Process understanding lead to more efficient pharmaceutical milling
Milling Process Solutions

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Case study: Milling overload reduced by process understanding

Problems of variable product quality were being experienced with a pharmaceutical mill. Material was fed to the mill by a rotary valve, and ground using turbulent air jets. Ground material exited via the dynamic classifier.

Understanding the milling process.

Periodic sampling of the product took place at a fixed period of D t. Samples measured in this way showed the process to be acceptably close to the setpoint.

Manual milling samples.

However, when real-time particle size measurements were undertaken, using an Insitec on the outlet of the mill, the mill was revealed to be much less stable.

Milling process with real-time particle size measurement.

The graph below shows that the median size (Dv 50) (red line) fluctuates significantly. This corresponds to a periodic fluctuation in the measured transmission (blue line), which is related to the mill loading. 100% transmission corresponds to there being no particles present, and 0% means that so many are present that no light is reaching the detector.

Milling process size fluctuations.

Periodic fluctuations that are the consequence of forcing material through the mill, lead to "short-circuiting" of the classifier. This affects the Dv 50 of the particles. The higher the loading, the more pronounced is the effect, resulting in a larger (coarser) median size. As more and more particles are forced through the classifier, the classifier motor draws more and more current (green line). 

The remedy to this situation was to use the measured transmission value to drive the feed rotary valve.

Milling process with measured transmission valve.

As a result transmission oscillates around a mean value (indicative of a simple control scheme) and critically the median size is continually close to the setpoint.

Milling overload reduced.

The numerous benefits of these actions included:


  • much higher product quality
  • smoother mill operation
  • less attention required for sampling, or for mill control
  • higher yield
  • less wastage and re-work