Biermann Ch. Handbook of Pulping and Papermaking

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274 10. FffiER FROM RECYCLED PAPER

pulp.

The reject rate of any system depends on

how the ink-foam complex is removed. The fate

of the collector chemical seems to be of great

importance in some paper grades. The amount of

deinking chemical in the pulp accepts is called the

carryover. Ideally very little collector must leave

with the stock accepts. Flotation deinking is done

at lower consistency than washing and is done at

about 1% consistency.

One type of flotation cell is shown in Fig.

10-12 and Plate 39. In this system, water and air

are pumped into various points of the cell. The

surface is skimmed to remove the ink. In 1988,

Beloit introduced pressurized ink flotation (Fig.

10-13) with the claim that it removes more ink

over a wider particle size range. Details of this

method are found in Carroll and McCool (1990).

The collector system is based on fatty acids

such as stearic acid with calcium or nonpolar

surfactants. Occasionally cationic or ampholytic

compounds have been used. In the first system,

fatty acid salts

(0.5-5%

on pulp with a known

Fig. 10-12. Verticel™ deinking flotation cell. Air and water are introduced under pressure. The

inserts show the skimmer and water inlet holes (an empty cell on the left). Courtesy of Fiberprep.

RECYCLED FIBER RECOVERY 275

Mixing

Separation

Fig. 10-13. Pressure Deinking Module™. Re-

drawn from a photograph supplied courtesy of

Beloit.

degree of unsaturation, that is, number of double

bonds as determined by the iodine number) and

calcium ions are used. Calcium (like taking a

shower in hard water) causes the fatty acids to

precipitate, thereby supplying a hydrocarbon

nucleus upon which the nonpolar ink particles can

collect. Calcium is used at about 200 ppm as

CaCOj.

The nonionic collector systems use an

alkyl (C-9 is common) phenol or relatively large

primary alcohols linked to oligomers (6-16 mono-

mers) of ethylene oxide. The presence of inorgan-

ic fillers such as clay or talc assists in the ink

flotation method and thus they are usually used.

Without the use of inorganic fillers, additional,

larger bubbles may be required, lower rise dis-

tance should be used, and larger flotation cell

capacity may be needed. However, most of the

filler will not be removed by flotation.

Flotation methods are also used to recover

the water from waste water streams. This is

described below.

Hybrid

deinking

processes

New deinking mills may use combined ink

flotation and washing, especially for recovery of

bleached chemical pulps. The bulk of the ink is

removed by flotation, and residual ink is removed

by washing. In this system, the dispersants are

added after the pulper but before washing, so that

residual ink that is not removed by ink flotation is

emulsified and removed during the washing pro-

cess.

This process is the best of both worlds and

is particularly useful for paper wastes that contain

many different types of ink formulations. As ink

formulations become more difficult to disperse by

saponification and emulsification, such as radiation

cured resins, ink flotation is necessary, but remov-

al of residual ink during washing is best accom-

plished by emulsification. Mills in Europe and

Japan even use flotation-wash-flotation systems.

About 10-20% of mills using a dual system use

washing followed by ink flotation. This method

has the disadvantage that many large ink particles

that might have been removed by flotation would

be dispersed prior to flotation, but has the advan-

tage of more favorable treatment of various wash-

ing solutions in some mills.

Post-treatment processes are developing as

well for even fiirther ink removal. One method is

to use disk dispersion above 20% consistency with

about 80-140 kWh/t (4-6 hp-day/ton) to reduce

particle sizes from several hundred microns to less

than 50 microns so they are not visible as individ-

ual specks to the eye. This is followed by addi-

tional washing or flotation. Another approach is

acidification of the pulp slurry followed by addi-

tional treatment of cleaning, very fine screening,

etc.

The state of the art for deinking newsprint is

92-95%

or higher ink removal.

Solvent extraction of

ink

At the risk of confiising the issue, there is a

third method for ink removal, but it is generally

uneconomical. The method is solvent extraction

by use of nonpolar solvents (Hiraoka, 1990). The

process resembles dry cleaning of clothing and

was used, although not fundamentally for

deinking, by Riverside Paper Co. in the U.S. and

Tagonoura Sangyo in Fuji City, Japan for recov-

ery of waxed paper, milk carton grades, etc.

where both the paper and polyethylene resin are

reusable. Riverside recovered 99% of its trichlo-

roethylene solvent, and Tagonoura recovers 90%

of its hexane solvent.

Water recycling and sludge recovery

The water containing the ink particles that

have been removed from the fiber must

cleaned

for reuse in the mill. One method of doing this is

to use a flotation method similar to that used in

flotation deinking.

Krofta Engineering Corp. is one manufactur-

er of a device that does this (Fig. 10-14). The

raw liquid is pressurized with air under 60-80 psig

276 10. FIBER FROM RECYCLED PAPER

1) RAW WATER INLET

2) CLARIFIED WATER OUTLET

3) FLOATED SLUDGE OUTLET

4) CLARIFIED WATER RECYCLE OUTLET

5) PRESSURIZED WATER INLET

6) ROTARY JOINT

7) RUBBER PIPE CONNECTION

8) PRESSURIZED WATER PIPING

9) PRESSURIZED WATER DISTRIBUTION HEADER

10) RAW WATER DISTRIBUTION HEADER

11) DISTRIBUTION HEADER OUTLET PIPES

12) FLOW CONTROL CHANNELS

13) TURBULENCE REDUCTION BAFFLES

14) ADJUSTABLE HEIGHT BAFFLE ATTACHMENT

15) FLOW CONTROL CHANNEL OUTER WALL

16) ROTATING CARRIAGE GEARMOTOR DRIVE

17) CARRIAGE DRIVE WHEEL

18) WHEEL SUPPORT RIM

20) TANK WALL

21) TANK FLOOR SUPPORT STRUCTURE

22) ROTATING CLARIFIED WATER CONTAINMENT WALL

23) SLUDGE WELL

24) LEVEL CONTROL OVERFLOW WEIR

25) ROTATING CARRIAGE STRUCTURE

26) REVOLVING SPIRAL SCOOP

27) SPIRAL SCOOP GEARMOTOR DRIVE

28) CLARIFIED WATER EXTRACTION PIPES

29) ELECTRICAL SLIP RING

30) TANK WINDOW

31) SEDIMENT REMOVAL SUMP

32) FINAL DRAIN

33) SEDIMENT PURGE OUTLET

34) LEVEL CONTROL ADJUSTMENT

Fig. 10-14. Dissolved air flotation deinking effluent clariflcation. Courtesy of Krofta Eng. Corp.

RECYCLED FIBER RECOVERY 277

Fig. 10-15. A sludge press in the foreground.

The mill's primary and secondary effluent

treatment is in the background.

pressure. When the raw liquid is introduced into

the tank, it goes to ambient pressure so that small

air bubbles are formed that attach to the suspended

solids to be removed. The lightweight sludge then

floats to the surface. Since the air was dissolved

in the water, the process is called dissolved air

flotation. The retention time is typically 3-4

minutes.

Polymer systems are used to help the separa-

tion. The sludge is skimmed from the top by the

rotating skimmer and collected in the middle for

removal. The treated water escapes underneath.

The sludge collector rotates around the cleaner in

this continuous process. The sludge that is ob-

tained from the water to be recycled must be

pressed to remove much of the water so that the

sludge can be sent to a landfill or disposed of

otherwise. Fig. 10-15 shows a sludge press.

Some problems with recycling paper are

mentioned by Hoekstra (1991). For example,

using 210 t/d old magazine (OMG) paper to

produce 150 t/d secondary fiber generates 3001 of

waste at 21% solids content. With high tipping

fees at the landfill, this is quite a problem. Burn-

ing of the sludge gives 6.5 t ash if 22% of the

solids are ash. Analysis of sludge from five

deinking plants gave the following level of heavy

metals (parts per million): cadmium, <0.2; chro-

mium, 16-118; copper, 31-400; lead, 3-210;

manganese, 31-880; nickel, 1-25; and zinc, 36-

1200.

These were generally below those of

municipal sludge. Also, the printing industry is

accepting new ink formulations without heavy

metals. BOD load is about 15-30 kg/t (30-60

lb/ton) pulp and TSS is about 10-50 kg/t (20-100

lb/ton) for deinked grades with washing methods

producing more than flotation methods. Effluent

volume is about 5 mVt.

Evaluation of

the deinking

process

The process of deinking may be evaluated by

brightness or ink speck counts of the pulp or

handsheets from the pulp. Solvent extraction of

deinked pulp gives a quantitative method of ink

content provided interfering compounds are not

present; however, appropriate selection of spectro-

scopic methods of analysis (IR, UV, VIS, NMR,

etc.) of the extract could easily give accurate ink

determinations.

Brightness alone cannot be used to quantify

ink since brightness depends on a variety of

factors including the distribution of the ink on the

fibers. Small particle sizes decrease brightness

much more than large particle sizes of the same

amount of ink. This is due to the increased

surface area to volume ratio of smaller objects.

This also accounts for the decrease in brightness

often observed to occur with mechanical disinte-

gration. Still, the overall pulp brightness gives a

good indication of deinking effectiveness, but does

little to indicate the source of trouble should the

brightness go down if the system is not operating

well. Furthermore, one should not allow any sort

of washing of the pulp during formation of the

brightness pad. Finally, brightness does not

indicate the presence of ink specks visible to the

unaided eye, i.e., larger than 50-70 ftm (about

0.003 in.) in diameter, which may alone be unac-

ceptable.

Image analysis techniques offer a good

method to observe the deinking process. Modem

278

10.

FIBER FROM RECYCLED PAPER

image analysis systems controlled by computers

offer a reproducible method for characterizing

deinked furnish without undue operator time and

bias which can be quite significant in an otherwise

quite time-consuming and subjective procedure.

Not only can ink speck counts be made, but ink

speck size distributions can be determined so that

information on the degree of emulsion or floccula-

tion in the deinking process can be gathered over

a period of

time

at a given

mill.

Fig. 10-16 shows

an automated system used in a mill for quality

control.

Image analysis systems are available that

require very little theoretical knowledge on their

operation. Earlier models are not as automatic

and require attention to the light source which

must not vary in intensity and spectral composi-

tion. Image analysis has also been used in several

studies (Hacker, 1991) to measure lightweight

contaminants. Here it is necessary to dye the

paper with a water-based black ink (such as Parker

Super Quink Ink) to enhance contrast by leaving

the lightweight (nonpolar) contaminants amber in

color while the paper turns black.

Slurry concentration

After deinking and cleaning, the dilute pulp

slurry must be concentrated for further processing

and storage. Fig. 10-17 shows a large disk filter

used to concentrate a pulp slurry from 0.6 to 10%

consistency at a mill that recovers 500 tons/day of

secondary fiber.

Bleaching of secondary fiber

The philosophy of bleaching deinked pulp is

very similar to that of bleaching virgin pulps

(Chapter 5). Mechanical pulps (or so called

wood-containing pulp) are bleached with peroxide

(about 1% on pulp, with 4% sodium silicate,

50°C) or dithionite (hydrosulfite, about 1% on

wood, 50-60°C at pH 5-6 to mitigate air oxidation

of dithionite). Wood free pulps are usually

bleached with a single stage hypochlorite treatment

(1%

on wood as chlorine), although an initial

chlorination before hypochlorite has also been

used. Sometimes these bleaching agents are added

to the pulper to help with ink removal, but more

efficient chemical usage is realized if the pulp is

bleached after cleaning and screening.

"f cocAP^MEcgy^iJmM

Fig. 10-16. An automated counter at a deinking mill for determining ink spot size distributions.

RECYCLED FffiER RECOVERY 279

Fig. 10-17. A large disk filter for concentrating the pulp slurry at a deinking mill (500 t/d).

inserts show the filter disks when cleaned, with fiber, and with fiber removal.

The

Example of a process

In order to summarize this chapter, a recy-

cling plant is presented in Fig. 10-18. This is a

news to news plant using 70% news and 30%

magazine for the feedstock. Numerous plant

layouts are possible; this represents one choice.

Notesy 1991 Chemical

Processing

Aids Short

Course, Tappi Press, pp 85-105. This is an

excellent, highly recommended reference on

deinking chemistry with

literature citations

that includes information on inks, printing,

deinking chemistry, and bleaching.

10.4 ANNOTATED BIBLIOGRAPHY

Crow, D.R. and R.F. Secor, The ten steps of

deinking, Tappi J,

70(7):

101-106(1987).

General deinking

Woodward, T.W.

Deinking chemistry,

Shrinath, A., J.T. Szewczak, and L.J. Bow-

en, A review of ink-removal techniques in

lU-

ANNOTATED BffiLIOGRAPHY 281

current deinking technology, Tappi J,

74(7):85-93(1991). The information in this

reference is better presented in the articles it

cites or those cited in this chapter. The

article's strong point is the coverage of

printing processes and ink removal strategies

for specific types of ink.

Contaminants

Horacek, R., Deinking, Beloit (Jones Div.),

Dalton, Mass., 1978. This reference is con-

sidered out of date by Beloit and is no longer

available from them.

Centrifugal

cleaners

LeBlanc, P.E. and M.A. McCool, Recycling

and Separation Technology, 1988 TAPPI

Pulping

Conf.

Proceedings, pp 661-667.

6. Bliss, T., Through-flow cleaners offer good

efficiency with low pressure drop, Pulp &.

Pa;?^r59(3):61-65(1985).

Ink washing

and flotation

Gilkey, M.W. and C.E. McCarthy, A new

device for high efficiency washing of deink

furnishes.

Proceedings,

1988 Pulping

Confer-

ence, TAPPI Press, pp 649-654.

8. Carroll, W.P. and M.A. McCool, Pressur-

ized deinking module. Proceedings, 1990

TAPPI Pulping

Conference,

pp 145-152.

Surfactants

9. Mah, T., Deinking of waste newspaper,

Tappi J, 66(10):81-83(1983), no references.

Mill experience is given regarding the

deinking of waste newspaper. Paper of 48-

52%

brightness was obtained by washing.

The two problems were foaming and

deinking waste news; both problems are

related to the surfactant. The results of

several alkyl and alkyl phenol oligo-

ethyleneoxides are presented.

10.

Okada, E., Deinking agent and its research

development, Japan Pulp Paper 27(4):45-

49(1990). Deinking surfactants for flotation

deinking were studied. Also the fatty acid

method was compared to nonionic

surfactants, with the fatty acid method gener-

ally working better.

11.

Balos, B, J. Patterson, K. Hornfeck, and M.

Liphard, Flotation deinking chemistry,

PaperAge 106(Recycl.

Annual):

15-17(1991).

This study compares classes of surfactants in

flotation deinking based on a laboratory

setup.

Here again the fatty acid soaps

compared well to compounds of 6 other

classes. Water hardness of at least 180 ppm

CaCOa was recommended for complete

flotation.

Solvent extraction

ink

12.

Hiraoka, M. Latest waste paper treatment

process, Japan Pulp Paper

27(4):

33-

42(1990).

Sludge,

environmental considerations

13.

Hoekstra, P.L., Paper recycling creates its

own set of environmental problems, Amer.

Papermak.

54(4):30,33(1991). The problems

of sludge disposal are considered.

Metals in ink

14.

Donvito, T.N., T.S. Turan, and J.R. Wilson,

Heavy metal analysis of inks: a survey,

Tappi

75(4):

163-170(1992).

Heavy metals

in inks are an environmental concern, espe-

cially in deinking sludge. This paper inves-

tigates methods of determining heavy metals

in inks. Many states will require the sum of

Pb,

Cd, Hg, and Cr to be below 100 ppm by

1994.

Decolorizing methods

15.

Cheek, M.C., A practical review of paper

decolorizing methods - present and future, in

Proceedings,

1991 Papermaker's

Conference,

TAPPI Press, pp 71-78. This is a useful,

general reference on paper decolorizing

chemicals and processing conditions

(i.

e., for

decoloring broke for reuse within the mill).

Ink

removal

test

methods

16.

McKinney, R.W.J., Evaluation of deinking

performance, a review of test methods, Tappi

282 10. FIBER FROM RECYCLED PAPER

71(1):

129431(1988). A brief evaluation

is made of the following test

methods:

bright-

ness measurement, ink removal-ink speck

analysis, yield, and solvent extraction.

Image analysis

17.

M.P. Hacker, 1991 Pulping

Conference

Pro-

ceedings, pp 455-468. This study measured

lightweight contaminants in paper with a

water-based black ink (such as Parker Super

Quink Ink) to enhance contrast by leaving the

lightweight (nonpolar) contaminants amber in

color while the paper turns black.

18.

McCool, M.A. and L. Silveri, Removal of

specks and nondispersed ink from a deinking

furnish,

TappiJ,

70(11):75-79(1987).

19.

Zabala, J.M. and M.A. McCool, Deinking at

Papelera Peninsular and the philosophy of

deinking system design, Tappi J.

71(8):

62-

68(1988)

20.

Klungness, J.H, L.E. Fernandez, and P.L.

Plantings, Image analysis for measuring

adhesive contaminants in pulp, Tappi J.

71(1):89-93(1989).

21.

Lowe, G., B.H. Licht, and G. Leighton,

Deinking plant optimization using image

analysis,

TappiJ,

74(1):

125-129(1991).

22.

Dallard, D.H and CM. Brown, Computer

Vision, Prentice-Hall, Englewood Cliffs,

New Jersey, 1982, 523 p. This is an often

cited general text on image analysis.

23.

Gonzalez, R.C. and P. Wintz, Digital Image

Processing, 2nd ed., Addison-Wesley,

Reading, Mass., 1987, 503 p. This is an

often cited general text on image analysis.

Flotation clarification of effluents

24.

Mahony, L.H., The role of flotation clarifi-

cation in de-inking and secondary fiber oper-

ations, PaperAge 107(Recycling Update):46-

47(1991). This paper describes the Krofta

Engineering Corp. dissolved air flotation

system.

Screening stickles

25.

Heise, O., Screening foreign materials and

stickles, Tappi J. 75(2):78-81(1992). This

article is a review of the current technology.

Charts show the screening efficiency versus

reject rate, slot velocity, rotor velocity, and

screen plate surface.

EXERCISES

What types of contaminants are encountered

in recycled paper?

What are some of the tools used to remove

contaminants from paper?

What is the relationship between contaminant

removal and fiber reject rate?

What are the two mechanisms of deinking in

widespread use?

Deinked newsprint generally would not be

bleached with sodium hypochlorite. Why

not?

6. How do through-flow cleaners operate? What

type of contaminants do they remove?

ENVIRONMENTAL IMPACT

11.1 INTRODUCTION

Pollution is a subjective term; therefore,

pollution "is in the eye of the beholder". First

what does one mean by a pollutant. Generally,

one might say a pollutant is a material which has

an adverse effect on the environment. What is an

adverse effect? For example, much color in the

effluent of a pulp and paper mill may not cause

"harm" to the environment in terms of toxicity per

se,

but it is considered aesthetically unpleasing.

Does this make it a pollutant? It doesn't matter;

it is still undesirable.

Toxicity is also relative. At one time, a toxic

material was considered one that in small amounts,

perhaps less than a gram, was enough to kill half

of the people who might be exposed to it. The

LD50 is the required dose, often expressed as

milligrams toxin per kilogram body weight, that is

lethal to 50% of the population exposed to such a

dose.

Usually the population is made up of rats

when the LD50 is determined in the laboratory.

For example, sodiimi cyanide has an LD50 of 15

mg/kg (orally in rats), which means a 100 kg (220

lb) person would need about 1.5 g to cause death,

presuming that a human reacts proportionally to a

rat. Sodium chloride, or table salt, has an

LD50

3.75 g/kg (orally in rats), which means our 100 kg

person would need to ingest 375 g of salt (over

three-fourths of a pound) to kill him. Table 11-1

shows some compounds with their LD50 doses.

Some people (read mercenaries who are hired

to have a particular view) say "the dose makes the

poison" and would argue sodium chloride (a nutri-

ent) and sodium cyanide are both toxic! While

literally true, this is not grounds for discharging

toxins into the environment.

More recently, government organizations are

considering exposure to a material over a lifetime

that causes more than a single cancer in one

million people as an unacceptable level of toxicity.

For example, let us say an individual consumes six

ounces of fish caught from a lake every day for 20

years.

If it is estimated that doing so would cause

Table 11-1. LD50 doses in mice or rats of some

materials^

Chemical

1 Na cyanide, oral

1 NaCl, oral

1 TCDD, dioxin, oral

arsenic acid, i.v.

1 2,4,5 T herbicide

phenol, human

nicotine, i.v.

caffeine, oral

LD50

mg/kg

3750

0.02

390

0.3

130

LD50

g/100 kg II

1.5 1

375 1

0.6 1

39 1

1-25 1

0.03 1

13 1

^Data from the Merck Index, 10* Edition.

two or more people per million people to get

cancer from this water (or more precisely, the

contaminants in that water), it is an unacceptable

risk. It is absurd that decisions are made on the

basis of such calculations. Dioxin discharges in

water effluents at the parts per quadrillion or even

parts per quintillion basis have become a major

concern to government agencies, and therefore the

industry, in the late 1980s. There are numerous

difficulties with such regulations.

First, long term toxicity is often determined

in animal studies at levels many orders of magni-

tude higher than actual exposure would be. If one

is looking at cancer rates of one in a million, over

100 million rats would have to be used (unless

toxicity is extrapolated

firom

higher doses). Clear-

ly this is not possible. More likely the researcher

is using 100-1000 rats and an exposure over 1000

times the expected ambient level.

Second, do the results on rats at high levels

correspond to anything meaningful in humans at

low levels. The fact is that these estimates must

be considered accurate within only a few orders of

magnitude. Anyone who tells you they can predict

283