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Recycling of paper products
Conservatron & Recycling, Printed m Great Britain.
0361- 3658/86 $3.001 .W Pergamon Journals Ltd.
Vol. 9. No. 4, pp. 351- 357. 1986.
REVIEW ARTICLE RECYCLING
OF PAPER
NO. 11 PRODUCTS
F. W. LOREY and W. C. MARTIN Garden State Paper Co., subsidiary of Media General, Park 80. Plaza East, Saddle Brook, NJ 07662, U.S.A.
INTRODUCTION Wastepaper can be classified into the following major categories: corrugated boxes, newspapers, mixed papers, pulp substitutes and high-grade stock suitable for deinking. Of these, newspapers are the easiest to segregate, mixed papers are the lowest grade, and highgrade stock suitable for deinking and pulp substitutes are the highest grades of waste. The latter includes such materials as clippings and office waste (e.g. computer printouts and files). Table 1 shows the quantities of each of these grades of wastepaper used in the manufacture of paper and paperboard in the U.S.A. in 1981. Paper properties are altered by recycling, with some characteristics being improved and others deteriorating. Contaminants such as adhesives, hot melts, and plastic film are potentially troublesome to all recycling operations, and removal methods, although partially successful, need to be developed further. I. GRADES OF WASTEPAPER 1.1. Corrugated boxes Used corrugated containers are mainly accumulated by commercial establishments such as supermarkets and department stores. After being repulped, they are recycled into several different grades of paper. Corrugated boxes made from virgin fiber are composed of linerboard containing unbleached kraft softwood pulps (long fiber, 3 - 5 mm) and corrugated medium (fluted element) containing mostly high-yield semichemical hardwood pulps (short fiber, 0.5 - 1 mm). The linerboard must be reasonably puncture resistant whereas the fluted medium, which maintains the structure of the box, must be resistant to crushing. During recycling, the hardwood and softwood fibers are mixed together and the resultant mixture does not have the individual characteristics of either original component. Recycled products include bogus linerboard and fluting medium made completely from used corrugated containers. Although the quality in terms of bursting strength of the linerboard and crush strength of the medium are generally not as good as the virgin products, the economic advantages nevertheless create a market for the recycled products. Recycled containers are also used in the filler plies of solid-folding boxboard to provide good bending and strength
Article published in Encyclopedia of Materials Science and Engineering (Pergamon Press, 1986). 351
352
f- M’. i_OKf:\rand \I;. C. MARi‘ih Table
1. U.S. consumption
End product
of wastepaper
Total wastepaper ionsumption (kt)
111paper
manufacitw
)I) 198i
Grades of paper stock (ktj
Mixed papers
Paper Newsprint Printing and writing Packaging and industrial converting Tissue Total
930 877 289 1 793 3 889
7 32 65 104
Paperboard Kraft, bleached and unbleached Semichemical Recycled Total
1 124 1 323 7 539 9 986
Construction paper and board Total (all grades)
and paperboard
Newspapers
Corrugated boxes
894
PIlIp substitutes
High-grade deinking
7 154 1 056
2 77 162 242
36 560 166 651 I 404
13 1 389 1 462
9 1 248 1 258
1 008 1217 4 098 6 324
114 23 690 827
113 115
I 342
667
238
344
66
27
15 217
2 233
2 552
6 910
2 307
1 216
307
759
1 074 2
Source: Paper, Paperboard, Wood PU(DCapacity, 1982 0 American Paper Institute. New York. Reproduced with permission.
properties, in absorbent grades of paper such as industrial towelling, and for industrial wrapping paper. The closure for a corrugated box can be staples, asphalt tape, or hot-melt adhesive, resulting in contaminants in the defibered wastepaper. Staples, and to a lesser extent hot-melt materials, are removed by cyclone cleaning or screening. Traditionally, mills processing waste corrugated containers have used asphalt dispersion systems in which pulped stock is thickened to 35% consistency, steamed in the pressure vessel at 150°C to melt any asphalt present, and then refined under pressure to disperse the asphalt before the pulp stock is discharged. 1.2 Newspapers Newspapers for recycling are obtained by the collection of post-consumer material or from overissue, the cleanest source. When not deinked, they are used mostly for the production of inner plies or backing of solid paperboard such as folding boxboard, and also construction board (e.g. gypsum and insulation board). When deinked, newspapers are used for recycled newsprint and to a lesser extent for tissue and towelling paper. One system (Homasote) which is used for producing insulation board, walboard or deck board does not incorporate a traditional fourdrinier or cylinder machine, but instead the sheets are formed with 8 x 12 ft (2.44 x 3.66 m) vacuum molds into panels 0.5 in. (1.27 cm) thick. The waste newspapers are repulped at 5% consistency (5 parts solids per 100 parts slurry), after which coarse contaminants are removed, residual paper flakes are defibered with disk refiners, and various chemicals are added for water resistance, strength and fungus control. The panels are formed at 2.5% consistency and 57°C. The molded panels at 25% solids are pressed to 40% solids with platen-type presses, and dried in tiered hot-air dryers. Finishing consists of either painting, or laminating with decorative plastic paper, or other sheet material. Deinking of newspaper is accomplished either with froth flotation cells, as used, for example, in mine ore separation, or by washing, using repetitive dilution and thickening techniques. Flotation is more common in Europe whereas washing systems are more common in the U.S.A. A typical flotation deinking system is illustrated in Fig. 1. Since 1961, mills have been established in the U.S.A. for the manufacture of newsprint from deinked newspapers, and in 1984 the production totalled 585 kt or about 15% of domestic
RECYCLlNG
OF PAPER
353
PRODUCTS
newsprint production. In the recycling process, ink is removed by washing with a proprietary chemical. Since dilution ratios for washing are necessarily high, clarification systems for water reuse and heat retention are an integral part of the process. These mills have paper machines very similar to modern virgin newsprint mills, but of course the fiber preparation system is vastly different. Newsprint is relatively lightweight, and good printing-press runability is a prime requirement. Therefore, to minimize contaminants only well-sorted collected newspapers or overissue are used as raw materials. The principal contaminant problem, adhesives, comes mostly from pressure-sensitive splicing of paper rolls in the pressroom. 1.3. Mixed papers Mixed papers contain grades of paper of vastly different characteristics and are not readily subject to sorting; they are therefore the lowest grade and are the most economical source of waste paper. They have little use in papermaking, but wide use in multi-ply boxboard and construction board. Mixed papers are usually heavily contaminated, and a series of screening and cyclone-cleaning steps are necessary for the separation of plastic, sisal, fabric and metal objects. Adhesives are also a problem, and are mostly in the form of hot-melt materials. With wide interest in the use of waste products as fuel for generating electricity, processes have been developed for separating off paper from wastes with lower calorific values. Most of these separation systems utilize trommel rotary screens followed by air separation which maintains the material in a dry form. One system has been developed where trash is slushed in a conventional pulper developed for waste paper, the fiber separated by screening and cleaning, and then pressed to somewhat less than 50% solids. The calorific value is less for dry material, which has 90% solids. Before either of these systems for obtaining fuel are used, valuable corrugated containers and newspapers are usually separted at the source for paper recycling. 1.4. Pulp substitutes and high-grade stock for deinking Pulp substitutes represent premium material usually in the form of shavings, clippings and sheet waste from converting operations, such as the production of envelopes and paper cups. These residuals are used directly, without being deinked, in paper mills which make the original grade. Conventionally, a mill reuses its own waste, known as broke, and the pulp substitute that it buys from converting operations is similar to this. The pulp substitute ranges from envelope cuttings to various publication blanks, and also includes kraft-bag waste. These recycled
To paper
math
‘1~
Fresh wter 3% Caustic
I 5% sodturn I 0 % H,O
soda silicate
Water
consistency
Froth Flotation
Fig. 1. Flotation
deinking
system
Froth ccl Is
I W. LOREY and W. <. MARTIii
354
Fiber
storage
Clean
storage
Fig. 2. Commercial system for processing polyethylene-coated
waste.
materials are also widely used in the manufacture of tissue and multi-ply boxboard (especially for the liner plies). Such grades of waste have a high market value and a very high recycling rate. High-grade stock suitable for deinking includes printed materials such as business papers, tab cards and computer printouts. The fiber components of these materials are bleached chemical pulps having good strength and aging properties. Deinking of high grades is usually achieved by washing rather than flotation: the basic chemical is caustic soda which is added in the pulping stage. Deflaking, screening and cleaning are similar to other recycling operations. High-grade deinked pulps are used for printing and writing grades, tissue (where the softness and whiteness of chemical pulps are important), and, to a lesser extent, for liner plies of boxboard. 1S. Other waste materials Another source of high-grade recycled chemical pulp (primarily sold to manufacturers of sanitary tissue) is polyethylene-coated food board. This material is processed in pulpers with very fine (0.3175 cm) extraction plates, and with careful control of temperature (82°C) to maintain the plastic film at its original size without either melting (at high temperatures) or fracturing (at low temperatures). The recovered material is screened, cleaned and then dewatered by pressing. A commercial system for cleaning polycoated waste is shown in Fig. 2. An alternative approach to the recovery of plastic-coated wastepaper is a solvent extraction process for the plastic using trichloroethylene or other solvents. 2. EFFECT OF RECYCLING
ON PAPER PROPERTIES
The surfaces of fibers are composed of windings of fibrils which upon refining are raised from the surface, thereby providing greater surface area for hydrogen bonding. These fibrils collapse somewhat on drying, and upon rewetting during recycling, the refibrillation is less extensive. Thus, strength properties which rely on bonding such as burst, tensile and fold resistance are decreased with recycling. With less bonding, the density of the paper tends to be lower, resulting in higher caliper, higher stiffness, and greater resistance to tear. Also, recycled paper has greater permeability to air and is more opaque. The effects of the number of repulping steps on paper properties are shown in Figs 3 - 5, indicating the effect on swelling (i.e. change in dimension of wood fibers due to the absorption of water), tension characteristics, and strength, respectively. Among these properties, tear strength, which relates to improved printing-press runability, is improved by repulping. The changes in characteristics are greater with the first recycling, diminishing with each subsequent recycling.
3.55
RECYCLING OF PAPER PRODUCTS
3. CONTROL OF CONTAMINANTS Contaminants are a common problem in all recycling processes, regardless of whether the end product is tissue, paper or paperboard. Heavy and large contaminants are easily removed, but those which are small in size and similar in density to wood fibers are not easy to remove
Number of repulpings
Fig. 3. Effect of repulping on swelling of paper.
I1
5ccQ0
I 1
i 2
I 3
I
I
4
5
J
6
Number of repulplngs
Fig. 4. Effect of repulping on tension characteristics of paper: 0, tensile breaking length; 0, stretch.
356
o bursting strength ??teorlng strength
Number of repulplngs
Fig. 5. Effect of repulping on paper strength.
with existing equipment. The best control is in regulating the quality and cleanliness of the waste paper used. The most troublesome contaminants are those which are tacky; they can either cause breaks in paper machine operation or breaks when rolls of paper are continuously unwound (e.g. on a printing press). These sticky contaminants are derived from adhesive materials, typically either pressure-sensitive adhesive used in splicing paper rolls, or hot-melt adhesives used as sealers with paperboard. Some variations in processing equipment design, mainly slotted screens and reverse centrifugal cleaners, have been designed for improved removal of sticky contaminants. Although this new equipment does not completely solve the problem, these devices are being increasingly used for contaminant removal. Screen plates can be made either with round or slotted holes. The smallest practical round hole has a diameter of about 0.125 cm, whereas a slotted screen may have slot widths as small as 0.025 cm. A globular-shaped sticky particle of just under 0.125 cm in diameter would be passed by a round hole but would be rejected by the narrower slot. Reversed centrifugal cleaners are conically shaped, similar to conventional centrifugal cleaners, but are designed to bring lightweight contaminants into the central core where they are then rejected from the large end of the cone. The clean fibers are removed from the small end of the cone, which is the reverse of a conventional cleaner. Usually, the pulp is first processed with conventional cleaners for removal of heavy contaminants such as sand and grit, before being run through reverse cleaners. Other light fraction cleaners have been developed in recent years of cylindrical design which eliminate water imbalance typical of conical cleaners.
REFERENCES 1. A. M. Altieri and J. W. Wendell Jr, Deinking of Waste Paper, TAPPI Monograph Series No. 31. Technical Association of the Pulp and Paper Industry, Atlanta, Georgia (1967). 2. A. M. Altieri and J. W. Wendell Jr, Deinking, MacDonald R. G., Franklin J. N. (eds.) Pu/p and Paper Manufacture, 2nd edn, Vol. 2, pp. 94- 131. McGraw-Hill, New York (1969).
RECYCLING OF PAPER PRODUCTS
357
3. D. E. Brooks, Economical reclamation of old corrugated containers. Tappi 61, 11 - 13 (1978). 4. D. J. Elliott, Selecting a recycled fiber system: a case history. Tappi 64, 35 - 37 (1981). 5. F. W. Lorey, As I see it: secondary fiber recycling. Tuppi 61, 7 (1978). 6. R. C. McKee, Effect of repulping on sheet properties and fiber characteristics. Pup. Trade J. 155, 34- 40 (1971). 7. M. MacLeod, Renovation of a recycling mill, Bergstrom ups Neenah capacity. Pap. Trade J. 157, 26 - 28 (1973).
0361- 3658/86 $3.001 .W Pergamon Journals Ltd.
Vol. 9. No. 4, pp. 351- 357. 1986.
REVIEW ARTICLE RECYCLING
OF PAPER
NO. 11 PRODUCTS
F. W. LOREY and W. C. MARTIN Garden State Paper Co., subsidiary of Media General, Park 80. Plaza East, Saddle Brook, NJ 07662, U.S.A.
INTRODUCTION Wastepaper can be classified into the following major categories: corrugated boxes, newspapers, mixed papers, pulp substitutes and high-grade stock suitable for deinking. Of these, newspapers are the easiest to segregate, mixed papers are the lowest grade, and highgrade stock suitable for deinking and pulp substitutes are the highest grades of waste. The latter includes such materials as clippings and office waste (e.g. computer printouts and files). Table 1 shows the quantities of each of these grades of wastepaper used in the manufacture of paper and paperboard in the U.S.A. in 1981. Paper properties are altered by recycling, with some characteristics being improved and others deteriorating. Contaminants such as adhesives, hot melts, and plastic film are potentially troublesome to all recycling operations, and removal methods, although partially successful, need to be developed further. I. GRADES OF WASTEPAPER 1.1. Corrugated boxes Used corrugated containers are mainly accumulated by commercial establishments such as supermarkets and department stores. After being repulped, they are recycled into several different grades of paper. Corrugated boxes made from virgin fiber are composed of linerboard containing unbleached kraft softwood pulps (long fiber, 3 - 5 mm) and corrugated medium (fluted element) containing mostly high-yield semichemical hardwood pulps (short fiber, 0.5 - 1 mm). The linerboard must be reasonably puncture resistant whereas the fluted medium, which maintains the structure of the box, must be resistant to crushing. During recycling, the hardwood and softwood fibers are mixed together and the resultant mixture does not have the individual characteristics of either original component. Recycled products include bogus linerboard and fluting medium made completely from used corrugated containers. Although the quality in terms of bursting strength of the linerboard and crush strength of the medium are generally not as good as the virgin products, the economic advantages nevertheless create a market for the recycled products. Recycled containers are also used in the filler plies of solid-folding boxboard to provide good bending and strength
Article published in Encyclopedia of Materials Science and Engineering (Pergamon Press, 1986). 351
352
f- M’. i_OKf:\rand \I;. C. MARi‘ih Table
1. U.S. consumption
End product
of wastepaper
Total wastepaper ionsumption (kt)
111paper
manufacitw
)I) 198i
Grades of paper stock (ktj
Mixed papers
Paper Newsprint Printing and writing Packaging and industrial converting Tissue Total
930 877 289 1 793 3 889
7 32 65 104
Paperboard Kraft, bleached and unbleached Semichemical Recycled Total
1 124 1 323 7 539 9 986
Construction paper and board Total (all grades)
and paperboard
Newspapers
Corrugated boxes
894
PIlIp substitutes
High-grade deinking
7 154 1 056
2 77 162 242
36 560 166 651 I 404
13 1 389 1 462
9 1 248 1 258
1 008 1217 4 098 6 324
114 23 690 827
113 115
I 342
667
238
344
66
27
15 217
2 233
2 552
6 910
2 307
1 216
307
759
1 074 2
Source: Paper, Paperboard, Wood PU(DCapacity, 1982 0 American Paper Institute. New York. Reproduced with permission.
properties, in absorbent grades of paper such as industrial towelling, and for industrial wrapping paper. The closure for a corrugated box can be staples, asphalt tape, or hot-melt adhesive, resulting in contaminants in the defibered wastepaper. Staples, and to a lesser extent hot-melt materials, are removed by cyclone cleaning or screening. Traditionally, mills processing waste corrugated containers have used asphalt dispersion systems in which pulped stock is thickened to 35% consistency, steamed in the pressure vessel at 150°C to melt any asphalt present, and then refined under pressure to disperse the asphalt before the pulp stock is discharged. 1.2 Newspapers Newspapers for recycling are obtained by the collection of post-consumer material or from overissue, the cleanest source. When not deinked, they are used mostly for the production of inner plies or backing of solid paperboard such as folding boxboard, and also construction board (e.g. gypsum and insulation board). When deinked, newspapers are used for recycled newsprint and to a lesser extent for tissue and towelling paper. One system (Homasote) which is used for producing insulation board, walboard or deck board does not incorporate a traditional fourdrinier or cylinder machine, but instead the sheets are formed with 8 x 12 ft (2.44 x 3.66 m) vacuum molds into panels 0.5 in. (1.27 cm) thick. The waste newspapers are repulped at 5% consistency (5 parts solids per 100 parts slurry), after which coarse contaminants are removed, residual paper flakes are defibered with disk refiners, and various chemicals are added for water resistance, strength and fungus control. The panels are formed at 2.5% consistency and 57°C. The molded panels at 25% solids are pressed to 40% solids with platen-type presses, and dried in tiered hot-air dryers. Finishing consists of either painting, or laminating with decorative plastic paper, or other sheet material. Deinking of newspaper is accomplished either with froth flotation cells, as used, for example, in mine ore separation, or by washing, using repetitive dilution and thickening techniques. Flotation is more common in Europe whereas washing systems are more common in the U.S.A. A typical flotation deinking system is illustrated in Fig. 1. Since 1961, mills have been established in the U.S.A. for the manufacture of newsprint from deinked newspapers, and in 1984 the production totalled 585 kt or about 15% of domestic
RECYCLlNG
OF PAPER
353
PRODUCTS
newsprint production. In the recycling process, ink is removed by washing with a proprietary chemical. Since dilution ratios for washing are necessarily high, clarification systems for water reuse and heat retention are an integral part of the process. These mills have paper machines very similar to modern virgin newsprint mills, but of course the fiber preparation system is vastly different. Newsprint is relatively lightweight, and good printing-press runability is a prime requirement. Therefore, to minimize contaminants only well-sorted collected newspapers or overissue are used as raw materials. The principal contaminant problem, adhesives, comes mostly from pressure-sensitive splicing of paper rolls in the pressroom. 1.3. Mixed papers Mixed papers contain grades of paper of vastly different characteristics and are not readily subject to sorting; they are therefore the lowest grade and are the most economical source of waste paper. They have little use in papermaking, but wide use in multi-ply boxboard and construction board. Mixed papers are usually heavily contaminated, and a series of screening and cyclone-cleaning steps are necessary for the separation of plastic, sisal, fabric and metal objects. Adhesives are also a problem, and are mostly in the form of hot-melt materials. With wide interest in the use of waste products as fuel for generating electricity, processes have been developed for separating off paper from wastes with lower calorific values. Most of these separation systems utilize trommel rotary screens followed by air separation which maintains the material in a dry form. One system has been developed where trash is slushed in a conventional pulper developed for waste paper, the fiber separated by screening and cleaning, and then pressed to somewhat less than 50% solids. The calorific value is less for dry material, which has 90% solids. Before either of these systems for obtaining fuel are used, valuable corrugated containers and newspapers are usually separted at the source for paper recycling. 1.4. Pulp substitutes and high-grade stock for deinking Pulp substitutes represent premium material usually in the form of shavings, clippings and sheet waste from converting operations, such as the production of envelopes and paper cups. These residuals are used directly, without being deinked, in paper mills which make the original grade. Conventionally, a mill reuses its own waste, known as broke, and the pulp substitute that it buys from converting operations is similar to this. The pulp substitute ranges from envelope cuttings to various publication blanks, and also includes kraft-bag waste. These recycled
To paper
math
‘1~
Fresh wter 3% Caustic
I 5% sodturn I 0 % H,O
soda silicate
Water
consistency
Froth Flotation
Fig. 1. Flotation
deinking
system
Froth ccl Is
I W. LOREY and W. <. MARTIii
354
Fiber
storage
Clean
storage
Fig. 2. Commercial system for processing polyethylene-coated
waste.
materials are also widely used in the manufacture of tissue and multi-ply boxboard (especially for the liner plies). Such grades of waste have a high market value and a very high recycling rate. High-grade stock suitable for deinking includes printed materials such as business papers, tab cards and computer printouts. The fiber components of these materials are bleached chemical pulps having good strength and aging properties. Deinking of high grades is usually achieved by washing rather than flotation: the basic chemical is caustic soda which is added in the pulping stage. Deflaking, screening and cleaning are similar to other recycling operations. High-grade deinked pulps are used for printing and writing grades, tissue (where the softness and whiteness of chemical pulps are important), and, to a lesser extent, for liner plies of boxboard. 1S. Other waste materials Another source of high-grade recycled chemical pulp (primarily sold to manufacturers of sanitary tissue) is polyethylene-coated food board. This material is processed in pulpers with very fine (0.3175 cm) extraction plates, and with careful control of temperature (82°C) to maintain the plastic film at its original size without either melting (at high temperatures) or fracturing (at low temperatures). The recovered material is screened, cleaned and then dewatered by pressing. A commercial system for cleaning polycoated waste is shown in Fig. 2. An alternative approach to the recovery of plastic-coated wastepaper is a solvent extraction process for the plastic using trichloroethylene or other solvents. 2. EFFECT OF RECYCLING
ON PAPER PROPERTIES
The surfaces of fibers are composed of windings of fibrils which upon refining are raised from the surface, thereby providing greater surface area for hydrogen bonding. These fibrils collapse somewhat on drying, and upon rewetting during recycling, the refibrillation is less extensive. Thus, strength properties which rely on bonding such as burst, tensile and fold resistance are decreased with recycling. With less bonding, the density of the paper tends to be lower, resulting in higher caliper, higher stiffness, and greater resistance to tear. Also, recycled paper has greater permeability to air and is more opaque. The effects of the number of repulping steps on paper properties are shown in Figs 3 - 5, indicating the effect on swelling (i.e. change in dimension of wood fibers due to the absorption of water), tension characteristics, and strength, respectively. Among these properties, tear strength, which relates to improved printing-press runability, is improved by repulping. The changes in characteristics are greater with the first recycling, diminishing with each subsequent recycling.
3.55
RECYCLING OF PAPER PRODUCTS
3. CONTROL OF CONTAMINANTS Contaminants are a common problem in all recycling processes, regardless of whether the end product is tissue, paper or paperboard. Heavy and large contaminants are easily removed, but those which are small in size and similar in density to wood fibers are not easy to remove
Number of repulpings
Fig. 3. Effect of repulping on swelling of paper.
I1
5ccQ0
I 1
i 2
I 3
I
I
4
5
J
6
Number of repulplngs
Fig. 4. Effect of repulping on tension characteristics of paper: 0, tensile breaking length; 0, stretch.
356
o bursting strength ??teorlng strength
Number of repulplngs
Fig. 5. Effect of repulping on paper strength.
with existing equipment. The best control is in regulating the quality and cleanliness of the waste paper used. The most troublesome contaminants are those which are tacky; they can either cause breaks in paper machine operation or breaks when rolls of paper are continuously unwound (e.g. on a printing press). These sticky contaminants are derived from adhesive materials, typically either pressure-sensitive adhesive used in splicing paper rolls, or hot-melt adhesives used as sealers with paperboard. Some variations in processing equipment design, mainly slotted screens and reverse centrifugal cleaners, have been designed for improved removal of sticky contaminants. Although this new equipment does not completely solve the problem, these devices are being increasingly used for contaminant removal. Screen plates can be made either with round or slotted holes. The smallest practical round hole has a diameter of about 0.125 cm, whereas a slotted screen may have slot widths as small as 0.025 cm. A globular-shaped sticky particle of just under 0.125 cm in diameter would be passed by a round hole but would be rejected by the narrower slot. Reversed centrifugal cleaners are conically shaped, similar to conventional centrifugal cleaners, but are designed to bring lightweight contaminants into the central core where they are then rejected from the large end of the cone. The clean fibers are removed from the small end of the cone, which is the reverse of a conventional cleaner. Usually, the pulp is first processed with conventional cleaners for removal of heavy contaminants such as sand and grit, before being run through reverse cleaners. Other light fraction cleaners have been developed in recent years of cylindrical design which eliminate water imbalance typical of conical cleaners.
REFERENCES 1. A. M. Altieri and J. W. Wendell Jr, Deinking of Waste Paper, TAPPI Monograph Series No. 31. Technical Association of the Pulp and Paper Industry, Atlanta, Georgia (1967). 2. A. M. Altieri and J. W. Wendell Jr, Deinking, MacDonald R. G., Franklin J. N. (eds.) Pu/p and Paper Manufacture, 2nd edn, Vol. 2, pp. 94- 131. McGraw-Hill, New York (1969).
RECYCLING OF PAPER PRODUCTS
357
3. D. E. Brooks, Economical reclamation of old corrugated containers. Tappi 61, 11 - 13 (1978). 4. D. J. Elliott, Selecting a recycled fiber system: a case history. Tappi 64, 35 - 37 (1981). 5. F. W. Lorey, As I see it: secondary fiber recycling. Tuppi 61, 7 (1978). 6. R. C. McKee, Effect of repulping on sheet properties and fiber characteristics. Pup. Trade J. 155, 34- 40 (1971). 7. M. MacLeod, Renovation of a recycling mill, Bergstrom ups Neenah capacity. Pap. Trade J. 157, 26 - 28 (1973).