Aggregate Production
SCREENING
Screens are use to:
Separate material into
particle size ranges.
To scalp off oversize rock
To screen out fines.
SCREENING
The screening process is based
upon the premise that:
Particle sizes smaller than
the screen cloth opening size will
pass through the screen.
Oversize pieces will be
retained on the screen.
SCREENING
Screens consists of 2, 3, or 4 decks
(layers) of open mesh screen wire cloth mounted one above the other in a rectangular box.
MESH
The term “mesh”
refers to the number
of openings per lineal
inch. To count the
number of openings
to an inch, measure
from the center of a
wire to a point one
inch distance.
MESH
The term “mesh” refers to the
number of openings per lineal in.
CLEAR OPENING
Clear opening
or “space” is the
distance
between the
inside edges of
two parallel
wires.
CLEAR SQUARE &
MESH OPENING
These screen openings are the
most commonly accepted in the
industry.
FLAT TOP SQUARE
OPENING
Flat top cloth is used as a subscreen
which supports a finer wire cloth.
SCREEN CAPACITY
Capacity is the rate in tons per hr
that 1 sf of screen produces
(passes) of the desired size.
Capacity varies with many factors.
Desired efficiency
Deck
Aggregate-size
SCREEN CAPACITY
Capacity, Fig. 14.18.
Efficiency, Table 14.7
Deck, Table 14.8
Aggregate-size, Table 14.9
SCREEN CAPACITY
The minimum screen
area to provide a
given capacity is
determined by
Equation 14.5, page 436.
SCREENING
EXAMPLE
The product from a 36- by
42-in. jaw crusher is
passed over a 2½ and a
1½ in. opening vibrating
screen.
CAPACITY OF
CRUSHER
42- by 48-in jaw crusher
Closed setting of 3 in.
Capacity of crusher
Table 14. 3, page 413
190 tph
CAPACITY ADJUSTED
Capacity of crusher 190 tph
Stone unit weight of 110 lb/cf
11cflb100
cflb110AdjustmentWeightUnit .
/
/
190 tph 1.1 = 209 tph
Fig. 14.5, read down the 3” line to the size.
SIZE DISTRIBUTION
Percent
passing
is on
the left
side of
the
figure.
QUANTITY
Output of crusher, 209 tph
Size,
in.
%
passing
% in
range
Total
output
Produced
tph
Over 2½ 100-73 27 209 56.4
2½- 1½ 73-46 27 209 56.4
1½- 0 46-0 46 209 96.2
Total 100 209
SCREENING EXAMPLE
Output of 42 X 48-in. jaw crusher.
Size,
in.
Produced
tph
Over 2_ 56.4
2_- 1_ 56.4
1_- 0 96.2
209
SCREENING EXAMPLE
The permissible screen
efficiency is 85%. Determine
the minimum-size screen, in
sq ft, required to handle the
crusher output. The screening
unit is 4 ft wide.
SCREEN WIDTH
STEP 1. QUANTITY
The quantity that will strike the 2 1/2 in. screen is the total output of the crusher:
209 tph
STEP 1. QUANTITY
If the 2½ in. screen was 100%
efficient (56.4 + 96.2) 152.6 tph would pass through to the 1½ in. screen.
Size,
in.
Produced
tph
21/2 - 11/2 56.4
11/2 - 0 96.2
152.6
STEP 1. QUANTITY
But the 2½ in. screen has an 85% efficient,
therefore only 129.7 tph strike the 1½ in. screen.
tph7129856152 .%.
STEP 2. SCREEN
CAPACITY
Screen capacity, Fig. 14.18
2½ inch -
4.2×1.1= 4.6 tph per sf
1½ inch -
3.4×1.1= 3.7 tph per sf
STEP 3. EFFICIENCY
Efficiency factors, Table
14.7
The permissible screen
efficiency is 85%.
Efficiency factor - 1.5
STEP 4. DECK
Deck factors, Table 14.8
2½ inch - Deck 1
Deck factor 1.00
1½ inch - Deck 2
Deck factor 0.90
STEP 4 -
AGGREGATE-SIZE
Aggregate-size factors, Table 14.9
Need % of aggregate less than 1/2
the size of the screen opening.
One-half size for 2 1/2 inch screen:
...
in12
openingscreenin524
1
STEP 4.
AGGREGATE-SIZE
One-half size for 2½ inch
screen is 1¼ in.
Figure 14.5 percent passing
1¼ in., for 3 in. closed
setting:
= 39%
STEP 4.
AGGREGATE-SIZE
Percent passing 1¼ in.,
39%
Quantity less than 1¼ in.:
209 tph 0.39 = 81.51 tph
STEP 4.
AGGREGATE-SIZE
%..
390tph209
tph5181
Percent of aggregate less
than one-half size for 2½ in.
screen.
In this case it was obvious, lets go on to the 1½ in. screen.
STEP 4.
AGGREGATE-SIZE
Aggregate size factor for
2½ inch screen with 39%
passing 1¼ in. Table 14.9
980910
200800 .
..
STEP 4.
AGGREGATE-SIZE
One-half size for 1½ inch screen:
...
in2
openingscreenin514
3
Figure 14.5, percent passing ¾ in., for 3 in. closed setting:
26%
STEP 4.
AGGREGATE-SIZE
Percent passing ¾ in., 26%
Quantity: 209 tph 0.26 = 54.3
However, this must be adjusted
because of the 85% efficiency of
the 2½ in. screen.
STEP 4.
AGGREGATE-SIZE
Adjusted because of 2½ in. screen 85% efficiency, the
one-half size for 1½ inch screen is:
54.3 tph 0.85 = 46.2 tph
STEP 4.
AGGREGATE-SIZE
%..
.350
tph7129
tph246
Percent of aggregate less
than one-half size for 1½
in. screen.
STEP 4.
AGGREGATE-SIZE
Aggregate size factor for
1½ inch screen with 35%
passing ¾ in. Table 14.9
900510
200800 .
..
STEP 5. NET SCREEN
SIZE
Formula 14.5, p. 436
2½ in. screen Q = 209 tph C = 4.6 tph/sf D = 1.0 E = 1.5 G = 0.98
sf9309800151sftph64
tph209A .
.../.
STEP 5. NET SCREEN
SIZE
Formula 14.5
1½ in. screen Q = 129.7 tph C = 3.7 tph/sf D = 0.90 E = 1.5 G = 0.90
sf92890090051sftph73
tph7129A .
.../.
.
STEP 6. GROSS
SCREEN SIZE
Prudent (p. 434) to have gross capacity 10 - 25% greater than net capacity.
2½ in. screen 30.9 1.1 = 34.0 sf minimum
STEP 6. GROSS
SCREEN SIZE
Gross capacity 10 - 25% greater than net capacity.
1½ in. screen 28.9 1.1 = 31.8 sf minimum
STEP 7. SCREEN SIZE
ft 8.5=
widthft4
sf34
Therefore, a 4 X 8.5 screen
Screen size must match
screening unit box width.
2½ in. screen
STEP 7. SCREEN SIZE
ft 8.0=
widthft4
sf831.
Therefore, a 4 X 8 screen
Screen size must match
screening unit box width.
1½ in. screen
SCREENING
How about that stone!