How to Decide: Which Mill System Should I Buy?

With the large variety of milling equipment on the market, it can be difficult to know which type of mill is appropriate for your process. The basic mechanical process of milling, also known as grinding, granulating, comminuting, etc., is generally used either:

to break apart clumps or agglomerates– that may have developed during storage, transportation, or handling of a given material without altering the average/mean size of the particles which make up the material, a process often referred to as deagglomeration;
to reduce the average/mean size of the particles in a given material, also known as particle size reduction.
Because of the relatively simple nature of these two processes, as well as the range of industries in which they are employed, milling machines can differ dramatically in terms of their fundamental principles of operation; this is especially true of size reduction machines. Jet mills, impact mills, screening mills, cone mills, ball mills and hammer mills are just a few of the different operational classes of mills available, and if you’re unfamiliar with the terminology involved, this can be especially confusing. Here are a few things to look for to help you determine which mill might be best for your application.

1. Versatility

If you’re looking to purchase a new mill, chances are good that you already know exactly which types of material you’re going to be processing with it. However, given the high cost of pharmaceutical-grade equipment, even mills, it is imperative to consider the future potential of any new piece of equipment. What projects might you take on in the next 5-10 years? If you plan to use the mill for a new R&D application, will you still have a use for it in the future, after the R&D stage is complete? Will you require particle sizing as well as deagglomeration capabilities?

Certain systems, like Fluid Air’s GRANUMILL® Jr., are classified under the FDA’s SUPAC (Scale-Up and Post-Approval Changes) guidelines as both a hammer-class mill (Fitzmill®) and a screening mill (Quadro Comil®). This means that the machine is suitable for both high-speed fine grinding for size reduction as well as low-speed screening for simple deagglomeration. Whatever your current requirements are, be sure to look for a system that won’t limit you to any one project, if possible.

2. Scale-up

If you’re purchasing a unit for an R&D application, do you plan to scale up your current process? Certain rotor speeds, tip speeds, blade types and other parameters that are effective for small-scale processing may not be feasible at a pilot or small production scale. Check to see if these parameters are scalable for larger batch sizes. If you already own a large-scale mill system and are looking for an R&D unit, determine whether the specifications of the smaller system will be readily scalable to meet your existing system. Even if you do not anticipate the need to scale-up now, it is worth investigating beforehand instead of scrambling to find a solution later.

3. Safety/exposure needs

Make sure that the equipment is able to meet any occupational exposure limits to ensure safe levels of operator exposure, even if you do not intend to process hazardous materials/APIs. Some manufacturers offer optional containment enclosures that can be removed when they are not in use. For explosion-critical applications, nitrogen gas purging and cryogenic milling with liquid nitrogen injection systems are two other options to look for.

Those processing volatile compounds should ensure that any new unit is equipped with the appropriate safety mechanisms and constructed to prevent, withstand, or mitigate any potential explosions that may occur.

4. Sanitation/cleaning requirements

Is the system compliant with your cleaning requirements? Many machines, especially R&D systems with small parts, can be difficult or impossible to adequately clean. Milling systems equipped with quick-release housing clamps are ideal for applications where sanitation is critical, as they allow the entire system to be broken down and submerged in a cleaning tank, greatly simplifying the cleaning process and reducing maintenance time. Aseptic processing using liquid nitrogen is another option for applications where sterility is critical. At a minimum, check that all of the system’s product-contact surfaces can be readily accessed and inspected to ensure thorough cleanliness.