coal reverse flotation. part ii. batch flotation tests

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Coal Reverse Flotation. Part II.Batch Flotation TestsMarek Pawlik a & Janusz S. Laskowski aa Department of Mining Engineering, The Universityof British Columbia, Vancouver, British Columbia,CanadaVersion of record first published: 15 Sep 2010.

To cite this article: Marek Pawlik & Janusz S. Laskowski (2003): Coal ReverseFlotation. Part II. Batch Flotation Tests, Coal Preparation, 23:3, 113-127

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Coal Reverse Flotation. Part II. Batch

Flotation Tests

Marek Pawlik and Janusz S. LaskowskiThe University of British Columbia,Department of Mining Engineering,Vancouver, British Columbia, Canada

Coal reverse flotation, a process in which coal mineral matter is floatedinstead of the organic matter, was studied through a series of batchflotation tests on artificial coal=silica mixtures using two coals varying inrank. Dodecyltrimethyl ammonium bromide (DTAB) was utilized as amineral matter collector, while humic acids (sodium salt) were added asa coal depressant.

It was shown that the separation of silica from coal by reverse flotationis a kinetic process in which silica floats first followed by clean coal. Theselective recovery of silica was possible only in a very narrow range ofDTAB dosages and the amount of DTAB needed for best selectivity wasa function of coal rank. A lower rank coal required much higher aminedoses than a bituminous coal (depressed with humic acids), which wasattributed to the higher amine adsorption on the more hydrophilic coal.

Keywords Coal flotation; Reverse flotation; DTAB; Humic acids;Surfactant adsorption

INTRODUCTION

The concept of coal reverse flotation is not entirely new. So far, theprocess has been tested to desulfurize coal [1–5]. A range of natural (dext-rins, starches) and synthetic (carboxymethyl cellulose) polysaccharides

Received 1 September 2002; accepted 15 February 2003.

Address correspondence to Janusz S. Laskowski, Department of Mining, University of

British Columbia, Vancouver, B.C. V6T 1Z4, Canada. E-mail: [email protected]

Coal Preparation, 23: 113–127, 2003

Copyright # Taylor & Francis Inc.

ISSN: 0734–9343 print

DOI: 10.1080/07349340390197542

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have been used as coal depressants. The separation of inorganic sulfur(typically in the form of pyrite) from depressed coal is usually achievedwith the use of xanthate collectors.

Laskowski et al.[6] and Liu and Laskowski [7] discussed the effect ofhumic acids on coal flotation. In the presence of humic acids the zetapotential values became more negative and almost complete coal depres-sion was observed, although staged reagent addition and manipulation ofpH improved the flotation recovery. Similar results were obtained by Laiet al. [8] who found that humic acids, even at low concentrations(� 10 ppm), decreased the flotation recovery and that the adjustment ofpH to more alkaline values improved the flotation response of the testedcoals. Consistent with these observations are the findings of Laskowskiand Yu [9], who showed that the depressing effect of humic acids was mostpronounced at low pH values, most likely due to the precipitation andincreased adsorption of the free humic acids. Other researchers alsoreported the depressant action of humic acids in the flotation of otherhydrophobic minerals such as graphite [10] and molybdenite [11].

Humic acids can be conveniently used as a model substance to modifycoal surface properties since their content in coal is a function of coalrank [12, 13]. This, combined with the fact that humic substances depressthe floatability of naturally hydrophobic minerals, makes them suitablefor a fundamental study on coal reverse flotation.

Since humic acids were selected in this project as coal depressantsalong with dodecyltrimethyl ammonium bromide used as a collector, thefirst part of this paper [14] dealt with adsorption of those compounds ontwo coals of different rank and silica. The objective was to study themechanism of interactions of these two compounds with coal and silica.It was shown that the adsorption mechanism of DTAB on a bituminouscoal surface involves hydrophobic interactions. In contrast, a hydro-philic=oxidized coal adsorbs DTAB through interactions between theanionic surface acidic groups and the cationic amine head group. DTABadsorption on silica proceeds through weak electrostatic attractionbetween the negatively charged sites on the silica surface and the cationicamine group of the surfactant. This leads to a different effect of DTABadsorption on the wettability of the studied coals and silica.

In this part, separation of silica from artificial coal-silica mixtures byreverse flotation with the use of the same reagents is tested.

EXPERIMENTAL

Materials

Subbituminous (LS43), bituminous (F4), and oxidized bituminouscoals were used throughout the work. Their proximate analyses, particle

114 M. Pawlik and J. S. Laskowski

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size distributions, and other characteristics were given in the first part ofthis paper [14].

As in the work of Stonestreet and Franzidis on coal reverse flotation[15–17], fine silica (SilcoSil 395, Ottawa Silica Company) was used as amodel mineral component of coal to prepare artificial coal=silica mix-tures for reverse flotation tests. The mixtures had an ash content of 40%.

Dodecyltrimethyl ammonium bromide (DTAB) was obtained fromAcros Organics. The sample was 99% pure (as reported by the manu-facturer), and was used without any further purification. The sodium saltof humic acids (HA) of technical grade was provided by Aldrich Che-mical Company. Typically, 1 g=L stock solutions in tap water were pre-pared daily (pH ¼ 9–9.1). Methyl-isobutyl carbinol (MIBC) was alsoused in some flotation tests as a frother. This reagent was added as a1 g=L stock solution in tap water.

Batch Flotation Tests

All flotation experiments were carried out at room temperature(�23�C) in a standard Denver laboratory flotation machine. A modified2-L flotation cell, designed by Roberts et al. [18], was used throughout.The froth was removed by means of a scraper designed to cover the fullwidth of the cell at a fixed depth. The size (or the reach) of the scraperwas such that its bottom edge would just touch the pulp surface once thecell was filled.

Another modification was the addition of a side water reservoir forconstant volume control. This was done by connecting the side vessel,using PVC tubing, through a valve at the bottom of the cell. The vesselcould be freely moved up or down along a laboratory burette stand tomaintain a constant pulp volume in the cell. During a test, the reservoirwould be constantly refilled with water directly from a tap.

In all tests, the feed weight was 200 g, the pulp volume was approxi-mately 2 L, and the impeller’s speed was 1100 rpm, which produced anaeration rate of 2L=min. All experiments were carried out in tap water.The pH of the pulp was also measured but was not adjusted to reflectnatural processing conditions.

Flotation commenced after a total of 30min of pulp conditioning inthe cell (15min of ‘‘wetting’’ with tap water plus an additional 15min ofconditioning after adding HA, DTAB, or MIBC) and the froth wasscraped manually every 5 s. The cumulative concentrates were collectedafter total flotation times of 30 s, 1min, 2min, and 5min (i.e., afterexactly 60 ‘‘scrapings’’) to investigate the kinetics of flotation. The solidsremaining in the pulp were treated as tailings. The wet concentrates, andthe tailings slurry, were directly collected into metal pans, dried overnightin a convection oven at 100–110�C, weighed, and sent for ash assays.

Coal Reverse Flotation. Part II 115

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RESULTS AND DISCUSSION

As expected, the ‘‘forward’’ flotation response of the high rank F4 coalto MIBC as a frother is very good, while the flotation yield of the sub-bituminous LS43 coal is very low (Figure 1).

It should be noted that the natural floatability of the F4 coal (noMIBC) is high, as indicated by a relatively high flotation yield of 33%achieved without the aid of a frother. In fact, the yield for the F4 coal wasover 60% after 10min of such frothless flotation.

However, this good natural floatability of the F4 coal can be depressedwith humic acids, as shown in Figure 2. It can also be seen that at higherdosages of HA increasing amounts of the depressant start appearing inthe pulp. Since a quaternary amine was to be used as a collector for silica,a HA dosage of 400 g=t was selected in order to avoid excessive inter-actions between the free anionic HA and the cationic amine in the pulp.

Figure 3 presents the flotation response of silica and all the coals toDTAB at their natural pH values. During the flotation tests with the F4-oxidized coal, it was observed that the pH of the flotation pulp decreased

FIGURE 1. Flotation of LS43 and F4 coals with MIBC as a frother.


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from about 5.8 to 3.8, which was attributed to the presence of acidicoxygen functional groups on the coal surface [14]. The slightly alkalinepH of the LS43 coal flotation pulps was most likely due to a rather highcarbonate content in the coal (2.1% CaCO3 plus 1.0% MgCO3).

It is quite obvious from Figure 3 that the flotation of the F4 coal is notdepressed until DTAB dosages approach the critical micelle concentra-tion (cmc). At the same time, the LS43 coal floats very poorly up to adosage of about 1.5 kg=t. The LS43 exhibits only a narrow floatabilitywindow at very high DTAB doses, but its flotation declines again nearthe cmc of DTAB. The addition of 400 g=t of humic acids to the F4 coalshifts the yield curve towards the flotation response of the LS43 coal.It was demonstrated in our earlier work [19] that in the presence of humicacids, the surface of a bituminous coal ‘‘looks’’ like the surface of a lowrank=oxidized coal—the bituminous coal surface becomes hydrophilicand negatively charged as a result of the adsorption of these highlyanionic macromolecules.

As expected, silica floats rather easily with DTAB. As discussed inthe first part [14], despite a low adsorption density of DTAB at thesilica=water interface, the hydrophobicity of the silica surface is veryhigh.

FIGURE 2. Flotation of F4 bituminous coal in the presence of humic acids (noMIBC).


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The flotation of the F4-oxidized coal with DTAB appears to followthe flotation response of the F4 coal depressed with humic acids. Itshould be noted, however, that the pH of the flotation pulps containingthe F4-oxidized coal was quite acidic (3.7–4.0), and it can be found fromFigure 4 that the flotation yield of the oxidized coal decreases withincreasing pH towards neutral values. Our adsorption studies [14]revealed that both the LS43 and F4-oxidized coals completely sorbedDTAB from solutions below an initial DTAB concentration of 150mg=Lat neutral pH values. It is noteworthy that this ‘‘limiting’’ DTAB con-centration of 150mg=L in the adsorption tests corresponds to a DTABdosage of 1500 g=t in the flotation tests. As can be seen from Figure 3, theflotation of the LS43 coal is very poor up to a dosage of approximately

FIGURE 3. Flotation of F4 coal, LS43 coal, and silica with the use of DTAB.

Flotation of F4-oxidized coal and F4 coal in the presence of 400 g=t of humicacids is also shown.


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1500 g=t. Only at higher dosages is there some amount of free DTABin the pulp whose presence also manifests itself by improved frothing.It also appears that this narrow window of good LS43 floatability at highDTAB dosages matches the range of the enhanced hydrophobicity of thecoal’s surface [14]. As Table 1 shows, the LS43 coal is indeed mildlyhydrophobic after conditioning with 1500 g=t of DTAB (complete DTABadsorption) at the onset of flotation, as evidenced by significantly higherflotation yields with MIBC as a frother. These results are in line withthe findings of Sun [20] and Wen and Sun [21], who reported thatamines can act as collectors in the flotation of low rank=oxidized coals.

FIGURE 4. Effect of pH on the flotation of F4 and F4-oxidized coals at a DTABdosage of 1500 g=t.

TABLE 1 Flotation of LS43 coal in the presence of DTAB and MIBC

LS43 only LS43þ 1500 g=t of DTAB

MIBC dosage [g=t] 100 200 100 200Flotation yield (2min) [%] 3.7 7.7 23.9 33.5


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It is noteworthy that these ‘‘improved’’ flotation yields in Table 1 are notparticularly high, indicating that DTAB is actually a weak collector foroxidized=low rank coals at neutral pH values.

It is important to observe that the flotation of fresh F4 coal withDTAB does not depend on pH, as opposed to the flotation of the oxi-dized sample (Figure 4). This reinforces the notion that DTAB interactswith a bituminous coal through hydrophobic, charge-independent forcesbetween the hydrocarbon chain and the hydrophobic surface. Such anattachment would cause amine molecules either to assume a flat orien-tation at the coal surface or to assume the orientation with the hydro-philic head groups pointing away from the surface and rendering thesurface hydrophilic. As our results demonstrate, DTAB alone is unable todepress the floatability of the F4 bituminous coal unless extremely highamine doses are utilized. At such high amine concentrations approachingthe cmc entire micelles adsorb on the solid surface, effectively increasingthe hydrophilicity of the surface. Our results agree well with the con-clusions of Elton [22] who postulated that it is very likely that a layer ofweakly adsorbed surfactant molecules, oriented tail-to-surface, will bestripped off together with the receding liquid from a strongly hydro-phobic surface as a result of collisions with air bubbles, thus exposing thenaturally hydrophobic surface and facilitating subsequent particle-bubbleattachments.

The separation of silica from coal by reverse flotation is a kineticprocess. As our flotation results show (Figures 5–7), only the first con-centrates contain significant quantities of silica, and this initial selectivestage is followed by the dilution of the concentrate with clean coal.Obviously, the addition of humic acids depresses the F4 coal quitestrongly and hence improves the selectivity of the process. Without humicacids, the separation of silica from the coal is rather poor.

A low rank coal does not require a depressant for reverse flotation.Interestingly, the trajectory of the yield curve for LS43=silica mixtures(Figure 8) follows very closely the yield curve for the LS43 coal (Figure 3).Significant amounts of solids in both cases can be floated only when theDTAB dosage is higher than about 1.5 kg=t. As in the case of theLS43=DTAB system, the poor flotation of LS43=silica mixtures wasaccompanied by the lack of frothing, suggesting absence of free amine inthe pulp. Since DTAB adsorption on silica is very low compared toadsorption on the LS43 coal [14], it may be concluded that the amount ofresidual amine in solution is determined by the magnitude of amineadsorption on the coal.

Because the adsorption of DTAB on the LS43 coal is so extensive thatthe coal is capable of completely adsorbing DTAB from solution, it canbe deduced that as increasing quantities of DTAB are introduced into thepulp, the amine preferentially adsorbs on the LS43 coal and not on silica.Only when the coal surface is saturated with DTAB can the free amine


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molecules adsorb on silica. Since even low amine adsorption on silicarenders its surface highly hydrophobic, it is silica that selectively floats atthe onset of flotation. In the presence of DTAB, silica is always morehydrophobic than the LS43 coal as can be seen from the contact angleresults [14]. The free amine molecules are also needed to provide mod-erate frothing.

FIGURE 5. Flotation kinetics of F4=silica mixtures at varying dosages of DTAB,pH¼ 7.0–7.2, no humic acids, and feed ash content¼ 40%.


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One aspect of the behavior of humic acids and DTAB deserves anadditional comment. It was concluded in the first part of this study thatboth the anionic HA and the cationic DTAB adsorb at the hydrophobiccoal=water interface in a qualitatively similar manner. As the flotationresults show, however, only humic acids can act as coal depressants.

FIGURE 6. Flotation kinetics of F4 (400 g=t of humic acids)=silica mixtures atvarying dosages of DTAB, pH¼ 7.1–7.3, and feed ash content¼ 40%.


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Humic acids are strongly hydrated molecules, as are other coal depres-sants such as dextrin. Even low concentrations of these hydrophilicmacromolecules on a hydrophobic coal surface can stabilize the surfacewater film and render the surface hydrophilic. In contrast, DTAB isquite hydrophobic due to the presence of the long hydrocarbon chain.

FIGURE 7. Flotation kinetics of LS43=silica mixtures at varying dosages ofDTAB, pH¼ 8.3–8.6, no humic acids, and feed ash content¼ 40%.


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Its adsorption at a hydrophobic coal=water interface does not, therefore,affect coal floatability so dramatically.

As our surface tension and adsorption data show [14], DTAB sharplyreduces the water surface tension which, according to the Gibbs equation,is equivalent to a high adsorption density at the air=water interface. Theadsorption of DTAB at the air=water interface appears to be only slightly

FIGURE 8. Yields and ash contents of concentrates after 2min. of flotation forthe tested coal=silica mixtures at varying DTAB dosages.


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higher than the adsorption density at the F4 coal=water interface. Thetendency to adsorb on the surfaces of air bubbles gives DTAB its frothingcapabilities.

In contrast, the effect of humic acids on the surface tension of water isvery weak, indicating that humic acids adsorption at the air=waterinterface is very low. The coal=water interface appears to be the primaryadsorption ‘‘area’’ for humic acids. Even upon introduction of air bub-bles to the pulp, these strongly hydrated macromolecules remain on thecoal surface.

It is also clear that the more hydrophilic the coal, the higher dosagesof amine are needed not only to initiate reverse flotation but also toimprove the separation efficiency. This trend can be attributed to thestrong adsorption of DTAB onto such hydrophilic low rank=oxidizedcoals.

CONCLUSIONS

Contrary to some literature reports, it was shown in this work thatdodecyltrimethyl ammonium bromide cannot be simultaneously usedas a coal depressant and silica activator (collector). DTAB is unableto depress the flotation of hydrophobic coals, such as the F4.Although our earlier contact angle measurements show that thebituminous coal surface becomes gradually less hydrophobic in thepresence of DTAB, the resulting reduction in hydrophobicity is notsufficient to depress coal flotation over a wide range of DTABdosages. Only near the critical micelle concentration of DTAB can theadsorbing micelles finally render the coal surface hydrophilic. At lowerconcentrations, the role of DTAB in the flotation of the F4 bitumi-nous coal appears to be simply that of a frother. High rank coalsrequire a depressant in reverse flotation to improve the separationefficiency.

The separation of silica from low rank coals, such as the LS43 coal (orcoals depressed with humic acids), is a kinetic process occurring in a verynarrow range of DTAB dosages. In a low rank coal=silica mixture,DTAB preferentially and completely adsorbs on the coal, leaving no freeamine to activate silica. Due to the mechanism of amine adsorption onsuch coals, the appearance of residual amine for the activation of silicatakes place when the coal is already weakly hydrophobic. Since both thecoal and silica can separately float under these conditions, it is the morehydrophobic component that selectively floats from a mixture. However,at a too-high amine concentration frothing is too intense, coal becomeseven more floatable, and the selectivity of reverse flotation dropsdramatically.


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ACKNOWLEDGMENTS

Funding for this project was provided by the Natural Sciences andEngineering Research Council of Canada through its Strategic Grant.

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coal reverse flotation. part ii. batch flotation tests

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