Deinking of Wastepaper By

Dr. Essam Saber Abd. El –Sayed March, 2007

Recycling Process Wastepaper Grades Wastepaper

Procurements Repulping Screening & Cleaning Deinking Bleaching Additives in each

stage

Deinking 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 magazines

2- Industrial wastes such as trimming and cutting paper mill, over-issued papers

3- Semi-industrial wastepaper, which are collected from supermarkets, department stores, office wastes

Recycling of Wastepaper

There are more than 80 types of wastepaper can be collected in five major categories:

1. Mixed Paper2. Old Newspapers

(ONP)3. Old Corrugated

Containers (OCC)4. High-grade Deinked5. Trimming paper

Wastepaper Grades

Wastepaper Grades

such as trimming, clipping, cull rolls ofconverting and printers mills, over-issue), which is generally clean and well sorted.

Wastepaper Sources

Pre- consumer Post- consumer

such as wastes from

home, office, markets,

which collected and

baled.

There 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 consumed

Utilization Rate: is the amount of secondary fibers used in paper/board production compared with the total fiber used.

Degree of RecyclingDegree of Recycling

Collection Sorting Classification Repulping Cleaning Screening Deinking Bleaching

Wastepaper Procurement

Common contaminants in wastepaper repulping systems

Types of contaminantTypical SourcesIn-Mill Problems

Hot MeltsAdhesives & coatingsCannot be handled in conventional systems, causes defects in products

Polystyrene foamBlocks and beads used in packaging

Difficult to remove, sticks to roll, causing sheet indentation

Wet Strength ResinsLaminated paper product

Slow down pulper process, causes sheet product defects

LatexAdhesives & coatingsDifficult to remove, causes products

defects

WaxesCoatings & laminatesDifficult to disperse, fouls equipment and degrades products

AsphaltLaminated productsSticks to fabrics, causes black spots in product

Foreign fibersVegetable & synthetic fibers

Causes product defects and web breaks

Deinking 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.

Deinking processDeinking process

Surfactants “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 formation Other chemicals, such as caustic soda, sod. Silicate and borax

are also used to enhance the action of the surfactants.

SurfactantsSurfactants

Deinking 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:

1. Removing ink from fiber (pulping)2. Removing ink from stock (cleaning/screening and

washing/flotation)3. Bleaching

1. (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.

1. Low-consistency pulper

2. High-consistency pulper

3. Drum pulper

4. Betonniere high-consistency pulper

5. Secondary pulper

Types of Pulpers

Schematic arrangement of an HC pulper (for slushing deinking materials, ONP and MGP)

Interior of an LC pulper(for manufacturing of packaging paper and board)

Types of Pulpers

Schematic arrangement of a drum pulper ( for low wet strength grades)

Drum pulper

Types of Pulpers

Cleaning 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 cleaners

Following 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

* MC (up to 2 %) cleaners. For removing small heavy particles.* LC cleaner (0.5-1.5 %) for light particles.

HC (2-5 %) cleaners. For precleaning for removing coarse and heavy particle that may cause damage in the processing system.

Types of Cleaners

Disk screen (opened) Coarse screen with rotating screen cylinder

Types of Screeners

After 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 0.8-1.2%).

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.

Flotation Process

Floatation cell

Flotation Process

In 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).

Washing Process

Belt filter type washer Schematic arrangement of a Dynamic Washer

Washer Machines

Combined 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.

Printing Inks components

pigments modifiers

vehicles

Pigments 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.

Pigments

Ink 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.

The ink Vehicle

BinderDescription

Rosin Ester

Esterification of rosin acids with glycerol or sorbitol hardened with a condensate of phenol and formaldehyde.

Petroleum ResinsPolymerization of unsaturated hydrocarbon fractions

Alkyd Resin

Reaction of polyfunctional acid with polyfunctional alcohols condensed with drying oil fatty acids (forms oil-modified polyester resins)

Common Binders

Radiation-Cured

Photo or electron beam initiated free radical polymerization of epoxy acrylates, urethane acrylates, or polyester acrylates.

Water-BasedAlkaline water soluble vinyl or styrene acrylate copolymers with amine addition for alkalinity.

Common Binders

Modifiers 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.

Modifiers

Types of Ink Drying Mechanisms

Ink is frequently classified according to its drying mechanism.

There are four general mechanisms for drying ink:

1-Absorption

2-Evaportion

3-Oxidation

4-Radiation curing

The chemistry of Deinking

The most important deinked chemical mechanisms are:

Fiber Swelling Saponification Wetting Emulsification / Solublization Sequestration / precipitation Antiredeposition Dispersion

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

The chemistry of Deinking

When 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.

Wetting

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.

Emulsification and Solubilization

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).

Sequestration and precipitation

Dispersion 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.

Dispersion

• Antiredeposition 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.

Antiredeposition

Deinking Processing Aids

Deinking Chemical

Structure/Formula

FunctionFurnish

Type

Dosage (%of

Fiber)

Sodium HydroxideNaOH

Fiber swelling

Ink break up

Ink dispersion

Wood-free

grades3-5

Sodium Silicates

Na2SiO3

Peptization

Ink dispersion

Alkalinity and buffering

H2O2stabilization

Ground -wood grades2-4

Sodium CarbonateNa2CO3

Alkalinity buffering

Ground -wood grades

2-5

Sod. or Pot. Phosphates

Na5P3O10

Na4P2O7

Metal ion sequestrantPeptization

Ink dispersion

All grades0.2-1

Nonionic surfactants

CH3(CH2)-CH2-

O(CH2CH2O)

xH

Ethoxylated Linear

alcohol/ Alkyl Phenols

Ink removal

Ink dispersion

Wetting

Emulsification

Solubilizing

Peptization

All grades0.2-2

Solvents

C12-C14

aliphatic saturated

hydrocarbons

Ink softening

SolvationWood-

free grades

0.5-2

Polymeric dispersants

-CH2-CHC=O

O-(Na+) n

Polyacrylate-Diisobutylene

Maleic Anhydride Copolymer

Ink dispersionAntiredeposition

Sequestration

All grades0.1-.5

Fatty Acid

(Soap)

CH3(CH2)16

COONa

Sodium Stearate

Ink flotation

AidAll

grades0.5-3

Peroxide

Sod. Hydrosulfite

Chlorine

H2O2

Na2S2O4

…Cl2

..OCl2

Bleach

Color Strip

Ground wood grades

1-2

0.5-1

0.5-3