State of the art Mechanical defibration methods and chemical defibration methods pertain to conventional methods for producing pulp from raw wood and they are still being used in the 21st century. Chemical defibration methods include i. a. the soda process, the sulphite process and the sulphate process. These methods have been consistently developed and further enhanced. In recent years there have been special efforts focussing on efficient energy use of the processes and on environmentally friendly processes. Thus, for instance, in chemical pulp production, the use of additives has increased and efforts have been made to minimise emissions both of malodorous reduced sulphur compounds and of sulphur dioxides. The emission limits imposed by air protection authorities are becoming increasingly stringent, so that nowadays there are efforts to recover the sulphur dioxide generated in malodorous sulphur compound combustion and other process steps. Sulphur dioxide can be purged from flue gases in gas scrubbers, for instance, in which sulphur dioxide is converted into sodium bisulphite by means of sodium hydroxide in a manner known per se.

The sulphate process is currently the most frequently used chemical pulp production method. In this process, the alkaline cooking liquor, i. e. white liquor, contains approximately three parts of sodium hydroxide, NaOH, and one part of sodium sulphide, Na2S. The process includes as an essential part the recovery of the cooking chemicals and certain by-products. After digestion, the pulp is scrubbed, the fatty and rosin acids derived from wood extractives contained in the pulp being separated from the wash liquor, i. e. the black liquor. The recovery of lighter organic components derived from extractives, such as turpentine, is also part of the overall recovery. During digestion, the major portion of wood lignin is dissolved in the cooking liquor, which is combusted in a soda recovery boiler, where the sulphur and sodium contained in the cooking liquor are also recovered. A great part of the

water of black liquor is evaporated, after which the residue is combusted in a soda recovery boiler. In the soda recovery boiler, the organic substance is removed by combustion and the inorganic portion of black liquor remains, which at this stage consists mainly of sodium carbonate, Na2C03, and sodium sulphide Na2S. The obtained molten salts are dissolved into water from the soda recovery boiler, resulting in so called"green liquor". The green liquor is fed to causticisation, where sodium carbonate reacts with calcium hydroxide in the aqueous solution: Na2CO3 + Ca (OH) 2- 2NaOH + CaCO3 (1) The produced calcium carbonate is further regenerated in a lime kiln by burning it into calcium oxide: CaCOs- CaO + C02 (2) The white liquor obtained from causticisation contains principally sodium hydroxide and sodium sulphide. The composition of such white liquor has a notable impact on the properties of the pulp to be cooked. Since the process is in principle closed in this respect as well, in other words, the cooking chemicals are not consumed during the process, it is crucial to maintain the sulphur: sodium ratio of the chemical cycle in balance. Nowadays balance control involves the major problem of excess sulphur introduced into the process in connection with sulphuric acid. Cellulose mills have several different subprocesses producing sulphur, one of the main ones being the tall oil production process. In addition sulphur reaches the process e. g. from the wood itself, from water, from the fuel oil of the lime kiln and from magnesium sulphate used in oxygen bleaching.

During sulphate cellulose digestion, among wood additives, resin fatty acids and rosin acids are saponified into sodium salts, i. e. soap, and there will also be residual unsaponified organic components, depending on the species of raw wood used.

These soaps are further micellised, with a certain amount of unsaponified substances dissolved into the micelles. Usually cellulose digestion is followed by a scrubbing step, the solution removed from this step, black liquor, comprising soap micelles rising to the surface of the liquor in the form of raw soap. The soap formed is separated from black liquor by decantation, after which it can be split into tall oil by means of sulphuric acid and heat. Consequently, during splitting, the soap separated from the surface of black liquor is cooked together with sulphuric acid,

corresponding acids being produced from the water-soluble sodium salts of fatty and rosin acids, the mixture of acids being referred to as raw tall oil. The reaction occurring during splitting has the following formula: H2SO4 + 2 R-COONa-> 2 R-COOH + Na2S04 (3) In their acid form, fatty acids are water-insoluble and are thus separated from soap.

During splitting, sulphur acid is consumed principally in the conversion of soap into tall oil, but also for neutralising the black liquor accompanying the soap and for regulating the pH to a level low enough for the fatty acids to be brought into acid form.

In practice, the sulphur acid addition should be such that the pH will decrease below 4. The tall oil obtained during splitting is separated from the pulp press water phase, which is recycled to the chemical circulation system of the pulp mill.

Waste acid from e. g. a chlorine dioxide reactor can be used as the splitting acid, which is in fact frequently done. Since the aqueous phase produced during splitting is recycled to the chemicals recovery, sulphur in dissolved form will also enter the chemicals recovery. Along with waste acid, a notable amount of sulphur will enter the process, because waste acid contains sodium sulphate or sodium sesquisulphate in considerable amounts besides sulphuric acid. Depending on the wood species and the site of growth, tall oil splitting may introduce sulphur into the process at a rate of 2 to 4 kg/ton of pulp. The sulphur loss of a modern cellulose pulp mill, i. e. the sulphur that is not recycled but is removed from the chemicals recovery, can be compensated with sulphurous chemicals generated during the preparation of bleaching chemicals, whose amount even exceeds the sulphur demand of the cellulose pulp mill. Especially splitting of tall oil soap introduces a considerable surplus of sulphur into the process. The excess sulphur has consequently to be removed from the process.

If the excess sulphur is removed from the process in the form of sodium sulphate, the removed sodium has to be compensated with fresh liquor, resulting in increased sulphur removal cost. In the control of the chemical balance of a sulphate cellulose mill, it is essential to control the ratio of sulphur to sodium, i. e. the sulphidity. Tall oil soap splitting is a major cause of imbalance in the sulphidity of a sulphate pulp process.

If excess sulphur is removed e. g. in the form of fly ash, sodium will escape simultaneously, requiring compensation with fresh liquor introduced into the process.

One solution to this problem would involve the use of sulphur-free acid.

Hydrochloric acid is inappropriate for this purpose due to the problems caused by chloride. Organic acids such as formic acid and acetic acid could perhaps partly replace sulphuric acid. On the other hand they are probably too weak acids, and also relatively expensive. Nitric acid is also inappropriate for the process, because it may produce nitrogen oxides during combustion in the soda recovery boiler.

WO patent specification 9411571 discloses a soap acidification by means of a large excess of sodium bisulphite solution relative to the soap amount for the recycling of sulphur with a view to reuse in tall oil preparation. The acidification can be performed in one single step or preferably in several steps. The bisulphite solution used preacidifies soap at an elevated temperature in the range from 90 to 150 °C and under atmospheric pressure or under slight overpressure. However, it will be necessary to further acidify soap by means of sulphuric acid in order to complete the cooking. A bisulphite solution as such is not very acidic, having a pH slightly below 5. The pH of splitting acids should be less than 4.

Further sulphur is additionally introduced into the process in the step of applying a soluble magnesium compound during oxygen bleaching. Magnesium sulphate is typically introduced into the process from outside the process. Instead of magnesium sulphate, it would be possible to use e. g. acetates or formiates, allowing a reduction of the sulphur load by the choice of chemicals, yet in practice, these options are too costly. Magnesium hydroxide and magnesium carbonate would be suitable alternatives with regard to their composition, however, they have the problem to be too insoluble.

Object of the invention The object of the present invention is to provide a simple and efficient recycling process of recovered sulphur originating from a chemical pulping process, especially from the chemical circulation system of the pulp mill, without the need of

introducing additional sulphur in any notable amount from the outside, whereby problems with the sulphidity of the process can be avoided.

Description of the drawings Figure 1 illustrates the pH decrease in an oxidation treatment in the course of oxidation Figure 2 is a schematic view of an apparatus used in the treatment of the invention Figure 3 is a schematic view of an application of the invention for the recycling of sulphur.

Figure 4 illustrates the pH change occurring in example 2.

Figure 5 illustrates the sulphur-sodium ratios compared in example 3.

Figure 6 illustrates the pH change occurring in example 4 in the course of oxidation.

Figure 7 illustrates the bisulphite oxidation of example 4 as a function of time.

Figure 8 illustrates the pH change occurring in example 4 in the course of oxidation.

Disclosure of the invention The method of the invention is based on the surprising finding that recycled sulphur is oxidised to a bisulphate solution by means of an oxidant. Surprisingly, the bisulphate solution obtained is considerably more acidic than e. g. a bisulphite solution, the acidity being sufficient for splitting soap into raw tall oil even in one single step. Thus raw tall oil can be cooked by using e. g. recycled sulphur obtained from the chemical circulation system of a pulp mill. The bisulphate solution can also be used for preparing sulphate compounds needed in the process, such as magnesium sulphate used in bleaching.

In this context, the expression"is then further used as recycled acid in various steps of the process"or"is then further used in various process steps"means that the bisulphate product obtained from the oxidation treatment is reused as a reagent, e. g. as an acidic agent ("acid source") in one or more process steps and/or as the raw material for preparing one or more reagents for use in the process, such as magnesium sulphate used in connection with pulp oxygen delignification or pulp bleaching.

The method of the invention can utilise the sulphur recovered from the process, especially malodorous gases containing sulphur. In one preferred embodiment, at least part of the sulphur recovered from the process, e. g. a sulphate process, preferably the malodorous gases, are combusted in order to convert the sulphur compounds contained in them into sulphur dioxide, and at least part of the sulphur dioxide obtained is used for preparing bisulphite, preferably an aqueous sodium bisulphite solution, by means of an alkaline agent, preferably sodium hydroxide, in a manner known in the art.

The invention relates to a process for preparing chemical pulp. Pulp obtained from the process can be used for producing paper or paperboard, for instance. Hence the terms"a preparation process of a pulp"or"process"in this context cover in a broad sense the process for producing the actual chemical pulp, preferably a sulphate process, including the various steps of pulp production, further pulp processing steps, such as processes for producing paper or paperboard, and in a manner known in the art, other processes related to a pulp process and different process steps related to the further processing of products other than pulp that are formed and obtained in connection with these particular processes, such as the conventional process step included into a sulphate process for the acid treatment of tall oil soap separated from the pulp, i. e. splitting in order to produce tall oil. Thus the bisulphate product obtained from the treatments of the invention can be used in any preparation step and/or treatment steps for which they are suitable.

The term"acid treatment"denotes in general a treatment performed with any acidic agent, typically an aqueous solution of an acidic agent having a pH below 5, preferably below 4, such as 2.

"Recycling"means that at least a portion of the sulphur or the bisulphate product treated by the method is recycled to the process cycle.

"Bisulphite scrubber"means the treatment of sulphur dioxide obtained from sulphur combustion with an alkaline agent, typically sodium hydroxide or an agent containing sodium hydroxide, in order to produce a bisulphite solution.

The bisulphate product obtained from the treatment method of the invention can be used in various steps of the process in order to partly or totally replace agents which would otherwise be necessary to introduce into the process cycle from the outside, e. g. to replace partly or totally any acid, e. g. sulphuric acid, introduced from outside the process into an acid treatment step.

In a preferred embodiment, the method includes a process step, in which tall oil soap is recovered and subjected to acid treatment in order to produce tall oil, wherein the acid treatment is performed by using a bisulphite solution obtained from an oxidation treatment, alone or together with a second acidic substance. The treatment can be carried out using methods known in the art.

In a second preferred embodiment the bisulphite solution obtained from oxidation treatment can be further used as a raw material for preparing sulphates used in the process, e. g. in the oxygen delignification step (bleaching step) for preparing magnesium sulphate used as a reagent.

The oxidant used in the method of the invention may be any conventional oxidant, such as hydrogen peroxide, oxygen, air, ozone or e. g. organic peracids, such as peracetic acid or perfonnic acid. Preferably oxygen, air or hydrogen peroxide is used, and suitably hydrogen peroxide is used, which is also the most convenient to use. Optionally it is also possible to use a peroxide-containing solution obtained from any other process step for oxidation, such a filtrate containing residual peroxide obtained from pulp bleaching.

It is obvious to those skilled in the art that the oxidation conditions and the amount of oxidant used are selected in conformity with the oxidant used and the desired oxidation degree in a manner known in the art.

The bisulphate formed in oxidation treatment decreases the pH of the bisulphite solution, having a very favourable effect on the further use of the solution in the process. Thus bisulphite can be partly or totally oxidised, preferably partly, in order to obtain the bisulphite amount, preferably the pH of the reaction mixture suitable for the purpose of use in each case.

In one embodiment the bisulphite solution is subjected to the oxidation until the desired pH is achieved for the reaction solution. The degree of oxidation required can be determined e. g. in terms of the pH desired for the reaction solution. The determination can be performed in a manner known in the art, by calculatory means and/or experimental means, depending i. a. on the used oxidant.

When oxygen or air is used as the oxidant the reaction is preferably conducted under overpressure, e. g. over 6 bar, preferably approx. 10 bar, in an autoclave and at raised temperature. Oxygen or air can further advantageously be fed through a mixer with a view to good contact between the gas and the liquid. The use of such pressurised oxidation results in a rapid reaction.

In the method of the invention, the bisulphite obtained from the process cycle can be oxidised to bisulphate with e. g. hydrogen peroxide as the oxidant. In that case, only water will be left as the additional reaction product. At the same time, the pH of the solution decreases substantially. Sulphurous acid (H2SO3) has the acid constants pKa = 1. 8 and pKa2 = 6. 8, whereas sulphuric acid has accordingly pKa = - 3 and pKa = 1. 8. Sodium bisulphite can be oxidised to sodium bisulphate e. g. by means of hydrogen peroxide following the equation (4): NaHSO3 + HsOz- NaHS04 + H2O (4) In other words, the method of the invention allows conversion of the recycled sulphur of the process from a sodium bisulphite form into a sufficiently acidic bisulphate form for it to be suitable for tall oil splitting, for instance. With bisulphite used as such, tall oil will be split only partly and final splitting will require the aid of acid introduced from the outside, e. g. sulphuric acid. The method of the present invention does not necessarily require any added sulphuric acid.

It is known that tall oil soap can be split by means of an external acid, such as sulphuric acid or waste acid from a chlorine dioxide process, providing sufficiently

low pH. The pH of bisulphite obtained directly from the process is not sufficient as such for this purpose. When a bisulphite solution is oxidised to bisulphate in accordance with the invention, this pH range can be reached. It is important to decrease the pH low enough for the fatty acids, which are weak acids, to get into acid form and separated. A bisulphate solution having a pH below 2 would bring a major portion of the fatty acids into acid form. When the pH is about 2 units below the pKa value of the fatty acid, the major portion of the fatty acids is in acid form, not in the form of salt.

In the method of the invention, hydrogen peroxide consumption for stoichiometric oxidation of bisulphite has the pulp ratio 1: 3 H202 : NaHSO3, i. e. peroxide consumption is of the order of 0.33 kg of H202/kg of NaHS03. However, it is preferably not necessary to oxidise bisulphite completely in order to achieve a sufficiently low pH, but partial oxidation may also be sufficient.

The pH decrease achieved as a result of the oxidation reaction of the invention is set forth by means of an example in figure 1. In the figure, the pH of a 10 g/l bisulphite solution decreases rapidly from its initial value of a little above four, when hydrogen peroxide is added. The pH of the bisulphite solution drops significantly even with a small peroxide amount. The pH of the bisulphate solution obtained can be brought to a level suitable for tall oil cooking, in the range pH <2, preferably pH<1. 5, the hydrogen peroxide amount being above 0. 1, preferably above 0. 2 times the amount required for stoichiometric oxidation of bisulphite. Hence the bisulphate solution is more acidic than the bisulphite solution, so that it is considerable more appropriate for use for tall oil splitting, for instance. This allows a substantial reduction at the mill of the amount of sulphuric acid introduced into the process from the outside regarding this purpose of use. Soap splitting by means of sulphuric acid introduced into the process entirely from the outside would produce approx. 2 to 4 kg/Adt of sulphur in the process, so that the sulphur amount would decrease significantly at least in this respect with the use of the method of the invention.

In the operation of the invention, the equipment needed for the oxidation reaction is simple, such as shown in figure 2, for instance. The equipment comprises substantially a mixing vessel 1, in which the reaction between the bisulphite containing recycled sulphur from the sulphate process S and the oxidant takes place.

The equipment may also comprise an oxidant dosing apparatus 2 and an intermediate storage container 3 for bisulphate produced as a reaction product, from

where the bisulphate liquor can be redirected to the desired process point, such as tall oil splitting or magnesium sulphate production, for instance.

Optionally, the bisulphate solution obtained in accordance with the invention can be prepared already in the bisulphite scrubber, producing sulphuric acid and bisulphate mixed with the bisulphite solution. In that case, the materials used in the process equipment shall resist any corrosion caused by the oxidant and the oxidation reaction SO2oSO3. However, this optional embodiment will be more costly than the option mentioned above. The bisulphate solution produced will also contain a certain amount of sulphuric acid.

The recycling, recovery and reuse of sulphur in a sulphate process in accordance with the invention are exemplified in figure 3.

The bisulphate solution is prepared in accordance with the invention by oxidising the sodium bisulphite obtained from the bisulphite scrubber 4 in the mixing vessel 1 by means of an oxidant, which is brought to the mixing vessel 1. The sodium bisulphate prepared in the mixing vessel is conducted e. g. to a raw tall oil cooking reactor 5 or to a reactor 6 for preparing magnesium sulphate. In reactor 6, magnesium sulphate is prepared from a magnesium reactant introduced in the process from the outside, and from sodium bisulphate prepared from recycled sulphur in accordance with the invention. The magnesium sulphate thus produced is used in oxygen bleaching 7. The filtrate from the oxygen bleaching is further conducted to an evaporating plant 8 and from there to the soda recovery boiler 9.

Raw soap is separated from the liquor coming from the washing plant 11 and going to the evaporating plant 8 during the preparation of raw tall oil, the raw soap being conducted to a cooking reactor 5, where it reacts into raw tall oil by means of bisulphate. Raw tall oil 12 is removed from the process from the tall oil cooking reactor 5 and the remaining sulphurous mother liquor is conducted through the evaporating plant 8 to the soda recovery boiler 9 for combustion of organic compounds and sulphur. In the soda recovery plant 9, the sulphur compounds are burned to sodium sulphide. From the soda recovery boiler 9, an aqueous solution of sodium carbonate and sodium sulphide, green liquor, is conducted to causticisation 13, where sodium carbonate is converted into calcium carbonate and the sodium hydroxide-sodium sulphide solution thus produced, white liquor, is recycled to the pulp digestion 10. The remaining calcium carbonate is further regenerated in the lime kiln 14, forming calcium oxide. The major sulphur dioxide source is the

malodourous gas combustion 15, where reduced sulphur compounds, mainly H2S, methyl mercaptane, dimethyl mercaptane and dimethyl dimercaptane, are incinerated to sulphur dioxide and are further conducted to the bisulphite scrubber 4. Likewise, besides malodorous gas combustion, sulphur dioxide from any other sources can be recovered. In the bisulphite scrubber 4, sulphur dioxide is absorbed into the NaOH solution, producing sodium bisulphite. The sodium bisulphite produced in the bisulphite scrubber is further fed to oxidation in the mixing vessel 1 in accordance with the invention.

The procedure of the invention allows the use of waste acid from the chlorine dioxide preparation 16 to be avoided either partly or completely and thus a reduction of sulphur introduced into the process from the outside.

In one embodiment of the invention, splitting is performed in one single step using a sodium bisulphate solution prepared in accordance with the invention as the splitting acid, the oxidant being used in a stoichiometric amount. In some cases it may be advantageous to use an excess of oxidant. Optionally, the bisulphate may be only partly oxidised. Splitting can also be performed with the use of a mixture containing the recycled bisulphate of the invention as a part, e. g. a mixture of sulphuric acid and bisulphate. This sulphuric acid can also be prepared from sulphur dioxide obtained by recycling by means of hydrogen peroxide, by oxidising first the sulphur dioxide to sulphur trioxide and further to sulphuric acid. It is previously known to prepare sulphuric acid by oxidising sulphur dioxide directly to sulphur trioxide. The procedure of the invention avoids investments in a separate sulphuric acid plant.

In an embodiment of the invention, tall oil splitting is performed in two or more steps. In a two-step method, the first step may comprise a bisulphate solution prepared in accordance with the invention and the second step some other acid, such as e. g. fresh sulphuric acid or waste acid from a chlorine dioxide reactor, or a mixture of acids. Optionally, the first step may involve the use of a sodium bisulphite solution obtained directly from the process or carbon dioxide, with the sodium bisulphate solution of the invention used only in the second step. Then a smaller amount of bisulphate and thus peroxide will be needed than in a single-step method. Optionally, in two-step tall oil soap cooking, the bisulphite solution characteristic of the invention is used in both the steps.

If the sulphur amount used for splitting is 2 to 4 kg s/Adt, the required stoichiometric amount of peroxide would be roughly the same, i. e. 2 to 4 kg.

However, the amount is not necessarily exactly stoichiometric, as can be seen in figure 1. Preferably, half of this amount, above 0.15 g H20/gof NaHS03will be sufficient. Most advantageously the amount of hydrogen peroxide addition is above 0.20 g of H2O/g of NaHSO3.

A magnesium compound used in oxygen bleaching can be prepared by utilising sulphur recovered in a sulphate process cycle, and then it will not be necessary to use magnesium sulphate from the outside, which would increase the sulphur load.

Recycled sulphur can be utilised as sulphur dioxide, bisulphite or sulphite, which is reacted into magnesium sulphate by means of an oxidant.

Sulphur-free raw materials of a magnesium compound may comprise e. g. magnesium hydroxide, magnesium oxide or magnesium carbonate. Technical MgO is preferably the magnesium source.

In one embodiment of the invention, the magnesium compound is dissolved in the bisulphate liquor obtained from the sulphate process using the reaction equation (5), yielding magnesium sulphate.

2 NaHS04 + Mg (OH) 2->Na2S04 + MgS04 + 2 E O (5) One optional way for preparing magnesium sulphate is performing the reaction already in the gas scrubber, where the gas containing SO2 is scrubbed with water containing an oxidant, such as hydrogen peroxide, and Mg (OH) 2. In that case, peroxide consumption will be in the range from 0.28 to 0. 33 kg/kg of MgS04.

A further optional way of preparing magnesium sulphate is dissolving the magnesium compound first in a bisulphite solution and subsequently oxidising the magnesium sulphite thus formed to magnesium sulphate, using hydrogen peroxide, for instance. Magnesium sulphite as such is a reducing compound, which is not usable in a bleaching process, because it consumes other chemicals unnecessarily.

Because MgS04 contains 26.6% of sulphur, the method of the invention produces a sulphur amount less than 0.26 to 0. 8 kg/ton of pulp, with the amount of MgS04 dosed in oxygen bleaching being 1 to 3 kg per ton of pulp.

The invention thus provides new very useful options for recycling sulphur in a pulp production process, e. g. a sulphate cellulose process.

The invention also provides for the use of the bisulphate liquor obtained from oxidation treatment as an acidic agent in acid treatment in different process steps, preferably in tall oil soap splitting. It further provides for the use of the bisulphate liquor obtained in the treatment as the raw material in the preparation of reagents used in the process, preferably magnesium sulphate.

The following is a detailed description of a number of features specific for the invention, however, without limiting the invention to these: Example 1 Examination of a sulphate process, from which excess sulphur is removed in the form of fly ash. Then the sodium entrained by the sulphur needs to be compensated with fresh liquor introduced in the process. The liquor demand is 2.5 kg of NaOH/kg of removed sulphur. If the amount of removed sulphur is 4 kg per ton of pulp, the equivalent NaOH demand is 10 kg per ton of pulp.

This is compared with the sulphate process of the invention. If the sulphur amount is decreased by recycling the sulphur contained in bisulphite through tall oil soap splitting, the amount of hydrogen peroxide needed for the oxidation of this sulphur amount is over 4 kg.

Calculated with current chemical prices, the price of hydrogen peroxide will be 2.1 . If sulphur is removed in the form of fly ash, the price of the sodium amount needed would be 3. 2, in other words, the cost effect is 1.5 fold compared to the process of the invention.

Example 2 A sodium bisulphite solution NaHSO3 obtained from the sulphate cellulose process in a concentration of 43 g/l and with pH 4 was oxidised by adding a 50% by weight hydrogen peroxide solution. Figure 4 shows the pH variation of the solution as a function of the peroxide addition. A rate of 12 g/l of Mg (OH) 2 was readily dissolved

in a bisulphite solution to which 11 g/1 of H202 had been added, the final solution containing 24.9 g/1 of MgS04. The solution also contained sodium sulphate.

Magnesium hydroxide appeared to be dissolved at a slightly slower rate in a non- oxidised bisulphite solution. Oxidation of magnesium bisulphite thus obtained by adding hydrogen peroxide at a rate of 11 g/l still yielded the same end result as oxidation of the bisulphite before the dissolution.

The magnesium sulphate solutions thus prepared have exactly the same function in oxygen bleaching and peroxide bleaching as pure magnesium sulphate introduced from the outside.

The accompanying sodium sulphate was not observed to cause any negative effects.

Example 3 A comparison of the ratio sulphur to sodium in two cases of a sulphate cellulose process A and B. In the first case A, sulphur was not recycled and the total amount of sulphuric acid required was introduced into the process from the outside, and sodium bisulphite was removed from the process. In the second case B in accordance with the invention, which is described above, bisulphite was oxidised to bisulphate by means of hydrogen peroxide using an addition of the required amount of sulphuric acid introduced from the outside.

Figure 5 illustrates these two cases A and B and their sulphur-sodium ratios at different stages. Step 1 illustrates the sulphur amount entrained by sulphur, step 2 illustrates the necessary sodium compensation (sodium hydroxide), step 3 introduces sulphur in the process along with oil, wood and MgS04, step 4 illustrates the removal of ash salt, fly ash, sulphur dioxide and soda precipitate, among other things, while step 5 illustrates wash losses and step 6 removal of other substances.

In case B, bisulphite was recovered using recycled white liquor, and then no fresh sodium was introduced into the system along with sodium bisulphate. The sulphur entrained by bisulphate entered the cycle normally and does not appear in the balance ratio.

With the procedure of case B, the sulphuric acid entering the system was decreased by compensating 2 kg of sulphur, i. e. 6.1 kg of sulphuric acid, with a bisulphate liquor prepared by oxidising bisulphite.

Figure 5 shows how the need for compensating NaOH decreases notably. With hydrogen peroxide used as the oxidant, the peroxide consumption depended on the oxidation degree of bisulphite. If bisulphite is oxidised to 100%, 2.1 kg of H202 will be required, and with a 50% oxidation degree the requirement will be 1.1 kg of H202 accordingly.

Example 4 4 1 of a sodium bisulphite solution obtained from the sulphate process, NaHS03, in a concentration of 250 g/l, was taken and placed in an autoclave with a volume of 8 1. The autoclave was equipped with a gas circulating stirrer, with oxygen used as the gas. The autoclave temperature was regulated to 80 °C and it was supplied with pure oxygen so as to generate an overpressure of 6 bar in the autoclave. A sample of the solution was taken at the initial moment t = 0 and several additional samples were taken during the oxidation process. The pH of the samples was measured and the sulphite and sulphate ion concentrations were analysed. Figure 6 shows the pH development in the course of oxidation as a function of time, and figure 7 the sulphate and sulphite ion concentrations, accordingly.

The pH of the bisulphite solution drops rapidly to below three at the initial stage of the oxidation and the decrease continues as the oxidation proceeds. The increased acidity is clearly due to the almost total oxidation of bisulphite to bisulphate. The test shows that the use of oxygen as the oxidant allows the appropriate pH range to be reached, the recycled solution being then apt for reuse in the splitting of tall oil soap.

Example 5 A 300 ml sample in a concentration of 44 g/1 was taken from a sodium bisulphite solution NaHS03 obtained from a sulphate cellulose process, the sample was placed into an open bottle having a glass sinter at its bottom. The bisulphate solution was supplied with oxygen through the glass sinter at a flow rate of 1 1/min of 02 as

measured by a rotameter. Figure 8 shows the pH variation of the solution as a function of time as oxidation proceeds.

The pH of the bisulphite solution dropped rapidly at the beginning of the oxygen feed and stabilised to slightly less than three.

Example 6 A process solution of sodium bisulphite having a volume weight of 1169 g/1, pH 5.6 and a 2-proportion of 15% was oxidised with hydrogen peroxide (50 w%) to sodium bisulphate. After oxidation, the solution contained 263 g/1 of sodium bisulphate. This solution was used for splitting tall oil soap.

400 g of tall oil soap was measured in a 2 1 vessel. When mother liquor was used for splitting, it was added at this stage. Using sulphuric acid, as in the comparative test, 300 g of water was also added. The insulated vessel was heated to a temperature of 100 °C and the acidic solution used was added until the pH dropped to below 3.

With the use of a sodium bisulphite solution, 50% of the required amount of sulphuric acid was compensated with this.

When the pH had dropped to below 3, the sample was boiled for about 15 minutes at a temperature of 100 °C. After this the product was poured into an extraction funnel. At the end of 30 minutes, three phases separated : tall oil, lignin and mother liquor. Tall oil was separated and recovered and the volume, volume weight and pH of the solution were measured, after which the tall oil yield was calculated. In part of the tests, the phases were separated by centrifugation. These test are marked with an asterisk.

In each test, about 400 g of tall oil soap and 233 ml of bisulphate liquor were used for splitting.

The tests used two different industrial tall oil soap samples, a and b. Table 1 shoes by means of comparison splitting using sulphuric acid and in Table 2 the splitting has been performed with a mixture of sodium bisulphate and sulphuric acid, in which bisulphate has been produced by oxidising the bisulphite solution from the process with oxygen. In Table 2, the oxygen ratio stands for the ratio of sulphuric acid obtained from sodium bisulphate to the added sulphuric acid.

Table 1 Test no Raw soap H2SO4 kg/t Yield % Acid Soap of raw soap number number mg value mg KOH/g KOH/g 1 a 94.2 68 152 0.6 2 a* 109 68 152 0.4 3 b 104 63 153 0. 6 4 b* 106 64 156 0.4 Mean value 103 66 153 0. 5

Table 2 Test no Raw Total Acid ratio Yield% Acid Soap soap H2SO4 NaHS04/H2S04 number number kg/t of value mg mg raw soap KOH/g 6 a* 122 0. 539 75 157 0.9 7 b 137 0. 539 74 157 1.5 b* 129 0. 539 74 158 0.9 Mean 129 0.539 74 158 1.1 value

The raw tall oil thus prepared had excellent acid number values corresponding to the fat and rosin acid content, which should have a value > 150. The average soap value, which corresponded to the soap amount in raw tall oil, meets the requirements on high-quality industrial tall oil. The yield was much better than with the use of sulphuric acid (cf. table 1).