CA2025522A1 - Explosion pulping - Google Patents

Abstract

ABSTRACT

In an explosion pulping process an inert gas is injected into the steamed pulp prior to explosion, to raise the pressure; the resulting exploded pulp displays improved mechanical properties and requires less refining time and refining energy.
By employing lime in a presoaking treatment and carbon dioxide in the inert gas, deposits of calcium carbonate are formed on the fibres, providing improved brightness characteristics.

Description

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This invention relates to an improved explosion pulping process.
Chemi-mechanical pulps (CMP) and chemi-thermomechanical pulps (CTMP) have been proposed as an alternative to costly chemical pulp5 and weaker mechanical pulps in the pulp and paper industry.
A problem with these proposed pulps is the relatively high energy required for defibration.
One approach which has been proposed as an alternative to the use of high defibration energy involves wood chip softening by steam at elevated pressures and temperatures, followed by an explosive decompression. It has been suggested that this approach will result in a better dispersion of the fibres such that the pulp requires less refining in subsequent operations, and thus will require less energy consumption in subsequent refining operations.
This process is referred to as the explosion pulping process.
In accordance with the present invention it has been found that benefits in the character of pulp are obtained by carrying out the explosion pulping process with injection of an inert gas into the pulp to increase the pressure, prior to the e~plosion s-tage.
The present invention seeks to provide an improved explosion pulping process exploiting an injection of inert gas prior to the explosion stage.
The present invention also seeks provide an improved explosion pulping process of this type which provides improved brightness in the fibres.

In accordance with the invention a process ~or the steam explosion of pulp comprises heating a pulp of wood chips with steam at an elevated temperature and pressure to produce a steamed wood chip pulp. An inert gas is injected into the steamed pulp to raise the pressure, and the pulp is exploded to disintegrate the Eibres in the pulp.
A pretreatment of the pulp prior to the steaming involves impregnation or soaking of the pulp 10 in an alkaline chemical solution. In one embodiment of the process, calcium oxide or calcium hydroxide is used as at least part of the alkaline component of the alkaline chemical solution and carbon dioxide is used as at least part of the inert gas. The carbon dioxide reacts with the calcium oxide or hydroxide forming deposits of calcium carbonate on the exploded fibres, thereby improving the brightness.
The pulp produced by the invention requires less refining energy in subsequent refining and 20 displays improved mechanical properties including improved burst strength, tear strength and breaking strength.
The pulp is, in particularl a chemi-mechanical pulp or a chemi-thermomechanical pulp.
The pretreatment of the pulp suitably comprises soaking the wood chips in a chemical solution containing sodium hydroxide, sodium sul-phite, sodium carbonate or sodium bicarbonate. Lime, in particular calcium oxide or calcium hydroxide may replace at least part of the sodium hydroxide for generating calcium carbonate deposits on the fibres, by reaction with carbon dioxide.

The s-teaming -typically comprises heating with steam at a pressure of 200-250 psig at a temperature of 170 to 210C., typically for up to 6 minutes.
The inert yas may be, for example, nitro-gen, carbon dioxide, argon or helium or mixtures thereof.
After the steaming the inert gas is injected into the steamed pulp to raise the pressure 10 typically to a pressure of 250 to 2000 psig. During this stage the temperature of the pulp may be main-tained without change.
The pulp, under the high pressure con-ditions, is then exploded, typically by exhausting it through an orifice to atmospheric pressure and this causes disintegration of the pulp fibres.
Upon this release to atmosphere the inert gas and steam separate from the pulp and may be recycled for reuse.
; 20 The inven-tion is illustrated in an especially preferred embodiment by reference to the accompanying drawing in which:
FIG. 1 is a flow sheet schematically illustrating the treatment of wood chlp pulp with the-improved explosion pulping of this invention.
With further reference to Fig. 1, wood chips are delivered by a screw conveyor 10 to a chemical soaking tank 12. The soaked or impregnated wood chip mixture or pulp is fed by line 14 to a press 15 in which excess soaking liquor is removed and recycled by line 18 to tank 12.

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~ 'he pressed chips are fed through line 20 to a vessel 22 containing a screw conveyor 24.
Saturated steam is introduced into vessel 22 through line 26, typically at a -temperature of 170 to 210C.
and a pressure of 200 to 250 psig.
After steaming in ~essel 22 the pulp is fed through line 28 to a reactor 30 having a restriction orifice 32.
An inert gas is injected into reactor 30 10 through line 34 to increase the pressure to 250 to 2000 psig while main-taining the same temperature.
Thereafter the restriction orifice 32 is opened so that the pulp in reactor 30 is suddenly released to atmospheric pressure through orifice 32.
Steam and inert gas may be recycled from reactor 30 through line 36 to screw conveyor 10.

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EXAMPLE
In order to demonstrate the benefits avallable from the improved explosion pulping process of the invention, explosion pulps of aspen were prepared using vapor phase steaming of aspen wood chips which had been presoaked in an alkaline chemical solution containing sodium sulphite. Pulps were obtained in high yield using steaming temperatures of 190C., 195C. and 200 C. and steam-ing times of 2 minutes, 1.5 minutes and 1 minute, respectively.
For comparison purposes two explosion conditions were used; in an ordinary explosion the chip pulp was exploded after the steaming, at a pressure proportional to the saturated steam pressure at the steaming temperature; in an explosion in accordance with the invention nitrogen gas was injected into the reactor to develop a pressure of 350 psi and the chip pulp was exploded.
There was no significant difference in yield between the nitrogen exploded pulps and the corresponding ordinary exploded pulps.
The nitrogen exploded pulps required less.
refining time and less refining energy than the ordinary exploded pulpsl and more especially provided savings of about 20%.
At the same freeness levels the nitrogen exploded pulps showed better burst strength, tear strength and brea~ing strength.
The brightness of the pulps was comparable.

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The nitrogen exploded pulps were easy torefine and more easily bleached to more than 80% MyO
brightness by single stage peroxide bleaching to provide proper-ties superior to those of mechanical pulps with the same yield.

Claims (6)

1. A process for the steam explosion of pulp comprising:
i) heating a pulp of wood chips with steam at an elevated temperature and pressure, ii) injecting an inert gas into the heated pulp to raise the pressure, and iii) exploding the pulp to disintegrate pulp fibres.

2. A process according to claim 1, wherein step i) comprises heating said pulp with saturated steam at a temperature of 170 to 210°C. and a pressure of 200 to 250 psig, and step ii) comprises injecting said inert gas to raise the temperature to 250 to 2000 psig.

3. A process according to claim 1, wherein said inert gas is selected from nitrogen, carbon dioxide, argon, helium and mixtures of two or more thereof.

4. A process according to claim 1, wherein said pulp is subjected to a pre-soaking in an alkaline chemical solution, prior to said heating step i).

5. A process according to claim 4, in which said chemical solution comprises calcium oxide or hydroxide and said inert gas comprises carbon dioxide, including allowing said carbon dioxide to react with said calcium oxide or hydroxide to form deposits of calcium carbonate on the disintegrated pulp fibres.

6. A process according to claim 1, wherein said step of exploding iii) comprises releasing said pulp to atmospheric pressure.
#16-09/13/1990