There are many industrial applications that require the enlargement of powder/particles for handling. Some objectives may be: enhancing flow properties, recycling, de-dusting, densifying, freezing ingredient distribution, improving mold filling properties, etc.
Size-enlargement is more commonly referred to as agglomerating, pelletizing, granulating and briquetting. This article deals with particle enlargement generally through a snowballing mechanism (pan/disc), with some shear action (high shear mixers), but without the application of high pressure or extrusion.
Most powders with the ability to fuse, or slightly dissolve in the presence of a liquid, can be granulated quite easily. Others may be coaxed to form granules by the introduction of some suitable binders. The binders may act as a medium for the solid particles to adhere to each other, some may form a matrix to capture the particles, and others may involve chemical reactions to achieve the objective.
Mix-Granulation versus Disc/Pan-Granulation:
Disc granulation (sometimes called pan granulation) is a continuous process. In its simplest form, powders and liquid are fed in steady streams and fixed proportions onto the rotating disc which is tilted at a certain angle to facilitate continuous overflow of the desired granules. Under theoretically steady-state conditions, particle residence time in the disc granulator is constant and relatively uniform-sized granules in the order of 1/2” diameter and larger are the output.
As with many continuous processes, truly steady-state conditions are fleeting for disc granulation, and actual yields are much lower than theoretical yields. Furthermore, disc granulation requires frequent operator attention to try and maintain a balance of mass flow.
Mix-granulation in a Lancaster Products K-series high shear mixing unit offers a much higher level of control and thus greater yields in the granulation process over disc granulation for several reasons.
- The mixing process is batch wise and tightly repeatable.
- The ability to produce products of the same characteristics batch-to-batch allows the operation to be automatic, with little operator attendance.
- The process produces a wider spectrum of granule sizes, from micro sizes to as large as 8 -10 mm depending on requirement and feed material.
- Because of higher shearing and configurable tool actions, granules are generally packed more tightly and require less liquid/binder.
- Other properties can be controlled to a significant degree according to objectives.
- Mix-granulation also has the flexibility of approaching the final moisture content either from the dry end or from the wet end.
The Lancaster Products mixer is capable of regulating intensity and energy, independent of mixing-granulating time, and a number of factors can be adjusted to influence granule characteristics. These mixer configurable parameters include: rotor design, rotor speed, direction and granulating time and pan speed. In addition, optimizing process parameters such as solids fineness, binder, liquid-to-solids ratio and liquid addition rate can enhance desired granule properties.
Lancaster Products K-series mixers are especially useful for processes where some multi-ingredient solids need to be well mixed, and then granulated into nominally spherical products. Mixer and process parameters can be controlled to achieve desired properties. Size distribution usually follows a bell-shaped curve, with approximately 90 percent of granules falling within one order of magnitude.
A door-to-door cycle can run anywhere between 3-4 minutes to over 10 minutes, depending on process and product requirements.
An example of mix-granulating plant may be as illustrated below: