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EP-4737639-A2 - METHOD FOR REDUCING AMOUNT OF BACTERIAL ENDOSPORES IN AN AQUEOUS FIBRE SUSPENSION

EP4737639A2EP 4737639 A2EP4737639 A2EP 4737639A2EP-4737639-A2

Abstract

The invention relates to a method for reducing bacterial endospores in an aqueous fibre suspension comprising recycled cellulosic fibres, wherein the fibre suspension has an original endospore amount, preferably of ≥10 000 CFU/ml. The method comprises adjusting the pH of the fibre suspension to a pH value of ≤6.5, adjusting the oxidation-reduction potential (ORP) of the fibre suspension to an ORP value of ≥200 mV with a first oxidizing agent, and introducing an amount of performic acid as a second oxidizing agent to the fibre suspension for reducing the bacterial endospores to an endospore amount of ≤1000 CFU/ml.

Inventors

  • KOLARI, MARKO
  • AHOLA, JUHANA

Assignees

  • Kemira OYJ

Dates

Publication Date
20260506
Application Date
20211129

Claims (15)

  1. Method for reducing bacterial endospores in an aqueous fibre suspension comprising recycled cellulosic fibres, wherein the fibre suspension has an original endospore amount, preferably of ≥10 000 CFU/ml, the method comprising (a) adjusting the pH of the fibre suspension to a pH value of ≤6.5, (b) adjusting the oxidation-reduction potential (ORP) of the fibre suspension to an ORP value of ≥200 mV with a first oxidizing agent, and (c) introducing an amount of performic acid as a second oxidizing agent to the fibre suspension for reducing the bacterial endospores to an endospore amount of ≤1000 CFU/ml.
  2. Method according to claim 1, characterised in that the fibre suspension comprises inorganic particles, such as calcium carbonate particles, and that the pH of the fibre suspension is adjusted to a range of 5 - 6.5, more preferably 5.5 - 6.5, even more preferably 6 - 6.5.
  3. Method according to claim 1 or 2, characterised in that the pH of the fibre suspension is adjusted by introducing an acidifying agent to the fibre suspension.
  4. Method according to claim 1, 2 or 3, characterised in that the temperature of the fibre suspension is at least 50 °C, preferably at least 60 °C, more preferably at least 70 °C.
  5. Method according to any of preceding claims 1 - 4, characterised in that the fibre suspension has a final conductivity value, measured after the adjustment of pH and the ORP value, in a range of 2 - 10 mS/cm, preferably 3 - 7 mS/cm.
  6. Method according to any of preceding claims 1 - 5, characterised in that the first oxidizing agent is H 2 O 2 or a percarbonate salt.
  7. Method according to any of preceding claims 1 - 6, characterised in that the ORP value is adjusted to a range from +100 mV to +500 mV, preferably from +200 mV to +400 mV, more preferably from +300 mV to +400 mV.
  8. Method according to any of preceding claims 1 - 7, characterised in that the performic acid is introduced to the fibre suspension in amount that reduces the endospore amount to ≤1000 CFU/ml, preferably ≤500 CFU/ml, more preferably ≤250 CFU/ml, even more preferably ≤150 CFU/ml.
  9. Method according to any of preceding claims 1 - 8, characterised in that the first oxidizing agent is introduced to the fibre suspension in amount of 300 - 1000 ppm, preferably 400 - 800 ppm, preferably 500 - 700 ppm, given as grams active agent per ton fibre suspension.
  10. Method according to any of preceding claims 1 - 9, characterised in that the performic acid as the second oxidizing agent is introduced to the fibre suspension in amount of 50 - 500 ppm, preferably 100 - 400 ppm, preferably 200 - 300 ppm, given as grams active agent per ton fibre suspension.
  11. Method according to any of preceding claims 1 - 10, characterised in that the adjustment of the pH and the ORP values, optionally also the introduction of the performic acid, are made in a pulper or after a pulper, preferably when the fibre suspension has a consistency of 1 - 5 weight-%, preferably 1 - 3 weight-%.
  12. Method according to any of preceding claims 1 - 11, characterised in that the adjustment of the pH and the ORP value, optionally also the introduction of the performic acid, are made in a process step or a process device where the fibre suspension has a consistency of 8 - 30 weight-%, preferably 15 - 25 weight-%.
  13. Method according to any of preceding claims 1 - 12, characterised in that after the bacterial endospore amount of the fibre suspension is reduced to ≤1000 CFU/ml, the fibre suspension is used to make a fibre stock, which is formed into a fibrous web or a fibrous layer in a multilayer fibrous web.
  14. Method according to claim 13, characterised in that the bacterial endospore amount in the formed fibrous web or the multilayer fibrous web is <5000 CFU/g of dry fibrous web, preferably <2500 CFU/g of dry fibrous web, more preferably <1000 CFU/g of dry fibrous web.
  15. Method according to claim 13 or 14, characterised in that the fibre stock comprises at least 10 weight-%, preferably at least 20 weight-%, more preferably at least 50 weight-%, of the said fibre suspension.

Description

The present invention relates to a method for reducing or decreasing amount of bacterial endospores in an aqueous fibre suspension according to the preamble of the enclosed independent claim. Bacterial cells are normally present in the aqueous environments of paper, board and tissue mills. Bacterial growth in the process is commonly monitored and limited by using various measures, e.g. feeding of biocides into the process. However, certain bacterial cells form endospores, which are highly resistant to typical bacterial destruction methods, such as heat, disinfectants, chemical biocides, desiccation, ultraviolet light and ionizing radiation. The bacterial endospores may remain viable but dormant for prolonged periods, even for years, until the external conditions become favourable, after which the transformation, i.e. germination, of bacterial endospores takes place. Recycled fibre material usually contains high amounts of bacteria as well as bacterial endospores. The fibre material collected from consumers and industrial sources for recycling often contain contamination, such as food or oil residues, which provide good growth medium for bacteria. Even relatively clean looking recycled fibre material, such as collected office waste paper, usually contains high amounts of endospores as the endospore amounts are not actively monitored in production of non-hygiene/non-food packaging paper or board grades. Furthermore, the collected fibre material is often stored baled in dirty, humid and/or warm conditions that increase the risk for extensive bacterial growth. Consequently, fibre suspensions produced from recycled fibre material usually contain high amounts of bacterial contamination in form of bacteria and bacterial endospores. The high bacterial contamination of the fibre suspensions produced from the recycled fibre material may cause problems in paper and board making process itself, or it may limit the uses of the produced paper or board. For example, if the paper or board product is intended for hygiene purposes or for packaging of food or beverages, the allowable amount of bacterial endospores in the final paper and board product is strictly limited in order to avoid possible contamination of packaged materials. These limitations have caused that the use of recycled fibre material has been practically non-existent in paper and board products that have been intended for food and beverage packaging. On the other hand, due to the constantly increasing demand for sustainability in all industrial production, there is an interest to increase and/or expand the use of recycled fibres in production of all paper and board grades. Consequently, there is a need for effective ways to decrease the amount of bacterial endospores in aqueous fibre suspensions comprising recycled cellulosic fibres. An object of this invention is to minimise or possibly even eliminate the disadvantages existing in the prior art. Another object of the present invention is to provide an effective method for reducing or decreasing the amount of bacterial endospores in aqueous fibre slurries comprising recycled cellulosic fibres. These objects are attained with the invention having the characteristics presented below in the characterising parts of the independent claims. Some preferred embodiments of the invention are presented in the dependent claims. In a typical method according to the present invention for reducing or decreasing amount of bacterial endospores in an aqueous fibre suspension comprising recycled cellulosic fibres, wherein the fibre suspension has an original endospore amount, preferably of ≥10 000 CFU/ml, the method comprises (a) adjusting the pH of the fibre suspension to a pH value of ≤6.5,(b) adjusting the oxidation-reduction potential (ORP) of the fibre suspension to an ORP value of ≥200 mV with a first oxidizing agent, and(c) introducing an amount of performic acid as a second oxidizing agent to the fibre suspension for reducing or decreasing the bacterial endospores to an endospore amount of ≤1000 CFU/ml. Now it has been surprisingly found that the high bacterial endospore amount in a fibre suspension comprising recycled cellulosic fibres can be effectively reduced when the pH and the oxidation-reduction potential (ORP) of the fibre suspension are first adjusted to the specific values of pH ≤6.5 and ORP ≥200 mV, followed by treatment of the fibre suspension with performic acid. It is assumed that when the performic acid is introduced to the fibre suspension as a second oxidant, at the right pH range and after the adjustment of the ORP value of the suspension with a first oxidant, the endospore destroying performance of the performic acid can be successfully employed, and even excessive endospore amount in the suspension can be significantly decreased with reasonable performic acid consumption. It has been observed that the present method may easily reduce the amount of bacterial endospores in the fibre suspension by 2 -