Nitrogen is widely used in milling applications within the pharmaceutical industry. The use of liquid nitrogen is predominantly for Inerting, Cryogenics and Preserving Product Quality. Hanningfield has been successful in demonstrating the effectiveness of cryogenic milling with a number of pharmaceutical and related companies.
Thermolabile materials are difficult to mill at ambient temperatures. They tend to soften, adhere in lumpy masses and clog screens under normal conditions. These properties are milling averse and prevent easy particle size reduction. Milling with Nitrogen can offer huge benefits to manufacturers when attempting to mill such materials.
The chosen material is frozen or cooled down to their point of friability using liquid nitrogen. Cryogenic cooling can deliver extremely low temperatures. Liquid nitrogen is the most ideal for cryogenic freezing of material before milling because it can reach near -320°F (-195°C), enabling the user to supply a constant flow of extremely cold liquid to freeze materials before conical milling.
Nitrogen can also be utilised to help preserve food and protect raw materials which can degrade or decompose if at any point they are brought into contact with oxygen. Processed items can be covered with nitrogen to protect the material throughout its milling and processing until it is packaged.
For example, materials containing proteins may often be processed using nitrogen to prevent the fat, oil and protein denaturing when exposed to the heat of milling. The loss of essential oils such as in expensive spices and medicinal herbs can be minimised during the milling process with the utilisation of liquid nitrogen. Flushing the packaging in so-called MAP modified atmospheric packaging equipment is now commonplace.
During the milling process, an oxygen analyser can be utilised to measure the level of oxygen within the milling chamber. Once the oxygen drops to an acceptably low level, the material can be charged for milling. For critical applications, the level of oxygen can be controlled by continuously monitoring oxygen levels in the milling system and modulating the entry of nitrogen to maintain the desired level. This process is highly effective and entirely automated.
The use of liquid nitrogen to inert a milling zone is a common method used to reduce the potential risk of an explosion. Size reducing materials can sometimes cause the material or mill contact parts to increase in temperature. In addition to this, there is also the unlikely chance for a potential spark from metal-to-metal contact.
(Note: the Hanningfield Under-Driven conical mill has a factory-set gap between the screen and impeller. The Over-Driven conical mill is supplied with spacers. Both of which ensures metal-to-metal contact does not occur).
Nitrogen inerting adds an added layer of protection to help remove all possible ignition sources. In typical applications, the inert nitrogen is fed into the milling chamber via nitrogen ports at the required pressure and volume. A number of additional components can be utilised at the same time including oxygen analysers, valves, temperature probes, pressure regulators and vents. These all aim to maintain safe and optimal levels of oxygen within the milling chamber.
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