Biermann Ch. Handbook of Pulping and Papermaking

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264 10. FffiER FROM RECYCLED PAPER

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FIBER REJECT RATE.

Fig. 10-1. Contaminant removal efficiency and

fiber reject rate.

contaminants from the useful fiber. It is useful to

compare various steps used for contaminant re-

moval by diagrams such as that in Fig. 10-1. The

efficiency of a process is the percentage of rejects

removed. The reject rate is the percentage of

useful fiber that is discharged with the rejects.

Consider a simple pipe split with equal flows to

rejects and accepts; 50% of the contaminants are

removed (50% efficiency), but with a fiber reject

rate of 50%, nothing has been accomplished. In

Fig. 10-1, two hypothetical systems are presented,

"A"

and "B". It should be apparent that system

"A"

is superior because it can be run at the same

efficiency for a lower fiber reject rate or can be

run at a higher efficiency with the same fiber

reject rate compared to "B". For any given sys-

tem, this diagram shows the trade-offs between

improved efficiency and fiber reject rates that one

will encounter with cleaning, screening, deinking,

and other processes.

Contaminants can be classified according to

their source. Stickles are a diverse group of

materials that are tacky to the touch. They may

enter the process through the polymeric contami-

nants of secondary fiber including hot melt adhe-

sives (typically consisting of ethylene vinyl ace-

tate-wax-hydrogenated resin acid combinations),

wax and polyethylene from coated boxes, contact

adhesives (polybutylene, natural rubber, etc.).

pressure sensitive adhesives [styrene-butadiene

rubber (SBR), carboxylated polybutadiene, and

vinyl acrylates], polystyrene, and so forth. Some

of these structures are shown in Fig. 10-2.

Stickles may also arise on the paper machine from

wood extractives that polymerize during pulping

and, especially, bleaching, additives used within

the mill such as fatty acids and other defoamers,

rosin and synthetic sizing agents, and polymers

used within the mill such as natural rubber splicing

tapes.

They tend to be soluble in nonpolar sol-

vents such as diethyl ether, toluene or methylene

chloride, have a tacky feel to them, and are very

difficult to remove from the process.

Fillers include calcium carbonate, which can

interfere with rosin/alum sizing, clays, and titani-

um dioxide. Films and laminates include polyeth-

ylene, aluminum foil, etc. High density materials

include materials such as glass, grit, sand, and

metallic objects. Other materials include small

amounts of unbleachable dyes, wet strength resins,

and stringy materials such as carpets.

Ink consists of pigments, such as carbon

black or titanium dioxide, to supply color and

opacity and a vehicle to carry the pigment and

bind it to the paper. The vehicle consists of

solvent and resin to bind the pigment to the paper.

Traditionally the vehicle was vegetable oil. This

is significant since vegetable oil is easily saponifi-

able with alkali, allowing the ink to be dispersed

during deinking. Saponification converts the

triglyceride plant oil to glycerol and the salts of

the three constituent fatty acids (see Fig. 2-31).

The type of resin depends on the type of ink

and how it attaches to the paper. Inks may "set"

on the paper by one of several mechanisms ac-

cording to Horacek (1978). 1) Absorption of the

hydrocarbon vehicle into the paper substrate; this

type of ink is used in newsprint and tends to

smudge. 2) Evaporation of the ink vehicle; this is

used in magazine and catalog grades using letter-

press or offset printing with rosin esters and metal

binders. 3) Oxidation of a drying oil with

multifunctional carboxylic acids and alcohols left

on the paper surface after the vehicle is absorbed

into the paper. 4)

or electron beam radiation

curing of monomers or prepolymers in the vehicle

into polymers such as acrylics. Infrared harden-

ing,

precipitation of

binders,

gelation, and cooling

RECYCLED FIBER PREPARATION

265

CH3-(CH2CH2)H:H3

Polyethylene (wax)

CH3(CH2)^CH3 n = 23 - 38

Paraffin wax

0-C-CH,

H/-^—CH/^-

CH2-CH-

Ethylene vinyl acetate copolynner (EVA)

Natural rubber (cis-1,4-polyisoprene)

CH,

^H ry CH/<

2 I

CH.

-im

Ethylene isobutylene copolymer

Polystyrene

Fig. 10-2. Several waste paper contaminaiits.

of hot thermoplastic inks used in electrostatic

printing (photocopy and laser printers) are other

mechanisms used less often, but are increasing.

The first two types of ink are readily

dispersible by emulsification. Oxidation cured

inks are often saponified by alkali, but radiation

cured inks do not easily break into pieces smaller

than 50-100 \im. The vehicles of inks that dry

consist of mineral oil, waxes, alkyd resins, and

other hydrocarbon resins; inks which cure by

infrared or ultraviolet radiation are made with

epoxy, urethane, vinyl, or polyol acrylates; xero-

graphic inks contain SBR, polyesters, and

acrylates. Latex binders are SBR, carboxylated

butadiene, polyvinyl acetate, etc. As ink, and the

paper to which it is attached, ages, it becomes

more difficult to remove. This should be kept in

mind when deciding how to handle wastepaper

inventory. Also, the size of the ink particles is

very important; typically, newsprint ink has very

small particles, on the order of 1-10 /xm, ledger

has large particles, on the order of 50-1000 /im,

and mixed news/magazine grades have intermedi-

ate particle sizes on the order of 1-50 jitm.

Aside from being classified by their source,

contaminants are often classified by their method

of removal. Some examples are large, dense

materials; small, dense materials; "ultradense"

materials; fibrous, light materials; small, light

flakes; and so on until you are sick. Such classifi-

cation schemes identify categories by density, size,

composition, shape, etc. Since these classifica-

tions are arbitrary and the contaminants change as

the fiber is processed, they are not mentioned

here.

Keep in mind, however, what types of

contaminants each process removes, to what

degree each is removed, and how does the process

alter the contaminants so that they may or may not

be removed in subsequent steps. For example, too

much agitation will break stickles apart so that

they may not be removed by screening; however,

remaining stickles are sometimes deliberately

dispersed so that they will have limited impact

during papermaking.

One way to limit the amount of contaminants

in the secondary fiber is to pay close attention to

the waste paper coming to the mill. The Paper

Stock Institute of America circular

PS-83

specifies

that newspapers for recycling should be almost as

266

10.

FIBER FROM RECYCLED PAPER

clean as when they were delivered to the consum-

er. Newspapers that are wrapped in plastic (even

strappings) or kraft paper or that have more than

rejects should be rejected. Ideally, little

useful fiber should be removed with the contami-

nants when processing waste paper.

10.3 RECYCLED FIBER RECOVERY

Introduction

Recycling fiber is the process of separating

useful fiber from the contaminants of waste paper.

A series of processes can be used to accomplish

this task. There are also many possible ways of

arranging these steps. Each mill has its own

needs,

its own hypotheses about what is the best

method, and its own operating experiences. In

this section we will consider many of the process-

es available and how they work.

Crow and Secor (1987) list 10 steps for

deinking, including pulping, pre-washing heat and

chemical loop, screening (coarse and fine),

through-flow and reverse cleaning, washing,

flotation, dispersion, bleaching, and water recircu-

lation and makeup, with brief descriptions of each

of these processes. While other methods are

available, these represent most of the tools avail-

able for fiber recovery from waste paper. Not all

steps are used in all recycled fiber plants.

Fig. 10-3. HI-CON^'^ high consistency pulper.

Courtesy of Black Clawson, Shartle Division.

Continuous

pulper

Recycled fiber recovery begins at the pulper,

Figs.

10-3 and 10-4, which is nothing more than

a large blender to disperse pulp into an aqueous

slurry. Pulping may be done at high or low con-

REJECTS

TO DUMPSTER

ACCEPTS TO SUCTION OF DUMP PUMP

WASH WATER RECIRCULATED TO PULPER

Fig. 10-4. Hydrapurge™ detrashing system. Courtesy Black Clawson, Shartle Division.

RECYCLED FffiER RECOVERY 267

sistencies using different rotors. Fig. 10-3 shows

a high consistency pulper and Fig. 10-4 shows a

low consistency pulper with trash removal.

During pulping, some of the gross con-

taminants are removed. A ragger is a chain which

is pulled out slowly over a long period of time

(perhaps a meter every few hours). The ragger

catches baling wire, wires, plastic sheeting, stringy

materials, tapes, wet strength paper, and other

long materials that wrap around it as the stock

rotates in the pulper. The accumulated material

forms a rag rope that is over a foot in diameter

upon removal. Fig. 10-5 shows a diagram of a

ragger with cutter.

Relatively large heavy contaminants such as

nuts,

bolts, rocks, pipe fittings, and other material

do not pass through the holes of the pulper and

eventually are forced outward to

the

junk chute at

the lower peripheral of the pulper. This material

is removed by one of several possible means. A

Black Clawson M-10

Automatic Ragger

(guard omitted for clarity)

Fig. 10-5. Rag rope cutter. Courtesy of Black

Clawson, Shartle Division.

junk tower is a long tube with an overhead grapple

that is used to manually remove debris that accu-

mulates at the bottom of the tower. A continuous

junk remover is an enclosed bucket conveyer used

to automatically remove heavy and floating debris

on a continuous basis (Fig. 10-6). It has often

been used when processing old corrugated con-

tainers (OCC). Because of the high maintenance

due to chain breakage and inability to keep the

pulper clean, bucket and chain trash-removing

configurations are no longer used in new equip-

ment. The junk trap consists of a vertical tube

with valves on the top and bottom to isolate it

from the pulper. The top valve is normally open

to allow junk to enter, and the bottom valve is

closed to retain the contents. The junk trap is

emptied by closing the top valve and opening the

bottom valve to let the debris out. The bottom

valve is then closed, the top valve opened, and the

processes repeated as necessary.

On the order of 18 kWh/t (1 hp-day/ton) is

required to disperse the fibers, and about the same

amount is used for cleaning and screening. The

pulp from the pulper then usually goes to pressure

screens and high density centrifiigal cleaners.

Sometimes high temperatures and agitation in

the pulper might break up contaminants into very

small particles and must be avoided. These

conditions would be avoided if fine screening is

the only subsequent method of contaminant remov-

al.

These conditions might also be avoided if ink

flotation is being used. In other cases, as with ink

washing, these conditions might be desirable.

Mdst deinking plants operate their pulpers in

batch

mode.

Deinking conditions in the pulper are

typically 6-15% consistency, 40-70°C, pH 9-11

(generally below 10 for groundwood pulps to limit

yellowing), for one hour. The trend is towards

higher consistency pulping, which has the advan-

tages of enhanced ink separation, reduced pulping

time,

and lower specific energy consumption.

Pulping at high consistency requires dilution

of the pulp to about 5% consistency before it will

pass through the extraction plate at the bottom of

the pulper (Plate 38) or other coarse screening

device [6-16 mm (1/4 to 5/8 in.) hole

size].

A tub

extension on the pulper is used so dilution water

can be added, or the pulper contents are dumped

into a stock chest. Typically white ledger paper is

268 10. FIBER FROM RECYCLED PAPER

Removable cover

Cage-type floating trash

strainer

Chain,

sprockets

and

buck-

ets furnished in either

chrome

steel or

malleable

iron

Safety feature: shear pin

to protect chains and

buckets

Gear reducer drive—5 HP

motor on opposite side

Perforated bottom

junk

box permits draining for

removal of trash—quick

opening cleanout door at

opposite side of hopper

for quick removal of trash

Pipe

connections

for white

water

wash to junk

hopper

and for draining hopper

to remove trash—either

connection can be used

for wash or drain

Baffle directs floating re-

jects to restricted area at

stock level in upper part

of junk remover—uptra-

veling buckets trap float-

ing rejects and discharge

them to junk hopper

Junk chute from pulping

unit

Junk remover boot

Cast shoe for guiding

chains

Standard junk removers for small

and medium tonnage operations

are furnished with buckets spaced

on 36" centers.

Larger junk removers are specified

with buckets on 18" centers—with

buckets perforated (as shown) to

facilitate removal of floating trash.

above showing conveyor mechanism

Fig. 10-6. Junk remover. Courtesy of Black Clawson, Shartle Division.

pulped at 15-20% consistency, newsprint at 10- 50%, the primary screen rejects are often sent to

12%

consistency, and OCC at 4-5% consistency. a secondary screen to recover usable fiber.

Pressure screens are coming into widespread use

Screening for this purpose. The basket has a life of 6-12

The stock is then sent to a screening system, months for deinking grades and 2-4 months for

Because primary screen rejects may be as high as nondeinking grades, which tend to have more

RECYCLED FIBER RECOVERY 269

abrasive particles in them. Wear occurs at the

rejects end where the consistency is higher and

water does not lubricate as well. The trend is

towards modular assembly of pressure screens so

that the reject end section can be replaced while

reusing most of the basket. Fiberprep uses an

inward-flow design and claims that centrifugal

force keeps the heavy debris away from the screen

and thereby reduces screen basket wear and

increases throughput. More information on pres-

sure screens may be found in Section 8.2.

Centrifugal, vortex, or cyclone cleaners

The basic principles of low density vortex

cleaners for cleaning pulp prior to the paper

machine were described in Section 8.2. In for-

ward cleaners a tangential input flow causes a

vortex to form inside the cleaner; heavy particles

move to the outside of the vortex and eventually

drop to the bottom of the cleaner with the rejects

while the pulp accepts move to the center of the

Fig. 10-7. Two high density cleaners.

vortex where they are removed at lower pressure.

By this means, small heavy debris such as sand,

glass,

and metal fragments are removed.

Medium (1-3%) and high (2-5%) density

centrifugal cleaners are used at 2-5% consistency

and low pressure drops, 70-210 kPa (10-25 psi),

to remove heavy contaminants of particle size 50-

1000 jLtm (0.002-0.040 in.) that escape with the

pulp through the pulper coarse screen and other

screens. Fig 10-7 shows two high density cleaners

used in a 500 ton/day secondary fiber mill. Fig.

10-8 shows a diagram of a high density cleaner

and its operation. Rejects consist of small pieces

of tramp metal, glass, and stones. Reject rates are

below

1 %.

Water (10-40 gal/min) may be used in

the throat of the cleaner to decrease the consisten-

cy in this region and help the heavy particles

settle. These cleaners are typically about 2.5 to 6

m (8-20 ft) tall (although low profile cleaners are

available) with a diameter of 0.20-0.65 m (8-26

in.) at the wide portion. The reject trap consists

of a pair of valves that are manually or automati-

cally operated. The top valve is left open to

collect debris while the bottom valve remains

closed. When the trap gets full, the top valve is

closed and the bottom valve is opened to remove

debris. The bottom valve is then closed, and the

top valve is then opened. A single high density

cleaner can process several hundred tons of pulp

per day.

If very fine grit remain, a low density clean-

ing system is used that operates below

1 %

consis-

tency (high pressure, 200-275 kPa or 30-40 psi) as

described in Section 8.2.

Black Clawson's trademark for high and

medium density cleaners is the Ruffclone'^'^. The

high density cleaners use 5-10 kWh/t (0.3-0.5

hp/d/t) pulp. Their fine paper forward cleaners

are Ultra-clones that use about 30-80 kWh/t (1.7-

4.5 hp/d/t).

Reverse

cleaners,

through

or parallel flow

cleaners

Lightweight contaminants were first removed

by reverse-cleaners, where normal forward clean-

ers were used and merely operated in reverse.

These cleaners required very large pressure drops

on the order of 500 kPa (70 psi) and used large

amounts of energy. Recent developments allow

both the rejects and the accepts to be removed at

270 10. FIBER FROM RECYCLED PAPER

1000

800

600

500

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[ 200

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Fig. 10-8. Ruffclone™ and Liquid Cyclone^^ high density

cleaners with capacity chart and recirculation

loop

for constant

discharge pressure. Courtesy Black Clawson, Shartle Division.

REJECTS

Fig. 10-9. A through-flow cleaner

designed to remove low density

contaminants. Courtesy of Beloit.

the bottom. Such cleaners, while called reverse

cleaners, are better termed through or parallel

flow cleaners. The reject rates have been reduced

from 20-30% for the original reverse cleaners to

2-3%

for modern designs. This means that nu-

merous stages are not required. Fig. 10-9 shows

the operation of a through flow cleaner.

Beloit (LeBlanc and McCool, 1988) has

developed a number of centrifugal cleaners that

are as examples of recent improvements. Beloit

developed these cleaners using image analysis to

track contaminants in the isolated fiber, trans-

parent cleaners with high speed video cameras to

observe the separation

process,

and analysis of the

internal flow patterns. The Uniflow'^'^ cleaner,

which appeared in 1982, is a through flow cleaner

that removes lighter-than-water contaminants such

as plastics and other polymers, stickles, and pitch.

The feed of the cleaner is at the top, and both the

rejects and accepts flow through the cleaner and

RECYCLED FIBER RECOVERY 271

out the bottom, hence the term uniflow. An inter-

nal core with a small annulus around the air core

collects the rejects. This circumvents the upward

moving, inward spiral flow that interfered with

lightweight reject separation and led to high reject

rates in earlier reverse flow cleaners. These

changes were claimed to reduce the rejects from

20%

to 1-2% with a two-fold improvement in

lightweight contaminant removal.

Black Clawson's through flow cleaners are

the 3 in. X-clone'^^ cleaners, which were first

operational at the Stone Container mill in Flor-

ence,

South Carolina in May of 1984, and have

been described by Bliss (1986). These cleaners

use

1 %

consistency pulp with a 60-100 kPa (10-15

psi) pressure drop. The power requirements are

6-10 kWh/t (0.3-0.5 hp/d/t). The reject rate is

normally 1-4% at a consistency of 0.1 to 0.4%.

High stock temperatures improve the efficiency.

Two banks of primary cleaners in series may be

used, and the rejects from the primary bank are

sometimes treated in secondary cleaners. The

Beloit Posiflow"^^ centrifugal cleaner, introduced

in 1986, is

forward cleaner that removes heavi-

er-than-water contaminants. It is claimed that in

some systems the conventional three-stage cleaning

systems are replaced with a single stage.

Gyrodean™

An alternative to reverse cleaners is the

Gyroclean of Fiberprep shown in Fig. 10-10. The

unit was developed by Lamort and Centre Tech-

nique du Papeterie (CTP) in France. The manu-

facturer claims the unit can operate at up to 2%

consistency. Fiber loss is said to be minimal, and

no secondary units are required. Units are avail-

able to process 70-520 mVhr (300-2300 gal/min).

The acceleration experienced by the stock is said

to be up to 700 times normal gravity.

Deinking chemistry

Less than 20% of secondary fiber is deinked

in the U.S. In addition to high wood costs, recent

legislation will change this picture as many states

will require recycled fiber in new newsprint as a

means of reducing the large quantities of material

sent to landfills, of which about 40% is paper.

The ink is about 0.5-2% of the mass of the waste

paper to be deinked.

The overall deinking process from the point

of view of the fiber can be broken down into four

steps.

1) Repulping with concomitant ink removal

from the fibers, 2) cleaning and screening opera-

tions to remove the bulk of the ink from the stock,

3) separation of residual ink contaminants from the

fiber stock, and 4) bleaching, if necessary. Other

steps are necessary for the process such as recov-

ering the wastes from the effluents to allow water

reuse.

A variety of chemicals are used in ink remov-

al including the sodium salts of hydroxide (for

fiber swelling, saponification of ester-containing

resins,

and ink dispersion), carbonate (as a buffer-

ing agent), silicate (peroxide stabilizer via metal

ion sequestering, wetting agent, pH buffer, and

ink dispersant), polyphosphate

(0.2-1 %

on pulp as

a metal ion sequestering agent and ink dispersant),

peroxide or hydrosulfite, fatty acid soaps, nonionic

surfactants, and other materials. The nonionic

surfactants are usually alkyl phenol or linear

alcohol ethylene oxides as described in Section

8.5. Petroleum ether, a mixture of Cg-Cio

alkanes, is sometimes used in small amounts to

soften the ink vehicle. If any of these materials

carry over into the papermaking system, there may

be difficulties with foaming, scale formation, wet

end chemistry, or surface sizing at the size press.

There are two commonly used methods of ink

removal: ink washing and ink flotation ov froth

flotation. Ink can also be removed by solvent

extraction in a process that resembles dry-clean-

ing, but it is too expensive to have widespread

use.

Ink washing

Ink washing involves ink removal by washing

it from the fiber using sodium hydroxide, sodium

silicate, and hydrogen peroxide with a suitable

dispersant in the pulper. Often the dispersant is

stearic acid and micelle formation occurs in the

classic mechanism by which soap is able to make

grease and oils water "soluble". A water-ink

emulsion system is formed with particle sizes

averaging below 1 fim. The emulsion is washed

from the pulp, and the ink is removed from the

washwater by flocculation so the wash water may

be reused. Hard water should not be used during

ink washing since this will precipitate the soap and

272 10. FIBER FROM RECYCLED PAPER

the complex will not be water soluble. The

clarified water is then reused to wash more pulp.

This general method has been used since the nine-

teenth century. It is suitable for traditional inks

with vegetable oil vehicles that are readily saponi-

fiable and dispersible.

Fig. 10-11 shows the theoretical ink removal

for a washing system. This graph indicates the

percentage of water removed in a single stage of

washing based on the water going in and the water

going out. For example, consider 100 lb of a pulp

slurry with an inlet consistency of 3% and an

outlet consistency of

12%.

There are 3 lb of fiber

and 97 lb of water going in. The 3 lb of fiber

corresponds to 25 lb of slurry at 12% outlet

consistency or 22 lb of water going out. This

means that 75 lb of the original 97 lb were re-

moved to give a washing efficiency of

77.3%.

In fact, this is only true for materials that are

water soluble and have no affinity for the fibers.

Ink particles of finite size will be trapped by fiber

networks that form as the slurry is concentrated on

a screen. Larger ink particles are more easily

retained by this mechanism.

Fig. 10-10. Gyroclean™ system for removal of lightweight contaminants. Three units in service

(top) and the operating principle (bottom) are shown. Courtesy of Fiberprep.

RECYCLED FIBER RECOVERY 273

100-

82-

78-

76'

74-

72-

-l..L4.YN^

N-^

\ \

\ ^

\2%

• ^

w \

5%.

\8%

'x.

V •

10%

OUTLET CONSISTENCIES, % 1

^^^40%

\^%

"\20%

\15%

0.5

1.5 3.5

4.5

2 2.5 3

Inlet Consistency, %

Fig.

10-11.

Theoretical washing efficiency based on inlet and outlet consistencies.

Washing equipment may take on a variety of

forms such as rotary vacuum washers and double

belt washers discussed in brown stock washing.

Some manufacturers have specific products for this

task, such as Black Clawson's DNT belt washer

(Gilkey and McCarthy, 1988). This washer uses

a endless synthetic wire with headbox, breast roll,

and couch roll to dewater and thicken pulp up to

15%

consistency.

Ink flotation

Ink flotation borrows from technology devel-

oped by the metallurgy industry for mining, which

started being used to deink paper in the 1960s.

Perry*s Chemical Engineer's Handbook, 6th

edition is a good reference for the basic principles

of this process. Flotation has been used extensive-

ly in Japan and Europe, but only recently came

into widespread use in the U.S. Flotation is a

process that separates materials based on the

property of wetability. Under appropriate condi-

tions,

non-polar (hydrophobic) materials are able

to adhere to air bubbles and rise to the surface.

The process is carried out in ink flotation

cells (several cells may be used in series) using

sodium hydroxide, sodium silicate, and hydrogen

peroxide with a collector system consisting of a

surfactant. With ink flotation, large ink particle

sizes are desired (at least 5 /im, but 10-50 ixm is

ideal) so the ink can agglomerate and be skimmed

from the slurry. Many new inks and inks on

office papers incorporate crosslinked resins and

flake off the fibers in relatively large particles,

making flotation the preferred method. Since the

ink is already flocculated, it is fairly easily sepa-

rated from the process water.

Ink flotation can be divided into three phases:

collision of ink particles with air bubbles, attach-

ment of ink particles to air bubbles, and separation

of the ink particle-air bubble complex from the