The interaction between diesel and surfactant Triton X-100 and their adsorption on coal surfaces with different degrees of oxidation

Accepted Manuscript The interaction between diesel and surfactant Triton X-100 and their adsorption on coal surfaces with different degrees of oxidation

Ziyong Chang, Xumeng Chen, Yongjun Peng PII: DOI: Reference:

S0032-5910(18)30893-3 doi:10.1016/j.powtec.2018.10.047 PTEC 13822

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Powder Technology

Received date: Revised date: Accepted date:

19 August 2018 20 October 2018 23 October 2018

Please cite this article as: Ziyong Chang, Xumeng Chen, Yongjun Peng , The interaction between diesel and surfactant Triton X-100 and their adsorption on coal surfaces with different degrees of oxidation. Ptec (2018), doi:10.1016/j.powtec.2018.10.047

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ACCEPTED MANUSCRIPT The interaction between diesel and surfactant Triton X-100 and their adsorption on coal surfaces with different degrees of oxidation Ziyong Chang, Xumeng Chen, Yongjun Peng* School of Chemical Engineering, The University of Queensland, St. Lucia, Brisbane, QLD 4072, Australia.

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*Corresponding author. Tel.: +61 7 3365 7156; fax: +61 7 3365 4199.

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E-mail address: [email protected] (Y Peng)

ACCEPTED MANUSCRIPT Abstract Poor flotation of oxidised coals is a major challenge confronting coal preparation plants worldwide. Despite a number of studies to improve the flotation of oxidised coals using surfactants to adsorb on oxidised coal surfaces, the industry continues to use diesel as the dominant collector to float oxidised coals with a low efficiency. Following the previous study employing a composite collector

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consisting of diesel and surfactant Triton X-100 to improve the surface hydrophobicity of oxidised coals with diesel targeting un-oxidised surface areas while Triton X-100 targeting oxidised surface

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areas [1], the current study investigated the synergistic interaction between Triton X-100 and diesel

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and their adsorption on coal surfaces with different degrees of oxidation in flotation. It was found that the superior performance of the composite collector in the flotation of oxidised coals was attributed to

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the emulsification of diesel by Triton X-100 and the adsorption of Triton X-100 on oxidised surface areas. Triton X-100 molecules which adsorbed at the oil/water interfaces emulsified the diesel,

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significantly decreasing the size of diesel droplets and facilitating the adsorption of diesel on unoxidised surface areas. On oxidised surface areas, Triton X-100 adsorbed through a “head-on”

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adsorption, rendering the hydrophilic surface hydrophobic.

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Keywords: Coal oxidation; Adsorption; Surfactant; Diesel; Flotation

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1. Introduction In coal flotation, diesel is often used as an oil collector to increase the hydrophobicity of coals so that the hydrophobic coal particles can attach to air bubbles to be recovered to the concentrate. The adsorption of diesel on coal surfaces is through the hydrophobic attraction between hydrocarbons and

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the carbonaceous matters on coal surfaces [2]. Normally, coal particles are of high hydrophobicity and

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easy to be floated with diesel as a collector. However, diesel is not efficient in the flotation of oxidised coals. The oxidation of coals generates oxygen-containing groups which prevent the

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adsorption of diesel on coal surfaces [1, 3-6]. In the previous study, we used X-ray photoelectron

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spectroscopy (XPS) to determine the degree of coal surface oxidation as the atomic percentage (at.%) of oxidised carbon on coal surfaces [1]. We found that coal flotation decreased with an increase in the

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degree of coal surface oxidation until a critical degree of coal surface oxidation, above which the true flotation of coals using diesel was impossible [1]. Wang et al. [7] studied the flotation behaviour of two coal samples obtained from an Australian coal preparation plant with 19.0 at.% and 11.0 at.%

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oxidation was much worse.

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surface oxidation and also found that the flotation of coal sample with a higher degree of surface

To improve the flotation of oxidised coals, some amphiphilic surfactants consisting of hydrophilic

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polar groups and hydrophobic hydrocarbon chains have been used as polar collectors to enhance coal surface hydrophobicity through their adsorption on oxidised surfaces [8-11]. For example, Qu et al. [9]

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used non-ionic surfactant 2-ethylhexanol in the flotation of oxidised coal with 42.68% increase in combustible recovery. Wang et al. [10] reported that the sliding time between oxidised coal particles and air bubbles was significantly reduced in 2-ethylhexanol solution, suggesting that the hydrophobicity of coal surface was enhanced by 2-ethylhexanol. The molecular dynamic simulation (MDS) conducted by Lyu et al. [11] indicated that non-ionic surfactant ethylene oxide nonylphenol adsorbed on oxidised coal surfaces with the ethylene oxide chains attached to the coal surfaces, which repelled water molecules and enhanced coal surface hydrophobicity. However, coal surfaces are heterogeneous with both hydrophilic and hydrophobic areas. Even on a heavily oxidised coal surface,

ACCEPTED MANUSCRIPT hydrophobic areas still occur. As shown in the previous study, the coal which exhibited no true flotation consisted of 13.20 at.% oxidised carbon and 86.80 at.% un-oxidised carbon [1]. It has been documented that surfactants can adsorb on both hydrophilic and hydrophobic solids. On hydrophilic solids, surfactant adsorption occurs with the polar groups attached to the solids and the hydrocarbon chains oriented outwards, which is termed as “head-on adsorption”, and then the hydrophobicity of

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the solids is increased. On hydrophobic solids, surfactant adsorption occurs with the hydrocarbon chains attached to the solids and the polar groups oriented towards the liquid phase, which is termed

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as “head-out” adsorption, and then the hydrophobicity of the solids is reduced [12]. Therefore, the

adsorb on unoxidized surfaces making them hydrophilic.

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flotation of oxidised coals with polar collectors may be problematic since polar collectors may also

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In the previous study, we used a composite collector consisting of diesel (a nonpolar collector) and Triton X-100 (a polar collector) to improve the flotation of oxidized coals with diesel targeting

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the hydrophobic areas while Triton X-100 targeting the hydrophilic areas on coal surfaces [1]. We found a close relationship between the degree of coal surface oxidation and the proportion of Triton

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X-100 in the composite collector used to achieve the optimal coal flotation. In the current study, the

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interaction between diesel and Triton X-100 and their adsorption on coal surfaces with different degrees of oxidation were studied.

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While adsorbing on hydrophilic parts of oxidised coals to increase coal surface hydrophobicity, the surfactant in the composite collector may also serve as an emulsifier of the oil collector to increase

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the contacts between the oil collector and the hydrophobic areas of oxidised coals. It has been reported that the efficiency of coal flotation is closely dependent on the dispersion of oil collectors which can be influenced by the addition of surfactants, such as Tween-80 and Triton X-100 [13]. The addition of surfactants can significantly reduce the size of oil droplets and generate spherical and well-defined droplets [14]. This is achieved by the orientation of surfactant molecules at the oil-water interface with their hydrocarbon chains attached to the oil droplets and the polar groups oriented outwards. This orientation of surfactant molecules decreases the oil-water interfacial tension [15, 16] and inhibits the coalescence of oil collectors by generating steric or electric barriers [17]. In the

ACCEPTED MANUSCRIPT current study, the possible emulsification of the oil collector by the surfactant in the composite collector was also examined. 2. Materials and experimental methods 2.1 Raw materials The coal sample used in this work was a high volatile bituminous coal with the combustible

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matter content, moisture content, and ash content being 89.95 wt%, 2.12 wt% and 7.93 wt%,

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respectively. The coal sample was crushed and ground to 80 wt% <200 μm which is the same particle size that used in most coal flotation plants. Based on the previous investigations [18, 19], this coal

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was oxidised at 150 ℃ for 4 hours, 24 hours and 72 hours to obtain coal samples with 8.23 at.%,

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13.20 at.% and 18.35 at.% surface oxidation, respectively. Flotation tests indicated that 8.23 at.%, 13.20 at.% and 18.35 at.% oxidised carbon corresponded to a slightly, intermediately and heavily

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oxidised coal surface, respectively [1].

De-ionized (DI) water with a resistivity of 35 Ωm was used in this study. Diesel was used as the

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conventional oil collector for coal flotation. The gas chromatography-mass spectrometry (GC-MS)

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analysis indicates that diesel is a mixture of different hydrocarbons and its composition is shown in Table 1 where n represents the number of carbon in the hydrocarbon chain. Table 1 suggests that the average chemical formula of diesel used in this study is C17H35. MIBC (Methyl Isobutyl Carbinol,

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C6H12O) was used as the frother and non-ionic surfactant Triton X-100 consisting of polyethylene

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oxide chain and hydrocarbon chain was used as the polar collector [1]. 2.2 Flotation tests

Flotation tests were carried out in a 1.5 L JK batch flotation cell using DI water. The details of flotation tests have been reported elsewhere [1, 20]. In this study, the pH of coal slurry was kept at 8.0 in all the flotation tests by adding 1 wt% NaOH or HCl solution. Four concentrates were collected for each flotation test after a cumulative flotation time of 1 min, 2.5 min, 5 min and 10 min. Triton X-100 was added first to condition with the coal slurry, followed by the addition of diesel and MIBC. The conditioning time of Triton X-100, diesel and MIBC was 3 min, 3 min and 2 min, respectively.

ACCEPTED MANUSCRIPT Flotation concentrates and tailings were filtered, dried at 80 ℃ and weighed to calculate the flotation yield. 2.3 Diesel adsorption measurements Because diesel is insoluble in water, its adsorption on coal surface was determined by extracting the adsorbed amount, followed by the measurement using a gas chromatography-flame ion detector

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(GC-FID). For each test, 2 g coal sample was conditioned with or without 50 mL 110-5 mol/L Triton

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X-100 at 25℃ and then diesel was added and conditioned for 2 hours. The coal slurry was transferred into a centrifuge tube and centrifuged at 6000 rpm for 15 min to achieve solid-liquid separation. The

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liquid was taken out and the sediment was filtered. 1.5 g filter cake together with 10 g anhydrous

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sodium sulphate and 20 mL hexane was added into a clean centrifuge tube. The centrifuge tube was then placed in an ultrasound bath for 15 min. Then the tube was shaken and put back to the ultrasound

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bath for 1min. After extraction, the mixture was centrifuged again and the supernatant solution was transferred into a clean tube. The extraction process was repeated again by adding 20 mL more hexane. Upon completion of extraction, the extracted solution was combined and analysed by GC-FID

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as reported in literature [21, 22]. For each test, duplicate samples were prepared following the same process. The moisture content of the filter cake was determined by weighing 10 g filter cake and dried

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overnight at 105 ℃. The moisture content of the filter cake was then calculated as follows: (1)

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where ω (%) is the moisture content of the filter cake, mdry (g) is the mass of the dry sample and mwet (g) is the mass of the wet sample. As shown in Table 1, diesel used in this study is a mixture of hydrocarbons with the number of C atoms ranging from 12 to 25. Because the length of hydrocarbon chains may influence their adsorption on solids, in this study, dodecane (C12), hexadecane (C16), and docosane (C22) were used to represent short hydrocarbons, middle hydrocarbons and long hydrocarbons, respectively. The adsorption density of C12, C16 and C22 on coal surfaces was calculated as follows: (2)

ACCEPTED MANUSCRIPT where Г (mg/g) is the adsorption density, C (ppm) is the concentration of C12, C16 or C22 in the extracted solution, V (L) is the volume of the extracted solution and w is the mass of the dry coal used for extraction. 2.4 Diesel droplets size measurements The influence of Triton X-100 on the emulsification of diesel was studied by measuring the size

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of diesel droplets in the presence of Triton X-100. The concentration of Triton X-100 used ranged

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from 0 to 110-4 mol/L which covered the concentration of Triton X-100 used in the flotation. For each test, 0.4 mL diesel was added into 200 mL DI water and conditioned at 900 rpm for 15 min. The

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stirring speed was the same as that used in flotation. Upon the completion of emulsification, the size

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distribution of diesel emulsions was measured using Malvern Mastersizer 2000. Each measurement was repeated 7 times and an average value was reported.

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3. Results and discussion

3.1 Adsorption of diesel on oxidised coal surfaces

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The effect of diesel on the surface hydrophobicity of oxidised coals was studied first through

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flotation tests. Three coal samples with 8.23 at.%, 13.20 at.% and 18.35 at.% surface oxidation were chosen to represent the slightly oxidised coal (SOC), intermediately oxidised coal (IOC) and heavily

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oxidised coal (HOC), respectively. Fig. 1 shows the flotation yield of the three oxidised coals as a function of diesel concentration. In this study, the flotation yield was used to represent the flotation

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performance of oxidised coals since the coal sample used in this study had a low ash content. As shown in Fig. 1, the degree of surface oxidation affected coal flotation significantly in the absence and presence of diesel. For the coal with 8.23 at.% surface oxidation, the flotation yield increased from 56.05% to 74.34% when diesel concentration increased from 0 mol/L to 1.3410-4 mol/L, then this increase slowed down with a further increase in diesel concentration, and the coal flotation yield reached 80.67% at 3.3510-4 mol/L diesel concentration. The results indicate that for SOC, it is still possible to achieve a satisfactory surface hydrophobicity by increasing diesel concentration in flotation. However, the flotation of IOC with 13.20 at.% surface oxidation and HOC with 18.35 at.%

ACCEPTED MANUSCRIPT surface oxidation was quite different. The flotation yield of IOC increased from 15.23% in the absence of diesel to 26.51% at 1.3410-4 mol/L diesel. Then the flotation yield increased slowly with a further increase in diesel concentration and reached only 36.57% at 3.3510-4 mol/L diesel. A similar flotation behaviour was observed for HOC with the flotation yield increasing slowly with diesel concentration. The flotation results indicate that a high flotation yield of IOC and HOC could not be

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achieved by diesel, suggesting that diesel was not effective to restore the surface hydrophobicity of IOC and HOC as the only collector.

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As shown in Table 1, Diesel is a mixture of a variety of hydrocarbons ranging from C11 to C25. In

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this study, the adsorption of C12, C16 and C22 from diesel on coal surfaces was studied to understand

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the interaction of short chain, middle chain and long chain hydrocarbons with oxidised coals. Fig. 2 shows the adsorption densities of C12, C16 and C22 on coals with different degrees of surface

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oxidation as a function of diesel concentration. As shown in Fig. 2, the adsorption isotherms of C12, C16 and C22 on the oxidised coals were similar. In general, the adsorption density initially increased rapidly with diesel concentration and then this increase slowed down with a further increase in diesel

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concentration. At each diesel concentration, the adsorption density of hydrocarbons follows the

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sequence of ГC16 > ГC12 > ГC22 on each coal. The fraction of C16 in diesel is 13.96% which is much higher than the fraction of C12 and C22 being 5.07% and 3.26%, respectively. Apparently, the fraction

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of hydrocarbons in diesel plays an important role in their adsorption on coals. Fig. 2 also indicates that the adsorption density of C12, C16 and C22 on oxidised coals decreased with an increase in the degree

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of coal surface oxidation. This is expected since a more oxidised coal provides a smaller hydrophobic surface for the adsorption of hydrocarbons. To investigate the effect of hydrocarbon length on the adsorption efficiency of hydrocarbons on oxidised coals, the adsorption density of the three hydrocarbons as a function of their own concentrations on oxidised coals was determined and the results are shown in Fig. 3. In general, on the three oxidised coals, regardless of the degree of coal surface oxidation, the adsorption density of C12, C16 and C22 followed the sequence of ГC16 > ГC12 > ГC22. The results suggest that middle hydrocarbons had a higher adsorption on the coal surfaces than short and long hydrocarbons. It is

ACCEPTED MANUSCRIPT known that with an increase in hydrocarbon chain length, on the one hand, the hydrophobic attraction between the hydrocarbon chain and the carbonaceous matter increases [23], which promotes the adsorption of hydrocarbons, but on the other hand, the dispersion of hydrocarbons decreases [24], which inhibits the adsorption of hydrocarbons. How these two opposite effects affect the adsorption of hydrocarbons has not been reported in literature. The adsorption results in this study suggest that the

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balance of these two opposite effects may lead to the highest adsorption of middle hydrocarbons on oxidised coal surfaces.

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Both Fig. 2 and Fig. 3 indicate that the adsorption of hydrocarbons did not level off at a high

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diesel concentration. It seems that after covering the hydrophobic area on oxidised coals, hydrocarbons may continue to adsorb through the lateral interaction with the adsorbed hydrocarbons.

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Although studies on the oil adsorption process are limited, the adsorption behaviour of surfactants indicates that when the monolayer head-on adsorption of surfactants saturates, more surfactants are

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adsorbed through the chain-chain interactions with the adsorbed surfactant tails [25, 26]. A schematic of diesel adsorption on oxidised coals with different degrees of surface oxidation in

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flotation is shown in Fig. 4. Hydrocarbons only adsorb on un-oxidised areas through hydrophobic

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attraction, but do not adsorb on oxidised areas. Therefore, the flotation of oxidised coals can only be improved to a certain extent using diesel as a collector. Due to the higher fraction and stronger

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adsorption efficiency, more middle hydrocarbons adsorb on coal surfaces. In flotation, since the diesel concentration is lower than 3.3510-4 mol/L, the adsorption of diesel on oxidised coals through the

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lateral interaction with the adsorbed hydrocarbons may not be expected. 3.2 The interaction between diesel and Triton X-100 and their adsorption on oxidised coal surfaces Fig. 5 shows the flotation of SOC, IOC and HOC as a function of diesel concentration in the presence of Triton X-100. The concentration of Triton X-100 ranges from 0 mol/L to 5.3610-5 mol/L. Apparently, the presence of Triton X-100 improved the flotation performance of oxidize coals. In general, in the presence of Triton X-100, the flotation yield of oxidised coals initially increased

ACCEPTED MANUSCRIPT rapidly when diesel concentration increased from 0 mol/L to 6.710-5 mol/L, and then almost levelled off with a further increase in diesel concentration. It seems that 6.710-5 mol/L diesel was sufficient to cover the un-oxidised areas of oxidised coals with different degrees of surface oxidation in the presence of Triton X-100. At the same diesel concentration, flotation yield increased with the concentration of Triton X-100. For example, for IOC, the flotation yield was 37.4% at 1.3410-5

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mol/L Triton X-100 and increased to 74.85% at 4.0210-5 mol/L Triton X-100 when 6.710-5 mol/L

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diesel was used.

The adsorption of diesel on coals with different degrees of surface oxidation in the presence of

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Triton X-100 was measured. In this measurement, C16 was chosen to represent the hydrocarbons in

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diesel and the adsorption of C16 on SOC, IOC and HOC was determined in the presence of a low Triton X-100 concentration, 110-5 mol/L, which was in the same range of the concentrations used in

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flotation. As shown in Fig. 6, in the presence of 110-5 mol/L Triton X-100, the adsorption densities of C16 on SOC, IOC and HOC were higher than those in the absence of Triton X-100, suggesting that

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the adsorption efficiency of diesel on coal surfaces was enhanced by Triton X-100. This explains the

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faster response of coal flotation to diesel concentration after Triton X-100 was added with diesel. As shown in Fig. 5, the flotation yield continued to increase slowly even at a high diesel concentration in

of Triton X-100.

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the absence of Triton X-100, but it reached a maximum at a low diesel concentration in the presence

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The increased diesel adsorption on coal surfaces by Triton X-100 may be associated with the emulsification capability of Triton X-100. It is known that surfactants with a hydrophilic-lipophilic balance (HLB) value between 8 and 16 can serve as oil emulsifiers in water to enhance the dispersion of oil droplets and their adsorption on coal surfaces in flotation [27]. The HLB value can be calculated based on the equation as follows [28]: ∑

(4)

where m is the number of hydrophilic groups in the molecule, Hi is the value of the ith hydrophilic group, and n is the number of lipophilic groups. The HLB value of Triton X-100 was calculated to be

ACCEPTED MANUSCRIPT 13.5, suggesting that Triton X-100 is a good emulsifier of oils. In this study, the influence of Triton X-100 on diesel emulsification in water was studied and the results are shown in Fig. 7. The Triton X100 concentration ranged from 0 to 110-4 mol/L which covered the concentration of Triton X-100 used in flotation. The diesel concentration used in the emulsification study was 6.710-3 mol/L to meet the minimum detection limit. Fig. 7 shows d10, d50 and d90 of diesel droplets as a function of Triton X-

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100 concentration. As can be seen, in the absence of Triton X-100, diesel existed as large oil droplets. The addition of Triton X-100 decreased the size of diesel droplets, suggesting that the arrangement of

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Triton X-100 molecules at the oil-water interface emulsified diesel in water. It is clear that the diesel

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droplet size decreased significantly with an increase in Triton X-100 concentration below 210-6 mol/L, then the decrease slowed down with a further increase in Triton X-100 concentration and

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finally the diesel droplet size levelled off at 110-5 mol/L Triton X-100. The results confirm that Triton X-100 can effectively emulsify diesel and produce smaller diesel droplets at a low Triton X-

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100 concentration, which will enhance diesel adsorption on coal surfaces and produce the maximum coal flotation even at a low diesel concentration.

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Fig. 7 shows that 110-5 mol/L Triton X-100 produced the maximum emulsification of 6.710-3 mol/L diesel. Therefore, the concentration of Triton X-100 required to emulsify diesel in flotation should be much lower than 110-5 mol/L. Because 1.3410-5 mol/L, 410-5 mol/L and 510-5 mol/L

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Triton X-100 were required to produce a flotation yield of 80% in the flotation of SOC, IOC and HOC, respectively, when 6.710-5 mol/L diesel was used (Fig. 5) and higher than the concentration required

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to emulsify the diesel, it is clear that in addition to emulsifying the diesel and enhancing diesel adsorption on un-oxidised areas of the coals, Triton X-100 also adsorbed on coal surfaces to further increase the surface hydrophobicity to achieve satisfactory flotation of oxidised coals. Triton X-100 is an amphiphilic surfactant consisting of both hydrocarbon chains and polyethylene oxide chains. Based on the report on the adsorption of surfactants in literature [12], it may be able to adsorb on oxidised areas through hydrogen bonding, increasing coal surface hydrophobicity, or on unoxidised areas through hydrophobic attraction, decreasing coal surface hydrophobicity. However, flotation results in Fig. 5 show that Triton X-100 at the concentrations used in flotation in this study

ACCEPTED MANUSCRIPT did not decrease coal surface hydrophobicity in the presence of diesel. For example, the flotation yield of IOC and HOC increased significantly as the concentration of Triton X-100 increased from 1.34×105

mol/L to 5.36×10-5 mol/L, at which the maximum diesel emulsification was already achieved. This

suggests that the surface hydrophobicity of oxidised coals increased with the further adsorption of Triton X-100 on oxidised surface areas.

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A schematic of the synergistic effect between diesel and Triton X-100 on oxidised coal surfaces in flotation is shown in Fig. 8. Diesel primarily adsorbs on the un-oxidised surface area and most of

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Triton X-100 molecules adsorb on the oxidised surface area with tails oriented outwards. A small

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amount of Triton X-100 molecules adsorb at the oil/water interfaces emulsifying the diesel, which significantly decreases the size of diesel droplets and facilitates the adsorption of diesel. Although at

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the oil/water interfaces, the polar groups of Triton X-100 are oriented towards the liquid phase, their

increasing the surface hydrophobicity.

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4. Conclusions

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concentration is small and does not offset the reduced diesel droplets on the un-oxidised area in

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In the flotation of oxidised coals, a high flotation yield could not be achieved for intermediately and highly oxidised coals when diesel was used as the only collector even at a high concentration. Diesel adsorbed on un-oxidised surface areas and its adsorption decreased with an increase in the

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degree of coal surface oxidation. The middle hydrocarbons in diesel had a higher adsorption density

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and efficiency on coal surfaces than the short and long hydrocarbons. The addition of Triton X-100 with diesel significantly increased the flotation yield of oxidised coals. The adsorption of diesel on coal surfaces increased in the presence of Triton X-100, due to the emulsification of diesel by Triton X-100, significantly reducing the size of diesel droplets. This benefits the adsorption of diesel on un-oxidised surface areas and their hydrophobilization. At the same time, Triton X-100 adsorbed on oxidised surface areas through a head-on adsorption, rendering the hydrophilic surface areas hydrophobic. Acknowledgements

ACCEPTED MANUSCRIPT The authors would like to acknowledge financial support from ACARP (Australian Coal Association Research Program) through the research project C23039 and C26008. The support from Glencore Coal and Anglo American Coal providing coal samples, suggestions and assistance from Dr. Jennifer Wannders in measuring diesel adsorption using GC-FID, and technical discussions from the coal preparation plants are greatly appreciated. The first author would like to acknowledge the

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scholarships provided by CSC (China Scholarship Council) and The University of Queensland. The second author would like to acknowledge the support from Queensland Government through Advance

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Queensland Research Fellowship funding scheme.

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Davies, J., A quantitative kinetic theory of emulsion type. I. Physical chemistry of the emulsifying agent. Proc. 2nd Intern. Congr. Surface Activity, Butterworths Scientific

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Publication, London, 1957. 426.

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vaccines. Vaccine, 2001. 19(17-19): p. 2666-2672.

ACCEPTED MANUSCRIPT Table 1. The composition of diesel (%). C13

C14

C15

C16

C17

C18

C19

C20

C21

C22

C23

C24

C25

1.78

5.07

10.11

11.29

12.05

13.96

10.63

9.13

8.02

5.89

4.47

3.26

2.29

1.40

0.65

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C12

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Mole percentage

C11

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ACCEPTED MANUSCRIPT Figure captions

Fig. 1. The flotation yield of oxidised coals as a function of diesel concentration. Fig. 2. The adsorption density of C12, C16 and C22 on coal surfaces as a function of the initial diesel

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concentration: (a) SOC, (b) IOC and (c) HOC. Fig. 3. Adsorption density of C12, C16 and C22 on the oxidised coals as a function of their initial

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concentrations: (a) SOC, (b) IOC and (c) HOC.

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Fig. 4. The schematic of diesel adsorption on oxidised coals with different degrees of surface oxidation in flotation.

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Fig. 5. Flotation yield of SOC (a), IOC (b) and HOC (c) as a function of diesel concentration in the

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absence and presence of Triton X-100. Fig. 6. Adsorption density of C16 on SOC (a), IOC (b) and HOC (c) in the absence and presence of

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Triton X-100.

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Fig. 7. The emulsification of 6.710-3 mol/L diesel in DI water by Triton X-100: d10, d50 and d90 of diesel droplets as a function of Triton X-100 concentration.

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Fig. 8. Schematic of the synergistic interactions between diesel and Triton X-100 on oxidised coal surfaces in flotation.

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Fig. 3.

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Fig. 4.

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Fig. 5.

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Fig. 6.

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Fig. 7.

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Fig. 8.

ACCEPTED MANUSCRIPT Highlights Diesel was not effective in the flotation of oxidised coals;

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Diesel adsorption on coals decreased with the degree of coal surface oxidation;

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The addition of Triton X-100 with diesel significantly improved the flotation;

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Triton X-100 emulsified diesel and facilitated its adsorption on un-oxidised areas;

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Triton X-100 adsorbed on oxidised areas rendering the surface hydrophobic.

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