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Ploughshare Mixers (or plow mixers) operate on the principle of a mechanically generated fluid bed with three dimensional movement of the product. They provide excellent solutions for mixing materials like powders, powders with liquids, and heavy pastes such as dough molding compounds (DMC) and putties. Homogenous mixing of materials with varying densities, flow-properties, particle sizes and structures is achieved in quick time. Mix ratios of up to 1:20,000 are possible. The ploughshare mixer can mix the material in less than 5 minutes with 95 to 98 percent homogeneity. The mixer can handle material with viscosities in the range of 600,000 centipoises.
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Theory of Fluidized Bed Mixing
The mechanically fluidized plough mixer provides quick, intense mixing without the application of high shear. This is done using particle fluidization, which by definition is the separation and agitation of solid particles using a rising stream of process gas. Mechanical fluidization is the process of separation and agitation of the particles using impingement of the material on mixing elements mounted on a rotating shaft located within a cylindrical horizontal vessel. The plough shaped mixing elements are designed to lift and separate the product within the vessel. The design of the ploughs, their number and spacing, and the speed of rotation all contribute to fluidization. With the right design of the mixing elements and appropriate selection speed of rotation, the ploughshare mixer provides intense mixing in a gentle manner, with very little shear.
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Design, Construction, and Operation
The ploughshare mixer consists of a cylindrical drum containing plough shaped mixing elements that are mounted on a horizontal shaft. The agitator's shaft is positioned in the centre of the drum and has welded / bolted arms on which the “Plough" shaped mixing elements are mounted. The ploughshare mixer gets is name because of these plough shaped mixing elements. Independently driven high-speed choppers may be installed to disperse agglomerations and control granulation during the mixing process.
The plough shaft is powered by a drive system comprising of a motor, gearbox, and couplings. The shaft exits the mixer drum at either end through the end plates bolted or welded to the cylindrical shell. The area where the shaft exits the container is provided with a sealing arrangement to ensure that material does not travel from the container to the outside and vice-versa. The mixer assembly along with the drive system components viz. motor, gearbox, couplings, and bearing supports is mounted on a supporting frame.
In operation, material is loaded through the top inlet. The working volume (volume up to which material should be loaded) of the ploughshare mixer ranges from 30% to 70% of the total drum volume. The shaft rotates at higher speed, producing a plough tip speed of more than 200 metres per minute to effect a fluidizing action. The ploughs throw the material upward, and the material settles in a random pattern. The size, shape, positioning, and peripheral speed of the ploughs are coordinated in such a way that they produce a whirling, three-dimensional product movement. The turbulence and material movement within the mixer container prevents formation of dead-spots or low-movement zones and ensures fast and homogenous mixing. The ploughs are designed to lift the material from the inside wall of the container without squashing the particles against the wall
The profile and arrangement of the ploughs ensure that after mixing, the material is directed towards the centrally located discharge valve. For larger mixers, more than one discharge valve can be provided depending on the requirement of the downstream process. Flush bottom discharge valves are preferred to avoid any dead pockets within the mixer. These valves may be manually, pneumatically of hydraulically operated. Large side doors may be provided for cleaning access. Figure 1 showsa 600 litre ploughshare mixer.
Ploughshare Mixers can be fitted with optional high speed chopper units for applications which require breaking down lumps or when the mixer is to be used for wet granulation. Single or multiple choppers are mounted in the lower portion of the mixing container. The mounting arrangement is such that the choppers are positioned in between the flights of two adjacent mixing ploughs. The ploughs move beneath the rotating choppers (see figure 3). Tulip-shaped choppers and X-mass tree chopper designs are common. The choppers help to remove lumps in the raw material, chop pasty additives, and prevent formation of agglomerates on liquid addition. The operation of the choppers is independent of the operation of the plough shaft. Choppers are generally operated towards the start and the end of the mixing cycle, or when undesired agglomerate formation occurs during the mixing process. The influence of the chopper action on the product mix can be controlled.
Equipment capacities vary from small laboratory size units to large production size mixers with capacities of 30 cubic metres. The specific power for the mixer drive generally range from 30 to 40 kW/m3, depending on the properties of the materials to be mixed. For mixers equipped with choppers, the chopper motor rating would depend on the size of the mixer and properties of the material. Additional features such as vacuum, heating/cooling jackets, and liquid spray systems can be provided if required.
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Applications of Ploughshare Mixer
Ploughshare mixers have applications in the following industries:
- Brake linings
- Confectionery products
- Waste treatment
Ploughshare mixers are available in batch modes and for continuous operations.
The batch ploughshare mixer is the most versatile mixer type available with an extensive range of applications:
- Dry powder mixing – e.g. Food flavoring/additives, Sponge/cake mixes, Cosmetics, Pharmaceuticals, Cement, Powdered rubber
- Paste mixing – e.g. Biscuit creams, dough, PVC pastes, Car body fillers/putties, Sugar/marzipan paste, Toiletry pastes, Tile adhesives and grouts
- Mixing of wet slurries – e.g. Filter cake re-slurrying, dough, Fiber glass resin dough putty
- Wet granulation
- Liquid coating of powders and granules
- Fat and oil incorporation
- Vacuum drying
- Hot air drying
Continuous ploughshare mixers differ from batch ploughshare mixers in that the mass flow of the product is from the inlet of the container to the discharge at the opposite end.
Typical applications of continuous ploughshare mixers are
- Dry cement blends
- Sludge conditioning
- Wetting out industrial dusts
- Coal reclamation
- Blending of minerals
The ploughshare mixer can be used as a multi-stage process vessel eliminating the need for additional specialist equipment. It can be used as a mixer-dryer, mixer-granulator, de-aerator, reactor and cooler.
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Photographs - Courtesy - Unique Mixers & Furnaces Pvt. Ltd., India (www.uniquemixer.com)