This invention relates to chlorine free bleaching of pulp from lignocellulosic material. The method comprises treat¬ ment of the pulp with chelating agent and subsequent bleach¬ ing in at least one step with chlorine free bleaching agents .

Modern chlorine free bleaching of chemical papermaking pulp is carried out to a large extent with hydrogen per¬ oxide in sequences, which normally also include the bleach¬ ing chemicals oxygen and at times even ozone. It is in future possible that also so-called peroxides, such as per- acetic acid and peroxymonosulfuric acid will be used. The use of several bleaching chemicals is due to the the fact that the lignin, which must be eliminated from the papermaking pulp in order to achieve a high pulp ISO- brightness, is strongly bonded. For carrying out the bleach¬ ing in a lenient way and with a minimum use of chemicals, it is favourable to use a combination of several chemic¬ als. The individual chemicals are added step-by-step with a washing of the pulp between the steps. When the bleach¬ ing is not required to be driven to full brightness, i.e. 89-90% ISO, the bleaching sequence usually includes only the chemicals oxygen and hydrogen peroxide. The greatest charge, counted in kg per ton pulp, normally is that of hydrogen peroxide, with a magnitude of about 15-45 kg per ton papermaking pulp. Hydrogen peroxide is a highly effective bleaching chemical when the bleaching conditions are the right ones. By using only oxygen and hydrogen peroxide, brightness levels pf substantially full brightness, i.e. almost 90% ISO, have been achieved on mill scale. This presupposes, however, very special con¬ ditions, such as that the papermaking pulp is subjected

CONFIRMATION

to far-reaching digestion and oxygen bleaching , ^' i . e . below kappa number 10, and that the pulp prior to the peroxide step is made substantially entirely free of disturbing metals, such as manganese, copper, iron and other. The presence of such metals implies, that parts of the hydrogen peroxide are degraded without any bleach¬ ing effect. This tendency increases with increased metal content, and the effect becomes more obvious with in¬ creased charge of hydrogen peroxide. As in many cases the hydrogen peroxide step is driven very far in order to bring about a high brightness level, the hydrogen peroxide steps carried out today are very sensitive even to low contents of disturbing metals. It has been learned, thus, by experience that the manganese content in pulp entering the hydrogen peroxide step must not exceed 1 g/ton pulp, because this would affect the final brightness level after the peroxide step. The pulp is desired to have high brightness, because it can then be sold at a higher price .

A low metal content in the pulp prior to the hydrogen peroxide step is obtained by treating the pulp with a chelating agent, for example EDTA or DTPA , and there¬ after to wash the pulp in order to eliminate the released metal ions before the hydrogen peroxide step. This method is described in the patent SE 467 006 (EKA Nobel). Accord¬ ing to the patent, the treatment with chelating agent (the so-called Q-step) shall be carried out between 10-100°C f° ^{r a} period of up to 6 hours at a pH between 3.1 and 9. EKA Nobel has at a later date, a.o. at the- International Pulp -Bleaching Conference in Stockholm 1991, shown more explicitly how the chelating age-nt: treat- -• ment is to be carried out. It was shown there that

the pH value should be about 5-6, and that the longer the treatment time and the higher the temperature are during the Q-step, the lower the disturbing metal cont¬ ent will be in the pulp before the hydrogen peroxide step, and that a higher pulp brightness can be achiev¬ ed at a given hydrogen peroxide charge. The viscosity of the pulp after the P-step was also shown to be high¬ er when the pH value in the Q-step was about 5-6.

The pulpmaking industry of today manufactures paper¬ making pulp mostly by digesting and oxygen bleaching the pulp to a far-reaching degree, whereafter the blea¬ ching proper is carried out by using all towers and tanks in the existing plants, so that a very long stay- time in the chelating agent step, and especially in the hydrogen peroxide step, is obtained. The pulp is also washed very accurately between the different bleaching steps. Such washing, however, requires large amounts of water, and therefore it cannot be used if and when the bleach plants must be closed at their effluent, which in most pulp mills will be required in order to meet various environmental obligations. It is also desirable to close the bleach plants in such a way , that, owing to increased contents of organic materials and process disturbing compounds and metals, the consumption of bleaching chemicals does not increase significantly over that of a system which is not closed, and in such a way, that incrustr- ations -.can be prevented at any costs. This latter problem is strongly connected to increased contents of a.o. calcium and barium, which readily form precip¬ itations (incrusts) of carbonate, sulphate and oxalate type. The risk of incrustration. ^■ ■ increases very much a.o.

when the system also is exposed to large pH and temp¬ erature changes, because then metals etc. can be releas¬ ed from the pulp and accumulate to a high content in the surrounding liquid phase.

The present invention has the object to solve the aforesaid problems. It implies, that papermaking pulp can be bleached with chlorine free chemicals in an effectively closed system without forming disturbing in- crustrations .

The characterizing features of the invention are app¬ arent from the attached claims.

The invention is described in greater detail in the following by way of some embodiments thereof. The invention is directed to chlorine free bleaching pf pulp, where pH is held above 9, suitably between 9 and 11.5, during the entire delignification and bleaching process. This implies, that the chelating agent step (Q-step) is carried out at a considerably higher than normal pH prior to a subsequent peroxide bleaching (P-step). It was found that this is possible by using chelating agents, which are active at a con¬ siderably higher pH value than that which normally is used at bleaching according to SE-patent 467 006 (see above). The chelating agent EDTA normally used is a poor chelating agent at alkaline conditions and can . be replaced, for example, by DTPA, which is active evwn at higher pH.

By carrying out the entire digestion and bleaching process at a relatively uniform pH, and so that the pH never turns acid, the risk of precipitations of in- crustrations can be reduced substantially. Incrustrat- ions normally become a problem difficult solve when the bleach plant is closed, because the concentrations

of a.o. calcium, barium, oxalate, carbonate and sul¬ phate can increase above the solubility maximum, which results in precipitations to begin to form. Normally, some type of initiation of the precipitation process is also required, for example via a pH or temperature change, in order to cause the precipitation to start, but in that case the precipitation can proceed very rapidly. Another advantage is, that the metals and a.o. calcium and barium, which are bound to the pulp prior to the bleaching, will remain on the pulp and are drained off from the closed bleach plant without being released, which implies that a build-up of cont¬ ent is avoided. The transistion metals dangerous for the process, i.e. especially manganese, further can be oxidized by adding an oxidation agent, so that their capacity of degrading peroxide is reduced, alter¬ natively entirely eliminated. The oxidation of these metals is carried out preferably with oxygen gas and/or hydrogen peroxide at a temperature of above 75 C prior to the bleaching with chlorine free bleaching agents, such as hydrogen peroxide.

A suggested suitable sequence comprises: digestion - Q-step - 02~delignification - Q-step - P-step. An initiating chelating agent step and an oxygen step, thus, can be carried out directly after the digestion. Thereafter the treatment proper with chelating agent is carried out prior to the hydrogen peroxide bleach¬ ing. In this case the pH value shall be held above 9, suitably at 9-11, preferably at 9-11.5, through the entire sequence. Both Q-steps can advantageously be carried out in the presence of oxidation agent, preferably oxygen and/or hydrogen peroxide for the oxidation of metals present. The metals become thereby

less disturbing for the bleaching. As an alternative or a compliment to the final hydrogen peroxide ^μ .each- ing, other chlorine free bleachings can be carried out where oxygen and/or peracids can be used. The bleach¬ ing preferably is carried out in at least two oxidiz¬ ing steps, where hydrogen peroxide and oxygen can be used in the same bleaching step.

When DTPA is used as chelating agent, the charge should be 1-10 kg (100%-product) per ton pulp, preferably 2-5 kg per ton pulp.

The invention also implies that an effluent-free pro¬ cess can be obtained. The process is preferably closed in such a way, that washing steps are carried out after every treatment step (Q-step and, respectively, bleaching step), and the filtrate from every washing step is recycled as washing liquid in preceding washing steps. A high and uniform pH level being maintained during the entire process, no precipitations of in- crustrations occur, in spite of the closing of the bleaching process.

The invention, of course, is not restricted to the em¬ bodiments described, but can be varied within the scope of the invention idea.