- Email: [email protected]
An evaluation of the role of particle size in the flotation of coal using different cell technologies
Minerals Engineering, Vol. 5, Nos 10-12, pp. 1225-1238, 1992 Printed in Great Britain
0892-6875/92 $5.00+0.00 © 1992 Pergamon Press Ltd
AN EVALUATION OF THE ROLE OF PARTICLE SIZE IN THE FLOTATION OF COAL USING DIFFERENT CELL TECHNOLOGIES M.C. HARRIS, J-P. FRANZIDIS, C.T. O'CONNOR and P. STONESTREET Department of Chemical Engineering, University of Cape Town, Rondebosch 7700, South Africa
ABSTRACT A large variety of new flotation cells has been introduced in the last f e w ),ears, probably as a result o f the successful introduction of column flotation in the minerals processing industry, hr common with the column cell. a number of these new cells employ an essentially quiescent separation zone. However, a number o f novel cell designs have been introduced that use agitation mechanisms similar to those employed on conventional flotation cells. The aim o f this investigation was to evaluate flotation behaviour in both an agitated and non-agitated environment, particularly with respect to particle size. A hybrid 'agitated' column cell was designed for the investigation, and the operation of this unit was compared to that o f a column ceil, and to a batch flotation cell. on a laboratory scale. The testwork was conducted on coal fines, as problems with the flotation o f coarse coal particles in a column cell had previously been identified. It was demonstrated that the addition o f an agitated stage to a column cell can significantly improve the coarse particle recovery in comparison to the conventional column cell, while maintaining good selectivity in the fine sizes.
Keywords Fine coal flotation, cell design, particle size INTRODUCTION Flotation equipment technology has made considerable advances in the last decade. A large variety of new flotation machines has been introduced, as an alternative to the conventional flotation cells that predominated in the industry for so many years. This proliferation of new technology has almost certainly been sparked by the successful introduction of the column cell (now mostly referred to as the 'conventional' or 'Canadian' column) on many mines since the early 1980's. It has been shown that column cells can achieve both improved performance and reduced cost in many applications. In particular, the addition of wash water to a deep froth bed, resulting in a net downward flow of water through the froth (positive bias), enabled the column cell to produce substantially higher product grades than conventional cells for equivalent recoveries. This success has lead to the design of a wide variety of new cell types that have typically sought to improve on one or more aspects of the conventional column cell design, while maintaining the advantages of improved grade that result from operating with a deep, well washed froth. Flotation cells such as the Jameson cell, the Bahr (pneumatic) cell, the Free jet flotation cell and the Filblast cell have introduced feed slurry aeration systems designed to
1225
1226
M . C . HARms et al.
substantially improve collection efficiency, which, along with a number of claimed advantages, results in a considerable reduction in height compared to the conventional column cell (1 to 2 m compared to 10 to 15 m) [1,2,3,4]. Other novel cell designs have maintained similar dimsions to that of the conventional column cell, and modified the internal characteristics for improved performance in specialised applications. The packing employed in the packed column enables the formation of a deep, stable froth bed for efficient cleaning of very fine particles [5]. The Hydrochem cell uses impellers mounted on a central shaft to induce varying degrees of agitation down the length of the cell, with the aim of creating zones in which optimum conditions exist for the collection of different size particles within the feed [6]. The reaction of the conventional cell manufacturers has been to introduce a number of so called 'column cells' which are characterised by dimensions and agitator configurations similar to conventional cells. These cells include modifications which allow for reduced gangue entrainment by improved cleaning in the froth zone. The Leeds column, marketed by Wemco, contains rods of varying density in the froth zone, which are designed to squeeze out the lightly held gangue particles from the mineralised bubbles [7]. The O u t o k u m p u high grade cell employs a conventional OK mechanism, combined with froth crowding and washing to achieve improved performance [8]. The use of agitation in cells such as the Leeds cell and the OK cell, and to a lesser extent in the H y d r o c h e m cell, is in clear contrast to the essentially quiescent operation of the conventional column cell. While the manufacturers of non-agitated cells point to the advantage of reduced maintenance and power requirements [9], the manufacturers of machines based on a more conventional design can point to the proven effectiveness of flotation in an agitated system over many years, and the familiarity of plant operators with such systems [8]. One of the main functions of agitation is to suspend the solids in the celt, and allow equal time for collection of particles of all sizes. In systems in which no agitation is employed, the residence time of a particle decreases in proportion to its size and density, owing to gravity. Thus the residence time of a coarse particle is often a small fraction of that of a very fine particle in a non-agitated system. This can lead to substantial losses of valuable mineral to the tailings stream when significant quantities of coarse particles are present in the feed. This can apply in a column cell, where particle collection occurs inside the cell, and in the cells that employ feed slurry aeration systems where particle collection occurs outside the cell. In both cases, losses can arise as a result of particle-bubble detachment inside the cell, which becomes increasingly probable with increasing particle size [10]. After detachment, some of the coarse particles will be lost to the tails due to insufficient time for recollection. Thus, the aim of this investigation was to evaluate the effect of agitation on flotation performance as a function of particle size, with particular emphasis on coarse particle recovery. The flotation response of a coal sample was investigated on a laboratory scale in three types of flotation cell: a conventional batch flotation cell; a 2" diameter column cell; and a specially designed hybrid cell that was essentially a combination of the two cells, which was adapted for continuous operation. The work represents part of a broad programme that has been undertaken at the University of Cape Town to identify the optimum type of flotation equipment for specific applications.
BACKGROUND It was decided to conduct this investigation on coal, as previous testwork on a number of South African coals had shown that while the column cell was extremely efficient at cleaning coal fines below 150 microns in size, the collection efficiency decreased significantly for larger particles [11]. Other work has shown that the recovery of coarse particles by flotation was particularly poor when large quantities of ultrafine material (-25 micron) were present [12]. This is frequently the case with the cyclone overflow that either
The role of particle size in coal flotation
1227
represents, or would represent, the feed to flotation on most South African collieries, which have been found to consist of up to 50 % ultrafine material and as much as 20 % +150 micron material. This is despite the widespread introduction of spirals to treat the l mm to 0.15 mm fines fraction. The study was conducted on a sample of coal fines from a colliery situated in the Natal coalfield in South Africa, that produces coking coal for domestic steel production. The sample consisted of naturally arising fines that form the feed to the conventional flotation plant operated at the colliery. EXPERIMENTAL
Sample Characterisation The feed sample was characterised using the following techniques: size and ash distribution; float and sink analysis; flotation release analysis; proximate analysis and petrographic analysis. The float and sink analysis was performed using a centrifugal technique developed at the University of Cape Town that enables rapid and accurate analysis of very fine coal samples [13]. Flotation release analysis aims to establish the optimum performance that can be achieved by flotation [14]. The technique used in this investigation consisted of subjecting the sample to incremental flotation in a 3 litre Leeds batch flotation cell. Initially, the minimum reagent addition was used, and the most gentle operating conditions employed, so that only the most hydrophobic material reported to the concentrate. Gradually, by increasing the reagent addition and air rate, more and more material was recovered, until the cell was barren of floatable material. The results of the coal characterisation are presented in Table 1. The sample had an ash content of 31%, and consisted of about 46% ultrafines, and about 15% 'coarse' (+150 micron) material. The majority of the ash was concentrated in the ultrafine fraction. The float and sink analysis and the release analysis indicated that a yield of about 60 % could be obtained at 10% ash, rising to about 68% at 13% ash. The predominant maceral in the sample was vitrinite, at 85% by volume.
Equipment Three types of equipment were used in the investigation. These are presented schematically in Figure l, and can be described as follows: I.
2.
.
A 3 litre bottom driven Leeds-type batch flotation cell was used to conduct batch flotation experiments in a completely mixed environment. A 2 inch diameter column cell was used to conduct continuous experiments in a quiescent environment. A column height of 2 metres was used. The feed point was 0.5 metres from the top of the column. An external USBM type bubble generator was used, and the interface level was controlled using a conductivity probe. A hybrid agitated column cell was designed specifically for this testwork. It consisted of the top half metre section of the 2" column cell, attached via a conical adaptor to the L e e d s - t y p e batch flotation cell used for the batch flotation tests. The cell was adapted for continuous operation by placing a tails outlet at the base of the cell. The feed point and wash water system were identical to those on the column cell. Thus the cell consisted of a quiescent top (column cell) section, and completely mixed bottom (Leeds batch cell) section.
Both the column cell and the agitated column cell had the same total volume, to enable a comparison at equivalent liquid residence times.
1228
M. C. HARRIS et al.
TABLE 1 Feed characterisation
Size Analysis:
Proximate Analysis:
Size Fraction (micron)
Mass (%)
Ash (~)
+150 + 75-150 + 25-75 -25
15.8 14.6 23.9 46.1
22.1 23.9 29.1 36.9
100.0
30.8
Total
H20 (%)
Cum. mass (%)
21.0 48.9 23.9 1.4
Sulphur (%)
Petrographic Analysis: Vitrinite (% vol) Exinite (% vol) Inertinite (% vol)
Float and Sink Analysis: Rel. Density
1.7
Volatiles (%) Fixed Carbon (%) CV (Md/kg)
Cum. Ash (%)
1.30 1.35 1.40 1.45 1.50 1,55 1.65 1.75 1.80
18.9 30.6 41.6 51.6 58.0 60.6 69.9 75.0 78.5
3.8 4.1 5.8 7.5 9.0 9.5 13.8 16.1 19.1
Total
I00.0
30.8
85.0 4.0 II.0
Release Analysis: Cum. Yield (%) 5.6 15.3 30.1 40.2 50.1 55.4 63.1
Cum. Ash (%) 5.5 6.0 7.1 7.8 8.5 9.1 10.0
Testwork A range of scouting tests was conducted on each of the cells to establish the correct range of the various operating conditions to achieve satisfactory performance. Operating conditions were then selected to obtain a range of flotation yields between 40 and 70%. Four tests in this range were selected for detailed analysis for each cell type. The concentrate and tailings samples generated in these experiments were analysed for size and ash distribution. The conditions that were used on each of the cells are presented in Table 2. RESULTS The overall results obtained in the tests on each cell type are presented in Figure 2, which shows the performance with respect to yield and concentrate ash content. The results of the float and sink and the release flotation analyses are included for comparison. The results of a number of the preliminary scouting tests are also included.
The role of particle size in coal flotation
BATCH FLOTATION CELL
CONVENTIONAL COLUMN CELL
~
1229
AGITATED COLUMN CELL
sh water
Concentrate v ~
Fr°th
i Wash water
Concentrate
~ ~Froth O01ESCE NT
Feed
Froth ,
"t
Feed
Concentrate
OUIESCE~VT
Air ~ j ~ X E D
[
Air
[ i I
Conventional bottom driven Leeds-type flotation cell Mode of operation: Batch Volume: 3 litres Air injected directly onfo impeller
Impeller Water Air
"-~ ~Tails
2" diameter column flotation cell
Mode of operation: Continuous Height: 2 metres Collection zone volume: 4 litres Sparger: external bubble generator
Fig.l
MIXED
I
!
Impeller (1200 rpm)
~
Tails
(1200rpm)
Hybrid agitated column cell
0,5 m top section of 2" column attached to .5 I Leeds-type flotation cell Mode of operation: Continuous Collection zone volume: 4 litres Air injected directly onto impeller
A schematic diagram of the equipment used in the investigation.
It can be seen that the performance achieved in the batch cell was substantially poorer than that obtained in both the column cell and the agitated column cell. The latter cells were both able to achieve a performance that closely approached the release flotation line. Only at a product ash of around 14% did all three cells converge on the optimum performance line defined by the float and sink curve. The coal recovery by size results obtained in the tests performed on the batch flotation cell are presented in Figure 3 on a dry, ash free basis (daf), along with the overall recovery achieved in each of the tests. The preferential flotation of the -75 micron fines is clearly apparent; the coarser fractions were only recovered after most of the fines had been collected. Ultimately, however, a coarse particle recovery very close to that obtained on the other size fractions was achieved. The analogous results for the tests performed on the column cell and the agitated column cell are presented in Figures 4 and 5. In the tests performed on the column cell, recovery was more pronounced in the intermediate size fractions, although below 150 micron the m a x i m u m recoveries obtained were very similar to the batch results. However, significant losses occurred in the +150 micron fraction, reducing the overall coal recovery that could be achieved by about 5%. In the tests performed on the agitated column, an almost uniform increase in the recovery from each size fraction was observed with increasing overall recovery. In the test with the highest overall recovery, the coal recovery obtained in the intermediate fractions was particularly high. In addition, the maximum coarse particle recovery was almost the same as that achieved in the batch cell. However, the recovery in the ultrafine fraction was
1230
M.C.
et al.
HARRIS
marginally lower than in either the batch or the column cell, giving a maximum overall recovery about 2% lower than the batch cell result. 18
/
/ 16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-" ....................................................... II .+.-
/ .+..
14 .........................................................................................
I I+ q -. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
+
+
•
m
qb
m
-
m < IdJ
/ 12 ................................................................................. IB m
II ............ [SS3 E 3
II
I-
ZI i i
10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
~
{=) Z
O
.:7 •, ~,+/ / . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~" ........... ~ .7 FLOAT & SINK
............
•Jt-[~
s
.............................
.............~..++,.~.:::~" /:~ ......................
+
J__,o..o.+-'+'"+°°'°°
t 0
10
20
30
.. J
40
50 YIELD
.E,~se
~LOTAT,ON
BATCH CELL
.,, '"" ,'"'+
+ I
-'"°'"
.......
- ........................................................ -
++" .... c+ C O L U M N CELL
60
! 80
70
90
100
(%)
Fig.2 Summary of the overall performance of the each of the cell types in comparison to the performance predicted by float and sink, and flotation release analyses.
100 908070-
.....................~ .......
?
.
+
........................:...
~
.............................................................................. ...................
~:;I;I;I;IIIIIIII~,~.~YlI.I::~:~,.,.,.,=~
............
~
.......................................................................................
.................. " " ~ 3 +,::,.................... "i??:::,....................................................................................................... ~ IZ3
............................................................................
60\
"\
500
0 J,i c~ .J
40-
0 0
30-
\
\
........................................................................................................................................":~......... X ......................................................................
20 ......................................................................................................................................................................... 1 0 ...........................................................................................................................................................................................................................
I ,,+c, F,O,,,.o,c~,, 0
-25
25,~75
L 75"150 + 150 SIZE FRACTION (micron)
'
OVERALL I~ECOVERY
Fig.3 The coal recovery by size results obtained in the tests performed on the batch flotation cell on a dry, ash free basis.
The role of particle size in coal flotation
1231
TABLE 2 Flotation conditions employed in the testwork 1. Conditions common to a l l three c e l l s : Shellsol A (95 % aromatic) Frother: H i - f l a s h TEB (supplied by SENMIN for f l o t a t i o n of coal fines) Pulp density: 10 % (mass/vol)
Collector:
2. Batch c e l l conditions: Frother concentration: 80 g/t Impeller speed: 1200 rpm Froth height: 2 cm Test No 1 2 3 4
Air rate: 6 I/mln Flotation time: 6 min
Collector conc. (g/t) 50 100 250 750
3. Column c e l l conditions: Feed rate: 2.7 kg/h Bias rate: 0.2 cm/s (+ or - 0.03 cm/s) Liquid residence time: 5 min Test No
Air rate (I/mln)
1 2 3 4
2.0 2.5 2.9 3.5
Froth height: 30 cm Collector conc: 1000 g / t Feed point: 0.5 m from top
Frother conc. (g/t) 130 130 130 180
4. Agitated column cell conditions: Feed rate: 2.7 Kg/h Bias rate: 0.2 cm/s (+ or - 0.03 cm/s) Liquid residence time: 5 min Impeller speed: 1200 rpm Test No
A i r rate (]/mtn)
1
2 3
1.6 1.8 2.0
4
2.4
Froth height: 30 cm Collector conc: 1000 g / t Feed point: 0.5 m from top
Frother conc: 100 g / t
The relationship between coal recovery and concentrate ash content for the +150 micron size fraction is presented in Figure 6 for each of the cell types. For this size fraction, the selectivity achieved by the batch and column cells was very similar, while some improvement in selectivity was observed in the agitated column cell. Figures 7, 8 and 9 show the analogous results for the 75 to 150 micron size fraction, the 25 to 75 micron size fraction and the -25 micron size fraction respectively. In the 75 to 150 micron fraction, the selectivity of the batch cell was significantly poorer than either of the column cells. Once again, however, the agitated column demonstrated improved selectivity in comparison to the column cell. In the 25 to 75 micron size fraction, the margin of the difference in selectivity between the batch cell and the two column cells was even greater. For this fraction, however, improved selectivity was achieved by the column cell relative to the agitated column.
M. C. HARRISet al.
1232
QOI 80-
-IA
70-
0
605040-
%
30-
\.
.“..
10 COLUMN CELL 0
! -25
I 25*75
I I 75*150 +150 SIZE FRACTION (micron)
,
/ I OVERALL RECOVERY
Fig.4 The coal recovery by size results obtained in the tests performed on the column cell on a dry, ash free basis.
\ .,
50
_“.,s. ‘*.
i
,oJ
0 -.
............................ .........
. AGITATED COLUMN CELL
0
1 -25
1 25*75
1 +150 75*150 SIZE FRACTION (micron)
I
I OVERALL RECOVERY
Fig.5 The coal recovery by size results obtained in the tests performed on the agitated column cell on a dry, ash free basis.
The role o f particle size in coal flotation
1233
18 [PLUS 1,50 MICRON SIZE FRACTION I
14........................................................................................................................................................................................................................... EE
lZ
........................................................................................................................................................................................................................... -...........-...--~
12
LU lZ 0 L) ..r (n ,<
................
. ........
......................................................... :::~........................;::~o" ":~:..................................................................................................................
r--i" ~ ° ' ' ' ' ' ' ' ' ' "
4
0
1'0
20
:30
%,CNCELL
I
COLUMN CELL
..............
"0"
40
gO
60
7'0
UMN
80
90
100
COAL RECOVERY (daf, %)
Fig.6 Concentrate ash content as a function of coal recovery for the +150 micron size fraction, for each of the cell types.
18 I 7 5 T O 150 MICRON SIZE FRACTION I 16 ...........................................................................................................................................................................................................................
........................................................................................................................................................................................................................... ./.../....
14
/
lZ
12-
,,...,'' /
U.l lz 0 0 'l- 1 0 u) <
,
/
:;~.~:~ ................. r7 •.4-. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-
Ell TCN CELL 6
....................................................................................................................................... . ~ ..................................................................................
C tOLUMN
CELL
...............
AGITATED COLUMN 4
50
,
,
,
,
,
,
55
60
65
70
75
80
85
90
95
COAL RECOVERY (daf, %)
Fig.7 Concentrate ash content as a function of coal recovery for the 75 to 150 micron size fraction, for each of the cell types.
100
1234
M.C.
HARRIS et al.
18
[ 25 TO 75 MICRON SIZE FRACTION I
Z LIJ IZ 0 0 3: (/)
~..-t~_...../ " /."...............................
1 2 ........................................................................................................................................................ ,,o"
oo,-
-
/
BATCH CELL
r-7
6 ..........................................................................................................................................................................................
C O L U M N CELL
......
"lAGITATED C O L U M N
4
50
55
Fig.8
(~0
65
10 15 80 COAL RECOVERY (daf, %)
8'5
90
95
100
Concentrate ash content as a function of coal recovery for the 25 to 75 micron size fraction, for each of the cell types.
18
16-
~
25 MICRON SIZE FRACTION]
i
/
rTn
I. Z 12- ...................................................................................................................................................................... uJ lZ ...e ................ i,i.~.i,:.;.............. 0 10.I< ..................................................
~.:;.;~..~..
......
..~.~ .......................
[~..,,~
'-I--
.,'"
~.-::;;~'~/~"
........................
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8 ...........................................................................................................................................................................................................................
BATCH CELL
D 6 .....................................................................................................................................................................................................
COLUMN CELL
..............
-IAGITATED C O L U M N
4
so
~s
eo
~s
lo
is
8'o
8's
~o
9s
COAL RECOVERY (daf, %) Fig.9 Concentrate ash content as a function of coal recovery for the -25 micron size fraction, for each of the cell types.
100
The role of particle size in coal flotation
1235
In the -25 micron size fraction, a further decrease in selectivity with respect to the batch
cell was observed, while the column cell again demonstrated the best performance. The significant differences between the rate of coarse particle recovery in the agitated column in comparison to the other two cells is clearly shown in Figure 10. Here the ratio of the coarse particle coal recovery (+150 micron) to the overall coal recovery is plotted against the overall coal recovery. In can be seen that for the batch and column ceils, the proportion of coarse material recovered with increasing overall recovery was very similar. However, in the agitated column, the coarse fraction formed a far greater fraction of the coal recovered with increasing overall recovery, indicating that the kinetics of coarse particle flotation were substantially increased in the presence of agitation.
1
,.H0.9-I-
~ .'e~ "'°'°
uJ 0.8> O ¢..) uJ n- 0 . 7 _J _J < re LU >
0.60.5-
re
@
w
/,/
> 0.40 0 LU re 0 . 3 LIJ U)
re < 0
0.2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0.1 ..................................................................................................................................................................
0
50
ss
go
6s
Cs
8o
gs
BATCHCELL r-] COLUMNCELL '4AGITATEDCOLUMN
90
95
100
OVERALL COALRECOVERY(daf,%)
Fig.10 The ratio of coarse particle recovery to the overall coal recovery plotted against the overall coal recovery, as an indication of the difference in the rate of coarse particle flotation in the different cell types. DISCUSSION
The nature of a typical coal flotation plant feed, such as that examined in this investigation, makes it necessary to provide conditions conducive to both fine and coarse particle recovery to achieve optimum performance. In the conventional batch flotation cell, it was observed that coarse particles could be recovered efficiently if enough collector was added to render the particles sufficiently hydrophobic. However, under these conditions, the selectivity achieved in the other size fractions became progressively poorer with decreasing particle size. The loss of selectivity in the fine sizes can be attributed to the entrainment of fine gangue particles into the concentrate. Overall, the concentrate that was produced in a single stage had an unacceptably high ash content, indicating that additional stages of cleaning would be required to achieve a satisfactory product.
1236
M . C . HARRIS el al.
The column cell demonstrated a significant improvement in selectivity in comparison to the batch cell, with the difference particularly marked in the finer size fractions. This can be attributed to the addition of wash water to the froth at a positive bias rate, and to the increased froth depth in comparison to the conventional cell. It has been shown in many applications that under these conditions, the entrainment of fine gangue can be practically eliminated [15]. However, the performance of the column cell with respect to coarse particle recovery was relatively poor. Observation of the column cell during operation indicated that most of the collection took place in the first 10 cm below the feed inlet. The batch flotation results suggest that the fine particles were recovered in this region, and therefore any coarse particle recovery took place lower down in the column. This, in combination with the reduced residence time due to the high particle settling velocity, suggests that the probability of coarse particle collection was substantially reduced. In addition, the increased distance over which the particle must be carried to reach the froth would increase the probability of particle-bubble detachment. The probability of re-collection after detachment would again be reduced due to the high particle settling velocity. The agitated column cell demonstrated the ability to provide conditions conducive to the selective recovery of both coarse and fine particles. The quiescent column section, operated identically to that of the conventional column cell, provided conditions suitable for the selective flotation of the fine particles. Once again, observation of the cell during operation indicated that most of the collection took place within the first few centimetres below the feed point, or just above the agitated section. Thus it is probable that most of the coarse particle collection occurred in the agitated section of the cell. The suitability of the agitated zone for coarse particle recovery can be related to the requirements of a large particle for successful collection. It has been well established that coarser particles are best collected by larger bubbles [10]. It has also been shown that the effectiveness of larger bubbles is enhanced by increased turbulence [16]. In addition, the suspension of the coarse particles by agitation eliminates the problem of reduced residence time due to settling. These factors suggest that the improved performance can be attributed to the increased probability of coarse particle collection by bubbles of a suitable size to allow successful transportation into the froth zone. The improved efficiency of coarse particle collection in an agitated environment is further supported by the reduced air requirement of the agitated column cell in comparison to the conventional column cell for similar levels of performance (2.4 vs 3.5 l/rain). In addition, the results obtained on the agitated column cell indicate that coarse particle recovery need not be adversely affected by a relatively high wash water addition. The marginally lower coal recovery obtained in the -25 micron size fraction by the column cell (87%) compared to the batch cell (90%) was probably due to the removal of some high ash composite particles due to the addition of wash water. However, the lower recovery in this fraction obtained by the agitated column (84%) in comparison to the column cell indicates that the length of the quiescent collection zone could require optimisation to achieve maximum fine particle recovery. The length of the quiescent collection zone of the agitated column probably also plays a role in explaining the differences in selectivity that were observed between the agitated and conventional column cells in the coarse and fine size fractions. The improved selectivity in the coarse fraction demonstrated by the agitated cell can probably be ascribed to fewer large 'clean' particles being lost to the tails as a result of their higher settling velocity. The decrease in selectivity observed in the finer size fractions in the agitated column can probably be accounted for by the fact that in a well mixed system some material will experience a residence time very much shorter than the mean residence time. Thus it is probable that some fine 'clean' particles arrived uncollected in the agitated zone, and were
The role of particle size in coal flotation
1237
lost to the tailings before collection could occur. Increasing the length of the quiescent collection zone would help to rectify this. CONCLUSIONS The beneficiation of material that contains significant quantities of both ultrafine and coarse material by flotation, such as the fines fraction produced by many collieries, can present many difficulties in achieving optimum performance for the full range of particle sizes that are present. In this investigation, good performance with coarse and fine particles using a conventional was achieved in the finer size fractions was gangue. This resulted in an unacceptably high
respect to recovery was obtained for both flotation cell. However, the selectivity that very poor, due to the entrainment of fine ash content in the final product.
The column cell demonstrated excellent performance with respect to recovery and selectivity in the size fractions below 150 micron, but this was achieved at the cost of high losses in the coarse +150 micron size fraction. The results obtained on the hybrid agitated column cell in the investigation indicated that the addition of an agitated section to a conventional column cell can significantly improve the performance of the unit with respect to the recovery of coarse particles, while maintaining good recovery and selectivity in the fine sizes. A number of aspects of the agitated column require further investigation, such as the effect of the quiescent collection zone length; and the effect of the degree of agitation employed. In addition, the performance characteristics on a metalliferous ore would be of considerable interest. ACK NOWLE DG EM ENTS The authors would like to thank GENMIN and SENMIN for their generous support of flotation research at UCT. In addition thanks are due to Mrs H. Divey and Mrs L. Wall for performing all the characterisation and batch flotation testwork, and to Mr A. M. Barker for construction of the agitated column cell. The proximate and sulphur analysis of the coal sample was performed by RICHLAB, while the petrographic analysis was performed by Falcon Research Laboratories, both based in Johannesburg.
REFERENCES .
Jameson, G.J., A new concept in column flotation design. Column Flotation '88 (K. V. S. Sastry, ed) SME Annual Meeting, Phoenix, Arizona, 281, (1988).
.
Bahr, A., lmhoff, R., Changgen, L. & Muller, \V., Development and progress in the application of the pneumatic flotation cell. Column FIolalion '91. Proceeding of an International Conference on Column Flotation (G. E. Agar, B. J. Huls, D. B. Hyma, eds) Sudbury, Ontario, 703, (1991).
.
Guney, A., Onal, G. & Dogan, M., Beneficiation of Uckopru chromite gravity tailings by free jet type flotation system. Column Flotation '91. Proceeding o f an International Conference on Column Flotation (G. E. Agar, B. J. Huls, D. B. Hyma, eds) Sudbury, Ontario, 609, (1991).
ME ~-10/12--K
1238
.
.
.
.
.
M.C.
HARRIS et al.
Introducing the Filblast froth column. Product description issued by Atomaer Technology Pty Ltd, Como, Western Australia, (1991). Yang, D. C., A new packed column flotation system. Column Flotation '88 (K. V. S. Sastry, ed) SME Annual Meeting, Phoenix, Arizona, 257, (1988). Schneider, J. C. & Van Weert, G., Design and operation of the hydrochem flotation column. Column Flotation '88 (K. V. S. Sastry, ed) SME Annual Meeting, Phoenix, Arizona, 287, (1988). Degner, V. R. & Sabey, J. B., WEMCO/Leeds flotation column development. Column Flotation '88 (K. V. S. Sastry, ed) SME Annual Meeting, Phoenix, Arizona, 267, (1988). Ulan, W. W., Green, D. & Kosick, G. A., In-plant testing of the Outokumpu high grade flotation cell. Column Flotation '91. Proceeding of an International Conference on Column Flotation (G. E. Agar, B. J. Huls, D. B. Hyma, eds) Sudbury, Ontario, 689, (1991).
.
Murdock, D. J., Tucker, R. J. & Jacobi, H. P., Column cells vs conventional flotation, a cost comparison. Column Flotation '91. Pm~ceediH,,, of an Inlernational Conference on Column Flotation (G. E. Agar, B. J. Huls, D. B. Hyma, eds) Sudbury, Ontario, 647, ( 1991 ).
10.
Glembotskii, V. A., Klassen, V. 1. and Plaksin, !. N., Flmatiotl, 230-235, Primary Sources, New York, (1963).
11.
Franzidis, J.-P., Harris, M. C. & O'Connor, C. T., Review of column flotation practice on South African mines. Column Flotatimz '91. Proceeding of an International Conference on Column Flotation (G. E. Agar, B. J. Huls, D. B. Hyma, eds) Sudbury, Ontario, 479, (1991).
12.
Firth, B. A., Swanson, A. R. & Nicol, S. K., The influence of feed size distribution on the staged flotation of poorly floating coals. Proc. Au.s'/ralas. ln.st. Min. Metall. No 167, 49 (1978).
13.
Franzidis, J.-P. & Harris, M. C., A new method for the rapid float-sink analysis of coal fines. J. S. Aft. hist. Min. Metall., 86, No. 10, 409 (1986).
14.
Dell, C. C., Release analysis, a new tool for ore dressing research, htst. ,,1Iin. Melall. S),mposium on Rec'ettl Developmenls in Mitwra/ Dres.sm,,, l.,ondon (I 952).
15.
Espinosa-Gomez, R., Finch, J. A. & Johnson, N. \V., Column flotation of very fine particles. Minerals Engineeri/zg, 1, No 1, 3, (1988).
16.
Jameson, G. J., Physical aspects of fine particle flotation. Principles of Mineral Flotation, Jones M. H. and Woodcock J. T., Australasian Institute of Mining and Metallurgy, Victoria, Australia, 215, (1984).
0892-6875/92 $5.00+0.00 © 1992 Pergamon Press Ltd
AN EVALUATION OF THE ROLE OF PARTICLE SIZE IN THE FLOTATION OF COAL USING DIFFERENT CELL TECHNOLOGIES M.C. HARRIS, J-P. FRANZIDIS, C.T. O'CONNOR and P. STONESTREET Department of Chemical Engineering, University of Cape Town, Rondebosch 7700, South Africa
ABSTRACT A large variety of new flotation cells has been introduced in the last f e w ),ears, probably as a result o f the successful introduction of column flotation in the minerals processing industry, hr common with the column cell. a number of these new cells employ an essentially quiescent separation zone. However, a number o f novel cell designs have been introduced that use agitation mechanisms similar to those employed on conventional flotation cells. The aim o f this investigation was to evaluate flotation behaviour in both an agitated and non-agitated environment, particularly with respect to particle size. A hybrid 'agitated' column cell was designed for the investigation, and the operation of this unit was compared to that o f a column ceil, and to a batch flotation cell. on a laboratory scale. The testwork was conducted on coal fines, as problems with the flotation o f coarse coal particles in a column cell had previously been identified. It was demonstrated that the addition o f an agitated stage to a column cell can significantly improve the coarse particle recovery in comparison to the conventional column cell, while maintaining good selectivity in the fine sizes.
Keywords Fine coal flotation, cell design, particle size INTRODUCTION Flotation equipment technology has made considerable advances in the last decade. A large variety of new flotation machines has been introduced, as an alternative to the conventional flotation cells that predominated in the industry for so many years. This proliferation of new technology has almost certainly been sparked by the successful introduction of the column cell (now mostly referred to as the 'conventional' or 'Canadian' column) on many mines since the early 1980's. It has been shown that column cells can achieve both improved performance and reduced cost in many applications. In particular, the addition of wash water to a deep froth bed, resulting in a net downward flow of water through the froth (positive bias), enabled the column cell to produce substantially higher product grades than conventional cells for equivalent recoveries. This success has lead to the design of a wide variety of new cell types that have typically sought to improve on one or more aspects of the conventional column cell design, while maintaining the advantages of improved grade that result from operating with a deep, well washed froth. Flotation cells such as the Jameson cell, the Bahr (pneumatic) cell, the Free jet flotation cell and the Filblast cell have introduced feed slurry aeration systems designed to
1225
1226
M . C . HARms et al.
substantially improve collection efficiency, which, along with a number of claimed advantages, results in a considerable reduction in height compared to the conventional column cell (1 to 2 m compared to 10 to 15 m) [1,2,3,4]. Other novel cell designs have maintained similar dimsions to that of the conventional column cell, and modified the internal characteristics for improved performance in specialised applications. The packing employed in the packed column enables the formation of a deep, stable froth bed for efficient cleaning of very fine particles [5]. The Hydrochem cell uses impellers mounted on a central shaft to induce varying degrees of agitation down the length of the cell, with the aim of creating zones in which optimum conditions exist for the collection of different size particles within the feed [6]. The reaction of the conventional cell manufacturers has been to introduce a number of so called 'column cells' which are characterised by dimensions and agitator configurations similar to conventional cells. These cells include modifications which allow for reduced gangue entrainment by improved cleaning in the froth zone. The Leeds column, marketed by Wemco, contains rods of varying density in the froth zone, which are designed to squeeze out the lightly held gangue particles from the mineralised bubbles [7]. The O u t o k u m p u high grade cell employs a conventional OK mechanism, combined with froth crowding and washing to achieve improved performance [8]. The use of agitation in cells such as the Leeds cell and the OK cell, and to a lesser extent in the H y d r o c h e m cell, is in clear contrast to the essentially quiescent operation of the conventional column cell. While the manufacturers of non-agitated cells point to the advantage of reduced maintenance and power requirements [9], the manufacturers of machines based on a more conventional design can point to the proven effectiveness of flotation in an agitated system over many years, and the familiarity of plant operators with such systems [8]. One of the main functions of agitation is to suspend the solids in the celt, and allow equal time for collection of particles of all sizes. In systems in which no agitation is employed, the residence time of a particle decreases in proportion to its size and density, owing to gravity. Thus the residence time of a coarse particle is often a small fraction of that of a very fine particle in a non-agitated system. This can lead to substantial losses of valuable mineral to the tailings stream when significant quantities of coarse particles are present in the feed. This can apply in a column cell, where particle collection occurs inside the cell, and in the cells that employ feed slurry aeration systems where particle collection occurs outside the cell. In both cases, losses can arise as a result of particle-bubble detachment inside the cell, which becomes increasingly probable with increasing particle size [10]. After detachment, some of the coarse particles will be lost to the tails due to insufficient time for recollection. Thus, the aim of this investigation was to evaluate the effect of agitation on flotation performance as a function of particle size, with particular emphasis on coarse particle recovery. The flotation response of a coal sample was investigated on a laboratory scale in three types of flotation cell: a conventional batch flotation cell; a 2" diameter column cell; and a specially designed hybrid cell that was essentially a combination of the two cells, which was adapted for continuous operation. The work represents part of a broad programme that has been undertaken at the University of Cape Town to identify the optimum type of flotation equipment for specific applications.
BACKGROUND It was decided to conduct this investigation on coal, as previous testwork on a number of South African coals had shown that while the column cell was extremely efficient at cleaning coal fines below 150 microns in size, the collection efficiency decreased significantly for larger particles [11]. Other work has shown that the recovery of coarse particles by flotation was particularly poor when large quantities of ultrafine material (-25 micron) were present [12]. This is frequently the case with the cyclone overflow that either
The role of particle size in coal flotation
1227
represents, or would represent, the feed to flotation on most South African collieries, which have been found to consist of up to 50 % ultrafine material and as much as 20 % +150 micron material. This is despite the widespread introduction of spirals to treat the l mm to 0.15 mm fines fraction. The study was conducted on a sample of coal fines from a colliery situated in the Natal coalfield in South Africa, that produces coking coal for domestic steel production. The sample consisted of naturally arising fines that form the feed to the conventional flotation plant operated at the colliery. EXPERIMENTAL
Sample Characterisation The feed sample was characterised using the following techniques: size and ash distribution; float and sink analysis; flotation release analysis; proximate analysis and petrographic analysis. The float and sink analysis was performed using a centrifugal technique developed at the University of Cape Town that enables rapid and accurate analysis of very fine coal samples [13]. Flotation release analysis aims to establish the optimum performance that can be achieved by flotation [14]. The technique used in this investigation consisted of subjecting the sample to incremental flotation in a 3 litre Leeds batch flotation cell. Initially, the minimum reagent addition was used, and the most gentle operating conditions employed, so that only the most hydrophobic material reported to the concentrate. Gradually, by increasing the reagent addition and air rate, more and more material was recovered, until the cell was barren of floatable material. The results of the coal characterisation are presented in Table 1. The sample had an ash content of 31%, and consisted of about 46% ultrafines, and about 15% 'coarse' (+150 micron) material. The majority of the ash was concentrated in the ultrafine fraction. The float and sink analysis and the release analysis indicated that a yield of about 60 % could be obtained at 10% ash, rising to about 68% at 13% ash. The predominant maceral in the sample was vitrinite, at 85% by volume.
Equipment Three types of equipment were used in the investigation. These are presented schematically in Figure l, and can be described as follows: I.
2.
.
A 3 litre bottom driven Leeds-type batch flotation cell was used to conduct batch flotation experiments in a completely mixed environment. A 2 inch diameter column cell was used to conduct continuous experiments in a quiescent environment. A column height of 2 metres was used. The feed point was 0.5 metres from the top of the column. An external USBM type bubble generator was used, and the interface level was controlled using a conductivity probe. A hybrid agitated column cell was designed specifically for this testwork. It consisted of the top half metre section of the 2" column cell, attached via a conical adaptor to the L e e d s - t y p e batch flotation cell used for the batch flotation tests. The cell was adapted for continuous operation by placing a tails outlet at the base of the cell. The feed point and wash water system were identical to those on the column cell. Thus the cell consisted of a quiescent top (column cell) section, and completely mixed bottom (Leeds batch cell) section.
Both the column cell and the agitated column cell had the same total volume, to enable a comparison at equivalent liquid residence times.
1228
M. C. HARRIS et al.
TABLE 1 Feed characterisation
Size Analysis:
Proximate Analysis:
Size Fraction (micron)
Mass (%)
Ash (~)
+150 + 75-150 + 25-75 -25
15.8 14.6 23.9 46.1
22.1 23.9 29.1 36.9
100.0
30.8
Total
H20 (%)
Cum. mass (%)
21.0 48.9 23.9 1.4
Sulphur (%)
Petrographic Analysis: Vitrinite (% vol) Exinite (% vol) Inertinite (% vol)
Float and Sink Analysis: Rel. Density
1.7
Volatiles (%) Fixed Carbon (%) CV (Md/kg)
Cum. Ash (%)
1.30 1.35 1.40 1.45 1.50 1,55 1.65 1.75 1.80
18.9 30.6 41.6 51.6 58.0 60.6 69.9 75.0 78.5
3.8 4.1 5.8 7.5 9.0 9.5 13.8 16.1 19.1
Total
I00.0
30.8
85.0 4.0 II.0
Release Analysis: Cum. Yield (%) 5.6 15.3 30.1 40.2 50.1 55.4 63.1
Cum. Ash (%) 5.5 6.0 7.1 7.8 8.5 9.1 10.0
Testwork A range of scouting tests was conducted on each of the cells to establish the correct range of the various operating conditions to achieve satisfactory performance. Operating conditions were then selected to obtain a range of flotation yields between 40 and 70%. Four tests in this range were selected for detailed analysis for each cell type. The concentrate and tailings samples generated in these experiments were analysed for size and ash distribution. The conditions that were used on each of the cells are presented in Table 2. RESULTS The overall results obtained in the tests on each cell type are presented in Figure 2, which shows the performance with respect to yield and concentrate ash content. The results of the float and sink and the release flotation analyses are included for comparison. The results of a number of the preliminary scouting tests are also included.
The role of particle size in coal flotation
BATCH FLOTATION CELL
CONVENTIONAL COLUMN CELL
~
1229
AGITATED COLUMN CELL
sh water
Concentrate v ~
Fr°th
i Wash water
Concentrate
~ ~Froth O01ESCE NT
Feed
Froth ,
"t
Feed
Concentrate
OUIESCE~VT
Air ~ j ~ X E D
[
Air
[ i I
Conventional bottom driven Leeds-type flotation cell Mode of operation: Batch Volume: 3 litres Air injected directly onfo impeller
Impeller Water Air
"-~ ~Tails
2" diameter column flotation cell
Mode of operation: Continuous Height: 2 metres Collection zone volume: 4 litres Sparger: external bubble generator
Fig.l
MIXED
I
!
Impeller (1200 rpm)
~
Tails
(1200rpm)
Hybrid agitated column cell
0,5 m top section of 2" column attached to .5 I Leeds-type flotation cell Mode of operation: Continuous Collection zone volume: 4 litres Air injected directly onto impeller
A schematic diagram of the equipment used in the investigation.
It can be seen that the performance achieved in the batch cell was substantially poorer than that obtained in both the column cell and the agitated column cell. The latter cells were both able to achieve a performance that closely approached the release flotation line. Only at a product ash of around 14% did all three cells converge on the optimum performance line defined by the float and sink curve. The coal recovery by size results obtained in the tests performed on the batch flotation cell are presented in Figure 3 on a dry, ash free basis (daf), along with the overall recovery achieved in each of the tests. The preferential flotation of the -75 micron fines is clearly apparent; the coarser fractions were only recovered after most of the fines had been collected. Ultimately, however, a coarse particle recovery very close to that obtained on the other size fractions was achieved. The analogous results for the tests performed on the column cell and the agitated column cell are presented in Figures 4 and 5. In the tests performed on the column cell, recovery was more pronounced in the intermediate size fractions, although below 150 micron the m a x i m u m recoveries obtained were very similar to the batch results. However, significant losses occurred in the +150 micron fraction, reducing the overall coal recovery that could be achieved by about 5%. In the tests performed on the agitated column, an almost uniform increase in the recovery from each size fraction was observed with increasing overall recovery. In the test with the highest overall recovery, the coal recovery obtained in the intermediate fractions was particularly high. In addition, the maximum coarse particle recovery was almost the same as that achieved in the batch cell. However, the recovery in the ultrafine fraction was
1230
M.C.
et al.
HARRIS
marginally lower than in either the batch or the column cell, giving a maximum overall recovery about 2% lower than the batch cell result. 18
/
/ 16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-" ....................................................... II .+.-
/ .+..
14 .........................................................................................
I I+ q -. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
+
+
•
m
qb
m
-
m < IdJ
/ 12 ................................................................................. IB m
II ............ [SS3 E 3
II
I-
ZI i i
10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
~
{=) Z
O
.:7 •, ~,+/ / . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~" ........... ~ .7 FLOAT & SINK
............
•Jt-[~
s
.............................
.............~..++,.~.:::~" /:~ ......................
+
J__,o..o.+-'+'"+°°'°°
t 0
10
20
30
.. J
40
50 YIELD
.E,~se
~LOTAT,ON
BATCH CELL
.,, '"" ,'"'+
+ I
-'"°'"
.......
- ........................................................ -
++" .... c+ C O L U M N CELL
60
! 80
70
90
100
(%)
Fig.2 Summary of the overall performance of the each of the cell types in comparison to the performance predicted by float and sink, and flotation release analyses.
100 908070-
.....................~ .......
?
.
+
........................:...
~
.............................................................................. ...................
~:;I;I;I;IIIIIIII~,~.~YlI.I::~:~,.,.,.,=~
............
~
.......................................................................................
.................. " " ~ 3 +,::,.................... "i??:::,....................................................................................................... ~ IZ3
............................................................................
60\
"\
500
0 J,i c~ .J
40-
0 0
30-
\
\
........................................................................................................................................":~......... X ......................................................................
20 ......................................................................................................................................................................... 1 0 ...........................................................................................................................................................................................................................
I ,,+c, F,O,,,.o,c~,, 0
-25
25,~75
L 75"150 + 150 SIZE FRACTION (micron)
'
OVERALL I~ECOVERY
Fig.3 The coal recovery by size results obtained in the tests performed on the batch flotation cell on a dry, ash free basis.
The role of particle size in coal flotation
1231
TABLE 2 Flotation conditions employed in the testwork 1. Conditions common to a l l three c e l l s : Shellsol A (95 % aromatic) Frother: H i - f l a s h TEB (supplied by SENMIN for f l o t a t i o n of coal fines) Pulp density: 10 % (mass/vol)
Collector:
2. Batch c e l l conditions: Frother concentration: 80 g/t Impeller speed: 1200 rpm Froth height: 2 cm Test No 1 2 3 4
Air rate: 6 I/mln Flotation time: 6 min
Collector conc. (g/t) 50 100 250 750
3. Column c e l l conditions: Feed rate: 2.7 kg/h Bias rate: 0.2 cm/s (+ or - 0.03 cm/s) Liquid residence time: 5 min Test No
Air rate (I/mln)
1 2 3 4
2.0 2.5 2.9 3.5
Froth height: 30 cm Collector conc: 1000 g / t Feed point: 0.5 m from top
Frother conc. (g/t) 130 130 130 180
4. Agitated column cell conditions: Feed rate: 2.7 Kg/h Bias rate: 0.2 cm/s (+ or - 0.03 cm/s) Liquid residence time: 5 min Impeller speed: 1200 rpm Test No
A i r rate (]/mtn)
1
2 3
1.6 1.8 2.0
4
2.4
Froth height: 30 cm Collector conc: 1000 g / t Feed point: 0.5 m from top
Frother conc: 100 g / t
The relationship between coal recovery and concentrate ash content for the +150 micron size fraction is presented in Figure 6 for each of the cell types. For this size fraction, the selectivity achieved by the batch and column cells was very similar, while some improvement in selectivity was observed in the agitated column cell. Figures 7, 8 and 9 show the analogous results for the 75 to 150 micron size fraction, the 25 to 75 micron size fraction and the -25 micron size fraction respectively. In the 75 to 150 micron fraction, the selectivity of the batch cell was significantly poorer than either of the column cells. Once again, however, the agitated column demonstrated improved selectivity in comparison to the column cell. In the 25 to 75 micron size fraction, the margin of the difference in selectivity between the batch cell and the two column cells was even greater. For this fraction, however, improved selectivity was achieved by the column cell relative to the agitated column.
M. C. HARRISet al.
1232
QOI 80-
-IA
70-
0
605040-
%
30-
\.
.“..
10 COLUMN CELL 0
! -25
I 25*75
I I 75*150 +150 SIZE FRACTION (micron)
,
/ I OVERALL RECOVERY
Fig.4 The coal recovery by size results obtained in the tests performed on the column cell on a dry, ash free basis.
\ .,
50
_“.,s. ‘*.
i
,oJ
0 -.
............................ .........
. AGITATED COLUMN CELL
0
1 -25
1 25*75
1 +150 75*150 SIZE FRACTION (micron)
I
I OVERALL RECOVERY
Fig.5 The coal recovery by size results obtained in the tests performed on the agitated column cell on a dry, ash free basis.
The role o f particle size in coal flotation
1233
18 [PLUS 1,50 MICRON SIZE FRACTION I
14........................................................................................................................................................................................................................... EE
lZ
........................................................................................................................................................................................................................... -...........-...--~
12
LU lZ 0 L) ..r (n ,<
................
. ........
......................................................... :::~........................;::~o" ":~:..................................................................................................................
r--i" ~ ° ' ' ' ' ' ' ' ' ' "
4
0
1'0
20
:30
%,CNCELL
I
COLUMN CELL
..............
"0"
40
gO
60
7'0
UMN
80
90
100
COAL RECOVERY (daf, %)
Fig.6 Concentrate ash content as a function of coal recovery for the +150 micron size fraction, for each of the cell types.
18 I 7 5 T O 150 MICRON SIZE FRACTION I 16 ...........................................................................................................................................................................................................................
........................................................................................................................................................................................................................... ./.../....
14
/
lZ
12-
,,...,'' /
U.l lz 0 0 'l- 1 0 u) <
,
/
:;~.~:~ ................. r7 •.4-. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-
Ell TCN CELL 6
....................................................................................................................................... . ~ ..................................................................................
C tOLUMN
CELL
...............
AGITATED COLUMN 4
50
,
,
,
,
,
,
55
60
65
70
75
80
85
90
95
COAL RECOVERY (daf, %)
Fig.7 Concentrate ash content as a function of coal recovery for the 75 to 150 micron size fraction, for each of the cell types.
100
1234
M.C.
HARRIS et al.
18
[ 25 TO 75 MICRON SIZE FRACTION I
Z LIJ IZ 0 0 3: (/)
~..-t~_...../ " /."...............................
1 2 ........................................................................................................................................................ ,,o"
oo,-
-
/
BATCH CELL
r-7
6 ..........................................................................................................................................................................................
C O L U M N CELL
......
"lAGITATED C O L U M N
4
50
55
Fig.8
(~0
65
10 15 80 COAL RECOVERY (daf, %)
8'5
90
95
100
Concentrate ash content as a function of coal recovery for the 25 to 75 micron size fraction, for each of the cell types.
18
16-
~
25 MICRON SIZE FRACTION]
i
/
rTn
I. Z 12- ...................................................................................................................................................................... uJ lZ ...e ................ i,i.~.i,:.;.............. 0 10.I< ..................................................
~.:;.;~..~..
......
..~.~ .......................
[~..,,~
'-I--
.,'"
~.-::;;~'~/~"
........................
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8 ...........................................................................................................................................................................................................................
BATCH CELL
D 6 .....................................................................................................................................................................................................
COLUMN CELL
..............
-IAGITATED C O L U M N
4
so
~s
eo
~s
lo
is
8'o
8's
~o
9s
COAL RECOVERY (daf, %) Fig.9 Concentrate ash content as a function of coal recovery for the -25 micron size fraction, for each of the cell types.
100
The role of particle size in coal flotation
1235
In the -25 micron size fraction, a further decrease in selectivity with respect to the batch
cell was observed, while the column cell again demonstrated the best performance. The significant differences between the rate of coarse particle recovery in the agitated column in comparison to the other two cells is clearly shown in Figure 10. Here the ratio of the coarse particle coal recovery (+150 micron) to the overall coal recovery is plotted against the overall coal recovery. In can be seen that for the batch and column ceils, the proportion of coarse material recovered with increasing overall recovery was very similar. However, in the agitated column, the coarse fraction formed a far greater fraction of the coal recovered with increasing overall recovery, indicating that the kinetics of coarse particle flotation were substantially increased in the presence of agitation.
1
,.H0.9-I-
~ .'e~ "'°'°
uJ 0.8> O ¢..) uJ n- 0 . 7 _J _J < re LU >
0.60.5-
re
@
w
/,/
> 0.40 0 LU re 0 . 3 LIJ U)
re < 0
0.2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0.1 ..................................................................................................................................................................
0
50
ss
go
6s
Cs
8o
gs
BATCHCELL r-] COLUMNCELL '4AGITATEDCOLUMN
90
95
100
OVERALL COALRECOVERY(daf,%)
Fig.10 The ratio of coarse particle recovery to the overall coal recovery plotted against the overall coal recovery, as an indication of the difference in the rate of coarse particle flotation in the different cell types. DISCUSSION
The nature of a typical coal flotation plant feed, such as that examined in this investigation, makes it necessary to provide conditions conducive to both fine and coarse particle recovery to achieve optimum performance. In the conventional batch flotation cell, it was observed that coarse particles could be recovered efficiently if enough collector was added to render the particles sufficiently hydrophobic. However, under these conditions, the selectivity achieved in the other size fractions became progressively poorer with decreasing particle size. The loss of selectivity in the fine sizes can be attributed to the entrainment of fine gangue particles into the concentrate. Overall, the concentrate that was produced in a single stage had an unacceptably high ash content, indicating that additional stages of cleaning would be required to achieve a satisfactory product.
1236
M . C . HARRIS el al.
The column cell demonstrated a significant improvement in selectivity in comparison to the batch cell, with the difference particularly marked in the finer size fractions. This can be attributed to the addition of wash water to the froth at a positive bias rate, and to the increased froth depth in comparison to the conventional cell. It has been shown in many applications that under these conditions, the entrainment of fine gangue can be practically eliminated [15]. However, the performance of the column cell with respect to coarse particle recovery was relatively poor. Observation of the column cell during operation indicated that most of the collection took place in the first 10 cm below the feed inlet. The batch flotation results suggest that the fine particles were recovered in this region, and therefore any coarse particle recovery took place lower down in the column. This, in combination with the reduced residence time due to the high particle settling velocity, suggests that the probability of coarse particle collection was substantially reduced. In addition, the increased distance over which the particle must be carried to reach the froth would increase the probability of particle-bubble detachment. The probability of re-collection after detachment would again be reduced due to the high particle settling velocity. The agitated column cell demonstrated the ability to provide conditions conducive to the selective recovery of both coarse and fine particles. The quiescent column section, operated identically to that of the conventional column cell, provided conditions suitable for the selective flotation of the fine particles. Once again, observation of the cell during operation indicated that most of the collection took place within the first few centimetres below the feed point, or just above the agitated section. Thus it is probable that most of the coarse particle collection occurred in the agitated section of the cell. The suitability of the agitated zone for coarse particle recovery can be related to the requirements of a large particle for successful collection. It has been well established that coarser particles are best collected by larger bubbles [10]. It has also been shown that the effectiveness of larger bubbles is enhanced by increased turbulence [16]. In addition, the suspension of the coarse particles by agitation eliminates the problem of reduced residence time due to settling. These factors suggest that the improved performance can be attributed to the increased probability of coarse particle collection by bubbles of a suitable size to allow successful transportation into the froth zone. The improved efficiency of coarse particle collection in an agitated environment is further supported by the reduced air requirement of the agitated column cell in comparison to the conventional column cell for similar levels of performance (2.4 vs 3.5 l/rain). In addition, the results obtained on the agitated column cell indicate that coarse particle recovery need not be adversely affected by a relatively high wash water addition. The marginally lower coal recovery obtained in the -25 micron size fraction by the column cell (87%) compared to the batch cell (90%) was probably due to the removal of some high ash composite particles due to the addition of wash water. However, the lower recovery in this fraction obtained by the agitated column (84%) in comparison to the column cell indicates that the length of the quiescent collection zone could require optimisation to achieve maximum fine particle recovery. The length of the quiescent collection zone of the agitated column probably also plays a role in explaining the differences in selectivity that were observed between the agitated and conventional column cells in the coarse and fine size fractions. The improved selectivity in the coarse fraction demonstrated by the agitated cell can probably be ascribed to fewer large 'clean' particles being lost to the tails as a result of their higher settling velocity. The decrease in selectivity observed in the finer size fractions in the agitated column can probably be accounted for by the fact that in a well mixed system some material will experience a residence time very much shorter than the mean residence time. Thus it is probable that some fine 'clean' particles arrived uncollected in the agitated zone, and were
The role of particle size in coal flotation
1237
lost to the tailings before collection could occur. Increasing the length of the quiescent collection zone would help to rectify this. CONCLUSIONS The beneficiation of material that contains significant quantities of both ultrafine and coarse material by flotation, such as the fines fraction produced by many collieries, can present many difficulties in achieving optimum performance for the full range of particle sizes that are present. In this investigation, good performance with coarse and fine particles using a conventional was achieved in the finer size fractions was gangue. This resulted in an unacceptably high
respect to recovery was obtained for both flotation cell. However, the selectivity that very poor, due to the entrainment of fine ash content in the final product.
The column cell demonstrated excellent performance with respect to recovery and selectivity in the size fractions below 150 micron, but this was achieved at the cost of high losses in the coarse +150 micron size fraction. The results obtained on the hybrid agitated column cell in the investigation indicated that the addition of an agitated section to a conventional column cell can significantly improve the performance of the unit with respect to the recovery of coarse particles, while maintaining good recovery and selectivity in the fine sizes. A number of aspects of the agitated column require further investigation, such as the effect of the quiescent collection zone length; and the effect of the degree of agitation employed. In addition, the performance characteristics on a metalliferous ore would be of considerable interest. ACK NOWLE DG EM ENTS The authors would like to thank GENMIN and SENMIN for their generous support of flotation research at UCT. In addition thanks are due to Mrs H. Divey and Mrs L. Wall for performing all the characterisation and batch flotation testwork, and to Mr A. M. Barker for construction of the agitated column cell. The proximate and sulphur analysis of the coal sample was performed by RICHLAB, while the petrographic analysis was performed by Falcon Research Laboratories, both based in Johannesburg.
REFERENCES .
Jameson, G.J., A new concept in column flotation design. Column Flotation '88 (K. V. S. Sastry, ed) SME Annual Meeting, Phoenix, Arizona, 281, (1988).
.
Bahr, A., lmhoff, R., Changgen, L. & Muller, \V., Development and progress in the application of the pneumatic flotation cell. Column FIolalion '91. Proceeding of an International Conference on Column Flotation (G. E. Agar, B. J. Huls, D. B. Hyma, eds) Sudbury, Ontario, 703, (1991).
.
Guney, A., Onal, G. & Dogan, M., Beneficiation of Uckopru chromite gravity tailings by free jet type flotation system. Column Flotation '91. Proceeding o f an International Conference on Column Flotation (G. E. Agar, B. J. Huls, D. B. Hyma, eds) Sudbury, Ontario, 609, (1991).
ME ~-10/12--K
1238
.
.
.
.
.
M.C.
HARRIS et al.
Introducing the Filblast froth column. Product description issued by Atomaer Technology Pty Ltd, Como, Western Australia, (1991). Yang, D. C., A new packed column flotation system. Column Flotation '88 (K. V. S. Sastry, ed) SME Annual Meeting, Phoenix, Arizona, 257, (1988). Schneider, J. C. & Van Weert, G., Design and operation of the hydrochem flotation column. Column Flotation '88 (K. V. S. Sastry, ed) SME Annual Meeting, Phoenix, Arizona, 287, (1988). Degner, V. R. & Sabey, J. B., WEMCO/Leeds flotation column development. Column Flotation '88 (K. V. S. Sastry, ed) SME Annual Meeting, Phoenix, Arizona, 267, (1988). Ulan, W. W., Green, D. & Kosick, G. A., In-plant testing of the Outokumpu high grade flotation cell. Column Flotation '91. Proceeding of an International Conference on Column Flotation (G. E. Agar, B. J. Huls, D. B. Hyma, eds) Sudbury, Ontario, 689, (1991).
.
Murdock, D. J., Tucker, R. J. & Jacobi, H. P., Column cells vs conventional flotation, a cost comparison. Column Flotation '91. Pm~ceediH,,, of an Inlernational Conference on Column Flotation (G. E. Agar, B. J. Huls, D. B. Hyma, eds) Sudbury, Ontario, 647, ( 1991 ).
10.
Glembotskii, V. A., Klassen, V. 1. and Plaksin, !. N., Flmatiotl, 230-235, Primary Sources, New York, (1963).
11.
Franzidis, J.-P., Harris, M. C. & O'Connor, C. T., Review of column flotation practice on South African mines. Column Flotatimz '91. Proceeding of an International Conference on Column Flotation (G. E. Agar, B. J. Huls, D. B. Hyma, eds) Sudbury, Ontario, 479, (1991).
12.
Firth, B. A., Swanson, A. R. & Nicol, S. K., The influence of feed size distribution on the staged flotation of poorly floating coals. Proc. Au.s'/ralas. ln.st. Min. Metall. No 167, 49 (1978).
13.
Franzidis, J.-P. & Harris, M. C., A new method for the rapid float-sink analysis of coal fines. J. S. Aft. hist. Min. Metall., 86, No. 10, 409 (1986).
14.
Dell, C. C., Release analysis, a new tool for ore dressing research, htst. ,,1Iin. Melall. S),mposium on Rec'ettl Developmenls in Mitwra/ Dres.sm,,, l.,ondon (I 952).
15.
Espinosa-Gomez, R., Finch, J. A. & Johnson, N. \V., Column flotation of very fine particles. Minerals Engineeri/zg, 1, No 1, 3, (1988).
16.
Jameson, G. J., Physical aspects of fine particle flotation. Principles of Mineral Flotation, Jones M. H. and Woodcock J. T., Australasian Institute of Mining and Metallurgy, Victoria, Australia, 215, (1984).