vibrating screen can be used to
replace a static sieve or a sieve bend.
It is extremely important that the
separation of raw coal in various
sizes to feed different process
circuits is done with the highest
possible efficiency. The separation
efficiency here is indicative of the
amount of undersize material
misplaced in the oversize and the
oversize material misplaced in
undersize. Sizing and classifying of
raw coal should always be done
with an objective of minimum
possible misplacement of undersize
and oversize materials. During the
design stage, the following process
design parameters need to be taken
in to account to achieve a high
degree of sizing and classifying
efficiencies:
n
n
Sizing of the equipment.
n
n
Proper concentration of solids in
the feed.
n
n
Correct amount of spray water,
where applicable
n
n
Proper pressure, as is the case
with classifying cyclones.
Impact of misplaced
materials on the
process circuits
Coarse coal circuit
Operation and efficiency of a coarse
coal circuit is not significantly
impacted with the presence of up to
5% of undersized material in the
feed, provided the size of this
material is limited to 2 or 3 mm and
the feed and refuse carrying
capacities of the washer are not
exceeded. Table 1 shows the size
distribution of feed to a heavy media
vessel circuit with excessive amount
of -
3
/
8
in. (9.5 mm) raw coal (9.50%
of the feed) or about 37 tph. In this
particular case, the amount of refuse
in 2 in. x
3
/
8
in. (50 x 9.50 mm) raw
coal exceeds the refuse carrying
capacity of the heavy media vessel.
An additional amount of -
3
/
8
in.
(9.5 mm) raw coal exasperates the
situation. The end result is the loss of
3 tph of 5.7% ash clean coal as it is
trapped in the refuse and exits the
vessel with it (Table 2).
Heavy media cyclone
circuit
Vibrating and static screens impact
the operation of a heavy media
cyclone circuit in two ways. First,
this circuit receives its feed as an
over product from deslime screens
where the raw coal is sized at 1 mm.
It is important that these screens
operate at a high efficiency in order
to achieve the following:
n
n
Minimise the misplacement of
-1 mm in the feed to the heavy
media cyclone circuit:
The
presence of excessive amount
of undersize material can result
in buildup of non-magnetics in
the circulating media, falsifying
its density. This reduces the
sharpness of separation in the
heavy media cyclone, which has
Table 4. Simulation carried out by Robert Moorhead, Technical Manager/Coal with FLSmidth Krebs
Micron Solids
(SG)
Feed
Overflow
Underflow
Recovery
CUM % + IND % + short tph CUM % + IND % + short tph CUM % + IND % + short tph
1000 1.45
5.0
5.0
11.3
0.0
0.0
0.0
5.6
5.6
11.3
100
595
1.45
25.3
20.3
45.6
0.0
0.0
0.0
28.3
22.7
45.63
100
297
1.43
52.88
27.60
62.11
0.02
0.02
0.0
59.15
30.87
62.10
100
149
1.43
74.32
21.44
48.23
4.33
4.31
1.03
82.61
23.46
47.20
97.9
105
1.53
81.76
7.44
16.73
9.28
4.94
1.18
90.34
7.73
15.55
93.0
74
1.53
87.22
5.46
12.30
20.78
11.50
2.74
95.09
4.75
9.55
77.7
44
1.53
92.60 5.38
12.10
46.62
25.83
6.16
98.05
2.96
5.95
49.1
25
1.53
95.97
3.37
7.59
68.85
22.23
5.30
99.19
1.14
2.29
30.2
-25
1.53
100
4.03
9.06
100
31.15
7.42
100
0.81
1.64
18.1
Total
225
23.83
201.17
89.4
Table 5. Size distribution of material in spiral circuit
Spiral circuit
Spiral feed at spirals
(38.15 % Solids)
Spiral product at spirals
(34.04% Solids)
Spiral refuse feed to HF screen
(63.15% Solids)
Size
% Wt
% Ash
% Wt
% Ash
% Wt
% Ash
+ 1.5 mm
5.29
23.51
6.29
7.35
12.60
89.64
1.5 x 1 mm
10.53
20.09
10.79
4.70
21.05
88.95
+ 1 mm composite
15.82
21.23
17.08
5.68
33.65
89.21
1 mm x 100 mesh
64.55
19.75
63.77
3.77
56.42
81.21
+ 100 composite
80.37
20.04
80.85
4.17
90.07
84.20
100 mesh x 0
19.63
48.26
19.15
37.31
9.93
80.63
Composite
100
25.58
100
10.52
100
83.84
30
|
World Coal
|
June 2016
