US20040211533A1

US20040211533A1 - Method for bleaching and color stripping recycled fibers

Info

Publication number: US20040211533A1
Application number: US10/832,182
Authority: US
Inventors: Qiang Huang; Rangamannar Goda; Thomas Crowley; Joseph Bettano
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-04-28
Filing date: 2004-04-26
Publication date: 2004-10-28

Abstract

A method for bleaching and color stripping recycled fibers by treating the fibers with a mixture of sodium borohydride and sodium bisulfite.

Description

This invention relates generally to a method for bleaching and color stripping recycled fibers with sodium borohydride and sodium bisulfite.

One type of reductive bleaching process involving sodium borohydride and sodium bisulfite for bleaching recycled fibers is disclosed in U.S. Pat. No. 5,562,803. This reference describes a sequential addition of sodium bisulfite and sodium borohydride in a two-stage process, with sodium bisulfite being added first, followed by addition of sodium borohydride to generate hydrosulfite. This process is referred to as the “DBI” process.

The problem addressed by this invention is to find a more efficient process using bisulfite and borohydride for reductive bleaching and color stripping of recycled fibers.

STATEMENT OF THE INVENTION

This invention is directed to a method for bleaching and color stripping recycled fibers by treating the fibers with a mixture of sodium borohydride and sodium bisulfite.

In a first embodiment of the invention, the method comprises the steps of: (a) adding sodium bisulfite to a recycled fiber slurry to produce a bisulfite-treated slurry; and (b) adding to the bisulfite-treated slurry a mixture produced by combining: (i) an aqueous solution comprising sodium borohydride and sodium hydroxide; and (ii) an aqueous solution comprising sodium bisulfite.

In a second embodiment of the invention, the method comprises combining: (i) an aqueous solution comprising sodium borohydride and sodium hydroxide; and (ii) an aqueous solution comprising sodium bisulfite, to produce a combined solution, and adding the combined solution to an aqueous slurry of recycled fibers; wherein the ratio of (moles bisulfite - moles hydroxide)/moles borohydride is from 0 to 25.

DETAILED DESCRIPTION OF THE INVENTION

All percentages are expressed as weight percentages, unless specified otherwise. The term “recycled fibers” refers to paper that has been subjected to reuse for the purpose making paper or paper products again. The term “pre-mix” refers to a process in which borohydride and bisulfite are mixed prior to addition to the pulp.

Dithionite ion, also referred to as hydrosulfite, can be produced by the reaction between bisulfite and borohydride ions, according to the following theoretical equation:

BH₄ ⁻+8HSO₃ ⁻+H⁺→4S₂O₄ ⁻²+B(OH)₃+5H₂O

The yield is somewhat less than 100% due to competing reactions, including that of borohydride with water, but is most often better than 85%. Since the exact mechanism of the reaction has not been fully characterized, this invention is not limited to reduction by dithionite ion, and other species present in the reaction mixture also may act as reducing agents. When the amount of bisulfite is at 8 moles per mole of borohydride, the theoretical reaction proceeds to completion. Without wishing to be bound by theory, it is believed that use of more than the theoretical amount of bisulfite or less than the theoretical amount of bisulfite results in a mixture containing hydrosulfite, sodium bisulfite, borohydride and possibly other species.

In the first embodiment of the invention, in which sodium bisulfite pre-treatment is performed, the pre-treatment is carried out by adding the bisulfite to the recycled pulp slurry to maintain a pH of the slurry between 5.0 and 8.5. Preferably, the pH is no more than 8.0, more preferably no more than 7.8, and most preferably no more than 7.5. Preferably, the pH is at least 5, more preferably at least 5.5, and most preferably at least 6.0. Preferably, the amount of bisulfite added to the pulp slurry as a pre-treatment, measured as the percentage of sodium bisulfite relative to the dried fiber content of the pulp, is at least 0.025%, more preferably at least 0.15%, and most preferably at least 0.25%. Preferably, the amount of bisulfite added to the pulp slurry, measured as the percentage of sodium bisulfite relative to the dried fiber content of the pulp, is no more than 5%, more preferably no more than 3%, more preferably no more than 2% and most preferably no more than 1%. In a preferred embodiment of the invention, a 38% aqueous sodium bisulfite solution is used. Use of bisulfite pre-treatment is preferred for lower-brightness fibers, i.e., fibers having an ISO brightness less than 75%, and especially for fibers at less than 65%. For lower-brightness fibers, preferably at least 0.15% bisulfite is added for pre-treatment.

In a preferred embodiment of the invention, borohydride is added in the form of an aqueous solution containing sodium borohydride and sodium hydroxide. Also aqueous sodium bisulfite solution is mixed with aqueous solution containing sodium borohydride and sodium hydroxide. In this embodiment, some of the bisulfite is consumed in a neutralization reaction with the hydroxide ion. In some applications, hydroxide ion present in borohydride solutions is neutralized by acid added to the bisulfite solution. In such a case, to the extent that the hydroxide initially present in the borohydride solution has been neutralized, it will not consume bisulfite, and will not be included in the ratio calculation. As described above, the theoretical reaction of borohydride and bisulfite requires 8 moles of unconsumed bisulfite per mole of borohydride, i.e., the ratio (moles bisulfite-moles hydroxide)/moles borohydride is approximately 8. The present invention uses a ratio from 0 to 25. Preferably, the ratio is no more than 23, more preferably no more than 20, and most preferably no more than 18. Preferably, the ratio is at least 2, more preferably at least 3, and most preferably at least 4.

In the first embodiment of the invention, in which the fiber slurry is pre-treated with sodium bisulfite, followed by borohydride and bisulfite solutions which are combined and then added to the slurry, the ratio (moles bisulfite-moles hydroxide)/moles borohydride combined for addition to the pulp (i.e., excluding the bisulfite used for pre-treatment) preferably is at least 3, more preferably at least 5, more preferably at least 6, and most preferably at least 7; preferably the ratio is no more than 15, more preferably no more than 14, more preferably no more than 13, and most preferably no more than 12. The time interval between addition of sodium bisulfite and addition of the pre-mixed solutions is not believed to be critical. Preferably, a time interval sufficient to ensure thorough mixing of the bisulfite and the pulp is allowed, preferably at least 30 seconds, more preferably at least one minute, more preferably at least five minutes, and most preferably at least ten minutes. Preferably, the time interval is no longer than 5 hours, more preferably no longer than 2.5 hours, and most preferably no more than 1 hour.

In the second embodiment of the invention, in which there is no sodium bisulfite pre-treatment, the ratio (moles bisulfite-moles hydroxide)/moles borohydride preferably is from 2 to 7 or from 9 to 16, more preferably from 2 to 6 or from 10 to 15, and most preferably from 2 to 5 or from 11 to 15. The data provided below in the Examples demonstrates, unexpectedly, improved performance at lower or higher than the theoretical 8 moles of unconsumed bisulfite per mole of borohydride.

In one embodiment of the invention, combining water and sodium metabisulfite, Na ₂S₂O₅, generates bisulfite. The aqueous sodium bisulfite preferably is about 2% to about 45% active by weight. A preferred borohydride composition for use in accordance with the methods of the invention is in liquid form and comprises about 1% to about 36% active sodium borohydride and about 30 to about 40% NaOH or Na₂CO₃(also known as soda ash), all by weight. A particularly preferred borohydride composition comprising 12% active sodium borohydride and 40% NaOH is commercially available from Rohm and Haas Company under the trademark ChromaClear™ solution. (For example, 100 g of ChromaClear™ solution contains 12 g sodium borohydride, 40 g NaOH, and 48 g H₂O). When the sodium borohydride solution contains sodium hydroxide, e.g., ChromaClear™ solution, the theoretical equation for reaction with bisulfite is as follows:

[NaBH ₄+3.2 NaOH]+11.2 NaHSO₃→4 Na₂S2O₄+(NaBO₂+3.2 Na₂SO₃+9.2H₂O)

In this case, where there are 3.2 moles of hydroxide per mole of borohydride, and the hydroxide has not been neutralized with a mineral acid, the ratio of bisulfite unconsumed by hydroxide to borohydride is (11.2−3.2)/1=8.0, i.e., the theoretical ratio.

In the first embodiment of this invention, preferably, the addition of sodium bisulfite for pretreatment is carried out by adding bisulfite solution to the pulp slurry after the slurry has been screened, cleaned, thickened and made ready for papermaking i.e., after the deckers in a typical pulp mill. In one preferred embodiment of the invention, the bisulfite is added to the MC standpipe in which pulp slurry accumulates prior to being pumped to the up-flow tower or the chest. Preferably, the total of the sodium bisulfite added for pre-treatment and the sodium bisulfite combined with sodium borohydride is at least 0.05%, more preferably at least 0.1%, more preferably at least 0.2%, and most preferably at least 0.5%; preferably the total amount is no more than 5%, more preferably no more than 3%, and most preferably no more than 2%.

In both the first and second embodiments of this invention, a borohydride solution and a bisulfite solution are combined to produce a mixture which is then added to the pulp slurry. Preferably, the solutions are combined by mixing them in an in-line static mixer, a T-pipe mixer, a pump recirculator, an agitator, the suction side of a pump impeller, a high-shear mixer, or a low-shear mixer. Most preferably, the mixer is an in-line static mixer. Typical in-line static mixers have from 2 to 24 internal elements, preferably from 2 to 6 internal elements. The length of the piping from the mixer to the point of addition to the pulp slurry also may affect the mixing; preferably this length is at least 3.28 ft (1 m), more preferably at least 4.92 ft (1.5 m). The number of elements, the diameter of the mixer and the length of piping required to achieve good mixing, i.e., to produce a substantially homogeneous mixture, can be determined easily from the flow parameters and fluid properties of each particular system. For example, in one method dye is added to one of the solutions and good mixing is assessed by visible determination that the color of the output is uniform. In another method, the pH of the pulp slurry after addition of the mixed borohydride and bisulfite solutions is measured; a stable pH value is an indication of good mixing, as are consistent bleaching results. Preferably, if mixing is insufficient, the borohydride and bisulfite solutions are diluted. Preferably, the borohydride and bisulfite solutions are mixed at a temperature in the range from 4° C. (39.2° F.) to 50° C. (122° F.), more preferably from 10° C. (50° F.) to 35° C. (95° F.).

Preferably, the mixed borohydride and bisulfite solutions are added to the pulp slurry directly, or by storing the output in a vessel for later addition to the pulp slurry. In one preferred embodiment, the output of the mixer is stored in a vessel and added to the pulp slurry within 12 hours of mixing, more preferably within 6 hours, more preferably within 3 hours, more preferably within 1 hour, and most preferably within ½ hour of mixing. In another preferred embodiment, the mixer output is added directly through piping which carries the output to the pulp slurry in less than 15 minutes, more preferably less than 10 minutes, and most preferably less than 5 minutes.

Preferably, the amount of borohydride combined with sodium bisulfite, measured as the percentage of sodium borohydride relative to the dried fiber content of the pulp, is at least 0.003%, more preferably at least 0.006%, and most preferably at least 0.009%. Preferably, the amount of borohydride, measured as the percentage of sodium borohydride relative to the dried fiber content of the pulp, is no more than 0.12%, more preferably no more than 0.08%, and most preferably no more than 0.04%. In a preferred embodiment of the invention, a 12% aqueous sodium borohydride solution is used, e.g., ChromaClear™ solution. In this embodiment, the weight of the solution used, measured as a percentage of the dried fiber content of the pulp, is at least 0.025%, more preferably at least 0.05%, and most preferably at least 0.075%. Preferably, the weight of solution used, measured as a percentage of the dried fiber content of the pulp, is no more than 1%, more preferably no more than 0.67%, and most preferably no more than 0.33%.

Preferably, the mixed borohydride and bisulfite solutions that are the output of the mixer are added to the pulp slurry after pre-treatment of the pulp slurry with bisulfite. In another preferred embodiment, the mixed solutions containing higher/lower than stoichiometric amount of bisulfite are added to fibers in the de-fibering stage, for example in the refiners or grinders or in the above locations without any bisulfite pretreatment.

EXAMPLES

Example 1

Laboratory Studies [0022]
Table 1 shows the results of a comparison between pre-mix and bisulfite pre-treated pre-mix processes on a sorted office waste furnish. Both pre-mix and bisulfite-pretreated pre-mix were performed with the solutions of bisulfite and borohydride combined at a bisulfite to borohydride molar ratio of 13.3:1. Bisulfite pretreatment was conducted at 10 lb/ton (0.5% based on dry fiber) of sodium bisulfite, followed immediately after thorough mixing of the bisulfite into the pulp by pre-mix addition to the pre-treated pulp. Bleaching conditions are listed in Table 1. It is clear that bisulfite-pretreated pre-mix performed significantly better than pre-mix especially at lower pre-mix treat rates. Table 2 shows the results of pre-mix at 12:1, 15:1, and 18:1 bisulfite to borohydride molar ratios on a mixed office waste furnish. Bleaching conditions can be seen in Table 2. Pre-mix at the higher ratios of 15 and 18 performs better than pre-mix at the lower ratio of 12. Table 3 shows the results of pre-mix at 2.9:1 and 12:1 bisulfite to borohydride molar ratios on a sorted color furnish. Bleaching conditions can be seen in Table 3. It is clear that in this case, pre-mix at lower SBS ratio of 2.9:1 unexpectedly performs better than pre-mix at higher SBS ratio of 12:1. Table 4 compares the pre-mix process to the DBI process at various bisulfite to borohydride ratios on a mixed office waste furnish, and demonstrates that the pre-mix process performs better than the DBI process. [0023]
Pre-mix Bleaching Process [0024]
Based on consistency, 7 g O.D. pulp was placed in heavy gauge polyethylene bags. The bags were sealed under nitrogen, shaken vigorously to disperse the pulp fiber, and preheated in a constant temperature bath to get to the desired bleach temperature. Pre-mix solutions were generated from ChromaClear™ solution and NaHSO[0025] ₃(SBS). In the method of generating the pre-mix solution, sodium bisulfite powder was added to water in a round bottom flask and stirred until the sodium bisulfite powder had completely dissolved. ChromaClear™ solution was then immediately added under an inert atmosphere and under rapid stirring to generate a completely formed pre-mix solution. Based on the borohydride concentration of each solution, the required volume of pre-mix solutions was calculated. Bleaching was carried out by adding the pre-mix solution under nitrogen and keeping the pipette below the surface of the pulp.

Each bag was resealed, shaken thoroughly to mix, and returned to the constant temperature bath for the desired time. At the end of the bleaching period each bag was removed from the bath and the pH was measured and recorded. The pulp was then diluted to 1% using deionized water. One handsheet was made from each run and air dried overnight at 50% relative humidity. Brightness readings were measured using a Technibrite™ ERIC 950 and are the average of five readings from each 7 g O.D. handsheet.

TABLE 1


Laboratory bleaching response of pre-mix and bisulfite
pretreated pre-mix processes.

			SBS:CC
			Mole	pH	pH	Bright.
Process	CC, %	SBS, %	Ratio¹	Initial	Final	% ISO	L*	a*	b*

No	—	—		9.1	—	78.8	92.8	0.69	2.91
bleach
Pre-mix	0.05	0.275	13.3	9.1	8.0	83.5	95.3	−0.69	3.62
Pre-mix	0.1	0.55	13.3	9.1	7.6	84.6	95.6	−0.71	3.36
Pre-mix	0.15	0.825	13.3	9.1	7.4	85.3	95.8	−0.78	3.10
Pre-mix	0.2	1.1	13.3	9.1	7.3	85.6	96.0	−0.80	3.25
Pre-SBS²+ Pre-mix	0.05	0.275	13.3	7.1	7.2	85.2	95.6	−0.37	2.84
Pre-SBS²+ Pre-mix	0.1	0.55	13.3	7.1	7.2	85.5	95.7	−0.43	2.84
Pre-SBS²+ Pre-mix	0.15	0.825	13.3	7.1	7.1	85.6	95.7	−0.5	2.81
Pre-SBS²+ Pre-mix	0.2	1.1	13.3	7.1	7.1	86.0	95.8	−0.48	2.75

[0027]

TABLE 2

Laboratory bleaching comparison at higher molar ratio bisulfite

SBS:CC

Mole pH pH

Process CC, % SBS, % Ratio¹ Initial Final Bright. % ISO L* a* b*

Pre-mix 0.25 1.25 12 7.5 7.0 63.8 87.8 −0.70 7.5

Pre-mix 0.25 1.5 15 7.5 6.9 64.3 87.9 −0.80 7.5

Pre-mix 0.25 1.75 18 7.5 6.8 64.3 87.9 −0.90 7.5
[0028]

TABLE 3

Laboratory bleaching comparison at lower and higher molar ratio of

(bisulfite - hydroxide)/borohydride

SBS:CC

Mole pH pH

Process CC, % SBS, % Ratio¹ Initial Final Bright. % ISO L* a* b*

No 5 5 22.3 61.2 11.8 11.1

Bleach

Pre-mix 0.5 1 2.9 5 6.7 28.3 69.5 11.8 16.5

Pre-mix 0.5 2.5 12 5 5.4 27.1 67.2 12.7 14.3

TABLE 4


Laboratory bleaching comparison of pre-mix and DBI processes

			SBS:CC
			Mole	pH	pH
Process	CC, %	SBS %	Ratio¹	Initial	Final	Bright. % ISO	L*	a*	b*

No	—	—		8.3	—	74.2	90.8	1.4	3.18
bleach
DBI	0.05	0.20	8.9	8.40	8.42	74.6	91.2	1.01	3.52
	0.10	0.40	8.9	8.40	8.23	78.2	92.8	0.41	3.32
	0.15	0.60	8.9	8.40	8.05	79.3	93.1	0.36	3.12
	0.20	0.80	8.9	8.40	7.90	80.1	93.4	0.31	2.91
	0.30	1.20	8.9	8.40	7.90	80.7	93.5	0.31	2.65
	0.40	1.60	8.9	8.40	7.70	81.2	93.6	0.33	2.42
DBI	0.05	0.35	18.0	8.54	8.06	75.3	91.5	0.88	3.47
	0.08	0.30	18.0	8.54	7.86	78.8	92.8	0.42	3.04
	0.10	0.70	18.0	8.54	7.70	79.5	93.1	0.28	2.98
	0.15	1.05	18.0	8.54	7.53	80.3	93.4	0.25	2.78
	0.20	1.40	18.0	8.54	7.39	80.3	93.3	0.19	2.74
	0.25	1.75	18.0	8.54	7.30	80.5	93.4	0.15	2.72
	0.30	2.10	18.0	8.54	7.22	80.6	93.4	0.11	2.72
Pre-Mix	0.050	0.20	8.9	8.40	7.90	78.1	92.7	0.41	3.36
	0.075	0.30	8.9	8.40	7.86	79.0	93.1	0.31	3.25
	0.088	0.35	8.9	8.40	7.79	79.4	93.2	0.24	3.24
	0.100	0.40	8.9	8.40	7.76	79.8	93.4	0.28	3.12
	0.150	0.60	8.9	8.40	7.63	80.2	93.5	0.22	2.99
	0.200	0.80	8.9	8.40	7.54	80.5	93.6	0.27	2.85
	0.250	1.00	8.9	8.40	7.47	81.0	93.7	0.27	2.73
	0.300	1.20	8.9	8.40	7.40	81.3	93.7	0.22	2.58
Pre-Mix	0.050	0.25	12.0	8.50	7.86	78.3	92.8	0.31	3.41
	0.075	0.38	12.0	8.50	7.81	79.2	93.2	0.24	3.33
	0.100	0.50	12.0	8.50	7.70	79.7	93.3	0.22	3.19
	0.150	0.75	12.0	8.50	7.54	80.4	93.6	0.17	3.03
	0.200	1.00	12.0	8.50	7.44	80.3	93.4	0.15	2.93
	0.250	1.25	12.0	8.50	7.37	80.7	93.5	0.15	2.77
	0.300	1.50	12.0	8.50	7.28	81.0	93.6	0.12	2.72
Pre-Mix	0.050	0.30	15.0	8.41	7.79	77.5	92.3	0.68	3.17
	0.075	0.45	15.0	8.41	7.69	79.3	93.1	0.40	3.04
	0.088	0.53	15.0	8.41	7.66	79.5	93.2	0.37	3.02
	0.100	0.60	15.0	8.41	7.59	80.0	93.4	0.36	2.94
	0.150	0.90	15.0	8.41	7.43	80.6	93.5	0.27	2.74
	0.200	1.20	15.0	8.41	7.32	80.0	93.3	0.3	2.75
	0.300	1.80	15.0	8.41	7.13	80.9	93.5	0.26	2.53
	0.500	3.00	15.0	8.41	6.99	81.3	93.6	0.24	2.32

Example 2

Mill Trial [0030]
The results comparing the pre-mix process and the SBS pretreated pre-mix process are presented in Table 5. It was demonstrated that the SBS pretreated pre-mix performance is superior to pre-mix alone and the higher molar ratio (12:1) pre-mix run is also better than the pre-mix run at a molar ratio of 9.4:1. [0031]
Pre-mix Bleaching Process: [0032]
Based on the ChromaClear™ solution dosage for the bleaching application and the selected molar ratio during the pre-mix process the sodium bisulfite dosage was determined. The molar ratio of (bisulfite-hydroxide)/borohydride in the pre-mix process without bisulfite pre-treatment varied from 9.4 to 12.0 during the trial. The required flow of the ChromaClear™ solution and sodium bisulfite solution was calculated based on the tonnage rate of the pulp bleached with pre-mix solution. The ChromaClear™ solution was supplied in poly totes and the sodium bisulfite solution was stored in a tank truck. Both the ChromaClear™ and sodium bisulfite solutions were controlled by variable speed drives and adjusted manually. The flow rate was checked using a calibration column type setup. ChromaClear™ solution was diluted to about 2% of its original concentration and the sodium bisulfite solution was diluted to about 5% bisulfite. The diluted ChromaClear™ solution in the main stream was blended with diluted sodium bisulfite solution in the side stream in a T-type connection. The chemicals flowed through a Koflo™ in-line static mixer (model 1-40-2-6-2, 6 elements, 1 inch (2.54 cm) diameter, and 9 inch (22.9 cm) length) just prior to injecting the pulp on the discharge of the 2nd stage washer. The distance between the T-type connection, at which the bisulfite solution side stream was introduced into the borohydride stream, and the static mixer was less than 1 feet (0.30 m) (estimated). The distance between the static mixer and the bleaching injection point was about 6 feet (1.8 m) (estimated). [0033]

Base-line data were collected first without any pretreatment. Immediately following the baseline data collection, the SBS pretreatment was started and the pre-mix process was run at a 9.4:1 molar ratio after 90-120 minutes and the results compared with the base line. In another trial, the pre-mix process without bisulfite pre-treatment was run at a 12:1 molar ratio. The ChromaClear™ solution dosage was 0.16-0.18% on bone dry fiber. Brightness was measured on the unbleached sample collected at the brown stock tank and the bleached pulp sample collected just before the pulp entering the third washer. The retention time was about 90-120 minutes at 170° F. Samples were collected every 60 minutes during the trial. The data in the table were averaged during the trial run (refer to Table 5).

TABLE 5


Mill trial

			SBS:CC		Bright.	Bright.
			Molar	Run	Inlet	Outlet	Brightness
Process	CC, %	SBS, %	Ratio¹	Time	% ISO	% ISO	Gain

Pre-	0.18	0.75	9.4:1	10 hrs	61.1	68.8	7.7
mix
Pre-	0.18	0.75	9.4:1	10 hrs	60.0	69.7	9.7
SBS²+ Pre-mix
High	0.17	0.85	12:1	27 hrs	62.4	71.4	9.0
SBS:CC
ratio
Pre-mix

Claims

1. A method for bleaching and color stripping recycled fibers; said method comprising steps of:

(a) adding sodium bisulfite to a recycled fiber slurry to produce a bisulfite-treated slurry; and

(b) adding to the bisulfite-treated slurry a mixture produced by combining: (i) an aqueous solution comprising sodium borohydride and sodium hydroxide; and (ii) an aqueous solution comprising sodium bisulfite.

2. The method of claim 1 in which an amount of bisulfite used for pretreatment ranges from 0.025% to 5% based on weight of dry fiber.

3. The method of claim 2 in which a ratio of (moles bisulfite-moles hydroxide)/moles borohydride combined to produce said mixture is from 0 to 25.

4. The method of claim 3 in which an amount of sodium borohydride used to produce said mixture is from 0.003% to 0.12%, based on the weight of dry fiber.

5. The method of claim 4 in which a ratio of (moles bisulfite-moles hydroxide)/moles borohydride combined to produce said mixture is from 3 to 15.

6. A method for bleaching and color stripping recycled fibers; said method comprising combining: (i) an aqueous solution comprising sodium borohydride and sodium hydroxide; and (ii) an aqueous solution comprising sodium bisulfite, to produce a combined solution, and adding said combined solution to an aqueous slurry of recycled fibers;

wherein a ratio of (moles bisulfite - moles hydroxide)/moles borohydride is from 0 to 25.

7. The method of claim 6 in which an amount of sodium bisulfite added is from 0.05% to 5%.

8. The method of claim 7 in which the ratio of (moles bisulfite-moles hydroxide)/moles borohydride is from 2 to 7 or from 9 to 16.

9. The method of claim 8 in which an amount of sodium borohydride added is from 0.003% to 0.12%, based on the weight of dry fiber.

10. The method of claim 9 in which the ratio of (moles bisulfite-moles hydroxide)/moles borohydride is from 9 to 16.