The present invention relates to a method of bleaching lignocellulosic fiber material in a combination step with ozone and peracid, such as peracetic acid, Z _HC Paa _MC . The invention preferably is directed to the bleaching of sulphate pulp. The pulp is treated with ozone in an HC step in an ozone reactor, where after the pulp is diluted with water or white water, and peracid is added prior to a MC bleaching tower, with subsequent washing.

At commercial operation, ozone has recently been introduced as a bleaching chemical at the bleaching of pulp, in order to minimize the use of bleaching chemi- cals, and, respectively, to increase the bleaching with¬ out using chlorine chemicals by using a strong delignifi- cation chemical. Owing to its good delignification capa¬ bility and relatively low price, in relation to its high specific oxidation potential, ozone is a costeffective complement to other chemicals in bleaching sequences, in which no chlorine chemicals are used. It is difficult by these sequences to achieve long range bleaching without serious carbohydrate degradation, which deteriorates the strength of the fiber material . Ozone, on the whole, causes considerable degra¬ dation, which distributes itself between moderate direct carbohydrate degradation and alkali sensitivity, due to the introduction of carbonyl groups into the cellulosa, which causes rapid degradation in a subsequent alkaline step. Due to the alkali sensitivity, the value obtained at the viscosity analysis is lower than what corresponds to real polymerisation degree of the carbohydrate mate¬ rial. The higher real polymerisation degree is determined simply by a boron hydride reduktion of the pulp before the viscosity analysis. The total degradation in the ozone step would be reduced substantially, if boronhyd- ride steps are introduced into the bleaching sequence

after the ozone step. For several reasons, this is prac¬ tically not possible. Until now no applicable solution has been proposed, by which this alkali instability could be avoided. Alternatives to ozone are peracids, such as per- formic acid, peracetic acid and persulphate (Caros acid) , which at suitable conditions yield low carbohydride de¬ gradation. Bleaching with these can be carried out in existing equipment found in traditional bleach plants, contrary to ozone. A serious disadvantage, however, is that they are considerably more expensive than ozone, in relation to their oxidation capability.

At the present intention the costeffective de¬ lignification of ozone has been combined with a treatment with a small amount of peracid, which was found to sub¬ stantially reduce the alkali instability caused by the initial ozone treatment of the pulp. The intention builds on the fact that the combination step can be adapted to the apparatur solution, which was presented for HC ozone steps, see USA 5 181 989. It was shown that the relati¬ vely limited peracid treatment, which is a prerequisite of the actual combination step, can be carried out in the normal treatment volume for more diluted pulp, in order to extract soluble lignin at slightly increased pH and, respectively, to treat the pulp with chelating agent in, order to remove disturbing transition metals after the ozone reactor, see patent SE 501 836.

Such combination step involves the difficulty that the ozone treatment must be carried out at low tem- perature and low pH, preferably 2-3, in order to be se¬ lective, while the peracid treatment requires a relati¬ vely high temperature and a higher pH, preferably 4-6. Owing to the fact that the combination step, which is carried out in a HC ozone step with subsequent dilution to a MC system, requires smaller amounts of peracid, because the emphasis is put on the ozone, it has been possible to adapt the conditions to existing HC ozone

step equipment.

Figure 1 describes the system for this combina¬ tion step. The apparatus equipment is entirely identical with that of a simple HC ozone step, with the exception of the charging equipment for peracid. Figure 1 includes ozone equipment 2, peracid charging equipment 3, a first press 1 and a second press 4.

Figures 2 and 3 illustrate the liquid and tempe¬ rature balance according to the conditions used according to the invention.

Figures 4 and 5 show the effect of the kappa numbers (which illustrate the lignin content in the pulp) and, respectively, the effect on the pulp viscosity, both directly on the pulp and after a boron hydride reduction of the pulp.

The ozone treatment takes place at a pulp con¬ centration of > 35% and a temperature somewhere between 20 and 60°C. This is a compromise between selectivity requirements, which are furthered by a very low tempera- ture, and to cool the hot white water, without seriously deteriorating the washing efficiency etc. The ozone reac¬ tion is extremely rapid and, therefore, the dwell time in the ozone reactor is only one or a few minutes . The pulp is diluted with hot water of a higher pH to a pulp con- centration of about 12%, and peracid is added. After a dwell time in a vessel of between 10 and 30 minutes, the pulp is washed. Charged peracid is consumed at a pH bet¬ ween 4 and 6 and a temperature of 60-80°C. Figure 4 shows the effect on the kappa number as a function of the con- sumption of ozone and, respectively, peracetic acid, both expressed as OXE (1 OXE = the substance amount, which takes up one mole of electrons at the reduction of the substance, 1 kg peracetic acid = 26.3 OXE, 1 kg ozone ^■ = 125 OXE) . The peracetic acid yields a smaller, but more selective delignification contribution and substantially decreased alkali sensitivity, which at normal viscosity analysis, see Figure 5, yields an increased pulp visco-

sity, especially when the pulp is not boron hydride re¬ duced before the analysis.