GB2176812A - Bleaching of cellulosic material - Google Patents

Abstract

Process for the delignification and bleaching of lignocellulosic material wherein the material is treated with chlorine dioxide in combination with hypochlorite in the presence of an acid. The hypochlorite treatment may precede or follow that with ClO2.

Description

SPECIFICATION Bleaching of cellulosic materials This invention relates generally to improvements in processes for bleaching cellulosic materials in the form of pulp, such as wood pulp, and particularly to the rapid bleaching of pulps.

Pulping is the changing of wood chips or other lignocellulosic material to fibrous form. Chemical. pulping requires cooking of such material in solution with a chemical in a "digester". The process includes partial removal of the colouring matter such as lignin associated with the wood.

Pulp, as it comes from the digester, whether produced from hardwood or softwood or from non-woody materials, contains residual coloring matter. While unbleached pulp may be used for the manufacture of certain grades of paper, for example, heavy wrapping paper and paper for use in bags, pulp which is to be used for printing or paper for sanitary paper must be bleached.

Depending upon the nature of the raw pulp and the end use of which the pulp will be employed, various chemical bleaching stages and various sequences of these stages which have been used heretofore. Among the principal chemical bleaching stages which have been used are the chlorination stage (designated "C"), the caustic extraction stage (designated "E"), the hypochlorite stage (designated "H"), and the chlorine dioxide stage (designated "D"). In addition, both chlorine and chlorine dioxide may be used in the same stage (designated "C,") or ("doc") and the chemicals may be used as a mixture or added sequentially. Various combinations of the above stages have been employed depending upon the specific conditions and bleaching requirements.For example, common bleaching sequences may include the following: CEH, CEHD, CEHED, CEHDED, and CEDED. Of these, the CDEDED and CDEHDED are the recognized standard sequences for producing 88+GE Brightness market pulp.

The CEDED sequence produces a high brightness pulp with a minimum of vicosity loss or cellulose degradation to the pulp. This results in a pulp which has high strength properties. The CEDED sequence also produces high brightness, but heretofore the hypochlorite stage has caused degradation of the cellulose albeit in a controlled fashion. This results in some loss in paper strength, but the pulp requires less mechanical beating in order to develop its maximum strength, compared with pulp bleached by the CDEDED sequence. The CDEHED sequence is used to make the same type of pulp as the CDEHDED sequence, but it has one fewer stage for control of brightness. The CEH sequence is used for semibleached pulp in the brightness range 65 to 75 GE Brightness Standard.The CDEHD sequence has not normally been used for pulp requiring brightness greater than 86 GE because bleaching to higher brightness with this sequence generally results in a severe loss in viscosity and strength.

In the initial chlorination stage, chlorine is added to the washed pulp received from the digester. Ordinarily, the chlorination stage (C) is performed at temperatures in the range of about 30"C to 500C, with a pulp consistency of about 3 percent. Under these conditions, the reaction time in the chlorination tower is about 30 to 60 minutes. The chlorine reacts directly with the lignin and other impurities in the pulp. Chlorine dioxide may be used in conjunction with chlorine (CD) or in place of chlorine for the initial chlorination stage (D).

Following the chlorination stage, a caustic extraction stage (E) using a dilute aqueous solution of sodium hydroxide (0.5 to 5.0 percent NaOH based on oven-dry weight of pulp) is performed to dissolve the chlorinated and oxidized lignin as well as some of the resin. The extraction stage is usually performed at tempertures of about 50"C to 80"C for a period of about 60 to 120 minutes with a pulp consistency of 10 to 20 percent.

The next stage of bleaching is commonly a hypochlorite stage (H), although a chlorine dioxide stage is sometimes preferred. In the hypochlorite stage, either sodium hypochlorite (NaOCI) or calcium hypochlorite (Ca(OCI)2) is used to further oxidize the remaining lignin and other impurities in the pulp. Some degradation of the pulp as a result of shortening the chain length of the cellulose molecule usually occurs in the hypochlorite stage.

Following the hypochlorite stage, there may be a second alkaline or caustic extraction stage (E) or a chlorine dioxide stage (D).

According to the present invention there is provided an improved process for the delignification and bleaching of lignocellulosic material which comprises treating the material with chlorine dioxide (D) in combination with hypochlorite (H) in the presence of an acid.

In accordance with one embodiment of the present invention the chlorine dioxide treatment is carried out prior to the addition of hypochlorite.

According to another embodiment of the present invention the hypochlorite treatment precedes the addition of chlorine dioxide. The acid is perferably carbonic, acetic, nitric hydrochloric, sulfuric or phosphoric acid, with carbonic acid being particularly preferred.

The treatment is preferably carried out at a temperature between 48"C and 60"C.

Bleaching is the treatment of cellulosic fibers to remove or alter the coloring matter associated with the fibers to allow the fiber to reflect white light more truly.

The present invention is concerned with the degradation of lignin and hence "delignification" and "bleaching" are used interchangeabiy.

Consistency is the weight of pulp fibers in a pulp suspension usually expressed as a percentage. For example, one pound oven dry fiber in one hundred pounds of a mixture of water and fibers would be a pulp suspension of 1% consistency. The consistency of the pulp will depend upon the type of dewatering equipment used.

One measure of the efficacy of a bleaching process is the degree of delignification. There are many methods of measuring the degree of delignification of the pulp, but most are variations of the permanganate test.

The permanganate test used herein provides a Permanganate number (KNO) a measure of potassium permanganate solution consumed by oven dry pulp under specified conditions. The Permanganate number is determined in accordance with TAPPI standard test method T 214 SU71.

Brightness in the examples which follow was measured with an Elrepho brightness meter in accordance with TAPPI standard T2 1 7M. Viscosity measurements were made using a capillary viscometer method as outlined in TAPPI procedure T-230. The viscosity value (centipoise or "CPS") is useful to determine the degree of cellulose degradation during bleaching.

In the Tables, "oven dried" is abbreviated "OD", "hypochlorite" is abbreviated "hypo" and "centipoise" is abbreviated "cps".

The present invention may be further illustrated by reference to, but is in no manner limited by, the following Examples.

Example 1 An 85 gram sample of unbleached Northern softwood kraft pulp having a Permanganate number of 17.5 was mixed with chlorine water at a charge level of 5.6% chlorine based on oven dry weight of the fibers in a closed glass bottle in a consistency of 3.5%. The bottle was immersed in a water bath maintained at 290C for 45 minutes. After the retention, the chlorinated pulp was washed well with tap water on a vacuum drain Buchner funnel employing filter paper.

The chlorinated pulp was then subjected to caustic extraction employing the following conditions: 2.5% NaOH based on oven dry pulp, 65.5"C, 90 minutes and 11% consistency. The extracted pulp was washed in the same manner described above for the chlorinated pulp 5g of the chlorinated and caustic extracted pulp (CE pulp) was used for CEK number determination (the Permanganate number after the CE stage). The determined CEK number was 3.3 which was in the normal range practiced in a pulp mill (3.0-6.0). The remaining 80 grams of the CE pulp were equally divided into two 40 gram batches. The first batch was treated with chlorine dioxide alone as a control of first chlorine dioxide bleaching stage (called Dl stage).The control D, bleaching was carried out under the following conditions: 1.1% Cl02 charged based on OD pulp, 63"C, 150 minute retention time and 10% consistency. In this ClO2 bleaching experiment the CE pulp mat was broken up in a Hobart mixer with addition of the required amount of dilution water for consistency adjustment. The well broken-up pulp slurry was then transferred into a glass bottle, equipped with air-tight cap. Prior to addition of chlorine dioxide solution the pulp slurry in glass bottle was preheated to 57-63"C. The required amount of chlorine dioxide solution was added to the pre-heated pulp slurry in the bottle.Upon adding chlorine dioxide solution the bottle opening was closed immediately with an air-tight cap and was shaken vigorously to promote a thorough mixing between the chlorine dioxide solution and the pulp slurry. After said mixing the glass bottle was immersed in a water bath maintained at 63"C. At the end of the bleaching reaction, the pulp was checked for pH and residual chlorine dioxide and then washed with tap water in the same manner described before. A handsheet weighing 5 grams was prepared for brightness measurement of the D1 stage. After air drying the sheet, the initial brightness was measured. The reverted brightness was read after aging the same sheet in a hot air circulating oven at 40.6"C for 60 minutes.

The second 40 grams of CE treated pulp mat was broken up in a Hobart mixer. Thereafter, 100ml of carbonic acid, which was prepared by saturating CO2 gas into deionized water, was added while agitating. Immediately after the carbonic acid addition, a sodium hypochlorite solution was added at a charge level of 0.55% available chlorine based on oven dry weight of fiber with agitation. Upon the hypchlorite addition, the carbonic acid and hypochlorite mixed pulp slurry was transferred to a glass bottle equipped with an air-tight cap and preheated to 57-63"C. After the preheating, a chlorine dioxide solution was added at 50% of the charge level in the control bleaching, 0.55% ClO2 based on ovendry weight of fiber in the pulp. The tightly closed glass bottle containing pulp fibers, carbonic acid, sodium hypochlorite and chlorine dioxide solution was immersed in a water bath maintained at 63"C for 90 minutes. The pulp consistency in the bottle was 10%, just like the control D1 stage bleaching. The bleached pulp was washed and measured for brightness in the same manner of the first batch.

After the first chlorine dioxide bleaching, the two pulp samples were further subjected to the second caustic extraction (E2) and second chlorine dioxide bleaching (D2) under the same conditions as follows: E2 stage: 0.5% NaOH based on OD pulp 74"C, 60 minutes retention time 11% consistency D2 stage: 0.4% ClO2 based on OD pulp 74 C, 200 minutes and 10% consistency After the D2 stage, initial and reverted pulp brightness and viscosity of the fully bleached pulp were measured.

The first batch thus was bleached in multi-stage bleaching sequence CED,E2D2 and the second batch was bleached in multistage bleach sequence CE(H+D)E2D2. The chlorine dioxide bleaching with a combination of acid and sodium hypochlorite solution is referred to here as (H+D). The bleaching results of control CEDED and CE(H+D)ED bleaching are summarized in Table 1.

TABLE 1 Batch 1 Batch 2 Bleaching Sequence CED,E2D2 CE(H + D)E2D2 D, or (H+D) stage % CIAO, on OD Pulp 1.1 0.55 % Hypochlorite on OD Pulp 0.0 - 0.55 % Substitution of Hypochlorite for C102 0.0 50.0 % Total Available Chlorine Used 2.90 2.00 D, Stage Brightness Initial 81.1 81.2 Reverted 77.2 76.5 D2 Stage Brightness Initial 88.6 88.4 Reverted 84.8 84.3 D2 Pulp Viscosity (cps) 14.1 14.6 From the results shown in Table 1, it is evident that sodium hypochlorite can be substituted for 50% of the ClO2 normally used in the first chlorine dioxide stage in the multistage bleaching sequence CEDED. The chlorine dioxide bleaching with a combination of carbonic acid and hypochlorite gave the same brightness obtained with 100% chlorine dioxide in the first chlorine dioxide bleaching stage.Contrary to the teachings of the conventional chlorine dioxide and hypochlorite bleaching art, the addition of acid and hypochlorite at the high temperature did not cause the expected viscosity drop during the (H+D) bleaching.

In this example, one part by weight of hypochlorite replaced one part by weight of chlorine dioxide even though one part by weight of hypochlorite is equivalent to 0.38 part by weight of chlorine dioxide in terms of oxidizing equivalent. In this novel bleaching process, one part by weight of hypochlorite reacts like one part of chlorine dioxide in the presence of acid. The (H+D) bleaching therefore reduced the total available chlorine usage in the first chlorine dioxide bleaching stage from 2.9% to 2.0%, which was a 31% reduction.

Example 2 A 100 gram sample of unbleached Southern pine kraft pulp having a Permanganate number of 18.5 was treated, as described in Example 1, with an aqueous solution of chlorine so that 5.94% chlorine was added based on the weight of the fibers. The chlorination treatment was carried out at a consistency of 2.8% for 30 min at 32"C. The chlorinated pulp was washed in the manner described in Example 1. The chlorinated pulp, at 3.5% consistency, was then treated with sodium hypochlorite solution and caustic soda solution for 20 minutes at 380C. The hypochlorite solution was added at a charge level of 1.5% available chlorine based on oven dry weight of fiber, at the same time the caustic solution was added at a charge level of 2.0% NaOH based on oven dry weight of fiber.

The chlorinated and hypochlorite treated (CH) pulp was equally divided and one batch was bleached with chlorine dioxide alone as a control D, bleaching and the other was bleached with a combination of carbonic acid and sodium hypochlorite and chlorine dioxide. The chemical charge level was 0.9% Cl02 based on oven dry weight of fiber in the first batch and for the second batch sodium hypochlorite solution was added at a charge level of 0.45% which is 50% of the 0.99/0 ClO2 added in the first batch, along with 60 cc carbonic acid solution and 0.45% ClO2. The addition was made in the same manner as in Example 1.

After the D, bleaching for the first batch and the (H+D) bleaching for the second batch, air dried brightness was measured for the two bleached samples. The two batches were then further treated with second caustic extraction stage with addition of sodium hypochlorite (EH stage) and then with chlorine dioxide bleaching in the second D stage under the same conditions. The bleaching results on Southern pine kraft pulp are shown in Table II.

TABLE II CHD1EHD2 CH(H + D)EHD2 Batch 1 Batch 2 D, or (H+D) -Stage % Cl02 on OD Pulp 0.9% 0.45 % Hypochlorite on OD Pulp 0.0 0.45 Volume of Carbonic Acid Added 0.0 60cc D, Stage-End pH 3.3* 2.6 Brightness 71.2 71.3 Total Available Cl2 Used (%) 2.37 1.63 D2 Stage Initial Brightness 87.7 87.8 Reverted Brightness 82.4 83.0 Pulp Viscosity, cps 7.7 7.7 0.25% NaOH was added to the D, stage as pH buffer.

EH conditions: 0.7% NaOH+0.45% sodium hypochlorite, 71"C, 60 Min.

D2 conditions: 0.4% Cl02 on OD pulp, 74"C, 90 Min.

As the result in Table II shows, it is again evident that 50% of the chlorine dioxide used in the first dioxide bleaching stage can be replaced by hypochlorite at an equal weight basis without affecting the D, stage brightness as well as the final brightness after the D2 stage in multistage bleaching process CHD,EHD2. It is again demonstrated that addition of hypochlorite and acid did not cause any pulp viscosity drop. Example 2 also proves that the (H+D) bleaching is applicable whether the chlorinated pulp is treated with caustic extraction or with alkaline hypochlorite. It should be noted that the total available chlorine usage in the first chlorine dioxide bleaching stage was reduced by 31% in the (H+D) bleaching as compared to the control D, bleaching.

Example 3 The unbleached pulp employed in this example was Canadian softwood kraft pulp obtained from a mill located on the east coast of Canada. The Permanganate number of the unbleached pulp was 16.3.

After treating the pulp by chlorination with 4.63% Cl2 at 2.8% consistency for 35 minutes at 38"C, the chlorinated pulp was then subjected to alkali/oxygen (E/o) treatment in a modified laboratory pressurized reactor under the following conditions: % NaOH 2.315% on OD weight of fiber Consistency 15% Retention Time 45 minutes Temperature 74"C 2 Pressure 100 PSIG (7 Kg/cm2) pH In/Out . 12.4/10.5 Permanganate number after CE/O: 2.5 80 grams of the oxygen treated pulp s ample was equally divided into two batches; one was treated with 0.9% ClO2 (based on OD pulp) and the other was treated with 0.45% ClO2 plus 0.45% sodium hypochlorite with addition of 50cc of carbonic acid. After the D, and the (H+D) bleaching, the pulp was further treated with caustic extraction and chlorine dioxide bleaching under the same conditions. The results of bleaching sequences CE/ODED and CE/O(H+D)ED are shown in Table Ill.

TABLE 111 Sequence CE/OD,E2D2 CE/O(H+D)E2D2 D1 or (H+D) Stage % Cl02 on OD Pulp 0.9 0.45 % Hypo on OD Pulp 0.0 0.45 Temperature ("C) 60 60 Time (Min.) 120 110 Consistency (%) 8 8 D, Brightness Initial 77.6 78.6 Total Available Chlorine (% Cl2) 2.37 1.63 D2 Stage Initial Brightness 87.2 . 87.8 Reverted Brightness 83.7 83.7 E2 stage conditions: 0.35% NaOH on OD pulp, 71"C, 120 Min., 12% consistency D2 stage conditions: 0.2% Cl02 on OD pulp, 71"C, 165 Min., 10% consistency From the results of Example 3 shown in Table Ill, it may be seen that in (H+D) bleaching the chlorine dioxide normally used after the E/O stage can be replaced to the extent of 50% with sodium hypochlorite at an equal weight basis.As compared to the D, bleaching the (H+D) bleaching even gave slightly higher initial brightness after the D, and D2 stage with the oxygen treated pulp with a 31% reduction of the available chlorine usage.

Example 4 A 175 gram sample of unbleached Southern pine kraft pulp having a Permanganate number of 16.3 was treated with 5.2% chlorine in the same manner as in Example 2. The chlorinated pulp was then neutralized with alkali for an extraction step at 3.5% consistency for 20 minutes at 38"C. The Permanganate number of the chlorinated and neutralized pulp (CN pulp) was determined to be 4.3. The CN pretreated pulp was equally divided into 5 batches for the first chlorine dioxide bleaching. In order to determine how much hypochlorite can substitute ClO2 in the first chlorine dioxide bleaching stage, percent substitution level was increased from 0% (control), 20%, 50%, 70% and 100% (only hypochlorite).After the D, or (H+D) bleaching each pulp was further treated with alkali extraction with addition of caustic and hypochlorite (EH) and chlorine dioxide bleaching (D2) under the same manner described in Example 2. The bleaching results of increased percent substitution of hypochlorite in multi-stage bleaching process CND,EHD2 are shown in Table IV. The changes in Dl brightness and D2 brightness and pulp viscosity by increasing the percent substitution are illustrated therein. As the table shows, a maximum substitution level is near 70% of the chlorine dioxide used in the control D1 stage. Up to the 70% substitution neither final brightness nor pulp viscosity was adversely affected.

The Table IV also shows that chemical savings can increase as percent substitution increases, since at 1983 prices the cost of sodium hypochlorite is about 50% less as expensive as the cost of chlorine dioxide at an equal weight basis.

TABLE IV EFFECTS OF SUBSTITUTION OF HYPOCHLORITE FOR CHLORINE DIOXIDE IN THE FIRST CHLO RINE DIOXIDE BLEACHING STAGE IN CND,EHD2 BLEACHING OF SOUTHERN PINE KRAFT PULP Dl or (H+D STAGE % Substitution of Hypo 0% 30% 50% 70% 100% % Hypo 0 0.36 0.6 0.84 1.2 % Cl02 1.2 0.84 0.6 0.36 0 Carbonic Acid 0.0 40cc 80cc 120cue 160cue Temp., "C 57 57 57 57 57 Time, Min. 90 90 90 60 35 % Consistency 8 8 8 8 8 EH Stage % NaOH 0.75 0.75 0.75 0.75 0.75 % Hypo 0.50 0.5 0.5 0.5 0.5 D2 Stage, % ClO2 0.4 0.4 0.4 0.4 0.4 CN KNO 4.3 4.3 4.3 4.3 4.3 D, Brightness (Initial) 66.3 68.9 68.9 66.4 70.8 (% Residual) (0.15) (0.11) (0.06) (0.0) (0.0) D2 Brightness Initial 86.9 87.8 87.9 87.2 86.7 Reverted 83.3 83.6 83.1 82.4 82.0 (% Residual) (0.012) (0.083) (0.085) (0.072) (0.073) D2 Viscosity (CPS) 8.0 8.6 8.3 8.3 6.2 Total Avail.Cl2 Charged (%) 9.903 9.3 8.93 8.54 7.85 Total Avail. Cl2Consumed (%) 9.251 8.81 8.55 8.29 7.76 EXAMPLE 5 The Northern softwood kraft pulp used in Example 1 was treated with chlorination and caustic extraction under the following conditions: C stage: 5.2% chlorine based on OD pulp 3.5% consistency, 29"C and 45 minutes E stage: 2.5% NaOH based on OD pulp 10% consistency, 65.5"C and 90 minutes CE-Permanganate number=4.3 The CE pulp was equally divided into three batches. The first batch was treated with only chlorine dioxide at 1.1 % CIO2 charge level based on oven dried pulp as a control first chlorine dioxide bleaching.The second batch was treated with the (H+D) bleaching with addition of carbonic acid at chemical charge levels of 0.55% ClO2 and 0.55% hypochlorite (50% substitution level). The third batch was treated with the (H+D) bleaching without addition of carbonic acid.

All three batch bleachings were carried out in 8% consistency at 63"C. When hypochlorite was added to chlorine dioxide, the retention time had to be shortened from 180 minutes to 60 minutes because the addition of acid and hypochlorite accelerated chlorine dioxide bleaching reaction. The three batches were further treated with the second caustic extraction and the second chlorine dioxide bleaching as in Example 1. The results of this experiment are given in Table V.

Comparing the result of batch 1 with that of batch 2. the 50% substitution of hypochlorite for chlorine dioxide with addition of carbonic acid gave higher reverted brightness of the resulting pulp, indicating the (H+D) bleaching improved the brightness stability. As the final brightness and viscosity of the batch 3 are compared with those of batch 2, without the acid addition, the resulting pulp will have lower reverted brightness. less stable brightness and lower pulp viscosity indicating some cellulose degradation during the (H+D) bleaching. From the results, it is evident that the acid addition is required to prevent the pulp viscosity loss and to improve the brightness stability in the (H+D) bleaching process.

Table V EFFECTS OF CARBONIC ACID ADDITION NORTHEASTERN SOFTWOOD-KRAFT PULP Batch 1 Batch 2 Batch 3 D, (H+D) (H+D) 1.1% CIAO, 180cc Carbonic Acid 180cc tap water 0.55% hypo 0.55% hypo 0.55% ClO2 0.55% ClO2 Temp. ("C) 63 63 63 Time (Min.) 180 60 60 Consistency (%) 8 8 8 E2, % NaoH 0.5 0.5 0.5 D2, % Cl02 0.35 0.35 0.35 D2 Bright.

Initial 87.2 87.5 87.6 Reverted 83.0 84.4 83.5 D2 Viscosity, cps 18.7 18.9 17.2 EXAMPLE 6 The CE prebleached pulp was prepared from the Northern softwood kraft pulp as discussed in Example 1. The CE pulp prepared was equally divided into three batches for the (H+D) bleaching followed by E2 and D2 bleaching. The first batch was treated with carbonic acid and 0.55% hypochlorite followed by 0.55% ClO2. The second batch was treated with the same amount of hypochlorite and chlorine dioxide but without addition of carbonic acid. The third batch was first treated with a 0.55% CIAO, for 120 minutes at reaction temperature of 63"C and, at the end of the ClO2 reaction, 0.55% hypochlorite was injected with carbonic acid.After a few minutes of retention at 63"C the chlorine dioxide and hypochlorite bleached (D+H) pulp was washed. The results of these bleaching experiments are shown in Table VI.

As may be seen from a comparison of the result of batch 1 with that of batch 2, it is again evident that without carbonic acid, the reverted brightness suffered as shown in Example 5. The result of batch 3 indicates that in this novel bleaching process the addition of hypochlorite and acid can be reversed; instead of the addition prior to Cl02 addition the hypochlorite and acid can be added at the end of ClO2 bleaching reaction in the same bleaching stage. The reverse addition may improve further the final brightness of the resulting pulp as shown in the Table VI.

TABLE VI (H+D) (D+H) Batch 1 Batch 2 Batch 3 Mode of Treatment Sequence 1. Carbonic Acid & 1. No carbonic acid 1. 0.55% 0.55% hypo 0.55% hypo ClO2 2. 0.55% ClO2 2. 0.55% ClO2 2. 2 hours reaction at 63"C 3. 0.55% hypo and carbonic acid E2 (0.5% NaOH) E2 (0.5% NaOH) E2 (0.5% NaOH) D2 (0.4% ClO2) D2(0.4% C102) D2 (0.4% ClO2) Brightness D1 (Initial) 76.4 75.6 75.9 D2 (Initial) 89.1 88.9 90.3 (Reverted)86.8 82.8 87.0 EXAMPLE 7 In this example, different modes of addition of carbonic acid/hypochlorite/ClO2 solution in the (H+D) bleaching stage were evaluated. Four different modes were evaluated: Mode 1. Simultaneous addition of carbonic acid and hypochlorite solution followed by 30 minutes retention at room temperature and then addition of ClO2 solution without washing step after the hypochlorite addition and retention.

Mode 2. The same mode of addition as Mode 1 but with a washing step prior to the ClO2 addition.

Mode 3. Simultaneous addition of carbonic acid and hypochlorite solution and ClO2 solution while agitating.

Mode 4. Sequential addition of carbonic acid first and hypochlorite solution and ClO2 solution last without retention time and washing between each addition.

The CE pulp sample was prepared by the same manner as in Example 1.

All four (H+D) bleaching experiments were carried out at 63"C in 10% consistency but with varying retention times. The effects of different addition modes were monitored by reading initial and reverted brightness of the resulting pulp after the (H+D) stage. The results are shown in Table VII. From the result, it is obvious that this novel bleaching with a combination of hypochlorite and chlorine dioxide does not require retention in time or heating and washing step between the hypochlorite addition and chlorine dioxide addition, and that the addition of acid, hypochlorite and chlorine dioxide solutions can be added simultaneously or sequentially. However, the sequential addition may be more effective than the simultaneous addition.

Mode of Addition 1 2 3 4 -Carbonic Acid+ -Carbonic Acid+ Simultaneous Sequential hypo (simult.) hypo addition of addition carbonic acid/ -30 minutes -30 minutes hypo/ClO2 (Carbonic acid retention at retention room temp. -Hypochlorite -No washing washing ClO2) -ClO2 addition -ClO2 addition -No retention and washing after hypo Final pH 2.0 2.3 2.5 2.3 ClO2 Residual (titre) 0.3cc 0.2cue 0.3cue 0.3cue Ret. time in (H+D) stage 120 min. 120 min. 80 min. 90 min.

(H+D) Brightness Initial 76.6 75.1 74.1 75.2 Reverted 73.2 72.6 71.5 72.6 EXAMPLE 8 A CE prebleached pulp was prepared in the same manner described in Example 5 for Northern softwood kraft pulp. The CE pulp was equally divided into three batches for the first chlorine dioxide bleaching stage in multistage bleaching CED,ED2. The first batch was bleached with chlorine dioxide only at a charge level of 1.1% ClO2 based on OD pulp at 63"C for 180 minutes.

The second batch was bleached with a combination of carbonic acid and hypochlorite and chlorine dioxide at a 50% substitution of hypochlorite for chlorine dioxide (0.55 hypochlorite plus 0.55% ClO2) at the same temperature of the first batch but with reduced retention time due to its faster bleaching reaction. The third batch was treated with the same chemical level of the second batch but at lowered temperatures. The dioxide bleaching temperature was lowered from 63"C to 54"C. After the D, or (H+D) bleaching those batches of pulp were further treated in the E2 and D2 stage by the same manner in Example 1.The result of lowered temperature in the (H+D) stage is shown in Table VIII. From the result it is obvious that in this novel (H+D) bleaching the bleaching temperature can be lowered by 9"C from 63"C to 54"C without adversely affecting the final brightness of the resulting pulp of multistage bleaching CE(H+D)E2D2.

TABLE VIII Northern Softwood Kraft Pulp CE pulp (CEnO 4.2) D1 or (H+D) Batch 1 (Control) Batch 2 (H+D) Batch 3 (H+D) % ClO2 1.1 0.55 0.55 % Hypo 0.0 0.55 0.55 Temp. "C 63 63 54 Retention time (Min) 180 120 150 Residual, titre (ml) 1.0 none none pH 1.7 2.4 2.6 E2, % NaoH 0.5 0.5 0.5 D2, % ClO2 0.4 0.4 0.4 Brightness D, (Initial) 77.6 72.1 73.1 D2 initial 90.1 90.1 90.1 D2 reverted 85.6 85.5 85.4 EXAMPLE 9 Effects of different temperature of the (H+D) stage were again investigated in the multistage bleaching process CN(H+D)EHD2 using Southern pine kraft pulp. A chlorinated and alkali neutral ized pulp was prepared by the same manner in Example 4. The K number of the CN pulp was 5.2. The CN pulp was equally divided into four batches.Each batch was bleached with the (H+D) bleaching at different temperatures, 48"C, 54"C, 60"C and 71"C. After the (H+D) bleaching, all three pulps were further treated with second caustic extraction with hypochlorite (EH) and second chlorine dioxide bleaching under the same conditions. The results are shown in Table IX.

From the result, it is again evident that in the novel (H+D) bleaching, its bleaching termperature can be lowered to 48"C without any adverse effect on the final brightness and pulp viscosity from the normal temperature range of first chlorine dioxide bleaching, 60"C-71"C. The lowering of bleaching temperature means a substantial energy savings at the bleach plant.

TABLE IX Batch 1 Batch 2 Batch 3 Batch 4 Temperature in (H+D) Stage, "C 48 54 60 71 Retention time, min. 180 120 100 80 Consistency, % 8 8 8 8 End pH in (H+D) 2.25 2.25 2.2 2.1 % Residual in (H+D) 0.0 0.0 0.0 0.0 (H+D) Brightness 73.2 70.1 71.8 73.2 (Initial) D2 Brightness, Initial 87.9 87.4 87.9 88.3 Reverted 84.3 83.9 84.0 84.9 D2 Viscosity, cps 13.3 14.0 13.5 13.7 EXAMPLE 10 Using a Northern softwood kraft pulp the CE prebleached pulp sample was prepared under the following conditions: C stage: 5.2% chlorine based on OD pulp, 3.5% consistency, 29"C and 45 minutes E stage: 2.5% NaOH based on OD pulp, 11% consistency, 71"C and 90 minutes The prepared CE pulp was equally divided into six batches for the (H+D)E2D2 bleaching using six different acids in the (H+D) bleaching stage.The six different acids evaluated are carbonic acid (saturated CO2 water), acetic acid (10% concentration), nitric acid (10%); hydrochloric acid (1N solution), sulfuric acid (4N solution) and phosphoric acid (10% solution). Each acid was added on an amount which pH of the (H+D) bleaching reached in the range of 2.0-2.5. The temperature and retention time were held constant at 57"C and 60 minutes respectively. After the (H+D) bleaching each pulp was further treated with 0.5% NaOH in E2 stage and 0.35% ClO2 in D2 stage under the same conditions described in Example 1. The results of different acids are given in Table X. From the results shown it is obvious that in the noble (H+D) bleaching process besides carbonic acid other organic and inorganic acid can be used as long as the end pH reaches in the range of 2.0-2.5.Regardless of the acid used, the same final brightness can be achieved without affecting pulp viscosity.

However, considering practical application of this novel (H+D) bleaching process under mill environments, weak acids having ionization constant of 4-7 are preferred such as acetic, phosphoric acid and carbonic acid in order to avoid any danger of acid cellulose degradation in case of overfeeding strong acids such as sulfuric, nitric and hydrochloric acids. Among the weak acids evaluated, carbonic acid is preferred since it has the following advantages over other acids: 1. Carbonic acid can be readily prepared at the mill by injecting CO2 gas into mill water.

2. It does not impose corrosion problems related to chloride and sulfate ions.

3. It does not impose any hazard in working environments because CO2 gas is not toxic.

4. Any overfeeding will not cause an undue cellulose degradation.

5. It may be less expensive than other acids in neutralizing or acidifying alkaline pulp slurry.

TABLE X Bleaching Sequence: CE1(H+D)E2D2 End pH of (H+D) D2 D2 Acid (H+D) Stage Bright. Brightness Viscosity(cps) Carbonic 2.45 69.1 88.3/84.7 20.4 Acetic 2.50 68.8 88.1/84.3 20.5 Nitric 2.30 69.3 88.5/84.5 21.1 Hydrochloric 2.30 68.5 87.9/84.0 21.2 Sulfuric 2.25 68.7 88.7/84.8 20.4 Phosphoric 2.30 68.5 88.7/84.9 20.7

Claims (7)

1. A process for the delignification and bleaching of lignocellulosic material which comprises treating the material with chlorine dioxide in combination with hypochlorite in the presence of an acid.

2. A method as claimed in claim 1 in which the chlorine dioxide treatment is carried out prior to the addition of hypochlorite.

3. A method as claimed in claim 1 in which the hypochlorite treatment precedes the addition of chlorine dioxide.

4. A method as claimed in any of claims 1 to 3 in which the acid is selected from carbonic, acetic, nitric, hydrochloric, sulfuric and phosphoric acid.

5. A method as claimed in claim 4 in which the acid is carbonic acid.

6. A method as claimed in any of claims 1 to 5 in which the treatment is carried out at a temperature between 48"C and 60"C.

7. A process for the delignification and bleaching of lignocellulosic material substantially as hereinbefore described with particular reference to any of the foregoing Examples.