WO2000079040A1

WO2000079040A1 - Process and device for oxygen delignification giving improved kappa reduction

Info

Publication number: WO2000079040A1
Application number: PCT/SE2000/001243
Authority: WO
Inventors: Petter Tibbling; Erik Nilsson; Andreas Christoffersen
Original assignee: Kvaerner Pulping Ab
Priority date: 1999-06-17
Filing date: 2000-06-15
Publication date: 2000-12-28
Also published as: SE522161C2; SE9902285L; SE9902285D0

Abstract

The present invention relates to a process and a device for the oxygen delignification of cellulose pulp suspensions giving improved kappa reduction. According to the process in accordance to the invention, the return liquor (40) which is recovered from a displacement or washing device (20) after an oxygen stage (/17) is subjected to wet oxidation under pressure (30). The wet oxidation process is accelerated by the liquor being pressurized in an oxygen-containing media and preferably at an elevated temperature. The wet-oxidized return liquor is supplied to the cellulose pulp such that the wet-oxidized return liquor constitutes the bulk of the liquid part of the cellulose pulp suspension before the latter is conducted to an oxygen delignification stage. Sodium hydroxide is also supplied (35) in a quantity which is such that the oxygen delignification can take place in a buffered medium such that the pH is kept essentially constant during the whole of the oxygen delignification. The invention provides an optimum kappa reduction at minimum cost.

Description

Process and device for oxygen delignification giving improved kappa reduction

The present invention relates to a process and a device for oxygen delignification giving improved selective kappa reduction in accordance with the preamble of Claim 1 and the preamble of Claim 14, respectively.

STATE OF THE ART Several different methods for improved selective delignification in conjunction with oxygen bleaching have been disclosed.

SE,C, 223066 presents a method in which lignin dissolution and bleaching of the pulp take place under the influence of oxygen in an alkaline medium, with a catalyst such as calcium carbonate being added to the pulp in a quantity corresponding to from 0.5 to 3% by weight. SE,C, 314581 presents a similar method to that of SE,C, 223066 in which different additives are supplied to the process. This patent also mentions that it is possible for oxyliquor/return liquor to be recirculated from a washing stage after the oxygen treatment, after which the washing filtrate is recirculated for the purpose of diluting the cellulose pulp suspension prior to the oxygen treatment .

SE,C, 341520 presents another method for lignin dissolution and the bleaching of pulp in which the alkaline pulp is firstly treated with an acid medium, followed by a wash, prior to the oxygen treatment. It is demonstrated in this patent, too, that it is possible to recirculate oxyliquor/return liquor from a washing stage after the oxygen treatment, after which the washing filtrate (the oxyliquor/return liquor) is recirculated for the purpose of diluting the cellulose pulp suspension prior to the oxygen treatment. The recirculation of oxyliquor takes place for economic reasons with regard to recovering chemicals and minimizing discharges. No advantages for the properties of the pulp are claimed and it is only stated that the recirculation did not result in any harmful ( ! ) effect on the final brightness.

SE,C, 363138 presents yet another method for lignin dissolution and pulp bleaching under the influence of oxygen, in an alkaline medium and using a catalyst.

SE, C, 418202-1 also demonstrates recirculation of oxyliquor while oxygenating under atmospheric conditions and that the cellulose pulp suspension which has been diluted with this oxygenated oxyliquor is then pretreated in a pressure vessel at a temperature of about 73-149°C and under a pressure of up to 21 bar. This pressurized treatment will therefore have an effect both on the recirculated oxyliquor and on the cellulose pulp suspension which is mixed in with it. SE,C, 360.128 presents yet another variant which has the aim of improving the oxygen bleaching and in which a relatively small quantity of spent cooking liquor is retained in the cellulose pulp suspension and a relatively large part of the liquor liquid is replaced with recirculated bleaching liquor/oxyliquor . It is stated that, in a preferred embodiment, the spent cooking liquor can be treated with a preoxidation, involving the addition of oxygen and expediently under pressure, after which the oxidized spent cooking liquor is finally displaced/replaced with the spent bleaching liquor.

A problem with using oxygen is that it is not possible to efficiently admix more than a certain quantity of oxygen with the cellulose pulp suspension. The above- disclosed solutions have not been sufficiently successful and, for that reason, attention has focused on double oxygen stages.

Examples of such double oxygen stages are presented in SE,C,505.141, SE,C, 507.870 and SE, C, 507.871, in which different combinations of pressure, temperature, dwell times and additions of oxygen and alkali to the two stages are claimed to achieve special effects. However, it is a well known fact that the second reactor makes a very modest additional contribution to the delignification task, frequently only 1-3 units when calculated as a kappa number.

A problem associated with all these solutions is that only a limited quantity of oxygen can be admixed with the cellulose pulp suspension. This quantity of oxygen reacts with released organic substances which are present m the cellulose pulp suspension and with organic substances which have not been released from the lignin. Consequently, only a part of the maximum quantity of oxygen which can be supplied will contribute actively to the delignification task, thereby resulting in an inferior delignification effect and increasing the requirement for extended delignification times, frequently resulting in the introduction of double oxygen reactors.

A related problem is the efficiency m the pulp wash or the drainage properties of the liquor before bleaching the pulp m the oxygen reactor. Carbon dioxide can be added to the cellulose pulp suspension prior to washing the pulp with the aim of improving the wash, see, for example EP, C, 296.198 ; this improves the oxygen delignification as a result of the COD being lower in the pulp entering the oxygen delignification stage and as a result of selectivity being increased. While the underlying causes are uncertain, both the pH and ionic strength of the liquor and the precipitation of resins and calcium carbonate are considered to have a powerful effect. While the addition of carbon dioxide to the wash also results in the COD in the pulp entering the bleaching department being reduced and n the requirement for bleaching chemicals decreasing, it also results in there being a decrease m the need for evaporation and in the need for cleaning deposits in the process equipment .

BRIEF DESCRIPTION OF THE INVENTION An overall aim of the present invention is to improve the kappa reduction in the pulp which is being treated in an oxygen stage while maintaining an optimally high viscosity, i.e. to achieve an increased selectivity in the oxygen delignification. Using the invention, it is possible to achieve a good 70 per cent degree of delignification from a kappa number of 30 without any undue loss of viscosity.

Another aim is to treat return liquor, which is recovered from washing equipment after an oxygen stage, in such a way that the bulk of the readily oxidized part of the return liquor which is recovered from the washing equipment can be oxidized with oxygen- containing gas in a treatment vessel, hereinafter termed oxygenator, before it is admixed once again with the cellulose pulp suspension. This oxygenator can also be termed a mineralizer since its function is to partially convert organic carbon into inorganic carbon, for which reason the term carbonator also describes its function in all essentials.

The oxygenator is expediently fabricated as a small pressure vessel which is capable of being pressurized in the range 1.5-15 bar, preferably 10-12 bar. The volume of the oxygenator is determined such that the dwell time of the return liquor which is recirculated from the washing stage in question is 5-30 minutes, preferably 5-20 minutes. The return liquor is preferably heated in the oxygenator to a temperature of 80-120°C, preferably 95-100°C.

Yet another aim is that the wet oxidation of the return liquor in the oxygenator should result in carbon dioxide being primarily generated internally in a quantity which is such that this substantially reduces the need to add external carbon dioxide or carbonates and may entirely eliminate the need for the expensive addition of external C0₂, something which is otherwise practised widely in some 20-30 fibre lines.

The return liquor which is wet-oxidized in this way is used as washing or displacement liquid in the pulp wash, with the washing resulting being improved to a degree which corresponds to that achieved by processes in which external carbon dioxide is added. This improvement in the pulp wash has direct effects on the oxygen delignification.

Yet another aim is to reduce the need for double oxygen stages, which take up a great deal of space and are also expensive. At least the same effect, and frequently a substantially superior effect, on the delignification task can be achieved in only one oxygen stage which is combined with an efficient oxygenator for returning extracted washing liquid. In certain cases, an improved kappa reduction of 4-6 units is obtained with only one oxygen stage which is combined with an oxygenator, whereas a second oxygen stage only achieves a kappa reduction of 1-2 units. The possibility is accordingly provided of improving kappa reduction by up to 200% as compared with using a second oxygen reactor.

An aim of an advantageous embodiment is for the oxygen delignification to be able to take place under an essentially constant pH which is at an optimally high level as a result of the wet-oxidized liquor being carbonated, with the carbonates being generated internally by means of oxidation.

Yet another aim is to enable existing installations to be converted in a very cost-effective manner, with only a relatively small oxygenator being required as an additional appliance, and it being possible to obtain a kappa reduction of 4-6 units.

Yet another aim is for the deligibility of the pulp production, and its interaction with the cooking department, to increase, since the kappa number of the pulp can be regulated in a simple manner by adjusting the NaOH concentration and/or the temperature in the oxygen reactor. For example, if a higher viscosity should be required in association with a normal kappa number, the temperature can instead be lowered in the oxygen reactor while otherwise maintaining an unaltered, constant pH. The pressure in the oxygen reactor as well as in the oxygenator should be kept at a maximally high level.

Yet another aim is to allow a very suitable process position for expelling manganese, calcium, magnesium and resin, etc., since the oxygenator, with its process conditions, promotes the precipitation of unwanted substances by means of floatation, sedimentation or filtration. This in turn results in a decrease in encrustation formation in the process equipment along the whole of the fibre line and consequently in the subsequent bleaching department as well. The system will give rise to an improved 0₂ bleaching process, resulting in an essentially resin-free and more pure intermediate product, which will provide good bleaching economy, high yield and strength in association with ECF/TCF final bleaching. This latter stands in relation to the virtually complete elimination of transition metals such as manganese, decisively decreasing peroxide consumption and increasing the carbohydrate content of the pulp in the final bleaching. The solubility of fatty acids and resin acids decreases markedly as the ionic strength, i.e. first and foremost the sodium carbonate content, of the liquor increases. This enables resins such as liquid-crystalline soft soap and salted-out sodium soap to be separated off mechanically from the wet-oxidized liquor.

The abovementioned advantages lead to the pulp process being improved in a number of respects. Under special circumstances, it is possible to obtain a 10% higher production when using externally added carbon dioxide or, in accordance with the invention, by using internally generated bicarbonate. At the same time, the requirement for bleaching chemicals can be reduced. If the washing stages are rendered more effective by using wet-oxidized return liquor as displacement liquid, this then results in 20 and 30% lower COD in the pulp entering and leaving the oxygen gas stage, respectively.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described below with reference to the figures, of which: Fig. 1 shows process equipment for producing cellulose pulp; Fig. 2 shows a titration curve with a linear scale, and Fig. 3 shows a titration curve which is in accordance with Figure 2 but which has a logarithmic scale .

DETAILED DESCRIPTION OF THE FIGURES

The concept of the invention will be described below with reference to process equipment which illustrates the principle of the concept. The essential features are wet-oxidation of oxyliquor/return liquor/return liquor/spent liquor in a pressurized, oxygen-containing environment, followed by impregnation/leaching of the cellulose pulp in this wet-oxidized oxyliquor, thereby amplifying the degree of delignification and the selectivity in the actual oxygen stage. The impregnation and leaching are preferably carried out under atmospheric conditions in order to achieve maximum selectivity and can be inserted separately or in combination with each other.

Process equipment Figure 1 shows a possible configuration of process equipment, which comprises a thick-pulp tower 10, a screening department 11, a screening department press 12, a steam mix 13 and a thick-pulp pump 14, followed by an intermediate storage tower 15, which holds the cellulose pulp suspension for 10-20 minutes. The cellulose pulp from the screening department press 12 can, of course, fall freely down to the said intermediate storage tower 15. There then follows the true oxygen stage (one or more oxygen stages coupled in series), with mixing equipment 16 for metering liquor, oxygen and steam into the cellulose pulp suspension before it is conducted to the oxygen reactor 17. After the oxygen stage (16+17), there then follows a flash tank 18 in which residual gases are blown off. After the flash tank, the cellulose pulp suspension is pumped onwards, using a pump 19, to two washing appliances 20, 21. The first washing appliance can be in the form of an atmospheric diffuser 20 and the second washing appliance can be in the form of a wash press 21. The washing appliances can be of different types, and the atmospheric diffuser can, for example, be replaced with a pressure diffuser.

According to the invention, the washing liquid, flow 40 in Figure 1, which is extracted from the first washing appliance, which washing liquid is hereinafter termed returned liquor (also oxyliquor) and in the main consists of the liquid part of the cellulose pulp suspension leaving the oxygen reactor, is to be dealt with separately.

Wet oxidation

The return liquor 40 is conducted to what is hereinafter termed an oxygenator 30. The oxygenator receives the extracted return liquor which is obtained from the washing appliance 20, and pressurizing to a pressure within the range 1.5-15 bar, preferably 10-12 bar, takes place in the oxygenator. In addition, oxygen is introduced into the oxygenator, with this expediently taking place using an in-line mixer 31 or a sparger (sparger = addition of oxygen through a tube made of porous, frequently sintered material) directly prior to, or in the inlet part, of the oxygenator. A pump 32a can be used for pressurizing the oxygenator to the requisite pressure, against the action of a regulatable throttle valve 32b, which is arranged in the outlet of the oxygenator. The throttle valve 32b can in particular be a pressure-reducing valve which opens at a preset pressure, for example 10-12 bar. The volume of the oxygenator is adjusted such that the dwell time in the oxygenator of the return liquor 40 which is extracted from the washing stage 20 is approximately 5-30 minutes, preferably 5-20 minutes. In order to improve the reaction conditions in the oxygenator still further, a heat exchanger 33 can be employed to heat the return liquor which is present in the oxygenator using low-pressure or high-pressure steam from a suitable source 34 within the cellulose pulp process. Preferably, the return liquor is heated in the oxygenator to a temperature of 80-120°C, preferably 95-100°C. The heating can also be effected by supplying steam directly to the pulp suspension. The oxygenator operates in the pH interval 8.5-12.5, preferably 10-11.7, and its performance is characterized by the degree of mineralization of the liquor and the mean valence number of the carbon in released organic compounds.

The degree of mineralization of dissolved and released substance is defined as TIC/ (TIC+TOC) , i.e. "Total Inorganic Carbon" and "Total Organic Carbon", quantities which are determined in a special laboratory analyser. TIC can be converted to Na₂C0₃ by multiplying by the factor 106/12. It can be noted that the wet-oxidized liquor has a TIC content which is approximately 2.5 times higher than that of conventional return liquor. This ^' corresponds to 15 grams of Na₂C0₃, or 0.14 mol of Na₂C0₃, per litre and explains the buffering effect of the liquor below pH 11.7, which is the pH of a pure solution of sodium carbonate whose concentration is 0.1-0.2 mol/litre. The mean valence number of the carbon of a dissolved substance is defined as 4* (TOC-COD) /TOC . It is directly related to the combustion value of a substance as well as to the 0₂ consumption of the liquor during oxygen delignification. The value is -1 for an extraction liquor, while it is between -0.3 and -0.5 for a conventional oxygen spent liquor. In the case of wood substances, the value for released lignin is about -1 while that for carbohydrates is about 0.

Buffering with sodium hydroxide

In order to obtain effective delignification in the oxygen reactor 17 at a high and essentially constant pH, the liquor system should be buffered and adjusted to a pH level within the interval 11.7-13.0. The compounds concerned here are readily oxidized organic compounds which, in association with wet oxidation, have been converted from organically bound carbon, i.e. TOC (Total Organic Carbon) , to inorganically bound carbon, i.e. TIC (Total Inorganic Carbon), which inorganic carbon is recovered in bicarbonates and/or carbonates, which bring about the desired buffering. The addition of NaOH provides the desired pH adjustment.

The figure shows an admixing device 35 which is placed in close conjunction with, and preferably directly after, the oxygenator and in which sodium hydroxide is admixed. In certain embodiments, the admixing device, indicated in dashed outline as 35b, can be located in a first part flow 41 from the oxygenator, with a second part flow 42 being conducted directly to a washing appliance in order to obtain an optimum washing result. As a result of this admixing of sodium hydroxide, water and sodium carbonate are formed when the sodium hydroxide reacts with the carbon dioxide which was formed on reaction with organic material when the return liquor was treated with oxygen in the oxygenator . The reaction follows the process steps: a) C + 0₂ = C0₂ in which the released organic material in the return liquor reacts with the oxygen and forms carbon dioxide, b) C0₂ + OH^" = HC0₃ ^" in which the hydroxide which has been supplied reacts with the carbon dioxide and forms bicarbonate; c) HC0₃ ^" + OH^" => CO3^2" + H₂0 in which the carbonate and bicarbonate ions give the liquor a buffering capacity and the pH value can be kept essentially unaffected despite the fact that OH ions are being consumed continuously as a result of the lignin in the cellulose pulp being oxidized during the delignification.

The carbon dioxide which is generated in the oxygenator is present as CO3^2", HC0₃ ^" or H₂C0₃ depending on the pH, in accordance with the following equations;

at pH 6.3, when a 50/50 molar equilibrium exists between H₂C0₃ and HC0₃ ^"; and

at pH 10.3, when a 50/50 molar equilibrium exists between HC0₃ ^" and C0₃ ^2"; when the pH then increases to 11.6, the equilibrium is displaced such that essentially all the organic carbon is present in the form of carbonate ions.

The quantity of sodium hydroxide which is added during the reaction process b above is to be determined so as to ensure that a buffering concentration of formed Na₂C0₃ which at least corresponds to 0.1 mol of Na₂C0₃ per litre, and which gives rise to a resulting pH of more than 11.6 in the liquor liquid when the pulp suspension is conducted to the oxygen reactor, is maintained in the wet-oxidized return liquor.

Foam separation

The oxygenator affords a splendid opportunity for separating off unwanted constituents, frequently in the form of foam or flocks which contain manganese, other transition metals, calcium, resin or other fine material, and also of separating off a bulk of difficultly soluble calcium/magnesium together with carbonate/hydroxide/silicate. Taken together with resin, so-called null fibre and fibre dust constitute an important organic constituent in this bulk which, when finally divided in the pulp, gives rise to problems with regard to bleachability, purity, resin content and drainage. The use of effective means for separating off these foamed constituents in the oxygenator substantially reduces the risks of clogging in the remaining parts of the pulp process. Adding magnesium sulphate to the oxygenator promotes the process of precipitating unwanted constituents, since adding this substance forms seeds for growing the organic/inorganic sediment. A certain quantity of magnesium sulphate is normally added prior to the oxygen reactor, and half of this magnesium sulphate should preferably be added to the oxygenator.

The oxygenator operates at an overpressure of about 10-12 bar, at 90-100°C and at a pH of between 8.5 and 12.5, which promotes a floatation process. Mn, together with Ca, Mg and resin, are enriched in flocks which float on the surface of the wet-oxidized return liquor. The foam which has formed is blown or scraped off at the top of the oxygenator; alternatively, it is separated off by means of a subsequent filtration or sedimentation in a backwater tank. For example, unwanted material can be separated off without using auxiliary chemicals, such as polyethylene oxide, which are only added to encourage the separation tendency. The use of complexing agents for expelling manganese m peroxide bleaching (EDTA, DTPA) can also be reduced, or entirely eliminated, in the final bleaching.

The following table shows the effects of the wet oxidation according to the invention on the possibility of filtering off manganese. Studies carried out on untreated return liquors show that the manganese content can vary between 20 and 80 mg/1; nevertheless, there has been no difference m the content of manganese in filtered and unfiltered liquors m any of the tests performed. It has not been possible to observe any effect of mechanical separation, for example any decrease m the manganese content following filtration. By contrast, both TIC and the degree of mineralization increase as soon as the liquor is wet-oxidized, with mechanical separation by means of filtration (see the last line m the table) becoming what is essentially a complete process for separating off manganese. Very probably, the increased concentration of carbonate is able to generate readily separable, crystalline coprecipitates of Mn/Mg/Ca, with CaC0₃, at least, being substantially less soluble than Ca(OH)₂.

There are several conceivable alternatives for separating off unwanted constituents which have been precipitated in the oxygenator or directly after the oxygenator. This can take place by;

- flotation processes, in which unwanted material is flocculated on the surface

- sedimentation processes, in which unwanted material is allowed to sink to the bottom of sedimentation vessels, or

- filtration processes, in which the wet-oxidized return liquor is caused to pass through some type of filter.

Filtrates are recirculated as washing after the cooking department, while thickened sludge can be pressed or treated with the remaining fibre-containing extraction from the cooking department.

Figure 1 shows this separation process as a filtration through the filter 50, where the unwanted material which has been separated off is removed in the flow 51.

Gas separation

Oxygen which does not react but which instead forms an accumulation of gas at the top of the oxygenator can be separated off using suitable means (not shown) and added to admixing mixers prior to the oxygen reactor or reactors .

Return of liquor liquid which is wet-oxidized in the oxygenator

Liquor liquid which is wet-oxidized in the oxygenator can be returned either by a long loop or by a short loop, i.e. the part flows 42 and 41, respectively, which are shown in Figure 1. The short loop involves the wet-oxidized liquor liquid being introduced, as dilution liquid, at the bottom of an impregnation vessel 15, which is shown in the figure as an intermediate storage tower 15. This dilutes the pulp before it is conducted to an oxygen stage 16+17. The wet-oxidized liquor liquid is typically supplied to the cellulose pulp in the intermediate storage tower so as to obtain a ratio of 9 tons of liquid per ton of pulp in the pulp suspension which is conducted to the oxygen reactor. The consistency of the cellulose pulp suspension which is fed into the intermediate storage tower/impregnation vessel 15 is typically 25-35%. The atmospheric leaching and the equilibrium in this stage is a very important element for the selectivity, before the pulp is contacted with oxygen under pressure.

The long loop involves the wet-oxidized liquor liquid being introduced, as washing and/or displacement liquid, into a washing appliance or press 12 in such a way that the liquid which is present in the cellulose pulp suspension before the latter is supplied to the washing appliance 12 is displaced by the wet-oxidized liquor liquid. After the screen press, the pressed cellulose pulp suspension retains a substantially higher proportion of the wet-oxidized liquor liquid in its remaining liquid part, something which additionally increases the proportion of wet-oxidized return liquor in the liquid part of the pulp suspension which is conducted to the oxygen reactor.

Alkali distribution

An important advantage of the present invention is that the distribution of alkali, i.e. sodium hydroxide, can be optimized in dependence on the pH which is desired in each position in the process. For example, the washing result is promoted by a low pH and a high carbonate content, and consequently the wet-oxidized liquor liquid can be conducted directly to a screen department press or a wash press without any sodium hydroxide being added. In order to ensure that the pH does not decrease on the cooking department side, the amount of alkali added can also be increased in one of the cooking department circulations. The greater part of the quantity of sodium hydroxide which is required, together with the carbon dioxide which is formed in the oxygenator, for forming the buffering sodium carbonate content is added to the admixing appliance 35, alternatively 35b. Alkali is added to the mixture 16 with the principal aim of achieving the requisite alkaline medium for the oxygen stage.

The total quantity of NaOH entering the reactor amounts to a maximum of 50 kg ptp in total, preferably 20-35 kg ptp in total in the case of softwood, with approximately 5-10 kg of this amount preferably being added to the oxygenator and 20-30 kg being added to the mixer 16. However, in certain applications, the addition of alkali to the oxygenator can be minimal, especially m applications m which a less alkaline, bicarbonate-buffered medium is sought m the washing stage m the long loop.

Excess of alkali m the form of carbonate (C0₃ ^") will recirculate the return liquor. While the invention can to some degree result m a higher consumption of oxidized white liquor (and also oxygen) , depending on the mode of operation, the increased cost is only a fraction of the cost for a system where, for example, carbon dioxide is added (corresponding to EP, C; 296198) .

The somewhat higher consumption of oxidized white liquor, of the order of size of 10-25%, brings about some degree of reduction m the thermal loading on the recovery boiler since the oxygenator oxidizes a certain quantity, in the order of size of 0.5-4%, of organic dry substance which would otherwise have been carbonated m the recovery boiler. It should also be remembered that, with the higher consumption of oxidized white liquid, it is possible to add less white liquor to the last cooking department circulation. Carbonate instead of bicarbonate will of course be returned to the cooking department process.

First comparative experiments

5 different examples of delignification processes in alkaline medium will be compared below.

Unbleached craft pulp, consisting of softwood, principally pine, with a kappa number of 29.4 and a viscosity of 1180 dmVkg, was used in all the experiments .

The return liquor (oxyliquor/return liquor) which was used m some experiments for diluting and leaching the cellulose pulp suspension is taken from a washing appliance located immediately after the oxygen stage in a mill line which also included a continuous cooking department, from which the said pulp sample is taken.

Experiment A is a simulation of the process conditions of the mill, m which neither wet oxidation of return liquor nor leaching were practised. Oxygen delignification takes place by oxygen being added to the pulp under temperature/time conditions which correspond to approximately 105°C for 1 hour of effective reaction time in a continuous process, and the quantity of oxygen which is added to the pulp suspension is 20 kg of 0₂ ptp (ptp = per ton of pulp) . The partial pressure of the oxygen will to all intents and purposes follow approximately the same profile as in the continuous oxygen reactor, i.e. from 8-9 bar to approximately 3-4 bar, with the latter pressure being at the top of an oxygen reactor.

Experiment B includes leaching, and the pulp is therefore first treated with the return liquor and an addition of NaOH at atmospheric pressure. For the leaching, the liquor is mixed with pulp at 20% consistency and is placed in an autoclave at room temperature. The autoclave is then heated in a heating bath at a temperature of 110°C for a total time of 60 minutes. This probably corresponds to 15-30 minutes at approximately 100°C. After that, the pulp is diluted with the said return liquor to a consistency of 10%.

Experiment C is carried out as for experiment B but with an increased addition of NaOH. Experiment D corresponds to experiment B and is also in accordance with the invention, where the return liquor has been treated separately and wet-oxidized for 30 minutes at 10 bar of oxygen pressure and in the presence of OH^" ions to give a final pH of about 12-12.5.

Experiment E corresponds to experiment C and is in accordance with the invention, where the return liquor has been treated separately and wet-oxidized in an oxygenator for 30 minutes at 10 bar of oxygen pressure and in the presence of OH^" ions to give a final pH of about 12-12.5.

An obvious conclusion which can be drawn from the comparative experiment is that a substantial and improved reduction in the kappa number of the cellulose pulp which is fed out from the oxygen reactor is obtained if the liquor liquid which has been obtained in an extraction stage after the oxygen stage is subjected to selective wet oxidation before this wet- oxidized liquor liquid is allowed to impregnate the cellulose pulp prior to the latter being conducted to the oxygen stage.

A simultaneous adjustment of the pH with sodium hydroxide in, or immediately after, the oxygenator also gives a good buffering effect such that the delignification task can be kept under a constantly high, but not too high, pH through the whole of the oxygen stage. When a wet oxidation in accordance with the invention is implemented in experiments D and E, it is only possible to discern a marginal decrease of 0.1 units in the pH of the cellulose pulp leaving the oxygen reactor.

An obvious effect of the carbonation is that the TIC (Total Inorganic Carbon, corresponding to carbonated carbon) is very high relative to processes in which there is no wet oxidation of the return liquor together with pH adjustment. A quantity of carbon which is bound in the form of carbonates is kept high through the whole of the oxygen stage, and the degree of mineralization (a proportion of inorganically bound carbon compared to organic carbon) is also kept high through the whole of the oxygen stage. This means that most of the oxygen which is supplied to the oxygen stage is used for the delignification task, and the proportion of the oxygen which has to be used for oxidizing released organic carbon in the return liquor is substantially reduced.

Samples taken when using the wet oxidation technique in accordance with the invention show that the organic carbon in the pulp suspension leaving the oxygen stage undergoes a reduction of what is typically from 4.9 to 4.4 grams/litre as compared with an oxygen stage which is otherwise similar but is without wet oxidation of research-related return liquor. While this may appear to be a marginal reduction of 0.5-4% in the organic carbon in the pulp suspension, the facts show that the wet oxidation has succeeded in reducing the most reactive carbon in the return liquor. This 0.5-4% reduction in the organic carbon in the liquor from the oxygenator results in a corresponding, direct reduction in the thermal loading on the recovery boiler. It is in the recovery boiler that the organic material which has been dissolved out in the liquor liquid is combusted, and this recovery boiler frequently represents a limiting factor, i.e. a bottleneck, in any attempts to increase production in the pulp-producing process since the recovery boiler is already being utilized to its maximum extent .

Other comparative experiments/verification of the effect of the pressure on the results

The effect of the pressure during the wet oxidation was verified in a second set of comparative experiments. Unbleached craft pulp, consisting of softwood, principally pine, and having a kappa number of 32.3 and a viscosity of 1276 dm³/ g, is used in all the experiments F-I. The return liquor which is used for diluting the pulp is taken from a wash which is located immediately after the oxygen stage in a mill line which also includes a continuous cooking department, from which the said pulp sample is taken.

Experiments F and G are a simulation of the process conditions of the mill for oxygen bleaching and were carried out like the experiment A (described previously) . The addition of NaOH was extreme in experiment G, corresponding to 100 kg ptp. It should be noted that the initial pH was approximately 1 unit higher than m experiment A (when the pH was approx . 12), and consequently on the high side.

In experiments H and I, the oxygen bleaching was carried out as m experiments F and G but with wet oxidation of the return liquor under a pressure of approx. 10 bar (m accordance with the invention) and under atmospheric conditions, respectively. The temperature/time treatment corresponds to 20-30 minutes at approx. 100°C. The addition of NaOH m association with the wet oxidation resulted m an initial pH of 13.2. On the other hand, there was no atmospheric impregnation and leaching with the return liquor prior to the oxygen delignification, as was carried out in experiments D and E.

The results of this second comparative test can be compiled in the following table:

It can be concluded that, despite a total alkali addition of (100+25) kg of NaOH Ptp, in the wet oxidation and oxygen delignification, respectively, and an initia 1 pH of 30.3, experiment I, involving wet oxidation under atmospheric conditions, is unable to delignify to the same level as is achieved in experiment G, involving a total addition of 100 kg of

NaOH ptp and a pH of 13.2. The large amount of alkali added, i.e. 100 kg, in the wet oxidation is necessary in laboratory experiments in order to build up instantaneously the alkali level which is normally built up in the continuous operation process by carry over from the cooking department, in particular. In continuous operation, the alkali level is built up gradually by the continuous recirculation of cooking liquids to preceding process stages. By contrast, it is possible, using pressurized wet oxidation, as in experiment H, to lower the kappa number a further 2 units while more or less maintaining the viscosity.

The fact that both TOC and COD increase by approx. 30% when delignifying in accordance with experiment H, but only by 20% when delignifying in accordance with experiment I, is also in line with this.

This demonstrates that the delignification can be improved 2-5 units by wet oxidation, of which approximately 2 units can be attributed to the pressurization during the wet oxidation.

Other experiments show that, without wet oxidation and the addition of sodium hydroxide, the pH falls drastically, i.e. by 1.9-2.0 units, which results in the delignification not taking place at an optimally high pH.

This effect has been partly responsible for the necessity of having to introduce double oxygen stages into many installed processes in order to improve, or obtain adequate, delignification and bleaching.

Additional experiments which confirm the above results have been carried out. All of 16 experiments which were performed confirm that the kappa number of the cellulose pulp after the oxygen stage decreases by a further 1-6 units if the return liquor has been subjected to a wet oxidation in accordance with the invention.

The liquid/pulp ratios which pertain in a process according to the invention as shown in Figure 1 are as follows (ton of liquid per ton of pulp) ; - pulp entering the storage tower: 9 - pulp leaving the storage tower: 19 consequently an addition of 10 tons of wet-oxidized return liquor per ton of pulp in the storage tower.

Stabilizing effect of the wet oxidation on the pH level .

Figure 2 shows a titration curve which has a linear scale and which illustrates how pH is altered when NaOH is added to the return liquor with and without preceding pressurized wet oxidation at 95-100°C. The curve in bold is a reference curve and shows how adding NaOH alone increases the pH, i.e. a completely unbuffered liquor. If a normal return liquor, curve 1 (triangles), and a pH of 12.4 are used as the starting points, approximately 25 kg of NaOH are consumed ptp, when the pH falls to 10.8 when the pulp leaves the oxygen reactor. This can be calculated from; 0.07 mol/L ^* 40 molecular weight ^* 9 tons of liquor/ton of pulp = 25 kg of NaOH ptp.

If pressurized return liquor (curve 2 or 3) in accordance with the invention is used instead, and if the same pH of 12.4 is used as the starting point, the same quantity of NaOH has been consumed when the final pH is 11.9, corresponding to a final NaOH concentration of 70 mmol/1. This results in some delignification potential remaining in the treated return liquor. It is evident from the reference curve using NaOH alone that it is necessary to raise the initial pH to 12.9 and that a final pH of about 11 is achieved when 25 kg of NaOH are consumed ptp. This means, on the one hand, that the delignification is carried out at an unnecessarily high pH, i.e. less selectively, but also that, during the process, the pH finally becomes too low and the reaction rate becomes unacceptably low. Large parts of the reactor volume are therefore not being used in an optimal manner. If the conventional oxygen delignification case (curve 1) is compared with the case where wet-oxidized liquor is used (curve 2 or 3), it is seen that, in a continuous steady state condition, the alkali distribution of the pulp can detect constant at a higher mean pH value.

Return washing liquor from the oxygenator which is conveyed backwards (countercurrent to the process flow) to the cooking department with a net flow of 2-3 tons of liquid per ton of pulp is pH-adjusted as desired within the interval from bicarbonate to carbonate. In the latter case, more oxidized white liquor is needed in total for the oxygen stage, something which can, however, be to some extent compensated for by adding less alkali to one of the cooking department circulations.

The buffering, or the stabilization of a higher pH level, is consequently a property of the return liquor which can be amplified by means of pressurized wet oxidation. A pH which is stabilized at a higher level during the oxygen delignification signifies a higher average pH and an accelerated reaction rate for the delignification process at what is in principle a constant consumption of alkali as far as the pulp is concerned. In reality, the reaction rate becomes negligible at a pH of less than 11.7, a fact which has also been documented by the STFI (Svgenska Tra Forsknings Insitutet (the Swedish Forest Products Research Laboratory) ) in their studies of the kinetics of oxygen delignification.

The reaction time in oxygen bleaching is doubled from 45 to 90 minutes if the OH concentration is lowered from 20 to 5 mmol/L, i.e. a lowering of the pH from 12.3 to 11.7. The present degree of delignification is then 65%, the temperature is 110 °C and the oxygen pressure is 1 MPa. STFI's validation of the kinetics was carried out at a pH which was kept constant. Below pH 11.7, the reaction times became unrealistically long.

Figure 3 shows a corresponding titration curve but using logarithmic scales. This figure shows even more clearly the improvement which the wet-oxidized return liquor (the oxygen spent liquor) brings about.

Possibilities of exchanging a reduction in kappa number for an improvement in viscosity

In experiments D and E, the viscosity of the pulp can be improved, while achieving the same kappa number level as the kappa number levels which were obtained in experiments A-C, by modifying the process conditions in the oxygen stage simply in a manner known per se, i.e. by lowering the temperature. STFI has documented the fact that, if "a constant pH" is raised from 12 to 12.3 and the temperature is lowered by 10 degrees C in an existing reactor, the viscosity can be improved by 50- 60 SCAN unit while retaining a degree of delignification of approximately 65%. Alternately, the kappa number can be lowered by 2-3 units, at the same viscosity, if the higher temperature is retained.

The device according to the invention is not restricted by the above-described embodiments but can be varied within the scope of the subsequent patent claims. That which is important is that the oxygenator is allowed to act only on a well-separated liquor flow, with no regard having to be paid to effects on the properties of the pulp, and that, in the actual oxygen stage, care is taken to avoid the bulk of the supplied oxygen and sodium hydroxide being consumed for oxidizing releasable organic material, thereby lowering the pH in the liquor surrounding the pulp.

Claims

PATENT CLAIMS

1. Process for the oxygen delignification of pulp suspensions giving improved selective reduction, characterized in that return liquor (40) , which is extracted from the cellulose pulp suspension in at least one washing stage (20) in the pulp-producing process after an oxygen stage (16, 17), is treated in a pressurized oxygenator (30) in the presence of an oxygen-containing gas,

- in which oxygenator organic material which is present in the liquor is subjected to wet oxidation,

- after which the return liquor which has been wet- oxidized in the vessel is mixed together with the pulp suspension before this mixture is supplied to an oxygen reactor,

- in that the return liquor is held and treated in the oxygenator for a shortest time period within the interval 5-30 minutes, preferably 5-20 minutes, - in that sodium hydroxide (NaOH) is supplied to the pulp suspension in conjunction with the formation of carbon dioxide in the oxygenator, and in that the supply of sodium hydroxide is secured before this mixture is supplied to an oxygen reactor.

2. Process according to Claim 1, characterized in that the oxygen-containing gas is supplied in the form of oxygen.

3. Process according to Claim 2, characterized in that the oxygen is supplied to the oxygenator while maintaining a lowest positive pressure of the oxygen within the interval 1.5-15 bar, preferably a lowest positive pressure within the interval 10-12 bar.

4. Process according to Claim 3, characterized in that the return liquor which is present in the oxygenator is heated, preferably by being directly or indirectly heated with steam.

5. Process according to Claim 4, characterized in that the heating of the return liquor in the oxygenator takes place at a temperature within the interval δ-120°C, preferably 95-110°C.

6. Process according to any one of the precedeing claims, characterized m that the wet-oxidized return liquor which is obtained from the oxygenator is at least partially (42) used as displacement liquid in a device (12) for washing the cellulose pulp suspension, with the wet-oxidized return liquor from the oxygenator replacing preceding cooking liquor, or another liquid which at least partially does not contain wet-oxidized liquor, m the pulp suspension well before the cellulose pulp suspension is conducted to a reactor (17) for oxygen delignification.

7. Process according to any one of Claims 1-5, characterized m that the wet-oxidized return liquor which is obtained from the oxygenator is at least partially (41) used as impregnation liquid in an atmospheric stage (15) for impregnating the cellulose pulp suspension, with the wet-oxidized return liquor from the oxygenator impregnating and in the main replacing a preceding cooking liquor, or another liquid which at least partially does not contain wet-oxidized liquor, in the pulp suspension well before the cellulose pulp suspension is conducted to a reactor (17) for oxygen delignification.

8. Process according to Claim 1, characterized in that the sodium hydroxide is added in a quantity which is such that a buffering concentration of sodium carbonate (Na₂C0₃) which is formed by reaction with the organic material which is present in the liquor, in particular, and which corresponds to a minimum of 0.02 ol, preferably 0.10-0.20 mol, per litre of pulp suspension, resulting in a pH exceeding 11.0, preferably exceeding 11.6, is reached in the pulp suspension.

9. Process according to Claim 1 or 8, characterized in that the sodium hydroxide is added to, or immediately after, the oxygenator.

10. Process according to any one of the preceding claims, characterized in that constituents which are precipitated in the oxygenator, which precipitation is promoted by the wet oxidation process, are separated off using a separation device in conjunction with the oxygenator.

11. Process according to Claim 10, characterized in that the separation takes place by means of either a filtration process, a sedimentation process or a floatation process.

12. Process according to Claim 10, characterized in that magnesium sulphate is added in order to be present during the wet oxidation in the oxygenator, with the precipitation process primarily being improved, but secondary with the level of the viscosity of the pulp after the oxygen delignification also being increased.

13. Process according to any one of Claims 10-12, characterized in that the precipitation process is amplified by adding a flocculant, for example of the polyethylene oxide type.

14. Device for the selective oxygen delignification, in at least one oxygen reactor (17), of pulp suspensions giving improved selective kappa reduction of the pulp suspension after the oxygen delignification, characterized in that a washing device

(20) is arranged in the cellulose pulp manufacturing process, after the oxygen reactor (17), from which washing device extracted return liquor is separated and conducted to an oxygenator (30) , - with the oxygenator (30) constituting a pressurized vessel having a device (31) for supplying oxygen- containing gas, in which oxygenator wet oxidation of the liquor takes place, - after which the wet-oxidized liquor is returned in a part flow (41 or 42) via a supply line to an exchange unit or dilution unit (12 or 15) in the manufacturing process before the cellulose pulp is supplied to the oxygen reactor (17), - which exchange unit or dilution unit (12 or 15) comprises means for replacing a liquor/cooking liquid, which is present in the cellulose pulp suspension before the exchange unit, with a wet-oxidized return liquor, or for diluting with the wet-oxidized return liquor,

- in that the volume of the oxygenator is such that it allows all the return liquor which is recycled to have a dwell time of 5-30 minutes, preferably 5-20 minutes, in the oxygenator,

- in that a device 35 for supplying sodium hydroxide is arranged in close conjunction with the oxygenator, preferably integrated with the oxygenator at least and its discharge end.

15. Device according to Claim 14, characterized in that the oxygenator is a pressure vessel which is capable of withstanding pressures of the order of size of 1.5-15 bar, preferably 1-12 bar, and in that means (32a, 32b) are present for obtaining and maintaining this pressure in the oxygenator during essentially the whole of the wet oxidation process, and preferably also during continuous feeding-in and feeding-out the return liquor.

16. Device according to either of the preceding Claims 14- 15, characterized in that the exchange unit comprises a washing device (12) in which the liquor/cooking liquid which is present in the cellulose pulp suspension before being conducted to the washing device is displaced/replaced by/with the return liquor which is wet-oxidized in the oxygenator (30) .

17. Device according to either of the preceding Claims 14-15, characterized in that the exchange unit comprises a storage tower/impregnation vessel (15) in which the cellulose pulp suspension is impregnated with the return liquor which is wet-oxidized in the oxygenator (30) .

18. Device according to Claim 15, characterized in that the device (35b) for supplying sodium hydroxide is arranged to supply sodium hydroxide to a first part flow (41) from the oxygenator (30), with a second part flow (42) not being supplied with sodium hydroxide and instead being conducted directly to a washing device (12) .

19. Device according to any one of the preceding device claims, characterized in that a device (50) for separating of precipitated material is arranged in close conjunction with the oxygenator.

20. Device according to Claim 19, characterized in that the separation device (50) comprises at least one of the following: a) a filter, b) a sedimentation device, or c) a floatation device.

21. Device according to Claim 19, characterized in that the separation device (50) is arranged directly after the oxygenator.

22. Device according to Claim 19, characterized in that the separation device (50) is arranged such that it is integrated with the oxygenator.