For the finer specifications - d50's less than 3 microns - the M12 Spiral Jet Microniser (300 mm diameter) was chosen to match high specific surface area samples produced on test. At these finenesses a great deal of variability was not required so the simplicity of the Spiral Jet Microniser, with no moving parts, was perfect for the application.

Fineness can be affected however, by adjustment of milling pressure, typically between 4 and 8 bar g. Feed rate is also influential and jet rings can be substituted to vary the angle of the jet streams within the milling chamber, and nozzle diameters can be changed to influence air flow and impact velocity of the particles in the mill chamber.

Features of the jet mill system for this application:

• To avoid choking blockages in the inlet venturi the internal conical surface was highly polished



• To avoid risk of contamination, the transition from the feeder to the mill inlet was completely enclosed with filtered air inlets to provide a natural flow for the suction effect of the inlet venturi. This also helps to alleviate bridging



• The Microniser was specially designed with quick release clamps to be easily dismantled for cleaning, and internal surfaces were highly polished



• Clean filtered air to pharmaceutical standards at a supply pressure of 6 bar g was to be available at up to 400 Nm³/h

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.

To produce certain special products with adjusted particle size distribution curves an independent Dynamic Air Classifier capable of cutting powder flows below 10 microns was required. This was to both reduce fines in coarse products from the Dynamic classifier Mill and, to adjust the distribution shape of finer products by removing the coarsest particles.

Features of the Air Classifier:

• Fine classification zone, incorporating variable speed inverter controlled classifier wheel from GMP construction, with safety locked access door for cleaning and inspection



• Secondary vortex classification zone to separate coarse particles before presentation to the fine classifier fed with around 33% of the total air flow



• Pneumatic feed inlet to fluidise particles in the classifier to maximise separation efficiency

In all 3 lines the following features were incorporated:

• All incoming air entered each of the 3 lines through a pharmaceutical specification filter combination in a packaged stainless steel casing with easy cartridge replacement facility



• All product feeds and discharges were via rotary airlock valves with adjustable blades, special scrapers and air purged seals



• The ductwork had to be in easily-handled lengths with removable bends and all joints secured by quick release clamps



• Product Collection Filters with conical discharge sections and anti-static stainless steel weave, PTFE-coated, polyester needle-felt filter media



• PLC control system hardware with MODBUS interface as preferred by customer, programmed by Atritor



• Control panels supplied by local contractor to Atritor design



• The 3 line plant was designed, constructed, installed and commissioned by Atritor Limited of Coventry, England to the Quality Standards required by an ISO 9000 accredited supplier

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