CROATICA CHEMICA ACTA CCACAA 61 (1) 51-63 (1988)

CCA-1779YU ISSN 0011-1643

UDC 541.183.OriginaL Scientific Paper

Wettability of Coal Surface and its Surface Free EnergyComponents

Bronislaw Jaiiczuk; Wieslaw W6jcik, and Tomasz BialopiotrowiczDepartment of PhysicaL Chemistry, Institute of Chemistry, Maria Curie-Sklodowska

University, 20-031 LubHn, Poland

Received .Tanuary, 1987

Contact angles were measured for various coal ranks - waterdrop - n-hexane, n-dodecane and cis-decalin systems. Using the'results obtained the values of the dispersion and nondispersioncomnonents of the surface free energy of Polish coal of variousranks, from 31.1 to 35, were calculated. These values (Ysđ and YsP)were compared with those calculated on the basis of measurementsof contact anzles conducted earlier for various coal ranks - airbubble or hvdrocarbons dron - water systems. The results obtai-ned allowed us to conclude that, f'irstlv, the surface free energyof coals results from both the dispersion and nondispersion inter-molecular interactions and. secondlv, that disnersion componentsof this energy depend on the coal ranks. The value of nondisner-sion comnonents of the surf'ace free enerzv does not depend on thecoal •.anks and should not exceed 10 mJ/m2.

It was also found that takinz into account the values of cJis-persion comnonents of the studied coals surface free energy wecan classify them into three grOUDS:1) coal rank 31, 2) coal rank32 and 3) coal rank 33, 34 and 35.

INTRODUCTION

The wettability phenomenon of solids of which contact anzle is a visiblemeasure, plavs an important role both in every day life and in many indu-strial branches.t-" Amonz them beneficiation of miner als and ores by thefroth flotation method is based mostly on different wettabilities of the surfaceof useful and waste minerals.š"

Froth flotation is also used to enrich coa! slime." It is possible to learnthe mechanism of this process and its management when the surface pro-perties of coa! are known. Wettabi1ity is closely connected with these pro-perties. Therefore, many attempts are made to determine the coal surfacefree energy as well as its changes under the influence of many differentflotation reagentsš", on the basis of contact angle measurementsš'<-" or onthe basis of adsorption of nonpolar and polar liquids on coal surface."

Aplan et aI.5,6 measured the contact angles in the systems: various coa1ranks - 1iquids whose values of surface tension are different (nondispersioncomponent of surface tension is not equal to zero) - air. Using Zisman's10,1tmethod, they determined the critical surface tension of wetting of coalsin the range from 43 to 50 mJ!m2 (the average value being 45 m.L'm").

52 B. JANCZUK ET AL.

According to this finding, the critical surface tension of wetting doesnot practically depend on the coal rank, furthermore, measuring the contactangle of methylene iodide droplet on the surface of coal of various ranks",they calculated the dispersion component (Ysd) of the surface free energy ofcoals. These calculations were done assuming, firstly, that the methylene iodidesurface tension results from dispersive intermolecular interaction. Secondly,that the adsorption of vapour methylene iodide molecules on the surfaceof coals behind the sessile liquid drop does not change the surface freeenergy of coals. They found that the ysd value was identical with theaverage value of the critical surface tension of coal wetting (Ye) and it didnot depend on the coal ranko The identity between ysd and Ye may provethat the surface free energy of coal originates only from dispersive inter-molecular interaction.

Studies conducted by Staszczuk et al.s on n-octane and n-propanoladsorption on the surface of coal from the Lublin Coal Basin showed thatits surface free energy originated from dispersive and nondispersive inter-molecular interactions. The dispersion component of the surface free energyof coal calculated from the adsorption isotherm of n-octane is 50 m.f/m",which is close to the value reported by Aplan et al.6 The nondispersioncomponent of the surface free energy of the same coal calculated from theadsorption isotherm of n-propanol is 215 m.L'm".

Our studies described in the latest papers,12,13 carried out on thewettability of coal of various ranks by liquids (of a different part of di-spersion and the nondispersion components in its surface tension) did notconfirm such a big value of the nondispersion component (YsP) of the surfacefree energy of coals. These studies12,13showed that ysd was nearly the sameas reported by Aplan et al.6 and by Staszczuk et al.s but the ysP valueshould be smaller than 10 mJ/m2.

Taking into account the above controversies concerning the surfaceenergy components of coals we think that this problem has not been com-pletely solved. Therefore, we attempted to determine the dispersion andnondispersion components of the surface free energy of Polish coals on thebasis. of the contact angle measured in systems chosen so that it would bepossible to eliminate the influence of the liquid film pressure on the contactangle or to define precisely the changes of the surface free energy of coalunder the influence on this film.

For this purpose contact angle measurements were made in the systems:coals. of various rank - water drop - n-alkanes and analysis of valuesof the contact angle? in the systems: coals of various rank - air bubblesor n-alkanes drop - water.

THEORY

Dispersion and nondispersion components of the surface free energy ofsolids may be calculated, among other things, on the basis of contact anglemeasurements in suitably chosen systems.14-23Such systems may be 1) solid

air bubble - water, 2) solid - hydrocarbon drop - water and 3) solidwater drop - hydrocarbon. The equilibrium state in solid - air buble:"';<i.tersystem may be estimated by the Young equation in the form:

WETTABILITY OF COAL 53

where: Ysv = Ys - ne, Ysv is the surface free energy of the solid coveredwith a water film, ne is the value of changes of the solid surface freeenergy (Ys) caused by water film pressure, Yi;w is the interfacial free energyos solid-water, Yw is the surface tension of water, e is the contact anglemeasured through water phase.

For the solid - hydrocarbon drop - water system the equilibrium stateis described by the Young equation in the form:

YSVH - Ysw = YWH COS 8* (2)

where: YSVH = YSH- ne!> YSVH is the interfacial free energy of solid/waterfilm - hydrocarbon, nel is the value of the changes of interfacial freeenergy of solid-hydrocarbon (YSH) caused by water film pressure on thesolid surface, YWH is the interfacial tension of water-hydrocarbon and e*is the contact angle of a sessile hydrocarbon drop on the solid surfaceimmersed in water.

For the system solid - water drop hydrocarbon the equilibrium stateis described by the Young equation in the form:

YSH - YSfW = YWH COS 81 (3)

where: YSfW is the interfacial free energy of solid/hydrocarbon film-water,el is the contact angle of a sessile water drop on the solid surface immersedin hydrocarbon.

Aronson et al.24 found that in quartz-hydrocarbon drop - water systema durable water film was formed between the surface of the quartz andn-tetradecane drop. The thickness of this film is comparable with the waterfilm between quartz and air bubble in quartz - air bubble - water systemif the disjoining pressure is the same." Applying this statement to othersystems we may assume that ne ~ nel.

Taking it into account and using the geometric mean to express theinterfacial free energy of solid-hydrocarbon as a function of the dispersioncomponents of the surface free energy of solid and hydrocarbon-v-š fromEqs. (1) and (2) we obtain the expression:

Yw cos 8 - YWH COS 8* + YH( d)I/2 _Ys - 2 (YHd)I/2

(4)

Thus, measuring the contact angle in solid - air bubble - water and solid- hydrocarbon drop - water systems the dispersion components of thesurface free energy may be calculated from Eq. (4). The nondispersion com-ponents of the surface free energy may be calculated from Eq. (1) 01' (2)if we can define precisely the value of ne or ne].

Our studies on wettability of quartz and marble-'"?" have shown thata water film is formed on these solids, whose pressure changes from Ysto Yw. Assuming that Ys - ne ~ Yw, for the system described by Eq. (1),and using the geometric mean for the dispersive'<?" and nondispersive-v+"intermolecular interaction at solid-water interface we may rewrite Eq. (1)in the form:

YsP-2 vysPywp + Ysd- 2 VYsđYwđ + )'WCOS 8 = O (5)

54 B. JANCZUK ET AL.

where ysd, ywd, ysP and ywP are the dispersion and nondispersion componentsof the surface free energy of the solid (Ys) and water Yw, respectively.

The dispersion component of the surface free energy may be calculatedfrom Eq. (4). The nondispersion component of the surface free energy maybe calculated from Eq. (5) if the condition Ys- Tee = Yw is fulfilled.

The dispersion and nondispersion components of the solid surface freeenergy may be calculated from the contact angle of the water drop on solidsurface immersed in two hydrocarbons whose surface tensions (YH],and YHJare markedly different." Assuming, after Tamai et al.2:l, that the non di-spersion interactions at solid-water interface for two different hydrocarbonsare equal, and using the geometric mean of dispersive intermolecular inter-actions at solid-hydrocarbon interface, Eq. (3) may be transformed into:

(YH, - Yu,) - (YWH,cos el -- YWH,cos e2)

2 (v'ydH1 - v'ydH,)(6)

where: index »1« and »2« refer to two different hydrocarbons. In the caseswhere the hydrocarbon film occurs under a water drop in, solid - waterdrop - hydrocarbon system and the dispersive'<t'' and nondispersive'P+š-'"intermolecular interactions may be expressed by the geometric mean, wemay write:

YWH cos el = YH - Yw + ne2 - 2 v' Ysd YHd + 2 v' (Ys d - ne3) Ywd +(7)

where: Tee3 and Tee4 are the values of the change of the dispersion and non-dispersion components of the solid surface free energy, caused by the hydro-carbon film.

The nondispersion component of the solid surface free energy may becalculated from Eq. (7) if the value of Ysd is known and we can estimatethe value of Tee2 = Tee3 + Tee4.

Eq. (7) may be easily solved in two cases: first, when it is possible toassume that Tee2 = O and then, when Tee4 = 0, and Tee2 = ne, ~ Ysd- YHd.

EXPERIMENTAL

Contact angles were measured at 200C ± 0.1 -c by the sessile drop methodusing the microscope-goniometer system at magnification X 25. For the contact anglemeasurements various coal ranks were chosen. These coals origtnated from thecollieries: Siersza, Jankowice, Gotwald, Kleofas, Szczyglowice, Marcell and Gliwice,the ranks of which according to the polish classification'" were: 31.1, 31.2, 32,1, 32.2,33, 34 and 35, respectively. A comparison of the Polish classification'" of coal withASTM30is difficult because they are based on quite different principles.

All coal specimens were carefully selected, excluding those with cracks, mineralmatter, different macerals, particles of pyrite, occlusions ete. Then the most regularcoal specimens were examined under the microscope. The selected pieces werestored for a few months in a desiccator filled with a mixture of molecular sieves(4 A + 5 A).

After that, the pieces were roughly polished with emery paper, obtaining asize of about 2 X 2 X 1 cm. Next, for a given piece a plane parallel to the beddingplane was chosen and it was polished in air. Polishing was performed slowly byhand to avoid local overheating and oxidations. The pieces were polished with aseries of emery paper (Carborundum grif from 400 to 00). The final polishing wasmade with white typewriting paper until reflecting surfaces was obtained. All po-

WETTABILITY OF COAL 55Iishing operations were carried out in a special chamber filled with molecular sievesmixture (4 A + 5 A).

After polishing the coal plates were placed in a beaker filled with a hydro-carbon and it was placed in ultrasonic bath for 10 minutes. Next, the plates wereimmersed in the same fresh hydrocarbon in a quartz cell for 2 hours and then theywere placed into the measuring chamber.

On thus prepared plates contact angles were measured in the systems: coals ofvarious rank - water drop - n-hexane, n-dodecane and cis-decalin.

In the eanlier stage of contact arigle measurements it appeared that the obtainedresults strongly depended on the way a drop settled on asurface of the coal plate.A little shock changed the value of the contact angle, sometimes even by about20 or 30 degrees. Therefore, we decided to measure the contact angle in two ways.First, a 2 mm" water drop was very carefully settled on the surface of the coal plateat a distance from this surface and the contact angle was read out on both sides.Next, on the same plate the second, the third and other following drops of waterwere settled (of course each drop in a different place), and the contact angle wasread out in each case on both sides of each drop.

When the contact angles on the right and left sides of the drops differed bymore than 2°, the results were rejected. This procedure of contact angle measure-ment was repeated for all the studied systems at least for ten different plate s (fora given coal rank). On analyzing the resu Its obtained in this way, it appeared thatabout 90010for each rank lay within an interval of 4°. The average value of contactangles for a given coal rank lying in this interval is called the maximai contactangle (e~,J.

The second way of contact angle measurernents differed from the first one onlyin the procedure of water drop settling. In this case 2 mm" water drops were settledon the surface of the coal plate at a higher but constant distance from this sur faceand sometirnes a settling drop was trapped, as Aplan et al. reported."

However, this time discrepancies among the contact angle results were higherand about 85010 of the obtained valu es lay in the 4° interval. The average value ofa contact angle for a given coal rank lying in this interval is called the minimalcontact angle (8~;n).

RESULTS AND DISCUSSION

The average values of contact angles denoted as E>max and E>min for thestudied systems are listed in Tables I and II, respectively.

TABLE I

Maximal Valu es of Contact Angles (em•.•) for Water Drops Settled on the Surfaceof Coals Immersed in: n-Hexane, n-Dodecane and cis-Decalin and Values of theDispersion and Nondispersion Components of the Suritu:e Free Energy Calculated

from Eqs (6) and (8), Respectively

~C Hexane Dodecane cis-Decalin'" Average••ul ul 1 2 3 values;:s ••..••o'"oi:: 2"' ..... f8J •••• rs'1 rs· e.: rsđ rs· 8max rsđ rs· rsđ rs·:>0

31.1 139.5 50.0 0.51 145.8 51.9 0.52 149.4 50.9 0.51 50.9 0.5131.2 141.6 46.5 0.41 147.5 48.8 0.42 150.6 47.6 0.41 47.6 0.4132.1 140.6 59.0 0.44 148.9 47.0 0.39 151.7 53.0 0.44 53.0 0.4232.2 143.7 42.9 0.33 149.1 51.3 0.35 152.9 46.9 0.32 47.0 0.3333 141.9 55.7 0.35 149.3 64.2 0.40 156.0 59.6 0.37 59.8 0.3734 141.0 62.5 0.37 150.1 64.6 0.38 157.1 63.5 0.37 63.5 0.3735 142.2 55.3 0.35 150.1 56.6 0.35 155.2 55.8 0.35 55.8 0.35

56 B. JANCZUK ET AL.

TABLE II

Minimal Values of Contact Angles (emi") for Water D1'OPS Settieii on the Suriaceof Coals Immersed in: n-Hexane, n-Dodecane and cis-Decalin and Values of theDispersion and Nondispersion Components of the Suriace Free Energy of Coal

Catculateti from Eqs (6) and (8), Respectively

~s:: Hexane Dodecane cis- Decalinol AverageI-<

ul ul 1 2 3 values;:l •.....OOl.....oI-< ()Ol ••..•

emin ysd ysP emi. ysd ys" emin ys" ys" ysd ys">031.1 107.4 59.0 5.49 112.5 53.9 5.42 115.3 56.5 5.50 56.5 5.4731.2 108.6 53.4 5.18 113.1 58.5 5.25 1Hi.4 55.8 5.18 55.9 5.2032.1 110.7 68.7 4.48 117.0 64.5 4.42 121.0 66.6 4.48 66.6 4.4632.2 111.2 64.8 4.39 117.1 62.6 4.37 120.9 63.7 4.39 63.7 4.3833 109.2 77.1 4.73 116.3 67.5 4.66 120.6 72.4 4.78 72.3 4.7434 113.1 68.4 3.79 119.5 85.2 3.98 125.7 76.3 3.80 76.6 3.8635 112.8 70.4 3.82 119.4 90.4 4.04 126.1 79.8 3.83 80.2 3.90

Table I shows that the Gm•x values are in the ranges from 139.5° to 143.7°,145.8° to 150.1° and 149.4° to 157.1° for coal ranks from 31.1 to 35 immersedin n-hexane, n-dodecane and cis-decalin, respectively.

The highest Gm•x values are for coals immersed in cis-decalin, and thelowest for coals immersed in n-hexane. The biggest difference in Gmax valuesis for coal rank 34, and it sometimes reaches 16° (see Table I). It can alsobe seen from Table I that the changes of contact angles (Gm•x) for the studiedcoals immersed in hydrocarbons are small. The changes for n-hexane anddodecane did not exceed 5°, and for cis-decalin the difference was 7.7°.

The minimal values of contact angles (Gmin) are in the ranges from107.4° to 113.1° and 112.5° to 119.5° and 115.3° to 126.10 for the studied coalsimmersed in n-hexane, n-dodecane and cis-decalin, respectively. The mea-sured Gmin values as in the case of Gro•x are lowest for coals immersed inn-hexane and the highest for coals immersed in cis-decalin. The biggestdifferences among Gmm values for all coal ranks immersed in n-hexane,n-dodecane and cis-decalin are 5.7°, 7.0° and 10.8°, respectively. It shouldbe emphasized that the difference between Gmin of coal of the same ranksimmersed in n-hexane and cis-decalin (Table II) is smaller than for Gm•x(Table I). The differences between the valu es of Gmin (Table II) and Gm•x

(Table I) is in the range from 27.9° (coal rank of 34 - water dropletn-hexane) to 35.4° (coal rank of 33 - water droplet - cis-decalin).

Assuming that the nondispersive interactions at interface of coaln-hexane, coal - n-dodecane and coal - cis-decalin are the same afterTamai et aP3 and using the results listed in Tables I and II, the dispersioncomponent of the surface free energy of the studied coals may be calculatedfrom Eq. (6). Thus, using Eq. (6) we calculated the ysd values for coal ranksfrom 31.1 to 35, employing in calculations the values of Gro•x (Table I) andGroin (Table II), as well as literature values31,32 of the surface tension ofhydrocarbons (YH(J,2») and water-hydrocarbons interface tension (Table III),

WETTABILITY OF COAL 57

TABLE III

Values of the Surjace Tension of Water, cis-Decalin and Homological Series ofHydrocarbon from n-Hexane to n-Hexadecane and Valu es of the Interfacial Tension

of Water-Hydrocarbons14.31.32

Various rank of coalsLiquid YH /,WH 31.1 31.2 32.1 32.2 33 34 35

G* G* G* G* G* G* G*

Hexane 18.49 51.10 85.4 85.5 94.4 96.1 110.3 111.4 114.0Heptane 20.30 51.10 86.3 86.7 97.2 98.8 112.5 114.3 116.3Octane 21.80 51.00 87.2 88.1 99.0 99.9 114.0 116.2 117.5Nonane 22.91 51.02 88.9 88.7 99.8 102.0 114.3 117.5 118.7Decane 23.90 51.10 88.8 89.9 101.5 103.2 115.1 117.6 120.3Undecane 24.70 51.10 88.7 90.2 102.5 103.4 119.3 120.5 121.6

Dodecane 25.08 51.12 88.7 90.2 102.0 104.5 117.6 121.0 122.1Tridecane 25.38 51.14 89.3 91.0 101.4 104.2 118.2 120.8 121.6

Tetradecane 25.60 51.20 89.6 91.4 102.2 102.8 117.8 121.0 122.6Pentadecane 25.80 51.20 89.3 90.3 102.7 103.0 119.3 120.2 123.1Hexadecan 26.35 51.22 90.2 91.1 103.1 105.6 119.7 121.1 122.4

cis-decalin 32.18 52.00

Yw Yw G G G G G G GWater

72.8 21.8 48.1 48.6 51.8 52.2 56.0 56.5 57.0

Values of the contact angles:ameasured by us in various coal ranks-hydrocarbon-water system,"" measured by us in various coal ranks-air bubbles-water system,"

Valu es of ysd were calculated for all possible combinations of the hydro-carbons used, i. e. 1) n-hexane - n-dodecane, 2) n-dodecane - cis-decalinand 3) n-hexane - cis-decalin. The ysd values calculated from Gmax andGmin for these combinations of hydrocarbons are given in Tables I and II,respectively. It can be seen from Table I that the ysd values (calculatedfrom Gmax values) for the coals studied are in the range: 1) from 41.5 mJ/m2

(coal rank 31.2) to 62.5 mJ/m2 (coal rank 34), 2) 47.0 mJ/m2 (coal rank 32.1)to 64.6 mJ/m2 (coal rank 34) and 3) 46.9 mJ!m2 (coal rank 32.2) to 63.5 mJ/m2

(coal rank 34) calculated for three early reported combinations of hydro-carbons. The average ysd values for the examined coals are in the rangefrom 47 m.L'm" (coal rank 32.2) to 63.5 mJ/m2 (coal rank 34).

The dispersion components of the surface free energy of low-energeticcoals (coal ranks 31 and 32) is sornewhat smaller than for high-energeticcoals (coal ranks 34 and 35). For all the coals examined here the averageysd value is 53.9 mJ/m2 and it is higher by about 14 mJ/m2 than ysd deter-mined from the adsorption isotherm of n-octane on the surface of coal fromthe Lublin Coal Basin." This average value is also greater by about 9 mJ/m2

58 B. JANCZUK ET AL.

than that of ysd determined by Aplan et a1.6 on the basis of the contactangle measurements for methylene iodide on the surface of various coalsand than the value determined by US12 for the same ranks of coal fromcontact angle measurements of various liquid.

As seen from Tables I and II, the ysd values calculated from Bmax aresmaller than those calculated from Bmin. Appropriate yscl values calculatedfrom Bmin for the pairs of hydrocarbons reported earlier 1) n-hexane -n-dodecane, 2) n-dodecane - cis-decalin and 3) n-hexane - cis-decalin arein the ranges: 1) 53.4 mJ/m2 (coal rank 31.2) to 77.1 mJ/m2 (coal rank 33),2) 53.9 mJ/m2 (coal rank 31.1) to 90.4 mJ/m2 (coal rank 35) and 3) 55.8 m.Izm"(coal rank 31.2) to 79.8 mJ/m2 (coal rank 35). Hence, the average valuesof ysđ calculated from Bmin are in the range from 55.9 mJ/m2 to 80.2 m.L'rn".Table II shows that in contrast to the ysđ values listed in Table I the ysdvalues increase as the rank of coal changes from 31.1 to 35.

Taking into account the ysd values, we may divide the coals studiedinto three groups: 1) coal rank 31, 2) coal rank 32 and 3) coal ranks 33, 34and 35. In this case, as it can be seen from Tables I and II, the ys..l valuesmarkedly differ from those reported by Aplan et a1.6 as well as from ourearlier results-" obtained for the same coal ranks.

Therefore, one may ask why such different contact angle values aremeasured for coals of the same ranks and in the same system and whywe obtained such different ysd values from the measured Bmax and Bmin inthe same systems.

Eq. (6), us ed here for the calculation of ysd of coals, was derived byTamai et aL.23 with, the assumption that there is no hydrocarbon film undera water droplet settled on the surface of the solid immersed in hydro-carbons (HI and H2) as well as that nondispersive interactions at solid -water interface in hydrocarbons HI and H2 are the same. In turn, our earlierstudiesš" did not exclude the possibility that a hydrocarbon film may occourund er a water droplet in solid - water droplet -- hydrocarbon system.

The question whether a hydrocarbon film may be present under a waterdroplet may be answered by solving Eq. (7). Assuming that no hydrocarbonfilm exists under the water droplet and that ne2 = ne3 + ne4 = O, Eq. (7)can be transformed into:

/9 YWHcos el + Yw- YH+ 2 (Ysd)I/2[(YHd)I/2- (Ywd)I/~l

(Ys")! - = -'-'-=-----.-.::-----"--.-.::'----.-.::------=-::----'-'--2 (Yw")1/2

Nondispersion components of the surface free energy of the studied coalswere calculated by putting into Eq. (8) the value of Bmax (Table I) or Bmin(Table II) instead of BI and values of YWH,YH,Yw, and YwP (Table III) fromIiterature.v-'" The ysd values used for calculations of Ysp were those theaverage values of which were figured out from Eq. (6) for the three cornbi-nations of hydrocarbons used earlier.

The ysP valu es calculated from Eq. (8) by using Bmax or Bmin are listedin Tables I and II, respectively. The ysP values shown in Table I show thatthey are in the range from 0.32 m.L'm" (coalrank of 32.2 for cis-decalin)to 0.51 mJ/m2 (coal rank of 31.1 for cis-decalin) for the studied coals. Hence,

(3)

WETTABILITY OF COAL

it appears that the value of nondispersive interactions at coal-water interfacedoes not depend on the kind of hydrocarbon in which coal plates wereimmersed to measure Bmax and the rank of coal.

In the case of using Bmjn (for calculations of the YsP) (Table II) onecannot see any great differences in the values of Ysp (Table II) of the studiedcoal ranks. The ysP values are in the range from 3.79 mJ/m2 (coal rank 34for n-hexane) to 5.50 m.I/rn" (coal rank 31.1 for cis-decalin).

The ysP for coal ranks 31.1 and 31.2 are alittle bigger in comparisonwith those for the other studied coal ranks. We did not find any correlationbetween the coal ranks and the changes of ysP calculated from Bmjn•

The results of the ysP presented in Table II also confirmed the assumptionof Tamai et al.23 that nondispersive interactions at solid-water interface areidentical regardless of the kind of hydrocarbons in which the solid wasimmersed. The yPs values for a given coal rank are nearly equal for theused hydrocarbons (Tables I and II).

It should be emphasized that the Ysp values calculated from Eq. (8) usingBmjn are very close to those we previously figured outJ2 applying anothermethod.

From the comparison of the Ysp values insert ed in Tables I and II itappears that the Ysp values calculated from Bmjn (Table II) are ten timeshigher than those calculated by using Bmax (Table I). This may suggest,according to our earlier studies.š" that in the coal water droplet-hydrocarbonsystem one cannot exclude the existence of hydrocarbon film under thewater droplet at least in the case of the water droplet settled very carefullyon the coal surface. Hence, the resulting ysd and Ysp values calculated fromEqs. (6) and (8) by using Bmax may be alittle smaller.

In order to verify the ysd and ysP valu es calculated from Eqs. (6) and(8) on the basis Bmax and Bmjn, we calculated these valu es for the studiedcoals using Eqs. (4) and (5).

In these calculations we used the contact angle values measured in thesystem coals - air bubble (B - Table III) ar hydrocarbon droplet (B* -Table III) - water? and the literature values of Yw, YH and YWH which arealso listed in Table III.14,31,32The contact angles B* were measured for drop-lets of hydrocarbons in the homological series from n-hexane to n-hexadecane.The ysd calculated from Eq. (4) and ysP calculated from Eq. (5) are listedin Table IV. To calculate the ysP values we used the ysd calculated fromEq. (4) and the contact angles measured in coal - air bubble - watersystem. The values of Yw = 72.8, ywd = 21.8 and Ywp = 51 mN/m were takenfrom Iiterature.v'

It should be emphasized that Eq. (5) is a quadratic one and for each ysdvalue we obtain two different values of Ysp. Table IV shows that for thegiven coal rank similar values of ysd were obtained using 11 aliphatichydrocarbons, and for the studied coal ranks from 31.1 to 35 the dispersioncomponents of the surface free energy increased from 53.2 m.L'm" to 83.6m.L'm",

cl:>TA

BLE

IVO

Values

ojthe

Dispersion

and

Nondispersion

Component

ofCoal

Surface

Free

Energy

CaLCulated

from

Eqs

(4)and

(5),Respectively

Var

ious

rank

ofco

als

Liq

uid

31.1

31.2

32.1

32.2

3334

35

Ys'l

/,sP

)ls·

)lsP

)ls"

rs!)

)ls·

rs!)

)ls·

)lsP

)ls"

)lsP

)lsa

)ls"

Hex

ane

53.7

127.

253

.012

9.2

61.5

128.

863

.512

7.7

80.1

117.

880

.911

8.2

84.3

114.

99.

038.

518.

608.

9011

.77

11.6

412

.71

Hep

tane

53.0

127.

953

.112

9.1

63.4

126.

765

.212

5.7

80.0

117.

981

.911

6.8

84.0

115.

38.

858.

549.

169.

4211

.73

12.0

912

.57

Oct

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WETTABILITY OF COAL 61Taking into account the average values of ysd, the studied coals may

be divided into three groups, i. e.: 1) coal ranks 31.1 and 31.2; 2) coal ranks32.1 and 32.2; and 3) coal ranks 33 , 34 and 35. Coals within the same groupspossessed similar values of the dispersion components of their surface freeenergy. The data in Table IV asIo show that the ysd calculated from Eq. (4)are practically equal to those calculated from Eq. (6) using emin (Table II).Differences between the ysd values did not exceed 5.5 m.Izrn", except forcoal rank 33 (Ysd - 72.3 and 80 m.L'm"). In the case of the ysd calculatedfrom Eq. (6) but using emax the difference reached 27.8 mJ/m2 for coalrank 35.

From Table IV it also results that lower ysP values are in the rangefrom 8.51 m.L'm" to 12.81 mJ/m2 and the higher ones from 114.3 m.L'm" to129.2 m.L'm".

It appears that the lower ysP values are from 1.5 to 3 times bigger thanthose calculated from Eq. (8) (Table II), and they are almost the same asit was reported;" However, the higher ysP values are nearly twice as low asthose calculated on the basis of the adsorption isotherm of n-propanol forcoal from the Lublin Coal Basin."

The differences among ysP values for each rang of coal calculated on thebasis of measured contact angles in the systems coal - water drop - hydro-carbon (Table II) and coal - air bubble - water (lower - Table IV) pro-bably result from the existence of a hydrocarbon film under the waterdrop. This film can retard polar interactions between coal and water. Hencethe ysP values calculated from Eq. (8) (Table II) are lower.

As regards the other values of ysP (higher - Table IV), we think thatthey are rather too high for coal because it is very difficult to find arealistic explanation for such high values which are only a consequence ofalgebraic solution of Eq. (5). However, additional studies should be madeto clarify this point.

On the basis of the calculations presented above we can also concludethat the surface free energy of the studied coals resulted from the dispersiveand nondispersive interface interactions, which supports our earlier studies.P-"However, in contrast to the earlier suggestions,5,6,12,13we think that the sur-face free energy of coal depends on its ranko

The presence of water molecules on coal surface can probably show»a real picture« of the surface free energy of the studied coals, which wasindicated by Staszczuk'" in his studies of coal properties using the methodof thermal analysis.

The value of the critical surface tension of wetting of the istudied" coalsis 45 m.L'm" and is equal to the critical surface tension of wetting of quartzand iron ores.'! It is commonly known that the surface of these solids ishighly hydrated and the value of the surface free energy distinctly exceedsits critical surface tension of wetting.

From the performed calculations it can also be concluded that for thecoals studied the nondispersion components of the surface free energy arepractically the same and in our opinion the most proper value should beabout 10 mJ/m2 or less. This component may result from the existence ofsuch polar groups as hydroxyl, carboxyl, carbonyl etc. on coal surface.

62 B. JANCZUK ET AL.

REFERENCE S1. A. D. Zi m o n, Adhesion of Liquids and WettabHity, Chem., Moscow (1974)

[in Russian].2. A. W. Ada m s o n, PhysicaL Chemistry of Surfaces, Interscience, New York

(1960).3. J. Le ja, Surjace Chemistry of Froth FLotation, Plenum Press, New York-

-London (1982).4. W. I. K 1a s s e n, Coai FLotation, Slask, Katowice (1966). [Polish Translationj.5. B. K. Par e k h and F. F. A p l a n, Recent DeveLoprnents in Separation

Science, [N. N. Li, R. B. L o n g, S. A. Ste r n, and P. Sam o s u n dar a nEds.J Vol. IV, pp. 107-111, CRC Press, West Palm Beach, FL (1978).

6. J. A. G u t i e r r e z - Rod r i q u e s, R. J. P ure e Il, and F. F. A p La n,CoHoids Suriaces 12 (1984) 1.

7. B. Ja il c z u k and T. B i a l o P i o tro w i c z, Przem. Chem. 64 (1982) 119.8. P. S tas z c z u k, L. H o 1 Y s z, T. B i a l o P o tro w i c z, and B. B i 1 i il s k i,

Przem. Chem. 65 (1986) 158.9. J. S a b l i k, PoLish J. Chem. 59 (1985) 433.

10. H. W. Fox and W. A. Z i s man, J. CoHoid Sci. 7 (1952) 428.11. W. A. Z i s man, Contact AngLe WettabiLity and Adhesion Advances in Che-

mistry, [F. M. F o w k e s Ed.] Series 43 pp. 1-51, American Chemical Society,Washington (1964).

12. W. W 6 j cik, T. B i a l o P i o tro w i c z, and B. Ja il c z u k, Submitted forpublication.

13. T. B i a l o P i o tro w i c z, B. Ja il c z u k, and W. W 6 j cik, submitted forpublication.

14. F. M. F o w k e s, J. Phys. Chem. 66 (1962) 382.15. F. M. F o w k e s, Ind. Eng. Chem. 56 (1964) 40.16. D. K. O w e n s and R. C. W end t, J. AppL PO/lImer Sci. 13 (1969) 1741.17. D. H. Ka e 1b 1e and C. U s, J. Adhesion 2 (1970) 50.18. D. H. Ka e 1b 1e, J. Adhesion 2 (1970) 66.19. Y. Kit a z a k i and H ata, J. Adhesion 4 (1972) 123.20. S. W u, J. PoLym. Sci. Part C 34 (1971) 19.21. S. W u, in PoLymer BLends [D. R. P a u 1 and S. New man Eds.] Vol. 1,

pp. 243-293, Acad Press, New York (1978).22. K. L. Mit t a 1, in Adhesion Science and TechnoLogy [L. H. Lee Ed.] Vol.

9A, pp. 129-168, Plenum Publishing Corporation, New York (1976).23. Y. T ama i, K. Ma k u u c h i, and M. S u z u k i, J. Phys. Chem. 71 (1967) 41.24. M. P. Ar o n s o n, M. F. Pet k o, and H. M. P r i n c e n, J. CoHoid Inter-

face Sci. 65 (1978) 296.25. J. F. Pad day, in Thin Liquids FiLms and Boundary Layers, p. 64, Academic

Press, London-New York (1970).26. B. J a il c z u k, E. C h i b o w s k i, and T. B i a l o p i o tro w i c z, J. CoHoid

Interjace Sci. 102 (1984) 533.27. B. Jailczuk and T. Bialopiotrowicz, J. Mater. Sci. 21 (1986) 1151.28. B. Ja il c z u k, E. C h i b o w s k i, and T. B i a l o p i o tro w i c z, Chem. Pap.

40 (1986) 349.29. PoLish Standard, PN!G-97002, CLassification of Coais, CoaLs Ranks (1950).30. AS TM D 388-77, Standard CLassification of Cools by Rank, (1981) AnnuaL Book:

of ASTM Standards, American Society for Testing Materials, Philadelphia DEPart 26 (1981).

31. B. Ja il c z u k and E. C h ib o w s k i, J. CoHoid Interface Sci. 95 (1983) 268.32. R. J. G o o d and E. El b im g, Ind. Eng. Chem. 62 (1970) 54.33. B. J a il c z u k and E. C h i b o w s k i, PoLish J. Chem. 59 (1985) 1251.34. P. St a s z c z u k, Private communication.

WETTABILITY OF COAL 63SAŽETA-K

Kvasivost površine ugljena i pripadne komponente površinske slobodne energije

Bronislaw Jaiiczuk, Wieslaw W6jcik i Tomasz Bialopiotrowicz

Mjereni su kontaktni kutovi za sisteme: različite vrste ugljen/vodena kap/n-heksan, n-dodekan ili cis-dekalin. Iz dobivenih rezultata izračunane su disperzijskei nedisperzijske komponente površinske slobodne energije različitih vrsta ugljenaiz Poljske.

Te vrijednosti (Ysd i ys") uspoređene su s vrijednostima izračunanima na temeljumjerenja kontaktnih kutova provedenih ranije za sisteme: ugljen/mjehurić zraka ilikap ugljikovodika/voda. Dobiveni rezultati ukazuju na to da površinska slobodnaenergija ugljena potječe iz disperzijskih i nedisperzijskih međumolekulskih inter-akcija, a da disperzijska komponenta te energije ovisi o vrstama ugljena. Vrijednostnedisperzijskih komponenata površinske slobodne energije ne ovisi o vrsti ugljenai ne prelazi vrijednost 10 mJ/M2.

Na temelju tih rezultata može se zaključiti da razlog neslaganjima u vrijedno-stima površinske slobodne energije ugljena što su ih dobili razni autori, može bitiprisutnost vodenog filma na površini ugljena. Vodeni film može također biti razlo-gom što su razni autori dobili gotovo jednake vrijednosti za površinsku slobodnuenergiju za različite vrste ugljena.