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!