WO1994005851A1

WO1994005851A1 - Novel method of bleaching of lignocellulosic pulp using persulphate

Info

Publication number: WO1994005851A1
Application number: PCT/CA1993/000362
Authority: WO
Inventors: Alfred Wong; Stanley Wu; Chen Chiu
Original assignee: Arbokem Inc.
Priority date: 1992-09-09
Filing date: 1993-09-09
Publication date: 1994-03-17
Also published as: AU5892094A; CA2080026A1

Abstract

A process for the bleaching of lignocellulosic pulps using persulphate chemicals are admixed with conventional oxidizing agents selected from hydrogen peroxide and chlorine dioxide and combinations thereof. A transition metal salt catalyst is added to effect improved pulp delignification with the combined pulp bleaching reagents of persulphate and chlorine dioxide.

Description

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NOVEL METHOD OF BLEACHING OF LIGNOCELLϋLOSIC PULP

USING PERSULPHATE BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates to a process for the bleaching of lignocellulosic pulps using persulphate chemicals.

2. Description of the Prior Art

Within increasing environmental demand for clean- manufacture of papermaking pulp, there is a need for improved method of bleaching pulp with little or no use of chlorine containing compounds.

Strong oxidizing agent such as monoperoxysulphuric acid salts are used in the treatment of organic wastes, bleaching of textiles and wood pulps.

Both sodium and potassium persulphate are commercially available as stable solid material. Persulphate (S₂0₈ ⁼) is chemically different from peroxymonosulphate (HS0₅ ^") . The former is derived principally from the electrochemical treatment of acidic sulphate. The latter is typically produced from the reaction of sulphuric acid with hydrogen peroxide. KHS0₅ is used in the form of commercially available mixed salt of KHS0₅, KHS0₄ and I^SO^, as marketed by DuPont Co., Wilmington, Delaware, under the trade name of OXONE. This particular mixture is commonly known as Baeyer's reagent (Ber. Vol. 34, page 853, 1901). Kennedy and Stock (J. Org. Che . , Vol. 25, page 1901, 1960) have shown that the oxidation of organic substances by the mixture of KHS0₅, KHSO and K_jSO^ proceeds under in situ acidic conditions by the presence of the bisulphate salt.

TITUTE SHEET Kringstad et al. (Nord. Pulp Pap. Res. J. , Vol. 3, No. 2, 1988) have reported the use of sodium persulphate for the oxidative sulphonation of kraft lignin. Huynh (U.S. Patent 4,773,966) teaches the use of a mixture of persulphate anion and transition metal cation to effect the catalytic oxidation of model lignin compounds. It is generally recognized that the chemical structure of model lignin compounds are very different from that of poly-dispersed lignin macromolecules present in wood pulp and non-wood pulp. For example, the average molecular weight exceeds 20,000 Daltons. It is essentially not soluble in water. In contrast, the lignin model compounds as used by Huynh are already soluble in water. The molecular weight of these lignin model compounds is of the order of less than 500. They are already lignin degradation products. See for example, Fengel and Wegner, "Wood-Chemistry, Ultrastructure, Reactions", pp. 131-181, De Gruyter, New York, 1984. It is unreasonable to extrapolate that the oxidative degradation technique as disclosed in Huynh can be applied to the effective removal of lignin of wood and non-wood pulp. Roesch et al. (Ger. Offen. DE3218889) have disclosed the simultaneous desizing and bleaching of textile material from textile fibres by using persulphate as a peroxide activator.

Zakis et al. (Khim. Drev. Riga), Vol. 9, pp. 109- 117, 1971) have studied the delignification action of ammonium persulphate on spruce wood.

Cael (U.S. Patent 4,404,061) teaches the use of monopersulphuric acid and its salts for the bleaching of Hgnocellulosic materials. In particular, wood chips are delignified with a mixture of persulphate

ITUTE SHEET salts under alkaline kraft-type cooking conditions.

Springer et al. (U.S. Patent 4,756,800) teaches the use of cupric ions to produce salts of monoperoxysulphuric acid, which in turn is used for the bleaching of pulp. The effectiveness of this prior art is limited to delignified pulp, i.e., low- lignin pulp which has already been previously chlorinated and alkali-extracted.

Unfortunately, these processes are limited to uses with delignified pulps or to use with wood chips prior to conventional pulping procedure.

Moreover, the relatively high costs of persulphates as well as salts of monoperoxysulphuric acid have limited the widespread commercial application of this approach. And in many cases, the pulp can only be bleached to acceptable brightness levels, with considerable concomitant loss of physical strengths.

There is a need for improving the oxidative bleaching technology to provide effective and economical manufacture of pulp for papermaking.

SUMMARY OF INVENTION

A process has been discovered for the effective delignification of wood and non-wood pulp, which comprises the steps of adding an alkali metal persulphate and an oxidizing agent, and reacting the said reagents with any lignocellulosic wood or non- wood pulp, under conditions of moderate temperatures and times. When the present invention is practiced, lignin- containing pulp is delignified to reduce the usage of subsequent chemical reagents for bleaching or brightening of the pulp. The spent liquor containing

SUBSTITUTE SHEET residual chemicals from the present invention may be recycled to improve the economy of pulp treatment of the present invention. Another advantage attendant to the present invention is the significant pulp bleaching or brightening effect achieved without the use of any chlorine containing chemicals. In particular, multi-stage non-elemental chlorine bleaching of pulp can be effected efficiently with the inclusion of a persulphate treatment step, to achieve satisfactory brightness levels. The non-persulphate bleaching stages of the multi-stage bleaching scheme of the present invention may be conventional in nature.

DETAILED DESCRIPTION OF THE INVENTION

Persulphate is a known candidate oxidative bleaching agent for pulp. To date, its relatively high cost has precluded any commercial interest within the pulp bleaching. We have now discovered that when persulphate is used in combination with such bleaching agent as hydrogen peroxide and chlorine dioxide, it can enhance the delignifying effect of these bleaching chemicals.

The alkali metal persulphate used in the present invention is available commercially. Either sodium, potassium or ammonium salt may be used in the chemical charge range of 0.5% to 5.0% based on oven-dry weight of pulp. The oxidizing agent used with persulphate as set forth in the present invention may be selected from, among others, hydrogen peroxide and chlorine dioxide, and combinations thereof. The chemical charge of oxidizing agent to be used with persulphate is in the range of 0.1% to 10%, based on oven-dry weight of pulp.

SUBSTITUTE SHEET When chlorine dioxide is selected as the oxidizing agent to be used with persulphate, a transition metal catalyst, in the range of 10 to 200 milligrams per kilogram of oven-dry pulp, is to be added to effect satisfactory delignification of the pulp. The transition metal catalyst may be salts of iron, copper or manganese, and combinations thereof.

Pulp which has been treated with a combination of persulphate and selected oxidizing agent in mixture may be bleached subsequently in one or more conventional bleaching stages to effect high pulp brightness.

In order to disclose more clearly the nature of the present invention, the following laboratory pulp treatment examples illustrating the invention are given. The specific conventional bleaching techniques used herewithin are well known to those skilled in the art.

f i t \n "^~~~~* f ^~~~~~- * z^~~-^~~- ^~~~- c * - ^{~ ~~~~}" EXAMPLE 1

Pulp: Unbleached mixed Canadian northern softwood kraft pulp; Kappa number 29.3; viscosity 40.5 mPa.s; brightness 28.2% Photovolt

Treatment conditions:

oven-dry pulp, g.

Na₂S₂0₈, % OD pulp H₂0₂, % OD pulp NaOH, % OD pulp Na₂Si0₃, % OD pulp MgS0₄.7H₂0, % OD pulp End pH of treatment

Kappa number Viscosity, Pa.s Brightness,% PV

Test Runs M283 and M294 clearly shows the benefit of the addition of sodium persulphate to enhance the activity of hydrogen peroxide on the decrease in Kappa number. The pulp viscosity is still maintained within acceptable range with the treatment procedure of the present invention.

SUBSTITUTE SHEET EXAMPLE 2

Pulp: Unbleached mixed Canadian northern softwood kraft pulp; Kappa number 30.0; viscosity 39.0 mPa.s; brightness 27.0% Photovolt

Lab treatment conditions: Temperature 50 deg. C; consistency 10.0%; time 30 minutes

oven-dry pulp, g.

Na₂S₂0₈,% OD pulp ClO,,% OD pulp Fe^+2f,mg/kg OD pulp End pH of treatment

Kappa number Viscosity,mPa.ε Brightness,% PV

Test Run M413 illustrates that sodium persulphate may be used beneficially with chlorine dioxide in presence of iron catalyst. As given in Test Run M386, absence of iron catalyst provides negligible beneficial effect of persulphate-chlorine dioxide treatment of pulp. Significant improvement in Kappa number reduction is achieved with the present invention.

SUBSTITUTE SHEET EXAMPLE 3

Lab treatment conditions: Temperature 50 deg.C; consistency 10.0%; time 30 minutes

These tests have unexpectedly shown that there is an optimum range of catalyst addition in the combined persulphate-chlorine dioxide treatment of pulp, to achieve low Kappa number, high viscosity and high brightness of the treated pulp.

SUBSTITUTE SHEET EXAMPLE 4

Pulp: Unbleached mixed Canadian northern softwood kraft pulp; Kappa number 30.0; Viscosity 38.0 mPa.s; Brightness 30.0% Photovolt

Lab treatment conditions: Consistency 10.0%; Time 90 minutes

Treatment sequence: D277/86 followed by D293, then by

D299 and finally by D301

DTMPA = sodium salt of diethylenetriamine pentamethylene phosphonic acid, chelatant

Test Runs D277/86, D293, D299 and D301 show the continued benefit of sodium persulphate treatment stage in a 4-stage bleaching sequence to achieve bleached pulp with a brightness of 68.8%. The residual hydrogen peroxide was 73% (of input) in Stage 3 and 76% (of the input) in Stage 4. The residual bleaching liquor may be recycled for reuse.

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SUBSTITUTE SHEE1 EXAMPLE 5

Pulp: Unbleached mixed European softwood kraft pulp; Kappa number 35.3; viscosity 39.0 mPa.s; brightness 25.9% Photovolt

Lab treatment conditions: Consistency 10.0%;

Treatment sequence: M687, followed by M688.

Test Run

Test Runs M687 and M688 show the inclusion of a combined sodium persulphate-hydrogen peroxide treatment stage in a 2-stage bleaching sequence to achieve bleached pulp with a brightness of 59.0%. The residual hydrogen peroxide was 77% (of the input) in Stage 1 and 89% (of the input) in Stage 2. The residual bleaching liquor may be recycled for reuse.

SUBSTITUTE SHEET

Claims

1. A process in which delignification of any cellulosic pulp is by the use of alkali metal persulphate with one or more oxidizing agents.

2. The process of claim 1 wherein the alkali metal persulphate may be selected from sodium, potassium or ammonium, and combinations thereof.

3. The method of claim 2 wherein the concentration of said persulphate is 0.5% to 5.0% based on oven-dry weight of pulp.

4. The process of claim 1 wherein the oxidizing agent is hydrogen peroxide, as used commonly in the alkaline regime.

5. The method of claim 4 wherein the concentration of hydrogen peroxide is 0.1% to 10% based on oven-dry weight of pulp.

6. The process of claim 1 wherein the oxidizing agent is chlorine dioxide, as used commonly in the acidic regime.

7. The method of claim 6 wherein the concentration of chlorine dioxide is 0.1% to 10% based on oven-dry weight of pulp.

8. The process of claim 1 wherein the alkali persulphate is used admixed with chlorine dioxide in the presence of a transition metal catalyst.

SUBSTITUTE SHEET

9. The method of claim 8 wherein the metal catalyst may be selected from iron, copper and manganese, and mixtures thereof.

10. The method of claim 8 wherein the ionic concentration of transition metal catalyst ion is between 10 milligrams per kilogram of oven-dry pulp to 200 milligrams per kilogram of oven-dry pulp.

11. The process of claim 1 wherein the residual treatment liquor extracted from pulp is recycled for reuse.

12. J?he process of claim 1 wherein subsequent one or more stages of known bleaching methods are used to achieve high pulp brightness.

SUBSTITUTE SHEET