WO1989008165A1

WO1989008165A1 - Process for the pre-treatment of chips

Info

Publication number: WO1989008165A1
Application number: PCT/SE1988/000702
Authority: WO
Inventors: Sture Backlund; Per-Olov Lindblad; Ove Bodin; Solveig NORDÉN
Original assignee: Sca Pulp Ab
Priority date: 1988-03-01
Filing date: 1988-12-27
Publication date: 1989-09-08
Also published as: SE466964B; SE8800713D0; SE8800713L; AU2933089A

Abstract

In view of the environment requirements of today it is essential to be able to delignify cellulose pulp to a low kappa number prior to the bleaching of the pulp. According to the invention, this is achieved in that the wood raw material in the form of chips is subjected to a pre-treatment with sulphur dioxide. According to the invention, this is carried out by using the sulphur dioxide at the treatment in gaseous state at an overpressure of 0-5 bar.

Description

PROCESS FOR THE PRE-TREATMENT OF CHIPS

This invention relates to a method at the pre-treat ent of wood chips, which are intended to be used for the manufacture of cellulose pulp according to alkaline delignification methods, more precisely a pre-treatment with sulphur dioxide.

At the manufacture of mechanical pulps it is, of course, desired to be able to delignify the pulp to a low kappa number. However, this is not possible without substanti¬ ally deteriorating certain properties of the pulp, especially its viscosity and paper properties, and without reducing the yield.

It was now found according to the invention, that by pre-treating the chips in a special way the chips can be .delignified so that a pulp -with substantially improved viscosity is obtained.

This is achieved according to the invention, in that the chips are subjected to a pre-treatment with sulphur dioxide.

It is per se known since long to pre-treat chips with sulphur dioxide.

According to FI-PS 22 575, for example, it is previously known to pre-treat chips with liquid sulphur dioxide, in order to remove from the wood rosin and other constit¬ uents detrimental at cellulose digestion as well as other substances soluble in liquid sulphur dioxide.

At the method according to said patent, the chips are subjected to an extraction with liquid sulphur dioxide either in a separate p-rocessing step or in the cellulose digester. The extraction is carried out by stirring the extraction liquid or causing it to circulate. The method is primarily intended for use at the manufacture of sulphite cellulose. In order at this method to maintain the sulphur dioxide in liquid state, it is required,, however, that the method is carried out at low temperatures of 20-30 C or at high pressure. At 80 C, for example, an overpressure of 19 bar is required. Moreover, when using liquid sulphur dioxide it must be charged in a very great quantity per ton of wood, for rendering it possible to distribute the liquid reasonably uniformly to the chips. Probably at least 1000 kg sulphur dioxide are required per ton of wood. The method, therefore, is entirely unreasonable to be used in practice. Further¬ more, the high concentration of sulphur dioxide implies a very great risk of hydrolysis of the wood material and that, therefore, the yield at a subsequent alkaline digestion would be very low.

In FI-PS 45 771 a method of impregnating chips is described which is especially intended for the manufacture of pulp according to the sulphite method. At the impregnation a solution is used, which contains a base and sulphur dioxide. The specification discloses that at the digestion of chips from softwood at least 8 % sulphur dioxide counted on dry weight must be used. It also is essential, that the chemical amount must be caused to penetrate into the digesting chips com¬ pletely. For this purpose, an impregnating step is carried out prior to the digestion proper.

It refers, thus, to an impregnation method at sulphite digestion, which has the object to impregnate the chips at optimum with base and sulphur dioxide before the digestion, and it does not refer to a pre-treatment with sulphur dioxide before an alkaline digestion, as is the case with the present invention.

According to the invention, the treatment is carried out with the sulphur dioxide in gaseous state. The treatment should occur at an overpressure of 0-5 bar, preferably 0.5-3 bar, suitably 1-2 bar.

According to an especially advantageous embodiment of the method according to the invention, the pre-treatment is carried out at a temperature of βθ-120 C during a period of 10-300 minutes.

According to one embodiment, the chips are washed after the pre-treatment. According to another embodiment, the sulphur dioxide charged to the chips is allowed to follow along to the subsequent delignification.

The invention is described in greater detail in the following by way of an embodiment thereof.

EXAMPLE

At this experiment softwood chips were used which in an amount of 100 g were charged into a 1-litre autoclave. Sulphur dioxide was supplied to the autoclave at an overpressure of 1 bar during a period of 40 min at a temperature of 80 C. The charged amount of sulphur dioxide was 38 kg per ton of wood material.

After completed pre-treatment with sulphur dioxide, sulphate digesting liquor heated to 80 C was injected into the autoclave, and the chips were digested accord- in to the scheme as follows: ^"

At 80°C 20 min

80-170°C 120 min

170°C 65 min

The charging counted as active alkali was 2β0 kg NaOH per ton of wood material, the sulphidity was 5$, and the liquor/wood ratio was 3.5:1»

As reference, the same type of chips was digested according to the same digestion scheme and conditions, but without pre-treatment. The two pulps obtained were analysed in respect of kappa number and viscosity according to the following standards:

Kappa number SCAN-C 1:77

Intrinsic viscosity SCAN-C 15-16:62

The results obtained appear from the Table below.

Treatment

Kappa number Intrinsic , viscosity dm /kg

According to the invention 29,9 1200

Reference 29,4 1090

As shown in the Table, for the pulp manufactured according to the invention at a kappa number of 29.9 an intrinsic viscosity of 1200 was obtained, while for the reference pulp the intrinsic viscosity was only 1090 at substantially the same kappa number, viz. 29.4.

At experiments with oxygen bleaching of the pulp according to the invention, it was found that the vis¬ cosity advantage of 110 units to a large extent remains at a bleaching all the way down to a kappa number of 12.

In view of the environment requirements of to-day, it is important to be able to delignify the pulp to a low kappa number prior to the chlorine bleaching while maintaining the pulp quality. A reduction of the kappa number to about 12 from normally about 20 at oxygen bleaching implies, that the chlorine demand at the bleaching can be decreased substantially and consequently the formation of dioxines and similar substances toxic for the environment can be reduced.

The invention is not restricted to the embodiment described, but can be varied within the scope of the invention idea.

Claims

C L A I M S

1. A method at the pre-treatment of chips with sulphur dioxide for the manufacture of celllulose pulp according to the sulphate method, c h a r a c t e r i z e d i n that the sulphur dioxide is added to the chips in gaseous state at an overpressure of 0-5 bar, preferably at 0.5-3 bar, suitably at 1-2 bar, at a temperature of 60-120 C, preferably at 70-100 °C, suitably at 80-90 °C , during a period of 1-300 min., preferably 10-120 min, suitably 20-60 min.

2. A method as defined in claim 1, c h a r a c t e r ¬ i z e d i n that the amount of charged sulphur dioxide at the pre-treatment is 5-60, preferably 20-40 kg per ton of pulp .

3. A method as defined in claim 1, c h a r a c t e r¬ i z e d i n that the chips are washed after the pre- treatment.

4. A method as defined in claim 1, c h a r a c t e r¬ i z e d i n that the sulphur dioxide charged to the chips from the pre-treatment is allowed to follow along to the subsequent- delignification.