The present invention relates to a process for delignification of lignocellulose material, preferably wooden material, for the production of papermaking pulp. An aqueous solution of phenols and pyrazolidones is added to the cooking liquor.

At the production of papermaking pulp in a conventional sulphate cook with softwood, the aim is to drive the delignification to as low a kappa number as possible with as high a carbohydrate yield as possible.

During the past decades, a variety of chemical additives have been added to the cook¬ ing liquor in order to increase the yield of cellulose among other things. Such an additive is anthraquinone (AQ). Anthraquinone is present in the stabilisation of carbohydrates as well as in the lignin extraction through a cyclic process.

carbohydrates anthraquinone 1 fragmented

AQ lignin

oxidised * anthrahydro- lignin (end group quinone stabilised) AHQ carbohydrates

Through the course of this cyclic reaction, very small quantities of anthraquinone are required in the cooking liquor. Batches in the order of 0.1-0.3 per cent by weight of wood are sufficient. Thus, an addition of anthraquinone implies that the pulp yield can be increased and that the lignin extraction is facilitated at the cooking of pulp. The anthraquinone and the anthrahydroquinone exist in a redox equilibrium. It is this redox equilibrium which is used in the process.

Since the anthraquinone either reduces or oxidises the end groups of the carbohydrates to alkali resistant groups, the attack from the peeling reaction is decreased, whereby a higher carbohydrate yield is obtained.

The price of anthraquinone per kilo is high, even when purchased in bulk. Despite the fact that only small quantities of anthraquinone, calculated on the wood substance, are used in the cook, costs are very high. Consequently, other alternatives are desired.

Art known previously

The American patent US, A, 4, 213, 821 discloses a process, using this redox equilibrium. Anthraquinone compounds together with a reduction agent, such as sodium dithionite, zinc dithionate, sodium borohydride or zinc powder and sodium hydroxide are added to the cooking liquor. In another American patent, US, A, 4, 786, 365 pulp is produced by adding anthraquinone and small quantities of sulphide of sodium to the cooking liquor. Quinones and anthraquinones are efficient additives and they are stabile during the pulp process. In the present invention, a similar redox equilibrium to the one mentioned above is used. The compound of special interest are a variety of phenols, such as quinone, anthra¬ quinone, aminophenols etc., which exist in a redox equilibrium with the corresponding keto forms. Preferred phenols are those having groups capable of oxidising in a para position. Such phenols can have structure formulas I or II:

where R, is either H, OH, NH ₂ , NHR, S0 ₃ \ Cl, or Br,

R ₂ is either H, methyl, ethyl, phenyl, bensyl, or CH ₂ COOH.

The present invention comprises an aqueous solution containing, among other things, phenols and pyrazolidones, which are added to the cooking liquor and are present in the cook.

The aqueous solution is preferably alkaline and is a residual product from the photography industry, and is used for developing films and photos. By using these residual products, it is possible to lower additive costs in an efficient way, at the same time as the residual product enters a new industrial cycle, and thereby does not further burden the environment. Pyrazolidones are substances which co-operate with phenols and strengthen their effect through an improved redox equilibrium.

By improving the redox equilibrium and by finding raw material at a low cost, conditions are considerably improved for the pulp industry, since enormous quantities of chemicals are required in order to meet the demand for production. According to a preferred embodiment of the invention, an aqueous solution of phenols is added in the amount of between 1 to 50 grams per litre, preferably between 10 to 30 grams per litre, most preferably between 15 to 25 grams per litre, and the quantity of phenols is added in a quantity of between 0.001% to 10%, preferably between 0.01% to 3%, by weight of wood. The amount of pyrazolidone in the aqueous solution is between 0.1 to 30 grams per litre, preferably between 0.1 to 20 grams per litre, most preferably between 0.1 to 10 grams per litre.

In another preferred embodiment of the invention, the phenols comprise 1,4- benzenediol. hydroquinone monosulphonate, hydroquinone disulphonate, 2-chloro- hydroquinone, 2-bromo-hydroquinone, 4- aminophenol, N- (4-hydroxyphenyl)-glycine monomethyl-p-amino-phenolsulphate or oxidised forms thereof and the pyrazolidones comprise l-phenyI-3-pyrazolidone, 4-methyl-l-phenyl-pyrazolidone, 4-methyl-4-hydroxy- methyl-l-phenyl-3-pyrazolidone, 4,4-dimethyl-l-phenyl-3-pyrazolidone or redox products thereof.

In a further preferred embodiment of the invention, the aqueous solution is alkaline or acidic.

In a further preferred embodiment of the invention, the aqueous solution also comprises glutaraldehyde. di- or tri-ethyleneglycol and /or acetic acid

In the most preferred embodiment of the invention, the aqueous solution is a residual product from the development of photographic film or photographic paper.

The residuai product can also include inorganic substances such as sodium sulphite, sodium hydroxide and carbonate of sodium, which are already included in the cooking liquor and are therefore of use to the cooking process ln order to examine the effect of the additive, a comparative study was performed, where a variety of quantities of added additive were compared to a convenuonal sulphate cook Two comparative cooking series were performed and the results are shown in tables 1 and 2 The experiment series are not limitative of the invention, but simply illustrate it

Series 1 300 g of dried industrial coniferous wood chips, with a drying level at 92.5% were impregnated at a pressure of 5 bar. followed by cooking for 1 15 minutes Tne cooking was performed in a rotating autoclave and was effected with a liquor and wood ratio of 4 1, an efficient alkalinity of 20% calculated on wood and a suiphidity of 35% Varying quantities of the additive were added to the cooking liquor A cooking temperature of 170 °C was reached after a raise of temperature of 1 °C/mιn. After each terminated cook, the pulps were washed and dehydrated, after which they were analysed The data of the analyses were summarised in table 1

Table

Series 2

Experiment series 2 was carried out under the same conditions and with the same course as experiment series 1. The result of the analyses is shown in table 2

Table

Cook Additive Kappa Silate yield Shives Total yield Viscosiπ no ml no % % % ml/g

1 0 21 9 43 1 0.5 43.6 1030

2 1 5 23 7 43.5 0 9 44 4 .

15 22.3 43.5 0.9 44 4 1050

4 30 22.8 43.6 0.9 44 4 .

5 75 25.2 43.6 1.1 44.7 .

6 150 27.0 42.6 1.6 44.2 -

Both of the experiment series show that the additive containing phenols and pyrazolidone increases the vield by 0 4% Additives containing phenols and pyrazolidones are therefore interesting wnen producing wood pulp and are useful alternatives compared to the expensive conventional additives, such as anthraquinone