Cone mills don’t process every type of powder in exactly the same way, and most of that is down to the properties of the material being fed into the mill.
While one material can be fed easily into a mill and clear the screen as soon as the particles reach the right size, another may start sticking to the tooling or binding.
These differences add up, increasing the frequency with which production has to be stopped so mills can be cleaned.
While ‘cohesive’ and ‘free flowing’ are useful terms, there are other factors that affect the behavior of these materials. Any good powder process starts by understanding said behaviors before selecting the type of mill set-up, especially if the powders are cohesive.
We’ll explain here how the flow of powder can affect cone milling as well as show you what should be examined when a process becomes unstable.
A free-flowing powder is just that, one which moves easily under its own gravity because the particles move around with relatively little resistance. This behavior means it's more likely to leave the conical mill screen once it's reached the required size.
More cohesive powders hold their shape, cling together and will resist movement through a screen even if those particles are small enough to pass through it.
The best way to separate them is to simply look at how they behave. If the powder pours consistently and forms an arc as it's poured, it's free-flowing. If it compacts during storage and clings to surfaces, it's likely cohesive.
Interestingly, fine particles have a higher surface area relative to their mass, which means that surface forces have more of an influence on their movement. This can reduce the speed with which material flows around and then out of the screen.
Particles that are irregular or plate-like may interlock with greater ease and be harder to move out of a chosen screen. The shape of these particles, then, can have a direct impact on feeding and packing even when they have a similar average size.
Increased moisture can also affect the flow. An increase can cause adhesion to parts of the mill, creating soft agglomerates in even reliably dry powders.
While a cone mill is in operation, the rotating impeller moves the fed material across a perforated screen, which could be square, grated, or round-holed in design. Any particles that are smaller than the aperture of the screen will pass through while larger materials remain there until they’ve been reduced.
Free-flowing materials typically reach the screen and then leave it via the holes fairly readily, which is ideal if the goal is to stabilize capacity and ensure as little time as possible in the chamber.
By contrast, cohesive materials may enter the chamber unevenly and travel as masses, sticking to the screen surface as they move around. This clinginess not only increases how long the materials remain in the chamber but also makes the particle size distribution (PSD) less consistent.
It can also increase the temperature and the number of fines present on the screen wall. All this is why engineers should consider the flow behaviors of a powder alongside the target PSD, as the mill must, at the same time, reduce the material and allow the correctly sized particles to leave efficiently.
The characteristics of round hole screens mean they offer a useful balance of capacity and PSD control and are preferred by engineers because they limit the amount of fines found on the screen. They are used in several procedures, including dry milling and blending.
The screen’s hole size also has a big influence on particles and capacity. Holes with larger apertures allow coarser products to come through, while smaller openings create finer particles and may make processing take longer.
When powders already move with ease, the aim is to keep that consistent flow going without applying any more milling energy than is needed or required by the PSD specification.
Smaller hole screens may not always be the best choice when small particles are needed from the milling process because materials with a high level of moisture may quickly coat the mill before it can eject the correctly sized material.
Screens that have larger apertures can help wet or more cohesive materials pass through without too much obstruction. In our experience, we often choose square-hole screens as the most appropriate choice for wet milling because their high open areas reduce the levels of binding.
Raised cutting edges found in grater-style screens are better for harder materials that need a more proactive cutting action over a passive mill procedure.
Ultimately, the choice of screen remains specific to its intended purpose. Factors like stickiness and target PSD should all be considered rather than relying on a general assumption that all cohesive powders need the same screen.
While the screen does decide which particles leave the mill, the impeller has a big say in this also. Its speed and profile impact how the material is presented to the screen.
Impellers for free-flowing powders are often designed to move the material evenly across the screen without reducing or causing any costly breakages to particles that are already the right size. Cohesive materials need a more proactive impetus so that they don’t accumulate in the chamber.
This accumulation, however, isn’t always solved by just increasing the speed of the impeller. While a faster impeller increases how quickly the particles break up, this same speed can also increase fines and temperatures. If the product is prone to being sticky as the temperature rises, then the additional speed may actually worsen the obstruction.
The clearance of the impeller along with its profile and the feed condition should all be reviewed together rather than assuming it is one element causing the entire problem.
The main goal of deagglomeration is not to grind every particle down to a smaller size but to break up unwanted clusters so the feed is more consistent and can be easily used in the next process stage.
Removing those clumps of agglomerate ensures the powder is uniform and helps in later processes such as tableting. However, over-milling can undermine the improvement if the knock-on effect is an increase in fine fraction or changes in the PSD beyond the required formulation.
Quadro’s cone mills are used for size reduction and deagglomeration, with the components - screen and impeller, for example - controlling how the materials are broken and classified.
The best results ultimately depend on the condition the powder needs to be in for the downstream process, not the most aggressive size reduction the mill can achieve.
Cohesive and free-flowing powders are very different in how they are fed into and move across the screen. While the latter flows more evenly, the former often needs closer attention paid to it.
No matter the issue currently being faced, engineers must first identify where the restriction occurs before changing a single parameter.
The most dependable configuration is found by testing the material against desired PSDs and downstream performance rather than relying solely on the powder type.