The influence of reagent type on the kinetics of ultrafine coal flotation

The influence of reagent type on the kinetics of ultrafine coal flotation

Powder Technology, 59 (1989) 153 - 162 153 The Influence of Reagent Type on the Kinetics of Ultrafiue Coal Flotation R. B. READ*, L. R. CAMP, M. S...

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Powder

Technology,

59 (1989) 153 - 162

153

The Influence of Reagent Type on the Kinetics of Ultrafiue Coal Flotation R. B. READ*, L. R. CAMP, M. S. SUMMERS and D. M. RAPP Illinois State Geological

Survey,

Champaign,

IL 61820

(U.S.A.)

(Received September 6, 1988; in revised form June 21, 1989)

SUMMARY

A kinetic study has been conducted to determine the influence of reagent type on flotation rates of ultrafine coal. Two ultrafine coal samples, the Illinois No. 5 (Springfield) and Pittsburgh No. 8, have been evaluated with various reagent types in order to derive the rate constants for coal (k,), ash (k,), and pyrite (k,). The reagents used in the study include anionic surfactants, anionic surfactant-alcohol mixtures, and frothing alcohols. In general, the surfactant-alcohol mixtures tend to float ultrafine coal at a rate three to four times faster than either pure alcohols or pure anionic surfactants. Pine oil, a mixture of terpene alcohols and hydrocarbons, was an exception to this finding; it exhibited higher rate constants than the pure aliphatic alcohols or other pure anionic surfactants studied; this may be explained by the fact that the sample of pine oil used (70% alpha-terpineol) acted as a frother/collector system similar to alcohol/kerosene. The separation efficiencies of ash and pyrite from coal, as evidenced by the ratios of k, /k, or k,/k,, tend to indicate, however, that commercially available surfactant-alcohol mixtures are not as selective as pure alcohols such as 2-ethyl-I-hexanol or methylisobutylcarbinol. Some distinct differences in various rate constants, or their ratios, were noted between the two coals studied, and are possibly attributable to surface chemistry effects. INTRODUCTION

Developments of new, advanced physical coal cleaning technologies are on the horizon. *Current address, Rockwell ville, AL 35806 (U.S.A.). 0032-4910/89/$3.50

International, Hunts-

Many of the processes require extensive comminution to unlock enclosed pyrite and ash from coal and depend upon separation of ultrafine product from mineral matter via wet processes such as flotation. Yet a significant problem exists in floating ultrafine coal, namely loss in recovery as a result of decreased rate of flotation [ 1, 21. Increasing the rate of flotation with the use of microbubbles has been reported for ultrafine size coal [ 11. The development of new cells which rely upon hydrodynamic factors are also appearing [3, 41. These concepts address the differences in particle and bubble diameter dependencies [ 51; however, for ultrafine size coal (-400 mesh or less), surface charges and chemical interactions with reagents become important. Emerging research addresses chemical approaches to the control of flotation for fine [6 - 91 and ultrafine [9 - 121 coal. Klimpel and Hansen [9] have suggested a number of modifications to propylene oxide adducts that improve selectivity and recovery for flotation of fine (-170 mesh) coal. They also found that methylisobutylcarbinol was more effective for slightly finer sized particles. Read et al. [ 11,121 have shown that anionic surfactant-alcohol mixtures may also provide an advantage for flotation of ultrafine coal; however, a relationship between frother chemical type (non-ionic, anionic) and kinetic properties exhibited in ultrafine coal flotation has not been shown. The objective of this study was to establish the influence of reagent type on the flotation rate of ultrafine coal, and to evaluate the relative degrees of selectivity of such reagents by means of a kinetic approach. Klimpel [13 - 151 has emphasized the interactive nature between chemistry, equipment and operating conditions; Meyer and Klimpel [ 161 have concluded that the chemistry com@ Elsevier Sequoia/Printed in The Netherlands

154

ponent is of much greater significance than often realized. This can be of particular importance when considering the flotation of ultrafine particles where adsorption of reagent and bubble/particle attachment phenomena are critical [17,18]. The importance of frother structure/particle size dependency has been previously established [ 191, but little work has been reported for multiple comparisons in ultrafine coal with different chemical types under essentially constant flotation operating conditions. It is not the intention of this paper to dismiss the other important factors of equipment and operating parameters; nor would it be proper to propose mechanisms of surface interactions without further fundamental study. A complete scientific explanation of the frother behavior discussed in this paper is not possible and as Hanson and Klimpel [19] noted “the scientific fundamentals involved in froth development and stability in the presence of particles are extremely complicated and not yet sufficiently developed so that an a priori prediction of a particular chemical’s behavior as a frother in any given system can be made”. Thus the emphasis in this study was to evaluate the significance of reagent type, based on time-recovery profiles, and to demonstrate the importance of frother selection in flotation of ultrafine coal. In this study, an Illinois No. 5 (Springfield) and a Pittsburgh No. 8 coal were examined due to their widespread mining in the Midwestern and Eastern coal fields and use for coal-fired electrical generating plants. Commercial reagents used included three anionic surfactants, three non-ionic alcohols and two anionic surfactant-alcohol blends. Two additional experimentally developed (non-commercial) anionic surfactant-alcohol blends were also chosen for examination in the Pittsburgh No. 8 coal. To study the influence of multiple reagent types on time-recovery profiles requires use of equivalent dosages [ 191, otherwise measurable changes in the derived rate constants may occur. Thus in this study, to accurately compare and derive such rate data, a constant dosage was chosen which produced acceptable Btu recovery (generally >80% and <95%) while avoiding overdose conditions. This criterion may have resulted in one or two cases, however, where slight underdose conditions occurred.

THEORY

Meyer and Klimpel [ 161 and Klimpel et al. [20] have reported that flotation of fine coal generally follows a first-order rate equation (eqn. (1)). dC = klC (1)

-dt

where C = concentration of floatable carbonaceous matter; t = time, in seconds; k1 = rate constant, in s-l. A plot of the logarithm of weight per cent coal remaining in the flotation cell uersus time (linear) yields a straight line with slope equal to the negative value of the rate constant for coal, k,. This first-order rate equation has been found to be in good agreement with results presented by Bushel1 for quartz [21] and other investigators for different ores [ 221. Aplan et al. [23] have noted that the rate constants for pyrite and ash can also be obtained in a similar manner, with such rate constants being an indicator of separation efficiencies for a given frother/coal system. While this analogy does appear to hold true for typical particle sizes encountered in conventional coal flotation, the advent of ultrafine flotation processes makes applicability of this approach to ultrafine sizes of interest. These rates, which may control the overall efficiency of separation of coal from ash and pyrite, could possibly be used as predictors in novel coal cleaning processes. A kinetic evaluation could be applied not only to assessing selectivity as related to new hydrodynamic factors, but also to those processes which utilize a chemical approach to control flotation of ultrafine coal. In this study, the firstorder rate equation was assumed for evaluation of the rate constants.

EXPERIMENTAL

Coal samples used in this study were from the Illinois No. 5 (hvb-c) and Pittsburgh No. 8 (hva) seams. Typical analyses are given in Table 1. The 2” X 0 preparation plant products, or laboratory-derived clean coal samples, were air-dried and roll-crushed to produce -6-mesh material, with riffled splits of the -6-mesh feed serving as samples for further size reduction by wet stirred-ball

155 TABLE 1 Typical analysis of coals used in the studya

Illinois No. 5 Pittsburgh No. 8

Ash (%I

Volatile matter (%I

Pyritic sulphur (%I

Total sulfur (%I

Btu/lb

12.1 6.7

35.1 35.8

1.14 0.77

2.87 1.52

12 860 14 000

aAll results on a dry basis.

milling. Unused portions of the -_-mesh sample were stored under a nitrogen atmosphere to prevent further oxidation. For the flotation tests, feed sizes of 80% -400 mesh were employed. Internal studies have shown the use of more dilute slurries of 3.5% to 4.5% solids to afford better separation characteristics for ultrafine coal than those typically used in conventional flotation studies, i.e., 8% to 10% solids. Thus, a pulp density ranging from 3.5 wt.% to 4.5 wt.% solids was prepared and a measured volume of slurry was placed in a 4-&e, Denver D-2 sub-aeration cell, and the impeller speed set to 1075 rev./mm For thorough mixing, the slurry was agitated for 5 min. After this time, a fixed quantity of reagent was added and mixed for 2 min. At the end of the conditioning cycle, air was introduced into the cell at a rate of 8 standard cubic feet per hour. A coal-laden froth was produced and collected by scraping it by hand from the top of the cell into collection receivers. A constant water level was maintained within the cell during this procedure. (While some researchers have questioned the manual collection method and automated this procedure [15,19], others [23,24] have reported the relative reproducibility of the manual collection method, especially if one operator performs such tests. In tests reported here, use of the same operator, collecting method and instrument conditions were rigorously maintained to avoid bias.) Collection of the concentrate continued for a total of 8 min with fractions being collected at selected time-intervals. The solids and liquids from the froth concentrates and the tailings were filtered, dried, weighed, and analyzed for per cent moisture, ash, total sulfur, forms of sulfur (pyritic, sulfatic and organic (by difference)), and heating value (Btu/lb). Yield, in weight per cent, Btu

recovery, and ash and pyrite rejection values were then calculated. Commercially available surfactants used in the study included three anionic types: an ammonium salt of an ethoxylated sulfonate (ES), magnesium lauryl sulfate (MLS) and sodium lauryl sulfate (SLS); three frothing alcohols: methylisobutylcarbinol (MIBC), 2ethyl-l-hexanol (2EH), and pine oil (PO); two proprietary anionic surfactant-alcohol blends: Gafoam-AD (GAD) and Airfoam (AF). Studies on the Pittsburgh No. 8 coal also included use of two cisdialkylethenesulfonate salts which were mixed with 2ethyl-1-hexanol in a ratio of 1:2.2, respectively. These two experimental blends had been used in previous flotation studies [ll, 121 for ultrafine Illinois coals and it was anticipated that positive results could be found for an Eastern coal as well. The two non-commercial anionic surfactants, one being a potassium &Gdodecene-6-sulfonate, and the other a potassium _E-Li-octene-4-sulfonate, were prepared, characterized, and utilized as described elsewhere [ll, 121. In this paper, these mixtures with 2ethyl-lhexanol have been designated as AS1 and AS2, respectively. RESULTS

AND DISCUSSION

In evaluating the rate constants, Btu recovery was used as an indication of carbonaceous coal recovered. The reader is reminded, however, that results reported are on a dry basis; the heating content does include pyrite or other minerals which may contribute slightly to the Btu values. Use of ash and pyrite rejection values provided a precise indication of the amount of these components still remaining in the cell. The rate constants for flotation of coal (k,), ash (k,), and pyrite (k,), therefore, reduce

156 TABLE 2 Recovery of rougher concentrates as a function of time for various reagents with the resulting ash, pyrite rejection and Btu recovery (Illinois No. 5 coal) ES

Time (s)

MLS

PO

MIBC

2EH

SLS

AF

GAD

0 - 30

ARa 96.8 PRb 95.5 BtuRC 10.1

92.8 90.5 17.0

87.9 82.0 31.0

97.4 96.2 9.0

91.9 89.8 25.0

90.0 88.4 21.8

91.0 87.7 21.2

91.4 86.5 22.2

30 -60

AR PR BtuR

93.2 90.4 21.7

85.2 80.4 34.0

78.5 69.3 54.2

95.1 93.3 17.5

88.4 84.9 36.2

81.2 76.7 40.9

81.7 74.9 42.7

82.6 75.6 43.5

60 - 120

AR PR BtuR

88.9 84.2 36.5

80.7 74.2 44.0

65.8 52.6 79.4

91.3 88.5 30.5

81.9 74.9 56.3

71.7 64.1 60.8

68.8 57.4 69.8

68.9 58.7 72.3

120 - 180

AR PR BtuR

84.9 78.2 48.7

72.0 62.2 64.0

59.4 43.7 88.7

88.2 84.1 40.9

77.0 67.7 69.3

66.9 54.1 71.2

61.3 47.8 83.4

60.5 47.2 86.2

180 - 240

AR PR BtuR

79.6 70.2 58.2

70.0 59.3 69.0

55.4 38.8 92.8

84.3 78.4 52.1

73.1 62.0 78.1

64.0 50.7 76.8

56.3 41.0 90.6

56.0 40.7 91.8

240 - 480

AR PR BtuR

71.7 59.1 76.7

65.7 52.5 79.0

51.7 35.8 94.1

68.9 60.3 83.4

58.7 43.9 96.2

58.7 44.7 85.1

47.7 30.4 95.5

49.1 31.9 97.0

aAR = %Ash Rejection bPR = %Pyrite Rejection ‘BtuR = %Btu recovery Dosages of each reagent are constant at 1.6 lb/ton dry solids.

to eqns. (2), (3), and (4). Computer programs were utilized to mathematically derive these constants. In all cases, the linear portion of the curve (the first four data points) was used to calculate the slope. By using this segment of the data, where the rate constant of the recovery dominates the system at initial flotation times, relatively accurate values of k,, k,, and k, should be attainable. To have included all data points, especially those at longer flotation times, would have possibly invalidated the first-order assumption. (See Klimpel [14, 161 for a thorough discussion of R, K tradeoff.) k,(s-‘) _\ , = 2.303

log,,( 100% - Btu recovery at ti) t2-

-

log,,( 100% - Btu recovery at t2) t2-

k,(s- ‘) = 2.303

t1

(2)

log&Ash Rejection at ti) t2 -

-

fl

t1

log,,,(Ash Rejection at t2) t2--tl

(3)

k&l)

= 2.303

log,,(Pyrite Rejection at tl) t2-t1

-

logi,,(Pyrite Rejection at t2) t2 -

t1

(4)

Illinois No. 5 Coal

The calculated Btu recoveries, and ash and pyrite rejections for the Illinois No. 5 coal, as a function of reagent type, are given in Table 2. Based on yield (wt.% recovery) alone, as shown in Fig. 1, and for the first 4 min of data, pine oil (PO) showed the most rapid rate of recovery. The surfactant-alcohol blends (GAD, AF) showed slightly slower rates. The order of recovery for the other reagents tested were B-ethylhexanol (2EH), followed by the pure anionic surfactants SLS, MLS, ES, and MIBC. Further comparison of the data at equivalent Btu recovery, although at differing time-intervals, suggests the resulting ash and pyrite rejections are approximately equal. The values of the rate constants k,, k, and k, are obtained from Figs. 2, 3, and 4, respectively. From Fig. 2 it is apparent that

157

pine oil (PO), and the surfactant-alcohol blends tend to float ultrafine coal nearly twice as fast as 2EH, SLS, or MLS; the pure anionic ethoxylated sulfonate surfactant (ES) floats coal nearly three times slower than the ‘O-

,o

I 30

60

120

160

Time

240

300

360



(sets)

!

180

Fig. 1. Yield (wt.%) of coal as a function of time (s) for various reagents (Illinois No. 5 Coal).

0

2EH

W

MISC

ES

GAD

AF

SLS

MLS

blends and MIBC floats this coal only one-third as fast. A similar ordering is found for the rate constants k, (Fig. 3) and k, (Fig. 4). Here the indications are that MIBC, ES, and 2EH are slowest in floating ash and pyrite from this ultrafine coal. Rates for the surfactantalcohol blends, or pine oil, while relatively fast for floating ultrafine coal, appear to float ash and pyrite at rates about two to four times faster than those of the pure alcohols. To ascertain an efficiency index for the different types of reagents, the ratios of kc/k, and k,/kp were calculated and are shown in Figs. 5 and 6. The data suggest that 2EH and PO are most effective with respect to separating coal from ash (kc/k,), followed by the MIBC and the ES surfactant. The surfactant-alcohol blends are intermediate with the pure anionic surfactants SLS and MLS being least effective. With respect to k,/k,, the pure alcohol 2EH is most efficient; MIBC and GAD perform equally well, followed by PO and AF. The pure anionic surfactants MLS and SLS tend to perform poorly with respect to separation of pyrite from coal. This may, however, be the result of preferential adsorption of the anionic surfactants onto pyrite and mineral surraces. These findings suggest optimizing and matching appropriate anionic surfactants, such as ES, (slower rates of ash and pyrite flotation) with alcohols (high rates of carbonaceous selectivity and

AS2

AS1

Reagent Fig. 2. Rate constants (k,)

for flotation of ultrafine coal as a function of reagent type.

158

6

0

2EH

PO

MIBC

ES

GAD AF Reagent

SLS

MLS

AS2

AS1

Fig. 3. Rate constants (k,) for flotation of ash as a function of reagent type.

0

2EH

PO

MIBC

ES

GAD

AF

Reagent

SLS

MLB

AS2

i

Fig. 4. Rate constants (kp) for flotation of pyrite as a function of reagent type.

faster flotation) in order to design more effective reagents for flotation of both fine and ultrafine Illinois Basin coals. Pittsburgh

No. 8 Coal

The calculated Btu recoveries, and ash and pyrite rejections for the Pittsburgh No. 8 coal as a function of reagent type are given in Table 3. Reduction of the data into terms of

the rate constants k,, k,, and k, are presented in Figs. 2, 3, and 4, respectively. With the exception of the AS1 mixture, which floats coal at a much higher rate than all other reagents examined, a similar ordering as that observed in the Illinois coal is found in k, for this coal (see Fig. 2); surfactantalcohol blends tend to float coal at greater rates than either the aliphatic alcohols or

159

qIllinoisNo. 5 •j

0 2EH

PO

MIBC

ES

GAD

AF

PittsburghNo. 0

SLS

MLS

AS2

AS1

Reagent

Fig. 5. Ratios of kc/k,

as a function of reagent type.

a

IllinoisNo. 5

qPittsburghNo. 0



2EH

Reagent

Fig. 6. Ratios of kc/k,., as a function of reagent type.

pure surfactants SLS, or MLS. An exception to this finding is again exhibited for the case of pine oil (PO). For this coal, the pure ethoxylated sulfonate (ES) floats coal at a rate equal to 2EH. Surfactant-alcohol blends show a greater affinity for floating ash at a higher rate (Fig. 3). The exception found here is for the AS2 mixture which is nearly as selective as

pure 2EH, and also better than PO. The pure anionic surfactant MLS is essentially equivalent to MIBC, but SLS tends to float ash at a rate measurably lower than either of these two. The rate constants for flotation of pyrite (kp), as shown in Fig. 4, indicate a trend similar to that found for (k,). The values of kc/k, for pine oil or the pure alcohols MIBC and 2EH, the pure

160 TABLE 3 Recovery of rougher concentrates as a function of time for various reagents with the resulting ash pyrite rejection and Btu recovery (Pittsburgh No. 8 coal). Time (s)

Reagent

ES

MLS

PO

MIBC

2EH

AS2

AS1

SLS

AF

GAD

0 - 30

ARa PRb BtuRC

91.5 94.1 16.9

94.8 95.5 10.9

91.5 92.5 20.3

96.6 97.0 9.5

92.7 93.4 17.4

81.2 80.1 32.7

88.9 89.6 25.9

98.1 98.6 5.1

87.2 85.1 22.4

86.9 86.5 22.6

30 - 60

AR PR BtuR

85.4 89.1 30.3

90.6 91.9 20.0

83.6 85.8 38.6

91.1 93.0 23.5

87.0 88.3 31.3

62.6 60.8 60.2

80.7 82.0 43.5

96.2 97.1 10.2

74.4 72.6 43.0

72.3 80.3 47.1

60-120

AR PR BtuR

74.3 78.1 53.1

83.8 86.7 34.1

69.2 72.9 67.9

85.3 89.4 39.0

77.6 80.0 53.8

42.7 34.7 85.7

69.4 71.4 64.7

93.6 95.1 16.2

58.5 56.3 67.8

53.7 51.6 74.2

120 -240

AR PR BtuR

60.7 65.0 77.8

75.9 80.4 50.5

54.6 59.0 90.6

76.8 83.1 60.6

63.6 68.9 79.6

31.9 23.7 96.5

54.8 56.4 86.0

89.9 92.6 26.0

48.4 45.3 81.9

39.9 36.4 90.7

240-

AR PR BtuR

53.2 58.8 87.8

69.2 74.6 62.0

49.3 53.6 95.6

67.4 76.0 77.7

55.5 61.9 90.0

29.0 20.7 98.2

47.4 48.9 92.9

86.3 89.2 34.5

38.0 33.6 93.2

35.0 30.5 94.9

360

aAR = % Ash Rejection bPR = % Pyrite Rejection ‘BtuR = % Btu Recovery Dosages of each reagent are constant at 1.2 lbs/ton dry solids.

surfactant ES, or the AS2 surfactant-alcohol blend, are essentially equivelent. The remaining pure surfactant or surfactant-alcohol blends tend to show less selectivity as evidenced by lower ratios. For h,/k,, as shown in Fig. 6, MIBC undoubtedly shows a much higher degree of selectivity of coal over pyrite when compared with the other alcohols, pure surfactants or surfactant-alcohol blends. However, the pure ES, SLS, or the AS1 blend shows measurable slectivities over the commercial blends, AF or GAD, or the AS2 mixture. Differences

in rates due to rank effects

When one compares the actual values of k, for the two different ranks of coal (Illinois No. 5, hvb-c; Pittsburgh No. 8, hva), some slight, measurable differences exist for the surfactant-alcohol blends, PO, or 2EH. SLS, however, had much higher rate constants in the Illinois coal than in the Pittsburgh samples (6.5 us. 1.5) as did MLS (5.2 us. 2.9). MIBC had higher rate constants for the Pittsburgh coal (4.3 us. 2.8). This difference suggests a possible preference for certain surfactants in flotation of ultrafine lower rank bituminous coals. For the Illinois No. 5 coal, MIBC was the slowest floating reagent,

but for the eastern coal, the pure anionic surfactants SLS, and MLS, tend to be slowest. In these cases, the relative hydrophobicity of the two different samples, i.e., the chemical composition of the two coal surfaces, may play a role in how two different reagents (one non-ionic, the other two anionic) adsorb or ‘chemically’ interact with such surfaces. If bubble size were the single rate controlling factor [ 171, one might have expected to see no difference in the relative results for the two different samples. Particularly, MIBC, which produces the coarsest bubbles, should have had the lowest relative rate constants for both coals. The ordering of the rate constants for flotation of ash for the Pittsburgh coal also appears to be similar to those found for the Illinois No. 5 coal, but the surfactant-alcohol blends have constants which are roughly twice those found for the Illinois coal. This is particularly noticeable for the AF or GAD blends, 2EH, MIBC or the ES surfactant; MLS floats ash in both coals at approximately the same rate, but SLS floats ash in the Illinois coal at a rate three times faster. This latter effect is not readily explicable. Comparison of the rate constants for flotation of pyrite for the two coals indicates

161 TABLE 4 Effect of particle size on flotation rate constants Coal

Particle size

k,’

ka

kP

Illinois No. 5

400 mesh 200 mesh

6.0 19.0

1.1 6.5

Pittsburgh No. 8

400 mesh 200 mesh

6.5 22.8

1.2 2.4

ak

a

k&s

kc&

1.8 9.1

5.4 3.0

3.3 2.1

1.7 5.6

3.3 3.1

3.8 4.1

x lo3 s-l

GAD and PO float pyrite approximately one and one-half times faster in the Illinois coal; AF floats pyrite at near equivalent rates in both coals. The ES, MIBC and 2EH have constants which are approximately equal for either coal as well, but the SLS floats pyrite in the Pittsburgh coal at a rate eight times slower than that found for the Illinois coal. When comparing the ratios kc/k, or kc/k, for the two coals, again as shown in Figs. 5 and 6, these data suggest that 2EH is possibly a preferred alcohol for lower rank coals. This is evidenced by the higher ratios which predict faster flotation of coal with lower ash and pyrite contents in the concentrate. For higher-rank eastern coals, MIBC may be best, especially with respect to pyrite rejection. This is particularly noticeable when comparing the kc/k, ratios for the two different coals; again, indications are that 2EH may be a better choice for lower-rank Illinois Basin coals. Also for the Pittsburgh No. 8 coal, the surfactant-alcohol blends tend to be much poorer with respect to kc/k, or kc/k, when compared with those results for the ratios obtained in the Illinois coal. This also implies that surfactant-alcohol blends, while providing advantages for flotation of ultrafine mid-Western coal, may not be extremely useful for flotation of ultrafine eastern coals, unless mixtures which include SLS or ES with an alcohol, such as MIBC, are used.

EFFECT RATES

OF PARTICLE

SIZE ON FLOTATION

It is known that flotation rate increases with increasing particle size [25]. The data in Table 4 also support this fact when considering the differences found in k,. For both the Illinois No. 5 and Pittsburgh No. 8 coals at particle sizes of 80% passing 200 and

400 mesh, the rate constants are roughly three to three and one-half times greater for the larger particles; however, when one considers differences in either k, or k,, the Illinois coal floats ash and pyrite at five to six times faster for the larger particles, whereas the Pittsburgh coal floats ash and pyrite at about three and one-half times faster. This relationship, however, is possibly due to the differences in the degree of dissemination of ash and pyrite for the two respective coals. Even at 80% passing 200 mesh, a relatively high proportion of mineral matter may not have been liberated in the Illinois coal and thus floated with the concentrate. Further evidence which supports this conclusion is obtained when the ratios of k, /k, or k, /k, for both samples are compared. For the Illinois coal, separation efficiencies are much greater for the 400-mesh samples, whereas for the Pittsburgh coal, only slight differences are noted in kc/k,, but the kc/k, is actually greater for the 200-mesh sample. This tends to suggest that for the Pittsburgh coal, no significant differences exist in results between fine (200 mesh) and ultrafine (400 mesh) feed. However, for the Illinois coal, a beneficial reduction in pyritic sulfur and ash is possible when utilizing ultrafine feed. CONCLUSIONS

Effects of reagent type on the flotation rate constants of coal, ash, and pyrite can be used to ascertain appropriate choices which will enhance flotation of ultrafine coal. Anionic surfactant-alcohol blends or pine oil tend to be most influential in increasing the rates of coal flotation (k,) for the lower-rank Illinois coal. For the higher-rank Pittsburgh coal, alcohols, or perhaps selected anionic surfactants such as sodium lauryl sulfate or ethoxylated sulfonates, if mixed with alcohols,

162

may be appropriate choices. When examining the efficiencies of separation, as indicated by the ratios of kc/k, or kc/k,, the pure alcohols tend to be best for either type of coal, but the surfactan-alcohol blends were better with the Illinois coal than with the Pittsburgh coal; in the latter case, the pure anionic surfactants were more effective than the commercial surfactant-alcohol reagents. The& findings do point out a practical application for existing fine coal flotation circuits. If increased throughput for such circuits is required, it may be possible to substitute ‘faster’ floating reagents which lead to shorter times of flotation (recovery). This could effectively reduce required retention time and increase the overall effective cell volume, eliminating the need for new cell capacity. A kinetic evaluation can be useful in determining the influence of reagent type on flotation of ultrafine coal, and such an evaluation could be useful in delineating advantages related to hydrodynamic factors.

10

11 12

13

14 ACKNOWLEDGEMENTS

Financial support by the Center for Research on Sulfur in Coal through grants from the Illinois Department of Energy and Natural Resources and its Coal Development Board and the Illinois Coal Industry is gratefully acknowledged.

15 16

17 18 19 20

REFERENCES R. H. Yoon, Microbubble Flotation of Fine Coal, Dept. of Mining and Minerals Engineering, Virginia Polytechnic Institute, Blacksburg, Va., 24061, DOE Grant Report DE-FG228OPC30234, 1984. R. E. Zimmerman, AZME Technical Publication, 2397 (1979) 18. D. C. Yang, U.S. Patent 4 592 834, June 3,1986. H. Huettenhain and M. V. Chari, Advanced Physical Fine Coal Cleaning Microbubble Flotation, paper presented at Coal Utilization and

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