EP0652321A1

EP0652321A1 - Chemical pulp bleaching

Info

Publication number: EP0652321A1
Application number: EP94307702A
Authority: EP
Inventors: Lawrence J. Guilbault; Maurice Hache; David C. Munroe; David L.K. Wang; Graziella Teodorescu
Original assignee: Morton International LLC
Current assignee: Morton International LLC
Priority date: 1993-11-04
Filing date: 1994-10-20
Publication date: 1995-05-10
Anticipated expiration: 2014-10-20
Also published as: FI945202A0; FI945202A; EP0652321B1

Abstract

Wood pulp, such as kraft, chemical or hybrid pulp, is subjected to a bleaching process wherein it is treated first with one or more oxidising agents such as oxygen, ozone or hydrogen peroxide, and then with a reducing agent such as sodium borohydride or sodium hydrosulfite. A further treatment with an oxidising agent can be carried out after the reducing treatment. Brightness may be further enhanced by treatment with an enzyme such as xylanase or with a chelant such as diethylenetriamine pentaacetic acid (DTPA).

Description

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to a process for bleaching wood pulp to enhance its brightness, to improve brightness stability and generally to improve the physical properties of the pulp.
Wood pulp can be produced by numerous different processes which can be classified under the broad headings of mechanical, chemical and hybrid processes. Among the chemical processes are the kraft process wherein the wood chips or the like are treated at elevated temperature and alkaline pH with sodium sulfide, sodium hydroxide and sodium carbonate, and the sulfite process wherein the wood chips are treated at acid pH, again at elevated temperature, with sodium and magnesium bisulfites. The process of the present invention has been found to be particularly advantageous in relation to pulps produced by these chemical processes, but is not limited to any particular type of wood pulp.

Description of the Prior Art

Hitherto, different chemical bleaching processes have been used for different types of wood pulp, and in the past these have generally comprised treatments with various combinations of chlorine, chlorine dioxide, sodium hydroxide and sodium hypochlorite. For example kraft pulp has conventionally been treated with chlorine followed by two successive cycles of sodium hydroxide and chlorine dioxide. Sulfite pulp on the other hand has been treated successively with chlorine, sodium hydroxide, sodium hypochlorite and chlorine dioxide.
Because of the pollution risks associated with the use of chlorine, and in particular the risk to the ozone layer, in recent years ways have been studied to replace chlorine in such bleaching treatments and ultimately to replace chlorine compounds as well. To this end, various combinations of oxidising agents have been tried and in particular oxygen itself, ozone and hydrogen peroxide have been used in various combinations.
Kassepi et al (TAPPI) Pulping Conf, (Toronto) Proc, 327-340 (Oct 1992) disclose the effect of ozonisation on kraft-oxygen (K-O) and kraft oxygen-peroxide (K-OP) pulps. The K-OP pulps generally responded better to ozone bleaching than K-O pulps. It was found that too high an ozone concentration gave rise to carbohydrate degradation and decreased viscosity of the pulp.
According to Patt et al Holzforschung 45 (Suppl) 87-92 (September 1991), ozone can be used effectively in both pre-bleaching and final bleaching but both lignins and carbohydrates tend to be attacked. Carbohydrate degradation has to be suppressed by preacidification of the pulp, low-temperature bleaching, slow addition of ozone to the pulp and proper mixing. Under these conditions, an ozone concentration of 0.1% can reduce the pulp kappa number by one unit.
Numerous similar studies have been carried out in recent years.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a pulp bleaching process, preferably totally chlorine free, which gives a pulp of high and stable brightness without adversely affecting the physical properties of the pulp and in particular without degrading the polysaccharides or reducing the viscosity of the pulp.
This object is achieved according to the present invention in that there is provided a process for bleaching wood pulp which comprises treating the pulp, first with one or more oxidising agents and then with a reducing agent. A further oxidising treatment may optionally be carried out after the treatment with the reducing agent.
One preferred reducing agent is sodium borohydride. Another is sodium hydrosulfite, which may be generated in situ by a reaction between sodium bisulfite and sodium borohydride.
Preferred oxidising agents for the initial treatment of the pulp are oxygen, ozone and hydrogen peroxide. The various bleaching stages are preferably carried out at alkaline pH, for which purpose an alkaline metal hydroxide such as sodium hydroxide may be added.
Where a final oxidising step is carried out, this is preferably done with hydrogen peroxide and sodium hydroxide.
Brightness may be further enhanced by the use of enzymes such as xylanase and chelants such as ethylene diamine tetraacetic acid (EDTA) or, more preferably, diethylene triamine pentaacetic acid (DTPA).
The sodium borohydride is preferably used in alkaline solution, and a particularly advantageous form is an aqueous solution of 10 to 25 wt.% of sodium borohydride and 15 to 45 wt.% of sodium hydroxide. One commercially available solution of this type comprises 12% of sodium borohydride and 40% of sodium hydroxide, and is supplied by Morton International, Inc under the trade mark Borol. Another such solution developed more recently comprises 20% of sodium hydroxide and 20% of sodium borohydride.
Further objects and advantages of the invention will become apparent from the following detailed description and examples.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate results obtained in some of the examples. In the drawings:

FIGS. 1 and 2 illustrate the brightness gains obtained with varying borohydride addition rates in Example 1;
FIG. 3 illustrates the relationship between tensile strength and beating degree for treated and untreated kraft pulps in Example 2;
FIG. 4 illustrates the relationship between tear index and beating degree for the pulps of Example 2;
FIGS. 5 and 6 illustrate the brightness gains obtained in Example 3;
FIG. 7 illustrates the brightness gains and reversion obtained in Example 7; and
FIGS. 8 and 9 illustrate the brightness gains and reversion obtained in Example 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention will now be described in more detail and exemplified. For simplicity, the various different treatment steps which can be used in accordance with the invention are given the following letter codes:
The following bleaching sequences in accordance with the invention have been found particularly advantageous:
OZRD;
OZRP;
OXPRP;
OXPR;
OQPR;
OQPRP;
(OP) (OP_N) ZRP
The following processes are preferred for bleaching particular types of pulp:

SULFITE PULP (SOFTWOOD OR HARDWOOD)

ECF Processes (free of elemental chlorine):

OZRD OZRDY

OZYD OZDR

OZDY

TCF Processes (totally chlorine-free)

PY OZPY QPY

ZPY OZPR (EOP) PY

ZPR OZRP QPR

ZRP OZYP (EOP) PR

ZYP OZRPY

KRAFT PULP (SOFTWOOD OR HARDWOOD)

ECF Processes:

D(EOP)D(ER)D OZR(EOP)D

D(EOP)D(EOP)DY OZY(EOP)D

D(EOP)D(ER)DY

TCF Processes:

OQPY OXYP OZPY O(OP)ZRP

OQYP OXPY OZYP O(OP)ZPY

OQRP OXRP OZRP O(OP)ZRPY

OQRPY OXRPY OZPR O(OP)ZYP

OQPR OXPR OZRPY
In the following examples concentrations are based on ODP (oven-dried pulp) unless otherwise stated.

EXAMPLE 1

Totally chlorine free (TCF) bleaching process sequences OXPR and OXPRP were carried out on kraft pulp of NCC pine.
The oxidative bleaching sequence was as follows:

O:: O₂ at 3%, NaOH at 2.0-2.5%;
X:: Xylanase;
P:: H₂O₂ at 2.0 to 3.0%, NaOH at 2.0 to 3.0%, DTPA at 0.4%.

The pulp was taken out after SO₂ addition after the last peroxide stage and had a pH value of 4.5 and a consistency of 31.8%. The initial ISO brightness was 73.9% but was not stable and varied between 70.9 and 71.8% several days later. The pulp was diluted down to 10% consistency with distilled water and a series of reductive bleaching trials was carried out at 70°C, for a retention time of 60 minutes in each case.
The trials were conducted using sodium borohydride, alone and in combination with sodium bisulfite and with hydrogen peroxide.
A 2% wt. aqueous borohydride solution (NaBH₄:NaOH ratio 3:10, pH = 12.8) and 4% wt. NaHSO₃ solution (pH = 4.2) were prepared for the treatment. Aging tests were conducted according to TAPPI standard method (4 hrs at 105°C).
Aqueous solutions of 11.5% H₂O₂ and 10% NaOH were used for the peroxide bleaching. A 32% DTPA solution considered as 100% was diluted with distilled water to a 1% solution to be used for the peroxide stage.
After the pH adjustment to 4.5 with 1% H₂SO₄, handsheets were prepared.
The trials were performed with 20g samples in sealed polyethylene bags plunged into an agitating water bath.
The first series of the trials was conducted with different amounts of alkaline borohydride solution (NaBH₄ 12%, NaOH 40%, water 48%). The bleaching conditions and the results are listed in Table 1 below and the brightness response versus the NaBH₄ solution addition rate is illustrated in Figure 1, which also illustrates the brightness after aging. Figure 1 shows that the brightness response is increased very fast by adding the borohydride solution to 0.6%. A further increasing of chemical charge to 1% can achieve 0.5% brightness extra. A maximum brightness gain of 2.6% ISO points can be achieved by adding 1% of the borohydride solution.
The brightness reversion of the untreated kraft pulp was 1.1 points. The reductive treatment with sodium borohydride solution can offer a relatively stable brightness gain, if one compares the brightness gain before and after aging.
The second series of the trials was performed with 1% of the borohydride solution and subsequently with 1% hydrogen peroxide with and without washing between these two stages. As reference, the pulp was also bleached with 1% hydrogen peroxide alone. The bleaching conditions and the results are compiled in Table 2. The combined treatment was carried out for 180 min. and the peroxide-only treatment for 240 min.
A treatment with 1% hydrogen peroxide can increase the final brightness by 3.6 points. A reductive treatment with 1% of the borohydride solution increases the brightness by 3 points. A subsequent peroxide bleaching can increase the brightness by 2.8 points more.
The brightness response is slightly reduced (0.2 points) if the pulp is not washed between the borohydride solution stage and the peroxide stage. For the mill practice, this combination is very interesting, because the mill can not only save the investment cost for the washer, but also use residual NaOH of the borohydride solution.

Constant bleaching conditions :

	R-Stage	P-Stage
Temperature, °C	70	70
Consistency, %	10	10
Time. min.	60	180 (n° 6)
		240 (n° 7-8)
Consumption of H₂O₂, %	-	49.8 (n° 6)
		64.2 (n° 7)
		61.6 (n° 8)

The third series of the bleach trials was carried out with sodium bisulfite and a combination of sodium bisulfite and the sodium borohydride solution. The addition rate of NaHSO₃ was kept constant by 1.5%/o.d. pulp, while the NaBH₄ addition rate was varied between 0.1% and 0.5%. The treatment rates and brightness gains are given in Table 3.

Constant bleaching conditions

Temperature :	70°C
Time :	60 min.
Consistency :	10 %

Sodium bisulfite is a weak reductant. A reductive treatment of the TCF bleached kraft pulp with 1.5% NaHSO₃ can increase the final brightness by 0.3 points. A combination of sodium bisulfite and alkaline borohydride solution can improve the bleaching efficiency to gain 1.9 brightness points. In comparison with adding 0.6% borohydride solution only, little synergistic bleaching effect is obtained by adding two reductants serially. This has already been established in the case of mechanical pulp and deinked pulp. The reason might be due to the lower residual lignin content of the bleached chemical pulp. The brightness gains before and after aging are illustrated in Figure 2 which shows that the brightness stability of the treated only with 1.5 NaHSO₃ pulp is inferior to that of the combination of NaHSO₃ and borohydride solution.

EXAMPLE 2

To investigate the physical properties of the reductive treated pulp, 100g oven dry pulp as used in Example 1 were treated with 1% of an alkaline NaBH₄ solution having the composition of the Borol solution described above. The reaction time was prolonged to 90 min. to compensate for the difficult mixing of the pulp at the medium consistency of 10% in the laboratory. The treatment temperature was 70°C as in Example 1, with an initial pH of 11.8 and a final pH of 11.6
The yield of the borohydride treated pulp was 99.7% and the final brightness before and after aging was 74.6% and 73.3% ISO respectively. The physical properties of the untreated and treated pulps are set out in Tables 4 and 5 respectively. In comparison with the untreated TCF bleached pulp, the beating energy as well as the strength properties, especially tensile strength and burst index of the borohydride treated pulp, were positively influenced, whereas the tear index of the treated and untreated pulps remained at the same level, as can be seen from Figures 3 and 4.
The optical properties, i.e. opacity, absorption coefficient and light-scattering coefficient of the borohydride-treated pulp were reduced slightly due to the higher final brightness of the treated pulp (2.8 points higher). The viscosity of the borohydride treated pulp was increased from 748 dm³/kg to 762 dm³/kg. This can explain the improvement of the strength properties after the borohydride treatment.
Kappa number and all strength properties were determined according to ZELLCHEMING standards. Brightness, viscosity and light-scattering,were tested according to SCAN-C 11:75, SCAN-CM : 88 AND SCAN-C 27:76, respectively.

EXAMPLE 3

Bleaching trials similar to those of Example 1 but using a sequence OQPR were carried out on 20g samples of spruce kraft pulp. The O and P stages were as in Example 1 and the Q stage was carried out with EDTA. The pulp was taken out after SO₂ addition, at a pH value of 5.5.
The treatment conditions were the same as those of Example 1. The pH value of the pulp was adjusted to 5.5 with 0.1% H₂SO₄ before handsheets were prepared for brightness determination. The initial ISO brightness of the untreated pulp was 75.6% before aging and 74.2% after aging for 4 hours at 105°C.
The first series of the trials was performed by adding the alkaline borohydride solution only. The addition rate of this solution was varied between 0.2% to 1%.
Another series of the bleach trials was carried out with a serial combination of sodium bisulfite and the borohydride solution. The addition rate of NaHSO₃ was kept constant at 1.5%/o.d. pulp, while the borohydride solution addition rate was varied between 0.2% to 0.6%.
2% alkaline borohydride solution and 4% sodium bisulfite solution were prepared daily.
The results are shown in Table 6 and illustrated in Figures 5 (NaBH₄ solution only) and 6 (NaBH₄/bisulfite).
The maximum achieved brightness response was 3.6% ISO points by adding 0.6% of the NaBH₄ solution. A further increase of the amount of NaBH₄ solution reduced the bleaching efficiency of this solution, most probably because the high alkalinity of the solution causes a negative alkaline darkening effect.
The use of 1.5% sodium bisulfite improved the brightness response by 1.3 brightness points.
A combination of sodium bisulfite and alkaline NaBH₄ solution could increase the brightness gain by 3 points (see Fig. 6) but concerning the brightness response, there was little evidence of the synergistic bleaching effect of adding two reductants, which has been observed when bleaching mechanical and deinked pulp.

Constant conditions :

Temperature :	70 °C
Time :	60 min.
Consistency :	10 %
Pulp sample :	20 g oven dry pulp

The aging test was conducted for 4 hours at 105°C, according to the TAPPI-method. The brightness of the untreated and reductively treated pulps decreased after the aging.
Fig 5 and 6 show the brightness reversion varies between 1.4 and 2.5 points. The brightness reversion of the OP bleached pulp was 1.4 points. The brightness reversion of the reductively treated pulps varied between 1.4 and 2.5% ISO. However the final brightness of reductively treated pulps (after aging) was still 2.8 - 3.1 points higher than the final brightness of untreated pulp.

EXAMPLE 4

To investigate the physical properties after the reductive treatment, 100g o.d. pulp as used in Example 3 were treated with 1% of the Borol NaBH₄ solution described above. The results before treatment are listed in Table 7 and those after are listed in Table 8. The conditions were the same as in Example 2 and the pH was 11.7 before and after treatment. The physical properties were measured as in Example 2.
The yield of the borohydride-treated pulp was 99.8% and the final brightness was 78.5% ISO before aging and 77.8% after. In comparison with the untreated kraft pulp, the beating energy as well as the strength properties of the borohydride-treated pulp were not influenced significantly.
Due to the higher final brightness, the optical properties like opacity, absorption coefficient and light-scattering coefficient were reduced a little. The viscosity of the treated pulp was increased slightly as expected.
The variations of tear index and tensile strength with beating degree were similar to those obtained in Example 2 (see Figs. 3 and 4).

EXAMPLE 5

Reductive bleaching trials using sequences OQPR and OQPRP were carried out on samples of kraft hardwood (birch) pulp, the oxidative sequence being carried out as in Example 3. The pulp samples came from Sweden. The kappa number after the cooking process was 15 and 10 after the O-stage. The residual kappa number of the OQP bleached pulp was determined as 5.4 in the laboratory.
The trials were conducted by using the Borol NaBH₄ solution described above, directly or in combination with hydrogen peroxide. Two pulps, namely wet pulp (ledge) and pulp sheet, with an initial brightness of 79.7% ISO and 81.6% ISO were used for the laboratory bleach trials.
A first series of the trials was performed by adding the NaBH₄ solution to both pulp samples. The addition rate was varied between 0.2% and 1.2% (the solution contains 12% wt NaBH₄, 40% wt NaOH and 48% wt H₂O).
The second series of the bleach trials was done by adding 1% peroxide or 0.5-1% NaBH₄ solution and 1% peroxide with and without washing between these two stages.
The bleach trials were performed in sealed PE-bags plunged into an agitating water bath.
The following conditions were applied:

Temperature : 70°C

Consistency of pulp : 10% (15% for R-stage)

Time : 60 min for NaBH₄ and combination stage

240 min for peroxide stage

Sample weight : 10 g and 100 g o.d. pulp
A 2% wt NaBH₄ solution (Borol, described above, pH = 12.8) was prepared freshly each time for the treatment.
Aging tests were conducted according to TAPPI standard method (4 hrs at 105°C). The results are shown in Table 9.

Constant conditions :

Temperature :	70°C
Time :	60 min
Consistency :	10%
Pulp sample :	10 g oven dry pulp

The second series of the laboratory bleach trials was carried out with peroxide as well as with a combination of the NaBH₄ solution and peroxide. The bleaching conditions and the results are listed in Table 10. The physical properties of the treated and untreated pulp are listed in Tables 11 to 13.
1% H₂O₂ can increase the brightness of 2.6 points. Reductive treatments with 0.5 and 1% of the NaBH₄ solution increased the brightness response of 2.1 and 2.8 points respectively. Subsequently the pulps were bleached with 1% H₂O₂. The final brightness was 85.1% ISO and 85.4% ISO respectively. If the pulp was not washed between the R-stage and the P-stage, the brightness response was reduced by 0.2 and 0.4 points.
With respect to the viscosity, the initial pulp had a viscosity of 770 dm³/kg. After the NaBH₄ treatment, the viscosity of the pulp was increased to 800 dm³/kg. But the viscosity of the pulp dropped again to 756 dm³/kg after the P-stage. In comparison with peroxide bleaching the viscosity difference was not significant.
The yield of the peroxide treated and with the combination of NaBH₄ solution and peroxide treated pulp ranged between 99.8-99.7%.

Constant bleaching conditions :

	R-Stage	P-Stage
Temperature. °C	70	70
Consistency. %	15	10
Time. min.	60	240
Consumption of H₂O₂		45.2 (N° 30)
		64.2 (N° 31)
		67.0 (N° 32)
		63.0 (N° 33)
		77.3 (N° 34)

With an increasing chemical addition rate of NaBH₄ solution to 1%/o.d. pulp, a brightness gain between 2.5 and 3.3% ISO was achieved. A further increase of the NaBH₄ solution addition rate did not improve the bleaching efficiency due to the high alkalinity of the NaBH₄ solution.
After the aging test (4 hours at 105°C), a higher brightness loss on the pulp sheet than on the wet pulp was determined. The reason was that the latter pulp had already lost its brightness before the bleach performance. However we were able to increase the final brightness (after aging) of the pulp with 2.1%-2.4% ISO by using 1% of the NaBH₄ solution.
In the combined treatment, total brightness increases of 3.5 and 3.8% ISO were gained by using 0.5 and 1% of alkaline NaBH₄ solution in R-stage and 1% H₂O₂ in P-stage. The brightness response was reduced slightly to 3.3-3.4% ISO if the pulp was not washed between R- and P-stage.
By applying 1% H₂O₂ for the final bleaching stage, a brightness gain of 2.6 points was achieved, which was 0.9-1.2% ISO lower than with the combination.
The physical properties of the P and RP treated pulps did not change in comparison with the untreated pulp.
The optical properties, i.e. opacity, absorption coefficient and light-scattering coefficient of the P and RP treated pulps were reduced slightly due to the higher final brightness of the treated pulps.

EXAMPLE 6

Bleaching sequences with and without a borohydride treatment stage were carried out to compare the brightness level and viscosity of the resulting pulp. One pulp sample was subjected to the bleaching sequence (OP) (OP_N) ZRP with the results shown in Table 14.

TABLE 14

Stage No.	Treatment Step	Residual kappa No.	Brightness % ISO	Viscosity dm³/kg
O	Unbleached
	15	28	886
1	OP (O₂,NaOH,H₂O₂)	4.5	50.7	765
2	OP_N(O₂, NaOH, H₂O₂, nitrilamine	2.9	63.7	734
3	Z (O₃ at 0.2%)	1	80.7	639
4	R (1% Borol at 70°C)	-	83	680
5	P (1%H₂O₂, 1% NaOH at 70°C)	-	87.3	662

A second sample was subjected to steps 1 to 3 and 5 in Table 14, the R stage 4 being omitted. The resulting brightness was also 87.3% ISO, but the viscosity was only 568 dm³/kg. Thus, the process of the invention gave an equivalent brightness level to that achieved with oxidative bleaching only, but with a much lower viscosity loss. Indeed, it can be seen from Table 14 that the NaBH₄ bleaching stage itself gives a substantial gain in viscosity.

EXAMPLE 7

Bleaching trials were carried out on the pulp of Example 1, but using an OXPY bleaching sequence using varying amounts of borohydride-generated sodium hydrosulfite (BGH) and a procedure similar to that of Example 1. The hydrosulfite was produced by the following procedure, using NaBH₄ in the form of the Borol solution supplied by Morton International, Inc.

Reaction

Chemicals

With a yield that is considered 85% of Na₂S₂O₄, we need for 400 ml of a 2% hydrosulfite solution the following chemicals :

Borol solution : 3.06 ml

Bisulfite : 10.3 g

Sulfuric acid : 14 ml of a 4 N sol.

Water : 370 ml (dist.)

Preparation

10.3 g sodium bisulfite (98% dry) are weighed in a beaker to 0.1 g exact.
The bisulfite is washed into a 500 ml Erlenmeyer flask with the 370 ml distilled water.
A magnetic stirrer is used to mix the solution in the flask, and it is cooled continuously in an ice bath.
While stirring, the sulfuric acid is slowly added.
After cooling down again, the Borol solution is added very slowly (drop by drop).
The reaction will be vigorous during the addition of the borohydride solution. The reaction vessel cannot be closed because of the escaping hydrogen. The colour of the solution will change from orange to clear again, because of the pH change during the Borol solution addition.
After the reaction, the pH of the solution is adjusted between 9 and 10 with 25% NaOH and a pH meter.
Then the Erlenmeyer is closed with a cork, to exclude any oxygen.
The concentration of this solution is measured according to TAPPI standard method T622 CH-84.
It is advisable to use a fresh solution for every bleach trial.

The bleaching conditions as well as the results are illustrated in Table 15. The brightness response, before and after aging, is illustrated in Fig. 7.

TABLE 15

RUN N°	CHEMICAL (%/ o.d. pulp		pH		BRIGHTNESS (% ISO)		Δ-BRIGHTNESS (% ISO)
	B G H	NaOH	Initial	Final	Before aging	After aging	Before aging	After aging
blank	-	-	4.5	4.5	70.9	70.7	-	-
15	0.50	-	3.9	3.4	71.9	71.4	1.0	0.7
16	0.75	-	3.7	3.3	74.0	72.8	3.1	2.1
17	1.00	0.2	5.9	5.8	75.3	74.3	4.4	3.6
18	1.25	0.18	5.6	5.5	74.8	74.0	3.9	3.3
19	1.50	0.20	5.7	5.2	74.2	73.3	3.3	2.6
20	1.75	0.22	5.9	5.5	74.5	73.5	3.6	2.8
21	2.00	0.22	5.7	4.9	73.7	72.7	2.8	2.0

BGH solution

Concentration : 2.198 %

pH : 9.2

Constant bleaching conditions

Temperature :	70°C
Time :	45 min.
Consistency :	10 %

Fig. 7 illustrates that the brightness response is increased with an increasing BGH addition rate to a maximum of 4.4 brightness points, then is reduced again if the BGH addition rate is beyond 1%. The high amount of hydrosulfite decomposes more quickly before it can be used for the bleaching. In practice, a high amount of hydrosulfite is not only uneconomical but also can bring the corrosion problem due to the high sodium thiosulfate concentration.
The aging test shows that the brightness reversion of the BGH treated pulp is higher than that of the borohydride-treated pulp. The brightness reduction is kept lower than 1 point. A maximum brightness achievement of 3.6 points can be gained by adding 1% Na₂S₂O₄ after aging.

EXAMPLE 8

Bleaching trials were carried out on the pulp of Example 3, using an OQPY bleaching sequence, with varying amounts of borohydride-generated hydrosulfite and a procedure similar to that of Example 3. The BGH was produced by the procedure described in Example 7. The bleaching conditions and results are shown in Table 16 and the brightness response, before and after aging, is illustrated in Figure 8, which shows the results for a wet pulp sample, and Figure 9 which shows the results for a pulp sheet.
Figure 8 illustrates that the brightness response of the wet pulp sample is increased with an increasing BGH addition rate to a maximum of 2.4 brightness points, then is reduced again if the BGH addition rate is beyond 0.5%. On the pulp sheet the maximum brightness response of 2.6 points is achieved by adding 0.75% BGH (see Figure 9).
To optimize the bleaching conditions of BGH, the effects of pH value of the pulp and of adding chelant DTPA were examined in more detail. The results in Table 16 show that the pH value of the pulp and adding 0.2% DTPA cannot significantly influence the brightness response of the BGH bleach performance.

Constant conditions :

Temperature :	70°C
Time :	45 min
Consistency :	10%
Pulp sample :	10 g oven dry pulp

The aging test shows that all reductive treatments have a lightly better brightness stability than that of the untreated pulp, but that the brightness reversion of the BGH treated sheet pulp is higher than that of the borohydride-treated sheet pulp. But for the wet pulp sample, the brightness response with BGH is lower than that with borohydride solution. Therefore the brightness reversion of the BGH treated pulp is also less than that of the borohydride-treated pulp.

Claims

A process for bleaching wood pulp which comprises treating the pulp first with at least one oxidising agent and then with a reducing agent.
A process as claimed in claim 1 wherein the reducing agent is sodium borohydride.
A process as claimed in claim 2 wherein the sodium borohydride is used in an aqueous alkaline solution.
A process as claimed in claim 3 wherein said aqueous solution comprises 10 to 25 wt.% of sodium borohydride and 15 to 45 wt.% of sodium hydroxide.
A process as claimed in claim 4 wherein said aqueous solution comprises about 12% sodium borohydride and about 40% sodium hydroxide and is used in an amount of 0.5 to 2.0% based on oven dried pulp (ODP).
A process as claimed in claim 1 wherein the reducing agent is sodium hydrosulfite.
A process as claimed in claim 6 wherein the sodium hydrosulfite is generated in situ by reaction between sodium bisulfite and sodium borohydride.
A process as claimed in any preceding claim wherein said oxidising is selected from oxygen, ozone and hydrogen peroxide.
A process as claimed in any preceding claim wherein a further oxidation treatment is carried out after the treatment with said reducing agent.
A process as claimed in claim 9 wherein said further oxidising treatment is carried out using hydrogen peroxide.
A process as claimed in any preceding claim wherein said treatments with oxidising and reducing agents are carried out at alkaline pH.
A process as claimed in any preceding claim wherein said pulp is treated with an enzyme to enhance brightness.
A process as claimed in claim 12 wherein said enzyme is xylanase.
A process as claimed in any preceding claim wherein said pulp is treated with a chelant to enhance brightness
A process as claimed in claim 14 wherein said chelant is diethylenetriamine pentaacetic acid (DTPA).