There is evidence that energy efficiency in comminution processes improves with impact velocity (Sadrai et al Minerals Engineering 24 (2011) 1053-1061). This paper claims that energy efficiency improves by factors of two or three if you can get impact velocities over 250 m/s. or close to sonic. Should comminution testing allow for impact velocity? Does a small mass accelerated to a high velocity give better better energy efficiency than a heavy mass accelerated to a low velocity but releasing the same energy input at impact?
Yes, I belive that the impact velocity certainly (Kinetic energy=1/2mv2) increases the efficency during the comminition process. We had some experience with HPGR performance using a die-press system. with increasing loading rate, the size reduction enhanced.However, increasing impact velocity needs a specific set up to test it.
Typically comminution testing does factor in impact velocities for grinding mills (velocity due to gravity and rotational velocity of the mill) however it is not directly measured, as the effect at the low velocities is probably insignificant.
Achieving high velocities can be difficult on the industrial scale as the cost and equipment needed to provide a robust solution addressing severe abrasion wear etc would probably out weight any savings. Starting particle size would also be an issue, as large rocks at 250 m/s have a lot of energy which has to go somewhere and the particle competency will greatly effect breakage characteristics.
Is there a particular problem or process you had in mind?
Yes, Justin. I've seen vortex grinding demonstrations: 20 mm particle to -53 micron, but no hard data. Do a Web search on Vortex grinding - I'd welcome your comments
old question but rather interesting! Yes, impact velocity is taken into consideration in comminution testing. but it seems that it does not have any significant effect on the efficiency of particle fragmentation. When doing single particle breakage tests, the work by Saeidi et al. (Minerals Engineering, Volume 100, January 2017, Pages 211-222, Investigating the effect of applied strain rate in a single breakage event) showed that the absorbed energy to cause fragmentation is not changed for different impact or compression velocities (rate at which the strain is applied) for the typical range of impact velocities used in comminution testing (from few mm/s to couple of m/s in drop weight like experiments). They found that the strain rate rather affects the classification during the breakage event, i.e. the re-capture of the fragmentation that will undergo further breakage. This is also consistent with my findings in Bonfils, B. et al. (The Effect Of Loading Rate In Single Impact Particle Breakage Using High Velocity Breakage Device, ESCC15, Sweden) where higher impact velocities were applied, but significant secondary breakage occurred due to the high kinetic energy transferred to the fragments at high impact velocities. More research is still required to decorrelate the energy used for primary fragmentation from the energy used in secondary breakage in the high-velocity breakage device.
Thanks Benjamin for the feedback - greatly welcomed. I recently received some feedback on this issue from Malcolm Powell. He suggested setting up a gas cannon to hurl individual particles at an anvil with velocities approaching Mach 1 and beyond.
There are plans to design and build such a device at the Aeronautical Engineering Department (Wits University, Joburg (Randall Paton). Some interesting comments were also recently made regarding the Veroliberator (Gregor Borg) with impact velocities up to 166 m/s. .Gregor's paper (presented at Comminution 2017) can be downloaded from the Research Gate site.
I have operated our gas gun up to 150 m/s with a hardened steel projectile of 200g or so, but nothing like Mach 1 or more! But it sounds very exciting. I have heard about the Veroliberator, but still waiting to see real experimental data on product size distribution and liberation characteristics.
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