Chemical Engineering November 2013 - 53

100
Cumulative breakage for CaO material
100
10
20
30
40
50
60
70
80
90
Initial
breakage
Fine
abrasion
Final
breakage
Medium
abrasion
Average
breakage
Fracture
Figure 16. Abrasion and fracture are
the dominant breakage mechanisms for
lime (CaO)
be determined by averaging the respective
B values. In addition, the average
fraction that shifts to create the finest
material can also be computed from the
B value data. This gives an indication of
the amount of fracture and abrasion the
material may be sensitive to (Figure 16).
10
20
30
40
50
60
70
80
90
50 100 150 200 250 300 350 400
Time, s
Bin 1 = 991 to 2584 μm
Bin 4 = 123 to 254 μm
Bin 7 = 16 to 33 μm
Bin 2 = 501 to 991 μm
Bin 5 = 66 to 123 μm
Bin 3 = 254 to 501 μm
Bin 6 = 33 to 66 μm
Figure 17.
Cumulative particlesize
distributions as
a function of time for
lime (CaO) subjected
to repeated impacts
at 5.4 ft/s impact velocities
Chemical
Eng.1-4pgKrytox Ad 2012:Layout 2 12/11/12 5:08 PM Page 1
Initially, almost 70% of the lime breakage
is due to fracture with only 10% due
to true abrasion events. However, after
60 s of milling, 33% is due to fracture and
28% is due to abrasion. Lime is sensitive
to both fracture and abrasion, but its
sensitivity is much greater for fracture
events. Now that the modes of breakage
have been quantitatively determined,
the optimal mill can be selected. In this
case, the optimal mill will be the one
where the fracture events induced in
the mill are occuring at about 1.5 times
the frequency of the abrasion events. Of
the mills considered in this article, the
impact mill rankings most closely match
the propensity for lime to break. Thus,
an impact-style mill is a good choice
for milling this material. It should be
pointed out that this is a major consideration
in the selection of a mill, but not
the only consideration. The tendency
of material to adhere to mill surfaces
should also factor in mill selection. The
exact impact mill presented above has
many moving parts that may be covered
with fine adhesive material. If lime material
is adhesive, then another impactstyle
mill, such as the air-jet impact mill,
may be a better choice.
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ChemiCal engineering www.Che.Com november 2013 53
Total breakage, %
Percent retained
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