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Effect of particle size on the reverse flotation of subbituminous coal
Powder Technology 301 (2016) 323–330
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Effect of particle size on the reverse flotation of subbituminous coal Yonggai Li a, Rick Honaker b,⁎, Jianzhong Chen a,⁎, Lijuan Shen a a b
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China Department of Mining Engineering, University of Kentucky, Lexington, KY 40506-0107, USA
a r t i c l e
i n f o
Article history: Received 15 May 2015 Received in revised form 20 March 2016 Accepted 12 June 2016 Available online 14 June 2016 Keywords: Froth flotation Subbituminous coal Oxidation Depressant Collector Particle size
a b s t r a c t The surface of subbituminous coal contains a significant amount of oxygen containing functional groups which contribute to a low degree of hydrophobicity. As such, upgrading of the fine fractions is difficult using conventional flotation practices which rely on bubble-coal particle attachment and separation based on density. Alternatively, reverse flotation provides a viable option whereby the mineral matter is made hydrophobic through the addition of a collector while a depressant is added to make the coal particle non-floatable. Bench-scale reverse flotation experiments were conducted using artificial mixtures of subbituminous coal and quartz of varying particle size to assess the flotation performance. The process provided a reduction on the ash content from values around 35% to while recovering nearly 85% of the combustible material. The performance was achieved using mixtures of coarse coal with fine quartz and medium-size coal with fine silica. The performance was significantly better than that achieved by the conventional practice of floating the coal and rejecting the mineral matter to the underflow stream. The separation efficiency obtained when evaluating a mixture of fine coal and quartz particles was not as favorable due to the effect of hydraulic entrainment. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Subbituminous coal is a low rank coal with low carbon contents and relatively high moisture contents with values ranging from 20%–30% by weight. It makes up 47% of U.S. coal production by weight and 41% by energy intensity [1]. It is generally used for electricity generation. Properties that limit the ability of the conventional froth flotation process to upgrade the heat content value of subbituminous coal include a high number of oxygen-containing functional groups, elevated humic acid levels and high porosity with pores filled with water [2–3]. The oxygen-containing groups such as hydroxyl, carboxyl, and carbonyl cause the formation of a hydration film on coal surface, which prohibits bubble-coal particle attachment. The presence of humic acid reduces the slurry pH and deteriorates the solution chemistry in a manner that suppresses the floatability of low rank coals. All of these issues typically result in low to no flotation recovery when the coal particles are the component selected to be floated [2–3]. Alternatively, the mineral matter associated with low rank coals is typically comprised of primarily silica and clays [4]. These minerals are routinely floated in the minerals industry. As such, floating the mineral matter under conditions that suppress coal flotation, which is a process referred to as reverse flotation, provides a potential option for upgrading fine low rank coal.
⁎ Corresponding authors. E-mail addresses: [email protected] (R. Honaker), [email protected] (J. Chen).
http://dx.doi.org/10.1016/j.powtec.2016.06.019 0032-5910/© 2016 Elsevier B.V. All rights reserved.
Coal reverse flotation is not a new concept as indicated by the results of numerous investigations including those focused on coal desulfurization [5–6]. In reverse coal flotation, dextrin is typically used as depressant whereby the chemical is adsorbed by hydrophobic bonding on coal surface [6]. The process was systematically studied by Stonestreet and Franzidis [7–9] for the separation of coal organic matter from gangue minerals with emphasis on reducing entrainment in conventional flotation. In their experiments, various types of amines were tested using a model system of bituminous coal and silica. The reverse flotation process achieved a mineral matter recovery of 92% to the froth concentrate with a corresponding coal recovery of 73%. The researchers found that staged additions of amines effectively reduces coal entrainment to the froth product. Pawlik and Laskowski [10–12] also conducted tests using artificial mineral and coal mixtures using dodecyltrimethyl ammonium bromide (DTAB) as a mineral matter collector and humic acids as a depressant for coal. The study found that DTAB adsorbs on the coal preferentially but high amine concentrations decrease selectivity. Ding and Laskowski [13–15] further used a zero conditioning method along with a blinder (a polyacrylamide) and dispersant (tannic acid) to reduce collector consumption which decreased reagent dosage from over 6 kg/t down to 1.375 kg/t. Zhang et al. found that particle size of lignite coal strongly affects the reverse flotation kinetics [16]. Maximum flotation rate constant was achieved at grinding fines (− 74 μm size fraction) content of 42.34% and with the − 250 + 150 μm fraction [16]. The effect of a broad range of flotation conditions was investigated including grinding time.
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Table 1 Particle size distribution of subbituminous coal. Particle size/μm
Yield/%
Ash dry/%
150–200 120–150 74–120 53–74 38–53 −38 Total
15.71 12.92 15.3 10.52 3.51 42.04 100.00
6.42 6.96 7.05 7.3 8.7 11.3 8.81
The optimum flotation performance was obtained using a grind that produced 86% of the material finer than 0.1 mm [17]. Xia and Yang discussed the effects of dextrin and HTAB dosages on reverse flotation behavior of Taixi oxidized coal for different particle size fractions and found that both depressant and collector dosages had a significant effect on coarse size fractions but limited effect on fine coals [18]. Even though the reverse flotation of low rank coal has been previously studied [4], the availability of flotation performance data for a number of particle size fractions is limited. In this study, the effect of particle size on separation performance achieved on both coal and mineral matter by the reverse flotation process was evaluated through a detailed investigation. The interactive particle size effects of both coal and mineral matter were also studied. 2. Materials and methods 2.1. Materials The subbituminous coal used for this study was collected at a surface mining operation located in the Powder River coal basin of the U.S. Upon receiving the coal from the mine in drums, the coal sample was split using standard sampling procedures to obtain representative samples which were subsequently crushed to below 200 μm by hammer mill. After crushing, the samples were sieved using lab vibrating screens to obtain samples representing a range of particle size fractions. Afterwards, the samples were stored in sealed plastic bags and kept in a freezer to prevent the loss of moisture and preserve the surface chemistry properties. The particle size distribution and the proximate analysis of a representative sample of the ground subbituminous coal are provided in Tables 1 and 2. X-ray diffraction (XRD) was conducted on the coal sample, which revealed that the primary component of the mineral matter in the Powder River basin coal was quartz. As such, quartz was used in the flotation study as a model mineral. The ASTM quartz c-778 was purchased from ebay at a purity level of 98.8%. The quartz was crushed to below 200 μm using a hammer mill and then ground in a ball mill to minus 56 μm. The particle size distributions of both coarse and fine quartz are shown in Figs. 1 and 2. Flotation feed was prepared by mixing subbituminous coal and quartz of different size fractions using a coal-to-quartz ratio of 7:3, which represents a typical ratio for coal flotation feed materials. Conventional flotation feed was comprised of raw coal (− 200 μm) and fine quartz (− 56 μm). The composition and characteristics of the reverse flotation feed materials are shown in Table 3, where CC represents coarse coal (150–200 μm), MC medium-sized coal (74–120 μm), FC fine coal (−38 μm), RC raw coal (−200 μm), FS fine silica (−56 μm), and CS coarse silica (−200 μm).
Fig. 1. Particle size distribution of coarse quartz.
2.2. Chemical reagents Diesel fuel oil No. 2 and SPP were used as collectors in the conventional flotation tests. SPP contained 90% fuel oil and 10% petroleum sulfonate [19]. A complex reagent, Lilaflot D817M provided by AKZO NOBEL has been widely used in reverse flotation of hematite or magnetite involving the flotation of silica [20–21]. In the reverse flotation of coal, the chemical was used as a collector for quartz. The chemical composition of the Lilaflot D817M is shown in Table 4. The collector was added as a 1% solution made by diluting 1 g as-received reagents to 100 ml with distilled water. Dextrin was purchased from Fisher Scientific and used as a coal depressant. Its chemical formula is (C6H10O5)x and molecular weight is 162.067 g/mol. A 1% dextrin solution was made with distilled water and then kept in 60 °C thermostatic water bath during the flotation process. Methyl isobutyl carbinol (MIBC, lab grade) was used as the frother at a feed slurry concentration of 20 ppm. Distilled water was used throughout the flotation test program and surface characterization studies to provide fundamental control of the solution chemistry.
Table 2 Proximate analysis of subbituminous coal. Sample
Moisture/% Ash dry/%
Volatile/% Fixed carbon/%
Sulfur/%
Subbituminous coal
27.72
46.62
0.72
8.81
16.85
Fig. 2. Particle size distribution of fine quartz.
Y. Li et al. / Powder Technology 301 (2016) 323–330 Table 3 Composition and characteristics of feed for reverse flotation. Feed
Coal particle size/μm
Quartz particle size/μm
Coal dry wt./g
Coal ash dry/%
Quartz wt./g
Feed ash/%
CC&FS MC&FS FC&FS RC&FS RC&CS
150–200 74–120 −38 −200 −200
−56 −56 −56 −56 −200
40.00 40.00 40.00 40.00 40.00
6.42 6.96 11.30 8.81 8.81
17.00 17.00 17.00 17.00 17.00
34.33 34.71 37.75 36.01 36.01
Table 4 Composition of Lilaflot D817M reagent. Chemical name
Concentration/%
N-(3-(Tridecyloxy)propyl)-1,3-propane diamine, acetate N-(3-(Tridecyloxy)propyl)-1,3-propane diamine
50–60 40–50
Sodium hydroxide (NaOH, 99.99% grade), hydrochloric acid (HCl, 25.00% v/v) and potassium chloride (KCl, 100.00% grade) bought from Fisher Scientific were used to adjust the solution chemistry for the zeta potential measurements.
2.3. Methods and procedures 2.3.1. FTIR spectrum Fourier transform infrared spectroscopy (FTIR) was used to evaluate the surface composition of the coal surfaces with a focus on the degree of oxidation. The FTIR unit was manufactured by NEXUS Inc. The FTIR spectrum of subbituminous coal was obtained with KBr pellets, compressed together with around 10 mg ground coal sample and 1.2 g analytical grade KBr. FTIR spectrum of KBr was obtained as a baseline. The FTIR spectrum was processed using OMNIC Software developed by Thermo Scientific.
2.3.2. Zeta potential measurement For the zeta potential distribution measurements, both the coal and quartz were ground using agate pestle to reduce the particle size to smaller than 30 μm. HCl and NaOH were used for pH adjustment while 10− 3 M KCl was used as a supporting electrolyte in distilled water. All measurements were performed using a Zeta Potential Analyzer manufactured by Brookhaven Instruments Corporation and the procedure described in the instruction manual.
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2.3.3. Flotation tests For both conventional flotation and reverse flotation, the tests were carried out in a 1-L Denver laboratory cell under natural pH conditions. A mixture consisting of 40 g coal (dry weight) and 17 g quartz was used as flotation feed and mixed with distilled water to obtain a solid concentration of 5.4% by weight. The impeller speed was 1200 rpm and the air valve was kept at maximum during all the tests. The slurry was initially conditioned for 3 min to ensure complete dispersion of the particles. The pH of the pulp was measured before the addition of dextrin and amine and ranged from 6.83 to 6.97. For the conventional flotation tests, collector was added and conditioned for an additional 3 min followed by the injection of the frother to a 20 ppm concentration and conditioning for another 1 min. The froth product was collected over a period of 5 min. When preparing for reverse flotation tests, dextrin and amine were each conditioned in the coal-quartz slurry for 3 min. Next, frother was added followed by 1 min of conditioning. The froth concentrate was removed continuously over a 5-minute flotation period. In these tests, the coal product was the material remaining in the cell while the reject was the froth concentrate material. The concentrate (the product remains in the cell) and the tailing (the froth product) were filtered, dried and weighed for recovery calculations. The ash contents of all the products were determined using Thermogravimetric Analyzer 701 from LECO Corporation following ASTM E1131 [22]. For both conventional flotation and reverse flotation, combustible matter recovery in concentrate and silica recovery in tailing were calculated by Eqs. (1) and (2), respectively. Separation efficiency was calculated by Eq. (3) [23].
Combustible matter recovery ð%Þ ¼ Mc ð100−AcÞ=½M F ð100−A F Þ 100 ð1Þ
Silica recovery ð%Þ ¼ MT AT =ðM F A F Þ 100
ð2Þ
Flotation efficiency index ð%Þ ¼ 100 MC ðA F −AC Þ=½M F ð100−A F Þ A F 100 ð3Þ
where MC, MT, MF are the weights of the concentrates, tailings and feed (g), respectively, AC, AT and AF are the ash contents of the concentrates, tailings, and feed by weight (%), respectively.
Fig. 3. FTIR spectrum of subbituminous coal.
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Fig. 6. Effect of dextrin dosage on reverse floatation of coarse coal and fine silica mixture (collector 1 kg/t, feed ash 34.33%). Fig. 4. Zeta potential distributions of subbituminous coal and silica.
3.2. Results of zeta potential distribution 3. Results and discussion
Given that the coal sample in this study is a low rank coal, it was expected that the coal surface was heavily oxidized. In the FTIR spectrum shown in Fig. 3, the high absorbance peak at about 3342 cm−1 for OH on the surface of subbituminous coal means that it has a high moisture content [24]. The absorbance at a peak of 1605 cm−1 for COOH and those between 1150 cm− 1 and 1380 cm− 1 for C\\O and O\\H in phenoxy groups are also high, indicating the extent of the oxygen-containing functional groups on the surface which contribute to its poor flotation performance in conventional tests [11,12,24]. The oxygen-containing functional groups on the subbituminous coal surface interact with water and generated a hydrophilic surface or very weakly hydrophobic at best. The peak at 1699 cm−1 represents unsaturated C_C which does not exist in bituminous coal, but does exist in low rank coal.
As shown in Fig. 4, the zeta potential characteristics of both subbituminous coal and silica were similar as indicated by negative values throughout the pH values studied and a decline in the absolute value with a decrease in the solution pH value. The coal surfaces had a zeta potential that was around 5 to 10 mV lower than the quartz particles over the range of pH values. The electronegativity of the coal surfaces for pH values over 3 because of the negatively charged sites provided by the oxygen-containing functional groups on the coal surface, such as OH and COOH [25]. The high surface potential of the coal particles results in a significant electrostatic repulsive force between the coal particles and air bubbles, which prevents or significantly slows the attachment process upon collision, thereby limiting combustible recovery. Electrokinetic potential of quartz is also negative in the same pH range as a result of the dissociation of\\Si\\OH on silica surface. Given the negative charge of the coal particles, a depressant is necessary when a cationic amine collector is used in reverse flotation [25].
Fig. 5. Conventional flotation results of subbituminous coal (raw coal and fine silica mixture) with diesel and SPP (feed ash 36.01%).
Fig. 7. Effect of dextrin dosage on reverse floatation of medium coal and fine silica mixture (collector 1 kg/t, feed ash 34.71%).
3.1. Result of FTIR
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Fig. 8. Effect of dextrin dosage on reverse floatation of fine coal and fine silica mixture (collector 1 kg/t, feed ash 38.06%).
3.3. Conventional flotation of subbituminous coal and quartz mixture
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Fig. 10. Effect of collector dosage on reverse floatation of coarse coal and fine silica mixture (dextrin 1 kg/t, feed ash 34.33%).
3.4. Reverse flotation of subbituminous coal with different size ranges mixed with fine silica
According to Fig. 5, the combustible matter recovery was increased by an elevation in the amount of collector. The trend was found to be true for both collector types. However, to achieve the relatively poor recovery performance, a very high amount of collector was required which is reflective of the low degree of surface hydrophobicity associated with the coal particles. The high collector dosages also lead to high ash content values in the concentrate, which was most likely due to hydraulic entrainment of the fine silica. SPP performed better than diesel at the same dosage as indicated by higher combustible recovery and lower product ash content values. Combustible recovery reached 72.40% when utilizing high collector dosages (10 kg/t SPP). At this dosage, the product ash content was very high (34.57%). The weak performance results obtained from the conventional flotation process clearly indicates poor floatability of the low rank coal which led to the consideration of using reverse flotation as a means of efficiently upgrading the low rank coal.
Fig. 6 shows the effect of the depressant dextrin on the performances achieved using the reverse flotation process when treating the coarse coal and fine silica mixture. With an increase in dextrin dosage, combustible recovery and the ash content of the concentrate fluctuate slightly. A dextrin dosage of 2 kg/t suppressed the floatability of the coal to a level that resulted in 82.84% of the combustible matter reporting to the concentrates of the process. The combustible matter remaining in the cell after 5 min of flotation contained 6.61% ash. Silica rejection remained relatively constant at around 88%. It indicates that dextrin in the pulp is enough to depress coarse coal flotation. Fig. 7 shows nearly the same trends when treating medium-sized coal and fine silica. Silica recovery to the float fraction was very high at 87% while recovery of the combustible material to the sink material was around 85%. These results show a very high level of selectivity and efficiency when using the reverse flotation process. In contrast to the findings obtained using coarse and medium size coal particles, the results obtained with a mixture of coal finer than 38 μm and fine silica did not show promise as shown in Fig. 8. Combustible recovery to the flotation underflow was only around 55.5% using a dextrin dosage of 4 kg/t and a collector dosage of 1 kg/t. The low
Fig. 9. Separation efficiency of reverse floatation for mixtures of coal with different particle sizes and fine silica (collector 1 kg/t).
Fig. 11. Effect of collector dosage on reverse floatation of medium coal and fine silica mixture (dextrin 1 kg/t, feed ash 34.71%).
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Fig. 12. Effect of collector dosage on reverse floatation of fine coal and fine silica mixture (dextrin 1 kg/t, feed ash 38.01%).
recovery values reported in Fig. 8 was not likely due to the ineffectiveness of the dextrin to suppress the coal flotation. Coal particles reported to the flotation froth product due to hydraulic entrainment caused by the relatively low coal density and small particle size in addition to the elevated impact of Brownian motion energy. A more interesting finding is that the presence of ultrafine coal particles suppressed the flotation of the fine quartz. As shown in the previous figures, quartz recovery values were above 85% with coarse and medium-sized coal. However, in the presence of ultrafine coal, quartz recovery was less than 50%. This may be due adsorption of a significant portion of the amine collector unto the negative charged and high-surface area coal particles thereby leaving an insufficient amount to properly coat the quartz particles. The results show that fine silica has different flotation behaviors when mixed with subbituminous coal of different particle size ranges. When mixed with coarse coal (150–200 μm) or medium coal (74– 120 μm), the highest silica recovery was 88.68% and 86.92%, respectively (with 1 kg/t dextrin and 1 kg/t collector), while the recovery decreases to 44.50% when mixed with fine subbituminous coal (− 38 μm). All other conditions were held constant. It indicates that the fine silica can float better when mixed with coarse and medium coal particles,
Fig. 13. Separation efficiency of reverse floatation for mixtures of coal with different particle sizes and fine silica (dextrin 1 kg/t).
Fig. 14. Effect of dextrin dosage on reverse floatation of raw coal and fine silica mixture (collector 1 kg/t, feed ash 36.01%).
while its floatability worsens in the presence of fine subbituminous coal most likely due to competitive adsorption of the amine collector. Separation efficiency of the reverse flotation tests of coal particles with different size ranges and fine silica was plotted in Fig. 9. Much higher separation efficiency can be achieved when dealing with a coarse or medium coal and fine quartz mixture than with a fine coal and fine quartz mixture. Figs. 10 and 11 shows the reverse flotation results of coarse coal and medium coal with fine silica mixture with different collector dosage. The combustible recovery in concentrate for both feeds decrease when dextrin dosage increases from 1 kg/t to 4 kg/t. It means that with the increase of dextrin, coal particles are also transferred to froth because of adsorption with collector. Silica recovery in tailing for both feeds increases as more silica particles are floated when more collector is used, which makes the ash content in concentrate decrease. For fine coal and fine silica mixture, the reverse flotation results with different collector dosage are illustrated in Fig. 12. It can be seen that the combustible recovery in concentrate is much lower than that of coarse coal or medium coal. With the increase of collector dosage, the combustible recovery falls markedly. It might because fine coals get entrained easily. The silica recovery increases with the increase of collector dosage, but it is much lower than that with coarse coal or medium coal at the same collector dosage. The ash content of concentrate is much higher.
Fig. 15. Effect of dextrin dosage on reverse floatation of raw coal and coarse silica mixture (collector 1 kg/t, feed ash 36.01%).
Y. Li et al. / Powder Technology 301 (2016) 323–330
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Fig. 18. Effect of collector dosage on reverse floatation of raw coal and coarse silica mixture (dextrin 1 kg/t, feed ash 36.01%). Fig. 16. Separation efficiency of reverse floatation for mixtures of raw coal and silica with different particle sizes (collector 1 kg/t).
The separation efficiency of these reverse flotation tests is shown in Fig. 13. It can be concluded that the separation efficiency with coarse coal or fine coal is much higher than that with fine coal. The increase of collector dosage has larger effect on the separation efficiency with finer coal particles than coarser ones. 3.5. Reverse flotation of raw subbituminous coal mixed with fine silica or coarse silica Fig. 14 shows the effect of dextrin on the reverse flotation of raw coal mixed with fine silica. With the increase of dextrin dosage, the combustible recovery increases while the silica recovery remains at a constant level of around 80%. The ash content of the coal product decreased from 13.78% to 11.33% while the ash content of the tailings or froth product increases from 57.88% to 71.89% when the dextrin dosage was elevated from 1 kg/t to 4 kg/t. For raw coal and coarse silica mixture, the effect of dextrin dosage on the reverse flotation is shown in Fig. 15. When dextrin dosage increases, combustible recovery to the coal product improved significantly. However, the recovery of the coarse silica to the froth product was only about 20% which is likely due to the particle density and coarseness of the particles targeted for flotation.
Fig. 17. Effect of collector dosage on reverse floatation of raw coal and fine silica mixture (dextrin 1 kg/t, feed ash 36.01%).
With an increase in dextrin dosage, the ash content of the concentrate is much higher than that achieved with fine silica while that of tailing is lower which reflects a reduction in separation efficiency. The separation efficiency of raw coal and silica particles as a function of particle size is shown in Fig. 16. The separation efficiency increases with an increase in dextrin dosage which indicates a positive impact of dextrin on the separation efficiency achieved using the reverse flotation process. The reverse flotation results of raw coal and fine silica with different collector dosage are shown in Fig. 17. It indicates that the silica recovery in tailing increases with the increase of the collector dosage, which reaches 90.48% when the collector dosage is 4 kg/t. The combustible matter recovery is not as high as the flotation results of silica mixed with only coarse coal or medium coal, as there are also some fine coal particles that need more depressant and easily get entrained in the feed. The highest combustible matter recovery is 70.18% when the used amine is 2 kg/t but then it decreases when more collectors were added. It means that some coals were lost in the froth product by interaction with the amine collector. The ash content of concentrate reaches 7.56% with 4 kg/t collector. As shown in Fig. 18, the silica recovery in the concentrate increases from 19.99% to 41.82% with the increase of collector dosages, indicating
Fig. 19. Separation efficiency of reverse floatation for mixtures of raw coal and silica with different particle sizes (dextrin 1 kg/t).
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collector will be consumed. Fine silica has a better flotation performance than coarse silica. In general, mineral particles are always finer than coal particles in coal samples [7], so reverse flotation is a potential alternative method to upgrade difficult-to-float coals, such as oxidized coal and low rank coal. The solution chemistry of real coal pulp is more complex, so more research about reverse flotation for real coal is necessary.
References
Fig. 20. Separation efficiency of reverse floatation for mixtures of coal and silica with different particle sizes (dextrin 1 kg/t, collector 1 kg/t).
coarse silica does not respond well to the collector due to the larger mass. This is because the buoyancy of bubbles cannot overcome the weight of coarse silica particles. Combustible recovery decreases from 71.22% to 57.20% because of the interaction with the cationic collector, as the surface of subbituminous coal is electronegative at the neutral pH value. More dextrin is needed to reduce the loss of coal particles. As also can be seen from Fig. 19, the separation efficiency of reverse flotation for the mixture of raw coal and fine silica is much higher than that of raw coal and coarse silica. The separation efficiency indexes of all the reverse flotation tests are compared in Fig. 20. It indicates that the selectivity of reverse flotation for mixtures of coarse or medium coal and fine silica is much higher than the others when 1 kg/t dextrin and 1 kg/t collector were used. As such, reverse flotation can perform well for coal upgrading when the material consists of coarse or medium size coal and fine quartz but appears to fail when the particle size of the coal is mostly below 38 μm. 4. Conclusions Subbituminous coal is a low rank coal with high content of moisture and surface oxygen-containing functional groups such as hydroxyl and carboxyl. Conventional flotation cannot perform well on subbituminous coal effectively even with large doses of collectors. Good reverse flotation results can be achieved when the flotation feed is made up of coarse or medium subbituminous coal particles and fine quartz particles. Entrainment also plays an important role on reverse flotation performance. The presence of fine subbituminous coal particles deteriorates the flotation behavior of fine silica, and a large dosage of dextrin and
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Effect of particle size on the reverse flotation of subbituminous coal Yonggai Li a, Rick Honaker b,⁎, Jianzhong Chen a,⁎, Lijuan Shen a a b
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China Department of Mining Engineering, University of Kentucky, Lexington, KY 40506-0107, USA
a r t i c l e
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Article history: Received 15 May 2015 Received in revised form 20 March 2016 Accepted 12 June 2016 Available online 14 June 2016 Keywords: Froth flotation Subbituminous coal Oxidation Depressant Collector Particle size
a b s t r a c t The surface of subbituminous coal contains a significant amount of oxygen containing functional groups which contribute to a low degree of hydrophobicity. As such, upgrading of the fine fractions is difficult using conventional flotation practices which rely on bubble-coal particle attachment and separation based on density. Alternatively, reverse flotation provides a viable option whereby the mineral matter is made hydrophobic through the addition of a collector while a depressant is added to make the coal particle non-floatable. Bench-scale reverse flotation experiments were conducted using artificial mixtures of subbituminous coal and quartz of varying particle size to assess the flotation performance. The process provided a reduction on the ash content from values around 35% to while recovering nearly 85% of the combustible material. The performance was achieved using mixtures of coarse coal with fine quartz and medium-size coal with fine silica. The performance was significantly better than that achieved by the conventional practice of floating the coal and rejecting the mineral matter to the underflow stream. The separation efficiency obtained when evaluating a mixture of fine coal and quartz particles was not as favorable due to the effect of hydraulic entrainment. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Subbituminous coal is a low rank coal with low carbon contents and relatively high moisture contents with values ranging from 20%–30% by weight. It makes up 47% of U.S. coal production by weight and 41% by energy intensity [1]. It is generally used for electricity generation. Properties that limit the ability of the conventional froth flotation process to upgrade the heat content value of subbituminous coal include a high number of oxygen-containing functional groups, elevated humic acid levels and high porosity with pores filled with water [2–3]. The oxygen-containing groups such as hydroxyl, carboxyl, and carbonyl cause the formation of a hydration film on coal surface, which prohibits bubble-coal particle attachment. The presence of humic acid reduces the slurry pH and deteriorates the solution chemistry in a manner that suppresses the floatability of low rank coals. All of these issues typically result in low to no flotation recovery when the coal particles are the component selected to be floated [2–3]. Alternatively, the mineral matter associated with low rank coals is typically comprised of primarily silica and clays [4]. These minerals are routinely floated in the minerals industry. As such, floating the mineral matter under conditions that suppress coal flotation, which is a process referred to as reverse flotation, provides a potential option for upgrading fine low rank coal.
⁎ Corresponding authors. E-mail addresses: [email protected] (R. Honaker), [email protected] (J. Chen).
http://dx.doi.org/10.1016/j.powtec.2016.06.019 0032-5910/© 2016 Elsevier B.V. All rights reserved.
Coal reverse flotation is not a new concept as indicated by the results of numerous investigations including those focused on coal desulfurization [5–6]. In reverse coal flotation, dextrin is typically used as depressant whereby the chemical is adsorbed by hydrophobic bonding on coal surface [6]. The process was systematically studied by Stonestreet and Franzidis [7–9] for the separation of coal organic matter from gangue minerals with emphasis on reducing entrainment in conventional flotation. In their experiments, various types of amines were tested using a model system of bituminous coal and silica. The reverse flotation process achieved a mineral matter recovery of 92% to the froth concentrate with a corresponding coal recovery of 73%. The researchers found that staged additions of amines effectively reduces coal entrainment to the froth product. Pawlik and Laskowski [10–12] also conducted tests using artificial mineral and coal mixtures using dodecyltrimethyl ammonium bromide (DTAB) as a mineral matter collector and humic acids as a depressant for coal. The study found that DTAB adsorbs on the coal preferentially but high amine concentrations decrease selectivity. Ding and Laskowski [13–15] further used a zero conditioning method along with a blinder (a polyacrylamide) and dispersant (tannic acid) to reduce collector consumption which decreased reagent dosage from over 6 kg/t down to 1.375 kg/t. Zhang et al. found that particle size of lignite coal strongly affects the reverse flotation kinetics [16]. Maximum flotation rate constant was achieved at grinding fines (− 74 μm size fraction) content of 42.34% and with the − 250 + 150 μm fraction [16]. The effect of a broad range of flotation conditions was investigated including grinding time.
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Table 1 Particle size distribution of subbituminous coal. Particle size/μm
Yield/%
Ash dry/%
150–200 120–150 74–120 53–74 38–53 −38 Total
15.71 12.92 15.3 10.52 3.51 42.04 100.00
6.42 6.96 7.05 7.3 8.7 11.3 8.81
The optimum flotation performance was obtained using a grind that produced 86% of the material finer than 0.1 mm [17]. Xia and Yang discussed the effects of dextrin and HTAB dosages on reverse flotation behavior of Taixi oxidized coal for different particle size fractions and found that both depressant and collector dosages had a significant effect on coarse size fractions but limited effect on fine coals [18]. Even though the reverse flotation of low rank coal has been previously studied [4], the availability of flotation performance data for a number of particle size fractions is limited. In this study, the effect of particle size on separation performance achieved on both coal and mineral matter by the reverse flotation process was evaluated through a detailed investigation. The interactive particle size effects of both coal and mineral matter were also studied. 2. Materials and methods 2.1. Materials The subbituminous coal used for this study was collected at a surface mining operation located in the Powder River coal basin of the U.S. Upon receiving the coal from the mine in drums, the coal sample was split using standard sampling procedures to obtain representative samples which were subsequently crushed to below 200 μm by hammer mill. After crushing, the samples were sieved using lab vibrating screens to obtain samples representing a range of particle size fractions. Afterwards, the samples were stored in sealed plastic bags and kept in a freezer to prevent the loss of moisture and preserve the surface chemistry properties. The particle size distribution and the proximate analysis of a representative sample of the ground subbituminous coal are provided in Tables 1 and 2. X-ray diffraction (XRD) was conducted on the coal sample, which revealed that the primary component of the mineral matter in the Powder River basin coal was quartz. As such, quartz was used in the flotation study as a model mineral. The ASTM quartz c-778 was purchased from ebay at a purity level of 98.8%. The quartz was crushed to below 200 μm using a hammer mill and then ground in a ball mill to minus 56 μm. The particle size distributions of both coarse and fine quartz are shown in Figs. 1 and 2. Flotation feed was prepared by mixing subbituminous coal and quartz of different size fractions using a coal-to-quartz ratio of 7:3, which represents a typical ratio for coal flotation feed materials. Conventional flotation feed was comprised of raw coal (− 200 μm) and fine quartz (− 56 μm). The composition and characteristics of the reverse flotation feed materials are shown in Table 3, where CC represents coarse coal (150–200 μm), MC medium-sized coal (74–120 μm), FC fine coal (−38 μm), RC raw coal (−200 μm), FS fine silica (−56 μm), and CS coarse silica (−200 μm).
Fig. 1. Particle size distribution of coarse quartz.
2.2. Chemical reagents Diesel fuel oil No. 2 and SPP were used as collectors in the conventional flotation tests. SPP contained 90% fuel oil and 10% petroleum sulfonate [19]. A complex reagent, Lilaflot D817M provided by AKZO NOBEL has been widely used in reverse flotation of hematite or magnetite involving the flotation of silica [20–21]. In the reverse flotation of coal, the chemical was used as a collector for quartz. The chemical composition of the Lilaflot D817M is shown in Table 4. The collector was added as a 1% solution made by diluting 1 g as-received reagents to 100 ml with distilled water. Dextrin was purchased from Fisher Scientific and used as a coal depressant. Its chemical formula is (C6H10O5)x and molecular weight is 162.067 g/mol. A 1% dextrin solution was made with distilled water and then kept in 60 °C thermostatic water bath during the flotation process. Methyl isobutyl carbinol (MIBC, lab grade) was used as the frother at a feed slurry concentration of 20 ppm. Distilled water was used throughout the flotation test program and surface characterization studies to provide fundamental control of the solution chemistry.
Table 2 Proximate analysis of subbituminous coal. Sample
Moisture/% Ash dry/%
Volatile/% Fixed carbon/%
Sulfur/%
Subbituminous coal
27.72
46.62
0.72
8.81
16.85
Fig. 2. Particle size distribution of fine quartz.
Y. Li et al. / Powder Technology 301 (2016) 323–330 Table 3 Composition and characteristics of feed for reverse flotation. Feed
Coal particle size/μm
Quartz particle size/μm
Coal dry wt./g
Coal ash dry/%
Quartz wt./g
Feed ash/%
CC&FS MC&FS FC&FS RC&FS RC&CS
150–200 74–120 −38 −200 −200
−56 −56 −56 −56 −200
40.00 40.00 40.00 40.00 40.00
6.42 6.96 11.30 8.81 8.81
17.00 17.00 17.00 17.00 17.00
34.33 34.71 37.75 36.01 36.01
Table 4 Composition of Lilaflot D817M reagent. Chemical name
Concentration/%
N-(3-(Tridecyloxy)propyl)-1,3-propane diamine, acetate N-(3-(Tridecyloxy)propyl)-1,3-propane diamine
50–60 40–50
Sodium hydroxide (NaOH, 99.99% grade), hydrochloric acid (HCl, 25.00% v/v) and potassium chloride (KCl, 100.00% grade) bought from Fisher Scientific were used to adjust the solution chemistry for the zeta potential measurements.
2.3. Methods and procedures 2.3.1. FTIR spectrum Fourier transform infrared spectroscopy (FTIR) was used to evaluate the surface composition of the coal surfaces with a focus on the degree of oxidation. The FTIR unit was manufactured by NEXUS Inc. The FTIR spectrum of subbituminous coal was obtained with KBr pellets, compressed together with around 10 mg ground coal sample and 1.2 g analytical grade KBr. FTIR spectrum of KBr was obtained as a baseline. The FTIR spectrum was processed using OMNIC Software developed by Thermo Scientific.
2.3.2. Zeta potential measurement For the zeta potential distribution measurements, both the coal and quartz were ground using agate pestle to reduce the particle size to smaller than 30 μm. HCl and NaOH were used for pH adjustment while 10− 3 M KCl was used as a supporting electrolyte in distilled water. All measurements were performed using a Zeta Potential Analyzer manufactured by Brookhaven Instruments Corporation and the procedure described in the instruction manual.
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2.3.3. Flotation tests For both conventional flotation and reverse flotation, the tests were carried out in a 1-L Denver laboratory cell under natural pH conditions. A mixture consisting of 40 g coal (dry weight) and 17 g quartz was used as flotation feed and mixed with distilled water to obtain a solid concentration of 5.4% by weight. The impeller speed was 1200 rpm and the air valve was kept at maximum during all the tests. The slurry was initially conditioned for 3 min to ensure complete dispersion of the particles. The pH of the pulp was measured before the addition of dextrin and amine and ranged from 6.83 to 6.97. For the conventional flotation tests, collector was added and conditioned for an additional 3 min followed by the injection of the frother to a 20 ppm concentration and conditioning for another 1 min. The froth product was collected over a period of 5 min. When preparing for reverse flotation tests, dextrin and amine were each conditioned in the coal-quartz slurry for 3 min. Next, frother was added followed by 1 min of conditioning. The froth concentrate was removed continuously over a 5-minute flotation period. In these tests, the coal product was the material remaining in the cell while the reject was the froth concentrate material. The concentrate (the product remains in the cell) and the tailing (the froth product) were filtered, dried and weighed for recovery calculations. The ash contents of all the products were determined using Thermogravimetric Analyzer 701 from LECO Corporation following ASTM E1131 [22]. For both conventional flotation and reverse flotation, combustible matter recovery in concentrate and silica recovery in tailing were calculated by Eqs. (1) and (2), respectively. Separation efficiency was calculated by Eq. (3) [23].
Combustible matter recovery ð%Þ ¼ Mc ð100−AcÞ=½M F ð100−A F Þ 100 ð1Þ
Silica recovery ð%Þ ¼ MT AT =ðM F A F Þ 100
ð2Þ
Flotation efficiency index ð%Þ ¼ 100 MC ðA F −AC Þ=½M F ð100−A F Þ A F 100 ð3Þ
where MC, MT, MF are the weights of the concentrates, tailings and feed (g), respectively, AC, AT and AF are the ash contents of the concentrates, tailings, and feed by weight (%), respectively.
Fig. 3. FTIR spectrum of subbituminous coal.
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Fig. 6. Effect of dextrin dosage on reverse floatation of coarse coal and fine silica mixture (collector 1 kg/t, feed ash 34.33%). Fig. 4. Zeta potential distributions of subbituminous coal and silica.
3.2. Results of zeta potential distribution 3. Results and discussion
Given that the coal sample in this study is a low rank coal, it was expected that the coal surface was heavily oxidized. In the FTIR spectrum shown in Fig. 3, the high absorbance peak at about 3342 cm−1 for OH on the surface of subbituminous coal means that it has a high moisture content [24]. The absorbance at a peak of 1605 cm−1 for COOH and those between 1150 cm− 1 and 1380 cm− 1 for C\\O and O\\H in phenoxy groups are also high, indicating the extent of the oxygen-containing functional groups on the surface which contribute to its poor flotation performance in conventional tests [11,12,24]. The oxygen-containing functional groups on the subbituminous coal surface interact with water and generated a hydrophilic surface or very weakly hydrophobic at best. The peak at 1699 cm−1 represents unsaturated C_C which does not exist in bituminous coal, but does exist in low rank coal.
As shown in Fig. 4, the zeta potential characteristics of both subbituminous coal and silica were similar as indicated by negative values throughout the pH values studied and a decline in the absolute value with a decrease in the solution pH value. The coal surfaces had a zeta potential that was around 5 to 10 mV lower than the quartz particles over the range of pH values. The electronegativity of the coal surfaces for pH values over 3 because of the negatively charged sites provided by the oxygen-containing functional groups on the coal surface, such as OH and COOH [25]. The high surface potential of the coal particles results in a significant electrostatic repulsive force between the coal particles and air bubbles, which prevents or significantly slows the attachment process upon collision, thereby limiting combustible recovery. Electrokinetic potential of quartz is also negative in the same pH range as a result of the dissociation of\\Si\\OH on silica surface. Given the negative charge of the coal particles, a depressant is necessary when a cationic amine collector is used in reverse flotation [25].
Fig. 5. Conventional flotation results of subbituminous coal (raw coal and fine silica mixture) with diesel and SPP (feed ash 36.01%).
Fig. 7. Effect of dextrin dosage on reverse floatation of medium coal and fine silica mixture (collector 1 kg/t, feed ash 34.71%).
3.1. Result of FTIR
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Fig. 8. Effect of dextrin dosage on reverse floatation of fine coal and fine silica mixture (collector 1 kg/t, feed ash 38.06%).
3.3. Conventional flotation of subbituminous coal and quartz mixture
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Fig. 10. Effect of collector dosage on reverse floatation of coarse coal and fine silica mixture (dextrin 1 kg/t, feed ash 34.33%).
3.4. Reverse flotation of subbituminous coal with different size ranges mixed with fine silica
According to Fig. 5, the combustible matter recovery was increased by an elevation in the amount of collector. The trend was found to be true for both collector types. However, to achieve the relatively poor recovery performance, a very high amount of collector was required which is reflective of the low degree of surface hydrophobicity associated with the coal particles. The high collector dosages also lead to high ash content values in the concentrate, which was most likely due to hydraulic entrainment of the fine silica. SPP performed better than diesel at the same dosage as indicated by higher combustible recovery and lower product ash content values. Combustible recovery reached 72.40% when utilizing high collector dosages (10 kg/t SPP). At this dosage, the product ash content was very high (34.57%). The weak performance results obtained from the conventional flotation process clearly indicates poor floatability of the low rank coal which led to the consideration of using reverse flotation as a means of efficiently upgrading the low rank coal.
Fig. 6 shows the effect of the depressant dextrin on the performances achieved using the reverse flotation process when treating the coarse coal and fine silica mixture. With an increase in dextrin dosage, combustible recovery and the ash content of the concentrate fluctuate slightly. A dextrin dosage of 2 kg/t suppressed the floatability of the coal to a level that resulted in 82.84% of the combustible matter reporting to the concentrates of the process. The combustible matter remaining in the cell after 5 min of flotation contained 6.61% ash. Silica rejection remained relatively constant at around 88%. It indicates that dextrin in the pulp is enough to depress coarse coal flotation. Fig. 7 shows nearly the same trends when treating medium-sized coal and fine silica. Silica recovery to the float fraction was very high at 87% while recovery of the combustible material to the sink material was around 85%. These results show a very high level of selectivity and efficiency when using the reverse flotation process. In contrast to the findings obtained using coarse and medium size coal particles, the results obtained with a mixture of coal finer than 38 μm and fine silica did not show promise as shown in Fig. 8. Combustible recovery to the flotation underflow was only around 55.5% using a dextrin dosage of 4 kg/t and a collector dosage of 1 kg/t. The low
Fig. 9. Separation efficiency of reverse floatation for mixtures of coal with different particle sizes and fine silica (collector 1 kg/t).
Fig. 11. Effect of collector dosage on reverse floatation of medium coal and fine silica mixture (dextrin 1 kg/t, feed ash 34.71%).
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Fig. 12. Effect of collector dosage on reverse floatation of fine coal and fine silica mixture (dextrin 1 kg/t, feed ash 38.01%).
recovery values reported in Fig. 8 was not likely due to the ineffectiveness of the dextrin to suppress the coal flotation. Coal particles reported to the flotation froth product due to hydraulic entrainment caused by the relatively low coal density and small particle size in addition to the elevated impact of Brownian motion energy. A more interesting finding is that the presence of ultrafine coal particles suppressed the flotation of the fine quartz. As shown in the previous figures, quartz recovery values were above 85% with coarse and medium-sized coal. However, in the presence of ultrafine coal, quartz recovery was less than 50%. This may be due adsorption of a significant portion of the amine collector unto the negative charged and high-surface area coal particles thereby leaving an insufficient amount to properly coat the quartz particles. The results show that fine silica has different flotation behaviors when mixed with subbituminous coal of different particle size ranges. When mixed with coarse coal (150–200 μm) or medium coal (74– 120 μm), the highest silica recovery was 88.68% and 86.92%, respectively (with 1 kg/t dextrin and 1 kg/t collector), while the recovery decreases to 44.50% when mixed with fine subbituminous coal (− 38 μm). All other conditions were held constant. It indicates that the fine silica can float better when mixed with coarse and medium coal particles,
Fig. 13. Separation efficiency of reverse floatation for mixtures of coal with different particle sizes and fine silica (dextrin 1 kg/t).
Fig. 14. Effect of dextrin dosage on reverse floatation of raw coal and fine silica mixture (collector 1 kg/t, feed ash 36.01%).
while its floatability worsens in the presence of fine subbituminous coal most likely due to competitive adsorption of the amine collector. Separation efficiency of the reverse flotation tests of coal particles with different size ranges and fine silica was plotted in Fig. 9. Much higher separation efficiency can be achieved when dealing with a coarse or medium coal and fine quartz mixture than with a fine coal and fine quartz mixture. Figs. 10 and 11 shows the reverse flotation results of coarse coal and medium coal with fine silica mixture with different collector dosage. The combustible recovery in concentrate for both feeds decrease when dextrin dosage increases from 1 kg/t to 4 kg/t. It means that with the increase of dextrin, coal particles are also transferred to froth because of adsorption with collector. Silica recovery in tailing for both feeds increases as more silica particles are floated when more collector is used, which makes the ash content in concentrate decrease. For fine coal and fine silica mixture, the reverse flotation results with different collector dosage are illustrated in Fig. 12. It can be seen that the combustible recovery in concentrate is much lower than that of coarse coal or medium coal. With the increase of collector dosage, the combustible recovery falls markedly. It might because fine coals get entrained easily. The silica recovery increases with the increase of collector dosage, but it is much lower than that with coarse coal or medium coal at the same collector dosage. The ash content of concentrate is much higher.
Fig. 15. Effect of dextrin dosage on reverse floatation of raw coal and coarse silica mixture (collector 1 kg/t, feed ash 36.01%).
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Fig. 18. Effect of collector dosage on reverse floatation of raw coal and coarse silica mixture (dextrin 1 kg/t, feed ash 36.01%). Fig. 16. Separation efficiency of reverse floatation for mixtures of raw coal and silica with different particle sizes (collector 1 kg/t).
The separation efficiency of these reverse flotation tests is shown in Fig. 13. It can be concluded that the separation efficiency with coarse coal or fine coal is much higher than that with fine coal. The increase of collector dosage has larger effect on the separation efficiency with finer coal particles than coarser ones. 3.5. Reverse flotation of raw subbituminous coal mixed with fine silica or coarse silica Fig. 14 shows the effect of dextrin on the reverse flotation of raw coal mixed with fine silica. With the increase of dextrin dosage, the combustible recovery increases while the silica recovery remains at a constant level of around 80%. The ash content of the coal product decreased from 13.78% to 11.33% while the ash content of the tailings or froth product increases from 57.88% to 71.89% when the dextrin dosage was elevated from 1 kg/t to 4 kg/t. For raw coal and coarse silica mixture, the effect of dextrin dosage on the reverse flotation is shown in Fig. 15. When dextrin dosage increases, combustible recovery to the coal product improved significantly. However, the recovery of the coarse silica to the froth product was only about 20% which is likely due to the particle density and coarseness of the particles targeted for flotation.
Fig. 17. Effect of collector dosage on reverse floatation of raw coal and fine silica mixture (dextrin 1 kg/t, feed ash 36.01%).
With an increase in dextrin dosage, the ash content of the concentrate is much higher than that achieved with fine silica while that of tailing is lower which reflects a reduction in separation efficiency. The separation efficiency of raw coal and silica particles as a function of particle size is shown in Fig. 16. The separation efficiency increases with an increase in dextrin dosage which indicates a positive impact of dextrin on the separation efficiency achieved using the reverse flotation process. The reverse flotation results of raw coal and fine silica with different collector dosage are shown in Fig. 17. It indicates that the silica recovery in tailing increases with the increase of the collector dosage, which reaches 90.48% when the collector dosage is 4 kg/t. The combustible matter recovery is not as high as the flotation results of silica mixed with only coarse coal or medium coal, as there are also some fine coal particles that need more depressant and easily get entrained in the feed. The highest combustible matter recovery is 70.18% when the used amine is 2 kg/t but then it decreases when more collectors were added. It means that some coals were lost in the froth product by interaction with the amine collector. The ash content of concentrate reaches 7.56% with 4 kg/t collector. As shown in Fig. 18, the silica recovery in the concentrate increases from 19.99% to 41.82% with the increase of collector dosages, indicating
Fig. 19. Separation efficiency of reverse floatation for mixtures of raw coal and silica with different particle sizes (dextrin 1 kg/t).
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collector will be consumed. Fine silica has a better flotation performance than coarse silica. In general, mineral particles are always finer than coal particles in coal samples [7], so reverse flotation is a potential alternative method to upgrade difficult-to-float coals, such as oxidized coal and low rank coal. The solution chemistry of real coal pulp is more complex, so more research about reverse flotation for real coal is necessary.
References
Fig. 20. Separation efficiency of reverse floatation for mixtures of coal and silica with different particle sizes (dextrin 1 kg/t, collector 1 kg/t).
coarse silica does not respond well to the collector due to the larger mass. This is because the buoyancy of bubbles cannot overcome the weight of coarse silica particles. Combustible recovery decreases from 71.22% to 57.20% because of the interaction with the cationic collector, as the surface of subbituminous coal is electronegative at the neutral pH value. More dextrin is needed to reduce the loss of coal particles. As also can be seen from Fig. 19, the separation efficiency of reverse flotation for the mixture of raw coal and fine silica is much higher than that of raw coal and coarse silica. The separation efficiency indexes of all the reverse flotation tests are compared in Fig. 20. It indicates that the selectivity of reverse flotation for mixtures of coarse or medium coal and fine silica is much higher than the others when 1 kg/t dextrin and 1 kg/t collector were used. As such, reverse flotation can perform well for coal upgrading when the material consists of coarse or medium size coal and fine quartz but appears to fail when the particle size of the coal is mostly below 38 μm. 4. Conclusions Subbituminous coal is a low rank coal with high content of moisture and surface oxygen-containing functional groups such as hydroxyl and carboxyl. Conventional flotation cannot perform well on subbituminous coal effectively even with large doses of collectors. Good reverse flotation results can be achieved when the flotation feed is made up of coarse or medium subbituminous coal particles and fine quartz particles. Entrainment also plays an important role on reverse flotation performance. The presence of fine subbituminous coal particles deteriorates the flotation behavior of fine silica, and a large dosage of dextrin and
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