Review
Enhancement in hydrophobicity of low rank coal by surfactants —A critical overview
Shobhana Dey ⁎
Council of Scienti fic and Industrial Research-National Metallurgical Laboratory,Jamshedpur,India
a b s t r a c t
a r t i c l e i n f o Article history:
Received 30December 2010
Received in revised form 14September 2011Accepted 15October 2011
Available online 9December 2011Keywords:Low rank
Hydrophobicity Oxidation Wettability Surface charge
The flotation of fine (−0.5mm)low rank or oxidized coal is dif ficult to achieve with the common coal flota-tion collectors like kerosene,fuel oil or diesel oil (oily collector).The presence of small amounts of oxygen is enough to cause oxidation.The oxidation of coals starts with the physical adsorption of oxygen on the surface to form an oxycomplex followed by chemical adsorption of oxygen to form polar phenolic –OH,carbonyls,phenols and peroxide type oxygenated moieties by the rupture of cyclic rings.The addition of promoter,sur-factant or oxygenated functional groups to the collector molecule markedly enhances the flotation of lower rank and oxidized coals due to the hydrogen bonding with the polar part of the coal surface and the reagent.The performance of these reagents is compared with that of oily collectors,namely kerosene,dodecane,non-ylbenzene and polar part of the surfactant having an oxygen atom.The mode of addition of non-ionic surfac-tant with oily collector also has a major role in the flotation response.The addition of non-ionic surfactant after the oily collector has shown a positive effect on yield and grade.
©2011Elsevier B.V.All rights reserved.
Contents 1.Introduction ..............................................................1512.
Literature review............................................................1522.1.Forms of oxygen on coal surface .................................................1522.2.Generation of surface charge at coal/water interface ........................................1522.3.Wettability of oxidized coal ...................................................1532.4.Wettability of oxidized coal in presence of surfactants .......................................1532.5.Effect of coal particle –promoter interactions on flotation......................................1542.6.Flotation of oxidized coals ..........................
..........................1542.6.1.Mechanism of adsorption ................................................1552.6.2.Collector spreading ...................................................1552.6.3.Interaction of the oxygenated non-ionic part of collectors on coal surface ...........................1562.6.4.Structural effect of reagents on flotation response...........
..........................1563.Summary ...............................................................157References .......................................
..........................
157
1.Introduction
Coal flotation makes use of the natural hydrophobicity of the car-bonaceous matter in coal.To enhance the hydrophobicity of the coal particles,oily collectors,such as diesel oil and kerosene are usually added.For higher-rank coals,the reagent consumption in flotation is low because of the natural hydrophobicity of the coal.However,
the oxidized coals or low rank coals are dif ficult to float with com-monly used fuel oil or kerosene and large amounts of collectors are required to achieve satisfactory yields [1].The poor floatability is due to the presence of greater amounts of oxygen content and abun-dance of hydrophilic surface functional groups [2,3,4,5].It was reported by Quast and Readett [6]that sub-bituminous coals have an average oxygen content of 18%,with carboxylic groups constitut-ing about one third of this amount.Therefore,the amount of adhesion of oil droplets onto low rank coals is very small,and the use of oil alone does not improve flotation.
Fuel Processing Technology 94(2012)151–158
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There are many coal processing plants that are facing problems in floating oxidized coal.The source can be from outcrop coal,strip mines—where the overburden was removed several years before mining started or even from improper stockpiling over a period of time.Oxidation alters both physical and chemical properties of the coal surface and reduces thefloatability.The surface oxidation of coal is reflected by decrease in pH of the pulp.To solve this problem reagents containing promoter are recommended,along with large amount of fuels[7].
Infine coal,oxidation by weathering or coals kept at mine site or during storage and transportation results in the formation of oxygenat-ed functional groups.Carboxyl,phenolic and carbonyl are the most commonly found functionalities on the coal surface and their concen-tration can be determined[8].This reduces the hydrophobicity of the coal surface by increasing the number of sites that form hydrogen bonds with water molecules.It is generally accepted that oxidation of coal(whether it be exposed artificially or to the atmosphere)proceeds in three stages.Stage I is the superficial oxidation characterized by the formation of coal–oxygen complexes with acidic properties.In Stage II,the organic components of coal form alkaline soluble hydroxy car-boxylic acids called humic acids.In Stage III,the humic acids degrade to simple water-soluble acids[9].Adsorption of oxygen on the coal sur-face is exothermic and,besides the moieties formed on the coal surface, the reaction products like CO,CO2and H2O may be released from the structure[10].The most susceptible linkages to oxidation were found to be theα-CH2groups to polyaromatics[11,12,13,14].Mitchell et al.
[15]revealed an interesting point on oxidation that blue-light irradia-tion is also a strong agent to oxidize the vitrinite surfaces.Sarikaya et al.[16]observed that upon oxidation theflotation yield reduces from95%to24%for a bituminous coal using alcohol type frother only. The oxidation of the surface makes the coal more difficult tofloat with oily collectors[17–22].An oily collector cannot spread on the surface of the coal particles.The present paper highlights the study of oxidation of the coal surface,adsorption behavior of surfactants on oxidized or low rank coal,its effect onfloatability and improvement of their hydro-phobicity by adding reagents in different sequences.
2.Literature review
Coalflotation is a complex phenomenon involving several phases (particles,oil droplets and air bubbles).These phases then simulta-neously interact with each other and with other species viz,molecules of surfactants/promoting reagents and dissolved ions in water.The structure of surfactants/collector molecules,the type of electrical charge possessing the collector,its dispersion ability and the type of bonding(physisorption or chemisorption)that forms with the coal sur-face are all important factors for imparting hydrophobicity on the coal surface.Molecular orientations of the reagent molecules also play an important role on the adsorption process[23–25].Theflotation studies performed with low rank coals indicated that the mixtures of oils could be used to improveflotation recovery and selectivity[26].As it is known,oil emulsions are thermodynamically unstable due to increased interfacial area.Therefore,to stabilize the emulsions,mechanical energy or a surfactant is needed.Mechanical emulsification is usually stable only for a short period of time,while emulsions produced with surfac-tants are stable over a long period of time[27].It was shown that the emulsification of the kerosene produced by the high intensity stirring shaped droplets of about20μm in size,while emulsification with the addition of surfactants(anionic and cationic)reduced the droplet size to about1.5–2.0μm[28,29].This stabilization of the oil droplets using surfactants is due to the decrease in surface free energy with adsorption at the oil/water interface of the surfactants.
Surfactants,when added together with oil,are likely to improve coal flotation by several mechanisms.When added as emulsifiers,they aid dispersion of the oil intofine droplets[27].The increased numbers of oil droplets assistflotation kinetics by increasing the probability of coal particle–oil droplet collisions[30].Surfactants also lower the ener-gy required to spread the collector oil[31]across the coal surface through adsorption at the coal/water interface and coal/oil interface. At low concentrations,such surfactants make coal hydrophobic,where-as,at high concentration,they make it hydrophilic[32].If the surfactant promotes the spreading and adhesion of oil,the hydrophobicity of coal will increase,leading to a high probability of adhesion of the air bubble to the coal particle[31,33].In contrast,if the wetting of the coal by water is increased by the surfactant,which usually happens at high con-centrations,the recovery of the coal will decrease.
Chander et al.[34]carried outflotation of low rank coal of−150μm size with dodecane and it was shown that substantial amount of reagent was required to achieve the satisfactory yield.They also performed coal flotation with non-ionic and ionic surfactants.Some of the water soluble surfactants make coal more hydrophilic or wettable when present in sufficient quantities.In contrast,the same surfactants increase the hydrophobicity when present in small amount.Therefore,theflotation characteristics of coals largely depend on these surface properties.
2.1.Forms of oxygen on coal surface
Quast and Readett[6]reported the wettability of sub-bituminous coal surface by the presence of oxygen content of18%and the carboxylic groups constituting about one third of this amount.Considerable con-troversy exists regarding the forms of oxygen in coal and the means of analyzing for them.The values in Table1for carboxyl(–COOH)and hy-droxyl(–OH,both phenol and alcohol)are cited from Penn State Coal Data Bank and a variety of other sources[3,35–38].It was found that the oxygen functionalities also include ether(C–O–C),ketone(C=O) and methoxy(–OCH3)group in addition to–OH and–COOH.It was observed by Korobetskii et al.[39]that small amounts of residual oxygen are sufficient to bring about oxidation.Natural oxidation mainly affects the external surfaces of coal,hence,for betterflotation results the size reduction must be retarded as long as possible[40].Weathering of the coal particles developed cracks whose extent was a function of coal rank.Low rank coal particles developed extensive cracks whereas high rank coals hardly seem to be affected physically[41,42].
2.2.Generation of surface charge at coal/water interface
Since coal is a very heterogeneous substance,the mechanism of sur-face charge generation is very complex.Campbell and Sun[43]have proposed a simplified model based on the earlier work of Glembotskii [44].This model assumes that when a freshly crushed coal surface is ex-posed to the atmosphere,the exposed carbon atoms in the lattice react with atmospheric oxygen and form oxidized surfaces with charging characteristics similar to oxide minerals.In this case,hydroxyl and hy-dronium ions will be the potential determining ions.
Kelebek et al.[45]investigated the generation of charging mechanism for low rank lignitic coals would be different due to the large amounts of oxygen-containing functional groups.The charging mechanism for lignite was explained based on the presence of oxygen-containing weakly acidic groups(e.g.,phenolic OH and COO).Depending on the moisture content, the surfaces possess these groups in dehydrated(oxide form)or in hydrated forms.The surface charge can be determined mainly by the
Table1
Approximate content of various oxygen functional groups in coal(based on the data of Blom et al.[35]).
%C Total O C–O–Cðer–OH C=O–COOH–OCH3
9511----856232--75166711-
65a28107281
a A European brown coal.
152S.Dey/Fuel Processing Technology94(2012)151–158
degree of dissociation of the weakly acidic groups at the surface.The dis-sociation takes place until equilibrium is reached.
2.3.Wettability of oxidized coal
The wettability of coal,which is better described by the degree of oxidation,can also be determined by the electrokinetic properties and contact angle measurements [46].As the hydronium and hydroxyl ions are the potential determining ions for both oxidized and non-oxidized coals,the addition of hydroxyl ions (OH −)increases the nega-tive charges on the coal surface.Conversely,when the pH of the pulp decreases,the adsorption of hydronium (H 3O +)ions increases on the coal surface until the negative charge gets neutralized and as the acid groups increase,the surface becomes more positive.Hence,concentra-tion of the hydronium and hydroxyl ions present in the pulp not only changes the magnitude but also the sign.Sarikaya and Ozbayoglu [16]carried out the electrokinetic potential (Fig.1)and contact angle mea-surement (Fig.2)for correlating the flotation recovery on Turkish coal.The Iso-electric point (IEP)of un-oxidized coal was found to be 4.2.As the oxidation increases,IEP decreases and increases the negative value on the surface of coal.The relationship between the electrokinetic potential and surface charge developed during the oxidation for differ-ent length of time at 200°C are shown in Fig.3.When the cationic collectors are used,the IEP's of oxidized coal shift towards the more alkaline pH,i.e.in the range of 9.3to 10.9,depending upon the type and concentration of the collectors.As the oxidized coal have negatively charged surface and the number of hydrophobic sites presence on the surface decreases due to oxidation,it is assumed that the positive amine ions are adsorbed electrostatically on oxidized coal surfaces in a wide range.This shifts the IEP's of oxidized coal towards the higher pH values.Adsorption increases as the concentration of the amine increases.
Vamvuka and Agridiots [47]investigated the behavior of reverse flo-tation on dif ficult-to-float lignites.They found that the hydrophobicity of these coals can be increased either by reduction of the oxygen functional at the coal surface before the treatment [48],or by using appropriate reagents [34,49].Both ionic and non-ionic surfactants may alter the surface characteristics of these coals.Blending of hydrocarbons and a non-hydrocarbon collector,such as copolymers,long chain amines and fatty acid amides improve the floatability [2,50,51,52,54].
2.4.Wettability of oxidized coal in presence of surfactants
It was found that the presence of surfactant with oily collector improves the flotation of coal.The contact angle for un-oxidized coal in distilled water was found to be 59°.The oxidation of the coal deteriorated the natural floatability by reducing the contact angle from 59°to 7°and thus become more readily wetted by water after oxidation.Floatability of lignite was studied using kerosene along with different types of surfactants like cationic,anionic and non-ionic surfactants.The addition of suitable cationic collectors,like fatty alkyl propelene diamine (Flotigam,PA)or alkyl ether amine part of it neutralized by acetic acid (Flotigam,ENA),modi fies the sur-face charge and increases the contact angle.Even with the oxidation of coal,the contact angle increased up to 75°[55].Cebeci used Acorga M5640(alkyl hydroxyl aryl aldoxime)as an emulsi fier,Hosta flot LIP (sodium dialkyl-dithiophosphate)as anionic and Flotigol CS (mixture of crysylic acids)as non-ionic collectors during flotation studies.It was found from his study that the combination of both kerosene+emulsi fier and mixture of kerosene,emulsi fier,surfactant were stable (except anionic),and easily dispersible in water.It formed very
Z e t a P o t e n t i
a l , m v
pH
Fig.1.Effect of oxidation on zeta potential of coal oxidized at 200°C and at different pH (after [16]).
C o n t a c t a n g l e d e g r e e , θ
Oxidation time, hours
Fig.2.Effect of 200°C oxidation on the contact angle of coal in distilled water and in the presence of 15mg I-1Flotigam PA at pH 7.4(after [16]).
pH
Z e t a P o t e n t i a l ,m v
R e c o v e r y (W t %), C o n t a c t a n g l e , θ
Fig.3.Correlation of the electrokinetic potential and contact angle of oxidized coal and its flotation response in the presence of Flotigam PA (after [16]).
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S.Dey /Fuel Processing Technology 94(2012)151–158
small oil droplets compared to the kerosene.It was envisaged that all of the collectors except kerosene investigated in this study increased thefloatability of lignite.However,when the combustible recovery andflotation efficiency index were considered,the best results were obtained from the combination of both kerosene–emulsifier and kerosene–emulsifier–Flotigol CS(non-ionic).It was expected due to the high binding and spreading tendency of both collectors over the coal surface.The coal concentrates having relatively low ash content were obtained with the kerosene+emulsifier+non-ionic surfactant. It was further observed that the oxidized coal surface became hydro-phobic after adding Flotigam PA(fatty alkyl propelene diamine),even when the oxidation is extensive.It has been shown that as the contact angle increases,theflotation recoveries also increase.The order of ad-dition of non-ionic surfactant with oily collector plays an important role on theflotation response.Chander et al.[34]observed that the ex-tent of adsorption on coal was expected to vary with dodecane.If the surfactants are added before any collector,it would adsorb on the coal surface,and not be available for emulsification.Addition of surfac-tant before the dodecane lowered the yield.This was attributed to the adsorption of the surfactant molecules on the surface of coal,making the coal hydrophilic.Therefore,the adsorbed surfactants are no longer available for emulsification.Ash rejection was also found to be poor when the surfactant was added prior to the addition of oily collector. They also compared theflotation results of the anionic surfactant with the oily collector.It was found that the yields were very similar for both mode of surfactant addition.The reason for this was attributed to a vast change in the froth characteristics due to this surfactant's well known foaming tendency.The anionic surfactant might have rapidly migrated into the aqueous phase.
Jia et al.[56]performedflotation of low rank/oxidized coal with dodecane,nonyl benzene and nonyl phenol(Table2).Nonyl benzene was found to be a betterflotation collector than dodecane for two coals,indicating strong interaction of the benzene ring with aromatic sites on the coal surface.This is due to strong p-bonding between the aromatic component of the coal matrix and the benzene ring of the reagent[22,57].
2.5.Effect of coal particle–promoter interactions onflotation
Promoters act as surface modifiers and may alter hydrophobicity depending on its concentration and the rank of coal[17,34,41,42,47,58-66].A change in the surface properties of the coal particles largely affects their attachment and detachment characteristics with other dispersed phases inflotation pulp.In theflotation of low rank or oxidized coals with highly negative surfaces in the pH range of3–5,and the use of cat-ionic promoters,enhance theflotation performance[47,67-69].Busta-mante and Woods[70]found that adsorption of dodecylammonium on non-polar parts of the coal surface decreased its hydrophobicity,while the adsorption on the mineral matter caused an increase in hydrophobic-ity.With weathered coal,where both the carbonaceous and the mineral matter were extensively negatively charged,dodecylammonium adsorbed with the polar group interacting with the surface and therefore all types of composite grains became hydrophobic.Non-ionic surfactants (NIS)and water-soluble polymers are found to modify the coal surface.Li et al.[71]used a comb-like polymer and found that the coal became more hydrophobic with increasing promoter concentration regardless of its originalfloatability.The triblock poly(ethylene oxide)(PEO)and poly(propylene oxide)(PPO)based systems,PEO-PPO-PEO were also found to improve coalflotation and the mechanism of polymer action was a function of the coal rank[18,41,42,64-66,72].They have amphi-philic characteristics and self-assemble into micelles to form a variety of close packed structures.By varying the block composition(PEO/PPO ratio)and the molecular weight,it is possible to tailor thefinal properties of these systems to meet the specific application needs.These reagents had double effect onflotation.They modified the coal surface and also improved the emulsification of the oily collector.For high rank coals, which usually require relatively small amounts of the collector,the sur-face modifier function of the polymers was dominant over their emulsi-fier function.It is generally accepted that the collector disperses into droplets in the pulp and these droplets collide with,adhere to and spread on the coal particles to render them more hydrophobic.Size distribution of oil droplets depends on dispersion and coalescence sub-processes, which were determined by the intensity of mechanical agitation and presence of promoters andfine solid particles in the system[42,66,73]. Polat et al.[66]investigated the effect of the addition of the PEO/PPO tri-block copolymeric promoters on the dispersion kinetics of oil(dode-cane).Addition of promoter reduced the size of median oil droplets sig-nificantly and the extent of this reduction was a strong function of promoter concentration,as can be seen in Fig.4.
2.6.Flotation of oxidized coals
Most coals are susceptible to weathering or oxidation.With oxida-tion,the number of oxygenated functional groups increases,thus making the coal surface hydrophilic and less amenable toflotation [17,20,74].Cebeci[55]studied thefloatability of Yozgat Ayridam lignite using kerosene and mixture of kerosene,emulsifier and surfactant
C
HO C
H O)4O C
O CH2O
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L-64 Conc., M
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110100
Time, minutes
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Fig.4.Kinetics of emulsification of n-dodecane(0.1%by volume)at various concentra-
tions of Pluronic acid L-64,a PEO/PPO/PEO tri block polymer(after[52]).
154S.Dey/Fuel Processing Technology94(2012)151–158
(both cationic and anionic).It was shown that a mixture of kerosene and an emulsifier,and kerosene,emulsifier and surfactant is easily dispersible in water and produce stable emulsions with smaller oil droplets,when compared to kerosene alone.The effectiveness of the surfactants forflotation of oxidized or sub-bituminous coals was also studied by many researches.The performance of non-ionic surfactant (NIS)is found to be more effective than the cationic(CS)/anionic surfac-tant(AS).The non-ionic surfactants possess effective hydrogen bonding at one end of the alkyl chain which confers on them the required water-solubility.The interaction of the oily collector in the presence of a cationic surfactant that had a polar nature with the coal surface may be due to hydrogen bonding and the electrostatic attraction with the negatively charged coal surface.This type of adhesion would result in a tendency for a residual waterfilm to be retained between the coal surface and oil.Therefore,the coal surface would be to a lesser extent, covered with the oil as obtained from NIS.Similarly,the presence of an-ionic collector with oily collector was due to the hydrogen bonding as a result of their polar nature.However,the electrostatic repulsion between the negatively charged oil droplets and coal surface was also indicated.Therefore,the surface of coal would be,to a lesser extent, covered with kerosene.It was also possible that the residual water film remained between the coal surface and kerosene droplets.As a result of the combining of these two effects,the yields were lower than those obtained from NIS[55].This is attributed to the pore pene-tration phenomenon with short chain oils and to insufficient spreading of long chain hydrocarbon oils due to viscosity[33].Theflotation of this type of coal was carried out also by using derivatives of tetrahydrofuran (THF)as collector[56].The oxidized coal possesses large number of hy-drophilic oxygenated functional groups and it is expected that the THF series might be even more effective asflotation collectors due to the hydrogen bonding between the reagents and the coal surface.This hypothesis was tested by Jia et al.[56]employing two types of coals, i.e.Illinois No.6and Pittsburgh No.8.These two coals were oxidized andflotation tests were carried out using dodecane and some of the THF surfactants as collectors under two fairly similar laboratory condi-tions.Fig.5(A)and Fig(B)present plots of the combustible matter re-covery as a function of collector dosage for theflotation of lab-
oxidized Illinois No.6coal and Pittsburgh No.8respectively.From Fig.5,it is found that recovery of combustible matter is very low when dodecane was used as the collector,but with THF collectors a much lower dosage was required for the same combustible matter.It appears that a higher dosage of dodecane is required tofloat the oxidized coal,whereas a lower dosage of THF-11is required for the flotation of oxidized coal in comparison to theflotation of as-received coal.These results indicate that the surfactants used have the ability to restore thefloatability of oxidized coal.Harris et al.[22]showed that the tetrahydrofurfuryl butyrate designated(THF-3)is an effective collector forflotation of bituminous coal.These authors indicated that the THF series of reagents might be effective alternatives as collectors in theflotation of both low rank and oxidized coal.Jia et al.[56]investi-gated the efficacy of the collecting ability of THF compounds having various hydrocarbon chain configurations to function as collectors for theflotation of un-oxidized coals,laboratory-oxidized coals,and natu-rally weathered coals(see Fig.5).The results show that THF-17en is more effective than the other collectors for both coals.The plots also show that surfactants with an oxygenated group are generally more effective than those without one and that nonylbenzene is more effec-tive than dodecane.This indicates that there might be two mechanisms for the interaction between the surfactants and the coal surface.
2.6.1.Mechanism of adsorption
It is known that surface of coal consists of inherently hydrophobic areas and also sites containing oxygenated moieties viz.as carboxyl, carbonyl,phenolic and ester groups[75,76].Thefirst mechanism of in-teraction between the surfactants and the coal surface appears to be through the polar groups of the reagent interacting with the oxygenated functional groups on the coal surface by hydrogen bonding. The second mechanism involves the interaction of the non-polar chain with the carbonaceous sites on the coal surface by dispersing water molecules from the coal surfaces.The interaction between an aliphatic chain and the coal surface is less pronounced than that of a benzene ring with the aromatic sites on the coal surface.This may be due to strong p-bonding interaction between the aromatic component of the coal matrix and the benzene ring of the reagent[22,57].Reagents con-taining a benzene ring showed better collecting ability compared to dodecane.This indicates that although the benzene ring has a stronger interaction with the aromatic coal surface than an aliphatic chain,the interaction is not as strong as that between the polar group of the re-agents and the coal surface.This also suggests that hydrogen bonding of the oxygenated groups in the reagents is stronger than the van der Waal interaction of the aliphatic chains with the carbonaceous portions of the surface.This explains why the reagents containing oxygen func-tional groups are much better collectors than hydrocarbon oil,such as dodecane,for theflotation of bituminous coals(Fig.6).It was expected that THF series would adsorb onto the oxygenated surface sites on the coal through hydrogen bonding,hydrophobic bonding of the aliphatic hydrocarbon chain with the hydrophobic sites on the coal surface and p-bonding of the benzene ring on the hydrocarbon chain of the collector with aromatic sites on the coal surface and thus making the subsequent spreading of dodecane on the coal surface more efficient.
2.6.2.Collector spreading
Generally coalflotation studies are carried out using oily collectors. The insoluble oils used as collectors for theflotation of coal must spread over the coal surface for effectiveflotation.If the coal is somewhat Illinois No 6
MIBC 0.52Kg/t
C
o
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GH4
Nonyl Phenol
THF-17en
Nonyl Benzene
Dodecane
GH4
Nonyl Phenol
THF-17en
Nonyl Benzene
Dodecane
Pittsburg No. 8 a
b
parison of the collecting ability of the nonionic THF-17en with that of nonyl-phenol,GH4,nonylbenzene,and dodecane for theflotation of−74μm Illinois No.6(A) and Pittsburgh No.8coals(B)(after[56]).
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S.Dey/Fuel Processing Technology94(2012)151–158。
