Communition

Question 1

What are the types of primary crushers?

Answer 1

~ jaw crusher (like a nut cracker)
~ rotary crusher
~ roll crusher

Question 2

How do ball mills work?

Answer 2

~ rotating cylinder partially filled with hard, wear-resistant media in the form of rods or balls

~ radius of mill should be large so teh media falls and impacts with more force

~ media should be heavy for more crushing force, and small to increase # of crushing points of contact

~ crushing is less efficient as particle size decreases, a point of diminishing returns is reached

~ critical speed of rotation is that required to carry media to the top of the mill before falling, mill is operated at 75% of this value

~ 50% of the volume is filled by media, keeping it from falling on the mill liner

~ 40% of mill is slurry with a solids content of 25-40vol%, must have enough powder so the media isn’t grinding on itself

~ deflocculant is added, a chemical that gives particle surfaces like charges so they repel in aqueous suspension, used since agglomeration would otherwise keep media from crushing individual particles

Question 3

What is the emperical law for the rate of milling for ball mills?

Answer 3

~ rate = (csqrt(a)p*d)/r

~ c: constant
~ sqrt(a): radius of mill
~ density of media
~ d: particle size
~ r: radius of the media

Question 4

What is a vibratory mill?

Answer 4

~ impact energy is much greater than a ball mill; finer media can be used

~ much more rapid communition than ball mixing

Question 5

What is a planetary mill?

Answer 5

~ media flung about by centripital force

Question 6

What is an attrition mill?

Answer 6

~ suspension of particles and 3-10mm media vigorously stirred (20-30revs/sec)

~ intense impacts via differential velocity of media moving around the agirator arm into the cavity the trailing edge

~ significant heating occurs, water cooling is required

Question 7

What is jet (fluid energy) mill?

Answer 7

~ mutual impacts and attrition between high speed particles entrained in a gas stream moving in a circular orbit

~ oversize particles remain in a peripheral grinding zone, fines are drawn off in a central collector

~ mills are designed so that particles are kept away from the wall, minimizing contamination

Question 8

How do we eliminate contamination?

Answer 8

~ having milling media and container being the same composition

Question 9

What are milled powders?

Answer 9

~ log-normal

Question 10

What is particle size distribution, narrowest to widest?

Answer 10

~ jet mill–>attrition mill–> vibratory mill–> ball mill