Unlike fracture which is breakage or failure of a material system at one point comminution involves continual breakage until specific size is reached.
Material fracture studies are usually applied to structural engineering problems. Fracture mechanics gives engineers a number beyond which a material will fail to perform its role and consequently result in failure of the component.
In comminution however it is required to break the material into particles of desired size.This seems simple enough but the process is barely efficient why?
[Losses Kinetic, acoustic, thermal ,wear resistance due to debris, no leverage]
Firstly the amount of losses in a mill performing comminution are significant and unavoidable these machines make a lot of noise leading to acoustic losses, they vibrate ,generate heat.
Many mills especially the ones which rely on impact with the different materials like hammer, ball, vibratory loose even more energy in kinetic collisions
Balls, hammers collide with material and instead of transmitting all the impact energy to the particles, rebound, creating losses
Another cause of inefficiency is that as size reduces the defects in particle becomes smaller or vanish increasing their strength and thus energy required to break them.
As surface area of the particles increases cohesive forces begin to take over and lump them into aggregates making size reduction difficult. Most importantly though for smaller particles it is difficult to effectively apply force
Because of reduced lengths smaller particles can only be compressed, bending is not possible ,no leverage, no length multipliers (T=fxl) are available. Bulk modulii are typically several tens of GPA and may not be easily overcome in impact conditions.
Then there are limitations imposed by machines. These machines have to use large forces to mill the material but ensure that they themselves are not broken down as a consequence impact velocities have to decrease and regular up keep of machine components have to be done
The maximum amount of energy that a material can supply is its own elastic/ plastic energy.Beyond this both the impactor and the material housing the mill will break.
Friction is an effective tool to break down surfaces. Pressures in GPA range are routinely reached due to small asperity contact but here too limitations exist. Once the surface has been broken it acts as a powder lubricant preventing further break down by reducing friction.
As things stand comminution is a highly energy and material intensive process not only is high energy required to mill but regular replacement of milling components need to be done.
The actual numbers in terms of power requirement would depend upon the material to be milled and the size to which it needs to be broken down. Typically kilowatt hours to mega watt hours in power is needed for 100 micrometer to 1 micrometer/nanometer range
Notes and questions
[Strong vs tough; strong=resist deformation,tough=resist fracture]
Could Jet Mills prove to be effective in breaking down The particles below micrometer size?
Jet Mills are quite good and milling particles to less than 10 micrometer diameter by impacting them against one another at high velocities.
size reduction is achieved by both frictional wear and impact breakdown in addition to fatigue
they used compressed air as input and are promising alternative to regular milling tech
.
Problems :limited feed size , can only work with smaller particles so the feed stock needs to be preprocessed by some other mill significantly reducing advantages.
Is it possible to design an efficient mill with common materials?
Is it possible to reduce milling energy needed to achieve particle size reduction?
How can we begin to optimize milling?
What parameters need to be optimized?
Do we need a general purpose mill or a mil to break down specific material?
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