INTRODUCTION TO SIGMA (KNEADER) MIXER AND PLOUGH SHEAR MIXER:
Mixing of semisolid materials (especially if voluminous) can be challenging. Many a times, industries (especially food, polymers, resins, rubber and chemical manufacturing) require mixing of highly viscous, as well as, tough or dense materials. This require special mixers which are not only sturdy but also makes use of special type of agitators that can achieve the most efficient mixing. On the other hand, there are a few slurries which are soft but highly viscous and require the homogenization. In case of such type of materials also, special mixers are needed.
Sigma Or Kneader mixer is totally different from the Plough Shear Mixer in all aspects such as size, shape and utility. While the former is used for the breaking down and thorough mixing of the toughest semisolid lumpy mass, the later is used for mixing and homogenizing of the relatively softer lumpy slurries. Since the bulk densities of both kinds of materials would be different, the types of agitators too are very much different and unique.
Plough Shear Mixer:
Plough Shear Mixer is a horizontal cylindrical drum type stationary mixer, where the mixing is achieved by the unique blades mounted on the concentric horizontal agitator. As mentioned in my other post on Vacuum Shovel Dryer, the ploughs of the mixer (because of their unique shape) not only breaks the lumps (if any), but continuously keeps lifting and pushing the mass out towards the inner cylindrical wall of the the drum). This continuous action at a high RPM initiates the mechanical fluidization of the material to be mixed. Based of the factors like the size and shape of the ploughs, distance between the adjacent ploughs and the RPM of the horizontal agitator, a centrifugal vortex motion takes place and hence the mixing mass starts getting thrown in all directions. This effect brings out a through mixing of the material (especially all the particles of different sizes and bulk densities).
Sigma Or Kneader Mixer:
Sigma Or Kneader Mixer is a horizontal (with body partially hemispherical drum shaped and partially rectangular shaped) type mixer, with specially designed low rpm twin blades (parallel blades) which can be either of overlapping nature Or are of tangentially scraping nature with respect to each other. The most popular types of blades can be S type, Z shaped or Profiled ones. However, the twisted S and Twisted Z types are common too. The sigma mixer with tangential type blades usually has one blade rotating at double the speed of the other. Whereas, the overalapping types blades has same rpm in both the blades, ensuring their orientation is such that they do not colloid with each other, however, does cover the maximum volume of the mixer.
Major Components of the Mixers:
1. Mixer Drum Or Bowl: As discussed above, both the mixers are absolutely different from each other, hence their mixing drums are also different in size, shape etc. Plough Shear mixer consists of a horizontal cylindrical stationary drum, which has the concentric agitator shaft (meaning parallel to the horizontal). Whereas, the Sigma Or Kneader Mixer has a specially designed drum which appears like a rectangle from the top, whereas, has a hemispherical bottom Or two halves which are hemispherical in nature. Here too both the blades are parallel to the horizontal.
Bowls in both the cases may be jacketed also (incase heating Or cooling is needed during mixing).
2. Agitator and Blades: Incase of a Plough Shear Mixer is a single hollow shaft with hollow branches welded Or bolted to the main shaft and the specially designed shovels Or ploughs are welded at the end of each branch. The entire unit ie central agitator along with the branches and ploughs rotate in one direction only (clockwise Or anticlockwise). Incase of the Sigma Or Kneader Mixer, usually have two shafts with solid machined Or profiled-cut heavy duty blades (either S type Or Z type Or some other profile). If one rotates clockwise, the other rotates anticlockwise, such that each blade pushes the mixing mass to the other blade.
3. Seal Or Gland packing, Bearings, Couplings and Gearbox: The bearings are couplings in both types of mixers depends on the type of the shaft used (hollow Or solid), shaft diameter etc. Usually the glandrope packing is used instead of mechanical seal, as it is more sturdy and long lasting. The gearboxes can be two in numbers incase of Sigma Mixer (one of each blade) Or can be one (with external gears for maintaining ration of 1:2 between the rpm of both the blades). However, incase of the Plough Shear Mixer, just a single gearbox suffices.
4. Motors: AC frequency motors of suitable capacity (based on the size of the mixer, torque calculations based on the properties of the mass to be mixed and the ratio of the gearbox) with a variable frequency drive are chosen. Sigma Mixer may use two separate motors for two different blades, whereas, Plough Shear Mixer does use only one motor.
5. Bottom Discharge Valve: Depending on the requirement, the bototm discharge valve can be of either flush type ball valve Or knife gate valve Or some other valve. Most of the times, the sigma mixer come with the bowl tilting mechanism, where the entire assembly (bowl with agitators) can be tilted upto ninety degrees so as to unload the material from the charging end itself.
6. Support Or Stand: Plough Shear Mixer can be supported with a cantilever skirt Or legs supports, however, sigma mixers are mostly both side supported.
7. Panel: Just like the other mixers Or dryers, both Plough Shear Mixer, as well as, Sigma Mixer can come with either a relay based push button type operating panel Or seldom with PLC based HMI operated touch screen panel. PLC/HMI is not so common, as the indications and commands are not many (viz. Start, Stop, Emergency Stop etc).
Material of Construction:
Both Plough Shear Mixer, as well as, Sigma Or Kneader Mixer are more common in Chemical and Food industry than in Pharmaceuticals. Also, they are majorly used for mixing higher Or voluminous batches. Therefore, GMP compliance may Or may not be needed. The product contact parts can be SS316 Or higher grade and Non Contact parts can be SS304 Or Mild Steel with epoxy painted. The process may demand the contact parts to be hastelloy C22 Or C276 Or Stainless Steel coated with Halar Or PU Or PVDF.
Surface Finish:
In case of the GMP compliance, the product contact surface needs to be extremely smooth. This helps in preventing the surface corrosion due to electrolytic cell formation. Hence the Product Contact surface is no less than 240 grit Mirror Finish and Non Contact surface is 180 grit matt finish. Incase of a Non-GMP set up, if Mild Steel is used (for Product Non Contact in some cases), the surface is either epoxy painted Or coated with protective layer.
Please feel free to contact incase of any queries, questions Or suggestions on this Or any other topic at shg.techo@gmail.com
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