Method for bleaching paper pulp

Abstract

The present invention relates to a method for bleaching an unbleached or pre-bleached paper pulp comprising at least the following consecutive steps: a) preparing an unbleached or pre-bleached paper pulp having a pH of at least 8, b) bringing the paper pulp obtained at the end of step a) into contact with chlorine dioxide, c) when the pH of the paper pulp obtained from step b) is lower than 10, adding at least one Brønsted base to the paper pulp, d) adding hydrogen peroxide to the paper pulp obtained at the end of step c), e) maintaining the paper pulp obtained at the end of step d) in a first bleaching tower, f) optionally, adding sulphuric acid to the paper pulp obtained at the end of step e) and maintaining the obtained paper pulp in a second bleaching tower, the method dispensing with the need for a step of washing the paper pulp before the end of step e), and, if applicable, before the end of step f).

Claims

1. A process for bleaching unbleached or pre-bleached paper pulp consisting of the following successive steps: a) preparing an unbleached or pre-bleached paper pulp with a pH greater than or equal to 8, b) placing the paper pulp obtained at the end of step a) in contact with chlorine dioxide, c) when the pH of the paper pulp at the end of step b) is less than 10, add at least one Brønsted base to the paper pulp, d) adding hydrogen peroxide to the paper pulp, e) keeping the paper pulp obtained at the end of step d) in a first bleaching tower, where the process has no step of washing the paper pulp before the end of step e).

2. The process according to claim 1, wherein during step a), a Brønsted base is added to the paper pulp, this Brønsted base being chosen from the group comprising: alkaline metal hydroxides, alkaline earth metal hydroxides, alkaline; oxides metal oxides, alkaline earth metal oxides; and their mixtures.

3. The process according to claim 1, wherein the paper pulp at the end of step a) has a pH between 8 and 13.

4. The process according to claim 1, wherein a quantity of active chlorine between 0.1% and 10%, by weight relative to the weight of dry paper pulp, is introduced during step b).

5. The process according to claim 1, wherein step b) has a contact time between the paper pulp from step a) and chlorine dioxide of at least 10 seconds.

6. The process according to claim 1, wherein step b) is carried out at a temperature greater than 20° C.

7. The process according to claim 1, wherein step b) is carried out at a temperature of between 25° C. and 90° C.

8. The process according to claim 1, wherein a quantity of hydrogen peroxide between 0.1% and 5%, by weight relative to the weight of dry paper pulp, is introduced during step d).

9. The process according to claim 1, wherein during step e), the paper pulp is held in the first bleaching tower for a duration of between 30 minutes and 180 minutes at a temperature between 40° C. and 95° C.

10. A process for bleaching unbleached or pre-bleached paper pulp consisting of the following successive steps: a) preparing an unbleached or pre-bleached paper pulp with a pH greater than or equal to 8, b) placing the paper pulp obtained at the end of step a) in contact with chlorine dioxide, c) when the pH of the paper pulp at the end of step b) is less than 10, add at least one Brønsted base to the paper pulp, d) adding hydrogen peroxide to the paper pulp, e) keeping the paper pulp obtained at the end of step d) in a first bleaching tower, f) at the end of step e), acidifying the paper pulp and keeping it in a second bleaching tower, where the process has no step of washing the paper pulp before the end of step e).

11. The process according to claim 10, wherein during step a), a Brønsted base is added to the paper pulp, this Brønsted base being chosen from the group comprising: alkaline metal hydroxides, alkaline earth metal hydroxides, alkaline; oxides metal oxides, alkaline earth metal oxides; and their mixtures.

12. The process according to claim 10, wherein the paper pulp at the end of step a) has a pH between 8 and 13.

13. The process according to claim 10, wherein a quantity of active chlorine between 0.1% and 10%, by weight relative to the weight of dry paper pulp, is introduced during step b).

14. The process according to claim 10, wherein step b) is carried out at a temperature greater than 20° C.

15. The process according to claim 14, wherein step b) is carried out at a temperature of between 25° C. and 90° C.

16. A process for bleaching unbleached or pre-bleached paper pulp consisting of the following successive steps: a) preparing an unbleached or pre-bleached paper pulp with a pH greater than or equal to 8, wherein at least one cellulose protective agent and a chelating and/or sequestering agent are added during step a); b) placing the paper pulp obtained at the end of step a) in contact with chlorine dioxide; c) when the pH of the paper pulp at the end of step b) is less than 10, add at least one Brønsted base to the paper pulp; d) adding hydrogen peroxide to the paper pulp; e) keeping the paper pulp obtained at the end of step d) in a first bleaching tower, wherein the process has no step of washing the paper pulp before the end of step e).

17. The process according to claim 10, wherein during step f), the paper pulp is acidified with sulfuric acid to a pH between 2 and 5.

18. The process according to claim 10, wherein during step f), the paper pulp is held in the second bleaching tower for between 10 and 180 minutes at a temperature between 50° C. and 90° C.

19. A process for bleaching unbleached or pre-bleached paper pulp consisting of the following successive steps: a) preparing an unbleached or pre-bleached paper pulp with a pH greater than 8, wherein at least one cellulose protective agent and a chelating and/or sequestering agent are added during step a; b) placing the paper pulp obtained at the end of step a) in contact with chlorine dioxide; c) when the pH of the paper pulp at the end of step b) is less than 10, add at least one Brønsted base to the paper pulp; d) adding hydrogen peroxide to the paper pulp; e) keeping the paper pulp obtained at the end of step d) in a first bleaching tower; f) at the end of step e), acidifying the paper pulp and keeping it in a second bleaching tower, wherein the process has no step of washing the paper pulp before the end of step e).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates steps a) to e) of a specific embodiment of the treatment according to the invention (D.sub.alk.sup.P).

(2) FIG. 2 illustrates steps a) to f) of a specific embodiment of the treatment according to the invention (D.sub.alk.sup.PA).

(3) FIG. 3 illustrates the mechanical properties (hand) of paper pulp obtained according to the invention (D.sub.alk.sup.PA) and according to prior art (D.sub.1nD.sub.2) as a function of the number of towers used in a PFI refiner.

(4) FIG. 4 illustrates the mechanical properties (traction index) of the paper pulp obtained according to the invention (D.sub.alk.sup.PA) and according to prior art (D.sub.1nD.sub.2) as a function of the number of towers used in a PFI refiner.

(5) FIG. 5 illustrates the mechanical properties (bursting index) of the paper pulp obtained according to the invention (D.sub.alk.sup.PA) and according to prior art (D.sub.1nD.sub.2) as a function of the number of towers used in a PFI refiner.

(6) FIG. 6 illustrates the mechanical properties (tear index) of pulp obtained according to the invention (D.sub.alk.sup.PA) and according to prior art (D.sub.1nD.sub.2) as a function of the number towers used in a PFI refiner.

EXAMPLE EMBODIMENTS OF THE INVENTION

Example 1: Process for Bleaching a Pre-Bleached Pulp According to the Invention (D.SUB.alk..SUP.P.) and a Conventional Control Sequence D.SUB.1.nD.SUB.2 .(Counter-Example 1) Therefore

(7) Process for Bleaching a Pre-Bleached Pulp According to the Invention (D.sub.alk.sup.P).

(8) The device shown in FIG. 1 has been used to implement this example.

(9) From pre-bleached softwood kraft paper pulp (100 grams of dry lignocellulose fibers per liter of fibrous suspension) using a sequence of molecular oxygen, hydrogen peroxide, sodium hydroxide and a chelating agent (wherein the Kappa number is 4.3; brightness is 82% ISO, and the viscometric average degree polymerization 1028) is treated in laboratory with 0.4% magnesium sulfate by weight relative to the dry paper pulp, then with 0.55% sodium hydroxide by weight relative to the dry paper pulp (step a, the pH of this step is 9.5).

(10) The pulp thus obtained is placed into a plastic polyethylene bag to which 2% chlorine dioxide by weight of active chlorine in relation to the dry paper pulp is added. The polyethylene bag containing the mixture obtained is immersed in a water bath thermostatically controlled at 45° C.

(11) After 5 minutes of reaction, the plastic bag is removed from the thermostatically-controlled water bath, then 0.2% sodium hydroxide by weight relative to the dry paper pulp is added to the pulp (step c, the pH in this step is 10.4).

(12) Then, 0.3% hydrogen peroxide by weight relative to the dry paper pulp is introduced (step d).

(13) Then the pulp contained in the polyethylene bag is again plunged into the thermostatically-controlled water bath for 90 minutes at 75° C. (Step e).

(14) The pulp is then washed on a no. 2 porosity filter funnel with 10 L of water.

(15) During step b), the concentrations of chlorate ions, chlorite ions and hypochlorite ions are measured in the bleaching effluents (Table 1).

The Process for Bleaching a Pre-Bleached Pulp, Conventional Control Sequence D.SUB.1.nD.SUB.2 .(Counter-Example 1)

(16) A D.sub.1nD.sub.2 (chlorine dioxide/neutralization/chlorine dioxide) type process is conventionally done in a pre-bleached paper pulp mill with the same sequence as in the previous paragraph (using molecular oxygen, hydrogen peroxide, sodium hydroxide and a chelating agent) on kraft paper pulp from softwood (100 grams of lignocellulosic fibers per liter of fibrous suspension).

(17) The paper pulp is put in contact with 0.06% sodium hydroxide by weight relative to the dry paper pulp of, then with 2.6% active chlorine by weight relative to the dry paper pulp, for 115 minutes at 75° C., and at a 10% consistency (D.sub.1) (the pH in this step is 7).

(18) The paper pulp is then washed on a no. 2 porosity filter funnel and with 10 L of water then put in contact with 0.06% sodium hydroxide by weight relative to the dry paper pulp and 0.65% active chlorine by weight relative to the dry paper pulp for 115 minutes at 80° C. and 10% consistency (D.sub.2) (the pH in this step is 7.5).

(19) The pulp is then washed on a no. 2 porosity filter funnel with 10 L of water.

(20) During steps D.sub.1 and D.sub.2, the concentrations of chlorate ions, chlorite ions and hypochlorite ions are measured in the bleaching effluents (Table 1).

Bleached Pulp Properties According to Invention (D.SUB.alk..SUP.P.) or the Conventional Control Sequence D.SUB.1.nD.SUB.2 .(Counter-Example 1)

(21) The washed pulp properties were analyzed according to the four following standards (Table 2): brightness according to ISO Standard 2470-1, 2009 Kappa number of pulp according to ISO Standard 302, 2015 viscometric average degree of polymerization of the cellulose according to the standard TAPPI T230-OM-13 chemical oxygen demand (COD) according to a method analogous to the ISO standard 15705, 2002.

(22) The chlorite and chlorate ions are assayed after step D.sub.alk of stage (D.sub.alk.sup.P); in fact, assaying of these species by iodometry after the stage D.sub.alk.sup.P would be distorted by the presence of residual hydrogen peroxide, also reacting with iodide ions. It would not be possible, then, to obtain the quantities of chlorite, chlorates and hypochlorites ions separately.

(23) TABLE-US-00001 TABLE 1 Bleaching effluents Concentration Concentration Concentration of COD of of chlorite hypochlorite (kg/t of chlorate ions ions ions dry Treatment (Mol/L) (Mol/L) (Mol/L) pulp) D.sub.alk.sup.P 2.15 × 10.sup.−3 5.63 × 10.sup.−3 0 3.92 (invention) D.sub.1nD.sub.2 4.42 × 10.sup.−3 3.87 × 10.sup.−4 0 5.90 (counter- example 1)

(24) TABLE-US-00002 TABLE 2 Properties of the bleached pulp Brightness, Kappa Treatment % ISO number DPv D.sub.alk.sup.P 89.4 3.4 828 (invention) D.sub.1nD.sub.2 89.0 0.8 850 (counter-example 1) DPv: Viscometric average degree of polymerization of the cellulose

(25) In relation to the D.sub.1nD.sub.2 process, the process (D.sub.alk.sup.P) according to the invention consumes 38% less chlorine dioxide. Furthermore, the water consumption is greatly reduced (10 L for washing the pulp between D.sub.1 and D.sub.2). In addition, reaction time (when in contact with ClO.sub.2) is decreased by 135 minutes (90+5 minutes vs 2×115 minutes) while maintaining the final pulp brightness and without any significant increase in the depolymerization of the cellulose.

(26) The bleaching process according the invention is performed continuously with no intermediate washing step needed. Thus the bleaching process according to the invention consumes less chlorine dioxide, reduces reaction time and eliminates a washing step and does so without engendering loss in brightness and without increasing depolymerization of the cellulose. In addition, the bleaching process according to the invention makes it possible to reduce the pollutant load of the effluents (COD) (Table 3) (−33%).

Example 2: Process for Bleaching a Pre-Bleached Pulp According to the Invention (D.SUB.alk..SUP.P.A) and According to the Conventional Control Sequence D.SUB.1.nD.SUB.2 .(Counter-Example 2)

(27) Process for treating a pre-bleached pulp according to the invention (D.sub.alk.sup.PA).

(28) The device shown in FIG. 2 has been used to implement this example.

(29) From pre-bleached kraft paper pulp from softwood (100 grams of lignocellulosic fibers per liter of fibrous suspension) using a sequence using molecular oxygen, hydrogen peroxide, sodium hydroxide and a chelating agent. Said pulp has a Kappa number of 5.2 as well as a brightness of 78.6% ISO and a viscometric average degree polymerization of 812. It is treated with 0.4% magnesium sulfate by weight relative to the dry paper pulp, then with 0.63% sodium hydroxide by weight relative to the dry paper pulp (step a, the pH is in this step is 9.5).

(30) The pulp thus obtained is placed into a polyethylene bag to which 2.2% of chlorine dioxide by weight of active chlorine relative to the dry paper pulp is added. The polyethylene bag containing the mixture obtained is plunged into a water bath thermostatically controlled at 75° C. (step b).

(31) After 5 minutes of reaction, the plastic bag is removed from the thermostatically-controlled water bath, then 0.2% sodium hydroxide by weight relative to the dry paper pulp is added to the pulp (step c, pH of this step is 10.2).

(32) Then, 0.3% hydrogen peroxide by weight relative to the dry paper pulp is introduced (step d, pH of this step is 10.2).

(33) Then the pulp contained in the polyethylene bag is again plunged into the thermostatically-controlled water bath for 90 minutes at 75° C. (Step e).

(34) After 90 minutes of reaction, the plastic bag is removed from the thermostatically-controlled water bath, then 0.15% sulfuric acid by weight relative to the dry paper pulp is added to the pulp (step f, the pH in this step is 3.8).

(35) Then the pulp contained in the polyethylene bag is plunged into the thermostatically-controlled water bath again for 60 minutes at 75° C.

(36) The acid step is necessary for this. Compared to Example no 1, the amount of lignin in the pulp is higher.

(37) The pulp is then washed on a no. 2 porosity filter funnel with 10 L of water.

A Process for Bleaching a Pre-Bleached Pulp According to the Control Sequence D.SUB.1.nD.SUB.2 .(Counter-Example 2)

(38) This counter-example was carried out under the same conditions as counter-example 1 but from the same pulp as that for Example 2.

The Bleached Pulp Properties According to Invention (D.SUB.alk..SUP.P.A) or the Counter-Example (DnD)

(39) The washed bleached pulp properties were analyzed according to the following 3 standards (Table 3): brightness according to ISO Standard 2470-1, 2009 Kappa number of pulp according to ISO Standard 302, 2015 viscometric average degree of polymerization of the cellulose according to TAPPI norm T230-om-13.

(40) Two sets of sheets have been produced according to ISO Standard 5269-1, 2005, with the paper pulp obtained according to the invention and according to the counter-example. Measurement of the mechanical properties were taken according to the following standards (Table 4): measurement of the hand according to ISO Standard 536, 2012 and ISO Standard 534, 2011 burst strength according to ISO Standard 2758, 2014 tear strength according to ISO Standard 1974, 2012 tensile strength according to ISO Standard 1924-2, 2008

(41) The paper pulp obtained according to the invention and according to the counter-example have also been refined with a PFI laboratory refiner (refiner with a rotating cylindrical bowl) according to Standard NF EN ISO 5264-2, 2011. During this refining, the draining index of the paper pulp was measured according to ISO Standard 5267-1, 1999 and the mechanical properties were measured according to the standards previously cited (FIGS. 3, 4, 5 and 6).

(42) The bleaching effluent was also analyzed according to the following 2 standards (Table 5): chemical oxygen demand (COD) according to a method analogous to ISO standard 15705, 2002. adsorbable halogenated organic compounds (AOX) according to standard 9562, 2004

(43) TABLE-US-00003 TABLE 3 Properties of the bleached pulp Brightness, Kappa Treatment % ISO number DPv D.sub.alk.sup.PA 87.4 2.9 775 (invention) D.sub.1nD.sub.2 87.3 1.0 789 (counter-example 2)

(44) TABLE-US-00004 TABLE 4 Mechanical properties of the bleached pulp Brightness Traction Hand Tear Index index index Treatment (cm.sup.3/g) (mN .Math. m.sup.2/g) (kPa .Math. m.sup.2/g) (N .Math. m/g) D.sub.alk.sup.PA 1.77 14.4 1.40 19.1 (invention) D.sub.1nD.sub.2 1.77 14.5 1.27 18.5 (counter-example 2)

(45) TABLE-US-00005 TABLE 5 Quality of effluents following various treatments COD AOX, Treatment (kg/t of dry pulp) (kg/t of dry pulp) D.sub.alk.sup.P 3.44 0.033 (invention) D.sub.alk.sup.PA 4.67 0.075 (invention) D.sub.1nD.sub.2 5.88 0.26 (counter-example 2)

(46) In relation to the D.sub.1nD.sub.2, the D.sub.alk.sup.PA process according to the invention consumes 32% less chlorine dioxide. Furthermore, the water consumption is greatly reduced (10 L for washing the pulp between D.sub.1 and D.sub.2). In addition, reaction time is decreased by 75 minutes (5+90+60 minutes instead of 2×115 minutes) while maintaining the final brightness, the mechanical properties of the pulp and without increasing the cellulose depolymerization (DPV).

(47) The treatment process according the invention is performed continuous, with no intermediate washing step needed. Thus the bleaching process according to the invention can thus consume less chlorine dioxide, reduce the reaction time and eliminate a washing step and does so without leading to a loss of brightness, loss of mechanical properties (before and after refining), and without increasing depolymerization of the cellulose. In addition, the bleaching process according to the invention makes it possible to reduce the amount of chlorinated organic compounds (AOX) formed (−71.1%) and to reduce the pollutant load of the effluents (COD) (−20.6%).