an adverse impact on clean coal
quality and yield.
n
Minimise surface moisture in
raw coal reporting to the heavy
media cyclone circuit:
This
becomes extremely important
when the required media density
exceeds 1.65 s.g. As the amount
of water in the feed increases,
amount of bleed of media from
heavy media cyclone circuit to
the dilute media circuit also
increases to maintain the level in
the media sump. This, in turn,
increases the magnetite loading
in the magnetic separators,
where a small percentage of it
is lost in the tailings. Failure
to control the amount of water
in the feed to the heavy media
cyclone circuit can make it
impossible to achieve media
densities of 1.70 s.g. and higher
– if required.
The second dual function of the
vibrating and static screens in the
heavy media cyclone circuit is to drain
media and diluted media from the
refuse and clean coal. Clean coal is
separated in more than one size
fraction in order to produce stoker and
speciality coal. Normally, a double
deck screen is required for this
application. A flexible screen surface
made of polyurethane with zig zag
opening, such as the one sold by
Polydeck Screen Corp., does an
excellent job of sizing coal down to
3 – 4 mm without blinding. Typically,
stoker and speciality coals have strict
limits on over and under size materials
allowed in the product. It is important
to select the right size screens with
proper opening and the material of
construction of the screen surface.
It is imperative that screen
surfaces of vibrating and static
screens are maintained close to the
design specifications during the
plant operation.
Table 3 shows that through
product of raw coal deslime screen
has 12.54% solids larger than 1 mm.
The presence of these solids is
contributing to higher than desired
concentration of solids (15.82%) in
the feed to the classifying cyclones
and to overloading of the spirals
(4 tph/start; 38.15% solids in feed).
A note of caution should be
added here, however: screen
surfaces of 1 mm or smaller
openings made of polyurethane do
provide longer wear life, but at the
expense of reduced efficiency. This
is because of a less open area as
compared to screen surfaces made
up of stainless steel profile wire.
Fine coal circuit (spirals,
hydro cyclones and reflux
classifiers)
Normally, the top size of the feed to a
fine coal circuit is limited to 1 mm.
This is done because heavy media
cyclones can process raw coal down
to 1 mm in size with a very high level
of efficiency, which cannot be matched
by water based beneficiation
machines. In many cases, these
machines process +1 mm and
-0.15 mm raw coal at a poor efficiency,
while achieving high efficiency in
processing of 1.0 x 0.15 mm size
fraction. Therefore, it is important that
deslime screens are sized and
operated to minimise the
misplacement of + 1 mm raw coal in
the screen through product. Normally,
this misplacement should be limited
to a maximum of 2% – 3%.
Classifying cyclones are most
commonly used to separate
1 mm x 0.15 mm size fraction from
1 mm x 0 mm raw coal. Typically with
a correct configuration of classifying
cyclone, its operating pressure and
concentration of solids in the feed,
cyclone overflow should have no
more than 3% – 5% of +0.15 mm
material in it. Cyclone underflow will
contain 15% – 20% of -0.15 mm
relatively high ash material (Table 4).
Table 3 shows an excessive amount
of misplaced -0.15 mm high ash solids
(40.94%) present in the classifying
Table 6. Spiral circuit performance (flotation sample 1 mm x 100 m )
Gravity
Feed
Product
Refuse
Recon feed
% Wt
% Ash % Wt
% Ash % Wt
% Ash % Wt
% Ash
1.60 float
79.62 3.55
98.61
3.10 10.73
17.59 80.48 3.50
1.80 float
1.73
38.33
0.76
36.93
1.50 40.29 0.91
38.07
2.00 float
1.26
57.03
0.24
51.36 2.34
55.22 0.67
54.13
2.20 float
1.26
67.84 0.14
61.68 2.94
67.54 0.72
66.63
2.20 sink
16.13
90.99 0.25
87.19 82.49 91.46 17.22
91.41
Composite 100
19.75
100
3.77
100
81.21
100
19.75
Table 7. Froth flotation circuit
Flotation samples
Feed
(11.86 % Solids)
Product
(16.91 % Solids)
Tailings
(4.40 % Solids)
Size
% Wt
% Ash % Wt
% Ash % Wt
% Ash
+ 1 mm
0
0
0
0
0
0
1 mm x 60 mesh
3.08
4.17
3.32
2.11
6.82
9.82
60 x 100 mesh
8.48
4.15
13.11
2.26
6.07
13.73
+100 mesh composite 11.56
4.16
16.43
2.23
12.89
11.66
100 x 325 mesh
37.75
9.87
58.68
3.18
14.45
53.41
325 mesh x 0
50.69
54.17
24.89
13.71
72.66
80.28
Composite
100
31.67
100
5.64
100
67.55
100 mesh x 0 composite 88.44
35.26
83.57
6.32
87.11
75.82
+325 mesh composite 49.31
8.53
75.11
2.97
27.34
33.73
Table 8. Clean coal
Size
Ash
After StackSizer installation
+0.15 mm – 83.67%
15.15, 17.38
9.38
0.15 mm x 0 – 16.37%
26.72
9.21
32
|
World Coal
|
June 2016
