Bleaching Method for Alkaline Sulphate Pulp.

TECHNICAL FIELD:

The present invention relates to a process for bleaching alkaline sulphate pulp whereby, due to environmental reasons, chemicals other than chlorine are used.

PRIOR ART:

Chlorine, hypochlorite and chlorine dioxide have been used for bleaching of cellulose pulps. During the last 20 years chlorine and hypochlorite have been replaced by oxygen gas, chlorine dioxide and peroxide with the aim to decrease the environmental problems which have been accredited to chlorinated organic compounds. The result is that today it is doubtful if remaining environmental problems can be accredited to chlorinated substances. The opinion that chloride compounds are dangerous, however, has resulted in extreme market demands on bleaching completely without chlorine chemicals, that means also a replacement also of chlorine dioxide. For qualified bleaching the chemicals oxygen gas, peroxide and ozone will then be actual. It is, however, doubtful if these chemicals will give any appreci¬ able environmental improvement above pure chlorine dioxide bleaching if the introduction of these chemicals are not combined with external purification or a very extensive closing of the systems so that the amount of dissipated substance to the drainage system is strongly reduced. One important development question is therefore to adapt the bleaching technique so that closing is facilitated.

Peroxide reacts rather slowly and is only moderately delignifying but strongly bleaching on the remaining

lignin. The peroxide steps are therefore characterized by relatively extreme conditions such as high temperature, long reaction time and often high pulp concentration. The environment in the peroxide step is alkaline. The bleaching effect of peroxide does not cease in the same forcefull way as the conventional chlorine containing bleaching chemi¬ cals, which to some extent creates a tendency to run the bleaching longer than what should be the case in a corre¬ sponding conventional bleaching step. With moderate bleaching activities the peroxide step can be extremely selective with regard to bleaching relative to carbohydrate degradation.

With regard to reactivity ozone is completely contrary to peroxide. It reacts extremely quickly with the lignin and unfortunately also quickly with most compounds including the carbohydrates. A relative low pH for the bleaching is regarded as suitable. The ability to delignify is very good whereas the bleaching effect primarily seems to be due to delignifying. With this working profile ozone complements the peroxide rather well.

TECHNICAL PROBLEM:

The above mentioned partial processes have their limits. The peroxide step is for example more sensitive to dis¬ turbances of different kinds and the result of the bleach¬ ing is therefore determined to a great extent by kinetic conditions. Though a moderate effort of bleaching can be very selective, a prolonged bleaching with more strict conditions will, however, give rise to an appreciable risk for degradation of carbohydrates, which will make it necessary to limit the work of the bleaching step.

With ozone bleaching, a problem with degradation of carbohydrates exists to an even larger extent, which leads to a decrease in strength of the cellulose products which

are produced from such pulp. This limits greatly the addition level for the ozone. It has therefore long been a problem associated with the above-mentioned processes to be able to control these and combine them in such a way that the disadvantages with the different steps will not occur. For pulp having high brightness and high strength, a multi- step process with accurate optimising of every single step is therefore required. In turn, this will lead to an extensive apparatus and high investment costs. A far reaching simplification of the process steps is therefore desirable.

THE SOLUTION:

By means of the present invention the above-mentioned problems associated with the known processes have been solved and a process is provided for bleaching of alkaline sulphate pulp which has been delignified and bleached to a lignin content of below approximately 1,3% (kappa value 9,4) in a previous step which process is characterized in that peroxide is added and is allowed to bleach the pulp in a first step whereupon the pulp having a solid content of 8-16% is supplied with gaseous ozone concurrently in an amount of less than 0,5 kg/t in a second step and that the pulp, possibly without any intermediate washing, is alkalised and supplied with peroxide for final bleaching in a third step.

According to the invention it is suitable that the ozone with its carrier gas of oxygen is added to the system in a shear mixer.

The temperature in the ozone step should according to the invention, be below 50°C.

According to the invention it is suitable that the pulp is stabilized with Mg ⁺⁺ ions in a step before the first peroxide step.

The first peroxide step, can according to the invention be performed with a pulp concentration of above 20%.

Further according to the invention, it is suitable that the pulp, after the first peroxide step, is washed in a press giving solid a content of the pulp of above 20%, preferably above 30%, whereupon the pulp is diluted with water having a temperature of below 50°C to a solid content of 8-16% during simultaneous acidifying.

Further according to the invention, it is suitable that in the first peroxide step 10-50 kg peroxide per ton pulp is added and that the temperature is maintained at 70-100°C, preferably 80-90°C. In the second peroxide step it can be suitable to add 5-30 kg peroxide per ton pulp and to maintain a somewhat lower temperature, 40-90°C, preferably 50-60°C.

The last peroxide step should, according to the invention, be performed in a downstream tower having an initial upstream conduct.

It can, however, according to the invention be suitable that the last peroxide step is performed in an upstream tower having an intermediate upstream tower and wash between the ozone step and the last upstream tower before the peroxide step.

According to the invention it is finally suitable that the bleaching backwater is used for washings and dilutions whereby the backwater is fed in strict counter current and

possible release thereof occurs from the wash immediately before the first peroxide step.

A suitable embodiment of chlorine chemical free bleaching according to the present invention consists accordingly of a multistep technique and a combination of ozone in acid solution and peroxide in alkaline solution. The bleaching according to the present invention presupposes as mentioned above that the pulp shall be delignified and prebleached to a lignin content of below approximately 1,3% (kappa value 9,4) in an earlier step. The bleaching should then be started with oxygen delignifying and ozone bleaching in a closed system with release of dissolved substance for destruction, for example, according to a process which is described in the international patent application no. WO 91/18145.

Stabilizing of the pulp with magnesium before the first peroxide bleaching is, besides giving a favourable influ- ence on the bleaching itself, advantageous partly because one can thereby simplify the apparatus, partly in that one therethrough facilitates for further closing of solved substance in the system.

The bleaching itself of the pulp in three steps with peroxide, ozone and peroxide as chemicals in the three steps could possibly require a rather comprehensive apparatus. According to the present invention one has brought about a technical solution for these three steps which substantially simplifies the design of the appar¬ atuses of the bleaching plant. The design has also been adapted to allow for consequent counter current reuse of backwater and such a substantial closing of the system as the stability of the peroxide steps in the relatively pure system allows for. The dissipation flow from the system can be made very small, for example less than 10 m ³ /t pulp. The

temperature has also been made such that if more tradi¬ tional final bleaching with chlorine dioxide, should be desired, this can in principle be carried out.

Most importantly, with the present invention the delignify¬ ing which occurs in the process, for example in the oxygen and ozone steps in the closed system and the low rest lignin content in the pulp resulting therefrom, makes it possible that only a very small amount of ozone is needed to activate the pulp between the peroxide steps and to make sure that the peroxide steps can be carried out with reasonably lenient conditions even when a very high final brightness is aimed at. This means in turn that the gas volume in the ozone step will be so small that the addition of ozone in the pulp suspension can occur in a normal shear mixer without any special pressure due to the fact that the amount of gas (ozone + carrier gas from the ozone manufac¬ turing) will be so small that the mixture of gas, water and fibers does not become unstable.

The ozone bleaching at this stage neither requires any special wash before the transfer to the final peroxide step. It is therefore sufficient that the pulp is alkalized in a mixer to a suitable level for peroxide bleaching at the same time as peroxide is added. The relatively inert carrier gas in the ozone step is thus transferred to the peroxide step and can there to some extent complement the peroxide as a bleaching agent due to the alkalic environ¬ ment. The speed of reaction in the ozone step is so high that the small amounts of ozone which have been added can be consumed in a normal pulp transporting pipe between the mixers. The ozone step will therefore with regard to the apparatus not constitute any separate step, but consists virtually only of an extra mixture in a normal bleaching step with, for example, an upstream tower (upstream to house the carrier gas in a suitable way analogue to that

which occurs with traditional so-called EOP-step) . The possible remainders of ozone, which exist after the ozone step and which also, though they are very small, can give rise to hygienic risks, are destructed in the following peroxide step with its relatively long time and high temperature.

Due to the activation of the ozone step, the peroxide step does not have to be driven so hard which allows the possibility that the peroxide bleaching can be readily performed in conventional bleaching equipment at middle concentration. The first peroxide step according to the invention can, however, be performed at a high pulp concentration whereas the second peroxide step due to apparatus reasons should be carried out at a middle concentration.

Because the washings according to invention are performed only after the peroxide steps and the systems, due to the initial delignification in for instance oxygen and a following ozone step, are very extended and the wash is therefore good, a possible recirculation of backwater concurrently through the pulp is therefore made possible. A recirculation of backwater within the stages is also conceivable, though with the complication that the backwater from the final peroxide step then must be acidified due to the conditions of the ozone step. A possible presence of rest peroxide in that case can, however, influence the selectivity of the ozone step in a positive way.

FIGURE DESCRIPTION:

The invention will in the following be described more in detail in connection with the figures where

Fig. 1 schematically shows a preferred embodiment of the process according to the invention comprising the prebleaching step;

Fig. 2 shows a diagram of obtained brightness as a func¬ tion of added oxidation equivalents, and

Fig. 3 shows a diagram of the intrinsic viscosity as a function of added oxidation equivalents.

In Fig. 1 the path of the pulp is shown from the point where it has been boiled and until it has been finally bleached. After washing on filters or the like and addition of chemicals such as sodium hydroxide it is prebleached and delignified with oxygen in step 1. Thereafter it is treated in a known way by washing and subjected to a new prebleach¬ ing and delignifying step with ozone in step 2. After the ozone step, any acid liquid can be released to avoid building up of metal contents in the pulp. Thereafter the pulp is washed and pH adjusted whereupon in step 3 a magnesium salt is added for stabilizing the pulp before the final bleaching step. This commences with a peroxide bleaching in step 4. As mentioned above, the pulp in this step may have a high or medium concentration. After the peroxide bleaching in step 4, the pulp is washed in step 5 and pumped into a shear mixer 6. Ozone is added to this shear mixer in a carrier gas of oxygen which, due to the small gas amount, does not need any special pressure.

The ozone bleaching takes only some minutes and takes place in the pipe from the shear mixer 6 until the pulp is fed into the second peroxide step. As appears from the figure, the peroxide is added via the pump in the bottom of an upstream conduit 7. Most of the ozone is consumed at this point in the process but possible remainders are consumed finally in connection with the peroxide bleaching. This

last bleaching is, according to the figure, carried out in a downstream tower, whereupon the pulp is finally washed in the last wash 8.

Instead of carrying out the peroxide bleaching in a downstream tower which follows an upstream conduit 7, bleaching can also be performed in an upstream tower. In an existing bleaching plant having a greater number of steps, according to the invention it can be suitable to perform the ozone bleaching in one step with a normal shear mixer and a subsequent upstream tower and a wash and thereafter carry out the final peroxide bleaching in one single step. The ozone bleaching can then be expected to occur in the mixture and the conduit to the tower which is then mainly used to destruct the ozone remainders either only through extended time or through an addition of a sulphate solution in the tower mixer at the bottom of the tower. After this upstream tower, the pulp is washed whereupon the tempera¬ ture is adjusted and the pulp is fed into the bottom of a peroxide bleaching tower where in this case the pulp is bleached in upstream.

The prebleaching of the pulp before the first peroxide step can be performed in another way. Most important is that the prebleaching results in a pulp having a lignin content below 1,3 or a kappa value below 9,4. According to the above-described preferred bleaching method, the bleaching sequence will then be totally OA (Z, E) X _MG PZP where the letters have the following meaning: O = oxygen

A = acid

Z = ozone

E = alkali

X = addition of magnesium ions P = peroxide bleaching

Fig. 2 shows the effect of bleaching according to the present invention. The diagram which depicts the brightness in percent ISO according to known standard as a function of added oxidation equivalents OXE shows two curves, one of which describes the process for magnesium stabilized pulp and the other one without magnesium stabilized pulp. PI is the curve for peroxide bleaching in the first peroxide stage and P2 the peroxide bleaching in the second stage. Between these stages a connection exists between the curves called Z which shows the strong bleaching ability of the ozone step. It appears clearly from the figure that magnesium stabilized pulp and an XPZP bleaching gives very high brightness.

OXE = oxidation equivalents Ozone: 1 kilo = 125 OXE Peroxide: 1 kilo = 58,8 OXE

Fig. 3 shows the same bleaching procedure as Fig. 2 but here the intrinsic viscosity is given as a function of added oxidation equivalents. As clearly appears from this curve, the smallest decrease of the intrinsic viscosity occurs when using magnesium stabilized pulp and bleaching according to the sequence PZP.

As mentioned above, and which is commonly known, the ozone reacts very quickly and gives a good bleaching but at the same time it attacks the cellulose molecules and gives a lower viscosity. It is therefore important that the ozone step is carried out quickly and with as little ozone as possible and the peroxide should therefore do most of the bleaching work and be the main part of the added oxidation equivalents. According to the present invention, the ozone step is carried out quickly and only in a feeding pipe so that the result will be very favourable giving a low decrease of viscosity and very high bleaching.

The invention is not limited to the above embodiment but can be varied in different ways within the scope of the patent claims.