Carefully conceived concept

Fine milling in compliance with Atex

Carefully conceived concept

Milling products is one matter. Finely milling products to a particle size of 75 µm in compliance with Atex is another. In order to ensure perfect Atex compliance when different products are milled finely, the overall concept must be conceived carefully .

A French pharmaceutical manufacturer had already validated a system for fine-milling granulated sugar (sucrose), but one final aspect was still lacking: Atex compliance. No one had imagined that they would have to take such an arduous path and resort to the services of an outside supplier. Granulated sugar was originally milled with a vibrating-sieve machine. In order to simplify the process and make it more reliable, a hammer mill was purchased from one of Frewitt’s competitors. The product to be milled was fed via a pneumatic conveyor and then filled manually. The system worked perfectly but the considerable manual handling was thought to be sub-optimal. The validation results were satisfactory yet the Atex compliance step caused problems. Realising the existing system could not fulfil the Atex standards, the manufacturer embarked on a search for an alternative solution and hit on Frewitt. Not only did the Frewitt solution conform to Atex, it also eliminated many manual processes.

A process expert from Frewitt visited the pharmaceutical manufacturer in order to obtain relevant, on-site information and also to analyse the existing system, the process and finally the product. The existing process was not meant to be completely revised; the aim was to find a way to enable the same product to be finely milled with the same sieve width, the same parameter settings and the same rotor tip speed. Various Frewitt milling processes were subsequently tried out. The Hammerwitt achieved similar results to the competitor’s hammer mill, while providing significant improvements in terms of granulometry and throughput. The first and most important step had been taken, namely defining the hammer mill as the size reduction process.

Next, the concept for the new process had to be developed. A lift and swivel column, a pneumatic conveying system and the Hammerwitt with the containers to be filled are all located in the Atex zone, whereas the switch cabinet and control panel are located outside the hazardous area. The milling chamber and the milling head area conform to Atex 3D (Zone 22).

The characteristics of the individual system components contributed to a substantial performance improvement throughout the system. The Hammerwitt made a major contribution by doubling the throughput and by using a 24-blade rotor (the competitor’s rotor is fitted with 16 blades) to mill a finer product (75 µm).

The Profi-Valve, another component, replaced the feed screw. The Profi-Valve reduces the risk of blockage to zero due to its special rotary valve design while the volumetric dosing capacity – explicitly matched to the capacity of the Hammerwitt milling chamber – provides a constant product feed. Finally, the Profi-Clean filter, which regulates the pressure in the milling chamber, ensures a constant filtered exhaust through the particle separator. It monitors and regulates the exhaust air to prevent the product from heating up, facilitating a constant, high throughput. The Profi-Clean system is equipped with an H13 filter which stops very fine dust or powder from escaping. The filter is continuously cleaned by an efficient back-blow system.

Making life easier for users was another important priority. The lift column can accommodate containers up to 2700 mm in height, so that capacities of 300, 600, 1200 and 2000 l can be filled quickly and safely in compliance with GMP regulations. The special mill design affords unparalleled flexibility, with only a few minutes needed to retool from the Hammerwitt hammer mill to the Turbowitt cylindrical mill. It is now child’s play to mill product A and then switch to deagglomerating product B.

The customer was thoroughly satisfied with the course of the entire project, from planning and engineering to installation and the FAT. The operator was delighted with the clarity of process monitoring as well as the simple handling and reliability. Despite the difficulty of milling sucrose (it is sticky and heat-sensitive with a tendency to smear), the mill runs without blockage and achieves the high, trouble-free throughput normally only associated with dry products.

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Roy Housh

Director of Sales, Frewitt