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Chapter 72 - Pulp and Paper Industry

GENERAL PROFILE

Kay Teschke

Evolution and Structure of the Industry

Papermaking is thought to have originated in China in about 100 A.D. using rags, hemp and grasses as the raw material, and beating against stone mortars as the original fibre separation process. Although mechanization increased over the intervening years, batch production methods and agricultural fibre sources remained in use until the 1800s. Continuous papermaking machines were patented at the turn of that century. Methods for pulping wood, a more abundant fibre source than rags and grasses, were developed between 1844 and 1884, and included mechanical abrasion as well as the soda, sulphite, and sulphate (kraft) chemical methods. These changes initiated the modern pulp and paper manufacturing era.

Figure 72.1  illustrates the major pulp and paper making processes in the current era: mechanical pulping; chemical pulping; repulping waste paper; papermaking; and converting. The industry today can be divided into two main sectors according to the types of products manufactured. Pulp is generally manufactured in large mills in the same regions as the fibre harvest (i.e., mainly forest regions). Most of these mills also manufacture paper - for example, newsprint, writing, printing or tissue papers; or they may manufacture paperboards. (Figure 72.2  shows such a mill, which produces bleached kraft pulp, thermomechanical pulp and newsprint. Note the rail yard and dock for shipping, chip storage area, chip conveyors leading to digester, recovery boiler (tall white building) and effluent clarifying ponds). Separate converting operations are usually situated close to consumer markets and use market pulp or paper to manufacture bags, paperboards, containers, tissues, wrapping papers, decorative materials, business products and so on.

Figure 72.1 Illustration of process flow in pulp and paper manufacturing operations

Figure 72.2 Modern pulp and paper mill complex situated on a coastal waterway

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There has been a trend in recent years for pulp and paper operations to become part of large, integrated forest product companies. These companies have control of forest harvesting operations (see the Forestry chapter), lumber milling (see the Lumber industry chapter), pulp and paper manufacturing, as well as converting operations. This structure ensures that the company has an ongoing source of fibre, efficient use of wood waste and assured buyers, which often leads to increased market share. Integration has been operating in tandem with increasing concentration of the industry into fewer companies and increasing globalization as companies pursue international investments. The financial burden of plant development in this industry has encouraged these trends to allow economies of scale. Some companies have now reached production levels of 10 million tonnes, similar to the output of countries with the highest production. Many companies are multinational, some with plants in 20 or more countries worldwide. However, even though many of the smaller mills and companies are disappearing, the industry still has hundreds of participants. As an illustration, the top 150 companies account for two-thirds of pulp and paper output and only one-third of the industry’s employees.

Economic Importance

The manufacture of pulp, paper and paper products ranks among the world’s largest industries. Mills are found in more than 100 countries in every region of the world, and directly employ more than 3.5 million people. The major pulp and paper producing nations include the United States, Canada, Japan, China, Finland, Sweden, Germany, Brazil and France (each produced more than 10 million tonnes in 1994; see table 72.1).

Table 72.1 Employment and production in pulp, paper, and paperboard operations in 1994,  selected countries.

Country *

Number employed in industry 

Pulp 

 

Paper and paperboard

 

 

 

Number of mills

Production (1,000 tonnes)

Number  of mills

Production (1,000 tonnes)

Austria

10,000

11

1,595

28

3,603

Bangladesh

15,000

7

84

17

160

Brazil

70,000

35

6,106

182

5,698

Canada

64,000

39

24,547

117

18,316

China

1,500,000

8,000

17,054

10,000

21,354

Czech Republic

18,000

9

516

32

662

Finland

37,000

43

9,962

44

10,910

Former USSR**

178,000

50

3,313

161

4,826

France

48,000

20

2,787

146

8,678

Germany

48,000

19

1,934

222

14,458

India

300,000

245

1,400

380

2,300

Italy

26,000

19

535

295

6,689

Japan

55,000

49

10,579

442

28,527

Korea, Republic of

60,000

5

531

136

6,345

Mexico

26,000

10

276

59

2,860

Pakistan

65,000

2

138

68

235

Poland**

46,000

5

893

27

1,343

Romania

25,000

17

202

15

288

Slovakia

14,000

3

304

6

422

South Africa

19,000

9

2,165

20

1,684

Spain

20,180

21

626

141

5,528

Sweden

32,000

49

10,867

50

9,354

Taiwan

18,000

2

326

156

4,199

Thailand

12,000

3

240

45

1,664

Turkey

12,000

11

416

34

1,102

United Kingdom

25,000

5

626

99

5,528

United States

230,000

190

58,724

534

80,656

Total worldwide

approx 3,500,000

9,100

171,479

14,260

268,551

* Countries included if more than 10,000 people were employed in the industry.

** Data for 1989/90 (ILO 1992).

Source: Data for table adapted from PPI 1995.

Every country is a consumer. Worldwide production of pulp, paper and paperboard was about 400 million tonnes in 1993. Despite predictions of decreased paper use in the face of the electronic age, there has been a fairly steady 2.5% annual rate of growth in production since 1980 (figure 72.3). In addition to its economic benefits, the consumption of paper has cultural value resulting from its function in the recording and dissemination of information. Because of this, pulp and paper consumption rates have been used as an indicator of a nation's socioeconomic development (figure 72.4).

Figure 72.3 Pulp and paper production worldwide, 1980 to 1993

Figure 72.4 Paper and paperboard consumption as an indicator of economic development

The main source of fibre for pulp production over the last century has been wood from temperate coniferous forests, though more recently the use of tropical and boreal woods has been increasing (see the chapter Lumber for data on industrial roundwood harvesting worldwide). Because forested regions of the world are generally sparsely populated, there tends to be a dichotomy between the producing and using areas of the world. Pressure from environmental groups to preserve forest resources by using recycled paper stocks, agricultural crops and short-rotation plantation forests as fibre sources may change the distribution of pulp and paper production facilities throughout the world over the coming decades. Other forces, including increased paper consumption in the developing world and globalization, are also expected to play a role in relocating the industry.

Characteristics of the Workforce

Table 72.1  indicates the size of the workforce directly employed in pulp and paper production and converting operations in 27 countries, which together represent about 85% of world pulp and paper employment and over 90% of mills and production. In countries which consume most of what they produce (e.g., United States, Germany, France), converting operations provide two jobs for every one in pulp and paper production.

The labour force in the pulp and paper industry mainly holds full-time jobs within traditional management structures, though some mills in Finland, the United States and elsewhere have had success with flexible working hours and self-managed job-rotation teams. Because of their high capital costs, most pulping operations run continuously and require shift work; this is not true of converting plants. Working hours vary with the patterns of employment prevalent in each country, with a range from about 1,500 to more than 2,000 hours per year. In 1991, incomes in the industry ranged from US$1,300 (unskilled workers in Kenya) to US$70,000 per year (skilled production personnel in the United States) (ILO 1992). Male workers predominate in this industry, with women usually representing only 10 to 20% of the labour force. China and India may form the upper and lower ends of the range with 35% and 5% women respectively.

Management and engineering personnel at pulp and paper mills usually have university-level training. In European countries, most of the skilled blue-collar workforce (e.g., papermakers) and many of the unskilled workforce have had several years of trade-school education. In Japan, formal in-house training and upgrading is the norm; this approach is being adopted by some Latin American and North American companies. However, in many operations in North America and in the developing world, informal on-the-job training is more common for blue-collar jobs. Surveys have shown that, in some operations, many workers have literacy problems and are poorly prepared for the life-long learning required in the dynamic and potentially hazardous environment of this industry.

The capital costs of building modern pulp and paper plants are extremely high (e.g., a bleached kraft mill employing 750 people might cost US$1.5 billion to build; a chemi-thermomechanical pulp (CTMP) mill employing 100 people might cost US$400 million), so there are great economies of scale with high-capacity facilities. New and refitted plants usually use mechanized and continuous processes, as well as electronic monitors and computer controls. They require relatively few employees per unit production (e.g., 1 to 1.2 working hours per tonne of pulp in new Indonesian, Finnish and Chilean mills). Over the last 10 to 20 years, output per employee has increased as a result of incremental advances in technology. The newer equipment allows easier change-overs between product runs, lower inventories and customer-driven just-in-time production. Productivity gains have resulted in job losses in many producing nations in the developed world. However, there have been increases in employment in developing countries, where new mills being constructed, even if sparsely staffed, represent new forays into the industry.

From the 1970s to 1990, there was a decline of about 10% in the proportion of blue-collar jobs in European and North American operations, so that they now represent between 70 and 80% of the workforce (ILO 1992). The use of contract labour for mill construction, maintenance and wood-harvesting operations has been increasing; many operations have reported that 10 to 15% of their on-site workforce are contractors.

FIBRE SOURCES FOR PULP AND PAPER

Anya Keefe and Kay Teschke

The basic structure of pulp and paper sheets is a felted mat of cellulose fibres held together by hydrogen bonds. Cellulose is a polysaccharide with 600 to 1,500 repeated sugar units. The fibres have high tensile strength, will absorb the additives used to modify pulp into paper and board products, and are supple, chemically stable and white. The purpose of pulping is to separate cellulose fibres from the other components of the fibre source. In the case of wood, these include hemicelluloses (with 15 to 90 repeated sugar units), lignins (highly polymerized and complex, mainly phenyl propane units; they act as the “glue” that cements the fibres together), extractives (fats, waxes, alcohols, phenols, aromatic acids, essential oils, oleoresins, stearols, alkaloids and pigments), and minerals and other inorganics. As shown in table 72.2 , the relative proportions of these components vary according to the fibre source.

Table 72.2 Chemical constituents of pulp and paper fibre sources (%)

 

Softwoods

Hardwoods

Straw

Bamboo

Cotton

Carbohydrates

     

  α-cellulose

38–46

38–49

28–42

26–43

80–85

  Hemicellulose

23–31

20–40

23–38

15–26

nd

Lignin

22–34

16–30

12–21

20–32

nd

Extractives

1–5

2–8

1–2

0.2–5

nd

Minerals and other inorganics

0.1–7

0.1–11

3–20

1–10

0.8–2

nd = no data available.

Coniferous and deciduous trees are the major fibre sources for pulp and paper. Secondary sources include straws from wheat, rye and rice; canes, such as bagasse; woody stalks from bamboo, flax and hemp; and seed, leaf or bast fibres, such as cotton, abaca and sisal. The majority of pulp is made from virgin fibre, but recycled paper accounts for an increasing proportion of production, up from 20% in 1970 to 33% in 1991. Wood-based production accounted for 88% of worldwide pulp capacity in 1994 (176 million tonnes, figure 72.5); therefore, the description of pulp and paper processes in the following article focuses on wood-based production. The basic principles apply to other fibres as well.

Figure 72.5 Worldwide pulp capacities, by pulp type

WOOD HANDLING

Anya Keefe and Kay Teschke

Wood may arrive at a pulp mill woodyard in the form of raw logs or as chips from a lumber mill. Some pulp mill operations have on-site sawmills (often called “woodrooms”) which produce both marketable lumber and stock for the pulp mill. Sawmilling is discussed in detail in the chapter Lumber. This article discusses those elements of wood preparation which are specific to pulp mill operations.

The wood preparation area of a pulp mill has several basic functions: to receive and meter the wood supply to the pulping process at the rate demanded by the mill; to prepare the wood so that it meets the mill’s feed specifications for species, cleanliness and dimensions; and to collect any material rejected by the previous operations and send it to final disposal. Wood is converted into chips or logs suitable for pulping in a series of steps which may include debarking, sawing, chipping and screening.

Logs are debarked because bark contains little fibre, has a high extractives content, is dark, and often carries large quantities of grit. Debarking can be done hydraulically with high-pressure water jets, or mechanically by rubbing logs against each other or with metal cutting tools. Hydraulic debarkers may be used in coastal areas; however, the effluent generated is difficult to treat and contributes to water pollution.

Debarked logs may be sawn into short lengths (1 to 6 metres) for stone groundwood pulping or chipped for refiner mechanical or chemical pulping methods. Chippers tend to produce chips with a considerable size range, but pulping requires chips of very specific dimensions to ensure constant flow through refiners and uniform cooking in digesters. Chips are therefore passed over a series of screens whose function is to separate chips on the basis of length or thickness. Oversized chips are rechipped, while undersized chips are either used as waste fuel or are metered back into the chip flow.

The requirements of the particular pulping process and chip conditions will dictate the duration of chip storage (figure 72.6 ; note the different types of chips available for pulping). Depending on fibre supply and mill demand, a mill will maintain a 2 to 6 week unscreened chip inventory, usually in large outdoor chip piles. Chips may degrade through auto-oxidation and hydrolysis reactions or fungal attack of the wood components. In order to avoid contamination, short-term inventories (hours to days) of screened chips are stored in chip silos or bins. Chips for sulphite pulping may be stored outside for several months to allow volatilization of extractives which may cause problems in subsequent operations. Chips used in kraft mills where turpentine and tall oil are recovered as commercial products typically proceed directly to pulping.

Figure 72.6 Chip storage area with front end loaders

George Astrakianakis

PULPING

Anya Keefe, George Astrakianakis and Judith Anderson

Pulping is the process by which the bonds within the wood structure are ruptured either mechanically or chemically. Chemical pulps can be produced by either alkaline (i.e., sulphate or kraft) or acidic (i.e., sulphite) processes. The highest proportion of pulp is produced by the sulphate method, followed by mechanical (including semi-chemical, thermomechanical and mechanical) and sulphite methods (figure 72.5). Pulping processes differ in the yield and quality of the product, and for chemical methods, in the chemicals used and the proportion that can be recovered for reuse.

Mechanical Pulping

Mechanical pulps are produced by grinding wood against a stone or between metal plates, thereby separating the wood into individual fibres. The shearing action breaks cellulose fibres, so that the resulting pulp is weaker than chemically separated pulps. The lignin connecting cellulose to hemicellulose is not dissolved; it merely softens, allowing the fibres to be ground out of the wood matrix. The yield (proportion of original wood in pulp) is usually greater than 85%. Some mechanical pulping methods also use chemicals (i.e., the chemi-mechanical pulps); their yields are lower since they remove more of the non-cellulosic materials.

In stone groundwood pulping (SGW), the oldest and historically most common mechanical method, fibres are removed from short logs by pressing them against a rotating abrasive cylinder. In refiner mechanical pulping (RMP, figure 72.7), which gained popularity after it became commercially viable in the 1960s, wood chips or sawdust are fed through the centre of a disc refiner, where they are shredded into finer pieces as they are pushed out through progressively narrower bars and grooves. (In figure 72.7 , the refiners are enclosed in the middle of the picture and their large motors are on the left. Chips are supplied though the large diameter pipes, and pulp exits the smaller ones.) A modification of RMP is thermomechanical pulping (TMP), in which the chips are steamed before and during refining, usually under pressure.

Figure 72.7 Refiner mechanical pulping

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One of the earliest methods of producing chemi-mechanical pulps involved pre-steaming logs before boiling them in chemical pulping liquors, then grinding them in stone grinders to produce “chemi-groundwood” pulps. Modern chemi-mechanical pulping uses disc refiners with chemical treatment (e.g., sodium bisulphite, sodium hydroxide) either prior to, during or after refining. Pulps produced in this manner are referred to either as chemi-mechanical pulps (CMP) or chemi-thermomechanical pulps (CTMP), depending on whether refining was carried out at atmospheric or elevated pressure. Specialized variations of CTMP have been developed and patented by a number of organizations.

Chemical Pulping and Recovery

Chemical pulps are produced by chemically dissolving the lignin between the wood fibres, thereby enabling the fibres to separate relatively undamaged. Because most of the non-fibrous wood components are removed in these processes, yields are usually in the order of 40 to 55%.

In chemical pulping, chips and chemicals in aqueous solution are cooked together in a pressure vessel (digester, figure 72.8) which can be operated on a batch or continuous basis. In batch cooking, the digester is filled with chips through a top opening, the digestion chemicals are added, and the contents cooked at elevated temperature and pressure. Once the cook is complete, the pressure is released, “blowing” the delignified pulp out of the digester and into a holding tank. The sequence is then repeated. In continuous digesting, pre-steamed chips are fed into the digester at a continuous rate. Chips and chemicals are mixed together in the impregnation zone at the top of the digester and then proceed through the upper cooking zone, the lower cooking zone, and the washing zone before being blown into the blow tank.

Figure 72.8 Continuous kraft digestor, with chip conveyor under construction

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The digesting chemicals are recovered in most chemical pulping operations today. The principal objectives are to recover and reconstitute digestion chemicals from the spent cooking liquor, and to recover heat energy by burning the dissolved organic material from the wood. The resulting steam and electricity supplies some, if not all, of the mill’s energy needs.

Sulphate Pulping and Recovery

The sulphate process produces a stronger, darker pulp than other methods and requires chemical recovery to compete economically. The method evolved from soda pulping (which uses only sodium hydroxide for digestion) and began to gain prominence in the industry from the 1930s to 1950s with the development of chlorine dioxide bleaching and chemical recovery processes, which also produced steam and power for the mill. The development of corrosion-proof metals, such as stainless steel, to handle the acidic and alkaline pulp mill environments also played a role.

The cooking mixture (white liquor) is sodium hydroxide (NaOH, “caustic”) and sodium sulphide (Na2S). Modern kraft pulping is usually carried out in continuous digesters often lined with stainless steel (figure 72.8). The temperature of the digester is raised slowly to approximately 170°C and held at that level for approximately 3 to 4 hours. The pulp (called brown stock because of its colour) is screened to remove uncooked wood, washed to remove the spent cooking mixture (now black liquor), and sent either to the bleach plant or to the pulp machine room. Uncooked wood is either returned to the digester or sent to the power boiler to be burned.

The black liquor collected from the digester and brown stock washers contains dissolved organic material whose exact chemical composition depends on the wood species pulped and the cooking conditions. The liquor is concentrated in evaporators until it contains less than 40% water, then sprayed into the recovery boiler. The organic component is consumed as fuel, generating heat which is recovered in the upper section of the furnace as high-temperature steam. The unburned inorganic component collects at the bottom of the boiler as a molten smelt. The smelt flows out of the furnace and is dissolved in a weak caustic solution, producing “green liquor” containing primarily dissolved Na2S and sodium carbonate (Na2CO3). This liquor is pumped to a recausticizing plant, where it is clarified, then reacted with slaked lime  (Ca(OH)2), forming NaOH and calcium carbonate (CaCO3). The white liquor is filtered and stored for subsequent use. CaCO3 is sent to a lime kiln, where it is heated to regenerate lime (CaO).

Sulphite Pulping and Recovery

Sulphite pulping dominated the industry from the late 1800s to the mid-1900s, but the method used during this era was limited by the types of wood which could be pulped and the pollution created by discharging untreated waste cooking liquor into waterways. Newer methods have overcome many of these problems, but sulphite pulping is now a small segment of the pulp market. Although sulphite pulping usually uses acid digestion, both neutral and basic variations exist.

The cooking liquor of sulphurous acid (H2SO3) and bisulphite ion (HSO3–) is prepared on-site. Elemental sulphur is burned to produce sulphur dioxide (SO2), which is passed up through an absorption tower that contains water and one of four alkaline bases (CaCO3, the original sulphite base, Na2CO3, magnesium hydroxide (Mg(OH)2) or ammonium hydroxide (NH4OH)) which produce the acid and ion and control their proportions. Sulphite pulping is usually carried out in brick-lined batch digesters. To avoid unwanted reactions, the digester is heated slowly to a maximum temperature of 130 to 140°C and the chips are cooked for a long time (6 to 8 hours). As the digester pressure increases, gaseous sulphur dioxide (SO2) is bled off and remixed with the raw cooking acid. When approximately 1 to 1.5 hours of cooking time remains, heating is discontinued and the pressure is decreased by bleeding off gas and steam. The pulp is blown into a holding tank, then washed and screened.

The spent digestion mixture, called red liquor, can be used for heat and chemical recovery for all but calcium-bisulphite-base operations. For ammonia-base sulphite pulping, the dilute red liquor is first stripped to remove residual SO2, then concentrated and burned. The flue gas containing SO2 is cooled and passed through an absorption tower where fresh ammonia combines with it to regenerate the cooking liquor. Finally, the liquor is filtered, fortified with fresh SO2 and stored. The ammonia cannot be recovered because it is converted into nitrogen and water in the recovery boiler.

In magnesium-base sulphite pulping, burning the concentrated pulping liquor gives magnesium oxide (MgO) and SO2, which are easily recovered. No smelt is produced in this process; rather MgO is collected from the flue gas and slaked with water to produce magnesium hydroxide (Mg(OH)2). SO2 is cooled and combined with the Mg(OH)2 in an absorption tower to reconstitute the cooking liquor. The magnesium bisulphite (Mg(HSO3)2) is then fortified with fresh SO2 and stored. Recovery of 80 to 90% of the cooking chemicals is possible.

Recovery of sodium-base sulphite cooking liquor is more complicated. Concentrated spent liquor is incinerated, and approximately 50% of the sulphur is converted into SO2. The remainder of the sodium and sulphur is collected at the bottom of the recovery boiler as a smelt of Na2S and Na2CO3. The smelt is dissolved to produce green liquor, which is converted to sodium bisulphite (NaHSO3) in several steps. The NaHSO3 is fortified and stored. The regeneration process produces reduced sulphur gases, in particular hydrogen sulphide (H2S).

BLEACHING

George Astrakianakis and Judith Anderson

Bleaching is a multi-stage process that refines and brightens raw pulp. The objective is to dissolve (chemical pulps) or modify (mechanical pulps) the brown-coloured lignin that was not removed during pulping, while maintaining the integrity of the pulp fibres. A mill produces customized pulp by varying the order, concentration and reaction time of the bleaching agents.

Each bleaching stage is defined by its bleaching agent, pH (acidity), temperature and duration (table 72.3). After each bleaching stage, the pulp may be washed with caustic to remove spent bleaching chemicals and dissolved lignin before it progresses to the next stage. After the last stage, the pulp is pumped through a series of screens and cleaners to remove any contaminants such as dirt or plastic. It is then concentrated and conveyed to storage.

Table 72.3 Bleaching agents and their conditions of use

 

Symbol

Concentration  of agent (%)

pH

Consistency*  (%)

Temperature  (°C)

Time (h)

Chlorine (Cl2)

C

2.5–8

2

3

20–60

0.5–1.5

Sodium hydroxide (NaOH)

E

1.5–4.2

11

10–12

<80

1–2

Chlorine dioxide (ClO2)

D

approx 1

0–6

10–12

60–75

2–5

Sodium hypochlorite (NaOCl)

H

1–2

9–11

10–12

30–50

0.5–3

Oxygen (O2)

O

1.2–1.9

7–8

25–33

90–130

0.3–1

Hydrogen peroxide (H2O2)

P

0.25

10

12

35–80

4

Ozone (O3)

Z

0.5–3.5

2–3

35–55

20–40

<0.1

Acid washing (SO2)

A

4–6

1.8–5

1.5

30–50

0.25

Sodium dithionite (NaS2O4)

Y

1–2

5.5–8

4–8

60–65

1–2

* Concentration of fibre in water solution.

Historically, the most common bleaching sequence used to produce market-grade bleached kraft pulp is based on the five-stage CEDED process (see table 72.3 for definition of symbols). The first two stages of bleaching complete the delignification process and are considered extensions of pulping. Because of environmental concerns about chlorinated organics in pulp mill effluents, many mills substitute chlorine dioxide (ClO2) for a portion of the chlorine (Cl2) used in the first bleaching stage (CDEDED) and use oxygen (O2) pre-treatment during the first caustic extraction (CDEODED). The current trend in Europe and North America is towards complete substitution with ClO2 (e.g., DEDED) or elimination of both Cl2 and ClO2. Where ClO2 is used, sulphur dioxide (SO2) is added during the final washing stage as an “antichlor” to stop the ClO2 reaction and to control the pH. Newly developed chlorine-free bleaching sequences (e.g., OAZQP, OQPZP, where Q = chelation) use enzymes, O2, ozone (O3), hydrogen peroxide (H2O2), peracids and chelating agents such as ethylene diamine tetracetic acid (EDTA). Totally chlorine-free bleaching had been adopted at eight mills worldwide by 1993. Because these newer methods eliminate the acidic bleaching steps, acid washing is a necessary addition to the initial stages of kraft bleaching to allow removal of metals bound to the cellulose.

Sulphite pulps are generally easier to bleach than kraft pulps because of their lower lignin content. Short bleaching sequences (e.g., CEH, DCEHD, P, HP, EPOP) can be used for most paper grades. For dissolving-grade sulphite pulps used in the production of rayon, cellophane and so on, both hemicellulose and lignin are removed, requiring more complex bleaching sequences (e.g., C1C2ECHDA). The final acid wash is both for metal control and antichlor purposes. The effluent load for dissolving-grade sulphite pulps is much greater because so much of the raw wood is consumed (typical yield 50%) and more water is used.

The term brightening is used to describe bleaching of mechanical and other high-yield pulps, because they are whitened by destroying chromophoric groups without dissolving the lignin. Brightening agents include H2O2 and/or sodium hydrosulphite (NaS2O4). Historically, zinc hydrosulphite (ZnS2O4) was commonly used, but has been largely eliminated because of its toxicity in effluent. Chelating agents are added before bleaching to neutralize any metal ions, thereby preventing the formation of coloured salts or the decomposition of H2O2. The effectiveness of mechanical pulp bleaching depends on the species of wood. Hardwoods (e.g., poplar and cottonwood) and softwoods (e.g., spruce and balsam) that are low in lignin and extractives can be bleached to a higher brightness level than the more resinous pine and cedar.

RECYCLED PAPER OPERATIONS

Dick Heederik

The use of waste or recycled paper as the raw material for pulp production has increased during the last several decades, and some paper plants depend almost completely on waste paper. In some countries, waste paper is separated from other household waste at the source before it is collected. In other countries separation by grade (e.g., corrugated board, newsprint, high-grade paper, mixed) takes place in special recycling plants.

Recycled paper can be repulped in a relatively mild process which uses water and sometimes NaOH. Small metal pieces and plastics may be separated during and/or after repulping, using a debris rope, cyclones or centrifugation. Filling agents, glues and resins are removed in a cleaning stage by blowing air through the pulp slurry, sometimes with the addition of flocculating agents. The foam contains the unwanted chemicals and is removed. The pulp can be de-inked using a series of washing steps which may or may not include the use of chemicals (i.e., surfactant fatty acid derivatives) to dissolve remaining impurities, and bleaching agents to whiten the pulp. Bleaching has the disadvantage that it may reduce fibre length and therefore lessen final paper quality. The bleaching chemicals used in recycled pulp production are usually similar to those used in brightening operations for mechanical pulps. After the repulping and de-inking operations, sheet production follows in a manner very similar to that using virgin fibre pulp.

SHEET PRODUCTION AND CONVERTING: MARKET PULP, PAPER, PAPERBOARD

George Astrakianakis and Judith Anderson

End products of pulp and paper mills depend on the pulping process, and may include market pulp and various types of paper or paperboard products. For example, the relatively weak mechanical pulp is converted into single-use products such as newspapers and tissue. Kraft pulp is converted into multi-use paper products such as high-quality writing paper, books and grocery bags. Sulphite pulp, which is primarily cellulose, can be used in a series of diverse end-products including specialty paper, rayon, photographic film, TNT, plastics, adhesives, and even ice cream and cake mixes. Chemi-mechanical pulps are exceptionally stiff, ideal for the structural support needed for corrugated container board. The fibres in pulp from recycled paper are usually shorter, less flexible and less water permeable, and can therefore not be used for high-quality paper products. Recycled paper is therefore mainly used for the production of soft paper products like tissue paper, toilet paper, paper towelling and napkins.

To produce market pulp, the pulp slurry is usually screened once more and its consistency adjusted (4 to 10%) before it is ready for the pulp machine. The pulp is then spread onto a travelling metal screen or plastic mesh (known as the “wire”) at the “wet end” of the pulp machine, where the operator monitors the speed of the moving wire and the water content of the pulp (figure 72.9 ; the presses and the cover of the drier can be seen in the upper left; in modern mills, operators spend a great deal of time in control rooms). Water and filtrate are drawn through the wire, leaving a web of fibres. The pulp sheet is passed through a series of rotating rolls (“presses”) that squeeze out water and air until the fibre consistency is 40 to 45%. The sheet is then floated through a multi-storey sequence of hot-air dryers until the consistency is 90 to 95%. Finally, the continuous pulp sheet is cut into pieces and stacked into bales. The pulp bales are compressed, wrapped and packaged into bundles for storage and transport.

Figure 72.9 Wet end of pulp machine showing fibre mat on the wire

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Although similar in principle to making pulp sheets, paper making is considerably more complex. Some mills use a variety of different pulps to optimize paper quality (e.g., a mix of hardwood, softwood, kraft, sulphite, mechanical or recycled pulps). Depending on the type of pulp used, a series of steps is necessary prior to forming the paper sheet. Generally, dried market pulp is rehydrated, while high-consistency pulp from storage is diluted. Pulp fibres may be beaten to increase the fibre-bonding area and thereby improve paper sheet strength. The pulp is then blended with “wet-end” additives (table 72.4) and passed through a final set of screens and cleaners. The pulp is then ready for the paper machine.

Table 72.4 Papermaking additives

Additive

Location applied

Purpose and/or examples of specific agents

Most commonly used additives

Talc

Wet end

Pitch control (prevent deposition and accumulation of pitch)

Filler (make brighter, smoother, more opaque)

Titanium dioxide

Wet end

Pigment (brighten sheet, improve printing)

Filler (make brighter, smoother, more opaque)

"Alum"(Al2(SO4)3)

Wet end

Precipitates rosin sizing onto fibres

Retention aid (fix additives to fibres, improve pulp fibre retention)

Rosin

Wet end

Internal sizing (resist liquid penetration)

Clay (kaolin)

Wet/dry

Filler (make brighter, smoother, more opaque)

Pigment or surface coating (impart colour)

Starch

Wet/dry

Surface sizing (resist liquid penetration)

Dry strength additive (increase strength, reduce surface lint)

Retention aid (bind additives to paper, improve pulp fibre retention)

Dyes and pigments

Wet/dry

e.g., acid, basic or direct dyes,colour lakes, CaCO3, may also include solvent vehicles

Latex

Dry end

Adhesive (reinforce sheet, bind additives to paper, fill pores)

Waterproofing (resist liquid penetration)

Other additives

Slimicides

Wet end

e.g., thiones, thiazoles, thiocyanates, hiocarbamates, thiols, isothiazolinones, formaldehyde, glutaraldehyde, glycols, naphthol, chlorinated and brominated organics, organic mercury compounds

Defoamers

Wet end

e.g., pine oil, fuel oil, recycled oils, silicones, alcohols

Wire treatment agents

Wet end

e.g., imidazoles, butyl diglycol, acetone, turpentine, phosphoric acid

Wet and dry strength additives

Wet end

e.g., formaldehyde resins, epichlorohydrin, glyoxal, gums, polyamines, phenolics, polyacrylamides, polyamids, cellulose derivatives

Coatings, adhesives and plasticizers

Dry end

e.g., aluminium hydroxide, polyvinyl acetate, acrylics, linseed oil, gums, protein glues, wax emulsions, azite, glyoxal, stearates, solvents, polyethylene, cellulose derivatives, foil, rubber derivatives, polyamines, polyesters, butadiene-styrene polymers

Others

Wet/dry

Corrosion inhibitors, dispersants, flameproofing, antitarnish agents, drainage aids, deflocculants, pH control agents, preservatives

The flow spreader and headbox distribute a thin suspension (1 to 3%) of refined pulp onto a moving wire (similar to a pulp machine, only at a much higher speed, sometimes in excess of 55 km/h) which forms the fibres into a thin felted sheet. The sheet moves through a series of press rolls to the dryer section, where a series of steam-heated rolls evaporate most of the remaining water. Hydrogen bonds between the fibres have fully developed at this stage. Finally, the paper is calendered and reeled. Calendering is the process by which the paper surface is ironed smooth and its thickness reduced. The dried, calendered paper sheet is wound onto a reel, labelled and transported to the warehouse (figure 72.10 ; note waste paper under reel, and unenclosed operator control panel). “Dry-end” additives can be added before calendering on the paper machine or in separate “off-machine” coating operations in the converting sector of the industry.

Figure 72.10 Dry end of a paper machine showing full paper reel and operator using air slitter to cut end

George Astrakianakis

A variety of chemicals are used in the papermaking process to provide the paper with specific surface characteristics and sheet properties. The most commonly used additives (table 72.4) are typically used at the per cent level, though some such as clay and talc may contribute as much as 40% to the dry weight of certain papers. Table 72.4  also indicates the diversity of chemical additives which may be used for specific production purposes and products; some of these are used at very low concentrations (e.g., slimicides are added to process water in parts per million).

The process of making paperboard is similar to that of making paper or pulp. A suspension of pulp and water is dispersed onto a travelling wire, the water is removed, and the sheet dried and stored as a roll. The process differs in the way that the sheet is formed to give thickness, in the combining of multiple layers, and in the drying process. Board can be made from single or multi-layered sheets with or without a core. The sheets are usually high-quality kraft pulp (or kraft and CTMP blend), while the core is made from either a blend of semi-chemical and low-cost recycled pulp or from entirely recycled pulp and other waste material. Coatings, vapour barriers and multiple layers are added according to the end use to protect the contents from water and physical damage.

POWER GENERATION AND WATER TREATMENT

George Astrakianakis and Judith Anderson

In addition to liquor recovery, pulp mills recover a significant portion of energy from burning waste materials and by-products of the process in power boilers. Materials such as bark, wood waste and dried sludge collected from effluent treatment systems may be burned to provide steam to power electrical generators.

Pulp and paper mills consume vast amounts of fresh water. A 1,000 tonne per day bleached kraft pulp mill may use more than 150 million litres of water a day; a paper mill even more. In order to prevent adverse effects on mill equipment and to maintain product quality, the incoming water must be treated to remove contaminants, bacteria and minerals. Several treatments are applied depending on the quality of the incoming water. Sedimentation beds, filters, flocculants, chlorine and ion exchange resins are all used to treat water before it is used in the process. Water that is used in the power and recovery boilers is further treated with oxygen scavengers and corrosion inhibitors such as hydrazine and morpholine to avoid deposits forming in the boiler tubes, to reduce metal corrosion, and to prevent carry-over of water to the steam turbine.

CHEMICAL AND BY-PRODUCT PRODUCTION

George Astrakianakis and Judith Anderson

Because many bleaching chemicals are reactive and hazardous to transport, they are produced on-site or nearby. Chlorine dioxide (ClO2), sodium hypochlorite (NaOCl) and peracids are always produced on-site, while chlorine (Cl2) and sodium hydroxide or caustic (NaOH) are usually produced off-site. Tall oil, a product derived from the resin and fatty acids that are extracted during kraft cooking, may be refined on- or off-site. Turpentine, a lighter fraction kraft by-product, is often collected and concentrated on-site, and refined elsewhere.

Chlorine Dioxide

Chlorine dioxide (ClO2) is a highly reactive greenish-yellow gas. It is toxic and corrosive, explodes at high concentrations (10%) and is quickly reduced to Cl2 and O2 in the presence of ultraviolet light. It must be prepared as a dilute gas and stored as a dilute liquid, making bulk transport impossible.

ClO2 is generated by reducing sodium chlorate (Na2ClO3) with either SO2, methanol, salt or hydrochloric acid. The gas leaving the reactor is condensed and stored as a 10% liquid solution. Modern ClO2 generators operate at 95% efficiency or greater, and the small amount of Cl2 that is produced will be collected or scrubbed out of the vent gas. Side reactions may occur depending on the purity of the feed chemicals, the temperature and other process variables. By-products are returned to the process and spent chemicals are neutralized and sewered.

Sodium Hypochlorite

Sodium hypochlorite (NaOCl) is produced by combining Cl2 with a dilute solution of NaOH. It is a simple, automated process that requires almost no intervention. The process is controlled by maintaining the caustic concentration such that the residual Cl2 in the process vessel is minimized.

Chlorine and Caustic

Chlorine (Cl2), used as a bleaching agent since the early 1800s, is a highly reactive, toxic, green-coloured gas which becomes corrosive when moisture is present. Chlorine is usually manufactured by the electrolysis of brine (NaCl) into Cl2 and NaOH at regional installations, and transported to the customer as a pure liquid. Three methods are used to produce Cl2 on an industrial scale: the mercury cell, the diaphragm cell, and the most recent development, the membrane cell. Cl2 is always produced at the anode. It is then cooled, purified, dried, liquefied and transported to the mill. At large or remote pulp mills, local facilities may be constructed, and the Cl2 can be transported as a gas.

The quality of NaOH depends on which of the three processes is used. In the older mercury cell method, the sodium and mercury combine to form an amalgam that is decomposed with water. The resulting NaOH is nearly pure. One of the shortcomings of this process is that mercury contaminates the workplace and has resulted in serious environmental problems. The NaOH produced from the diaphragm cell is removed with the spent brine and concentrated to allow the salt to crystallize and separate. Asbestos is used as the diaphragm. The purest NaOH is produced in membrane cells. A semi-permeable resin-based membrane allows sodium ions to pass through without the brine or chlorine ions, and combine with water added to the cathode chamber to form pure NaOH. Hydrogen gas is a by-product of each process. It is usually treated and used either in other processes or as fuel.

Tall Oil Production

Kraft pulping of highly-resinous species such as pine produces sodium soaps of resin and fatty acids. The soap is collected from black liquor storage tanks and from soap skimming tanks that are located in the evaporator train of the chemical recovery process. Refined soap or tall oil can be used as a fuel additive, dust control agent, road stabilizer, pavement binder and roofing flux.

At the processing plant, soap is stored in primary tanks to allow the black liquor to settle to the bottom. The soap rises and overflows into a second storage tank. Sulphuric acid and the decanted soap are fed into a reactor, heated to 100°C, agitated and then allowed to settle. After settling overnight, the crude tall oil is decanted into a storage vessel and allowed to sit for another day. The top fraction is considered dry crude tall oil and is pumped to storage, ready for shipment. The cooked lignin in the bottom fraction will become part of the subsequent batch. The spent sulphuric acid is pumped to a storage tank, and any entrained lignin is allowed to settle to the bottom. The lignin left in the reactor is concentrated for several cooks, dissolved in 20% caustic and returned to the primary soap tank. Periodically, the collected black liquor and the residual lignin from all sources are concentrated and burned as fuel.

Turpentine Recovery

Gases from the digesters and condensate from black liquor evaporators may be collected for recovery of turpentine. The gases are condensed, combined, then stripped of turpentine, which is recondensed, collected and sent to a decanter. The top fraction of the decanter is drawn off and sent to storage, while the bottom fraction is recycled to the stripper. Raw turpentine is stored separately from the rest of the collection system because it is noxious and flammable, and is usually processed off-site. All the non-condensable gases are collected and incinerated either in the power boilers, the lime kiln or a dedicated furnace. The turpentine can be processed for use in camphor, synthetic resins, solvents, flotation agents and insecticides.

OCCUPATIONAL HAZARDS AND CONTROLS

Kay Teschke, George Astrakianakis, Judith Anderson, Anya Keefe and Dick Heederik

Table 72.5  provides an overview of the types of exposures which may be expected in each area of pulp and paper operations. Although exposures may be listed as specific to certain production processes, exposures to employees from other areas may also occur depending on weather conditions, proximity to sources of exposure, and whether they work in more than one process area (e.g., quality control, general labour pool and maintenance personnel).

Table 72.5 Potential health and safety hazards in pulp and paper production, by process area

Process area

Safety hazards

Physical hazards

Chemical hazards

Biological hazards

Wood preparation

    

Log pond

Drowning; mobile equipment;  slipping, falling

Noise; vibration; cold; heat

Engine exhaust

 

Wood room

Nip points; slipping, falling

Noise; vibration

Terpenes and other wood extracts; wood dust

Bacteria; fungi

Chip screening

Nip points; slipping, falling

Noise; vibration

Terpenes and other wood extracts; wood dust

Bacteria; fungi

Chip yard

Nip points; mobile equipment

Noise; vibration; cold; heat

Engine exhaust; terpenes and other wood  extracts; wood dust

Bacteria; fungi

Pulping

    

Stone groundwood  pulping

Slipping, falling

Noise; electric and magnetic  fields; high humidity

  

RMP, CMP, CTMP

Slipping, falling

Noise; electric and magnetic  fields; high humidity

Cooking chemicals and by-products; terpenes  and other wood extracts; wood dust

 

Sulphate pulping

Slipping, falling

Noise; high humidity; heat

Acids and alkalis; cooking chemicals and  by-products; reduced sulphur gases; terpenes  and other wood extracts; wood dust

 

Sulphate recovery

Explosions; nip points; slipping,  falling

Noise; heat; steam

Acids and alkalis; asbestos; ash; cooking  chemicals and by-products; fuels; reduced  sulphur gases; sulphur dioxide

 

Sulphite pulping

Slipping, falling

Noise; high humidity; heat

Acids and alkalis; cooking chemicals and  by-products; sulphur dioxide; terpenes and  other wood extracts; wood dust

 

Sulphite recovery

Explosions; nip points; slipping,  falling

Noise; heat; steam

Acids and alkalis; asbestos; ash; cooking  chemicals and by-products; fuels; sulphur dioxide

 

Repulping/de-inking

Slipping, falling

 

Acids and alkalis; bleaching chemicals and  by- products; dyes and inks; pulp/paper dust;  slimicides; solvents

Bacteria

Bleaching

Slipping, falling

Noise; high humidity; heat

Bleaching chemicals and by-products;  slimicides; terpenes and other wood extracts

 

Sheet forming and  converting

    

Pulp machine

Nip points; slipping, falling

Noise; vibration; high  humidity; heat; steam

Acids and alkalis; bleaching chemicals and  by-products; flocculant; pulp/paper dust;  slimicides; solvents

Bacteria

Paper machine

Nip points; slipping, falling

Noise; vibration; high  humidity; heat; steam

Acids and alkalis; bleaching chemicals and  by-products; dyes and inks; flocculant; pulp/paper  dust; paper additives; slimicides; solvents

Bacteria

Finishing

Nip points; mobile equipment

Noise

Acids and alkalis; dyes and inks; flocculant;  pulp/paper dust; paper additives; slimicides;  solvents

 

Warehouse

Mobile equipment

 

Fuels; engine exhaust; pulp/paper dust

 

Other operations

    

Power generation

Nip points; slipping, falling

Noise; vibration; electric and  magnetic fields; heat; steam

Asbestos; ash; fuels; terpenes and other wood  extracts; wood dust

Bacteria; fungi

Water treatment

Drowning

 

Bleaching chemicals and by-products

Bacteria

Effluent treatment

Drowning

 

Bleaching chemicals and by-products; flocculant;  reduced sulphur gases

Bacteria

Chlorine dioxide  generation

Explosions; slipping, falling

 

Bleaching chemicals and by-products

Bacteria

Turpentine recovery

Slipping, falling

 

Cooking chemicals and by-products; reduced  sulphur gases; terpenes and other wood extracts

 

Tall oil production

  

Acids and alkalis; cooking chemicals and  by-products; reduced sulphur gases; terpenes  and other wood extracts

 

RMP = refining mechanical pulping;  CMP = chemi-mechanical pulping;  CTMP = chemi-thermomechanical pulping.

Exposure to the potential hazards listed in table 72.5 is likely to depend on the extent of automation of the plant. Historically, industrial pulp and paper production was a semi-automatic process which required a great deal of manual intervention. In such facilities, operators would sit at open panels adjacent to the processes to view the effects of their actions. The valves at the top and bottom of a batch digester would be manually opened, and during the filling stages, gases in the digester would be displaced by the incoming chips (figure 72.11). Chemical levels would be adjusted based on experience rather than sampling, and process adjustments would be dependent on the skill and knowledge of the operator, which at times led to upsets. For example, over-chlorination of pulp would expose workers downstream to increased levels of bleaching agents. In most modern mills, progress from manually controlled to electronically controlled pumps and valves allows for remote operation. The demand for process control within narrow tolerances has required computers and sophisticated engineering strategies. Separate control rooms are used to isolate the electronic equipment from the pulp and paper production environment. Consequently, operators usually work in air-conditioned control rooms which offer refuge from the noise, vibration, temperature, humidity and chemical exposures inherent to mill operations. Other controls which have improved the working environment are described below.

Figure 72.11 Worker opening cap on manually controlled batch digester

MacMillan Bloedel archives

Safety hazards including nip points, wet walking surfaces, moving equipment and heights are common throughout pulp and paper operations. Guards around moving conveyors and machinery parts, quick clean-up of spills, walking surfaces which allow drainage, and guard-rails on walkways adjacent to production lines or at height are all essential. Lock-out procedures must be followed for maintenance of chip conveyors, paper machine rolls and all other machinery with moving parts. Mobile equipment used in chip storage, dock and shipping areas, warehousing and other operations should have roll-over protection, good visibility and horns; traffic lanes for vehicles and pedestrians should be clearly marked and signed.

Noise and heat are also ubiquitous hazards. The major engineering control is operator enclosures, as described above, usually available in wood preparation, pulping, bleaching and sheet-forming areas. Air-conditioned enclosed cabs for mobile equipment used in chip pile and other yard operations are also available. Outside these enclosures, workers usually require hearing protection. Work in hot process or outdoor areas and in vessel maintenance operations requires workers to be trained to recognize symptoms of heat stress; in such areas, work scheduling should allow acclimatization and rest periods. Cold weather may create frostbite hazards in outdoor jobs, as well as foggy conditions near chip piles, which remain warm.

Wood, its extracts and associated micro-organisms are specific to wood preparation operations and the initial stages of pulping. Control of exposures will depend on the particular operation, and may include operator booths, enclosure and ventilation of saws and conveyors, as well as enclosed chip storage and low chip inventory. Use of compressed air to clear wood dust creates high exposures and should be avoided.

Chemical pulping operations present the opportunity for exposures to digestion chemicals as well as gaseous by-products of the cooking process, including reduced (kraft pulping) and oxidized (sulphite pulping) sulphur compounds and volatile organics. Gas formation may be influenced by a number of operating conditions: the wood species used; the quantity of wood pulped; the amount and concentration of white liquor applied; the amount of time required for pulping; and maximum temperature attained. In addition to automatic digester capping valves and operator control rooms, other controls for these areas include local exhaust ventilation at batch digesters and blow tanks, capable of venting at the rate the vessel’s gases are released; negative pressure in recovery boilers and sulphite-SO2 acid towers to prevent gas leaks; ventilated full or partial enclosures over post-digestion washers; continuous gas monitors with alarms where leaks may occur; and emergency response planning and training. Operators taking samples and conducting tests should be aware of the potential for acid and caustic exposure in process and waste streams, and the possibility of side reactions such as hydrogen sulphide gas (H2S) production if black liquor from kraft pulping comes into contact with acids (e.g., in sewers).

In chemical recovery areas, acidic and alkaline process chemicals and their by-products may be present at temperatures in excess of 800°C. Job responsibilities may require workers to come into direct contact with these chemicals, making heavy duty clothing a necessity. For example, workers rake the spattering molten smelt that collects at the base of the boilers, thereby risking chemical and thermal burns. Workers may be exposed to dust when sodium sulphate is added to concentrated black liquor, and any leak or opening will release noxious (and potentially fatal) reduced sulphur gases. The potential for a smelt water explosion always exists around the recovery boiler. Water leaks in the tube walls of the boiler have resulted in several fatal explosions. Recovery boilers should be shut down at any indication of a leak, and special procedures should be implemented for transferring the smelt. Loading of lime and other caustic materials should be done with enclosed and ventilated conveyors, elevators and storage bins.

In bleach plants, field operators may be exposed to the bleaching agents as well as chlorinated organics and other by-products. Process variables such as bleaching chemical strength, lignin content, temperature and pulp consistency are constantly monitored, with operators collecting samples and performing laboratory tests. Because of the hazards of many of the bleaching agents used, continuous alarm monitors should be in place, escape respirators should be issued to all employees, and operators should be trained in emergency response procedures. Canopy enclosures with dedicated exhaust ventilation are standard engineering controls found at the top of each bleaching tower and washing stage.

Chemical exposures in the machine room of a pulp or paper mill include chemical carry-over from the bleach plant, the papermaking additives and the chemical mixture in the waste water. Dusts (cellulose, fillers, coatings) and exhaust fumes from mobile equipment are present in the dry-end and the finishing operations. Cleaning between product runs may be done with solvents, acids and alkalis. Controls in this area may include complete enclosure over the sheet drier; ventilated enclosure of the areas where additives are unloaded, weighed and mixed; use of additives in liquid rather than powder form; use of water-based rather than solvent-based inks and dyes; and eliminating the use of compressed air to clean up trimmed and waste paper.

Paper production in recycled paper plants is generally dustier than conventional paper production using newly produced pulp. Exposure to micro-organisms can occur from the beginning (paper collection and separation) to the end (paper production) of the production chain, but exposure to chemicals is less important than in conventional paper production.

Pulp and paper mills employ an extensive maintenance group to service their process equipment, including carpenters, electricians, instrument mechanics, insulators, machinists, masons, mechanics, millwrights, painters, pipefitters, refrigeration mechanics, tinsmiths and welders. Along with their trade-specific exposures (see the Metal processing and metal working and Occupations chapters), these tradespeople may be exposed to any of the process-related hazards. As mill operations have become more automated and enclosed, the maintenance, cleaning and quality assurance operations have become the most highly exposed. Plant shutdowns to clean vessels and machines are of special concern. Depending on mill organization, these operations may be carried out by in-house maintenance or production personnel, although subcontracting to non-mill personnel, who may have less occupational health and safety support services, is common.

In addition to process exposures, pulp and paper mill operations entail some noteworthy exposures for maintenance personnel. Because pulping, recovery and boiler operations involve high heat, asbestos was used extensively to insulate pipes and vessels. Stainless steel is often used in vessels and pipes throughout pulping, recovery and bleaching operations, and to some extent in papermaking. Welding this metal is known to generate chromium and nickel fumes. During maintenance shut-downs, chromium-based sprays may be applied to protect the floor and walls of recovery boilers from corrosion during start-up operations. Process quality measurements in the production line are often made using infrared and radio-isotope gauges. Although the gauges are usually well shielded, instrument mechanics who service them may be exposed to radiation.

Some special exposures may also occur among employees in other mill-support operations. Power boiler workers handle bark, waste wood and sludge from the effluent treatment system. In older mills, workers remove ash from the bottom of the boilers and then reseal the boilers by applying a mixture of asbestos and cement around the boiler grate. In modern power boilers, this process is automated. When material is fed into the boiler at too high a moisture level, workers may be exposed to blow-backs of incomplete combustion products. Workers responsible for water treatment may be exposed to chemicals such as chlorine, hydrazine and various resins. Because of the reactivity of ClO2, the ClO2 generator is usually located in a restricted area and the operator is stationed in a remote control room with excursions to collect samples and service the saltcake filter. Sodium chlorate (a strong oxidizer) used to generate ClO2 can become dangerously flammable if it is allowed to spill on any organic or combustible material and then dry. All spills should be wetted down before any maintenance work may proceed, and all equipment should be thoroughly cleaned afterward. Wet clothing should be kept wet and separate from street clothing, until washed.

INJURIES AND NON-MALIGNANT DISEASES

Susan Kennedy and Kjell Torén

Injuries

Only limited statistics are available on accident rates in general in this industry. Compared to other manufacturing industries, the 1990 accident rate in Finland was below the average; in Canada, the rates from 1990 to 1994 were similar to other industries; in the United States, the 1988 rate was slightly above average; in Sweden and Germany, the rates were 25% and 70% above the average (ILO 1992; Workers’ Compensation Board of British Columbia 1995).

The most commonly encountered risk factors for serious and fatal accidents in the pulp and paper industry are the papermaking equipment itself and the extreme size and weight of pulp or paper bales and rolls. In a 1993 United States government study of occupational fatalities from 1979 to 1984 in pulp, paper and paperboard mills (US Department of Commerce 1993), 28% were due to workers being caught in or between rotating rolls or equipment (“nip-points”) and  18% were due to workers being crushed by falling or tumbling objects, especially rolls and bales. Other causes of multiple deaths included electrocution, hydrogen sulphide and other toxic gas inhalation, massive thermal/chemical burns and one case of heat exhaustion. The number of serious accidents associated with paper machines has been reported to decrease with the installation of newer equipment in some countries. In the converting sector, repetitive and monotonous work, and the use of mechanized equipment with higher speeds and forces, has become more common. Although no sector-specific data are available, it is expected that this sector will experience greater rates of over-exertion injuries associated with repetitive work.

Non-Malignant Diseases

The most well documented health problems encountered by pulp mill workers are acute and chronic respiratory disorders (Torén, Hagberg and Westberg 1996). Exposure to extremely high concentrations of chlorine, chlorine dioxide or sulphur dioxide may occur as a result of a leak or other process upset. Exposed workers may develop acute chemical-induced lung injury with severe inflammation of air passages and release of fluid into the air spaces, requiring hospitalization. The extent of damage depends on the duration and intensity of the exposure, and the specific gas involved. If the worker survives the acute episode, complete recovery may occur. However, in less intense exposure incidents (also usually as a result of process upsets or spills), acute exposure to chlorine or chlorine dioxide may trigger the subsequent development of asthma. This irritant-induced asthma has been recorded in numerous case reports and recent epidemiological studies, and current evidence indicates that it may persist for many years following the exposure incident. Workers similarly exposed who do not develop asthma may experience persistently increased nasal irritation, cough, wheezing and reduction in airflow rates. Workers most at risk for these exposure incidents include maintenance workers, bleach plant workers and construction workers at pulp mill sites. High levels of chlorine dioxide exposure also cause eye irritation and the sensation of seeing halos around lights.

Some mortality studies have indicated increased risk of death from respiratory disease among pulp mill workers exposed to sulphur dioxide and paper dust (Jäppinen and Tola 1990; Torén, Järvholm and Morgan 1989). Increased respiratory symptoms have also been reported in sulphite mill workers who are chronically exposed to low levels of sulphur dioxide (Skalpe 1964), although increased airflow obstruction is not normally reported among pulp mill populations in general. Symptoms of respiratory irritation are also reported by workers exposed to high air concentrations of terpenes in turpentine recovery processes often present at pulp mill sites. Soft paper dust has also been reported to be associated with increased asthma and chronic obstructive pulmonary disease (Torén, Hagberg and Westberg 1996).

Exposure to micro-organisms, especially around wood chip and waste piles, debarkers and sludge presses, creates an increased risk for hypersensitivity responses in the lungs. Evidence for this appears to be limited to isolated case reports of hypersensitivity pneumonitis, which can lead to chronic lung scarring. Bagassosis, or hypersensitivity pneumonitis associated with exposure to thermophylic micro-organisms and bagasse (a sugar cane by-product), is still seen in mills using bagasse for fibre.

Other respiratory hazards commonly encountered in the pulp and paper industry include stainless steel welding fumes and asbestos (see “Asbestos,” “Nickel” and “Chromium” elsewhere in the Encyclopaedia). Maintenance workers are the group most likely to be at risk from these exposures.

Reduced sulphur compounds (including hydrogen sulphide, dimethyl disulphides and mercaptans) are potent eye irritants and may cause headaches and nausea in some workers. These compounds have very low odour thresholds (ppb range) in individuals not previously exposed; however, among long-time workers in the industry, odour thresholds are considerably higher. Concentrations in the range of 50 to 200 ppm produce olfactory fatigue, and subjects can no longer detect the distinctive “rotten eggs” odour. At higher concentrations, exposure will result in unconsciousness, respiratory paralysis and death. Fatalities associated with exposure to reduced sulphur compounds in confined spaces have occurred at pulp mill sites.

Cardiovascular mortality has been reported to be increased in pulp and paper workers, with some exposure-response evidence suggesting a possible link with exposure to reduced sulphur compounds (Jäppinen 1987; Jäppinen and Tola 1990). However, other causes for this increased mortality may include noise exposure and shift work, both of which have been associated with increased risk for ischaemic heart disease in other industries.

Skin problems encountered by pulp and paper mill workers include acute chemical and thermal burns and contact dermatitis (both irritant and allergic). Pulp mill workers in kraft process mills frequently experience alkali burns to the skin as a result of contact with hot pulping liquors and calcium hydroxide slurries from the recovery process. Contact dermatitis is reported more frequently among paper mill and converting workers, as many of the additives, defoaming agents, biocides, inks and glues used in paper and paper-product making are primary skin irritants and sensitizers. Dermatitis may occur from exposure to the chemicals themselves or from handling freshly treated paper or paper products.

Noise is a significant hazard throughout the pulp and paper industry. The US Department of Labor estimated that noise levels over 85 dBA were found in over 75% of plants in the paper and allied products industries, compared to 49% of plants in manufacturing in general, and that over 40% of workers were exposed regularly to noise levels over 85 dBA (US Department of Commerce 1983). Noise levels around paper machines, chippers and recovery boilers tend to be well over 90 dBA. Conversion operations also tend to generate high noise levels. Reduction in worker exposure around paper machines is usually attempted by the use of enclosed control rooms. In converting, where the operator is usually stationed next to the machine, this type of control measure is seldom used. However where converting machines have been enclosed, this has resulted in decreased exposure to both paper dust and noise.

Excessive heat exposure is encountered by paper mill workers working in paper machine areas, with temperatures of 60°C being recorded, although no studies of the effects of heat exposure in this population are available in the published scientific literature.

CANCER

Kjell Torén and Kay Teschke

Exposures to numerous substances designated by the International Agency for Research on Cancer (IARC) as known, probable and possible carcinogens may occur in pulp and paper operations. Asbestos, known to cause lung cancer and mesothelioma, is used to insulate pipes and boilers. Talc is used extensively as a paper additive, and can be contaminated with asbestos. Other paper additives, including benzidine-based dyes, formaldehyde and epichlorohydrin, are considered probable human carcinogens. Hexavalent chromium and nickel compounds, generated in stainless-steel welding, are known lung and nasal carcinogens. Wood dust has recently been classified by IARC as a known carcinogen, based mainly on evidence of nasal cancer among workers exposed to hardwood dust (IARC, 1995). Diesel exhaust, hydrazine, styrene, mineral oils, chlorinated phenols and dioxins, and ionizing radiation are other probable or possible carcinogens which may be present in mill operations.

Few epidemiological studies specific to pulp and paper operations have been conducted, and they indicate few consistent results. Exposure classifications in these studies have often used the broad industrial category “pulp and paper”, and even the most specific classifications grouped workers by types of pulping or large mill areas. The three cohort studies in the literature to date involved fewer than 4,000 workers each. Several large cohort studies are currently under way, and IARC is coordinating an international multicentric study likely to include data from more than 150,000 pulp and paper workers, allowing much more specific exposure analyses. This article will review the available knowledge from studies published to date. More detailed information may be obtained from earlier published reviews by IARC (1980, 1987, and 1995) and by Torén, Persson and Wingren (1996). Results for lung, stomach and haematological malignancies are summarized in table 72.6 .

Table 72.6 Summary of studies on lung cancer, stomach cancer, lymphoma  and leukaemia in pulp and paper workers

Process  description

Location  of study

Type of  study

Lung  cancer

Stomach  cancer

Lymphoma  NHL/HD§

Leukaemia

Sulphite

Finland

C

0.9

1.3

X/X

X

Sulphite

USA

C

1.1

0.7

0.9

Sulphite

USA

C

0.8

1.5

1.3/X

0.7

Sulphite

USA

PM

0.9

2.2*

2.7*/X

1.3

Sulphate

Finland

C

0.9

0.9

0/0

X

Sulphate

USA

C

0.8

1.0

2.1/0

0.2

Sulphate

USA

PM

1.1

1.9

1.1/4.1*

1.7

Chlorine

Finland

C

3.0*

Sulphite/paper

Sweden

CR

2.8*

Paper dust

Canada

CR

2.0*

Paper mill

Finland

C

2.0*

1.7

X/X

Paper mill

Sweden

C

0.7*

Paper mill

USA

C

0.8

2.0

2.4

Paper mill

Sweden

CR

1.6

Paper mill

USA

PM

1.3

0.9

X/1.4

1.4

Board mill

Finland

C

2.2*

0.6

X/X

X

Power plant

Finland

C

0.5

2.1

Maintenance

Finland

C

1.3

0.3*

1.0/X

1.5

Maintenance

Sweden

CR

2.1*

0.8

Pulp and paper

USA

C

0.9

1.2

0.7/X

1.8

Pulp and paper

USA

C

0.8

1.2

1.7/X

0.5

Pulp and paper

Sweden

CR

0.8

1.3

1.8

1.1

Pulp and paper

Sweden

CR

2.2/0

Pulp and paper

Sweden

CR

1.1

0.6

Pulp and paper

USA

CR

1.2*

Pulp and paper

USA

CR

1.1

Pulp and paper

USA

CR

—/4.0

Pulp and paper

Canada

PM

1.2

3.8*/—

Pulp and paper

USA

PM

1.5*

0.5

4.4/4.5

2.3

Pulp and paper

USA

PM

0.9

1.7*

1.6/1.0

1.1

Pulp and paper

USA

PM

0.9

1.2

1.5/1.9*

1.4

Pulp and paper

USA

PM

1.7*

1.4

1.6*

C = cohort study, CR = case-referent study, PM = proportionate mortality study.  * Statistically significant. § = Where separately reported, NHL = non Hodgkin lymphoma and  HD = Hodgkin’s disease. X = 0 or 1 case reported, no risk estimate calculated, — = No data reported.

A risk estimate exceeding 1.0 means the risk is increased,  and a risk estimate below 1.0 indicates decreased risk.

Source: Adapted from Torén, Persson and Wingren 1996.

Respiratory System Cancers

Maintenance workers in paper and pulp mills experience an increased risk of lung cancer and malignant mesotheliomas, probably because of their exposure to asbestos. A Swedish study showed a threefold increased risk of pleural mesothelioma among pulp and paper workers (Malker et al. 1985). When the exposure was further analysed, 71% of the cases had been exposed to asbestos, the majority having worked in mill maintenance. Elevations in lung cancer risk among maintenance workers have also been shown in Swedish and Finnish pulp and paper mills (Torén, Sällsten and Järvholm 1991; Jäppinen et al. 1987).

In the same Finnish study, a twofold increased risk of lung cancer was also observed among both paper mill and board mill workers. The investigators made a subsequent study restricted to pulp mill workers exposed to chlorine compounds, and found a threefold increased risk of lung cancer.

Few other studies of pulp and paper workers have shown increased risks for lung cancer. A Canadian study showed an increased risk among those exposed to paper dust (Siemiatycki et al. 1986), and US and Swedish studies showed increased risks among paper mill workers (Milham and Demers 1984; Torén, Järvholm and Morgan 1989).

Gastro-intestinal Cancers

Increased risk of stomach cancer has been indicated in many studies, but the risks are not clearly associated with any one area; therefore the relevant exposure is unknown. Socio-economic status and dietary habits are also risk factors for stomach cancer, and might be confounders; these factors were not taken into account in any of the studies reviewed.

The association between gastric cancer and pulp and paper work was first seen in a US study in the 1970s (Milham and Demers 1984). The risk was found to be even higher, nearly doubled, when sulphite workers were examined separately. US sulphite and groundwood workers were also found in a later study to run an increased risk of stomach cancer (Robinson, Waxweiller and Fowler 1986). A risk of the same magnitude was found in a Swedish study among pulp and paper mill workers from an area where only sulphite pulp was produced (Wingren et al. 1991). American paper, paperboard and pulp mill workers in New Hampshire and Washington state ran an increased mortality from stomach cancer (Schwartz 1988; Milham 1976). The subjects were probably a mixture of sulphite, sulphate and paper mill workers. In a Swedish study, threefold increased mortality due to stomach cancer was found in a group comprising sulphite and paper mill workers (Wingren, Kling and Axelson 1985). The majority of pulp and paper studies reported excesses of stomach cancer, though some did not.

Due to the small number of cases, most studies of other gastrointestinal cancers are inconclusive. An increased risk of colon cancer among workers in the sulphate process and in paper board production has been reported in a Finnish study (Jäppinen et al. 1987), as well as among US pulp and paper workers (Solet et al. 1989). The incidence of biliary tract cancer in Sweden between 1961 and 1979 was linked with occupational data from the 1960 National Census (Malker et al. 1986). An increased incidence of cancer of the gallbladder among male paper mill workers was identified. Increased risks of pancreatic cancer have been observed in some studies of paper mill workers and sulphite workers (Milham and Demers 1984; Henneberger, Ferris and Monson 1989), as well as in the broad group of pulp and paper workers (Pickle and Gottlieb 1980; Wingren et al. 1991). These findings have not been substantiated in other studies.

Haematological Malignancies

The issue of lymphomas among pulp and paper mill workers was originally addressed in a US study from the 1960s, where a fourfold increased risk of Hodgkin’s disease was found among pulp and paper workers (Milham and Hesser 1967). In a subsequent study, the mortality among pulp and paper mill workers in the state of Washington between 1950 and 1971 was investigated, and a doubled risk of both Hodgkin’s disease and multiple myeloma was observed (Milham 1976). This study was followed by one analysing mortality among pulp and paper union members in the United States and Canada (Milham and Demers 1984). It showed almost a threefold increased risk for lymphosarcoma and reticulum cell sarcoma among sulphite workers, while sulphate workers had a fourfold increased risk of Hodgkin’s disease. In a US cohort study, sulphate workers were observed to have a twofold risk of lymphosarcoma and reticulosarcoma (Robinson, Waxweiller and Fowler 1986).

In many of the studies where it was possible to investigate the occurrence of malignant lymphomas, an increased risk has been found (Wingren et al. 1991; Persson et al. 1993). Since the increased risk occurs both in sulphate and sulphite mill workers, this points towards a common source of exposure. In the sorting and chipping departments, the exposures are rather similar. The workforce is exposed to wood dust, terpenes and other extractable compounds from the wood. In addition, both pulping processes bleach with chlorine, which has the potential to create chlorinated organic by-products, including small amounts of dioxins.

Compared with lymphomas, studies on leukaemias show less consistent patterns, and the risk estimates are lower.

Other Malignancies

Among US paper mill workers with presumed exposure to formaldehyde, four cases of urinary tract cancer were found after 30 years’ latency, although only one was expected (Robinson, Waxweiller and Fowler 1986). All of these individuals had worked in the paper-drying areas of the paper mills.

In a case-control study from Massachusetts, central nervous system tumours in childhood were associated with an unspecified paternal occupation as a paper and pulp mill worker (Kwa and Fine 1980). The authors regarded their observation as a random event. However, in three subsequent studies, increased risks were also found (Johnson et al. 1987; Nasca et al. 1988; Kuijten, Bunin and Nass 1992). In studies from Sweden and Finland, two- to threefold increased risks of brain tumours were observed among pulp and paper mill workers.

ENVIRONMENTAL AND PUBLIC HEALTH ISSUES

Anya Keefe and Kay Teschke

Because the pulp and paper industry is a large consumer of natural resources (i.e., wood, water and energy), it can be a major contributor to water, air and soil pollution problems and has come under a great deal of scrutiny in recent years. This concern appears to be warranted, considering the quantity of water pollutants generated per tonne of pulp (e.g., 55 kg of biological oxygen demand, 70 kg of suspended solids, and up to 8 kg of organochlorine compounds) and the amount of pulp produced globally on an annual basis (approximately 180 million tonnes in 1994). In addition, only about 35% of used paper is recycled, and waste paper is a major contributor to total worldwide solid waste (about 150 million of 500 million tonnes annually).

Historically, pollution control was not considered in the design of pulp and paper mills. Many of the processes used in the industry were developed with little regard for minimizing effluent volume and pollutant concentration. Since the 1970s, pollution abatement technologies have become integral components of mill design in Europe, North America and other parts of the world. Figure 71.12  illustrates trends over the period 1980 to 1994 in Canadian pulp and paper mills in response to some of these environmental concerns: increased use of wood waste products and recyclable paper as fibre sources; and decreased oxygen demand and chlorinated organics in wastewater.

Figure 72.12 Environmental indicators in Canadian pulp and paper mills, 1980 to 1994, showing use of  wood waste and recyclable paper in production, and biological oxygen demand (BOD) and  organochlorine compounds (AOX) in wastewater effluent

This article discusses the major environmental issues associated with the pulp and paper process, identifies the sources of pollution within the process and briefly describes control technologies, including both external treatment and in-plant modifications. Issues arising from wood waste and anti-sapstain fungicides are dealt with in more detail in the chapter Lumber.

Air Pollution Issues

Air emissions of oxidized sulphur compounds from pulp and paper mills have caused damage to vegetation, and emissions of reduced sulphur compounds have generated complaints about “rotten egg” odours. Studies among residents of pulp mill communities, in particular children, have shown respiratory effects related to particulate emissions, and mucous membrane irritation and headache thought to be related to reduced sulphur compounds. Of the pulping processes, those with the greatest potential to cause air pollution problems are chemical methods, in particular kraft pulping.

Sulphur oxides are emitted at the highest rates from sulphite operations, especially those using calcium or magnesium bases. The major sources include batch digester blows, evaporators and liquor preparation, with washing, screening and recovery operations contributing lesser amounts. Kraft recovery furnaces are also a source of sulphur dioxide, as are power boilers which use high-sulphur coal or oil as fuel.

Reduced sulphur compounds, including hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide, are almost exclusively associated with kraft pulping, and give these mills their characteristic odour. The major sources include the recovery furnace, digester blow, digester relief valves, and washer vents, though evaporators, smelt tanks, slakers, the lime kiln and waste water may also contribute. Some sulphite operations use reducing environments in their recovery furnaces and may have associated reduced sulphur odour problems.

Sulphur gases emitted by the recovery boiler are best controlled by reducing emissions at the source. Controls include black liquor oxidation, reduction in liquor sulphidity, low-odour recovery boilers and proper operation of the recovery furnace. Sulphur gases from digester blow, digester relief valves and liquor evaporation can be collected and incinerated - for example, in the lime kiln. Combustion flue gases can be collected using scrubbers.

Nitrogen oxides are produced as products of high-temperature combustion, and may arise in any mill with a recovery boiler, power boiler or lime kiln, depending on the operating conditions. The formation of nitrogen oxides can be controlled by regulating temperatures, air-fuel ratios and residence time in the combustion zone. Other gaseous compounds are minor contributors to mill air pollution (e.g., carbon monoxide from incomplete combustion, chloroform from bleaching operations, and volatile organics from digester relief and liquor evaporation).

Particulates arise mainly from combustion operations, though smelt-dissolving tanks can also be a minor source. More than 50% of pulp mill particulate is very fine (less than 1 mm in diameter). This fine material includes sodium sulphate (Na2SO4) and sodium carbonate  (Na2CO3) from recovery furnaces, lime kilns and smelt-dissolving tanks, and NaCl from burning by-products of logs which have been stored in salt water. Lime kiln emissions include a significant amount of coarse particulates due to entrainment of calcium salts and sublimation of sodium compounds. Coarse particulate may also include fly ash and organic combustion products, especially from power boilers. Reduction of particulate concentrations can be achieved by passing flue gases through electrostatic precipitators or scrubbers. Recent innovations in power boiler technology include fluidized bed incinerators which burn at very high temperatures, result in more efficient energy conversion, and allow burning of less uniform wood waste.

Water Pollution Issues

Contaminated wastewater from pulp and paper mills can cause death of aquatic organisms, allow bioaccumulation of toxic compounds in fish, and impair the taste of downstream drinking water. Pulp and paper wastewater effluents are characterized on the basis of physical, chemical or biological characteristics, with the most important being solids content, oxygen demand and toxicity.

The solids content of wastewater is typically classified on the basis of the fraction that is suspended (versus dissolved), the fraction of suspended solids that is settleable, and the fractions of either that are volatile. The settleable fraction is the most objectionable because it may form a dense sludge blanket close to the discharge point, which rapidly depletes dissolved oxygen in the receiving water and allows the proliferation of anaerobic bacteria which generate methane and reduced sulphur gases. Although non-settleable solids are usually diluted by the receiving water and are therefore of less concern, they may transport toxic organic compounds to aquatic organisms. Suspended solids discharged from pulp and paper mills include bark particles, wood fibre, sand, grit from mechanical pulp grinders, papermaking additives, liquor dregs, by-products of water treatment processes and microbial cells from secondary treatment operations.

Wood derivatives dissolved in the pulping liquors, including oligosaccharides, simple sugars, low-molecular-weight lignin derivatives, acetic acid and solubilized cellulose fibres, are the main contributors to both biological oxygen demand (BOD) and chemical oxygen demand (COD). Compounds which are toxic to aquatic organisms include chlorinated organics (AOX; from bleaching, especially kraft pulp); resin acids; unsaturated fatty acids; diterpene alcohols (especially from debarking and mechanical pulping); juvabiones (especially from sulphite and mechanical pulping); lignin degradation products (especially from sulphite pulping); synthetic organics, such as slimicides, oils and greases; and process chemicals, papermaking additives and oxidized metals. The chlorinated organics have been of particular concern, because they are acutely toxic to marine organisms and may bioaccumulate. This group of compounds, including the polychlorinated dibenzo-p-dioxins, have been the major impetus for minimizing chlorine use in pulp bleaching.

The amount and sources of suspended solids, oxygen demand and toxic discharges are process-dependent (table 72.7). Due to the solubilization of wood extractives with little or no chemical and resin acid recovery, both sulphite and CTMP pulping generate acutely toxic effluents with high BOD. Kraft mills historically used more chlorine for bleaching, and their effluents were more toxic; however, effluents from kraft mills which have eliminated Cl2 in bleaching and use secondary treatment typically exhibit little acute toxicity if any, and subacute toxicity has been greatly reduced.

Table 72.7 Total suspended solids and BOD associated with the untreated (raw) effluent  of various pulping processes

Pulping Process

Total Suspended Solids (kg/tonne)

BOD (kg/tonne)

Groundwood

50–70

10–20

TMP

45–50

25–50

CTMP

50–55

40–95

Kraft, unbleached

20–25

15–30

Kraft, bleached

70–85

20–50

Sulphite, low-yield

30–90

40–125

Sulphite, high-yield

90–95

140–250

De-inking, non-tissue

175–180

10–80

Waste paper

110–115

5–15

Suspended solids have become less of a problem because most mills utilize primary clarification (e.g., gravity sedimentation or dissolved air flotation), which removes 80 to 95% of the settleable solids. Secondary wastewater treatment technologies such as aerated lagoons, activated sludge systems and biological filtration are used for reducing BOD, COD and chlorinated organics in the effluent.

In-plant process modifications to reduce settleable solids, BOD and toxicity include dry debarking and log conveying, improved chip screening to allow uniform cooking, extended delignification during pulping, changes to digestion chemical recovery operations, alternative bleaching technologies, high-efficiency pulp washing, fibre recovery from whitewater and improved spill containment. However, process upsets (particularly if they result in intentional sewering of liquors) and operational changes (particularly the use of unseasoned wood with a higher percentage of extractives) may still cause periodic toxicity breakthroughs.

A relatively recent pollution control strategy to eliminate water pollution entirely is the “closed mill” concept. Such mills are an attractive alternative in locations that lack large water sources to act as process-supply or effluent-receiving streams. Closed systems have been successfully implemented in CTMP and sodium-base sulphite mills. What distinguishes closed mills is that liquid effluent is evaporated and the condensate is treated, filtered, then reused. Other features of closed mills are enclosed screen rooms, counter-current washing in the bleach plant, and salt control systems. Although this approach is effective at minimizing water pollution, it is not yet clear how worker exposures will be affected by concentrating all contaminant streams within the mill. Corrosion is a major issue facing mills using closed systems, and bacteria and endotoxin concentrations are increased in recycled process water.

Solids Handling

The composition of solids (sludges) removed from liquid effluent treatment systems varies, depending on their source. Solids from primary treatment principally consist of cellulose fibres. The major component of solids from secondary treatment is microbial cells. If the mill uses chlorinated bleaching agents, both primary and secondary solids may also contain chlorinated organic compounds, an important consideration in determining the extent of treatment required.

Prior to disposal, sludges are thickened in gravity sedimentation units and mechanically dewatered in centrifuges, vacuum filters or belt or screw presses. Sludges from primary treatment are relatively easy to dewater. Secondary sludges contain a large quantity of intracellular water and exist in a matrix of slime; therefore they require the addition of chemical flocculants. Once sufficiently dewatered, sludge is disposed of in land-based applications (e.g., spread on arable or forested land, used as compost or as a soil conditioner) or incinerated. Although incineration is more costly and can contribute to air pollution problems, it may be advantageous because it can destroy or reduce toxic materials (e.g., chlorinated organics) that could create serious environmental problems if they were to leach into the groundwater from land-based applications.

Solid wastes can be generated in other mill operations. Ash from power boilers can be used in road beds, as construction material and as a dust suppressant. Waste from lime kilns can be used to modify soil acidity and improve soil chemistry.

REFERENCES

Canadian Pulp and Paper Association. 1995. Reference Tables 1995. Montreal, PQ: CPPA.

Food and Agriculture Organization (FAO) of the United Nations. 1995. Pulp and Paper Capacities, Survey 1994-1999. Rome: FAO.

Henneberger, PK, JR Ferris, and RR Monson. 1989. Mortality among pulp and paper workers in Berlin. Br J Ind Med 46:658-664.

International Agency on the Research of Cancer (IARC). 1980. Monographs on the Evaluation of Carcinogenic Risks to Humans: Wood, Leather and Some Associated Industries. Vol. 25. Lyon: IARC.

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Jäppinen, P. 1987. Exposure to Compounds, Cancer Incidence and Mortality in the Finnish Pulp and Paper Industry. Thesis, Helsingfors, Finland.

Jäppinen, P and S Tola. 1990. Cardiovascular mortality among pulp mill workers. Br J Ind Med 47:259-261.

Jäppinen, P, T Hakulinen, E Pukkala, S Tola, and K Kurppa. 1987. Cancer incidence of workers in the Finnish pulp and paper industry. Scand J Work Environ Health 13:197-202.

Johnson, CC, JF Annegers, RF Frankowski, MR Spitz, and PA Buffler. 1987. Childhood nervous system tumors—An evaluation of the association with paternal occupational exposure to hydrocarbons. Am J Epidemiol 126:605-613.

Kuijten, R, GR Bunin, and CC Nass. 1992. Parental occupation and childhood astrocytoma: Results of a case-control study. Cancer Res 52:782-786.

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Malker, HSR, JK McLaughlin, BK Malker, NJ Stone, JA Weiner, JLE Ericsson, and WJ Blot. 1985. Occupational risks for pleural mesothelioma in Sweden, 1961-1979. J Natl Cancer Inst 74:61-66.

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Milham, SJ and P Demers. 1984. Mortality among pulp and paper workers. J Occup Med 26:844-846.

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Schwartz, B. 1988. A proportionate mortality ratio analysis of pulp and paper mill workers in New Hampshire. Br J Ind Med 45:234-238.

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Workers’ Compensation Board of British Columbia. 1995. Personal communication.

OTHER RELEVANT READINGS

Bascom, R and P Raford. 1994. Upper airways disorders. In Textbook of Clinical Occupational and Environmental Medicine, edited by L Rosenstock and MR Cullen. Philadelphia: WB Saunders Co.

Bernhart, S. 1994. Irritant bronchitis. In Textbook of Clinical Occupational and Environmental Medicine, edited by L Rosenstock and MR Cullen. Philadelphia: WB Saunders Co.

Chan-Yeung, M and J Malo. 1995. Forestry products. In Occupational and Environmental Respiratory Disease, edited by P Harber, MB Schenker and JR Balmes. St. Louis: Mosby-Yearbook Inc.

Food and Agriculture Organization (FAO) of the United Nations. 1994. 1993 Yearbook of Forest Products. Rome: FAO.

Rix, BA and E Lynge. 1996. Industrial hygiene measurements in a new industry: The repulping and deinking of paper waste. Am J Ind Med 30:142-147.

Schwartz, DA. 1994. Acute inhalational injury. In Textbook of Clinical Occupational and Environmental Medicine, edited by L Rosenstock and MR Cullen. Philadelphia: WB Saunders Co.

Smook, GA. 1989. Handbook for Pulp and Paper Technologists. Atlanta, GA: Technical Association for the Pulp and Paper Industry.

Springer, AM. 1986. Industrial Environmental Control Pulp and Paper Industry. New York: John Wiley and Sons.

United Nations. 1995. Statistical Yearbook, 1993. New York: UN.