Method for reducing the amount of bacterial endospores in an aqueous fiber suspension

Abstract

The present invention relates to a method for reducing bacterial endospores in an aqueous fiber suspension containing recycled cellulose fibers, wherein the fiber suspension has an initial endospore amount, preferably ≥10000 CFU / ml. The method comprises adjusting the pH of the fiber suspension to a pH value of ≤6.5, adjusting the oxidation-reduction potential (ORP) of the fiber suspension to an ORP value of ≥200 mV using a first oxidizing agent, and introducing an amount of performic acid as a second oxidizing agent into the fiber suspension for reducing bacterial endospores to an endospore amount of ≤1000 CFU / ml.

Description

Field of the Invention

[0001] The present invention relates to a method for reducing the amount of bacterial endospores in an aqueous fiber suspension according to the preamble of the appended independent claims. Background Art

[0002] Bacterial cells are commonly present in the aqueous environments of paper mills, board mills, and tissue mills. Bacterial growth in such processes is typically monitored and restricted by using various means, e.g., by feeding biocides into the process. However, certain bacterial cells form endospores, which are highly resistant to typical sterilization methods such as heat, disinfectants, chemical biocides, drying, ultraviolet light, and ionizing radiation. Bacterial endospores can remain viable but dormant for long periods of time, even years, until external conditions become favorable, whereupon the bacterial endospores transform, i.e., germinate.

[0003] Recycled fiber materials typically contain large amounts of bacteria as well as bacterial endospores. Fiber materials collected for recycling from consumer and industrial sources typically contain contaminants such as food or oil residues, which provide an excellent growth medium for bacteria. Even recycled fiber materials that appear relatively clean, such as collected office waste paper, typically contain large amounts of endospores because the amount of endospores is not actively monitored in products of non-hygienic / non-food packaging paper or board grades. In addition, the collected fiber materials are typically stored in bales under dirty, wet, and / or warm conditions, which increases the risk of substantial bacterial growth. Consequently, fiber suspensions produced from recycled fiber materials typically contain substantial bacterial contamination in the form of bacteria and bacterial endospores.

[0004] The high bacterial contamination of fiber suspensions produced from recycled fiber materials can cause problems in the paper and board manufacturing process itself, or it can limit the use of the produced paper or board. For example, if paper or board products are designed for hygienic purposes or for the packaging of food or beverages, the amount of bacterial endospores tolerated in the final paper and board products is strictly restricted to avoid possible contamination of the packaging material. These restrictions have led to the virtual non-use of recycled fiber materials in paper and board products designed for food and beverage packaging. On the other hand, due to the increasing demand for sustainability in all industrial production, increasing and / or expanding the use of recycled fibers in all paper and board grades remains a concern. Therefore, there is still a need for an effective way to reduce the amount of bacterial endospores in aqueous fiber suspensions containing recycled cellulose fibers. Summary of the Invention

[0005] The object of the present invention is to reduce or possibly even eliminate the deficiencies existing in the prior art.

[0006] Another object of the present invention is to provide an effective method for reducing or decreasing the amount of bacterial endospores in a fiber aqueous slurry containing recycled cellulose fibers.

[0007] These objects are achieved by the present invention having the features provided in the characterizing part of the independent claims.

[0008] Some preferred embodiments of the present invention are provided in the dependent claims.

[0009] In a typical method for reducing or decreasing the amount of bacterial endospores in an aqueous fiber suspension containing recycled cellulose fibers according to the present invention, wherein the fiber suspension has an amount of original endospores of preferably ≥ 10000 CFU / ml, the method comprises

[0010] (a) adjusting the pH of the fiber suspension to a pH value of ≤ 6.5,

[0011] (b) using a first oxidant to adjust the oxidation-reduction potential (ORP) of the fiber suspension to an ORP value of ≥ 200 mV, and

[0012] (c) introducing an amount of performic acid as a second oxidant into the fiber suspension for reducing or decreasing the amount of bacterial endospores to an amount of ≤ 1000 CFU / ml.

[0013] Currently, it has been unexpectedly found that when the pH and oxidation-reduction potential (ORP) of the fiber suspension are first adjusted to specific values of pH ≤ 6.5 and ORP ≥ 200 mV, and then the fiber suspension is treated with performic acid, the amount of high bacterial endospores in the fiber suspension containing recycled cellulose fibers can be effectively reduced. It is believed that when performic acid is introduced as a second oxidant into the fiber suspension, within the correct pH range and after adjusting the ORP value of the suspension with a first oxidant, the endospore-destroying property of performic acid can be successfully utilized, and even the amount of excessive endospores in the suspension can be significantly reduced by a reasonable consumption of performic acid. It has been observed that the method of the present invention can easily reduce the amount of bacterial endospores in the fiber suspension by 2 - 4 logarithmic units, possibly even higher. The combination of the adjustment of the pH and ORP values and the introduction of performic acid provides an unexpected improvement in endospore control for fiber suspensions containing recycled fibers and a large amount of bacterial endospores. The reduction in the amount of bacterial endospores enables the treated fiber suspension containing recycled cellulose fibers to be used even for the production of paper and board grades designed for hygienic purposes and / or for food or beverage packaging.

[0014] In the context of the present invention, the term "oxidation-reduction potential" (abbreviated as ORP) denotes the oxidation or reduction potential of an aqueous fiber suspension. The ORP value of an aqueous fiber suspension can be determined by using a chemically-inert electrode immersed in the suspension and measuring its potential relative to a reference electrode. Some commercial sensors for ORP value measurement are available.

[0015] In the context of the present invention, the term "bacterial endospore" will be understood as a dormant and non-reproductive structure formed by bacteria. Bacterial endospores contain bacterial DNA and a part of its cytoplasm enclosed by a protective outer covering. Bacterial endospores can germinate into a metabolically active state, i.e., a vegetative state, under favorable conditions. According to one embodiment of the present invention, the method of the present invention is used to reduce the amount of bacterial endospores, for example, from the genus Bacillus, Brevibacillus, and / or Paenibacillus, which are known to grow under the operating conditions of paper, cardboard, and toilet paper paper machines, etc. These bacterial genera are capable of producing heat-resistant endospores that are tolerant of the heat in the dryer section used in the process of producing cellulose fibrous webs such as paper, cardboard, toilet paper, etc.

[0016] In this context, all bacterial endospore values in CFU / ml are provided for a fiber suspension having a fiber consistency of 4 wt-% (provided as dry fiber). The bacterial endospore value is determined for a fiber suspension having a fiber consistency of 4 wt-%, or if determined according to another consistency, the bacterial endospore value is calculated to correspond to the endospore value for a 4 wt-% fiber consistency.

[0017] The fiber suspension comprises a liquid phase, which is usually water, and a solid phase suspended in the liquid phase, which comprises at least recycled cellulose fibers and optionally inorganic particles. The initial aqueous fiber suspension for carrying out the bacterial endospore reduction treatment in the method of the present invention comprises recycled cellulose fibers, and it has an amount of initial bacterial endospores of at least 1000 CFU / ml, usually at least 5000 CFU / ml, more usually at least 10000 CFU / ml. The fiber suspension comprises an aqueous liquid phase and a solid phase comprising recycled fibers. The recycled cellulose fibers in the fiber suspension can be bleached, unbleached or a mixture of bleached and unbleached recycled fibers. The recycled cellulose fibers in the fiber suspension are non-synthetic natural fibers initially obtained by any mechanical or chemical pulping method or by any combination of mechanical and chemical pulping methods. The recycled cellulose fibers can be wood and / or non-wood fibers, preferably wood fibers, such as hardwood, softwood or any combination thereof. The recycled fibers can be derived from any available recycled industrial and / or consumer fiber materials. The recycled fibers can be derived from, for example, old corrugated cartons (OCC), office waste paper, mixed office waste paper, sorted office waste paper or any mixture thereof. The recycled fibers can be derived from, for example, pre-consumer recycled fiber materials and / or post-consumer recycled fiber materials. The recycled fibers can even be secondary fibers from the production process of paper, cardboard, toilet paper, etc., such as broke. The amount of recycled cellulose fibers in the fiber suspension to be treated is at least 60 wt.-%, preferably at least 80 wt.-%, more preferably at least 90 wt.-% or at least 95 wt.-% based on the total dry fiber weight of the suspension. According to one embodiment, the cellulose fibers in the fiber suspension consist of recycled cellulose fibers.

[0018] The aqueous fiber suspension can have an amount of initial bacterial endospores of at least 5000 CFU / ml, usually at least 10000 CFU / ml, more usually at least 15000 CFU / ml, even more usually at least 20000 CFU / ml. The amount of initial endospores of the fiber suspension comprising recycled cellulose fibers can be in the range of 1000 - 500000 CFU / ml, more usually 10000 - 500000 CFU / ml or 15000 - 400000 CFU / ml, even more usually 20000 - 350000 CFU / ml.

[0019] The aqueous fiber suspension usually has a negative initial oxidation-reduction (ORP) value, for example, in the range of -500 mV to -50 mV, more usually -400 mV to -100 mV, even more usually -300 mV to -200 mV.

[0020] According to one embodiment of the present invention, the fiber suspension may comprise inorganic particles, such as particles of calcium carbonate, kaolin, talc, gypsum, etc. The inorganic particles typically originate from internal fillers, inorganic coatings, labels, fixing agents, etc., which are already present in the fibrous material collected for recycling. The amount of inorganic particles provided as the ash of the used recycled cellulose fibrous material may be in the range of 5 - 30 wt-%, 5 - 25 wt-% or 10 - 20 wt-%. During the repulping of the fibrous material, it is generally difficult and / or uneconomical to completely remove the inorganic particles from the recycled cellulose fibrous material, which means that generally at least some inorganic particles enter the fiber suspension with the recycled cellulose fibrous material.

[0021] The fiber suspension may generally comprise at least some dissolved carbonate ions.

[0022] During the pH value and / or ORP value adjustment and / or peracetic acid introduction, the fiber consistency of the fiber suspension may be at least 1 wt-% calculated as dry fiber, preferably at least 3 wt-%. According to one embodiment of the present invention, the fiber consistency of the fiber suspension may be 1 - 30 wt-%, preferably 3 - 20 wt-%, more preferably 4 - 10 wt-% calculated as dry fiber. According to another embodiment of the present invention, the fiber consistency of the fiber suspension may be 1 - 15 wt-%, preferably 2 - 10 wt-%, more preferably 2 - 5 wt-% calculated as dry fiber.

[0023] The adjustment of the pH and ORP values and optionally the introduction of peracetic acid can be carried out in or after a beater where the recycled fibrous material is disintegrated and diluted with water to a fiber consistency of generally 1 - 5 wt-%, preferably 1 - 3 wt-%, or in or after a pulping step. As an alternative, the pH and ORP values can be adjusted, and optionally peracetic acid can be introduced, in a process step or process device where the fiber consistency of the fiber suspension is relatively high, such as 8 - 30 wt-%, preferably 15 - 25 wt-%. It is possible to adjust the pH and ORP values at the same fiber consistency as or different from the introduction of peracetic acid, i.e., the fiber suspension can be concentrated or diluted between different process steps, especially between step (b) and step (c).

[0024] Typically, the fiber suspension formed in the beater / refining step contains in particular hydrophobic contaminants such as plastics, tapes and / or glue residues. These contaminants as well as other impurities, large particles etc. are removed in one or more screening steps after the beater / refining step, where contaminants and / or impurities with a size larger than 200 microns are typically removed. According to one embodiment of the invention, the adjustment of the pH and ORP values and optionally the introduction of performic acid can be carried out after said screening step, for example, to optimize chemical consumption. For example, at least the pH and ORP values can be adjusted, preferably also the performic acid can be introduced, in a separate mixing tank located after said screening step and before a possible thermal disperser etc.

[0025] The fiber suspension is typically subjected to fiber classification, where the fibers are separated into at least a long fiber fraction and a short fiber fraction according to their length. According to one embodiment of the invention, the adjustment of the pH and ORP values and optionally the introduction of performic acid can be carried out after said classification step. For example, at least the pH and ORP values can be adjusted, preferably also the performic acid can be introduced, for at least one, preferably all fiber fractions obtained from the fiber classification and used for the preparation of fiber stock.

[0026] According to one embodiment of the invention, the pH of the fiber suspension is preferably adjusted to a pH value of ≤6.5 by introducing an acidifying agent into the fiber suspension. The pH of the fiber suspension can be adjusted in a pH range of 4 - 6.5, preferably 4.5 - 6.5, more preferably 5 - 6.5, even more preferably 5.5 - 6.3. The acidifying agent can be any compound suitable for adjusting the pH value of the pulp suspension to the desired level, for example, polyaluminum chloride, alum etc. The acidifying agent can be an organic acid such as citric acid, formic acid etc., or an inorganic acid such as hydrochloric acid, sulfuric acid etc., or a mixture of organic and / or inorganic acids. The acidifying agent can be an acidifying gas such as carbon dioxide gas. When the acidifying agent is an acid in liquid form, it is added to the fiber suspension, and when the acidifying agent is in gas form, for example, carbon dioxide gas, it is introduced into the fiber suspension. Preferably, the acidifying agent is introduced or added to the fiber suspension in an amount such that the pH of the fiber suspension is adjusted to the desired pH value without causing a significant increase in the conductivity of the fiber suspension. Effective mixing is advantageous when adding the acidifying agent to the fiber suspension.

[0027] Preferably, the pH of the fiber suspension is adjusted to a less acidic pH value, e.g., to a pH value ≥ 4, preferably ≥ 4.5, more preferably ≥ 5. An overly acidic pH can cause various components, such as at least some of the inorganic particles present in the fiber suspension, to dissolve, which can subsequently lead to an increase in conductivity and some problems during the production of the fibrous web. In particular, if the fiber suspension contains inorganic particles, such as calcium carbonate particles, the pH of the suspension is preferably adjusted to a pH within the range of 5 - 6.5, more preferably 5.5 - 6.5, and even more preferably 6 - 6.5. The acidifying agent can be selected based on the nature of the fiber suspension to be treated, particularly the buffering capacity. For example, a fiber suspension with a high buffering capacity, such as a fiber suspension containing recycled fibers from sorted office waste and having a high calcium carbonate particle content, can be treated with an acidifying agent selected from the group consisting of the organic or inorganic acids or mixtures thereof to obtain a consumption of the acidifying agent that is suitable in terms of economy and to avoid large pH changes that can cause an undesired change in the conductivity of the fiber suspension.

[0028] According to a preferred embodiment, during the reduction of bacterial endospores in the fiber suspension of the method according to the invention, the conductivity of the fiber suspension does not change significantly. This means that the fiber suspension generally has an initial conductivity value in the range of 2 - 10 mS / cm, preferably 2 - 7 mS / cm, measured before the pH and ORP value adjustment and the introduction of performic acid, and a final conductivity value in the range of 2 - 10 mS / cm, preferably 3 - 7 mS / cm, measured after the pH and ORP value adjustment and the introduction of performic acid. After the pH and ORP value adjustment and the addition of performic acid, the conductivity of the fiber suspension remains at a level that preferably allows the sizing and retention chemicals to perform effectively during the subsequent preparation of the final fibrous web.

[0029] The redox potential (ORP) of the fiber suspension is adjusted to an ORP value ≥ 200 mV by adding or introducing a first oxidizing agent to the fiber suspension. Preferably, the ORP value of the fiber suspension can be adjusted to ≥ 250 mV, more preferably ≥ 300 mV. It has been observed that performic acid can effectively eliminate and destroy the bacterial endospores present in the fiber suspension when the ORP value of the fiber suspension is adjusted to a level ≥ 200 mV. According to one embodiment, the ORP value of the fiber suspension can be adjusted to a range of +100 mV to +500 mV, preferably +200 mV to +400 mV, more preferably +300 mV to +400 mV.

[0030] The first oxidant for adjusting the ORP value is different from performic acid, that is, the first oxidant is not performic acid. Preferably, the first oxidant used does not contain performic acid. Possibly, organic peracids other than performic acid can be used as the first oxidant for adjusting the ORP value. Preferably, however, the first oxidant can be hydrogen peroxide, H 2 O 2 or percarbonate, preferably sodium percarbonate. Hydrogen peroxide and percarbonate are readily available on an industrial scale, and they can effectively adjust the ORP value of the fiber suspension to the desired level. The first oxidant can be considered a sacrificial treatment agent, through which the consumption of performic acid can be kept as low as possible. The use of the first oxidant indeed provides an effective way to adjust the ORP value to a specific level, where the full potential of performic acid can be achieved.

[0031] The first oxidant can be introduced into the fiber suspension in an amount that provides the desired ORP value to the fiber suspension containing recycled cellulose fibers. According to one embodiment of the present invention, the first oxidant can be introduced into the fiber suspension in an amount of 300 - 1000 ppm, preferably 400 - 800 ppm, preferably 500 - 700 ppm, in terms of grams of active agent per ton of fiber suspension.

[0032] The addition of the first oxidant to the fiber suspension does not provide a bleaching effect to the fiber suspension. This means that the ISO brightness of the fiber suspension does not change significantly after the addition of the first oxidant. Generally, the change in the ISO brightness of the fiber suspension is less than 5 ISO%, preferably less than 3 ISO%, more preferably less than 1 ISO%, as measured by using standard ISO 2470 - 1:2016.

[0033] According to the present invention, for the fiber suspension to be treated, performic acid is introduced, that is, added to the fiber suspension, in an amount that reduces the amount of bacterial endospores to ≤1000 CFU / ml, preferably ≤500 CFU / ml, more preferably ≤250 CFU / ml, even more preferably ≤150 CFU / ml, and sometimes even ≤100 CFU / ml. According to one embodiment of the present invention, performic acid can be introduced into the fiber suspension in an amount that reduces the amount of bacterial endospores to ≤50 CFU / ml, even reduced to ≤30 CFU / ml or even ≤10 CFU / ml. Performic acid can be introduced, that is, added to the fiber suspension, in an amount of 50 - 500 ppm, preferably 100 - 400 ppm, preferably 200 - 300 ppm, in terms of grams of active agent per ton of fiber suspension. The amount of bacterial endospores can be determined by using conventional techniques known to those skilled in the art in this regard.

[0034] Performic acid CH 2 O 3 is introduced into the fiber suspension as an aqueous solution. Performic acid can be prepared by mixing an aqueous hydrogen peroxide solution with an aqueous formic acid solution and optionally a catalyst, such as sulfuric acid. Preferably, an aqueous performic acid solution is used as an equilibrium solution containing performic acid, formic acid, water, hydrogen peroxide and optionally a catalyst. The performic acid solution typically has a concentration of at least 10%, usually about 13.5% or 14% performic acid calculated as weight per volume ratio.

[0035] Preferably, performic acid is made to interact with the bacterial endospores in the fiber suspension at an elevated temperature of the fiber suspension. According to one embodiment, the temperature of the fiber suspension can be at least 50 °C, preferably at least 60 °C or sometimes even at least 70 °C. The temperature of the fiber suspension can preferably be <100 °C, more preferably <85 °C, even more preferably <75 °C. In particular, when adjusting the pH of the fiber suspension in the range of 5.5 - 6.5, when performic acid is introduced into the fiber suspension and / or when performic acid interacts with the bacterial endospores, it will be advantageous if the temperature of the fiber suspension is in the range of 30 - 120 °C, preferably 30 - 99 °C, more preferably 50 - 80 °C, even more preferably 60 - 80 °C. It has been observed that in this way, the maximum destruction of endospores can be achieved. Without wishing to be bound by any theory, it is believed that high temperature can further sensitize the bacterial endospores and make them receptive to destruction when performic acid is introduced into the fiber suspension. Advantageously, effective reduction or destruction of endospores can be obtained at a fiber suspension temperature below 100 °C. Thus, expensive and complex processing devices, such as pressurized hot steam treatment tanks, can be avoided.

[0036] By carrying out in a separate process step, the temperature of the fiber suspension can be increased to a desired value by heating the fiber suspension to the desired temperature. For example, the fiber suspension can be transferred to a separate tank located after the beater, preferably after the screening step. In the separate tank, the fiber suspension can be heated to the desired temperature. As an alternative, pH adjustment, ORP value adjustment, and / or at least peracetic acid introduction can be carried out during a process stage in which the temperature of the fiber suspension is at the desired level. Examples of process stages, possibly process steps in a RCF plant, can be (for example) the dispersion of the recycled fiber suspension in a thermal disperser. Thermal dispersers are commercially available and are typically used in the pulping of recycled fibers to break down adhesives and homogenize dirt. Possibly, the temperature of the fiber suspension is adjusted to the desired level shortly after or immediately after introducing peracetic acid into the fiber suspension. For example, the introduction of peracetic acid can be carried out immediately before the fiber suspension is pre-dewatered, heated, and dispersed in the thermal disperser. It is also possible to introduce peracetic acid into the fiber suspension while heating the fiber suspension to the desired temperature, for example, heating to the desired temperature in a thermal disperser. The consistency of the fiber suspension at the inlet of the thermal disperser can be 20 - 40 weight-% calculated as dry fiber, and the temperature of the fiber suspension in the disperser can be 60 - 120 °C.

[0037] Preferably, before introducing peracetic acid into the fiber suspension, the temperature of the fiber suspension is adjusted to a high temperature as described above.

[0038] It is also possible to adjust the pH and ORP value and introduce peracetic acid into the fiber suspension in a separate tank located immediately after the disperser. It is even possible to heat the fiber suspension to the desired temperature in the same separate tank.

[0039] According to one embodiment, the pH and ORP value can be adjusted, optionally with the introduction of peracetic acid, for the fiber suspension in a separate tank, such as a storage tank, etc., in which the fiber suspension for preparing the fiber stock is stored. The fiber suspension can contain recycled fibers or (for example) fibers obtained by mechanical pulping or consist thereof.

[0040] In the case where the pH of the fiber suspension is in the pH range of 4 - 5.5, preferably 4.3 - 5, even a lower fiber suspension temperature, such as 25 - 60 °C, preferably 30 - 55 °C, more preferably 35 - 55 °C, can be sufficient. Without wishing to be bound by any theory, it is believed that a lower pH can sensitize bacterial endospores in a manner similar to high temperature and make them receptive to the destructive action of peracetic acid.

[0041] According to one embodiment of the invention, after treating the fiber suspension with performic acid, at least a portion of the aqueous liquid phase of the fiber suspension is separated from the solid phase of the fiber suspension comprising recycled cellulose fibers, and the separated aqueous liquid phase is recycled back to the process and reused to form the initial fiber suspension. The separated aqueous phase typically contains a base concentration of unconsumed first and / or second oxidizing agent. This means that in the method, recycling of the aqueous phase provides the possibility of reducing the amount of the first and / or second oxidizing agent required to adjust the ORP value to the desired level and to destroy or reduce bacterial endospores.

[0042] After having allowed performic acid to interact with the fiber suspension and having reduced the amount of bacterial endospores in the fiber suspension, if desired, the pH of the fiber suspension can be adjusted (e.g., neutralized) to the desired value by using a strong base such as, for example, NaOH or sodium bisulfite. Typically, neutralization is carried out after a suitable treatment or interaction time has elapsed after the introduction of performic acid. The treatment time required for the interaction between performic acid and bacterial endospores can be, for example, 15 min or 30 min.

[0043] The present invention is suitable for use in any production process in which cellulose fibrous webs are produced from recycled fibers, such as in the production of paper, cardboard, toilet paper, etc. The present invention is particularly suitable for the production of molded pulp products. The method of the present invention is particularly suitable for reducing the amount of bacterial endospores in a fiber suspension containing recycled cellulose fibers and designed for the production of liquid packaging boards, food packaging boards, etc.

[0044] According to a preferred embodiment of the invention, after the introduction (i.e., addition) of performic acid, the treated fiber suspension can be used to prepare a fibre stock which forms a fibrous web or a fiber layer in a multi-layer fibrous web. The fiber suspension can be diluted with water to a consistency suitable for forming the fibre stock and for forming the fibrous web or fiber layer. The fibre stock can also be used for the production of molded pulp products. The fibre stock can contain any conventional chemicals such as retention aids, sizing agents, wet strength and / or dry strength agents commonly used in the preparation of fibrous webs such as paper, cardboard, toilet paper, etc.

[0045] In addition to recycled fibers, the fibre stock can also contain virgin fibers. Preferably, the fibre stock contains at least 10 wt.-%, preferably at least 20 wt.-%, more preferably at least 30 wt.-% or at least 50 wt.-% of a fiber suspension comprising recycled cellulose fibers treated according to the method of the present invention.

[0046] Preferably, the amount of total bacterial endospores in the formed fibrous web or multi-layer fibrous web can be <5000 CFU / g dry fibrous web, preferably <2500 CFU / g dry fibrous web, more preferably <1000 CFU / g dry fibrous web, preferably <500 CFU / g dry fibrous web, and even more preferably <250 CFU / g dry fibrous web.

[0047] According to a preferred embodiment of the present invention, the method is used for producing food and / or liquid packaging grade paper or cardboard. Generally, the grammage of packaging grade cardboard can be 150 - 400 g / m 2 , preferably 200 - 360 g / m 2 , more preferably 240 - 300 g / m 2 . For barrier properties, the paper and cardboard grades for food and / or liquid packaging can be polymer-coated or foil-laminated. Suitable polymers for coating are (for example) polyolefins such as polyethylene or polypropylene; polyvinyl alcohol; polyvinylamine; polyethylene terephthalate; polybutylene terephthalate. Detailed Description

[0048] Experiment

[0049] Example 1

[0050] Example 1 was a laboratory test to study the killing effect of performic acid on bacterial endospores in a recycled fiber pulp sample at adjusted pH and redox potential (ORP).

[0051] In Example 1, 14% performic acid calculated as weight to volume ratio was used as a pre-formed conventional equilibrium solution of performic acid, formic acid, water, and hydrogen peroxide.

[0052] 1 liter of true recycled fiber (RCF) pulp was collected from an RCF mill that produces packaging cardboard from classified office paper RCF pulp. The RCF pulp sample had a consistency of 4.4 weight-%, pH 6.3, and an initial ORP value of -431 mV.

[0053] The collected RCF samples were divided into 6 sub-samples, each with a volume of 100 ml. Sub-samples 1, 2, 3 were heated to +30 °C by keeping them in a 30 °C water bath for 45 minutes. Sub-samples 4, 5, 6 were heated to +80 °C by keeping them in an 80 °C water bath for 45 minutes.

[0054] The pH in sub-samples 2, 3, 5, and 6 was lowered to pH <6.0. In sub-samples 2 and 5, the pH was adjusted by polyaluminum chloride (PAC, as 1600 ppm active), and in sub-samples 3 and 6, by 10% citric acid (as 100 ppm active).

[0055] After pH adjustment, sub-samples 2, 3, 5 and 6 were treated with hydrogen peroxide (at 600 ppm activity) and performic acid (14% PFA, at 200 ppm activity).

[0056] Sub-samples 1 and 4 were kept without any chemical treatment and they were used as reference samples.

[0057] For all samples, pH, ORP value and conductivity measurements were carried out as well as quantification of bacterial endospores (plate count agar, +32 °C, incubation for 2 days). For sub-samples 2, 3, 5 and 6, the measurements and endospore quantification were carried out after chemical treatment. Before aerobic spore determination, the samples were pasteurized at +82 °C for 10 min.

[0058] The results of Example 1 are shown in Table 1.

[0059] The results obtained in Table 1 show that regardless of whether the sub-samples were kept at 30 °C or 80 °C, the amount of bacterial endospores in the chemically untreated reference sub-samples 1 and 4 was high, 200000 CFU / ml or 500000 CFU / ml, and the ORP value was extremely low, -431 mV or -227 mV. It was observed that after contact at +30 °C and at +80 °C for 45 min, the chemical treatment with PAC / citric acid + H 2 O 2 + PFA drastically increased the final ORP value to a level above +300 mV and significantly reduced the amount of bacterial endospores by 1 - 4 logs (logarithms). In addition, it was observed from the results of sub-samples 3 and 6 that the chemical treatment including citric acid used for pH adjustment only caused a slight change in conductivity.

[0060] Example 1 shows that at +80 °C, an effective endospore killing effect was obtained by chemical treatment including citric acid, H 2 O 2 and PFA. This chemical treatment reduced the bacterial endospore level from 200000 spores / ml to 10 spores / ml, while the conductivity of the fiber suspension did not change significantly. Therefore, the results obtained clearly show that chemical treatment with performic acid, including pH and ORP adjustment, can be effectively used to reduce the amount of bacterial endospores in highly reducing RCF fiber suspensions without significantly changing the conductivity of the pulp. This can ultimately help RCF mills aim at the production of hygienic cardboard grades, where the bacterial endospore content in the final cardboard is a key hygiene standard.

[0061] Table 1 Results of Example 1.

[0062]

[0063] Although the present invention has been described with reference to what presently appears to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the described embodiments, but on the contrary, is intended to cover various modifications and equivalent technical solutions within the scope of the appended claims.

Claims

1. A method for reducing bacterial endospores in an aqueous fiber suspension comprising recycled cellulose fibers, wherein the fiber suspension has an initial endospore amount of ≥ 10000 CFU / ml, the method comprising (a) adjusting the pH of the fiber suspension to a pH value of ≤ 6.5, (b) adjusting the oxidation-reduction potential (ORP) of the fiber suspension to an ORP value of ≥ 200 mV using a first oxidizing agent in an amount of 300-1000 ppm, calculated as grams of active agent per ton of fiber suspension, the first oxidizing agent being H2O2, an organic peracid other than performic acid or a percarbonate, and (c) introducing performic acid as a second oxidizing agent into the fiber suspension in an amount of 50-500 ppm, calculated as grams of active agent per ton of fiber suspension, for reducing the bacterial endospores to an endospore amount of ≤ 1000 CFU / ml.

2. The method according to claim 1, characterized in that The fiber suspension contains inorganic particles, and the pH of the fiber suspension is adjusted to a range of 5-6.

5.

3. The method according to claim 2, characterized in that The inorganic particles are calcium carbonate particles.

4. The method according to claim 1 or 2, characterized in that The pH of the fiber suspension is adjusted by introducing an acidifying agent into the fiber suspension.

5. The method according to claim 1 or 2, characterized in that The temperature of the fiber suspension is at least 50°C.

6. The method according to claim 1, characterized in that The fiber suspension had a final conductivity value in the range of 2-10 mS / cm measured after adjusting the pH and ORP values.

7. The method according to claim 1, characterized in that The ORP value was adjusted to a range of +200 mV to +400 mV.

8. The method according to claim 1, characterized in that The first oxidizing agent is introduced into the fiber suspension in an amount of 400-800 ppm, calculated as grams of active agent per ton of fiber suspension.

9. The method according to claim 1, characterized in that Performic acid is introduced as the second oxidizing agent into the fiber suspension in an amount of 100-400 ppm, calculated as grams of active agent per ton of fiber suspension.

10. The method according to claim 1, characterized in that When the fiber suspension has a consistency of 1-5 wt-%, the adjustment of pH and ORP value is carried out in the beater or after the beater.

11. The method according to claim 1, characterized in that The adjustment of the pH and ORP value is carried out in a process step or process device in which the fiber suspension has a consistency of 8 to 30 wt-%.

12. The method according to claim 1, characterized in that After the amount of bacterial endospores of the fiber suspension has been reduced to ≤ 1000 CFU / ml, the fiber suspension is used to prepare a fiber stock, which is formed into a fibrous web.

13. The method according to claim 1, characterized in that After the amount of bacterial endospores of the fiber suspension has been reduced to ≤ 1000 CFU / ml, the fiber suspension is used to prepare a fiber stock, which is formed into a fiber layer in a multi-layer fibrous web.

14. The method according to claim 12, characterized in that The amount of bacterial endospores in the formed fibrous web was <5000 CFU / g dry fibrous web.

15. The method according to claim 13, characterized in that The amount of bacterial endospores in the formed multi-layer fibrous web was <5000 CFU / g dry fibrous web.

16. The method according to claim 12 or 13, characterized in that The fiber stock comprises at least 10 wt.-% of the fiber suspension.

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