SE507870C2 - Two=stage oxygen delignification of paper pulp - Google Patents

Abstract

The pulp is fed (1) to a first stage vessel after mixing (2) with oxygen and alkali. Vessel (3) operates at a pressure of over 3 bar. Additional oxygen as hydrogen peroxide with more alkali is mixed (9) and fed to second vessel (4) with low pressure steam for heating purposes. Vessel (4) operates at 90-120 deg C and the pressure is adapted to the operating temperature. Preferably most oxygen and alkali is added to mixer (2) just after pump means (1). Hydrogen peroxide at up to 5Kg per ton and more alkali are added before the downward flowing second treatment vessel (4). Low pressure steam can be used to heat said vessel (4) to a temperature in the range 90-120 deg C. A gas space at the top of vessel(4) allows degassing (6) of the pulp with heat recovery by flashing (5,7) or by heating dilution liquids to further improve the process economy.

Description

507 870 2 the peroxide reactions. With this peroxide addition to the downstream reactor, the pulp can be delignified to a greater degree and the brightness of the pulp increased before the pulp is taken to final bleaching.

This further facilitates the bleaching connection. The more detailed features of the invention appear from the claims.

The process according to the invention also has several advantages: no separate pump is needed between the reactors; the heating of the pulp is done at the lowest pressure level of the system, which means that low-pressure steam can be used for heating the pulp; mixing of hydrogen peroxide and adjustment of the alkali level before the second stage is done in a mixer on top of the second reactor vessel; - because the downstream reactor is not filled with mass, deaeration (degassing) of the system can easily be done in the top of this reactor vessel; the steam recovered by flashing the mass 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, which allows the heat that does not flash off to be recovered; the downstream reactor is a buffer in that it functions as both a reactor and a storage tower.

In the following, the invention will be described in more detail with reference to the figure which shows an example of a plant for carrying out the method according to the invention.

In the plant shown, unbleached pulp of average concentration (8-20%) is pumped by means of a pump 1 to a mixer 2 for mixing oxygen and alkali into the pulp. Thereafter, the pulp is passed to a first oxygen delignification step which consists of an upstream pressurized reactor source 3. The pulp is thus fed to the bottom of the vessel and taken out at the top. From there, the pulp is transferred to a second oxygen delignification stage which consists of a downstream pressurized reactor vessel 4 where pulp is supplied at the top and taken out at the bottom of the vessel. Hydrogen peroxide in an amount of up to 5 kg per tonne of pulp and a small amount of alkali for adjusting the alkali level are mixed in between the steps by means of a second mixer 9.

After this second step, the oxygen delignified pulp is passed on to subsequent treatment step 5, which may consist of a blowing tank where the pulp is flashed and recovered steam can be used for heating process water. Step 5 can also consist of a washing step. To cool the pulp, the pulp is diluted at the bottom of the second reactor using washing filtrate which is cooled in a heat exchanger 7. At the same time, hot water is heated in this heat exchanger and the obtained hot water is used as washing liquid in the process.

A container 6 is arranged for venting the mass in the vessel 4.

The chemicals added to the mixer 2 (oxygen and alkali) should make up the bulk of the required amount. Preferably, the entire amount of chemicals is invested already in the first step. Any additional chemical batch is made between the steps.

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

Since the pulp is pumped upwards through the first reactor vessel 3 and then transferred to a downstream reactor vessel 4, no further pump is required between the reactor vessels.

In the second step, a temperature between 90-120 ° C is maintained and a pressure adapted to the temperature is selected, low enough for low-pressure steam 8 to be used for heating the mass, preferably not more than 2 bar. The heating of the pulp is suitably done between the steps at the lowest pressure level of the system with low-pressure steam.

In the second stage reactor vessel 4, a gas space is maintained at the top. Thus, degassing of the system can be performed in the top of this vessel. The reactions that take place with oxygen in the second stage only take place with dissolved oxygen in the mass and with the oxygen that remains after the degassing. In this case, the pulp leaving the second stage will contain a smaller amount of gas than otherwise, which means that subsequent washing steps can work better.

Furthermore, the temperature of the pulp can be lowered at the bottom of the second stage reactor vessel 4 by using a colder liquid for dilution. This provides the opportunity to recover the heat that is not flashed off.

The method also means that the second vessel 4, in addition to acting as a reactor for oxygen delignification, can also constitute a storage tower for the pulp and thereby serve as a buffer in the system.

The two-stage system for oxygen delignification described above means a considerably simpler and cheaper procedure for delignification. In particular, the operating economy can be improved by heating the mass between the steps by means of low-pressure steam instead of intermediate pressure steam required in previous systems.

The method according to the invention thus means that both investment costs and operating costs can be significantly reduced. In particular, significant energy savings can be made over conventional two-stage oxygen delignification systems.

The invention is of course not limited to the reported embodiment but can be varied within the scope of the inventive concept.

Claims (4)

507,870 Patent claims Process for oxygen delignification of chemical pulp in two stages, characterized in that the first stage is carried out in an upstream reactor vessel (3) in which a pressure exceeding 3 bar is maintained and that the second stage is carried out in a downstream reactor vessel (4 ) in which the temperature is between 90 and 120 ° C and where the pressure is adapted to the temperature, that heating of the pulp is carried out between the steps by means of low-pressure steam and that hydrogen peroxide in an amount of up to 5 kg per tonne of pulp with corresponding adjustment of alkali level is added to the second stage in a mixer (9) before the second stage reactor vessel (4). Process according to Claim 1, characterized in that the majority of the chemicals required for the delignification are mixed into the pulp before the first step. Method according to one of the preceding claims, characterized in that a gas space is maintained at the top of the reactor vessel (4) at the second stage, degassing taking place from this space. 4. A method according to any one of the preceding claims, characterized in that the heat content of the pulp after the second step is recovered by flashing or heating of diluent.