Process chemicals are all additives which are used to solve or prevent problems in the paper manufacturing process, to improve its efficiency and/or to provide eco¬logical advantages. For example these additives allow reduction in the consump¬tion of fresh water and energy, the prevention of foam and deposits, the improve¬ment of drainage and/or reduction in fiber losses. Their proportion related to all chemical additives is only 10 % (Fig. 3.3).
Retention and Drainage Aids (RDA)
The term retention refers to the holding back of the papermaking stock on the wire during sheet formation and dewatering. The fibers are retained on the wire better than the fillers and the fines, which may be washed through the wire and even through the fiber mat formed. The most common retention parameters can be defined as first-pass retention (FPR) and overall retention. First pass retention is the ratio of the amount of solid material that leaves the headbox slice to the amount of solid material that is contained in the paper web leaving the couch roll (typical values are 40–70 %). Overall retention is the ratio of the amount of solid material that is sent to the wet-end of a paper machine to the amount that goes onto the reel at the dry end of the machine (typical values are 90–98 %).
126.96.36.199 Retention Aids
These increase adsorption of pigments (fillers) and fine particles onto the fibers so that they are retained with the fibers. This adsorption must be able to resist the high shear forces which arise in pipes and different apparatuses of modern high-speed paper machines. In the early days of papermaking, common retention aids were based on alum, which neutralizes charges on the furnish components. Later, single polymers such as PEI (polyethylenimine) were introduced with patching as the dominant mechanism. In single polymer and dual polymer systems (e. g. PEI + PAM), high molecular weight polymers like PAM (polyacrylamides) cause bridg¬ing – which is considered to be the major mechanism.
However, the latest develop¬ments, micro- and nanoparticle systems (e. g. PAM/bentonite, PEI/PAM/bento-nite, silica gel/PAM), follow a complex flocculation mechanism. Since their in¬troduction in the early 1980s, the latter systems have been successfully applied. Recently, organic microparticle systems have been introduced. Systems based on polyethylene oxide (PEO) and phenolic resin or polyvinylformamide (PVF) copoly¬mers and polyvinglamine (PVAm) function according to the network flocculation mechanism or at least by hydrogen bond interactions.
Retention aids can be divided into three main product groups according to their chemical composition: inorganic salts such as aluminum sulfate and polyalumi¬num chloride (PAC), natural polymers such as cationic starch, and synthetic poly¬mers, high-molar-mass polyelectrolytes (Fig. 3.16). The most important members of the last group are cationic, anionic or nonionic polyacrylamides and modified polyethylenimines.
There are also many smaller groups of products such as vari¬ous polyamines, poly-DADMAC and, most recently, polyvinylamines. The above mentioned microparticle systems need, as a further compound, inorganic particles like bentonite or silica gel. The worldwide consumption of RDAs is 2 V 106 tons sizing, where alum is needed, still plays a very important role, leading to the market share for aluminium compounds being about 50 %.
Cationic starch has a market share of 36 % or 720 000 tonnes p.a. Again, it must be remembered that cationic starch is also used to increase the dry strength of paper as well as to boost retention. The market share for synthetic polymers is 6 % or 120 000 tonnes p.a. This figure seems very low, but the high efficiency of these polymers means that they only need to be applied at rates as low as 100–500 g solids per tonne of paper. From the point of view of their chemistry, all retention aids are polymers, even aluminum sulfate and PAC are present in the form of polynuclear complexes, the structure of which depends on the pH. Polyacrylamide and polyethylenimine illus¬trate the common underlying chemical principle of all these products: a polar structure with hydrogen bonds and ionic bonds and a high molar mass (Fig. 3.17).
188.8.131.52 Drainage Aids
These are chemical additives that improve the dewatering of the paper web along the process, in forming, pressing and drying. The potential benefits include in¬creased machine speed and higher production rate, improved formation, and lower dryer steam consumption. Improved drainage can lead to decreased headbox consistency. This, in turn, decreases fiber flocculation. In general, most of the agents that serve as retention aids and charge neutralizers can also be used as drainage aids, but their charge density (meq g–1) and their molar mass (g mol–1) in interaction with the other wet-end conditions influence their drainage efficiency significantly. Therefore drainage aids are mainly based on the same chemistry as retention aids and are expressed in the common term RDA. The specific benefits that can be obtained can be illustrated with reference to a modified polyethyleni¬mine with a charge density of 6.5 meq g–1 and a molar mass of 2 V 106 gmol–1. Besides the improvement of fiber, fines and filler retention on the wire, the drain¬age acceleration on the wire and especially the water release in the press and dryer section lead to an increase of 3 to 10 % in productivity and a similar saving of specific energy.
RDAs also act in synergy with other chemical additives such as deaerators, which boosts the overall performance of other wet-end chemicals and reduces their consumption. With improved retention as well as with the fixation of colloi¬dal dissolved substances much cleaner white-water is obtained with a COD reduc¬tion and over all less deposits are achieved. Consequently a higher degree of proc¬ess water closure, less volume of waste water and an improved PM runnability can be obtained. The waste water is also more easily treated.
In the production and processing of chemical, semichemical and mechanical pulps and recovered paper, various inorganic and organic substances are accumu¬lated in dissolved or colloidal dissolved form. Other water-soluble substances enter with the fresh water, fillers, recycled uncoated and coated paper broke and also by chemical additives. As the process water circuits are increasingly closed, the con¬centration of these water-soluble and colloidal substances and finely dispersed particles increases considerably and an additional contaminant load is thus im¬posed on the waste water. These substances interfere with the production process by increasing build-ups and deposits, they reduce the efficiency of the chemical additives, and impair the quality of the produced paper. Therefore these sub¬stances are also classed as detrimental substances (Fig. 3.18).
The use of highly cationic polymers is a common possibility to remove the load of dissolved anionic substances by complex formation, fixation to the fibers and subsequent discharge with the freshly produced paper. Besides the dissolved an¬ionic substances there is a variety of nondissolved, hydrophobic substances (parti¬cles) mainly coming from the raw materials, e. g. wood extractives, coating ad¬ditives, adhesives. By fixation of the particles to the fibers they can be removed from the system before they build uncontrolled reactions with the paper stock and chemical additives or before they have a chance to form sticky agglomerates. The most commonly used fixing agents in the paper making process can be divided into six categories (Table 3.8).
Apart from their chemical nature and molecular structure, these polymers mainly differ through their charge density and molec¬ular weight. In general it can be stated that polymers with a combination of high cationic charge density and high molecular weight perform best in terms of elim¬inating dissolved anionic material from a contaminated system. However, the den¬sity of accessible anionic charge, located on the surface of white pitch particles, is usually much lower than the accessible anionic charge density of dissolved sub¬stances.
Therefore a fixing reacting exclusively by a charge mechanism, is often not efficient in controlling the bonding strength between white pitch particles and fibers and minimizing the formation of white pitch agglomerates. Besides the charge density, there is another molecular parameter which can be used to control the bonding strength between fibers and particles. The modification of cationic polymers with nonpolar hydrophobic groups is likely to influence the interaction of the polymer with the hydrophobic particles. The modification of a partly hydro¬lysed PVAm (DH=30 %) and a PEI with hydrophobic functional groups is depicted schematically in Fig. 3.19.
In many cases, the use of a fixing agent in a mill has to be seen in combination with other chemical additives e. g. retention aid, sizing agent, dry strength resin, where their effectiveness will be significantly improved by the fixative.
The most popular tests for quantifying the general contamination of a paper machine system are COD (chemical oxygen demand), cationic demand and con¬ductivity. For specific insights regarding pitch contaminants there are other useful tests. Turbidity of stock filtrate can serve to quantify the presence of colloidal dis¬solved material. Soxhlet extraction with dichloromethane (DCM) is commonly used for quantitative pitch analysis of deposit and paper samples. The hemacyt¬ometer is frequently used to measure dispersed pitch.
An optical laser counter can be used to measure particle size and volume of hydrophobic particles. Information about the deposition tendency of hydrophobic particles, untreated and treated with fixing agents, can be evaluated by the impinging jet method in which the filtrate of a stock or white water sample is pumped at a certain speed through an impinging jet cell. The deposition of the particles will be measured by the surface coverage on the collector plate after a defined time period.
Additives for Pitch and Deposit Control
The very heterogeneous compounds of detrimental substances which can be pre¬sent in the papermaking process, are the reason why only a part of the pitch and deposit problems can be solved by fixing agents. Besides the dissolved anionic substances there is a variety of nondissolved, hydrophobic substances (particles) coming from many different sources and determined by the raw materials used
. • rosin and wood extractives from high yield chemical and mechanical pulps (wood pitch)
. • binders and coating additives from recycled coated broke (white pitch)
. • adhesives and hot melts from recovered papers (stickys).
These substances affect paper machine runnability and paper quality. They can be found on paper machine wires, felts, vacuum boxes, dryer cans, calender rolls and in the finished paper. The consequences vary from early replacement of wire and felts, to web breaks, dirt and even holes in the finished paper. In order to under¬stand the mechanism of fixation for particles within this size range, one has to understand the role of shear forces acting on particles fixed to the fiber. When a particle is fixed to a fiber, it can be detached by shear forces and pushed back into the system. Whether the particle is detached or not depends on the balance be¬tween the strength of the fixing bond and the shear force. For a given strength of fixing bond larger particles will be detached more easily then smaller particles.
Taking into account the collision probabilities of particles and fibers under prac¬tical conditions, one can imagine that tear off and fixation take place numerous times before the particle finally gets fixed. For particles undergoing several succes¬sive fixation and tear-off cycles, a net transfer of cationic polymer from the fiber surface onto the particle surface will take place (Fig. 3.20). When the amount of transferred cationic polymer is high enough, this will lead to destabilisation. Even¬tually collisions between destabilised particles will occur and they will form even larger aggregates. As a result, poor fixing performance is observed when particle size increases.
In practice large aggregates will be mechanically entrapped in the fiber mat causing sheet defects and machine deposits. In order to avoid the forma¬tion of aggregates and the deposition of destabilised particles, one has to avoid polymer transfer between fibers and hydrophobic particles. Therefore the bonding strength between fibers and particles, as well as their colloidal stability, has to be controlled by adapting the chemistry of the fixing agents to the chemical and physicochemical characteristics of the white pitch particles.
The vast majority of disturbing particles, by number and by volume, can be found in the critical size range between 0.2 and 5 mm, where mechanical separa¬tion techniques like pressure screens and cleaners are no longer effective. Chem¬ical additives have different working mechanisms, and it is not always possible to predict which type of chemical is best suited to each case. Very often, it is worth¬while to carry out some laboratory tests before deciding to use a new chemical.
Anionic dispersants increase the colloidal stability by adsorption of anionic groups onto the particle surfaces. At the same time, the pitch particle dimensions are decreased and the possibility of building larger agglomerates decreases. Typical dispersants are polynaphthalene sulfonates and lignosulfonates. It should be noted that this mechanism is the opposite to the fixing of pitch onto the fibers. One consequence of using anionic dispersants is that, if fixing agents are used in a later stage, the amount of fixing agent to achieve the desired result has to be increased.
The use of aluminum sulfate (alum) is a classical way to solve pitch problems on paper machines. But only in an acid environment (pH 4.5–5.5) is alum a strongly cationic product that effects coagulation and fixation of dissolved and colloidal material onto the fiber. At higher pH (5.5–7), alum can be replaced by poly-alumi-num chloride. Alum lowers the pH, hence, the use of alum in the presence of calcium carbonate should be considered very carefully.
In addition to the fully synthetic fixatives (Section 3.7.2.), starch-based, semi¬synthetic strongly cationic polymers are available. Not only do these starch-based fixing agents have a better biodegradability, but they are also capable of forming hydrogen bonds from hydroxy groups in their anhydroglucose repeating units. In paper production, the hydroxy groups can form hydrogen bonds, not only with cellulose fibers but also with interfering low-molecular weight anionic carbohy¬drate compounds, thus decreasing the concentration of such substances in the water circuit .
To control pitch and other hydrophobic substances in the stock-water-system, adsorbents can also be used. These additives are pigments with a high specific surface area in water, like bentonites, micro crystalline talc or mica. The surface of the micro talc particles is hydrophobic and thus hydrophobic substances tend to adhere to it. The size of the particles to be adsorbed, however, must be smaller than that of the adsorbent used.
To prevent pitch or coating broke residues from adhering to felts, wires, or cylinders, it is possible to use some special wire or cylinder protective agents. These chemicals normally contain a cationic polymer and a surface active agent.
Use of the enzyme lipase has, in some cases, proved to be successful in the control of deposits. Lipase hydrolyzes triglycerides into fatty acid and glycerine, which are less hydrophobic and less inclined to build up tacky deposits. An addi¬tion of 3 ppm lipase calculated on pulp should be added to thick stock, e. g. ground¬wood, when about 70 % of the triglycerides will be hydrolyzed. The resultant fatty acids will be dispersed into the pulp and fixed onto the fibers with alum. Pitch deposits on the machine chest wall – which are very often a problem – will almost disappear and the amount of pitch deposits on wire, press, and dryer sections will decrease by 30 to 50 %.
Another possibility is to add a chemical for pitch removal in mechanical pulp production. Polyethyleneoxide (PEO) has been used for this purpose. PEO builds hydrogen bonds to pitch and has a natural affinity to pitch particles.
The first step of a pitch/deposit control program should be the evaluation of different treatment procedures in the laboratory with the test methods described in Section 3.7.2.
Slimicides and Biocides
Microbial problems in paper and board mills have significantly increased with the enclosure of white water systems to reduce the use of fresh water. Environmental constraints have also contributed to the need for reducing the amount of effluents. The recirculation of white waters has created serious slime problems because of the growth of microbes in the systems. Increased paper recycling levels increase the amount of nutrients in solid or dissolved form. It is noteworthy also that additives used in the production process themselves represent an ideal nutrient source (starch), or contain impurities with quite a lot of nutrient material (kaolin), for microbes.
The slurries of fillers and coating pigments can contain considerable amounts of phosphorus and nitrogen, which along with the carbon source are the principal nutrients for most microbes. The incoming raw water can also contain considerable amounts of nutrients for microbes. The amount of nutrients present varies to a marked extent according to the season of the year. Moreover, the tem¬perature in the process water circulation system has increased, thus providing a more suitable environment for the microbes.
The growth of microbes can cause many problems in the mills either in the paper product (spots, holes, spores, odor) or in the process (runnability, corrosion, deposits). Thus the microbiological state of the whole paper production process has to be under control. Problems caused by microbes can, to a large extent, be avoided by maintaining machine cleanliness and by taking notice of the microbe-favorable places in the local system as well as environmental factors. Equally im¬portant is the control of incoming raw materials and the purity of the chemicals used, and monitoring slurry preparation at the mill properly. Storage chests for coated broke require particular attention.
The first steps to minimize microbial growth should be good housekeeping, prevention of deposit formation and an evaluation of the design and size of stock and water chests and pipes. The whole volume of stock suspension in the stock preparation and storage plant should be as small as possible and there should be no corner without flow.
Despite proper attention to the items described above, conditions recorded as slime problems (caused by microbial growth together with organic and inorganic contaminants) occur and are then offset by dosage with biocides. Biocides and products called slimicides or microbicides are chemicals used to prevent the growth of microbes. A number of biocides with different active ingredients are available. The efficiency of some biocides is based on destroying the cell mem¬brane function, thereby inhibiting the metabolism and the growth of microbes.
Other types of biocides penetrate the membrane and react with essential cell com¬ponents (enzymes, proteins, etc.). The selection of the proper biocide for a partic¬ular use will depend on many factors, e. g., temperature, pH levels, and type and properties of the actual microbial fauna present which can even change from time to time. Typically, the major classes of biocides are organobromides, organosul¬furs, isothiatzolinones, thiocyanates, thiocarbamates, metallics (substances con¬taining copper and tin), chlorinated phenols, and phenates. Awareness of poten¬tially dangerous effects upon the environment is important, and slime control programs must be planned accordingly.
New types of biocides are being developed continuously to meet changing regulatory demands in regard to reduction of, e. g., toxicity and environmental impact. The main problems to note under practical conditions are environmental security, the manner of dosage (concentration, points of input in the system), and efficiency under process conditions. No one biocide will effectively kill all the bacteria present in the water system. This has led to the development of wide spectrum biocides.
Strategies used in biocidal control of microbiological slime problems include narrowing the spectrum of the bacterial population, thereby also reducing the formation of biofilms. To preclude the development of resistant microbe popula¬tions, a biocide should be periodically substituted by another one functioning by a different mechanism. The required dosage of biocides can be greatly reduced by determination and selection of the correct biocides for the particular species of microbes causing the problem. It should be kept in mind, however, that the re¬quired dosage can depend greatly upon pH. For example, thiocarbamates are effec¬tive in an acidic environment; however, as pH is raised toward neutral, the effect falls off dramatically due to the shorter half-life (about 18 h at pH 6 but less than 2 h at pH 7).
Knowledge of the composition of slime deposits and the formation mechanisms of biofilms has led to more selective slime control agents, in combination with the use of chemicals that are capable of either penetrating the biofilms or dispersing the deposits. So less toxic substances can be used in the system and keeping machine surfaces is easier.
In slime control without application of biocides, chemicals are avoided as far as possible. Some examples of elimination of slime without biocides are bacteriphage application, enzyme application and removal of nutrients. Slime-decomposing en¬zyme systems and specific viruses that kill specific bacteria have been developed. The Biochem method for example, uses modified lignosulfonate as a complex former. It neutralizes metabolites by electrostatic discharge, thereby making the nutrients nonusable by microbes. It also chelates essential trace elements present in the circuit system. Another product group to reduce deposit build-up, primarily microbiological slime, in the circuit system of a paper machine is the so-called “biological dispersing agents” or biodispersants.
Principally, they are combinations of surfactants that have been optimized to dissolve pockets of slime, but they are usually also capable of breaking up other (primarily hydrophobic) deposits. These products, which are sometimes also used in combination with enzymes, are based on natural terpenes (oil of oranges), paraffin, lignosulfanates, or various deter¬gents (tensides). Which product to use depends on the type of raw materials em¬ployed in the paper manufacturing process and the specific problem to be solved. If numerous microorganisms are introduced into the process (e. g. through the use of surface water), there is sometimes no alternative to the additional use of bio¬cides. Biodispersants are effective in three ways. First they infiltrate the deposit (the so-called “creep effect”), then break it apart. Finally, they envelop the deposit in a chemical coating (surface passivation). These processes can be observed by measuring the surface coating of piezoelectric crystals, for example, or on metallic surfaces introduced into the paper machine as testing probes and examined using an electron microscope.
Biocide research today has to consider the toxicological and environmental im¬pacts. The research therefore is focused on making biocides more effective at lower concentrations as well as on developing nontoxic and environmentally friendly products. In fact, many agents already known for years to be effective as biocides or as disinfectant chemicals, but considered to be costly, have been put into reuse. Examples are hydrogen peroxide, glutaraldehyde, ozone, and peracetic acid.
To control the microbiological situation, sometimes a full count of microbe spe¬cies at different locations is required, rather than determination of the presence of certain microbial groups like slime-forming species, fungi, yeasts, anaerobic bacte¬ria, etc. Conventional methods for identification of microbes include enumeration and screening of individual species isolated from count plates on selective media. Ready-made plates are available for the selection of different types of microbes. Commercial identification systems for industry and medical purposes, based on numerous biochemical tests or other characteristics, are available, including a da¬tabase that covers practically all groups of bacteria.
Defoamers and Deaerators
Foam arises from dispersed gas in a liquid in a ratio such that the bulk density of the mixture approaches that of a gas rather than a liquid. When foam collects at the air/liquid interface, it is called surface foam. When foam is mixed into the liquid and is only slightly or not visible on the surface, it is usually called entrained air. Along the production processes of pulp, paper and board more or less foam is built up and gas is entrained. The violent motion of water during filtration and drainage and in the white water system, especially at high paper machine speeds, produces a stable foam, in the presence of surface active agents and colloidal surface active film formers with a negative charge, such as hemicelluloses, proteins, and polysac¬charides. On the fiber surface there are always hydrophobic patches from wood resins, and positively charged patches, e. g. from alum, where the air/gas bubbles then accumulate (Fig. 3.21). The increased use of calcium carbonate as filler and coating pigment may lead to a further significant source of gas/foam by the de¬composition of CaCO3 to CO2
High air/gas contents may be the source of severe problems in both paper man¬ufacturing and paper coating and should be kept below 0.1 to 1.0 %, depending on the paper grade. Too high air/gas content in the headbox suspension has a negative effect on the retention of fibers, fines and fillers as well as on the paper formation and dewatering on the wire and press sections (Fig. 3.22). At high gas contents the number of pinholes (with their negative effect e. g. in coating and printing) and the average pore diameter of the paper sheet are increased, and also the surface rough¬ness. For all pumping and suction processes more energy will be consumed. Paper machine runnability, and consequently productivity, may be lowered. Also, some process chemicals, such as reductive bleaching agents and retention aids, perform significantly worse in the presence of entrained air and gas. In coating, high gas contents of the color give rise to runnability problems and quality loss.
Gas makes coating color foam in the machine circulation loop and leads to tank overflow and to an irregular pump/metering performance. The most evident negative phenome¬non is, however, the uncoated spots on the paper surface caused by the entrapped air. Together with an insufficient amount of applied color, gas bubbles are the main cause of such a coating defect called skipping. In paper coating, high gas contents are closely linked with the color formulation, color preparation and circulation system, the applicator system and the coater speed (see Sections 184.108.40.206.4.5 and 7.7.6). Problems of excessive gas contents also originate from the use of hydro¬phobic substances (talc) and stabilizer chemicals for synthetic binders as well as from certain chemical reactions e. g. decomposition of calcium carbonate at re¬duced pH. Deaeration by thermomechanical means only does not provide the desired persistency of gas-free stock.
Chemical deaerators permit steady and prolonged deaeration by triggering bub¬ble coalescence. This is achieved by means of hydrophilic emulsion particles, which penetrate the contaminated gas bubble surfaces, thereby facilitating their coalescence (Fig. 3.23). The difference in mechanism between deaeration by both thermomechanical and chemical means give the best results. The suitability of a defoamer and deaerator for certain mill conditions depends first on the hydro¬phobicity or hydrophilicity of its components. This means that a very hydrophobic product is a good “foam killer” and a more hydrophilic one is an efficient deaerator.
Defoamers and deaerators are derived from hydrocarbons that contain substi¬tuted polar groups. The active substances contained in products supplied in the form of 25–30 % aqueous emulsions are mainly higher fatty alcohols, fatty acids, and fatty acid esters and their ethoxylates (Table 3.9). They may contain anionic or nonionic emulsifiers. The active substances contained in so-called oil-type defoa¬mers are mainly fatty alcohol ethoxylates, fatty acid ethoxylates or mixtures of fatty alcohols. They can also contain emulsifiers in order to aid dispersion. It is im¬portant to note that the term oil-type defoamer refers to the oily consistency of this group of products, and has nothing to do with the use of mineral oil as an active substance. Emulsion-type defoamers account for half of the worldwide consump¬tion of defoamers and deaerators, expressed as solids. Synthetic oils represent 40 % and mineral oils 10 %. It seems that mineral oils are no longer in use in Europe.
Controlled deaeration of stock suspensions and coating colors requires regular measurement of the air/gas content. The mainly used methods are based on the compressibility of air/gas (e. g. Brecht-Kirchner equipment, EGT, Celleco). The measurement measures the change in the gas volume by the change in the applied pressure using a vacuum (expansion method) or using pressure (compression method). A continuous in-line measurement based on ultrasonics is also possible. In water dispersed little gas bubbles are excellent scatterers of sound. The on-line apparatus measures the ultrasonic damping.
Again, with increased usage of recovered paper, solid board and corrugated box board, enclosure of mill water systems, and use of plastic wires and felts, many new challenges have arisen for keeping the surfaces of chest walls, pipes, rolls, filters and drying cylinders and different paper machine clothings clean for opti¬mum production. As is shown in Fig. 3.24 there are different necessities for clean¬ing. The major cleaning application, 41 %, is so-called system cleaning e. g. boiling out the whole stock and white water system which is directly connected to the wire part of the paper machine. One third of the cleaner consumption is used for felt cleaning and 15 % for wire cleaning.
To prevent deposition or to remove deposits from wires and felts, so-called con¬ditioners are continuously applied. The following dirt and deposits occur:
. • inorganic substances e. g. lime and calcium sulfate • organic substances e. g. resin/pitch, adhesives
. • residues of chemical additives e. g. dyes, coating binders, starches, wet strength resins
. • microbiological substances, fines from paper stock
. • compounds of fresh process water e. g. humic acid, iron, manganese
Cleaning agents are characterised by a great number of commodities which can differ considerably in their chemical composition as well as in their cleaning effi¬ciency. In principal, the cleaning agents belonging to the group of process chem¬icals can be subdivided into inorganic and organic products. As inorganic cleaning agents, sodium hydroxide as an alkaline agent and hydrochloric, sulfuric and phos¬phoric acids as acid agents, are mostly used. The number of organic cleaning agents available on the market is much higher. Simple formulations in most cases consist of tensides and sodium hydroxide or inorganic acids respectively. In more complex mixtures dispersing agents, chelating agents and solvents can be found. Solvent cleaning agents mostly consist of aliphatic and aromatic hydrocarbons, in certain cases as mixtures with tensides.
In the German paper industry in 1999, an average of 0.35 kg of cleaners per ton of paper was consumed. The proportion of organic cleaning agents was around 60 %. Cleaning agents are applied continuously and, more frequently, discontinu¬ously. Referring to the mass balances of discontinuous cleaning of paper machine clothings with tenside containing acid and alkaline cleaning agents, the process water load resulting from the application in the production of corrugating papers bears no risk for anaerobic or for aerobic waste water treatment plants when the cleaning agents are used in an appropriate manner and the washing liquor is buffered sufficiently.
Flocculants and Coagulants for Clarification of Different Water Sources
In the production of pulp, paper and board a large amount of water is generally necessary. Therefore different quantities and portions of fresh water (taken from river/lake nearby or from fountains), and recycled process water have to be used. For high production efficiency and good paper quality, clean water is required. On the other hand the effluent has to be environmentally friendly and has to meet the official legislation.
Therefore all three water sources (fresh water, recycled process water, effluent) have to be treated and clarified, mechanical treatment (filtration, sedimentation, flotation) always having first priority over chemical treatment. In practice, in most cases, a combination of mechanical and chemical treatment leads to an efficient solution.
220.127.116.11 Fresh or Raw Water
This can be colored, as turbid surface water may contain humus as well as iron and manganese, either in the oxidized form or bound to organic matter. Water of this type is treated by aeration + coagulation + addition of alkali + clarification + sand filtration. The removal of color and organic compounds usually requires a method based on chemical precipitation. Some reduction in organic matter con¬tent, of course, can be achieved by simple filtration. The removal of iron and manganese, on the other hand, usually requires:
1. 1. Oxidation of iron (II) and manganese (II) to oxidation state III e. g. by chlorine or permanganate
2. 2. Hydrolysis of the trivalent cation to produce the hydrated hydroxide
3. 3. Coagulation of the hydrated hydroxide
4. 4. Removal of the coagulate.
The next step is usually chemically-induced coagulation and flocculation. The se¬quence of events is addition of chemical, rapid mixing, and finally gentle stirring. Here aluminum sulfate is one of the commonly used chemicals. A suitable rate of addition can be calculated from zeta potential measurements. Aluminum sulfate reacts with the alkaline compounds naturally present in the water and also with any added lime or sodium carbonate. Another commonly used coagulant is sodium aluminate. This is normally employed as a supplementary coagulant to ensure proper treatment of cold waters or to coagulate residual aluminum sulfate. Supple¬mentary coagulants are used in cases where floc formation would otherwise be unsatisfactory. Here the most common agents are activated silicic acid, certain nat¬ural organic compounds and synthetic polyelectrolytes, mainly modified polyacry¬lamides with high molecular mass.
Their ionogenic character depends on the individual mill conditions. Other chemicals used, mainly for pH adjustment, include certain calcium and sodium compounds. The addition rates of aluminum com¬pounds are in the range 10 to 100 g m–3 fresh water and those of polymers ca. 0.1 to 1 g m–3.
18.104.22.168 Recycled Process Water
Recycled process water allows one to reduce fresh water demand and to reduce the effluent loadings from paper and board manufacture. In papermaking water con¬sumption has been reduced dramatically in the last decades. Printing papers, for example, are manufactured at a specific fresh water consumption of about 8–10 m3 t–1, packaging papers at about 3 to 5 m3 t–1. Hereby, at least for graphic papers, solids and dissolved organic matter are removed. If lowering water con¬sumption down to 2–4 m3 t–1 salt concentrations have to be managed. In packag¬ing paper production e. g. for testliner and corrugated medium, in more and more cases fresh water consumption is down to the evaporated water in the dryer section of the paper machine. This means a specific fresh water consumption of 1–1.5 m3 t–1 and no more waste water to the effluent. These results can only be obtained by a very efficient mechanical and chemical treatment of the recirculated process water. To separate and remove solids and dissolved substances from this water, nonsalt building flocculants, e. g. polyethylenimine, polyamine and/or poly¬acrylamides, have to be used. The addition of these polyelectrolytes (solid) is in the range 10 to 100 g m–3 recycled process water.
In completely closed water circuits biological treatment (anaerobic-aerobic) re¬move dissolved organic matter even more efficiently. However, recycling the bio¬logically treated water can be restricted by the resulting reduction in product brightness, which does not matter for packaging papers. With nano- and ultra¬filtration very small solid particles and some dissolved high molecular mass mate¬rial are removed. For good results the surface chemistry during the manufacturing process has to be thoroughly watched. Internal process measures reduce loadings at source, and direct savings can be made in raw material costs (fiber, fillers, additives) and also in energy consumption.
Effluents from pulp, paper and board mills contain wood materials in solid, colloi¬dal and dissolved form. The effluents also contain some chemicals used in the production process, either in their original or modified form. The principal me¬chanical methods used to remove these compounds are filtration (including mem¬brane and nanotechnology), sedimentation and/or flotation. All these methods require very good flocculation and coagulation of these undesirable substances in the effluent. Only then can they be separated from the clear water. The main chemicals used for coagulation are aluminum salts (e. g. Al2(SO4)3), iron salts (e. g. FeCl3 or Fe2(SO4)3 or FeSO4) and lime with addition rates of 100–500 g m–3 efflu¬ent. To achieve optimum flocculation results, it is often necessary to feed a suitable polymer, e. g. modified polyacrylamide with high molecular mass and a certain charge density during slow mixing. Addition rates of the polyelectrolyte (solid) are 1–50 g m–3 effluent.
The primary purpose of chemical coagulation is to neutralize the electrical charges on the particles and hence prevent repulsion. This means adding metal cations because the suspended organic material is nearly always negatively charged. In most cases, the coagulant itself also precipitates (for exam¬ple, as the hydroxide), making the treatment more effective. For this reason, pH control is important. Organic polyelectrolytes, on the other hand, normally bind together dirt particles and in this way promote floc formation. Typical charge measurements are anionicity determination, zeta potential measurement, and cal¬culation of particle sizes. In effluent treatment, chemical coagulation is applied mainly to the separate treatment of concentrated effluent fractions (e. g., those from pulp bleaching, deinking of recovered printing papers, coating color prepara¬tion, and broke). The flocs formed are removed from the water by sedimentation, flotation, or filtration. The sludge produced is either fed back to stock preparation or after dewatering by polyelectrolyte addition and specific presses goes to in¬cineration, depending on the paper grade produced.
Biological treatment of the effluent is particularly useful for removal of low molec¬ular mass organic matter. The process is based on the ability of microbes to live and reproduce in effluents. In doing so, microbes break down dissolved and colloi¬dal substances by using them as nutrients. In this way, waste is converted partly into biomass and partly into carbon dioxide and water. For effective biological treatment pretreatment of the effluent may be necessary.
In particular, tempera¬ture, pH, and oxygen and nutrient contents must be suitable for microbial growth. For biological effluent treatment many different kinds of microbes, ranging from the simplest bacteria to protozoa and even worms, can be simultaneously used. Quite a large number of pulp, paper and board mills already use biological treat¬ment, very often an aerobic one, and, depending on the specific conditions, also a combination of anaerobic and aerobic treatments.
3.8 General Remarks on the Application of Chemical Additives
A range of chemical additives are used along the process chain of paper and board manufacturing and paper coating to provide technological, economical and eco¬logical advantages (Tables 3.10 and 3.11). They allow the reduction of fresh water and energy consumption, as well as enabling the paper industry to increase the recycling rate in paper and board production. Furthermore, savings in raw mate¬rial can be achieved by reducing the basis weight of paper without losing certain quality properties. The use of new chemical additives allows one to produce paper at neutral pH/slightly alkaline conditions instead of in an acid environment which was mainly in use until the 1970s. This results in less corrosion of machinery parts and also in improved paper properties, e. g. higher paper strength as well as much better permanence of the paper. Papermaking chemicals allow large sums of capital tied up in plant and equipment to be utilized more efficiently, and they can boost productivity by reducing downtime and increasing production rates. The productivity of subsequent coating and converting processes can also be improved by selecting chemical additives according to their specific properties.
Whether by chance or design, many papermaking c
hemicals have some useful side-effects in addition to the effects for which they have been primarily developed (Fig. 3.25). The intelligent combination of different chemical additives can im¬prove quality, economy and ecology. On the other hand they can also interact in a negative way so the potential interactions of all chemicals applied have to be con¬sidered (Fig. 3.26). Some integrated systems have already been established accord¬ing to the “lock and key” principle, such as combinations of fixing agents/retention aids/drainage aids, fixing agents/retention aids/internal sizing agents and combinations of sizing agents/coating binders. Figure 3.27 shows a combination of a binder and a co-binder in a coating formulation, which results in improved ma¬chine runnability and better printability of the paper.
Attention has to be paid to the chemical and physical conditions of the white water circuit and to the measurement and control of the effectiveness of various chemical additives (Table 3.12). On-line measurements and integrated process control give the best and constant performance of multifunctional chemical com¬binations. Of similar importance are the methods and positions for metering chemical additives to the paper production system as well as the sequence of their addition (Fig. 3.28).
Nowadays, chemical additives are an integral part of advanced papermaking technology. Maximum performance can be achieved by tailoring their properties in order to obtain the desired effects and by adapting products to local manufacturing conditions. The development of innovative chemicals and intelligent formulations and combinations of chemicals will play an even more important role in the future in responding to the challenges facing the paper industry in terms of higher pro¬ductivity, improved and/or new quality properties and minimal environmental im¬pact. This depends on taking an integrated approach to paper manufacturing, coating and converting and strengthening the cooperation between the paper in¬dustry, machinery manufacturers, the chemical industry and the paper converters and printers.