Method of bleaching pulp with ozone wherein the acidity is maintained with an acid solution prepared by electrolysis or electrodialysis

When closing the process of manufacturing cellulose pulp of high brightness, i.e. including the recovery of essentially all waste liquor, there is an untenable enrichment of certain basic elements, such as sodium and sulphur. The present invention provides a partial solution to this problem and is concerned with a method in bleaching with ozone lignocellulosic material, e.g. cellulose pulp, which has been at least partially manufactured chemically, in the presence of water at a degre of acidity, expressed as pH, of 2-5. The method is characterized in that the acidity is maintained during the ozone bleaching process by adding a sulphuric acid solution or an acid sodium sulphate solution produced by electrolysis or electrodialysis of an essentially neutral sodium sulphate solution obtained by recycling chemicals in a system to which waste liquor from the ozone bleaching process is added.

TECHNICAL FIELD 
The present invention relates to a method in bleaching of lignocellulosic 
material (cellulose pulp) with ozone. 
The cellulose pulp can be produced both chemically and chemimechanically. 
Chemical cellulose pulps are a clearly defined category of cellulose pulps 
and are comprised, for instance of sulphite pulp sulphate pulp and 
polysulphide pulp. Particularly suitable chemical cellulose pulps are 
those that have a high viscosity at relatively low Kappa numbers. 
Belonging to this group of cellulose pulps are sulphate pulps which have 
been cooked or digested at a high sulphidity, and modified sulphate pulps 
that have been cooked or digested in accordance with a counterflow method 
in which white liquor is added also during an advanced stage of the 
cooking process, and alkaline cellulose pulps which are cooked in the 
presence of one or more catalysts, for instance in the presence of a 
quinone compound, such as anthraquinone. Other suitable cellulose pulps 
are those having the designations MSS-AQ (mini--sulphide--sulphite 
anthraquinone), Si--Sa--Si (sulphite-sulphate-sulphite) and PS-Si 
(polysulphide-sulphite), all of which are referred to in the journal 
"Paperi ja Puu", 5/1989, pp. 509-513. 
Particularly suitable chemimechanical pulps are those in which the 
digestion or cooking process is relatively far reaching and is followed by 
a mechanical defibering process. 
The ozone bleaching process can be applied to unbleached cellulose pulp or 
cellulose pulp which has been delignified/bleached in one or more stages. 
PRESENT STANDPOINT OF TECHNIQUES 
In recent times, the- use of chlorine as a bleaching agent in the 
delignification of cellulose pulp has decreased drastically for 
environmental reasons. Instead, chlorine-free bleaching chemicals, such as 
oxygen and hydrogen peroxide, have been used to bleach cellulose pulp. 
Ozone bleaching has also become relevant for application on a full scale, 
i.e. for use in practice. The use of ozone has previously been inhibited 
among other things because ozone as a bleaching agent has not been 
sufficiently selective, i.e. the carbohydrates of the cellulose pulp have 
been seriously damaged from the attack of free radicals, particularly, at 
the large ozone additions (10-15 kg ozone per tonne of pulp), that are 
often required by such processes. It has been realized that an advantage 
is afforded in the manufacture of, e.g., chemical cellulose pulp when the 
pulp is subjected to an extensive, or far reaching, digestion process. 
i.e. are digested to an extent such that the lignin contents, expressed in 
Kappa numbers, will be much lower than has earlier been usual and/or to 
greatly reduce the lignin content of the cellulose pulp in a delignifying 
stage, for instance an oxygen bleaching stage following the cooking 
process. Since the cellulose pulp to be ozone bleached has a relatively 
low lignin content, it is possible to use a comparatively small ozone 
charge. Furthermore, it has been found possible to control the formation 
of free radicals by the ozone, by maintaining the ozone treatment process 
at a low pH. low temperature and a relatively high pulp consistency (for 
instance, a consistency within the range of 10-35%) and by adding radical 
capturers. 
The desire to maintain a low pH in the ozone bleaching process, i.e. a high 
degree of acidity, particularly following an alkaline cooking process, 
will put high demands on the washing equipment for removing of cooking 
liquor from the cellulose pulp and optionally for removing of oxygen 
bleaching spent liquor from the cellulose pulp. Despite thorough washing 
of the cellulose pulp there is a certain amount of chemically bound alkali 
in the cellulose pulp and that alkali consumes hydrogen tons when 
acidifying the cellulose pulp prior to the ozone treatment process. In 
addition comes the need of acid (hydrogen ions) in order to obtain the 
hydrogen ion concentration desired when bleaching the cellulose pulp with 
ozone. Normally, only sulphuric acid is considered for acidification of 
the cellulose pulp. The use of, for instance, hydrochloric acid may result 
in enrichment of chloride in the chemical recovery system, thereby 
creating corrosion problems. 
DISCLOSURE OF THE INVENTION 
Technical Problem 
The waste liquors deriving from the final pulp bleaching stage have 
hitherto not generally been returned to the chemical recovery system, but 
they have been discharged to the recipient. The ever increasing demands 
placed on emissions to the environment have meant that the release of 
these liquors to the recipient must be reduced to a minimum. This also 
applies to relatively harmless compounds in the form, for instance, of non 
chlorinated but oxygen consuming substances. 
When substantially all waste liquors from the bleaching process are 
returned to the chemical recovery system, a number of basic substances, 
such as sodium and sulphur, become enriched. Some of these substances must 
then be ejected from the system, either to air or water, in some form. In 
the worst of cases, this will result in harm to the environment, for 
instance when sulphur is ejected in the form of sulphur dioxide and/or 
hydrogen sulphide. In other case, for instance, sodium sulphate is 
obtained for which a market cannot be readily found. 
Solution 
The present invention solves this problem partially and relates to a method 
in bleaching with ozone lignocellulosic material (cellulose pulp) which 
has been produced at least partially by chemical processes. In the 
presence of water and at an acidity expressed as pH, of 2-5. The method is 
characterized in that the acidity is sustained during the ozone bleaching 
process by adding to the system either a sulphuric acid solution or an 
acid sodium sulphate solution prepared by electrolysis (or 
electrodialysis) of a substantially neutral sodium sulphate solution 
obtained by recycling chemicals in a system to which rest solution (waste 
liquor) derived from the ozone bleaching process is passed. 
According to the invention, sodium sulphate is taken from the chemical 
recovery system at some appropriate location. An example of an appropriate 
location is the electric precipitator in the recovery -boiler included in 
the chemical recovery system, where the separated -dust contains 
essentially sodium sulphate and minor quantities of sodium carbonate and 
sodium chloride. 
According to one preferred embodiment of the invention the waste liquor 
from the ozone bleaching process is mixed with other waste liquor from the 
pulp manufacturing process, this waste-liquor being recovered and returned 
to the evaporation and combustion plant (the recovery boiler) of the 
chemical system. Part of the sodium sulphate collected in the electric 
precipitator is removed therefrom and dissolved in water and the 
subsequent solution is then subjected to an electrolysis or an 
electrodialysis process. 
According to another preferred embodiment of the invention, ozone bleaching 
waste liquor is mixed with waste liquor from an alkaline treatment stage, 
for instance in the bleaching department, so as to obtain an essentially 
neutral solution, which is then cooled so that sodium sulphate (Na.sub.2 
SO.sub.4) is precipitated, this sodium sulphate precipitate being 
separated and led away and dissolved in water and the resulting solution 
is then subjected to an electrolysis or an electrodialysis process. 
The remaining mother liquor, which contains essentially organic materials, 
is transferred to the chemical recovery system. When the sodium sulphate 
solution contains metals, such as calcium, magnesium or manganese, in 
harmful quantities, it is appropriate to treat the solution with a 
hydrogen saturated cation exchanger prior to the electrolysis or 
electrodialysis stage. 
The sodium sulphate solutions obtained, for instance, in the aforedescribed 
manner, are subjected to electrolysis or electrodialysis in a cell which 
is provided with anode, cathode and one or more membranes. 
The treatment process can be carried out in at least three ways. According 
to the simplest alternative, the electrolysis is carried out in a cell 
that is provided with anode, cathode, and a cation selective membrane, 
wherein sodium sulphate solution is delivered to the anode chamber and 
water is delivered to the cathode chamber so that an acid solution is 
formed and oxygen gas is generated at the anode and so that a sodium 
hydroxide solution is formed and hydrogen gas is generated at the cathode. 
According to another alternative embodiment, the electrolysis is carried 
out in a cell that is provided with anode, cathode an anion selective and 
a cation selective membrane, wherein sodium sulphate solution is 
introduced between the two membranes and water is introduced to the anode 
chamber and the cathode chamber respectively, so that a sulphuric acid 
solution is formed and oxygen gas generated at the anode and so that a 
sodium hydroxide solution is formed and hydrogen gas generated at the 
cathode. 
According to a third, more advanced alternative, the treatment is carried 
out in a multi chamber cell that is provided with anode, cathode and anion 
selective, cation selective and bipolar membranes, wherein the anode 
chamber is delimited by an anion selective membrane and the cathode 
chamber is delimited by a cation selective membrane, and wherein bipolar 
membranes are disposed between said membranes and wherein sodium sulphate 
solution is delivered to those chambers that are delimited by bipolar 
membranes, so that a sulphuric acid solution is formed in at least the 
membrane chamber which is placed nearest the cathode and hydrogen gas is 
generated at said cathode, and so that a sodium hydroxide solution is 
formed at least in the membrane chamber which is placed nearest the anode, 
and oxygen gas is generated at said anode. 
According to this third alternative, oxygen gas and hydrogen gas are formed 
in smaller quantities than in the two preceding alternatives, since these 
electrode reactions do not take place at the bipolar membranes. This third 
method is referred to as electrodialysis. 
The first and the second alternative methods, i.e. the methods which 
describe conventional electrolysis, are particularly preferred because of 
their simplicity. 
The acid solution (sulphuric acid solution) and the sodium hydroxide 
solution respectively formed in the aforesaid cells are removed from the 
cell individually and the acid solution (the sulphuric acid solution) is 
delivered to the cellulose pulp so as to obtain the acidity required and 
desired for the ozone bleaching process. Sodium hydroxide is also a 
valuable chemical which can be used, for instance, in the alkaline stage 
of the cellulose pulp treatment process, this stage often following the 
ozone bleaching stage. If desired, the intrinsically valuable chemicals 
oxygen and hydrogen can also be collected. 
Advantages 
The invention enables the consumption of externally produced (purchased) 
acid and externally produced (purchased) alkali to be greatly reduced in 
the manufacture of cellulose pulp that is bleached with ozone. The 
secondary products oxygen gas and hydrogen gas can be used in the 
cellulose pulp manufacturing process, for example for oxygen gas bleaching 
and steam generation respectively. The invention primarily leads to the 
possibility of mastering the environmental problems in a practically 
complete closing of the pulp manufacturing process, i.e. also including 
final bleaching waste liquors. When commercial sulphuric acid is used to 
achieve the necessary acidity of the cellulose pulp in an ozone bleaching 
process, it would mean, for instance, that 12,000-18,000 tonnes of sodium 
sulphate must be ejected per year from the process in an average sized 
sulphate pulp mill, when this alternative is chosen for maintaining a 
chemical balance in the system.

BEST MODES OF CARRYING OUT THE INVENTION 
Preferred embodiments of the invention are described in the following with 
reference to the drawings. Two exemplifying embodiments are also 
described. In connection herewith, the description includes more detailed 
information as to how the inventive method is carried out. 
According to FIG. 1 pine chips are delivered through the conduit 1 to the 
digester 2, which contains a cooking liquor composed essentially of sodium 
hydroxide and sodium sulphide. Subsequent to digesting the wood and 
exposing the fibres thereof, the exposed fibres--the pulp--is transported 
through the pulp conduit 3 (which extends through the entire pulp mill) to 
the washing section 4, where the major part of the consumed cooking liquor 
(the cooking waste liquor or the black liquor) is extracted from the pulp. 
The cooking waste liquor is transported through conduits 5 and 6 to the 
chemical recovery system of the mill, this system including, among other 
things, an evaporation plant 7 and a combustion plant (recovery boiler) 8. 
which is provided with an electric precipitator 9. 
Subsequent to washing of the pulp in the washing department 4 and 
subsequent to optional screening of the pulp (not shown in the drawing), 
the pulp is passed to an oxygen bleaching stage where the pulp is supplied 
with oxygen gas, through the conduit 11, and also with alkali, normally 
sodium hydroxide, and optionally also with a protector. The oxygen 
bleached pulp is passed to a washing stage 12 in which the pulp is freed 
from the major part of the waste liquor deriving from the oxygen bleaching 
stage. The aforesaid treatment process is carried out advantageously in a 
conventional press or in a washing press to which washing liquid is added. 
The pulp is brought to the desired consistency for the following ozone 
bleaching process of the pulp, by adjusting the pressure at which the pulp 
is pressed. The oxygen bleaching waste liquor pressed from the pulp is 
passed through the conduits 13 and 6 to the traditional chemical recovery 
system. Part of the oxygen bleaching waste liquor can be returned to the 
bottom of the oxygen bleaching reactor 10 through the conduit 14. The 
pulp, which has a comparatively high pulp consistency is then delivered to 
the mixer 15 in which an acid sodium sulphate solution is added to the 
still alkaline pulp, this sodium sulphate solution being prepared in the 
electrolysis cell 16 and transported through the conduit 17. The acid 
solution is added in a quantity which will ensure that the pH value of the 
pulp/water mixture will be suitable for the ozone bleaching of the pulp, 
i.e. within the range 2-5. 
When the pulp leaves the mixer 15, it may, for instance, have a medium pulp 
consistency (10-15%) or a high pulp consistency (30-35%), depending on 
requirements. The pulp is then delivered to the ozone bleaching reactor 18 
to which oxygen gas containing a given low ozone content is delivered, in 
addition to pulp, through the conduit 19. The ozone bleaching stage is 
maintained at a comparatively low temperature, for instance a temperature 
within the range of 40-60.degree. C. The pulp reacts completely with the 
ozone in this stage, and residual oxygen gas is led from the reactor 
through the conduit 20, for transportation to an apparatus for generating 
fresh ozone (not shown in the Figure). The pulp is then transported to a 
further washing stage 21, for instance in the form of a washing press. The 
ozone bleaching waste liquor recovered is passed back to the system, 
through the conduit 22. The waste liquor is divided into two flows, of 
which one flow is passed through the conduit 23 back to the ozone 
bleaching reactor 18 for the purpose of flushing pulp from the reactor. 
The other flow is passed to a mixing and crystallizing vessel 25, through 
the conduit 24. 
The pulp is then passed to a mixer 26 in which alkali, essentially sodium 
hydroxide, is added to the pulp. A significant part of the alkali is 
recovered in the form of a sodium hydroxide solution in the electrolysis 
cell 16, and the solution is added to the pulp in the mixer 26 through the 
conduit 27. The remainder of the sodium hydroxide required is supplied 
externally (purchased alkali) through the conduit 43. The pulp is then 
passed to an alkalizing or extraction tower 28. In addition to using a 
plain alkali stage in this stage of the process, it is possible, and even 
advantageous, to supply also oxygen or peroxide or both of these 
chemicals. Finally, the pulp is washed in the washing stage 29. It is 
quite possible to terminate the pulp manufacturing process at this stage 
and, for instance, to transport the pulp to a paper mill and/or to a 
dewatering and drying machine for the manufacture of commercial pulp. 
However, it is often desired to increase both the cleanliness and the 
brightness of the pulp by undertaking one or more additional bleaching 
stages while using bleaching chemicals such as, e.g., dithionite, 
peroxide, chlorine dioxide or additional ozone. 
When the bleaching process is terminated at the aforesaid alkali stage, 
clean water is normally added to the pulp through the conduit 30. The 
alkaline waste liquor is returned in the system through the conduit 31. 
The recovered waste liquor is divided into three parts. A first part is 
added through the conduit 32 to the mixer 26, where it is mixed with the 
pulp so as to increase its alkali content. A second part is delivered 
through the conduit 33 to the conduit 24 where the alkaline waste liquor 
is mixed with the acid ozone bleaching waste liquor so that the waste 
liquor mixture delivered to the crystallization vessel 25 is generally 
neutral, i.e. has a pH of 7 or a value in the vicinity thereof. The 
remaining quantity of waste liquor is passed through the conduit 31 to the 
pulp washer 4, immediately downstream of the digester 2. 
The waste liquor mixture in the vessel 25 is cooled to a temperature of, 
e.g., 15.degree. C. or therebelow. A major part of the sodium sulphate 
present in the waste liquor will then precipitate in the form of crystals, 
which settle on the bottom of the vessel 25. The crystals are then passed 
through the conduit 34 to a washing filter 35, with the aid of an 
appropriate feeding out device. The waste liquor--mother liquor--freed 
from the sodium sulphate crystals in the vessel 25 is discharged through 
the conduit 36 and mixed with oxygen bleaching waste liquor and cooking 
waste liquor (black liquor) for transportation into the evaporator plant 
7. 
The sodium sulphate crystals are washed and cleaned on the filter 35 with a 
small quantity of liquid, and are thereafter passed to the dissolving 
vessel 38 through the conduit 37. The liquid removed from the sodium 
sulphate crystals on the filter 35 may, for instance, be introduced in the 
conduit 36 (not shown in the Figure). Preferably, clean water is passed 
through the conduit 39 to the dissolving vessel 38 in a quantity such that 
essentially all sodium sulphate crystals are dissolved. The dissolving 
process is facilitated by using a temperature which is slightly higher 
than room temperature and which, for instance, reaches 35.degree. C. The 
solution containing a large quantity of dissolved sodium sulphate is 
transported through .the conduit 39 to the anode chamber of the 
electrolysis cell 16. Preferably clean water is delivered to the cathode 
chamber in the electrolysis cell 16 through the conduit 40. During the 
process of electrolysis, there is formed at the anode an acid solution 
which is recovered in the manner earlier described, while at the cathode 
there is formed a sodium hydroxide solution which is recovered in She 
aforedescribed manner. Oxygen gas is also generated in the anode chamber, 
this gas being led away through the conduit 41, while hydrogen gas is 
generated in the cathode chamber and led away through the conduit 42. 
In FIG. 2 is a flow sheet shown, which coincides totally with the flow 
sheet of FIG. 1 with regard to the pulp manufacturing (and pulp refining) 
process. A large part of the liquid and waste liquor transportation is 
also effected in direct agreement with the two flow sheets. Consequently, 
those reference signs used in FIG. 2 that are in agreement with the FIG. 1 
illustration are used with the addition of +50 except with respect to 
those few deviations which exist when making a comparison between the two 
embodiments of the invention. 
In order to avoid unnecessary repetition, only those part stages in the 
flow sheet of FIG. 2 which differ from what is shown in the flow sheet of 
FIG. 1, will here be described and commented on. 
According to this embodiment of the invention, the waste liquor is 
conducted from the washing stage 71, which is located immediately 
downstream of the ozone bleaching stage 68, through the conduits 72 and 74 
to the conduit 63, where the mentioned ozone bleaching waste liquor is 
mixed with oxygen bleaching waste liquor from the washing (press) stage 
62. The mixture concerned is transported to the evaporation plant 57 
through the conduit 56. The waste liquor concentrated by evaporation, the 
thick liquor, is then passed to the recovery boiler 58 in which it is 
burned. The organic material content of the thick liquor is now converted 
to carbon dioxide and water, whereas its inorganic content is essentially 
recovered as sodium carbonate and sodium sulphide in the bottom part of 
the recovery boiler. However, a significant part of the inorganic material 
accompanies the flue gases and is separated as sodium sulphate in the 
electric precipitator 59. All or a part of the sodium sulphate recovered 
is passed in powder form to the dissolving tank 83, through the conduit 
75. Preferably, clean water is added through the conduit 84 in an amount 
such that all sodium sulphate will be dissolved. If it is necessary to 
purify the solution from foreign chemicals, this can be effected in a 
following treatment stage (not shown in the Figure). The resultant sodium 
sulphate solution is transported through the conduit 85 to the anode 
chamber of the electrolysis cell 66. Preferably clean water is introduced 
to the cathode chamber of the electrolysis cell 66, through the conduit 
86. Oxygen gas formed in the anode chamber is led away through the conduit 
87 and hydrogen gas formed in the cathode chamber is led away through the 
conduit 88. The acid solution generated in the process of electrolysis is 
transported through the conduit 67 to the mixer 65, for necessary 
acidification of the pulp prior to the pulp coming into contact with ozone 
in the bleaching stage 68. The sodium hydroxide solution produced in the 
process of electrolysis is transported through the conduit 77 to the mixer 
76, in which the pulp is made alkaline. 
These two flow sheets illustrate applications of the invention when 
bleaching sulphate pulp with ozone, this pulp having been subjected to an 
oxygen bleaching process prior to the ozone bleaching stage. Reference has 
earlier been made to suitable final bleaching stage or stages when 
desiring to further increase the cleanliness and brightness of the pulp. 
Similarly, it is possible to introduce further delignifying and/or 
bleaching stages between the digestion stage, i.e. the manufacture of the 
original pulp, and the ozone bleaching stage. Advantageously, an acid 
treatment stage can be included, for instance by allowing the pulp to 
react with a gas that contains nitrogen dioxide (this process is known as 
PRENOX), immediately prior to the oxygen bleaching stage. In addition to 
an oxygen bleaching stage in the mentioned position, there can be applied 
a plain alkali stage, or a peroxide reinforced alkali stage, or an alkali 
stage which is reinforced with both peroxide and oxygen. Irrespective of 
whether an acid treatment stage, for instance in accordance with the 
PRENOX method, is introduced in the mentioned position or not, it may be 
beneficial to treat (bleach) the pulp with chlorine dioxide, with or 
without intermediate washing of the pulp, immediately prior to the ozone 
bleaching stage. 
As previously mentioned, the inventive method is in no way limited to the 
ozone bleaching of sulphate pulp, but can be applied to the ozone 
bleaching of any chemical pulp whatsoever and also to the ozone bleaching 
of chemimechanical pulp. When bleaching such pulps, the bleaching 
sequences can be quite different to the aforedescribed and mentioned 
bleaching sequences. With regard, for instance, to sulphite pulp, this 
pulp will have after the digestion stage a much lower lignin content and a 
much higher brightness than sulphate pulp, which means that only two 
bleaching stages and at most three bleaching stages, of which one is an 
ozone bleaching stage, are required in order to obtain a highly clean and 
bright pulp. 
In FIG. 3 shows in more detail the construction of an electrolysis cell 
similar to that shown in FIGS. 1 and 2 and illustrates how decomposition 
of the sodium sulphate takes place. 
The electrolysis cell 100 is comprised of two chambers, the anode chamber 
101 and the cathode chamber 102. An anode 103 is arranged in the first 
mentioned chamber and a cathode 104 is arranged in the other chamber. The 
two chambers are mutually separated by a cation selective membrane 105, 
which will only allow sodium ions to pass through. Sodium sulphate 
solution is added t6 the anode chamber 101 through the conduit 106 and 
water is added to the cathode chamber 102 through the conduit 107. As a 
result of applying an electric voltage across the cell, hydrogen ions are 
formed at the anode 103 while generating oxygen gas at the same time, this 
gas being led away from the cell through the conduit 108. Hydroxide ions 
are formed at the cathode 104 at the same time as hydrogen gas is 
generated, this gas being led away from the cell through the conduit 109. 
The current efficlency in the electrolysis process is greatly dependent on 
the concentrations of hydrogen ions and hydroxide ions respectively in 
respective chambers. Consequently, it is not possible in this type of 
electrolysis cell to produce two hydrogen ions=H.sup.+, for each sulphate 
ion=SO.sub.4.sup.2- and normally it is necessary to be satisfied with a 
yield of one hydrogen ion or slightly more with each sulphate ion. A part 
of the sodium sulphate is therefore still not decomposed. Consequently, 
the solution removed from the anode chamber 101 through the conduit 110 is 
designated an acid sodium sulphate solution. A sodium hydroxide solution 
is removed from the cathode chamber 102 through the conduit 111. The 
destination of these solutions will be apparent from the earlier 
description. 
When desiring to produce two hydrogen ions for each sulphate ion, i.e. to 
produce sulphuric acid, an electrolysis cell 120 having three chambers can 
be used, as shown in FIG. 4. 
This cell also includes an anode chamber 121. and a cathode chamber 122. An 
anode 123 is arranged in the anode chamber while a cathode 124 is arranged 
in the cathode chamber. Located centrally between these two chambers is a 
further chamber 125 which is encircled by a cation selective membrane 126 
which allows sodium ions to pass through, and an anion selective membrane 
127 which allows sulphate ions to pass through. Sodium sulphate solution 
is delivered to the cell through the conduit 128 and is recycled back to 
the conduit (in a lower concentration) through the conduit 129. Water is 
delivered to the cell through the conduit 130, to both the cathode chamber 
122 and the anode chamber 121. 
When an electric voltage is applied over the cell 120, oxygen gas is 
generated at the anode 123, this gas being led away through the conduit 
131, at the same time as sulphuric acid (H.sub.2 SO.sub.4 or 2H.sup.+ 
+SO.sub.4.sup.2-) is formed in the anode chamber 121. Hydrogen gas is 
generated at the cathode 124 and is led away from the cell through the 
conduit 132, at the same time as sodium hydroxide (NaOH or Na.sup.+ 
+OH.sup.-) is formed in the cathode chamber 122. Respective solution is 
led away from the cell 120 through the conduits 133 and 134 and the 
chemicals concerned are used in the manner earlier mentioned. 
When desiring to reduce the amount of oxygen gas and hydrogen gas formed in 
relation to the major products acid and alkali in comparison with those 
electrolysis processes described with reference to FIGS. 3 and 4, a cell 
140 of the type illustrated in FIG. 5 can be used. 
This cell also has an anode chamber 141 and a cathode chamber 142 having 
respectively an anode 143 and a cathode 144. The anode chamber 141 is 
delimited on one side by an anion selective membrane 145, while the 
cathode chamber 142 is delimited on one side by a cation selective 
membrane 146. Arranged between these two membranes are two bipolar 
membranes 147 and between these a further cation selective membrane 146 
and an anion selective membrane 145 are arranged. Sodium sulphate solution 
is added to the anode chamber 141 and the cathode chamber 142 respectively 
through the conduit 148, and also to each third chamber between the 
bipolar membranes, which can be included in a greater or smaller number. 
Water is added to remaining cell chambers through the conduit 149. When an 
electric voltage is applied across the cell, oxygen gas is generated at 
the anode 143 and led away through the conduit 150, and hydrogen gas is 
generated at the cathode 144 ahd led away through the conduit 151. 
Furthermore, sodium ions migrate in a direction towards the cathode 144 
and sulphate tons migrate in a direction towards the anode 143. The water 
is dissociated to hydrogen ions and hydroxide ions at the same time. 
Surphuric acid (H.sub.2 SO.sub.4 or 2H.sup.+ +SO.sub.4.sup.2-) and sodium 
hydroxide respectively are formed in this way. Sulphuric acid solution is 
led away from two of the membrane chambers and transported through the 
conduit 152 for use in accordance with the earlier description. 
Sodium-hydroxide solution is led away from two other membrane chambers and 
transported through the conduit 153 for use in accordance with the earlier 
description. Sodium sulphate solution (of lower concentration) is led away 
from the bottom of the three chambers where fresh such solution is added 
to the upper part of the cell and recycled back to the conduit 148, 
through the conduit 154. The decomposition of sodium sulphate and water 
illustrated in FIG. 5 is normally designated electrodialysis. 
It will be understood that other processes of electrolysis respectively 
electrodialysis can also be applied when carrying out the inventive 
method. 
A number of experiments have been carried out with the method according to 
the present invention. The manner in which these experiments were carried 
out and the results obtained will be evident from the following working 
examples. The experiments concerned were carried out on a pilot-plant 
scale. 
EXAMPLE 1 
The experiment was carried out with a pine sulphate pulp having a Kappa 
number of 29.0 and a viscosity of 1,250 dm.sup.3 /kg. The pulp was oxygen 
bleached at a pulp consistency of 12% and at an oxygen pressure of 5 
kg/cm.sup.2. 2% sodium hydroxide and 0.3% magnesium in the form of 
magnesium sulphate (MgSO.sub.4) had earlier been supplied to the pulp. 
Upon completion of the oxygen bleaching process, the pulp had a Kappa 
number of 15.0 and a viscosity of 1,010 dm.sup.3 /kg. The pulp was then 
pressed to a pulp consistency of 40%. Inorganic chemicals (washing losses) 
corresponding to 12 kg sodium sulphate per tonne of pulp were found to 
remain in the pulp. An acid sodium sulphate solution was then added. to a 
pulp flow of 100 kg/min. in an amount of 10 litre/min. The liquid 
contained 80 g/l of sodium sulphate and hydrogen ions, expressed as 
sulphuric acid, in an amount of 160 g/l. The pulp concentration fell 
therewith to 33% and the pH of the pulp-became 2.5. 
The pulp was fluffed and introduced into an ozone bleaching reactor, where 
the pulp was allowed to react with ozone added in a quantity of 0.4 kg/min 
in a flow of 5.7 kg/min oxygen gas. The temperature was 50.degree. C. and 
the treatment time 30 minutes. The oxygen gas freed from ozone was removed 
from the reactor. At the end of the treatment time, liquid was added so as 
to flush away the pulp. Dilution liquid in the form of water was added in 
an amount of 540 l/min. The pulp was then pressed to a pulp consistency of 
30%. The resultant ozone bleaching waste liquor was thereby recovered. The 
pulp was then transferred to a mixer, to which 2.7 kg of sodium 
hydroxide/min were added. The pulp was allowed to react with the alkali 
for 120 minutes at a temperature of 65.degree. C. and a pulp concentration 
of 14%. The pulp was then washed with clean water so as to obtain an 
alkaline waste liquor. 
The Kappa number of the pulp after this treatment stage was 7.3, its 
viscosity 903 kg/dm.sup.3 and its brightness 53% ISO. 
The Kappa numbers, viscosities and brightnesses recited in this patent 
application have been determined in accordance with SCAN-C 1:77, SCAN-CM 
15:88 and SCAN-C 11:75 respectively. 
The ozone bleaching waste liquor in an amount of 15 l/min., containing 310 
g/l sodium sulphate, hydrogen tons, calculated as H.sub.2 SO.sub.4, in an 
amount of 40 g/l and organic material in an amount of 90 g/l. was mixed 
with the alkaline waste liquor in an amount of 12 l/min so that the 
resultant pH of the mixture was 7.1. The mixture was transferred to a 
crystallization vessel, in which the mixture was cooled to 10.degree. C. 
Sodium sulphate crystals in an amount of 3.3 kg/min were sepatated from 
the mixture (the mother liquor) and the sodium sulphate content thereof 
fell to 90 g/l. 
These crystals were then dissolved in a vessel in clean water in a quantity 
such that the content of dissolved sodium sulphate became 430 g/l. This 
solution had a temperature of 35.degree. C. and was added to the anode 
chamber of an electrolysis cell of the type illustrated in FIG. 3. Clean 
water was added to the cathode chamber of the cell. The cell temperature 
was 50.degree. C., the voltage was 3.8 V. the current density was 25 
A/dm.sup.2 and the power consumption was 240 kW. 
During the electrolysis process, oxygen gas was generated in the anode 
chamber in an amount of 190 l/min (0.27 kg/min) at the same time as an 
acid sodium sulphate solution was formed in which the hydrogen ions, 
calculated as sulphuric acid, rose to 1.7 kg/min (75% conversion). 
Hydrogen gas developed in the cathode chamber in an amount of 380 l/mmn 
(0.034 kg/min) at the same time as a sodium hydroxide solution was formed 
in an amount of 14 l/mmn at a concentration of 10%. 
As previously mentioned, the resultant acid sodium sulphate solution 
contained 80 g/l sodium sulphate and hydrogen ions, expressed as sulphuric 
acid, in an amount of 160 g/l. The solution concerned was used fully to 
acidify the pulp prior to the ozone bleaching stage, as earlier described, 
so that the pulp had a pH of 2.5. The resultant sodium hydroxide solution 
was added to the pulp, as earlier mentioned, in the mixer prior to the 
alkalization treatment process. The sodium hydroxide content of this 
solution covered 50% of the sodium hydroxide addition to the pulp. 
The following chemical savings were achieved in this experiment: 
Acidifying chemical, calculated as sulphuric acid=17 kg per tonne of pulp; 
Alkalizing chemical, sodium hydroxide=14 kg per tonne of pulp; 
Oxygen gas=2.7 kg per tonne of pulp. 
As earlier mentioned, when practicing the inventive method on a full scale, 
the oxygen gas can be used in an alklaline treatment stage, for instance 
in an introductory oxygen bleaching stage. 
EXAMPLE 2 
This experiment was carried out with a pine sulphate pulp having a Kappa 
number of 26.0 and a viscosity of 1,202 dm.sup.3 /kg. The pulp was 
bleached with oxygen gas at a pulp consistency of 12% and at an oxygen gas 
pressure of 6 kp/cm.sup.2. 1.5% sodium hydroxide and 0.3% magnesium in the 
form of magnesium sulphate (MgSO.sub.4) had earlier been added to the 
pulp. Upon completion of the oxygen bleaching process, the pulp had a 
Kappa number of 13.0 and a viscosity of 990 dm.sup.3 /kg. The pulp was 
then pressed to a consistency of 40%. Inorganic chemicals (washing losses) 
corresponding to 9 kg sodium sulphate per tonne of pulp were then found to 
remain in the pulp. A sulphuric acid solution of 10% concentration was 
then added to the pulp in a flow of 100 kg/min. The pulp consistency then 
fell to 38% and the pulp was found to have A pH=2.7. 
The pulp was fluffed and introduced into an ozone bleaching reactor, where 
the pulp was allowed to react with ozone, which was added in a quantity of 
0.3 kg/min in an oxygen gas flow of 4.3 kg/min. The temperature was 
50.degree. C. and the treatment time 30 minutes. 
The oxygen gas freed from ozone was withdrawn from the reactor. At the end 
of the treatment time, liquid was added so as to flush away the pulp. 
Dilution liquid in the form of water was added in an amount of 500 l/mmn. 
The pulp was then pressed to a pulp consistency of 30%. The resultant 
ozone bleaching waste liquor was recovered. The pulp was then transferred 
to a mixer to which 2.5 kg sodium hydroxide/min was added. The pulp 
reacted with the alkali for 130 minutes at a temperature of 60.degree. C. 
and a pulp consistency of 14%. The pulp was then washed with clean water 
so as to obtain analkaline waste liquor. Subsequent to this treatment 
stage, the pulp was found to have a Kappa number of 6.7. a viscosity of 
900 kg/dm.sup.3 and a brightness of 55% ISO. 
Precipitator dust from the sulphate mill from which the experiment pulp was 
taken was dissolved in an amount of 1.8 kg/min in clean water. The amount 
of water used was such that the resultant solution had a sodium sulphate 
content of 360 g/l. The dissolving took place at a temperature of 
55.degree. C. In this experiment, there was used an electrolysis cell 
which had three chambers of the type Illustrated in FIG. 4. The cell 
included two membranes. The sodium sulphate solution was added to the 
centre chamber, i.e. the chamber located between the two membranes. Clean 
water was added to the anode chamber and the cathode chamber respectively. 
The cell temperature was 55.degree. C., the voltage 4.2 V, the current 
density 20 A/dm.sup.2 and the power consumption 190 kW. 
During the electrolysis process, oxygen gas developed in the anode chamber 
in an amount of 135 l/min (0.19 kg/min) at the same time as sulphuric acid 
was formed in an amount of 1.2 kg/mn in a concentration of 10%. Hydrogen 
gas was developed in the cathode chamber in an amount of 270 l/min (0.024 
kg/min) at the same time as a sodium hydroxide solution was formed in an 
amount of 1.0 kg/min at a concentration of 10%. The electrolyzed sodium 
sulphate solution was removed from the bottom of the centre chamber and 
was found to still contain a given content of sodium sulphate, this 
solution being recycled and mixed with fresh sodium sulphate solution, 
which was added to the cell at the upper part of the centre chamber. 
The sulphuric acid solution obtained was used fully to acidify the pulp 
prior to he ozone bleaching stage, as previously mentioned, such that the 
pH of the pulp was 2.7. The sodium hydroxide solution obtained was added 
to the pulp in the mixer, as earlier described, prior to the alkalizing 
treatment stage. The sodium hydroxide solution obtained was added to the 
pulp in the mixer as earlier described prior to the alkalizing treatment 
stage. The sodium hydroxide content of this solution covered 40% of the 
sodium hydroxide charged to the pulp. 
The following chemical savings were achieved with this experiment: 
Sulphuric acid=12 kg per tonne of pulp 
Sodium hydroxide=10 kg per tonne of pulp 
Oxygen gas=1.9 kg per tonne of pulp