
Figure 4.2.2c: Parts of a Sturtevant
- Fines Chamber 9. Inside Drum Cover 17. Intake Cone
- Tailings Cone 10. Inside Drum Cover Liner 18. Intake Cone Line
- Ring Liner 11. Valve & Valve Rod 19. Fan Cone
- Air Vane 12. Gear Reducer 20. Packing Ring
- Outside Casing 13. Main Shaft 21. Upper Dist. Plate
- Outside Casing Liner 14. Distributing Hub 22. Upper Dist. Plate Liner
- Inside Drum 15. Dist. Hub Liner 23. Selector Blade
- Inside Drum Liner 16. Lower Dist. Plate 24. Main Fan Blade
Figure 4.2.2d: Air (Gas) Flow Paths
The following items should be checked regularly:
- Clearance between the top of the selector blades and the underside of the drum cover should be 1/4 to 3/8 inch.
- Selector blades should pass a minimum of 2 inches under the drum cover (overlap) – not the edge of the diaphragm.
- Check for holes between the fines chamber and the tailings cone (use a good light). Also check the intake cone and the seal.
- Check for leaks on the inside drum cover, with the valves pushed in all the way and out all the way.
- Check for wear on selector blades, upper and lower distribution plates (especially holes), valves, air vanes and all liners.
- Make sure that the positioning of selector blades is properly balanced.
- Check that the position of all valves is synchronized in all positions or opened by the same amount.
- A separator functions better with long selector blades rather than short. This implies that for long blades, fewer are required for the same fineness. The possible fineness range by means of diaphragm adjustment then, is always less.
1) Maximize the number of fan blades then the number selector blades that gives you the largest diaphragm opening.
2) Increasing the main fan diameter or the shaft speed will dramatically increase motor power drawn exponentially. Watch your motor power carefully. See also sections on Qf/Qa and Modification Ideas.
A: Selector Blade Wear and Blade Shapes
1) The flow pattern along a selector blade is shown above, where most of the material is concentrated at the blade edge nearest the upper distribution table. Material travels straight up, but curves away as it begins to travel outward. This pattern is sometimes etched into the blade steel, or appears as build-up, or can be replicated using a blade that has been spray painted.
2) The general rules for selector blades is as follows:
- a) Increasing the number of blades increases the % passing 325 mesh.
- b) Increasing the blade total area increases the product blaine.
Caution: in both cases it appears to be a non-linear relationship. There’s insufficient data to say exactly how they relate.
3) Keeping these general rules in mind, St. Constant developed the Trapezoid blade and Demopolis developed the Hockey Stick blade. In each case, each plant has trimmed the excess steel (the latter more extremely) in order to maximize the number of blades for a better 325, but at the same time keeping the blaine about the same. To get the right shape a lot of experimenting is required.
B: Controlling Selector Bypass or Gap Reverse Flow
1) A minimum clearance of 1/4″ must be maintained between the top the blade and the underside of the inside drum cover. As well a 2″ overlap should be maintained between the inside diameter of the drum cover (valves fully retracted) and the outside diameter of the mounted selector blades.
2) Due to the high pressure that exists just beyond the selector blades and the low pressure around the center of the main fan, air will try to flow in the reverse direction through the gap between the blade and drum cover. The wider the gap the more reverse air flow. This reverse air flow will drag coarse particles along, contaminating the finish product.
3) To improve performance despite tight clearances there are two modification ideas to try:
- a) A 1/4″ X 1″ flat bar can be welded on the top edge and on the side facing the direction of travel as shown above, (the correct length requires experimenting). The flatbar or lip prevents coarse particles from leaving the top edge and getting re-entrained with the reverse air flow. This idea was first developed in Demopolis and tried successfully in Woodstock.
- b) A wear ring can be welded to the underside of the inside drum cover all of the way around. This blocks the reverse air flow and thus prevents any re-entrainment. This idea was developed by St. Constant.
- c) Exshaw found that one works just as well as the other. Trying both did not improve on the first.
C: Ideas from Sturtevant
1) On many Sturtevants, operators have found that the upper table diameter is too small giving less than satisfactory performance. One idea from Sturtevant is to install a 6″ table extension, shown above which does help improve the 325 mesh on fines product. WATCH that no gap exists between the table and new extension.
2) Another idea from Sturtevant is to install a deflector ring in behind the blade all of the way around the upper table, as shown above. The idea works in that it does improve mesh and blaine performance, but it is not recommended in that it appears to increase separator bypass as well. In addition, material builds up in behind which adds a lot of extra weight to the table.
G: Internal Airflow Considerations
- A) If the air intake duct is not being used for drying or cooling then according to Demopolis experience, this duct (and other protrusions) should be cut flush to the inner cone. The extended duct(s) into the inner cone disturbs the cyclonic action and re-entraining coarse particles when it is no longer necessary. Fluidizing the coarse particles at the bottom of the inner cone serves only to increase the chance that they will flow out through the return air vanes contaminating the finish product. Note that excessive reverse air flow through the rejects duct will do the same thing. In the latter case it may be necessary to install a flap valve of some sort.
- B) Some effort has been spent testing different configurations on pilot scale models. Unfortunately nothing conclusive came out of this. Clearly this is a source of separator bypass (which is avoided in high efficiency cage rotor type separators) – an inherent flaw in this design that we can do little about. However recognize that as the table and fan turns, the air flow in both the inner and outer cones will turn in the same direction. Thus it is important that the return air vanes be oriented such that air flows from outer to inner cone. This does increase bypass but that’s better than having coarse particles flowing from inner to outer cone which will happen if the these blades are turned the wrong way.
- C) In some instances, it is possible to over vent the separator. In such cases over venting will pull coarse particles from the inner cone into the outer cone, decreasing the fines product mesh and blaine. However this is difficult to detect unless the dust collector is dedicated to just the separator. In this case the dust collector fines should equal or better the separator fines.
4.2.3 Raymond Separators
Operation (see also figures 4.2.3a & 4.2.3b)
- Material is introduced into the top of the separator and falls on the distribution plate. The material is slung outward into an air stream, created by the fan, and lifted to and through the separation blades.
- The double whizzers knock down the coarse particles where they are collected and discharged from the inner cone back to the mill inlet for regrinding.
- The finer particles are lifted through the whizzers passing by the selector vanes and collected and discharged from the outer cone to a transport devise.
- The selector vanes determine how much air and material is directed through the double whizzer blades. If all the material were to be directed through the whizzer blades there would be too many returns. This means higher production costs. If too little material is directed through the whizzer blades there is not proper segregation and the proper amount passing a 325 mesh screen is not achieved.
- The selector vanes can be adjusted and serve the same function as the adjustment plates in a Sturtevant separator.
- Since the system is not of the air through type there is no need for a large dust collector. The only air generated is from the transportation system (air gravity conveyor) and thermal displacements.
- Water cooling jackets are fixed on the exterior of the outer cone in case the finished product is excessively hot.
Figure 4.2.3a: Raymond Double Whizzer Separator
Material Flow and Air Sweep Through The Air Separator (schematic)
Figure 4.2.3b: Raymond Air Classifier
4.2.4 High Efficiency Separators
Principle of Operation
High efficiency separators (HES) are sometimes called CAGE ROTOR separators. One of the most obvious differences between HES and the older Sturtevant and Raymond separators is that the older machines had internal fans while the HES has no fan and relies on an external fan to supply air for separation and transport of material. Within Lafarge in North America there two types of HES presently in service. O-SEPA’s are built and sold by Fuller under license from Onoda Cement in Japan. SEPOL’s are built and sold by Polysius. Note also that the major difference between the O-Sepa and the Sepol, is that air is down-drafted in the Sepol where as it is up-drafted in the O’Sepa. Operational installations are as follows:
O-SEPA: Bath FMA, FMB
Richmond FM1, FM2
SEPOL: Alpena FM19, FM20, FM21
Classification Process (see figure 4.2.4a)
The only moving part within the separator is the cage rotor. The rotor is driven by a variable speed motor controlled from the control room. Clean air enters the volute housing and is forced to travel in a circular path by the shape of the housing. The air encounters the inlet vanes which are arranges in a circle completely around the cage rotor. The air next enters the classification zone which is the space between the inlet vanes and the cage rotor vanes. The air now enters the cage rotor through its vanes. The air then exits through the bottom of the cage rotor in Sepol’s; or through the top of the cage rotor in O-Sepa’s; and then out of the separator through the exit elbow. Unclassified material enters the separator at the top and falls by gravity to the top of the cage rotor and lands on the distribution table. Since the table is rotating, centrifugal force propels the material outwards and off the rim of the distribution table where it impacts a wear ring and begins to fall into the classification zone.
At this time, the material encounters the air entering through the inlet vanes and begins to fall through the classification zone in a circular path induced by the air currents. The larger, heavier particles tend to fall through the circular air currents while the smaller lighter particles tend to flow with the air currents into the cage rotor and out of the classifier as finished product. Particles which are marginal in size may enter the cage rotor vanes but may also be rejected if their velocity is not great enough to pass between adjacent vanes without being struck by a vane. One way to visualize this operation is to consider that there is specific amount of time between the passing of one vane and the next vane. If a particle can travel through the rotor vanes in less time than this passing time, it can penetrate the cage and flow out of the separator. If the particle is traveling too slowly, it will be struck by a vane and be rejected.
Understanding of the previous described classification process leads to an understanding of the manner in which the fineness is controlled. Two methods are available for controlling the fineness:
- A) By varying the volume of air flowing through the separator, the velocity of the air entering the cage is also varied. As the volume and velocity is increased, the PRODUCT becomes coarser. As the volume and velocity is decreased, the PRODUCT becomes finer.
- B) By varying the speed of the cage rotor, the blade passing time is varied. As the rotor speed is increased, the PRODUCT becomes finer. As the rotor speed is decreased, the PRODUCT becomes coarser.
(Table speed refers also to cage rotor speed.)
It is readily seen that product fineness control is much easier with the HES than with the older types of separators. In addition, there are no selector blades to change. All fineness control can be done from the control room.
The finer material that leaves the HES in the air stream is considered finished product. This product is carried in the air stream and on to the dust collection system. It is here that the dust laden air entering the collector is cleaned and the finish product is collected and transported via air slides and air lock feeders to the cement pump which pumps it to the storage silos.
ALPENA HES DESIGN DATA
FM 19 FM 20,21
Separator horsepower 200 500
Cage rotor diameter (m) 2.0 3.1
Fan HP 300 700
Draft rating (ACFM) 59,000 143,000
Rotor speed range (RPM) 47-280 31-190
Design feed rate (TPH) 165 514
|Figure 4.2.4b: Fuller O-SEPA|
High efficiency separators for the most part run trouble free. The dispersion plate, impact plate, guide vanes and rotor cage need to be inspected occasionally. Of particular importance are the intake duct. For best performance, the gas flow velocity must be as uniform as possible over the entire height of the cage. Damaged dampers and dust build-ups have caused skewed intake flows, resulting in poor performance.
The cage seal is another source of problems. The seal between the cage and the outlet duct must be in good shape to prevent coarse particles from contaminating the finished product. Newer OSEPA’s have a better seal design.
Research by ONODA suggests that the different sizes of separators may not have been scaled up correctly. Cage height, gap width (between the cage and guide vanes) and especially cage bars or blade spacing are important factors. Apparently there is a critical optimum spacing width between cage blades or bars. Too wide or too narrow results in poor 325 mesh values. The spacing varies with unit size. Larger units appear to have spacing that’s too wide.
OSEPA vs SEPOL
We only have experience with these models in Lafarge Corporation. In France, Five-Cail Babcock’s (FCB) TSV high efficiency units have been installed with very good success. Other models include FLS SEPAX and Sturtevant SD Classifier. OSEPA’s are characterized by a top discharge duct, twin feed chutes and a bladed rotor cage. They also have both a primary and secondary air intake ducts. The tertiary ducts have been eliminated from newer units. SEPOL’s are bottom discharge, with a single feed chute, single intake duct and rotor cage with bars. To date there is no discernible difference in performance.
Figure 4.2.4c: High Efficiency Separator: O-SEPA Cross Section
Figure 4.2.4d: High Efficiency Separator: Polysius SEPOL