"September 1991.""To be presented at the 1991 AIChE annual Meeting, November 17-22, 1991, Los Angeles, CA.

Stratton, Robert A.

Scholarly Materials And Research @ Georgia Tech

Institute of Paper Science and TechnologyAtlanta, GeorgiaIPST TECHNICAL PAPER SERIESNUMBER 398THE FLOTATION OF STICKY CONTAMINANTSFROM RECYCLED FIBER STREAMSR.A. STRATTONSEPTEMBER, 1991The Flotation of Sticky Contaminants from Recycled Fiber StreamsR.A. StrattonTo be presented at the1991 AIChE Annual MeetingNovember 17-22, 1991Los Angeles, CACopyright 1991 by The Institute of Paper Science and TechnologyFor Members OnlyNOTICE & DISCLAIMERThe Institute of Paper Science and Technology (IPST) has provided a high standard of professional serviceand has put forth its best efforts within the time and funds available for this project. The information andconclusions are advisory and are intended only for internal use by any company who may receive thisreport. Each company must decide for itself the best approach to solving any problems it may have andhow, or whether, this reported information should be considered in its approach.IPST does not recommend particular products, procedures, materials, or service. These are included onlyin the interest of completeness within a laboratory context and budgetary constraint. Actual products,procedures, materials, and services used may differ and are peculiar to the operations of each company.In no event shall IPST or its employees and agents have any obligation or liability for damages including, butnot limited to, consequential damages arising out of or in connection with any company's use of or inabilityto use the reported information. IPST provides no warranty or guaranty of results.The Flotation of Sticky Contaminants from Recycled Fiber StreamsRobert A. StrattonPrepared for presentation at the AIChE Annual Meeting/November 17-22, 1991; ForestProducts Symposium, Recycling Session.Copyright Robert A. StrattonDate: August 1991UNPUBLISHEDAIChE shall not be responsible for statements or opinions contained in papers or printed inits publications.INTRODUCTIONRemoval of contaminants from secondary fiber ("deinking") is accomplished by washing,flotation, or a combination of the two. Washing is effective for removing particles smallerthan about 10-20 im in extent1, while flotation works best for particles in the range 10-2001,2. Newsprint inks can be readily dispersed into small particles which can be removedby washing. Pulping of secondary fiber containing sticky contaminants such as hot-melt orpressure-sensitive adhesives typically results in the production of larger particles. We haveexamined the application of surface chemistry principles to the flotation of such stickycontaminants.The process of removal of one type of solid particle from the presence of another by (froth)flotation has been used in ore beneficiation and deinking of paper for many years.Separation is dependent on there being a difference in surface free energy between the twotypes of solids (i.e., fiber and contaminant). When an air bubble attaches to a solidsuspended in a liquid, a contact angle 0 can be defined as shown in Figure 1, i.e., the anglebetween the solid surface and the tangent to the air/liquid interface at the intersection withthe solid surface. The angle is measured through the liquid phase.1Fig 1. Schematic Representation of Bubble Attachment to a Sticky.2* a * * * * * *a *a*a*a* * *a*a* aa****C*. .* a .....aa . . . . .. *. ....... ....... . , --. ................. . . .... ... . .a _ aC ..... aa a a....... aC a.i. . . ... . . . . .. . ..a C. . . . . . .. ............ a. .a a a a a a . . . .a..a. .a a . . .. ......... a. .. ..... . .. . ..a.a.a.a. . . . aaa......... aa . C. c.. . .. C . . . .. .. . . ...... .......a... . .. a. ..... Ca. ...^..^... *,^ ^^^^ ^^^^ ^^. ^^^^ ^^^^ ^^*............................. ^^ .. ^**.. .............. ^'I· · · · · · ·* ''^^^* ^^^'^^ . ^^*'^ ^ *^^^*^.... - ^ *.^^^^^^^.^.^....^^..^^^ ^^^i^^^.-^^t H ^^^**'-^;^C'5^*ir*iC r.z~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 5't 15*~ · 4.* ^ ^ ^^... r * ̂^.^^..r r . . . .'-·- ' ' ,_In order for a liquid to spread on a solid, the work of adhesion, WSL, must be greater thanthe work of cohesion, WLL, of the liquid. These quantities are related to the interfacialtensions as follows:WsL = sv + YLV YSL (1)WLL= 2YLV (2)where the subscripts indicate the solid, liquid, or vapor phase. The spreading coefficient,SL/S, is given bySL/ = WSL -WLL (3)or, in terms of the interfacial tensions, as(4)SL/S = sv - LV -SL (4)A necessary condition for flotation is that the air bubble be able to displace the liquid inorder to become attached. This is equivalent to requiring that the liquid not spread on thesolid or SL/S < 0. The forces created by the interfacial tensions are shown in Figure 2. Aforce balance (in the plane of the surface of the solid) results3 in Young's equation,YSV= YSL + YLV Cos (5)3YSL YSV/LVFig 2. Balance of Surface Tensions at the Air-Liquid-Solid Interface.4When equation 5 is substituted into 4, the spreading coefficient becomesSLS =YLV (Cos 0 - 1) (6)and the necessary condition for bubble attachment (S < 0) is Cos 0 < 1 or 0 > 0. Althoughnecessary, this is not a sufficient condition for practical applications. In a flotation cell, theair bubble/solid attachment must be able to withstand the forces caused by the differentialbuoyancy of the two and the shearing forces produced by the bubble ascension and theturbulent mixing.The strength of the attachment Fa is equal to the product of the perimeter of the bubble/solidinterface and the liquid surface tension.Fa = 7dyLV Sin 0 (7)Here, d is the diameter of the circle formed by the attachment (see Figure 1), and theexpression is multiplied by Sin 0 to produce the component of the surface tensionperpendicular to the solid surface.This can be rewritten in terms of the preattached bubble diameter D and the ratio d/D asFa = 7 D (d/D) YLV Sin 0 (8)5d/DI,b PI00rS O n o 1 \ \1 X aboo mo ° >c> C\ m30 o0 \/1).. D 05 Q. \06Assuming that the volume of the bubble does not change upon attachment and that theattached bubble has the shape of a spherical segment, we can express the equality of thetwo volumes asV = (7/6) D = (7/24) h (4h + 3d 2) (9)where h is the height as defined in Figure 1. By expressing h in terms of d and the contactangle 0, equation 9 can be rearranged tod/D = 2 Sin 0/ (1 + Cos 0) +3(1 + Cos 0) Sin 2 0](10)This function is shown in Figure 3 where the almost linear dependence of d on contactangle for a given bubble size is seen. (Strong positive curvature occurs at still higherangles, but the latter are of no practical importance.) Over the more limited range 0 < 0 <65° found experimentally in the present work, equation 10 can be closely approximated byd/D = Sin (11)The exact and approximate relations are compared in Figure 4.Substituting the result of equation 11 into equation 8, we findFa s D YLV Sin 2 (12)7d /D00C'.4.1-O 3CDCTxcn.ro3§F000O0CoCL(tCrA0-o00,00m400.0*DO)0a)00, _Lenz0x-=0z8orF a - D 'LV ( 1 - Cos2 e) (13)The strength of bubble attachment is thus strongly dependent on contact angle which in turnis determined by the surface energy of both the solid and the medium (equation 5).To predict values of the contact angle from equation 5, the interfacial tension, YSL, betweenthe solid and the medium is needed. Girifalco and Good4 showed that this parameter canbe calculated from the surface energies of the solid and the liquid, Ysv and yLV,respectively.2 (ysvy~v) 1/2 (14)SL = Ysv + YLV -2 (s 'LV (14)Fowkes 5 observed that, for the interaction of polar materials with nonpolar ones, only theLondon dispersion component of the intermolecular forces of the polar material wassignificant. He suggested separating the surface energy of the polar material intodispersion (yd) and polar (yp) contributions and that these were additive.Y=¢+ (15)For the interaction of an aqueous phase, for example, with a hydrophobic solid, Fowkessuggests that equation 14 be written asYSL= S+ yL2(s 1) (16)9Here, YL represents the combined contributions (equation 15) of the dispersion and polarcomponents of the aqueous phase; L is the dispersion component, and Y is the surfaceenergy of the solid having only a dispersion component. The subscript V indicating theinterface with air has been omitted for simplicity. Equation 16 can now be combined withthat of Young (equation 5) to give an expression known as the Girifalco-Good-Fowkes-Young (G-G-F-Y) equation 3 .Cos 0 = -1 + i2 s) / YLcose=1+2('~Y~J/yL (17)This equation gives explicitly the dependence of the contact angle on surface energy of thesolid, Us, and that of the aqueous phase, yL. The dispersion component of the aqueousphase, Y'L, is expected to be a constant. We will test the validity of equation 17 forflotation systems by independently measuring Cos 0, s, and YL. These results will thenbe combined with the concepts behind equation 7 to interpret flotation efficiency data. Theexperimental results were taken6 from research contracted by the American Paper Institute.EXPERIMENTALFour "stickies" materials were chosen for study and are listed in Table 1. Theircompositions were determined from FTIR spectra. The surface energy of the stickies wasdetermined by measuring the contact angle of a 4AL drop of methylene iodide placed on thesolid. The surface tension of methylene iodide is primarily due to dispersion forces withvalues of XL and L of 50.8 and 50.4 mN/m, respectively. The measured contact angle andthese values for yL and dL were substituted into equation 17 which was rearranged to yields- The calculated surface energies are listed in Table 1.10Table 1. Composition of StickiesSurface Energyergs/cm 2Wax-Coated Kraft (WCK)Composition28.0 Paraffin waxHot-Melt Adhesives27.1 Polyvinylacetate +Polystyrene +ParaffinicHydrocarbon(Wax orPolyethylene)50.7 Glycerol Esterof polymerizedrosin41.7 Polyethylenebase11StickyHMDHMBHMEContact angles of drops of several simulated deinking liquors on the stickies weremeasured immediately (five seconds) and as a function of time of contact up to 20 minutes.(The measurements were carried out in a humidified chamber to prevent drop evaporation.)Values of the contact angle between four different liquors and the four stickies measured atfive seconds are listed in Table 2. The time dependence of the contact angle will bereported elsewhere.The deinking liquors contained 0.1% by weight sodium silicate and sufficient nonionicsurfactant (Triton X-100) to achieve the desired surface tensions listed in Table 2. The pHwas adjusted with sodium hydroxide.Table 2.Liquor12.412.410.110.1Contact Angles at Five Seconds0. deerL,mN/m39.229.939.429.7WKC76.347.775.047.3HMD71.343.069.343.7HMB HME54.0 58.722.0 29.054.0 63.023.3 28.3Flotation studies were conducted using a pneumatic cell developed at the Institute of PaperChemistry. The critical parameters are listed in Table 3.121245Table 3. Flotation ParametersTemperature:pH:Average Bubble Size:Sticky Particle Size:Time of AgitationPrior to Flotation:Time of Flotation:73°F10.1 or 12.4260 gm (YL = 29.7 mN/m)350 lgm (YL = 39.4 mN/m)57 - 420 gm1/4 - 60 min.1 min.The results for the three (hot-melt adhesive) stickies with four deinking liquors andvarious agitation times are presented in Table 4.Table 4. Flotation ResultsSurfaceTension,mN/m39.439.429.729.739.429.739.439.429.729.729.739.239.239.439.230.130.1Time of AgitationPrior to Flotation,min.62062020201720441/46020202021/4ApparentContactAngle. deg62.560.038.537.560.037.542.542.010.010.0-20.0-32.0-32.058.047.018.024.0StickyRemoval%93.494.940.867.696.237.896.992.92.73.56.390.384.598.096.32.111.113StickyHMDHMDHMDHMDHMDHMBHMBHMBHMBHMBHMBHMBHMEHMEHMEHMEDISCUSSIONValidity of the G-G-F-Y EquationThe G-G-F-Y equation is usually used to relate the surface energy interactions betweenpure liquids and pure solids. To use it in the flotation studies, it is necessary to show that itholds also for aqueous solutions of surfactants.Of the four stickies considered here, the wax-coated kraft has the simplest composition. Itsinteractions with the liquors should consist of only dispersion forces. Equation 17 wassolved for L and the measured values of 0 (five seconds), YL, and s for the WCK, andthe four liquors (from Tables 1 and 2 above) were inserted. The average value for ~L forthe four liquors was 21.9 mN/m with a standard deviation of 0.6 mN/m. This value is inexcellent agreement with the value found 3 for the dispersion component of water, 22mN/m, and suggests that the surfactant liquors have dispersion interactions similar towater. Thus, the G-G-F-Y equation holds for the system WCK/deinking liquors with aconstant value for ~L of 22 mN/m.For the hot-melt adhesives, the situation is more complex. From the compositional andsurface energy information in Table 1, it is evident that polar components are present. Itwould appear that the interaction term in equation 14 should also contain polarcontributions rather than just the dispersion contributions shown in equation 16. Thesecontributions would then lead to a modified value for ~L in the G-G-F-Y equation.14The applicability of the G-G-F-Y equation to the hot-melt adhesives was tested using theprocedure outlined for the WCK and the data in Tables 1 and 2. The values calculated forL are given in Table 5. Surprisingly, they do not differ greatly from the value for water.It can be argued that the correct value for s to be used in the calculation is not that listed inTable 1 (obtained from interaction with methylene iodide) but should be modified to reflectadsorption of surfactant molecules onto the solid from the deinking liquors. In the absenceof detailed information on this process and in order to follow relative changes, we willassume in subsequent calculations using the G-G-F-Y equation that the value for water isapplicable to stickies/deinking liquors systems.Table 5. Calculated Values for LSticky Liquor. 'LimN/mHMD 1 24.72 24.74 26.25 24.2HMB 1 19.12 16.44 19.35 16.0HME 1 21.32 18.84 19.75 18.715Flotation StudiesFlotation tests were carried out on particles of the three hot-melt adhesives. A variety ofconditions of pH, liquor surface tension, and time of agitation prior to flotation were used.From the data in Tables 3 and 4, the force of attachment Fa was calculated from equation 8using the exact expression (equation 10) for the factor d/D. The percent sticky removedduring one minute of flotation is shown plotted against the corresponding force ofattachment in Figure 5. Removal efficiency improves with increasing strength ofattachment with most of the change occurring over a narrow range of forces. (The curveshown is a least squares fit to the hyperbolic tangent function.)Although the dependence of removal efficiency on attachment force is believed to be ageneral relation, the particular "critical" force will be a function of the specific parametersof the flotation system. These parameters would include the ratio of bubble-to-particle size,mixing conditions in the flotation cell and the duration of flotation. The sensitivity to thesefactors needs to be examined in detail.To gain a better appreciation of the influence of liquor and sticky surface energies on theattachment force, the G-G-F-Y equation can be substituted into equation 13 to giveFa -47 D (x - x2) (18)whereX = S 1/2 (19)16'1% Removed00 Q > o 0 o- -> mino ( 3 U C )3 -0§ . OcSf M " 4 U'g' . 0c-m17IThe ratio (s)J /YL is seen to be the critical parameter.Upon substituting the exact equation 10 into equation 8 and simplifying, the force ofattachment can be found as a function of the Cos 0.Fa= 27rD(1 - Cos2/4 +6 Cos - 2 C3] (20)Equation 17 can then be used to evaluate this function. A map showing the dependence ofthe attachment force on the sticky surface energy is shown in Figure 6. The curvesrepresent conditions of constant liquor surface tension at the indicated values obtained fromequation 20 and the G-G-F-Y equation. The dashed lines indicate levels of the attachmentforce required to reach the given recovery efficiencies. These levels were obtained fromFigure 5. For a particular sticky with a given surface energy, a vertical line can be drawn atthat energy to intersect the level of recovery desired. The deinking liquor must then have asurface tension equal to (or greater than) that at the intersection. Intermediate values for YLcan be interpolated (approximately linearly) between the values for the indicated curves.It is emphasized that these levels (dashed lines) pertain only to the particular flotationconditions used to generate the data in Figure 5. For a different set of operatingparameters, the attachment force required to achieve a given recovery level would need tobe determined experimentally. Once this is done and the level of attachment force is enteredon the map in Figure 6, the necessary conditions for liquor surface tension for particulartypes of stickies can be determined.18CONCLUSIONS1. The G-G-F-Y equation can be used to correlate the experimental data onstickies/deinking liquors systems.2. The flotation efficiency results can be explained by a theory based on the need forstrong air bubble-sticky attachment. It includes the effects of bubble size, liquorsurface tension, and the surface energy of the sticky.19Attachment Force,1)200dyneC1]CO-imz0C)-I0O0Io >0 gl0 C00 C O O03 - -C-Dcoo"0rD~.5gsCo.CDS2.00 LC,ocna0 og o3 0C,0z0z0'1lzICoCOr-a0rrmzmC)CoCCDrDm=>I-mC-iACKNOWLEDGEMENTSThe design and execution of the experimental program was performed by Joseph J. Becherand Jack D. Hultman. The author thanks the American Paper Institute for permission topublish the experimental results.21LITERATURE CITED1. McCool, M. A. and Silveri, L., Tappi J. 7 (11), 75 (1987).2. Leja J., "Surface Chemistry of Flotation," Plenum Press, New York, 1982.3. Adamson, A.W., "Physical Chemistry of Surfaces," Wiley-Interscience,New York, 4th Ed., 1982.4. Girifalco, L.A. and Good, R.J., J. Phys. Chem. 61, 904 (1957).5. Fowkes, F.M., J. Phys. Chem., 67, 2538 (1963).6. Hultman, J.D., Becher, J.J., Swanson, J.W., and Bowers, D.F., Removalof "Sticky" Contaminants from Recycled Fiber, Progress Report One,Project 3428, Appleton, WI, The Institute of Paper Chemistry,June 18, 1980.22

The flotation of sticky contaminants from recycled fiber streams

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The flotation of sticky contaminants from recycled fiber streams

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