Method for producing bleached wood fibre material

Abstract

In a process for producing bleached mechanical woodpulp, said process comprising the steps of a) delaminating comparatively large particles of wood, which have optionally been pretreated with chemicals and/or water, into modified particles of wood, b) grinding the modified particles of wood from a) in one or more refiners, c) optionally treating the stalk obtained in step b) with oxidative or reductive bleaching agents, a composition Z is present during step a) and/or step b), said composition Z comprising one or more of the following components (Z1) to (Z3): a salt of dithionous acid H.sub.2S.sub.2O.sub.4 (Z1), a dithionous acid or dithionous acid derivative generator compound (Z2), a salt of sulfurous acid (sulfite) plus sodium tetraborohydride (Z3) and also optionally additives (Z4).

Claims

1. A process for producing bleached mechanical woodpulp, said process comprising the steps of a) delaminating large particles of wood, which have optionally been pretreated with chemicals and/or water, into modified particles of wood, b) grinding the modified particles of wood from a) in one or more refiners, and c) treating the stock obtained in step b) with oxidative or reductive bleaching agents, wherein a composition Z is present during step a) or both step a) and step b), said composition Z comprising a carbonate or hydrogencarbonate of alkali or alkaline earth metal and one or more of the following one or more of the following components (Z1) to (Z3): a salt of dithionous acid H.sub.2S.sub.2O.sub.4(Z1), a dithionous acid or dithionous acid derivative generator compound (Z2), a salt of sulfurous acid (sulfite) plus sodium tetraborohydride (Z3) and also optionally additives (Z4) and wherein step a) comprises the large, optionally pretreated particles of wood being first (i) exposed to mechanical pressure and/or shearing forces and then (ii) ground in a refiner, wherein refiner stage a) (ii) pressure and/or energy consumption are lower than the corresponding parameters for the refiner in step b) and wherein Z is metered into the refiner of stage a) (ii), or both stage a) (ii) and b), and said large particles of wood, are a size of about (15-50) mm×(15-50) mm×about (6-12) mm, wherein the amount of component(Z1) or (Z2) or (Z3) per kilogram of modified particles of wood is in the range from 1 to 50 grams.

2. The process according to claim 1, wherein said composition Z comprises a salt of dithionous acid H.sub.2S.sub.2O.sub.4(Z1).

3. The process according to claim 2, wherein the salt of dithionous acid H.sub.2S.sub.2O.sub.4 is an alkali metal salt of dithionous acid H.sub.2S.sub.2O.sub.4.

4. The process according to claim 2, wherein the salt of dithionous acid H.sub.2S.sub.2O.sub.4 is sodium dithionite.

5. The process according to claim 4, wherein the large particles of wood are of debarked coniferous or nonconiferous wood.

6. The process according to claim 5, which requires additives Z4.

7. The process according to claim 1, wherein the large particles of wood are of debarked coniferous or nonconiferous wood.

8. The process according to claim 1, wherein which requires additives Z4.

9. The process according to claim 1, wherein said composition Z comprises Z1 wherein Z1 comprises calcium or magnesium salts, of dithionous acid or mixtures thereof.

10. The process according to claim 1, wherein said composition Z comprises Z2.

11. The process according to claim 1, wherein said composition Z comprises Z2 wherein Z2 comprises thiourea dioxide in combination with lye.

12. The process according to claim 10, said composition Z comprises Z3.

13. The process according to claim 1, wherein said composition Z comprises Z3 wherein Z3 comprises calcium or magnesium salt, of sulfurous acid (H.sub.2SO.sub.3), in combination with sodium tetraborohydride.

14. The process according to claim 1, wherein said composition Z comprises Z3 wherein Z3 comprises the combination of sodium sulfite (Na.sub.2SO.sub.3) with sodium tetraborohydride (NaBH.sub.4).

Description

EXAMPLES

(1) Black spruce wood (Picea mariana) and turpentine pine wood (Pinus taeda) were used.

(2) The corresponding wood was debarked and hogged by customary mechanical methods into chips measuring about 5 cm×5 cm×1 cm.

(3) A) ATMP Variant (in Accordance with the Present Invention)

(4) This raw material was further processed in the so-called ATMP process of Andritz AG (Austria) as described hereinbelow.

(5) The chips were treated in a chip press (Impressafiner screw machine from Andritz AG, Austria) at a pressure of about 1.4 bar. The material thus treated was treated with water on emerging from the screw machine and fed into a refiner (Andritz 36-1CP from Andritz AG, Austria), a fiberizer having a single grinding disk (diameter 0.91 m), where it was converted into a fibrous material at a grinding disk speed of 1800 rpm and a pressure of 2.4 bar.

(6) The material thus fiberized was fed into a first main refiner (Andritz 36-1CP) and converted therein at a grinding disk speed of 2300 rpm and a pressure of 5.2 bar in the presence of composition Z as described hereinbelow into mechanical woodpulp.

(7) An embodiment (III) solution of composition Z in water, comprising 10 wt % of sodium dithionite and 2 wt % of sodium carbonate, each based on the mass of the solution, was metered virtually directly into the grinding mechanism of the first main refiner, at a rate of 15 grams of pure sodium dithionite per kilogram of fiberized material (oven dry “OD”).

(8) This mechanical woodpulp was ground further in a second main refiner having two grinding disks (Andritz 401) at atmospheric pressure.

(9) B) TMP Variant (for Comparison)

(10) The comparative tests (conventional TMP process) were carried out similarly to the inventive tests (variant A) except that inventive step a) was not performed and the chips (see above) were ground directly into mechanical woodpulp in a first main refiner (Andritz 36-1CP from Andritz AG, Austria) at a pressure of 3.45 bar and a disk speed of 1800 rpm, in the presence of a composition Z-in-water solution as described above under A). This mechanical woodpulp was ground further in a second main refiner having two grinding disks (Andritz 401 from Andritz AG, Austria) at atmospheric pressure.

(11) C) General

(12) Specific energy consumption is reported in kWh per OD metric ton (to), where OD is oven dry. Specific energy consumption was determined as follows: The power consumption of the refiner within a given period was measured and divided by the mass of the fiberized OD material.

(13) The mechanical parameters of the mechanical woodpulp samples and the brightness were measured using standard TAPPI test methods: http://www.tappi.org.

(14) Brightness was determined using Tappi T 452.

(15) Tensile Index was determined using Tappi T 456.

(16) Tear Index was determined using Tappi T 414.

(17) Tensile Energy Absorption (TEA) was determined using Tappi T 494,

(18) The Light Scattering Coefficient was determined using ISO 9416.

(19) Mechanical woodpulp fractionation was carried out using a Bauer Mc Nett Classifier.

(20) The analysis for shiver was carried out using a Pulmac Shive Analyzer equipped with a 0.10 mm sieve plate.

Example 1

(21) ATMP Variant A) and Comparative Variant B) Using Black Spruce Wood

(22) The mechanical woodpulp obtained from black spruce wood by variant A) as described was processed with a standard laboratory sheet former to TAPPI T 205 into test paper and certain mechanical properties determined thereon, and the optical properties (brightness for example) were measured on sheets of paper which were produced to TAPPI T 218.

(23) For comparison, mechanical woodpulp obtained by variant B) as described was processed into test paper as described above and tested using the methods described above.

(24) The results are shown in table 1.

(25) Mechanical woodpulp properties were standardized to a freeness of 200 ml for the aqueous pulp.

(26) TABLE-US-00001 TABLE 1 Variant B) (for Parameter Units Variant A) comparison) Specific Energy kWh/to 1648 1984 Consumption Tensile Index Nm/g 41.8 33.9 Tear Index mNm.sup.2/g 9.8 8.1 Tensile Energy J/m.sup.2 37.9 25.6 Absorption (TEA) Light Scattering m.sup.2/kg 54.0 52.5 Coefficient Shives % 1.6 1.2 Brightness % 68.6 65.5

Example 2

(27) ATMP Variant A) and Comparative Variant B) Using Turpentine Pine Wood

(28) The mechanical woodpulp obtained from turpentine pine wood by variant A) as described was used to produce test paper as described in Example 1 and to determine specific properties thereon using the methods described in Example 1.

(29) For comparison, mechanical woodpulp obtained from turpentine pine wood by variant B) as described was processed into test paper as described in Example 1 and examined using the methods described in Example 1.

(30) The results are shown in Table 2

(31) Mechanical woodpulp properties were standardized to a freeness of 200 ml for the aqueous pulp.

(32) TABLE-US-00002 Variant B) (for Parameter Units Variant A) comparison) Specific Energy kWh/to 1440 1648 Consumption Tensile Index Nm/g 25.7 25.6 Tear Index mNm.sup.2/g 8.7 8.7 Tensile Energy J/m.sup.2 18.3 17.0 Absorption (TEA) Light Scattering m.sup.2/kg 42.6 43.5 Coefficient Shives % 0.15 1.16 Brightness % 61.4 58.9

(33) The examples show that the process of the present invention is more energy-saving while at the same time leading to bleached mechanical woodpulp having higher brightness and better mechanical properties.