The Effects of Drum Speed, Shake Frequency and Shake Amplitude ...

The Effects of Drum Speed, Shake Frequency and Shake Amplitude Combinations
on Cleaning of Lignite Fines by Multi Gravity Separator
Derya Oz Aksoy, Sabiha Koca
Eskisehir Osmangazi University, Eskisehir, Turkey.
Huseyin Koca
Anadolu University, Eskisehir, Turkey.
ABSTRACT: Significant amounts of fine coal are lost every year all over the world, because modern
processing techniques are unable to recover very fine materials. Moreover many coals also contain high
levels of sulphur and ash, which results in particulate matter and sulphur dioxide emissions when these
materials are burned in coal fired power stations. These emissions cause severe environmental pollutions
and have to be reduced to an acceptable level. Physical processing methods are utilized widely to clean
lignite fines. Gravity concentration techniques are preferred because of cheapness and easiness. Multi
gravity separator is one of the recently designed gravity concentration device and preferred in fine coal
cleaning due to its high performance. The variables of multi gravity separator such as drum speed, shake
frequency and shake amplitude affect the separation performance significantly. In this work, the
combinations of these parameters were studied to determine the optimum conditions of separation.
INTRODUCTION
In order to improve the recovery of fine coal, and at the same time to reduce the sulphur and ash content to
an acceptable level, a number of fine coal cleaning techniques are available. Cleaning of coal by physical or
chemical methods prior to combustion; Removing SO2 gasses by post-combustion control methods;
Synthetic fuel production; and removing SO2 during combustion techniques can be utilized for this purpose
(Rowson, 1986).
Amongst these methods cleaning of coal prior to combustion involves either physical or chemical
techniques or a combination of these. Physical processing methods such as gravity concentration are used in
coal preparation plants widely despite their low efficiency particularly on desulphurization (Khory, 1981;
Shah et al., 2002). Gravity concentration methods are the cheapest therefore they are preferred for economic
reasons. The major limiting factor in gravity concentration is the dependence on particle size of coal. Pyritic
sulphur is generally disseminated finely and can be liberated only by fine grinding. Therefore fine coal
cleaning should be applied.
Multi Gravity Separator (MGS) was originally designed and used in fine metallic minerals
concentration during 1990’s (Chan et al., 1991; Ozdag, et al., 1993, Udaya Bhaskar, et al., 2002). Then
MGS was applied to fine coal cleaning (Erdem et al., 2008, 2010; Koca et al., 2000; Oz et al., 2009).
MGS can increase gravity force up to 25 g that improves its ability to process finer materials
Drum speed, shake frequency, shake amplitude, washwater flowrate and tilt angle are the most
important variables that affect the separation of minerals in a MGS. As the particle size of the minerals
gets finer, the interaction between these parameters gets more complicated and reduces the efficiency of
the cleaning process. Therefore studies were conducted to determine the interactions of these variables.
In another work done by the same authors, washwater flowrate and drum speed combinations of the
MGS was studied in details to improve the performance of the process (Aksoy, et al., 2011). In this work,
other important parameters of the MGS, namely shake frequency and shake amplitude were studied to
determine the interaction with drum speed.

MATERIAL AND METHOD
Lignite samples were taken from a nearby open pit coal mine. Samples were dry ground in open air and
ground to -0.212 mm in size. The ash content of the lignite sample is 42.56 %.
The Mozley laboratory C 900 MGS were used in the experimental work. Its structure and operating
conditions were given elsewhere (MGS Application Guide 1991; Chan et al. 1991).The feed to the MGS
was prepared by mixing 500 grams of dry sample with one liter of water that gives 33 % solids
concentration (by weight). The mixture was stirred continuously to maintain uniform suspension. The
MGS variables were adjusted at the required level as per the experimental design. The feed slurry was
then fed to the MGS feed vessel at the required flow rate as the MGS was in operation. The separator was
kept running until the material flow was finished, which took about 5 minutes, and MGS was stopped.
Upper cover of the separator was removed, and remaining material in the drum was taken into equivalent
fractions. Heavy product, which was collected through front launder, referred to as tailings, and light
product, which was collected through back launder, referred to as concentrate.
RESULTS AND DISCUSSION
The effects of drum speed, shake frequency and shake amplitude combinations on the cleaning of lignite
fines were studied. The other experimental conditions were kept constant as following:
Drum speed : 220-260 rpm
Shake frequency : 4-5 cps
Shake amplitude : 10-20 mm
Washwater flowrate : 1 l/min
Feed density : 33 %
Feeding duration : 90 sec
Washing duration : 5 min
Tilt angle : 0 o
Results are given in graphs in terms of ash content and combustible recovery in the concentrate
fraction. There are a number variables which affect the separation of minerals in a multi gravity separator.
It is thought that drum speed, shake frequency and shake amplitude are the most important ones. There
should be an optimum combination of these variables for a given minerals mixture.
The effects of drum speed and shake frequency at 10, 15 and 20 mm shake amplitude are given in
Figure 1-3.
Ash, %
44
42
40
38
36
34
32
4 cps
4.5 cps
5 cps
210 220 230 240 250 260 270
Drum Speed, rpm
75
70
65
4 cps
4.5 cps
5 cps
210 220 230 240 250 260 270
Drum Speed, rpm
(a) (b)
Figure 1. The effects of drum speed and shake frequency on ash content (a) and combustible recovery (b)
at 10 mm shake amplitude.
Combustible Recovery, %
100
95
90
85
80

(a) (b)
Figure 2. The effects of drum speed and shake frequency on ash content (a) and combustible recovery (b)
at 15 mm shake amplitude.
Ash, %
44
42
40
38
36
Ash, %
44
42
40
38
36
34
210 220 230 240 250 260 270
Drum Speed, rpm
4 cps
4.5 cps
5 cps
4 cps
4.5 cps
5 cps
34
210 220 230 240 250 260 270
Drum Speed, rpm
Combustible Recovery, %
85
4 cps
4.5 cps
5 cps
80
210 220 230 240 250 260 270
Drum Speed, rpm
(a) (b)
Figure 3. The effects of drum speed and shake frequency on ash content (a) and combustible recovery (b)
at 20 mm shake amplitude.
Drum speed of the MGS is one of the most important parameter that affects the separation
significantly. Drum speeds between 160 and 300 rpm increase the g force from 7 to 25 g at the internal
drum surface. The increased g force allows the separation of minerals that have smaller density
differences and finer particle sizes (MGS Application Guide, 1991; Chan et al., 1991).
The shake amplitude and shake frequency are important both in mechanism of bed dilation and
stratification, and for particle transportation. (Burt, 1984). The separation of particles in the drum takes
place under the effects of shearing forces. Changes in amplitude and shake frequency of the MGS affect
these forces as well as separation significantly. It was claimed in literature that there should be an
optimum combination of shake amplitude and shake frequency for a given mineral mixtures (Ozdag et al.,
1993).
As can be seen from Figures 1-3, ash content of the concentrates decreased significantly with an
increase in the drum speed at all studied combinations. The decrease in ash content in the concentrate was
attributed to the increased g force at the internal drum surface. It should be noticed that separation was
very poor at low drum speeds and most of the material was reported to the lights fraction. As the drum
speed increases, the quality of the separation improves that causes ash reduction in the concentrate. This
also causes the decrease in combustible recovery in the concentrate. Therefore 260 rpm drum speed was
chosen as optimum value.
Longer amplitude causes more material to report the lights fraction. Therefore, an increase in shake
amplitude increases both ash content and combustible recovery significantly. Shake frequency affected the
Combustible Recovery, %
100
95
90
100
95
90
85
80
4 cps
4.5 cps
5 cps
75
210 220 230 240 250 260 270
Drum Speed, rpm

esults in a different manner. At the shortest amplitude, lower frequency produced the best results while
higher frequency produced better results at higher amplitudes.
CONCLUSIONS
Drum speed, shake frequency and shake amplitude are the most important variables that affect the
separation of minerals in a MGS. As the particle size of the minerals gets finer, the interaction between
these parameters gets more complicated and reduces the efficiency of the cleaning process. Therefore
studies were conducted to determine the interactions of these variables. After MGS experiments, the
optimum conditions were determined as drum speed of 260 rpm, 10 mm shake amplitude and 4 cps shake
frequency.
At optimum conditions, ash content and combustible recovery of the concentrate were obtained as
32,92% and 77,93%, respectively.
The results and discussions indicate that drum speed affects the separation quality significantly. Higher
drum speed gives the better results at all studied combinations. There is a strong relationship between
shake amplitude and shake frequency. Better results were obtained at slow frequency when amplitude was
short. While the amplitude was longer, higher frequency produced better results.
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