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Selective detachment process in column flotation froth
Minerals Engineering 19 (2006) 687–695 This article is also available online at: www.elsevier.com/locate/mineng
Selective detachment process in column flotation froth Rick Q. Honaker *, Ahmet V. Ozsever, B.K. Parekh University of Kentucky, Department of Mining Engineering, 234-B Mining and Mineral Resources Building, Lexington, KY 40506-0107, USA Received 14 July 2005; accepted 6 September 2005 Available online 20 October 2005
Abstract The selectivity in flotation columns involving the separation of particles of varying degrees of floatability is based on differential flotation rates in the collection zone, reflux action between the froth and collection zones, and differential detachment rates in the froth zone. Using well-known theoretical models describing the separation process and experimental data, froth zone and overall flotation recovery values were quantified for particles in an anthracite coal that have a wide range of floatability potential. For highly floatable particles, froth recovery had a very minimal impact on overall recovery while the recovery of weakly floatable material was decreased substantially by reductions in froth recovery values. In addition, under carrying-capacity limiting conditions, selectivity was enhanced by the preferential detachment of the weakly floatable material. Based on this concept, highly floatable material was added directly into the froth zone when treating the anthracite coal. The enriched froth phase reduced the product ash content of the anthracite product by five absolute percentage points while maintaining a constant recovery value. 2005 Elsevier Ltd. All rights reserved. Keywords: Coal; Fine particle processing; Froth flotation; Column flotation; Froth kinetics
1. Introduction The froth flotation process is comprised of two separate and distinctly different zones, i.e., the collection zone and the froth zone. In the collection zone, the separation of the valuable minerals from the non-valuables is achieved based on the bubble–particle attachment process. Due to hydraulic entrainment, a portion of the non-valuable minerals is carried from the collection zone into the froth zone with the mineral–bubble aggregates. Selectivity of the process can be enhanced in the froth zone by providing drainage of the feed pulp and utilizing the selective detachment of the more weak hydrophobic particles as a result of bubble coalescence and the resulting bubble surface area reduction. The importance of the latter sub-process was the subject of recent investigations (Van Deventer et al., 2004; Honaker and Ozsever, 2003; Ata et al., 2002; Ralston et al., 1999; Falutsu, 1994; Hewitt et al., 1994; Yianatos
*
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0892-6875/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.09.019
et al., 1988). On the other hand, decades of research focused on the froth flotation process have resulted in a clear understanding of the processes and sub-processes involved in the selectivity achieved in the collection zone (Yoon and Mao, 1996; Mao and Yoon, 1997). The separation of the desired mineral from the other mineral components in the collection zone is based on differential flotation rates. The collection zone flotation rate of a mineral, k ci , can be quantified by the expression: 3 Vg k ci ¼ P cP aP d ð1Þ 2 Db in which Vg is the superficial gas velocity, Db the bubble diameter, Pc the probability of collision, Pa the probability of attachment and Pd the probability of detachment. As shown in Eq. (1), a change in gas velocity or bubble size affects the flotation rate of each mineral species equally and thus does not improve selectivity. Furthermore, bubble– particle collision is not a selective process and should be maintained at maximum efficiency to ensure a high recovery of the floatable mineral. The probability of detachment is a function of both particle size and density and thus may
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play a minor role in selectivity. However, the bubble–particle attachment process as quantified by Pa is the principle mechanism defining the ability to effectively separate minerals in the collection zone of a froth flotation process. After collision and attachment, the bubble–particle aggregate moves upward through the collection zone and into the froth zone. The transition into the froth zone is characterized by an increase in the air fraction from around 20–30% to 70–80% (Finch and Dobby, 1990). In the froth zone, the water filling the voids between the bubbles drain back into the collection zone and the water layer surrounding each bubble thins. Upon approach of the liquid–air interfaces, bubbles coalesce thereby causing a reduction in bubble surface area. If the reduced amount of bubble surface area available in the froth zone is insufficient to carry the solids reporting from the collection zone, particles detach and potentially move with the fluid into the collection zone. As such, froth zone recovery may have a significant influence on overall flotation recovery. To assess the effect on overall flotation recovery for a given mineral, linear analysis concepts developed by Meloy (1983) can be applied to the mass transport processes illustrated in Fig. 1. As proposed by Finch and Dobby (1990), an expression for the overall flotation recovery RO for a mineral component i can be derived as a function of the collection zone recovery RC and froth zone recovery RF (Eq. (2)), i.e., RO ¼
RC RF RC RF þ ð1 RC Þ
ð2Þ
One should note that, if RF equals 100%, RO is equivalent to RC. The collection zone recovery of particle type i can be determined using the axial dispersion model described by Levenspiel (1972), i.e., Rci ¼ 1
where sp is the particle retention time and Pe the axial dispersion coefficient. Perfectly mixed conditions, which resemble the conditions in a conventional flotation cell, are characterized by a Pe value of zero. The Pe value approaches infinity for plug flow conditions. An empirical expression for quantifying Pe was developed by Mankosa et al. (1992), which accounts for the length-to-diameter (L/D) ratio of the collection zone and the counter velocities of the gas (Vg) and liquid (Vl), i.e., 0:53 0:35 L Vt ð5Þ Pe ¼ D ð1 eÞV g in which e is the air fraction. By substituting Eqs. (3)–(5) into Eq. (2), the overall recovery can be quantified as a function of the collection zone recovery for particle i having a flotation rate ki and a corresponding froth zone recovery, RFi . Using a typical L/D ratio of 2 for an industrial column, overall recovery values reveal that froth zone recovery has a relatively small impact on overall recovery for particles with a high flotation rate or collection zone recovery as shown in Fig. 2. Highly floatable particles that are detached in the froth zone are efficiently recovered in the collection zone. However, more weakly detached hydrophobic particles have a lower collection zone recovery and thus are less likely to be recovered in the collection zone. As a result, reductions in froth zone recovery have a significant impact on the overall recovery of particles that have moderate-to-weak flotation characteristics. Therefore, the differential effect of froth zone recovery values between particles of varying floatability indicates an additional selectivity mechanism provided by the reflux action between the collection and froth zones.
4A expfPe=2g 2
2
ð1 þ AÞ expfðA=2ÞPeg ð1 AÞ expfðA=2ÞPeg ð3Þ
in which rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pe þ 4k ci sp A¼ Pe
1.00 0.90 0.80
ð4Þ
0.70 0.60
RCRF
Froth Zone
Concentrate
Overall 0.50 Recovery 0.40 0.30 1.00 0.93
0.20
RC (1-RF)
RC
0.10
0.43 0.30
1-RC
Tailings
Fig. 1. Interaction between zones in a column flotation cell.
Froth Recovery
0. 74
80
75
0.
0.07 0.
92
86
0.
Collection Recovery
0.
97
0.22 95
Collection Zone
0.
0.
Feed = 1
1. 00
0.00
Fig. 2. The effect of froth recovery on overall flotation recovery over a range of collection zone recovery values resulting from the collection zone flotation rates of particles with varying hydrophobicity; L/D = 2.0, Vg = 2.0 cm/s, e = 0.25.
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For systems that contain multiple mineral species or unliberated particles with varying degrees of hydrophobicity, the detachment process in the froth zone may also enhance the overall selectivity achieved by froth flotation. Particles with a low degree of surface hydrophobicity are likely to be weakly bound to the bubble surface and thus will be preferentially detached. Moys (1978) and later Yianatos et al. (1988) confirmed by experimental evidence that the detachment process is selective. From Yianatos et al. (1988), the detachment rate for chalcopyrite under a given set of conditions was 0.30 min1 whereas pyrite obtained a higher rate at 0.41 min1. As such, differential detachment rates can be exploited to improve the selectivity between hydrophobic species. Detailed investigations utilizing modified column apparatus designs have provided promising quantifiable evidence of the selective detachment and drop-back processes (Rubio, 1996; Ata et al., 2002). For example, Ata et al. (2002) used a unique cell design to show that highly hydrophobic particles added directly into the froth zone can selectively replace particles of lower hydrophobicity. Specifically, the addition of hydrophobic silica particles in the froth reduced the recovery of hematite particles that were added in the collection zone by 10 percentage points. Based on this finding, it is plausible that the addition of highly hydrophobic particles in the froth zone could assist in the selective rejection of the more weakly hydrophobic material (e.g., low grade particles) reporting from the collection zone, thereby enhancing the overall selectivity between particles of varying hydrophobicity. Results supporting this hypothesis are presented and discussed in this publication. As far as it could be ascertained, the use of a continuous flotation column to quantify the selective detachment and froth recovery is minimal to date. This paper details the work performed to assess froth recovery and the corresponding selective detachment process under various operating conditions for fine coal cleaning using column flotation. 2. Experimental 2.1. Sample preparation A portion of the investigation involved the addition of a more hydrophobic material into the froth zone of a flotation system that was treating a material with lower floatability characteristics. The objective was to quantify the potential separation performance improvements due to the exploitation of selective detachment. A requirement of the two materials was that the solid densities be relatively close but with a distinct difference so that a density separation of the flotation products and tailings could be achieved. Materials meeting this requirement are anthracite and medium-volatile bituminous coal. Anthracite coal is moderately hydrophobic and has a relative solid density greater than 1.5 whereas medium-volatile bituminous coal
689
is highly hydrophobic and has a mean relative density (RD) below 1.5. The anthracite coal sample was obtained from the fine circuit feed of an operating coal preparation plant in Pennsylvania (USA) while the bituminous coal sample was collected from the product stream of a spiral concentrator circuit of a preparation plant in West Virginia (USA). The bituminous coal sample was air dried at room temperature conditions and floated at 1.3 RD to recover the most hydrophobic fraction, which was used in the froth enrichment tests. The 1.3 RD float material was then washed with hot water to remove the heavy liquid (LMT) used in the density fractionation treatment and again washed with double distilled water to ensure that the surface of the coal was cleaned from contaminants. After air drying, the 1.3 RD float fraction of the bituminous coal was pulverized to a particle size below 212 lm. A particle size fraction of 212 · 75 lm was used in all the flotation tests since coal flotation is optimum and entrainment is minimal in this size range. The flotation of coal particles greater than 212 lm generally results in a substantial decrease in flotation rate and thus recovery due to an elevated probability of detachment (Brown and Smith, 1954; Aplan, 1976). Unlike conventional flotation cells, wash water is added in a relatively deep flotation froth to eliminate entrainment. In addition, the natural flow of the pulp is in the downward direction and provides a near plug-flow condition due to the large length-to-diameter ratio. As a result, particles having a size of 75 lm or greater, which possess a downward settling velocity, would not be hydraulically entrainable under the conditions described in this publication. Column flotation results from tests using oxidized anthracite coal confirmed this hypothesis. Both the anthracite and the 1.3 RD coal were screened using a 75 lm sieve in a laboratory scale Sweco unit. The ash contents of the 212 · 75 lm anthracite and 1.3 RD float bituminous coal were 50.72% and 3.56%, respectively. The density fractionation results for the 212 · 75 lm anthracite coal shown in Table 1 indicate that the majority of the material exists in the relative density range of 1.5– 1.7. The analysis revealed that no material floated at a relative density of 1.5.
Table 1 Density fractionation analysis of the 212 · 75 lm anthracite coal Relative density fractions
Fractional Weight (%)
1.5 · 1.6 1.6 · 1.7 1.7 · 1.9 1.9 · 2.2 2.2 Sink Total
Ash (%)
17.69 17.51 9.03 11.32 44.46
2.65 7.23 23.43 53.49 91.81
100.00
50.72
690
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2.2. Sample characterization 2.2.1. Induction time measurements Based on work reported by Arnold and Aplan (1989, 1990), the floatability of the various maceral groups that comprise coal varies significantly. In general, a decrease in floatability is in the following order: liptinites, vitrinite and inertinites. Since the solid density of the liptinite macerals is below 1.2, the vitrinites between 1.25 and 1.40 and inertinites between 1.40 and 1.60 for bituminous coals, floatability of coal improves with a decreasing particle density as shown by Olson and Aplan (1984). Therefore, the investigation concentrated on the froth zone behavior of five density fractions in the anthracite coal. Using an electronic induction timer (MCT-100), the time intervals required for particles in the five density fractions and the 1.3 RD bituminous coal to attach to a bubble after collision were measured (Yoon and Yordan, 1991; Ye et al., 1989). Measurements of induction time were accomplished by moving a captive bubble toward a bed of particles for successfully longer periods of time until the particles become attached to the bubble. The inductiontime measurement technique monitors attachment percentage with a given number of tests at a controlled contact time to obtain a distribution of percent attachment versus contact time. The induction time then is chosen to be the value of a particular contact time at which 50% of the tests resulted in bubble–particle attachment. This procedure allows the actual contact time between the bubble and particle bed to be measured. The results were used as an indicator of floatability. Induction times were measured with and without collector (i.e., fuel oil no. 2) at pH values of 3, 5 and 7. As shown in Table 2, induction time values for the anthracite density fractions decreased substantially when the pH was adjusted to values below 7, thereby indicating that optimum flotation performance would be achieved using medium pH values in the acidic range. Also, the addition of fuel oil significantly reduced the induction time, which is in agreement with the floatability findings of Olson
and Aplan (1984). An unexpected finding was the relatively good floatability of the 2.2 RD sink material, which was comprised of mostly hydrophilic mineral matter that results in an ash content of 91.81%. Microscopic petrographic analysis of the 2.2 RD sink material revealed the presence of ÔboneÕ particles, which are a complex mix of coal and mineral matter. As previously stated, the purpose of the 1.3 RD float bituminous coal was to evaluate the effect of enriching the froth phase with a more hydrophobic material with nearly the same density as the coal being added in the collection zone. The induction times in Table 2 confirms that the 1.3 RD float material is more floatable than the anthracite coal, which is likely due to a greater degree of hydrophobicity. 2.2.2. Determination of the collection zone kinetic rate To measure the collection zone flotation rate ðk ci Þ of the particle density fractions, tests were performed using only the collection zone section of a 5-cm diameter flotation column. The froth zone was separated from the main body of the column leaving only the collection zone section attached to the main frame of the flotation apparatus. The goal was to ensure that the operating characteristics of the collection zone used in determining the collection rate constant, kc, was identical to the environment in which the froth zone evaluation tests were performed. The pulp level was controlled by manipulation of a control valve on the tailings stream, which maintained the level at the overflow lip of the column. The duration of the test was equivalent to approximately three particle retention times in an effort to ensure a steady-state operating condition. Under this condition, samples of the feed, product (overflow) and tailings (underflow) streams were collected, dried and weighed. Density fractionation was performed to determine the amount of mass from each density fraction reporting to each process stream. Using the test data, the k ci value for each density fraction was determined using the plug-flow recovery model, i.e., Rci ¼ 1 exp½k ci sp
Table 2 Induction time values obtained for each density fraction in the anthracite coal and the 1.3 RD float material in bituminous coal over a range of medium pH conditions Relative density fractions
Collector (fuel oil) pH = 3 None
pH = 5 1 lbs/ton
None
pH = 7 1 lbs/ton
None
Induction time (milliseconds)—212 · 75 lm anthracite coal 1.6 Float 2.70 <2 2.70 <2 6.60 1.6 · 1.7 3.00 <2 3.90 <2 7.00 1.7 · 1.9 4.15 2.35 7.44 2.86 10.07 1.9 · 2.2 6.44 4.99 9.27 6.20 12.72 2.2 Sink 10.30 9.40 12.10 8.50 15.60
1 lbs/ton 1.85 1.95 3.54 6.93 7.80
Induction time (milliseconds)—212 · 75 lm bituminous coal 1.3 Float <2 <1 <2 <1 <2 <1
ð6Þ
in which Rci is the collection zone recovery of density fraction i measured from the experimental data and sp is the particle retention time. The use of the plug-flow model is justified by the relatively large length-to-diameter ratio of about 40:1, which has been shown by Finch and Dobby (1990) to provide near plug-flow conditions. Using test conditions that were identical to the froth evaluation tests, the recovery values for each density fraction were determined from collection zone tests at four different volumetric flow rates. Recovery values were calculated based on a mass balance of the weights and ash contents in the feed, product and tailing samples. Using recovery values and Eq. (6) yielded the best-fit estimate of the collection zone flotation rate shown in Table 3 for each density fraction.
R.Q. Honaker et al. / Minerals Engineering 19 (2006) 687–695
Table 3 Collection zone flotation rate and recovery values for each density fraction in the 212 · 75 lm anthracite coal sample
691
trol valve, which is connected to the tailings stream of the column. 2.4. Column flotation procedure
Relative density
kc
Collection zone recovery (RC) 912 cc/min
1089 cc/min
1272 cc/min
1451 cc/min
1.6 Float 1.6 · 1.7 1.7 · 1.9 1.9 · 2.2 2.2 Sink
1.138 0.941 0.789 0.508 0.242
0.989 0.975 0.955 0.865 0.614
0.977 0.955 0.926 0.813 0.550
0.960 0.930 0.893 0.763 0.496
0.940 0.903 0.859 0.716 0.451
2.3. Column flotation apparatus A flotation column was constructed that employed a unique design to separate the froth and pulp during continuous operation. The column was split at the froth–pulp interface by a valve as shown in Fig. 3, so that instantaneous separation of the froth and pulp phases could successfully be achieved. The flotation column also incorporated the ability to provide instantaneous stoppage of the process streams by using solenoid valves after ensuring steady-state operation of the column. The collection zone section (182 cm high) was constructed of cylindrical glass tubing having an inside diameter of 5 cm. The froth zone section (45 cm high) was derived from clear, cylindrical Plexiglas tubing with the same inside diameter. Feed slurry was injected into the column at a distance of 70 cm below the product overflow lip, using a variable speed pump. Pulp level was maintained by utilizing a con-
The evaluation of the selectivity mechanisms in the froth zone of flotation column was conducted over a range of volumetric feed rates and feed solid concentrations using the anthracite coal. As a result of the varying feed mass rates, flotation data were obtained under kinetic-limiting and carrying-capacity limiting conditions. All other parameters were maintained at constant values, which are provided in Table 4. A polyglycol reagent was used as a frother while fuel oil no. 2 was employed as the collector. For the tests investigating froth zone enrichment, 1.3 RD float bituminous coal with a 212 · 75 lm particle size was added in the wash water distributor at concentrations that are expressed as a percentage of the total anthracite feed added in the collection zone. The distribution ring was Table 4 Flotation column operating conditions Feed rate Feed solids concentration by weight External feed concentration by weight Frother concentration (Poly-630) Collector concentration (fuel oil no. 2) Froth depth Superficial gas velocity Volumetric wash water rate
912–1451 cc/min 10–15% 5–10–15% 30 ppm 0.45 kg/ton 45 cm 2 cm/s 400 cc/min
FINAL CONCENTRATE
Divider valve, separating the collection and froth zones
pH-meter
pH-meter
Flow-meters
Frother Tank
PUMP
PUMP
Feed (Anthracite coal) Slurry Tank
TAILINGS
Fig. 3. Schematic illustration of the flotation column apparatus.
External Feed (Bituminous coal) Slurry Tank
692
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located at a distance from the top of the column that was 1/ 3 the total froth depth. The flotation column tests were conducted for a period equivalent to three particle residence times to ensure steady-state operating conditions. Once this condition was achieved, timed samples were collected from the feed, product and tailing streams. Subsequently, the process streams were instantaneously stopped and the material in both the froth and collection zones isolated. The slurry collected from the two zones and the process stream samples were filtered, dried, weighed, and subjected to density fractionation. Ash analyses were also performed on all samples. For tests during which 1.3 RD bituminous coal was added into the froth zone, the samples collected from the process streams were subjected to density fractionation using at a 1.4 RD. Since the anthracite coal did not contain any 1.4 RD float material, the 1.4 RD sink was subjected to ash analyses to evaluate the separation performance achieved on the anthracite coal. 2.5. Froth zone characterization methodology The froth zone recovery and reflux values for each flotation test were quantified using a simulation that follows the flow diagram in Fig. 1 and the overall recovery expression described by Eq. (2). The main steps of the simulation included: 1. Calculate the RC values using the kc values and the residence time values, which are based on the volumetric column feed rate and the volumetric flow rate of material returning from the froth zone (Eqs. (3) and (4)). 2. Calculate the amount of mass M reporting to the product streams: M Feed i RCollection i ¼ M Froth zone i M Froth zone i RFroth i ¼ M Concentrate i M Feed i ð1 RCollection i Þ ¼ M Tailings i 3. Calculate the drop-back from the froth zone to the collection zone:
quired constraints. The correct Rf i values provided the mass flow rates for each density fraction, the total mass flow rate, and the overall ash contents in the product and tailings streams that were experimentally obtained from each column flotation test. 3. Results and discussion 3.1. Internal reflux Based on induction time measurements (Table 2) and several exploratory flotation experiments, the treatment of the 212 · 75 lm anthracite coal by column flotation was conducted using a medium pH value of around 3.0. The separation performances achieved by varying volumetric feed rate and feed solids concentration were comparable to those achieved by release analyses at medium pH values of 3 and 5 as shown in Fig. 4. However, both the release and column flotation performances were significantly inferior to the results obtained from density fractionation or washability analysis. At similar recovery values, a clean coal ash content of 6% was produced from density-based separations whereas froth flotation produced ash contents of around 16%. An investigation conducted to reveal the reason for the large ash content difference found the existence of unliberated particles having a very high ash content value that possess a relatively high flotation rate. As such, the elevated recovery rates of the high ash content material limited the ash reduction potential of the flotation process for the anthracite coal. The increase in the feed volumetric flow rate and solids concentration resulted in a rise in the feed mass flux rate, which initially provided a corresponding linear increase in the product mass flow rate. Under this condition, selectivity is based on differential flotation rates while froth zone recovery is nearly 100%. However, as shown in Fig. 5, the product mass flow rate reached a peak and began to decrease for feed mass flux rates greater than 5.65 tons/h/ m2. Above this mass flux value, the column operated under
M Froth zone i ð1 RFroth i Þ ¼ M Dropback i 100
4. Calculate the new mass entering the collection zone: M 0Collection zone i ¼ M Feed i þ M Dropback i
By using a Visual Basic Macro Algorithm (embedded in Microsoft Excel), steps 1–5 were continually repeated in a looping manner until steady-state conditions (mass in = mass out) were realized, which included the mass balance for each relative density fraction. Simultaneously, the SOLVER routine embedded in Microsoft Excel was used to determine the best fit value for the froth zone recovery corresponding to each density fraction, Rf i . The SOLVER application is set to satisfy specific conditions with ac-
Recovery (%)
5. Repeat steps 1–4 until steady-state conditions are reached.
80
60
40 pH = 3, Release Test pH = 5, Release Test Washability Internal Refluxing Tests
20
0
2
6
10
14
18
22
26
30
Product Ash (%) Fig. 4. Column flotation separation performances achieved on 212 · 75 lm anthracite coal as compared to those achieved by release and washability analyses.
R.Q. Honaker et al. / Minerals Engineering 19 (2006) 687–695 1.0
0.8 2.5
10% 10% 10% 10% 15% 15% 15%
2.0
1.5
1.0 2.0
3.0
4.0
5.0
Overall Recovery
Concentrate (tons/hr/m2)
3.0
solids, 912 cc/min solids, 1089 cc/min solids, 1272 cc/min solids, 1451 cc/min solids, 1089 cc/min solids, 1272 cc/min solids, 1451 cc/min
6.0
7.0
a carrying-capacity limited condition for which there is insufficient bubble surface area to carry all the particles reporting to the froth zone to the product launder. A decrease in the product mass flux is a result of the selective detachment of the coarsest particles. The flotation column operated under kinetic-limited conditions for each of the volumetric flow rates tests at a feed solids concentration of 10 wt.%. Evidence of this fact is provided in Fig. 6 which shows that the froth zone recovery for each of the density fractions (i.e., represented by their respective flotation rates) was nearly 100% with the exception being the particles having the lowest collection zone rate constant. Due to the heterogeneity of the highash particles, it is likely that the binding energy between the particle and bubble was sufficiently weak to be detached upon passage through the transition from the collection zone to the froth zone and during bubble coalescence despite the availability of bubble surface area. This type of selective detachment has been reported by Jameson (2002) for coarse particles, which led to improved
1.0 10% Solids, 912 cc/min 10% Solids, 1089 cc/min 10% Solids, 1272 cc/min 10% Solids, 1451 cc/min 15% Solids, 1089 cc/min 15% Solids, 1272 cc/min 15% Solids, 1451 cc/min
0.6
0.4
0.2
0.0 1.138
0.941
0.789
0.508
10% Solids, 912 cc/min 10% Solids, 1089 cc/min 10% Solids, 1272 cc/min 10% Solids, 1451 cc/min 15% Solids, 1089 cc/min 15% Solids, 1272 cc/min 15% Solids, 1451 cc/min
0.4
0.0 8.0
Fig. 5. Relationship between the feed mass flux and the product mass flux as the flotation process approaches carrying-capacity limited conditions.
0.8
0.6
0.2
Feed (tons/hr/m2)
Froth Recovery (%)
693
1.14
0.94
0.79
0.51
0.24
Collection Zone Flotation Rate (min-1) Fig. 7. Experimental froth zone recovery values as a function of the collection zone flotation rates corresponding to the various density fractions in the anthracite coal.
coarse particle flotation by the introduction of the coarse components of a given feed directly into the froth zone. As the column approaches and exceeds the maximum mass flux in the concentrate, froth zone recovery for all components in the feed decreases to values of around 50% or less. As indicated in Fig. 6 by the differences in the froth zone recovery values between the various components having different flotation rate values, the detachment process is selective. For example, the froth recovery difference between particles having a k ci value of 1.138 min1 and those with a value of 0.508 min1 was about 20% whereas the difference under kinetic-limited conditions was around 5%. However, increasing the feed mass flux above the carrying-capacity limit appears to deteriorate the selectivity of the detachment process. As shown by theoretical simulations in Fig. 2, froth zone recovery in a flotation column should have minimal impact on overall recovery for highly floatable material. Fig. 7 verifies this observation with experimental evidence. The overall recovery values were obtained from density fractionation of samples collected from the process streams of each column flotation test and mass balancing of each component in the feed. For the 10% feed solid concentration tests, the change in recovery for a feed component having a specific k ci value was strictly a function of decreasing residence time since froth zone recovery for most components was nearly 100% (Fig. 6). However, as the system approached carrying capacity limiting conditions at a feed solid content of 15%, particles having a k ci value of 0.51 min1 exhibited a significantly greater drop in overall recovery compared to the decrease observed for the other particle types. This observation supports the presence of selectivity in the detachment process, which appears to decrease after exceeding the carrying-capacity limit.
0.242
Collection Zone Flotation Rate (min-1) Fig. 6. The effect of an increase in the feed mass flux on the froth zone recovery realized for density fractions characterized by measured collection zone flotation rates.
3.2. Froth zone enrichment Based on the evidence of selective detachment provided in Figs. 6 and 7, it was hypothesized that the addition of
R.Q. Honaker et al. / Minerals Engineering 19 (2006) 687–695
Table 5 Bituminous coal distribution during the external froth enrichment tests conducted using a feed volumetric flow rate of 1451 cc/min and a feed solids concentration of 15 wt.% Process stream
Product Tailings Feed
Percent bituminous coal addition 5% of Anthracite feed
10% of Anthracite feed
15% of Anthracite feed
98.26 1.74
99.08 0.92
98.84 1.16
100.00
100.00
100.00
100
80
Recovery (%)
material into the froth phase that has a greater degree of surface hydrophobicity than the feed coal could enhance selectivity by replacing the lower quality particles during bubble surface area reduction events. In fact, this hypothesis was the basis of the research reported by Ata et al. (2002) in which hydrophobized silica was added to a flotation froth recovering hematite. However, the density difference between the bituminous and anthracite coals is significantly smaller than the system studied by Ata et al. (2002), which minimizes the density effect on detachment. For the tests in which the column was operated under rate limiting conditions (i.e., feed volumetric flow rates <1451 cc/min), 100% of the 1.3 RD bituminous coal reported to the column product stream. A slight loss of bituminous coal to the tailings stream occurred when the volumetric feed rate was increased to 1451 cc/min, which provided carrying capacity conditions as indicated in Fig. 5. Table 5 outlines the distribution path of the externally added bituminous coal in the flotation column. Less than 2% of the bituminous coal added in the froth phase was lost to the tailings stream while operating under carrying-capacity limiting conditions. This finding indicates that the bituminous coal particles are replacing the weaker hydrophobic anthracite particles on the surface of the bubbles. The addition of the 1.3 RD float fraction of a highly floatable bituminous coal into the froth phase provided significant improvements in the selectivity achieved from the treatment of the anthracite coal as shown in Fig. 8. The bituminous coal was removed from the product and tailing samples by performing a density fractionation at a 1.40 RD. As such, the data in Fig. 8 represents the improved performance on the anthracite coal only. At a given recovery value, the product ash content was reduced from 19% to nearly 14% by the addition of 1.30 bituminous coal at a concentration equal to 15% of the feed mass flow rate. As shown in Fig. 9, the impact on selectivity occurred as a result of a decrease in the overall recovery of particles having a collection zone rate smaller 0.941 min1. This finding correlates well with the induction time measurements in Table 2. The 1.30 RD float material only replaced particles that possessed a significantly lower degree of floatability. Similar to the findings of the carrying-capacity limited tests, the most significant reduction in recovery was realized by particles which had a collection zone rate
pH = 3, Release Test 60
pH = 5, Release Test Internal Refluxing Tests
40
15% solids with bituminous coal (5% of anthracite feed) 15% solids with bituminous coal (10% of anthracite feed) 15% solids with bituminous coal (15% of anthracite feed)
20
0 10
14
18
22
26
30
Product Ash (%) Fig. 8. Improvement in the separation performance achieved on the anthracite coal as a result of the addition of the 1.30 RD bituminous coal into the froth phase.
1.0
0.8
Overall Recovery
694
0.6
0.4
1089 cc/min
0.2
1089 cc/min with bituminous coal (15% of feed) 1272 cc/min 1272 cc/min with bituminous coal (15% of feed)
0.0 1.138
0.941
0.789
0.508
0.242
Collection Zone Flotation Rate (min-1) Fig. 9. Effect of froth zone enrichment on the overall recovery of each density component in the anthracite coal.
of 0.51 min1 and an induction time of 4.9 ms as compared to less than 1.0 ms for the bituminous coal. 4. Conclusions The bubble coalescence that is known to occur in the froth zone of flotation systems results in a reduction in the bubble surface area needed to carry particles to the product stream. Under high solid loading conditions, an insufficient amount of surface area exists at the top of the froth zone and, thus particles are detached from the bubble surfaces. The detachment process is selective in that particles having the weakest bond with the bubble surface are preferentially detached. An evaluation of the detachment process in the froth phase and its associated selectivity has been conducted to quantify froth zone recovery and the corresponding effect on the overall process selectivity. A well-known fundamental column flotation model and experimental data were used to assess the selectivity achieved under kinetic-limited
R.Q. Honaker et al. / Minerals Engineering 19 (2006) 687–695
and carrying-capacity limited conditions. A narrowly sized anthracite coal has been used due to its unique characteristics that allow timely and accurate assessments of the desired rate and recovery values. The particle sizes cover a range that are known to provide optimum coal flotation recovery and sufficiently coarse to eliminate recovery due to hydraulic entrainment. Test results revealed that increasing the feed solids content from 10 to 15 wt% provided a reduction in product ash content by about 14 absolute percentage points while maintaining relatively high recovery values. For tests involving 10% solids, the selectivity improvement was due to the exploitation of differential flotation rates. However, a feed solid concentration of 15% provide carrying-capacity limited conditions under which the selectivity improvement is due to selective detachment as a result of bubble surface are reduction and the reflux process that occurs between the collection and froth zones. Differential froth flotation recovery values as high as 20 absolute percentage points were obtained between highly and weakly floatable particles. Improved selectivity was obtained for the treatment of the anthracite coal by the addition of bituminous coal into the froth phase. As a result of the higher degree of floatability, the bituminous coal particles replaced a portion of the particles that were weakly to moderately hydrophobic. The result was a five absolute percentage point decrease in the product ash content at equal recovery values. References Aplan, F.F., 1976. Coal flotation. In: Fuerstenau, M.C. (Ed.), Flotation: A.M. Gaudin Memorial Volume. SME Inc., Littleton, pp. 1235–1264. Arnold, B.J., Aplan, F.F., 1989. The hydrophobicity of coal macerals. Fuel 68 (5), 651–658. Arnold, B.J., Aplan, F.F., 1990. Use of petrologic analysis and flotation kinetics to evaluate froth flotation products. Coal Science and Technology 16, 221–235. Ata, S., Ahmed, N., Jameson, G.J., 2002. Collection of hydrophobic particles in the froth phase. International Journal of Mineral Processing 64, 101–122.
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Brown, D.J., Smith, H.G., 1954. The flotation of coal as a rate process. Transactions of the Institute of Mining Engineers 113, 1001–1020. Falutsu, M., 1994. Column flotation froth characteristics-stability of the bubble–particle system. International Journal of Mineral Processing 40, 225–243. Finch, J.A., Dobby, G.S., 1990. Column Flotation. Pergamon Press, Oxford. Hewitt, D., Fornasiero, D., Ralston, J., 1994. Bubble–particle attachment efficiency. Minerals Engineering 7, 657–665. Honaker, R.Q., Ozsever, A.V., 2003. Evaluation of the selective detachment process in flotation froth. Minerals Engineering 16 (10), 975–982. Jameson, G.J., 2002. Personal communication. Levenspiel, O., 1972. Chemical Reaction Engineering. Wiley, New York. Mankosa, M.J., Luttrell, G.T., Adel, G.T., Yoon, R.H., 1992. A study of axial mixing in column flotation. International Journal of Mineral Processing 35, 51–64. Mao, L., Yoon, R.H., 1997. Predicting flotation rates using a rate equation derived from first principles. International Journal of Mineral Processing 51 (1–4), 171–181. Meloy, T.P., 1983. Analysis and optimization of mineral processing and coal-cleaning circuits—circuit analysis. International Journal of Mineral Processing 10 (1), 61–80. Moys, M.H., 1978. A study of a plug-flow model for flotation froth behavior. International Journal of Mineral Processing 5, 21–28. Olson, T.J., Aplan, F.F., 1984. The floatability of locked particles in a coal flotation system. In: Proc. 2nd International Congress Applied Mineralogy in Minerals Industry, pp. 367–393. Ralston, J., Fornasiero, D., Hayes, R., 1999. Bubble–particle attachment and detachment in flotation. International Journal of Mineral Processing 56 (1–4), 133–164. Rubio, J., 1996. Modified column flotation of mineral particles. International Journal of Mineral Processing 48, 183–196. Van Deventer, J.S.J., Feng, D., Burger, A.J., 2004. Transport phenomena at the pulp-froth interface in a flotation column: II Detachment. International Journal of Mineral Processing 74 (1–4), 217–231. Ye, Y., Khandrika, S.M., Miller, J.D., 1989. Induction time measurements at a particle bed. International Journal of Mineral Processing 25 (3–4), 221–240. Yianatos, J.B., Finch, J.A., Laplante, A.R., 1988. Selectivity in column flotation froths. International Journal of Mineral Processing 23, 279– 292. Yoon, R.H., Mao, L., 1996. Application of extended DLVO theory, IV— Derivation of flotation rate equation from first principles. Journal of Colloid and Interface Science 181 (2), 613–626. Yoon, R.H., Yordan, J.L., 1991. Induction time measurements for the quartz-amine flotation system. Journal of Colloid and Interface Science 141 (2), 374–383.
Selective detachment process in column flotation froth Rick Q. Honaker *, Ahmet V. Ozsever, B.K. Parekh University of Kentucky, Department of Mining Engineering, 234-B Mining and Mineral Resources Building, Lexington, KY 40506-0107, USA Received 14 July 2005; accepted 6 September 2005 Available online 20 October 2005
Abstract The selectivity in flotation columns involving the separation of particles of varying degrees of floatability is based on differential flotation rates in the collection zone, reflux action between the froth and collection zones, and differential detachment rates in the froth zone. Using well-known theoretical models describing the separation process and experimental data, froth zone and overall flotation recovery values were quantified for particles in an anthracite coal that have a wide range of floatability potential. For highly floatable particles, froth recovery had a very minimal impact on overall recovery while the recovery of weakly floatable material was decreased substantially by reductions in froth recovery values. In addition, under carrying-capacity limiting conditions, selectivity was enhanced by the preferential detachment of the weakly floatable material. Based on this concept, highly floatable material was added directly into the froth zone when treating the anthracite coal. The enriched froth phase reduced the product ash content of the anthracite product by five absolute percentage points while maintaining a constant recovery value. 2005 Elsevier Ltd. All rights reserved. Keywords: Coal; Fine particle processing; Froth flotation; Column flotation; Froth kinetics
1. Introduction The froth flotation process is comprised of two separate and distinctly different zones, i.e., the collection zone and the froth zone. In the collection zone, the separation of the valuable minerals from the non-valuables is achieved based on the bubble–particle attachment process. Due to hydraulic entrainment, a portion of the non-valuable minerals is carried from the collection zone into the froth zone with the mineral–bubble aggregates. Selectivity of the process can be enhanced in the froth zone by providing drainage of the feed pulp and utilizing the selective detachment of the more weak hydrophobic particles as a result of bubble coalescence and the resulting bubble surface area reduction. The importance of the latter sub-process was the subject of recent investigations (Van Deventer et al., 2004; Honaker and Ozsever, 2003; Ata et al., 2002; Ralston et al., 1999; Falutsu, 1994; Hewitt et al., 1994; Yianatos
*
Corresponding author. Tel.: +1 859 257 1108; fax: +1 859 323 1962. E-mail address: [email protected] (R.Q. Honaker).
0892-6875/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.09.019
et al., 1988). On the other hand, decades of research focused on the froth flotation process have resulted in a clear understanding of the processes and sub-processes involved in the selectivity achieved in the collection zone (Yoon and Mao, 1996; Mao and Yoon, 1997). The separation of the desired mineral from the other mineral components in the collection zone is based on differential flotation rates. The collection zone flotation rate of a mineral, k ci , can be quantified by the expression: 3 Vg k ci ¼ P cP aP d ð1Þ 2 Db in which Vg is the superficial gas velocity, Db the bubble diameter, Pc the probability of collision, Pa the probability of attachment and Pd the probability of detachment. As shown in Eq. (1), a change in gas velocity or bubble size affects the flotation rate of each mineral species equally and thus does not improve selectivity. Furthermore, bubble– particle collision is not a selective process and should be maintained at maximum efficiency to ensure a high recovery of the floatable mineral. The probability of detachment is a function of both particle size and density and thus may
688
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play a minor role in selectivity. However, the bubble–particle attachment process as quantified by Pa is the principle mechanism defining the ability to effectively separate minerals in the collection zone of a froth flotation process. After collision and attachment, the bubble–particle aggregate moves upward through the collection zone and into the froth zone. The transition into the froth zone is characterized by an increase in the air fraction from around 20–30% to 70–80% (Finch and Dobby, 1990). In the froth zone, the water filling the voids between the bubbles drain back into the collection zone and the water layer surrounding each bubble thins. Upon approach of the liquid–air interfaces, bubbles coalesce thereby causing a reduction in bubble surface area. If the reduced amount of bubble surface area available in the froth zone is insufficient to carry the solids reporting from the collection zone, particles detach and potentially move with the fluid into the collection zone. As such, froth zone recovery may have a significant influence on overall flotation recovery. To assess the effect on overall flotation recovery for a given mineral, linear analysis concepts developed by Meloy (1983) can be applied to the mass transport processes illustrated in Fig. 1. As proposed by Finch and Dobby (1990), an expression for the overall flotation recovery RO for a mineral component i can be derived as a function of the collection zone recovery RC and froth zone recovery RF (Eq. (2)), i.e., RO ¼
RC RF RC RF þ ð1 RC Þ
ð2Þ
One should note that, if RF equals 100%, RO is equivalent to RC. The collection zone recovery of particle type i can be determined using the axial dispersion model described by Levenspiel (1972), i.e., Rci ¼ 1
where sp is the particle retention time and Pe the axial dispersion coefficient. Perfectly mixed conditions, which resemble the conditions in a conventional flotation cell, are characterized by a Pe value of zero. The Pe value approaches infinity for plug flow conditions. An empirical expression for quantifying Pe was developed by Mankosa et al. (1992), which accounts for the length-to-diameter (L/D) ratio of the collection zone and the counter velocities of the gas (Vg) and liquid (Vl), i.e., 0:53 0:35 L Vt ð5Þ Pe ¼ D ð1 eÞV g in which e is the air fraction. By substituting Eqs. (3)–(5) into Eq. (2), the overall recovery can be quantified as a function of the collection zone recovery for particle i having a flotation rate ki and a corresponding froth zone recovery, RFi . Using a typical L/D ratio of 2 for an industrial column, overall recovery values reveal that froth zone recovery has a relatively small impact on overall recovery for particles with a high flotation rate or collection zone recovery as shown in Fig. 2. Highly floatable particles that are detached in the froth zone are efficiently recovered in the collection zone. However, more weakly detached hydrophobic particles have a lower collection zone recovery and thus are less likely to be recovered in the collection zone. As a result, reductions in froth zone recovery have a significant impact on the overall recovery of particles that have moderate-to-weak flotation characteristics. Therefore, the differential effect of froth zone recovery values between particles of varying floatability indicates an additional selectivity mechanism provided by the reflux action between the collection and froth zones.
4A expfPe=2g 2
2
ð1 þ AÞ expfðA=2ÞPeg ð1 AÞ expfðA=2ÞPeg ð3Þ
in which rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pe þ 4k ci sp A¼ Pe
1.00 0.90 0.80
ð4Þ
0.70 0.60
RCRF
Froth Zone
Concentrate
Overall 0.50 Recovery 0.40 0.30 1.00 0.93
0.20
RC (1-RF)
RC
0.10
0.43 0.30
1-RC
Tailings
Fig. 1. Interaction between zones in a column flotation cell.
Froth Recovery
0. 74
80
75
0.
0.07 0.
92
86
0.
Collection Recovery
0.
97
0.22 95
Collection Zone
0.
0.
Feed = 1
1. 00
0.00
Fig. 2. The effect of froth recovery on overall flotation recovery over a range of collection zone recovery values resulting from the collection zone flotation rates of particles with varying hydrophobicity; L/D = 2.0, Vg = 2.0 cm/s, e = 0.25.
R.Q. Honaker et al. / Minerals Engineering 19 (2006) 687–695
For systems that contain multiple mineral species or unliberated particles with varying degrees of hydrophobicity, the detachment process in the froth zone may also enhance the overall selectivity achieved by froth flotation. Particles with a low degree of surface hydrophobicity are likely to be weakly bound to the bubble surface and thus will be preferentially detached. Moys (1978) and later Yianatos et al. (1988) confirmed by experimental evidence that the detachment process is selective. From Yianatos et al. (1988), the detachment rate for chalcopyrite under a given set of conditions was 0.30 min1 whereas pyrite obtained a higher rate at 0.41 min1. As such, differential detachment rates can be exploited to improve the selectivity between hydrophobic species. Detailed investigations utilizing modified column apparatus designs have provided promising quantifiable evidence of the selective detachment and drop-back processes (Rubio, 1996; Ata et al., 2002). For example, Ata et al. (2002) used a unique cell design to show that highly hydrophobic particles added directly into the froth zone can selectively replace particles of lower hydrophobicity. Specifically, the addition of hydrophobic silica particles in the froth reduced the recovery of hematite particles that were added in the collection zone by 10 percentage points. Based on this finding, it is plausible that the addition of highly hydrophobic particles in the froth zone could assist in the selective rejection of the more weakly hydrophobic material (e.g., low grade particles) reporting from the collection zone, thereby enhancing the overall selectivity between particles of varying hydrophobicity. Results supporting this hypothesis are presented and discussed in this publication. As far as it could be ascertained, the use of a continuous flotation column to quantify the selective detachment and froth recovery is minimal to date. This paper details the work performed to assess froth recovery and the corresponding selective detachment process under various operating conditions for fine coal cleaning using column flotation. 2. Experimental 2.1. Sample preparation A portion of the investigation involved the addition of a more hydrophobic material into the froth zone of a flotation system that was treating a material with lower floatability characteristics. The objective was to quantify the potential separation performance improvements due to the exploitation of selective detachment. A requirement of the two materials was that the solid densities be relatively close but with a distinct difference so that a density separation of the flotation products and tailings could be achieved. Materials meeting this requirement are anthracite and medium-volatile bituminous coal. Anthracite coal is moderately hydrophobic and has a relative solid density greater than 1.5 whereas medium-volatile bituminous coal
689
is highly hydrophobic and has a mean relative density (RD) below 1.5. The anthracite coal sample was obtained from the fine circuit feed of an operating coal preparation plant in Pennsylvania (USA) while the bituminous coal sample was collected from the product stream of a spiral concentrator circuit of a preparation plant in West Virginia (USA). The bituminous coal sample was air dried at room temperature conditions and floated at 1.3 RD to recover the most hydrophobic fraction, which was used in the froth enrichment tests. The 1.3 RD float material was then washed with hot water to remove the heavy liquid (LMT) used in the density fractionation treatment and again washed with double distilled water to ensure that the surface of the coal was cleaned from contaminants. After air drying, the 1.3 RD float fraction of the bituminous coal was pulverized to a particle size below 212 lm. A particle size fraction of 212 · 75 lm was used in all the flotation tests since coal flotation is optimum and entrainment is minimal in this size range. The flotation of coal particles greater than 212 lm generally results in a substantial decrease in flotation rate and thus recovery due to an elevated probability of detachment (Brown and Smith, 1954; Aplan, 1976). Unlike conventional flotation cells, wash water is added in a relatively deep flotation froth to eliminate entrainment. In addition, the natural flow of the pulp is in the downward direction and provides a near plug-flow condition due to the large length-to-diameter ratio. As a result, particles having a size of 75 lm or greater, which possess a downward settling velocity, would not be hydraulically entrainable under the conditions described in this publication. Column flotation results from tests using oxidized anthracite coal confirmed this hypothesis. Both the anthracite and the 1.3 RD coal were screened using a 75 lm sieve in a laboratory scale Sweco unit. The ash contents of the 212 · 75 lm anthracite and 1.3 RD float bituminous coal were 50.72% and 3.56%, respectively. The density fractionation results for the 212 · 75 lm anthracite coal shown in Table 1 indicate that the majority of the material exists in the relative density range of 1.5– 1.7. The analysis revealed that no material floated at a relative density of 1.5.
Table 1 Density fractionation analysis of the 212 · 75 lm anthracite coal Relative density fractions
Fractional Weight (%)
1.5 · 1.6 1.6 · 1.7 1.7 · 1.9 1.9 · 2.2 2.2 Sink Total
Ash (%)
17.69 17.51 9.03 11.32 44.46
2.65 7.23 23.43 53.49 91.81
100.00
50.72
690
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2.2. Sample characterization 2.2.1. Induction time measurements Based on work reported by Arnold and Aplan (1989, 1990), the floatability of the various maceral groups that comprise coal varies significantly. In general, a decrease in floatability is in the following order: liptinites, vitrinite and inertinites. Since the solid density of the liptinite macerals is below 1.2, the vitrinites between 1.25 and 1.40 and inertinites between 1.40 and 1.60 for bituminous coals, floatability of coal improves with a decreasing particle density as shown by Olson and Aplan (1984). Therefore, the investigation concentrated on the froth zone behavior of five density fractions in the anthracite coal. Using an electronic induction timer (MCT-100), the time intervals required for particles in the five density fractions and the 1.3 RD bituminous coal to attach to a bubble after collision were measured (Yoon and Yordan, 1991; Ye et al., 1989). Measurements of induction time were accomplished by moving a captive bubble toward a bed of particles for successfully longer periods of time until the particles become attached to the bubble. The inductiontime measurement technique monitors attachment percentage with a given number of tests at a controlled contact time to obtain a distribution of percent attachment versus contact time. The induction time then is chosen to be the value of a particular contact time at which 50% of the tests resulted in bubble–particle attachment. This procedure allows the actual contact time between the bubble and particle bed to be measured. The results were used as an indicator of floatability. Induction times were measured with and without collector (i.e., fuel oil no. 2) at pH values of 3, 5 and 7. As shown in Table 2, induction time values for the anthracite density fractions decreased substantially when the pH was adjusted to values below 7, thereby indicating that optimum flotation performance would be achieved using medium pH values in the acidic range. Also, the addition of fuel oil significantly reduced the induction time, which is in agreement with the floatability findings of Olson
and Aplan (1984). An unexpected finding was the relatively good floatability of the 2.2 RD sink material, which was comprised of mostly hydrophilic mineral matter that results in an ash content of 91.81%. Microscopic petrographic analysis of the 2.2 RD sink material revealed the presence of ÔboneÕ particles, which are a complex mix of coal and mineral matter. As previously stated, the purpose of the 1.3 RD float bituminous coal was to evaluate the effect of enriching the froth phase with a more hydrophobic material with nearly the same density as the coal being added in the collection zone. The induction times in Table 2 confirms that the 1.3 RD float material is more floatable than the anthracite coal, which is likely due to a greater degree of hydrophobicity. 2.2.2. Determination of the collection zone kinetic rate To measure the collection zone flotation rate ðk ci Þ of the particle density fractions, tests were performed using only the collection zone section of a 5-cm diameter flotation column. The froth zone was separated from the main body of the column leaving only the collection zone section attached to the main frame of the flotation apparatus. The goal was to ensure that the operating characteristics of the collection zone used in determining the collection rate constant, kc, was identical to the environment in which the froth zone evaluation tests were performed. The pulp level was controlled by manipulation of a control valve on the tailings stream, which maintained the level at the overflow lip of the column. The duration of the test was equivalent to approximately three particle retention times in an effort to ensure a steady-state operating condition. Under this condition, samples of the feed, product (overflow) and tailings (underflow) streams were collected, dried and weighed. Density fractionation was performed to determine the amount of mass from each density fraction reporting to each process stream. Using the test data, the k ci value for each density fraction was determined using the plug-flow recovery model, i.e., Rci ¼ 1 exp½k ci sp
Table 2 Induction time values obtained for each density fraction in the anthracite coal and the 1.3 RD float material in bituminous coal over a range of medium pH conditions Relative density fractions
Collector (fuel oil) pH = 3 None
pH = 5 1 lbs/ton
None
pH = 7 1 lbs/ton
None
Induction time (milliseconds)—212 · 75 lm anthracite coal 1.6 Float 2.70 <2 2.70 <2 6.60 1.6 · 1.7 3.00 <2 3.90 <2 7.00 1.7 · 1.9 4.15 2.35 7.44 2.86 10.07 1.9 · 2.2 6.44 4.99 9.27 6.20 12.72 2.2 Sink 10.30 9.40 12.10 8.50 15.60
1 lbs/ton 1.85 1.95 3.54 6.93 7.80
Induction time (milliseconds)—212 · 75 lm bituminous coal 1.3 Float <2 <1 <2 <1 <2 <1
ð6Þ
in which Rci is the collection zone recovery of density fraction i measured from the experimental data and sp is the particle retention time. The use of the plug-flow model is justified by the relatively large length-to-diameter ratio of about 40:1, which has been shown by Finch and Dobby (1990) to provide near plug-flow conditions. Using test conditions that were identical to the froth evaluation tests, the recovery values for each density fraction were determined from collection zone tests at four different volumetric flow rates. Recovery values were calculated based on a mass balance of the weights and ash contents in the feed, product and tailing samples. Using recovery values and Eq. (6) yielded the best-fit estimate of the collection zone flotation rate shown in Table 3 for each density fraction.
R.Q. Honaker et al. / Minerals Engineering 19 (2006) 687–695
Table 3 Collection zone flotation rate and recovery values for each density fraction in the 212 · 75 lm anthracite coal sample
691
trol valve, which is connected to the tailings stream of the column. 2.4. Column flotation procedure
Relative density
kc
Collection zone recovery (RC) 912 cc/min
1089 cc/min
1272 cc/min
1451 cc/min
1.6 Float 1.6 · 1.7 1.7 · 1.9 1.9 · 2.2 2.2 Sink
1.138 0.941 0.789 0.508 0.242
0.989 0.975 0.955 0.865 0.614
0.977 0.955 0.926 0.813 0.550
0.960 0.930 0.893 0.763 0.496
0.940 0.903 0.859 0.716 0.451
2.3. Column flotation apparatus A flotation column was constructed that employed a unique design to separate the froth and pulp during continuous operation. The column was split at the froth–pulp interface by a valve as shown in Fig. 3, so that instantaneous separation of the froth and pulp phases could successfully be achieved. The flotation column also incorporated the ability to provide instantaneous stoppage of the process streams by using solenoid valves after ensuring steady-state operation of the column. The collection zone section (182 cm high) was constructed of cylindrical glass tubing having an inside diameter of 5 cm. The froth zone section (45 cm high) was derived from clear, cylindrical Plexiglas tubing with the same inside diameter. Feed slurry was injected into the column at a distance of 70 cm below the product overflow lip, using a variable speed pump. Pulp level was maintained by utilizing a con-
The evaluation of the selectivity mechanisms in the froth zone of flotation column was conducted over a range of volumetric feed rates and feed solid concentrations using the anthracite coal. As a result of the varying feed mass rates, flotation data were obtained under kinetic-limiting and carrying-capacity limiting conditions. All other parameters were maintained at constant values, which are provided in Table 4. A polyglycol reagent was used as a frother while fuel oil no. 2 was employed as the collector. For the tests investigating froth zone enrichment, 1.3 RD float bituminous coal with a 212 · 75 lm particle size was added in the wash water distributor at concentrations that are expressed as a percentage of the total anthracite feed added in the collection zone. The distribution ring was Table 4 Flotation column operating conditions Feed rate Feed solids concentration by weight External feed concentration by weight Frother concentration (Poly-630) Collector concentration (fuel oil no. 2) Froth depth Superficial gas velocity Volumetric wash water rate
912–1451 cc/min 10–15% 5–10–15% 30 ppm 0.45 kg/ton 45 cm 2 cm/s 400 cc/min
FINAL CONCENTRATE
Divider valve, separating the collection and froth zones
pH-meter
pH-meter
Flow-meters
Frother Tank
PUMP
PUMP
Feed (Anthracite coal) Slurry Tank
TAILINGS
Fig. 3. Schematic illustration of the flotation column apparatus.
External Feed (Bituminous coal) Slurry Tank
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located at a distance from the top of the column that was 1/ 3 the total froth depth. The flotation column tests were conducted for a period equivalent to three particle residence times to ensure steady-state operating conditions. Once this condition was achieved, timed samples were collected from the feed, product and tailing streams. Subsequently, the process streams were instantaneously stopped and the material in both the froth and collection zones isolated. The slurry collected from the two zones and the process stream samples were filtered, dried, weighed, and subjected to density fractionation. Ash analyses were also performed on all samples. For tests during which 1.3 RD bituminous coal was added into the froth zone, the samples collected from the process streams were subjected to density fractionation using at a 1.4 RD. Since the anthracite coal did not contain any 1.4 RD float material, the 1.4 RD sink was subjected to ash analyses to evaluate the separation performance achieved on the anthracite coal. 2.5. Froth zone characterization methodology The froth zone recovery and reflux values for each flotation test were quantified using a simulation that follows the flow diagram in Fig. 1 and the overall recovery expression described by Eq. (2). The main steps of the simulation included: 1. Calculate the RC values using the kc values and the residence time values, which are based on the volumetric column feed rate and the volumetric flow rate of material returning from the froth zone (Eqs. (3) and (4)). 2. Calculate the amount of mass M reporting to the product streams: M Feed i RCollection i ¼ M Froth zone i M Froth zone i RFroth i ¼ M Concentrate i M Feed i ð1 RCollection i Þ ¼ M Tailings i 3. Calculate the drop-back from the froth zone to the collection zone:
quired constraints. The correct Rf i values provided the mass flow rates for each density fraction, the total mass flow rate, and the overall ash contents in the product and tailings streams that were experimentally obtained from each column flotation test. 3. Results and discussion 3.1. Internal reflux Based on induction time measurements (Table 2) and several exploratory flotation experiments, the treatment of the 212 · 75 lm anthracite coal by column flotation was conducted using a medium pH value of around 3.0. The separation performances achieved by varying volumetric feed rate and feed solids concentration were comparable to those achieved by release analyses at medium pH values of 3 and 5 as shown in Fig. 4. However, both the release and column flotation performances were significantly inferior to the results obtained from density fractionation or washability analysis. At similar recovery values, a clean coal ash content of 6% was produced from density-based separations whereas froth flotation produced ash contents of around 16%. An investigation conducted to reveal the reason for the large ash content difference found the existence of unliberated particles having a very high ash content value that possess a relatively high flotation rate. As such, the elevated recovery rates of the high ash content material limited the ash reduction potential of the flotation process for the anthracite coal. The increase in the feed volumetric flow rate and solids concentration resulted in a rise in the feed mass flux rate, which initially provided a corresponding linear increase in the product mass flow rate. Under this condition, selectivity is based on differential flotation rates while froth zone recovery is nearly 100%. However, as shown in Fig. 5, the product mass flow rate reached a peak and began to decrease for feed mass flux rates greater than 5.65 tons/h/ m2. Above this mass flux value, the column operated under
M Froth zone i ð1 RFroth i Þ ¼ M Dropback i 100
4. Calculate the new mass entering the collection zone: M 0Collection zone i ¼ M Feed i þ M Dropback i
By using a Visual Basic Macro Algorithm (embedded in Microsoft Excel), steps 1–5 were continually repeated in a looping manner until steady-state conditions (mass in = mass out) were realized, which included the mass balance for each relative density fraction. Simultaneously, the SOLVER routine embedded in Microsoft Excel was used to determine the best fit value for the froth zone recovery corresponding to each density fraction, Rf i . The SOLVER application is set to satisfy specific conditions with ac-
Recovery (%)
5. Repeat steps 1–4 until steady-state conditions are reached.
80
60
40 pH = 3, Release Test pH = 5, Release Test Washability Internal Refluxing Tests
20
0
2
6
10
14
18
22
26
30
Product Ash (%) Fig. 4. Column flotation separation performances achieved on 212 · 75 lm anthracite coal as compared to those achieved by release and washability analyses.
R.Q. Honaker et al. / Minerals Engineering 19 (2006) 687–695 1.0
0.8 2.5
10% 10% 10% 10% 15% 15% 15%
2.0
1.5
1.0 2.0
3.0
4.0
5.0
Overall Recovery
Concentrate (tons/hr/m2)
3.0
solids, 912 cc/min solids, 1089 cc/min solids, 1272 cc/min solids, 1451 cc/min solids, 1089 cc/min solids, 1272 cc/min solids, 1451 cc/min
6.0
7.0
a carrying-capacity limited condition for which there is insufficient bubble surface area to carry all the particles reporting to the froth zone to the product launder. A decrease in the product mass flux is a result of the selective detachment of the coarsest particles. The flotation column operated under kinetic-limited conditions for each of the volumetric flow rates tests at a feed solids concentration of 10 wt.%. Evidence of this fact is provided in Fig. 6 which shows that the froth zone recovery for each of the density fractions (i.e., represented by their respective flotation rates) was nearly 100% with the exception being the particles having the lowest collection zone rate constant. Due to the heterogeneity of the highash particles, it is likely that the binding energy between the particle and bubble was sufficiently weak to be detached upon passage through the transition from the collection zone to the froth zone and during bubble coalescence despite the availability of bubble surface area. This type of selective detachment has been reported by Jameson (2002) for coarse particles, which led to improved
1.0 10% Solids, 912 cc/min 10% Solids, 1089 cc/min 10% Solids, 1272 cc/min 10% Solids, 1451 cc/min 15% Solids, 1089 cc/min 15% Solids, 1272 cc/min 15% Solids, 1451 cc/min
0.6
0.4
0.2
0.0 1.138
0.941
0.789
0.508
10% Solids, 912 cc/min 10% Solids, 1089 cc/min 10% Solids, 1272 cc/min 10% Solids, 1451 cc/min 15% Solids, 1089 cc/min 15% Solids, 1272 cc/min 15% Solids, 1451 cc/min
0.4
0.0 8.0
Fig. 5. Relationship between the feed mass flux and the product mass flux as the flotation process approaches carrying-capacity limited conditions.
0.8
0.6
0.2
Feed (tons/hr/m2)
Froth Recovery (%)
693
1.14
0.94
0.79
0.51
0.24
Collection Zone Flotation Rate (min-1) Fig. 7. Experimental froth zone recovery values as a function of the collection zone flotation rates corresponding to the various density fractions in the anthracite coal.
coarse particle flotation by the introduction of the coarse components of a given feed directly into the froth zone. As the column approaches and exceeds the maximum mass flux in the concentrate, froth zone recovery for all components in the feed decreases to values of around 50% or less. As indicated in Fig. 6 by the differences in the froth zone recovery values between the various components having different flotation rate values, the detachment process is selective. For example, the froth recovery difference between particles having a k ci value of 1.138 min1 and those with a value of 0.508 min1 was about 20% whereas the difference under kinetic-limited conditions was around 5%. However, increasing the feed mass flux above the carrying-capacity limit appears to deteriorate the selectivity of the detachment process. As shown by theoretical simulations in Fig. 2, froth zone recovery in a flotation column should have minimal impact on overall recovery for highly floatable material. Fig. 7 verifies this observation with experimental evidence. The overall recovery values were obtained from density fractionation of samples collected from the process streams of each column flotation test and mass balancing of each component in the feed. For the 10% feed solid concentration tests, the change in recovery for a feed component having a specific k ci value was strictly a function of decreasing residence time since froth zone recovery for most components was nearly 100% (Fig. 6). However, as the system approached carrying capacity limiting conditions at a feed solid content of 15%, particles having a k ci value of 0.51 min1 exhibited a significantly greater drop in overall recovery compared to the decrease observed for the other particle types. This observation supports the presence of selectivity in the detachment process, which appears to decrease after exceeding the carrying-capacity limit.
0.242
Collection Zone Flotation Rate (min-1) Fig. 6. The effect of an increase in the feed mass flux on the froth zone recovery realized for density fractions characterized by measured collection zone flotation rates.
3.2. Froth zone enrichment Based on the evidence of selective detachment provided in Figs. 6 and 7, it was hypothesized that the addition of
R.Q. Honaker et al. / Minerals Engineering 19 (2006) 687–695
Table 5 Bituminous coal distribution during the external froth enrichment tests conducted using a feed volumetric flow rate of 1451 cc/min and a feed solids concentration of 15 wt.% Process stream
Product Tailings Feed
Percent bituminous coal addition 5% of Anthracite feed
10% of Anthracite feed
15% of Anthracite feed
98.26 1.74
99.08 0.92
98.84 1.16
100.00
100.00
100.00
100
80
Recovery (%)
material into the froth phase that has a greater degree of surface hydrophobicity than the feed coal could enhance selectivity by replacing the lower quality particles during bubble surface area reduction events. In fact, this hypothesis was the basis of the research reported by Ata et al. (2002) in which hydrophobized silica was added to a flotation froth recovering hematite. However, the density difference between the bituminous and anthracite coals is significantly smaller than the system studied by Ata et al. (2002), which minimizes the density effect on detachment. For the tests in which the column was operated under rate limiting conditions (i.e., feed volumetric flow rates <1451 cc/min), 100% of the 1.3 RD bituminous coal reported to the column product stream. A slight loss of bituminous coal to the tailings stream occurred when the volumetric feed rate was increased to 1451 cc/min, which provided carrying capacity conditions as indicated in Fig. 5. Table 5 outlines the distribution path of the externally added bituminous coal in the flotation column. Less than 2% of the bituminous coal added in the froth phase was lost to the tailings stream while operating under carrying-capacity limiting conditions. This finding indicates that the bituminous coal particles are replacing the weaker hydrophobic anthracite particles on the surface of the bubbles. The addition of the 1.3 RD float fraction of a highly floatable bituminous coal into the froth phase provided significant improvements in the selectivity achieved from the treatment of the anthracite coal as shown in Fig. 8. The bituminous coal was removed from the product and tailing samples by performing a density fractionation at a 1.40 RD. As such, the data in Fig. 8 represents the improved performance on the anthracite coal only. At a given recovery value, the product ash content was reduced from 19% to nearly 14% by the addition of 1.30 bituminous coal at a concentration equal to 15% of the feed mass flow rate. As shown in Fig. 9, the impact on selectivity occurred as a result of a decrease in the overall recovery of particles having a collection zone rate smaller 0.941 min1. This finding correlates well with the induction time measurements in Table 2. The 1.30 RD float material only replaced particles that possessed a significantly lower degree of floatability. Similar to the findings of the carrying-capacity limited tests, the most significant reduction in recovery was realized by particles which had a collection zone rate
pH = 3, Release Test 60
pH = 5, Release Test Internal Refluxing Tests
40
15% solids with bituminous coal (5% of anthracite feed) 15% solids with bituminous coal (10% of anthracite feed) 15% solids with bituminous coal (15% of anthracite feed)
20
0 10
14
18
22
26
30
Product Ash (%) Fig. 8. Improvement in the separation performance achieved on the anthracite coal as a result of the addition of the 1.30 RD bituminous coal into the froth phase.
1.0
0.8
Overall Recovery
694
0.6
0.4
1089 cc/min
0.2
1089 cc/min with bituminous coal (15% of feed) 1272 cc/min 1272 cc/min with bituminous coal (15% of feed)
0.0 1.138
0.941
0.789
0.508
0.242
Collection Zone Flotation Rate (min-1) Fig. 9. Effect of froth zone enrichment on the overall recovery of each density component in the anthracite coal.
of 0.51 min1 and an induction time of 4.9 ms as compared to less than 1.0 ms for the bituminous coal. 4. Conclusions The bubble coalescence that is known to occur in the froth zone of flotation systems results in a reduction in the bubble surface area needed to carry particles to the product stream. Under high solid loading conditions, an insufficient amount of surface area exists at the top of the froth zone and, thus particles are detached from the bubble surfaces. The detachment process is selective in that particles having the weakest bond with the bubble surface are preferentially detached. An evaluation of the detachment process in the froth phase and its associated selectivity has been conducted to quantify froth zone recovery and the corresponding effect on the overall process selectivity. A well-known fundamental column flotation model and experimental data were used to assess the selectivity achieved under kinetic-limited
R.Q. Honaker et al. / Minerals Engineering 19 (2006) 687–695
and carrying-capacity limited conditions. A narrowly sized anthracite coal has been used due to its unique characteristics that allow timely and accurate assessments of the desired rate and recovery values. The particle sizes cover a range that are known to provide optimum coal flotation recovery and sufficiently coarse to eliminate recovery due to hydraulic entrainment. Test results revealed that increasing the feed solids content from 10 to 15 wt% provided a reduction in product ash content by about 14 absolute percentage points while maintaining relatively high recovery values. For tests involving 10% solids, the selectivity improvement was due to the exploitation of differential flotation rates. However, a feed solid concentration of 15% provide carrying-capacity limited conditions under which the selectivity improvement is due to selective detachment as a result of bubble surface are reduction and the reflux process that occurs between the collection and froth zones. Differential froth flotation recovery values as high as 20 absolute percentage points were obtained between highly and weakly floatable particles. Improved selectivity was obtained for the treatment of the anthracite coal by the addition of bituminous coal into the froth phase. As a result of the higher degree of floatability, the bituminous coal particles replaced a portion of the particles that were weakly to moderately hydrophobic. The result was a five absolute percentage point decrease in the product ash content at equal recovery values. References Aplan, F.F., 1976. Coal flotation. In: Fuerstenau, M.C. (Ed.), Flotation: A.M. Gaudin Memorial Volume. SME Inc., Littleton, pp. 1235–1264. Arnold, B.J., Aplan, F.F., 1989. The hydrophobicity of coal macerals. Fuel 68 (5), 651–658. Arnold, B.J., Aplan, F.F., 1990. Use of petrologic analysis and flotation kinetics to evaluate froth flotation products. Coal Science and Technology 16, 221–235. Ata, S., Ahmed, N., Jameson, G.J., 2002. Collection of hydrophobic particles in the froth phase. International Journal of Mineral Processing 64, 101–122.
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