P. Somasundaran, Lei Zhang, S. Krishnakumar and Richard Slepety8

ABSTRACT ing landfill space, diminishing 8upply of wood and environmental concern over deforestation. Deinkingis one of the most important steps in waste fibers can be recycled and reused in paper making. and a variety of techniques includ- Since is one of the major deleterious contami- ing conventional and wa,hinc are nants in the recovered paper due to its darkness, being used currently to deink secondary fibers. This the quality of the secondary is detennined review encompasaes the surface chemical and hy- largely by the extent of ink removal. The most com- drodynamic principles underlying the flotation monly used techniques for ink removal are wash- deinking process. and considers various reagent ing, flotation, cleaning and screening. Among them, schemes and parameters for controlling the pro- flotation is a process that utilizes the differences cess. The effects of surfac~ts. ion con- in surface physicochemical properties of various centration, pH, temperature, ink particles SIze, particles to separate them. In this procesa, hydro- bubble size and other parameters on the deinkin~ phobic particles, or hydrophilic particles that are efficiency are discussed- Specific3lly; problems in made hydrophobic by surface-active reagents (sur- deinking the flexographic and toner are ad. factants>, attach to the gas bubbles in the pulp and dressed- Possibilities of modifying existing flotation are carried with the froth and separated from other techniques to achieve better deinking of fibers are particles which remain in the bulk. The former par- indicated. ticles can then be separated from the matrix by removing the froth from the bulk pulp U>- Com- pared to other physical separation methods. flota- KEYWORDS tion features high selectivity and relatively low cost and is an economical process for deinking. Bubbles, Contact angle, Deinking, Flotation, Ink Particle size, , Toners, Waste paper. Flotation is essentially controlled by the physico- cllemical and hydrodyn~c properti~ of the pulp, which is ~eW'ed in this paper along with the ef- INTRODUCTION fect of various parameters in flotation deinking. Possible extenaion of other techniques for improv- Recycling of recovered paper has become 8. major ing dein k-1ngefficiency will also be discu8sed. issue in the paper ind~try as a retult of decreas-

DISCUSSION

Unit Processes in Deinking SOm4sun@ran and Zhanc are with NSF lUCR Cerat8r for Surfactant., Colu~bia Ulliuer.ity, 500 Wfd 120th The main task in the deinking DCprinted is Strut, 911 SW Mudd Bwildin,g, N.w Y~, NY 10027. to detach the ink ft-om the paper fiber and then to USA separate the ink from it. Deta.chment of the ink from the fiber is achieved by a combination of physi- Kru1l.nakumar i. with Un,leuer Research US. 45 Riwr Rd, E~watcl; NJ 07020. USA cal and chemical actioD$ '2. a). The first step, pulp- ing. involves mechanical agitation of the recovered paper in a mixing unit at oonsistencies ranging from below this range washing is more effecti"e cr. w. A 8-16% solids, with the addition of a few chemicals combination of the two processesis found to be ideal to facilitate ink detachment from the fiber. A com- for most type of furnishes (.9:.12). Dispersion is ef- bination of physical and chemical actions causes fective in detaching difficult inks such as toners or the ink to be detached from the fiber surface. In UV-cured inks froro fiber. The ink particles are dis- deinking, the pulping unit is u!tually oper.ated in persed in thi8 case by chemical addition. steam in- bat&:hesat temperatures ranging from 55-70 °C and jection or vigorous mechanical mixing units. at pH 9-11, which have been found to be the opti- mwn for ink det3chment. The pulper is also an ideal Large q\lantities of water used in the deinking pro- point for the addition of chemicals required in the cess are \lSually re\l.ged by employing water recir- subsequent stages as the mechanical action helps cUlation techniques. Control of water hardness and to mix them inU) the pulp for maximum effective- pH is critical for the recirculbtion ofwa.ter in deink- ness. A following steP. screening, IS designed to re- move contaminants like staples, paperclips, gums ing plants. etc.. that are present in the pulp- Coar&e screens are used to remove st$ples. paperclips and stones physical Chell1istry of Flotation Deinking while fine pressUJ'escreens are used to remOvelight l1le principles of flotation deink.ing are similar to contaminanu like plaatics, , etc. that aTe those of conventional mineral flotation wherein the commonly found in recovered ONP, OMG, MOW, particles to be removed are naturally hydrophobic etc. or are rendered hydrophobic by the adsorption of Dlolecules of surface active reaaents () The most commonty used techniques for ink sepa- on their surfaces- Adsorption of surfact.nts at the ration, which is the key step in the deinking pro- solid-liquid interface can alter the surface proper- cess, are washing and flotation. In the washing ties sufficiently to affect hydrophobicity and thus process.. the fine ink particles in suspension are the flotation. Separation of one particle from an- washed aw~y in a water stream and the ink is 9Ub- other is dependent on selective adsorption of 6ur- sequently removed from the filtrate by flocculation factants on only' the particles which are to be using polymers. W~shinl is efficient only for re- moying the finer ink particles. It uses large yol- floated. wnes ofwater ~nd the fiber yield can be low. Flota- Adsorption is essentially selective partitioning of tion deinking i9 similar tocollventional mineral flo. the surfactant into the interfacial region. If the tation <1. 4> where the hydrophobic particles are interactions between the surfactant and the solid removed by their attach1\\ent to a stream of risjng are more energetically favorable than th06e be- air bubbles. Flotation is usually less sensitive to tween the surfactant and the solution, adsot'ption particle size and mults in a h~her fiber yield ~ can take place. The contributinc forces for adsorp- §). As can be seen from Fig. 1, flotation is more tion include CO1/alentbonding, couloD1bic interac- effective in the size range of 15-150 microns, and tion, ion exchange, desolvation of'the polar group of' the surfactant, desolvation of the surface, hy- -I F-~ 11- drogen bonding, hydrophobic interaction and v~n der Waals interactions. For each surfactant-solid Systelll several of'the abo1/efactors can be contrib- i~:il ,-- uting depending on the 6Olid and the surfactant }-..!!~~~~~ -{ type, surfactant concentration, temperature. etc. An - - understanding of the mechanisms of adsorption is I--~:'" ~ tlterefore essential in order to select appropriate r-~ ~- conditions for flotation separation (~lfi} ~ The ink requires specific cllemi- ,. tal conditions which are created by adding the ap- propris.te chemicals prior to flotation. Flotation w..,.. reagents added in the deidkmg pr0ce56 include col- I... lector, £rother, pH regulator, particle 6ize control- ... .. ler and dispers~nt. Depeddlng on the type of raw >- ---""-- ~ ~ - 0 ~ material to be deinked, the nature and do6age of ~ wemicals are varied. Typical chemicals used for p IIRr'K:J.;ESIZE. ~ deinking flotation are shown in Table 1 ~ong with Fi8urc 1 General scheme of particle 8ize dependeRce of their function and dosage (1.6, 11)- For enhanced cM ~OYal.«icienCJ' by -varioua deinkillC operat.ioDA (6). 23 Progress in Paper Recyclin£' I May 1999 . Page 24 Progress in Paper Recycling I May 1999 carboxymethylcellulo6e. Fatty 3cida, which are deinking performance. the flotation machines used primarily blends of 16-18 C atom chains, are widely for deinking are also modified from those used for used collector chemicals in ONP/OMG deinking. mineral proceaing (l.a). They complex .,ith the Ca2. ions in the system to Caustic soda is one of the most importAnt chemicals form calcium which can adsorb on to the ink used in the deinking process. The high surface and provide the collector action. CaClz is concentration of the alk3li hydrolyzes the ink often added to provide sufficient Ca'. activity in vehicles and causes swelling of the fibers whicl"l the systexn.Low molecular weivht cationic polymers ~elps to release the ink particles from the fiber. are often used as coagulants in conjunction with The QJnount of added 6hould be high molecular weight anionic polymers in the optimized to provide sufficient alkalinity for good water recycling stage for clarification purposes . saponification and hydrolysis of the while Clays are added in the pulp to improve the ink minimizing: the formation of chromophores that rexnoval, although theiT function is not fully cause yellowing of the fibers (W. Mechanical or understood (D. Agglomerating chemicals are used groundwood-containing pulps such as ONP and now to treat the electrostatic inks (toner OMG are vulnerable to the yellowing effects of inks) (1.9). excessive alkali. Bleaching agents like peroxides Calcium- forDIation is the most important step are also added to improve the brightness of the fiber in deinking flotation of old . The mecha- and to neutralize the yeUo~ng effect of certain nism by which calcium improves the flotation effi- added chemicals (zg). Sodium carbonate and sodium ciency of ink particles has been a subject of consid- silicate are often used in conjunction with 'NaOH erable research (2.6=:l.i>-Under the alkaline condi- to provide the duired alkalinity to the medium with tiODBexisting in the flotation cells. the ink particles minimum fiber damage. The use of silicate5 and. are negatively charged due to the ionization of the carbonates allows de;nking to be achieved at a lower organic .cids on their surface layers. It has been pH which reduces yellowing of the fiber. Other suggested that Ca2..may interact with the nega- alkalis which are more environmentally benign tively charged particles through carboxylic bridg- than NaOH such as MgO, Mg(OH)z' NaHCO3, ing mechanism and reduce the surface charge,lead- Ca

and th\l~ reduce the problem of unwanted scale r;;-~;:;1 r~~~~~])~ - deposition in the mill. Newer flotation eollector r;-~.-1 .--~c I~~I., I I I -.-&.-~ ebemicals based on modified polyester resina have c.J" ~ (!oI" also been developed. They tend to act in low d06age9 ~:c.z.?- .-'-c(?- 02-- ~ : c.2- as foaming collectors and do not require the presence of hardnes6 ions (9). 01+ 1c.1+ ,-_",~I Hydrodynamics of Deinking Flotation --. c-'"'- e.J+ c.2.- ~ ca2+ The basic mechanisms of flotation include the col- -.-,..- (>2+ c.'+ ~ iM.-;d08 ~ {/'8 lision between the particles and bubbles in the liq- uid suspension, the attachlnent of the particles to \=.. , the bubbles and the rising of the parliclcJbubble .. aggregates to the surface of the suspension. The .,. ~~ collision, attachment and stability of the aggregates '-' 1 '\ depend on the hydrodynamiCS in the flotation cell. ~ -~ j~c Thus. flotatiOll is a dynamic process that involves Fi8un2 Schela-aticNPr'eeeDtatioa of ~ble -~~i.-s many hydrodynamic phenomena in a system con- of calcium-fatty ~id interaction in d.,uinc flOQUOD taining solid, liquid aDd gas in a state of varyillg (z.7). turbulence. Apart fI'Om the physical chemistry of the interfaces, parameter6 such as pulp aeration, conditions. the precipitated calciuUl soap can I ; bubble mineralization. agitation intensity, bubble heterocoagulate with the ink particles, resulting size and particle size have aD effect on the flota- in their coating with a hydrophobic layer. 'these hydrophobic aggregates can easily attach to ajr tion efficiency. bubbles .The overall and ca1cium-fatty acids system, it i5 to be noted probability P of particle-bubble attechment can be that these experiments were done either with Ulodel systems or in the ab&enceof fibers and other chemi- represented by the product of three separated prOb- cals in the pulp. In real deinking pulp, interactions ability tenns <1): among ink particles, calcium ions and fatty ~cids p = PcX P& X P4 [1] .re expected to be more complicated. where Pc i& the probability of collision between the Advances in the understanding of the mechanisms particle and the bubble, P. is the probability of underlying de inking ha"e helped in the adhesion after collision, and P. ia the probability of development of special chemicals for maximum maintaining the adhesion. The first terDi, pc. benefit. One type of sueh chemicals called depends Ir\ainly on the hydrodynamic c:onditiom "displectors. (dispers..nt/colleetor), usually of the flotation cell. For large particles and large proprietary formulations of alkoxylated fatty acid bubbles, collision will depend greatly on the inertial deri"atives, dUplayS properties ofbotb dispersants forces.For small particles and mediwn size bubbles, and collectoTS thereby improving the performance particle-bubble collision will occur with the sinking in both the flotation and the washing stages of particles encountering rising bubbles. ~tremel,y deinking <.3.0).They do not require calciwn addition

Progress in Paper Recycling I Moy 1999 Page 26 to cause the detachment of particle/bubble aggre- fine particles are carried along the streamlines and gates and the collapse of bubbles. Also, in deinking, will not make a close enoounter with bubbles, 11nless the floated material is often an aggregate of ink they can crOSs the streamline by diffuaion (3.!>. particles rather thaQ a dispersed particle. High Thus, for different systems, hydrodynamic shear rate must be avoided in such cases in order conditions in the cell playa vital role in affecting to keep from destroyiQg the aggregates- A kinetic the panicle/bubble collision in different ways. model that takes into account both particle/bubble adhesion and breakage has heeQ developed by The relationship between P a8 a function of bubble size (D~) and particle size (D). can be expressed as Bloom et al. for the deinking flotation (.32). (11Q.): Effects of Process Variables in Flot:ation In {2] Deinking Pc:= a In the following section, effects of 1/ariou6 physico- chemical and hydrodynamic parameters on nota- tion deinking are discussed. in which Cl and PI are parameters that vary with a) Surfoctant type flow conditions. It is clear that Pc varies approxi- A comparative study of several types of surfactant5 mately with the square of panicle size, D , and reveals that soaps of fatty acids yield good ink re- inversely as the square of the bubble sue, D:. moval and xninimal fiber 10S8 . Tests have also indi- can be predicted by the theory (3.6),for particles of cated maximwn brightness levels to be obtained smaller than 100 ~m the force of attachment is 300- with C14 to CI8 fatty acids (Fig. 4) <42>. 400% higher than that predicted (31). b) Surf~tallt I soop con«ntra.tion Flotation is most effective for the removal of ink It has been observed that in the presence of Ca"', particles in the range of 10-1SOJ11D. A theoretical the floatability or ink particles increases with analysis oftbe particle/bubble attachment by colli- sion suggests that when particles are large enough to be unaffected by Brownian motion, the flotation - "'~ rate should increase roughly with the square of the 58.2 particle size (W. Similar results have been reported sa for the flotation of model ink particles with the flo- tation rate proportional to the ink particle diam- ~ 55.- eter raised to the power of 1.8 (W. The flotation removal of fine particles, based on Equation 2, can 54 be enhanced by reducing the bubble size. 521 --' ------~ The turbulent conditions in the flotation cell also ~ control the particle/bubble attachment and detach- So8p LAS f.-oS f~ QAC AM"" NPG ment.. For deinking cells. due to the small size of F~ S A comp.ri.oa of deinkiGg .mcieaoy ot diffe...n~ SUI't_taa~s: Soap' f.tt.y acid.. LAS - linear alkylbeaaene ink pa;rticles and the lower concentration of solid5. ~onaU; FAS-,.tty .leohol su.lIate; FAES. f.tty .Icobol a comparatively quiescent condition is usually used. ether 8alt_&-; Q-'C -quateT'Dary am.onium compoWid; While good mixing is necessary for efficient par- AMPH - _mpholytie surfact...ts; NPG ~ non)'lpbenol ticle/bubble collision, it should not be so strong as ethoxyla&e (~. PAge 27 Progress in Paper Recycling / May 1999 t. MAK.LUULlI

fiber i& impaired by the retention of the Ca soap in the pulp. In this regard. alkyl polyglycol ether can retard collector retention in the fibers. Soap reten- tion can also be minimi2ed by optimizing the de- wateriJ1g conditions during which the Ca soap de- posited on the fibers can be .ashed away. How- ever, experiments have shown that under normal deinking conditions only a small n-action of the Ca soap ia actually retained on the fiber, and the high retention found in the mills has been attributed to mechanical entrapment (4.8..4D). The quality of re- cycled paper is also affected by the residual fatty increasing addition of soap (2.6.)(Fig. 5). There is an acid on fibers. It has been suggested that fatty acid may be at least partly responsible for the low coef- optimum soap concentration beyond which bilayer adsorption of surfactants may occur rendering the ficient of friction (COF) of recycled pa- particles hydrophilic and hence less floatable (~) pers. Other collectors such as have been pro- The soap concentrations used are usually well below posed for raising the COF of papers made from re- the critical micelle concentrations. Micelles, cycled pulp (OD). however, are u6eful in dispening pigment particles of low water solubility by 60lubilizjng them. In such c) Effect of calcium and water hardlteSS . casesnonionic sw-factants which have a.much lower Flotation response was observed to be inadequate critical micelle concentration than the ionic at hardne6s levels below 180 ppm, above which surfactants may be used (i.Q)- Hiib dosages of maximum flotation efficiency .u obtained- The nonionic 8urfactants have been found to decrease brightness of fiber is also affected by water hard- the ink flotation efficiency, possibly due to the ness with the brightness increasing with water hardness (.42. ru

- \ M. D. ~ oC .. 0 .. :: .. . ~ 0 0 . - - . 0 .- U 4 C .

. aD 1~O '.0 ,TEUAfC CO.CC.TRATIOR ... Figun 6: Effect of soap CoDcentratioa on the brichtueM Figu"~ 5: Influenc;e of soap coneentratioD on the at different h.rdneH leveu (~). ftoa~bility of ink partiolea (!I). Prnsne8in Paper Recycling I May 1999 Page 28 ~~~lUUl.1

increase in temperature (Fig. 10) (5.6.).Generally, an incrcase in temperature can facilitate the de- tachment Qfink particles from fibers which contrib- ~ utes to an increase in deinking efficiency. However, "!~-'.. -- elevation in temperatures can al9o increase the solu- bility of various chemicals that can either enhance or hamper the deinking process. - 0 8$ 170 - 110 - 11(» Cloud point is a property of nonionic ethoxylated ~~-~ surfactants above which the surfactant solutions _N-es --,,-~ become cloudy as a result of phase separation. It F'6Un 7: Efre« of.-terhardnen OD the brightneS6 oCthe baa been found that deinking efficiency in wash- deinked fiber' (.i1). ing is strongly dependent on the cloud point of the surfactants used. The highest deinked abeet bright- ness values were obtained when the process tem- perature was within 5 .C of the surfactant cloud point <51.58). The importance of this relationship is to be noted in deinkina flotation when nowonic ethoxyiated surfactanta are present in the pulp. f) Role of clay and other mineroi additivea' It h.., been found that in traditional deinking flo- tation scheme (Ca soap-fatty acids), ash in the feed ~- & .. ~ ,. --~ ~~ i 55 .,- ~------. # , --- am

~

u u u ... 1U flotation cell. Also. it bas been suggested that cal- , ciwn ions can attach themselves to the fibers and ~pI nnder the fiber hydrophobic. Thus. the presence of P..- A ftOt.a.. ~ calcium ions can contribute to the 1066of fiber in .-.~1.5 ,"~5.5 the flotation

e) Effect of temperature Deinking flotation is usually done at 40-60 ~C. Larsson et 81. have reported floatability of model ink. particles to decrease slightly with increase in temperature (2.6,).Mal1:hildon et aI-. on the contrary, Flgl£re10: Deere.ae of inked 81Jrlace.. a f\lnctioG of toJ8. - ~L - :-1.. -"'1\V~1 to be enhanced with . peraCure at different eo.p concentJ'ationS Ca>. ~~OO2l1

can be optimized for ink removal. It ueed to be the practicc to add ash by blending coated materials ... 30 such as magazines with the newsprint (1). Due to u... rapid growth in newsprint deinking by flotation ~ 20 ~ A technique3, the demand for coated material i9 fast .. .. exceeding the supply. Moreover the uh content in 0 the coated materials varies widely giving rise to 10 slgnificant swings in the ash content of the feed. Direct addition of clays or some other minerals such 0 . as calcite and uolite in the flotation cell has been 5 '0 50 ,- explored. These materials have been found to have DI_~ a positive effect on ink flotation Schriver et al. re. ported that among 10 different mineral additives, calcined filler clays give maximwn brightness when used in conjunction with either fatty acids or di6- . 20 u~ persantJcoJIectorc:ombinations (~). It is to be noted t: that role of ash in flotation deinking is also not very c :: B well understood. Grant et al. speculated that ink 0.. 10 k particles may adsorb onto the surface of the inor- ~ . ganic material. The resulting particle complex is 2 ~ larger in size and more hydrophobic and hence Z floated more efficiently (6..0.).This phenomenon is 0 l1_I_J~1 ~ - - akin to carner flotation or piggy-back flotation used . .0 .. ,- in mineral proceaaing in which very fine particles ~~. are attached to bigger carrier particles and floated Figure J 1 Effect of pal"tiele ,i- on flotation kiDeti~. <.61>.Grant et al. also showed that size of agglom- A) Ink particle ~iz. distribution before flotation; B) Ink erate formed by ink particles and clay particles is particle..- diatributiop after 10 lDin flotation (G). dependent on the type of minerals and the ratio of inks to mineral fillers.

Letac:her et al., on the other hand, reported that 1. fillers do not playa significant role in improving Nu.-- 1. the flotation deinking (u meas~ed by brightness) at (6.2).Retention ofkaolin clay tillers did not improve ;-.~ 1. the brightness of the stock. The increase of bright- 1. ness with silica filler was due to filler retention instead of improved flotation efficiency. However, their tests were carried out with a displector chem- istry that is quite different from fatty acid chemis- 70 try. It may be one of the reason as to why fillers are 8G not beneficial for efficient deinking. $a B :-~- 40 oJ Effect of particle s~ . It is well known that for small particles in the range . of I-SO J1Dl,floatability decreases with particle size 10 ~-, . , , . , (6.3).The larger particles are usually easily floated .., 1.8 18 ,. "- 1"~ and can be separated in a short time (~ The (1 -t (1 0..1 flotation rate of small particles is rather slow and PAA1n.ESIZE.,.m they cannot be easily removed even by prolonged F~ 12: Eft'eot of particle size on brigbtae.. of deiAked flotation. Figure 11 shows the ink particle size fiber (7). dutribution before and after flotation and clearly indicates the higher floatability of the coarse decrea&es the particl~bubble attachment efficiency. particles- (The number of particles as shown in the The presence of small particles affects th~ figure is a particle count on a fixed area after brightne65 of the recovered fiber adversely. Figure deposition on a filter.) The main reason for the poor 12 indicates the effect of particle si~e on the flotation response of the small particles js the brightnesa of the fiber. As can be 6een, 85 the 6ize hydrodynamic effect (reduced momentum) that of the ink particle6 generated become6 swaner the

Pace 30 Progress in Paper Recycling I May 1999 MAR.2002.1

B particles of various sizes ~ A , -, ~~--{\)1., 8 ~. . . .. :'- in the deinking pulp. It is i;f? reported that when plate or disk shape particles ; interact with a bubble, ~ larger particles collide , with the bubbles edge-on y- T- and bounce off before ~. ..- attachment can take -- place, while sznall fl ': particle. can flip onto the . ~ side resulting in a larger - - s- - . - - . "-~ contact area. which ~~~~ ~ ~ ~ : .:s ~ .." ..~.d 11.1 ~ cannot drain and rupture .- .- during the tizne of contact. In both cases, the particles ha~e a. c much 5!naller probability ~ : ~..~~---~- T'---~ ,-- -. Figure 13, A) Tr~ectorie8 of two particl~ drawn of attachment than does W sc-.le, ftO-'Dg throu~h a bubble witt. one ~~-e .sphere attac;hing ~o the bubble. Eaeh Uawn a similar size sphere (6.4) sphere represents one frame of film or 1.4 me of (Fig. 13)- lapsed time. B) Disk, drawn to sc..le. collidiDr -ith a bubble, bouDoinC', t~c, and fto.rinc . around the bubble. Oiek path i. iD the same h) Effect of bubble 5.ze I plane 88 the figure. C) S-all diak fracme..t, Equation 2 states that dra- to _Ie, ftippin~ to ride aud miMin~ the collision probability P T~ bubble

have a higheT efficiency due to the hi&her specific

-I - . .., LS surface. It is shown in Fig. 14 that small bubbles .- bring the largest brightneas gain (6n). However,

small bubbles can also cause an increase in fiber

loss due to the entrainment (00). reflectance of the fiber also decrease.. Thus, for The bubble size is affected to a greater extent by efficient deinking, the process variables should be the chemistry of the deinking pulp. Rao et al. .djuated to D\inimize the generation of fine particles showed that as the surface tension of the pulp de- and also to achieve maximum removal of the creases,average bubble size as well as D\axiIntlm particles s-enerated. Small particles are formed bubble sm decrea6e8.Thus, the chemicals u.gedin when the inks are aged or when strong mechanical the deinking process can be expected to affect flo- action is used to detach the difficult to remo1le,offset tation by affecting bubble size distribution (ti1). inks. Repulping of the flexographic printing papers also generates rme particles under the common alkaline conditions used. The probability for producing small particles increase5 with the I~] 15. --- difficulty of ink separation. HowC1/erthe presence L ~IU~- J of both calcium and 6oap in the pulp induces z ~_..~""'-- .. agglolneration of offset ink particles and increases 0 10 ~~ the average particle size. Also, the po6sibility of ... using polymers to flocculate the fines prior to ! . r ue.u~ -, flotation needs to be further in1lestigated- 1- S8Z£ -1 The sh.pe of the particles .Iso plays. role in A. .. c.- - determining their floatability by affecting the a 200 ~ 800 probability of particle~bubble collisions. 'l11e toner AtH RATIO (%) particles are usually present as plate or di6k-like Figu~ 14; Effect or bubbl~ size on ink notation (Ii>.

Pa~ 31 Pr(O&-cdS in Paper Recycling I May 1999 [iI I i; Treatment of Flexographic and Toner Inks Although the toner particles are generally hydro- phobic ~). the flotation removal of them has been Water-based flexographic inks and electrostatic inks reported to be poor compared to that of conven- (toner) are relatively new types of printing inks which tional inks. One reason may be the attachment of OCCUrmOTe and more often in recycled paper, espe- fiben to the toner particles after repulping. as ton- cially the mixed office papen. 'nley have qujte differ- ers are fused to the paper surfaf:e and are diffiClllt ent composition hom traditional printing inks and to detach from the fiber. The attached fiber makes are not readily removed by flotation using conven- it hydrodynamically very difficult for particle! ~onal flotation chemistry discussed above, e.g., cat. bubble attachment to occur. Furthennore, the at- ciwn -soap chemistry. The presence of these inks has tached fiber renders the toner/ink aggregates ml)re become one of the major problems in the deinking hydrophilic and thus less floatable (11. 18). flotation and pt"OCe6SeSdependent on different spe- cific physicod1emical properties are required to treat The printing parameters for toners such as the papers printed with these inks. thickness of print, degree of bonding to paper, and the speed o(printing are found to affect subsequent Flexographic printing WleS water.based inks and is deinking. Slow.printing machines usually produce environmental-friendly due to reduced emission of larger toner particles that contain attached fibers volatile organic compounds d\lrlng the printing pro- upon repulping and nee:atively affect the flotation c:ess.It is also economical compared wid1 the tradi- deinking (W. It is hard to break up thick. prints in bona] offset and letterpress printing. After repulp- the npulping stage due to their 6b-onger cohesive- mg, flexographic inks fonn very fine particles « 5 ness (8Q). As mentioned in the above sectioD6, the J1ID)and due to the small particle size, plus the hy- shape of the toner particles may also play an im- drophilic nature of the inks, it is rather difficult to portant role in affecti,ng the flotation of toners (6.4). remove them by flotation. They form very stable col- loidal dispersion. It is reported that the conventional A number of techniques have been proposed to treat calci.\l1nsoap of fatty acids are effective for the labo- the toner particles. It is sugge6ted that toner flota- ratory flotation of model flexo inks- Calcium soap and tion can be enhanced by simply improving the flexo inks can fonn aggregates (§B. §.2).The problem chemical and hydrodynamic conditions of the pulp with flexo inks xnay thus be due to d1ecomplex diem- to generate smaller ink particles that ue ~ of istry involved in the actual mill with fibers and other attached fiben

entrapment (12>. ~Ol MSEO ~ ~ As flexographic inks contain acrylic resins that are - water soluble under alkaline conditions, repulping the nICOVeredpaper under acidic or neutral condi- tions can reduce the ink dispersion. It can be seen from F"ag.15 that with a.standard deinking chemical mixture, the brightness of the pulp increases with the decrease in pulp pH (1.1). Based on this. a two 8tac8 deinking proce&&has been proposed to treat .. mixed ~vered paper, with a neutral stage before the conventional alkaline deinking stage. This pro- I ~ cause&less dispersion offlexo inks and reduces ~ ink redepoeition (12. ~). Other approaches include , 'U;XO p~ c... addins- a washing stage to the notation ~ or I .. ~ p~ F£8) modif)-ing the composition of flexographic inks to improve their flotation deinkability ~. 1.5). ~J~ArEl';i;m ~ - Photocopying and laser printing processesuse ther- L~~~ moplastic powder8, t.enn.edtoners, instead of conven- .-.WASt£.-5 tional inks. They are electt'OStatically placed on the --~~p~ paper and then fused in place. Upon repulping, Ule .. _L I I , I I toner particles break into plata or disk-like particles . 7 . . .. ,t u of various sizes. They are not easily removed by con- ~.-N.~ ventional washing, flotation, cleaning or screening processes. . Fis..,.e 15: Effect of decreasing pH on ftexou-phic in deillkinc <71). as mineral oil and polymers to agglomerate the ton- LITERATURE CITED en into larger particles and their subsequent re- l.Gaudin,A.M. Flotation. McGraw Hill, New York, moval by fine screening and centrifugal cleaning have been proposed (8.'1.M). Pulping under acidic NY (1951). conditions could cause toner to be released as small 2. Crow, D.R.; Secor, R.¥. The ten steps of deinking. particles and increase the effectiveness of toner Tappi J. 70 (7): 101 (July 1987). removal (85.).Two-stage treatment (washing/flota- tion and two stage flotation) (86). densification and 3. Secondary Fiber Recycling (Spangenberg, R. J.. forward cleaning also gives better flotation results. Ed.) 'fAPPI Press, Atlanta, GA (1993). Despite the advances in understanding the deiDk- ing of fles:ographic and toner inks, eJ:perimental results reported for flexographic and toner deink- ing often are conflicting, probably due to the differ- ences in the printing process and the composition of the printing inks. Clearly more work is needed 5. Read,B.R- Selectionof chemi~ ,.,;.thin . mod- to elucidate interactions between these types of inks ern deinking plant. Paper Technologyand Indus- and deinking chemicals and to derive a general try, p. 339 (Nov. 1985). protocol for deinking based on 6ucll information. 6. Harri80n, A. Flotation deinking is critical in unit process method of deinking. Paper &. Pulp, p. 60 SuMMARY (March 1989). Flotation ia an effective method for deinking and 7. Zabala, M.; McCool, J. Deinking at Papelera when co.nbined suitably with other processes like Peninsular and the philosophy of deinking systeJn washing, can remove a ,.ride variety of inks in a de$ige- Tappi J 71 (8): 62 (Aug. 1988). large range of sizes. However, flotation is a com- plex process U1.atinvolves many physicochemical 8. Ferguson, L. D. Deinking chetnistry: Part n and hydrodynamic parameters at the 8olid/liquid/ Tappi J. 75 (8): 49 (Aug. 1992). gas interface. The flotation deinking efficiency can be affected by a large number of factors. Process 9. &hrlver, KE. Mill cliemistry must be consid- variables such as collector type. collector concen- ered before making deink line decision. Pulp &. Pa- tration, Ca- concentration, pH, temperature, clay minerals, particle size and shape and'bubble size per, p. 76 ('M~ 1990). ~ shown in this discussion to have significant ef. 10. Koftinke, R A. Modern newsprint system COIn- fects. The mechanisll1 of calciwn-soap fonnation biDes flotation and washing deinking:. Tappi J. 68 and ita effect on flotation deinking has been re- viewed.lt is clear that clarification of the underly. (2): 61 (Feb. 1985)- ins mechani5lI1s in deinking flotation offers an op- 11. Borchardt. J. K; Matalalnaki. D. W. Ne'lRsprint portunity to optimize the deinking process and help deiDking: unit operations studies of flotation-wash to develop special cheU\icals for the maximum ben- deinking, a comparison of commercial and propri- efit. etary surfactants. Pulp and Paper Canada 95 (10): Although significant advances have been made in T374 (1994). elucidating the mechanisms of deinldng conven- tional newsprint and pr0ce5S 6d1eJlle&for treating such inks are well established, flotation deinking of flexographic and toner inks still face difficulties particularly due to lack of understanding of the processes involved.

ACKNOWLEDGMENT 14. Goddard, ED.; Somasundaran. P. Adso.-ption The ~utbol"s acknowledge financial support of the of surfactants on solids. Cr~tica Chemica Acta 48: National Science Foundation (CTS-962278I, CTS- 451 (1976) 9632479 and EEC-98M618). p~33 progrese in Paper Recycling I May 1999 7. MAR.200211

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65. Julien Saint Amand, F.; Perrin, B The effect on 76. Snyder, B. A.; Schmjdt, D. C.; Berg, J. C. Charac- particle size on ink and speck removal efficiency of teritation and flotation studies of electrostatic inka the deinkiog &teps- Proc- 1st Recycling Forum,p. Frog. in Paper Recyci. 3 (1): 17-26 (1993). 39, Technical Section CPPA. Montreal, PQ. Canada (1991) 77 Johnson, fi A.; Thomp8on, E. V. Fiber and tone.- detachment during repulping of D\ixed office waste 66. ~ersch, M.; Pelton, R. Mechanism of pulp 1065 containing photocopied and laser-printed paper. in flotation deinking. Journal of Pulp and Paper Tappi J. 78, p. 41 (1995). Science 22, p. J338 (1996). 78 Vidotti. R. M.; Johnson, D. A.; Thompson, E. V 67. Rao, R.; Steniu8, P.The effect of flotation deink- Influence of toner detachment during mixed office ing C'.hemicalson bubble formation. Journal of Pulp waste paper repulping on flotation efficiency. Proc. and Paper Science 24 (5): 1!j6 (1998). 3rd Recycling Forum, p. 257, Technical Section CPPA, Montreal, PQ, Cauda (1995). 68. Dorris, G. M.; Nguyen, N- Flotation of model inks, part n. Flexo ink dispersion without fibres. 79. Snyder, B. A.; Berg, J. C. Effect of particle size Proc. 2nd Recycling FOnItn. p. 13. Technical Sec- and density in flotation deinking of electrostatic tion CPPA, Montreal, PQ. Canada (1993). papers. Tappi J. 77 (7): 157 (July 1994).

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70. Chabot, B.; Daneault, C.; Darns, G. M. Entrap- 81. Dorris, G. M.; Sayegh, N. N. The role or print ment of water-hued inks in fibre mats. J. of Pulp layer thickness and cohesiveness on deinking of and Paper Science 21, p. J296 (1995). toner-printed papers. Tappi J. 80 (4): 181 (April 1997). 71.Mak, T.; Reid, F.; Yau, T. Deinking offlexograpruc ink by flotation process. 79'" Annual Meeting, Tech- 82. Snyder, B. A.; Berg, J. C. Liquid bridge agglom- nical Section CPPA. Montreal, PQ. Canada. (1993). ing.eration: Tappi a J.fundamental 77 (5); 79 (1994). approam ' to toner deink- 12. Galland, G.; Vernac. Y. Deinking of wastepaper containing waterbaaed flexographic newsprint. 83. Darlington, W. B. A new process for deinking Proc. 1st Recycling Forum, p. 31, Technical Section electrostatically-printed secondary fiber. Tappi J. CPPA, Montreal, PQ, Canda (1991). 72 (1): 35 (Jan 1989).

73. Galland, G.; Vemac. Y.; Carre, B. The advan- 84. Olson, C. R.; Richmann, S- K.; gutman, F- J.; tase8 of combining neutral and alkaline deinking. Letacher, M- B. Deinking of laser-printed stock us- Proc. 3rd Recycling Fonlm, p. 235, Technical Sec- ing chemical densification and forward cleaning. tion CPPA, Montreal, PQ, Canada (1995). - Tappi J. 76 (1): 136 (Jan. 1993).

74. Ackermann, C.; Putz, H.; Gottsching, L 85. Azevedo, M. A. D.; YalaDianchili, M. R.; Drelich, Deinkability of waterborne tlexo inks by flotation J.; Miller, J. D. Flotation of toner particles and min- Proe. 2nd Recycling Forwn, p- 201. Technical Sec eral filler particles during de--inkingoflaser printed tion CPPA. Montreal, PQ. Canad. (1993). wastepaper. SME preprint, pp. 97-112, SME, Den- ver, CO (1997). 75. Upton, B. H.; Krishnagopalan. G. A.; Abubakr. S. Deinking flexographie neW6pnnt: using ultrafil- 86. Quick, T. H.; Hodgson. K T. Xerography deinking tration to close th~ ~ter loop. Tappi J. 80 (2): 155 - a fundamental approaclL Proc.TAPPI Pulpin8' Conf.. (Feb. 1997). p. 561, TAPPI Press, Atlanta. GA (1985). .

.ginal M~U6CriPtReceived for Revi~ on Octob;; lO ~ ~ l998 Revised Manuscripted - Accepted on January 29. 1999

Pare 36 Progress in PapeT Recycling I Me-y 1999