The invention relates to an improved process for bleaching chemical pulp or Kraft pulp. In particular the invention relates to such a process in which ethylendiamine disuccinate (EDDS) is used as a chelating agent during the treatment of the pulp.

During the preparation of pure white chemical pulp, resins and lignin are removed completely. Delignification can be performed by using chlorine gas but this process is abandonned because of the environmental problems related to it. Today, delignifiction is performed with oxygen.

Further refining is then done by Elementary Chlorine Free (ECF) or Total Chlorine Free (TCF) bleaching processes.

During these bleaching processes hydrogen peroxide is one of the chemicals used. Because transition metal ions catalyse the decomposition of hydrogen peroxide, they must be removed from the bleaching operation. Typical transition metal ions present in paper pulp, and catalysing the decomposition of peroxides, are Mn2+, Fe3+ and Cu2+. The most efficient way to remove these metal ions is by chelation. The chelates formed can then be removed by washing.

Addition of chelating agents is normally done after the oxygen delignification step. Also addition of chelating agents during the bleaching step is described.

In a chemical pulp bleaching operation, first an oxygen delignification is performed under alkaline conditions.

Alkaline conditions here mean a starting pH above 12, decreasing during the delignification step to pH 10-11.

The pulp after oxygen delignification can then be rinced. This washed pulp, after water addition, still has a pH

around 9-10. The chelation pretreatment is normally performed with EDTA (ethylenediaminetetra acetate) and DTPA (diethylenetriaminepenta acetate)-see Tappi Journal, 1996, vol 79, n°10, p. 161-166. This means that the pH of the pulp has to be decreased by acid addition to reach values of 5-6, the optimal chelating conditions of EDTA or DTPA. After washing out the metal-complexes, peroxide bleaching is then performed under alkaline conditions, which means that additional chemicals are needed to increase pH again to 10-11.

It is clear that elimination of these intermediate washing and/or neutralisation steps would simplify the process and reduce the salt content in the washing waters.

Such a process modification was already suggested but the consequence thereof is that EDTA and (to a lesser extent) DTPA can no longer be used as chelating agent, because they loose complexing activity at pH-values above 9, the optimal peroxide bleaching conditions.

Under such alkaline conditions use of diethylenetriamine- pentamethylene phosphonic acid (DTPMPA) as complexing agent was described in Tappi J. 1996, vol 79, p. 161-166.

Also other phosphonates could be used alone or in combination with other reagents, as suggested in US- 4752354. The major disadvantage is however the fact that these reagents are not well biodegradable.

It has also been suggested to use monoamino polycarboxylic acid derivatives as biodegradable alternatives for the chelating reagents. Thus for instance 6-alanine-N, N- diacetate as disclosed in DE-4128084, and N- carboxymethylaspartic acid, N- (1,2-dicarboxyethyl)-aspartic acid, N- (1, 2-dicarboxy-2-hyroxyethyl)-glycine and N- (1,2- dicarboxy-2-hydroxyethyl)-aspartic acid, as disclosed in

WO-9514808.

There is however no specific information regarding the complexation conditions during the chelating step (Q step) after the oxygen delignification of chemical pulp. <BR> <BR> <BR> <BR> <BR> <BR> <BR> <P>WO-9428464 suggests the use of polyamino disuccinic acids (EDDS) in pulp and paper processing to enhance or maintain bleaching effectiveness.

It was however shown by the applicants of the present invention that these derivatives do not form sufficiently strong complexes with Fe3+, Mn2+ and Cu2+, under the conditions where EDTA and DTPA are normally used.

In a more recent patent application, WO-9730209, a bleaching process for pulps was described, using EDDS as chelating agents during the pretreatment of chemical pulp, before peroxide bleaching. It is stated that the use of EDDS instead of EDTA or DTPA allows higher pH conditions during the Q-step, namely pH 4-8, preferabbly 6.5-7.5, instead of the standard values of 5-6. This results in a reduction of chemicals to be added to neutralise and/or alkalinise the pulp.

In their effort to find an improved solution, extensive experimentation has been performed by the applicants of this invention, which resulted in the unexpected finding that EDDS was able to bind Mn2+, Fe3+ and Cu2+ at pH-values between 10 and 11, i. e. well above the pH-values of 6,5- 7,5 as tought by WO-9730209.

Based on these findings a new and improved process for perioxide bleaching of alkaline oxygen delignified pulp was developed.

According to the invention a process for bleaching alkaline

oxygen delignified pulp is provided which is characterised in that a polyamino disuccinic acid derivative is used as the chelating agent at pH values above 9,5 before and/or during the peroxide bleaching step.

The expression polyamino disuccinic acid derivates as used herein comprises the different optical isomers such as the R, S, S, S and R, R-isomers.

More in particular the process according to the invention uses ethylenediamine disuccinate (EDDS) species.

Because chelation is performed at pH > 9,5 the oxygen delignified pulp doesn't need to be neutralised, and afterwards realkalinised. Thus salt load in effluents, as well as chemicals consumption can be strongly reduced. At the same time it is possible to perform the bleaching operation without having to rinse the pretreated oxygen delignified pulp with fresh water. Thus fresh water consumption can also be reduced.

In a preferred embodiment of the invention, the pretreated oxygen delignified pulp is washed with alkaline rinse waters coming from later alkaline peroxide bleaching steps, this before the first bleaching step. Rinse waters from the bleaching operation can thus be used in counter current.

The polyamino disuccinic acid derivative chelating agent, most suitably EDDS, is preferably used at pH-values varying between 10 and 11. The concentration of polyamino disuccinic acid derivative on pulp dry substance may vary between 0,05-1% w/w, preferably between 0,2 and 0,8% w/w, and more preferably between 0,4 and 0,7% w/w.

The invention will be further illustrated in the following examples.

Example 1 : Pretreatment of paper pulp containing metal cations : The pulp used in this test is a resinous Kraft pulp, oxygen delignified, with the following characteristics : -Kappa-index : 14,7 -DPV = 1220 (polymerisation degree of cellulose) The chelation stage Q was performed on a 10% consistency pulp at 80°C, during 60 minutes, in a polyethylene bag.

When chelation was performed under alkaline conditions (with EDDS), pH was adjusted with NaOH on a pulp already containing the chelating agent. After the Q step the pulp is washed with deionised water. Then cation content in the pulp after the chelating step Q was determined by the following method : the organic matter is incinerated at 425°C. The ashes are then dissolved in hydrochloric acid and the cations are determined by Inductive Coupled Plasma (ICP) spectrometry. Kappa-index and Dpv are determined according to the ISO standards (ISO R302). Results of these determinations are give in table 1.

Table 1: Effect of chelating agent on metal cation content of pulp, and on Kappa-index and DP, of the pulp after peroxide bleaching stage P Type of chelating agent None EDTA DTPA EDDS Chelating agent content 0,5 0,5 0,5 Cation content after Q in ppm Iron 20 4 5 4 Manganese 66 <1 <1 2 Copper <1 <1 <1 <1 Kappa-Index after P 8,8 7,6 8,1 7,5 DPvafter P 1020 1220 1250 1150

Example 2 Peroxide bleaching of paper pulp pretreated with chelating agent: After the chelating step as described in example 1, the pulp is washed with deionised water. Then the pulp is brought to a consistency of 10%. After addition of hydrogen peroxide and alkali to the hot pulp (90°C) the bleaching step is performed for two hours in a polyethylene bag. After the P step the pulp is washed with deionised water. Then brightness is determined according to the ISO standard (IS02470).

Results of the peroxide bleaching step are given for different chelating agents at different concentrations in table 2.

Table 2: Pulp brightness in function of chelating conditions and bleaching conditions of different concentrations of EDTA, DTPA and EDDS versus blanco. None EDTA DTPA EDDS Q stage chelant added, % 0 0,2 0,5 1 0,2 0,5 1 0,2 0, 5 1 NaOH added, % 0 0 0 0 0 0 0 0, 3 0,8 1, 3 initialinitialpH 5,5 5,5 5,5 5,5 5,5 5,5 10,2 11 10 Pstage H202 added, % 2 0,2 2 2 2 2 2 2 2 2 NaOH added, % 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 final pH 11,7 11,7 11,6 11,9 11,8 11,7 12 11,5 11,6 11,6 H202 consumed % 2 1,88 1,6 1,79 2 1,82 2 Brightness ISO, % 44,5 57.9 60,5 52,8 58,9 60,2 55,8 56,2 63,9 54,6

Example 3 Determination of oxygen consumption: EDDS is compared to EDTA and DTPA in the following test.

Of each of the above mentioned complexing agents 7,5 mmol is dissolved in water and brought to pH=10 with diluted NaOH. This solution is transferred to a calibrated flask of 100 ml, 850 mg of 33% hydrogen peroxide is added and the volume is adjusted to 100 ml.

The test solutions are then left alone at 40°C during two hours. After that period hyrogen peroxide loss is determined by adding excess potassium iodide and backtitration with thiosulfate.

The following results were obtained. Complexant Oxygen consumption in % EDTA 40% DTPA 73% EDDS 42%