A Technical Article by James Ellis (Hanningfield) and Stefano Butti (F.P.S. Food and Pharma Systems S.r.l.)

Abstract

Cone milling is one of the most prevalent methods of size reduction in the process industries. Due to the ever-increasing potency of process material (APIs etc) containment becomes an essential element of modern day processing considerations.

One suggested method for the containment of such fine powder during milling, is the use of an isolator to ensure all excess material remains contained, ensuring the fine dust particles are not exposed to either atmosphere or operator. Understanding when to use an isolator, and how to use it, can be essential to ensuring the milling process meets required and advised processing guidelines.

1. Introduction

The increasing popularity and potency of new pharmaceutical API’s, HAPI’s and sterile products makes the need for tailor-made containment a priority. This is necessary to continuously meet the required safety standards and to ensure an appropriate level of advanced technology is in place to support research activities, product developments and final production phases.

‘The increasing potency of API’s has made containment during milling a hot topic.’

This concern is particularly relevant in relation to mechanical milling methods, such as cone milling. Milling is a process which can generate fine dust particles - this is a problem which needs to be deeply analysed and investigated to ensure proper levels of containment, ergonomy and process accuracy are maintained. For this reason, choosing to integrate a mill with an isolator is a matter of acute technical importance.

Cone milling is one of the most common methods of milling in the pharmaceutical and allied industries. Although cone mills often produce less dust than alternative forms of milling, there is still some level of dust generation. An excellent solution for containing this is to use an isolator for keeping the mill, and therefore an excess dust, enclosed within the booth.

However, in order to properly evaluate when to use a containment booth, we must first gain an in-depth understanding of both cone milling and isolation chambers.

2. Cone Milling - Dust Generating Process

Cone milling is an extremely effective machine for size reduction. Material is fed into the cone mill through an in-feed chute. This can either be charged into the mill using a vacuum or gravity feed. The material passes to a rotating impeller which forces the material through the holes in the screen (without metal-to-metal contact). Once the material has passed through the screen, the finished product falls from the bottom of the mill to a receptacle beneath.

However, one common problem in cone milling, similar to all other forms of size reduction, is the generation of dust. The dust is formed during the grinding stage of milling, and can become air-borne if mishandled.

To protect both the operator and the process environment, it is often advisable to consider methods for keeping this dust contained. This is particularly pertinent to many modern processes, which use increasingly potent API’s or other potentially harmful substances. Often, the best solution is to completely contain the mill by means of housing it.

This can be achieved inside a transparent isolator, which completely contains the product (and any excess generated dust). The flexibility of a cone mill makes this easy to achieve, as tooling changes are simple to undertake, even when using isolator gloves. Hence the isolator continues to offer process flexibility to allow for screen/impeller changes and dismantling for cleaning.

Another important consideration is ATEX. By housing the mill inside an isolator, one can achieve ATEX requirements inside the isolator, even if the process room itself does not conform to ATEX. Hence, an isolator offers a simple method for achieving an ATEX process, without needing to adapt the entire process environment.

3. Integrating Containment Methods

Cone mill integration within the isolator is performed by means of through the wall fixing flange. This fixing flange and particular configuration of the cone mill allow for a physical division of the cone mill head by the technical area that is left outside the isolator. Thanks to this special configuration all cone mill cleaning operation are performed within the isolator by means of gloves or half-suit, reducing any risk of exposure for the operator and avoiding any transport to cleaning room.

For this reason, ergonomy within the isolator is one of the most important features, which should always be properly checked in a preliminary study at the design stage. Often, the best way to achieve the optimal design is a combination of technical drawings and prototypes. A prototype can be used for the simulation of all operations to be performed within the isolator (charging into mill, milling, discharging, weighting, cleaning and maintenance).

‘An isolator can be used to house an cone mill, for containment during milling.’

One of the foremost considerations when contemplating the use of an isolator is the product charging phase. From a containment perspective, it is important to understand where the product is coming from and in which kind of container (rigid, flexible, IBC etc.). in order to define a suitable isolator introducing method (pre-chamber, RTP, Split Valve, direct connection on process equipment etc.)

Proper planning at this stage will make the product charging/discharging phase easier. A proper connection system is required for the mill charging/discharging phase to reduce dust generation and increase product yield. Accurate weighting is also available at the cone mill discharging stage, by means of a custom designed and built packing-off station.

For specific product conditioning requirements, it is possible to integrate various milling techniques to achieve the desired outcome. For example, inerting of the mill head, or cryogenic milling with an individual control system for exhaust gas by means of dedicated ventilation. Sterile configuration of the system may also prove beneficial within the isolator. Sterilisation media by means of steam and VHP may also be considered.

Finally, an isolator can prove highly flexible, as it can be used for processes other than cone milling. It is possible to use the same isolator to integrate pin-mills (PSD 50-150 μm), Q-mills (PSD 20-100 μm) and spiral jet-mills (PSD 1-20 μm).

4. Conclusion

In conclusion, there are many considerations when cone milling a product. However, one of the foremost considerations is the issue of dust generation.

An isolator offers a simple and effective solution to this problem, by containing the dust within the isolator, ensuring it does not escape to atmosphere. Meanwhile, the isolator draws upon the flexibility offered by a cone mill, enabling quick and easy tooling changes.

Another advantage of an isolator is that it is an excellent means of achieving an explosion-proof process. The isolator itself creates an ATEX environment for milling, ensuring the entire process does not need to be changed to comply with the necessary requirements.

To integrate a cone mill with an isolator requires many variables to be considered. However, by properly evaluating the process, one can easily achieve an optimal process, which conforms to modern processing guidelines, simply by integrating these two pieces of highly flexible processing equipment.

A ‘Shared-Knowledge’ Article between Hanningfield (UK) and F.P.S. (Italy)