Method for manufacturing a fibrous web

Abstract

A method includes forming an aqueous fibre suspension including cellulosic fibres from one or more raw material flows, and applying at least one chemical and/or physical control measure to the aqueous fibre suspension or at least one of its raw material flows for control of microbial activity in the aqueous fibre suspension or the raw material flow before an inlet of an intermediate residence entity. In this manner a starting ORP value for the aqueous fibre suspension is obtained. The aqueous fibre suspension is in the intermediate residence entity at least a minimum delay time. A final ORP value is measured for the aqueous fibre suspension after an outlet of the said intermediate residence entity before the formation of the fibrous web. An ORP difference value between the starting ORP and final ORP values is calculated. Finally, the aqueous fibre suspension is formed into a fibrous web and dried.

Claims

1. A method for manufacturing a fibrous web, such as web of a paper, a board, a tissue or the like, the method comprising: forming an aqueous fibre suspension comprising cellulosic fibres from one or more raw material flows, applying at least one chemical and/or physical control measure to the aqueous fibre suspension or at least one of its raw material flows for control of microbial activity in the aqueous fibre suspension or the raw material flow before an inlet of an intermediate residence entity, such as a storage tower or a broke tower, which has a minimum delay time of at least one hour, preferably at least two hours, and obtaining a starting ORP value for the aqueous fibre suspension, maintaining the aqueous fibre suspension in the intermediate residence entity at least the minimum delay time, measuring a final ORP value for the aqueous fibre suspension after an outlet of the said intermediate residence entity but before a formation of the fibrous web, calculating an ORP difference value between the starting ORP and the final ORP values, and if the ORP difference value exceeds a predetermined threshold value, then adjusting the applied chemical and/or physical control measure(s) until the ORP difference value falls below the predetermined threshold value, and forming the aqueous fibre suspension into a fibrous web and drying the fibrous web.

2. The method according to claim 1, wherein the predetermined threshold value for the ORP difference value is less than 100 mV.

3. The method according to claim 1, wherein the ORP difference value is below the predetermined threshold value at least for 90% of an observance period of 24 hours.

4. The method according to claim 1, wherein the predetermined threshold value for the ORP difference value is less than 90 mV, preferably less than 75 mV, more preferably less than 50 mV.

5. The method according to claim 1, wherein the final ORP value is in a range of 0-+350 mV, preferably 0-+200 mV, more preferably +50-+175 mV, even more preferably +100-+150 mV.

6. The method according to claim 1, further comprising determining a first rH value of the aqueous fibre suspension after the intermediate residence entity, wherein the first rH value is in a range of 21-32, preferably 21-27, more preferably 22-26, even more preferably 24-26.

7. The method according to claim 6, further comprising determining a second rH value for the aqueous fibre suspension before the intermediate residence entity, wherein a difference between the aqueous fibre suspension's rH values before and after the intermediate residence entity is less than 3, preferably less than 2.5, more preferably less than 1.5.

8. The method according to claim 1, wherein after the intermediate residence entity, the aqueous fibre suspension has a bacterial endospore content less than 400 CFU/ml, preferably less than 200 CFU/ml, more preferably less than 100 CFU/ml.

9. The method according to claim 1, further comprising determining a bacterial endospore content value of the aqueous fibre suspension before and after the intermediate residence entity, whereby a difference between the determined values is less than 100 CFU/ml.

10. The method according to claim 1, wherein a bacterial endospore content in the dried web is 1000 CFU/g, preferably 500 CFU/g, more preferably 250 CFU/g.

11. The method according to claim 1, wherein the aqueous fibre suspension is maintained in the intermediate residence entity for 1-12 h, typically 1-8 h, more typically 2-7 h.

12. The method according to claim 1, wherein the chemical control measure comprises feeding of a microbial control chemical to the aqueous fibre suspension or to at least one of its raw material flows.

13. The method according to claim 12, wherein the microbial control chemical is a biocide, a reductive chemical or an oxidative chemical.

14. The method according to claim 13, wherein the biocide is a non-oxidative biocide, preferably selected from glutaraldehyde, 5-chloro-2-methyl-4-isothiazolin-3-one (CMIT) and 2-methyl-4-isothiazolin-3-one (MIT).

15. The method according to claim 13, wherein the biocide is an oxidative biocide, preferably selected from monochloramine (MCA), chlorine dioxide, performic acid, or an N-containing compound combined with an oxidant, e.g. urea reacted with an oxidant, preferably hypochlorite.

16. The method according to claim 1, wherein the obtained starting ORP value for the aqueous fibre suspension is measured before its entry to the intermediate residence entity or at the entry to the intermediate residence entity.

Description

EXPERIMENTAL

Example 1

[0049] This laboratory test compared efficacy of two oxidizing biocides, namely free active chlorine and stabilized active chlorine, in killing of vegetative bacterial cells and in controlling of bacterial spore formation. Test was performed with authentic bacterial population of a broke sample taken from couch pit of a board machine making 3-ply food-packaging board. Broke sample was divided in equal proportions. Two reference samples were stored as such, the free active chlorine sample was treated with sodium hypochlorite, and the stabilized active chlorine sample was treated with sodium hypochlorite stabilized by 5,5-dimethyihydantoin (mixed in 1:1 molar ratio to form monochloro-5,5-dimethylhydantoin, MCDMH). Both forms of active chlorine were dosed at 10 ppm (=mg/l as total active chlorine Cl.sub.2). Broke samples were stored at +45 C. without mixing. Total aerobic bacteria and aerobic bacterial spores were quantified by using conventional agar plate cultivation methods (Plate Count Agar, incubation at +37 C. for 2 days) at the beginning of the test (untreated reference samples) and after 1 and 2 days of contact time. Broke pH and Redox (mV) values were also monitored. Results are shown in Table 1.

[0050] Results in Table 1 show that in the beginning of the experiment the untreated reference broke samples 1 and 2 contained relatively low amounts of aerobic bacteria (810.sup.2 and 110.sup.3 CFU/ml), and a small amount of aerobic bacterial spores (80 and 70 CFU/ml). During two days of storage the total aerobic bacteria level in the reference samples 1 and 2 increased up to 1-210.sup.7 CFU/ml, whereas the aerobic bacterial spore counts decreased down to 10 and 30 CFU/ml. This indicates, surprisingly, that the aerobic storage conditions (pH 7.5-7.9, redox 120-149 mV) at a typical board machine temperature favoured vegetative bacterial growth but did not cause any increase in bacterial sporulation. Broke sample treated with free active chlorine (sodium hypochlorite, no stabilizer) showed almost equal content of vegetative bacteria after 1 day storage time. This indicates that at a 10 ppm dosing level the free active chlorine did not demonstrate any longer-term killing effect in the broke sample. However, the treatment with free active chlorine caused a 10-fold increase in the quantity of aerobic spores (60 CFU/ml.fwdarw.670 CFU/ml) due to the stress caused by the free active chlorine. Treatment of broke with a stabilized chlorine (MCDMH, 10 mg/l as active chlorine) showed 1 log unit stronger reduction of total aerobic bacteria content compared to free chlorine (610.sup.4 CFU/ml compared to 410.sup.5 CFU/ml) after 1 day of storage. Further, the MCDMH did not cause any new spore formation in the broke and spore counts remained at 30-60 CFU/ml level during the 2 days experiment. After 2 days of storage all samples contained bacteria 1-210.sup.7 CFU/ml indicating that none of the oxidizer treatments showed a long-lasting killing effect.

TABLE-US-00001 TABLE 1 Results of Example 1. Start of the test 1 days storage time 2 days storage time Total aerobic Bacterial Total aerobic Bacterial Bacterial bacteria spores redox bacteria spores redox Total aerobic spores (CFU/ml) (CFU/ml) pH (mV) (CFU/ml) (CFU/ml) pH (mV) bacteria (CFU/ml) (CFU/ml) pH redox (mV) Reference broke 1, 1 10.sup.3 80 7.8 149 7 10.sup.5 70 7.8 146 2 10.sup.7 10 7.5 126 no added Active Chlorine Reference broke 2, 8 10.sup.2 70 7.9 138 6 10.sup.5 60 7.8 140 1 10.sup.7 30 7.5 120 no added Active Chlorine Broke, ND ND ND ND 4 10.sup.5 60 7.8 108 2 10.sup.7 670 7.6 68 Na-hypochlorite treatment, 10 mg/l Broke, ND ND ND ND 6 10.sup.4 30 7.9 86 2 10.sup.7 60 7.6 78 MCDMH treatment, 10 mg/l

[0051] Example 1 shows, surprisingly, that a biocide treatment is not absolutely necessary for preventing bacterial spore formation in machine broke. Test demonstrated that if broke from of a board machine is stored under suitable conditions, the spore formation can be minimized. This example also showed that if such aerobic broke is treated with free active chlorine, at dosages not providing a complete kill of bacterial cells, it can irritate remaining bacteria to spore formation. Surprisingly, treating the broke in similar manner with stabilized active chlorine is not causing bacterial spore formation.

Example 2

[0052] This laboratory test was performed with broke sample taken from an alkaline board machine producing 3-ply food-packaging board and the sample included the mill's authentic bacterial population in it. The sample was divided in two different containers, one stored as such and the second one amended with biocide, 50 mg/l of glutaraldehyde as active agent. Containers were closed and stored at +45 C. without mixing i.e. under conditions that simulate situation in broke storage tower during a machine shutdown. Total aerobic bacteria and aerobic spore contents were determined by using conventional agar plate cultivation methods (plate count agar, 2 days incubation at +37 C.) at the beginning of the test and after 3 days of storage time, along with pH and redox measurements.

[0053] Results are shown in Table 2.

TABLE-US-00002 TABLE 2 Results of Example 2. At start of the test After 3 days storage time Total aerobic Total aerobic bacteria Bacterial spores redox bacteria Bacterial spores (CFU/ml) (CFU/ml) pH (mV) (CFU/ml) (CFU/ml) pH redox (mV) Untreated broke sample 2 10.sup.7 125 7.9 137 5 10.sup.7 850 6.9 23 Broke, treated with ND ND ND ND 2 10.sup.7 80 7.4 145 glutaraldehyde

[0054] Results in Table 2 show that during 3 days of storage time, in the untreated broke sample, pH value (7.9.fwdarw.6.9) and redox value (+137 mV.fwdarw.23 mV) dropped markedly indicating that conditions in the broke turned from aerobic to fermentative during the storage time. Total aerobic bacteria counts increased from 210.sup.7 CFU/ml to 510.sup.7 and amount of aerobic spores increased from 125 CFU/ml to 850 CFU/ml.

[0055] Broke treated with 50 mg/l of glutaraldehyde biocide contained total aerobic bacteria 210.sup.7 CFU/ml after 3 days of storage, i.e. 40% of the untreated reference, indicating that this biocide treatment did not have a long-lasting killing effect. However, the treatment effectively prevented development of anaerobic fermentative conditions, i.e. redox (145 mV) and pH (7.4) remained at high level. Conditions were not triggering any spore formation in the broke sample and the broke contained only a low amount (80 CFU/ml) of spores after 3 days of storage.

[0056] Example 2 demonstrates that a biocide treatment which is not causing an intensive and long-lasting killing effect of bacterial cells can surprisingly well control spore formation in broke, as long as the biocide treatment is successful in preventing development of anaerobic conditions in the broke.

Example 3

[0057] This example compares technical performance of two different biocide programs in the broke system of a 3-ply board machine producing food-packaging board. Broke system is a part of the wet-end of the board making process. This board machine has set a hygiene target for the final board that it should contain aerobic bacterial spores less than 1000 CFU, and preferably less than 250 CFU, per gram of dry board.

[0058] In this experiment, for the first period (Days 1-10) the machine was running a biocide program consisting of stabilized active chlorine (MCDMH) and glutaraldehyde. For the second period, the machine was running chlorine dioxide, a non-stabilized oxidizer, as the biocide. It was running for 10 days starting from a shutdown (Days 15-25). Third period (Days 26-47) was run with the same MCDMH and glutaraldehyde program as the first period. During this experiment technical performance of the two different biocide programs was monitored at selected dates by several means: on-line Redox monitoring system collecting Redox values at every 10 minutes (results are shown as daily average mV values); measuring aerobic spore content of the final board samples; and by measuring aerobic bacterial spore quantities from different process locations by using agar plate cultivation methods (pasteurization at 82 C. for 20 min, followed by cultivation on Plate Count Agar for 2 days at +37 C.).

[0059] Results are shown in Table 3.

TABLE-US-00003 TABLE 3 Results of Example 3. Aerobic Bacterial Spores (CFU/ml) in process Bacterial Low High Spores in Redox (mV) consistency consistency Spore content Final in Couch Pit (1-4 w/w-%) (4-6 w/w-%) within the Incoming Board (daily avg) broke tower broke tower broke system Pulp (CFU/g) Day 1 197 110 270 +160 80 220 Day 2 192 150 200 +50 40 240 Day 10 191 80 110 +30 30 230 Day 15 387 250 440 +190 20 520 Day 19 492 610 1100 +490 60 5530 Day 21 477 ND ND ND ND 9580 Day 25 471 850 950 +100 ND 3210 Day 43 186 280 210 70 40 680 Day 47 173 50 170 +120 ND 210

[0060] Results in Table 3 show that during the first period (Days 1-10) the produced final board had spore content always <250 CFU/g and thus the board met the hygiene targets. During days 1-10 redox level of couch pit (=tank collecting and sending material to low consistency broke tower) was stable at +190-200 mV range. It is seen that during days 1-10 the broke system had stable aerobic conditions and Aspore content within the broke system (=difference between inlet and outlet) was generally low, indicating that intensive formation of new spore did not occur. Also other areas of the process, treated with the stabilized oxidizer MCDMH, contained generally low amounts of spores. For example, pulp transportation water (15 CFU/ml) and incoming pulp (30-80 CFU/ml) possessed low quantities of spores, indicating that the MCDMH biocide program did not trigger intensive spore formation.

[0061] During second period (Days 15-25) the system was treated with chloride dioxide. Dosing of this non-stabilized oxidizer increased Redox values in the system dramatically, e.g. in broke system from +190 mV range up to +492 mV. Interestingly, spore quantities also showed a strong increase, for example up to 1100 CFU/ml in the high consistency broke tower. Also spore content in the final board increased dramatically, to a magnitude higher values than what is the set hygiene target for final board, the highest value being as high as 9580 CFU/g. This indicates that the strong oxidative stress caused by non-stabilized oxidizer triggered intensive spore formation in the broke system of this board machine.

[0062] During the third period (Days 26-47) the process was treated with MCDMH and glutaraldehyde, similarly as during the first period. With a small delay the process conditions stabilized back to similar Redox range as during first experimental period, and interestingly, also spore values in the final board returned back to target level.

[0063] Results from Example 3 support the surprising finding that for the production of food-packaging board with a low content of aerobic bacterial spores, it is more effective to treat the system with biocides such as stabilized-oxidizers and glutaraldehyde in a manner providing stable aerobic conditions with moderate Redox values, compared to treating the system with oxidizing biocides and targeting high +380 to +500 mV Redox values in the broke system.

[0064] Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.