This invention relates to oxygen delignification of chemical pulp and, more precisely, oxygen delignification in two stages .

The Swedish development in the bleaching of chemical pulp has implied an increase in the interest to extend delignificaiton with oxygen prior to the final bleaching, in order thereby to improve the pulp quality of totally chemical free bleached pulp, i.e. TCF-pulp (Totally Chlorine Free) , and in order to facilitate the closing of both TCF-systems and ECF-systems (Elementary Chlorine Free) .

Several variations of two-stage systems of oxygen delignificati on are currently in operation, which use, for example, extended retention time, re-mixing, intermediate treatment for chemical distribution, which have become possible by a two-stage system. Both stages are carried out upstream and pressurized in various degrees.

The following conditions have been found expecially advantageous for the delignification:

- in the first stage the chemical concentration (oxygen and liquour) should be high, i.e. it is an advantage to charge all chemicals to the first stage. In other words, it is important for the reactions of oxygen to maintain initially a high pressure.

- for extended delignification in the first phase of the oxygen system it is above all important to maintain a high temperature. If the alkalinity is sufficiently high for a good reaction, no additional chemicals need be charged to the second stage (final phase) .

These two-stage systems have the disadvantage above all cf requiring much too high investment expenditures, but also that only high-quality intermediate pressure steam can be used for heating the pulp between the two stages.

The present invention has the object to solve the aforesaid problems by carrying out the two oxygen delignification stages under pressure, in that the first stage is carried out in an upstream reactor vessel and the second stage in a downstream reactor vessel. Further extended delignification and increase of pulp brightness are obtained by supply of hydrogen peroxide, up to 5 kg per ton pulp, to a mixer located prior to the downstream reactor. A minor adjustment for the increased alkali demand of the peroxide reactions is made in this mixer. By means of this peroxide supply to the downstream reactor the pulp can be delignified to a greater extent and the brightness of the pulp can be increased prior to the final bleaching. Thereby the closing of the bleaching plant is further facilitated.

The characterizing features of the invention are apparent from the attached claims .

The method according to the invention offers in addition several advantages :

- no separate pump between the reactors is required;

- the pulp is heated at the lowest pressure level of the system, which implies that low pressure steam can be used for heating the pulp;

- possible admixture of additional chemicals such as hydrogen peroxide and adjustment of the alkali level take place before the second stage in a mixer at the top of the second reactor vessel;

- due to the fact that the downstream reactor is not filled with pulp, the system can easily be de-aired (degassed) in the top of the reactor vessel;

- the steam recovered by flashing the pulp after the second stage can be used for heating purposes, for example water heating;

- the temperature of the pulp at the bottom of the downstream reactor can also be lowered by dilution with colder liquid, thereby making it possible to recover the heat not flashed off;

- the downstream reactor constitutes a buffer by acting both as reactor and storage tower.

The invention is described in greater detail in the following, with reference to the accompanying figure, which illustrates an embodiment of a plant for carrying out the method according to the invention.

In the plant shown unbleached pulp of medium concentration (8-20%) is pumped by a pump 1 to a mixer 2 for the admixture of oxygen and alkali to the pulp. The pulp is thereafter directed to a first oxygen delignification stage consisting of an upstream pressurized reactor vessel 3.

The pulp is charged at the bottom of the reactor and removed at the top. The pulp is fed therefrom to a second oxygen delignification stage consisting of a downstream pressurized reactor vessel 4 where the pulp is supplied at the top and removed at the bottom of the vessel. Additional chemicals possibly can be admixed between the stages by means of a mixer 9. For instance hydrogen peroxide in an amount of up to 5 kg per ton pulp and a small amount of alkali for adjustment of the alkali level. After said second stage the oxygen delignified pulp is directed to subsequent processing stages 5, which can consist of a blow tank where the pulp is flashed and recovered steam can be utilized for heating the process water. Stage 5 may also be a washing stage. For cooling the pulp, it is diluted at the bottom of the second reactor by washing filtrate cooled in a heat exchanger 7. Warm water is simultaneously heated in this heat exchanger, and the resulting hot water is used as washing liquid in the process.

A container 6 is provided for de-airing the pulp in the vessel 4.

The chemicals (oxygen and alkali) added in the mixer 2 should form the main part of the amount required. Preferably the entire chemical amount is charged already in the first stage. Possible additional chemical charges can be made between the stages.

In the first stage a pressure above 3 bar, preferably 3-10 bar, is maintained. The temperature should be 75-100°C.

Owing to the fact that the pulp is pumped upward through the first reactor vessel 3 and thereafter transferred to a downstream reactor vessel 4, no additional pump is required between the reactor vessels.

In the second stage a temperature between 90-l20°C is maintained, and a pressure adapted to the temperature is chosen which is sufficiently low for rendering it possible to use low pressure steam 8 for heating the pulp, preferably at maximum 2 bar. The pulp preferably is heated with low pressure steam between the stages at the lowest pressure level of the system.

In the reactor vessel 4 of the second stage a gas space is maintained upwardly. Degassing of the system can thereby be carried out at the top of the vessel. The reactions taking place with oxygen in the second stage take place only with oxygen solved in the pulp and with the oxygen remaining after degassing. The pulp leaving the second stage will thereby contain a smaller amount of gas than otherwise and, therefore, subsequent washing stages can have a better function.

The temperature of the pulp at the bottom of the reactor vessel 4 of the second stage can be lowered by using a colder liquid for dilution. Thereby the heat not flashed off can be recovered.

The method also renders it possible that the second vessel 4, besides its function as a reactor for oxygen delignification, also can be a storage tower for the pulp and thereby serve as a buffer in the system.

The two-stage system for oxygen delignification described above is a much simpler and cheaper method of delignification. Especially the operation economy can be improved by heating the pulp between the stages by low pressure steam instead of medium pressure steam required in previous systems.

The method according to the invention, thus, implies that both investment and operation costs can be reduced substantially. It is above all possible to save much energy in relation to conventional systems for oxygen delignification in two stages.

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