Adsorption behavior of surfactants on lignite particles with different densities in aqueous medium

Transcription

1 Physicochem. Prob. Miner. Process. 53(2), 2017, Physicochemica Probems of Minera Processing IN (print) IN (onine) Received February ; reviewed; accepted March 14, 2017 Adsorption behavior of surfactants on ignite partices with different densities in aqueous medium Jie Wang *, Yaqun He *,**, Baofeng Wen *, Xiangyang Ling *, Weining Xie *, huai Wang * * choo of Chemica Engineering and Technoogy, China University of Mining and Technoogy, Xuzhou, Jiangsu , China ** Advanced Anaysis and Computation Center, China University of Mining and Technoogy, Xuzhou, Jiangsu, , China. Corresponding author: yqhe_cumt@126.com (Yaqun He) Abstract: Lignite is we known for its strong hydrophiicity and hard-to-foat properties. However, the surface free energy of the soid is made up of two components, that is the Lifshitz-van der Waas and acid-base interaction energy. Differences in these two components between the ow ash (ower density) and high ash fraction (higher density) provide a benefit for improving the separation efficiency through introducing surfactants in fotation. In this paper, thermodynamic characterization of three density ignite fractions was measured by a Washburn dynamic method. Combining the Washburn equation and Van Oss-Chaudhury-Good theory, the surface free energy components of three sampes were cacuated according to the wetting process by n-hexane, diiodomethane, deionized water and ethyene gyco. The Lifshitz-van der Waas part of surface free energy reduced with the coa density increase, especiay between fractions of g/cm 3 and g/cm 3, whie the Lewis base part increased sighty. The interfacia interaction free energies between the surfactant and ignite in aqueous medium indicated that the ow hydrophiic index benefited for the stronger adsorption intensity. Increase of the surfactant Lifshitz-van der Waas component increases the adsorptive intensity on ower density ignite and the repusive intensity on higher density ignite, which is beneficia for separation. Keywords: interfacia surface free energy, adsorption, ignite, surfactant, Washburn equation Introduction Lignite is we known for its strong hydrophiicity and hard-to-foat properties. In aqueous media, the minera-rich partices in ignite are aso hydrophiic. In practice, surfactants have been appied to pretreat coa sampes to adjust the surface hydrophobicity of the coa-rich or minera-rich partices. Cationic surfactants have been proved to have an essentia function in reverse fotation of hard-to-foat coa, such as either ignite or oxidized coa (Wen and un 1977; arikaya and Özbayoğu

2 Adsorption behavior of surfactants on ignite partices with different densities ; Zhang and Liu 2015). A arge number of nonionic surfactants has been used to investigate the fotation efficiency more widey (Ceik and Yoon 1991; Vamvuka and Agridiotis 2001; Zhang and Tang 2013). urfactants can pay a roe of either frother, coector or both. The frother may act as coector when the coa surface has some poar moieties. Athough, it is cear that inorganic siicate mineras have the highenergy surface subject to faciitate strong adsorption by the cationic surfactant, such as fatty amine (Wen and un 1977; Xia and Yang 2013), the adsorption behavior of nonionic surfactant on coa-rich/minera-rich ignite partices is sti uncear. This work is mainy focused on the interfacia interaction free energies between ow moecuar weight non-ionic surfactants and ow ash (ower density) or high ash (higher density) fractions of Inner Mongoia ignite. Low rank ignite is characterized by more oxygen functiona groups on the partice surface, which resuts in the hydrophiicity of ignite. However, the mineras are mainy hydrophiic siicates, carbonates, sufates and quartz. In terms of the interfacia interaction free energy, it is essentia to confirm surface free energy components of the ower/higher density ignite sampes. Van Oss- Chaudhury-Good theory indicates that the tota surface free energy of substance is composed of the non-poar (Lifshitz-van der Waas γ LW ) and poar parts (Lewis acidbase, γ + and γ ) (Van Oss et a., 1979; Van Oss et a., 1980; Van Oss, 2006). Using the combined Washburn equation (Grundke et a., 1996; Chau 2009), the surface free energy and their poar/non-poar components of the ow and high ash ignite can be determined. Zou et a. (2013) investigated the surface free energy components of two fine iberated midding bituminous coas through such method. The surface free energy components of coas are consistent with the regressive reease fotation test resuts. The cacuated interfacia interaction free energy is concerned with the surface free energy components of soid and surfactant. Wang et a. (2011) studied the surface free energy components of frothers through contact ange measurements. Based on the measured surface free energy components of the soids and surfactants, their interaction free energies were cacuated. In this paper, the raw ignite partices (-0.5 mm) were separated into three densities, incuding g/cm 3, g/cm 3 and g/cm 3 fractions. The surface functiona group compositions were determined by FTIR. n-hexane with ow surface tension of 18.4 mn m -1 was the probe to measure the geometric factor of packed powder beds. Other three types of probe iquids, incuding two poar (deionized water and ethyene gyco) and one non-poar (diiodomethane), were used in the Washburn wetting tests in order to cacuate their contact anges. A the contact ange measurements were performed at 20 C. The surface free energy of different density ignite sampes was determined. Furthermore, the reationship between the surface chemica structure and surface free energy of three ignite sampes was discussed. Based on the resuts, the interfacia free energy between the surfactant and ignite surface in the aqueous medium was cacuated. In addition, the effect of reagent structure on different adsorption behaviors was aso concerned.

3 998 J. Wang, Y. He, B. Wen, X. Ling, W. Xie,. Wang Materias and methods Materias A ignite sampe was obtained from Inner Mongoia Province, China. ink and foat tests for the fine fraction (-0.5 mm) were carried out in dense medium iquids with densities of 1.45 and 1.8 g/cm 3, respectivey. After sink and foat separation, each product was washed, rinsed and dried at 105 C for 6 h in a vacuum oven. ampes of the products were weighed and then ground to -74 μm separatey to determine their ash content and wetting behavior. The probe iquids, incuding n-hexane, ethyene gyco, deionized water and diiodomethane were of anaytica purity. urfactants, incuding terpineo (TP), methy isobuty carbino (MIBC), tripropyene gyco n-buty ether (TPnB) and diethy o- phthaate (DOP) were aso of anaytica purity. Methods Contact ange measurement A KRU tensiometer K100 was used to measure the contact ange of coa sampes with various densities in different iquids. Temperature was kept stricty around 20 ± 0.5 C. A sma round-shape fiter paper was paced at the bottom of the Washburn tube. Different density fractions of g was weighed and put into a cean Washburn tube. Then Washburn tube was fixed on the hook of the Tensiometer K100. A 30 cm 3 probe iquid rose sowy unti about 2 mm from the bottom of the tube. Before the measurement, the physica parameters of the probe iquid, incuding density ρ, surface tension γ and iquid viscosity η, were inserted in the measuring process. With the surface of iquid got in touch with the bottom of the tube automaticay, the tensiometer started to record the mass increment w with the contact time t with the set of 200 s. According to the Washburn equation, inserted in measuring software, the contact ange can be obtained from the sope of w 2 t: t cos 1 (1) 2 2 w c where c is the geometric factor of the packed bed, c R 2 r 2 2 eff, R eff (μm) is the effective radius of capiary, ε is the porosity, r (mm) is the radius of packed bed, ρ (g cm -3 ) is the density of the iquid, γ (mn m -1 ) is the surface tension of iquid, η (mn m -2 s -1 ) is the iquid viscosity and θ ( ) is the contact ange. Firsty, the geometric factor c of different density ignite soids was confirmed before contact ange measurement. In the tests, n-hexane was used as the reference iquid (Zou et a., 2013). Therefore, the geometric factor c of the typica density ignite soid can be determined from Eq. (1). In order to obtain the repeatabiity, the geometric factor c of each soid sampe was tested twice. Probe iquids used in the

4 Adsorption behavior of surfactants on ignite partices with different densities 999 experiment were n-hexane, ethyene gyco, deionized water and diiodomethane. The parameters of the surface free energy of these iquids are shown in Tabe 1 (Li et a., 1993; Iveson et a., 2000). Probe iquid Tabe 1. Physica parameters of probe iquids at 20 C ρ g/cm 3 η mpa s LW n-hexane deionized water ethyene gyco diiodomethane Cacuation of surface free energy Van Oss et a. (1980) supposed that the surface free energy of soid/iquid is made up of two components, that is the Lifshitz-van der Waas γ LW and the acid-base interaction energy AB, which is composed of Lewis acid component and base component. The surface free energy can be expressed as foows: LW AB (2) whie the reationship between i (soid) and j (iquid) interface interaction free energy can be expressed as: LW LW G 2( ). (3) (3) ij i j i j i j The Young equation characterizes the reationship between the soid surface energy, iquid surface free energy L, soid-iquid interaction free energy L and the equiibrium contact ange (θ) expressed by a formua: 1 cos L G (4) L The reationship between the surface free energy components and their content ange can be expressed as: LW LW 1 cos L 2 L L L. (5) From the contact ange of three probe iquids on the soids, the surface free energy components of the soids, incuding Lifshitz-van der Waas γ s LW, Lewis acid component and base component can be cacuated by using Eq.(5). The interaction free energy of two-phase, soid and iquid (L), is expressed by Eq.(3). The interaction free energy of the ignite partices and surfactant in water (W) can be expressed as:

5 1000 J. Wang, Y. He, B. Wen, X. Ling, W. Xie,. Wang 2 G G G LW AB LW AB LW LW LW LW iwj iwj iwj i W W j 2 W i j W W i j W i j i j. (6) Resuts and discussion urface functiona group anaysis Tabe 2 shows the ash content of the coa sampes with different densities. The FTIR are shown in Fig. 1. The FTIR signas deriving from chemica functiona groups are isted in Tabe 3 (Painter and nyder 1981; Liang et a., 2015). Tabe 2. Ash content of coa sampes with different densities Coa sampe g/cm g/cm g/cm 3 Ash content, % Tabe 3. Band assignments for FTIR spectra of coas (Painter and nyder 1981; Liang et a., 2015) Wavenumber (cm -1 ) Functiona groups OH(mineras) OH Aromatic CHx stretching Aiphatic CH x stretching COOH Aromatic C=C ring stretching Aiphatic CH x bending C-O-C stretching i-o stretching Aromatics CHx Out-of-pane deformation -650 A-O formation, i-o Figure 1 presents the surface functiona groups of three different density ignite sampes. For the g/cm 3 coa sampe, the wide organic -OH peak occupies a quite high proportion of the coa surface at 3400 cm -1. Despite of the wide organic -OH, the absorbance of C=C peak, is presented by the strongest peak at 1600 cm -1 (Fig. 1a). Besides, the characteristic peaks of aiphatic CH X are found at the ranges of cm -1 and cm -1. On the other hand, the comparativey weak peaks of i-o are aso seen at neary 1000 cm -1 and the range of -650 cm -1 from the FTIR spectrum of the coa sampe g/cm 3. However, for g/cm g/cm 3 ignite coa sampes, the organic peaks of -OH and C=C are decreased significanty in the comparison with the resuts presented in Fig. 1a. On the contrary, the minera peaks of i-o are increased remarkaby at the ranges of cm -1 and -650 cm -1. In

6 Adsorption behavior of surfactants on ignite partices with different densities 1001 addition, -OH peaks at 3400 cm cm -1 are presented aongside the organic -OH. From Figure 1c, the organic peaks are further decreased for g/cm 3 ignite sampe, whie the inorganic peaks are increased remarkaby. The absorbance of the inorganic peaks are much higher than the organic peaks. a) b) c) Fig. 1. FTIR of three density ignite coa sampes, (a) g/cm 3, (b) g/cm 3, (c) g/cm 3 Contact ange and surface free energy cacuation Tabe 4 shows the average geometric factor c and contact ange between the probe iquid and ignite from the Washburn method. For the g/cm 3 ignite sampe, the contact anges were 60, 65 and 87 for diiodomethane, ethyene gyco and deionized water, respectivey. From the physica parameters, shown in Tabe 1, the poarity of deionized water is stronger than that of ethyene gyco. It is shown that non-poar diiodomethane can wet the ower density ignite easiy, whie more poar deionized water spreads more difficuty than ethyene gyco. For g/cm 3 ignite, the contact anges were 70, 68 and 86 for diiodomethane, ethyene gyco and deionized water, respectivey. The deionized water spreads on the g/cm 3 ignite with more difficuty than non-poar diiodomethane. In the case of the g/cm 3 ignite, the wetting phenomenon was simiar with that of the g/cm 3 ignite sampe. On the other hand, diiodomethane and ethyene gyco wet the ignite more easiy with

7 1002 J. Wang, Y. He, B. Wen, X. Ling, W. Xie,. Wang the decrease of ignite sampe density, whie deionized water wets ignite more difficut. In concusion, with the poarity increase of the probe iquid, the contact ange increases obviousy for the ower density ignite. However, for the medium and the higher density coa sampes, the medium poar iquid wets two sampes more easiy than the non-poar/ strong poar iquid. ignite density, ρ (g cm -3 ) Tabe 4. Geometric factor c and contact ange of ignite sampes with different densities c (10-6 cm -5 ) Contact ange, θ ( ) n-hexane diiodomethane ethyene gyco deionized water By combining the parameters given in Tabes 2 and 4, the surface free energy components of the ignite sampes can be cacuated from Eq. (5), and the resuts are isted in Tabe 5. Tabe 5. urface free energy components of ignite sampes with different densities (mn/m) ignite density, ρ (g cm -3 ) LW AB * * is hydrophiic index (Wang et a., 2011). From the resuts shown in Tabe 5, it is indicated that the main surface free energy component for the g/cm 3 LW density ignite is Lifshitz-van der Waas. The Lewis acid component is comparativey ow and equa to mn/m, whie the base component is 7.83 mn/m. imiar vaues of the Lewis acid s and base components are shown in Tabe 5 for g/cm 3 and g/cm 3 fractions. Besides, the Lifshitz-van der Waas γ LW is found to be the main component in the medium and high density ignite sampes. However, the Lifshitz-van der Waas γ LW decreases with the increase of the ignite density, which indicates that the non-poarity of ower density ignite is stronger than that of the higher density sampe. The Lewis acid components are sma for a the three density sampes, whie the Lewis base component increases for the g/cm 3 coa sampe. The Lewis acid-base AB components of ignite show no cear reguarity. However, the hydrophiic index increases with the density increase. The surface free energy demonstrates

8 Adsorption behavior of surfactants on ignite partices with different densities 1003 the decreasing trend with the increase of the density due to the absoute predominance of the Lifshitz-van der Waas component. By combining the resuts of the surface functiona groups shown in Fig. 1, it is concuded that the inorganic oxygen functiona groups have the stronger hydrophiicity than the organic oxygen groups. Interfacia interaction free energy in aqueous medium The surface tension parameters and hydrophiic index of four types of surfactants, incuding TPnB, DOP, TP and MIBC are isted in Tabe 6 (Wang et a., 2011). Tabe 6 urface free energy components of surfactants urfactant LW Chemica structure TP MIBC OH HO TPnB O O HO O DOP O O O O From Tabe 6, there is a notabe positive correation between the number of carbon atoms in the reagent moecue and the Lifshitz-van der Waas component. Besides, the aromatic ring enhances the Lifshitz-van der Waas component DOP than that of TPnB despite the number of carbon atoms of the former is ess than that of the atter. In genera, the hydrophiic index increases with the increase of the number of oxygen atoms in the reagent moecue (Tabe 6). However, the hydrophiic indexes are different though the same four oxygen atoms in the TPnB and DOP moecue. In detais, three ether bonds and one hydroxy are in TPnB, whie two ester groups are formed in DOP. Hence, ether bond and hydroxy are more hydrophiic than the ester group in the reagent moecue. For the moecues of TP and MIBC, one hydroxy is shown in each moecue, but their hydrophiic indexes are aso different. Therefore, the cycic structure resuts in the more hydrophiic property of TP than that of MIBC. In concusion, not ony the number of oxygen atoms and structure of LW LW of

9 1004 J. Wang, Y. He, B. Wen, X. Ling, W. Xie,. Wang oxygenated groups but aso the chemica structure of hydrocarbon have significant infuence on the hydrophiic index of non-ionic surfactants. The adsorption performance of tested reagents on different density ignite sampes are characterized by their cacuated interaction free energy (Eq. 6). The resuts are shown in Figure 2 to 4. LW AB Fig. 2. Tota interaction free energy G iwj between surfactant and ignite in aqueous medium From Figure 2, it is shown that different types of surfactants adsorb on ignite sampes with various densities with different intensities. For the fraction of g/cm 3, the tota interaction free energies are , , and mj/m 2 for TPnB, DOP, TP and MIBC, respectivey. imiary, the tota interaction free energies for other two sampes dispay the increasing trend for four surfactants in the above order. The typica vaues are , , and mj/m 2 for the medium density ignite sampe and , , and mj/m 2 for g/cm 3 ignite. The negative vaues of these interaction free energies indicate that the adsorption processes are spontaneous. Obviousy, the adsorption intensities of these surfactants are in the order MIBC > TP >DOP >TPnB for three density sampes. On the other hand, the surfactant adsorbs on the ignite surface more easiy and strongy with the decrease of partice density. ince the tota interaction free energy is consisted of the Lifshitz-van der Waas and Lewis acid-base component, it is necessary to separate it to these two parts in order to distinguish their effects. AB Figure 3 shows the Lewis acid-base interaction free energy G iwj between the surfactant and ignite in the aqueous medium. Apparenty, the trend in Fig. 3 is simiar with that in Fig. 2. For the g/cm 3 ignite sampe, Lewis acid-base interaction free energies are , , and mj/m 2 for TPnB, DOP, TP and MIBC, respectivey. On the other hand, the Lewis acid-base interaction free energies for the

10 Adsorption behavior of surfactants on ignite partices with different densities 1005 other two sampes are , , and mj/m 2 for the medium density ignite and , , and mj/m 2 for the higher density ignite sampes for TPnB, DOP, TP and MIBC, respectivey. Comparison of the tota interaction free energy with Lewis acid-base interaction free energy shows that the absoute vaue of each sampe is decreased sighty for the density fractions of g/cm 3 and g/cm 3, whie it increases sighty for the g/cm 3 ignite. Hence, the Lewis acid-base interaction pays a dominant roe in the adsorption performance of these non-ionic surfactants on the ignite surface in the aqueous medium. Fig. 3. Lewis acid-base interaction free energy between surfactant and ignite in aqueous medium AB G iwj Figure 4 shows the Lifshitz-van der Waas interaction free energy between surfactant and ignite in the aqueous medium. Firsty, it is quite different from the circumstances in Figs. 2 and 3. For the fraction of g/cm 3, Lifshitz-van der Waas interaction free energies are -1.35, -1.89, and mj/m 2 for TPnB, DOP, TP and MIBC, respectivey. For the other two coa sampes, the typica vaues are -0.23, , and mj/m 2 for the medium density ignite and 0.37, 0.51, 0.32 and 0.17 mj/m 2 for the higher density ignite. The absoute vaues of these interaction free energies are much smaer than Lewis acid-base interaction free energies, which indicates the comparativey weak interaction of Lifshitz-van der Waas. On the other hand, these interaction energies for the higher density ignite are a positive, which means the repusive Lifshitz-van der Waas force between the surfactant and higher density ignite in aqueous medium. In addition, arger Lifshitz-van der Waas force is observed obviousy in the ower density than that in the medium density.

11 1006 J. Wang, Y. He, B. Wen, X. Ling, W. Xie,. Wang AB Fig. 4 Lifshitz-van der Waas interaction free energy G iwj between surfactant and ignite in aqueous medium A schematic diagram of interfacia interaction between non-ionic surfactant and partice in water is shown in Fig. 5. The modes of ower and higher density ignite partice surfaces are derived from the FTIR anaysis (Figs. 1a and 1c). From the above resuts, two components of the interaction between surfactant and ower density ignite are both attractive in the aqueous medium, whie the Lifshitz-van der Waas interaction is repusive for higher density ignite. This repusive interaction between surfactant and higher density ignite surface resuts in the adsorption priority to the ower density sampe for the non-ionic surfactant. For the Lewis acid-base interaction free energy (Fig. 3), MIBC, with the smaest hydrophiic index, presents reativey stronger adsorption intensity on the ignite surfaces. Among the other three reagents, this adsorption intensity component decreases with the increase of the hydrophiic index. In addition, the Lewis acid-base interaction intensity increases sighty with the decrease of the ignite hydrophiic index (Fig. 3). Therefore, the ow hydrophiic index, regardess of surfactant or ignite, benefits for the Lewis acid-base interaction intensity. However, the Lifshitz-van der Waas interaction intensity shows a positive reationship with the Lifshitz-van der Waas components of surfactants for the ower and medium density ignite sampes. For the higher density ignite sampes, arger Lifshitz-van der Waas components of surfactants ead to the stronger repusive intensity. In concusion, the non-ionic surfactant has the priority to adsorb on the ower density ignite coa partice surfaces, which benefits for improving the hydrophobicity of the ower density ignite coa. Besides, to increase the adsorption difference, the surfactant shoud increase the Lifshitz-van der Waas component but decrease the Lewis acid-base component.

12 Adsorption behavior of surfactants on ignite partices with different densities 1007 Fig. 5. chematic diagram of interfacia interaction between non-ionic surfactant and partice in water, surfactant in red round, water in bue round, (eft) ower density ignite (right) higher density ignite Concusions This research focused on surface free energy components of three density ignite sampes and the interfacia interaction free energy between ow moecuar non-ionic surfactant and the ignite surface in water as a medium. The Lifshitz-van der Waas LW components were 28.58, and mn/m for g/cm g/cm 3 and g/cm 3 ignite sampes, respectivey, which decreased with the increase of the ignite density. However, the hydrophiic indexes were 2.97, 3.23 and 3.43, which were reated to the density positivey. Therefore, inorganic oxygen functiona groups were more hydrophiic than organic oxygen functiona groups. The interfacia interaction free energy resuts indicated that a non-ionic surfactant had the priority to adsorb on the ower density ignite surface. In addition, the Lewis acid-base interaction free energy payed a predominant roe in the adsorption interaction. This type interaction intensity increased with the decrease of the reagent hydrophiic index for each density sampe. On the other hand, the hydrophiic index of the surfactant was affected by its oxygen functiona group types and hydrocarbon chains. Ether bonds and hydroxy groups were more hydrophiic than ester groups though the same oxygen atoms were maintained in the reagent moecues of TPnB and DOP. In addition, a cycic hydrocarbon structure ed to the stronger hydrophiicity of surfactant. Hence, the ow hydrophiic index, regardess of surfactant or ignite, benefited for the Lewis acid-base interaction intensity. However, the Lifshitz-van der Waas interaction intensity showed the positive reationship with the Lifshitz-van der Waas components of the surfactants for the ower and medium density fractions of ignite. However, arger Lifshitz-van der Waas components of the surfactants ed to the stronger repusive intensity for the g/cm 3 ignite fraction. In concusion, to increase the adsorption intensity discrepancy between coa-rich and minera-rich ignite partices, it may provide benefit to increase the Lifshitz-van der Waas component of the nonionic surfactant.

13 1008 J. Wang, Y. He, B. Wen, X. Ling, W. Xie,. Wang Acknowedgements This work was supported by the Nationa Natura cience Foundation of China (No , ). We aso want to thank the support of cience Innovation Research of Coege Graduate in Jiangsu Province of China (No. KYLX15_1415). We woud ike to thank Advanced Anaysis and Computation Center of China University of Mining and Technoogy for their technica support. References ARIKAYA M., ÖZBAYOGLU G., 1995, Fotation characteristics of oxidized coa, Fue, 74(2): WEN W.W., UN.C., 1977, Eectro kinetic study on the amine foatation of oxidized coa, Transactions of the ociety of Mining Engineers of AIME, 262(2): ZHANG H., LIU Q., 2015, Lignite ceaning in Nac soutions by a reverse fotation technique, Physicochemica Probems of Minera Processing, 51(2): CELIK, M.., YOON, R.H., 1991, Adsorption of poy (oxyethyene) nonypheno homoogues on a ow ash coa, Langmuir, 7(8): VAMVUKA D., AGRIDIOTI V., 2001, The effect of chemica reagents on ignite fotation. Internationa Journa of Minera Process, 61(3): ZHANG W., TANG X., Fotation of ignite pretreated by sorbitan monooeeate. Physicochemica Probems of Minera Processing. 2013, 50(2): XIA W., Yang J., 2013, Reverse Fotation of Taixi Oxidized Coa, Energy Fues, 27, VAN O C.J., 2006, Interfacia Forces in Aqueous Media, New York: Marce Dekker, VAN O C.J., OMENYI.N., NEUMANN A.W., 1979, Negative Hamaker coefficients, Cooid and Poymer cience, 257(7): VAN O C.J., ABOLOM D.R., NEUMANN A.W., 1980, The hydrophobic effect: essentiay a van der Waas interaction, Cooid and Poymer cience, 258(4): CHAU T.T., 2009, A review of techniques for measurement of contact anges and their appicabiity on minera surfaces, Mineras Engineering, 22, GRUNDKE K., BOGUMIL T., GIETZELT T., JACOBACH H.J., KWOK D.Y., NEUMANN A.W., 1996, Wetting measurements on smooth, rough and porous soid surfaces, Progress in Cooid and Poymer cience, 101, ZOU W., CAO Y., LIU J., LI W, LIU C., 2013, Wetting process and surface free energy components of two fine iberated midding bituminous coas and their fotation behaviors, Powder Technoogy, 246, WANG H., GUO C., FU J., HE Z., LIANG W., CHEN X., ZHUANG C., 2011, Adsorption behavior of weak hydrophiic substances on ow-energy surface in aqueous medium, Appied surface science, 257, IVEON.M., HOLT., BIGG., 2000, Contact ange measurements of iron ore powders, Cooids and urfaces A: Physicochemica and Engineering Aspects, 166(1-3): LI Z., GIEE R.F., VAN O C.J., YVON J., CAE J., 1993, The surface thermodynamic properties of tac treated with octadecyamine, Cooid and Interface cience, 156(2): PAINTER P.C., NYDER, R.W., 1981, Concerning the appication of FT-IR to the study of coa: a critica assessment of band assignments and the appication of spectra anaysis programs. App. pectrosc, 35, LIANG Y., TIAN F., LUO H., TANG H., 2015, Characteristics of coa re-oxidation based on microstructura and spectra observation, Internationa Journa of Mining cience and Technoogy, 25,