Explosion-safe Milling System for API Small Particles and High Safety

Author / Editor: Matthias Honisch* / Manja Wühr

Due to their high fineness and throughput hammer mills are enormously popular. But if production calls for explosion-safe processing in a cleanroom special technical Know-how must be assured.

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Hammer Witt achieves fine milling down to 30 µm with high throughput.
Hammer Witt achieves fine milling down to 30 µm with high throughput.
(Picture: Frewitt)

The basics of explosion protection apply not only to refineries and paint manufacturers in terms of hazardous explosive areas, milling operations are also concerned. Hammer mills users find themselves on the front line of explosion prevention, as the powders must be very finely milled using high RPM's.

Frewitt customers are knowledgeable of the risk factors, and that's why they've chosen for their process engineering Frewitt hammer mills — fine milling down to 30 µm with high throughput. One of these customers, a manufacturer of drugs to treat coronary pathologies, vascular disorders as well as cancer, has formerly milled its pharmaceutical active ingredients on the hammer mill of well-known Frewitt competitors.


Due to new requirements of safety, and efficiency, the drug manufacturer needed to modernize his operations, move his production of active pharmaceutical ingredient (API) to a cleanroom; hence a milling system fulfilling the increased requirements for process safety and efficiency was necessary to procure.

Not just any hammer mill was required — a hammer mill which could meet all the guidelines and could produce in a clean room with high throughput, was the only tool that would allow the project to succeed. The pharmaceutical manufacturer could not find this with it former, well-known suppliers; it looked elsewhere for a viable solution, and found it in Frewitt's Hammer Witt.


The pharmaceutical manufacturer was required to mill a heat-sensitive product, achieve a particle size of 100 µm, while ensuring a high throughput. In order to prevent the formation of an ignitable mixture during the conveying, dosing, milling and filling processes, the process environment room had to be inerted.

The most important measures employed to prevent explosions in certain areas are to inert the system, monitor oxygen levels, temperature and to regulate nitrogen addition. Having these measures, in place and monitored: a leak could quickly recognized, and the required protective actions could be taken.

In addition to the aforementioned safety precautions, the calculation of the highest allowable oxygen level is paramount. In this case, it was three percent, and this level was programmed as the alarm threshold. If the alarm threshold level were to be reached: the machine would stop in order to prevent a potential explosion. Only if the oxygen concentration returned under the non-critical threshold of three percent the milling process could be continued.


The process is summarily described as follows: The product is conveyed via a vacuum system into an inerted atmosphere. The feed funnel on the Hammerwitt milling system must be continuously coated with the product. The product moves from the inerted feed funnel into the inerted milling chamber where it is milled. The milled material is then removed with a dosing system, and gravity fed into a container where it is precisely weighed, and thereafter packed.

The solution which was discussed in meetings and implemented by Frewitt is comprised of different elements, which together ensure the explosion-safe conveyance, dosing, milling and filling of the product. A level sensor is placed in the feed funnel of the vacuum conveyor, monitoring product feed and regulates it in order to ensure a continuous process. In order to keep the milling chamber temperature at a constant, cool temperature: the milling housing is equipped with a water-cooled system. The temperature of the milling chamber, as well as the drive bearings, is constantly monitored.


The oxygen concentration level, with a programmed threshold of three percent, is monitored from the product feed to the final filling of the milled product. The alarm threshold of three percent is programmed, and held constant under the pre-set value of three percent. This maintains the milling chamber at Atex II G/D (Zone 0/20) and the area of the milling head at Atex II 2G/D (Zone 1/21).

Monitoring and regulating the exhaust air ensures that the product is not heated, hence allowing for a constant, high throughput. The Profi-Clean 250 system, equipped with an H13 filter, prevents even the finest dust or powder leakage from the system. The filter is continuously cleaned by an efficient, blow-back system.

Bag-changings is done with special attention. When the bag reaches the prescribed weight; a valve stops the product flow. The remaining nitrogen is aspirated from the bag. Thereafter, the Profi-Bant bag holder is disengaged, the filled bag removed, and a replacement bag is installed. The system remains inerted during the bag exchange.

Saving Raw Material

This inerting design is not only technically complete (fulfilling GMP standards, Atex guidelines and all rules of the country-specific Institution of Quality), it also has considerable economic advantages. Continuous monitoring of oxygen contents reduces the use of nitrogen, a very expensive raw material, to a minimum.

* The author is Regional Sales Manager at Frewitt, Granges-Paccot/Switzerland.