SE522161C2 - Method and device for oxygen delignification with improved copper reduction - Google Patents

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

Oxylute is then pretreated in a pressure vessel at a temperature of around 73-149 ° C and a pressure up to 21 bar. This pressurized treatment will therefore affect both the recycled oxylute and the cellulosic pulp suspension mixed therein.

In SE, C, 360. 128 shows another variant with the aim of improving the oxygen bleaching, where a smaller amount of the boiling liquor is retained in the cellulose pulp suspension and a larger part of the lye liquid is replaced with recycled bleach / oxyl liquor. In a preferred embodiment it is stated that the boiling liquor can be treated with a pre-oxidation with the addition of oxygen and suitably under pressure, after which oxidized boiling liquor is finally displaced / replaced with the bleaching liquor.

A problem with the use of oxygen is that the oxygen cannot be mixed efficiently into the cellulose nasal suspension over a certain amount of oxygen. The solutions known above have not given sufficient results, which is why the focus has been on double oxygen steps.

Examples of such double oxygen steps are shown in SE, C; 505. 141, SE, C, 507,870 and SE, C, 507,871 where different combinations of pressure, temperature, residence times and investments of oxygen and alkali to the two steps are claimed to give special effects. However, it is a well-known fact that the second reactor makes a very modest addition to the delignification work, often only 1-3 units in kappa. A problem with all these solutions is that only a limited amount of oxygen can be mixed into cellulose pulp suspension. Released organic substances contained in the cellulose pulp suspension and organic substances not released from the lignin. Thus, only a subset of the maximum amount of oxygen supplied will effectively contribute to the delignification work, which results in a lower delignification effect and increases the need for extended delignification times, often leading to the introduction of double oxygen reactors.

A related problem is the efficiency of the pulp washing or lye drainage properties before pulp bleaching in the oxygen reactor. In order to improve the pulp washing, carbon dioxide can be added to the cellulose pulp suspension before washing, see for example EP45 C, 296. 198, whereupon oxygen delignment is improved by lower COD in the pulp to the oxygen delignment and increased selectivity.

The underlying causes are uncertain, but both the pH and ionic strength of the lye as well as the precipitation of resins and calcium carbonate are considered to have a strong effect. The carbon dioxide additive in the laundry also means that the COD in the pulp for the bleaching plant is reduced and that the need for bleaching chemicals decreases, but also that the need for evaporation decreases and that the need for cleaning deposits in the process equipment decreases.

BRIEF DESCRIPTION OF THE INVENTION An overall object of the present invention is to improve the capping reduction in the pulp treated in an oxygen step with optimum high viscosity. i.e. increased selectivity in the oxygen delignition. With the invention it is possible to achieve a degree of more than 70 percent delignification from kappa 30 without undue loss of viscosity.

A further object is to treat recovered liquor recovered from washing apparatus after an oxygen step in such a way that a major part of the slightly oxidized part of the recycled liquor recovered from the washing apparatus can be oxidized with oxygen-containing gas in a treatment vessel, hereinafter referred to as oxygenator. This oxygenator can also be called a mineralizer, as its function is to partially convert organic carbon to inorganic carbon, which is why the term carbonator also essentially describes the function.

The oxygenator is suitably manufactured as a small pressure vessel capable of being pressurized in the range 1.5-15 bar, preferably 10-12 bar. The volume of the oxygen atom is determined so that the return liquor recirculated from the current washing step obtains a residence time of 5-30 minutes, preferably 5-20. Preferably, the return liquor in the oxygen atom is heated to a temperature of 80-120 ° C, preferably 95-100 ° C. Yet another object is that the wet oxidation of the recycled liquor in the oxygen atom causes carbon dioxide to be primarily intergenerated in such an amount that this greatly reduces the need for the addition of extreme carbon dioxide or carbonates, possibly completely eliminating the need for costly 9945se.doc 99-06-15 10 15 20 25 30 522 161 external CO 2 addition, which is otherwise widely applied in about 20-30 ñberlines.

The thus oxidized return liquor is used as washing or displacing liquid in the pulp washing, whereby the washing result is improved to a corresponding degree as processes with the addition of external carbon dioxide. This improvement in mass washing has direct effects on oxygen delignification. Another purpose is to reduce the need for double oxygen steps, both of which are space-consuming and costly. At least the same effect and usually considerably improved effect on the delignification work can be obtained in only one oxygen step, which is combined with an efficient oxygenator for the return of extracted washing liquid. In some cases a capping reduction of 4-6 units is obtained with only one oxygen stage combined with an oxygenator, while a second oxygen stage gives only a capping reduction of 1-2 units. This enables an improvement of up to 200% in capping reduction compared to the use of a second oxygen reactor.

An object of an advantageous embodiment is that the oxygen degeneration can take place below a substantially constant pH value at an optimally high level, by a carbonation of the wet oxidized lye, where the carbonates are internally generated by oxidation. Another purpose is to allow a very cost-effective rebuilding of existing facilities, where only a smaller oxygenator is required as an additional device, and 4-6 units of jacket reduction can be obtained. Another object is that the controllability and interaction of the pulp production with the coker increases, as the cutting agreement on the pulp can be easily adjusted by adjusting the NaOH concentration and / or temperature in the oxygen reactor. If, for example, a higher viscosity with a normal kappa number is desired, the temperature in the oxygen reactor can instead be lowered at an otherwise unchanged constant pH. The pressure in the oxygen reactor as well as in the oxygenator must be kept maximally high. 9945en.doc 99-06-15 15 15 15 25 25 522 161 5 Another object is to allow a very suitable process position for the expulsion of manganese, calcium, magnesium and resin etc., as the oxygenator with its process conditions promotes a precipitation by fl otation, sedimentation or filtration of undesirable substances. This in turn results in reduced crust formation in the process equipment in the entire fiber line and thus also in subsequent bleaching.

The system will provide an improved Oz bleaching process, which results in a substantially resin-free and cleaner intermediate product that will provide good bleaching economy, high yield and strength in ECF / TCF final bleaching. This latter is related to the almost complete elimination of transition metals such as manganese, which decisively reduces peroxide consumption and increases the carbohydrate content of the pulp in the final bleaching. The solubility of fatty acids and resin acids decreases markedly with the ionic strength of the lye, ie mainly the sodium carbonate content. This allows resins such as liquid crystalline soap and salted sodium soap to be mechanically separated from the wet oxidized liquor.

The above-mentioned advantages mean that the pulp process is improved in a number of respects.

Under special circumstances, a 10% higher production can be obtained using externally added carbon dioxide or in accordance with the invention by internally generated bicarbonate. At the same time, the need for ink chemicals can be reduced. If the washing steps are made more efficient by using wet oxidized return liquor as a displacing liquid, this gives 20 resp. 30% lower COD in the mass into and out of the oxygen stage.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in the following with reference to the figures, of which: Fig. 1 shows a process equipment for the production of cellulose pulp; Fig. 2 shows a titration curve with a linear scale, and Fig. 3 shows a titration curve according to Fig. 2 but with a logarithmic scale.

DETAILED DESCRIPTION OF THE DRAWINGS The concept according to the invention will be described in the following with reference to a process equipment of principle. The essential thing is a wet oxidation of oxylut / return liquor / return liquor / waste liquor in a pressurized oxygen-containing environment, followed by an impregnation / leaching of the cellulose pulp in this wet-oxidized oxyl liquor, which enhances the degree of delignification and selectivity in the actual oxygen step. Impregnation and leaching are preferably carried out under atmospheric conditions for maximum selectivity and can be applied individually or in combination with each other.

Process equipment A possible configuration of process equipment is shown in Figure 1, which contains the thick mass 10, siler 11, siler press 12, steam mix 13, thick mass pump 14 followed by an intermediate storage tower 15 with a holding time of 10-20 minutes for the cellulose mass suspension.

The cellulose mass from the silage press 12 can of course fall freely to said intermediate storage tower 15. This is followed by the actual oxygen step (one or more oxygen steps connected in series) with mixer equipment 16 for dosing lye, oxygen and steaming the cellulose mass suspension before it is led to the oxygen reactor 17.

After the oxygen step (16 + 17) follows a flash tank 18, in which residual gases are blown off. After the ash tank is pumped by means of a pump 19, cellulose pulp suspension is passed on to two washing machines 20,21. The first washing apparatus may be in the form of an atmospheric diffuser 20, and the second washing apparatus in the form of a washing press 21. The washing apparatus may be of different types and the atmospheric diffuser ear may, for example, be replaced by a pressure diffuser.

The washing liquid extracted from the first washing apparatus, fl fate 40 in fi gur l, which washing liquid is hereinafter referred to as return liquor (also oxyl liquor) and which mainly consists of the liquid part in the cellulose nasal suspension from the oxygen reactor, must according to the invention be handled individually.

Wet oxidation The return liquor 40 is passed to a oxygenator 30, hereinafter referred to as the oxygenator.

Furthermore, oxygen is introduced into the oxygen atom, which suitably takes place with an in-line mixer 31 or a sparger (asparagus addition of oxygen through a tube of porous often sintered material) directly before or in the inlet portion of the oxygen atom. A pump 32a can be used to pressurize the oxygenator to the required pressure, against the action of a controllable throttle valve 32b arranged in the outlet of the oxygen atom. The throttle valve 32b may preferably be a pressure reducing valve which opens at a preset pressure, for example 10-12 bar.

The volume of the oxygenator is adjusted so that the return liquor 40 extracted from the washing step 20 obtains a residence time of about 5-30 minutes, preferably 5-20 minutes, in the oxygen atom.

To further improve the reaction conditions in the oxygenator, the return liquor present in the oxygen atom can be heated with a heat exchanger 33 with low or high pressure steam from a suitable source 34 in the cellulose pulp process. Preferably, the return liquor in the oxygen atom is heated to a temperature of 80-120 ° C, preferably 95-100 ° C. The heating can also take place by direct supply of steam into the pulp suspension.

The oxygen atom operates in a pH range of 8.5 - 12.5, preferably 10-117, and its performance is characterized by the degree of mineralization of the lye and the average valence number of the carbon in triggered organic compounds.

The degree of mineralization for solutes and released substances is defined as TIC / (TIC + TOC), ie "Total Inorganic Carbon" and "Total Organic Carbon", quantities that are determined in a special laboratory analyzer. TIC can be converted to Na2CO3 by multiplying by the factor 106/12.

It can be noted that the wet oxidized liquor has a TIC content of about 2.5 times higher than the conventional return liquor. This corresponds to 15 grams of Na2CO3 or 0.14 mol NaZCO, per liter and explains the buffer effect of the lye below pH 11.7, which is the pH for a pure soda solution with a concentration of 0.1-0.2 mol / Liter.

Carbon average valence value for solids is defined as 4 * (TOC-COD) / TOC.

It is directly related to the calorific value of a substance as well as the Oz consumption of lye in oxygen depletion. For a deduction end, the value is -1 and for a conventional oxygen outlet between -O, 3 and -0.5. For wood substances, released lignin is around -1 while carbohydrates are around O. 9945se.doc 99-06- 15 10 20 25 30 522 161 8 Buffering with sodium hydroxide value, the liquor system must be buffered, and adjusted to a pH level within the range ll.7-13.0. This means that easily oxidised organic compounds which during wet oxidation have been transferred from organically bound carbon, TOC (Total Organic Carbon), to inorganic carbon, TIC (Total Inorganic Carbon), which inorganic carbon is recovered in bicarbonates and / or carbonates, which provide the desired buffering.

The addition of NaOH provides the intended pH adjustment.

The mixture shows a mixing device 35 located in close proximity to and preferably directly after the oxygen atom in which mixing device a mixture of sodium hydroxide takes place. In some embodiments, the mixing device, dashed as 35b, may be located in a first subflow 41 from the oxygenator, where a second subflow 42 is directed directly to a washing apparatus to obtain optimal washing results.

Through this mixture of sodium hydroxide, water and sodium carbonate are formed, as the sodium hydroxide reacts with the carbon dioxide formed in reaction with organic material in the oxygen treatment of the recycled liquor in the oxygen atom.

The reaction follows the process steps; a) C + O 2:> CO 2 where the released organic material in the recycled liquor reacts with the oxygen to form carbon dioxide, b) CO2 + OH ':> HCO,' where the added hydroxide reacts with the carbon dioxide to form bicarbonate; c) HCO; + OH ':> CO 22' + H 2 O where the carbonate and bicarbonate ions give the liquor a buffering capacity and the pH value can be kept substantially unaffected even though OH ions are continuously consumed due to oxidation of the equation in the cellulose pulp during the delignification.

Depending on the pH, the carbon dioxide generated in the oxygen atom is present as COf, HCO; , or H2CO2 according to; H2CO, + OH '<:> HCO; + H 2 O at pH 6.3 when the molar equilibrium 50/50 is between H 2 CO 2 and HCO; ; 9945se.doc 99-06-15 10 15 20 25 30 522 161 9 and HCO3 '+ OH' <:> CO32 '+ H20 at pH 10.3 when molar equilibrium 50/50 is present between HCO3' and CO32; When the pH then rises to 1.6, the equilibrium is shifted so that essentially all organic carbon is formed in the form of carbonates.

The amount of sodium hydroxide charged during the reaction process b above should be determined so that a buffering concentration of formed Na 2 CO is maintained in the wet oxidized recycle liquor corresponding to at least 0.1 mol

Foam separation The oxygenator offers an excellent possibility for separation of undesirable constituents, often in the form of foam or fl oxides, containing manganese, other transition metals, calcium, resin, other fines and also separation of a bulk of sparingly soluble calcium / magnesium together with carbonate / hydroxide / silicate . So-called zero samt ber as well as fl berdarnm together with resin constitute an essential organic component in this bulk which fi n distributed in the pulp causes problems with bleachability, purity, heat content and drainage.

With effective means for separating these foamed constituents in the oxygenator, the clogging risks in the other parts of the pulping process are significantly reduced.

An addition of magnesium sulphate in the oxygenator promotes the precipitation process of undesirable constituents as this addition forms sprouts for growth of the organic / inorganic sediment. Normally a certain amount of magnesium sulphate is added before the oxygen reactor, and preferably half of this charge of magnesium sulphate should already take place in the oxygenator.

The oxygen atom works at an overpressure around 10-12 bar, 90-100 ° C and at a pH between 8.5-12.5, which promotes an fltation process. Mn together with Ca, Mg and resin are enriched in oxides liquid on the surface of the wet oxidized recycle liquor. The foam formed is blown or scraped off on top of the oxygen atom, or alternatively separated by a subsequent filtration or sedimentation in a backwater tank.

For example, separation of undesired material can take place without the use of auxiliary chemicals such as polyethylene oxide, which auxiliary chemicals are only added to promote the tendency to separate. The use of complexing agents for manganese emissions in peroxide bleaching (EDTA, DTPA) can also be reduced or completely eliminated in the final bleaching. .

The following table shows the effects of the recoverable wet oxidation on the possibility of filtering out manganese. Studies on untreated recycled liquors show that the manganese content can vary between 20-80 mg / l, but in all tests the level of manganese content has been the same for filtered and unfiltered lye. No effect of mechanical separation, for example reduction of the manganese content after a filtration, has been observed.

On the other hand, as soon as the liquor is wet oxidized, both the TIC and the degree of mineralization increase, whereby mechanical separation by filtration (see the last row in the table) becomes an almost complete process for separating manganese.

Probably the elevated concentration of carbonate can generate easily separable, crystalline coprecipitates of Mn / Mg / Ca, where in each case CaCOB is significantly more sparingly soluble than Ca (OH) 2.

Return liquor Wet- Wet- Wet- Wet- Wet- Wet- oxidized oxidized oxidized oxidized oxidized oxidized NaOH in - - - Yes Yes Yes Yes oxygenator 100 ° & - 10min 30min 10 min 30min 10min 30 min 60mg Mg / 1 Pressure - 10 bar 10 bar 10 bar 10 bar atm atm pH 10.8 10.3 10.2 12.3 12.3 12.3 12.3 TOM, g / 1 18.1 rs, g / 1 31.7 TIC, g / l 1.45 1.29 1.37 2.37 2.36 2.56 2.51 TOC, g / l 9.45 7.70 8.64 7.90 7.70 7.57 8.36 COD, g / l 25.0 20.4 20.7 20.4 20.3 20.8 22.4 Mineral,% 13.3 14.3 13.7 23.1 23.5 25.3 23.1 Valence carbon -0.3 0 0.4 0.1 O -0.1 0 9945sedoc 99-06-15 10 20 25 522 161 11 Mn, mg / 1 79.5 41.5 40.3 27.5 28.5 28.5 28.5 unfiltered Mn, mg / 1 78.4 0.95 1.0 1.2 1.4 0.42 1.5 filtered There are possible alternatives for the separation in the oxygenator or directly after the oxygenator of precipitated unwanted constituents. This can be done through; fl otation processes, where unwanted material fl is increased on the surface - sedimentation processes, where unwanted material is allowed to sink to the bottom of sedimentation vessels, or - filtration processes, where the wet oxidized return liquor is allowed to pass through some type of filter.

Filtrate is recycled as washing after cokery, while pre-coated sludge can be pressed or treated with the other fiber-containing deduction from the cokery. Figure 1 shows this separation process as a filtration through the filter 50, where the separated unwanted material is separated in the flow 51.

Gas separation Oxygen that does not react but forms an accumulation of gas in the top of the oxygen atom can be separated by suitable means (not shown) and charged to admixture mixers before the oxygen reactor or reactors. Recirculation of wet oxidized lye in the oxygenator The recirculation of liquefied oxidized liquor in the oxygen atom can take place in either a long or short loop, in Figure 1 shown in partial flows 42 and 41, respectively.

The short loop means that the wet-oxidized lye liquid is introduced into the bottom of an impregnation vessel 15, shown in the figure as an intermediate storage tower, which dilutes the bottom of the tower 15. This dilutes the mass before it is led to an oxygen stage 16 + 17.

Typically, the wet oxidized liquor is fed to cellulose pulp in the intermediate storage tower so that a ratio of 9 tons of liquid per ton of pulp is obtained in the pulp suspension which is fed to the oxygen reactor.

The cellulosic pulp suspension fed to the intermediate storage emptying / embossing vessel 15 typically has a 25-3 5% consistency. The atmospheric leaching and equilibrium in this step is a very important step for the selectivity, before the mass is contacted with oxygen under pressure.

The long loop means that the wet-oxidized lye liquid is introduced as washing and / or narrowing liquid in a washing apparatus or press 12, in such a way that the liquid contained in the cellulose pulp suspension before supply to the washing apparatus 12 is displaced by the wet-oxidized lye liquid. After the screen press, the pressed cellulose pulp suspension obtains a significantly higher proportion of the wet oxidized liquor in its remaining liquid part, which further increases the proportion of wet oxidized return liquor in the liquid part of the pulp suspension which is fed 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 depending on the desired pH in the respective position in the process. In order not to lower the pH on the coker ice side, the alkali batch can also be raised in one of the coker circulations.

The amount of sodium hydroxide needed to form the buffering sodium carbonate content together with the carbon dioxide formed in the oxygenator is charged in its main part into the mixing apparatus 35, alternatively 35b.

The charge of alkali in the mixer 16 is made mainly for the purpose of reaching the required alkaline environment for the oxygen step.

The total amount of NaOH to the reactor amounts to a maximum of 50 kg ptp in total, preferably 20-35 kg ptp in total for softwood, preferably distributed over about 5-10 kg in the oxygenator and 20-30 kg in the mixer 16. In some applications, however, the investment can of alkali in the oxygenator be minimal, especially in applications where a less alkaline bicarbonate buffered environment is sought in the washing step in the long loop. Excess alkali in the form of carbonate (CO3 ") will recycle with the recycled liquor.

Depending on the mode of operation, the invention can give a certain higher consumption of oxidized white liquor (also oxygen) but the increased cost is only a fraction of the cost of a 9945se.doc 99-06- 15 10 15 20 25 30 522 151 :: ° - .. ï ° ~ .. = is. s 'i' 13 systems with, for example, carbon dioxide additive (corresponding to EP, C; 296198).

The slightly higher consumption of oxidized white liquor, in the order of 10-25%, entails a certain reduction in the thermal load on the recovery boiler, as the oxygenator oxidises a certain amount, in the order of 0.5-4%, of organic dry matter that would otherwise have been carbonated in the recovery boiler. It should also be remembered that with the higher consumption of oxidized white liquor, less white liquor can be invested in the final cooker circulation. Carbonate instead of bicarbonate will of course be returned to the cooking process.

First comparative experiment In the following, a comparison will be made between 5 different examples of delignification processes in an alkaline environment.

In all experiments, unbleached sulphate pulp of softwood, mainly pine, with a kappa number of 29.4 and a viscosity of 1,180 dmS / kg has been used.

The return liquor (oxyl liquor / recycled liquor) used in certain experiments for dilution and leaching of the cellulosic pulp suspension is taken from a washing machine located immediately after the oxygen step in a factory line e including also a continuous cookery, where said pulp sample is taken.

Experiment A is a simulation of the factory's process conditions, where neither wet oxidation of recycled liquor nor leaching has been practiced. Oxygen delignment takes place by charging oxygen to the pulp under temperature-time conditions corresponding to approximately 105 ° C for 1 hour effective reaction time in a continuous process and the amount of oxygen invested in the pulp suspension is 20 kg 02 ptp (ptp = per tonne of pulp / per tonne of pulp) . The partial pressure of the oxygen will essentially follow approximately the same profile as in the continuous oxygen reactor, ie. from 8-9 bar to about 3-4 bar, the latter pressure at the top of an oxygen reactor.

Experiment B includes leaching and the pulp is therefore first treated with the recycled lye and a batch of NaOH at atmospheric pressure. For the leaching, the lye is mixed with pulp at 20% consistency and placed in an autoclave at room temperature. The autoclave is then heated in a hot bath at a temperature of 110 ° C for a total time of 60 minutes. This should correspond to 15-30 minutes at about 100 ° C. Thereafter, the pulp is diluted to 10% consistency with said return liquor.

Experiment C takes place as for experiment B, but with an increased investment of NaOH.

Experiment D corresponds to Experiment B and in accordance with the invention where the recycled liquor has been treated separately and wet oxidized for 30 minutes at 10 bar oxygen pressure and the presence of OH 'ions to a final pH around 12-12.5.

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

MASS Attempt Attempt Attempt Attempt Attempt A B C D E kappa number, mass out from oxygen step 13.7 13.2 12.1 §.¿l _8 '._ 0 viscosity, mass out fr. oxygen stage 960 980 940 780 770 Return liquor Norm. Standard. Standard. Wet- Wet- oxides. oxides.

NaOH, kg / h 22 22 30 22 30 pH in, to OZ reactor 12.1 12.1 12.3 12.0 12.0 pH out, from Oz reactor 10.1 10.1 10.4 11.9 11.9 pH fall, in Oz reactor 2.0 2.0 1.9 0._1 2.1 TIC in, g / l 0.62 0.62 0.73 1.40 1.70 TIC out, g / l 0.70 0.70 0.73 1.60 1.80 COD in, g / l 14.9 14.1 14.2 13.7 14.0 COD out, g / l 18.0 17.1 18.0 19.7 18.8 mineral. % in ll 12 14 25 27 mineral. % out 10 11 11 19 20 valence carbon in -0.3 -0.5 -0.8 -0.8 -0.5 valence carbon out -0.4 -05 -0.6 -0.3 +0.1 A clear conclusion to be drawn from the comparative experiments is that a significantly improved shear reduction is obtained on the cellulose pulp discharged from the oxygen reactor, if a selective wet oxidation takes place of the liquor liquid obtained in an extraction step after 9945se.doc 99-06-15 ... nu ... o _ .. 10 l5 20 25 30 522 161 15 the oxygen step , before this wet oxidized liquor is allowed to impregnate the cellulosic mass before the cellulosic pulp is led to the oxygen step.

With a simultaneous pH adjustment with sodium hydroxide in or immediately after the oxygenator, a good buffering effect is also obtained so that the delignification work can be maintained during a constant high, but not too high, pH value throughout the oxygen step. Only a marginal decrease in the pH value of 0.1 units on the cellulose mass from the oxygen reactor can be discerned when applying an recoverable wet oxidation in experiments D and E.

A significant effect of the carbonation is that TIC (Total Inorganic Carbon, corresponding to carbonated carbon) is very high relative to processes without wet oxidation of the recycled liquor with pH adjustment. The amount of carbon bound in the form of carbonates is maintained high throughout the oxygen step and the degree of mineralization (the proportion of inorganically bound carbon relative to organic carbon) is also maintained high throughout the oxygen step.

This means that the oxygen supplied in the oxygen step is largely used for the delignification work, and that a significantly reduced proportion of the oxygen needs to be used to oxidize the released organic carbon in the recycled liquor.

Samples using the wet oxidation technique according to the invention show that the organic carbon in the pulp suspension from the oxygen step undergoes a reduction typically from 4.9 to 4.4 grams / liter, when compared with an otherwise similar oxygen step but without wet oxidation of recycled recycled liquor. This may seem like a marginal reduction of 0.5-4% of the organic carbonaceous mass suspension, but reality shows that wet oxidation has succeeded in reducing the most reactive carbon in the recycled lye. This O.5-4% reduction of the organic carbon in the liquor from the oxygenator results in a direct reduction of the thermal load of the recovery boiler to a corresponding degree. It is in the recovery boiler that the organic material dissolved in the lye liquid is burned, and this recovery boiler often constitutes a limiting factor, an fl ash neck, in the event of any production increases in the pulp production process, as the recovery boiler is already utilized to the maximum.

Second Iron-bearing experiments / verification of the effect of pressure on results In a second iron-bearing experiment, the effect of the pressure during the wet oxidation was confirmed. In all experiments F-I, unbleached softwood pulp sulphate pulp was used, mainly pine with kappa number 32.3 and viscosity 1276 dm3 / kg. The recycled liquor used to dilute the pulp is taken from a wash located immediately after the oxygen step in a factory line including also a continuous boiler, where said pulp sample is taken.

Experiments F and G are a simulation of the factory's process conditions for oxygen bleaching and were carried out as Experiment A (previously described). The loading of NaOH was extreme in Experiment G, corresponding to 100 kg ptp. Notably, the initial pH was about 1 unit higher relative to Experiment A (when the pH was about 12), and thus on the high side.

In experiments H and I, the oxygen bleaching was carried out as in experiments F and G, but with hydrogen oxidation of the recycled liquor under a pressure of about 10 bar (according to the invention) and under atmospheric conditions, respectively. The temperature-time treatment corresponds to 20-30 minutes at about 10 ° C. Due to the charge of NaOH in the wet oxidation, an initial pH of 13.2 was obtained. In contrast, there was no atmospheric impregnation and leaching with the return liquor before the oxygen delignification as performed in experiments D and E.

The results of this second comparative test can be summarized in the following table; MASS Attempt Attempt Attempt Attempt F G H In kappa number, mass out from oxygen step 17.5 14.3 12.7 14.6 viscosity, mass out fr. oxygen stage 1059 966 900 912 Return liquor Norm. Standard. Wet tox Wet tox + pressure atm NaOH, kg / h 25 100 25 25 pH in, to 02 reactor 12.7 13.2 13.2 13.3 pH out, from 02 reactor 10.3 13.0 13.1 13.2 TIC in, g / l 0.9 0.9 1.4 1.5 TIC out, g / l 1.1 1.5 1.7 1.7 TOC in, g / l 7.1 7.1 7.0 7.1 TOC ut g / l 8.6 8.9 9.3 8.7 9945561100 sis-ons 10 15 20 25 522 161 17 COD in, g / l 18.8 18.8 18.7 19.3 COD out, g / l 22.3 23.2 23.8 23.1 mineral. % in 11 11 17 17 mineral. % out 11 16 16 16 It can be concluded that experiment I with atmospheric wet oxidation, despite a total alkali rate of (100 + 25) kg NaOH ptp in wet oxidation resp. oxygen delignification, as well as an initial pH of 13.3, are not able to delign to the same level as in experiment G with a total batch of 100 kg NaOH ptp and a pH at 13.2.

The high investment of alkali, l00kg, in the wet oxidation is necessary in laboratory experiments to momentarily build up the alkali level that is normally built up in the process during continuous operation by transfer from mainly the boiler. During stationary operation, the alkaline level is built up gradually through the continuous recirculation of cooking liquids to the previous process steps.

With pressurized wet oxidation, on the other hand, as in experiment H, the kappa number can be further reduced by 2 units while the viscosity is maintained at approximately.

In line with this is also that both TOC and COD increase by about 30% in the delignification according to experiment H, but only 20% according to experiment I.

This shows that the delignification can be improved 2-5 units by wet oxidation, of which about 2 units can be attributed to the pressurization during the wet oxidation. Other experiments show that without wet oxidation and the addition of sodium trihydroxide, the pH drops drastically, 1.9-2.0 units, which has the consequence that the delignification work does not take place at an optimally high pH value.

This effect has partly led to many installed processes being forced to introduce double oxygen steps to improve or obtain sufficient delignination and bleaching.

Additional experiments have been performed which show the above results. Of 16 experiments performed, all evidence that kappa decreases by an additional 1-6 units on the cellulose mass after the oxygen step 9945se.doc 99-06-15 10 20 25 30 522 161 18 if a recoverable wet oxidation has been performed on the return liquor.

The liquid / pulp conditions prevailing in an inventive process according to Figure 1 as follows (tonnes of liquid per tonne of pulp); -mass in to storage tower z 9 -mass out of storage tower: 19 thus an addition of 10 tons of wet oxidized return liquor per ton massai the storage tom.

The stabilizing effect of wet oxidation on the pH level.

Figure 2 shows a titration curve with a linear scale, illustrating how the pH changes when NaOH was added to the recycled liquor with and without prior pressurized wet oxidation at 95-100 degrees C. The bold curve is a reference and shows how pure NaOH addition raises the pH value, ie. a completely unbuffered lye.

If one starts from normal return liquor, curve 1 (triangles) and a pH value of 12.4, approximately 25 kg of NaOH ptp is consumed when lowering down to pH 10.8 as the mass learns the oxygen reactor.

This can be calculated from; 0.07 mol / L * 40 mol weight * 9 ton lye / ton mass = 25 kg NaOH ptp.

If instead use oxidized recycled liquor (curve 2 or 3) according to the invention and start at the same pH at 12.4, the same amount of NaOH has been consumed when the final pH reaches 11.9, corresponding to a final concentration of NaOH of 70 mmol / l. This means that a certain delignification potential remains with the treated return liquor.

It appears from the reference curve with pure NaOH, that one must raise the initial pH to 12.9 and reach a final pH around 1 l with a consumption of 25 kg NaOH ptp.

This means partly that the delignification is carried out at unnecessarily high pH, ie less selectively, but also that the pH during the process eventually becomes too low and the reaction rate is unacceptably low. Large parts of the reactor's volume are therefore not used in an optimal way.

If one wants to compare the conventional oxygen delignification case (curve 1) with the case of wet oxide-clad liquor (curve 2 or 3), it is seen that the alkali distribution of the pulp in a continuous steady state can be kept constant at a higher average pH value. 9945en.doc 99-06- 15 10 15 20 25 30 522 161 I, Return wash liquor from oxygenator which is carried backwards (countercurrent process fl fate) to the boiler with a net flow of 2-3 tonnes of liquid per tonne of pulp, pH is adjusted as desired in the range from bicarbonate to carbonate. In the latter case, a total of more oxidized white liquor is needed for the oxygen step, which, however, can to some extent be compensated with a lower alkali charge for one of the cooker circulations.

Buffing, or the stabilization of a higher pH level, is thus a property of the recycled lye that can be enhanced with pressurized wet oxidation. Stabilized pH at a higher level during the oxygen degeneration means a higher average pH and an accelerated reaction rate for the delignification process with basically constant alkali consumption in terms of the pulp. In reality, at pH below 11.7, the reaction rate becomes negligible, which fact has also been documented by STF I (Swedish Wood Research Institute) in their studies of the kinetics of oxygen depletion.

The reaction time for oxygen bleaching is doubled from 45 to 90 minutes if the OH concentration is lowered from 20 to 5 mmol / L, ie a pH reduction from 12.3 to 11.7. The actual degree of delignification is then 65%, the temperature 1 10 degrees C and the oxygen pressure 1 MPa. STFIs validation of the kinetics was performed at constant pH. Below pH 11.7, the reaction times then became unrealistically long.

Figure 3 shows a corresponding titration curve, but with logarithmic scales. This shows even more clearly the improvement that the wet-oxidized recycled liquor (oxygen liquor) brings.

Opportunities to switch kappa number reduction to viscosity improvement Through a simple modification of the process conditions in the oxygen step in a manner known per se by lowering the temperature, experiments D and E can obtain an improved viscosity of the pulp at the same kappa number level corresponding to the kappa number levels A obtained in experiment C. It is documented by STFI that if in an existing reactor, "constant pH" is raised from 12 to 12.3 and the temperature is lowered by 10 degrees C, with a degree of delignment of about 65% the viscosity can be improved by 50-60 SCAN units. Alternatively, you can lower the coat by 2-3 units at the same viscosity, if the higher temperature is maintained.

The device according to the invention is not limited by the embodiments described above, but can be varied within the scope of the following claims. The essential thing is that the oxygenator is allowed to operate only on a well-separated liquefied liquid de fate, where no effect needs to be taken into account on the properties of the pulp, and to in the oxygen step itself avoid that added oxygen and sodium hydroxide are mainly consumed to oxidize released organic material and lower pH in the mass of surrounding liquor. 9945se.doc 99-06-15

Claims (9)

1. 0 15 20 25 30 l. 9945 se4.doc 522 161 21 c o n u c v Q | A method for oxygen delignification of pulp suspensions with improved selective capping reduction is characterized in that return liquor (40) extracted from the pulp pulp suspension in at least one washing step (20) in the pulp adjustment process after an oxygen treatment step (16) is treated with oxygen. ) in the presence of oxygen-containing gas, - that the oxygen gas is supplied to oxygenator while maintaining a minimum overpressure of the oxygen gas in the range 1.5-15 bar, preferably a minimum overpressure in the range 10-12 bar, in which oxygenator a wet oxidation of the liquor is present organic material takes place, after which the recycled liquor oxidized in the vessel is mixed with the pulp suspension before this mixture is fed to an oxygen reactor, N aOH) is added to the pulp suspension in connection with the formation of carbon dioxide id in the oxygen atom and where the carbonate and bicarbonate ions formed give the liquor a buffering capacity and -that the oxygen atom operates at a pH between 8.5 and 1.2.5 and that the supply of sodium hydroxide is ensured before this mixture is fed to an oxygen reactor. 2. A method according to claim 1, characterized in that the oxygen-containing gas is supplied in the form of oxygen gas. Process according to Claim 2, characterized in that the return liquor present in the oxygen atom is heated, preferably by direct or indirect heating with steam. Process according to Claim 3, characterized in that the return liquor in the oxygen atom is heated to a temperature in the range 80-120 ° C, preferably 95-110 ° C. A method according to any one of the preceding claims, characterized in that the wet oxidized return liquor obtained from the oxygen atom is used at least in part (42) as a displacement liquid in a washing device (12) for the cellulose pulp suspension. , wherein the oxygen-oxidized return liquor from the oxygen atom replaces the previous cooking liquor or other at least partially non-wet-oxidized liquor-containing liquid in the pulp suspension well before the cellulose pulp suspension is fed to a reactor (17) for oxygen delignment. Process according to one of Claims 1 to 4, characterized in that the wet-oxidized return liquor obtained from the oxygen atom is used at least in part (41) as an impregnation liquid in an atmospheric impregnation step (15) for the cellulose pulp suspension, the oxygen-oxidized return liquor impregnating from the oxygen atom. and essentially replaces the preceding cooking liquor or other at least partially non-wet oxidized liquefied liquid in the pulp suspension well before the cellulose pulp suspension is fed to a reactor (17) for oxygen delignment. Process according to Claim 1, characterized in that the sodium hydroxide is added in an amount so that in the pulp suspension a buffering concentration of sodium carbonate (mainly Na 2 CO , 20 mol per liter of pulp suspension, having a pH value exceeding 11.0, preferably exceeding 1 1.6. characterized in that components precipitated in the oxygen atom, which precipitate is promoted by the wet oxidation process, are separated by a separating device adjacent to the oxygen atom 10. A method according to claim 9 9945 se4.doc 01-10-05 10 15 20 25 30 ll. 12. 1 3. 9945se 4.doc 522 161 23 of: eo Process according to claim 9, characterized in that magnesium sulphate is added to be present during the wet oxidation in the oxygen atom, whereby primarily the precipitation process is improved, but secondarily also that the level of viscosity of the pulp after oxygen alignment is increased. Process according to one of Claims 9 to 1, characterized in that the precipitation process is enhanced by the addition of a thickener, for example of the polyethylene oxide type. Device for selective oxygen delignment in at least one oxygen reactor (17) of pulp suspensions with improved selective capping reduction of the pulp suspension after the oxygen delignment, characterized in that a washing device (20) is arranged in the manufacturing process of cellulose pulp extra to an oxygenator (30), where the oxygen atom (30) is a pressurized vessel with an oxygen-containing gas supply device (31) in which oxygenator wet oxidation of the liquor takes place, the oxygen atom being a pressure vessel capable of withstanding pressures of the order of 1.5 15 bar, preferably 10-12 bar, and that means (32a, 32b) are provided for obtaining and maintaining this pressure in the oxygen atom during substantially the entire wet oxidation process and preferably also during continuous input and discharge of the return liquor - after which the wet oxidized liquor is returned in part fl desolate (41 or 42) via a supply line t an exchange unit or diluent unit (12 or 15) in the manufacturing process prior to the supply of the cellulosic pulp to the oxygen reactor (17), which exchange unit or diluent unit (12 or 15) comprises means for exchanging a lye / boiling liquid present in the cellulosic pulp suspension with the return exchange unit respectively dilution with the wet oxidized return liquor. -that the oxygen atom has a volume that allows all recycled liquor that is recycled to receive 10 15 20 25 30 14. 1 5. 1 6. 1 7. 1 8. 1 9. 9945se4.doc 522 161 24 o u u u o o n | ø v; a residence time of 5-30 minutes, preferably 5-20 minutes, in the oxygenator, so that the carbonate and bicarbonate ions formed there give the liquor a buffering capacity, and that in close proximity to the oxygenator, preferably integrated with the oxygenator at least at its discharge end, a sodium hydroxide supply device 35 is provided. Device according to claim 13, characterized in that the replacement unit comprises a washing device (12) in which the liquor / boiling liquid is suspended in cellulose pulp suspension before conduction to the washing device is displaced / replaced by the return oxidized wet liquor in the oxygenator (30). Device according to any one of the preceding claims 13-14, characterized in that the exchange unit comprises a storage empty / impregnating vessel (1 5) in which cellulose pulp suspension is impregnated with the return oxidized wet liquor in the oxygenator (30). Device according to claim 13, characterized in that the supply device (35b) for sodium hydroxide is arranged to supply sodium hydroxide to a first part fl desolate (41) from the oxygenator (30), a second part fl desolate (42) not being supplied with sodium hydroxide and instead being fed directly to a washing device ( 12). Device according to one of the preceding device requirements, characterized in that a separating device (50) for precipitated material is arranged in close connection with the oxygenator. Device according to claim 17, characterized in that the separation device (50) comprises at least one of a) a filter, b) a sedimentation device, or c) an flotation device. Device according to claim 17, characterized in that the separating device (50) is arranged directly after the oxygen atom. Device according to claim 17, characterized in that the separating device (50) is arranged integrated with the oxygenator. 9945se4.doc 01-10-05