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FINE SCREENING WITH ELECTRICALLY HEATED SCREEN DECKS Report over
initial tests and their evaluation
Bertil I. Pålsson1 and Johan Bucht2 1 Mineral Processing, Luleå
University of Technology, Sweden 2 Technology and Business
Development, LKAB/Minelco, Sweden ABSTRACT Minelco by LKAB,
Technology and Business Development, has commissioned Mineral
Processing at LTU to conduct a series of investigations with a new
screening device hitherto not used by LKAB. The screen, Mogensen
Sizer G054 with electrically heated decks, was during the trials
located in the pilot plant at Mineral Processing, where a test
station was built for it. The purpose of the investigations was to
test some specific applications, and to prepare a standard test
procedure for evaluating screening tests with a Mogensen Sizer. One
application was the production of magnetite -1 mm from sinter
fines, where very strong consolidation effects were discovered and
had to be taken into consideration. Specifically, consolidated
sinter fines with moisture content more than 1 % gave high losses.
The tests show that the best way to evaluate the screening results
is to plot the proportion of finished product, and the loss of the
correct fraction as a function of feed rate and moisture content.
These being quantity results, they need to be supplemented with
quality parameters, and the best seem to be the weight percentage
of too coarse and too fine particles in the finished product. The
direct effect of the level of heat is not yet clear. There appears
to be some threshold level for the heating, but to what extent this
is related to the screen deck type, the heat capacity and density
of the material, or the particle size, is not known.
1. Introduction Minelco by LKAB, Technology and Business
Development, commissioned Mineral Processing at LTU to conduct a
series of investigations with a new screening device hitherto not
used by LKAB, a Mogensen Sizer G054 with electrically heated decks.
It was during the trials located in the pilot plant at Mineral
Processing, where a test station was built for it. The purpose of
the test program was, besides testing some specific applications,
to establish a standard test procedure for tests with a Mogensen
Sizer. In the work reported here, the aim was to evaluate the
screen’s applicability for producing a -1 mm magnetite product from
sinter fines (MAF) coming from the ordinary production at
Malmberget. The screen with transformer is shown in Figure 1. Note,
that this is a full production unit, albeit the smallest one. It is
equipped with four sloping wire-
155
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mesh screens, 500 x 1300 mm (0.65 m²). The screens are coupled
to be the resistors in a low-voltage, high-current electrical
circuit. Note, the connection board for current cables to the left
of the screen. The transformer must be located on this side, and
close to the connection board.
Figure 1. General drawing for the Mogensen Sizer G054.
The transformer, rated at 96 A, is fed with single-phase
alternating current 400 V, however in the Mineral Processing pilot
plant it was connected to a 63 A feed. This was sufficient for the
tested applications, where the maximum power was circa 10 kVA,
i.e., approx. a 25 A load.
2. Test station The screen got elongated support legs, approx.
1.3 m, to be able to fit 200 litre drums placed under it, cf.
Figure 2. Two work platforms were built around the screen. The one
on the side of the connection plate carries the transformer, and
has in the front a small detachable staircase, to have access to
heating controls and inspection hatches. One of the support legs
has on the front side a switch for the motor power. Screen heating
may also be turned on by the safety box on the left side of the
transformer. The platform at the back of the screen is used for
deck changes, and for reconnecting cables in the transformer. A
separate staircase goes to this platform.
156
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A table feeder owned by Mineral Processing is used for feeding
the screen by a moveable feed pipe. It may be moved to the side for
capacity sampling by a pipe on the right side of the screening
station.
Figure 2. Photo of the screening station with table feeder on
the upper floor, feed pipe,
screen, and the transformer (partly hidden by the exhaust hose).
The connection between transformer and screen is shown in Figure 3.
The coupling to the transformer’s current rails is done according
to Figure 4. Note, that it for practical reasons it not possible to
have more than three screens connected to the same current rail,
since each screen deck requires two connections where only seven
are available.
157
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Figure 3. Close-up of the cables between screen and transformer.
Two cables to each
screen carry the current through the wooden blocks. Off-current
is taken by the cables connected to the metal rails (partly hidden)
on the screen side. The signal cable for detecting an open
back-door is seen on the top, and goes to the safety box for the
screen heating.
158
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Figure 4. Sketch of the transformer. In the present test
station, the primary current feed is
on the left side. Under the box for the primary cable entrance,
the safety box for heating current is mounted.
3. Material The reported tests were done on a sinter fines
quality, MAF, from the LKAB Malmberget operations. Test sieving at
LTU gave that the sample had circa 85 weight percent
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4. Test procedures
4.1. Basics Totally, twenty-five tests with MAF were run. They
were planned according to a statistical research plan. In all
cases, the screen deck set-up was: 2.4 – 1.6 – 1.35 – 1 mm. It was
soon discovered that the correct product quality (max. 3% >1 mm)
was achieved already after the third screen deck (1.35 mm).
Therefore, the major results are calculated on the fines from the
1.35 mm deck and a combined coarse product from the three
upper-most decks. Initially, the test parameters were intended to
be:
• Feed rate (ton/h) – 3 levels (1 – 5, centre point 3 ton/h), •
Moisture (%) – 3 levels (1 – 3, centre point 2 % moisture).
During the test runs, it was discovered that the results were
different if the material came directly from a stored drum, or if
they were run on material combined from an earlier test. An extra
parameter Rerun was, therefore, added to the research plan.
4.2. Evaluation For each test, the data are summarised in an
Excel-worksheet, and the particle size distributions are plotted in
the LKAB standard diagram, cf. Fig. 4. For each screen deck a
number of key numbers are established: solids split, recovery by
sieve fraction, d50, the imperfection and the screening efficiency.
Furthermore, these are also calculated as Total, which means that
the fines from deck 4 is calculated with a combined coarse product
from all decks in the test. In the same way, Total3deck is
calculated on the fines from deck 3 and the combined coarse product
from decks 1-3. The same logic applies for Total2deck.
4.2.1. Capacity -1.35 mm screen (ton/h) Measured capacity, i.e.,
on the -1.35 mm deck (ton/h) is calculated from the weighed deck
fines product and divided with the test time.
4.2.2. Proportion produced -1.35 mm screen The proportion by
weight of the fines from deck 3 (1.35 mm) as a weight proportion of
the feed to the whole screen.
4.2.3. Imperfection 3deck The imperfection calculated over the
three upper-most decks (Total3deck) as defined by the d75, d50 and
d25 of the separation curve, which is computed from the fractional
sieve recoveries. The values are obtained by moving the reading
rulers in Figure 5. The expression for the Imperfection (I) is
50
2575
2 dddI
⋅−
=
160
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0.000.10
0.200.30
0.400.50
0.600.70
0.800.90
1.00
10 100 1000 10000
Figure 5. Separation curve with reading rulers.
4.2.4. Efficiency 3deck Since a Mogensen Sizer belongs to a
group of screens that are usually referred to as probalistic
screens, the screen efficiency is calculated as if it was a
classifier. It means that, contrary to normal screens, there is a
part of the material coarser than the cut size that will be
misplaced and report to the fines product. The efficiency is
calculated for the upper-most three decks by putting a line for the
d50 in a simple particle size distribution diagram where coarse,
feed, and fines products are plotted, cf. Figure 6. In this the
reading rulers are moved to get, +−+− dodudfdf CCCC , where
−dfC = proportion less than the cut-size in the feed size
distribution
+dfC = proportion larger than the cut-size in the feed size
distribution
−duC = proportion less than the cut-size in the coarse product
size distribution
+doC = proportion larger than the cut-size in the fine product
size distribution These readings are used together with the weight
proportion of coarse product g to calculate:
• Proportion of the feed finer than the cut-size that ends up in
the coarse product
−
−
−
⋅=φ
df
dudu C
Cg
• Proportion of the feed coarser than the cut-size that ends up
in the fine product
+
+
+
⋅−=φ
df
dodo C
Cg)1(
161
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• The sorting (screen) efficiency η φ φ= − −− +1 du do
1.0
10.0
100.0
10 100 1 000 10 000
Figure 6. Particle size diagram with line for d50 and reading
rulers.
4.2.5. Cut-size 3deck The cut-size for Total3deck is had
directly from the separation curve.
4.2.6. Proportion >1 mm in finished product The proportion
>1 mm in the finished product is found from the particle size
diagram.
4.2.7. Loss of -1 mm material to other products The loss, in per
cent, of -1 mm material to other products is calculated as
FinIn
CAndelIn mm1100⋅
− , where
AndelIn is the weight proportion finished product, C1mm is
weight per cent less than 1 mm in finished product, FinIn is weight
per cent less than 1 mm in the feed.
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5. Results The total 25 tests are divided in three
sub-groups:
• 3 pre-test (became so good that they were included in the
results), • 18 main tests with variations of moisture and feed rate
(heat circa 70 C), • 4 extra test on high heat level (heat 90 – 110
C).
Since it was not possible to keep the feed and moisture levels
constant with the wanted precision, and that the high moisture-high
feed combination was not doable, the original statistical research
plan is compromised, and this limits the ability for a full
statistical analysis. Instead, the resulting key numbers are
plotted as functions of feed rate and moisture content. The dot
size for each observation varies with the test value and gives an
easy visual indication how the test went. The colour code used is,
pink for the pre-tests, light blue for the main tests (normal
heat), and red for the extra tests with extra heat. In the extra
tests with high heat the cables were re-connected so they were on a
stronger current rail. This gave screen deck temperatures in the
order of 100 C instead of the normal approx. 70 C.
Proportion produced -1.35 mm screen
0.85
0.49
0.84
0.38
0.81
0.32
0.72
0.82
0.28
0.63
0.79
0.82
0.840.84
0.070.19
0.06
0.69
0.10
0.81
0.43
0.33
0.77 0.75
0.30 0.44
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 1 2 3 4 5 6 7 8
Feed rate (ton/h)
Moi
stur
e (%
)
Pre-testNormal heatHigh heat
Figure 7. Proportion of fines produced from 1.35 mm screen.
The weights proportion of finished product shows a clear
pattern, where the influence of the moisture content is very
strong. It is hard to distinguish any effect of the heat
levels.
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Imperfection 3 deck
0.33
0.17
0.67
0.18
0.75
0.17
0.18
0.95
0.25
0.18
0.18
0.180.17
4.960.72
4.86
0.18
4.86
0.18
0.77
0.60
0.180.19
0.60 1.09
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 1 2 3 4 5 6 7 8
Feed rate (ton/h)
Moi
stur
e (%
)
Pre-testNormal heatHigh heat
Figure 8. Imperfection over three decks.
The imperfection becomes worse for high moisture levels, but the
variations are not entirely systematic.
Efficiency 3 deck
100.0
51.7
77.6
35.1
76.4
30.0
71.1
74.6
13.7
64.4
69.5
70.7
76.478.8
5.19.3
4.8
70.0
6.1
73.7
27.5
43.6
70.2 70.1
10.5 46.8
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 1 2 3 4 5 6 7 8
Feed rate (ton/h)
Moi
stur
e (%
)
Pre-testNormal heatHigh heat
Figure 9. Efficiency over three decks.
The sorting efficiency largely mirrors the production of -1.35
mm material.
164
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Cut-size 3 deck
1200
783
1079
630
1016
586
927
1047
426
859
995
1030
9951079
500500
510
885
510
1000
745
552
984 984
365 780
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 1 2 3 4 5 6 7 8
Feed rate (ton/h)
Moi
stur
e (%
)
Pre-testNormal heatHigh heat
Figure 10. Cut-size over three decks.
The cut-size is greatly worsened (step-wise) at higher moisture
contents.
% +1 mm in product
6.00
0.44
1.81
0.43
1.56
0.66
1.30
1.56
1.01
1.06
1.41
1.67
1.771.62
0.711.14
0.89
0.86
0.68
1.25
0.51
0.86
0.871.33
0.811.11
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 1 2 3 4 5 6 7 8
Feed rate (ton/h)
Moi
stur
e (%
)
Pre-testNormal heatHigh heat
Figure 11. Proportion of +1 mm material in finished product.
The proportion of +1 mm material in the finished product is
within specification (
-
%Loss -1 mm
42.1
2.8
55.7
6.4
62.0
16.4
4.5
67.5
27.0
8.4
4.7
3.2
91.477.8
92.6
19.8
88.4
5.5
49.3
61.9
2.5
10.312.7
65.448.8
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 1 2 3 4 5 6 7 8
Feed rate (ton/h)
Moi
stur
e (%
)
Pre-testNormal heatHigh heat
Figure 12. Loss of -1 mm material to other products.
Figure 12 is together with Figure 7 the ones that show the best
systematic variation for the quantitative production. Here, a high
value for one of the pre-tests might be attributed to the run being
without any heating. For the test with high heat it is hard to say
that they are essentially better. A possible interpretation is that
the effect of the heating has a threshold that ones reached doesn’t
improve further.
6. Iso-curves with consideration of the reruns Since it during
the test runs became apparent that it had a deciding influence,
whether the material was consolidated or dispersed (loosened by
earlier tests) contour plots are made for the parameter Rerun
(Omkör). The plots are done with the research planning program
MODDE ver. 8, but only for the tests with normal heat, and the
pre-tests in heat position 4. The test data is now fitted to an
equation Konstant + Mat + Fuk + Omk + Mat² + Mat•Fuk + Mat•Omk +
Fuk•Omk + Mat•Fuk•Omk + ε, where ε = the error in the estimation
Mat = feed rate (ton/h) Fuk = moisture (%) Omk = quality variable
with 0 for no rerun and 1 for rerun The plots to the left in
Figures 13 and 14 are for drums with consolidated material. The
ones to the right are the results of one or several reruns. A
166
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consolidated drum might be compared to material from an open-air
storage, while a rerun material is something that will come out of
the current production.
Figure 13. Iso-curves for the weight proportion of material
produced.
Figure 14. Iso-curves for the loss of -1 mm material.
It is obvious that the consolidation (compaction) of the
material is a serious problem in the production of MAF -1 mm. One
way to counter that in a full-scale operation would be to have the
material to pass through a harp to loosen it.
7. Discussion - Conclusions The investigation indicates that the
best way to evaluate the screening results is to plot the
proportion of finished product and the percentage loss of the
desired sieve fraction as a function of feed rate and moisture
content. These are the two most important quantity parameters. For
the quality, plots of weight per cent of plus or minus material in
the finished product will suffice. MAF is an extremely
hard-screened material, and that is known since earlier on.
Consolidated material with moisture content of more than 1% gives
high losses. In practice, this means that the material should
always pass a simple harp to break the agglomerates apart. Another
way would be to put an extra coarse screen deck in the top-most
position in the Mogensen Sizer.
167
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Further work must be concentrated on elucidating the influence
of the screen heating effect. In the tests at LTU, no discernible
difference was observed between the pre-tests, run at no or low
heating, and the test with normal and high heat. It might be an
effect of the tests being run with batches limited to one drum,
approx. 400 kg, and run times of 5 to 60 minutes. It is possible
that differences in the build-up of blocking fines material on the
decks do not manifest itself on the scale that the LTU tests
represent. Tests on a larger scale should, therefore, run for
longer times – at least for a shift – to be able to see if the
heating has any influence on the screening results. Another rather
surprising finding is that very little is written about heated
screen decks in the literature. The very few references found are
old, mostly some patents from the 1950’s, and they refer back to an
even older patent from 1921. There is a short article/technical
note in a journal (Riesbeck, 1973), and a short reference (Anon.,
2007) pointing to screens from Deister. The patents in turn
acknowledge technical notes from, among others, Allis Chalmers. A
more recent literature survey turned up additonal references in the
German literature. However, in conclusion, it seems that heated
screen decks are a known technique, which is applied as a solution
of last resort.
8. Acknowledgement In the development of the sampling and
evaluation techniques, valuable help was provided by Lennart
Karlsson from Fredrik Mogensen AB.
9. References Anon., 2007. Technical Note. Pit & Quarry, No.
1. 9 s. (Pointing to Deister) Hannon, T.W., 1955. US Patent
2,704,155. Electrically heated screen
construction. 5 Figs., 6 s. Hannon, T.W., 1958. US Patent
2,825,461. Electrically heated screen
construction. 6 Figs., 6 s. Riesbeck, L.J., 1973. Why select
heated screens? Rock Products 76, No. 10
(October), p. 52-53. Scanlon, J.J. and Schroth, J.J., 1958. US
Patent 2,850,163. Electrically heated
vibrating screen. 6 Figs., 10 s. (Assigned to Link-Belt Co.)
168
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