Prior art The development of bleaching technique towards a more environment friendly process has in principle had two different lines of development, one line towards the use of chlorine free chemicals and one line towards increased closing of the bleaching plants. Both development lines have resulted in TCF sequences and so called light ECF sequences. In both of these sequences ozone bleaching is involved as an alternative. One problem with ozone bleaching is that the selectivity evaluated as the pulp viscosity (cellulose degradation) in relation to the lignin release is quite poor. Even if the strength of the pulp per se is not changed to the same extent, this has led to a comparatively low interest for ozone bleaching.

In TC and light ECF sequences, a peroxide step is usually included. This step demands removal of certain metal ions and especially manganese ions. Efficient metal removing steps are either acid steps or complex forming steps, also being active at low pH values (however at a higher pH value compared to a pure acid step).

In bleaching, oxalic acid is formed, especially in oxygen delignification, in ozone steps and in bleaching with chlorine dioxide. The combination of acid steps wherein metals are washed out and a high oxalic acid production has led to problems when closing the process, among other problems calcium oxalate is deposited on the equipment. Calcium oxalate will precipitate at about pH 4-8, a pH range in which, on the one

hand, the complex forming effect is most efficient and, on the other hand, this is a pH range which is easily obtained when mixing acid and alcaline bleaching filtrates.

In an article in J. Pulp & Paper Sci. 23 (1997); 5 of J.

Liden, a mechanism for the positve effect of the Mg addition in oxygen and peroxid bleaching is explained. Magnesium forms a complex with dissolved organic compounds, which in its turn can dissolve Mn (II) and Fe (II). Thus, these divalent ions should be dissolved before they can be oxidized to higher valence states in the bleaching steps. The compounds referred to by Lidén et al are formed at alcaline pH.

In laboratory experiments the applicant has made comparisons between additions of different acids to the pulp for obtaining one and the same pH value of 3 before ozone bleaching, wherein the best selectivity was obtained with oxalic acid addition. A subsequent analysis of the different pulps disclosed that the pulp treated with oxalic acid still maintained almost the entire original amount of both calcium and manganese ions, despite the low pH value and despite that also the selectivity was the best in this case.

Without being bound to any specific theory it is believed that the oxalic acid added to this step forms complexes with metal ions so that they cannot be washed out and also bind the deleterious divalent metal ions contributing to the decomposition of the ozone.

Description of the invention The object of the present invention is to improve the efficiency in relation to prior art, while at the same time the effluents from the bleaching process are diminished or at least are not increased.

This is obtained with the method disclosed in the

introduction, which according to the present invention is characterized in that bleaching filtrate, which is washed out of the pulp after the ozone step is completely returned to the pulp before bleaching and in that the filtrate from the washing step before the ozone step is used as washing liquid after the ozone step, or alternatively, in case only dewatering is performed before the ozone step, that the filtrate from the washing step after the ozone step is used as dilution liquid before the ozone step, for the formation of a closed loop.

In order to reach down to the correct pH level for ozone bleaching, i. e. about pH 3, a strong acid is also added, preferably sulphuric acid, together with said washing filtrate.

Short description of the drawing The invention will now be described more in detail with reference to the attached drawing, in which the Figure shows a closed so called oxalic acid loop according to the invention.

Description of preferred embodiments The Figure shows a schematic outline of the method according to the invention, and more specifically the part which is denominated"the oxalic acid loop". Incoming pulp A is fed into a washing step 1, from there further to an ozone bleaching step 3, a subsequent washing step 2 and from there to subsequent bleaching.

The pulp is diluted before washing step 1, which lies ahead of the ozone step 3, with filtrate from washing step 2, lying after the ozone step 3. The filtrate from washing step 2 is used also as washing liquid on washing step 1. The filtrate is acid and only requires cooling (4) in a degree corresponding to the heat release in the ozone step (exothermic rection).

Filtrate from washing step 1 is used as washing liquid on washing step 2 and dilutes the pulp before washing step 2. In this way a closed loop is obtained, wherein high concentrations of oxalate and metal ions is circulating.

When filtrate from washing step 2 is added to the pulp in washing step 1 the content of oxalic acid/oxalate on the pulp is increased, resulting in that the recirculated metal ions such as Mn and Fe can be fed to the ozone step without negative effect on the ozone bleaching. A certain pH adjustement is made on filtrate from washing step 1 with the aid of sulphuric acid.

The pH will be lowest in washing step 2 because of the pulp rection with ozone and because of the pH adjustment with sulphuric acid, which facilitates the dewatering on the washing step and makes the washing-out of free metal ions effective before the pulp is transported to subsequent bleaching steps, which for example can be an alcaline peroxide step. Namely, in peroxide bleaching it is important to keep the content of free Mn ions low. Compared to a conventional system the pulp can probably contain higher amounts of manganese when it has been preceeded by an ozone step with a high content of dissolved organic compounds, which can render manganese harmless by forming organic complexes with the ion.

A great advantage with the proposed process solution according to the invention is that this part of the bleaching line is kept completely closed on the liquid side.

Example In order to evaluate a possible oxalic acid concentration in the effluent pulp the material balances have been studied for bleaching with the closed oxalic acid loop according to the invention in comparison with a conventional HC ozone step.

In the example, hardwood pulp having a kappa number of 9

was introduced into the ozone (Z) step and it had a kappa number of 5.4 after ZE (ozone step folloowed by alcaline extraction) according to Table 3 in the CTP report"Formation of oxalic acid during ozone bleaching, origin and quantification". In the tests 5 kg 03/t and 2.4 kg 03/t was consumed. This results in an oxalate formation of 270 mg/kg pulp, and, converted into BDT, this equals 0.243 kg/BDT.

The oxalate content when using the method according to the invention compared to the conventional method, increases from 0.014 to 0.050 kg/BDT, which is only a fraction of the amount formed.