Milling heat sensitive powders can often present difficulties in processing and production within our industry. Often, heat is generated during conical milling within the gearbox of the mill and the screen located in the machine housing. The temperature of these parts can rise as they run for long periods of time. The effect of heat generation on the material during milling can result in damaged tooling, ruined batches and, in severe cases, can even cause an explosion.
Heat generation however, can be closely monitored and controlled with the appropriate steps and management methods. This can minimise the risk to the product and most importantly to the operator. Hanningfield offer a number of innovative solutions to enable our customers to mill heat-sensitive materials with confidence.
The Problem(s)
Product Quality
Heat generation during the milling process can have a detrimental effect on product quality and destroy batches of highly valuable APIs. Materials containing high levels of fats or oils often prove to be highly heat sensitive. Thus, even a small temperature increase during processing can have a huge effect on overall outcomes. Hence, it is essential to keep temperature rises as low as possible or even to remove this variable altogether during production.
Tooling
Severe damage can be caused to the processing equipment if the incorrect tooling is selected during the milling of heat sensitive materials. For example, screens can break and impellers can become impaired. In severe cases, a blinded screen can cause damage to the gearbox as the motor works increasingly harder as it attempts to mill the product. It is crucial that material passes through the screen easily and quickly, while still achieving the PSD the customer requires. If material remains in the screen too long, it is exposed to more heat, which increases with every revolution of the impeller.
The Solution
We offer a number of innovative solutions to the milling of heat sensitive products.
Choosing the Correct Screen
One of the first steps when milling this type of material is to ensure the selection of a mill screen with the correct hole size. This ensures the product passes quickly through the screen. Not only does this give the customer a better particle size, tighter PSD and less fines, but also reduces the chance the particle will be affected by heat from remaining in the screen too long. Our experts are on hand to guide you through selecting the correct conical mill screen for your specific process. We are also able to hold in-house trials on customer supplied product to ascertain the correct tooling prior to any order being placed. This ensures the ‘trial and error’ stage is not carried out on your time and expense.
Considering the Type of Conical Mill
A second consideration which may be suggested based on your application or following trials, is the use of an over-driven conical mill instead of its under-driven counterpart. As the impeller of an over driven mill is powered by a belt drive from above, there is little to no heat generation to the screen or impeller.
For customers with more heat sensitive products, or products that have the potential to explode, there are some additional options available.
Addition of a ‘Cooling Jacket’
The addition of a ‘cooling jacket’ to the outside of the machine housing can be retrofitted and provides a constant stream of cold water which is passed through the jacket. This brings the standing temperature of the machine housing down and keeps it low even during the milling process. The temperature of the water can be changed with ease to bring the machine housing down to an acceptable level where the customer finds the most optimum milling conditions. A cooling jacket should ensure the screen and impeller do not rise in temperature, and as long as the water temperature can be changed, the housing can be cooled in line with any rises in temperature during the milling process.
Liquid Nitrogen (LN2)
The use of liquid nitrogen is predominantly for Cryogenics and Preserving Product Quality. 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.
Inclusion of a Temperature Probe
The addition of a temperature probe to the machine housing can also provide customers more confidence when milling heat sensitive products as it offers increased control over any temperature increases. A temperature probe can be fitted against the screen and/or gearbox. This delivers a cost effective method to easily monitor temperature rises which may affect product quality or operator safety. When the surface temperature of the screen/gearbox reaches a point which is considered potentially harmful or dangerous, the machine will cut out and the impeller will stop within three seconds. When a product is highly sensitive, even the smallest amount of time can cause permanent damage, thus this fast reaction time is crucial. At Hanningfield, we understand that each process is unique and so our customers can request the pre-set cut out temperature of the probe, providing greater control over individual applications.
Conclusion
The milling of heat sensitive materials can often present many difficulties in processing and production within our industry. Heat generation however, can be closely monitored and controlled with the appropriate steps and management methods. This can minimise the risk to the product and most importantly, to the operator.
We are able to work closely with each customer, providing a bespoke solution to their process. This can ensure a working system that is both highly productive and safe. This in turn, provides our customers with increased confidence when milling heat sensitive materials.
Get in Touch
If you would like to speak to one of our experts regarding our Cone Mills or would like a quote on one of our other products, send us a message using our Contact Form or call us on +44 (0)1702 549 777.
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