This invention relates to the bleaching of chemical pulp with chlorine free bleaching chemicals. The invention, more precisely, refers to a method of treating the pulp with chelating agent in connection with the bleaching.

Chlorine free bleaching of chemical paperma ing pulp is carried out to a large extent with hydrogen peroxide in sequences which normally also include the bleaching chemicals oxygen gas and at times ozone. In future probably also so- called peracids, such as peracetic acid and persulphuric acid, will be used. The lignin, which must be removed from the paper¬ making pulp in order to achieve high ISO-brightness of the pulp, is firmly bound, and for performing the bleaching in a lenient way and with economic use of chemicals it is favourable to use a combination of several bleaching chemicals. The individual chemicals are added in steps, and the pulp is washed between the steps.

Hydrogen peroxide is a highly effective bleaching chemical, if the bleaching conditions are right ones, and by using only oxygen gas and hydrogen peroxide brightness levels of almost full brightness, i.e. al onst 90% ISO, have been achieved on mill scale. This presupposes, however, very special conditions, such as that the papermaking pulp is digested and oxygen bleached to a far-reaching extent, i.e. below kappa number 10, and that the pulp prior to the peroxide step substantially entirely has been freed of disturbing metals, such as manganese, copper, iron and others. The presence of such metals causes parts of the hydrogen peroxide to be degraded without any bleaching effect, which tendency increases with increased metal content and increased charging of peroxide. As the peroxide step is driven very far, i.e. precisely to the limit at which the chemical is capable to bring about a maximum brightness level, the peroxide steps of today are highly sensitive even to low metal contents. The manganese content in pulp entering the peroxide step, for example, must not exceed 1 g/ton pulp when highest brightness is to be achieved. This level, however, is difficult to attain on mill scale. A high brightness of the pulp is desired, because such pulp can be sold at a higher price.

A low metal content in the pulp prior to the peroxide step is achieved by treating the pulp with a chelating agent, for example EDTA or DTPA and thereafter washing the pulp to eliminate the released metal ions prior to the peroxide step. Such a method is reported, for example, in EP-A-402335, the so- called Lignox-method. It is there stated that the chelating agent treatment shall be carried out between 10°C and 100°C, preferably 40-90°C, for a period of up to 6 hours and at a pH between 3.1 and 9.0.

The pulp industry manufactures today chlorine free papermaking pulp with high brightness by a far-reaching digestion and oxygen bleaching of the pulp, by using existing bleach plants and towers so that a very long stay-time in the chelating step and peroxide step is achieved, and by washing the pulp very carefully between different bleaching steps. Such washing, however, requires large amounts of water and, therefore, cannot be used when the bleach plants have to be closed, which probably will be the case when various environment commitments must be fulfilled. At the construction of new plants, or the reconstruction of existing ones, it is also desired that the stay-times in different process steps are as short as possible, in order thereby to minimize the building volume and, thus, also the investment costs. It is, finally, important to minimize the amount of added chelating agent, because it is an expensive chemical and, so far, it has not been possible to dismiss its possible damage to the environment.

The present invention offers a solution for the aforesaid problems. The invention, thus, results in a more effective chelating agent treatment which is carried out prior to the peroxide step, in that the chelating agent is activated by treating the pulp with an oxidation agent. By activation is meant that the efficiency of the chelating agent is increased by oxidation or that the metals, which should be removed from the pulp, are oxidated. In both cases the metal binding is facilitated and the metal content in the pulp is reduced. According to the invention, it is possible, for example, to reduce the manganese content to below 1 g per ton pulp.

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

The invention is described in greater detail in the following with reference to some embodiments thereof.

The pulp from a preceding process step, for example oxygen delignificaiton is wahsed in a pulp washer room. Thereafter the pH value is adjusted by the addition of acid or liquor. The acid/liquor shall be admixed in a homogenous way, for example in connection with a dilution of the pulp or by means of a pump or mixer. When a mixer is used in this position, a very intense and uniform admixing is ensured, which in certain cases can be of decisive importance. The pH value shall there be adjusted to between 2 and 7, preferably 4-6.

Subsequent to the pH adjustment, the chelating agent, for example EDTA or DTPA, is added in a separate mixer which causes an effective admixing of the chelating agent to the pulp. The amount of chelating agent suitably is 1-5 kg per ton pulp, preferably 1.5-2.5 kg/t.

An oxidizing agent is also added to the pulp. As oxidizing agent oxygen-containing gases such as air or oxygen gas can be used. Alternatively, hydrogen peroxide or ozone can be used. The charging of oxidizing agent, which contains oxygen gas, shall be 1-10 kg per ton pulp, preferably 3-7 kg/t, counted as pure oxygen gas. When hydrogen peroxide is added, the amount shall be 0.5-3 kg, preferably 1-2 kg/t. At the use of ozone, an ozone/air mixture with 0.5-3 kg ozone per ton pulp, preferably 1-2 kg/t, is added.

The pulp is thereafter pumped to a reactor vessel, in which a temperature of 40-130°C, preferably 100-130°C, and a pressure of 1-10 bar, suitable 1.5-10 bar, preferably 3-8 bar, are maintained. The stay-time of the pulp in the vessel shall be between 5 minutes and 2 hours, preferably 10-60 minutes.

The pulp treated with chelating agent is directed from the reactor vessel to a pulp washer room where the temperature is held above 80°C whereby the metal ions, which are bound by chelating agent and thereby can be washed out, are washed out effectively.

This pulp washer can be a pressurized filter or preferably a roll press, which is very effective and has low water consumption (4-5 m ³ /ton pulp) .

At the chelating agent treatment according to the invention, the oxidizing agent has the object to activate the added chelating agent so that it binds undesired metal ions, such as manganese, copper and iron, more actively. The oxidizing, besides, facilitates the separation of metals by releasing them more easily from the pulp. The metals thereby can be bound better by the chelating agent and be washed out from the pulp in the subsequent washing step. The pulp will thereby be purer from a metal aspect.

The pulp is directed after the washing to a subsequent bleaching step where chlorine free bleaching is carried out with peroxide-containing bleaching agent. The low metal content in the pulp renders it possible to bleach more effectively to maximum brightness.

EXAMPLE

An oxygen bleached coniferous wood pulp with kappa number 12.9 and brightness 40.0% ISO was treated with a chelating agent (Q) , washed and bleached with peroxide (P) .

The Q-step was performed at 10% pulp consistency in 1 hour and final pH 5.0-5.2. The Q-step was performed with and, respectively, without addition of oxygen (Q ₂ ) . Then the pulp was washed and bleached in a pressurized P-step to which oxygen was added, a so called (PO)-step. The conditions in the (PO) - step was 12% pulp consistency, 2 hours retention time, 100-110°C and final pH 10.0-10.4. The results is shown in the table below.

This result shows that the final brightness of the pulp is increased if the chelating step is activated by addition of oxygen while the peroxide bleaching conditions are unchanged.

The invention, of course, is not restricted to the embodiments shown, but can be varied within the scope of the invention.