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This invention relates to the delignification of waste paper with a high lignin content, preferably corrugated cardboard containing chemical high-yield pulp. It has been proved possible by this processing to restore to a certain extent the original properties of the fibres in respect of flexibility and swelling degree, without increasing the dewatering resistance.

Waste paper of the aforesaid kind often contains aluminium in an amount of approximately 5-10 kg/ton bone-dry pulp. This aluminium usually has been added in the form of alum as retention agent at previous paper aking.

It is known that aluminium causes problems with incrustration in evaporation plants. Magnesium compounds can be added for reducing these problems. Aluminium can hereby be precipitated in the form of aluminium-magnesium compounds. In this way the amount of solved aluminium in the pulp suspension can be decreased.

Due to the relatively high aluminium content in waste paper, however, this measure is not sufficient for rendering it possible to recycle white water from the delignification step to the evaporation plant. Too great an amount of solved aluminium would in this case be returned to the evaporation and give rise to incrustration problems.

The recycling of the white water from the delignification step, however, is desirable and necessary in view of the chemical and COD-content in the white water.

_/ » The present invention offers a _. solution to the aforesaid

^ problems. After pretreatment by slushing and screening,an

~ oxygen delignification is carried out, whereafter the pulp is transferred for papermaking. According to the invention, the pulp is dewatered before the delignification, whereby

he main part of the white water is transferred directly as dilution liquid in the paper mill, preferably for diluting the stock. Substantially all solved aluminium is thereby removed from the pulp and transferred to the paper mill without being mixed into the circulation system of the delignification step. This implies that the white water from the dewatering after the delignification can be recycled to evaporation and chemical recovery with a minimum aluminium content.

The invention is described in greater detail in the follow¬ ing, with reference to an embodiment thereof, the flow dia¬ gram of wBicrh-i-a shown in the accompanying Figure.

Waste paper is charged into a pulper 1 and finely distributed therein to pulp. At the dilution a certain amount of NaOH can be added. The waste paper preferably consists of corr¬ ugated cardboard made of chemical pulp with a high lignin content, but also mechanical pulp can be used to a certain extent. After the pulping, the pulp is cleaned in screens 2 whereby the concentration of the pulp is relatively low, for example about 1% .

Thereafter the pulp is dewatered in a dewatering press 3 and thereafter steam-treated. Alkali and oxygen are added and intensively admixed. The subsequent oxygen delignific¬ ation is carried out in a pressurized reactor 4 at a pulp concentration of 10-12% or lower. Subsequent to the delign¬ ification washing is carried out in a washing press 5, from which the pulp is transferred for papermaking in a paper mill 6. In connection therewith the return fibre pulp is mixed with other pulps, for example unbleached kraft pulp for the making of, for example, kraft liner. It is, of course, also possible to make other paper qualities. Additional bleaching and treatment of the return fibre pulp can then be carried out. By the aforesaid method, the properties of the return fibre pulp are so improved that a greater amount of return fibres can be used without deteriorating the properties of the final product.

The white water is transported as follows. Prom the dewatering press 3 the main part of the white water 7 is directed directly to the paper mill 6. A certain amount, however, can be recycled for dilution in the pulper 1. Prom the washing press 5 the white water 8 is directed to evaporation and chemical recovery 9. White water from the paper mill ^β can be utilized for diluting the pulp after the washing press 5.

The method, thus, implies that the main part of the solved aluminium is transferred via the white water flow 7 to the paper mill 6 past the oxygen delignification. This aluminium together with added alum will serve as retention agent in the papermaking machine. The white water 8 from the delignification thereby can be recycled to the recovery cycle. The aluminium content in this white water 8 can be kept so low, that no problems with incrustration in the evaporation plant will arise.

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