
Nitrogen is widely used in milling applications within the pharmaceutical industry, predominantly for inerting, product quality and cryogenics. It is selected as the ‘gas of choice’ due to its excellent properties perfectly aligning it with the aforementioned uses.
Some basic facts about nitrogen;
> Nitrogen makes up the majority of the air we breathe
> It is colourless and odourless
> Does not support combustion
> Does not support life (hence its use to keep food fresh – like within crisp packets)
> Has a boiling point of -196°C, and freezes at -210°C








Inerting
Nitrogen generally does not support combustion by replacing the oxygen within a given space. With a potential liquid to gas expansion ration of almost 1:700, it will quickly replace the air present within a given space. For milling applications which require explosion proofing (E.g. ATEX, Class and Division or IECEx), nitrogen is a commonly used method of inerting the milling zone to reduce the potential for an explosion. Milling of materials will generally cause the material and contact parts (gearbox, screen) to increase in temperature, and although very unlikely there is always the potential for a spark from metal-to-metal contact (the Hanningfield Under-Driven conical mill range have a factory set gap between the screen and impeller, and the Over-Driven range are supplied with spacers, which ensure metal to metal contact does not occur).
Nitrogen inerting however adds an extra layer of protection to help eliminate all possible ignition sources. Generally the nitrogen is fed into the milling chamber via nitrogen ports at the desired pressure and amount. A variety of different components can be used alongside nitrogen ports, such as temperature probes, oxygen analysers, pressure relief valves and pressure regulators, all used to help guarantee a safe and steady flow of nitrogen into the mill.
Product Integrity
Nitrogen is often used to help preserve food and protect materials which may degrade when in contact with oxygen. As such, entire processes may be blanketed with nitrogen to protect the material throughout until it is eventually packed. In milling, it is common to use an oxygen analyser to measure the level of oxygen within the milling chamber, and once the oxygen drops to an acceptably low level, the material can be charged for milling.
Cryogenics
Nitrogen is the most commonly used gas for cryogenic freezing of material before milling. Nitrogen remains as a gas until -196°C enabling the user to supply a constant flow of extremely cold gas to freeze materials before milling. Materials that have high fat content or plant materials are excellent examples where nitrogen can greatly improve the milling operation. This works by bringing the material far away from a temperature where it may melt or release oils, by embrittling products that are waxy, or freeze drying plant materials containing a high moisture content.
Cryogenic milling is widely employed within the pharmaceutical cannabis industry, where cannabis exhibits many material properties which can create problems during milling, including moisture content, oil content and fibrosity. In cannabis milling applications it is common to apply nitrogen in its liquid form, sometimes even ‘nitrogen bathing’ the material before milling. This is due to the size of the infeed material, where the worse the surface area of the material the longer it will take to freeze.
Cryogenic freezing of material can also help to protect material which degrades when it is exposed to heat increase. Materials that contain proteins may often be processed using nitrogen to prevent the proteins denaturing when exposed to the heat of milling.









Author: Alexander Ellis, Sales Manager
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Hanningfield have experience of a wide range of nitrogen applications, including use in milling and vacuum conveying applications.
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