Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F1/00—Wet end of machines for making continuous webs of paper
  • D21F1/66—Pulp catching, de-watering, or recovering; Re-use of pulp-water
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21B—FIBROUS RAW MATERIALS OR THEIR MECHANICAL TREATMENT
  • D21B1/00—Fibrous raw materials or their mechanical treatment
  • D21B1/38—Conserving the finely-divided cellulosic material
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H21/00—Non-fibrous material added to the pulp, characterised by its function, form or properties; Paper-impregnating or coating material, characterised by its function, form or properties
  • D21H21/02—Agents for preventing deposition on the paper mill equipment, e.g. pitch or slime control
  • D21H21/04—Slime-control agents

Definitions

• the present invention relates to a process for controlling microbial growth in a production line for cellulosic products with the aid of gases.
• the invention also relates to the use of gases such as carbon dioxide, nitrogen, a noble gas and/ or non-naturally occurring mixtures thereof for controlling microbial growth.
• cellulosic products such as cellulose, paper and board (hereinafter referred to as paper)
• cellulosic fibers are treated in aqueous suspensions under varying conditions.
• the amount of water in the production line is huge and the water is continuously recirculated in smaller or larger loops.
• the conditions in the production line are often susceptible to microbial growth. This is especially so during storage of the aqueous material.
• the microbes For breeding the microbes need time to grow and propagate in the system. The retention time must be long enough; otherwise, the cells will be washed out of the system. In a papermaking system the microbes can easily find locations with a sufficient retention time, e.g. water storage tanks, stock chests, treatment of broke and long pipelines.
• Normal papermaking conditions are suitable for the growth of many kinds of microbes.
• the recirculating white water in the short and long circulation contains enough carbohydrates and other essential ingredients such as inorganics and trace elements.
• chemicals used in the papermaking themselves represent an ideal nutrient source, e.g. starch, or contain as impurities quite a lot of nutrient material, e.g. kaolin.
• the incoming raw water can also contain considerable amounts of nutrients.
• the microbes found in a papermaking system can be divided into three main groups, bacteria, fungi and algae.
• the bacteria are either spore forming (anaerobic) or non spore forming (aerobic); the fungi comprise moulds and yeasts; and the prevailing algae are blue-green or green algae.
• microbes Unless microbial control agents are added to the papermaking system, the general growth requirements for microbes are usually well satisfied in papermaking systems. Modern microbial control agents can roughly be divided into groups, which act in the following ways: oxidative decomposition of microbes (O2, CIO2, O3, peroxides); biocides, which inhibit or kill the microbes (organic, synthetic chemicals); and enzymes.
• Oxidative decomposition of microbes O2, CIO2, O3, peroxides
• biocides which inhibit or kill the microbes (organic, synthetic chemicals); and enzymes.
• Oxygen is primarily used to prevent anaerobic conditions, while chlorine dioxide, ozone and peroxide work as biocides and disinfectants. They are quite effective but considered to be costly.
• Conventional biocides can be used alone or in combination with oxidative biocides. They are effective, but toxic. They can also be environmentally dangerous and hazardous for the working environment.
• a relatively new method of slime control is the use of enzyme. Enzymes are active at pH 3.5-10, which is an advantage compared with conventional biocides. However, enzymes have limited effect on bacteria.
• Oxygen and oxygen-rich gases have primarily been used for preventing the formation of hydrogen sulfide and other volatile gases by anaerobic bacteria in waste waters.
• Robichaud, . T. , Tappi Journal, Feb. 1991, pp 149-153 has reported on the use of aeration for controlling anaerobic bacteria to improve product quality and mill safety in papermaking systems.
• gases have been used in the packaging of foodstuffs in order to retain the foodstuff's original taste, texture and appearance.
• the gas mixtures usually consist of carbon dioxide, nitrogen and oxygen, but also other gases such as nitrous oxide, argon and hydrogen have been used.
• Carbon dioxide inhibits microbial activity on the foodstuff by reducing the pH and by penetrating biological membranes, causing changes in permeabili- ty and function.
• Nitrogen is primarily used to replace oxygen in packaging. Oxygen helps to preserve the oxygenated form of myoglobin, which gives meat its red colour. Oxygen is also required for fruit and vegetable respiration.
• a gaseous inhibitor in the form of carbon dioxide, nitrogen, a noble gas or a gas mixture containing one or more of said gases may be used for controlling microbial growth in a papermaking system.
• the present invention provides a process for controlling microbial growth in a production line for cellulosic products, which comprises providing an aqueous material containing water as well as suspended pulp fibers and/or additives therefor, maintaining said aqueous material under conditions susceptible to microbial growth, providing a gaseous inhibitor comprising a gas selected from carbon dioxide, nitrogen, noble gases and non-natural gas mixtures containing the same, and introducing said gaseous inhibitor to said aqueous material in an amount sufficient to significantly retard or inhibit the growth of microorganisms therein.
• the gaseous inhibitor is preferably added immediately prior to and/or during storage of said aqueous material, since storage provides a sufficient time for the microbes to propagate.
• the gaseous inhibitor should preferably consist of or contain a significant amount of carbon dioxide, nitrogen and/or a noble gas such as helium, neon, argon, krypton, xenon and/or radon.
• Carbon dioxide is the preferred gas according to the present invention.
• argon is preferred.
• a gas mixture should be a non-natural one.
• the gases of the mixture may be mixed before introduction into the aqueous material, or they may be added separately, simultaneously or sequentially.
• the gas mixture may contain oxygen, it should not have the composition of air, since air is known to inhibit only the growth of anaerobic microbes.
• the gaseous inhibitor of the present invention may be used in combination with oxygen either by combining oxygen in the mixture of carbon dioxide, nitrogen and/or argon, or by adding oxygen separately from the other gases.
• oxygen When oxygen is added to the gas mixture, the amount of oxygen may vary from 10 to 90% of the total gas volume.
• an oxygen-rich gas is introduced into the aqueous material separately from the gaseous inhibitor of the present invention. Such an oxygen-rich gas may be added either before or after the addition of the inhibiting gas mixture.
• a production line for cellulosic products comprises a line for the production of pulp, paper, board or the like.
• the production line will typically include at least a portion of reprocessed recovered paper and/or broke and will include loops of recirculating waters.
• the production line has a more or less closed water system, it being understood by those skilled in the art that the problems with microbial growth are prone to escalate in closed systems with an increasing and accumulating mass of microbes circulating in the system.
• the temperature in the papermaking systems usually varies from 30 to 60 °C. Because of recycling, the temperature in the white water often exceeds 50 °C. Fungi and yeasts generally do not tolerate temperatures above 40 °C. Contrary to this, many bacteria thrive well in the high temperature range. pH usually varies from 3 to 10. Acidic conditions, pH 3-6, are very convenient for fungi and yeasts. Bacteria dominate under neutral and alkaline conditions, viz. at pH levels from 7 to 10. Anaerobic conditions can be found in many places throughout the production line, such as storage after dithionite bleaching, pulp chests and white water tanks.
• Slime helps microbes to adhere onto surfaces and provides a food reserve.
• Microbial growth causes operating problems by plugging filters and screens, by reducing wire and felt life and by causing a decline of productivity due to breaks, wash-ups, etc. Corrosion induced by microbes is a consequence of vigorous microbial activity on surfaces.
• the most important microbial species in this area include sulphate-reducing bacteria, which are anaerobic in character. There are, however, also a number of aerobic species which are harmful. Additives, such as starch, can deteriorate due to microbial activity, not to mention that a contaminated starch slurry can constitute a heavy inoculation of the white water system. When masses of microbes get loose from the actual growth place, the result may be seen as spots, holes or dirt specks in the final paper product. Spore-forming bacteria tolerate much heat and usually survive the drying stage. Thus they remain alive in the product and can be harmful later on.
• microbe or microorganism as used in the context of the present invention is intended to mean bacteria, fungi and/or algae such as described above. It should be understood that all of the microorganisms present in a papermaking system will not be influenced by the gaseous inhibitor of the present invention and that the gaseous inhibitor of the invention may therefore be used in combination with other inhibitors, such as oxygen-rich gases and biocides of various forms, as long as these do not interfere with the working of the invention itself.
• the gaseous inhibitor of the present invention is a gas or a gas mixture capable of inhibiting, partly or totally, the growth of microorganisms present in the papermaking system.
• the preferred single gas is carbon dioxide.
• the gaseous inhibitor may also comprise nitrogen and/or argon. Said gases may also be used alone, but they are more preferably used in combination with carbon dioxide.
• a suitable gaseous inhibitor consists of a mixture of carbon dioxide, nitrogen and argon. The mixture preferably contains at least 10% carbon dioxide.
• the gaseous inhibitor may additionally contain oxygen.
• the oxygen should not be used in a combination resembling air, since such a mixture is effective only against anaerobic microorganisms.
• the proportion of carbon dioxide should be between 90% and 10% and the proportion of oxygen should be between 10% and 90% . It should be noted that normal air contains about 21 % oxygen and about 0.03 % carbon dioxide.
• a preferred embodiment of the invention comprises the use of a gaseous inhibitor consisting essentially of carbon dioxide, nitrogen, argon or mixtures thereof, which is introduced into a liquid flow of the aqueous material entering a storage tank for said aqueous material or into said storage tank itself.
• An oxygen-free gaseous inhibitor is preferably added in an amount sufficient to purge said aqueous material of oxygen and thereby inhibiting the growth of aerobic bacteria contained therein. Most aerobic bacteria are sensitive to the lack of oxygen and will eventually be killed off by such a procedure while the carbon dioxide will adversely affect many of the anaerobic species.
• oxygen is used separately from the gaseous inhibitor of the present invention.
• the introduction of the gaseous inhibitor is followed by and/or preceded by an introduction of an oxygen containing gas into said storage tank in an amount sufficient to kill anaerobic bacteria in the aqueous material.
• the introduction of the gaseous inhibitor and the introduction of oxygen may be repeated in an alternating manner during storage of said aqueous material.
• the inhibitor is preferably added in a position where the risk for microbial growth is largest, i.e. in storage tanks for aqueous pulp suspensions or aqueous additives susceptible to microbial attack.
• the gaseous inhibitor may also be added to liquid flows of the aqueous material, to recirculating waters and to fresh water prior to its entering the system.
• the main principle of the present invention is to reduce the microbial growth at any position where it would otherwise rise to harmful proportions. It is not necessary to kill all the microbes but it is essential to reduce the microbial growth to such proportions that the harmful accumulations are minimized in the production line and in the final product.
• the gaseous inhibitor is introduced into a liquid flow of the aqueous material or into a liquid flow of a diluent or additive for said material just prior to storage thereof.
• the gaseous inhibitor may also be introduced into any storage tank containing said aqueous material by bubbling the gas into the aqueous material and/ or by filling the void space above the fluid.
• the aqueous material comprises a pulp suspension in a papermaking system and the suspension is treated with the gaseous inhibitor before it enters and/or as it is retained in a pulp storage tower, a stock chest, a broke tower or the like storage tank.
• the pulp suspension may also be stock in the stock preparation of a paper making system.
• the aqueous material to be treated comprises white water in a papermachine, preferably white water stored in the long circulation.
• the aqueous material comprises a slurry of an additive chemical such as starch, coating, pigment, filler, or the like.
• additives are usually retained in aqueous suspension in readiness for use in the papermaking process and many of the additives contain nutrients making them susceptible to microbial attack.
• starch which in itself is a nutrient for many microorganisms.
• Many other additives although inert in themselves, contain sufficient amounts of impurities to make them, with time, susceptible to microbial attacks. Treating such additive tanks with intermittent introductions of the gaseous inhibitor will effectively reduce the amount of microbes entering the system that way.
• the gaseous inhibitor is added at a late point, preferably just prior to the point where microbial attack is expected to be most severe, such as in a storage tower. Additional gaseous inhibitor (carbon dioxide/nitrogen/argon) should, if necessary, be added to the head space of the tower.
• oxygen should preferably be added directly after a pump feeding the aqueous material to the storage tower to make use of any turbulence to achieve a high rate of mixing. Additional oxygen should, if necessary be added to any of the tower's recirculation pipes to avoid creating any anaerobic areas in the storage tower.
• gaseous inhibitor of the present invention comprising carbon dioxide, nitrogen or argon alone or in a non-natural gas mixture has not previously been used in papermaking systems for controlling microbial growth.
• the aqueous materials of the cellulosic production line are processed to cellulosic products such as paper, board, dried pulp or the like material in a manner which is conventional in all other ways except for the biocidal treatment of the present invention.
• a set of representative bacterial strains was isolated from a white water sample from a Swedish recycled pulp mill. The sample originally yielded 70 bacterial strains which each represented a group of different bacteria, which grew under similar conditions. The strains were tested on different media and their oxygen demands were studied. The results gave an indication of eight main groups. Further investigations showed that three out of the eight main groups seemed to be almost similar. Those three groups were put together to one large group.
• a mill produced wood-free paper from chemical pulp. At the time of the trials, the mill stored pulp for long periods of time in storage towers prior to the stock preparation. The storage caused problems with bad smell and black spots in the pulp, which was believed to be caused by high microbial activity during storage.
• a pulp storage tower having problems with an excess of microbial growth during a production stop is fed by a pump until the storage tower has been filled to about 80% of its capacity.
• a gaseous inhibitor comprising a mixture of carbon dioxide/nitrogen/argon in the ratio 70/25/5 is added to the feed line just before the pulp enters the tower.
• the gas is fed into the feed line at a rate of 1.5 kg gas per ton of pulp.
• the gas feeding is continued about 5 min after the feeding pump has stopped feeding pulp to the tower, in order to fill the head space of the tower with gaseous inhibitor.
• an oxygen-rich gas air
• Addition of oxygen-rich gas is continued until a significant amount of oxygen is found to be present in the vent from the tower.
• the treatment is repeated by first feeding gaseous inhibitor of the present invention into the gas distribution tube, and after a residence time of about 2 hours, oxygen-rich gas is fed through the tube.
• the microbiocidal treatment is repeated every day of the production stop.
• a paper web is formed from the stored pulp in the normal way and only a minimal amount of dirt specks are seen in the paper.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Paper (AREA)
• Micro-Organisms Or Cultivation Processes Thereof (AREA)
• Preparation Of Compounds By Using Micro-Organisms (AREA)
• Polysaccharides And Polysaccharide Derivatives (AREA)

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• 2000
  • 2000-05-04 FI FI20001040A patent/FI110533B/fi not_active IP Right Cessation
• 2001
  • 2001-04-27 CA CA002407898A patent/CA2407898A1/en not_active Abandoned
  • 2001-04-27 AU AU2001258445A patent/AU2001258445A1/en not_active Abandoned
  • 2001-04-27 AT AT01931742T patent/ATE259448T1/de not_active IP Right Cessation
  • 2001-04-27 US US10/275,149 patent/US20030155090A1/en not_active Abandoned
  • 2001-04-27 EP EP01931742A patent/EP1287201B1/de not_active Expired - Lifetime
  • 2001-04-27 JP JP2001580489A patent/JP2003531973A/ja active Pending
  • 2001-04-27 DE DE60102015T patent/DE60102015T2/de not_active Expired - Fee Related
  • 2001-04-27 WO PCT/FI2001/000410 patent/WO2001083886A1/en active IP Right Grant
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• 2002
  • 2002-11-01 NO NO20025242A patent/NO20025242D0/no not_active Application Discontinuation

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