2Deinking of Wastepaper By Dr. Essam Saber Abd. El –Sayed March, 2007
3Deinking of Wastepaper Recycling ProcessWastepaper GradesWastepaper ProcurementsRepulpingScreening & CleaningDeinkingBleachingAdditives in each stage
4Recycling of Wastepaper Recycling of waste paper refers to the process of recovering fibers from used paper and converting them back into usable paper and paperboard.Fibers reclaimed are called “secondary fibers”Secondary fibers can be classified into:1- Collected wastepaper, which collected from houses and markets, such as corrugated containers, newspapers and magazines2- Industrial wastes such as trimming and cutting paper mill, over-issued papers3- Semi-industrial wastepaper, which are collected from supermarkets, department stores, office wastes
5Wastepaper GradesThere are more than 80 types of wastepaper can be collected in five major categories:Mixed PaperOld Newspapers (ONP)Old Corrugated Containers (OCC)High-grade DeinkedTrimming paper
6Wastepaper Sources Pre- consumer Post- consumer such as trimming, clipping, cull rolls ofconverting and printersmills, over-issue),which is generallyclean and well sorted.such as wastes fromhome, office, markets,which collected andbaled.
7Degree of RecyclingThere are two primary indices used to compare the level of recycling in various countries:Recovery Rate: is the amount of wastepaper recovered for reuse compared with paper consumedUtilization Rate: is the amount of secondary fibers used in paper/board production compared with the total fiber used.
9Common contaminants in wastepaper repulping systems In-Mill ProblemsTypical SourcesTypes of contaminantCannot be handled in conventional systems, causes defects in productsAdhesives & coatingsHot MeltsDifficult to remove, sticks to roll, causing sheet indentationBlocks and beads used in packagingPolystyrene foamSlow down pulper process, causes sheet product defectsLaminated paper productWet Strength Resins
10Difficult to remove, causes products defectsAdhesives & coatingsLatexDifficult to disperse, fouls equipment and degrades productsCoatings & laminatesWaxesSticks to fabrics, causes black spots in productLaminated productsAsphaltCauses product defects and web breaksVegetable & synthetic fibersForeign fibers
11Deinking processDeinking of pulp is essentially a laundering or cleaning process, where the ink is considered to be the dirt. Chemicals along with heat and mechanical energy, are used during repulping to dislodge the ink particles from the fibers and disperse them in the stock suspension.The ink particles are then separated from the so-called “grey stock” by flotation or washing techniques or by applying a hybrid process that utilizes the both separation techniques.
12SurfactantsSurfactants “surface active agents” are the chemicals used in stock deinking, which affect the surface tension of liquids & solids.Surfactants are molecules having a dual character, part of each molecule is hydrophilic and the other is hydrophobic.Typically, these agents are chemically modified mineral oils, where hydrophilic groups have been added to the molecular structures to make them partly soluble.There are three specific types of surfactants, are important in deinking application:1-Detergents: to remove the ink from fiber2-Dispersents: to keep the ink particles dispersed and prevent re-deposition onto the fibers3-Foaming agents: to reduce the surface tension of water and to promote foam formationOther chemicals, such as caustic soda, sod. Silicate and borax are also used to enhance the action of the surfactants.
13Deinking Process System Design A deinking system is designed according to the type of wastepaper used, quality of deinked stock desired and the furnish type to be deinked.A deinking system should be designed to meet the individual needs of paper mill.All systems have some basic characteristics in common. These are:Removing ink from fiber (pulping)Removing ink from stock (cleaning/screening and washing/flotation)Bleaching
14(Repulping) Repulping is the first stage in the deinking process. In this stage, the secondary fiber is defiberated and the ink is removed from the fiber and dispersed.Pulping may be achieved by batch or continuous methods.Pulping consistencies are usually between 4-6%(low-consistency pulper) or 12-15% (in high-consistency pulper).The amount of mechanical energy generated by the pulper is important in determining the rate of defibering and the rate of ink removal and dispersion.The mechanical energy is dependent upon the pulper configuration and pulping consistency.
16Types of PulpersSchematic arrangement of an HC pulper (for slushing deinking materials, ONP and MGP)Interior of an LC pulper(for manufacturing of packaging paper and board)
17Types of PulpersSchematic arrangement of a drum pulper ( for low wet strength grades)Drum pulper
18Cleaning and Screening Forward (conventional) centrifugal cleaners remove particulates having specific gravities greater than wood fibers.Reverse cleaners are used for removal of light contaminants.Particle size and shape have some influence on ink removal by centrifugal cleanersFollowing centrifugal cleaning, the stock is screened with either pressure screens or open vibrating screens.Ink removal by screening is poor because the ink particles tend to align themselves with fibers and pass through screen
19Types of Cleaners* MC (up to 2 %) cleaners. For removing small heavy particles.* LC cleaner ( %) for light particles.HC (2-5 %) cleaners. For precleaning for removing coarse and heavy particle that may cause damage in the processing system.
20Types of Screeners Coarse screen with rotating screen cylinder Disk screen (opened)
21Flotation ProcessAfter cleaning and screening, the remaining ink particles are separated from the stock by washing or flotation process.In the flotation process, a series of flotation cells is used. A flotation cell is tank supplied with air bubbles.In this process, chemicals (surfactants) are introduced during the pulping operation to promote flocculation of the ink particles and the foam, and then aerated at law consistency (typically at %).The chemistry of flotation process depend on adhering ink particles to the air bubbles. These bubbles rise to the surface as a froth and are skimmed off as rejects.A series of secondary cells is used to increase fiber yield.Ink removal effectiveness decreases as ink particle size falls below 40-50μm.
24Washing ProcessIn the washing process, the detergents and dispersants are utilized in the pulper to remove the ink constituents from fibers, break them down, and disperse them into very fine particles.The ink dispersion is subsequently separated from the pulp, typically by a multistage dilution and thickening washing sequence to produce a clean pulp.The separation of ink is achieved during washing process by washing equipment or screens.Depending upon the type of washer, a wide range of consistencies can be used.The ink particles in this process are extremely small (less than 15μ).
25Washer Machines Belt filter type washer Schematic arrangement of a Dynamic Washer
26Combined washing & flotation process The objective in washing is to break the ink down into particles under 15μ, render them hydrophilic, and keep them finely dispersed.For effective flotation removal, the ink particles must form hydrophobic flocs, ideally in the size range from 30 to 60μ.In the two-stage system (washing-flotation), washing serves to remove fines and fillers along with the smaller ink particles.Washing also appears to enhance the flotation stage by removing some contaminants elements from the furnish which inhibits attachment of ink particles to bubbles.
28PigmentsPigments are insoluble, colored materials, within the vehicle and therefore, must be dispersed.The pigment type is determined the desired color.Dyes are soluble colored materials within the vehicle, and not generally used because of their low resistance to light and a tendency to migrate to fibers.Deinkability is not normally influenced by pigment type.
29The ink VehicleInk Vehicle is the most important component in determining ease of ink removal.A vehicle is composed of a resin (binder), which binds pigment particles together and to the surface of the paper, and a solvent, which provides the ink with proper fluidity.Binders, when dried form polymerized films which vary greatly in their chemical resistance.
30Common Binders Description Binder Rosin Ester Esterification of rosin acids with glycerol or sorbitol hardened with a condensate of phenol and formaldehyde.Rosin EsterPolymerization of unsaturated hydrocarbon fractionsPetroleum ResinsReaction of polyfunctional acid with polyfunctional alcohols condensed with drying oil fatty acids (forms oil-modified polyester resins)Alkyd Resin
31Common BindersPhoto or electron beam initiated free radical polymerization of epoxy acrylates, urethane acrylates, or polyester acrylates.Radiation-CuredAlkaline water soluble vinyl or styrene acrylate copolymers with amine addition for alkalinity.Water-Based
32ModifiersModifiers are materials which give inks specific chemical or physical properties.Examples are waxes, plasticizers, drying agents and co-solvents.They are added in small quantities and do not impact the deinkability of inks.
33Types of Ink Drying Mechanisms Ink is frequently classified according to its drying mechanism.There are four general mechanisms for drying ink:1-Absorption2-Evaportion3-Oxidation4-Radiation curing
34The chemistry of Deinking The most important deinked chemical mechanisms are:Fiber SwellingSaponificationWettingEmulsification / SolublizationSequestration / precipitationAntiredepositionDispersion
35The chemistry of Deinking Fiber Swelling occurs when immersed cellulosic fibers in water or electrolyte solution.The breaking of interfiber bonds and swelling of fibers are important steps in deinking as they greatly facilitate loosening and removal of inks and coatings from fiber surfaces.Saponification involves hydrolysis of esters in aqueous alkali. This reaction will convert the ester into its component acid and alcohol.Many of the resins used as ink binder are esters and therefore can be broken up in hot alkali solution.This is one of principle reactions occurring in deinking of conventional offset and gravure inks.Phenolic modified rosin often esters can be saponified under severe conditions of pH and temperature
36WettingWhen a liquid surface is in contact with a solid, the molecules at interface may be more attracted to the solid than the bulk liquid. If so, the molecules tend to spread out over the solid and surface area (surface tension or energy) of the liquid is increased. This phenomenon is called wetting.In deinking, with liquid (water) and solid (ink and fiber), proper wetting allows more rapid penetration of chemicals into fiber network and ink-fiber contact area and helps ink break up and separate from fiber.Surface energies influence ink collection in flotation cells.
37Emulsification and Solubilization Emulsification is the dispersion of liquid phase to another to form a significant stable suspension.Emulsification is an important chemical mechanism in deinking only when there are oils present in ink.Adsorption of emulsifying agents (surfactants) at the oil-fiber interface release the oil from the fiber (with the pigment particles) and forms an oil-in-water emulsion.Solublization, simply put, is the dissolving of substances in a medium in which they are normally insoluble.Solublization differs from emulsification in that solubilized material is in the same phase as the solution while emulsified material is a dispersion.
38PeptizationPeptization is the conversion of an insoluble solid to a colloidal state whereby the particles are electrically stabilized.The important of peptization as deinking mechanism is unknown. But it probably occurs, for example, as a secondary reaction following saponification of ester-based resin binders or as ink particles are mechanically and thermally broken up.
39Sequestration and precipitation The presence of polyvalent cations notably calcium, magnesium, and iron- can be detrimental to the deinking process even, when nonionic surfactants are used.These cations can reduce negative surface charges on both fiber and ink leading to agglomeration and redeposition. Catioins also may act as linkages between –ve fiber and –ve ink particles.These ions inter the system in the water or paper stock and can be removed by sequestration (formation of a water soluble complex) and precipitation (formation of an insoluble precipitate).
40DispersionDispersion is the phenomenon of adjusting the surface characteristics of particles (suspension or emulsion) to prevent reagglomeration.Adsorption of negatively charged dispersing agents (surfactant or inorganic ions) onto detached ink or emulsified oil particles causes mutual repulsion and prevents agglomeration.
41AntiredepositionAntiredeposition refers to preventing the deposition of solid ink and oily particles back onto fibers.They function by sterically inhibiting the approach of ink particles to fibers and can be quite effective as wash aids in washing deinking.