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

ABSTRACT ing landfill space, diminishing 8upply of wood andenvironmental concern over deforestation. Paperfibers can be recycled and reused in paper making.Since ink is one of the major deleterious contami-nants in the recovered paper due to its darkness,the quality of the secondary pulp is detenninedlargely by the extent of ink removal. The most com-monly used techniques for ink removal are wash-ing, flotation, cleaning and screening. Among them,flotation is a process that utilizes the differencesin surface physicochemical properties of variousparticles to separate them. In this procesa, hydro-phobic particles, or hydrophilic particles that aremade hydrophobic by surface-active reagents (sur-factants>, attach to the gas bubbles in the pulp andare carried with the froth and separated from otherparticles which remain in the bulk. The former par-ticles can then be separated from the matrix byremoving the froth from the bulk pulp U>- Com-pared to other physical separation methods. flota-tion features high selectivity and relatively low costand is an economical process for deinking.

Deinkingis one of the most important steps in wastepaper recycling and a variety of techniques includ-ing conventional froth flotation and wa,hinc arebeing used currently to deink secondary fibers. Thisreview encompasaes the surface chemical and hy-drodynamic principles underlying the flotationdeinking process. and considers various reagentschemes and parameters for controlling the pro-cess. The effects of surfac~ts. calcium ion con-centration, pH, temperature, ink particles SIze,bubble size and other parameters on the deinkin~efficiency are discussed- Specific3lly; problems inde inking the flexographic and toner inks are ad.dressed- Possibilities of modifying existing flotationtechniques to achieve better deinking of fibers areindicated.

KEYWORDS

Bubbles, Contact angle, Deinking, Flotation, InkParticle size, Surfactant, 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-fect of various parameters in flotation deinking.Possible extenaion of other techniques for improv-ing dein k-1ng efficiency will also be discu8sed.

INTRODUCTION

Recycling of recovered paper has become 8. majorissue in the paper ind~try as a retult of decreas-

DISCUSSION

Unit Processes in DeinkingSOm4sun@ran and Zhanc are with NSF lUCR Cerat8rfor Surfactant., Colu~bia Ulliuer.ity, 500 Wfd 120thStrut, 911 SW Mudd Bwildin,g, N.w Y~, NY 10027.USA

The main task in the deinking DC printed papers isto detach the ink ft-om the paper fiber and then toseparate the ink from it. Deta.chment of the inkfrom the fiber is achieved by a combination of physi-cal and chemical actioD$ '2. a). The first step, pulp-ing. involves mechanical agitation of the recoveredpaper in a mixing unit at oonsistencies ranging from

Kru1l.nakumar i. with Un,leuer Research US. 45 RiwrRd, E~watcl; NJ 07020. USA

below this range washing is more effecti"e cr. w. Acombination of the two processes is found to be idealfor most type of furnishes (.9:.12). Dispersion is ef-fective in detaching difficult inks such as toners orUV-cured inks froro fiber. The ink particles are dis-persed in thi8 case by chemical addition. steam in-jection or vigorous mechanical mixing units.

8-16% solids, with the addition of a few chemicalsto facilitate ink detachment from the fiber. A com-bination of physical and chemical actions causesthe ink to be detached from the fiber surface. Indeinking, the pulping unit is u!tually oper.ated inbat&:hes at temperatures ranging from 55-70 °C andat pH 9-11, which have been found to be the opti-mwn for ink det3chment. The pulper is also an idealpoint for the addition of chemicals required in thesubsequent stages as the mechanical action helpsto mix them inU) the pulp for maximum effective-ness. A following steP. screening, IS designed to re-move contaminants like staples, paperclips, gumsetc.. that are present in the pulp- Coar&e screensare used to remove st$ples. paperclips and stoneswhile fine pressUJ'e screens are used to remOve lightcontaminanu like plaatics, stickies, etc. that aTecommonly found in recovered ONP, OMG, MOW,

etc.

Large q\lantities of water used in the deinking pro-cess are \lSually re\l.ged by employing water recir-cUlation techniques. Control of water hardness andpH is critical for the recirculbtion ofwa.ter in deink-

ing plants.

physical Chell1istry of Flotation Deinking

l1le principles of flotation deink.ing are similar tothose of conventional mineral flotation wherein theparticles to be removed are naturally hydrophobicor are rendered hydrophobic by the adsorption ofDlolecules of surface active reaaents (surfactants)on their surfaces- Adsorption of surfact.nts at thesolid-liquid interface can alter the surface proper-ties sufficiently to affect hydrophobicity and thusthe flotation. Separation of one particle from an-other is dependent on selective adsorption of 6ur-factants on only' the particles which are to be

floated.

The most commonty used techniques for ink sepa-ration, which is the key step in the de inking pro-cess, are washing and flotation. In the washingprocess.. the fine ink particles in suspension arewashed aw~y in a water stream and the ink is 9Ub-sequently removed from the filtrate by flocculationusing polymers. W~shinl is efficient only for re-moying the finer ink particles. It uses large yol-wnes ofwater ~nd the fiber yield can be low. Flota-tion deinking i9 similar tocollventional mineral flo.tation <1. 4> where the hydrophobic particles areremoved by their attach1\\ent to a stream of risjngair bubbles. Flotation is usually less sensitive toparticle size and mults in a h~her fiber yield ~§). As can be seen from Fig. 1, flotation is moreeffective in the size range of 15-150 microns, and

Adsorption is essentially selective partitioning ofthe surfactant into the interfacial region. If theinteractions between the surfactant and the solidare more energetically favorable than th06e be-tween the surfactant and the solution, adsot'ptioncan take place. The contributinc forces for adsorp-tion include CO1/alent bonding, couloD1bic interac-tion, ion exchange, desolvation of'the polar groupof' the surfactant, desolvation of the surface, hy-drogen bonding, hydrophobic interaction and v~nder Waals interactions. For each surfactant-solidSystelll several of'the abo1/e factors can be contrib-uting depending on the 6Olid and the surfactanttype, surfactant concentration, temperature. etc. Anunderstanding of the mechanisms of adsorption istlterefore essential in order to select appropriateconditions for flotation separation (~lfi}

-~ -I F 11-

i~:il ,--}-..!!~~~~~ -{ - -

I--~:'" ~r-~ ~-

The ink separation process requires specific cllemi-tal conditions which are created by adding the ap-propris.te chemicals prior to flotation. Flotationreagents added in the deidkmg pr0ce56 include col-lector, £rother, pH regulator, particle 6ize control-ler and dispers~nt. Depeddlng on the type of rawmaterial to be deinked, the nature and do6age ofwemicals are varied. Typical chemicals used fordeinking flotation are shown in Table 1 ~ong withtheir function and dosage (1.6, 11)- For enhanced

~

I........>-0

~~

,.w..,..

~~ --- ""-- ~ -

p IIRr'K:J.;E SIZE. ~Fi8urc 1 General scheme of particle 8ize dependeRce ofcM ~OYal.«icienCJ' by -varioua deinkillC operat.ioDA (6).

Page 23Progress in Paper Recyclin£' I May 1999

.

Page 24 Progress in Paper Recycling I May 1999

carboxymethylcellulo6e. Fatty 3cida, which areprimarily blends of 16-18 C atom chains, are widelyused collector chemicals in ONP/OMG deinking.They complex .,ith the Ca2. ions in the system to

Caustic soda is one of the most importAnt chemicals form calcium soaps which can adsorb on to the inkused in the deinking process. The high surface and provide the collector action. CaClz isconcentration of the alk3li hydrolyzes the ink often added to provide sufficient Ca'. activity invehicles 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 withThe QJnount of sodium hydroxide added 6hould be high molecular weight anionic polymers in theoptimized to provide sufficient alkalinity for good water recycling stage for clarification purposes <a>.saponification and hydrolysis of the resins while Clays are added in the pulp to improve the inkminimizing: the formation of chromophores that rexnoval, although theiT function is not fullycause yellowing of the fibers (W. Mechanical or understood (D. Agglomerating chemicals are usedgroundwood-containing pulps such as ONP and now to treat the electrostatic printing inks (toner

OMG are vulnerable to the yellowing effects of inks) (1.9).excessive alkali. Bleaching agents like peroxidesare also added to improve the brightness of the fiberand to neutralize the yeUo~ng effect of certainadded chemicals (zg). Sodium carbonate and sodiumsilicate are often used in conjunction with 'NaOHto provide the duired alkalinity to the medium withminimum fiber damage. The use of silicate5 and.carbonates allows de;nking to be achieved at a lowerpH which reduces yellowing of the fiber. Otheralkalis which are more environmentally benignthan NaOH such as MgO, Mg(OH)z' NaHCO3,Ca<OH)2 can also be used for the 6a.me pl1rpose (W.In addition to providing alkalinity, silicates aid indeinking by providing an ink dispersant action andby preventing ink redeposition on the fibers.Silicates also help to stabilize the peroxide bycomplexing with the metal ions that wouldotherwise decompose them (5., 1.6.. 2.2.. 2.3).Polypbosphates are added to sequester excessiveMg." and Ca." ions. They also form uncoloredcomplexes with cations such as iron (.16) andthereby help to improve the brightness of the finalfiber. Chelating agents like DTPA(diethylenetriaminepentaacetic acid) and EDTA(ethylenediaminetetraacetic acid) are also used tofonn colDplexes with the heavy metal ions whichimpair the performance of the peroxide (~ 25).Nonionic surfactant6like ethoxylated alkyl phenolsand ethoxylated linear alcohol6, function in thedeinking system by lowering the surface teDBion ofwater, enabling efficient wetting of the fibers whichin turn aids the remo"al of ink from the fibers andits dispersion by solubilization and emulsification.Solvents are available for dis601ving most inks. buttheir use is restricted by their prohibitive costs andenvironmental concern. Also, they have to berelatively in6oluble in water in order that they mayform solvent-water emulsions. Hydrophilicpolymers aid in dispersion and can also act as anti-redepOsition agents. The two most common typesof polymers used are polyacrylates and

deinking performance. the flotation machines usedfor deinking are also modified from those used for

mineral proceaing (l.a).

Calcium-soap forDIation is the most important stepin deinking flotation of old newspapers. The mecha-nism by which calcium improves the flotation effi-ciency of ink particles has been a subject of con sid-erable research (2.6=:l.i>- Under the alkaline condi-tiODB existing in the flotation cells. the ink particlesare negatively charged due to the ionization of theorganic .cids on their surface layers. It has beensuggested that Ca2.. may interact with the nega-tively charged particles through carboxylic bridg-ing mechanism and reduce the surface charge,lead-ing to bridginK between adsorbed Caso and anionicfatty acids. Another possibility is the bulk precipi-tation of calcium soap that c.n cau8e heterocoagu-lation of ink particl68 and calcium soap particlesand thus result in the buildup of calcium soap onink particles by amicroencapswation lDechani61Jl.The ink particles become hydrophobic and henceare readily floatable (26). The third mechanisminvolves reduction of the negative charge of the ink:particles by Cazo and result.nt les8 repulsive forcebetween the ink particles and euy aggregation.These aggregates acquire hydrophobic propertiesupon fatty acid adsorption and are easily floatedout. These proposed mechanisms are summarizedin Fig. 2 W.

Rutland et aI. used a 6urface force apparatus tostudy the interaction of fatty acid collector and cal-cium ions with a negatively charged mica sub6trateat high pH to 6i1nulate the flotation deinking pro-cess. They found no evidence of the bridging mecha-nisxn by the calcium ions under alkaline conditions.The calcium dehydration/de!ttabiliution mecha-ni6m was observed only at low concentrations of(atty acid (below calcium 60ap solubility limit)where calciwn species adsorb on ink particles andlQwer the surface charge of them- At higher fattyacid and calcium concentrations. calcium SQap wuprecipitated in the bulk solution. Under high shear

Page 25Pro~ in Pe.per Recycling I May 1999

,. MAK. lUUlI11

and th\l~ reduce the problem of unwanted scaledeposition in the mill. Newer flotation eollectorebemicals based on modified polyester resina havealso been developed. They tend to act in low d06age9as foaming collectors and do not require the

presence of hardnes6 ions (9).

r~~~~~]) ~ -.--~c-.-&.-~c.J" ~ (!oI"02-- ~ : c.2-

1 01+ c.1+

c-'"'- e.J+

c.2.- ~ ca2+

~(>2+ c.'+

~ {/'8

r;;-~;:;1I I

.-'-c(?-

r;-~.-1I~~I., I

~:c.z.?-

Hydrodynamics of Deinking Flotation,-_",~I --.-.-,..-iM.-;d08

The basic mechanisms of flotation include the col-lision between the particles and bubbles in the liq-uid suspension, the attachlnent of the particles tothe bubbles and the rising of the parliclcJbubbleaggregates to the surface of the suspension. Thecollision, attachment and stability of the aggregatesdepend on the hydrodynamiCS in the flotation cell.Thus. flotatiOll is a dynamic process that involvesmany hydrodynamic phenomena in a system con-taining solid, liquid aDd gas in a state of varyillgturbulence. Apart fI'Om the physical chemistry ofthe interfaces, parameter6 such as pulp aeration,bubble mineralization. agitation intensity, bubblesize and particle size have aD effect on the flota-

tion efficiency.

\=.. ,.. ~.,. ~'-' 1 '\

~ -~ j~cFi8un 2 Schela-atic NPr'eeeDtatioa of ~ble -~~i.-sof calcium-fatty ~id interaction in d.,uinc flOQUOD

(z.7).

conditions. the precipitated calciuUl soap canheterocoagulate with the ink particles, resultingin their coating with a hydrophobic layer. 'thesehydrophobic aggregates can easily attach to ajrbubbles <m. A study on ink particles suspensionwithout fiber shows that ink agglomeration is to-tally dependent on the calcium-soap formation andprecipitation with the heterocoagulation startingol1;ly after cslcium soap formation. The adsorptionof free fatty acid anions on ink particles, on theother hand, is neg1i£ible as most fatty acid is pre-cipitated as calcium soap (28)- Harwot et al stud-ied the deposition of calcium oleate particles onlatex particles with carboxylated charge groups onthe &urface to simulate the ink-ca1ciulll soap inter-action. They also found heterocoagulation to be themain interaction mechanism between calcium ole-ate and carboxylated latex particles (2a)- While allthe above studies suggest heteroeoagulation to bethe mechanism of interaction between ink particlesand ca1cium-fatty acids system, it i5 to be notedthat these experiments were done either with Ulodelsystems or in the ab&ence of fibers and other chemi-cals in the pulp. In real deinking pulp, interactionsamong ink particles, calcium ions and fatty ~cids.re expected to be more complicated.

I ;

The attacllment of particles to bl1bbles has beentreated using the DLVO theory involving electri-cal double layer forces between the particle andthe bubble (witb adsorbed surfactant on ~th) andvan der Waals forces, modified to incll1de hydro-phobic attractive force arising from the transfer ofthe surfactant chains froom the solid-liquid inter-face to the liquid-air interface and repulsive inter-actions due to possible steric hindrance betweenadsorbed surfactant layers on the interactingbubbles and particles (J1).

The above treatment d~ not consider the hydro.dynamic conditions of the cell. Particlelbubble col-lision is indeed affected by the hydrody»amic char-~ristiC8 of the flotation cell <32. Ja>. The overallprobability P of particle-bubble attechment can berepresented by the product of three separated prOb-ability tenns <1):

p = Pc X P & X P4 [1]

where Pc i& the probability of collision between theparticle and the bubble, P. is the probability ofadhesion after collision, and P. ia the probability ofmaintaining the adhesion. The first terDi, pc.depends Ir\ainly on the hydrodynamic c:onditiomof the flotation cell. For large particles and largebubbles, collision will depend greatly on the inertialforces. For small particles and mediwn size bubbles,particle-bubble collision will occur with the sinkingparticles encountering rising bubbles. ~tremel,y

Advances in the understanding of the mechanismsunderlying de inking ha"e helped in thedevelopment of special chemicals for maximumbenefit. One type of sueh chemicals called"displectors. (dispers..nt/colleetor), usuallyproprietary formulations of alkoxylated fatty acidderi"atives, dUplayS properties ofbotb dispersantsand collectoTS thereby improving the performancein both the flotation and the washing stages ofdeinking <.3.0). They do not require calciwn addition

Progress in Paper Recycling I Moy 1999Page 26

to cause the detachment of particle/bubble aggre-gates and the collapse of bubbles. Also, in deinking,the floated material is often an aggregate of inkparticles rather thaQ a dispersed particle. Highshear rate must be avoided in such cases in orderto keep from destroyiQg the aggregates- A kineticmodel that takes into account both particle/bubbleadhesion and breakage has heeQ developed byBloom et al. for the deinking flotation (.32).

fine particles are carried along the streamlines andwill not make a close enoounter with bubbles, 11nlessthey can crOSs the streamline by diffuaion (3.!>.Thus, for different systems, hydrodynamicconditions in the cell playa vital role in affectingthe panicle/bubble collision in different ways.

The relationship between P a8 a function of bubble.size (D~) and particle size (D) can be expressed as

(11Q.): Effects of Process Variables in Flot:ation

DeinkingIn {2]

In the following section, effects of 1/ariou6 physico-chemical and hydrodynamic parameters on nota-

tion deinking are discussed.

Pc:= a

in which Cl and PI are parameters that vary withflow conditions. It is clear that Pc varies approxi-mately with the square of panicle size, D , andinversely as the square of the bubble sue, D:.

a) Surfoctant typeA com parative study of several types of surfactant5reveals that soaps of fatty acids yield good ink re-moval and xninimal fiber 10S8 <Fig. 3) (i.o).AInpholytic surfactants exhibit good filler flotationbut cause excessive fiber loss. Nonionie suxfactantsgive good foaming effect but inadequate flotation.The better collecting action of soaps eompared tothat of non ionic surfactanta is probably due to thecharge neutralization effect of c:aleiuro ions. Cat-ionic surfactants yield good tiller flotation but onlymoderate ink removal. The chain length of the sur-factant can also affect the flotation response. It hasbeen found that the absence of double bonds in thecarbon chain helps the removal of ink from the (l-berg while the presence of double bonds helps theflot.tion proceS8 itself <!l>. Tests have also indi-cated maximwn brightness levels to be obtainedwith C14 to CI8 fatty acids (Fig. 4) <42>.

For large particles, inertia force plaY6 a key role incausing particles to deviate from fluid atreamlinesaround the bubble and cau&e the collision. The largeinertia force may cause detachment of the particlesas well. Fine particles have very small inertia andfollow the streamline around the bubbles unlesstheir hydrodynamically determined trajectoriesco~e within one particle radius of the bubble SUI'-face. On the other hand, fine particles have a lowdetacllrnent probability. It has been reported th.twhile for ensembles of particles larcer than 100JA.In the att.chment of particle ensembles to bubblescan be predicted by the theory (3.6), for particles ofsmaller than 100 ~m the force of attachment is 300-400% higher than that predicted (31).

Flotation is most effective for the removal of inkparticles in the range of 10-1SO J11D. A theoreticalanalysis oftbe particle/bubble attachment by colli-sion suggests that when particles are large enoughto be unaffected by Brownian motion, the flotationrate should increase roughly with the square of theparticle size (W. Similar results have been reportedfor the flotation of model ink particles with the flo-tation rate proportional to the ink particle diam-eter raised to the power of 1.8 (W. The flotationremoval of fine particles, based on Equation 2, canbe enhanced by reducing the bubble size.

b) Surf~tallt I soop con«ntra.tionIt has been observed that in the presence of Ca"',the floatability or ink particles increases with

- "'~58.2sa

~ 55.-

54

521

The turbulent conditions in the flotation cell alsocontrol the particle/bubble attachment and detach-ment.. For deinking cells. due to the small size ofink pa;rticles and the lower concentration of solid5.a comparatively quiescent condition is usually used.While good mixing is necessary for efficient par-ticle/bubble collision, it should not be so strong as

~--' -- - -- - ~So8p LAS f.-oS f~ QAC AM"" NPG

F~ S A comp.ri.oa of deinkiGg .mcieaoy ot diffe...n~SUI't_taa~s: Soap' f.tt.y acid.. LAS - linear alkylbeaaene~onaU; FAS-,.tty .leohol su.lIate; FAES. f.tty .Icobolether 8alt_&-; Q-'C - quateT'Dary am.onium compoWid;AMPH - _mpholytie surfact...ts; NPG ~ non)'lpbenol

ethoxyla&e (~.

PAge 27Progress in Paper Recycling / May 1999

t. MAK. LUULlI

fiber i& impaired by the retention of the Ca soap inthe pulp. In this regard. alkyl polyglycol ether canretard 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 normaldeinking conditions only a small n-action of the Casoap ia actually retained on the fiber, and the highretention found in the mills has been attributed tomechanical entrapment (4.8. .4D). The quality of re-cycled paper is also affected by the residual fattyacid on fibers. It has been suggested that fatty acidmay be at least partly responsible for the low coef-ficient of friction (COF) of recycled newsprint pa-pers. Other collectors such as tall oil have been pro-posed for raising the COF of papers made from re-

cycled pulp (OD).

increasing addition of soap (2.6.) (Fig. 5). There is anoptimum soap concentration beyond which bilayeradsorption of surfactants may occur rendering theparticles hydrophilic and hence less floatable (~)The soap concentrations used are usually well belowthe critical micelle concentrations. Micelles,however, are u6eful in dispening pigment particlesof low water solubility by 60lubilizjng them. In suchcases nonionic sw-factants which have a. much lowercritical micelle concentration than the ionicsurfactants may be used (i.Q)- Hiib dosages ofnonionic 8urfactants have been found to decreasethe ink flotation efficiency, possibly due to thereverse orientation of the adsorbed surfactant (~,if)) a8 well as increased ateric repulsion betweenthe ink particles and the bubbles and increasedelasticity of thin films between ink. particles andbubbles. They also decrease the surface tension ofthe pulp and reduce the air-liquid-particle contactangle (j.§). Since soap has to act together with Ca'.to be effective, the optimum soap concentration i5also dependent on the water hardness. Figure 6shows the effect of soap concentration on thebrightness of the deinked fibers at different waterhardness levels (!1).

c) Effect of calcium and water hardlteSS .Flotation response was observed to be inadequateat hardne6s levels below 180 ppm, above whichmaximum flotation efficiency .u obtained- Thebrightness of fiber is also affected by water hard-ness with the brightness increasing with waterhardness (.42. ru <FiC- 7). Generally higher waterhardness level i6 required for flotation deinkingwith fatty acids. Other metal ion6 such as Mgaa.AI).. also contribute to enhaneed ink flotation (51).

In deinking pulp the hardne&S is regulated by theaddition of calcium ion in the form ofCaC11" It hasbeen c.learly shown in the past that in the absenceof calcium the ink particles do not float and evensmall additions of it improve the flotation consid-erably (Fig- 8). The mechanism by whieh calciumimproves the flotation effic.iency of ink particlesusing fatty acid a6 collector has been discus6ed inthe previous section. Excessive Ca.. c.oncentrationsmust be avoided as it can promote scaling in the

Although calcium soap formation is essential forthe deinking notation, the quality of the deinked

..::

~0--

- \M.D.~oC0.....0.

.0.-U4C.

Figun 6: Effect of soap CoDcentratioa on the brichtueMat different h.rdneH leveu (~).

. aD 1~O '.0

,TEUAfC CO.CC.TRATIOR ...Figu"~ 5: Influenc;e of soap coneentratioD on the

ftoa~bility of ink partiolea (!I).

Prnsne8 in Paper Recycling I May 1999Page 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 enhanceor hamper the deinking process.

~"!~-'.. --

-8$ 170 - 110 - 11(»~~-~

_N-es --,,-~

F'6Un 7: Efre« of.-terhardnen OD the brightneS6 oCthedeinked fiber' (.i1).

Cloud point is a property of nonionic ethoxylatedsurfactants above which the surfactant solutionsbecome cloudy as a result of phase separation. Itbaa been found that deinking efficiency in wash-ing is strongly dependent on the cloud point of thesurfactants used. The highest deinked abeet bright-ness values were obtained when the process tem-perature was within 5 .C of the surfactant cloudpoint <51. 58). The importance of this relationshipis to be noted in deinkina flotation when nowonicethoxyiated surfactanta are present in the pulp.

0

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 ...~pI

1Uflotation cell. Also. it bas been suggested that cal-ciwn ions can attach themselves to the fibers andnnder the fiber hydrophobic. Thus. the presence ofcalcium ions can contribute to the 1066 of fiber inthe flotation <U. 5"-).

~,"~5.5

"."A..'.'

A ftOt.a...-.~1.5

P..-

.Fieu~ 9: ~ft'~ of pH ora ink notation (D).

d) Effect of pHA high pH is necessary for fiber swelling whichimproves ink detachment. Generally a pH of 8-10u reported to be the optimum for the ink flotation,above which the floatability ofink particles has beenfound to decrease (Fig. 9) U9. 26.~, W. This isprobably due to the increased surface charge whichkeepS the ink particles in a highly dispersed stateand difficult to float. pH also affects flotation dueto its effects on solubility of fatty acids and otherchemicals.

e) Effect of temperatureDeinking flotation is usually done at 40-60 ~C.Larsson et 81. have reported floatability of modelink. particles to decrease slightly with increase intemperature (2.6,). Mal1:hildon et aI-. on the contrary,- . ~L - :-1.. -"'1\V~1 to be enhanced with Flgl£re 10: Deere.ae of inked 81Jrlace .. a f\lnctioG of toJ8.

peraCure at different eo.p concentJ'ationS Ca>.

~~OO2l1

can be optimized for ink removal. It ueed to be thepracticc to add ash by blending coated materialssuch as magazines with the newsprint (1). Due torapid growth in newsprint deinking by flotationtechnique3, the demand for coated material i9 fastexceeding the supply. Moreover the uh content inthe coated materials varies widely giving rise toslgnificant swings in the ash content of the feed.Direct addition of clays or some other minerals suchas calcite and uolite in the flotation cell has beenexplored. These materials have been found to havea positive effect on ink flotation Schriver et al. re.ported that among 10 different mineral additives,calcined filler clays give maximwn brightness whenused in conjunction with either fatty acids or di6-persantJcoJIector c:ombinations (~). It is to be notedthat role of ash in flotation deinking is also not verywell understood. Grant et al. speculated that inkparticles may adsorb onto the surface of the inor-ganic material. The resulting particle complex islarger in size and more hydrophobic and hencefloated more efficiently (6..0.). This phenomenon isakin to carner flotation or piggy-back flotation usedin mineral proceaaing in which very fine particlesare attached to bigger carrier particles and floated<.61>. Grant et al. also showed that size of agglom-erate formed by ink particles and clay particles isdependent on the type of minerals and the ratio ofinks to mineral fillers.

30......u~ 20~....0

10

A

05 '0

DI_~50

.,-

B

. 20~ut:c::..0 10k~.2~Z

0 l1_I_J~1 ~. .0

~~.

- -.. ,-

Figure J 1 Effect of pal"tiele ,i- on flotation kiDeti~.A) Ink particle ~iz. distribution before flotation; B) Inkparticle..- diatributiop after 10 lDin flotation (G).

Letac:her et al., on the other hand, reported thatfillers do not playa significant role in improvingthe flotation deinking (u meas~ed by brightness)(6.2). Retention ofkaolin clay tillers did not improvethe brightness of the stock. The increase of bright-ness with silica filler was due to filler retentioninstead of improved flotation efficiency. However,their tests were carried out with a displector chem-istry that is quite different from fatty acid chemis-try. It may be one of the reason as to why fillers arenot beneficial for efficient deinking.

1.1.1.1.

Nu.--at

;-.~

70

8G

$a

:-~- 40..10

B

oJ Effect of particle s~It is well known that for small particles in the rangeof I-SO J1Dl, floatability decreases with particle size(6.3). The larger particles are usually easily floatedand can be separated in a short time (~ Theflotation rate of small particles is rather slow andthey cannot be easily removed even by prolongedflotation. Figure 11 shows the ink particle sizedutribution before and after flotation and clearlyindicates the higher floatability of the coarseparticles- (The number of particles as shown in thefigure is a particle count on a fixed area afterdeposition on a filter.) The main reason for the poorflotation response of the small particles js thehydrodynamic effect (reduced momentum) that

~-, . , , . ,.., 1.8 18 ,. "- 1"~

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F~ 12: Eft'eot of particle size on brigbtae.. of deiAkedfiber (7).

decrea&es the particl~bubble attachment efficiency.The presence of small particles affects th~brightne65 of the recovered fiber adversely. Figure12 indicates the effect of particle si~e on thebrightnesa of the fiber. As can be 6een, 85 the 6izeof the ink particle6 generated become6 swaner the

Pace 30Progress in Paper Recycling I May 1999

MAR. 2002.1

particles of various sizesin the deinking pulp. It isreported that when plateor disk shape particlesinteract with a bubble,larger particles collidewith the bubbles edge-onand bounce off beforeattachment can takeplace, while sznallparticle. can flip onto theside resulting in a largercontact area. whichcannot drain and ruptureduring the tizne ofcontact. In both cases,the particles ha~e a.much 5!naller probabilityof attachment than doesa similar size sphere (6.4)(Fig. 13)-

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Figure 13, A) Tr~ectorie8 of two particl~ drawnW sc-.le, ftO-'Dg throu~h a bubble witt. one.sphere attac;hing ~o the bubble. Eaeh Uawnsphere represents one frame of film or 1.4 me oflapsed 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.zeplane 88 the figure. C) S-all diak fracme..t, Equation 2 states thatdra- to _Ie, ftippin~ to ride aud miMin~ the collision probability Pbubble <W. .' 1 .th ' th c' vaneSlnverse yWl e

square of tile bubble size. D~. The decte3se of bubble

SIze would help to improve the rem~al of fine ink

particles. Aleo, at the same air rate. small bubbles

have a higheT efficiency due to the hi&her specific

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).

c~ : ~..~~---~- T'---~ ,-- -.

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I

T~ --..I

, I

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reflectance of the fiber also decrease.. Thus, forefficient deinking, the process variables should be.djuated to D\inimize the generation of fine particlesand also to achieve maximum removal of theparticles s-enerated. Small particles are formedwhen the inks are aged or when strong mechanicalaction is used to detach the difficult to remo1le, offsetinks. Repulping of the flexographic printing papersalso generates rme particles under the commonalkaline conditions used. The probability forproducing small particles increase5 with thedifficulty of ink separation. HowC1/er the presenceof both calcium and 6oap in the pulp inducesagglolneration of offset ink particles and increasesthe average particle size. Also, the po6sibility ofusing polymers to flocculate the fines prior toflotation needs to be further in1lestigated-

The bubble size is affected to a greater extent bythe chemistry of the deinking pulp. Rao et al.showed that as the surface tension of the pulp de-creases, average bubble size as well as D\axiIntlmbubble sm decrea6e8. Thus, the chemicals u.ged inthe deinking process can be expected to affect flo-tation by affecting bubble size distribution (ti1).

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.The sh.pe of the particles .Iso plays. role indetermining their floatability by affecting theprobability of particle~bubble collisions. 'l11e tonerparticles are usually present as plate or di6k-like

-a 200 ~ 800

AtH RATIO (%)

Figu~ 14; Effect or bubbl~ size on ink notation (Ii>.

Pa~ 31Pr(O&-cdS in Paper Recycling I May 1999

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[I iIi; Treatment of Flexographic and Toner Inks Although the toner particles are generally hydro-

phobic ~). the flotation removal of them has beenreported to be poor compared to that of conven-tional inks. One reason may be the attachment offiben to the toner particles after repulping. as ton-ers are fused to the paper surfaf:e and are diffiCllltto detach from the fiber. The attached fiber makesit hydrodynamically very difficult for particle!bubble attachment to occur. Furthennore, the at-tached fiber renders the toner/ink aggregates ml)rehydrophilic and thus less floatable (11. 18).

Water-based flexographic inks and electrostatic inks(toner) are relatively new types of printing inks whichOCCUr mOTe and more often in recycled paper, espe-cially the mixed office papen. 'nley have qujte differ-ent composition hom traditional printing inks andare not readily removed by flotation using conven-~onal flotation chemistry discussed above, e.g., cat.ciwn -soap chemistry. The presence of these inks hasbecome one of the major problems in the deinkingflotation and pt"OCe6SeS dependent on different spe-cific physicod1emical properties are required to treatpapers printed with these inks.

The printing parameters for toners such as thethickness of print, degree of bonding to paper, andthe speed o(printing are found to affect subsequentdeinking. Slow.printing machines usually producelarger toner particles that contain attached fibersupon repulping and nee:atively affect the flotationdeinking (W. It is hard to break up thick. prints inthe npulping stage due to their 6b-onger cohesive-ness (8Q). As mentioned in the above sectioD6, theshape of the toner particles may also play an im-portant role in affecti,ng the flotation of toners (6.4).

Flexographic printing WleS water. based inks and isenvironmental-friendly due to reduced emission ofvolatile organic compounds d\lrlng the printing pro-c:ess. It is also economical compared wid1 the tradi-bona] offset and letterpress printing. After repulp-mg, flexographic inks fonn very fine particles « 5J1ID) and due to the small particle size, plus the hy-drophilic nature of the inks, it is rather difficult toremove them by flotation. They form very stable col-loidal dispersion. It is reported that the conventionalcalci.\l1n soap of fatty acids are effective for the labo-ratory flotation of model flexo inks- Calcium soap andflexo inks can fonn aggregates (§B. §.2). The problemwith flexo inks xnay thus be due to d1e complex diem-istry involved in the actual mill with fibers and otherconstituents, redeposition of the inks onto fibers, andentrapment (12>.

A number of techniques have been proposed to treatthe toner particles. It is sugge6ted that toner flota-tion can be enhanced by simply improving thechemical and hydrodynamic conditions of the pulpto generate smaller ink particles that ue ~ ofattached fiben <W- The addition of chemicals such

~OlMSEO ~ ~

As flexographic inks contain acrylic resins that arewater soluble under alkaline conditions, repulpingthe nICOVered paper under acidic or neutral condi-tions can reduce the ink dispersion. It can be seenfrom F"ag. 15 that with a. standard deinking chemicalmixture, the brightness of the pulp increases withthe decrease in pulp pH (1.1). Based on this. a two8tac8 deinking proce&& has been proposed to treatmixed ~vered paper, with a neutral stage beforethe conventional alkaline deinking stage. This pro-~ cause& less dispersion offlexo inks and reducesink redepoeition (12. ~). Other approaches includeaddins- a washing stage to the notation ~ ormodif)-ing the composition of flexographic inks toimprove their flotation deinkability ~. 1.5).

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Photocopying and laser printing processes use ther-moplastic powder8, t.enn.ed toners, instead of conven-tional inks. They are electt'OStatically placed on thepaper and then fused in place. Upon repulping, Uletoner particles break into plata or disk-like particlesof various sizes. They are not easily removed by con-ventional washing, flotation, cleaning or screeningprocesses. .

L~~ ~ ~J~ArEl' ;i;m ~ -

.-.WASt£.-5--~~p~

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Fis..,.e 15: Effect of decreasing pH on ftexou-phic indeillkinc <71).

LITERATURE CITEDas mineral oil and polymers to agglomerate the ton-en into larger particles and their subsequent re-moval by fine screening and centrifugal cleaninghave been proposed (8.'1.M). Pulping under acidicconditions could cause toner to be released as smallparticles and increase the effectiveness of tonerremoval (85.). Two-stage treatment (washing/flota-tion and two stage flotation) (86). densification andforward cleaning also gives better flotation results.

l.Gaudin,A.M. Flotation. McGraw Hill, New York,

NY (1951).

2. Crow, D.R.; Secor, R.¥. The ten steps of deinking.Tappi J. 70 (7): 101 (July 1987).

3. Secondary Fiber Recycling (Spangenberg, R. J..Ed.) 'fAPPI Press, Atlanta, GA (1993).

Despite the advances in understanding the deiDk-ing of fles:ographic and toner inks, eJ:perimentalresults reported for flexographic and toner deink-ing often are conflicting, probably due to the differ-ences in the printing process and the compositionof the printing inks. Clearly more work is neededto elucidate interactions between these types of inksand deinking chemicals and to derive a generalprotocol for deinking based on 6ucll information.

5. Read, B.R- Selection of chemi~ ,.,;.thin . mod-ern deinking plant. Paper Technology and Indus-try, p. 339 (Nov. 1985).

6. Harri80n, A. Flotation deinking is critical in unitprocess method of deinking. Paper &. Pulp, p. 60

(March 1989).SuMMARYFlotation ia an effective method for deinking andwhen co.nbined suitably with other processes likewashing, can remove a ,.ride variety of inks in alarge range of sizes. However, flotation is a com-

plex process U1.at involves many physicochemicaland hydrodynamic parameters at the 8olid/liquid/gas interface. The flotation deinking efficiency canbe affected by a large number of factors. Processvariables such as collector type. collector concen-tration, Ca- concentration, pH, temperature, clayminerals, particle size and shape and'bubble size~ shown in this discussion to have significant ef.fects. The mechanisll1 of calciwn-soap fonnationand ita effect on flotation deinking has been re-viewed.lt is clear that clarification of the underly.ins mechani5lI1s in deinking flotation offers an op-portunity to optimize the deinking process and helpto develop special cheU\icals for the maximum ben-

efit.

7. Zabala, M.; McCool, J. Deinking at PapeleraPeninsular and the philosophy of deinking systeJnde$ige- Tappi J 71 (8): 62 (Aug. 1988).

8. Ferguson, L. D. Deinking chetnistry: Part nTappi J. 75 (8): 49 (Aug. 1992).

9. &hrlver, KE. Mill cliemistry must be consid-ered before making deink line decision. Pulp &. Pa-

per, p. 76 ('M~ 1990).

10. Koftinke, R A. Modern newsprint system COIn-biDes flotation and washing deinking:. Tappi J. 68

(2): 61 (Feb. 1985)-

11. Borchardt. J. K; Matalalnaki. D. W. Ne'lRsprintdeiDking: unit operations studies of flotation-washdeinking, a comparison of commercial and propri-etary surfactants. Pulp and Paper Canada 95 (10):

T374 (1994).Although significant advances have been made inelucidating the mechanisms of deinldng conven-tional newsprint and pr0ce5S 6d1eJlle& for treatingsuch inks are well established, flotation deinkingof flexographic and toner inks still face difficultiesparticularly due to lack of understanding of the

processes involved.

ACKNOWLEDGMENT14. Goddard, ED.; Somasundaran. P. Adso.-ptionof surfactants on solids. Cr~tica Chemica Acta 48:

451 (1976)

The ~utbol"s acknowledge financial support of theNational Science Foundation (CTS-962278I, CTS-

9632479 and EEC-98M618).p~33

progrese in Paper Recycling I May 1999

7. MAR. 200211

28. Johansson, B.; Wickman, M.; Strom, G. Themechanism of offset ink particles agglomeration ina calcium-fatty acid collector system. Proc. 3rd Re-cycling Forum,p.51, Technical Section CPPA,Montreal, PQ, Canada (1995).

15. Sornasundaran. P. Physico-chernical aspects ofsurface active agents on minerals. CroaticaChemica Acta 52, p 67 (1919).

16. Woodward, T- W. Appropriate chemical additivesare key to iJXIproved deinking operations. Pulp andPaper, p. 59 (Nov- 1986) 29. Harwot, P-; van de Ven, T. G. M. Effects of'so-

dium oleate and calcium chloride on the depositionof latex particles on an air/w.ter interf'ace- Colloidsand Surlace$ A. 121, p. 229 (1997).

17. Woodward, T- W. Deinking cheD1i6try- 1991Chemical Processing Aids Short Course, No. 281,TAPPI Press, Atlanta, GA (1991).

30. Horacek. R. G.; Jarrehult, B. Chemical applica-tion expands in washing/Dotation deitlking systems.Pulp &. Paper, p. 97 (March 1989).

18. McCool, M. A. Flotation deinking. In Second-ary Fiber Rec;ycling. p. 141-162 (R. J. Spangenberg,Ed.) TAPPI Press, Atlanta, GA (1993).

31. Somasundaran, P.; Chardar, Po; Chtri. K. A studyof the interactions between particles and bubblesin surfactant solution- Colloids and SUJOface A, 8. p.121 (1983).

19. Ferguaon, L. D. Deinking chemistry: Part ITappi J. 15 (7): 15 (July 1992).

20. Renders, A. Hydrogen peroxide and relatedchemical additives in deinking processes- Tappi J.76 (11): 155 (Nov. 1993).

32. Schulze, H. J. . The fundamentals of flotationdeinknig in comparison to mineral flotation. PrQc.1st Recycling Forum, p. 161, Techwcal SectionCPPA, Montreal. PQ, Canada (1991).21- Nystrom, M.; Pykalainen, J-; Lehto, J. Peroxide

blea<:hing of mechanical pulp using different typesof alkali. Paper and Timber 75 ( 6): 419 (1993). 33. Paulsen. F- G.; Pan, R-; Bousfield, D. W.; Th-

ompson, E- v. The dynamics of bubble/particle ap-proach and attachUlent during flotation and theinfluence of' short-ranae nonhydrodynamic fon:eson diajoinin& film ruptUJ'e. Proc- 2nd RecyclingForum, p. 1, Technical Seetion CPPA, Montreal, PQ.Canada (1993)

22.Ali, T.; Mcl1ellan, F.; Adiwinita, J.; May, M. Func-tional and perfonnance characteristics of solublesilicates in deinking. Part 1: alkaline deinking ofnewsprint magazine. Proc. 1st Recycling Forum, p.21, Technical Section CPPA. Montreal. PQ. Canada(1991).

34. M.tis. K.A; Zouboulia, A. I. The role ofbubblelparticle DU. In Flotation Science and Engineer-ing, p- 63 (Matis, K. A, Ed.) MaTcel Dekker, NewYork, NY (1995).

23. Colodette, J. L.; Rothenberg, S.; Dense, C.W.Hydr'Ogen peroxide stabi1ity in presence of 80-dium silicate. JPPS 15 1:J3 (1989).

35. Yoon, R. H-; Luttl'eU, G. H.; Adel, G. T Advancesin fine particle flotation- In Challenges in :MineralProcessmg, p- 487 (K V. S. Sastry, M. C. Fuerstenau,Eds.) SQ, Denver, CO (1989).

24. Bambrick, D. R. The effect of DTPA on reduc-ing peroxide dec:oxnposition. Tappi J. 68 (6): 96 (June1985).

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26. Larason. A.; Stenius, P.; Odberg, L- Surfacechemistry in flotation deinking - Perl I. SvenskPapperstidning 18, p. 158 (1984).

37 - Somasundaran, P.; Varoonov, R-; Tch.liovska.S.; Nishkov, I. Ensemble effects in the detaclunentif floating microspheres- Colloids and Surface A,64. p. 35 (1992).27. Rutland, M.; Pugh, R. J. Calcium soap& in flota-

tion deinking; fundBmental studies using surfaceforce and coagulation techniques. Colloids and Sur-faces, A 125, p. 33 (1997).

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Progress in Paper Recycling I May 1999Page 34

39. Bloom, F.; Ht:indel, T- J. A theoretical model offlotation deinking efficiency. Journal of Colloid andInterface Science 190, p. 197 (1997).

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sorption of n-dodecyl.B-D-maltoside on solids. J.Colloid and Interface Science 191, p. 202 (1991). 59. Schriver, K.E.; Bingham. S.J.; Fraizer, M. The

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47. Bechstein, G.; Unger, E. VerfahrenstechnischePrabl.Dle der Entf"arbung von Altpapier nac:h demDeinkinc-FlotationsprozeO. Chem. Techn- 23, p 731(1971)-

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MAR. 200211 /98 _f

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detachment during repulping of D\ixed office wastecontaining photocopied and laser-printed paper.Tappi J. 78, p. 41 (1995).

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78 Vidotti. R. M.; Johnson, D. A.; Thompson, E. VInfluence of toner detachment during mixed officewaste paper repulping on flotation efficiency. Proc.3rd Recycling Forum, p. 257, Technical SectionCPPA, Montreal, PQ, Cauda (1995).

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68. Dorris, G. M.; Nguyen, N- Flotation of modelinks, part n. Flexo ink dispersion without fibres.Proc. 2nd Recycling FOnItn. p. 13. Technical Sec-tion CPPA, Montreal, PQ. Canada (1993).

79. Snyder, B. A.; Berg, J. C. Effect of particle sizeand density in flotation deinking of electrostaticpapers. Tappi J. 77 (7): 157 (July 1994).

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85. Azevedo, M. A. D.; YalaDianchili, M. R.; Drelich,J.; Miller, J. D. Flotation of toner particles and min-eral filler particles during de--inkingoflaser printedwastepaper. SME preprint, pp. 97-112, SME, Den-ver, CO (1997).

74. Ackermann, C.; Putz, H.; Gottsching, LDeinkability of waterborne tlexo inks by flotationProe. 2nd Recycling Forwn, p- 201. Technical Section CPPA. Montreal, PQ. Canad. (1993).

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86. Quick, T. H.; Hodgson. K T. Xerography deinking- a fundamental approaclL Proc. TAPPI Pulpin8' Conf..p. 561, TAPPI Press, Atlanta. GA (1985). .

~. ginal M~U6CriPt Received for Revi~ on Octob;; lO~ l998

Revised Manuscripted Accepted on January 29. 1999

-

Progress in PapeT Recycling I Me-y 1999Pare 36