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The demineralization of a weathered coal by froth flotation
Powder
Technology.
35 (1983)
The Demineralization E_ T_ WOODBURN,
1 - 15
of a Weathered
D. J. ROBBINS
December
14.1981;
of Manchester
Flotation
Institute
of Science
and Technology.
Sacis~lle
in revised form April 26.1982)
SUMMARY
Froth flotation tests of weathered coal are reported in which pretreatment of the coal slurries by the absorption of SO2 gas improved the beneficiation remarkably- This was associated with the production of unstable froths and experiments were done to determine the relationship between froth stability and improved performance_ The experimental measurement of performance was based on gravimetric measurements of ashes but in addition microscopic analyses were done on selected runs providing number counts of coal and mineral particles in various size ranges for different flotation fractions_ The microscopic data provided useful information on the dminage of coal and mineral particles in the deep froths_
INTRODUCTION
The usage of low-rank and weathered coals is restricted because of the nature and amount of their associated minerals. These coals are difficult to beneficiate by conventional washing techniques because of the presence of a large amount of fine coal and shale and also, in the case of weathered coal, particularly because of their hydrophilic surfaces_ There is, however, a developing interest in the use of ultra-fine coals both in combustion and for conversion by entrained bed gasification and for liquefaction by catalytic pressure hydrogenation. At the particle sizes envisaged for these processes, the mineral will to a large extent be liberated from the coal. As a consequence, it is conceptually possible to separate mineral from coal by physical processes only_ 0032-5910/83/$3_00
Coal by Froth
and S. A_ FLYNN
Deportment of Chemical Engineering. Lmicersity Street. Manchester 3160 I QD (Gt. Britain)
(Received
1
Physical cleaning processes such as froth flotation partly demineralize coal, thus reducing the adverse effects of contaminants considerably, while not eliminating them completely, without adversely affecting the combustion characteristics of the cleaned material. In general, these processes require a high degree of liberation of mineral, which usually requG%es fine grinding_ The separation of ultra-fine particulates cannot be achieved by sink-and-float procedures but requires a process whose selectivity is based on surface effects_ Froth flotation is the oldest physical separation process of this type and has the advantages of a relatively low power consumption per unit mass of material processed and a proven capacity to handle higher tonnage rates of solids, than the newer oil agglomeration proceduresThe obvious disadvantages of froth flotation are its tendency to entrain very fine materials and the production of a wet froth_ Both effects can potentially be reduced by a cleaner float using a hydrocarbon entrainer, but first it is necessary to establish the best performance of the roughing floatIt was postulated that mineral entrainment could be minimized by the use of a deep butrelatively unstable froth in the roughing floats. The development of a treatment process which would recover a marketable fraction from coal waste is obviously attractive in the short term_ In the longer term, it will be necessary to develop analogous processes for low-rank coal of the NCB902 type, of which ’ the U-K. has large unesploited deposits_ Low-rank coals have a high volatile content and a high H[C ratio_ A demineralized lowrank coal would thus appear to be an excel0 ?%evier sequoiafinted
in The Netherlands
2
feed-stock for liquefaction. It is also particularly suitable for combustion because of its ease of ignition, and the most promising of several possible uses would be as the solid component in a coal-oil mixture. Law, Law and Lee [l] have discussed the combustion of these mixtures and have indicated the need for high combustion rates of the residual particulates after the liquid has been burned.
.Lent
PERFORMANCE
MEASURE
It is desirable to have a single measure of the degree of separation achieved which incorporates both the purity of the product and the fraction of dry ash-free coal recovered. Several measures have previously been suggested, e.g. Gaudin [2] and that used by Firth, Swanson and Nicol 131, both of which have the disadvantage of being unbounded at perfect separation, making it impossible to relate efficiency to .the limiting condition. If, however, a separation measure based on the entropy of mixing is defined for a binary mixture WE-C
i=2 i=
Xi 1nXi
1
then the separation of a feed stream of mixing entropy Hr into a fraction c of concentrate and 1 -c of tailings, each with its own entropy of mixing Hc and H, respectively, will be associated with an entropy reduction. AH=H,-cHc-(I-cc)H,
Fig_ l_ Beneficiation Deurbrouck
of
Baker, Miller and
S(t)
ash(t) in cumulative feed ash x 100 where t is a param eter indicating the degree of processing of the feed coal_ The recovery R(t) is the percentage of dry ash-free coal in the original feed which has been recovered in the concentrate_ The grade-recovem curve unambiguously defines by its upper bound the local optimal treatment, irrespective of technical requirements associated with grade, and of recovery, which is primarily an economic consideration,
FROTH
Since for a perfect separation WC = H, = 0, the limiting reduction in the mixing measure is
circuit
Moys
STABILITY
AND
143 defined
FLOTATION
KINETICS
a froth parameter
*=1-Z!? QP
AH=Hrr, A separation
efficiency
can then be defined
as HE=$xlOO f
Performance data is also reported in terms of grade-recovery curves (Figs. 2, 3, 4) in which the grade is defined by
where q,, is the air flux leaving the top surface of the froth by bubble breakage and qp is the air flux entering the froth from the pulp- 01 is clearly a function of froth height, there being a limiting height h, in a given cell representing a stationary froth from which no concentrate is being withdrawn. At this limiting co~ndition, qb = qp and CY= 0. For a froth from which concentrate is to be withdrawn, the initial formation time tp,
Point
Fig_ 2. Grade-recovery
curves,
nominal
-45
F10 Fll F12 F13 F14
0 l-71 ll_i7 3529 129-41
Point
Run
SOI (mglg
nominal
coal 1
pm fraction_
1
curves,
SO2 (m&g
1 2 3 -l 5
2 3 -I 5 6 i
Fig_ 3. Grade-recovery
Run
+-.I5 pm -105
which is the time lapse between the first admission of air to the first withdrawal of concentrate, is an ind’rect measure of (Y_ Assuming the froth volume is largely determined by its air content, then during the formation period
pm
FO Fl F2 F3 F-l F1/2 F212
coal)
0 3-71 ll_ii 35.29 129-11 3-71 ll_i7
fraction_
The simplest relationship incorporating dependence of cr on the froth height h is
a
4 h ~=l---
ha for which h, is a measure of froth stabilitySubstituting in eqn. (1)
(2) Equation (2) can be solved implicitly for h, under initial conditions. Moys has also derived a relationship linking the withdrawal rate of species i over the concentrate weir MC; with the rate at which the species entered the froth from the pulp
For beneficiation to take place, the ratio of froth return factors of mineral to coal should be high even though the individual values may be small_ The condition h, + h, can be achieved experimentally either by controlling h, to be just less than h, or by operating the concentrate withdrawal in a periodic manner. This latter is discussed further in B(ii) in the section on Experimental Procedure_ Under conditions of high froth stability (large h,) but with a small operating froth height hF, CY+ 1.0, the relative recoveries become
.&lpi_ CK
Mci =
Exqp 1 hFkFi
exp
(3)
lM,i
The ratio hc/qp is the minimum residence time of air Tmin in the froth- Equation (6) differs from eqn. (5) in that the relative separation depends on the difference between the magnitude of the return rate constants rather than their ratio. It is of interest to note that for a stable froth the first operating conditions should give good cleaning and the second only poor separation_
LYQP
+ II
-CYar6]
In eqn_ (3), h, is the constrained froth height, i.e. that between the pulp surface and the level of the weir, and hF is the operating froth height. Other froth parameters are 6, the ratio of h, to hF, and E, the fraction of nonstagnant froth_ As defined earlier, qp is the air flux entering the froth from the pulp in m-3m-2s-l_
The selectivity of separation in the froth of coal (species c) with respect to mineral (species m) is given by
M, -
= Flq,,
Mcxn
fiF
how hFs hc, kc> bi,,]
pm
When h, *
F[q,,
0, hF will be small and
h-> hF> h,, kFc, k,]
(4)
+ 1-o
The effect of hF on the cleaning action can be determined by observing the measured ratios nz= n& -M, M-+-o I I provided that conditions are such that the other significant variables, qp, h,, kFi and k pm, are unchanged. Under conditions when hF + h, a -+ 0 and @Fc
-
f’+&k qP
X-
KC
M cmI h,-o
GENERAL
INFERENCES
FROM
PREVIOUS
WORK
II X
(5)
Plant scale tests reported by Muller and Brockhoff [5] and Kind [6] indicate that concentrates of 7% ash can be achieved .by continuous operation in -14 m3 cells at separation efficiencies ranging from 30 to 50%. It is important to establish under which conditions the higher figure can be consistently obtained. In particular, it is necessary to have more detailed information relating to separation efficiency of the coal processed as a function of size_ This is particularly important because mineral particles of the order of 5 pm are difficult to detect gravimetrically in the presence of much larger particles, yet their presence may certainly be the source of unacceptable pollution, scaling and corrosion effects. It. is necessary, therefore, that the presence of these particles be estimated separately.
5
Point
Run
SO, (mgig
1 2 3 4 5
F5 F6 F7 FS F9
coat)
0 J-71 11.75 Xi-29 129_11
RECOVERY
Fig_ 4_ Grade-recovery
cm-ves. nominal
+105
pm
-250
It may be inferred from Firth, Swanson and Nicol [3] and Baker, Miller and Deurbrouck [7] that sequential flotation processing including secondary cleaning or scavenging is beneficial_ The technique of Baker et al. results in significant reduction in pyritic sulphur as shown in Fig. l_ Several authors - Pefukhov [S] , Aplan [ 91 and Baker and Miller [lo] - have discussed chemical conditioning and froth stabilizers but only pyrite depression has been considered in any detail. Little attention has been paid to the possibility of depressing the clay and shale components and to the effect of the chemical addition and solids on froth stability. Very little information is also available regarding the beneficiation of weathered coal, although it may be inferred from Firth, Swanson and Nicol [3] and Petukhov [El] that low separation efficiencies of the order of 20% are the best achievable currently. Wen and Sun [ll] showed that the effect of oxidation on the coal surface is to increase the negative zeta potential_ This has the adverse effect of making the surface more hydrophilic, thus reducing both the recovery and selectivity. Stachuski, Fijal and Michalek [12] attribute this increase of negative zeta potential to an increase in OH CO and COOH functional groups on the coal surface.
pm
fraction-
Nimerik and Scott [ 131 suggest the use of cationic collectors such as amines, which should certainly improve the recovery but. not necessarily the selectivity.
E_XPERlMENTAL
PROCEDURE
The objective of the set of esperiments reported here was to investigat.e the limiting separations achievable by conventional froth flotation techniques using deep froth layers for weathered fine (-250 pm) coal. Particular attention was paid to the effect of coal size on separation efficiencyA. Conventional fiotation tests All flotation tests were done with a lahoratory Denver D-12 cell using 425 g of coal in an initial water volume of 7000 cm3_ The raw coal was obtained from a Power Station stock pile. Its properties are reported in Table 1. The raw coal was milled using a ChristyNorris fixed hammer mill and screened on a Lockers shaking screen into the following nominal size ranges: -250 pm +105 pm, -105 pm +35 pm and -45 Mm. (i) Size analysis of feed fkactions A wet sieve analysis of the individual cuts and the original feed was done on a Gilson
Parlid@
CompawnE
Cumulativ@ wet 5%under ciiae, Sample
upper siw
sieve analysis
Lockers
feed
(Pm)
-150 -90 -53 -32 -22
G6
Feed
nominal size -250 pm
+10-I
91.9 84.4
71,4 56.3 42.2
a0
siussran@!
millcld samples, hlw
GO Feed
G7 Feed
wminal size
naminal ske =-I5 pm
-10-l +I5
pm
94.8 .72.1 45.7 90,s 20.7
pm
pm
99,9 75.1 56.5 36.5
97.8 76.4
sieving apparatus. These data are reported in Table 2. Samples were prepared from the individual wet-screened fractions available from the sieve analysis of runs G5 and G6 feed. These samples were examined microscopically with the -data processed on a Quantimet Image Analyser from which the total area of particles Sr and the total intercept IT in a field were reported. Using these two figures, together with the number count of recorded particles, three diameters of equivalent circles could be deduced: wet
(1) area mean diameter: (2) mean chord length: (3) circumscribed
6, = [4Sr/n,7r]“2 d2 = Sr/Ir
diameter:
a3 = IT/nT
are
lhwarly
independent.
are based on number
The
mean diam&ers
counG and will be Mgnilicxmtly amalkr than if thq wwe d&w= miwd on a rncw basis such as those obtained from the n~iwwMve~, In ardor to convert from a number to a mass basis, the cumulative
number sine distributians must be known, which are currently not available from the existing data processing system. (ii) SO; prereduction of slurry Preliminary tests showed that the beneficiation of coal was sensitive to pretreatment by oxidation or reduction (Ramdja [ 14 1). the coal treatment improving with increase in the
degree of reduction.
Ramdja used an SnC12
solution as his reducing agent, but SO, was considered to be potentially more useful, largely because it is present in significant amounts in many waste gases, but also because it could be generated cheaply from pyrite residues separated in the beneficiation process. The SO2 -was bubbled slowly into the
agitated feed slurry, from a cylinder on a toppan balance with digital read-out so that the absolute amount added could be closely controlled and reproduced. After the addition, the slurry was left stirring for an hour. The F series of tests were done on slurries of the three nominal size fractions produced
0~) Cell opemtior? T!w cell was run with a cs?~atant agitator speed af ISQQ rpin uild an indnced air flnw of 9 Ilmin, During the course of each run, concentrate was collected and filtered, the Oiltrate being replaced in the cell discantinuously. the solids being weighed. dried and analysed. In
the later stages, the froth stability decreased to a point when the liquid level in the cell had to be raised by the addition of water in order that the concentrate could be withdrawn. The flotation test was continued until the pulp colour changed from black to a dirty grey. Careful records were kept of the mass withdrawals and additions, enabling solid and water inventories to be kept during the run. This enabled an estimate to be made of the volume between the pulp level and the level corresponding to that of the weir. Subsequently, in runs G5 to GlO. the level of the top of the froth layer was also observed. Each individual test included the measurement of ash in the feed, all concentrates and in the residual tailings. (v) Additional
measurements
Subsequent measurements include analysis for total iron using the Lab-X Telsec X-ray Fluorescence Analyser. Microscopic analysis of selected runs were done using a Quantimet Image Analyser to assist with characterization. Electrokinetic measurements were made on a selected run G5 using a Rank hlicroelectrophoresis Apparatus Mark II to determine the
In t!?c! firtit si(4t!(i. convc~nti~uia! fimtim toatfi wre done in lx?i?x undue id43nticai conditions except that the initial wt~w volrrmes wre 7.0 wid Q,Q I respectively. As the volume of liquid in 11~ cell wit11 air at 9 l/min and an agitator s!~ed af 33OQ rwn was measured as 9.5 ! to ths lxxtam of CIW overfIow weir, the latter run corresponded to the conditions for which h, - 0 and tlw initial ratio of rates M,/Jl, tended to the ratio of tlie rates. leaving the pulp 4V,/.lf,.
(ii, Dminage
tests
the second test series, the froth was allowed to build LIP and be removed in a periodic manner. In each period, the froth espanded to reach its limiting volume without concentrate being withdrawn. then at the end of the period all froth above the overflow weir was removed rapidly by scraping. This corresponded to the condition where hF - h,. III
RESULTS r\ND DISCUSSION (i) Screening
efficiency
and sieve analysis
The dry screening is clearly inefficient, as reported in Table 2_ It is anticipated that improvements in controlling the rate of feeding the screen will significantly improve screening efficiency, although there is clearly a decrease in separation efficiency at the smallest size. Table 3 shows that the Gilson wet sieve analysis correlates well with size distribution obtained by microscopic esamination- The Gilson apparatus does not directly measure sizes below 22 flrn and information about the
8 TABLE
3
Comparison
of the mean sieve diameter with the three microscopic
Run
G5 Feed nominal size +I04 -250
G6 Feed nominal size +45 -104
diameters
Size range
Mean size
Number
(cm)
Wm)
of particles in field
Area mean diameter Wm)
Mean chord length Wm)
Circumscribed diameter (pm)
-22 -32 -53 -90 -150
116 27.0 42.5 71.5 120-O
1036 121 64 64 58
8.09 31.60 45.62 78.45 127.74
7.88 23.92 38.18 70-83 101.46
6.52 32.83 42.82 68.25 126.32
-22 +22 -32 +32 -53 +-53 -90 +90 -150
11.0 27.0 42.5 71.5 120-O
1153 234 153 66 63
4.72 29.01 47.89 69.66 113.29
3-32 24-91 38.85 53.25 102.57
5.29 26.55 46.36 71.59 98.28
+22 +32 +53 +90
TABLE 4 Summary of final separations achieved ss a function of size fraction and SO2 pretreatment Size fraction (Pm) -45
-105
-250
1-45
+105
Cumulative concentrate
WI
Recovery daf coal (%I
PI
WI
61.6 48-S 48-6 45.5 57-3
88.2 76-l 69.3 67.7 81.8
19.0 16.7 10-s 11-4 11.9
12.0 10-2 16.2 14-S 20.5
76.4 76.4 68.4 75-6 57.5 77.7 62.0
92.7 93.8 88.0 92.3 81.4 91.8 86-5
il.12 18-01 9.45 12.78 7.99 9.85 7-22
28-5 15.0 28.1 25.8 24.9 34.2 31.5
95.0 83.5 76.9 83.4 80.5
97.5 93.3 94.1 91.6 91.4
17.39 10.78 8.95 8.25 6.71
29.7 38.5 38.6 44.6
SO? pretreatment (mglg c0a.U
Final taiiings ash
FlO Fll F12 F13 F14
-
Feed ash WI
Mixing measure of feed
Run No.
28.04
0.593
4.71 11-77 35.29 129-41
23.08 25.01 23.08 25.01 23.08 25.01 23.08
0.540 0.562 0.540 0.562 0.540 0.562 0.540
FO Fl F1/2 F2 F2/2 F3 F4
-
28.66
0.599
F5 F6 F7 F8 FS
-
4.71 4.71 11-77 11-77 35.29 129.41 4.71 11.71 35.29 129.4
presence of extremely small particles has to be based at the present time on microscopic examinations alone- These indicated that there was a significant amount of extremely small particles, estimated to have a size of <5 pm, present in the -22 pm fraction. {ii) Effect of SO,pretreatment on beneficiation Table 4 summarizes the experimental runs in terms of the final cumulative separations achieved_ More detailed information relating
ash
Separation efficiency
46-4
the variation in cumulative concentrate ash during a particular run may be inferred from the grade-recovery curves shown in Figs. 2,3 and 4. The cumulative concentrate ash can be calculated from the grade and feed ash using the definition of grade given in the section on Performance Measure. to
Concentrateash=feedash(l-g) The separation achieved is expressed as separation efficiency, as discussed in the same
9 TABLE
5
Initial froth stability cIata.Air rate: 9.0 llmin. Total froth volume I+: 3.0 I. Constrained volume 2-S I_ b = O-714_ Size range
Run
(Pm)
No.
SOaddition (mglgcoal)
FlO Fll F12 F13 F14
-45
-105
-250
+45
+105
-
4.71 11-77 35.29 129-41
FO Fl F112 F2 F212
0 4-71 4.71 11.77 11-77
F3 F4
35-29 129.41
F5 F6 F7
-
F8 F9
35.29 129-4
4-71 11-77
Initial frother addition
Total frother addition
initial water removal rate
Initial froth formation time tf
(mglg)
(mgig)
(gmin-')
(s)
volume (1)
O-19 0.19 O-38 0.56 0.75 094
056 O-56 l-60 4-04
164-3 55-O 13.4 12-6
4.71
55-7
45 55 51 55 120* 136
3.52 3.26 3.34 3-26 < 3.0 3.002
0.095 O-38 0.19 0.38 O-19 0.28 0.38 o-47
2-07 2.45 2-16 2.82
15-S 680-l 5-l 751.1
50 20 115 20
3.37
1.51 2-45 2.16
64.1
0.38 O-38 O-38 0.47 0.56 O-75 O-94
2.54 l-69
82-S 89.6
2-07
49.8
3-20
30-9
1201;
section, which can be related to the three commonly used measures, final tailings ash, recovery of dry ash-free coal and the cumulative concentrate ash_ These three figures are not independent and as those reported are derived directly from experimental measurements, checks for their consistency provide an estimate of experimental accuracy_ For the -250 pm +105 pm size fraction, there is a significant drop in final concentrate ash together with an increase in efficiency for increasing levels of SO2 pretrc:atment_ The individual grade-recovery curves for particular runs are shown on Fig. 4. The upper boundary of the experimentally achieved grade-recovery space corresponds to that of a single run involving pretreatment with 55 g of SOz. This run therefore represents the optimal treatment whereby the best grade will be achieved for any specified recoverypm +45 pm fraction, the For the -105 runs Fl and F2 are anomalous. From Table 5 it can be seen that the initial froth expansion for these runs corresponded with that associated with no bubble breakage. Very high
2i.i 46-i
135 38 33 38 39 110* 125 120f 110* 230
Limiting
Initial
froth
3-01 <3_0
3.002 3-90 4.45 3-90 3.84 < 3.0 3.006 c3.0 r; 3.0 3.001
X7_
QF 1 -a&
O-17 O-084 0.109 0.084 o-0 0.0007 0.119 3.50 O-003 3.5 0.0 O_OOOi O-28 O-42
O-28 O-26 o-0 0.002 O-0 o-0 0.0001
initial concentrate waterremoval rates were associated with these extremely stable froths. As these runs were atypical with respect to initial concentrate water removal rates, runs F1/2 and F2/2 were repeated under conditions corresponding to Fl and F2 respectively, but at half the initial frother amounts_ Figure 6 shows kinetic data for the runs Fl and F1/2 and F2, F2/2_ _Q the repeated runs had more typical water removal rates, their grade-recovery curves can be compared with FO, F3 and F4 on Fig. 3. Several interesting points arise from a comparison between the overall separation efficiencies reported for the -105 pm +45 pm size fraction and the grade-recovery curves on Fig_ 3. Firstly, although the single treatment corresponding to 55-O g of SOz (run F4) forms the upper boundary of the graderecovery space and therefore represents the optimal treatment. it has, however, a lower overall separation efficiency than for a pretreatment corresponding to 15.0 g of SO2 (run F3). This is because a higher coal recovery was achieved in the latter run leaving a
10 .o.-
1-O
$I<-.
y,yy---____‘_____ \ 1
\
wo
_‘
3.
‘. wt
1---
3
‘.
!. >
.\A_
-‘\ :\
‘.
‘\
‘3.
‘.
I.
-I-,
x-_3
.l .OC
o-1
I _
=\
5 \ 6-
i 0
lo Time
.mmi
Fig_ 5. Kinetics, untreated Size (Pm)
Run
FIO
-45 -250
Gl +105
F5 G2
1
I
20
30
.1
coal. Coal
7.0
3 1 5 7
7.0 9.0
I
1
30
20
rune.
mm5
Fig. 6. Kinetics. treated coal.
Initial volume (1)
9.0
t
10
Mineral
Size
Run
Initial volume (1)
Goal
Mineral
Fl FIl2 F2 F2/2
7.0 7.0 7-O 7.0
1 3 5 7
2 4 6 8
(Pm) 4 2 6 8
tailings richer in minerals. Also, despite the decrease in overall separation efficiency for runs FO, F1/2 and F2/2 reported in Table 4, these runs enclose each other in inverse order to the above sequence on the grade-recovery curve, and clearly reflect an improvement in beneficiation with increasing SO2 pretreatment in this size range. The separation efficiency has, therefore, to be used with some caution as it is clearly biased towards high recoveries. For the -45 jxm fraction, the upper bound in graderecovery space is again represented by a single test equivalent to 55.0 g of SO2 pretreatment. Once again, the separation efficiency sequence does not quite mirror that of the grade-recovery curves and the importance of Coal recovery as a factor in evaluating separa-
+215 -105
tion efficiency can be seen by comparing runs FlO and Fll. Finahy, there is a marked decrease both in separation efficiency and grade-recovery for the best pretreatment (55.0 g of SOz) for the sequence of size fractions -250 pm +105 pm, -105 pm +45 pm, and -45 pm. (iii) Froth
stability and kinetics
Figure 5 shows a comparison between the kinetics of flotation for untreated -45 pm and -250 pm +105 pm fractions. The tests were done at two initial pulp volumes for each material, Le. 7.0 and 9.0 1. At the latter value under agitated and aerated conditions, there x&s virtually no unconstrained froth volume and according to the section on
11 TABLE
6
Flotation
kinetics for two untreated
coal fractions
Particle size (m-d
Run No.
Initial PUiP volume (1)
Initial frother added (mglg coal)
Total frother added (mglt3 coal)
Initial water rate (cm3 min‘-r)
-45
Gl FlO
9.0 7-O
0.19 O-19
0.19 0.56
G2 F5
9.0 7-c
0.38 O-38
1.52 2-54
-250
+105
TABLE
Run No.
1261.0 164.3
0.4 1 0.18
0.15 0.085
2-73 2.12
850-S 82.8
O-44 O-16
O-087 O-04 1
5.08 3.90
G
+45
Total frother added (mglg coal)
Initial water rate (cm3 min-‘)
Initial first-order rate constants % (mm-‘)
i-srn (min-r)
kc -E‘ IXl
4.71 4.71
0.38 0.19
2-45 2.16
0.77
0.25
3.08
11.77 11.77
0.38 0.28
2.82 1.51
680-l 5.1 29.2 751-l 64-l
0.12 0.77 0.117
0.025 O-27 O-022
4-80 2.85 5.32
Fl F1/2 F2 F212
Froth Stability and Flotation Kinetics the flotation kinetics approach those characteristic of the pulp processes. These show decreasing first-order rate constants which have been frequently observed in mineral processing and discussed, e.g. Woodbum and Loveday [ 151. The lower volume runs were part of the F series and a comparison with Gl and G2 provided an estimate of the cleaning effectiveness of the froth layer First-order rate constants were extracted from Fig. 5 and Fig. 6 under initial conditions using the relationship In
W:(t) I
L
rvi(0)
__ elf
species
1.
Initial frother added (m&z coal)
SO-. addition (mglg coal)
(pm)
where
%n (min-‘)
kinetic data for two coal fractions pretreated with SO=. All initial volumes i-0
Particle size
hi=-d
%
& (min-‘)
7
Flotation
-105
Initial first-order rate constants
I
if-o
i may
be coal
(c) or mineral
(m).
Rate constants obtained in this wtiy are tabulated in Tables 6 and 7. For very high initial rates noted on Fig. 6, differentiation is obviously liable to error and the initial rate was taken as the average rate over the first short time interval. From Fig. 5 and Table 6, the initial rates for Gl and G2 show that the coal rate constant is not size dependent while
of the minercll is. This may be esplained by the importance of inertial effects in entrainment, which could be more significant for the dense mineral than for the light coal. Comparison between the ratio of apparent first-order rate constants for the runs with a deep froth layer and those in which pulp effects only are important (Gl and FlO, G2 and F5) show that there is no appreciable cleaning in the froth for those unreduced coals. Figure 6 shows the effect of initial frother addition on particles in the -105 pm +45 pm size, which have been treated with 2 g of SO2 (Fl, F1/2) and 5 g of SO+ (F2, F2/2)_ The effect of a -relatively high level of frother addition (0.38 mg g-’ of coal) is shown for runs Fl and F2 to produce a very high initial water rate almost equivalent to that observed for the runs Gl and G2 at low froth volumes At half the initial frother addition, the initial water rates for runs F1/2 and F2/2 were much lower than for Fl and F2 and more comparable with other runs in this series (i.e. FO, F3, F4). The apparent first-order coal rate constants for runs F1/2 and F2/2 are that
12
similar to those for the untreated coal FlO and F5, but the rate constants for the mineral differ appreciably between the two series. This suggests that the effect of SO2 pretreatment is primarily to reduce mineral flotation rates, probably by improving the drainage of mineral particles from unstable froths. The high apparent flotation rate constants both for coal and mineral in the stable froth runs indicate very little drainage from these froths, and the low ratios of kc/k,,, reported for Fl and F2 reflect the poor beneficiation observed, which is comparable with those noted for unreduced coal. in terms of the flotation model, the runs %Z and F2 correspond to a condition where h, S- hF and the poor grades observed indicate low absolute values for the species froth return rate constants kFc and kh. This corresponds to eqn. (6). (iv) Fmth dminage The comparison between a conventional run G5 and run G8 which was carried out under periodic conditions. as described in B(C) in the section on Esperimental Procedure, offers further information about the effect of drainage from limiting froths in which hF + h, as described by eqn. (5). Details of the chemical treatment used in these runs are given in Table 8 and gravimetrically determined ashes for various size fractions in the feed material are given in Table 9. The weighted average ash computed from individual fractions compares well with that determined on the feed sample as a whole. Number and area counts of mineral particles observed by microscopic esamination under transmitted light are also reported. As the mineral is heavier than the coal: it might be TABLE
TABLE 8 Experimentaldetailsfor conveationnlrlrd periodic runs G6 and G8 respectively Nominal
particle size -250
pm +106 pm
Initial charge Run GS 425 g coal Run G8 425 R coal
7000 cm> water 7000 cmJ water
Both runs treated witb 55.0 g SOz. Run GS operated by allowing froth to build up periodically. removing the accumulated concentrate. Chemical treatment 1.13 m 6-l mixed crcsols dextrin 0.223 mg 6-l 1.76 rng 0-l IWe(CNk Montanr.1 361 used as frothcr Feed ash 24.13% Run G5 Initial froth build-up-time Run G8
24 s
Feed ash 24.09%
espected that the area fraction would be lower than the gravimetric ash content. This is not so for the -22 pm particles. which probably reflects the difficulty in distinguishing between extremely small coal and mineral particles under transmitted light. The low mineral fraction reported for the +90 Mm -150 pm fraction is not statisticaliy significant because of the small total number of particles which were counted in this size range. Microscopic examination also revealed the presence of large numbers of very fine particles (-5 pm) in the -22 pm fraction. Mineral contents of individual size fractions are reported in Table 10 for a cumulative concentrate corresponding to a recovery of about 15% of dry ash-free coal. These early concentrates usually have the lowest ash content within a run. There are significant
9
Analysisof feed coal. Runs G5 and GS. Total ash = 24.1%. Size range Wm)
Mass fraction in range Xi
Microscopic
-22 +22 -32 +32 -53 +53 -90 +90 -150. +150 -250
0.23 0.05 0.12 0.24 0.30 0.06
0.381 0.168 0.114 0.119 0.017
Fractional
analysis area mineral
Fractional 0.741 0.314 0.260 0.078 0.017
number
mineral
Gravimetric (%I
ash Ai
29.31 18.94 17.95 20.95 22.16 37.89 XzqA J = 23.5%
13 TABLE
10
Ash nnnlyscs of concentrntcs. Iluns G5 nnd GS. Run G5: 1st conccutmtc corresponding to 0.286 frnctionnl recovery; nsh = 5.4Ct. Run G8: 2nd couccntrntc corresponding to 0.138 frnctiannl recovery; nsh = 5.19%
Run G5 Grnvimctric (%I
-22
0.093
+22 -32 +32 -53 453 -90 +90 -150 +150 -250
0.080 0.205 0.350 0.24 0 9.032
11.4 5.52 4.39 4.33 4.31 4 -4 1
TABLE
\‘Xi:\ i
Run G8
Mnss frnction Si
nsh r\ i
Mnss frnction Xi
Grnvimetric (XI
0.128 0.101 0.158 0.267 0.323 0.023
7.43 4.63 3.79 3.60 3.90 4.35
ash di
G.5 = 5.09%
GY = 4.35%
11
Ash nnnlyscs of fiunl tailings. Runs G5 nm3 G8. Run G5: Frnctionnl con1 left in tailings; nsh - Gl.lW. Ruu GS: Frnctionnl con1 left in tailings; nsh = 53.2%. Run G5
-22 +22 -32 +32 -53 +53 -90 +90 -150 +150 -250
xxi,1 i
Run G8
Mnss frnction -Vi
Grnvimrtric (%I
0.50 0.04 0.07 0.16 0.20 0.03
51.8 72.9 77.8 80.9 79.9 73.1
nsh A i
differences between the weighted average ash contents and the overall ash determined on the whole concentrate. These discrepancies may be due to the loss of ultrafine material (-2 pm) from the -22 flrn fraction by passing through the filter paper under the smallest sieve. Ashes for these samples may additionally be liable to esperimental error because of the small amounts used in the analysis. There is a significant improvement in the ash content of all size fractions for the deep froth as opposed to the conventional float, indicating that improvements by drainage can be anticipated, producing concentrates of about 3.5 to 4.5% ash for all fractions escept the finest. The limiting ash for this fraction appears to be of the order of lo%, assuming the error in weighted averages is caused by an underestimation of ash in this range. Table 11 contains ash analysis of the residual unfloated tailings at the end of both. runs. It is interesting to note the high ash
Mnss fmction Xi
Grnvimctric (“ml
0.402 0.035 0.076 0.202 0.24 7 0.035
44.9 55.1 58.9 G-1.6 57.2 47.1
nsh Ai
G5 = 65.45
GS = .53_4F~
content of the -22 pm material in both tailings, which indicates that even within a range where entrainment. is significant. some selective separation of coal from mineral occurs. Both runs show more complete removal of coal in the middle size region (-105 pm +32 pm) with the conventional run showing better final tailings ashes. It was not possible to maintain the high static frot.11 levels required for GS towards the end of the run, which reduced overall recovery.
(v) Ptztrogmphic analysis Table 12 shows pet.rographic analysis for the conventional run G5 for two size fractions. While the difference between feed and first concentrate appears small. the ratio of vitrinite-esinite]inertinite becomes noticeably more favourable in the concentrate than in the feed. The analysis also indicates that of the non-coal components the shale is least -efficiently removed.
14 TABLE 12 Petrographicanalysis Component
Vitrinite Exinite Inertinite Shale Pyrites Other minerals ‘Coke’
Size range -150
pm +90 pm
Size range -32
pm +22 pm
G5 Feed
G5 1st Concentrate
G5 Feed
G5 1st Concentrate
61.0 6.8 13.2 7-s 0.4 6.2 4.6
72.2 10.4 10.2 3.4 0.2 1.2 2.4
67.0 3.6 15.6 5.8 0.6 5.0 2.4
75.0 6.8 il.8 3.0 0.4 0.8 2.2
CONCLUSIONS (i) It is possible to produce a cumulative concentrate from weathered coal with an ash of approximately 7% with dry ash-free coal recoveries of approximately 85 to 90%, in the size range -250 pm +45 pm, by pretreating the coal slurry with SO2 gas followed by froth flotation. (ii) In the size range -250 m -1-105 m, concentrates containing less than 5% ash can be prepared at 40% recoveries of dry ash-free coal. (iii) The effect of froth stability on beneficiation is complex. Very stable froths can give poor grades if they are allowed to produce very high initial rates of water removal in the concentrate_ Periodic tests show, .however, that stable froths which are given the opportunity to drain well; before removal over the concentrate weir, produce low ashes. (iv) Froth stability decreases during the course of a run and for a constant size fraction with increasing levels of SO1 pretreatment. Frother addition in limited amounts helps to increase stability. (v) Periodic flotation tests showed that ashes of about 3.5 to 4-0s can be obtained in early concentrates in the size range -150 pm +32 pm for deep well-drained froths. Ash levels in the range -22 pm were high, in the range 8 to 11%. Tailings assays showed that the most complete removal of coal occurred in the range -150.1.rm +32 pm. (vi) The residual tailings contained a higher mass fraction of -22 JUXImaterial than the feed.’ The high residual mineral in this fraction in the tailhgs~and the lower mineral (8 to 11%) in the early concentrates shows that
even for the ultra-fine material there is some selective removal of coal(vii) The flotation model proposed by Moys has proved a useful conceptual basis for understanding .flotation kinetics. In particular, by determining three limiting conditions where h, + 0, h, + h, and when h, + 00 for a finite hF, there is a framework for the estimation of froth parameters in their region of masimum sensitivity-
ACKNOWLEDGEMENT We wish to acknowledge with thanks the assistance of Dr_ A. Smith of the Yorkshire Regional Laboratories of the N.C.B. in performing the petrographic analyses_ LIST OF SYMBOLS
-
55
da h
hi+ hc, km
HE
H,, Hc, HT IT
mass fraction feed as concentrate area mean diameter equivalent circle, pm mean chord length, pm circumscribed diameter, pm transient froth height during formation, m height parameters. describing froth height operating froth, constrained height between pulp and weir, maximum constrained height, m separation efficiency, % mixing measure. Feed, concentrate and tailings respectively sum of intercepts of leading edges of all particles in Quantimet field, pm
15
k
kFi
nT qb
apparent first-order rate constant relating to the transfer of species i from pulp over concentrate weir, min-’ return first-order rate constant of species i from froth to pulp, min-’ mass flow of species i over concentrate weir, kg s-r mass flow of species i entering froth from pulp, kg s-l total number of particles observed by a microscopic count air flux leaving top surface of froth due to bubble breakage, m3
9P
R(t)
ST
t tF
vF wj(t)
-Ti
m--2
s-
T min
Subscripts iaf m
Greek symbols CY froth parameter = 1 ratio of constrained 6 froth heights = h,/hF fraction of stagnant (1 - e) remote from weir percentage grade of 5 concentrate
- q&p to operating froth surface cumulative
of time in
and abbreviations coal dry ash free mineral
REFERENCES
1
air flus entering froth from pulp, m3 m-* s-’ cumulative percentage daf coal recovered at time t, % projected area particles in Quantimet field, pm’ time, s time of formation of initial froth, s froth volume, cm3 mass of species i remaining in pulp after time t, kg mass fraction component i in mixture, -
minimum air retention froth = h,/q,
8 9 10 11 12 13 11 15
C. K. Law, K_ H. Law and C. Ii_ Lee. Encq~y. 4 (1979) 329 - 339_ A_ M_ Gaudin. Nofafion. McGraw-Hill. New York. 1932. p_ 636. B. A_ Firth, A_ R_ Swanson and S. K_ Nicol. ZnL J. of Min. Process.. S (19C9) 321 - 33-t_ %I_ H. Moys. PhD. Thesis. Univ. or Satal, Durban. 1979_ 3_ %I_ Miilier and D. J. Brockhoff. _-l;IfZwreilungs Tcchnik.. 3 (1976) 129 - 13-t_ P. Kind, Aufbcrcilungs Technil:.. 3 (1918) 119 - 12s. A. F_ Baker. K. J_ Miller and _a_ W. Deurbrouck, 6th Znst_ Coal Prep. Congress. Paris. March (1973). Paper 27E. Lr_ N_ Petukhov, Khirn. Tccr. TopI_. 13 (19i9) 12 - 20. F_ F_ Xplan, FZolotation 2. Am. Inst. of Min. Jfet_ & Pet_ Eng.. New York. S.Y. (1976) 1235 - 1264. ;I_ F. Baker and K_ J. Miller. Z_?S_ Bureau of Illines Report. XI_ 751s (1971) 1 - 21_ W. W. Wen and S. C_ Sun. Sot. of 3fin. Eng of _-%.Z.AZ_E. Trans.. 262 (19T7) 17-l - lSO_ J_ Stachuski. T_ Fijal and 11. Nichalek, _4rch. Gorn.. 25 (1950) 269 - “Sl_ K. H. Simrrik and B. E. Scott. _)Zining Consr_ Journal. 66 (19SO) 31 - 22,3i_ A_ F. Ramdja. M_Sc_ Dissertation. U31IST_, 3Ianchaster. 19SO. E. T_ Woodburn and B_ K_ Loveday, 2. South African Inst. of Jlining & Net_. 65 (1965) 612 6%.
Technology.
35 (1983)
The Demineralization E_ T_ WOODBURN,
1 - 15
of a Weathered
D. J. ROBBINS
December
14.1981;
of Manchester
Flotation
Institute
of Science
and Technology.
Sacis~lle
in revised form April 26.1982)
SUMMARY
Froth flotation tests of weathered coal are reported in which pretreatment of the coal slurries by the absorption of SO2 gas improved the beneficiation remarkably- This was associated with the production of unstable froths and experiments were done to determine the relationship between froth stability and improved performance_ The experimental measurement of performance was based on gravimetric measurements of ashes but in addition microscopic analyses were done on selected runs providing number counts of coal and mineral particles in various size ranges for different flotation fractions_ The microscopic data provided useful information on the dminage of coal and mineral particles in the deep froths_
INTRODUCTION
The usage of low-rank and weathered coals is restricted because of the nature and amount of their associated minerals. These coals are difficult to beneficiate by conventional washing techniques because of the presence of a large amount of fine coal and shale and also, in the case of weathered coal, particularly because of their hydrophilic surfaces_ There is, however, a developing interest in the use of ultra-fine coals both in combustion and for conversion by entrained bed gasification and for liquefaction by catalytic pressure hydrogenation. At the particle sizes envisaged for these processes, the mineral will to a large extent be liberated from the coal. As a consequence, it is conceptually possible to separate mineral from coal by physical processes only_ 0032-5910/83/$3_00
Coal by Froth
and S. A_ FLYNN
Deportment of Chemical Engineering. Lmicersity Street. Manchester 3160 I QD (Gt. Britain)
(Received
1
Physical cleaning processes such as froth flotation partly demineralize coal, thus reducing the adverse effects of contaminants considerably, while not eliminating them completely, without adversely affecting the combustion characteristics of the cleaned material. In general, these processes require a high degree of liberation of mineral, which usually requG%es fine grinding_ The separation of ultra-fine particulates cannot be achieved by sink-and-float procedures but requires a process whose selectivity is based on surface effects_ Froth flotation is the oldest physical separation process of this type and has the advantages of a relatively low power consumption per unit mass of material processed and a proven capacity to handle higher tonnage rates of solids, than the newer oil agglomeration proceduresThe obvious disadvantages of froth flotation are its tendency to entrain very fine materials and the production of a wet froth_ Both effects can potentially be reduced by a cleaner float using a hydrocarbon entrainer, but first it is necessary to establish the best performance of the roughing floatIt was postulated that mineral entrainment could be minimized by the use of a deep butrelatively unstable froth in the roughing floats. The development of a treatment process which would recover a marketable fraction from coal waste is obviously attractive in the short term_ In the longer term, it will be necessary to develop analogous processes for low-rank coal of the NCB902 type, of which ’ the U-K. has large unesploited deposits_ Low-rank coals have a high volatile content and a high H[C ratio_ A demineralized lowrank coal would thus appear to be an excel0 ?%evier sequoiafinted
in The Netherlands
2
feed-stock for liquefaction. It is also particularly suitable for combustion because of its ease of ignition, and the most promising of several possible uses would be as the solid component in a coal-oil mixture. Law, Law and Lee [l] have discussed the combustion of these mixtures and have indicated the need for high combustion rates of the residual particulates after the liquid has been burned.
.Lent
PERFORMANCE
MEASURE
It is desirable to have a single measure of the degree of separation achieved which incorporates both the purity of the product and the fraction of dry ash-free coal recovered. Several measures have previously been suggested, e.g. Gaudin [2] and that used by Firth, Swanson and Nicol 131, both of which have the disadvantage of being unbounded at perfect separation, making it impossible to relate efficiency to .the limiting condition. If, however, a separation measure based on the entropy of mixing is defined for a binary mixture WE-C
i=2 i=
Xi 1nXi
1
then the separation of a feed stream of mixing entropy Hr into a fraction c of concentrate and 1 -c of tailings, each with its own entropy of mixing Hc and H, respectively, will be associated with an entropy reduction. AH=H,-cHc-(I-cc)H,
Fig_ l_ Beneficiation Deurbrouck
of
Baker, Miller and
S(t)
ash(t) in cumulative feed ash x 100 where t is a param eter indicating the degree of processing of the feed coal_ The recovery R(t) is the percentage of dry ash-free coal in the original feed which has been recovered in the concentrate_ The grade-recovem curve unambiguously defines by its upper bound the local optimal treatment, irrespective of technical requirements associated with grade, and of recovery, which is primarily an economic consideration,
FROTH
Since for a perfect separation WC = H, = 0, the limiting reduction in the mixing measure is
circuit
Moys
STABILITY
AND
143 defined
FLOTATION
KINETICS
a froth parameter
*=1-Z!? QP
AH=Hrr, A separation
efficiency
can then be defined
as HE=$xlOO f
Performance data is also reported in terms of grade-recovery curves (Figs. 2, 3, 4) in which the grade is defined by
where q,, is the air flux leaving the top surface of the froth by bubble breakage and qp is the air flux entering the froth from the pulp- 01 is clearly a function of froth height, there being a limiting height h, in a given cell representing a stationary froth from which no concentrate is being withdrawn. At this limiting co~ndition, qb = qp and CY= 0. For a froth from which concentrate is to be withdrawn, the initial formation time tp,
Point
Fig_ 2. Grade-recovery
curves,
nominal
-45
F10 Fll F12 F13 F14
0 l-71 ll_i7 3529 129-41
Point
Run
SOI (mglg
nominal
coal 1
pm fraction_
1
curves,
SO2 (m&g
1 2 3 -l 5
2 3 -I 5 6 i
Fig_ 3. Grade-recovery
Run
+-.I5 pm -105
which is the time lapse between the first admission of air to the first withdrawal of concentrate, is an ind’rect measure of (Y_ Assuming the froth volume is largely determined by its air content, then during the formation period
pm
FO Fl F2 F3 F-l F1/2 F212
coal)
0 3-71 ll_ii 35.29 129-11 3-71 ll_i7
fraction_
The simplest relationship incorporating dependence of cr on the froth height h is
a
4 h ~=l---
ha for which h, is a measure of froth stabilitySubstituting in eqn. (1)
(2) Equation (2) can be solved implicitly for h, under initial conditions. Moys has also derived a relationship linking the withdrawal rate of species i over the concentrate weir MC; with the rate at which the species entered the froth from the pulp
For beneficiation to take place, the ratio of froth return factors of mineral to coal should be high even though the individual values may be small_ The condition h, + h, can be achieved experimentally either by controlling h, to be just less than h, or by operating the concentrate withdrawal in a periodic manner. This latter is discussed further in B(ii) in the section on Experimental Procedure_ Under conditions of high froth stability (large h,) but with a small operating froth height hF, CY+ 1.0, the relative recoveries become
.&lpi_ CK
Mci =
Exqp 1 hFkFi
exp
(3)
lM,i
The ratio hc/qp is the minimum residence time of air Tmin in the froth- Equation (6) differs from eqn. (5) in that the relative separation depends on the difference between the magnitude of the return rate constants rather than their ratio. It is of interest to note that for a stable froth the first operating conditions should give good cleaning and the second only poor separation_
LYQP
+ II
-CYar6]
In eqn_ (3), h, is the constrained froth height, i.e. that between the pulp surface and the level of the weir, and hF is the operating froth height. Other froth parameters are 6, the ratio of h, to hF, and E, the fraction of nonstagnant froth_ As defined earlier, qp is the air flux entering the froth from the pulp in m-3m-2s-l_
The selectivity of separation in the froth of coal (species c) with respect to mineral (species m) is given by
M, -
= Flq,,
Mcxn
fiF
how hFs hc, kc> bi,,]
pm
When h, *
F[q,,
0, hF will be small and
h-> hF> h,, kFc, k,]
(4)
+ 1-o
The effect of hF on the cleaning action can be determined by observing the measured ratios nz= n& -M, M-+-o I I provided that conditions are such that the other significant variables, qp, h,, kFi and k pm, are unchanged. Under conditions when hF + h, a -+ 0 and @Fc
-
f’+&k qP
X-
KC
M cmI h,-o
GENERAL
INFERENCES
FROM
PREVIOUS
WORK
II X
(5)
Plant scale tests reported by Muller and Brockhoff [5] and Kind [6] indicate that concentrates of 7% ash can be achieved .by continuous operation in -14 m3 cells at separation efficiencies ranging from 30 to 50%. It is important to establish under which conditions the higher figure can be consistently obtained. In particular, it is necessary to have more detailed information relating to separation efficiency of the coal processed as a function of size_ This is particularly important because mineral particles of the order of 5 pm are difficult to detect gravimetrically in the presence of much larger particles, yet their presence may certainly be the source of unacceptable pollution, scaling and corrosion effects. It. is necessary, therefore, that the presence of these particles be estimated separately.
5
Point
Run
SO, (mgig
1 2 3 4 5
F5 F6 F7 FS F9
coat)
0 J-71 11.75 Xi-29 129_11
RECOVERY
Fig_ 4_ Grade-recovery
cm-ves. nominal
+105
pm
-250
It may be inferred from Firth, Swanson and Nicol [3] and Baker, Miller and Deurbrouck [7] that sequential flotation processing including secondary cleaning or scavenging is beneficial_ The technique of Baker et al. results in significant reduction in pyritic sulphur as shown in Fig. l_ Several authors - Pefukhov [S] , Aplan [ 91 and Baker and Miller [lo] - have discussed chemical conditioning and froth stabilizers but only pyrite depression has been considered in any detail. Little attention has been paid to the possibility of depressing the clay and shale components and to the effect of the chemical addition and solids on froth stability. Very little information is also available regarding the beneficiation of weathered coal, although it may be inferred from Firth, Swanson and Nicol [3] and Petukhov [El] that low separation efficiencies of the order of 20% are the best achievable currently. Wen and Sun [ll] showed that the effect of oxidation on the coal surface is to increase the negative zeta potential_ This has the adverse effect of making the surface more hydrophilic, thus reducing both the recovery and selectivity. Stachuski, Fijal and Michalek [12] attribute this increase of negative zeta potential to an increase in OH CO and COOH functional groups on the coal surface.
pm
fraction-
Nimerik and Scott [ 131 suggest the use of cationic collectors such as amines, which should certainly improve the recovery but. not necessarily the selectivity.
E_XPERlMENTAL
PROCEDURE
The objective of the set of esperiments reported here was to investigat.e the limiting separations achievable by conventional froth flotation techniques using deep froth layers for weathered fine (-250 pm) coal. Particular attention was paid to the effect of coal size on separation efficiencyA. Conventional fiotation tests All flotation tests were done with a lahoratory Denver D-12 cell using 425 g of coal in an initial water volume of 7000 cm3_ The raw coal was obtained from a Power Station stock pile. Its properties are reported in Table 1. The raw coal was milled using a ChristyNorris fixed hammer mill and screened on a Lockers shaking screen into the following nominal size ranges: -250 pm +105 pm, -105 pm +35 pm and -45 Mm. (i) Size analysis of feed fkactions A wet sieve analysis of the individual cuts and the original feed was done on a Gilson
Parlid@
CompawnE
Cumulativ@ wet 5%under ciiae, Sample
upper siw
sieve analysis
Lockers
feed
(Pm)
-150 -90 -53 -32 -22
G6
Feed
nominal size -250 pm
+10-I
91.9 84.4
71,4 56.3 42.2
a0
siussran@!
millcld samples, hlw
GO Feed
G7 Feed
wminal size
naminal ske =-I5 pm
-10-l +I5
pm
94.8 .72.1 45.7 90,s 20.7
pm
pm
99,9 75.1 56.5 36.5
97.8 76.4
sieving apparatus. These data are reported in Table 2. Samples were prepared from the individual wet-screened fractions available from the sieve analysis of runs G5 and G6 feed. These samples were examined microscopically with the -data processed on a Quantimet Image Analyser from which the total area of particles Sr and the total intercept IT in a field were reported. Using these two figures, together with the number count of recorded particles, three diameters of equivalent circles could be deduced: wet
(1) area mean diameter: (2) mean chord length: (3) circumscribed
6, = [4Sr/n,7r]“2 d2 = Sr/Ir
diameter:
a3 = IT/nT
are
lhwarly
independent.
are based on number
The
mean diam&ers
counG and will be Mgnilicxmtly amalkr than if thq wwe d&w= miwd on a rncw basis such as those obtained from the n~iwwMve~, In ardor to convert from a number to a mass basis, the cumulative
number sine distributians must be known, which are currently not available from the existing data processing system. (ii) SO; prereduction of slurry Preliminary tests showed that the beneficiation of coal was sensitive to pretreatment by oxidation or reduction (Ramdja [ 14 1). the coal treatment improving with increase in the
degree of reduction.
Ramdja used an SnC12
solution as his reducing agent, but SO, was considered to be potentially more useful, largely because it is present in significant amounts in many waste gases, but also because it could be generated cheaply from pyrite residues separated in the beneficiation process. The SO2 -was bubbled slowly into the
agitated feed slurry, from a cylinder on a toppan balance with digital read-out so that the absolute amount added could be closely controlled and reproduced. After the addition, the slurry was left stirring for an hour. The F series of tests were done on slurries of the three nominal size fractions produced
0~) Cell opemtior? T!w cell was run with a cs?~atant agitator speed af ISQQ rpin uild an indnced air flnw of 9 Ilmin, During the course of each run, concentrate was collected and filtered, the Oiltrate being replaced in the cell discantinuously. the solids being weighed. dried and analysed. In
the later stages, the froth stability decreased to a point when the liquid level in the cell had to be raised by the addition of water in order that the concentrate could be withdrawn. The flotation test was continued until the pulp colour changed from black to a dirty grey. Careful records were kept of the mass withdrawals and additions, enabling solid and water inventories to be kept during the run. This enabled an estimate to be made of the volume between the pulp level and the level corresponding to that of the weir. Subsequently, in runs G5 to GlO. the level of the top of the froth layer was also observed. Each individual test included the measurement of ash in the feed, all concentrates and in the residual tailings. (v) Additional
measurements
Subsequent measurements include analysis for total iron using the Lab-X Telsec X-ray Fluorescence Analyser. Microscopic analysis of selected runs were done using a Quantimet Image Analyser to assist with characterization. Electrokinetic measurements were made on a selected run G5 using a Rank hlicroelectrophoresis Apparatus Mark II to determine the
In t!?c! firtit si(4t!(i. convc~nti~uia! fimtim toatfi wre done in lx?i?x undue id43nticai conditions except that the initial wt~w volrrmes wre 7.0 wid Q,Q I respectively. As the volume of liquid in 11~ cell wit11 air at 9 l/min and an agitator s!~ed af 33OQ rwn was measured as 9.5 ! to ths lxxtam of CIW overfIow weir, the latter run corresponded to the conditions for which h, - 0 and tlw initial ratio of rates M,/Jl, tended to the ratio of tlie rates. leaving the pulp 4V,/.lf,.
(ii, Dminage
tests
the second test series, the froth was allowed to build LIP and be removed in a periodic manner. In each period, the froth espanded to reach its limiting volume without concentrate being withdrawn. then at the end of the period all froth above the overflow weir was removed rapidly by scraping. This corresponded to the condition where hF - h,. III
RESULTS r\ND DISCUSSION (i) Screening
efficiency
and sieve analysis
The dry screening is clearly inefficient, as reported in Table 2_ It is anticipated that improvements in controlling the rate of feeding the screen will significantly improve screening efficiency, although there is clearly a decrease in separation efficiency at the smallest size. Table 3 shows that the Gilson wet sieve analysis correlates well with size distribution obtained by microscopic esamination- The Gilson apparatus does not directly measure sizes below 22 flrn and information about the
8 TABLE
3
Comparison
of the mean sieve diameter with the three microscopic
Run
G5 Feed nominal size +I04 -250
G6 Feed nominal size +45 -104
diameters
Size range
Mean size
Number
(cm)
Wm)
of particles in field
Area mean diameter Wm)
Mean chord length Wm)
Circumscribed diameter (pm)
-22 -32 -53 -90 -150
116 27.0 42.5 71.5 120-O
1036 121 64 64 58
8.09 31.60 45.62 78.45 127.74
7.88 23.92 38.18 70-83 101.46
6.52 32.83 42.82 68.25 126.32
-22 +22 -32 +32 -53 +-53 -90 +90 -150
11.0 27.0 42.5 71.5 120-O
1153 234 153 66 63
4.72 29.01 47.89 69.66 113.29
3-32 24-91 38.85 53.25 102.57
5.29 26.55 46.36 71.59 98.28
+22 +32 +53 +90
TABLE 4 Summary of final separations achieved ss a function of size fraction and SO2 pretreatment Size fraction (Pm) -45
-105
-250
1-45
+105
Cumulative concentrate
WI
Recovery daf coal (%I
PI
WI
61.6 48-S 48-6 45.5 57-3
88.2 76-l 69.3 67.7 81.8
19.0 16.7 10-s 11-4 11.9
12.0 10-2 16.2 14-S 20.5
76.4 76.4 68.4 75-6 57.5 77.7 62.0
92.7 93.8 88.0 92.3 81.4 91.8 86-5
il.12 18-01 9.45 12.78 7.99 9.85 7-22
28-5 15.0 28.1 25.8 24.9 34.2 31.5
95.0 83.5 76.9 83.4 80.5
97.5 93.3 94.1 91.6 91.4
17.39 10.78 8.95 8.25 6.71
29.7 38.5 38.6 44.6
SO? pretreatment (mglg c0a.U
Final taiiings ash
FlO Fll F12 F13 F14
-
Feed ash WI
Mixing measure of feed
Run No.
28.04
0.593
4.71 11-77 35.29 129-41
23.08 25.01 23.08 25.01 23.08 25.01 23.08
0.540 0.562 0.540 0.562 0.540 0.562 0.540
FO Fl F1/2 F2 F2/2 F3 F4
-
28.66
0.599
F5 F6 F7 F8 FS
-
4.71 4.71 11-77 11-77 35.29 129.41 4.71 11.71 35.29 129.4
presence of extremely small particles has to be based at the present time on microscopic examinations alone- These indicated that there was a significant amount of extremely small particles, estimated to have a size of <5 pm, present in the -22 pm fraction. {ii) Effect of SO,pretreatment on beneficiation Table 4 summarizes the experimental runs in terms of the final cumulative separations achieved_ More detailed information relating
ash
Separation efficiency
46-4
the variation in cumulative concentrate ash during a particular run may be inferred from the grade-recovery curves shown in Figs. 2,3 and 4. The cumulative concentrate ash can be calculated from the grade and feed ash using the definition of grade given in the section on Performance Measure. to
Concentrateash=feedash(l-g) The separation achieved is expressed as separation efficiency, as discussed in the same
9 TABLE
5
Initial froth stability cIata.Air rate: 9.0 llmin. Total froth volume I+: 3.0 I. Constrained volume 2-S I_ b = O-714_ Size range
Run
(Pm)
No.
SOaddition (mglgcoal)
FlO Fll F12 F13 F14
-45
-105
-250
+45
+105
-
4.71 11-77 35.29 129-41
FO Fl F112 F2 F212
0 4-71 4.71 11.77 11-77
F3 F4
35-29 129.41
F5 F6 F7
-
F8 F9
35.29 129-4
4-71 11-77
Initial frother addition
Total frother addition
initial water removal rate
Initial froth formation time tf
(mglg)
(mgig)
(gmin-')
(s)
volume (1)
O-19 0.19 O-38 0.56 0.75 094
056 O-56 l-60 4-04
164-3 55-O 13.4 12-6
4.71
55-7
45 55 51 55 120* 136
3.52 3.26 3.34 3-26 < 3.0 3.002
0.095 O-38 0.19 0.38 O-19 0.28 0.38 o-47
2-07 2.45 2-16 2.82
15-S 680-l 5-l 751.1
50 20 115 20
3.37
1.51 2-45 2.16
64.1
0.38 O-38 O-38 0.47 0.56 O-75 O-94
2.54 l-69
82-S 89.6
2-07
49.8
3-20
30-9
1201;
section, which can be related to the three commonly used measures, final tailings ash, recovery of dry ash-free coal and the cumulative concentrate ash_ These three figures are not independent and as those reported are derived directly from experimental measurements, checks for their consistency provide an estimate of experimental accuracy_ For the -250 pm +105 pm size fraction, there is a significant drop in final concentrate ash together with an increase in efficiency for increasing levels of SO2 pretrc:atment_ The individual grade-recovery curves for particular runs are shown on Fig. 4. The upper boundary of the experimentally achieved grade-recovery space corresponds to that of a single run involving pretreatment with 55 g of SOz. This run therefore represents the optimal treatment whereby the best grade will be achieved for any specified recoverypm +45 pm fraction, the For the -105 runs Fl and F2 are anomalous. From Table 5 it can be seen that the initial froth expansion for these runs corresponded with that associated with no bubble breakage. Very high
2i.i 46-i
135 38 33 38 39 110* 125 120f 110* 230
Limiting
Initial
froth
3-01 <3_0
3.002 3-90 4.45 3-90 3.84 < 3.0 3.006 c3.0 r; 3.0 3.001
X7_
QF 1 -a&
O-17 O-084 0.109 0.084 o-0 0.0007 0.119 3.50 O-003 3.5 0.0 O_OOOi O-28 O-42
O-28 O-26 o-0 0.002 O-0 o-0 0.0001
initial concentrate waterremoval rates were associated with these extremely stable froths. As these runs were atypical with respect to initial concentrate water removal rates, runs F1/2 and F2/2 were repeated under conditions corresponding to Fl and F2 respectively, but at half the initial frother amounts_ Figure 6 shows kinetic data for the runs Fl and F1/2 and F2, F2/2_ _Q the repeated runs had more typical water removal rates, their grade-recovery curves can be compared with FO, F3 and F4 on Fig. 3. Several interesting points arise from a comparison between the overall separation efficiencies reported for the -105 pm +45 pm size fraction and the grade-recovery curves on Fig_ 3. Firstly, although the single treatment corresponding to 55-O g of SOz (run F4) forms the upper boundary of the graderecovery space and therefore represents the optimal treatment. it has, however, a lower overall separation efficiency than for a pretreatment corresponding to 15.0 g of SO2 (run F3). This is because a higher coal recovery was achieved in the latter run leaving a
10 .o.-
1-O
$I<-.
y,yy---____‘_____ \ 1
\
wo
_‘
3.
‘. wt
1---
3
‘.
!. >
.\A_
-‘\ :\
‘.
‘\
‘3.
‘.
I.
-I-,
x-_3
.l .OC
o-1
I _
=\
5 \ 6-
i 0
lo Time
.mmi
Fig_ 5. Kinetics, untreated Size (Pm)
Run
FIO
-45 -250
Gl +105
F5 G2
1
I
20
30
.1
coal. Coal
7.0
3 1 5 7
7.0 9.0
I
1
30
20
rune.
mm5
Fig. 6. Kinetics. treated coal.
Initial volume (1)
9.0
t
10
Mineral
Size
Run
Initial volume (1)
Goal
Mineral
Fl FIl2 F2 F2/2
7.0 7.0 7-O 7.0
1 3 5 7
2 4 6 8
(Pm) 4 2 6 8
tailings richer in minerals. Also, despite the decrease in overall separation efficiency for runs FO, F1/2 and F2/2 reported in Table 4, these runs enclose each other in inverse order to the above sequence on the grade-recovery curve, and clearly reflect an improvement in beneficiation with increasing SO2 pretreatment in this size range. The separation efficiency has, therefore, to be used with some caution as it is clearly biased towards high recoveries. For the -45 jxm fraction, the upper bound in graderecovery space is again represented by a single test equivalent to 55.0 g of SO2 pretreatment. Once again, the separation efficiency sequence does not quite mirror that of the grade-recovery curves and the importance of Coal recovery as a factor in evaluating separa-
+215 -105
tion efficiency can be seen by comparing runs FlO and Fll. Finahy, there is a marked decrease both in separation efficiency and grade-recovery for the best pretreatment (55.0 g of SOz) for the sequence of size fractions -250 pm +105 pm, -105 pm +45 pm, and -45 pm. (iii) Froth
stability and kinetics
Figure 5 shows a comparison between the kinetics of flotation for untreated -45 pm and -250 pm +105 pm fractions. The tests were done at two initial pulp volumes for each material, Le. 7.0 and 9.0 1. At the latter value under agitated and aerated conditions, there x&s virtually no unconstrained froth volume and according to the section on
11 TABLE
6
Flotation
kinetics for two untreated
coal fractions
Particle size (m-d
Run No.
Initial PUiP volume (1)
Initial frother added (mglg coal)
Total frother added (mglt3 coal)
Initial water rate (cm3 min‘-r)
-45
Gl FlO
9.0 7-O
0.19 O-19
0.19 0.56
G2 F5
9.0 7-c
0.38 O-38
1.52 2-54
-250
+105
TABLE
Run No.
1261.0 164.3
0.4 1 0.18
0.15 0.085
2-73 2.12
850-S 82.8
O-44 O-16
O-087 O-04 1
5.08 3.90
G
+45
Total frother added (mglg coal)
Initial water rate (cm3 min-‘)
Initial first-order rate constants % (mm-‘)
i-srn (min-r)
kc -E‘ IXl
4.71 4.71
0.38 0.19
2-45 2.16
0.77
0.25
3.08
11.77 11.77
0.38 0.28
2.82 1.51
680-l 5.1 29.2 751-l 64-l
0.12 0.77 0.117
0.025 O-27 O-022
4-80 2.85 5.32
Fl F1/2 F2 F212
Froth Stability and Flotation Kinetics the flotation kinetics approach those characteristic of the pulp processes. These show decreasing first-order rate constants which have been frequently observed in mineral processing and discussed, e.g. Woodbum and Loveday [ 151. The lower volume runs were part of the F series and a comparison with Gl and G2 provided an estimate of the cleaning effectiveness of the froth layer First-order rate constants were extracted from Fig. 5 and Fig. 6 under initial conditions using the relationship In
W:(t) I
L
rvi(0)
__ elf
species
1.
Initial frother added (m&z coal)
SO-. addition (mglg coal)
(pm)
where
%n (min-‘)
kinetic data for two coal fractions pretreated with SO=. All initial volumes i-0
Particle size
hi=-d
%
& (min-‘)
7
Flotation
-105
Initial first-order rate constants
I
if-o
i may
be coal
(c) or mineral
(m).
Rate constants obtained in this wtiy are tabulated in Tables 6 and 7. For very high initial rates noted on Fig. 6, differentiation is obviously liable to error and the initial rate was taken as the average rate over the first short time interval. From Fig. 5 and Table 6, the initial rates for Gl and G2 show that the coal rate constant is not size dependent while
of the minercll is. This may be esplained by the importance of inertial effects in entrainment, which could be more significant for the dense mineral than for the light coal. Comparison between the ratio of apparent first-order rate constants for the runs with a deep froth layer and those in which pulp effects only are important (Gl and FlO, G2 and F5) show that there is no appreciable cleaning in the froth for those unreduced coals. Figure 6 shows the effect of initial frother addition on particles in the -105 pm +45 pm size, which have been treated with 2 g of SO2 (Fl, F1/2) and 5 g of SO+ (F2, F2/2)_ The effect of a -relatively high level of frother addition (0.38 mg g-’ of coal) is shown for runs Fl and F2 to produce a very high initial water rate almost equivalent to that observed for the runs Gl and G2 at low froth volumes At half the initial frother addition, the initial water rates for runs F1/2 and F2/2 were much lower than for Fl and F2 and more comparable with other runs in this series (i.e. FO, F3, F4). The apparent first-order coal rate constants for runs F1/2 and F2/2 are that
12
similar to those for the untreated coal FlO and F5, but the rate constants for the mineral differ appreciably between the two series. This suggests that the effect of SO2 pretreatment is primarily to reduce mineral flotation rates, probably by improving the drainage of mineral particles from unstable froths. The high apparent flotation rate constants both for coal and mineral in the stable froth runs indicate very little drainage from these froths, and the low ratios of kc/k,,, reported for Fl and F2 reflect the poor beneficiation observed, which is comparable with those noted for unreduced coal. in terms of the flotation model, the runs %Z and F2 correspond to a condition where h, S- hF and the poor grades observed indicate low absolute values for the species froth return rate constants kFc and kh. This corresponds to eqn. (6). (iv) Fmth dminage The comparison between a conventional run G5 and run G8 which was carried out under periodic conditions. as described in B(C) in the section on Esperimental Procedure, offers further information about the effect of drainage from limiting froths in which hF + h, as described by eqn. (5). Details of the chemical treatment used in these runs are given in Table 8 and gravimetrically determined ashes for various size fractions in the feed material are given in Table 9. The weighted average ash computed from individual fractions compares well with that determined on the feed sample as a whole. Number and area counts of mineral particles observed by microscopic esamination under transmitted light are also reported. As the mineral is heavier than the coal: it might be TABLE
TABLE 8 Experimentaldetailsfor conveationnlrlrd periodic runs G6 and G8 respectively Nominal
particle size -250
pm +106 pm
Initial charge Run GS 425 g coal Run G8 425 R coal
7000 cm> water 7000 cmJ water
Both runs treated witb 55.0 g SOz. Run GS operated by allowing froth to build up periodically. removing the accumulated concentrate. Chemical treatment 1.13 m 6-l mixed crcsols dextrin 0.223 mg 6-l 1.76 rng 0-l IWe(CNk Montanr.1 361 used as frothcr Feed ash 24.13% Run G5 Initial froth build-up-time Run G8
24 s
Feed ash 24.09%
espected that the area fraction would be lower than the gravimetric ash content. This is not so for the -22 pm particles. which probably reflects the difficulty in distinguishing between extremely small coal and mineral particles under transmitted light. The low mineral fraction reported for the +90 Mm -150 pm fraction is not statisticaliy significant because of the small total number of particles which were counted in this size range. Microscopic examination also revealed the presence of large numbers of very fine particles (-5 pm) in the -22 pm fraction. Mineral contents of individual size fractions are reported in Table 10 for a cumulative concentrate corresponding to a recovery of about 15% of dry ash-free coal. These early concentrates usually have the lowest ash content within a run. There are significant
9
Analysisof feed coal. Runs G5 and GS. Total ash = 24.1%. Size range Wm)
Mass fraction in range Xi
Microscopic
-22 +22 -32 +32 -53 +53 -90 +90 -150. +150 -250
0.23 0.05 0.12 0.24 0.30 0.06
0.381 0.168 0.114 0.119 0.017
Fractional
analysis area mineral
Fractional 0.741 0.314 0.260 0.078 0.017
number
mineral
Gravimetric (%I
ash Ai
29.31 18.94 17.95 20.95 22.16 37.89 XzqA J = 23.5%
13 TABLE
10
Ash nnnlyscs of concentrntcs. Iluns G5 nnd GS. Run G5: 1st conccutmtc corresponding to 0.286 frnctionnl recovery; nsh = 5.4Ct. Run G8: 2nd couccntrntc corresponding to 0.138 frnctiannl recovery; nsh = 5.19%
Run G5 Grnvimctric (%I
-22
0.093
+22 -32 +32 -53 453 -90 +90 -150 +150 -250
0.080 0.205 0.350 0.24 0 9.032
11.4 5.52 4.39 4.33 4.31 4 -4 1
TABLE
\‘Xi:\ i
Run G8
Mnss frnction Si
nsh r\ i
Mnss frnction Xi
Grnvimetric (XI
0.128 0.101 0.158 0.267 0.323 0.023
7.43 4.63 3.79 3.60 3.90 4.35
ash di
G.5 = 5.09%
GY = 4.35%
11
Ash nnnlyscs of fiunl tailings. Runs G5 nm3 G8. Run G5: Frnctionnl con1 left in tailings; nsh - Gl.lW. Ruu GS: Frnctionnl con1 left in tailings; nsh = 53.2%. Run G5
-22 +22 -32 +32 -53 +53 -90 +90 -150 +150 -250
xxi,1 i
Run G8
Mnss frnction -Vi
Grnvimrtric (%I
0.50 0.04 0.07 0.16 0.20 0.03
51.8 72.9 77.8 80.9 79.9 73.1
nsh A i
differences between the weighted average ash contents and the overall ash determined on the whole concentrate. These discrepancies may be due to the loss of ultrafine material (-2 pm) from the -22 flrn fraction by passing through the filter paper under the smallest sieve. Ashes for these samples may additionally be liable to esperimental error because of the small amounts used in the analysis. There is a significant improvement in the ash content of all size fractions for the deep froth as opposed to the conventional float, indicating that improvements by drainage can be anticipated, producing concentrates of about 3.5 to 4.5% ash for all fractions escept the finest. The limiting ash for this fraction appears to be of the order of lo%, assuming the error in weighted averages is caused by an underestimation of ash in this range. Table 11 contains ash analysis of the residual unfloated tailings at the end of both. runs. It is interesting to note the high ash
Mnss fmction Xi
Grnvimctric (“ml
0.402 0.035 0.076 0.202 0.24 7 0.035
44.9 55.1 58.9 G-1.6 57.2 47.1
nsh Ai
G5 = 65.45
GS = .53_4F~
content of the -22 pm material in both tailings, which indicates that even within a range where entrainment. is significant. some selective separation of coal from mineral occurs. Both runs show more complete removal of coal in the middle size region (-105 pm +32 pm) with the conventional run showing better final tailings ashes. It was not possible to maintain the high static frot.11 levels required for GS towards the end of the run, which reduced overall recovery.
(v) Ptztrogmphic analysis Table 12 shows pet.rographic analysis for the conventional run G5 for two size fractions. While the difference between feed and first concentrate appears small. the ratio of vitrinite-esinite]inertinite becomes noticeably more favourable in the concentrate than in the feed. The analysis also indicates that of the non-coal components the shale is least -efficiently removed.
14 TABLE 12 Petrographicanalysis Component
Vitrinite Exinite Inertinite Shale Pyrites Other minerals ‘Coke’
Size range -150
pm +90 pm
Size range -32
pm +22 pm
G5 Feed
G5 1st Concentrate
G5 Feed
G5 1st Concentrate
61.0 6.8 13.2 7-s 0.4 6.2 4.6
72.2 10.4 10.2 3.4 0.2 1.2 2.4
67.0 3.6 15.6 5.8 0.6 5.0 2.4
75.0 6.8 il.8 3.0 0.4 0.8 2.2
CONCLUSIONS (i) It is possible to produce a cumulative concentrate from weathered coal with an ash of approximately 7% with dry ash-free coal recoveries of approximately 85 to 90%, in the size range -250 pm +45 pm, by pretreating the coal slurry with SO2 gas followed by froth flotation. (ii) In the size range -250 m -1-105 m, concentrates containing less than 5% ash can be prepared at 40% recoveries of dry ash-free coal. (iii) The effect of froth stability on beneficiation is complex. Very stable froths can give poor grades if they are allowed to produce very high initial rates of water removal in the concentrate_ Periodic tests show, .however, that stable froths which are given the opportunity to drain well; before removal over the concentrate weir, produce low ashes. (iv) Froth stability decreases during the course of a run and for a constant size fraction with increasing levels of SO1 pretreatment. Frother addition in limited amounts helps to increase stability. (v) Periodic flotation tests showed that ashes of about 3.5 to 4-0s can be obtained in early concentrates in the size range -150 pm +32 pm for deep well-drained froths. Ash levels in the range -22 pm were high, in the range 8 to 11%. Tailings assays showed that the most complete removal of coal occurred in the range -150.1.rm +32 pm. (vi) The residual tailings contained a higher mass fraction of -22 JUXImaterial than the feed.’ The high residual mineral in this fraction in the tailhgs~and the lower mineral (8 to 11%) in the early concentrates shows that
even for the ultra-fine material there is some selective removal of coal(vii) The flotation model proposed by Moys has proved a useful conceptual basis for understanding .flotation kinetics. In particular, by determining three limiting conditions where h, + 0, h, + h, and when h, + 00 for a finite hF, there is a framework for the estimation of froth parameters in their region of masimum sensitivity-
ACKNOWLEDGEMENT We wish to acknowledge with thanks the assistance of Dr_ A. Smith of the Yorkshire Regional Laboratories of the N.C.B. in performing the petrographic analyses_ LIST OF SYMBOLS
-
55
da h
hi+ hc, km
HE
H,, Hc, HT IT
mass fraction feed as concentrate area mean diameter equivalent circle, pm mean chord length, pm circumscribed diameter, pm transient froth height during formation, m height parameters. describing froth height operating froth, constrained height between pulp and weir, maximum constrained height, m separation efficiency, % mixing measure. Feed, concentrate and tailings respectively sum of intercepts of leading edges of all particles in Quantimet field, pm
15
k
kFi
nT qb
apparent first-order rate constant relating to the transfer of species i from pulp over concentrate weir, min-’ return first-order rate constant of species i from froth to pulp, min-’ mass flow of species i over concentrate weir, kg s-r mass flow of species i entering froth from pulp, kg s-l total number of particles observed by a microscopic count air flux leaving top surface of froth due to bubble breakage, m3
9P
R(t)
ST
t tF
vF wj(t)
-Ti
m--2
s-
T min
Subscripts iaf m
Greek symbols CY froth parameter = 1 ratio of constrained 6 froth heights = h,/hF fraction of stagnant (1 - e) remote from weir percentage grade of 5 concentrate
- q&p to operating froth surface cumulative
of time in
and abbreviations coal dry ash free mineral
REFERENCES
1
air flus entering froth from pulp, m3 m-* s-’ cumulative percentage daf coal recovered at time t, % projected area particles in Quantimet field, pm’ time, s time of formation of initial froth, s froth volume, cm3 mass of species i remaining in pulp after time t, kg mass fraction component i in mixture, -
minimum air retention froth = h,/q,
8 9 10 11 12 13 11 15
C. K. Law, K_ H. Law and C. Ii_ Lee. Encq~y. 4 (1979) 329 - 339_ A_ M_ Gaudin. Nofafion. McGraw-Hill. New York. 1932. p_ 636. B. A_ Firth, A_ R_ Swanson and S. K_ Nicol. ZnL J. of Min. Process.. S (19C9) 321 - 33-t_ %I_ H. Moys. PhD. Thesis. Univ. or Satal, Durban. 1979_ 3_ %I_ Miilier and D. J. Brockhoff. _-l;IfZwreilungs Tcchnik.. 3 (1976) 129 - 13-t_ P. Kind, Aufbcrcilungs Technil:.. 3 (1918) 119 - 12s. A. F_ Baker. K. J_ Miller and _a_ W. Deurbrouck, 6th Znst_ Coal Prep. Congress. Paris. March (1973). Paper 27E. Lr_ N_ Petukhov, Khirn. Tccr. TopI_. 13 (19i9) 12 - 20. F_ F_ Xplan, FZolotation 2. Am. Inst. of Min. Jfet_ & Pet_ Eng.. New York. S.Y. (1976) 1235 - 1264. ;I_ F. Baker and K_ J. Miller. Z_?S_ Bureau of Illines Report. XI_ 751s (1971) 1 - 21_ W. W. Wen and S. C_ Sun. Sot. of 3fin. Eng of _-%.Z.AZ_E. Trans.. 262 (19T7) 17-l - lSO_ J_ Stachuski. T_ Fijal and 11. Nichalek, _4rch. Gorn.. 25 (1950) 269 - “Sl_ K. H. Simrrik and B. E. Scott. _)Zining Consr_ Journal. 66 (19SO) 31 - 22,3i_ A_ F. Ramdja. M_Sc_ Dissertation. U31IST_, 3Ianchaster. 19SO. E. T_ Woodburn and B_ K_ Loveday, 2. South African Inst. of Jlining & Net_. 65 (1965) 612 6%.