Minerals Engineering, Vol. 12, No. 1, pp. 1-13, 1999
Pergamon 0892--6875(98)00116-2
© 1998 Elsevier Science Lid All rights reserved 0892-6875/99/$ - - see front matter
A COMPARATIVE EVALUATION OF THE LEADING ADVANCED FLOTATION TECHNOLOGIES
M.K. MOHANTY and R.Q. HONAKER Department of Mining Engineering, Southern Illinois University, Carbondale, Illinois 62901, USA E-mail:
[email protected] (Received 3 June 1998; accepted 2 September 1998)
ABSTRACT A comparative evaluation has been conducted using three leading advanced flotation technologies, which utilize different types of bubble-particle attachment environment. Based on a statistical evaluation of the test data the Packed-Column technology, which provides a near plug-flow flotation environment due to the presence of corrugated packing material in the cell, produced the best separation performance due to its ability to support an extremely deep froth zone. However, because of the absence of an air sparging system and the consequent larger bubbles, the froth carrying capacity was the minimum with the Packed-Column technology. On the other hand, the throughput capacity achieved by the Jameson Cell technology, which has a self air-inducing co-current system that provides an intimate bubble-particle attachment environment characterized by an extremely high air fraction and ultrafine bubbles, was found to be maximum. The Microcel TM technology achieved its maximum carrying capacity while providing a high energy recovery with a reasonably low reagent consumption. © 1998 Elsevier Science Ltd. All rights reserved
Keywords Froth flotation, Flotation froths, Column flotation, Coal, Flotation machines
INTRODUCTION Several advanced flotation technologies have been commercialized since the firstflotation column was patented in the early 1960%. The most significant difference among these technologies lies in their microbubble generation system and the size of the bubbles that are produced, which affects the amount of bubble surface area available for the flotationof coal particles.Some technologies also differ with respect to their bubble--particle contact environments, whereas others differ with respect to their constructional features to promote a superior axial mixing environment in the cell. The use of self-induced air and the generation of uniform microbubbles are novel concepts of other advanced flotationtechnologies. Several of these technologies were compared for fine coal cleaning on the basis of their ash and sulfur rejection performances by Killmeyer et al. (1989) as a part of an U. S. D O E sponsored study. The three different coal samples tested in this project were IllinoisNo. 6, Pittsburgh No. 8 and Upper Freeport seam
2
M.K. Mohantyand R. Q. Honaker
coals. Predictably, all the advanced flotation technologies produced metallurgical performances superior to the performance achieved by the conventional flotation process on all three coals tested. However, the Packed-Column was the only advanced technology which achieved all the target performance parameters, such as less than 4.0% product ash content, greater than 80% energy recovery and greater than 70% pyritic sulfur rejection for all three coals. The MicrocelTM column achieved the desired performances for both Illinois No. 6 and Pittsburgh coal samples, whereas the Ken-Flote column succeeded for only the Upper Freeport coal. The other competing technologies that achieved the desired performances for only one coal were the Flotaire column and the University of Utah's flotation technology. The aforementioned study accomplished an excellent assessment of the metallurgical performance of the various advanced flotation technologies for fine coal cleaning. However, because of the lack of common experimental conditions, such as feed coal particle size, reagent types and dosages as well as the number of cleaning stages, a meaningful performance comparison could not be extracted from the resulting experimental data. Therefore, a flotation study comparing six different commercial column technologies was conducted by Honaker et al. (1995) using the same coal and reagent types for each technology. The technologies included the Canadian Column, the Flotaire Column, the Jameson Cell, the MicrocelTM Column, the Packed-Column and the Turbo-air Column. The other popular column technologies such as the Ken-Flote and the Cominco's flotation columns were not included in this study because of the similarity of their design to some of the technologies studied in the investigation. The study concluded that Jameson Cell, MicrocelTM and Packed-Column have definite advantages that characterize their uniqueness among the column flotation technologies. The Jameson Cell technology was found to require the lowest amount of reagents, indicating the highest mechanical shear provided by its bubble generation system. On the other hand, the Packed Column technology, which does not utilize a bubble generator to provide the mechanical shear, was found to require the largest amount of reagent and compressed air among all the competing technologies. The Packed-Column appeared to provide the most optimum metallurgical performance and the lowest throughput capacity among the three leading technologies. However, the study used a coal preparation plant flotation feed sample that had a very low solid content. As reported in literature, a high feed solid content improves the metallurgical performance from the flotation columns significantly by allowing the column to operate in a carrying capacity limiting condition (Mohanty et al., 1998; Mohanty et al., 1997; Honaker & Mohanty, 1996; Finch et al., 1989; Yianatos et al., 1988; Yianatos et al., 1987; Moys, 1978). Thus, the most optimum performance from any column technology can be obtained only by operating the column at a high feed solids content to ensure that a carrying capacity type of flotation environment prevails in the flotation cell. This publication presents and discusses the results of a study which evaluated the leading advanced flotation technologies and compared the optimum metallurgical performances obtained by each of the competing technologies using a statistical analysis tool. The three advanced flotation technologies evaluated in the study include the Jameson Cell, the MicrocelTM and the Packed-Column. The throughput capacity values for each technology were investigated and compared for treating a fine coal sample.
EXPERIMENTAL Sample The sample treated in the flotation column experiments was collected from a thickener underflow stream of a coal preparation plant treating the Coalberg seam in West Virginia. Upon arrival, the slurry samples were stored in sealed 200-liter drums. The particle size-by-size characterization data for the sample provided in Table 1 indicate that about 70% of the coal has a particle size less than 37 ~rn (400 mesh) and contains about 42.6% ash and 0.86% total sulfur. Thus, the sample may be characterized as a relatively high ash and low sulfur coal. An advanced flotation washability (AFW) analysis was conducted on a representative portion of the sample to characterize its ultimate flotation response (Mohanty et al, 1998).
Comparative evaluation of advanced flotation technologies
TABLE 1
3
The particle size-by-size characterization data of the Coalberg coal seam sample used in the comparative evaluation of the column flotation technologies Size Class
Weight
Ash
Total Sulfur
+212 212 x 150 150 x 75 75 x 37 -37 Total
3.86 3.32 10.74 12.66 69.42 100.0
23.68 33.74 27.13 30.72 48.6 42.58
1.21 1.32 1.25 1.22 0.69 0.86
(~rn)
(%)
(%)
(%)
Flotation experiments The design and the general operating parameters utilized during the experimental programs for the three flotation technologies are provided in Table 2. Prior to each test series, the coal slurry was conditioned with 1 kg/ton of kerosene for a period of 5 minutes. For the Packed-Column and Jameson Cell tests, a polyglycol frother solution was continuously added to the feed stream at a pre-determined rate, whereas, for the Microcel TM tests, the frother solution was added to the bubble generation system. The tailings flow rates were controlled to maintain a desired froth height using a PID controller along with a pressure transducer placed at the bottom section of the flotation cell. The feed volumetric rates and solid contents were varied over a wide range to generate the optimum grade-recovery relationship from each technology. TABLE 2
Design and operating parameter values used for the tiotation experiments conducted utilizing three leading advanced flotation technologies
Design Parameters Cell Diameter Length Operating Parameters Aeration Rate (cm/s) Air Pressure (kPa) Collector (kg/ton) Frother (ppm) Froth Height (cm) Wash Water (crn/s)
Jameson Cell
Microcel l M Column
10 cm 150 cm
240 cm
10 cm 510 cm
2.10 138 1.0 10 - 20 36 - 66 0.25
3.00 - 4.65 138 1.0 10 - 25 366-396 0.25
0.8 - 2.30 1.0 5-10 23 - 41 0.27
5 cm
Packed-Column
For the experiments conducted to determine the throughput capacity of each technology, the feed solid contents were varied while maintaining a constant volumetric flow rate to allow the same retention time for each test. Although aeration to each cell was maintained at the highest possible rate, a positive bias rate was maintained in the flotation cells to eliminate the recovery of the entrained materials to the product launder.
RESULTS AND DISCUSSION As shown in Figures l(a) and (b), the column performance data-points are well within the ultimate flotation performance curve predicted by the advanced flotation washability (AFW) analysis procedure. The proximity of the column data-points to the AFW performance curve generated both on the basis of ash and total sulfur assays indicate the highly selective flotation environment prevailing in all the three flotation cells. However, it seems difficult to comparatively evaluate the performance obtained from the three competing technologies
4
M.K. Mohantyand R. Q. Honaker
based on the combustible recovery versus ash rejection and combustible recovery versus total sulfur rejection relationships shown in Figures l(a) and (b). As a matter of fact, all three sets of data-points appear to belong to the same population, which may erroneously suggest that the performances obtained from the three technologies are not significantly different. However, when plotted on the basis of product ash and product total sulfur over much smaller scale, as shown in Figures 2(a) and (b), the data exhibit a significant scatter. The variations among the data-points from the different technologies at the same combustible recovery value appear to indicate significant differences in their performances. It should be realized that the visible scatter in the data points obtained from the same technology resulted because of the variation of feed solid contents allowed in the experimental program. At high feed solid contents, the column tends to operate in a carrying capacity limiting condition, which promotes a selective reflux environment in the flotation cell, thus producing the optimum separation performance. On the other hand, the data-point generated at a low feed solid content is most likely produced in a kinetically limiting environment, which produces performances inferior to that obtained in the carrying capacity limiting environment.
100
(a)
.~
80
8
60
i
I
i
i
!
o
oL
40
AFW [] Jameson o Mierocel A Packed-Column
..O
E
20
00
I
I
I
20
40
60
I
80
100
Ash Rejection (%)
(b)
lO0
i 80 60
. 4o
AFW
:o
Q Jameson o Microcel A
0
,
0
I
20
o ~
-t~ Oo~k NAh
Packed-Column ,
I
I
I
40
60
80
,
1O0
Total Sulfur Rejection (%) Fig. 1
Metallurgical performances achieved by the three leading column flotation technologies and the advanced flotation washability (AFW) analysis on the basis of (a) ash rejection and (b) total sulfur rejection values.
Comparativeevaluationof advancedflotationtechnologies
5
As shown in Figure 2(a) and (b), it may appear that the Packed-Column technology provides the maximum selectivity followed by MicrocelTM and Jameson Cell technologies. However, since this finding is based on only a limited number of experiments, it is imperative to determine whether the difference in performance produced by the individual technologies are statistically significant. A comparison was conducted using the Chow test (Chow, 1960) to determine the difference in the regression equations developed to represent the ash and sulfur cleaning performance curves for each technology. Statistical comparison of performance curves using Chow test
Two key separation performance parameters, i.e., ash separation efficiency (combustible recovery-ash recovery) and total sulfur rejection performance, were selected to be compared to investigate the statistical
(a)
lO0
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o0 60
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-
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-.
[,-,-
0
dao
20
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o
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t
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4
6
8
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Microcd
A
Packed-Coltmm
I
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[
12
I
14
16
18
Product Ash (%) (b)
I00, 80
o0
'
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;
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[] Jameson o Microcei /A A Pacl~d-Colmnn/
60 ~
I
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%
o~oO
..q •~
40
AS; ~
°°oO
20
0
i
0.7
0.8
0.9
1.0
1.1
1.2
Product Total Sulfur (%) Fig.2
Metallurgical performances achieved by the three leading column flotation technologies and the advanced flotation washability (AFW) analysis on the basis of (a) product ash and (b) product total sulfur contents, feed ash: 42.6%; feed total sulfur: 0.86%.
6
M.K. Mohanty and R. Q. Honaker
significance of the differences in the performance obtained with the competing flotation technologies. The incremental mass yield values were used as the common reference points for comparing the performance parameters. The quadratic polynomial regression equations were fit to the best experimental data obtained from each technology to describe the separation efficiency and total sulfur rejection as a function of the mass yield values. The generalized fitted equation for each technology was: (1)
y = a + bx +cx 2
where y is the separation efficiency or total sulfur rejection, x the mass yield and a, b and c the fitted coefficients, which are parameter estimates. The regression coefficient values listed in Table 3 were obtained at coefficient of determination (r 2) values greater than 0.98. The general approach used to measure the significance of difference in performances was to statistically test the equality of the respective regression coefficients of the equations describing the separation efficiency and total sulfur rejection response for each competing technology using the Chow test in two phases. The first phase considered the comparison among all three technologies, whereas the comparison between each technology pair was conducted in the second phase. TABLE 3
A list of the coefficient values which best fit the polynomial regression equation describing the separation efficiency and total sulfur rejection as a function of the mass yield values
Technologies Separation Efficiency Jameson Cell Microcel TM
Best Fit Polynomial Regression Constants a b c -2.7683 1.7610 -0.0086 -2.1845 1.8603 -0.0105
Packed-Column
-2.1419
1.8968
-0.0096
Total Sulfur Rejection Jameson Cell Microcel TM
a 100.0625 100.0577
b -1.1846 -1.0259
c -0.0024 -0.0059
Packed-Column
99.5669
-0.8878
-0.0072
While comparing the three technologies in the first phase, if the fitted coefficients were found to belong to the same population at a significance level (a) of at most 0.10, it would be concluded that the separation performances obtained from the competing technologies are not statistically different and vice versa. The null hypothesis for this test can be written as:
I-lo:
Aj = A m = Ap Bj=Bm=B p C j = C m = Cp
where A, B and C represent the actual parameters and the subscripts j, m and p represent the competing technologies i.e., Jameson Cell, Microcel TM and Packed-Column, respectively. The alternative hypothesis (Ha) for the test is that any two parameters are not equal. The test statistic used was a special type of Fstatistic, which is provided in Equation 2. F
= (SSErestricted- SSEunrestricted)ldfi
(2)
SSEunrestricted[dfu
and dfi = d f r - d f u
(3)
where dfr, and dfu are the degrees of freedom associated with the restricted and unrestricted regression cases, respectively. The restricted regression case was the one where a regression analysis was conducted
Comparativeevaluation of advanced flotationtechnologies
7
using the experimental data obtained from all three technologies together, whereas the sum of square error (SSE) values obtained from the individual regression analysis conducted for each technology were summed up to determine the unrestricted SSE. The critical F -values at ~ value of 0.10 and 0.05 were obtained to compare the calculated F-value. If the null hypothesis was rejected in favor of the alternative hypothesis, a statistical difference was concluded from the aforementioned hypothesis testing and the next phase of analysis comparing the technology pairs was initiated. Utilizing the same procedure, the differences between the individual technologies were evaluated considering all the technology pair combinations, such as MicrocelTM--Jameson Cell, Jameson Cell-Packed Column and MicrocelTM-Packed Column. The results obtained from the Chow test are summarized in Table 4, which indicates that the separation efficiency and total sulfur rejection performances of each technology are statistically different from the other. The separation performance differences evaluated between the Packed-Column versus Microcel TM and Packed-Column versus Jameson Cell were found to be significant at an c( value of 0.05. On the other hand, the Microcel TM versus Jameson Cell differences were found to be statistically significant at a relatively higher o~ value of 0.10.
TABLE 4
A summarized list of the results obtained from Chow test comparing the difference in the separation performances achieved from the competing flotation technologies
Performance Comparison Separation Efficiency P-M-J P-M P-J M-J
Calculated F-statistic
Critical F at cx=0.05
Critical F at u~-O. 10
Degrees of Freedom
9.56 4.19 23.35 3.26
2.74 3.71 3.49 3.71
2.18 2.73 2.61 2.73
(6, 16) (3,10) (3,12) (3,10)
3.00 4.35 4.07 3.86
2.33 3.07 2.92 2.81
(6,12) (3, 7) (3, 8) (3, 9)
Total Sulfur Rejection P-M-J 6.93 P - M 4.36 P - J 12.81 M - J 2.96 • P: Packed-Column; M:
Microcel TM; J: Jameson Cell
The Packed-Column has also been found to provide the best metallurgical performance by past investigators (Killmeyer et al., 1989; Honaker and Paul, 1995). The ability to support an extremely deep froth zone, which enhances the selectivity among the particles of varying degrees of hydrophobicity, is the most likely cause that results in the superior separation performance of the Packed-Column. The near-plug flow flotation environment in the Packed-Column may be another factor aiding the most selective flotation performance achieved by the Packed-Column. As hypothesized by Honaker and Paul (1995), a steady regeneration of new bubble surface area occurs throughout the height of the Packed-Column due to a continuous coalescence and breakage of air-bubbles caused by the presence of the packing material in the cell. On the other hand, in any conventional open column, such as Microcel TM, the air-bubbles continuously coalesce with an increase in column height which ultimately decreases the available bubble surface area. As a result, particles in a Packed-Column are attached, detached and re-attached throughout the column height. These events strongly influence the selectivity in the Packed-Column due to its ability to support very deep froth zones. The new bubble surface area being generated throughout the column height combined with the reflux and selective detachment mechanisms can be represented by a rougher-multiple cleaner circuit as illustrated in Figure 3(b). The number of simulated cleaner units is a function of the froth depth. Using the concept of circuit analysis described by Meloy (1983), one can easily show that the proposed rougher-multiple cleaner circuit provided by the Packed-Column is more selective than the rougher-cleaner circuit simulated by conventional open columns as shown in Figure 3 (a) which was originally proposed by Finch and Dobby (1990). This provides a possible explanation for the superior selectivity achieved by the Packed-Column flotation technology.
8
M.K. Mohanty and R. Q. Honaker
(a)
Feed
Concentrate
t
Tailings
0 iO v ILh
Froth Phase o lUfll_
Feed -----P
gf TO 0 00,0 0
I -
,t-
u
Pulp Phase
Io o Froth DroilXBa~k
Concentrate
Tailings
(b)
Concentrate
Recov~
Froth Phase
Feed
0 I Nl~
Feed
Wc Fmtln o Dro~-I~
Tailings Pulp Phase
o o 0
o
Cle~.ner I o
o
C once ntrate Tailings Fig.3
Schematics illustrating the differences in selectivity mechanisms between the (a) conventional open column (such as MicrocelTM) and (b) Packed-Column (Honaker and Paul, 1995).
Another important finding obtained from the aforementioned statistical analysis is the performance superiority of the MicrocelTM over the Jameson Cell technology. The Jameson Cell, which provides an efficient bubble-particle contact environment in the downcomer, allows the recovery of many weakly hydrophobie middling and coal pyrite particles to the froth zone. In the kinetically limiting environment, these unwanted middling and coal pyrite particles report to the product launder, thus affecting the quality of the product. On the other hand, in a carrying capacity limiting condition, the Jameson Cell does not provide an efficient reflux environment unlike its flotation column counterparts. The particles, which are dropped back from the froth zone due to carrying capacity limitations, may or may not re-attach in the pulp
Comparativeevaluationof advanced flotation technologies
9
zone where the air fraction is less than 30% by volume. In addition, the dropped back particles remain in the separation chamber for a short period, which is insufficient to allow their complete recovery by the partially loaded fresh bubbles discharged at the bottom of the downcomer. As reported by Honaker and Paul (1995), the particle residence time in the separation chamber of the Jameson Cell is about half of other flotation columns. Thus, most of the particles dropped back from the froth zone report to the tailing stream of the Jameson Cell unlike other flotation columns where a selective reflux occurring between the froth zone and collection zone further enhances the selectivity.
Throughput capacity comparison The throughput capacity for the individual flotation technologies was determined by conducting the flotation experiments at the optimum operating condition over a range of feed solid contents from 3% to 21% by weight. The aeration rate was raised to the maximum possible level for each technology, while maintaining a significant positive bias factor during the experiments. The volumetric feed flow rate to the cell was maintained constant during the experiments to allow the same retention time, while the feed mass solid rate to the cell was increased gradually by raising the feed solid contents. With low feed solid contents, the available bubble surface area in the froth zone is sufficient to carry over all the attached coal particles to the concentrate launder. At lower feed solid contents, the increasing feed solid rate results in a commensurate increase in the concentrate solid rate. However, after increasing the feed solid rate up to a certain point when the bubble surface area available in the froth zone is fully loaded with the coal particles, the maximum concentrate solid rate is reached and is referred as the throughput capacity of the given technology. By increasing the feed solid rate beyond this point, a drop-back of particles from the froth zone to the collection zone occurs due to insufficient bubble surface area in the froth zone. The coarse coal particles having lower flotation rate constant values will have a higher probability of detachment from the forth zone. Thus, the coarser particles are more likely to be dropped back from the froth zone than the finer particles. Understandably, the finer particles require more bubble surface area than the coarse particles of the same mass. Thus, as the average size of the concentrate particles reduces with an increasing feed solid rate, the concentrate solid mass rate is also reduced. The relationship between the combustible recovery and feed solid rate for each of the competing flotation technologies is illustrated in Figure 4(a). As shown, the combustible recovery values reduce with an increase in feed solids rate for all three technologies. However, the concentrate solids rate gradually increases and reaches a maximum before beginning to reduce with an increase in feed solid rate as illustrated in Figure 4(b). The Jameson cell technology was found to provide the maximum throughput capacity of 1.76 g/mincm 2 followed by Microcel TM and Packed-Column technologies with throughput capacity values of 1.47 g/min-cm 2 and 1.23 g/min-cm 2, respectively. It should be realized that these throughput capacity values will be true only for the specific feed size distribution studied in this investigation, which had a ds0 size of nearly 70 lam. It is well understood that the carrying capacity of a cell is a function of the bubble size in its froth zone, which is in turn related to the initial size of the bubbles used in the collection zone. The Packed-Column technology, which operates without an air-sparging system, utilizes relatively large size bubbles in the collection zone even with a relatively higher frother concentration of nearly 25 ppm. The bubble size at the concentrate lip grows further due to froth drainage and coalescence caused by an extremely deep froth zone in spite of a high superficial gas velocity of greater than 4 cm/s at a 138 kPa (20 psi) of air pressure. This phenomenon results in the minimum throughput capacity provided by the Packed-Column technology. On the other hand, Microcel TM and Jameson Cell are known to utilize ultrafine bubbles produced from the respective bubble generation systems. In the Microcel TM, the microbubbles are produced by pumping a slurry-frother mixture along with a controlled amount of air through a static mixer. On the other hand, in the Jameson Cell a feed slurry-frother mixture is pumped through an orifice, which draws in air due to a venturi effect and ultrafine bubbles are produced in the downcomer because of a high shearing action provided by the jet action of the feed slurry. Due to the difference in the way the microbubbles are produced by both technologies, the mean size and the size distributions are different for the bubbles produced and used in both systems. The results from past studies on bubble size measurement, as illustrated in Figure 5, show that the mean size (ds0) of the microbubbles utilized in the collection zone by the
10
M.K. Mohanty and R. Q. Honaker
Jameson and MicrocelTM are 300 ~rn and 700 0rn, respectively, (Brake et al., 1996; Atkinson, 1994). The corresponding ds0 to d99 bubble size distribution for both technologies are 300 to 700 ~tm and 700 to 1800 0rn, respectively. The smaller bubble sizes produced with the Jameson Cell technology were also visually experienced by the authors while conducting the flotation experiments. The froth flooding phenomenon occurred with the Jameson Cell while conducting flotation tests with as little as 10 ppm of frother and about 2 cm/s of aeration rate, whereas Microcel TM required 15 to 20 ppm of frother for an efficient operation at a 2 cm/s of aeration rate. This finding suggests that the bubble sizes produced in the Jameson Cell are sufficiently fine due to the high shearing action resulting from the pressurized feed slurry for which even a small amount of frother causes a froth flooding condition in the cell. This can be considered as an indirect evidence of the finer bubble sizes produced in the downcomer of Jameson Cell in comparison to the collection zone of Microcel TM technology. The small bubble size in the collection zone of Jameson Cell results in relatively small bubble sizes in the forth zone, and thus, more bubble surface area to carry more floatable particle to the product launder.
(a)
loo
80 o
60
"4
40
E ~o
20
Coal Berg Seam = Jameson
"~.
0
0
-~9 x
0
~
A Packed-Column
Microeel
L
I
I
i
I
2
4
6
8
10
12
Feed Rate (gm/min-cn~)
b) 2.0
~o
1.6 1.2
o
0.8 6 o 8
0.4
• • 0.0
i
0
I
2
i
Microeel Packed-Column I
4
i
" ~.,
I
I
6
8
I
10
12
Feed Rate (gm/min-cn~') Fig.4
A comparison of throughput capacity achieved with each of the competing advanced flotation technologies.
Comparative evaluation of advanced flotation technologies
11
As shown in Figure 4(b), the Jameson Cell technology achieved the maximum product throughput capacity at a feed solid rate of about 5 g/min-cm 2. However, as shown in Figure 4(a), such a feed rate will produce a combustible recovery value of nearly 55%. In other words, if the Jameson Cell is operated to achieve its maximum throughput capacity, about 45% of the valuable combustible material will be rejected to the tailings stream, which may necessitate a scavenger stage treatment to recover most of the valuable combustibles. The low combustible recovery value may be resulting because of an increased by-pass of coal particles in the downcomer with an increase in feed solid rate due to the limited retention time, which is in the order of only a few seconds. On the other hand, the MicrocelTM technology, whose maximum throughput capacity was nearly 16% less than that of the Jameson Cell technology, achieved its maximum capacity at a feed solid rate of 3 g/min-cm 2. As shown, the Microcel TM technology will produce a reasonably high combustible recovery value close to 80%, indicating a minimal loss of combustibles to the reject stream, which may not require a scavenger treatment stage. Thus, MicrocelTM technology may be a more desirable flotation process in such scenarios.
100
,
O,
,Jr
4/i- •
| I I
tD I
,).(
I
10
I I I
•
M i c r o c e i TM ( B r a k e et al, 1996)
o
J a m e s o n (Atkinson, 1994)
I I I I I
i,
1
0
I
500
~
I
1000
~
I
1500
2000
Bubble Diameter (micron) Fig.5
Bubble size distributions measured in the collection zones of the Jameson Cell and Microcel TM column.
As explained in the previous paragraphs, the decrease in the concentrate solid rate at higher feed solid rate is caused due to a decrease in the average particle size in the concentrate. As shown in Figure 4(b), this decrease is more significant for the Microcel TM and Packed-Column than the Jameson Cell technology. The visual observation, while conducting the flotation experiments at very high solid contents, suggest that the bubble rise velocity in the froth zone is severely restricted with an increased bubble loading. The slowly rising bubbles in the froth zone allow significant drainage, which facilitates more bubble coalescence. As a result, the bubble surface area available in the froth zone is further reduced. The concentrate throughput at high feed solid rates from the Jameson Cell is the least affected among the three technologies. However, with other columns, especially with Packed-Column while operating above 20% feed solid content, the bubble rise velocity is so severely restricted that almost nothing reports to the product launder.
12
M.K. Mohantyand R. Q. Honaker CONCLUSIONS
The following conclusions on the flotation columns are based on the experimental data generated using laboratory-scale flotation columns and, thus, problems associated with large-scale units are not considered. .
One of the main conclusions is that the method of comparing performances based on the recovery versus rejection plot may result in erroneous conclusions. The recovery versus rejection curve normalizes the feed assay which may vary somewhat from test to test. However, the selectivity comparison using the recovery-rejection relationship may be misleading as illastrated by the results in this publication.
.
It can be stated at a significance level of 0.05 that the metallurgical performances obtained from the three leading advanced flotation technologies, such as the Jameson Cell, Microcel TM and Packed-Column, are statistically different from each other. For the Coalberg seam coal used in this investigation, the Packed-Column produced the best separation performance on the basis of both ash and total sulfur assays, followed by MicrocelTM and Jameson Cell technologies. A highly efficient selective reflux mechanism resulting from the extremely deep froth zone and a near plug flow mixing environment in the Packed-Column are the two apparent reasons behind the maximum selectivity obtained from this technology.
.
The Jameson Cell was found to achieve the maximum product throughput capacity followed by MicrocelTM and Packed-Column technologies. The respective throughput capacities were 1.76, 1.47 and 1.23 g/min-cm 2 for treating the Coalberg seam sample with a ds0 particle size of nearly 70 ~m. Past research indicates that among the three technologies, the Jameson Cell produces the finest bubble sizes in the collection zone. The smaller bubble size in the collection zone results in relatively smaller bubbles in the froth zone since the Jameson Cell is typically operated with a shallow froth zone, which causes less froth drainage and bubble coalescence. Thus, more bubble surface area is available in Jameson Cell to carry the coal particles to the concentrate launder in comparison to the MicrocelTM. On the other hand, the Packed-Column technology operating without a bubble generation system utilizes relatively larger size bubbles in the collection zone. The bubble size at the concentrate lip grows further due to froth drainage and coalescence caused by an extremely deep froth zone in spite of a high superficial gas velocity. This phenomenon results in the minimum throughput capacity for the Packed-Column technology.
.
At higher feed solid rates, a significant by-pass was noticed with the Jameson Cell technology especially while operating with high superficial gas velocity, resulting in a poor combustible recovery value. This may be occurring due to an insufficient particle retention time in the downcomer, which reduces the component recovery values and thus the separation efficiency. The Microcel TM technology, on the other hand was able to achieve high combustible recovery values even at high feed solid rate, which may render it more desirable in a rougher type application.
ACKNOWLEDGMENT The authors appreciate the guidance provided by Dr. Hasan Sevim, a professor in the Mining Engineering Department for the statistical analysis used in this publication.
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