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a linear relationship to the number of CSP particles
down to about 1 mm. Most CSP particles smaller
than 1 mm will dissolve in the glass melt. The prob-
lem CSP fraction measures from 1 mm to around 10
mm: for 90% removal of this fraction, the sensor has
to have a minimum resolution of 0.4 mm. This
means that the laser diodes must have a diameter of
0.4 mm, or that the camera in a CSP sorter has to
have a resolution of 0.4 mm. This is possible with a
camera sensor but rejects will increase in line with
the number of CSP particles . Also,
more glass will be rejected because of optical effects.
Table 1
shows the size distribution of CSP
and confirms that the 10-55 mm fraction accounts
for 70% by weight of the CSP. It is also clear that
the majority of CSP particles making up the
remaining 30% by weight fall into the 1-8 mm size
range where CSP is difficult be detected by lasers or
by camera sensors. Although constituting only 30%
by weight, CSP particles measuring between 1 and
8 mm make up 90% of all CSP particles in terms of
number (see Table 1).
Crushing
Clearly, it is necessary to crush the glass to liber-
ate corks, foils, etc. and also to meet the specifica-
tions laid down by the glass industry. But that same
crushing operation generates more particles and
thus more CSP particles; the result when compared
to the feed is even worse because CSP is more brittle
than glass. If we assume an average wall thickness of
4 mm for packaging glass and the size distribution of
Figure 5, we can generate Table 1. A single theoreti-
cal particle of 4 x 10 x 10 mm weighs around 1 gram.
From this theoretical example of CSP distribu-
tion, it is clear that, given a detection limit of 8 to 10
mm, over-grinding of the feed has a disastrous effect
on the number of particles smaller than 8 mm. Poor
materials handling in the processing plant has the
same impact on size distribution as crushing; for
example, discharging from one transport belt to
another will lead to further size reduction and more
fines will be generated.
With jaw and roll crushers, the gap and setting
can be altered mechanically to control size distribu-
tion. Most recycling plants are not designed to avoid
size reduction of the glass. During collection of the
glass, transportation and processing, a further 10-
15% fines (-10 mm) can be generated.
Processing the 1-10 mm fraction
To avoid production problems in the glassworks,
the fraction measuring between 1 and 10 mm can be
separated by screening. This fraction represents 10-
20% of the end product for which another applica-
tion has to be found. The value of glass cullet is gen-
erally around € 40-45 per tonne and landfilling costs
are € 20-100 per tonne, depending on the location.
The 1-10 mm fraction can be reduced by
impact crushing to a size smaller than 1 mm
. Screening to a size of 1 mm is an
expensive operation because a large screen surface
is needed and wear of the screen decks caused by
fine glass is considerable, to say the least. The glass
powder measuring less than 1 mm is mixed with
the 10-55 mm fraction and fed into the furnace. The
Figure 6
Figure 5
Recycling International • April 2005 50
d50
Ca 30 %
Ca 5 %
CSP after comminution Glass after comminution
N
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o
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d50
N
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CSP
100
75
50
25
0
0 10 20 30 40 50
Size (mm)
W
ei
g
h
t-
%
CSP
100
75
50
25
0
0 10 20 30 40 50
Size (mm)
W
ei
g
h
t-
%
Figure 5 Size distribution of CSP
size (mm)
0%
100%
1 10 100
GRINDING
Cum. Undersize (%)
Figure 6 Reducing the 1-10 mm fraction
Table 1
Size glass Average Weight % 1 kg Particles in
size mm CSP gram size glass
40 – 55 mm 47.5 2 20 1
30 – 40 mm 35 8 80 6
20 – 30 mm 25 15 150 23
10 – 20 mm 15 25 250 110
6 – 10 mm 8 25 250 400
4 – 6 mm 5 12 120 500
-4 mm – 13 130
100 1000 1040
T E C H N O L O G Y
