For the coarser specifications, with d50's from 70 down to 12 microns, ideal distributions were achieved on the Dynamic Classifier Mill with frequency inverter control of the rotational speed of both the mill rotor and the classifier wheel. In this way, the degree of milling and the cut-point of the particle size can both be varied to effect the shape of the distribution curve.

To add an extra element of control the process fan was also inverter controlled to give fingertip adjustment of the air flow through the classifier mill system. Air flow and the subsequent velocities of particles presented to the dynamic classifier affect the particle size cut-point which is influenced both by the speed of the classifier and the approach velocity of the particles. The finest cut points are achieved by the highest speed of the classifier combined with the lowest approach velocity of the particles ie. the lowest air flows.

To ensure a capacity of up to 2 t/h of crystalline feed a 45 kW mill (DCM45) with an 11 kW classifier drive was chosen. To further influence particle size distribution flexibility, optional mill elements were specified as follows:

• Short blade radial classifier wheel for coarser standard distributions



• Long blade tangential classifier wheel with air-rinsed gap facility for finer distributions
  
  The air-rinsed gap between the classifier top ring and the product outlet prevents coarse particle by-pass into the exiting product and air stream
  
  Long blades on the classifier wheel increase the flow area through the blades so reducing the particle approach velocity, to give finer cut points



• Smooth mill liner to reduce particle impact severity producing fewer fines below 10 microns ie. tighter distribution



• Easily removable rotor pins to further reduce impact severity and dummy plugs to neatly block the resultant holes

To reduce the turn-around time between batches, the mill had to be easily opened to access areas where Lactose powder might accumulate, for cleaning. To satisfy this requirement a hinged-body design of DCM was selected to give maximum access to the areas beneath and around the mill rotor. The unit was also 10 bar g Pressure Shock Resistant to prevent risk to operating personnel in the event of a powder explosion. To satisfy GMP guidelines, all product contact parts and inlet air contact parts were to be made from 304 stainless steel as a minimum with GMP finish eg. welds ground smooth, and polished with no inaccessible areas where powder might accumulate.

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