High-speed or high shear mixer/granulator is used extensively in pharmaceutics. The machines have a stainless steel mixing bowl containing a three-bladed main impeller, which revolves in the horizontal plane, and a three-bladed auxiliary chopper (breaker blade) which revolves either in the vertical or the horizontal plane.
The high shear mixers have been applied for high speed dispersion of dry powders, aqueous or solvent granulations, wet granulation, melt granulation, effervescent products and melt pelletization.

Working Principle of High Shear Mixer/Granulator

Blending and wet massing is accomplished by high mechanical agitation by an impeller and chopper. Figure shows a vertical high mixer, which is the most widely used version in the pharmaceutical industry. Mixing, densification, and agglomeration of wetted materials are achieved through shearing and compaction forces exerted by the impeller. The impeller rotates on the vertical shaft at a rotational speed corresponding to a radial blade tip speed of approximately 5-15 m/s. the chopper rotates at a similar tip speed which, because of its small diameter, corresponds to a very high rotation speed in revolutions per minute (rpm)(i.e. 1500-4000 rpm). The primary function of chopper is to cuts lumps into smaller fragments and aids the bowl or sprayed onto the powder to achieve a more homogeneous liquid distribution.

 Chopper and Impeller

The granulation is conventionally performed in the following process steps:
  1. The unmixed dry powders are placed in the bowl then the dry material is mixed at high impeller speeds for a few minutes (approx. 2-5 min).
  2. Addition of liquid binder (Granulating liquid)by pouring via a port in the lid of the granulator onto the powder, while impeller are running at a low speed (approx.1-2min.)
  3. The chopper is usually switched on when the moist mass is formed , as its function is to break up the wet mass to produce a bed of granular material.
  4. Wet massing with both agitators running at high speed (approx.1-5min.)
  5. Once a satisfactory granule has been produced, the granular product is discharged, passing through a wire mesh which breaks up any large aggregates, into the bowl of a fluidized-bed drier.
  6. Drying the granulate.
  7. Dry sieving the granulate

The process needs to be controlled with care as the granulation progresses so rapidly that a usable granule can be transformed very quickly into an unusable, overmassed system. Thus it is often necessary to use a suitable monitoring system to indicate the end of the granulation process, i.e. when a granule of the desired properties has been attained. The process is also sensitive to variations in raw materials, but this may be minimized by using a suitable end-point monitor.

Variables Occur in High Shear Mixer/Granulator
Process variables that affect granulation process are as follow:
  • Impeller rotation speed
  • Chopper rotation speed
  • Liquid flow rate
  • Load of the mixer
  • Liquid addition method
  • Wet-massing time (subsequent of liquid addition time)
Product variables that affect granulation process are as follow:
  • Amount of liquid binder
  • Characteristics of liquid binder
  1. Surface tension
  2. Viscosity
  3. Adhesiveness
  • Characteristics of the feed materials
  1. Particle size and size distribution
  2. Particle specific surface area
  3. Solubility in the liquid binder
  4. Wettability
  5. Packing properties
Apparatus Variables : The instrumental variables affecting the granulation characteristics are:
  • Size and shape of mixing chamber
  • Size and shape of impeller
  • Size and shape of chopper

Advantages and Disadvantages of Rapid Mixing Granulator

Advantages of rapid mixing granulator:
  • mixing, massing and granulation are all performed within a few minutes in the same piece of equipment.
  • Gravity loaded
  • Unit formula maintained
  • Forms desired wet granule rapidly
  • Less wetting and more rapid drying
  • Homogeneously dry mixes quickly: color distribution is excellent; can eliminate premixes of addition by geometric progression
  • Self-discharging
  • Improved coefficient of weight variation
  • Improved content uniformity
  • Sanitary construction
  • Relatively easy to clean
  • Mixing bowl may be jacketed
  • Option to dry granulation with mixer
  • Adequate safety devices
  • Conforms to good manufacturing practices(GMP)
Disadvantages of rapid mixing granulator:
  • Relatively high cost
  • High noise level
  • Adding material directly is not convenient
  • Temperature rise from head of friction
  • Non movable
  • Must be raised to working height
  • Foreign spare part