Process for bleaching paper pulp using melamine as a viscosity stabilizer

Abstract

A process for maintaining pulp viscosity while enhancing brightness during the bleaching stage or stages of pulp preparation in the manufacture of paper comprising adding an effective amount of melamine to paper pulp prior to or during the bleaching stage. One example adds melamine to one bleach stage of a chlorine bleaching sequence. One example adds melamine to one bleach stage of a chlorine bleaching sequence.

Description

Paper is made from wood pulp obtained from trees which undergoes a series of treatments. These treatments are described in various sources, among which the most useful is Casey, Pulp and Paper Chemistry and Chemical Technology, published by John Wiley and Sons, 1980. The Kirk-Othmer Encyclopedia of Chemical Technology, also published by John Wiley, second and third editions, is also a useful reference.

Logs are first cut into small sections and then into chips. The chips are digested to form paper pulp by a variety of processes. The pulping stage may be a thermo-mechanical, a semi-chemical, or a chemical operation. In the chemical processes, the wood chips are cooked in a closed digester tank filled with either a solution of a bisulfite salt (sulfite process), caustic soda and sodium sulfide (sulfate or kraft process) or in caustic soda solution (soda process) to dissolve the materials which hold the cellulose or paper-making fibers. After completion of the pulping process the pulp is bleached.

In one typical bleaching sequence the wood pulp is bleached with chlorine in solution (C stage), extracted with alkali (E stage) and then treated with chlorine dioxide (D stage). Another typical bleaching sequence requires: chlorination (C stage), alkali extraction (E stage) and two hypochlorite bleachings (H stage). This second sequence is written as C-E-H-H where washing between the stages is indicated by a hyphen (Casey, page 669). After bleaching, these as yet unmodified cellulose fibers next undergo a stock preparation or refining step in either a batch beater or a continuous refining procedure. The wet mass of fibers is then formed into a sheet and dried.

In the bleaching steps competing factors are balanced. The brightness of the pulp is increased while maintaining pulp viscosity at acceptable levels. During bleaching lignin removal must occur without excessive cellulose degradation.

Various additives have been used in the bleaching steps to maintain higher viscosities without interfering with lignin removal. Sulfamic acid at 1 to 5 pounds per ton of pine kraft pulp resulted in good viscosities when used in the chlorination stage (Aldrich, TAPPI, March, 1968, Volume 51, 3,71A). It is also known that the drop in viscosity during the chlorination stage can be decreased by adding chlorine dioxide (Fredericks, TAPPI, January, 1971, Volume 54, 1,87). In another study, sulfamic acid was effective in the hypochlorite stage on bamboo pulp obtained by the sulfate process (Jangalgi, IPPTA, January, 1971, Volume 8, 1,11). However, sulfamic acid, a white crystalline powder, has corrosive properties which can produce skin inflammation or blindness if permitted to enter the eye (Hernadi, Zellstoff und Papier 1975/5 p 147-149). A need for a replacement for sulfamic acid as viscosity stabilizer in bleaching is apparent.

It has been found that melamine may be used to replace sulfamic acid as a viscosity protector in the bleaching stages. Melamine may be used in smaller amounts than sulfamic acid while achieving similar viscosity protection. In one embodiment, this invention is a process for maintaining pulp viscosity while enhancing brightness during the bleaching stages of pulp preparation in paper manufacture comprising adding an effective amount of melamine to paper pulp prior to or during the bleaching stages of paper pulp manufacture.

In another embodiment, this invention is a process for maintaining pulp viscosity while enhancing brightness during the bleaching stages of pulp preparation in paper manufacture comprising adding up to 2 parts by weight melamine to 100 parts by weight dry paper pulp.

In another embodiment this invention is a process for maintaining pulp viscosity while enhancing brightness during the bleaching stages of paper pulp manufacture comprising adding from 0.10 to 1.0 parts by weight melamine to 100 parts by weight dry paper pulp.

In a preferred embodiment, this invention is a process for maintaining pulp viscosity while enhancing brightness during the bleaching stage or stages in paper manufacture comprising adding about 0.25 parts by weight melamine to 100 parts by weight dry paper pulp.

The following examples show the use of melamine as an additive in the bleaching stages using kraft process pulp in a laboratory test (Examples 1, 2 and 3) and bamboo/mixed tropical hardwoods pulp in a mill scale evaluation (Example 4).

The melamine used was the commercially available crystal sold by Melamine Chemicals, Inc., Donaldsonville, La.

The pulp and pulp strength properties are measured according to standard TAPPI methods (Technical Association of the Pulp and Paper Industry, Atlanta, GA.).

______________________________________ TAPPI Method______________________________________Kappa Number T236 os-76Viscosity (Cp) T230 os-76Laboratory Pulp Processing T248 pm-74(PFI MILL)Freeness T227 os-58Forming Handsheets for Physical T205 om-81TestingBrightness T452 os-77Physical Testing of Pulp Handsheets T220 os-71(caliper, burst index, tensileindex, tear index, foldingendurance)______________________________________

EXAMPLE 1

Melamine as a Viscosity Stabilizer in the Bleaching Stage of Pulp Preparation

Unbleached kraft process pulp having a Kappa number of 38.7 was treated with sulfamic acid, urea or melamine at various percentages on pulp according to the following bleaching procedure. The single capital letters, C, E, D are used to describe particular bleaching stages as described in Casey, Pulp and Paper Chemistry and Chemical Technology, John Wiley and Sons, 1980, Page 669. The removal of lignin during the bleaching stage is expressed as a reduction in the Kappa number.

The delignification/bleaching stages were accomplished as follows: Protective agents were added at the indicated percentages on pulp and chlorination with chlorine water (C stage) was at 9.66% chlorine on pulp at room temperature for one hour at 3% pulp consistency.

Caustic extraction with sodium hydroxide (E stage) was done at 4% NaOH on pulp at 70.degree. C. for one hour at 12% pulp consistency. Chlorine dioxide bleaching with ClO.sub.2 (D stage) was at 1.5% ClO.sub.2 on pulp at 70.degree. C. for 2.5 hours at 12% pulp consistency.

Table 1 presents the pulp properties after the C and E stage and after C, E and D stages when the various protective agents were added to kraft process pulp. About 0.25% melamine on pulp protects the viscosity after C and E stages to about the same extent that 0.50% sulfamic acid protects the viscosity.

TABLE I__________________________________________________________________________EFFECT OF CELLULOSE PROTECTORS IN CHLORINE DELIGNIFICATION% Pulp PropertiesProtective on C-E Stage C-E-DAgents Pulp KAPPA Number 5 Viscosity (Cp) Brightness % Viscosity (Cp)__________________________________________________________________________None -- 8.6, 9.0 24,5, 23.7 72, 71.3 17.1Sulfamic 0.5 9.5 35.8 71.3 23.7AcidUrea 1.0 8.2 33.8 71.0 22.0Melamine 0.10 8.8, 9.1 27, 27.2 71.5, 72.5 0.15 9.6 29.9 72.0 0.20 8.6 32.8 73.0 0.26 9.0 37.7 70.5 0.30 10.3 38.0 71.5 0.50 10.4 40.0 71.8 28.6 0.80 9.7 41.0 71.7 1.20 9.7 41.6 71.8 1.43 9.7, 9.4 41.6, 42, 41.2 72, 71.5 29.2__________________________________________________________________________

EXAMPLE 2

Melamine as a Viscosity Stabilizer When Used at Various Temperatures in the Chlorination Stage

Unbleached kraft process spruce pulp having a Kappa number of 38.7 was bleached in C-E-D sequences with and without the use of melamine and sulfamic acid in the chlorination stage at different temperatures. Table II contains a summary of the comparative pulp properties after C-E stages and Table III summarizes the comparative properties after C-E-D stages. It can be seen from these figures that melamine at a reduced level (0.25%) is as effective as sulfamic acid (0.5%) even at higher temperatures of chlorination.

TABLE II______________________________________Effect of Melamine at DifferentTemperatures of Chlorination (pulpproperties after C-E stages). PROTECTIVE AGENT Melamine None Sulfamic Acid (0.25% onProperties (Control) (0.5% on Pulp) Pulp)______________________________________Temp. 30.degree. C.Kappa Number 8.5 7.7 7.5Viscosity (Cp) 23.8 36.7 34.7Temp: 40.degree. C.Kappa Number 8.2 8.7 6.2Viscosity (Cp) 23.2 34.2 34.0Temp: 50.degree. C.Kappa Number 8.6 7.7 6.5Viscosity (Cp) 22.2 32.5 32.8______________________________________

Conclusions: At all temperatures the Kappa Number achieved is lowest with melamine. Viscosity values obtained with melamine are comparable to those achieved by using a higher % of sulfamic acid.

TABLE III______________________________________Effect of Melamine at DifferentTemperatures of Chlorination (pulpproperties after C-E-D stages). PROTECTIVE AGENT Melamine None Sulfamic Acid (0.25% onProperties (Control) (0.5% on Pulp) Pulp)______________________________________Temp: 30.degree. C.Kappa Number 3.2 2.5 1.8Viscosity Cp 17.5 24.5 24.0Brightness % 71.0 70.5 71.0Temp: 40.degree. C.Kappa Number 3.1 2.8 2.2Viscosity Cp 16.8 23.7 23.5Brightness % 70.5 71.0 70.5Temp: 50.degree. C.Kappa Number 2.3 3.2 2.5Viscosity Cp 16.3 21.3 21.5Brightness % 70.0 70.0 70.5______________________________________

At each temperature level the effect of melamine on viscosity is comparable to that of sulfamic acid but with a lower dosage level (0.25% melamine against 0.5% sulfamic acid).

EXAMPLE 3

Melamine as a Viscosity Stabilizer in the Hypochlorite Bleaching Stage of Pulp Preparation

Unbleached kraft process spruce pulp having a Kappa number of 38.7 was bleached in a C-E-H sequence with the use of sulfamic acid (0.5%) or melamine (0.25%) in the hypochlorite stage (H. stage) in two separate experiments. A third experiment was done without any additive as a control. The comparative results reported in Table IV show the effect of melamine as a viscosity stabilizer in the hypochlorite stage of bleaching.

TABLE IV______________________________________Effect of Melamine in the Hypochlorite StageProtectiveAgents Usedin Properties After Properties AfterChlorination C-E Stages C-E-H StagesHypochlorite Kappa Viscosity Kappa Viscosity Bright-Stage Number (Cp) Number (Cp) ness %______________________________________None/None 6.0 18.3 1.7 7.5 75.3(Control)None/ " " 1.3 9.4 76.5SulfamicAcid (0.5%)None/ " " 1.5 9.2 75.1Melamine(0.25%)______________________________________

Results indicate comparable viscosities for melamine in comparison with those obtained for sulfamic acid at higher dosage levels.

EXAMPLE 4

Mill Trial

Melamine as a Viscosity Stabilizer in the Bleaching Stage of Pulp Preparation

A mixture of bamboo and mixed tropical hardwoods kraft pulp was fed to a continuous bleach plant operating with C-E-HH bleaching sequence at the rate of 35-40 kg per hour. The chlorination (C. Stage) reaction time was 1.5 hours at 25.degree. C. After alkali extraction (E. stage retention time: 1.25 hours) hypochlorite bleaching using calcium hypochlorite (H. Stage) required 2 hours in each of the two stages. Melamine was used in two independent trials--(1) in the chlorination stage and (2) in the first hypochlorite stage. Where the melamine additive was used, it was at the rate of 0.25% relative to the rate of feed of unbleached pulp.

Tables V and VI provide a comparison of control values with the values obtained with the chlorine stage use of melamine. Table V reports the effect on pulp properties of chlorine stage addition of melamine. The effect on pulp strength properties of chlorine stage addition of melamine is summarized in Table VI.

Table VII and VIII provide a comparison of control values with the values obtained with the use of melamine additive in the first hypochlorite bleaching stage. While Table VII reports the effect on basic pulp properties, Table VIII summarizes the effect on pulp strength properties of the melamine addition.

TABLE V______________________________________The Effect on Pulp Properties ofChlorine Stage Addition of Melamine. Without WithPulp Properties Melamine Melamine______________________________________Unbleached pulpKappa Number 23.7.sup.1 26.5.sup.1Viscosity, Cp 19.0.sup.1 16.2.sup.1C-E pulpKappa Number 8.8 .+-. 1.5.sup.2 6.7 .+-. 0.6.sup.2Viscosity, Cp 12.8 .+-. 1.0.sup.3 12.8 .+-. 0.4.sup.3Hypo-I pulpBrightness % 75-77 76-77Viscosity, Cp 6.9 .+-. 0.2 7.5 .+-. 1.4Post color number (16 h) 6.58 .+-. 0.32 6.38 .+-. 0.46Plant conditionsChlorine tower overflow pulppH 2.4-2.8 2.2-2.7Temperature .degree. C. 27/28 28/29Residual chlorine, gpL .sup. .021 .+-. 0.14.sup.4 .sup. .115 .+-. .058.sup.4Alkali extraction pulppH 10.8-11.8 11.0-11.3Alkali tower 59/42 59/41temperature .degree.C. (top) (bottom) (top) (bottom)Hypo-I stage overflow pulppH 7.7-9.4 7.4-8.9Hypo-I tower 42 .+-. 1 42 .+-. 2temperature .degree.C.______________________________________ .sup.1 Differences are due to variations in incoming pulp. .sup.2 Kappa Number with additive is lower even when Kappa Number of feed is higher. .sup.3 Viscosities are identical even when viscosity of control pulp is initially higher. .sup.4 Implies that less chlorine is needed to achieve same level of delignification. TABLE VI______________________________________The Effect on Pulp Strength Propertiesof Chlorine Stage Addition of Melamine.Pulp Strength Without With 95%Properties Melamine Melamine ConfidenceFreeness, mL 500 300 500 300 Limits______________________________________Unbleached pulpTensile index, 57.9 68.3 60.9 67.8 .+-.2.2Nm/gBurst index, 4.05 5.03 4.52* 4.84 .20kPam.sup.2 /gTear index, 14.1 12.3 13.3 11.8 1.2mNm.sup.2 /gFolding 302 851 479 832 23%endurance.sup.2C-E pulpTensile index, 50.0 67.1 60.0* 67.2 2.7Nm/gBurst index, 3.45 4.92 4.55* 5.17* .19kPam.sup.2 /gTear index, 11.0 10.7 13.1 11.3 1.2mNm.sup.2 /gFolding 120 724 398* 795 16%endurance2C-E-H pulpTensile index, 46.6 64.2 55.6* 63.6 2.2Nm/gBurst index, 3.68 3.78 4.33* 5.20* .17kPam.sup.2 /gTear index, 9.74 9.50 11.8 11.7 1.5mNm.sup.2 /gFolding 72 537 209* 692 16%endurance.sup.2______________________________________ .sup.1 Handsheets were conditioned at 50% RH and 73.degree. F. .sup.2 FoIding endurance is the number of folds measured using a MIT tester set at 0.5 kg tension Indicates statistical significance at the 95% confidence level. TABLE VII______________________________________The Effect on Pulp Properties ofHypochlorite Stage Addition of Melamine. Without WithPulp Properties Melamine Melamine______________________________________C-E pulpKappa Number 8.7 8.2Viscosity, Cp 12.3 11.8Hypo-I pulpBrightness % 76-77 74-77Viscosity, Cp .sup. 7.0 .+-. 1.3.sup.1 .sup. 9.6 .+-. 0.4.sup.1Post color number (16 h) 6.52 .+-. 1.23 --Hypo-II pulpBrightness, % 78-80 78-81Viscosity, Cp 6.2 .+-. 0.8 6.6 .+-. 0.3Post color number (16 h) 6.64 .+-. 1.25 --Plant conditionsAlkali extraction pulppH 9.5-10.8 9.8-10.3Alkali tower 58/44 57/42temperature .degree.C. (top) (bottom) (top) (bottom)Hypo-I stage overflow pulppH 6.8-8.2 7.6-9.0Hypo-I tower 42-43 40-42temperature .degree.C.Residual chlorine, gpL .sup. .001 .+-. .004.sup.2 .sup. .167 .+-. .030.sup.2Hypo-II stage overflow pulppH 7.6-8.2 8.4-8.6Hypo-II tower -- --temperature .degree.C.Residual chlorine, gpL trace .006 .+-. .001______________________________________ .sup.1 Demonstrates improvement in pulp viscosity due to melamine .sup.2 Implies that less chlorine is used up to achieve same level of brightness TABLE VIII______________________________________The Effect on Pulp StrengthProperties of Hypochlorite StageAddition of MelaminePulp Strength Without With 95%Properties Melamine Melamine ConfidenceFreeness, mL 500 300 500 300 Limits______________________________________C-E pulpTensile Index, 64.8 67.2 61.0 69.7 .+-.2.4Nm/gBurst Index, 4.73 5.38 4.95 5.65 .18kPam.sup.2 /gTear Index, 12.0 10.5 12.1 7.40* 1.1mNm.sup.2 /gFolding 302 1047 597 1148 22%endurance.sup.2C-E-H pulpTensile Index, 51.3 61.5 58.7* 67.2* 2.4Nm/gBurst Index, 3.20 3.83 4.0* 5.25* .14kPam.sup.2 /gTear Index, 8.13 7.45 11.0* 10.5* 1.3mNm.sup.2 /gFolding 59 251 269* 759* 23%endurance.sup.2C-E-H-H pulpTensile Index, 46.3 57.9 54.4* 66.2* 3.0Nm/gBurst Index, 2.70 3.53 3.63* 5.00* .18kPam.sup.2 /gTear Index, 8.35 7.40 8.85 9.75* 1.0mNm.sup.2 /gFolding 35 138 100* 550* 19%endurance.sup.2______________________________________ .sup.1 Handsheets were conditioned at 50% RH and 73.degree. F. .sup.2 Folding endurance is the number of folds measured using a MIT tester set at 0.5 kg tension. *Indicates statistical significance at the 95% confidence level.

Without wishing to be bound by theory it is currently thought that the flat molecular structure of melamine together with its facility for forming hydrogen bonds with cellulose are the main factors responsible for its surprising and unexpected performance as a viscosity stabilizer. The covering of the cellulose by the flat melamine molecules may act like armor plate, helping to keep the bleach away from direct contact with the cellulose.

Claims

1. A process for bleaching paper pulp comprising: adding melamine to paper pulp prior to or during at least one stage of a chlorine bleaching sequence wherein the melamine added is in an amount effective to maintain the pulp viscosity while enhancing brightness during said at least one bleach stage.

2. The process of claim 1 wherein up to 2 parts by weight melamine are added to 100 parts by weight dry pulp.

3. The process of claim 2 wherein from 0.10 parts to 1.0 parts by weight melamine are added to 100 parts by weight dry paper pulp.

4. The process of claim 3 comprising adding about 0.25 parts by weight melamine to 100 parts by weight dry paper pulp.

5. The process of claim 1 wherein said paper pulp is digested in a chemical process selected from the group consisting of the sulfate process, the sulfite process and the soda process prior to said bleaching step.

6. The process of claim 1 wherein melamine is added during a chlorine bleaching stage.

7. The process of claim 1 wherein melamine is added to a hypochlorite bleaching stage.